Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications 3030362671, 9783030362676

Table of contents :
Preface
Contents
About the Editors
Contributors
Part I: Fundamentals of Nanomaterials and Nanocomposites
1 Nanomaterials and Nanocomposites: Classification and Toxicity
Introduction
Nanomaterial Classification
Nanomaterial Size
Nanomaterial Morphology
Nanomaterial Dimensionality
Composition, Crystallinity, Uniformity, and Aggregation
Nanocomposite Classification
Nanofiller-Reinforced Macroscale Matrix
Composites with Both Matrix and Fillers at Nanoscale
Release of Nanoparticles from Nanocomposites
Airborne Nanoparticles During Handling of Nanofillers
Release of Nanoparticles from Cutting of Nanocomposites
Drilling of Nanocomposite and Release of Nanoparticles
Sanding of Nanocomposites and Release of Nanoparticles
Crushing of Nanocomposite and Release of Nanoparticles
Thermal Stress of Nanocomposites and Release of Nanoparticles
Fracture of Nanocomposites and Release of Nanoparticles
Nanocomposite Photodegradation and Release of Nanoparticles
Nanocomposite Biodegradation
Diffusion, Desorption, or Dissolution of Nanoparticles from Nanocomposites into Liquid
Incineration of Nanocomposites and Release of Nanoparticles
Toxicity of Nanoparticles
Conclusions
References
2 Recent Progress in All-Inorganic Hybrid Materials for Energy Conversion Applications
Introduction
Fabrication and Components of all-Inorganic Perovskite Solar Cells
Progress in all-Inorganic Perovskite Solar Cells
Conclusion and Future Prospective
References
Part II: Types of Nanomaterials and Nanocomposites
3 Carbon-Fiber Composites: Development, Structure, Properties, and Applications
Introduction
Carbon Fibers
Synthesis of Carbon Fibers
Spinning
Stabilization
Carbonization
Natural Fiber
Carbon Fiber Composites
Carbon Fiber Ceramic Composite (CFCC)
Carbon Fiber Metallic Composites
Carbon Fiber Polymeric Composite
Applications, Properties, and Structure of CFCs
Conclusions
References
4 Carbon Fiber Composites
Introduction
Carbon Fiber Composites
Polymer-Matrix Composites
Metal-Matrix Composites
Carbon-Matrix Composites
Ceramic-Matrix Composites
Hybrid Composites
Applications of Carbon Fiber Composites
Biomedical Applications
Automobiles Applications
Aerospace Applications
Electromagnetic Shielding Applications
Civil Construction Applications
Conclusion and Future Outlook
References
5 Nanomaterials: Applications in Biomedicine and Biotechnology
Introduction
Nanoparticles as Drug Carriers
Characterization of Nanoparticles for Drug Formulations
Targeted Drug Delivery
Diagnostic Nanomedicine
Nanopharmaceuticals
Characteristics
Applications
Categorization
Organic Nanoparticles
Liposomes
Polymeric Nanoparticles
Micellar Nanoparticles
Dendrimers
Inorganic Nanoparticles
Carbon-Based Nanostructures
Quantum Dots
Gold Nanoparticles
Superparamagnetic Iron Oxide Nanoparticles
Conclusion and Future Outlook
References
6 Application of Different Porous Materials
Introduction
Characterization Methods
Porosity
Gravimetric Method
Gas Expansion Method
Pore Morphology
Scanning Electron Microscopy (SEM)
Transmission Electron Microscopy (TEM)
Pore Size Distribution
Specific Surface Area
Permeability
Tensile Strength
Classifications of Porous Material
Ceramics
Membranes
Hydrogels
Foam
Broad Area of Application
Building Material
Water Treatment
Biomedical Applications
Reservoir Engineering
Conclusion
References
7 Bio-composites: Eco-friendly Substitute of Glass Fiber Composites
Introduction
Why Bio-composites?
Applications of Bio-composites
Automotive
Aerospace
Packaging
Structural
Miscellaneous
Natural Fibers
Selection of Fibers for Bio-composites
Plant-Based Reinforcement
Bast Fibers
Leaf Fibers
Seed Fibers
Agro-waste Fibers
Animal-Based Reinforcement
Hair Fibers
Silk Fibers
Hybrid Bio-composites
Biodegradable Polymers
Limitations and Remedies
Chemical Treatments
Mechanical Performance
Conclusions and Outlook
References
8 Biodegradable Polymer Composite Films for Green Packaging Applications
Introduction
Natural Fiber
Classification of Natural Fibers
Chemical Composition of Natural Fibers
Biodegradable Polymer Composites Based on Natural Fiber
Strategies for Enhancing the Interfacial Interaction, Preparation Techniques, and Testing Methods for Polymer/Natural Fiber Co...
Strategies for Enhancing the Interfacial Interaction for Polymer/Natural Fiber Composites
Physical Pre-treatment
Chemical Pre-treatment
Biological Pre-treatment
Physicochemical Pre-treatment
Preparation Methods for Green Packaging Film
Hand Layup
Injection Molding
Compression Molding
Extrusion
Solvent Casting Method
Melt Blending
Testing Methods
Water Vapor Transmission Rate and Water Vapor Permeability
Mechanical Test
Tensile Test
Impact Test
Optical Characteristic Test
Water Absorption Test
Thermal Stability Test
Conclusions and Future Outlook
References
9 Cellulose Nanofibers for Development of Green Composites
Introduction
The Structure of the Cellulose
Cellulose Nanofiber: A Brief Introduction
Processing and Extraction Methods of Cellulose Nanofibers
High-Pressure Homogenization
Micro Fluidizer
Grinding
Cryo-crushing
High-Intensity Ultrasonication
Commonly Used Surface Pretreatment Methods
Pulping Process
Bleaching Process
Alkaline Treatment
Enzyme Treatment
Steam Explosion
Mechanical Properties of Cellulose Nanofiber-Reinforced Polymer Composites
Conclusion
References
10 Cellulose-Based Nanomaterials for Water Pollutant Remediation: Review
Introduction
Types of Cellulose-Based Nanomaterials (CBNs)
Cellulose Nanocrystals (CNCs)
Cellulose Nanofibers (CNFs)
Cellulose Nanowhiskers (CNWs)
Bacterial Nanocellulose (BNC)
Algal Nanocellulose (ANC)
Wastewater Treatment by CBNs
Filtration-Processed Wastewater Treatments of CBNs
Sorption-Mediated Wastewater Treatments of CBNs
Removal of Heavy Metals
Removal of Organic Pollutants
Domestic Wastewater Treatments
Conclusion and Future Outlook
References
11 Development of Glass Ceramics from Agricultural Wastes
Introduction
Commercial-Type Glass Ceramics, Composition, and Characteristic Features
Potential Resources of Agricultural Waste to Be Used as Glass Ceramic Components
Conclusion and Further Outlook
References
12 Difficulties in Thin Film Synthesis
Introduction
Difficulties in Thin Film Synthesis
Material-Related Difficulties
Difficulties in Synthesis Stage
Characterization-Related Difficulties
Application-Related Difficulties
Conclusions and Further Outlook
References
13 Smectite Clay Nanoarchitectures: Rational Design and Applications
Introduction
Clays
From Natural Clays to Clays with More Open Structures
Acid-Activated Clays
Pillared Interlayered Clays (PILCs)
Porous Clay Heterostructures (PCHs)
Silica-Clay Nanocomposite
The Parameters to Obtain Clays with More Open Structures
Type of Clay Mineral
Surfactant and Cosurfactant
Pillaring Agent: Inorganic Precursor
Insertion of Heteroatoms with the Silica Structure
Removal of Organic Content
Application
Catalysts and Catalytic Support
Adsorbents
Filler Agent
Inorganic Host for Drug Encapsulation
Conclusions and Further Outlook
References
14 Greener Composites from Plant Fibers: Preparation, Structure, and Properties
Introduction
Green Composites
Processing Methods
Properties
Electronic and Optical Properties
Magnetic Properties
Mechanical Properties
Thermal and Adsorption Properties
Catalytic Property
Applications
Conclusion
References
15 Highly Stable Boron Carbide-Based Nanocomposites
Introduction
Experimental
Materials
Technologies
Characterization
Basic (Boron Carbide B4C) Component
Production from Elements
Chemical Synthesis from Liquid Charge
Synthesis in Electric Arc
Boron Carbide Based Metal-Ceramics
Binary Binder Alloys CuMn and CuTi
Ternary Binder Alloy CoNiTi
Boron Carbide Based Ceramic Composites
Ceramic Composite B4C-TiB2
Ceramic Composite B4C-ZrB2
Ceramic/Metal-Ceramic Composite B4C-TiB2-WC-Co
Conclusions and Further Outlook
References
16 Influence of a Twisting-Helical Disturber on Nanofluid Turbulent Forced Convection
Introduction
Geometry and Solution
Results and Discussion
Conclusion
References
17 Advanced Research Developments and Commercialization of Light Weight Metallic Foams
Introduction
Methods for Producing Metallic Foams
Casting
Methodology of Making Foams Followed by Investment Casting Technique
Manufacturing from Amorphous Alloys
Foams Made from Melt
Foaming of Metallic Melts
Foaming by Filler Materials
Foaming Through Sandwich Technology (Aluminum Foams)
Manufacturing of AFS and Their Products
Properties of Metallic Foams
Microstructural Properties
Mechanical Properties
Thermal Conductivity
Absorption Property (Sound Absorption)
Permeability
Application and Commercialization
Structural Applications
Filters
Industrial Applications
Constructional Uses
Other Applications
Conclusions and Further Outlook
Future Works
References
18 Mesoporous Nanomaterials: Properties and Applications in Environmental Sector
Introduction
Mesoporous Nanomaterials
Silica-Based Mesoporous Nanomaterials
Activated Carbon-Based Mesoporous Nanomaterials
Mesoporous α-Fe2O3 Nanoparticles
Mesoporous TiO2 Nanoparticles
Synthesis of Mesoporous Nanomaterials
Template-Based Synthesis
Hard-Template Synthesis
Soft-Template-Based Synthesis
Hybrid Templating/Soft- and Hard-Template Synthesis
Template-Independent Synthesis
Applications in Environmental Remediation
Adsorption of the Pollutants
Photocatalytic Cleavage of Dyes
Conclusions and Further Outlook
References
19 Nanocellulose for Sustainable Future Applications
Introduction
Sources of Cellulose
Plants
Tunicates
Algae
Bacteria
Nanocellulose
Applications of Nanocellulose
Applications of Nanocellulose in Environment
Industrial Effluents and Contaminated Waters
Conclusion
References
20 Nanoclay as Carriers of Bioactive Molecules Applied to Agriculture
Introduction
Chemical Structure and Methods of Modification of Nanoclays
Nanoclays as Vehicles of Bioactive Substances
Nutrients and Fertilizers
Biostimulants
Toxicity: Impact on Soil and Plants
Conclusions and Further Outlook
References
21 Nanoclays as Eco-friendly Adsorbents of Arsenic for Water Purification
Introduction
A Brief History of Arsenic: The Past and the Present
Global Distribution of Arsenic
Arsenic Removal
Adsorption Techniques
Current Developments in Use of Natural and Functionalized Nanoclays for Removal of Arsenic by Adsorption Processes
Conclusions and Further Outlook
References
22 Nanomaterials from Agrowastes: Past, Present, and the Future
Introduction
Silver Nanoparticles from Agrowastes
Gold Nanoparticles (AuNPs) from Agrowastes
Synthesis of Graphene Oxide from Agrowastes
Production of Solar Grade Silicon Nanoparticles from Agrowastes
Production of Amorphous Silica Nanoparticles from Agrowastes
Carbon Nanomaterials from Agrowastes
Synthesis of Zinc, Nickel, Platinum, Palladium, and Magnetic Iron Oxide Nanoparticles from Agrowastes
Conclusion
References
23 Nanoporous Metallic Foams for Energy Applications: Electrochemical Approaches for Synthesizing and Characterization
Introduction
Nanoporous Electrodes Used for Fuel Cells
Pt-Based Electrodes for Fuel Cells
Pt-Transition Metal Oxides Electrodes for Fuel Cells
Non-noble Transition Metals as Electrodes for Fuel Cells
Nanostructured Electrodes for Supercapacitors
Electrodes for Electric Double-Layer Capacitors
Electrodes for Pseudocapacitors
Electrodes for Asymmetric Supercapacitors
Nanostructured High-Capacity Electrodes for Li-Batteries
Anode Materials
Cathode Materials
Conclusions and Future Outlook
References
24 Plant Fibers-Based Sustainable Biocomposites
Introduction
Types of Composite Materials
Classification Based on Matrix
Metal Matrix Composites
Ceramics Matrix Composites
Polymer Matrix Composites
Classification Based on Reinforcement
Glass Fiber-Reinforced Composites
Natural Fiber-Reinforced Composites
Cellulose Whisker-Based Composites
Pretreatments of Natural Fiber
Physical Methods of Pretreatments
Chemical Methods of Pretreatment
Mercerization of Natural Fiber
Acetylation of Natural Fibers
Peroxide Treatment of Natural Fibers
Graft Copolymerization of Natural Fibers
Modification of Natural Fibers by Using Coupling Agents
Natural Fiber Versus Synthetic Fiber
Natural Fiber-Based Composites
Jute-Based Composites
Sisal-Based Composites
Cotton-Based Composites
Kenaf-Based Composites
Hemp-Based Composites
Abaca-Based Composites
Flax-Based Composites
Bagasse-Based Composites
Fabrication Techniques
Problems with Green Composites
Carbon Footprint and Green House Gas Emission
Conclusion and Future Prospect
References
25 Polymeric TiO2 Nanocomposites for Development of Fouling-Resistant Membranes for Wastewater Treatment
Introduction
Classification of Membranes
Classification Based on Pore Size
Classification Based on the Composition of Materials
Classification Based on Module/Configuration
Classification Based on Materials Employed for the Synthesis of Membranes
Classification Based on Driven Force
Classification Based on Membrane Flow Configurations/Operation Mode
Polymeric Materials Used for the Preparation of Membranes
General Characteristics of Polymeric Materials
Membrane Fouling
Types of Membrane Fouling
Quantification of Membrane Fouling
Consequences of Membrane Fouling
Methods to Improve Membrane Fouling Characteristics
Strategies to Improve Membrane Performance
Surface Modification of the Membranes with Other Hydrophilic Polymeric Materials
Surface Modification of Membranes by Incorporation of NPs: Formation of Nanocomposite Membranes
Polymer-TiO2 Nanocomposite (PTN) Membranes
Blending of TiO2
Surface Deposition of TiO2
Entrapping of TiO2 in Polymer Matrix
Improvement in Stability and Performance of Polymer Nanocomposite Membranes
Strategies Adopted for the Preparation of Stable Polymer-TiO2 Nanocomposite Membranes
In Situ Preparation of TiO2
In Situ Polymerization of Membranes
Self-Assembly of TNPs
Modification of TNPs Surface
Chemical Modification of TNPs for Membrane Preparation
Recent Approaches to Improve the Stability and Antifouling of Polymer-TiO2 Nanocomposite Membrane
Conclusions and Further Outlook
References
26 Porous Materials for Applications in Energy and Environment
Introduction
Porous Materials
Peculiarity of Aerogels
Synthesis of Aerogels by the Sol-Gel Method
Hydrolysis and Condensation
Aging
Drying
Supercritical Drying
Ambient Pressure Drying
Freeze Drying
Applications: Energy and Environment
Conclusions and Further Outlook
References
27 Recycled Plastics and Nanoparticles for Green Production of Nano Structural Materials
Introduction
Polymers
Polyethylene
Polypropylene
Epoxy Resin
Polymer Additives
Fillers
Plasticizers and Lubricants
Antiaging Materials
Flame Retarders
Colorants and Blowing Agents
Cross-linking Agents
Properties of Plastics
Recycling of Plastic Wastes
Nano Structural Materials
Conclusions
References
28 Sustainable Conversion of Coconut Wastes into Useful Adsorbents
Introduction
Heavy Metal Pollution
Agriculture Waste as Adsorbent for Heavy Metal Removal
Coconut Adsorbent
Coconut Coir
Coconut Copra Meal
Coconut Fiber
Coconut Husks
Coconut Pith
Coconut Shell
Conclusion
References
29 Roadmap of Nanomaterials in Renewable Energy
Introduction
Artificial Photosynthesis
Batteries
Biofuels
Carbon Dioxide Capture
Energy Storage
Fuel Cells
Hydrogen Energy
Phase Change Materials
Solar Cells
Thermoelectric Generators
Conclusions and Further Outlook
References
30 Nanostructured Composite Modifying Coatings for Highly Efficient Environmentally Friendly Dry Cutting
Introduction
Tool Materials with Wear-Resistant Coatings for Environmentally Friendly Dry Cutting
Coatings for Environmentally Friendly Dry Cutting: Specifics of the Structures and Phase Compositions of the Coatings
Influence of the Coating Thickness on the Cutting Properties of the Tools
Influence of the Thicknesses of the Coating Nanolayers on the Cutting Properties of the Tools
Influence of the Composition of the Coating on Its Performance Properties
Cutting System with the Compensation for CF´s Physical Effects
Conclusion
References
31 Titanium Dioxide Nanoparticles
Introduction
Chemical and Physical Properties
Synthesis
Application Areas
Health Effects
Exposure Routes and Limits
Measurement Techniques of Titanium Dioxide
Conclusions and Further Outlook
References
32 Wear-Resistant Metals and Composites
Introduction
Recent Developments in the Production of Wear-Resistant Materials and Metal Matrix Composites
Fabrication of Wear-Resistance Composites and Nanocomposites
Powder Metallurgy (PM)
Diffusion Bonding
Stir Casting
Disintegrated Melt Deposition
Liquid Metal Infiltration
Semisolid Casting (SSC)
Semisolid Hot Pressing
Thermal Spraying
Additive Manufacturing (AM)
Application of Wear-Resistance Materials
Conclusion
References
Part III: Synthesis of Nanomaterials
33 Biosynthesized Gold and Silver Nanoparticles in Cancer Theranostics
Introduction
Cancer, Statistics, Conventional Therapy, and Challenge
Nanomedicine, an Alternative Approach
Biosynthesis of Nanoparticles: Advantages over Chemical Synthesis
Biosynthesized Gold Nanoparticles in Cancer Theranostics: Recent Advancements and Patents
Biosynthesized Silver Nanoparticles in Cancer Theranostics: Recent Advancements and Patents
Challenge and Possible Solution
Conclusions and Further Outlook
References
34 Sustainable Synthesis of Greener Nanomaterials: Principles, Processes, and Products
Introduction
Economic Contributions of Greener Nanomaterials for Sustainable Growth
Green Synthesis of Nanoparticles and Applications
Green Synthesis of Biomass Nanocomposites
Green Synthesis of Metal and Metal Oxide Nanoparticles
Green Synthesis of Metallic Nanoparticles by Plants
Green Synthesis of Iron Nanoparticles
Metallic and Metal Oxide Nanoparticles from Agricultural Waste
Conclusions and Further Outlook
References
35 Nanomaterials Synthesis and Their Eco-Friendly Applications
Introduction
Nanoparticle Synthesis Methods
Chemical Approach
Green Approach
Biological Approach to Green Synthesis
Viruses
Bacteria
Fungi
Algae
Plants
Other Natural Materials
Factors Affecting Synthesis
Decontamination Properties of Nanoparticles/Nanocomposites
Toxicity and Removal of Heavy Metals
Toxicity of Anions and Their Removal
Hazards of Dyes and Their Removal
Removal of Dyes with Metal Nanoparticles
Removal of Dyes with Natural Adsorbents
Antibiotics and Antimicrobial Properties of Nanoparticles/Nanocomposites
Copper Nanoparticles
Zinc Nanoparticles
Silver Nanoparticles
Gold Nanoparticles
Iron Nanoparticles
Other Metal Nanoparticles with Their Antimicrobial Activity
Conclusions and Further Outlook
Conclusion
Future Aspects
References
36 Green Synthesis and Application of Metal and Metal Oxide Nanoparticles
Introduction
Numerous Synthesis Methodologies of Metal NPs
Green Synthesis Definition
Nanomaterial Characterization Techniques
Raman Spectroscopy
Fourier Transform Infrared Spectroscopy
Sample Preparation and Measurement for FTIR
X-Ray Diffraction (XRD)
UV-Vis Spectroscopy
Photoluminescence Spectroscopy
Light Scattering Method
Surface Area Analysis (BET Method)
Transmission Electron Microscopy
Principles of TEM
Sample Preparation for TEM
Scanning Electron Microscopy
Principles of SEM
Sample Preparation for SEM
Zeta Potential
Conclusions and Further Outlook
References
37 Nanomaterials Through Powder Metallurgy: Production, Processing, and Potential Applications Toward Energy and Environment
Introduction
Nanomaterials and Their Classification
Nanoparticles: Metals, Metal Oxides, and Ceramics
Metal Nanoparticles
Metal Oxide Nanoparticles
Ceramic Nanoparticles
Nanoparticles: Production via Powder Metallurgy and Mechanical Alloying
Powder Production via Physical Route: Atomization Technique
Powder Production via Chemical Route: Reduction of Metal Oxides
Powder Production via Mechanical Route: Mechanical Milling/Alloying
Nanoparticles: Processing via Powder Consolidation
Pressure-Based Densification
Sintering Densification Process
Hybrid Densification Process
Characterization Techniques for Nanoparticles
X-Ray Diffraction
Microscopic Techniques
Atomic Force Microscopy
Applications of Nanoparticles in Energy and Environment
Solar Energy Harvesting
Environmental Applications
Conclusions and Future Outlook
References
38 Scalable Synthesis of Nanomaterials
Introduction
Top-Down Approach
Mechanical Processes
Laser Ablation
Nanolithography
Bottom-Up Approach
Solvothermal Method
Sol-Gel Methods
Sonochemical Method
Aerosol-Based Processes
Gas Condensation
Chemical Vapor Deposition
Arc Discharge Generation
Plasma Process
Bioreduction
Hybrid Approaches
Nanofabrication with Templates
Conclusions and Further Outlook
References
39 Production and Applications of Biomass-Derived Graphene-Like Materials
Introduction
Importance to Prepare Graphene from Different Sources
Conversion Method
Activation Parameters
Different Biomass Precursors
Rice Husk
Coconut Shell
Peanut Shell
Glucose-Derived Graphene
Chitosan
Sugarcane Bagasse
Seeds
Applications
Energy Storage
Supercapacitors
Dye Adsorption
Gas Adsorption
Conclusion and Further Outlook
References
40 Gas-Phase Synthesis for Mass Production of TiO2 Nanoparticles for Environmental Applications
Introduction
Reactor Configurations
Particle Formation and Kinetics
Oxidation of TiCl4
TTIP Decomposition and Hydrolysis
Particle Formation
Size, Phase, and Structure Control
Operational Conditions
Additives
Recent Works on TiO2 in Gas-Phase Synthesis
Conclusions and Further Outlooks
References
41 Laser Additive Manufacturing of Nanomaterials for Solar Thermal Energy Storage Applications
Introduction
Solar Thermal Energy Storage
Sensible Heat Storage
Thermochemical Storage
Latent Heat Storage
Phase Change Materials (PCMs)
Thermal Stability and Heat Transfer
Nanomaterials
Fabrication Techniques of Nanomaterials
Laser Additive Manufacturing
SLM Process Parameters
SLM Scan Strategies
Meander Scanning Strategy
Chessboard/Island Scanning Strategy
Hull and Core Strategy
Pre-sintering Strategy
Challenges of the SLM Process
Cracks
Density
Surface Quality Issues
Microstructural Properties
Dislocations
Pores
Residual Stresses
Casting Segregation
Conclusion
References
42 Mechanical Performance of Nanocomposites and Biomass-Based Composite Materials and Its Applications: An Overview
Introduction
Nano Composites Fabrication Methods
Mechanical Performance of Nano Composite Materials
Biomass-Based Composites: Types and Fabrication Methods
Mechanical Performance on Biomass-Based Composite Materials
Applications of Nano Composite Materials
Applications of Biomass-Based Composite Materials
Conclusions and Further Outlook
Further Outlook
References
43 Nanomaterials from Biomass: An Update
Introduction
Nanomaterials
Nanomaterial Synthesis from Renewable Biomass
Physical Activation
Chemical Activation
Hydrothermal Carbonization
Nanomaterials from Plants, Microbes, and Actinomycetes
Cellulose Nanostructures
Starch Nanocrystals
Carbon Nanostructures
Applications
Conclusions and Further Outlook
References
44 Nanomaterials from Marine Environments: An Overview
Introduction
Engineered Nanoparticles in Aquatic Ecosystem
Metal Sulfide and Polysaccharide Based Colloids and Nanoparticles in Aquatic Environment
Analytical Characterization and Fractionation Techniques of Natural Aquatic Colloids
Characterization of Polysaccharide-Based Metallic Nanoparticles
Electrochemical Characterization of Nanoparticles in Marine Water Conditions
Voltammetric Measurements
Amperometric Measurements
Chronoamperometric Evaluation
Atomic Force Microscopy in Marine Biophysics
Conclusions and Further Outlook
References
45 Nanostructured Heterogeneous Catalysts for Biomass Conversion in Green Solvents
Introduction
Pretreatment of Lignocellulosic Feedstock
Cellulose
Hydrolysis of Cellulose to Glucose
Isomerization of Glucose to Fructose
Saccharides to HMF
Hemicellulose
Dehydration of Xylose to Furfural in Homogeneous Acidic Catalyst
Dehydration of Xylose to Furfural by Heterogeneous Catalyst
Furfural to Further Value-Added Chemicals
Lignin
Conclusion and Further Outlook
References
46 Thermochemical Conversion of Biomass Waste-Based Biochar for Environment Remediation
Introduction
Biomass Resources and Selection
Primary Resources
Secondary Resources
Tertiary Resources
Biomass Conversion Technologies
Biochemical Conversion
Anaerobic Digestion
Fermentation
Mechanical Extraction
Thermochemical Conversion
Combustion
Gasification
Pyrolysis
Pyrolysis Products
Bio-oil
Biochar
Biochar for Agro-environmental Benefits
Biochar for Agronomic Benefits
Biochar as an Adsorbent for Pollutant Removal
Mechanism of Heavy Metal Removal by Biochar
Conclusions and Further Outlook
References
47 Microbial Synthesis of Gold Nanoparticles and Their Applications as Catalysts
Introduction
Gold Nanoparticles as Catalysts
Green Synthesis of Gold Nanoparticles Using Microbial Cell Factories
Nitroaromatics and Dyes: Environmental Problem
Catalytic Performance of Gold Nanoparticles Prepared from Microbial Sources
Supported Gold Nanoparticles Catalysts
Importance of Aromatic Amines and their Preparation
Supported Gold Nanoparticles Catalyst for the Hydrogenation/Reduction of Nitroarenes
Mechanistic Details
Bio-Supported Metal Nanoparticles as Heterogeneous Catalysts
Bio-Supported Gold and Palladium Nanoparticles for the Catalytic Reduction of Nitroarenes
Conclusions and Further Outlook
References
Part IV: Main Processes Using Nanomaterials
48 Trends on Synthesis of Polymeric Nanocomposites Based on Green Chemistry
Introduction
Use of Ultrasound in the Processing of PNCs
Pre-dispersion/Exfoliation of NPs
Dispersion/Exfoliation of NPs with the Polymer
Embedding of NPs on Finished Products
Use of Microwaves in the Processing of PNCs
Treatment of NPs Before PNCs Processing
During the Mixing or Synthesis of the Polymer
After Processing the PNCs
Use of Plasma in PNCs Processing
Treatment of NPs Prior to PNCs Processing
Plasma Treatment After Processing of the PNCs
Perspectives for Plasma in NPCs Applications
Technical Considerations to Define the Use of Green Technologies in the Synthesis of Polymer Nanocomposites
Functionalized NPs
Polymer Degradation
Damage on the NPs
Conclusions and Additional Perspectives
References
49 Artificial Photosynthesis
Introduction
Artificial Photosynthesis Principle and Mechanism
Artificial Photosynthesis Applications and Different Design Aspect
Production of Hydrogen by Artificial Photosynthesis
Production of Hydrogen Peroxide by Artificial Photosynthesis Process
Application of Artificial Photosynthesis for CO2 Capture and Production of Biofuel
Conclusion and Future Outlook
References
50 Environmentally Benign Synthesis of Nanocatalysts: Recent Advancements and Applications
Introduction
Methods of Fabrication
Phytogenic Fabrication
Bacterial-Assisted Fabrication
Mycogenic Fabrication
Phycogenic Fabrication
Microwave/Sonochemical-Assisted Fabrication
Ionic Liquid-Assisted Fabrication
Catalytic Degradation of Various Organic Pollutants
Synthetic Azo Dyes
Pesticides
PAHs and Aromatic Amines
Phenols
Mechanism of Degradation and Fate of Contaminants
Conclusions and Further Outlook
References
51 Improving the Performance of Engineering Barriers in Radioactive Waste Disposal Facilities: Role of Nano-materials
Introduction
Design Criteria for Engineering Barriers in Radioactive Waste Disposal Facilities
Wasteform and Container Design Criteria
Backfill and Buffer Design Criteria
Structural Materials and Other Barriers
Role of Nano-materials in the Safety of Radioactive Waste Disposal Facilities
Intrinsic Nanoparticles in Engineering Barriers
Pseudo Nanoparticles in Engineered Barriers
Nanoparticles in Natural Barriers
Synthetic Nanoparticles in Engineered Barrier
Conclusion and Further Outlook
References
52 Nanomaterials for Water Splitting: A Greener Approach to Generate Hydrogen
Introduction
Water Splitting in Nature: Photosynthesis
Water Splitting: From Natural to Artificial Photosynthesis
Photocatalytic Water Splitting: Fundamental Aspects
Chemistry of Photocatalytic Water Splitting
Routes of Photocatalytic Water Splitting
Nanomaterials in Water Splitting: Green and Sustainable Applications
Future Challenges and Water Splitting
Conclusions and Further Outlook
References
53 Nanomaterials in Soil Health Management and Crop Production: Potentials and Limitations
Introduction
Nanomaterials
Natural Nanomaterials (NNMs)
Manufactured Nanomaterials (MNMs)
Synthesis of Nanoparticles
NonsolidNanomaterials
Solid Nanomaterials
Titanium Dioxide (TiO2)
Silver (Ag)
Silica (SiO2)
Nanomaterials as Nanofertilizers
Delivery of Fertilizers
Chemical
Biofertilizer and Micronutrient
Seed Treatment
Classification of Nanofertilizers on the Basis of Requirement
Macronutrient NFs
Nitrogen (N)-NMs
Phosphorus (P)-NMs
Potassium (K)-NMs
Calcium (Ca)-NMs
Magnesium (Mg)-NMs
Micronutrient Nanofertilizers
Iron (Fe)-NMs
Manganese (Mn)-NMs
Zinc (Zn)-NMs
Copper (Cu)-NMs
Molybdenum (Mo)-NMs
Nanocarrier-Based Fertilizers
Nutrient-Augmented Zeolites
Other Nanocarriers
Plant-Growth-Promoting NMs
Titanium Dioxide (TiO2)-NMs
Carbon Nanotubes
Silver (Ag)-NMs
Gold (Au)-NMs
Silicon Dioxide (SiO2)-NMs
Fate and Behavior of the Nanomaterials in Soil
Toxicity and Bioaccumulation of Metal-Based Nanomaterials
Aluminum Oxide
Gold
Copper
Titanium Dioxide
Limitations
Conclusion and Further Outlook
References
54 Photocatalysis for Wastewater Treatment with Special Emphasis on Plastic Degradation
Introduction
Comparison of Photocatalysis with Other AOPs
Photocatalytic Activity Under Different Sources
Visible Light
Solar Light
UV Radiation
Types of Wastewater Treated Under Photocatalysis
Textile Wastewater
Examination of Photocatalytic Detoxification
Mechanism of Photocatalytic Degradation
Method of Photocatalytic Degradation
Carrier Molecules
Parameters Manipulating Photocatalytic Membrane Reactor
Photobioreactor
Photodegradation of Pollutant
Types of Photobioreactor
Photocatalysis for Plastic Degradation
Mechanism of Catalyst in Degrading Plastic
Information of Plastic Degradation by Photocatalysis
Conclusion
References
55 Lanthanide-Based Compounds for Environmental Remediation
Introduction
Methods for Fabricating Lanthanide-Based Compounds
Solid State Reaction
Hydrothermal
Combustion
Precipitation
Pechini Approach
Photocatalytic Applications
Conclusions
References
56 Removal of Radioactive Wastes Using Nanomaterials
Introduction
Removal of Radioactive Wastes Using Engineered Nanomaterials
Nanocomposite Membranes
Titanate-Based Nanofibers and Nanotubes
Nanoscale Silver Iodide
Ag2O Grafted Titanate Nanolamina
Sodium-Copper Hexacyanoferrate
Biogenic Gold Nanomaterials
Prussian Blue Magnetic Nanoparticles
Conclusion
References
57 Visible Light-Driven Photocatalysts for Environmental Applications Based on Graphitic Carbon Nitride
Introduction
g-C3N4-Based Heterojunction as Photocatalysts for Pollutant Remediation
Construction of Heterojunctions of g-C3N4 with Longer Bandgap Semiconductors
Designing the Heterojunctions Between g-C3N4 and Similar or Smaller Bandgap Semiconductors
Architecture of Z-Scheme-Type Heterojunction Between g-C3N4 and Other Semiconductors with Pertinent Band-Edge Potential
Conclusions and Further Outlook
References
58 Highly Efficient Electrocatalytic Water Splitting
Introduction
Fundamentals of Electrocatalytic Water Splitting
Reaction Mechanism, Kinetics, Descriptor, and d-Band Theory
Parameters for Catalytic Performance Evaluation
Methodology for Electrochemical Measurements
Relationship Between Material Structure and Catalytic Activity
Metal/Heteroatoms Doping
Oxygen Vacancies
Constructing an Interface
Edge-Defect Engineering
Strain Engineering
Compositing with Conductive Substrates
Micromorphology and Porous Structure
State-of-the-Art HER Electrocatalysts
Noble Metal-Based Nanomaterials
Transition Metal-Based Materials
Metal-Free Materials
State-of-the-Art OER Electrocatalysts
Precious Metals and Their Oxides
Transition Metal Oxides
Layered Double Hydroxides (LDHs)
Metal-Organic Frameworks (MOFs)-Based Materials
Other Metal-Based Materials
Metal-Free Materials
Conclusion and Further Outlook
References
59 Recent Engineering Approaches for Lead-Free Piezoelectric Harvesters Design
Introduction
Eco-friendly Lead-free Piezoelectric Oxides and Their Composites with Polymers
Electronic Circuits for Interfacing the Piezoelectric Harvesters and the Load
Conclusion and Further Outlook
References
Part V: Main Products and Devices Obtained with Use of Nanomaterials
60 3D Printing of Fiber-Reinforced Polymer Nanocomposites: Additive Manufacturing
Introduction
Technologies in 3D Printing
Fused Filament Fabrication (FFF)
Composite Material Printing Methods
Types of FFF Printing Machines
Stereolithography (SLA)
Laminated Object Manufacturing (LOM)
Composite-Based Additive Manufacturing (CBAM)
Selective Laser Sintering (SLS)
Materials for 3D Printing
Various Testing Methods
Applications of 3D Printing
Conclusions
References
61 Advanced Functional Nanomaterials for Explosive Sensors
Introduction
Fabrication of Electrochemical Sensor
Nanomaterial-Based Electrochemical Sensors
Graphene Oxide as Electrode Modifier
Polyaniline-Decorated rGO as Electrode Modifier
MnO2/rGO as Electrode Modifier
Silver Nanoparticle-Decorated rGO as Electrode Modifier
Polyelectrolyte-Functionalized Graphene as Electrode Modifier
rGO/SrTiO3 as Electrode Modifier
Conclusion and Future Prospective
References
62 Application of Perovskite-Based Nanomaterials as Catalysts for Energy Production Fuel Cells
Introduction
General Structure of Perovskites
Properties and Applications of Perovskites
Hydrogen Evolution Reaction
Oxygen Evolution Reaction
Methanol Electrooxidation
Hydrazine Oxidation
Conclusions and Further Outlook
References
63 Appraisal of Solar Radiation with Modelling Approach for Solar Farm Design
Introduction
Electrical Generation, Climate, and Sun Power Potential in Konya-Elazğ
Solar Radiation Intensity Calculation
Horizontal Surface
Daily Total Solar Radiation
Daily Diffuse Solar Radiation
Momentary Total Solar Radiation
Momentary Direct and Diffuse Solar Radiation
Calculating Solar Radiation Intensity on Inclined Surface
Momentary Direct Solar Radiation
Momentary Diffuse Solar Radiation
Reflecting Momentary Solar Radiation
Total Momentary Solar Radiation
Method
Solar Radiation Attributes
The Selection of Third-Generation Solar-Cell Materials in Solar Farm Design
Conclusions and Further Outlok
References
64 Biobased and Biodegradable Polymer Nanocomposites
Introduction
Polymer: Biobased and Biodegradable Polymers
Biobased Polyamides
Polysaccharides
Polylactic Acid (PLA)
Polyhydroxyalkanoate (PHA) Polyhydroxybutyrate (PHB)
Proteins
Succinate Polymers
Biobased Polyethylene (Bio-PE)
Biobased Poly(Ethylene Terephthalate) (PET) and Poly(Trimethylene Terephthalate) (PTT)
Cashew Nutshell Liquid and Vegetable Oils
Bionanocomposites
Classification of Bionanocomposites
Bionanocomposites: Processing and Properties
Conclusion and Futher Outlook
References
65 Biobutanol: A Promising Alternative Commercial Biofuel
Introduction
Properties of Butanol
Different Methods of Butanol Production
Chemical Process
Biological Process
Microorganisms in the Production of Biobutanol
Clostridia
Escherichia coli
Cyanobacteria
Biomasses for Biobutanol Production
Other Different Methods to Enhance the Yield of the Product
Immobilization of Cells Increase the Cell Density
In Situ Product Removal
Synergies with Bioethanol and Biodiesel
Applications of Biobutanol as Biofuel
Butanol as an Alternative for Gasoline in Spark-Ignited (SI) Engines and Compression Ignition (CI)
Merits and Demerits of Butanol
Future Prospects
Nanotechnology in the Conversion of Biomass into Biofuel
Nanomaterials in Pre-processing of Raw Biomass
Use of Nanoparticles as Additives for Biofuel Applications
Performance Analysis of Nanoadditives with Biofuel Blend
The Effect of Nanoadditives on Brake Thermal Efficiency
The Effect of Nanoadditives on Brake-Specific Fuel Consumption
The Effect of Nanoadditives on Brake Power
Factors Affecting the Performance of Nanoparticles in Biofuel Production Processes
Types of Approach
Temperature
Pressure
pH
Size of Nanoparticles
Future Direction
Adverse Effects of Nanoadditives on Release into the Atmosphere
Conclusion and Further Outlook
References
66 Biolubricants with Additives in Malaysia for Tribological Applications
Research Trends on Biolubricants with Additives in Malaysia for Tribological Applications
Biolubricants with Lubricant Additives
Biolubricants with Nanoparticle Additives
Research Trends on Palm-Based Oil with Additives in Malaysia for Tribological Applications
Palm-Based Oil with Lubricant Additives
Palm-Based Oil with Nanoparticle Additives
Research Trends on Jatropha Oil with Additives in Malaysia for Tribological Applications
Jatropha-Based Oil with Lubricant Additives
Jatropha-Based Oil with Nanoparticle Additives
Research Trends on Bio-Based Oil with Additives in Malaysia for Tribological Applications
Bio-Based Oil with Lubricant Additives
Bio-Based Oil with Nanoparticle Additives
Conclusions and Further Outlook
References
67 Dye-Sensitized Solar Cells
Introduction
Working Principle
Components and Materials
Substrate
Photoanode Materials
Morphology
Light Scattering
Plasmonic Nanoparticles
Doping
Composites
New Oxides
Sensitizers
Metal Complex Dyes
Metal-Free Organic Dyes
Natural Dyes
Electrolyte
Counter Electrode
Pt Counter Electrodes
Carbonaceous Materials
Conducting Polymers
Alloys
Transition Metals
Conclusions
References
68 Ecofriendly Composite/Nanocomposite from Discarded Addition and Condensation Polymers
Introduction
Most Common Polymer Recycling Technologies
Recycling of Addition Polymers
High-Density Polyethylene (HDPE)
Low-Density Polyethylene (LDPE)
Polypropylene (PP)
Polystyrene (PS)
Poly(Vinyl Chloride) (PVC)
Recycling of Condensation Polymers
Poly(Ethylene Terephthalate) (PET)
Polycarbonate (PC)
Conclusion
References
69 Nanocatalysts for Biofuels Production
Introduction
Nanocatalysts in the Production of Liquid Fuels from Biomass
First-Generation Biofuel
Biodiesel Derived from Triglycerides (Vegetable Oils/Animal Fats)
Biodiesel Production
Bioethanol from Starch-Rich Biomass
Second-Generation Biofuels
Composition of Lignocellulosic Biomass
Principal Routes of Conversion of Lignocellulosic Biomass
Hydrolysis
Conversion of Lignocellulosic Biomass to HMF
Conversion of Lignocellulosic Biomass to Furfural
Production of 2,5-Dimethylfuran (DMF) and 2-Methylfuran (MF)
Thermochemical Processes
Liquefaction
Pyrolysis of Biomass
Gasification of Biomass
Conclusion and Further Outlook
References
70 Nanocomposites for Supercapacitor Application
Introduction
Types of Electrical Energy Storage Devices
Battery
Fuel Cell
Electrostatic Capacitor
Supercapacitor
Electric Double Layer Capacitor (EDLC)
Pseudocapacitor
Hybrid Supercapacitor
Electrode Materials
Carbon Materials
Composite Materials
Conducting Polymers
Metal Oxide
Ruthenium Oxide
MnO2
Cobalt Oxide
NiO/Ni(OH)2
ZnO
TiO2
Electrolytes
Synthesis Techniques
Sol-Gel
Hydrothermal/Solvothermal Technique
Coprecipitation Technique
In-Situ Polymerization
Vacuum Filtration Technique
Chemical Vapor Deposition (CVD)
Electrochemical Deposition Technique
Applications
Automobiles and Transportation
Public Sector
Medical and Industrial Applications
Defense and Military Applications
Conclusions and Further Outlook
References
71 Nanopesticides, Nanoherbicides, and Nanofertilizers: The Greener Aspects of Agrochemical Synthesis Using Nanotools and Nano...
Introduction
Nanoagroparticles
Nanoagroparticles Applications
Green Synthesis of Nanoagroparticles
Biological Synthesis
High-Pressure Homogenization
Ionic Pregelation and Polyelectrolyte Complexation
Oil-in-Water Method
Conclusions and Further Outlook
References
72 Nanotechnology for Electrical Energy Systems
Introduction
Energy Storage
Supercapacitor
Types of Supercapacitor
Nano Supercapacitor
Lithium-Ion Battery
Merits
Demerits
Nano-Lithium Batteries
Utilizing Nanotechnology in the Fabrication of Batteries Offers the Below Advantages
Comparison Between Li-Ion Battery and Supercapacitor
Hydrogen Fuel Battery
Pressurized Tank Storage
Cryogenic Liquid Hydrogen Storage
Hydrogen Acceptance in Metal-Based Compounds
Nano Hydrogen Storage
Efficient Utilization
Light-Emitting Diode (LED)
PlaCSH Light-Emitting Diode
PC Light-Emitting Diode
Nanosensors
Identification of Bacteria That Are Magnetically Isolated Using Surface-Enhanced Raman Spectroscopy (SERS)
Fabrication and Utilization of Gold/Reduced Grapheme Oxide (rGO) Nanocomposite-Based Biosensor
Bacteria Identification Through Magneto-Fluorescence Approach
Bacterial Identification Through Mechanical Nanosensors Method
Nanosensors in Water Monitoring
Conclusion
References
73 Perovskite Materials in Photovoltaics
Introduction
What Is Perovskite?
Perovskite Solar Cells
Basic Principle and Fabrication of PSC Device
Origin of Perovskite Solar Cells
Conclusion and Future Prospective
References
74 Natural Polymer Composites for Environmental Applications
Introduction
Natural Polymer Composites
Cellulose-Based Composites
Chitosan-Based Composites
Alginates-Based Composites
Starch-Based Composites
Soy Protein-Based Composites
Hemicellulose-Based Composites
Wastewater Treatment Mechanisms
Adsorption
Catalytic Degradation
Conclusion and Further Outlook
References
75 Second Life of Polymeric-Based Materials: Strategies and Performance
Introduction
Current Waste Management Practices in Composites
Recycling Methods
Mechanical Recycling
Chemical Recycling
Thermal Recycling
Comparison
Quality of Recycled Composite Materials
Quality of Products Made with Recycled Thermosetting Matrix/Fiber Composites
Quality of Products Made with Recycled Thermoplastic Matrix/Fiber Composites
Legislation for Recycling Waste Composites
Conclusions and Further Outlooks
References
76 Super Capacitance of Metal Oxide Nanoparticles
Introduction
Principles of Supercapacitor
Electric Double-Layer Capacitor
Pseudocapacitor
Metal Oxide-Based Supercapacitor Electrode
Aqueous Asymmetric Supercapacitor
Ruthenium Oxide
Co3O4
Nickel Oxide
Fe2O3 and Fe3O4
VO2 and V2O5
MnO2 and Mn3O4
CuO
Metal-Ion Hybrid Supercapacitor
Li-Ion Hybrid Supercapacitor
Sodium-Ion Hybrid Supercapacitor
K-Ion Hybrid SuperCapacitor
Zn-Ion Hybrid Supercapacitor
Conclusions and Further Outlook
References
77 Synthetic Fibers from Renewable Sources
Introduction
Polylactic Acid
Chemical Description of the PLA and Its Generation
Environmental Impact of the PLA
Applications of the PLA
Properties and Processability of PLA to Make Textiles
Cellulose
Viscose
Modal
Cupro
Acetate
Lyocell Process (N-methylmorpholine-N-oxide - NMMO)
Celsol
Celulose Carbamate (Carbacell Technology)
Polyhydroxyalkanoates
Advantages and Disadvantages of the Use of PHA
Textile Processing of PHAs
Alginate
Polylactic Fe3O4/Graphene Oxide Composite Conductive Fiber
Polylactic-Hydroxyapatite Biocompatible Scaffolds
Polylactic-Polyhydroxyalkanoates Nonwoven for Biodegradable Mulches
Cellulose-Noble Metals Fibers with Color Fastness and UV Blocking Properties
Luminescent Cellulose Fibers for Anti-Counterfeiting Articles
Polyhydroxyalkanoate Medical Textiles and Fibers
Biodegradable and Anti-Flame Alginate Fibers
Carbon-Based Nanoparticle Sodium Alginate Fibers for Medical Applications
Antimicrobial Alginate Wound Dressing Containing Silver/Zinc Nanoparticles
Conclusions and Further Outlook
References
78 Fabrication of Electrochemical Sensors for the Sensing of Hazardous Compounds
Introduction
How to Prepare the Electrochemical Sensors?
Electrochemical Sensing of Hazardous Compounds
Sensing of Catechol
Sensing of Hydroquinone
Sensing of Hydrazine
Sensing of 2-Phenylphenol and Chlorophenol
Sensing of Hydrogen Peroxide
Sensing of Nitrite
Conclusions and Further Outlook
References
79 Multi-junction Polymer Solar Cells
Introduction
Materials for Multi-Junction Polymer Solar Cells
Device Structures of Multi-Junction Polymer Solar Cells
Normal Structure
Inverted Structure
Self-Passivating Structure
Parallel Structure
Advances in Multi-Junction/Tandem Polymer Solar Cells
Conclusions and Further Outlook
References
80 Current State and Prospective of Supercapacitors
Introduction
Principle and Device Structure of Supercapacitor
Conclusions
References
81 Enzyme Catalyzed Glucose Biofuel Cells
Introduction
Device Structure of Glucose Biofuel Cells
Preparation of Anode for Glucose Biofuel Cell Applications
SWCNT/FRT/Glucose Oxidase/GCE as Anode
Polypyrrole/FRT/Di/NADH/GDH/GCE as Bioanode
Graphene/FRT/GOx/GCE as Bioanode
A Hybrid Biocatalyst for Biofuel Cells
MnO2/PSS/Gph/FRT/GOx/GCE as Bioanode
Conclusions
References
82 Properties of Diamonds and Their Application in Photodetectors
Introduction
Properties of Diamond
Crystal Structure
Electrical Properties
Extrinsic Diamond
Optical Transmission and Absorption
Metal-Diamond Contacts
Exploiting Photodetection Mechanisms
Diamond Photoconductors
Diamond Schottky Barrier Photodiodes
Schottky Barrier
Dark Current
Electrode Structure of Diamond Detectors
Diamond UV Photoconductors
Spectral Responsivity
External Quantum Efficiency
Diamond Radiation Detectors
Charge Collection Efficiency
Charge Collection Distance
Energy Resolution
Radiation Hardness
Conclusions
References
Part VI: Applications of Nanomaterials and Nanocomposites
83 3D Printing for Energy-Based Applications
Introduction
Fundamentals of Additive Manufacturing
Fused Deposition Modeling
Digital Light Processing 3D Printing
Carbon Nanomaterials Materials in 3D Printing
Carbon Nanotubes
Fullerenes
Carbon Black
Graphene
Metal Nanoparticles and Nanowires
3D Printing Batteries and Energy Storage Devices
Fuel Cells
Solar Cells and Supporting Technologies
Conclusions and Further Outlook
References
84 Adsorption-Based Removal of Heavy Metals from Water Using Nano-akaganéites
Introduction
As and Se as Water Pollutants
Adsorption
Selection of an Appropriate Adsorbent
Selection of Nano-akaganéite
Synthesis Methods of Nano-akaganéites
General Characteristics of Fe Oxy-Hydroxides
Adsorption Experimentation and Characteristics with NA as Adsorbents
Conclusions and Further Outlook
References
85 Agro Wastes/Natural Fibers Reinforcement in Concrete and Their Applications
Introduction
Potential of Agro-Wastes
Agro Waste Usage in Construction Materials
Agro-Wastes: A Short Introduction
Some Common Types of Agro-Wastes and Natural Fibers
Cellulose Fibers
Jute Fibers
Bagasse Fiber
Coconut Coir
Hemp Fiber
Nettle Fiber
Bamboo Fiber
Mechanical Properties of the Natural Fiber-Reinforced Concrete
Conclusions/Future Recommendations
References
86 Applications of Photochemical Oxidation in Textile Industry
Introduction
Treatment Methods for Textile Wastewater
Chemical Treatment
Advanced Oxidation Processes
Photochemical Oxidation Processes
Mechanism of Photo-Oxidation
Homogeneous Photochemical Oxidation Processes
Vacuum UV (VUV) Photolysis
UV/H2O2
UV/O3
UV/O3/H2O2
Fenton and Photo-Fenton Techniques
Mechanisms of Dark Fenton and Photo-Fenton
Fenton´s Reaction
Photo-Fenton
Sono-Photo-Fenton Process
Sono-Electro-Fenton Process
Photo-Electro-Fenton Process
Combination or Sequential Treatment
Some Proposed Mechanisms of Different AOPs
Photocatalytic Processes
Photocatalytic Applications for Dye Degradation
Heterogeneous Photocatalysis
UV/TiO2
UV/ZnO
Transition Metal-Doped TiO2
Transition Metal-Doped ZnO
Conclusions
References
87 Application of Fly Ash for Oil-in-Water Emulsion Separation
Introduction
Fly Ash Material
History
Physical Properties
Chemical Properties
Applications
Overview of Membrane Processes
Materials Based Classification
Separation Process Based Classification
Industrial Applications of Membrane Technology
Polymeric Membranes
Ceramic Membranes
Ceramic Membrane Fabrication Methods
Preparation of Ceramic Membrane Supports
Ceramic Membrane Based Microfiltration of Oil-in-Water Emulsions
Designed Procedure for the Conversion of Fly Ash into Ceramic Membrane
Microfiltration of Oil-in-Water Emulsions
Analysis
Conclusions and Outlook
References
88 Current Water Treatment Technologies: An Introduction
Introduction
Purification Technologies for Drinking Water
Coagulation and Flocculation
Sedimentation and Filtration
Adsorption
Disinfection
Wastewater Treatment Processes
Primary Treatment
Secondary Treatment
Tertiary Treatment
Emerging Technologies of Water Treatment
Membrane Technology
Advanced Oxidation Processes
Water Reuse Technology
Conclusions and Further Outlook
References
89 Application of Iron Oxide Nanomaterials for the Removal of Heavy Metals
Introduction
Advantages of Iron Oxide Nanomaterials
The Development of Iron Oxide Nanomaterials as Adsorbent for Heavy Metals
Synthesis of Iron Oxide Nanoadsorbents
Goethite (α-FeOOH)
Hematite (α-Fe2O3)
Maghemite (γ-Fe2O3)
Magnetite Fe3O4(FeIIFe2IIIO4)
Ferrihydrite (Fe5HO84H2O)
Practical Application of Iron Oxide Nanomaterials in Effluent Treatment
Electroplating Wastewaters Treatment
Arsenic Treatment
Conclusions and Further Outlook
References
90 Application of Nanosilicon and Nanochitosan to Diminish the Use of Pesticides and Synthetic Fertilizers in Crop Production
Introduction
Applications of Nanosilicon and Nanochitosan in Agriculture
Silicon Nanomaterials
Definition
Obtaining Process
Differences between the Bulk Si and the NSi
Agricultural Use of Si NMs
Use of Si NMs to Modify Soil Properties and as Fertilizer Carriers
Si NMs as Pesticide Carriers to Increase Their Efficiency and Decrease Soil, Water and Atmosphere Pollution
Si NMs as Biostimulants to Increase Metabolic Efficiency and Tolerance to Biotic and Abiotic Stress
Use of Si NMs as Nanosensors
Nano-Chitosan
Definition
Obtaining Process
Differences between the Bulk Chitosan and NCs
Nanochitosan
Conclusions and Further Outlook
References
91 Degradation and Removal of Petroleum Hydrocarbons from Contaminated Environments Using Nanotechnologies and Nanomaterials
Introduction
Remediation
Nanotechnology-Based Methods for the Cleanup of Oil and Petro-chemical Wastes
Magnetic Nanocomposites
Aerogels
Micro- and Nano-TiO2
Nanosorbents and Dispersants
Carbon Nanostructures
Nanomembranes
Conclusions and Further Outlook
References
92 Degradation of Plastics Using Nanomaterials
Introduction
Photooxidative Degradation
Thermal Degradation
Ozone-Induced Degradation
Mechanochemical Degradation
Catalytic Degradation
Biodegradation
Nanoparticles as Enhancer for Degradation of Plastics
TiO2-Based Nanomaterials
Paramagnetic Iron Oxide Nanoparticles
Fullerene 60 Nanoparticles
Nanobarium Titanate Nanoparticles
Nanoplastics
Conclusion
References
93 Devising and Exploiting Functionalities of Nanocomposites for Removal of Organic Pollutants and for Disinfection
Introduction
Why Nanomaterials and Nanocomposites?
Functionalities and What to Expect from Different Functionalities?
Water Treatment
Water Disinfection
Wastewater Treatment: Removal of Pollutants
Removal of Organic Pollutants: Dye and Pigments
Removal of Organic Pollutants: API Pollutants
Removal of Organic Pollutants and Heavy Metal
Removal of Organic Pollutants: Hydrocarbon Removal
Removal of Organic Pollutants: Pesticides Removal
Removal of Endocrine Disrupters Synthetic Compounds
Nanotechnology and Nanocomposites: Futuristic Applications
Toxicological Aspects of Nanomaterials
Conclusion and Further Outlook
References
94 Doped Semiconductor Nanomaterials: Applications in Energy and in the Degradation of Organic Compounds
Introduction
Semiconductors
Doping of Semiconductor Nanomaterials
Methods of Synthesis and Doping of Semiconductor Nanomaterials
Sol-Gel
Coprecipitation
Hydrothermal
Combustion
Mechano-synthesis
Chemical Deposition in Steam Phase
Applications of Doped Semiconductor Nanomaterials
Degradation of Organic Compounds
H2 Production
Solar Cells
Up-Conversion Materials
Perspectives
Conclusions
References
95 Eco-friendly Nanomaterials in Agriculture: Biofortification, Plant Growth Promotion, and Phytopathogen Control
Introduction
Engineered Nanomaterials
Nano-encapsulation in Biodegradable Polymer for Slow and Controlled Release of Agrochemicals
Green Synthesis of Nanoparticles for Agriculture Using Plants and Microbes
Viruses as Nanopesticides for Phytopathogen Control
Carbon Nanomaterials
Nanomaterials as Biosensors
Pathogen Detection
Pesticide Detection
Conclusions and Further Outlook
References
96 Eco-friendly Nano-adsorbents for Pollutant Removal from Wastewaters
Introduction
Metallic Ions in Wastewaters
Organic Contaminants in Aqueous Solutions
Adsorbents for Pollutant Removal
Synthesis and Characterization of Eco-friendly Nano-Adsorbents
Direct Activation Method
Fusion Method
Ultrasound Method
Removal of Pollutant from Wastewaters Using Eco-friendly Nano-Adsorbents
Conclusion
References
97 Engineered Nanomaterials for Emerging Contaminant Removal from Wastewater
Introduction
Techniques
Batch
Continuous
Fixed Bed
Fluidized Bed
Nanofiltration
Engineered Nanomaterials for Adsorption and Nanofiltration
Nanoparticles as Adsorbent Devices
Metal Oxide (MO)-Based NPs
Clays
Carbon-Based Nanomaterials
Nanocomposite Materials as Adsorbent Devices
Polymer-Based Nanocomposites as Filtration Devices
Cost and Efficiency
Conclusions and Further Outlook
References
98 Environmentally Friendly Wastewater Treatment Methods for the Textile Industry
Introduction
Coloration of Textiles
Role of Water in Textile Wet Processing
Wastewater from Textile Wet Processing
Textile Wastewater Treatment
Primary Treatment
Secondary Treatment
Tertiary Treatment
Sustainable Textile Wastewater Treatment
Adsorption
Membrane Separation
Flotation
Ozonation
Ion-exchange
Evaporation
Solar Evaporation
Mechanical Evaporation
Coagulation and Electro-coagulation
Granular Activated Carbon (GAC)
Advanced Oxidation Process (AOP)
Rotating Biological Contactor (RBC)
Sequencing Batch Reactor (SBR)
Microorganisms, Clay, and Chitosan
Clay
Chitosan
Bioadsorbents in Wastewater Treatment
Citrus Fruits
Banana Fiber and Stem
Coir Fiber
Cotton Boll Peel
Coconut Shell Activated Carbon
Tea Waste
Other Bioadsorbents
Sustainable Sludge Management
Methods of Sludge Treatment
Anaerobic Digestion
Aerobic Digestion
Enzyme Treatment
Thermal Hydrolysis
Chemical Stabilization
Conclusion and Future Trends
References
99 Green Fiber-Reinforced Concrete Composites
Concrete
Types of Concrete
Concrete Properties
Problems of Concrete
Concrete Ingredients and Their Properties
Binding Material/Cement
Ordinary Portland Cement
Functions of Cement Manufacturing Materials
Sulfate-Resistant Cement
Low Heat Cement
Rapid Hardening Cement
Extra Rapid Hardening Cement
High Alumina Cement
Portland Slag Cement
Portland Pozzolana Cement
Super Sulfated Cement
Air Entraining Cement
White Cement
Masonry Cement
Quick Setting Cement
Aggregates
The Purpose of Aggregates
Types of Aggregates
Water
Admixtures
Fiber-Reinforced Concrete (FRC)
Effect of Fibers in Concrete
Factors Affecting the Properties of FRC
Types of Fibers Used in FRC
Fiber-Reinforced Concrete/Cement Composite
Effect of Different Fibers on the Properties of FRC
Glass Fibers
Polyolefins (Polypropylene and Polyethylene)
Steel Fibers
Natural Fibers
Fabrication Methods of FRC Composites
Problems During Fabrication of FRC
Applications of Fiber-Reinforced Concrete
Green Fiber-Reinforced Concrete (GFRC)
Advantages of FRC as Compared to Plain Concrete
References
100 Green/Eco-friendly Micropunching Techniques for Energy Application
Introduction
Micropunch
Punch-Foil Interaction
Microholes Distributed in Turbine Blades for Transpiration Cooling
Microhole Variations During the Micropunching
Conclusions and Further Outlook
References
101 High Entropy Alloys: Advance Material for Landing Gear Aerospace Applications
Introduction
Basics of Landing Gear
Shock Absorber
Brakes
An Aircraft Accident Related to Shimmy
Gear Walk
Suppression of Shimmy
Shimmy Dampers
Materials Selection for the Landing Gear
High-Entropy Alloys as Landing Gear Materials
High-Entropy Alloy Properties and Manufacturing Methods
Core Effects
High Entropy Effect
Sluggish Effect
Lattice Distortion Effect
Cocktail Effect
Crystal and Phase Formation
Simple Solid Solution
Simple Solid Solution of Crystal Structure
Elevated Temperatures of Phases
Powder Metallurgy of High-Entropy Alloy Composition
Powder Development
Conventional Consolidation Approaches
Mechanical Behavior of PM HEAs
High Strain Rate Distortion and High-Temperature Performance
Compression Performance
Distinct Properties of PM HEAs
Corrosion and Oxidation Effect
Friction Behavior
Alloying Strengthens Opportunities
Conclusion
References
102 Hydroxyapatite Nanomaterials for Environmental Applications in Wastewater Treatment
Introduction
Hydroxyapatite
Structure
Properties
Hydroxyapatite Nanomaterials
Nano-hydroxyapatite
Hydroxyapatite Nanocomposites
Applications of Hydroxyapatite Nanomaterials
Removal of Organic Compounds
Adsorption of Dyes
Photodegradation of Dyes
Another Species
Removal of Ionic Species
Fluoride
Radioactive Species
Metals
Conclusions and Further Outlook
References
103 Manganese Oxides: Synthesis and Application as Adsorbents of Heavy Metal Ions
Introduction
Synthesis de MnO by Soft Chemistry Techniques
MnO Synthesis by Sol-Gel Method
MnOx Synthesis by Pechini Method
MnOx Synthesis by Co-precipitation Method
MnOx Synthesis Using Microorganisms
Heavy Metal and Other Pollutants Removal by Mean of MnOx
Conclusions and Further Outlook
References
104 Materials for CO2, SOx, and NOx Emission Reduction
Introduction
CO2 Emission Reduction
CO2 Capture Technologies
Pre-combustion CO2 Capture Technologies
Post-combustion CO2 Capture Technologies
Oxyfuel Combustion
Materials for CO2 Adsorption
Materials for Chemical CO2 Adsorption
Materials with Grafted Amines or Impregnated with Amines
Metal Oxides and Salts
Materials for Physical CO2 Adsorption
Activated Carbon
Mesoporous Silica
Zeolites
Metal-Organic Frameworks
CO2 Storage Technologies
Opportunities for Using CO2 as a Raw Material for Chemical Synthesis
SOx Abatement Technologies
SOx Scrubber Technology
Post-combustion SOx Removal
Catalytic Reduction of SOx
SOx Adsorption
NOx Elimination Technologies
Nitrogen Oxides: Sources of Their Occurrence and Problems Associated with Emissions
Catalysis for NOx Removal
Conclusions and Further Outlook
References
105 Materials from Agricultural Wastes
Introduction
Manufacturing Silica from Agricultural Waste
Silica from Agricultural Residues
Synthesis of Nanosilica Materials
Silica Powders and Nanowires
Adsorbents from Agricultural Wastes
Heavy Metal and Dye Adsorption
Biosorption Using Agricultural Wastes
Energy Storage Materials from Agricultural Wastes
Supercapacitors
Manufacturing Concrete from Agricultural Wastes
Nano Additives and Binders
Lightweight Fiber-Reinforced Concrete
Biofuel from Agricultural Wastes
Cellulose Nanofibers from Agricultural Wastes
Synthesis of Nanoparticles from Agricultural Wastes
Conclusions and Further Outlook
References
106 Metal-Organic Framework-Derived Catalysts for Zn-Air Batteries
Introduction
Assembling the ZABs
Fundamental Cathodic Reactions
Mass Blocking for ORR/OER
Merits of MOFs for ZABs
MOF-Derived Catalysts for Zn-Air Battery
Metal-Coordinated N-Doped Carbons (M-N-C)
MOF-Derived Metal Oxide/Carbon Composites
Strategies to Enhance the Catalytic Performance
Conclusions and Challenges
References
107 Micro- and Nanotechnology Applied on Eco-friendly Smart Textiles
Introduction
Micro- and Nanofinishes Applied on Smart Textiles
Micro-/Nanocapsules
Cyclodextrins
Liposomes
Nanoparticles
Latest Developments in Nanotechnology for Eco-Friendly Smart Textiles
Self-Cleaning Properties
UV Protective Characteristic
Antioxidant, Antimicrobial, and Repellent Capacity
Conclusions and Future Trends
References
108 Nanotechnology in Textile Finishing: Recent Developments
Introduction
Nanofinishing Classification
Nanostructures
Application of Nanofinishing
Coating
Electroless Deposition
Sol-Gel
Factors Affecting the Sol-Gel Reaction
Precursor Properties
Temperature and Time
Solvent
Properties and Concentrations of Catalyst
Ratio of Water to Metal Alkoxide
Textile Finishing Based on Nanotechnology
Self-Cleaning (Lotus Effect)
Nano Material Based Self-Cleaning by Photocatalytic Action
Superhydrophobic
Using Electrospun Nanofibers
Oil/Water Separation
UV Protection
Soil/Stain Repellent
Finishing Based on Nanometals/ Metal Oxides
TiO2
TiO2 in Photocatylytic
ZnO
Copper
Sensing Nanofinishes for Textiles
Photochromic
Conclusion and Further Outlook
References
109 Nano-catalyst Production Using Nano-biotechnology
Introduction
Types of Nanoparticles
One Dimension
Two Dimension
Three Dimension
Classification of Nanoparticles Based on Their Properties
Nanoparticles Production
Naturally
Environmentally
Biologically
Artificially
Role of Bio-nanotechnology
Characterization of Nanoparticles
Nanotoxicology
Conclusions and Further Outlook
References
110 Nanoinsulation Materials for Energy Efficient Buildings
Introduction
Nanoinsulation Materials for Buildings
Nanoporous Insulation Materials: Aerogels
Origin and Properties
Preparation Method
Aerogel Building Applications
Fiber-Reinforced Aerogel Blankets
Manufacturing and Performances
FRABs Building Applications
Vacuum Nanoporous Insulation Panels
Specifications and Performances
VIPs Building Applications
Transparent Aerogel Insulation Materials
Conclusions and Future Trends
References
111 Nanoagriculture: A Holistic Approach for Sustainable Development of Agriculture
Introduction
Nanotechnology and Agriculture
Precision Farming
Nanotechnology and Animals
Nanotechnology and Water Purification
Nanofood
Nanofood Processing
Nanofood Packaging
Nanotechnology and Its Toxicity in Foods
Nanotechnology-Soil Nutrition and Plants
Nanopesticides
Nanofertilizers
Conclusion and Further Outlook
References
112 Nanomaterials and Nanocoatings for Alternative Antimicrobial Therapy
Introduction
Synthesis and Properties of Organic and Inorganic Nanoparticles
Nanomaterials Applications in Antimicrobial Therapy
Potential Antimicrobial Nanomaterials
Titanium Dioxide (TiO2) NPs
Zinc Oxide (ZnO) NPs
Silver (Ag/Ag2O) NPs
Copper (Cu/CuO) NPs
Gold (Au) NPs
Silica (SiO2) NPs
Aluminum Oxide (Al2O3) NPs
Limitations of Cell Toxicity of Nanoparticles
Conclusion
References
113 Nanomaterials and Nanocomposites for Energy-Efficient Building Envelopes
Introduction
Transparent Solar Reflective Nanocoatings
Metal-Based Nanomaterials
Metal-Oxide-Based Nanomaterials
Zinc Oxide (ZnO)
Gallium-Doped Zinc Oxide (GZO) and Aluminum-Doped Zinc Oxide (AZO)
Tin Oxide (SnO2)
Indium-Doped Tin Oxide (ITO) and Antimony-Doped Tin Oxide (ATO)
Nano-Based Daytime Radiative Cooling
Nanoparticle-Based Radiative Coolers
Reflective Top Layer-Missive Bottom Layer
Emissive Top Layer-Reflective Bottom Layer
Nanophotonic-Based Radiative Coolers
Applications for Energy-Efficient Building Envelopes
Cool Windows
Cool Roofs
Conclusions and Future Outlook
References
114 Nanomaterials and Nanoprocesses for the Removal and Reuse of Heavy Metals
Introduction
Nanomaterials as Adsorbents
Nanomaterials for the Elimination of Heavy Metals
Carbon-Based Nanomaterials
Silica-Based Nanomaterials
Zerovalent Metal-Based Nanomaterials
Metal-Oxide Based Nanomaterials
Nanocomposite Nanomaterials
Inorganic Nanocomposites
Organic Polymer-Supported Nanocomposites
Magnetic Nanocomposites
Conclusions and Further Outlook
References
115 Nanomaterials for Latent Thermal Energy Storage
Introduction
PCMs Employed in LTES (Latent Heat Energy System)
Classifications of the PCMs
Heat Transfer Enhancement Techniques
Nanomaterials embedded PCMs (NEPCMs)
Effect of Morphology
Effect of Size
Effect of Concentration (Weight/Volume Fractions)
Effect of pH Value
Effect of Types of Nanomaterials
Thermal Characteristics of NiPCMs
Latent Heat Versus Thermal Conductivity
Thermal Conductivity Versus Viscosity
Solidification and Melting Characteristics of NiPCMs
Applications
Heating and Cooling of Buildings
Conclusion and Further Outlook
References
116 Nanomaterials for Arsenic Remediation with Boosted Adsorption and Photocatalytic Properties
Introduction
Metal-Oxide-Based Nanomaterials for the Adsorption of As Species
TiO2 Based Materials for the Adsorption of As Species
Iron-Oxide-Based Materials for the Adsorption of As Species
The Use of Photocatalytic Nanomaterials in As Removal
TiO2-Based Nanomaterials for Arsenic Removal Through a Photocatalysis Process
Metal Oxide Nanomaterials for the Photocatalytic Removal of Arsenic Compounds Present in Wastewater
Carbon-Based Nanomaterials for Removing Arsenic from Wastewater and Groundwater: Adsorption and Photo-Oxidation Capabilities
Organic Carbon-Based Nanomaterials (O-CBNMs)
Synthetic Carbon-Based Nanomaterials (SCBNMs)
Concluding Remarks
References
117 Nanomaterials for Environmental Engineering and Energy Applications
Introduction
The Aim and Objective of This Study
The Scientific Doctrine Behind Nanomaterials and Engineered Nanomaterials
Scientific Vision and Deep Scientific Ingenuity Behind Nanotechnology Applications in Human Society
What Do You Mean by Nanomaterials and Engineered Nanomaterials?
Nanomaterials for Environmental Protection and the Vision for the Future
Nanomaterials in Water Disinfection
Nanotechnology Applications in Drinking Water and Industrial Wastewater Treatment
Nanomaterials and Environmental Remediation
Recent Scientific Endeavor in the Field of Nanotechnology
Significant Research Pursuit in the Field of Nanomaterials Applications in Environmental Protection
Significant Research Pursuit in the Field of Nanomaterials Applications in Energy Industry
The Scientific Sagacity, the Scientific Subtleties, and the Vision of Environmental Sustainability
Water Purification, Industrial Wastewater Treatment, and the Application of Nanotechnology
Arsenic and Heavy Metal Groundwater Remediation and the Vision of Nanotechnology
The Scientific Doctrine of Renewable Energy Technology
Modern Science, Modern Technology, and the Sagacity of Science
Future Recommendations of This Study and the Future Flow of Scientific Thoughts
Conclusion, Outlook, and Scientific Perspectives
Important Websites for Reference
References
118 Nanomaterials in Urban-Architectonic Production
Introduction
Nanomaterial Applied to Produce and Store Energy, Water, and Foods in Cities and Urban Centers
The Need to Use Nanomaterials in Urban-Architectonic Production
Nanomaterials and Energy Efficiency in Buildings and Cities
Nanomaterials for Water Efficiency in Buildings and Cities, Including Purification and Desalinization
Nanomaterials and Efficiency in Food Production for Urban Centers
Conclusions and Additional Perspectives
References
119 Nanomaterials: Recent Advances for Hydrogen Production
Introduction
Nanomaterials for PC and PEC Water Splitting
Metal Oxide Based Nanoarrays
TiO2 Arrays
ZnO Arrays
WO3 Arrays
BiVO4 Arrays
Fe2O3 Arrays
Metal Sulfide Nanocomposites
2D Nanomaterials
2D Metal Chalcogenides Nanosheets
Metal-Free Nanosheets
Nanomaterials for Hydrogen Storage
Nanostructured Metal Hydrides
Carbonaceous Nanomaterials
Nanocomposite Conducting Polymers
Other Nanomaterials
Nanomaterials for PEMFCs
Conclusion and Further Outlook
References
120 Nanoremediation and Nanobioremediation in Water Treatment: The Search for an Eco-friendly Alternative
Introduction
Nanoremediation
Zero-Valent Iron Nanoparticles
Carbon-Based Nanoparticles
Common Minerals
Plasmonic Nanoparticles
Nanobioremediation
Biofunctionalized Nanoparticles
Biologically Synthesized Nanoparticles
Permeable Reactive Barrier
Reuse Capacity
Compatibility with the Environment
Conclusions and Further Outlook
References
121 Nanoremediation of Polluted Environment: Current Scenario and Case Studies
Introduction
Engineered Nanomaterials for Environmental Remediation
Remediation of Wastewater and Groundwater
Remediation of Soils
Nanophytoremediation
Sustainable and Ecosafe Nanoremediation
Conclusion
References
122 Nanostructured Semiconductors for Photocatalytic CO2 Reduction
Introduction
Fundamentals in Photocatalytic CO2 Reduction Reaction
Mechanism of Photocatalytic CO2 Reduction Reaction
Challenges in CO2 Photoreduction
Design of a Photocatalytic System for CO2 Reduction
Composition of Photocatalytic System
Detection of Photocatalytic CO2 Reduction Products
Semiconductor-Based Photocatalyst in CO2 Reduction
Metal-Based Semiconductors
Metal Oxides
Metal Chalcogenides
Metal-Organic Frameworks (MOFs)
Other Metal-Based Semiconductors
Nonmetal-Based Semiconductors
Graphene and Graphene Oxide
Graphitic Carbon Nitrides (g-C3N4)
Other Nonmetal-Based Semiconductors
Strategies for Catalysts Improvement
Crystal Structure Engineering
Surface Engineering
Elemental Doping
Cocatalyst Loading
Nanostructure Construction
Hybrid Nanostructure Construction
Conclusions and Further Outlook
References
123 New Generation of Eco-Friendly Adsorbents for Future Water Purification
Introduction
Adsorption
Removal of Ions from Wastewater by Nanomaterials
Nano-Adsorbent Materials
Nano-Composites Adsorbents
Removal of Organic Pollutants from Wastewater by Nanomaterials
Removal of Emerging Pollutants from Wastewater by Nanomaterials
Mathematical Analysis of the Adsorption Balance
Models of Adsorption Isotherms
Reversibility of the Adsorption Process
Conclusions and Further Outlook
References
124 Potential Agrifood Applications of Novel and Sustainable Nanomaterials: An Eco-friendly Approach
Introduction
Nanofertilizers
Nanofertilizers: Production and Use
Micronutrient Nanofertilizers
Macronutrient Nanofertilizers
Nanobiofertilizers
Limitation of Nanofertilizers
Nanomaterials: Uptake and Translocation
Nanoparticles Transformation
Nanoparticle Accumulations
Conclusions and Further Outlook
References
125 Polymer/Nanocarbon Nanocomposite-Based Eco-friendly Textiles
Introduction
Eco-Friendly Polymers
Eco-Polymer-Based Textile Materials
Eco-Friendly Polymer/Nanocarbon Nanocomposite-Based Textile Materials
Technical Platform for Eco-Friendly Textile Nanomaterials
Electronic Textiles
Military and Defense Industry
Antimicrobial Relevance
Self-Healing Eco-Textiles
Conclusions and Further Outlook
References
126 Application of Heterogeneous Nanocatalysis-Based Advanced Oxidation Processes in Water Purification
Introduction
Heterogeneous Fenton Catalysis and Its Application in Water Purification
Overview of Heterogeneous Fenton Oxidation Technology
Strategies for Enhanced Heterogeneous Fenton Catalysis Activity and Related Reaction Mechanisms
Improving the Number of Active Sites of Catalysts
Preparation Catalysts with Large Surface Areas
Morphological-Controlled Crystal Active Facet Exposure
Enhancing the Interface Electron Transfer Process
Direct Injecting Electrons from Zero-Valent Metal
Synergistic Effect Between Different Metal Components
Direct Injecting Electrons from Carton Materials
Direct Injecting Electrons from Chelating/Reducing Reagents
Direct Injecting Electrons from Metal Sulfides
Direct Injecting Electrons from Surface Defects
Application of Fenton Catalysis in Coal Chemical Wastewater Treatment
Heterogeneous Catalysis of Peroxymonosulfate and Its Application in Water Purification
Overview of Heterogeneous Catalysis of Peroxymonosulfate
Strategies for Enhanced Heterogeneous Peroxymonosulfate Catalytic Activity and Related Reaction Mechanisms
Improving the Number of Active Sites of Catalysts
Preparation Catalysts with Large Surface Areas
Morphological-Controlled Crystal Active Facet Exposure
Enhancing the Process of Interface Charge Transfer
Direct Injecting Electrons from Zero-Valent Metal
Synergistic Effect Between Different Metal Components
Direct Injecting Electrons from Carton Materials
Direct Injecting Electrons from Metal Sulfides
Direct Injecting Electrons from Chelating/Reducing Reagents
Direct Injecting Electrons from Surface Defects
Application of Persulfate Oxidation Technology in Industrial Wastewater Treatment
Heterogeneous Catalysis Ozonation and Its Application in Water Purification
Overview of Heterogeneous Catalysis Ozonation
Strategies for Enhanced Heterogeneous Catalysis Ozonation Activity and Related Reaction Mechanisms
Improving the Number of Active Sites of Catalyst
Preparation Catalysts with Large Specific Surface Area
Morphological-Controlled Crystal Active Facet Exposure
Regulation of Catalysts with Surface Acid-Base Sites
Regulation of Catalysts with Surface Hydroxyl Group
Enhancing the Interface Charge Transfer Process
Direct Injecting Electrons from Zero-Valent Metal
Synergistic Effect Between Different Metal Components
Direct Injecting Electrons from Carton Materials
Direct Injecting Electrons from Surface Defects
Application of Catalysis Ozonation in Practical Water Treatment
Full-Scale Application of Catalytic Ozonation for Drinking Water Treatment
Combination of O3 and H2O2 Oxidation Technology for MTBE Remediation in Las Vegas
Heterogeneous Photocatalysis and Its Application in Water Purification
Overview of Heterogeneous Photocatalysis Technology
Strategies for Enhanced Heterogeneous Photocatalysis Activity and Related Reaction Mechanisms
Improving the Number of Active Sites of Catalyst
Preparation Catalysts with Large Specific Surface Area
Morphological-Controlled Crystal Active Facet Exposure
Bandgap Engineering with Efficiency Electron-Charge Separation and Wide Light Absorption
Element Doping in Semiconductors
Coupling of Different Semiconductors
Coupling of Semiconductors and Noble Metal
Surface Defect Engineering
Application of Photocatalysis Technology in Practical Water Treatment
Application of Photocatalysis Technology for the Removal of Amoxicillin
Combination of UV-H2O2 Oxidation Technology for the Removal of Active Pharmaceutical Ingredients
Conclusions and Outlooks
References
127 Recent Progress of Gold Nanomaterials in Cancer Therapy
Introduction
Gold Nanoparticle-Enhanced Radiotherapy
Radiotherapy Chain
Gold Nanoparticles
Radiation Interactions of Gold Nanoparticles
Advantages of Adding Gold Nanoparticles in Radiotherapy
Monte Carlo Simulation
Concept of Simulation
Macroscopic and Microscopic Simulation
Recent Progress in Monte Carlo Simulation
Dose and Imaging Contrast Enhancement in Gold Nanoparticle-Enhanced Radiotherapy
Dose Enhancement Using Kilovoltage Photon Beams
Dose Enhancement Using Megavoltage Electron Beams
Imaging Contrast Enhancement Using Kilovoltage Photon Beams
Dose Enhancement Using Megavoltage Flattening-Filter-Free and Flattening-Filter Photon Beams
Imaging Contrast Enhancement Using Megavoltage Flattening-Filter-Free and Flattening-Filter Photon Beams
Conclusions and the Further Outlook
References
128 Shape Memory Nanomaterials for Damping Applications
Introduction: Shape Memory Materials as Smart Materials
A Brief Overview of some Shape Memory Systems
Shape Memory Alloys
Shape Memory Polymers
Application of Shape Memory Materials
SMA Systems
NiTi SMAs
Cu Based
Fe-Based SMAs
Functionality of Shape Memory Materials
Techniques for Enhancing the Usability of SMAs
Thermal Processing
Alloying
Shape Memory Nanomaterials
Nanomaterials
Nanostructured Shape Memory Alloys: SPD Techniques
Equal Channel Angular Pressing/Equal Channel Angular Extrusion
High Pressure Torsion
Accumulative Roll-Bonding (ARB)
TiNi-Based Nanostructured SMAs
Cu-Based and Fe-Based Nanostructured SMAs
Summary on SPD Processes
Nanostructured Shape Memory Alloys: Other Techniques
Nanoscale Shape Memory Alloys
Damping in Shape Memory Materials
Damping Mechanisms in SMAs
Internal Friction during Thermoelastic Martensitic Transformation
Martensite Twin re-Orientation
Stress Induced Martensite
Damping in NiTi/TiNi SMAs
Damping in Low Cost SMAs
Damping in Shape Memory Nanomaterials
Conclusion
References
129 TiO2/Fly Ash Nanocomposite for Photodegradation of Organic Pollutant
Introduction
Organic Pollutants in Waste Waters
Synthesis and Characterization of Nanophotocatalysts
Sol-Gel Method
Hydrothermal Method
Coprecipitation method
TiO2/Fly Ash Photocatalytic Nanocomposites
Removal of Pollutant from Wastewaters Using Nanophotocatalysts
Conclusion and Outlook
References
130 Two-Dimensional Transition Metal Chalcogenides for Hydrogen Evolution Catalysis
Introduction
Fundamentals of the HER
The Principle of Hydrogen Evolution Reaction
Parameters and Descriptors for HER Process
Onset Potential and Overpotential ()
Electrochemical Surface Areas
Tafel Slope
Faradic Efficiency
Stability
Descriptors
Selection Criteria for HER Catalysts
Two-Dimensional Transition Metal Chalcogenides (2D-TMCs) for HER
Structures and Properties of 2D-TMCs
Geometry Structures
Band Structures and Density of State Analysis
2D-TMCs as HER Catalysts (The Intrinsic Activity of 2D-TMCs for HER)
Modification Strategies for Boosting the HER Activity of TMCs
Increasing Edge Sites and Optimizing Edge Activity
Increasing Defect Density on the Basal Plane
Phase Transition Engineering
Conclusions and Future Outlook
Conclusions
Future Outlook
References
131 Type of Soil Pollutant and Their Degradation: Methods and Challenges
Introduction
Type of Pollutants
Organic Pollutants
Polycyclic Aromatic Hydrocarbons (PAHs)
Agricultural Chemicals
Biological Pollution
Inorganic Pollutants
Arsenic Pollution
Management of As Toxicity
Cadmium Pollution
Management of Cadmium Pollution
Chromium Pollution
Lead Pollution
Management of Lead Toxicity
Mercury Toxicity
Management of Mercury Toxicity
Nitrogen Pollution
Management of N Pollution
Phosphorus Pollution
Effect of Pollution on Soil Health
Effect on Water Bodies
Effect on Air
Pollutant Removal Methods
Physicochemical Methods
Precipitations
Adsorption Kinetics
Ion Exchange
Biosorption
Biological Techniques
Microbial Method for Pollutant Removal
Phytoremediation
Phytoextraction
Phytodegradation
Rhizofiltration
Rhizodegradation
Phytostabilization
Method of Analysis
Organic Pollutant Analysis
Biological Pollutant Analysis
Inorganic Pollutant
Heavy Metal Analysis
Phosphorus Analysis Method
Nitrate Pollution Determination
Challenges of During Analysis of Pollutant
Conclusions
Further Outlook
References
132 Wastewater Treatment by Photocatalytic Biosynthesized Nanoparticles
Introduction
Outline of Photocatalysis
Basic Principle of Photocatalysis
Biosynthesized Nanoparticles
Types of Biosynthesized Nanoparticles
Bacterial Synthesis
Fungal and Algal Synthesis
Plant-Mediated Synthesis
Photocatalytic Properties of Biosynthesized Nanoparticles
Photocatalytic Metal and Metal Oxide Nanoparticles
Photocatalytic Carbon-Based and Polymer Nanoparticles
Photocatalytic Nanocomposites
Photocatalytic Nanoparticles in Wastewater Treatment
Photocatalytic Decomposition of Dyes
Photocatalytic Detoxification of Heavy Metals
Photocatalytic Decomposition of Other Harmful Organic Compounds
Photocatalytic Antimicrobial Property
Conclusion and Future Outlook
References
133 Water Treatment and Desalination Using the Eco-materials n-Fe0 (ZVI), n-Fe3O4, n-FexOyHz[mH2O], and n-Fex[Cation]nOyHz[Ani...
Introduction
Part 1: Basic Concepts
Iron Corrosion
Iron Corrosion Water Remediation Strategies
Morphology of n-Fe0 Aggregates
Nernst Equation
Estimating the Amount of Cation Removed from the Measured Current
Estimating the Current in the Water Created by the n-ZVI from the Amount of Pollutant Removed
n-ZVI Aggregate Morphology
Evolution of n-ZVI Aggregates
Role of Reductants, Antioxidants, and Partial Pressure Changes
ZVI Operation
ZVI Rate Constants
Pollutant Removal
ZVI Surface-Based Reactor Types
The Three-Stage ZVI/n-FexOyHz Water Remediation Process
Stage 1
Stage 2
Stage 3
Other Processes Used by ZVI to Remove Pollutants
Pollutants Removed
ZVI Water Treatment Markets
Potable Household Water, Potable Municipal Water, Livestock Feed Water, and Emergency Relief Water
Agricultural Grade, Industrial Grade, Riparian Grade Water
Remediation of Munitions, Chemical, Industrial, and Sewage Polluted Water
Desalination to Produce Partially Desalinated Water
Hydrogen Production
Gasoline, Fuel Gas Production
Production of n-ZVI from Wastewater
Extraction of Minerals and Metals from Water Using ZVI to Enable Commercial Resale
Production of Heavy Water from Water Using ZVI
Part 2: ZVI Water Treatment Plants and Processes
Fixed Bed Commercial Municipal Water Treatment Processes
10,000 m3 d-1 Fixed Bed Bischof Process Plant
Permeability Loss
Anderson Moving Bed Commercial Water Treatment Process
Roeske Moving Bed Commercial Water Treatment Process (RP)
Fixed Bed Reactor Advances since 1908
B&F: Fenton Process Reactor (US Patents 7,479,717 and 7,445,717)
B&F: Steel Wool Reactor (US Patents 8,025,800 and 8,101,087)
B 7,854,682; and 8,042,696)
B&F: ZVI-Bauxite Reactor (US Patents 8,206,586 and 8,673,152)
B&F: Porous Ceramic Bed Containing ZVI (US Patent 9,988,285)
B 3,681,021; 3,789,112; and 3,888,974)
Heavy Water Market
B&F: Process to Remove Heavy Metals from Wastewater (US Patent 3,931,007)
B&F: Removal of Halogenated Organic Compounds from Wastewater (Sweeny Process) - US Patents 4,219,419 and 6,531,065
B&F: Surface-Based Removal of Halogenated Organic Compounds from Groundwater (US Patent 7,252,771)
B&F: Removal of Organic Contaminants from Soil (US Patents 6,039,882 and 6,207,073)
B&F: Removal of Viruses from Water (US Patent 8,114,279)
B&F: Treatment of Sewage Effluent (US Patents 8,758,616 and 8,758,617)
B&F: Water Treatment Using a Composite Fe + Ag Filter Bed (US Patent 10,173,196)
B&F: Water Treatment Using a ZVI a Reactive Proppant During Groundwater Remediation (US Patent 5,733,067)
B&F: Water Treatment Using a ZVI a Reactive Proppant for the Subsurface Processing of Flowback Water (US Patent 5,641,020)
B&F: Water Treatment Using a ZVI for the Subsurface Processing of Flowback Water (US Patent 7,179,381)
B&F: Water Treatment in a Borehole (US Patent 5,803,174)
B&F: Adsorbent Activated Carbon Mat Containing n-ZVI for Water Treatment (US Patent 6,787,034)
B&F: Method of Cleaning a Filter Bed Containing n-FexOyHz Precipitates (US Patent 471,811)
B&F: Oxygenated Sewage Treatment
BP: Household Iron Filter Reactors (US Patents 8,404,210 and 8,636,909)
BP: Commercial Sewage Treatment Plant (US Patent 3,522,173)
BP: Creation of a Reactive Filter Bed Within an Aquifer Using n-ZVI (US Patent 5,857,810)
BP: Creation of a Reactive Filter Bed Within an Aquifer Using n-ZVI Emulsions (US Patents 6,664,298; 7,037,946; 7,008,964; 7,2...
BP: Combined ZVI and Bioremediation of Groundwater Polluted with Chlorinated Solvents (US Patents 7,129,388; 7,531,709; and 8,...
BP: ZVI Permeable Reactive Barriers for Groundwater Remediation: Gilham Process (US Patent 5,266,213)
PRB Remediation Using the RP (US Patent 5,868,941)
PRB Remediation of TCE Using Fe, Fe/Pd, or Fe/Cu (US Patents 5,611,936; 5,616,253; and 5,758,389)
PRB Remediation of Arsenic (US Patents 6,132,623 and 6,387,276)
PRB Remediation: Replacement of Fe with FeO (US Patent 6,602,421)
PRB Remediation: Replacement of Fe with Fe3O4 or Fe Pillared Clay (US Patent 5,750,036)
PRB Remediation: Construction of a PRB Using n-Fe/Fe (US Patent 5,975,798)
PRB Remediation: Reducing the ZVI Permeability Loss (US Patent 7,347,647)
PRB Remediation: Removal of Fe Ions Downstream of the PRB (US Patent 6,254,786)
PRB Remediation: Replacement of the ZVI and Reactive Weirs (US Patents 5,624,552 and 10,125,034)
PRB Remediation: PRB Construction Using Geocomposite Sacks (US Patents 7,670,082 and 8,262,318)
PRB Remediation: PRB Construction Using Horizontal Wells (US Patents 8,210,773; 8,366,350; 8,596,351; 9,061,333; and 9,943,893)
PRB Remediation: Direction of Subsurface Water Flow into the PRB (US Patents 5,833,388; 5,975,800; 6,207,114; and 9,884,771)
BP: Treatment of Mine Wastewater (US Patent 10,239,769)
Moving Bed Reactor Advances
AP/CS: Rotating Drum Reactor (US Patent 6,893,862)
FB: Two-Stage Process for Perchlorate Removal (US Patent 9,085,469)
FB: Entrained Flow Arsenic Removal Process (US Patent 9,656,890)
FB: Spouted Bed Reactor for Arsenic Removal (US Patent 9,718,713)
FB: Entrained Flow Treatment System for Chlorinated Liquids (US Patent 8,764,988)
FB: Entrained Flow Treatment System for the Removal of Microbiota (US Patent 8,048,317)
FB: Entrained Flow Treatment System for the Removal of NaCl, KCl, and LiCl (Spanish Patent ES2,598,032)
BP/VP: Vibrating Bed Reactor for Arsenic and Chromium Removal (US Patent 6,942,807)
BP/VB: Ultrasonic PRB Reactor for Organo-Halide Removal (US Patent 6,013,232)
FB: Rotating Paddle Reactor for the Removal of Organic Pollutants (US Patent 1,284,488)
FB: Entrained Flow Treatment System Using ZVI Foam Emulsions (US Patent 9,624,113)
Eagle Ford Shale Water (US Patent 9,624,113)
Contaminated Soils (US Patent 9,624,113)
Cleaning Drill Cuttings (US Patent 9,828,258 B2)
Remediating Mine Water (US Patent 2018/0009678A1)
BP/FB: Fluidized Bed Wastewater Treatment (US Patents 10,329,179 and 10,377,648)
Diffusion Reactor Advances
Water Remediation
OS: TCE Removal (US Patent 7,008,964)
OS: Removal of Nitrates, Metals, and Organo-Halides by Redox Shift
OS, COS, CCS: Co-removal of Metals, Anions, and NaCl from Water
Hydrogen Production from Wastewater
CS: Closed Couple Hydrogen Production Reactors (UK Patent GB2520775 A)
CS: High Temperature Hydrogen Production Reactors
CS: Hydrogen Production by n-ZVI Catalysis at 26 C [27]
CCS: Continuous Hydrogen Production at

Citation preview

Oxana Vasilievna Kharissova Leticia Myriam Torres-Martínez Boris Ildusovich Kharisov Editors

Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications

Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications

Oxana Vasilievna Kharissova • Leticia Myriam Torres-Martínez • Boris Ildusovich Kharisov Editors

Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications With 1329 Figures and 303 Tables

Editors Oxana Vasilievna Kharissova Department of Chemistry Science Universidad Autónoma de Nuevo León San Nicolás de los Garza, Mexico

Leticia Myriam Torres-Martínez Instituto de Ingeniería Civil Universidad Autónoma de Nuevo León San Nicolás de los Garza, Nuevo León, Mexico

Boris Ildusovich Kharisov Facultad de Ciencias Químicas Universidad Autónoma de Nuevo León San Nicolás de los Garza, Mexico

ISBN 978-3-030-36267-6 ISBN 978-3-030-36268-3 (eBook) ISBN 978-3-030-36269-0 (print and electronic bundle) https://doi.org/10.1007/978-3-030-36268-3 © Springer Nature Switzerland AG 2021 All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Nanomaterials and nanocomposites for energy generation/storage and environmental applications (including so-called ecomaterials) are those enhancing the environmental improvement throughout the whole life cycle and maintaining accountable performance. They can minimize environmental impacts to increase energy and material efficiency and to enhance the recyclability of materials. These materials should possess some of following superior properties: energy and resource saving ability, reusability and recyclability, structural reliability, chemical and physical stability, substitutability, cleanability, and biological safety ability. Among several classifications, one includes “cyclic” materials, materials for ecology and environmental protection, materials for society and human health, and materials for energy based on the two main criteria as their sources and functions. The materials for the above applications could be further divided into recycled materials, renewable materials, materials for efficiency, materials for waste treatment, materials for reduction of environmental load, materials for easy disposal or recycle, hazard-free materials, materials for reducing human health impact, materials for energy efficiency, and materials for green energy. Their production should follow minimal environmental impacts; they are frequently “greener,” with high productivity, environmental treatment efficiency, and recyclability, and contain minimal hazardous substances. Among them, we note the eco-cement; synthetic wood; advanced ceramics for exhaust gas purification and energy-saving heat reservoirs; a variety of catalysts for water splitting under solar energy to produce H2 as energetic vector and photocatalysts for distinct chemical processes, such as photoreduction of CO2 to produce fuels with low carbon content; special ultra-light steels; hydrogen storage alloys; sophisticated materials for solar cells; and much more. This is a relatively hot field in materials science, belonging the applied sections of nanoscience and nanotechnology. In the conditions of present strong demands for energy, material, and money savings, as well as heavy contamination problems, we observe a gap which needs to be filled by the creation of our handbook. The main objective of applications of this handbook is to raise greater awareness of these materials among the international R&D community.

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Preface

We are extremely grateful to all authors for their hard work in creation of the present handbook and hope that this collection will be a useful guide for developing novel materials for a sustainable future. San Nicolás de los Garza, Mexico June 2021

Oxana Vasilievna Kharissova Leticia Myriam Torres-Martínez Boris Ildusovich Kharisov

Contents

Volume 1 Part I 1

2

Fundamentals of Nanomaterials and Nanocomposites . . . . .

1

Nanomaterials and Nanocomposites: Classification and Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ivan Pacheco and Cristina Buzea

3

Recent Progress in All-Inorganic Hybrid Materials for Energy Conversion Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Khursheed Ahmad, Praveen Kumar, and Shaikh M. Mobin

41

Part II 3

Types of Nanomaterials and Nanocomposites . . . . . . . . . . .

61

Carbon-Fiber Composites: Development, Structure, Properties, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sefiu Adekunle Bello

63

4

Carbon Fiber Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anand Babu Perumal, Reshma B Nambiar, Periyar Selvam Sellamuthu, and Emmanuel Rotimi Sadiku

5

Nanomaterials: Applications in Biomedicine and Biotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saher Islam, Devarajan Thangadurai, Charles Oluwaseun Adetunji, Olugbenga Samuel Michael, Wilson Nwankwo, Oseni Kadiri, Osikemekha Anthony Anani, Samuel Makinde, and Juliana Bunmi Adetunji

6

Application of Different Porous Materials . . . . . . . . . . . . . . . . . . Subhajit Patra and S. Suresh

7

Bio-composites: Eco-friendly Substitute of Glass Fiber Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Khubab Shaker, Yasir Nawab, and Madeha Jabbar

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135

151

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8

Biodegradable Polymer Composite Films for Green Packaging Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shobhit Dixit and Vijay Laxmi Yadav

9

Cellulose Nanofibers for Development of Green Composites . . . . Deepak Verma, Gaurav Joshi, and Teekam Singh

10

Cellulose-Based Nanomaterials for Water Pollutant Remediation: Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohd Yusuf

177 195

213

11

Development of Glass Ceramics from Agricultural Wastes . . . . . Ranjana Das and Chiranjib Bhattacharjee

229

12

Difficulties in Thin Film Synthesis . . . . . . . . . . . . . . . . . . . . . . . . Barış Şimşek, Özge Bildi Ceran, and Osman Nuri Şara

251

13

Smectite Clay Nanoarchitectures: Rational Design and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bruna Pes Nicola, Katia Bernardo-Gusmão, and Anderson Joel Schwanke

14

Greener Composites from Plant Fibers: Preparation, Structure, and Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Devarajan Thangadurai, Suraj Shashikant Dabire, Jeyabalan Sangeetha, Abdel Rahman Mohammad Said Al-Tawaha, Charles Oluwaseun Adetunji, Saher Islam, Arun Kashivishwanath Shettar, Muniswamy David, Ravichandra Hospet, and Juliana Bunmi Adetunji

275

307

15

Highly Stable Boron Carbide–Based Nanocomposites . . . . . . . . . Levan Chkhartishvili, Otar Tsagareishvili, Archil Mikeladze, Roin Chedia, Vakhtang Kvatchadze, and Vakhtang Ugrekhelidze

16

Influence of a Twisting-Helical Disturber on Nanofluid Turbulent Forced Convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Sheikholeslami, M. Jafaryar, A. Arabkoohsar, and Ahmad Shafee

353

Advanced Research Developments and Commercialization of Light Weight Metallic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . Kumar Harshit and Pallav Gupta

373

17

18

Mesoporous Nanomaterials: Properties and Applications in Environmental Sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Devarajan Thangadurai, Vishal Ahuja, Jeyabalan Sangeetha, Jarnain Naik, Ravichandra Hospet, Muniswamy David, Arun Kashivishwanath Shettar, Anand Torvi, Shivasharana Chandrabanda Thimmappa, and Nivedita Pujari

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19

Nanocellulose for Sustainable Future Applications Ihsan Flayyih Hasan AI-Jawhari

20

Nanoclay as Carriers of Bioactive Molecules Applied to Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Danila Merino, Bárbara Tomadoni, María Florencia Salcedo, Andrea Yamila Mansilla, Claudia Anahí Casalongué, and Vera Alejandra Alvarez

21

22

23

Nanoclays as Eco-friendly Adsorbents of Arsenic for Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Estefanía Baigorria, Leonardo Cano, and Vera Alejandra Alvarez Nanomaterials from Agrowastes: Past, Present, and the Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Devarajan Thangadurai, Jarnain Naik, Jeyabalan Sangeetha, Abdel Rahman Mohammad Said Al-Tawaha, Charles Oluwaseun Adetunji, Saher Islam, Muniswamy David, Arun Kashivishwanath Shettar, and Juliana Bunmi Adetunji Nanoporous Metallic Foams for Energy Applications: Electrochemical Approaches for Synthesizing and Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Randa Abdel-Karim

24

Plant Fibers-Based Sustainable Biocomposites . . . . . . . . . . . . . . . Priya Yadav, Chandra Mohan Srivastava, and Dipti Vaya

25

Polymeric TiO2 Nanocomposites for Development of Fouling-Resistant Membranes for Wastewater Treatment . . . . . . Priti Bansal and Dhiraj Sud

26

27

28

29

Porous Materials for Applications in Energy and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tzipatly A. Esquivel-Castro, Antonia Martínez-Luévanos, Sofía Estrada-Flores, and Lucía F. Cano-Salazar Recycled Plastics and Nanoparticles for Green Production of Nano Structural Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sefiu Adekunle Bello and Maruf Yinka Kolawole Sustainable Conversion of Coconut Wastes into Useful Adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abdul Rahman Abdul Rahim, Khairiraihanna Johari, Norasikin Saman, and Hanapi Mat Roadmap of Nanomaterials in Renewable Energy . . . . . . . . . . . . Ricardo Beltran-Chacon

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455

471

489 513

549

579

599

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Nanostructured Composite Modifying Coatings for Highly Efficient Environmentally Friendly Dry Cutting A. A. Vereschaka and S. N. Grigoriev

.......

679

31

Titanium Dioxide Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . Şana Sungur

713

32

Wear-Resistant Metals and Composites . . . . . . . . . . . . . . . . . . . . B. P. Aramide, Abimbola Patricia I. Popoola, Emmanuel Rotimi Sadiku, F. O. Aramide, T. Jamiru, and S. L. Pityana

731

Volume 2 Part III 33

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35

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Synthesis of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . .

Biosynthesized Gold and Silver Nanoparticles in Cancer Theranostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Susheel Kumar Nethi, Anubhab Mukherjee, and Sudip Mukherjee Sustainable Synthesis of Greener Nanomaterials: Principles, Processes, and Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Devarajan Thangadurai, Lokeshkumar Prakash, Jeyabalan Sangeetha, Abdel Rahman Mohammad Said Al-Tawaha, Muniswamy David, Saher Islam, Charles Oluwaseun Adetunji, and Juliana Bunmi Adetunji

757

759

775

Nanomaterials Synthesis and Their Eco-Friendly Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mubashra Afroz, Saumya Agrahari, and Praveen K. Tandon

799

Green Synthesis and Application of Metal and Metal Oxide Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohamed A. Hassaan, Ahmed El Nemr, and Safaa Ragab

831

Nanomaterials Through Powder Metallurgy: Production, Processing, and Potential Applications Toward Energy and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Kaviarasu and M. Ravichandran

38

Scalable Synthesis of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . Cesar Maximo Oliva González, Oxana Vasilievna Kharissova, Lucy T. González, Miguel A. Méndez-Rojas, Thelma Serrano Quezada, and Yolanda Peña Méndez

39

Production and Applications of Biomass-Derived Graphene-Like Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nur Fatihah Tajul Arifin and Norhaniza Yusof

859 899

923

Contents

40

41

42

xi

Gas-Phase Synthesis for Mass Production of TiO2 Nanoparticles for Environmental Applications . . . . . . . . . . . . . . . . . . . . . . . . . . Sovann Khan, Ken-ichi Katsumata, Vicente Rodríguez-González, Chiaki Terashima, and Akira Fujishima

953

Laser Additive Manufacturing of Nanomaterials for Solar Thermal Energy Storage Applications . . . . . . . . . . . . . . . . . . . . . Modupeola Dada and Patricia Popoola

975

Mechanical Performance of Nanocomposites and Biomass-Based Composite Materials and Its Applications: An Overview . . . . . . V. Arumugaprabu, R. Deepak Joel Johnson, and S. Vigneshwaran

991

43

Nanomaterials from Biomass: An Update . . . . . . . . . . . . . . . . . . 1005 Jeyabalan Sangeetha, Arun Kashivishwanath Shettar, and Devarajan Thangadurai

44

Nanomaterials from Marine Environments: An Overview Charles Oluwaseun Adetunji, Osikemekha Anthony Anani, Saher Islam, Oseni Kadiri, Devarajan Thangadurai, Wilson Nwankwo, Samuel Makinde, Jeyabalan Sangeetha, Juliana Bunmi Adetunji, and Abdel Rahman Mohammad Said Al-Tawaha

45

Nanostructured Heterogeneous Catalysts for Biomass Conversion in Green Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1041 Tripti Chhabra and Venkata Krishnan

46

Thermochemical Conversion of Biomass Waste-Based Biochar for Environment Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065 Sudipta Ramola, Tarun Belwal, and Rajeev Kumar Srivastava

47

Microbial Synthesis of Gold Nanoparticles and Their Applications as Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1081 Saravanan Krishnan and Anju Chadha

Part IV

. . . . . 1023

Main Processes Using Nanomaterials . . . . . . . . . . . . . . . . .

1109

48

Trends on Synthesis of Polymeric Nanocomposites Based on Green Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1111 Pablo González-Morones, Ernesto Hernández-Hernández, Roberto Yañez-Macias, Zureima García-Hernández, Gustavo Soria-Arguello, and Carlos Alberto Ávila-Orta

49

Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1143 Ranjana Das

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50

Environmentally Benign Synthesis of Nanocatalysts: Recent Advancements and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 1163 Pavan Kumar Gautam, Saurabh Shivalkar, and Sintu Kumar Samanta

51

Improving the Performance of Engineering Barriers in Radioactive Waste Disposal Facilities: Role of Nano-materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183 R. O. Abdel Rahman, S. S. Metwally, and A. M. El-Kamash

52

Nanomaterials for Water Splitting: A Greener Approach to Generate Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201 Santosh Bahadur Singh

53

Nanomaterials in Soil Health Management and Crop Production: Potentials and Limitations . . . . . . . . . . . . . . . . . . . . 1221 Pratibha Singh and A. P. Singh

54

Photocatalysis for Wastewater Treatment with Special Emphasis on Plastic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . 1247 Karthika Arumugam, Swaminathan Meenkashisundaram, and Naresh Kumar Sharma

55

Lanthanide-Based Compounds for Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1269 Sahar Zinatloo-Ajabshir

56

Removal of Radioactive Wastes Using Nanomaterials . . . . . . . . . 1291 Jeyabalan Sangeetha, Muniswamy David, Jarnain Naik, Devarajan Thangadurai, Suraj Shashikant Dabire, and Shivasharana Chandrabanda Thimmappa

57

Visible Light-Driven Photocatalysts for Environmental Applications Based on Graphitic Carbon Nitride . . . . . . . . . . . . . 1309 Waseem Raza and Khursheed Ahmad

58

Highly Efficient Electrocatalytic Water Splitting . . . . . . . . . . . . . 1335 Mengjie Liu, Lawrence Yoon Suk Lee, and Kwok-Yin Wong

59

Recent Engineering Approaches for Lead-Free Piezoelectric Harvesters Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1369 Mariya Aleksandrova

Contents

xiii

Part V Main Products and Devices Obtained with Use of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1391

60

3D Printing of Fiber-Reinforced Polymer Nanocomposites: Additive Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393 Borra N. Dhanunjayarao, N. V. Swamy Naidu, Rajana Suresh Kumar, Y. Phaneendra, Bandaru Sateesh, J. L. Olajide, and Emmanuel Rotimi Sadiku

61

Advanced Functional Nanomaterials for Explosive Sensors Khursheed Ahmad and Shaikh M. Mobin

62

Application of Perovskite-Based Nanomaterials as Catalysts for Energy Production Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . 1445 Shimaa M. Ali

63

Appraisal of Solar Radiation with Modelling Approach for Solar Farm Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465 Lutfu S. Sua and Figen Balo

64

Biobased and Biodegradable Polymer Nanocomposites . . . . . . . . 1493 Tri-Dung Ngo

65

Biobutanol: A Promising Alternative Commercial Biofuel D. Priscilla Mercy Anitha, S. Periyar Selvam, and Emmanuel Rotimi Sadiku

66

Biolubricants with Additives in Malaysia for Tribological Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1541 T. V. V. L. N. Rao, Ahmad Majdi Abdul Rani, Mokhtar Awang, Masri Baharom, and Yoshimitsu Uemura

67

Dye-Sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1555 Susana Borbón, Shadai Lugo, and Israel López

68

Ecofriendly Composite/Nanocomposite from Discarded Addition and Condensation Polymers . . . . . . . . . . . . . . . . . . . . . 1589 Bruno de Paula Amantes, Daniela de França da Silva Freitas, Sibele Piedade Cestari, Gerson Alberto Valencia Albitres, Danielle de Mattos Mariano, and Luis Claudio Mendes

69

Nanocatalysts for Biofuels Production . . . . . . . . . . . . . . . . . . . . . 1613 Gerardo Antonio Flores-Escamilla, José Julián Cano-Gómez, José Pablo Ruelas-Leyva, Sergio Aarón Jimenez-Lam, and Iván Alonso Santos-López

70

Nanocomposites for Supercapacitor Application . . . . . . . . . . . . . 1639 P. Anandhi, V. Jawahar Senthil Kumar, and S. Harikrishnan

. . . . 1423

. . . . . 1521

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71

Nanopesticides, Nanoherbicides, and Nanofertilizers: The Greener Aspects of Agrochemical Synthesis Using Nanotools and Nanoprocesses Toward Sustainable Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1663 Jeyabalan Sangeetha, Ravichandra Hospet, Devarajan Thangadurai, Charles Oluwaseun Adetunji, Saher Islam, Nivedita Pujari, and Abdel Rahman Mohammad Said Al-Tawaha

72

Nanotechnology for Electrical Energy Systems . . . . . . . . . . . . . . 1679 Subramanian Amuthameena, Balraj Baskaran, and Easwaramoorthy Nandakumar

73

Perovskite Materials in Photovoltaics . . . . . . . . . . . . . . . . . . . . . . 1703 Khursheed Ahmad and Shaikh M. Mobin

74

Natural Polymer Composites for Environmental Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1725 Mohd Shabbir and Xiaogang Luo

75

Second Life of Polymeric-Based Materials: Strategies and Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743 Caren Rosales, Vera Alejandra Alvarez, and Leandro Nicolas Ludueña

Volume 3 76

Super Capacitance of Metal Oxide Nanoparticles . . . . . . . . . . . . 1759 P. Kamaraj, R. Vennila, M. Sridharan, and P. A. Vivekanand

77

Synthetic Fibers from Renewable Sources . . . . . . . . . . . . . . . . . . 1773 Leticia Melo-López, Christian Javier Cabello-Alvarado, Marlene Lariza Andrade-Guel, Diana Iris Medellín-Banda, Heidi Andrea Fonseca-Florido, and Carlos Alberto Ávila-Orta

78

Fabrication of Electrochemical Sensors for the Sensing of Hazardous Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1799 Khursheed Ahmad and Waseem Raza

79

Multi-junction Polymer Solar Cells . . . . . . . . . . . . . . . . . . . . . . . 1817 Khursheed Ahmad and Qazi Mohd Suhail

80

Current State and Prospective of Supercapacitors . . . . . . . . . . . . 1835 Khursheed Ahmad and Waseem Raza

81

Enzyme Catalyzed Glucose Biofuel Cells . . . . . . . . . . . . . . . . . . . 1855 Khursheed Ahmad and Qazi Mohd Suhail

82

Properties of Diamonds and Their Application in Photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1871 Qilong Yuan, Cheng-Te Lin, and Kuan W. A. Chee

Contents

Part VI

xv

Applications of Nanomaterials and Nanocomposites . . . .

1897

83

3D Printing for Energy-Based Applications . . . . . . . . . . . . . . . . . 1899 Steve F. A. Acquah

84

Adsorption-Based Removal of Heavy Metals from Water Using Nano-akaganéites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1925 Sarah Geo, Gurijala Sai Kedar Reddy, Surabhi Yadav, Mokhtar Ali Abduh Mohammed, and Vadali V. S. S. Srikanth

85

Agro Wastes/Natural Fibers Reinforcement in Concrete and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1953 Deepak Verma and Irem Sanal

86

Applications of Photochemical Oxidation in Textile Industry . . . 1975 Mohamed A. Hassaan, Marwa R. Elkatory, and Ahmed El Nemr

87

Application of Fly Ash for Oil-in-Water Emulsion Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2005 K. Suresh and S. Suresh

88

Current Water Treatment Technologies: An Introduction . . . . . . 2033 Na Tian, Yulun Nie, Xike Tian, and Yanxin Wang

89

Application of Iron Oxide Nanomaterials for the Removal of Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2067 Tiantian Luo, Chao Yang, Xike Tian, Wenjun Luo, Yulun Nie, and Yanxin Wang

90

Application of Nanosilicon and Nanochitosan to Diminish the Use of Pesticides and Synthetic Fertilizers in Crop Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2093 Armando Robledo-Olivo, Marcelino Cabrera-De la Fuente, and Adalberto Benavides-Mendoza

91

Degradation and Removal of Petroleum Hydrocarbons from Contaminated Environments Using Nanotechnologies and Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2121 Devarajan Thangadurai, Vishal Ahuja, and Jeyabalan Sangeetha

92

Degradation of Plastics Using Nanomaterials . . . . . . . . . . . . . . . . 2139 Muniswamy David, Lokeshkumar Prakash, Jeyabalan Sangeetha, Jarnain Naik, Devarajan Thangadurai, and Shivasharana Chandrabanda Thimmappa

93

Devising and Exploiting Functionalities of Nanocomposites for Removal of Organic Pollutants and for Disinfection . . . . . . . 2153 Vinay M. Bhandari and Shobha Shukla

xvi

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Doped Semiconductor Nanomaterials: Applications in Energy and in the Degradation of Organic Compounds . . . . . . . 2179 Sofía Estrada-Flores, Antonia Martínez-Luévanos, Tzipatly A. Esquivel-Castro, and Tirso E. Flores-Guia

95

Eco-friendly Nanomaterials in Agriculture: Biofortification, Plant Growth Promotion, and Phytopathogen Control . . . . . . . . 2203 Gauri A. Achari, Reshma N. Zakane, and Meenal Kowshik

96

Eco-friendly Nano-adsorbents for Pollutant Removal from Wastewaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2225 Maria Harja and Gabriela Ciobanu

97

Engineered Nanomaterials for Emerging Contaminant Removal from Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247 Romina Paola Ollier, María Emilia Villanueva, Guillermo Javier Copello, Vera Alejandra Alvarez, and Laura Mabel Sanchez

98

Environmentally Friendly Wastewater Treatment Methods for the Textile Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2269 Aishwariya Sachidhanandham and Aravin Prince Periyasamy

99

Green Fiber-Reinforced Concrete Composites . . . . . . . . . . . . . . . 2309 Muhammad Umair, Muhammad Imran Khan, and Yasir Nawab

100

Green/Eco-friendly Micropunching Techniques for Energy Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2341 Kelvii Wei Guo

101

High Entropy Alloys: Advance Material for Landing Gear Aerospace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2359 Ayodeji Ebenezer Afolabi, Abimbola Patricia I. Popoola, and Olawale M. Popoola

102

Hydroxyapatite Nanomaterials for Environmental Applications in Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . 2387 G. Amor, A. Vázquez, Lucy T. González, and Boris Ildusovich Kharisov

103

Manganese Oxides: Synthesis and Application as Adsorbents of Heavy Metal Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2409 Tirso E. Flores-Guia, Lucía F. Cano Salazar, Antonia Martínez-Luévanos, and J. A. Claudio-Rizo

104

Materials for CO2, SOx, and NOx Emission Reduction . . . . . . . . 2429 Marina G. Shelyapina, Inocente Rodríguez-Iznaga, and Vitalii Petranovskii

Contents

xvii

105

Materials from Agricultural Wastes . . . . . . . . . . . . . . . . . . . . . . . 2459 V. Dharini, S. Periyar Selvam, and Emmanuel Rotimi Sadiku

106

Metal-Organic Framework-Derived Catalysts for Zn-Air Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2475 Syed Shoaib Ahmad Shah, Tayyaba Najam, and Mohammed Muzibur Rahman

107

Micro- and Nanotechnology Applied on Eco-friendly Smart Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2491 María José Romagnoli, Jimena Soledad Gonzalez, María Alejandra Martinez, and Vera Alejandra Alvarez

108

Nanotechnology in Textile Finishing: Recent Developments . . . . 2509 Aravin Prince Periyasamy, Jiri Militky, Aishwariya Sachidhanandham, and Gopalakrishnan Duraisamy

109

Nano-catalyst Production Using Nano-biotechnology . . . . . . . . . . 2541 Roohi, Zernab Fatima, Mohammed Rehan Zaheer, and Mohammed Kuddus

110

Nanoinsulation Materials for Energy Efficient Buildings . . . . . . . 2559 Marco Casini

111

Nanoagriculture: A Holistic Approach for Sustainable Development of Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2587 A. Shafi, Jasmine Qadir, Suhail Sabir, Mohammad Zain Khan, and Mohammed Muzibur Rahman

112

Nanomaterials and Nanocoatings for Alternative Antimicrobial Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2603 Saher Islam, Devarajan Thangadurai, Charles Oluwaseun Adetunji, Wilson Nwankwo, Oseni Kadiri, Samuel Makinde, Olugbenga Samuel Michael, Osikemekha Anthony Anani, and Juliana Bunmi Adetunji

113

Nanomaterials and Nanocomposites for Energy-Efficient Building Envelopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2621 Kwok Wei Shah and Teng Xiong

114

Nanomaterials and Nanoprocesses for the Removal and Reuse of Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2649 Devarajan Thangadurai, Vishal Ahuja, and Jeyabalan Sangeetha

115

Nanomaterials for Latent Thermal Energy Storage . . . . . . . . . . . 2661 S. Harikrishnan and A. D. Dhass

xviii

Contents

116

Nanomaterials for Arsenic Remediation with Boosted Adsorption and Photocatalytic Properties . . . . . . . . . . . . . . . . . . 2681 Laura Hinojosa-Reyes, Aracely Hernández-Ramírez, Mariana Hinojosa-Reyes, and Vicente Rodríguez-González

117

Nanomaterials for Environmental Engineering and Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2723 Sukanchan Palit and Chaudhery Mustansar Hussain

118

Nanomaterials in Urban-Architectonic Production Silverio Hernández-Moreno

. . . . . . . . . . . 2747

Volume 4 . . . . 2767

119

Nanomaterials: Recent Advances for Hydrogen Production Elsa Nadia Aguilera González, Sofía Estrada Flores, and Antonia Martínez Luévanos

120

Nanoremediation and Nanobioremediation in Water Treatment: The Search for an Eco-friendly Alternative . . . . . . . . 2793 Verónica González

121

Nanoremediation of Polluted Environment: Current Scenario and Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2821 Devarajan Thangadurai, Mohima Chakrabarty, and Jeyabalan Sangeetha

122

Nanostructured Semiconductors for Photocatalytic CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2839 Xiandi Zhang, Chui-Shan Tsang, and Lawrence Yoon Suk Lee

123

New Generation of Eco-Friendly Adsorbents for Future Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2875 J. Botello-González, N. E. Dávila-Guzmán, and J. J. Salazar-Rábago

124

Potential Agrifood Applications of Novel and Sustainable Nanomaterials: An Eco-friendly Approach . . . . . . . . . . . . . . . . . 2899 Charles Oluwaseun Adetunji, Oseni Kadiri, Saher Islam, Wilson Nwankwo, Devarajan Thangadurai, Osikemekha Anthony Anani, Samuel Makinde, Jeyabalan Sangeetha, and Juliana Bunmi Adetunji

125

Polymer/Nanocarbon Nanocomposite-Based Eco-friendly Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2917 Ayesha Kausar

126

Application of Heterogeneous Nanocatalysis-Based Advanced Oxidation Processes in Water Purification . . . . . . . . . 2941 Chu Dai, Xike Tian, Chao Yang, Yulun Nie, and Yanxin Wang

Contents

xix

127

Recent Progress of Gold Nanomaterials in Cancer Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2989 James Chun Lam Chow

128

Shape Memory Nanomaterials for Damping Applications . . . . . . 3019 Ea Okotete, Ak Osundare, J. L. Olajide, D. Desai, and Emmanuel Rotimi Sadiku

129

TiO2/Fly Ash Nanocomposite for Photodegradation of Organic Pollutant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3051 Lidia Favier and Maria Harja

130

Two-Dimensional Transition Metal Chalcogenides for Hydrogen Evolution Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3075 Shuwen Niu and Gongming Wang

131

Type of Soil Pollutant and Their Degradation: Methods and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3103 M. L. Dotaniya, C. K. Dotaniya, Kuldeep Kumar, R. K. Doutaniya, H. M. Meena, A. O. Shirale, M. D. Meena, V. D. Meena, Rakesh Kumar, B. P. Meena, Narendra Kumawat, Roshan Lal, Manju Lata, Mahendra Singh, Udal Singh, A. L. Meena, B. R. Kuri, and P. K. Rai

132

Wastewater Treatment by Photocatalytic Biosynthesized Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3135 Jaison Jeevanandam, Saikumar Manchala, and Michael K. Danquah

133

Water Treatment and Desalination Using the Eco-materials n-Fe0 (ZVI), n-Fe3O4, n-FexOyHz[mH2O], and n-Fex[Cation]nOyHz[Anion]m [rH2O] . . . . . . . . . . . . . . . . . . . . . . 3159 David D. J. Antia

134

Room Temperature Gas Sensor Based on Reduced Graphene Oxide for Environmental Monitoring . . . . . . . . . . . . . 3243 Waseem Raza and Khursheed Ahmad

135

Lead-Free Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . 3263 Khursheed Ahmad and Shaikh M. Mobin

136

Cross-Linking Method-Based Nanogels for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3289 Mohamed Mohamady Ghobashy

137

Reduced Graphene Oxide-Supported Hybrid Composites for Electrochemical-Sensing Applications . . . . . . . . . . . . . . . . . . . . . 3307 Khursheed Ahmad and M. A. Gondal

xx

Contents

138

Nanostructured Materials for Simultaneous Determination of Ascorbic Acid, Uric Acid, and Dopamine . . . . . . . . . . . . . . . . . . . 3331 Khursheed Ahmad and Haekyoung Kim

139

Target-Specific Applications of Fly Ash Cenosphere as Smart Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3349 Diwakar Z. Shende, Kailas L. Wasewar, and Shraddha S. Wadatkar

Part VII

Toxicological Aspects of Nanomaterials . . . . . . . . . . . . . .

3371

140

Ecotoxicology: Methods and Risks . . . . . . . . . . . . . . . . . . . . . . . . 3373 Amneesh Singla, Krishna Moorthi Sankar, and Yashvir Singh

141

Nanostructured Polymers for Thermoelectric Conversion . . . . . . 3393 José M. Mata-Padilla, Carlos Alberto Ávila-Orta, Víctor J. CruzDelgado, and Juan G. Martínez-Colunga

142

Nanotechnology and the Sustainability: Toxicological Assessments and Environmental Risks of Nanomaterials Under Climate Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3421 Devarajan Thangadurai, Muniswamy David, Suraj Shashikant Dabire, Jeyabalan Sangeetha, and Lokeshkumar Prakash

Part VIII

Nanomaterials on the Basis of Natural Products . . . . . . .

3443

143

Sustainable Lignocellulosic Nanomaterials for Future Green Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3445 Tri-Dung Ngo, Richard Chandra, and Behzad Ahvazi

144

Recent Advances in Thermoplastic Starch Biodegradable Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3465 María Paula Guarás, Leandro Nicolas Ludueña, and Vera Alejandra Alvarez

145

Compatibility of Natural Fiber and Hydrophobic Matrix in Composite Modification . . . . . . . . . . . . . . . . . . . . . . . . 3489 Oludaisi Adekomaya and Thokozani Majozi

Part IX Nanomaterials and Nanomembranes for Remediation and Waste/Pollution Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3509

146

Nanomaterials: Green Synthesis for Water Applications . . . . . . . 3511 Alaa El Din Mahmoud

147

Producing Nanomembranes by Novel Methods . . . . . . . . . . . . . . 3533 Oleg Figovsky

Contents

xxi

148

Fabrication of Polyamide Thin Layer Membranes for Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3551 Nilufar Roozbehani and Yaghoub Mansourpanah

149

Incorporation of Cellulose Nanomaterials into Membrane Materials for Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 3581 Amos Adeniyi, Alice O. Oyewo, Emmanuel Rotimi Sadiku, and Maurice S. Onyango

Part X Polymer-Based Materials, Composites, and Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3603

150

Surface Modification of Polymeric Membranes Using Nanomaterials for Water Applications . . . . . . . . . . . . . . . . . . . . . 3605 Mei Qun Seah, Yılmaz Yurekli, and Woei Jye Lau

151

Polymer Composites: Smart Synthetic Fibers Approach in Energy and Environmental Care . . . . . . . . . . . . . . . . . . . . . . . . . 3637 Christian Javier Cabello-Alvarado, Marlene Lariza Andrade-Guel, Diana Iris Medellín-Banda, Leticia Melo-Lopez, and Carlos Alberto Ávila-Orta

152

Development of Bio-based and Biodegradable Plastics . . . . . . . . 3663 Kelvin Adrah, Daniel Ananey-Obiri, and Reza Tahergorabi

153

Bio-based and Biodegradable Plastic Materials: Life Cycle Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3689 Oludaisi Adekomaya, Thokozani Majozi, and Sulaiman Adedoyin

154

The Use of Eco-friendly Recycled Polymer Composites in Boat Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3707 R. O. Okpuwhara, B. O. Oboirien, Emmanuel Rotimi Sadiku, Suprakas Sinha Ray, and S. A. Akinlabi

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3733

About the Editors

Dr. Oxana Vasilievna Kharissova (born in 1969 in Ukraine, former USSR, has lived in Mexico since 1995, and naturalized in Mexico in 2004) is currently a professor and researcher at the Universidad Autónoma de Nuevo León (UANL). Degrees: M.Sc. in crystallography from Moscow State University, Russia, and Ph.D. in materials from the Universidad Autónoma de Nuevo León, Mexico. Memberships: National Researchers System (SNI, Level II), Materials Research Society, and Mexican Academy of Science. She is co-author of 10 books, 12 book chapters, and 105 articles and holds 8 patents. Specialties: materials, nanotechnology (carbon nanotubes, graphene, nanostructurized metals, fullerenes), microwave irradiation, and crystallography as well as nanotechnology-based methods for petroleum treatment. Dr. Kharissova holds the awards “Flama, Vida y Mujer 2017” and “Tecnos” (2004). She is an expert of the National Council for Science and Technology of Mexico (Conacyt). Prof. Dr. Leticia Myriam Torres-Martínez (born in 1955 in Mexico). She is currently the general director of the Centro de Investigación en Materiales Avanzados (CIMAV-CONACYT). Degrees: Ph.D., industrial chemistry from the Universidad Autónoma de Nuevo León and Ph.D. in advanced ceramic materials from the University of Aberdeen, Scotland, UK Certified leader in applied renewable energy and energy efficiency by Harvard University, USA. Full-time professor and researcher at Universidad Autónoma de Nuevo León (UANL) for 39 years. Deputy director of scientific development at CONACYT (2011–2013). Memberships: xxiii

xxiv

About the Editors

Mexican Academy of Science and National Researchers System (SNI, Level III), president the Mexican Academy of Materials (2000–2002). She has more than 70 national and international awards and recognitions, which we highlight: (1) “Premio Nacional de Ciencia 2018 en el área de Tecnología, Innovación y Diseño” (The National Science and Technology Gold Medal, awarded by the President of México in Los Pinos), for her lifetime contributions on the field. (2) Twenty-four awards for UANL Best Research Work in Exact Sciences and Engineering and Technology areas. The most recent one, in 2020, was awarded in Exact Sciences area. The following are her scientific, academic, and technological achievements: more than 200 indexed published articles, supervised more than 70 postgraduate theses, 2 authorized patents and 4 registered patents, 7 chapters book, 3 books, 400 international lectures, 9 proceeding books, 27 innovations and technological developments, 58 research projects, and 3000 citations to her publications. She has been the leader of five research groups, three national scientific networks, and five designed and implemented postgraduate programs. Two of these programs designed and offered as UNI-Enterprise (Vitro and Cemex). Her research leadership is distinguished for developing new materials (in powder and thin films) with semiconductor properties that present a favorable combination of electronic structure and property of absorbing light, in addition to efficiently absorbing reagents, to achieve high efficiencies of photo (electro)catalysts in (a) H2O conversion reactions to produce clean H2, (b) CO2 photoconversion to generate solar-based fuels with carbon content, and (c) the degradation of organics in water. The scientific and technical novelty of her current work involves the development and coupling of advanced photocatalysts for their use in photoinduced processes with the objective of obtaining alternative clean and renewable energy sources to avoid the use of fossil fuels. Her group has experience in new phase diagram studies, different synthesis methods, crystallochemistry of materials, and electrical, catalytic, and photo(electro)catalytic properties.

About the Editors

xxv

Dr. Boris Ildusovich Kharisov (born in 1964, in Russia, has lived in Mexico since 1994, and naturalized in Mexico in 2003) is currently a professor and researcher at the Universidad Autónoma de Nuevo León (UANL). He took part in the liquidation of the consequences of the Chernobyl accident, working in the contaminated zone in 1987. Degrees: M.Sc. in radiochemistry in 1986 and a Ph.D. in inorganic chemistry in 1993, from the Moscow State University, Russia; Dr. Hab. in physical chemistry in 2006 from Rostov State University, Russia. Specialties: materials chemistry, coordination and inorganic chemistry, phthalocyanines, ultrasound, nanotechnology, chemical treatment of petroleum, and environmental remediation. Memberships: Mexican Academy of Science, National Researchers System (SNI, Level III). He is co-author of 16 books, 191 articles, and 17 book chapters and holds 8 patents. Co-editor: three invited special issues of international journals. He is member of the editorial board of four journals. His biography was published in: Who is Who in the World, Outstanding People of the Twentieth Century, and other books.

Contributors

R. O. Abdel Rahman Hot Laboratory Center, Atomic Energy Authority of Egypt, Cairo, Egypt Randa Abdel-Karim Department of Metallurgy, Faculty of Engineering, Cairo University, Giza, Egypt Abdul Rahman Abdul Rahim Department of Chemical Engineering, Faculty of Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia Gauri A. Achari Department of Biological Sciences, Birla Institute of Technology and Science Pilani, KK Birla Goa Campus, Zuarinagar, Goa, India Department of Biotechnology, DCT’s Dhempe College of Arts and Science, Miramar, Goa, India Steve F. A. Acquah Department of Chemistry, Digital Media Lab, University of Massachusetts Amherst, Amherst, MA, USA Sulaiman Adedoyin Mechanical Engineering Department, Faculty of Engineering, Olabisi Onabanjo University, Ago-Iwoye, Nigeria Oludaisi Adekomaya Sustainable Process Engineering, School of Chemical and Metallurgical Engineering, Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, South Africa Mechanical Engineering Department, Faculty of Engineering, Olabisi Onabanjo University, Ago-Iwoye, Nigeria Amos Adeniyi Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology (TUT), Pretoria, South Africa Charles Oluwaseun Adetunji Applied Microbiology, Biotechnology and Nanotechnology Laboratory, Department of Microbiology, Edo University Iyamho, Auchi, Edo State, Nigeria Juliana Bunmi Adetunji Nutrition and Toxicological Research Laboratory, Department of Biochemistry Sciences, Osun State University, Osogbo, Nigeria xxvii

xxviii

Contributors

Kelvin Adrah Food and Nutritional Sciences, North Carolina Agricultural and Technical State University, Greensboro, NC, USA Ayodeji Ebenezer Afolabi Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Mubashra Afroz Department of Chemistry, University of Allahabad, Prayagraj, India Saumya Agrahari Department of Chemistry, University of Allahabad, Prayagraj, India Elsa Nadia Aguilera González Department of Advanced Ceramic Materials and Energy, School of Chemical Sciences, Autonomous University of Coahuila, Saltillo, Coahuila, Mexico Khursheed Ahmad Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea Vishal Ahuja Department of Biotechnology, Himachal Pradesh University, Shimla, Himachal Pradesh, India Behzad Ahvazi Bio-Industrial Research and Development, InnoTech Alberta, Edmonton, AB, Canada Faculty of Engineering, Department of Mechanical Engineering, 10-203 Donadeo Innovation Centre for Engineering, University of Alberta, Edmonton, AB, Canada Ihsan Flayyih Hasan AI-Jawhari Department of Biology, College of Education for Pure Sciences, University of Thiqar, AL-Nasiriya, Iraq S. A. Akinlabi Department of Mechanical Engineering, Walter Sisulu University Butterworth, Butterworth, Eastern cape, South Africa Gerson Alberto Valencia Albitres Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil Mariya Aleksandrova Department of Microelectronics, Technical University of Sofia, Sofia, Bulgaria Shimaa M. Ali Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Abdel Rahman Mohammad Said Al-Tawaha Department of Biological Sciences, Al-Hussein Bin Talal University, Maan, Jordan Vera Alejandra Alvarez Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata (UNMdP) – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata, Argentina

Contributors

xxix

G. Amor Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico P. Anandhi Department of Electronics and Communication Engineering, College of Engineering Guindy, Anna University, Chennai, India Daniel Ananey-Obiri Department of Computational Science and Engineering, College of Engineering, North Carolina Agricultural and Technical State University, Greensboro, NC, USA Osikemekha Anthony Anani Laboratory of Ecotoxicology and Forensic Biology, Department of Biological Science, Faculty of Science, Edo University, Iyamho, Edo State, Nigeria Marlene Lariza Andrade-Guel Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México David D. J. Antia DCA Consultants Ltd, Falkirk, UK A. Arabkoohsar Department of Energy Technology, Aalborg University, Aalborg, Denmark B. P. Aramide Department of Mechanical Engineering, Mechatronics and Industrial Design, Tshwane University of Technology, Pretoria, South Africa F. O. Aramide Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Department of Metallurgical and Materials Engineering, Federal University of Technology Akure, Akure, Nigeria Nur Fatihah Tajul Arifin Advanced Membrane Technology Centre (AMTEC), Universiti Teknologi Malaysia, Johor, Malaysia School of Chemical and Energy Engineering (FCEE), Universiti Teknologi Malaysia, Johor, Malaysia Karthika Arumugam Department of Microbiology, The Standard Fireworks Rajaratnam College for Women, Sivakasi, India Centre for Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India V. Arumugaprabu School of Automotive and Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil, Srivilliputhur, Tamil Nadu, India Carlos Alberto Ávila-Orta Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México Mokhtar Awang Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia

xxx

Contributors

Masri Baharom Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia Estefanía Baigorria Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)– Universidad Nacional de Mar del Plata (UNMdP), Mar del Plata, Argentina Figen Balo Industrial Engineering Department, Firat University, Elâzığ, Turkey Balraj Baskaran Department of Electrical and Electronics Engineering, K. Ramakrishnan College of Technology, Tiruchirappalli, India Priti Bansal YCoE, Punjabi University Guru Kashi Campus, Talwandi Sabo, India Sefiu Adekunle Bello Department of Materials Science and Engineering, Kwara State University, Malete, Nigeria Ricardo Beltran-Chacon Departamento de Energía, Centro de Investigación en Materiales Avanzados (CIMAV), Chihuahua, Chihuahua, Mexico Tarun Belwal Centre for Biodiversity Conservation and Management, G.B. Pant National Institute of Himalayan Environment and Development, Kosi-Katarmal, Almora, Uttarakhand, India Adalberto Benavides-Mendoza Department of Horticulture, Autonomous Agricultural University Antonio Narro, Saltillo, Mexico Katia Bernardo-Gusmão Laboratório de Reatividade e Catálise (LRC), Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre, RS, Brazil Vinay M. Bhandari Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pune, India Chiranjib Bhattacharjee Chemical Engineering Department, Jadavpur University, Kolkata, India Susana Borbón Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico J. Botello-González Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Cristina Buzea IIPB Medicine Corporation, Owen Sound, ON, Canada Christian Javier Cabello-Alvarado CONACYT-Centro de Investigación y de innovación del Estado de Tlaxcala (CITLAX), Tlaxcala de Xicoténcatl, Tlaxcala, México Centro de Investigación en Química Aplicada (CIQA), Departamento de Materiales Avanzados, Saltillo, Coahuila, México Marcelino Cabrera-De la Fuente Department of Horticulture, Autonomous Agricultural University Antonio Narro, Saltillo, Mexico

Contributors

xxxi

Leonardo Cano Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)– Universidad Nacional de Mar del Plata (UNMdP), Mar del Plata, Argentina José Julián Cano-Gómez Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León (UANL), San Nicolás de los Garza, México Lucía F. Cano-Salazar Department of Advanced Materials and Energy, School of Chemical Sciences, Universidad Autonoma de Coahuila, Saltillo, Coahuila, Mexico Claudia Anahí Casalongué Grupo de Fisiología del Estrés en Plantas, Facultad de Ciencias Exactas y Naturales, Instituto de Investigaciones Biológicas (IIB), CONICET-Universidad Nacional de Mar del Plata, Mar del Plata, Argentina Marco Casini Department of Urban Planning, Design and Technology of Architecture, Sapienza Università di Roma, Rome, Italy Özge Bildi Ceran Department of Chemical Engineering, Çankırı Karatekin University, Çankırı, Turkey Sibele Piedade Cestari Department of Architecture, Queen’s University of Belfast, Belfast, UK Anju Chadha Laboratory of Bio-organic Chemistry, Department of Biotechnology, Indian Institute of Technology Madras, Chennai, India National Centre for Catalysis Research, Chemistry Department, Indian Institute of Technology Madras, Chennai, India Mohima Chakrabarty Department of Biotechnology, University School of Biotechnology, Guru Gobind Singh Indraprastha University, Delhi, India Richard Chandra Bio-Industrial Research and Development, InnoTech Alberta, Edmonton, AB, Canada Roin Chedia P. Melikishvili Institute of Physical and Organic Chemistry, I. Javakhishvili Tbilisi State University, Tbilisi, Georgia Kuan W. A. Chee Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of China, Hefei, P.R. China Laser Research Institute, Shandong Academy of Sciences, Qingdao, P.R. China Tripti Chhabra School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, India Levan Chkhartishvili Boron-Containing and Powder Materials Laboratory, F. Tavadze Metallurgy and Materials Science Institute, Tbilisi, Georgia Engineering Physics Department, Georgian Technical University, Tbilisi, Georgia

xxxii

Contributors

James Chun Lam Chow Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada Gabriela Ciobanu Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical University of Iasi, Iasi, Romania J. A. Claudio-Rizo Department of Advanced Materials, School of Chemical Sciences, Universidad Autónoma de Coahuila, Saltillo, Coahuila, Mexico Guillermo Javier Copello Facultad de Farmacia y Bioquímica, Departamento de Química Analítica y Fisicoquímica, (UBA), Universidad de Buenos Aires (UBA), Buenos Aires, Argentina Instituto de Química y Metabolismo del Fármaco (IQUIMEFA), CONICET – Universidad de Buenos Aires, Buenos Aires, Argentina Víctor J. Cruz-Delgado Departamento de Procesos de Transformación de Plásticos, Centro de Investigación en Química Aplicada, Saltillo, Mexico N. E. Dávila-Guzmán Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Suraj Shashikant Dabire Department of Zoology, Karnatak University, Dharwad, Karnataka, India Modupeola Dada Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Chu Dai Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, People’s Republic of China Michael K. Danquah Chemical Engineering Department, University of Tennessee, Chattanooga, TN, USA Ranjana Das Chemical Engineering Department, Jadavpur University, Kolkata, India Muniswamy David Department of Zoology, Karnatak University, Dharwad, Karnataka, India Daniela de França da Silva Freitas Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil Danielle de Mattos Mariano Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil Bruno de Paula Amantes Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil D. Desai Department of Mechanical Engineering, Mechatronics and Industrial Design, Tshwane University of Technology, Pretoria, South Africa

Contributors

xxxiii

Borra N. Dhanunjayarao Department of Mechanical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh, India Department of Mechanical Engineering, Vignan’s Institute of Information Technology (A), Visakhapatnam, Andhra Pradesh, India V. Dharini Department of Food and Process Engineering, School of Bioengineering, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India A. D. Dhass Department of Mechanical Engineering, PACE Institute of Technology and Sciences, Ongole, Andhra Pradesh, India Shobhit Dixit Department of Chemical Engineering and Technology, IIT BHU, Varanasi, Uttar Pradesh, India C. K. Dotaniya College of Agriculture, SKRAU, Bikaner, India M. L. Dotaniya ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur, India R. K. Doutaniya OPJS University, Churu, India Gopalakrishnan Duraisamy Department of Fashion Technology, PSG College of Technology, Coimbatore, India Ahmed El Nemr Environmental Division, National Institute of Oceanography and Fisheries, NIOF, Alexandria, Egypt A. M. El-Kamash Hot Laboratory Center, Atomic Energy Authority of Egypt, Cairo, Egypt Marwa R. Elkatory Advanced Technology and New Materials and Research Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), Alexandria, Egypt Tzipatly A. Esquivel-Castro Department of Advanced Ceramic Materials and Energy, School of Chemical Sciences, Universidad Autonoma de Coahuila, Saltillo, Coahuila, Mexico Sofía Estrada-Flores Department of Advanced Ceramic Materials and Energy, School of Chemical Sciences, Universidad Autonoma de Coahuila, Saltillo, Coahuila, Mexico Easwaramoorthy Nandakumar Department of Electrical and Electronics Engineering, Sasurie College of Engineering, Vijayamangalam, India Zernab Fatima Protein Research Laboratory, Department of Bioengineering, Integral University, Lucknow, India Lidia Favier Univ Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, Rennes, France Oleg Figovsky Israel Association of Inventors, Haifa, Israel

xxxiv

Contributors

Gerardo Antonio Flores-Escamilla Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León (UANL), San Nicolás de los Garza, México Tirso E. Flores-Guia Department of Advanced Materials, School of Chemical Sciences, Universidad Autonoma de Coahuila, Saltillo, Coahuila, Mexico Heidi Andrea Fonseca-Florido Centro de Investigación en Química Aplicada (CIQA), Boulevard Enrique Reyna 140, Saltillo, México Akira Fujishima Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan Zureima García-Hernández Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México Pavan Kumar Gautam Department of Applied Sciences, Indian Institute of Information Technology Allahabad, Allahabad, India Sarah Geo College of Integrated Studies and School of Chemistry, University of Hyderabad, Hyderabad, India Mohamed Mohamady Ghobashy Radiation Research of Polymer Chemistry Department, National Center for Radiation Research and Technology, Atomic Energy Authority, Cairo, Egypt M. A. Gondal Laser Research Group, Department of Physics, Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia K.A.CARE Energy Research and Innovation Center, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Cesar Maximo Oliva González Department of Chemistry Science, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Jimena Soledad Gonzalez Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA) (CONICET-UNMdP), Mar del Plata, Argentina Lucy T. González Tecnológico de Monterrey, Escuela de Ingeniería y Ciencias, Monterrey, Mexico Verónica González Universidad Autónoma de Nuevo León, UANL, Facultad de Ciencias Químicas, Laboratorio de Materiales I, Av. Universidad, Cd. Universitaria, San Nicolás de los Garza, Mexico Pablo González-Morones Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México S. N. Grigoriev Moscow State Technological University STANKIN, Moscow, Russia

Contributors

xxxv

María Paula Guarás Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata (UNMdP) – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata, Argentina Kelvii Wei Guo Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong Pallav Gupta Department of Mechanical Engineering, A.S.E.T., Amity University Uttar Pradesh, Noida, India S. Harikrishnan Department of Mechanical Engineering, Kings Engineering College Irungattukottai, Sriperumbudur, Tamil Nadu, India Maria Harja Department of Chemical Engineering, Faculty of Chemical Engineering and Environmental Protection, “Gheorghe Asachi” Technical University of Iasi, Iasi, Romania Kumar Harshit Department of Mechanical Engineering, School of Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom Mohamed A. Hassaan Environmental Division, National Institute of Oceanography and Fisheries, NIOF, Alexandria, Egypt Ernesto Hernández-Hernández Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México Silverio Hernández-Moreno Universidad Autónoma del Estado de México, Toluca, Mexico Aracely Hernández-Ramírez Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, UANL, San Nicolás de los Garza, Mexico Laura Hinojosa-Reyes Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, UANL, San Nicolás de los Garza, Mexico Mariana Hinojosa-Reyes Facultad de Ciencias, Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico Ravichandra Hospet Department of Botany, Karnatak University, Dharwad, Karnataka, India Chaudhery Mustansar Hussain Department of Chemistry and Environmental Sciences, New Jersey Institute of Technology, University Heights, Newark, NJ, USA Saher Islam Institute of Biochemistry and Biotechnology, Faculty of Biosciences, University of Veterinary and Animal Sciences, Lahore, Pakistan Madeha Jabbar Textile Composite Materials Research Group, National Center for Composite Materials, National Textile University, Faisalabad, Pakistan M. Jafaryar Renewable Energy Systems and Nanofluid Applications in Heat Transfer Laboratory, Babol Noshirvani University of Technology, Babol, Iran

xxxvi

Contributors

T. Jamiru Department of Mechanical Engineering, Mechatronics and Industrial Design, Tshwane University of Technology, Pretoria, South Africa V. Jawahar Senthil Kumar Department of Electronics and Communication Engineering, College of Engineering Guindy, Anna University, Chennai, India Jaison Jeevanandam CQM - Centro de Quimica da Madeira, MMRG, Universidade da Madeira, Campus da Penteada, Funchal, Portugal Sergio Aarón Jimenez-Lam Facultad de Ciencias Químico-Biológicas, Universidad Autónoma de Sinaloa (UAS), Ciudad Universitaria, Culiacán, México Khairiraihanna Johari Department of Chemical Engineering, Faculty of Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia R. Deepak Joel Johnson School of Automotive and Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil, Srivilliputhur, Tamil Nadu, India Gaurav Joshi Department of Mechanical Engineering, Graphic Era Hill University, Dehradun, India Oseni Kadiri Department of Biochemistry, Faculty of Basic Medical Sciences, Edo University Iyamho, Auchi, Edo State, Nigeria P. Kamaraj Department of Chemistry, Bharath Institute of Higher Education and Research, Chennai, India Ken-ichi Katsumata Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan Research Center for Space Colony, Tokyo University of Science, Chiba, Japan Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, Tokyo, Japan Ayesha Kausar Nanosciences Division, National Center for Physics, Quaid-iAzam University Campus, Islamabad, Pakistan C. Kaviarasu Department of Mechanical Engineering, Arasu Engineering College, Tamilnadu, India Muhammad Imran Khan Textile Composite Materials Research Group, National Center for Composite Materials, Faculty of Engineering and Technology, National Textile University, Faisalabad, Pakistan Sovann Khan Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan Boris Ildusovich Kharisov Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico

Contributors

xxxvii

Oxana Vasilievna Kharissova Department of Chemistry Science, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Haekyoung Kim School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea Maruf Yinka Kolawole Department of Mechanical Engineering, Kwara State University, Malete, Nigeria Meenal Kowshik Department of Biological Sciences, Birla Institute of Technology and Science Pilani, KK Birla Goa Campus, Zuarinagar, Goa, India Saravanan Krishnan Laboratory of Bio-organic Chemistry, Department of Biotechnology, Indian Institute of Technology Madras, Chennai, India Venkata Krishnan School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, India Mohammed Kuddus Department of Biochemistry, University of Hail, Hail, Kingdom of Saudi Arabia Kuldeep Kumar ICAR Indian Institute of Soil and Water Conservation, Dehradun, RS Kota, India Praveen Kumar Discipline of Chemistry, Indian Institute of Technology Indore, Indore, MP, India Rakesh Kumar Division of Crop Research, ICAR-RCER, Patna, Bihar, India Rajana Suresh Kumar Department of Mechanical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh, India Narendra Kumawat College of Agriculture, Indore, Madhya Pradesh, India B. R. Kuri School of Agriculture, Suresh Gyan Vihar University, Jaipur, India Vakhtang Kvatchadze E. Andronikashvili Institute of Physics, I. Javakhishvili Tbilisi State University, Tbilisi, Georgia Roshan Lal Shri Bhawani Niketan Law College, Jaipur, India Manju Lata Barkatullah University, Bhopal, India Woei Jye Lau Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Malaysia Lawrence Yoon Suk Lee Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China

xxxviii

Contributors

Cheng-Te Lin Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China Mengjie Liu Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hong Kong, China Israel López Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Leandro Nicolas Ludueña Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata (UNMdP) – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata, Argentina Shadai Lugo Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Tiantian Luo Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Wenjun Luo Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Xiaogang Luo School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei Province, PR China Alaa El Din Mahmoud Environmental Sciences Department, Faculty of Science, Alexandria University, Alexandria, Egypt Thokozani Majozi NRF/DST Chair: Sustainable Process Engineering, School of Chemical and Metallurgical Engineering, Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, South Africa Samuel Makinde Informatics and CyberPhysical Systems Laboratory, Department of Computer Science, Edo University Iyamho, Auchi, Edo State, Nigeria Saikumar Manchala Department of Chemistry, National Institute of Technology, Warangal, Telengana, India Andrea Yamila Mansilla Grupo de Fisiología del Estrés en Plantas, Facultad de Ciencias Exactas y Naturales, Instituto de Investigaciones Biológicas (IIB), CONICET-Universidad Nacional de Mar del Plata, Mar del Plata, Argentina Yaghoub Mansourpanah Membrane Research Laboratory, Lorestan University, Khorramabad, Iran Juan G. Martínez-Colunga Departamento de Procesos de Transformación de Plásticos, Centro de Investigación en Química Aplicada, Saltillo, Mexico

Contributors

xxxix

Antonia Martínez-Luévanos Department of Advanced Ceramic Materials and Energy, School of Chemical Sciences, Universidad Autonoma de Coahuila, Saltillo, Coahuila, Mexico María Alejandra Martinez Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA) (CONICET-UNMdP), Mar del Plata, Argentina Hanapi Mat Advanced Materials and Process Engineering Laboratory, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, Skudai, Johor, Malaysia José M. Mata-Padilla Consejo Nacional de Ciencia y Tecnología-Centro de Investigación en Química Aplicada, Saltillo, Mexico Diana Iris Medellín-Banda Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México A. L. Meena ICAR-Indian Institute of Farming Systems Research, Modipuram, India B. P. Meena ICAR-Indian Institute of Soil Science, Bhopal, India H. M. Meena ICAR-Central Arid Zone Research Institute, Jodhpur, India M. D. Meena ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur, India V. D. Meena ICAR-Indian Institute of Soil Science, Bhopal, India Swaminathan Meenkashisundaram Nanomaterials Laboratory, International Research Centre, Kalasalingam Academy of Research and Education, Srivilliputhur, India Leticia Melo-Lopez CONACYT-Centro de Investigación y de innovación del Estado de Tlaxcala (CITLAX), Tlaxcala de Xicoténcatl, Tlaxcala, México Centro de Investigación en Química Aplicada (CIQA), Departamento de Materiales Avanzados, Saltillo, Coahuila, México Luis Claudio Mendes Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil Miguel A. Méndez-Rojas Departamento de Ciencias Químico-Biológicas, Escuela de Ciencias, Universidad de las Américas Puebla, Puebla, Mexico Yolanda Peña Méndez Department of Chemistry Science, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Danila Merino Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), CONICET-UNMdP, Mar del Plata, Argentina

xl

Contributors

S. S. Metwally Hot Laboratory Center, Atomic Energy Authority of Egypt, Cairo, Egypt Olugbenga Samuel Michael Cardiometabolic Research Unit, Department of Physiology, College of Health Sciences, Bowen University, Iwo, Nigeria Archil Mikeladze Boron-Containing and Powder Materials Laboratory, F. Tavadze Metallurgy and Materials Science Institute, Tbilisi, Georgia Jiri Militky DMI, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic Shaikh M. Mobin Discipline of Chemistry, Indian Institute of Technology Indore, Indore, MP, India Discipline of Biosciences and Biomedical Engineering (BSBE), Indian Institute of Technology Indore, Indore, MP, India Discipline of Metallurgy Engineering and Material Science (MEMS), Indian Institute of Technology Indore, Indore, MP, India Mokhtar Ali Abduh Mohammed School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad, India Presently at Faculty of Engineering and Information Technology, Taiz University, Taiz, Yemen Anubhab Mukherjee Esperer Onco Nutrition Pvt Ltd., Hyderabad, Telangana, India Sudip Mukherjee Avatar Biopharma Pvt Ltd, Hyderabad, Telangana, India Department of Bioengineering, Rice University, Houston, TX, USA N. V. Swamy Naidu Department of Mechanical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh, India Jarnain Naik Department of Zoology, Karnatak University, Dharwad, Karnataka, India Tayyaba Najam Institute for Advanced Study, Shenzhen University, Shenzhen, China Reshma B. Nambiar College of Animal Science, Zhejiang University, Hangzhou, China Yasir Nawab Textile Composite Materials Research Group, National Center for Composite Materials, Faculty of Engineering and Technology, National Textile University, Faisalabad, Pakistan

Contributors

xli

Susheel Kumar Nethi Fels Institute for Cancer Research and Molecular Biology, Lewis Katz School of Medicine, Temple University, Philadelphia, PA, USA Tri-Dung Ngo Bio-Industrial Research and Development, InnoTech Alberta, Edmonton, AB, Canada InnoTech Alberta Inc., Edmonton, AB, Canada Department of Civil and Environmental Engineering, Faculty of Engineering, 7-203 Donadeo Innovation Centre for Engineering, University of Alberta, Edmonton, AB, Canada Bruna Pes Nicola Laboratório de Reatividade e Catálise (LRC), Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre, RS, Brazil Yulun Nie Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Shuwen Niu Hefei National Laboratory for Physical Science at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, P. R. China Wilson Nwankwo Informatics and CyberPhysical Systems Laboratory, Department of Computer Science, Edo University Iyamho, Auchi, Edo State, Nigeria B. O. Oboirien Department of Chemical Engineering, University of Johannesburg, Johannesburg, South Africa Ea Okotete Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria R. O. Okpuwhara Department of Chemical Engineering, University of Johannesburg, Johannesburg, South Africa J. L. Olajide Department of Mechanical and Automation Engineering, Tshwane University of Technology, Pretoria, South Africa Romina Paola Ollier Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), CONICET-UNMdP, Mar del Plata, Argentina Maurice S. Onyango Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology (TUT), Pretoria, South Africa Ak Osundare Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria Alice O. Oyewo Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology (TUT), Pretoria, South Africa

xlii

Contributors

Ivan Pacheco Department of Pathology, Grey Bruce Health Services, Owen Sound, ON, Canada Department of Pathology and Laboratory Medicine, Schülich School of Medicine and Dentistry, Western University, London, ON, Canada IIPB Medicine Corporation, Owen Sound, ON, Canada Sukanchan Palit Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, India Subhajit Patra Department of Chemical Engineering, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India S. Periyar Selvam Department of Food Process Engineering, SRM Institute of Science and Technology, Potheri, Kattankulathur, Chengalpattu District, Tamil Nadu, India Aravin Prince Periyasamy Departmental of Material Engineering, DMI, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic Anand Babu Perumal College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou, China Vitalii Petranovskii Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Ensenada, Mexico Y. Phaneendra Department of Mechanical Engineering, Vignan’s Institute of Information Technology (A), Visakhapatnam, Andhra Pradesh, India S. L. Pityana National Laser Centre, Council for Scientific and Industrial Research, Pretoria, South Africa Abimbola Patricia I. Popoola Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Olawale M. Popoola Centre for Energy and Electric Power, Tshwane University of Technology, Pretoria, South Africa Patricia Popoola Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Lokeshkumar Prakash Department of Zoology, Karnatak University, Dharwad, Karnataka, India D. Priscilla Mercy Anitha Department of Biotechnology, SRM Institute of Science and Technology, Potheri, Kattankulathur, Chengalpattu District, Tamil Nadu, India Nivedita Pujari Department of Bioinformatics, Karnataka State Akkamahadevi Women’s University, Vijayapura, Karnataka, India Jasmine Qadir Sheri Kashmir University of Agriculture Sciences and Technology, Jammu, India

Contributors

xliii

Thelma Serrano Quezada Department of Chemistry Science, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico Safaa Ragab Environmental Division, National Institute of Oceanography and Fisheries, Alexandria, Egypt Mohammed Muzibur Rahman Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia P. K. Rai ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur, India Sudipta Ramola Department of Environmental Science, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India Ahmad Majdi Abdul Rani Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia T. V. V. L. N. Rao Department of Mechanical Engineering, SRM Institute of Science and Technology, Kattankulathur, India M. Ravichandran Department of Mechanical Engineering, K. Ramakrishnan College of Engineering, Tamilnadu, India Suprakas Sinha Ray Centre for Nanostructures and Advanced Materials, DSTCSIR Nanotechnology Innovation Centre, Council for Scientific and Industrial Research, Pretoria, South Africa Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa Waseem Raza Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India Gurijala Sai Kedar Reddy College of Integrated Studies and School of Physics, University of Hyderabad, Hyderabad, India Armando Robledo-Olivo Department of Food Science and Technology, Autonomous Agricultural University Antonio Narro, Saltillo, Mexico Vicente Rodríguez-González Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan División de Materiales Avanzados, IPICYT, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, Mexico Inocente Rodríguez-Iznaga Instituto de Ciencia y Tecnología de Materiales (IMRE), Universidad de La Habana, La Habana, Cuba María José Romagnoli Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA) (CONICET-UNMdP), Mar del Plata, Argentina

xliv

Contributors

Roohi Protein Research Laboratory, Department of Bioengineering, Integral University, Lucknow, India Nilufar Roozbehani Membrane Research Laboratory, Lorestan University, Khorramabad, Iran Caren Rosales Grupo de Ciencia e Ingeniería de Polímeros (CeIP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA, CONICETUNMdP), Mar del Plata, Argentina José Pablo Ruelas-Leyva Facultad de Ciencias Químico-Biológicas, Universidad Autónoma de Sinaloa (UAS), Ciudad Universitaria, Culiacán, México Suhail Sabir Environmental Research Lab, Department of Chemistry, Aligarh Muslim University, Aligarh, India Aishwariya Sachidhanandham Department of Textiles and Clothing, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, Republic of India Emmanuel Rotimi Sadiku Department of Chemical, Metallurgical and Materials Engineering, Institute of NanoEngineering Research (INER), Tshwane University of Technology (TUT), Pretoria West Campus, Pretoria, South Africa Department of Mechanical Engineering, Maharashtra Institute of Technology, Pune, India J. J. Salazar-Rábago Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico María Florencia Salcedo Grupo de Fisiología del Estrés en Plantas, Facultad de Ciencias Exactas y Naturales, Instituto de Investigaciones Biológicas (IIB), CONICET-Universidad Nacional de Mar del Plata, Mar del Plata, Argentina Norasikin Saman Advanced Materials and Process Engineering Laboratory, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, Skudai, Johor, Malaysia Sintu Kumar Samanta Department of Applied Sciences, Indian Institute of Information Technology Allahabad, Allahabad, India Irem Sanal Department of Civil Engineering, Bahcesehir University, Istanbul, Turkey Laura Mabel Sanchez Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), CONICET-UNMdP, Mar del Plata, Argentina Jeyabalan Sangeetha Department of Environmental Science, Central University of Kerala, Periye, Kasaragod, India

Contributors

xlv

Krishna Moorthi Sankar Department of Mechanical Engineering, University of Petroleum and Energy Studies, Dehradun, India Iván Alonso Santos-López Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León (UANL), San Nicolás de los Garza, México Osman Nuri Şara Department of Chemical Engineering, Faculty of Engineering and Natural Sciences, Bursa Technical University, Bursa, Turkey Bandaru Sateesh Department of Mechanical Engineering, Vignan’s Institute of Information Technology (A), Visakhapatnam, Andhra Pradesh, India Anderson Joel Schwanke Laboratório de Reatividade e Catálise (LRC), Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre, RS, Brazil Mei Qun Seah Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Malaysia Periyar Selvam Sellamuthu Department of Food Process Engineering, School of Bio-engineering, SRM Institute of Science and Technology, Chennai, Tamil Nadu, India Mohd Shabbir School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei Province, PR China Ahmad Shafee Institute of Research and Development, Duy Tan University, Da Nang, Vietnam A. Shafi Environmental Research Lab, Department of Chemistry, Aligarh Muslim University, Aligarh, India Kwok Wei Shah Department of Building, School of Design and Environment, National University of Singapore, Singapore, Singapore Syed Shoaib Ahmad Shah Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur, Pakistan Hefei National Laboratory for Physical Sciences at the Microscale, School of Chemistry and Material Science, University of Science and Technology of China, Hefei, Anhui, People’s Republic of China Khubab Shaker Textile Composite Materials Research Group, National Center for Composite Materials, National Textile University, Faisalabad, Pakistan Naresh Kumar Sharma Centre for Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India M. Sheikholeslami Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Islamic Republic of Iran Renewable Energy Systems and Nanofluid Applications in Heat Transfer Laboratory, Babol Noshirvani University of Technology, Babol, Iran

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Marina G. Shelyapina Department of Nuclear Physics Research Methods, Saint Petersburg State University, Saint Petersburg, Russia Diwakar Z. Shende Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India Arun Kashivishwanath Shettar Department of Applied Genetics, Karnatak University, Dharwad, Karnataka, India A. O. Shirale ICAR-Indian Institute of Soil Science, Bhopal, India Saurabh Shivalkar Department of Applied Sciences, Indian Institute of Information Technology Allahabad, Allahabad, India Shobha Shukla Department of Metallurgical Engineering and Materials Sciences, Indian Institute of Technology Bombay, Mumbai, India Barış Şimşek Department of Chemical Engineering, Çankırı Karatekin University, Çankırı, Turkey A. P. Singh Agricultural Services, Indian Farmers Fertiliser Cooperative Limited, Jaipur, India Mahendra Singh Bihar Agricultural University, Sabour, India Pratibha Singh Soil Science and Agricultural Chemistry, Rajasthan Agricultural Research Institute, SKN Agriculture University, Jobner, India Teekam Singh Department of Mathematics, Graphic Era Hill University, Dehradun, India Santosh Bahadur Singh Department of Chemistry, National Institute of Technology, Raipur, Chhattisgarh, India Udal Singh College of Agriculture, Lalsot, India Yashvir Singh Mechanical Engineering Department, Graphic Era University, Dehradun, India Amneesh Singla Department of Mechanical Engineering, University of Petroleum and Energy Studies, Dehradun, India Gustavo Soria-Arguello CONACYT, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México M. Sridharan Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, India Vadali V. S. S. Srikanth School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad, India Chandra Mohan Srivastava Amity School of Applied Sciences, Centre for Polymer Technology, Amity University Haryana, Gurugram, India

Contributors

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Rajeev Kumar Srivastava Department of Environmental Science, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India Lutfu S. Sua Elazig, Turkey Subramanian Amuthameena Department of Electrical and Electronics Engineering, Sri Krishna College of Technology, Coimbatore, India Dhiraj Sud Department of Chemistry, Sant Longowal Institute of Engineering and Technology (Deemed to be University), Longowal, India Qazi Mohd Suhail Saiyyid Hamid Senior Secondary School (Boys), Aligarh Muslim University Aligarh, Aligarh, India Şana Sungur Department of Chemistry, Faculty of Art and Science, Mustafa Kemal University, Hatay, Turkey K. Suresh Department of Chemical Engineering, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India S. Suresh Department of Chemical Engineering, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India Reza Tahergorabi Food and Nutritional Sciences, North Carolina Agricultural and Technical State University, Greensboro, NC, USA Praveen K. Tandon Department of Chemistry, University of Allahabad, Prayagraj, India Chiaki Terashima Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan Research Center for Space Colony, Tokyo University of Science, Chiba, Japan Devarajan Thangadurai Department of Botany, Karnatak University, Dharwad, Karnataka, India Shivasharana Chandrabanda Thimmappa Department of Microbiology and Biotechnology, Karnatak University, Dharwad, Karnataka, India Na Tian School of Environmental Studies, China University of Geosciences, Wuhan, People’s Republic of China Xike Tian Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Bárbara Tomadoni Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones de Ciencia y Tecnología de Materiales (INTEMA), CONICET-UNMdP, Mar del Plata, Argentina

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Anand Torvi Centre of Nano and Material Science, Jain University, Bangalore, Karnataka, India Otar Tsagareishvili Boron-Containing and Powder Materials Laboratory, F. Tavadze Metallurgy and Materials Science Institute, Tbilisi, Georgia Chui-Shan Tsang Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China Yoshimitsu Uemura Centre for Biofuel and Biochemical Research, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia Vakhtang Ugrekhelidze Quality Management Department, National High Technology Center of Georgia, Ltd, Tbilisi, Georgia Muhammad Umair Textile Composite Materials Research Group, National Center for Composite Materials, Faculty of Engineering and Technology, National Textile University, Faisalabad, Pakistan Dipti Vaya Department of Chemistry, Biochemistry and Forensic Sciences, Amity School of Applied Sciences, Amity University Haryana, Gurugram, India A. Vázquez Centro de Investigación en Biotecnología y Nanotecnología, Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico R. Vennila Department of Chemistry, Adhiyaman Arts and Science for Women (Autonomous), Krishnagiri, India A. A. Vereschaka IDTI RAS, Vadkovsky per. 18-1a, Moscow, Russia Deepak Verma Department of Mechanical Engineering, Graphic Era Hill University, Dehradun, India S. Vigneshwaran School of Automotive and Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil, Srivilliputhur, Tamil Nadu, India María Emilia Villanueva Facultad de Farmacia y Bioquímica, Departamento de Química Analítica y Fisicoquímica, (UBA), Universidad de Buenos Aires (UBA), Buenos Aires, Argentina Instituto de Química y Metabolismo del Fármaco (IQUIMEFA), CONICET – Universidad de Buenos Aires, Buenos Aires, Argentina Departamento de Ciencias Básicas, Universidad Nacional de Luján (UNLu), Buenos Aires, Argentina P. A. Vivekanand Department of Chemistry, Saveetha Engineering College, Chennai, India

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Shraddha S. Wadatkar Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India Gongming Wang Hefei National Laboratory for Physical Science at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, P. R. China Yanxin Wang School of Environmental Studies, China University of Geosciences, Wuhan, China Kailas L. Wasewar Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India Kwok-Yin Wong Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hong Kong, China Teng Xiong Department of Building, School of Design and Environment, National University of Singapore, Singapore, Singapore Roberto Yañez-Macias Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México Priya Yadav Department of Chemistry, Biochemistry and Forensic Sciences, Amity School of Applied Sciences, Amity University Haryana, Gurugram, India Surabhi Yadav College of Integrated Studies and School of Life Sciences, University of Hyderabad, Hyderabad, India Vijay Laxmi Yadav Department of Chemical Engineering and Technology, IIT BHU, Varanasi, Uttar Pradesh, India Chao Yang Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Qilong Yuan Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China Yılmaz Yurekli Bioengineering Department, Engineering Faculty, Manisa Celal Bayar University, Manisa, Turkey Norhaniza Yusof Advanced Membrane Technology Centre (AMTEC), Universiti Teknologi Malaysia, Johor, Malaysia School of Chemical and Energy Engineering (FCEE), Universiti Teknologi Malaysia, Johor, Malaysia Mohd Yusuf Department of Chemistry, YMD College, Maharshi Dayanand University, Nuh, Haryana, India

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Mohammed Rehan Zaheer Department of Chemistry, Gagan College of Management and Technology, Aligarh, UP, India Mohammad Zain Khan Environmental Research Lab, Department of Chemistry, Aligarh Muslim University, Aligarh, India Reshma N. Zakane Department of Biological Sciences, Birla Institute of Technology and Science Pilani, KK Birla Goa Campus, Zuarinagar, Goa, India Xiandi Zhang Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China Sahar Zinatloo-Ajabshir Department of Chemical Engineering, University of Bonab, Bonab, Iran

Part I Fundamentals of Nanomaterials and Nanocomposites

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Nanomaterials and Nanocomposites: Classification and Toxicity Ivan Pacheco and Cristina Buzea

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial Dimensionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composition, Crystallinity, Uniformity, and Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofiller-Reinforced Macroscale Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites with Both Matrix and Fillers at Nanoscale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Release of Nanoparticles from Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Airborne Nanoparticles During Handling of Nanofillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Release of Nanoparticles from Cutting of Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drilling of Nanocomposite and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sanding of Nanocomposites and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crushing of Nanocomposite and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Stress of Nanocomposites and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . Fracture of Nanocomposites and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Photodegradation and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion, Desorption, or Dissolution of Nanoparticles from Nanocomposites into Liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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I. Pacheco (*) Department of Pathology, Grey Bruce Health Services, Owen Sound, ON, Canada Department of Pathology and Laboratory Medicine, Schülich School of Medicine and Dentistry, Western University, London, ON, Canada IIPB Medicine Corporation, Owen Sound, ON, Canada e-mail: [email protected] C. Buzea IIPB Medicine Corporation, Owen Sound, ON, Canada e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_1

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Incineration of Nanocomposites and Release of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toxicity of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter overviews nanomaterial and nanocomposite classification together with the main aspects of their toxicity. The classification of nanoparticles and nanofibers according to their size, composition, and aspect ratio together with exemplified electron microscopy images facilitates understanding the role of nanoparticles in determining the properties of a nanocomposite. Nanocomposites themselves encompass a very broad category of materials. The classification of nanocomposites according to the dimensionality of their component phases shows the existence of two main categories of materials: those having a matrix phase at macroscale and a filler at nanoscale or both matrix and filler at nanoscale, such as core–shell nanoparticles or decorated fibers. Nanocomposites are shown to release fragments during their manufacturing, use, and disposal. Cutting, drilling, and sanding of nanocomposite pose an occupational exposure risk to workers. Thermal and mechanical stress, photodegradation, interaction with liquids, and incineration processes are all shown to result in the release of fragments at nano- and microscale. The released nanoparticles and nanofibers that are airborne are potentially toxic due to the possibility of inhalation. Their toxicity resides in the ability of nanoparticles to be pervasive, bypass organisms’ defense systems, travel through the pores of fenestrated tissues, become systemic, enter cells, and disrupt cellular processes leading to a gamut of diseases. In order to create safe nanocomposites, it is important to determine the types of nanomaterials that pose a health risk and to limit their use or mitigate their toxicity.

Introduction This chapter discusses the topics of nanomaterials and nanocomposites, their classification, and toxicity. Nanocomposites are materials composed of several phases, at least one of them showing dimensions in the nanometer range [1]. The nanoscale phase, or nanofiller, confers the nanocomposite material unique property combinations compared to that of the matrix alone. There are also nanocomposites that are entirely at nanoscale, such as core–shell nanoparticles and coated nanoparticles, among others. Therefore, in order to understand nanocomposites, one must understand first nanofillers as materials with dimensions at the nanoscale and their general classification according to their size, morphology, dimensionality, composition, uniformity, and agglomeration. With a better comprehension of the gamut of nanofillers, the classification of nanocomposites brings into focus the myriad of possibilities for improving material properties. Whereas the fabrication of nanocomposites has received significant research interest, there are far fewer studies that assessed the release of fragments from nanocomposites and their environmental and toxicological

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implications. These studies demonstrate the release of nanoparticles and microparticles from nanocomposites during different stages of their production, use, and disposal. As the released particles become airborne, they have the potential to cause adverse health effects to exposed subjects, such as workers and consumers. While there is a scarcity of studies related to the toxicity of fragments released from nanocomposites, a substantial volume of research on the toxicity of nanoparticles made of specific materials is already available, which one can ultimately extrapolate to the materials that compose the fragments of nanocomposites or be a starting point for further research.

Nanomaterial Classification Nanomaterials are materials that have particles or components at the nanoscale. Technically speaking, most of granular polycrystalline materials that have grains smaller than a micron would qualify as a nanomaterial; however, here we will focus on nanoparticles and nanofibers that have at least one dimension at nanoscale. The classification of nanomaterials is essential for a better understanding of their use as nanofillers in nanocomposites. Nanomaterials can be classified according to their dimensionality, size, morphology, composition, uniformity, and agglomeration state, among others, as seen in Fig. 1 [2]. Each of these nanoparticle properties, especially size, morphology, and

Fig. 1 Nanomaterial classification. (Reprinted with permission from Ref. [2]. Copyright [2007], American Vacuum Society)

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composition, is essential in dictating their electromagnetic and chemical properties [3]. What makes nanoparticles different from their larger counterparts of the same material is the existence of quantum size effects and an increased surface area for the same volume of material in nanoparticulate form compared to microparticulate form or even bulk [2]. Strange things happen at the nanoscale! Let’s take gold, for example: • Bulk gold is diamagnetic but becomes ferromagnetic when in the form of small nanoparticles [3]. • Gold melting temperature in bulk form is 1,337 K and decreases hundreds of degrees to about 1000 K for gold nanoparticles with a size of 3.8 nm. • Gold nanoparticles with different sizes have different colors in solution as a result of surface plasmon resonance; when gold nanoparticles are subjected to light, the electrical component of the field displaces the conduction-band electrons creating uncompensated charges at the nanoparticle surface [3].

Nanomaterial Size First of all, they are called nanomaterials because they have at least one dimension in the nanoscale, more precisely between one nanometer and hundreds of nanometers. Nanomaterials with all dimensions in the nanometer range are called nanoparticles. The smallest nanoparticles would be large molecules, while larger nanoparticles could also qualify as small microparticles. The upper size limit for nanoparticles is somehow of a controversy and in our opinion not such a relevant matter, the reason being that it is just an arbitrary number [2]. The size at which the properties of nanomaterials become different from those of micromaterials is not a size fits all but depends on material type, temperature, and other factors [3, 4, 5, 6, 7]. Also, this size threshold of nanoparticles is not related to their toxicity in living beings either. Very small nanoparticles are usually called clusters, being formed of several hundreds up to thousands of atoms. The size of clusters can go up to several nanometers, intermediate between that of molecules and that of relatively small nanoparticles. Interestingly, small nanoparticles or clusters are special in the sense that they exhibit properties which are distinct from both of molecules or bulk of the same material [8]. In addition, small atomic clusters containing about a hundred atoms exhibit a stronger size dependence of their properties compared to larger particles [9]. As the size of a nanoparticle becomes smaller, both surface and quantum confinement effects become important in dictating their physicochemical properties [8]. An important question follows: what is the size of nanoparticles for which their properties are comparable to those of bulk? There is no magic size for this transition, the answer depending on which property and material type are being considered [8]. Table 1 shows the size at which specific properties change compared to bulk of the same material. For a nanoparticle with a size of 3 nm, the number of bulk atoms is equal to the number of atoms at the surface of nanoparticle. If we decrease the size, then the surface effects are predominant compared to the bulk,

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Table 1 Physicochemical and mechanical properties that change at a specific threshold size for nanoparticles compared to larger counterparts and bulk of the same material. The change can be gradual, as in melting temperature, or sudden, as in catalytic activity. The change may be due to quantum effects or to surface effects Physicochemical or mechanical property Number of bulk atoms equal to number of surface atoms Catalytic activity Acquired magnetism of materials that are nonmagnetic in bulk form (au, Pt, Pd) Quantum-dot fluorescence Quantization of energy band metals (spacing of 1 K) Decrease in melting temperature compared to bulk Changes from ferromagnetism to superparamagnetism Modified hardness Optical properties Au nanorod luminescence Crossing cell membranes Lung alveolar deposition

Nanoparticle threshold size 3 nm 5), while absorption of As (III) displays parabolic behavior, with maximum adsorption at a pH of around 8.5 [2]. Strategic functionalization of nanoclays allows optimization of the process of absorption of some polluting metals because modifications of the original clays allow control of their composition and change several parameters – such as the surface area, interlaminar space and volume, and porosity – that are responsible for the adsorption and desorption properties of the clays. In this sense, pretreatment of clay materials with chemicals such as inorganic and organic reagents can enhance As removal from aqueous media over the full pH range [2, 7]. Surface modifications of nanoclays by iron(III) [Fe(III)] oxide and hydroxide or Fe(III) species intercalation generate superficial changes in the clays [5, 7]. Such modifications can induce major changes in the surface and pores of the materials and therefore can change the adsorption and desorption properties of the clays [2, 5, 7]. This aim of this chapter is to review current developments in the use of natural and functionalized nanoclays for removal of As by adsorption processes. The outlook and future prospects for this methodology are also discussed.

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A Brief History of Arsenic: The Past and the Present For 2400 years, the natural substance called arsenic was used for therapeutic purposes. Arsenic was mainly used to fight diseases of infectious origin – such as syphilis, ulcers, and anemia – without consideration of the consequences of this treatment. However, its use for prolonged treatment caused deterioration in human health [11]. Its medical uses were therefore suspended, but with advancements in science, different chemical substances containing As were formulated that did not have harmful effects on human health [11]. Salvarsan (also known as arsphenamine) was the first chemical compound synthesized in the laboratory, was not toxic for patients, and was effective in curing diseases. This important development led to Paul Ehrlich and Ilya Ilyich Mechnikov winning the Nobel Prize in Physiology or Medicine in 1908 [12, 13]. Over time, the popularity of arsenic increased as a result of its promising physicochemical properties. However, because arsenic is a painless, colorless, and tasteless substance, it began to be used as a poison [11]. Detection of the cause of arsenic-related deaths was confounded because the symptoms were similar to those of cholera, which was a common disease [11]. From around 1800, arsenic started to become well known as a “silence poison” because it was implicated in cases of murder and deaths of unknown causes, including the death of Napoleon Bonaparte [1]. On the other hand, different types of farm animals were being fed dietary supplements containing arsenic [1, 14]. Such substances promoted rapid growth in chickens and other farm animals. However, this practice was discontinued because of concerns about exposure to arsenic in humans who ate bird meat [1, 14]. For thousands of years, different uses of arsenic have triggered problems worldwide. These have impacted the environment and, in particular, human health [1].

Global Distribution of Arsenic Arsenic is an essential element and a complex phenomenon [4, 7]. This element causes both immediately toxic effects and longer-term problems in humans. Longterm exposure to arsenic present in the environment has prejudicial effects on health worldwide [4, 5, 7]. Different types of cancers (bladder, lung, skin, and kidney), neurological disorders, and muscular weakness are among them. Arsenic can also cause loss of appetite, nausea, a dry throat, sharp pains, jaundice, erythema, and tingling in the extremities. Arsenic toxicity principally affects the airways, gastrointestinal tract, and cardiovascular and nervous systems [1, 4, 5, 7]. This element is found in the atmosphere, rocks and soils, organisms, and natural waters. However, combinations of natural processes – such as biological activity, volcanic emissions, and climatic conditions, among others – generate environmental problems through arsenic mobilization [6, 10, 15]. Furthermore, human activity has had an important impact on arsenic distribution with anthropogenic causes such as mining activity, use of pesticides and herbicides, combustion of fossil fuels, and use of arsenic as an additive in livestock feed (Fig. 3). Because of this, the global

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Fig. 3 Sources of arsenic contamination in the environment

Fig. 4 Chemical structures of arsenite and arsenate

distribution of arsenic has very important implications for both the environment and human health [1, 4–7, 16]. In natural waters, arsenic is found in a variety of forms: soluble, particulate, and organic bound [1, 4, 5, 7]. Inorganic arsenic compounds are approximately 100 times as toxic as organic arsenic compounds [1, 4, 7]. Inorganic arsenic species exist mainly in two predominant forms: arsenite As(III) and arsenate As(V) (Fig. 4) [1, 4, 5, 7, 16]. Trivalent arsenic is more toxic than pentavalent arsenic and causes more biological damage to humans and the environment [4, 5, 7, 16]. Arsenic contamination of water is a major global problem, affecting both underground aquifer basins and drinking water [1, 3]. The danger of exposure comes mainly from drinking water extracted through excavation in regions where sedimentation rich in As predominates [1, 3]. The available information on arsenic-affected populations is incomplete, but it is estimated that more than 200 million people in more than 120 countries are probably at risk of exposure to water contaminated with arsenic [1, 3, 6, 10, 17] (Fig. 5). High arsenic content has been detected in underground aquifer basins in countries such as the USA, Chile, Mexico, Bolivia, Peru, Argentina, Cambodia, China, Vietnam, Bangladesh, India, Thailand, Nepal, and

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Fig. 5 Global distribution of arsenic in groundwater. (Adapted from Litter et al. [10])

Ghana [1, 3, 10, 18]. In the Americas, 16 countries have been affected by this problem. The populations most affected have been low-income groups. However, many regions in first-world countries have also been affected [1, 3, 6, 18]. The biogeochemistry of arsenic in seawater is complex [19]. The total arsenic concentration in open ocean waters is 0.5–3 μg/L [19] and varies between different. Depending on which areas of the sea. Microbial activities contribute to distribution of As, in conjunction with photochemical reactions [19]. However, rainwater usually contains an average arsenic concentration of 19 ng/L [15, 19]. The arsenic cycle is caused by interactions between natural water and rock, with atmospheric influence [15, 19]. Different rivers with low arsenic content have been found in North America, South America, Europe, and Asia [15]. Among the main rivers affected globally, the Padma River and Meghna River in Bangladesh have been found to contain arsenic concentrations of up to 2 μg/L [15]. The aquifer basins affected by this problem include rivers in China. Some rivers that originate in the Andes Mountains flow into the north of Chile, Bolivia, and Peru, and arsenic concentrations are high in these rivers [15]. Furthermore, in Ghana, rivers in a region of 1600 km2 have been shown to be contaminated by arsenic concentrations of up to 7900 μg/L [15]. The distribution of arsenic in different water beds worldwide generates numerous problems in the environment and in society. The presence of As in drinking water is a global public health problem [4, 5, 10]. The WHO accepts arsenic content of up to 0.01 mg/L in drinking water [10], but individual countries apply this regulation according to their own laws [10]. Because the As content exceeds the levels allowed by the WHO in many parts of the world, new research into methodologies to reduce the content of this metalloid in water has been initiated [2, 5, 7, 10].

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Arsenic Removal Arsenic removal methodologies are principally aimed at decontamination of water resources for intake. According to the WHO, the daily requirement for safe water is 20 L per person [10, 18]. In this respect, different methods – such as coagulation, flocculation, sedimentation, filtration, inverse osmosis, ion exchange, and sorption techniques – have been used to decrease or eliminate As in aqueous systems [2, 5, 7, 8, 18]. Table 1 summarizes some of the advantages and disadvantages of each technique. Most techniques are more efficient when the initial arsenic concentration is high (>100 mg/L). However, these methodologies fail to reduce the amount of As present in water to the permitted limits, leaving harmful residual concentrations of arsenic [2, 7, 8, 18]. Adsorption techniques are considered more efficient for removal of low arsenic concentrations in aqueous systems [2, 7, 8].

Table 1 Advantages and disadvantages of arsenic (As) removal methodologies Techniques Coagulation, flocculation, sedimentation, filtration

Advantages Ideal for waters with Fe and Mg content Low capital investment

Inverse osmosis

95% As removal pH non dependent High efficiency in water with high concentrations of As and a high pH Good at nitrate and chromate removal

Ion exchanges

Adsorption

>95% As removal Low costs Simple operation Selective technique for As Durability

Disadvantages five folds of oxygen/TiCl4 ratio) at high temper
ature (700~1000 C) [33].

TTIP Decomposition and Hydrolysis TTIP is thermal decomposed to form TiO2, hydrocarbon, and water via equation (9). The activation energy varies from 40 to 164 kJ/mol at temperature ranging from 623
C to 873
C [34]. However, hydrolysis was found to have faster reaction rate than that of thermal decomposition due to reaction of activation energy by water molecules (see Equation (10)). Activation energy of hydrolysis of TTIP was reported at 8.43 kJ/mol [34]. It was reported that kinetics growth rate of TiO2 film by decomposition of TTIP under N2 condition was expressed by 4.0  1011 exp(40 [kJ.mol1]/RT)[TTIP][N2] [33–35].

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TiðOC3 H7 Þ4 ➔TiO2 þ 4C3 H6 þ H2 O

ð9Þ

TiðOC3 H7 Þ4 þ 2H2 O➔TiO2 þ 4C3 H7 OH

ð10Þ

Particle Formation TiO2 particle formation follows by three important steps: precursor’s feeding, reaction/nucleation, and growth (collision-coalescence). Liquid-fed route: In general, TTIP and TBT are used in this feeding method. Precursor solution is prayed into the reaction zone. Then, solvent is evaporated and precursor encounters precipitation and reaction (decomposition and/or hydrolysis) to form small particles inside droplet-like spheres (see Fig. 4a). At lower feed flow rate, droplets are free of aggregation. Therefore, particles grow homogeneously to form individually spherical particles. However, at high feed flow rate, droplets and nucleated particles commonly agglomerate each other. If the reaction temperature is low, final products composted of clusters of aggregated spherical particles. If the reaction temperature is higher than the melting point of materials, particles collapse and form dense spherical particles with bigger sizes [2, 3]. Gas-fed route: In most case, TiCl4 was used in this route because it has lower boiling point comparing to TTIP and TBT. Liquid precursor is evaporated to gasphase precursor by heating method. At the reaction zone, homogeneous reaction and condensation of gas-phase precursor happened, subsequently [34]. TiO2 nucleates formed cluster of smaller crystallites, called primary particles. The later step is growing step, which happens similar to that of liquid feeding process. The primary particles grow by surface reaction/deposition to form bigger particles. At slow flow rate, particles grow homogeneously to form very fine particulates with homogeneous size dispersion. At high feed flow rate, nucleation is fast. Surface deposition growth is less probable than formation of individual particles, which results in existing of many numbers of particles aggregated together. If the reactor is performed at high temperature or slow cooling rate, cluster of particles will sinter and collapse to form bigger particle. When reactor is performed at low temperature or rapid cooling, final products compost of aggregated structures of particles (see Fig. 4b).

Size, Phase, and Structure Control Operational Conditions Simple controlled parameters in gas-phase synthesis are operational conditions such as types of precursors, precursor’s feeding rates/methods and concentrations, reaction temperatures, and gas flow rates. Types of precursors were not found to have significant effects on phase and size of particles. However, the morphological structures of particles differed by types of precursor selected. Generally, using

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(a) Liquid-Phase Feeding

Homogeneous growth

w Slo Hi g

h fe

d fee

ed

flo

te wra

flow rate

e Low t Low

Droplet

Droplet evaporation

ture mpera

tem p

erat

Nucleation Reaction

ure

Particle Growth

(b) Gas-Phase Feeding Homogeneous growth

ee d wf Slo Hig

hf e ed

rate flow

flo w

rat e

g e r in Sint

Ra

Homogeneous reaction/condensation

Reaction and Nucleation

pid

le /coa

s ce n

ce

coo l in g

Particle Growth

Fig. 4 Particle formation during gas-phase synthesis

TTIP and TBT as precursors produced spherical particles, while using TiCl4 produced particulates (polyhedral) [19, 38]. Precursor concentration is one of the important operational parameters, which give great impacts to phases, sizes, and structures of final products. Primary particle sizes generally increase with the increase of precursor concentration. Increase of the concentration leads to enhance nucleation rate and surface reaction [34, 38]. In rare cases, increase of the concentration decreases primary particle size due to increase numbers of nuclei, which revolutionize to form individual particles (not surface growth). In contradictory to primary particles, secondary particle sizes always increase with increase of concentration of precursor due to aggregation effects [34, 38]. However, precursor

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concentration was reported to have only minor effects of crystal phase of TiO2 from TTIP decomposition. By using hot-wall reactor, Chin et al. (2011) increased the TTIP concentration from 9.65  106 M to 1.37  105 M, and anatase fraction varied from 95.4% to 94.7%, which is almost unchanged [39]. However, precursor concentration has significant effects in TiCl4 oxidation in high oxygen atmospheres but less effects in low oxygen atmospheres [40]. Temperature is very important parameter for controlling phases, structures, and sizes of particles because temperature affects reaction, nucleation, collision, and sintering rate. Generally, increase temperature enhances the rutile formation. Rutile
starts forming at reaction temperature higher than 900 C for TiCl4 oxidation in hotwall reactor [40]. The maximum temperature for rutile formation was around 1100–
1300 C. Rutile was not further increase when temperature increased further [40, 41]. At that high temperature, anatase particle’s size growth faster than phase transformation, as result of decrease of rutile phase. Suyama et al. (1975) suggested that anatase nucleation took place at the early stage, while rutile one at the latter stage of reaction. As result, rutile particle sizes are smaller than those of anatase [40]. Rutile phase of final products was believed to form from phase transition from anatase clusters along to reaction zone. Temperature also strongly affects sizes and the structures of TiO2 nanoparticles. The change of primary particle sizes with temperature is varied depending on level of temperature ranges. In most case, primary particle size decreases with the increase of reaction temperatures because nucleation rate becomes faster than growth rate [8, 42]. In difference from primary particles, secondary particles always increased with the increase of temperature. Increase temperature leads to particle’s growth by sintering/coalescence of clusters of primary particles [34]. Gas composition and gas flow rate also account for change of particle’s properties. Faster gas flow rate reduces the resident time of reaction and gives rise to less chance of particle’s growth, less agglomeration, and less rutile formation. Gas composition was found to have effects during TiCl4 oxidation. Increase of the oxygen concentration was found to reduce the particles size. This attributes to faster nucleation rate than growth rate with the increase of oxygen concentration [8]. Rutile was increased with the increase of oxygen concentration. There are two possible reasons: increased oxygen reduces the size of anatase (size effect on rutile transition) but increases the nucleation rate of rutile [40].

Additives The effects of water contents were investigated on the effects of particle sizes, structures, and phases. Nucleation and surface reaction compete each other depending on the temperature at the contact of water and precursor. It was found that preheated H2O at close to reaction temperature before contacting the precursor enhanced the nucleation rate, which resulted in reducing particle sizes with the increase of H2O [34, 42]. It was observed that addition of water vapor created spherical shapes of particles [42]. Numbers of works were proceeded to control phases of produced TiO2 nanoparticles by addition of other elements. Addition of some elements such as Fe3+, Al3+, and Sn4+ were reported to increased rutile phases [11]. In contrast, addition of Si4+ was shown to retard the rutile phase formation [34].

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Recent Works on TiO2 in Gas-Phase Synthesis Gas-phase synthesis has been already utilized for large-scale production of TiO2 nanoparticles for commercialized purposes. However, recent research is still undergoing for synthesis of functional TiO2 nanoparticles with complexed structures such as heterostructures with other materials and doping multiple elements. Reactor designing and simplifying the process also minimize the energy consumption and production cost. Rahiminezhad-Soltani et al. (2019) were able to synthesize anatase
TiO2 at low temperatures (~400 C) using H2O-assisted atmospheric pressure chemical vapor synthesis [14]. It was interesting that particles obtained by liquid-phase H2O addition were smaller than that of gas-phase H2O addition (13 nm vs. 41 nm). High heat capacity of liquid-phase H2O reduced the temperature of TiO2 monomers and primary particles, and it also reduced the coagulation, coalescence, agglomeration, and sintering of the nanoparticles. This low-temperature gas-phase synthesis would give an important technique for lower-energy-consumed process. Besides, recent trends have been also focused on heterostructures. Various types of TiO2 composites such as WO3/TiO2 [43], TiO2/CuO [15], TiO2/Pd [23], TiO2/Pt [22], TiO2/Pt/Co [44], and TiO2/Au/Pd [24] have been synthesized. Ramadhan et al. (2019) synthesized WO3/TiO2 by single-step FSP from mixtures of TTIP, ammonium metatungstate hydrate, and dimethylformamide, as a precursor [43]. From CH4-O2 combustion flame, FSP-synthesized WO3/TiO2 showed very low crystal

linity, which required further annealing from 800 C to 1000 C. Annealed WO3/ TiO2 composite was proposed as good materials for electrochromic devices. Chen et al. (2019) used co-flow FSP to produce CuO/TiO2 photocatalysts for combustion of lean CO [20]. A mixture of precursor composted Cu(NO3)2•3H2O, ethanol, and TBT was injected into burner of CH4/O2 flame by syringe pump. Introduction of copper precursor affected the crystallinity and phase of TiO2. Specific surface area (SSA) was reduced from 98.96 to 70.07 m2/g with increase of Cu precursor from 2 at.% to 20 at.%. The sharpen XRD peak with increase of Cu contents demonstrated the increase of crystal sizes. More interestingly, rutile phase was increased from 27.6% to 56.2% with increase of Cu content from 2 at.% to 20 at.%, respectively. It was observed that Cu2+ preferentially doped into TiO2 lattices due to its small radius size and less valent state than Ti4+, and created oxygen vacancies by reducing the Cu2+. Oxygen vacancy formation enhanced the oxygen diffusion rate and further increased the sintering rate of TiO2. In addition, defective TiOx species should be an important nucleation center for rutile. Therefore, rutile phase increased with the increase of Cu2+ doping contents. However, at high concentrations, Cu could not dope into TiO2 lattice anymore. Excessive Cu2+ aggregated on the surface of TiO2 and formed CuO nanoparticles on the surface of TiO2. Beside heterostructures with semiconductor materials, composite structures with metal nanoparticles and/or clusters were also reported in single-step gas-phase synthesis. Fujiwara et al. (2016) synthesized Pd sub-nanocluster/TiO2 composites by FSP from solution precursor of 2-ethylhexanoic acid, acetonitrile, TTIP, and palladium acetylacetonate under CH4/O2 flame [23]. Pd cluster (50%) nitrogen-doping concentration was not further increased. The highest N-doping concentration was up to 0.96 at.%. They claimed that reduced flame enthalpy by addition of water improved the N-doping into TiO2 lattice [37]. Recent work reported by Bi et al. (2019) used NH3 as nitrogen source for N-doping in FSP-synthesized TiO2 [18]. NH3 was carried by N2 and supplied to the reactor by second nozzle placed on the top of the flame. This procedure is similar to N-doping prepared by conventional NH3-annealing method. The dominant doping is interstitial sites at the surface of TiO2 particles. This in-situ N-doping procedure was also used to synthesize composite of Pt/TiO2 heterostructures, and it showed higher photocatalytic CO oxidation and high thermal stability due to Pt-N bond formation [18]. In gas-phase synthesis, multi-structures of TiO2 such as doping, heterostructures, and combined doping and heterostructures are able to synthesize within single-step approach by feeding methods of precursors. Figure 5 summarizes the feeding methods in FSP to obtain various composite particles. In doping system, metal precursors were generally premixed and sprayed into the same flame (Fig. 5a). In this procedure, homogenous mixture of metal precursors is very important to ensure the homogeneous doping behavior. The types of precursors and solvents need to be carefully selected to make sure that all metal precursors dissolve well into the solvents used. Based on the solid solution properties of materials, doping concentration is limited. Therefore, at high concentration of doping elements secondary phases always formed. This unintentional binary structure can be a good heterostructure that enhanced activities of TiO2. Sometimes, the deposition of inactive secondary phase on TiO2 could reduce the activities due to coverage of active sites on TiO2 for reaction. Premixed precursors also limit with TiO2/metals

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(a)

969

(b)

Precursor 2 Precursor 1

Precursor 1 + 2

(c)

Precursor 1

Precursor 2

Fig. 5 Feeding modes for synthesis of composite materials by FSP: (a) All precursors are mixed and feed together; (b) precursors are feed separately into different pots of the same nozzle (burner); (c) precursors are feed separately into different nozzles

heterostructure synthesis. Metal nanoparticles (active materials) are covered or encapsulated by TiO2 nanoparticles, which result in lower catalytic activities. Second approach of separate feedings of precursors was developed. Metal precursors are fed separately into the same flame or different flames. Figure 5b shows that metal precursors are fed through different route into different pots of the same flame. Lin et al. (2019) used this route to synthesize PtTiO2 composites from TTIP and Pt(acetate) precursor [22]. Due to lower decomposition temperature of TTIP, TiO2 monomers formed at the early stage, while Pt (acetate) decomposes and formed Pt vapors at the latter stage along the hightemperature flame. At downstream of lower temperature flame, Pt vapors condensed either on TiO2 particles/clusters and/or nucleated homogeneously in the gas phase, followed by subsequent deposition by collision with TiO2 nanoparticles. Alternatively, 2-nozzle FSP was also reported as an effective method to produce

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different components of oxide-based materials with minimizing the alloy formation (and/or doping). Figure 5c represents the 2-nozzle FSP for synthesis of two metal oxides. Precursors are sprayed into two different nozzle flames. This method ensures the well mixtures of multicomponent nanoparticles and facile control of individual components. 2-nozzle FSP was also utilized for synthesis of other catalytic materials such as Pt-FeOx/CeO2, MnOx/Al2O3, and FeOx/Al2O3 [24].

Conclusions and Further Outlooks With its long history, gas-phase synthesis has been considered as a potential method for synthesis of TiO2 nanoparticles ranging from lab scales to industrial scales. Direct conversion of TiO2 particles from titanium precursors without any separation process or post-thermal annealing allows this method to produce TiO2 products with low cost, compared to wet chemical methods. General reactors used in gas-phase synthesis were classified based on the thermal sources of reaction: hot-wall reactor, flame reactor, and plasma reactor. Within these tree types of heating procedure, gas-phase synthesis enables to produce TiO2 nanoparticles with controllable product’s properties based on simple operation parameters such as precursors and feeding types, precursor concentrations and compositions, gas flow rates and gas compositions, reaction atmospheres and additives, and reaction temperatures. So far, TiO2 nanoparticles with various morphological structures such as spherical, particulate, and rod shapes were synthesized. The common crystal phases of TiO2 such as anatase and rutile were easily tuned. Particle sizes ranging from several nanometers to several micrometers can be controllable. Advanced structures become the current trends of phase-phase synthesis to improve the properties of TiO2 nanoparticles according to the target applications. Typically, to deliver the TiO2 nanoparticles as promising candidate to sustainable world, TiO2 must effectively works under solar light, which is abundant and clean energy on earth. Defect and bandgap engineering such as doping and heterostructures is demonstrated for powerful way to improve the activities of TiO2. However, solving technological barriers in the gas-phase synthesis is undergoing. Acknowledgments This research was financially supported by a Grant-in-Aid for JSPS Fellows (18F18337) from the Japan Society for the Promotion of Science (JSPS). S. Khan would like to thank JSPS for supporting his postdoctoral research (standard-P18337). V. Rodríguez-González thanks CONACyT, Mexico, for the Visiting Scientists and Scientists on Sabbatical Leave Fellowship Program 2018–2019.

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Laser Additive Manufacturing of Nanomaterials for Solar Thermal Energy Storage Applications

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar Thermal Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensible Heat Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermochemical Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Latent Heat Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Stability and Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fabrication Techniques of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser Additive Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SLM Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SLM Scan Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges of the SLM Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the energy industry, solar energy is extracted from the sun, the principal source of energy among other workable power sources. Given the sun’s indeterminate and sporadic nature, optimization of the thermal conversions in sunlight-based energy by integrating thermal energy storage systems to reserve and store available and/or surplus energy for power production and cooling applications is crucial. Also, materials used for storage must be hazard-free, lightweight, and cost-effective and have an excellent performance of hydrogen absorption and proficiency to withstand extreme environmental conditions. Nanophase change materials with carbon-based nanoparticles among other nanomaterials show most M. Dada (*) · P. Popoola Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_178

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of these attributes and are often used for solar thermal energy storage; however, the low thermal conductivity of phase change materials causes a slow discharge rate attributed to the manufacturing process. Other challenges of using conventional methods of fabricating phase change materials are time consumption in machining the part geometry and difficulty in fabrication leading to a 90% waste volume. Selective laser sintering, a laser additive manufacturing technique, is versatile, customizable, and the most efficient technology in fabricating complexshaped parts, layer by layer, promising no excesses or waste, and the thermal properties of sintered parts can be improved by optimizing process parameters. Hence, laser additive manufacturing is an effective advanced manufacturing technique for fabricating nanomaterials with excellent thermal conductivity for solar thermal storage applications. Keywords

Solar energy · Nanomaterials · High entropy alloys · Thermal stability and heat transfer · Thermal energy storage · Laser additive manufacturing

Introduction The primary consumption of energy has been estimated to continue thriving according to Asia/World Energy Outlook 2016 [1]. Globally, the demand is expected to increase on average by 1.2% each year at 13.7 billion tons of oil equivalent from year 2014 to 18.9 billion tons of oil equivalent in year 2040 [2]. In no particular order, the oil will increase by 29%; coal will increase by 24% and natural gas by 25%. This means fossil fuels which have a combined share of 78% will make up the bulk of the world’s energy supply, and this will remain one of the most dominant sources of energy [3]. Coal with its cost competitiveness and abundance in diverse places contributes to a stable power supply and security [4]. The nonconventional natural gas also with its abundant resource and stable supply through LNG supplies and pipelines has the cleanest environmental attributes [5], while oil in its liquid state is the most economically efficient source of energy to handle and transport. However, these fossil fuels have serious environmental issues including CO2 emissions [6]. The industrial age has seen an increase in the CO2 levels approximately by 30% because the biological cycle experiences difficulties in adjusting rapidly and sufficiently in absorbing the amount of CO2 produced by fossil fuels and released to the atmosphere [7]. Furthermore, the population growth coupled with a concomitant industrial and agricultural growth has coincided with an increase in greenhouse gases and atmospheric CO2 levels [8, 9]. This has ultimately led to the increase in the global temperature accelerating the climate changes and sea levels beyond natural means and resulting in adverse global changes in agricultural productivity, diseases, and inundations of low-lying coastal areas [10].To solve these problems, natural and artificial innovative energy technologies such as solar, hydrogen, wind, bioenergy, fusion, artificial photosynthesis, and fission are alternative renewable

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Table 1 Other renewable energy sources are the following Source Hydropower Wind power Geothermal

Other sources are ocean waves and tides, biomass

Advantages Fueled by water, clean source of fuel, renewable, available, cheap Unlimited, natural resource, renewable, clean Renewable, environmentally friendly, stable

Renewable, low cost of maintenance, clean source of fuel, lasts long, predictable, no environmental impact

Disadvantages Disrupts ecosystem, water pollution, may cause drought Costly, technology immaturity, dangerous to wildlife, noise pollution Requires electricity for heat pumps, requires lots of water, costly, environmental damage, location dependent Restrictions in the amount of energy supplied, expensive technology to build, location dependent

energy sources. Nature replenishes renewable energy sources through the wind, water, sun, and plants; therefore, these sources are expected to manage the climate change challenges by limiting the effect of CO2 emissions to adaptable levels [11]. Solar radiation is currently the most effective renewable energy technology for global development; it safeguards the security of energy supplied, meets the increasing demand for energy, reduces CO2 emissions, and creates jobs (Table 1).

Solar Thermal Energy Storage The sun is the main supply for solar technologies that are used to produce light, heat, and electricity [12, 13]. Thermal energy is a change in internal energy, supplied, transported, and stored as latent, thermochemical, or sensible heat. Thermal energy storage systems supply the internal energy stored within the system immediately when needed. These storage systems are applied for both high and low temperatures with characteristic properties such as lifetime usage, no thermal losses, efficiency, and capacity [14]. When the energy required per time is smaller than what is required, the storage system supplies additional energy through physical and chemical discharge processes distinguished by the chemical composition of the system [15].

Sensible Heat Storage In sensible heat storage units, oil and water are liquid storage materials, while concrete, brick, earth, and iron are solid storage materials with the temperature of the material changing without a change in the material’s phase while it is charging and discharging [16, 17]. Sensible heat is achieved during the rise in temperature of the material and then released when necessary with a decrease in temperature.

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Therefore, the mass of the heat storage medium, the temperature change, and the average specific heat all depend on the storage size which can be expressed as ℚ ¼ m:Cp :ΔΤ

ð1Þ

where Cp is the specific heat of the material, m is the mass of the material, and ΔΤ is denoted as the difference in temperature from the initial to the final conditions. The capacity needed to store this thermal energy is extremely large; thus, the large storage size and difference in high temperature restrict this storage system from further usage.

Thermochemical Storage The thermochemical storage system stores heat during an endothermic path and releases heat during an exothermic path while using chemical bonds during this reversible chemical reaction expressed as Α þ Heat⟷Β þ C

ð2Þ

The transformation of chemical A into two new chemicals B and C occurs when the heat is absorbed in the storage system. However, the new chemicals must be stored at room temperature in separate vessels, and whenever heat is required, chemical B reacts with chemical C to form chemical A, and then the heat stored is released. The volume of energy stored depends on the storage materials, the extent of conversion, and the reaction heat. This chemical storage system can be characterized by the amount of energy stored per unit mass, and the energy is stored at longer periods at room temperature and/or moved from one place to another without any loss of energy compared to other systems. However, this system is still being developed with limited commercial applications attributed to high costs, reactor configurations, and insufficient storage material development [18, 19].

Latent Heat Storage Latent heat compared to sensible heat is released when the material experiences a phase change from one crystalline form, solid to solid, solid to liquid, or liquid to gas, without a phase change and then releases energy when the material experiences a reverse-phase change [20]. In simple terms, it is the heat absorbed when the substance changes from one crystalline phase to another, and during these changes, the temperature remains constant. The materials used for latent heat systems are called phase change materials. The solid-liquid phase change material is widely used for its large storage capacity during the heating and cooling process, small volume, and pressure changes [21]. The energy stored in this material can be expressed as

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ℚ

Ð

T pcm

latent¼

T1

ðm:Cp,s :dT Þþm:Hlatent þ

ÐT 2 T pcm

ðm:Cp,l :dT Þ

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ð3Þ

where the latent and sensible energy stored is ℚlatent; Cp,s and Cp,I are the material’s specific heats at solid and liquid states, respectively; and Hlatent is denoted as the heat of fusion at a phase change temperature of Tpcm.

Phase Change Materials (PCMs) Conventional materials such as pressurized water storage systems are used to harness solar energy, which is an abundant alternative energy source [22]. However, the unavailability of the energy extraction anytime when needed restricts this storage system and makes phase change materials a better alternative attributed it being an effective latent heat thermal energy storage system at a constant temperature and higher heat [23]. This storage system should be kept at atmospheric pressures to decrease the cost of containment. Furthermore, these PCMs can be classified into three: inorganic, organic, and eutectics. Organic PCMs such as n-Octadecane and paraffin wax exhibit good chemical stability and a large temperature range [24]. Eutectic PCMs are a mixture of organic materials like acids and paraffin and inorganic materials like metallic and salt hydrates. These PCMs also have a fixed phase change temperature but are available at extreme temperatures with a large storage capacity for thermal energy, while inorganic solid to liquid PCMs are corrosive and cool rapidly with good thermal conductivity. Generally, PCMs have several advantages over other solar thermal storage materials such as they are low cost with sharp melting points. They self-nucleate and are chemically inert and stable [25]. They have high volumetric storage densities and no phase segregation. They are recyclable and nonflammable. They are available in large temperature ranges and have low volume changes [21]. However, the overall thermal conductivity of PCMs is very low compared to other storage materials with ranges from 0.1 W/m K to 0.6 W/m K attributed to the corrosion between the PCMs and the container or the poor stability of the materials’ properties used to develop the PCMs and the manufacturing process [26]. This is a significant drawback for this material because the only mode of heat transfer is by conduction with adverse insulating effect when the PCM coagulates onto the heat exchanger surfaces during the heat removal phase process. Since natural convection doesn’t aid the solidification process, this results in longer heat absorptions and heat removal of the storage material [27].

Thermal Stability and Heat Transfer Energy storage systems are efficiently utilized when there is thermal stability. Therefore, knowledge of the thermal stability and heat transfer process is essential because the thermal stability of PCMs is determined after many thermal

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cycles by measuring the changes in the thermophysical properties of the material in liquid and/or solid states [26]. The two commonly used methods in measuring the melting temperature and heat of fusion of PCMs are differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In both techniques, the sample and reference are heated simultaneously at a constant rate. The recommended reference material to be used is alumina (Al2O3), and the difference in temperature between the sample and the reference is directly proportional to the difference in heat flow. Research has been widely carried out on the various heat transfer enhancement techniques. According to Agyenim et al. [28], the use of heat pipes and fins is more effective in heat transfer enhancements than other design options available. Farid et al. [29] reported that hydrated salts due to their moderate costs, high volumetric storage density, and high thermal conductivity are more attractive especially Glauber salt (Na2SO4.H2O) which has a high latent heat of 254 kJ/kg; however subcooling and phase segregation limit the application of this material. Telkes [30] investigated using borax as a nucleating agent to minimize the subcooling effect. However, thickening agents are also required to stop the high-density borax from settling. The densities of salt hydrates have been reported to be denser than paraffin; and therefore, salt hydrates are reported to be more effective PCMs per volume basis. On the other hand, paraffin has also been reported to have excellent thermal properties compared to other PCMs attributed to its nondegradation even after many cycles or contact with metals. Jegadheeswaran and Pohekar [31] reviewed the implementation of enhancement techniques in a different configuration of latent heat thermal storage systems. The authors argued that thermal conductivity of PCMS can be enhanced by adding high conductive porous materials or high conductive but low-density materials, placing metal structures in the PCMs, and dispersing high conductive particles in the PCMs. Since thermal energy storage systems require good thermal conductivity to store thermal energy at a fast rate; researchers have investigated the possibilities of adding several conductive materials to PCMs. Sari et al. [32, 33] examined adding high-density polyethylene and expanded graphite with paraffin wax by the transient hot-wire method to improve the thermal conductivity of the PCM. Fang et al. [34] examined a PCM composite prepared with hexagon boron nitride nanosheets to improve the thermal conductivity of the PCM. The authors concluded that h-BN nanosheets are promising fillers for the preparation of high conductive PCM composites for thermal energy storage applications. Sanusi et al. also examined the influence of adding graphite nanofiber to paraffin to improve the thermal performance of the PCMs, and the authors also concluded that adding nanofibers to PCMs was effective in improving the thermal performance of the PCMs. With this discovery, nanomaterials became desirable as a suitable addition to PCMs to improve its thermal conductivity properties. Other materials used for thermal enhancements of PCMs are paraffin composites, carbon fibers, Metal foam, graphite, fins, HPs, and polymers [35, 36].

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Nanomaterials Nanomaterials are substances that occur naturally; however, engineering nanomaterials are designed and characterized by an ultrafine grain and have at least one dimension of the material approximately 100 nm or fewer, that is, one-millionth of a millimeter [37]. Nanomaterials can either be irregular, spherical, or tubular, and they are of great interest due to their various properties with applications in the medical and electronic industries [38]. The various types of nanomaterials are dendrimers, nanotube, fullerenes, and quantum dots. These materials are classified into zero-, one-, two-, and three-dimensional nanostructures. Zero-dimensional nanostructures can be created using atomic clusters, clusters assemblies, and filaments [39]. Onedimensional nanostructure like surface films is created using multilayers, while two-dimensional nanostructures like fibers are created using buried layers or ultrafine-grained over layers. On the other hand, three-dimensional nanostructures like particles are created using nanophase materials with equiaxed nanometer grains [40]. Compared with conventional materials, nanomaterials have innovative quantum effects and increased surface-area-to-volume ratio and excellent chemical reactivity. Nanomaterial applications are in textiles, sunscreens, cosmetics, automobile tires, stain-resistant products, paints, and electronics [41]. Nanostructure semiconductors have quantum refinement effects, giving it a nonlinear optical property and used in solar cells. Nanophase ceramics are ductile at high temperatures, and nanosized metallic powders are used as porous coatings, dense parts, and gastight materials.

Fabrication Techniques of Nanomaterials There are two major methods of nanoparticle synthesis, namely, top-down and bottom-up. The top-down technique positively influences the material’s grain refinement, increasing the aggregation of the particles and surface energy. This process breaks up solid substances by grinding them either in a wet or dry form. The topdown wet grinding is achieved through mechanical alloying, with a vibrating ball mill, a centrifugal fluid mill, a planetary ball mill, a flow conduit bead mill, a wet jet mill, an annular gap bead, or an agitating bead mill. This method prevents nanoparticles from condensing, thus getting nanoparticles that are highly dispersed [42]. The wet grinding method, on the other hand, is ground by compression, by friction, or by shock using a hammer mill, jet mill, roller mill, shearing mill, tumbling mill, and/or ball mill [43]. The bottom-up technique is divided into the liquid and the gaseous phase methods. The gaseous phase methods minimize organic impurities in particles, and it involves a chemical and physical vapor deposition; the chemical vapor deposition involves a chemical reaction which produces ultrafine particles of 1 μm or less, while the physical vapor deposition is achieved when the evaporated materials are cooled [44]. Other fabrication routes include solvothermal synthesis, supercritical methods, spray drying, and spray pyrolysis [45]. Evaporation of materials in the physical

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vapor method is achieved using arc discharge method, whereas the chemical vapor deposition requires a heat source such as a laser [46]. Generally, the fabrication of nanoparticles requires the use of a device that improves purity: stabilize structures, reactants, and physical properties [47]. The effect of the fabrication process on nanoparticles added to PCMs to improve the thermal conductivity most especially is significant. Synthesis requires a device at lower costs, which will give high reproducibility, scale-up, and mass production. Conventional methods mentioned above not only require an increase in production time; it results in a large amount of waste and restricts the geometry of the part built to cylinders and cubes.

Laser Additive Manufacturing Contrary to other manufacturing techniques, laser additive manufacturing involves joining materials to make geometries from 3D model data, via a layer-by-layer technique. In recent years, additive manufacturing (AM) has been an alternative to the conventional subtractive or formative manufacturing techniques owing to its many advantages. AM allows industries to generate products using fewer parts and fabricating products that are less vulnerable to wear and tear, weak points, and stress [48]. It reduces new product cost by 70% and marketing time by 90% through employing the recently available rapid prototyping and associated manufacturing techniques. Once the shape and dimensional tolerances of a component or product have been created as a computerized 3D image, a solid replica can be created in hours anywhere in the world [49]. AM is versatile, flexible, and customizable, making it preferred by most sectors of production. There is no need for storage as parts can be made on demand from a CAD file, and therefore, there is no need to change the production line to make one part. Parts are built layer by layer reducing excesses while human production errors are minimal. More complex parts can be produced in shorter time frames, and higher product quality is also assured because parts developed are without residual porosity [50]. Furthermore, to fully realize these benefits, AM methods that can process large materials at high speed and low cost containing useful properties that meet or exceed those attained by other conventional routes must be created [51]. During the creation of these routes, AM processes may be susceptible to errors due to the complexity of AM data; however, accuracy is imperative to reduce reproduction and improve product quality. Therefore, modeling and testing approaches are needed to ensure the accuracy of AM design models. According to history, AM as a manufacturing process started in the 1980s as a rapid prototyping technology with the invention of stereolithography (SLA) by Charles Hull. Also, ASTM et al. [52] defined AM as joining materials to make objects from a computer-aided design (CAD) model data, usually layer upon layer, as opposed to subtractive manufacturing. This method traditionally begins with an idea; the idea is then expressed through the creation of a three-dimensional (3D) model with the use of CAD software. The CAD-based 3D model is saved as a standard tessellation language file, which is a triangulated representation of a model. The software slices the data file

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Fig. 1 The 3D printing classification

into numerous horizontal sections that create a tool path for each individual layer. The tool path is followed by the laser in the AM device, creating the object by adding layers of the material, one on top of the other until the idea in physical form is created [53]. Coating and fusing are the two fundamental steps in the fabrication process. The material is being laid over the working surface forming a thin layer, and the thickness of the layer depends on the technology used during the coating process and while the fusing step is the action of the source energy. Figure 1 below shows the AM fabrication classification, namely, stereolithography (SL), Polyjet, fused deposition modeling (FDM), laminated object manufacturing, 3D printing (3DP), Prometal, selective laser sintering (SLS)/selective laser melting (SLM), laser engineering net shaping (LENS), and electron beam melting (EBM). The additive manufacturing methods are divided into seven categories: • Binder jetting: this is an additive manufacturing process where a liquid bonding agent is selectively deposited to join powder materials, for example, inkjet 3D printing. • Direct energy deposition: this uses focused thermal energy to fuse materials by melting as they are being deposited, for example, laser engineering net shaping (LENS). • Material extrusion: in this process, materials are selectively dispensed through a nozzle, for example, fused deposition molding (FDM). • Material jetting: this process uses thermal energy selectively to fuse regions of a powder bed, for example, drop on demand (DOD). • Sheet lamination: this is a process, where sheets of material are bonded to form an object, for example, laminated object manufacturing. • Powder bed fusion: this uses thermal energy selectively to fuse regions of a powder bed, for example, selective laser melting (SLM) and selective laser sintering (SLS). • Vat photopolymerization: this is the process, where liquid photopolymer in a vat is selectively cured by light-activated polymerization, for example, stereolithography (SLA).

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SLM is an additive manufacturing process where the base material is a powder or granulated material and this material is delivered in a series of thin layers. Each layer is selectively irradiated by a laser beam which completely melts the material in the selected regions. A new layer is applied on top of the previous one, and the process repeats itself until the 3D object is fully constructed. This technique involves partial melting and resolidification by cooling. This process can fabricate ceramic, polymer parts and metals without a binder [54]. SLM is the most versatile AM process as it can produce a wide spectrum of materials. Nowadays, typical standard materials of SLM technologies widely used are tool steel (H13), stainless steel (316 L), pure titanium, titanium alloy (Ti-6Al-4 V), cobalt-chromium alloys (ASTM75) and two aluminum alloy powders (Al-12Si-Mg and Al-10Si-Mg), Inox904L, Inconel, gold, high entropy alloy, and more recently nanomaterials. SLM has the ability to tune properties during the processing of parts and increase functionality at relatively low costs, and it fabricates near-net-shaped components ready to use if the surface level roughness is acceptable. Most importantly, SLM is advantageous to produce components with high density, good mechanical properties, and complicated shapes [55].

SLM Process Parameters The SLM process is capable of building fully dense components; however, to achieve this, the process parameters should be optimized to produce parts that are near-net-shaped with 99.9% relative density: • Layer thickness: The building platform downward vertical movement at each layer of completion. • Laser power: The maximum nominal laser power was reported to be at 200 W [56]. • Temperature: Building platform temperature is 200  C [57]. • Hatch spacing: The distance between two adjacent scan vectors. • Nominal scan speed: The ratio between the point distance and the laser dwelling time on each point; however, the laser off time in between two consecutive pulses is not counted. • Laser focus position: The distance from the laser focus plane and the building platform. • Laser scan strategy: The pattern followed by the laser during each layer deposition.

SLM Scan Strategies There are various laser scanning strategies that influence the grain structure and overall quality of the parts built using SLM. Some of these strategies are the following.

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Meander Scanning Strategy This strategy is used for hatching. The neighboring vectors of the laser path are scanned with a constant hatching distance in the opposite direction over the entire layer. The scanning of larger areas with a meander strategy could induce higher residual stresses due to high thermal differences in the opposite ends of the scanning vectors [58, 59]. Chessboard/Island Scanning Strategy This strategy eliminates stresses and overheating of local areas. It is achieved by splitting the entire area into numerous local areas combined with a different scanning order as a cross-sectional area of the sample divided into several subareas, where each subarea is scanned with the meander strategy [60]. Hull and Core Strategy This strategy is used when there are different process parameters within a single layer. The samples are divided into two areas; on the sample’s outer contour, there is hull area, and in the center of the sample is the core area. For each of these areas, different contour and hatch parameters can be set. This allows even better distribution of energy in the core layer of the sample [61]. Pre-sintering Strategy It is used to eliminate defects. The first exposure is treated with lower laser power to preheat the layer while maintaining a uniform powder height. Then there is a second exposure with full power resulting in lower temperature gradients and energy while maintaining the same scan speed [62].

Challenges of the SLM Process Cracks Materials processed using the laser additive manufacturing process must have good weldability, avoiding cracks during rapid solidification. Due to the rapid cooling rate, components fabricated using additive manufacturing have microstructures that differ from other conventional methods; however, the formation of cracks after synthesis has been recorded. Reduction of thermal gradient and fabrication under elevated temperatures at optimized parameters show promising features in eliminating cracks [63]. Density The SLM process has no mechanical pressure applied during fabrication compared to the formative and subtractive manufacturing technologies; gas bubbles can become trapped in the material during solidification attributed by a decrease in solubility of dissolved elements in the melt pool. Optimizing the process conditions and parameters using a bidirectional scan pattern and laser remelting after each layer deposition can help prevent these issues [64].

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Surface Quality Issues Surface quality may be reduced when the edges of the component produced are not flat but are contoured. Post-processing such as abrasive sandblasting, acid etching, plasma spraying, oxidation, and laser surface remelting can be used to resolve this issue when it occurs [65]. Microstructural Properties The direction of heat transfer determines the orientation of grains. Therefore, using a unidirectional scan pattern and adjusting the process parameters and scan strategies can reduce defects in the microstructural properties of the material. Dislocations Dislocations are sometimes caused by the thermal contraction stresses during rapid solidification of the fabrication process. This rapid solidification happens under conditions of anisotropic heat removal. Although refined grain size contributes to an increase in strength, however, the dislocation densities will as well contribute to the strength of the material. Pores Metastable phase, nonequilibrium microstructures, and solute trapping are among the metallurgical defects known to cause porosity attributed to gas entrapment and unmelted powders in the additive manufacturing process. Porosity forms in areas that are overmelted induced by excessive energy, unmelt areas induced by insufficient energy, and areas that have been overheated induced by high laser power and low scan speed [66]. Therefore, optimizing the process parameters will hinder the formation of pores. Residual Stresses Residual stresses are caused by localized heating, thermal contraction, and expansion, and changing the scan strategy will help reduce residual stresses on the component manufactured [67]. Optimizing the process parameters will reduce residual stresses built up during fabrication of parts. Casting Segregation Dislocation cells formed during thermal contractions interact with the solute distribution and lead to microscale segregation [68].

Conclusion Phase change materials are used for storage with heat exchangers, for a solid-solid transition from heat transfer, for storage of solar energy, for moving the peak heating load during electricity consumption, and for building applications. These materials are one of the most proficient storage materials for thermal energy applications

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attributed to PCMs having a larger storage capacity and density. More so, the energy is stored without increasing the temperature of the material beyond its melting point. However, PCMs generally exhibit a low thermal conductivity restricting its usage. Therefore, new technologies were developed to improve the thermomechanical property of PCMs, and the combinations of conductive nanomaterials with PCMs are currently the most viable method of enhancement. Different fabrication techniques were employed to develop these composites consisting of nanomaterials and PCMs called nanophase change materials showing promising properties of thermal improvement. Nevertheless, most techniques employed required a large amount of processing time and produce too much waste, and the fabricated materials are mostly difficult to machine. On the other hand, laser additive manufacturing via selective laser melting is flexible, fast, and versatile. SLM is automated with little or no waste material. This advance manufacturing technique with its limitless possibilities can be used to fabricate nanophase change materials for thermal storage applications with improved thermal properties.

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Mechanical Performance of Nanocomposites and Biomass-Based Composite Materials and Its Applications: An Overview

V. Arumugaprabu, R. Deepak Joel Johnson, and S. Vigneshwaran

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano Composites Fabrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Performance of Nano Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass-Based Composites: Types and Fabrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Performance on Biomass-Based Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nano Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Biomass-Based Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The development of composite materials and its usage in modern-day scenario goes on increasing because of the excellent property enhancement they are providing. Researchers focuses their attention toward making of new materials for the benefit of society. One such move is the development of nanocomposites for various engineering applications. Nano composites are prepared through incorporation of nano-sized particles into different matrices such as polymer, metal, and ceramics. The nano-sized particles to be added into the matrices generally range from 0.5 wt. % to 5 wt. %. When compared to the existing enormous number of composite materials, the nanocomposites offer valuable merits such as excellent mechanical strength and very good toughness, and in addition to this, it offers potential thermal conductivity, electrical resistance, and also dimensional stability. A few applications of these nanocomposites include V. Arumugaprabu (*) · R. D. J. Johnson · S. Vigneshwaran School of Automotive and Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil, Srivilliputhur, Tamil Nadu, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_123

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automotive parts, packaging sectors, batteries, capacitors, etc. This chapter’s first part presents the complete overview on how nanocomposites fabricated its mechanical performance and its applications. The second part of this chapter discusses about biomass-based composite materials. Throughout the world, four types of biomass are used such as wood and agricultural products, solid waste products, landfill gas/biogas, and alcohol fuels. Among this, the composites are developed in more numbers with researchers focusing on first two types of biomass. This chapter also discusses in detail about the mechanical performance studies on composite materials prepared using solid wastes, wood, and agricultural products such as rice husk and its potential applications. The major advantage offered is the drastic reduction in environmental pollution by reuse of these wastes for composite fabrication. Keywords

Nano composite · Biomass · Matrixes · Mechanical performance · Industrial waste and Agricultural waste

Introduction One of the familiar concepts across the globe is “composite materials” which evolve as an effective replacement for the existing conventional materials because of its easy availability, low cost, and optimized properties. A third new material with innovative performance characteristics produced through the assembly or joining of two different materials is said to be a composite material. With the rapid evolvement in technologies over a decade, usage of composites in various forms and different applications goes on an increasing trend till date. One such thing derived from the composite materials concept is the nanocomposite. A nanocomposite as usual consists of a reinforcement phase and a matrix phase; the main thing is in which anyone of the phase is in nano-scale size. The nano-scale size should be normally in the range of 10 3 m–10 9 m [1]. Depending on the application, fillers are used in the nanocomposites production which is used as a hybrid along with the matrix or reinforcement. Fibers are normally used as the reinforcement. The addition of fillers along with this normally enhances the properties of the composites further. Based on their engineering applications point of view, the nanocomposites were classified as follows: 1. 2. 3. 4.

Structural materials or functional materials Polymer-based nanocomposites Non-polymer-based nanocomposites Based on the filler types

The structural or functional nanocomposites involves the usage of crystals in nano size for various mechanical, electrical, and optical properties behavior.

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Polymer nanocomposites are further classified broadly based on the usage of varying reinforcements such as polymer/ceramic, polymer and polymer, inorganic and organic, and biocomposite. Non-polymer nanocomposites are further classified broadly based on the usage of varying reinforcements such as metal and metal nanocomposites, metal and ceramic composites, and ceramic and ceramic composites. Depending on the types of filler used such as metal oxide, carbon, and nano-metal, nanocomposites were further used for various applications. Although many composite materials are existing and researchers reported on the same the key specialty of nanocomposites is with respect to their physical structure in which because of the nano size surface to volume ratio will be more which in term enhance the properties when comparing to the existing ones. This makes the nanocomposites more special ones. The nanocomposites are more advantageous in many ways; still they have a demerit that some of them are heavy toxic in nature which causes huge environmental threat so the same has been addressed. The development of nanocomposites based on biomass is another interesting area where the focus is in the reusage of various industrial wastes which seems to be a huge threat for the environment. This chapter discusses about the fabrication of nanocomposites with various combinations and their applications. Also the different kinds of composites manufactured using biomass and their applications are discussed in detail.

Nano Composites Fabrication Methods The nanocomposites depending on the reinforcement and the matrix classified are very vast, and the fabrication method also varies accordingly. The fabrication depends on the reinforcement and matrix in which some of the common familiar processing techniques used are [2]: 1. 2. 3. 4.

Mechanical processing Thermomechanical processing Non-equilibrium processing Chemical processing/electrical processing

The fabrication of nano structures/composites by means of mechanical processing methods includes severe plastic deformation, shock wave consolidation, and transformation-assisted consolidation. The next fabrication method of nanocomposites by means of thermomechanical process includes hot pressing, hot isostatic pressing, hot extrusion, sinter forging, and sintering. Non-equilibrium processes of nanocomposite fabrication methods are microwave sintering, thermal spraying, spark plasma sintering, and laser-based techniques. The chemical or electrical processing includes electro-deposition and self-assembly nano structured consolidation. Among all the available methods, the more advantageous is the template-free methods where the production is more easy and highly reproducible. Also this method is very simple and more purified one.

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Mechanical Performance of Nano Composite Materials This section discusses in detail about some of the nanocomposite materials that have been prepared by using various combinations of reinforcements and matrices. The mechanical property analysis of the various combinations of nanocomposite materials prepared was also discussed as follows: Influence of size of nano-particles and its interphase thickness on the mechanical properties of polymer nanocomposite is investigated by Ashraf et al. [3] and Zare et al. [4]. It is inferred that larger nano-particle produces poor tensile strength irrespective to the interphase thickness. It is evident from the study that only a high-thickness nano-particle interphase cannot produce the good mechanical properties. Size of the nano-particle plays significant role on deciding the mechanical properties of the nanocomposites. Response of mechanical performance for the carbon nano-tube treatment was studied by Ferreira et al. [5]. Carbon nano-tube (CNT) treated with various acids like HCl and H2SO4/HNO3 was used as the reinforcing material for the high-density polyethylene composite. Mechanical properties of untreated and chemical-treated CNT reinforced composite were analyzed, and it was observed that chemically treated CNT reinforced polyethylene composite showed superior properties compared with the untreated. It is due to the fact that chemical treatment to the CNT decreases the crystallinity and enhances the dispersion rate of CNT to the polyethylene matrix. Mechanical responses like compressive modulus, rate-dependent modulus, and stress relaxation behaviors of boron-doped and undoped CNT sponge reinforced polydimethylsiloxane (PDMS) composites were investigated. It is inferred that carbon nano-tube sponges doped with boron can be a satisfying reinforcing material to fabricate a tough and ultra-light PDMS composite material. It is observed that there was a 70% increase in compressive modulus when the composite is reinforced with boron-doped carbon nano-tubes [6]. Pristine, coupled, and acetylated cellulose nano-fibers (CNF) were reinforced with polylactic acid (PLA) material using injection molding and solution casting methods to fabricate CNF/PLA composites. Fabricated composites were investigated for its mechanical stability. Among the three different treatments to the fibers, acetylated CNF shows better compatibility with PLA polymeric material, and the acetylated cellulose nano-fiber reinforced composites showed better mechanical stability [7]. Multi-walled carbon nano-tubes are used as primary reinforcement, and silica/ alumina nano-particles were used as the secondary reinforcement for the epoxy polymeric material. The composites were fabricated using solution dispersion method. The fabricated nanocomposite is tested for its mechanical behavior using monotonic uniaxial test. The weight percentage of the nano-fillers, namely, silica and alumina, was maintained below 1 wt. % for proper dispersion. It is observed that silica/alumina-filled MWNT reinforced epoxy composite shows good mechanical behavior compared to the unfilled composite [8]. The effect of inorganic nano-filler on flexural strength and interlaminar shear strength of the glass fiber reinforced

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polymer composite was investigated by Ramesh et al. [9]. The investigation is performed with titanium dioxide nano-fillers in two different environmental conditions, namely, dry and hydrothermal condition. Also, the filler content is varied as 0, 0.1, 0.3, and 0. 7%. From the experiments it is revealed that 0.1% of filler addition to the glass fiber reinforced composite showed 19% improvement in flexural and 18% improvement in interlaminar shear strength compared to all other fabricated combinations. It is also observed that TiO2 nano-filler reinforcement makes the composite more compatible for hydrothermal environment. Incorporation of organically modified montmorillonite (OMMT) to the acrylonitrile butadiene styrene (ABS) nano composites for any improvement in the composite’s mechanical property was investigated by Weng at al. [10]. The composites were fabricated using fused deposition modeling (FDM) 3D printing process and injection molding method. Mechanical properties like tensile and flexural properties were analyzed for the fabricated composite specimens. It is observed that specimens fabricated with FDM 3D printing process showed 43% improved tensile property whereas the specimens with injection molded method showed approximately 29% enhanced tensile property when adding 5 wt. % of OMMT to the ABS composite. Mechanical properties of functionalized graphene and pristine graphene reinforced epoxy nanocomposite were analyzed. Functional groups, namely, COOH and NH2, were functionalized to graphene, and it was checked using FTIR spectroscopy. It was inferred from the results of mechanical testing that COOH- and NH2-functionalized graphene exhibits better mechanical properties compared to the other specimens fabricated. Also, it was observed that 0.1 wt. % of graphene to the composite can be more compatible to have satisfying mechanical behavior [11]. Various composites by varying the nano-fillers and polymer matrix materials were developed using simulation by molecular dynamic method (MD). Graphane (GA), graphene (GE), and carbon nano-tubes were the various nano-fillers used as reinforcing materials. Polyethylene (PE), polymethylmethacrylate (PMMA), polytetrafluoroethylene (PTFE), and polyvinylidene chloride (PVDC) were the various polymeric materials used for simulation. Interfacial mechanical properties and fiber pullout were performed to derive the best combination of reinforcing and matrix to develop a composite material. It is observed that GE reinforced PE composite possesses better mechanical behavior [12]. Investigation on the influence of functionalized cellulose nano-crystals from Pseudobombax munguba on the mechanical properties of poly (butylene adipateco-terephthalate) (PBAT)-based nano composites was carried out by Pinheiro et al. [13]. Different wt. % (3, 5, and 7 wt. %) of nano-crystals were used for fabricating the PBAT nanocomposite. Functionalization of nano-crystal was done with octadecyl isocyanate, and the process was ensured with FTIR spectroscopy. The study suggests the incorporation of functionalized nano-crystals from munguba fiber can be a good reinforcing material with increased mechanical properties. Also, it is observed that the fabricated composite maintains the biodegradability which makes the composite a better replacement for the non-biodegradable polymer composites.

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Using chemical vapor deposition (CVD) method, carbon nano-fibers were grown on carbon fiber surfaces. Investigation on the CVD process parameters was carried for producing better mechanical properties from the fabricated composites of carbon nano-fiber-coated carbon fiber reinforced polypropylene (PP) composites. It is observed that thickness and surface area of carbon nano-fiber coating the carbon fiber plays a significant role in enhancing the mechanical properties of PP composite [14]. Effect of graphene nanoplatelet (GNP) for its content and type on the mechanical behavior of epoxy composites was analyzed by Salom et al. [15]. The GNP filler is functionalized with NH2. The fabricated composite is tested in universal testing machine for its deformation at break, Young’s modulus, and mechanical strength at below 25  C. It is observed that only at lower level of filler content tensile strength of the composite was stable; when increasing the nano-filler content, tensile strength tends to decrease; it should be due to agglomeration of graphene. Also, it was inferred that Young’s modulus of the composite increases as the nano-filler content increases.

Biomass-Based Composites: Types and Fabrication Methods In the era of development and usage of composite materials, another vital factor is the biomass-based composites/nanocomposites for various applications. This biomass-based composites finds a separate place in replacing existing conventional materials with a huge potential of reducing the environmental risk. The reusage of various industrial wastes and biowastes by means of producing biomass drastically reduces the pollution caused by the wastes. Biomass has been derived from various sources such as industrial wastes, animal wastes, agricultural wastes, and municipal solid wastes. One of the major reasons for the potential usage of biomass in the composite production is due to its chemical composition that includes cellulose, hemi cellulose, lignin, proteins, alkali, etc. The typical biomass is extracted from agricultural wastes by means of technologies such as pyrolysis, compositing, pelletizing, pressing, and enzymatic digestion [16]. The production of natural fiber reinforced biomass composites for various applications is in increasing trend because of the potential advantage such as processing temperature, cycle time, and density when compared with the thermoplastic reinforcements. One such type of biomass extracted from agricultural wastes by means of pyrolysis process is known as biochar. Biochar is a high-carbon, fine-grained residue that is produced through pyrolysis processes; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids called biochar. Biochar can be used as the reinforcement in polymer composite to enhance the properties of the composite, since it has good carbon content. Biochar was prepared from the Zea mays cob (which is the agro-waste) as shown in Fig. 1 using pyrolysis process, and the prepared biochar is ball milled to produce uniform sized particle to be used as secondary reinforcement in natural fiber polymer composites.

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Fig. 1 Biomass from agricultural waste Zea mays cob undergoes pyrolysis process and obtained as biochar in powder form Table 1 Elemental composition of biochar from corncob (wt. %) Corncob Corncob biochar

Carbon

Oxygen

Hydrogen

Nitrogen

Sulfur

Ash

Ref

47.35 77.6

38.07 5.11

5.90 3.05

0.69 0.85

0.18 0.02

1.94 13.34

[17]

The elemental composition of biomass from agricultural wastes compared with the composition of ordinary corncob in which enhancement in all the properties is clearly identified is shown in Table 1. Coats et al. [18] studied the performance of biomass prepared from the microorganism reinforced in thermoplastic with wood flavor and also reported that the composite materials property gets improved by using this biomass and interestingly noted that the biomass serves as plasticizer which in turn propagates the material properties. Usmani et al. [19] studied about the utilization of biomass for production of advanced functional materials instead of typical natural fiber composites. It was noted that the natural fiber polymer composites along with biomass as a filler show very good performance enhancement, but on the other hand, the problems related to the structural integrity, solubility, and processing persist in order to avoid the same ionic liquids are proposed as best alternative. Biomass materials extracted from rice husk, rice straw, and paper sludge are used as a potential replacement for wood particles in wood composites [20]. Among the different wastes, the paper sludge of 10% wt was found to be best suited ones for making wood paper sludge composite green pallets. The utilization of biomass also creates more interest in the construction applications especially used for insulations. The biomass thermal insulations proved to be a potential one in buildings evident through earlier researches [21]. The increasing worldwide waste problem focuses on lignocellulosic biomassbased composite materials in which it has to be pre-treated and used for making

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composite panels. For the same pre-treatment, the ionic liquids were used from which the composite boards were manufactured [22]. Another important research has been done in such a way that nanocomposites were prepared by using biomass [23]. The cost-effective and environment-friendly nanocomposites have been prepared with the removal of phosphate. Wheat straw material was used from which the biomass was extracted and utilized for the nanocomposites. Saba et al. [24] analyzes in detail about the usage of biomass from oil palm as filler reinforcement in polymer composite making. The dynamic mechanical analysis, thermal stability, and mechanical performance of the composites drastically increased by using this biomass as an effective reinforcement. Biomass is also used as a vital replacement for electrochemical storage systems. One of the recent advances is the usage of biomasses as materials for making electrode, binders, and separators [25].

Mechanical Performance on Biomass-Based Composite Materials This section discusses in detail about mechanical performance of the composite materials that have been prepared by using various types of biomasses. Essabir et al. [26] fabricated the coir shell particle and coir fiber reinforced polypropylene (PP) hybrid composites. Compared to the non-hybrid composites, the developed coir shell particle and coir fiber reinforced polypropylene hybrid composite increased the modulus of the composites by 35–50%. Wood polypropylene composite with 20 wt. % coffee ground powder is fabricated and characterized by Garcia et al. [27]. The flexural modulus of the composites was increased with the coffee ground powder addition. The results reported that the smaller size of the coffee powder particle aided in developing good boding features, which became the main reason for the increased strength. Zhang et al. [28] fabricated a novel composite with rice straws and coir fibers. The hybridization improved the bonding properties of the composites. More importantly when increasing the coir fiber content, the fiber to fiber connection was increased, correspondingly increasing the strength. Starch-based biomass composites’ mechanical and antimicrobial properties were investigated by Spiridon et al. [29]. The additions of lignin to the biocomposite showed enhance tensile strength. Lignin reinforcement at 5% showed 90 to 130% increase in the tensile strength of the biomass composites developed. Nabinejad et al.’s [30] investigation on oil palm shell-based composites revealed that the biomass compatibility with the polymer matrix can be enhanced through the chemical treatment process. Further it is necessary to optimize the chemical concentration during treatment process. At high concentration the biomass may lose its weight, and surface degradation occurs which reduces the bonding ability of the biomass reinforcement which correspondingly alters the mechanical performance. Polycaprolactone biocomposite having 28 wt. % of date palm sheath biomass as reinforcement exhibited tensile strength of 25 MPa, whereas pure polycaprolactone

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has tensile strength of 19 MPa only. Biomass reinforcement also increased the modulus of the composites from 140 MPa to 282 MPa [31]. Binshan Mu et al. [32] explored the mechanical performance of the polyethylene composite having four different biomass reinforcements (hardwood, softwood, wheat straw, and bamboo). The tensile and flexural strength of the hardwoodbased biocomposites was found to be maximum due to the presence of high cellulose content. Bo Chen et al. [33] compared the mechanical strength of the lignocellulosic biomass composite having natural fiber (tropical maize bagasse, sweet sorghum bagasse, and sugarcane bagasse) and wood reinforcement. The composite with tropical maize bagasse reinforcement showed maximum tensile strength (26.8 MPa) and flexural strength (46.1 MPa). Ahankari et al. [34] reported that the green composite on agricultural waste mix reinforcement exhibited increased tensile and modulus strength by 256% and 308% compared to pure green composite. The biocomposite’s mechanical performance can be enhanced through the biomass fiber reinforcement. However, the increment in the composite strength depends upon the type of fiber reinforcement [35]. Rozman et al. [36] reported that the size of rice husk is the important factor to be considered for increasing the mechanical properties. The high surface area of contact at smallersized rice husk increases the bonding between matrix and rice husk; this leads to increase the strength. Fracture toughness of the polyurethane composite having biomass sisal fiber and coconut fiber was investigated by Silva et al. [37]. The alkaline-treated sisal fiber absorbs more energy during fracture test than the coconut fiber composite. Compared to coconut fiber, sisal fiber exhibits good interaction with matrix which promotes the energy absorption of the composite during fracture. Haafiz et al. [38] segregated microcrystalline cellulose from the biomass oil palm and reinforced in the polylactic acid. The addition of microcrystalline cellulose increased the tensile modulus of the composite to 30%; however in the case of tensile and elongation, decrement is noted. Miléo et al. [39] reported that it is possible to increase the stiffness of the polymer composites through reinforcing the biomass cellulose fibers. Castro et al. [40] stated that the type of fabrication process has significant influence on the biomass-filled composites. The fiber distribution was found to be more homogeneous on composites fabricated through injection molding process; these composites exhibited good resistance to impact than the composites fabricated through press molding.

Applications of Nano Composite Materials The nano composite materials find its applications in very vast area that includes industries, medical applications, coatings, and automotive sectors. In the case of automobiles, various parts such as door handle, engine cover, belt cover, and manifolds are made by using nanocomposites. Some of the industrial applications include blades, fuel tanks, impeller, and electronic products. In addition to that, some of the notable applications of nanocomposites were biomedical equipment, sensors, and

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household items. Seema Tiwari et al. [41] carried out research on potential usage of nanocomposites technology for diagnosis and delivering the drugs which creates a huge impact in the biomedical field. It has a potential advantage in such a way it is biodegradable. The polymer nanocomposites with varying reinforcements and matrices play a vital role in various applications such as skeletal repair, optical devices, dental field, catalysis, abrasion coating, lithium battery, aerospace applications, food packaging applications, containers, and fuel cells [42]. In the case of high-performance nanocomposites production, graphene is used as a potential reinforcement. This type of composites is used in thermal, refractory, and military applications as well as prosthetics [43, 44]. Further potential application studies continued on the usage of nanocomposites on equipment used in oil production and petrochemical industries [45] where polytetrafluoroethylene-based nanocomposites show better wear resistance in the friction process. The usage of polymer and ceramic matrix nanocomposites in aerospace applications [46] was suggested due to the multifunctional properties that has been generated due to the dispersion of fine nano-particles. The major areas in which this type of nanocomposite is involved in aerospace are coatings, shielding, health monitoring, and various structures. In addition to the abovementioned applications, nanocomposites were employed in solar panel generation, wastewater treatment process, light-emitting diodes, and photovoltaic devise [47].

Applications of Biomass-Based Composite Materials The biomass-based composites find its applications in major sectors due to its low cost, environment safety, and excellent performance. Rowell et al. [48] discusses in detail about the application of biomass-based biocomposites for the purpose of alternating building materials. In this they suggested that the biomass generated from the agro-wastes serves as a potential replacement for existing building materials; also biomass is a renewable one which will replace wood. High-performance supercapacitors were developed by using the renewable biomass composites [49]. Biomass from chitson added with carbon/PANI to make composites which possess very high specific capacitance when comparing to the existing ones. Another interesting usage of the biomass-based composite is in the manufacturing of supercapacitors and aerogels [50] with higher mechanical strength and more electrical conductivity. The outdoor performance of the composites improved by means of effective testing, and modified biomass will be used as the reinforcement which enhances the usage in various structural applications [51].

Conclusions and Further Outlook A brief tertiary review on this nanocomposites and biomass-based composites draws the following conclusions:

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• The potential of nanocomposites as an effective replacement for conventional materials exists almost a decade with a possibility to address the difficulties in fabrication method as well as the mass-scale production. • The combination of few types of reinforcement and matrix for the production of nanocomposites found to be toxic in nature has to be addressed in the future by researchers as a critical factor. • The nanocomposites explore its horizon throughout all aspects in the world has the potential to widen its usage in the near coming future also because of its low cost, availability, flexibility, and high performances that decides the industrial growth. • Biomass-based composites find its application in various sectors which is understood from the various researches that has been carried out in earlier years between 2007 and 2018. • Biomass is the need of the hour since the wastes generated during the various manufacturing processes go on in an increasing trend throughout the world so reusage of the same is needed. Based on this aspect, researchers develop a lot of innovative materials using biomass. • The interesting factor to be noted is in most of the composites researchers carried out using biomass reported that the addition of this as a filler or as a reinforcement improves the mechanical, thermal, electrical, and chemical properties drastically.

Further Outlook • At present and in the coming future, the researcher’s focus may be toward the drastic development of bio-nanocomposites which are more environmentalfriendly. • The usage of bio-nanocomposites in medical applications and electronic applications in a drastic manner protects the flora and fauna from the e-waste which is more hazardous at present scenario. • Biomass-based composites are used for making advanced functional materials in a mass production scale to completely utilize the large scale of agro-wastes, biowastes, and industrial wastes generated by means of day-to-day activity. • In the future biomass-based composites which are completely biodegradable may find its place as an excellent replacement.

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Nanomaterials from Biomass: An Update Jeyabalan Sangeetha, Arun Kashivishwanath Shettar, and Devarajan Thangadurai

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial Synthesis from Renewable Biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrothermal Carbonization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials from Plants, Microbes, and Actinomycetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Increasing global population led to energy crisis, food demand, pollution impacts, and climate change that have significantly affected the quality survival and sustainability of the environment. Modern technologies with greater properties are needed to face all these issues. In this context, researchers designed a powerful tool as nanomaterials which could be the better alternative for the present scenario. Especially, biomass-based nanomaterials are gaining much importance among the researchers and stakeholders due to its cost-effective, eco-friendly, and J. Sangeetha Department of Environmental Science, Central University of Kerala, Periye, Kasaragod, India A. K. Shettar Department of Applied Genetics, Karnatak University, Dharwad, Karnataka, India D. Thangadurai (*) Department of Botany, Karnatak University, Dharwad, Karnataka, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_23

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replacement capability in environmental, agricultural, and industrial applications. Biomass is widely distributed and readily available material from agricultural, forestry, and industrial sectors, which is rich in renewable carbon source. Carbon nanomaterials are at present in extensive application at various fields. From the last few decades, researchers are working upon the production of nanomaterials such as carbon nanotubes, nanocrystalline cellulose, and starch nanostructures from various types of biomass. This chapter describes the production, types, and applications of various types of nanomaterials derived from biomass of plants, microorganisms, and actinomycetes. Keywords

Biomass · Nanomaterials · Carbon nanotubes · Nanocellulose · Nanostructures

Introduction Energy demand is increasing day by day; nonrenewable resources will certainly get exhausted in the near future. Hence, the use of renewable biomass is a promising strategy for producing nanomaterials. In recent years, nanomaterials have gained a wide attention in the field of biomedical applications. Sources of renewable biomass mainly include plants and microorganisms like bacteria, yeast and molds, actinomycetes, etc. Cellulose, starch, and carbon nanostructures are important nanomaterials that are synthesized by renewable biomass. Activation is important part of production of nanomaterials. Using biomass as source for production of nanomaterials includes several methods such as physical activation, chemical methods, and hydrothermal activation. Nanomaterials synthesized from biomass are potentially being used in several biomedical applications such as wound dressing, tissue engineering, biocompatible agents, etc. This chapter gives insights into sources, methods, and applications of nanomaterials synthesized from renewable biomass.

Nanomaterials Nanotechnology has evolved over time for its wide range of applications. By the very fact that the technology aims at producing nanomaterials, the size of the particles avails many advantages over conventional ones. An ideal nanomaterial is approximately 1–100 nm. Due to its tiny size, it provides a large surface area and also increases the number of active sites, thus helping in enhancing reaction process [1]. Nanomaterials have proved their efficacy in various fields for their use. Nanomaterials are engineered particles designed to have extremely small size to make use of unique physicochemical properties that exist at nanoscale [2]. Due to their unique size, nanomaterials’ physicochemical properties differ from those of their largerscale particles and thus are extremely used in various biological applications.

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Production of biomass for nanomaterials has gained much attention recently due to their biodegradability property and eco-friendly nature and has diverse bioresources from which they can be isolated.

Nanomaterial Synthesis from Renewable Biomass Synthesis of nanomaterials from renewable biomass has been extensively studied during recent years. Nanomaterials with diverse sizes, morphology, and properties are derived from both organic and inorganic matters. Liposomes, polymersomes, polymer constructs, and micelles are some of the organic materials from which synthesis of nanomaterials is studied [3]. Biomass being one of the greatest sources of energy, a wide range of nanomaterials is synthesized from multiple sources. Sources of biomass from which nanomaterials can be synthesized include agriculture wastes, municipal solids, forestry crop residues, industrial residues, sewage sludge, and animal residues. Agricultural waste makes up a large amount of organic matter and serves as good source for synthesis of nanomaterials. Rice husk is an agricultural residue that is known to synthesize nanomaterials of size 80–85 nm [4]. Production of nanomaterials makes use of several methods for it to become available with beneficial activity. The methods employed depend on the properties of nanoparticle, like size, shape, morphology, etc. (Fig. 1). Some of the methods employed for synthesis of nanomaterials are discussed below.

Physical Activation Activation of organic matter is mainly done to increase the porosity and enhance other physical properties to achieve ideal nanoparticle. Physical methods mainly include gas condensation and vacuum deposition and vaporization. For example, synthesis of carbon nanostructures can be done by physical activation of biomass, where the organic matter is first carbonized into carbon component at low temperatures, and then the resultant carbon undergoes activation process at higher temperatures in the presence of suitable activator [6]. The physical activation method is known to produce nanomaterials with specific surface area and pore distribution [7]. Activated nano-carbons can also be produced by physical activation of biomass from agricultural waste by using water steam that has shown considerable results with significant increase in specific surface area, cavity volume, and the iodine number of the final product [8].

Chemical Activation Chemical activation makes use of chemicals in order to activate biomass for production of nanostructures. Chemical activation method is often referred to as one-step method. The raw material (biomass) is mixed with an activating agent,

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Fig. 1 Biological synthesis and process optimization for the production of nanoparticles. (Copyright © Elsevier 2016, adapted from Singh et al. [5] with permission)

the resulting mixture is dried, and later it is heated in an inert atmosphere [9]. Some of the commonly used activating agents are potassium hydroxide, potassium carbonate, sodium carbonate, magnesium chloride, and some acids such as phosphoric acid, sulfuric acid, aluminum chloride, and zinc chloride. Among these, activation by KOH is known to be the best method, as it introduces pores onto biomass-derived carbon materials due to several potentially beneficial features like mild activation temperature, higher production rate, and porosity with larger surface area [10]. The role of activating agent here is to remove water from the structure of the primary material and lower the temperature necessary for carbonization, which contributes to the creation of a porous structure in the product [11]. Nanomaterials synthesized by chemical activation methods can increase surface area of nanomaterials that may be useful for application in gas storage, electrochemical supercapacitors, etc. [12]. However, most chemical activation methods suffer from significant environmental disadvantages because of the use of large amounts of corrosive activating agents [13]. When compared with physically activated carbons, chemically activated carbons have advantage in producing much higher surface areas and, thus, generally show relatively higher CO2 adsorption capacities [14].

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Hydrothermal Carbonization Hydrothermal carbonization (HTC) method involves various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures, thus known as hydrothermal method. Two types of hydrothermal carbonization can be classified based on pyrolysis of biomass as high-temperature HTC and low-temperature HTC [15]. Cellulose produced by citric acid catalyzed by HTC resulted in dispersed micrometer-sized carbon nanospheres, and the diameter of carbon nanospheres became wider with the increase of citric acid concentration, reaction temperature, and reaction time [16]. Hydrothermal carbonization of waste sugar solution is efficient in preparing porous carbon nanospheres with superior rate capability [17]. HTC method sounds to be efficient chemical route to treat raw materials from biomass of the intrinsic advantages such as benign environment, versatile chemistry, enhanced reaction rate, and economic cost. Hydrothermal carbonization of biomass can be again divided into two categories according to the conditions and products: (i) hydrothermal gasification in subcritical water (the solid carbonaceous materials are converted into flammable gas mixture with or without the help of catalysts) and (ii) hydrothermal carbonization (regarded as hydrothermal liquefaction) in hot compressed water [18].

Nanomaterials from Plants, Microbes, and Actinomycetes Nanomaterials synthesized from biomass have various advantages because of their biodegradability and eco-friendly nature. Biomass is obtained from numerous sources, among which plants, microbes, and actinomycetes. Plant biomass, an abundantly available organic matter, constitutes agricultural waste and forest residues. Cellulose, lignocelluloses, hemicelluloses, lignin, etc., are the main source of plant biomass from which nanomaterials are synthesized [19]. Nanomaterials synthesized from cellulose and lignocelluloses are widely used biopolymers from plants for synthesis of biofuel [20]. Reusable nanomaterials synthesized from plant composites like orange peel are used in the removal of methylene blue from water (Table 1). Microorganisms are known to be potential agents in many fields for their wide range of applications. Microbial-mediated biosynthesis of nanomaterials is an efficient nano-manufacturing process that has bought revolution in conventional methods for synthesis of nanomaterials [35]. Nanomaterials synthesized from microbes have proved with potential advantages with defined chemical composition, size, and morphology, and also biosynthesis can be done at lesser physiochemical conditions [36]. Bacteria like Gluconacetobacter are proved to be potential agents for biosynthesis of bacterial nanocellulose. Yeasts species like S. cerevisiae have been also used for biosynthesis of Au-Ag alloy nanoparticles for electrochemical sensor fabrication. Microalgae such as Tetraselmis kochinensis, Scenedesmus, and Desmodesmus have been exploited for metal nanoparticle synthesis for their

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Table 1 Synthesis of nanoparticles using biomass of various plantsa

a

Plant Aloe vera Aloe vera Camellia sinensis

Nanoparticle Au, Ag In2O3 Ag, Au

Size (nm) 50–350 5–50 30–40

Citrullus colocynthis Curcuma longa Diospyros kaki Eucalyptus macrocarpa Eucalyptus macrocarpa Mangifera indica

Ag Pd Pt Au

31 10–15 15–19 20–100

Ag

10–100

Ag

20

Rhododendron dauricum Psidium guajava Pyrus sp.

Ag

25–40

Au Au

Terminalia catappa

Au

25–30 200– 500 10–35

Shape Spherical, triangular Spherical Spherical, triangular, irregular Spherical Spherical Crystalline Spherical, triangular, hexagonal Spherical, cubes

References [21] [22] [23] [24] [25] [26] [27] [28]

Spherical, triangular, hexagonal Spherical

[29] [30]

Spherical Triangular, hexagonal

[31] [32]

Spherical

[33]

Adapted and modified from Shah et al. [34]

potential applications as antimicrobial agents and in drug delivery, catalysis, and electronics [37]. Actinomycetes are gram-positive bacteria that have gained attention of researchers due to various industrial and biomedical activities. Both extra- and intracellular nanoparticles can be synthesized by actinomycetes (Tables 2, 3, and 4). Intracellular synthesis occurs on the surface of the mycelia due to the electrostatic binding of Ag+ ions to the negatively charged carboxylate groups in the enzyme present on the cell wall of mycelia. The Ag+ ions are then reduced by the enzymes in the cell wall forming silver nuclei. The accumulation of the silver nuclei leads to the formation of nanoscale silver particles. The microbe-based synthesized silver nanoparticles mainly using actinomycetes are potential antibacterial agents, and it was found that smaller silver nanoparticles synthesized by microbial route have greater antibacterial activity when compared to their chemical moieties [98] (Fig. 2).

Cellulose Nanostructures Cellulose, the most widely distributed polymers on Earth, is synthesized mainly by plants and also other life forms like microorganisms, algae, and marine forms. For instance, a single cotton fiber (thickness, 20–30 nm) consists of superfine fibrils of 2–20 nm diameter and is affected by many factors such as source, climate, soil, etc. Nanometer dimension of cellulose is referred as cellulose nanostructures or nanocellulose. Cellulose nanostructures are proved to be availing several attractive

Yarrowia lipolytica Pseudomonas aeruginosa Rhodopseudomonas capsulata Shewanella algae Brevibacterium casei Trichoderma viride Phanerochaete chrysosporium Bacillus licheniformis Escherichia coli Corynebacterium glutamicum Trichoderma viride Ureibacillus thermosphaericus

Microorganisms Sargassum wightii Rhodococcus sp. Shewanella oneidensis Plectonema boryanum Plectonema boryanum UTEX 485 Candida utilis V. luteoalbum Escherichia coli 30 37 30 25 37 27 37 37 37 30 10–40 60–80

Au Au, Ag Ag Ag

Ag Ag Ag

Ag Au

37 37 37

Au Au Au

Au Au Au

Culturing temperature (°C) – 37 30 25–100 25

Nanoparticle Au Au Au Au Au

Table 2 Synthesis of metal nanoparticles using biomass of microorganismsa Location Extracellular Intracellular Extracellular Intracellular Extracellular Intracellular Intracellular Extracellular Extracellular Extracellular Extracellular Intracellular Intracellular Extracellular Extracellular Extracellular Extracellular Extracellular Extracellular Extracellular

Shape Planar Spherical Spherical Cubic Octahedral – – Triangles, hexagons Triangles – Spherical – Spherical Spherical Pyramidal – – Irregular – –

(continued)

[56] [57]

[53] [54] [55]

[48, 49] [50] [51] [52]

[45] [46] [47]

[43] [43] [44]

References [38] [39] [40] [41] [42]

Nanomaterials from Biomass: An Update

2–4 50–70

50 50 5–50

10–20 10–50 5–40 50–200

15 15–30 10–20

ZrO2 (4 mg g
1). LaFeO3 was the best catalyst among all four catalysts to form phenolic units from the polymeric lignin [15]. As lignin has a very complex structure and a number of products obtained from lignin are also in excess, it is very tough to identify the structure of the products. The solution to this problem is to first study the lignin model compound and then apply that results directly to the polymeric lignin to convert it into value-added chemicals. Structure of lignin is having C-C and C-O specially β-O-4 linkage which contributes 50% to 60% of the ether bond in lignin structure. So, Wang and co-workers [13] have taken 2-phenoxy-1-phenylethanol (PP-ol) as a lignin model compound containing β-O-4 linkage and Cα-hydroxyl group and performed the oxidative conversion of the PP-ol and commercial lignin to aromatic compounds. The oxidative conversion of PP-ol in the presence of oxides like SiO2, Al2O3, MgO, and CeO2 with methanol as a solvent in oxygen atmosphere results in the formation of phenol, acetophenone, methyl benzoate and benzoic acid. The conversion of PP-ol and the yield of the abovementioned compounds were very low. After that, the catalytic conversion of

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2-phenoxy-1-phenylethanone (PP-one) having β-O-4 linkage and Cα-ketonic group in methanol under oxygen atmosphere has been done with above mentioned oxides. With the blank catalyst, 21% conversion of PP-one to various aromatic compounds has been observed. The conversion of PP-one in the presence of Al2O3, SiO2, MgO, and CeO2 were 23%, 26%, 67%, and 76%, respectively. The conversion of PP-one in case of blank catalyst and in presence of SiO2 and Al2O3 was almost same, so these two oxides did not contribute in the oxidative conversion of PP-one. The conversion of PP-one in the presence of MgO was high, but the yield of the aromatic monomers obtained was low in quantity. The reason behind the low yield is because of the basic nature of this oxide due to which the possibility of side reactions also increases, and also it reacts with the acidic product, i.e., benzoic acid, which further stops the cyclic catalytic activity of the oxide. In case of CeO2, phenol and methyl benzoate were formed as a major products, and acetophenone and benzoic acid were also formed, but the yield of these two products were very low. By increasing the reaction time from 2 h to 4 h, the catalytic conversion of PP-one further increases from 76% to 96%. The major products obtained with CeO2 as catalyst were phenol and methyl benzoate with a yield of 87% and 77%, respectively, after 4 h of reaction time. Acetophenone and benzoic acid were the minor products with the yield of 7% each over CeO2 in 4 h of reaction time. With the above experimental observation, CeO2 was the best oxide for the cleavage of the β-O-4 bond in the PP-one having α-ketonic group. But this oxide cannot activate the breakage of β-O-4 bond in PP-ol which is having an α-hydroxyl group. Therefore, the oxidative conversion of Cα-hydroxyl group to Cα-ketonic group can enhance the conversion of PP-ol to value-added chemicals. From the literature, it is clear that noble metals nanoparticles like Pd can promote the oxidation of alcohols to aldehydes [16]. Metal oxide supported Pd nanoparticles, e.g., Pd-SiO2, Pd-Al2O3, Pd-MgO, and Pd-CeO2 have been synthesized by using formaldehydereduction technique [21]. With the introduction of Pd nanoparticles over various metal oxides, the oxidative conversion of PP-ol has been increased. The conversion of PP-ol in the presence of Pd-Al2O3, Pd-SiO2, Pd-MgO, and Pd-CeO2 were 34%, 10%, 22%, and 64%, respectively. The oxidation of hydroxyl group in PP-ol to ketonic group in PP-one has been observed in all the Pd-supported metal oxide. The highest yield of PP-one has reached 30% in presence of Pd-Al2O3. With the usage of Pd-CeO2 as catalyst, conversion of PP-ol to PP-one and other aromatic products was maximum. Methyl benzoate, acetophenone, and phenol with respective yields of 14%, 38%, and 48% were the major products obtained by using PP-ol as substrate, Pd-CeO2 as catalyst, methanol as solvent medium under O2 atmosphere. Cleavage of Cα-Cβ bond along with breakage of β-O-4 bond resulted in the formation of methyl benzoate. Acetophenone formed by the cleavage of β-O-4 bond and PP-one was synthesized by the oxidation of Cα-hydroxyl group present in PP-ol ketonic group. As the loading of Pd over CeO2 increased from 0.5 wt% to 1.5 wt%, the conversion of PP-ol increases from 12% to 68%. Similarly, the yield of PP-one also increases significantly with the increase in the loading of palladium nanoparticles over CeO2. In the catalytic conversion of PP-one, by using CeO2 as catalyst, methyl benzoate and phenol were the major product, and acetophenone was the minor product. But in

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the presence of Pd over CeO2, selectivity of acetophenone increases which is a result of β-O-4 cleavage but without Cα-Cβ cleavage. This observation confirms that the loading of Pd nanoparticles over CeO2, enhances the breakage of β-O-4 bond more than the Cα-Cβ bond. Native lignin not only contains β-O-4 linkage but also many functional groups, specially methoxy group (-OCH3) in its structure. These functional groups present in the neighborhood of β-O-4 bond change the activation energy needed for the cleavage of this bond. Therefore, oxidation of methoxy functionalized PP-ol, i.e., 2-(4-methoxyphenoxy)-1-phenylethanol has been done by using Pd-CeO2 under the same conditions. The conversion of functionalized PP-ol was 90% with the methyl benzoate, acetophenone, and 4-methoxyphenol as products with a yield of 40%, 38%, and 82%, respectively. The above results show that the presence of an electron donating functional group like methoxy group in the neighborhood of β-O-4 bond promotes the cleavage of β-O-4 bond and favors the synthesis of valuable aromatic compounds. The oxidative cleavage of organosolv lignin by using Pd/CeO2 under O2 atmosphere at 185 °C for 24 h was performed. The obtained products were vanillin, guaiacol, and 4-hydroxybenzaldehyde with a yield of 5.2%, 0.87%, and 2.4%, respectively. Some of the complex products were not possible to be identified. Out of the four catalyst, Pd-Al2O3, Pd-SiO2, Pd-MgO, and Pd-CeO2, Pd-CeO2 was a stable heterogeneous catalyst with high efficiency that comes out to be the best catalyst for the oxidative conversion of model compound of lignin and native lignin.

Conclusion and Further Outlook Renewable biomass sources have become great challenge on our planet due to increase in industrialization and depletion of non-renewable sources. Replacement of non-renewable sources, like crude oil, petrol, diesel, etc., by renewable biodiesel, biopolymers, etc. are in high demand in today’s world. This can be made possible by using heterogeneous catalysts for the treatment of lignocellulosic biomass. Homogeneous catalyst can also be used for this process, but it cannot be recycled and also homogeneous catalysts which are acidic in nature will corrode the reactor very quickly thereby increasing the maintenance and overall cost of the production of various useful products from lignocellulose biomass source. Heterogeneous catalysts have unique characteristics of high surface area, tunable pore size, etc. which make them suitable and efficient for various biomass-related reactions. Each and every reaction related to biomass conversion demands for different characteristics, e.g., conversion from xylose to furfural demands the presence of acidic sites in heterogeneous catalysts. Heterogeneous catalysts have the property to tune its properties by doing various kind of functionalization like sulfonation, amination, etc. which make its nature appropriate for the different types of reaction of biomass. Day by day, the demand for the renewable sources is increasing, so industries have to increase the production of these value-added chemicals. In addition, solvents play a very crucial role for the biomass-related reactions. Non-Green solvents like methanol, dimethylformamide, dichloromethane, etc. which are hazardous in nature

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are to be avoided for the biomass process, as these create more pollution and have dreadful effects on the human beings. Green solvents have low or no toxicity which makes the biomass-related synthesis process less hazardous. Therefore, Green solvents like water, ethanol, isopropyl alcohol, dimethyl carbonate, etc. are of utmost interest for the pretreatment of lignocellulosic biomass and also for the conversion of various components of biomass to values added chemicals. Therefore, heterogeneous catalysts are being used for the biomass conversion processes in the presence of green solvents for the synthesis of value-added chemicals. Extensive researches are going on for the biomass-related reactions. But more research are required for the pretreatment of lignocellulosic biomass to separate the three major components of biomass. Further usage of cellulose, hemicellulose, and lignin for the synthesis of biodiesel, biopolymers, and various other useful products also requires catalysts. Thus, there is a strong need for the design and development of highly efficient heterogeneous catalysts that can operate in Green solvent medium under mild conditions. This will pave the way for a more sustainable future.

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Thermochemical Conversion of Biomass Waste-Based Biochar for Environment Remediation Sudipta Ramola, Tarun Belwal, and Rajeev Kumar Srivastava

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass Resources and Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Primary Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tertiary Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass Conversion Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochemical Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermochemical Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochar for Agro-environmental Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochar for Agronomic Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochar as an Adsorbent for Pollutant Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Heavy Metal Removal by Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Huge quantity and a wide spectrum of agriculture wastes are generated as a result of various agriculture operations. These wastes include crop residues, manures from farms and poultry houses, fertilizers, and pesticides that either run off or infiltrate to pollute water and soil. There are mainly three main

S. Ramola (*) · R. K. Srivastava Department of Environmental Science, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India e-mail: [email protected] T. Belwal Centre for Biodiversity Conservation and Management, G.B. Pant National Institute of Himalayan Environment and Development, Kosi-Katarmal, Almora, Uttarakhand, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_122

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processes of biomass conversion technology, i.e., biochemical, thermochemical, and physicochemical, that convert biomass-based waste for further resource recovery. Biochar is a black carbonaceous product prepared by thermochemical conversion of biomass-based waste including crop residues and animal manures. It is a multi-functional product with distinct structural, physicochemical, and biological properties which are function of parent feedstock and pyrolysis conditions. Different types of biochar have been successfully investigated for their role in agronomic benefits as well as for environmental remediation. Biochar has multi-dimensional properties such as porous structure, large surface area, aromatic structure, presence of functional groups (mainly C¼O containing groups that further enhance adsorption properties), and mineral components which facilitate broad-spectrum application of biochar. Some of the applications of biochar include soil carbon sequestration, soil amendment, and pollutant removal from aqueous and soil medium. The present chapter focuses on different biomass conversion technologies, use of biochar as an adsorbent, its mechanism for wastewater remediation and highlights the ongoing state-of-the-art research and development. Keywords

Agro-waste · Thermochemical conversion · Biochar · Environmental remediation · Wastewater treatment

Introduction Biomass is a renewable source of energy which is available as organic material prepared by all terrestrial and aquatic plants via photosynthesis. The solar energy captured during photosynthesis is stored in the form of chemical energy as chemical bonds between C (carbon), H (hydrogen), and O (oxygen). During biomass conversion, the chemical bonds between C, H, and O are broken, and stored chemical energy is released from biomass [28]. The main advantages of bioenergy are its reproducibility, resource universality, and less emission of NOX (oxides of nitrogen), SO2 (sulfur dioxide), and soot [16, 54]; however, it has lower energy density per volume/mass as compared to equivalent fossil fuels. A vast range of biomass can be used for bioenergy generation, and selecting a specific biomass depends on various factors such as economic and technological feasibility. Before a biomass is converted to bioenergy, it has to undergo a number of stages such as harvesting, drying, storage, transportation, and processing. Availability of biomass, quality, cost, location, collection, transport, storage, handling, location of consumer, type of energy service, and specific co-product required are also taken into consideration. The annual worldwide biomass resources are estimated to be 146 billion tons. As an estimate, by using 10% of the total biomass (residues from forest harvest, wood processing, and agriculture sector) at 50% conversion efficiency, 3.1 trillion

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tons of oil equivalent energy, i.e., 200 times the world energy consumption in 2015 can be produced. For same amount of biomass, at 10% conversion rate, 1.6 billion tons of chemical feedstock can be produced [35]. The primary, secondary (byproducts), and tertiary (post-consumption) biomass resources are mainly used as feedstock for the biomass conversation using biochemical, physicochemical, and thermochemical techniques. These techniques are used depending on the feedstock type and required end products, and each technique has its own advantages and limitations. This chapter mainly discusses about the plant-based biomass resources, different biomass conversation methods, and use of biochar as a product of pyrolysis for environmental remediation.

Biomass Resources and Selection The Green Revolution has brought enormous gain in crop production coupled with the generation of huge amount of agro-waste. These agro-wastes are used as potential resources for making valuable products by different conversion methods for their application in pollutant removal, soil improvement, and enhanced plant growth [43]. Worldwide, about 4000 MT/year crop residue is produced from 27 food crops having a share of 75% lignocellulosic residue produced by cereal crops [21]. In Asia, the contribution of rice and wheat crops for global lignocellulosic biomass generation is around 30% [11]. Lignocellulosic agro-waste is the most abundant renewable energy resource with potential for further agronomic and environmental use. It mainly consists of cellulose (38–50%), hemicellulose (23–32%), lignin (15–25%), and extractives [32, 43]. Beside environmental pollution, there are several other adverse effects of burning crop residues in open fields, such as loss of nutrients. It is estimated that in general crop residues contain 80% of nitrogen (N), 25% of phosphorus (P), 50% of sulfur (S), and 20% of potassium (K) which get lost on open burning. Depletion of beneficial soil microorganisms due to elevated soil temperature and reduced level of N and C in the top 0–15 cm soil profile are also serious issues raised due to open burning. Burning of crop residues in open causes generation of greenhouse gases and other chemically and radiative important trace gases and aerosols. It is estimated that upon burning, C present in rice straw is emitted as CO2 (70% of carbon present), CO (7%), and CH4 (0.66%), while 2.09% of N in straw is emitted as N2O. Generation of huge amount of agro-waste is also a problem for waste management. Open burning of crop residues is very common practice which leads to non-recovery of potential resources as well as release of air pollutants which interfere with radiative-convective coupling of the Sun-Earth system and significantly contribute to Asian brown clouds [37]. The air pollutants that are mainly released during open burning are particulate matter (PM2.5 and PM10), i.e., ash, polycyclic aromatic hydrocarbons (PAH), and soot, as well as non-methane hydrocarbon compounds (NMHCs) [17, 43]. Biomass can be categorized as primary, secondary, and tertiary resources depending on the stage at which they are used. This is explained as follows.

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Primary Resources Primary resources are those produced by photosynthesis and derived directly from land resources. These include perennial short rotation woody crops and herbaceous crops, seeds of oil crops, agriculture residues, and forestry residue.

Secondary Resources Secondary resources are byproducts derived from processing of primary biomass resources. The main crop is not harvested for energy purposes, but residues such as straws, husks, shells, and logging byproducts are used to derive energy. This include physically derived biomass (saw dust production in mills), chemically derived biomass (black liquor production from pulping process), biologically derived biomass (manure production by animals) and thinning and logging byproducts from woody biomass.

Tertiary Resources Tertiary resources are generated after post-consumer consumption. These are mainly the end products of life materials which are discarded, e.g., animal fats and grease, used vegetable oils, packaging waste, and waste and debris generated at construction and demolition sites.

Biomass Conversion Technologies At present, extensive agricultural waste disposal methods not only fail to effectively convert and utilize agricultural resources but also cause serious environmental pollution. In livestock and poultry manure, there are organic matter, pathogenic bacteria, parasitic eggs, and heavy metal which are directly discharged into the water without any proper treatment, causing serious contamination of groundwater and surface water systems. Pollution caused by agricultural wastes has become a serious concern over the years. Developments have been made to use these wastes for product recovery, i.e., as feedstock for energy production, chemical recovery, and pollutant adsorption. Use of appropriate biomass conversion technology depends on factors such as biomass type, availability and quantity, desired form of required energy, and economic feasibility. There are mainly three processes for biomass conversion, i.e., biochemical (anaerobic digestion and fermentation), physicochemical (extraction with esterification), and thermochemical (combustion, pyrolysis). Different biomass conversion technologies are depicted in Fig. 1 and further explained in below sections.

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Fig. 1 Different biomass conversion technologies and their end products as resource

Biochemical Conversion Anaerobic Digestion It includes biological conversion of organic material to biogas and other gaseous products. This process takes place with the help of bacteria that convert organic materials to heat, CH4, CO2, H2S, H2, and energy content (~20–40%) of lower heating value of feedstock [29]. This is a relatively slow process that takes place for several days in tanks where ideal conditions are maintained. The bacteria can convert about 90% of the feedstock into two main products, i.e., (i) biogas (~50–60% CH4, ~40– 50% CO2, and traces of H2S) and (ii) biofertilizer (digestate valuable plant nutrients and organic humus which is separated into liquid fraction and solid fractions). CH4 can be upgraded to a purer methane form, i.e., biomethane. This process requires additional energy (electricity) and some inputs, such as chemicals, water, or membranes, depending on the type of technology employed. This involves removal of CO2 and purification of other trace gases to get versatile high-value fuel for application. Anaerobic digestion also acts as a treatment process for industrial organic waste with higher moisture content like moisture sewage waste and sewage sludge with 80–90% moisture content [29] to get versatile high-value fuel. This energy requirement and release of methane may also increase greenhouse gas emission compared to biogas. However, CO2 stream can be captured and used in industrial process, and methane slip can be reduced to almost zero through the use of thermal oxidizer. Fermentation It is also an anaerobic process that converts simple sugars of feedstock to alcohol and CO2 by different microorganisms, mainly yeast. It is used commercially on large

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scale and mainly utilizes sugar crops (sugarcane, sugar beets) and starch crops (maize, wheat) to prepare ethanol, which is separated from other components by distillation [1, 29]. Distillation is an energy-intensive process and requires higher energy so the net energy balances become less in case of fermentation [22]. The solid residue from fermentation can also be used further as an animal feed and fuel for boiler or in gasification. Bio-based ethanol or bioethanol is used as an extender in petrol, and flexible fuel vehicles, i.e., higher blends (85% ethanol and 15% gasoline E85), are used in countries such as Australia, Sweden, the USA, and Brazil [1]. Currently most of the fermentation for bioethanol production is done by using sugarcane and maize as raw materials. Other feedstock such as wheat and sugar beet can also be utilized, provided they are available in larger quantities as they are primarily grown for food purposes.

Mechanical Extraction Mechanical extraction or physical conversion process that is used to extract oil from different feedstock. Feedstock used for this process mainly include waste fats and oil, rapeseeds, cottonseeds, linseed, and groundnuts. After the process of extraction, the end products are oil and a residual solid cake which can be used as fodder for animals [29]. The liquid fuel produced after mechanical extraction can further undergo esterification, which converts oil to fatty acid methyl ester, more widely known as biodiesel. Biodiesel is blended with diesel and is used as a fuel for transportation.

Thermochemical Conversion Thermochemical conversion has higher efficiency and takes less time than biochemical conversion process. Lignin breakdown is relatively slow and incomplete in biochemical conversion, whereas they are effectively decomposable via thermochemical methods. It produces better calorific values and cleaner fuel source byproduct as compared to fossil fuels and hence promoted over other technologies. Following are different types of thermochemical conversion.

Combustion Combustion is the simplest thermochemical conversion process where biomass is burnt in the presence of oxygen to produce heat, mechanical power, or electricity as main products. Combustion of any biomass can be understood in four stages, i.e., drying, pyrolysis, volatile combustion, and char combustion. In drying process water is evaporated from feedstock with increased temperature. This stage requires heat to evaporate and dry all the water content from feedstock. It is desirable to have feedstock with minimal moisture to increase the combustion efficiency and minimize

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smoke emission. In the second stage, pyrolysis occurs, and volatile gases, tars, and hydrocarbons are emitted out from feedstock. The third stage is volatile combustion which is marked by release of CO2 and H2O. Last is char combustion that forms CO2 and CO as the main emission products.

Gasification In gasification an organic feedstock is converted into gaseous product, termed as producer gas/product gas/syngas which consists of CO, H2, CO2, and CH4 and other hydrocarbon species [35]. Syngas can be used to produce heat or synthesized to produce liquid transportation fuels [12]. Operating conditions of gasification include moderate to high temperature (500–800  C), 10–20 s of residence time, and heat transfer rate of 20–30  C/min. Decomposition of biomass generates about 5 wt% of liquid product, 10 wt% of solid product, and 85 wt% of gaseous product. Pyrolysis The word pyrolysis is derived from the Greek words “pyro” meaning fire and “lysis” meaning breakdown. Pyrolysis is done in limited oxygen, and heat is provided externally which breaks the biomass to gas, liquid, and solid products. It is flexible in operation with different heating temperatures, heating rate, and time. Production of higher fraction of gas, liquid, or solid products depends mainly on pyrolysis temperature and vapor residence time. A low pyrolysis temperature and longer vapor residence time favor higher yield of solid product, i.e., biochar, whereas higher temperature and longer vapor residence time increase the yield of gaseous products, and moderate temperature of pyrolysis and short hot vapor residence time are optimum conditions for producing liquids [1]. It is more favored and acceptable process due to rationale yields of smaller compounds formed by decomposition, i.e., volatiles (bio-oil and syngas) and biochar produced from the condensation/ polymerization of lignocellulosic biomass [6, 34]. Depending on the operating conditions, pyrolysis process is divided into fast pyrolysis and slow pyrolysis. In fast pyrolysis, operating conditions include temperature of 500  C, shorter residence time of 2 s, and heat transfer rate of 300  C/min. Biomass decomposes very fast in this pyrolysis process and generates bio-oil (75 wt%), char (12 wt%), and gas (13 wt%) [4]. After cooling and condensation, a dark brown homogenous mobile liquid is formed that has heating value about half of conventional fuel oil [1]. Operating conditions in slow pyrolysis include low to moderate temperature, i.e., in range of 300–500  C, longer residence time of 5–30 min, and heat transfer rate from 5  C/min to 20  C/min. It generates about 30–50 wt% of bio-oil, 25–30 wt% of char, and 35 wt% of gas. Pyrolysis Products Pyrolysis produces biochar (a black carbonaceous product); volatile matter such as condensable vapors (pyrolysis oil); non-condensable gases such as CO, CO2, H2, and CH4; and other lightweight hydrocarbon gases called syngas. It offers more

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scope for recovering products from agro-forestry waste. In contrast to simple combustion, pyrolysis provides different products that can be used as feedstock for petrochemicals and other applications such as adsorbent for pollutant removal, soil amendment, and soil carbon sequestration. Bio-oil Bio-oil is a complex oxygenated compound made up of water, water-soluble compounds such as acids, esters, and water-insoluble compounds coming from depolymerization and fragmentation of cellulose, hemicelluloses, and lignin. The properties of bio-oil depend on feedstock, procedure employed for bio-oil preparation, and operating conditions [9]. It is high oxygenated compound and hence has less heating values, typically half the heating value of fossil crude such as heavy fuel oil. The specific gravity of the liquid is about 1.10–1.25 which makes it heavier than water and fuel oil. Water content is typically between 15% and 35%, and heating value is below than those for conventional fuel oil. It is acidic with pH range of 2–4 and is therefore unstable and corrosive. Bio-oil can be refined and upgraded for its potential use as liquid transportation fuel. Upgraded biofuel may also be used for power generation, production of resins and chemicals [46], and production of anhydrosugar like levoglucosan wood preservative [45]. Bio-oil possesses higher viscosity that leads to aging problem while it is stored and it counts for higher pressure drop, hence increasing cost of equipment maintenance. Technological advancement is needed in the future so that bio-oil can be used as a potential material for heating.

Biochar Biochar is a black carbonaceous product prepared from pyrolysis of biomass. The physicochemical, structural, and functional properties of biochar are influenced by biomass used and pyrolysis conditions employed during biochar preparation. Different types of biochar have been successfully investigated for their role as agronomic entity as well as for environmental remediation. Some of this include its use for carbon storage, soil carbon sequestration, soil amendment, enhancing crop yield, and pollutant removal from aqueous and soil medium [38–40, 48]. Biochar has been successfully tested as an efficient adsorbent for many organic and inorganic pollutants from aqueous solution and different soils. Different properties of biochar enable it to remove pollutant from target medium. Some of these properties include high specific surface area, microporous structure, alkaline pH, high cation exchange capacity, and presence of minerals such calcite, dolomite, periclase, sylvite, montmorillonite, and quartz [39, 40] and functional groups such as carbonyl, hydroxyl, phenolic, and carboxyl that play an important role in pollutant removal. Among different pyrolytic conditions, temperature is a key parameter that affects the adsorption properties of biochar. Pyrolytic temperature alters the structure of biomass and helps in its carbonization. At higher temperature, the organic matter of biomass is carbonized, and surface area and porosity increase that further facilitates the chances of pollutant adsorption. Similar results are reported for adsorption of trichloroethylene [56], Cd [18], methylene blue [26], and Pb [40]. Increased

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temperature also renders alkaline pH, increase in mineral components such as K, Ca, Mg, and P that further enhances the chances of metal adsorption. However, with increased temperature, due to more carbonization, functional groups are reduced that minimizes the chances of complexation between metals and functional groups. Higher temperature increases the chances of CO2 release that leads to secondary reaction between released CO2 and carbon present in biochar matrix. This results in opening of closed pores and hence increased porosity with better chances of adsorption. Also, loss of oxygen and hydrogen at higher pyrolysis temperature lowers hydrophobicity of biochar and hence forms less water clusters on its surface, leading to improved adsorption [52].

Biochar for Agro-environmental Benefits Biochar for Agronomic Benefits Many studies have been done on investigating the effect of biochar on improving soil nutrient content and pollutant remediation. The conversion of invasive and weed species of plants such as Parthenium hysterophorus to biochar has agronomic benefits, e.g., improved seedling vigor index of maize as compared to control. Soil dehydrogenase activity, catalase (increased up to 3 g/kg), active microbial biomass carbon, and basal soil respiration increased progressively with addition of Parthenium biochar with no adverse effect on soil microbial activity even at the highest rate of 20 g/kg [20]. Similarly, as a sustainable weed management practice, water hyacinth (Eichornia crassipes) was converted to biochar at varied temperature (200–500  C) and residence time (30–120 min), and its effect on soil biochemical properties was studied. It was observed that maize seedling vigor index increased up to 1.61 in biochar treatment compared to 1.0 at control at a dose of 20 g/kg. In the same dose of biochar, activity of soil enzymes like acid phosphatase, alkaline phosphatase, and fluorescein hydrolases increased by 32%, 22.8%, and 50%, respectively. Water hyacinth biochar treatment also enhanced soil biological activity such as increased active microbial biomass and soil respiration by 1.9 times, suggesting that biochar can be seen as a sustainable soil ameliorant [27]. This also implies that preparation of biochar from bioresources is also a solution for improving waste management and environment remediation. Cole et al. [8] used freshwater macroalga Oedogonium intermedium to recover dissolved nitrogen and phosphorus from municipal wastewater and recycled these nutrients to agriculture fields in order to test a novel approach to close anthropogenic nutrient cycle. The cultivated algae were converted into compost (combined with sugarcane bagasse) as well as biochar (slow pyrolysis at 450  C) and were found to contain total N and P content of 2.5% and 0.6% and 5.5% and 2.5%, respectively. Corn biomass was also found to be four to nine times more in treatments receiving 50% and 100% compost in comparison to synthetic fertilizer. Compost in conjugation with biochar treatment caused additional increased crop productivity by 15% because of the ability of biochar to hold labile N and P in soil and prevent its loss [8].

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Seaweed (marine macroalgae) aquaculture is a very rapidly growing industry which is primarily used for the production of food and hydrocolloids with huge amount of waste. This waste can be used for the production of biochar. Biochar produced from seaweeds has higher yield in comparison to the land-based lignocellulosic biomass. In comparison to the biochar prepared from high lignocellulosic biomass, the C content is lower in biochar prepared from seaweeds. However, seaweed biochar has a high concentration of exchangeable nutrients particularly N, P, K, Ca, and Mg and cation exchange capacity (CEC) which makes them very effective in nutrient retention and biosorbent of heavy metals for environmental remediation [3, 41, 42].

Biochar as an Adsorbent for Pollutant Removal Activated carbon has been used globally for many decades as an efficient adsorbent for removal of wide spectrum environmental pollutants. Biochar has similar porous structure as that of activated carbon but is much cheaper, often equally or at times more efficient adsorbent, as well as more versatile in terms of environmental and agronomic applications. For instance, Kołodyńska et al. [19] investigated the adsorption capacity of commercial active carbon Purolite AC20 (obtained from the bituminous coal mineral) and biochar for heavy metal ions of Cu(II), Zn(II), Cd (II), Co(II), and Pb (II) at different operating factors such as adsorbent dose, contact time, solution pH, initial concentration, and temperature. It was found that biochar was more effective than commercial active carbon Purolite AC20 in removing heavy metal ions from aqueous solutions with the highest adsorption capacity of biochar at pH 5.0 to be 37.80 mg/g for Pb(II), 33.90 mg/g for Cd(II), 24.95 mg/g for Cu(II), 23.26 mg/g for Zn(II), and 20.23 mg/g for Co(II). Biochar loaded with ammonium, phosphate, nitrate, as well as other valuable nutrients can be a source of slowreleasing carbonaceous product for soil and hence improved yield. The energy requirement in terms of higher temperature and activation for activated carbon is much more than that of biochar. Biochar on the other hand is often produced as a byproduct of pyrolysis of abundantly present low-cost agro-waste and other solid waste with no intended further resource use. Also, the feedstock used for preparation of biochar is often not limited to agro-waste but also includes wood biomass, animal litter, algae, and invasive plant species. Table 1 describes the lists of biomass feedstock for biochar preparation and removal of target pollutants.

Mechanism of Heavy Metal Removal by Biochar Adsorption mechanism of biochar is dependent on various properties such as surface functional groups, surface area, porosity, and presence of mineral components. Different metals are adsorbed by different mechanisms, and the role of biochar properties in this is metal dependent. Mainly five mechanisms are proposed for metal removal from aqueous solution by biochar, i.e., precipitation, cation exchange,

Spartina alterniflora biochar Mg-enriched tomato leave biochar MgO-sugar beet tailing biochar nanocomposites

Cigarette waste-bentonite composite Cigarette waste-calcite composite Cotton wood biochar-Fe2O3 composite Sludge-derived biochar Digested dairy waste biochar Digested whole sugar beet biochar Miscanthus sacchariflorus biochar Cactus fiber biochar

Bamboo biochar

Tyre biochar

Iron-modified tyre biochar

Biochar Anaerobically digested sugar beet tailing Iron-modified bagasse biochar

Pb Pb As(V) Pb Pb Pb Cd Cu Cu Phosphate Phosphate

400  C 600  C, 1 h 600  C, 10C/min, 1 h

Cu

Hg

Pb

Pyrolysis conditions 600  C, 10  C/min, 2 h Indigenous kiln preparation Indigenous kiln preparation Indigenous kiln preparation Indigenous kiln preparation 700  C, 15  C/h, 1.5 h 700  C, 15  C/h, 1.5 h 600  C, 1 h 550  C, 10  C/min, 2 h 600  C, 2 h 600  C, 2 h 500  C, 10  C/min, 1 h 600  C, 1 h

Target pollutant Phosphate Phosphate

nd 88.50% 66.70%

99.9% 99.9% nd nd 99% >97% nd nd

81.3%

89.4%

95%

Thermochemical Conversion of Biomass Waste-Based Biochar for Environment. . . (continued)

[23] [51] [53]

[40] [40] [55] [24] [13] [13] [18] [10]

qmax – 500 mg/g qmax – 500 mg/g 3147 mg/kg 30.88 mg/g nd nd qmax – 13.24 mg/g qmax – 1.4 mol/kg

Langmuir model Langmuir model Langmuir model – Freundlich model Langmuir model Langmuir model DubininRadushkevich qmax – 48.49 mg/g Langmuir model nd nd 835 mg/g Langmuir model

[39]

qmax – 26.31 mg/g Langmuir model

[39]

Reference [50] [39]

[39]

Temkin model

Adsorption isotherm nd Freundlich model

qmax – 52.63 mg/g Langmuir model

qmax – 4.97 mg/g

Adsorption Removal capacity 73% nd 71.7% qmax – 2.32 mg/g

Table 1 Lists of biomass feedstock for biochar preparation and removal of target pollutants

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Pyrolysis conditions 600  C, 10C/min, 1 h 100  C, 25  C/min, 4 h 200  C, 25  C/min, 4 h 200  C, 25  C/min, 4 h 450  C, 1.5 h

nd is not determined

Bamboo biochar-montmorillonite composite 400  C, 5  C/min, 1 h Bamboo biochar-montmorillonite composite 400  C, 5  C/min, 1 h Bentonite-cassava peel hydrochar composite 500  C, 10  C/min, 60 min Jacobsite–tea branch biochar nanocomposite 500  C, 25  C/min, 90 min Jacobsite–tea branch biochar nanocomposite 500  C, 25  C/min, 90 min Magnetic corn stover biochar 500  C, 30 min

Biochar MgO-peanut shell biochar nanocomposites Dairy manure solid biochar Dairy manure solid biochar Dairy manure solid biochar Ball-milled sugarcane bagasse biochar

Table 1 (continued)

nd nd nd nd nd nd

Sb Cd Fluoride

Removal 11.7 93% 100% 77% nd

Target pollutant Nitrate Pb Pb Atrazine Methylene blue NH4+ Phosphate NH4+

Adsorption isotherm Langmuir model nd nd nd Langmuir model

4.11 mg/g

181.49 mg/g

237.53 mg/g

nd

Langmuir model

Langmuir model

qmax – 12.52 mg/g Langmuir model qmax – 105.28 mg/g Langmuir model qmax – 123.67 mg/g nd

Adsorption capacity 95 mg/g nd nd nd 354 mg/g

[30]

[47]

[47]

[7] [7] [15]

Reference [53] [5] [5] [5] [25]

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complexation, electrostatic interactions, and chemical reduction. These mechanisms may work solely or in synergetic integration with one another, solely depending on properties of biochar and target heavy metal [2, 14, 31, 33, 36, 44, 49]. Insoluble precipitation of heavy metal can occur on biochar surface or as coprecipitation between heavy metal and mineral components such as carbonate, phosphate and silica present on the biochar surface. Cation exchange can occur between heavy metals in the aqueous solution and protons or exchangeable metal ions such as Ca2+, K+, Na+, and Mg2+ present on the biochar surface. Another mechanism involved forms complexation of heavy metals with functional groups and π electron-rich domain on the surface of biochar. Biochar can also be removed via electrostatic interaction, and depending upon pHpzc, it can be anionic metal attraction (pH < pHpzc) or cationic metal attraction (pHpzc < pH). Another method of metal removal is by its reduction followed by subsequent sorption of the reduced metal species.

Conclusions and Further Outlook A wide range of biomass conversion technologies are available for the conversion of biomass waste into a number of valuable products. These technologies have advanced with time to efficiently utilize the resources to gain economic and environmental benefits. However, the use of these technologies depends on various factors, including feedstock type, desired end products, and economic and technological feasibility. Biochar has emerged as a valuable resource from thermochemical conversion of biomass and is used for agronomic and environmental benefits. Advantages of biochar include improving soil nutrient, carbon sequestration, and removal of toxic heavy metals from aqueous and soil medium. Modifications of biochar can also be done further by different methods for improvement of its functional properties. Large-scale applications of biochar also need to be tested to know its environmental fate and long-term applicability.

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Microbial Synthesis of Gold Nanoparticles and Their Applications as Catalysts

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gold Nanoparticles as Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Synthesis of Gold Nanoparticles Using Microbial Cell Factories . . . . . . . . . . . . . . . . . . Nitroaromatics and Dyes: Environmental Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Performance of Gold Nanoparticles Prepared from Microbial Sources . . . . . . . . . . . . Supported Gold Nanoparticles Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of Aromatic Amines and their Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supported Gold Nanoparticles Catalyst for the Hydrogenation/Reduction of Nitroarenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-Supported Metal Nanoparticles as Heterogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-Supported Gold and Palladium Nanoparticles for the Catalytic Reduction of Nitroarenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nanoparticles of noble metals such as gold, silver, and platinum are useful for important applications in catalysis and biomedical science. Biological methods of nanofabrication include the use of easily available plants, microbes, and enzymes. This chapter presents an overview of microbial methods of preparing S. Krishnan (*) Laboratory of Bio-organic Chemistry, Department of Biotechnology, Indian Institute of Technology Madras, Chennai, India A. Chadha (*) Laboratory of Bio-organic Chemistry, Department of Biotechnology, Indian Institute of Technology Madras, Chennai, India National Centre for Catalysis Research, Chemistry Department, Indian Institute of Technology Madras, Chennai, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_201

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gold nanoparticles and the use of these gold nanoparticles in catalytic applications. Mainly, the catalytic performance of biosynthesized gold nanoparticles toward protecting the environment from aromatic toxic pollutants and textile dyes is discussed with relevant examples. The method of preparation of supported gold nanoparticles catalysts using conventional chemical and microbial methods for the hydrogenation/reduction of nitrobenzene to aniline is presented. Toward sustainable catalysis, the application of bio-supported nanoparticles (gold nanoparticles, palladium nanoparticles, and the combination), as heterogeneous catalysts for various chemical reactions like oxidation, reduction, and coupling, to yield value-added fine chemicals and intermediates under environmentally benign conditions is elaborated. Keywords

Biosynthesis · Gold nanoparticles · Supported nanoparticles · Catalysts · Toxic dyes · Nitroarenes

Introduction Nanotechnology has contributed to the tremendous improvement in the field of catalysis, mainly in chemical processing, pollution control, catalyst manufacture, and fuel cells [1]. Even before the actual onset of nanotechnology, nanoparticles did exist, but their role in catalysis was discovered later. A suitable example is the HaberBosch process developed in 1931 which involves the synthesis of ammonia from gaseous nitrogen and hydrogen using iron catalyst (BASF-S6–10) made of Fe (40.5%), K (0.35%), Al (2.0%), Ca (1.7%), and O (53.2%). In this process, iron ore catalyst consisting of iron-based nanoparticles with a surface area of 20 m2g
1 was employed [2]. Metal nanoparticles are synthesized by different methods and used as catalysts in various chemical reactions. Metal nanoparticles such as Au, Pd, Rh, Pt, and Ir are used as catalysts in Fischer-Tropsch, isomerization, hydroformylation and other reactions, where the catalytic activity of nanoparticles is dependent on particle size. Chemical reactions catalyzed by metal nanoparticles are classified into two types, i.e., homogeneous or heterogeneous catalysis. Homogeneous catalysis involves the use of unsupported metal nanoparticles, whereas heterogeneous catalysis involves the use of supported metal nanoparticles as catalysts. Unsupported metal nanoparticles are homogeneous catalysts with intrinsic advantages in catalysis such as efficiency and selectivity. Supported nanoparticles catalyst primarily consists of metal nanoparticles as the active phase which is anchored to a carrier/matrix. In few examples, promoters are also added during the preparation of heterogeneous catalysts. Different supports used in the preparation of supported catalysts are metal oxides, polymers, composites, and layered double hydroxides, among others. Unlike unsupported metal nanoparticles, supported nanoparticles have unique advantages of catalyst reusability, no

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leaching of metals, and no loss of catalytic activity caused by nanoparticles aggregation. Sustainable catalysts possess characteristics such as high activity, selectivity, efficient recovery, recyclability, and cost-effectiveness [3].

Gold Nanoparticles as Catalysts With the advancement of nanotechnology, nanoparticles of noble metals are produced and have been successfully used for different applications. Due to their intrinsic electron structures, the noble metal nanoparticles have numerous advantages in catalysis and surface plasmon resonance (SPR)-based optical detection. Noble metals are relatively inert in air and moisture and therefore do not get oxidized or corroded easily. That is why these elements are preferred for producing nanostructured materials. Based on their electronic configurations, noble metals can be categorized as 4d series of transition metals like Ag, Ru, Rh, Pd, and 5d series like Au and Pt. Gold (Au) is one of the most widely used noble metals and is chemically nonreactive in the bulk state. Gold clusters and gold nanoparticles exhibit catalytic activity depending on the particle size. Historically, colloidal solutions of noble gold particles were used to stain glass for decorative purposes during ancient Roman times (since AD 400), for example, the Lycurgus cup which is made up of glass with colloidal alloy of gold (40 ppm) and silver (300 ppm). The cup’s dichroic glass appears red in color when viewed under transmitted light and appears green in the presence of reflected light. A colloidal solution of gold nanoparticles was first prepared by M. Faraday in 1857 where a two-phase system was employed for the reduction of aqueous solution of chloroaurate (AuCl4
) under an atmosphere of phosphorus in CS2. Richard Zsigmondy received the Nobel Prize in Chemistry in 1925 for his contribution to modern colloidal chemistry of gold sols and the subsequent invention of the ultramicroscope. Among metal nanoparticles, gold nanoparticles (Au NPs) are considered “green” because they are nontoxic and benign as compared to transition metals such as palladium, ruthenium, rhodium, and others. Compared to bulk gold which is a chemically inert material, nanoscale gold is catalytically active, due to electronegativity, oxidation potential, and large number of surface atoms with low coordination sites on the surface. Earlier, gold nanoparticles of size smaller than 5 nm supported on α-Fe2O3 are known to act as good catalysts for the oxidation of carbon monoxide. Later, this concept was used to manufacture gas masks which oxidize carbon monoxide to carbon dioxide using gold nanoparticles catalysts (Aurolite), developed by a South African metal and mineral processing company, Mintek. Another interesting example is the direct synthesis of hydrogen peroxide from gaseous hydrogen and oxygen using Au-based catalysts where high atom economy and prevention of waste were achieved. An interesting example of gold nanoparticles toward greener catalysis was the aerobic oxidation of methanol to methyl formate where atmospheric air acts as the oxidant and water was formed as a by-product.

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Green Synthesis of Gold Nanoparticles Using Microbial Cell Factories The advantages and applications of nanoparticles have led to devising sustainable strategies to synthesize metal nanoparticles. In the interest of developing environmental-friendly and sustainable alternatives to existing synthetic approaches, biogenic methods have received wide attention in recent years [4]. The important factors which led to the development of bio-inspired methods to produce nanoparticles are based on six criteria involved in the design of nanomaterials [5]. These design principles of greener nanotechnology can be applied at different stages of the life cycle of nanomaterials, namely, origin of materials, manufacturing, distribution, use, and end of life [6]. From a synthetic perspective, the main advantages of biological methods over chemical methods are the use of mild reaction conditions such as ambient temperature and standard pressure, no toxic solvents or chemical reductants or stabilizers, reduced impacts of purification, and elimination of reaction steps which are considered benign [7]. Biological synthesis of gold nanoparticles is possible using plants, microbes (bacteria, fungi/yeast, actinomycetes, and algae), or isolated biomolecules. Among the natural sources, plants are most extensively studied for the biogenesis of gold nanoparticles. Plant-mediated synthesis of gold nanoparticles involves the use of living plants or plant extracts or phytochemicals. In recent years, microbial synthesis of metal nanoparticles has gained more attention [8]. Microbes which are employed for the synthesis of nanoparticles can be in used in many forms, i.e., as whole cells or derived sources such as cell-free filtrate and cell-free extract, culture supernatant of fungal mycelial cells, and, in a few cases, isolated enzymes and selected protein fractions. Biosynthetic methods reported to produce metal nanoparticles using the culture supernatants or filtrates of microbes are generally timeconsuming, and those which employ the isolated/purified proteins involve a set of rigorous and expensive steps to purify the proteins required in large amounts. Culture supernatants consist of spent and unspent medium used for microbial growth.

Nitroaromatics and Dyes: Environmental Problem Environmental buildup of toxic contaminants such as heavy metals, azo dyes, aromatic nitro compounds, and halogenated herbicides is a serious concern in recent years. Most of these refractory pollutants are being released into water bodies by chemical industries. Owing to environmental concerns, there is an urgency to degrade these nitroaromatic compounds especially nitrated biphenyls, fluorenes, and naphthalenes which impose genotoxicity and mutagenic and carcinogenic properties to humans. The major impact of environmental pollution caused by textile industry is confined to the nonbiodegradable dyes used. Textile dyes composed of azo and nitroaromatic structures possess cancer causing threat to human population. Examples of this class of dyes include Azure B, Disperse Red 1, Sudan I, Basic Red 9, and crystal violet as shown in Fig. 1.

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Fig. 1 Examples of the hazardous dyes used in textile industry

The polluting dyes also affect the microbial community in the soil, growth of plants, and marine life. To overcome the complications caused by these pollutants, it is important to degrade these toxic compounds to nontoxic residuals. The catalytic degradation of nitroaromatic compounds and textile dyes using gold nanoparticles prepared by microbes is discussed in the next section.

Catalytic Performance of Gold Nanoparticles Prepared from Microbial Sources The ultimate application is the important stage in the life cycle analysis of nanostructured materials. Out of the six design principles, five (safer nanomaterials, reduced environmental impact, waste reduction, materials, and energy efficiency) address the importance of potential applications of nanoparticles toward safer health and environment. Continuous efforts to synthesize catalytically active materials at the nanoscale have been recognized as important developments toward “green” chemistry. The utilization of renewable resources such as microbes, plants, and other biological sources for the preparation of catalytic nanomaterials is a good option as these methods are green and more economical [9, 10]. Within the classification of

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microbes, preparation of gold nanoparticles using bacteria, fungi/yeast, actinomycetes, and algae is established [11]. The use of biosynthesized gold nanoparticles prepared from microbes for the reduction of nitroaromatic compounds and dyes is listed in Table 1. Metal nanoparticles catalyze the reduction of nitroaromatic compounds in the presence of a hydride source such as sodium borohydride which can be qualitatively monitored using a spectrophotometer. For example, 4-nitrophenol, in the presence of excess of NaBH4, turns yellow which is observed as a strong absorbance peak around 400 nm. The resulting reduced product, p-aminophenol, is not yellow, thus allowing monitoring of the reaction rate of the reduction by monitoring the concentration of 4-nitrophenol as the change in NaBH4 concentration is negligible during this reaction. In this pseudo-first-order reaction, the decrease in absorbance intensity (A) at 400 nm can be measured with respect to time (t). A graphical plot of ln (At/A0) versus time gives a straight line with a negative slope of the apparent rate constant (kapp). The lower the rate constant (kapp) values are, the better the catalytic activity of the nanoparticles. For better comparison of catalysts, a kinetic parameter known as normalized rate constant (knor) is calculated by dividing the apparent rate constant by the concentration of metal nanoparticles. Likewise, the catalytic degradation of textile dyes was studied by monitoring the decrease in the absorbance intensity in the visible region. Each dye has a characteristic absorbance peak due to the presence of chromophores. For example, Congo red shows absorption peaks at 350 nm (n ! π*) and 450 nm (π ! π*). Krishnan et al. have reported that gold nanoparticles biosynthesized using the cell-free extract of yeast Candida parapsilosis ATCC 7330 showed improved dispersion of gold nanoparticles in a pH 12 solution [12]. These dispersed nanoparticles (32 nm) demonstrated their size-dependent catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol using sodium borohydride (Scheme 1) as a hydride source with an apparent rate constant of 0.138 min
1 and normalized rate constant (knor) of 4.18 x 107 mol
1 min
1. Similarly, fungal mycelia and cell-free extracts of Mariannaea sp. HJ were reportedly used to prepare gold nanoparticles for the catalytic reduction of 4-nitrophenol with rate constants of 0.342 min
1 (37.4 nm) and 1.48 min
1 (11.7 nm), respectively [16]. Recently, gold nanoparticles (9.8 nm) prepared using the cell-free extract of Trichoderma sp. WL-Go have been used for the reduction of aromatic pollutants [17]. Significantly, these gold nanoparticles successfully reduced nitroaromatic compounds like 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitroaniline, and 3-nitroaniline to their corresponding products with kinetic rate constants of 0.444 min
1, 0.618 min
1, 0.294 min
1, 0.348 min
1, and 0.9 min
1, respectively, which were determined spectrophotometrically. Based on the reduction rate calculated from this cuvette-based assay, the effect of these substituted nitroarenes on the reactivity of the catalyst cannot be defined precisely. Among the nitroarenes studied, ortho-substituted amino nitrobenzenes were reduced rapidly as compared to meta-substituted nitrobenzenes under identical experimental conditions. This could be due to the positive inductive effect seen in 2-nitroaniline. Importantly, azo dyes such as Cation Red, Weak Acid Red GRS, Acid Orange G,

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Table 1 Examples of microbe synthesized gold nanoparticles as catalysts

S. no Classification Microorganism Reduction of nitroaromatics 1. Yeast Candida parapsilosis ATCC 7330 2. Yeast Magnusiomyces ingens LH-F1 3. Fungi Rhizopus oryzae 4. Fungi Aspergillus sp. WLau 5. Fungi Mariannaea sp. HJ 6.

Fungi

7.

Bacteria Yeast

8.

Fungi Fungi

9.

Fungi

10.

Bacteria

Trichoderma sp. WL-go Labrys sp. WJW Trichosporon montevideense WIN Aspergillus sp. Pycnoporus sanguineus Trichoderma sp.

Shewanella oneidensis MR-1 11. Seaweed Lobophora variegata 12. Marine Turbinaria brown algae conoides, Sargassum tenerrimum Reduction of nitro and azo dyes 13. Fungi Mucor indicus CBS 226.29 ET 14. Fungi Trichoderma harzianum 15. Marine Padina macroalgae tetrastromatica 16. Fungi Aspergillus fischeri 17. Bacteria Streptomyces griseoruber 18. Bacteria Staphylococcus epidermidis 19. Bacteria Enterobacter aerogenes

Microbial source for nanoparticles synthesis

Particle size

Reference

Cell-free extract

32 nm

[12]

Cell-free extract

[13]

Protein extract Cell-free extract

20.3– 28.3 nm 20 nm 29 nm

Cell biomass and Cell-free extract Cell-free extract

37.4 nm, 11.7 nm 9.8 nm

Cell-free extract Cell-free extract

18.8 nm 22.2 nm

Cell-free extract Protein extract

9.5 nm 6.07 nm

Cell-free filtrate Bacterial culture

20– 30 nm –

[21]

Aqueous extract

11.69 nm

[22]

Aqueous extract

5–57 nm

[23]

Cell biomass

–

[24]

Whole biomass

[25]

Seaweed extract

26– 34 nm 11.4 nm

Cell-free filtrate Culture supernatant

50 nm 5–50 nm

[27] [28]

Culture supernatant

20– 25 nm 4–6 nm

[29]

Culture supernatant

[14] [15] [16] [17] [18]

[19] [20]

[26]

[30] (continued)

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Table 1 (continued)

S. no 20.

Classification Bacteria

21.

Marine brown algae

Microorganism Bacillus marisflavi YCIS MN5 Turbinaria conoides, Sargassum tenerrimum

Microbial source for nanoparticles synthesis Cell-free filtrate

Particle size 14 nm

Reference [31]

Aqueous extract

5–57 nm

[23]

Scheme 1 Catalytic reduction of 4-nitrophenol using gold nanoparticles (Au NPs) prepared using Candida parapsilosis ATCC 7330 and sodium borohydride (NaBH4) as a hydride source [12]

Reactive Yellow 3RS, Reactive Green KE-4B, Reactive Black, and Acid Black 10B were also catalytically degraded by gold nanoparticles with overall decolorization efficiency of 82.2–97.5%. Apparently, Au NPs (29 nm) prepared using the cell-free extracts of Aspergillus sp. WL-Au show remarkable catalytic activity for the reduction of 4-nitrophenol with a normalized rate constant (knor) of 4.04 x 105 mol
1 min
1 [15]. In another study, the catalytic performance of gold nanoparticles biosynthesized using cell-free extracts of bacteria Labrys sp. WJW, yeast Trichosporon montevideense WIN, and fungus Aspergillus sp. WL-Au for the reduction of 4-nitrophenol was investigated. Results revealed the catalytic activity follows the order, i.e., WL-Au-Au NPs (0.37 min
1) > WJW-Au NPs (0.27 min
1) > WIN-Au NPs (0.23 min
1) [18]. Dong et al. reported the preparation of bioreduced graphene oxide (rGO)/gold nanoparticles (Au NP) composite using Shewanella oneidensis MR-1 [21]. As compared to bio-Au NPs or bio-rGO or chemically prepared Au NP/rGO composite, these reusable Au NP/rGO biohybrids efficiently catalyzed the reduction of 4-nitrophenol under identical conditions. Clearly, the bio-Au NPs/rGO prepared by the biogenic approach demonstrated almost four times higher catalytic activity than the chemically prepared nanocomposite, chem-Au NPs/rGO, with normalized rate constants of 138.45 s
1 M
1 and 31.99 s
1 M
1, respectively. Normalized k values of bio-Au NPs/rGO (138.45 s
1 M
1) were found to be much higher than gold chemically synthesized nanocomposites such as Au NPs/TWEEN/GO (1.96 s
1 M
1), Au NPs/GO (13.75 s
1 M
1), G/C-Au (18.27 s
1 M
1), Au NPs/ rGO (31.30 s
1 M
1), and Au@PZS@CNTs (33.54 s
1 M
1) reported elsewhere [32–36].

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Scheme 2 Catalytic reduction of (a) methylene blue and (b) Congo red using gold nanoparticles (Au NPs) prepared using Bacillus sp. in the presence of sodium borohydride (NaBH4) [31]

Recently, the aqueous extract of seaweed Lobophora variegata was used to prepare gold nanoparticles of size 11.69 nm and used for the catalytic reduction of –o,
m,
p nitrophenols to their corresponding aminophenols [22]. Gold nanoparticles prepared by two different species of marine brown algae Turbinaria conoides and Sargassum tenerrimum have demonstrated catalytic activity for the reduction of 4-nitrophenol and 4-nitroaniline [23]. Besides, dye molecules, namely, Rhodamine B and Sulforhodamine 101 hydrate, were also shown to undergo reductive transformation in the presence of gold nanoparticles. Gold nanoparticles (50 nm) biosynthesized using Aspergillus fischeri showed good catalytic degradation of methylene blue as compared to chemically prepared Au NPs [27]. The same research group also explored the catalytic performance of gold nanoparticles biosynthesized using different bacterial species for the reduction of methylene blue [28–30]. Very recently, gold nanoparticles prepared using Bacillus sp. have been shown to catalyze the reduction of Congo red and methylene blue as shown in Scheme 2, with the kinetic rate constant of 0.222 min
1 and 0.246 min
1, respectively [31].

Supported Gold Nanoparticles Catalysts Advantages of supported metal nanoparticles catalysts such as more reuse cycles, lack of catalyst leaching, and nanoparticles aggregation have made them choice materials for a range of applications. Supported metal catalysts have been used in fundamental processes of energy production, oil refining, petrochemistry, and environmental protection. Supported gold nanoparticles catalysts have been extensively studied for various organic reactions such as hydrogenation, oxidation, and C-C coupling, among others.

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Importance of Aromatic Amines and their Preparation It is inevitable that green and sustainable methods be developed for industrially relevant processes and organic reactions of commercial value. One of the best examples is aryl amines. Aromatic amines and related compounds are widely used in the synthesis of pharmaceuticals, polymers, and rubber and also in textile industries. Aryl amines form the core of many of the important drugs prevailing in the current market, and some of them are shown in Fig. 2. Erlotinib, a tyrosine kinase inhibitor used for the treatment of lung and pancreatic cancer, is composed of macetylenyl aniline and quinazoline. Bicalutamide, an oral anticancer drug used to treat prostate cancer, has p-cyano-m-trifluoroaniline as a structural amine component. Nilutamide, a drug used to treat advanced stage prostate cancer, is made of p-nitro m-trifluoromethylaniline. Morniflumate is a nonsteroidal anti-inflammatory drug which has m-trifluoroaniline as amine core. Paracetamol, a widely prescribed drug for analgesic and antipyretic action, is an acetylated p-aminophenol. Fosamprenavir consists of a p-sulfonamidoaniline unit as an aryl amine core and is used as an antiHIV drug. Aryl amines such as benzenediamine and its derivatives are used in the textiles industry as a dye intermediate. It is also known that valuable chemicals involve the use of aniline and their derivatives for the preparation of amides, imines, azo compounds, isocyanates, and diazonium salts. The classical approach to prepare aryl amines from nitroarenes is the “Bechamp reduction” which involves excessive use of metals and a strong mineral acid [37].

Fig. 2 Examples of commercially important compounds with aryl amines

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These harsh conditions affect the selectivity of reduction of a nitro group in the presence of other functional groups. Other methods, namely, direct hydrogenation, catalytic hydrogenation, hydride transfer reduction, and metal-free reduction, are also well established [38]. Biological methods developed using enzymes like nitroreductases isolated from various microbial species are known to reduce nitroarenes using NAD(P)H as a cofactor. However, in most cases, these enzymes catalyze the reduction of nitroaromatic compounds to hydroxyl amines instead of aryl amines. With the advent of nanotechnology, synthesis of aryl amines from nitroarenes using supported metal nanoparticles as catalyst is known [39].

Supported Gold Nanoparticles Catalyst for the Hydrogenation/ Reduction of Nitroarenes Supported gold nanoparticles catalysts are prepared in many ways [40] which are summarized in Fig. 3. Deposition-precipitation (DP) method is the most preferred method which involves bringing the support and the gold precursor in contact with each other, followed by subsequent thermal treatment under suitable conditions to form and deposit gold nanoparticles onto preformed catalytic supports. Synthesis of

Fig. 3 Typical steps in the preparation of supported gold nanoparticles catalyst for hydrogenation reactions [40]

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Au/CeO2 catalyst by the deposition-precipitation method involves multiple steps, high-temperature heating (300 °C), and inert conditions (Argon) [41]. Coprecipitation is quite similar to the DP method, which involves the generation of a catalytic support and gold nanoparticles from their respective precursors. In a report by Carlos et al. gold nanoparticles supported on hydrotalcite (magnesiumaluminum hydroxycarbonate) prepared for the hydrogenation of nitroaromatic compounds were obtained by the coprecipitation procedure. During this multistep process, heating (80 °C), isopropanol (solvent), and nitrogen atmosphere were employed [42]. Impregnation is a technique where the support material immersed in an aqueous solution of the metal precursor is allowed to evaporate. Subsequently, the metal precursor impregnated support is dried, subjected to reductive atmosphere with or without pre-calcination at high temperature. In a recent study, the preparation of gold nanosphere and nanorod catalysts supported on SBA-15 was achieved by the impregnation method which involves multiple steps and excessive heating during calcination (350 °C) [43]. Grafting is a process where a strong covalent bond is formed between the carrier matrix and gold precursor. For instance, graphene oxide grafted chitosan stabilized gold nanoparticles catalyst was prepared by multiple steps using excessive amounts of NaBH4 (0.05 M) [44]. Ion adsorption process occurs because of electrostatic interactions between the support and the Au species. For example, γ Al2O3 supported Au NPs were prepared by anion adsorption in multiple steps. Here, heating is required for reduction (70 °C) and calcination (300 °C) [45]. Similarly, the ionic exchange method involves the replacement of support hydroxyl groups by the localized charges on the Au species. Azizi et al. report the preparation of γ Al2O3 supported Au NPs by direct anionic exchange (DAE) which is a multiplestep process where the reduction and calcination occurs at high temperature [46]. Sol-gel methodology involves the metal alkoxide hydrolysis step, followed by reduction of metal precursors to generate the catalytically active amorphous alloy. In a report by Claus et al., supported gold nanoparticles catalyst was prepared by the sol-gel technique using methanol as solvent, and high temperature was required for the reduction of gold(III) precursor (450 °C) and calcination (400 °C) process [47]. On the whole, preparation of supported gold nanoparticles catalysts is a multistep process and requires numerous unit operations (Fig. 3), employing energy-intensive techniques and harsh reaction conditions such as heating, solvents, and inert atmosphere. The catalytic performance of the supported gold nanoparticles catalyst depends on many factors. The important factors are particle size, particle morphology, and nature of supports, charge transfer, and contact area. A metal oxide framework is the widely used carrier system for gold nanoparticles. For example, Au/TiO2 has gold nanoparticles supported onto a mesoporous form of TiO2. Different support matrices, namely, magnesium oxide, rutile (TiO2), SiO2 coated branched polyethyleneimine, and chitosan-grafted graphene oxide, are used for the preparation of heterogeneous catalysts for the hydrogenation of nitroarenes. Recent methodologies reported to synthesize aryl amines show the requirement of toxic metal catalysts [48], inflammable toxic solvents [49–51], heating [52, 53], and inert conditions [41, 42] which are not environmentally benign.

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Mechanistic Details The mechanism of electron transfer by metal nanoparticles is referred to as electron relay where the nanoparticles relay the electrons from donor (hydride) to the acceptor (nitroarene) based on the redox potential of the gold cluster [54]. Thermodynamically, the minimum criteria required for electron transfer is that the redox potential of nanoparticles should be less than the electron donor but it should be greater than that of the electron acceptor. For instance, nitrophenol (E° ¼
0.76 V) is reduced by NaBH4 (E° ¼
1.33 V) in the presence of gold nanoparticles whose redox potential should be in between, i.e.,
1.33 < Au NPs [Nanoparticles] <
0.76. Transfer hydrogenation (reduction) is one of the most convenient approaches where metal nanoparticles act as a catalyst in the presence of a hydride source such as sodium borohydride [55]. In principle, there are two well-established mechanisms involved in the reduction of nitroarenes to aryl amines, i.e., Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanism. In the LH mechanism, the nitroarene and the reducing agent get adsorbed on the catalyst surface. Then the hydride ion from the reducing agent is transferred to the nitroarene, and as a result, the nitroarene gets reduced. The nitroarene reduction due to surface-adsorbed hydride species is the rate-determining step. ER mechanism involves the reduction of nitroarene which occurs by collision between the unadsorbed nitroarenes and the surface-adsorbed hydrogen atoms. Besides, semiconductor, defect-mediated, and photocatalytic mechanisms are also proposed for the hydrogenation of nitroarenes under specific reaction conditions but are less common. Noble metal nanoparticles such as gold nanoparticles catalyze the reduction of nitrobenzene (Ar-NO2) to aryl amines (Ar-NH2) through two pathways based on the electrochemical mechanism proposed by Haber. The direct pathway involves the formation of an Ar-NO (Nitroso) compound obtained by the removal of a water molecule from Ar-NO2 in the presence of gold-hydride species which is later reduced to give Ar-NHOH. Subsequent reduction and removal of water lead to the formation of Ar-NH2. On the other hand, the indirect (condensation) pathway shows the presence of azoxybenzene and diazobenzene as major intermediates. Azoxybenzene (Ar-NO¼N-Ar) is formed by the condensation of Ar-NO with Ar-NHOH which gets reduced to diazobenzene (Ar-N¼NAr). Gold-hydride species (Au-H) subsequently reduces diazobenzene to give two molecules of aniline (Ar-NH2).

Bio-Supported Metal Nanoparticles as Heterogeneous Catalysts Compared to conventional catalytic supports, microbes provide biological supports which are environmentally advantageous and renewable as they can also be used to synthesize metal nanoparticles without addition of external reducing agents [56]. Different microbial species are reportedly used in the preparation of bio-supported metal nanoparticles. Alternatively, microbes are used as bio-supports for pre-synthesized metal nanoparticles. In one example, De Corte et al. demonstrate the

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formation of palladium nanoparticles on the surface of bacterial cells using hydrogen or formate as the electron donor [57]. Bio-supported metal nanoparticles are reportedly used as heterogeneous catalysts [58] for various chemical reactions which are summarized in Table 2. Microorganisms can recover toxic metal waste such as palladium and transform them into catalytically active Pd(0) nanoparticles. In a particular study, bio-recovered palladium waste was chemically oxidized to soluble Pd(II), which can be further converted into bio-Pd nanoparticles using Cupriavidus necator ATCC 4329. The bio-nanocatalyst acts as a sustainable catalyst and mediates carbon-carbon bond formation through Mizoroki-Heck reactions [61] as shown in Scheme 3a. These authors have also studied the catalytic role of bio-supported palladium nanoparticles prepared using C. necator and P. putida, for Suzuki-Miyaura and Heck reactions (Scheme 3b & c). Among the bio-nanocatalysts, C. necator supported Pd nanoparticles efficiently catalyzed Suzuki coupling reactions for a wide range of substituted aryl iodides tested, and the formation of respective bi-aryl products up to 100% yield was also evidenced. The catalytic activity of the commercial palladium nano-powder (25 nm) was comparable to that of C. necator bio-Pd for few aryl iodides studied. Bennett et al. reported that bio-Pd nanocatalysts prepared using Desulfovibrio desulfuricans NCIMB 8307 demonstrated the C-C bond formation between iodobenzene and styrene with high reaction rate (0.17 mmol
1min) and quantitative conversion (85%) as compared to commercial Pd/C catalyst (0.15 mmol
1min, 70%) [71]. In addition, these bio-Pd nanocatalysts were reusable with minimal loss in conversion (5%) after six catalytic runs. In a separate study, palladised cells of E. coli, D. desulfuricans, and C. metallidurans demonstrated superior catalytic activity in C-C bond formation between phenyl iodide and ethyl acrylate, as part of a Heck coupling reaction during industrial testing by AstraZeneca [75] (Scheme 4). Particle size plays a crucial role in determining the catalytic activity of the nanocomposite. Sobjerg et al. reported that size of the Pd nanoparticles formed in bio-nanocatalyst was greatly influenced by the ratio of biomass concentration to metal precursor used [64]. It was shown with two different microbes, Staphylococcus sciuri and Cupriavidus necator, as case I, increased biomass-Pd ratio, yielded small Pd nanoparticles of size 80% [63].

Bio-Supported Gold and Palladium Nanoparticles for the Catalytic Reduction of Nitroarenes Most of the bio-supported nanoparticles catalysts reported so far are composed of palladium nanoparticles as the active phase. As compared to palladium-based bionanocatalysts, bio-supported gold nanoparticles catalysts are regarded as better sustainable catalysts due to their benign characteristics [89]. The first report is the fungus Cylindrocladium floridanum supported gold nanoparticles (25 nm) used for the reduction of 4-nitrophenol in the presence of NaBH4 with a reaction rate constant of 0.027 min
1 which was qualitatively monitored using a spectrophotometer [65]. Increase in the reaction rate was observed during the increase in the concentration of gold nanoparticles used and also with decrease in particle size. This reusable heterogeneous catalyst was prepared by treating the as-synthesized bio-supported gold nanoparticles onto borosilicate glass beads at 80 °C for 4 h. The same research group also prepared bio-supported gold nanoparticles ( 20 nm) using the mushroom Flammulina velutipes which demonstrated catalytic activity for the reduction of methylene blue and 4-nitrophenol with rate constant of 0.0529 min
1 and 0.1236 min
1, respectively [90]. Gold nanoparticles associated with membrane-bound fraction of E. coli K12 can act as excellent bio-nanocatalyst which was demonstrated for the reduction of nitroaromatics in aqueous solution [72]. To this end, the reduction of 4-nitrophenol to 4-aminophenol was achieved using these bio-nanocomposite as a heterogeneous catalyst with a rate constant of 1.24  10
2 min
1. The use of microbes as bio-supports to prepare bio-Au nanocatalyst was also established. For example, a yeast Saccharomyces cerevisiae was used to prepare biosupported gold nanoparticles over a reaction time of 72 h [84]. A similar study by Lin et al. described the in situ formation and immobilization of Au NPs (14.8 nm) on the cell surface of yeast Pichia pastoris [73]. Also, these Au NP-immobilized cells were used as benign and reusable catalysts for the reduction of 4-nitrophenol to 4-aminophenol using sodium borohydride as reducing agent with apparent rate constant of 0.48 min
1. Similarly, bimetallic bio-Pd/Au nanocatalysts were prepared by reductive precipitation and deposition of metal nanoparticles on the cell surface of bacteria Cupriavidus necator H16 using formate [70]. These bio-supported Pd/Au alloy nanoparticles demonstrated catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol which was monitored using a spectrophotometer. Recently, bio-supported Pd/Au nanoparticles catalyst prepared using Bacillus sp. in the presence of sodium lactate has successfully catalyzed the reduction of 4-nitrophenol to 4-aminophenol [91]. Bio-Pd/Au bimetallic nanoparticles catalysts prepared by bioreductive coprecipitation method using Shewanella oneidensis showed improved reaction rate and selectivity in the C-C bond formation through Suzuki coupling as compared to

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Scheme 7 Suzuki coupling reaction mediated by bio-Pd or bio-Pd/Au nanocatalyst and ethanol/ water as a solvent [69]

bio-Pd nanocatalyst [69]. In case of aryl halides studied, bio-Pd/Au nanocatalyst showed improved conversion of coupling products as compared to bio-Pd and bioAu catalyst (Scheme 7). A similar report by Tuo et al. demonstrated the catalytic activity of Au/Fe3O4 or Pd/ Fe3O4 or PdAu/ Fe3O4 nanocomposites biosynthesized using S. oneidensis MR-1, for the reduction of several nitroaromatic compounds using sodium borohydride as hydride source [76]. Based on the observations, reactivity of the catalysts can be arranged in the given order PdAu/Fe3O4 > Pd/Fe3O4 > Au/Fe3O4. Rate constant (kapp) of the bio-Pd/Au catalyst was 0.3282 min
1 which was comparable to the conventional Au/Pd catalysts such as Pd@Au core-shell nanotetrapods (0.1390 min
1) [92] and Au-Pd carbon spheres (0.8760 min
1) [93]. In an isolated study, bimetallic Au/Pd bio-nanocatalyst prepared using Escherichia coli MC4100 displayed better selectivity toward oxidation of benzyl alcohol, which is a key step in organic synthesis of valuable chemicals and intermediates, mainly in the perfume industry [67]. Unlike commercial 5% Pd/C catalysts, bio-Pd nanocatalyst demonstrated oxidation of benzyl alcohol to benzaldehyde with 98% selectivity, and no traces of toluene or benzyl benzoate were observed after 7 h. Moreover, bio-Au/Pd nanocatalyst showed improved catalytic activity (two–threefold) as compared to others, which was due to the synergistic effect of bio-Pd and bio-Au nanocatalysts which was supported by the rate of conversion recorded after 180 min (Scheme 8). Bio-supported gold nanoparticles prepared from soil fungus Aspergillus japonicus AJP01 were used as versatile catalyst for high-yielding synthesis of propargylamines using A3 coupling reaction. Propargylamines form the building block for the synthesis of bio-active compounds and pharmaceuticals. The reaction conditions were optimized which involve the use of tetrahydrofuran as a solvent, temperature of 80 °C, and reaction time of 24 h (Scheme 9) [77]. Besides, the catalytic activity of this bio-Au nanocatalyst was also explored for the reduction of 4-nitrophenol and hexacyanoferrate(III) which was monitored using a spectrophotometer. Noticeably, about 58% reduction of 4-nitrophenol occurred within 1 min, and the total reduction up to 90% was achieved within 5 min. In case of hexacyanoferrate(III), the reduction rate was 14.28% per minute and 78% got reduced within 10 min.

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Scheme 8 Oxidation of benzyl alcohol to benzaldehyde using bio-Au/ Pd versus Pd/C catalyst [67]

Scheme 9 Bio-supported gold nanoparticles (Au NPs) for the synthesis of propargylamines by A3 coupling reaction [77]

Numerous researchers have reported the catalytic performance of microbe supported gold nanoparticles for the same reaction, i.e., reduction of 4-nitrophenol to 4-aminophenol using a spectrophotometric assay. Recently, bio-supported gold nanoparticles prepared using the yeast Candida parapsilosis ATCC 7330 were used as sustainable catalysts for the reduction of nitroarenes to aryl amines in good yields under benign conditions, and this work was recently patented [85].

Conclusions and Further Outlook As a step closer toward environmental sustainability, metal nanoparticles prepared using microbial resources were used as benign catalysts for the degradation of nitroaromatic pollutants and azo dyes. In addition, the significance of bio-supported metal nanoparticles as a sustainable catalyst for the preparation of various bulk

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chemicals and industrially important compounds under mild conditions is presented. Also, the impact of biological supports on stability, reactivity, and reusability of the bio-supported metal nanoparticles in various chemical reactions is discussed with suitable examples. Especially, this chapter also discusses the preparation of conventional gold-supported nanoparticles catalyst for the hydrogenation/reduction of nitroarenes to aryl amines. Noticeably, cell-free metal nanoparticles or bio-supported metal nanoparticles prepared using microbes displayed higher catalytic reactivity for the reduction of nitroaromatic compounds with appreciable kinetic rate constants which are comparable or in few cases better than the nanocatalysts prepared by conventional methods. Most of the supported metal nanoparticles catalysts prepared by chemical methods showed that the supported nanoparticles catalyst can be reused for at least five cycles and in a few cases for up to ten cycles for the hydrogenation of nitroarenes. Like chemically prepared supported metal nanoparticles, the biosupported metal nanoparticles are reusable for multiple catalytic cycles. Another important criterion is to understand the selectivity of bio-nanocatalyst in heterogeneous catalysis which has a substantial impact in sustainable catalysis. Improvement in catalyst selectivity will significantly reduce the chemical waste generated during organic reactions and thus augment the environmental sustainability. Importantly, leaching of metals from the supported metal nanoparticles catalyst during heterogeneous catalysis is a serious concern. It is recommended that minimum acceptable concentration of toxic metals such as Pd in the preparation of pharmaceuticals should be SnO2 > BiVO4 > WO3 was reported for H2O2 production in HCO3  electrolyte. CaSnO3 has been reported as the most efficient material for H2O2 formation through the water oxidation reaction by Park et al. [70]. Despite the immense potential of the hydrogen peroxide as liquid fuel, the commercial exploitation of the artificial photosynthesis process for hydrogen peroxide synthesis is still in infancy stage due to lack of efficient catalyst-electrolyte combination.

Fig. 7 Schematic illustration of H2O2 and S from O2 and H2S on n-type electrode. (Reprinted with permission from Ref. [62])

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Application of Artificial Photosynthesis for CO2 Capture and Production of Biofuel As a measure to control global warming due to emission of the greenhouse gas CO2, the sequestration and storage of CO2 has become a trusted area of present research and technology development. Photosynthesis is the most promising way to capture CO2 and to get fuel as the product of photosynthesis. But, severe loss of greenery has reduced the capture of CO2 from the environment. Researchers have designed artificial photosynthesis process to capture CO2 and to produce products of fuel applications. CO2 storage through AP represents a serious technological challenge to utilize it as raw material for the production of renewable fuel. As per research publications, carbon-based fuels produced by artificial photosynthesis are simpler molecules such as methane, methanol, and carbon monoxide [71, 72]. The renewable hydrogen produced in AP is used mostly to reduce CO2 to respective fuels. Carbon dioxide-based products of artificial photosynthesis have been summarized by Fukuzumi [73], and all possibilities have been presented in Eqs. 1, 2, 3, 4, and 5. CO2 þ 2e þ 2Hþ ¼ CO þ H2 O, E0 ¼ 0:11 V

ð1Þ

CO2 þ 2e þ 2Hþ ¼ HCOOH, E0 ¼ 0:25 V

ð2Þ

CO2 þ 4e þ 4Hþ ¼ HCHO þ H2 O, E0 ¼ 0:07 V

ð3Þ

CO2 þ 6e þ 6Hþ ¼ CH3 OH þ H2 O, E0 ¼ 0:02 V

ð4Þ

CO2 þ 8e þ 8Hþ ¼ CH4 þ 2H2 O, E0 ¼ 0:17 V

ð5Þ

The production of carbon-based fuels from CO2 can help to alleviate the shortage of fossil fuels and reduce our overall contribution to atmospheric CO2, and these socalled drop-in fuels allow the use of renewable sources of energy without the need to modify current energy infrastructure [74]. For CO2 reduction, Au–Cu bimetallic nanoparticle catalyst and cobalt metal catalysts have been studied by several researchers for structure–activity correlation [75]. In case of methanol formation, Cu on ZnO, CeO2, and Ni–Ga intermetallic oxides have been studied [75]. Tandem catalyst system (CeO2, Pt, and SiO2) using an assembly of nanostructures has been studied for selective formation of propanal from methanol and ethylene in sequence. Kim and Kwon [76] have presented a review on recent research developments related to oxide semiconductors, III-V semiconductors, and perovskites materials emphasizing on the modification of physical and chemical properties of used materials for application in CO2 reduction. This group of researchers has also highlighted photovoltaic cell-biased and light-assisted photo-electrochemical systems with detailing about the prospect of CO2 photo conversion systems. Fung et al. [77] have illustrated recently emerged two-dimensional (2D) materials (graphenebased, transition metal dichalcogenide and graphitic carbon nitride-based photo catalysts) for their distinctive features of ultrathin characteristic, self-adjusting bandgap, abundant active centers, large surface-to-volume ratio, unique electrical

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and optical absorption properties, and ample coordinated unsaturated surface sites. All these unique physicochemical properties represent 2D materials as promising material for carbon-based fuels production from CO2 by AP. These groups of researchers also have reviewed some recent developments, emerging strategies related to tailoring of functional behavior of 2D materials, comprising heterostructure fabrication, elemental doping, and surface defect modification to improve CO2 photo-reduction phenomenon over 2D materials. Jain et al. [78] have illustrated 2D graphene, as an emerging and efficient photocatalyst for CO2 reduction. This group of authors has also focused on the efficiency of coupling graphene oxide / reduced graphene oxide with various semiconductors to fabricate hybrid nanocomposites, suitable as visible light active photoredox catalysts for CO2 reduction to carbon-based solar fuels. Though research and developments in artificial photosynthetic pathway is all-embracing, still some scientific challenges need to be solved for commercial exploitation of AP in terms of abundance of required materials, improvement of product quality to be utilized as electrode material, stability of organic catalyst, appropriate design, and storage of energy in carbon-based fuels scientific challenge.

Conclusion and Future Outlook In designing the commercial scale artificial photosynthesis system, the primary challenge has been to lower the cost of the materials, such as catalyst systems (platinum and iridium) and specialized membrane materials, which are too expensive to do artificial photosynthesis at large scales. Another critical challenge is to improve stability of the catalyst system, i.e., to develop materials that are stable in acidic and basic medium with long life span, as the movement of electrons and hydrogen ions during reactions needs either an acidic or basic medium. Future development researchers are planning for encapsulating photo electrochemical electrodes, to withstand in wet environment, in basic or acidic conditions of reactors. Future studies are also oriented to ease the complexity of reducing CO2 by designing a catalyst system which can juggle eight electrons. The artificial photosynthesis systems will also likely integrate new materials, which are usually more expensive to manufacture and involve greater complexity because they have to handle light. The future prospect of AP “bionic leaf” concept is emerging. Incorporation of nanotechnology is a promising avenue to enhance efficiency of AP. In bionic leaf, semiconductor nanowires absorb light while the catalysts coated on the wires split water to make hydrogen or electrons. Modification of the system with bacteria has also been attempted, which has shown good life span on these wires and takes up electrons or hydrogen to fix CO2 and finally to reduce CO2 to carbon-based fuel. Multiple bacterial species in the bionic system have produced some useful chemicals such as natural gas and butanol. Hence, the future developments of the AP are not limited to product development, but integration of biotechnology and nanotechnology is expected to present a next-generation energy solution.

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62. Fukuzumi S (2016) Artificial photosynthesis for production of hydrogen peroxide and its fuel cells. Biochim Biophys Acta Biophys Incl Photsynth 1857:604–611 63. Hong D, Murakami M, Yamada Y, Fukuzumi S (2012) Efficient water oxidation by cerium ammonium nitratewith [IrIII(Cp*)(4,40 -bishydroxy-2,20 -bipyridine)(H2O)]2+ as a precatalyst. Energy Environ Sci 5:5708–5716 64. Isaka Y, Kato S, Hong D, Suenobu T, Yamada Y, Fukuzumi S (2015) Bottom-up and topdown methods to improve catalytic reactivity for photocatalytic production of hydrogen peroxide from water and dioxygen with a ruthenium complex and water oxidation catalysts. J Mate Chem A 3:12404–12412 65. Shiraishi Y, Kanazawa S, Kofuji Y, Sakamoto H, Ichikawa S, Tanaka S, Hirai T (2014) Sunlight-driven hydrogen peroxide production from water and molecular oxygen by metalfree photocatalysts. Angew Chem Int Ed 53:13454–13459 66. Cui Y, Ding Z, Liu P, Antonietti M, Fu X, Wang X (2012) Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants. Phys Chem Chem Phys 14:1455–1462 67. Liu J, Zou Y, Jin B, Zhang K, Park JH (2019) Hydrogen peroxide production from solar water oxidation. ACS Energy Lett 4:3018–3027 68. Izgorodin A, Izgorodina E, MacFarlane DR (2012) Low overpotential water oxidation to hydrogen peroxide on a MnOx catalyst. Energy Environ Sci 5:9496–9501 69. Shi X, Siahrostami S, Li G-L, Zhang Y, Chakthranont P, Studt F, Jaramillo TF, Zheng X, Nørskov JK (2017) Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide. Nat Commun 8:701–706 70. Park SY, Abroshan H, Shi X, Jung HS, Siahrostami S, Zheng X (2019) CaSnO3: an electrocatalyst for two-electron water oxidation reaction to form H2O2. ACS Energy Lett 4:352–357 71. Su J, Vayssieres L (2016) A place in the sun for artificial photosynthesis? ACS Energy Lett 1:121–135 72. Lewis NS, Nocera DG (2006) Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U S A 103:15729–15735 73. Fukuzumi S (2017) Production of liquid solar fuels and their use in fuel cells. Joule 1:689–738 74. Zhang H, Li C, Piszcz M, Coya E, Rojo T, Rodriguez-Martinez LM, Armand M, Zhou Z (2017) Single lithium-ion conducting solid polymer electrolytes: advances and perspectives. Chem Soc Rev 46:797–815 75. Nocera DG (2012) The artificial leaf. Acc Chem Res 45:767–776 76. Kim J, Kwon EE (2019) Photoconversion of carbon dioxide into fuels using semiconductors. J CO2 Util 33:72–82 77. Fung CM, Tang JY, Tan LL, Mohamed AR, Chai S-P (2020) Recent progress in two-dimensional nanomaterials for photocatalytic carbon dioxide transformation into solar fuels. Mater Today Sustain 9:100037 78. Jain SL, Szunerits S, Boukherrou R (2018) Graphene-based photocatalytic materials for conversion of carbon dioxide to solar fuels. In: Reference module in chemistry, molecular sciences and chemical engineering, pp 396–412. Encyclopedia of Interfacial Chemistry, Surface Science and Electrochemistry, Elsevier Inc.

Environmentally Benign Synthesis of Nanocatalysts: Recent Advancements and Applications

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Pavan Kumar Gautam, Saurabh Shivalkar, and Sintu Kumar Samanta

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phytogenic Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacterial-Assisted Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mycogenic Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phycogenic Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microwave/Sonochemical-Assisted Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionic Liquid-Assisted Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Degradation of Various Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Azo Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAHs and Aromatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Degradation and Fate of Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Contamination of aquatic bodies by synthetic organic pollutants has been a serious issue of concern around the globe. Organic pollutants mainly encompass a wide range of pollutants like polychlorinated biphenyls (PCBs), synthetic azo dyes, pesticides, phenolics, polycyclic aromatic hydrocarbons (PAHs), surfactants, etc. Due to the complex chemical structure, they are highly soluble and persistent and cause severe damage to the ecosystem. Recently, sustainable and P. K. Gautam · S. Shivalkar · S. K. Samanta (*) Department of Applied Sciences, Indian Institute of Information Technology Allahabad, Allahabad, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_52

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green synthesis of nano-sized catalysts is gaining substantial appreciation from the scientific community due to their high catalytic efficiency, economic feasibility, and nontoxic nature. Development of environmentally benign routes for the manufacturing of nanocatalysts includes the substitution of volatile chemicals by green and nonvolatile reactants and nonhazardous solvents. Cellular extract of plants, bacteria, fungi, and algae contains several active biomolecules that serve as a natural reducing and capping agent and can form stable nanoparticles for the catalytic removal of recalcitrant organic pollutants in aqueous matrices. Similarly, green reaction media like ionic liquids can also be utilized to produce nanocatalysts for the purpose of water purification. It has been evident from the previous and recent studies that the nanomaterials synthesized by eco-friendly protocols bear high surface area, commendable tunability, exceptional recyclability, and remarkable catalytic activity and can be successfully applied to produce high quality of decontaminated water. In the present chapter, we have discussed different ecologically sound methods to synthesize nanomaterials for catalytic applications with special reference to different classes of organic pollutants. Keywords

Water pollution · Nanocatalysts · Green synthesis · Organic pollutants · Water treatment

Introduction Water is one of the most essential commodities for the survival of living organisms on planet Earth. In addition to household consumptions, water is vigorous for agriculture, industry, fishery, tourism, etc. Growing population, urbanization, and unplanned industrialization have led to the decreased accessibility of potable water. Among different class of contaminants, organic pollutants encompassed a wide group of chemical pollutants chiefly made up of carbon and hydrogen with other atoms such as halogens, phosphorus, nitrogen, and sulfur [1]. Synthetic azo dyes, phenols, organopesticides, detergents, surfactants, aromatic amines, polycyclic aromatic hydrocarbons (PAHs), pharmaceuticals, and other xenobiotic compounds are called as persistent organic pollutants (POPs) as they are extremely resilient to chemical and biological degradation. Additionally, they increase the biological oxygen demand and create anoxic condition. This causes severe destruction of the aquatic ecosystem. Release of aqueous effluent from sewage and industries into water bodies has risen the problem of organic contamination of water manifold high. Recently, nano-sized materials for the catalytic treatment of POPs are attaining significant attention from the academic community due to their exceptional capability, tunability, and rapidity. The unique features of nanocatalysts such as high surface area, structural stability, and tremendous recyclability for repetitive uses make them most suitable candidate for water purification [2]. However, involvement of

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toxic and volatile reactants and solvents used in the production of nanocatalysts is really a severe problem as it exerts secondary pollution. This issue captivated the researchers to develop environmentally benign protocols for the synthesis of efficient nanocatalysts, capable of removing organic pollutants from water. In this regard, biological synthesis of nanomaterials for catalytic applications has received considerable importance [3]. Biogenic preparation of nanometered catalyst involves the use of active biomolecules obtained from the cellular extract of living organisms. Cellular extract of plants, bacteria, fungi, and algae contains different kinds of proteins, vitamins, alkaloids, phenolics, nucleic acids, and flavonoids. These biomolecules can serve as reducing as well as capping and stabilizing agents to form highly stable nanoparticles. Furthermore, these biomolecules add functional groups to the surface of nanocatalysts. These functional groups are important for the initiation of catalytic reactions. The aqueous extract of living organisms is able to replace toxic and volatile man-made reducing agents like NaBH4 and hydrazine hydrate. This may reduce the cost of synthesis. Plenty of workers have reported successful biological synthesis of Fe, Ag, Au, Pd, Zn, Cu, Mn, and Ti nanoparticles to explore for the catalytic degradation of tenacious organic pollutants. Biological manufacturing of nanocatalysts has proved to be the best alternative of conventional chemical route due to its simplicity, ease of synthesis, and nontoxic character. Besides biological synthesis, some other green and hazard-free protocols are also being adopted for the production of nanocatalysts. Application of microwave and ultrasound irradiation for the very quick synthesis of ultrafine and monodispersed nanocatalysts is now a forefront area of scientific research. Similarly, use of ionic liquids for the synthesis of nanocatalysts is also growing as ionic liquids are nonvolatile and green solvents [4]. We have designed this chapter with the objective to deliver recent advancement in the utilization of greenly synthesized nanomaterials for catalytic removal of hazardous organic pollutants. Various green synthetic routes and their advantages with appropriate examples have been provided in the chapter. Here, we have discussed the possible application of greenly developed nanocatalysts for the degradation of major and important organic contaminants.

Methods of Fabrication Nanoparticles behave differently from their respective bulk materials in terms of their physical, chemical, and biological properties. Obtaining entirely different properties from the same material by altering its size domain has attracted many researchers. This fact revolutionized the innovative application of these nanoparticles in various domains such as biomedical, environmental, electronics, catalysis, cosmetic, food packaging and preparation [5]. Nanocatalysts are functionalized nanomaterials/nanoparticles. These nanomaterial-based heterogenous catalysts can be synthesized from several metal nanoparticles. Nanocatalysts have evident potential for speeding up the catalytic process in the biological/chemical reactions. The enhanced ability of nanocatalysts is due to their higher surface area, thus making

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more catalytic reactions at the same time. Possible areas of applications of these functionalized nanoparticles are medicine, nanozymes, alternative fuels, contaminant degradation, etc. Ease of synthesis and diversity of applications of the nanoparticles have developed it as a new research area since the last couple of decades. Various physiochemical methods have been employed to synthesize the nanoparticles from the noble metals. Some of the physical methodologies of synthesis include lithography, laser ablation, aerosol technologies, and ultraviolet irradiation. While chemical method mostly includes reduction of metal salts to synthesize nanoparticles, sometimes electrochemical methods and radiolysis have also been reported. Most commonly sodium borohydride, trisodium citrate dihydride, ascorbate, elemental hydrogen, and potassium bitartrate are used as a reducing agent. However, these are the conventional techniques for the synthesis of nanoparticles and have several demerits in terms of their cost-effectiveness, biocompatibility, and release of hazardous components as by-products. To overcome these shortcomings, the idea of synthesis of reliable and biocompatible nanoparticles using sustainable methodologies has flourished among the researchers since last decade. This is the advent of biogenic synthesis of nanoparticles. The objective is to develop the nanomaterials of desired shape, size, and dispersity from biological sources with eco-friendly nature in order to minimize the negative impacts on human and environment. Biogenic approach is the use of various plants and microorganisms to synthesize nanoparticles. Microorganisms like bacteria, algae, and fungi are the potential sources of several intracellular and extracellular enzymes, while plant extracts are the potential source of antioxidants. These active biomolecules are the basic requirement for the synthesis of metal nanoparticles. Biogenic nanoparticles have higher stability and adaptability to get functionalized for specific applications. The nanoparticles of silver, platinum, gold, zirconia, titania, etc. and quantum dots have been synthesized using microorganisms. Post-synthesis standard procedure for the establishment and optimization of the nanoparticles is required for its ultimate use such as nanocatalysts. In this section various biogenic sources with their potential ability to synthesize low toxic and biocompatible nanoparticles and possible environmental remediation have been discussed.

Phytogenic Fabrication Phytogenic method is the plant-mediated synthesis of functionalized metal nanoparticles. Reduction of metals for the biosynthesis of nanocatalyst is carried out using the biomolecules such as vitamins, alkaloids, flavonoids, ketones, tannins, terpenoids, aldehydes, polysaccharides, phenolics, and amino acids obtained from plant extract [2]. These plant extracts can be obtained from almost all of its part such as stem, seed, plant tissues, flowers, leaf, etc. Reduction capability of extracts from different plants is different and depends on the diversity and content of biomolecules. The basic protocol for nanoparticle synthesis plants is very simple. Metal salt solution is mixed with plant extract at desired pH either with or without

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agitation. If required, nanoparticles are stabilized by mild heating, and synthesis process is completed. Later, the synthesized nanoparticles are characterized using TEM, SEM, XRD, XPS, FTIR, etc. in order to obtain the information about their physicochemical characteristics. The whole process of synthesis is illustrated in Fig. 1. Plenty of work has been reported concerning the successful synthesis of metallic nanoparticles by using cellular extract of plants. For instance, silver nanoparticles were synthesized using the stem and leaf extract of Verbesina encelioides [6]. It was found that there was a difference of rate of synthesis, faster from stem extract compared to leaf extract. In Another example, gold nanoparticles were synthesized from powdered cumin seeds and rose petals. The reduction of tetrachloroaurate salt from flower extract consisting of various sugars and proteins at pH 3 and at 30  C produces gold nanoparticles as reported. Several plants have accumulated metals in their epidermis, vascular tissue, and cortex. And these accumulated metals get reduced intracellularly to form the nanoparticles. Brassica juncea and Medicago sativa, are metallophytes which are known to accumulated gold when exposed to aqueous KAuCl4 solution. Later, stored Au nanoparticles were found throughout the vascular tissues, cortex, and epidermis. Other plant-mediated biosyntheses of functionalized nanoparticles are tabulated in Table 1. The use of phytogenic synthesis of nanoparticles is much more reliable as per the application point of view. The merits of using phytogenic nanoparticles over synthetically synthesized nanoparticles are listed below. • • • • • • • • • • •

Cost-effectiveness. Eco-friendly. No side effects. Ease of availability. Simple synthesis protocol. Safe to handle. Efficient control over size and shape of nanoparticles. More stable nanoparticles. Higher rate of synthesis. By-products of synthesis are not hazardous. Good for large-scale synthesis of nanoparticles.

Bacterial-Assisted Fabrication Bacteria are omnipresent unicellular living organisms and form a major group of prokaryotes. Few bacteria have special ability to quell stress-like metal ion toxicity as their first-order defense mechanism, while some of them even have been observed to be adapted to survive under heavy metal concentration such as Pseudomonas aeruginosa and Pseudomonas stutzeri [30]. To resort to the metallic ion stress, they secrete several inorganic compounds either extracellularly or intracellularly, which in turn takes the form of biosynthesis of nanoparticles. Resistance to heavy metals is

Stable and capped NPs are produced with high yield.

-Mixing Ratio -Temperature -pH -Aeration

Add bacteria cultrue to Metal salt solution

Inoculate Bacteria

Formation of Metal NPs in approx. 72 h

Fungal Hyphae in 1 mM Metal ion solution.

Washed Fungal Hyphae

Fungal Hyphae washed in distilled water for 48 h

Fungal Hyphae with increased biomass

Fungal Hyphae in growth media

Fig. 1 General illustration of different biogenic methods of nanoparticles synthesis

NPs are precipitated

Mild Heating

Adjust pH

Add Plant Extract to Metal salt Sol.

Plant extract

Optimization

Mycogenic method

Add Algal Extract to Metal salt solution

Algal extract

Phycogenic method

NPs are precipitated

Reverse Micelles Firmation

Bacteria assisted

Microwave/Ultrasonic wave of high intensity acting on metal salt sol.

Microwave/ Sonochemical assisted

NPs are precipitated due to nucleation

Cavitation

Phytogenic method

Metal Salt Solution

NPs are precipitated

Ionic Liquids

Ionic Liquid assisted

Solvothermal Process

Methods of Fabrication

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Table 1 List of various nanoparticles synthesized from different biogenic sources with their size range S. no. 1. 2. 3. 4. 5. 6. 7. 1. 2. 3. 4. 5. 6. 7. 8. 1. 2.

Biogenic source Phytogenic Phytogenic Phytogenic Phytogenic Phytogenic Phytogenic Phytogenic Bacteria Bacteria Bacteria Bacteria Bacteria Bacteria Bacteria Bacteria Mycogenic Mycogenic

3. 4. 5. 1. 2. 3.

Mycogenic Mycogenic Mycogenic Phycogenic Phycogenic Phycogenic

Species Cinnamomum camphora Alfalfa sprouts Aloe vera Lemongrass plant Geranium leaves Azadirachta indica (neem) Avena sativa (oat) Escherichia coli Pseudomonas stutzeri Plectonema boryanum Clostridium thermoaceticum Acinetobacter spp. Rhodopseudomonas capsulata Klebsiella pneumoniae Shewanella oneidensis Fusarium oxysporum Fusarium oxysporum and Verticillium Phanerochaete chrysosporium Trichoderma asperellum Phoma sp. 3.2883 Sargassum wightii Plectonema boryanum Chlorella vulgaris

Nanoparticles Au and Ag Ag Au Au Ag Au and Ag Au CdS Ag Ag CdS Magnetite Au Ag Uranium (IV) Au Magnetite

Size (nm) 55–80 2–20 50–350 Unknown Unknown 50–100 2–5 ~200 1–10 Unknown 10–40 10–20 5–32 Unknown 20–40 20–50

Ref [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Ag Ag Ag Au Pt Au

50–200 13–18 71–75 8–12 Unknown 9–20

[24] [25] [26] [27] [28] [29]

an ability of bacteria to detoxify chemicals. This is an energy-dependent process and requires ATPase for the chemiosmotic efflux of metal ions from special membrane proteins of bacteria. The methods through which bacteria can detoxify metallic ions are reduction, precipitation, intracellular bioaccumulation, extracellular complexion, and biosorption. Detoxification through reduction of toxic and soluble inorganic metal ions produces nontoxic and insoluble metal nanoparticles. But to avoid the polydispersity and to achieve monodispersity of nanoparticles, the procedures of biogenic synthesis need to be optimized furthermore. The bacterial genera most studied for this biosynthesis of functionalized nanoparticles are Pseudomonas and Bacillus [31]. Bacterium isolated from the municipal sewage waste, Bacillus licheniformis, has remarkable ability to synthesize silver nanoparticles of 50 nm approximately. Likewise, extracellular complexion of Pseudomonas aeruginosa synthesizes gold nanoparticles. Thiobacillus ferrooxidans, Sulfolobus acidocaldarius, and Thiobacillus thiooxidans have an incredible potential to reduce ferric to ferrous ion when allowed to obtain energy and grow on the sulfur. Thiobacillus ferrooxidans reduces ferric ion due to the

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frequent re-oxidation of ferrous ion in presence of the oxygen, while Thiobacillus thiooxidans has significant ability to reduce ferric to ferrous ion aerobically at low pH. And unlike T. ferrooxidans reduced ferrous ion, it is highly stable to auto-oxidation. Even dead or inactive bacterial biomasses with certain organic functional group in their cell wall can reduce metallic ions to from respective nanoparticles [32]. Bacteria have been known for the bioaccumulation of some metal ions which is further explored into its multiple applications in bioremediation as nanocatalysts. Pedomicrobium has been reported to have gold accumulation ability. It produces gold nanoparticles of size range 5–25 nm having octahedral structures. Another notable example is Geobacter sulfurreducens which forms gold nanoparticles by reducing ferric ion present in their periplasmic space. Gold nanoparticles are also formed in the periplasmic space of Shewanella algae. Shewanella algae are facultative anaerobic, metal-reducing cyanobacteria, thus having enormous potential to synthesize metal and metal oxide nanoparticles. The biomineralization phenomenon occurring in some of the bacteria is responsible for the synthesis of nanoparticles. For example, the enzymatic reduction occurring within the resting cells of the Shewanella putrefaciens and Geobacter metallireducens produces technetium nanoparticles. Other examples are Enterobacter cloacae, Rhodospirillum rubrum, and Desulfovibrio desulfuricens have potential to reduce selenite to selenium and E. coli K12 in the formation of tellurium nanoparticles.

Mycogenic Fabrication Fungi are the primitive eukaryotic, non-phototrophic microorganisms having rigid cell wall. Fungi can also synthesize nanoparticles from aqueous metal ions both intracellularly and extracellularly. Synthesis rate is faster in intracellular synthesis, while nanoparticles of bigger size are obtained via extracellular synthesis. This size difference of nanoparticles is due to its possible nucleation inside the fungi. Some of the fungi synthesizing nanoparticles extracellularly are Penicillium fellutanum, Fusarium solani, Rhizopus nigricans, Aspergillus oryzae, and Phoma glomerata. The studies for the fungal species synthesizing nanoparticles intracellularly have been limited. Yarrowia lipolytica and Saccharomyces cerevisiae are the yeasts belonging to Ascomycetes class which are found to have better potential for synthesis of nanoparticles intracellularly [33]. The techniques for the mycogenic synthesis of nanoparticles involve culture preparation and media variation depending upon the fungal species involved. Commonly nanoparticles are synthesized using the fungal hyphae. The first step is to increase the biomass of fungal hyphae, for which the hyphae is cultured in the growth media and incubated with shaking until the mass of hyphae increased. Later fungal hyphae is removed from the growth media and washed with distilled water. Now again this washed fungal hyphae is placed in the distilled water and incubated with shaking for a maximum of 48 h. After incubation the fungal hyphae is removed from the distilled water and placed in the 1 mM solution of metal ion. This solution

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with fungal hyphae is monitored for 48–72 h until the formation of metal nanoparticles. Metal salts are used for the continuous source of metal ions. For example, silver nitrate is the most commonly used silver ion source, chloroauric acid for gold ions, and cadmium sulfide for cadmium ion [34]. Fungi have been used to synthesize diverse range of nanoparticles, but the most common ones are gold and silver. Other nanoparticles include magnetite, cadmium sulfide, zirconia, zinc oxide, silica, platinum, and titanium. Mycogenic synthesis produces dynamic gold nanoparticles with varying shape and size with respect to change in the pH of the solution. V. luteoalbum produces 10 nm gold nanoparticles of spherical shape at pH 3; triangular at pH 5, hexagonal at 5, and spherical gold nanoparticles of larger shape are formed at pH 5 to 7. Above pH 7, the largest gold nanoparticles are formed with no definite shape and size. In case of silver nanoparticles, the opposite effect of pH is reported by Soni and Prakash [35]. The size of the silver nanoparticles synthesized from F. oxysporum and C. tropicum has shown to increase with the decrease of pH of metal ion solution. Changes in the size of gold and silver nanoparticles with respect to temperature is a common phenomena. Generally high temperature results smaller nanoparticles [35].

Phycogenic Fabrication Algae are the primitive plants which lack most of the distinct tissues and structures of typical terrestrial plants. These are either microscopic which float on the water surface due to high content of lipid (phytoplankton) or macroscopic which are found as an attachment over rocks (seaweeds). Due to their abundance in nature and richness in bioactive molecules with amine, carboxyl, and hydroxyl functional groups, they can be used to reduce metallic salts to their respective nanoparticles and further functionalized to form nanocatalysts. Thus, phycogenic approach for the synthesis of the nanoparticles is a promising field of nano-biotechnology. Researchers are dedicatedly exploring this area of biosynthesis to develop various functionalized nanoparticles and their potential applications. And one of the most focused applications of phycogenic biosynthesis of nanoparticles is bioremediation, a process for the removal of potentially toxic contaminant from the environment. The very simple procedure is employed for the algae-mediated synthesis of metal nanoparticles. Algae cell culture or biomass is introduced to metal salt solution. And the process of reduction begins due to the presence of bioactive compounds in algae, leading to the formation of metal or metal oxide nanoparticles. Recently Siddiqi and Husen summarized a general method for the synthesis of metal and metal oxide nanoparticles using algae and characterized their properties and features for their suitable application in nano-remediation [36]. Most frequently and widely synthesized nanoparticles from algae are gold and silver. But synthesis of palladium and platinum nanoparticles have also been reported using the complete intact cells of Plectonema boryanum. Seaweeds on the other hand undergo additional process of biosorption before reduction for the synthesis of nanoparticles. Red algae, brown algae, and green algae instigate rapid biosorption of gold ion (Au[III]) on the surface

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of seaweed for almost an hour, and later slow reduction of surface gold (Au[III]) to noble gold (Au[0]) occurs [37]. Other examples of synthesis of nanoparticles from algae are palladium nanoparticles from powdered biomass of Chlorella vulgaris and gold and palladium nanoparticles of size range 8–12 nm, respectively, from brown marine algae such as Sargassum wightii and Sargassum boyinum [38]. Palladium nanoparticles from the Sargassum boveanum have been tested for electrocatalytic ability in the reduction of hydrogen peroxide. So along with different other functionalized nanoparticles, it can be explored successfully for the application in nano-remediation as nanocatalysts.

Microwave/Sonochemical-Assisted Fabrication Microwave-assisted synthesis is considered as a green and environment-friendly method to synthesize metallic nanocatalysts for environmental remediation purpose. Recently microwave-assisted fabrication of NPs is getting a great deal of attention due to its advantages over conventional chemical synthetic routes due to the convenience and reproducible results. Microwave irradiation synthesis is a pollution-free and energy-efficient method to produce nanoparticles with enhanced surface characteristics, morphology, narrow size distribution, high surface area, as well as high yield by averting the side reactions [39]. Moreover, the reaction time is manifold faster, and sometimes materials are synthesized within few minutes or even in seconds. For instance, Lai et al. [40] demonstrated rapid dandelion extract-mediated synthesis of silver nanocatalyst assisted by microwave irradiation. TEM images revealed that the fabricated NPs were 20 nm in size with high degree of uniformity in shape. XRD spectra divulged that the NPs were well crystalline. The developed NPs were applied for the mineralization of rhodamine B and methyl orange. Almost complete removal was observed at 90 s and 40 s only for methyl orange and rhodamine B, respectively. Similarly, Francis et al. [41] manufactured Ag NPs by using E. scaber leaf extract under the microwave irradiation of 2450 MHz. The average particle size of the NPs was estimated to be 37.86 nm by TEM. The synthesized NPs successfully degraded various organic nitro compounds, namely, 4-nitrophenol, 2-nitroaniline, and 4-nitroaniline, and toxic dye eosin Y. Like microwave, ultrasound-assisted fabrication of NP synthesis is also a green alternative of chemical route. Using high intensity of ultrasonic waves, the NPs can be synthesized in a very short time without high temperature and pressure. In addition, ultrasound prompted cavitation facilitates the synthetic reactions by nucleation and particle development. Ultrasonic irradiation also causes defragmentation of solid slurries to form monodispersed and smaller-size NPs. As a result of these unique advantages, sonochemical synthesis of nanomaterials is being accepted. In a report, Pd NPs were synthesized biologically by using Andean blackberry leaf extract under ultrasonic irradiation. TEM analysis verified the creation of decahedron-shape Pd NPs with a diameter of 55–60 nm, and XRD spectra inveterate its crystalline nature. Synthesized NPs effectively sequestrated methylene blue containing wastewater via photocatalytic degradation. The degradation process was well explained by first-order kinetic model.

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Ionic Liquid-Assisted Fabrication Ionic liquids are organic salts made up of cations and anions that are liquid at conditions at ambient temperature. They are considered as the green solvent as they have negligible pressure. They bear some exceptional characteristics like nonvolatility, non-flammability, high latent heat capacity, remarkable structural and thermal stability, and high solubility for a wide range of solvents. Therefore, they have been a suitable alternative of toxic and volatile solvents for synthetic reactions and catalytic applications. In a novel approach, ionic liquid-assisted solvothermal method was adopted to synthesize bismuth oxyhalide nanosheets (BiOX) for the photocatalytic degradation of rhodamine B and 5-fluorouracil [42]. The BiOX were synthesized by using three different types of ionic liquids, namely, imidazolium, pyridinium, and pyrrolidinium, to observe their effect on morphology, surface properties, and photocatalytic activity of BiOX nano-sized semiconductors. Catalytic experiment revealed that bismuth oxide having Cl and Br(BiOCl, BioBr) as a halides (BiOCl) showed the highest removal efficiency. BioCl synthesized with pyridinium ionic liquid resulted in 93% removal for 5-fluorouracil. 100% degradation of rhodamine B occurred after 30, 45, and 60 min for BiOClpyridinium, BiOCl-imidazolium, and BiOCl-pyrrolidinium nanocatalyst, respectively. In a novel approach, 3D Cu2S-MoS2 nanocomposites were fabricated by a one-step hydrothermal method with the help of the ionic liquid [BMIM]SCN [43]. Different ratios of Cu2S and MoS2 were taken in order to observe the shape, size, and morphology of the nanocomposite. Time-dependent experiments were carried out to elucidate the effect of ionic liquid (IL) in the synthesis process. Among the different Cu2S-MoS2 nanocomposites, the Cu2S-MoS2 (1:1) composite exhibited the highest photocatalytic activity and cycling stability. Recycling study deciphered that the Cu2S-MoS2 (1:1) had the good degradation ability for methylene blue up to four repetitive cycles.

Catalytic Degradation of Various Organic Pollutants Synthetic Azo Dyes Minimization of the quantity of azo dyes in aqueous industrial effluents is important for ecological point of view. Very small amount of dyes can severely affect the aesthetic prettiness of water. In addition, a thin layer of these azo dyes formed over the surface of aquatic reservoirs restricts the penetration of solar light, which reduces the photosynthetic activity, and anoxic condition is created. Furthermore, they exert mutagenic, carcinogenic, and allergic reactions to the humans and aquatic organisms if they enter into the food chain. Recently, greenly synthesized nanocatalysts have shown astonishing potential to degrade the complex aromatic ringed structured dyes into simpler or nontoxic intermediates. Plenty of workers have developed biological and other green protocols to synthesize metallic nanocatalysts for scavenging dyes in aqueous solutions. Among various metallic nanocatalysts, iron NPs have proven

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their utility for the catalytic removal of dyes. Huang et al. [44] used tea polyphenols to fabricate Fe NPs and applied them for catalytic treatment of malachite green. Occurrence of polyphenols in tea extract was deciphered by FTIR spectrum. Optimization study showed that maximum removal of malachite green (90.56%) was achieved with 1:1 ratio of Fe2+ and tea extract at pH 6.0. In a report, P. sativum peel extract was used to synthesize Fe3O4 NPs and further explored for the catalytic removal of methyl orange [45]. The surface area of the synthesized composite was estimated to be 17.6 m2 g 1 with the help of BET surface area analyzer. Synthesized nanocatalysts were agglomerated and 20–30 nm in size. Almost 96% removal for methyl orange was achieved at pH 6.0 with 0.3 g/L of catalyst dose. Gold nanocatalysts are also known for their high catalytic activity against different azo dyes. It has been reported that P. alba flower extract-mediated Au NPs were manufactured by using two different concentrations (1% and 5%) of flower extract and applied for the catalytic removal of methylene blue, methyl red, Congo red, and eosin yellow. Paul et al. [46] also utilized P. benghalensis (B) O. Ktz as a green reducing and stabilizing agent to produce Au NPs for the photocatalytic degradation of methylene blue. Almost complete removal was acquired and the degradation followed pseudo-first-order reaction kinetics. In a worthy attempt, gold nanoparticles, having size ranging from 20 to 40 nm, were developed from the cellular extract of Aspergillum sp. [47]. The synthesized Au NPs showed remarkable catalytic activity for several toxic azo dyes. Catalytic experiments revealed that 91.0–96.4% removal was achieved for Cationic Red X-GRL, Acid Orange II, and Acid Scarlet GR within 7 min of reaction time. Silver nanocatalysts synthesized from eco-friendly protocols have also been magnificently applied for the catalytic sequestration of azo dyes. For instance, Ag NPs were synthesized using P. alba flower extract and efficiently applied for the catalytic removal of methylene blue, methyl red, Congo red, 4-nitrophenol, and eosin Y. Significant degradation of all the undertaken dyes was achieved within 40 min of time span. In another notable attempt, palm shell extract-mediated Ag NPs were used for the significant degradation of Reactive Blue-21, Reactive Red-141, and Rhodamine-6G [48]. Additionally, the Ag NPs exhibited noteworthy catalytic activity for their binary mixtures as well. Recycling study showed that the spent Ag nanocatalysts could be applied many times for the decolorization purpose without any substantial loss in their catalytic ability. Other metallic nanoparticles have proven their applicability as a potential catalyst against azo dyes. Kora and Rastogi [49] adopted an ecofriendly and facile protocol to synthesized palladium nanoparticles by using biological polymer obtained from B. serrata. These Pd NPs were employed for the NaBH4assisted catalytic removal of several anthropogenic dyes such as Coomassie Brilliant Blue G-250, rhodamine B, methylene blue, and 4-nitrophenol.

Pesticides Pesticides are applied to protect crops against insects, weeds, fungi, and other pests. They also play a chief role in food production. They are persistent organic pollutants and very noxious to humans and can have both serious and prolonged health effects,

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depending on the quantity and the ways in which a person is exposed. More importantly, pesticides can bioaccumulate and biomagnify in the environment. Recently, nano-enabled treatment has provided an attractive solution to tackle this challenge. In a novel approach, magnetic NPs were produced by using magnetotactic bacteria by the process of biomineralization. Produced NPs showed astonishing ability to degrade pesticide ethyl-paraoxon. Recycling study showed that the spent NPs could be reused to repetitive applications. In a noteworthy attempt, highly crystalline metal hexacyanoferrate nanoparticles of Cu, Zn, and Ni were synthesized by S. mukorossi. The catalytic reactions were performed under sunlight, and significant reduction for some of the hazardous pesticides, namely, chlorpyrifos, thiamethoxam, and tebuconazole, was noticed. Total 71–98% degradation was achieved for the pesticides selected for experiment. Mass spectroscopy analysis revealed that hydroxylation of aromatic or pyridine ring, dechlorination followed by dealkylation, oxidation of P=S or =NH bond, and cleavage of ring structure were the degradation pathways that resulted the generation of nontoxic intermediate compounds. Shi et al. [50] produced Au nanocatalysts by utilizing porous chemical pretreated S. cerevisiae yeast strain for the aqueous phase degradation of an herbicide quinclorac. Degradation of quinclorac was assisted by NaBH4 along with Au nanocatalysts. The catalyst worked proficiently, and complete removal of quinclorac was acquired on 24 h of contact time. Chromatographic study revealed that quinclorac degraded into 7-chloroquinoline-8-carboxylic acid which was further degraded into 3,7-bichlorid quinolone, 3-chlorocatechol, and 2-chloro-1,6benzenedicarboxylic acid. In a report, Au-modified TiO2 nanocatalysts were prepared sustainably by using nontoxic cyclodextrin. Synthesized nanocomposite was applied as an effective catalyst for the aqueous phase removal of a pesticide phenoxyacetic acid (PAA). Fu et al. [51] followed a green route to synthesize Fe2O3 NPs and demonstrated the aqueous phase photocatalytic degradation of a hazardous pesticide acetochlor. Total 91% degradation of acetochlor was achieved. GC-MS study concluded that 1-methyl-3-ving-benzene, allyl-methyl-amine, and 1-chloro-2-ethoxy-ethane were the by-products. These intermediates were further oxidized into NO3, CO2, and H2O.

PAHs and Aromatic Amines PAHs are a class of diverse organic compounds containing two or more fused aromatic rings of carbon and hydrogen atoms. PAHs are important environmental pollutants because of their universal presence and carcinogenicity. Recently, nanotechnology-assisted catalysis for the treatment of PAHs containing wastewater is gaining popularity due to the significant results. In this context, Shanker et al. [52] reported green synthesis of hexagonal, rod, and spherical-shaped hexacyanoferrate NPs by using a natural surfactant Sapindus mukorossi, with size ranging from 10 to 60 nm. Synthesized NPs were successfully employed for the photocatalytic removal of several hazardous PAHs, namely, anthracene, phenanthrene, chrysene, fluorene, and benzo(a)pyrene, in water. Catalytic experiment showed that under

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optimized experimental conditions (PAHs, 50 mg L 1; catalyst dose, 25 mg; neutral pH; and solar irradiation), all the PAHs compounds were converted into their nontoxic moieties. 80–90% degradation was achieved for anthracene and phenanthrene. The degradation of chrysene, fluorene, and benzo(a)pyrene was found to be 70–80%. In a novel attempt, Fe NPs were fabricated by applying tea extract as a green reducing agent and applied for the catalytic removal of phenanthrene [53]. It was noticed that the Fe NPs initiated photo-Fenton-like oxidative reaction. The synthesized NPs showed outstanding ability and resulted in 100% degradation of phenanthrene within 90 min of reaction time. GC-MS analysis identified trans-3-(O-hydroxyphenyl)-1-phenyl-2-propen-1-one as a degradation by-product. Muthukumar et al. [54] followed a green procedure and synthesized Fe-doped ZnO NPs (Fe-ZnO) by applying aqueous extract of A. dubius leaf. This biologically produced Fe-ZnO nanocomposite was applied as a nanocatalyst for the aqueous phase remediation of naphthalene-contaminated water. FTIR spectra of leaf extract confirmed the presence of amaranthine and other phenolics responsible for the capping of synthesized NPs. The highest degradation (92.3%) was achieved with the 40 ppm of naphthalene at pH 4.0 under visible light within 240 min of contact time. Aromatic amines are particular class of organic compounds consisting of an aromatic ring attached to an amine. They are widely used as a reagent intermediate in various industries like oil refining, dyes, cosmetics, medicines, rubber, textiles, agrochemicals, explosives, synthetic polymers, dyes, adhesives, pharmaceuticals, and pesticides. They can easily contaminate the surface and groundwater due to their high aqueous solubility. Mineralization of these hazardous pollutants by nano-sized catalysts can be an attractive alternative technique to minimize their quantity in the aquatic environment. Rani and Shanker [55] synthesized Zn- and Cu-doped hexacyanoferrate (HCF) NPs by S. mukorossi extract. Both types of metallic NPs were applied for the catalytic treatment of several carcinogenic aromatic amines from simulated wastewater. It was observed that under optimized reaction conditions, ZnHCF removed 90% of p-anisidine, 76% of p-toluidine, 71% of aniline, and 70% of p-chloroaniline, while in the case of CuHFC, the percent degradation was found to be 88% for p-anisidine, 72% for p-toluidine, 69% of aniline, and 64% for chloroaniline. GC-MS investigation established the formation of small, nontoxic by-products such as benzoquinone, hydroquinone, but-2-enal, 4-oxobut2-enoic acid, and malealdehyde.

Phenols Contamination of aquatic bodies by phenolic pollutants occurs due to the polluted wastewater from industrial, agricultural, and domestic activities into water bodies, because they are widely used in the manufacturing of fertilizers, explosives, paints and paint removers, drugs, pharmaceuticals, textiles, coke, etc. Phenol enters into the body via ingestion, skin absorption, and inhalation and causes local and systemic toxic effects. Recently, nanometered catalysts have been successfully

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applied for the preconcentration of phenolic pollutants from aqueous streams. Khan et al. [56] fabricated Ag NPs by using L. chinensis fruit juice as a reducing and capping agent and applied them for the photocatalytic removal of 4-nitro phenol. The developed bionanocatalyst was spherical shaped having an average size of 10 nm. The degradation was assisted by NaBH4, and 4-nitrophenol was converted into 4-amino-phenol. In an eco-friendly approach, CuO NPs were produced biologically from F. japonica fruit extract for the catalytic removal of 4-nitrophenol [57]. FTIR spectra of fruit extract confirmed the affluence of active biomolecules such as phenolics, proteins, enzymes, and vitamins that acted as reducing and capping agents. Total 87% of degradation of 4-nitrophenol was achieved on only 30 min of reaction span. Degradation kinetic was well explained by pseudo-first-order kinetics. Among phenolic pollutants, water pollution from bisphenol (BPA) is a serious concern due to its detrimental health effects on human and wildlife. BPA disrupts endocrine system and causes severe damage to aquatic life. Successful attempts have been made to remove bisphenol from water using nanocatalysts. Rani and Shanker [58] synthesized ZnO and ZnO-doped zinc-hexacyanoferrate (ZnO@ZnHCF) nanocomposites by employing A. indica leaf extract that was used for the catalytic removal of BPA. FTIR analysis detected the presence of several active phytochemicals that served as reducing and stabilizing agent. SEM images displayed that the synthesized nanocatalysts were cubical in shape. It was observed that semiconducting and intercalative nature of ZnO and zinc-hexacyanoferrate exerted synergistic effect and high removal of BPA was achieved. 97% removal of BPA was achieved with 2 mg L 1 of initial BPA amount and 25 mg of catalyst dose at a neutral pH. Surprisingly, the reusability of spent catalyst was found to be excellent, and the nanocomposite was ten times recyclable with satisfactory removal efficiency.

Mechanism of Degradation and Fate of Contaminants A water treatment technology is considered successful if the degraded by-products are nontoxic and less toxic in nature. It is a well-known fact that organic pollutants are highly resistant to degradation due to their structural complexity. Sometimes partial degradation results in the generation of more toxic intermediates than parent molecule. Nanoparticle-mediated catalysis includes two main catalytic pathways, i.e., oxidative and reductive degradation. Electron donor and acceptor moieties generated from the surface of nanocatalyst play a major role for the initiation of catalytic reactions. According to the available literature, nontoxic intermediates like oxalic acid, formic acid, or acetic acid are formed during the breakdown of organic contaminants having one or more ring structures. These intermediates are formed due to the ring cleavage, dechlorination, and demethylation reactions initiated by the free radicals generated by radiation sources. These intermediates are further transformed into gaseous products such as CO2, SO2, NH4, etc. through the process of mineralization.

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Conclusions and Further Outlook Over the last decade, nano-enabled water treatment technology has provided a fruitful solution because of their proficiency to remediate tenacious organic pollutants. Now we are more able to manipulate and comprehend nano-sized materials for the removal of target specific pollutants. It has been possible due to the modern and sophisticated analytical techniques. It was revealed that the nanocatalysts synthesized from green routes are highly proficient for the minimization of organic contaminants. It was observed that biologically produced nanocatalysts are able to degrade almost all sorts of organic contaminants. They can effectively transform the complex organic molecules into their nontoxic and/or less toxic intermediates. Some nanocatalyst showed superb catalytic ability by complete mineralization of the organic molecules. It was also concluded that sustainably produced nanocatalysts are able to remove almost every class of organic pollutant in an effective way. These nanocatalysts can be successfully harnessed for the modern water purification processes, effluent treatment plants, and sewage treatment plants to produce high quality of water for reuse. Due to their environmental benignity, they can replace the nanocatalysts synthesized by traditional routes. Acknowledgments The author PKG is grateful to the Science and Engineering Research Board (SERB), Government of India, for its NPDF program (Grant no. PDF/2016/002910). SS is thankful to the DBT for its JRF scheme (Grant no. BT/IN/Indo/US/Foldscope/39/2015).

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Improving the Performance of Engineering Barriers in Radioactive Waste Disposal Facilities: Role of Nano-materials

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design Criteria for Engineering Barriers in Radioactive Waste Disposal Facilities . . . . . . . . . . Wasteform and Container Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Backfill and Buffer Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural Materials and Other Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of Nano-materials in the Safety of Radioactive Waste Disposal Facilities . . . . . . . . . . . . . . Intrinsic Nanoparticles in Engineering Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pseudo Nanoparticles in Engineered Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles in Natural Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Nanoparticles in Engineered Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The last step in any radioactive waste management practice is the disposal activity which is designed to ensure isolation of these wastes under controlled conditions over extended time scale. Current designs of radioactive waste disposal facilities rely on the containment and confinement strategy through the use of multi-barrier concept in which engineering and natural barriers function to ensure long-term safe practice of the disposal activity. The main target of this work is to present the current efforts in understating the role nanoparticles in enhancing or retarding the transport of radionuclides in engineering and natural barriers. Within this context, the radioactive waste classification system and its relation to the disposal option will be introduced. The features and limitations of different disposal options will be summarized. The safety functions and the design criteria for different engineering barriers will be overviewed and the performance of different materials R. O. Abdel Rahman (*) · S. S. Metwally · A. M. El-Kamash Hot Laboratory Center, Atomic Energy Authority of Egypt, Cairo, Egypt e-mail: [email protected]; [email protected]; [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_79

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used in these barriers will be summarized. Recent efforts in characterizing and testing the role of natural nanoparticles either intrinsic or pseudo will be summarized. The role of synthetic nano-materials to improve the performance of cement-based wasteform will be highlighted. Finally, some concluded remarks on the required future investigations to decide on the application of synthetic nanomaterials in the disposal environment will be drawn. Keywords

Engineering barrier · Natural barrier · Nanoparticles · Colloid · Disposal safety

Introduction During the last century, the nuclear industry started and developed to serve human civilization. Several milestones were achieved through the application of nuclear science and technology in different economical sectors [1]. These applications extended to include power generation, medical diagnosis and treatment, sterilization and gauging activities in industrial, health, and agricultural sectors, and assessment of environmental problems [1–3]. As a result of these applications, different waste streams that contain radioactivity amounts higher than those identified by the national regulatory authorities were generated. These wastes should be managed within a holistic approach to mitigate any negative impact on the human health or the environment. The first and most important step in managing these wastes is to have a clear classification and categorization systems to allow efficient and economical management practice. The international atomic energy agency (IAEA) proposed a waste classification system based on the linkage between the radiological and thermal characteristics of the wastes into very short-lived wastes (VSLW), very low-level wastes (VLLW), low-level wastes (LLW), intermediate-level wastes (ILW), high-level wastes (HLW), and the end point of their life cycle, i.e., disposal [3–5]. This system identifies five end point options, namely, storage for decay, landfill disposal, near surface disposal, intermediate depth disposal, and deep geological disposal facilities. Despite this classification system is not mandatory, different countries adopted it [3]. It should be noted that within this system, the wastes are categorized based on their chemical and physical properties to ease their pre-disposal activities [2, 3]. Pre-disposal activities include pre-treatment and treatment activities that are performed to reduce the volume of the generated wastes, and conditioning that aims to stabilize the treated wastes and facilitate their handling and transport from a waste management facility to another [2, 3]. These activities are summarized in Fig. 1 [6]. The selection of the end point of life cycle option for these wastes is not only bounded by the waste classification but also with several national requirements including radiological performance requirements, national geological and hydrogeological conditions, and the sociopolitical acceptance [2, 3]. Storage for decay is an option for VSLW; in this option, the waste is stored in a radioactive

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Fig. 1 Pre-disposal activities in radioactive waste management system Table 1 Features and limitation of different disposal options. (Copyrighted from Ref. [7]) Option Near surface with engineered barrier system Borehole and cavities at intermediate depth

Geological disposal (including deep borehole)

Features Multi-barrier approach to enhance the safety of disposal Suitable for most LILW Long experience of operation The depth is adequate to eliminate the risk of erosion, intrusion, and rainwater percolation Possibility to use existing disused cavities, mines Use of reinforced concrete and drainage system Simple and not expensive (boreholes) Suitable for all waste categories. Enhanced confinement

Limitations Limited amount of long-lived waste. Erosion, intrusion, and percolation of rainwater may affect the performance Geological barriers are site dependent

Site-dependent geological formations High cost Complex technology involved Extensive safety and performance analyses

storage facility at the generators. As a result of their low radioactivity and short half lives, they reach exempt level within short time. For waste classes containing longer lived radionuclides with higher radioactivity content, disposal in designated facilities is favorable. Table 1 summarizes the features and limitations of different disposal options [7]. The radiological hazard confinement in radioactive waste disposal facilities, as in any other radioactive facility, relies on the application of the defense in depth strategy. In this strategy, the safety of the facility is ensured via the application of multi-barrier concept that integrates the engineering and natural barriers performances to inhabit any release of radio-contaminate from the facility [1].

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Engineered barriers are those installed in and around the facility; they include the wasteform, structural walls, drainage system, liner, and closure systems [2, 3, 7–9]. On the other hand, natural barriers include all the geological barriers that host the facility. They act as secondary barriers in case of a failure in the engineering barrier, where their confinement performances rely on the intrinsic ability of the geological medium to limit radionuclide transport [10]. In the following sections, the design criteria for different engineering barriers will be presented and the role of natural nanoparticles in the safety of radioactive waste disposal will be summarized. The effort in this work focus on the role of the natural nanoparticles in enhancing the performance of engineering and natural barriers in radioactive waste disposal facility and highlight the effect of artificial nano-materials that act as additive for cementitious wasteform.

Design Criteria for Engineering Barriers in Radioactive Waste Disposal Facilities The main aim of designing a radioactive wastes disposal facility is to ensure the confinement of the radiological hazard of these wastes over extended time scales under current and anticipated conditions. In geological disposal, i.e., Posiva and KBS, the ultimate goal of confinement is assured by relaying on the following measures [11]: 1. Favorable engineering barrier conditions and their proven technical quality 2. Favorable predictable natural barrier performance 3. Sufficient depth of the facility In near surface disposal, same measures are used to ensure confinement excluding the facility depth, which is not a factor that contributes to the radio-contaminant confinement. Engineered barriers include the wasteform, container (i.e., canister in geological disposal), structural barriers, and the closure structures. To ensure the long-term safety of the disposal facilities, these barriers should be designed to sustain effective implementation of their safety functions. Table 2 illustrates the safety functions of these barriers in a geological and near surface disposal facilities and the potential materials that could be used to design these barriers [7–9, 11, 12]. It should be noted that the backfill and cover barriers are considered as essential parts of the closure system in near surface disposal facilities. The analysis of the disposal safety is usually conducted by assuming failure in some engineering barriers that led to intrusion of water into the facility. The failure in the barriers might be due to the natural evolution of these barriers over long periods of time or as a result of a defect in one of these barriers. For near surface facility, surface water infiltration and groundwater penetration could occur depending on the hydrogeological conditions of the facility. For geological disposal, groundwater penetration is the only source for intruded water. The contact between the intruded water and the engineering and natural barriers will lead to further weathering in these

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Table 2 Safety function of engineered barrier in different disposal facility [7–9, 11, 12] Geological disposal Barrier Wasteform

Safety function Retain radionuclide

Material Glass

Canister

Mechanical strength, corrosion resistance

Copperiron

Buffer

Provide favorable mechanical, geochemical, and hydrogeological conditions for the canister, Protect the canister from external process Retain and retard potential radiocontaminant release Provide favorable mechanical, geochemical, and hydrogeological conditions for the canister Retain and retard potential radiocontaminant release in case of failure Contribute to the mechanical stability of the rock adjacent to the deposition tunnels Physical stability

Bentonite

Deposition backfill

Structural materials

Liner

NA

Cover

NA

Near surface disposal Safety function Material Provide Cement, structural bitumen, stability polymer, Limit water glass ingress Concert, Retain metal radionuclides NA

Bentonite

Void filling Limit water infiltration Radionuclide sorption Gas control

Cement based, clay based

Concrete

Physical stability containment barrier Containment barrier

Concrete, steel

Limit water infiltration Control of gas release Erosion barrier Intrusion barrier

Cement based, clay based, geotextile Clay, gravel/ cobble, geotextile

NA not applicable

barriers and the water will act as a carrier that can facilitate the transport of the radiocontaminants through different engineering and natural barriers to reach the accessible environment. Figure 2 illustrates a disposal failure scenario based on the

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Fig. 2 Schematic diagram of radio-contaminant release scenario from near surface disposal facility

Waste package

Backfill

Natural barriers

infiltration of surface water that leads to leaching of radio-contaminants from the waste package and subsequently the downward migration of these contaminants through the backfill and the structural walls into the natural barriers [13]. The retention and retardation performances through different barriers are evaluated by calculating the distribution and the retardation coefficients, respectively. These coefficients are determined by conducting batch and continuous laboratory experiments or field experiments according to standardized methodologies. The distribution coefficient (Kd, ml/g) is the ration between the radionuclide activity sorbed onto the barrier (Ao
Ae, Bq) to the radioactivity (Ae, Bq) that remains free in the contacting liquid phase, i.e., intruded water, and is given as follows [14]:  Kd ¼

Ao
Ae Ae

  V  103 m

ð1Þ

where V is the solute volume (l) and m is the barrier mass in (g). The retardation coefficient (Rf) measures the delay that occurs to the radionuclides with respect to the carrier [13].   ρ δq Rf ¼ 1 þ θ δC

ð2Þ

δq where ρ is the density (g/ml), θ is the porosity, and δC is the derivative of the isotherm equation. For linear sorption, Eq. 2 is reduced to [13]:

  ρ R f ¼ 1 þ Kd θ

ð3Þ

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Fig. 3 Relationships between different low-level radioactive waste disposal design phases and quality of information. (Copyrighted from Ref. [15])

The design of a radioactive waste disposal facility, either geological or near surface, is dependent on the design requirement provided by the national regulatory authority. The design process is a multistage iterative process that starts during the planning phase of the disposal project and evolve during the development of the project throughout its different life cycles. In the planning phase, the conceptual design of the facility is defined based on the information on the types and amount of the generated and projected wastes and host environment and engineering barriers characteristics needed to comply with regulatory requirements [15, 16]. In subsequent project phases, basic and detailed designs are developed based on the data obtained from the sitting and detailed design phases including data that are used to evaluate the confinement and retardation performances of different engineering barriers. The design also may be modified to account for some construction and operational updated procedures [16]. Figure 3 illustrates the relation between the quality of data and the disposal phase [15].

Wasteform and Container Design Criteria IAEA defined the wasteform as “The physical and chemical form after treatment and/or conditioning (resulting in a solid product) prior to packaging” [9]. The waste container is considered as the last barrier to protect the wasteform from external intrusion and limits the radionuclide releases. These barriers are designed to ensure successful execution of their safety functions. This is achieved by evaluating their performance against the regulatory standards according to internationally approved

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methods and standards that test the performance under conservative conditions [7]. The use of these standards minimizes the experimental uncertainty and allows intercomparison between the materials tested in different institutions. The role of the wasteform in maintaining the structural stability, retaining radionuclide, and limiting water ingression is determined based on the measurements of the compressive strength, normalized release rate (NRi, mg.m
2.D
1) or leachability index (L), and permeability, respectively [17]. The radionuclide retention performance is quantified by assuming conservative failures as a result of the natural evolution of the engineering barriers that leads to the saturation of the disposal facility with the intruded water [18, 19, 20]. The leaching rates and leaching indices are measured based on static short-term (i.e., MCC1), static long-term (i.e., IAEA/ISO 6961-82), semi-dynamic (i.e., ANS-2009), or dynamic (i.e., ASTM column extraction method) standardized leaching test [19]. The normalized release rates are determined as follows: NRI ¼

Ci V f i SΔt

ð4Þ

where Ci is the released radionuclide (i) concentration in leachant (g/m3), V and S are the leachant volume (m3) and sample surface area (m2), respectively, fi is the fraction of the element in the sample, and Δt is the time change. In addition to the above mentioned safety functions, the thermal and radiation stability of the wasteform, ease of operation and cost, and the maturity, reliability, and robustness of the immobilization technology are used to select the appropriate immobilization matrices [3]. Table 3 lists the main features for using different matrices in producing the wasteform. It should be noted that glass-based matrices are applied to immobilize HLW, ILW, and LLW in some countries [3, 17, 18]. Cement-based matrices are widely applied to immobilize different types of LILW and hazardous wastes in many countries [3, 9, 17, 18, 20, 21, 22].

Backfill and Buffer Design Criteria Monolith (cement based) and granular (clay based) materials were proposed as backfill in near surface disposal facilities and as deposition backfill and buffer in the proposed geological disposal facilities [18, 23–26]. The conceptual design for Table 3 Main features for using cement, bitumen, and glass as waste matrices Performance indicators Compressive strength Normalized leaching rate, gm/cm2/day Radiation stability Thermal stability Permeability

Cement Good 10
5–10
1 High Good Low

Bitumen Low 10
6–10
3 Low Low Low

Glass – 10
8–10
4 High Good Very low

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this barrier starts with the materials selection. In general, material selection criteria are divided based on the target safety function to primary and supportive criteria [27]. Primary criteria have direct effects on the confinement of the radionuclides, i.e., related to radio-contaminant retardation and limit the carrier transport into and through the disposal facility. The supportive criteria are required to ensure the sustainability of the primary criteria and include [27]. • Self-sealing of any induced cracks in facility or at the interface with the natural barriers • Retain the safe performance at different anticipated temperatures and radiation levels. • Retain mechanical properties and provide resistance to applied mechanical forces • Retain morphological stability and compatibility with structural barriers and with the host geology for required period of time Subsequently, in near surface disposal facilities, the selection of the appropriate backfill material is derived mainly based on their sorption properties and low permeability [8]. In geological disposal, candidate materials as buffer and deposition backfill should have notable sorption properties, low permeability, swelling ability, thermal and mechanical stability, and easy workability. The bearing capacity and heat conductivity are the performance indicators for the mechanical and thermal stability, respectively. Table 4 lists a comparison between the features of using different granular materials as backfill material. Trends in enhancing the performance of granular backfill are directed to enhance the sorption capacity of bentonite towards the anionic radionuclides [28–30]. Montmorillonite surface and pore structure modification by using Fe(III) species in the interlayer or by using Fe(III) (hydr)oxide surface coatings was found to increase the anion sorption capacities [29, 30]. Zeolites were proposed to retain some of the most mobile elements including 137Cs, 129I, 99Tc, 237NP, and 226Ra [21, 31]. A mixture of clay, sand, and graphite was tested as a backfill material to enhance the overall thermal, mechanical, and retention performance of the barrier in deep geological disposal [32]. In that backfill system, clays contribute to low permeability, high swelling pressure, and plasticity. Sand or crushed host rock contributes mechanical strength and higher thermal conductivity, and the graphite was proposed to enhance the thermal performance.

Table 4 Features of different granular backfill materials Features Sorption ability Permeability Swelling ability Mechanical stability Thermal stability

Quartz Small Large NA Good Good

Bentonite Large Small Large Good Good

Zeolite Large Large NA Low Good

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Cement-based backfill materials are applied in different near surface disposal practices and also proposed for borehole and intermediate and geological facilities. The widespread of this backfill category is attributed to the favorable chemical conditions that are created in the facility which limit the radionuclide migration. These conditions are [33]: 1. Maintain the high alkaline porewater environment in the disposal; this condition will reduce the steel corrosion and lower the radio-contaminant, i.e., lanthanides and actinide, and solubility. 2. Maintain suitable chemical and physical conditions to allow strong sorption of radionuclide. Also it prevents gas buildup in the facility and could be designed to allow retrievability if required [33].

Structural Materials and Other Barriers Disposal facility structure provides physical stability and containment barrier [8]. The design criteria for these barrier includes effective permeability, compressive strength, and shear strength. The use of conventional and low-pH cements in the construction of different disposal facilities were proposed and applied. In some cases, both types of cements could be used. Cut-off walls might be used to [34]: 1. Limit horizontal migration of groundwater into or out of the disposal facility 2. Provide structural integrity to a disposal facility during its operational phase 3. Retard radionuclides migration via sorption Materials used as cut-off walls are concrete curtain walls, secant pile walls, cement-clay cut-off walls, and steel sheeting. The design criteria include effective permeability and service life. Drains are designed for water management, particularly during the operational and institutional control in the post-closure phases. Typical drainage systems are either combinations of clay and gravel blankets or conventional ceramic or concrete drains. The key design criterion is the volumetric flow. Cover is used in near surface disposal facility (Table 2). The selection of the cover concept is dependent on the site climate and the main design criteria include vertical permeability, water shedding capacity, plasticity, slope stability, and barrier cohesion [15].

Role of Nano-materials in the Safety of Radioactive Waste Disposal Facilities The interaction of infiltrated or groundwater with the engineering barriers can lead to the degradation of these barriers and formation of colloids. The generated colloids have particle size distributions that extend from nano- to micro-scale, i.e.,

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1–1000 nm depending on the duration of the contact between cement and groundwater, backfill and water composition, nature of the flow, and temperature [35]. Colloids remain suspended in water due to their surface properties and represent a potential sorption surface for the radionuclides. This section intended to clarify the role of nano-colloid in the safety of radioactive waste disposal facilities and highlights the research that target wasteform improvement using nano-materials.

Intrinsic Nanoparticles in Engineering Barriers Intrinsic (eigen)-nanoparticles are formed by condensation of the hydrolyzed actinides (i.e., plutonium, americium, and neptunium) as a result of waste corrosion under disposal conditions [36, 37]. Tetravalent plutonium has a strong tendency to form polymeric complexes and colloids. Small polymers (i.e., dimer, trimer, and tetramer) are composed of plutonium of mixed oxidation states. In natural barrier environment (i.e., pH 6–8), plutonium exist in four oxidation states (III, IV, V, VI). Tetravalent plutonium has high tendency for deposition and sorbed onto the rocks if their particle size exceeded 220 nm [37]. The corrosion of metallic uranium in an anaerobic reaction with 0.001 MNaOH at 50  C for 200 days led to the formation of crystalline UO2 nanoparticles (5–10 nm) that tend to aggregate to form clusters of 20 nm. A small fraction of these particles (with particles sizes greater than 200 nm) were stabilized in a colloidal suspension due to the presence of silicate [38]. The mobility of these intrinsic particles could be significant and affect the safety of the disposal practice under the following conditions [39]: 1. Significant particles concentration is generated. 2. Stable particles suspension is attained. To reduce the mobility of these nanoparticles, the use of filter barrier was suggested for geological disposal sited in crystalline rock. This filter barrier could compose of high density bentonite to allow efficient filtration. For geological disposal sited in clay formation, it was found that natural clay barrier can limit the mobility of these particles, where only colloids with particles size less than 10 nm can diffuse through these natural barriers [39].

Pseudo Nanoparticles in Engineered Barriers In cementitious disposal facility, i.e., cement-based backfill and wasteform, the contact between cement and water initiate chemical reactions. As a result of these reactions, the porewater pH started to reduce from 13 to 11 in three stages as a result of portlandite and calcium silicate hydrate (CSH) phases dissolution. This dissolution leads to the formation of cement-derived colloids that can agglomerate and precipitate or disaggregate and remain mobile [35]. These particles can act as carrier

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or pseudo nanoparticles, which react with the radionuclides, and depending on their mobility, the nanoparticles can lead to enhanced or retarded radio-contaminant migration. The enhanced radio-contaminant migration is dependent on the significance and stability of the nanoparticles suspension, as mentioned above, and on the stable binding between the radionuclides and the nanoparticles [40]. In the case of pseudo nanoparticles, the evaluation of the role of these particles is conducted by studying the sorption –desorption characteristics between them and potential radiocontaminants. Subsequently, the characterization of the concentration, particle size, surface properties, and sorption potential of the pseudo nanoparticles towards potential radio-contaminants are crucial topics to understand their role in the safety of the disposal practice. The characterization of the nano-colloid in groundwater is conducted by following low-flow passive sampling technique at different sites, then collect the colloids using suitable membrane, and analyze the nanoparticles size distribution, surface properties, radioactivity, and structure using suitable standardized techniques. The characterization of groundwater samples collected from the near- and far-field of Drigg disposal facilities showed that the colloid populations in the samples were in the range 1010–1012 colloid/dm3. The nanoparticles of size less than 2 nm are mainly silica- or iron-containing colloids with minor amounts of alumina-, calcium-, and chromium-containing colloids. The fraction of the detected alpha activity (20%) in samples collected from the disposal trench groundwater (10 m below groundwater table) was found to be associated with nanoparticles having sizes in the range 1–2 nm. No activity was detected in the far field indicating that the radioactivity is confined in the facilities. 137Cs and 60Co detected in the trench groundwater were not sorbed onto the nanoparticles [41]. The presence of iron nanoparticles, of different chemical structures, is attributed to the degradation of the iron canister and embedded steel in the engineering barriers. These nanoparticles were found to enhance the safety of the disposal practice by reducing the mobility of uranium. This role is controlled by the chemical structure of the iron nanoparticles and the pH value of the groundwater. These two factors were found to highly affect the sorption efficiency, reduction degree, and speciation in the solid [42]. Nano-magnetite, mica, siderite, hematite, and ferrihydrite were found to retard uranium migration via sorption and reduction mechanisms. Magnetite was reported to enhance the formation of the reduced U3O8 and UO2 as a result of the stoichiometric ration of Fe(II)/Fe(III) and its surface defect sites, where the formation of secondary uranium (V) phases having urinate-like structure was detected on the magnetite surface. The sorption of uranium on hematite nanoparticles was deduced based on the detection of mononuclear surface complex of uranyl binding to hematite. Hexavalent uranium is reduced considerably by surface catalyzed Fe(II) in ferrihydrite, where hexavalent uranium can be incorporated into Fe(III)-oxyhydroxides as hexavalent species or as pentavalent species by substituting Fe(III). Goethite-containing Fe(III)-oxy-hydroxides were found to sorb uranium hexavalent by surface complexation [42]. Iron sulfides exist in the disposal environment; mackinawite and greigite nanoparticles can play an important role in the retardation of uranium hexavalent due to their large surface area and high solubility.

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Characterization of the nanoparticles generated in quartz-mortar (M1) backfill proposed for the Swiss repository indicated that the dominant contributors to the colloidal inventory are nanoparticles ( titanium dioxide (TiO2) > silver > silica. Formulations of nanomaterials are solid particles or as nonsolid structures. Nonsolids are lipid or polymer (natural or synthetic) structures or oil water (O/W) emulsions. In most of patented formulations the size of such structures varied mostly between 100 and 300 nm and in published formulation it varies between 300 and 2000 nm. In few patents, the size range was recorded over 2 μm. A considerable fraction of the formulations indicated to be “nano” which includes size fractions more beyond the “nano-range” (i.e., >100 nm) [8].

NonsolidNanomaterials The most evident fraction of NM is nonsolid which is comprised of nanostructures. Active substances have generally poor solubility in water and at room temperature even solid or crystalline therefore to convert into solution form organic solvents are used. O/W-based emulsions may be used to overcome the problem of solubility [9] which

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increases the solubility and loading capacity of the formulation of active substance which in turn enhance coverage of leaf surface resulting in penetration of the active substance via cuticula. Nanoemulsions are metastable, thus prone to crystallization, agglomeration, and sedimentation. Addition of suitable surfactants and additional protective colloids can achieve stabilization. Monoglycerides, that is, hydrolyzation products from natural fats, may be used as an amphiphile to form liquid crystal and microemulsion structures which are able to incorporate up to 30–40% of water in the oil phase. Amorphous solid organic NM may be produced by spray drying an O/W emulsion into a redispersible powder. After redispersion in an O/W emulsion they become amorphous nanoparticles of novaluron. The nanoformulation of novaluron does not affect the activity of a commercial non-nanoemulsion. In encapsulation techniques most abundant natural amino polysaccharide used is chitosan which is produced by deacetylation of chitin, a major component of the cell wall of common soil fungi. These nanocapsules have various applications in cosmetics, food and nutrition, pollutant capture from wastewater, drug delivery, and many more and also needs worth further research.

Solid Nanomaterials Titanium Dioxide (TiO2) Fujishima and Honda (1972) [10] discovered the phenomenon of photocatalytic splitting of water on TiO2 electrode under ultraviolet light. Many studies have been conducted to explore the potential of this material in its nano form which is used in “energy” and “environment.” Photocatalytic activity is the unique property of TiO2 and it is also known as a highly efficient photocatalyst [11]. Silver (Ag) Silver nanomaterials have unique optical and physical properties. These are used for construction of highly sensitive and selective detectors, optical (bio) labeling, and conductivity elements in electronics, catalysis, and sensing [12]. Antibacterial properties have been reported in Ag and even nano-Ag [13]. Silica (SiO2) In agricultural formulations mesoporous silica present is responsible for main mode of action NMs which are used as controlled release carriers in drug delivery. These are potentially suitable for various controlled release applications due to their unique properties, such as greater surface area (>900 m2 g 1), more pores volume (>0.9 cm3 g 1), tunable pore size with a narrow distribution (2–10 nm), and good chemical and thermal stability of these materials [14].

Nanomaterials as Nanofertilizers Degradation of soil health is due to indiscriminate use of agricultural inputs, viz. fertilizer, other agricultural chemicals, degradation of water resources, and uneven distribution of weather parameters resulting in low input use efficiency. Nutritional

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deficiency is the third most important limiting factor for plant growth and productivity next to drought and salinity. To overcome these problems and sustaining the agricultural production and maintaining soil health and environment, there is urgent need to focus on advanced knowledge, tools, and techniques for enhancing the use efficiency of applied inputs. Nanotechnology extends the possibility of exploring nanostructured materials as fertilizer carrier or controlled-release vectors for building of smart fertilizers and for enhancing the nutrient use efficiency thereby reducing environmental pollution [15]. Development of Nano materials may provide an effective solution for systematic release of chemicals to specific sites of plants which care nutrient deficiency in soil plant system. “Smart delivery system” refers the combination of targeted, controlled, remotely regulated, and involving many functional characteristic for escaping from biological barriers to get successful targeting [3]. Nanotechnology applications of nanomaterials in agriculture have pivotal role in enhancing input use efficiency. These nanoparticles possess greater surface area to volume ratio which enhances more reactivity, and their small size provides higher penetration of particles into soil and plants. Large-scale production of nanoparticles of important metals due to advancement of technology has enhanced fertilizer formulation by reducing nutrient losses and higher uptake in plants [16]. The properties like more surface area, sorption capacity, and controlled release of nanofertilizers (NFs) toward specific sites have attributed them as system for smart delivery. The NFs are the NMs that serve as the source of essential nutrients (macro- or micronutrients) for crop plants or serve as carriers of the traditional fertilizers (nanocarriers) for better utilization. Surface coatings of nanomaterials on fertilizer particles hold the material more strongly due to greater surface tension than the conventional surfaces and thus help in controlled release [17]. Tarafdar and Adhikari (2015) [4] described the role of various nanomaterials in soil and plant system (Table 1). As mentioned in Table 2, nanofertilizers control release of agrochemicals, site targeted delivery, reduction in toxicity, and increased nutrient utilization of fertilizers [18] due to their large surface area, increased solubility, and specific targeting due to small size, more mobility, and reduced toxicity [19]. Nanofertilizers are extracted from different vegetative or reproductive parts of the plant by different chemical, physical, mechanical, or biological methods with the help of technological interventions of nanotechnology for improving soil fertility, productivity, and quality of agricultural produces. Tarafdar and Adhikari (2015) [4] reported the properties of nanofertilizers having greater surface area due to very Table 1 Nanomaterials and their functions Nanomaterials Carbon nanotubes Nanonutrients Nanopesticides Nanaoscale carriers Nanosensor

Functions Germination of seed Plant/animal/human nutrition Plant protection Efficient delivery of fertilizer and pesticides Detection of nutrients and contaminants

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Table 2 Comparison of nanotechnology-based formulations and conventional fertilizers applications [30] S. No. 1

Properties Solubility and dispersion of mineral micronutrients

2

Nutrient uptake efficiency

3

Controlled release modes

4

Effective duration of nutrient release

5

Loss rate of fertilizer nutrients

Nanofertilizers-enabled technologies Nano-sized formulation of mineral micronutrients may improve solubility and dispersion of insoluble nutrients in soil, reduce soil absorption and fixation, and increase the bioavailability Nanostructure formulation might increase fertilizer efficiency and uptake ratio of the soil nutrients in crop production and save fertilizer resource Both release rate and release pattern of nutrients for water soluble fertilizers might be precisely controlled through encapsulation in envelope forms of semipermeable membranes coated by resin-polymer, waxes, and sulfur Nanostructure formulation can extend effective duration of nutrient supply of fertilizers into soil Nanostructure formulation can reduce loss rate of fertilizer nutrients into soil by leaching and/or leaking

Conventional technology Less bioavailability to plants due to large particle size and less solubility

Bulk composite is not available for roots and decrease efficiency

Excess release of fertilizers may produce toxicity and destroy ecological balance of soil

Used by the plants at the time of delivery, the rest is converted into insoluble salts in the soil High loss rate by leaching, rain off, and drift

small size of particles which provide more sites of facilitation for different metabolic process resulting in higher production of metabolites. Comparative efficacy of nanofertilizers has been proved greater over the conventional chemical fertilizers as these have novel mechanism of action, increased use efficiency, reduced nutrient loss, and minimum deterioration of the environment. Foliar application of nanoformulations increased production levels of crops; mechanism involved behind this is the small size and increased surface area of these fertilizers which make plants to absorb them efficiently [20]. Nanofertilizers are developed by either encapsulation of nutrient inside the nonporous material or nutrient can be mounted on nanomaterials. Rai et al. (2012) [21] described that encapsulation of nutrient within the nanomaterials is one of the new and dynamic technique performed in different ways particularly, (i) encapsulation of nutrient inside the nonporous material, (ii) coating with thin film of polymer, (iii) delivery of particle or emulsion of nanoscale dimension. For increasing fertilizer use efficiency naturally occurring nano clays and zeolites minerals are also used

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[15]. Zeolites have been used in fertilizer delivery mechanism as intermingled with primary nutrients, viz. nitrogen, potassium, phosphorous, and calcium [22]. Nanofertilizer supplies nutrient as per requirement at specified time, site, and rate. Nanofertilizers are cost effective to produce than the fertilizers that depend upon manufactured coating materials. Nanofertilizers might be good alternative for enhancing nutrient use efficiency and mitigating the problems of eutrophication. Slow and controlled release of nanofertilizer is important in improving soil fertility status by decreasing toxic effects associated with traditional fertilizers. Like conventional fertilizers, nanofertilizers are also soluble in the soil solution, and the plants can absorb them directly. The solubility of these nanofertilizers is more than that of bulk solids which is present in the rhizosphere as these are very small in size. Nanofertilizers reduce losses of nitrogen due to leaching, emissions, and incorporation for long term by soil microorganisms. These are more efficient compared to the ordinary fertilizers, as Nanofertilizers are developed for regulating the release of nutrients according to the requirement of crop plants [23]. Nanofertilizers are capable to enter directly into the cells due to their small size, thus reduces the mechanisms of their uptake in the plant cell which require very high energy [24].

Delivery of Fertilizers Bulk application of fertilizer, in the form of ammonium salts, urea, and nitrate or phosphate compounds is very harmful and unavailable for the plants as most of these are lost as run-off causing pollution [25]. Nanomaterials have smart delivery system of supplying the nutrients to the plants [17].

Chemical Encapsulation of fertilizers with nanomaterials and their slow release may be beneficial to meet the soil and crop requirements. The stability of the coating decreases the rate of dissolution of fertilizer and controls the sustained release of fertilizer. Kaolin and polymeric biocompatible nanomaterials have potential application in the field of agricultural nanotechnology. Corradini et al. (2010) [26] studied on biodegradable polymeric chitosan NPs (~78 nm) to observe the slow release of the NPK fertilizer as urea, calcium phosphate, and potassium chloride. Biofertilizer and Micronutrient Polymeric NMs are applied as coating on biofertilizer preparations to make desiccation-resistant formulations. In horticultural crops foliar application of micronutrients increased uptake of micronutrients [27]. Peteu et al. (2010) [28] reported that soil application of slow release of micronutrients by nanomaterials promoted plant growth and soil health. Seed Treatment Nutrient use efficiency can be improved by improving the delivery system of micronutrients. Clay nanocomposite superabsorbent has been identified for seed

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treatment or coating before sowing. In the rainfed ecosystem nano clay composite can supply moisture to the seeds and seedlings in the rhizosphere of crop for their survival under water stress situation, particularly in the period of long dry spell after a rain. Hence, use of nano clay composite superabsorbent may be a potential technology for conservation agriculture semi-arid region. Yadav et al. (2010) [29] studied that Zn enriched urea enhanced grain yield and quality of aromatic rice. Coating of seeds with teprosyn-ZnP or teprosyn-Zn has been reported to correct Zn deficiencies in wheat, maize, sunflower, groundnut, and soybean.

Classification of Nanofertilizers on the Basis of Requirement Nanofertilizers provide essential nutrients required for growth and development of plants in controlled manner. These are classified as follows.

Macronutrient NFs These are prepared from nano-sized macronutrients that are required by crop plants in large quantities. Macronutrients include N, P, K, Ca, Mg, and S. Their requirement is related with increase in the demand of food for the ever growing population of the world. Macronutrient demand is expected to increase to 263 Mt. by 2050. Development of nanofertilizer macronutrients may play a vital role to satisfy the requirement of macronutrients in the form of their increased use efficiency resulting in an increased growth and productivity of crops compared to conventional fertilizers with very low use efficiency around 20%. Macronutrients nanofertilizers have been developed with the help of nanotechnology all over the world and reported for tremendous increase in resource use efficiency resulting in enhanced growth and productivity of crops. The use of nanofertilizers as the source of nutrients in agriculture also reduced the cultivation cost thus these may be an economical alternative to the existing conventional chemical fertilizers. Nitrogen (N)-NMs N is the most important nutrient involved in many processes of crop plants. Nitrogenous fertilizer is more vulnerable with groundwater contamination and hazard. Dynamics of absorbed form of nitrogen releasing is slower than for the ionic form. Release of nutrients from the fertilizer capsule is regulated by type of coating and binding of slow release nanofertilizers and subnano-composites. Nanofertilizers explore great opportunity for improving economy, energy, and the ecology by reducing leaching loss of nitrogen, losses through emissions, and continuing amalgamation by soil microbial population. For example, slow release of N has been observed when zeolite chips were coated on urea [30]. Similarly, slow and sustainable release of N into the soil was observed when urea-modified hydroxyapatite NMs were encapsulated under pressure into the cavities of soft wood of Gliricidia sepium. Nitrogen supply by this method was observed optimum up to 60 days compared to traditional sources of nitrogen resulting in N supply to the plants in the initial stage and very less supply at later stage of crop [31].

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Phosphorus (P)-NMs Phosphorus is an integral constitute of many metabolites and involved in many processes of metabolism. P containing chemical fertilizers has very low fertilizer use efficiency and only up to 20% is available to crops and the remaining gets deposit in the soil and/or goes as run off causing eutrophication of water bodies. Nanotechnology has important role in increasing the nutrient use efficiency (NUE) and decreasing environmental threats. To increase P use efficiency, hydroxyl apatite [Ca5(PO4)3OH] NMs were prepared using wet chemical method and compared with traditional phosphatic fertilizers and crop performance of soybean under greenhouse condition. Growth rate (33%) and seed yield (20%) were significantly increased in comparison to the conventional phosphatic fertilizers [32]. Tarafdar et al. (2012) [33] developed microbially synthesized nanomaterials of phosphorus from tricalcium phosphate by using Aspergillus sp. The experiment was conducted under arid environment and revealed that the use of nanomaterial substantially reduces the fertilizer quantity. Foliar application of nanophosphorus @ 640 mg ha 1gave equivalent yield to 80 kg ha 1 phosphorous in cluster bean and pearl millet [34]. Potassium (K)-NMs Potassium NMs have been developed using carrier as base material and tested under controlled conditions. Anjuman et al. (2017) [35] reported the increased trend in respect of growth of Kalmi; its uptake and concentration of phosphorus (P) and potassium (K) were better in nanofertilizer treatments than in the conventional fertilizer treatments indicating the fact that there is a bright possibility of nanofertilizer in agriculture. Calcium (Ca)-NMs Ca plays important role in cell elongation and strengthens cell wall structure by formation of calcium pectate in plants. Peanut grown in sand were tested with CaCO3 NMs (20–80 nm) 160 mg l 1 as Ca in Hoagland solution as a source of Ca for 80 days and were compared with control (without Ca) and with soluble source of Ca as Ca(NO3)2 (200 mg l 1). Fresh biomass was significantly improved in comparison to the control; similarly on dry weight basis in comparison to the soluble source of Ca [Ca(NO3)2]. Ca uptake was also enhanced in seedling stem and roots compared to the control [36]. Magnesium (Mg)-NMs Mg plays important role in photosynthesis as it is an integral component of chlorophyll. It is also involved in the production of amino acids and proteins of cell. This also helps in phosphorus uptake and migration and induces resistance for biotic and abiotic stress in plants. Delfani et al. (2014) [37] investigated the combined effect of foliar application of Mg-NMs and Fe-NMs (0.5 g l 1) on the photosynthetic efficiency of black-eyed pea (Vigna unguiculata). The results revealed that photosynthetic efficiency significantly improved with the combined application of Fe- and Mg-NMs resulted in enhanced growth and yield; however, their application resulted

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in decrease in grain yield by 8%. It was also observed that in different plant tissues, uptake of Mg increased compared to the control and Mg application on regular basis suggests that the application of Mg-NMs increases the uptake of Mg.

Micronutrient Nanofertilizers Widespread deficiencies of micronutrients due to overexploitation of soils has now become challenging, even though plant nutrition requirement of these micronutrients is very less but their deficiency causes hidden hunger. These deficiencies should be corrected by supplying of appropriate nutrient with advanced efficient techniques like use nanoparticles which have been reported to enhance the uptake pattern by the plants. Micronutrients are responsible for many important roles in plant physiology. Their requirement is very less (  100 ppm) but important for different metabolic processes of plants. Micronutrients include iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chloride (Cl), and nickel (Ni). These are also applied to the crop plants with different composite fertilizers containing different grades of NPK. Micronutrients provide enough nutrients and have very low environmental risks. Minor changes in soil properties like pH, soil texture, and organic matter severely affect availability of micronutrients to plants resulting in very less fertilizer use efficiency of micronutrient containing fertilizers [38]. Their requirement can be fulfilled through the application of nanofertilizers containing micronutrients. Agronomic efficiency of micronutrient fertilizers may be increased by altering the particle size. Reduction in particle size results in more number of particles per unit weight of applied fertilizer and also extends the specific surface area of particles of fertilizers. For example, zinc sulfate (ZnSO4) (1.4 to 2 mm) was found to be somewhat less effective than fine ZnSO4 (0.8 to 1.2 mm), whereas granular ZnO (2.0 or 2.5 mm) has been found completely ineffective [39]. Use of smaller size granules resulted in a better distribution of Zn, and the more contact area of surface with Zn fertilizer showed better uptake of Zn [40]. Therefore, more research and studies should be conducted with major emphasis on the particle size of nanomaterials for increasing the efficiency of the fertilizers for getting enhanced uptake and greater yield. Iron (Fe)-NMs In hydroponic system under greenhouse study, chlorophyll contents of the sub-apical leaves of soybean were found to be significantly increased with the application of dilute concentrations of Fe-nanomaterials (30, 45, and 60 mg l 1) compared to the regular application of Fe-EDTA. The study revealed that Fe-nanomaterials may be an efficient source than Fe-EDTA applied at 3 g kg 1 in soil. This may be because manufactured nanomaterials may interact with soil resulting in decrease of their toxic effects and uneven distribution of manufactured nanomaterials in soils, reduced contact of the organisms with MNPs may further result in the observed reduced risks. Some studies reported that the endpoints used in these studies were not sensitive enough to detect the negative effects. Alternative sensitive endpoints may be used to detect adverse effects at much smaller concentrations. For example, in Lumbricus terrestris L. the toxicity of manufactured nanomaterials was detected with the sensitive endpoint of apoptotic at 15 mg kg 1 in soil for TiO2-NMs and 4 mg kg 1 in soil for Ag-NMs. It is clear from one study that the toxicity was less in water (100 mg l 1 for TiO2-NMs) than in soil (15 mg kg 1 for TiO2NMs), which indicates less bioavailability of manufactured nanomaterials in water. In another study more bioavailability of these nanomaterials was recorded in water than in soil. Soil biodiversity is the important indicator for sustainable agro ecosystem. Protease, catalase, and peroxidase activities of soil are the indicators of toxic effect of TiO2- and ZnO-NMs in soil. To study the toxicity of Al-nanomaterials in soils, carbon dioxide production and the total mineralization of glucose were measured. These are important parameters for characterization of soil microbial activities. Besides this, the evaluation of size of soil microbial population can be done from total phospholipid-derived phosphate and from both fatty acid profiles and total genomic DNA. It is very difficult to select different variables for characterizing the activities of an organism. There is urgent need for standardized system for selecting key target measurements toxicological studies in manufactured nanomaterials. There is still no solid reports are available for nanocarrier-based fertilizers regarding their role in enhancing the nutrient use efficiency by improving the nutrients transport into the plant tissues/cells or decreasing the environmental risks associated with the use of conventional fertilizers. Certain NMs like SiO2-NPs, Fe2O3-NPs, and CNTs are more economic and efficient than previously available carriers like zeolite. These would regulate the controlled uptake of active substances, thus decreasing the inputs and also the waste produced. Nanocarriers should be designed so that these can anchor the soil structure, roots of plants, and organic matter. This may be done only by the understanding of molecular and conformational mechanisms between the nanoscale delivery structure and soil materials.

Conclusion and Further Outlook The indiscriminate use of agrochemicals to increase agricultural production is resulting in soil and groundwater pollution due to their low efficiency and high environmental risk. For sustaining the agricultural productivity and conserving the soil health it is necessary to identify environment friendly approach with high nutrient use efficiency. Nanotechnology is becoming important for the agricultural sector day by day. Different studies reported promising results and applications in

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the areas of pesticides delivery, biopesticides, fertilizers, and genetic material for plant transformation. Use of nanomaterials is expected to reduce the dosage and controlled release of fertilizers. Nanotechnology may also provide solutions for degrading persistent chemicals into harmless useful components. This approach has promising impact in the use of natural resources such as chitosan and diatomite for production of coating material which are useful for many agricultural applications and in mitigation of drought stress tolerance of crops. The requirement of nitrogenous and phosphatic fertilizers is in large quantities as these are primary nutrients and play very important role in growth and development of plants. These fertilizers also have problem of low nutrient use efficiency and losses are more. Thus more efforts might be focused on N- and P-nanofertilizers. Comparison studies should also be conducted between Ca-NMs from CaCO3 with other Ca sources like CaCl2 or CaSO4. Research should also be focused on multinutrient comparison like that of Ca- and N-NMs using soluble Ca(NO3)2 as control. Intensive research might be conducted on micronutrients to reduce the effect of factors affecting their availability under field conditions. Biofortification studies may also be conducted to investigate the role of micronutrients in enhancing the quality of food in comparison to conventional fertilizers. Application of nanomicronutrients through fertigation should also be tested to compare the efficacy of these nanoformulations over commercially available micronutrient fertilizers. Toxicity of nanomaterials is the main constraint with these formulations therefore efficient and effective dose should be calculated for each and every crop without any toxic effects may also be investigated with the help of research studies. Hence, nanotechnology may be a powerful tool for addressing the current issues of environmental challenges and benefits of human being or mankind.

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Photocatalysis for Wastewater Treatment with Special Emphasis on Plastic Degradation

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Karthika Arumugam, Swaminathan Meenkashisundaram, and Naresh Kumar Sharma

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of Photocatalysis with Other AOPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Activity Under Different Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Visible Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UV Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Wastewater Treated Under Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examination of Photocatalytic Detoxification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Photocatalytic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method of Photocatalytic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carrier Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters Manipulating Photocatalytic Membrane Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photobioreactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photodegradation of Pollutant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Photobioreactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalysis for Plastic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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K. Arumugam Department of Microbiology, The Standard Fireworks Rajaratnam College for Women, Sivakasi, India Centre for Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India S. Meenkashisundaram Nanomaterials Laboratory, International Research Centre, Kalasalingam Academy of Research and Education, Srivilliputhur, India e-mail: [email protected] N. K. Sharma (*) Centre for Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_41

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Mechanism of Catalyst in Degrading Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Information of Plastic Degradation by Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Advanced oxidation process such as photocatalysis is seen as a potential future technology to offer clean water for various human needs. Through this process, different organic pollutants and recalcitrant chemicals are effectively removed from both water and wastewater. Some of the AOPs, such as ozonation (O3), catalysis by iron ions, electrodes, metal oxides, UV/Fenton, UV/hydrogen peroxide (H2O2), etc., generate harmful intermediates which restrict their full-scale implementation. Alternatively, irradiation techniques such as photocatalysis are a viable option and have shown to treat wastewater contaminated with hazardous chemicals, organic dyes, pesticides, antibiotics, viruses, bacteria, protozoa, etc., without producing toxic levels of by-products. This chapter reviews emerging aspects of photocatalysis for treatment of various recalcitrant pollutants with a special emphasis on polyethylene degradation. It summarizes the source, types, mechanism, and parameters of wastewater treatment using photocatalytic activity. Polyethylene is well known as serious cause of threats to human health and environment. Polyethylene, polystyrene, plastic film, and polypropylene degradation has been shown to occur via photocatalysis. This could be achieved using TiO2, ZnO, and doped and undoped metal oxides as photocatalysts. Also, photocatalytic degradation is compared with microbial techniques for polyethylene degradation along with brief reports on novel photobioreactors; a combination of microbial and photocatalytic reactors in degradation of polyethylene. Keywords

Photocatalysis · Organic pollutants · Polyethylene · Microbial degradation · Photobioreactors

Introduction Clean water is vitally important for human survival and daily activities such as mining, manufacturing, growing crop, etc. Many methods are followed for wastewater treatment but are not well efficient to remove the organic pollutant and require high technological operation with high volume of chemicals. Recently effective results are gained through advanced oxidation process (AOP). Through this process, organic pollutant and recalcitrant chemicals were effectively mineralized in the wastewater. Though many photochemical/UV reactions with ozone or hydrogen peroxide were carried out in some AOPs to remove organic pollutants, the main disadvantages are the rise of harmful intermediates [1]. This can be overcome by photocatalytic degradation. In photocatalytic degradation, organic pollutants present in wastewater are completely mineralized into carbon dioxide,

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water, and other nontoxic products. Nowadays UV-based photocatalysts are commonly used which can enhance the degradation process effectively [2]. The principle of photocatalysis involves excitation of electrons from valence band to conduction band, thereby forming electron hole pair. Holes generated in the valence band react with H2O/OH /H2O2 to form free radicals, which lead to possible mineralization of organic pollutant. Wastewater contaminated with hazardous chemicals, pesticides, phenols, chlorophenols, and other pollutants was effectively treated using photocatalysis [3]. It also inactivates the viruses, bacteria, and protozoa residuals. Recently photocatalysis was also used to treat plastic waste material. To avoid the toxic by-product formed from other disposal methods, photocatalysis could be carried out to degrade the plastic waste with the help of suitable catalyst.

Comparison of Photocatalysis with Other AOPs Advanced oxidation process is a kind of chemical treatment which could be applied to remove organic pollutant from the water and wastewater. The mechanism involved in AOP is the production of hydroxyl radicals (HO.) that is responsible for the degradation of organic pollutant and converts them into carbon dioxide and water. This AOP utilizes various oxidants such as H2O2, O3, and Fe2+ to produce reactive oxidizing species (ROS). Various advanced oxidation processes are shown in Fig. 1 and Table 1

Photocatalytic Activity Under Different Sources Visible Light In visible light the photonic energy is low. In order to activate it, light harvesting is the basic need to initiate photocatalysis. Photocatalysis under visible light follows three main important steps: (i) examining the band gap semiconductors with essential visible light [4], (ii) introducing external atoms to turn the band structure [5], and AOP

UV/O3

UV/H2O 2

O3/H2O 2

O3/UV/H2O2

Photocatalysis

Fe2+

H2O2

Ozone

Fe2+/H2 O2

Fig. 1 Various advanced oxidation processes

Photo/ Fe2+

TiO2/UV

Fe2+/H2O

TiO2/ H2O2

2

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Table 1 Various oxidation processes carried out in advanced oxidation process. √- Yes; X- No AOP Ozone H2O2 Fe2+ Photocatalysis

Bromated Toxicity by-product √ √ X X X X X

Recycle and reuse X X X √

Sludge Cost-effectiveness formation Disinfectant X X √ X X √ √ √ √

(iii) enhancing the visible light absorption with the help of loading dyes, quantum dots, or noble metals. The key factors that determine the absorption of visible light are charge transport and redox potential of charge carrier involved in photocatalysis.

Solar Light Solar light acts as an effective source for photocatalytic degradation of organic pollutant. pH of the aqueous solution is considered to be an important parameter for the photocatalytic activity. During photocatalytic action, solar energy is converted into chemical energy by reducing CO2 to CH4, CO, and CH3OH. Harvesting solar light based on photo-generated redox process is an important footstep to increase the efficiency of photocatalytic process. It also depends on the band structure of semiconductor needed for the photocatalytic reaction [6].

UV Radiation The UV light-based photocatalyst is more efficient than the other two. During the process of photocatalysis, the energy created in ultraviolet light is very high and thus transfers electrons to conduction band and creates electron hole pair to initiate the degradation process. It is one of the high photonic energy techniques which reduce CO2 to CH4 [7] (Table 2).

Types of Wastewater Treated Under Photocatalysis Wastewater toxicity can be removed by the catalysis in photodegradation process. Most important types of wastewater treated under photocatalysis are municipal wastewater (MWW) and industrial wastewater (IWW). Around 40% of MWW are treated under photocatalytic method which include gray water effluent for about 6%. Industrial wastewater contributes about 60% of total wastewater generation, industries like pharmaceuticals, pulp and paper, textile, tannery, and citrus processing. Overall, by analyzing these types of wastewater, some are safe to discharge in the environment, some will be sent to water treatment plant, and some will increase the biodegradability depending upon the toxic efficiency [11].

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Table 2 Source of photocatalytic degradation activity Sources Solar light

Visible light

UV light

Aqueous pollutant Ofloxacin, ibuprofen, carbendazim, propiconazole, acephate 4-Nitrophenol, metronidazole, amoxicillin, 2,4dichlorophenol, bisphenol A, diclofenac, 4chlorophenoxyacetic acid, and 2,4,6-trichlorophenol Pharmaceuticals, pesticides, personal care products, TSS, TDS, BOD, COD

Nanoparticle Metal/nonmetal doped TiO2 with Fe, N, Bi, and Ni

Reference [1, 8]

Doped TiO2 with Fe, Sn, Zr, Ni, Cu, Ag, and N

[9]

TiO2 and ZnO doped with Mo, Cr, La, Er, and Ce and Mn, Fe, Co, Ni, and Cu, respectively

[10]

Textile Wastewater Among the photocatalytic treatment of wastewater, textile wastewater was most promisingly practiced nowadays. Some of the reports were discussed below. In textile industry, large quantity of water is utilized for processing like printing and dyeing units. As it contains many chemicals and reactive dye, it is necessary to go for treatment before it reaches the environment. Photocatalytic treatment uses effective catalyst to remove dye from textile wastewater; thereby it could be applicable to release into the environment. Cotton bandage processing textile industry consumes significant amounts of water during manufacturing, creating high volumes of wastewater needing treatment. The organic pollutant in the wastewater remains a significant environmental issue. The photocatalytic degradation practice to treat cotton bandage textile wastewater is rarely investigated. COD analysis was carried out to quantify the amount of oxidizable pollutant in the wastewater. Different nanoparticles like TiO2, ZnO, and BiVO4 were used in photocatalytic degradation. Comparing the three nanoparticles, TiO2 is proven to be the efficient one, which reduced the COD of the effluent well. Photocatalytic method is considered to be a more efficient and time-consuming way for treating organic waste in real cotton bandage processing effluent. Similarly by using TiO2 and ZnO, the photocatalytic decolorization was performed for real and synthetic textile wastewater. Various parameters like COD, TDS, pH etc., were investigated and examined spectrophotometrically under solar irradiation. The progress of treatment stages was followed spectrophotometrically at different wavelength. After the decolorization process of 10 to 100 min, 100% were obtained. It depends upon the catalyst and the dye they used. The results indicate clearly that titanium dioxide and zinc oxide could be used efficiently in photocatalytic treatments of textile industrial wastewater. Photocatalytic degradation can be performed by immobilized TiO2 particle on glass fiber fabric (non-woven) against the reactive dye of azoïc and metal phthalocyanines. About 74% of degradation can be obtained under solar irradiation with

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removal of 0.2 and 0.9 g COD/h/m2. Global composition of wastewater and chemical structure of dye are the main interpretation in degradation process. The main successful method is the dip-coating technique under visible radiation. Photocatalytic degradation using TiO2 reduces the physicochemical parameters like BOD, COD, turbidity, TDS, and alkalinity. This process can be also carried out in lab-scale as well as larger-scale process.

Examination of Photocatalytic Detoxification Various examination can be carried out to conclude the effect of photocatalysis; some of the important test are as follows: bioluminescence assay [12], immobilization test [13], estrogen test, genotoxicity assessment (cell line test) [14] and zone of inhibition (Kirby-Bauer method), and phytotoxicity test [15].

Mechanism of Photocatalytic Degradation During photocatalytic treatment of wastewater, electrons from generated light energy get excited from valence band to conduction band, thereby leaving the H+ ions. The ROS generated react with H+ ions to form active free hydroxyl radicals on the surface of the catalyst. These free hydroxyl radicals easily attack the contaminants present in the wastewater. It is well understood that the reduction of conduction band and oxidation of valence band initiate the degradation of pollutants present in wastewater (Mamba and Mishra [16]).

Method of Photocatalytic Degradation There are four main important methods of photocatalytic degradation. (i) Sol-gel: It is a process of producing colloidal materials by converting monomers at low temperature. It has uniform nanostructure. Photocatalysis depends upon the metal precursors and the reagent used [17]. (ii) Precipitation: It is a process of transformation of metal precursor into lowsoluble substance as a precipitate using various catalysts like ZnO. It is one of the effective proven technologies with no specific solvent necessary. (iii) Hydrothermal synthesis: It is possible to grow good-quality crystal compound at high melting points with expensive autoclave [18]. (iv) Solution combustion synthesis: It is a well-known method that is able to synthesize crystal in a desired structure. It takes place in the presence of metal cations between fuel and oxidation. It is fully based on redox reaction with high energy consumption [19].

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(v) Hydrothermal synthesis: Ultrasound is the most efficient technology with increased reaction selectivity. The main advantage is its low energy consumption and eco-friendly manner in synthesizing nanocomposite particles [20].

Carrier Molecules The carrier molecule is important to get contact with the pollutant. It should have high porosity and better biocompatibility to get attached with the biofilm. At the same time, the microbes involved in photocatalytic degradation can also grow in pores free from the influence of ROS and light radiation [21]. The types of carrier molecules are as follows: (i) Cellulose carrier: The carrier material is made of cellulose porous material with positive charge. It has an efficient adsorption capacity in pollutant water. The microbes involved in biodegradation process could be easily loaded inside these porous matrix to increase photocatalytic reduction and thereby efficient pollutant removal. (ii) Ceramic carrier: The carrier molecules are made of ceramic material with rough surface. It has capability to load maximum photocatalyst while degrading the pollutant. At the same time, it is able to protect growth of microorganism from UV and free radicals [22]. (iii) Polyurethane sponge: The carrier molecule is made of a sponge having specific surface area with high porosity and high compatibility. It is one of the efficient carrier molecules to adhere microbes as well as photocatalytic materials [23]. (iv) Polyurethane foam: The carrier molecules are made of foam with high mesoporous structure. It is efficient to treat wastewater because of the coating of hydrogels and aerogels [24]. (v) Biofilms: This type of carrier molecules utilizes photocatalytic materials along with their adherent microorganism to treat the wastewater. It is one of the best carrier molecules which protect the microbes from radial attachment during the biodegradation process [25].

Parameters Manipulating Photocatalytic Membrane Reactor (i) Concentration: The concentration of photocatalyst should remain constant. The rate of reaction in photocatalytic mechanism is indirectly proportional to increased concentration of catalyst and directly propositional to the increased concentration of substrate. Hence catalyst concentration should remain constant [26]. (ii) Types: The type of photocatalyst should be chemically and physically stable. It must be nontoxic, and band gap energy should be most importantly considered while choosing the photocatalyst [27].

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(iii) Temperature: The perfect temperature for the photocatalytic reaction should be 20–80 ° C. High temperature will affect the adsorption of the reactant. Similarly low temperature will affect the desorption of the product during photocatalysis [28]. (iv) pH: Similar to temperature change, pH disturbs the catalyst surface charges. Optimum pH should be from 5 to 7 [29]. (v) Wavelength: The wavelength of the photons should be lower than the adsorption edge. Most probably, the wavelength around 280–400 nm worked out effectively [29]. (vi) Light intensity: Intensity of light is one of the most important factors, where an increase in intensity increases the energy consumption, thereby affecting the degradation. Hence constant light intensity must be maintained during the process. (vii) Membrane: Membrane size, properties, permeability, types, and fouling should be most importantly considered while carrying out the photocatalytic reaction [30].

Photobioreactor Photobioreactor may be a closed or open type reactor, which uses light source to cultivate plants, mosses, macroalgae, microalgae, cyanobacteria, and purple bacteria. These organisms do photosynthesis with the help of light and carbon dioxide to generate biomass. This chemical energy is however strenuously converted as biomass in an activated sludge treatment plants containing bacterial consortium. Nevertheless, this secondary sludge is of very little importance and in many cases is only to be dumped in the soil. Not only are algae shown to degrade varieties of emerging pharmaceutical drugs, textile dyes, and persistent pollutants and remove heavy metals; they are capable of generating biofuels, feedstocks, and secondary metabolites such as pharmaceuticals and nutraceuticals and can also be used in CO2 sequestration. The benefits from algae-based wastewater treatment however come with several challenges which still let bacterial-based methods to dominate over the wastewater treatment plants. The emerging organic contaminants (EOCs) include a wide range of xenobiotic and hazardous pollutants such as plastics, pharmaceuticals, healthcare products, surfactants, and pesticides. In recent days the conventional wastewater treatment is insufficient and unable to remove emerging and resistant contaminants. EOCs have been reported to be toxic and responsible for the reduction of macroinvertebrate diversity in water bodies (including rivers and ocean). Highrate algal ponds (HRAPs) are algae-based effluent treatment systems widely researched recently due to their potential to offer high resource recovery from algae biomass. HRAPs have been tested with several wastewaters mostly of domestic and municipal origin and high value-added products such as fertilizer, protein-rich feed, and fodder; biofuel and biohydrogen could be produced. The major setback is the separation of microalgae from treated water; several investigations are currently ongoing for effective separation of microalgae from

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treated water. In some of the studies, to effectively remove algal biomass, bacterial cells have been added; these types of systems are however complex and need strict maintenance. Another potential domain for degradation of recalcitrant is the combination of advanced oxidation process (AOPs) such as Fenton’s process, catalytic wet air oxidation, supercritical water oxidation, etc., along with biological treatment systems (including algae-based photobioreactors). Many researchers have independently investigated the effect of zerovalent iron (ZVI) and Fenton oxidation on degradation of various toxic pharmaceuticals and other xenobiotics. Synthetic and real wastewaters have been tested for treatment using ZVI process. With a dose of 10 g/L active carbon and 30 g/L iron at pH 4.0, the COD removal of 43% was observed in many of the industrial wastewaters. The predominant removal mechanisms such as oxidation-reduction, precipitation, and coagulation were observed during COD removal using ZVI processes. ZVI process was also found to increase the biological susceptibility of chemicals present in the wastewater by increasing the BOD/COD ratio from 0.07 to 0.6, thereby enhancing the biodegradability of various effluents. The setbacks observed during ZVI process was the accumulation of residual contaminants although low molecular weight compounds ( Ni2P > Ni12P5 [24], CoP > Co2P [25], and MoP > Mo3P [26]. Another advantage of heteroatom doping is to provide moderate free energy for the adsorption of HER intermediates, further facilitating the bonding to Hads or desorption of H2. The doping of heteroatoms is also known to retard the metal corrosion, especially the surface oxidation in acidic electrolytes [27]. Such stabilization of electrode surface largely contributes to the improved stability of catalysts, one of the most critical problems in practical applications. On the other hand, the strong electronegativity of heteroatoms can also restrict the electron delocalization in metals. The heteroatom doping on metals thus may render them semi-conductive or even insulating. Appropriate amount of doping has to be considered to ensure good conductivity of catalyst during the material design and synthesis. However, the effect of heteroatoms on OER process is rather difficult to define, as the main active center is metal cation. One key factor influencing the OER activity is the interaction between metal d-band center and p-band center of oxygen. The existence of heteroatoms with strong electronegativity in the vicinity of active metal site will prevent the coordination of hydroxide ligand (OH*) on the catalytic surface, resulting in the deactivation. Once the peroxide intermediates (OOH*) are formed, however, the heteroatoms can accelerate the dissociation of O2 molecule due to the 3p-2p repulsion between heteroatom and OOH*, which is a positive effect.

Oxygen Vacancies Oxygen vacancy engineering is a widely adapted tactic for oxide-based materials to promote their catalytic activity. It is usually achieved by harsh synthetic methods such as the reduction by strong reducing agent, plasma activation, and pyrolysis. The created oxygen vacancy can activate the neighboring atoms and adjust the density of state near to Fermi level. As a consequence, the electron transfer rate and intermediate replacement are accelerated. Additionally, two lone pair electrons from the defect site can be exited into the conduction band, contributing to the improved conductivity. In the mixed valence metal oxides, the appropriate control of metal ion ratio can modify their electronic properties and thus catalytic activities. Dai et al. designed an efficient Co3O4-based OER electrocatalyst by a one-step plasma engraving that generated abundant oxygen vacancies on the surface, forming more Co2+ than Co3+ species [28]. The created oxygen defects apparently contributed to the enhanced electronic conductivity, which is a great benefit for the electrocatalytic activities. The specific activity normalized by BET area can exclusively specify the role of oxygen vacancies. Compared to the untreated one, the plasma-engraved Co3O4 presented a more efficient OER catalyst, as manifested by a lower onset potential of 1.45 V and a much higher current density of 44.44 mA cm2 at 1.6 V vs. RHE.

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Fig. 7 OER elementary steps in (a) adsorbate evolution mechanism (AEM) and (b) lattice oxygenmediated OER mechanism (LOM) [29]. Lattice species and electrolyte ions are shown in red and blue, respectively. In PDOS diagrams, the left of energy axis is electrolyte species and crystal PDOS is on the right. (Copyright © 2016, Springer Nature)

In addition to the conductivity gain, the lattice oxygen vacancy is also considered to take part in the OER process. Stevenson’s group reported a series of cobaltite perovskites (La1-xSrxCoO3-δ) with the controllable Co-O bond covalency and concentration of oxygen vacancies [29]. A lattice oxygen-mediated OER mechanism (LOM) is hypothesized through density functional theory (DFT) modeling. In the traditional adsorbate evolution mechanism (AEM), the energy of metal 3d band is higher than that of O 2p in PDOS diagram. All oxygen intermediates are generated from the electrolyte and Co4+ can be produced by redox reactions. In contrast, according to LOM, a higher potential will not generate Co4+ but oxidizes a ligand hole in O 2p band, causing the lattice oxygen to yield superoxide ion O2. DFT results showed that the surface oxygen intermediates could offer a reaction pathway with the higher stability in LOM than AEM (Fig. 7).

Constructing an Interface The complexity of hybrid nanocomposites makes the investigation of their structureactivity relationship more challenging. Creating an interface between appropriate component materials not only affects the morphology of electrocatalysts but also creates the interactions that accelerate the charge transfer and electrochemical kinetics. The strong electronic interaction can reconstruct the active sites and enhance the activity towards the water splitting. The specific influences are concluded as the following aspects: First, the interface can accelerate the HER/OER kinetics by optimizing the chemisorption of reactant molecules and reaction

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Fig. 8 (a) Interface engineering of hybrid materials showing the possible effects caused by the interface under operating conditions. (b) LSV curves of FeOOH/LDH samples. Inset is the schematic depiction of the interfacial interaction via the formation of oxygen bridges between the FeOOH nanoparticles and Ni–Fe LDH. (Reprinted with permission from Ref. [33]. Copyright © 2018, American Chemical Society)

intermediates. Second, proper compositions can promote different elementary reaction steps of HER/OER process, namely, the synergistic effects [30, 31]. Recently, Gong et al. reported that a nanoscale NiO/Ni/CNT heterostructure exhibited an overpotential of 80 mV towards HER at the current density of 10 mA cm2, which is much better performance than that of individual Ni/CNT (200 mV) and NiO/ CNT (450 mV) catalysts [32]. On the NiO/Ni interface, the OH generated by the splitting of H2O was shown to preferentially attach to NiO site rather than Ni metal. This was caused by the strong electrostatic affinity and less filled d orbitals in Ni2+ species compared to Ni metal. Meanwhile, the nearby Ni site can facilitate the H adsorption and lead to the synergistic effect toward HER catalysis. In contrast, the single active sites were lacking the adsorbed H or OH species. Sun’s group synthesized a nanocomposite of FeOOH/N-Fe LDH where the FeOOH particles of average size ranging from 2 to 18 nm were anchored on the NiFe LDH surface (Fig. 8b) [33]. Various analytical techniques such as X-ray absorption near edge structure (XANES), direct current (DC) voltammetry, and large amplitude Fourier transformed alternating current (AC) voltammetry were engaged to prove the formation of interfacial oxygen bridge (Fe-ONi) between ultrafine FeOOH particles and NiFe LDH nanosheets. Unsaturated coordination of FeOOH was claimed to cause a stronger interfacial interaction. The Fe-O-Ni couples and highly oxidative Fe(3 + δ)+ species on the interface were shown to reform the local electronic structure and promote the oxidation of Ni2+ in NiFe LDH, thus enhancing its OER activity. Particularly, the composites with 2 nm FeOOH nanoparticles demonstrated an excellent performance with an overpotential of 174 mV at the current density of 10 mA cm2 in 1 M KOH.

Edge-Defect Engineering Edge defects in nanomaterials are recognized as another active center for electrocatalysis. For instance, it has been proved that exposing edge sites of graphene can

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greatly tune the chemical reactivity, because their unique electronic density of state (DOS) differs from that of in-plane sp2 carbon sites [34, 35]. Shu et al. explored the edge defects on MoSe2 catalyst for HER by first-principle calculations [17]. Compared to the pristine MoSe2 basal plane, the reconstructed Mo and Se zigzag edges modified the electronic structures of materials, leading to a decreased value of ΔGH*, which contributed to a higher HER activity. In their studies, the exchange current density that reflects the proton transfer rate was also calculated to compare the catalytic ability. The edge sites of MoSe2 surface enabled a higher exchange current density (1017 A/site) even compared with Pt surface. Similarly, a Co3O4 catalyst with abundant edge dislocations in the (011) facets displayed a great activity toward OER, only requiring 183 mV to achieve current density of 10 mA cm2 in 1 M KOH [36].

Strain Engineering According to the d-band theory, the compressive strain would cause a weaker binding while the tensile strain leads to a stronger binding. This conclusion is still ambiguous, since the surface reactivity with strain varies depending on the adsorbate species, catalytic surface sites, and the crystal orientation of the surface. To be specific, the surface-adsorbed species can induce a compression or tension in the adjacent atoms by either pushing them outward or pulling them inward. Once an external expansive or compressive strain is applied, the eigenstress can be relieved or coupled, causing the downshift of d-band under the Fermi level, thus tuning the binding energy [37]. Cheng et al. developed an OER/ORR bifunctional electrocatalyst based on the lattice-strained NiFe-metal organic frameworks (MOFs) [38]. The high-valance Ni4+ and superoxide OOH* were believed as both the critical active site and intermediate, exhibiting an overpotential of 300 mVat the current density of 2000 A gmetal1. Under the additional ultraviolet-light radiation on pristine material, the unreactive π-conjugated ligand molecules were selectively dissociated, and NiFe-MOFs with different lattice expansion ratios were fabricated. According to the X-ray absorption fine structure (XAFS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) results, both the electronic and atomic configurations of Ni nodes were redistributed. First, partial electrons were transferred from Ni to the adjacent O and a stronger Ni-O bond was formed after lattice swelling due to the Ni 3d-O 2p hybridization. Second, the valence-band maxima decreased, and the Fermi level shifted negatively towards Ni 3d bands. Third, the delocalization of Ni 3d orbitals lowered the O2 adsorption energy and formation energy barrier of OOH* species. As a consequence, the intermediates (O* radical) formation on Ni atoms were much facilitated, as well as the kinetics of O-O coupling for enhanced OER activity. Li and co-workers synthesized highly efficient Ag particles for HER catalysis in acidic electrolyte, which displayed a low overpotential of 32 mV at 10 mA cm2 in 0.5 M H2SO4 [39]. Using the laser ablation in liquid, a great amount of stacking faults and surface steps were generated in Ag nanoparticles. Unsaturated

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coordination of Ag surface and high tensile strain due to the stacking faults were proved by simulation and geometric phase analysis. Based on the computational calculations, both the strain and low coordination number were responsible for adjusting the adsorption energy (ΔGH*) and the promotion of the HER performance.

Compositing with Conductive Substrates The aim of fabricating a composite with conductive substrates is obviously to improve the conductivity of the whole system, as well as to accelerate the transfer rate of protons and electrons. It has some similarities to the interfacing engineering. By simply changing the supports, the electronic properties and catalytic activity of surface catalysts can be adjusted. The synthesis of a composite is usually achieved by in situ growth to minimize the aggregation of nanomaterials, thereby exposing the maximum number of active sites. The suitable supports should possess the features of excellent conductivity, high specific surface area, mechanical strength, and stability. They can be either carbon-based materials such as carbon nanotubes and graphene or metallic current collectors such as nickel/copper foams. Recently, a novel three-dimensional nickel foams/porous carbon/anodized nickel electrode was fabricated by a research group led by Zhang, in which the Ni foam framework and a ZIF-8-derived porous carbon membrane served as the conductive support and interlayer, respectively [40]. The conductive carbon membrane was able to protect the unstable inner nickel foam and support the outer oxygen-evolving nickel electrocatalyst. This configuration endowed the electrode with an excellent OER activity and high stability. The significant improvement in their OER performance was attributed to the high activity of anodized nickel skeleton, the interconnected porous conductive network, and the enhanced stability of the electrode. Chen et al. reported an efficient and stable HER electrocatalysts composed of Ru/Co bimetallic nanoalloys encapsulated in N-doped graphene [41]. This composite catalyst showed a remarkable activity toward HER under alkaline conditions with an overpotential of only 28 mV at 10 mA cm2, which was stable for over 10,000 cycles. The DFT calculations indicated that the C atom next to the doped N atom was the active site for H adsorption, pointing out that the N doping decreased ΔGH*. Encapsulated Ru/Co nanoalloy core, on the other hand, facilitated the electrons transfer to graphene shell and strengthened the C-H bond, which also significantly reduced ΔGH*.

Micromorphology and Porous Structure High electrochemical surface area and porous microstructure are crucial for enhancing the electrocatalytic performance. These two factors are considered and put into the design and fabrication of nanomaterials as well as nanocomposites. The control of morphology is a direct way to provide the catalysts with high activity, including specifically exposed facets for the desired reaction, more active redox sites, a large area for full contact with the electrolyte, as well as a short pathway for electron

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transfer. To date, dozens of morphologies of electrocatalysts have been designed and prepared by various approaches. Based on the morphologic dimension, the nanomaterials can be categorized as zero-, one-, two-, and three-dimensional, such as nanoparticles, nanofibers, nanosheets, and nanocages. By rationally assembling a simple unit structures, a complex and sophisticated structure, such as an aligned morphology or self-supported hierarchical structure, can be constructed. For example, Gao et al. reported the self-assembled hierarchical NiCo2O4 hollow microcuboids built by 1D nanowires subunits, as bifunctional electrocatalysts for both HER and OER reactions [42]. This hierarchical-structured electrode showed an excellent activity toward overall water splitting with the current density reaching up to 10 m Acm2 by applying just 1.65 V across the two electrodes. The three-dimensional (3D) hierarchical hollow structure holds a great promise in offering enough space to facilitate the penetration of electrolyte into active sites and the subsequent electrochemical reactions, enabling a facile release of gas bubbles to improve the reaction interface. An interconnected electrolyte-filled porous network with high accessible surface area and large pore volumes can enable the sufficient contact with the reactant, facilitate diffusion of products, and realize the fast charge transport behavior, making the maximum number of active sites participate in the electrocatalytic reaction. Recently, the single atomic Co on ordered porous N-doped carbon was prepared by Sun and co-workers [43]. Excellent bifunctional performance was reported for oxygen reduction reaction (ORR) and HER, which can be attributed to both isolated Co-N4 moiety and 3D porous ordered carbon architecture. The superior HER performance were mostly arising from the single Co-N4 sites which provided a favorable hydrogen adsorption-desorption energy barrier (ΔGH* ¼ 0.08 eV). The unique N-doped carbon structure with ordered macropores and mesopores also played a critical role, not only serving as the framework for stabilizing the isolated Co-N4 site but also providing the electrochemical surface area.

State-of-the-Art HER Electrocatalysts Noble Metal-Based Nanomaterials So far, variety of precious metal-based catalysts, such as Pt, Pd, Ru, and Rh, have been utilized for hydrogen evolution reaction. Among them, Pt has long been recognized to own the ideal ΔGH* with the excellent performances, both in acidic and neutral media (see volcano plot in Fig. 1). Its activity in base, however, is rather poor, due to the strong M-OHad interaction and large energy barrier of H2O dissociation [15]. Not to mention the high cost and scarcity, there are other critical challenges for using Pt-based catalysts for water splitting. How can we manipulate the nanostructure so as to expose more active centers? How can we decrease the loading amount of Pt in the alloy or composite with secondary metals (mostly 3d transition metals)? How do we increase the intrinsic site-specific activity of noble metal? [44]. In this section, recent progress in the research on noble metal-based nanomaterials for HER will be discussed with the strategies used to enhance their specific activity.

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The control of facet and morphology of Pt nanocrystals is one of the most effective approaches to tune its electrocatalytic properties. Strmcnik et al. summarized the different HER activity of single crystal Pt facets (hkl) in alkaline media [45]. They showed that the degree of activity follows the order of Pt(110) > Pt (100) > Pt(111) and thus high density of surface defects on Pt(110) can offer a large amount of active sites for water dissociation. To minimize the use of costly noble metals, Esposito and Chen deposited a monolayer of Pt on tungsten carbide surface (ML Pt-WC) [46]. The Pt monolayer on WC showed similar chemical and electronic properties to bulk Pt for HER, displaying almost same catalytic activity to Pt foil in 0.5 M H2SO4. In their DFT calculations, ML Pt-WC was proven to bind to the reaction intermediate species, Had, with a moderate strength (10.1 kcal/mol), nearly identical to that of Pt(111) (10.8 kcal/mol). This finding and strategy can be further developed to a 3D coreshell structure with high surface area to generate a higher current density. Xiong et al. prepared a PtFeCo trimetallic nanostructure for catalytic HER application [47]. The enhanced HER activity of this trimetallic nanostructure was attributed to two aspects. First, the Co integrated into PtFe alloy can adjust the charge density and electronic structures. PDOS of Pt d orbitals shifted as the Co content increased, thus optimizing the energy barrier of H adsorption and bond-breaking at the best Co ratio. Second, the low-coordination stepped atom on the edges and the (422) facet on the top/bottom surface provided plenty of active sites during the HER process. This is a good example of the combined advantages of surface and composition engineering, where the PtFeCo alloy displayed super-high current density of 1325 mA cm2 at 0.4 V (Fig. 9).

Transition Metal-Based Materials The most famous transition metal-based electrocatalyst for HER is molybdenum disulfide (MoS2). Although the bulk form of MoS2 displays very poor catalytic activity, its nanostructure has been demonstrated to be a great candidate for HER catalyst. The edge sites of MoS2 were found to be reactive and the theoretical ΔGH* (+0.08 eV) on the metallic edges is close to that of Pt. Chorkendorff et al. showed that the electrocatalytic activity of MoS2 is linearly correlated with the number of edge sites, not the basal plane sites [48]. However, DFT calculations suggested that there is only 1/4 H-coverage on the edge, compared to ~1 monolayer H-coverage on Pt(111). It was suggested that tuning the electronic structure and bond strength of Had of MoS2 edge sites is the key for future development of MoS2 for HER application. Following this work, plenty of strategies have been applied to move the position of MoS2 close to the vertex on the volcano plot, including the exfoliation of layered MoS2, the modification of edge sites by bimetallic sulfides, and increasing the conductivity by forming nanocomposites. For example, Cui et al. reported the synthesis of vertically aligned MoS2 and MoSe2 layers, maximizing the exposure of the edge sites [49]. The exchange current density was dramatically increased due to the high density of exposed edge sites. Zhang and colleagues improved the sluggish

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Fig. 9 (a) HER/HOR behavior, atomic structural model, and STM images of Pt(hkl) surfaces ([45]. Copyright © 2016 Elsevier Ltd.) (b) Illustration of planar ML-Pt WC and core-shell Pt WC catalysts. (Reproduced from Ref. [46] with permission from The Royal Society of Chemistry)

HER kinetics by doping Ni atoms onto the lattice of MoS2 nanosheets. It turned out that the introduced Ni sites were able to reduce the energy barrier of water dissociation and desorption of OH in alkaline media [50]. Selenium (Se), just below sulfur in group IVA of the periodic table, also endows their metal selenides similar properties to metal sulfides, such as similar valance state and same outermost shell electron number (n ¼ 6) [51]. Compared with S, however, Se has more metallic properties, and thus better conductivity for its compounds. The HER activity of metal selenides also has been intensively investigated. Chen et al. converted CoMoO4 nanowires to the hierarchical MoSe2-CoSe2 nanotubes by the selenization under hydrothermal conditions. The CoSe2 nanoparticles and fewlayered MoSe2 nanosheet exhibited superior and stable HER performance both in acid and base [52]. Other transition metal compounds including nitrides and phosphides have also received vast attention for HER activity in a wide pH range due to their stable and conductive natures. For example, hundreds of transition metal nitrides have been so far identified to be active HER catalysts, some of which are comparable with Pt. The N atoms are reported to modify the d-band of metals in metal nitrides, making their catalytic activity close to that of noble metal. Sun et al. reported the extraordinary HER activity of Ni3N/Ni electrocatalyst which approached nearly 0 mV onset potential and 12/19 mV overpotentials in alkaline/neutral media. Using DFT

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calculations, they revealed that such an exceptional activity originated from the optimal ΔGH*, which is very close to the ideal value (0.01 eV). Additionally, two active phases, Ni3N and Ni, were found to favor the adsorption step and H2O dissociation step, respectively, demonstrating the importance of creating the ideal interface between nitrides and Ni metal for HER process [53].

Metal-Free Materials Metal-free materials such as pure carbon-based materials (graphene, carbon nanotubes, etc.) are usually intrinsic HER-inert materials. Although some literatures claimed that so-called pristine carbon catalysts also own electrocatalytic HER activity, it is more likely due to the metal impurities from solutions or materials themselves. Generally, carbon materials are engaged as the conductive supports to couple with other active components. In this case, the outer carbon layer can also offer active sites and exert the synergetic effects. Besides, the modification of pure carbon materials, such as heteroatom doping, will endow them special electronic properties and conductivity. Carbon nitride (C3N4) has been studied in recent years as another metal-free HER candidate. The C3N4/N-doped graphene hybrid HER catalyst was prepared by annealing graphene oxide and dicyandiamide [54], which showed a comparable performance close to metallic catalysts, although not as good as Pt. This well-designed metal-free hybrid definitely opened a new direction to find alternatives for the noble metal-based HER catalysts.

State-of-the-Art OER Electrocatalysts Precious Metals and Their Oxides Precious metals have been studied for the OER catalysis since the early days of OER research. The OER activity sequence in acid follows Ru < Ir < Pd < Rh < Pt. These pure metals tend to form an oxidized surface under high anodic potentials. Among them, rutile-type RuO2 is the most active in both acidic and alkaline media. The overpotentials of 200 mV in acid and 300 mV in base are required to reach the reference current density of 10 mA cm2. According to DFT calculations and experimental evidences, the activity of RuO2 is greatly affected by physiochemical properties such as local electronic structure, microstructure, and crystallinity [8]. Iridium oxide (IrO2) and their alloys/mixed oxides are also regarded as promising candidates for water oxidation. To reduce the use of Ir, one of the precious metals, Guo’s group synthesized IrCoNi hollow nanocrystals and applied them as an OER catalyst in acidic electrolyte [55]. The IrCoNi hollow nanocrystals showed the current density of 10 mA cm2 at an overpotential of 303 mV in 0.1 M HClO4, which is ten times better than commercial Ir/C (1 mA cm2). DFT calculations showed that alloying Ir with 3d transition metals shifted the PDOS to the left, indicating that the d-band electrons

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moved far away from Fermi level, and thus forming a weaker bond with OER intermediates. This implies the adsorption of oxygen species was weakened compared to that of pure Ir, leading to a smaller energy barrier for OER.

Transition Metal Oxides Due to the remarkable activity and durability comparable to RuO2 and IrO2, transition metal oxides (TMO) have been one of the most widely studied electrocatalysts for OER in alkaline solutions in the past half a century. In the early studies, the first class of TMO electrocatalysts, for instances Ni-, Co-, Fe-, Mn-based or their mixed oxides, were in bulk forms or electrochemically deposited films. With the development of nanotechnology, the controllable synthetic technique for nanomaterial has brought TMO back to the front line of OER research [5]. Their activity is mainly controlled by M-OH bond strength according to the work of Rüetschi and Delahay [56, 57]. The weaker the bond is, the higher the activity is. Too strong M-OH will result in the poison effect, like Mn-OH. The trend of OER activity is NiOx > CoOx > FeOx > MnOx. Compared to other mixed oxides, NiFeOx and CoFeOx have been regarded as the best catalysts on the volcano plot. Spinel structure Co3O4 that consists of Co2+ at tetrahedral sites and Co3+ at octahedral sites is a good example. Under high oxidation potentials, cobalt oxide surface will be firstly oxidized to a high valance state CoOOH, similar to other first-row TMOs. The anodic peak of CoO2/CoOOH redox couple occurs before water oxidation starts. Therefore, the Co4+ is widely accepted as an essential species for OER [58]. Recently, perovskite oxides (POs) with A1-xA’xByB’1-yOz structure, where A/A’ is rare-earth or alkaline-earth metal and B/B0 is transition metal, have emerged as a potential OER catalysts. The filling degree of metal 3d antibonding orbital (eg) in POs adjusts the spin state, the M-O bond covalence, and the shift of d-band center, further affecting the OER kinetics [59]. For example, Ba0.5Sr0.5Co0.8Fe0.2O3-δ POs with an optimum eg value (1.2) has become one of the excellent OER electrocatalysts superior to IrO2 and RuO2.

Layered Double Hydroxides (LDHs) In alkaline and neutral media, the LDHs of first-row transition metals (Ni, Co, Cu, Fe, Mn, Zn, Ga, etc.) that possess the hydrotalcite-like structure display excellent activities towards water oxidation and have been widely investigated for decades. Huang et al. fabricated a 2D atomically thin γ-CoOOH nanosheet (1.4 nm) as an OER electrocatalyst that displayed a good mass current density of ca. 67 A g1 at low overpotential of 300 mV [60]. The γ-CoOOH layer owns half-metallic properties due to the tuning of electronic structure, exhibiting 52 times better conductivity than its bulk counterpart. Proved by first-principle calculations and characterizations, the unsaturated coordinated CoO6-x octahedrons on the surface formed a σ-bonding eg orbital overlapped with the adsorbates, which contributed to the interfacial electron transfer between Co and OOH* intermediates. Moreover, the hole states in the t2g orbitals enhanced the electrophilicity of O* and water molecules.

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Among various transition metal LDHs, the NiFe bimetallic hydroxide has been recognized as a potential candidate comparable or even better than metal oxidebased OER electrocatalysts [61–63]. With the compound formula expressed as  2þ 3þ xþ Ni1x Fex ðOHÞ2 ðAn Þx=n ∙yH2 O , the incorporation of Fe3+ into the Ni(OH)2 layers makes the layers positively charged, which is compensated by intercalated anions between the interlayers, such as CO32, H2O, and F. The arrangement and amount of Fe3+ ions greatly affect the kind of hydroxide sites which can act as OER active centers. The optimization of incorporating Fe content is difficult because the Fe addition to form inactive Fe oxyhydroxides phases varies among different synthetic methods. However, it is commonly accepted the best Fe content ranges from 10 to 50%, while pure Fe or Ni (oxy)hydroxides exhibit much worse OER performance due to the poor conductivity. Boettcher’s group investigated the electronic properties, structure, and OER performance of Ni1–xFex(OH)2/Ni1–xFexOOH as a function of Fe incorporation level [64]. As expected, pure NiOOH showed very poor OER activity t regardless of the degree of order, anodic polarization, or aging. It is somewhat contradictory to the previous reports which concluded that the Fe impurities in electrolyte can highly enhance the OER activity. It turned out that the inclusion of Fe into NiOOH can increase >30-fold conductivity, but it is not the only factor for the activity enhancement. The effect of Fe on the electronic structure by a partial charge transfer and activation of Ni centers seems to play a more important role (Fig. 10). Besides, the intercalation of anions and a third metal incorporation can also influence the OER performance. Recent researches showed that the OER activity can be correlated with the pKa of conjugate acid of the intercalated anions and interlayer distance for OH- exchange in alkaline medium [65, 66].

Metal-Organic Frameworks (MOFs)-Based Materials In recent years, metal-organic frameworks (MOFs) have attracted growing attention in water electrolysis field. Their unique structure of repeated metal-ligand units bridges the gap between homogenous and heterogenous catalysts. The high porosity and rigid open frameworks provide enough pathways for electrolyte penetration and charge/mass transfer. It is also advantageous to have the tunable ordered structure with metal nodes and ligands, offering more flexibility in catalyst design [67]. In particular, the ultrathin 2D MOFs have become an area of research focus due to the unsaturated-coordinated surface sites and rapid electron transfer of the ultrathin films. For example, Zhao and his colleagues [68] reported an ultrathin NiCo bimetallic nanosheets (thickness ¼ 3 nm) by a simple hydrothermal synthesis followed by ultrasonication. This electrocatalyst deposited on a glassy carbon electrode only required an operating overpotential of 250 mV for water oxidation. The excellent catalytic performance was attributed to the coordinatively unsaturated metal Ni centers, which interacted with intermediates more actively than bulk MOFs. Besides, there is the electron transfer from Ni to Co through bridging O2 since Co2+(d7) sites owns more unfilled eg orbitals compared to Ni2+(d8), which also benefits the high performance.

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Fig. 10 (a) SEM image of Ni1-xFexOOH LDH; (b) crystal structures of NiFeLDH viewed roughly along the [110] direction; And (c) CV scans of Ni1–xFex(OH)2/ Ni1–xFexOOH films deposited on interdigitated array (IDA) microelectrodes ([64]. Copyright © 2014, American Chemical Society)

The poor conductivity of most MOFs inspired researchers to develop the MOFs/ conductive support composites (e.g., MXene and graphene) and MOF-derived carbon-based nanomaterials. The ordered open frameworks with abundant metal/ organic species can provide promising self-sacrificing templates and precursors to produce various derivates, such as metal/metal oxides/metal nitrides decorated carbons and doped porous carbon. These MOF-based derivates are intrinsically similar to traditional heterogenous metal-based catalysts. However, with tunable morphologies and compositions of original templates, as well as the pyrolysis conditions, they become a kind of newly emerging nanomaterials for highly efficient electrocatalysis of water oxidation [69].

Other Metal-Based Materials Other types of metal-based compounds, including carbonate hydroxides, carbides, nitrides, phosphides, sulfides, and borates, have also been studied for the OER

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Table 2 Summary of some state-of-the-art nonmetal oxide OER electrocatalysts Catalyst N-doped WC Fe3C@NG800–0.2 Co4N/CC FeNi3N/Ni NiCoP/NF Fe/CoP/Ti O-NiFeS Fe0.1NiS2 NA/Ti Fe-Ni3S2/FeNi Co9S8/NOSC-900 MoS2/Ni3S2 NiCoSe0.85/NiCo LDH NixFe1-xSe2/DO EG/CoSe-NiFe-LDH

Electrolyte 0.5 M H2SO4 0.1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH

η10 (mV) ~245 361 257 202 280 310 259 (η20) ~205 282 340 218 216 195 250

Tafel slope (mV dec1) / 62 44 40 87 67 / 43 54 68 88 77 28 57

Reference [72] [73] [74] [75] [76] [77] [78] [79] [71] [80] [81] [82] [83] [84]

catalysis because of their unique properties. Some state-of-the-art nonmetal oxide electrocatalysts in past few years are summarized in Table 2. Xie et al. reported the metallic Ni3C nanoparticles/carbon composites for water oxidation catalysis. The metallic character of Ni3C and supportive carbon facilitated the fast electron transfer inside and charge transfer outside. This dual electric behavior regulation boosted the electrochemical water oxidation. By post-reaction characterizations such as XAFS and HRTEM, the NiOx/Ni3C/C heterogeneous structure was demonstrated to be real active species [70]. Xu et al. first synthesized Fe-Ni sulfide (Fe-Ni3S2) on FeNi foil and revealed that the high OER activity was closely related with the Fe doping. The incorporation of Fe resulted in an increased density of state (DOS) in large energy range and more metallic states, which facilitated the electron transport and OER kinetics. The Fe-Ni3S2 electrode displayed a low overpotential of 282 mV at 10 mA cm2 and a small Tafel slope of 54 mV dec1 in strong alkaline solution [71]. These compounds are actually thermodynamically unstable under high potentials and oxidizing conditions. Compositional stability of these types of materials is still a concern even though they do not display apparent activity decay after long-term test. It is commonly believed that the surface of catalysts is oxidized to the corresponding oxides/hydroxides and form core-shell-type structures. Conducting an operando characterization such as in situ Raman spectroscopy or in situ IR spectroscopy can provide a deeper insights to the catalytic process and true active centers in OER researches.

Metal-Free Materials To reduce the cost of metal and increase the activity per mass, low atomic mass metal-free electrocatalysts have started to gain wide interest. Since the oxygen

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molecules are mostly generated on the metal species, the direct application of metalfree nanomaterials in OER is relatively difficult, compared to the opposite reaction, oxygen reduction reaction (ORR). Metal-free materials have been thus usually employed as the substrates coupled with other phases. Recent researches, however, discovered that the active centers and activity origins in OER and ORR are similar, leading to the rapid development of metal-free OER catalysts, for example, heteroatom-doped carbon nanotubes (CNTs), graphene, and C3N4 [85]. Dual-element-doped carbon nanomaterials have been demonstrated to be more efficient bifunctional catalysts than noble metals and applied to catalyze the OER/ ORR in fuel cells and metal-air batteries [86]. Guo’s group demonstrated the bifunctionality of N,P co-doped graphene framework (PNGF) on OER/ORR by computational design and experimental evidences. They showed that the active P-N bonds were responsible for high OER performance with an operating potential of 1.55 V at 10 mAcm2. Large-sized P atom can only be doped at the edge sites rather than the graphitic surface of carbon. The oxidized pyridine-like P site was shown to stabilize the graphitic N and activate the neighboring C atom. Hence, the electronic structure and the binding energy of C atom were modified. The N,P co-doped graphene showed nearly ideal bond strength for the relative intermediates and the rate-determining step in the OER reaction [87].

Conclusion and Further Outlook To date, plenty of nanomaterials and nanocomposites have shown remarkable performances for electrochemical water splitting. Enormous efforts have been devoted on the rational design and optimization of non-precious electrocatalysts, such as non-noble metal-based materials and metal-free materials. In this chapter, we first introduced the fundamentals on HER/OER processes, then discussed the factors affecting the material activities towards water splitting, with highlights on the recent research progress of electrocatalysts. Here, three important advices in the research of electrochemical water splitting are: (1) Standardized testing protocols and benchmarks are needed to be established in order to minimize the experimental errors and correctly estimate the properties of electrocatalysts. (2) The mechanism of HER/ OER processes from both thermodynamic and kinetic aspects, as well as the reaction descriptor and d-band theory have to be understood in-depth to figure out the relationship between intermediates and catalytic surface at the molecular level. (3) The electronic/atomic/structural effects on catalyst activity, such as size, defect, composition, and interface, should be comprehended to reveal the structure-functionality principle that provides rational guidance for catalyst design. Over the last decade, we have seen splendid improvements in water-splitting performances of well-designed electrocatalysts. Particular spotlight have shined on the low-cost electrocatalysts of which performance even surpass that of precious metals. However, the stability of the electrocatalyst remains as a big challenge to meet the industrial requirement. There are also other important questions out there waiting for the answers: How to construct the catalytic centers? Will the active

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centers go through structural evolution during the catalytic process? To settle these problems, more in situ techniques should be involved during the characterizations to evaluate the catalyst properties under real reaction conditions and analyze the reaction mechanism. DFT theory calculations should be also combined with experimental evidences to turn the current “material fabrication-blind test” to more rational and predictable designs.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eco-friendly Lead-free Piezoelectric Oxides and Their Composites with Polymers . . . . . . . . . Electronic Circuits for Interfacing the Piezoelectric Harvesters and the Load . . . . . . . . . . . . . . . . Conclusion and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter provides up-to-date information about the piezoelectric converters of mechanical to electrical energy, involving lead-free materials as key functional films. Oxide and polymer-oxide hybrid and composite-based harvesters are reviewed. The main mechanisms, parameters, and characteristics of variety designs are compared and discussed from an engineering point of view regarding application in real devices requiring network independent or battery-less power supply. Flexible and wearable harvesting elements, with their specifics in terms of stability and reliability, are included in the review. Nanostructuring approaches for enhancement of piezoelectric properties are one of the important sections, considering their cost-effectiveness and technological compatibility with the conventional fabrication technologies. Finally, the circuits making the converted signals useful for low-power consumers are taken into account as a final stage of the harvesting system design and completeness. Keywords

Lead-free piezoelectric materials · Vibrational harvesters design · Nanostructured piezoelectric generators · Flexible and wearable electronics M. Aleksandrova (*) Department of Microelectronics, Technical University of Sofia, Sofia, Bulgaria e-mail: m_aleksandrova@tu-sofia.bg © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_109

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Introduction Piezoelectric energy harvesters convert mechanical deformations and vibrations into an electric charge due to the piezoelectric effect. Different constructions and materials have been studied during the last few years, with the largest yield of electric charge being ensured by the so-called beams (cantilevers) realized with piezoelectric ceramics. The volume of the piezoelectric crystal was relatively large (on the order of a few cubic centimeters), and therefore the electrical energy was relatively high – on the order of a few hundred milliwatts at a load of only 30–50 g. However, modern miniaturization trends require these materials and constructions to be integrated in other miniature devices and to become part of a monolithic, compact micro- or nanosystem, being in the form of nanostructures. Technically, it is not difficult to obtain piezoelectric coatings of nanoscale thickness, but in this case the distance between the charges in the dipole cannot be changed significantly, so the piezoelectric response of the elements is too weak. These problems still require efficient solutions. For this reason, such types of elements continue to be subject to intensive research. So far, there is insufficient experimental data showing the influence of substrate type, deposition technology, and films topology on the efficiency and robustness at deformation in a real work environment. One of the possible approaches to enhance their performance is to produce hybrid structures, combining piezoelectric oxides with piezoelectric polymers, to study the possibility of using template-assisted growth of piezoelectric nanowires in nanoporous oxides, or to grow a highly revealed surface of the film. This chapter summarizes the most recent achievements in the: • eco-friendly lead-free piezoelectric oxides and their composites with polymers; • design of devices for efficient energy conversion (beam, membrane, and concentrators approaches); • nanostructuring as a tool for increasing the yield; • flexible piezoelectric micro- and nanogenerators and their integration in the wearable electronics, requiring such kind of power supply. • electronic circuits that further process the signals generated from the piezoelectric harvester.

Eco-friendly Lead-free Piezoelectric Oxides and Their Composites with Polymers The efficiency of piezoelectric materials can be evaluated by their piezoelectric coefficients, giving the relation between the applied stress and the electrical charge generation ability. These coefficients are directionally dependent according to the applied stress axis and the measured plane electrical charge. Piezoelectric coefficients are directly related to the output power, which is a crucial factor for using the piezoelectric thin films for energy harvesting. Typically, the piezoelectric coefficients are high for lead-containing materials, such as lead zirconium titanate (PZT),

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which is one of the most studied for different electronic device applications such as pressure sensors and energy harvesting [14]. However, due to the recent worldwide demand for environmentally friendly technologies and materials, PZT must be replaced by a suitable alternative with competitive piezoelectric properties. It was found that the following are compatible with the microfabrication technology and nanostructuring approaches: zinc oxide (ZnO) [9], barium strontium titanate (BaSrTiO3 or BST) [6, 28], potassium niobate (KNbO3) [19, 26], some organic materials (polymers PVDF and PVDF-TrFE) [9], as well as organic/inorganic hybrids or composites [23]. Interestingly, ultrathin ZnO films grown by Atomic Layer Deposition (ALD), with thickness ranging from 50–90 nm, also exhibit piezoelectric behavior, although the dipole dimensions are very small in terms of opposite charges distance. However, they can still be kept in polarized condition [8, 29]. The films were grown on solid (silicon) and flexible (polyethylene naphtalate PEN) substrates at three different temperatures, affecting the crystallinity of the resulting ALD films. X-ray diffraction study revealed hexagonal ZnO with wurtzite-type polycrystalline structure (Fig. 1a, b). Depending on the electrode types (aluminum or gold), the generated peak piezoelectric voltage from the ALD ZnO thin films varied in the range 5–15 mV. The

Fig. 1 X-ray diffraction patterns for ZnO films on (a) p-Si(100), (b) PEN substrates at different growth temperature [8]. Piezoelectric response of (c) PEN/PEDOT:PSS/Au/ZnO/Al and (d) PEN/ Al/ZnO/Al devices at high temperature growth of ZnO [8]

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smoothest signals with relatively small distortions and good periodicity were achieved for ZnO grown at high temperatures (Fig. 1c, d). Produced flexible piezoelectric generator samples were found that work optimally in the low frequency range of 20–50 Hz. Greater capacity and lower dielectric loss reported, in comparison with studies of ZnO nanostructures (nanowires, nanorods, etc.) can be ascribed to the better contact with the well-arranged ALD film. Mechanical enhancement of the piezoelectric devices, especially thin film ones, is important in terms of reliability at multiple bending cycles. Therefore, bacterial cellulose/barium titanate composite structures are the preferred solution [31]: in addition to their flexibility, the devices based on this composition benefit from biocompatibility at a reasonable price. The fabrication procedure is simple and consists in adding nanoparticles with different concentrations in aqueous suspensions with nanocellulose, and then depositing them by the casting-evaporation method to produce 30 μm coating (Fig. 2 up). Increase of the BaTiO3 in the composition led to dielectric constant increase and dielectric losses decrease, making the coating suitable for implementation also in the charge storage element. Cyclic application and removal of pressure leads to the generation of a signal with a sign (positive or negative) that follows the direction of the pressure

Fig. 2 Fabrication procedure of nanocellulose/BaTO3 composite coating (up) and time-dependent voltage generated from device with different BaTiO3 content, charging capacitors with different values (down) [31]. (Reprinted from H. Yeol, et al. Compos.Part B: Eng., Vol. 168, pp. 58–65, Copyright (2019), with permission from Elsevier)

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change. Optimum electrical characteristics at the output were achieved at content of 40 wt% BaTiO3, where the highest electrical parameters were 2.86 V, 262.35 nA, and respectively resultant power of 378.24 nW at compression of 5 kPa. The piezoelectric response of BaTiO3/nanocellulose composite films were tested by charging microcapacitors after rectification that was successful (Fig. 2 down). Based on the idea that the composite materials combine the advantages of the involved components, the researchers have developed BaTiO3/Polydimethylsiloxane (BT/PDMS) composite films for energy-harvesting devices [25]. In this case the piezoelectric response was found to be strongly dependent on the mass weight ratio of the oxide related to the polymeric matrix. To demonstrate the performance of the composite films, a low-frequency cantilever element has been designed and flexible polyethylene terephthalate (PET) substrate was used with indium-tin oxide (ITO) and copper (Cu) as bottom and top electrodes, respectively (Fig. 3a). The composite films have been prepared by casting of a solution consisting of liquid PDMS and dispersed BT nanoparticles with concentration varying between 5% and 30%. The element was activated by low-frequency

Fig. 3 Mechanical harvester with BT/PDMS film (a) cantilever design for element testing, dependence of the generated voltage on the BT:PDMS ratio, (c) contribution of piezo- and triboelectric effects for the electric output, (d) photo of the prepared eco-friendly flexible nanogenerator [25]. (Reprinted with permission from Suo et al. ACS Appl. Mater. Interfaces, 8, 50, 34335–34341 Copyright (2016) American Chemical Society)

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Fig. 4 (a) flexible fiber-based energy harvester PVDF/BT/GrNSs; (b) SEM image of PVDF fiber; SEM image of PVDF fiber with BT nanoparticles and GrNSs [21]. (Reprinted from K. Shia et al. Nano Energy, Volume 52, 153–162, Copyright (2018), with permission from Elsevier)

vibration (20 Hz). As can be seen in Fig. 3b, the highest produced voltage with an average magnitude of ~10 V was obtained for 20% ratio of the components, classified as optimal. Higher content of nanoparticles suppresses the piezoelectric response as a result of different polarization due to agglomeration of nanoparticles. An additional benefit of using this composite is the presence of the triboelectric effect, which proved to cause extra charges, respectively extra voltage of ~2.2 V, which increased the overall voltage output (Fig. 3c). The ability to flex the fabricated element is shown in Fig. 3d. Performance of the BaTiO3 nanoparticles can be enhanced in combination with graphene nanosheets (Gr NSs) and PVDF nanofibers (Fig. 4a) [21]. It has been found that both types of nanoparticles contribute to enhancement of the electrical output of the device, and the polymeric fibers with diameters ~160 nm (Fig. 4b) and length in the micrometers range is responsible for their capsulation in the inner open part of the fiber (Fig. 4c), making the entire nanostructure flexible and durable for long-term cyclic bending. PVDF solution was prepared for electrospinning. Uniform dispersion of both nanoparticles was achieved by ultrasound treatment. This is also the approach for inserting the nanoparticles in the nanofibers along their length. Тhe ratio between BT nanoparticles and graphene nanosheets was 15 wt%, but related to the third component it ranged from 0.05–0.25%. Membrane from the fibers was adhesive-taped to aluminum-coated plastic substrate in order to stick to the nanogenerator functional element. To find the optimum nanogenerator performance, a few sets of experiments were conducted and the resulting piezoelectric voltages were measured as a function of the nanoparticles content into the PVDF fibers (Fig. 5a), as a function of the bending radius (Fig. 5b), and as a function of the press-release frequency (Fig. 5c). From the first type of relation it was found that 0.15% composite nanoparticles embedded in the fiber result in the greatest measured voltage on average 12 V (~20 V peak-topeak). In the range of strain causing bending from 4 mm to 6 mm, the voltage remained almost constant, at an average of 10 V. In terms of cyclic bending, the optimum was found at frequency of 2 Hz. It was reported for long-term stability of the voltage, which didn’t exhibit significant deviations after 1800 cycles of pressrelease.

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Fig. 5 Basic relations affecting the performance and produced voltage of the fabricated energy harvesting element (a) dependence on the nanoparticles concentration; dependence on the bending radius; (c) dependence on the frequency of bending [21]. (Reprinted from K. Shia et al. Nano Energy, Volume 52, 153–162, Copyright (2018), with permission from Elsevier)

A similar approach, but with a different combination of materials, has been reported in which BaTiO3 is mixed with PVDF, and silver nanowires are dispersed in this solution [11]. Barium titanate particles were produced as a result of an ecofriendly and low-cost chemical reaction at temperatures higher than 1000 °C. For the composite, PVDF solution was prepared first, and barium titanate particles were later added to the solution, varying the concentration in the range 0–7.5 wt%. Additionally, silver nanowires (Ag-NWs) were added to the homogenous solution (Fig. 6a). Relatively thick films, greater than 100 μm, were produced by a molding process and sandwiched between aluminum electrodes for piezoelectric generator fabrication. Figure 6b shows BTO/PVDF composite films at a different concentration of BTO at pressure 3 N and frequency of 5 Hz. PVDF-based generator without any additives exhibited the lowest open circuit voltage of 2 V. The introduction of BTO resulted in an increase of the potential owing to the specific asymmetry of the oxide constituent, which resulted in higher voltage than the sample with pristine PVDF, namely ~4.4 V to ~11 V, according to the components ratio. The electrical output is further optimized after inserting Ag-NWs, causing better distribution of the BTO and the best piezoelectric response of 14 V at the same pressure and frequency (Fig. 6c). Thus, the optimum performance was found to be at 7.5 wt% BTO and 7.5 vol % of Ag-NWs. To demonstrate the practical applicability of the produced device, it was attached to the human body and activated from hand-folding (Fig. 6d). The measurements confirm that no voltage is generated when there is no folding, and that the short pulses gradually increased their voltage values depending on the degree of folding. In addition to the preparation of composites, doping is another suitable method for gaining piezoelectric voltage. If, for example, KNaNbO3 (KNN) is doped with BaTiO3 to affect the orthorhombic phase of the host, the piezoelectric response could be doubled in comparison with the response produced from each material in a single undoped layer [30]. After doping, the bi-component mixture was dissolved and was spin-coated with polydimethylsiloxane (PDMS) on electroconductive plastic substrate. As can be seen from the SEM image (Fig. 7a), the film thickness was ~150 μm, with fine grains in the range 100–200 nm. The piezoelectric film was sandwiched between

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Fig. 6 (a) SEM image of the composite BTO NP/Ag-NWs; (b) piezoelectric voltage generated at various ratio of the composite BTO/PVDF; (c) comparison of the performance of generators using PVDF only, BTO/PVDF and Ag-NWs/BTO/PVDF; (d) demonstration of the sample application in the everyday life (attached to a hand) [11]. (Reprinted from B. Dudem et al. Applied Energy 230, 865–874, Copyright (2018), with permission from Elsevier)

ITO electrodes. The presented relation between the components’ ratio and the piezoelectric voltage is for applied force of 10 N. According to the measurements, the films with doping levels of KNN-0.02 BTO were optimal and exhibited maximum electrical output parameters of 48 V and 450 nA (Fig. 7b). It means electrical power of 21 μW from an area of 9 cm2. Doping of BaTiO3 with strontium (Sr) with Ba and Sr in equal proportion provide excellent piezoelectric voltage and current [7]. The BaSrTiO3 (BST) films can be deposited by conventional methods for the microfabrication technology of vacuum radiofrequency sputtering and to achieve the desired stoichiometry by simply tuning the sputtering voltage in narrow ranges (0.5–0.7 kV). In this way, thermally sensitive substrates like plastic foils and textiles could be also coated. The relations among the BST films’ microstructure, functional groups, electronic state of elements and surface morphology, the charge generation ability, and produced power in respect to the deposition conditions, are investigated. Identification of the chemical bonds and functional groups was performed by Fourier-transform infrared spectroscopy (FTIR) in transmission mode. It could be noted that some typical absorption features

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Fig. 7 Piezoelectric nanogenerator with KNN film doped by BT (a) SEM top view and crosssection of the piezoelectric layer, together with schematic representation of the harvesting device and photo of the prepared sample; (b) comparison between the piezoelectric voltages generated produced from the doped layer and from KNN and BT layers separately [30]. (Reprinted from V. Vivekananthan et al. Materials Letters 249, 73–76, Copyright (2019), with permission from Elsevier)

for the barium titanate lattice, reported also as fundamental also for barium strontium titanate, appeared in the BST films grown at sputtering voltage of 0.7 kV, but they are not revealed in the films prepared at lower voltages of 0.5 kV and 0.6 kV. Examples include the IR lines at 476 cm 1 (Ti-O stretching in SrTiO3) and at 517 cm 1 (related to Ti-O stretching vibrations along the polar axis in BaTiO3) (Fig. 8a). The electromechanical testing was performed in cantilever beam mode with laboratory-made vibrational setup. The oscillograms presented in Fig. 8b, c are related to vibration with frequency of 50 Hz applying mass loading equivalent to 100 g/cm2. While the films with typical perovskite piezoelectric features deposited at 0.7 kV are able to produce clearly periodical signal with high symmetry (Fig. 8c) and the same frequency as the excitation stimulus, the films lacking most of the piezoelectric phase bonds and sputtered at 0.5 kV produced ~100 mV lower voltage with lack of periodicity and deviation from the excitation frequency (Fig. 8b). Although the symmetry of the pulses seems the same in value, the width of the pulses has random distribution and the non-linear distortions are dominant. The produced electrical power densities were compared to estimate the ability of both types of elements to harvest and convert mechanical to electrical energy after applied strain (Fig. 8d). Although similar behavior could be noted for both films (maximum produced power at 10% strain, decreasing to minimum at 20%), the power density for energy harvesting with BaSrTiO3 sputtered at 0.7 kV showed 40% higher maximum power density.

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Fig. 8 Characterization of sputtered BaSrTiO3: (a) FTIR spectra of the films sputtered at different voltages; (b) piezoelectric voltage produced from films deposited at low sputtering voltage (0.5 kV); (c) piezoelectric voltage produced from films deposited at higher sputtering voltage (0.7 kV); (d) output electrical power of both energy harvesters [7]

Device design is another requirement for efficient energy conversion. Suitable construction of the substrate, active piezoelectric films, or electrodes’ shapes are of key importance for increasing the electromechanical coupling and decreasing energy losses. In this regard, rectangular substrates with low aspect ratio and interdigital electrodes were found to be the optimal solution for energy-harvesting purposes. Recently Sun et al. reported that a harvester working at low frequency of 1 Hz produced on polyamide (PI) flexible substrate with the above-mentioned design [24]. Unprecedented voltage of 110 V was achieved. Piezoelectric oxides involved in the composition (bismuth oxide, lanthanum oxide, ferric oxide) were synthesized by solid-state reaction and powder was produced, dissolved, and spin-casted onto sputtered electrodes (Fig. 9a). The interdigital design allows better poling of the piezoelectric film, making this film thinner and the device as a whole lighter and more flexible. As was shown in Fig. 9b, the open circuit voltage and short circuit current of the harvester, when it is attached to the finger and its motion is not restricted, are rather high considering the small activation force applied at finger-bending. The values are

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Fig. 9 Flexible energy harvesting based on PI and lab made piezoelectric oxides: (a) step-by-step fabrication; (b) performance at weak low frequency bending [24]. (Reprinted from Y. Sun et al. Nano Energy 61, 337–345, Copyright (2019), with permission from Elsevier)

respectively 11.5 V/87 nA and 15.4 V/93 nA according to the direction of joining – along or across the poling direction with respect to the length of the finger. In any case, this design in combination with the piezoelectric composite piezoelectric constant of 90 pC/N and dielectric constant of ~60 resulted in electrical outputs competitive to battery power supply, and could serve as an independent source of energy. Some efforts to increase the surface-to-volume ratio and gain in this way the piezoelectric output have been related to nanostructuring of the piezoelectric materials, like nanobranches obtained by seed amorphous sublayer [5] and nanowires, obtained by nanoporous template-assisted growth [27]. In the first case, ZnO thin films were produced by reactive RF sputtering with oxygen deficit over conductive polymer film poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), which is amorphous. This film performs several key functions in the harvester design: first, it is a seed layer promoting further growth of the ZnO like a 3D nanobranched structure; second, it provides the mechanical durability due to the greater elasticity, as compared to the ZnO; third, it demonstrates the possibility of

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Fig. 10 SEM image of the obtained nanostructures ZnO sputtered on amorphous seed layer of conductive PEDOT:PSS film [5]. (M. Aleksandrova et al., Appl. Sci. 2017, 7, 890 published under open access Creative Common CC BY license)

building a piezoelectric generator with nonmetallic electrodes, which is the first step toward development of a fully polymeric piezoelectric nanogenerator. Although the sheet resistance of the PEDOT:PSS films is 85 Ω/sq (approximately 10 times higher than the resistance of the aluminum electrodes, for example), the elements still can show very good electrical performance due to the unique nanostructure, which is similar to dendrite network (Fig. 10). Wires with high aspect ratio and diameter of ~150–200 nm, having side-grown dendrites, are organized like a network. Each secondary dendrite has a different length, varying from 500 nm to 1.5 μm, but their thickness is ~100 nm. Interesting properties can be observed in the energy harvesting device behavior due to the specific morphology of the piezoelectric film and the hybrid organic/ inorganic interface. Among them are the frequency and temperature dependences of the dielectric permittivity and losses (Fig. 11a, b), piezoelectric voltage (Fig. 11c, d), and mechanical stability. The variation of the permittivity after multiple bending (>10,000 cycles or equivalent 17 h of intensive bending) decreases by ~13%, following the same trend as a function of the operational temperature, ranging from 10 °C to 40 °C. The stability in the dielectric parameters is related to the mechanical stability because of the involvement of the PEDOT:PSS films as electrodes. It was proved that PEDOT:PSS material is responsible for stable interfaces with other layers and low junction capacitance in flexible electronic devices, even after bending [4]. Dielectric

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Fig. 11 ZnO/PEDOT:PSS harvester electrical characterization after bending – frequency and temperature dependence of (a) dielectric permittivity; (b) dielectric losses; and piezoelectric voltage at (c) low dielectric losses region and (d) high dielectric losses region [5]. (M. Aleksandrova et al., Appl. Sci. 2017, 7, 890 published under open access Creative Common CC BY license)

losses after multiple bending increased with one order of magnitude and shifted to the lower frequencies, but they are still smaller than those reported in the literature for devices with nanostructured piezoelectric films [3]. This is due to the low-density network of nanobranches, allowing fluent polarization. The generated open circuit voltage at 350 g/cm2 and bending radius of 12 mm was maximum 1 V in the lowfrequency region. At frequencies higher than 100 Hz, due to the increased dielectric losses and worsening electromechanical coupling, it reaches only 300 mV. This is evidence for the suitability of the element to work as a low-frequency energyharvesting element that could be activated from processes of mechanical loading like human motions. Considering that one of the possible ways to increase the piezoelectric performance is stress concentration, nano-objects serving as concentrators of mechanical tension were created. Intermediate coating that can be used as a supplementary material in this case is anodic aluminum oxide (AAO), which provide an array of well-ordered nanopores that could be initiators of nanowires growth. At a later stage, the template could be removed and the tips of the nanowires (their top sides, which can be smaller than 80 nm in diameter) can be considered as concentrators. In this way, lead-free material potassium niobate (KNbO3) having piezoelectric properties

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Fig. 12 Comparison of the structure and electrical performance of KNbO3 (a) SEM image of cross-section of produced nanowires; (b) piezoelectric voltage generated from nanostructured KNbO3; (c) SEM image of top view of nonstructured KNbO3; (d) piezoelectric voltage generated from nonstructured KNbO3 [27]. (© [2019] IEEE. Reprinted, with permission, from T. Tsanev et al. Proceedings of the 31st International Conference on Microelectronics (MIEL), p. 125–128)

was successfully nanostructured and clearly demonstrated after electrical characterization ability to produce greater piezoelectricity in comparison with the nonstructured coating of the same material with same thickness [27]. Sputtering was conducted from both sides of the AAO to ensure better distribution of piezoelectric material in depth in the nanopores (Fig. 12a). The corresponding electrical signal is characterized with rms value of 410 mV, produced from an active area of 3 cm2 (Fig. 12b). For the nonstructured film the SEM image shows fine granular, uniformly distributed coating with high density (Fig. 12c). The corresponding electrical signal is much weaker (Fig. 12d). AAO is widely used as a nanostructuring template not only for growing nanowires by filling it with vacuum deposition process, but also by filling it with liquid medium in the form of a piezoelectric precursor that is vacuum-sucked out through the AAO nanopores. In this way, BaTiO3 nanowires were successfully obtained for energy-harvesting purposes [2]. Ba(CH3COO)2 (barium acetate) and C12H28O4Ti

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Fig. 13 SEM images of the produced nanowires after high temperature annealing for calcination of the precursor components [2]. (K. Aisu et al. Journal of Materials Science & Nanotechnology, vol. 1, p. 1–6, 2014, open access published under Creative Commons Attribution License)

(tetraisopropyl orthotitanate) were used as a source of Ba and Ti, and the piezoelectric precursor was synthesized by chemical reaction. The AAO template consisted of nanopores with a diameter of ~200 nm. SEM images of the nanowires calcined at different temperatures revealed their quality and deviation from the expected geometry of wires (Fig. 13). It was demonstrated that although the BaTiO3 crystallinity increased with the temperature, some nanowires are broken owing to partial filling of the nanowires with precursor solution. This trend is more noticeable for the temperatures 700 °C and 800 °C and appears because of the greater shrinkage of the unfilled nanowires. It was found that 900 °C is not a suitable temperature, as nanowires completely disappear. Measurements of the dielectric response of the nanowires showed a stable dielectric permittivity (εr > 800) in a broad frequency range and dielectric losses lower than 5%, which are close values to the commercial BaTiO3 nanoparticles. This is a proof of their applicability for energy-harvesting devices, but with the caveat to keep in mind the high temperature of treatment, which limits the pool of possible substrates (for example, the plastic ones should be excluded from the competition). High-vacuum die casting was used to fill AAO pores with a diameter of ~ 80 nm with ZnO and to produce ZnO nanowires after etching of the AAO template (Fig. 14)

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Fig. 14 Schematic representation of the die casting mold [16]. (C.G. Kuo et al., Sensors (Basel), 2016;16(4):431, published under Creative Commons by Attribution (CC-BY) license)

Fig. 15 SEM image of the produced ZnO nanowires (a) top view and (b) cross-section [16]. (C.G. Kuo et al., Sensors (Basel), 2016;16(4):431, published under Creative Commons by Attribution (CC-BY) license)

[16]. The method consists of insertion of liquid Zn metal into the nanopores by applying preliminary calculated hydraulic force, considering the surface area and surface tension of the liquid, as well as the contact angle between the Zn metal and the AAO and the diameter of the nanopores. After high-temperature treatment in ambient environment oxidation process takes place and Zn nanowires are transformed to ZnO nanowires (the template is then partially dissolved). As can be noted from the SEM images (Fig. 15a, b) the produced ZnO nanowires are characterized by high density (good pores filling), uniformity in length, good vertical alignment, and uniform diameter. These characteristics are preconditions for high-quality electrical contact with the electrode, which is of great importance for the small amount of the energy loss at the contact interfaces. Measurement of the piezoelectric current in nanowire samples of different lengths, and with the AFM probe in contact mode, showed that longer nanowires generate higher current (the highest measurement was 69 pA).

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Electronic Circuits for Interfacing the Piezoelectric Harvesters and the Load Piezoelectric energy-harvesting elements are unworkable without power processing circuits. These serve as units for impedance matching, by converting the AC signal, produced at the piezoelectric element output, into DC signal, or by converting AC signal into another AC signal with suitable magnitude, or shape [1, 15]. In this way, they make the produced energy suitable for direct usage by from low-power consumers. The power processing circuit should be lightweight and compact enough to be integrated with (or mounted on) the same substrate as the piezoelectric converting element. The core components of the power processing circuits are the specially designed integrated circuits with active or passive conversion and low-voltage drop over the elements like rectifying diodes. Du et al. proposed a passive circuit to increase the rectified power at the piezoelectric element’s output of cantilever-type harvester [10]. The most widely spread solution consists of four passive diodes connected in parallel to the storage capacitor and to the vibrational harvester. A great amount of energy is lost in this case, however, when the voltage flips. To increase the rectifier throughput, synchronized switch harvesting on an inductor (SSHI) has been proposed for synchronously flipping this voltage. As a result, the loss of energy is significantly reduced [13]. An approach of segmentation of the electrodes activated by the same proof mass and serial connection of the segments is used to gain the generated voltage and overcome the threshold voltage of the rectifier’s diodes. Synchronous electric charge extraction (SECE) is a technique that has been developed for piezoelectric transducers that work without the use of a rectifier [22]. Specialized integrated circuits with ultra-low power consumption are required for power management without significant impact in the energy losses. It is recommended to have standby current of ~550 nA at 2.7 V at the input (coming from the transducer’s output). For implementation of the theoretical design, Bidirectional Switching Converter has been used, consisting of series-connected PMOS and NMOS transistors. The voltage drop over the Schottky junctions is 0.32 V. RC low-pass filter, peak detector, and zero-crossing detector are among the other building blocks. The initial start-up of the system is realized by a back-up charged capacitor or battery. The specialized integrated circuits MAX9030 from Maxim Integrated and LMV761 from Texas Instruments were found to be very suitable for this purpose. A next step in the development of these kinds of systems is the design of a power-conditioning interface circuit called “parallel synchronized triple bias-flip” (P-S3BP) [17]. The feature in common with the previously reported designs is the assumption that the piezoelectric energy harvesting can be represented as a current generator, although the current in the piezoelectric materials is ultra-low due to their dielectric nature. It is realized in a way to fulfil the four stages of the signal transformation: first, to make the piezoelectric voltage synchronous in phase with the current; second, to apply passive circuit for bias flip for the voltage enhancing; third, to apply active bias-flip circuit for additional enlarging of the voltage magnitude; and fourth, to create a balance between the output electrical power and dissipated power.

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Recently, a charge-pump converter based on a complementary metal–oxide– semiconductor (CMOS) technology has been developed [12]. It is easy for integration with the microelectromechanical (MEMS) components, such as cantilevers, for example. Details about the electric circuits behind each building unit can be found in Duque et al. [12]. Here, after the rectifier, there is block for creating periodic square wave, taking the rectifying voltage from the first capacitor, and serving as accumulator. Then, a second rectifier with additional capacitors has formed a voltage doubler, and the final voltage is boosted (Fig. 16). Because the consumption of this circuit is greater than the produced power from the energy harvesting (1.875 μA), the first capacitor is crucial for the work of the entire system. The efficiency of energy converting by this power management circuit has been estimated at ~60%. A less complex circuit has been proposed in Sarker et al. [20] with a simple single-stage AC-DC convertor, and without a supporting capacitor, serving as battery. MOSFET thyristors have been used to build the AC-DC full-wave rectifier bridge in the interface circuit to produce usable DC voltage without electrical losses from voltage drops over the rectifying diodes (Fig. 17). A DC-DC step-up converter is the essential building block for the boosting of the output voltage. Thus, although the DC voltage after the AC-DC converter can be as low as 300 mV, the final voltage can be raised to 1.67 V after the DC-DC boost circuit. The efficiency of energy converting by this power management circuit has been estimated to be ~80%, although the ripple of the output signal is relatively high (greater than 500 mV), as can be seen in the right side of Fig. 17. Optimized power conditioning passive circuit with reduced output voltage’s ripple has been proposed in Pandiev et al. [18]. This type of monolithic regulator combines a low-powered boost regulator with a management controller adopting storage elements, making the design more reliable (Fig. 18). The storage element can be either rechargeable battery or supercapacitor (SC). When the system operates under conditions in which the stored electrical energy or harvested energy is periodically insufficient, the energy storage element can be

Fig. 16 Block diagram of the circuit for AC-DC conversion and the converted voltage [12]

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Fig. 17 Block diagram of the proposed interface circuit and the resulting converted DC voltage [20]

Fig. 18 AC/DC power processing circuit diagram for thin film PEH, generating small signals [18]. (© [2019] IEEE. Reprinted, with permission, from I. Pandiev et.al, IEEE 31st International Conference on Microelectronics (MIEL))

used as a back-up energy source. What is interesting here is the selected type of diodes for the rectifying bridge to avoid great voltage loss as a forward drops: Schottky diodes, type 1 N5711 or 1 N5817, are chosen. For them, the maximum value of the forward voltage drop is up to 0.2 V at forward current up to 1 mA. To implement the DC/DC stage, a development board EVAL-ADP5090 has been used. By using the internal cold start-up circuit, the regulator can start operating at an RMS input voltage as low as 380 mV. After cold start-up, the regulator can operate with RMS input voltage ranging from 80 mV to 3.3 V. The efficiency of energy

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converting by this power management circuit has been estimated to be ~80%; however, the output voltage can be double, as compared to the circuit reported in Sarker et al. [20]. Additionally, the ripple of the voltage is lower than 10 mV.

Conclusion and Further Outlook This chapter provides an overview of the most important and basic approaches for optimizing piezoelectric energy harvesting systems, implemented as thin-film elements, published in the last few years. Materials science, device engineering, and power management strategies have been considered as possible tools for extracting the maximum possible electrical power to power supply grid-independent and battery-free electronic devices. Currently, the problem of low conversion efficiency is not yet fully solved and this is the reason why there are still a small number of commercial energy harvesters of this type and only prototypes are reported at the current stage of research. It can be noted that while there is a field for further development of the materials engineering for gaining the harvester performance, there is a limited number of solutions for the constructive design of the elements (mostly MEMS compatible cantilever type, or nanostructured arrays) and interface circuits (mostly passive converters). Therefore, the main tool for control of the conversion efficiency remains the continuously developed synthesis of materials with superior piezoelectric properties. The main challenges are expected to remain in the production of sufficient current to get useful power from the thin-film and small-area harvesting elements for the needs of portable devices and wearable electronics. It can be also noted that the literature is saturated with writing about with nanomaterials implemented in the harvesters for enhancement of the piezoelectric response. Therefore, in the next few years, it is possible to switch the focus from the nanomaterials to the metamaterials as advanced structures for improving the electromechanical coupling. It will be interesting to see in future the cost-efficiency versus energy efficiency of the proposed solutions in this new emerging branch. Acknowledgment The author acknowledges Bulgarian National Science Fund, grant DH 07/13.

References 1. Asthana P, Khanna G (2020) Power amplification interface circuit for broadband piezoelectric energy harvester. Microelectron J 98:104734 2. Aisu K, Osada M, Suzuki Y (2014) Synthesis of BaTiO3 nanowires via anodic aluminum oxide template method assisted by vacuum-and-drop loading. J Mater Sci Nanotechnol 1:1–6 3. Aepuru R, Bhaskara BV, Kale SN, Panda HS (2015) Unique negative permittivity of the pseudo conducting radial zinc oxide-poly(vinylidene fluoride) nanocomposite film: enhanced dielectric and electromagnetic interference shielding properties. Mater Chem Phys 167:61–69 4. Aleksandrova M, Dobrikov G, Singh A K, Videkov V, Kolev G (2017) Flexible optoelectronic device with polymer based electrode on hybrimer substrate. In: Proceedings of the IEEE 40th international spring seminar on electronics technology – ISSE2017, Sofia, Bulgaria, 10–14 May 2017

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5. Aleksandrova M, Kolev G, Vucheva Y, Pathan H, Denishev K (2017) Characterization of piezoelectric microgenerator with nanobranched ZnO grown on a polymer coated flexible substrate. Appl Sci 7:890–897 6. Aleksandrova M, Dobrikov G, Kolev G, Marinov Y, Vlakhov T, Denishev K (2018) Flexible and lead-free barium strontium titanate based generatrors, In: Proceedings of the 41st IEEE international spring seminar on electronics technology (ISSE), Zlatibor, Serbia, 16–18 September 2018 7. Aleksandrova M, Ivanova T, Koch S, Hamelmann F, Karashanova D, Gesheva K (2020) Study of sputtered barium strontium titanate films for energy harvesting applications. Adv Mater Lett 11(10) article number 20101567 8. Blagoev BS, Aleksandrova M, Terziyska P, Tzvetkov P, Kovacheva D, Kolev G, Mehandzhiev V, Denishev K, Dimitrov D (2018) Investigation of the structural, optical and piezoelectric properties of ALD ZnO films on PEN substrates. J Phys Conf Ser 992:012027 9. Chauhan SS, Bhatt UM, Gautam P, Thote S, Joglekar MM, Manhas SK (2020) Fabrication and modeling of β-phase PVDF-TrFE based flexible piezoelectric energy harvester. Sens Actuat A: Phys 304:111879 10. Du S, Jia Y, Zhao C, Amaratunga GAJ, Seshia AA (2018) Passive design scheme to increase the rectified power of piezoelectric energy harvesters. IEEE Trans Ind Electron 65:7095–7105 11. Dudem B, Kim DH, Bharat LK, Yu JS (2018) Highly-flexible piezoelectric nanogenerators with silver nanowires and barium titanate embedded composite films for mechanical energy harvesting. Appl Energy 230:865–874 12. Duque M, Leon-Salguero E, Sacristán J, Esteve J, Murillo G (2019) Optimization of a piezoelectric energy harvester and design of a charge pump converter for CMOS-MEMS monolithic integration. Sensors 19:1895–1908 13. Gasnier P, Willemin J, Boisseau S, Despesse G, Condemine C, Gouvernet G, Chaillout J-J (2014) An autonomous piezoelectric energy harvesting IC based on a synchronous multi-shot technique. IEEE J Solid State Circuits 49:1561–1570 14. Gupta R, Rana L, Tomar M, Gupta V (2018) Characterization of lead zirconium titanate thin films based multifunctional energy harvesters. Thin Solid Films 65230:39–42 15. Ganesh RJ, Kodeeswaran S, Kavitha M, Ramkumar T (2020) Performance analysis of piezoelectric energy harvesting system employing bridgeless power factor correction boost rectifier. Mater Today Proc. In press. Available online 25 Feb 2020 16. Kuo C-G, Chang H, Wang J-H (2016) Fabrication of ZnO nanowires arrays by anodization and high-vacuum die casting technique, and their piezoelectric properties. Sensors 16:431–441 17. Liang J, Zhao Y, Zhao K (2019) Synchronized triple bias-flip interface circuit for piezoelectric energy harvesting enhancement. IEEE Trans Power Electron 34:275–286 18. Pandiev I, Aleksandrova M, Kolev G (2019) Analysis and design of power processing circuits for thin film piezoelectric energy harvesters on flexible polyethylene terephthalate substrates. In: IEEE 31st international conference on microelectronics (MIEL), Nis, Serbia, 1618 September 2019 19. Rubio-Marcos F, Fernandez JF, Ochoa DA, García JE, Ramajo L (2017) Understanding the piezoelectric properties in potassium-sodium niobate-based lead-free piezoceramics: interrelationship between intrinsic and extrinsic factors. J Eur Ceram Soc 37:3501–3509 20. Sarker MR, Ali SHM, Othman M, Islam S (2013) Designing a battery-less piezoelectric based energy harvesting Interface circuit with 300 mV startup voltage. J Phys Conf Ser 431:012025 21. Shia K, Sunab B, Huanga X, Jiang P (2018) Synergistic effect of graphene nanosheet and BaTiO3 nanoparticles on performance enhancement of electrospun PVDF nanofiber mat for flexible piezoelectric nanogenerators. Nano Energy 52:153–162 22. Shareef A, Goh WL, Narasimalu S, Gao Y (2019) A rectifier-less AC–DC Interface circuit for ambient energy harvesting from low-voltage piezoelectric transducer Array. IEEE Trans Power Electron 34:1446–1457 23. Sabry RS, Hussein AD (2019) PVDF:ZnO/BaTiO3 as high out-put piezoelectric nanogenerator. Polym Test 79:106001 24. Sun Y, Chen J, Li X, Lu Y, Zhang S, Cheng Z (2019) Flexible piezoelectric energy harvester/ sensor with high voltage output over wide temperature range. Nano Energy 61:337–345

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25. Suo G, Yu Y, Zhang Z, Wang S, Zhao P, Li J, Wang X (2016) Piezoelectric and triboelectric dual effects in mechanical-energy harvesting using BaTiO3/Polydimethyl-siloxane composite film. ACS Appl Mater Interf 8:34335–34341 26. Tsanev T, Aleksandrova M, Ivanova T, Dobrikov G (2019) Investigation of Lead-free Potassium Niobate Thin Films on Silicon for Piezoelectric Transducers. In: Proceedings of the X IEEE National Conference with International Participation (ELECTRONICA), Sofia, Bulgaria, 14–15 May 2019 27. Tsanev T, Aleksandrova M, Videkov V (2019) Study of nanoporous anodic aluminum oxide as a template filled with piezoelectric materials. In: Proceedings of the IEEE 31st international conference on microelectronics (MIEL), Nish, Serbia, 16–18 September 2019 28. Tariverdian T, Behnamghader A, Milan PB, Barzegar-Bafrooei H, Mozafari M (2019) 3Dprinted barium strontium titanate-based piezoelectric scaffolds for bone tissue engineering. Ceram Int 45:14029–14038 29. Tao K, Yi H, Tang L, Wu J, Wang P, Wang N, Hu L, Fu Y, Miao J, Chan H (2019) Piezoelectric ZnO thin films for 2DOF MEMS vibrational energy harvesting. Surf Coat Technol 35915:289–295 30. Vivekananthan V, Chandrasekhar A, Alluri NR, Purusothaman Y, Kim WJ, Kang C-N, Kim S-J (2019) A flexible piezoelectric composite nanogenerator based on doping enhanced lead-free nanoparticles. Mater Lett 249:73–76 31. Yeol H, Young C, Jeong G (2019) Microstructures and piezoelectric performance of ecofriendly composite films based on nanocellulose and barium titanate nanoparticle. Compos Part B Eng 168:58–65

Part V Main Products and Devices Obtained with Use of Nanomaterials

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3D Printing of Fiber-Reinforced Polymer Nanocomposites: Additive Manufacturing

Borra N. Dhanunjayarao, N. V. Swamy Naidu, Rajana Suresh Kumar, Y. Phaneendra, Bandaru Sateesh, J. L. Olajide, and Emmanuel Rotimi Sadiku

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Technologies in 3D Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fused Filament Fabrication (FFF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stereolithography (SLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laminated Object Manufacturing (LOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composite-Based Additive Manufacturing (CBAM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selective Laser Sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials for 3D Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Various Testing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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B. N. Dhanunjayarao Department of Mechanical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh, India Department of Mechanical Engineering, Vignan’s Institute of Information Technology (A), Visakhapatnam, Andhra Pradesh, India N. V. S. Naidu (*) · R. S. Kumar Department of Mechanical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh, India e-mail: [email protected] Y. Phaneendra · B. Sateesh Department of Mechanical Engineering, Vignan’s Institute of Information Technology (A), Visakhapatnam, Andhra Pradesh, India J. L. Olajide Department of Mechanical and Automation Engineering, Tshwane University of Technology, Pretoria, South Africa E. R. Sadiku Department of Chemical, Metallurgical and Materials Engineering, Institute of NanoEngineering Research (INER), Tshwane University of Technology (TUT), Pretoria West Campus, Pretoria, South Africa Department of Mechanical Engineering, Maharashtra Institute of Technology, Pune, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_166

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Applications of 3D Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1413 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1419

Abstract

Continuous fiber-reinforced polymer composite materials fabrication using additive manufacturing (AM) technology like 3D printing is an active research area to be taken up. The research not only focusses on the determination of mechanical, electrical, and thermal properties of a component but also enhancing the same individual or in a combination of all. In addition to these, improved mechanical performance can be achieved by employing 3D printing technology. The incorporation of fiber properties with altered additives mixing via thermosets/thermoplastic filaments/matrices, the performance enhancement of the resultant composites is achieved. Apart from the automotive and aerospace industries, studies have shown that this technology is well-suited for the manufacture of a wide variety of customized products. This technology is highly capable of optimizing the material along with the structure, volume fraction, and the fiber direction that can be controlled at every location in a composite by threedimensional computer-aided design (CAD) data. The present book chapter focuses on the technology that can be used to fabricate a composite by using continuous fiber by 3D printing technology. Undoubtedly, this technique can be the next-generation composite fabrication route that will be very beneficial to many manufacturing industries in terms of speed, efficiency, and reduction in production cost. The first section of this chapter presents the latest trends in 3D printing technology for possible enhancement of the properties of the composite components. The second segment of the chapter is devoted to the methodology and material requirements and their suitability for application-specific. The third section elaborates on the various testing methods required for the characterization of the resultant composite. In the end, the chapter concludes with observations ranging from the fabrication, characterization, and testing of the composites. Also, the suggestions for future scope are made towards the end of the chapter. Keywords

Nanofiber reinforcements · Continuous reinforcement fibers · Polymer matrix materials · 3D Printing · Fused filament fabrication

Introduction Three-dimensional (3D) printing is the technology of additive manufacturing where materials are added layer by layer unlike the conventional molding or cutting processes to fabricate the components. Earlier, 3D printing technology was used only for prototyping the complex products but later due to its versatility, it became a powerful method owing to the need for fewer components in an assembly resulting

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in an end-user product with the use of functional materials [1]. The main drawback of this technology is the deficient mechanical performance and lack of structural integrity of the products fabricated by the conventional method because of the solo performance of polymer resins [2, 3]. The major goal of the proposed new method of 3D printing technology is to produce mechanically strong structures for automobile and aviation industry applications [4] by a combination of mechanical properties of structural components. 3D printing is the most trending next-generation composite fabrication technology that does not require any fabrication tools unlike conventional manufacturing methods [5]. For so many decades the researchers focused on fiber-reinforced composites because of its supreme mechanical characteristics such as high strength, low density, high stiffness, low cost, and relatively easy to manufacture and suitable for a wide range of applications [6]. The composite consists of constituents to meet the requirements of functional or structural components. With a huge number of abilities, Fiber-reinforced polymer composites (FRPCs) are unique in the material era because they exhibit characteristics like high specific strength and performance, better anti-fatigue, and anti-aging abilities [4, 5]. The novel usage of FRPC’s is becoming more and more in aviation, automobile, wind farms, and other industries. Composite parts fabricated by the presently available machinery is very costly, difficult to fabricate complex shapes, and more time-consuming as it requires pre-processing, processing, and post-processing. Hence, there is a huge demand for new methods of fabrication of FRPCs very effectively and easily without compromising on the above factors. FRPCs 3D printing has become the representative technology in the new stage of intelligent manufacturing [7]. A number of manufacturing methods currently available include extrusion injection molding, compounding, molding, pultrusion, impregnating and winding, etc. Among these, the most widely used process for FRPCs production is by material lay-up followed by consolidation (i.e., two-stage process) which requires costly equipment to apply pressure on the whole part which increases the cost of manufacturing [8]. According to the fiber length present in the matrix polymer, the FRPCs are categorized as short fiber (0.2–0.4 mm), long fiber (10–25 mm), and continuous fiber-reinforced polymer composites [9]. These polymer composites are of two kinds, namely, the thermosetting and the thermoplastic material. The thermosetting plastics include epoxy and polyurethane while the thermoplastics include PP, PA, PEEK, etc. [10]. The traditional composite manufacturing method has been reconstructed into a robust AM of polymer composites and it enables the product to produce with considerably improved properties and highly customizable when compared to un-reinforced polymers and conventional composite materials [1]. 3D printing is more advantageous for the industries where the manufacturing capability of highly customizable parts, complex structures, and personalized devices are possible with medium production volume. The example of such a technique is rapid prototyping. Therefore, considering the advantages offered by the CFRPs and 3D printing technology both in unison can be used to lightweight, more complex, and high-performance components effectively and efficiently [7, 11]. This chapter presents an insight into the latest advancements in 3D printing technology of polymer composite materials by AM techniques. Also, the possible methods of testing for characterization of the

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3D printed composite specimens have been addressed. The latest trends to existing AM methods, challenges posed along with their developments are also presented in detail.

Technologies in 3D Printing Reinforcing fiber can improve the properties of polymer composites prepared by the AM technique largely. Void content and fiber orientation are primary concerns for composite material by 3D printing, but the majority of the 3D printing technologies available commercially intend to benefit from fiber reinforcement. The different AM technologies for the fabrication of FRPCs are vat photopolymerization, material extrusion, powder bed fusion, and sheet lamination [9]. The classification of various AM techniques for fiber-reinforced polymer composites are shown in the flow chart displayed in Fig. 1.

Fused Filament Fabrication (FFF) FFF also called fused deposition modelling (FDM) is a very popular 3D printing technology among AM developed in the 1980s is extensively used in many industries. FFF fabricates the parts using a spool of a filament made of thermoplastic material where the filament is pushed through the heated chamber by means of filament drive gear [1, 10, 14]. The filament then gets heated/melted in the heating chamber and extruded through the nozzle on to the build-plate. The nozzle deposits the material layer by layer with the help of a print head which moves continuously to precise locations of the build plate by following the predetermined path resulting in final 3D object buildup. The extrusion process is very similar to the manual hot glue extrusion from glue gun and syringe dispensing. Many of the FFF 3D printers are capable of producing FFPC objects layer by layer [7, 9]. The printing procedure of FFF is explained using a block diagram with the sequence of steps shown in Fig. 2.

Composite Material Printing Methods There are three methods available for introducing a reinforcing fiber into a matrix material by the FFF method depicted in Fig. 3 [17]. Single nozzle head: Printing by single nozzle requires a variety of filaments that are made of the composite material like the reinforcing materials (i.e., rubber particles, glass powder or carbon fiber and wood flour, etc.) are incorporated in matrix material during filament preparation. So, the spool itself contains a composite material filament. It is prepared by introducing the fibers in a matrix material and wound round like a spool of filament for the fabrication process. This is the procedure followed before actually using the filament for the printing process [15]. Duel nozzle head: It has two independent nozzles heads with separate reinforcing spool and filament spool. Reinforcing fiber is incorporated with matrix

Fig. 1 Classification of various 3D printing technologies for printing FRPC [9, 12, 13]

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Fig. 2 Block diagram of FFF. (Reproduced with permission from [15, 16])

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Fig. 3 Different methods of 3D printing composite material. (Reproduced with permission from [15, 19])

filament (layered composite structure) by depositing on the build plate in the form of one material over the other material alternatively in the form of printing [15]. In-Nozzle Impregnation: A spool of matrix filament and continuous reinforcing fibers are separately placed and supplied to the head consisting of a printer nozzle. The reinforcing fiber automatically is supplied to the nozzle by the motion of the matrix filament. The continuous reinforced fibers are heated before impregnation to increase the bonding capability with polymer material which is later plasticized with the matrix material [12, 13, 18].

Types of FFF Printing Machines Based on the configuration of the FDM 3D printers, they are classified into four different types: Cartesian printer, Polar printer, Delta printer, and Robotic arm printer, and details are schematically shown in Fig. 4. FFF is very is economical which can be used in an office environment for creating long-lasting, durable, and structurally strong parts. Cartesian FDM 3D printers: FFF 3D printing technology with Cartesian configurations consists of links connected by linear joints. Three mutually perpendicular coordinates X Y Z with the help of which the 3D modeled component is printed by positioning correctly the direction of the print head. The build plate travels in the Z direction to meet the required thickness of the 3D geometry while the print head moves in the XY plane to meet the length and width of the geometry. Polar 3D FDM Printers: When the position and the orientation information pertaining to the built-up object is given, FFF 3D printing technology with polar configuration is considered. The build plate rotates about the Z axis and translating in the XY plane while the print head moves along the fixed spherical workspace in Z axis. Delta FDM Printers: Delta technology machines apply the Cartesian system where the arm moments are used to determine the position and orientation of the

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Fig. 4 Classification of different FFF processes

printer head. Delta printing machines are equipped with a fixed round build plate and three arms supported by rails that move up and down independently. The extruder print head is controlled and is connected to the three arms which can move in any direction while the build plate is immovable. The only difference between the Cartesian and delta printers is this particular feature of a fixed build plate. These printers are constructed to accelerate the printing process as it only facilitates the extruder head movement in all the directions to fabricate a component. FDM 3D Printing with Robotic Arms: Commonly robotic arms are known to be used in industries like a large automobile assembly line and production line, etc. Now robotic arms are playing a vital role in 3D printing technology for the fabrication process. The extruder is placed at the end effector of the robotic arm while the fiber spool is placed just before extruder. Here the position and orientation of the print head are completely determined by the robotic arm configuration. The print head gets heated to melt the filament which is then extruded on to the build plate to prepare 3D components. Unlike the delta FDM printers, the build plate in this case is not fixed. This technology is now being used for the automated fiber placement (AFP) to print composite material which is reinforced by continuous fiber. This process is more flexible and easier to produce very complex geometries. Liquid Deposition Modelling (LDM): Liquid Deposition Modeling (LDM) is one of the favorable technologies in AM. It is developed to print viscous materials such as clay, porcelain, alumina, wood paste, zirconium, ceramic, and other advanced materials. This technology is gaining more popularity where selfproduction and digital handcrafting are employed creating 3D end-use materials.

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Since 2012, LDM technology is available as a special entity developed by Delta printers on the world’s advanced saving project (WASP’s) which can be configured on the already available ones. This technology uses a self-made special kind of extruder; it can be configured with so many 3D printers available in the market like standard Cartesian 3D printer, delta printer, etc. Compressed air is used to apply force on a piston to pump the material to the extruder. Extruder moves and prints the material on a build plate according to the geometry given to the printer. LDM can control the flow of material accurately that optimizes material usage.

Stereolithography (SLA) SLA is one of the popular 3D printing methods of the Vat Polymerization technique. It works on the principle of the photo-initiated polymerization process whose details are shown in Fig. 5. SLA uses ultraviolet (UV) laser radiation or lamp to cure liquid photopolymer upon which the reaction causes a photopolymer to solidify via crosslinking [1]. A resin tank or reservoir filled with a UV curable resin is a liquid photopolymer. A movable build platform is submerged in the resin tank and moves down by piston to create a sufficient number of layers to reach the required height of the object. The laser source is used to emit the single point UV light and it falls on the XY scanning mirror to reflect the UV light on a build platform. Upon exposer of resin to UV light which is present on the build plate, the resin starts curing and solidify as the first thin layer. Next, the movable build platform moves down to provide uncured liquid resin above to the first layer and second subsequent thin layer build on the application of the UV light. This layer-by-layer process is repeated to produce a solid part where the movable build platform will be lifted by the elevator to make the 3D object out of the resin tank. The final 3D part exhibit poor mechanical properties which are typically post-cured by exposure to UV light to get enhancement in the same. For preparing composites, the fibers are blended with

Fig. 5 Schematic of Stereolithography 3D printing process. (Reproduced with permission from [20])

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photopolymer resin. The primary compounds of liquid resin are monomers and oligomers while the secondary ones are photo-initiators binders and additives. Many types of additives can be used to blend or modify the resin material, and this is where nanoparticles play a vital role which can be introduced to improve the properties [20].

Laminated Object Manufacturing (LOM) LOM is a layer by layer AM printing technique which is first developed in 1991 by the Helisys, Torrance CA [1] and the process schematic diagram is shown in Fig. 6. LOM can be used for fabricating a variety of materials like fabrics, papers, synthetic materials, plastics, and composites. The basic principle behind LOM is that the thin laminate sheets are cut to the desired shape by the laser cutter where these layers are then glued together using adhesive paper to form a stack of a laminated 3D object. LOM is equipped with the thin laminate sheet which is supplied from the material supply roll that is passed through the build platform on to another roll. Then laser cuts the thin laminate sheet by translating the XY moving head to the desired shape and size of the object. After cutting is complete, the remaining material is collected by the waste take-up roll. Build platform moves vertically which is used to achieve the desired height of the 3D object by placing a stack of layers. A heat-sensitive adhesive is provided on top of each laminate to glue each laminate together while the compaction of laminates is done by application of force and rolling action of heated roller time and time on to the stack of laminate sheets till, they get solidified. In some cases, the post LOM process is required to consolidate the stack of laminates for strengthening the layer-to-layer interface and to minimize the voids from the final product. The methodical procedure of the LOM process is explained with the block diagram shown in Fig. 7. The strengths of the LOM technique are high volumetric

Fig. 6 Schematic of LOM. (Reproduced with permission from [1])

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Fig. 7 Block diagram of the LOM [21]

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build rate, relatively low cost, and good interfacial bonding between layers that allow for combinations of metal foils included in the components. The typical materials used for LOM are paper, plastic sheets, and metal foils/tapes.

Composite-Based Additive Manufacturing (CBAM) CBAM is very similar to the LOM and it is particularly designed for fiber-reinforced composite 3D printing. CBAM can be used to fabricate a variety of reinforcing and matrix material combinations that include nonwoven mats of fiberglass, carbon fiber, and Kevlar bonded with thermoplastic matrix materials ranging from nylon/polyamide to polyethylene, PEEK and many more [11]. The merits of CBAM are the stronger fabricated parts and lightweight compared with other 3D printing technologies. Also, CBAM offers more design freedom, eliminates tooling time, quicker than traditional composite techniques, cost savings, can print complex geometries, and has a broader selection of materials. CBAM is used to print functional parts and tools that have huge advantages in aerospace, automotive, defense, electrical, medical device, athletics, and electronic industries due to more than 10 times stronger compared to unreinforced 3D printed plastic parts. The CBAM process shown schematically in Fig. 8 where a CAD file is loaded to slice it to the number of layers that are then converted into individual bitmaps. The print head follows the bitmap shape to leave an aqueous solution on each substrate layer prepared by reinforcing material. Subsequently, thermoplastic matrix material powder poured on to the substrate sheet in which matrix material sticks to the wet surface only (surface in contact with the aqueous solution) removing the excess dry powder. This process is repeated to all the layers of the 3D part and the same is produced after all the layered sheets are stacked one over the other. Heat is given to reach melting temperature and then applying some force to compress to reach the final height and thereafter the same is removed [22].

Selective Laser Sintering (SLS) SLS is one of the powder bed fusion AM methods with the basic idea of this is to produce solid plastic parts using a laser [1]. SLS can be used for printing wax, metals, ceramics, polymers, and composites. SLS can work with many types of polymers starting from nylon (PA), polyethylene (PE), PEEK and PCL, etc. SLS is equipped with a build cylinder moving along Z direction and an attached high energy laser source within the workspace. Laser source emits laser which travels to the build cylinder through the mirror and XY deflector. There are two containers for housing the composite material powder in which one of them is a feed container while the next one is overflow container [20]. The composite powder deposited on to the build area will be spread and rolled with the help of roller on the surface of the

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Fig. 8 Process steps of CBAM. (Reproduced with permission from [11, 22])

work-table. Once the roller starts rolling on the build area for the first layer, the laser then starts to focus on the material powder which then gets sintered and the process repeats for the next and subsequent layers. Finally, when all the layers are sintered some loose powder occupied surrounding the sintered object will act as a support material. The part is then slowly removed from the powder, cleaned, and then is made ready for use in post-processing for further action [23]. The systematic procedure of the SLS is shown in Fig. 9. SLS can be classified into four types, namely, solid-state sintering, liquid-state sintering, full melting, and chemically induced bonding. The melting temperature of the specimen is the temperature of the solid-state sintering. Partial melting takes place in liquid state sintering during which the binder particles become a liquid, while the structural material remains solid. The entire powder melts in full melting and offers comparable properties to bulk material properties. Various latest technologies with specifications for FRPC’s 3D printing are summarized in Table 1.

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Fig. 9 Schematic of selective laser sintering. (Reproduced with permission from [1])

Materials for 3D Printing The commonly used polymer materials for 3D printing are shown in Fig. 10. Currently, the use of polymers like PLA and ABS are very high in comparison with those of general-purpose polymers. There is a growing demand for strong functional materials that can withstand harsh environments and high temperatures. The development of high-performance thermoplastics is crucial for the progression of AM for industrial applications. They support the transition of the technology from prototyping to advanced applications in critical industries like medical and aerospace. In connection to the above, many companies are working on promoting the situation to prepare high-performance thermoplastic polymer composite filaments and filament fibers for 3D printing technology [34]. High-performance composites are prepared by the use many polymer matrix materials such as ULTEM, PEEK, PEKK, HSHT, PETG, TPU, PC, ASA SBS, ABS, Nylon, Elastomer polymers by reinforcing the continuous fibers, short fibers, and particulate fibers such as Carbon fiber, Fiberglass, Kevlar fiber, Basalt fiber, HSHT fiberglass, etc. [35].

Various Testing Methods To characterize the 3D printed composite material some of the standard tests are required to be performed. The tests are tensile test [36], compression test [37], flexural (3-Point and 4-point bending) test [16], Charpy impact strength test, interlaminar shear strength test, in-plane shear test [37], single-lap joint test [5], thermogravimetric analysis (TGA) [2], static and kinetic coefficients of friction, microscale abrasion test, water absorptions test, and fiber volume fraction test [38].

1524 / – / 450

10–80 / 60 / 270

FFF

FFF & CFC FFF & CFC

4000 / – / 400

–/–/–

FFF

RAFFF

150 / 60 / 400

– / – / 450

(continued)

Continuous Composite CF3DP

Anisoprint СOMPOSER [27] Anisoprint PROM IS 500 [28] CEAD CFAM Prime [29]

9 T Labs Red Series [25] THERMWOOD LSAM [26]

Specifications Max printing speed / Min ‘z’ / Max Build volume, mm  printing Temp, Material capability Technology (mm/sec) / μm / 0C mm  mm DLP & – / 25 / – 203.7  114.6  330 Range of resins, reinforcing DCM fibers, and particles qualified by Fortify.

FFF

Table 1 Summary of Latest technologies

Make & model Fortify Flux One [24]

3D Printing of Fiber-Reinforced Polymer Nanocomposites: Additive. . .

Others Continuous kinetic mixing to suspend the functional additives in the resin matrix, magnetic alignment of fibers is possible for optimized microstructures, designed for end-use production parts. 350  270  250 Hybrid plastic and Heated print bed, heated build chamber, automatic CF/plastic calibration, closed frame, dual extruder. 3048  1524  6096 Thermoplastic composite Print heads are capable of printing large parts such materials as 500 pounds per hour and it is provided with print and trim on a single machine, equipped to print both horizontally and vertically. 420 х 297 х 210 Any plastic as a matrix, Equipped with dual nozzle such as FFF and CFC CCF and CBF extruders with fiber cutting system 600  420  300 PLA, ABS, PETG, PA, Up to 4 print heads possible, TPU, PC, ASA, SBS, any Plastic with high processing temperatures is CF/GF filled plastics. possible 4000  2000  1500 PLA, PEEK, ULTEM, ABS, Large scale fiber reinforced polymer-based 3D other plastics, Carbon fiber printing. and composites Robot work envelope Carbon, glass, aramid, Tool head coats the strand of fiber with a optical, metallic fibers photopolymer as it’s deposited and then immediately cures it with a powerful UV light print both structural fibers and functional fibers in a single-step process.

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– / 100 / –

320  132  154

330  270  200

FFF & CFF – 50 / –

FFF

330  270  200

FFF & CFF – 50 / –

Others A new process called micro automated fiber placement (μAFP), robotic tool changer with two printheads. Up to 4 times stronger than FFF parts and 2 times as MJF parts made with PA12.

Laser bed leveling, active print calibration, industrial applications. Onyx, Onyx FR, Laser bed leveling, active print calibration, Fiberglass polymer filament industrial applications, second-generation extruder, out-of-plastic and out-of-fiber detection. Onyx, Onyx FR, Nylon Industrial applications, White, Second-generation extruder, out-of-plastic and outCarbon fiber, fiberglass, of-fiber detection. Kevlar, HSHT fiberglass filaments Onyx Desktop printer, Second-generation extruder, out-of-plastic detection.

Specifications Max printing speed / Min ‘z’ / Max Build volume, mm  printing Temp, Material capability Technology (mm/sec) / μm / 0C mm  mm μAFP & – / 50 / 250 310  240  270 PEEK + CF, PEKK + CF FFF PA6 + CCF, PA6 + CFG, PEEK + CCF, PEKK + CCF CBAM –/1/ 305  406  102 CF/PEEK, CF/Nylon 12, GF/PEEK, GF/Nylon 12, CF/Nylon 6, CF/GF/ Nylon 12, CF/Elastomer, GF/Elastomer FFF – / 50 / 140 330  270  200 Plastic Composite Filament

Table 1 (continued)

MARKFORGED Onyx One [32]

MARKFORGED X7 [22]

MARKFORGED X3 [22] MARKFORGED X5 [22]

Impossible objects CBAM-2 [31]

Make & model Desktop metal Fiber [30]

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300  200  200

280  220  200

70 / – / 450

– / 25 / 300

FFF

FFF

Nylon White, Onyx, Fiberglass, Carbon Fiber, HSHT, Kevlar Fiberglass filaments PEEK and ULTEM, AM9085F, carbon PEEK and carbon PA PEEK, ABS, ULTEM, Carbon fiber and composites PLA, other plastics, ABS, Carbon fiber, and composites

Onyx

Beltless system increases the print speed, equipped with heat reflected chamber to keep the heat generated in the printing phase. Designed it to bring the heat to the exact point, capable of reaching high temperatures.

Auto-leveling build plate Double extruder with double gear system.

Desktop printer, Second-generation extruder, out-of-plastic detection. Desktop printer, Second-generation extruder, out-of-plastic detection.

ROBOZE ONE +400 XTREME [22] ROBOZE ONE [22]

ROBOZE ARGO 500

MARKFORGED Mark Two [22]

MARKFORGED Onyx Pro [33]

FRPC’s Fiber Reinforced Polymer Composites, DLP Digital Light Processing, DCM Digital Composite Manufacturing, FFF Fused Filament Fabrication, CFC Composite Fiber Coextrusion, RAFFF Robotic-arm assisted Fused Filament Fabrication, μAFP Micro Automated Fiber Placement, CBAM Composite-Based Additive Manufacturing, CFF Continuous Filament Fabrication, LSAM Large Scale Additive manufacturing, CF3D Continuous Fiber 3D Printing, CCF Continuous Carbon Fiber, CBF Continuous Basalt Fiber, PLA Polylactic Acid, ABS Acrylonitrile Butadiene Styrene, PEEK Poly Ether Ketone Ketone, ULTEM polyetherimide, PA polyamide (Nylon), AM9085F polyetherimide blend, Onyx Nylon blend loaded with Chopped carbon fiber, HSHT High-Strength HighTemperature, CF Carbon fiber, CFRC Continuous fiber-reinforced composites, PETG Polyethylene Terephthalate Glycol, TPU Thermoplastic polyurethane, PC Polycarbonate, ASA acrylonitrile styrene acrylate, SBS Styrene-Butadiene rubber, CF/GF Carbon Fiber/ Glass Fiber, PEKK Poly Ether Ketone Ketone, PA6 + CF Chopped carbon fiber-filled Nylon, PA6 + GF Chopped fiberglass filled Nylon

500.38  500.38  500.38

– / 25 / 450

320  132  154

FFF & CFF – / 100 / –

FFF

320  132  154

FFF & CFF – / 100 / –

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Fig. 10 Common types of 3D printing materials

The standard test procedures and methods help in preparing the specimens to conduct a particular test correctly. Most of the researchers followed the ASTM standards for the polymer composites while very few researchers followed the ISO or Chinese standards based on their requirements and the details are summarized in Table 2. For conducting the tensile, compression, and flexural tests, a self-alignment gripped Universal Testing Machine (UTM) has to be used to avoid specimen mounting errors or faults. The UTM with a load cell of 10 kN is enough considering the matrix material being a polymer. The constant crosshead speed (mm/s) to be considered to achieve a constant strain rate based on the requirement and strain gauge or laser extensometer is to be used to measure the strain values. Tabs are used to overcome the slipping of specimens from the grips and stress concentration at grip zones. These tabs are bonded to the specimens using epoxy. In addition to these, specimen slipping can be minimized by placing a two-sided sandpaper sheet in between the specimen and the grips of the holder. Generally, it is recommended to

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Table 2 Summary of testing methods of fiber reinforced polymer composite 3D printing specimens Test specimen Tensile test

Compression test 3-point bending test

4-point bending test Charpy impact strength test Interlaminar shear strength test

Standard ASTM D3039 ASTM D638-14 Type-I

Specimen size 250 mm  15 mm  1.4 mm 165 mm  19 mm  4 mm

ASTM E8M-15a ASTM D412 C ISO 527:1997 (ISO527-2:1993 E) GB/T1040.1-2006 ASTM D695-02 ASTM D6641 ASTM D7264 ASTM D790 ASTM D1184 ASTM D 7905 (end notched specimen) ISO 14125:1998 GB/T449:2005 ASTM D6272-17 ASTM D6110-18

– 115 mm  19 mm  3.3 mm 250 mm  25 mm  2 mm

Reference [2, 36–40], [11, 14, 16, 18, 19, 38– 45] [46] [47] [48]

150 mm  15 mm  3 mm 80 mm  12.5 mm  2 mm

[49, 50] [37, 39, 40]

160 mm  11 mm  4 mm 65 mm  12 mm  4 mm 30 mm  19 mm  3 mm –

[37, 39, 42– 45, 48–51],

ISO 14130:1997 ASTM D2344 In-plane shear test ASTM D3518 Single lap joint test ASTM D2093-03 TGA – Fiber volume fraction test ASTM D3171-15 Coefficients of friction ASTM D1894 Microscale abrasion test ASTM 52100 Water absorptions test ASTM D570-98

100 mm  15 mm  2 mm 80  15  5 mm 130 mm  15 mm  3.5 mm 89 mm  10.4 mm  4 mm 20  10  2 mm 40 mm  12 mm  6 mm 175 mm  20 mm  2 mm – 7.6  2.5 mg – – – –

[42] [42, 48] [37] [5] [2] [39] [52] [52] [43]

ASTM American Society for Testing and Materials, ISO International Standards Organization, GB/T Chinese Standard, TGA Thermogravimetric analysis

use end tabs and sandpapers to mitigate sample slipping. The plots of tensile test data can be exploited to get tensile modulus value [14, 17]. The tensile testing specimen as per ASTM standards is shown in Fig. 11. A compression test can be performed in the direction 00 and 900 orientation of the fibers and one can find elastic modulus and strength. If require tabs are used and compression fixtures can be procured as required following the standards. The modulus value can be obtained from the tensile test from the stress-strain curves. The test can be started by adjusting the compression fixture and specimen. Failure can occur at the free zone in between the tables [37, 39]. The flexural test can be

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Fig. 11 Dog-bone shaped specimen for tensile testing. (Reproduced with permission from [14])

Fig. 12 Flat shaped specimen for flexural testing. (Reproduced with permission from [14])

performed to determine the modulus and strength values of the specimens displayed in Fig. 12. The flexural test can be conducted by 3-point and 4-point bending test. According to the standard method a special attachment is attached to the machine where force is applied vertically at mid-point by considering the length of the specimen at sufficient crosshead speed usually 2 mm/min. The recorded response of load-displacement is then converted to a stress-strain plot for further analysis. Compressive failure is observed for most of the flexural specimens as a result of poor interface bonding, high void content, and interfacial matrix cracking [42, 48]. Charpy Impact test is performed to determine the impact strength of the specimen which gives the amount of energy absorbed by the specimen [42]. The In-plane shear test can be conducted by performing the tension and compression tests at 45° to the fiber direction to find the strength and modulus values. The specimens for the same are cut from plates at 45° to the fiber direction [37]. An interlaminar shear test can be conducted to determine the apparent interlaminar shear strength using the short-beam shear test method. The vertical load can be imposed on the specimen at mid-portion at sufficient loading rates [42, 48]. The density measurement is possible by considering the Archimedes principle for which the specimen samples are immersed in a glass tube containing water at a known level so that volume of displaced fluid can be determined. Also, the densities can be measured using an analytical balance tool by obtaining the mass of each specimen. Resin burnout and acid digestion method can be used to estimate the fiber volume ratio [39]. Thermal stability such as decomposition temperature and weight loss can be determined by TGA representative curves. The test is performed in a nitrogen atmosphere by placing a TGA specimen in an aluminum crucible at elevated temperatures by the thermal analyzer [2]. The static and dynamic coefficient of friction and wear rate of the composite specimens can be measured by testing the samples in the tribology testing machine under test conditions [52]. The water absorptions for 3D printed composites can be measured according to the standard procedure. The printed specimens placed in a hot air oven at a specified temperature for the required duration are then cooled down at the drying chamber at room temperature. After this, each sample weight is measured and is placed in a boiling distilled water and after socking for some time, samples are then transferred to the

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distilled water at room temperature. Finally, the samples are then removed wiped with tissue papers then the weight of the same is measured revealing the rate of absorption of the composite specimen [43]. The quality of the specimens can be investigated by the optical microscope and on completion of all the destructive tests, the fractured surfaces can also be examined by the same. The main purpose of the optical microscope study is to observe the internal structure of the material. The preparation of the sample for this study is by cutting the specimens at required places, embedding them in resin and polished it for getting proper results. The image analysis software tools can be used to assess the obtained pictures such as Image analyzer.

Applications of 3D Printing Numerous novel 3D printing AM techniques have been developed over the past 20 years. The initial research is focused on the application of AM techniques to the prototyping, educational, digital art and hobby printing, etc. At first, the 3D printed components proved to be poor in mechanical performance and having lesser strength as compared to the same made by conventional processes. Also, the layer by layer deposition of material in Y-direction resulted in unreinforced 3D printing. Hence unreinforced polymer 3D printing part strength is not enough to meet the requirements of industrial products especially in the applications of fully functional and load-bearing parts. With the advent of the preblended filament concept, the filament is reinforced with the fibers to form a composite resulting in the improvement of mechanical performances of the component. Later these are extended to 3D printing of continuous fiber-reinforced polymer composites and functional materials. 3D printing has proven to be an emerging technology with an exponential increase among all AM technologies. Within 20 years, it has been expanded to many more applications ranging from single material printing to multi-material printing and composite material printing to functional material printing [53]. 3D printing technology has grown to uncompromising heights due to the following advantages. 1. Increased specific strength 2. More creative to produce complex monolithic and customized structural parts and geometries 3. Can be tailormade for application-specific. to meet load requirements 4. Faster delivery time, lower material consumption and production cost 5. Stronger parts with no tooling or machining cost and intricate micro-meter resolution, individualized production decentralized 6. On-demand manufacturing is possible Undoubtedly, 3D printing has become the modern representative technology in the field of intelligent manufacturing which helped AM grow to a multi-billiondollar industry. The applications of 3D printing not only limited to the aerospace and automotive industries. It has also expanded too many more industries as follows:

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1. Medical such as printing of medical devices, biomedical such as printing of scaffolds 2. Marine, wind farms such as printing of lightweight components 3. Military, electronics, and bioelectronics such as sensors 4. Rapid tooling, jigs, and fixtures such as etching fixtures 5. Architecture, and constructions such as replicating the building 6. Clean energy, structural health monitoring 7. Robotics such as end effectors 8. Transportation and telecommunications 9. Sports like racing bicycles 10. Food packaging applications 11. Education for effective teaching methods and working models, toys 12. Consumer products such as lightweight wheelchairs 13. Rapid prototyping applications [10, 33]. Earlier AM was very expensive and therefore the technology did not reach most of the people. The decade of research and the technologies that came into existence led to AM becoming more affordable and is now seen in a wide range of industries. Now the 3D printing technology is utilized by many industries to benefit end-use parts [54]. Some of the 3D printed components are shown in Fig. 13.

Fig. 13 (a) Engine Plenum and intake system built to replace the existing component, (b) Engine injector bases printed to replace the metal part, (c) Arms and supporting base of UAV quadcopter are printed, (d) 3D Printed Components & Carbon Fiber Plenum [54, 55]

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Many researchers have taken success steps in the field of composite 3D printing and they made the expertise available to us for carrying out further research. Presently most of the work in this area is focused on nylon polymeric matrix with different reinforcements. There is a lot of scopes for other researchers to make their efforts in making this technology suit to other polymeric matrix materials. An overview of the mechanical performance (tensile, bending, and impact strength) of 3D printed Fiber Reinforced Polymer composites using FFF are shown in Table 3.

Conclusions 3D printing has a vast history with nearly 35 years of existence. But, most of the focus of this technology over the decades has been on platforms and materials. The composite material 3D printing technology is the novel segment in AM. Researches have a lot of challenges to offer but at the same time, this creates numerous opportunities. This book chapter is thoroughly attempted to provide a sweeping summary of the past technologies and present novel technological advancements and their capabilities. The present work brings and insight on the use of a different combination of polymer and reinforcing materials. Also, the work also focusses on the availability of various possible testing methods for the composite material by AM techniques. The AM technologies existent for 3D printing of composite materials, namely, FFF, SLS, SLA, LOM, and CBAM. FFF is the 3D printing technique used most widely among other techniques involved in composite material preparation. True polymers are weak in terms of mechanical performance and require some doping hard material to blend with the polymer to get improved properties. The in-depth development of polymer for AM of composite material has driven to the next-generation materials which are fast replacing the conventional materials and components made from the same. In recent decades, fabrication of composite materials by 3D printing technology proved to be the most promising although not completely replacing conventional manufacturing methods. However, 3D printing technology enables more design possibilities in producing the multifunctional parts (i.e., industrial as well as consumer products). The material developments, process optimization, and new 3D printing techniques of composite material thrived to the next level of manufacturing in the field of AM and are rapidly expanding their market in wide areas. It is to be concluded that incorporation of fiber reinforcements (continuous or discontinuous or particulate fibers or a combination these) are an excellent choice to enhance the properties of a polymer composite and is considered to be a favorable resolution for creating more structural applications. On the other hand, care needs to be taken on optimized deposition parameters like raster angle, infill density, infill pattern, print speed, nozzle diameter, printing temperature, build plate temperature, layer thickness, and fan speed which can greatly influence the overall properties of the component.

103 74 – 50.1 52 41 47 701.41 574.58

0.4 / – 0.9 / – –

0.6 / – 1/– 0.6 / – 1/– –

–

0.5 / 0.0125

Onyx

PLA

CEGF

CCF

CEGF

PP

PLA

CEGF

NCMW

CCF

CMW

PA

ABS

CCF

0.5 / 0.22

634.3 [00] 15.5 [900]

–

CCF

478

252

1478.11

–

TER E-20 PA

CCF

Reinforcement GPNP

Tensile strength (MPa) 63.2 – Flat

Matrix material PLA

Nozzle ϕ (mm) / layer height (mm) 0.4 / 0.12

–

–

67.77

– – – – –

– – 13 GPa

38.1 [450]

858.05

Flexural strength (MPa) 94.3 on-edge

–

– – – – Compressive strength: 233.06 MPa Compressive strength: 82 MPa –

– – –

Compressive strength: 316.8 [00] MPa

Other properties Impact strength: 34.3 KJ/m2 ILSS: 17.1 MPa ILSS: 48.75 MPa

Table 3 Summary of the properties of 3D printed Fiber Reinforced Polymer composites using FFF

Mark forged Mark Two 3D printer [40] Customized 3D Printer [41]

Mark forged Mark One 3D printer [37]

RepRap Prusa 13 3D printer [51] Prusa MK3 i3 modified 3D printer [36]

Customized 3D Printer [48] Mark forged Mark One 3D printer [39] Customized 3D Printer [18]

3D Printing machine [Ref.] Wit Box desktop 3D printer by BQ [42]

1416 B. N. Dhanunjayarao et al.

– – –

Onyx PA PLA

PA

PLA

PLA PA ABS

PEEK

TPI TPI PA

CHCF CCF CCF

CCF

SCF

CCF CCF CCF

IF-WS2

CCF SCF CEGF

83.4 36.6 –

– 23.8 91  3.43 (Rec fill, 8 layers) 75  0.99 (Rec fill, 8 layers) 110  2.09 (Rec fill, 8 layers)

–

–

CKF

CCF

1 / 0.9 0.4 / 0.3 –

–

–

–

–

–

164.5 118.3 –

148.7 132.5 –

1/– – 0.4 / 0.2

–

–

–

–

–

– – –

–

–

53

–

– –

– – – –

81.3

– – 152.9

– – – –

2 / 0.4

153.62

15.2 304.3 111

–

SCFRPA

1.5 / 1

254.8  4.266 150.2  3.744 48  2.165 1366.15  28.78

– / 0.1

PA

CCF CKF CKF  450 CCF

(continued)

Mark forged Mark Two 3D printer [16]

Lulzbot Tez 6 printer [57] Ultimaker [52] Customized 3D printer [43]

COMBOT-I 3D printer by ShaanXi Fibertech [44] Customized 3D printer [50] MakerBot Replicator 2X 3D printer [56] Customized 3D printer [49]

Mark forged Mark X 3D Printer [2] Mark forged Mark Two commercial 3D printer [19]

Mark forged Mark One 3D printer [38]

60 3D Printing of Fiber-Reinforced Polymer Nanocomposites: Additive. . . 1417

PA

PA

Reinforcement CCF

CCF

CCF

200

–

–

Tensile strength (MPa) 5.19 GPa

Nozzle ϕ (mm) / layer height (mm) –

–

–

Flexural strength (MPa) 270.63

–

–

Other properties –

3D Printing machine [Ref.] Mark forged dual nozzle 3D Printer [45] FDM (Mark forged and Arevo Labs) [11] Mark forged Mark One 3D printer with two nozzles [5]

CCF Continuous Carbon Fiber, CEGF Continuous E-Glass Fiber, NCMW Nickel-Chromium Metal Wire, CMW Copper Metal Wire, CKFU Continuous Kevlar Fiber, CKF Continuous Kevlar Fiber, CHCF Chopped Carbon Fibers, GPNP Graphene nanoplatelets, SCF Short Carbon Fiber, IF-WS2 Inorganic Fullerene Tungsten Sulfide, ABS Acrylonitrile Butadiene Styrene, PP Polypropylene, PLA Polylactic Acid, PA polyamide (Nylon), Onyx Nylon blend loaded with Chopped carbon fiber, SCFRPA Short Carbon Fiber Reinforced Polyamide, TER Thermosetting Epoxy Resin, PEEK Poly Ether Ketone Ketone, TPI Thermoplastic polyimide, ILSS Inter Laminar Shear Strength

Matrix material Onyx

Table 3 (continued)

1418 B. N. Dhanunjayarao et al.

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Advanced Functional Nanomaterials for Explosive Sensors

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Khursheed Ahmad and Shaikh M. Mobin

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fabrication of Electrochemical Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterial-Based Electrochemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphene Oxide as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyaniline-Decorated rGO as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MnO2/rGO as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silver Nanoparticle-Decorated rGO as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyelectrolyte-Functionalized Graphene as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . rGO/SrTiO3 as Electrode Modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1424 1425 1425 1428 1430 1431 1435 1437 1440 1442 1443

Abstract

This chapter reviews the recent advances, difficulties, and challenges in the detection of explosives. The nitro-based explosive materials such as K. Ahmad Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] S. M. Mobin (*) Discipline of Chemistry, Indian Institute of Technology Indore, Indore, MP, India Discipline of Biosciences and Biomedical Engineering (BSBE), Indian Institute of Technology Indore, Indore, MP, India Discipline of Metallurgy Engineering and Material Science (MEMS), Indian Institute of Technology Indore, Indore, MP, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_90

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dinitrotoluene (DNT), trinitrotoluene (TNT), and picric acid are used for the production of explosives. There are various methods such as mass spectrometry (MS), ion mobility spectrometry (IMS), and gas chromatography (GC) which are used for the detection of such compounds; however, most of these devices are bulky and expensive and require time-consuming procedures. The electrochemical method is considered the most reliable, highly sensitive, selective, and low cost which possesses advantages over conventional methods. To construct a highly sensitive and selective electrochemical sensor, metal oxide-based nanomaterials have been immobilized onto the active surface area of the different electrodes (glassy carbon, gold, FTO, ITO, platinum, and graphite). The goal of this chapter is to review recent advances in electrochemical detection of nitrocontaining explosive at working electrode consisting of different nanomaterials; specially, we will discuss the role of graphene and graphene-containing metal oxide composites for the detection of nitroaromatic compounds.

Introduction In the present scenario, today’s world has some major challenges which need immediate attentions. These challenges are the environmental pollution, security threats, and energy crisis [1–7]. In recent years, explosive material-based weapons which are simple and easy to install have enormous damage to the world [8–10].Thus, the detection of toxic and explosive materials has become the prime issue of international concern [9]. The explosive material can be defined as a reactive material (nuclear or chemical) which has a high amount of potential energy. This may provide initiation to undergo rapid self-propagating decomposition and produce an explosion [10]. Nitroaromatic compounds possess highly volatile nature and are considered an important class of explosives. These nitroaromatic compounds are composed of benzene ring functionalized by nitro groups. The electron-rich molecules may react with these compounds due to their electron-deficient structure and also form stacking complexes via π-π bonding or interactions [11, 12]. The explosive materials may also cause mutagenic and toxic effects and affect the biological systems. Moreover, few explosive materials can penetrate the skin rapidly which resulted in mutagenicity, toxicity, and carcinogenicity in animals as well as humans. According to the US Environmental Protection Agency, the maximum allowed limit for trinitrotoluene (TNT) in drinking water is ~2 ppb [13]. Thus detection of explosive/toxic compounds is necessary for environmental, homeland, and military issues. In the last few decades, traditional methods such as trained canine teams, mass spectrometry, surface-enhanced Raman spectroscopy, X-ray imaging, mass spectrometry, chromatographic separation techniques, gas chromatography, infrared absorption spectroscopy, and electrochemical procedures have been used for the detection of explosive nitroaromatic compounds [13]. Although these methods have shown some advantages, they suffered from certain features such as time-consuming, lacking portability, expensive, etc. In recent years, the electrochemical methods such as voltammetry have attracted much attention for the detection of explosives/toxic compounds due to their low cost, easy fabrication, high sensitivity, selectivity, and repeatability [14–16]. Moreover, the

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Scheme 1 Schematic diagram of the fabrication of electrochemical sensor

presence of electrochemically active functionalities (such as –NO2) which can be reduced to nitroso/hydroxylamine derivatives motivated the researchers to employ electrochemical techniques for sensing applications. It has been also seen that nanotechnology played a pivotal role in the production of electrochemical sensors with portability, ability for fast detection, specificity, and low cost.

Fabrication of Electrochemical Sensor There are different kinds of electrodes such as FTO, ITO, glassy carbon, graphite, pencil or paper electrodes, etc. that have been widely explored as working substrate to fabricate the electrochemical sensors. Initially, electrode modifiers (materials such as ZnO, rGO, MnO2, TiO2, SnO2, polymers, etc.) dispersed in the DI water with the help of a sonicator, and few drops of this dispersed to be drop casted onto the cleaned electrode substrate and dried in air for few hours (Scheme 1).

Nanomaterial-Based Electrochemical Sensors Matter shows different physiochemical properties in nanoscale even in some cases improvements has been takes place in physicochemical properties. This improvement may be attributed from various factors such as surface area or confinement effects [3]. Nanotechnology, the art of design, engineering, and synthesis of matter in nanoscale, provides a new platform for the detection of analytes. These nanomaterials are used as electrode materials (electrocatalysts) for the fabrication of electrochemical sensors to detect a variety of analytes. In electrochemical method, the electrochemical reactions take place between the working electrode and analytes producing some detectable electronic response by the flow of electrons and ions. The nitroaromatics exhibit redox behavior which is due to the presence of nitro groups, and this phenomenon makes them detectable by electrochemical approach.

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Fig. 1 SEM images of MnO2 nanorods at different magnifications. (Adapted with permission [16])

Nowadays, carbon-based materials such as graphene oxide, reduced graphene oxide (rGO), carbon nanotube and carbon nanoparticles, and carbon dot are the most popular and efficient electrode materials for electrochemical sensor applications. Graphene which is a two-dimensional (2D) carbon material has sp2-bounded carbons discovered by Novoselov et al. [17] that possess excellent properties such as high elasticity, high surface area, rapid heterogeneous rate transfer, high mechanical strength, and fast conductivity. In 2010, Tang et al. [10] have introduced graphene as electrode material for the construction of trinitrotoluene (TNT) sensor. Later, Li et al. have prepared a glassy carbon electrode modified graphene oxide-based nitrophenol sensor [8]. This sensor showed a very good detection limit of 0.02 μM. Gao and coworkers [16] have synthesized MnO2 nanorods by hydrothermal method (Fig. 1). These MnO2 nanorods were employed as electrode modifier for the electrochemical sensing of para-nitrophenol. The voltammetric investigations are the most important approaches for the determinations of the toxic or explosive compounds. In this regard, authors have employed cyclic voltammetry (CV) to detect the para-nitrophenol ( p-NP), and the recorded CV has been shown in Fig. 2. Authors have recorded CV in the presence and absence of p-NP, and the obtained results revealed the oxidation-reduction peaks corresponded to the nitro group only. The effect of different concentrations was also investigated, and the authors observed that the current increases with increasing the

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Fig. 2 Cyclic voltammetry of GC/MnO2 nanorods in the absence (black) and presence (red) of p-NP. (Adapted with permission [16])

Fig. 3 CVof GCE (black), zinc glycerolate/GCE (red), and ZnO/GCE (blue) in the absence (a) and presence (b) of p-NP. (Adapted with permission [11])

concentration of the p-NP. Moreover, this modified electrode (GCE/MnO2) showed good sensitivity for p-NP with average detection limit. Sinhamahapatra et al. have synthesized three-dimensional flower-shaped zinc glycerolate driven with ZnO microstructures for p-NP sensing application [11]. The CV curves for GCE (black), zinc glycerolate/GCE (red), and ZnO/GCE (blue) in the absence (a) and presence (b) of p-NP were recorded and are presented in Fig. 3. The ZnO microstructure modified GCE electrode showed good sensitivity of 404.35 μA/ mMcm2 with the limit of detection of 0.013 mM. Previously, polymers, carbon nanotubes, metal oxides, graphene, etc. have been used to modify the surface of glassy carbon electrode for nitroaromatic sensing applications [18–27]. Numerous electrode materials have been studied which revealed that the performance of any electrochemical sensor largely depends on the presence of the electrode materials and their structural properties such as surface morphology or surface area. So, the preparation of novel electrode materials for hazardous/explosive materials is an important tool. In this chapter we have focused on the graphene-based electrode materials for the determination of nitroaromatics.

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Graphene Oxide as Electrode Modifier Nitroaromatics such as nitrotoluene, nitrophenols, and nitrobenzene have high toxic effects on humans as well as plants and animals. The presence of these compounds (even in trace level) in water also changes the odor and test of the drinking water. Detection of such nitroaromatic compounds is important. Thus, Li et al. have introduced graphene oxide as electrode modifier for the determination of 4-nitrophenol [8]. Authors have prepared graphene oxide by using modified Hummers method. The synthesized graphene oxide was characterized by various techniques such as FTIR, SEM, and AFM. The recorded FTIR spectrum, SEM image, and AFM image have been presented in Fig. 4. The FTIR spectrum (Fig. 4a) showed the well-defined absorption bands which were corresponded to the O–H stretching (3300 cm
1), C–O stretching (1750 cm
1), C–O (epoxy; 1225 cm
1), and C–O (alkoxy; 1035 cm
1) vibrations. The SEM image (Fig. 4b) clearly showed the flakes-like surface morphology which was also confirmed by AFM analysis (Fig. 4c). Further, an electrochemical sensor was fabricated using glassy carbon electrode as working electrode substrate. The obtained CV and EIC graphs of bare GCE (blue) and GO/GCE (red) in the presence of [Fe(CN)6]3
/4
redox couple have been inserted in Fig. 5. The high current response was observed for bare GCE with a well-defined redox peaks, while

Fig. 4 FTIR (a), SEM image (b), and AFM (c) of graphene oxide. (Adapted with permission [8])

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Fig. 5 CV curves (A) and EIS (B) of bare GCE (a) and GO/GCE (b) in the presence of [Fe(CN)6]3
/4
redox solution. (Adapted with permission [8])

the lower current response appeared for GO/GCE which suggested the blocking diffusion of [Fe(CN)6]3
/4
redox and electrode surface (Fig. 5A). The obtained EIS results were also consistent with CV results (Fig. 5B). Although current response was lower for GO/GCE relatively in [Fe(CN)6]3
/4
redox electrolyte compared to the bare GCE, an improved current was observed in the presence of 4-NP. This suggested the electrocatalytic reduction of 4-NP at GO/GCE surface. The poor current response was clearly observed for bare GCE in the P-NP solution compared to the modified GO/GCE which may be due to the presence of structural defects in GO.

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Fig. 6 Working mechanism of 4-NP at GO/GCE. (Adapted with permission [8])

The probable electrochemical reduction of 4-NP using GO/GCE was found to be a two-electron and three-proton process which has been shown in Fig. 6. In the first mechanistic step, free radical of OH• can be seen rather than OH
group. In the second working mechanistic step, O• also appeared. However, the fabricated GO/GCE has shown excellent detection limit, but the performance depends on the presence of electrode modifiers. So researchers have employed some other novel electrode modifiers for the developments of electrochemical sensors.

Polyaniline-Decorated rGO as Electrode Modifier rGO is widely used to enhance the conductive behavior of the nanoparticles. Saadati et al. have synthesized a molecularly imprinted polymer (MIP)/rGO composite for electrochemical sensing of p-NP [28]. FTIR spectra of GO (a) and p-NP-MIP-PANI (polyaniline)/GO (b) have been presented in Fig. 7. The well-defined absorption bands (C¼O, C¼C, O–H, etc.) of GO were observed, whereas in the case of p-NPMIP-PANI/GO, similar absorption band appeared with additional C-N vibration bands of PANI. The electrochemical sensor was fabricated by employing carbon paste electrode (CPE) as electrode substrate. MIP-PANI/GO was employed as electrode modifier. The control electrode (CPE/GO) was also prepared for comparison purposes. The electrochemical activity of the p-NP at CPE was recorded by recording CV curve. The recorded CV curve has been presented in Fig. 8. The pH of the acetate buffer solution was adjusted to 5.0 (0.2 mol/L). Two reduction peaks (C1, C2) and two oxidation peaks (A1, A2) appeared as cathode and anode, respectively (Fig. 8). The reduction peak (C1) was irreversible reduction process of the nitro groups to result the amine moiety which could be oxidized (A2) an anodic scan.

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Fig. 7 FTIR spectra of (a) GO and (b) p-NP-MIPPANI/GO. (Adapted with permission [28])

Fig. 8 CV of 1 mmol/L p-NP at carbon paste electrode (CPE) in acetate buffer solution (pH ¼ 5.0) at a scan rate of 100 mV/s. (Adapted with permission [28])

Further the electrochemical investigation was also carried out for the CPE, CPE/ non-imprinted polymer/(NIP), and CPE/MIP in 0.06 mmol/L p-NP (acetate buffer solution pH ¼ 5.0; scan rate ¼ 100 mV/s). The obtained results have been depicted in Fig. 9. The highest current response was observed for CPE/MIP compared to the other two control electrodes (CPE, CPE/NIP). This suggested the excellent electrocatalytic behavior of MIP-PANI. However, in this case one oxidation peak (A2) and one reduction peak (C1) were observed. The fabricated electrode has revealed a decent detection limit of 20 μM for the determination of p-NP.

MnO2/rGO as Electrode Modifier In past few years, substantial research efforts have been made to prepare the graphene/MnO2 composite for electrochemical devices or sensors. Haldorai et al. prepared graphene/MnO2 composite using a three-dimensional (3D) precursor (manganese benzoate dihydrazinate) [29]. The formation of graphene/MnO2 composite was confirmed by Raman and XRD analysis, whereas the surface area was

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Fig. 9 CV of 0.06 mmol/L p-NP at CPE (a), CPE/NIP (b), and CPE/MIP (c) in acetate buffer solution (pH ¼ 5.0) at a scan rate ¼ 100 mV/s. (Adapted with permission [28])

Fig. 10 Raman (a), XRD (b), and BET (c) analysis of the MnO2/rGO composite. (Adapted with permission [29])

calculated by BET analysis. The obtained results from Raman, XRD, and BET have been shown in Fig. 10. Figure 10a shows the strong Raman bands at 179, 371, 518, 573, and 653 cm
1, whereas two weak bands also appeared at 329 and

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749 cm
1. These bands suggested the presence of a-MnO2. Additionally, D and G bands also appeared at 1349 cm
1 and 1575 cm
1 which corresponded to the presence of graphene (Fig. 10a). The XRD pattern showed the diffraction peaks which corresponded to the (110), (200), (310), (400), (211), (301), (510), (411), (600), (521), (002), and (541) planes. The obtained XRD pattern suggested the tetragonal structure of a-MnO2 and found to be well-matched with previous JPCDS no. 44-0141 (Fig. 10b). The other diffraction peak at 23 also appeared which corresponded to the reflection of rGO (002) and found to be well-matched with JPCDS no. 75-1621. This also suggested the conversion of GO to rGO which confirmed the formation of MnO2/ rGO. The specific surface area of the composites plays a crucial role for electrochemical applications. Thus, the authors have recorded the nitrogen adsorptiondesorption isotherm of the MnO2/rGO composite which has been incorporated in Fig. 10c. The extremely high specific surface area of 323m2/g was observed with an average pore size of 120 nm. This suggested the presence of the macroporous nature of MnO2/rGO composite. Further, the electrochemical investigations were carried out to check the performance of the MnO2/rGO composite using impedance spectroscopy. The impedance spectra of the MnO2/rGO composite were recorded in 0.5 M Na2SO4 solution in the frequency range of 0.1–100 Hz, and equivalent circuit has been presented in inset of Fig. 11. The impedance spectra showed the good electrochemical behavior of MnO2/rGO composite in 0.5 M Na2SO4 solution. This excellent behavior may be due to the high specific surface area of the MnO2/rGO composite. Further the CV of the MnO2/rGO composite was recorded in the presence and absence of 4-NP (0.1 mM), and the obtained results have been shown in Fig. 12a. The CV curve

Fig. 11 Nyquist plot of MnO2/rGO composite in 0.5 M Na2SO4 solution. Inset revealed the equivalent circuit and enlarged view of Nyquist plots. (Adapted with permission [29])

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Fig. 12 CV curves of the composite in the absence and presence of 0.1 mM 4-NP (a), CV curves (b) of different scan rates (10–350 mV/s), calibration plot of peak currents (R1 and O1) versus scan rate (c), and Epc versus log V (d). (Adapted with permission [29])

clearly showed one oxidation peak (O1) and two reduction (R1 and R2) peaks in the presence of 0.1 mM 4-NP. This indicated the oxidation and reduction reactions taking place at the electrode surface toward the nitro group present in 4-NP. However, no oxidation or reduction peaks were observed in the absence of 4-NP. Furthermore, investigation was also carried out to observe the impact of scan rates on the electrochemical performance of MnO2/rGO composite in the presence of 4-NP. The CV curves of the MnO2/rGO composite recorded in 0.1 mM 4-NP at different scan rates (10–350 mV/s) have been shown in Fig. 12b. The current response in terms of oxidation/reduction peak increases with increasing the scan rates under constant concentration of 4-NP. This increased current response was found to be linear, and the calibration plot between the peak current and square root of the scan rate has been presented in Fig. 12c, and the other calibration plot between Epc and log V has been shown in Fig. 12d. The fabricated electrochemical sensor of 4-NP exhibited an excellent detection limit of 0.017 μM. The prepared composite has a potential for other electrochemical applications too. The linear range was found to be 1–100 μM. This sensor was also used to detect 4-NP in river and tap water with good recovery.

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Silver Nanoparticle-Decorated rGO as Electrode Modifier rGO has been a potential material to prepare the composite with improved physiochemical properties. Noor et al. synthesized silver (Ag)-decorated rGO composite (Ag-rGO) under benign approach [30]. The UV-Vis absorption and Raman spectra of the GO (a), rGO-Ag-30 s (b), rGO-Ag-1 min, and rGO-Ag-3 min have been presented in Fig. 13. The two absorption peaks of GO appeared at 277 and 299 nm which can be assigned to the π-π (C–C transition) and n-π transitions (C¼O). A distinct absorption peak was detected at 402 nm for Ag-rGO composite samples (Fig. 13A), while the peak at 227 nm disappeared. This detected peak was assigned to the characteristic peak (surface plasmon resonance ¼ SPR) of Ag nanoparticles. Thus the presence of SPR peak (Ag+ nanoparticles) and disappearance of the peak at 227 nm (GO converted to rGO) suggested the successful formation of Ag-rGO composite.

Fig. 13 UV-Vis (A), Raman (B), and XRD (C) of GO (a), rGO-Ag-30 s (b), rGO-Ag-1 min, and rGO-Ag-3 min. (Adapted with permission [30])

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Further, the formation of Ag-rGO composites was confirmed by Raman analysis. The two Raman bands D and G are well-known bands for GO. The D and G bands were assigned to the Ag1g and E2g (sp2-bonded carbon atoms). The G band in the Ag-rGO slightly shifted toward the lower wavelength, and this indicated the reduction of GO to rGO (Fig. 13B). The XRD pattern of GO showed the diffraction peak at ~10 , and this can be assigned to the (001) plane of GO (Fig. 13C). The XRD patterns of the Ag-rGO composites showed that the diffraction peaks at 38.1 , 44.1 , 64.2 , and 77.2 were assigned to the (111), (200), (220), and (311) planes of face-centered cubic (fcc) of silver nanoparticles, respectively (Fig. 13C). The obtained XRD was wellmatched with JPCDS no. 89-3722. The diffraction peak at 10 shifted to 13 which suggested the reduction in GO. These observations revealed the simultaneous formation of Ag-rGO composite under microwave irradiation conditions. Further, the bare GCE was modified with GO and Ag-rGO composites for electrochemical investigations. The Nyquist plot of the bare and modified electrodes GO/GCE (b) and Ag-rGO composites/GCE (c ¼ 30 s, d ¼ 1 min, and e ¼ 3 min) in the presence of 2.5 mM K3[Fe(CN)6] in 0.1 M KCl has been depicted in Fig. 14A, whereas the CV curves of the bare and modified electrodes GO/GCE (b) and Ag-rGO composites/GCE (c ¼ 30 s, d ¼ 1 min, and e ¼ 3 min) in the presence of 2.5 mM K3[Fe(CN)6] in 0.1 M KCl have been presented in Fig. 14B. The GO/GCE showed the large semicircle and suggested the larger charge transfer resistance (Rct) which may be attributed to the non-conductive nature of GO. However, in the case of Ag-rGO composites, Rct value largely decreased due to the presence of Ag nanoparticles and rGO. The lower Rct value also suggested the better electron transport ability of the Ag-rGO composites. The CV investigation also showed the higher current response (with well-defined oxidation-reduction peaks) for the Ag-rGO composite compared to the bare GCE or GO/GCE electrodes which may be due to the better electron transfer ability of Ag-rGO composite, and the obtained results were consistent with EIS investigations (Fig. 14B). Further, the bare GCE was modified with GO and Ag-rGO composites for electrochemical investigations.

Fig. 14 Nyquist (A) and CV curves (B) of bare GCE (a), GO (b), and Ag-rGO composites (c ¼ 30 s, d ¼ 1 min, and e ¼ 3 min) in the presence of 2.5 mM K3[Fe(CN)6] in 0.1 M KCl. (Adapted with permission [30])

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Fig. 15 CV curves (A) of bare GCE (a), GO (b), and Ag-rGO composites (c ¼ 30 s, d ¼ 1 min, and e ¼ 3 min) modified GCE in the presence of 100 μM 4-NP in 0.1 M PBS (pH 6) at scan rate ¼ 50 mV/s. CV of Ag-rGO composites (f ¼ 3 min) in the absence of 4-NP. CV curves (B) of Ag-rGO composite modified GCE in 100 μM 4-NP in 0.1 M PBS at different pH (pH ¼ 2–9). (Adapted with permission [30])

The CV graphs of the bare GCE (a), GO (b), and Ag-rGO composites (c ¼ 30 s, d ¼ 1 min, and e ¼ 3 min) modified GCE in the presence of 100 μM 4-NP in 0.1 M PBS (pH 6) at scan rate of 50 mV/s have been presented in Fig. 15A. The highest current response for Ag-rGO composite (e ¼ 3 min) modified GCE with reduction peak of nitro group present in 4-NP was obtained compared to the other bare or modified electrodes (Fig. 15A). The CV of Ag-rGO composite (e ¼ 3 min) modified GCE was also recorded in the absence of 4-NP, and this current response was lower compared to the presence of 4-NP. Further, the effect of pH was also investigated using Ag-rGO composite (e ¼ 3 min) modified GCE in 100 μM 4-NP in 0.1 M PBS at different pH (2–9). The obtained results have been incorporated in Fig. 15B. The reduction peak shifted with increasing the pH of the solution. The wider linear range of 1–1100 μM and detection limit of 0.34 μM were achieved toward the sensing of 4-NP using Ag-rGO composite modified GCE. This electrode was also used to determine 4-NP in real water samples with decent recovery.

Polyelectrolyte-Functionalized Graphene as Electrode Modifier Graphene has excellent features such as high conductivity and provides conductive support for semiconducting metal oxides or poor conductive materials. On the other hand, poly(diallyldimethylammonium chloride) (PDDA) which is an electronic conductive polymer and has strong ionic polymeric structural properties has been used to combine with other materials and acted as a functional macromolecule (especially employed to functionalize graphene). This PDDA also helps the graphene to maintain its electronic structure. The solubility, superior conductivity, and biocompatibility of the PDDA are the other beneficial features. Thus, Peng et al. have prepared PDDA-functionalized graphene (PDDA-G) composite for

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Fig. 16 TEM (A) image of PDDA-G; XRD (B) of graphite (a), GO (b), and PDDA-G (c); UV-Vis (C) and FTIR (D) of GO (a) and PDDA-G (b). (Adapted with permission [31])

electrochemical determination of 4-NP [31]. The physicochemical properties of the PDDA-G were investigated by TEM, XRD, UV-Vis, and FTIR measurements. The TEM images of the PDDA-G clearly showed the slightly folded nanosheets, and some aggregations of the sheets could be seen in HRTEM image (Fig. 16A). The XRD patterns of the graphite and GO showed the diffraction peaks at ~26.4 and ~10 which are the characteristic peaks of graphite and GO, respectively (Fig. 16B). The XRD pattern of the prepared PDDA-G exhibited the broader diffraction peak at ~25 which indicated the successful formation of PDDA-G composite. The absence of diffraction peak at 10 in the PDDA-G sample also showed the complete reduction of GO (Fig. 16B). The UV-Vis spectra of the GO showed the absorption peak at 230 nm with a shoulder peak at 300 nm, whereas the PDDA-G exhibited the absorption peak at 270 nm (Fig. 16C). This confirmed the formation of PDDA-G composite. The FTIR spectra of the GO and PDDA-G have been presented in Fig. 16D which also confirmed the formation of GO and PDDA-G with well-defined absorption bands. Finally GCE was modified with GO and PDDA-G using drop cast technique. The performances of the non-modified electrode and modified electrodes (GO/GCE and PDDA-G/GCE) were determined by recording CV curves. The PDDA-G/GCE electrode exhibited the highest current response compared to the other two electrodes (non-modified GCE and GO/GCE).

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Fig. 17 CV of PDDA-G/ GCE at different pH in 4-NP, pH ¼ 4.0 (a), pH ¼ 5 (b), pH ¼ 6 (c), pH ¼ 6.5 (d), pH ¼ 7 (e), and pH ¼ 8 (f) at scan rate ¼ 100 mV/s. Inset showing the calibration plot of peak current versus pH values. (Adapted with permission [31])

Fig. 18 CV of 0.1 mM 4-NP at PDDA-G/GCE at different scan rates in 0.2 M PBS (pH ¼ 7.0). Inset showing the calibration plot of peak current versus scan rates. (Adapted with permission [31])

Further the CV curves of the PDDA-G/GCE were recorded in the presence of 4-NP under different pH. The obtained results have been depicted in Fig. 17. The highest current response was deduced to be at pH 7, whereas the least current response was observed at pH 8 as shown in Fig. 17. Furthermore, the CV of PDDAG/GCE was also recorded in the presence of 0.1 mM 4-NP in 0.2 M PBS (pH ¼ 7.0) at different scan rates, and the obtained results are shown in Fig. 18. From the observations, it is clear that the current increases linearly with increase in the scan rates and the calibration curve of the peak current against the square root of the scan rate has been inserted in the inset of Fig. 18. The modified electrode (PDDA-G/GCE) exhibited a detection limit of 0.02 μM with a wider linear range (0.06–110 μM). This electrode also exhibited good stability, accuracy, and reproducibility. The real samples were also carried out for practical application purposes.

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Scheme 2 Schematic synthetic route of rGO/SrTiO3 composite. (Adapted with permission [15])

rGO/SrTiO3 as Electrode Modifier In recent years, perovskite materials have been widely explored in different applications. The perovskite structures have been considered the key material as a light absorber for solar cell applications. On the other hand, perovskite oxides have been employed in photocatalysis and sensing applications. Ahmad et al. prepared a novel rGO-decorated SrTiO3 composite by using solvothermal method (Scheme 2) [15]. In this work, authors adopted in situ approach for the simultaneous preparation of rGO/SrTiO3 composite under facile conditions. The synthesized rGO/SrTiO3 composite was characterized by advanced physicochemical characterization techniques (XRD, SEM, EDX, and TEM). After the confirmation of the formation of rGO/SrTiO3 composite (denoted as MGCE), a glassy carbon electrode was modified with rGO/SrTiO3 composite using drop cast method. For control experiments or to check the synergy effect, rGO and SrTiO3 were also prepared and deposited onto the glassy carbon electrodes. The poor current response was observed for rGO-modified GCE and SrTiO3-modified GCE compared to the rGO/SrTiO3-modified GCE (MGCE) as confirmed by electrochemical investigations. The recorded CV graphs of the bare GCE and MGCE in the presence of p-NP (A), DNP (B), DNT (C), and TNP (D) have been presented in Fig. 19. The MGCE showed the higher electrocatalytic current response toward the sensing of p-NP compared to the bare GCE (Fig. 19A). The CV graph clearly showed the oxidation (O1) and reduction peaks (R1 and R2). Similarly the current response of the MGCE was found to be higher compared to the bare GCE in the

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Fig. 19 CV for p-NP (A), DNP (B), DNT (C), and TNP (D) at bare GCE and MGCE in the presence of PBS at scan rate ¼ 100 mV/s. (Adapted with permission [15])

presence of DNP (B), DNT (C), and TNP (D) at bare GCE and MGCE in the presence of PBS at scan rate ¼ 100 mV/s. The higher current response of MGCE may be attributed to the synergistic effects between SrTiO3 and rGO. From the obtained results, it was also observed that with increasing the nitro groups, reduction peaks increase which suggested the reduction of nitro groups present on the different nitroaromatic compounds (Fig. 19). Further, the effect of concentration was also investigated on the performance of MGCE at constant scan rates. The obtained CV graphs of the MGCE in the presence of p-NP (A), DNP (B), DNT (C), and TNP (D) at different concentrations at a scan rate of 100 mV/s have been depicted in Fig. 20. The current response in terms of oxidation or reduction peaks increases with increasing the concentrations of p-NP (A), DNP (B), DNT (C), and TNP (D) as shown in Fig. 20. This increased current response was found to be linear, and the linear calibration plots of the peak current versus concentrations of the p-NP (A), DNP (B), DNT (C), and TNP (D) have been presented in the inset of their corresponding CV curves (Fig. 20). The MGCE exhibited excellent detection limit and sensitivity toward the determination of different nitroaromatic compounds ( p-NP, DNP, DNT, and TNP).

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Fig. 20 CV of MGCE for p-NP (a), DNP (b), DNT (c), and TNP (d) at different concentrations at a scan rate of 100 mV/s. (Adapted with permission [15])

Conclusion and Future Prospective In recent years, the detection of toxic or explosive nitroaromatic compounds attracted much attention due to the environmental and security issues. Nitro group containing aromatic compound possesses explosive nature, and previously various techniques have been widely explored to detect these explosive compounds at trace levels. Electrochemical approach displayed many advantages over other methods for the determination of such explosive compounds. The performance of these electrochemical sensors depends on various factors such as the type of electrode modifiers, pH of the medium, working substrates, structural properties of the electrode materials, etc. Moreover, electrochemical sensor may also be miniaturized in small devices to detect the hidden explosive compounds. The performance of such electrochemical can be further improved, and the focal points are given below: (i) By understanding the structural and surface morphological and electronic properties of the electrode modifiers for the construction of explosive sensors.

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(ii) Since the electrode modifiers (nanomaterials) play a crucial role in the electrochemical sensing of explosive compounds and the properties of the nanomaterials could be altered by the synthetic procedures. Thus it is important to develop the novel synthetic approaches to improve the physicochemical properties of the nanomaterials. (iii) Some novel composites of transition metal oxides with graphene or other conductive materials could be an efficient approach for the sensitive determination of explosive compounds. (iv) The surface morphology of the nanomaterials may also affect the performance of the electrochemical sensors. Therefore, it would be beneficial to develop/ design and synthesize the novel surface morphology.

References 1. Colton RJ, Russell JN (2003) Making the world a safer place. Science 299:1324–1325 2. Ahmad K, Mobin SM (2019) High surface area 3D-MgO flowers as the modifier for the working electrode for efficient detection of 4-chlorophenol. Nanoscale Adv 1:719–727 3. Senesac L, Thundat TG (2008) Nanosensors for trace explosive detection. Mater Today 11:28–36 4. Ahmad K, Mohammad A, Rajak R, Mobin SM (2016) Construction of TiO2 nanosheets modified glassy carbon electrode (GCE/TiO2) for the detection of hydrazine. Mater Res Express 3(074005):1–13 5. Singh S (2007) Sensors-an effective approach for the detection of explosives. J Hazard Mater 144:15–28 6. Ahmad K, Mobin SM (2019) Synthesis of MgO microstructures for Congo red dye adsorption and peroxide sensing applications. J Environ Chem Eng 7:103347 7. Ahmad K, Mobin SM (2019) Construction of PANI/ITO electrode for electrochemical sensing applications. Mater Res Express 6:085508 8. Li J, Kuang D, Feng Y, Zhang F, Xu Z, Liu M (2012) A graphene oxide-based electrochemical sensor for sensitive determination of 4-nitrophenol. J Hazard Mater 201–22:250–259 9. Ahmad K, Mohammad A, Ansari SN, Mobin SM (2018) Construction of graphene oxide sheets based modified glassy carbon electrode (GO/GCE) for the highly sensitive detection of nitrobenzene. Mater Res Express 5:078005 10. Tang L, Feng H, Cheng J, Li J (2010) Uniform and rich-wrinkled electrophoretic deposited graphene film: a robust electrochemical platform for TNT sensing. Chem Commun 46:5882–5884 11. Sinhamahapatra A, Bhattacharjya D, Yu J-S (2015) Green fabrication of 3-dimensional flower-shaped zinc glycerolate and ZnO microstructures for p-nitrophenol sensing. RSC Adv 5:37721–37728 12. Ahmad K, Mohammad A, Mobin SM (2017) Hydrothermally grown α-MnO2 nanorods as highly efficient low cost counter-electrode material for dye-sensitized solar cells and electrochemical sensing applications. Electrochim Acta 252:549–557 13. Chen TW, Sheng ZH, Wang K, Wang FB, Xia XH (2011) Determination of explosives using electrochemically reduced graphene. Chem Asian J 6:1210–1216 14. Giribabu K, Oh SY, Suresh R, Kumar SP, Manigandan R, Munusamy S, Gnanamoorthy G, Kim JY, Huh YS, Narayanan V (2016) Sensing of picric acid with a glassy carbon electrode modified with CuS nanoparticles deposited on nitrogen-doped reduced graphene oxide. Microchim Acta 183:2421–2430 15. Ahmad K, Mohammad A, Mathur P, Mobin SM (2016) Preparation of SrTiO3 perovskite decorated rGO and electrochemical detection of nitroaromatics. Electrochim Acta 215:435–446

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16. Wu J, Wang Q, Umar A, Sun S, Huang L, Wang J, Gao Y (2014) Highly sensitive p-nitrophenol chemical sensor based on crystalline α-MnO2 nanotubes. New J Chem 38:4420–4426 17. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA (2004) Electric field effect in atomically thin carbon films. Science 306:666–669 18. Huang J, Wang L, Shi C, Dai Y, Gu C, Liu J (2014) Selective detection of picric acid using functionalized reduced graphene oxide sensor device. Sens Actuators B Chem 196:567–573 19. DelMar RM, Rodríguez IN, Palacios-Santander JM, Cubillana-Aguilera LM, HidalgoHidalgo-de-Cisneros JL (2005) Study of the responses of a sonogel-carbon electrode towards phenolic compounds. Electroanalysis 17:806–814 20. Xu Y, Wang Y, Ding Y, Luo L, Liu X, Zhang Y (2013) Determination of p-nitrophenol on carbon paste electrode modified with a nanoscaled compound oxide Mg(Ni)FeO. J Appl Electrochem 43:679–687 21. Abaker M, Dar GN, Umar A, Zaidi SA, Ibrahim AA, Baskoutas S, Hajry A (2012) CuO nanocubes based highly-sensitive 4-nitrophenol chemical sensor. Sci Adv Mater 4:893–900 22. Huang W, Yang C, Zhang S (2003) Simultaneous determination of 2-nitrophenol and 4-nitrophenol based on the multi-wall carbon nanotubes Nafion-modified electrode. Anal Bioanal Chem 375:703–707 23. Chu L, Han L, Zhang X (2011) Electrochemical simultaneous determination of nitrophenol isomers at nano-gold modified glassy carbon electrode. J Appl Electrochem 41:687–694 24. Ndlovu T, Arotiba OA, Krause RW, Mamba BB (2010) Electrochemical detection of o-nitrophenol on a poly(propyleneimine)-gold nanocomposite modified glassy carbon electrode. Int J Electrochem Sci 5:1179–1186 25. Yang C (2004) Electrochemical determination of 4-nitrophenol using a single-wall carbon nanotube film-coated glassy carbon electrode. Microchim Acta 148:87–92 26. Gu Y, Zhang Y, Zhang F, Wei J, Wang C, Du Y, Ye W (2010) Investigation of photoelectrocatalytic activity of Cu2O nanoparticles for p-nitrophenol using rotating ring-disk electrode and application for electrocatalytic determination. Electrochim Acta 56:953–958 27. Casella IG, Contursi M (2007) The electrochemical reduction of nitrophenols on silver globular particles electrodeposited under pulsed potential conditions. J Electrochem Soc 154:697–702 28. Saadati F, Ghahramani F, Shayani-jam H, Piri F, Yaftian MR (2018) Synthesis and characterization of nanostructure molecularly imprinted polyaniline/graphene oxide composite as highly selective electrochemical sensor for detection of p-nitrophenol. J Taiwan Inst Chem Eng 86:213–221 29. Haldorai Y, Giribabu K, Hwang S-K, Kwak CH, Huh YS, Han Y-K (2016) Facile synthesis of a-MnO2 nanorod/graphene nanocomposite paper electrodes using a 3D precursor for supercapacitors and sensing platform to detect 4-nitrophenol. Electrochim Acta 222:717–727 30. Noor AM, Kumar PR, Yusoff N, Ming HN, Sajab MS (2016) Microwave synthesis of reduced graphene oxide decorated with silver nanoparticles for electrochemical determination of 4-nitrophenol. Ceram Int 42:18813–18820 31. Peng D, Zhang J, Qin D, Chen J, Shan D, Lu X (2014) An electrochemical sensor based on polyelectrolyte-functionalized graphene for detection of 4-nitrophenol. J Electroanal Chem 734:1–6

Application of Perovskite-Based Nanomaterials as Catalysts for Energy Production Fuel Cells

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Structure of Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and Applications of Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Evolution Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxygen Evolution Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol Electrooxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrazine Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nowdays, where the pollution is increasing with the increase of the world’s population and the demand of power is growing drastically. With various types of the recently popping types of renewable energies, the major fact for each research team working in the energy field is the concern of power efficiency. Efficiency for electric sources may be improved by utilizing nanomaterials as alternative to commonly used expensive catalysts similar to platinum black. These nanomaterials are used to increase the efficiency for power production in fuel cells. In this chapter, a unique class of mixed oxide nanomaterials, known as perovskites, will explored. Perovskites, which have the general formula ABO3 where A is an alkali, an alkaline earth metal, or a lanthanide and B is a transition metal, can be prepared through simple chemical methods similar to coprecipitation, combustion, or microwave-assisted methodologies. With the emergence of these nanocatalysts that both are cost-effective and have high

S. M. Ali (*) Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_36

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catalytic activities, perovskites have been successively applied as nanocatalysts for fuel cell reactions, such as methanol oxidation, hydrogen evolution, and many other reactions. The catalytic activity can be evaluated by electrochemical measurements such as potentiodyamic polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA). In this phase, many combinations of metal ions can be used in the perovskite synthesis. This will enable researchers to obtain a variety of useful properties. The presented chapter will include a screened recent advanced literature concerning the efficiency evaluation of perovskites, ABO3, and doped perovskites, A1-xA0 xBO3, AB1-yB0 yO3, or A1-xA0 xB1-yB0 yO3 oxides, nanocatalysts for fuel cell reactions. Keywords

Nanomaterials · Perovskites · Catalyst · Fuel cell reactions · Electrochemical techniques

Introduction General Structure of Perovskites In 1839, Gustav Rose discovered a naturally occurring mineral, calcium titanate (CaTiO3), in the Ural Mountains of Russia and named it after Russian mineralogist Lev Perovski. Perovskites are materials of the general chemical formula ABO3, where A is an alkali, an alkaline earth metal, or a lanthanide and B is a transition metal. Perovskites are also of the same crystal structure as CaTiO3, which can be obtained as orthorhombic crystals, where Ti4+ sites are coordinated octahedral and Ca2+ sites are coordinated cuboctahedral in a cage of oxygen, to form a primitive cubic unit cell. The ideal cubic perovskite structure is realized, such as SrTiO3 (Fig. 1), but not familiar. The distortion can be due to size effect, composition change, or the Jahn-Teller effect [1]. Based on ion size, the degree of the distortion can be estimated by Goldschmidt’s tolerance factor, τ, given in Eq. (1) [3, 4]: ðr þ r O Þ τ ¼ pffiffiffi A 2ð r B þ r O Þ

ð1Þ

where, rA, rB, and rO are ionic radii of A, B, and oxygen ions, respectively. The cubic perovskite structure results if 0.89 > τ > 1, in the case of the ideal cubic SrTiO3, τ ¼1. Any deviation in the value of the tolerance factor will affect the symmetry of the crystal structure, for example, the crystal structures are orthorhombic for GdFeO3 (τ ¼ 0.81) and hexagonal for BaNiO3 (τ ¼ 1.13) [3, 4]. The change in composition can also lead to a deviation from the ideal cubic structure, for example, if the transition metal ion, B ion, has several possible

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Fig. 1 The ideal cubic perovskite structure. Large open circles denote the oxygen atoms, and smaller open and solid circles denote the metal cations with A and B sites, respectively [2]

oxidation states that can be altered with the temperature change, such as Fe or Ni. The oxygen content can be consequently changed with temperature, resulting in several oxygen vacancy orderings or geometries, AFeO3-δ or ANiO3-δ [1]. The Jahn-Teller effect can be described as a structure distortion due to the presence of an odd number of electrons in the eg orbital of the transition metal ion, such as Mn, due to the elongation of the coordinated B–O bonds [1].

Properties and Applications of Perovskites There is an unlimited possibility of combinations of most of metal ions in the periodic table to form stable perovskite structures. Therefore, perovskites can be considered as multifunction materials due to the chemical flexibility and the complex nature of the transition metal ions at the B site of the perovskite. There is a wide range of interesting properties such as dielectric, optical, absorption, electrical, and magnetic properties, which permit its applications in many potential fields such as light-emitting diodes, field-effect transistors, photodetectors, photovoltaics, and optoelectronics devices, as shown in Table 1 [5]. The electrocatalytic activity of perovskites for many potential reactions such as energy production fuel cell reactions has attracted researchers’ attentions for many years. Several fuels can be used to produce energy, which can be further used in many applications, as shown in Fig. 2. In this chapter, recent literature concerning the catalytic performance of ternary perovskites, ABO3, and doped perovskites, A1-xA0 xBO3, AB1-yB0 yO3, or A1-xA0 xB1-yB0 yO3, for fuel cell reactions will be summarized and discussed. The most important fuel cell reactions, to be considered here, are hydrogen evolution reaction (HER), oxygen evolution reaction (OER), methanol oxidation reaction (MOR), and hydrazine electrooxidation.

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Table 1 Properties and applications of perovskites [6] Property Proton conductivity

Ionic conductivity Mixed conductivity

Ferroelectric/ piezoelectric Catalytic Electrical/dielectric

Magnetic Optical

Superconductivity

Application SOFC electrolyte Hydrogen sensor H2 production/extraction Solid electrolyte SOFC electrode

Piezoelectric transducer Thermistor, actuator Catalyst Multilayer capacitor Dielectric resonator Thin film resistor Magnetic memory Ferromagnetism Electro-optical modulator Laser Superconductor

Material BaCeO3, SrCeO3 BaZrO3

Ref. [7–9]

(La,Sr)(Ga,Mg)O3-δ La(Sr,Ca)MnO3-δ, LaCoO3 (La,Sr)(Co,Fe)O3-δ BaTiO3, Pb(Zr,Ti)O3 Pb(Mg,Nb)O3 LaFeO3, La(Ce,Co)O3 BaTiO3, BaZrO3

[10] [11–13]

GdFeO3, LaMnO3

[19– 21] [22– 25]

(Pb,La)(Zr,Ti)O3 YAlO3, KNbO3 Ba(Pb,Bi)O3, BaKBiO3

[14, 15] [16, 17] [18]

[26– 28]

Hydrogen Evolution Reaction The HER in an acidic medium starts with a proton charge, Volmer step Eq. (2), to produce an adsorbed hydrogen. Then an electro-desorption, Heyrovsky step Eq. (3), or a recombination of two physically desorbed hydrogen, Tafel step Eq. (4), occurs to yield a hydrogen molecule, as shown in the following equations: M þ H3 Oþ þ e ! MHads þ H2 O

ð2Þ

MHads þ H3 Oþ þ e ! H2 þ M þ H2 O

ð3Þ

MHads þ MHads ! H2 þ 2 M

ð4Þ

The method of preparation can affect the catalytic performance of perovskites. In general, perovskite materials prepared by the hydrothermal methods showed a better catalytic performance than those prepared by the solid-state reactions. The reasons can be the enhanced morphological properties in terms of porosity and surface area and the disappearance of undesirable secondary phases. SrRuO3 was synthesized by three different methods: microwave-assisted citrate, traditional heating citrate methods, and coprecipitaion, and the catalytic activity for the HER was examined via electrochemical techniques in an acidic medium [30]. Based on the calculated electrochemical parameters, it was found that the citrate-

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Fig. 2 (a) Electrochemical reactions occurring in different types of fuel cell and (b) fuels and applications for fuel cells [29]

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nitrate method provided a better route than the coprecipitaion for preparing a more efficient SrRuO3 catalyst for the HER and it is better to use microwave radiation than the traditional heating during the synthesis. SrRuO3 synthesized by malic acid sol-gel method exhibited also a higher catalytic activity for the HER than the stateof-the-art Pt/C in an alkaline medium, with a value of HER overpotential of 0.11 V [31]. The effect of the synthesis method was related to the surface area, morphological feature, and the particle size of the prepared catalyst. The microwave synthesis method was further optimized as it resulted in the catalyst with the highest performance. LaNiO3 was prepared by the microwave-assisted citrate nitrate method at different operating powers and irradiation times [32]. An intermediate value of the operating microwave power, 720 W, resulted in the perovskite with the highest electrocatalytic performance, as compared to low and high operating power values. The catalytic activity was also enhanced with increasing microwave irradiation time; again, these findings were related to the high surface area of the prepared perovskite catalysts. The B-site metal ion is responsible for the electrocatalytic activity of perovskites; therefore, the catalytic activity can be affected by changing the B-site metal ion type. A study of the catalytic activity La-transition metals perovskites; LaBO3, B ¼ Ni, Co, Fe, or Mn, for the HER [33]. The order of the electrocatalytic activity agreed with the values of the tolerance factor, and a better activity is noticed with increasing tolerance factor, provided that the transition metal itself has a catalytic activity for the HER (Fig. 3). Among different transition metal-based perovskites, LaFeO3 catalyst showed the highest performance with the lowest activation energy of 41.2 kJ.mol1. A study of the effect of doping at the B site was performed by examining the electrocatalytic activity of LaNi1-xCoxO3, x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1. The catalytic activity was decreased with increasing fraction of the doped Co because Co itself has a lower activity than Ni for the HER, with the exception of

Fig. 3 (a) Linear Tafel polarization curves for the HER recorded on CPEs modified with 10% (w/w %) (—) LaNiO3, (. . .) LaCoO3, (- - -) LaFeO3, and (-..-..-) LaMnO3 prepared by the microwaveassisted citrate method at 720 W for 30 min in 0.1 mol L1 H2SO4 and scan rate ¼ 1 mV s1. (b) Dependence of the HER rate on the tolerance factor [33]

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Fig. 4 SEM micrographs of (a) CaRuO3, (b) SrRuO3 with a magnification of 20,000 times, and (c) BaRuO3 with a magnification of 15,000 times, prepared by coprecipitation method [34]

LaNi0.4Co0.6O3, which showed a better performance than the undoped LaNiO3 due to its unique morphology [33]. The A-site metal ion is not only responsible for the stability of the whole perovskite configuration, but it can also contribute to the catalytic activity by an interaction with the B-site metal ion. The catalytic activity of ARuO3, A ¼ Ca, Sr, or Ba, for the HER in an acidic medium was investigated [34]. Changing the A-site metal ion affects the structure and morphological feature of the prepared ruthenates, as shown in Fig. 4. BaRuO3 exhibited the highest catalytic activity, in spite of its largest surface area among prepared ruthenates, because the presence of Ba2+ at the A site resulted in the strongest Ru-Ru bond as compared to Ca2+ and Sr2+ ions. The doping effect at the A site was examined by testing the catalytic activity of a series of SrxCa1-xRuO3, x ¼ 0.25, 0.50, and 0.75. All prepared doped perovskites showed higher catalytic activity than undoped ones, SrRuO3 and CaRuO3, which indicated the importance of the doping concept at the A site for the enhancement of the catalytic activity. Increasing the fraction of the Ca2+ ion dopant resulted in a decreased particle size, an increased surface area, and thus an enhanced catalytic activity.

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The lanthanide ion type affects the catalytic activity of LnFeO3, Ln ¼ La, Nd, Sm, or Gd, for the HER in an acidic medium [35]. According to calculated exchange current density values and activation energies of different lanthanide-ferrites catalysts, the order of the electrocatalytic activity was NdFeO3 > LaFeO3 > SmFeO3 > GdFeO3. This can be explained on the basis that the type of the lanthanide ion affected the perovskite structure in terms of Fe-O bond length and therefore its strength. NdFeO3 contained the longest and thus the weakest Fe-O bond; therefore the loosely bound lattice oxygen can contribute to the highest NdFeO3 catalytic activity. A series of LaxSm1-xFeO3, x ¼ 0.25, 0.50, or 0.75, was tested as catalyst for the HER. Doped ferities exhibited higher activities for the HER as compared to LaFeO3 and SmFeO3 perovskites, and the catalytic activity was decreased with increasing fraction of the doped Sm. The electrocatalytic activity of Ln0.3Sr1.7PdO3 (Ln ¼ La, Nd, or Gd) perovskites for the HER was studied by polarization and impedance measurements [36]. The lanthanide doping at the Sr2+ site enhanced the activity in the case of Gd0.3Sr1.7PdO3, showed no difference in the case of Nd0.3Sr1.7PdO3, and decreased the activity for La0.3Sr1.7PdO3, as compared to the ternary undoped Sr2PdO3. A correlation between the lanthanide dopant size and the catalytic activity of lanthanide-doped Sr2PdO3 showed that the increased surface area with decreasing dopant ion size is not the only factor contributing to the electrocatalytic activity, but also the formation of other Pd phases, which can also participate to the catalytic activity.

Oxygen Evolution Reaction The oxygen evolution reaction (OER) is the anodic reaction of the water splitting process. The kinetics of OER involves complicated multi-electron steps, several accompanying processes can co-exist such as the dissolution of the metal oxide catalyst, and thus, it is very confusing to understand the OER mechanism clearly (5) and (6). The general mechanism for the OER in an alkaline medium involved three steps; first and second electron transfer processes, to form adsorbed hydroxyl group and oxygen atom, respectively. In the final step, recombination of two desorbed oxygen atoms results in the evolution of an oxygen molecule. This can be simply expressed as follows [37–40]: S þ HO ! S  OH þ e

ð5Þ

S  OH ! S  O þ Hþ þ e

ð6Þ

S  O þ S  O ! 2S þ O2

ð7Þ

where S is the active site. On the other hand, the mechanism of the oxygen evolution at a metal oxide electrode in an acidic medium involves first a discharge of a water molecule to form adsorbed hydroxyl radical at the electrode surface, as shown in Eqs. (8) and (9)

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MOx þ H2 O ! MOx ðH2 OÞ

ð8Þ

MOx ðH2 OÞ ! MOx ð: OHÞ þ Hþ þ e

ð9Þ

According to the adsorption type, the physically adsorbed hydroxyl group can be oxidized to form an oxygen molecule, as shown in Eq. (10), or the chemically adsorbed hydroxyl group can interact with the metal oxide to form a higher oxide which decomposed to the lower oxide with the oxygen evolution, as shown in Eqs. (11) and (12): MOx ð: OHÞ ! MOx þ 1=2 O2 þ Hþ þ e

ð10Þ

MOx ð: OHÞ ! MOxþ1 þ Hþ þ e

ð11Þ

MOxþ1 ! MOx þ 1=2O2

ð12Þ

The electrocatalytic activity for the OER of recently prepared Sr2PdO3 perovskite in HClO4 medium was tested [41]. The value of the current density was increased by about 50 times by casting the perovskite over the working electrode surface. The high catalytic activity of Sr2PdO3 can be due to its electronic structure which can undergo a transition from low to high spin state at a given temperature, similar to LaCoO3. The perovskite catalyst exhibited a high operation stability over a period of 3 and a half hours and showed a self-activation character under potentiostatic conditions. The effect of the type of B-site metal ion on the catalytic activity of transition metal-based perovskites, LaBO3, B ¼ Ni, Fe, Mn, Cr, or Co, for the OER in HClO4 medium was examined via electrochemical techniques [42, 43]. The order of the electrocatalytic activity was LaNiO3 > LaFeO3 > LaMnO3, while LaCrO3 and LaCoO3 perovskites exhibited no activity for the OER in the acidic medium due to the dissolution of the first and the formation of a passive film of Co3O4 over the second. The order of the electrocatalytic activity was explained based on the molecular orbital theory proposed by Gasteiger et al. [44], where the filling degree of the eg orbital of the B-site transition metal ion can greatly affect the covalency between the B-site ion and the adsorbed radicals, which in turn, affected the catalytic activity for the OER. The B-site metal ion type largely affected the morphology and the particle size of different transition metal-based perovskites. The doping effect at the A site was also investigated through testing the catalytic activity of a series of LaxSr1-xNiO3, x ¼ 0.2, 0.4, 0.6, and 0.8. It was found that the partial substitution of the trivalent La ion with the divalent Sr ion resulted in a partial valence transformation at the Ni site from Ni3+ to Ni2+, as indicated by the formation of NiO as a secondary phase. The electrocatalytic activity was increased with increasing doping extent due to the increased surface area and the increased amount of formed NiO, which was reported as an efficient catalyst for the OER. On the other hand, Yamada et al. examined the catalytic activity for the OER for a wide variety of perovskites (ABO3, A ¼ Ca, Sr, Y, or La; B ¼ Ti, V, Cr, Mn, Fe, Co, Ni, or Cu) by linear sweep voltammetry, as shown in Fig. 5 [45]. It was found that the eg electrons for the transition metal ions and oxygen 2p-band center were not helpful

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Fig. 5 Linear sweep voltammograms of (a) CaBO3 (B ¼ Ti, V, Cr, Mn, Fe, Co), (b) SrBO3 (B ¼ Ti, V, Cr, Mn, Fe, Co), (c) YBO3 (B ¼ V, Cr, Mn, Fe, Co, Ni), and (d) LaBO3 (B ¼ V, Cr, Mn, Fe, Co, Ni, Cu). The yellow lines represent current densities of 0.05 mA cm2, which determine onset OER potentials. The upper and bottom panels display entire and enlarged ranges, respectively [45]

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descriptors for the OER activity. However, the charge-transfer energy calculated from DFT model was an appropriate descriptor for the design of the perovskite catalysts. The co-doping concept, at both A and B sites, was used to increase the catalytic activity of the perovskites for the OER [46]. A nano-sized La0.4Sr0.6Ni0.5Fe0.5O3, prepared by the sol-gel method, showed a superior catalytic activity for the OER in KOH medium than those of undoped perovskites LaNiO3, LaFeO3, and partially doped perovskite LaNi0.5Fe0.5O3. La0.4Sr0.6Ni0.5Fe0.5O3 catalyst exhibited a lower overpotential and a smaller Tafel slope due to the optimized eg filling. Suntivich et al. reported that a co-doped or a dual functional perovskite can proceed a high catalytic activity for the OER if the eg filling of the transition metal ion in the perovskites reached 1.2, the optimum value that yielded the best catalytic performance [44]. The degree of eg filling greatly affected the bonding between the lattice oxygen in the perovskite and the adsorbed intermediates of the OER, thus affecting the electrocatalytic activity. Ba0.5Sr0.5Co0.8Fe0.2O3  δ (eg occupancy close to unity) exhibited a better catalytic performance than IrO2 catalyst. In the same way, the electrocatalytic activity for the OER was found to be enhanced by a lattice oxygen (perovskite) participation [47]. Silicon-incorporated strontium cobaltite perovskite electrocatalyst exhibited a 12.8-fold increase in oxygen diffusivity, which corresponded to a ten-fold enhancement in the OER catalytic activity, indicating lattice oxygen participation. A series of La0.6Sr0.4CoxFe1-xO3-δ (x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1, denoted as LSF, LSCF-28, LSCF-46, LSCF-64, LSCF-82, and LSC, respectively) showed almost the same crystal structure and morphology. However, changing Co/Fe ratio, the value of x, strongly affected the catalytic activity for the OER [48]. Table 2 showed a comparison of different catalysts prepared in this series and also with other reported benchmark catalysts for the OER. The optimal LSCF-82 catalyst exhibited the best OER activity as indicated by a low onset potential of 1.541 V, a small Tafel slope of 80.56 mV dec1, in 0.1 M KOH solution. The type of the A-site metal ion strongly affected the catalytic activity of perovskites for the OER. The electrocatalytic activity of CaCoO3 and SrCoO3 for the OER in an alkaline medium was compared [55]. Both perovskites showed a comparable electronic structure, with a shorter Co–O bond in CaCoO3 and a greater oxygen 2p-band character, as shown in Fig. 6. This difference has no effect on the onset potential of the OER, but it affected the catalyst stability. The shorter Co–O bond facilitated a faster mechanism of the OER over CaCoO3 than SrCoO3. Double perovskites (Ln0.5Ba0.5)CoO3-δ (Ln ¼ Pr, Sm, Gd, or Ho) were reported as efficient catalysts for the OER [56] due to the optimum eg filling degree, close to unity. In addition, it was found that the catalytic activity can be increased as the oxygen 2pband center approached the Fermi level. However, if the oxygen 2p-band center is too close, the catalyst stability started to decreased. Therefore, the high catalytic activity and stability of these double perovskites resulted because the oxygen 2p-band center was neither too close nor too far to the Fermi level, i.e., located at optimum distance from the Fermi level. The oxygen 2p-band center relative to the Fermi level was computed by density functional theory, and the order of the catalytic performance of double perovskites was Pr > Ho > Sm > Gd-doped BaCoO3-δ.

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Table 2 Comparison on OER performances between our LSCF samples and the catalysts in other previously reported literatures [48] Sample LSF LSCF-28 LSCF-46 LSCF-64 LSCF-82 LSC BCFSn-721 RuO2 LSCF-10% BSCF/NC CoFe2O4 NF 1D-Co3V2O8

E10 mA cm2 (E/V vs. RHE) 1.661 1.624 1.614 1.603 1.586 1.652 1.65 1.558 1.643 1.58 1.67 1.58

Tafel slope (mV dec1) 87.92 91.25 88.25 87.18 80.56 92.02 69 47.6 81.59 65 82.15 42

Ref. [48]

[49] [50] [51] [52] [53] [54]

Fig. 6 Electronic spin states of the octahedral site Co ions of ACoO3 (A ¼ Ca, Sr) and Co3O4 and a schematic band diagram of ACoO3 (A ¼ Ca, Sr). IS, intermediate spin [55]

Methanol Electrooxidation Methanol oxidation reaction (MOR) is an important reaction in many industrial applications such as treatment of gas pollutants and exhaust effluents produced by alcohol-fueled vehicles. Methanol is the used fuel in the direct methanol fuel cell (DMFC), and the efficiency of such cell is low, about 10%, which constituted a

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challenge for researches to develop new catalytic materials that can yield a higher conversion efficiency. A series of perovskites (ABO3; A ¼ Ba, Ca, Sr, La; B ¼ Fe, Ru) were synthesized and tested as catalysts for the MOR [57]. It was found that perovskites with Ru in the B site showed a high catalytic performance for methanol electrooxidation. Figure 7 showed the cyclic voltammograms for methanol oxidation over SrFeO3, SrRuO3, and LaRuO3, There were two characteristic anodic peaks (1 and 2) and one cathodic peak (3) appeared only for La and Sr ruthenates. In addition, the catalytic performance for oxidation of ethanol, formic acid, and carbon monoxide for ruthenates was tested and compared with Pt catalyst as shown in Table 3. Data showed that the oxidation onsets occurred at much lower potentials for perovskites as compared to Pt catalyst, with the exception of formic acid. Perovskite materials have been successfully used as efficient catalysts for methanol conversion into carbon dioxide. The catalytic activity of LaBO3, B ¼ Co, Mn, or Fe, prepared by the reactive grinding, for methanol oxidation, was examined by Levasseur et al. [58]. There are two types of adsorbed oxygen associated with the perovskite materials. The first is the weakly bounded oxygen to the perovskite surface, α-oxygen. The second is β-oxygen which is assigned to the lattice oxygen and can be taken as an indicator for the oxygen mobility within the perovskite lattice. It was found that the rate of the methanol oxidation was dependent on α-oxygen and there were two possible mechanisms based on the α-oxygen amount. In case of

Fig. 7 CV for three different perovskites obtained with Nafion electrolyte (temperature 80 °C; sweeping rate 6 mV/s) [57]

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Table 3 Potential for the onset of oxidation of different fuels on perovskite and Pt catalyst (data were obtained at 80 °C, 6 mV s1) [57]

Catalyst SrRuO3 LaRuO3 Pt

Onset potential of methanol (V vs. DHE) 0.28 0.30 0.48

Onset potential of ethanol (V vs. DHE) 0.33 0.33 0.50

Onset potential of formic acid (V vs. DHE) 0.30 0.32 0.28

Onset potential of CO (V vs. DHE) 0.33 0.35 0.50

excess α-oxygen amount, a monodentate carbonate intermediate was formed which then decomposed into carbon dioxide. As the reaction proceeded, the concentration of α-oxygen became low, and a bidentate carbonate was the intermediate, which led to protonation of anion vacancies required for the oxidation process. According to temperature-programmed desorption of oxygen experiments, LaMnO3 contained the highest amount of α-oxygen and showed the highest activity for the MOR, while both LaCoO3 and LaFeO3 showed a deactivation behavior. Mesoporous perovskites, LaBO3, B ¼ Co, Mn, or Fe, were prepared by nanocasting method over a mesoporous silica and tested as catalysts for the MOR [59]. The nanocast mesoporous LaMnO3 catalyst showed the highest conversion efficiency for methanol as compared with both LaCoO3 and LaFeO3, as shown in Fig. 8. This can be due to the highest surface area value reported for LaMnO3 catalyst. After one catalytic run, the change in the BET specific surface area was very small, and LaMnO3 perovskite structure was retained with no impurity peaks observed in the XRD spectrum. Also, a second catalytic run was found to be reproducible, similar to that of the fresh catalyst. The electrocatalytic activity of a nano-scale perovskite LaMnO3, synthesized by a microwave-assisted coprecipitation method, for the MOR was improved by functionalizing the perovskite with Pt nanoparticles and multi-walled carbon nanotubes (MWCNTs) [60]. The proposed catalyst showed higher catalytic activity than its individual components due to the catalytic effect of the transition metals (La and Mn), the anti-poisoning effect provided by Pt, and the enhanced electrical conductivity for the oxygen ion supported by MWCNTs, as shown in Fig. 9. The oxidation methanol peak appeared at +1.4 V, in the case of the pure perovskite catalyst, curve b, which was not suitable for quick start-up of fuel cell. The reduction in the onset potential values resulted in introducing Pt nanoparticles, curve d, and both Pt nanoparticles and MWCNTs, curve e. The same improvement procedure was applied to enhance the electrocatalytic activity of NdFeO3 for the MOR and showed good results.

Hydrazine Oxidation Hydrazine offers a promising use in fuel cells, where it can be oxidized and split into hydrogen and nitrogen. Hydrazine can be also used in fuel cells, and due to its high hydrogen content, hydrazine fuel cells are characterized by high electrical energy

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Fig. 8 Methanol conversion profiles as a function of temperature over LaMnO3, LaCoO3, and LaFeO3 perovskites synthesized by using the nanocasting method [59]

80 b e

J / mA.cm–2

62 If

44

Ib

26

d c

8 –10 –0.4

a

0

0.4

0.8

1.2

1.6

E / V vs. SCE Fig. 9 Cyclic voltammograms of methanol oxidation on (a) GC and GC/Chitosan (CH), (b) GC/ LaMnO3NPs/CH, (c) GC/PtNPs/CH, (d) GC/PtNPs/LaMnO3NPs/CH, and (e) GC/PtNPs/CNTs/ LaMnO3NPs/CH electrodes in 1 M H2SO4 and 0.8 M methanol at scan rate of 50 mV s1 [60]

output and the absence of carbon monoxide emissions. SrPdO3 showed a high electrocatalytic activity for hydrazine oxidation with an increased rate, current density, by about 10 times compared with electrochemically deposited Pd [61]. The superior activity is limited to a strong acidic condition, which indicates a fiverather than a four-proton mechanism, as shown in the following equation:

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Fig. 10 A mechanism for the electrooxidation of hydration at GC/LaCoO3 electrode in alkaline medium [62]

NH2 NH3 þ ! N2 þ 5 Hþ þ 4 e

ð13Þ

SrPdO3 perovskite showed a high operation and storage stability after a storage time of 15 days at room temperature, as indicated by the small relative current decrease, 3.9%, as compared to the freshly prepared material. The electrocatalytic activity for transition metal-based perovskites, LaBO3, B ¼ Ni, Mn, Cr, Co, or Fe, for the hydrazine electrooxidation was examined by Ali and Al-Otaibi [62]. It was found that no hydrazine oxidation peak was observed in alkaline hydrazine solution except for LaCoO3. On the other hand, under neutral or acidic conditions, LaCoO3 perovskite exhibited low activity. Therefore, alkaline condition was essential for the high electrocatalytic activity of LaCoO3 for hydrazine oxidation. This can be explained on the basis that the chemisorbed oxygen species at the perovskite surface which increased its catalytic activity for the electrooxidation reactions. The amount of these active species is increased by the presence of oxygen vacancies in the perovskite lattice, which are more promoted for LaCoO3, with respect to other transition metal-based perovskites, due to the spin state transition of Co from 3+ to 2+ state. A possible mechanism for the hydrazine electrooxidation at LaCoO3 is shown in Fig. 10.

Conclusions and Further Outlook Perovskites are a distinct family of crystalline nanomaterials, with the general formula ABO3, due to the infinite possibilities by which it can be formed from the recombination of most of the elements in the periodic table. This leads to a great

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diversity of properties that have many useful applications. This chapter focuses on its applications as catalysts for energy-producing reactions such as fuel cell reactions. • The catalytic activity for the HER is dependent on the synthesis method by which the perovskite was prepared. Hydrothermal methods present better catalysts than solid-state reactions, and it is more favored by microwave heating. Also, changing the synthesis parameters can affect the structural and morphological features of the prepared materials which in turn can affect its activity. The metal ion at the B site, transition metal ion, is responsible for the catalytic activity. Reported studies showed that changing the type of the B ion can alter the reaction mechanism and therefore the rate by which the HER proceeds. The A metal ion also contributes to the catalytic activity and can cause an enhancement by a synergetic effect. Introducing dopant ions at A and/or B sites, A1-xA0 xBO3, AB1-yB0 yO3, or A1-xA0 xB1-yB0 yO3, usually improves the activity at optimum dopant size and ratio. • The catalytic activity for the OER is largely affected by the filling degree of eg orbital of the transition metal ion, at the B site. The activity is higher with increasing covalence between the metal ion, eg electrons, and adsorbed oxygen, 2p band. Recent studies showed the additional roles of the calculated chargetransfer energy and the distance between the oxygen 2p band and the Fermi level in evaluating the electrocatalytic activity. • High catalytic activity for the MOR is observed when Ru at the B site, ruthenates. The amount of the α-oxygen affects the activity as in the presence of excess amount, the methanol is directly oxidized to carbon dioxide through the formation of a monodentate carbonate intermediate. • Recent studies show that hydrazine can be oxidized at perovskite surfaces efficiently either at acidic or alkaline conditions, with different reaction mechanisms. At the end, the reader should consider the amazing variation extent and the great diversity in properties of perovskite nanomaterials that have opened the door to millions of researchers over the past years to discover materials with important applications in many fields. This manipulation of chemical structures and properties should be continued, as there are still many distinctive properties to be discovered.

References 1. Johnsson M, Lemmens P (2007) Crystallography and chemistry of perovskites. In: Handbook of magnetism and advanced magnetic materials. Wiley, Chichester 2. Cowley RA, Gvasaliya SN, Lushnikov SG et al (2011) Relaxing with relaxors: a review of relaxor ferroelectrics. Adv Phys 60:229–327 3. Wells AF (1995) Structural inorganic chemistry. Oxford Science Publications, LibraryThing, online service 4. Müller U (1993) Inorganic structural chemistry. Wiley, Chichester

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5. Assirey EA (2019) Perovskite synthesis, properties and their related biochemical and industrial application. Saudi Pharm J 27:817–829 6. Souza ECC, Muccillo R (2010) Properties and applications of perovskite proton conductors. Mater Res 13:385–394 7. Iwahara H, Esaka T, Uchida H et al (1981) Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production. Solid State Ionics 3–4:359–363 8. Uchida H, Maeda N, Iwahara H (1983) Relation between proton and hole conduction in SrCeO3-based solid electrolytes under water-containing atmospheres at high temperatures. Solid State Ionics 11:117–124 9. Iwahara H, Uchida H, Ono K et al (1988) Proton conduction in sintered oxides based on BaCeO3. J Electrochem Soc 135:529–533 10. Feng M, Goodenough JB (1994) A superior oxide-ion electrolyte. Eur J Sol State Inorg Chem 31:663–672 11. Teraoka Y, Zhang HM, Okamoto K et al (1988) Mixed ionic-electronic conductivity of La1xSrxCo1yFeyO3δ perovskite-type oxides. Mater Res Bull 23:51–58 12. Carter S, Selcuk A, Chater RJ et al (1992) Oxygen transport in selected nonstoichiometric perovskite-structure oxides. Solid State Ionics 53:597–605 13. Fukui T, Ohara S, Kawatsu S (1998) Conductivity of BaPrO3 based perovskite oxides. J Power Sources 71:164–168 14. Dimos D, Mueller C (1998) Perovskite thin films for high-frequency capacitor applications. Annu Rev Mater Res 28:397–419 15. Shaw T, Trolier-McKinstry S, McIntyre P (2000) The properties of ferroelectric films at small dimensions. Annu Rev Mater Sci 30:263–298 16. Spinicci R, Tofanari A, Delmastro A et al (2002) Catalytic properties of stoichiometric and nonstoichiometric LaFeO3 perovskite for total oxidation of methane. Mater Chem Phys 76:20–25 17. Forni L, Rossetti I (2002) Catalytic combustion of hydrocarbons over perovskites. Appl Catal B Environ 38:29–37 18. Kishi H, Mizuno Y, Chazono H (2003) Base-metal electrode-multilayer ceramic capacitors: past, present and future perspectives. Jpn J Appl Phys 42:1–15 19. Jonker GH (1956) Magnetic compounds with perovskite structure IV. Conducting and nonconducting compounds. Physica 22:707–722 20. DeTeresa J, Ibarra M, Algarabel P et al (1997) Evidence for magnetic polarons in the magnetoresistive perovskites. Nature 386:256–259 21. Moritomo Y, Asamitsu A, Kuwahara H et al (1996) Giant magnetoresistance of manganese oxides with a layered perovskite structure. Nature 380:141–144 22. Moret M, Devillers M, Worhoff K et al (2002) Optical properties of PbTiO 3, PbZrxTi1–xO3, and PbZrO3 films deposited by metalorganic chemical vapor on SrTiO 3. J Appl Phys 92:468–474 23. Jona F, Shirane G, Pepinsky R (1955) Optical study of PbZrO3 and NANBO3 single crystal. Phys Rev 97:1584–1590 24. Weber M, Bass M, Demars G (1971) Laser action and spectroscopic properties of Er3+ in YAlO3. J Appl Phys 42:301–305 25. Rao K, Yoon K (2003) Review of electrooptic and ferroelectric properties of barium sodium niobate single crystals. J Mater Sci 38:391–400 26. Ihringer J, Maichle J, Prandl W et al (1991) Crystal-structure of the ceramic superconductor BaPb0.75Bi0.25O3. Z Phys B Condens Matter 82:171–176 27. Cava RJ, Batlogg B, Krajewski JJ et al (1988) Superconductivity near 30-K without copper-the Ba0.6K0.4BiO3 perovskite. Nature 332:814–816 28. Sampathkumar T, Srinivasan S, Nagarajan T et al (1994) Properties of YBa2Cu3O7–δ-BaBiO3 composite superconductors. Appl Supercond 2:29–34 29. Rand DAJ (2011) A journey on the electrochemical road to sustainability. J Solid State Electrochem 15:1579–1622 30. Galal A, Darwish SA, Atta NF et al (2010) Synthesis, structure and catalytic activity of nanostructured Sr–Ru–O type perovskite for hydrogen production. Appl Catal A Gen 378:151–159

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31. Sugawara Y, Kamata K, Yamaguchi T (2019) Extremely active hydrogen evolution catalyst electrochemically generated from a ruthenium-based perovskite-type precursor. ACS Appl Energy Mater 2:956–960 32. Galal A, Atta NF, Ali SM (2011) Optimization of the synthesis conditions for LaNiO3 catalyst by microwave assisted citrate method for hydrogen production. Appl Catal A Gen 409– 410:202–208 33. Galal A, Atta NF, Ali SM (2011) Investigation of the catalytic activity of LaBO3 (B ¼ Ni, Co, Fe or Mn) prepared by the microwave-assisted method for hydrogen evolution in acidic medium. Electrochim Acta 56:5722–5730 34. Atta NF, Galal A, Ali SM (2012) The catalytic activity of ruthenates ARuO3 (A¼ Ca, Sr or Ba) for the hydrogen evolution reaction in acidic medium. Int J Electrochem Sci 7:725–746 35. Atta NF, Galal A, Ali SM (2014) The effect of the lanthanide ion-type in LnFeO3 on the catalytic activity for the hydrogen evolution in acidic medium. Int J Electrochem Sci 9:2132–2148 36. Ali SM, Al lehaibi HA (2020) Catalytic activity of lanthanide-doped Sr2PdO3 for the hydrogen evolution reaction in fuel cells. J Electrochem Soc 167:026501 37. Eigeldinger J, Vogt H (2000) The bubble coverage of gas-evolving electrodes in a flowing electrolyte. Electrochim Acta 45:4449–4456 38. Katsuki N, Takahashi E, Toyoda M (1998) Water electrolysis using diamond thin-film electrodes. J Electrochem Soc 145:2358 39. Vogt H, Balzer R (2005) The bubble coverage of gas-evolving electrodes in stagnant electrolytes. Electrochim Acta 50:2073–2079 40. Singh R, Singh N, Singh J (2002) Electrocatalytic properties of new active ternary ferrite film anodes for O2 evolution in alkaline medium. Electrochim Acta 47:3873–3879 41. Ali SM, Eskandrani AA, Al-Otaibi HM (2020) Study of the oxygen evolution reaction at strontium palladium perovskite electrocatalyst in acidic medium. Int J Mol Sci 21:3785–3797 42. Ali SM, Abd Al-Rahman YM, Galal A (2012) Catalytic activity toward oxygen evolution of LaFeO3 prepared by the microwave assisted citrate method. J Electrochem Soc 159:F600–F605 43. Ali SM, Atta NF, Galal A et al (2016) Catalytic activity of LaBO3 for OER in HClO4 medium: an approach to the molecular orbital theory. J Electrochem Soc 163:H81–H88 44. Suntivich J, May KJ, Gasteiger HA et al (2011) A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 334:1383 45. Yamada I, Takamatsu A, Asai K et al (2018) Systematic study of descriptors for oxygen evolution reaction catalysis in perovskite oxides. J Phys Chem C 122:27885–27892 46. Guo Q, Li X, Wei H et al (2019) Sr, Fe Co-doped perovskite oxides with high performance for oxygen evolution reaction. Front Chem 7:224–232 47. Pan Y, Xu X, Zhong Y et al (2020) Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation. Nat Commun 11:2002 48. Wang Z, Tan S, Xiong Y et al (2018) Effect of B sites on the catalytic activities for perovskite oxides La0.6Sr0.4CoxFe1-xO3-δ as metal-air batteries catalysts. Prog Nat Sci Mater 28:399–407 49. Xu X, Su C, Zhou W et al (2016) Co-doping strategy for developing perovskite oxides as highly efficient electrocatalysts for oxygen evolution reaction. Adv Sci 3:1500187 50. Li M, Liu T, Bo X et al (2017) A novel flower-like architecture of FeCo@NC-functionalized ultra-thin carbon nanosheets as a highly efficient 3D bifunctional electrocatalyst for full water splitting. J Mater Chem A 5:5413–5425 51. Wang Z, Li M, Liang C et al (2016) Effect of morphology on the oxygen evolution reaction for La0.8Sr0.2Co0.2Fe0.8O3δ electrochemical catalyst in alkaline media. RSC Adv 6:69251–69256 52. Vignesh A, Prabu M, Shanmugam S (2016) Porous LaCo1–xNixO3δ nanostructures as an efficient electrocatalyst for water oxidation and for a zinc–air battery. ACS Appl Mater Interfaces 8:6019–6031 53. Wang Z, Zuo P, Fan L et al (2016) Facile electrospinning preparation of phosphorus and nitrogen dual-doped cobalt-based carbon nanofibers as bifunctional electrocatalyst. J Power Sources 311:68–80 54. Wang J, Zhao H, Gao Y et al (2016) Ba0.5Sr0.5Co0.8Fe0.2O3δ on N-doped mesoporous carbon derived from organic waste as a bi-functional oxygen catalyst. Int J Hydrog Energy 41:10744–10754

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55. Li X, Wang H, Cuil Z et al (2019) Exceptional oxygen evolution reactivities on CaCoO3 and SrCoO3. Sci Adv 5:eaav6262 56. Grimaud A, May KJ, Carlton CE et al (2013) Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat Commun 4:2439 57. Lan A, Mukasyan AS (2007) Perovskite-based catalysts for direct methanol fuel cells. J Phys Chem C 111:9573–9582 58. Levasseur B, Kaliaguine S (2008) Methanol oxidation on LaBO3 (B ¼ Co, Mn, Fe) perovskitetype catalysts prepared by reactive grinding. Appl Catal A Gen 343:29–38 59. Nair MM, Kleitz F, Kaliaguine S (2012) Kinetics of methanol oxidation over mesoporous perovskite catalysts. Chem Cat Chem 4:387–394 60. Yavari Z, Noroozifar M, Khorasani-Motlagh M (2016) The improvement of methanol oxidation using nano-electrocatalysts. J Exp Nanosci 11:798–815 61. Ali SM, Al lehaibi HA (2018) Smart perovskite sensors: the electrocatalytic activity of SrPdO3 for hydrazine oxidation. J Electrochem Soc 165:B345–B350 62. Ali SM, Al-Otaibi HM (2019) The distinctive sensing performance of cobalt ion in LaBO3 perovskite (B ¼ Fe, Mn, Ni, or Cr) for hydrazine electrooxidation. J Electroanal Chem 851:113443

Appraisal of Solar Radiation with Modelling Approach for Solar Farm Design

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Generation, Climate, and Sun Power Potential in Konya-Elazığ . . . . . . . . . . . . . . . Solar Radiation Intensity Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontal Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calculating Solar Radiation Intensity on Inclined Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar Radiation Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Selection of Third-Generation Solar-Cell Materials in Solar Farm Design . . . . . . . . . . . . . . Conclusions and Further Outlok . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Sun irradiation is a significant consideration in solar power systems’ feasibility. Many models were suggested to estimate solar radiation concentrations because there is a lack of meteorological data available in the field. On the contrary, the efficiency of these models depends heavily on climate and ecological variables. This is one of the main reasons for increasing the amount of models available. This study aims to determine the most appropriate models for two specific cities chosen in a specific climatic region and to design a photovoltaic system for maximum effectiveness under particular weather conditions in Turkey; for these cities’ solar energy potentials, a comparative analysis in the third climatic region

L. S. Sua Elazig, Turkey F. Balo (*) Industrial Engineering Department, Firat University, Elâzığ, Turkey e-mail: fi[email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_133

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is also offered. It aims at extending the study to other towns in the country’s remaining climatic areas and generating a detailed model of solar radiation. Keywords

Solar energy · Solar farm · Global solar radiation · Renewable energy · Data analysis · Photovoltaic

Introduction In parallel with the increasing energy demand worldwide and diminishing fossilbased resources, the ability of such resources meeting the energy demand is continously decreasing. Based on the current consumption trends, taking advantage of renewable resources is critical. Solar energy potential among the others is greater in terms of application areas. However, measuring the potential of solar energy has great importance for installation projects. Solar radiation value has significant role as it provides essential information about environmental issues such as climate change. Partial world map of daily mean of surface irradiance is displayed in Fig. 1 [1]. Solar energy keeps increasing its popularity as a renewable source of energy on which detailed researches have been conducted. The main factors leading to this popularity are elimination of hazardous waste and the need for complex technologies. Global energy budget is exhibited below in Fig. 2 [2]. Researchers have started to concentrate on local sun irradiation models linked to the design of photovoltaic systems in latest years. Many papers suggest that the method of ANNs is better than the empirical models [3–5]. Kasra et al. provided four fractional sunlight length models with information for 9 years for Isfahan in Iran. The 4 years of information are used to evaluate the information. They altered their root-mean-square error (RMSE) from 1.18 to 1.1 MJ/m2 day [6]. For 17 towns in Iran, Behrang et al. used particle swarm optimization method to research for 11 models [7]. In Iran, Hargreaves and the cloud-based model were evaluated by Fariba et al. 17-year information are used to gain empirical constants. The RMSE varied from 1.12 to 0.71 MJ/m2 day [8]. For Saudi Arabia, El-Sebaii et al. carried out three average fractional sunlight duration (MSDF) models, three fractional sunlight duration (SDF) models, and non-sunlight length (NSDF) models to predict monthly mean worldwide sun irradiation. Cloud cover, relative humidity, and temperature were the features grouped into average sunlight duration fraction models. Nine-year information is used to determine new empirical coefficient values. Nine models of RMSE ranged from 0.02 to 0.15 MJ/m2 day [9, 10]. Chelbi et al. investigated five empirical models for four locations in Tunisia [11]. Yao et al. assessed 89 monthly mean irradiation models for Shanghai in China. Many models are implemented with the identical mathematical expressions utilizing different coefficients. They obtained fresh fitting coefficients for five sunshine fraction models in Shanghai [12]. Ahmet et al. studied quadratic, linear, and cubic empirical models for four regions in Turkey [13]. Khorasanizadeh et al. evaluated three MSDF models and three NSDFs for six provinces in Iran in order to compute monthly worldwide sun

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400 W/m2

60 350 W/m2

40 300 W/m2

Latitude (°)

20

250 W/m2 200 W/m2

0

150 W/m2

–20

100 W/m2

–40 50 W/m2

–60

Daily mean of surface irradiance (2008-05-18) –60

–40

–20

0

20

NaN

40

60

Longitude (°)

Fig. 1 Partial world map of daily mean of surface irradiance [1]

Fig. 2 Global energy budget [2]

irradiation. The temperature and relative humidity are added as characteristic properties in the average fractional sunlight duration models. The all models’ root average square mistake altered from 0.82 to 0.47 MJ/m2 day compared to sunlight duration fraction models [14]. Senkal suggested an ANN model using latitude, altitude, longitude, ground

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temperature, and two different ground emissions as inputs. Using satellite information, the last three features were obtained. One year of information from ten locations is used to train the ANN. The RMSE is recorded as 0.32 and 0.16 MJ/m2 day in the testing and preparation stage [15]. Manzano et al. evaluated the linear model Angstrom–Prescott for 25 locations in Spain. For calibration purposes, the 10-year information are analyzed. The RMSE has altered from 0.8 to 0.36 MJ/m2 day except for four sites [16]. For the input layer, Ozgoren et al. utilized the ANN model of the multi-nonlinear regression to acquire adequate independent features. They chose ten features including year’s month, latitude, length of sunlight, cloudiness, altitude, average atmospheric temperature, the land’s temperature, minimum-maximum atmospheric temperature, and wind velocity. To train the ANN, the Levenberg-Marquardt optimization algorithm is used [17]. For 22 locations in South Korea, Park et al. searched for linear empirical model [18]. Mohandes utilized particle swarm optimization for Saudi Arabia to train the ANN. The longitude, altitude, latitude, sunlight’s length, and the year’s month were used as inputs. Thus, the forecast was for worldwide sun irradiation on a monthly mean. Thirty-one sites’ information are used to train ANNs. The mean percentage total error of 8.85% is obtained [19]. Hacer et al. explored five fractional sunlight duration models for seven places in Turkey to estimate monthly mean irradiation [20]. Shahaboddin et al. assessed two SDF, two MSDF, and a NSDF modellings for Shiraz in Iran. RMSE altered from 1.55 to 1.3 MJ/ m2 day [21]. Adaramola researched six non-sunlight length models for Akure in Nigeria to estimate mean Angstrom-Page and long-term monthly sun irradiation model. Precipitation, ambient temperature, and relative humidity were utilized in non-sunlight duration models. For the linear model, the root average square error shifted from 8.25 to 4.78 MJ/ m2 day [22]. Li et al. evaluated eight fractional sunlight duration models in China for four stations. For calibration, data for 11 years are utilized. The 4-year information are used for validation. The RMSE is utilized as an indicator for statistics. The linear modelling RMSE shifted from 1.26 to 0.72 MJ/m2 day. RMSE of the eight models shifted from 1.33 to 0.7 MJ/m2 day [23]. These steady parameters are usually depending on the inquiry fields, however, according to the findings of many articles. The values of the coefficient are derived individually. The sunlight length’s fraction varied from 1,634 to 1,636 MJ/ m2 day [24]. Bakırcı researched 60 empirical models created to estimate worldwide monthly with daily mean sun irradiation, in which many of the estimates only had the identical formulations with various continuous regressive parameters. These steady parameters are usually based on the inquiry fields, however, according to the findings of many articles [25]. Seventy-eight empirical models were studied by Fariba et al. They categorized them into four groups: sun-sourced, cloud-sourced, weather-sourced, and other temperature-sourced models. They used a couple of models of each class to develop a case study for Iran. A sun-sourced model with exponential expression determines the highest output [26]. To select the appropriate input features, Jiang et al. conducted main association rules and Pearson correlation coefficients. The parameters are selected for wind velocity, precipitation, complete mean opaque sky coverage, opaque sky coverage, average temperature, relative humidity, minimum-maximum temperature, coolingheating degree days, and daylight temperature [27]. Katiyar et al. used annual information to research for quadratic, linear, and cubic models for the forecast of the monthly mean irradiation for four provinces in India. The values varied between 0.8 and

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0.43 MJ/m2 day [28]. Qin et al. utilized Levenberg-Marquardt algorithm with entries containing differences in field temperature between nighttime and daytime, average field temperature, air pressure days’ number, monthly precipitation, and vegetation index. For Tibetan Plateau, 7-year information are utilized to train ANNs from 22 sites [29]. Shahaboddin et al. utilized the ANN and the intense learning machines’ algorithm in Iran. The percentage of relative humidity, mean air temperature, difference in temperature, and sunlight length is implemented as entries. Three years of information are used for testing. RMSE ranged from 0.93 to 0.86 MJ/m2 day [21]. Senkal et al. studied the model of artificial neural networks for 12 provinces in Turkey. Average irradiation beam, average diffuse irradiation, latitude, altitude, and longitude were used as entries. It is suggested the satellite-source technique for predicting the mean monthly irradiation. The root average square error from 2.75 to 2.32 MJ/m2 day has altered [30]. By utilizing the energy balance between neighboring atmosphere and soil layer, Antonio et al. built a linear formulation to correlate sun irradiation with the sunlight product length and variation in temperature on a daily basis [31]. Yadav et al. conducted the simulation program of the Waikato environment to achieve the most effective input features for prediction. As entry features, they obtained the maximal and minimal temperature, mean temperature, altitude, and sunlight length, while longitude and latitude were the features with the lowest effect. The forecast was for worldwide sun radiation on a monthly mean. The maximal average percentage total error is achieved by ANNs as 6.89% [32, 33]. Data are utilized to train the specific models for 10 years. The parameters are utilized to distinguish between minimal and maximal atmospheric temperatures, relative humidity, fraction of sunshine length, water vapor pressure, extraterrestrial worldwide solar irradiation, and mean ambient temperature. The RMSE ranged from 1.81 to 1.79 MJ/m2 day. Janjai et al. studied a satellite-sourced model for four locations in Thailand and five areas in Cambodia. The RMSE was 1.13 MJ/m2 day [34]. Zang et al. investigated the identical model for 35 locations in China by decreasing 2 coefficients [35]. Amit et al. researched countless papers using ANN in three reviews to estimate solar irradiation and horizontal surfaces of sun irradiation. They indicated that there were better ANN models than empirical models [36]. Sun et al. evaluated the autoregressive moving mean model’s impact to predict solar irradiation. They explored information from two locations in China over 20 years [37]. For 35 locations, the average percentage total error and the RMSE varied from 16.22% to 4.33% and from 1.88 to 1.10 MJ/m2 day, respectively. Zhao et al. investigated the linear model for nine locations in China. The RMSE ranged from 1.72 MJ/m2 day to 5.24 MJ [38]. Ayodele et al. applied a function to enable the dispersion of the clearness index. By utilizing 7 years of information, daily solar irradiation information was obtained by the coefficient values. With the exception of October, all months’ efficiency values are acquired. RMSE ranged from 0.221 to 0.213 MJ/m2 day [39]. In order to predict monthly average clearness values, Iranna et al. researched 16 non-sunlight length models. As entries, the features associated with moisture, longitude, altitude, wind speed, relative moisture, and five other temperatures are utilized. Analyzing the models will assess data for 875 locations [40]. With measured data from 18 Turkish locations, Kadir analyzed 7 diverse sunlight length fraction modellings. On a monthly basis, he utilized few modellings containing logarithmic, exponential, linear, and quadratic formulations to estimate the long-term mean daily

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global sun irradiation. The performance of the performed modellings is accomplished with slight diversities for the identical locations [41]. Lanre et al. utilized the adaptive neuro-fuzzy inference system and ANN in Nigeria. Length of sunlight, peak, and minimal temperature as inputs were utilized. Data were used to train the modelling for 6 years, while information were used for testing purposes for 15 years. In testing and training phases, the RMSE ranged from 1.76 to 1.09 MJ/m2 day, respectively [42]. Chen et al. studied five sunlight fraction modellings for three locations in Liaoning Province, China. The data were acquired from each location for 35 years, while 70% were searched for empirical coefficient values. Thirty percent of data was utilized for testing. The empirical coefficient values for each of stations are obtained. The RMSE ranged from 1.98 to 2.73 MJ/m2 day in Chaoyang [43]. The hybrid modelling set researched in order to predict the daily sun irradiation by Koike and Yang [44]. In Tibet, Pan et al. researched the exponential modelling depending on temperature for 11 meteorological locations. The difference in temperature is utilized as input. Data have been implemented for 35 years to calibrate the modelling. Data have been submitted for testing for 5 years. The RMSE of the modelling altered from 2.54 to 3.24 MJ/m2 day for all stations [45]. The modelling computed the worldwide clear index and the clear radiation of the sky. These modellings utilized sunlight length, Angstrom turbidity, ozone layer thickness, temperature pressure of air, ground elevation, and relative humidity as input characteristics. In China, the radiation information acquired from 2000 to 1993 is used to verify the hybrid modelling for 97 aerological stations. RMSE obtained 1.3 and 0.7 MJ/m2 day [46]. In China, Wan et al. used the linear Angstrom-Prescott modelling for 41 locations to estimate worldwide solar irradiation on a daily basis. Relying on various criteria, these sites were split into seven solar climate areas and nine heat climate areas. Using altitude, latitude, longitude, number of days, fraction of sunshine length, and average temperature on a daily basis as inputs, they implemented the ANN modelling [47]. The average percentage total error ranged between 15.43% and 4.00%, while the RMSE ranged from 1.03 to 1.83 MJ/ m2 day. Khorasanizadeh et al. evaluated six modellings for four regions in Iran [48, 49]. Li et al. used a mixed modelling (cosine and sine functions) for 79 locations in China with information for 10 years [50]. The initial modelling depends on exponential. On the features of cosine and sine, the secondary on polynomial and four other modellings, RMSE ranged between 1.26 and 0.72 MJ/m2 day, and the average percentage total error rose between 5.72% and 3.38%, respectively. Jamshid et al. investigated one MSDF framework and three SDF modellings for two locations in Iran. They utilized the technique of regression of the support vector. The RMSE changed from 2.14 to 3.70 MJ/m2 day. As inputs for kernel function, the maximal-minimal temperature, relative moisture, and sunlight length were chosen [51]. Liu et al. researched three non-sunlight period, three MSDF, and two SDF modellings for Gaize in the Tibetan Plateau. One thousand and eighty five days were evaluated for calibration, while information was used for validation purposes for 701 days. The RMSE varied between 1.68 and 3.13 MJ/ m2 day. They argued for different seasons that it was pointless to derive coefficient values respectively [52]. Wan Nik et al. evaluated six mathematical expressions of the solar radiation proportion between on a daily basis and on an hourly basis. The forecast was produced for monthly mean irradiation on an hourly basis. From Malaysia’s three

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locations, information was used to test the modellings for 3 years. They found that the relative RMSE ranged from 26.49% to 8.22% [53]. Kevin et al. achieved the greatest output by using general features. This may be because there are several locations in each of the areas. The mean data from different locations can diminish the amplitude of changes in solar radiation and other characteristic properties [47, 54]. Janjai et al. obtained a model based on the satellite to estimate mean sunlight on an hourly basis. At 15:00 and 9:00, the RMSE ranged from 10.7% to 7.5% [55]. Yao et al. examined the 11 mathematical expressions of the solar ratio on an hourly basis to solar radiation on a daily basis. To confirm the models, they used data from 10,979 pairs for 3 years. The RMSE ranged from 0.321 to 0.481 MJ/m2 hour and correspond to 88.33–142.22 W/m2 [56]. Jamshid et al. studied two modellings of vector regression assistance for two locations in Iran. As inputs, sunlight length, relative moisture, and the minimal and maximal temperature were utilized. The RMSE were determined between 4.47 and 1.63 MJ/m2 day [57]. Li et al. searched for 12 NSDFs for Chongqing in China. The characteristic properties fog, precipitation, and average temperature of the dew point were linked in the initial three modellings. The minimal-maximal temperature values have been combined in the last nine designs. The 2,552 days were utilized in test. Two thousand nine hundred and twenty one days have been utilized to set the modellings. The RMSE altered from 5.18 to 6.24 MJ/m2 day for the initial three modellings. The RMSE of the last nine modellings ranged from 3.05 to 2.52 MJ/m2 day [58].

Electrical Generation, Climate, and Sun Power Potential in KonyaElazığ Due to equipment constraints along with their high costs of upkeep and the number of sun irradiation measurement stations being restricted, weather variables are frequently used to calculate the quantity of solar radiation [59–61]. Sunshine length and land-related variables are essential for establishing installations based on sun power. In this scenario, comprehensive search on sun power potential, climate, and current infrastructure needs to be conducted. The country’s majority displays elevated potential for sunlight. A research by the Directorate-General for Electrical Power Resources (EIE) demonstrates that the mean yearly sunlight is about 2,640 h (7.2 h/day) and that the mean irradiation intensity is 1,311 kWh/m2year (3.6 kWh/ m2/day). The sun power systems installed in the country are projected to rise to 3,360 MW by 2017 when unlicensed and licensed systems have been confirmed or are being constructed. From the sun, there will be 4 billion and 905 million kWh of renewable energy every year. This manufacturing rate amounts to upward 1.85% of the country’s energy needs [62]. Sun irradiation map for Konya and Elazig is displayed in Fig. 3. In terms of sunpower potential, both cities are grouped under secondary climatic zone. Phase shift in the function of radiation, average solar irradiation, latitude, and radiation function frequency values for both provinces are given in Table 1.

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Fig. 3 Solar radiation map for Konya and Elazig Table 1 Values of radiation Province Konya Elazig

FGI (MJ/m2day) 7.36 8.29

Iort (MJ/m2day) 13.1 13.4

Latitude 37.52 38.40

FKI 4.72 2.35

For both provinces to show their solar radiation features and potential, a comparative assessment is performed on the Matlab simulation program in the next chapter.

Solar Radiation Intensity Calculation Due to the differences in climatic conditions and geographic characteristics, calculating amount of solar radiation in a specific region requires the selection of most appropriate modelling among many that exist in the literature. Lewis (1992) obtained data from long-term solar radiation, relative humidity, weather temperature, and sun-hour measurements in Tennessee, USA. Various functions are generated using these data to be used for solar radiation estimations [63]. Oliveira et al. developed various mathematical modellings for São Paulo, Brazil, to be able to calculate monthly, daily, and hourly solar radiation on horizontal surfaces based on total and diffuse radiation values between 1994 and 1999 [64]. Che et al. introduced solar radiation modellings by using 40 years of radiation values obtained from 14

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different stations in China [65]. It is argued that second-degree polynomial functions give the best solutions due to the fact that the country covers a wide geographical area. Statistical comparison of the measured data with the developed modellings is also provided.

Horizontal Surface Daily Total Solar Radiation On a horizontal surface, total solar radiation per day can be obtained through Eq. 1 [66]: I ¼ I ort  FGI cos

h

i 2π ðn þ FKI Þ 365

ð1Þ

where FGI is radiation function frequency; FKI, radiation function phase shift; Iort, annual average of daily total radiation; and n, days.

Daily Diffuse Solar Radiation On horizontal surfaces, daily total diffuse solar radiation can be obtained through Eq. 2 [67]:   Iy ¼ I ð1  BÞ2 1 þ 3B2

ð2Þ

where Io is out-of-atmosphere radiation and B is transparency index.

Momentary Total Solar Radiation On horizontal surfaces, momentary total solar radiation can be determined using Eq. 3 [68, 69]: Io ¼

24 I ðCosðeÞCosðd ÞSinðwsÞ þ wsSinðeÞ sin ðdÞÞf π s

ð3Þ

where Is (W/m2) is solar constant; e, latitude angle; ws, sunrise hour angle; d, declination angle; and f, solar constant correction factor, which can be determined utilizing the related equations and tables. Out-of-atmosphere radiation can be obtained utilizing Eq. 4 [66–69]: 

π I ts ¼ Ats Cos ðt  12Þ tgi

 ð4Þ

where Ats is solar radiation and tgi is imaginary day length.

Momentary Direct and Diffuse Solar Radiation Amount of momentary diffuse and direct solar radiation on horizontal surfaces can be calculated using Eqs. 5 and 6 [68–72] where Ays is the function frequency:

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π I ys ¼ Ays Cos ðt  12Þ tg



I ds ¼ I ts ¼ I ys

ð5Þ ð6Þ

Calculating Solar Radiation Intensity on Inclined Surface Momentary Direct Solar Radiation The momentary direct sun irradiation can be determined on inclined superficies (30 , 60 , 90 angles) utilizing the formula [66–72]: I be ¼ I b ¼ Rb Rb ¼

Cos θ Cos θz

ð7Þ ð8Þ

cos θz ¼ sin d sin e þ cos d cos e cos w

ð9Þ

cos θ ¼ sin d sin ð e  βÞ þ cos d cos ð e  βÞ cos w

ð10Þ

Momentary Diffuse Solar Radiation On an inclined superficies, the momentary value of diffuse irradiation can be determined by the formula [66–68]: I ye ¼ Ry I ys

ð11Þ

Ry (conversion factor) for diffuse radiation can be obtained by the formula [66–68]: Ry ¼

1 þ cos ðaÞ 2

ð12Þ

Ry parameter ensures the superficies’ slope. For vertical superficies (a = 90 ), Ry is exactly 0.5. Thus, the momentary values of the diffuse irradiation on inclined superficies (30 , 60 , 90 angles) for 24 h can be obtained.

Reflecting Momentary Solar Radiation On inclined superficies, the reflecting irradiation can be obtained using the formula [66–68]: I ya ¼ I ts p

1 þ cos ðaÞ 2

ð13Þ

The ecologic reflection rate is given with ρ characteristic and utilized as about ρ = 0.2 in computations.

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Total Momentary Solar Radiation On inclined superficies, the momentary total irradiation [66–68] can be obtained by the formula: I t ¼ I de þ I ye þ I ya

ð14Þ

Method Figure 4 shows the values for: (a) 24 h, alteration in the current yearly complete value of sun irradiation (b) Per hour, change in the current yearly diffuse values of solar radiation (c) 24 h, change in the current annual horizontal value of direct sun irradiation Figure 5 guarantees daily alterations: (a) The sun irradiation’s total values per day; (b) the declination angle; (c) hourly angle for sunrise; (d) for factor of correction, sun constant; (e) the sun values of radiation out of atmosphere; (f) (Ays) function periodicity graph; (g) (Ats) diffuse irradiation from sun; and (h) index of accountability (B) on horizontal surface. In Fig. 6, the momentary direct radiation values with three diverse angles (30 ,  60 , and 90 ) for 24-h time period are exhibited. For all three angles, the peak values are reached on the 355th day at 12:00, and the lowest values are recorded on the same day at 03:00. The yearly momentary values of diffuse irradiation for three angles (30 , 60 , and  90 ) are ensured in Fig. 7. In Fig. 8, the total momentary sun irradiation’s yearly values for 24-h periods are ensured. For annual angle and hours, the total momentary sun radiation is ensured in Fig. 9.

Solar Radiation Attributes Depending on the above assessment, it is possible to assess the real potential of both provinces through the calculations of solar features given in Table 2. On horizontal and inclined surfaces, sun radiation levels are obtained by Matlab simulation program. Depending on the calculations, the indicator values indicate that in both provinces the potential for photovoltaic installations corresponds to the anticipated concentrations. Maximum value of the total radiation reaches 8.4694 W/m2 in Elazig, while it reaches 5.7864 W/m2 in Konya. Amount of direct and diffuse radiation is also considerably higher in Elazig. Comparing the anticipated values with the real ones is an essential part of planning the PV systems. System efficiency relies on different parameters. To design the optimum system, it is very important to use realistic radiation values. This study aims to organize a reference to select the most effective solar panel based on the actual

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Fig. 4 Change in annual sun irradiation values on a horizontal superficies for 24 h

solar radiation values determined for the most effective design of the photovoltaic system. The solar radiation concentrations are found to be at viable efficiency levels to design a photovoltaic system. Up to this point in this study, the degree of solar efficiency for a solar farm is attempted to be determined through the selection of best performing modelling. As a result of the analysis, it is observed that the solar values obtained from the modellings are appropriate for both of the regions to design solar farms. In the next section of the study, best-performing third-generation solar cell material is determined in these regions. For this purpose, the most popular nanomaterials are investigated first. In the light of the studies conducted on this topic, the best material is attempted to be found using a multi-criteria decision-making model.

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Fig. 5 Horizontal sun irradiation

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Fig. 6 The annual momentary values of direct irradiation for 24 h on inclined superficies

The Selection of Third-Generation Solar-Cell Materials in Solar Farm Design For the earth, the most abundant energy source is sun. The solar energy collection and its transformance to electrical power have broad implication and important effect on our society. For this reason, it has attracted the researchers’ attention. Increasing conversion efficiency and decreasing cost are the primary tasks to make PV energy competitive and able to replace conventional petroleum-based energies. In Fig. 10, world electricity demand and world PV market in 2019 are given.

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Fig. 7 Annual momentary diffuse radiation values for inclined surfaces

The solar panel market has enjoyed a great expansion these recent years because of the move toward clear energies like PVs. Different technologies and materials are being employed to generate solar cell production with high conversion efficiency and low cost. Figure 11 shows best cell performance. The nanomaterial sector seems to be the way by which PVs can be improved, whether in organic-sourced or inorganicsourced solar panels. Figure 12 shows an organic-sourced (TiO2) nanomaterial solar cell.

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Fig. 8 Annual total momentary radiation values for inclined surface

The combination CdSe/CdTe, CuInSe2 or CIS (copper indium diselenide), CdTe (cadmium telluride), CdS (cadmium sulfide), and silicon can be used as nanomaterials in inorganic-sourced solar cells. In the near-infrared zone, the composition CdSe/CdTe in shell/core forms have the property to radiate, which doesn’t have for CdTe or CdTenano-particles taken separately [74, 75]. The shell/core form’s success relies clearly on the minimum

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Fig. 9 Total momentary radiation for annual angle and hours

cage mismatch between the utilized materials [75]. The thin CuInSe2 layers show absorbant’s another form broadly utilized in PVs because of the fact that because of the fact that they collect the maximal solar spectrum excellently [76]. This material’s form has the superiority compared to silicon of matter’s quantity: CuInSe2 is deposited as thin films with around 1 mm thickness, while for silicon thick bottom layer of approximately 300 mm is needed. The CdTe’s bandgap is 1.45 Ev. It is one of the absorbant photovoltaic materials. Cadmium telluride is also combined with cadmium sulfide (CdS). PV panel form CNTs/CdS (carbon nanotubes)/CdTe/ITO (indium tin oxide) acts as rear contact/n-type semiconductor/p-type semiconductor/front electrode. At orthogonal azimuthal angle and 45  solar incidences, the conversion efficiency can be changed from 3.5% to 7% [77]. The CdS enters the solar cells’ fabrication by fluorine-reinforced tin oxide Au/ (FTO)/TiO2/CdS polysulfide and photoanode electrolyte. The gold nanoparticles have been utilized as an interface sheet between TiO2 and FTO. For the forms Au/ FTO/TiO2/CdS and CdS/TiO2/FTO, the η (conversion efficiency) rises from 0.86% to 1.62%, respectively. By developing silver-screen printed contiguity, rise in silicon solar panel efficiency can be acquired. This can be obtained by offering silver-based nanoparticles in the fixate to to increase contact closeness between the η (conversion efficiency) and the FF (fill factor) accordingly. In solar cells, silicon can be also utilized as nanowires. The photoanodes for dye-sensitized solar cell (i.e., TiO2, ITO, ZnO, etc.) can be used as nanomaterials in organic-sourced solar cells (also called photoelectrochemical solar cell). The organic-sourced solar cells have drawn considerable attention recently. In solar cell fabrication, the key phenomena were eventually dominated as light trapping, exciton production impact, and one-dimensional material [78]. The organic-sourced solar cells are composed of counter and photoactive electrode engrossed. The nano-based materials on TiO2 can be consisted by combining chemical synthesis with dealloying process. With a thickness of around 10 mm, a hierarchical nano-form was determined. TiO2 has the nanorods’ and nanoflower arrays’ shape [79]. For a PV cell, a conversion performance of 1.30% is obtained.

Mom. direct rad.

Mom. dif. rad.

Mom. tot. rad.

Function freq.

Total diffuse radiation

Transp. index

Out-of-atmosphere radiation

Sunrise hour angle

Declination angle

Attributes Total radiation

Bmin Iy(max) W/ m2 Iy(min) W/m2 Ats(max) Ats(min) It(max) It(min) (Ays)max (Ays)min Id(max) Id(min) Ib(max) Ib(min)

Imax W/m2 Imin W/m2 dmax dmin wmax wmin Io(max) W/ m2 Io(min) W/m2 Bmax

Table 2 Solar radiation attributes 8.4694 8.4200 23.4498 23.4498 110.0186 60.9814 278,010 176,900 0.1630 0.0044 8.8522 8.4200 1.5075 1.0104 1.5075 1.5044 1.4253 0.9028 1.4253 1.4165 0.2463 0.2551

176,900 0.0230 0.0034 5.7822 5.7400 1.0356 0.6804 1. 0175 1.5044 0.97 0.62 0.8253 1.2165 0.1613 0.2551

Elazig

5.7864 5.7400 24.4498 24.4498 109.0186 71.9814 278,010

Konya

Mom. reflecting rad.

Mom. dif. rad.

Attributes Mom. dir. rad.

IrBmax(30 ) IrBmin(30 ) IrBmax(60 ) IrBmin(60 ) IrBmax(90 ) IrBmin(90 )

Idbmax(30 ) Idbmin(30 ) Idbmax(60 ) Idbmin(60 ) Idbmax(90 ) Idbmin(90 ) IbBmax (30 ) IbBmin(30 ) IbBmax (60 ) IbBmin(60 ) IbBmax (90 ) IbBmin(90 )

0.0606 0.0605 0.2261 0.2257 0.4522 0.4513

0.2545

0.2545 0.0406 0.0605 0.1461 0.2257 0.2742 0.4513

0.2549 0.2458

0.2545 0.2461

Elazig 1.3299 1.3216 1.0690 1.0624 0.7127 0.7082 0.2457

0.2549 0.1458

0.2545 0.1461

Konya 0.7599 1.2216 0.6290 0.8624 0.4727 0.5082 0.1557

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Fig. 10 (a) World electricity demand, 2017–2019 [72]. (b) World PV market in 2019

ITO is organic-sourced solar cell. By utilizing electrochemistry method, it has been obtained. Bilayer heterojunction consisted of a two-stage solution methodology. In the first stage, electrodepositing polythiophene is applied. In the second stage, [6,6]-phenyl-C61-butyric acid methyl ester’s spin-coating chloroform solution is applied on PTH sheet [80]. The second sheet plays the donor material’s role because of its high hole mobility. ITO’s transformation performance is about 0.1% [80, 81].

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Fig. 11 Best cell performances [73]

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Fig. 12 An organic-sourced (TiO2) nanomaterial solar cell [74]

The ZnO-sourced hybrid solar cells on the form FTO (fluorine-doped tin oxide)/ TiO2 (titanium dioxide formed as nanotube sequences)/N719 (ruthenium(II)dye)/ P3HT poly(3-hexylthiophene)/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester)/ Au have been produced. A performance around 0.656% is founded [81]. In solar cell sector advancement, this section of study will focus on nanomaterials’ contribution. By using processes sourced substantially on chemistry, the nanoelectrochemistry appears to be a non-negligible option for broadly utilized, lowcost, and high-performance solar cell production. For PV cell fabricating, it is due to the fact that these continuums are realized at minimum or ambient temperatures, which considerably decreases the energy invoice. In spite of the fact that the transformation performance found through traditional solar cells is comparatively high in comparison with nanomaterial-sourced solar cells, they stay more appealing due to their potential wide application and low fabrication expense in humans’ daily life. By solid-state physics, this shift to nanoelectrochemistry tends to overcome the boundaries faced. The elevated opportunities to raise photocarrier collection and light trapping have been attained. Interesting and diverse forms were executed like nanorods, nanowires, nanocones, nanosprings, nanopillars, nanotubes, nanopagodas, nanobelts, nanopetals, nanoflowers, and others. However, the actual recuperation is monitored in inorganic-sourced solar cells rather than the organic-sourced solar cell. The organic-sourced solar cells are promising “future-generation” solar cells but cannot be a competition for the inorganic-sourced ones. Their real transformation performance is very small to ensure and to have electrical big-scale implementation. Endeavors should be made to make this feasible.

Conclusions and Further Outlok In this study, a set of nanomaterials are evaluated based on a set of criteria, and then the most appropriate material is aimed to be determined based on these criteria. Table 3 presents the alternatives that are considered for the purpose of this study.

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Analytic hierarchy process (AHP) methodology is utilized in the process of choosing the best alternative among the others based on a predetermined set of criteria. AHP technique is one of the multi-criteria decision-making methods, and its purpose is to choose the best alternative among a list of others based on certain criteria. The method is composed of a hierarchical structure, and the objective of the method is evaluated based on the criteria, while the criteria are evaluated based on the alternatives. For various reasons, AHP method is a decision support system used in decision-making problems. The ability of evaluating many criteria at the same time, being able to determine the ideal alternative, providing the decision-makers with the flexibility to reflect their opinions, avoiding the uncertain factor in situations where uncertainty exists, and chance of restructuring in the cases of disagreements can be listed as some of the reasons contributing to the popularity of the method. AHP methodology enables the evaluation of all alternatives by considering many different criteria. Doing this task manually can be very challenging, and it consumes considerable amount of time. The users can express their opinions in a flexible manner with AHP method and make more detailed and different evaluations by adding various criteria and objectives. It also provides an analysis and information utilization process to re-evaluate the conflicts and produce solutions for them. The method helps in avoiding the uncertainty in situations where risk and uncertainty exist. AHP method is composed of several steps. First, the definition of the problem to be investigated within the study is provided; then a decision matrix is generated among the criteria effecting the selection of the alternatives, and relative priorities of these criteria are determined. After these calculations, percentage priority values are determined by evaluating the alternatives for each criterion, and the best alternative is determined using the resulting distribution. The criteria are listed in Table 4 below. It was mentioned that the earlier steps of AHP method involve calculation of criteria priorities and ranking the alternatives based on these priority values. In this context, criteria priorities that were obtained from the analysis are exhibited in Table 5. The table indicates that the most important criterion is “Efficiency,” and it is followed by “FF (%)” criterion. On the other hand, the criterion with the lowest priority value is “Area.” The alternatives are evaluated according to these criteria priorities, and the obtained results are shown in Table 6. The table indicates that the best material based on the predetermined criteria is “GaAs” which is followed by “Si (crystalline).” It was observed that the material with the lowest score is “Organic.” As a result of the two analysis described in above sections, the modellings that are used for two sample regions can constitute a reference for future researches. Coupled with feasibility studies, determining regions with optimum solar energy potential will make it possible to build better-performing solar farms.

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Table 3 Alternatives considered within the study

Si (crystalline) Si (multicrystalline) GaAs (thin film) CIGS (Cd free)

Table 4 Criteria considered within the study

Efficiency (%) Area V I (A) FF (%)

Table 5 Priority values of the criteria

0.415014 0.054 0.143535 0.143535 0.243915

Table 6 Priority values of the alternatives

Criterion Si (crystalline) Si (multicrystalline) GaAs (thin film) CIGS (Cd free) CdTe (thin film) CIGS(large) a-Si/nc-Si (tandem) Organic

CdTe (thin film) CIGS(large) a-Si/nc-Si (tandem) Organic

Efficiency (%) Area V I (A) FF (%)

Priority value 0.1986 0.1453 0.2497 0.0729 0.0815 0.1064 0.1126 0.0329

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer: Biobased and Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biobased Polyamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic Acid (PLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyhydroxyalkanoate (PHA) Polyhydroxybutyrate (PHB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Succinate Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biobased Polyethylene (Bio-PE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biobased Poly(Ethylene Terephthalate) (PET) and Poly(Trimethylene Terephthalate) (PTT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cashew Nutshell Liquid and Vegetable Oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bionanocomposites: Processing and Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Futher Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Plastics are polymers, typically versatile, cost-effective, and require less energy to produce than alternative materials such as ceramics, glasses, and metals. Polymers can be manufactured to have many different properties, and the use of these polymers in various fields has surpassed other materials. Petroleum T.-D. Ngo (*) Bio-Industrial Research and Development, InnoTech Alberta, Edmonton, AB, Canada InnoTech Alberta Inc., Edmonton, AB, Canada Department of Civil and Environmental Engineering, Faculty of Engineering, 7-203 Donadeo Innovation Centre for Engineering, University of Alberta, Edmonton, AB, Canada e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_142

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resources are being extensively used to produce polymers; however, the excessive usage of these petro-based polymers raise ecological and environmental issues. Over the last decades, green and natural renewable materials have increasingly become a research focus due to environmental issues and nonrenewable petroluem resources. Biobased and biodegradable nature of biopolymers makes them one of the favorite choices to be utilized for the preparation of environmentfriendly materials. This chapter discusses and highlights biobased and biodegradable polymers and common factors affecting polymers as well as nanocomposites preparation and their utilization in a diverse range of products.

Introduction Plastics based on synthetic polymers are widely used to make various materials and applications for our daily lives. Petroleum serves as a comparatively recent source for chemicals and plastics. Their accelerated production began with the rapidly developing petroleum industry in the 1950s [1]. Some inadequate properties, such as insufficient mechanical strength, high gas and water vapor permeability, low heat degradation temperature, etc., restricted more wide-range applications of polymers of biological origin. Therefore, improvement of their properties is needed and is frequently done through the addition of reinforcements or fillers, so-called composite materials [2]. In the 1970s, many of better polymers and improved reinforcing materials were developed for composite applications; therefore, the composites industry began to mature. Although polymer-clay nanocomposites appeared before the 1980s, the development of thermoplastic nanocomposites began to appear in the literature at the end of this decade. At that time, the use of polyamide 6 and organoclay for preparation of nanocomposites was developed by Toyota [3, 4]. The polymers, fuel, chemicals, and materials that we use everyday are mostly derived from fossil oils made through the refinery or chemical process. However, the polymers, fuel, chemicals, and materials are used very quickly in comparison to the fossil generation. These resources are exhaustible, being finite in quantity. In addition, the fossil resources are not renewable, and the usage of carbon-containing materials mainly ends up with CO2 emissions which is another side of the problem. These factors influence on global processes and Earth’s environment. In 1970s, the polymer plastic pollution in the ocean was first reported by scientists [5]. Polymer plastics also have negative health impacts on humans and the environment. The materials harm human health because they release toxic chemicals throughout the life cycle of the products [6]. Due to the concerns over environmental pollution from the polymers, composite materials, and the depletion of fossil oils, intensive research is being conducted for developing biobased and biodegradable polymers and nanocomposites from renewable natural resources [7]. There have been many research achievements in biobased and biodegradable polymers, including naturally occurring biodegradable polymers, biodegradable polymers derived from renewable resources, and biodegradable polymers based on petroleum, although several biobased polymers may not be biodegradable. The advances in chemical processing and biotechnology have allowed the

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production of novel biobased, biodegradable polymers and bionanocomposites [8]. This chapter is devoted to some aspects of the biobased and biodegradable polymers and nanocomposites. Biobased and biodegradable polymers nanocomposites are an important class of hybrid materials, comprised of biopolymers and nanoparticles or nanoreinforcements. This chapter discusses and highlights the biobased and biodegradable polymers and common factors affecting of biobased and biodegradable polymers as well as nanocomposites preparation and their utilization in a diverse range of products. Finally, the future perspective and current challenges related to biobased and biodegradable polymer nanocomposites are outlined.

Polymer: Biobased and Biodegradable Polymers Polymers can be found almost in every material used in our daily life. They are widely used materials and play an important role in our society and industry. They are used in automotives, clothing, communication, contruction, food packaging, health care equipment and supplies, household appliances, transportation, sports, etc. Polymeric materials can be classified into either thermoplastics or thermosetting polymers. Though thermoset and thermoplastics sound similar, they have very different properties and applications. Thermoplastic polymers consist of wellpacked, noncovalently bound polymer chains. The materials can melt and flow when heated above their melting point. Thermosetting polymers consist of networks of polymer chains interconnected through covalent bonds. They are materials that undergo a chemical reaction or curing, and normally transform from liquid to solid. The latter structures do not melt when heated and cannot be dissolved in a solvent [9, 10]. These differences between thermoplastic and thermosetting polymers have a direct impact on polymer recyclability. Thermoplastics can be recycled through melting process. The melting process can be repeated, with the plastic melting and solidifying as the temperature climbs above and drops below the melting temperature, respectively. On the other hand, subsequently, exposure of the thermoset to high heat after solidifying will cause the material to degrade, not melt. These materials typically degrade at a temperature below where it would be able to melt. In current industrial uses, majority of thermoplastics and thermosetting polymers are synthetic material and derived from petroleum. This is imposing a limitation to the polymer industry due to the continuous depletion of crude oil, frequent oscillation in oil price, environmental concerns with sustainability, gas emissions, disposal, and recyclability [11, 12]. Many studies have opined that the new biobased and biodegradable polymers have the potential to replace petroleum-based polymers and help solve some of the most urgent problems caused by the overuse of petroleumbased polymers, such as water and soil pollution, deleterious influence to human health, and overdependence on petroleum. Therefore, biobased and biodegradable materials have gained more attention from industry. This is due to several reasons including their environmentally friendly characteristics, vast variety and availability, low toxicity, as well as their potential for biodegradability, conservation of petroleum demands, accessibility, economic efficiency, and low carbon footprint. The industry

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for biopolymers is still in its infancy and growing fast. However, there is a lot of confusing terminology in the area of biopolymers these days, due to the fact that not all biobased polymers or biobased plastics are biodegradable. Biobased polymers are polymers made from biological sources. They are sustainable polymers synthesized from renewable resources such as plants, corn, and other biomass-starting materials instead of the conventional fossil resources such as petroleum oil and natural gas, preferably based on biological and biochemical processes [13]. Some of these biobased polymers are formed directly in the polymeric form within the producing organisms such as microorganisms, algae, or plants, while the other biobased polymers are manufactured from biobased monomers. Biobased polymers can be thermoset, thermoplastic, and elastomer. Biobased polymers are consisting of monomeric units that are covalently bonded to form larger structures. On the other hand, biodegradable polymers are a special class of polymer. They can be derived from either renewable or nonrenewable resources. In other words, they can be either natural or synthetic. The materials break down after their intended purpose by bacterial decomposition process, leading to lower molecular weight products that can then be used in other processes, by other organisms. They can break down to result in natural byproducts such as CO2 and N2 gases, water, biomass, and inorganic salts. The biodegradable polymers have undergone extensive investigation since the 1970s. The use of natural or renewable materials for the preparation of biobased or biodegradatble products may result in materials with similar and sometimes possibly improved properties as compared to petroleum-based resouces. In addition, the ultilization of sustainable polymeric materials including biobased and biodegradable polymers is believed to contribute to the preservation of natural resources and protection of the global environment because of its carbon offset or carbon neutral nature and ought to be essential for attaining better human life in this century [13]. Biobased and biodegradable polymers are classified into three classes. The first class is naturally derived biomass polymers. In this class of biobased polymers, biomass is directly utilized as polymeric material including chemically modified ones such as cellulose, cellulose acetate, starches, chitin, modified starch, etc. Cellulose and starch are the simplest biobased polymers. They have been known and widely used for centuries. However, the use of these materials was restricted to a few applications such as packaging, textiles, and construction. The second class of the biobased polymer is bioengineered polymers. The biosynthesized for produce the class of biobased polymer by using microorganisms and plants. For example, poly(hydroxy alkanoates) (PHAs) and poly(glutamic acid) are considered as the second class of biobased polymer. The two classes of biobased polymer play an important role in situations that require biodegradability. The third one is synthetic biobased polymers. Monomers used in this class of polymers are produced from naturally derived molecules or by the breakdown of naturally derived macromolecules through the combination of chemical and biochemical processes. Polylactide (PLA), poly(butylene succinate) (PBS), biopolyolefins, biopolyethylene (bio-PE), and biopoly(ethylene terephthalic acid) (bio-PET) are considered as synthetic biobased polymers. It is practically possible for using the monomers in third class polymers into the existing petroleum-derived polymers production system. Therefore, among the three classes of biobased and biodegradable polymers, the third

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class is the most promising polymer for production and utilization as compared to the first and second ones. However, some of the third class polymers such as bio-PET and biopolyolefins are not biodegradable after use. Thus, their contribution for reducing environmental impact is mainly derived from reducing the carbon footprint [14–16]. One can find a vast range of applications of biopolymers in different fields such as agricultural films, automotive, medical and pharmaceutical, food packaging, hygiene, and protective clothing.

Biobased Polyamides Polyamides are polymers which contain repeating amide, -CO-NH- linkages on their structure. Figure 1 shows the chemical structure of the polyamides. Proteins are examples of naturally occurring polyamides. The best-known manufactured polyamides are often called nylons. They are aliphatic polyamides. The monomers for polyamides may be aliphatic, semiaromatic, or aromatic (aramids) as shown in Table 1. Polyamides are widely used in markets such as automotive and transportation, electrical and electronics, consumer goods, packaging, etc. The use of renewable raw materials substantially improves the carbon footprint and has a positive impact on the life-cycle assessment of plastic products, thus making the properties of biobased polyamides similar to those of petroleum-based polyamides such as 6 and 6.6. This is a reasonable milestone to set when creating realistic development strategies [17]. The monomers from sebacic acid, which comes from castor oil, are being used for the production of PA6,10; PA10,10; and PA10,12. One of the most important biobased polymers is polyamide 11 (PA 11), which is a commercial aliphatic polyamide that is produced from castor oil [18, 19]. Fig. 1 Chemical structure of polyamides

Table 1 The monomers for polyamides Polyamide Polyamide 46 Polyamide 6 Polyamide 12 Polyamide 66 Polyamide 69 Polyamide 6–10 Polyamide 6–12 Polyamide 1212

Monomer(s) 1,4-Diaminobutane/adipic acid Caprolactum Laurolactam Hexamethylene diamine/adipic acid Hexamethylene diamine/azelaic acid Hexamethylene diamine/1,12-dodecanedioic acid Hexamethylene diamine/sebacic acid 1,12-Dodecanediamine/1,12-dodecanedioic acid

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Polysaccharides Polysaccharides include well-known polymers, such as cellulose, starch, and their derivatives, as well as more exotic polymers, such as chitosan and pectin. Starch or amylum is the most common carbohydrate in human diets. It is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. This material is contained in large amounts in staple foods such as cassava, maize or corn, potatoes, rice, and wheat. The chemical structure of starch is shown in Fig. 2. This polysaccharide is produced by most green plants as energy storage. Pure starch is a white, tasteless, and odorless powder. It is insoluble in cold water or alcohol. Depending on the plant, starch generally contains 20–25% amylose and 75–80% amylopectin by weight [20, 21]. Starches can be used in various industries including adhesive and binding applications, paper making, corrugating, construction, paints and coatings, textiles, and oilfield applications. Cellulose is an organic compound with the formula (C6H10O5)n. It is a polysaccharide consisting of a linear chain of several hundreds to many thousands of β(1!4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants. Due to its abundance in plant fibers, cellulose is important to the polymer industry. It is not used directly as a polymer matrix to bind the other materials, but it is utilized as an additive. It is used as reinforcement in the composites. For instance, the incorporation of cellulose fibers or natural fibers such as flax, hemp, and wood into a polymer matrix improves the mechanical properties of the final product. Cellulose is odorless and has no taste. It is hydrophilic with the contact angle of 20–30°. Cellulose is insoluble in water and most organic solvents. It is chiral and biodegradable. The chemical structure of cellulose is shown in Fig. 3 [22–24]. CH2 OH

Starch

OH

CH2 OH

OH OH

CH2 OH

OH OH

CH2 OH

OH OH

OH

Fig. 2 Chemical structure of starch

CH2 OH

Cellulose

OH

CH2 OH

OH OH

OH OH

OH OH

Fig. 3 Chemical structure of cellulose

CH2 OH

OH

CH2 OH

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Cellulose for industrial use is mainly obtained from wood pulp and cotton [25]. Cellulose has versatile uses in many industries such as cosmetic and pharmaceutical industries, fibers and clothing, paper and paperboard, veterinary foods, wood and paper, etc. However, cellulose is mainly used to produce paper and paperboard. Smaller quantities are converted into a wide variety of derivative products such as cellophane and rayon. Cellulose derivatives are still used on a wide scale in film, cigarette filters, and biomedical applications [26]. The current focus of cellulose research is nanocellulose, including cellulose nanocrystals and cellulose nanofibrils. The nanocellulose materials are produced from cellulose fibers after mechanical, physical, chemical, and separation procedures [27–29]. In addition, conversion of cellulose from energy crops into biofuels such as cellulosic ethanol is under development as a renewable fuel source. Chitin is the most abundant natural amino polysaccharide. Chitin is also the most important polysaccharides. The material is estimated to be produced annually almost as much as cellulose. Chitosan is a close derivative of chitin. It is the second most abundant biopolymer in the world. Chitin has become of great interest not only as an underutilized resource, but also as a new functional material of high potential for various applications. Natural green products will continue to win more market share from synthetic, and often toxic, chemicals as consumers and industries are becoming more aware of their environmental impact. Chitin and chitosan are considered as useful biocompatible materials to be used in a medical device to treat, augment, or replace any tissue, organ, or function of the body [30]. The materials can be used in a variety of areas such as biotechnology, cosmetics, food processing and agriculture, pharmaceutical and medical applications, paper production, and textile wastewater treatment [31].

Polylactic Acid (PLA) Polylactic acid (PLA) has received much attention from many industries for several years. It is a thermoplastic aliphatic polyester derived from renewable resources (Lactic acid: C3H6O3). The thermoplastic is the most important biobased and biodegradable polymers derived from renewable resources. The monomers for producing the PLA is typically from fermented plant starch such as from cassava, corn, sugar beet pulp, or sugarcane. Figure 4 shows the chemical structure of the PLA. There are several different types of Polylactic acid including racemic PLLA (poly-L-lactic acid), regular PLLA (poly-L-lactic acid), PDLA (poly-D-lactic acid), and PDLLA (poly-DL-lactic acid). Each of these PLAs has slightly different characteristics, but they are produced from a renewable resource.

Fig. 4 Chemical structures of PLA

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PLA polymers can have range from amorphous glassy polymer to semicrystalline and highly crystalline polymer. The glass transition temperature is in the range of 60–65 °C, and a melting temperature is in the range of 130–180 °C. The tensile modulus of PLA is 2.7–16 GPa. As such, the mechanical properties and processability of these polymers are quite similar to some petroleum-based polymers. PLA is the most widely used thermoplastic polymer filament material in 3D printing. PLA can degrade into innocuous lactic acid, so it is used as medical implants in the form of anchors, screws, plates, pins, and rods and as a mesh. PLA can also be used as a decomposable packaging material, either cast, injection-molded, or spun [32, 33].

Polyhydroxyalkanoate (PHA) Polyhydroxybutyrate (PHB) Polyhydroxyalkanoates or PHAs are members of polyester family. These polymers consist of hydroxyalkanoate monomers. They are produced by numerous microorganisms, through bacterial fermentation of sugars or lipids [34]. Polyhydroxybutyrate (PHB) is a polymer belonging to the polyesters class that is of interest as bioderived and biodegradable plastics [35]. Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA). The PHB polymers are produced by a variety of organisms. These polymers include poly-3-hydroxybutyrate (PHB or P3HB), poly4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO), and their copolymers. Among PHAs, polyhydroxybutyrate (PHB) and poly(hydroxybutyratecohydroxyvalerate) (PHBV) are probably the most common types of polyhydroxyalkanoate. They are frequently considered as biodegradable polymers in the type of PHAs. Molecular weights of polyhydroxyalkanoates (PHAs) depend on the producing bacteria and growth medium conditions. Figure 5 shows the chemical structures of PHAs. PHA has also received much attention from many industries. The degradation of the PHA polymer does not correspond to an overall increase of atmospheric CO2 levels. PHA is associated to a carbon-neutral footprint. The polymers are produced by numerous microorganisms and are therefore biobased and biodegradable through enzymatic activity [36]. The polymer can be utilized in various areas of biomedical R group -CH3

Poly(3-hydroxyalkanoates)

-CH2 -CH3

Poly(3-hydroxyvalerate)

PHV

-(CH2 )2 -CH3

Poly(3-hydroxyhexanoate)

PHHex

-(CH2 )4 -CH3

Poly(3-hydroxyoctanoate)

PHO

-(CH2 )6 CH3

Poly(3-hydroxydecanoate)

PHD

-CH2 -

Poly(3-hydroxy-5phenylvalerate)

PHPV

Fig. 5 Chemical structures of PHAs

PHA

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applications, such as sutures, stents, dialysis media, and drug delivery devices. The PHA is also being evaluated as a material for compost bags, disposable tableware, food packaging, loose-fill packaging, and tissue engineering.

Proteins Proteins are polymers which are composed of amino acids. They are organic compounds made of carbon, hydrogen, nitrogen, oxygen, or sulfur. Amino acids are the building blocks of proteins. Protein is a macronutrient that is essential to building muscle mass. Protein is commonly found in animal products, though is also present in other sources, such as nuts and legumes. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids [37]. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. We sometimes hear that there are three types of protein foods: complete proteins, incomplete proteins, and complementary proteins. Proteins play a role in nearly every biological process. Their functions are varying widely. In a body, the main functions of proteins are to build, strengthen, and repair or replace things, such as tissue [38]. Protein can be used in the plastic industry as a biobased polymer as well. The addition of keratin to synthetic elastomers results in materials with good thermal, mechanical, flame resistant, and thermo-oxidative properties [39]. The addition of soy protein to petroleum-based latexes results in a material with properties comparable to carbon black-filled elastomers [40]. The present and interaction of vegetable oils and proteins in peanut oil emulsions revealed that protein-coated droplets are stabilized via disulfide crosslinking [41]. Figure 6 shows protein chain with peptide bond.

Succinate Polymers More biobased succinate polymers are being developed since biobased succinic acid (SA) becomes more commercially available. In this group of polymers, polybutylene succinate (PBS) is an interesting material from an application standpoint. PBS is produced by direct polycondensation of SA and butanediol (BD) and is one of the most well-known succinate polymers. PBS is the biodegradable plastic that decomposes into water and carbon dioxide with the microorganism under the soil. The other is Poly(ethylene succinate) (PES) which is produced via polymerization of

NH2 Fig. 6 Protein chain with peptide bond, R ¼ amino acid group

OH

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Poly(butylene succinate) or PBS

Poly(ethylene succinate) or PES

Poly(butylene succinate-co-butylene adipate) or PBSA

Poly(butylene succinate-co-butylene terephthalic acid) or PBST

Z

Poly(butylene succinate-co-butylene furandicarboxylate) or PBSF

Fig. 7 Chemical structures of succinate polymers

succinic acid, and ethylene glycol is biodegradable and could also be sourced from biobased building blocks [42–45]. Figure 7 shows the chemical structures of succinate polymers.

Biobased Polyethylene (Bio-PE) While the traditional polyethylene is produced by fossil-sourced raw materials such as oil or natural gas, the biobased polyethylene (bio-PE) is produced by bioethylene which is chemically converted from bioethanol. Due to soaring oil prices, bioethanol produced by fermentation of sugar streams attracted the fuel industry in the 1970s [46]. Bioethanol is produced from sugar cane using a fermentation process. The bioethylene monomer can then be used in traditional polyethylene polymerization processes to make the various grades of PE including HDPE, LDPE, LLDPE, etc. The advantage of bio-PE is the fact that its properties are identical to fossil-based PE, which has a complete infrastructure for processing and recycling. Biobased polyethylene is a great example of a polymer made using a renewable feedstock. However, the biobased PE is not biodegradable.

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Biobased Poly(Ethylene Terephthalate) (PET) and Poly(Trimethylene Terephthalate) (PTT) Biobased PET is a polyester thermoplastic which is partially produced from renewable biomass such as sugars obtained from agricultural activities. The biobased PET is fully recyclable, biodegradable, compostable, and renewable bioplastic material. The material can be composed of 30% plant-derived ethanol glycol and 70% fossilderived terephthalic acid (TPA). Biobased PET is effectively “drop-in” products for petroleum PET. Biobased poly(trimethylene terephthalate) (PTT) is prepared from biobased 1,3-propoane diol (Bio-PDO). PTT fibers are used in the carpet and textile industries. The polymer has a good shape recovery property due to its unique chain conformation. It can be processed in current production facilities and can be recycled in current PET recycling facilities.

Cashew Nutshell Liquid and Vegetable Oils The cashew nutshell liquid (CNSL) is a mixture of phenol. The most important constituents in CNSL are anacardic acid, cardanol, and cardol. CNSL is an agricultural byproduct of cashew nut and cashew apple production, produced by the cashew nut tree (Anacardium occidentale) [47]. Naturally occurring, CNSL contains mainly four components including cardanol, cardol, anacardic acid, and 6-methyl cardol. Table 2 shows the chemical structures of these compounds. Cashew nutshell liquid is a dark brown viscous liquid present inside a soft honeycomb structure of the cashew nutshell. Cashew nutshell liquid is opaque when applied as a thin film, with a reddish-brown color. Several methods can be used to extract the CNSL including hot oil process, roasting method, miscellaneous methods, mechanical extraction, solvent extraction, vacuum distillation, or supercritical fluids processes that mainly utilize hot-oil and the local roasting in which the CNSL flows out from the shell. The aromatic content and crosslink density are features and potential properties of the CNSL. CNSL has been utilized in flame-retardant applications due to its chemical structure, which includes an aromatic ring. The material has also been used in various applications, which include synthetic polymers such as resole and novolac resins, free radical and ionic thermosets, and novel CNSL-formaldehyde resins. CNSL can be polymerized by various techniques. The presence of the aliphatic side chain gives these resins pronounced hydrophobicity, which is a valuable property for many applications. On the other hand, the unsaturation in the side chain of CNSL can be utilized in free radicals or ionic initiators for addition polymerization [48–50]. The CNSL has phenolic properties, and these properties have been utilized to make condensation polymers by reaction with formaldehyde, furfural, HMTA, etc. Currently, there is no evidence that CNSL-based materials are biodegradable [36]. Vegetable oil in the modern day is not just a cooking oil but has now turned into various polymeric products. In addition, an effort of replacing petroleum with green resources is urgently required of its cheap price, and abundantly available.

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Table 2 Some fatty acids in natural oils [54, 55] Name α-Eleostearic acid

Formulation C18H30O2

Licanic acid

C18H28O3

Linoleic acid

C18H32O2

Linolenic acid

C18H30O2

Myristic acid Oleic acid Palmitic acid Palmitoleic acid Ricinoleic acid

C14H28O2 C18H34O2 C16H32O2 C16H30O2 C18H33O3

Stearic acid Vernolic acid

C18H36O2 C18H32O3

CNSL Anacardic acid, cardol, cardanol, and 2-methyl cardol Components of the side chain

Chemical structure CH3-(CH2)3-CH¼CH-CH¼CH-CH¼CH (CH2)7COOH

CH3(CH2)4CH¼CH-CH2-CH¼CH (CH2)7COOH CH3-CH2-CH¼CH-CH2-CH¼CH-CH2CH¼CH(CH2)7COOH CH3(CH2)12COOH CH+(CH2)7CH¼CH(CH2)7COOH CH3(CH2)14COOH CH3(CH2)5CH¼CH(CH2)7COOH

CH3(CH2)16COOH

CH3

C15 H31-n

C15H31-n

C15 H31-n

C15 H31-n

C15 H31-n n=0 n=2 n=4 n=6

Triglycerides, the primary components of vegetable oils, are an abundant, renewable, and widely investigated alternative feedstock for polymeric materials. Triglycerides from plants, such as palm trees, soybeans, rapeseeds, cotton, sunflower, palm kernel, olives, and coconuts can be utilized as well [51–53]. These triglycerides contain several reactive sites, such as double bonds and ester groups. This opens up various possibilities to tailor new structures. The chain lengths of the fatty acids in naturally occurring triglycerides can be of varying lengths; 16, 18, and 20 carbon atoms are the most common. Table 2 shows the chemical structure of some fatty acids and CNSL. Over the past decades, various polymeric systems have been developed based on the crosslinking of vegetable oils through free radical or cationic polymerization reactions. Table 3 shows the fatty acids composition in various vegetable oils. To increase the reactivity of the vegetable oils with polymers, the carbon-carbon double bonds in the fatty acid chains of the vegetable oils can be modified to undergo various reactions to attach different polymerizable functionalities, such as acrylates. For example, the acrylated epoxidized soybean oil (AESO), which is synthesized from the reaction of acrylic acid with epoxidized soybean oil, has been utilized in polymers and composites [53, 57].

Fatty acid α-Eleostearic acid Licanic acid Linoleic acid Linolenic acid Myristic acid Oleic acid Palmitic acid Palmitoleic acid Ricinoleic acid Stearic acid Vernolic acid Others

Linseed oil (%) –

– 17.0 52.0 – 22.0 5.0 –

4.0 – –

Castor oil (%) –

– 4.0 0.5 – 5.0 1.5 –

87.5 0.5 – –

– 9.0 – – 45.0 39.0 – – 5.0 – 2.0

– 4.0 – –

Palm oil (%) –

74.0 8.0 – – 8.0 6.0 –

Oiticica oil (%) –

Table 3 Fatty acids composition in various vegetable oils [56]

– 2.0 – 2.0

– 26.0 10.0 – 56.0 4.0 –

Rape seed oil (%) –

– 43.0 – –

– 35.0 12.0 – 46.0 4.0 –

Refined tall oil (%) –

– 4.0 – –

– 53.0 7.0 – 24.0 12.0 –

Soybean oil (%) –

– 4.0 – –

– 47.0 – – 42.0 6.0 –

Sunflower oil (%) –

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There are several types of polymers in the market. The most common, which are thermoplastic, thermoset, and biopolymers, are summarized in Table 4. In general, the biobased polymers can be either biodegradable or nondegradable. Likewise, while many biobased polymers such as starch are biodegradable, not all biodegradable polymers (e.g., polycaprolactone) can be recognized as biobased. The global biobased polymers market size is expected to reach $9.6 billion by 2025, rising at a market growth of 6.6% compound annual growth rate (CAGR) during the forecast period [66].

Bionanocomposites Classification of Bionanocomposites The combining two or more materials which have quite different properties and do not dissolve or blend into each other will form composites. The materials in the composites work together to give the composites unique properties. Composites offer many benefits; key among them are corrosion resistance, design flexibility, durability, light weight, and strength. Composites have permeated our everyday lives such as products that are used in aerospace, constructions, household appliances, medical applications, oil and gas, transportation, sports, and many more. As the demand for “green” materials and products is growing, the use of renewable resources and recycled materials proves to be greatly attractive. Biocomposites from biobased and biodegradable polymers with reinforcements have appeared as low-cost, light materials for different applications and have attracted much interest. Biobased and biodegradable polymers have the potential to replace petroleum-based polymers in many applications. However, the biobased and biodegradable polymers are generally not competitive with polymers in mechanical strength. Therefore, reinforcements are usually added to improve their performance. Recently, nanotechnology is a rapidly evolving technology as science; engineering and technology have merged to bring nanoscale materials that much closer to reality. Nanotechnology is helping improve products’ properties that we use every day. This technology is creating new, exciting products for future utilizations. Nanofibers, nanotubes, nanoparticles, nanowhiskers, and nanoplates are examples of a few substances that are within the nanoscale (less than 100 nm) in at least one dimension which have been utilized in the nanocomposite developments [67, 68]. Innovation in polymer nanocomposites leads to diverse applications of biobased and biodegradable polymers in automotive, biosensors, bone regeneration, drug delivery, packaging, solar cells, super capacitors, etc. Combining benefits from biobased and biodegradable polymers and nanoreinforcements could be a step toward sustainable development. Biomimetic approach has been taken into consideration in which a vital role is played by the integration of nanoreinforcements in biobased and biodegradable polymers. Nanocomposites are made of polymers and nanoreinforcements, having dimensions in the nanometer range (1–100 nm). In the present scenario, the biobased and biodegradable polymers are facilitated by the functionalization of the nanomaterials.

Thermoplastic

Density (g/cm3) 1.1–1.2 1.0–1.1 1.3 1.3–1.4 0.9 0.9–1.0 0.9–1.0 1.0 0.9 1.1 1.1 1.2 1.5–1.6 1.3–1.5 1.2–1.4 1.1–1.2 0.9–1.3 1.0–1.1 2.1 1.3–1.5 1.1

Polymers Acrylic (metacrylate) Acrylonitrile butadiene styrene (ABS) Cellulose acetate Cellulose nitrate Crosslinked polyethylene (PE) Ethylene vinyl acetate (EVA) High-density polyethylene (HDPE)

High-impact polystyrene (HIPS)

Low-density polyethylene (LDPE)

Nylon 6 (PA 6) Nylon 6,6 (PA 6,6)

Polycarbonate (PC) Polyethylene terephthalate (PET) Polyether ether ketone (PEEK) Poly ether ketone (PEK) Poly methyl methacrylate (PMMA) Polypropylene (PP) Polystyrene (PS)

Polytetrafluoroethylene (PTFE) Polyvinyl chloride (PVC)

Rigid thermoplastic Polyurethane (RTPU, PUR-RT)

Table 4 Properties of polymers and biopolymers [58–65]

96.0 120.0– 180.0 210.0– 270.0 105.0– 115.0 220.0 260.0– 270.0 155.0 260.0 343.0 343.0 160.0 160.0 210.0– 249.0 327.0 210.0– 260.0 145.0

Tm (°C) – – 493.0 160.0

75.0

13.0 52.0–90.0

69.0 55.0–159.0 92.0–95.0 100.0–110.0 72.4 35.8 34.0–48.0

81.4 82.7

10.0–11.6

42.0

Tensile strength (MPa) 20.0–30.0 47.0 40.0 48.0 18.0 17.0 32.0–38.2

4.0

(continued)

0.3 3.0–4.0

2.3 2.3–9.0 3.7–24.0 3.5 3.0 1.6 3.0–3.5

2.8 2.8

0.2–0.3

2.1

Young’s modulus (GPa) 0.7 2.1 1.4 1.4 0.5 0.02 1.3

Biobased and Biodegradable Polymer Nanocomposites

5.0

100.0 50.0–200.0

200.0 300.0 1.6–50.0 20.0 2.5 80.0 1.6–3.0

60.0 60.0

400.0

2.5

Elongation (%) 6.0 270.0 10.0–60.0 40.0 350.0 750.0 150.0

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Biobased and biodegradable polymers

Thermoset

Table 4 (continued)

1.25 1.1 1.2–1.3 1.2 1.3 1.2–1.3

Poly(butylene succinate) (PBS) Polycaprolactone (PCL)

Polyhydroxyalkanoates (PHA)

Polyhydroxybutyrate (PHB)

Poly-3-hydroxybutyrate (P-3-HB) Poly-3-hydroxybutyrate-co-3-hydroxyvalerate (P-3-HB-3 HV) Poly(hydroxybutyrate-cohydroxyvalerate) (PHBV) Polylactic acid (PLA) Poly-l-lactic acid or poly-l-lactide (L-PLA)

Poly-dl-lactic acid or poly-dl-lactide (DL-PLA) Starch (film to injection molding grades)

1.1

Polyamide 410 (PA410)

1.5

1.3

1.2–1.3 1.2–1.3

1.2–1.3

Density (g/cm3) 1.2–1.3 1.5–1.6 1.2 1.2 1.1 1.5–1.6 1.2–1.3 1.2–1.3

Polymers Epoxy (EP) Melamine formaldehyde (MF) Phenol formaldehyde (PF) Rigid thermoset polyurethane (RPU) Unsaturated polyester (UPE) Urea formaldehyde (UF) Polyurethane rigid Polyurethane rubber

100.0– 190.0 115.0 170.0– 200.0 170.0– 180.0 110.0– 115.0

255.0– 260.0 115.0 58.0– 65.0 40.0– 180.0 140.0– 190.0 180.0 140.0

Tm (°C) – – – – – – – –

20.0– 1000.0

2.0–10.0

2.1–30.7 3.0–10.0

7.0–15.0

0.4–6.0 1.6–20.0

1.6–6.0

21.5 300.0– 1000.0 2.0–1200.0

Elongation (%) 1.3 0.6 1.2 90.0 2.0 0.8 78.0 300.0– 580.0 100.0

20.0–80.0

27.6–50.0

5.9–72.0 15.5–150.0

25.0–30.0

40.0 23.0–40.0

24.0–43.0

10.0–39.0

26.5–35.0 13.0–42.0

75.5

Tensile strength (MPa) 600.0 65.0 45.0 60.0 60.0 65.0 0.08–103.0 39.0

0.1–2.0

1.0–3.5

1.1–3.6 2.7–4.1

0.6–1.0

3.5 3.5

3.5–7.7

0.3–3.8

48.0 0.2–0.5

2.9

2.0–10.0

Young’s modulus (GPa) 80.0 12.0 6.5 2.2 3.4 9.0

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Fig. 8 Nanoreinforcements classification

These novel nanostructure materials show specific and unique conductivity, mechanical, thermal properties, and permeability. These materials have vast applications due to their multidimensional properties such as biocompatibility, antimicrobial activity, and biodegradability [69]. Nanoreinforcements can be classified into different categories as shown in Figure 8. The first one is nanoplatelets which is a material that has only the thickness in the nanometer scale range. The nanomaterials in this category include phyllosilicates, silicic acid (magadiite), layered double hydroxides [M6Al2(OH)16CO3nH2O; M ¼ Mg, Zn], zirconium phosphates [Zr(HPO42H2O], and di-chalcogenides [(PbS)1.18(TiS2)2, MoS2]. The most popular nanoplatelets applied in various nanocomposite formulations from thermoset and thermoplastic polymers are layered or plate-like clay minerals [70– 81]. The second class of the nanoreinforcements is two dimensions of nanoparticulates which are in the nanometer scale range. The nanofibers, nanotubes, nanorods, and nanowhiskers are two dimensions of nanoparticulates which are in the nanometer scale range. Along with inorganic carbon nanotubes, nanosized fibers of biological origin that come from plants and animals are also considered as two dimensions of nanoparticulates. The nanomaterials such as nanocellulose, cellulose nanocrystals (CNC), cellulose nanofibrils (CNF), and starch nanofibers are nanosized fibers of biological origin. Plants such as cotton, wood, soy, hemp, flax, jute, sisal, banana, and kenaf serve as a source of the cellulose and starch nanofibers. The third category is nanoparticles in which all three dimensions of the materials are of nanometers scale. They are three-dimensional particulates and are called “isodimensional.” The materials can be spherical, cubic, and shapeless nanoparticles with a size up to 100 nm. Nanosized hydroxyapatite (active component of scaffolds and implants), special case (the hydroxyapatite Ca10(PO4)6(OH)2), a major mineral component of bones) are belong to this group of nanoparticulates [69]. Preparation of biobased products from biobased and biodegradable polymers with different nanoreinforcements and bionanoreinforcements, such as nanocelluloses (cellulose nanocrystals, cellulose nanofibrils), nanolignin, nanohemicellulose, etc., has been considered to have substantial environmental and economic benefits.

Bionanocomposites: Processing and Properties In recent years, the development of environmentally friendly polymeric nanocomposite materials, which are primarily from biobased polymers and biodegradable polymers with balanced properties, has become the focus of considerable research

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attention. The polymers can be based on both fossil fuels and natural resources. The key factors to incorporate nanofillers are to increase the efficiency of biobased and biodegradable polymers due to their high aspect ratio, biocompatibility, low density, and high mechanical strength. The utilization of various nanoreinforcements in environmentally benign biobased and biodegradable polymers has exhibited considerable promise for designing and manufacturing green polymeric nanomaterials with desired properties. This can be done by various methods which can result in the enhancement on thermal, mechanical, and electrical properties of biobased and biodegradable polymers. The principal factors that result in the properties of nanomaterials to differ significantly from other materials are the increased relative surface area and quantum effects [82]. Considering the manufacturing methods for biobased and biodegradable polymers nanocomposites, the widely used methods for manufacturing conventional composite parts could be potentially utilized for manufacturing the materials. There are several methods for both thermoset and thermoplastic polymers nanocomposites such as casting, hand lay-up, spay-up, wet lay-up, pultrusion, resin transfer molding (RTM), vacuum-assisted resin transfer molding (VARTM), autoclave processing, resin film infusion (RFI), the prepreg method, filament winding, pultrusion, fiber placement technology, compression molding, extrusion, injection molding, 3D printing, etc. [83] By incorporating nanofillers in biobased and biodegradable polymers, they can prove a wide range of substantial enhancement in electrical, mechanical, optical, thermal, and chemical properties. Multiwall carbon nanotubes (MWCNT), carbon nanofibers, nanoclays, predisposed nanosilica, nanoalumina, nano-SiC particles, HNT™ (Halloysite nanotubes), nanocellulose, and nanolignin have been used in biobased and biodegradable polymers nanocomposites to enhance mechanical/thermal properties of the polymers. Biobased nanocomposites have been prepared by the cationic polymerization of conjugated soybean oil (CSOY) or conjugated LoSatSoy oil (CLS) with styrene (ST) and divinylbenzene (DVB), and a reactive organomodified montmorillonite (VMMT) clay as a reinforcing phase [84]. Authors reported a significant improvement in the dynamic bending storage modulus, the compressive modulus, the compressive strength, the compressive strain at failure, the thermal stability, and the vapor barrier performance for the CSOY- and CLS-based nanocomposites with 1–2 wt% VMMT loading. They also noticed that CLS-based nanocomposites with 1–2 wt% VMMT exhibits increases of 100–128%, 86–92%, and 5–7% in compressive strength, compressive modulus, and compressive strain at failure, respectively. In addition, the CLS with higher unsaturation and reactivity affords nanocomposites with higher thermal stability and higher mechanical properties than the CSOY. With a small amount of nanoclays in the formulation of biobased unsaturated polyester resin (UPRs), the mechanical, thermal, and barrier properties of the UPRs enhanced to exhibit good enhancements [85, 86]. Liu et al. prepared graphene-reinforced tung oil (TO)-based unsaturated polyester nanocomposites via in situ melt polycondensation integrated with Diels-Alder addition [87]. They reported that the mechanical and thermal properties of the TO-based unsaturated polyester resin (UPR) were greatly improved by the incorporation of GO. Only 0.10 wt% content of GO, the obtained biobased nanocomposite, showed tensile strength and modulus of 43.2 MPa and 2.62 GPa, and Tg of 105.2 °C,

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which were 159%, 191%, and 49.4% higher than those of the unreinforced UPR/TO resin, respectively. In addition, the biobased UPR nanocomposite with 0.1 wt% of GO demonstrated superior comprehensive properties as compared to neat UPR. Therefore, the developed biobased UPR nanocomposites are very promising to be applied in structural plastics. Johnson and his team reported the utilization of TEMPO-oxidized celluloses in biobased nanocomposites. The TEMPO-oxidized wood pulps (carboxylate content 1.1 mmol/g cellulose) were fibrillated to varying degrees using a high-intensity ultrasonic processor. They reported that the thermal degradation temperature of unfibrillated oxidized pulps was only minimally affected (6 °C decrease) by the fibrillation process. Dynamic mechanical analysis of the nanocomposites revealed strong fibril-matrix interactions as evidenced by remarkable storage modulus retention at high temperatures and a suppression of matrix glass transition at “high” (~5 wt%) nanofibril loadings. Creep properties likewise exhibited significant (order of magnitude) suppression of matrix flow at high temperatures [88]. Barrierimproving effects of CNCs have also been reported, and ascribed to an increased path length for diffusing molecules by tortuosity effect [89, 90]. Biopolymer nanocomposites of polyhydroxybutyrate and polylactic acid biopolymers comprised of cellulose nanofibrils (CNFs) and nanoclay (NCs) were prepared by a twin-screw extruder. The addition of nanofillers to matrixes decreased the density of the BNCs, whereas the mechanical properties generally increased [91]. Al and his team reported that the reinforcing effect of CNFs was more than that of NCs. Thermal stability of the BNCs was reported to improve incorporation with fillers; the addition of the fillers showed improvement to T10%, T50%, and T85%. Zhou et al. prepared bionanocomposite of poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) and cellulose nanocrystals (CNCs) by the cast-film method. They reported that the crystallization behavior of PHBH changed with the addition of CNCs, resulting in an improvement in the thermal stability and mechanical properties of the bionanocomposite. Compared with PHBH, the water-vapor permeability coefficient of bionanocomposite increased by 207.8% and the oxygen permeability coefficient decreased by 18.5% [92]. Table 5 presents the tensile properties of some biobased and biodegradable polymers nanocomposites. The effect of CNC addition on the optical transparency, water vapor permeability (WVP), and tensile properties of bionanocomposite films was studied by Miri et al. They reported that bionanocomposite films remain transparent with the presence of CNC dispersion at the nanoscale. The WVP was significantly reduced, and the elastic modulus and tensile strength were increased gradually with the addition of CNC [93]. Bahmani et al. studied the extrudable hydroxyapatite in plant oil-based biopolymer nanocomposites for biomedical applications [102]. The nanocomposites comprised of a vegetable oil-based resin (soybean oil epoxidized acrylate (SOEA)) or SOEA+2-hydroxyethyl acrylate (HEA) which was reinforced using silanized nanohydroxyapatite (Si-nHA). They reported better particle alignment and relatively greater augmentation of filament mechanical properties relative to the base resins. The global bioplastics production capacities are difficult to estimate. They are usually based on forecast because of continuously emerging range of the materials and rising interests on investing in bioplastics sector. Nanocomposites exhibit

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Table 5 Properties of biobased and biodegradable polymers nanocomposites [65, 93–108] Materials APPE APPE+1 wt% Cloisite 30B APPE+3 wt% Cloisite 30B APPE+5 wt% Cloisite 30B CMC CMC-ST CMC-ST + 0.5% CNC CMC-ST + 2.5% CNC CMC-ST + 5.0% CNC Gelatin Gelatin+1 wt% Cloisite Gelatin+3 wt% Cloisite Gelatin+5 wt% Cloisite Gelatin+10 wt% Cloisite PCL PCL-NC2.5 PCL-NC5.0 PCL-NC10.0 PCL-0 PCL + 0.5 wt% PCL + 1.0 wt% PCL + 2.0 wt% PA410 PA410 + 1 wt%CNT PA410 + 2 wt%CNT PA410 + 3 wt%CNT PA410 + 4 wt%CNT PA410 + 5 wt%CNT PA410 + 6 wt%CNT PHB PHB+ 15% CTS PHB+ 20% CTS PLA + HNTs PLA + HNTs-ZrO2 1% PLA + HNTs-ZrO2 3% PLA + 4 wt% silane-modified HNT PLA PLA + 1% ZnO PLA + 3% ZnO PLA + 5% ZnO PLA PLA + 1 wt% LNC

Elongation (%) 12.5 12.3 12.0 11.4 23.0 23.9 21.3 20.9 19.0 9.6 8.0 3.6 3.0 0.9 125.0 78.0 26.1 6.3 478.0 465.0 402.0 368.0 100.0 7.0 7.0 5.0 7.0 5.0 6.0 7.30 1.5 0.7 4.3 3.6 1.5 – 9.3 13.0 11.1 12.8 70.0 30.0

Tensile strength (MPa) 131.7 139.0 144.1 149.8 76.2 66.8 83.6 99.1 110.8 88.9 88.1 97.4 97.4 110.8 2.7 3.7 4.8 6.5 13.9 20.7 26.5 35.4 75.5 74.3 75.6 74.3 75.1 74.5 76.3 16.2 8.7 2.3 36.0 28.0 24.0 70.0 42.0 41.0 39.0 35.0 40.0 45.0

Young’s modulus (GPa) 1.1 1.1 1.1 1.2 1.1 0.9 1.2 1.4 1.7 3.3 3.5 4.7 5.9 8.3 0.01 0.02 0.03 0.2 0.4 0.5 0.6 0.7 2.9 2.9 2.9 2.9 2.9 3.0 3.0 0.2 0.2 78.5 1.1 0.8 0.8 2.8 2.7 2.9 3.0 2.8 1.8 1.7 (continued)

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Table 5 (continued) Materials PLA + 3 wt% LNC Pectin-polyethylene glycol blend+30 wt % HNT Potato starch+5 wt% HNT PVA PVA + 3 wt% CW PVA + 6 wt% CW PVA + 9 wt% CW PVA + 12 wt% CW SOEA SOEA+HEA 10% Si-nHA/SOEA 20% Si-nHA/SOEA 25% Si-nHA/SOEA 30% Si-nHA/SOEA 7% Si-nHA/SOEA + HEA 17% Si-nHA/SOEA + HEA 25% Si-nHA/SOEA + HEA 27% Si-nHA/SOEA + HEA ESO ESO + 10% POSS ESO ESO + 5 wt% Cloisite 30B ESO + 8 wt% Cloisite 30B

Elongation (%) >230.0 –

Tensile strength (MPa) 26.0 27.0

Young’s modulus (GPa) 1.1 4.2

– 30.0 12.0 17.0 9.0 10.0 – – – – – – – – – – – – 127.0 133.0 151.0

9.8 105.0 123.0 115.0 110.0 130.0 3.9 1.0 9.1 14.0 19.5 22.8 3.6 14.1 19.3 20.4 – – 1.3 3.0 4.5

0.4 1.8 2.2 2.3 2.4 2.7 0.08 0.01 1.1 2.1 2.6 3.6 0.2 0.4 0.5 0.6 0.2 700.0 0.001 0.003 0.004

Aliphatic polyester (APPE), carboxymethyl cellulose (CMC), polycaprolactone (PCL), poly(vinyl alcohol) (PVA), soybean oil epoxidized acrylate (SOEA), starch (ST), cellulose nanocrystals (CNC); cellulose whiskers (CW), chitosan (CTS), cloisite nanoclay, Graphene oxide (GO) nanosheets, Halloysite nanotubes (HNTs), Lauryl functionalized nanocellulose (LNC), multiwall carbon nanotube (CNT), Nanofibrillated chitosan (NC), Polyhedral oligomeric silsesquioxanes (POSSs), silanized nanohydroxyapatite (Si-nHA) particles with and without diluent hydroxyethyl acrylate (HEA), and Zirconium dioxide or Zirconia (ZrO2)

various properties, such as noncorrosiveness, lightweight, high mechanical strength, and high-temperature capability. They are used in different applications and fields, such as aerospace and defense, automotive, electronics and semiconductors, coating, constructions, energy, marine, and packaging. The superior properties offered by nanocomposites are enabling its usage in various end-use industries. The production of biobased polymers has become much more differentiated in recent years. The major biomass feedstock used for biobased polymer production is biogenic by-products (46%), especially the by-product glycerol from the biodiesel production, used for epoxy resin production. In 2019, the total production volume of biobased polymers was 3.8 million tonnes, which is 1% of the production volume of fossil-based polymers and about 3% more than in 2018. This CAGR is expected to

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continue until 2024 [109]. On the other hand, the nanocomposites market size is estimated to be $4.1 billion USD in 2019 and is projected to reach $8.5 billion USD by 2024 [110]. With properties and high potentials, the nanocomposites continue to become increasingly popular and important for a wide range of applications. However, there are challenges in understanding, predicting, and managing potential adverse effects, such as toxicity and the impact of exposure on human beings and the environment.

Conclusion and Futher Outlook There is a wide variety of sources of biological origin to fabricate the biobased and biodegradable polymers as well as nanocomposites. They are highly versatile, and they have a broad range of physical, mechanical, and chemical properties suitable for diverse applications. Nevertheless, biobased and biodegradable polymers themselves have the potential to replace petroleum-based polymers. However, the biobased and biodegradable polymers are generally not competitive with polymers in mechanical strength. They have high gas and water permeability, low heat degradation temperature, etc. Therefore, nanosized particles or nanoreinforcements are introduced to improve the properties and functionalities of the biobased and biodegradable polymers. However, nanoparticles aggregation is frequently a problem, because of the bonding interactions between functional and active groups on the surface of nanoparticles. Therefore, the first step will be the design and the choice of a right method for fabrication of biobased and biodegradable polymers nanocomposites. In addition, widespread applications of nanomaterials might have enormous potential to both positively and negatively affect human health and the environment. Therefore, biobased and biodegradable polymers nanocomposites play an important role for future applications.

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Biobutanol: A Promising Alternative Commercial Biofuel D. Priscilla Mercy Anitha, S. Periyar Selvam, and Emmanuel Rotimi Sadiku

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Butanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different Methods of Butanol Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microorganisms in the Production of Biobutanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomasses for Biobutanol Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Different Methods to Enhance the Yield of the Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Biobutanol as Biofuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Merits and Demerits of Butanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Growing demand for energy, depletion of fossil fuel supplies, and global warming have evoked the production of alternative renewable and environmentally friendly energy sources, such as bioethanol and biodiesel derived from crops. Nevertheless, there is a concern over the use of fertile land for D. Priscilla Mercy Anitha Department of Biotechnology, SRM Institute of Science and Technology, Potheri, Kattankulathur, Chengalpattu District, Tamil Nadu, India S. Periyar Selvam (*) Department of Food Process Engineering, SRM Institute of Science and Technology, Potheri, Kattankulathur, Chengalpattu District, Tamil Nadu, India E. Rotimi Sadiku Department of Chemical, Metallurgical and Materials Engineering, Institute of NanoEngineering Research (INER), Tshwane University of Technology (TUT), Pretoria West Campus, Pretoria, South Africa Department of Mechanical Engineering, Maharashtra Institute of Technology, Pune, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_163

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food agriculture for biofuel production. Therefore, the use of microbes, particularly bacteria, in biofuel production especially biobutanol has gained importance in the production of biofuel. Glycerol acts as a great carbon source in the production of biobutanol. Microbes such as Clostridia, Cyanobacteria, algae, and other genetically engineered microbes that are used in biofuel production also aid in bioremediation under stressed conditions. Due to its high importance in bioremediation and its ability to reduce carbon emissions, production of microbial biofuel could substitute the commercially available fuels. Various kinds of nanoparticles such as iron oxide, nickel cobaltite, zinc oxide and different nanocomposites have been used in the production of biofuels. The inclusion of these nanoparticles in various processes of bioconversion offers a feasible means of lowering the processing methods of raw biomass and indeed the cost of production and the detrimental ecological effects.

Introduction Currently, the prevailing conception around the global community is to minimize the consumption of fossil fuels and reduce the effect of global warming. The main global issues are increasing consumption of fossil fuels and rising air pollutants through their use. Energy demand is expected to increase to 53% by 2030 and petroleum consumption will increase to 136.80 million barrels per day by 2030. It is currently receiving considerable attention as a suitable replacement of fossil fuel. Owing to population growth and industrialization across the world, the requirement for fuel is expanding constantly. Population growth and industrialization have imposed greater pressure on the use of non-renewable resources. Biofuel can be categorized as first-generation (edible vegetable oils), second-generation (nonedible vegetable oils), and third-generation (microbes and algae) biofuels based on the sources used. However, due to higher food demand, lower yield, increased land use, and competitive price, there are some drawbacks in using these vegetable oils as alternative fuels [1]. Thus, the use of bacteria is a good choice for biofuel production. Butanol is one of the most promising biofuels of all biofuels and is more equivalent to gasoline. It is widely used because of its high energy density (29.2 MJ/ L for butanol versus 19.6 MJ/L for ethanol and 32 MJ/L for gasoline). Bioethanol can be derived from any organic waste materials, which include mono-, oligo-, and polysaccharides. Butanol can be kept in humid environments because of its greater hydrophobicity. It is also noncorrosive and can be used in combustion engines, in fuel mixtures up to 30% (v/v) [2, 3]. Butanol also has potential to be used in various chemical processes as the starting material, resulting in isoprene, isobutene, butane, and others [4]. Currently, butanol is synthesized by two chemical processes namely, oxo synthesis and reppe synthesis. Nonetheless, the materials obtained under these methods cannot currently be accepted as alternative fuel materials for economic reasons. One promising method is fermentation that uses specific bioresources and Gram-positive bacteria [5]. Gram-negative bacteria (E. coli) also play a vital role in butanol fuel production. Louis Pasteur first documented about

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microbial butanol production in 1861. Another approach to improve the properties of biofuels is by introducing nanoparticles; however, there are currently concerns that nanoparticles do not burn inside the engine cylinder and also concerns on their effect on exhaust gases, fuel pumps, fuel filters, and fuel injection specifications. Nanoparticles have a higher volume to surface area ratio which makes them a highly reactive catalyst. The field of nanotechnology has grown during the last few decades due to its ability to use different nanoscale materials, with sizes ranging from 1 to 100 nm. As a result, nanoparticles are used in a wide variety of applications, including agriculture sector, food industry, personal care products, and pharmacological and industrial applications. The use of nanobiotechnology in all these diverse sectors is majorly attributed to the remarkable properties of nanomaterials that include their nanoscale volume, structure/composition, and high reactivity. The extremely small size of nanoparticles allows wide area-tovolume ratio and enables an enhanced active surface area that are important in order to produce various reactions and mechanisms; secondly, it allows the nanoparticles to demonstrate various properties has also broadened their applications in numerous areas such as therapeutics, biosensors and water-environmental remediation. In addition, nanoparticles possess many desirable features like high crystallization, catalytic properties, chemical stability, and high potential for adsorption. These distinctive features have made nanoparticles to be the desirable candidates for advanced biofuel processing. They are often being used as catalytic substances and play a significant role in electron transfer, reduction of inhibitory compounds, and increasing anaerobic activity.

Properties of Butanol Biobutanol properties generated by fermentation has the capacity to offer a new generation of biofuel, which has various striking properties like a liquid fuel for transport. It is because butanol has drawn growing interest and provides multiple benefits compared to ethanol. It is a four-carbon alcohol and is commonly used as an industrial solvent and as an intermediate for the processing of a number of organic chemicals in fuel source. Butanol is a colorless and flammable alcohol chained straight into four carbons. Butanol can be combined with ethanol, ether, and other organic solvents. It has a four carbon structure, and the carbon atoms can either form a straight-chain or a branched structure, resulting in various properties [6]. Depending on the position of the -OH structure and the carbon chain, various isomers exist with various applications. Compared to its equivalents such as isobutanol, 2-butanol, and tertiary butanol and other fuels such as gasoline and ethanol, butanol exhibits good properties. Butanol is not much volatile and reactive and has greater flash point and lesser vapor pressure, thus making handling safer and easier. It can be transported through conventional fuel pipelines. Butanol also has a decreased water sensitivity (7.8%) compared to ethanol (100%), which gives it more resistance and prevents it from being contaminated by water and moisture. It makes ethanol less corrosive, and because it is easier to handle than ethanol or gasoline, it could be effectively mixed with petrol and distributed through fuel

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channels. Ethanol should be separately handled and combined with the fuel source. Butanol can be used directly or combined with petrol or diesel without refurbishing the engine [7].

Different Methods of Butanol Production Chemical Process Butanol can be synthesized by chemical process. Oxo-synthesis is one such process, involving the reaction of propylene along with CO and H in the presence of a catalyst. Another approach is aldol condensation that includes the condensation and dehydration of two acetic aldehyde molecules. Of the two approaches, oxosynthesis is more advantageous, yielding higher ratio of n-butanol to isobutyl alcohol when compared with other process. Oxo-synthesis process is therefore the principal industrial method for the production of butanol. Other raw materials obtained from fossil oil such as carbon monoxide, ethylene, hydrogen, and propylene are also used in butanol production.

Biological Process The biological process offers several positive effects than the chemical process. It utilizes renewable energy sources; it is easier to separate the by-products as they are fewer in number [8]. Bacterial species such as Clostridium thermocellum, C. saccharobutylicum, C. cellulolyticum, and C. acetobutylicum are cellulolytic and solventogenic [9]. They have the ability to ferment carbohydrates into acetone, butanol, and ethanol (ABE) by integrated bioprocessing, which is economically and environmentally favorable. Thermoanaerobacterium saccharolyticum is another thermophilic anaerobic Clostridium capable of directly fermenting primary sugars into biofuels. Clostridia are of prime interest in fermenting biomass because of the production of cellulase and hemicellulase enzymes by thermophilic anaerobic bacteria, without the use of expensive hydrolytic enzymes. This makes the process of biomass conversion more cost-effective. Bacterial strain selection in biobutanol production is usually determined based on the types of feedstock, targeted efficiency, appropriate nutrient demand, butanol tolerance, and resistance to bacteriophage. More attempts should be rendered to separate novel species with desirable characteristics and develop novel strains using different techniques for genetic engineering. In the first step of fermentation, bacteria multiply exponentially and produce acids (mostly acetate and butyrate). This decreases the pH to around 4.5. The production percentage tends to fall at the end of acidogenesis as the bacterial cells change their metabolic activity from acidogenic to solventogeneic in response to the reduced pH. However, fermentation is not practically effective. Yields are poor and are very expensive to separate. To achieve higher butanol yields, the metabolic networks using metabolic and genetic engineering should be modified [10, 11].

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Microorganisms in the Production of Biobutanol Clostridia Clostridia are rod-shaped, spore-forming Gram-positive bacteria and usually strict anaerobes which naturally produce butanol. Clostridium strains provide a range of benefits in that they can yield a variety of naturally occurring hemicellulase and cellulase enzymes, though they cannot ferment crystalline cellulose. Another advantage is that they have the potential to ferment hexose and pentose sugars, while industrial yeast can only ferment hexose sugars. Some Clostridial strains have a strong cellulase enzyme complex, called a cellulosome. Genetic engineering study aims at converting this substance to butanol-synthesizing strain to increase the efficacy of cellulose degradation. Clostridial processing of solvents is a biphasic fermentation process. The first phase is the acid phase, during which the pathway-forming acids are initiated and the prime products acetate, butyrate, hydrogen, and carbon dioxide are released. This acid phase typically occurs only during the exponential growth phase. The second step is the solventogenic process, wherein the acids are reassimilated and used in acetone, butanol, and ethanol production. Solventogenesis is strongly associated with sporulation. The transcription factor liable for sporulation initiation also starts the solvent production. To improve the solvent production, the positive impacts of sporulation initiation on solvent formation need to be exclusively utilized. This is because the influence of sporulation initiation on the formation of solvents serves as a balance in controlling gene expression for sporulation towards solvent formation [12]. Escherichia coli Escherichia coli or E. coli is a microorganism that is widely used for commercial isobutanol production [13]. Genetically engineered E. coli produced the highest yield of butanol compared to any other microorganism. Approaches such as elementary mode analysis were used to enhance E. coli’s metabolic efficiency such that sufficient butanol could be achieved [14]. It has the potential to use lignocellulose in the synthesis of butanol (waste plant matter leftover from agriculture). Cyanobacteria Cyanobacteria have gained great interest among the various isobutanol producers due to their photosynthetic metabolism which uses carbon dioxide from the atmosphere [15]. In addition, cyanobacteria have low nutritional specifications, high tolerance to complex environments, and less complicated potential for genetic engineering [16]. Cyanobacteria are suitable for isobutanol biosynthesis if genetically engineered to synthesize butanol and its associated aldehydes. Cyanobacteria species producing butanol render several benefits [17]. Researchers have introduced a codon-optimized acyl carrier protein thioesterase and have developed six consecutive genetic modifications in Cyanobacterium synechocystis sp. PCC6803 for increased production of butanol.

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Biomasses for Biobutanol Production High raw material costs are considered one of the significant challenges in the processing of commercial butanol. The economic viability of the process can be improved by utilizing cheap and plentiful feedstocks, such as corn stovers [18]. In addition to the traditional feedstock such as sugar and starch (sucrose based), other organic materials such as barley, rice, bagasse, maize core, soy molasses, whey permeate, fruit-processing industry’s waste, and others may also be used [19]. Figure 1 shows the use of various agricultural wastes (biomass) and treatment process in the production of butanol [20]. The most important criterion for fermentation feedstock is its economic feasibility. Instead of glucose, metabolic engineering can be applied to enable the organisms to use a cheaper substratum such as glycerol. Since fermentation processes involve food-derived glucose, butanol production can have an adverse effect on food supply [21]. Glycerol is a great substitute for producing butanol. Although glucose is useful, they are available in small quantity. Whereas glycerol is plentiful and has a lower market rate, as it is a biodiesel waste product. The use of glycerol, which is a byproduct obtained from biodiesel production for butanol processing, has been documented in various studies. In one such research, fermentation of glycerol by C. pasteurianum was identified. Approximately 60 g/L glycerol was used, and up to 17 g/L butanol was produced at the end of the fermentation process. The yield achieved was considerably higher than the yield obtained (0.15–0.20 gg 1) during glucose fermentation with C. acetobutylicum. These findings showed that crude glycerol obtained from biodiesel can be used as a promising, cost-effective, sustainable raw material for butanol production. Moreover, mutant C. pasteurianum MBEL GLY2 was able to produce butanol by utilizing glycerol in a continuous fermentation process with reduced by-product formation. The mutagenesis strain was able to generate 11 g/L butanol from 80 g/L glycerol, compared with 7.6 g/L butanol produced by the parent strain [22].

Cotton stalk

Acid pretreatment

C.tyrobutyricum Δack-adhE2

Sugarcane bagasse H2C

OH

n-Butanol Soybean hull Enzyme hydrolysis

Fibrcus-bed bioreactor

Corn fiber

Fig. 1 Various agricultural wastes (biomass) in the production of butanol

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Algae nowadays have become convincing and promising feedstock for the synthesis of butanol. Many algal species have approximately 50% high oil content making them suitable candidates for the production of biofuel. The excess raw materials in algae may also be used for butanol production, as they consist of reduced levels of protein and lipid substance but increased levels of carbohydrates [23].

Other Different Methods to Enhance the Yield of the Product Immobilization of Cells Increase the Cell Density Immobilization of cells comparatively increases the cell count, viability, and eventually decreases cell loss in comparison with suspension cultures. This contributes to increased cell density and higher production during the fermentation cycle. Clostridium acetobutylicum DSM 792 when immobilized on wood pulp fibers with the mixture of sugar and glucose as substrate yielded approximately 10.0 g/L butanol [24]. C. pasteurianum cells immobilized using glycerol as a carbon source on amberlite produced around 9.00 g/L of butanol [25]. In Situ Product Removal The most common approach employed in butanol processing is distillation; however, it is very energy consuming and economically unsuitable. Hence, various modern in situ product removal techniques including gas stripping, dilution cell recycling, bleeding, and solvent removal – solvent extraction are used to eliminate the substances from the fermentation broth which would otherwise result in solvent accumulation toxicity [26]. Synergies with Bioethanol and Biodiesel Biobutanol has synergies with bioethanol as well as biodiesel. Another characteristic property that made biobutanol appealing is its potential to be used with ethanol and petrol as a co-mixing factor. Additionally, biofuels based on butanol ester have the ability to be a complete substitute for gasoline, diesel, and potentially in aviation as jet fuels. This is generally considered acceptable because its combustion does not emit any harmful substances, such as SOX, NOX, or carbon monoxide, in an internal combustion engine. It is generated using corn, wheat, sugarcane, sugarbeet, sorghum, cassava, and other plant materials or the same agricultural feedstocks used to produce ethanol. Current bioethanol plants can be reconfigured cost-effectively for the production of biobutanol involving comparatively minimal improvements to the fermentation and distillation facilities. There is indeed a vapor pressure co-blend combination including biobutanol in bioethanol-comprising petrol that promotes mixing of ethanol and offers stronger fuel efficiency than blends of petrol ethanol alone. Butanol can even be mixed with petrodiesel and plant oils to promote biodiesel synthesis. Biobutanol has a lesser viscosity relative to biodiesel, which can provide more reliable consistency and potentially higher production costs.

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Biobutanol, in general, helps the biomass producers and biofuel converters with the possibility of converting to a bio-molecule of greater importance. It is also associated with the incorporation of bioethanol into the fuel pool and enhances that. Biobutanol is currently used in European petrol up to 10% v/v and in US petrol up to 12% v/v. Butanol has the efficiency to significantly increase the permissible limit use in petrol. Improving the output of fuel blends through this form may boost the global market growth for biofuels, along with the agricultural sector that facilitates it [27].

Applications of Biobutanol as Biofuel Butanol is a significant raw chemical and organic solvent, extensively used in chemical industry, pharmaceuticals, and also food industry. As butanol has strong water insolubility, lower vapor pressure, higher calorific value, and many other properties, it has wide application prospects than ethanol. In consequence, several research groups have performed investigations on butanol as an alternative biofuel, where butanol is mixed either with petrol or with diesel for engine use and for fundamental burning reactors.

Butanol as an Alternative for Gasoline in Spark-Ignited (SI) Engines and Compression Ignition (CI) Based on the previous experiments and observations, if the fuel is contaminated with water, butanol is less likely to detach from the mixture than ethanol. This makes storage and distribution of blended fuels simpler. Butanol increases the efficiency and produces 25% more energy and therefore the consumption of fuel rate for butanol would be lower than that for ethanol. Butanol can either be used in internal combustion engine alone, or combined with gasoline without any alteration. With these advantages, studies on the utilization of butanol in SI engines and CI engines have increased. Several groups have conducted studies on the use of butanol as CI engine fuel. Scientists previously have examined the influence of butanol-diesel mixture on the performance and emission characteristics of multi-injection, heavy-duty direct-injection diesel engine and various EGR ratios. At an optimized engine speed of around 1800 rpm and load around 12 bar brake mean effective pressure, EGR levels were modified to maintain the NO emission at 2.0 g/kWh through a variable nozzle turbocharger. It was noted that butanol-diesel blend would significantly reduce soot emissions over the medium EGR range (approximately over 50% EGR ratio) and was also found to have the capacity to reduce NO emissions by allowing higher EGR levels. Additionally, the introduction of n-butanol decreased CO emissions and had no effect on total hydrocarbon emissions. Ten percent butanol mixture displayed no effect on the indicated specific fuel consumption (ISFC), while when gradually increased up to 20%, it increased the fuel consumption by 2% below 50% in the EGR ratio. Butanol additive can also tolerate a higher EGR ratio without the penalty of HC, CO, soot emissions, and ISFC [28].

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Table 1 Merits and Demerits of Butanol Merits Bio-butanol could be used directly in pure state or combined with gasoline at any concentrations Handling is easier, as it has a lower vapor pressure compared to ethanol It is immiscible with water and therefore less prone to contaminate groundwater It has a better fuel efficiency/gasoline mixture ratio, based on its improved energy output It is less corrosive and thus could be used in infrastructure such as pipelines, tanks, filling stations, pumps, and so on

Demerits Production rate is low by 30 times than ethanol

Bio-butanol may lead to higher greenhouse gas emissions per unit of extracted motive energy When used in spark-ignition engines, it can lead to a potential corrosive or aggradation problem due to its high viscosity Heating value is relatively low Causes gas gauge reading errors at times

Merits and Demerits of Butanol See Table 1.

Future Prospects Although biobutanol is a preferable substitute biofuel for petrol or diesel engines, butanol applications depend on its production and quality. Hence, it is much more important to increase biobutanol production in the future. First, the system-level metabolic engineering of strains is likely to become a competitive method based on the current data of complete genome sequences and new metabolic engineering methods. Second, by integrating fermentation and downstream processes compared with strain growth, the optimized bioprocess for the production could be developed. In addition, the engineered biomass is not only limited to sugars extracted from lignocellulosic biomass but also includes unutilized resources for effective use, including organic compounds and glycerol. As described earlier, future ABE fermentation work will require not only molecular biological methods and methods of fermentation engineering but also substratum modification techniques to increase butanol production and productivity.

Nanotechnology in the Conversion of Biomass into Biofuel The use of nanoparticles in the bioenergy sector has received a tremendous attention for sustainable energy supply and long-term ecological treatment. Applications of nanotechnology by using nanoparticles during the production of alcohol enhance the overall efficiency of the process by increasing pretreatment performance, enzymatic hydrolysis, and increasing the rate of reaction in the course of fermentation process. The key factors required for producing end products and enabling effective delivery of the reaction rate are particle size and structure,

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surface area, nanoparticles, and the form of biomass used. The key drawbacks of traditional biobutanol production methods are low reaction rate, high biomass processing costs, and low product yield. Nanoparticles have been used effectively for the production of biobutanol to solve these problems and are on their way to increase the productivity. As for the production of biodiesel, acidic and basic nanocatalytic applications can take the place of synthetic chemicals like sodium methoxide by interacting with the oils and free fatty acids. Such nanocatalysts have significant advantages of recyclability and strong economic effects. In addition, reactions can occur at low temperature and pressure. This strategy limits the emission of pollutants in the environment caused by sodium methoxide. Experimental research cited that the mesoporous nanocatalyst of silica, Ti-loaded SBA15, had ten times the free fatty acids content and water resistance levels compared to other catalyst for the production of biodiesel from vegetable oil, and this nanocatalyst performed three times better than other prominent nanocatalysts like titanium silicalite-1 and titanium dioxide silicate. On the other hand, Ti-loaded Santa Barbara Amorphous-15 (SBA-15) nanocatalyst decreased the chemical cost of transesterification process by recycling the nanocatalyst. This process is more efficient and environment friendly [29]. Figure 2 represents the function of acidfunctionalized magnetic nanoparticles (nanocatalyst) for the pretreatment of lignocellulosic biomass [30]. Microbial stability is considered vital in the fermentation of acetone, butanol, and ethanol (ABE) associated with the butanol-producing solvent extraction method. Nanocellulose obtained through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)catalyzed oxidation, defined as TEMPO-oxidized cellulose nanofibers (TOCNs), seems to have a high surface density of negative charges. Hence, its use in microbial fermentation systems would aid in the stability of the microbial processes. TOCN is also used in the immobilization process along with bacterial cells in alginate beads. It was observed that the influence of TOCN in the immobilization process increased the mechanical and chemical strength of alginate by cross linking the acidic groups of TOCNs and alginating by Ca2+ ions, thereby contributing to the development of a 3D cell trapping network while retaining high bacterial viability [31]. The addition of nanoparticles (NPs) to the nutritional medium affected the synthesis of butanol which altered the growth rates of bacteria. In particular, strain Clostridium acetobutylicum IMV B-7807 in the presence of iron, gold, and cerium nanoparticles in the medium produced a stimulating effect on the synthesis of biobutanol at the end of the processing and production method. Contrarily, for the strain Clostridium beijerinckii IMV B-7806, all investigated nanoparticles inhibited the synthesis of butanol. Also, a general decrease in the synthesis of biobutanol in the presence of all the nanoparticles was observed. Significant inhibition of the final product synthesis was detected in the presence of iron and cerium oxides. It was noted that nanoparticles also have a tendency to suppress butanol synthesis in this strain. The intensity of effect depended on the concentration of nanoparticles in the culture medium. Therefore, the effect of NPs was shown to be strain-specific. Efficacy of NPs action is concentration-dependent. Hence, selection of appropriate nanoparticles is highly important [32].

Fig. 2 Representation of acid-functionalized magnetic nanoparticles (nanocatalyst)-mediated pretreatment of lignocellulosic biomass

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Nanomaterials in Pre-processing of Raw Biomass Biomass pretreatment is an inevitable process but expensive step, and a substantial advancement in biomass pre-processing is the most critical step for economically generating biobutanol. The use of nanoparticles in combination with various alternative approaches to raw biomass pretreatment makes the mechanism more efficient. During pretreatment, nanoparticles effectively improve the reaction at cellular level and facilitate definite and specific alterations of biocatalysts in addition to removing the pollution caused by the chemical pretreatment. Because of their small size, metal nanoparticles are extremely able to penetrate the raw biomass cell wall effectively and associate easily with biomolecules to liberate carbohydrates for producing biobutanol. High nanoparticle [perfluoroalkyl sulfonic (PFS) and alkyl sulfonic (AS) magnetic nanoparticles] concentration significantly reduces the incubation time to perforate large segments of the cell wall area to release intracellular (carbohydrate/lipid) components. A successful destruction of the cell wall was observed due to the significant association between nanoparticles and cell wall components (cellulose/proteins) which provided a broad surface area for nanoparticles to function on the cell. Biomass pre-processing is achieved via an enzymatic hydrolysis step. Cellulase is the main enzyme necessary for the enzymatic hydrolysis of biomass during alcohol fermentation. Srivastava et al. [33] reported the effect of NiCo2O4 nanoparticles (1 mM) on the production of cellulase enzymes from Aspergillus fumigatus NS and showed that the thermal stability was enhanced significantly. In addition, the use of metal nanoparticles serves as co-factors to improve enzymatic stability and immobilize enzymes onto a supporting substrate for enzymatic action. Efficiency of the process can be also improved by using various other nanoparticles as they offer enzymes with broad immobilization surface, prolonged self-life, and stability.

Use of Nanoparticles as Additives for Biofuel Applications In several investigations, the effect of nanoparticles being added to fluids is explored. The nanoadditives are properly and technically blended with the fuel by means of ultrasonication. During this process, sound waves with high frequency pressure are produced to distribute microparticles within the fuel base. Due to the desirable heat release rate, they are often used as additives to increase the combustion efficiency of fuels and the incorporation of nanomaterials to the fluid would enhance their structural characteristics, such as heat capacity, vapor pressure, reduced delay and secondary atomization. Proper nanoparticles, nanodroplets with biofuel, and fossil fuel have shown to increase the fuel quality, lubricity, cetane number, burning rate, chemical reaction, catalytic efficiency, flash point, water co-solvency, heat, and mass transfer [34, 35]. Application of nanoadditives such as alumina, carbon nanotubes, cobalt tetraoxide, zirconium dioxide, lanthanum oxide, cerium oxide, silicon dioxide, zinc oxide, iron oxide, copper oxide, and cerium-zirconium-mixed oxides are some of the nanoadditives that are currently used in boosting engine power, torque, and brake thermal performance of biofuel [36]. Aluminum nanoparticles are more favorable to the development of explosions during combustion, resulting in a mixture of A-F and a complete combustion. Cobalt nanoadditives minimize the

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amount of NOx synthesis and magnalium particles serve as a heat exchanger within the combustion chamber by significantly lowering temperature, eliminating hotspots and lowering the output of NOX. The fuel additives are used to boost the fuel performance; they often serve as a catalyst to improve the chemical reaction rate, supplying more oxygen for the full combustion of fuel, thus improving the engine properties. Similarly, the effect of nanoadditives could be considered as catalytic effects. Liquid nanoadditive in the form of nanosuspension demonstrates more efficiency than micro-suspension. It also displays excellent outcome by providing higher suspension than n-decane-based fuels. The main objective of adding metal-based nanoadditives to biodiesel engine is to maximize the engine efficiency by increasing fuel properties. The addition of Al2O3, CNT, CeO2, Al, Ag, and graphene nanoparticulate to biodiesel decreases the flash point values by increasing the viscosity and density values. The pH of nano-biodiesel significantly affects the distribution of nanoparticles such as aluminim oxide and copper oxides. The aluminim oxide/diesel and copper oxide/ diesel suspension provide strong stability at a pH of around 7.5. Mass concentration of 25–100 ppm in diesel; correspond to no substantial modifications in the parameters of flow rate, cold filter plugging point, sulfur, volume, and purification [37]. Performance Analysis of Nanoadditives with Biofuel Blend The Effect of Nanoadditives on Brake Thermal Efficiency

Impacts of fuel blend on the engine quality is measured by BTE. Incorporating nanoparticles with biofuel emulsions facilitates rapid and thorough combustion processes due to improved heat mass transport mechanisms, resulting in significant enhancement in combustion performance. Many researchers discovered that the CIME nano-emulsions mixture appears to display a higher BTE at peak load compared to neat CIME; this could be due to the micro-explosion of water molecules in the fuel and catalytic effect of metallic oxides. The water molecules aid in better blending of the fuel atoms with air and also in quick evaporation. The emulsion nanoparticles have a large surface to volume ratio which results in rapid evaporation and improved atomization. In addition, the nanoparticles also assist in separating the hydrogen atom from water that results in combustion [38]. The addition of 150 ppm carbon nanotubes (CNT) supplemented with water-biofuel emulsion resulted in a cumulative BTE of 30.5% due to high surface area/volume ratio of carbon nanotubes and also contributed more efficiently to the combustion. It is also because of the CNT reactive surfaces that acted as a potential catalyst in the process of combustion. The effect of organic molecules on engine performance was evaluated for the first time and glycerine was used as a nanoadditive. The glycerine was coupled with fuel containing biofuel-water and screened on a CI engine. The blend of biofuel-glycerine showed a greater improvement in the BTE with an increase in engine speed compared to that of neat biofuel. The BTE increased by 7.8% and 14.2% for E10 (10% conc. H2O) and E15 (15% conc. H2O) at constant speed condition and at full load. Copper oxide-mixed fuel contributes to increased BTE with a minimal reduction in specific fuel consumption by 2.2%. Copper oxide

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nanoadditives also reduce exhaust emissions of hydrocarbon, carbon monoxide, and smoke by up to 5.4%, 32%, and approximately 13%. The emissions of NOx, however, had a negative impact and increased by 3.2% according to previous investigations. The Effect of Nanoadditives on Brake-Specific Fuel Consumption

BSFC is the “ratio of the fuel expended by the engine to the engine power generated with respect to a time in a given frame.” It usually decreases with increase in the load and hence its correlation with engine load is a key factor to be evaluated. It was revealed that the BSFC was reduced at all load conditions with the inclusion of cobalt oxide nanoparticles and with the inclusion of magnalium nanoparticles the energy consumption decreasedand the thermal efficiency was increased. Metalbased nanomaterials are usually broken down prior to fuel and water vaporization, releasing active metal atoms (cerium) that would thus minimize the advancement of incomplete combustion carbon deposits on the inner surface of the cylinder, which might result in reduced engine friction. The Effect of Nanoadditives on Brake Power

The “output of power of a drive shaft of an engine lacking any power loss instigated by the gear, transition friction,” etc., is known as brake power. It was reported that with addition of aluminum oxide and copper oxide to the neat biofuel, there was a marginal increase in the brake power and torque of the configured engine. The addition of 50 ppm of aluminum oxide and copper oxide to the neat biofuel enhances the torque by a maximum value of 1% and power by 3.3% [39]. Factors Affecting the Performance of Nanoparticles in Biofuel Production Processes The efficiency of nanoparticles in biofuel production methods is affected by numerous parameters. These include the types of approach, temperature, pressure synthesis, pH, etc. The following sections summarize some of these operating parameters [40]. Types of Approach

Previous research works have established numerous techniques for nanoparticle synthesis that include co-precipitation, thermal degradation, microemulsion, hydrothermal synthesis, formulation using microbial organisms (fungi and algae), synthesis using plant materials, etc. Each technique has its own advantages and disadvantages. However, biological strategies are extremely preferred as they use low-toxic and environmentally friendly raw material, they also have relatively low toxic activities on biocatalysts during the processes of fuel production. Moreover, it is preferred to synthesize nanoparticles from plants and bacteria, since this method consumes less energy and is extremely cost-effective.

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Temperature

Factors like temperature are one of the major key parameters that should be considered during nanoparticle synthesis. Based on the process, the calcination temperature of metallic nanoparticles varies from 100 °C to 700 °C. Physical and chemical methods typically use extreme temperatures (>300 °C), whereas biological methods use mild ( Pt > Pd > Ru > Ni [83]. However, their high cost makes it impossible for their use at high scales. Other transition metals have been usefully used for biomass gasification such as Cu, Co, Fe, and Zn with high activity. However, the activity and stability are still lower compared with the nickel-based catalysts [87]. Alkaline catalysts. It is known that alkali metals are very reactive. In the form of carbonates (Na2CO3, K2CO3, CaCO3, CsCO3) or supported on alumina can be used directly like catalysts within the primary gasifiers. They have demonstrated to be very effective in reactions of gasification. However, their use has some disadvantages: its agglomeration leads to loss of activity, and its recovery is difficult and expensive [90]. But they have an advantage. Some alkaline species are contained in the biomass; therefore, when the temperature increases, these species could be liberated and can act as catalysts in the gasification reactions. It means, a kind of auto-catalytic system. Natural catalysts. We can find several minerals in nature that could be used as catalysts in biomass gasification. They are used as primary catalysts; it means, minerals are placed in contact directly with the biomass in the gasifiers [90]. Some examples of natural catalysts are calcined rocks such as calcites, magnesites, and calcined dolomites. On the other hand, the commonly non-calcined minerals used are limestone (CaCO3), magnesium carbonate (MgCO3), and dolomite (CaCO3/ MgCO3) [90]. In general, the mineral catalysts are used because of their low cost, abundancy, and good performance in tar remotion. Among the mineral catalysts, the dolomite is the most studied in biomass gasification due to it can increase the remotion of tar in the primary stage leading to improve the quality of the effluent gas. Also, some authors have reported the use of dolomite in the secondary reactor achieving the high conversion of tar into syngas [90]. Olivine is another mineral that has attracted attention in the last years. It is a silicate that contains magnesium and iron (Mg, Fe)2SiO4. Some studies have demonstrated similar activity (or even superior) than dolomite with better mechanical stability [88]. Zeolites. Zeolites are aluminosilicates with a well-defined crystalline structure. In general, they are microporous solids with uniform cavities that could be used as molecular sieves. The natural or synthetic zeolites are used as catalysts in many industrial catalytic processes. In the case of gasification, acid zeolites type Y, type

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beta, MCM-41, and ZSM5 have been used as support of nanoparticles of Ni, Fe, and Rh [87]. These catalysts have demonstrated to be effective in steam reforming of tar due to acid properties and high-temperature resistance. However, the catalysts suffer rapid deactivation because of the formation of coke. Therefore, there is a challenge in designing zeolite-supported catalysts with adequate textural properties, promotors, or dopants that inhibit the formation of coke to increase the stability of the catalysts. Activated carbon. Activated carbon (biochar or mineral) is widely used for the elimination of tar. Activated carbon could be used as a catalyst or as support for supported metal catalysts. It has ideal textural properties to promote the metal dispersion on its surface and the mass transfer of organic molecules [87]. Activated carbon could be deactivated by coke formation which blocks the pores and reduces the specific surface area. Another disadvantage is that char could be lost due to the gasification by the reactions of steam reforming and dry reforming [90].

Conclusion and Further Outlook This chapter gives an understanding of the application of nanocatalysts in biofuels production. The examples given above indicate the nanocatalysts feasibility in lignocellulosic biomass conversion. Catalytic methods as pyrolysis and gasification processes are of vital importance in the conversion of biomass into biofuels. Attention should be paid on the biomass feedstocks because they play a key role during its conversion to biofuels by nanocatalysts, and the use of advanced characterizations are helpful to design better catalysts with the specific active sites where the biomass conversion reactions will occur.

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Nanocomposites for Supercapacitor Application P. Anandhi, V. Jawahar Senthil Kumar, and S. Harikrishnan

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Electrical Energy Storage Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrostatic Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ruthenium Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MnO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cobalt Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NiO/Ni(OH)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol–Gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrothermal/Solvothermal Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coprecipitation Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In-Situ Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vacuum Filtration Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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P. Anandhi · V. Jawahar Senthil Kumar Department of Electronics and Communication Engineering, College of Engineering Guindy, Anna University, Chennai, India S. Harikrishnan (*) Department of Mechanical Engineering, Kings Engineering College, Irungattukottai, Sriperumbudur, Tamil Nadu, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_96

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Chemical Vapor Deposition (CVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Deposition Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automobiles and Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Public Sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Medical and Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Defense and Military Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In this chapter, it is focused to highlight the importance of fabricating nanocomposites as active materials for supercapacitor applications. This chapter begins with a general introduction about the necessity of energy storage, and it presents the types of electrical energy storage devices. Supercapacitor is reckoned to be the best storage device when compared with other storage devices (electrostatic capacitor, battery, fuel cell) due to the following features such as high power density, higher cycle life, high rate capability, and less maintenance. Various electrode materials used in supercapacitors, such as carbon-based materials, metal oxides, and conducting polymers, are discussed. Besides, it emphasizes the use of nanocomposite towards the enhancement of the electrochemical performance of the supercapacitors. Next, various synthesis techniques for preparing nanocomposites are reported, and the chapter ends with applications of the supercapacitor.

Introduction Renewable energy sources are greatly recommended for the production of energy, mainly to reduce the use of fossil fuels and ensure green environment to the next generation. Albeit renewable energy sources for the generation of electrical energy are suggested, their energy conversion efficiency is poor. Also, continuous generation of electrical energy from them is dubious as the availability of these sources is intermittent and some of them rely on climatic condition. So, in order to ascertain the stable and continuous supply, energy storage is essential so that reasonable, reliable, and efficient energy supply could be achieved in the post-fossil era. Improving energy storage systems is undoubtedly important to store the energy processed for different applications.

Types of Electrical Energy Storage Devices Capacitor, battery, fuel cell, and supercapacitor are the four main storage devices. The construction and working principle of these devices are discussed in the following sections [50].

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Battery Battery is reckoned to be a commonly used power source for many applications in industrial and domestic electronics. It is a storage device which produces electrical energy from chemical energy by means of redox reaction. It comprises one or more electrochemical cells and each cell contains two electrodes linked electrically by electrolyte with cations and anions. Based on the transport of the ions, the polarity of the cell can be determined. Depending upon the charging capability, batteries are classified into two types and they are as follows: rechargeable battery and disposable battery. In rechargeable battery, during discharging, the chemical energy is converted into electrical energy, and during charging, the electrical energy is utilized to restore the original chemical composition. Some of the examples for disposable batteries are zinc-carbon battery and alkaline battery. Lead-acid, lithium-ion cell, nickel-cadmium, nickel metal hydride, and nickel-zinc belong to rechargeable battery type. Between these two types, disposable batteries have greater specific energy. Li-ion battery (shown in Fig. 1) is considered to be the best one in the electrochemical cells as it has certain features like high energy density in the range between 120 and 170 Wh kg 1, reasonable weight, and negligible memory effect [28]. Despite these features, it has two major drawbacks namely, less power density and low charge/discharge rates. Usually, the anode and cathode of Li-ion cell are made up of carbon-based materials and lithium-intercalated compounds like manganese oxide, iron phosphate, nickel oxide, and cobalt oxide, respectively. Lithium ions are migrated between the anode and the cathode. At the time of charging, lithium ions are travelled from the cathode to the anode and during discharging, the ions are travelled back to the cathode from the anode through the connected load. The lithium ion causes hydrogen gas and lithium hydroxide as it reacts with water present in the electrolyte. Hence, the Li-ion batteries are made up of well-sealed container filled with the organic electrolyte and this arrangement would be advantageous to restrain the possibility of hazardous reactions. Fig. 1 Schematic diagram of Li-ion battery [50]

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Fuel Cell It is a device which converts the chemical energy derived from the fuel into electrical energy. Unlike batteries, fuel cells do not require recharging and the byproducts derived from the reaction could not cause any kind of harms to the environment. While comparing to thermomechanical techniques, they do not have combustion, which gives rise to maximum energy conversion efficiency of 60%. Owing to these facts, fuel cells are considered to be eco-friendly, cost-effective, and more reliable for power sources. As far as the energy densities of the fuel cells are concerned, they have highest energy density of 500 Wh kg 1 and above. Even though they have high energy density, they suffer due to low power density and because of this, they are recommended for low power applications only. Based on the electrolytes, fuel cells are classified into ceramic oxide, alkaline solution, and polymer membrane. Among them, much attention is received by the proton exchange membrane (PEM) type. The simplified block diagram of fuel cell representing the construction and working is seen in Fig. 2 [17]. Fuel cell comprises an electrolyte and two electrodes on both sides of the electrolyte layer. In fuel cell, hydrogen fuel is continuously supplied to anode and oxygen is supplied to cathode. Hydrogen at the anode electrode is decomposed into negative and positive ions. Electrolyte membrane of the fuel cell could allow only positive ions to move from anode to cathode and it would never allow negative ions (electrons). These negative ions grouped on other side of the membrane and they would move freely to the cathode via external load.

Fig. 2 Block diagram of PEM fuel cell [50]

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Electrostatic Capacitor It is one of the passive devices in electrical system and it has very simple construction. It consists of two conducting parallel plates separated by a dielectric. It has a tendency to store and release the charge at a faster rate compared with batteries and fuel cells and it has higher power density and on account of this, it can not store higher energy. Figure 3 shows the electrostatic capacitor in which, if an external power source is connected then, positive and negative charges would be accumulated on the two plates separately. The performance of the capacitor can be assessed by means of input voltage and the charge accumulated on the conducting plates. In order to improve the performance of the capacitor, high permittivity of the dielectric materials, shorter distance between the conducting plates, and larger surface area of the plates are required.

Supercapacitor It has drawn much attention in the field of energy storage device because it is reckoned to be a bridge between battery and capacitor. It is well known that capacitors have tendency to charge and discharge at a faster rate, whereas batteries have the capability of storing greater energy. Supercapacitors have certain significant advantages such as high power density (greater than 10 kW kg 1), greater cycle life (more than 1,000,000 cycles), high rate capability, less maintenance required while working for long time, and low cost. Supercapacitors are used as temporary energy storage device when they are worked with fuel cells or batteries. According to the energy storage mechanism and transfer of ions from the electrolyte to the electrode surface, supercapacitors can be categorized as electric double layer capacitor (EDLC), pseudocapacitor, and hybrid capacitor.

Electric Double Layer Capacitor (EDLC) EDLCs are developed using two carbon-based electrode materials, an electrolyte and a separator. EDLCs can store charge either electrostatically or through a non-faradaic method that does not require transfer of charge between the electrode and the Fig. 3 Electrostatic capacitor [50]

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Fig. 4 Electric double layer capacitor (EDLC)

electrolyte (shown in Fig. 4). The energy storage concept used by EDLCs is the electrochemical double layer. When voltage is applied to the supercapacitor, ions in the electrolyte diffuse into pores of the electrode of opposite charge. Charge accumulates at the electrode/electrolyte interface, forming two charged layers (double layer) with just an incredibly small distance of separation. This is the distance from the surfaces of the electrode to center of the ion layer. The carbon materials used for these capacitors provides a high surface area with a distance of only a few angstroms (0.1 nm), owing to their porosity. Here, the capacitance is proportional to the surface area and inversely proportional to distance between the two layers. Due to this very small space, high capacitance values could be achieved. No chemical or compositional variations are involved with non-faradaic operations, because there is no transfer of charge between electrode and electrolyte. For this purpose, charge storage in EDLCs is greatly reversible and allows them to obtain high cycling stability. EDLCs typically operate too many charge/discharge cycles. But, on account of the electrostatic surface charging process, EDLC experience a low energy density. Of late, the researchers have been making efforts on new materials to enhance energy performance and the operating temperature range of EDLC.

Pseudocapacitor It is another type of supercapacitor between a battery and double-layer capacitor. They store charge through faradaic process that involve charge transfer between electrode and electrolyte. It is also made up of two electrodes separated by electrolyte (shown in Fig. 5). When voltage is given to a pseudocapacitor, reduction and oxidation occur at the electrode, which allows the transfer of charge across the

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Fig. 5 Pseudocapacitor

double layer, resulting in faradaic current flowing through the supercapacitor. The charge is stored electrostatically with a chemical reaction at the electrode, without an interaction of the electrode with ions. The capacitance values of pseudocapacitors are higher due to multiple processes involved to store charge. When compared to EDLC, it provides high specific capacitance and energy density due to a faradaic system. The choice of supercapacitor, however, depends on application and availability. Owing to multiple oxidation states, transition metal oxide materials also exhibited high-specific capacitance with pseudocapacitive behavior and are used for many applications. Some other materials used for pseudocapacitors are conducting polymers. Conway stated that many faradaic mechanisms can lead to electrochemical capacitive properties [10]. In the redox pseudocapacitance, the charge transfer from electrode to electrolyte takes place by redox reactions (faradaic reaction). Redox reactions imply reactions of reduction-oxidation. At the time of oxidation and reduction process, the oxidation state of the materials gets changed. Reduction takes place when the electrons are accepted while oxidation state gets reduced. Oxidation means electrons are released and the oxidation state increases. For ruthenium oxide (hydration) with pseudocapacity, redox occurred through the accepting and releasing of electrolyte protons. It also receives electrons and reduces its oxidation state from +4 to +3 during the

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acceptance of protons [42]. Intercalation in the pseudocapacitor occurs when the ions insertion (or) intercalate in a layer of redox-active material followed by faradaic charge transfer without any crystallographic phase change. An adequate number of electrons could be passed to the host while insertion to ensure an electrical neutrality of electrode. Insertion should be constrained by ion’s ability to diffuse via electrode material.

Hybrid Supercapacitor As discussed above, EDLCs provided excellent cyclic stable, higher power density while pseudocapacitor exhibited greater specific capacitance. However, EDLC exhibit low energy density, and at the same time, pseudocapacitor has low cyclic stability. So, two factors decide the energy density of SCs: the capacitance of the electrode and the voltage of the cell. An alternative method to increase the capacitance value is to prepare a nanosized and porous material as electrode with improved energy density. Hybrid supercapacitors show better performance by overcoming the disadvantages of both EDLCs and pseudocapacitors. Hybrid electrode configuration comprises two different electrodes made up of different materials, whereas composite electrodes consist of one type of material which is integrated within the same electrode into another. Hybrid supercapacitors exhibit improved electrochemical behavior when consisting of two different electrodes made from different materials than that of the individual ones. Hybrid systems were fabricated with energy source of a battery (faradaic) like electrode and a power source of a capacitor (non-faradaic) like electrode (seen in Fig. 6). The combined structure has higher operating potential and gives greater specific capacitance that value is two to three times greater than conventional capacitors, EDLC and pseudocapacitors. It stores charge by both faradaic and nonfaradaic process which allowed high energy and power densities than EDLC and greater cyclic stability than pseudocapacitor.

Fig. 6 Hybrid supercapacitor

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Table 1 Comparisons between capacitors, batteries, fuel cells, and supercapacitors Parameters Power density (W/kg) Energy density (Wh/kg) Charge/discharge time

Capacitors >5000 0.01–0.3 1 ps – 1 ms

Batteries 100–3000 30–265 1–10 h

Operating voltage range (V) Cycle life (cycles)

6–800 More than one million 20 to +100

1.2–4.2 150–2000

Operating temperature range ( C) Cost per Wh

0.10–1

20 to +65 1–2

Fuel cells 1–1000 500–2000 Not applicable 0.6–0.7 Not applicable +50 to +1000 0.035–0.05

Supercapacitors 5000–10,000 0.5–20 1 ms – 1 s 1.0–4.5 Up to one million 40 to +85 10–20

There are three types of hybrid capacitors namely, asymmetric hybrids, battery-type hybrids and composite hybrids and the electrode configuration of each type has differed. Asymmetric hybrids supercapacitor integrates faradaic and non-faradaic reactions by combining a pseudocapacitive electrode with an EDLC electrode [15]. Generally, the carbon-based materials are used as negative electrode and the pseudocapacitor materials as positive electrode. Battery-type hybrid supercapacitor has made up of integrating a battery electrode with supercapacitor electrode. This type of hybrid capacitors exhibits the improved performance characteristics of both batteries and supercapacitors such as energy density of batteries and recharge time, cycle time and power density of supercapacitors. Composite hybrids combine carbon-based materials with either conducting polymers or metal oxides. The electrode materials could enhance corrosion stability, enhanced the specific capacitance and the working potential. At the end, various functional parameters of the above discussed energy storage devices are furnished in Table 1.

Electrode Materials The capacitance and charge storage of supercapacitor depend strongly on the materials used for the electrode. Hence the most important approach to address these challenges is the further development of new materials with high capacity and improved performance compared to existing electrode materials. The capacitance of supercapacitor actually depends on the particular electrode surface area. Since the material contacts with an electrolyte and not all precise surface area are electrochemically accessible, the measured capacitance of the different materials is not linearly increased with the increasing surface area. Depending on the required energy storage type for the many application and the appropriate capacitance ranges, supercapacitor can be constructed from different materials. Electrode materials of supercapacitor may be divided into three different types based on their use for EDLCs, pseudocapacitors, and hybrid supercapacitors (Table 2). For supercapacitors, a significant number of materials are currently available; carbon is the main commercial material that is generally used and converted into many structures.

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Table 2 Types of supercapacitors and their electrode materials Electric double layer capacitors

Pseudocapacitors Hybrid capacitors Asymmetric Composite Battery-type

Activated carbons Carbon aerogels Carbon nanotubes Carbon fibers Metal oxides Conducting polymers Carbon materials, metal oxides Carbon materials, conducting polymers

The metal oxide or conducting polymer used as electrode material for over a certain number of cycles (300 miles or 500 km) without interruption into vehicle load or traveler space. For building up the hydrogen energy storage materials, many research centers have been created all over the world. These research centers mainly focus on metal hydrides and the carbon-based storage materials [18]. In recent times, the research has its focus on independent storage materials. In the following 10–15 years, there will be movement to hydrogen powered fuel cell vehicles (Fig. 7). In the meantime, the petroleum and fossil fuel usage will be

Fig. 6 Structure of hydrogen fuel cell. (Courtesy Battery University)

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Fig. 7 Working principle of hydrogen fuel cell. (Courtesy Toppr)

diminished due to the improved performance of the battery or gasoline hybrid cars [46]. Hydrogen storage will be done by pressurized tank storage, cryogenic liquid hydrogen storage, and hydrogen acceptance in metal-based compounds techniques.

Pressurized Tank Storage Hydrogen can be stored in pressurized tanks. These tanks are made up of cylinders that are wrapped with carbon fibers. The hydrogen has been compressed at the pressure of 34 MPa with a mass of 32.5 Kg and the volume of 186 L for the range of 500 km. For bigger individual vehicles, the volume of the tank is about 90 Q/o. On board storage tank utilizing Quantum Technology was certified in Germany in the year 2002 [44]. It isn’t suitable to store the fluid hydrogen for the ordinary vehicle use at lower temperatures. It makes the storage system lose up to 1% of its storage volume every day by bubbling. So to reduce this reason, it is vital for high refrigeration to keep the hydrogen at 20 K [45]. Cryogenic Liquid Hydrogen Storage The hydrogen stored in a liquid form at
259.2 °C temperature is called as a cryogenic liquid hydrogen storage [43]. Liquid hydrogen (LH2) possesses low density. It weighs only about 71.37 x 10
3 kg per liter. One liter of hydrogen can provide electricity of about 8.52 MJ. Since this kind of storage needs to be done at a much lower temperature, it is highly challenging. To meet this challenge, it needs much more insulation than normal, thereby increasing the cost of maintenance. When mixed with other gases, liquid hydrogen becomes an explosive. Therefore, before refilling the tank, all the necessary protective measures have to be set up [47].

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Hydrogen Acceptance in Metal-Based Compounds This procedure of metal-based hydration can be utilized to store hydrogen underneath 3/4 MPa at room temperature; in any case, the metals prompt an excess of extra weight for most vehicles and are likewise costly [47]. It has been discovered that a lot of hydrogen can be stored by lithium nitride. This material stores hydrogen quickly in the temperature scope of 170–210 °C and acquired a 9.31 wt% take-up when the sample was held at 255 °C for 30 min. Under high vacuum (10
9 MPa) around 66.1% of the hydrogen was discharged at temperatures below 200 °C. The remaining 34% of the stored hydrogen required temperatures over 320 °C for discharge [43]. The hydrogen was taken up as lithium hydride (LiH) and lithium amide (LiNH2). The researchers proposed that the related metal-NH systems ought to be concentrated to track down progressively practical weights and temperatures for a hydrogen storage framework [47]. Nano Hydrogen Storage Electrochemical method of hydrogen storage has been done. For the storage of proficient energy, Nanotubes have been set up by utilizing several new nanotechnological strategies. Since it has the property of storing energy, clearly it will have the property of charging and discharging. A discharge limit of 260 mAh/g has been estimated at 50 mA/g and 20 °C. It was discovered that the cyclic voltammetry (CV) reaction showed great electrochemical activity. The direct reaction between (NH4)2MoS4 and hydrogen resulted in the production of nanotubes. Firstly, the sample powder is pretreated by ball milling under hydrogen air and is treated with 5 M KOH solution at 50 ° C for 1 h. Nextly, it has been rinsed with distilled water and then dried at 80 °C for about an hour. Brunauer-Emmett-Teller (BET) estimations have been made by nitrogen gas adsorption/desorption technique which gives the value of SSA-specific surface area of the nanocomposite [48]. Depending on this value, it is concluded that whether to do the alkaline treatment for the proficient improvement of the particular surface area. The sample powder was blended in with Teflon Acetylene dark powder in a slurry to locate its electrochemical properties. And then it is glued on a nickel foam network and later dried and squeezed to build a working electrode. Thus the electrochemical properties were noted [18]. The electrode electrochemical characteristics (Fig. 8) were estimated by utilizing a sintered counter terminal (around 1000 mAh) and a reference electrode in 5 M KOH arrangement at 20 °c. Figure 8 shows the cyclic voltammetry (CV) bends for the main hydrogen reduction (adsorption) and oxidation (desorption). Correspondingly formed voltammograms are additionally received for the coming five-possible cycling. To begin with, when the electrode is scanned cathodically, a cathodic pinnacle showed up with the peak position at - 0.955 V vs counter electrode, which is ascribed to hydrogen decrease on the sample site. An anodic pinnacle is seen at - 0.645 V versus counter electrode during the accompanying anodic polarization and was allocated to the hydrogen oxidation. Nextly, the estimations of peak flows of hydrogen oxidation/reduction for nanotubes, suggestive of those for carbon nanotubes, are detailed before. Of these, 18 were a lot higher than that of the

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Fig. 8 Cyclic voltammetric analysis (reproduced with the permission of author [3])

polycrystalline terminal, showing great electrochemical action of the nanotube electrodes. Thirdly, the peak match between the hydrogen reduction and oxidation shows that the electrochemical hydrogen adsorption/desorption continues reversibly, and this is an attractive trademark for electrodes in rechargeable batteries [47]. Finally, the peak currents for hydrogen oxidation and reduction are very subtle to SSA, and they increment as the SSA increments. Therefore, apparently the electrochemical charge and discharge system happening in nanotubes is somewhere close to the carbon nanotubes (physical procedure) and metal hydride terminals (chemical procedure) and comprises of the charge transfer response (Red./Ox.) and dispersion step (Diff.) It has been examined that the carbon nanotubes can electrochemically store a lot of hydrogen. The electrodes that are made of clean and open SWNTs show high limit up to 800 mAh/g, which relates to a hydrogen storage capacity limit of 2.9 wt %. The prepared three terminals are charged at 100 mA/g for 5 h and released at 50–200 mA/g [12]. The electrode is subjected to a starter trial of 30 successive cycles, wherein it is observed that its capacity gets decreased to only about 2%. Therefore, it is evident that the integration of nanotube creates a critical impact in improving the electrode activity. The another interesting discovery is that when compared with the polycrystalline electrodes, the electrodes with nanotubes have much higher discharge voltages. Also, a few inclines that are found in the charge/ discharge curves demonstrate the presence of various adsorption/desorption sites. These slopes are similar to the carbon nanotube electrodes and also show the formation of possible hydrated medium phases. The sorption of hydrogen into

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bulk sample is a much complicated process. The current outcomes recommend that the measure of hydrogen on nanotubes acquired by the electrochemical method is the highest among all the studies reported in the research works. Also, this measure is emphatically reliant on the specific surface areas. The nanotubes are sometimes treated with alkalines, those kind results in a moderately low hydrogen adsorption on the surface as well as at the interstitial sites. A physicochemical association is liable for higher rate of hydrogen adsorption of nanotubes. In any case, more examination is as yet expected to comprehend the exact idea of the association among hydrogen and nanotubes [49].

Efficient Utilization Light-Emitting Diode (LED) Light-emitting diode (LED) is one of the electronic devices that is used for energy utilization. Here, this device emits light whenever the electrons move between the two terminals from one end to the other. When compared to incandescent and fluorescent lights, LED has longer life cycle and more efficiency. When a LED is installed in swimming pool, water gets trapped inside the device, and hence the energy efficiency decreases and hence also the life cycle. Due to this reason, metal reflectors are used to protect the LED and to extract more light. For traditional procedures, the extraction limit is up to 38% that decreases the complexity that makes the image to look foggy. This issue can be redressed by infusing a few nanotechnology techniques. To extract more light and to increase the efficiency, Chou’s group invented a new kind of LED known as PlaCSH LED which means plasmonic cavity with sub wavelength hole array [19]. Through this invention, the team achieved nearly 60% of efficiency, which is higher than ordinary LEDs. Heating problem that is caused by the light that got trapped in ordinary LEDs is also rectified by higher brightness of PlaCSH LEDs. The below schematic defines the characteristics of ordinary and nanotechnology LEDs. Many LEDs are available that uses nanotechnology. Here, PlaCSH and PcLED are focused.

PlaCSH Light-Emitting Diode Though the concept behind PlaCSH is difficult, it has a basic and humble structure. PlaCSH has a layer of light emitting material that is 100 nm thick (Fig. 9). This layer is set inside a cavity whose one surface is constructed with a flimsy metal film. The other surface is constructed with a metal mesh whose thickness is about 15 nm. Each wire has a width of 20 nm and is separated by 200 nm from one center to the other. PlaCSH can concentrate greater amount of light toward the observer, since it focuses more amount of light out of the LED. The framework becomes more adaptable when compared to conventional displays by replacing the ordinary fragile transparent electrode. Also, PlaCSH is more ductile and flexible that it can be weaved into a mesh. The other important advantage is its cost. It is cost-friendly for manufacturers (Fig. 10). Chou invented a cheap and very simple procedure for

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Fig. 9 Constructional difference between conventional and PlaCSH LED. (Courtesy technology.org)

Fig. 10 Working principle of PlaCSH light-emitting diode. (Courtesy technology.org)

making PlaCSH organic LEDs, known as nanoimprint, in the year 1995, that make nanostructures, in a style like a print machine creating papers in a printing press. Chou and his group are presently leading examinations to show PlaCSH in red and blue natural LEDs, notwithstanding the green LEDs utilized in the current investigations. They, additionally, are showing the framework in inorganic LEDs.

PC Light-Emitting Diode Another new LED innovation incorporated with nanotechnology in order to increase energy efficiency is known as PC LED (phosphor conversion light-emitting diode) [20]. These LEDs are not only used for decoration but also plays an essential role in several gadgets such as mobiles, electronic devices, and other lighting devices. This nanotechnology incorporated LED lighting examines the fundamental necessities of both phosphorous and quantum wells for the productive generation of white light. In order to improve the intensity of emission and colors, the technology uses metallic nanostructures to empower the solid light. In many applications, light is coordinated in just a single direction. But in specific applications, light has to be produced in forward and reverse directions. This bidirectional emission results in huge amount of energy losses (Fig. 11). Therefore, by integrating an array of nanostructures, this discharge symmetry of planar structures can be eliminated. This array of

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Fig. 11 Illustration of pcLED

Fig. 12 Process for the generation of an enhanced magneto electric response of the nanopyramid array

nanostructures has to be incorporated onto the fluorescent layers in a pyramidal shape. The schematic of PCLED is shown below. At ~650 nm, i.e., localized surface plasmon resonance (LSPR) wavelength, the nanopyramid exhibits more light toward the base of the pyramid. At ~585 nm, i.e., satellite laser ranging (SLR) wavelength, the reverse occurs (Fig. 12). These impacts are because of the excitation of magnetic dipole moments through the electric field of light. This process creates an enhanced magneto electric response of the nanopyramid array that starts from the height and pyramidal shape of nanostructures [21]. In general terms, the efficiency of a light-emitting device is the product of three partial efficiencies: η ¼ ηexc ηrad ηext. ηexc is the excitation efficiency. It represents the part of infused bearers that recombines in the dynamic region, on account of electrically driven gadgets. It represents the absorption efficiency of phosphor, in case of optically pumped

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gadgets. ηrad is the radiative efficiency, frequently alluded to as the internal quantum efficiency or QY. It speaks to the division of the energized or infused electron-hole pairs that recombine to discharge a photon, and it is characterized as the proportion of the radiative rate to the complete recombination rate. ηext is the extraction efficiency. It is the fraction of red/blue/green light that gets away externally into free space from the device. Since due to total internal reflection, more amount of light gets trapped inside the device. Many LEDs rely on the integration of lightemanating devices in order to reduce the trapping of emitted light within the device and also to increase the ηext efficiency. There are also other parameters that define the color of the emitted light. Those factors are also to be evaluated during the manufacture of LEDs.

Nanosensors Nanosensors are employed in distinguishing nanoparticles as well as to identify some of the physical properties on the nanoscale. Nanosensors may be a mechanical sensor or a chemical sensor. These nanosensors find applications in both engineering and clinical fields. An example is water quality monitoring system. The use of nanosensors here plays a vital role in detecting the low pollutant concentration and to reduce the complexity of the matrix. Just not many such sensors, only rapid check and water safe have made into showcase. Here, it is discussed about the present condition of nanosensors for water quality monitoring. This technology has been created to identify natural, inorganic synthetic substances and pathogens. It is also used to detect other parameters of water quality such as pH, turbidity, and hardness of the water. Nanoprobes are used to spot microbes with high sensitivity. The sensor stage utilizes the remarkable optical, magnetic, or electrical properties of nanomaterials to improve the analyte location. The nanoprobe measures an analyte and needs fast and flexible binding. The basic nanosensor consists of: 1. A nanomaterial 2. A recognition element 3. A mechanism for signal transduction The nanomaterials have to undergo several processes before utilization. It needs to be purified and also undergo various methodologies such as centrifugation, filtration, microfluidic partition, and confinement [22].

Identification of Bacteria That Are Magnetically Isolated Using SurfaceEnhanced Raman Spectroscopy (SERS) This investigation deals with the detection of magnetically separated bacteria from a sample of interest. This method involves treating the sample with gold-coated magnetic core nanoparticles. Firstly, these nanoparticles are functionalized with bacterium explicit antibodies (MnFe3O4@Au-antibody). Secondly, the nanoprobe bacteria are detached magnetically and treated with gold nanorods that are antibody

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treated (AuNR-antibody). Finally, the gold-treated bacteria are probed by SERS. This is an optical method of identification. Investigation on an optical sensor proves that when the target interacts with a nanomaterial, a signal is produced. Few advantages of this approach are practical affectability, fast readout, and general effortlessness of activity. Due to this features, the optical sensor transduction is highly preferable. Of many types of optical signal transduction, surface plasmon-enabled spectroscopy and fluorescence spectroscopy are the most predominant methods. The principle of fluorescence is based on that when a light is excited on a particle, a fluorophore is emitted when it returns to its ground state. The measurement of emission gives the value of fluorescence. Some of the fluorescent nanoparticles are upconversion nanoparticles, quantum dots, and metal nanoparticles. These are explored for their sensing applications. A significant number of the fascinating improvements with regard to the utilization of fluorescent nanoparticles for sensing applications are summed up by Ng et al. When gold and silver nanoparticles, that is noble metal nanoparticles, are energized by light, it can generate surface plasmons, for instance, oscillating surface electrons. These electrons create solid electromagnetic fields inside the region which is of few nanometers from the surface of the nanoparticle. Localized surface plasmon resonance (LSPR) is an absorption band that occurs at the frequency at which the surface electronics resonate. LSPR occurs as a function of size, identity, shape, and nearby condition that encompasses the nanoparticle. LSPR is utilized by two different classifications of signal transduction such as surface-enhanced Raman spectroscopy (SERS) and colorimetric or absorption methods. As seen before, LSPR creates an improved electromagnetic field in the region within the noble metal surface. The Raman cross sections are enhanced by a few significant degrees at the point when target analytes are inside this improved electromagnetic field. This enhancement is called as SERS. A SERS spectrum mirrors the characteristic covalent bonds of the analyte since it is a vibrational spectroscopic technique. This is also known as a fingerprint spectrum. This tends to be legitimately assessed to demonstrate the presence of an analyte in a complex matrix only if the signal from this spectrum is adequately strong. In the intrinsic detection mode, SERS has indicated the limit with regard to single-particle detection, though normally for non-ecologically important analytes under requesting instrumental working conditions. The need for the hotspots, such as graphene, carbon nanotubes, and inorganic 2-D nanosheets, that are fundamentally responsible for the SERS enhancement makes complications in the utilization of SERS for water contaminant quantitation.

Fabrication and Utilization of Gold/Reduced Grapheme Oxide (rGO) Nanocomposite-Based Biosensor Here, in this method, a glassy carbon electrode (GCE) is treated with rGO, followed by electro polymerization of pyrrole. Then the resultant byproduct is electrodeposited with gold and then co-deposited with silica and acetyl cholinesterase (AChE). Within the sight of an OP, the electrical flow is decreased comparative with its nonattendance. The procedure is demonstrated below. It is an electrical

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method. Here, SDBS represents sodium dodecylbenzene sulfonate and ATCl represents acetylthiocholine chloride. Electrical signal transduction-based nanosensors come under three various classes. They are chemiresistors, electrochemical sensors, and field-impact transistors. Chemiresistor has an electrical circuit that operates based on the principle, and when an analyte interfaces with the surface of the sensor, its resistance is changed. When there occurs a reaction of electron transfer between surface of the sensor and a target analyte, voltage or current is produced. This current or voltage that is produced as a resultant of electron transfer reaction is estimated by electrochemical nanosensors. In field-effect transistors, the analyte is exposed to an electric field that is constrained by a conductive gate electrode. FETs measure the mobility of such charge carriers, that is, analytes when they travel through a channel. A change in signal is distinguished when the analyte enters the channel. This change is measured by the FETs. Cui and Lieber in 2013 made their studies on these FETs; only after that, FET-based nanosensors have begun its remarkable development in the society. Sensitive identification of waterborne contaminants like Escherichia coli27, hepatitis C28, etc. can be made with few modifications in chemiresistors, electrochemical sensors, and FETs.

Bacteria Identification Through Magneto-Fluorescence Approach Magneto-fluorescent nanosensors (MFNs) are used for both fluorescence detection as well as magnetic detection of bacteria. Fluorescence and magnetic bacterial recognition can be done by combining magneto fluorescent nanosensors with E. coli-specific antibodies [23]. Magnetic nanomaterials can be property of immediately responding to an external magnetic field. Such type of nanomaterials along with analyte specific biomolecules exhibits the ability to magnetically isolate the analytes, and later these analytes can be identified. Several materials such as iron, oxides of iron, nickel, cobalt, etc. are utilized to generate magnetic particles. Ironbased nanoparticles are mostly preferred because their synthesis cost is comparatively low and it has apparent compatibility to life. Three general classes that come under magnetic signal recognition. They are magnetoresistance, hydrodynamic property estimations, and T2 relaxation nuclear magnetic resonance. Magnetoresistance nanosensors are recently used for the identification of influenza and mycobacterium. When the folding of magnetic nanoparticle-based analytes occurs, the sensors show a change in electrical resistance. In the presence of an analyte, the magnetic nanoparticles combine together. During this process, there occurs a change in T2 relaxation time of water protons. Magnetic resonance imaging (MRI) technique is used to monitor this change in T2 relaxation time. This methodology is being used to identify Salmonella enterica and Newcastle disease virus sensitively. Bacterial Identification Through Mechanical Nanosensors Method Mechanical nanosensors are those that recognize the presence of biomolecules depending on the principle of mechanics. When compared with other methods like optical, electrical, and magnetic nanosensors, these mechanical nanosensors have better points of interest for the estimation of mechanical properties in nanoscale. Some of the leading mechanical nanosensors are cantilever sensors and CNT-based fluidic shear stress sensors. There are some devices such as atomic force microscope

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Fig. 13 Basic schematic of AFM [Courtesy Ques10]

(AFM) that identifies surface structures of the image in a nm scale or even on a subnm scale. This is used to estimate surface forces. The standard AFM has a minute tip appended to a cantilever spring [24]. When the cantilever spring is subjected to a force, it bends to an extent equal to the force applied. This bending of the spring is identified by the AFM. The force between the tip and surface is of the order of upto 1 nN. Some AFM has carbon nanotubes (CNT) at its tip where CNTs are molecular structure tubes made of graphitic carbon. Figure 13 depicts the operation of AFM.

Nanosensors in Water Monitoring Water supply in the world nowadays relies on the integrated treatment and delivery. Based on this hypothesis, using several physical and chemical processes, water is being treated at a centralized plant before going to delivery stage. In most of the countries, to reduce the quantity of pathogens, a disinfectant is added instantaneously before the water leaves from the plant. After treating with disinfectant, the water enters the delivery system. Here, the treated water may be exposed to interference with waste, surface, or ground water. Hence, its eminence is tarnished by percolating of the contaminants such as lead, copper, or polymers that is present in the plumbing tools or by bacterial re-growth. The noticeable parts of centralized treatment systems are water towers, taps, and water treatment plants. But then, their significance as sampling areas is reduced by large size and spatial difference of the water distribution network and pipes inside buildings. In centralized treatment plants, variety of water quality samples are regularly gathered either at the plant or at the specified locations within the system or may be from inside the buildings. This method is known as premise plumbing. Because of huge variations in geographical structure, age of water, and others that may not have been measured during the sampling system design, the above said approach oversights the region of concern. Also, these factors can make a way for biasing that wrongly denotes the quality of water. In the same way, in Flint, Michigan, a lead crisis occurred, and it was documented that the water treatment staff did not properly sample sufficient amount of homes with known service lines. It was also

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Fig. 14 Nanosensors for water quality monitoring [25]

legally declared that these homes were at greatest risk and also needed utmost care. To acquire real-time data from sensor nodes through various regions, sensor networks are utilized. Growth of IoT assures the proper utilization of these sensors networks [25]. These sensor networks analyze all the properties of distributed water in a more detailed manner from water treatment plant to the tap. So this sensor network theme is interesting for water distribution and management. The next-generation IoT will be the combination nanoscale devices and IoT that is Internet of Nano-Things (IoNT). The nanosensors are integrated into an IoT in this technology (Fig. 14). In the process of water distribution, this system will have a combined array of nanosensors that give the proper monitoring of both water quality and quantity in real time, at all the geographic scales and remote places such as service lines, premise plumbing, dead end pipes, etc., which are unreachable for existing methods. IoT is a fastly developing technology in the recent days. There are also many challenges that have to be faced by IoT technology. These challenges include collection of data, communication, and storage of data. These have to be overcome to make the goal of IoT to bring in the reality. After the proper optimization of the nanosensors, they are incorporated into the water monitoring system. These nanosensors require improvement and need to be reconfigured for the following reasons [25]. • To make the device accurately work under highly changing environmental circumstances • To make the replacement of in-field sensor so simple

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• To enhance communication of data • To reduce the possibility for sensor entangling Of these, reducing the possibility for sensor entangling is a challenging issue for embedded sensors. Because the fouling that occurs by salts, biological evolution and organic matter should be taken in to account in the beginning of the enhancement [23]. To face all these challenges and also to develop the operation of the water monitoring system, these nanosensor networks have to be designed with maximum care for data security.

Conclusion In order to improve the efficiency of electrical energy storage (EES) devices such as supercapacitors and batteries, nanotechnology techniques are being used which turns out to be absolutely challenging. Lithium-ion technology is viewed as the most encouraging variation of EES due to its remarkable energy and power density as well as due its high cell voltage. Through heat-safe, new ceramic, active electrode materials and adaptable separators, the capacity and security of lithium-ion batteries can be enhanced with nanotechnology applications. For stationary energy storage and also for utilization in hybrid and electric vehicles, the organization Evonik forces the commercialization of such systems that employ nanotechnologies. Due to its environment friendly nature, hydrogen accounts to be a challenging energy storage method. Hydrogen storage is viewed as a serious aspect aside from essential nanostructure change. The automotive industry that requires a minimum of 10% wt, of hydrogen storage limit, doesn’t meet its request from the present materials in chemical hydrogen storage method. Different nanomaterials, dependent on nanoporous metal-organic compounds, give improvement possibilities, which appear to be financially feasible at any rate with respect to the activity of energy components in convenient electronic gadgets. Thermal energy storage is another significant area. The energy request in buildings might be fundamentally decreased by utilizing phase change materials, for example, latent heat storage methods. In heating grids at district level or in industries, adsorption storage methods could be implemented as heat storage methods. These adsorption stores are based on nanoporous materials such as zeolites. This method is feasible from the view of economic point.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . What Is Perovskite? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic Principle and Fabrication of PSC Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Origin of Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the past few years, the synthesis and development of perovskite materials have been focused due to the excellent physiochemical properties of the perovskites. The organic-inorganic hybrid metal halide perovskites have been proven a most efficient light harvester or light absorber for the development of perovskite solar cells. The solar cells based on these organic-inorganic hybrid metal halide K. Ahmad Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] S. M. Mobin (*) Discipline of Chemistry, Indian Institute of Technology Indore, Indore, MP, India Discipline of Biosciences and Biomedical Engineering (BSBE), Indian Institute of Technology Indore, Indore, MP, India Discipline of Metallurgy Engineering and Material Science (MEMS), Indian Institute of Technology Indore, Indore, MP, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_39

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perovskites have led the power conversion efficiency to a level needed for commercialization which makes them superior among thin-film solar cells. In 2017, organic-inorganic hybrid metal halide perovskites have crossed the power conversion efficiency of 23% which is believed to be due to the long charge diffusion length, tunable bandgap, controlled electron/hole transport behavior, and high absorption coefficient. However, these perovskite solar cells suffer from serious drawbacks such as low stability and presence of highly toxic lead (Pb2+). Currently, the researchers have now focused to develop the strategies toward the fabrication of lead-free perovskite solar cells with high device stability and performance in aerobic condition.

Introduction Sustainable energy is the major concern for the building of our future world [1–5]. Currently, the society around the major world relies on the energy supplied by fossil fuels which released a high amount of carbon dioxide which causes the greenhouse effect [3]. The researchers all over the world are seeking to develop the most efficient and low-cost devices to fulfill the energy requirements in the future. Among the renewable energy sources, solar energy is considered the most suitable source due to its neverending process. Photovoltaic is the most promising technology which converts the solar energy into electricity based on photovoltaic effect [4]. Thus photovoltaic technology is considered the most suitable and efficient technique over the other renewable energy sources such as fuel cells and biofuel cells. The first solar cell device was developed in 1883, while the silicon-based solar cells were prepared in 1954 with a power conversion efficiency of 6% [1]. The solar cell field has been divided into three generation categories (Flowchart 1). The first-generation solar cells have been developed based on p-n junction using mono- and polycrystalline silicon (Si). The second generation is based on amorphous silicon, cadmium telluride, or copper (gallium) indium selenide/sulfide [6]. The third-generation solar cells are interesting and composed of semiconducting materials. The third-generation solar cells have been developed to overcome the issues raised in the first- and secondgeneration solar cells. However, the dominated solar cell devices are expensive and need the complicated manufacturing process. Therefore, scientists have worked to find out the alternative technique and materials to reduce the cost with simple fabrication procedures. M. Gratzel has invented dye-sensitized solar cells (DSSCs) which have attracted the researchers due to its simple fabrication procedures and decent performance [7–26]. However, the limited power conversion efficiency and the use of expensive ruthenium-based dyes as light sensitizer restrict its practical applications [27]. Therefore, it is of great interest for scientific community to develop the costeffective and highly efficient new light absorbers/light harvesters for DSSCs. Thus, in 2009, Miyasaka and coworkers have discovered a new light sensitizer (methylammonium lead halide) named perovskite for DSSCs [28]. The developed device showed excellent performance in terms of open circuit voltage but suffers from the leakage of liquid electrolyte and the sensitivity of the methylammonium lead

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Flowchart 1 Types of solar cells based on different generations

halide with liquid electrolyte. Further novel strategies were developed by various research groups to further improve the performance of the perovskite-based solar cells. Previously, various electron transport layers and hole transport materials were employed to improve the charge collection efficiency, and finally in 2019, the highest power conversion efficiency of more than 23% was reported. Therefore, it is clear that methylammonium lead halide perovskite-based solar cells could be commercialized. But unfortunately, the presence of highly toxic lead and low aerobic stability of methylammonium lead halide perovskite restrict its practical applications. In this chapter, we have discussed the role and advantages/disadvantages of methylammonium lead halide perovskite-based solar cells and the recent approaches toward the development of lead-free perovskite solar cells.

What Is Perovskite? Generally, the perovskite term is used to describe any material which has an ABO3 type structure (where A = Ca2+, Sr2+, Ba2+ and B = Ti2+, Sn2+). The term perovskite was originated from the Russian mineralogist L. A. Perovski [28]. The first time perovskite term was given to CaTiO3. These kinds of perovskites have wide bandgap, high electrical conductivity and ferroelectricity, and high thermal stability and have been used in many applications such as solid oxide fuel cells, sensors, electro-catalysis, and photocatalysis [29, 30]. Another class of hybrid perovskites adopts the general molecular formula ABX3, where A represents the

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Fig. 1 Crystal structure of perovskites with a chemical formula of ABX3. (Adapted with permission [35])

organic cations such as (CH3NH3+) = MA and HC(NH2)2+ = FA, B is the metal cations (Pb2+, Sn2+), and X is the halide anions (F
, Cl
, or I
). In a typical perovskite structure, A is coordinated with X anions by 12-fold cuboctahedrally, while B occupies the center of [BX6]4
cluster as shown in Fig. 1 [31–34]. The formation of such hybrid perovskites can be determined by Goldschmidt tolerance factor (t) which is given in Eq. 1. rA þ rX ðtÞ ¼ pffiffiffi 2 ðrB þ rX Þ

ð1Þ

where rA, rB, and rX are effective ionic radii for A, B, and X ions, respectively. Moreover, octahedral factor (μ) is used to evaluate the stability of the perovskites and the equation given below: ðμÞ ¼

rB rX

ð2Þ

It is considered that perovskites are stabilized when the (t) lies between the range of 0.813 and 1.107 and (μ) between the range of 0.442 and 0.895. This unique structure of perovskite possesses high absorption coefficient, tunable bandgap, excellent absorption range, long electron and hole diffusion lengths, extended charge carrier lifetime, low exciton binding energy, and high ambipolar charge nobilities. The most important properties of these organic-inorganic hybrid metal halide

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perovskites are the low bandgap and high absorption coefficient which make their possibility for photovoltaic applications [35].

Perovskite Solar Cells Basic Principle and Fabrication of PSC Device The principle of PSCs is based on dye-sensitized solar cells which work on photosynthesis process. The perovskite solar cells are composed of four different components which are given below: 1. 2. 3. 4.

Electron transport layer Light absorber Hole transport material Back contact

To fabricate a PSC device, firstly, a thin layer of electron transport layer (ETL) composed of conducting metal oxides such as TiO2, ZnO, or SnO2 is deposited onto the patterned conductive glass substrate (ITO or FTO). The FTO/ETL was further annealed at 450–500  C for 20–30 min inside the vacuum furnace which is cooled down to RT naturally. Now a thin layer of perovskite light absorber (MAPbI3) is deposited onto the FTO/ETL and annealed at 70–100  C for 30–120 min on a hot plate, and subsequently a hole transport layer (HTL) of spiro-MeOTAD is deposited onto the FTO/ETL/MAPbI3, and finally a metal contact layer of gold (Au) is deposited onto the FTO/ETL/MAPbI3/HTM to complete the device (FTO/ETL/MAPbI3/HTM/Au) as shown in Scheme 1. The PSCs simply consist of sandwich-like architecture in which MAPbI3 absorbs the sunlight and the electron-hole pairs are generated. The generated electron inside the MAPbI3 injected the electron into the conduction band of the ETL of metal oxide which is collected at the FTO. On the other hand, the hole inside the MAPbI3 get transferred through the hole transport materials to the Au to complete the working process of the PSC device.

Origin of Perovskite Solar Cells The idea of perovskite solar cells was originated by Miyasaka and co-workers in 2009 [28]. They have introduced the CH3NH3PbI3 perovskite as visible light sensitizer in DSSCs and obtained an efficiency of ~3.8%. Further, Im et al. found that the bandgap of CH3NH3PbI3 perovskite is too narrow (1.5 eV) with a high absorption coefficient compared to the ruthenium-based dye (N719) [36]. They have prepared CH3NH3PbI3 perovskite quantum dots using simple protocols which are employed as light absorber in quantum dot-sensitized solar cells (Fig. 2). They have fabricated solar cells with three

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FTO/ETL

FTO

Spin coating

Spin coating

Future

FTO/ETL/MAPbl3 /HTM/Au

Thermal evaporation

FTO/ETL/MAPbl3

Spin coating

FTO/ETL/MAPbl3 /HTM

Spin coating

Scheme 1 Schematic representation of the fabrication of the perovskite solar cells

different configurations: (i) quantum dot-sensitized solar cell, (ii) quantum dot-arrayed (p-i-n junction) solar cell, and (iii) quantum dot-embedded polymer solar cells. From the developed solar cells, it was observed that device having quantum dots as light absorber is more efficient compared to the other two devices. This high performance may be due to the presence of metal oxide supports which provide high surface area to utilize the quantum dots as light absorbers. The quantum dot-sensitized solar cells are also similar to dye-sensitized solar cells, and the difference between these two types of solar cells is only the presence of different light absorbers. The PCE of these works was interesting, but the presence of liquid electrolyte destroyed the stability of the developed solar cell devices. Therefore, it was necessary to overcome this issue; in this regard, Kim et al. have introduced a solid-state hole transport material (spiro-MeOTAD) [37]. A new device architecture (FTO/TiO2/CH3NH3PbI3/spiro-MeOTAD/Au) named perovskite solar cells (PSCs) was introduced, and the digital image of the fabricated solar cell is presented in Fig. 3. The cross-sectional SEM image of the fabricated solar cell device was recorded which clearly shows the presence of different components of PSCs in the device architecture (Fig. 3b–c). The developed PSCs showed the enhanced PCE of 9.7% and excellent photocurrent density of 17.6 mA/cm2 including high open circuit voltage of 888 mV. The fill factor (FF) for this developed device was also found to be 62% (Fig. 4). The authors also concluded that the replacement of liquid redox electrolyte with solid-state HTM, i.e., spiro-MeOTAD, not only resolved the issue of dissolution of CH3NH3PbI3 perovskite nanoparticles in the liquid redox electrolyte but also improved the

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Fig. 2 TEM images of (a) CH3NH3PbI3 perovskite deposited TiO2, (b) magnified image of CH3NH3PbI3 perovskite deposited TiO2, (c) pure CH3NH3PbI3 perovskite and (d) TiO2. (Adapted with permission [36])

Fig. 3 (a) Digital image, (b) schematic of cross-sectional, (c) SEM image for cross-sectional view, and (d) interfacial junction structure. (Adapted with permission [37])

efficiency of the perovskite solar cells. PSCs consist of different components, and each component has important role, but electron transport layer plays a crucial role. The performance of PSCs was further improved by introducing new electron transport layers and charge extraction layers. To date, titanium dioxide (TiO2) has been widely employed as electron transport layer in PSCs [36–42].

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Fig. 4 (a) Photocurrentvoltage characteristics of CH3NH3PbI3 perovskitesensitized solar cell. (Adapted with permission [37])

a

b

2

Glass

Spiro-OMeTAD

3 e– ITO ZnO Ag

Energy (eV)

4 ZnO 5

Ag

CH3NH3Pbl3

ITO h+

6

CH3NH3Pbl3

7

Spiro-OMeTAD

Fig. 5 Device architecture and energy level diagram of PSCs. (Adapted with permission [43])

The electrons from the perovskite light absorber are rapidly injected into the conduction band of TiO2, but recombination rate is also high due to the poor electron mobility/transport activities. To further resolve this issue, ZnO-based electron transport layer was introduced by Liu et al. (Fig. 5) [43]. The developed PSCs have shown improved PCE of 11 to 15.7%, but ZnO is also having an issue of chemical instability. Thus, researcher also employed another emerging electron transport layer of SnO2 and the highest PCE of 13% was achieved [44–49]. Further, numerous efforts have been made by researchers to improve the performance of PSCs by introducing novel electron transport layers such as ZrO2, WOx, PbZrTiO3, Nb2O5, Zn2SnO4, nanocomposites, etc. with different surface morphology/ architectures [50–58]. The effects of annealing temperature, doping with metal/nonmetal elements on electron transport layers, and preparing composites with carbon-based

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Scheme 2 Schematic illustration for the preparation of CH3NH3PbI3 perovskite film via two-step spin coating method

Fig. 6 SEM images of CH3NH3PbI3 at different loading times for the (a) 0.038 M CH3NH3I concentration and 0.063 M CH3NH3I concentration. (b) Insets: digital photographs of the prepared samples used for SEM analysis. Scale bars = 500 nm and (c) schematic representation of the perovskite solar cell. (Adapted with permission [59])

materials have also been employed to improve the performance of PSCs [58]. The CH3NH3PbI3 perovskite also plays an important role. The film thickness and surface morphology also affect the performance. So, in this regard, Im et al. [59] have developed a two-step spin coating method for the deposition of CH3NH3PbI3 perovskite thin films to improve the quality of perovskite films (Scheme 2). They have found that two-step deposition method leads to the formation of CH3NH3PbI3 cuboids. The size and shape of these cuboids could be tuned by varying the CH3NH3I concentration as confirmed by SEM images. The highest PCE of more

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Fig. 7 (a) Schematic illustration of the preparation of (CH3NH3PbI3-x(SCN)x)

Fig. 8 (a) XRD, (b) crystal structure, (c) SEM, and (d) cross-sectional SEM image of CH3NH3SnI3-based lead-free perovskite solar cell device. (Adapted with permission [61])

than 15% was recorded (Fig. 6). Furthermore, various strategies have been developed to improve the performance of the PSCs, but the presence of highly toxic lead (Pb) and poor aerobic stability or moisture sensitivity remain a challenge. Tai et al. have introduced a new lead precursor (lead(II) thiocyanate (Pb(SCN)2)) to improve the moisture resistivity of the PSCs [60].

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In this work they have also employed a two-step deposition method to fabricate the PSCs. The fabricated PSCs showed enhanced air stability (Fig. 7) with improved PCE, but the presence of Pb still remained a challenge for scientific community. In this regard, for the first time, Snaith et al. have introduced a Pb-free perovskite (CH3NH3SnI3) light absorber for solar cell applications [61]. They have developed the Pb-free PSCs which have shown the highest PCE of 6.4% (Fig. 8). However, serious issue of instability of Sn2+ in perovskite light absorber in air destroyed the overall performance. Under atmosphere, Sn2+ rapidly oxidized to Sn4+ which forbade the use of this CH3NH3SnI3 as light absorber for the development of leadfee perovskite solar cells [61]. Therefore, it is of great importance to find out some nontoxic/low-toxic new light absorbers with aerobic stability. Bismuth is a nontoxic and nonradioactive element and has the potential to replace the Pb2+ from the perovskite structure [62–65]. Bismuth-based perovskite light absorbers could be the efficient light harvester in the development of Pb-free perovskite solar cells. Ahmad et al. [24] have prepared a novel 1D polymer of methylammonium bismuth chloride ([(CH3NH3)3Bi2Cl9]n) perovskite and explored its optoelectronic properties. The [(CH3NH3)3Bi2Cl9]n was crystallized in centrosymmetric orthorhombic Pnma space group [24]. The crystal structure, unit cell, and stacking view of the [(CH3NH3)3Bi2Cl9]n have been presented in Fig. 9.

a

b

a

b a

c

c C14

2.572(10) Å

c

d

C14

C12

2.835 Å

C13

C12 93.30 Å C11

C15

Bi2 C16 C16

173.86 Å Bi1

C16

C16

Bi Cl N H C

Fig. 9 Crystal structure: perspective view (a), unit cell (b), geometry view (c), and stacking view (d) of [(CH3NH3)3Bi2Cl9]n. (Adapted with permission [24])

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Bismuth (Bi) atoms were coordinated by three terminal and three bridging chlorine (Cl) atoms which extended to form a 1D polymer chain in two different fashions along the b-axes only (Fig. 9a). The unit cell of the [(CH3NH3)3Bi2Cl9]n can be seen in Fig. 9b where the complex anions contain two alternate face-sharing octahedra with two bridging Cl atoms. The environment around the Bi atoms was found to be distorted octahedral geometry (Fig. 9c). However, the stacking view can be seen in Fig. 9d. Further the XRD of the [(CH3NH3)3Bi2Cl9]n bulk crystal was recorded and compared with the simulated XRD of [(CH3NH3)3Bi2Cl9]n (Fig. 10a). The recorded XRD of [(CH3NH3)3Bi2Cl9]n bulk was found to be similar to the simulated XRD which also suggested the formation of [(CH3NH3)3Bi2Cl9]n perovskite. The bulk crystal of [(CH3NH3)3Bi2Cl9]n was exposed to the air, and the XRD was recorded (Fig. 10b). The obtained results showed the high aerobic stability of [(CH3NH3)3Bi2Cl9]n perovskite. The UV-vis absorption spectra of [(CH3NH3)3Bi2Cl9]n were recorded, and the spectrum is presented in Fig. 10c. The optical bandgap was deduced to 2.85 eV using Tauc relation (Fig. 10d). Furthermore, a thin film of [(CH3NH3)3Bi2Cl9]n perovskite layer was deposited onto the ITO glass substrate, and the recorded XRD is presented in Fig. 11a. Further to check the aerobic stability of the prepared film was air exposed, and the XRD was recorded. The recorded XRD was found to be similar which suggested its good

b Intensity (a.u.)

Intensity (a.u.)

a [(CH3NH3)3Bi2Cl9]n measured

[(CH3NH3)3Bi2Cl9]n calculated

30

40 50 2θ(°)

60

70

80

c

d

400

500 600 700 Wavelength (nm)

800

[(CH3NH3)3Bi2Cl9]n non-exposed

10

(αhν)2 (eV/cm)2

20

Absorbance (a.u.)

10

[(CH3NH3)3Bi2Cl9]n air exposed

20

30

40 50 2θ(°)

60

70

80

Eg= 2.85eV

2.4

2.6

2.8

3.0

3.2

3.4

3.6

hν (eV)

Fig. 10 Characterization of [(CH3NH3)3Bi2Cl9]n: XRD (a, b), UV-vis spectrum (c), and Tauc plot (d). (Adapted with permission [24])

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aerobic stability (Fig. 11b). The surface morphology of the prepared film was checked by recording SEM image (Fig. 11c). Finally a PSC device was fabricated which exhibited the decent open circuit voltage of 430 mV (Fig. 11d). However, a poor current and efficiency were observed. This poor performance may be due to the wide bandgap (2.85 eV) of [(CH3NH3)3Bi2Cl9]n perovskite light absorber. The bandgap tuning is necessary to improve the performance of Pb-free PSCs. Thus, Zhang et al. have proposed a novel strategy to tune the bandgap and developed the highly stable lead-free perovskite solar cells [65]. The authors have employed the two-step vacuum deposition method to prepare the high-quality thin films of (CH3NH3)3Bi2I9 (Fig. 12). The introduced (CH3NH3)3Bi2I9 perovskite light absorber has a bandgap of 2.1 eV which is lower than that of the [(CH3NH3)3Bi2Cl9]n with higher aerobic stability. These properties of the prepared (CH3NH3)3Bi2I9 perovskite make it a suitable candidate for photovoltaic applications. Typically, FTO glass substrate coated with TiO2 was annealed at 500  C for 30 min. Further, a 50–800 nm-thick film of BiI3 was deposited at higher vacuum degree (10
4 Pa). Subsequently the BiI3 converted to MBI (CH3NH3)3Bi2I9 homogeneously in a ceramic vessel with middle perforating PTFE septum on the evaporation of CH3NH3I (Fig. 12a). Finally, the hole transport material and gold layer were deposited to complete the device. The

Fig. 11 XRD of [(CH3NH3)3Bi2Cl9]n thin film (a, b), SEM (c), and J-V curve (d) of the PSC device. (Adapted with permission [24])

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surface morphology was checked, and the SEM results revealed that the MBI films are well distributed with uniform surface morphology and covered the whole surface area. The schematic device architecture and energy level diagram of the fabricated lead-free perovskite solar cells are presented in Fig. 12b, c, respectively. The device structure has been depicted in Fig. 12d. The phase purity as well as formation of (CH3NH3)3Bi2I9 perovskite was confirmed by X-ray diffraction study, and the obtained pattern is shown in Fig. 13. The obtained diffraction peaks are almost the same to the simulated diffraction peaks. The proposed MBI perovskite has a bandgap of 2.1 eV which is beneficial for light absorption studies. Therefore, it can be said that the MBI has a potential to be used as light-harvesting materials for photovoltaic applications. Moreover, their high moisture tolerance power makes it extraordinary as compared to Pb2+-based perovskites. The device was fabricated with mesoscopic architecture, and the performance was checked for the fabricated devices, and the photocurrent-voltage curves are shown in Fig. 14. The thickness of the MBI perovskite was tuned which influences the performance of the fabricated devices perovskite solar cells, and the obtained results are presented in Table 1. The observations suggested that the MBI perovskite film with thickness of 300 nm shows the highest efficiency of 1.64% with excellent VOC of 830 mV among all the fabricated devices. The authors have demonstrated that the MBI perovskite could be used as the light absorber for the development of lead-free

Fig. 12 (a) Schematic, (b) SEM, (c) device structure, and (d) energy level diagram of the MBI perovskite solar cell components. (Adapted with permission [65])

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Fig. 13 XRD patterns for the MBI perovskite and BiI3 films, FTO glass substrate, and the calculated MBI. (Adapted with permission [65])

Fig. 14 Photocurrent-voltage curves recorded for MBI perovskite solar cell device with different thicknesses (T120, T300, T500, and T700) of MBI film. (Adapted with permission [65])

perovskite solar cells. Moreover, it was observed that MBI perovskites possess excellent moisture tolerance compared to the Pb2+-based perovskites. Due to the excellent stability and nontoxic nature of bismuth, Khadka et al. [66] prepared new bismuth-based perovskites (CsBi3I10 = CBI-1 and Cs3Bi2I9 = CBI-2) for the fabrication of Pb-free PSCs. The prepared perovskites have a bandgap of 2.08 eV and 1.8 eV for Cs3Bi2I9 and CsBi3I10, respectively. The thin films of CBI-1 and CBI-2 were prepared at different temperatures using anti-solvents (Fig. 15). The surface morphologies of the prepared thin films of CBI-1 and CBI-2 were checked by recording SEM images. The recorded SEM images of CBI-1 and CBI-2

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Table 1 Photovoltaic parameters for solar cell devices with different thicknesses. (Adapted with permission [65]) Solar cellsa T120

No. of devices best 18 cells T300 best 20 cells T500 best 18 cells T700 best 20 cells single-parameter bestc

Jsc (mA cm
2) 1.20 1.13  0.09 2.95 2.82  0.10 2.61 2.43  0.19 1.56 0.65  0.38 3.00

Voc (V) 0.61 0.50  0.06 0.81 0.78  0.03 0.72 0.64  0.08 0.52 0.50  0.13 0.83

FF 0.62 0.52  0.06 0.69 0.64  0.09 0.70 0.57  0.15 0.39 0.33  0.08 0.79

PCE (%) 0.46 (O.38)b 0.30  0.06 1.64 (1.57) 1.40  0.21 1.31 (1.17) 0.91  0.35 0.32 (0.28) 0.11  0.08 1.64

Data were reversely scanned by a rate of 100 mV s
1 under AM 1.5G, 100 mW cm
2. The active area is 24 mm2 b Forward-scanned PCEs are listed in parentheses, and the hysteresis can be roughly reflected by the forward–reverse values and J–V curves as exhibited above c Some of the best parameters do not come from the same cell, and so are listed individually as single-parameter best a

Fig. 15 Pictorial view for the preparation of CBI-1 and CBI-2 perovskite film using different steps. (Adapted with permission [66])

have been presented in Fig. 16a, b, respectively. The XRD patterns of the BiI3, CBI-1, and CBI-2 have been shown in Fig. 16c, whereas the Raman spectra of the BiI3, CBI-1, and CBI-2 have been depicted in Fig. 16d. The optical properties of the BiI3, CBI-1, and CBI-2 were investigated by recording UV-vis absorption spectra, and the spectra have been shown in Fig. 16e. The optical bandgaps were found to be 2.25 eV, 2.08 eV, and 1.80 eV, respectively.

Fig. 16 SEM (a, b), XRD (c), Raman (d), UV-vis spectra (e), and PL of CBI-1 and CBI-2 perovskite film prepared using different approach (f). (Adapted with permission [66])

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Fig. 17 Schematic diagram of PSC device architecture. (Adapted with permission [66])

However, the PL spectra of the BiI3, CBI-1, and CBI-2 have been depicted in Fig. 16f. These investigations suggested the potential of CBI-1 and CBI-2 for solar cell applications. Thus, Khadka et al. have fabricated the PSC devices using CBI-1 and CBI-2 as light absorbers. The schematic device architecture is presented in Fig. 17. The authors have employed new strategies such as use of anti-solvents (chlorobenzene), morphology tailoring, interface engineering, and different annealing temperatures to obtain high-quality thin films. The fabricated device exhibited the best power conversion efficiency of 1.26% with good open circuit voltage of 740 mV. These results revealed that surface chemistry control, morphology tailoring, interface engineering, etc. are the important parameters which further need to be investigated for the further improvements of CBI- or MBI-based PSCs.

Conclusion and Future Prospective In recent years, CH3NH3PbI3 light absorber-based perovskite solar cells have been the most studied research area due to the excellent optical behavior of CH3NH3PbI3 perovskite. The hysteresis, presence of toxic Pb2+, and lower stability lead the researchers toward the design and development of a new perovskite material for Pb2+-free and highly stable perovskite solar cells. More recently, another class of perovskite (CH3NH3)3Bi2I9/Cs3Bi2I9 which is highly stable in air has been introduced for lead-free perovskite solar cells. However, to date (CH3NH3)3Bi2I9-/ Cs3Bi2I9-based perovskite solar cells show poor efficiency which needs to be improved. Therefore, the performance of (CH3NH3)3Bi2I9-/Cs3Bi2I9-based perovskite solar cells could be improved by the following points: (i) By understanding the basic structure, crystal growth, and optoelectronic properties of (CH3NH3)3Bi2I9-/Cs3Bi2I9-based perovskite solar cells. (ii) Since the physical properties (absorption coefficient, dielectric constant, refractive index, charge diffusion lengths, and effective mass) of perovskite materials

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are related to the charge transport/collection and light absorption, it is necessary to develop the methods to improve their physical properties. (iii) The efficiency of the (CH3NH3)3Bi2I9-/Cs3Bi2I9-based perovskite solar cells may be improved by developing the fabrication procedures and film thickness and incorporating different charge extraction layer, hole transport materials, and/or different methods such as two-step deposition method, vapor method, etc.

References 1. Kitano M, Hara M (2010) Heterogeneous photocatalytic cleavage of water. J Mater Chem 20:627–641 2. Ahmad K, Ansari SN, Natarajan K, Mobin SM (2019) A two-step modified deposition method based (CH3NH3)3Bi2I9 perovskite: lead free, highly stable and enhanced photovoltaic performance. Chem Electro Chem 6:1–8 3. Sum TC, Mathews N (2014) Advancements in perovskite solar cells: photophysics behind the photovoltaics. Energy Environ Sci 7:2518–2534 4. Ahmad K, Mohammad A, Mathur P, Mobin SM (2016) Preparation of SrTiO3 perovskite decorated rGO and electrochemical detection of nitroaromatics. Electrochim Acta 215:435–446 5. Reddy VS, Kaushik SC, Ranjan KR, Tyagi SK (2013) State-of-the-art of solar thermal power plants. Renew Sust Energ Rev 27:258–273 6. Chen GY, Seo J, Yang CH, Prasad PN (2013) Nanochemistry and nanomaterials for photovoltaics. Chem Soc Rev 42:8304–8338 7. O’Regan B, Grätzel M (1991) A low cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353:737–740 8. Chen Y-Z, Wu R-J, Lin LY, Chang WC (2019) Novel synthesis of popcorn-like TiO2 light scatterers using a facile solution method for efficient dye-sensitized solar cells. J Power Sources 413:384–390 9. Prabavathy N, Balasundaraprabhu R, Balaji RG, Malikaramage AU, Prasanna S, Sivakumaran K, Kumarad GRA, Rajapakse RMG, Velauthapillai D (2019) Investigations on the photo catalytic activity of calcium doped TiO2 photo electrode for enhanced efficiency of anthocyanins based dye sensitized solar cells. J Photochem Photobiol A Chem 377:43–57 10. Motlak M, Hamza AM, Hammed MG, Barakat NAM (2019) Cd-doped TiO2 nanofibers as effective working electrode for the dye sensitized solar cells. Mater Lett 246:206–209 11. Wang C-T, Wang W-P, Lin H-S (2018) Niobium and iron co-doped titania nanobelts for improving charge collection in dye-sensitized TiO2 solar cells. Ceram Int 44:18032–18038 12. Shakir S, Abd-ur-Rehman HM, Yunus K, Iwamoto M, Periasamy V (2018) Fabrication of un-doped and magnesium doped TiO2 films by aerosol assisted chemical vapor deposition for dye sensitized solar cells. J Alloy Compound 737:740–747 13. Hao NH, Gyawali G, Hoon JS, Sekino T, Lee SW (2018) Cr-doped TiO2 nanotubes with a double-layer model: an effective way to improve the efficiency of dye-sensitized solar cells. Appl Surf Sci 458:523–528 14. Dong YX, Jin B, Lee SH, Wang XL, Jin EM, Jeong SM (2019) One-step hydrothermal synthesis of Ag decorated TiO2 nanoparticles for 2 dye-sensitized solar cell application. Renew Energy 135:1207–1212 15. Zhang X, Liu F, Huang QL, Zhou G, Wang Z-S (2011) Dye-sensitized W-doped TiO2 solar cells with a tunable conduction band and suppressed charge recombination. J Phys Chem C 115:12665–12671 16. Tran VA, Truong TT, Phan TAP, Nguyen TN, Huynh TV, Agrestic A, Pescetellic S, Le TK, Carlo AD, Lund T, Le S-N, Nguyen PT (2017) Application of nitrogen-doped TiO2 nano-tubes in dye-sensitized solar cells. Appl Surf Sci 399:515–522

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56. Li Y, Zhao L, Xiao M, Huang Y, Dong B, Xu Z, Wan L, Li W, Wang S (2018) Synergic effects of up conversion nanoparticles NaYbF4:Ho3+ and ZrO2 enhanced the efficiency in holeconductor-free perovskite solar cells. Nanoscale 10:22003–22011 57. Zhang Y, Wang J, Liu X, Li W, Huang F, Peng Y, Zhong J, Cheng Y, Ku Z (2017) Enhancing the performance and stability of carbon-based perovskite solar cells by the cold isostatic pressing method. RSC Adv 7:48958–48961 58. Mahmood K, Sarwar S, Mehran MT (2017) Current status of electron transport layers in perovskite solar cells: materials and properties. RSC Adv 7:17044–17062 59. Im JH, Jang IH, Pellet N, Grätzel M, Park N-G (2014) Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat Nanotechnol 9:927–932 60. Tai Q, You P, Sang H, Liu Z, Hu C, Chan HLW, Yan F (2016) Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity. Nat Commun 7:11105 61. Noel NK, Stranks SD, Abate A, Wehrenfennig C, Guarnera S, Haghighirad AA, Sadhanala A, Eperon GE, Pathak SK, Johnston MB, Petrozza A, Herza LM, Snaith HJ (2014) Lead-free organic–inorganic tin halide perovskites for photovoltaic applications. Energy Environ Sci 7:3061–3068 62. Eckhardt K, Bon V, Getzschmann J, Grothe J, Wasser FM, Kaskel S (2016) Crystallographic insights into (CH3NH3)3(Bi2I9): a new lead-free hybrid organic–inorganic material as a potential absorber for photovoltaics. Chem Commun 52:3058–3060 63. Singh T, Kulkarni A, Ikegami M, Miyasaka T (2016) Effect of electron transporting layer on bismuth-based lead-free perovskite (CH3NH3)3 Bi2I9 for photovoltaic applications. ACS Appl Mater Interfaces 8:14542–14547 64. Shin J, Kim M, Jung S, Kim CS, Park J, Song S, Chung K-B, Jin S-H, Lee JH, Song M (2018) Enhanced efficiency in lead-free bismuth iodide with post treatment based on a hole-conductorfree perovskite solar cell. Nano Res 11:6283–6293 65. Zhang Z, Li X, Xia X, Wang Z, Huang Z, Lei B, Gao Y (2017) High-quality (CH3NH3)3Bi2I9 film-based solar cells: pushing efficiency up to 1.64%. J Phys Chem Lett 8:4300–4307 66. Dhruba B, Shirai Y, Yanagida M, Miyano K (2019) Tailoring the film morphology and interface band offset of cesium bismuth iodide-based Pb-free perovskite solar cells. J Mater Chem C 7:8335–8343

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natural Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginates-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soy Protein-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hemicellulose-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wastewater Treatment Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Polymers from natural resources established an important position owing to their easy availability, abundance, biocompatibility, and sustainability. Nowadays, human beings are aware about the environmental concerns and ecofriendly practices in every aspects of life, hence demand for such materials have increased more. Polymeric composites in macro, micro, and nano scale are popular for various applications such as energy production and storage, environmental cleaning, sensing, and packaging. This book chapter is all about to review the development of natural polymers-based composite materials for environmental applications and their mode of actions. What can be the further possibilities in this area are discussed by the end of chapter. M. Shabbir (*) · X. Luo School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei Province, PR China e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_89

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Keywords

Polymers · Composites · Environment · Wastewater · Sustainability

Introduction Polymers play important role in our day to day life. Our basic needs are all dependent on polymers from proteins as building blocks of our body, carbohydrates as food, and various polymers such as cellulose, wool, silk, nylon, polyester etc. are used for textiles. Rubber and plastics have been of high importance which are chemically polymers. These are long-chain molecules made up of repeating units called as monomeric units, e.g., proteins from amino acids, cellulose from glucose units, and so on. Polymers are classified on various bases, and these come from both natural and synthetic origin. Polymers have been employed in many industrial applications, waste-water treatment, electrical and electronics applications for their unique properties. With the exponential advent of new materials, certain limitations in terms of durability, universality, and efficiency of polymers came into existence. Polymer composites evolved to fulfill the modern day needs which the polymers alone were not capable of and their significance environmental applications can be evaluated by the trend of research published on them recently (about 50% of published research of polymer composites in 2019 is related to environmental applications) (Fig. 1). Polymer composites are made up of two or more than two components, among which one is polymer, having different physical

Fig. 1 Number of research publications on polymeric composites (Y ¼ year; PCEA ¼ polymeric composites for environmental applications). (Source: Science Direct on 28 March 2020 with search terms “polymeric composites”)

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and chemical properties. If one of the components is of nano size, then they are named as polymer nanocomposites. Usually polymer acts as matrix and other component as dispersed phase in polymer composites. Polymer composites have characteristics different or improved from their individual components. Nanomaterials have been highly studied as dispersion phase in polymer matrices for their chemical, magnetic, optical, electronic, catalytic, and photo electrochemical properties [1]. Nanoparticles have important role in industries such as textiles, food packaging, pharmaceuticals, and coatings for their antimicrobial activity. Catalytic properties of nanoparticles make them utilized for chemical industries and environmental applications. Nanoparticles such as Ag and Au for antimicrobial, Pt, Pd, ZnO, and TiO2 for catalytic properties have been explored well in literature. Polymer matrices are used for the fabrication of composites to enhance recyclability and reusability of these materials. There may be many methods to fabricate polymer composites depending on the involvement of each component during fabrication process: • Pre-prepared fillers introduced into polymer matrix and stabilized by various means • Fillers simultaneously developed (in situ) in dissolved polymers and designed into a specific morphology • Dipping polymer into nanosols of fillers and simultaneous fabrication and fixation of nanoparticles Availability of clean water is the global priority, without which there are serious challenges for the survival of various species including human beings. Only a fraction of water available on earth is pure and can be utilized for drinking purpose. Industries, pesticides, municipal waste, agricultural activities, heavy metals, and biomedical waste are the major contributors of the water pollution. Toxic pollutants accumulate in the food chain disturb animal metabolism lead to severe diseases [2]. Depending on the pollutants, approaches are applied for environmental remediation such as adsorption, degradation, etc. Inorganic pollutants (heavy metal ions, phosphates, sulphates, etc.) are usually removed via adsorption process while, organic pollutants (organic dyes, pharmaceuticals, agricultural compounds, etc.) mostly large compounds and in low concentrations required to be degraded in wastewater. Industries, particularly textiles and pharmaceuticals, have put much pressure on water bodies and polluted to a great extent, result of which, environmental problems are highly studied with the application of polymer composites as per data presented in Fig. 1. Organic dyes and pharmaceutics remain in traces, so it is not easy to remove them by adsorption because of cost, energy, and time. Photocatalytic degradation has been an alternative for such pollutants with the application of nanomaterials and polymer composites. Polymer composites, for photocatalytic degradation, are primary choice owing to high sensitivity, reusability, high efficiency and stability. A few pollutant examples of dyeing and pharmaceutical industry are enlisted in Table 1 [3, 4].

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Table 1 Some examples of pharmaceutical drugs and organic dyes in wastewaters Pharmaceuticals Naproxen, Ibuprofen, Diclofenac, Mefenamic acid, Acetaminophen, Chloramphenicol, Fluoxetine, Diazepam, Metronidazole, Chlortetracycline, Propranolol, Azithromycin, Tylosin A, Sulfamethoxazole, Sulfamethazine, Ciprofloxacin

Organic dyes Acid orange, Acid red, Methylene blue, Malachite green, Crystal violet, Indigo, Cresol red, Direct orange, Direct yellow, Direct black, Reactive orange, Rhodamine B, Reactive blue, Methyl orange

Natural Polymer Composites Cellulose-Based Composites Cellulose is a natural polymer composed of glucose as monomeric units linking the C-1 of one pyranose ring and the C-4 of the next ring. Cellulose provides a strong matrix for immobilization of dispersion phase for the development of cellulosebased composites. Cellulose reinforced polymer composites are gaining high attention due to low cost, easy availability, biodegradability, and biocompatibility of cellulose. Polymeric composites designed from cellulose with some filler can be molded into various shapes and sizes as per the required application areas. Polymeric composite sheets, membranes, and beads of micro or nano sizes have been developed in recent decades for environmental, energy, catalytic, and sensing applications. Cellulose-based composites find their role in wastewater treatment via adsorption, catalytic degradation of pollutants evidenced since years in literature. For the removal of inorganic pollutants such as metal ions, adsorption via magnetic nanoparticle-polymer composites is highly popular. Iron oxides are very well known for the adsorption properties and easy magnetic separation of these pollutant loaded materials (Fig. 2). Applying coprecipitation approach, polygonal magnetite nanoparticles (MNPs), of 6–14 nm size, were fabricated using nontoxic bacterial cellulose (BC), biosynthesized by Gluconacetobacter xylinus (ATCC ® 10245). Composites were mesoporous with mostly < 60 nm pore sizes and pore size distribution centered around 27 nm. The fabricated BC/ MNPs composite was used for the removal of antimony (Sb (III)) from aqueous solution and this adsorption was found controlled by chemical forces (electrostatic attraction) in homogenous monolayer revealed by fitting of experimental data with adsorption and kinetic models. The fabricated composite showed significant regeneration capability and high performance for many cycles, with good efficiency (88.7–93%) [6]. In a study, design of cellulose microfibrils (CMF)-based magnetic cellulose composite catalyst was presented for wastewater treatment purpose. CMF improved the dispersion of metal organic framework and iron oxide (Fe3O4) nanomaterials and resulted high catalytic performance. Reusability and durability of efficiency in reusing are another positive aspect of these catalysts [7]. Magnetic cellulose composites designed by various methods and into different morphologies

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Fig. 2 A scheme of the fabrication of Fe3O4@cellulose composites [5]

are well described by Liu et al. [8]. Magnetic cellulose composites of various shapes such as fibers, films, microspheres, and hydro- or aerogels have been explored for practical applications. Magnetic nanoparticles can be embedded into cellulose matrix in many ways including Coprecipitation Method, Sol-Gel Reactions, High-Temperature Decomposition, and Solvothermal Method. Since magnetic nanoparticles are known for adsorption of pollutants from wastewater, their presence on the surface of cellulose matrices or materials is of high importance and for this, an approach of dipping cellulose into nanosols of iron oxide and simultaneous fabrication and fixation of nanoparticles can be applied, which also extends its limits to many metal or metal oxide nanomaterials embedded into or on cellulose materials [9]. Fillers, other than iron oxides, have also been studied well for cellulose composites. Cellulose acetate-tin (IV) phosphate nanocomposite (CA/TPNC) ion exchangers designed in a study for adsorption and photocatalytic degradation of methylene blue from wastewater simultaneously, as organic-inorganic nanocomposite ion exchangers are considered good candidates for environmental remediation owing to high selectivity and specificity [10]. Semiconductor photocatalysts (e.g., ZnO, TiO2) are popular for their catalytic properties on wastewater pollutants. Photocatalytic efficiency of these photocatalysts can be improved via composite development to make the pollutants adsorbed on the surface. Achieving the adsorption and catalytic properties in a single material could get high efficiency materials in wastewater treatments. ZnO nanoparticles are relatively cost effective, nontoxic materials having various morphologies make them a good choice for development of cellulose composites. Zinc oxide/cellulose nanocrystal (ZnO/CNC) hybrids with modulated morphologies were prepared by using bamboo CNC as templates via green one-step technique. Based on the synthesis parameters, characteristics

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Fig. 3 Cross-linking reaction mechanism of epichlorohydrin (ECH) with graphene oxide (GO) and cellulose [16]

and applications of composites were controlled for high efficiency. These hybrids showed outstanding and low-cost adsorbent capacity for efficient removal of cationic dyes (methylene blue (MB) and malachite green (MG)) in wastewater [11]. A lot of research has been done based on cellulose-based composites with varying dispersion phases such as ZnO, TiO2, Cu nanoparticles, graphene oxide, etc. for large range of environmental applications. For example, various functional groups (hydroxyl, carboxyl, and epoxy groups) in graphene oxide’s extended layered structure (Fig. 3) provides dispersibility and affinity to many pollutants in water with strong adsorption abilities. Hence, many studies are based on graphene oxide as a filler with cellulose as well as chitosan polymer [12–17]. Some polyphenolic

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compounds can also help providing chelating functionalities to remove metal ions via chelation. In such an effort, solution casting approach applied to fabricate calcinated tea and cellulose composite (CTCC) films aimed to enhance mechanical properties, thermal stability, and dielectric constant of the neat cellulose films. The CTCC films showed adsorption behavior towards Pb2+ ions which is directly related to the calcinated tea content and pH level [18]. Nanofibrous anatase-titania-cellulose composite (having deposition of 20–50 nm size anatase titania (TiO2) nanoparticles) prepared for efficient photocatalytic degradation of methylene blue and methyl orange aqueous solutions under UV light irradiation. Results of photocatalytic degradation has been impressive even when methylene blue was dropped onto the surface of the composite which was decomposed after 1 h outdoor sunlight irradiation [19]. Cellulose for its unique characteristics has been studied very well for composites design in wastewater applications.

Chitosan-Based Composites Chitosan, partly acetylated (1–4)-2-amino-2-deoxy-β-D-glucan obtained from chitin (Fig. 4), is another important natural polymer like cellulose known for biodegradability, biocompatibility, nontoxicity, and having absorption properties. Wide application range of chitosan products also depends on the presence of functional amino groups in chemical structure that may lead to various modifications. Effective approaches are there to broaden the application range of chitosan by forming organic-inorganic composites of chitosan through incorporation of fillers (e.g., metal nanoparticles) to improve physical and mechanical properties [20]. Easy separation, owing to the magnetic properties, of magnetic-chitosan composites make them of high importance in removal of pollutants by adsorption process. Iron oxides are the commonly used magnetic particles in composites. The particles have favorable characteristics such as high magnetization, low cost, low environmental impact and high chemical stability. Hence, many reports are available in literature about chitosan-based magnetic composites: magnetic chitosan composites (Fe3O4@chitosan) were synthesized with high potential of Cr(VI) removal from water [21], magnetic composite particles with a core

*

O

HOH2C

HO

O

HOH2C

*

NH2

O NH2

O

HO

n Fig. 4 Chitosan chemical structure

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containing iron oxide encapsulated in cross-linked chitosan and a functionalized synthetic copolymer shell [obtained by graft-copolymerization of GMA (Glycidyl methacrylate), EGDMA (ethylene glycol dimethacrylate) and PEGMMA (poly(ethylene glycol) methacrylate) using a water soluble azo initiator] are prepared by in situ oxidation of the ferrous ions for the removal of copper (II) ion in wastewater [22], magnetic thiolated/quaternized-chitosan composite adsorbent for removal of metal ions (As(V), As(III), Cu2+, Hg2+, Zn2+, Cd2+, Pb2+), as the economy development polluted the environment with heavy metals create problems due to their high toxicity, carcinogenicity and biological accumulation [23], and many more. A simple solution mixing-evaporation method was applied to develop high-performance Fe3O4/multiwalled carbon nanotubes (MWNT)/Chitosan nanocomposite of enhanced electrical conductivity, mechanical properties, and thermal stability [24]. Graphene oxide is another important filler used in both cellulose and chitosan-based composites. Many oxygen functional groups such as –COOH and –OH in graphene oxide (GO) are responsible for high sorption of heavy metal ions as well as allows GO to participate in bonding interactions in composite materials. But it is not easy to separate out it from treated water, this leads to exploration of its properties with polymers in composites. Magnetic chitosan and graphene oxide-ionic liquid (MCGO-IL) composites as biosorbents for the removal of Cr(VI) were synthesized by impregnating MCGO with ionic liquid [25]. For selective adsorption of lead ions, magnetic chitosan/graphene oxide (MCGO) materials of high stability and easily recovery was fabricated by Fan et al., [26]. Besides heavy metal ions, magnetic chitosan-graphene oxide composites show great adsorption capacity towards organic dye pollutants also and hence, functionalization of graphite oxide (GO) with magnetic chitosan (Chm) was investigated to prepare a nanocomposite material (GO–Chm) for the adsorption of a reactive dye (Reactive Black 5) in a study [27]. Ngah et al. [28] reviewed chitosan composites (e.g., chitosan/montmorillonite, chitosan/polyurethane, chitosan/activated clay, chitosan/bentonite, chitosan/ceramic alumina, chitosan/perlite, chitosan/cotton fiber, chitosan/sand, chitosan/calcium alginate, and many more) for environmental applications [28]. Recent years evidenced about polymer blending as a method for providing polymeric materials (e.g., polyvinyl alcohol, polyethylene glycol) with desirable properties for practical applications. Phosphate ions have been removed using polyethylene glycol/chitosan and polyvinyl alcohol/chitosan composites of high adsorption capacity [29]. Chitosan, mostly comes from sea food wastes, have great value with low cost production, high adsorption properties and this motivates environmentalists to utilize it highly in wastewater treatment applications.

Alginates-Based Composites Alginates are anionic polysaccharides derived from natural sources such as brown algae (Phaeophyceae) cell walls (e.g., Macrocystis pyrifera, Laminaria hyperborea) and several bacteria strains (Azotobacter, Pseudomonas). These are

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Fig. 5 Chemical structures of (a) 1,4 α-L-guluronic acid, (b) 1,4 β-D-mannuronic acid

O

OH -OOC

O

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O

(a)

-OOC OH

O

OH O O

(b)

linear biopolymers composed of 1,4-linked β-D-mannuronic acid and 1,4 α-Lguluronic acid (Fig. 5) residues in homogeneous or heterogeneous manner. Alginates usually exist in form of salts of metal ions such as sodium and calcium [30]. Composites based on alginates, having free hydroxyl and carboxyl groups, also play important role in environmental applications and evidenced by various studies published. Owing to the functional groups, alginate can capture metallic ions via ion exchange between the crosslinking cations and target pollutants such as heavy metals or dyes. Alginate-based composites, fabricated by combining alginates with activated carbon (AC), biochar, carbon nanotube (CNT), graphene oxide (GO), nanoparticle, magnetic materials, and microorganism, are superior to simple alginate materials with respect to their physicochemical properties. These properties mainly depend on the synthetic methods, i.e., ionic crosslinking, emulsification, electrostatic complexation, and self-assembly [31]. Alginate composites are well efficient for the removal of pollutants via both adsorption and catalytic degradation mode of actions. Organic/inorganic hydrogel nanocomposite of titania incorporated sodium alginate cross-linked polyacrylic acid (SA-cl-poly(AA)-TiO2) with high swelling capacity were prepared to act as superabsorbent for the removal of methylene blue dye. High efficiency of the absorbent is verified by very high adsorption capacity (Qmax ¼ 2257.36 (mg/g)) and a correlation coefficient of 0.998 calculated from isotherm equations [32]. Alginate-based TiO2 macro-bead photocatalysts developed for the advantages of easy recovery, mechanical stability, enhanced adsorption capacity, and recyclability. UV or ozone-UV cleaner treatments easily can get all adsorbed dye from beads which is an important advantage of such materials [33]. Thus, alginatebased composites are good candidates for adsorption of heavy metals and dyes, a good contribution to the research on natural polymer composites for environmental applications.

Starch-Based Composites Starch (general formula (C6H10O5)n) is a polysaccharide of glucose monomeric units made of two types of α-d-glucan chains, amylose and amylopectin. Free hydroxyl groups at position C-2, C-3 and C-6 provide reactivity to starch and hence various chemical modifications can be made to achieve specific characteristics as well as strong adsorbing functional groups. Starch composites have been studied for the

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removal of organic dyes from wastewater, however large-scale applications of starch are yet to be studied due to difficult practical operation and high cost of it. Starch/ polyaniline nanocomposite synthesized by chemical oxidative polymerization of aniline for removal of up to 99% of Reactive Black 5 and up to 98% of Reactive Violet 4 via adsorption mechanism. Composite particles are in the size range of 70–90 nm and spectroscopic studies revealed the broken inter-molecular hydrogen bonds created exposed groups for dye interaction [34]. Plasma and radiations play important role in polymer grafting, modifications, and hence, contribute to polymer composites development. With the use of γ-induced grafting of acrylamide and acrylic acid onto maize starch, bifunctional starch composites were prepared for adsorptive removal of both anionic dyes (direct blue 21 and direct brown 2) and a cationic dye (methylene blue) [35]. As far as grafting of polymers (polyaniline, acrylamide, etc.) have been carried out with starch for environmental applications, some inorganic components also explored. Polyaluminum ferric chloride-starch graft copolymer with acrylamide and dimethyl diallyl ammonium chloride was prepared to treat textile wastewater via coagulation-flocculation process [36], starch/SnO2 nanocomposite (SnO2 nanoparticles (5–12 nm size) were agglomerated to the starch matrix) was synthesized as an effective adsorbent for the removal of Hg2+ from aqueous medium with maximum monolayer adsorption capacity of 333 mg/g and the removal percentage of up to 97% [37], for methyl orange dye removal via adsorption, starch-NiFe-layered double hydroxide composite was prepared via coprecipitation method [38].

Soy Protein-Based Composites Soy proteins are one of the most abundant plant proteins on this planet earth. With high content of essential amino acid and desirable functional properties, soy proteins have attracted persisting interest in food and pharmaceutical industry. Soy proteins are classified into three major types: Soy protein isolates (SPI) with >90% purity, soy protein concentrate (SPC) with >70% purity, and fractionated 11S/7S globulins with >90% protein content and > 85% fraction purity [39]. Pure soy protein isolates have very low adsorption capacity to metal ions so various physical or chemical methods are required to improve adsorption characteristics. Composite hydrogels were prepared by soy protein isolate as a matrix and polyethyleneimine as a functional component through chemical cross-linking method for selective adsorption of Cu (II) ions from aqueous solution [40]. Porous composite beads were prepared by chemical immobilization of soy protein isolate on deacetylated konjac glucomannan with high adsorption capacity for methylene blue dye [41]. Along with heavy metal ions and organic dyes, soy protein-based composites proved to be effective for pharmaceutics removal from wastewater. Tetracycline antibiotic have been removed by adsorption on aerogel composites formed of graphene oxide with soy protein with significant efficiency [42]. Soy protein are known for composite developments, but less fraction of this used in environmental applications.

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Hemicellulose-Based Composites Hemicellulose (Fig. 6) is a smaller branched carbohydrate made up of different monosaccharides D-Xylose, mannose, L-arabinose, galactose, and glucuronic acid bonded with 1,4-Glycosidic bonds in main chains, and 1.2-, 1.3-, and 1.6-glycosidic bonds in side chains while cellulose is made of very long unbranched fibrils composed exclusively of glucose, bonded with 1,4-Glycosidic bonds, held together by hydrogen bonding. Hemicellulose does not contribute much to the environmental research to date, particularly for wastewater treatment, still there are some reports on application of hemicellulose contributing via composite formation with components such as metal oxides, chitosan, and cellulose. For the removal of heavy metals (e.g., Ni(II), Cd(II), Cu(II), Hg(II), Mn(VII), and Cr(VI)) via adsorption, a ternary composite of carboxymethyl chitosan, hemicellulose, and nanosized TiO2 was prepared. TiO2 nanoparticles incorporated into presynthesized carboxymethyl chitosan-hemicellulose polysaccharide network creating chelating groups in the composite structure. Easy regeneration of composites by EDTA and reusability for many cycles are advantages of these hemicellulose composites [43]. Carboxymethyl chitosan-hemicellulose resin was synthesized in another such study by thermal cross-linking process for adsorptive removal of heavy metal ions [44]. A stimuli-responsive porous hydrogel of hemicellulose, a promising material for wastewater treatment, was synthesized using CaCO3 as porogen for adsorptive removal of methylene blue dye [45]. Chitosan has been an important co-partner to hemicellulose in designing functional composites for adsorptive removal of both heavy metals and organic dyes. Bio-sorbent of hemicellulose and chitosan were prepared, and hemicelluloses used were pre-grafted with penetic acid (diethylene triamine pentaacetic acid). These biosorbents were used to remove heavy metals Pb2+, Cu2+, and Ni2+ [46]. Another composite material for Congo red dye removal was prepared based on dialdehyde HO

HO O

O

OH

O

O

OH

OH O OH

O HO

O O

OH OH

OH Fig. 6 Hemicellulose chemical structure

OH

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hemicellulose (hemicellulose oxidized by NaIO4 and aldehyde groups generated at C-2&3) and chitosan-Fe3O4 composite by the Schiff’s base reaction [47]. Hemicelluloses are natural polymers of secondary importance in comparison to cellulose, studied highly.

Wastewater Treatment Mechanisms Environmental pollutants, particularly in wastewater, are classified majorly into organic and inorganic pollutants according to their chemical nature (Fig. 7). Heavy metals, inorganic phosphates, sulfates, etc. are put under inorganic pollutant class and organic compounds from industrial effluents, pharmaceuticals, pesticides, textile dyes, etc. are categorized as organic pollutants. Many methods such as chemical oxidation, biological treatment, coagulation, flocculation, adsorption, electrochemical, precipitation, adsorption, and photocatalysis have been used for the removal of dyes from wastewater. Most of them are costly and not used effectively except adsorption and photocatalysis studied well [10]. Depending on pollutant’s chemical nature, approaches are used to remove them from wastewater such as adsorption and catalytic degradation. As earlier discussed, usually inorganic pollutants can be removed by adsorption process on polymer composites, while organic pollutants are generally large compounds and difficult to adsorb so requires different approach, i.e., catalytic degradation.

Adsorption Adsorption is the most common approach towards wastewater treatment and applied at large-scale to heavy metal ions and some textile dyes. Polymer composites designed into morphologies provides large surface area to maximize adsorption and subsequently high efficiency. Spherical shapes of varying sizes, porous structures are common examples of high surface area morphological designs to materials. Surface functionalizations to impart ion exchanging moieties, chelating groups, etc. can be carried out for specificity towards pollutants (e.g., heavy metal ions) and decide about the adsorption mechanism. Physicochemical properties studied by various adsorption isotherm and kinetics models to correlate the adsorption data and to study the actual Fig. 7 Pollutants classification and treatment approaches

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Fig. 8 Representation of adsorption (physisorption and chemisorption) of pollutants on spherical and sheets composites

mechanism of adsorption. Thermodynamic studies and parameters provide vital information to design composites of high specificity and efficiency. For example, Langmuir adsorption isotherm model most widely applied to wastewater treatment applications which applies to adsorption on specific sites. Pseudo first- and pseudo second-order kinetics models are common kinetics models used to correlate adsorption data, among which pseudo second-order mostly fitted to studies and justifies chemisorption phenomena involving chemical interactions responsible for adsorption. Physisorption doesn’t need chemical interactions and only physical forces make pollutants adsorbed on composite materials (physisorption and chemisorption are represented in Fig. 8 showing small spheres as pollutants adsorbed on composites of shapes of spherical and sheets, in chemisorption zig-zag lines represent chemical interactions between pollutants and composites).

Catalytic Degradation As earlier discussed, large organic compounds including textile dyes and pharmaceutics are difficult to remove via adsorption and then recovery of the adsorbents and adsorbates is another difficult task. Hence degradation of these compounds into small fragments which are considered harmless via catalysis is a good approach for wastewater treatment. Natural polymer composites having fillers

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Fig. 9 Proposed photocatalytic mechanism of ZnO-NPs for the catalytic degradation of methylene blue dye [48]

of metal (e.g., Pt, Pd, Au) catalysts perform well in this approach. Another catalytic approach is photocatalysis which needs support from electromagnetic radiations (sunlight, UV light). Photocatalysis is the approach applied to degrade large compounds such as organic dyes and pharmaceuticals. This approach needs semiconductors such as ZnO and TiO2 highly cited in literature. Polymer composites of such semiconductors shows superiority in terms of reusability, easy recovery and efficiency. Both solar radiation and ultraviolet radiation play important role in electron-hole pairs creation responsible for degradation reaction. Final degraded products generally go to smaller molecules including carbon dioxide and water. An example of photocatalytic degradation of methylene blue dye by ZnO nanoparticles is shown in Fig. 9 by using ultraviolet radiations and as such the polymer composites of natural polymer and ZnO can be explained. Oxidation, reduction, and complete mineralization reactions result into end products confirmed by spectroscopic techniques. Semiconductor ˑ photocatalysts, under the solar irradiation, create reactive species e.g., O 2 , OH to react with organic pollutants to produce small molecules such as CO2, H2O etc.

Conclusion and Further Outlook Nanomaterials have been of high importance for almost all industrial applications including environment management. Adsorbents and photocatalysts of nano size have already been used for wastewater treatments since long. Bare nanoparticles for water treatment have issues in their recovery and poses environmental danger by leaching into water ecosystem. Nanomaterial’s incorporation into polymer matrices provides stability to nanomaterials and risk of leaching can be avoided [49]. Natural polymers discussed above, owing to their biodegradability and biocompatibility, can

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perform that function quit well. Cost effective adequate availability, regeneration ability of natural polymers provides them superiority in the field of composites design. Cellulose and chitosan polymers are among highly studied for composite design for environmental applications. Morphology of materials plays crucial role in water treatment applications. Current research in this area needs innovative approaches to get the most efficient morphology of material. Composites of sizes ranging from nano to centimeter (nanocomposites, microspheres, beads, membranes/sheets) are in trend as per their mode of actions for environment. Search of new polymers in combination with morphologies to composites is the current demand in environmental research. Forest biomass and agricultural byproducts could be resources of natural polymers, a lot of work can be done in this area to find new sources of polymer stock. Other chemical components, e.g., graphene oxide can enhance the efficiency of composites by increased entrapment of pollutants as well as activity of semiconductor photocatalysts can be enhanced. Some copartners, e.g., Au can support the functionality of semiconductor catalysts by enhancing photo-absorption properties and results into higher efficiency. Such innovative ideas collaborated with research fundamentals lead to development of novel composites superior in efficiency. Natural polymers such as starch, hemicellulose, and soy protein are relatively less studied for environmental applications. These polymers have a wide scope and also search for some new natural polymers are to be explored in future.

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Second Life of Polymeric-Based Materials: Strategies and Performance Caren Rosales, Vera Alejandra Alvarez, and Leandro Nicolas Ludueña

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Waste Management Practices in Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of Recycled Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of Products Made with Recycled Thermosetting Matrix/Fiber Composites . . . . . . Quality of Products Made with Recycled Thermoplastic Matrix/Fiber Composites . . . . . . Legislation for Recycling Waste Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Wastes from polymeric materials could be considered as resources to manufacture new products by recycling. The properties of the as-obtained materials should be comparable to those of the virgin fossil-based plastics and also biopolymers. Several strategies are useful to achieve the required properties in order to introduce the recycled products into the market having competitive performance. Scientific knowledge related to different aspects of polymeric materials is a relevant tool to assure the performance of recycled products for new applications. C. Rosales Grupo de Ciencia e Ingeniería de Polímeros (CeIP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA, CONICET-UNMdP), Mar del Plata, Argentina V. A. Alvarez (*) · L. N. Ludueña Grupo de Materiales Compuestos Termoplásticos (CoMP), Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata (UNMdP) – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata, Argentina © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_87

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Diverse strategies, able to upgrade the morphology and the properties of recycled materials, have been used in order to reach the desired quality/performance in those new applications. The aim of this chapter is to review the current developments in the recycling of plastics, giving ideas on how producing materials with desired quality from wastes contributes to a more sustainable management of energy and resources. Keywords

Recycling · Polymers · Wastes · Quality · Competitive performance · Strategies

Introduction Polymers display several advantages; they are lightweight and durable inexpensive materials mainly derived from the petroleum industry. They can be easily molded into a huge variety of commodities that are then used in a really wide variety of applications [1]. Due to this, the manufacture of polymers has remarkably increased over the past decades. Composite materials are considered engineering materials that can replace metals in structural applications. Polymer composite materials have a large applicability in different industries (Fig. 1).

Fig. 1 Industries using polymeric composite materials

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The use of composite materials has been growing with the global business sector for composite products expected to reach £73 billion by the actual year [2]. However, the extensive use of such kind of materials has produced environmental problems, mainly related to their disposal. In a circular economy, resources are kept in use for as long as possible, extracting the maximum value from them while in use and then recovering and regenerating products and materials at the end of each service life [2]. Recycling is one of the most extensive alternatives to reduce environmental problems derived from the accumulation of polymeric wastes. The use of polymer-based products is continually increasing, frequently in construction and packaging industries. Despite the considerable developments that have been made in recycling, the case of polymer composite-based materials remains a challenge. Polymer composites are made of, at least, two materials in different phases: mainly the matrix and the fillers. These phases are precisely combined within the composites, and, therefore, it is commonly very difficult to separate them. Nevertheless, researchers have done many efforts to develop technically feasible, economically viable, and environmentally acceptable recycling processes for polymer composites [1, 3].

Current Waste Management Practices in Composites There are two main current topics of waste managements for polymeric composites: landfilling and recycling technologies (thermal, chemical, and mechanical recycling) (Fig. 2). For thermosetting matrix composite-based materials, mechanical recycling techniques comprise grinding, used to comminute the scrap material and then generate

Fig. 2 Current waste management practices in composites

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Fig. 3 Recycling methods used for thermosetting composites

recycled products in dissimilar size ranges than can be employed as fillers or, in some cases, as partial reinforcement in the manufacture of composite materials. On the other hand, thermal recycling of thermosetting-based composites involves heat that is used to break down the scrap composite (Fig. 3). In the case of thermoplastic matrix composite materials, washing, grinding, remelting, and remolding are the most common recycling techniques. Thermal processing is used when washing or grinding is not technically and/or economically feasible or for old scrap. In this case energy is recovered by incineration or combustion. Chemical recycling can be also applied to thermoplastic composites, but it is not widely used in the industry because solvents used are expensive and cause other environmental problems (Fig. 4) [4].

Recycling Methods Mechanical Recycling This recycling method, which is one of the most widely used recycling methods for fiber-reinforced polymer composites, is commonly known as physical recycling. In this method, the polymer composites are ground, reprocessed, and compounded to produce a new product, raw material, or another product meant for the same or new application [5]. It is necessary to identify the material of matrix composite to be recycled. Thermoplastic polymers can be reprocessed several times, but

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Fig. 4 Recycling methods used for thermoplastic composites

thermosetting polymers cannot be remolded due to their cross-linked structure. A complete recycling process is not possible for thermosetting materials, although this can be reused as filler or reinforcement in a new composite piece [6, 7]. The mechanical technique as much for thermoplastic as for thermosetting matrix composite is initialized by sorting process, where manual and automated technique is the initial step. Then, scrap composite components are reduced in size usually in some primary crushing process [8]. This would typically involve the use of a slowspeed cutting or crushing mill to reduce the material to pieces of 50–100 mm in size. This facilitates the removal of metal inserts, and, if done in an initial stage where the waste arises, the volume reduction assists transport. The main size reduction stage would then be in a hammer mill or other high-speed mill where the material is ground into a finer product ranging from typically 10 mm in size down to particles less than 50 μm in size [9, 10]. In the case of thermoplastic composite, this grinding material can be reprocessed by extrusion or injection molding. Mechanical recycling method is especially suitable for short fiber-reinforced polymers. This is because the decrease in mechanical properties induced by fiber breakage has a minor effect which was investigated for both glass fiber and carbon fiber-reinforced composites [8, 11, 12]. For thermosetting resin matrix, the process is more complex. After the reduction processes, the ground composite material is classified by size into coarse or powder particles. The coarse particles are reused to prepare composites by bulk molding compound (BMC), using the particles as reinforcement. The fine powder can be reused

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in BMC or sheet molding compound like active fillers [9, 13, 14]. The advantage of mechanical recycling is the fact that both fibers and resin can be recovered [13]. Nevertheless, this technique has a few disadvantages. The first one is related with the length degradation suffered on recovered fibers, which diminishes their mechanical properties. The second one is related to the heterogeneity of the collected material and the deterioration because of the polymer chain scission reaction which takes place during each reprocessing cycle. The reclaimed material can be blended with pristine material to obtain greater results [7, 15, 16].

Chemical Recycling Chemical recycling, also known as feedstock recycling, is defined as the process that targets chemical degradation of polymer scraps into their monomers or other basic chemicals [5]. This resulted material can be used for new polymerization to obtain a new or related product [15]. The difference in the process between thermoplastic and thermosetting matrix composite consists in the suitable choice of chemicals and solvents based on the nature of the polymer matrix [14]. Solvolysis is the principal process and can be divided into two types: low temperature and sub-/supercritical solvolysis. Low-temperature solvolysis consists in applying atmospheric pressure below 200
C with acidic medium or other reactive solvents (water, alcohol, ammonia, or glycol) to remove or break down chemical bonds of the composite matrix. Once the process is completed at the reactor machine, valuable final products are obtained. The obtained product is a mixture of fibers, organic liquid (monomers), and solvents. The advantage of this technique involves effective control over all chemical reactions and does not allow secondary reactions [9, 13]. Supercritical fluids have been used as emerging technologies related with green processes like biomass conversion and waste treatment and recycling. Sub-/supercritical solvolysis is a technique that involves the use of supercritical fluids in a reactor container, at 400
C for approximately 30 min. The efficiency and reactivity of supercritical fluids make them a better option to depolymerization in a rapid and selective reaction. When the process is finalized, different types of monomeric components with fiber elements are obtained [9, 13].

Thermal Recycling Thermal recycling can be classified into three types: pyrolysis, fluidized bed pyrolysis, and microwave pyrolysis [3, 5, 17, 18]. All three processes emphasize the reclamation of all types of fibers, fillers, and other inserts at the expense of the valuable matrix. This is because in all these methods, the polymer matrix is volatilized into lowmolecular-weight polymers and gases such as carbon dioxide, hydrogen, and methane and an oil fraction. The advantage of thermal recycling processes over chemical recycling processes is that the former is tolerant to more contaminated scrap materials. Pyrolysis process for recycling fiber-reinforced composites based on both thermosetting and thermoplastic matrices involves a thermal degradation of the

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matrix, keeping a temperature between 450
C and 700
C in the near absence of oxygen. While the process is carried out, end products are obtained: gaseous (hydrogen, methane, and other hydrocarbons), oily, and solid (smaller molecular organic substances and glass or carbon fibers) components. Working temperatures have an important effect on the mechanical, electrical, and surface properties of the recovered fibers [13]. At low pyrolysis temperatures, residues of decomposed resin matrix may cover the fiber surface (pyrolytic carbon). Mechanical properties of the fibers remain unchanged with respect to the virgin ones, but the interfacial adhesion with a new matrix polymer is reduced. At higher pyrolysis temperatures, the amount of pyrolytic residues is decreased, and also fiber surface was found to be greatly activated. Nevertheless, the strength of the recovered fibers at high temperatures was deteriorated. It is possible to remove the pyrolytic carbon completely at lower temperature, when the process is carried out in oxygen-containing atmosphere [9, 13, 19] or in a secondary combustion to oxidize minor resin impurity formation on the fibers [20, 21]. The pyrolysis technique has some advantages such as the following: there is no use of solvents, and the produced gases can be reused to supply the energy needed for the process. Recovered glass fibers are able to be reused in the same way as the coarse particles obtained from mechanical recycling technique. In the case of recovered carbon fibers, mechanical properties are similar to virgin ones and are capable to be remanufactured. However, as has been mentioned above, the mechanical and surface properties of recovered recycled fibers are especially sensitive to recycling parameters [9, 13]. Fluidized bed technique is a process developed to recover glass or carbon fibers from scrap or waste fiber composite materials. The process begins with a reduction in size of the scrap to about 25 mm. The ground particles are fed into a bed of silica sand and fluidized with a hot stream of air (ranging temperatures, 450–550
C). At this moment, the matrix composite is vaporized, whereas fibers are released as individual particles and carried out in the gas stream. A cyclone machine is responsible for sorting the fibers from the gas stream and making them pass through a second combustion chamber which operates at a higher temperature (1000
C). This final stage is necessary for cleaning fiber surfaces from organic constituents, considering that the separation at previous chamber occurred at low temperature [9, 13].

Comparison When pyrolysis and fluidized bed methods are compared to low-temperature solvolysis process, the last one is more convenient because it recovers valuable chemicals from the composite matrix and fibers with great mechanical properties. In spite of this, chemical solvents can be dangerous and noxious to the environment. At this point, supercritical fluids are considered better reaction media for recycling process of composite materials [22]. The main issue to overcome is to transform laboratory-scale solvolysis into a fully functional commercial scale. Nowadays mechanical recycling technique is the most widely used in the industry, also becoming a pre-recycling process for thermal and chemical recycling [3].

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Commercial-scale polymer composite recycling has been used in several industries in France, the USA, Germany, and the UK for the recycling of fibers along with products such as gas and liquid, which can be further used as feedstocks [23–26]. However, it is still a challenge to keep mechanical properties as both virgin carbon and glass fibers. Usually the recycled fibers need a secondary treatment to eliminate char and resin impurities [27]. From the economic point of view, literature states that the energy consumption in fiber recycling using any recycling technique is lower when compared to the energy required to manufacture virgin fibers. The selling price for both recycled carbon and glass fibers with minor detriments in the mechanical properties can be competitive with the expensive virgin fibers which makes highly possible the replacement of virgin fibers with recycled fibers in various applications [28]. Pyrolysis method for recycling composite materials would be the most environmentally favorable option for waste management compared to incineration and landfilling [29]. The same study has to be done to evaluate the other available recycling techniques and compare them against each other [29]. Overall, the use of recycled fibers to replace virgin ones is a sustainable way to manage the cumulating CFRP and GFRP waste [4].

Quality of Recycled Composite Materials In this section we show case study examples related with the use of recycled composites to produce the same or new products for different applications. In the case of the manufacture of the same product, different strategies will be shown to keep properties as pristine composites. The cases of study were selected based on recent developments and their potentiality to be applied in commercial products.

Quality of Products Made with Recycled Thermosetting Matrix/Fiber Composites The manufacture of a product using recycled composite materials based on thermosetting resins with properties comparable with those of virgin composites requires the use of high fiber volume fraction (usually >40%). This can be done by obtaining recycled composites with highly aligned fibers [30]. Based on this strategy, Longana et al. [30] have used the concept of circular economy to produce epoxy/short fiber composites. Previous studies showed results about recycled composites remanufactured with fibers that have undergone only one recycling step. They studied the effect of multiple short carbon fiber recycle loops of the performance of remanufactured epoxy/carbon fiber composites. The fiber recycling process consisted of polymer pyrolysis. They used the high-performance discontinuous fiber (HiPerDiF) method [31] to produce epoxy preforms with highly aligned short carbon fibers. Then, the prepregs were cured by vacuum bag molding in an autoclave. The recycling process by polymer pyrolysis was performed two times repeating, at each one, the remanufacture of the recycled

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composites. They found a significant drop on the stiffness, tensile strength, and deformation at break after the second recycling process. It was attributed to the shortening of the fibers and to residual epoxy resin on the fiber surface loosing matrix/fiber interfacial strength. They concluded that the reclaiming process of the fibers by polymer pyrolysis should be further optimized avoiding fiber degradation. They also suggest the use of a sizing agent to reduce fiber degradation and to consider the hybridization with virgin fibers. The recycling of blades for wind turbines, usually made of thermosetting resin/ glass fiber composites, is nowadays a great challenge due to the increasing demand for the generation of wind energy [32]. Mamanpush et al. have studied the manufacture of composite panels from mechanically grinded blades bound by a thermosetting adhesive. They studied the effect of grinding size, moisture content, and binder level on the mechanical and physical properties of the panels. The second composite generation showed improved water resistance and mechanical properties than their wood-based counterparts. They emphasize the low-cost and eco-friendly characteristics of the grinding process in contrast with chemical recycling techniques. They proposed that the finishing product made by this recycled material may have a variety of applications from floor tiles to plastic road barriers. The nondestructive retrieval of expensive carbon fibers (CF) during the recycling process of high-performance carbon fiber/thermosetting composites is a difficult task [33]. Yuan et al. [34] proposed the development of degradable thermosetting matrices with stable covalent bonds to solve this problem. Their novel developed thermosetting resin was a poly(hexahydrotriazine) (PHT). The properties of this resin were comparable to those of epoxy resins, so they suggest to use it in high-performance carbon fiber composites. PHT/CF cross-ply and unidirectional laminates were manufactured by prepreg preparation and the subsequent compression molding. The properties were compared with those of commercial epoxy/CF advanced composites of Hexcel Corporation with the same fiber structure and content. PHT/CF composites showed the same mechanical performance and improved heat and chemical resistance than its epoxy counterparts. In addition, undamaged CF from PHT composites were successfully recovered after multiple recycling steps. Resin depolymerization with a solution of tetrahydrofuran (THF) and HCl was the recycling method used. PHT monomer could also be recovered to be used for further laminate manufacture. The key issue was the optimization of the wet ability of the depolymerization solution. Yu et al. [35] proposed an easier chemical method to recycle epoxy/CF composites recovering undamaged CF fibers. The process consisted in dissolving the epoxy resin with ethylene glycol (EG) increasing temperature via a transesterification process which involves the exchange of an organic group of an ester with an organic group of an alcohol. When epoxy is immersed in EG at a certain temperature, the hydroxyl group of EG participates in the transesterification process breaking down the long polymer chains into small pieces. The reverse reaction is strongly influenced by the EG content in the epoxy dissolution. If sufficient EG is provided, it starts attacking the epoxy surface and then proceeds as the broken polymer segments leave the solvent-polymer interface. If EG tends to evaporate due to the temperature of the environment, then the repolymerization process can dominate. So, temperature and

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EG content are the key tasks to control the dissolution process. After finding the optimal dissolution conditions, CF are separated from the composite. Then, EG is evaporated and separated from the dissolved polymer solution. Finally, both the recycled CF fabric and resin solution are put in a mold that fit the fabric size and resin repolymerization is carried out. A remanufacture composite using 100% of both recycled CF and resin is obtained applying this procedure. Both recycled CF and composites retained more than 95% of the mechanical properties after four recycling steps in comparison with their fresh counterparts. The relevant characteristics of this recycling method in comparison with traditional chemical ones are as follows: low cost, easy implementation, zero pollution, fully recyclable since it involves simple heating and the use of a solvent, eco-friendly, and easy to manage.

Quality of Products Made with Recycled Thermoplastic Matrix/Fiber Composites Automotive is the main industry for thermoplastic polymer/fiber composites [36]. The commodities for this application are short and long glass fiber (GF)-reinforced polypropylene (PP) and polyamides (PA) [37]. Due to the ability of thermoplastic polymers to be remelted, grinding and reprocessing by extrusion techniques is the most popular recycling technique for these materials [38]. The main problem is fiber breakage as a function of repeated recycling steps which detriment the final mechanical performance of the products [39]. Colucci et al. [40] recycled PP/GF composites from automotive parts by grinding and reprocessing by injection molding. The recycling procedure was performed only once. The recycled materials showed detriments in the mechanical properties which were attributed to three main reasons, (1) shortening of fibers; (2) fiber agglomeration; and (3) poor fiber distribution, all in comparison with the pristine composite. They also showed that polymer molecular weight was not affected concluding that polymer degradation is not the mechanism for loosing properties in thermoplastic polymer composites. Tapper et al. [41] showed the effect of two closed loop recycling procedures on the final properties of PP/CF composites. The recycling process consisted of the preparation of composites by compression molding, grinding, dissolution of PP in xylene, fiber recovery, PP precipitation into powder, and reprocessing by compression molding using the reclaimed fiber and recycled polymer. In contrast with the previous results shown by Colucci et al. [40], Tapper et al. [41] showed improvements in the mechanical properties of the recycled products which were attributed to improved interfacial adhesion due to residual PP in the fiber surface after reclamation. It is concluded that grinding and reprocessing is a simple, economical, and ecofriendly recycling technique, but final properties are significantly affected by fiber length reduction and worsened both distribution and dispersion of the fibers within the polymer matrix. On the other hand, chemical recycling may be conducted to a final product with mechanical performance even improved in comparison with the pristine composite but with the disadvantages of the use of costly and toxic organic solvents and time-consuming processes.

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Table 1 Advantages and disadvantages of recycling methods for wind turbine blades made of thermoplastic composites Recycling method Grinding and injection Molding Pyrolysis

Advantage Simple Mature technology Fiber length maintenance Mature technology

Dissolution/distillation/ extrusion

Fiber length and mechanical property maintenance Recover polymer matrix Fiber length and mechanical property maintenance Recover polymer matrix

Dissolution/ evaporation/extrusion

Fiber length reduction Fiber mechanical property degradation Loose polymer matrix Expensive to separate solvents Volatile solvent required Volatile solvent required

Recent works have dealt with the fabrication of wind turbine blades with thermoplastic fiber composites [42–44]. The main reason is the feasibility of thermoplastic resins to be recycled in contrast with their thermosetting counterparts. Other advantages of using thermoplastic composites for this application are cost savings due to non-heated tooling and shorter manufacturing cycle times [45]. Cousins et al. have developed turbine blades based on thermoplastic polymer/fiber composites. They fabricated a section of the blade by the VARTM infusion method using a liquid thermoplastic resin in the family of methacrylates, a peroxide as initiator, and 50 plies of fiberglass woven roving. They applied four recycling methods to the manufactured part: (1) cutting a section of the part for the fabrication of a skateboard deck by thermoforming; (2) grinding and injection molding of ASTM type IV dog bones; (3) dissolution of matrix in chloroform, fiber recovery, and finally precipitation of the polymer using methanol; and (4) fiber recovery by polymer pyrolysis. Table 1 shows the advantages and disadvantages of each method used. These results are also validated by the results shown by Tapper et al. [41] and Colucci et al. [40].

Legislation for Recycling Waste Composites Increased polymer matrix composite utilization mainly driven by transport and energy industries has led to increasing pressure to resolve composite waste management [46–48]. Consumers, scientific community, industries, and governments are now responsible for applying the circular economy concept for using polymer composite materials. First attempts are often driven by governments creating legislations. One of the largest consumers of composite materials is the automotive industry in Europe. Europe has created the End-of-Life Vehicle (ELV) Directive (European Commission 2000) [55] which is a legislation dealing with the recycling of materials [49]. The ELV Directive states that by 2015, 85% of ELVs will have to be reused or recycled (excluding energy recovery), with only 10% incinerated for energy recovery and only 5% going to landfill [50]. Legislations which have also

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been put to cover other industries using polymer composites in Europe include Waste Electrical and Electronic Equipment (WEEE) Directive (European Commission 2003) [56] and the impending directive on “construction and demolition waste” [51]. In the case of blades for wind turbines, Europe has included a tax for waste disposal in landfill by legislation (Directive 1999/31/EC) [52]. China has a great trouble in resolving waste electrical and electronic devices not only because of domestic generation but also because of illegal import of e-waste. China government has launched laws to control e-waste. A section of this law encourages industries to produce value-added products based on plastic composites using recycled polymers from e-waste [53]. Some regulations are not directly related with recycling of polymer composites but lead to technological developments on this topic. For example, since 2004 both the US Environmental Protection Agency (EPA), Land Disposal Restrictions (LDR), and European regulation (EU Directive 99/31/EC) have imposed constraints on disposal in landfills of organic materials which involves plastic composite materials [54]. These legislations indirectly require recycling solutions for composite materials as an economic strategy to fit the legislation requirements. It is concluded that technological developments about composite recycling techniques are mainly driven by government legislations encouraging industries to apply the circular economy concept to their products.

Conclusions and Further Outlooks Recycling of polymer-based materials is one of the most important approaches for end-of-life waste management of scrap plastic products. Nevertheless, the most remarkable challenges restricting the recycling of polymer-based materials are related to maintain the quality of the material, the high cost of the process, and the effects on the environment. In addition, polymer crosses contamination; added additives, nonpolymeric impurities, and possibility of degradation over time are of real concern. Additional essential challenge when recycling plastic wastes is the absence of compatibility of utmost plastics with other ones that can be overcome by using different processing techniques. The main barriers that avert the polymeric-based materials circularity are technical, financial, and policy ones. Furthermore, the increase on polymer-based material recycling produces a decrease on the rate of composite manufacture directly positively impacting on CO2 emission reduction. More effort should be done in order to increase the polymer-based material circularity.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric Double-Layer Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pseudocapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide-Based Supercapacitor Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aqueous Asymmetric Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Ion Hybrid Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Metal oxide nanoparticles-based supercapacitor devices have received considerable attention for application as energy storage devices due to their environmental friendly and earth-abundant nature. However, many bulk metal oxide-based supercapacitors still remain a big challenging task because of their poor electrical conductivity and need for high-capacitance supercapacitors. To solve this problem, metal oxide nanoparticles have been subjected to change in their morphology and induce their electrical conductivity. Bimetallic oxides exhibit good electrochemical performance due to their electrical conductivity higher than P. Kamaraj (*) Department of Chemistry, Bharath Institute of Higher Education and Research, Chennai, India R. Vennila Department of Chemistry, Adhiyaman Arts and Science for Women (Autonomous), Krishnagiri, India M. Sridharan Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, India P. A. Vivekanand Department of Chemistry, Saveetha Engineering College, Chennai, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_120

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single metal oxide. Herein we discuss the single and bimetallic oxide-based supercapacitor devices and metal oxide hybrid with carbon-based materials like nano-structured carbon, graphene, and graphene oxide and also metal-ion hybrid supercapacitor to further understand energy storage mechanism.

Introduction Environmental concerns are on the energy demand due to the growing population whose usage of the fossil fuel produces global climate change. Therefore, worldwide research focuses on finding environment-friendly and renewable energy sources. Firstly, solar cell and fuel cell are the most important sources ahead of other renewable energy sources. These sources ensure energy security, and handling the energy system has some limitations such as intermittent availability and storage problems [1–3]. To overcome this problem, most of researchers concern themselves with Li-ion batteries and supercapacitors. Battery is the most important energy storage device, but it has some problems like high energy density and requiring large pulse/impulse [4–8]. To solve these problems, metal oxide nanoparticles have been widely used as suitable materials for electrodes of energy storage device, due to their unique properties like high surface area and high theoretical capacitance. However, many bulk metal oxides still remain a big challenging task, because of their poor electrical conductivity and further development is needed for high-capacitance supercapacitors. The fabrication of metal oxide nanoparticles with suitable composition, the synthesis of excellent morphology, their electrical conductivity and surface defects have enhanced the specific capacitance of metal oxide nanoparticles. High surface area and high electrical conductivity, electro-active area, and stability are playing essential roles in metal oxide-based supercapacitor system [9]. Single metal oxide nanoparticle shows lower capacitance. Therefore, single metal oxide nanoparticles are not suitable for practical usage. To address this issue, various modified technologies are used to enhance the specific capacitance of the single metal oxide. Furthermore, the modified technologies afford the enhancement of redox reaction and minimize the charge transfer resistance of electrochemical supercapacitor. The specific capacitance of modified metal oxides is significantly higher than that of single metal oxide nanoparticle and also manages the charging-discharging kinetics. These modified nano-structured metal oxide nanoparticles have high surface area, which can create useful contact between electrode and electrolyte. Also, the ions transportation between active electrodes and electrolyte is enhanced. Recently, the use of carbon-based materials like graphene, carbon nanostructure, and carbon nanofiber composited with metal oxides has resulted in significant improvement in electrical conductivity [10–13]. The creation of oxygen vacancies on metal oxides forms interfacing layer. This layer induces the charge storage capacity and pseudocapacitance of metal oxide nanoparticles. Hence, in this book chapter, we take into account the recent improvement in metal oxide-based supercapacitor in detail and the basic principle of metal oxide supercapacitor devices, metal oxide composite, metal-ion hybrid, and metal oxide supercapacitor.

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Principles of Supercapacitor Supercapacitors can be classified into two types based on their working and energy storage mechanism, namely, electric double-layer capacitor and pseudocapacitor.

Electric Double-Layer Capacitor The first type is electric double-layer capacitor (EDLC) in which electrochemical reactions do not occur. In this type of supercapacitors, electrode/electrolyte interfaces are not involved in charge transfer reaction. Based on the performance of the EDLC and the capacitance of the double layer, its storage mechanism is similar to conventional capacitor [14, 15]. The energy storage mechanism of EDLC involves the movement of ions into the surface of the electrodes. The overall configuration of the EDLC has two electrodes, and each electrode/electrolyte acts as a capacitor; therefore, the overall configuration of EDLC is considered as two capacitors. Carbon-based materials such graphene and carbon nanotube are widely used as electrode material for EDLC, because of their high surface area and high conductivity [16, 17].

Pseudocapacitor Pseudocapacitor is a type of supercapacitor in which the charge storage process of pseudocapacitance is different from double-layer capacitance and its charge storage depends on electron transfer reaction, but EDLC charge storage depends on electrode surface accumulated electrolyte ions. Basically, electrode/electrolyte surface of the pseudocapacitor involves reversible redox reaction. The storage capacity of pseudocapacitor linearly responds to applied potential. The different types of pseudocapacitance are (i) redox pseudocapacitance, (ii) intercalation pseudocapacitance, and (iii) underpotential deposition. Electrochemical adsorption with charge transfer resulted in redox pseudocapacitance. The electrode surface adsorbed metal ions resulted in a monolayer s called underdeposition potential. Intercalation pseudocapacitance occurs by transfer reaction of faradaic charge with intercalated layer of redox active electrode [18, 19].

Metal Oxide-Based Supercapacitor Electrode Aqueous Asymmetric Supercapacitor This type of capacitor has two different electrodes. The electric double-layer absorption/desorption takes place on one electrode, and faradaic reaction takes place on another electrode. Aqueous asymmetric supercapacitor has high ionic conductivity

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and high specific capacitance, but its energy density is not suitable for practical applications. The low-cost and efficient electrode fabrication is a challenging task for supercapacitor application. Metal oxides acted as an anode and cathode material for aqueous asymmetric supercapacitor to result in high energy densities [4, 20]. In recent years, many metal oxides such as V2O5, RuO2, MoO3, Fe2O3, WO3, and MnO2 and intercalated metal oxides such as LiCoO2 and LiMn2O4 have been used as an electrode material for aqueous asymmetric supercapacitor.

Ruthenium Oxide Ruthenium oxide has some important properties like good electrical conductivity, reversible redox ability, high theoretical capacitance and good corrosion resistance in basic or acidic medium. Ruthenium oxide has two phases: (i) RuO2.XH2O (amorphous) and (ii) RuO2 (crystal phase). Hydrated RuO2 has superior electrochemical performance which involves electron hopping between electrode material and current collector, between particles, and within the particles. Buzzanca et al. reported that hydrous RuO2 was used as a pseudocapacitor electrode and acidic electrolyte system [21]. But during the cycling reaction, the agglomeration occurs. To control agglomeration process is problematic for RuO2-based electrode. Cao et al. designed sheet-like graphene and carbon nanotube decorated with RuO2 which effectively controls the agglomeration [22]. Co3O4 Co3O4 material has been used as a cathode material for aqueous asymmetric supercapacitor. In aqueous electrolyte, the electrochemical process is controlled by Co3O4 cathode. It is used to develop high specific theoretical capacitance and boost redox reversibility. Yu et al. studied Co3O4 decorated with N-doped carbon hollow sphere which is used as a cathode material for aqueous asymmetric supercapacitor and also resulted in high energy density [23]. Nickel Oxide Environmental friendly and naturally abundant NiOX is widely used as a supercapacitor. It exists in multiple oxidation states, involves charge storage process, and enhances redox reactions. It has high theoretical specific capacitance and poor electrical conductance, but it shows less stability at charge-discharge process due to its volume expansion that can destroy the active electrode and reduce the electrical conductance [24]. Even without usage or mixing of other materials, bare nickel oxide has high specific capacitance of 1337 F/g. Morphology of NiOx depends on solvent, and it enhances the electrochemical performance. The morphology of NiOx was changed through addition of ethanol into the aqueous solution. Higher volume of ethanol is added to fragment the nanoflake morphology [25]. NiO is grown on carbon cloth and nickel foam substrate results in high electrical conductivity and induce mechanical property. The NiO was grown on Ni foam by hydrothermal method (Fig. 1). It shows specific capacitance value of 674 F/g [26]. NiOX successfully grown on carbon cloth through chemical bath deposition method showed high specific capacitance value of 660 F/g. Both carbon cloth- and Ni foam-grown NiO

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Fig. 1 SEM images of NiO nanosheet

film shows high cyclic stability. After 5000 cycles, 93% of initial capacitance were retained for nickel foam-grown NiO. After 4000 cycles 82% of its starting capacitance was retained [27]. Nanosheet and nanosheet morphologies show no significant changes in electrochemical performance. Nickel-cobalt oxyhydroxide successfully synthesized on carbon nanotube using chemical bath deposition method resulted to have high specific capacitance (940 F/g) compared to bare nickel oxide, bare carbon nanotube, and nickel oxide-decorated carbon nanotube (660 F/g) synthesized by chemical bath deposition method. Nickel-cobalt oxyhydroxide@CNT shows a unique structure. Nickel-cobalt oxyhydroxide nanoflake is well grown on carbon nanotubes. This unique structure densely allows the penetration of electrolyte ions and induces higher specific capacitance [28]. Electrodeposited NiOx on Ni foam resulted in high capacitance of 950 F/g, which controlled the morphology. The exfoliation of NiOx on substrate is done by using graphene. It acted as a binder between Ni foam and NiOx. Without the usage of graphene, electrodeposited single NiOx shows poor performance. The additional materials acted as binder and also controlled the morphology and overgrowth of NiOx. Various methods such as solvothermal, hydrothermal, and electrodeposition method were used to fabricate NiOx-based electrode fabrication [29]. Ni(OH)2 composited with RGO synthesized by solvothermal method resulted in 1886 F/g. It has an interconnected porous structure [30]. Ternary nickel oxide composite like PANI coated on nickel oxide@graphene was successfully synthesized. PANI was uniformly coated on

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nickel oxide@graphene with no significant change in morphology. It exhibited high specific capacitance compared to single nickel oxide and graphene [31]. Binary Ni-based metal oxide (NiCo2O4) was widely used for supercapacitor application, due to its excellent electrochemical performance. NiCo2O4 was prepared by coprecipitation and hydrolysis methods. NiCo2O4 was coupled with graphene oxide by using coprecipitation method, and it resulted in specific capacitance of 1211 F/g. The usage of sodium dodecyl sulfate for synthesis of NiCo2O4/graphene oxide, can help to change the morphology of mesoporous structure to flowerlike morphology [32]. NiCo2O4 coupled with single-walled carbon nanotube (SWCNT) by simple hydrolysis method exhibited 1642 F/g. The water and ethanol ratio play essential role in deciding the morphologies of NiCo2O4@SWCNT as different ratio shows different morphology. The optimal ratio of 1:4 of water and ethanol used to produce the electrode shows better electrochemical performance [33].

Fe2O3 and Fe3O4 Fe-based oxides like Fe2O3 and Fe3O4 are widely used as an electrode material for aqueous asymmetric supercapacitor, due to their high specific capacitance and environment friendliness. But their large volume expansion and lower electrical conductivity hinder the practical application. But it is incorporated with other highly conductive material to overcome the abovementioned problem. Yan et al. fabricated graphene nanosheet decorated with Fe3O4 nanosphere by facile solvothermal method. Graphene/Fe3O4//graphene/MnO2 (Fig. 2) was used as anode material for aqueous asymmetric supercapacitor, resulting in high energy density of 87.6 W h/kg, and after 10,000 cycles, 93.1% of capacitance remained at its initial capacitance [34]. Li et al. designed Fe2O3 nano-needle arrays used as an AASCD, and it demonstrates excellent performance. The resulting power density is 3.5 kW/kg and specific energy density is 103 W h/kg, and after 5000 cycles 86.6% remains from their starting capacitance [35]. VO2 and V2O5 Vanadium is abundant in nature and has high theoretical specific capacitance and high power density. But its poor electrical conductivity and less structural stability hinder vanadium oxide for practical usage of storage devices. Vanadium exists in different forms like VO2, V2O3, and V2O5, but +5 oxidation state is the most stable form and +4 is the worst. V2O5 is the mostly used cathode for AASCD. Carbonbased materials are combined with vanadium oxide to induce the conductivity. Lee et al. designed mixed form of V3+ and V4+ with VO2 acting as an anode for AASCD with working potential of 1 to 0 V versus Ag/AgCl. VO2 anode and Mn3O4 cathode resulted in high power density and energy density, 1.1 kW/kg and 42.7 W h/kg [36]. MnO2 and Mn3O4 Manganese oxides are widely used as a cathode material for AASC. Both MnO2 and Mn3O4 have considerable attention for energy storage application due to their earthabundant nature, low cost, and high operating potential. Zhang et al. fabricated

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Fig. 2 (a) Schematic representation of Fe3O4@graphene nanosheet. (b–e) SEM images of the bare Fe3O4 (b), 30% Fe3O4@graphene (c), 60% Fe3O4@graphene (d), 90% Fe3O4@graphene (e)

β-MnO2 used as an AASC; it shows high energy density of 40.4 W h/kg and high power density of 17.6 kW/kg. Several groups reported the insertion of some metal cations (Na+ and K+) into the MnO2. It enhances the electrochemical performance based on the redox behavior [37]. Xia et al. designed AASC by using Naincorporated MnO2 like Na0.5MnO2 nanowall arrays as a cathode and carbon@Fe3O4 as anode delivering high energy density of 81 W h/kg [38].

CuO CuO is an environmental friendly, earth-abundant, and biocompatible material for energy storage applications. Kaner et al. designed three-dimensional and nano sized wall of CuO frame work and the activated carbon was used as another electode which exhibited an energy density of 19.7 W h/kg and excellent cycling stability [39].

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V. Sharma et al. designed hollow sphere and solid copper oxide by one-step synthesis. The hollow structure morphology shows approximately 70% higher than solid copper oxides. The long-term cycling stability due to high surface area helps to electrolyte ion adsorption-desorption. This resulted in specific capacitance which is much higher than carbon nanotube, graphene, and rGO coupled with copper oxide [40].

Metal-Ion Hybrid Supercapacitor Traditional supercapacitors are not efficient for energy storage devices due to their lower power density and energy density. A metal-ion hybrid supercapacitor plays an essential role in energy storage devices due to its high energy and power density. Metal-ion hybrid supercapacitor is a type of asymmetric supercapacitor device. Metal-ion hybrid supercapacitor consists of two electrodes, one is battery-type faradaic electrode and another one is a type of capacitive electrode [41, 42]. Nowadays, most of the researchers focus on novel design to provide metal-ion hybrid supercapacitor devices.

Li-Ion Hybrid Supercapacitor Most of the researchers focused on Li-ion hybrid supercapacitors, due to their unique properties like high electrical conductivity and energy density. Nanosized and different structural metal oxides, hybrid metal oxide, and carbon-based material hybrid metal oxide are used for metal-ion hybrid supercapacitor [43, 44]. Niobium oxide is incorporated with carbon-based materials to produce significant improvement in electrochemical performance. Chen et al. designed N-doped T-Nb2O5/ tubular carbon (Fig. 3) as an anode; Nb2O5 was uniformly dispersed on the surface of the tubular carbon [45]. This novel hybrid provides porous structure and induces electrical conductance to enhance Li-ion storage. The as-synthesized on Li-ion hybrid supercapacitor based Nb2O5 electrode resulted high energy density of 86.6 W h/kg and power density 6.09 kW/kg. Li et al. constructed SnO2 anchored with mesoporous tubular carbon and achieved high energy density of 110 W h/kg and power density of 2960 W/kg [46]. Zhou et al. reported MnO@graphene composite and hierarchical porous N-doped carbon electrodes provide attractive energy density of 127 W h/kg [47]. In recent years, bimetallic oxides are applied for lithium ion hybrid supercapacitor devices (LIHSCDS). Lei et al. constructed TiNb2O7 coupled with holey graphene (Fig. 4) acted as an anode material for LIHSCDS; this electrode exhibited a high energy density of 86.3 W h/kg at 237.7 W/kg and power density of 3.88 KW/kg at 28.7 W h/kg and longtime cycling stability. After 3000 cycles, 90.2% of their initial capacitance were retained, and it shows electrochemical results and retention capacity of 73.5% [48]. Sodium-Ion Hybrid Supercapacitor Sodium-ion hybrid supercapacitor devices (NIHSDs) are receiving a considerable attention from researchers due to their low cost sodium sources and sodium’s low redox potential (2.71 V) approximately similar to redox potential of Li (3.0 V).

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Fig. 3 Schematic illustration of T-Nb2O5/tubular carbon fabrication

Fig. 4 Schematic illustration of TiNb2O7@graphene fabrication

But NIHSDs have imbalance in the charge storage mechanism of faradaic and non-faradaic cathode. Lee et al. reported that the use of Nb2O5@carbon core shell nanocomposite and Nb2O5@rGO as anode and activated carbon electrode as cathode resulted in high energy density and power density [49]. Fan et al. designed Na2Ti2O7 composite with carbon nanofiber (Fig. 5) used as a high energy density anode material and active carbon cathode-assembled NIHSDs provide high energy and power density [50]. Cathode materials also play essential role in energy storage performance of NIHSCDs. Chen et al. designed NaMnO2 cathode with Na2SO4 electrolyte and potassium manganese hexacyanoferrate which induce stability. This device shows an excellent energy density and better cycling stability. After 1000 cycles, 86.7% of specific capacitance retained their initial capacitance [51].

K-Ion Hybrid SuperCapacitor Goodenough et al. had designed K+-ion-based hybrid supercapacitor which provides an excellent electrochemical performance [52]. Zhang et al. fabricated K2Ti6O13

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Fig. 5 (a) Schematic representation of Na2Ti2O7@activated carbon fiber fabrication and (b) schematic illustration of fabrication of sodium-ion hybrid capacitor

anode and N-doped nanoporous carbon and potassium based electrode design which resulted in a battery of like high energy density and exhibited high power density 7200 W/kg. After 5000 cycles 75.5% retained their initial specific capacitance [53].

Zn-Ion Hybrid Supercapacitor The low cost and environment friendliness of Zn-ion based precursor widely used in energy storage application. Xu et al. designed Zn2+ inserted MnO2 nanorod as a cathode and carbon anode exhibit adsorption/desorption on the ions and this shows an excellent energy storage performance. Mn2+ cation was added into the ZnSO4 to enhance the energy storage performance through anion replacement of SO4 by CF3SO3. The resulting MnO2//2M ZnSO4//AC exhibited excellent capacity and high energy density. After 5000 cycles 93.4% retained their initial capacitance [54].

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Conclusions and Further Outlook An overview of recent improvements in metal oxide-based supercapacitors has been done. Fundamental researches on supercapacitor in the field of energy conversion and storage devices need to be further explored. We may even explore different morphologies of metal oxides synthesized by different synthesis methods, synergistic effect of different materials, and the addition of metal oxides to improve the performance of supercapacitor. However, for further understanding on the charge transfer mechanism, various composites aid in the improvement of electrochemical performance of supercapacitor. The authors hope that this book chapter will inspire more interest of researchers in the improvement and application of metal oxide materials. Also, authors believe that the metal oxide-based materials will facilitate the efficient improvement of these energy conversion and storage devices.

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nH2O in potassium chloride (KCl) aqueous solution, J Solid State Chem 148:81–84 53. Dong S, Li Z, Xing Z, Wu X, Ji X, Zhang X (2018) Novel potassium-ion hybrid capacitor based on an anode of K2 Ti6 O13 microscaffolds, ACS Appl Mater Interfaces 10:15542–15547 54. Ma X, Cheng J, Dong L, Liu W, Mou J, Zhao L, Wang J, Ren D, Wu J, Xu C, Kang F (2019) Multivalent ion storage towards high-performance aqueous zinc-ion hybrid supercapacitors, Energy Storage Mater 20:335–342

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Leticia Melo-Lopez, Christian Javier Cabello-Alvarado, Marlene Lariza Andrade-Guel, Diana Iris Medellín-Banda, Heidi Andrea Fonseca-Florido, and Carlos Alberto A´vila-Orta

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Description of the PLA and Its Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impact of the PLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of the PLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and Processability of PLA to Make Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viscose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cupro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lyocell Process (N-methylmorpholine-N-oxide – NMMO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Celsol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Celulose Carbamate (Carbacell Technology) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyhydroxyalkanoates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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L. Melo-Lopez · C. J. Cabello-Alvarado CONACYT-Centro de Investigación y de innovación del Estado de Tlaxcala (CITLAX), Tlaxcala de Xicoténcatl, Tlaxcala, México Centro de Investigación en Química Aplicada (CIQA), Departamento de Materiales Avanzados, Saltillo, Coahuila, México e-mail: [email protected]; [email protected] M. L. Andrade-Guel · D. I. Medellín-Banda · C. A. Ávila-Orta (*) Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México e-mail: [email protected]; [email protected]; [email protected] H. A. Fonseca-Florido Centro de Investigación en Química Aplicada (CIQA), Boulevard Enrique Reyna 140, Saltillo, México e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_145

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Advantages and Disadvantages of the Use of PHA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile Processing of PHAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic Fe3O4/Graphene Oxide Composite Conductive Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic-Hydroxyapatite Biocompatible Scaffolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic-Polyhydroxyalkanoates Nonwoven for Biodegradable Mulches . . . . . . . . . . . . . . . . . . Cellulose-Noble Metals Fibers with Color Fastness and UV Blocking Properties . . . . . . . . . . . Luminescent Cellulose Fibers for Anti-Counterfeiting Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyhydroxyalkanoate Medical Textiles and Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable and Anti-Flame Alginate Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-Based Nanoparticle Sodium Alginate Fibers for Medical Applications . . . . . . . . . . . . . . Antimicrobial Alginate Wound Dressing Containing Silver/Zinc Nanoparticles . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Man-made fibers or synthetic fibers made from fossil sources possess properties superior over natural fibers such as wool, sisal, cotton, etc., due to their durability and chemical resistance. Nevertheless, the raw materials for the production of synthetic fibers are harmful to the environment or based on unsustainable processes. As a consequence man-made fibers from renewable sources have constantly gained more attention because it has been seen that they can be a good alternative for garments, disposable clothes, even fashion design. And above all, because it is possible to control their life cycle, avoiding that when they are discarded they add to the large amount of persistent waste that reaches landfills and the sea, in addition to that during the generation of the raw materials to fabricate this type of synthetic fibers, there is little or no environmental damage. The raw material that is used to generate textiles from renewable sources is obtained from plants, animals, and microorganisms, which by their very chemical nature are made up of attached atoms by chemical bonds that in certain process conditions can be broken or modified generating the material degradation. That is why it is extremely important to know the properties of these materials from renewable sources, in order to process them in the best way, in addition to being able to improve them.

Introduction To date, the world is constantly searching for new plastic materials or the modification of existing plastic materials so that they have the ability to biodegrade. This need arises as part of the search for the solution to the serious problem of plastic contamination that we are experiencing worldwide, where once the articles fulfill their function, they are discarded either as packaging, damaged clothing, broken plastic parts, etc. Most of these wastes have been manufactured with plastic materials that for many years were viewed favorably as they are extremely durable materials; however, over time that durability has become a problem. That is why now the search for biodegradable materials or biopolymers is constantly increasing. Currently, the generation of any product is being done with greater awareness regarding its final disposition, as well as the processes that involve its manufacture. As a

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consequence, more and more it is sought to generate and improve biodegradable plastics. Among the sources of raw material to generate bioplastics are those from plant sources, from microorganisms, obtained by conventional synthesis process (derived from agro-resources), etc. Below are some of the biopolymers that are most commonly used for the generation of textiles, as well as their characteristics, advantages, and disadvantages of the products generated with them.

Polylactic Acid Polylactic acid (PLA) or polylactide is a sustainable, biodegradable, and recyclable polymer, so its characteristics perfectly fit the current world needs; however, it is not a recently invented material. The chemist Théophile-Jules Pelouze synthesized the PLA in 1845, it was formulated by Bishoff and Walden in 1893 [1], and patented by Dupont in 1932 [2]. In 2005 the company NatureWorks LLC began to manufacture the first corn-based melt-processable natural-based polymer and fiber under the NatureWorks ® PLA and Ingeo ™ fiber brand names, employing technology similar to that used to manufacture polyester type [3]. NatureWorks LLC actually is the world’s largest PLA manufacturing plant because the company has developed a patented, low-cost continuous process for the economic production of PLA polymer for packaging and fiber applications [4].

Chemical Description of the PLA and Its Generation PLA is a thermoprocessable aliphatic polymer with helical structure [5] belonging to the ester family. PLA is industrially obtained through the polymerization of lactic acid (LA) by condensation of an acid with an alcohol, like PET as forming a polyester, or by the ring opening polymerization (ROP) of lactide (the cyclic dimer of lactic acid, as an intermediate) [3, 6, 7]. Lactic acid can be produced by chemical synthesis or by bacterial fermentation [6]. The chemical synthesis produces the racemic 1 to 1 mixture of D- and L-lactic acid [8] while by bacterial fermentation of sugars or polysaccharides from renewable sources, including annually grown crops such as corn, sugar beets, cane molasses, and potato [7, 9, 10] allows the generation of optically pure D- or L-lactic acid when the appropriate microorganism is selected. Almost all lactic acid is today produced by a fermentative route, starting in general from corn-derived dextrin [8]. However, when it is desired to obtain PLA of high molecular weight, it is generally produced by the lactide ring-opening polymerization route, since it is an economically viable process (Fig. 1). There are two kinds of fermentative processes of hexoses and pentoses sugars; the homofermentative route that converts a sugar molecule into two lactic acid molecules with good purity, plus it is a sustainable process thanks to high selectivity. The other type of fermentative process is the heterofermentative process, with which a mixture of lactic acid, carbon dioxide, and ethanol or acetic acid is produced from sugar, making it necessary to purify the monomer. The fermentation processes have

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Fig. 1 Synthesis of PLA from L- and D-lactic acids. (Reprinted from [11] by permission from Springer Nature: Springer Publishing Company. Bio-based Polymers and Nanocomposites. Preparation, Processing, Properties & Performance by Muhammed Lamin Sanyang, Mohammad Jawaid (2019))

the advantage of using cheap renewable raw materials, but it also has the disadvantage of needing very long conversion times and low concentration of active cell, so the productivity of lactic acid is very low [8].

Environmental Impact of the PLA PLA production is sustainability and its environmental impact is much less than the production of petroleum-based polymers. Although the PLA generation needs fossil fuels for processing of the raw material and in producing the polymer, it used 20– 50% less fossil fuel resources, less water, in addition to generating less greenhouse gas quantity [4, 12]. Furthermore, the use of PLA reduces society’s dependency on fossil fuels and plastic waste accumulation [13] because PLA can be reused by chemical recycling, or it can degrade. The use of PLA in the textile industry is an alternative that favors the environment, especially the ocean. The International Union for Conservation of Nature (IUCN) estimated that between 0.8 and 2.5 Mton/year of primary microplastics are released into the ocean, and 34.8% of the releases are generated during the laundry of synthetic textiles [14]. One of the problems that microplastics cause both on the surface and on the seafloor is that they enter the food chain when they are ingested by living beings that are subsequently eaten by their predators, including humans. PLA is a recyclable and biodegradable (compostable) material, but that is not its only benefit, the raw materials from which lactic acid is generated are plants, which during their growth absorb carbon dioxide from the atmosphere and use it during photosynthesis [4]. PLA is a fully compostable polymer unlike other synthetic fibers, it is a material that degrades easily by hydrolysis [10], as long as the process is

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catalyzed by temperature and humidity, together with an attack of microorganisms (bacteria and fungi) that degrade to PLA initially on a macroscopic scale and later on a molecular scale, to form lactic acid. Lactic acid is a compound that many organisms can metabolize and use as an energy source, generating carbon dioxide, water, and some humus as a by-product [15]. The composting process allows the earth’s carbon present in the PLA to complete its cycle during biodegradation, which is why globally it is said that the production of PLA generates less carbon dioxide compared to other petroleum-based fibers. It is worth mentioning that although PLA fibers are compostable, the PLA fabrics are durable enough to meet the various market and supply chain requirements [4]; this kind of material combines ecological advantages with excellent performance in textiles [16]. Chemical recycling of PLA is carried out in a similar way to the PET recycling process, that is, the depolymerization of PLA happens by hydrolysis carried out with water at a range of temperatures 100–250 °C and catalyzed with nitric acid, whereby the waste PLA is back into fully functional lactic acid. The chemical recycling of PLA is economically and environmentally viable, thereby verifying the total sustainability of PLA production [4].

Applications of the PLA Previously PLA uses were limited to biomedical applications (mainly sutures), implant devices, tissue scaffolds, and drug delivery systems due to availability and cost of manufacture. Nowadays new economically viable techniques of production of high molecular weights broadened its uses [2]; therefore currently PLA is used to make fibers as packaging, composites, fabrics with excellent properties and other consumer products [3, 16]. Despite PLA being a biodegradable polymer, its application as a functional biomaterial is limited because the degradation of the polymer chains makes lactic acid, which provides acidity to the medium where it is generated [17]. The reduction of the pH in a region of tissue can generate inflammation, which could become a chronic disease while lactic acid is decreasing the pH of the area and the surrounding tissue; however, it is possible to prevent the decrease in pH by adding to the polymer a biocompatible buffering [18]. Table 1 shows the characteristics of the PLA regarding the processability as textiles and its main properties.

Properties and Processability of PLA to Make Textiles There are parameters to take into account the melt spinning of synthetic biopolymers: molecular weight, moisture sensitivity, crystallization rate of the fibers, high glass transition temperature narrow melting profiles, and low ductility. The processability of PLA is similar to that of petroleum-based synthetic thermoplastic polymers such as polyester [12]; therefore it is possible to process PLA using the same machinery

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Table 1 PLA fiber attributes, applications, and different forms of textiles [4, 16] Attributes Good flame resistance and retardant (Continues to burn for 2 min after flame removed and LOI of 26%) Very low smoke generation (63 m2/kg) Low toxic gas on burning Good moisture management (Moisture regain 0.4–0.6%) Faster drying Low odor retention Good loft High resilience Comfort properties Unique soft hand touch UV stability. Superior strength retention than polyester and far superior resistance to discoloration than acrylics Does not support bacterial growth Processability Excellent wicking Good elastic recovery (5% strain) 93 Tenacity similar to PET fibers (6.0 g/d) Unaffected by dry-cleaning solvents Disadvantage Susceptible to hydrolysis during spinning, drying, and finishing without adequate care Poor resistance to hydrolysis under alkaline conditions that generate structural damage in terms of tensile strength Low melting point (approx 170 °C) results in a low domestic ironing temperature, leading to limitations for end users Remove quickly fiber tenacity diminution with the temperature increasing

Applications Fabrics from sportswear, active, underwear, and fashion wear with very good moisture management Fiberfill for pillows, duvets, mattresses, and comforters Compostable agricultural and geotextiles for soil erosion control and crop protection (nonwovens) Industrial applications: as carpet and furnishings fiber by its stain resistance properties PLA fibers can be used alone or in blends with cellulosic fibers (cotton, lyocell) and wool (such blends are still biodegradable compared to their PET counterparts)

Forms of PLA textiles Staple fiber Monofilament Multifilament Trilobal fiber Bicomponent fiber Knitted structure Woven structure Spun bonded nonwoven Needle punched nonwovens Composite materials

required to generate PET fiber spinning (long- or short-staple fiber) and nonwoven processes, with the advantage of a reduced processing temperature required. PLA pellets can absorb moisture up to a content of 400 ppm and its extrusion and melt spinning is very sensitive to moisture. The PLA chains may experience "depolymerization" by hydrolytic degradation during processing because of the attack of the hydroxyl groups of water on the ester linkages of the PLA chain, reducing the molecular weight. Due to this, it is important to control the humidity and keep it at 100 ppm to avoid degradation problems [4]. Furthermore, similarly for the melt spinning process, PLA fabric generation uses conventional dyeing (disperse dyes), drying, heat setting, softening processes, and finishing machinery employed to PET [4, 16, 19]. However, despite being able to use the same machinery, when processing PLA it is necessary to use at relatively mild processing conditions (pre-treatment, dyeing, and finishing treatments) in comparison to PET fabrics [20, 21].

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It is necessary to mention that after different finishing stages, the difference between handle of PLA and PET fabrics has been significantly reduced after dyeing, drying, heat setting, and softening processes [19]. PLA fibers are commercially available and have acceptable textile properties [22], despite being a material with limitations of processability as low melt strength, narrow processing window, low crystallization, and inherent brittleness [23–25]. However, as consumers are increasingly tending toward alternatives that cause less harm to the environment, PLA fabrics acceptance is likely to continue in a similar way to what happened with Lyocell, despite presenting many technical difficulties in processing [26]. The characteristics of the macromolecules of PLA that are used for the procedure of melt spinning are: average molecular weight (Mn) of 50,000–120,000 g per mole, the polydispersity of 1.2–1.8, melting temperature in an interval of 190–240 °C, and the melt extrusion temperature must be at least 20 °C higher than the melting point of the polymer [27]. Polylactic acid is a slow crystallizable polymer; it is possible to increase the PLA fiber crystallinity by a mechanical alternative. The filaments can be spun at velocities of 4000 m/min and up to 6000 m/min with a drawn ratio of up to 4000, as long as this spun velocity do not affect fiber ductility and elongations at the break because of an excessively biased fiber orientation along the machine direction of the production process. It is also possible to accelerate the crystallization process by adding plasticizers that facilitate the molecular mobility and nucleating agents that accelerate the process [28–30].

Cellulose The polysaccharide cellulose is the most common organic polymer and important skeletal component in plants; it representing about 1.5
1012 tons of the total annual biomass production, is an excellent source of raw material for the increasing demand for ecological products. Cellulose is a homopolymer composed of repeating β-dglucopyranose molecules that act as building blocks, therefore it has characteristics of carbohydrate and polymeric macromolecule, that is, it contains a large number of hydroxy groups, it is highly functionalizable, hydrophobic, chiral, and biodegradable, with broad chemical modifying capacity (Fig. 2). It generates specificity and diverse architectures and it is sensitive toward the hydrolysis and oxidation of the chain-

Fig. 2 Representation of a cellulose molecule. (Reprinted from [34] by permission from Springer Nature: Springer Publishing Company. Cellulose Chemistry and Properties: Fibers, Nanocelluloses and Advanced Materials by Orlando J. Rojas (2016))

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forming acetal groups. All these properties are the result of intermolecular interactions, cross-linking reactions, chain lengths, and chain length distribution by the distribution of functional groups on the repeating units and along the polymer chains [31–33]. When cellulose was first used as a polymer, the wood and pulp processing involved highly polluting methods such as the chemical sulfate (Kraft) and sulfite pulping methods characterized by the emission of dangerous substances (carbon disulfide and hydrogen sulfide). However, from 1980 to the present, biotechnological methods and biological transformation have been introduced into the pulp and paper industries, many of them through the use of selected types of enzymes [35, 36] that perform molecular, supramolecular, and morphological structure modifications of the cellulose [37]. The chain length of cellulose or degree of polymerization (DP) varies with the origin and treatment of the raw material. The wood pulp DP is between 300 and 1700. Cotton and other plant fibers have DP values between 800 and 10,000, and the DP values of bacterial cellulose are similar. Regenerate fibers from cellulose contain 250–500 repeating units per chain [31]. The cellulosic fibers production and manufacture are based on the processing of dissolving pulp and cotton linters [38]. Man-made fibers based on cellulose are produced by any of the following processes: viscose, modal, cupro, lyocell, and acetate. Currently, the cellulose dissolving methods used on an industrial scale are cuprammonium, viscose, and NMMO methods, despite the fact that cuprammonium and viscose methods are energy-intensive and produce effluent streams containing significant amounts of toxic materials. Other cellulosic fibers production methods such as Celsol have also been developed, however, it is only carried out in the pilot stage, and similarly, the Cellulose Carbamate method, which is carried out in semi-industrial stage.

Viscose The viscose method has been produced for more than 100 years. This is an efficient process regarding the consumption of raw materials, and it dominates the production methods. This methodology involves the use of carbon disulfide (CS2) in conjunction with cellulose to convert it to xanthogenate as a metastable intermediate. Is possible to form a viscose solution by dissolving the xanthogenate in aqueous sodium hydroxide in a wet process. Through the precipitation of the xanthogenate solution in a shaped product, the substituent is cleaved off, and high-purity cellulose is regenerated. This method presents several environmental risks; the use of carbon disulfide, heavy metal compounds in the precipitation process and toxic resultant by-products [38].

Modal Compared with viscose, modal operate with the cellulose of higher alpha content (non-soluble in 18% sodium hydroxide) and degree of polymerization (DP), lower cellulose concentration, a higher degree of filtration and chemical additives in the

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spinning bath, and lower spinning speed [39]. Furthermore, unreacted hemicellulose (solid waste into the water) and exhaust air by carbon disulfide are higher in the case of Modal.

Cupro The cupro process is based on the application of direct dissolution systems followed by the production of regenerated cellulose filaments by spinning the solution of cellulose and the complex tetraamminecopper (ii) hydroxide (cuprammonium hydroxide), which is formed from the mixture of copper (ii) hydroxide and aqueous ammonia [40]. The dissolution process is carried out for cuprammonium via weakening the intra/intermolecular hydrogen bonds and complex formation.

Acetate Cellulose secondary acetate fibers are manufactured from cotton linters by steeping in glacial acetic acid and sulfuric acid-catalyzed reaction with acetic anhydride. The reaction is exothermic and the final product in a maximum of 20 h is cellulose triacetate, which is converted to secondary acetate by adding sufficient water. The hydrolysis is stopped when 1/6 of the acetate groups have been randomly changed to hydroxyl groups. The precipitated polymer flakes are dissolved in acetone containing small amounts of water or alcohol [4].

Lyocell Process (N-methylmorpholine-N-oxide – NMMO) The Lyocell process is an alternative more ecological and environmentally friendly than the cuprammonium and viscose processes. The cellulose pulp is directly dissolved in a mixture of water and N-methylmorpholine-N-oxide at high temperature and the cellulose fibers are formed and regenerated by extruding the cellulose/NMMO/water solution into an aqueous precipitation bath. If a decrease of crystallinity and orientation of Lyocell fibers is desired, precipitation must be done in alcoholic baths. On the other hand, to obtain fibers with a firm, highly oriented core and a soft, nonfibrillating shell, it is preferable to produce the fibers through two-stages; precipitation in alcohol and water [31]. Stabilizers that suppress the radical separation of the NMMO and scission of the cellulose chain can be used in the NMMO process [31, 41]. Furthermore, in this process, it is possible to use cellulose of lower purity, that is, with higher hemicellulose content (less expensive cellulose) [42]. One downside is that the solvent is expensive, however, in a closed circle it is almost completely recycled (99%) [31].

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Celsol Celsol is enzymatic biotransformation of pulps into direct soluble cellulose with solubility in an aqueous sodium hydroxide; solution up to 99 wt% [43, 44]. With this method, it is possible to produce regular type regenerated cellulose fibers, reaching a fiber production on a level of 93–95% to the weight of the initial pulp. The regenerated cellulose fibers (Celsol) are spun [45] using the alkaline solution of biotransformed pulp and acidic regeneration bath.

Celulose Carbamate (Carbacell Technology) In the cellulose carbamate method, initially, the cellulose is alkalized and partially degraded, then it is dissolved in sodium hydroxide solution, the solution is filtered and deaerated prior to wet spinning in an acidic precipitation bath. Then the carbamate groups are hydrolyzed at elevated temperature in a salt-containing alkaline decomposition bath. The technical sequence of the cellulose carbamate method is similar to the viscose method but without the use of sulfur-containing hydroxide solution, innocuous urea as a substitute for the toxic CS2 is used. Another advantage of the cellulose carbamate method is that cellulose carbamate can be processed on viscose spinning machines [46].

Polyhydroxyalkanoates Polyhydroxyalkanoates (PHAs) are polymers with a polyester chemical structure that are mostly produced from a wide range of gram-negative and gram-positive bacteria from fermentation processes, accumulating PHAs in the cytoplasm, to act as storage materials inside its cells [47]. A wide variety of prokaryotic organisms accumulate PHAs upto 30–80% of their cellular dry weight [48]. As the name implies, the repeating unit of the PHA polymers contains an alkyllike structure with a pendant group R, which can vary depending on the microorganism or substrate used. If the group R is the same in all the repetitive unities in the polymeric chain, it is a homopolymer. Nevertheless, if the pendant group R differs in the monomeric repetitive units, the material is a copolymer [49], as shown in Fig. 3. Besides, depending on the number of carbon atoms in the monomer units, PHAs are classified into three types: short-chain length PHA (3–5 carbon atoms), mediumchain length PHA (6 to 13 carbon atoms), and long-chain length PHA (14 carbon atoms or more) [49, 50]. The polyhydroxybutyrate (PHB) is the most well-known polymer of the polyhydroxyalkanoates family, where its properties as homopolymer are similar to synthetic plastics, specifically that of polypropylene ((crystallinity ¼ 60 and 50%, melting temperature ¼ 175 °C and 176 °C, respectively. However, it is also produced as a copolymer with hydroxyvalerate, hydroxyoctanoate, hydroxyhexanoate, or hydroxyoctadecanoate [51].

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Fig. 3 Structure of polyhydroxyalkanoates. (Reprinted from [49] by permission from Springer Nature:Springer Publishing Company. Materials Horizons: From Nature to Nanomaterials by Vimal Katiyar, Amit Kumar, Neha Mulchandani (2020))

Advantages and Disadvantages of the Use of PHA This type of polymers is considered as a potential substitute for conventional polymers obtained from fossil sources because, unlike the latter, PHAs can be generated from fully renewable sources, in addition to having properties that allow their use in various areas. One of the most important properties of PHAs is that they are biocompatible materials, so they can be used for applications in the health sector; they are also materials with excellent strength and stiffness in addition to being completely biodegradable [52–55]. PHAs have some disadvantages in addition to the high price of this type of material, among them low melt elasticity; the parts that are generated with these polymers have low-impact resistance, brittleness, and low toughness. The biggest disadvantage related to their thermal stability is that there are very few degrees of difference between its melting temperature and thermal degradation temperature, which hinders its processability [51, 53–55].

Textile Processing of PHAs The difficulties in the processing of PHA are mainly its low viscosity, low melt elasticity, and thermal degradation that causes a decrease in the molecular weight of the polymer, and that usually occurs at temperatures just above the melt temperature. In addition to the above processes, PHAs also tend to exhibit slow crystallization, so

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polymer chains have the opportunity to accommodate enough to form large crystallites. This crystallization phenomenon can negatively affect the processes in which the polymer is subjected to stretching, for example, in flat film extrusion or in the generation of fibers by melt spinning. Specifically, for melt spinning, the slow crystallization after spinning is a disadvantage because the fibers become extremely brittle and the brittleness increases during storing due to physical aging. Among the processes that have been used to generate PHA fibers are melt spinning and gel-spinning mainly. The melt spinning process is a process that can produce a large number of fibers in a short time, in addition to the fact that once the fibers are formed they are ready to be used, that is, no purification process is necessary to remove residual solvent; this is why melt spinning is a desirable process. The melt spinning process has been used to produce PHA fibers, which without proper care, are made up of a phase made up of large spherulitic type crystallites and an amorphous phase, which together results in fibers that lack adequate physical performance. The crystallization process is responsible for the inadequate properties of the PHA fibers, so it is necessary to alter the crystallization process, although not to avoid it, but to make it happen differently. Among the alternatives is the addition of plasticizers, these substances enter between the polymer chains preventing large crystallites from forming; in addition, the incorporation of plasticizers decrease the melting temperature of the polymers by 15–20 °C, although this requires up to 30 wt % the plasticizer [56]. Another option is the addition of nucleating agents to the polymer to increase its crystallization temperature compared to the neat polymer. Among the nucleating agents that have been used are boron nitride, talc, terbium oxide, lanthanum oxide, calcium carbonate, etc., with boron nitride being the most efficient nucleating agents for PHB [57, 58]. Furthermore, it has been seen that the remnants of proteins and lipids from the culture media (contaminants) can also act as efficient nucleating agents [59]. A further alternative to modify the textile physical properties (elongation at break, tensile strength, and Young’s modulus) of the PHA fibers is by modifying the type of crystals that form in them by applying mechanical stresses to the fibers while simultaneously they are kept at a specific temperature. The PHA fibers’ physical properties depend on the type of crystals present, be they spherulite or fibril-like. After the PHA is spinning, the fibers are drawing, and if the drawn ratio was low, many large orthorhombic crystals (α modification) or spherulites form at the surface of melt-spun fibers, causing the fibers to have poor textile physical properties because the fibers are brittle and had no significant elongation at break and tensile strength. But if the fibers still hot are drawing at high drawing rate, and annealing under tension, their surface will be smooth because of the perfect fibrillary structure formed in the core and in the surface, and this effect increases with a rising tension load during annealing. The fibril-like crystal structure (hexagonal crystal or β modification) results in good textile mechanical properties like high tenacity, high modulus, and an acceptable elongation at break [60– 62]. Inclusively, in some cases, the drawn materials exhibit a rubber-like elastic behavior, indicating that drawing leads to changes similar to Cold Rolling [60].

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The gel-spinning process has an advantage over the melt-spinning process, which is that in the first the processing temperature is lower than the one in the melt spinning process, which could reduce the thermal degradation of PHA [63]. For example, starting with a polymer of original molecular weight of 550,000, it dropped down to 175,000 while employing the melt spinning process;while employing the gel-spinning process, it is expected that the original molecular weight will drop down to around 330,000. The gel-spinning process consists of obtaining a gel by dissolving the polymer in a solvent, trying to obtain a concentration as high as possible, for example, for a PHB of MW about 300,000 g mol–1, the 1,2-dichloromethane is recommended as the best solvent and the concentration is about 20 wt% for PHB. The gel is extrudable at about 170 °C to obtain the fibers with tensile strength about double that of melt-spun material of similar parameters [64].

Alginate Alginate is a polyanion obtained from brown seaweed and identified since 1880 by Stanford, which can be used to generate textiles that have the advantage of being biocompatible, of low toxicity, and whose raw material has low cost [65]. Alginate is a water-soluble polysaccharide composed by units of β-D-mannuronic acid (M) and 1–4 linked α-L-guluronic acid (G) linked together as G-block regions and M-block regions [66] (Fig. 4). The physical and mechanical properties of the alginate fibers are dependent on the length and molecular weight, however, for commercial applications, the highviscosity alginate consists of chains of 150,000 molecular weight [65]. The procedure commonly used to generate alginate fibers is by the wet spinning of a sodium alginate solution, similar to those used for spinning regenerated cellulose fibers. The alginate is obtained from the brown seaweed, for this the dried and crushed algae undergo dissolution and precipitation treatments by varying the pH to basic and acidic values, thereby eliminating unwanted components, including cellulose. This process is possible because the alginate is a substance sensitive to changes in pH, where its viscosity in the solution can be altered depending on its pH. For example, alginate in solution with a certain molecular weight will precipitate at pH 3 or less but will remain in the solution if the pH is kept between 5 and 10 [67]. During the extraction process, sodium hypochlorite is also used to remove the remaining color and to sterilize the aligning solution. To generate the alginate fibers, it is necessary to have the pure sodium alginate, with which a viscous solution is generated that is spun by the wet spinning method into a bath in which the coagulation of the fibers is carried out. The coagulation process occurs due to the presence of polyvalent cation salts (Ca2+ being the most commonly used ion) that are exchanged with sodium ion from the alginate or when it comes into contact with the bath composed of a mixture of inorganic acid (hydrochloric acid), oil, and a cationic surface agent, where the hydrogen ions from the solution are exchanged with sodium ion from the sodium alginate, which is converted to solid alginic acid.

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Fig. 4 The conformation of monomers and blocks distribution of alginate salt

The alginate fibers obtained by this method can be wound onto bobbins or be used to generate nonwoven fabrics [68]. Divalent cations such as Calcium Ca2+ are usually used because they give rise to polymers of reduced solubility in water, as long as the molecular weight and the concentration of the alginate is not very high because otherwise, the presence of the polyvalent ion will generate crosslinks and in turn gels. These crosslinks are the result of the association of a polyvalent ion with two or more carboxylic groups that are in different alginate chains so that their mobility is reduced forming the gels. The formation of alginate fibers is the result of taking advantage of the difference in solubility of the alginate chains according to the counterion that is associated with their carboxylic groups, but also with the extrusion process, that is, if a sodium alginate solution is poured into a coagulation bath without being extruded, a crosslinked gel will be generated with a large amount of water trapped inside [4]. As with other polymer solutions, the viscosity of a sodium alginate solution is affected by temperature, where increasing the temperature of the solution decreases the viscosity of the solution. However, if the temperature increase is drastic, a chemical change of the polymer chains can be generated (depolymerization of the chains), which would also result in a decrease in the viscosity of the solution, but due to the generation of polymer chains of smaller size [67]. Because the solubility of alginates is affected by various factors such as pH variation, it has sought to obtain crosslinking of a diverse nature. Among the covalent crosslinks is that formed by using formaldehyde, which reacts with hydroxyl groups to form cross-chain crosslinks, which in turn reduces the solubility of alginates in water. Coordination and ionic crosslinks can also be present when beryllium and chromium metallic ions are used; with these ions a greater number of coordination bonds are generated, which are more stable, resulting in an increase in the toughness of the fibers formed with the chromium and beryllium alginates, as

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well as a decrease in their extensibility. This happens because when these ions are present, the chelation of two or more carboxylic groups is generated with a single metal ion, thereby reducing the mobility of the polymer chains and their solubility. By using this type of metallic alginates to produce textile fibers, materials resistant to washing cycles and basic pH are generated. Coordination and ionic crosslinking are responsible for the formation of the fibers once the sodium alginate comes into contact with the coagulation bath that has these ions, however, this same ionic crosslinking limits the possibility of achieving an effective stretching compared to stretching achieved by subjecting cellulose chains to a similar procedure [69]. There are also ionic crosslinks and crosslinks by coordination of alginates with a metal ion. In this type of crosslinking, chelation occurs between two or more carboxylic groups with a metal ion, reducing the mobility of the polymer chains and their solubility. This type of crosslinking is formed when the sodium alginate solution is extruded and comes into contact with the coagulation bath that has the metal ions. The type of metal ion used in the coagulation bath is very important, the most used ion is Ca2+ because the fibers formed with this ion can be stretched, generating high crystallinity as a consequence of the high orientation of the polymer chains in the fibers. When other ions are used, such as beryllium and chromium, the fibers that are formed are highly stable in their physical and chemical properties, making them resistant to washing cycles and basic pH. However, these fibers have high toughness and low extensibility, which limits a stretching that is effective compared to the stretching achieved by subjecting the cellulose chains to a similar procedure. Furthermore, when beryllium is used, the resulting alginate is brittle and toxic, while when the chromium ion is used, its commercial application of the fibers is conditioned because they have a green coloration [69]. Alginate fibers are an option when looking to obtain textile fibers from renewable resources and that which are also biodegradable by factors such as light, air composition, as well as humidity together with the action of microorganisms (fungi and bacteria), but not by superior beings such as mammals, because their digestive system does not have the capacity to degrade the polymeric chains of alginate [70]. Alginate fibers can be used in various areas, especially health care because their properties are totally appropriate for this area, they are biocompatible, hemostatic, highly absorbent, nontoxic, nonallergenic, noncarcinogenic. This type of fiber allows moisture control in wounds because it is highly absorbent and it is possible to incorporate drugs in this type of material. The technology regarding the types of dressings used to heal chronic wounds has changed to favor and accelerate epithelization, granulation, and tissue healing. Unlike conventional dressings, alginate dressings, in addition to serving as a barrier against pathogens that could infect the wound, can also absorb or provide moisture to the wound. When alginate dressings are in the dry state, they have the ability to absorb moisture, toxins, and substances secreted by the wound, but in cases where the wound is dry and should be in a wet state, the dressings of alginate gels can provide the necessary moisture [71].

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Below are examples of functional fibers made with polymers generated from renewable monomers. In them, it can be seen that by incorporating additives and/or appropriate chemical modifications of the materials, textiles with extraordinary and novel properties can be obtained.

Polylactic Fe3O4/Graphene Oxide Composite Conductive Fiber The purpose of the invention is to generate a new type of functional fiber based on polylactic acid electrically conductive. Polylactic acid as a raw material is biocompatible and biodegradable, however, it is a poor electrical conductor, this deficiency can be modified by incorporating to the PLA micro- or nanofillers to improve its conductivity. Generally, as the conductive phase of the polymeric composite increases, the nanocomposite becomes more conductive, however, the incorporation of additives tends to negatively affect the mechanical properties of the material, making its processability difficult. One way to obtain a conductive material based on PLA with the ability to be spun is through the use of small amounts of additives that, despite these low percentages, provide the conductive property to the polymeric matrix. An example of this type of additive is the Fe3O4/graphene oxide nanocomposite with a spherical structure; this nanocomposite is prepared through a sequence of steps. The first step consists of the oxidation of the FeCl3 salt to form Fe3O4 spheres whose surface is modified with various types of amines that provide a positive charge to its surface. These modified spheres are added to a dispersion of negatively charged graphene oxide to obtain Fe3O4/graphene oxide composite material that is subsequently put in contact with a solution of PLA in dichloromethane to prepare the spinning solution. To prepare the PLA composite conductive fibers with good conductivity, wet spinning or electrostatic spinning has been used. There are several advantages to using PLA related to it, the most relevant is that the PLA is a low energy consumption polymer and it is an easy processability polymer. Conductive PLA fibers can be used in energy applications, optoelectronic devices, information, sensors, molecular wires, and molecular devices, as well as electromagnetic shielding and metal anti-corrosion applications [72].

Polylactic-Hydroxyapatite Biocompatible Scaffolds An application in which the properties of PLA can be exploited is in tissue engineering structures. It has been seen that electrospun nanocomposite scaffolds based on PLA-hydroxyapatite nanocomposite maintain their biodegradability characteristic, but also, compared to the scaffold of raw PLA, nanocomposite scaffolds present greater biological compatibility. This improvement is due to the fact that the hydroxyapatite particles promote better cell adhesion and proliferation, so it is convenient that high concentrations of hydroxyapatite are added to the nanocomposites. The inclusion of up to 6% of hydroxyapatite in a PLA matrix is possible while maintaining the mechanical properties of the nanocomposite fibers, whose

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stiffness and Young’s moduli ranging values are similar to the values of the raw PLA fibers, possibly because the hydroxyapatite particles act reinforcing the polymeric matrix [73].

Polylactic-Polyhydroxyalkanoates Nonwoven for Biodegradable Mulches Polyhydroxyalkanoates are of great interest due to their processing properties (thermoplastics), by their obtaining source, but above all for their biodegradability capacity. The biodegradability of PHAs makes them optimal materials for applications where it is required that once they have fulfilled their function, they can be degraded by the action of microorganisms and environmental conditions in which they are found. For example, for agricultural applications, conventional mulches are made of polyethylene due to the low cost of this polymer, however, once they have fulfilled their function (prevention of weeds, evaporative loss of soil moisture, and soil erosion), they become an environmental problem. An alternative to solve this problem is the use of biodegradable polymers such as PLA and PHA to manufacture the mulches, where the use of raw PLA or a mixture of both polymers allows generating nonwoven mulches with different biodegradation rates. This difference in biodegradability allows the materials to be used in different applications, for example, a spunbond PLA mulch can be used in for long-term agricultural applications (row covers or landscape fabrics) due to its low degradation and good mechanical stability for more than 7 months, even if they have been in direct contact with the soil. In contrast, the meltblown-PLA+PHA nonwoven is useful as a potentially biodegradable agricultural mulch; its rapid degradation is directly associated with the presence of PHA in the polymeric mixture since this polymer is more susceptible to attack by soil microorganisms [74].

Cellulose-Noble Metals Fibers with Color Fastness and UV Blocking Properties An innovative way to obtain sustainable, innovative, and permanent colored man-made cellulose fibers (MMCFs) with Au and/or Ag nanoparticles is described below. In this invention the nanoparticles are embedded and homogenously distributed in the polymeric matrix, thereby reducing their release into the environment. The spin dope employed was prepared by dissolving in an ionic liquid the bleached birch pre-hydrolyzed kraft pulp covered with noble metal nanoparticles, and then spinning the mixture to generate the staple fibers. The nanoparticle coated pulp was obtained by reacting kraft pulp with an acidic auric salt (HAuCl4) and/or a silver salt (AgNO3) to generate the nanoparticles over the surface of the pulp. The nanocomposite cellulose staple fibers formed by the dry-jet wet-spun technique show breaking tenacities and elongation values similar to those of the fibers formed without metallic nanoparticles, including the properties of the nanocomposite

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cellulose staple fibers that are comparable to commercial lyocell fibers. These characteristics are possible because the fiber spinning process is not harmed by the presence of nanoparticles, since they are present in minimal quantities, as a consequence of negligible quantities of noble metallic salts. However, the low amounts of metallic salts give rise to bright color fibers with a good wash and color fastness and promising UV blocking properties. Its characteristics and low-cost production make cellulose fibers with incorporated gold and silver nanoparticles commercially competitive [75].

Luminescent Cellulose Fibers for Anti-Counterfeiting Articles The Lyocell process has recently been used to generate cellulose fibers modified with luminescent inorganic nanoparticles doped with lanthanide ions (LaF3: Ce3+, Gd3+, Eu3+, CeF3: Tb3+, CePO4: Tb3+) by incorporating the nanoparticles into the polymer matrix of the fibers. This way of incorporating the nanoparticles provides advantages compared to the finishing processes that only generate surface fiber modification. When the nanoparticles are inside the polymeric matrix, the properties of the fibers are wash-proof, ensuring long-lasting use. The obtaining process of the fibers begins with the preparation of the environmental friendly spinning dope, which consists of an aqueous solution of cellulose, nanoparticles doped with lanthanide ions, and N-methyl morpholine-N-oxide (NMMO). With the spinning dope, the fibers were generated using the wet-dry spinning method employing a solidifying water bath. The fibers were spun, then washed in hot water, and finally dried. The resulting fibers presented excellent characteristics as bright, multicolor, efficient, and narrow emission bands upon UV irradiation and long luminescent lifetimes, these being optimal characteristics to generate endowed fabrics with a singular luminescent “code.” These fibers could be introduced in small quantities into textiles and paper to produce special anti-counterfeiting clothing like official uniforms and to provide protection to official documents. In addition to the excellent functionality of the fibers, they also showed good mechanical properties because of the absence of agglomerates of nanoparticles, and a good distribution of them across the entire volume of the fibers [76].

Polyhydroxyalkanoate Medical Textiles and Fibers In the practice of the medical sector, there exists a need for different types of absorbable textiles such as patches, surgical meshes, fibers, and monofilament fiber with prolonged strength retention. This type of textiles and fiber-based medical devices can be derived from poly-4-hydroxybutyrate and its copolymers. Poly-4-hydroxybutyrate is a polymer that belongs to the polyhydroxyalkanoates (PHAs) polyesters. In general, PHAs are of commercial interest because of their thermoplastic properties and relative ease of production by a fermentation process and at a competitive cost. The disadvantage of this type of polymer is its low molecular

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weight, however, the poly-4-hydroxybutyrate and its copolymers have enough characteristics to generate absorbable textile and fiber-based medical devices. The absorbable surgical meshes could be used as an absorbable hernia mesh with prolonged strength retention, capable of providing a mechanically stable closure, while reducing the incidence of adhesions (fistula formation), pain, risks of infection, restriction of physical capabilities, minimal inflammatory response, minimal risks of disease transmission, reduced risks of infection, or other post-operative problems resulting from absorption and eventual elimination of the device, leaving a healthy natural tissue structure. All these benefits are desirable when the patients treated are people with diabetes, obesity, nutritional impairment, compromised immune systems, or other conditions that compromise wound healing. The absorbable surgical meshes are also suitable for use in pediatric patients, where the nonabsorbable synthetic meshes are not ideal because they could hinder growth. Another important application of this type of absorbable suture material is in pericardial repair to prevent adhesions between the sternum and heart following open-heart surgery. There are also similar needs to prevent adhesions in spinal and gynecology procedures. The devices may additionally be combined with autologous, allogenic, and/or xenogenic tissues to provide implants with improved mechanical, biological, and handling properties [77].

Biodegradable and Anti-Flame Alginate Fibers This example shows the use of alginate fibers to generate products for use as insulation or flame-retardant materials in industry and construction purposes that are degradable so that their application does not become a problem for the environment in the future or is associated with several problems like skin irritation, as happens with conventional inorganic materials such rockwool and glass fibers. The invention consists of a composite consisting of inorganic nanoparticles contained within a network of a natural polymer formed by means of inorganic interactions. The preferable polymer is of the alginate type, but other types of polymer can be used, such as starches, celluloses, chitosans, insulins, pectins, caseins, whey proteins, gelatines, mixtures of these, or even derivatives as acetylated starch and carboxymethylated cellulose. These polymers often contain acetyl, alkyl carboxyl, or aryl carboxyl ester groups, while the inorganic nanoparticles may have been derivated from montmorillonite, smectite-like clay material, kaolinite, hydroxyapatite, or other biocompatible calcium phosphates, a hydrotalcite or a metal oxide. The external ions of the nanoparticles must be exchanged with a modifying agent, such as ammonium, phosphonium, or sulfonium ions to make compatible the external layer of the nanoparticles, for example of the clay with the natural polymer. The properties of the resulting composite are: excellent isolation and flame retardant, do not produce toxic fumes during thermal degradation, they are easy to make, their production is less energy-intensive when compared to conventional materials, are biodegradable, flexible, and maintain their strength in addition to not causing skin irritation (user-friendly).

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As a result of these excellent properties, these types of nanocomposites are suitable to manufacture materials like fibers, coatings, films, and packaging materials. They are useful in isolation and/or flame-retardant materials. These fibers can be used not only in the building industry, but also to manufacture medical articles wherein the inorganic nanoparticle is derivated from hydroxyapatite or another biocompatible calcium phosphate (for instance, for sutures). From the fibers obtained with such a solution spinning process, various types of articles can subsequently be made, such as building elements for isolation and/or flame retardancy purposes [78].

Carbon-Based Nanoparticle Sodium Alginate Fibers for Medical Applications The fibers that will be described next belongs to the field of preparation of marine biomass composite fibers In them a synergistic effect was observed between the one-dimensional carbon nanotubes and the two-dimensional carbon material (graphene oxide) that originates from the multifunctional high- performance carbon-based nanoparticles/sodium alginate composite fibers. The composite fibers are formed from an aqueous solution that contains sodium alginate, graphene oxide, and carbon nanotubes. The carbon nanotube/graphene oxide/sodium alginate solution is then spun (by wet spinning, dry spinning, or electrospinning), and the nascent spinning fibers are subjected to a preheating bath, followed by drawing, setting, and oiling steps. When a coagulation bath is employed, it contains calcium chloride and divalent salt including calcium chloride, zinc chloride, and barium chloride. The produced carbon nanotube/graphene oxide/sodium alginate fibers have uniform nanoparticle dispersion, the formation of a network structure, and the carbon nanoparticles are in an effective orientation the favors their mechanical properties. The resulting fibers have high tensile strength, excellent toughness, good electrical conductivity, increased resistance to degradation, high anti-radiation, and adsorption performance. The carbon-based nanoparticle sodium alginate composite fibers can be used to adsorb heavy metal ions and dyes in aqueous solutions. However, due to its special properties and functions, it can be used in health care and medical treatment or military industry, in the field of materials and environment [79].

Antimicrobial Alginate Wound Dressing Containing Silver/Zinc Nanoparticles The alginate wound dressing containing silver/zinc nanoparticles can promote wound healing, tissue repair, and regeneration and absorb wound exudate. The composite silver-zinc antibacterial material works synergistically with the alginate, which is beneficial to slow release the bactericidal factor for a long time, maintaining

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the long-term effective bactericidal concentration. Silver and zinc have the effect of stopping bleeding and promoting wound healing. The wound dressing is prepared by needling or spunlacing the generated fibers from the solution of alginate (sodium alginate, potassium alginate, ammonium alginate, propylene glycol alginate, and/or calcium alginate) that contain the antibacterial additive (silver and zinc nanoparticles) and inert nanocarrier (calcium carbonate, hydrotalcite, titanium dioxide, bentonite, diatomaceous earth, micronized silica gel, and kaolin). The antibacterial alginate-silver-zinc fibers are formed by the wet spinning technique; they are coagulated in a bath containing metal ion aqueous solution or alcohol solution, the fibers are then pulled (stretched), washed with water, rolled, and dried under hot air. The alginate wound dressing containing silver/zinc nanoparticles has good stability, low toxicity, does not cause irritation, no sensitization, has good biocompatibility, high moisture absorption, is easy to remove, has high oxygen permeability, as well as biodegradability and biocompatibility characteristics. The antibacterial and sterilization efficiency is high and the treatment effect is good. The alginate antibacterial dressing has antibacterial, anti-inflammatory, absorbing, and repairing properties, which promotes wound healing, maintaining moisture, so it is ideal to relieve pain. This alginate dressing has a simple manufacturing process at low cost, it is suitable for industrial production, it is used in the field of wound healing, and it has a wide range of applications. Silver-loaded dressings existing on the market generally contain the silver attached to fiber products by dipping, coating, spraying, and adsorption. In this preparation method, the antibacterial alginate fiber dressing is formed from a mixture of the composite silver-zinc antibacterial agent with the alginate to form a spinning dope, which is then spin processed. The alginate antibacterial fiber can not only be employed directly as a dressing, but it can also be used to make other antibacterial fiber products such as antibacterial surgical thread, antibacterial clothes, hats, and socks [80].

Conclusions and Further Outlook Synthetic textiles have a wide field of application in any of its presentations (woven, nonwoven, thread, reinforcement, etc.). Textiles are present in practically all areas, their use is unlimited due to its excellent chemical and mechanical performance. However, these characteristics are a double-edged sword, because once textiles are discarded at the end of their useful life, it will remain almost unchanged for years to even centuries. Or if the textile begins to degrade, the deterioration gives rise to microplastics, which is a problem for all living beings. Hence the importance of tending more and more to the generation and use of polymers from renewable sources, which produce the least amount of pollutants or toxic substances during their generation, and which once their application or utility has been completed, they have the ability to biodegrade.

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Generating materials with good performance and resistance, at the same time biodegradable is a difficult task. To achieve this, it is of utmost importance to know the properties of polymers, especially those properties that interfere with the processability or durability of the pieces or textiles that are manufactured with them, in order to create new ways to improve them, taking into account the processes by which is carried out its degradation mechanisms.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How to Prepare the Electrochemical Sensors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Sensing of Hazardous Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Catechol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Hydrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of 2-Phenylphenol and Chlorophenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Hydrogen Peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Environmental pollution is a big threat for the world. There are various inorganic/ organic pollutants which derailed out from the different industries and have toxic nature. Many compounds such as toluene, hydrazine, nitrite, hydrogen peroxide, resorcinol, 4-chlorophenol, hydroquinone, catechol, phenol, nitro-phenol, and nitrobenzene are widely used in the industries and these compounds are showed hazardous effects on humans, animals, as well as environments. Some of these compounds even in trace amount may harm the human beings. Therefore, the K. Ahmad (*) Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] W. Raza Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_195

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determinations of such compounds are of great importance. Previously various approaches have been made to detect these organic/inorganic compounds. Various research groups have employed different detection techniques which showed good results. Recently, electrochemical approach attracted much attention of the researchers due to its excellent sensitivity, good detection limit, reproducibility, repeatability, simple fabrication procedure, low cost, and high selectivity. In this chapter, fabrication and advantages of electrochemical sensor for the sensing of different analytes have been discussed. Moreover, the role of newly designed and different electrode modifiers for the fabrication of electrochemical sensors has also been discussed.

Introduction In present time environmental pollution is a great threat for the whole world [1–9]. Environmental pollution rapidly increasing, and various factors are responsible for this enhanced environment pollution. There are various organic, inorganic, or other toxic compounds which possess hazardous impact on animals as well as humans and environment [8–25]. Phenol family is toxic in nature and widely used in various applications. Catechol has been used in chemical, textiles, oil refinements, plastic industries, and agricultural fields [26]. Similarly, hydroquinone which is also isomer of catechol has been applied in various applications such as paints, cosmetics, antioxidants, pesticides, oil industries, photography and pharmaceuticals etc. [26]. There is also another form of phenol derivative known as resorcinol (1,2-benzenediol) has been used in food, dye, and pharmaceutical industries. These phenolic compounds such as catechol and resorcinol have hazardous impact on the environment as well as plants, animals, and human beings. Another form of phenol family is 2-phenylphenol and 4-chlorophenol [1, 5] which also has toxic nature. These phenolic compounds are also used as disinfectant in nursing homes, households, fungicides, food processing plants, hospitals, barbershops, industries, and laundries [5]. These phenolic compounds are also hazardous to the skin, eyes, and responsible for other health issues. Nitrite is another source of nitrogen for green plants and known as intermediate byproduct in the nitrogen cycles. Nitrite is also present in soil, water, and environment. Nitrite is also employed in food industries as preservative. Although nitrite does not cause harmful effects in moderate concentrations but at higher concentrations it may interact with hemoglobin to produce methemoglobin which inhibited the hemoglobin to transport the oxygen throughout the human body and can cause tissue hypoxia [27]. Nitrite may also interact with amide, secondary amines, and tertiary amines to produce nitrosamines which are a carcinogenic compound. Thus, detection of nitrite is important for human as well as environmental concerns. Another compound urea is an organic compound which is used as fertilizer and converted to ammonia and polluted the environment. Urea is also present in protein metabolism and its presence in high concentration in the blood or urine may cause urinary tract obstruction, dehydration, shock, burn, kidney

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damage, and gastrointestinal bleeding. However, its presence in low concentration may also cause nephritic syndrome, cachexia, and hepatic failure. Urea is also used in milk and its higher concentration in the milk can causes ulcers, indigestion, acidity, etc. Therefore, the detection of urea is important for its use in dairy products, clinical diagnostics, fertilizer plants, or environment monitoring. Hydrazine is also an unstable and highly toxic compound and widely used in rocket fuels, duel cells, chemical reactions, catalyst, and other applications [7]. The long-term contact with hydrazine may have carcinogenic effects due to containing of neurotoxin. Hydrogen peroxide (H2O2) plays crucial role physiological processes. H2O2 is an analyte used in clinics, drugs, chemical, and food industry [4]. H2O2 also has negative impacts on the environments and human beings. So, the detection of H2O2 is also a necessary task. The nitro group containing aromatic compounds such as nitrophenol and picric acid (2,4,6 trinitrophenol) are highly toxic and explosive in nature [15, 17]. Picric acid is used in pharmaceutical, dye, and leather industries [15]. It has toxic nature and has hazardous effects on the environment, plants, and human beings due to its biotoxicity. Thus, the above-discussed compounds are highly toxic and have hazardous effects. Therefore, the determinations of such compounds are important task. In previous reports, various approaches have been used to the determination of such toxic compounds. The conventional methods such as high-performance liquid chromatography, spectrophotometry, quartz crystal microbalance, spectrofluorometry, surface plasmon resonance, electrophoresis, and flow injection chemiluminescence have been employed for the determination of toxic compounds [9, 28]. In last few years, electrochemical methods have attracted the scientific community for the sensing of toxic and hazardous compounds due to its simple fabrication, cost effectiveness, high sensitivity, selectivity, and repeatability [29–31]. Hence, in this chapter, we have discussed the recent advances in the sensing of different toxic and hazardous compound employing electrochemical methods.

How to Prepare the Electrochemical Sensors? The fabrication of the electrochemical sensors is a simple task. Generally a glassy carbon electrode (GCE) or screen printed electrode (SPE) has been widely used as electrode substrate. The working area of the screen printed or glassy carbon electrode cleaned with alumina slurry and the electrode modifier has to be deposited on to the cleaned active area of the working electrode substrate. Further, this modified electrode to be dry in air for several hours and further used as working electrode. In general, screen-printed electrode is used as working electrode whereas two other electrodes (reference electrode ¼ Ag/AgCl and counter electrode ¼ platinum wire) also used in the three electrode assembly for the determination of the toxic compounds using electrochemical approach. The schematic illustration of the fabrication of the electrochemical sensor using screen-printed electrode substrate has been presented in Scheme 1.

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Scheme 1 Schematic illustration of the fabrication of the electrochemical sensor and three electrode system

Electrochemical Sensing of Hazardous Compounds There are numerous toxic and hazardous compounds which are harmful to the humans, plants, animals, and environment. Herein, we have summarized the recent advances in the field of electrochemical sensing of different toxic compound using electrochemical methods.

Sensing of Catechol Catechol is a derivative of the phenol and is highly toxic in nature. Nazari et al. prepared an electrochemical sensor for the sensing of catechol using ZnO-Al2O3 ceramic nanofibers electrode modifier while glassy carbon electrode was used as working substrate [32]. The prepared ZnO-Al2O3 ceramic nanofiber was checked by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Energy-dispersive X-ray spectroscopy (EDX). The surface was calculated using Brunauer–Emmett–Teller (BET) investigations. Further electrochemical investigation were carried out in presence of 5 mM K3Fe (CN)6 in 0.1 M KCl. The cyclic voltammograms (CVs) of the AuNP/GO/chit/GCE (a), ZnO/Al2O3/GO/chit/GCE (b), and AuNP/ZnO/Al2O3/GO/chit/GCE (c) in 5 mM K3Fe(CN)6 in 0.1 M KCl have been displayed in Fig. 1. The recorded CV curves showed the high current activity for the AuNP/ZnO/Al2O3/GO/chit/GCE compare to the AuNP/GO/chit/GCE or ZnO/Al2O3/GO/chit/GCE. Further, the differential pulse voltammetry (DPV) curves of the bare GCE (a), GO/chit/GCE (b), AuNP/GO/chit/ GCE (c), and AuNP/ZnO/Al2O3/GO/chit/GCE (d) in catechol (1 mM) have been displayed in Fig. 2a. The higher current was appeared for the AuNP/ZnO/Al2O3/GO/ chit/GCE in 1 mM catechol. Further the DPV curves in presence of 50 μM catechol were also recorded at different pH (Fig. 2b). The recorded DPV graphs showed the shifting in the potential with change in the pH values.

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Fig. 1 CV curve of the AuNP/GO/chit/GCE (a), ZnO/Al2O3/GO/chit/GCE (b), and AuNP/ZnO/ Al2O3/GO/chit/GCE (c) in 5 mM K3Fe(CN)6 in 0.1 M KCl. (Adopted with permission [32])

Fig. 2 DPV (a) of the bare GCE (a), GO/chit/GCE (b), AuNP/GO/chit/GCE (c), and AuNP/ZnO/ Al2O3/GO/chit/GCE (d) in catechol (1 mM) and effect of pH (b) on the DPV curves in 50 μM catechol. (Adopted with permission [32])

The obtained results showed good electrochemical activity of the AuNP/ZnO/ Al2O3/GO/chit/GCE towards determination of catechol. The detection limit of 3.1 μM was obtained for catechol sensing using AuNP/ZnO/Al2O3/GO/chit/GCE. In another report, Liu et al. employed F, N-doped carbon dots/laccase composite for the sensing of catechol [33]. The authors of this work have prepared F, N-doped carbon dots decorated laccase using benign approach. The formation of the F, Ndoped carbon dots/laccase composite was confirmed by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectroscopy, and photoluminescence (PL) spectroscopy which clearly showing the formation of F, N-doped carbon dots/laccase composite.

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Fig. 3 XPS of the F, N-doped carbon dots: Survey scan (a), high resolution C1s (b), N1s (c), and F1s (d). (Adopted with permission [33])

The X-ray photoelectron spectroscopy (XPS) measurements were also carried out for the F, N-doped carbon dots. The recorded XPS curve of the F, N-doped carbon dots has been displayed in Fig. 3. The full survey scan of the F, N-doped carbon dots has been shown in Fig. 3a whereas the high resolution scan of the C1s, N1s, and F1s have been depicted in Fig. 3c–d. The observations confirmed the presence of F and N elements in the carbon dots and this suggested the formation of F, N-doped carbon dots. Furthermore, electrochemical was fabricated, and electrochemical detection of catechol was investigated using cyclic voltammetry. The CV curves of the F, N-CDs/lac/GCE in presence of catechol (1 mM) at different scan were recorded. The obtained CV results have been displayed in Fig. 4a whereas the linear calibration plot has been depicted in Fig. 4b. The obtained results revealed good electrochemical activity of the F, N-CDs/lac/GCE and the electrochemical current response increases linearly. The excellent detection limit of 0.014 μM was obtained using F, N-CDs/lac/GCE. The fabricated F, NCDs/lac/GCE also exhibited high sensitivity of 219.17 μM μAcm2 mM1 towards the sensing of catechol.

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Fig. 4 CV (a) curves of F,N-CDs/lac/GCE in presence of catechol (1 mM) at different scan rates (10-250 mV/s) and linear calibration plot (b). (Adopted with permission [33])

Fig. 5 XRD patterns of CNT and Cu-CNT. (Adopted with permission [34])

Sensing of Hydroquinone Hydroquinone is another isomer form of catechol and derivative of phenol compound which is also toxic in nature and widely used in cosmetics. The accurate detection is necessary to avoid the carcinogenic effect of the hydroquinone. Yao et al. prepared a composite of copper nanoparticles (CuNPs)/multi-walled carbon nanotubes (MWCNTs) using microwave method [34]. The confirmation of the formation of the CuNPs/MWCNTs composite was carried out by X-ray diffraction (XRD) and SEM analysis (Fig. 5).

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The recorded XRD pattern of the CNT showed the diffraction planes of (002) and (100) while the XRD pattern of the Cu-CNT exhibited the diffraction planes of (002), (111), (200), and (220) which confirmed the formation of CNT and Cu-CNT composite. Further, SEM images were also recorded to observe the morphological features of the CNT and Cu-CNT composite. The electrochemical sensor was prepared using GCE as working electrode. Since, electrochemical impedance spectroscopy (EIS) is an important tool to know the electrochemical activity of the electrode materials. Thus, Nyquist plot of the bare GCE, MWCNTs, and Cu-MWCNTs in 5 mM [Fe(CN)6]3/4in 0.1 M KCl were recorded and presented in Fig. 6. The equivalent circuit for the plotted EIS result has been presented in inset of Fig. 6. The EIS results revealed that Cu-MWCNTs modified electrode has higher electrochemical activity compare to the other two electrodes. This may be due to the synergistic effects between MWCNTs and CuNPs. Furthermore, the chitosan (Chi) was also used in the fabrication of electrodes for the sensing of hydroquinone using cyclic voltammetry. The CV curves of the bare (a) GCE, (b) MWCNTs@Chi/GCE, and (c) CuMWCNTs@Chi/GCE in the presence of hydroquinone have been displayed in Fig. 7. The CV pattern of the bare GCE showed least current whereas Cu-MWCNTs@Chi/ GCE exhibited the highest current towards the sensing of hydroquinone using cyclic voltammetry. This higher current activity or electrochemical behavior of the CuMWCNTs@Chi/GCE may be due to the synergistic interactions. The lowest detection limit of 0.04 μM was obtained with excellent reproducibility. The prepared CuMWCNTs@Chi/GCE was also employed for real sample analysis and obtained results showed satisfactory performance for practical applications.

Fig. 6 Nyquist plots of the bare GCE, MWCNTs, and Cu-MWCNTs in 5 mM [Fe(CN)6]3/4- in 0.1 M KCl. Inset: equivalent circuit. (Adopted with permission [34])

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Fig. 7 CV curves of the bare (a) GCE, (b) MWCNTs@Chi/GCE, and (c) Cu-MWCNTs@Chi/ GCE in the presence of hydroquinone. (Adopted with permission [34])

Sensing of Hydrazine Hydrazine is a hazardous compound and the determination of hydrazine is an important tool. Therefore, Beduk et al. have designed and fabricated a novel sensor for the detection of hydrazine [35]. In this work, authors developed inkjetprinted paper using poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) ¼ PEDOT:PSS decorated ZnO with nafion matrix sensor for the sensing of hydrazine. The obtained results showed increased current response with increasing concentration of hydrazine. The schematic diagram of the developed sensor has been displayed in Fig. 8a while the digital picture of the sensor has been displayed in Fig. 8b. Further CV investigations were carried out in different concentrations of hydrazine in 0.1 M PBS (Fig. 8c). The fabricated sensor showed good electrochemical activity with improved hydrazine detection. The current was enhanced with increased concentration of the hydrazine. The sensitivity and stability of the developed sensor was also improved with the decoration of the PEDOT:PSS by ZnO particles. The layer by layer deposition method was applied for the fabrication of the electrode which was characterized by XRD, SEM, and atomic force microscopy (AFM) techniques. The developed sensor exhibited the lower detection limit of 5 μM with a wide linear range (10 to 500 μM). This printed electrode (sensor) was also applied for real sample analysis using mineral, sea, and tap water for further explore in the practical applications.

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Fig. 8 Schematic of the sensor (a), digital image of the developed sensor (b), and CV curves of PEDOT:PSS/ZnO/nafion in 0, 0.1, 0.5, 1, 5 mM of hydrazine in 0.1 M phosphate buffer solution (PBS). pH ¼ 7.4. (Adopted with permission [35])

Sensing of 2-Phenylphenol and Chlorophenol Karimi-Maleh et al. developed the electrochemical sensor for the sensing of 2-phenylphenol (water pollutant) in presence of 4-chlorophenol using voltammetric measurements [36]. Karimi-Maleh et al. synthesized Fe3O4 nanoparticles decorated n-hexyl-3methylimidazolium hexafluorophosphate composite. The n-hexyl-3-methylimidazolium hexafluorophosphate is denoted with HMPF6 whereas carbon paste electrode denoted with CPE. The CPE was modified with Fe3O4-NPs/HMPF6 denoted as Fe3O4-NPs/HMPF6/CPE. This modified Fe3O4-NPs/HMPF6/CPE was further employed for the sensing of 2-phenylphenol using voltammetry investigations. The fabricated sensor showed descent electrochemical performance. In another work, Zhu et al. [37] also demonstrated the role of electrochemical sensor for the detection of 4-chlorophenol. A novel composite was prepared composed of hydroxylated carbon nanotubes (CNTs-OH) decorated with platinum nanoparticles (PtNPs)/rhodamine B (RhB) composite. The schematic illustration for the fabrication of electrode modifier CNTs-OH/PtNP/RhB has been displayed in Scheme 2. Further the electrochemical sensor with electrode modifier (CNTs-OH/PtNP/ RhB) was developed and its electrochemical activity was checked in a redox electrolyte solution (5 mM K3[Fe(CN)6] in 0.1 M KCl). The CV curves of GCE (a), CNTs-OH/GCE (b), CNTs-OH/RhB/GCE (c), CNTs-OH/PtNPs/GCE (d), and CNTs-OH/PtNP/RhB/GCE in 5 mM K3[Fe(CN)6] in 0.1 M KCl at scan rate ¼ 50 mV/s have been shown in Fig. 9. The electrode modified with CNTs-OH/PtNP/RhB showed the highest electrochemical activity compare to the other four electrodes (GCE, CNTs-OH/GCE, CNTs-OH/RhB/GCE, and CNTs-OH/PtNPs/GCE). This improved electrochemical activity attributed to the synergistic effects. Furthermore, the prepared electrodes GCE (a), CNTs-OH/GCE (b), CNTs-OH/ RhB/GCE (c), CNTs-OH/PtNPs/GCE (d), and CNTs-OH/PtNP/RhB/GCE (e) was explore to the sensing of 4-chlorophenol and 2,4,6-trichlorophenol. The recorded

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Scheme 2 Schematic illustration of the synthesis of CNTs-OH/PtNP/RhB. (Adopted with permission [37])

Fig. 9 CV curves of GCE (a), CNTs-OH/GCE (b), CNTs-OH/RhB/GCE (c), CNTs-OH/PtNPs/ GCE (d), and CNTs-OH/PtNP/RhB/GCE in 5 mM K3[Fe(CN)6] in 0.1 M KCl at scan rate ¼ 50 mV/s. (Adopted with permission [37])

DPV curve of GCE (a), CNTs-OH/GCE (b), CNTs-OH/RhB/GCE (c), CNTs-OH/ PtNPs/GCE (d), and CNTs-OH/PtNPs/RhB/GCE (e) in 50 μM 2,4,6 TCP and 100 μM 4-CP in 0.1 M PBS (pH ¼ 6.0) have been displayed in Fig. 10. The CNTs-OH/PtNPs/RhB/GCE exhibited higher current response compare to the other four electrodes (GCE, CNTs-OH/GCE, CNTs-OH/RhB/GCE, and CNTsOH/PtNPs/GCE). However, GCE showed the least current response which is due to the poor and bare surface area of the electrode. The developed sensor showed the detection limit of 3.69 μM for 4-CP whereas 1.55 μM for 2,4,6-TCP, respectively.

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Fig. 10 DPV curve of GCE (a), CNTs-OH/GCE (b), CNTs-OH/RhB/GCE (c), CNTs-OH/PtNPs/ GCE (d), and CNTs-OH/PtNPs/RhB/GCE (e) in 50 μM 2,4,6 TCP and 100 μM 4-CP in 0.1 M PBS (pH ¼ 6.0). (Adopted with permission [37])

This electrode CNTs-OH/PtNPs/RhB/GCE also showed potential for real sample analysis. Thus it can be said the proposed electrode CNTs-OH/PtNPs/RhB/GCE possess excellent electrochemical activity and can be further employed in the detection of other hazardous compounds.

Sensing of Hydrogen Peroxide Hydrogen peroxide (H2O2) has an important role in physiological processes. The determination of H2O2 is an important task [4]. Thus, Hang et al. developed the hierarchical graphene/nanorods-based hydrogen peroxide sensor applying electrochemical method [38]. The developed sensor showed reasonable detection limit and sensitivity. In other work, Dang et al. have prepared a novel electrode material to prepare the electrochemical electrode for the detection of H2O2 [39]. The author has synthesized copper (Cu) metal-organic-framework (Cu-MOF) decorated ammoniated Au nanoparticles (AuNPs-NH2). The schematic representation for the preparation of AuNPs-NH2/Cu-MOF has been illustrated in Scheme 3. The XRD pattern of simulated and experimental of Cu-MOF, standard XRD of AuNPs and experimental XRD of AuNPs-NH2/Cu-MOF (a) and FTIR spectra of Cu-MOF and AuNPs-NH2/Cu-MOF (b) have been presented in Fig. 11. The simulated and experimental XRD patterns of the Cu-MOF were well-matched which suggested the successful formation of the Cu-MOF in bulk. Further, the standard XRD of the AuNPs was also compared with the experimental XRD data of the

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Scheme 3 Schematic synthesis process of the AuNPs-NH2/Cu-MOF. (Adopted with permission [39])

Fig. 11 XRD pattern of simulated and experimental of Cu-MOF, standard XRD of AuNPs and experimental XRD of AuNPs-NH2/Cu-MOF (a). FTIR spectra of Cu-MOF and AuNPs-NH2/CuMOF (b). (Adopted with permission [39])

AuNPs (Fig. 11a). The obtained results were well matched which also confirmed the successful synthesis of AuNPs. The XRD of AuNPs-NH2/Cu-MOF showed the diffraction peaks corresponded to the Cu-MOF and AuNPs. This suggested the formation of XRD of AuNPs-NH2/Cu-MOF composite. The FTIR of the Cu-MOF and AuNPs-NH2/Cu-MOF also showed the vibration bands corresponded to the OHC¼O, C¼O, C¼C, and C-H groups. However, the absorption band at 1650 cm1 was attributed to the NH2 groups (Fig. 11b). Furthermore, electrochemical sensor was developed using AuNPs-NH2/Cu-MOF as electrode material. The amperometric current responses were recorded to evaluate the performance of the electrochemical sensors. For comparison, Cu-MOF was also deposited on the working substrate and its electrochemical performance was also checked using amperometric measurements. The amperometric curves of the CuMOF and AuNPs-NH2/Cu-MOF in 0.5 mM H2O2 in 0.1 M PBS at pH ¼ 7.4 and amperometric curve of the AuNPs-NH2/Cu-MOF with different concentration of H2O2 have been presented in Fig. 12a–b. The observation showed that the lower current was appeared for the Cu-MOF modified electrode whereas the higher current response was obtained for the AuNPs-NH2/Cu-MOF modified electrode (Fig. 12a). Furthermore, authors also recorded the amperometric curve using AuNPs-NH2/CuMOF electrode with successive addition of H2O2. The current enhanced with spike of the H2O2 as shown in Fig. 12b.

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Fig. 12 Amperometric curves of the Cu-MOF and AuNPs-NH2/Cu-MOF in 0.5 mM H2O2 in 0.1 M PBS at pH ¼ 7.4 (a) and amperometric curve of the AuNPs-NH2/Cu-MOF with different concentration of H2O2 (b). (Adopted with permission [39])

The modified electrode with AuNPs-NH2/Cu-MOF showed the wide linear range from 5 μM to 850 μM. The detection limit using AuNPs-NH2/Cu-MOF modified electrode was calculated to be 1.2 μM. The obtained results were really impressive and could be further employed for practical applications.

Sensing of Nitrite Nitrite ions NO2 which is also known as preservative are usually used in the drink and food industries. The NO2 may discharge to the environment and has hazardous effects on human health and ecosystem. The detection of NO2 is important and many approaches have been used to detect the NO2 ions. Han et al. prepared a novel composite (rose-like AuNPs/MoS2/graphene) using hydrothermal method [40]. The formation of the rose-like AuNPs/MoS2/graphene composite was checked using XRD, XPS, and TEM measurements. The schematic illustration of the preparation of rose-like AuNPs/MoS2/graphene has been shown in Scheme 4. The TEM results showed that MoS2 possess flower-like surface morphology. The survey scan XPS spectrum (a), C1s (b), O1s (c), Mo3d (d), S2p (e), and Au4f (f) XPS spectrum of the rose-like AuNPs/MoS2/graphene have been displayed in Fig. 13a–f. The XPS data clearly showed the presence of C1s, O1s, Mo3d, S2p, and Au4f which indicated the formation of AuNPs/MoS2/graphene composite. Further, the electrodes were prepared using different electrode modifier to investigate the effects of the materials. The CV curves of the bare GCE, GN/GCE, MoS2NF/GCE, and AuNPs/MoS2/GN/GCE in 1 mM NaNO2 in 0.1 M PBS were recorded and the obtained results have been depicted in Fig. 14a. The observations revealed that bare GCE has poor electrochemical activity while the AuNPs/MoS2/GN/GCE possesses excellent electrochemical activity.

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Scheme 4 Schematic representation of the synthesis of rose-like AuNPs/MoS2/graphene and working of modified electrode towards the sensing of nitrite. (Adopted with permission [40])

Fig. 13 XPS full scan spectrum (a), C1s (b), O1s (c), Mo3d (d), S2p (e), and Au4f (f) XPS spectrum of the rose-like AuNPs/MoS2/graphene. (Adopted with permission [40])

The highest current response was obtained for the AuNPs/MoS2/GN/GCE in 1 mM NaNO2 in 0.1 M PBS at pH ¼ 4. Further the effect of different scan rates was also investigated in 1 mM NaNO2 in 0.1 M PBS. The recorded CV graphs have been inserted in Fig. 14b. The obtained results showed that the current enhanced in a linear way with increase in the scan rate. This suggested the diffusion controlled process for the sensing of nitrite. The detection limit of 1 μM was obtained with wide linear range using AuNPs/MoS2/GN/GCE. The obtained detection limit for the sensing of nitrite was quite interesting and showed the potential use of AuNPs/MoS2/GN/GCE as a suitable electrode material for electrochemical sensing applications.

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Fig. 14 CV curves of the bare GCE, GN/GCE, MoS2NF/GCE, and AuNPs/MoS2/GN/GCE in 1 mM NaNO2 in 0.1 M PBS at scan rate ¼ 50 mV/s: pH ¼ 4.0 (a). CV curves of the AuNPs/MoS2/ GN/GCE in 1 mM NaNO2 at different scan rates (b). (Adopted with permission [40])

Conclusions and Further Outlook There are so many toxic and hazardous compounds which are frequently used in various industries, cosmetics, and also used as preservatives. These compounds like catechol, hydroquinone, hydrazine, hydrogen peroxide, nitrite, chlorophenol etc. has negative impacts on the human beings and environment including animals and plants. There were conventional approaches to detect these toxic compounds but electrochemical has shown excellent performance with good detection limit and sensitivity. However, there are few challenges for the electrochemical approaches for the sensing of such hazardous compounds. Since the performance of the electrochemical detecting devices depends on the working substrate and electrode modifier, a highly efficient sensor needs to be developed. The electrochemical sensitivity and detection limit can also be improved by applying new electrode materials such as highly conducting metal oxides, twodimensional materials such MXene decorated metal oxides or polymer decorated metal oxides. Moreover, some new working electrode substrate consists of highly conducting materials are desirable for the construction of the sensors for electrochemical sensing of toxic or hazardous compounds.

References 1. Ahmad K, Mobin SM (2019) High surface area 3D-MgO flowers as the modifier for the working electrode for efficient detection of 4-chlorophenol. Nanoscale Adv 1:719–727 2. Senesac L, Thundat TG (2008) Nanosensors for trace explosive detection. Mater Today 11:28– 36 3. Colton RJ, Russell JN (2003) Making the world a safer place. Science 299:1324–1325 4. Ahmad K, Mobin SM (2019) Synthesis of MgO microstructures for Congo red dye adsorption and peroxide sensing applications. J Environ Chem Eng 7:103347

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5. Li J, Kuang D, Feng Y, Zhang F, Xu Z, Liu M (2012) A graphene oxide-based electrochemical sensor for sensitive determination of 4-nitrophenol. J Hazard Mater 201-22:250–259 6. Sinhamahapatra A, Bhattacharjya D, Yu J-S (2015) Green fabrication of 3-dimensional flowershaped zinc glycerolate and ZnO microstructures for p-nitrophenol sensing. RSC Adv 5:37721– 37728 7. Ahmad K, Mohammad A, Rajak R, Mobin SM (2016) Construction of TiO2 nanosheets modified glassy carbon electrode (GCE/TiO2) for the detection of hydrazine. Mater Res Express 3(074005):1–13 8. Tang L, Feng H, Cheng J, Li J (2010) Uniform and rich-wrinkled electrophoretic deposited graphene film: a robust electrochemical platform for TNT sensing. Chem Commun 46:5882– 5884 9. Singh S (2007) Sensors-an effective approach for the detection of explosives. J Hazard Mater 144:15–28 10. Ahmad K, Mobin SM (2019) Construction of PANI/ITO electrode for electrochemical sensing applications. Mater Res Express 6:085508 11. Ahmad K, Mohammad A, Ansari SN, Mobin SM (2018) Construction of graphene oxide sheets based modified glassy carbon electrode (GO/GCE) for the highly sensitive detection of nitrobenzene. Mater Res Express 5:078005 12. Giribabu K, Oh SY, Suresh R, Kumar SP, Manigandan R, Munusamy S, Gnanamoorthy G, Kim JY, Huh YS, Narayanan V (2016) Sensing of picric acid with a glassy carbon electrode modified with CuS nanoparticles deposited on nitrogen-doped reduced graphene oxide. Microchim Acta 183:2421–2430 13. Wu J, Wang Q, Umar A, Sun S, Huang L, Wang J, Gao Y (2014) Highly sensitive p-nitrophenol chemical sensor based on crystalline α-MnO2 nanotubes. New J Chem 38:4420–4426 14. Chen TW, Sheng ZH, Wang K, Wang FB, Xia XH (2011) Determination of explosives using electrochemically reduced graphene. Chem Asian J 6:1210–1216 15. Ahmad K, Mohammad A, Mobin SM (2017) Hydrothermally grown α-MnO2 nanorods as highly efficient low cost counter-electrode material for dye-sensitized solar cells and electrochemical sensing applications. Electrochim Acta 252:549–557 16. Huang J, Wang L, Shi C, Dai Y, Gu C, Liu J (2014) Selective detection of picric acid using functionalized reduced graphene oxide sensor device. Sensors Actuators B Chem 196:567–573 17. Ahmad K, Mohammad A, Mathur P, Mobin SM (2016) Preparation of SrTiO3 perovskite decorated rGO and electrochemical detection of nitroaromatics. Electrochim Acta 215:435–446 18. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA (2004) Electric field effect in atomically thin carbon films. Science 306:666–669 19. Xu Y, Wang Y, Ding Y, Luo L, Liu X, Zhang Y (2013) Determination of p-nitrophenol on carbon paste electrode modified with a nanoscaled compound oxide Mg(Ni)FeO. J Appl Electrochem 43:679–687 20. DelMar RM, Rodríguez IN, Palacios-Santander JM, Cubillana-Aguilera LM, Hidalgo-Hidalgode-Cisneros JL (2005) Study of the responses of a sonogel-carbon electrode towards phenolic compounds. Electroanalysis 17:806–814 21. Ndlovu T, Arotiba OA, Krause RW, Mamba BB (2010) Electrochemical detection of onitrophenol on a poly(propyleneimine)-gold nanocomposite modified glassy carbon electrode. Int J Electrochem Sci 5:1179–1186 22. Chu L, Han L, Zhang X (2011) Electrochemical simultaneous determination of nitrophenol isomers at nano-gold modified glassy carbon electrode. J Appl Electrochem 41:687–694 23. Abaker M, Dar GN, Umar A, Zaidi SA, Ibrahim AA, Baskoutas S, Hajry A (2012) CuO nanocubes based highly-sensitive 4-nitrophenol chemical sensor. Sci Adv Mater 4:893–900 24. Yang C (2004) Electrochemical determination of 4-nitrophenol using a single-wall carbon nanotube film-coated glassy carbon electrode. Microchim Acta 148:87–92 25. Huang W, Yang C, Zhang S (2003) Simultaneous determination of 2-nitrophenol and 4-nitrophenol based on the multi-wall carbon nanotubes Nafion-modified electrode. Anal Bioanal Chem 375:703–707

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26. Ahmad K, Kumar P, Mobin SM (2020) A highly sensitive and selective hydroquinone sensor based on a newly designed N-rGO/SrZrO3 composite. Nanoscale Adv 2:502–511 27. Zhou Y, Ma M, He H, Cai Z, Gao N, He C, Chang G, Wang X, He Y (2019) Highly sensitive nitrite sensor based on AuNPs/RGO nanocomposites modified graphene electrochemical transistors. Biosens Bioelectron 146:111751 28. Ahmad K, Kumar P, Mobin SM (2019) Hydrothermally grown SnO2 flowers as efficient electrode modifier for simultaneous detection of catechol and hydroquinone. J Electrochem Soc 166:B1577–B1584 29. Mohammad A, Ahmad K, Qureshi A, Tauqeer M, Mobin SM (2018) Zinc oxide-graphitic carbon nitride nanohybrid as an efficient electrochemical sensor and photocatalyst. Sensors Actuators B Chem 277:467–476 30. Raza W, Ahmad K (2018) A highly selective Fe@ZnO modified disposable screen printed electrode based non-enzymatic glucose sensor (SPE/Fe@ZnO). Mater Lett 212:231–234 31. Inamuddin, Ahmad K, Naushad M (2014) Optimization of glassy carbon electrode based graphene/ferritin/glucose oxidase bioanode for biofuel cell applications. Int J Hydrog Energy 39:7417–7421 32. Nazari M, Kashanian S, Moradipour P, Maleki N (2018) A novel fabrication of sensor using ZnO-Al2O3 ceramic nanofibers to simultaneously detect catechol and hydroquinone. J Electroanal Chem 812:122–131 33. Liu L, Anwar S, Ding H, Xu M, Yin Q, Xiao Y, Yang X, Yan M, Bi H (2019) Electrochemical sensor based on F,N-doped carbon dots decorated laccase for detection of catechol. J Electroanal Chem 840:84–92 34. Yao Y, Liu Y, Yang Z (2016) A novel electrochemical sensor based on glassy carbon electrode modified with Cu-MWCNTs nanocomposite for determination of hydroquinone. Anal Methods 8:2568–2575 35. Bedük T, Bihar E, Surya SG, Robles ANC, Inal S, Salama KN (2020) A paper-based inkjetprinted PEDOT:PSS/ ZnO sol-gel hydrazine sensor. Sensors Actuators B Chem 306:127539 36. Karimi-Maleh H, Fakude CT, Mabuba N, Peleyeju GM, Arotiba OA (2019) The determination of 2-phenylphenol in the presence of 4-chlorophenol using nano-Fe3O4/ionic liquid paste electrode as an electrochemical sensor. J Colloid Interface Sci 554:603–610 37. Zhu X, Zhang K, Wang D, Zhang D, Yuan X, Qu J (2018) Electrochemical sensor based on hydroxylated carbon nanotubes/platinum nanoparticles/rhodamine B composite for simultaneous determination of 2,4,6-trichlorophenol and 4-chlorophenol. J Electroanal Chem 810:199–206 38. Hang T, Xiao S, Yang C, Li X, Guo C, He G, Li B, Yang C, Chen H, Liu F, Deng S, Zhang Y, Xie X (2019) Hierarchical graphene/nanorods-based H2O2 electrochemical sensor with selfcleaning and anti-biofouling properties. Sensors Actuators B Chem 289:15–23 39. Dang W, Sun Y, Jiao H, Xu L, Lin M (2020) AuNPs-NH2/Cu-MOF modified glassy carbon electrode as enzyme-free electrochemical sensor detecting H2O2. J Electroanal Chem 856:113592 40. Han Y, Zhang R, Dong C, Cheng F, Guo Y (2019) Sensitive electrochemical sensor for nitrite ions based on rose-like AuNPs/MoS2/graphene composite. Biosens Bioelectron 142:111529

Multi-junction Polymer Solar Cells Recent Trends and Challenges

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Khursheed Ahmad and Qazi Mohd Suhail

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials for Multi-Junction Polymer Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Device Structures of Multi-Junction Polymer Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inverted Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-Passivating Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parallel Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advances in Multi-Junction/Tandem Polymer Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

There is a need of neat and clean energy sources particularly comparable with fossil fuels. Solar energy is a neat and clean form of energy which is available in huge amount. Solar energy can be directly transformed to the electrical energy using photovoltaic device. These photovoltaic devices also called solar cells which are based on photoelectric effect. Previously different solar cells have been developed like dye sensitized solar cells, organic solar cells, quantum dot solar cells, perovskite solar cells, and polymer solar cells. In last few years, K. Ahmad (*) Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] Q. M. Suhail Saiyyid Hamid Senior Secondary School (Boys), Aligarh Muslim University Aligarh, Aligarh, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_196

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polymer solar cells attracted more attention because of their light weight, excellent mechanical flexibility, and cost effectiveness. Polymer solar cells have shown good power conversion efficiency of 7.9% and showed the potential of polymers in solar cell applications. There is a need of unique design and new device structures with novel materials to further improve the performance of the polymer solar cells. This will be helpful to make this technology for practical applications. The polymer multi-junction solar cells comprised of two or more subcells in a series or parallel connections. This reduces the current loss in the single-junction polymer solar cells. In this chapter, we have discussed the recent advances and challenges in polymer tandem/multi-junction solar cells.

Introduction Researchers are curious and highly interested to find out the different forms of energy [1– 3]. Particularly renewable energy source is one of the desirable forms of neat and clean energy [4–7]. Sunlight has been compared with other forms of the energy sources and conventional fossil fuel and found to be most significant and safe energy source. Sunlight directly converted to the electricity using electrical devices known as solar cells [8–13]. The utilization of the small amount of the sunlight can provide the sufficient energy to fulfill the requirement of the energy around the world. It is unfortunate that only 0.1% of the energy supply from the sunlight consumed by the whole world due to the lack of efficient and cheap solar cells devices [2]. The practically used silicon solar cells need sophisticated processing, complex engineering, and high quality silicon to develop the solar cells [2]. The silicon solar cells also suffer from the poor mechanical flexibility. Previously, solar cells with different materials have been fabricated to resolve the above discussed issues remains in the silicon solar cells. In previous years, polymer has been used in various optoelectronic applications especially in photovoltaic applications. Polymer-based solar cells also called polymer solar cells and showed various advantages like good mechanical flexibility and low weight [14–30]. Moreover, polymer solar cells can be cost effective due to the use of low-cost polymers compare to the silicon-based solar cells. In 1992, a polymer solar cell has attracted attention when Sariciftci et al. [14] find out the electron transfer (photo-induced electron) from a polymer semiconducting material (conjugated polymer) to the electron acceptor (C60). Thus, it was believed that conducting polymer played important role in polymer solar cells. Hence, the power conversion efficiency of the polymer solar cells reached to 7.9% by utilizing the conducting polymer-based electron donors. The band gap and energy levels of the conjugated polymers can be engineered by tuning their chemical structures. Thus, conjugated polymers offer promising and efficient role in polymer solar cells. The polymer solar cells suffer from some serious issues like short carrier diffusion lengths and poor carrier mobility present in the conjugating polymers [31–40]. In single junction polymer solar cells, the thickness of the active layer cannot be increased which resulted to poor absorption of sunlight. Therefore, it is believed that polymer solar cells can show better performance when fabricated and developed in multi-junction/

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tandem architecture. In multi-junction polymer solar cells, polymers with variable band gaps have been used with multiple subcells. The subcells are usually connected in a series of electrical connections in multi-junction solar cells. This helps the polymer solar cells to improve the open circuit voltage by utilizing the subcells. In previous years numerous strategies were developed to improve the performance of the multi-junction polymer solar cells. In this chapter, we have reviewed the recent advances and trend in the multijunction polymer solar cells. The challenges of the tandem/multi-junction polymer solar cells have also been described.

Materials for Multi-Junction Polymer Solar Cells There are different types of materials used for multi-junction polymer solar cells. These materials are the donor, acceptor, and interfacial materials [2]. In case of donor materials, the polymers especially conjugated polymer with band gap between 1.5 eV to 3 eV possess high absorption coefficient. This kind of polymers also showed semiconducting nature which comes from their framework of carbon-carbon single and double bonds. These polymers can have good absorption properties for solar cell applications. These polymers may suffer from limit of photocurrent generation due to their narrow absorption range. The donor polymer materials should possess higher hole mobility for better charge transport. Thus, the selection of suitable donor polymer material is crucial to develop the highly efficient multijunction polymer solar cells. The polymers with band gap lower than 1.8eVare expected to show better light harvesting activity. On the other hand, acceptor materials should have better electron affinity than that of the donor polymer materials. It can also be said that acceptor material should have LUMO and HOMO levels below the LUMO and HOMO levels of the donor polymer materials which help for better charge transfer between the interfaces and makes them suitable for good exciton dissociation at the interface. The acceptor materials should also have better ability to accept the electron rapidly and high electron mobility which improve the performance of the multi-junction polymer solar cells. Previously transition semiconducting metal oxides such as ZnO and TiO2, indium tin oxide, and other metallic nanoclusters such as gold and silver have been used as recombination or interfacial layers in polymer solar cells. This interfacial layer protects the base for top cell with bottom subcell. The transparency of the applied interfacial layer along with its electrical properties affects the performance of the polymer solar cells. This is because interfacial layers present in the polymer solar cell reduce the light which will be absorbed by the available back subcells. Generally, metal oxide with good transparency act as electron transport and collecting layers for the present first subcells while act as stable base for the prepared subcell on its top. On the other hand, conducting polymers such as modified poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) along with transition metal oxides like molybdenum oxide (MoO3) and vanadium oxide (V2O5) etc. act as the working hole transport layers. Two or more

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than two polymer subcells can be stacked by utilizing multilayered semitransparent hole transport or electron transport interlayers. The ZnO or TiO2 may also be employed as optical spacer or electron transport layer (transparent) between the active layer and top metal electrode. The spacer can also improve the charge carrier mobility.

Device Structures of Multi-Junction Polymer Solar Cells There were multi-junction polymer solar cells developed with different device structures. In this section, we have reviewed the different device structures of the multi-junction polymer solar cells.

Normal Structure This is the conventional and basic device structure of the multi-junction polymer solar cells called normal structure multi-junction polymer solar cells. The device structure of the normal multi-junction polymer solar cell has been displayed in Scheme 1. Normal device structure composed of subcells (one multi-junction cell with polymeric semiconducting materials of different band gap) where polymer with wide band gap has been used as optical front cell (See Scheme 1; bottom cell). The narrow band gap polymer has been applied as optical black cell (See Scheme 1; top cell). The Scheme 1 showed the normal device structure multi-junction polymer solar cells consists of two subcells. The high energy photon was absorbed by bottom cell (with wide band gap materials) and passes the low energy photon which can be absorbed by the top cell with low band gap materials.

Scheme 1 Schematic device structure of normal multi-junction polymer solar cells

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Inverted Structure The polymeric materials with narrow band gap are applied as bottom cell (front optical subcell) whereas the polymers with wide band gap have been used as top cell (optical back subcell) in the fabrication of inverted multi-junction polymer solar cells. The schematic representation of the device structure has been shown in Scheme 2 and 3.

Self-Passivating Structure There is also another device structure (self-passivating) of the multi-junction polymer solar cells.

Scheme 2 Schematic picture of the inverted structure of the multi-junction polymer solar cells device Scheme 3 Schematic picture of the device structure (selfpassivating) of multi-junction polymer solar cells

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In this type of solar cells, PbSe quantum dot nanocrystals are used as optical front subcell while poly(3-hexylthiophene) (P3HT)/6,6-phenyl-C61-butyric acid methyl ester (PCBM) has been used as back subcell. This type of multi-junction polymer solar cells has the following advantages: 1. 2. 3. 4.

Solution-based processing Better protection (UV degradation protection) for back subcell Better UV conversion efficiency Better electric field to back subcell to extract the charges

Parallel Structure Parallel structure of the multi-junction polymer solar cells were developed by Zhang et al. which provide a unique way to connect the two subcells in parallel manner [41]. The schematic picture of the parallel structure of the multijunction polymer solar cells has been displayed in Scheme 4. In this type of solar cell, PCBM was employed as dual functional to form bulk hetero-junction with P3HT and a bilayer hetero-junction subcell with CuPc (Scheme 4). The performance of the fabricated device was found to be very poor and this may be due to the creation of excitons within the exciton diffusion lengths at the interface of CuPc and PCBM which were able to diffuse for the separation of free carriers. Further some other device structures such as stacked structure and folded reflective structure were also developed to improve the performance of the multi-junction polymer solar cells.

Scheme 4 Schematic picture of the parallel multi-junction polymer solar cell device structure

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Advances in Multi-Junction/Tandem Polymer Solar Cells In 2009, Chen et al. [42] employed a series of poly[4,8-bis-substituted benzo[1,2b:4,5-b’]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b]thiophene-2,6-diyl] (PBDTTT) polymers for the construction of polymer solar cells. In this work, authors also prepared a copolymer of benzo[4,8-bis-substituted-benzo[1,2-b:4,5b’]dithiophene and thieno[3,4-b]thiophene and denoted this as PBDTTT-E. In other case, alkyloxy group present in the carbonyl of the thieno[3,4-b]thiophene was changed with alkyl side chain and denoted as PBDTTT-C. Further PBDTTT-C was also modified with fluorine and denoted as PBDTTT-CF. The structures of the PBDTTT-E, PBDTTT-C, and PBDTTT-CF have been shown in Fig. 1a. The obtained structure of the PBDTTT-E, PBDTTT-C, and PBDTTT-CF showed the different HOMO and LUMO energy values (Fig. 1b). This change in the HOMO and LUMO energy values was attributed to the modification of PBDTTT polymer to form the PBDTTT-E, PBDTTT-C, and PBDTTT-CF. The UV-vis spectra of the PBDTTT-E, PBDTTT-C, and PBDTTT-CF have been displayed in Fig. 1c. Furthermore, polymer solar cells with three different polymers (PBDTTT-E, PBDTTT-C, and PBDTTT-CF) were developed. The developed polymer solar cells device with PBDTTT-E showed the power conversion efficiency of 5.15% while PBDTTT-C and PBDTTT-CF based polymer solar cell devices exhibited the efficiency of 6.58% and 7.73% respectively. This performance of the polymer solar cells using PBDTTTCF was impressive and best among the other reported polymer solar cells. Thus Chen et al. have applied this performance to the NREL for the authentication. The NREL have certified this performance of the PBDTTT-CF based polymer solar cell device. The certified parameters were found to be as:

Fig. 1 Chemical structures (a), energy level values (b), and absorption spectra (c) of three different polymers (PBDTTT-E, PBDTTT-C, and PBDTTT-CF). (Reproduced with permission [42])

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Fig. 2 Photocurrent densityvoltage (IV) curve of the PBDTTT-CF based polymer solar cell device certified by NREL. (Reproduced with permission [42])

Fig. 3 EQE curves of the polymer solar cells fabricated with three different polymers (PBDTTT-E, PBDTTT-C, and PBDTTT-CF). (Reproduced with permission [42])

Photocurrent density ¼ 13.364 mA/cm2; open circuit voltage (Voc) ¼ 0.763 V; fill factor (FF) ¼ 66.39%, and efficiency ¼ 6.77%. The certified IV curve of the PBDTTT-CF based polymer solar cell device by NREL has been shown in Fig. 2. Further external quantum efficiency (EQE) was also measured of the developed polymer solar cells. The EQE curves of the polymer solar cells fabricated with three different polymers (PBDTTT-E, PBDTTT-C, and PBDTTT-CF) have been displayed in Fig. 3. The polymer solar cells fabricated with PBDTTT-C exhibits the least quantum efficiency while with PBDTTT-CF showed highest quantum efficiency. The EQE results were consistent with obtained IV results.

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Fig. 4 The schematic picture of the tandem polymer solar cells. (Reproduced with permission [43])

In 2013, Andersen et al. [43] have developed polymer tandem solar cells by utilizing various unique strategies. The schematic picture of the developed tandem polymer solar cell has been depicted in Fig. 4. Andersen et al. [43] developed the multi-junction polymer solar cells without using ITO glass substrate and vacuum evaporation techniques. The stack layers were prepared using coating or printing approach. The number of layers prepared exceeds ten to complete the device as depicted in Fig. 4. The effect of thicknesses of the layer was also investigated. The P3HT and PCBM comprising of ZnO was used for electron selectivity, while PEDOT:PSS for the preparation of hole selectivity, electrode, and recombination layers. A unique slanted comb like Ag grid electrode was applied to reduce shunts. The IV curves of the fabricated polymer solar cells devices with different thickness of the PEDOT:PSS were recorded and have been presented in Fig. 5a. The IV curves of the polymer solar cell devices with different thickness of the ZnO layers were also recorded. The IV curves have been displayed in Fig. 5b. The depicted IV curves in the Fig. 5a and b clearly revealed that the thickness of the layers affect the photovoltaic performance of the polymer solar cells. The IV curves of a device comprising F010-ALP4083-ZnO intermediate layer: single junction and tandem solar cell device have been depicted in Fig. 6. The single junction polymer solar cells device exhibited the power conversion efficiency of 1.94% whereas tandem solar cells device displayed a power conversion efficiency of 1.33%. Mitul et al. [44] prepared low temperature processes interconnecting layer for the construction of polymer tandem solar cell applications. Tandem polymer solar cells efficiently utilize the photon energy and other advantage is the double open circuit voltage compare to the non-tandem polymer solar cells. A chemically stable and low

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Fig. 5 IV measurements of the developed devices with different thickness (a) of PEDOT:PSS and devices fabricated with different thickness (b) of ZnO layer. (Reproduced with permission [43])

Fig. 6 IV curves of a device comprising F010-ALP4083-ZnO intermediate layer: single junction and tandem solar cell device; table showed the photovoltaic parameters of the tandem and single junction solar cells. (Reproduced with permission [43])

temperature processed PEDOT:PSS/aluminum-doped zinc oxide (AZO)/ethoxylated poly-ethylenimine (PEIE) interconnecting layer was prepared for tandem polymer solar cells. The schematic picture of the polymer tandem solar cell device developed by Mitul et al. [44] has been displayed in Scheme 5a which is clearly showed the components of the polymer tandem solar cells. From the Scheme 5a, it can be seen that PEDOT:PSS/AZO/PEIE has been employed as interconnecting layer in the polymer tandem solar cell device. The energy level values of the different components of the polymer tandem solar cells were also discussed by Mitul et al. for better understanding of the working mechanism. The schematic energy level picture of the different components of the polymer tandem solar cells has been displayed in Scheme 5b. The energy level values of the PEDOT:PSS, AZO, and PEIE were found to be well-matched which helps to for better electronic communications. This activity can be seen from the Scheme 5b.

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Scheme 5 Schematic picture of the polymer tandem solar cell device (a) and energy level value diagram (b). (Reproduced with permission [44])

The atomic force microscopy (AFM) is one of the best techniques to understand surface properties of the prepared layer and thin films. This also provides the surface roughness and topology of the prepared layers for solar cell applications. Mitul et al. employed AFM approach for understanding the top surface properties of the prepared different layers. The obtained results of the prepared layers using AFM technique has been displayed in Fig. 7. The topography image of the bottom active layer (P3HT:PCBM) indicate that fully covered surface roughness with a value of 6.17 nm (Fig. 7a). However, the grain boundaries were not found significant by AFM analysis. The topology of the coated active layer (PEDOT:PSS) on top of P3HT:PCBM was also checked using AFM (Fig. 7b). The root mean square (rms) value was fount 6.1 nm. Further PEIE

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Fig. 7 AFM image of bottom active layer; P3HT:PCBM (a), PEDOT:PSS (b) on top active layer, PEIE on top (c) of PEDOT:PSS, AZO (d) on top of PEDOT:PSS, PEIE on AZO/PEDOT:PSS (e), and P3HT:PCBM (top cell) on top of ITO/bottom (f). (Reproduced with permission [44])

was coated on PEDOT:PSS and the recorded AFM image has been depicted in Fig. 7c. In this case, some small holes were observed along with the presence of porous surface. The charge transport may be affected by the presence of this porous surface. This porous nature was due to the partial dissolution of PEDOT:PSS due to the presence of PEIE. In case of the coating of AZO on the top of PEDOT:PSS, grains of 20 nm in size was observed (Fig. 7d). The surface roughness of the coated AZO was found to be 2.62 nm. The interfacial area, conductivity can be increased

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Fig. 8 IV curves of the single-junction and tandem polymer solar cell devices having different interconnecting layers. (Reproduced with permission [44])

with increased large grain size. The use of PEIE as interfacial layer (Fig. 7e) between active layer/electrode improves the efficiency of the device and smoothness of the metal oxide layer. The AFM image of the P3HT:PCBM (top cell) on top of ITO/ bottom has been displayed in Fig. 7f. The AFM investigations showed that PEDOT: PSS/AZO/PEIE has efficient surface morphology for better charge transport/collection at the interfaces (between metal anode and polymer film). Further tandem polymer solar cells were developed and their photovoltaic investigations were carried out to check the performance of the devices. The IV curves of the fabricated tandem polymer solar cells using three different interconnecting layers have been presented in Fig. 8. The single junction polymer solar cells were also developed. The single junction polymer solar cells showed the power conversion efficiency of 3.26%. The polymer solar cell device having PEDOT:PSS/AZO interconnecting layer showed the efficiency of 3.20% whereas the device having PEDOT:PSS/PEIE interconnecting layer exhibited the efficiency of 0.15%. The improved efficiency of 3.58% was obtained for the polymer solar cell device having PEDOT/AZO/PEIE as interconnecting layer. Interconnection layers plays important role in polymer solar cells. The interconnection layers secure the charge transport between front and rear cells in the polymer solar cells. In 2019, Seo et al. [45] have applied a new hybrid interconnection layers (MoO3/ Ni/ZnO:PEOz; where PEOz ¼ poly(2-ethyl-2-oxazoline)) for the construction of tandem polymer solar cells. The hybrid electron collecting buffer layer (PEOz embedded ZnO ¼ ZnO:PEOz) was synthesized using sol-gel approach on to the nickel (Ni) interlayers. The Ni interlayers were prepared on MoO3 buffer layers. The Ni interlayer acts as a protection layer for MoO3 and electrical conductor inside the polymer tandem solar cells.

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Scheme 6 Schematic picture of polymer tandem solar cell (a) device and energy level diagram (b). (Reproduced with permission [45])

Fig. 9 Schematic device structure of front cell (a) and rear cell (b) of the polymer tandem solar cells. IV curves (c) and EQE (d) of the front and rear cell of the polymer tandem solar cells. (Reproduced with permission [45])

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The schematic picture of the polymer tandem solar cell has been depicted in Scheme 6a whereas the energy level values of the different layers of the polymer tandem solar cell device has been presented in Scheme 6b. The Ni worked as interlayer between the two cells and this can be seen in Scheme 6b. The individual schematic pictures of the front and rear polymer solar cell have been shown in Fig. 9a and b respectively. The photovoltaic performance of these two front and rear cells were investigated. The front cell showed lower performance compare to the rear cell. The front cell exhibited the power conversion efficiency of 6.13% as revealed by IV curve present in Fig. 9c. The rear cell has shown excellent and enhanced power conversion efficiency of 11.83% which is quite higher than that of the front cell (Fig. 9c). The front cell showed the lower efficiency but the obtained open circuit voltage was quite interesting (890 mV). The EQE curves of the fabricated tandem polymer solar cells were also recorded and have been displayed in Fig. 9d. The obtained EQE results revealed that rear cell has the ability to absorb the light in longer wavelength whereas the front cell can absorber light in relatively shorter wavelengths. The thickness of the Ni played an important role in these fabricated polymer solar cells. The highest efficiency of 14.22% was obtained with Ni (thickness 6 nM) and this thickness was found to be best for interconnections due to the semitransparent. The obtained performance was really impressive and highest among other polymer tandem solar cells with different device structures.

Conclusions and Further Outlook Polymer solar cells have attracted enormous attention and scientific interest because of their cost effectiveness, light weight, flexibility, and good performance. Multijunction polymer solar cells were more found to be more efficient and attracting the researchers. In previous few years, different polymer solar cells with single junction, double-junction, triple-junction, and multi-junction device structures have been developed. Multi-junction/tandem polymer solar cells showed higher open circuit voltage compare to that of the single junction polymer solar cells. Multi-junction may achieve the highest efficiency of 19–24% according to the theoretical assumptions. Although various efforts were made to enhance the performance of the tandem/multi-junction polymer solar cells with designing and introducing novel acceptor/donor polymers but still further novel strategies are needed to improve their performance. There are few challenges in polymer solar cells which need to be resolved such as: 1. Optical absorption of the active layers present in polymer solar cells need to be improved. 2. Complete absorption of visible and infrared light by more donor polymer layers. 3. Novel interfacial layers should also be developed to further enhance the performance of the polymer tandem/multi-junction solar cells.

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23. Muhlbacher D, Scharber M, Morana M, Zhu Z, Waller D, Gaudiana R, Brabec CJ (2006) High photovoltaic performance of a low-bandgap polymer. Adv Mater 18:2884–2889 24. Brabec CJ, Winder C, Sariciftci NS, Hummelen JC, Dhanabalan A, van Hal PA, Janssen RAJ (2002) A low-bandgap semiconducting polymer for photovoltaic devices and infrared emitting diodes. Adv Funct Mater 12:709–712 25. Li G, Shrotriya V, Huang J, Yao Y, Moriarty T, Emery K, Yang Y (2005) High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat Mater 4:864–868 26. Brabec CJ, Shaheen SE, Winder C, Sariciftci NS, Denk P (2002) Effect of LiF/metal electrodes on the performance of plastic solar cells. Appl Phys Lett 80:1288–1290 27. Chen M-H, Hou J, Hong Z, Yang G, Sista S, Chen L-M, Yang Y (2009) Efficient polymer solar cells with thin active layers based on alternating polyfluorene copolymer/fullerene bulk heterojunctions. Adv Mater 21:4238–4242 28. Wang X, Perzon E, Delgado JL, Cruz l Pd, Zhang F, Langa F, Andersson M, Inganas O (2004) Infrared photocurrent spectral response from plastic solar cell with low-band-gap polyfluorene and fullerene derivative. Appl Phys Lett 85:5081–5083 29. Bundgaard E, Krebs FC (2007) Low band gap polymers for organic photovoltaics. Sol Energy Mater Sol Cells 91:954–985 30. Campos LM, Tontcheva A, Gunes S, Sonmez G, Neugebauer H, Sariciftci NS, Wudl F (2005) Extended photocurrent spectrum of a low band gap polymer in a bulk heterojunction solar cell. Chem Mater 17:4031–4033 31. Brabec CJ, Sariciftci NS, Hummelen JC (2001) Plastic solar cells. Adv Funct Mater 11:15 32. Spanggaard H, Krebs FC (2004) A brief history of the development of organic and polymeric photovoltaics. Sol Energy Mater Sol Cells 83:125–146 33. Bundgaard E, Krebs FC (2007) Low band gap polymers for organic photovoltaics. Sol Energy Mater Sol Cells 91:954–985 34. Günes S, Neugebauer H, Sariciftci NS (2007) Conjugated polymer-based organic solar cells. Chem Rev 107:1324–1338 35. Li Z, Xu X, Zhang W, Meng X, Genene Z, Ma W, Mammo W, Yartsev A, Andersson MR, Janssen RAJ, Wang E (2017) 9.0% power conversion efficiency from ternary all-polymer solar cells. Energy Environ Sci 10:2212–2221 36. Ma W, Yang C, Gong X, Lee K, Heeger AJ (2005) Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology. Adv Funct Mater 15:1617–1622 37. Forrest SR (2005) The limits to organic photovoltaic cell efficiency. MRS Bull 30:28–32 38. Koster LJA, Mihailetchi VD, Blom PWM (2006) Ultimate efficiency of polymer/fullerene bulk heterojunction solar cells. Appl Phys Lett 88(093511):1–3 39. Scharber MC, Mühlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, Brabec CJ (2006) Design rules for donors in bulk-heterojunction solar cells-towards 10% energy-conversion efficiency. Adv Mater 18:789–794 40. Colsmann A, Junge J, Kayser C, Lemmer U (2006) Organic tandem solar cells comprising polymer and small-molecule subcells. Appl Phys Lett 89(203506):1–3 41. Zhang C, Tong SW, Jiang C, Kang ET, Chan DSH, Zhu C (2008) Simple tandem organic photovoltaic cells for improved energy conversion efficiency. Appl Phys Lett 92:083310 42. Chen H-Y, Hou J, Zhang S, Liang Y, Yang G, Yang Y, Yu L, Wu Y, Li G (2009) Polymer solar cells with enhanced open-circuit voltage and efficiency. Nat Photon 3:649–653 43. Andersen TR, Dam HF, Andreasen B, Hösel M, Madsen MV, Gevorgyan SA, Søndergaard RR, Jørgensen M, Krebs FC (2014) A rational method for developing and testing stable flexible indium- and vacuum-free multilayer tandem polymer solar cells comprising up to twelve roll processed layers. Solar Sol Energy Mater Sol Cells 120:735–743 44. Mitul AF, Mohammad L, Venkatesan S, Adhikari N, Sigdel S, Wang Q, Dubey A, Khatiwada D, Qiao Q (2015) Low temperature efficient interconnecting layer for tandem polymer solar cells. Nano Energy 11:56–63 45. Seo J, Moon Y, Lee S, Lee C, Kim D, Kim H, Kim Y (2019) High efficiency tandem polymer solar cells with MoO3/Ni/ZnO:PEOz hybrid interconnection layers. Nanoscale Horiz 4:1221–1226

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Current State and Prospective of Supercapacitors Design, Synthesis, and Fabrication of Novel Electrode Materials for Energy Storage Applications Khursheed Ahmad and Waseem Raza

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principle and Device Structure of Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The design and preparation of novel electrode materials for energy storage applications is of great importance. Supercapacitors are the energy storage devices which attracted the enormous attention because of the fast charge/discharge ability, high power density, and excellent cyclic life. The performance of the supercapacitor devices depends on the presence of electrode materials. Thus, it is important of great importance to design and develop the high surface area and nanostructured electrode materials for the preparation of supercapacitor electrodes. Surface morphology, concentration, and electrical properties of the electrode materials also affect the electrochemical activity of the supercapacitor electrodes. Previously, different materials like conducting polymers, carbon materials, metal oxides/hydroxides, and metal sulfides have been employed as electrode modifiers for energy storage applications.

K. Ahmad (*) Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] W. Raza Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_197

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In this chapter, we have reviewed the construction of different electrode modifier for energy storage application. The future prospect and current states have also been reviewed. Keywords

Energy storage · Supercapacitors · Electrode materials and nanostructures

Introduction World need cheap and safe energy and the energy requirements have been increased in the last few decades [1–7]. The whole world depend on different energy sources especially petroleum, wood, coal, and natural gases. However, these energy sources are limited and they also have some negative impacts on environment. It was believed that some other renewable energy sources such as wind energy, geothermal, hydropower, and marine energy may fulfill the energy requirements [8–20]. However, these kinds of energy sources are sporadic and may not be used for long time. Therefore, it was proposed that energy storage devices such as supercapacitors, batteries, and capacitors may be helpful to store the energy for later times. The widely used batteries with rechargeable properties have the potential to store the energy and the charge transfer process involves the redox reactions [9]. The rechargeable batteries have shortcomings such as spark hazard, lower power density, and poor life span which need to be overcome [2]. On the other hand supercapacitors gained enormous attention due to the some advantages such as long life cycle and fast charge/discharge process [15–30]. For the first time in 1957, Becker developed the capacitor using carbon electrodes [1]. The supercapacitors can be classified in two categories: 1. Electrochemical double layered capacitors 2. Pseudocapacitors The double layered capacitors work on the principle of specific surface area electrode material and solution interface. The pseudocapacitors work on the occurrence of strong reversible chemical adsorption/desorption, oxidation/reduction reactions, electrochemical under-potential deposition, and generation of the charging potential related to the capacitor. The electrochemical capacitors have important role in the development of electric vehicles [1]. The performance of the supercapacitors can be affected by various factors but depends especially on the electrode materials. The nature of the electrode materials (also known as electrode modifiers) plays crucial role in achieving the high power density and charge/discharge activity of the supercapacitors. Therefore, in previous years various transition metal oxides, conducting polymers, porous materials, nano-materials with specific surface morphology, perovskite oxides, and hybrid composite materials have been utilized for the construction of electrochemical supercapacitors [31–44]. In this chapter we have described the principle of supercapacitor, types of supercapacitors, and different components of supercapacitors. Moreover, we have

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reviewed the current state and challenges in the preparation of electrode materials for high performance supercapacitors.

Principle and Device Structure of Supercapacitor In general, supercapacitor consists of two electrode separated by the separator usually used ion-permeable film. However, both the electrodes are connected by an electrolyte ionically. The schematic picture of the supercapacitor has been presented in Scheme 1. The electrolyte has been used as a medium which contains both positive and negative ions. The collector (metallic film) also deposited on the surface (outer surface) of the electrodes. This helps to connect the electrodes with the electric circuit to complete the device (Scheme 1). Based on the charge storage mechanism, supercapacitors can also be classified in three categories: 1. Electrostatic double layered capacitor 2. Pseudocapacitor 3. Hybrid capacitor In this chapter, we have reviewed the advancements in the design and synthesis of electrode materials for the construction of supercapacitors. Zheng et al. have designed and synthesized a novel electrode material for the construction of supercapacitors [42]. The authors have prepared nickel oxide (NiO)/ nickel sulfide (NiS)@carbon nanotube (CNT) nanocomposites using one step microwave approach. The schematic picture (Scheme 2) showed the complete synthetic procedure of the NiO/NiS@CNT nanocomposite. Furthermore the formation of the prepared samples was confirmed by X-ray diffraction (XRD) investigations. The XRD data of the CNT, NiO@CNT, and NiO/NiS@CNT nanocomposite are depicted in Fig. 1.

Scheme 1 Schematic illustration of charge transfer mechanism in supercapacitor

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Scheme 2 Schematic diagram of synthesis of NiO/NiS@CNT nanocomposite. (Reproduced with permission [42])

Fig. 1 XRD patterns of the CNT, NiO@CNT, and NiO/NiS@CNT nanocomposite. (Reproduced with permission [42])

The XRD spectra of CNT showed the strong diffraction peak at around ~25
, while other peaks with low intensity were also observed at ~44
and ~45
. This observed diffraction peaks were consistent with the JCPDS no. 89-8487 and confirmed the CNT with good phase purity. The XRD patterns of the NiO@CNT and NiO/NiS@CNT nanocomposites showed the diffraction peaks which were found to be matched with JCPDS no. 47-1049 and 02-1280. The obtained XRD pattern confirmed the formation of NiO@CNT and NiO/NiS@CNT nanocomposites as

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shown in Fig. 1. Further authors also employed scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDX) techniques to understand the morphological and elemental compositions of the prepared NiO@CNT and NiO/NiS@CNT nanocomposites. The SEM image of the CNT showed the tube like surface morphology and NiO@CNT revealed the presence of NiO nanoparticles on the nanotubes surface. In case of NiO/NiS@CNT nanocomposite, two different kinds of nanoparticles were grown on CNT surface. The particle size of the NiS was found to be larger compare to the NiO particles. The electrodes were prepared using the synthesized nanoparticle composites. The electrochemical investigations were carried out to further check the performance of the developed electrodes for supercapacitor applications. Cyclic voltammetry (CV) was used to check the electrochemical capability of the prepared electrodes. The CNT modified electrode exhibits poor electrochemical behavior and current response compare to the NiO/NiS@CNT modified electrode. The CV curves of the NiO@CNT modified electrode were recorded at different scan rates (Fig. 2a). The recorded CV curves of NiO/NiS@CNT modified electrode at different scan rates are depicted in Fig. 2b. The NiO/NiS@CNT modified electrode showed enhanced current compare to the NiO@CNT modified electrode. This was due to the better electronic communication between the electrode and electrolyte interface and synergistic effects. Moreover, the larger area in the CV curve of the NiO/NiS@CNT modified electrode was observed compare to the

Fig. 2 CV curves (a, b) and charge/discharge (c, d) curves of the NiO@CNT and NiO/NiS@CNT modified electrodes. (Reproduced with permission [42])

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NiO@CNT modified electrode. This suggested the presence of better electrochemical activity in the NiO/NiS@CNT modified electrodes which makes it suitable for supercapacitor applications. The charge/discharge curves of the NiO@CNT and NiO/NiS@CNT modified electrodes were also recorded at different current densities. The NiO@CNT modified electrode exhibited the lower charge/discharge behavior compare to the NiO/NiS@CNT modified electrode. The obtained charge/ discharge curves of the NiO@CNT and NiO/NiS@CNT modified electrodes are shown in Fig. 2c and d, respectively. The NiO/NiS@CNT modified electrode exhibited the high specific capacity of 809.7 F/g at current density of 1 A/g with excellent cyclic stability up to the 20,000 cycles. Yang et al. [43] have synthesized NiCo2O4@NiO composite on carbon fiber paper using facile approach. In the first step, carbon fiber was modified with NiCo2O4 using hydrothermal approach. In the second step, the prepared NiCo2O4 was decorated with NiO to prepare the NiCo2O4@NiO composite using hydrothermal approach. The schematic illustration of the synthesis of NiCo2O4@NiO composite has been displayed in Scheme 3 for better understanding of synthetic process. Further the physiochemical properties of the NiCo2O4@NiO composite were determined by XRD, SEM, EDX, and high resolution transmission electron microscopy (HR-TEM). The SEM analysis suggested that NiO sheets have anchored on the NiCo2O4 nanowires and formed three dimensional (3-D) core-shell hierarchical structures (Fig. 3a, b). The TEM (Fig. 3c) and HR-TEM (Fig. 3d) revealed that NiO sheets fully covered the NiCo2O4 nanowires. The core-shell hierarchical structures are useful materials for supercapacitors due to their higher accessible surface area which may enhance the specific capacitance. Further the electrochemical activity of the prepared NiCo2O4 and NiCo2O4@NiO electrodes was determined in 1 molar potassium hydroxide (KOH) using CV and charge/discharge investigations. The CV graphs of the NiCo2O4 and NiCo2O4@NiO electrodes in KOH solution are presented in Fig. 4a. The recorded CV results showed the high electrochemical performance of the NiCo2O4@NiO electrode compare to the NiCo2O4 electrode (Fig. 4a). This may be due to the better electronic communication between the electrolyte and electrode surface and 3-D core-shell structure of the NiCo2O4@NiO. Later, CV graphs of the NiCo2O4 electrode were also recorded at different scan rates and the current response was found to be increased with increase in the scan rates (Fig. 4b).

Scheme 3 Schematic synthetic illustration for NiCo2O4@NiO composite. (Reproduced with permission [43])

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Fig. 3 SEM (a, b), TEM (c), and HR-TEM (d) images of the NiCo2O4@NiO on carbon fiber. (Reproduced with permission [43])

The CV results showed the potential of the NiCo2O4 electrode for supercapacitors applications. Thus, charge/discharge graph was also recorded in presence of KOH at current density of 2 Ag1 to explore the prepared electrodes for supercapacitors applications. The charge/discharge graphs of the NiCo2O4 and NiCo2O4@NiO electrodes at current density of 2 Ag1 are presented in Fig. 4c. The NiCo2O4@NiO electrode showed the better charge/discharge activity compare to the NiCo2O4 electrode and found to be consistent with CV results. Furthermore, authors also recorded the charge/discharge curves of the NiCo2O4@NiO electrode under different current densities. The higher specific capacitance of 1188 Fg1 was observed at 2 Ag1 using NiCo2O4@NiO electrode. Thus, it is clear that NiCo2O4@NiO electrode has potential for supercapacitors applications. Since the electrochemical activity of the supercapacitors can be affected by the presence of electrode materials, various nanostructured, porous, and hetero-structure materials have been utilized for the fabrication of electrodes for supercapacitors applications. Thus, in 2020, Zhang et al. have prepared MnO2/MoS2 hetero-structure using ultrasonic assisted shear exfoliation and magnetron sputtering approaches [44]. The Synthetic scheme of s-MnO2 is displayed in Fig. 5a, whereas the SEM images of c-MnO2, s-MnO2 and s-MnO2/MoS2 composite are displayed in Fig. 5b–e. The SEM results showed that c-MnO2 (Fig. 5b) has solid piece structure while s-MnO2 (Fig. 5c) has powdery structure with more uniform distribution. The

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Fig. 4 CV graphs of NiCo2O4 and NiCo2O4@NiO electrodes (a). CV graphs of NiCo2O4@NiO electrode at different scan rates (b). Charge/discharge graphs of NiCo2O4 and NiCo2O4@NiO electrodes (c) at current density of 2 Ag1. Charge/discharge graphs of NiCo2O4@NiO electrode at varied current densities (d). (Reproduced with permission [43])

SEM pictures of the s-MnO2/MoS2 composite showed the well distribution of the s-MnO2 on to the surface of the MoS2 structure (Fig. 5e, f). Further the presence of different elements was also investigated by employing EDX approach. The recorded EDX results of cross-sectional s-MnO2/MoS2 are displayed in Fig. 5g. The obtained EDX results revealed the presence of Mn, O, Mo, and S. This confirmed the phase purity of the s-MnO2/MoS2 hetero-structure. The TEM images of the c-MnO2 and s-MnO2 are displayed in Fig. 6a and Fig. 6b, respectively. The TEM image of the s-MnO2/MoS2 is displayed in Fig. 6c. The XRD patterns of the c-MnO2 and s-MnO2 are displayed in Fig. 6d. The XRD spectra showed the strong diffraction peaks which suggested the crystalline nature. The XRD patterns were well matched with JCPDS no. 14-0644 and showed the presence of tetragonal phase of MnO2. The Ns adsorption/desorption isotherms of the c-MnO 2 and s-MnO2 are depicted in Fig. 6e. The c-MnO2 has specific surface area of 64.99 m2/g, while s-MnO2 has surface area of 87.05 m2/g. The nanostructured electrode materials with high surface area have been considered suitable candidate for supercapacitors applications. Further X-ray photoelectron spectroscopy (XPS) was also conducted

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Fig. 5 Synthetic scheme of s-MnO2 (a), SEM images of c-MnO2 (b) and s-MnO2 (c), s-MnO2/ MoS2 composite (d, e) and EDX results of cross-sectional s-MnO2/MoS2 (f–g). (Reproduced with permission [44])

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Fig. 6 TEM pictures of c-MnO2 (a), s-MnO2 (b), s-MnO2/MoS2 (c). XRD patterns (d) and N2 adsorption-desorption isotherms (e) of c-MnO2 and s-MnO2. XPS (Mn 2p) spectra of s-MnO2, c-MnO2, and s-MnO2/MoS2 (f). (Reproduced with permission [44])

and XPS (Mn 2p) spectra of s-MnO2, c-MnO2 and s-MnO2/MoS2 are presented in Fig. 6f. The XPS spectra confirm the formation of s-MnO2, c-MnO2, and s-MnO2/MoS2. Further the electrochemical activity of the s-MnO 2, c-MnO2, and s-MnO2/MoS2 was analyzed using CV approach. The recorded CV graphs of the s-MnO2, c-MnO2, and s-MnO2/MoS2 are presented in Fig. 7. The CV graphs of the c-MnO2 and s-MnO2 modified electrodes were recorded at different applied scan rates and the data are displayed in Fig. 7a and c, respectively. The observations revealed that s-MnO2 modified electrode has enhanced

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Fig. 7 CV graphs of c-MnO2 (a), s-MnO2 (c), s-MnO2/MoS2 (e). Charge/discharge curve of the c-MnO2 (b), s-MnO2, (d) and s-MnO2/MoS2 (f). (Reproduced with permission [44])

electrochemical activity compare to the c-MnO2 modified electrode. The charge/ discharge graphs of the c-MnO2 and s-MnO2 modified electrodes at different current densities are presented in Fig. 7b and d, respectively. The observations also showed improved charge/discharge activity for the s-MnO2 modified electrode compare to the c-MnO2 modified electrode. Thus, a composite of s-MnO2/MoS2 hetero-structure was designed and prepared for further improvements in the electrochemical activity. The s-MnO2/MoS2 hetero-structure modified electrode was used to record the CV graphs for electrochemical investigations. The CV graphs of the s-MnO2/MoS2 hetero-structure modified electrode at different scan rates are depicted in Fig. 7e. The obtained results exhibited better current response compare to the c-MnO2 and

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s-MnO2 modified electrodes. The charge/discharge graphs of the s-MnO2/MoS2 hetero-structure modified electrode were also recorded at different current densities as shown in Fig. 7f. The highest specific capacitance of ~224 mF/cm2 was observed at current density of 0.1 mA/cm2 using s-MnO2/MoS2 hetero-structure modified electrode. This may be due to higher conductivity and specific surface area of the s-MnO2/ MoS2 hetero-structure. The unique and new nanostructured materials with improved surface properties are desirable factors for suitable electrode materials for electrochemical devices. Thus, Hou et al. [45] have fabricated the electrode modifier under facile conditions by utilizing zeolitic imidazolate frameworks (ZIF-67). Authors have prepared Co3S4@NiO hollow dodecahedral using ZIF-67 as precursor material. The Co3S4 was prepared using ZIF-67 at 120
C for 4 h followed by the

ZIF-67

120 ºC

140 ºC

4h

16h Co3S4

Co3S4@NiO

Scheme 4 Schematic graph shows synthesis of Co3S4@NiO hollow dodecahedral. (Reproduced with permission [45])

Fig. 8 XRD (a) of Co3S4 (pink) and Co3S4@NiO (blue). EDX of Co3S4@NiO (b). (Reproduced with permission [45])

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decoration of Co3S4 with NiO at 140
C for 16 h. The synthetic procedure of ZIF-67 derived Co3S4@NiO hollow dodecahedral has been illustrated in Scheme 4. The XRD, EDX, SEM, and TEM investigations were performed to check the physiochemical/structural properties of the prepared Co3S4 and Co3S4@NiO. The XRD patterns of the Co3S4 and Co3S4@NiO are depicted in Fig. 8a. The XRD pattern of Co3S4 exhibits strong diffraction peaks and found to be matched with JCPDS no. 47-1738. The XRD pattern of the Co3S4@NiO good diffraction peaks and the diffraction planes were found to be matched with JCPDS no. 47-1049. The presented XRD data in Fig. 8a confirmed the successful preparation of Co3S4 and Co3S4@NiO. Further, authors have employed EDX approach to find out the elemental composition of Co3S4@NiO. The EDX spectrum of the Co3S4@NiO is presented in Fig. 8b. The EDX spectrum revealed the presence of Co, Ni, S, and O with atomic percentage of 21.19%, 24.53%, 28.61%, and 25.67%, respectively. This also confirmed the formation Co3S4@NiO with high phase purity. Authors also recorded SEM and TEM images of the Co3S4 and Co3S4@NiO. The SEM investigations suggested the presence of dodecahedral like surface properties of the Co3S4@NiO,

Fig. 9 CV graphs of Co3S4@NiO modified electrode (a) at different applied scan rates. CV graphs of Co3S4 and Co3S4@NiO modified electrodes at applied scan rate ¼ 5 mV/s (b). Charge/discharge graphs of the Co3S4@NiO modified electrode (c) at different applied current densities (c). Charge/ discharge graphs of the Co3S4 and Co3S4@NiO modified electrodes at applied current density¼ 1 A/g (d). (Reproduced with permission [45])

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whereas TEM analysis showed that Co3S4@NiO has hollow dodecahedral structure. Further, XPS was also used to find out the phase purity and presence oxidation states. The XPS results showed the formation of Co3S4@NiO composite. Furthermore, electrodes were fabricated using Co3S4 and Co3S4@NiO hollow dodecahedral composite. The CV graphs of the Co3S4@NiO modified electrode at different applied scan rates are depicted in Fig. 9a. The CV graphs showed that current response increases with increase in the applied scan rate. The CV graphs of Co3S4 and Co3S4@NiO modified electrodes at applied scan rate ¼ 5 mV/s are depicted in Fig. 9b. The higher electrochemical activity/current response was appeared for the Co3S4@NiO modified electrode compare to the Co3S4 modified electrode (Fig. 9b). Further, charge/discharge curves of the Co3S4@NiO modified electrode at different applied current densities were also recorded and presented in Fig. 9c. The charge/discharge results showed the higher specific capacitance at the applied current density ¼1 A/g (Fig. 9c). However, the charge/discharge graphs of the Co3S4 and Co3S4@NiO modified electrodes at applied current density ¼ 1 A/g was also recorded for comparison purpose. The obtained results are displayed in Fig. 9d. The charge/discharge results showed the better specific capacitance for the Co3S4@NiO modified electrode than that of the Co3S4 modified electrode (Fig. 9d). The highest specific capacitance of 1877.93 F/g was obtained in 6 M KOH solution using Co3S4@NiO modified electrode at applied current density of 1 A/g. It is believed that two-dimensional (2-D) materials possess excellent properties and have potential for electrochemical applications. In last few years a new class of 2-D materials metal carbide/metal nitride also called MXene has attracted the researchers. Wang et al. have fabricated a new material comprised of graphene wrapped MXene using plasma exfoliation approach [46]. In general MXene has been prepared by its parent phase MAX. In 2019, Wang et al. have prepared MXene graphene wrapped MXene for supercapacitors applications [46]. The MAX (titanium aluminum carbide¼Ti3AlC2) was etched using HF to form the MXene phase (Ti3C2Tx). Further, graphene oxide (GO) binding was done with MXene to form the MXene@GO. Finally the MXene/rGO was obtained using plasma delamination. The schematic picture showed the synthetic procedure of the MXene@rGO (Scheme 5). The formation of MXene@rGO was checked by XRD and XPS investigations. The XRD pattern of the MXene@rGO showed the poor crystalline nature which may be due to the amorphous nature of rGO. The XPS data also suggested the successful

Scheme 5 Schematic graph of the preparation of graphene wrapped MXene composite. (Reproduced with permission [46])

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formation of MXene@rGO composite. Further SEM pictures were also recorded to find out the effect of exfoliation on the formation of MXene@rGO composite. The recorded SEM pictures of MAX, MXene, MXene@GO, and MXene@rGO are displayed in Fig. 10. The MAX showed dense layered chunks like surface morphology (Fig. 10a), whereas MXene showed layered structure with spacing (Fig. 10b) which probably due to the removal of Al after etching process. This confirmed the formation of multilayered MXene phase.

Fig. 10 SEM pictures of (a) MAX, (b) MXene, (c, d) MXene@GO, and (e, f) MXene@rGO. (Reproduced with permission [46])

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Fig. 11 CV graphs of MXene (a), MXene@rGO (c) and charge/discharge curves of MXene (b), MXene@rGO (d) modified electrodes. (Reproduced with permission [46])

The SEM investigations of MXene@GO showed the flakes like surface morphology (Fig. 10c, d). In case of MXene@rGO, powder-like pieces were observed as confirmed by SEM images (Fig. 10e, f). The elemental composition was investigated by EDX analysis. Further authors TEM approach also employed for further structural investigations. Later, authors prepared the electrodes using MXene and MXene@rGO for electrochemical investigations using CV approach. The CV graphs of the MXene and MXene@rGO at different scan rates were recorded and the data are presented in Fig. 11a and c, respectively. The MXene@rGO modified electrode exhibited the better electrochemical activity and higher current response with larger CV area compare to the MXene modified electrode. The charge/discharge graphs of the MXene and MXene@rGO modified electrodes at different current densities have been displayed in (Fig. 11b, d), respectively. Similar to CV investigations, charge/discharge investigations also showed the better performance of the MXene@rGO modified electrode than that of the MXene modified electrode. The specific capacitance of 54 mF/cm2 at an applied current density of 0.2 A/cm2 using MXene@rGO modified electrode.

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Conclusions In this chapter basic principle of supercapacitors and types of supercapacitors has been reviewed. The electrode materials or electrode modifiers directly affect performance of the supercapacitors. So, we have described different kind of electrode materials have also been discussed for the construction of electrodes for supercapacitors applications. Supercapacitors are the most efficient energy storage devices and have the potential to store the energy for later uses purposes. For better performance of supercapacitors, electrode materials should have high surface area, porosity, better electrochemical properties, and high conductivity. The supercapacitors may show higher specific capacitance by introducing novel nanostructured materials, hybrid composited, metal doped transition metal oxide, and metal alloys, etc. Thus, it is required to fabricate the novel electrode modifiers by utilizing benign approaches and synthetic strategies for high performance supercapacitors. 2-D materials like graphene and MXene have the promising properties for the construction of highly efficient electrodes for supercapacitors. Thus, researchers should develop the novel hybrid materials with MXene to construct the high performance supercapacitors.

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13. Ahmad K, Kumar P, Mobin SM (2020) A two-step modified sequential deposition methodbased Pb-Free (CH3NH3)3Sb2I9 perovskite with improved open circuit voltage and performance. Chem Electro Chem 7:946–950 14. Simon P, Gogotsi Y, Dunn B (2014) Where do batteries end and supercapacitors begin? Science 343:1210–1211 15. Ahmad K, Ansari SN, Natarajan K, Mobin SM (2019) A two-step modified deposition method based (CH3NH3)3Bi2I9 perovskite: lead free, highly stable and enhanced photovoltaic performance. Chem Electro Chem 6:1–8 16. Iro ZS, Subramani C, Dash SS (2016) A brief review on electrode materials for supercapacitor. Int J Electrochem Sci 11:10628–10643 17. Ahmad K, Mobin SM (2020) Organic–inorganic copper (II)-based perovskites: a benign approach toward low-toxicity and water-stable light absorbers for photovoltaic applications. Energy Technol 8:1901185 18. Ahmad K, Ansari SN, Natarajan K, Mobin SM (2018) Design and synthesis of 1D-polymeric chain based [(CH3NH3)3Bi2Cl9]n perovskite: a new light absorber material for lead Free perovskite solar cells. ACS Appl Energy Mater 01:2405–2409 19. Lee G, Cheng Y, Varanasi CV, Liu J (2014) Influence of the nickel oxide nanostructure morphology on the effectiveness of reduced graphene oxide coating in supercapacitor electrodes. J Phys Chem C 118:2281–2286 20. Wang J, Zhang X, Li Z, Ma Y, Ma L (2020) Recent progress of biomass-derived carbon materials for supercapacitors. J Power Sources 451:227794 21. Lima RMAP, de Oliveira HP (2020) Carbon dots reinforced polypyrrole/graphene nanoplatelets on flexible eggshell membranes as electrodes of all-solid flexible supercapacitors. J Energy Storage 28:101284 22. Muzaffar A, Ahamed MB, Deshmukh K, Thirumalai J (2019) A review on recent advances in hybrid supercapacitors: design, fabrication and applications. Renew Sust Energ Rev 101:123–145 23. Vangari M, Pryor T, Jiang L (2012) Supercapacitors: review of materials and fabrication methods. J Energy Eng 139:72–79 24. Ke Q, Wang J (2016) Graphene-based materials for supercapacitor electrodes – a review. J Mater 2:37–54 25. Wen S, Jung M, Joo O-S, Mho S-i (2006) EDLC characteristics with high specific capacitance of the CNT electrodes grown on nanoporous alumina templates. Curr Appl Phys 6:1012–1015 26. Xie Y, Du H (2015) Electrochemical capacitance of a carbon quantum dots-polypyrrole/titania nanotube hybrid. RSC Adv 5:89689–89697 27. Jian X, Yang H-m, Li J-g, Zhang E-h, Liang Z-h (2017a) Flexible all-solid-state high performance supercapacitor based on electrochemically synthesized carbon quantum dots/polypyrrole composite electrode. Electrochim Acta 228:483–493 28. Sun X, Lei Y (2017) Fluorescent carbon dots and their sensing applications. Trends Anal Chem 89:163–180 29. Wang J, Qiu J (2016) A review of carbon dots in biological applications. J Mater Sci 51:4728–4738 30. Khan S, Gupta A, Verma NC, Nandi CK (2015) Time-resolved emission reveals ensemble of emissive states as the origin of multicolor fluorescence in carbon dots. Nano Lett 15:8300–8305 31. Kalytchuk S, Wang Y, Poláková KI, Zbořil R (2018) Carbon dot fluorescence-lifetimeencoded anti-counterfeiting. ACS Appl Mater Interfaces 10:29902–29908 32. Pang L, Ming J, Pan F, Ning X (2019) Fabrication of silk fibroin fluorescent nanofibers via electrospinning. Polymers 11:986 33. Li L, Li L, Wang C, Liu K, Zhu R, Qiang H, Lin Y (2015) Synthesis of nitrogen-doped and amino acid-functionalized graphene quantum dots from glycine, and their application to the fluorometric determination of ferric ion. Microchim Acta 182:763–770

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Enzyme Catalyzed Glucose Biofuel Cells Advancements Towards the Fabrication of Electrodes for Biofuel Cell Applications

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Device Structure of Glucose Biofuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Anode for Glucose Biofuel Cell Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SWCNT/FRT/Glucose Oxidase/GCE as Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polypyrrole/FRT/Di/NADH/GDH/GCE as Bioanode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphene/FRT/GOx/GCE as Bioanode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Hybrid Biocatalyst for Biofuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MnO2/PSS/Gph/FRT/GOx/GCE as Bioanode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In last decades, glucose biofuel cells gained enormous attention of the researchers working on the development of glucose biofuel cells. Glucose biofuel cells have the potential to provide the energy for the electrical vehicles and implantable devices. In last few years, fuel cell based vehicles were also developed and have attracted the people. Moreover, glucose biofuel cells are environmental friendly and may also be used in implantable devices. The performance of the glucose biofuel cells can be affected by the quality of the fabricated electrodes. Thus, in last K. Ahmad (*) Discipline of Chemistry, Indian Institute of Technology, Indore, MP, India School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea e-mail: [email protected] Q. M. Suhail Saiyyid Hamid Senior Secondary School (Boys), Aligarh Muslim University, Aligarh, Aligarh, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_198

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few years, researchers have developed different electrode materials for the construction of electrodes for high performance glucose biofuel cells. In general different nano-materials and polymers have been used as electro-catalysts in the construction of electrodes for glucose biofuel cells. The higher electro-catalytic activity of the electro-catalysts accelerates the electrochemical oxidation of glucose in biofuel cells. In this chapter, we have reviewed the advances in the preparation of electrodes for the developments of glucose biofuel cells. Moreover, the challenges and prospective of the glucose biofuel cells have also been discussed.

Introduction The utilization of renewable energy sources to produce electricity is one of the important tasks. Solar cells and fuel cells are the efficient devices which produce the electrical energy [1–8]. Biofuel cells are the devices which produced the electrical energy by the conversion of chemical energy using electro-catalysts [9–13]. The biofuel cells have the potential for powering the implantable and portable electronic devices. The electrical energy can be generated by biofuel devices via the occurrences of electrochemical oxidation reactions at the surface of the modified electrodes. The biofuel cells devices are different from the other conventional energy devices. The biofuel cell devices involve the oxidation of renewable energy substances/fuels such as methanol, ethanol, and glucose. The biofuel cell devices involve the use of electro-catalysts for the acceleration of oxidation reaction at the electrode surface during the generation of electrical energy by utilizing the chemical energy. In conventional devices, expensive metals such as Ru, Pt, or Pd were used as electro-catalysts to increase the rate of the oxidation reaction. However, these are precious metals and their presence makes the devices expensive. Thus, it is good to find out other low cost electro-catalysts for the development of highly efficient biofuel cells. The biofuel cells also have potential to be used in various applications such as remote sensing, biosensors, pacemakers, drug delivery systems, and nano-batteries. Glucose biofuel cells provide the electrical energy by the oxidation of glucose by utilizing enzymes as catalysts. The hydrogenase/oxidase enzymes have widely used in the preparation of electrodes for glucose biofuel cell applications. The enzymes complexes has three-dimensional (3D) matrix which limit the electronic communications between the electrode and redox active cites present on the enzyme surface. Therefore, it is necessary to design and develop the mediators to provide the better electrical communications between the electrode and redox active cites present on the enzyme surface. Nanomaterials and conducting polymers have excellent properties and have been widely explored in various electrochemical applications such as fuel cells, sensors, solar cells, pollutants adsorption, and supercapacitors. Two-dimensional (2D) materials such as graphene and conducting polymers such as polypyrrole, polyaniline, and polythiophene have been used as efficient mediators in the construction of anodes for glucose biofuel cells applications. The anodes of the glucose biofuel cells influence the performance of the glucose biofuel cell devices. Thus, numerous approaches were made to develop the highly efficient anodes/bioanodes for high performance glucose biofuel cells [14–27].

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In this chapter, the preparation of electrodes/anodes and recent advances in the development of anode materials have been reviewed. The future prospective of the glucose biofuel cells has also been discussed.

Device Structure of Glucose Biofuel Cells The glucose biofuel cell consists of two electrodes anode and cathode, whereas the oxidation and reduction reactions takes place, respectively. The oxidation reactions occur at the anode, whereas the reduction reactions take place at the cathode. The schematic representation of the glucose biofuel cells has been displayed in Scheme 1. In the glucose biofuel cells, glucose oxidase is widely used to catalyze the oxidation of glucose at the surface of anode, whereas laccase was employed for the construction of cathode to reduce the oxygen. The oxidation of glucose produced gluconolactone at anode, whereas laccase produced water by the reduction of oxygen. The oxidation and reduction reactions can be seen in the Scheme 1.

Preparation of Anode for Glucose Biofuel Cell Applications In this section we will discuss the fabrication of anode/bioanode and design and synthesis of electrode materials for the construction of anode/bioanode for glucose biofuel cell applications. In general, researchers widely used glassy carbon electrode as working substrate for the preparation of anode for glucose biofuel cells applications. The glassy carbon electrode to be drop casted with mediator materials such as ferritin and enzymes like glucose oxidase for the construction of anode for glucose biofuel cells applications.

SWCNT/FRT/Glucose Oxidase/GCE as Anode In 2011, Shin et al. [28] employed carbon nanotubes for the preparation of anode for glucose biofuel cells. The authors have prepared a novel anode using single walled

Scheme 1 Schematic picture of the glucose biofuel cells

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Scheme 2 Schematic of working mechanism of anode towards glucose oxidation. (Reproduced with permission [28])

carbon nanotubes/ferritin (FRT)/glucose oxidase/glassy carbon electrode (SWCNT/ FRT/GOx/GCE) as electrode modifiers. The SWCNT/FRT composite was used as electron transfer mediator between the enzyme and electrode. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) methods were employed to check the performance of the prepared anode. The scanning electron microscopy (SEM) was also employed to check the surface of the prepared electrode. The SEM results exhibited the well distribution of the SWCNT and GOx on the surface of glassy carbon electrode. The schematic picture of the prepared anode has been displayed in Scheme 2. The Scheme 2 shows that SWCNT/FRT increased the rate of electron transfer from the active sites of the GOx to the electrode. This SWCNT/FRT also improved the electro-catalytic oxidation of glucose. The electrode surface of three glassy carbon electrodes was modified with GOx, SWCNT/GOx, and SWCNT/FRT/GOx using drop casting technique. The electrochemical investigations of the modified electrodes (GOx, SWCNT/GOx, and SWCNT/FRT/GOx) were carried out using CV approach and the recorded data have been depicted in Fig. 1. The CV graph of the GOx/GCE showed the lowest current response and electrochemical activity, whereas this current was slightly improved by introducing SWCNT to the surface of GOx/GCE. However, the highly efficient and improved current was observed when FRT was introduced between SWCNT and GOx. This indicated that FRT has worked as an efficient electron transfer mediator between the electrode and enzyme. This better and improved current was due to the efficient electron transfer from the active sites of the enzyme (GOx) to the electrode. The SWCNT/FRT/GOx/GCE showed better electrochemical response in absence of glucose (Fig. 1) compare to the other two electrodes. Thus, authors employed this SWCNT/FRT/GOx/GCE for further electrochemical measurements. The CV and LSV graphs of the SWCNT/FRT/GOx/GCE in presence of different concentrations of glucose were taken to investigate the effect of concentration on the electrochemical activity of SWCNT/FRT/GOx/GCE. The CV graphs of the SWCNT/FRT/GOx/

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Fig. 1 CV graphs of the GOx/GCE (a), SWCNT/GOx/ GCE (b) and SWCNT/FRT/ GOx/GCE in PBS of 7.4. (Reproduced with permission [28])

Fig. 2 CV (a) graphs of SWCNT/FRT/GOx/GCE in 1 mM (a), 5 mM (b), 10 mM (c), 20 mM (d), and 40 mM (e) of glucose. Current density versus concentration plot (b). (Reproduced with permission [28])

GCE in presence of 1 mM, 5 mM, 10 mM, 20 mM, and 40 mM glucose are depicted in Fig. 2a. The observations indicated that SWCNT/FRT/GOx/GCE exhibited the enhanced electro-catalytic current in presence of 1 mM glucose. This obtained current of SWCNT/FRT/GOx/GCE in 1 mM glucose was higher compared to the obtained current for SWCNT/FRT/GOx/GCE in absence of glucose. This suggested the electrochemical oxidation of glucose. Further the CV graphs of the SWCNT/FRT/GOx/GCE in presence of 5 mM, 10 mM, 20 mM, and 40 mM glucose were also taken and the obtained CV results have been displayed in Fig. 2a. The observations revealed that the current response increases with the increase of the glucose concentration. The current density versus glucose concentration plot has been presented in Fig. 2b. The highest current density of 4.6 mA/cm2 was achieved using SWCNT/FRT/GOx/GCE. The obtained current density was really impressive for the electrochemical oxidation of glucose using SWCNT/FRT/GOx/GCE.

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Polypyrrole/FRT/Di/NADH/GDH/GCE as Bioanode Conducting polymers have been used in electrochemical devices due to their excellent conductive properties. Kim and co-workers have investigated the electrochemical properties of the polypyrrole for the construction of anode for glucose biofuel cells [29] (Scheme 3). The authors prepared polypyrrole using by the polymerization of pyrrole monomer. Further FRT was deposited into the polyrrole followed by the deposition of diaphorase (Di) and nicotinamide adenine dinucleotide (NADH). The glucose dehydrogenase (GDH) was also coated followed by the coating of cross linker glutaraldehyde. Polypyrrole worked as electron transfer enhancer while FRT acted as electron transfer mediator during the oxidation of glucose. The electrochemical approach was used to investigate the performance of the constructed anode. The recorded CV graphs of the polypyrrole/FRT/GCE, polypyrrole/FRT/NADH/GCE, polypyrrole/FRT/Di/NADH/GCE, and polypyrrole/Di/ NADH/GCE in PBS (pH ¼ 7.4) at scan rate ¼ 100 mV/s have been presented in Fig. 3. The highest electrochemical activity was observed for the polypyrrole/FRT/ Di/NADH/GCE anode which suggested the excellent electrochemical activity of the polypyrrole and FRT. Further authors have recorded the CV graphs of the polypyrrole/FRT/Di/NADH/GDH/GCE in PBS at different scan rates. The CV data has been shown in Fig. 4 which is showing that current increases rapidly with increasing the scan rates. Moreover, the CV results indicated the better oxidation process compare to the reduction (Fig. 4). Thus, authors applied this electrode for the oxidation of glucose. The CV graphs of the polypyrrole/ FRT/Di/NADH/GDH/GCE in presence and absence of 45 mM glucose were also recorded. The obtained results showed the better electrocatalytic current was observed on the oxidation of glucose. The highest current density of 1.2 mA/cm2 was obtained on the oxidation of 45 mM glucose.

Scheme 3 Schematic picture shows the working mechanism of polypyrrole/FRT/Di-NADH-GDH electrode. (Reproduced with permission [29])

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Fig. 3 CV curves of Polypyrrole/FRT/GCE (a), Polypyrrole/FRT/NADH/GCE (b), Polypyrrole/ FRT/Di/NADH/GCE, (c) and Polypyrrole/Di/NADH/GCE (d) in PBS (pH ¼ 7.4) at scan rate ¼ 100 mV/s. (Reproduced with permission [29])

Fig. 4 CV curves of polypyrrole/FRT/Di/NADH/GDH/GCE in PBS (pH ¼ 7.4) at different scan rates (10-100 mV/s). (Reproduced with permission [29])

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Graphene/FRT/GOx/GCE as Bioanode Graphene has received enormous attention due to its excellent properties and has been used in electrochemical applications. Thus, Inamuddin et al. [30] constructed a bioanode using graphene as electron transfer enhancer while FRT as electron transfer mediator. Authors also used glucose oxidase (GOx) as enzyme to catalyze the oxidation reactions taking place at the surface of bioanode. The surface of GCE was modified with graphene, FRT, and GOx followed by the addition of glutaraldehyde. The schematic illustration of the working mechanism of the constructed bioanode towards the oxidation of glucose has been displayed in Scheme 4. The schematic graph clearly showed the transfer of the generated electron from the active sites of the GOx to GCE. The FRT acted as an electron transfer mediator, whereas the graphene has acted as electron transfer enhancer. Further electrochemical investigations were carried out by using LSV and CV approaches. The CV curves of the bioanode (graphene/FRT/GOx/GCE) in presence of 45 mM glucose in PBS (pH ¼ 7.0) at different scan rates (20, 40, 60, 80, and 100 mV/s) have been recorded. The obtained CV data has been displayed in Fig. 5. The observations revealed that electrochemical oxidation of glucose occurs at the surface of graphene/FRT/GOx/GCE. The electrochemical current response has been increases continuously with increasing the applied scan rates. The highest current was obtained at scan rate of 100 mV/s. Furthermore, LSV method was also used to further check the electrochemical performance of the constructed bioanode. The LSV curves of the graphene/FRT/GOx/GCE in presence of 5.0 mM (a), 15.0 mM (b), 25.0 mM (c), 35.0 mM (d), and 45.0 mM (e) glucose at scan rate ¼ 100 mV/s were also recorded. The LSV data has been shown in Fig. 6a. The LSV results showed the improved electrochemical oxidation of glucose at the

Scheme 4 Schematic representation of bioanode for glucose oxidation. (Reproduced with permission [30])

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Fig. 5 CV curves of graphene/FRT/GOx/GCE in 45 mM glucose in PBS (pH ¼ 7.0) at different scan rates (20, 40, 60, 80, and 100 mV/s). (Reproduced with permission [30])

Fig. 6 LSV (A) curves of the graphene/FRT/GOx/GCE in presence of 5.0 mM (a), 15.0 mM (b), 25.0 mM (c), 35.0 mM (d), and 45.0 mM (e) glucose at scan rate ¼ 100 mV/s. Calibration plot between the current versus glucose concentration (B). (Reproduced with permission [30])

surface of bioanode. The current response was enhanced with increasing the concentration of the glucose and the highest current response was obtained at 45.0 mM glucose. The obtained current response for the constructed bioanode towards the oxidation of glucose at different concentrations was found to be linear as confirmed by calibration plot (Fig. 6b). This improved performance of the bioanode was attributed to the better redox properties and synergistic effects of the FRT and graphene. The highest current density of 66.5 mA/cm2 was obtained for the electrochemical oxidation of 45.0 mM glucose.

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A Hybrid Biocatalyst for Biofuel Cells Christwardana et al. [31] designed biocatalysts for biofuel cell applications. In this work authors have synthesized a composite consist of silver (AgNP) nanoparticles, naphthalene-thiol based couplers (Naph-SH), and GOx which was further bonded with polyethyleneimine (PEI) and CNT (CNT/PEI/AgNPs/Naph-SH/GOx). This CNT/PEI/AgNPs/Naph-SH/GOx was used for the electrochemical oxidation of glucose. Enzymatic biofuel cells may be applied in insulin pumps, hearing devices, and pace-maker, etc. However, enzymatic biofuel cells have some drawbacks such as short stability, poor catalytic activity, and low performance/sluggish charge transfer rate. Hence, to overcome such issues of the enzymatic biofuel cells immobilization approach has been introduced. Christwardana et al. [31] applied CNT as conductive support material for GOx for better electron transfer and to improve catalytic activity. The developed glucose biofuel cells showed good power density. Chung et al. [32] also applied enzyme-incorporated copper nanoflowers for glucose biofuel cells. The sonication method was applied for the preparation of enzyme-inorganic hybrid nanoflowers. Authors have developed the mediator-less glucose biofuel cells using novel strategies. The nanoflowers incorporating enzymes (GOx, laccase, catalase with copper phosphate) were mixed with MWCNT. The schematic graph of the glucose biofuel cells has been represented in Scheme 5. The oxidation of glucose yielded gluconolactone at the anode surface (Scheme 6). Morphological features of the electrode materials also influence the catalytic activity. Thus, the morphological investigations of the GOx, laccase, catalase, and bovine serum albumin (BSA) were analyzed by SEM analysis. The SEM images of the SEM pictures of GOx, laccase, catalase, and BSA have been displayed in Fig. 7. The SEM investigations indicate the presence of nanoflowers like surface morphology of the GOx, laccase, catalase, and BSA (Fig. 7). The CV method was applied to evaluate the performance of the bioelectrodes (bioanode and biocathode). The CV curves have been shown in Fig. 8. The CV

Scheme 5 Schematic graph of the biofuel cells. (Reproduced with permission [32])

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Scheme 6 Synthetic procedure of MnO2/PSS/Gph. (Reproduced with permission [33])

Fig. 7 SEM pictures of GOx nanoflowers (a), laccase nanoflowers (b), catalase nanoflowers (c), and BSA nanoflowers (d). (Reproduced with permission [32])

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Fig. 8 CV curves of the bioanode (a) and biocathode (b). (Reproduced with permission [32])

Scheme 7 Working mechanism of bioanode (MnO2/PSS/Gph/Frt/GOx/ GCE) towards the oxidation of glucose. (Reproduced with permission [33])

curves of the bioanode with GOx and free GOx have been depicted in Fig. 8a which exhibited the poor current response for GOx nanoflowers based bioanode. In case of biocathode, the poor current response was also observed for laccase nanoflowers based biocathode.

MnO2/PSS/Gph/FRT/GOx/GCE as Bioanode Sofia et al. [33] also designed a novel bioanode for the electrochemical oxidation of glucose for glucose biofuel cell applications. Authors have synthesized the manganese dioxide-polystyrene sulfonate-graphene (MnO2/PSS/Gph) composite. The schematic illustration of the synthesis of MnO2/PSS/Gph has been represented in Scheme 7. The physiochemical properties were analyzed by SEM, EDX, and Fourier transform infrared (FTIR) spectroscopy.

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Fig. 9 CV curves of the MnO2/PSS/Gph/Frt/GOx/GCE in 40 mM glucose in absence (a) and presence (b) of nitrogen purging at scan rate ¼ 100 mV/s. (Reproduced with permission [33])

Further a novel bioanode was developed (MnO2/PSS/Gph/Frt/GOx/GCE) for glucose biofuel cell application. The working mechanism of the MnO2/PSS/Gph/ Frt/GOx/GCE towards the oxidation of glucose has been illustrated in Scheme 6. Further electrochemical methods CV and LSV were applied to check the electrocatalytic activities of the MnO2/PSS/Gph/Frt/GOx/GCE for glucose oxidation reactions. The CV curves of the MnO2/PSS/Gph/Frt/GOx/GCE in 40 mM glucose in absence and presence of nitrogen purging were recorded and are shown in Fig. 9. The obtained CV data showed the excellent current response towards the oxidation of glucose using MnO2/PSS/Gph/Frt/GOx/GCE bioanode. The higher current response was observed in presence of nitrogen purging. The lower current response for the MnO2/PSS/Gph/Frt/GOx/GCE bioanode in absence of nitrogen purging may be due to the generation of side products (hydrogen peroxide). The current density of 2.7 mA/cm2 was obtained using MnO2/PSS/Gph/Frt/GOx/GCE bioanode for the oxidation of glucose.

Conclusions Glucose biofuel cells have the potential for implantable/portable miniaturized devices and pacemaker, etc., applications. Generally glucose biofuel cells involve enzymes to catalyze the glucose oxidation reaction and electron transfer enhancer and mediators to accelerate the rate of electron transfer. Ferritin, which has redox properties, has been widely used as electron transfer mediator, whereas glucose oxidase is an enzyme which is used to catalyze the glucose oxidation reactions at the anode surface in the glucose biofuel cells. The electron transfer enhancer also plays a vital role in glucose biofuel cells. Previously conducting polymers and

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nano-materials have been widely explored as electron transfer enhancer for the construction of anodes for biofuel cell applications. Therefore, the following strategies would be helpful to improve the performance of the enzyme catalyzed glucose biofuel cells: 1. Since the electron transfer enhancer materials influence the performance of the anodes/bioanodes, thus novel electron transfer enhancers to be developed for the construction of anodes/bioanodes for biofuel cell applications. 2. The design and fabrication of new hybrid composite materials with high surface area would be great to develop the anodes for biofuel cell applications. 3. The nanomaterials with new morphological features would also be useful for the construction of anodes. 4. Polymeric composite materials with high surface area and conductivity will also be helpful to enhance the electron transfer in biofuel cells. 5. MXene (metal carbide or metal nitride) based electron transfer enhancer may also be useful for biofuel cell applications. Acknowledgments K.A. acknowledged Discipline of Chemistry, IIT Indore.

References 1. Ahmad K, Mohammad A, Mobin SM (2017) Hydrothermally grown α-MnO2 nanorods as highly efficient low cost counter-electrode material for dye-sensitized solar cells and electrochemical sensing applications. Electrochim Acta 252:549–557 2. Ahmad K, Mobin SM (2017) Graphene oxide based planar heterojunction perovskite solar cell under ambient condition. New J Chem 41:14253–14258 3. Ahmad K, Kumar P, Mobin SM (2020) A two-step modified sequential deposition methodbased Pb-free (CH3NH3)3Sb2I9 perovskite with improved open circuit voltage and performance. ChemElectroChem 7:946–950 4. Ahmad K, Ansari SN, Natarajan K, Mobin SM (2019) A two-step modified deposition method based (CH3NH3)3Bi2I9 perovskite: lead free, highly stable and enhanced photovoltaic performance. ChemElectroChem 6:1–8 5. Ahmad K, Ansari SN, Natarajan K, Mobin SM (2018) Design and synthesis of 1D-polymeric chain based [(CH3NH3)3Bi2Cl9]n perovskite: a new light absorber material for lead free perovskite solar cells. ACS Appl Energy Mater 01:2405–2409 6. Ahmad K, Mobin SM (2020) Organic–inorganic copper (II)-based perovskites: a benign approach toward low-toxicity and water-stable light absorbers for photovoltaic applications. Energ Technol 8:1901185 7. Katz E, Shipway AN, Willner I (2003) In: Vielstich W, Gasteiger HA, Lamm A (eds) Handbook of fuel cells-fundamentals, technology and applications, fundamentals and survey of systems, vol 1. Wiley, Chichester, p 355 8. Topcagic S, Minteer SD (2006) Development of a membraneless ethanol/oxygen biofuel cell. Electrochim Acta 51:2168–2172 9. Ivnitski D, Branch B, Atanassov P, Apblett C (2006) Glucose oxidase anode for biofuel cell based on direct electron transfer. Electrochem Commun 8:1204–1210 10. Haque SU, Inamuddin AN, Rajender B, Khan A, Asiri AN, Ashraf GM (2017) Optimization of glucose powered biofuel cell anode developed by polyaniline-silver as electron transfer enhancer and ferritin as biocompatible redox mediator. Sci Rep 7:12703

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11. Gobel G, Beltran ML, Mundhenk J, Heinlein T, Schneider J, Lisdat F (2016) Operation of a carbon nanotube-based glucose/oxygen biofuel cell in human body liquids, performance factors and characteristics. Electrochim Acta 218:278–284 12. Basu D, Basu S (2011) Synthesis and characterization of PteAu/C catalyst for glucose electrooxidation for the application in direct glucose fuel cell. Int J Hydrog Energy 36:14923–14929 13. Gu Y, Yang H, Li B, An Y (2016) A ternary nanocatalyst of Ni/Cr/Co oxides with high activity and stability for alkaline glucose electrooxidation. Electrochim Acta 192:296–302 14. Wan J, Mi L, Tian Z, Li Q, Liu S (2020) A single-liquid miniature biofuel cell with boosting power density via gas diffusion bioelectrodes. J. Mater. Chem. B 8:3550–3556 15. Marinoiu A, Gatto I, Raceanu M, Varlam M, Moise C, Pantazi A et al (2017) Low cost iodine doped graphene for fuel cell electrodes. Int J Hydrog Energy 42:26877–26888 16. Bandapati M, Dwivedi PK, Krishnamurthy B, Kim YH, Kim GM, Goel S (2017) Screening various pencil leads coated with MWCNT and PANI as enzymatic biofuel cell biocathode. Int J Hydrog Energy 42:27220–27229 17. Odetola C, Trevani L, Easton EB (2015) Enhanced activity and stability of Pt/TiO2/carbon fuel cell electrocatalyst prepared using a glucose modifier. J Power Sources 294:254–263 18. Aslan S, Tutum M, Tepeli Y, Anik U (2016) Comparison of influence of nanomaterials on a glassy carbon paste electrode-based bioanode in biofuel cells. Turk J Chem 40:698–705 19. Mishra P, Jain R (2016) Electrochemical deposition of MWCNTMnO2/PPy nano-composite application for microbial fuel cells. Int J Hydrog Energy 41:22394–22405 20. Tsang ACH, Kwok HYH, Leung DYC (2017) The use of graphene based materials for fuel cell, photovoltaics, and supercapacitor electrode materials. Solid State Sci 67:A1–A14 21. Janardhanan VM, Deutschmann O (2011) Modeling diffusion limitation in solid-oxide fuel cells. Electrochim Acta 56:9775–9782 22. Perveen R, Inamuddin, Haque UIS, Nasar A, Asiri AM, Md Ashraf G (2017) Electrocatalytic performance of chemically synthesized PIn-au-SGO composite toward mediated biofuel cell anode. Sci Rep 7:13353 23. Lv Z, Xie D, Li F, Hu Y, Wei C, Feng C (2014) Microbial fuel cell as a biocapacitor by using pseudo-capacitive anode materials. J Power Sources 246:642–649 24. Kalathil S, Nguyen VH, Shim J-J, Khan MM, Lee J, Cho MH (2013) Enhanced performance of a microbial fuel cell using CNT/MnO2 nanocomposite as a bioanode material. J Nanosci Nanotechnol 13:7712–7716 25. Hou J, Liu Z, Zhang P (2013) A new method for fabrication of graphene/polyaniline nanocomplex modified microbial fuel cell anodes. J Power Sources 224:139–144 26. Zhang C, Liang P, Yang X, Jiang Y, Bian Y, Chen C et al (2016) Binder-free graphene and manganese oxide coated carbon felt anode for high-performance microbial fuel cell. Biosens Bioelectron 81:32–38 27. Cosnier S, Gross AJ, Giroud F, Holzinger M (2018) Beyond the hype surrounding biofuel cells: What’s the future of enzymatic fuel cells? Current Opinion Electrochem 12:148–155 28. Shin HJ, Shin KM, Lee JW, Kwon CH, Lee S-H, Kim SI, Jeon J-H, Kim SJ (2011) Electrocatalytic characteristics of electrodes based on ferritin/carbon nanotube composites for biofuel cells. Sensors Actuators B Chem 160:384–388 29. Inamuddin K, Shin M, Kim SI, So I, Kim SJ (2009) A conducting polymer/ferritin anode for biofuel cell applications. Electrochim Acta 54:3979–3983 30. Inamuddin K, Ahmad MN (2014) Optimization of glassy carbon electrode based graphene/ferritin/ glucose oxidase bioanode for biofuel cell applications. Int J Hydrog Energy 39:7417–7421 31. Christwardanaa M, Kim D-H, Chung Y, Kwon Y (2018) A hybrid biocatalyst consisting of silver nanoparticle and naphthalenethiol self-assembled monolayer prepared for anchoring glucose oxidase and its use for an enzymatic biofuel cell. Appl Surf Sci 429:180–186 32. Chung M, Nguyen TL, Tran TQN, Yoon HH, Kim IT, Kim MI (2018) Ultrarapid Sonochemical synthesis of enzyme-incorporated copper Nanoflowers and their application to Mediatorless glucose biofuel cell. Appl Surf Sci 429:203–209 33. Haque SU, Nasar A, Inamuddin, Asiri AM (2019) Preparation and characterization of a bioanode (GC/MnO2/PSS/Gph/Frt/GOx) for biofuel cell application. Int J Hydrog Energy 44:7308–7319

Properties of Diamonds and Their Application in Photodetectors

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Qilong Yuan, Cheng-Te Lin, and Kuan W. A. Chee

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Extrinsic Diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Transmission and Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Diamond Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exploiting Photodetection Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond Photoconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond Schottky Barrier Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrode Structure of Diamond Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond UV Photoconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spectral Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . External Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond Radiation Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Charge Collection Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Charge Collection Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiation Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1872 1874 1874 1875 1875 1876 1876 1878 1878 1880 1882 1882 1882 1884 1884 1886 1887 1888 1889 1892 1893

Q. Yuan · C.-T. Lin Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China K. W. A. Chee (*) Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of China, Hefei, P.R. China Laser Research Institute, Shandong Academy of Sciences, Qingdao, P.R. China e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_189

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Abstract

Diamond is not an unfamiliar material for most people as it is being widely used in jewelry over the past several centuries. Besides being metastable and having a shiny appearance, diamond has the highest mineral hardness and thermal conductivity among natural materials while posessesing extraordinary properties among ultra-wide bandgap semiconductors. Compared with traditional semiconductors, such as Si, GaAs, SiC, and so on, diamond has superior carrier mobility, good optical transparency, and high radiation hardness. As a result, radiation detector applications in high-energy physics experiments began to exploit natural diamond materials from as early as in the middle of the last century. However, the highly variable and uncontrollable defect content and concentration of impurities in natural diamonds constrain the advanced and reliable application of diamond electronic devices. In recent years, the development of diamond synthesis techniques via chemical vapor deposition (CVD) has led to investigations into diamond electronic applications, as large-area (centimeter scale) production of single-crystal diamond can be realized, and the impurity concentration can be controlled. Diamond radiation detectors therefore represent one of the research projects that underwent some decades of investigation and are still being under study. In this chapter, we summarize the detection mechanisms in two different types of diamond detectors: 1) diamond photoconductors and 2) Schottky barrier photodiodes, which depend strongly on the contact properties between the electrode materials and diamond surface. Meanwhile, the main features of diamond detectors, like responsivity, quantum efficiency, charge collection efficiency, charge collection distance, and others, are introduced and discussed. The remarkable properties of diamond promise several opportunities in a diverse range of application fields, whereas the daunting technical challenges, such as relating to size, doping, and manufacturing scalability, are still in the way of high technology commercialization, especially in electronics. Nevertheless, with new developments in materials synthesis, and research and practical applications, the outlook for diamond is exciting. Keywords

Dopant impurities · Diamond electronics · Photoconductor · Schottky barrier photodiode · Quantum efficiency · Responsivity · Microwave plasma chemical vapor deposition · Radiation detector

Introduction Diamonds have been admired for over thousands of years for their gemological characteristics. In contrast to graphite, the other carbon allotrope, the stable covalent bonding of each carbon atom with four identical neighbors in the diamond crystal lattice is highly symmetric, thus exhibiting the highest scratch hardness and a relatively

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high optical dispersion. Besides these, diamond has several other important attributes, such as a wide bandgap (
5.47 eV) [1], high carrier mobility [2, 3], high thermal conductivity [4], good optical transparency [5], and high radiation hardness [6]. Other special optical properties include a high index of refraction and a high lustre. Laboratory-scale usage of natural diamond can be traced back to the middle of the last century, for example, in its application to radiation detection and particle physics [7, 8]. However, due to the exorbitance of natural diamonds and the existing high impurity concentrations, diamond-related research and technology developments have been slow, until the production of synthetic diamonds became possible [9]. Natural and synthetic diamond can be classified according to their nitrogen impurity content into Types I and II. Based on distinct differences in thermal, electronic, and optical properties, diamond can be further classified into Types Ia, Ib, IIa, and IIb, as shown in Table 1 [5]. For Type I diamonds, the main impurities are nitrogen atoms, at a concentration of about 0.1%, while Type II diamonds have no measurable nitrogen impurities. The pale yellow or lack of color of the Type Ia diamond is due to the absorption of blue light by nitrogen clusters within the carbon lattice, and the more intense yellow or brown hue of the Type Ib diamond is due to the absorption of green and blue light by the more diffuse dispersion of the nitrogen impurities. About 98% of all natural diamonds are Type Ia, whereas almost all highpressure high-temperature (HPHT) synthetic diamonds are of Type Ib. A small percentage (up to 2%) of natural diamonds exist as Type II, when they have been formed under extremely high pressure, and over longer periods of time. Occasional crystal imperfections can result in various color characteristics (violet, brown, yellow, orange, pink or red), but which can be alleviated by a HPHT process that removes most or all of the color. Like some 1–2 % of natural diamonds, synthetic Table 1 Comparison of the different types of diamond [5] Diamond type Color

Electrical conductivity type Thermal conductivity (W/cmK)

Impurity specie

Nitrogen

Boron

Percentage of all natural diamonds

I a Transparent/ colorless/pale yellow Highly insulating

b Dark yellow/ brown

~7

~10–18

Up to ~0.3 % (~3000 ppm) Below detectable limits ~98 %

Up to ~0.05 % (~500 ppm) Below detectable limits ~0.1 %

II a Transparent/colorless

~20 (expected to have the highest thermal conductivity) Below detectable limits Below detectable limits ~1–2 %

b Blue/bluegray Highly conductive ~20

Below detectable limits 62%

[74]

> 90%

[74] [75]

[77]

MB dye

Synthetic wastewater

DO61 dye

Synthetic wastewater Synthetic wastewater (deionized and double distilled water) Synthetic wastewater

Electro-Fenton process

100% of degradation and mineralization 100% of degradation was observed after 30 min 100% after 6 h

Electro-Fenton process

100% after 360 min

[78]

Electro-Fenton process called “pyrite electroFenton” (pyrite-EF)

100%

[79]

Synthetic wastewater, bidistilled water Ultra-pure, water type I, according to ASTM-D119399ε Synthetic wastewater, distilled water.

Electro-Fenton process

Electro-Fenton process

[80]

Electrochemical and photo-electrochemical Fenton process

74% after 50 min

[81]

Electro-Fenton method in batch recirculation mode

91% of COD removal.

[82]

AR14 dye

(4-amino-3hydroxy-2-ptolylazonaphthalene1-sulfonic acid Alizarin red dye DY-52 dye

4-Nitrophenol dye

[76]

(continued)

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Table 3 (continued) Dye MO dye

Matrix –

Methods Electro-Fenton process with H2O2 generation in a rotating disk reactor

Orange G dye

–

Rhodamine B dye

Synthetic wastewater, rectangular threephase, 3D electrode reactor Synthetic wastewater, cylindrical reactor with a working volume of 0.15 L Aqueous synthetic wastewater Aqueous synthetic solution

Electro-photocatalytic Fenton- Mesoporous TiO2/stainless steel mesh photo-electrode Electro-Fenton system (3D-E-Fenton)

RB5 dye

RR120 dye

Orange II dye

Azure B dye

RB 19 dye Rhodamine B dye MO dye

MB dye

CR dye

Synthetic wastewater, double distilled water Synthetic wastewater Synthetic wastewater, in an ultrasound bath. Synthetic wastewater, in pyrex photoreactor Synthetic wastewater, batch mode in a thermostatic water bath Synthetic wastewater, batch mode in a thermostatic water bath

Removal efficiency 100% of color removal of MO after 15 min, while TOC removal 58.7% after 2 h 78% of color removal after 3 h

Ref. [83]

99%

[85]

81% after 120 min.

[86]

98%

[87]

Three-dimensional electro-Fenton (3D/EFenton) system Sono-photo-Fenton and photo-Fenton

98.89 and 71.57% of decolorization and COD removal 100%

[88]

[89]

Sono-Fenton

78%

[90]

Sonophotocatalytic degradation

100% after 180 min

[91]

Sonophotocatalytic

100%

[92]

Fenton, photo-Fenton, sono-Fenton, and sonophoto-Fenton methods

100% under UVand 93% under VIS after 120 min

[93]

Fenton, photo-Fenton, sono-Fenton, and sonophoto-Fenton methods

100% under UVand 97% under VIS after 120 min

[93]

Electro chemical Fenton processes – Homogenous and heterogeneous electroFenton treatment Heterogeneous electroFenton reaction

[84]

(continued)

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Table 3 (continued) Dye Malachite green Acridine orange MB dye

Matrix –

Methods Fenton processes

Removal efficiency 100% after 50 min

Ref. [94]

–

Fenton processes

98.4% after 60 min

[95]

99.7%

[96]

Acid dye bath effluent

Dye bath effluent

Heterogeneous Fenton Fenton processes

[97]

Levafix CA dye MR dye RR 120 dye RY 84 dye Eosin Y dye Reactive brilliant orange X-GN AO 7 dye

Synthetic wastewater – – – – –

Electro-Fenton treatment Electro-Fenton Photo-Fenton Photo-Fenton Solar photo-Fenton Heterogeneous photoFenton

92% and 24% of color and COD removal 100% 80% 99% 99% 90% 98,6%

[99] [100] [100] [101] [102]

Sono-Fenton

91%

[103]

Sono-Fenton

82%

[104]

Real dyeing wastewater AR 14 dye AO 7 dye BY 28 dye

Synthetic wastewater Synthetic wastewater Real dyeing wastewater – – –

Electro-Fenton

75% COD removal

[105]

97% 84.9% 97%

[106] [107] [108]

DY12 dye

–

Photoelectro-Fenton Sono-photo-Fenton Photoelectro-Fenton/ ZnO Photoelectro-Fenton/ ZnO

90%

[109]

AO 7 dye

[98]

Some Proposed Mechanisms of Different AOPs An example of sono-photo-Fenton and sonication mechanisms for azure-B can be shown below (Eqs. 22–31) [89]: By sono-photo-Fenton: Fe3þ þ H2 O þ hv ! Fe2þ þ  OH þ Hþ

ð22Þ

Fe2þ þ H2 O2 ! Fe3þ þ OH þ H

ð23Þ

HO2 þ OH ! H2 O þ O2

ð24Þ

Fe2þ þ OH ! Fe3þ þ OH

ð25Þ

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HO2 þ HO2 ! H2 O2 þ O2

ð26Þ

OH þ OH ! H2 O2

ð27Þ

H2 O þ US ! OH þ H

ð28Þ

H þ H2 O2 ! OH þ H2 O

ð29Þ

Fe3þ þ H ! Fe2þ þ Hþ

ð30Þ

Azure-B þ OH ! Products

ð31Þ

By sonication:

Other example of photo-Fenton-mechanisms can be shown in the Fig. 7 [21]. Other example of Fenton, photo-Fenton, and sono-photo-Fenton mechanisms can be shown in the Fig. 8 [93]. Other examples of AOPs (UV/O3) mechanisms study using HPLC-MS/MS for AR-17, DY-12, and AY-11 were reported [12, 28, 30]. For AR-17 dye, the analysis suggested a sequenced oxidation mechanism, in which the O3 and or hydroxyl radical preferentially attacked the chromophore centers of the dye molecules

Fig. 7 Photo-Fenton oxidation mechanism for RB5 dye [21]

Fig. 8 Fenton, photo-Fenton, and sono-photo-Fenton mechanism [93]

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(–N¼N–) cleaving them into the lateral substituted naphthalene ring [28]. The monitoring of the AR-17 dye after 230 min of treatment showed complete disappearance of fragments and no aromatic amine or phenol was detected. While for DY12 dye, the DY-12 dye decolorization process was analyzed by HPLC-MS/MS at the interval times of 5–750 min. The monitoring of the DY-12 dye after 750 min of treatment showed manly complete disappear of fragments and no aromatic amine or phenol are detected during or at the end of the treatments. The appearance of new minor peaks and disappearance of the major peak in the decolorized dye products elution profile support the biodegradation of DY-12 dye [30]. The AY-11 dye degradation intermediates were identified using the LC-MS/MS technique. An investigation of the reaction mechanism of the ozonation of AY-11 dye decomposition was carried out from the initial step to the final products. It was shown that radical AY-11 dye underwent partial and complete oxidation of the dye ring to form final products, namely maleic acid, malonic acid, oxalic acid, and formic acid [12].

Photocatalytic Processes Photocatalysts are compounds that are triggered without being absorbed by photon absorption and help intensify a reaction [110]. A photocatalyst should not be involved or consumed directly in the reaction and must create other process routes from current photo reactions and accelerated reaction rates [111]. The most of photocatalysts are semi-conductor to perform a photo-induced oxidation cycle to degrade organic pollutants beside deactivation of viruses and bacteria [112, 113]. Numerous forms of semiconductors were tested as VIS active photocatalysts, for example, plasmonic and metal oxides (MOs) photocatalysts, and polymeric C3N4, have been utilized for the selective redox organic alterations, [114–116]. The heterogeneous photocatalysis contains the photo-activation of a semiconductor under UV or VIS radiation, with energy equivalent to or bigger than the energy of band gap, encouraging oxidation reduction (OR) reactions on the surface of catalyst, and thus lead to the breakdown of organic contaminants. Some of the most used catalysts are TiO2, CdS, ZnO, ZnS, WO3, SnO2, and Fe2O3.

Photocatalytic Applications for Dye Degradation Diverse types of dyes are available as coloring items in the markets. Classification of dye materials is based on the molecule component structure, color, and application process. The general classification of dyes originated as quinine-amine, acridine, azo, nitro, xanthene, and anthraquinone dyes, and so on, depend on the chromophoric group in the molecular moiety [117]. The studies recorded on degradation of photocatalytic dye are mainly concerned with variables such as dye concentration, amount of photocatalyst used, effect of irradiated light intensity, irradiation time, and influence of dissolved O2 and other organisms. The photocatalytic color decay is regarded to the first order pseudo reaction with kinetic data equipped with the

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equation of heterogeneous photocatalytics is considered one of the most recent approaches of color degradation or bleaching [118]. The breakdown of contaminants comprises three main stages: (i) charge carriers the photogeneration (electron–hole pairs) then (ii) transferring of charge carriers to the surface, and finally (iii) subsequent breakdown in dye with the photo generated electron to harmless compounds of low molecular weight.

Heterogeneous Photocatalysis MOs perform a significant part in various types of applications due to their simple composition and stoichiometric multiplicity. Water splitting using photo catalysis was investigated first using TiO2 [119], and it was commercialized because of its low charge, no contrary consequence, less chemical reactivity, and high photocatalytic action. Many other metal oxides like WO3 [120], ZnO [121], MoO3 [122], SnO2 [123], ZrO2 [124], SrTiO3 [125], ZnTiO3 [126], Fe2O3 [127], CeO2 [128], FeVO4 [129], etc., emerged as photocatalysts as a result of their appropriateness for photocatalysis.

UV/TiO2 TiO2 photocatalysis has relatively small price and is much cheaper than other AOPs (UV/O3; UV¼H2O2, photo-Fenton). The common photo oxidizing and photocatalytical properties of TiO2 semiconducting particle suspensions are extremely recognized in the literatures. Since TiO2 has the potential to totally oxidize a huge quantity of harmful compounds to harmless compounds, photochemical steadiness, and economical. It has been developed into a benchmark semiconductor for use in various photocatalysis processes of water detoxification schemes (Fig. 9) [130]. Consequently the technique of TiO2 photocatalysis has been efficaciously operated

Fig. 9 Photo-induced processes by TiO2

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in recent years to breakdown contaminants [131]. The initial step in photocatalysis with TiO2 comprises the production of an electron-hole pair (e/h+), because of the creation of. OH, O2 and OOH radicals, as shown below in (Eqs. 10–14) [132]. As a result of that TiO2 can absorb only a lesser part of the solar spectrum in the nearUV (band gap 3.0 eV [rutile] to 3.2 eV [anatase]), it is necessary to transfer its absorption edge to the red spectrum area or to find alternate partner. A photochemical reaction rate (r) is proportional to the light intensity the photocatalyst (IA) absorbs. The light that the catalyst absorbs is determined by the difference between the light which reaches Total sample (Io) and light absorbed by a coloring. The radicals produced through the mechanisms below are attacking and oxidizing the organic pollutants. Additionally to. OH, O2 and, in certain cases, the positive holes (h+) are also recommended as potential oxidizing species that may attacked organic pollutants exit on or near the surface of TiO2 [133]. It was found that the adsorption level on the unmodified TiO2 in photocatalytic degradation is higher for dyes with a positive charge (cationic) than for those with a negative charge (anionic). As the charge relies on the pH of a specific solution, it follows that pH as well as the quality of a specific dye affect the photocatalyst surface. Around the same time, their photocatalytic degradation is more rapid than that of anionic dyes. TiO2 þ hvþ ! TiO2 ðHþ þ e Þ

ð32Þ

H2 OðadsÞ þ Hþ !  OH þ Hþ

ð33Þ

OH þ Hþ !  OH

ð34Þ

O 2 þ e !  O 2 

ð35Þ

O2ðadsÞ þ e þ Hþ ! HO 2

ð36Þ

UV/ZnO Because of its chemical steadiness, nontoxic, and inexpensive nature, ZnO has attained considerable attention from researchers (Fig. 10). It also absorbs a significant fraction of the solar spectrum, which makes it a good candidate for photocatalysis. For instance, ZnO and transition metal nanoparticles doped from ZnO. In many studies, ZnO can be as effective as TiO2 and in some cases with higher photocatalytic activity than TiO2 in the photocatalytic degradation of certain organic matter [134]. The photocatalytic degradation of the simple yellow 2 dye was studied by Poulios et al. [135]. Under the conditions applied in this study, the solution was degraded by a TiO2 P-25 catalyst by 95% after 60 min of UV light exposure, while the solution was degraded by about 100% at the end of 60 min under ZnO. Muruganandham et al. [136] studied the photocatalysis degradation of reactive yellow14 color in an aqueous solution, and the superior efficiency of ZnO over TiO2 was also demonstrated. The analysis showed ZnO > TiO2-P25 > TiO2 (anatase), the order of reactivity following. Tian et al. [137] investigated the effectiveness of ZnO semiconductors synthesized by calcination of zinc acetate dihydrate (Zn

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Fig. 10 Removal of pollutants by the formation of photo induced charge carriers (e/h+) in a semiconductor ZnO2 particle surfaces

(Ac)2•2H2O) and TiO2-P25 (Degussa) in the photodegradation of the methyl orange dye, accomplishing that the degradation rate using the ZnO was 4 times greater when contrasted to the usage of the TiO2-P25 (Degussa). Using the heterogenous photocatalytic cycle, Kansal et al. [138] studied methyl orange degradation and rhodamine 6 G dyes. The experimental findings indicate that with usage of a ZnO and basic pH, the highest decolorization of the dyes occurs (above 90 percent); in addition, it was found that the ZnO/VIS system works much more efficiently than the ZnO/UV system. Figure 10 describes the schematic mechanism of photocatalytic activity of synthesized ZnO nanoparticles for different organic pollutants under UV irradiation [21]. Table 4 shows examples of different photochemical processes such as UV, UV/O3, UV/H2O2, Fenton processes such as (Fe2+/H2O2/UV), and other heterogeneous photochemical process such as UV/ZnO, UV/TiO2 and the decolorization efficiencies of each processes.

Transition Metal-Doped TiO2 Greater energy than band gap is required for photoexcitation. However, almost 45% VIS and 5% UV light is present in the solar spectrum. To use much of the spectrum, scientists have investigated doping transition metal such as Cu, Mn, V, Ni, Fe, Ag into the TiO2 to move its band gap toward the VIS region because of change in the electronic properties and crystal structure. Co, Mn, and Mn–Co doping into TiO2 (Mn, Co –TiO2) was studied using co-precipitation technique for methylene blue dye (MB) degradation [139]. Doping with Mn, Co, and Mn–Co into TiO2 lead to shift in the XRD peak position to lower angle, which specifies the increase in the lattice constant [140]. Dopants have been proposed as electron hunter to improve the isolation of the electron hole or the existence of charges. This marks charge carriers

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Table 4 Degradation of dyes by different photocatalytic process Technique TiO2/UV TiO2/UV TiO2/UV TiO2/UV TiO2/UV TiO2/ H2O2/UV ZnO/UV ZnO/UV ZnO/UV

Dye Direct red 80 dye Eosin yellow Basic yellow 2 Acid yellow 73 BG yellow Procion navy Hexl, Procion crimson H-exl and Procion yellow H-exl) RB 5 Direct red 80 dye Methyl Orange

ZnO/UV ZnO/UV ZnO/UV

Basic yellow 3 Acid yellow 73 BG yellow

Removal efficiency 96% of color removal after 15 min, pH 3 95.17% of color removal after 60 min 94.65% of color removal 60 min 95.57% of color removal 60 min 99% of color removal 90 min 100% of complete decolorization is achieved after only 8 min at H2O2 0.5% w/w 90% of color removal after 60 min 96% of color removal after 15 min pH 3 92% of color removal after 9 hours – pH 3 95.48% of color removal after 60 min 96.22% color removal after 60 min 99% of color removal after 60 min

Ref [148] [149] [149] [149] [150] [151]

[21] [148] [152] [149] [149] [152]

accessible for redox reactions. Ag-TiO2 has been expansively investigated. Yu et al. [141] described Ag-TiO2 showing enhanced photocatalytic activity. Khairy and Zakaria [142] examined the outcome of Cu and Zn-TiO2 on their photocatalytic performances under UV radiation toward the decolorization of methyl orange (MO) solutions and COD. The small crystallite size and doping ions (Cu and Zn) limited any phase transformation and encouraged the development of the TiO2 anatase phase. The optical study exhibited that doping ions lead to an improvement in the absorption edge wavelength and a reduction in the band gap energy of TiO2. The doped TiO2 in common exposed greater photocatalytic performances than the pure ones. The Cu-TiO2 exhibited the best photocatalytic performance depend on the determined COD values. Sharma et al. [143] examined the Photocatalytic degradation of direct blue 71 dye by Ag-TiO2. Improvement in the photocatalytic degradation under UV radiation of the dye was noticed with Ag-TiO2 at 2.0% mole fraction in comparison to undoped TiO2. While examining the photoctalytic performance, the reaction mixture pH was reserved unchanged with doped TiO2, where in with undoped TiO2 required acidic pH to have more photocatalytic performance.

Transition Metal-Doped ZnO ZnO-based photocatalysts have been operated for the organic contaminants degradation under UV or solar irradiation. ZnO is a very well-known semiconductor photocatalyst demonstrating a wide band gap of 3.36 eV with great exciton binding energy of 60 meV at room temperature [144]. Doping with numerous transition metals has been investigated by scientists to improve the photocatalytic performance of ZnO semiconductor. Fe-ZnO has been operated for photocatalysis by [145]. With the increase of Fe -ZnO, the full width at half maximum of the XRD peaks increased. It clues about the decrement in the crystallinity of the composite as well as a decrease in crystallite size. Introduction of lattice disorder and strain induced by interstitial Fe

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atoms causes’ size decrease [140]. As the Fe content increases, lattice strain increases and inhibits the grain growth. Distribution of lattice constants resulting from crystal imperfections leads to lattice strain. Saleh and Djaja [146] studied UV light photocatalytic degradation of methyl blue and methylene orange with Fe-ZnO nanoparticles. The impacts of numerous factors, such as pH, dopant concentrations, and photocatalytic amount, were examined. In general, Fe-doped ZnO nanoparticles are better catalysts for the degradation of methyl orange under UV irradiation than for the degradation of methylene blue. The results also showed that the adding of a dopant atom expressively enhanced the photocatalytic performance. Mittal et al. [147] studied the UV–VIS light induced photocatalytic of Cu-ZnO. Outcomes demonstrate that Cu-ZnO NPs have showed greater level of degradation in comparison to other undoped samples at pH 8. Doping has improved the degradation by trapping e and h+ and thus by decreasing electron–hole recombination.

Conclusions This chapter focuses on advanced oxidation treatment methods that are much stronger than others when the strong unsaturated bonds of coloring molecules collapse. The efficiency and ability of advanced oxidation processes to process almost all solid components in textile liquid has attracted attention. Advanced oxidation methods have shown surprising results in treating wastewater from dyeing factories, and these methods have demonstrated the ability to break dyes, remove colors, and reduce rates of dye contamination. Combined treatment with the UV and Fenton is known in the United States as the SPF (sono-photo-Fenton) method, which greatly enhanced the production of hydroxyl radicals in an aqueous environment so that they could break mixed dyes in the aqueous environment. Two different types of complex treatment methods can be used in the treatment of water contaminated with dyes, where the liquid wastes from treatment are subject to the Fenton method followed by biological treatment under optimal experimental conditions to produce an acceptable treated water. The examples of heterogeneous photocatalysts such as ZnO/UV and TiO2/UV and its applications in dye degradation are also investigated. Brief description of metals doped metal oxide and its applications in photo degradation of organic dyes are also mentioned.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fly Ash Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of Membrane Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials Based Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Separation Process Based Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Industrial Applications of Membrane Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymeric Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ceramic Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ceramic Membrane Fabrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Ceramic Membrane Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ceramic Membrane Based Microfiltration of Oil-in-Water Emulsions . . . . . . . . . . . . . . . . . . . . . . . Designed Procedure for the Conversion of Fly Ash into Ceramic Membrane . . . . . . . . . . . . Microfiltration of Oil-in-Water Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2006 2007 2010 2010 2012 2012 2014 2014 2015 2015 2019 2024 2026 2027 2028 2028

Abstract

This chapter describes the review on membrane processes, different types of membrane materials like ceramic, polymeric, etc., preparation methods, flow patterns, and effective ways to use fly ash ceramic membrane in oil-in-water emulsion effluent treatment. The scope of the fly ash for the fabrication of low cost membrane is reviewed. The schematic way of membrane preparation and effective usage of fly ash is covered. Investigations on basic concepts on emulsions, colloidal solutions, surfactants, oil-in-water emulsion preparation, stability, K. Suresh · S. Suresh (*) Department of Chemical Engineering, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected] © This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_118

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and separation studies are also covered. The correlation of permeate flux and oil rejection with its results is discussed. This chapter gives an insight of the applications of fly ash in the field of wastewater treatment by developing low cost ceramic membrane for microfiltration applications.

Introduction This chapter describes the effective ways to use fly ash in oil-in-water emulsion effluent treatment. Fly ash is the most abundant waste material produced by burning of coal in thermal power plants to produce power. Although its existence is more problematic to environment, it is used to produce value added products in various industries, paving, including concrete. Hence, one of the applications of fly ash is in the field of wastewater treatment by developing low cost ceramic membrane for microfiltration applications. The geometry of the fly ash particles is in the spherical shape as depicted in Fig. 1. From the SEM image shows floc particles which give advantage to produce porous inside ceramic materials phases such as northite, mullite, and cordierite. Fly ash have different components like CaO, Fe2O3, Al2O3, SiO2 and few other components. Normally, these components are depend on type of coal used in different industries. The main glassy phase contains the fly ash are anhydrite (CaSO4), gehlenite (Ca2Al2SiO7), hematite (α-Fe2O3), maghemite

Fig. 1 Spherical shape of fly ash from scanning electron microscope view

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(γ-Fe2O3), mullite, periclase (MgO), quartz and rutile (TiO2). In addition, cristobalite (SiO2), mullite (3(Al2O3)2SiO2), esseneite (CaFeAlSiO6) phases are also produced from crystallize glassy phase at high temperature. Huge difference in composition of membranes derived from fly ash can be observed due to variation in the additives used, sintering temperatures and material composition.

Fly Ash Material History Electrical power generating produce millions of tons of fly ash and related by-products by burning coal. With increasing electricity demand and with abundance of coal in nature, fly ash generation is increasing enormously. According to Ahmaruzzaman (2010), only 16% of fly ash is utilized by existing techniques [1]. Out of this, the cement industry utilizes about 30% of fly ash and the remaining portion is disposed to landfill and ash ponds. Due to the existence of transition metal oxides, fly ash is known to provide adverse environmental effect. Among various viable alternatives for fly ash utilization, the fabrication of efficient ceramic membranes is an important alternative, owing to the utilization of fly ash to further address potential adverse environmental effects caused by discharged wastewater such as oil-in-water emulsions. Listed as sixth largest in terms of electrical power generation, India produces significant amount of fly ash. While majority of fly ash is being used for cement manufacture, roadways, and brick manufacture, large quantity of fly ash is stored in ponds. Currently, only 50% of the fly ash is used in the annual production capacity of 160 million tones in India. Fly ash is abrasive, refractory, and also alkaline. It consists of macronutrients (P, K, Ca, Mg) and micronutrients (Zn, Fe, Cu, Mn, B, and Mo) and can be adopted for plant growth. Lime binding capacity of the fly ash enables it for cement manufacturing, concrete building materials, and other relevant products. Chemical constitution of fly ash corresponds to significant proportion of silica (60–65%) followed by alumina (25–30%) and magnetite and Fe2O3 (6–15%). Due to this, Iyer and Scott (2011) have opined that it can be used for the synthesis of zeolite, alum, and precipitated silica [2]. With appropriate physiochemical properties (bulk density, particle size, porosity, water holding capacity, and active surface area), fly ash has been suggested to be adsorbent [3]. Further, agricultural and engineering materials can be also developed from fly ash. Physical Properties Table 1 presents the physical properties of fly ash. Typically, fly ash contains spherical solid or hollow amorphous powder particles. Further, fly ash also contains angular shaped particles of carbonaceous materials. The particle size distribution is about 1–150 μm [4]. Specific gravity and specific surface area vary from 2.1 to 3.0 and 170 to 1000 m2/kg, respectively. Depending upon unburned carbon content, the color of fly ash varies from tan to black.

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Chemical Properties Depending upon the type of coal burnt and handling/storage procedures, fly ash chemical properties vary significantly. Fundamentally, fly ash classification is based on the type of coal (anthracite, bituminous, subbituminous, and lignite) used during combustion [1–3]. While bituminous coal based fly ash consists of silica, alumina, iron oxide, calcium, and carbon, the fly ash obtained from subbituminous coal consists of higher constitution of magnesium and calcium oxide and lower amounts of silica, iron oxide, and carbon. The American Society for Testing Materials (ASTM) classification (C618) of fly ash refers to class F and C categories (Table 2). This is based on the constitution of alumina, silica, iron oxide, and lime in fly ash. With low lime, class F fly ash possesses 70% overall weight percentage of alumina, silica, and iron oxide. However, for class C fly ash, the overall weight percentage of alumina, silica, and iron oxides is about 50–70% along with abundance of lime content. Generally, class C fly ash is obtained by low rank coals, that is, lignite and subbituminous coal with cementations properties. On the other hand, class F fly ash is obtained from the combustion of higher rank pozzolanic coals. The main differences between these ashes are in calcium, alumina, silica, and iron content. While calcium content in class F varies from 1% to 12%, it varies from 30 to 40% in class C. Another variation in the composition corresponds to alkali (sodium and potassium) and sulfate content. This is higher for class C fly ash but not class F fly ash. European standards based fly ash constitution is being presented in Table 3. The general classification of coal fly ash typically ignores the mineralogy aspects of the coal. Vassilev and Vassileva (2007) indicated that the coal fly ash classification does not systematic scientific basis, despite being addressed from industrial coal Table 1 Physical properties of fly ash

Properties Specific gravity Color Specific surface area (m2/kg) Particle size (μm)

Range 2.1–3.0 Grey black 170–1000 1–150

Table 2 Classification of fly ash by ASTM standards Class C F

SiO2 + Al2O3 + Fe2O3 (%) 50–70 >70

SO3 (%) dichloroacetaldehyde (DCAL) > trichloroacetaldehyde (TCAL). As a result of these disadvantages, numerous alternatives to chlorination for drinking water disinfection have been investigated and proposed in recent years. Various alternatives to chlorination include different oxidants such as ozone, hydrogen peroxide, ferrate, iodine, bromine, and potassium permanganate [97–99]. Besides, physicochemical means such as photocatalysis, electrochemical disinfection, and physical treatments such as ultraviolet irradiation, ultrasonication, and microwave systems also have been applied in the disinfection process [87, 100–106]. Ozone is widely used to disinfect drinking water and wastewater due to its strong biocidal oxidizing properties. Recently, it has been reported that hydroxyl radicals (•OH), resulting from ozone decomposition, play a significant role in microbial inactivation B. subtilis endospore inactivation in water containing NOM, as well as in pH-controlled distilled water [99]. Besides, peracetic acid is also a strong disinfectant with a wide spectrum of antimicrobial activity, thus it is used as a disinfectant for wastewater effluents has been drawing more attention in recent years [107]. The easy operation, high activity, nontoxic, no quenching requirement, and small dependence on pH are a benefit for drinking water disinfection. However, major disadvantages associated with peracetic acid disinfection are the increases of chemical cost and organic content in the effluent and thus the regrowth of the potential microbial.

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While ultrasound pretreatment combined with ultraviolet irradiation ultrasound pretreatment has gained application in the water treatment processes, which might be also useful in terms of cost-effectiveness. For example, Blume and Neis evaluate the scientific and economic potential of ultrasound application as a pretreatment step in combination with UV to optimize the disinfection process of wastewaters [108]. The process flow diagram is depicted schematically in Fig. 6. In their lab-scale tests, 30 s of UV treatment alone were required to reduce the number of fecal coliforms by 3.7 log units. When applied in combination, 5 s of ultrasonic followed by only 5 s of UV irradiation had the same result and energy consumption was only 43%. In recent years, effective electrochemical disinfection has emerged as one of the most promising alternatives to conventional water treatment, which is a convenient and highly efficient way to produce germ-free water [104, 105, 109]. The advantages of these procedures make them more attractive than other methods. The electrochemical technology is environmentally friendly, low-cost, easily operated, and known to inactivate a wide variety of microorganisms ranging from bacteria to viruses and algae. For instance, Kerwick et al. have reported on a series of experiments evaluating the disinfection efficacy of an electrochemical disinfection technology against Escherichia coli and bacteriophage MS2 [110]. The results of these experiments conclude that electrochemical disinfection can be effective without the generation of chlorine species. Furthermore, laboratory-scale electrochemical (EC) disinfection experiments were performed to investigate the disinfection efficiency of the wastewater electrolysis cell with four seeded microorganisms (Escherichia coli, Enterococcus, recombinant adenovirus serotype 5, and bacteriophage MS2) [111]. And the formation of organic DBPs trihalomethanes (THMs) and haloacetic acids (HAA5) at the end of the EC treatment was also investigated in this study. The results showed that at an applied cell voltage of +4 V, the WEC achieved 5-log10 reductions of all four seeded microorganisms in real toilet wastewater within 60 min. In contrast, chemical chlorination (CC) disinfection using hypochlorite (NaClO) was only effective for the inactivation of bacteria. Based on the energy consumption estimates, the wastewater

Fig. 6 Flow scheme of the ultrasonic combination with UV pretreatment disinfection process for wastewaters [108]

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Fig. 7 Schematic of a solar-powered mobile toilet using wastewater electrolysis cells for toilet wastewater treatment [111]

electrolysis cell system can be operated using solar energy stored in a DC battery as the sole power source (shown in Fig. 7).

Wastewater Treatment Processes Generally, industrial wastewater is an aqueous discharge due to the use of water or cleaning activities in an industrial manufacturing process [16, 112–114]. Hence, the characteristics of industrial wastewaters can differ considerably both within and among industries. Various contaminants widely exist in industrial wastewater, such as organic matters, heavy metals, biological particles, and some other toxic materials [17, 19, 44, 115]. Therefore, wastewater should be pretreated for discharge, or be treated completely at the plant to protect the environment, aquatic life, and humans. Besides, due to the continuing increase in water shortages and environmental protection concerns, industrial effluent treatment for reuse in the process has been accepted as a sustainable option. Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids and organic matters from wastewater [11, 18, 24, 114, 116]. According to the different degrees of treatment required, the general wastewater processes can be classified as primary, secondary, and tertiary wastewater treatment [117]. A typical flow diagram incorporating some of the units is shown in Fig. 8. The units are shown cover pretreatment, clarification, filtration, adsorption, and filtration, and they may use reverse osmosis, ion exchange, electrodialysis, and evaporation. The pretreatment is intended for removing floating solids and settling grit and sand, along with other sludge deposits. Tertiary treatment, properly, would be any treatment added onto or following secondary treatment. Depending on the need and type of wastewater and the pollutants, of course, other unit processes can be

Fig. 8 Wastewater treatment plant floc diagram [117]

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added. These treatments and their advantages as well as limitations in industrial applications are summarized in this section.

Primary Treatment Wastewater usually contains a relatively large amount of inorganic solids such as sand, cinders, and gravel, which are collectively called grit. The amount present in particular wastewater depends primarily on whether the collecting sewer system is of the sanitary or combined type. Thus, the rough filtering should consist of this wastewater treatment process, which can remove the coarse solids and other large materials often found in raw wastewater for protecting pumping equipment and facilitating subsequent treatment processes [9]. Preliminary treatment helps to remove or to reduce in size the large, entrained, suspended, or floating solids which consist of pieces of wood, cloth, paper, plastics, garbage, etc. Besides, heavy inorganic solids, such as sand and gravel as well as metal or glass, and excessive amounts of oils or greases are removed. Pre-aeration of wastewater is aeration before the primary treatment, which can be accomplished by introducing air into the wastewater for 20–30 min at the design flow. This process can obtain a greater removal of biochemical oxygen demand (BOD), suspended solids in sedimentation tanks, and grease/oil carried in the wastewater before further treatment. Primary treatment is the physical process with sedimentation and flotation to remove as much organic and inorganic solids as possible. The purpose of primary treatment is to reduce the velocity of the wastewater sufficiently to permit solids to settle and floatable material to surface. Sedimentation is one of the most common physical treatment processes for solids separation at the beginning and end of wastewater treatment operations and is easy to implement in this process. In addition, other aeration and filtration technologies are also commonly used in this stage. Here, wastewater is passed through a filter medium to separate solids. Approximately 25–50% of the incoming BOD, 50–70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation, but colloidal and dissolved constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent, therefore it contains mainly colloidal and dissolved organic and inorganic solids. It is still not suitable for discharge, and hence the secondary treatment shall be carried out. Generally, the size of the suspended solids has a considerable impact on separation processes such as sedimentation, impact on separation processes such as sedimentation, flocculation, and filtration. Hence, some studies have been to investigate the removal of the more efficient contaminants by carefully researching the particle size distribution in primary sedimentation and filtration [46, 51, 57]. In raw sewage and primary effluents 45–90% of COD and 35–80% of phosphorus were associated with suspended solids. The coagulation/flocculation process is commonly used as a pretreatment before secondary treatment to enhance the biodegradability of

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the wastewater during the biological treatment [44, 113, 118]. For example, Amuda and Amoo have simulated coagulation/flocculation process efficiency for beverage industrial wastewater treatment plant for removal of COD, TP, and TSS using ferric chloride and nonionic polyacrylamide and also to investigate optimum coagulant dosages, optimum coagulation pH, and effect of polyelectrolyte addition on the coagulation process [113]. The results have shown that the percentage removals of 73, 95, and 97 for COD, TP, and TSS, respectively, are achieved by the addition of 300 mg/L FeCl3·6H2O, whereas 91%, 99%, and 97% removal of COD, TP, and TSS, respectively, is achieved with the addition of 25 mg/L polyelectrolyte to 100 mg/L ferric chloride. And it can be concluded from this study that coagulation/flocculation may be a useful pretreatment process for industrial wastewater before secondary treatment.

Secondary Treatment The secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In terms of the size of the solids, the distribution is approximately 30% suspended, 6% colloidal, and about 65% dissolved solids. Recent effluent standards and water quality standards require a greater degree of removal of organics from wastewater than can be accomplished by primary treatment alone [119, 120]. Additional removal of organics can be accomplished by secondary treatment. The main purpose of the secondary treatment is to remove the colloidal and dissolved organics (BOD, COD) in the wastewater, and the removal rate of BOD and suspended solids can reach more than 90% and 95%, respectively [121]. Generally, the activated sludge method and biological treatment method are commonly used in this stage, and the secondary treated effluent can reach the discharge standard [121–124]. The key step in secondary treatment is aeration. Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter [125–127]. By aerating the wastewater, the number of aerobic bacteria is increased tremendously. This large number of bacteria consumes organic matter that is in the waste, and when the organic matter has been consumed, the bacteria will die off at a rapid rate. Besides, aerobic biological treatment in activated sludge systems is also an efficient method for reducing the organic content in secondary treatment [128–130]. The removal of 11 antibiotics of 6 classes in the activated sludge process was investigated using two series of batch reactors treating freshwater and saline sewage, respectively [129]. In this study, biodegradation and adsorption were the major removal routes for the target antibiotics, where volatilization and hydrolysis were neglectable. Among the 11 target antibiotics, cefalexin and the two sulfonamides were predominantly removed by biodegradation in both freshwater and saline sewage systems. Ampicillin, norfloxacin, ciprofloxacin, ofloxacin, tetracycline, roxithromycin, and trimethoprim were mainly removed by adsorption. The removal efficiency of pharmaceuticals from four Taiwanese

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wastewater treatment plants in secondary wastewater treatment processes has been evaluated in the recent study [131]. Removal efficiencies varied among WWTPs, and the total removal rates of mass flow of pharmaceuticals in all of the WWTPs studied ranged from 72% to 92%. The type of treatment units utilized across WWTPs, the physicochemical properties of individual pharmaceuticals, and environmental factors can greatly affect the removal rates. Recently, anaerobic membrane bioreactor (AnMBR) is becoming increasingly popular for municipal secondary wastewater treatment [123, 132–134]. The combination of membrane separation technology and an anaerobic bioreactor may allow for a sustainable municipal secondary wastewater treatment with complete biomass retention, the added benefits of lower sludge production, enhanced high-quality effluent, net energy production, and without the extra costs for aeration associated with the conventional activated sludge process [135]. A laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) has been operated for 106 days for municipal secondary wastewater treatment, and the schematic diagram is shown in Fig. 9 [124]. The results revealed that the SAnMBR for municipal secondary wastewater treatment was technically feasible in terms of COD removal, sludge production, and biogas yield. COD removal efficiency of approximately 90% with a methane yield rate of 0.26 LCH4/gCODremoval was achieved. In addition, this technology is very economical because the operational costs can be offset by benefits from biogas recovery.

Tertiary Treatment The performance of secondary treatment plants is almost always measured in terms of BOD and SS removal. A well-designed and operated secondary plant will remove from 85% to 95% of the influent BOD and SS. Since effluents from the secondary treatments continue to present a high number of pathogenic microorganisms, there is a need for tertiary treatments to reach the high-quality standards [136]. Tertiary treatment is supplementary to primary and secondary treatment for the purpose of removing the residual organic and inorganic substances in wastewater. The purpose of tertiary treatment is to further remove some special pollutants, such as fluorine and phosphorus removal, which belongs to advanced treatment and commonly used chemical method [137]. Further treatment of the refractory organic matter, nitrogen, and phosphorus can lead to eutrophication of water-soluble inorganic matter. The tertiary treatments mainly consist of biological denitrification and phosphorus removal, disinfection, oxidation, filtration, activated carbon adsorption, ion exchange, etc. [138–142]. Phosphorus in the secondary effluents is mostly soluble and is present as orthophosphate, thus tertiary treatment of secondary municipal effluents to remove orthophosphate has become increasingly necessary to meet environmental regulations worldwide [122, 130]. Acid mine drainage (AMD) sludge, a waste product from coal mine water treatment, was used in this study as an adsorbent to develop a costeffective treatment approach to phosphorus removal from municipal secondary

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Fig. 9 Schematic diagram of the laboratory-scale SAnMBR apparatus (a) and full-scale SAnMBR system (b) [124]

effluents [143]. Adsorption of orthophosphate onto AMD sludge particles followed the Freundlich isotherm model with an adsorption capacity ranging from 9.89 to 31.97 mg/g when the final effluent concentration increased from 0.21 to 13.61 mg P/L. P adsorption was found to be a rather rapid process and neutral or acidic pH enhanced phosphorus removal. Koivunen et al. have investigated the occurrence and removal of salmonellae and fecal indicators in four conventional municipal wastewater treatment plants (MWTP), and tested the efficiency of a semi-technical scale biological nutrient removal process and three pilot-scale tertiary filtration units in microbial removal [136]. Pilot-scale tertiary treatment by rapid sand contact filter, chemical contact filter, and biological-chemical contact filter reduced salmonella numbers below the detection limit and fecal coliform numbers on average by 99%, 39%, and 71%, respectively. These results indicate that tertiary filtration units can efficiently remove microorganisms and other pollutants from secondary treated wastewaters, especially rapid sand filtration in conjunction with polyaluminum chloride coagulant.

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Thus, the conventional municipal wastewater treatment without efficient tertiary treatment, like filtration or disinfection, may constitute a risk for public health. Recently, advanced oxidation processes (AOP) have been used to try to oxidize the organics present in wastewater [137, 140]. Vijayalakshmi et al. have explored the option of electrooxidation and advanced oxidation as tertiary treatment techniques for the purification of tannery wastewater [141]. The TOC removal of 85% was achieved by the UV/O3/H2O2 process, whereas it is hardly 50% by electrooxidation. To minimize power consumption, a two-stage process involving electrooxidation in the first stage and advanced oxidation in the second stage has been attempted. Besides, solar TiO2-photocatalysis was applied to water from a natural wastewater treatment plant [144]. Total disinfection was achieved in less than 60 min using the catalyst AC-TiO2 and ozone. And no bacteria can be observed at 24 or 48 h. The results obtained in this study have demonstrated that combining biological and photocatalytic treatments could greatly improve the depuration of residual water. Though these techniques are effective for the degradation of refractory molecules, the energy consumption was found to be prohibitively high for actual implementation.

Emerging Technologies of Water Treatment The conventional treatment technology removes the majority of BOD and suspended solids in wastewaters. However, with the increase of the demand for safe water, additional treatments have been added to wastewater treatment plants to provide for further removal of nutrients and/or toxic materials and ensure the safety of water sources [8, 22]. Therefore, various modern emerging technologies compared to traditional process engineering offer new opportunities for the development of advanced water and wastewater technology processes [145–147]. In the last years, new technologies for disinfecting wastewater have been developed to extend the possibilities of wastewater reuse, such as membrane technologies and advanced oxidation processes. Alternatives have presented themselves for classical and conventional water treatment systems. Advanced wastewater treatments have become an area of global focus as individuals, communities, industries, and nations strive for ways to keep essential resources available and suitable for use.

Membrane Technology Wastewater treatment by membrane technologies has increased significantly in the last decade [148–150]. In the past, these technologies were considered unsuitable owing principally to the high costs involved. However, as a result of the demand for wastewater reuse as well as the increasingly stringent norms, intensive efforts on advanced treatment processes using membrane systems have been made to determine the ability of the treatment to remove various contaminants from wastewater. Membrane technology allows the separation through a physical barrier to the pollutants present in the water, which is an effective method to remove inorganic/

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organic micropollutants from drinking water and wastewaters [151, 152]. Compared with conventional treatment methods, membrane technology can provide the excellent quality of treated water, minimize disinfectant demand, and generate less sludge. Moreover, it possesses the reliability, compactness, and less maintenance. Nevertheless, membrane technologies also present certain operational drawbacks. Continual backwashing is required to avoid system clogging, as well as periodic chemical cleansing to eliminate materials that build up irreversibly in the membrane and cause fouling, which in turn affects water flow and transmembrane pressure. These problems may be minimized by the application of pretreatments such as granular filtration. According to the pore sizes of the membrane, the commonly used membrane technologies can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) [153, 154]. The porosity of microfiltration is 0.1–10 μm, and microfiltration technology can ensure the separation of particles and most bacteria but an adequate virus. Ultrafiltration with a porosity of 0.01– 0.1 μm can completely remove the bacteria, viruses, and protozoa. Nanofiltration (0.001–0.01 μm) removes color, volatile organic compounds (VOC), pesticides, sulfates, and phosphates, whereas reverse osmosis ( Zn (II) > Cd (II) > Cu (II). The results can be fitted with a pseudo-second-order rate equation, which assumes that the adsorption rate on the surface of the adsorbent is the rate-determining step. Adsorption of Pb (II), Cd (II), and Cu (II) was an endothermic process while Zn (II) was exothermic. Pb (II) adsorption was the only spontaneous adsorption reaction. The results showed that nano-hematite was an effective adsorbent, which could remove multiple heavy metals from water simultaneously. Lately, superparamagnetic hematite nanoparticles were synthesized and used to treat the acid mine drainage (AMD) in a batch mode [27]. The synthesized hematite nanoparticles have shown a superparamagnetic character with a saturation magnetization of 5.6 emu g
1. The results showed that nano-hematite could totally remove Al (III), Mg (II), and Mn (II), and could remove over 80% of Ni (II) and Zn (II). In recent years, reports on modified nanomaterials, which combine the superiorities of both polymers and magnetic, are gaining increasing attention. For example, Su et al. [28] reported a magnetic α-Fe2O3/GO nanocomposite for the removal of As (III) and As (V) from aqueous solutions. In comparison with blank, the optimized α-Fe2O3/GO nanocomposites exhibited higher adsorption efficiency of As (III) and As (V) from aqueous solution. The adsorption capacity of As (III) and As (V) on α-Fe2O3/GO nanocomposite was found to be 147 and 113 mg/g at pH 7 and pH 3, respectively. Ravindranath et al. [29] reported cube-like heterostructures of Fe2O3/Al2O3 composites by the precipitation method. The cube-like heterostructures of Fe2O3/ Al2O3 acted as a good adsorbent for the removal of toxic Hg(II) in aqueous solution. The maximum adsorption capacity for Hg (II) reaches 216 mg g
1 at pH 7. Upon increasing the pH values, the negative charge density on the surface of Fe2O3/Al2O3 increased, leading to greater adsorption of Hg (II) through electrostatic interaction. At pH values >7.0, the formation of species such as HgO and Hg2O led to their decreased adsorption onto the surface. The adsorption of different heavy metals using α-Fe2O3-based nanomaterials are listed in Table 5.

Maghemite (γ-Fe2O3) Maghemite, γ-Fe2O3, is a red-brown, ferrimagnetic mineral isostructural with magnetite, but with cation deficient sites. It occurs in soils as a weathering product of magnetite or as the product of heating of other Fe oxides, usually in the presence of organic matter. Maghemite is a critical magnetic material.

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Table 5 Hematite (α-Fe2O3) nanomaterials for the removal of heavy metals Iron Oxide Hematite of different morphologies Nano-hematite NPs

Fe2O3- Al2O3 nanocomposite α-Fe2O3/GO α-Fe2O/ MWCNTs

Heavy metal Cr (VI)

Pb (II), Cd (II), Cu (II), and Zn (II) Cu, Pb, Ni and Hg As (III), As(V) Cu(II), Cr(VI)

Preparation/modification Acid hydrolysis, acid hydrolysis with base addition, sol–gel, and transformation of ferrihydrite methods Nanoparticles were commercially available

Nanocomposite fibers were synthesized via electrospinning method α-Fe2O3 and graphene composite α-Fe2O3 and Multi-walled carbon nanotubes composite

Efficiency/ adsorption capacity Range of 6.33– 200 mg·g
1

Ref. [5]

Pb (II) > Zn (II) > Cd (II) > Cu (II)

[30]

Adsorption Cu < Pb < Ni < Hg

[31]

As (III) 147 mg g
1, As(V) 113 mg g
1 Cu(II) 470 mg g
1, Cr(VI) 60 mg g
1

[28] [32]

Maghemite (γ-Fe2O3) nanoparticles have been reported extensively to treat heavy metals in wastewater. Maghemite (γ-Fe2O3) nanomaterials are very cheap and can be easily synthesized. The exclusive presence of Fe (III) in γ-Fe2O3 nanoparticles implies high chemical stability but also to the absence of any reducing activity. For this reason, their efficiency mainly refers to the chemisorption of pollutants with high affinity to Fe (III). Also, maghemite (γ-Fe2O3) nanoparticles can be applied as single-phase nanoadsorbents on their surface or as magnetically driven carriers of other active phases. (Fig. 4). Hu et al. [33] explored Cr (VI) removal by nano-maghemite and found that the equilibrium period was independent of initial Cr (VI) concentration, and the adsorptive capacity increased when pH decreased. Nano-maghemite emerged a high selectivity for Cr (VI) from water. The negligible competition was observed for many coexisting ions. The adsorption capacity of nano-maghemite for Cr(VI) is 19.2 mg g
1. Besides, the results indicated that no chemical redox reaction occurred during Cr (VI) retention. The adsorption mechanism of Cr (VI) onto γ-Fe2O3 is suggested to be the electrostatic attraction, particularly at a relatively low pH. As for heavy metal cations, maghemite adsorbents are also very useful. Maghemite nanoparticles (γ-Fe2O3) produced using a recently developed, single-step method proved to be highly effective for selective removal of Cu (II), Ni (II), Mn (II), Cd (II), and Cr (VI) from acid mine drainage (AMD) and simulated wastewater. The results showed that the affinity between maghemite nanoparticles and heavy metals were in the following order: Cu (II) > Cr (VI) > Mn (II) > Ni (II) > Cd (II). A further study was carried out to investigate the adsorption kinetics and mechanisms of multiple heavy metals, Cr (VI), Cu (II), and Ni (II) by maghemite nanoparticles. All the adsorption was highly pH-dependent. The optimal pH for the selective removal of Cr (VI), Cu (II), and Ni (II) were 2.5, 6.5, and 8.5,

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Fig. 4 Two application mode of maghemite in heavy metals removal

respectively. Under the optimal pH, their uptakes mainly resulted from the electrostatic attraction. Rajput et al. [34] synthesized superparamagnetic maghemite nanoparticles with a tunable morphology by employing a flame spray pyrolysis method. The maximum Langmuir adsorption capacities of these maghemite nanoparticles were 68.9 mg g
1 at 45 °C for Pb (II) and 34.0 mg
1 at 25 °C for Cu (II). Electrostatic interactions were mainly responsible for the metal ions adsorption. The surface of maghemite was covered with FeOH groups in water, which could form positive Fe-+OH2 or negative FeO
groups with the change of pH. More Fe (III) O
or Fe (III) OH sites formed with the increase of pH, thus improving the adsorption capabilities for Pb (II) and Cu (II). Maghemite nanoparticles are considered better adsorbents for As (V) than magnetite (Fe3O4). Their ability to bind with As (V) oxy-ions is attributed both to the adsorption on the γ-Fe2O3 structure or on the hydrolyzed surface formed after contact with water. In addition, γ-Fe2O3 may reach smaller sizes and, therefore, achieve a higher specific surface area. In fact, the formation of a thin γ-Fe2O3 layer on Fe3O4 nanoparticles most probably explicates the performance of these other phases against arsenic. In recent years, reports on polymer-modified maghemite nanomaterials, which combine the superiorities of both polymers and maghemite, are gaining increasing attention. For example, Chávez-Guajardo et al. [35] prepared a novel polypyrrole/ maghemite (PPY/γ-Fe2O3) and polyaniline/maghemite (PANI/γ-Fe2O3) magnetic nanocomposites (MNCs) as active agents for removal of heavy metals ions from aqueous media. In the Cr (VI) and Cu (II) case, we determined the value of qe as 209 and 171 mg g
1, for the PPY/γ-Fe2O3 MNC, and 196 and 107 mg g
1, for the PANI/γ-Fe2O3 MNC, respectively. In the PPY/γ-Fe2O3 and PANI/γ-Fe2O3 MNC case, the time necessary for attaining the qe saturation limit was of the order of 15 and 35 min for both Cr (VI) and Cu (II), respectively. The batch adsorption data could be well represented by the pseudo-second-order kinetic model. Moreover, maghemite nanoparticles have also been reported to be modified by poly (1-vinyl imidazole), polyrhodanine, polypyrrole, polyaniline, etc. and these polymer-modified maghemite nanoparticles have exhibited excellent removal capabilities and selectivity toward heavy metal ions.

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Table 6 Maghemite (γ-Fe2O3) nanomaterials for the removal of heavy metals

Iron Oxide γ-Fe2O3 nanoparticles

Heavy metal As

γ-Fe2O3 NPs

Cr

γ-Fe2O3 nanoparticles

Zn, Cu, Cr

γ-Fe2O3 nanoparticles

Cr

Preparation/modification Nanoparticles of different size were synthesized by co-precipitation method and polyol process Sol–gel method was used for the preparation of nanoparticles AMBERLITE cationic exchange resin batch adsorption/desorption studies were employed Nanoparticles were supported by montmorillonite clay by co-precipitation and hydrosol method

Efficiency/ adsorption capacity Adsorption capacity 34.93 mg g
1 Adsorption 97.3% Adsorption 78– 90% Adsorption Removal capacity 15.3 mg g
1

Ref. [5]

[33] [36]

[37]

In addition, Predescu et al. [36] have synthesized γ-Fe2O3 nanocomposites with AMBERLITE cationic exchange resin and used it for the adsorption of Zn, Cu, and Cr. The adsorption study has shown that a double effect of electrostatic attraction during the adsorption process and the ionic exchange was observed for maghemitecovered cationic resin. The different synthetic root and application of maghemite (γ-Fe2O3) nanostructures for the removal of heavy metals are listed in Table 6.

Magnetite Fe3O4(FeIIFe2IIIO4) Magnetite, Fe3O4(FeIIFe2IIIO4), is a black, ferrimagnetic mineral containing both FeII and FeIII. It has an inverse spinel structure. Magnetite is essential to iron ore. Together with titanomagnetite, it is responsible for the magnetic properties of rocks; these are the object of paleomagnetic studies. It is formed in various organisms in which it serves as an orientation aid. Other names for magnetite include black iron oxide, magnetic iron ore, ironII, III oxide, loadstone* (when natural polarity is present), tri-iron tetroxide, ferrous ferrite, Hercules stone, and Magneteisenerz (German). Magnetite, as magnetic materials, can be applied as single-phase nanoadsorbents or as magnetically driven carriers. (Fig. 5). (*Magnetite is attracted by a magnet but, ordinarily, it cannot itself attract iron particles.) Chemical co-precipitation has been widely used to prepare magnetite nanoparticles by adding alkaline carbonate into a solution containing Fe2+ and Fe3+ in a molar ratio of 1:2. It was found that the particle size was reduced when a surfactant (such as oleic acid) was used during the preparation [38]. Two methods were reported to prevent the change of the ratio caused by air oxidation. One is to conduct the reaction under an inert environment with nitrogen gas. Another is to set the initial Fe3+: Fe2+ molar ratio less than 2:1 so that after the oxidation of Fe2+ to Fe3+, the ratio approaches to 2:1. In addition, nano-Fe3O4 will be oxidized to nano-γ-Fe2O3.

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Fig. 5 Synthesis and some surface modification magnetite in removal processes

For the removal of heavy metals, nano Fe3O4 was commonly used as the magnetic core for composite sorbents [39]. As with the maghemite, magnetite-based nanomaterials could be easily separated from the aqueous solution after treatment by adding a magnetic field (Fig. 6). There have been numerous reports on their applications in heavy metals treatment [40]. Giraldo et al. [42] synthesized magnetite nanoparticles by using a co-precipitation method, and the obtained nanoparticles were used to treat Pb (II), Cu (II), Zn (II), and Mn (II) in a batch mode. The results show that the adsorption capacity of Fe3O4 nanoparticles is maximum for Pb (II) and minimum for Mn (II), due to a different electrostatic attraction between heavy metal cations and negatively charged adsorption sites, mainly related to the hydrated ionic radii of the investigated heavy metals. Besides, experimental results indicated that the adsorption is strongly influenced by pH and temperature, the effect depending on the different metal ion considered. Mayo et al. [43] have synthesized Fe3O4 NPs of different sizes and found that NPs of smaller size shows better efficiency for arsenic (As) removal. They have used a magnetic separating column for the adsorption of As, which contains Fe3O4 NPs. It has been observed that when particle size decreased from 300 to 12 nm, the adsorption capacities for both As (III) and As (V) increased by 200 times. Similar observations have been reported by Shen and coworkers [5]. Their results have further confirmed that the smaller sized Fe3O4 NPs (8 nm) have more efficiency to almost adsorb various metal ions, seven times higher as compared to coarse particles. Nevertheless, bare magnetite nanoparticles are easily oxidized with oxygen due to the existence of Fe (II) in their structures, and they also tend to be corroded by acids or bases. Thus, magnetite particles are usually surface-modified by functional

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Fig. 6 Photographs of Fe3O4 dispersion and magnetic separation [41]

groups like –NH2, –COOH, –SH, etc., or coated with a protective shell. Porous carbon@Fe3O4 magnetic composites (PCMCs) have been prepared by Mo and coworkers [44] using carbon-coated magnetic nanoparticles. Porous carbon has a high surface area, large pore volumes, and is mechanically and thermally stable (Fig. 9a). The experimental results have been compared with the as-prepared Fe3O4 NPs and PCMCs to detect the removal capacity of Cr (VI). Figure 7 has shown that the PCMCs had a better removal ability than that of the Fe3O4 NPs, with adsorption rates of 63.9% and 49.2% in 30 min, respectively. Fe3O4 NPs have been functionalized with different functional materials such as polymer, thiols, amines, chitosan, and organic acids to improve their adsorption efficiency. The free functional groups present on the surface provide lots of active sites as well as aqueous stability. The functional group consists of a hydrophilic head and a hydrophobic tail. The head and the tail group have been tailored according to the type of the contaminant to be removed. For example, Badruddoza et al. [45] have synthesized ionically modified magnetic nanoparticles (PPhSi-MNPs). Based on the zeta-potential and chemical analyses of the adsorbent surface, a synergy of electrostatic interaction and ion-exchange between anionic metals and the cationmodified magnetic nanoparticles allow highly efficient removal of the pollutants. With adsorption capacities for arsenic (50.5 mg g
1) and chromium (35.2 mg g
1), the results found that compared with bare magnetic NPs the adsorption capacity of modified NPs has been much higher than bare NPs. Huang et al. [46] have reported a novel magnetic nanoadsorbent has been developed by the covalent binding of polyacrylic acid (PAA) on the surface of

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Fig. 7 (a) SEM images of the as-produced PCMCs. (b) Adsorption rate of Cr(VI) on PCMCs and Fe3O4 NPs

Fe3O4 nanoparticles and the followed amino-functionalization using diethylenetriamine (DETA) via carbodiimide activation. The maximum adsorption capacity of this adsorbent to Cr (VI) was only 11.24 mg g
1. Moreover, the interaction between EDTA-functionalized Fe3O4 NPs and heavy metal ions was reversible; therefore, heavy metal ions could be released from NPs by ultrasound radiation. Generally, coating and immobilization technology could provide adsorbents with increased mechanical strength, porous characteristics, and high adsorption capacity. Adeli et al. [47] have synthesized SDS-coated Fe3O4 NPs and used them for the adsorption of Cu, Ni, and Zn. They have demonstrated that in the absence of SDS coating, metal ions have been hardly adsorbed. But by increasing the amount of SDS, adsorption increases remarkably by the gradual formation of SDS aggregates on the surface of Fe3O4 NPs (Fig. 8). The different synthetic roots and application of magnetite (Fe3O4) nanostructures for the removal of heavy metals are listed in Table 7.

Ferrihydrite (Fe5HO8•4H2O) Ferrihydrite has shown its high potential for removing heavy metals from wastewater due to their affinity toward heavy metals, large surface area, and their low cost. The removal mechanism consists of adsorption, ion-exchange, and co-precipitation. Since having adsorption properties, ferrihydrite can be effective for the remediation of acid mine drainage. Though possessing many advantages for treating heavy metals, ferrihydrite cannot be used directly in stationary bed or flow-through systems due to their poor mechanical robustness, low hydraulic conductivity, and excessive pressure drop [52]. Therefore ferrihydrite has limited practical application. It is often combined with porous nanomaterials to form composites. For example, Zhang, Y et al. [53] developed a new polymer-supported hybrid adsorbent (HFO-P(TAA/HEA)) for highly efficient removal of Pb (II), Cu (II), Cd (II), and Ni (II) from wastewater by supporting hydrous ferric oxide. The results

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Fig. 8 Schematic illustration of the probable adsorption mechanism of the metal ions on the surface of SDS-coated Fe3O4 NPs (zinc as a model) [47] Table 7 Magnetite (Fe3O4) nanomaterials for the removal of heavy metals

Iron Oxide Fe3O4 nanomagnet

Heavy metal As

Fe3O4 nanoparticles

Cd, Ni, Cu, Cr

Fe3O4 nanoparticles

Pb and Cr

Fe3O4 nanoparticles

Ni(II), Cu(II), Cd(II) and Cr(VI) Cr

Cr, Co, Ni, Cu, Cd, Pb, As

Carbon@ Fe3O4 nanocomposite Fe3O4 nanoparticles

Preparation/modification Nanoparticles were prepared by the homemade method using saponification method Nanoparticles were synthesized by co-precipitation method using D-sorbitol (prevent agglomeration) Nanoparticles were synthesized by hydrothermal method

Efficiency/ adsorption capacity Adsorption 99.2%

Ref. [48]

Adsorption 95–100%

[49]

[50]

Nanoparticles of different size were synthesized by co-precipitation method and polyol process

96.8 mg g
1 (Pb) 41.5 mg g
1 (Cr) Adsorption capacity 34.93 mg g
1

Nanocomposite was synthesized by in situ carbonization of galactose

Adsorption 63.9%

[44]

Carboxyl, amine, thiole functionalized nanoparticles were prepared

Adsorption 100% (at high pH)

[51]

[5]

showed that the removal order of this hybrid nanomaterial in the quaternary system was in the following order: Pb (II) > Cu (II) > Ni (II) > Cd (II).(Fig. 9). Moreover, HFO nanoparticles were also reported to be an extremely effective adsorbent toward As (III) with the uptake capacity of 92 mg·g
1 calculated from the Langmuir model.

Fig. 9 Diagram for synthesis of P(TAA/HEA) hydrogel, FTIR spectra of P(HEA), P(TAA/HEA), and HFO-P(TAA/HEA) hydrogels

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Xiong et al. [54] have developed a novel phosphorus adsorbent, ferrihydritemodified diatomite. The ferrihydrite-modified diatomite was made through surface modification treatments, including NaOH treatment and ferrihydrite deposition on raw diatomite. Because of the increased surface area and surface charge, the maximum adsorption capacity of ferrihydrite-modified diatomite at pH 4 and pH 8.5 was increased from 10.2 mg P g
1 and 1.7 mg P g
1 of raw diatomite to 37.3 mg P g
1 and 13.6 mg P g
1, respectively. Increased surface area and surface charge result in an increased phosphorus adsorption capacity of ferrihydrite-modified diatomite.

Practical Application of Iron Oxide Nanomaterials in Effluent Treatment Electroplating Wastewaters Treatment Ferrite process was first adopted to the treatment of the laboratory wastewaters of the heavy metal ions in the Tokyo Institute of Technology in 1979 [55]. In the ferrite process, heavy metal ions are removed into the lattice points of the ferrite. In the ferrite process, heavy metal ions are removed into the lattice points of the ferrite [55]. Since the heavy metal ions situate in the lattice points in the spinel structure, the incorporated metal ions are not readily leached. The ferrite sludge contains some amount of the heavy metal ions adsorbed on its surface. The amount of those adsorbed is very small but readily leached. Figure 10 shows the flow chart of the ferrite process, which is practically operated for the treatment of the plating wastewaters in Japan. At this facility, the plating wastewaters are treated in the flow process (3.5 m3/day). In recent years, the ferrite process method has made great improvements. For example, Galvanic treatment

Fig. 10 Flow chart of ferrite process (flow system) for the treatment of the plating wastewaters (Nissei Co., In Chiba in Japan) [55]

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(GT)-ferrite process method, ferrite process-High gradient magnetic separation (HGMS), ion exchangeferrite process, electrolysis-ferrite process, etc. have been widely used for ferrite process.

Arsenic Treatment The full-scale SORB 33 system (Fig. 11) applies a pump-and-treat process that sends pressurized water through filter vessels containing the media. As water is forced through the fixed bed, arsenic is attracted to the media, and the water is reduced to 8 μg L
1 of arsenic or less, a level agreed upon between the city and the Oklahoma Department of Environmental Quality for the pilot system. The city received proposals incorporating the use of several technologies, including ion exchange and several adsorption media. Ultimately, a proposal from the team of Urban Contractors, Garver Engineers, and De Nora Water Technologies using an iron oxide media was chosen. Adsorption with iron oxide-based media does not require chemical regeneration or flocculation, making the arsenic removal process simple and reliable, minimizing labor. Among its advantages is that the media not only adsorbs arsenate As(V), as do other adsorbents, but also adsorbs arsenite As(III), which is beyond the capability of other adsorbents. The Norman SORB 33 system was installed in spring 2008 and began full-scale test operation in June 2008. Water from Well No. 31 flows directly to the SORB system after first being pH adjusted. Groundwater’s pH level is an important variable affecting the arsenic treatment media’s adsorptive capacity.

Fig. 11 Norman SORB 33 arsenic treatment system in Oklahoma City

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Conclusions and Further Outlook In this chapter, the environmental and special purification of heavy metal from heavy metal contaminants by the applications of iron oxide nanomaterials, namely, goethite (α-FeOOH), ferrihydrite, maghemite (γ-Fe2O3), hematite (α-Fe2O3), and magnetite Fe3O4 were discussed. These iron oxide nanomaterials have been utilized in the purification of heavy metal-contaminated water with great success. The reason behind the successful application is due to their fascinating properties like high surface area and ease of recycling; only using a magnetic field can be recycled. In addition to these properties, the iron oxide nanomaterials can easily be fabricated with other nanomaterials and are easy to be functionalized, resulting in multifunctional nanoadsorbent. Iron-based materials are highly biocompatible with living organisms and the environment. However, the major obstacles in successful utilization are their production cost. Researchers are still working to reduce their production costs that will enable its practical use for wastewater treatment. Another problem is that most of the experimental data came from the laboratory. The full-scale plant operation outcomes are often missing. It is hard to determine their feasibility in practical application. Besides the production of iron oxide nanomaterial itself, no project is available for producing iron oxide-based nanomaterials in bulk. Therefore, the fate of these nanomaterials will depend on their commercialization. One more aspect that requires in-depth research is the discharge of iron oxidebased nanomaterials and their impact on aquatic environments. Minimal information is available at present with respect to their disposal in water bodies, which must be analyzed in detail. In this way, risk identification in this area will be exceedingly helpful in finding out the effects of nanotoxicity on the ecosystem. Furthermore, various new testing methods are also required to understand the effectiveness of these nanomaterials. The development of iron oxide-based nanocomposite with materials has modified physiochemical stabilities, and key magnetic characteristics also have provided amazing advantages in the application of these novel nanomaterials. Another problem is that the cost effect should be evaluated when the magnetic nanomaterials are introduced to the practical application in the environment field. Besides, the reasonable modification of the magnetic nanomaterials is required to avoid aggregation. In short, iron oxide nanomaterials have shown dramatic growth, and their diverse and multifunctional applications can bring about enormous changes in various areas, including environmental remediation.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nanosilicon and Nanochitosan in Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silicon Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Obtaining Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Differences between the Bulk Si and the NSi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agricultural Use of Si NMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Obtaining Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Differences between the Bulk Chitosan and NCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanochitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Modern agriculture and human nutrition depend on the use of agrochemicals. Among these are pesticides and synthetic fertilizers that cause a significant impact on ecosystems. It is desirable, therefore, to have alternatives that without reducing the production of food and its quality, reduce the amount and variety of pesticides and synthetic fertilizers used. Nanosilicon and nanochitosan are attractive materials due to their low environmental impact and their ability to induce positive responses in soils and plants. Compared with bulk materials, the use of nanometric materials significantly increases their effectiveness. The application of A. Robledo-Olivo Department of Food Science and Technology, Autonomous Agricultural University Antonio Narro, Saltillo, Mexico M. Cabrera-De la Fuente · A. Benavides-Mendoza (*) Department of Horticulture, Autonomous Agricultural University Antonio Narro, Saltillo, Mexico © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_47

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nanosilicon and nanochitosan to the soil, either individually or in combination, increases the bioavailability of mineral nutrients, reducing the need to apply copious amounts of fertilizers. On the other hand, the adverse effects of salinity, water deficit, heavy metals, and root pathogens are mitigated, reducing the need for pesticide use and increasing tolerance to environmental stress in plants. When applied by foliar spraying the impacts of nanosilicon and nanochitosan are equally positive, functioning as biostimulant compounds that induce and strengthen the defense mechanisms of plants against biotic and abiotic stresses, in addition to increasing their nutritional quality. The result obtained is a combination of stronger crops and an edaphic system that supports more productive plants. This chapter presents updated information about the agricultural application of nanosilicon and nanochitosan with the objective of reducing the use of pesticides and synthetic fertilizers, mitigating the environmental impact of the agricultural activity.

Introduction Food production for a growing population in a climate change scenario imposes unprecedented ecological, technological, social, and economic challenges. Among them, achieving higher yields and quality in crops in a changing environment and subjected to various types of stress. In parallel, to contribute to the efficient and sustainable use of water and soil, the use of pesticides and fertilizers that cause pollution and degradation should be reduced. Although practically all agricultural activities have a certain level of impact on ecosystems, in the case of pesticides and fertilizers, the level of attention and concern they generate in society is much higher. Part of the explanation for this more significant concern lies in the potential for toxicity and trophic transfer of pesticides, and in the ability of fertilizers to cause soil degradation through salinization, metal liberation, and degradation of organic matter. The other part of the explanation is that in many cases the fertilizers and pesticides applied are very inefficient, observing that a considerable part does not reach its biological target, but is lost towards the surrounding environment, causing pollution. Among the techniques that have been proposed to mitigate the ecological impact of agricultural activities, without diminishing the production of food, fiber, and other agricultural commodities, is the use of nanofertilizers (NFs) and nanomaterials (NMs) that show greater effectiveness on crop growth and quality than traditional bulk materials [1]. Similarly, while NFs and NMs, directly and indirectly, provide essential mineral nutrients for plants, they increase stress tolerance and improve their nutritional quality. The mechanism by which the NMs induce these positive responses of higher tolerance and quality in crops depends on the transference of information between plant cells (cell signaling) and is called priming, biostimulation, or elicitation [2]. The mechanism of biostimulation induced by NMs occurs through at least two well-differentiated processes or phases: (a) the initial biostimulation whose

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Fig. 1 (a) Interaction of surface charges of proteins or other organic molecules and the core of an NM. (b) Formation of the NM corona in a natural fluid. From [3]

occurrence depends on the interaction of the electric double layer (EDL) of the NMs corona (Fig. 1) with the integral proteins of cell walls and membranes; the latter react in response to surface energy, hydrophobicity, size, and the charges of the functional groups of the NM corona (Fig. 2); (b) The subsequent biostimulation resulting from the composition of the NM, where the process depends on the release of the functional groups or ions that make up the corona and the core of the particle (Fig. 3) [3]. The first phase of biostimulation by the NMs constitutes a physicochemical phenomenon that depends on the interaction between the surfaces of the materials and the surfaces of the cell walls and the plasma membrane system and the

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Fig. 2 Diagram of the interaction between surface charges and functional groups of a cell wall or cell membrane (left) and the NMs corona (right). EDL is the electric double layer of the cell surface. From [3]

organelles. This first phase is presented for virtually all types of NM, regardless of their composition. In the case of the second phase, which is mainly a biochemical phenomenon, the presence of essential elements or toxic elements and their interactions with ions or molecules in the environment is what explains most of the cellular responses after physicochemical biostimulation. As an example, consider the nanoparticles (NPs) of Cu. The surface charges, hydrophobic sectors, the size, and shape of Cu NPs will induce a first biostimulation response in plants. Subsequently, when Cu NPs are metabolized in the apoplast or are internalized towards the cytoplasm and organelles, the concentration and location of the Cu+ and Cu2+ ions and their

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Fig. 3 The interaction between NMs with cell surfaces. (a) NM binding to the cell wall or the membrane through to the charged groups or the free energy of the surface of the corona surrounding the NP or NM, as well as to hydrophobicity forces that induces aggregation between surfaces. (b) Afterwards to the binding to the membrane or cell wall, the NM is internalized to the cytoplasm due to direct transit across the biological surface, endocytosis, or pore formation. (c) Once the NP reaches the cytoplasm, contacts with cytoplasmic proteins, internal membranes, or organelles occur, which causes a series of metabolic changes and modifications in gene expression that are signaled to other parts of the plant. In this phase, the interaction of the NMs and the cellular media release the components (such as Cu2+ from a copper NP) that in turn elicit other cellular responses. From [3]

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interaction with other ions or molecules found in the external or internal environment of the plant will determine the subsequent reactions of the cells [3]. The positive impact resulting from the application of NMs of Si (NSi) and chitosan in crops is derived from processes of biostimulation of plants and chemical interactions with soil components and soil solution. These processes increase the bioavailability of mineral elements applied with bulk fertilizers or NFs, in addition to promoting the metabolism associated with plant defense responses. The application of NMs may increase the efficiency of the use of fertilizers. It would reduce the amount that should be applied to crops, leading to a decrease in ecological impact and pollution with fertilizers. Similarly, the amounts of pesticides used for crop protection could decrease, since plants treated with NSi and/or nanochitosan have a more active defensive system, which allows a better response to biotic and abiotic stress [1, 2]. This chapter describes the agricultural use of silicon and chitosan NMs. These NMs do not contain or contribute directly to any of the 14 essential mineral elements that are provided with the fertilizers: N, P, K, Ca, Mg, S, Fe, B, Cl, Zn, Mn, Cu, Ni, and Mo. Therefore, they are not considered NFs [1]. However, both the Si and chitosan NMs are particularly useful biostimulants capable of increasing the efficiency in the use of fertilizers and the tolerance to abiotic and biotic stress.

Applications of Nanosilicon and Nanochitosan in Agriculture Silicon and chitosan materials have a great diversity of applications in the medical, food, manufacturing, and environmental areas. The agricultural sector is no exception, and it is well known that Si and chitosan are used for the improvement of nutrition, water supply, and to make the use of agrochemicals more effective. Si and chitosan in their bulk versions work as excellent plant biostimulants [4, 5]. As with other kinds of materials, the nano version of Si and chitosan shows higher reactivity and effectiveness as a biostimulant of crops. When the bulk materials are transformed into their nano versions, new properties arise at the nano level, and these unique nano-features evoke much more intense and varied cellular responses [3]. Therefore, the NSi and nanochitosan show much greater effectiveness as biostimulants than the bulk Si and chitosan. The general rule for elements of biological importance is that superior results of biostimulation of crops are achieved by applying the elements in nano form than in bulk form. In other words, with much lower concentrations of nanoforms, results equivalent to those attained with larger quantities of bulk materials are obtained [2]. An important point that should be highlighted is that Si, in addition to functioning as a biostimulant agent and structural constituent in many plant species, constitutes an element whose consumption greatly benefits human health. Although its essentiality is a matter of debate, many experts agree on the need to include it as part of the fertilizers of food crops and in the diet of humans [4]. Consequently, an additional objective of the use of NSi in crops would be to ensure a higher concentration of the element in the parts of the plant that are consumed, whether these are leaves, stems, roots, or fruits. Research is still required

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on the issue of food crop safety whose production is carried out with NSi applications; however, experiences with the medical use of the Si element both in bulk- and nano-forms [6] seem to indicate in principle that its use can be safe.

Silicon Nanomaterials Definition Si NMs constitute any material formed by Si or SiO2 with at least one dimension smaller than 100 nm. Nanosilicon (NSi), nanosilica (NSiO2), nanozeolites, and nanoclays are the most commonly used Si NMs in medicine, agriculture, and industry. The most common forms of biological application are NPs, including 0D silicon or silica quantum dots and nanozeolites, 1D nanowires and nanorods, 2D silicon nanosheets, and nanoclays, and 3D NMs as zeolites. Its extensive biological use is the result of its properties as biocompatibility and a high surface-to-volume ratio [7–9].

Obtaining Process Many physical, chemical, and biological manufacturing processes have been described to fabricate Si NMs. Some reviews that describe this topic are [9, 10]. The obtaining of zeolites and nanozeolites from agricultural wastes can be highlighted [11, 12].

Differences between the Bulk Si and the NSi The bulk Si is a material widely used for a long time in construction. Similarly, since ancient times the optical properties of opal have been known, a type of NSi of abiotic or biological origin. In modern times, the bulk Si is used in electronics, optics, and the food industry as an additive [13] and agriculture as biostimulant and promoter of stress tolerance [14]. The porous silicon is used as a chemical catalyzer, adsorbent, and sensor [15], in the latter case, thanks to its optical and physicochemical properties [8]. As with other NMs, the differences between the bulk Si and the NSi originate as a result of the nanometric scale of the particles and their porous surface topography, which causes (a) a substantial increase in the surface-to-volume ratio, (b) a significant increase in its surface free energy and hydrophobicity, as well as (c) phenomena of electron confinement in one or more dimensions [16]. From the point of view of the agricultural application, the properties (a) and (b) determine the most studied responses of the plants when the Si NMs are applied. The high surface-to-volume ratio allows a greater capacity to absorb electromagnetic radiation to produce photoluminescence, as well as the adsorption of inorganic,

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organic, and biomolecule substances to form a corona that can be used to stabilize and biofunctionalize the Si NMs [17]; the greater surface free energy and hydrophobicity allow ample biological reactivity and greater bioavailability of Si in ionic form for plant cells [1]. In both cases, biostimulation and metabolic improvement phenomena are induced that have been widely described and will be examined in the next section. Regarding point (c) above, the confinement of electrons induces collective behaviors that are not observed in the bulk material or the discrete molecules, which explain the optical and semiconduction phenomena that are of broad interest in the industry of energy, electronics, and environmental engineering and food industry, in the latter for its ability to catalyze the detection and photodegradation of pollutants [18]. These optical and electronic phenomena can be useful to improve metabolic processes such as photosynthesis and respiration in plant cells [19] and to study and monitor, by biological and cellular imaging, different metabolic processes in plants such as nutrient transport [20]. However, their application in the field of agricultural production is still limited.

Agricultural Use of Si NMs This section will describe the different uses of Si NMs. The various applications listed have resulted in an improvement in the efficiency and effectiveness of different types of agrochemicals, which can result in decreasing the amount used and subsequent economic and environmental costs. Each subsection corresponds to a specific use that has been reported in the literature.

Use of Si NMs to Modify Soil Properties and as Fertilizer Carriers The use of NMs for the improvement of plant nutrition has been described for virtually all essential and beneficial elements applied in conventional fertilizers [2, 21]. Silicon is no exception, and among the materials derived from Si are zeolites and nanozeolites. Zeolites are harmless materials, useful as food additives, which have a long history of agricultural use [22], traditionally used as a material for fertility improvement, fertilizer use efficiency, and capacity of water retention of soils and substrates in open field, greenhouses, and nurseries. Zeolites are described as microporous crystalline aluminosilicates with 3D nanostructure, while nanozeolites are 0D nanoparticulate materials. Zeolite minerals show arrangements with three-dimensional tetrahedral structures, with dimensions of 0.02 to 0.08 nm, that make up many pores with a diameter of 0.02–0.12 nm running through the material, resulting in a large internal surface that accommodates forming a kind of molecular sieve (Fig. 4). The characteristic on which the utility of the zeolites is based is a large external and internal surface with negative charges that allow interaction with cations or polar molecules, high capacity of adsorption, and capillary absorption of water, gases, and vapors as well as catalytic properties. The cation exchange capacity of natural zeolites can take values from 216 to 550 Cmol 100 g
1 [23].

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Fig. 4 Schematic structure of a zeolite mineral. The pores are marked with orange circles that represent cations with charge n + that accommodates on the inner surface

Applied to the soil in quantities of 500 to 8000 kg ha
1 natural zeolites increase the cation exchange capacity of the soil (CEC), water retention capacity, and hydraulic conductivity, resulting from the large adsorption surface and the capillary action of the pores of the mineral. Crop responses are higher productivity observed in different plant species. Zeolites can also be used as fertilizer carriers to improve their efficiency in poor soils with extremes of pH or low level of organic matter; in this case, it can be applied in quantities around the 60 kg ha
1 [23]. The use of zeolites applied to the soil can also increase the amount of organic carbon soil aggregates. In a comparative study, nanozeolite presented better results than zeolite in terms of the effects above. Nanozeolite, on the other hand, shows a CEC twice as high as zeolite, so the impact on soil fertility or its nutrient carrying capacity is much more significant [24]. The use of nanozeolites to improve soil properties and make plant nutrition more efficient results in a decrease in economic and environmental costs, mitigating pollution by leaching and losses from soil fixation and volatilization [2]. Table 1 shows some agricultural application results obtained with the use of nanozeolites. Another category of silicon derived material is nanoclays (Fig. 5), widely used in the food industry. Nanoclays are natural edaphic constituents, along with nanoparticles (NPs) of organic matter and Fe oxides [2]. Many of the studies carried out with nanoclays have been carried out by synthesizing polymer-nanoclay composites, where the polymer can grant water absorption potential and nanoclay the adsorption of fertilizing elements or nitrification inhibitors or other active molecules to improve the growth of plants or potential for adsorption of toxic elements for the remediation of contaminated soils. Table 2 shows some agricultural application results obtained with the use of nanoclays.

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Table 1 Some results reported in soil fertility and crop nutrition when applying nanozeolites Target element(s) and objective Improve the bioavailability of S to crops

Material used Nanozeolite saturated with S (30 kg S ha
1)

Increase the efficiency in the use of the P diminishing losses by soil fixation

The nanozeolite (31– 65 nm) was obtained by grinding zeolite. The nanozeolite was saturated with NH4+

Increase the efficiency in the use of the Zn diminishing losses by soil fixation

Natural zeolite (clinoptilolite) ball milled to achieve NPs 90– 110 nm. The Nps were fortified with ZnSO4

In vitro growth of plant growthpromoting bacteria isolated from the soil

Nanozeolite (50 mg L
1)

Model under study In groundnut plants, nanozeolite improved growth and various biochemical variables associated with biostimulation Chamomile in calcareous soil. The nanozeolite was applied with nanohydroxyapatite, phosphoric rock, and triple superphosphate. Nanozeolite substantially increased the availability of P for plants In a percolation reactor, the sorption and desorption of Zn were examined. The controlled release of Zn by the nanozeolites was observed for 49 days, while ZnSO4 only provided Zn2+ for 9 days A positive impact on the growth of bacteria was detected

References [25]

[26]

[27]

[28]

Regarding the use of nanosilicon (NSi) and nanosilica (NSiO2), the mesoporous nature (pore size 2–50 nm) of these materials makes them ideal as biostimulants and carriers of inorganic, organic and biological molecules. Due to their biostimulant capacity resulting from the large active surface area, NSi and NSiO2 can produce per se a positive response from plants by increasing the concentration of beneficial phytochemicals [34] or they can efficiently provide the element Si [35] which is indispensable for some plants such as grasses or beneficial for other species. The Si or SIO2 NMs can be applied in amounts of 10 to 2000 mg L
1 solution or kg
1 substrate, by sprinkling the seeds, seedlings, adult plants or applied to the soil (up to 7.5 g per pot) or in the nutrient solution from 10, 100 or up to 1000 mg L
1. In many studies, a positive or neutral effect of Si NMs has been observed; only a few have obtained a negative response from plants [16]. The amount of NSiO2 that induced in vitro toxicity in the MG-63 human cell line was higher than 125 mg L
1 [34]. The NSi and NSiO2 function as efficient fertilizer carriers that contribute N and B. Is possible to mix the NMs with organic matter or with biomolecules such as urease

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Fig. 5 Scanning electron microscopy image of bentonite nanoclay particles dispersed in water (1% wt). Scale bar ¼ 40 μm. (Image from [29])

to improve the water retention capacity and the nutrient supply of the soil or substrate [16]. Urea adsorbed on NSiO2 was stabilized when applied to the soil, reducing losses from leaching or volatilization [36]. The NMs of Si and SiO2 used in the amount of 0.05 and 0.07 mg kg
1 soil were also shown to be compatible with beneficial microorganisms, such as P-solubilizing bacteria, allowing the improvement of crop P nutrition [37]. In a study carried out on maize, the NPs of SiO2 (20– 40 nm) increased the microbial biomass of the soil where the plants grew, also raising the amount of Si absorbed by the crop [38]. SiO2 NPs can also be loaded with fertilizers (NPK) and encapsulated with waterabsorbing polymers such as chitosan, alginate, and kaolin to obtain a hydrogel that also functions as a controlled-release fertilizer, lasting up to 6 months, useful in soils with low water supply or salinity problems [39]. Other materials such as carboxymethyl cellulose and acrylic acid composites have also shown good results when used as a coating of SiO2 NPs loaded with NPK fertilizers [40]. In another study, not related to agriculture but of potential application in the control of fertilizer release, aggregates of Si quantum dots amine-modified and loaded with biomolecules were constructed. The aggregates dispersed, releasing the biomolecules, against a decrease in pH [41], a phenomenon that occurs in the rhizosphere when plants solubilize soil nutrients. If materials analogous to these were used to load them with fertilizers, the release of nutrients would occur only in the vicinity (0.25 g/cm3) and lower porosity (50%) than aerogel. Freezing the solvent and sublimating it (freeze drying) instead leads to cryogel, which comes always in powder because the crystallization of the solvent in the pores destroys the gel network [17]. On the other hand, supercritical drying happens at pressure greater than the vapor pressure, allowing the liquid to turn into gas without the two phases coexisting at any time (supercritical fluid). Since in supercritical condition there are no solid–liquid bonds, superficial tension is nullified, avoiding compressive forces on the gel skeleton triggered by capillary tension. Supercritical drying may be carried out either at high temperature or, more commonly, low temperature. With this method, the aged gel sample is placed in an autoclave at 100 bar and 4–10  C is filled with liquid CO2 to replace the solvent within the pores. The gel is then heated up to 40  C at 100 bar pressure to evaporate CO2 at a supercritical stage. The aerogel sample is then isothermally depressurized and finally cooled at room temperature [16]. Additional substances can be integrated to aerogel preparation to improve performance of the final product or overcome possible shortcomings. For instance, silica aerogels exhibit sintering phenomena at temperature above 800  C, showing agglomeration of particles and collapse of their pores, with 20% volume shrinkage at 800  C, 30 min and 50% volume shrinkage at 900  C, 30 min [26]. Although relatively relevant to their use in building construction, this hampers their application in the industrial sector or their compatibility with specific manufacturing processes such as molding or stamping. Currently, the main method to improve heat resistance of silica aerogel consists in doping it with metallic oxides such as Al2O3, ZrO2, Y2O3, and TiO2 to reinforce the aerogel skeleton and maintain the nanoporous structure, at the cost of increased density and thus thermal conductivity. Another approach provides for the preparation of aerogel from monodispersed silica sol, instead of the traditional acid–base two step preparation, which allows to achieve larger particles with a narrower size distribution and improve heat resistance maintaining low thermal conductivity [27].

Aerogel Building Applications Silica aerogel is mainly available on the market in monolithic blocks or slabs, loose granular form, or as mats, rolls, or fiber-reinforced panels (rigid or semi-rigid). Aerogel in monolithic blocks or slabs is still difficult to manufacture and market because of its brittleness and the difficult of obtaining defect-free specimens from the production process, with applications still at the prototype stage. Nevertheless, monolithic aerogel shows promising characteristics in terms of thermal conductivity (λ ¼ 0.01 W/mK in moderate vacuum), visible light transmission and solar heat gain (respectively 90% and 75% for a 10 mm slab), making it particularly interesting as an insulator inside double glazing units. In

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order to address the brittleness issue, opaque composite monolithic aerogels have also been developed to obtain self-supporting products that can be directly applied for insulating the building envelope [28]. Aerogel in loose granular form with hydrophobic processing can be used for filling wall cavities or mixed in cement for the construction of internal or external insulating plaster. The latter are also available premixed with lime and hydraulic cement with aggregates in aerogel and mineral particles, with λ ¼ 0.028 W/mK and application thickness of 30–150 mm. Experimental testing of mortar prepared with various mixes of commercial silica aerogel particles (diameter 0.1–5 mm) and sand showed a thermal conductivity as low as 0.15 W/mK for a 60 vol.% aerogel mix, further reduced to 0.09 W/mK values achieved by the integration of common additives such as polypropylene fibers, air entraining agent, and latex powder [29]. Acoustic testing on plaster with aerogel granules (80–90% in volume of aerogel) shows a peak of the absorption coefficient equal to 0.29 at about 1050 Hz, compared to a value of about 0.1 of conventional plasters, and evidenced best performance for smaller granules [30]. Small aerogel particles, with size ranging from 2–40 μm to 1.2 mm, can be added to paints and coatings for specialized thin-film-insulative coating applications, such as reducing burn risks in hot piping or preventing condensation in cold ones, even in a few mm thickness. Due its optical properties, aerogel in granular form is also used in manufacturing Transparent Insulating Materials (TIM) [31, 32]. By placing aerogel particles in 0.7– 3.5 mm size range in the cavity between glass or polycarbonate panes, light diffusing panels of unmatched thermal performance are achieved. Silica aerogel light transmission is less desirable for thermal insulation of opaque building enclosures, since it also allows infrared radiation to pass through and escape heated spaces. For this reason, carbon black can be added as an opacifier to absorb infrared radiation and sometimes it can even add to the material mechanical strength. A 9% carbon black addition to the silica aerogel lowers thermal conductivity from 0.0170 to 0.0135 W/mK at ambient pressure [16]. The material is defined by its fragility, having reduced mechanic strengths: compression strength σc ¼ 80–100 kPa (at 10% deflection), tensile strength parallel with the fibers σt.II ¼ 200 kPa, and tensile strength perpendicular to the fibers σt.⊥ ¼ 100 kPa [33]. To improve its mechanical characteristics in order to ease its use in insulating opaque enclosures, aerogel is usually integrated in a fibrous support structure obtaining fiber-reinforced aerogel blankets (FRABs).

Fiber-Reinforced Aerogel Blankets Manufacturing and Performances In order to be used as insulating products for building application, silica aerogel are integrated in a fibrous support structure obtaining an insulation mat called “fiberreinforced aerogel blankets, FRABs” (Fig. 5).

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Fig. 5 FRAB (courtesy of Ama Composites ®)

FRABs manufacturing process first impregnates a fibrous felt (mainly glass fiber or PET and glass fibers) with silica sol-gel solution (sol-gel casting); when impregnation is complete, felts are rolled and placed in a controlled environment for chemical ageing of the gel. Finally, supercritical extraction of the solvent is carried out in large autoclaves with CO2 recovered from other external industrial processes. As the gel dries, aerogel particles are firmly embedded into the mat fibers. After a final drying step an aerogel saturated mat, easy to transport and install, is achieved (Fig. 6). Final FRAB composition is 40–55% silica, 20–45% PET/glass fiber, and 0–15% additives. FRABs have a very low thermal conductivity (λ ¼ 0.015 W/mK) with a constant thermal performance for wide operating temperatures (from
200 to +200  C), good noise insulation and good resistance to fire, from Euroclass C for FRABs with PET fibers matrix to A2 for FRABs with glass fiber matrix only. FRABS also show high hydrophobicity values while maintaining high water vapor permeability (see Table 3). Concerning noise insulation, acoustic measurements of a test fac¸ade before and after the internal application of a 2 cm-thick FRABs evidenced a remarkable 3 dB improvement for the sole FRAB, and a 7 dB improvement following the application of the interior finishing layer consisting of two gypsum wall boards of 12.5 mm [34]. Regarding FRABs behavior in high humidity conditions, independent testing evidenced aerogel’s excellent performance over time. Starting from a consolidated λ value of 0.017 W/mK, it was observed that after wetting a FRAB for 24 h at increasing relative humidity, thermal conductivity increased by 10% at 65% humidity and by 20% at 90% humidity, maintaining a remarkable value of 0.020 W/mK. This increase occurs firstly due to accumulation of water on the panel surface and subsequently by capillary condensation in the nanopores [35]. Concerning the behavior in extreme cold, freeze-thaw cycles simulated by freezing the FRAB for 25 day at
15  C and thawing it for measurements showed no variations in thermal resistance for dry samples, whereas freezing wet samples evidenced up to 25% increase in thermal conductivity for 4% moisture content, equivalent to 90% humidity levels [36].

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Fig. 6 Fiber-reinforced aerogel blankets (FRABs) manufacturing process Table 3 Main aerogel insulating product specifications

Aerogel product Thermal conductivity (W/mK) Thermal diffusivity (mm2/s) Operating temperature ( C) Vapor diffusion resistance μ Compressive strength @10% deformation (kPa) Density (kg/m3) Specific heat (J/kgK) Fire reaction (Euroclass) Primary energy PEI n.r. (MJ/m2) GWP (kg CO2/m2) Thickness for R ¼ 2 m2 K/W (cm) Cost for R ¼ 2 m2 K/W (% compared to EPS medium load)

FRAB 0.015 0.1
200/+200 5 80 150 1000 C, s1, d0 or A2, s1, d0 242 12.50 3 1000

EPS medium load 0.031 1.069
40/+85 70 100

Medium density rock wool 0.033 0.458 SNHS [48]. Apart from this, silica nanomaterials are utilized in the generation of composite materials.

Zerovalent Metal-Based Nanomaterials Major issues associated with magnetic polymer nanoparticles, especially in the case of metal (Fe, Co, and Ni) and alloy nanoparticles, are their stability and their metal alloys, towards oxidation in air and chemical degradation. For that reason, two major breakthroughs have been achieved first by developing zerovalent nanoparticle and second by adding a protection layer that prevents the contact between oxygen and surface of the magnetic particles [49, 50]. Figure 2 shows the different types of zerovalent nanoparticles.

Metal-Oxide Based Nanomaterials Similar to carbon and silica, metal oxides possess exceptional properties such high selectivity and high removal potential. These properties make them promising and efficient heavy metals adsorbers in waste treatment process. Metal oxides of aluminum, cerium, iron, manganese, magnesium, titanium, zinc, and zirconium have been well acknowledged for nanoparticle preparation. Since 1972, TiO2 nanoparticles are

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Fig. 2 Types of zerovalent nanoparticles

one of such nanostructures used for photocatalytic degradation [51]. In recent advancements, similar light-induced photodegradation technology has been applied by using TiO2 nanoparticles for wastewater treatment due to high photocatalytic potential, physical, chemical and biological stability, cost-effective construction. Photocatalytic action of metal oxide (TiO2) degrades organic contaminants in water and oxidizes them to Cl , CO2, H2O, NO3 , and PO43 in the presence of light. In contrast to the above-discussed nanomaterials, TiO2 is non-specific in its action; however, it will also be an advantageous for waste treatment. UV irradiation generates reactive oxygen species (ROS) from TiO2 which helps in the degradation of contaminants in very short reaction time [43]. Likewise, ZnO nanoparticles also degrade chemicals from waste effluents by photocatalysis [43, 52].

Nanocomposite Nanomaterials In the past few decades, researchers have developed nanostructures from different materials that have revolutionized the industries. Each kind of nanomaterial has offered its own advantages; however, they also have different disadvantages such as less user friendly, toxicity, low compatibility with system, aggregation, poor separation, and less physical, chemical, and biological stability [53]. This led to the evolution of hybrid nanocomposites to exploit the benefits of nanomaterials [54]. Different types of nanocomposites are discussed below.

Inorganic Nanocomposites Nanomaterials prepared with organic/natural materials like zeolite, montmorillonite, maleic anhydride, bentonite in addition to some inorganic materials like activated carbon (AC), philo-silicates, clay, CNTs, kaolinite are referred to as inorganic

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nanocomposite or inorganic-supported nanocomposite. Among these, AC is commonly used as it is user friendly, economic, and effective in treating wastewater and removing pollutants from effluents [55]. AC nanocomposites, made with CNTs/ magnetic acids/hydroxyapatite, are known for the removal of Cr(VI), Pb(II), and Cd(II) from wastewater. Among the natural organic molecules, chitosan is one of largest group of biopolymers, extracted from arthropods, mollusks, and exoskeletons, and can be used for the synthesis of “CNTs-supported nanocomposite.” Glutaraldehyde derivatives of CNTs-nanocomposites have been proved useful in the removal of Cd(II), Cu(II), Ni(II), and Zn(II) from aqueous sample solution [43].

Organic Polymer-Supported Nanocomposites Mechanical strength, adaptive functional groups, reusability and regeneration, eco-friendliness, and natural stability are the prime choice for nanocomposites selection and organic polymer-supported nano-composites make the competition complex (Fig. 3) [50, 56]. Likewise, synthetic organic polymers and some biopolymers including cellulose, chitosan, and alginate are exploited extensively for the synthesis of nanocomposites. These biopolymers contain a wide range of functional groups like cellulose molecules, which are rich in hydroxyl groups that provide space for coordination sites to chelate with heavy metal ions [57]. Nanocellulose (NC)-silver nanoparticle (AgNPs) composites have dual activity: removal of heavy metal and control of microbial loads from water [58].

Magnetic Nanocomposites Even within composites, magnetic nanocomposites (Fig. 4) (made of magnetic iron and iron oxides) occupy an important role in industrial applications as they possess high separation ability [43]. Besides wastewater treatment, magnetic nanocomposites have been used for energy storage devices, electronic devices, Fig. 3 Synthesis of organic polymer-supported nanocomposites

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Fig. 4 Fabrication of magnetic nanocomposites

microwave absorbers, and sensors. Magnetic NPs-collagen composites have shown increased oil absorption capacity with selectivity [59].

Conclusions and Further Outlook Due to rapid industrialization, ecological balance has been destabilized and natural resources are polluted. In special context to heavy metals, their presence even in small amount is lethal to living beings. After screening a number of methods for removing these heavy metals from water, researchers successfully have developed nanomaterials with different selectivity and applications as per the need. Nanomaterials are superior to other methods due to their large surface area, smaller size, and high selectivity and stability. Among the different nanocomposites, some composites have also proved useful in the elimination of microbial load and inorganic and organic pollutants. A lot of work has been done, but still efforts are needed to design an efficient system which is recyclable, cost–effective, and stable with respect to harsh environment of industrial effluents.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCMs Employed in LTES (Latent Heat Energy System) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classifications of the PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat Transfer Enhancement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials embedded PCMs (NEPCMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Concentration (Weight/Volume Fractions) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of pH Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Types of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Characteristics of NiPCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Latent Heat Versus Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Conductivity Versus Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solidification and Melting Characteristics of NiPCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heating and Cooling of Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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S. Harikrishnan (*) Department of Mechanical Engineering, Kings Engineering College Irungattukottai, Sriperumbudur, Tamil Nadu, India e-mail: [email protected]; [email protected] A. D. Dhass Department of Mechanical Engineering, PACE Institute of Technology and Sciences, Ongole, Andhra Pradesh, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_97

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Abstract

In this chapter, it is focused to emphasize the development of nanomaterials enhanced phase change materials for latent thermal energy storage systems. Here, it is started with the working principle of phase change materials, types, and the importance of improving the thermal conductivity of the phase change materials. Various heat transfer techniques employed for accelerating the melting and solidification of the phase change materials are briefly discussed. Among different enhancement techniques, nanomaterials used for improving the thermal conductivity of the phase change materials are highlighted herein. The effects of concentration, size, morphology, and types of the nanomaterials toward the thermal conductivity enhancement of the phase change materials are reported comprehensively. At the end of the chapter, applications of the nanomaterials embedded phase change materials are discussed one by one.

Introduction The high energy demand in many developing countries has resulted in the inconsistency between the demand and supply of electrical energy, particularly in peak load conditions. Therefore, it is necessary to increase power generation by installing new conventional power generating stations or renewable energy sources (solar, wind, biomass, etc.). Through this, it is possible to bridge the gap between power demand and supply. Currently, conventional power generation stations still depend on fossil fuels. Long-term generation of power through fossil fuels is a risk. Another alternative for generating electricity is through renewable energy sources, but power conversion efficiency is not up to the level desired. The energy level required is always at a maximum than the energy generated, resulting in the shortage of fossil fuels and, finally, energy crisis. Due to this mismatch, certain conditions can be modified to some degree without making any damage to the atmosphere as long as solar energy is effectively stored in the systems for thermal energy storage (TES). The constant rise in building heating loads during winter season is one of the main causes of increase in energy requirement. The method of latent thermal energy storage (LTES) is suggested to increase the energy savings of the heating system in domestic applications. During off-peak periods, the PCMs (phase change materials) used in the LHES process store heat energy from the heating device and release the accumulated heat energy for the duration of peak periods. In addition, solar energy can be accumulated in the PCMs during daytime, and the heat energy can be released during nighttime provided that the solar energy temperature is adequate for processing. In the LTES system, the PCM stores the cold energy supplied by the chilled water systems during part load operations and retrieves the stored energy during peak load operations. Mostly, organic PCMs are suggested over inorganic PCMs as they have outstanding features such as no supercooling effect, good thermal properties, chemically stable, and adjustable transition zone for heat energy harvesting. In this chapter, a systemic study of PCMs is

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employed in enhancement of heat storage and classification of PCMs for organic, inorganic, and eutectic categories. Different enhancement techniques are suggested to accelerate the melting and solidification processes of the PCMs.

PCMs Employed in LTES (Latent Heat Energy System) For the solid-liquid case, latent heat storage keeps a constant temperature during storage material’s state of stage transition. In addition, energy is retained as reactive heat. The temperature range at which the phase changes occurs below and above, but the product temperature increases. When the storage material is heated above its phase transition temperature solid to liquid phase change is taken place and it can be indirectly understood that it stores the heat energy. The liquid PCMs will again become solid-state materials when the ambient temperature decreases, releasing the heat previously stored in an exothermic cycle as shown in Fig.1 [1, 2]. The heat stored in the latent heat storage materials during phase change process can be determined by ΔQ ¼ ΔH The two possible application areas of PCMs can be separated into temperature control: heat can be supplied to or retrieved from the PCM without any temperature alteration. As a result, to stabilize the temperature, PCMs can be used. Large Heat storage as sensible heat material leads to temperature increases when heat is stored (e.g. brick)

Heat storage as latent heat material throughout the phase changing process; keeping the temperature constant

T [o C] Sensible

TmeltingPCM Latent Heat

Sensible

Solid

Solid + Liquid

Liquid Stored Heat [kJ.kg-1]

Fig. 1 Latent heat energy storage in solid-liquid phase [1]

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Phase Change Materials

Organic

Paraffin

Alchol

Inorganic

Non-Paraffin

Fatty Acids

Salt hydrate

Mg-Cl2

Eutectic

Organic -Inorganic

Metals

Organic eutectics

KCl

Carbonate Salts

Inorganic -eutectics

Fig. 2 Classifications of PCM materials [5]

quantities of energy (heat/cool) in PCMs can be stored at a relatively small change in temperature [3]. The selection of any material for latent heat storage is based on the end application as shown in Fig. 2. However, for proper selection of the right materials, definite attractive properties of PCMs including thermodynamic, kinetic, physical, and chemical properties as well as cost-effective viability and accessibility must be considered [4].

Classifications of the PCMs See Figs. 2 and 3.

Heat Transfer Enhancement Techniques Scientists have accomplished a lot of research works to overcome the problems of low thermal conductivity of the PCMs. It is shown in Fig. 3 that different strategies have been adopted to improve the thermal conductivity of PCMs. Carbon nanomaterials, ceramics, and metal foams/fins were used to boost PCM thermal conductivity [6–9]. Some researchers conversed this issue in depth and covered a quantity of methods in the direction of improving the thermal conductivity [10, 11]. Thermal performance of the composite PCM comprised eicosane and copper foam. The presence of copper form was able to increase the

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Latent Heat Storage System

PCM Type & Operating Temperature (°C)

Organic (4-150)

Inorganic (8-900)

Performance enhancement techniques

Heating strategies

Cooling strategies

Concentrated solar power

Insulation

Additives Macro, micro, nanoencapsulation

Parabolic trough thermal connection

Ventilation Evaporative cooling

Eutectics (12-600) Flat plate thermal collectors

Soaked Material Shape stabilization

Radiative night cooling

Process heat recovery Photovoltaic Thermal

Internal heat gain

Fig. 3 Various strategies for latent heat storage systems [5]

effectual thermal conductivity from 0.423 W/mK to 3.06 W/mK. This increase in thermal conductivity reduced the freezing time significantly. In another work, the composite PCM using paraffin and graphite foam with small pore size and thicker ligament increased thermal diffusivity. Graphite foams with larger pore size and thinner ligament could be expected to increase the heat storage capacity rates further [12, 13]. Thermal conductivity enhancement of conventional PCMs is the fastest-growing method of impregnating porous materials. This is mostly due in the direction of the greater magnitude of the porous material’s thermal conductivity than the pure PCMs [14]. The experimental results showing the heat transfer characteristics of composite PCM were studied with respect to the effect of form porosity and pore size. Test results showed that for larger pore size foams, the steady state was attained at a faster rate while comparing to smaller pore size foams. They reported that foam porosity and pore size affect the conductive as well as the convective heat transfer, and therefore, an optimal porosity and pore size quality could be preferred for better

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performance of the paraffin wax/aluminum foam composite [15]. Recently, significant progress has been made in the use of high-conductivity nanomaterials in combination with PCMs to improve their thermal conductivity. This includes the use of nanopowders (like Al, CuO, Cu, SiC), nanowires (NWs), and carbon nanotubes (CNTs). Compared to the base material, nanoparticles embedded in conventional PCMs have a better thermal conductivity [16]. Nanoparticles with PCMs are added to the techniques for improving latent thermal energy storage. The nanocomposites are generally prepared by dispersing carbon nanoparticles into pristine PCMs, resulting in form-stabilized PCMs. The disparity in the composites’ latent heat efficiency (%) is proportional to pure PCMs. There is no information available in some research on the thermal conductivity and latent heat capacity enhancement of composite PCMs promoted by adding nanoparticles. For composite PCMs, the change of latent heat capacity (%) is calculated by using Eq. 1: Latent heat capacity ð%Þ ¼

ΔHcomposite  100% ΔH PCM

ð1Þ

ΔHcomposite ¼ the latent heat capacity of PCMs/nanocomposites (melting or cooling) ΔHPCM ¼ the latent heat capacity of pristine PCM (melting or cooling) It is inevitable to enhance the thermal conductivity of the PCMs as it could be fruitful to develop the improved performance of the LTES system. The thermal conductivity of PCMs can generally be improved by impregnating high thermal conductivity porous materials and dispersing high-conductivity materials/nanoparticles and/or low-density materials into the PCM base. Impregnating porous materials into PCM increases their thermal conductivity and thus improves the thermal performance of LTES [7, 18–22]. Experiments were conducted on the thermal conductivity and viscosity of noctadecane/TiO2 nanoparticles. The results showed in both solid and liquid phases a non-monotonic nature of thermal conductivity. The maximum enhancement of thermal conductivity occurred in the solid phase at 3 wt% of nanoparticles. The influence on the thermal efficiency of a LTES system of carbon nanoparticles is dispersed into PCM. Different nanoparticles, namely, single-walled carbon nanotubes, multi-walled carbon nanotubes (MWCNT), and carbon nanofibers, were considered in the preparation of the composite PCMs, and as a result, a higher latent heat capacity of composites was achieved compared to pure wax [23, 24].

Nanomaterials embedded PCMs (NEPCMs) It is a well-known fact that PCMs have low thermal conductivity, and this could increase the time required to complete the melting and freezing processes. As a result, the energy storage/retrieval rates are reduced, and therefore, PCMs are restricted to small-scale applications only and not recommended for large-scale

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Table 1 Thermal conductivities of some commonly used PCMs and nanoparticles [11, 23, 25–35] S. No 1 2

Thermal conductivity value of PCM Thermal conductivity PCM (W/mK) RT27 0.13 Octadecane 0.17

3

RT25

0.26

4

Eicosane

0.36

5 6

Tetradecanol Capric acid

0.42 0.48

Thermal conductivity value of nanoparticles Thermal conductivity Nanoparticles (W/mK) Aluminum (Al) 237 Silver 429 nanoparticle (Ag) Copper 400 nanoparticle Titanium dioxide 8.4 (TiO2) Carbon nanotube 3000 Graphene 3000

applications. In the past, many techniques were developed to enhance the thermal conductivity of PCMs. But none of them have indeed helped to achieve larger enhancement. After the evolution of nanoscience and nanotechnology, a novel approach of addition of solid nanoparticles into the base fluid was implemented, and finally, it is reckoned to be the most effective enhancement technique. Table 1 presents some of the works demonstrated on the nanoparticle-enhanced PCMs.

Effect of Morphology Nanoparticle surface morphology plays an important role in determining their properties. Scanning electron microscope (SEM) can examine the surface texture. The specific surface area of Mg-41204 powder calcined at 1000  C was reported to be 40 m2g1, and the surface area decreased to about 25 m2g1 with higher calcination temperature. SEM analyses confirmed this heterogeneity and clarified it in terms of relatively large agglomerates of low density produced at different temperatures. In general, spinel powder had a small crystallite size with a large surface area at low sintering temperature. It is possible to see isolated and connected spherical particles with an increase in temperature. In an attempt to minimize surface energy, the particle assumes a spherical shape. Surface morphology depends on the preparation process. Nanocrystalline MgAI2O4 has various morphologies that match different methods of preparation.

Effect of Size Another important parameter of nanofluid thermal conductivity is the particle size. Nanoparticles of different sizes can be made, usually between 5 and 100 nm. Cu nanoparticle is coated with ethylene glycol, and the base fluid is ethylene glycol. Cu nanoparticles with less than 10 nm were synthesized by using a one-step method. For stabilization purposes, thioglycolic acid less than 1 vol.% was applied to some of

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the samples, and these samples demonstrated much better improvement as compared to samples without thioglycolic acid. A 40% increase in thermal conductivity was observed at 0.3% (with thioglycolic acid) particle size fraction [36]. Different sizes (36 and 47 nm) of Al2O3 nanoparticles were used in experiment to measure the thermal conductivity enhancement of Al2O3/water nanofluids. Thermal conductivity enhancement of these two particle sizes was found to be same even if the concentration of the nanoparticles was varied from 0 to 18% and the tempeature was varied from 20 to 50 C respectively at room temperature [37]. Various sizes of the partiles (38.4, 47 and 80 nm) have not made noticeable differences in the thermal conductivity ratio [38, 39]. The different nanoparticles Al2Cu and Ag2Al are immersed in water and ethylene nanofluids and their thermal conductivity behavior studied experimentally. The size of nanoparticles is varied from 30 to 120 nm. It was reported that the thermal conductivities of four types of nanofluids prepared in the study were increased with a decrease in particle size [40]. It should be noted that thermal conductivity increases with decreasing particle size. Brownian motion is considered to be the main mechanism for enhancing thermal conductivity, as the effect of Brownian motion increases with decreasing particle size, improving micro-convection around nanoparticles. Here, it can be understood that the size of the particles towards the thermal conductivity enhancement relies on the types of particles.

Effect of Concentration (Weight/Volume Fractions) Thermal conductivity of nanofluids containing Al2O3 (13 nm), SiO2 (12 nm) and TiO2 (27 nm) was determined with respect to volume fraction of particles. Water was used as the base fluid, and nanofluids were prepared using a two-step method. The effective thermal conductivity of 4.3 vol.% Al2O3/water nanofluid at 31.85% was up to 32.4%. It was established that the enhancement of thermal conductivity augmented linearly with the fraction of particle volume [41]. Particle volume fraction is a parameter which is investigated in virtually all experimental studies, and the results are generally qualitatively in agreement. Many researchers report increasing thermal conductivity with increasing fractionation of particle volume, and typically linear relationships are found. Some studies, however, also indicate nonlinear behavior [42]. It was observed that 4 vol.% of CuO/ ethylene glycol enhanced the thermal conductivity up to 20%. From this study, it can be understood that the relationship between the volume fraction and the thermal conductivity was found to be linear [41]. However some researchers explained that the relation between partial volume fraction and thermal conductivity is a nonlinear behavior. As the concentration of TiO2 nanoparticles dispersed in deionized water increases from 0.5 to 5% thermal conductivity of the nanofluid decreases and it was observed particularly at lower concentrations. The reason for the reduction in thermal conductivity might be attributed to surfactant, application of sonication for a longer period, or hydrophobic surface forces [43].

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Effect of pH Value Thermal conductivity of nanofluid (Water and TiO2) is decreased with increasing pH value. This decrease is not important, however, since only 2% shift was observed when the pH value increased from 3.4 to 9 [44]. The effect of pH on nanofluid thermal conductivity is studied. Cu/water and Al2O3/water are known as nanofluids. Sodium dodecylbenzene sulfonate has been added to the specimens as the dispersant. An optimal pH value of nanofluid could be helpful to achieve the larger enhancement of the thermal conductivity. It should also be noted that the thermal conductivity of the base fluid does not significantly change with pH. The reason for this fact is that at optimal pH value, surface charges of nanoparticles are increased and thereby, they could develop repulsive forces between nanoparticles. This effect prevents extreme clustering of nanoparticles (excessive clustering can result in sedimentation, which decreases thermal conductivity enhancement) [45]. When pH value of the nanofluid containing water and 0.5 vol.% Al2O3 is varied between 2.0 and 11.5 enhancement of thermal conductivity is decreased to 19% from 23%. It is explained that increase in pH value decreases gradually the corresponding thermal conductivity. The measurement of thermal conductivity for Al2O3 nanofluid with different base fluids like water, pump oil, and ethylene glycol was studied experimentally. Test results reveal that if the pH value of the nanofluids increases then, the thermal conductivity of the nanofluids decreases [46]. Many studies attempted in the past provide the information on the effect of pH value on the thermal conductivity of the nanofluids however, they did not give enough clarity on the understanding of pH value and enhancement of thermal conductivity. Further investigations are required to elucidate the clarity on the relationship between the thermal conductivity and pH value of the nanofluids [42].

Effect of Types of Nanomaterials It is well known that the thermal conductivity of metal NPs (nanoparticles) is higher than that of solid fluids. For example, at room temperature, copper thermal conductivity is about 700 times higher than water and about 3000 times higher than engine oil. Even oxides like alumina (Al2O3) have higher thermal conductivity than liquid. Thus, fluids containing suspended solid particles are expected to show significantly increased thermal conductivity compared to conventional heat transfer fluids. Nanofluids are formed by dispersing solid particles in liquids such as water, ethylene glycol, or oils from nanometric scales. It is expected that nanofluids will have superior properties compared to those of traditional heat transfer fluids and microscopic particle size fluids. Since heat transfer occurs on the particle surface, it is preferable to use the particles with a broad total surface area. The wide total area of the surface also raises the suspension for stability. Nanofluids consisting of CuO or Al2O3 NPs in water or ethylene have recently been shown to have advanced thermal conductivity [47–50].

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Thermal Characteristics of NiPCMs Two impact factors culminated in the variance pattern of enthalpy with the mass fraction of additives. Intermolecular forces between nanoparticles and PCM are the first impact factor. The low enthalpy of additives is another influencing factor, which would decrease the melting enthalpy of composites. When the mass fraction was less than 1 mL, the first impact factor affected the melting enthalpy of NDPCM (nanoparticle dispersed phase change material). The influence of the second impact factor increased by more than 1%, and then the melting enthalpy decreased [51]. Therefore, it is difficult to realign the PCM matrix molecules in the presence of heavily loaded nanoparticles, which is also a major reason for the reduction of the composite’s active latent heat [52]. Simply mixing theory, theoretical melting enthalpy NDPCM can be calculated by using [53] ΔHtheoretical ¼ ΔH PCM  ð 1  φÞ m m

ð2Þ

ΔHtheoretical ¼ theoretical melting enthalpy of composite m ¼ melting enthalpy for base PCM ΔHPCM m φ ¼ weight fraction of nanoparticle This section’s analysis investigated the impact of nanoparticles on the LHS unit’s efficiency. Figure 4 shows the PCM’s liquid fraction (β) curves for π ¼ 0%, 2.5%, and 5% at a constant HTF inlet temperature of 60  C and a flow rate of 0.25 m3/h. From the following formula, β can be calculated [54]:

Fig. 4 Volume-average liquid fraction profiles with various time periods [55]

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0.3 wt% NP 0.25 wt% NP 0.2 wt% NP

Thermal Conductivity ( W/mK)

0.3

0.15 wt% NP

0.1 wt% NP 0.05 wt% NP

0.25 0.2

0 wt% NP

0.15 0.1 0.05 0 124.2

124.9

125.6

126.3

127.2

128

128.8

Latent heat (kJ/kg) Fig. 5 Effect of nanoparticles on the thermal conductivity and latent heat of the composite PCMs

β¼

V liquidPCM V liquidPCM þ V solidPCM

ð3Þ

where V represents the volume of the nano-PCM in solid or liquid form as indicated in the subscript. The completion of the phase change cycle took 10,000 s for pure PCM (π ¼ 0%), 9120 s for π ¼ 2.5%, and 8080 s for π ¼ 5%. Dispersing nanoparticles at π ¼ 2.5% has a mild impact on the melting time (8.8% less), while the total melting time has been shortened by almost 19.6% for π ¼ 5%. As part of practical technology, therefore, the volume fraction at 5% was sufficient. The dispersed TiO2 nanoparticles into the base material have had an effect on the prepared composite PCM thermal conductivity and latent heat. In Fig. 5, it is clear that the thermal conductivity and latent heat of the composites are inversely proportional to each other for the addition of the nanoparticles. As the nanoparticle mass fraction increases the composites’ thermal conductivity, the composites’ latent heat decreases. This is because the PCM (base material) and nanoparticles (supporting material) interact. The increase in thermal conductivity and decrease in PCM latent heat strongly depend on the type, size, shape, and nanoparticle mass fraction.

Latent Heat Versus Thermal Conductivity The increase in thermal conductivity was found to be 70.52% for composites with 0.3 wt% TiO2 nanoparticles, whereas the reduction in latent heat was determined to be

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3.49%. From the LTES perspective, this small reduction of the composite PCM’s latent heat would have no significant impact on the system’s energy storage and release capacity, but would accelerate the system’s energy storage and release rates [56–59].

Thermal Conductivity Versus Viscosity The PCM’s thermal conductivity plays an important position in enhancing energy storage as well as discharge processes quicker than other parameters such as the HTF’s operating temperature, HTF’s volume flow rate, PCM’s specific heat capacity, and PCM mass. It was expected that composites with different mass fractions of CuO nanoparticles would achieve a higher thermal conductivity than the base fluid. The composites and base fluid thermal conductivity have been calculated at room temperature, and the result is shown in Fig. 6. This shows that the thermal conductivity often increases linearly as the concentration of CuO in the base fluid increases. The viscosity difference with the nanoparticles density in the composites base fluid and operating temperature is shown in Fig. 7. From Fig.7, it shows that as the nanoparticle concentration rises, the viscosity also rises correspondingly. Viscosity enhancement would not introduce any detrimental effect on the performance of the LTES system. In contrast to the LTES system, in the case of the dynamic flow system where the nanofluid flows either counter or parallel to the HTF, viscosity enhancement provides resistance to nanofluid flow, which in turn would affect the heat transfer rate between the nanofluids and HTF. CuO nanoparticles in the base fluids acted as the nucleating agent, and subsequently, they reduced the supercooling effect of the water-glycerol mixture [60].

Fig. 6 Effect of mass fractions of CuO nanoparticles on thermal conductivity

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Fig. 7 Effect of mass fractions of CuO nanoparticles on viscosity

Solidification and Melting Characteristics of NiPCMs As far as the LTES system is concerned, the thermal performance of the PCMs is an essential quality for long-term use. For investigating the thermal reliability, a nanofluid with 1.0 wt% CuO nanoparticles was considered. During the solidification and melting phases, the phase temperatures and latent heat of the composite PCMs are calculated in terms of the number of thermal cycles. Test results presented in Fig. 8 show that due to the addition of nanoparticles, the phase change temperatures of the base fluid were altered and the maximum changes in phase change temperatures for the solidification and melting were found to be 2.21% and 2.40%, respectively. Likewise, from Fig. 9 the average latent heat differences for solidification and melting are 0.91% and 0.76%, respectively. Such variations are clearly indicated to be marginal and therefore would not significantly affect the energy storage and release capacity of the LTES system. Therefore, more thermal stability has been shown by the nanofluids prepared as PCMs.

Applications Bulk materials are reduced to the scale ranging from 1 to 100 nm then, known as nanoparticles, whose properties are subjected to change [61]. The changes in the physiochemical properties are due in part to the mechanical motion of the surface layer of the nanoparticles. They exhibit greater surface area by weight than larger particles, resulting in greater reactivity and continuous random motion. The dispersion of nanoparticles in polymers, metals, ceramics, and fluids has opened up further

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Phase change temperature (°C)

5 Melting 4.5

Solidification

4 3.5 3 2.5 2 0

75

150

225

300

375

450

525

No. of cycles

Fig. 8 Variation of phase change temperature with respect to the number of cycles

310 Phase change temperature (°C)

Solidification 308

Melting

306 304 302 300 298 0

75

150

225

300

375

450

525

No. of cycles

Fig. 9 Variation of latent heat with respect to the number of cycles [60]

paths for more flexible composite engineering that has advantageous thermophysical properties [62]. These new hybrid nanofluids/composites are used in many technological applications ranging from biomedical to energy transportation. A recent area of research is the application of nanoparticles into TES base fluids. The amount of nanoparticles that need to be dispersed into the base fluids for specific TES applications is still being studied and investigated. To date, carbon nanotubes have been the most

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commonly used additives since their thermophysical properties have been identified and the benefits compared to other nanoparticles have been demonstrated. TES is a mixture of various technologies that store thermal energy in a reservoir for later use. The stored energy in TES system can be retrieved during on-peak load conditions, resulting in ensuring off-peak load operation of the energy sources (chillers or heaters). Heating and cooling applications, particularly with regard to buildings, are among the various technologies in TES. Transport (automotive), microelectronics, electrical smart systems, medicine, spacecraft, and agriculture (livestock) need heating and cooling systems to improve efficiency or thermal comfort. Several studies have been performed on efficient developments in heating and cooling systems for general applications, but with less research being carried out on the heating and cooling applications of nanomixtures using TES.

Heating and Cooling of Buildings Space cooling in industrial areas is a huge scientific challenge that also applies to many other production areas, including transport, manufacturing, microelectronics, sports arenas, etc. Traditionally, it has been in practice for many decades to heat and cool a building or space. In order to protect human beings from the hot weather, buildings can be constructed in the middle of trees or bricks can be used to build walls and floors. Depending on the desired comfort, the need for effective use of TES has led to the use of PCM in various building technologies to assist in cooling and heating. PCMs have a high latent heat storage capacity and were considered in building applications for thermal storage [63]. It is possible to use wallboards, shutters, under floor heating systems, and ceiling boards as part of the building for heating and cooling applications with the advent of PCM impregnated in Trombe walls [64]. Many PCMs have relatively low thermal conductivity which limits their further use, and so there is a need to integrate PCMs with nanomaterials to enhance their thermophysical properties. Several studies have demonstrated the possibilities of designing and using PCM nanocomposites to improve TES capability in application building. Nanocomposite enhanced PCMs are recommended to use in modern buidings or multi-storey buildings manily, to reduce the energy consumptions of the chillers and heaters during on-peak periods. During winter, they can store the solar energy in the day time and release the stored heat energy to the buildings/spaces during night-time; During summer, cool energy derived from the wind blowing in the night time and release the harvested cool energy to the buildings during daytime [65].

Conclusion and Further Outlook This chapter could provide information about the use of nanomaterials in PCMs. Nanomaterials are used to increase the thermal conductivity of the PCMs since the enhanced thermal conductivity of the PCMs could expedite the energy storage/ release processes. The degree of enhancement mainly relies on the size, shape,

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type, good dispersion, and orientation of the nanomaterials. Also, the addition of nanomaterials could cause little shift in the latent heats and phase change temperatures of the PCMs, and these small deviations could never affect the performance of the LTES systems. This enhancement technique is implemented in low-temperature heating application of the PCMs only. Hence, it is essential to explore the solution for successful implementation of dispersion of nanomaterials in high-temperature heating application of the PCMs.

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Nanomaterials for Arsenic Remediation with Boosted Adsorption and Photocatalytic Properties

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Laura Hinojosa-Reyes, Aracely Hernández-Ramírez, Mariana Hinojosa-Reyes, and Vicente Rodríguez-González

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Oxide-Based Nanomaterials for the Adsorption of As Species . . . . . . . . . . . . . . . . . . . . . . . . TiO2 Based Materials for the Adsorption of As Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron-Oxide-Based Materials for the Adsorption of As Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Use of Photocatalytic Nanomaterials in As Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2-Based Nanomaterials for Arsenic Removal Through a Photocatalysis Process . . . . . Metal Oxide Nanomaterials for the Photocatalytic Removal of Arsenic Compounds Present in Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-Based Nanomaterials for Removing Arsenic from Wastewater and Groundwater: Adsorption and Photo-Oxidation Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Carbon-Based Nanomaterials (O-CBNMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Carbon-Based Nanomaterials (SCBNMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Groundwater contamination with arsenic stems from natural and anthropogenic sources represents one of the most critical environmental and public health L. Hinojosa-Reyes (*) · A. Hernández-Ramírez Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, UANL, San Nicolás de los Garza, Mexico e-mail: [email protected] M. Hinojosa-Reyes Facultad de Ciencias, Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico V. Rodríguez-González Photocatalysis International Research Center, Research Institute for Science and Technology, and Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan División de Materiales Avanzados, IPICYT, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, Mexico © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_78

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problems in many countries. Inorganic and organic arsenic compounds can pose serious health risks to humans. Traditional techniques, such as the oxidation process, membrane filtration, adsorption, and chemical precipitation, among others, are used for removing both forms of arsenic species. However, effective remediation of arsenic-contaminated water remains a critical task. This chapter discusses the use of nanomaterials for the removal of arsenic species only by adsorption and photocatalytic processes, which are recognized as suitable technologies for this purpose. Adsorption is a preferable method for the elimination of As(V) species, and different types of nanomaterials have been evaluated. While, TiO2 nanoparticles, other metal oxides, carbonaceous materials, and nanocomposites have been described in the photocatalytic degradation of inorganic and organic compounds, including arsenic species. Moreover, photocatalysis has been referred to as an effective method for oxidizing As(III) to As(V). Here, a critical analysis of the most widely investigated nanoparticles that have exhibited suitable arsenic-removal properties for water remediation technologies is highlighted. Metal oxides (as TiO2, ZnO, and iron oxides), nanocomposites, and carbon-based nanomaterials have been extensively used in recent years for photocatalytic degradation and adsorption processes. The synthesis, material functionalization, and physicochemical properties of nanomaterials are discussed deeply as a function of the arsenic removal performance and removal mechanisms of these nanomaterials. This chapter reviews the potential and limitations of these nanomaterial-based treatments conceived for eliminating arsenic species. Keywords

Arsenic species removal · Photocatalytic oxidation · Adsorption process · Metal oxides nanomaterials · Nanocomposites · Carbon-based nanomaterials Abbreviations

AA AC AOP As CA CB CBNMs CNTs DMA EXAFS FT-IR GAC GFH GO HP

Activated alumina Activated carbon Advanced oxidation process Arsenic Cacodylic acid Conduction band Carbon-based nanomaterials Carbon nanotubes Dimethylarsenic acid Extended X-ray Absorption Fine Structure Fourier Transformed Infrared Granular activated carbon Granular ferric hydroxide Graphene oxide Heterogeneous photocatalysis

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HSC IBF LDHs MMA MMOs MOFs NPs PAA PAO p-ASA PLGA PZC RGO ROX SBA-15 TICB TOC UV UVA VB WHO XANES XPS

2683

Hydrothermal spherical carbon Ibuprofen Layered double hydroxides Monomethylarsenic acid Mixed metal oxides Metal-organic frameworks Nanoparticles Phenylarsonic acid Phenylarsine oxide p-Arsanilic acid Poly(lactic-co-glycolic) acid Point of zero charge Reduced graphene oxide Roxarsone Santa Barbara amorphous-15 (mesoporous silica) TiO2 supported onto chitosan bed Total Organic Carbon Ultraviolet Ultraviolet radiation in the range (315–400 nm) Valence band World Health Organization X-ray Absorption Near-edge Structure X-ray Photoelectron Spectrometry

Introduction Arsenic pollution of groundwater is a global concern since elevated concentrations (up to 4,000 μg L
1) affect over 200 million people worldwide. Significant As content in groundwater has been detected in different countries such as Argentina (Cordoba), Chile, Mexico, Peru, USA (Ohio, Western USA), Hungary, Romania, South-West Finland, Bangladesh, India (Calcutta, West Bengal), Nepal, Vietnam (Hanoi), China (Xinjiang, Shanxi, Inner Mongolia), Thailand (Ronpibool, Nakhon Si Thammarat Providence), Japan (Fukuoka), and Cambodia [1]. This type of pollution comes from natural sources such as the dissolution of As-rich minerals, volcanic activity, and anthropogenic origin. The main anthropogenic activities include mining and agriculture (use of herbicides, fungicides, pesticides, and food additives) with the oral route as the primary source of As exposure to humans. Arsenic mainly occurs in aquatic systems as inorganic As species in two oxidation states, i.e., 3 and 5; the very toxic oxyanions of arsenite (As(III)) and arsenate (As(V)) are the predominant As forms that occur in aqueous environments. As(III) is found predominantly as arsenite (H3AsO3), while As(V) appears in water as arsenates (H2AsO4
or HAsO42
). The As(III) species have exhibited higher mobility, approximately 60 times more solubility, and 20 times higher toxicity than the As (V) species [1, 2]. Arsenic also exists as organic forms such as methylated or

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aromatic organoarsenicals in the thermodynamically more stable pentavalent state (As(V)). Methylated organoarsenicals such as monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA) are used as herbicides and pesticides [2]. Aromatic organoarsenicals such as phenylarsonic acid (PAA), p-arsanilic acid (p-ASA), and roxarsone (ROX) are widely used as food additives in the livestock and poultry industry; and phenylarsine oxide (PAO) is used for biochemical applications or water analysis [3]. Chronic arsenic exposure via ingestion causes lung, bladder, and skin cancers. It has been associated with multiple noncancer health diseases, including cardiovascular disorders, neurological effects, and type 2 diabetes. In this sense, the World Health Organization (WHO) has reduced the guideline level recommended for arsenic in drinking water from 50 to 10 μg L
1, and most industrialized countries have also set out 10 μg L
1 as the limit value [4]. Thus, the concentration of inorganic and organic arsenic compounds must be reduced in the aqueous environment. Various technologies have been developed to remove these arsenic species from water, including adsorption, oxidation processes, chemical precipitation, membrane filtration, and ion exchange, among others. Most of the available removal technologies are more efficient for As(V) species given that As(III) is predominantly noncharged at pH below 9. Precipitation, adsorption, and ion exchange are less efficient for As(III) removal. Thus, methods that can simultaneously remove organoarsenicals and inorganic arsenic species are restricted to the combined use of technologies such as heterogeneous photocatalysis (oxidation process) followed by adsorption [1, 2].

Metal-Oxide-Based Nanomaterials for the Adsorption of As Species Adsorption involves the accumulation of substances at the interface of two phases, usually at the liquid-solid interface through a mass transfer process. The adsorbed substance is the adsorbate, and the adsorbing material is named the adsorbent. The mechanisms of the adsorption process can be physical and chemical. Physisorption takes place due to the physical interaction between the solid surface and the adsorbed molecules. This process is driven mainly by van der Waals and electrostatic forces, and hydrogen bonding between the adsorbate molecules and adsorbent surface atoms. On the other hand, chemisorption is due to chemical bonding such as covalent or strong electrostatic bonds between adsorbed molecules and the solid surface during the adsorption process [5]. Adsorption is a promising, simple, and efficient method for removing inorganic and organic arsenic species from water and wastewater. This process has shown high selectivity toward As(V) species. This method does not add undesirable by-products during the treatment, and the adsorbents can be regenerated and reused for several cycles with no significant loss of the adsorption capacity. Usually, the desorption step is carried out by changing the pH and/or the ionic strength of the eluent. Moreover, the adsorption technology can be used to treat water with high hardness,

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and no other chemicals are required. In this sense, nanosorbents have some unique properties such as large specific surface area, small size 100 nm, catalytic potential, high reactivity, easy separation, and a large number of active sites that enhance the arsenic removal efficiency in comparison with other available adsorbents. The application of nanomaterials for water purification requires the synthesis of costeffective materials; however, the main concern related to their application has to do with toxicity and environmental fate. For the preparation of nanosorbents, transition metal-based oxides of Fe and Ti and carbon-based nanomaterials (CBNMs) have been extensively described due to the low toxicity and high effectiveness of these materials [6, 7]. It has been described that metal-oxide-based nanosorbents show adjustable adsorption capacity for both As(V) and As(III) inorganic forms. However, the adsorption properties of nanoparticles for water remediation can be affected by the agglomeration of the nano adsorbents. This problem can be solved by using different methods to synthesize nanoparticles, such as impregnation with surfactants, polymers, polyelectrolytes, and deposition or doping with different metal or metal oxide nanoparticles, among others [7]. The functionalization of metal-oxide-based nanosorbents on CBNMs is also an important strategy to enhance the removal efficiency of arsenic species that will be discussed later in this chapter. Studies on the removal of arsenic species by the adsorption process using metal-oxide-based nanomaterials published in the last 10 years are shown in Table 1.

TiO2 Based Materials for the Adsorption of As Species Promising results on the use of TiO2 nanoparticles as adsorbents have been related to their chemical, thermal, and physical stability, large specific surface area, simple preparation procedure, reusability, and low-cost besides their low toxicity and environmentally friendly features. Literature works indicate that TiO2 based nanomaterials have been extensively used as adsorbents of As species in groundwater and wastewater, showing high affinity for both As inorganic forms (As(III) and As(V)); however, the highest selectivity at neutral pH has been described for the removal of As(V) species [9, 12]. For instance, Lescano et al. evaluated and compared the efficiencies of three different adsorbents for arsenic (As) removal from water: titanium dioxide (TiO2), granular ferric hydroxide (GFH), and activated alumina (AA). In this study, the authors compared the adsorption capacities at pH 7 of TiO2 (15 and 7 mg g
1 for As(V) and As(III)) and of GFH (18 and 13 mg g
1 for As(V) and As(III)). TiO2 and GFH with especific surface area values of 200 and 280 m2 g
1 showed higher adsorption capacities than AA, which adsorbed 8 and 5 mg g
1 of As(V) and As(III), respectively, even when the specific surface area of AA was 320 m2 g
1 [11]. The adsorption of As species in aqueous solutions using TiO2 materials was strongly affected by the effluent pH. Under alkaline conditions (pH ¼ 8), the As(III) adsorption capacity increased (As(III) pKa values ¼ 9.2, 12.1 and 13.4) in comparison with the one described for As(V) species due to its oxyanion characteristics

30

1.7 0.1 0.5

1

1 0.1

0.5

Hydrolysis

Commercial

Commercial

Commercial

Hydrolysis

Hydrolysis

Hydrothermal

Solvothermal

TiO2

TiO2 P25-Degussa

TiO2, GFH, and AA

Nano TiO2 (196 m2 g
1) Anatase bulk TiO2 (9.5 m2 g
1) (SigmaAldrich) TiO2 (196 m2 g
1)

TiO2 (196 m2 g
1)

TiO2 (octahedral nanocrystals with exposed facets {101}, {001}, and {100} TiO2 {201}

[Adsorbent] (g L
1) 0.2

Synthesis method Sol-gel

Material TiO2 Anatase

NaAsO2and Na2HAsO47H2O

As2O3 Na3AsO4.12H2O

Na3AsO412H2O

NaAsO2 and Na2HAsO47H2O

NaAsO2 and Na2HAsO47H2O As2O5

NaAsO2

As(III) speciesa

As compound (NaAsO2, Na2HAs O4.7H2O

Table 1 Ti and Fe metal oxide-based adsorbent nanomaterials for arsenic species removal

As(III) and As(V) (5 mg L
1/ each one) pH 9.0

pH 8.2 [As] 0.04 M NaClO4 [As(V)] ¼ 117 mg L
1 pH 4.0 0.2 mg L
1 As(V) pH 7.0

Conditions pH 7.0 [As (III) + As(V)] ¼ 1 mg L
1 [As] ¼ 3310 mg L
1 pH 7.0 wastewater pH 8.0 C0 ¼ 0.12 mg L
1 100–5000 mg L
1 pH 7.0 40–3100 μg L
1 pH 4.5

50.5 mg g
1 (As(III)) and 29.3 mg g
1 (As(V)

TiO2 {101}, As(III): 2.6 mg g
1 and As(V) 2.1 mg g
1

330 mg g
1

As(III) 93.0 mg g
1 As(V) 35.2 mg g
1

[16]

[15]

[14]

[13]

[12]

[11]

[10]

0.25 mg g
1 15 mg g
1 TiO2 (As V) 7 mg g
1 TiO2 (As III) 6.1 mg g
1 3.1 mg g
1

[9]

Ref [8]

110.3 mg g
1

Adsorption capacity 38.4 mg g
1 As(III) 19.7 mg g
1 As(V)

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0.1

2

Precipitation

Coprecipitation

Mesoporous bimetal oxide magnetic manganese ferrite nanoparticles (MnFe2O4)

Arsenate (As(V)), p-ASA, ROX, and DMA

As(III) and As(V)

Na3AsO412H2O As(III) and As(V)

1 0.4

Yttrium-doped iron oxide

MgO/TiO2/Ag Iron-oxide nanoparticles

As(V) and P(V) (~0.1 30 mg L
1/ pH 5.0) pH 7.0 10 mg L
1 arsenic solution, pH 7.0, platform shaker at 250 rpm, and 25  C for 12 h 20 mg L
1 arsenic solution, pH 7.0, and agitation for 24 h at room temperature 50 mg L
1 arsenic solution, pH 2.1, mechanical shaker at 160 rpm, and 25  C for 24 h

Na2HAsO47H2O and KH2PO4 and

1.0

Precipitation of Fe2O3 in commercial anatase TiO2 Precursor calcination Etching procedure of Fe-Si oxide composites

As(III) and As(V) (~1 mg L
1/ each one) pH 7.0

As(V) (0.1–10 mg L
1) pH 6.9 As(III) and As(V) (0.05– 20 mg L
1/ each one) pH 6.0 in batch reactor at 150 rpm, and 25  C As(III) and As(V) (0.1– 10 mg L
1/ each one)

NaAsO2 and Na2HAsO47H2O

0.32

NaAsO2 and Na2HAsO47H2O

6.25

Hydrothermal and solgel, followed by reduction in H2

NaAsO2and Na2HAsO47H2O

10

Magnetic Fe3O4@TiO2 sandwich-like sheets TiO2–Fe2O3 bicomposite

Na2HAsO47H2O

40

Cross-linking chitin with P25-Degussa Cross-linking of chitosan with nanoTiO2 anatase (25 nm) and Cu(NO3)2 Cross-linking of chitosan with nanoTiO2 anatase (25 nm)

Chitin-TiO2 hybrid material Nano titanium dioxide-enabled copper cross-linked chitosan TiO2-impregnated chitosan beads

[25]

[24]

[22] [23]

[21]

[20]

[19]

[18]

[17]

Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . . (continued)

As(V) ¼ 90.7 mg g
1 42 mg g
1 at pH 7 for As (III) and 83 mg g
1 at pH 3 for As(V) 84.2 mg g
1 for As(III) and 170.48 mg g
1 for As(V) Adsorption capacities of 68.3 mg g
1 for As(V), 59.5 mg g
1 for p-ASA, 51.5 mg g
1 for ROX, and 35.8 mg g
1 for DMA

12.3 mg g
1 (As(III)) (in presence of UV light and 12.3 mg g
1 (As(V) (in dark) As(III) ¼ 2.1 mg g
1 (pH ¼ 9.2) As(V) ¼ 2.0 mg g
1 (pH ¼ 7.7) 31.0 mg g
1 (As(III)) (in presence of UV light and 36.6 mg g
1 (As(V)) (in dark)) 12.14 mg g
1 (As(V)) and 30.28 mg g
1 (P(V))

3.1 mg g
1

116 2687

0.2

20 (Column)

Two step solvothermal

Precipitation method followed by aging with concrete (sand and Portland cement)

Magnetic Fe3O4@UiO-66 composite

Concrete/maghemite nanocomposites

[Adsorbent] (g L
1) 0.2

1

Synthesis method Precipitation

Chemical coprecipitation

Material Mesoporous silica modified by ironmanganese binary oxide (FeMnOx/SBA-15) Aluminum substituted nickel ferrite (Ni-Al-Fe)

Table 1 (continued)

As(V)

As(V)

As(III) and As(V)

As compound As(III) and As(V)

[29]

11.1 mg g
1

[27]

[28]

114 mg g
1 for As(III) and 103 mg g
1 for As (V)

57.6 mg L
1 for As(III) and 24 mg L
1 for As(V) solution, pH 7.0 in batch experiments at 180 rpm, and room temperature for 24 h 150 mg L
1 arsenic solution, pH 7.0, platform shaker at 250 rpm, and 25  C for 4 h 10 mg L
1 arsenic solution, pH 5.0, mechanical shaker, 30  C, and 60 h

Ref [26]

73.2 mg g
1

Adsorption capacity 61.3 mg g
1 for As(III) and 67.7 mg g
1 As(V)

Conditions 5 mg L
1 arsenic solution, pH 7.0, mechanical shaker at 180 rpm, and room temperature for 24 h

2688 L. Hinojosa-Reyes et al.

a

1

0.4

One-step coprecipitation

Template

Cellulose@Fe2O3 composite

Fe3O4@Zr(OH)4impregnated chitosan Beads

Raw water from copper smelting industry

3

Precipitation followed by immobilization

Hydrous iron oxide immobilized on porous alginate beads

As(III) and As(V)

As(III) and As(V)

As(III) and As(V)

57.6 mg L
1 for As(III) and 24 mg L
1 for As(V) solution, pH 7.0, mechanical rotary shaker at 100 rpm, and 23  C for 24 h 0–25 mg L
1 for As(III) and 0–50 mg L
1 for As (V) solution, pH 7.0, mechanical rotary shaker at 200 rpm, and 25  C for 4 h 0.2–59 mg L
1 As(III) and As(V) solutions, pH 6.8, mechanical rotary shaker at 140 rpm at room temperature for 3 h 35.3 mg g
1 As(III) and 35.7 mg g
1 As(V)

29.4 mg g
1 for As(III) and 36.5 mg g
1 for As (V)

68.4 mg g
1 for As(III) and 42.8 mg g
1 for As (V)

[32]

[31]

[30]

116 Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . . 2689

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L. Hinojosa-Reyes et al.

(As(V) pKa values of 2.2, 6.9, and 11.5) [13] enhancing the adsorption of As(V) under acid conditions [12, 14]. Among the TiO2 based adsorbent materials, the potential of using TiO2 nanoparticles for treating water, particularly for As(V), was demonstrated according to its distribution coefficient value (Kd value 3.3  108), exhibiting superior adsorption capacity with respect to the one allowed using TiO2 bulk particles (Kd value 3.1  103), whereas the presence of multiple metals (Zn, Cd, and Pb) in tap water did not affect the As(V) adsorption [12]. The applicability of TiO2 nanomaterials for the efficient removal of As(III) from highly contaminated wastewater generated at the copper smelting industry (3890 mg L
1 As(III)) was confirmed during successive treatment cycles. The As(III) adsorption behavior was described with the charge distribution multisite surface complexation model. The adsorption removal was not affected by the coexisting metals (Cd, Cu, and Pb) that were also eliminated from the effluent during the wastewater treatment [9]. The dependence of the surface adsorption capacity of pollutants such as As(III) and As(V) on the anatase TiO2 crystal facets has drawn increasing attention due to their unique surface properties [15, 16, 33]. The adsorption behavior of As(III) and As(V) species at neutral pH in three-faceted anatase TiO2 crystals was described by the Langmuir model with the following order of reactivity: {101} > {001} > {100}. The {101} exposed facet of anatase TiO2 is thermodynamically stable with the lowest surface energy (0.44 J m
2) in comparison with {100} (0.53 J m
2) and {001} (0.90 J m
2). These values were in good agreement with the surface structure and under-coordinated Ti and O atoms of TiO2 [15]. Despite the high adsorption capabilities of TiO2, the embedding of TiO2 nanoparticles on porous support materials, including biomaterials such as chitin and chitosan, has been reported to overcome the agglomeration of these nanoparticles, simplifying their regeneration [17–19]. Chitin and chitosan are waste by-products from the shellfish processing industry. Although chitin is less expensive and more stable chemically than its derivative chitosan, few reports have been described on the applicability of purechitin-derived-hydrogel materials as adsorbents. Peralta-Ramos reported the use of chitin hydrogel reinforced with TiO2 nanoparticles as adsorbent of As(V), showing an adsorption capacity of 3.1 mg g
1 at pH 6.9 [17]. On the other hand, chitosan has been described as an adsorbent material for the removal of cationic metals showing low adsorption capacities for As(V) and As(III) species. Thus, its adsorption capacity was improved by using TiO2 supported on a chitosan bed (TICB) as a novel sorbent capable of removing both As(III) and As(V) species with similar adsorption capacities of 2.2 mg As(III) g
1 TICB and 2.1 mg As(V) g
1 TICB, despite its low specific surface area of 0.56 m2 g
1 [19]. The adsorption behavior of TiO2 could be enhanced by preparing nanocomposites with other metal oxides or coatings with other transition metals [20–22]. The synthesis of magnetic sandwich-like Fe3O4@TiO2 sheet binary nanocomposites combining the sol-gel and hydrothermal assisted methods was evaluated on the adsorption of As(III) and As(V) species. The Fe3O4@TiO2 sheet nanocomposite with high specific surface area (89.4 m2 g
1) showed adsorption capacities of As(V) and As(III), assisted by

116

Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . .

2691

UV radiation, of 36.36 and 30.96 mg g
1, respectively, allowing residual concentrations for both species below the recommended limit of 10 μg L
1. Furthermore, the prepared material exhibited excellent stability over a wide pH range (3–9) and reusability for five regeneration cycles, keeping the efficient adsorption capacity. The applicability of magnetic Fe3O4@TiO2 sheets in real water effluents was demonstrated with the low effect of common coexisting ions in arsenic removal processes except for silicate and phosphate ions [20]. The adsorption performance of MgO/TiO2/Ag composites prepared by a simple precursor calcination method to remove As(V) showed an adsorption capacity of 90.66 mg g
1 at pH 7.0, which can be well described by the Freundlich model. Moreover, the As(V) removal was effective over a wide pH range (6–12) [22]. The effectiveness of the TiO2 adsorbent for its application in the remediation of arsenic in water has been associated with the formation of stable bidentate mononuclear or binuclear corner-sharing complexes between the As species and TiO2. Pena et al. studied the interaction between As species and TiO2 through different instrumental techniques. The results by zeta-potential measurements suggested the formation of negatively-charged inner-sphere surface complexes for both arsenic species since the adsorption of As(V) and As(III) species on the TiO2 surface decreased the pHPZC of TiO2 from 5.8 to 5.2; these results were in good agreement with the FT-IR study. On the other hand, the formation of bidentate binuclear surface complexes was evidenced by EXAFS analyses due to an average Ti-As(V) bond distance of 3.30 Å and Ti-As(III) bond distance of 3.35 Å [34]. Jegadeesan et al. determined the mechanisms of arsenic adsorption on anatase TiO2 by XANES and EXAFS spectroscopies and reported that the partial oxidation of As(III) occurred when As(III) was adsorbed onto amorphous TiO2, but no As(III) oxidation happened when As(III) was adsorbed onto commercial crystalline TiO2. Their data also indicated that As(V) formed binuclear bidentate inner-sphere complexes with amorphous TiO2 and commercial crystalline TiO2 at neutral pH [8]. Li et al. reported that the adsorption mechanisms of As(III) and As(V) on Ce-Ti bimetal oxide were mediated by the formation of negatively charged inner-sphere surface complexes as described for TiO2. Additionally, the FT-IR and XPS techniques allowed to confirm that the hydroxyl groups were bound to Ce/Ti on the adsorbent surface and involved in the formation of monodentate and bidentate complexes with As(V) and As(III), respectively [35]. On the other hand, the adsorption mechanism of MMA and DMA species on nanocrystalline TiO2 showed that MMA formed bidentate surface complexes while DMA formed monodentate complexes. The reduction of the TiO2 isoelectric point during the adsorption of MMA and DMA indicated the formation of negatively charged surface complexes. Thus, the adsorption mechanism could be explained by the electrostatic interaction between the TiO2 surface and As(V) species (MMA and DMA) related to the adsorbent surface charge (isoelectric point) and speciation of arsenic species at different pH values [36]. The adsorption process of As species (arsenite, arsenate, MMA, and DMA) onto TiO2 was related to the formation of surface complexes, as indicated in Fig. 1.

2692

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Fig. 1 Schematic illustration of the formation of complexes between various As species and TiO2 surface. (Reprinted from Ref. [36]. Copyright (2012), with permission from Elsevier)

Iron-Oxide-Based Materials for the Adsorption of As Species Iron-oxide nanomaterials are extensively used to remove As species from water and wastewater due to their high affinity toward arsenic species. The high natural abundance of iron and the low cost of the synthesis process to obtain iron oxide nanoparticles have supported the wide application of these materials in addition to their suitable magnetic properties for the separation and reusability of the prepared materials. Generally, the three most commonly used forms of iron oxides are magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3). The bare iron oxide nanoparticles have high chemical reactivity and are easily oxidized in the air (especially magnetite) due to their magnetic behavior and dispersibility. Among the iron oxide forms, hematite (α-Fe2O3) is the most stable and highly resistant to corrosion. To maintain the stability and enhance the adsorption capacity of iron oxide nanoparticles for the removal of arsenic species in water treatment procedures, metal, metal oxides, inorganic compounds such as porous silica, inorganic materials such as concrete, metal-organic frameworks, and polymers have been used to functionalize them [37]. Cheng et al. synthesized mesoporous iron oxide nanoparticles by an etching procedure of Fe-Si oxide composites in 1 M NaOH. The nanoparticles showed a large specific surface area (317 m2 g
1) and NPs (approximately 4 nm). The adsorption of arsenic species was pH-dependent, and pH values from 6 to 8 enhanced the adsorption of As(III) while the As(V) removal occurred at pH values from 2 to 4. The As(III) and As(V) adsorption could be well fitted by the Langmuir model, and the maximal adsorption capacities for As(III) and As(V) were 42 and 83 mg g
1, respectively. The presence of coexisting anions in water such as HCO3
, SO42
, Cl
, F
, and NO3
and the ionic strength did not have any effect on arsenic

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Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . .

2693

adsorption; however, the arsenic-species removal decreased up to ~25–35 and ~60– 70% in the presence of silicate and phosphate anions, respectively. The used adsorbent was regenerated in an aqueous alkali solution during five adsorptiondesorption cycles. The mechanism of arsenic removal on nano-iron oxide may occur through chemisorption via inner-sphere surface complexation, and hydroxyl groups on the iron oxide and arsenic species were involved in the adsorption process [23]. The yttrium-doped iron oxide adsorbent prepared by the precipitation method was designed to achieve superior adsorption affinity toward arsenic and excellent magnetic properties [24]. The nanosized crystals ( p-ASA > ROX > DMA. The adsorption of the arsenic species was through the formation of an inner-sphere complex with As-O-M (M ¼ Fe or Mn) linkages. However, the adsorption of As(V), p-ASA, ROX, and DMA was significantly reduced up to 60–80% by the presence of PO43
and SiO32
anions that could compete with the arsenic species for the adsorption sites [25]. An additional advantage of mixed iron oxides was described by Zhou et al., where the removal of As(III) species took place by both the oxidation and adsorption of As(III) species, representing an advantage for the treatment of As-contaminated drinking water. Thus, the binary FeMnOx-supported mesoporous silica (SBA-15) showed enhanced adsorption capacity and stability for both As(III) and As(V) species, allowing the regeneration of the adsorption material up to four consecutive cycles [26]. The water-stable MOFs of zirconium ions and terephthalic acid (UiO-66) are porous materials widely used for the adsorption of water contaminants and were employed to prepare the magnetic Fe3O4@UiO-66 composite by a two-step solvothermal method (Fig. 3). The core-shell composite exhibited a high specific surface area (124.8 m2 g
1) and abundant micro
/mesopores that enhanced the adsorption capacity of As(V) species at pH 7 (73.2 mg As g
1 adsorbent) in

Fig. 3 Facile fabrication of a magnetic composite based on UiO-66 for efficient removal of As(V) from the water with easy separation after adsorption. (Reprinted from [28]. Copyright (2019), with permission from Elsevier)

116

Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . .

2695

comparison with UiO-66 and pristine Fe3O4 materials [28]. On the other hand, mechanically stable nanocomposites have been evaluated on the removal of As(V) species from water samples. The nanoparticles of magnetite were prepared by the precipitation method, then were aged with a concrete mixture in the wt.%/wt.% ratio (sand (83.1): Portland cement (11.3): magnetite (5.6)) allowing augmented As adsorption capacity and decreasing the arsenic concentration in water from 10 mg L
1 to 10 μg L
1. Moreover, the nanocomposites can be used in continuous flow systems for arsenic removal from hard water [29]. The immobilization of iron oxide in polymers is a strategy used to disperse the NPs avoiding their aggregation and decrease of the effective adsorption area in the adsorption process. Among the polymers, cellulose, alginate, and chitosan have been described for the dispersion of iron oxide and NPs, showing enhanced adsorption capacities of As(III) and As(V) species compared with bare iron oxide NPs [30–32]. However, it has been described that the adsorption efficiency of As species in the polymeric/inorganic hybrid adsorbent could decrease gradually after successive cycles of regeneration due to the use of alkaline solutions that can produce the polymer mass loss. After 16 cycles of reuse, the hydrous iron oxide immobilized porous alginate beads preserved 50% of the absorption efficiencies of As(III) and As(V) [30].

The Use of Photocatalytic Nanomaterials in As Removal Heterogeneous photocatalysis (HP), which is an advanced oxidation process (AOP), is recognized as one of the most successful technologies in water decontamination. During the HP, the semiconductor is irradiated with light energy equal to or larger than its band-gap energy, resulting in the generation of electron-hole pairs (positive hole (h+)) in the valence band (VB) and an electron (e
) in the conduction band (CB) (Eq. 1). The photo-generated electrons could react with adsorbed oxygen to generate O2•
radical ions (photoreduction) (Eq. 2), while the holes with adsorbed water molecules promote the formation of •OH radicals (Eq. 3). HP can transform various toxic and harmful chemical pollutants into innocuous or slightly toxic compounds by oxidation mechanism (inorganic As(III) to As(V) species (Eqs. 5–7), and organic arsenic species (MMA, DMA, and aromatic organoarsenic molecules) to CO2, water and the formation of inorganic As(V) species) (Eq. 8) (Fig. 4) [36, 38]. The photo-generated holes could also interact with electron donors such as organic adsorbed molecules (photo-oxidation) and inorganic As(III) species (Eqs. 4, 6–8) [36]. In Fig. 4, the primary mechanisms of a TiO2 photocatalyst in the oxidation of As species are shown, where h+ and the O2•- and •OH reactive radical species are involved: Photocatalyst ðTiO2 Þ þ hυ ! e
þ hþ

ð1Þ

e
þ O2 ! O2 •

ð2Þ

2696

L. Hinojosa-Reyes et al.

Fig. 4 Schematic illustration of the photocatalytic process of a TiO2 semiconductor for the oxidation of As species. (Reprinted from Ref. [36]. Copyright (2012), with permission from Elsevier)

hþ þ H2 O!∙ OH þ Hþ

ð3Þ

hþ þ AsðIIIÞ ! AsðIVÞ

ð4Þ

OH þ AsðIIIÞ!AsðIVÞ þ HO


ð5Þ

O2 •
þ AsðIIIÞ ! AsðIVÞ þ H2 O

ð6Þ

hþ ðor ∙ OH, O2 ∙
Þ þ AsðIVÞ!AsðVÞ

ð7Þ

hþ ðor ∙ OH, O2 ∙
Þ þ organic As pollutant!CO2 þ H2 O þ AsðVÞ

ð8Þ

∙

TiO2-Based Nanomaterials for Arsenic Removal Through a Photocatalysis Process Titanium dioxide (TiO2) is the most-used semiconductor for UV-induced photocatalysis (band-gap of approximately 3.2 eV) due to its high efficiency. Its major advantages have been described in section “TiO2 Based Materials for the Adsorption of As Species”. The TiO2-based materials have been employed for the photocatalytic oxidation of arsenic species. Table 2 shows some applications of TiO2 based photocatalysts reported in the last few years for the oxidation of arsenic species with their reaction conditions. Although TiO2 metal oxide nanoparticles have been mainly used in photodegradation of organic pollutants, heterogeneous photocatalysis has been additionally described as an interesting alternative and effective method for oxidizing As(III) to As(V). Compared with other oxidation reagents such as permanganate, manganese oxide, ozone, gaseous chlorine, Fenton’s reagent (Fe/H2O2), and hypochlorite, the main advantages of photocatalysis as an oxidation technology for arsenic removal are a) the use of nontoxic photocatalysts, b) the use of soft oxidants, c) the operation under mild conditions, d) no generation of hazardous compounds, and

150 W, mercury lamp 300 W Xe-arc lamp

Xe lamp (500 W)

0.1

0.6

1 0.8

0.3

0.25 1.75

Commercial

Commercial

Commercial

Hydrothermal

Solvent evaporation-induced self-assembly Solvothermal

Sol-gel

Sol-gel

TiO2 P25-Degussa

TiO2 P25-Degussa

TiO2-P25-Degussa

Brookite TiO2-based materials Mesoporous TiO2

Cake-like TiO2 derived from MIL125(Ti) Fe doped TiO2

Ni doped TiO2

p-ASA

Sodium arsenite

Sodium arsenite

DMA

Sodium arsenite

PAO

p-ASA

ROX

PAA

As compound As2O3 or Na2HAsO47H2O

100%, 100 min 80%, 180 min

3.25 mg L
1 pH 5.0 [p-ASA] ¼ 10 mg L
1 pH 5.0

[46]

[45]

[44]

[43]

[42]

(continued)

75%, 150 min

100%, 70 min 100%, 30 min

10 min, 100%

[3]

[41]

100%, 10 min 100%, 20 min

[40]

Ref [39]

100%, 6 min

[As(III)] ¼ 6 mg L
1 pH 3.0

[DMA] ¼ 0.2 mg L
1 pH 7.5

Conditions [As(III)] ¼ 68 mg L
1 pH 9.0 Air flow O2 satured 7.7 mg L
1 PA pH 6.5 O2 satured 0.01 mg L
1 pH 7.5 O2 satured 0.0082 mg L
1 pH 3.5 [PAO] ¼ 0.25 mg L
1 O2 50 mL min
1 pH 6.3 [As(III)] ¼ 30 mg L
1

Percentage removal (%) 100%, 30 min

Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . .

450 nm, 358 W

400–700 nm, 25 W

UVA

350 nm

350 nm

0.1

Synthesis method Commercial

Material TiO2 P25-Degussa Radiation Hg UV-lamp, 400 W

[Catalyst] (g L
1) 2

Table 2 TiO2 based nanomaterials for the photocatalytic removal of As species

116 2697

375–380 nm, 10 W

365 nm, 125 W 365 nm, 125 W Xe lamp, 300 W

2 1 0.2 1 1 8 1

Hydrolysis

Chemical mixture

Hydrothermal

Sol-gel, impregnation Sol-gel, impregnation Hydrothermal

254 nm, 9 W

Xe lamp irradiation

360 nm, 20 W

UV lamp, 125 W

0.3

0.3

300 W Xe lamp with a 420 nm cutoff filter 500 W Xe lamp

0.8

Radiation 100 W, 365 nm

Solvothermal and hydrothermal Chemical precipitation Incipient wet impregnation Impregnation

Synthesis method Template

[Catalyst] (g L
1) 1

Sodium arsenite

As(III)

Sodium arsenite

Arsenic trioxide

Sodium arsenite

Sodium arsenite

Sodium arsenite

Sodium arsenite

As(III)

Sodium arsenite

As compound Sodium arsenite

[As(III)] ¼ 1.5 mg L
1 pH 11.0 [As(III)] ¼ 4 mg L
1 pH 7.0 [As(III)] ¼ 3 mg L
1 pH 9.0 [As(III)] ¼ 20 mg L
1

pH 7.0 [As(III)] ¼ 5 mg L
1 [As(III)] ¼ 1 mg L
1 pH 7.0 [As(III)] ¼ 10 mg L
1

Conditions [As(III)] ¼ 5 mg L
1 pH 7.0 [As(III)] ¼ 2 mg L
1 pH 4.0 [As(III)] ¼ 10 mg L
1 pH 4.0 5 mg L
1

100%, 30 min 85%, 120 min 100%, 400 min 100%, 90 min

92%, 80 min 62%, 120 min 100%, 60 min 100%, 120 min 100%, 500 min 89%, 10 h

Percentage removal (%) 93%, 4 h

[57]

[56]

[55]

[54]

[53]

[52]

[51]

[50]

[49]

[48]

Ref [47]

MoOx, molybdenum oxide; ACF, activated carbon fiber; GAC, granular activated carbon; SBA, Santa Barbara amorphous (mesoporous silica); HYB, hybrid system

g-C3N4 nanosheets/ TiO2 hollow microspheres

GAC/TiO2

TiO2/ACF

MoOx/TiO2 γ-Al2O3 ads SBA-15/c-Fe2O3– TiO2 TiO2-HYB (activated alumina) TiO2/ACF

MoOx/TiO2

Ag2O/TiO2

Material Fe-Ce/TiO2 flower like V2O5/TiO2

Table 2 (continued)

2698 L. Hinojosa-Reyes et al.

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Nanomaterials for Arsenic Remediation with Boosted Adsorption and. . .

2699

e) the possibility of a hybrid process with physical or chemical technologies [58]. The anatase and rutile crystalline phases of TiO2 have demonstrated efficiency for the degradation of organic and inorganic arsenic species. However, the anatase form has shown much higher photocatalytic activity compared to the rutile form. The prepared mixed phases of TiO2 (anatase and rutile) produce a class of photocatalysts with unusually high activity, such as the commercial P25-Degussa (80:20, anatase/ rutile) extensively described for the oxidation of arsenic species. The enhanced activity of the mixed phases relative to pure ones is that the transfer of electrons from the anatase conduction band to a lower energy rutile electron trapping site in mixed-phase TiO2 catalysts serves to reduce the recombination rate of anatase, leading to more efficient e
/hþ separation and higher catalytic reactivity [43]. Historically, Yang et al. reported for the first time the complete oxidation of As(V) to As(III) by using P25-Degussa under UV radiation (30 min), initial concentration of 68 mg L
1, and solution pH of 9. It was found that the redox potential of the As(V)/As(III) couple was lower than the valence band potential; in this sense, the photo-generated holes have enough thermodynamic potential to oxidize As(III) to As(V) [39]. The performance of P25-Degussa in the efficient photocatalytic degradation of aromatic organoarsenic compounds was also demonstrated. For example, Zheng et al. studied the photocatalytic degradation of PAA by using P25-Degussa. They found that pH exerts an effect on the adsorption and photocatalysis processes due to the TiO2 surface charge and the speciation of arsenic compounds as a function of pH. The main observed by-products were phenol, catechol, and hydroquinone. Through the addition of appropriate scavengers, it was elucidated that •OH radicals are responsible for the mineralization of phenylarsonic compounds [40]. Later on, Zheng et al. reported the p-ASA and ROX degradation by using P25-Degussa and found that under acidic and neutral conditions, a strong adsorption process took place. The hydroxyl radical played an essential role in the mineralization of arsenical compounds to arsenate at alkaline pH. The p-ASA did not undergo degradation in the absence of molecular oxygen, indicating that reductive processes also play an important role in the ROX degradation [41]. Miranda et al. reported significant effects by the pH and catalyst loading on the PAO degradation using P25-Degussa under UVA radiation; in the same context, previous researchers found that hydroxyl radicals played an important role in the oxidation of this molecule [3]. On the other hand, the brookite-TiO2 crystalline phase has been much less studied in comparison with those of anatase or rutile, and then, the potential of brookite for photocatalytic applications is poorly known. López-Muñoz et al. reported that the photocatalytic activity was highly dependent on the morphology of the catalyst; in the case of the oxidation of As(III) to As(V) species, the activity of pure brookite was lower than that of pure anatase. Nevertheless, the brookite/anatase samples showed enhanced activity compared with pure brookite; this effect was attributed to a synergistic effect achieved by the coupling of both semiconductors that would allow the interfacial electron transfer, therefore suppressing the e
/h+ recombination and promoting the photocatalytic efficiency [42]. UV light accounts for a small fraction of the solar spectrum (~5%). Thus, there is a need to develop titania-based photocatalysts to be active under the visible light

2700

L. Hinojosa-Reyes et al.

spectrum. An effective method for improving the performance of TiO2 nanomaterials is to increase their optical activity by shifting the response onset from UV to the visible region by doping them with metals and coupling with metal oxides; thus, several works have been focused on the application of metal-doped TiO2 and coupled mixed oxides with TiO2 for the removal of arsenic from aqueous solutions. The doping of transition metals such as Ni and Fe, and the codoping of transition metal ions like Fe and Ce into the nanocrystalline TiO2 to modify the band-gap energy, is a more cost-effective process [45–47]. The red shift in the band-gap absorption has been attributed to the charge transfer transition between the d and f electrons of the dopant and the CB (or VB) of TiO2. In this context, the nickel-doping of TiO2 shifted the band-gap energy from 3.2 to 2.8 eV, enhancing approximately 20% the oxidation percentage of p-ASA under visible light compared to the bare TiO2. The nickel-titanium photocatalyst acted as a mediator of interfacial charge transfer to inhibit the e
/h+ recombination [46]. Garza-Arevalo et al. described the use of iron-doped TiO2 for the oxidation of As(III) under visible radiation and the enhanced removal capacity of As(V) from an aqueous solution compared with the bare TiO2 and P25-Degussa. The potential application of iron-doped TiO2 in the treatment of As-contaminated groundwater was also demonstrated, showing chemical stability and reusability capacity. The incorporation of iron ions into the TiO2 lattice extended the absorption to the visible light region and created surface oxygen vacancies, which favored the photocatalytic oxidation reaction of As(III) using a small doping amount of Fe (1.0 wt.%) in TiO2 powder [45]. Visible-light-induced photoreactivity of TiO2 was also improved by coupling it with other metal oxides such as Ag2O, V2O5, and MoOx [48–50]. Ren et al. synthesized the 30% Ag2O–TiO2 composite, which was active under visible light. The composite exhibited much higher photochemical reactivity in the oxidation of As(III) compared with bare TiO2 or Ag2O pure oxides. The removal percentage of As(III) was 83% after visible light exposure for 120 min under optimal experimental conditions (pH 4.0, catalyst loading of 0.3 g L
1, and initial As(III) concentration of 10 mg L
1). The improved photocatalytic activity was attributed to the formation of a heterojunction between Ag2O and TiO2, the strong visible light absorption, and the high separation efficiency of photo-generated e
/h+ pairs resulting from the Schottky barriers at the Ag-Ag2O interface [49]. To restrict the e
/h+ recombination phenomenon of TiO2 and increase its photocatalytic efficiency under visible radiation for the efficient removal of As(III) species, the narrow band-gap semiconductor V2O5 was mixed with TiO2. The V2O5/TiO2 nanocomposites were prepared by the solvothermal and hydrothermal methods, showing both core-shell spherical and solid-sphere nanostructures. The V2O5/TiO2 material with core-shell sphere morphology exhibited enhanced visible photocatalytic activity in the oxidation of As(III) (approximately 92%) and improved charge separation efficiency compared with solid sphere nanostructures because of its larger specific surface area (see Fig. 5). The oxidation of As(III) in the V2O5/TiO2 photocatalytic system was mediated by the superoxide radicals and photo-generated holes [48].

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Fig. 5 Proposed mechanisms for the photocatalytic oxidation of As(III) over the V2O5/TiO2 composite under visible light (VB ¼ valence band, CV ¼ conduction band, h+ ¼ hole, and e
¼ electron). (Reprinted from Ref. [48]. Copyright (2016), with permission from Elsevier)

Vaiano et al. reported the synthesis of a new catalyst based on the mixed oxide MoOx (molybdenum oxide) with TiO2 for its application in the photocatalytic oxidation of As to arsenate under UV irradiation. The molybdenum oxide as an amorphous phase was anchored on the surface of TiO2. The increased efficiency of MoOx/TiO2 (0.3 g L
1) in the oxidation of As(III) compared to TiO2 could be ascribed to the superoxide generation induced by the surface molybdenum oxide that works as an electron shuttle between the conduction band of TiO2 and O2 present in the reaction medium. Complete oxidation of As(III) to As(V) was reached in 1 h of photocatalytic reaction. Moreover, during the photocatalytic treatment, the generated As(V) was released into the solution, avoiding the deactivation of the catalyst and enhancing its stability for three cycles compared to the traditional photocatalysts [50]. To allow the complete removal of the released As(V), this research group proposed a combined process by the photocatalytic oxidation of As(III) with a MoOx/TiO2 catalyst followed by the adsorption of As(V) using a γ-Al2O3 adsorbent as an efficient alternative for the removal of arsenic from drinking water compared with processes based on typical oxidants [51]. On the other hand, another trend is to enhance the specific surface area of the TiO2 based catalyst materials to reach the complete removal of As species, which has been done by evaluating composites of adsorbent materials with TiO2 and TiO2 mixed oxides. For example, Yu et al. reported the use of magnetic mesoporous silica (SBA15)/γ -Fe2O3 prepared by an internal hydrolysis method as a carrier to incorporate TiO2 nanoparticles uniformly. The SBA-15/γ -Fe2O3-TiO2 composite with a specific surface area of 431 m2 g
1 showed a synergistic effect allowing the photocatalytic oxidation of As(III) to As(V) and the adsorption of As(V). This composite also presented excellent desorption and stability properties up to five reuse cycles. The enhanced photocatalytic oxidation performance of As(III) under UV irradiation was ascribed to the high specific surface area, and high dispersion of γ-Fe2O3, and TiO2 on SBA-15 [52].

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In the last few years, MOFs have received considerable attention due to their high specific surface area and large adsorption capacity of pollutants, including As species [59]. Liu et al. reported for the first time the use of MIL-125(Ti) as a sacrificial template to prepare TiO2 at temperatures between 300 and 600  C in air atmosphere. These TiO2 based materials possessed a uniform cake-like cylindrical structure with a smooth surface. The material annealed at 380  C presented the highest photocatalytic performance in the oxidation of As(III) species under visible light in conjunction with adsorption capacity that can be attributed to the synergistic effect of a strong photo-generated e
/h+ separation and high specific surface area (115 m2 g
1) [44]. Carbonaceous materials are also considered as novel adsorbent materials due to their extensive use in adsorption processes and the available abundance of carbon on Earth. Combining TiO2 with carbonaceous nanomaterials is being increasingly evaluated for the removal of arsenic species [54–57]. In this context, Yao et al. synthesized a granular activated carbon (GAC)/TiO2 composite and evaluated its photocatalytic performance in the oxidation of As(III). The complete oxidation of the As(III) solution at pH 9 under UV radiation was carried out in 240 min, and the removal of the generated As(V) was reached in 400 min. The efficient performance of this material was due to the high specific surface area of the composite material (384 m2 g
1) [56]. In many regions where the arsenic pollution of water represents a severe problem, the low-cost technologies based on the use of solar light can be adapted by using photoactive semiconductors. Therefore, to effectively remove arsenic, a two-step process has to be designed: 1) As(III) oxidation and 2) elimination of As(V) [60]. Thus, the use of simulated solar radiation under experimental laboratory conditions represents a promising approach of the photocatalyst in the field of environmental remediation by using solar radiation. For instance, Dong et al. reported the photocatalytic degradation of DMA by using mesoporous TiO2 at pH 7 and simulated sunlight radiation, reaching 100% of oxidation of DMA in 30 min of reaction. The main advantage of using mesoporous TiO2 is that the high specific surface area enables in situ removal of As(V) species previously oxidized. This mesoporous TiO2 was synthesized by the solvent evaporation-induced self-assembly method, which provides oxygen vacancy modulation, which, in turn, enhances the oxidation of As(III) species, see Fig. 6 [43].

Metal Oxide Nanomaterials for the Photocatalytic Removal of Arsenic Compounds Present in Wastewater TiO2 has been recognized as the most effective photocatalyst for the degradation of many organic and inorganic pollutants in aqueous media, including arsenic compounds [38]. It is well-known that this semiconductor material needs to be activated by UV light ( 420 nm) as irradiation source, solution pH 3.0, [As(III)] ¼ 100 μM, 0.5 g L
1catalyst load

Arsenite

[67]

[68]

[69]

95%, 30 min

100%, 28 min

[66]

100%, 36 min

Non reported

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generated As(V) remains in the aqueous solution after 300 min of reaction. On the contrary, this by-product was highly adsorbed on the sol-gel ZnO surface, achieving its complete elimination from water. Structural defects produced on the zinc oxide during the synthesis by the sol-gel method could act as active sites, improving the performance of sol-gel ZnO during the photocatalytic process [61]. PAO, CA (cacodylic acid), and ROX compounds have also been degraded by photocatalysis using the ZnO semiconductor [3]. Thus, the behavior of commercial ZnO was studied through the removal of PAO and CA and also compared with the performance of P25-Degussa. The organic species were oxidized and transformed into As(V) by both catalysts. However, ZnO exhibited better activity than TiO2 when irradiated under UV radiation for 1 h. Under the optimal conditions (Table 3), the byproducts detected during the PAO oxidation were PAA, phenol, and inorganic arsenate. While methanol, and inorganic arsenate were found as the main degradation products of CA. Acuña et al. demonstrated that ROX degradation was accomplished using ZnO prepared by the wet chemical method. During the degradation process, the powerful •OH radicals attacked the organic molecule and generated the nitrophenol radical, which in turn was decomposed into arsenic acid. Therefore, the ROX concentration was reduced by 70% in 180 min, mainly generating As(V) and very low content of As(III) [62]. Metal doping and coupling ZnO with other metal oxides are some strategies to reach the ZnO activation by irradiating with visible light. The coupling of two metal oxides increases the separation of charge carriers during the photocatalytic process, and the optical absorption of the material could be extended to the visible-light region [71, 72]. In this sense, the coupled oxide TiO2-ZnO was evaluated in the degradation of As2O3 (synthetic solution at 0.1 mg L
1 of initial concentration) using as irradiation source UV and natural solar light [64]. Nanocomposites were prepared by the coprecipitation method with different Ti/Zn precursor ratios (90:10, 50:50, and 10:90). The efficiency in the As(III) degradation depended on the type of radiation and ratio of the prepared mixed oxide. Thus, the highest removal of As(III) (90% in 120 min) was attained with the sample 90:10 under UV light, and with the sample 50:50 under sunlight. The photocatalytic activity of the catalysts was also tested in industrial wastewater that contained 0.1 mg L
1 of arsenic. The rate conversion efficiency of As(III) to As(V) under solar irradiation decreased allowing 70%. This behavior was attributed to the presence of salts in the water matrix (phosphate and lead compounds) which affected the arsenic adsorption on the catalyst surface. Zinc oxide has been also modified by coupling with CuO [63] or doping with Cu [65] for the oxidation of arsenic species. In both cases, the photocatalytic activity in the removal of arsenic pollutants was higher than the one displayed by the unmodified ZnO. Nanopowders of CuO and ZnO were mechanically mixed at different weight ratios (5, 10, 20, and 50 wt.% of CuO) to prepare CuO/ZnO coupled materials [63]. Experimental results indicated that the CuO(20%)/ZnO sample exhibited the highest efficiency (95%) in the degradation of an As(III) solution at 30 mg L
1 of initial concentration. The performance of this binary composite was compared with bare ZnO and CuO when applied as catalysts under UV light. CuO

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showed reduced activity removing only 20% of As(III), while ZnO reached about 55% of arsenic oxidation. The improved activity of the CuO(20%)/ZnO photocatalyst was attributed to an effective separation of photo-generated e
and h+ to form O2•
and •OH radicals, respectively for the oxidation of As(III) to As(V). Recently, doping ZnO with Cu ions has been investigated in the photocatalytic degradation of arsenic as arsenite (AsO2
) [65]. ZnO doped with 1.08 mol.% of Cu (1.08Cu-ZnO) exhibited higher activity, achieving complete removal of As(III) in 30 min compared to unmodified ZnO when irradiated with visible light (400–600 nm). The removal tests were conducted in distilled and drinking water to evaluate the effect of the matrix on the efficiency of the As(III) oxidation to As(V). The results indicated that the efficiency of the 1.08Cu-ZnO catalyst was preserved in drinking water. The incorporation of copper into the ZnO structure decreased the band-gap value to 2.91 eV with respect to bare ZnO (3.18 eV) [65]. It has been reported in the literature that impurities in ZnO commonly cause a narrower bandgap, making the catalyst more active under visible light [73], and in some cases, depending on the dopant ion, it could act as an electron trapper, avoiding the e
/h+ recombination [74]. Other ZnO-based composites have been prepared from layered double hydroxides (LDHs), where ZnFe-LDH precursors have been used for preparing mixed metal oxides (MMOs) by calcination at 200  C, 300  C, 400  C, 500  C, and 600  C and varying the Zn/Fe molar ratio [75]. The results of XRD patterns indicated that ZnO and ZnFe2O4 crystalline phases were formed after the thermal treatment at 300  C. The sample obtained with Zn2+/Fe3+ molar ratio of four and calcined at 300  C (ZnFe-MMOs-4-300) retained a high specific surface area (82.6 m2 g
1) and was selected for simultaneous removal of arsenite and ibuprofen (IBF) in a mixture solution. Assays with the ZnFe-MMOs-4-300 catalyst were first conducted in water samples containing only As(III) at an initial concentration of 1000 mg L
1 at pH 6 and catalyst load of 0.2 g L
1. The adsorption and photocatalytic oxidation allowed that As(III) species were transformed into As(V) in 30 min under simulated solar radiation; afterward, the As(V) amount decayed due to the adsorption onto ZnFeMMOs-4-300 by forming surface complexes. When simultaneous degradation of IBF and As(III) was performed under similar conditions to those of individual arsenic degradation, the presence of IBF did not affect the removal of As(III). Fig. 7 depicts the simultaneous removal of IBF and arsenite by the ZnFe-MMOs4-300 composite under solar radiation. WO3 is an attractive metal oxide for photocatalytic applications because it possesses suitable band-gap energy for the absorption of visible light. However, the unfavorable position of the conduction band potential leads to the rapid recombination of e
/h+ pairs, decreasing the photocatalytic activity [74]. Kim et al. reported a study using Pt/WO3 for degrading arsenite (As(III)) in aqueous solution under visible light ( 45% of the tested concentration 2.3  10 4 mg/mL

TiO2 TMAOHa 0.48–1.01 7.5 EC50 > 45% of the tested concentration 0.016 mg/mL

Tetramethylammonium hydroxide Hexamethyl tetramine

b

a

Solvent Concentration range (lg/mL) Mean size (nm) Bioluminescence test (Microtox) Daphnia magna (standard test) Germination test (several seeds tested) Anaerobic consortium (biogas production) Aerobic consortium (oxygen uptake rate) Nitrification consortium (oxygen uptake rate)

Nanoparticle CeO2 HMTb 0.02–0.57 6.5 0.021 mg/ mL 0.012 mg/ mL 0%

No effect

No effect

18%

100–120%

Au Trisodium citrate Oct–62 10 EC50 > 45% of the tested concentration No data

33% inhibition at 0.13 mg Ag-NP/mL No effect

No effect

75–95%

Ag Sodium borohydrate 16–100 29 EC50 > 45% of the tested concentration No data

Table 2 Acute toxicological data on inorganic nanoparticles (NP). Concentration of NPs was the maximum reached in the laboratory without agglomeration. Since NP concentration was different in each toxicity test, the file concentration range corresponds to the range of concentrations used in the different tests. Bioluminescence, Daphnia magna, aerobic and nitrification consortia test results are expressed as EC50 values. The Germination test values are expressed as Germination Index. Anaerobic toxicity test is expressed as percentage of biogas reduction. (Reproduced from [64]. Copyright (2011) Elsevier)

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been demonstrated the dependence of the cluster size with the inhibition of DNA polymerase and the increase of cytotoxicity. An alternative to minimize damage could be the use of primary organic solvents insoluble in water so that once the remediation is done, the fullerenes can be easily removed from the aqueous medium [63]. In general, it is difficult to predict the response of the nanoparticles in the environment; this will depend on the physicochemical behavior of the surroundings. According to its properties, the nanoparticles could agglomerate into microparticles, become embedded in surfaces, being dissolved or suffer morphological alterations, among others unexpected behaviors. This situation has led researchers to develop toxicity test for inorganic nanoparticles, among which can be mentioned bioluminescence test, daphnia magna and other aquatic organisms (Danio rerio and Oncorhynchus mykiss), germination, earthworms, and common wastewater-based tests [64]. In Table 2 are presented some results of these interesting toxicity tests. Over the years, different studies have been carried out on human exposure to nanoparticles. Humans are exposed to nanoparticles in several ways, for example, trough inhalation, ingestion of water and food and dermal contact, result of the application of general personal care products. There are multiple health implications related to exposure of nanoparticles, Fig. 12 lists some of these. By its nature, cellulose nanofibers are a promising alternative for environmental remediation since these are highly biodegradable and have no toxicity [61, 65–68].

Fig. 12 Human health implications associated with exposure of variety of nanomaterials. (Reproduced from [46] . Copyright (2016) Elsevier)

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It is worth mentioning that there are research works on the use of plants to absorb nanoparticles that were previously used for remediation; this process is called phytoremediation [69–73]. In general, the materials used in nanobioremediation are chosen to achieve the compatibly with the environment.

Conclusions and Further Outlook Nanoremediation and nanobioremediation are two areas that are in constant innovation. The objective of this chapter was to present advantage and disadvantage associated to both areas as well as the most recent advances. The decision to choose one over another implies the analysis of multiples aspects, such as the pollutant to be treated, concentration, physical state, microorganisms present, among others. There still a lot to be studied in the remediation processes. The search for a material that is easy to synthesize, inexpensive, and that effectively removes contaminants without harming microorganisms in the aqueous medium, still prevails. However, the most important thing is to achieve remediation without side effects.

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Devarajan Thangadurai, Mohima Chakrabarty, and Jeyabalan Sangeetha

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Engineered Nanomaterials for Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Remediation of Wastewater and Groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Remediation of Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanophytoremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sustainable and Ecosafe Nanoremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Environmental remediation is the process of removal of pollutants or contaminants from various ecosystems using various in situ and ex situ technologies. Conventional methods are time consuming and target specific and may hinder by the by-products of degradation process. Hence, it leads the path of search to find a suitable alternative with high efficiency. Nanotechnology possesses high-potential applications in various fields. Environmental nanotechnology is gaining deep attention among researchers to protect the earth from pollutants and to enhance existing treatment techniques to combat the global pollution. It could be a better replacement capacity for on-site and off-site remediation. Enhanced properties like nanoscale size, less time consumption, highly flexible for in situ and ex situ D. Thangadurai (*) Department of Botany, Karnatak University, Dharwad, Karnataka, India e-mail: [email protected] M. Chakrabarty Department of Biotechnology, University School of Biotechnology, Guru Gobind Singh Indraprastha University, Delhi, India J. Sangeetha Department of Environmental Science, Central University of Kerala, Periye, Kasaragod, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_72

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techniques, high level of surface area-to-volume ratio for potential reactivity, and resistance to environmental factors make nanoparticles ideal for environmental applications. There are different nanomaterials and nanotools available to treat the contaminants. All these nanotechniques and nanotools are based on the properties of contaminants and the site of contamination. This chapter provides an insight to nanoremediation and its application in remediation of different polluted environments. Keywords

Nanoremediation · Pollutants · Contaminants · Degradation · Nanoparticles · Nanotools

Introduction Environmental contamination – a major crisis in the current world – is the introduction of large amounts of unwanted and hazardous materials in the air, soil, and water at levels which render them too toxic to be used by other organisms. The sources of pollution are varying like industries, agricultural sectors, sewages, and household wastes. Industrial wastes are composed of complex mixtures of organic and inorganic substances like chemicals, heavy metals, toxic gases, pesticides, and so on. These pollutants then enter into the various food chains through soil and water. A large number of pollutants are potent carcinogens resisting breakdown and accumulating in the food chain. Pollution by heavy metals is a global threat which poses various health and environmental hazards and needs to be taken care of. Heavy metals normally lead to acute and chronic toxicity of the liver and kidney damage which may lead to cancers of the lung, kidney, and liver [1, 2]. Environmental remediation is the process of removal of harmful contaminants from air, water, soil, and sediments by biological, chemical, and physical methods for cleaning up the environment with focus toward sustainability. There are various types of remediation technologies that have been developed recently such as thermal treatment, dumps and landfills, and biological waste treatment. These techniques can either be ex situ or in situ. In situ physical treatment of organically contaminated soil takes place in three categories like soil vapor extraction in the vadose zone, air sparging, and soil flushing. Soil vapor extraction takes place by applying vacuum pressure to the contaminated unsaturated subsurface area to induce the controlled flow of air through the contamination zone and removing the volatile organic compounds. In air sparging systems, the injected air flows through the water column past soil particles transferring the volatile compounds to the air bubbles which then into the vadose zone where they can be captured by soil vapor extraction system. Soil flushing is another technique in which water with added surfactants is injected into or applied to the contaminated zone. Ex situ physical treatment includes volatilization technologies and washing technologies. In volatilization technologies, soil contaminants are physically removed by heating them to temperatures sufficient

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to vaporize them and then collecting them as a concentrate for further treatment or disposal. This can be achieved by aeration, vacuum, steam, or heating. Water technologies include water washing and solvent washing. In water washing, excavated soil is washed with water to remove the organic contaminants, and in solvent extraction, the soil is first mixed with solvent and then separated into two phases. This is followed by the recovery of solvent for its reuse. In situ chemical treatment, a chemical oxidant especially hydrogen peroxide, is injected into a contaminated subsurface zone in order to oxidize the organics. It includes the lasagna process, dechlorination, and in situ solidification. Ex situ chemical treatments include chemical reduction and chemical oxidation techniques [3]. Another eco-friendly method used for remediation which exploits the capability of microorganisms to breakdown organic compounds is biological remediation, resulting in the formation of carbon dioxide and water or methane. Inorganic matter cannot be biodegraded but can be broken down into small compounds which are less toxic. Biodegradation often involves the attack on a particular molecular site by a microorganism. Advantages of this technology include low land requirement, low capital and operating costs, and good process control [4]. Though there are numerous advantages of using physical, biological, and chemical treatments for remediation, there are several disadvantages as well such as: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Requirement of large amounts of chemicals Resistance of some complex contaminants to oxidation Inability to reach low permeability soil and groundwater Reversion of target contaminants Inability to treat contaminated zones to utmost levels Less flexibility in design and operations of biological systems Longer process time required Difficult to extrapolate from laboratory scale Difficulty to degrade some compounds completely without the formation of recalcitrants

In the past few years, a lot of studies have been done on nanoremediation and its future prospects in environmental remediation. Chemical treatment itself may lead to contamination, and microbial remediation is target and external factors specific. However, nanoparticles provide a cost-effective and time-efficient solution for detection and decontamination of harmful pollutants [5] (Fig. 1).

Engineered Nanomaterials for Environmental Remediation Nanotechnology is the science of understanding and controlling of matter at dimensions of 1–100 nm. This field of science which has numerous applications in science, technology, and pharmaceuticals is now being extensively used for soil and groundwater remediation. The effectiveness and efficiency of nanoparticles make them almost ideal for environmental remediation also keeping in mind their high surface

Fig. 1 Challenges of environmental remediation and the applications of nano-sized materials. (Copyright © The Royal Society of Chemistry 2018, adapted with permission from Das et al. [6])

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area, associated high reactivity, and less toxicity toward microbial fauna. Some of the features of these materials which make them suitable for environment remediation are: (i) They have high surface-to-mass ratio which can be used in reactions involving solid-water or solid-gas interfaces and may lead to high reactivity; decreasing the size from mm to nm also increases there reactivity. (ii) Changes in size and structure of nanoparticles lead to materials with significantly new physical, chemical, and biological properties. (iii) They can arrange or self-assemble into highly ordered layers with special properties. (iv) Small size also enables them to be transported easily into the subsurface via injection or direct push in slurry form to the contaminated sites [5]. (v) It also provides a cost-effective way for environmental cleanup [5]. However, there are some disadvantages of nanoparticles which include its poor transport properties due to Brownian motion, density of particles, ionic strength, and long-range magnetic attractive forces increasing the aggregation of these nanoparticles leading to filtration on the subsurface and difficulty to distribute in the contaminated zone during remediation because of particle-particle (agglomeration) and particle-collector (deposition) interaction subsequently leading to the loss of reactivity, limiting their penetration in porous media, thereby decreasing their environmental mobility (Fig. 2). Nanomaterials are of diverse types such as inorganic, carbon-based, and polymerbased nanomaterials [7]. Inorganic nanomaterials can be further subdivided into metal or metal oxide-based nanomaterials which have high potential for efficient removal of toxic metals. Magnetic metal oxide-based nanoparticles also can therefore be used for remediation [8]. Some of the widely used synthetic techniques for the production of stable, shape-controlled, mono-disperse metal, and metal oxide nanoparticles are thermal decomposition or reduction and co-precipitation [7]. Silver and titanium oxides are examples of two most commonly used metal oxide nanoparticles. Silver oxide nanoparticles show antibacterial (both Gram positive and Gram negative), antifungal, and antiviral activities depending upon their size. Silver oxide nanoparticles are reported for its antibacterial activity against both Grampositive and Gram-negative bacteria, antifungal activity, and antiviral activity depending upon their size. Smaller-sized nanoparticles are more capable of binding to the cell wall of pathogens thus hampering their functions such as permeability [9] (Fig. 3). Titanium dioxide (TiO2) nanoparticles are useful compounds with diverse functions such as breaking down almost any organic compound when exposed to sunlight (photocatalyst), removing nitrogen oxide from the air, and converting them into harmless chemicals that could be washed down by rain and formation of self-cleaning fabrics. Just like silver oxide, titanium dioxide nanoparticles can also be for restraining viruses and bacteria. Titanium dioxide destroys the membrane of cells, stops protein formation, and hence restrains viral activation [10]. In addition to

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Fig. 2 Approaches in environmental remediation using nanoparticles and nanomaterials. (Adapted from Guerra et al. [7])

these, binary mixed TiO2-SiO2 materials have also been created and have shown to remove variety of pollutants [11]. Other examples of inorganic nanoparticles are bimetallic nanoparticles, iron-based nanoparticles, and titanate nanotubes. Carbonbased nanoparticles pertain to carbon materials family consisting of graphene, activated carbon, carbon nanotubes, and fullerene. They possess extraordinary electrical conductivity, heat conductivity, high surface area, low toxicity, and ecofriendly. These nanotubes can be of two types such as single-walled nanotubes and multi-walled nanotubes. Carbon being one of the most abundant elements on Earth is biocompatible and can form various nanoparticles. Graphene can remove fluoride from aqueous solutions, whereas modified graphene can adsorb various gaseous and water contaminants such as methane, hydrogen sulfide, heavy metals, etc., and even in the field of biosensing, carbon-based nanotubes have been used [12]. Activated carbon, zeolites, and clay have also been used to treat organic dye from wastewaters. Even though carbon nanotubes have significant advantages, their use on the industrial scale for practical applications is still not feasible because of high production costs [8]. TiO2-graphene, ZnO-graphene, and CdS-graphene nanocomposites have also been used for remediation of benzene and water contaminants [7]. Polymer-based NPs range from 10 to 500 nm in size which are solid, colloidal, and bioactive materials. These particles absorb or adhere on the surface of the particles such as chitosan, polyglutamic acid, and polylactic acid [13]. These

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Fig. 3 Removal of organic and inorganic pollutants using bionanomaterials through (a) adsorption, (b) transformation, and (c) photocatalysis. (Copyright © The Royal Society of Chemistry 2018, adapted with permission from Das et al. [6])

nanoparticles have high biological safety and biodegradability. Several polymers have been shown to remove contaminants such as chemicals, gases, and various biologicals. Their capability to do so is normally attributed with their high mechanical strength, thermal stability, durability, and recyclability of the material [7]. Polymeric nanomaterials can be divided into three types such as natural polymerbased nanomaterials, biosynthesized polymer materials, and chemically synthesized polymer materials. Natural polymers are renewable and biodegradable and have various sources. Most commonly used natural polymer materials are chitosan, alginate, starch, cellulose, and chondroitin sulfate. Chitosan is non-toxic and biocompatible polymer which can bind to protein or DNA and protect them from degradation. It is also suitable to be used as an adjuvant or delivery carrier. Starch being readily available, its non-toxicity, low immunogenicity, and good storage stability makes it widely used in the pharmaceutical industry. Alginate is also very useful in the medical field due to its good moisture absorption, easy removal, high oxygen permeability, gelatin

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obstructive, biodegradability, and biocompatibility. Biosynthesized polymers are made by enzyme hydrolysis and contain microbial polyesters and microbial polysaccharides. Poly-β-hydroxybutyrate is a polymer produced by microorganisms under unfavorable conditions. This polymer cannot just be used for drug delivery but can also be used in tissue engineering as a scaffold material and in surgical treatment as a bone repair material. Chemically synthesized polymers include polyester, polyvinylpyrrolidone (PVP), polyurethane (PU), silicone rubber, and polyvinyl alcohol. Polyurethane has high elasticity, high strength, wear resistance, excellent fatigue resistance, and good compatibility which make it suitable to be and polymeric drug capsules. Lignin, tannin, bark-derived PU, cellulose derivatives of PU, and starch derivative of PU are the currently available synthetic biodegradable polymers. Polymer-based nanoparticles have wide array of applications in the biomedical and pharmaceutical field such as vaccine adjuvant, drug delivery system, and an antibacterial agent. Nanomaterials controlled release properties also account for their biodegradability, temperature sensitivity, and pH [13]. Table 1 shows the different nanomaterials used in the degradation of environmental contaminants. Nanoscale zeolites, metal oxides, carbon nanotubes, and fibers are some nanomaterials that have been used for environmental cleanup [14]. Nanoscale zero-valent iron (nZVI) is used for the removal of arsenic (III) and arsenic (V) as a colloidal reactive barrier material which is followed by the rapid separation of zero-valent iron NPs and immobilization of chromium (VI) and lead (II) from aqueous solution, reducing the chromium to chromium (III) and lead to lead (0) while oxidizing the iron to goethite [15].

Remediation of Wastewater and Groundwater A major problem which accompanies industrialization and modernization is the scarcity of water. An aim of the upcoming technologies is to minimize the use of water and the correct disposal of harmful effluents which leads to water pollution. Domestic wastewater may be good for agricultural use due to its high nutrient content but is not fit for drinking and other purposes and hence needs to be treated. Quality of the effluent can be dramatically improved by using nanotechnology. Four classes of nanomaterials that can be used for treating wastewater are dendrimers, metal-containing nanoparticles, zeolites, and carbonaceous nanomaterials. For the treatment of water in situ, a commonly used nanomaterial is nZVI. nZVI particles are nanomaterials made from Fe(II) and Fe(III) using borohydride as the reductant whose size varies between 10 and 100 nm and exhibits a characteristic core shell structure. nZVI just like various polymeric nanomaterials has large surface area, dual property of adsorption and reduction, and more number of reactive sites than microsized particles. Also, these particles can be modified and they produce less amount of harmful waste during the treatment process. Another nanomaterial with exceptional adsorption properties that can be used for treating wastewater is carbon nanotube which can reduce the cost, time, and labor involved in the removal of organics, biological impurities, arsenic compound, and volatile organic compounds

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Table 1 Nanoparticles and their applications in remediation Nanomaterials Amphiphilic polyurethane nanoparticles Nanoscale zero-valent iron Nanocellulose Polymer nanocomposites Carbon nanotubes Dendrimer NP composite Titanium dioxide Bimetallic nanoparticles Magnetic nanoparticles Aerogels Graphene-based nanomaterials Plasmonic-based nanomaterials

Application Soil remediation

Pollutant Phenanthrene

Ref [15, 16]

Groundwater treatment, wastewater Treatment of water Remediation of soil and water Water treatment

Heavy metals, hydrocarbons, oil

[17–20]

Heavy metals, dyes Hydrocarbons, heavy metals

[21, 22] [23, 24]

Ethylbenzene, copper, nickel ions, cationic dye, oil Organic pollutants, metal ions

[20, 25–28] [15, 29]

Organic pollutants

[15]

Polybrominated diphenyl ethers, chlorine Heavy metals

[30, 31]

Water and wastewater treatment Water and soil disinfectant Water and soil treatment Water and soil treatment Wastewater and air treatment Water and air treatment Wastewater treatment

Oil, bromate, heavy metals, volatile organic carbon, CO2 Cationic compounds Organic pollutants, pathogenic microorganisms

[32] [20, 33, 34] [35] [36]

from wastewater [37]. Graphene oxide is another carbon nanotube which interacts strongly with chlorobenzene in the presence of chlorine groups [38]. Other nanoparticles which may also be used are pure and doped forms of titanium dioxide nanoparticles, nanosilver, gold nanoparticles, and zinc oxide (ZnO) nanoparticles which have been considered excellent photocatalysts for a wide range of organic pollutants [39]. It was also observed that dehalogenation and photodegradation processes are found to be suitable for a variety of water treatment applications. Some of the issues associated with nanoremediation for water is that (i) lot of energy is required, (ii) safe and environment friendly disposal of the organic matter after treatment, and (iii) reuse of wastewater with nanotechnology is still not feasible in industrial-scale applications [38].

Remediation of Soils Soil pollution is another serious problem arising due to the increase in usage of pesticides and herbicides which leads to soil contamination. A promising strategy to minimize the entry of contaminants in plants can be achieved by using nanoparticles

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Fig. 4 Remediation of soil contaminants using nanoparticles. (Copyright © Elsevier 2020, adapted with permission from Bakshi and Abhilash [40])

such as nZVI, ZnO, TiO2, carbon nanotubes, fullerenes, and BNPs. These nanoparticles help in remediation by increasing the conversion of soil heavy metals (like Cr (VI)) to less toxic forms and degradation of organic contaminants (e.g., DDT, chlorinated organic solvents, carbamates, and so on) [21]. Magnetic nanoparticles are also used to remove salts and metals and assist in the decomposition of organic pollutants by incorporating them into nanoscale resins or beads. This adsorption increases their specificity, absorption, and stability. It is also possible to use dendrimers for specific degradation of pollutants to provide clean water [32] (Fig. 4).

Nanophytoremediation Phytoremediation is the use of plants and plant-associated microorganisms to treat and control wastes in water, soil, and air. It uses green plants to metabolize and degrade the contaminants of soil, sludge, sediments, and water. The mechanism used by plants for phytoremediation includes phytoextraction, phytodegradation, phytostabilization, phytovolatilization, and rhizodegradation. Phytoextraction is a process by which plants take up contaminants into the plant shoots followed by their translocation to the shoots and leaves. Phytodegradation is the breakdown of organic contaminants by plant enzymes or enzyme cofactor, and phytostabilization is

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another strategy that could be used to minimize the migration of contaminants in soils. It basically aims in reducing the interaction between the contaminants and the associated biota. Phytovolatilization is another process involved in phytoremediation by which plants transform contaminant into volatile form, thereby removing it from soil. This has been observed to be a promising technology in the removal of mercury and selenium followed by their conversion to volatile forms and release into the atmosphere. Rhizodegradation is a biological process for the treatment of pollutants through enhanced microbial activity in the root soil rhizosphere of certain vascular plants. It involves the conversion/reduction of metal ions by rhizospheric organisms [41, 42]. Nanophytoremediation is an advanced technique in environmental remediation which uses both nanotechnology and phytoremediation for environmental cleanup. Some of the properties that affect nanophytoremediation are the physical and chemical properties of the contaminants, environmental factors, and plant characteristics. Nanoparticles such as nZVI, TiO2, BNPs, and magnetite nanoparticles have shown rapid degradation of several organic pollutants [43–46]. TiO2 nanoparticles also degrade pollutants such as diuron and phenanthrene [47, 48]. Uptake of nanoparticles by plant species depends mainly on the size of the nanoparticle since that is the major factor which decides if it is capable of moving from the roots to the other parts of the plant, but it also depends upon the type and chemical composition of the nanoparticle. Some factors that have to be kept in mind while choosing an ideal plant for a suitable nanoparticle are fast growth of plant, large biomass production, tolerance toward pollutants/contaminants that accumulate well-developed root systems for increased root surface area, easy to harvest, easy to genetically manipulate, readily available, nontoxic, and able to induce phytohormones and phytoenzymes [49] (Fig. 5). Nanophytoremediation can be brought about by using naturally occurring or genetically modified plants which fasten up the removal of contaminants in lesser time than usual. This interaction between the nanoparticle and the plant causes many physiological and morphological changes, and the plant’s response depends upon the nanoparticle type, dose, and speciation. In addition to their role in nanoremediation, nanoparticles such as silver nanoparticles have shown to induce abscisic acid and gibberellin production which helps the plant to tolerate stresses as well as increase the uptake of nutrients and water for improved growth. Even though nanophytoremediation makes up an excellent technique for the removal of contaminants in an eco-friendly and less time-consuming aspects, it has certain disadvantages like it is only suitable for sites which are moderately contaminated soils. It also depends mainly on the meteorological conditions and has the tendency to aggregate thereby reducing their mobility [42] (Fig. 6). Pradhan et al. [50] had used manganese nanoparticles on Vigna radiata (mung bean) and observed that MnNP-treated plants did not trigger oxidative stress. It was also seen that manganese released from the nanoparticles after 24 h of experiment along with MnNP was carried through to the leaf of the plant samples which interacted with chloroplast and augmented photosynthesis by higher oxygen splitting and generating a large number of electrons. MnNPs were also found to be

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Fig. 5 Factor influencing the phytoremediation of contaminants using nanoparticles and nanomaterials. (Copyright © Springer Nature 2018, adapted and modified with permission from Srivastav et al. [42])

biosafe toward beneficial soil microorganism such as Trichoderma viride, and also the microbial system did not exhibit growth retardation when treated with different concentrations of MnNP.

Sustainable and Ecosafe Nanoremediation The use of engineered nanomaterials for environmental cleanup has provided a quick and efficient method, but the human risk and economic investment associated with them is not well studied. Sustainable and eco-friendly nanoremediation is especially important during soil and water remediation since these nanomaterials may further be broken down into harmful substances, thereby accumulating in plants or in soil

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Fig. 6 Enhancing phytoremediation of soil pollutants using nanomaterials. (Copyright © Elsevier 2020, adapted with permission from Bakshi and Abhilash [40])

reducing their fertility. Biosafety of these materials is also important since many nanoparticles are used in agriculture, treatment of wastewater, drug delivery, and vaccine adjuvants. Polysaccharide/polymer-based nanomaterials are ideal for making eco-friendly nanoparticles as they combine good chemical reactivity and high biodegradability with negligible toxicity. Cellulose from sugarcane bagasse, fruit peel biomass, and rice husks is one such nanopolymer which is abundantly present, renewable, and low cost and can also be used for water remediation. Another feature that makes cellulose an attractive source for the design of advanced materials is its hierarchical structure. Cellulose fiber composites are composed of macrofibers of cellulose, hemicelluloses, and lignin. These macrofibers are further cleaved to produce cellulose nanofibers which have applications in wastewater treatment [51]. Similar to cellulose, more ecofriendly nanomaterials should be produced which support sustainability.

Conclusion Anthropogenic activities which are similar to mining, smelting operations, and agricultural use of metals or metal-containing compounds are major contributors of environmental contamination. Different physical, chemical, and biological methods have been employed for in the removal of contaminants from water, soil, and sediments. Among these, biosorption is considered as an innovative technology to remove the contaminants since it is cost effective and eco-friendly. Microbial systems characterized by high surface to volume ratios are considered to be better for

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bioremediation. Moreover, the microbial membranes harbor potentially active chemosorption sites and multiple functionally and structurally different proteins that help in the bioremediation process. But the effectiveness of microbial bioremediation depends largely on various biotic and abiotic factors. Microbial enzymes like reductases and oxygenases also influence the process of bioremediation. Phytoremediation, a process involving environmental cleanup by plants, has also emerged as an alternative technology for the management of toxic chemicals. Cell immobilization is another well-known technique, which increases the performance of metal uptake from the contaminated environment. From the past few years, the importance of nanomaterials has been studied in various fields. It offers notable advantages in some remediation applications, but these benefits are mainly site specific. Since the complete knowledge about nanoparticles is still not known, their use for remediation should be followed after further safety and appropriate researches. Further, studies are required in dealing with problems such as the biotoxicity of nanomaterials, effects on the environment, the investigation of required local environmental conditions, fate, transport and transformation of nanoparticles/nanomaterials, and study of the antagonistic or synergistic effects of nanoparticles and microbial activities in soils. Therefore, based on the above discussion, it can be concluded that nanoparticles have immense applications not just in the biomedical field as drug delivery systems and vaccine adjuvants but also in the field of remediation and environment cleanup. Due to its diverse applications, it is expected that their synthesis and applications will increase in the near future and play a critical role in sustainable development.

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Xiandi Zhang, Chui-Shan Tsang, and Lawrence Yoon Suk Lee

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals in Photocatalytic CO2 Reduction Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Photocatalytic CO2 Reduction Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges in CO2 Photoreduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design of a Photocatalytic System for CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composition of Photocatalytic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detection of Photocatalytic CO2 Reduction Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Semiconductor-Based Photocatalyst in CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Based Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonmetal-Based Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for Catalysts Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal Structure Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elemental Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cocatalyst Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanostructure Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hybrid Nanostructure Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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X. Zhang · C.-S. Tsang Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China L. Y. S. Lee (*) Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, SAR, China e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_103

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Abstract

The excessive use of fossil fuels during rapid industrialization of world has inevitably led to the continuous increase in the atmospheric CO2 level, which became one of the major concerns worldwide. Using solar energy to convert CO2 into useful fuels is a promising approach to address both energy shortage and environmental crisis, and this has recently emerged as one of the most important research areas. However, the stable nature of CO2 molecule under ambient conditions and the complicated reaction mechanism involved in its reduction reaction pose great challenges in both aspects of thermodynamics and kinetics for realizing the practical transformation of CO2. Nanostructured materials with large surface areas and unique optical and electronic properties are one of core elements in the development of photocatalytic CO2 conversion platform. In this chapter, a comprehensive understanding on photocatalytic CO2 reduction, including the reaction mechanism, challenges in both thermodynamic and kinetic aspect, the design principles of a photocatalytic CO2 reduction system, and the detection methods for various reduction products will be discussed. In addition, recent research developments in high-performance photocatalytic CO2 conversion systems based on nanomaterials design and their hybrids will be summarized with an outlook on the research trend.

Introduction Despite the ongoing research efforts on alternative fuels such as hydrogen, biofuels, and alcohol fuels, the fossil fuels are still the major source for global energy supply. The demand for energy has shown a steep increase over the past decade due to the rapid industrialization in developing countries. The carbon dioxide (CO2) emission from excessive use of fossil fuels consequently has shifted the atmospheric carbon balance, reaching a historic record atmospheric CO2 concentration. In 2017, the global temperature has increased by approximately 1
C compared to the preindustrial level due to human-related activities, and is expected to further rise by 1.5
C in 2030–2052 [1]. The bitter aftereffects of global warming, including the rise of sea level, ice melting, more acidic ocean, and more frequent and severe weather, are now threatening the sustainability of the Earth. Not to mention the depletion of fossil fuels in a foreseeable future, the search for clean, renewable, and sustainable energy source is one of the most important quests in our times. Among various measurements conducted so far to deplete the atmospheric CO2 concentration, the photocatalytic CO2 reduction to value-added chemicals is considered as a promising solution [2]. The successful implementation of solar-driven CO2 reduction reaction (CRR) would realize a sustainable cycle of consumption–regeneration of carbon-based fuels with no CO2 emission with the minimum change in the current infrastructures for energy generation plants based on fossil fuels. Photocatalytic CRR is a process mimicking the natural photosynthesis in which CO2 is transformed into

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various organic products with the aid of sunlight and catalysts. The reduction of CO2 involves the change in oxidation state of carbon from the highest +4 oxidation state to a lower oxidation state, thus affording different products that vary from gaseous product, for example, carbon monoxide (CO) and methane (CH4), to liquid products including alcohol, aldehyde, and carboxylic acids. As CO2 is an extremely stable molecule, high energy input is required in order to break the C¼O bond. To facilitate the conversion efficiency of CRR, it is necessary to incorporate a catalyst that can lower the activation energy for C¼O bond dissociation in a photocatalytic system. Nanostructured semiconductors have gained immense attention as a photocatalyst for CRR in recent years [2–4]. Compared with bulk counterparts, nanostructured semiconductors provide much larger surface areas, and thus much higher population of catalytically active sites. Besides, the diffusion path of photogenerated carriers from the interior to the surface in a nanostructured semiconductor is relatively shorter than its bulk material. The charge separation thus can be enhanced, together with the suppression of charge recombination, endowing greatly enhanced catalytic efficiencies [5]. Until now, numerous semiconductor-based materials have been explored as a photocatalyst for CRR [2, 5]; however, a catalyst with excellent conversion efficiency, turnover frequency, and product selectivity still remains to be found. The intrinsic properties of semiconductor need to be first considered, for example, band structure, charge separation and transportation, surface interaction with CO2 and reaction intermediates. The unclear reaction mechanism of CRR is another barrier that poses the difficulty in catalyst optimization. Until now, several possible pathways have been proposed for photocatalytic CO2 reduction. The first part of this chapter presents an overview of the reaction mechanism over semiconductor with the details on current challenges in photocatalytic CRR. In the second part, the intrinsic and extrinsic factors that greatly affect the CRR performance of semiconductor, as well as the qualitative and quantitative detection methods for reduction products will be discussed. Finally, the strategies for catalyst design to tackle the aforementioned challenges will be reviewed with the recent examples in brief. In the view of extensive literatures focused on the design and fabrication of CRR photocatalysts, the readers are referred to specific reviews, books, or original papers for more information [2, 6].

Fundamentals in Photocatalytic CO2 Reduction Reaction Mechanism of Photocatalytic CO2 Reduction Reaction The mechanism of photocatalytic CRR over semiconductor is illustrated in Fig. 1. The process involves three steps. Firstly, the electrons in the valence band (VB) of semiconductor are photoexcited to the conduction band (CB) after light absorption, forming the electron (e) and hole (h+) pairs. Some of these e and h+ would migrate to the surface of the semiconductor, while the others recombine and lose their energy before triggering the reaction. Finally, the e that migrated to the surface

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Fig. 1 Schematic illustration of the photocatalytic CO2 reduction mechanism over a semiconductor

reduces CO2 into various organic compounds via a series of redox reactions, while the h+ is used to oxidize the water molecule or another hole acceptor. All redox reactions in the process of CRR occur on the surface of semiconductor. Usually, H2O or a protic solvent is commonly used as a reductant in CRR. Using aqueous system as an example, H2O can react with h+ to produce hydrogen radical (H•) and molecular hydrogen (H2), as given in Eqs. (1), (2), and (3); H2 O þ hþ ! Hþ þ OH

ð1Þ

þ • e CB þ H ! H

ð2Þ

H • þ H • ! H2

ð3Þ

Meanwhile, e on the surface of semiconductor can undergo different pathway to generate various products, including carbon monoxide (CO), methane (CH4), methanol (CH3OH), and formic acid (HCOOH), and several mechanisms have been proposed for their formation. In 1995, Apno et al. [7] first proposed the mechanism for the production of CO, CH4, and CH3OH based on the use of TiO2 in photocatalytic CRR, where CO2 is initially converted to CO2• radical and further reduced to CO and C•. The C• radical finally combines with H•, and forms CH4 and CH3OH. Later, Subrahmanyam et al. proposed another mechanism for the production of formaldehyde (HCHO), HCOOH, CH3OH, CH4, and ethylene (C2H6) [8]. In their proposal, instead of forming the intermediate CO2•, CO2 is first combined with H• to form HCOOH, which could be further reduced to other product via step-by-step reactions. Shkrob et al. proposed another pathway which is known as glyoxal pathway [9]. Same as Apno’s proposal, the reaction starts from the formation of CO2• intermediate. It is, however, followed by the conversion of CO2• into glyoxal (OCHCHO), and further dissociation of OCHCHO to aldehyde. Until now, the transformation mechanism of CO2 is still controversial. Some studies show no trace of aldehyde produced in photocatalytic CRR, deviating from the mechanism proposed by Subrahmanyam and Shkrob [10]. Nonetheless, the possible redox reactions in CRR and its corresponding redox potentials obtained from the thermodynamic data are widely accepted. The equations are summarized in Table 1.

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Table 1 Summary of main products of CO2 reduction and their corresponding reduction potentials Product Hydrogen Methane Carbon monoxide Methanol Formic acid Ethane Ethanol Oxalate

Reaction 2H2O + 2e ! H2 + 2OH CO2 + 8H+ + 8e ! CH4 + 2H2O CO2 + 2H+ + 2e ! CO + H2O CO2 + 6H+ + 6e ! CH3OH + H2O CO2 + 2H+ + 2e ! HCOOH 2CO2 + 14H+ + 14e ! C2H6 + 4H2O 2CO2 + 14H+ + 14e ! C2H5OH + 4H2O 2CO2 + 2H+ + 2e ! H2C2O4

E
(V vs. NHE, pH ¼ 7) 0.41 0.24 0.51 0.39 0.58 0.27 eV 0.33 eV 0.87 eV

Challenges in CO2 Photoreduction Photocatalytic CRR has been intensively studied for more than a decade. The reported conversion efficiency of CO2 is, however, still low [11], and the apparent quantum efficiency is below 10%, which hinder practical applications. One of the major limitations in photocatalytic CRR is the intrinsic properties of semiconductor, which determine the light harvesting properties, rate of charge separation and transportation, and surface interaction with reactants, thus greatly affecting the conversion efficiency and product selectivity in photocatalytic CRR. CO2 is a stable and chemically inert molecule in linear geometry. The bond energy of C¼O is 806 kJ∙mol1 which is much higher than that of CC (336 kJ∙mol1) and CH (411 kJ∙mol1). Therefore, a considerable amount of Gibbs free energy (ΔG) is needed in order to break the C¼O bond and bend the linear structure, thereby making CRR a grand thermodynamic challenge. In the photocatalytic CRR, the source of energy is solar light. The light harvest efficiency of a semiconductor is determined by its band gap energy (Eg), which is determined by the potential difference between VB and CB. The Eg dictates the energy or wavelength of light that a semiconductor can absorb from the solar spectrum. The ultraviolet (UV), visible (Vis), and infrared (IR) lights take up 9%, 51%, and 43% of the solar spectrum, respectively, therefore those that can absorb visible light are more advantageous for the better utilization of solar light. For visible light absorption, relatively narrow (1.75–3.0 eV) band gap semiconductors are required. Figure 2 shows the band structure of some semiconductors and the standard redox potentials for CO2 transformation to various organic products. To proceed the transformation of CO2 to a target product, the band position of semiconductor is critical to fulfill the thermodynamic requirement. The CB edge must lie at a more negative potential while the VB edge at a more positive potential than the standard redox potential of a particular redox reaction. Notice that the redox potential for the formation of CO2•, the key intermediate at the initial step of CRR, is very negative (1.9 V vs. NHE) and it also lies above the standard redox potential of H2O/H2. This implies not only a high overpotential is needed for the transformation of CO2, but also the competing reaction of H2O reduction to H2 during CRR, which has the lower activation energy, has to be dealt with. Water reduction is a 2e transfer process, whereas the reduction

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Fig. 2 Band positions of some common semiconductor photocatalysts [11]. (Reproduced by permission of The Royal Society of Chemistry)

of CO2 involves multiple electron transfer. These thermodynamic barriers impose the difficulty on the choice of semiconductor as a photocatalyst in solar-driven CRR, and the limitation on the conversion efficiency of CO2 to carbonaceous products. The rate of charge separation and transportation in a semiconductor ascribes the quantum efficiency of a photocatalytic system, and thus the kinetic of CRR. The quantum efficiency of semiconductor in photocatalysis is defined by the percentage of photogenerated e–h+ pairs produced from the absorbed photons, and has a direct relationship with the rate of charge separation. The low charge separation rate leads to a low quantum efficiency, and thus lowers the kinetic of CRR due to insufficient amount of photogenerated e. The rate of charge transportation is another important factor that influences the kinetic of CRR. For CRR to proceed, the photogenerated e needs to migrate to the redox species at the surface of semiconductor. Otherwise, the photogenerated e would recombine with the h+, which is known to be a fast process in the absence of electric field, and go back to the ground state via a nonradiative relaxation, thus lowering the CO2 reduction rate. Therefore, it is important to have a shorter migration path and quick charge transportation rate for the photogenerated e. It is widely believed that CRR occurs on the catalyst surface where CO2 in solvent media can be adsorbed. The surface adsorption energy of species formed during the CRR, therefore, largely influences the efficiency of photocatalytic CRR, which is governed by the chemical constituents, surface microstructure, electron band structure, and crystal phase of semiconductor. However, due to the lack of understanding on the mechanism, the optimization of semiconductor photocatalysts for CRR is difficult. In addition to those intrinsic features of semiconductor, the

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solubility of CO2 in solvent is also a crucial factor. Although H2O is a better choice for green catalysis, the adsorption of CO2 molecule on semiconductor is not easy due to the low solubility of CO2 in H2O (0.033 mol L1 at 25
C under 1 atm). Also, the difference in polarities between CO2 and H2O (H2O: 1.85 D; CO2: 0 D) leads to easier adsorption of H2O on the semiconductor surface than CO2.

Design of a Photocatalytic System for CO2 Reduction Composition of Photocatalytic System A photocatalytic system for CRR contains three main components: photocatalyst, solvent media, and reductant. The catalytic reaction is conducted in solvent media, either at the interface of gas–solid phase or liquid phase. In a liquid-phase system, catalyst is dispersed in a solvent that contains dissolved CO2. As mentioned previously, due to the limited solubility of CO2 in water, additives such as NaOH, NaHCO3, or Na2CO3 will be added to water in order to improve the adsorption of CO2 molecules on semiconductor surface. Another option is to use a mixture of H2O and organic solvent as the reaction media. Organic solvents such as acetonitrile (ACN), ethyl acetate (EtOAc), and N,N-dimethylformamide (DMF) are usually employed in CRR owing to their better CO2 solubility [12]. Despite of the poor solubility with CO2, H2O plays an important role as the proton source in CRR. Example reported by Lin et al. showed that the addition of 30 vol% H2O in the reaction media led to a significant increase in the conversion of CO2 to CO [12]. However, further increase of H2O beyond 60 vol% resulted in a marginal decrease in the generation of CO with the increase in the production of H2. In a gas–solid phase system, photocatalytic CRR is performed under the presence of humidified CO2, which makes it more complicated than the liquid-phase system. The advantage of gas–solid phase system is that it requires much less H2O, thus suppressing the competing H2O reduction reaction. Photocatalyst has to be immobilized at the gas–solid interface. Although this may decrease the accessible area for both photons and reactants, this approach significantly facilitates the separation of products, reactants, and photocatalytic materials. Another drawback of aqueous suspensions is the low solubility of CO2, which can be overcome in gaseous mixtures of CO2 and H2O. The ratio of H2O to CO2, which can be easily tuned in the gas phases, was found to play an important role in determining the reaction rate. Xie et al. who investigated the difference between gas–solid and liquid phase CRR based on TiO2 showed that the product selectivity in gas–solid phase reaction was much better than that of the liquid phase reaction [13]. The reductant in a photocatalytic CRR system acts as a sacrificial agent to supply electrons for quick scavenging of the photogenerated holes, thus suppressing the recombination of charge carriers. Triethanolamine (TEOA), triethylamine (TEA), and CH3OH are the examples of commonly used sacrificial agents in photocatalytic CRR. In addition to the electron donating ability, the oxidation product of the sacrificial agent has to be considered. The sacrificial agent that produces O2 should

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not be used to avoid the subsequent gas separation step. One thing to bear in mind is that the use of sacrificial agent in CRR may affect the pathway of product generation, or even contribute to the yield of product. The study reported by Schneider et al. showed that the oxidation of CH3OH, which is used as the sacrificial agent in the catalysis, also transferred the electron to the CB of the photocatalyst, and made a contribution to the stoichiometric generation of product [14].

Detection of Photocatalytic CO2 Reduction Products Various gaseous and liquid products can be afforded from the photocatalytic CRR, either as a sole product or a mixture of products. Therefore, it is necessary to use different detection methods for accurate product identification, both qualitatively and quantitatively [11]. In a gas sample taken after photocatalytic CRR, gaseous products including CO, CH4, ethane (CH3CH3), and ethylene (CH2¼CH2) can be afforded, along with H2, the side product, and residual CO2 due to the low conversion efficiency. Unless the gas sample contains only CO and CO2 that can be identified with Fourier-transform Infrared spectroscopy (FTIR) or diffuse reflectance infrared Fourier-transform spectroscopy (DRIFT), the most commonly used technique for the identification of multicomponent gas product is gas chromatography (GC). Common practice for qualitative measurement of each component in the sample is to use a GC coupled with mass spectrometer (GC-MS). For quantitative measurements, the GC coupled with either thermal conductivity detector (TCD) or flame ionization detector (FID) is usually employed. TCD is a universal detector for most organic and inorganic compounds including permanent gases (e.g., H2, N2, and CO), whereas FID is mostly used to detect the compounds containing C–H bonds. Either Ar or N2 is employed as the reference gas in TCD because He would interfere the detection of produced H2 amount, one of the important parameters to evaluate for the performance of CRR catalyst, which will significantly reduce the sensitivity of the detector toward other compounds. In this case, FID is more recommended for a better sensitivity in the detection of CO and hydrocarbons of low concentration. For this, a methanizer is required to convert CO into CH4 for quantification CO that is missing C–H bond. Many types of column can be used for the analysis, including molecular sieves, carbon columns, Poraplot Q, and Al2O3. Nevertheless, the high concentration of CO2 in the product gas will lead to the deactivation of Ni catalyst in methanizer and shorten the lifetime of column. Therefore, it is necessary to first separate CO2 from the product gas before the analysis. Figure 3 shows an example of a customized GC instrument for the analysis of CRR products. For liquid product, different techniques are applied according to their functional groups. The analysis of alcohol products such as CH3OH, ethanol, 1-propanol, and 2-propanol is conducted by GC-FID with a commonly used column including HP-5, Porapak Q, DB-WAX, and PEG [15]. In order to protect the column, sample treatment, for example, the centrifugation or filtration through PTFE filter to remove any solid particles, is required before sample injection. Moreover, extra attention should be paid to the matrix effect due to sacrificial agent and organic solvent in the

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Fig. 3 The flow chart of GC for production analysis from CO2 reduction [15]. V1–V3: gas switch valves; CL1–CL3: separation columns. (Reproduced by permission of The Royal Society of Chemistry)

CRR as it may affect the accuracy of the analysis. Hong et al. revealed the effect of sacrificial agent and organic solvents including ACN, DMF, TEOA, and TEA on the analysis of alcohol products [15]. In addition to the effect on the peak position and peak area changes, extra peaks other than the target compounds also appeared due to the interaction of the matrix with the stationary phase of the column, thus interfering the proper interaction between the stationary phase and the target compounds. Several methods have been engaged for the analysis of aldehyde/ketone products such as formaldehyde, acetaldehyde, and acetone. One of them is colorimetric method. The sample is first mixed with either Nash reagent or chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) and analyzed by UV–Vis absorption spectroscopy with a specific wavelength monitored. This method offers relatively simple procedure with good sensitivity; however, it is applicable to only formaldehyde. Another method is GC-TCD/FID which is well suited for any aldehydes/ ketone. However, the method is useful when the concentration of aldehyde/ketone is high. The difficulties in this method are related to the high reactivity of these compounds which leads to either decomposition or chemical modification during sample handling and chromatographic separation, resulting in a weak response. In addition, FID has a lower sensitivity toward low molecular weight O- and N-containing compounds compared with hydrocarbons. Especially, the detection limit of GC toward aldehyde/ketone is about two orders of magnitude higher than that of alcohol. Considering the smaller amount of aldehydes/ketones than other organic products usually produced in photocatalytic CRR, GC may not be a good

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choice for their analysis. In contrast, more accurate quantitative measurement can be done by high-performance liquid chromatography (HPLC) after derivatization with 2,4-dinitrophenylhydrazine (DNPH) [15]. The reaction of aldehydes/ketones with the aromatic hydrazine group of DNPH in acidic media affords the formation of the corresponding hydrazones. These hydrazones are then separated by reversed-phase HPLC, and detected by diode array detector (DAD). Although the sample treatment takes relatively longer than the other two methods, HPLC delivers a much better sensitivity with very low detection limit, and is applicable in a wide range of aldehyde/ketone products. Carboxylic acids, including formic acid (HCOOH) and acetic acid (CH3COOH), are also one of the product types that can be obtained from CRR. The quantification can be done by either 1H NMR spectroscopy with an internal standard [16] or UV– Vis absorption spectroscopy with calibration curve prepared, presuming only one type of carboxylic acid in the sample solution. On the other hand, ion chromatography (IC) or HPLC is more suitable for the quantitative analysis of multiple carboxylic acids present in sample. IC has a good selectivity and sensitivity for the analysis of low molecular weight carboxylic acids in aqueous or water-miscible matrices. An anion-exchange column, Dionex ion exchange column that has alkanol quaternary ammonium function group on the stationary phase, is commonly used for the separation under the gradient elution of the mobile phase. Meanwhile, for HPLC, the reversed-phase HPLC method using a Hi-Plex H column is employed. It is an ion-exchange ligand-exchange column comprised of strong cation-exchange resin with sulfonated, cross-linked styrene-divinylbenzene copolymer in hydrogen form. The separated acid products can be detected with either DAD, various wavelength detector (VWD), or refractive index (RI) detector. Interference on the chromatogram caused by the sacrificial agent and organic solvents in CRR has been reported by Hong et al. where the peak areas of formic acid and acetic acid are greatly affected by the presence of DMF, TEA, and TEOA [15]. The interference may be related to the interaction of amine groups of DMF and TEA with the stationary phase material, affecting the retention of carboxylic acids. In the case of TEOA, the baseline of chromatogram needs be corrected because of severe tailing of TEOA peak which causes the uplift of baseline.

Semiconductor-Based Photocatalyst in CO2 Reduction The use of nanostructured semiconductors as photocatalyst in light-driven CRR has been of great interest since the pioneering work of Fujishima and Honda et al. [17]. Until now, various types of semiconductors have been explored for the potential application in photocatalytic CRR. These semiconductors can be classified into two main types: metal-based and nonmetal-based semiconductors. Metal-based semiconductors have been dominating in the field of photocatalytic CRR for more than a decade; however, numerous drawbacks including high cost, poor long-term stability, easy poisoning by environmental parameters such as oxygen, water, and pH, and limited earth abundance have hindered their practical usage in industry.

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Nonmetal-based semiconductors have started to gain a considerable attention in recent years as a possible solution for the sustainability of catalyst. This class of semiconductors is mostly earth-abundant, environmental friendly, and cheap. With the electronic conductivity comparable to metal-based semiconductor, they also demonstrated prominent catalytic activity, product selectivity, and durability in CRR. In the following section, both types of semiconductors will be discussed with the selected examples.

Metal-Based Semiconductors Metal Oxides Metal oxides are the most extensively studied type of semiconductor in photocatalytic CRR owing to their good photostability. Among them, titanium dioxide (TiO2) has been the most distinguished and widely studied one since its first report by Fujishima and Honda et al. [17], thanks to its availability, high resistance to photocorrosion, and nontoxic nature. However, the lower flat-band potential in the CB compared with the standard redox potential of CO2 to CO2•, and the wide bandgap of TiO2, exceeding 3 eV, allows only UV light absorption, limiting its application in photocatalytic CRR. Nevertheless, the potential of TiO2 as a CRR photocatalyst has triggered the exploration of other metal oxides. The research interest has been initially focused on the materials with a suitable band gap and more reductive CB. It includes materials that contain octahedrally coordinated transition metal ions in d0 configuration which are Zr4+, Nb5+, Ta5+, and W6+. These metals can exist as either binary oxide (e.g., ZrO2 and Nb2O5) or oxysalt including titanate, niobates, tantalates in perovoskite AMO3 structure (e.g., SrTiO2, NaNbO3, and NaTbO3). Also, materials contain main group metal ions in d10 configuration with a general formula of MyOz or AxMyOz where M is Ga, Ge In, Sn, Sb, or Zn, while A is electropositive cation such as alkali, alkaline earth, or rare earth metal ions. Recently, the interest has started to shift to the materials with a narrower band gap that can absorb visible light, which include Cu2O, BiVO4, InTaO4, Bi2WO6, and ZnGa2O4 [18–22]. Metal Chalcogenides Metal chalcogenides (sulfides, telluride, and selenide) have recently started to gain a great attention as the potential candidates for photocatalytic CRR [2]. In comparison with metal oxides, metal chalcogenides possess excellent properties such as tunable stoichiometric compositions, stable crystalline structures, rich metal sites, as well as higher electrical conductivity, which makes them a more interesting material. In addition, they usually have narrower band gaps and more reductive CB that theoretically can lead to a better conversion efficiency and richer product variety than their oxide counterparts. Examples of metal chalcogenide in photocatalytic CRR are dominated by sulfide materials, for example, simple sulfides including ZnS, Bi2S3, CuS, Cu2S, and CdS, and more complex sulfides such as CdIn2S4 and ZnIn2S4 [2]. The selenides and

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tellurides such as CdSe, CoTe, and ZnTe are also reported to have photocatalytic activities toward CRR [2, 23]. The major drawback of metal chalcogenides is the photocorrosion that causes the loss of anions. For examples, the illumination of sulfides in an aqueous dispersion would lead to the oxidation of S2 ion in the crystal lattice into elemental sulfur, and finally to sulfates (SO42). Therefore, reducing agents such as sulfite (SO32), thiosulfate (S2O32), hypophosphite anions (H2PO2) and tertiary amines such as TEA or alcohols will be used in the catalyst in order to scavenge the photogenerated holes.

Metal-Organic Frameworks (MOFs) MOFs are a class of crystalline micro- or mesoporous hybrid materials composed of inorganic metal ions or clusters and polydentate organic ligands connecting via metal–ligand bond. MOFs are relatively new as photocatalysts for CRR compared with other semiconductors; however, they have gained a lot of attentions in recent years and proved a promising potential as the photocatalyst for CRR [4], which arises from their unique structure and properties. First, with the appropriate choice of metal nodes and/or ligand strands, the resulting MOFs can function as a semiconductor. The photogenerated electrons from the excited ligand strands or metal node upon illumination can be transferred to the catalytically active sites. This charge transfer process can be facilitated by the highly crystalline structure of MOFs which directs the electrons migration to the catalytic site via the organic linker rather than to the surface of the MOF, thus reducing the recombination rate of photogenerated e–h+ pairs. Also, MOFs usually contain a considerable amount of coordinative unsaturated metal sites that are the active site for CRR. In addition, due to its highly tailorable nature, extra catalytically active site or adsorption site for CO2 can be incorporated into the MOF structure via ligand functionalization or post-synthetic method. Last but not least, the highly porous structure of MOFs with uniform and adjustable pore size is a desirable feature for catalysis. This characteristic structure of MOFs not only enhances the mass transport of CO2 to the active sites during the catalytic reaction but also endows a large surface area-to-volume ratio that helps to boost up the conversion efficiency. A number of MOFs have been reported to actively photocatalyze CRR, including MIL-125, MIL-101, MIL-53, MIL-88, UiO-66, and their amine functionalized derivatives [24, 25]. Moreover, the organic linkers of UiO-66 and UiO-67 were modified to host metal ions and photosensitizer, respectively [26]. Highly conjugated ligand strands were also engaged to construct MOFs such as Zn/PMOF, PCN-222, NNU-28, MOF-525, MOF-525-Zn (or Co), and Al PMOF which have been also reported to be active CRR photocatalysts [27, 28]. Nevertheless, the practical industrial application of MOFs on fuel production is limited by its low solubility in H2O and poor stability. Other Metal-Based Semiconductors Metal nitrides and phosphides, such as ZnGeNO, ZnGaNO, GaN, and GaP, have also been demonstrated as a photocatalyst in CRR [17, 29, 30]. Layered double hydroxides (LDH) are another new member recently added to the list of active CRR photocatalysts. LDH is a group of anionic clays with the general formula of

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[M(II)1xM(III)x(OH)2]x+[Ax/n]nmH2O, where M(II) and M(III) are the divalent and trivalent metal cations respectively, and A is an intercalate anion such as CO32, NO3, F, or Cl. Although they were known for more than 50 years, it is only recent that the photocatalytic CRR activity of LDHs is recognized. The photocatalytic ability of LDHs is governed by its structure. The layered structure of LDHs facilitates the diffusion and separation of photogenerated charge carriers. Meanwhile, the edge-shared MO6 octahedron at the host structure of the LDH not only enables electron transfer and avoids the recombination during the photocatalysis but also acts as the redox center for the catalysis [31]. Among various LDHs reported so far, only those composed of divalent cations of Mg, Co, Ni, Cu, and Zn, and trivalent cations of Al, Ga, and In have shown good performances in photocatalytic CRR because of the higher adsorptivity of CO2 and reasonable reducing position of the CB [32]. Much research effort is needed to synthesize highly crystalline LDHs with desired size and morphology, to expose large amount of highly dispersed active sites in the structure, and to improve long-term catalytic stability.

Nonmetal-Based Semiconductors Graphene and Graphene Oxide Both graphene and graphene oxide are considered as an effective and cheap photocatalyst for CRR [33]. Graphene has a two-dimensional nanosheet structure wherein a single layer of sp2-bonded carbon atoms are connected in a hexagonal honeycomb lattice. The potential application of graphene in photocatalytic CRR is of particular interest owing to its interesting structural, optical, and electrical properties, together with its good thermal and chemical stability. The nanosheet structure of graphene allows a large specific surface area which can promote its interaction with reactants during catalysis and light absorption efficiency. It is important to note that only monolayer graphene shows good light absorption properties, whereas the efficiency of light absorption drops as the number of stacking layer increases. Due to the zero band gap with a symmetric band structure, graphene has excellent conductivity and electron mobility that facilitate the charge separation and migration, and this is especially beneficial for the multi-electron reduction process in CRR. Graphene oxide, which is the oxidized form of graphene, is a small band gap semiconductor. The surface of graphene oxide consists of abundant functional groups such as hydroxyl, epoxy, carboxyl, and carbonyl groups. By manipulating the ratio of these functional groups on the surface, its band structure can be adjusted. In addition, the band gap of graphene oxide can be modified from 0.2 to 4.2 eV simply by changing the ratio of sp2 and sp3 hybridized atoms in the material. Graphitic Carbon Nitrides (g-C3N4) The potential application graphitic carbon nitrides (g-C3N4) in photocatalytic CRR has also been extensively studied [34]. g-C3N4 is a visible-light-responsive semiconductor due to its narrow band gap of ~2.70 eV. Similar to graphene, g-C3N4 exists

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as a two-dimensional nanosheet structure which is composed of tri-s-triazines interconnected via tertiary amines, together with the stacking of nanosheets via van der Waals interaction between the aromatic C–N heterocycles. Such structure contributes to its large specific surfaces area and excellent thermal and chemical stability. g-C3N4 only contains two earth abundant elements: C and N, making it a relatively cheap material than others, and is easily prepared by calcination of urea, thiourea, melamine, cyanamide, and any other compounds that contain carbon and nitrogen. It is of interest to note that g-C3N4 synthesized from different chemical sources shows different activity in a catalytic reaction due to their difference in pore distribution, surface area, surface functional group, and surface state. All these characteristics make g-C3N4 a promising and interesting material for photocatalytic CRR. Nevertheless, since bulk g-C3N4 has shortcomings of insufficient visible light utilization, low exposed surface area, and rapid recombination of photoinduced electron/hole pairs, further optimization is needed.

Other Nonmetal-Based Semiconductors Recently, carbon quantum dots [16], boron nitride (BN) [35], silicon carbide (SiC) [36], covalent organic frameworks (COFs), and covalent organic polymers (COPs) also started to gain popularity in the application of photocatalytic CRR [37, 38]. Among them, COFs are of particular interest. COF, which can be considered as the organic version of MOFs, is a porous material created by organic building units linked through covalent bonds. Similar to MOFs, COFs possess many advantages including high specific surface area, large and tunable porosity, and good thermal stability. Both framework structure and functionality of COFs can be customized by exploiting the size, symmetry, and binding parameters of the organic linker. Thus, a larger flexibility on the manipulation of product selective in photocatalytic CRR is provided.

Strategies for Catalysts Improvement Although many semiconductors have so far identified to show good photocatalytic CRR activities, both the conversion efficiency and product selectivity are still far from practical application requirement. Most catalytic systems are hampered by one of limitations/shortcomings of light harvesting, the rate of charge separation and transportation, and, importantly, the adsorption of CO2 on catalyst surface. As a result, various strategies have been employed to tackle these problems, which will be discussed in this section.

Crystal Structure Engineering The catalytic efficiency of semiconductor photocatalysts for CRR is highly influenced by their intrinsic properties, such as surface electronic and optical properties and the adsorptivity toward CO2. Because the intrinsic property of

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semiconductor is closely correlated with their crystal structure (e.g., nature, dimension, uniformity, termination of the crystal phase), it can be, in principle, systemically adjusted via crystal structure engineering. Li et al. reported that the cubic phase of NaNbO3 exhibited better photocatalytic activity toward CRR than the orthorhombic phase [39]. In addition to the advantages of the higher symmetry cubic structure in electron excitation and charge carrier migration, the higher surface area contributed to a better photocatalytic performance. Another example reported by Liu et al. demonstrated that the photocatalytic efficiency of TiO2 in the reduction of CO2 to CH4 is largely dependent on the crystal phase with the following order: brookite > anatase > rutile (Fig. 4a) [40]. The better performance of brookite phase was ascribed to its structure that allows facile formation of oxygen vacancies (VO), faster reaction rate of CO2 with adsorbed H2O or surface OH groups, and an additional reaction route involving the formation of HCOOH intermediate.

Surface Engineering The atomic structure of semiconductor, especially the surface structure, dictates the surface charge density, adsorption and activation of CO2, and the kinetic of charge recombination. Surface engineering is thus widely adapted method to improve the catalytic efficiency and product selectivity of photocatalyst for CRR. The vacancies on semiconductor surface not only change the properties in light absorption and charge separation and transfer but also affect the adsorption and activation of CO2 which can lower the CO2 activation energy or even alter the CRR reaction pathway. The importance of surface vacancy control for the photocatalytic CRR activity has been well illustrated with TiO2. Direct electron transfer from TiO2 to CO2 is thermodynamically prohibited due to its more positive CB comparing with the standard redox potential of CO2 to CO2•. It is, however, still possible because of the adsorbed CO2 species on TiO2 surface (Fig. 4b) [41]. CO2 can be adsorbed on the surface of TiO2 by bridging the oxygen atoms to the bridged-bonded oxygen vacancy site (VO) of TiO2. Such configuration allows the overlapping between the C–O π* antibonding orbital of CO2 and the 3d orbital of Ti, thus facilitating the electron transfer from Ti3+ to adsorbed CO2 to form CO2•. The interaction between VO on TiO2 surface and CO2 can also lower the activation energy of CO2 dissociation into neutral CO and O ion. The O ion heals the VO site, while CO can either be released from the surface or undergo further redox reaction and transformed to other organic products. Other reported examples of enhanced photocatalytic CRR activity by controlling VO include SrTiO3δ, ZnAl-LDH, Bi2WO6, and CeO2 [42, 43]. Other anion vacancies on catalyst surface also have been applied for photocatalytic CRR, including carbon vacancies (VC), nitrogen vacancies (VN), and sulfur vacancies (VS). The study of Tu et al. demonstrated that the introduction of VN into g-C3N4 (Fig. 4c) can induce the formation of mid-gap states that can extend the visible light absorption and minimize the recombination loss of photogenerated e–h+ pairs, thus improving the reduction of CO2 to CO about four times compared to the bulk g-C3N4 [44]. On the other hand, Vc-modified g-C3N4 was reported by

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Fig. 4 (continued)

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Shen et al. to show 2.3 times improvement in the photoreduction of CO2 to CO [45]. This enhancement was attributed to the improvement in CO2 adsorption/ activation, as well as the upshift of the CB and the enrichment in the charge carrier generation due to the introduction of Vc. Recently, Li et al. reported sulfur-deficient CuIn5S8 that is highly active for visible-light driven photoreduction of CO2 to CH4 (8.7 μmol g1 h1) with nearly 100% product selectivity [46]. Such excellent performances were reasoned with the formation of charge-enriched Cu–In dual site after introducing Vs, which not only lowered the overall activation energy barrier of CO2 but also altered the reaction pathway to form CH4 instead of CO. In addition to the anion vacancies, the introduction of cation vacancies has also been investigated. By introducing Zn vacancies (VZn) into ZnIn2S4, He et al. significantly enhanced the photoreduction of CO2 to CO, about 19 times better than its bulk counterpart [47]. They correlated this enhancement in photocatalytic efficiency to the increase in light response range, the rate of charger separation and transfer, and the population of surface active sites. Surface modification can also be achieved with functional species that can induce the change in light absorption, charge separation and transfer, and CO2 adsorption. For examples, Qin et al. demonstrated that the modification of g-C3N4 nanosheet with barbituric acid could facilitate the photocatalytic efficiency of CO2 reduction to CO by improving the optical absorption, and charge separation [48]. Liao et al. also reported an amine-functionalized TiO2 nanoparticles (NPs) that could effectively promote the production yields of CO and CH4 in photocatalytic CRR [49]. Because CO2 is a Lewis acid, the surface functionalization of TiO2 with amine increases the affinity of CO2 through chemisorption. This chemisorption is originated from the imine condensation between amine and CO2 to afford carbamate, which is considered as an activation process of CO2 because carbamate has the higher reactivity than linear CO2 molecule. Moreover, this chemisorption also helps to shorten the distance between the activated CO2 species and Ti cations, thus facilitating the charger transfer for the improved catalytic efficiency.

Elemental Doping The catalytic efficiency and product selectivity can be adjusted by elemental doping on the semiconductor. With foreign atoms doped in the crystal structure, the electronic structure of semiconductor can be modulated. As a result, the capability of semiconductor in light absorption and the binding energy of the surface toward CO2 ä Fig. 4 (a) HRTEM images of anatase, brookite, and rutile phase of TiO2, and their production of CO and CH4 after 6 h illumination [40]. (b) Schematic illustration of an oxygen vacancy defect (VO) and a CO2 molecule absorbed at VO on reduced TiO2 surface, and the STM image of TiO2 surface after CO2 adsorption [41]. (c) EPR spectra of bulk g-C3N4 and g-C3N4 with nitrogen vacancy (VN), and the graphical illustration of g-C3N4 with VN [44]. (The references [40, 41, 44] are reprinted with permission from American Chemical Society)

Fig. 5 (a) SEM and its corresponding HRTEM (inset) images of C-doped SnS2, and the illustration of the photocatalytic reaction mechanism and the band edge positions of C-doped SnS2 and SnS2 [51]. (Reproduced with permission from Springer Nature.) (b) HRTEM image of Au–Cu alloy NPs on P25 TiO2

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and the key intermediates in CRR can be adjusted accordingly. The concentration and types of doped element significantly influence the properties of semiconductor. The doping elements can be classified into two types: nonmetal and metal atoms. Semiconductors with nonmetal dopants usually show the improvements in light response range and the charge separation rate. The study by Wang et al. revealed that S-doping on g-C3N4 could highly promote the rate of photocatalytic reduction of CO2 to CH3OH [50]. The narrower band gap of S-doped g-C3N4 extended the light absorption to generate more electrons. In addition, the presence of S impurities in g-C3N4 created more defects to the structure, which can serve as a trap for the photogenerated electrons which promoted the charge separation and inhibited the recombination of e–h+ pairs. Shown et al. demonstrated that the CRR catalytic activity of SnS2 under visible light could be elevated upon the carbon doping, accompanying an excellent product selectivity (CO2 to acetylaldehyde) (Fig. 5a) [51]. The carbon doping induces micro-strains in the SnS2 lattice, which affects the electronic band structure and optical properties, thus improving the efficiency of light harvesting and charge separation and transfer. In addition, the carbon doping also facilitates the adsorption of CO2 on SnS2 surfaces with a relatively small dissociation barrier, therefore enhances the photocatalytic efficiency of CRR. On the other hand, metal doping generally modifies the CB of semiconductors and acts as a photogenerated electron trap that improves the charge separation and transfer during photocatalytic CRR. For example, the doping of V, Cr, Mn, Fe, and Ni atoms onto TiO2 leads to a significant red-shift of light absorption. The extent of this shift depends on the concentration of the cation dopants. Density functional theory (DFT) calculations suggested that the presence of cation dopants can lead to the formation of some localized states below the CB edge of Ti 3d orbital, and consequently narrows the band gap of TiO2. Wang et al. reported a mesoporous TiO2 doped with Co could conduct visible light-driven photoreduction of CO2 to CO and CH4 [52]. Their study demonstrated that Co dopants not only improved the visible light absorption of TiO2 but also adjusted the selectivity of the products (CO and CH4) by controlling the molar ratio of Co and Ti hence the amount of VO formed in Co-doped TiO2. Lu et al. reported a COF composed of 2,6-diaminoanthraquinone2,4,6-tryiformylphloroglucinol (DQTP) doped with various transition metal ions (DQTP COF-M; M ¼ Ni, Co, Zn) [53]. The transition metal ion dopants exerted an obvious influence on both reactivity and product selectivity of DQTP COF during the photocatalytic CRR. DQTP COF-Co showed high production rate of CO, while DQTP COF-Zn exhibited good selectivity toward the production of HCOOH (>90% over CO). A “two-pathway” mechanism was proposed to explain such distinct CRR products. In the DQTP-COF system, the ordered π-conjugated ä Fig. 5 (continued) [55]. (Reprinted with permission from America Chemical Society.) (c) Free energy diagrams for the photoreduction of CO on Bi3O4Br and Co-Bi3O4Br [58]. (Reproduced with permission from Springer Nature.) (d) Schematic illustration of synthetic process and HRTEM image of graphene/g-C3N4 nanocomposite [61]. (Reproduced by permission of The Royal Society of Chemistry)

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structure acted as a platform to provide an electron pathway for highly efficient transfer of charge carriers to the metal ion dopants. Since different metal ions have different coordination environment toward the reaction intermediates of CO2, different reaction pathways of CRR would be taken. Metal ion with electron-rich coordination environment can easily weaken and break the C–O bond to form CO, while the metal ion with electron-deficient environment enhances the C–O bonding and tends to form HCOOH. Co2+ is a good electron donor which usually tends to convert CO2 to CO, while Zn2+ is a poor electron donor which favors to convert CO2 to HCOOH. Similarly, Sun et al. demonstrated that NH2-MIL-125(Ti) MOF doped with different noble metals, Pt and Au, achieve different photocatalytic CRR performances under visible light irradiation [54]. The Pt-doped MOF displayed an enhanced reactivity in the CO2 reduction to formate, while the Au-doped MOF showed an opposite result. The observation was explained by the hydrogen spillover effect wherein the hydrogen can spillover from Pt to the bridging oxygen atom that linked to Ti atoms of NH2-MIL-125(Ti), thus facilitating the formation of Ti3+, which is the catalytic active site responsible for the reduction of CO2 to formate. In contrast, such effect is difficult to be achieved over the Au-doped MOF.

Cocatalyst Loading CO2 reduction is a highly sluggish reaction which involves multistep proton-coupled electron transfer. In some cases, even the photogenerated charge carriers migrated to the semiconductor surface may not be able to participate in the redox reactions. To tackle this problem, auxiliary cocatalyst may be introduced to the surface of semiconductor. The kinetic of the charge transfer process and the activation energy of CO2 can be remarkably changed with the loading of a suitable cocatalyst onto the surface of semiconductor. Thus, the photocatalytic efficiency and the product selectivity of semiconductor in CRR can be much enhanced. Moreover, the timely consumption of photogenerated charges on the cocatalyst can slow down the photocorrosion of semiconductor, thus improving the photostability of the semiconductor. The utilization of metal-based cocatalysts has been widely studied to boost up the catalytic reactivity of semiconductors in photocatalytic CRR. The Fermi level of metal and/or its oxidized species is generally lower than the CB minimum of semiconductor, and consequently results in the formation of Schottky barrier on the metal–semiconductor interface which can accelerate the charge transfer and separation, and thereby the rate of CO2 reduction. Noble metals including Pt, Pd, Au, and Ag are the most widely used cocatalyst in photocatalytic CRR. They are usually deposited onto the surface of semiconductor via either chemical reduction or photochemical reduction of its corresponding precursors. Especially, those with the localized surface plasma resonance (SPR) properties (Au and Ag) can serve as electron traps to promote photogenerated charge separation and suppress the recombination of e–h+ pairs. Moreover, the SPR effect of these plasmonic metals can function as photosensitizer that can extend the visible light response range of

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semiconductor and improve their photocatalytic performance in CRR. The effect of noble metal cocatalyst on photocatalytic CRR of TiO2 has been reported by Xie et al. who revealed that the formation rate of CH4 increased in the sequence of Pt > Pd > Au > Rh > Ag cocatalyst, corresponding to their efficiency in the photogenerated charge separation [13]. The loading of Pt, in particular, was found to largely enhance the electron density on TiO2, which can explain the product selectivity of CH4 by driving the eight-electron reduction of CO2 to CH4. In addition, the size of Pt NPs on TiO2 affects the product selectivity. The smaller the size of Pt NPs is, the better the selectivity of CH4 production becomes. Neaţu et al. reported that the deposition of Au–Cu alloy NPs on P25 TiO2 (Fig. 5b) also greatly enhances the production rate of CH4 (>2000 μmol g1 h1) and selectivity (>97%) under the irradiation of simulated sunlight [55]. The excellent selectivity toward CH4 was ascribed to the presence Cu atoms of the Au–Cu alloy bonding to CO on TiO2, while the visible light absorption is due to the SPR effect of Au. Recently, non-noble metal cocatalyst has gained increasing interests for photocatalytic CRR. Huang and coworkers reported the deposition of single Co2+ sites on C3N4 (Co2+@C3N4) for visible-light driven CO2 reduction [56]. In this system, Co2+ was activated via the coordination with the N atoms of C3N4 through Co–N bonding. This activated Co2+ was recognized as the active catalytic site for the reduction of CO2 to CO, while C3N4 was responsible for light harvesting. As a result, a good catalytic activity with turnover number >200 and selectivity of CO production >70% was achieved. Zhong et al. reported a synergistic design of 2, 20 -bipyridine (Bpy)-based COF with single Ni sites deposition (Ni-TpBpy) [57]. The single Ni sties were successfully anchored on the COF via the chelation with N atoms of Bpy units. Ni-TpBpy exhibited an excellent photocatalytic CRR activity with a CO production rate of 4057 μmol g1 and 96% selectivity over H2 upon visible light irradiation of 5 h. Both DFT calculations and experimental results revealed the important role of the COF which not only acted as a site for CO2 adsorption and coordination with single Ni sites but also facilitated the activation of CO2 and inhibited the competitive reduction of H2O to H2. Di et al. successfully demonstrated that the incorporation of single Co atoms in the Bi3O4Br atomic layers can facilitate the charge separation and transfer, as well as CO2 adsorption and activation, delivering a selective reduction of CO2 to CO with a formation rate of 107 μmol g1 h1 [58]. This rate recorded roughly 4 and 32 times higher than that of Bi3O4Br atomic layer and bulk Bi3O4Br, respectively. Theoretical calculations suggested that the single Co atoms can lower the CO2 activation energy barriers by stabilizing the COOH* intermediates and tune the rate-limiting step from the formation of adsorbed intermediate COOH* to CO* desorption, which enabled the excellent catalytic performance of the photocatalyst (Fig. 5c). Beside metal atom/ions or NPs, metal complexes and metal-free cocatalysts have also been used as either photosensitizer or cocatalyst in photocatalytic CRR. For example, Kuriki et al. constructed a hybrid system that consisted of a Ru complex and C3N4. The Ru complex acted as the catalytic active site in photoreduction of CO2 to formic acid while C3N4 played the role of light harvesting unit. Under optimized conditions, the system achieved a turnover number of >1000 and an

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apparent quantum yield of 5.7% at 400 nm [59]. Similar strategies of using Ru complexes coupled with C3N4 have been reported by other research groups. Yang et al. reported a hybrid system composed of a photoactive 2D triazine COF coupled with [Re(Bpy)(CO)3Cl] complex via post-synthetic modification, which efficiently reduced CO2 to CO under visible light illumination with high selectivity (98%) [60]. The 2D COF and Re complex was used as a photosensitizer and catalytic site, respectively. A sandwich-like nanocomposite of graphene and g-C3N4 was also reported by Ong et al. (Fig. 5d), which exhibited a 2.3 time enhancement in photoreduction of CO2 to CH4 under visible-light irradiation compared with the pristine g-C3N4 [61]. The enhancement is attributed to the role of graphene that contributed to the improvement in charger transfer and separation rates and the inhibition of e–h+ pair recombination.

Nanostructure Construction The morphology of a nanostructured semiconductor, such as the dimension, architecture, and exposed crystal facets, has profound impacts on its photocatalytic CRR performance. With the precise control on those parameters, it is possible to gain a deeper insight on the structure-catalytic activity relationships for the photocatalytic CRR performance, both conversion efficiency and product selectivity. Exposed crystal facets on semiconductor surface dictate its photocatalytic performance. The atomic arrangement of surface crystal facets can alter the absorptivity and activation of CO2, thus the catalytic activity and product selectivity in CRR. The redox abilities of photogenerated carriers are also closely related to the surface electronic band structure that is determined by the surface crystal facets. Furthermore, the efficiency of charge separation and transfer in a semiconductor is governed by the crystal orientation, and hence affects the surface charge density for redox reactions of CRR. Xu et al. reported an anatase TiO2 ultrathin nanosheets with 95% exposed {100} facet which showed about 2.8 times higher catalytic activity in the reduction of CO2 to CH4 than TiO2 cuboids with 53% {100} facet (Fig. 6a) [62]. They explained that the higher CB minimum resulted from the higher percentages of {100} facet resulted in more strongly reductive electrons for the CRR reaction. Besides, {100} surface is easier to produce oxygen vacancy (VO) due to the presence of large amount of non-saturated coordinated Ti and O atoms, which can offer a better CO2 adsorption and activation. Ye et al. also reported that the photocatalytic CRR activity order of {100} > {101} > {001} facets of anatase TiO2 [63]. This order is mainly related to the CO2 adsorption properties and charge separation efficiency on the different exposed facet. Interestingly, the study of Mao and coworkers showed that this order can be reversed with 1% Pt loading [64]. They found that in the presence of Pt, {001} facet could enhance the photoinduced carrier separation efficiency more efficiently than {010} facet, which afforded a higher conversion efficiency for {001} facet. Numerous nanostructured semiconductors of different dimensionalities have been developed for photocatalytic CRR. One-dimensional (1D) nanostructures

Nanostructured Semiconductors for Photocatalytic CO2 Reduction

Fig. 6 (a) Schematic illustration of anatase TiO2 crystal with different exposed crystal facets and the photocatalytic CRR result under UV-Vis irradiation [62]. (b) FESEM image of Zn2GeO4 nanoribbons [66]. (c) Schematic illustration of the formation of single-unit-cell Bi2WO6 layers, and its AFM image and the corresponding height profile [21]. (Reproduced permission from John Wiley and Sons.) (d) FESEM and TEM images of hierarchical yolk@shell microspheres of TiO2 [69]. (The references [62, 66, 69] are reprinted with permission from American Chemical Society)

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including nanorods, nanotubes, nanofibers, and nanobelts are far more interesting than the corresponding bulk materials owing to their larger surface area to volume ratio, facilitated electron transfer, and better light absorption. Ultrathin W18O49 nanowires prepared by Xi et al. showed an excellent photocatalytic CRR activity (production rate of CH4 ¼ 0.029 mmol L1 g1 h1) with 95% product selectivity, in contrast to inactive commercial bulk WO3 [65]. This drastic change in the catalytic property was explained with the presence of a large number of VOs in the nanowire as well as its large surface area. Liu et al. demonstrated that single crystalline Zn2GeO4 nanobelts with lengths of hundreds of micrometers (Fig. 6b), thicknesses as small as 7 nm, and aspect ratios of up to 10,000 could render a CH4 yield of ~1.5 μmol g1 at the first hour of illumination [66]. Considering its corresponding bulk material produces only trace amount of CH4 even with 16 h illumination, this also represents a striking change of catalytic property. Besides the higher specific surface area, the excellent catalytic activity of Zn2GeO4 nanobelts was attributed to its ultralong longitudinal dimension that enabled a sufficiently spacious transport channel for charge separation, together with the ultrathin geometry that allowed charge carriers to move rapidly from the interior to the surface to participate in the CRR. Comparing with 1D nanostructures, the charge carriers in two-dimensional (2D) structures are less localized. Even so, the interest in constructing 2D structured semiconductors has been triggered in recent years due to its high specific areas with the exposure of abundant active sites for CO2 adsorption and photocatalytic reaction. The study of Li et al. demonstrated the excellent photocatalytic CRR activity of Bi2WO6 atomic layer with a single-unit-cell thickness with good photostability (Fig. 6c) [21]. The total yield of methanol production with this Bi2WO6 atomic layer reached 451.7 μmol g1, ca. 125 times higher than that of bulk counterpart, which was ascribed to the higher CO2 adsorption capacity and better electronic conductivity for charge transfer and separation due to the ultra-large surface area and the single-unit-cell thickness, respectively. Similar synthetic strategy has been engaged on the formation of single atomic layer of BiVO4 which also showed an excellent photocatalytic CRR efficiency and photostability [19]. An ultrathin nanosheets of g-C3N4 was fabricated by Xie et al. by thermal exfoliation of the bulk [67]. This nanosheet exhibited a remarkably enhancement in light harvesting, stronger redox ability of photogenerated charge carriers, and better CO2 adsorption properties due to the higher population of surface active sites, as well as the enhancement in charge separation and transfer. As a result, the photoreduction of CO2 to CH4 and CH3OH of the nanosheet have increased by 10 and 5 times, respectively, compared to the bulk material. More complicated and hierarchical structures have also been developed and investigated for photocatalytic CRR. For example, a hollow sphere composed of TiO2 and graphene nanosheets was prepared by Tu et al. using a layer-by-layer assembly technique and showed a photoreduction of CO2 to CO and CH4 nine times better than commercial P25 TiO2 [68]. Such large increment was due to the facilitated charge separation and charge transfer related to the ultrathin nature of TiO2 nanosheet and the stacked structure of TiO2 and graphene. In addition, the hollow

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structure acted as a photon trap-well to allow the multi-scattering of incident light, thus enhancing the efficiency of light harvest. Liu et al. reported an interesting hierarchical yolk@shell microspheres of TiO2, which is synthesized by diethylenetriamine (DETA)-mediated anhydrous alcoholysis of titanium (IV) butoxide (Fig. 6d) [69]. DETA played an important role in the interlayer intercalation and surface grating of the structure, which endowed a strong visible light absorption ability and CO2 adsorption capacity to the microsphere. As a result, the photocatalytic activity of the microsphere in the reduction of CO2 to CH3OH is 10 times higher than the commercially available P25 TiO2. Yan et al. prepared a mesoporous ZnGa2O4 with a wormhole framework that exhibited a good photocatalytic activity in the conversion of CO2 to CH4 [70]. Its better CO2 adsorptivity and larger specific surface area due to the mesoporous structure has promoted the conversion rate by 40 times.

Hybrid Nanostructure Construction The coupling between two different semiconductors can endow superior physicochemical or optical properties to the resultant hybrid nanostructure compared to the original starting materials. The properties include the improved efficiency in the spatial separation of photogenerated e and h+, the increased population of catalytic active sites with enhanced accessibility, the improvement in the adsorptivity of reactant, and the creation of a novel band structure, all of which can be used to tune the photocatalytic CRR activity. There are several types of hybrid nanostructures developed for the photocatalytic CRR application, including homojunction, type II heterostrucutre, p–n heterojunctions, and Z-scheme. Homojunction is a hybrid structure formed with the same semiconductor materials, which is the result of the coexistence of either different crystal phases or surface facets that have different band edge position to allow efficient spatial separation of photogenerated charge carriers and the suppression of e–h+ pairs recombination. Zhao et al. reported a mixed anatase–brookite phase TiO2 NPs (Fig. 7a) which showed a better photocatalytic CRR reactivity than the corresponding single phase anatase and brookite, as well as the anatase–rutile phase P25 TiO2 [71]. The interfaces created between anatase and brookite nanocrystals in the hybrid structure was found to enhance the charge separation and interfacial charger transfer from brookite to anatase due to the slightly higher CB edge of brookite than that of anatase. With a single type phase TiO2, the catalytic property can be modulated by tuning the exposed facets. Yu et al. demonstrated that the coexistence of {001} and {101} facets on anatase TiO2 could enhance the photocatalytic reduction of CO2 to CH4 [72]. Since {001} and {101} facets possess different band structure and band edge position, their co-exposure can form a surface heterojunction that can facilitate the transfer of photogenerated charger carriers. In type II heterostructure, the coupling of two semiconductors with different band gaps and band structures, usually one semiconductor with the higher CB and VB positions (SC I) than the other one (SC II), results in a staggered band alignment.

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Fig. 7 (continued)

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Such difference in chemical potential between the semiconductors leads to the formation of band bending at the interface of the heterojunction, and hence induces a build-in electric field that drives the migration of photogenerated charge carriers between the semiconductors in opposite direction. In other words, the photogenerated electrons from the CB of SC I move to the CB of SC II, and the photogenerated holes from the VB of SC II move to the VB of SC I. Consequently, efficient spatial separation of charge carriers on different sides of heterojunction is afforded before its recombination. It is important to note that direct particle-to-particle contact with large contact surface area is beneficial for the promotion of charge separation, thus greatly enhancing the photocatalytic efficiency. Zhang et al. reported a hierarchical assembly of ultrathin ZnIn2S4 nanosheets on TiO2 electro-spun nanofibers with the intercalation of Ag metal [73]. The CO2 to CH4 photoreduction rate with this composite has increased 16 times compared to ZnIn2S4 nanosheets, due to the SPR effect of Ag that boosted up the formation of photogenerated charge carriers, together with the semiconductor heterojunction effect between ZnIn2S4 and TiO2 which facilitated the separation of charge carriers via interfacial electron transfer process. Li et al. constructed a core-shell structure by coupling Cu3(BTC)2 (BTC ¼ benzene-1,3–5-tricarboxylate) MOF with TiO2 (Fig. 7b) [74]. This Cu3(BTC)2@TiO2 heterostructure allowed efficient transfer of photogenerated electrons from TiO2 shell to Cu3(BTC)2 core, which not only facilitated the spatial separation of charge carriers in TiO2 to the Cu3(BTC)2 but also supplied the photogenerated electron to the adsorbed CO2 on the MOF for reduction reaction. As a result, much improved photocatalytic reactivity and product selectivity were achieved in the visible light-driven reduction of CO2 to CH4. Very recently, a composite of BiVO4 and Bi4Ti3O12 with type II heterojunction was reported by Wang et al. [75]. Compared to pristine Bi4Ti3O12, the photocatalytic CRR rates for CH3OH and CO productions were highly promoted by 10 and 3 times, respectively. Such a high catalytic activity was attributed to the type II heterojunction structure which significantly enhanced the separation of photogenerated carriers by regulating the transfer of charge carriers. The coupling between p- and n-type semiconductors in a system affords the formation of p–n heterojunction. The contact between p- and n-type semiconductors leads to the formation of a space charge region that results in the band bending and ä Fig. 7 (a) HRTEM image of mixed anatase–brookite phase of TiO2 NPs [71]. (Reproduced permission from The Royal Society of Chemistry.) (b) Production yields of CH4 and H2 from the photoreduction of CO2 using the core-shell structure of Cu3(BTC)2@TiO2, and the TEM image of the structure [74]. (c) Schematic illustration of the formation of hybrid CuO–TiO2  xNx hollow nanocube and its FESEM images [76]. (d) Z-schematic system of visible light-driven CO2 reduction of [Ru(dpbpy)]-modified (CuGa)1  xZn2xS2 hybrid photocatalyst, BiVO4 photocatalyst, and [Co (tpy)2]3+/2+ redox shuttle electron mediator [78]. (Reproduced permission from The Royal Society of Chemistry.) (e) TEM and HRTEM images of WO3/Au/In2S3 [79]. (Reprinted permission from American Chemical Society.) (f) HRTEM image, photocatalytic performance, and schematic illustration of direct Z-scheme for TiO2/CdS composite [83]. (The references [74, 76, 83] are reproduced by permission from John Wiley and Sons)

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the equilibration of the Fermi levels. Consequently, the built in electrical potential in the space charge region can direct the transfer of photogenerated e (from the CB of p-type to the CB of n-type side) and h+ (from the VB of n-type to the VB of p-type side), and thus permits effective separation of charge carriers and prohibits their recombination. Hence, the p–n heterojunction can improve the photocatalytic efficiency in CRR. For example, In et al. reported a p–n hybrid CuO–TiO2xNx hollow nanocube (Fig. 7c) for photocatalytic reduction of CO2 to CH4 under the irradiation of simulated sunlight, which exhibited 2.5 times faster production rate than P25 TiO2 [76]. Tang et al. also demonstrated that a ball-flower like NiO/g-C3N4 heterojunction composite catalyzed visible light-driven CO2 reduction to CO with 2.5 and 7.6 times increased production yields than the pristine g-C3N4 and NiO, respectively [77]. The enhancement is attributed to the perfect band matching and efficient internal charge transfer within the p–n junction which can efficiently separate the photogenerated e–h+ pairs, together with the stronger absorption of visible light and larger specific surface area. Artificial Z-scheme is another heterostructure that has been widely investigated to enhance the photocatalytic CRR performance of semiconductor. In the Z-scheme nanostructures, one of the two component semiconductors is an oxidation photocatalyst and the other is a reduction photocatalyst with suitable band alignment. Such arrangement of two semiconductors can extend the light-response range, effectively separate the photogenerated charge carriers, and facilitate the charge transfer, as well as simultaneously preserve the strong reduction and oxidation abilities of each photocatalyst, all of which can contribute to overall photocatalytic activities. There are three types of Z-scheme nanostructures: traditional Z-scheme, all-solid-state Z-scheme, and direct Z-scheme based on whether charge carrier mediator is present or not. For traditional Z-scheme, redox mediator is employed as the transfer medium of charge carriers. In a forward reaction, it consumes the photogenerated electrons in the CB of semiconductor II (SC II) and the photogenerated holes in the VB of semiconductor I (SC I). Consequently, the strong reductive and oxidative sites in the heterostructure are separated, thus enhancing the photocatalytic performance. Suzuki et al. constructed a visible light-driven Z-schematic CRR system using a simple mixture of a CRR photocatalyst [Ru(dpbpy)]-modified (CuGa)1xZn2xS2 (dpbpy ¼ 4,40 -diphosphonate2,20 -bipyridine), a water oxidation (OER) catalyst BiVO4, and a redox-shuttle electron mediator [Co(tpy)2]3+/2+ (tpy ¼ 2, 20 : 6, 200 -terpyridine) (Fig. 7d) [78]. Engaging H2O as an electron donor, a moderate product selectivity (> 60%) for CO and formate over H2 was achieved. The photogenerated electrons in BiVO4 were transferred from the CB of BiVO4 to [Co(tpy)2]3+/2+, and then to the VB of [Ru(dpbpy)]-modified (CuGa)1xZn2xS2. As a result, the photogenerated electrons in [Ru(dpbpy)]-modified (CuGa)1xZn2xS2 participated in the reduction of CO2, while the photogenerated hole in BiVO4 would undergo the water oxidation to generate O2. Similar to traditional Z-scheme, an electron transfer mediator is also required in all-solid-state (ASS) Z-scheme. Instead of ionic redox mediator, however, a solid conductive material is applied in ASS Z-scheme as an electron bridge to

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connect the interface of the semiconductors for the fast transfer of photogenerated charge carriers. The advantages of using solid conductive materials are that there is no back reaction and light shielding problems that often occur with ionic redox mediator. Also, both liquid and gas phase reaction can be conducted using the solid conductive materials. The most frequently used solid conductive materials in ASS Z-scheme are noble metals (e.g., Au and Ag). The metallic character of noble metals can lead to the formation of low-resistance Ohmic contact in the heterojunction region with semiconductors, which is beneficial for the transfer and separation of charge carriers. In recent year, carbon and copper-based solid conductive materials have started to gain the popularity, especially graphene and its derivatives owing to their high conductivity and electron mobility nature. For example, Li et al. synthesized a Z-scheme WO3/ Au/In2S3 nanowire arrays (NWAs) (Fig. 7e) which showed a good catalytic activity for the visible light-driven CO2 reduction to CH4 [79]. In this system, the embedded Au NPs provided an efficient charge transfer channel, facilitating the transfer of photogenerated charge carriers across the interface. As a result, the recombination of the photogenerated charge carriers in both WO3 and In2S3 could be suppressed, leaving the photogenerated electrons in In2S3 with a longer surviving time, and consequently improved the photocatalytic CRR activity of the heterostructure. Kim et al. also reported a three-dimensional (3D) Z-scheme NWAs of BiVO4/carbon-coated Cu2O for the reduction of CO2 to CO under visible light irradiation with the production rate enhancement of 9.4 and 4.7 times compared with Cu2O mesh and Cu2O NWAs, respectively [80]. The 3D NWAs structure apparently increased the surface area, and thus the light harvesting power. More importantly, the incorporation of conductive carbon layer in the heterostructure facilitated the photogenerated charge separation and transfer in the Z-scheme, and consequently achieved strong reduction and oxidation potentials. In addition, the BiVO4/carbon-coated Cu2O NWAs showed an excellent photostability during the CRR with 98% of the initial CO production rate being retained after 20 h irradiation. This was ascribed to the incorporation of protective carbon layer and the Z-schematic charge flow. An urchin-like CoZnAl-LDH/ reduced graphene oxide (RGO)/g-C3N4 Z-scheme heterostructure was demonstrated as a good photocatalyst for visible light-driven CO2 reduction to CO by Yang et al. [81]. This hybrid structure exhibited a CO production rate of 10.11 μmol1 g1 h1 which is 3.4 and 8.5 times better than that of CoZnAlLDH/g-C3N4 and bare COZnAl-LDH, respectively, with the selectivity toward CO over 96%. This significant enhancement on the catalytic performance was correlated to the 3D hierarchical structure of CoZnAl-LDH/RGO/g-C3N4 which boosted up the light harvesting capacity, and hence increased the population of photons that could participate in the catalytic reaction. Besides, the Z-scheme charge flow not only inhibited the recombination of photogenerated charge carriers but also promoted the oxidizability and reducibility of CoZnAl-LDH and g-C3N4. Last but not least, the structure facilitated the adsorption and activation of CO2, therefore reducing the activation barrier for the reduction of CO2 to CO.

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Different from traditional and ASS Z-scheme, an electron mediator is not required in direct Z-scheme because two semiconductors intimately contact with each other. As a result, direct Z-scheme allows the efficient separation and transfer of photogenerated charge carriers and the optimized redox potential for designated redox reaction, together with the suppression of the backward reaction and light shielding effect. The key features that differentiate direct Z-scheme from type II heterojunction and p-n junction are the existence of internal electronic field (IEF) and the charge transfer direction, respectively. In direct Z-scheme, the staggered band alignment of two semiconductors leads to the formation of IEF only if there is a difference between their work function. The free electrons preferably migrate from the semiconductor of smaller work function and higher CB and VB positions (electron donor, SC I) to the other one (electron acceptor, SC II) in order to equilibrate their fermi levels due to their close contact. This creates positive and negative charges at the heterostructure, leading to the formation of IEF and band edge bending. The IEF directs the recombination between the photogenerated electrons in the CB of SC II and the photogenerated holes in the VB of SC I, while the photogenerated electrons in the CB of SC I and the photogenerated holes in the VB of SC II are preserved and spatially separated, which can be used for the designated photocatalytic reduction and oxidation with enhanced catalytic performance. Heterostructures with direct Z-schematic charge transfer have been engaged for photocatalytic CRR. Wang et al. reported a marigold-like SiC@MoS2 nanoflower that can achieve an overall conversion of CO2 to CH4 and O2 in aqueous solution without any sacrificial agents under visible light [82]. The production rates of CH4 and O2 were 323 and 621 μL g1 h1, respectively, and the catalytic performance of SiC@MoS2 was stable for continuous 40 h. The excellent photocatalytic performance of SiC@MoS2 was explained with its Z-scheme heterostructure that afforded a more negative CB of SiC for CRR and a more positive VB of MoS2 for water oxidation. Moreover, the morphology of the heterostructure allowed more exposed catalytic surface on SiC and MoS2, and thus more readily available for the adsorption of CO2 and H2O. Low et al. also reported a recyclable direct Z-scheme composite film composed of TiO2 and CdS for photocatalytic CRR with high efficiency (Fig. 7f) [83]. Due to the significant enhancement in the redox abilities of the photogenerated charge carriers in the direct Z-scheme, the photocatalytic CO2 reduction to CH4 of TiO2/CdS was 3.5, 5.4, and 6.3 times higher than that of CdS, TiO2, and P25 TiO2, respectively. A direct Z-scheme composite of hydrogen-bond linked zinc phthalocyanine/BiVO4 nanosheet (ZnPcBiVO4-NS) has been demonstrated as a photocatalytically active catalyst in visible light-driven CO2 reduction to CO (main product) and CH4 with a certain amount of O2 produced [84]. The optimized ZnPcBiVO4-NS showed the improved photocatalytic performance, four times better than BiVO4 NS, and exhibited good photostability. Importantly, the quantum efficiency of ZnPcBiVO4-NS showed a ca. 16-fold enhancement compared with the previously reported BiVO4 NPs. The excellent photocatalytic performance of ZnPcBiVO4NS is linked to the improvement in the separation of photogenerated charge carriers and the extended visible light absorption due to direct Z-scheme.

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Conclusions and Further Outlook Photocatalytic CRR over nanostructured semiconductor for the production of valueadded chemicals and fuels provides not only a solution to lower the atmospheric CO2 concentration but also a clean and renewable alternative energy source to replace fossil fuels. Although various types of semiconductors have been reported as photocatalysts of CRR, including metal oxides, metal sulfides, MOF, graphene and graphene oxides, g-C3N4, etc., the conversion efficiency and the product selectivity are still far from the standard of practical applications due to the limitations of the intrinsic properties of semiconductors, such as unsatisfactory light harvest, low CO2 adsorption ability, inefficient charge carriers separation and transfer, and fast recombination of charge carriers. With the consideration of these limitations, many design strategies have been developed and explored in order to create a more active and selective photocatalyst for CRR. From relatively simple strategies, such as crystal phase engineering, introduction of surface vacancies and surface ligand modification, elemental doping, and cocatalyst loading, to more complex strategies including the construction of nanostructures and hybrid structures based on different charge transfer scheme, various strategies have been adapted with the goal to (1) improve the light harvesting power by extending the light response range, (2) boost up the adsorptivity of CO2 by enlarging the specific surface area and increasing the population of host sites, and (3) facilitate the separation and transfer of photogenerated charge carriers suppressing the recombination by creating an efficient electron and hole transfer tunnel. Some catalytic systems have already demonstrated remarkable catalytic activities and product selectivity in photocatalytic CRR. In the future, more attention needs to be paid on the long-term stability, scalability, and economic sustainability in order to meet the industrial requirements. Another challenge in the field of photocatalytic CRR is to understand the reaction mechanism, which would definitely shed lights on the enhancements of conversion efficiency and product selectivity. So far, several reduction pathways for the formation of different products have been proposed; however, the governing parameters that determine which pathway will be taken is still unclear. It is of great importance to uncover the reduction mechanism and optimum conditions for the catalysts to realize the long-waited energy revolution.

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New Generation of Eco-Friendly Adsorbents for Future Water Purification

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J. Botello-Gonza´lez, N. E. Da´vila-Guzma´n, and J. J. Salazar-Ra´bago

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Ions from Wastewater by Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-Adsorbent Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-Composites Adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Organic Pollutants from Wastewater by Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Emerging Pollutants from Wastewater by Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . Mathematical Analysis of the Adsorption Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Models of Adsorption Isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reversibility of the Adsorption Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Water is a vital substance for all living things on Earth, and its contamination is a real global problem that affects many countries that are increasingly moving toward industrialization as they seek prosperity for their citizens. There are several treatment methods to resolve water contamination, such as chemical precipitation, ultrafiltration, electrocoagulation, electrodialysis, and membrane separation, to name a few, but the cost of these methods is high. On the other hand, the adsorption process presents an interesting solution because its operation is simple and the adsorbents can be prepared to remove a specific contaminant. Adsorption is a separation process used successfully in recent decades to remove organic and inorganic contaminants from aqueous solutions. This process is based on the passive accumulation of compounds on the surface of materials. J. Botello-González (*) · N. E. Dávila-Guzmán · J. J. Salazar-Rábago Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Mexico e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_77

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The most important adsorbent used is activated carbon; however, in some cases, its production involves a high-cost process and a significant environmental impact. To remedy this situation, various natural materials have been used as adsorbents, including algae, and cellulose hierarchical materials such as sawdust and agro-industrial waste. Studies indicate that these adsorbents have a good adsorption capacity due to their surface chemical properties. They are also abundant and environmentally friendly. In recent research, adsorption studies are carried out with nanomaterials that have a great advantage; the surface area where the adsorption takes place can be increased. The purpose of this chapter is to point out that the future of adsorption is to investigate new generations of nanoadsorbents that are eco-friendly.

Introduction Water pollution is an environmental phenomenon that has been aggravated by industrial development, especially in recent decades. The sources of contamination can be very diverse (heavy metals, synthetic dyes, pesticides, pharmaceuticals, emerging contaminants); however, the most significant are industrial discharges and emissions that in some cases exceed the maximum permitted concentrations established in the international standards of wastewater discharges [1]. Effluents from industrial processes contain organic and inorganic pollutants which are mostly generated as by-products or waste, while the major sources of biological contamination are domestic activities, although sometimes agricultural processes participate in biological water contamination as well [2]. These effluents are discharged into the natural channels of rivers and lakes, adversely affecting living organisms in the aquatic environment and representing a danger to human and animal health. Substances such as aromatic compounds, pesticides, dyes, drugs, and potentially toxic metals pose a serious risk to human life. The sources of water contamination are classified as natural and anthropogenic. The first is derived from processes typical of nature such as volcanic eruptions, hurricanes, cyclones, and tornadoes, while anthropogenic pollution is caused by human activities such as agriculture, livestock, and mining activities, among others. Potentially toxic metals represent a health hazard because of their accumulation, non-biodegradability, and toxicity characteristics. Metals such as Pb2 +, Cd2 +, and Ag +, among others are toxic pollutants in surface and groundwater, even at concentrations below 10 ppm. Some metals such as zinc, chromium, and copper are essential for human health, but they are harmful when they are found at concentrations above the maximum permissible limits. Long-term exposure to some potentially toxic metals may cause nose, mouth, and eye irritations, headache, stomachache, diarrhea, dizziness, gastrointestinal complications, hypertension, fatigue, hemolytic anemia, abdominal pain, nausea, constipation, weight loss, cognitive dysfunction, and depression [3]. Wastewater that may contain dyes and pesticides could become part of the food chain, resulting in mutagenesis, carcinogenicity, and serious damage to health in living systems. Therefore, it is necessary to study and develop new alternatives for

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the elimination of all these pollutants present in water and, thus, reduce the risk of disease and the environmental impact. Their presence in wastewater can be removed by various methods; among these stand out ion exchange, reverse osmosis, chemical precipitation, and membrane separation, to name a few. These processes generally have the disadvantage of being costly, producing a plethora of waste, and having a low removal efficiency at low pollutant concentrations, in addition to requiring great control over operating conditions [4]. Adsorption is another method used for remediation and has proven to be highly efficient and profitable for the removal of toxic compounds present in municipal and industrial drinking water and wastewater. Adsorption is a superficial phenomenon with a common mechanism for the removal of organic (natural or synthetic) and inorganic contaminants. When a solution containing an absorbable solute comes into contact with a solid with a porous surface structure, liquid-solid intermolecular attractive forces cause some of the solute molecules in the solution to concentrate or deposit upon the solid surface. Regarding bulk material, all of the bonding requirements (whether ionic, covalent, or metallic) of their constituent atoms are filled with other atoms in the material. However, the atoms on the adsorbent surface are not surrounded by other of the adsorbent atoms and therefore can attract more atoms [5]. Among the most widely used adsorbents are activated carbon, zeolites, silica gel, and activated alumina. The cost of this process can be reduced using adsorbents of organic origin such as algae, bacteria, fungi, clay, chitin, peat, and agro-industrial residues, which are named biosorbents. The use of biosorbents is advantageous over conventional adsorbents; these biomasses have a low cost, large availability, and low environmental impact or, in other words, are eco-friendly [6, 7]. The types of substrates of biological origin that have been investigated for the preparation of biosorbents include microbial biomass (bacteria, archaea, cyanobacteria, filamentous fungi, yeast, and microalgae), seaweed (macroalgae), industrial waste (fermented and non-fermented food waste, activated sludge, and anaerobes), agro-industrial residues (fruit/vegetable residues, rice straw, wheat bran, sugar beet pulp, soybean hulls, coffee residues, etc.), natural residues (vegetable residues, sawdust, tree bark, herbs), other materials (chitosan, cellulose, etc.), and nanocellulose-based composites that have emerged as great potential adsorbents for the removal of dyes, potentially toxic metals, adsorption of toxic industrial effluents, and removal of fertilizers/pesticides [8–11]. Nanomaterials are effective adsorbents because the size of their particles can be adjusted for each type of adsorbate, their surface area is exceptionally large, and in addition to many advantages derived from their unique structural properties, they may have high porosity and a high capacity to attract contaminants due to their excellent adsorption capacity. These properties make it an excellent material that can be used for the adsorption of both organic and inorganic compounds and are therefore used to remove various contaminants from water. They can also regenerate after exhaustion through the desorption process that is achieved when the surface of the nanomaterial is practically free of contaminants and that allows its active sites to adsorb contaminants in another of the adsorption cycles. The advantage of

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nanomaterials is that they can be easily modified by any organic or inorganic residue, making it more selective for particular species. With all these unique characteristics, they are used and will continue to be used as adsorbents in water treatment [12]. The nanomaterial can be classified according to the type of material of which they are mainly made up and the different basic components that they may contain depending on the nanomaterial that is sought to be built. Those that are made from carbon and have carbon as their basic component are classified as carbon and graphene nanotubes. They are extremely small, have a unique hollow structure, and have a large surface area with more adsorption sites, making them a good candidate for wastewater treatment. Some other nanomaterials are formulated based on metals such as silver or gold nanoparticles and on metallic oxides such as zinc oxide, titanium dioxide, and iron oxide, which have metal as their basic component. Metal and metal oxides have properties such as chargeability, conductivity, electrical, and magnetic properties that make them a good choice of an adsorbent. Metalloids like silica nanoparticles have silica as the basic constituent and can be very low in toxicity. Carbon nanotubes are the rolled sheets of graphene. There are three types of carbon nanotubes depending on the number of sheets or layers that need to be folded to prepare them. There are carbon nanotubes made up of a single-rolled graphene sheet, whereas if there are two constituent sheets, they are called double-walled carbon nanotubes. If there are multiple sheets, they are multi-walled carbon nanotubes. They have a high surface area and more adsorption power compared to simple graphene and have been studied to remove various pollutants from wastewater. The types of interactions that take place between the adsorbate and the adsorbent are simple hydrophobic interactions such as the π-π interaction, van der Waal forces, the H-junction, ion exchange, electrostatic attraction, etc. These types of forces are useful for removing nonpolar organic substances from wastewater. Carbon nanotubes must be functionalized or modified by certain organic functional groups such as carboxylic or carbonyl groups for the removal of polar and/or ionic contaminants. These modifications help bind ionic or polar pollutants through electrostatic interactions. It has been observed that different toxic metal ions and charged dyes are removed with the help of these modified carbon nanotubes. The surface of the nanotubes can be positively or negatively charged, depending on the size and characteristics of the pollutant to be adsorbed. Silica or silicon dioxide is a very abundant element on Earth’s crust. Silica is the main component of sand and is cheap and light. Along with having many applications in electronics, in the manufacture of chips for computers and electronic devices, the most common use of silica in chemistry is as an adsorbent in column chromatography, which is used to distinguish various components present in a mixture. Silica, which is a biocompatible material, is widely used in drug delivery applications. Since normal silica has a large surface area and a high adsorption tendency, its nanoparticles further improve these two, so great interest should be placed in making use of it as a nanomaterial that can adsorb pollutants in water. Metal oxide nanoparticles include nano ferric oxides, nano titanium oxides, nano zinc oxides, nano aluminum oxides, and so forth; like the rest of the nanomaterials,

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these have a large surface area, but the most important characteristic is that they have an excellent adsorption capacity due to the presence of charges on their surface. Metal oxide nanoparticles have a high tendency to attract toxic substances such as ionic organic pollutants with great selectivity and to improve their capacity modified with activated carbon or polymeric supports [13]. Eco-friendly nano-adsorbents have emerged as a sustainable solution for the removal of pollutants from wastewater. The main approach to synthesize eco-friendly nano-adsorbents is through the use of plant extracts as stabilizers and reducing agents because its methodology is simple and does not require any harmful chemicals. Another approach is the use of nano-adsorbent composites based on biopolymers, not only to improve the adsorption efficiency but to reduce the environmental impact and their cytotoxicity. In this chapter, the current status of water decontamination by nanomaterials is reviewed, and the adsorption mechanisms are outlined. In the following sections, an overview of different types of nano-adsorbents material (conventional, composites, eco-friendly, and metal-organic frameworks) for the removal of pollutants from wastewater is presented. Finally, the chapter aims to provide readers with a basic understanding of the modeling of adsorption equilibrium and its reversibility.

Adsorption The word adsorption was first introduced in 1881 by the physicist Heinrich Kayser [14]. The adsorption phenomenon is the accumulation of substances (adsorbate) on the surface of solid material (adsorbent) through physical or chemical interactions [15]. The surface chemistry of the adsorbent as well as its textural properties (e.g., surface area, pore size distribution) are characteristics with great influence on the selectivity, removal efficiency, and adsorption kinetics of adsorbates. Normally, the surface area is increased by the reduction of the particle size of the adsorbents [16]. For this reason, nanomaterials have attracted a great deal of attention as nano-adsorbents. Additionally, the small particle size of the nano-adsorbents increases the availability of more active sites on the surface of the nanomaterials, which causes an enhancement on the adsorption capacity [17]. There are several adsorption mechanisms outlined for the removal of pollutants from wastewater that depends on the surface chemistry of the adsorbent and the chemical speciation of the adsorbate. In Fig. 1, some of the adsorption mechanisms that are responsible for the removal of pollutants from wastewater are depicted [18]. Electrostatic attraction occurs between particles of opposite charges, while ion exchange occurs as an ion with a high affinity for the active sites of the adsorbent exchanges with another ion of less affinity. Hydrogen bonding is an intermolecular force of attraction between hydrogen atoms bonded to a strongly electronegative atom and another electronegative atom, whereas the presence of functional groups on both the adsorbate and the adsorbent can lead to π-π stacking interactions. These interaction forces are the result of the alignment of the positive and negative electrostatic potential in one ring and another ring, respectively [19].

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Fig. 1 Schematic illustration of some of the adsorption mechanisms in the removal of pollutants from wastewater. (Adapted from Zhu et al. 2018)

The adsorption capacity of a nano-adsorbent material is influenced by several factors such as the adsorption experimental conditions (pH, temperature, initial concentration) and the physicochemical characteristics of the nano-adsorbent (surface area, surface chemistry, pore size distribution). Several characterization techniques have been employed to gain insight and understanding of the adsorption performance of a nano-adsorbent. For instance, N2 physisorption has been widely employed to determine the surface area of adsorbent materials, including nano-adsorbents. The amount of N2 adsorbed at 77 K is related to the surface area through the BET equation, introduced by Brunauer, Emmett, and Teller. Although other gases like carbon dioxide and argon have been used to determine the surface area, the most popular has been nitrogen because of its low cost and high availability [20]. The surface chemistry of nano-adsorbent materials has been qualitatively investigated using Fourier-transform infrared (FTIR) spectroscopy analysis, where the functional groups on the nano-adsorbent surface can be identified through the adsorption bands at specific wavenumbers. Since the functional groups are the active sites for the adsorption process, its identification is of great relevance to elucidate the interactions between the adsorbate and the nano-adsorbent [21]. It is important to mention that one characterization technique alone cannot provide enough information to elucidate the adsorption mechanism of nano-adsorbent materials. For this reason, other characterization techniques are employed. Accordingly, surface charge and the pH of the point of zero charge (PZC) of nano-adsorbent materials are determined from a zeta potential analysis. The PZC refers to the pH value where a positive charge equals a negative charge. Above the PZC, the surface is negatively charged, and the interaction with positive particles would be increased by electrostatic attraction [22]. For example, X-ray photoelectron spectroscopy (XPS) is used to identify the changes of binding energies in elements of the nano-adsorbent materials after the adsorption process [23]. In this manner, it is possible to identify the atoms that interact with the adsorbate. For instance, the XPS analysis allowed us to identify the formation of O-Mn-F on the surface of Fe3O4/g-MnO2, responsible

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for the adsorption of fluoride from aqueous solution [24]. Scanning electron microscopy (SEM) is a characterization technique that is useful for the analysis of the morphology of nano-adsorbents. Coupled with energy-dispersive X-ray (EDAX) analysis, it is possible to determine the element composition before and after the adsorption process [25]. For magnetic nanoparticles, the magnetization curve is obtained using a vibrating sample magnetometer. From this analysis, the saturation magnetization and the coercivity of the nano-magnetic adsorbent can be obtained [26].

Removal of Ions from Wastewater by Nanomaterials Environmental pollution is a global problem that has been aggravated in recent decades since the industrial revolution, as a result of the rapid growth of population, the excessive consumption of natural resources, and the emission of pollutants into the bodies of water and the atmosphere. For this reason, in 2010 the United Nations General Assembly recognized the human right to water and sanitation [27]. However, 2 million tons of wastewater, industrial, and agro-industrial wastes are discharged into bodies of water worldwide every day; this exposes people to preventable health risks [28]. Several pollutants such as heavy metal ions, rare earth elements, and radionuclide contaminants are released into the environment. Heavy metal ions such as lead, copper, chromium, among others are discharged by several industries, and they can cause severe health problems affecting the lungs, kidney, brain, and liver, depending on the type of heavy metal ion, concentration, and time of exposure [29]. On the other hand, the presence of rare earth elements (REE) on wastewater is due to the large scale of refining and mining activities. Long exposures to REE can cause lung-related diseases [30]. In the case of radionuclide contaminants, an upsurge in the activities of the nuclear industry has been reported, which has led to an increase in the amount of nuclear waste. Because of its radioactive nature, isolation and confinement are necessary for environmental protection and human health. Chemical toxicity, kidney, and lung cancer are some of the effects of uranium exposure on human health [31]. Currently, various types of technologies have been used to control environmental pollution, as well as for the treatment of water and the removal of pollutants from the atmosphere, among which the adsorption process stands out because of its easy operation and high efficiency [32]. The most commonly used adsorbent materials are activated carbon, zeolites, and silica gel [33]; however, high regeneration costs have led to the search of more energy-efficient materials. Nowadays, nanostructured adsorbents have been prepared and evaluated for the removal of various pollutants [34, 35]. Nano-adsorbent materials have unique chemical and physical properties owing to their small size, for instance, larger surface area and higher availability of active sites on the surface, which are responsible for the fast and selective adsorption [36]. Nano-adsorbent materials can be used as bare-naked nanoparticles or as composite materials [37], and the main advantages and disadvantages, as well as their adsorption mechanism, will be outlined below.

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Nano-Adsorbent Materials In recent years, substantial research efforts to use nanoparticles as adsorbents have been made. A detailed outline of the removal of pollutants from wastewater through nanoscale zerovalent iron (nZVI) has been provided by Stefaniuk et al. (2016) [38]. There are three reported pathways for the removal of metal ion species by nZVI that include adsorption, complexation, and chemical reduction (Fig. 2) [39]. For metal ions with standard electrode potentials (E0) that are close to or more negative than the E0 of zerovalent iron (Fe0), the main removal mechanism is adsorption/surface complexation. In the case of metal ions that are more positive than Fe0, the removal is mainly attributed to adsorption and a partial chemical reduction, while a complete chemical reduction can occur in the removal of metal ions with E0 higher than Fe0. Despite its advantages over its bulk counterpart, the proneness to agglomeration and oxidation of bare nZVI in the removal of pollutants through this method represents a disadvantage. In order to overcome these drawbacks, recent investigations have been conducted to obtain stabilized nano-zerovalence metal ions (nZVM), including nZVI [40–43]. The green synthesis approach using plant extract seems to be a useful methodology for reducing the agglomeration and cytotoxicity of nZVM. Plant extracts contain several phytochemicals such as catechin, phenolic acids (syringic acid, tannic acid), and hydroxycinnamic acids (ferulic acid), with polyphenolic functional groups. The reduction of nanoparticles as adsorbents can occur by the action of polyphenols extracted from green sources, which can also act as capping agents through adsorption on the nanoparticle surface [44] (Fig. 3). Anacardium occidentale (AO) testa has been reported to contribute to the reduction and stabilization of nZVI, nanoscale zerovalent nickel (nZVN), and nanoscale zerovalent copper (nZVC). For instance, Chandra et al. reported the efficient removal of uranium by nZVC [41]. The removal process was strongly dependent on pH, since, at pH values higher than 4, uranium-hydroxide complexes exist predominantly, reducing the adsorption of U(VI). Similarly, nZVI synthesized from the testa extract of AO was evaluated for the removal of uranium [46]. The study of the effect of pH in the adsorption of uranium revealed that acidic conditions

Fig. 2 Schematic representation of the coreshell model of nZVI for metal ion removal. (Redrawn from Sharma 2019 [39])

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Fig. 3 Schematic representation of nZVM production from green extracts. (Adapted from Vaseghi et al. 2018 [45])

improved the reduction process of uranium through H+ ions; hence pH 4 was considered as the optimum for maximum removal of uranium. In the removal of lead, nZVN synthesized from testa extract of AO was proven to be an efficient nanoadsorbent at pH values lower than 6 [40]. Further increase in pH (pH >6) can cause the complex formation of lead species, leading to precipitation and reducing the removal capacity of nZVN. Apart from the AO extract as a reducing and capping agent, mulberry, oak, and cherry leaf extracts have been reported to be helpful in the synthesis of nZVI and the removal of As(III) and Cr(VI) [42]. The nZVI produced by leaf extracts has shown a high affinity for both Cr(VI) and As(II). The adsorption mechanism of As(III) was proposed as deprotonation of arsenite complexes and surface complexation on nZVI. On the other hand, the removal of Cr(VI) was explained as a reduction of Cr(VI) to Cr(III), coupled with oxidation of Fe(0) to Fe(II) and Fe(III). Furthermore, sugar cane extract has been proved to be favorable for the green synthesis of ZnO nanoparticles [47]. The results indicated that ion exchange was the main mechanism for the removal of Cd(II) and Pb(II) ions. In [48], the removal of lead ions by CuO nanoparticles synthesized with leaves extract of Simarouba glauca was investigated. The Simarouba glauca plant is rich in metabolites such as flavonoids, tannins, phenolic compounds, and so on. Metabolites are responsible for the reduction of nanoparticles and can act as capping agents. The adsorption performance of the as-synthesized CuO nanoparticles was dependent on the surface charge of the nanoadsorbent and the degree of chemical speciation of lead ions.

Nano-Composites Adsorbents It has been detailed earlier that nano-adsorbent materials in their bare forms have some limitations in the removal of pollutants from wastewater. To surmount the limitations, the development of nano-composite adsorbent materials is envisioning to take advantage of both the nanoparticles and the host or support matrix. An example of this is the synthesis of an eco-magnetic biocomposite comprising Fe3O4 supported on nano-hydroxyapatite (n-HAp)/chitosan (Fe3O4@n-HApCS) [49], where the magnetic properties of Fe3O4 facilitate its separation from the liquid phase. On the other hand, nano-hydroxyapatite shows a high adsorption capacity for

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heavy metals through ion-exchange mechanisms and electrostatic attraction. Similar results were reported by [50] for the removal of fluoride by magnetic Fe3O4 supported on n-HAp/alginate biocomposite (Fe3O4@n-HApAlg). The maximum adsorption capacity was reported as 4.05 mg/g, which was superior to the values reported in the literature. In [51], magnetic hydroxyapatite nanoparticles (MNHA) were evaluated for the removal of uranium from wastewater. An increase in the pH favors the binding of the positively charged uranium ions (UO22+) to the phosphate groups of MNHA. Further increase of pH (pH >5) causes the electrostatic repulsion of the uranium complexes and the negatively charged surface of MNHA. The removal of rare earth elements (REEs) has been conducted using nano-Mg (OH)2 supported on sodium alginate matrix (SA@Mg(OH)2) [52]. Ion-exchange reaction of europium ions with magnesium ions released from the nano-Mg(OH)2 was exposed as the main adsorption mechanism. Another supporting material for nano-adsorbent particles is presented by [53]. Fe-exchanged nano-bentonite outperforms Fe3O4 nanoparticles on the removal of nitrate and bicarbonate ions from wastewater. The removal of both ions is due to electrostatic attraction with metal hydroxides (positively charged) formed in the nano-bentonite. Similarly, the surface of hydroxylated Fe3O4 nanoparticles might have produced positive changes in the aqueous environment, adsorbing nitrate and bicarbonate ions through electrostatic attraction. In [54], the solution combustion synthesis (SCS) of a composite of silver nanoparticles and yttrium oxide was proposed for the removal of Cu(II) and Cr(VI) from aqueous solution. Yttrium oxide was used to obtain stabilized nanoparticles since yttrium can prevent the agglomeration of nanoparticles and the reduction of the particle size. On the other hand, urea was used as a fuel in the SCS providing negative functional groups (NO
) on the surface of the nano-composite. The adsorption capacity obtained was threefold greater than the one reported in the literature for other nano-adsorbents. Paramagnetic calcium ferrite nanoparticles have also shown high removal levels of Cr(VI) from aqueous solutions [55]. At high acidic conditions, the protonated surface of the composite promotes the electrostatic attractions between the anionic species and positively charged surfaces of calcium ferrite nanoparticles, which favor the adsorption process. Also, the paramagnetic characteristic of calcium ferrite nanoparticles allows an easy and fast separation from the aqueous media. Recently, new nano-composites materials based on metal-organic frameworks (MOF) have been attracting a growing amount of attention because of their unique characteristics. MOF are crystalline porous materials composed of metal nodes coordinated with organic linkers [56]. It has been reported that MOF can achieve surface areas up to 10,000 m2/g. Besides the outstanding surface areas, tailoring the adsorption selectivity of MOF by tuning the properties of the metal nodes, and the functional sites of the organic linker, is attractive characteristics of these materials [57]. A comprehensive review of recent progress and challenges regarding the application of MOF-based nano-composites for the removal of toxic and radioactive ions was provided by [58].

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Removal of Organic Pollutants from Wastewater by Nanomaterials Most organic pollutants, including dyes, hydrocarbons and aromatics, pesticides, and pharmaceuticals/drugs, are toxic organic and bio-refractory compounds, as they are resistant to microbial degradation and large amounts of dissolved oxygen are required for their breakdown, causing exhaustion of the dissolved oxygen available to aquatic ecosystems [5]. Among all types of organic pollutants, dyes such as Congo red, Rhodamine B, malachite green, methylene blue, and methyl violet, to name a few, are the most common non-biodegradable pollutants present in wastewater. It is estimated that the total consumption of dyes in textile industries is over 10,000 t per year and about 10–15% of these dyes are released into the water. Dyes can be mutagenic, carcinogenic, and allergenic to humans; therefore, pollution of water caused by dyes can be dangerous for both aquatic and human organisms. They mainly originate from colored effluents in the textile and paper industries [59–62]. Among these organic pollutants that exist, many are pesticides, insecticides, and herbicides that have been used in agriculture and pest control. They have always been used to improve both the quality and the number of crops, but they have a collateral effect since they are poisonous and very dangerous for those who are in direct or indirect contact with them. A small amount in the human body is enough to cause cancer or other health diseases that eventually lead to death. Dichlorodiphenyltrichloroethane (DDT) is a pesticide that is very effective in mosquito control, but it is recalcitrant, and their metabolites are found in the blood of many people. Other pollutants were manufactured to be used in various industries, such as polychlorinated biphenyls (PCBs) and phthalates, which are plasticizers used in bottles, toys, and personal care products. PCBs are a large group of similarly structured compounds with variations in toxicity and persistence in the environment and the human body. Some forms are very similar to dioxin. Polybromated diphenyl esters (PBDEs) are fire retardants added to a wide variety of consumer items that seep into surrounding materials and can now be detected in many populations. Organic pollutants like PCBs and DDT are lipophilic; they are stored in fat cells and can be preserved for years. They are also found in dietary elements such as fish, meat, and dairy products [63]. Pharmaceutical drugs, which are produced to improve and even save the lives of human beings in some cases, become carriers of many diseases when there is an attempt to eliminate them without adequate treatment since they could directly affect vital organs like the kidney, liver, brain, and the central nervous system. There are also organic products used in multiple petrochemical industries for the synthesis of chemical products, where the by-products generated during the process pollute the environment. Some of these are released in a liquid state and others as volatile gases. These by-products are highly toxic, and their long-term exposure can cause death, so it is of utmost importance to eliminate them from aquatic effluents. These include aromatic hydrocarbons such as phenol, toluene, aromatic amines, etc. [64]

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In the interest of removing organic pollutants from water, there are several treatment methods, such as chemical precipitation, ultrafiltration, ion flotation, electrocoagulation, electrodialysis, sedimentation, adsorption, oxidation, distillation and membrane separation, to name a few. These methods have some disadvantages, as they are not sufficient for all types of contaminants when applied individually and many of them have a high operating cost, high waste generation, and low removal efficiency at low pollutant concentrations. The use of adsorption processes is one of the alternatives that has been investigated because of its effectiveness in the removal of contaminants at low concentrations, low cost, and ease of operation. The cost of this process can be reduced using adsorbents of organic origin such as algae, bacteria, fungi, and agro-industrial residues. These biomasses have some advantages such as low cost, large availability, and being eco-friendly [65]. To improve the adsorption efficiency of the pollutants in water, the adsorbents can be designed and modified in their compositions, structures, preparation methods, and, especially, their surfaces, where the sequestration of contaminants occurs to extract them from the effluents of contaminated water. These types of adsorbents can be modified in their physical, chemical, and biological properties to improve their adsorption capacity and directed so that they exhibit special characteristics for each reaction, transformation, and elimination of contaminants in the water. The capacity and efficiency of adsorption technologies in water treatment also depend on the characteristics and functions of the adsorbents [66]. Nano-technological adsorbents are what scientists are studying today because they provide a fairly effective tool for removing contaminants from water. As the size of any material decreases, its surface area increases and therefore provides more active sites that will ultimately improve the adsorption capacity. The other advantage of nanomaterials is their capacity for regeneration and reuse over several cycles. For this purpose, many nano-adsorbents such as carbon nanotubes, graphene oxide, and metal oxide nanoparticles, polymeric nanoparticles such as chitosan, and cellulose nanoparticles have been used. The adsorption capacities for organic dyes, aromatics, phenolic derivatives, pharmaceuticals, and antibiotics have been studied using nanoadsorbents [67]. Nanomaterials are the new proposals for the environment, but they can shortly become a serious problem if they are not eco-friendly. Some of them are not biodegradable and enter the human body [68]. There are no nanomaterials that are completely safe and nontoxic today. The probability of them causing environmental contamination arises when they are released when managed during the synthesis, application, and elimination processes. The systems that are most likely to be affected are all water, soil, and air, through which nanomaterials can enter the body of humans and other animals. Additionally, plants can also absorb nanomaterials from water and soil and accumulate them in their edible parts, affecting those who ingest them later on. Therefore, eco-friendly nanomaterials are the key to a bright future in water treatment technologies and can be prepared with their environmental sustainability and biodegradability in mind. There is a need to develop more efficient, selective, economic, and ecological nanomaterials [2].

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Removal of Emerging Pollutants from Wastewater by Nanomaterials The current demographic and industrial development has caused an exponential increase in the contamination of water resources, which motivates the development of processes and technologies that allow moving the physical, chemical, and biological parameters of wastewater back to their natural state. Among the toxic compounds that are discharged into the water environment, the so-called emerging pollutants have stood out in recent years. These are compounds that are not yet regulated, and because of their chemical nature, they can create adverse effects on the environment. Drugs fall within this category since they are designed to interact directly with living beings, and their presence in the environment can lead to generating resistance to elimination by microorganisms, and various health issues in humans, besides, many countries do not have regulations to limit their discharge into aquifers [69]. Table 1 summarizes the concentrations reported for various drugs according to their clinical category. Several studies have shown the permanence of drugs in wastewater even after going through conventional treatment. Wang et al. [70] pointed out that the type of water, the treatment, and the drug influence the removal rate since some molecules can achieve high removal rates while others remain persistent. Tertiary processes have been developed in response to removing emerging pollutants from water. Among these treatments, various physicochemical processes stand out, such as ion exchange, reverse osmosis, electrodialysis, adsorption, and some others [71].

Table 1 Concentrations of drugs in surface water and effluents from wastewater [85] Drug classification Antibiotics

Antiphlogistics and analgesics

Antihistamines

Beta-blockers

Antacids

Compound Doxycycline Chlortetracycline Sulfamethizole Tylosin Diclofenac Acetylsalicylic acid Paracetamol Morphine Antipyrine Ibuprofen Chlorpheniramine Fexofenadine Hydroxyzine Loratadine Atenolol Metoprolol Nadolol Propranolol Ranitidine Cimetidine

Concentration (ng/L) 1.0 690.0 130.0 280.0 1200.0 340.0 3.6 2.8 58 30.0 0.94 4.0 1.5 1.4 2.7 14.0 16.0 13.0–590.0 10.0–38.0 580.0

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Various studies have shown that adsorption is an efficient process in the removal of contaminants, especially pharmaceutical compounds. This has favored the evaluation of a great diversity of materials for the removal of pharmaceutical contaminants, highlighting the use of laminar materials such as clays and zeolites [11]; polymeric materials [72]; and, predominantly, activated carbon [73–75]. However, in recent years, a wide variety of nanostructured materials have also been applied to the removal of drugs present in the aqueous phase, most notably metal oxides [76], carbon nanotubes [77], and shell organometallic compounds (MOFs) [78, 79]. The physicochemical mechanisms involved in drug removal are diverse and depend on intrinsic factors such as the nature of the adsorbent; the structure and the conformation of the drug; and extrinsic factors such as the temperature and pH of the medium. Since most drugs have aromatic structures in their molecular arrangement, one of the most common interactions in removal when using activated carbon is the π- π* interaction between the adsorbate and adsorbent. However, these interactions can be affected by the presence of functional groups, such as halogenated, hydroxyl, alcoholic, and carboxylic groups, both on the surface of the carbonaceous material and in the molecular structure of the drug; some of these groups promote the over-activation of the electronic cloud or its deactivation; this is due to the ability to donate or accept electrons from some groups. The former is considered to promote the adsorption process, while deactivating groups will decrease the efficiency of the process. Because functional oxygen groups are generally deactivating, they remove electrons from the π band on the surface of activated carbon, causing an increase in their concentration and triggering a decrease in the adsorption capacity of the carbonaceous material [80]. On the other hand, some drugs can form metal-binder interactions, a property that has been exploited for their removal using metal oxides [81]. Other pharmaceutical compounds are hydrophobic, which has favored the use of polymeric resins that promote aliphatic interactions between adsorbate-adsorbent [72]. However, the mechanism that is always present in any adsorption process is the electrostatic interaction, which is considered to be weak and depends significantly on the conditions of pH and alkalinity of the medium. One of the advantages of using nanostructured materials in adsorption processes is that the mechanisms that govern drug adsorption can be predefined. For example, you can define the doping and dispersion percentages of a heteroatom of interest, or the molecular arrangement that will result in a specific molecular structure, and the textural properties that are suitable for the adsorption of the analyte. Nevertheless, one of the biggest drawbacks is the application on a larger scale; in this regard, activated carbon has a great advantage, but new advances arise every day bringing the possibility of using these materials closer to reality. Among the new materials that have been used to remove pharmaceutical compounds are also some biomasses such as algae and fungus residues and some others. The results have revealed that the use of this type of material is another alternative for the treatment of affected effluents. The adsorption capacity of these materials is attributable to the characteristic surface groups they present, which are generally

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identified through an FTIR analysis. Among these groups, the alcohol, carboxylic, and amino type emerge. One of the reported removal mechanisms by these materials is chemisorption, consisting of the formation of high-energy adsorbate-adsorbent interactions, which could lead to complications in the regeneration and reuse of these biomasses for the adsorption process. On the other hand, one of the main disadvantages in the use of these materials is the possibility that organic substances present in their structure (flavonoids, pectins, etc.) leach into the aqueous medium, causing an increase in the chemical oxygen demand in water as they would contribute with discharges of organic matter to the environment. In this regard, some researchers have used extraction methods to avoid this complication, obtaining prominent results, and in some cases, the biosorbent has been reused up to four times [82–84].

Mathematical Analysis of the Adsorption Balance The efficiency of the materials used in the adsorption of contaminants is evaluated, as a general rule, through the analysis of the adsorption balance. As a general principle, the adsorption isotherms are used and developed at different operating conditions such as pH, temperature, ionic strength, or presence of interferers; in this way, it is possible to know the response of the materials in the removal of a substance of interest. To build the isotherms, it is necessary to calculate the equilibrium adsorption capacity from a mass balance, as illustrated in Eq. 1: q¼

V ðC
Ce Þ m 0

ð1Þ

where q is the mass of the pollutant adsorbed per unit mass of adsorbent, generally expressed in mg g
1; C0 and Ce are the initial concentration and at the equilibrium of the pollutants, in mg L
1, respectively; m is the mass of the adsorbent in grams; and V is the volume of the experimental solution, in L. These experimental data are interpreted by the various mathematical models of the adsorption isotherms.

Models of Adsorption Isotherms The adsorption isotherm is the mathematical relationship between the mass of the adsorbed solute per unit mass of the adsorbent and the concentration of the solute in the solution when equilibrium has been reached. More than 40 mathematical models of adsorption isotherms have been developed to represent the adsorption balance in solid-liquid systems; however, the most widely used isotherm models are Freundlich, Langmuir, and Prausnitz-Radke; we will briefly analyze them below. The Freundlich isotherm model was first proposed empirically, and subsequently theoretically, considering that the adsorbent surface is energetically heterogeneous; in other words, the adsorbent surface has different types of active sites. The mathematical representation of this isotherm is [86]:

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q ¼ k f C =n

ð2Þ

where kf is an adjustment constant related to the adsorption capacity, expressed in mg1
1/n L1/n g
1, while n is related to the adsorption intensity. On the other hand, Langmuir’s model arises from the analysis of the dynamic equilibrium where the adsorption rate of the molecules is equal to the desorption rate of the adsorbed molecules. The model is expressed mathematically as follows: q¼

qm K L C 1 þ KLC

ð3Þ

where qm represents the maximum adsorption capacity of the material, in mg g
1, and KL is a constant associated with the thermodynamic equilibrium of the process, expressed in L mg
1. Finally, the Prausnitz-Radke model combines the characteristics of the Langmuir and Freundlich isotherms. The equation that represents this model is expressed below [87]: q¼

aC 1 þ bCβ

ð4Þ

where a and b are expressed in L g
1 and Lβ mg
β, respectively. One of the statistical tools to select the ideal model to represent the equilibrium of the process is the average deviation percentage, which is estimated using the following equation: %D ¼

! N  1 X qexp
qcal   100% N i¼1  qexp 

ð5Þ

Once the ideal mathematical model for our case study is known, it is possible to make estimates of the amount of adsorbent that is necessary to treat effluent or to estimate the final concentration of the effluent under specific conditions. Figure 4 shows as an example the adsorption isotherms of an analyte model at two pH values. Point A coincides with the initial condition of the process (C0, q ¼ 0). The process runs on the adsorption operation line until equilibrium is reached, represented by point B (Ce, qe). It is always convenient to use materials with a high adsorption capacity; however, availability to reuse the adsorbent materials is necessary to develop a more environmentally friendly process; hence, it is necessary to measure the reversibility of the adsorption process.

Reversibility of the Adsorption Process One of the most important analysis in the reuse of adsorbents is the reversibility of the adsorption process, for which it is necessary to carry out desorption experiments, consisting of placing the spent adsorbent in a solution free of the

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Fig. 4 Representation of the adsorption balance through isotherms. The solid curves represent the predictions of the Redlich-Peterson model. The dashed lines describe the operating lines of the adsorption and desorption processes

contaminant at controlled conditions, to promote the material to reach a new balance. If the process is completely reversible, the desorption balance should correspond with the adsorption isotherm at the conditions of the desorption solution. The mass of the compound that was not desorbed can be estimated from the following mass balance [88]: qd ¼

q0 m
V F CF m

ð6Þ

where q0 and qd represent the mass of the adsorbed pollutant before starting the desorption process and once the new equilibrium state is reached after desorption, respectively, both in mg g
1; VF is the volume of desorption, in L, and CF is the concentration of the contaminant in the new equilibrium, in mg L
1. The line of operation of the desorption process is illustrated in Fig. 4. Point C denotes the initial condition (C ¼ 0, q0) if we consider that adsorption is carried out at pH 7 and desorption at pH 4. The new equilibrium should be completely reversible at point E, which coincides with the isotherm at the new operating condition; however, sometimes the desorption is partial, leaving the new equilibrium at an intermediate point, as denoted by letter D. To quantify the scope of this process, it is possible to estimate a percentage of desorption, using the following equation:

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%Desorption ¼

q0
qd  100% q0
qd,rev

ð7Þ

where qd,rev represents the adsorbed amount on the adsorbent if the process was reversible. This value can be estimated from the mass balance (Eq. 1) and the model of the adjustment isotherm of the experimental data of the adsorption balance. In some cases, it may happen that point F presented in Fig. 4 is reached after the desorption process; in these cases, the adsorbent has regenerated since the amount adsorbed is equal to zero. The previously described mathematical models have been designed for gas-solid systems; therefore, the direct interpretation of the model parameters often does not correctly describe the phenomenon. This has led various authors to develop mathematical models that are more attached to phenomenological reality [89]. For the correct use of these models, a deep mechanistic analysis of the removal process must be carried out in order to achieve correct mathematical modeling.

Conclusions and Further Outlook In summary, the present review described the mechanism of elimination of nanoparticles and nanocomposites for the decontamination of wastewater. For ionic species, adsorption capacities were strongly dependent on pH. Green synthesis of nanoparticle adsorbents was achieved through the use of plant extracts, which provide several benefits such as reduction and stability and less cytotoxicity, so these nanomaterials can be considered as ecological adsorbents for environmental applications. However, there are some drawbacks in the use of nanoparticle adsorbents for wastewater remediation, including the low mechanical resistance, the difficult separation, and the high cost that the use of nano-adsorbents may represent. In this sense, the studies confirmed that the inclusion of nano-magnetic particles can help overcome these limitations by providing an easy separation through the use of a magnetic field. Furthermore, from the bibliographic survey, it has been established that nano-composites can be used successfully for the removal of contaminants from wastewater because of the synergistic effect of their components. On the other hand, given the detection of more and more drugs in the aqueous medium, the application of friendly and easy-to-operate tertiary processes such as adsorption acquires significant relevance. For this reason, the development of effective adsorbent materials becomes highly important. To achieve this, it is necessary to rely on new technological developments in the synthesis and characterization of materials. The development of nanostructured adsorbents, such as carbon nanotubes and MOFs for the treatment of water for human consumption, has proven to be a competitive option on a laboratory scale, given the type of interactions that will occur between the adsorbate, whether it is organic or inorganic, and the adsorbent. Nevertheless, it is necessary to scale the production and application of this type of material to a real level. All this must also be done in compliance with the principles of environmental sustainability, and although great progress has

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been made in recent years, more research is needed to improve the performance of nano-adsorbents for wastewater remediation, including their regeneration and reuse, as well as environmental impact and cytotoxicity assessment.

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Charles Oluwaseun Adetunji, Oseni Kadiri, Saher Islam, Wilson Nwankwo, Devarajan Thangadurai, Osikemekha Anthony Anani, Samuel Makinde, Jeyabalan Sangeetha, and Juliana Bunmi Adetunji Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofertilizers: Production and Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micronutrient Nanofertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Macronutrient Nanofertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanobiofertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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C. O. Adetunji Applied Microbiology, Biotechnology and Nanotechnology Laboratory, Department of Microbiology, Edo University Iyamho, Auchi, Edo State, Nigeria O. Kadiri Department of Biochemistry, Faculty of Basic Medical Sciences, Edo University Iyamho, Auchi, Edo State, Nigeria S. Islam (*) Institute of Biochemistry and Biotechnology, Faculty of Biosciences, University of Veterinary and Animal Sciences, Lahore, Pakistan W. Nwankwo · S. Makinde Informatics and CyberPhysical Systems Laboratory, Department of Computer Science, Edo University Iyamho, Auchi, Edo State, Nigeria D. Thangadurai Department of Botany, Karnatak University, Dharwad, Karnataka, India O. A. Anani Laboratory of Ecotoxicology and Forensic Biology, Department of Biological Science, Faculty of Science, Edo University, Iyamho, Edo State, Nigeria J. Sangeetha Department of Environmental Science, Central University of Kerala, Periye, Kasaragod, India J. B. Adetunji Nutrition and Toxicological Research Laboratory, Department of Biochemistry Sciences, Osun State University, Osogbo, Nigeria © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_45

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Limitation of Nanofertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials: Uptake and Translocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles Transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle Accumulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The nanosize of the fertilizer which confers a high volume to surface area ensures their effective action over conventional fertilizer. This fertilizer improves the efficient use of nutrient, thereby leading to highly productive agriculture. As a result, nanotechnology is expected to ensure the sustainability of the agriculture sector. The nanoforms of fertilizer are better and readily absorbed by plant when compared to ordinary fertilizer. Nanotechnology is a promising domain that can be explored in light of concerns for waste management. With the growing human population globally, there has been an increasing need for food preservative due to global decline in food production occasioned by drought and man-made disaster. Nanotechnology evolution has made a lot of impact and contribution to food industry for food preservation, processing, and packaging. Keywords

Nanoparticles · Nanomaterials · Agrifood applications · Nanofertilizers · Nanosensors · Nanocomposites

Introduction Research related to nanotechnology involves the development and use of nanomaterials of various compounds. Nanomaterials can be prepared by various biological and biomimetic origins, with the help of varied techniques like enzymes-based nanobiosensors; silver, gold, and titanium can aid in the accurate and quick diagnosis of spotting residues of pesticides in the agroecosystem. The utilization of nanopesticides can be used to boost the opposition of pesticides by plant, reduce pesticide noxiousness, increase the pesticides’ shelf life, and improve pesticides’ solubility. Nanofertilizer improves the efficient use of nutrient, thereby leading to highly productive agriculture [1, 2]. As a result, this technology is expected to ensure the sustainability of the agriculture sector [3, 4]. It is forecasted that the planet Earth would be populated with 9 billion people by 2050, which implies that in the next few years, there is likely going to be either a food crisis or excessive food production efforts and activities with resultant volumes of agricultural wastes. Nanotechnology is a promising domain that can be explored in light of concerns for waste management also. Nanotechnology is one of the potential remedies for management of the anticipated massive agricultural waste [5–8]. In recent times, the European Union has realized the capacity of nanotechnology as a prospective answer to management of agricultural wastes. Nanotechnology has attained an extraordinary limit in science

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and technology (bioremediation technology), as it has brought about a paradigm shift in the traditional method employed in the remediation of pollutants. The usefulness of nanotechnology could be regarded as novel way of relieving existing age-long resonant burdens. The field of nanotechnology has continually proven to be vital in the advancement of research in food science. In food production/processing, nanomaterial has been utilized to improve food quality and preservation. From shelf life improvement to contaminant tracing, smart packaging, and the incorporation of antibacterial or health supplements, nanotechnology has indeed made a giant stride in the food industry [9]. Some forms of the nanosystems which have found usefulness in food packaging, processing, and preservations include nanofibers, nanocapsules, and solid nanoparticles [10]. This chapter intends to highlight the potential applications of nanomaterials in agriculture and food industry along with their associated risks for an environment (Table 1 and Figs. 1 and 2).

Nanofertilizers The development and application of nanofertilizers is advanced and promising research field in the agricultural sector in recent times. The nanosize of the fertilizer which confers a high volume to surface area ensures their effective action over conventional fertilizer. There have been the developments of several types of nanofertilizers such as iron, silver, zinc, carbon nanotubes, silica, and molybdenum and their application in various crop systems. The nanoforms of fertilizer are better and readily absorbed by plant when compared to ordinary fertilizer [14]. Nanofertilizers due to its nanoform are able to move faster due to its higher entropy as a result of its colloid suspension state. Several studies have shown nanomaterials have a positive impact on plant biomass, root elongation, chlorophyll II content, and the germination of seeds at certain concentrations [15, 16]. This entropy energy was observed to be directly proportional to Gibbs energy, thereby facilitating quicker movement of the colloidal state of the nanofertilizer and invariably improving its penetration to plants’ cell membrane. In a study by Kottegoda et al. [17], efficient delivery of urea release using ureahydroxyapatite was reported. There was reported control of the release of urea for up to a week. In another study by Abdel-Aziz et al. [18], a smart nanofertilizer made from nanochitosan-nitrogen, phosphorus, and potassium (NPK) and its effect on wheat growth was investigated. This fertilizer was referred to as the nanochitosan-NPK fertilizers. The significant yield of wheat and reduction in the life cycle by this plant by 23.5% was estimated in the study. Previous studies by Ma et al. [19] have reported improvement in photosynthesis when the nano-NPK fertilizer was applied to plant. They highlighted the role this fertilizer plays in enhancing water and nutrient absorption. In other studies, plants’ exposure to nano-NPK fertilizer was reported to increase plasma membrane permeability as well as cell mortality [20, 21]. The applications of zinc oxide nanoparticles as plant fertilizer have been reported to show both beneficial and detrimental effect on plant growth, and this is dependent

Hydroponics

As fertilizers

Cyamopsis tetragonoloba

Cicer arietinum, Spinacia oleracea, Daucus carota, Brassica juncea, and Sesamum indicum Spinacia oleracea

Nicotiana tabacum

Arachis hypogaea, Zea mays

Spinacia oleracea

ZnO

FeS2

ZnO

Fe/SiO2

TiO2

CuO

Triticum aestivum

ZnO

Seed priming and foliar application

0.25% suspension

15 mg/kg

48 h and 35 days

3 days

21 days

0.2 μM and 1 μM

12–14 h

60 days

80–100 μg/mL

Growth cycle 6 weeks

Mixed with soils 200 mg/kg

Seed priming

Mixed with 20 mg/L growth substrate Foliar spray 10 mg/L

Foliar spray

45 days

Coffea arabica

ZnO

10 mg/L

Overnight

50 μg/mL

Triticum aestivum, Zea mays, Arachis hypogaea, Allium sativum

MWCNTs

Seed priming

Duration of treatments 24 h

Concentrations used 100 μg/mL

Mode of Nanomaterials Crop species application MWCNTs Hordeum vulgare, Glycine max, Zea mays Seed priming

Table 1 Effect of nanomaterials on crop physiology and plant protectiona

Improved photosynthesis and biomass production Positively affected growth physiology, increased metabolites, enzymatic activities, and anatomical properties of plants Enhanced plant growth and biomass accumulation Increased biomass accumulation, chlorophyll, nitrogen, and protein content

Responses Enhanced germination and growth of seedlings Improved and rapid germination, increased biomass accumulation and water absorption potential of seeds Enhanced growth, biomass accumulation, and net photosynthesis Increased grain yield and biomass accumulation Improved plant growth, biomass accumulation, and nutrient content Increased germination and crop yield

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a

Foliar application

Fruit spray Foliar application Foliar application

Oryza sativa

Prunus domestica fruits

Solanum lycopersicum

Vigna unguiculata

Solanum lycopersicum

Solanum lycopersicum

SiO2 NPs

ZnO, CuO, and Ag NPs Al2O3 NPs

Ag NPs

CuO

MgO

Trifoliate stage 40 days

7 days

50–100 μg/mL

7–10 μg/mL

7 days

150–340 μg/mL 11 days

20 days

4 days

70 days

100 and 1000 μg/mL 400 mg/L

2.5 mM/L

20 and 30 mg/L 55 days

50 mg/L and 75 mg/L 50 mg/L

Reproduced from Shang et al. [11] with slight modification, Creative Commons Attribution 4.0 International

Foliar application Drenching

Foliar application

Vigna sinensis

Ag NPs

Mixed with pot soils Foliar application

TiO2 and SiO2 Oryza sativa

Triticum aestivum

AgNPs

Improved growth and tolerance to heat stress Enhanced growth and biomass by stimulating root nodulation and soil bacterial diversity Mitigated Cd toxicity and improved growth by stimulating antioxidant potential and inhibiting Cd translocation Alleviated heavy metal toxicity and improved growth by decreasing bio-concentration and translocation in plants Suppressed gray mold symptoms caused by B. cinerea and soil-borne diseases Successfully controlled Fusarium root rot in tomato Showed no phytotoxicity, but could inhibit growth of Xanthomonas axonopodis pv. malvacearum and Xanthomonas campestris pv. campestris in vitro Effectively controlled late blight disease caused by Phytophthora infestans Controlled bacterial wilt disease by suppressing pathogen Ralstonia solanacearum

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Fig. 1 The impact of nanomaterials in agriculture, food, and environment. (Copyright © Springer Nature 2018, Reproduced with permission from Kaphle et al. [12])

on metal oxide concentration. In a recent study by Xu et al. [22], biomass and photosynthesis were reported to be enhanced when a concentration of 10 mg/kg zinc oxide nanoparticles was used. DiMario et al. [23] earlier reported the positive effect of zinc oxide nanoparticles through its role of improving the supply of carbon dioxide at the carboxylation site in chloroplast which enhances the enzymatic activities of carbonic anhydrase, an enzyme which facilitates photosynthesis. Previous studies have reported the beneficial effects of zinc nanofertilizer on plant growth such as oxidative stress reduction, promoted germination, induced antioxidative compound production, and increased chlorophyll content in different crops such as Cicer arietinum, Lupinus termis, Arachis hypogaea, Cyamopsis tetragonoloba, Solanum lycopersicum, Helianthus annuus, and Glycine max at different zinc oxide concentration [24–29]. Detailed information on the effects of zinc nanoparticles on the plant has been reviewed by Sturikova et al. [30].

Nanofertilizers: Production and Use Nanomaterials or nanoparticles used for fertilizer production can be synthesized using different approach which includes physical, chemical, or biological processes. Physical method is based on size reduction to nanoscales from the bulk materials. It is majorly based on the milling of the materials. Nanosize control and sustaining a higher purity index is a major limitation of this approach. The chemical approach begins at the molecular or atomic scale with build-up particles to the nanoscales using chemical reactions. This method has the advantage of better control over particle size and purities as it is a chemically synthetic process which is well controlled [31, 32]. Aside from these two processes of nanoparticle synthesis, synthesis can also be carried out through a synthetic biosynthesis process. Fungi, plants, and bacteria are some natural sources which can be used. This process has also reported for having the advantage of greater control over toxicity and particle size of the final product [33, 34] (Fig. 3).

Fig. 2 Applications of nanotechnology in agrifood systems. (Copyright © The Royal Society of Chemistry 2016, reproduced with permission from Rodrigues et al. [13])

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Fig. 3 Green synthesis of nanosized materials. (Reproduced from Thakur et al. [35], Creative Commons Attribution International)

Raliya et al. [36] and Singh and Rattanpal [31] noted that the chemical approach for nanosynthesis has better advantage over either the physical or the biological approach. Control in physiochemical properties resulting in target-specific nanoformulation which is homogenous in most instances was attributed for this effective approach used for nanofertilizer production. Scientists are under persistent pressure to develop technologies which not only guarantee unique technologies in crop productions but also fit the economic bills of the growers and the production industry [37]. Nanoscience, through the development of nanofertilizer, can offer lasting solution to these challenges. In a nanofertilizer formulation, the active ingredients are released at a pace which is in accordance with the plant needs as it grows. This fertilizer improves the efficient use of nutrient, thereby leading to highly productive agriculture. As a result, this technology is expected to ensure the sustainability of the agriculture sector [3, 4]. These fertilizers are required in minute quantity when compared to the conventional fertilizer which is required in large quantities, thereby saving the soil from nutrient pollution [33]. Solanki et al. [38] and Liu and Lal [1] reported the use of different elements such as nitrogen, phosphorus, potassium, iron, zinc, molybdenum, copper, manganese, and carbon nanotubes for the development of various nanofertilizers with effective nutrient delivery rates. As reported previously, nanoparticles are made from organic and inorganic nanomaterials. The inorganic nanomaterials include oxides of metal such as magnesium oxide, zinc oxide, silver oxide, titanium dioxide, and others. Likewise, organic nanomaterials include polymers, lipids, and carbon nanotubes. Nanofertilizer is fundamentally classified on the basis of its nutrient. There is the (i) micronutrient nanofertilizers and (ii) macronutrient nanofertilizer. Nanobiofertilizer is an emerging field of research [1, 2].

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Micronutrient Nanofertilizers Elements required by the plant in trace or low quantities are essential for the maintenance of vital metabolic processes in plants [39]. In a study by Khot et al. [3], the effect of foliar application of zinc and boron nanofertilizers was investigated. They reported that low amounts of boron or zinc nanofertilizers increased fruit yield by 30% in pomegranate trees. In a study by Mattiello et al. [40], fertilizer made from zinc nanomaterials increases shoot growth and fruit yield when compared with those in zinc sulfate commercial fertilizer. Crop production was reported to be enhanced by 38% when zinc nanoparticles as nanofertilizer were applied in pearl millet. Other crops where zinc nanoparticles act as a nutrient source improving yield include potato, wheat, rice, maize, sunflower, and sugarcane [41]. Chlorophyll II contents and growth rate improved in Brassica napus exposed to stabilize maghemite nanoparticles [40]. In a study to assess the effect of nanoparticles made from zinc, manganese, iron, and copper oxide on lettuce (Lactuca sativa) seed germination, manganese oxide and iron oxide nanoparticles were reported to stimulate lettuce seedling growth from 12% to 54% [42].

Macronutrient Nanofertilizers Chhipa [43] and Ditta and Arshad [44] reported the formulation of nanofertilizer from the infusion of macronutrients with nanomaterials. Macronutrients used for this formulation included nitrogen, phosphorus, potassium, magnesium, sulfur, and calcium and were in encapsulated form. There is a projected increase in NPK use in the agricultural sector in 2020 [45]. This increase is expected to be in the range of 265 million tons, thereby making the development of this form of fertilizer a priority over conventional fertilizer. Urea-modified hydroxyapatite, mesoporous silica, and zeolites have been reported as a slow or controlled release fertilizer with promising outcomes [1, 41]. A biosafe fertilizer developed from a nanostructured waterphosphorite suspension and source of phosphorus was previously reported by Patra et al. [46]. Increase in fresh fruit yield and production quality was also observed in the study.

Nanobiofertilizers Biofertilizers are formulations containing living organism for the purpose of fixing atmospheric nitrogen, synthesis of growth-promoting substances, and enhancing soil productivity. Nanofertilizers are fertilizers consisting of biofertilizers with nanoparticles. The sole aim is to improve plant growth [47]. Delivery, shelf life, and interaction between microorganisms and nanoparticles are vital factors to be considered in the development of nanobiofertilizers. The interaction between rhizobacteria and gold nanoparticles exerted positive effect in study by Malusá et al. [48] and Shukla et al. [49]. However, silver nanoparticles are not suitable for

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use with biofertilizer as reported by Duhan et al. [50]. They reported silver nanoparticles to result in harmful effect on microorganism’s biological process such as alteration to cell functions. Use of nanoparticles can help extend the shelf life and stability of biofertilizer [51]. Nanoformulation can also help to improve biofertilizer stability to UV inactivation, and heat [51]. Simarmata et al. [47] and Duhan et al. [50] reported the use of polymeric nanoparticle in the development of formulations resistant to desiccation. Though nanobiofertilizers can resolve the challenges posed by biofertilizers, this technology still requires studies and development.

Limitation of Nanofertilizers Nanofertilizers have undoubtedly been used for sustainable agriculture and enhanced crop productivity. However, this technology can result in unfavorable outcomes on agricultural activities and the ecosystem. Unintended health and environmental safety issues are some vital limitations to the use of this technology in agricultural crop productivity. Toxicity of nanomaterials to plant and the way different plants respond to nanomaterials of different types in a dose-dependent manner is another issue [50]. Variability and reactivity of nanomaterials are major sources of concerns to farmworkers as xenobiotic exposure can be of safety concerns to farmworkers. Identification of hazard factors and early risk assessments are critical for establishing priorities for toxicological research. FeregrinoPerez et al. [2] highlight the limitation of nanomaterials due to limitations associated with toxicity and bioavailability. This is of interest as previous studies have reported the phytotoxic effect of nanoparticles, accumulation and transformation (phytotoxicity) of nanoparticles in plants, dose and application method, and types of nanoparticles (shape, size, composition, surface properties) [52]. Determination of the degree of nanoparticle toxicity in plant is important for better understanding of how nanofertilizers are utilized by plants. Therefore, before market implementation of nanofertilizers, it is important to consider the advantage as well as its limitation.

Nanomaterials: Uptake and Translocation Nanofertilizers are absorbed through the roots or leaves of crops. It can penetrate through the root endodermis and epidermis and be transported to plant aerial part via the xylem vessels. Likewise, nanoparticles are transported to the other part of the plants through the phloem after its absorption by the leaf stomata. In either case, the pore size might range from 3 to 8 nm for nanoparticle penetration through cell walls to the cell plasma membrane. Nanoparticles or its aggregates higher than 8 nm cannot get into cells [53]. Zhang et al. [54] in their study observed that gold nanoparticles with size greater than 3.5 nm were not absorbed in tomato roots. Some other studies have also shown that cerium(IV) oxide and silver nanoparticles are absorbed and distributed in cucumber leaves and the lettuce plants [55, 56]. Plant physiology and diverse other mechanisms help determine nanoparticles uptake or

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Fig. 4 Applications of polymeric nanocarriers in agricultural sector. (Copyright © The Royal Society of Chemistry 2016, reproduced with permission from Shakiba et al. [60])

translocation [57]. However, in certain instance, plant activates its defense response against nanoparticles. Accumulation of metals ions and nanometal oxides occurred in carrot exposed to metal oxide nanoparticles of cerium(IV) oxide, zinc oxide, and copper oxide [58]. Such uptake and accumulation are major health risk to the health of humans [58]. Metal oxide nanoparticles were observed to be accumulated on the outer layer of carrot, while the metal ions were detected on the fleshy edible part and were of higher toxicity to human health. López-Moreno et al. [58] suggested this outer layer to restrict the penetration of nanoparticles in the edible tissues [59]. López-Moreno et al. [58] suggested reduction in this toxicity by peeling the outer root layer or vegetable (Fig. 4).

Nanoparticles Transformation Nanomaterials are highly reactive which interact with numerous components in the environment and are prone to changes and transformation of its physicochemical properties [61]. Several soil organic and inorganic substances interact with nanomaterial altering its fate, behavior, and toxicity. Nanofertilizer exposes root in rhizosphere to nanoparticles which greatly influences its heavy metal behavior and toxicity. Parsons et al. [62] demonstrated biotransformation process with mesquite plants treated with nickel(II) hydroxide nanoparticles. The plant roots were observed to have fragments of nickel(II) hydroxide, while its shoots and leaves contained nickel nanoparticles. Biotransformation has also been reported using cerium(IV) oxide nanoparticles in cucumber plants. Results show cerium(IV) to cerium(III) in the roots and shoots [63]. However, studies by Ma et al. [64] disapprove this claim as there was no biotransformation of cerium(IV) oxide nanoparticles in cucumber roots. It was suggested that for biotransformation to occur, certain conditions are required

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in the plant rhizosphere. In a study by Ma et al. [65] on the environmental and biological assessment of silver nanoparticle on lettuce plants, silver nanoparticles were transformed thiol-containing molecules and silver-glutathione, respectively.

Nanoparticle Accumulations Accumulation of nanoparticles in food parts is among the issues involved in the use of nanofertilizers in agricultural practice. Nanoparticle type and size, plant species, and organ or tissue part used as food or for food processing are among the factors which determine the accumulation of nanoparticles. Nanoparticle accumulations can result in toxicity to humans and variability in the interactions between plants, and nanomaterial can affect the way this toxicity accumulates [66]. Growth inhibition, synthesis of reactive oxygen species, and cell death are some induced multi-walled carbon nanotube phytotoxicity reported in red spinach [67]. In another study, cerium(IV) oxide nanoparticles accumulation inhibited the nitrogen fixation of soybean [68]. In other studies, C60 (fullerene) application to tomato, zucchini, and soybean increased dichlorodiphenyldichloroethylene accumulation [69]. Nanofertilizers have significant roles to play in enhancing agricultural productivity. Their advantages over conventional fertilizers have been reviewed in this study. Therefore, the potential application of nanofertilizer in the agrifood sectors cannot be overlooked. Nanofertilizers have the sole advantage of reduced volatilization and leaching which are associated with conventional fertilizers. However, there is the challenge toxicity posed by nanofertilizers and its potential effect on human health. These present interesting and novel fields in nanotechnology toward attaining sustaining agriculture in the nearest future. Considering the potentials and advantages of nanofertilizer, there is need to work out strategies to deal with the health and environmental risk posed with the toxicity accumulated with nanoparticle accumulations (Figs. 5 and 6).

Fig. 5 Nanomaterial applications in crop protection. (Reproduced from Worrall et al. [70], Creative Commons Attribution 4.0 International)

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Fig. 6 Role of nanotechnology in sustainable agriculture. (Copyright © Elsevier 2017, reproduced with permission from Vishwakarma et al. [71])

Conclusions and Further Outlook In agricultural production, nanoparticles have been employed to significantly reduce agricultural costs, improve soil fertility, destroy/incapacitate pests, prevent diseases and attacks by pests on crops, prevent uncontrolled leakage of nutrients from the soil, and prevent postharvest wastage of crops, integrated as part of veterinary drugs for animal health management. In conclusion, the authors suggested that nanomaterials should be scrutinized via further research for effective working as well as their mechanism of action, in order to understand the effects and possible application in other areas of biotechnology. There has been increasing use of nanomaterials in the food industry over the past years. As their application in food packaging, nanoparticles have been majorly applied as vehicles for conveying nutritional as well as nutraceutical ingredients in food materials. There have also been promising results in the aspect

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of food preservation. Though the advent of nanotechnology is changing the way we process and preserve food, there are still persistent challenges and likewise opportunities which are positive and negative in some regards. Safety issues are one of these concerns, while their impact on the environment is another aspect to be considered. There is a need for adequately testing of food material incorporating this technology by relevance agencies before their release to the consumers.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eco-Friendly Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eco-Polymer-Based Textile Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eco-Friendly Polymer/Nanocarbon Nanocomposite-Based Textile Materials . . . . . . . . . . . . . . . . Technical Platform for Eco-Friendly Textile Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Military and Defense Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antimicrobial Relevance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-Healing Eco-Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Eco-friendly materials are the subject of recent research interest in various academia and industry fields. Eco-materials or eco-friendly materials are also termed as the green materials. These are environment-friendly materials, usually obtained from renewable resources. Exploitation of the eco-materials has been tremendously increased for the production of eco-textiles. Use of the eco-friendly polymers has become widespread owing to growing interest in biodegradability, sustainability, recyclability, and environmental concerns. Polymeric eco-materials have various potential uses in textile industry. In addition to the eco-friendly polymers, the nanocomposite materials based on eco-polymers and nanofillers have been prepared. Recently, the polymer-based nanocomposite materials have also been focused for eco-textiles. This chapter deals with various essential aspects of the eco-friendly textile-related nanomaterials. The nanocarbon and A. Kausar (*) Nanosciences Division, National Center for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_57

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inorganic nanoparticle-based nanofillers have been employed in the polymeric nanocomposite for the textile purposes. The nanocarbons including carbon nanotube, graphene, and carbon black have been fixated for textiles. Similarly, the inorganic nanoparticles such as metal nanoparticles, metal oxides, and inorganic nanofillers have been used in eco-friendly textiles. The polymer/nanocarbon nanocomposite-based eco-friendly textiles have shown electrical conductivity, strength, strain, toughness, chemical stability, flame resistance, antibacterial, and other useful properties. The polymer-based nanomaterials have countless opportunities as textile constituents. Eco-friendly nanocomposite-based textile materials have found applications in the military and defense, electronic textiles, antibacterial, and self-healing materials. The chapter also throws light on future prospects of eco-friendly textile nanomaterials. The challenges and advancements in the application areas of the polymer/nanocarbon nanocomposite-derived ecofriendly textiles can be overcome by using the progressive nanoparticles, hybrid nanofillers in eco-polymers, advanced processing techniques, and processing conditions. Keywords

Eco-polymer · Nanocarbon · Nanocomposite · Defense · Military · Electronics · Antibacterial · Self-healing · Textile · Nonflammable · Strength

Introduction Nanotechnology has also modified the materials for the textile industries [1–3]. The existing cotton- and polyester-based textiles have been modified using the knowledge of nanotechnology [4–6]. The textile processing technologies have also been modified through the nanotechnological skills and advent. The nanotechnology has increased the light protection, antimicrobial, chemical stability, and strain resistance of the textile materials [7–9]. The multifunctional textiles emphasize the improvements in the processability and durability of these materials. In this regard, various techniques have been adopted to form textiles based on the nanocarbons. The research has increased the solicitations of the nanotech in the textile productions [10–12]. Both the natural and synthetic polymers have been used in the textiles such as starch, cellulose, protein, polyamides, polyesters, polyolefins, etc. The natural and synthetic polymers may be the biodegradable or non-biodegradable. The nanocarbons have been identified as very important type of the nanofillers for the polymers. Similarly, the nanocarbon-based polymeric materials have been used in the textile industry. The nanocarbon-derived eco-friendly nanocomposites have been used in the eco-textiles. The nanocarbon has been filled in the eco-polymer to form the eco-friendly nanocomposite textiles [13]. The nanofillers may also have unique eco-friendly characteristics for the eco-textiles. The applications of the eco-friendly textiles are wide-ranging from the military to biomedical fields. The eco-friendly textiles based on the polymer/nanocarbon nanocomposites have been employed in

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the electronics. Another important use of the polymer/nanocarbon eco-friendly textiles has been observed in the defense industry. Especially, the eco-friendly textiles have been used for the blast-free and nonflammable clothing. The ecofriendly textiles have also been employed for the antibacterial or antimicrobial application. The eco-friendly textiles have been applied in the self-healing materials. In this chapter, the fundamentals and potential forecasts of polymer/nanocarbonbased eco-friendly textiles are given here. Furthermore, the state-of-the-art progresses in the field of polymer/nanocarbon-based eco-friendly textiles have been discussed in detail. The polymer/nanocarbon-based eco-friendly textiles have gained the significant research interest owing to the brilliant properties and the technical importance. Future developments must focus on the structure-property relationships in the polymer/nanocarbon-based eco-friendly textiles for the advanced technical applications.

Eco-Friendly Polymers Eco-friendly polymer or eco-polymer is an environment sociable polymer [14, 15]. Eco-friendly polymers have been applied in the textiles, automobiles, construction, packaging, and medical sectors [16, 17]. The eco-friendly polymers have also been considered as the biopolymers obtained from the renewable resources. The derivation of the eco-friendly polymers from the renewable sources is given in Fig. 1. The eco-polymers have been acquired from the renewable assets such as the wood/plantbased biomass, fossils, and other biomaterials [18]. However, such polymers have also been attained from the petrochemical products. The eco-polymers are often deliberated as the biodegradable or decomposable plastics. The eco-polymers are least harmful to the environment when degraded. The eco-polymers can be broadly categorized as the non-biodegradable polymers and the biodegradable polymers, as given in Table 1. The non-biodegradable polymers include polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate), and other thermoplastics. The biodegradable polymers include several important materials such as poly(Ecaprolactone), poly(lactic acid), polyhydroxyalkanoates, and other biodegradable polymers. The cellulose-based biodegradable polymers have been employed in several applications [19–22]. Eco-polyesters were prepared through the biomass. Fig. 1 Eco-friendly polymers

2920 Table 1 Classification of polymers for eco-textiles

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Non-biodegradable polymer Polyethylene/bio-polyethylene Polypropylene Polystyrene Poly(ethylene terephthalate) Polyamide Derivatives of polyurethane Derivatives of polysaccharides Most of synthetic thermoplastics

Biodegradable polymer Poly(lactic acid) Poly(E-caprolactone) Polyhydroxyalkanoates Polysaccharides Proteins

The polyethylene glycol and biodegradable poly(ethylene terephthalate) have been produced from the biomass. Poly(lactic acid) is an important eco-polymer. It has been formed by the fermentation of glucose obtained from the sugar cane or the corn starch. The poly(lactic acid) can also be obtained from the plant oils. Furthermore, it is also formed in the microorganisms [23–26]. The new synthetic eco-friendly polymers should be developed and applied for the eco-textiles. The polymers of interest can be classified as the non-biodegradable polymers and biodegradable polymers. The synthetic polymers are mostly the non-biodegradable polymers. These polymers are lightweight with the durability properties. The nonbiodegradable synthetic polymers can be easily processed into various shapes and products. These polymers have range of applications in the biomedical fields such as tissue engineering, bio-implants, medical devices, drug delivery, etc. The novel non-biodegradable synthetic polymeric materials have substituted the traditional polymers. Polyethylene is a common type of non-biodegradable synthetic polymer. Polyethylene exists in two important forms as high-density polyethylene and low-density polyethylene. The high-density polyethylene has high molecular weight, and low-density polyethylene has low molecular weight. Another type of polyethylene is the ultrahigh-molecular-weight polyethylene. The Ziegler process was successfully used to form the ultrahigh-molecular-weight polyethylene. The ultrahigh-molecular-weight polyethylene has fine biocompatibility, stability, toughness, strength, etc. The high-density polyethylene and ultrahigh-molecular-weight polyethylene have been commonly employed in the bio-related materials. The nonbiodegradable high-density polyethylene has been used in the bio-implants. These bio-implants have revealed minimum inflammation in the human body. The nonbiodegradable high-density polyethylene has also been applied in the bone tissue scaffolds. Thus, the non-biodegradable synthetic polymers are well-suited for bioimplants. However, the research has focused the formation of biodegradable synthetic polyethylene. The biodegradable synthetic polyethylene is also useful for various biomedical-related applications. The polypropylene or polypropene is a nonpolar crystalline polymer. Polypropylene is from the group of the polyolefins. The polypropylene is a non-biodegradable synthetic polymers prepared through the chain growth polymerization. The polypropylene can be used similar to the polyethylene. However, the high-density polyethylene and ultrahigh-molecular-weight

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polyethylene have been more frequently used in the biomedical relevance, compared with the polypropylene. The polystyrene is also an important type of the nonbiodegradable polymer. It is a transparent plastic. The polystyrene is a noncytotoxic and nondegradable in cellular environs. The polystyrene can be effortlessly manufactured and can be easily surface functionalized. The polystyrene can form interactions with the in vivo systems. In the implants and tissues, the polystyrene has fine biocompatibility and biodispersal properties. The functional polystyrene has fine cell proliferation properties. The functional polystyrene has also tendency toward easy biodegradation. The polyester is an important category of non-biodegradable synthetic polymer. The polyesters also exist as biodegradable natural polymers. Usually, the ester bonding in polyesters is susceptible to hydrolysis and degradation. The aromatic polyesters are non-biodegradable and synthetic polymers. For example, the poly(ethylene terephthalate) is a well-known synthetic polymer. The poly(ethylene terephthalate) is a biocompatible polymers having fine mechanical properties. The fibers and structure made from the poly(ethylene terephthalate) are useful for making the heart valves and artificial blood vessels. The poly(ethylene terephthalate) is commonly used in the biomedical joint arthroplasties. Especially, the poly(ethylene terephthalate) has been used in the hip arthroplasties. However, the ultrahigh-molecular-weight polyethylene has shown better performance in the hip and knee joints. The polyamide in the aliphatic form is known as nylon. The nylon 6 and nylon 6,6 are commonly used forms of the aliphatic polyamides. The condensations of the diamines and the dicarboxylic acids have led to the formation of the nylons or the aromatic polyamides. The nylons can be the degradable polymers, while the aromatic polyamides are the non-biodegradable synthetic polymers. The nylons have been employed in the surgical sutures and bio-implants. The polyurethane is also a non-biodegradable synthetic polymer. It is usually formed through the condensation of the diisocyanates and diols. Nevertheless, the polyurethane is a biocompatible polymer. The synthetic polyurethanes can also exist in the biodegradable forms. The polyurethane with the aliphatic chains may form the degradable molecules. The polyurethanes have been applied for the bio-implants and the tissue engineering. The polyurethanes have low cost and hydrolytic constancy to be used in the human body. The polyurethane-based bio-implants and joints have good biocompatibility and fatigue resistance. The biodegradable polymers may easily decompose in the presence of certain bacteria, moisture, gases, etc. The biodegradable polymers are advantageous in terms of addressing the disposal hitches. The biodegradable polymers have been used in several fields of films, coatings, and useful structures. The petroleum-derived polymers have severe environmental problems. The biopolymers obtained from petroleum have same environmental issues. The biodegradable polymers having long life during the product functioning and then easy degradation are usually preferred. Such polymers are often used as the eco-polymers. The biodegradable polymers have been applied in the tissue engineering, bio-implants, and joints. The interactions of the biodegradable polymers with the body cells and the cellular components are essential to investigate the performance of the tissue engineering, bio-implants, and joints. In the applications such as the packaging, biodegradable

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polymers have been effectively applied for the environmental friendly relevance. The biodegradable polymers have also been useful in eco-friendly textiles. Such textiles have easy disposal, so these are environmental friendly. The chemical structure and the degradation mechanisms of the biodegradable polymers are indispensable to apprehend. The main benefit of the biodegradable polymers relative to the non-biodegradable polymers is the degradation of these materials. The biodegradable polymers may degrade naturally owing to the environmental benefits, compared with the traditional plastics. Among prominent biodegradable polymers, polyhydroxyalkanoates, poly(lactic acid), polysaccharides, poly(E-caprolactone), etc. have been studied. The poly(lactic acid) has been intensely used in the packaging industry. The poly(lactic acid) has several advantages over the traditional packaging non-biodegradable polymer-based packaging. Thus, polymers like poly (lactic acid) have contributed to lower down the environmental problems. The use of the biodegradable polymers and controlling their degradation path may lead to the regulation over the plastic waste disposal. Polysaccharides are important biopolymers obtained from the natural sources, for example, starch, chitin, chitosan, pectin, yeast, etc. Starch is the most important type of polysaccharide. This polymer is highly hydrophilic, so cannot be used in the moist environments. The polysaccharides have been widely applied in the food industry, drug delivery, and tissue engineering. Starch is suitable for the food industry. In addition, the starch-based biodegradable polymers have good processing and thermoplastic properties. Chitosan and chitin have been successfully applied in the drug delivery applications. Cellulose has been beneficially used in the form of the bio-composites. Polysaccharides usually have low toxicity and good biocompatibility and biodegradability properties. Moreover, these polymers have hydrophilicity, mechanical heftiness, optical properties, and thermal stability. Proteins are the condensation polymers formed from the amino acids. Proteins have high biocompatibility and biodegradability. The biodegradable protein group include collagens, keratin, fibroin, etc. Collagen is a very important biopolymer for the tissues and implants. Collagen fibers are the high strength materials for the tissue organization. Collagen is also useful for wound healing. From the microorganisms, several biodegradable polymers have been derived. The most commonly stated polymers in this regard are polyhydroxyalkanoate, polyhydroxybutyrate, and poly(hydroxybutyrate cohydroxyvalerate). Polyhydroxyalkanoates are usually gained from bacteria. Polyhydroxyalkanoates are the biodegradable polymers fashioned from the gram-negative and the gram-positive bacteria. Polyhydroxyalkanoates are usually stored in their cytoplasm. Polyhydroxyalkanoates have been applied in active tissues and other biomedical applications. Polyhydroxybutyrate and poly(hydroxybutyrate cohydroxyvalerate) are also the renowned polymers produced using microorganisms. The type and quantity of microorganisms, temperature, and pH may affect the production of the polyhydroxyalkanoate, polyhydroxybutyrate, and poly (hydroxybutyrate cohydroxyvalerate). Several biodegradable polymers have been adopted from the bio-derived monomers. The poly(lactic acid) and poly(trimethylene terephthalate) are the biodegradable polymers gained from the bioderived monomers. The lactic acid monomers have been derived from the several

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bio-sources. Poly(lactic acid) has been used to form the fibers through the melt processing and spinning. The fibers have been beneficially used in the eco-textiles to form the environmental friendly material.

Eco-Polymer-Based Textile Materials Textile industries have gained the attention of nanotechnological materials [27, 28]. Textile industries have used several synthetic polymers such as nylon, polyester, etc. and natural fibers including silk, wool, cotton, etc. [29]. Various chemical and physical methods have been applied to synthesize and modify the eco-polymers. Eco-polymers have been applied in expanded textile applications [30]. The ecopolymers such as cotton and silk having both the hydrophilic and hydrophobic properties have been preferred for textiles [31, 32]. The synthetic polymers and fibers need to be developed having such characteristics [33]. The utilization of ecopolymers and eco-fibers in eco-textiles has led to the developments in this field [34, 35]. In this regard, the chemical processing of eco-textiles has been focused [36, 37]. Incidentally, the lyocell has been used as eco-polymer and consequently as the ecotextiles. The cellulose, starch, cotton, silk, lyocell, and other eco-fibers need to be modified for the advanced textiles. Synthetic fibers are usually the nondegradable; however, the studies have been performed to improve the renewability and biodegradability of these materials. The textile industry may contribute huge amount of pollution to the environment. Accordingly, the sustainable eco-polymers are the most demanding materials in the textile industry. The use of the eco-polymers may reduce the emission of toxic gases and toxins to the environment during the textile processing. These pollutants are mainly released to the sea water or oceans. The textile pollutants in the water may destroy the aquatic life and the processes. Moreover, the burning of textile toxins may also release in the atmosphere. The biopolymers have been considered as the eco-polymers. The biopolymers such as proteins and polysaccharides are the sustainable and the renewable materials. The biopolymers are also termed as the biodegradable polymers or green polymers or eco-polymers. However, all the biodegradable polymers are not green polymers or biopolymers. These polymers can be easily obtained from the living species such as the animals or plants. Thus, the source materials for these polymers are environmentally friendly. The synthetic pathways for these polymers are also facile and ecological friendly. Consequently, the biomonomers can also be obtained from the natural sources to form the biopolymers. Among the biopolymers, the fibers are important type of natural materials. The natural fibers can be extracted from the natural sources adopting easy methods. The fibers can be extracted using the chemical or microorganisms. These fibers are essential source of the eco-textile materials. The chemical processing is involved at every step in the fabrication of the fiber, yarn, and fabrics. The natural fibers are usually biodegradable. The synthetic fibers have also been prepared using the eco-polymers. Both the biodegradable and non-biodegradable polymers have been used to form the synthetic fibers. Like the synthetic polymers, the synthetic fibers may be non-biodegradable.

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The use of biodegradable natural or synthetic fibers are eco-friendly both in terms of the production and environmental degradation. Such fibers may disintegrate naturally by the moisture, chemicals, bacteria, other microorganisms, or other environmental factors. The biodegradable natural or synthetic fibers also facilitate environmental sustainability. Among the biodegradable fibers, the sustainable polyester fibers have been preferably used. However, the polyesters are mostly the synthetic fibers. The polyesters may form the renewable and biodegradable fibers. The different types of the polyesters used are aromatic, aliphatic, and aromaticaliphatic polyesters. The polyesters such as poly(lactic acid), poly(ethylene terephthalate), polyhydroxybutyrate, polyhydroxybutyrate, poly(butylene succinate), etc. have been used to form the polyester fibers. Among these polyester fibers, aliphatic polyesters are more workable. These biodegradable fibers have fine chemical, mechanical, and thermal properties to be utilized in textiles. The eco-textiles obtained from the eco-polymers can be disposed and recycled in an environmentalfriendly manner. Similar to the synthetic polymers, the synthetic fibers are degradable in an environmental-friendly way, and least toxic gas emissions were observed. In addition to the polyester, the cellulose fibers and the starch fibers have also been produced and used for the textiles. The lyocell is also a biodegradable fiber used for the textile materials. The lyocell is usually obtained from the cellulose and consists of cellulose fibers. The industrial scale processes have been used to develop the textile materials using the lyocell. The fibers obtained and processed using lyocell are known as the rayon fibers. Rayon fibers have been commercially used for the textile production. Similarly, the cotton fibers have been obtained from the cellulose and have 1,4-Dglucopyranose backbone. The chemical resistance and tensile properties of cotton fibers can be enhanced through the surface treatments. Silk is an important protein fiber called the fibroin. The natural eco-polymer-based silk fibers are usually obtained from the cocoons of the silk worms. These fibers also need the treatments for the further commercial uses. Thus, the production and use of polyester fibers, cellulose fibers, and starch fibers are need to be processed for the textile industry. In this regard, the related bio-monomers have been continually focused and developed for the textiles. These eco-polymers and derived fibers have also been focused for the commercialization regarding the textile industry. In the textile industry, the eco-polymers have been used to substitute the traditional synthetic polymers and fibers. However, there are several disadvantages of using the eco-polymers and eco-fibers in the textile industry toward the commercialization. The disadvantages are related to the performance and the worth balance of the eco-polymers or eco-fibers.

Eco-Friendly Polymer/Nanocarbon Nanocomposite-Based Textile Materials The nanocarbons are mostly composed of the carbon-based nanoparticles. The nanocarbons possess small size and dimensions in the nm range. The nanocarbons can be broadly classified as the carbon nanotube, graphene, nanodiamond, etc. The nanocarbons possess various beneficial properties compared with the inorganic

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nanoparticles. Out of these, the carbon nanotubes have been used with the conjugated and nonconjugated polymers. The nanocarbons have facile surface modification, easy processing, and biocompatibility properties. The nanocarbons have unusual physical properties suitable for various application areas. The most feasible methods to form the nanocarbons are the chemical vapor deposition, physical vapor deposition, laser techniques, and plasma-related methods. In the nanocomposites, nanocarbons led to several unique morphological, conducting, and sensing properties. The nanocarbons-based nanocomposites have been employed in the drug delivery, biological imaging, fluorescence imaging, biosensing, capacitance, and electronics applications. The fundamentals and the potential prospects of the nanocarbons and specifically the nanocarbons-based textile materials have gained the immense research interests. The nanocarbons and their derived hybrid materials have gained the significant research curiosity in the textile industry owing to the exceptional features. The future developments must focus on the use of nanocarbonsbased nanostructures to form the functional textiles and investigation of their structure-property relationships. Eco-friendly nanocomposites are desired materials for several industries such as space, automobile, defense, and biomedical applications [38–40]. Eco-friendly nanocomposites have been used to prepare the nanocomposite nanofibers. In the ecofriendly nanocomposite nanofibers, various nanocarbons and inorganic nanofillers have been reinforced, and then the textile materials have been produced. The nanocarbon nanomaterials such as the carbon nanotube and carbon black have high surface area and may perform better as the nanofiber nanomaterials. These nanofibers have high strength, toughness, and chemical stability characteristics. The polymer/nanocarbon nanofiber textiles especially the polymer/carbon nanotube-based textiles have shown the lightweight and high electrical conductivity [41–43]. These nanocarbons have been filled in the polyamide, polyethylene, polyester, etc. to form the nanofibers for the eco-textiles [44]. Such textiles have been developed with the non-flammability properties for the defense and the fire-fighting clothing. The polymer/nanocarbon nanofibers have the high functional surface area. These nanofibers may have pores in their structures. The small pore size is usually looked-for for the sensing and antibacterial applications. The polymer/nanocarbon nanofiber-based textiles have been applied in the filtration membranes. Moreover, the polymer/nanocarbon nanofiber textiles have been applied for the wound dressings. These materials can not only filter the noxious gases but also the harmful bacteria. Zhang and co-researchers [31] coated the Spandex multi-filament yarn with thermoplastic polyurethane/carbon nanotube (TPU/CNT) nanocomposite. Figure 2 represents the whole coating procedure of the nanocomposite on the commercial yarn. The commercial yarn was dried according to the optimal hot conditions. The dried fibers were then coated with the TPU/CNT nanocomposite. Figure 3 shows the scanning electron microscopy (SEM) images of the conductive polymer/CNT nanocomposite coatings on the eco-textile yarns. It was experiential that with the cumulative amount of the nanocomposite, the coating thickness was evidently amplified. The nanocomposite-coated yarn performed as the smart eco-textile. The yarn was used for the strain sensing. The smart eco-textile had the resistance recovery and the cyclability.

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Fig. 2 Schematic illustration of fiber coating setup [31]. TPU/CNT thermoplastic polyurethane/ carbon nanotube

Fig. 3 SEM observation of cross-sectional areas of yarns coated with (a) 5 wt.%; (b) 6.3 wt.%; (c) 8 wt.%; and (d) 10 wt.% conductive polymer nanocomposite with 2 wt.% CNT [31]. CNT carbon nanotube

Song et al. [45] performed the preparation of reduced graphene oxide (RGO) and coated on silica textile. The RGO-coated silica textile was then infiltrated with the phenol formaldehyde (PF) resin. The PF solution was prepared in the ethanol to impregnate the RGO-coated silica textiles and develop the RGO/silica textile/PF composite as the eco-textile. The whole process for the formation of RGO/silica textile/PF

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PF Resin/EtOH Solution Fibers

Textile

Folded Textile Textile Immersion Oxidation & Exfoliation

Graphite

Hydroquinoe/GO Solution

RGO

Resin Immersion In Situ Creating RGO Framework

RGO/Textile

Curing

RGO/Textile/PF Composite

Fig. 4 Scheme for the formation of RGO/silica textile/PF composite [45]. PF phenol formaldehyde, RGO reduced graphene oxide, EtOH ethanol

composite is given in Fig. 4. The eco-friendly textile with the 4.1 wt.% RGO had the heat constancy of 225 °C. In addition, the eco-textile had the high strength of 40 MPa. Feller and co-workers [46] studied the eco-textiles based on the conducting polymeric nanocomposites. Three types of nanomaterials were prepared using the poly(ethylene-co-ethyl acrylate), poly(amide12-b-tetramethylene oxide), and poly (propylene) matrices and carbon black as the nanofiller. The eco-friendly textiles were capable of the sensing the vapors of the ethanol, methanol, chloroform, and toluene. Nilsson and co-workers [47] prepared the poly(vinylidene fluoride) and carbon black-derived melt-spun fibers for eco-textiles. The nanofiber yarns have shown high strain properties. The poly(vinylidene fluoride)/carbon black-based textiles were tested for the piezoelectric effects. The poly(vinylidene fluoride)/ carbon black eco-textiles were used as sensor to notice the human heartbeat. Han and co-workers [48] industrialized eco-textile based in the epoxy/carbon fiber. They also used the ethylene glycol monoethyl ether (EGME) for the carbon black dispersion in the eco-textile. The eco-friendly textile was composed of the layered structure (Fig. 5). The inner layer was self-possessed of the carbon black nanostructure formed in the EGME. The outer layer was made up of the epoxy/carbon fiber composite. In this material, the interlaminar spacing in the epoxy/carbon fiber was filled with the carbon black. The nanostructured portion may enrich the electrical conductivity properties. Later, the EGME was removed from the composite. Use of the 0.8 wt. % carbon black was sufficient to increase the electrical conductivity of the ecotextile material. Various polymer/nanocarbon combinations have been produced for the eco-friendly textiles aimed for the sensing and other applications.

Technical Platform for Eco-Friendly Textile Nanomaterials Electronic Textiles The polymer/nanocarbon nanocomposites have been employed for the electronic devices [49–51]. Consequently, these materials have been focused for the electronic textiles. The eco-textiles are lightweight and elastic materials [52]. The polymeric nanocomposites have high electrical performance, optoelectrical features,

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Carbon fiber

Nanostructured interlaminar interface

Fig. 5 Schematic of a nanostructured composite [48]

mechanical stability, chemical stability, and expedient properties to be employed in the electronic textiles. The polymer/nanocarbon nanocomposite-based eco-friendly textiles have wide scope in the electronics. The conducting nanofibers made up of the polymer/nanocarbon nanocomposite have strain-sensing properties [53–55]. Accordingly, the eco-friendly textiles were prepared from the polymer/nanocarbon nanocomposites. The electrical conductivity was rehabilitated upon the application of the external mechanical strain. The strain sensors based on the carbon nanotube, graphene, and the other carbon nanomaterials were prepared [56]. Du and coresearchers [57] prepared the wearable electronics based on the polymer/carbon nanotube eco-textile. The polymer/carbon nanotube eco-textile was prepared as the nonwoven fabrics. The wearable electronics have the repeated washability, the strain sensing, and the pressure-sensing properties. The strain-sensing polymer/nanocarbon eco-friendly textiles have been produced through using different techniques such as the dip coating, electrospinning, wet spinning, layering method, and so on. Each of the mentioned method has the relative advantages and disadvantages. The electronic textiles are also termed as the e-textiles. The electronic textiles or e-textiles are actually the textile fabrics having the electronics, sensing, and battery devices for the desirable uses. The smart textiles have been developed using several ways such as the use of electronically modified fibers during the textile manufacturing. The wearable e-textiles are demanded these days. There are several requirements for the wearable e-textiles such as the elasticity, toughness, gasping, and high sensing competences. The e-textiles are needed to be the comfortable, pleasant wearing, and soft touching. The conventional fabrics used in the textiles cannot conduct the electricity or energy. The entire potential of the e-textiles has not been comprehended. The electronic textiles are referred as the smart textiles when these materials are composed of the suitable efficient fabrics. The various advantages have been gained from the electronic textiles. The electronic textiles have been used to transfer the energy to the human body in order to enhance the performances for the military and sports applications. Similarly, the electronic textiles may act according to the heat or sound waves in the environment. The electronic textiles can be made for the purpose of the color changing. These types of e-textiles can be used for the interior designs. Thus, the fabrics used in electronic textiles must have the microelectronic features and the desired signal processing. In the biomedical industries,

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the smart textiles have been used to retain and release the drug, anti-inflammatory agents, and antiaging drug toward human skin. The electronic textiles have been studied for the various chemical, electronic, and biological factors. The polymer/ nanocarbon eco-textiles have high strength, stretchability, and conductivity properties. However, there are several design and processing challenges to expedite the eco-textile systems based on the polymer/nanocarbon nanocomposites.

Military and Defense Industry Textile materials have been widely used in the military and the defense industries [58–60]. In the defense industries, the textiles have been applied with the heat resistance, flame resistance, water confrontation, ballistic impact resistance, nuclear radiation shielding, and chemical opposition properties [61–63]. The military industry also needs camouflage properties for the protection of soldiers [64–66]. The textiles used in several military purposes are often known as military textiles. The performance of military textiles can be improved through the adoption of eco-friendly textiles based on the polymer/nanocarbon nanocomposites [67]. Such polymer/nanocarbon eco-friendly textiles have good frivolousness, strength, stretching, heat stability, water resistance, and camouflage performance. On commercial scale, a very famous strength fiber is the Kevlar. The Kevlar is a synthetic aromatic polyamide. It is used to form the bullet- and blast-proof materials [68]. The eco-friendly nature of Kevlar can be enhanced using the nanocarbons. Moreover, the Kevlar fibers can be modified to convert into the eco-friendly textiles. Similarly, the other polymers can be employed for the defense garments [69]. For high-performance textiles, the textile manufacturing industries need to incorporate the polymer/nanocarbon-based eco-friendly textiles. Consequently, the nanocomposite-based eco-textiles have shown innovations for the defense and the military industries [70]. The modern textiles have become famous in the contemporary military fields. The suitable and high-performance textile materials are very important for the military uniforms, the protective clothing, the wraps, the hand scarves, the sheets, the sand bags, etc. The modern nanocomposite textiles have been adopted better than the traditional clothing. The type of textiles may minimize the stress and injuries of the soldiers and military persons. The military textiles are also made to control the thermal environment to perform better during wars. Thus, the polymer/nanocarbon-based textile materials have been used to enhance the warrior performance against various war-related hazards. The soldier performance can be monitored and enhanced through using the sensors in the warfare clothing, the gloves, the helmets, the scarfs, etc. Use of the sensors may also monitor the skin temperature, respiratory rate, heart rate, and body motion. The body motion controlling the sensors have been incorporated in the military garments. These e-textiles are essential in the better war performances. The textiles with the high strength, heat, and chemical resistance have been applied for the protection against ballistic weapons, biological weapons, and nuclear weapons. To resist extreme weather conditions during war, the textiles clothing have been designed to give the

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protection against different environmental conditions. In the military and defense industries, the new strategies must be generated to implement the several new technologies.

Antimicrobial Relevance The polymer/nanocarbon nanocomposites-based eco-friendly textiles having the antimicrobial properties have been studied [71]. The polymer/nanocarbon nanocomposites have been included with the silver nanoparticles, silica, titania, zinc oxide (ZnO) and other metal nanoparticles, and metal oxides to increase the antibacterial effects. These materials have also shown the antifungal characteristics. In the eco-textile form, the polymer/nanocarbon nanocomposites are very useful as the antimicrobial materials [72]. The polymer with the nano-silver nanoparticles has high surface area and the confrontation toward the bacteria and fungi. Velmurugan and co-workers [73] introduced the silver nanoparticles into the eco-textiles. The materials have shown the antibacterial activity toward the Brevibacterium linens and Staphylococcus epidermidis bacterial strains. Perminova et al. [74] prepared the polymer/nanocarbon with titania nanoparticles with the activity toward the bacterial cells. Oh et al. [75] prepared the polymer/nanocarbon with the titania nanoparticles which have the antibacterial properties. The addition of ZnO nanoparticles in ecofriendly textiles may also increase the antibacterial properties [76]. In the polymer/ nanocarbon-based eco-friendly textiles, antibacterial effects can be achieved through the addition of the antimicrobial additives and agents [77]. The antimicrobial additives or the antimicrobial agents can be coated on the polymer/nanocarbon surface to gain the antimicrobial effects. One other way is to use the naturally antibacterial polymer in the eco-textiles. In this regard, the chitosan (eco-polymer) can be filled with the nanocarbons to form the eco-textiles. The chitosan has the inherent antibacterial and the antifungal properties.

Self-Healing Eco-Textiles Self-healing or self-repairing or auto-repairing eco-polymers form an imperative category of receptive materials. Self-healing eco-polymers have the ability to reversibly repair their damage. These eco-polymers have inbuilt ability to automatically overhaul any structural damage. Various thermoplastic and thermosetting eco-polymers have been used such as polyurethanes, poly(methyl methacrylate) (PMMA), acrylates, epoxies, etc. Self-healing eco-polymers have been applied in coatings, battery electrodes, supercapacitors, sensors, textiles, etc. The healing properties of these eco-polymers have been enhanced using various nanoparticles. Self-healing agents can also be incorporated in nanocomposites to enhance the healing properties. Self-healing nanocomposite textiles have been focused owing to fine healing characteristics and performance. The design of self-healing eco-polymeric nanocomposite textiles has led advantageous features. Self-healing eco-polymeric

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nanocomposite textiles have been adopted for coatings, sensors, anticorrosion, and anti-structural damage. The self-healing eco-polymers have been studied as an important class of materials [78–80]. The self-healing phenomenon is often associated with physical or chemical crosslinking. The self-healing behavior is usually controlled by the microphase separation and appropriate external stimuli. The selfhealing epoxies have been used for the corrosion protection of the metals. The poly (methyl methacrylate) doped with organic dyes have shown the reversible photodegradation. The self-healing mechanism and the effect of the structural designs on the healing properties have been investigated [81]. The self-healing eco-polymers have found applications in the coating, battery electrodes, supercapacitors, and textiles. However, the self-healing materials need to be produced on a large scale for the commercial applications. In this regard, the self-healing nanocomposites have been developed through incorporating nanoparticles in the self-repairing materials. The self-repairing nanocomposites had important utilization in the textile solicitations. In textile applications, various self-healing nanocomposite textiles have been researched. The self-healing nanocomposite textiles have been used for H2, H2S, SO2, and other gas separation. The self-healing textiles have also been reported for oil-water separation [82]. These textiles have mostly shown the thermally triggered self-healing process. The studies on the self-healing performance, morphology, and separation ability have been carried out [83]. The autonomous self-healing is generated using healing agents such as the nanocapsules. Various thermoplastic and thermosetting matrices have also been used for the formation of the nanocomposites. The progress in self-healing materials revealed several technical solicitations. The eco-polymers are lightweight materials having fine processibility, chemical constancy, and thermal performance [84]. For wide-ranging structural applications, physical properties of these eco-polymers need to be further enhanced. The eco-polymeric textile may have the adverse cracking problems. The damages in these materials are difficult to repair. An important phenomenon is the eco-polymer self-healing in textiles. The self-healable eco-polymer has the capability to convert physical energy to chemical/physical response to rebuild the damage. The intrinsic self-healing eco-polymers may repair the injured zone owing to the eco-polymer chain mobility. The inherent self-repairing materials have fine aptitude to reversibly renovate damages, exactly as its original form. The self-healing properties need certain external stimuli such as the temperature, pressures, etc. The damage healing involves the reformation of physical or chemical bonding between the eco-polymer chains. The molecular mechanism involved in the healing has also been explored. Mostly, the thermoplastic eco-polymers have shown the self-healing properties. However, the thermosetting eco-polymers such as the epoxies have also shown this phenomenon. The arena of self-healing eco-polymer is continuously emerging in terms of the textile applications [85]. The mechanism of self-healing also needs to be explored to attain the high potential of these eco-polymers. Garcia et al. [86] researched intrinsic self-healing eco-polymeric architectures. The eco-polymer designs affect the mechanical, thermal, and other physical characteristics. The damage influences and essential property losses can be minimized using an appropriately designed self-healing structure. The restorative competence of the self-

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Fig. 6 Hydrogen bonding in polyurethane (left) and network distortion plus repairing (right) [86]

healing eco-polymers still need to be studied. In polyurethanes, the healing affect is usually observed owing to the interactions between the urethane linkages (Fig. 6). The bonding may lead to the flexible network formation for the self-healing. The physical crosslinks are reversible and flexible. Hydrogen bonding effect is minimized at the high glass transition temperature. However, the eco-polymer chain movements allow the scratch recovery on the surface. Gao et al. [87] formed polyacrylate (PA)-based self-healing eco-polymer with furfuryl alcohol (FA) and bismaleimide (BMI) as PA/(FA-BMI). At high temperature >100 °C, PA/(FA-BMI) can be converted to the FA and BMI constituent, whereas at 50 °C the damaged constituents can be repealed. Consequently, PA/(FA-BMI) depicted self-healing performance for scratches. The thermogravimetric analysis (TGA) revealed enhancement in the heat resistance of PA/(FA-BMI) with FA and BMI. The self-healing eco-polymeric composite and nanocomposite have been developed in this regard. These materials have essential aptitude to recover the damage autonomically, with the external stimuli. The inclusion of nanoparticles has affected the intrinsic healing characteristics of the nano-systems. The self-repairing properties of the nanocomposite textiles may include the corrosion, fatigue, and impact recovery. The self-healing nanocomposites have been applied in the adhesives, coatings, aerospace, automotive, electronics, and other technical industries. Guadagno et al. [88] developed the self-healing nanocomposite based on the epoxy resin and polyhedral oligomeric silsesquioxane (POSS). The epoxy/POSS nanocomposite has shown reversible hydrogen bond formation leading to the selfhealing process. The epoxy and carbon nanotube-based nanocomposites have also shown the self-healing properties. These materials have revealed the fine reversible healing efficiency. Such nanocomposites have the potential for sensitive in situ sensors. The autonomous self-healing or the intrinsic self-healing in the nanocomposites can be achieved via the structural mobility through external stimuli. Final shape is fully restored owing to the physical/chemical crosslinking between the eco-polymer chains. The inclusion of the nanoparticles may easily fill the nano-crack of ~100 mm in the structures [89]. However, maintaining the multiple re-healing

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cycles in these systems is quite challenging. The crosslinking in the intrinsic selfhealing nanocomposites is feasible for the thermoplastic materials; however, the selfhealing in the thermosetting nanocomposite still need to be explored in detail. The self-healing nanocomposite textiles have the ability to repair the damage in the structures. An important use of the self-healing nanocomposite textiles is for the anticorrosion coatings. These textiles have been reinforced with the nanocapsules or nanofibrous fillers to promote the self-healing effects. In this regard, the nanofiller, type, aspect ratio, and concentration have enhanced the self-healing performance. Sometimes, a self-healing agent is also added for the fast crack or damage recovery, self-curing, and anticorrosion. The self-healing agents can be loaded in the nanocapsules and added inside the nano-systems. The controlled release of the corrosion self-healing agent to the damage zone may help to fast repair the harms or cracks. Pulikkalparambil et al. [90] developed the self-healing nanocomposite textiles of the epoxy with the nanocapsules and nanofibers. The selfhealing textiles were prepared using layer-by-layer method. These nanocomposites with nanocapsules have shown higher the self-healing properties, relative to the pristine material. In the self-repairing nanocomposite, the scratched metal surface has shown better corrosion protection. Figure 7 shows the process of self-repairing in epoxy and cellulose nanofiber (CNF)-based nanocomposite textiles. The self-healing effect was enhanced using the corrosion inhibitor. The nanoparticles amalgamation upgraded the wear rate of the coatings. Thus, the self-healing nanocomposite textiles are smart materials. These textiles have shown the autonomous self-repair toward the external stimuli such as the heat, light, moisture, etc. The self-healing nanocomposite textiles have also shown the long-term corrosion safety. The textiles have also revealed the sensing potential for the vapors, pressure, and temperature. Consequently, these materials have been applied for the environmental and biomedical uses. The self-healing eco-polymers and the self-healable textiles have led to the advantageous material aspects, mostly the polyethylene, polypropylene, poly(vinyl chloride), PMMA, polyurethane, epoxies, etc. However, in some systems, the self-healable properties have caused lowering in the mechanical performance. This problem has been well-treated with the addition of the nanoparticles. The strengthened textiles with the restored mechanical performance are needed for the technical uses. The self-healable textiles are the most demanding materials for Cellulose nanofiber (CNF)

Corrosion inhibitor Top layer

Middle layer Primer layer Carbon steel

Fig. 7 Self-repairing in epoxy/CNF nanocomposite [90]

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the coatings. The self-healable textiles are quite desirable for the microelectronic devices and systems. These textiles have been applied in the electronics components to show the high mechanical/electrical self-healing properties. The battery separators can be made using self-healable textiles. For sensing textile applications, new design innovations are needed such as the employment of the synthetic self-healable ecopolymers and nanocarbon particles for better sensing performance toward toxic gases. Nevertheless, its still challenging to fabricate intrinsic self-healing textiles with high performance. The new synthetic self-healable eco-polymers-based textiles need to be designed with functional nanoparticles for the quick healing response. The novel healing agents and nanocapsules must be designed to accelerate the restorative process. The long-term practical applications of self-repairing materials with minimum damage must be focused for the future advances. In short, self-retrieving eco-polymers, self-retrieving nanocomposites, and self-repairing nanocomposite-based textiles have been prepared. The auto-repairing materials have the capability to respond to the external stimuli to recover initial material shape or repair the damage. Recently, selfhealing textiles especially the nanocomposite textiles have attracted much attention for perspectives of the coatings, anticorrosion, strengthened materials, biomedical, and environmental relevance.

Conclusions and Further Outlook The polymer/nanocarbon-based nanomaterials have been reported for various eco-textiles-related applications. Accordingly, research has focused on the design and use of polymer/nanocarbon nanostructured materials in eco-textiles. The polymer/nanocarbon-based nanomaterials have been employed in the range of textile applications such as eco-friendly electronic textiles, eco-friendly defense or military clothing, and eco-friendly antibacterial clothing. The formation of the eco-friendly electronic textiles has been used to sense the strain and other effects of interest. The strain-sensing properties of e-textiles have been boosted with the increasing amounts of nanocarbons including the carbon nanotube, graphene, carbon black, etc. Similarly, the antibacterial effects can be increased using the higher contents of nanocarbon nanofiller. However, the use of functionalized carbon nanotube or other carbon nanomaterials may cause better effects on the sensing, electrical, flame resistance, and blast conflict properties. Adoption of the new fiber formation and the textile fabrication techniques may also be helpful for the advanced military, electronics, antibacterial effects, and self-healing materials. Consequently, the eco-polymers have been synthesized and modified for the formation of next-generation eco-friendly textiles. Nevertheless, the several functionalization, processing, and application challenges need to be overcome to get the full use of the polymer/nanocarbon eco-friendly textiles technology in these fields. In summary, the polymer/nanocarbon-based nanomaterials have been established for the eco-friendly textiles. The polymer/nanocarbon nanomaterials have been industrialized using the eco-polymers and the nanocarbons. The

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polymer/nanocarbon nanomaterials have been produced using carbon nanotube, graphene, etc. in the eco-friendly textiles. The polymer/nanocarbon-based ecofriendly textiles have been applied for the electronics, defense textiles, antibacterial, and self-healing textile applications. The polymer/nanocarbon-based eco-textiles have been successfully used in the electronics. Thus, the e-textiles have been used in the modern electronics technology. In the defense technology, the eco-friendly textiles with the flame and blast resistance have been produced. Addition of the metal nanoparticles and inorganic nanoparticles have enhanced the antibacterial effects of the eco-friendly textiles. The self-healing materials including the self-healing polymers, self-healing nanocomposites, and self-healing nanocomposite textiles have been discussed. The self-healing nanocomposite textiles have been designed using various nanoparticles including the nanocarbon nanofillers. The self-healing textiles have gained enormous interest for the myriad of applications such as the coatings, electronic devices, supercapacitor, and sensing materials. The challenges and advancements in the application areas of the ecofriendly textiles can be overcome by using the hybrid nanocarbon nanoparticle modification, using advanced fabrication methods, and using optimized process conditions.

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Application of Heterogeneous Nanocatalysis-Based Advanced Oxidation Processes in Water Purification

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An Introduction Chu Dai, Xike Tian, Chao Yang, Yulun Nie, and Yanxin Wang

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Fenton Catalysis and Its Application in Water Purification . . . . . . . . . . . . . . . . . . Overview of Heterogeneous Fenton Oxidation Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for Enhanced Heterogeneous Fenton Catalysis Activity and Related Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Fenton Catalysis in Coal Chemical Wastewater Treatment . . . . . . . . . . . . . . . Heterogeneous Catalysis of Peroxymonosulfate and Its Application in Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of Heterogeneous Catalysis of Peroxymonosulfate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for Enhanced Heterogeneous Peroxymonosulfate Catalytic Activity and Related Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Persulfate Oxidation Technology in Industrial Wastewater Treatment . . . . Heterogeneous Catalysis Ozonation and Its Application in Water Purification . . . . . . . . . . . . . . . Overview of Heterogeneous Catalysis Ozonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for Enhanced Heterogeneous Catalysis Ozonation Activity and Related Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Catalysis Ozonation in Practical Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Photocatalysis and Its Application in Water Purification . . . . . . . . . . . . . . . . . . . . . Overview of Heterogeneous Photocatalysis Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for Enhanced Heterogeneous Photocatalysis Activity and Related Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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C. Dai Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, People’s Republic of China e-mail: [email protected] X. Tian (*) · C. Yang · Y. Nie Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China e-mail: [email protected]; [email protected]; [email protected] Y. Wang School of Environmental Studies, China University of Geosciences, Wuhan, China e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_64

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Application of Photocatalysis Technology in Practical Water Treatment . . . . . . . . . . . . . . . . . 2980 Conclusions and Outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2982 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2983

Abstract

Advanced oxidation processes (AOPs) involving the production of strongly oxidizing active substances for elimination of various organic pollutants have shown great potential in current water purification. Among them, especially the heterogeneous nanocatalysis based on nanotechnology and nanomaterials provides a promising alternative due to the adjustable physicochemical properties for enhanced catalytic efficiency. Therefore, in this handbook, we will focus on and summarize the current heterogeneous nanocatalysis water treatment technologies according to our previous work and the latest literature. The characteristics of typical AOPs including photocatalysis, Fenton catalysis, persulfate activation, and ozone oxidation are firstly introduced briefly. Secondly, a comprehensive overview of the used nanotechnology and nanomaterials will be summarized such as their physiochemical characterization, catalytic activity, reaction mechanism, and the key factors that affect their performance. Finally, the application of typical heterogeneous AOPs in practical water purification and the present engineering projects will be given. We think this handbook can provide the quick reference for researchers of the historical development, research status, and development trend of AOPs for water treatment using nanomaterials.

Introduction During recent years, large numbers of organic pollutants have been identified as potential emerging contaminants in the environment. Industrialization and unrestrained use of consumer goods have led to havoc in terms of pollution and energy utilization. The contaminants usually do not get fully eliminated or degraded in the sewage treatment plant and then the water containing these pharmaceuticals is being released in treated effluents bringing out the contamination to ponds, rivers, lakes, and, ultimately, back to drinking water. This will cause serious threats to human health. Thus, efficient, practical, and cost-effective technologies are required for the treatment of pollutants in wastewater. Various methods have been developed to deal with contaminated water, such as adsorption, flocculation, biological methods, and advanced oxidation processes (AOPs). Among these methods, AOPs have shown great potential to oxidize most contaminants to harmless compounds or even complete mineralization to CO2, H2O, inorganic ions, owing to the generation of highly effective reactive oxygen species (ROS) such as hydroxyl radicals (•OH), superoxide anion (O2•
), singlet oxygen (1O2), and sulfate radical (SO4•
) during the catalytic process. And these involve four typical AOPs: Fenton oxidation technology, ozonation, photocatalysis oxidation technology, and newly developed peroxymonosulfate activation technology. Fenton oxidation technology with the longest research history has been widely used for kinds of wastewater treatment with the improvement of water quality and

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biodegradability due to the formation of nonselective oxidation of •OH. Thus, it is usually used for the treatment of refractory wastewater, for example, the steel wastewater, coking wastewater, pharmaceutical wastewater, petrochemical wastewater, leather wastewater, electronic wastewater, food wastewater, printing and dyeing wastewater, and paper wastewater. Ozone is also a powerful oxidant with a redox potential of 2.07 V. Therefore, catalytic ozonation is able to oxidize a lot of organic substances and realize their deep mineralization. It is generally used as the final stage for color and odor removal, but also for oxidation of specific pollutants and final disinfection. In addition, thanks to excellent disinfection and oxidation qualities, ozone can also be used for drinking water treatment. Photocatalysis represents an ideal environmental pollution control technology, which utilizes solar energy to realize self-purification of the environment and widely used in the degradation of organic dyes, purification of industrial wastewater, and treatment of indoor pollutants. While as SO4•
possesses relatively higher standard redox potential (2.5–3.1 V) and much longer half-life period (30–40 μs) than •OH (1.8–2.7 V, 20 ns) as well as a wide pH application range (about 2–8), peroxymonosulfate activation technology has been developed and widely used in in-situ chemical oxidation remediation of groundwater and soil and other aspects of environmental pollution control. Different oxidation technologies have their own advantages, while their common goal is to achieve efficient pollutant removal. Thus, how to improve the oxidation efficiency is the most concerned issue in water purification. It is well known that many transition metal ions exhibit excellent catalytic activity, while the narrow pH application range, secondary pollution caused by metal ions strictly limits its practical application. Heterogeneous catalysis with suitable catalysts can efficiently promote the conversion of various oxidants to more ROS, thus resulting from higher catalytic activity together with a wider pH application range. These catalysts can also be easily separated and reused to reduce the dissolution of metal ions. The catalytic performance of catalyst is dependent on its structure and surface properties. The development of nanotechnology makes it easier to adjust the composites, even the physical and chemical surface properties of catalysts. This greatly increases the possibility for application of heterogeneous nanocatalysis-based AOPs in practice engineering projects. Thus, a better understanding of the structure, composites, and related catalytic activity, as well as the related reaction mechanism will provide right directions for choosing suitable catalysts for the application of different environmental situations. Therefore, in this handbook, we will focus on and summarize the heterogeneous nanocatalysis technology for water purification according to our previous work and the latest literature. The typical AOPs including Fenton catalysis, persulfate activation, ozone oxidation, and photocatalysis will first be provided with a general overview. Then, the strategies with enhanced heterogeneous catalysis and related reaction mechanism will be detailed summarized. Finally, the examples for the application of heterogeneous catalysis in practical water purification will be listed. We think this handbook can provide a quick reference for researchers of the historical development, research status, and development trend of AOPs for water treatment using nanomaterials.

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Heterogeneous Fenton Catalysis and Its Application in Water Purification Overview of Heterogeneous Fenton Oxidation Technology Since 1894, Henry J. Fenton found that the reaction between Fe2+ and H2O2 can oxidize tartaric acid (TA), the H2O2-based oxidation technology has drawn great interest in wastewater treatment [1]. As H2O2 is a green oxidant considering that it can generate highly active •OH to achieve nonselective degradation of varies pollutants and its reaction by-products are only H2O and O2 [2]. Compared to the traditional Fenton reaction, the development of heterogeneous Fenton reaction possesses the advantages of recyclability, wide pH response range, easy solid–liquid separation, and non-production of iron sludge, which makes it the most promising technology in practical application [2]. Based on this Fenton catalytic technology, scientists put forward two different mechanisms to explain the strong oxidizing ability of Fenton reagent. One is the hydroxyl radical mechanism first proposed by Haber and Weiss in 1934. They pointed out that Fe2+ plays an important role in the reaction process and it can decompose H2O2 to produce a strong oxidizing •OH, the involved reactions are described in Eqs. 1, 2, and 3. Another is the constantly improved high valent iron oxide intermediates mechanism first proposed by Bray and Gorin. They think that the intermediate products with strong oxidation during the reaction are FeO2+ or FeO3+, rather than •OH, which can be list as Eqs. 4, 5, and 6. Up to now, there is no clear experiment to distinguish the mechanism of hydroxyl radical and high valent ferrite intermediates, so the debate about the two mechanisms is still ongoing. However, the •OH radical mechanism of Haber Weiss has been reported more in the literature.  FeII þ H2 O2 ! FeIII þ • OH þ OH
k ¼ 63
76 M
1 s
1




 FeIII þ H2 O2 ! FeII þ HO2 • þ Hþ k ¼ 0:001
0:01 M
1 s



1

ð1Þ ð2Þ

 FeIII þ HO2 • ! FeII þ O2 þ Hþ

ð3Þ

 FeII þ H2 O2 ! FeIV þ 2OH


ð4Þ

 FeIV þ H2 O2 ! FeII þ O2 þ 2Hþ

ð5Þ

 FeIV þ  FeII ! FeIII

ð6Þ

In addition to the above two controversial topics, how to improve heterogeneous Fenton catalytic efficiency is the most concerned issue and the production of •OH is the most efficient and essential step for the removal of contaminants. As the reduction of Fe(III) by H2O2 (Eq. 2) is always the rate-limiting steps due to the low rate constant (0.001–0.01 M
1 s
1), which determines the overall efficiency of

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the whole Fenton reactions. Besides, HO2• generated through Eq. 2 was also less reactive than •OH due to the lower oxidation potential (2.73 (•OH) vs. 1.50 V (HO2•)) [3]. Therefore, how to accelerate the redox cycling of Fe(III)/Fe(II) and promote the utilization efficiency of H2O2 in heterogeneous Fenton reactions is the core issue, motivating researchers to design more effective heterogeneous Fenton catalysts. And a series of metals, metal oxides, minerals modified with metals, carbon materials based on different synthesis strategies, and reaction mechanisms are designed towards various organic pollutants degradation. An understanding of the related reaction mechanism and synthesis strategy can provide theoretical support for selecting suitable catalysts to achieve higher pollutants removal efficiency. Therefore, the main objective of this section is to provide a summary of the accomplishments concerning heterogeneous Fenton catalytic to point out the major directions for choosing the catalysts in Fenton catalytic in the future.

Strategies for Enhanced Heterogeneous Fenton Catalysis Activity and Related Reaction Mechanisms Improving the Number of Active Sites of Catalysts Preparation Catalysts with Large Surface Areas Generally speaking, catalysts with larger specific surface area usually contains more active sites, which will beneficial for the catalytic reaction. For example, Jin et al. prepared Fedpa@SiO2 catalyst, which showed better catalytic activity for degradation of 2,4-dichlorophenol compared with the pure Fedpa (95% vs. 44.8%) [4]. The complexing ability of dpa reduced its redox potential and accelerated the electron transfer among H2O2 and iron to generate dpaFeV¼O. What’s more, the carrier SiO2 contributed to both separating the active sites on the surface of Fedpa@SiO2 and avoiding the self-decomposition of Fedpa. Xu et al. successfully prepared sepiolite-supported magnetite (Fe3O4-Sep) with large specific surface area, which provides sufficient active sites for effective adsorption of BPA and BPA can be further degraded by the generated •OH [5]. Thus, reducing the size of catalyst or loading catalysts to a large surface area carrier (mesoporous materials or mineral materials) can obtain more active sites, so as to improve the activity of catalytic reaction. Morphological-Controlled Crystal Active Facet Exposure It is well-known that different facets of a single-crystalline material display different geometric and electronic structures, thus endowing them with distinctive properties. Particularly, high-energy facets that contain abundant unsaturated coordination atoms and atomic steps and ledges usually exhibit high reactivity. In our previous work, CuFeO2 nanocubes and nanoplates with different surface facets of {110} and {012} are used to compare the effect of exposed facets on

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ofloxacin (OFX) degradation [6]. It was found that the degradation rate by CuFeO2 {012} is four times faster than that of CuFeO2 {110} (0.0408 vs. 0.0101 min
1). This is because of •OH is preferentially formed from the reduction of absorbed H2O2 by electron from CuFeO2 {012} due to suitable elongation of O-O (1.472 Å) bond length in H2O2. Compared to (101) surface, MIL-88A-Fe-(100) with lower energy barrier for H2O2 dissociated into •OH (0.58 eV vs. 0.8 eV) showed better catalytic activity for the removal of various organic pollutants [7] (Fig. 1). Therefore, morphological control of nanocrystals provides a way to tailor the physical and chemical properties of nanomaterials, which may shed light on the design of highly efficient heterogeneous Fenton catalysts.

Fig. 1 Optimized surface structures of CuFeO2 catalyst: (a) H2O2 adsorption on CuFe-012; (b) H2O2 adsorption on CuFe-110; (c) H2O2 decomposition as a function of reaction time over CuFe110 and CuFe-012; and (d) calculation of activation energy for CuFe-110 and CuFe-012 [6]

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Enhancing the Interface Electron Transfer Process Except for creating more catalytic active sites, enhancing the interface electron transfer process can also be effective strategies to achieve higher catalytic activity and utilization efficiency of H2O2. The method including direct electron transfer from zero-valent metal, substitution of different metal components, using electronrich materials (carbon-based materials, chelating/reducing reagents, metal sulfides) as cocatalysts or constructing catalysts with surface defects will give a detail introduction in this part. Direct Injecting Electrons from Zero-Valent Metal Since the use of zero-valent iron (ZVI, Fe0) for the remediation of groundwater contamination was first reported in 1990, ZVI has extensively applied in environmental fields by virtue of its high reducibility, easy accessibility, and environmental friendliness. Especially, the corrosive oxidation of Fe0 to Fe2+ could be accompanied by the two-electron transfer to O2 with in-situ generation of H2O2 (E0(Fe2+/ Fe0) ¼
0.44 vs. E0(O2/H2O2) ¼ +0.68 V NHE) [8]. Moreover, the presence of Fe0 can also reduce pollutants to less toxic or nontoxic species through direct electron transfer to pollutants or indirect electron transfer to accelerate the Fe (III)/Fe(II) cycle. Niu et al. prepared core/shell type Fe0@carbon inserted in the interlayer space of montmorillonites (Fe@C-MMT) and strengthened elimination of phenol and methyl orange with high mineralization efficiency (80%) was achieved. The reaction mechanism of IME-Fenton on the surface of Fe@C-MMT is shown in Fig. 2a [8]. Compared with Fe0, zero-valent zinc (Zn0), zero-valent aluminum (Al0), zerovalent magnesium (Mg0) with lower standard electrode potential (E0(Zn2+/ Zn0) ¼
0.763 V, E0 (Al3+/Al0) ¼
1.66 V, E0 (Mg2+/Mg0) ¼
2.372 V), can provide a greater thermodynamic driving force for electron transfer, which can also reduce O2 to in-situ generation of H2O2 and accelerate the electron transfer during the Fenton-like catalytic process. In Yang’s study, a novel heterogeneous Fenton-like system, composed of Zn0-CNTs-Fe3O4 composite and dissolved oxygen (O2) in solution, which can in-situ generate H2O2 and •OH, was used for the degradation of 4-chlorophenol (4-CP) with the removal efficiencies of 4-CP of 99% [9]. A novel

Fig. 2 (a) Reaction mechanisms of IME-Fenton on the surface of Fe@C-MMT. (b) Schematic diagram of the reaction mechanism of the Mg/Fe-O2 system [9, 10]

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bimetallic Mg0/Fe0-O2 heterogeneous Fenton-like system was also developed to degrade 4-CP as exhibited in Fig. 2b [10]. It was found that with the mole ratio of Mg to Fe of 32:1, the catalyst had the best performance for the 4-CP degradation and the maximum cumulative concentration of H2O2, the degradation efficiency of 4-CP and the removal efficiency of TOC in Mg0/Fe0-O2 system were 34.5 mg/L, 100% and 91.8%, respectively. Synergistic Effect Between Different Metal Components Co and Mn on the surface of CoMnAl-CMO-Al2O3 presented synergistic effects on the efficient generation of HO%, which was mainly responsible for the detoxification of pharmaceutical wastewater [11]. As transition metal ions like Co2+, Mn2+, and Ce3+ exhibit similar catalytic effect to Fe2+, thus can also improve the Fenton reaction activity. In addition, the electron transfer between Cu2+ and Ni2+ in CuNiFe-LDHs promotes the regeneration of Cu+, which can readily react with H2O2 to produce •OH may dominate the reaction [12]. And around 90% phenol can be mineralized in Cu0.5Ni2.5Fe LDH-H2O2 catalytic system at the H2O2 dosage close to the theoretical value. The possible reaction mechanism for the degradation of phenol over CuNiFe LDHs is shown in Fig. 3a. The existence of transition metal ions like Cu+, Cr2+, V2+ can accelerate the reduction of Fe3+ through a spontaneous oxidation-reduction process to achieve high catalytic efficiency. Therefore, substitution of different metal components with varied redox properties can also be an efficient strategy for improving their catalytic properties due to the synergy between different components, and the corresponding redox reactions are listed in Table 1. Direct Injecting Electrons from Carton Materials Many carbon-based materials, including graphene (GO), carbon nanotubes, activated carbon (AC) and carbon quantum dots (CQDs), biochar, and g-C3N4 have been widely applied in heterogeneous Fenton reactions due to their abundant electrons and ubiquitous existence in natural environments. For example, our group also reported that the Fenton-like activity of mesoporous iron oxide was enhanced by tailoring the surface electron transfer process via CQDs

Fig. 3 (a) Possible reaction mechanism for the degradation of phenol over CuNiFe LDHs. (b) The proposed heterogeneous Fenton-like reaction mechanism of CQDs/CuMIO [12, 13]

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Table 1 The standard electrode potentials of different redox couples No. 1 2 3 4 5 6 7 8

Redox couple Fe3+/Fe2+ Cr3+/Cr2+ V4+/V3+ Cu2+/Cu+ Mn3+/Mn2+ Co3+/Co2+ Ce4+/Ce3+ Ni3+/Ni2+

φ /V 0.77
0.408 0.361 0.15 1.54 1.92 1.44 1.74

Redox reaction – Cr2+ + Fe3+ ! Fe2+ + Cr3+ V3+ + Fe3+ ! Fe2+ + V4+ Cu++ Fe3+ ! Fe2+ + Cu2+ Mn3+ + Fe2+ ! Fe3+ + Mn2+ Co3+ + Fe2+ ! Fe3+ + Co2+ Ce4+ + Fe2+ ! Fe3+ + Ce3+ Ni3+ + Fe2+ ! Fe3+ + Ni2+

ΔE0/V 1.178 0.409 0.62 0.77 1.15 0.67 0.97

and Cu modification (CQDs/Cu-MIO) [13]. The as-prepared CQDs/Cu-MIO catalyst exhibited a high efficiency and excellent stability towards OFX degradation over a wide pH range (pH ¼ 3.6 to 10.0) without any extra energy input. Such excellent catalytic activity was ascribed to the spontaneous electron transfer between Cu+/Cu2+, Fe2+/Fe3+, and the excellent electron transformation/energy exchange property of CQDs, the proposed reaction mechanism of CQDs/CuMIO is shown in Fig. 3b. In addition, biochar, the carbon-rich residue of the artificial burning or pyrolysis of biomass, has shown a promising supporting material of iron/iron mineral nanoparticles in the preparation of composite catalysts for the Fenton-like removal of pollutants due to its function as an electron shuttle and abundant persistent free radicals. Except for enriching pollutants on the solid–liquid interface, biochar can also catalyze the Fentonlike oxidation of pollutants through direct reaction with H2O2 to produce the reactive oxygen species. Thus, 2,4-D was efficiently degraded in pyrite-BCQ (quinone-like structure of biochar)/H2O2 catalytic system. This could be mainly attributed to the biochar’s function as an electron shuttle for the transformation of O2•
into •OH, which is described in Eqs. 7 and 8. O2 •
þ BCQ ! BCSQ •
þ O2

ð7Þ

BCSQ •
þ H2 O2 ! BCQ þ • OH þ OH


ð8Þ

Therefore, combining carbon materials with a heterogeneous Fenton catalyst can not only accelerate electron transfer process at the interface with their excellent electrical conductivity, but also serve as active sites to direct activate H2O2 and generate more ROS for the degradation of refractory organic contaminants. Direct Injecting Electrons from Chelating/Reducing Reagents Wang et al. studied the effects of different chelating agents on the degradation of bisphenol A (BPA) in BiFeO3-H2O2 catalytic system [14]. It was found that ethylenediaminetetraacetic (EDTA) has the most significant effect on BPA degradation and the removal efficiency were increased from 20.4% (blank) to 91.2% (EDTA). This can be ascribed to the lower redox potential of Fe3+/Fe2+ when added nitrilotriacetic acid (NTA), EDTA from 0.77 V to 0.356 and 0.209 V, respectively. Recently, Sun et al. used an eco-friendly reducing ascorbic acid (AA) as

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reducing reagents in Fe3O4-H2O2 for the alachlor degradation [15]. The addition of AA could significantly accelerate the Fenton reaction by promoting the surface Fe3+/Fe2+ redox cycle of Fe3O4 and surface Fenton reaction was mainly responsible for the alachlor degradation with more than 62.6% of contribution. The related reaction mechanism is shown in Fig. 4a and Eqs. 9 and 10. FeIII
Fe4 O3 þ H2 O2 ! FeII
Fe4 O3 þ HO2 • þ Hþ k ¼ 2:5  10
3 M
1 s
1




ð9Þ

Hþ

AA
FeIII
Fe4 O3 þ H2 O2 ! FeIII
Fe4 O3 þ : OH þ H2 O
þ AAax k ¼ 35 M
1 s
1

ð10Þ

Fig. 4 (a) The possible reaction mechanism in the Fe3O4/AA/H2O2 system. (b) Proposed oxygen vacancy involved Fenton reaction mechanism of Cu doped Fe3O4@FeOOH

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Thus, the complex of iron-based catalysts with chelating/reducing reagents could effectively change the redox potential of Fe3+/Fe2+ and thus accelerates its cycle and promotes the generation of more active OH. Typical reagents including EDTA, NTA, AA, hydroxylamine (HA), tartaric acid (TA), citric acid (CA), oxalic acid (OA), p-hydroquinone (p-HQ), and so on. Direct Injecting Electrons from Metal Sulfides Xing et al. combined Fe3+ with various metal sulfides for the degradation of Rhodamine B and found that the efficiency of reduction of Fe3+ to Fe2+ followed the order of WS2 > CoS2 > ZnS > MoS2 > PbS > Cr2S3 > conventional Fenton [16]. Specifically, the reaction rate constant with MoS2 was 18.5 times higher than that without MoS2. The unsaturated S atoms on MoS2 surface can capture protons to form H2S with exposed reactive Mo4+, facilitating the reduction of Fe3+ to Fe2+ (Eq. 11). Besides, Mo6+ can be converted back to Mo4+ by the reaction with H2O2 (Eq. 12), which ensures the catalytic cycling of MoS2. Mo4þ þ Fe3þ ! Mo6þ þ Fe2þ

ð11Þ

Mo6þ þ H2 O2 ! Mo4þ þ 2Hþ þ O2

ð12Þ

Subsequently, magnetic Fe3O4-MoS2 nanocomposites (MF) with the addition of a small amount of H2O2 were used for efficiently inactivate bacteria and remove diclofenac (DCF) at a wide pH range from 3.5 to 9.5 [17]. During the catalytic process, MoS2 is found to facilitate the reduction of Fe3+ directly by exposed Mo4+ or indirectly by the generation of HO2•, which thus enhances the generation of •OH and improved the catalytic activity. MF is also employed to treat sanitary sewage and 66.2% and 88.9% of TOC with the initial concentration of 98.7 mg/L can be removed without and with sonication during the reaction in 4 h, respectively. Except for MoS2, some studies found that other metal sulfides (MoS2, WS2, Cr2S3, CoS2, PbS, or ZnS) can also serve as excellent cocatalysts to accelerate the rate-limiting step of Fe3+/Fe2+ conversion by the exposed reductive metallic active sites. Direct Injecting Electrons from Surface Defects The formation energy of oxygen vacancy, as well as the nature and the content of defect sites, greatly affects the catalytic activity. For example, an oxygen vacancy promoted heterogeneous Fenton-like reaction mechanism over Cu doped Fe3O4@FeOOH catalyst was reported in our group. Cu doped Fe3O4@FeOOH catalyst with abundant oxygen vacancy exhibits 10 times higher degradation rate compared to Fe3O4@FeOOH (9.04/h vs. 0.94/h). And the newly formed oxygen vacancy from in situ Fe substitution by Cu rather than promoted Fe3+/Fe2+ cycle was responsible for the ultra-efficiency of Cu doped Fe3O4@FeOOH at neutral and even alkaline pH [2]. Proposed oxygen vacancy involved Fenton reaction mechanism is

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shown in Fig. 4b. Subsequently, CuVOx bimetallic catalyst with abundant surface defects also reported and exhibited higher catalytic activity, wider pH applicability, and more satisfactory reusability than monometallic copper compounds [18]. By introducing V into Cu-based materials, several surface properties that are closely related to the reactivity of catalysts such as adsorption capacity, surface defects, and active sites concentration were improved. And the electron-rich center around Cu/V active sites and surface oxygen vacancies are responsible for the rapid dissociation of H2O2 and the efficient catalytic oxidation of fluconazole in CuVOx Fenton system. OV þ H2 O2 ! • OH þ OH


ð13Þ

As the OVs not only have significant effects on the electronic structure, electron mobility, and surface properties of the catalysts, but also lower the reaction activation energy of the activation and decomposition of small molecules around the OVs. Besides, OVs can also be some reaction active sites to directly react with H2O2 to produce active free radicals (Eq. 13). Therefore, we believe that the construction of OVs, which is the most common defects with easy accessibility, will be one of the most efficient strategies for designing efficient heterogeneous catalysts for potential application in practice engineering projects.

Application of Fenton Catalysis in Coal Chemical Wastewater Treatment McWong Environmental Technology Corp., Ltd. mainly applied heterogeneous catalysis, two-stage biological nest technology in the field of domestic coal chemical wastewater treatment and reuse [19]. The heterogeneous catalytic reactor uses Fe2+ as catalyst to catalytic activation of H2O2 to produce strong oxidant •OH, thus oxidize organic matters into CO2 and water. The core technology is to fill the reactor with the filler which has been treated by special technology. By adopting the expansion bed method, most of the Fe3+ produced in the reaction can be crystallized and grow on the filler to form MW-FeOOH crystal, which is also an excellent catalyst. This technology can reduce the use of Fe2+, greatly reduce the production of sludge, and greatly reduce the operation cost (Fig. 5). The principle is as follows: Fe2þ þ H2 O2 ! FeOOH þ OH • þ H2 O

ð14Þ

And the reuse-water project designed and built by McWong is currently the largest project of this kind in the world. 14,400 m3/day RO brine is treated to meet the requirements of the “Class-A” Integrated Wastewater Discharge Standards of the Yellow River Basin (Shanxi Section), with COD lower than 50 mg/L, ammonia nitrogen lower than 12 mg/L, and total nitrogen lower than 20 mg/L. The project is the largest RO concentrated water treatment system in China.

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Fig. 5 A picture for application of heterogeneous catalysis in coal chemical wastewater treatment [23]

Heterogeneous Catalysis of Peroxymonosulfate and Its Application in Water Purification Overview of Heterogeneous Catalysis of Peroxymonosulfate Nowadays, the development of sulfate radical (SO4•
)-based AOPs has attracted increasing attention from the academic and industrial communities. As SO4•
possesses relatively higher standard redox potential (2.5–3.1 V) and much longer half-life period (30–40 μs) compared to the counterpart of hydroxyl radicals (•OH, 1.8–2.7 V, 20 ns) [20]. Moreover, SO4•
can react with the refractory organic matter at a wide pH range (about 2–8). Peroxymonosulfate (HSO5
, PMS) and peroxydisulfate (S2O82
, PDS) are the precursors of SO4•
and have been increasingly recognized as alternative oxidants for the effective degradation of refractory organic contaminants. It has been reported that the distance of the O-O bonds in KHSO5 and (NH4)2S2O8 is 1.460 and 1.497 Å and the bond energy is 140–213.3 and 140 kJ/mol, respectively, indicating that PDS is easier to be cleaved than PMS [20]. However, other studies have suggested that PMS is more easily activated because of its unsymmetrical character. And various methods have been employed to activate PMS, including heating, alkali, ultraviolet (UV) light irradiation, transition metals catalysts, activated carbon, and so on. Among these, transition metals activation based on Fe, Co, Mn, Cu, Ni, Zn, and V has been widely researched for their non-negligible advantages of controllable and low energy.

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As alternatives to avoid the drawbacks in homogeneous reactions, such as the production of iron sludge and the secondary pollution result from the transition metal ions, the heterogeneous PMS activation catalyst, including zero-valent iron, metal oxides, and supported catalysts and carbonaceous materials have attracted great interest. The reaction between the transition metals ions and PMS usually produced the active radicals including SO4•
and •OH as shown in Eqs. 15, 16, and 17, which represents a kind of radical activation mechanism. Recently, our group found that besides radical mechanisms (SO4•
and •OH), a non-radical mechanism (i.e., 1O2) was also operative when using LaMnO3 to activate PMS based on the quenching experiments and ESR data [21]. 1O2, an excited state of O2, is a mild and selective oxidant with the lifetime of 1O2 is about 3 μs in water, which is much shorter than that in air. The generation of 1O2 can be summarized in four different reactions: (1) the self-decomposition of PMS (Eq. 18). Studies have shown that the typical triplet signals for 1O2 still remained when the reaction was conducted under nitrogen atmosphere, suggesting that 1O2 was generated from the decomposition of PMS, not from dissolved oxygen; (2) the recombination of •OH (Eq. 19); (3) the decomposition of lattice oxygen (Eq. 20); and (4) the energy trapping by surface adsorbed O2 (Eq. 21), which was reported in our group. The phenol degradation rate constant decreased from 0.106 to 0.03 min
1 when the reaction was conducted in air versus in nitrogen atmosphere [20, 22]. ðnþ1Þþ  Mnþ þ HSO
þ SO4•
þ OH
5 ! M

ð15Þ

ðnþ1Þþ •  Mnþ þ HSO
þ SO2
5 ! M 4 þ OH

ð16Þ

nþ  Mðnþ1Þþ þ HSO
þ SO5•
þ Hþ 5 ! M

ð17Þ

2

2
1 HSO
5 þ SO5 ! HSO4 þ SO4 þ O2

ð18Þ

2OH∙ ! 1=21 O2 þ H2 O

ð19Þ

Olatt ! 1 O2

ð20Þ

 O2

Energy 1

!

O2

ð21Þ

It has also been demonstrated that SO4•
and •OH and 1O2 were not involved in BPA degradation in PMS-amorphous MnOx. But the formed reactive complex between PMS and the MnOx might have mediated direct electron transfer between BPA and PMS, resulting in the BPA degradation. In addition to the different PMS activation mechanisms, various catalysts including metal oxides, metal sulfides, minerals modified with metals, carbon materials were developed, which aims to find out the cheap, highly efficient, and environmentally friendly catalysts for application in practical water purification. In depth understanding of the related reaction mechanism and different design strategy can provide theoretical supports to help readers for selecting suitable catalysts with

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higher catalytic performance in different water environments. Therefore, this section gives a comprehensive summary of the current achievements concerning heterogeneous PMS activation catalysis to help the readers better understand the current situation and future development trend for using nanomaterials in water treatment filed.

Strategies for Enhanced Heterogeneous Peroxymonosulfate Catalytic Activity and Related Reaction Mechanisms Similar to the Fenton-like reaction, improving the number of active sites of catalysts or enhancing the electron transfer rate at the heterogeneous catalytic interface is beneficial to the heterogeneous peroxymonosulfate catalytic activity.

Improving the Number of Active Sites of Catalysts Preparation Catalysts with Large Surface Areas Due to their low cost, chemical stability, and easy reusability, natural minerals including palygorskite, kaolinite, and bentonite have been extensively utilized as supports of nanocatalysts. Dong et al. successfully synthesized CuFe2O4/kaolinite composite catalyst, which showed higher degradation efficiency towards the removal of bisphenol A (BPA) when compared to the pure CuFe2O4 (97.0% vs. 65.9%) [23]. The higher specific surface area, larger pore volume, more hydroxyl groups, and more accessible reactive sites contributed to the greater catalytic activity. The radical generation mechanism and possible radicals’ transfer routes are shown in Fig. 6a. Except for the natural minerals, other metal oxides like Al2O3, SiO2 with abundant content, large surface area are also widely used as supports to improve the dispersion of catalyst. Zhu et al. successfully synthesized a magnetic Co-Fe/SiO2 layered catalyst (LC) as an effective PMS activator and exhibited superior catalytic performance with 98% CIP degradation efficiency in 10 min [24]. Thus, preparing catalysts with larger specific surface area will be beneficial for the catalytic reaction.

Fig. 6 (a) Schematic of the radical generation mechanism and possible radicals’ transfer routes. (b) Surface Ti5c-centered water adsorption, -OH generation, and PMS activation mechanisms on Co/Ti001 catalyst [27, 28]

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Morphological-Controlled Crystal Active Facet Exposure Zhang et al. loaded Co3O4 on TiO2{001}, which exhibited a much higher PMS activation capacity and pollutants (p-nitrophenol and rhodamine B) degradation efficiency with a much lower Co2+ leaching [25]. The polar {001} facet is characterized by a high density of atomic steps, edges and kinks of the low coordinate surface atoms with a large number of dangling bonds, all of which can serve as reactive sites for water adsorption and dissociation and thus improve Co-OH
complex formation dynamics in sulfate radical formation. The surface Ti5c-centered water adsorption, -OH generation, and PMS activation mechanism on the facettailored Co/Ti-001 is shown in Fig. 6b. It is believed that designing and fabricating high-reactive facets of metals or metal oxides would significantly enhance catalytic performance in the activation of PMS. But up to now, few people have studied the effect of crystal surface on the removal pollutants in the PMS activation process. Thus, it is a promising strategy to enhance the catalytic activity for catalysts by controlling the crystal surface.

Enhancing the Process of Interface Charge Transfer In previous studies, many transition ions have been investigated for their PMS activation properties, such as Co2+, Fe2+,Ce3+, Ag+, and Ni2+ [26]. While they are oxidized to a high valence state, they will exhibit poor catalytic performance as the generated SO5•
is not as active as SO4•
. Thus, enhance the interface charge transfer process has significant importance to improve their catalytic activity. Direct Injecting Electrons from Zero-Valent Metal With the property of high reducibility (E0 Fe0/Fe2+ ¼
0.44 V), easy accessibility, and environmental friendliness, ZVI is also extensively used for the activation of PMS to remove varies contaminants in environment. Similar to H2O2 catalytic reaction, the existence of zero-valent metals not only accelerates the metal ions redox cycling through the reduction of ZVI, but also provides a way that involves direct electron transfer to PMS to produce SO4•
. For example, CuO@FeOx@Fe0 catalysts were prepared by displacement plating and calcination to activate PMS for the degradation of sulfamethoxazole (SMX). EPR detection confirmed that massive SO4•
, •OH, O2•
, and 1O2 generated in this catalytic system were responsible for its superior capabilities for the removal of SMX [27]. Except for the nZVI, other zero-valent metal including zero-valent tungsten (nZVT), zero-valent zinc (nZVZn) are also developed based on the similar reaction principle. Owing to the high reduction potential of nZVZn (E0Zn/Zn2+ ¼
0.763 V), nontoxic nature and low cost is selected for activation of HSO5
into SO4•
and • OH. Shah et al. reported highly stable and efficient are synthesized Zn0 for efficiently removal of chlorpyrifos in the presence of PMS [28]. As shown in Fig. 7a, Ye et al. also put forward a promising strategy for utilizing nanoscale W0 to accelerate Cu(II)/Cu(I) redox cycling inducing the continuous production of reactive oxygen species (SO4•
, •OH, and O2•
) through activating PMS for the removal of tetracycline (TC) [29]. 70% of the TC can be efficiently removed within 15 min and the

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Fig. 7 (a) Proposed mechanism of ROSs generation in the nZVT/Cu(II)/PMS system. (b) Possible PMS activation mechanism over CuO-Co3O4@CeO2/PMS [29, 30]

intermediate Cu(I) was confirmed as the main effective activator for PMS, which derived from the direct reduction pathway driven by epigenetic low valence tungsten species and the indirect reduction pathway driven by active hydrogen atoms. Synergistic Effect Between Different Metal Components It was strongly demonstrated that the synergistic effect between two different metal ions can significantly improve PMS activation and enhance catalytic activity of catalysts. For example, complete degradation of 2,4-D was realized under CuO-Co3O4@CeO2/PMS catalytic system, which was much higher than CeO2(9.8%), CuO(10.1%), CuO@CeO2(12.2%), Co3O4(28.3%), CuO@Co3O4(42.1%), and Co3O4@CeO2(53.4%) [30]. The outstanding catalysis of CuO-Co3O4@CeO2 resulted from the synergy of Ce, Co, Cu, and SO4•
and •OH radicals were responsible for efficient removal of 2,4-D, which is illustrated in Fig. 7b. In addition, our group also reported the manipulation of generated ROS over Ce doped Mn2O3 during the PMS activation for efficient degradation of 2,4-DCP [31]. 100% of 2,4-DCP degradation can be obtained at 90 min over 4 wt. % Ce doped Mn2O3 at pH 7 (82.1% for Mn2O3 at 90 min). The reaction rate constant was also about 3.6 times higher than that of Mn2O3 (0.0668 min
1 vs. 0.0186 min
1). Besides 1 O2, •OH, and SO4•
were identified and involved in 2,4-DCP degradation process over Ce doped Mn2O3, while only 1O2 was detected over Mn2O3. Moreover, the contribution of •OH and SO4•
to 2,4-DCP degradation can be regulated by adjusting the Ce doping amount. Therefore, similar to Fenton-like reaction, utilizing the synergistic effect between different metal components can be the most common strategy to enhance the catalytic activity of catalysts. Direct Injecting Electrons from Carton Materials Metal-free carbonaceous materials can be used as a promising alternative due to the prevention of metal leaching and secondary contamination to the water environment. As aforementioned, carbonaceous materials such as GO, CNT, AC, and g-C3N4 with high specific area, good absorption property, and conductivity are promising metal

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supports in catalytic field. In such cases, the catalytic property of the metal species was likely to be affected by the carbon support. For example, Ding et al. fabricated a novel α-Fe2O3/MXene (FM) nanocomposite, which exhibited excellent degradation performance towards salicylic acid. The mineralization could reach 68.5%. As the efficient and fast conversion of Fe3+/Fe2+ redox pairs dramatically improved the catalytic performance of FM-2 nanocomposite, which was due to the synergistic effect of MXene and α-Fe2O3 [32]. The possible mechanism of PMS activation by FM nanocomposite is shown in Fig. 8a. Except for the transition metals activation, the metal-free carbonaceous materials, such as biochar, carbon nanotube, and activated carbon, can also activate PMS and effectively remove organic matters from water and show promising alternative catalysts for environmental remediation. Co-doping of N and S into industrial reduced graphene oxide (i-rGO-NS) was used for catalyzing oxidation methyl paraben (MP) and exhibited superior effectiveness than the classical metal catalysts (e.g., Co3O4 and Fe3O4) [33]. It was found that 1O2 was the main reactive oxygen species, revealing that MP degradation follows predominantly the nonradical oxidation pathway. The possible reaction mechanism is described in Fig. 8b. So as a kind of cheap and environmentally friendly materials, C-based materials will become a research prospect in the field of catalysis. Direct Injecting Electrons from Metal Sulfides Except as a cocatalyst, MoS2 was proposed as an emerging activator of both PMS and PS for the degradation of carbamazepine (CBZ) [34]. More than 95% CBZ could be degraded in 40 min from initial pH 3 to 9 in both MoS2/PMS system and MoS2/ PS system. Through the comparison with different Mo materials, the selectivity of Mo(V) towards PMS activation was highlighted. Not all the MxSy were capable of PMS and PS activation. The unexpected activity of MoS2 for PS activation enlarged

Fig. 8 (a) Schematic diagram of the possible mechanism of the PMS activation by the FM nanocomposite. (b) Mechanism of PMS activation on i-rGO-NS [32, 33]

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Fig. 9 Proposed activation mechanism of MoS2 activated PMS and PS reaction [38]

the source of sulfate radical generation. The related activation mechanism is shown in Fig. 9. Therefore, metal sulfides (MxSy) can not only serve as excellent cocatalysts to accelerate the rate-limiting step of Fe3+/Fe2+ conversion by the exposed reductive metallic active sites but also act as an emerging activator of PMS. Thus, MxSy also obtained increasing research attention in heterogeneous catalysis of PMS. The commonly used activators include WS2, CoS, and FeCo2S4. Direct Injecting Electrons from Chelating/Reducing Reagents As aforementioned, the chelating agents, such as EDTA, NTA, could decrease the Fe3+/Fe2+ or Co2+/Co3+ redox potential, thus greatly accelerate their cycle. Therefore, the addition of ligands during the PMS activated process can also enhance their catalytic activity. But the poor biodegradability and toxicity of traditional modifiers under natural environment are still hazardous to environmental safety. To satisfy both the environment safety and high catalytic performance at the same time, ascorbic acid (VC), epigallocatechin-3-gallate (EGCG) as nontoxic and economical antioxidant were applied to modify the widely used catalysts [35, 36]. Tan et al. synthesized two recyclable nanocomposites VC@Fe3O4 and EGCG@Fe3O4, which possessed a higher catalytic rate towards PMS compared with Fe3O4 for SD removal (7.24  10
2 min
1 for VC@Fe3O4, 5.41  10
2 min
1 for EGCG@Fe3O4, and 1.05  10
2 min
1 for Fe3O4) [35]. VC or EGCG would obviously boost the cycle of Fe2+-Fe3+ on catalyst surface and newly generated Fe2+ can continuously be consumed to activate PMS, which is the answer for the rapid generation of reactive radicals (•OH and SO4•
). The proposed mechanism of radical generation in EGCG@Fe3O4 activated PMS system is shown in Fig. 10.

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Fig. 10 Proposed mechanism of radical generation in EGCG@Fe3O4 activated PMS system [35]

Direct Injecting Electrons from Surface Defects Recently, Dong et al. fabricated a novel cost-effective Co3O4@illite catalyst with rich oxygen vacancies for catalyzing PMS to eliminate ATZ [37]. The OVs derived from the illite introduction reduced the coordination of some of the Co atoms in the surface octahedral sites, dramatically accelerated the electron-transfer, reduced the adsorption energy, and promoted the formation of more reactive oxygen species (SO4•
, •OH, and 1O2) (Fig. 11a). In our group, we also proposed an oxygen defect dependent PMS activation mechanism over perovskite with the 1O2 as the dominant ROS, which comes from the PMS spontaneous decomposition process [38]. Among the tested four perovskites, ofloxacin (OFX) degradation efficiency increased with the following order: LaFeO3 < LaZnO3 < LaMnO3 < LaNiO3, which agreed well with their oxygen defect amounts based on XPS and EPR analysis. The related reaction mechanism is shown in Fig. 11b. It is possible that the OVs not only have significant effects on the electronic structure, electron mobility, and surface properties of the catalysts, but also lower the reaction activation energy of the activation and decomposition of small molecules around the OVs. Thus, defect engineering plays an increasingly important role in catalytic reaction. And developing catalysts that possess rich surface oxygen vacancy are also an effective strategy for enhancing the catalytic activity of PMS activation.

Application of Persulfate Oxidation Technology in Industrial Wastewater Treatment One of the most difficult problems in industrial wastewater treatment is that the refractory organics contained in the wastewater cannot be removed by conventional process, resulting in the COD of effluent exceeding the standard. The development of sulfate radical (SO4•
)-based AOPs has unique advantages in the degradation of

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Fig. 11 (a) The schematic illustration of radical generation mechanism in Co@I/PMS system. (b) Illustration of related reaction mechanism over different perovskites [37, 38]

refractory organic compounds containing aromatic ring structure in industrial wastewater (Fig. 12). Professor Jinquan Wan’s team from South China University took the lead in building the world’s first persulfate and acid free advanced oxidation technology industrial wastewater treatment demonstration project in 2014, which relied on national water special project and Guangdong strategic emerging industry core technology research project [39]. It showed good operation effect when applied the SO4•
based AOPs in Zhongshan Yongfa Paper Co., Ltd., which produced 5000 tons paper wastewater every day. Under the pH value of wastewater nearly neutral, the removal rate of COD is kept at about 50–67%, and the COD of effluent is kept below 60 mg/L, which meets the requirements of national discharge standard of water pollutants for paper industry (GB3544-2008). At present, Professor Jinquan Wan’s team has successfully applied the technology to industrial wastewater treatment projects of many enterprises in Guangdong province, Hunan province in China.

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Fig. 12 SO4•
-based oxidation technology for application in industrial wastewater treatment project

Heterogeneous Catalysis Ozonation and Its Application in Water Purification Overview of Heterogeneous Catalysis Ozonation Since the discovery and naming of ozone by German scientist C.F. Schonbein in 1840, the application of ozone in water treatment field has developed quickly. It was well-known that ozone, as a strong oxidant (E0 ¼ 2.07 V) can react with numerous contaminants [40]. It is generally used as final stage for color and odor removal, but also for oxidation of specific pollutants and final disinfection. In addition, thanks to excellent disinfection and oxidation qualities, ozone is widely used for drinking water treatment. Ozone can be added at several points throughout the treatment system, such as during pre-oxidation, intermediate oxidation, or final disinfection. Usually, it is recommended to use ozone for pre-oxidation, before sand filters or with active carbon filters. However, the low solubility and stability in water and selective reaction with nucleophilic molecules for ozone alone limited its efficiency in water treatment. To solve the above problems, some advanced oxidation processes, such as O3/UV process, O3/H2O2 process, and the combination of O3 with biological treatment or other AOPs were investigated. Among these technologies, the catalytic ozonation processes have been widely studied and developed, as they can significantly promote the decomposition of O3 into more active species with a higher mineralization efficiency. And compared with ozonation using several transition metal ions (Co2+, Ti2+, Mn2+, etc), heterogeneous catalytic ozonation is cleaner, more economical, and more environmentally friendly. In general, ozonation of contaminants can proceed via two ways: (1) a direct molecular ozone reaction by destructing aromatic ring to form intermediates like small chain carboxylic acids, aldehydes and ketones through oxidation-reduction

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Fig. 13 (a) Possible reactions during the heterogeneous catalysis ozonation process. (b) Schematic representation of direct O3 reaction with unsaturated bond [40, 41]

reaction (Eqs. 22 and 23), cycloaddition reaction (Eq. 24 and Fig. 13b), electrophilic substitution reaction, and nucleophilic reaction; (2) an indirect active species mechanism involving the generation of highly oxidative hydroxyl radical (Eqs. 25, 26, 27, 28, and 29) [40, 41]. Regardless of the direct or indirect mechanism, how to improve the utilization efficiency of O3 is an essential step for achieving higher mineralization efficiency of pollutants. For instance, mono or mixed metal oxides, supported metals, and metal-free materials have been reported to be effective catalysts in accelerating ozone decomposition to generate reactive radicals. An understanding of the application can provide theoretical support for selecting suitable catalysts aimed at different kinds of wastewater to obtain higher pollutant removal efficiency. Therefore, the main objective of this section is to provide a summary of the accomplishments concerning catalytic ozonation to point to the major directions for choosing the catalysts in catalytic ozonation in the future.

Strategies for Enhanced Heterogeneous Catalysis Ozonation Activity and Related Reaction Mechanisms Catalytic reaction commonly takes place on the surface of the catalyst, so that the environmental function of most catalyst is associated with their structural characteristics. Thus, the typically used methods in Fenton-like reaction are also suitable for catalytic ozonation process. In addition, the resonance structure of ozone makes it possible to react with acid sites with its nucleophilicity as well as the base sites through its electrophilicity. Thus, adjust acid-base characteristics is another method to improve the catalytic activity with improved reactive sites. And a comprehensive summary of the obtained nanocatalysts based on the above design approaches with promoted catalytic activity will be given below.

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Improving the Number of Active Sites of Catalyst Preparation Catalysts with Large Specific Surface Area Zhu et al. fabricated ordered mesoporous Fe3O4 using KIT-6 as the hard template through a nanocasting route [42]. The mesoporous Fe3O4 presented superior catalytic activity for removing atrazine during ozonation compared to conventional Fe3O4 nanoparticles (82% vs. 25%). The redox cycles of Fe2+/Fe3+ were responsible for the generation of •OH and •OH dominantly contributed to the degradation of ATZ. In addition, some natural minerals or other metal oxides like Al2O3, SiO2 with abundant content, large surface area are also widely used as supports to improve the dispersion of catalyst. Liu et al. prepared CuFe2O4 loaded on natural sepiolite (CuFe2O4/SEP) for efficient catalytic ozonation of quinoline [43]. The mineralization efficiency over CuFe2O4/SEP/O3 was 90.3%, and it was 5.4 times higher than that of the uncatalyzed ozonation (16.8%). The excellent catalytic activity was attributed to the rapidly converted O3 and H2O2 into the •OH and O2•
during CuFe2O4/SEP/O3 catalytic progress. H2O2 was the product of the combination of two •OH radicals or the reaction of O3/•OH with unsaturated organics. The related reaction mechanism was described in Fig. 14. Morphological-Controlled Crystal Active Facet Exposure Different exposed crystal facets exhibit different atoms arrangement and coordination number, which decides the surface-active sites and reactivity. Wang et al. also reported morphology control studies of MnTiO3 nanostructures with exposed {0001}facets as a high-performance catalyst for water purification, and •OH, O2
•, and 1O2 were responsible for the 4-CP mineralization [44]. Due to the differences in the abundance of surface basic sites, defects densities OVs and coordination number of surface atoms in different exposed facets, ceria nanorods (R-CeO2), nanocubes (C-CeO2), and nano-octahedra (O-CeO2) with exposed facets of (110) + (100),

Fig. 14 Proposed degradation mechanism of catalytic ozonation by CuFe2O4/SEP for quinoline degradation [43]

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(100), and (111) exhibited different TOC removal efficiency for catalytic ozonation of p-nitrophenol [45]. Thus, the significance of the exposed crystal facets in interfacial reactions, ozone and organic intermediates may show different behavior on these surfaces which can lead to different catalytic activity. And selectively exposing crystal facets with coordinatively unsaturated sites is one of the most important and novel strategies to tailor the surface property. Regulation of Catalysts with Surface Acid-Base Sites As described in the section “Overview of Heterogeneous Catalysis Ozonation,” the resonance structure of ozone makes it possible to react with acid sites with its nucleophilicity as well as the base sites through its electrophilicity. For example, the non-transition metal catalyst nano MgO with many Lewis acid sites on the surface exhibits better degradation efficiency (91% vs. 54%) and TOC removal efficiency (46% vs. 33%) of quinoline than the pure O3 [46]. Thus, the MgO had a synergistic effect for the degradation of quinoline and the related reaction mechanism can be described in Eqs. 30, 31, 32, 33, 34, 35, and 36 and Fig. 15A. In addition, γ-Al2O3 was an amphoteric solid with Lewis acid AlOH (H+) sites and basic Al-OH sites resulted in a higher TOC and COD removal efficiency of 46%, 75% compared to O3 alone of 14%, 35%, respectively [47]. It is believed that only Al-OH basic sites in γ-Al2O3 are involved in catalytic ozonation in water and the hypothesis for the adsorption mechanism of carboxylic acids with the basic sites of -Al2O3 during ozonation is described in Fig. 15B. O3 þ Quiniline ! CO2 þ H2 O þ Intermediate

ð30Þ

OH þ Quiniline ! CO2 þ H2 O þ Intermediate

ð31Þ

MgO
O3 þ Quiniline ! CO2 þ H2 O þ Intermediate

ð32Þ

:

Fig. 15 (A) Mechanism of quinoline degradation in catalytic ozonation. (B) Expected interaction of O3 with (a) acid and (b) basic sites of -Al2O3 [46, 47]

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MgO
Quiniline þ O3 ! CO2 þ H2 O þ Intermediate •

MgO
S þ O3 ! MgO
SO þ O2 •

•

MgO
SO þ 2H2 O þ O3 ! MgO
SOH þ 3: OH þ O2 •

MgO
SOH þ Quiniline ! CO2 þ H2 O þ Intermediate

ð33Þ ð34Þ ð35Þ ð36Þ

Thus, catalysts with surface acid sites or basic sites can also serve as reactive sites promoting the decomposition of ozone into more active oxygen species and resulting in a higher mineralization efficiency. Regulation of Catalysts with Surface Hydroxyl Group Moreover, surface hydroxyl group of the catalyst has been proposed as active sites to initiate •OH formation. Wang et al. synthesized Ag-doped MnFe2O4, which showed excellent catalytic activity for the removal of di-n-butyl phthalate (75.3% (0.5%Ag/MnFe2O4) vs. 30%(O3)) [48]. It is believed that catalyst surface hydroxyl groups was a critical factor for ozone decomposition and subsequent • OH production. The related reaction mechanism and reactions are shown in Fig. 16, respectively. The above analysis shows that in addition to increasing the specific surface area of the catalyst, adjusting the number of acid-base sites and surface hydroxyl functional groups on the catalyst surface can also effectively increase the reaction active sites of the catalyst, thus enhancing the catalytic ozonation activity.

Fig. 16 The proposed reaction pathways during catalytic ozonation over Ag-doped MnFe2O4 catalyst and related reactions [48]

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Enhancing the Interface Charge Transfer Process Direct Injecting Electrons from Zero-Valent Metal Fe0 is an effective reductant (redox potential E0 ¼ 0.44 V) based on the in-situ generation of Fe2+ and atomic hydrogen (H) under anoxic condition. Combined Fe0 system with O3 can overcome the problems in traditional Fe0 system, such as the narrow working pH, precipitation of metal hydroxides and metal carbonates, and the low reactivity. Therefore, Xiong et al. used the micro-size Fe0/O3 (mFe0/O3) system for efficient removal of the p-nitrophenol and the COD removal efficiency was 89.5% after 60 min treatment. The COD removal efficiency was much higher than Fe0/air process (17.5%), MnO2/O3(45.5%), and Al2O3/O3(74.4%) [49]. The reaction mechanism of the mFe0/O3 process consists of four parts: (a) homogeneous catalytic ozonation of Fe2+/Fe3+; (b) heterogeneous catalytic ozonation of Fe3O4, Fe2O3 or FeOOH; (c) Fenton-like reaction; and (d) adsorption and precipitate, which are described in the following Fig. 17. In addition, Zn0, Cu0, Co0, and Al0 are also widely used in catalytic ozonation. Zhang et al. also compared the catalytic activity of aniline over Fe0, Co0, Al0, and Cu0 catalyst in the presence of O3 [50]. It was found that Cu0/O3 catalytic system exhibited the highest removal efficiency of 98% within 24 min, followed by the Co0/ O3, Fe0/O3, and Al0/O3 catalytic system. The reaction mechanism in Cu0/O3 catalytic system can be described in Eqs. 37, 38, 39, 40, and 41. O3 þ Cu0 þ 2Hþ ! O2 þ Cu2þ þ H2 O

ð37Þ

O3 þ 2Cu0 þ 2Hþ ! O2 þ 2Cuþ þ H2 O

ð38Þ

Fig. 17 Reaction mechanism of the mFe0/O3 process [49]

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O2 þ Cu0 þ 2Hþ ! O2 þ Cu2þ þ H2 O2

ð39Þ

O2 þ 2Cu0 þ 2Hþ ! O2 þ 2Cuþ þ H2 O2

ð40Þ

Cuþ þ H2 O2 ! Cu2þ þ : OH þ OH


ð41Þ

Synergistic Effect Between Different Metal Components An increasing number of bimetallic catalysts are becoming popular in heterogeneous catalytic ozonation, as bimetallic oxides usually present higher catalytic activity and stability compared with monometallic oxides. The synergistic effect of active sites on the surface Cu2+ and Al3+ in CuAl2O4 showed better catalytic ozonation activity for elimination of Acid Orange 7(AO7) than that of single CuO/O3 or Al2O3/O3 catalytic system and the reaction rate constant was 0.112, 0.071, 0.074 min
1, respectively [51]. It was worth mentioning that surface adsorbed •OH and O2•
played a vital role in the completely AO7 degradation and 87.2% of the COD removal within 25 min in CuAl2O4/O3 catalytic system. The related reaction mechanism is shown in Fig. 18. Besides, Fe3O4/Co3O4 composites were also reported to significantly improve the utilization efficiency of O3 and the mineralization of SMX by catalytic ozonation system, as more •OH was formed with the synergic effect between Fe3O4 and Co3O4 [52]. The TOC removal efficiency for SMX greatly increased from 16% (O3) to 60% in Fe3O4/Co3O4/O3 in 60 min. Thus, utilizing the synergistic effect between different metal components can be the most common strategy to enhance the catalytic activity of catalysts. Direct Injecting Electrons from Carton Materials Nanocarbons (rGO, MWCNTs, g-C3N4) have been demonstrated as promising environmentally benign catalysts for advanced oxidation processes upgrading metal-based materials. For example, the oxygen functionalized graphitic carbon nitride (O@g-C3N4) composite was successfully prepared and used as a metal-free catalyst for ATZ removal [53]. The introduction of O atoms into g-C3N4 framework

Fig. 18 Schematic diagram of synergistic effect between Cu2+ and Al3+ for efficient removal of AO7 (a) and related reactions (b) [51]

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could enhance the specific surface area, reshape the crystal growth, disrupt the sp2-hybridized carbon network, induce new electronic properties, and further create more active sites, resulting in an improvement of 29.76% on the degradation efficiency. The related reaction can be expressed as Eqs. 42, 43, 44, 45, 46, and 47. Moreover, the O@g-C3N4 showed excellent stability and reusability over multiple reaction cycles without obvious activity loss. O2 þ e ! O2 •


ð42Þ

2O2
• þ 2Hþ ! H2 O2 þ O2

ð43Þ

H2 O2 ! HO2
þ Hþ

ð44Þ

HO2
þ O3 ! HO2 • þ O3 •


ð45Þ

Hþ þ O3 ! HO3 •
! O2 þ • OH

ð46Þ

2O3 þ H2 O2 ! 2: OH þ 3 O2

ð47Þ

Moreover, Song et al. synthesized N, P, B, and S doped rGO for catalytic ozonation degradation of refractory organics with the order of P-rGO > N-rGO > B-rGO > rGO [54]. It was found that the delocalized π electron and surface functional groups of heteroatom-doped graphene played important roles in such high catalytic activity. The active free electron was able to be captured by O3 to form O3•
and HO3• that was rapidly transformed into •OH. Besides, the delocalized π electron also interacted with H2O to form hydroxide (OH
) and hydronium (H3O+) ions. Then, OH
stimulated ozone decomposition to form • HO2 and O2•
. Surface -OH group on the heteroatom-doped graphene interacted with the molecular ozone to form •OH and O2•
. The proposed mechanism of heteroatom-doped graphene-catalyzed ozonation was shown in Fig. 19. Direct Injecting Electrons from Surface Defects Oxygen vacancies were found to exhibit a strong affinity toward ozone adsorption, where ozone molecules spontaneously dissociated into reactive oxygen species such as O2•
and 1O2. Zhu et al. successfully prepared α-MnO2 nanofiber with controllable surface oxygen vacancy concentration by tuning the temperature and time of vacuum treatment [55]. The surface oxygen vacancy preferentially forms at sp3 oxygen site of (110) plane, would largely improve the adsorption and decomposition of ozone molecule on the surface of α-MnO2, resulting in that ozone removal rate at 20 h increased from 32.6% to 95%. The huge difference of ozone adsorption energy before and after oxygen vacancy forms indicates that the surface oxygen vacancy is likely to be the active site for ozone decomposition. Hierarchical Mn2O3/LaMnO3-δ perovskite composites were synthesized for catalytic ozonation over oxalic acid and benzotriazole [56]. It suggested that oxygen vacancy, the Mn3+/Mn4+ redox centers, and the surface hydroxyl groups were the potential active sites for ozone decomposition. DFT simulations and ESR data further revealed that ozone would be

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Fig. 19 Proposed mechanism of heteroatom-doped graphene-catalyzed ozonation [54]

spontaneously dissociated on oxygen vacancy to produce O2•
and 1O2, while Mn sites and surface hydroxyl groups facilitated ozone dissociation to •OH by stretching the O-O bonds.

Application of Catalysis Ozonation in Practical Water Treatment Full-Scale Application of Catalytic Ozonation for Drinking Water Treatment Liu et al. adopted a combination process of biofiltration, conventional treatment, catalytic ozonation, and upflow biological activated carbon for full-scale treating micropolluted raw water in Guanjinggang waterworks in China, and the present investigation mainly focuses on the effect of catalytic ozonation on the performance of BAC and water quality over a long period of 699 days [57]. The quality of finished water produced by the combination process is found to fully meet the drinking water standard of China. Catalytic ozonation is efficient in CODMn removal and can also enhance the biodegradability of the effluent. Variations of sedimentation effluent and water temperature have no influence on the CODMn removal by catalytic ozonation. Some ammonia can be removed and some organic nitrogen can be oxidized to NH4+-N during catalytic ozonation. In many cases, a slight increase of turbidity in water is found after catalytic ozonation, and the leaching of manganese from the ceramic honeycomb-supported Mn-Fe-K mixed oxide catalyst is also observed during catalytic ozonation. However, as an easy operation and a cost-effective technology for drinking water treatment, catalytic ozonation is promising for micropolluted raw water treatment in developing countries (Fig. 20).

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Fig. 20 Schematic of the drinking water preparation in Guanjinggang waterworks [57]

Combination of O3 and H2O2 Oxidation Technology for MTBE Remediation in Las Vegas In 2002–2003, MTBE concentrations of up to 42,000 ug/L were found near the former source area with an MTBE plume extending some 200 ft in most directions. In 2007, Broadbent completed a site assessment and remedial design for the project. A number of alternatives were evaluated including conventional pump and treat, air stripping with carbon adsorption, soil vapor extraction, in-situ chemical oxidation, as well as natural attenuation. The extent of MTBE plume was a limiting factor in the remedial selection as conventional alternatives proved costly due to the potential for long-term remediation and delays to project construction. APTwater’s PulseOx ® AOP process was identified as an excellent alternative for in-situ chemical oxidation of both BTEX and MTBE. Based on powerful O3 + H2O2 chemistry, the system’s AOP chemistry produces the •OH directly in the subsurface [58]. This provides 35–50% more powerful oxidation than ozone or peroxide-only systems. As a result, MTBE and BTEX compounds were rapidly destroyed to water and dissolved CO2 via an oxidative chain-reaction. A byproduct of the oxidation reaction is a tremendous increase in groundwater dissolved oxygen (DO) content. Overall, the PulseOx process effectively treats source area “hot spots” while also promoting natural attenuation across the site. The net result is efficient contaminant destruction and reduced clean-up times. Based on faster clean-up and more favorable economics, Broadbent selected the PulseOx process for remediation of the site. The alternative was approved by the Nevada Department of Environmental Protection (NDEP) subject to confirmatory pilot testing. Pilot testing was conducted in the fall of 2009 using a trailer-mounted PulseOx P-1000 unit capable of delivering 15 lb/day of ozone into eight injection points. The testing results showed a significant decrease in concentrations of MTBE and BTEX across the site. Within 3 months of testing, the size of the contaminant plume was reduced by nearly 75%, along with reductions in source area MTBE concentration from approximately 57,000 mg/L to below 15,000 mg/L (Fig. 21).

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Fig. 21 Interior view of P-1500 unit showing automatic ozone distribution manifold [58]

Heterogeneous Photocatalysis and Its Application in Water Purification Overview of Heterogeneous Photocatalysis Technology Photocatalysis oxidation technology, which utilizes the green, pollution-free, and inexhaustible solar energy to remove environmental contamination, has attracted tremendous interest due to their great potential in addressing increasingly severe global energy and environmental issues. Since Fujishima and Honda first reported photocatalysis on a TiO2 electrode in 1972, a large number of similar semiconductorbased photocatalysts, such as ZnO, BiOCl, WO3, Ag3PO4, and Bi2WO6 were developed to remove various organic pollutants in environment. As we all know, photocatalysis occurs when the photocatalyst absorbs a photon of energy equivalent to or higher than the band-gap energy (Eg) of the semiconductor, then photoinduced electron-hole pairs are then formed through migration of photogenerated electrons from valence band (VB) to conduction band (CB). The photogenerated e
/h+ pairs imitate oxidation and reduction reactions on the surface/ interface of photocatalyst, producing highly reactive oxygen species (e.g., O2•
, • OH), which can further oxidize organic contaminants into less hazardous molecules (CO2, H2O, and other inorganic products). The basic photocatalytic reaction mechanism is described in Fig. 22 [59].

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Fig. 22 Fates of photogenerated charge carriers in g-C3N4 upon absorbing solar [59]

However, there are still several pivotal questions concerning practical applications: (i) to reduce the cost of production for the purpose of commercialization and (ii) to improve the catalytic efficiency. The faster electron-hole pair recombination often results in lower photocatalytic activity. And most of the photocatalysts can only absorb the ultraviolet and visible light, which totally accounts for 56% of the whole solar spectrum. Thus, in order to improve the photocatalytic efficiency, developing strategies to improve the electronic hole separation efficiency and make full use of the residual 44% of nearinfrared light (NIR) are highly desirable. And many catalysts, such as noble metals, metal oxides semiconductors, and carbon-based materials based on different design strategies have been explored to achieve the above goal. Therefore, a comprehensive summary of the current achievements concerning heterogeneous photocatalysis will help the readers better understand the current situation and future development trend for using nanomaterials to solve energy-related and environmental issues worldwide.

Strategies for Enhanced Heterogeneous Photocatalysis Activity and Related Reaction Mechanisms As photocatalytic reactions are carried out on the surfaces of photocatalysts, the photocatalytic activity is closely related to the surface atomic configuration of semiconductors. Structure-reactivity relationships of materials are always the focus of research studies in heterogeneous catalysis. Hence, controllable synthesis of photocatalysts with specific morphology and structure has attracted widespread

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attention. Strategies including increase the catalyst surface or control crystal active facet exposure to improve more reactive sites or doped metals, coupling different semiconductors, combining semiconductors with noble metal or carton materials to improve the electron-hole separation efficiency, and extend the light absorption to longer wavelengths have been summarized in this part. In addition, constructing surface defects especially for the oxygen vacancy to enhance the photocatalytic reactivity were also introduced, which is of particular interest during recent research.

Improving the Number of Active Sites of Catalyst Preparation Catalysts with Large Specific Surface Area Arani et al. studied the effect of Co3V2O8 catalyst size on photocatalytic degradation of MB. It was found that the photocatalytic activity of Co3V2O8 nanostructures prepared in presence of H2salen Schiff-base is higher than bulk structures under UV light irradiation (87% vs. 25%, 120 min) [60]. By decreasing particle size of semiconductors, the surface area to volume ratio and finally available surface-active sites and interfacial charge-carrier transfer rates increased and thus leading to higher catalytic activities. Morphological-Controlled Crystal Active Facet Exposure Developing photocatalytic nanocrystals with preferable exposure of the high-energy facets and with visible light-driven catalytic activity has been an attractive but challenging topic. Typical examples include the nanocrystals of anatase TiO2 with preferable (001) exposure, ZnO with (0001) exposure. However, most of the photocatalytic reactions must be conducted under ultraviolet light irradiation due to their higher bandgaps. Thus, Ag3PO4 with preferable (110) exposure, WO3 with (001) facet exposure, ZnFe2O4 with {001}, {111} facet exposure are developed to further expand the light-responsive. For example, a synergetic effect of facet junction and specific facet activation for photocatalytic degradation of toluene was observed, where truncated octahedral ZnFe2O4 nanoparticles with both {001} and {111} facets exposed exhibited a superior performance than the others [61]. The formed surface facet junction between {010} and {100} facets was responsible for the improved activity by efficiently separating photogenerated e
/h+ pairs. Photogenerated electrons and holes were demonstrated to be immigrated onto {001} and {111} facets, separately. And both O2•
and •OH were the major reactive radicals involved in the photocatalytic process according to the ESR results. The related photocatalytic mechanism is shown in Fig. 23. Thus, rational design and facet-engineering of nanocrystal is an effective strategy to optimize the catalytic performance of abundant and economic semiconductor photocatalysts. As the interface usually serves as the channel for charge transfer and plays a crucial role in the separation of photoinduced e
/h+ pairs, which is largely dependent on the exposed specific facets of catalysts.

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Fig. 23 Illustration of the proposed photocatalytic degradation mechanism by ZnFe2O4 with {001} and {111} formed facet junction [61]

Bandgap Engineering with Efficiency Electron-Charge Separation and Wide Light Absorption Except for increasing the active sites, the photocatalysts with suitable bandgap can effectively suppress the combination of electrons and holes and broaden the range of light absorption, thus improve the photocatalytic activity. Element Doping in Semiconductors To date, different transition metal dopants, including Fe, Zn, Cu, Ni, Mn, Ba, and Co have been analyzed for their ability to enhance the photocatalytic performance. Owing to the expanded light absorption region and enhanced charge separation efficiency by forming a doping energy level in the bandgap of BiOCl. Co doped in BiOCl exhibited an outstanding photocatalytic activity for bisphenol A removal under visible light irradiation despite BiOCl is a typical UV-light-sensitive catalyst [62]. The reaction rate constant was seven times higher than that of ordinary BiOCl (0.021 vs. 0.003 min
1). The UV-vis-NIR DRS spectra of BiOCl, Co-BiOCl, and the BPA degradation mechanism over Co-BiOCl nanosheets is shown in Fig. 24a, b. Metal-doped is usually subjected to photo-corrosion, poor thermal stability, by contrast, non-metal (C, N, B, S, etc.) doping involving the higher photocatalytic activity, stability, and nontoxicity of dopant ions have been a popular doping technique. For example, the doped carbon element could distinctly induce the variation of physicochemical properties for Bi2MoO6, e.g., lattice expansion, particle size reduction, surface area enhancement, and CB edge upward shift [63]. Consequently, the as-prepared carbon doped Bi2MoO6 exhibited significantly enhanced photocatalytic performance in the removal of various organic pollutants, e.g., dyes and tetracycline hydrochloride. The enhanced photocatalytic performance could be

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Fig. 24 (a) The UV-vis-NIR DRS spectra of BiOCl, Co-BiOCl. (b) The BPA degradation mechanism over Co-BiOCl nanosheets. (c) The inner electric field and band edge bending at the interface of Cu2
xSe/CdS p-n junction; (d) The charge transfer mechanism between Cu2
xSe and CdS under visible light irradiation. (e) The proposed type-II staggered band alignment of Ag3VO4/ BiVO4 photocatalyst. (f) The photocatalysis enhancement mechanism of the Z-scheme Ag3VO4/ BiVO4 photocatalyst [62, 64, 65]

dominantly ascribed to the improved photogenerated carrier separation and transfer as well as the more photo-produced ROS. In respect of photocatalysis, the incorporated dopants would leave the catalysts with additional, isolated bands and excess charge carriers, which could extend the light absorption of photocatalysts and affect the band positions. Besides, the doped ions could also play the role of trap sites to capture photogenerated e
/h+, leading to reduced recombination of photogenerated charge carriers. Coupling of Different Semiconductors Besides, combining two or more metal oxide catalysts to construct semiconductor heterojunctions with suitable CB/VB position or building-in intrinsic field stands out as promising devices in dissociating photogenerated electron-hole pairs. For example, benefitting from the formation of Cu2
xSe/CdS p-n junction, the effective charge separation was obtained. The Rhodamine B (RhB) degradation rate in 10wt%Cu2
xSe/CdS is 7.8 times higher than that of pristine CdS [64]. The photocatalysis mechanism over Cu2
xSe/CdS junction is described in Fig. 24c, d. Due to the higher Fermi level of n-type CdS, electrons can be transferred from CdS to Cu2
xSe until their EF reach to the same level, thus formed an inner electronic field (Fig. 24c). Under visible light irradiation (Fig. 24d), the electron can be excited from VB to CB of both CdS and Cu2
xSe. Driven by inner electronic field in the junction, the electrons in the CB of Cu2
xSe will transfer to the CB of CdS and the hole in the VB of CdS will transfer to the VB of Cu2
xSe to achieve effectively electron-hole separation. The electron reacts with dissolved O2 to form superoxide

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O2•
, while the holes can react with surface-bound OH
to generate •OH. Finally, RhB can be efficiently degraded to H2O and CO2. Moreover, Ag3VO4/BiVO4 heterojunction was proven to be an efficient photocatalyst for the reduction of Cr6+ and oxidation of Bisphenol S under visible light irradiation [65]. And the highest reduction and oxidation efficiency were 74.9%, 94.8%, respectively. The enhanced photocatalytic performance is attributed to the built-in electric field assisted charge transfer between Ag3VO4 and BiVO4, and the increasing lifetime of the charge carrier confirmed by the results of time-resolved fluorescence spectra and photoelectrochemical measures, which follow a typical Z-scheme charge transfer mechanism, as described in Fig. 24e, f. Coupling of Semiconductors and Noble Metal The deposition of noble metal (e.g., Ag, Au, Pt, and Pd) nanoparticles (NPs) on photocatalyst surfaces offers a new way to improve the photocatalytic activity by the localized surface plasmon resonance (LSPR) effect. LSPR-induced improvement of semiconductor photocatalysts is mainly attributed to the extension of light absorption to longer wavelengths in the visible light spectrum and the resulting generation of more e
/h+ pairs. For example, due to the surface plasmon resonance (SPR) effect induced by Au NPs and the cooperative electronic capture properties of Au, Bi2MoO6, and TiO2, an extended absorption in visible light region, effective electron-hole pairs separation was achieved during the photodegradation of methylene blue [66]. Thus, despite a degree decrease compared with that in UV-light irradiation (82% in 120 min), Au/Bi2MoO6@TiO2 still possess a much higher degradation efficiency (68% in 120 min) than that of the pure TiO2 (8%), Bi2MoO6@ TiO2 (19%) in visible light radiation and the corresponding mechanism is shown in Fig. 25. However, most of the noble metals are usually expensive which will increase the cost of catalyst preparation and thus bring difficulties for the industrial applications. Owing to the unique characteristics of high electrical conductivity, high specific surface area, good chemical stability and good corrosion resistance, carbon

Fig. 25 Schematic diagram of electron-hole separation mechanism for Au/Bi2MoO6@TiO2 NTAs electrodes and related reactions [66]

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materials, such as graphene, CQDs, and g-C3N4 have been extensively used as cocatalyst to improve the visible light absorption and the transfer efficiency of photogenerated electron-hole pairs. Their unique structure and characteristics are obviously seen in Fig. 26a–c [67–69]. Especially for the g-C3N4 with a suitable bandgap to absorb visible-light radiation has been considered as an alternative candidate for practical applications. For example, compared to pure BiVO4, Ag3PO4, Ag/Ag3PO4/BiVO4/RGO displayed more superior photodegradation efficiency with 94.96% removal of TC in 60 min. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area, and desirable absorption capability, as well as the synergistic effects of RGO and Ag/Ag3PO4. The related reaction mechanism is shown in Fig. 26d [70]. Similarly, m-Bi2O4/NCDs photocatalyst exhibits excellent photocatalytic activity for the degradation of methylene orange (MO) and phenol under visible light irradiation. The pollutants of MO and phenol could be efficiently degraded by m-Bi2O4/NCDs within 30 and 120 min, respectively, which is much better than that of the single m-Bi2O4, indicating that the introduction of NCDs into m-Bi2O4 can effectively improve the photocatalytic activity [71]. Besides, the g-C3N4/MnO2 exhibited greatly improved photocatalytic activities for dye degradation and phenol removal in comparison to the single g-C3N4 or MnO2 component. The band structure and photocatalytic mechanism of g-C3N4/MnO2 is shown in Fig. 26f [72]. Surface Defect Engineering Recently, many researchers have been attracted to construct oxide semiconductor heterostructure surface oxygen vacancies is fabricated, which shows faster degradation rate towards the tetracycline removal under the visible-light irradiation compared to the ordinary ZnO (0.0487 vs. 0.0135 min
1) [73]. The enhanced photocatalytic activity can be attributed to the hexagonal porous structure offering more surface-active sites and the surface oxygen vacancy defects favoring the electron-hole separation and visible-light absorption. Moreover, surface vacancies can also expand light absorption at longer wavelengths even in the NIR light region. Bi2O4-Bi4O7-BiO2
x with rich oxygen vacancy has been successfully prepared and its absorption spectra is successfully extended to NIR light region compared with bare Bi2O4 and Bi4O7, implying that more charge carriers can be generated under NIR light irradiation (Fig. 27) [74]. The bandgap of Bi2O3, Bi2O4, Bi4O7, BiO2
x, Bi4O7-BiO2
x, and Bi2O4-Bi4O7-BiO2
x are calculated to be 2.82, 2.12, 1.77, 1.68, 1.65, and 1.70 eV, respectively. Besides, oxygen vacancies served as the e
-h+ mediator further promotes the interfacial charges migration, thus Bi2O4-Bi4O7BiO2
x exhibited significantly enhanced performance for the BPA removal under visible and NIR light irradiation, compared with single or mixed bismuth oxides (Bi2O4, Bi4O7, BiO2
x, and Bi4O7-BiO2
x). Thus, defect states derived from OVs can act as electron traps to inhibit the recombination of photogenerated electron-hole pairs as well as prolong photoinduced carriers’ lifetime, and further enhance the photocatalytic activity of the as-prepared samples.

Application of Heterogeneous Nanocatalysis-Based Advanced Oxidation. . .

Fig. 26 (a) The superior properties of pristine graphene. (b) Quantum confinement effect of carbon quantum dots and their associated πp* transition. (c) The redox potentials of the relevant reactions with respect to the estimated position of the g-C3N4 band edges at pH 7. The proposed mechanism for photogenerated charge carrier transfer: Ag/Ag3PO4/BiVO4/RGO (black sheet represents graphene in the diagram) (d); N-CQDs/Bi2O4 heterojunction (e); and g-C3N4/MnO2 nanocomposite (f) [68–73]

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Fig. 27 (a) UV-vis DRS and (b) plot of transferred Kubelka-Munk versus the energy of light of the as-prepared samples. (c) Schematic diagram of flowing of photoinduced carriers in the Bi2O4Bi4O7-BiO2
x heterojunction [74]

Application of Photocatalysis Technology in Practical Water Treatment Application of Photocatalysis Technology for the Removal of Amoxicillin Amoxicillin manufacturing site of GSK in Singapore is one of the biggest sites for the production of antibiotics worldwide. It is well known that biological treatment plants are not very capable of treating the strong wastewater from antibiotic manufacturing lines, due to the effect of its ingredients, which resulted at GSK Singapore in incinerating the wastewater. GSK-manufacturing site was searching for a more suitable process to be applied for the removal of several substances from the strong wastewater. A treatment system has been installed (see Fig. 28a, b) where the capacity of strong wastewater was increased from 54 m3/day stepwise to 100 m3/day [75]. The specific UV-process is removing toxicity and phenol-based active structures and is increasing the bioavailability of the strong wastewater resulting GSK stopping incineration of this wastewater (see Fig. 28c, d and Table 2). Due to pre-treatment by photo-oxidation, the existing biological system at GSK is also able to handle this wastewater. For all environmental activities of GSK including the installation of the photo-oxidation, GSK (together with other sustainability projects) was labeled with a Singapore Environment Achievement Award. Combination of UV-H2O2 Oxidation Technology for the Removal of Active Pharmaceutical Ingredients A leading American pharmaceutical company operates a production site that produces wastewater with high COD amounts containing various active pharmaceutical

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Fig. 28 (a) Treatment installation at GSK Singapore (Source: GSK). (b) Photo-oxidation installation placed at GSK for elimination of residual Amoxicillin and non-bioavailable organics in strong process wastewater. (c) Parameters TOC, COD, and BOD5 during the treatment at GSK Singapore. (d) Parameters phenols and bioavailability during the treatment at GSK Singapore [75]

Table 2 Data of treatment at GSK Singapore [75] Parameter Flow rate COD Phenol-based organics from amoxicillin process Bioavailability OPEX

Data 54–100 m3/day 58,000– 70,000 mg/L ≈1000–5000 mg/L

Result

≈0% ≈20 €/m3

≈70%. . .0.90%

≈40%. . .0.50% reduction rate 80%

Conclusions and Outlooks In this chapter, four typical AOPs including Fenton catalysis, persulfate activation, ozone oxidation, and photocatalysis are provided with a general overview. The strategies with enhanced heterogeneous catalysis, such as the common methods for adjustment of the surface area and facet exposure to improve the catalytic reactive sites; doped metals, composite metal oxides, introducing additional electrons (i.e., carton materials, chelating/reducing reagents) to promotes the decomposition of oxidants (H2O2, O3, PMS) into more ROS (•OH, O2•
and 1O2) in Fenton catalysis, persulfate activation, ozone oxidation or improve the separation ability of e
/h+ in photocatalytic reactions. Moreover, the surface defect engineering has demonstrated to be an effective strategy to enhance the catalytic activity in heterogeneous catalysis technology. Despite successes in laboratory, translation and commercialization of the technologies discussed herein has been generally slow, which points to the need for future research to rationally design nanomaterials for the intended purpose of water treatment. Significant hurdles must be overcome before bench-scale nanomaterials discoveries can be translated to water treatment practices.

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1. It is worthy to notice that the above-mentioned technology has its unique advantages and limits. For example, ozone is generally used as the final stage for color and odor removal, but also oxidation of specific pollutants and final disinfection. Thus, for practical application, it usually needs a combination of various treatment technologies and their synergistic effect will lead to more efficient water restoration quality. Thus, a combination of various oxidation technologies will still be a trend of future development in water treatment. 2. Innovative nanocatalysts not only present opportunities for applications outside the realm of traditional fields but they also broaden the utility of AOPs. Developing catalysts with the characteristics of low cost, high efficiency, and no secondary pollution will be the primary consideration for the application of heterogeneous catalysis in water remediation. For example, biochar from the artificial burning or pyrolysis of biomass, or sludge from the wastewater has shown promising catalysts for water remediation. Especially the development of defect engineering further improves the catalytic performance. 3. In addition, a critical issue frequently neglected in research focused on nanomaterials development is diminished performance in complex water matrices. Many studies test the oxidation efficiencies of nanomaterials using synthetic water containing only target pollutants (thus lacking in background organic content) and often at concentrations much higher than the norm for actual influents. In reality, however, natural organic matter such as humic substances from plant decomposition typically exists in much larger quantities than pollutants of concern and is known to decrease AOP efficiencies due to radical scavenging. To this end, nanomaterials can be developed with preferential adsorption or size exclusion properties.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gold Nanoparticle-Enhanced Radiotherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiotherapy Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiation Interactions of Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of Adding Gold Nanoparticles in Radiotherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concept of Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Macroscopic and Microscopic Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recent Progress in Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dose and Imaging Contrast Enhancement in Gold Nanoparticle-Enhanced Radiotherapy . . . Dose Enhancement Using Kilovoltage Photon Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dose Enhancement Using Megavoltage Electron Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Imaging Contrast Enhancement Using Kilovoltage Photon Beams . . . . . . . . . . . . . . . . . . . . . . . Dose Enhancement Using Megavoltage Flattening-Filter-Free and Flattening-Filter Photon Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Imaging Contrast Enhancement Using Megavoltage Flattening-Filter-Free and Flattening-Filter Photon Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and the Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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J. C. L. Chow (*) Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_2

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Abstract

In radiotherapy, ionizing radiation causes deoxyribonucleic acid (DNA) damage in cancer cells, so that the cancer can be controlled by terminating the reproduction of these affected cells. DNA is a biological molecule in nanometer scale in a living cell necessary for cell reproduction. Adding heavy-atom nanomaterial as radiosensitizer to the cancer cells can enhance the DNA damage in the cells, as the irradiated cancer cells can be more easily detected by medical imaging. This is because the addition of heavy-atom nanomaterial in the cancer cell increases the compositional atomic number of the cell, resulting in a photoelectric interaction enhancement with increased secondary electron yield for energy deposition. The gold nanomaterial addition to the cancer tumour benefits radiotherapy that produces a highly conformal dose distribution covering the cancer target, while sparing the surrounding critical organs. We use Monte Carlo simulation to study the nanodosimetry of DNA, guiding us to evaluate the effectiveness of nanomaterials such as gold nanoparticles transported to the cancer cells. Monte Carlo simulation is a mathematical method to model the interaction between the DNA and radiation beam. In this chapter, we review the basic concept and recent progress of Monte Carlo simulation used in predicting the dose and imaging contrast enhancement in gold nanoparticle-enhanced radiotherapy. We also review the dose and imaging contrast enhancement ratios among different nanoparticle materials using different photon beam energies in the kilovoltage range. In gold nanoparticle-enhanced radiotherapy using the flattening-filter-free and flattening-filter photon beams, we examine the recent results of dose and imaging contrast enhancement between these photon beams using Monte Carlo simulation.

Introduction Recently, there is a rapid progress of technology in nanomaterials and nanocomposites in various applications. Biomaterials such as gold nanoparticles used in cancer therapy can act as an effective radiosensitizer in radiotherapy [1]. To kill or control a cancer cell using ionizing radiation in radiotherapy, sufficient deoxyribonucleic acid (DNA) damage in the cancer cell should be produced by the radiation. This leads to the termination of the cancer cell reproduction, because DNA is essential in the cell division. Cancer cell stopped reproduction due to the DNA damage means that the tumour in the patient is controlled by radiation. In radiotherapy, if we can produce a highly conformal dose coverage at the tumour volume, we not only increase the cancer cell kill but also spare the normal tissues and critical organs surrounding the tumour. This treatment goal can be achieved by adding heavy-atom nanomaterials to the tumour [2–4]. When heavy-atom nanomaterials such as gold nanoparticles are added to the tumour, the compositional atomic number of the tumour volume is increased. As the photoelectric interaction depends on the atomic number (Z), increasing the

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atomic number leads to an increase of photoelectric interaction [5, 6]. This results in more absorption of incident radiation and secondary electron yield. The increase of incident radiation absorption due to gold nanoparticle addition causes an increase of imaging contrast of the tumour [7], while the increase of secondary electrons emitted from gold nanoparticles causes more energy deposition at the cancer cell, leading to more DNA damage [8]. Adding gold nanoparticles to the tumour enhances its imaging contrast. Therefore the tumour can be visualized more accurately in imaging module such as computed tomography in radiation treatment planning. On the other hand, tumour control is also enhanced, because the increase of secondary electron yield from gold nanoparticles provides extra DNA damages in the cancer cells in radiotherapy. Subsequently, a new radiotherapy option called gold nanoparticle-enhanced radiotherapy or nanoparticle-assisted radiotherapy is developed, and many cell and preclinical studies have been carried out. For cancer cell study, Zhang et al. [9] irradiated the human erythroleukemia K562 cells added with gold nanoparticles using a Cs-137 radioactive source. Zhang et al. [9] found that gold nanoparticles had a significant radiosensitization effect on the cell, and increasing the concentration of gold nanoparticles increased the cancer cell kill. Berbeco et al. [10] irradiated the HeLa cells using the clinical 6 MV photon beams. A significant increase of DNA damage was found when gold nanoparticles were added to the cell. Berbeco et al. [10] also found out the relationship between the irradiation depth and the low-energy photon from the beam. For preclinical study, Hainfeld et al. [11] treated mice with cancer (EMT-6 mammary carcinomas) using radiation with and without gold nanoparticle addition. They found that the results of 1-year survival of mice were 86% compared to 20%, when gold nanoparticles were added to the tumour. Both cell and preclinical results showed that adding gold nanoparticles to the tumour can increase the cancer cell kill. Optimizing the gold nanoparticle addition to the tumour requires a thorough understanding of the relationship between various nanoparticle parameters and dose/imaging contrast enhancement. The nanoparticle parameters include nanoparticle size, concentration, distribution pattern, material, and shape. Moreover, the dose/imaging contrast enhancement also depends on the energy, type, and quality of the radiation beam. To predict interaction between the incident radiation beam and nanoparticle in a cellular medium, an accurate and precise nanodosimetry model is needed [12]. Monte Carlo simulation is proved an effective method to predict the dose and imaging contrast enhancement, when gold nanoparticles are added to the cell under irradiation [13]. Monte Carlo method is a broad class of computational algorithm replying on repeated random sampling to determine a numerical result, and the accuracy depends on the number of trials (or histories) in the simulation. Monte Carlo simulation has been used as a benchmark to predict dosimetry, imaging contrast, internal organ motion, and patient waiting time in radiotherapy [14, 15]. Monte Carlo simulation was used here to predict dose and imaging contrast enhancement in gold nanoparticle-enhanced radiotherapy. In this chapter, we review some recent progress of Monte Carlo simulation results regarding dose and imaging contrast enhancement, using different energies and

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types of radiation beams. We also examine the dependence of dose/image contrast enhancement on nanoparticle material such as gold, platinum, silver, iron oxide, and iodine. Recently, flattening-filter-free photon beam is used in radiotherapy, taking advantage of its very high dose rate in intensity-modulated radiotherapy [16, 17]. We review effects of energy distribution of low-energy photon in the dose and imaging contrast enhancement, using the flattening-filter-free photon beams in gold nanoparticle-enhanced radiotherapy.

Gold Nanoparticle-Enhanced Radiotherapy Cancer is one of the main causes of death in human, and the number of patient increases rapidly due to the aging population. Cancer can generally be treated by surgery, chemotherapy, and radiotherapy delivered individually or in various combinations. Radiotherapy is important because more than 50% of cancer cases are treated by radiation [18]. Gold nanoparticle-enhanced radiotherapy is a novel radiotherapy option with gold nanoparticle addition to the patient’s tumour in the dose delivery. To understand this treatment method, we first review the general radiotherapy chain in cancer treatment. We then illustrate the physical and chemical characteristics of gold nanoparticles and explain how they interact with radiation in the cellular medium. Finally, we review the dose and imaging contrast enhancement due to gold nanoparticle addition in radiotherapy.

Radiotherapy Chain Radiotherapy chain is the cancer treatment process containing different steps from diagnosis and prescription to completion of treatment in a cancer center. A typical radiotherapy chain is shown in Fig. 1. When cancer in the patient has been diagnosed and the patient is sent to the cancer center, radiation dose prescription to the tumour/target is made by the radiation oncologist as per the routine radiotherapy protocol. To prepare the radiation dose delivery, the patient is sent to a simulation unit (usually computed tomography simulator) to acquire the anatomy information in form of a 3D image set. The patient is set up on the couch of the simulator with the geometrical position mimicking the condition in the real dose delivery inside the treatment room. Sometimes, immobilization device is used to help the positioning of patient if necessary. The acquired image set of the patient including the tumour and surrounding critical organs is then handled by the radiation oncologist to contour the target (tumour) and all organs-atrisk. This contoured image set is transferred to the treatment planning system. A dosimetrist or planner performs radiation treatment planning to create a plan which fulfills the dose prescription at the target and sparing of other organs-at-risk. Dose distribution in the patient is calculated by the planning system using dose calculation methods such as convolution-superposition algorithm or Monte Carlo simulation. The finished treatment plan and calculated dose are verified and

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Fig. 1 A radiotherapy chain showing different steps in the radiation treatment process

approved by the medical physicist and radiation oncologist. A verification setup of the patient can be carried out to validate if the treatment plan is deliverable in the treatment unit. This includes checking whether the gantry of the medical linear accelerator would collide with the patient or the treatment couch. Treatment delivery then starts and after all prescribed dose is delivered, the radiation treatment is complete.

Gold Nanoparticles Regarding the dimension of particles, nanoparticles have a very small size of 1–100 nm in diameter. Fine particles have size of 100–2500 nm, and coarse particles have size of 2500–10,000 nm. Since particle is a solid quantity of matter, it is seen that nanoparticle has a very large surface-to-volume ratio. This large ratio causes nanoparticle processing some nanoscale size-dependent properties compared to the bulk material. One example is that nanoparticle has special optical effect because it is small enough to confine electrons and produce quantum effects [19]. Long ago in the ninth century, it is well-known that gold nanoparticles could be used in form of colloidal suspension in water for staining glass coloring. Nowadays, one reason why gold nanoparticles are so popular in various applications such as therapeutic agent delivery, biological imaging, transmission electron microscopy, and fuel cell is that it is convenient to fabricate. Gold nanoparticles can be produced through a chemical reduction of chloroauric acid, H[AuCl4], by adding

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a reducing agent. The gold ions from the chloroauric acid are reduced to gold atoms, when the acid and reducing agent are mixed and stirred rapidly and become supersaturated. The uniformity and size of gold nanoparticles can be controlled by the degree of stirring in the resolution of chloroauric acid and reducing agent. Gold nanoparticles can also be fabricated using different chemical methods such as the Brust method, Martin method, and Turkevich method [20–22]. In the biomedical application of gold nanoparticles, transport of nanoparticles to the target (living cell) in the patient is important. In small-animal experiment, gold nanoparticles are delivered to the tumour through injection [11]. This is based on the increase of permeability within the tumour vasculature due to the angiogenesis. When the gold nanoparticle reaches the living cell, cellular uptake happens through endocytosis. That is, the cell engulfs the nanoparticle surrounded by the plasma membrane. It should be noted that the gold nanoparticle only entered the cell to form a vesicle but does not make contact with the cytoplasm. It is found that the cellular uptake of gold nanoparticles depends on the size and shape of the nanoparticles. It is also found that the cellular uptake rate depends on the aspect ratio of the gold nanoparticle. The cellular uptake rate of the rod-shaped gold nanoparticle was lower than that of the spherical-shaped [23]. Due to the high surface-to-volume ratio of gold nanoparticle, a protective layer is needed as a cover to lower down the very high reactivity of the nanoparticle. The material of the layer is used to be polymer such as anionic poly(acrylic acid), neutral poly(2,3-hydroxy-propylacrylamide), and thermoresponsive poly(N-isopropylacrylamide). There is a concern that the protective layer may affect the performance of the nanoparticle in radiotherapy. He et al. [8] compared the dose enhancement due to the gold nanoparticle addition with and without the protective layer. They found that the presence of the layer did not affect the secondary electron yield from the nanoparticle and hence the dose enhancement using Monte Carlo simulation.

Radiation Interactions of Gold Nanoparticles In radiobiology, a cancer cell kill depends on a critical DNA damage called doublestrand break that both strands are cleaved by interactions with ionizing radiation. As the cancer cell with that damaged DNA cannot continue to reproduce, after the current cell cycle, the cell is terminated or “killed”. Ionizing radiation is radiation that has enough energy to produce an ion pair (i.e., a free electron and positive ion) from an atom. For example, electromagnetic radiation having wavelength shorter than 10
8 m is recognized as ionizing radiation. When a clinical photon beam from the treatment unit irradiates the tumour, there are basically three photon interactions, namely, photoelectric, Compton, and pair production, that occurred in the irradiated volume. Compton effect is the predominant interaction in the therapeutic beam energy range of 50 kVp–25 MV. When the energy is below 50 kVp, the photoelectric interaction is most important. Pair production is the primary interaction at beam energy above 25 MV.

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In photoelectric interaction, the incident photon collides with a tightly bound electron in an atom. The photon transfers energy to the electron, which breaks away the atom and begins to ionize surrounding molecules in the medium. The photoelectric interaction depends on the energy of the incident photon and atomic number of the target. The lower the photon energy and the higher the atomic number of the target, the more likely that a photoelectric interaction happens. In Compton effect, the incident photon collides with an outermost electron of an atom. Both the photon and electron are therefore scattered, and the incident photon can continue to undergo additional interactions with lower energy. The emitted electron, on the other hand, begins to ionize with the energy transferred by the photon. The Compton crosssection is inversely proportional to the energy of the incident photon and is independent to the atomic number of the target. It is the most important photon-tissue interaction in radiotherapy. Pair production involves the nucleus of an atom interacting with a photon. The incident photon transfers energy to the nucleus and creates a pair of positron and electron. The positron ionizes until it combines with a free electron in the medium, which generates two photons scattering in opposite directions. Pair production only happens when the beam energy is higher than 25 MV. The probability of pair production is proportional to the logarithm of energy of the incident photon and the atomic number of the target. As it is rare to use radiation beam with energy higher than 25 MV in cancer treatment, pair production is not an important particle interaction in radiotherapy. Apart from the secondary electrons emitted by the photoelectric interaction, Compton effect, and pair production, electrons can also be produced by other physical processes such as beta ray, internal conversion, and Auger effect. These electrons interact in the medium and deposit energy as radiation dose. Since adding gold nanoparticles to the tumour increases its compositional atomic number, the photoelectric interaction is enhanced by the kilovoltage photon beam in gold nanoparticle-enhanced radiotherapy [12, 24]. Therefore, it is found that the benefit of gold nanoparticle addition to the tumour is larger when using kilovoltage photon beam compared to megavoltage.

Advantages of Adding Gold Nanoparticles in Radiotherapy Since adding gold nanoparticles to the tumour results in an increase of compositional atomic number, photoelectric interaction enhancement is possible when photon beam in the kilovoltage range is used in radiotherapy [3, 5]. The increase of photoelectric cross-section leads to two advantages in gold nanoparticle-enhanced radiotherapy, namely, imaging contrast enhancement and dose enhancement at the cancer target. Gold nanoparticles are well-known as an imaging contrast agent in radiotherapy [7]. In radiology imaging, contrast depends on the difference in photon beam absorption in the patient’s tissues. The absorption of human tissue is related to the attenuation and scattering of the photon beam in terms of mass attenuation coefficient. In computed tomography simulation using the kilovoltage photon beam,

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the mass attenuation coefficient contains components from the photoelectric interaction, Compton effect, and coherent scattering. Since photoelectric interaction is dominant in the kilovoltage energy range, the interaction is considerable when the photon beam energy is higher than or almost equal to the electronic binding energy. Although iodine has a relatively higher atomic number than human tissue and is a popular element used in x-ray imaging, more powerful imaging contrast agents are being developed. For example, gold nanoparticles are found better than iodine in the imaging contrast enhancement, because gold absorbs more photons than iodine [25]. In addition, gold nanoparticles are relatively nontoxic compared to iodine-based agent regarding adverse reactions such as anaphylactic shock and kidney failure. When gold nanoparticles are used as contrast agent, they are coated with a protective layer to maintain their stability [8]. The agent surface is further covered by a soluble biocompatible layer for water/solvent solubility. In preclinical model, Hainfeld et al. [26] found that the gold nanoparticle contrast agent can be applied to small animals through intravenous injection. Dose enhancement can be achieved by adding gold nanoparticles to the cancer target. The increase of photoelectric interaction using the kilovoltage photon beam produces extra secondary electrons depositing energy in the cancer cell [12]. This enhancement of energy deposition causes more lethal DNA damages such as doublestrand break and therefore cancer cell kill [8]. Dose enhancement due to gold nanoparticle addition has been studied by many groups, and encouraging results of cancer control were reported in the cell and preclinical model [27–29]. For example, Herold et al. [30] irradiated cancer cell uptaken gold nanoparticles with sizes of 1.5–3 μm diameter, using kilovoltage photon beams. They found a reduction of excised cell plating efficiency showing the irradiation was effective. Hainfeld et al. [11] irradiated the tumour of mice with gold nanoparticle addition; they found an increase of 1-year survival rate of 86% compared to 20% with irradiation alone. Apart from photoelectric electrons emitted by gold nanoparticles, some groups found that very low energy electrons (3–20 eV), produced by cascade effect of ionizing photon beams when interacting with the medium, played an important role in the DNA damage such as the double-strand breaks [31–33]. Their works prompted us to review our understanding of cancer cell kill, which happens when the DNA is interacted with the secondary electrons generated by the ionizing radiation [24].

Monte Carlo Simulation Developing a new generation of radiosensitizer using gold nanomaterials requires a robust nanodosimetry model, predicting the relationship between the physical parameters of the irradiated nanoparticle and energy deposition in the cellular medium [2]. The parameters of the nanoparticle include the nanoparticle size, shape, concentration, distribution pattern, coating, and material. The radiation beam parameters include the beam type, energy, and field size [13]. Moreover, the distance between the nanoparticle and the DNA and the distance between the

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radiation source and the nanoparticle both affect the secondary electron yield of the nanoparticle [8]. These complex relationships regarding the nanoparticle characteristics can be determined using Monte Carlo simulation, which greatly saves many resources in experimentation. In the following sections, we introduce the concept of Monte Carlo simulation, the macroscopic and microscopic approach of simulation and some recent progress regarding gold nanomaterial nanodosimetry.

Concept of Simulation Monte Carlo simulation or experiment is a computational method to predict a numerical result from a problem by repeating random sampling of an experiment. The greater the number of the experiment (or number of histories) carried out, the closer is the numerical result to the absolute solution. The simulation method is named “Monte Carlo” after the gambling spot in Monaco. It reflects the modelling technique depending on the chance and random outcomes like those gambling games in the casino such as dice, roulette, and slot machine. Since Monte Carlo simulation is based on random sampling and statistical analysis, the random sampling can be carried out using true or pseudorandom numbers. The true random number is generated by measuring some physical phenomenon, for example, cosmic background radiation and thermal noise. The pseudorandom number is generated by a computational algorithm as a pseudorandom number generator. This generator depends on the seed value input in the beginning, and the random sequence can be reproduced if the seed value is known. As it is pseudorandom, the sequence period is close to the theoretical upper bound of m factorial. Monte Carlo simulation is mainly used in three distinct problem classes, namely, optimization, numerical integration, and generating draws from a probability distribution. The simulation has a wide range of applications in different fields such as radiotherapy, financial market simulations, traffic flow simulations, environmental science, astrophysics, molecular modelling, and quantum field theory [34, 35]. In radiotherapy, Monte Carlo simulation can be used to predict the patient waiting time, internal organ motion, medical imaging, dose calculation for treatment planning, and DNA damage in nanoparticle-enhanced radiotherapy [12, 14, 26, 36]. In this chapter, we focus on using Monte Carlo simulation to predict the particle transport in a biological environment. The particle transport estimated by the simulation was used to determine the photon and electron interactions with gold nanoparticles in a cellular medium. In radiotherapy, perhaps the most popular application of Monte Carlo simulation is to calculate the radiation dose in a treatment plan. It is well-known that Monte Carlo simulation has been the benchmark of dose calculation on heterogeneous media such as bone, lung, air, and soft tissue [15]. Unlike other commercial dose calculation methods such as pencil-beam and convolution-superposition, Monte Carlo method does not depend on the assumption of charge particle equilibrium [37]. Therefore, it has a higher accuracy than other calculation methods especially for the dose deposited by small segmental photon fields. For particle

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transport in radiotherapy, there is a variety of Monte Carlo codes such as Geant4 [38], FLUKA [39], and PENELOPE [40], which can be used to predict the particle interactions and energy deposition.

Macroscopic and Microscopic Simulation Monte Carlo macrodosimetry predicts energy deposition of voxels in millimeter scale. This volume scale is within the patient size and grid resolution in a treatment planning system. Macrodosimetry is therefore very popular in predicting radiation dose distribution of a cancer patient in radiotherapy [41]. The macroscopic dosimetry concept is based on the kinetic energy released in matter (KERMA). The unit of KERMA is gray. The difference between KERMA and absorbed dose is that because the secondary electrons travel in the medium and deposit energy along their tracks, the energy absorption does not happen at the same locations as the energy transfer described by KERMA. The absorbed dose, however, only considers the sum of energy lost in collisions along the electron tracks within a volume as defined. In macrodosimetry, the concept of particle and energy fluence and definition of linear energy transfer are valid. There are many Monte Carlo codes such as the EGSnrc [42], PENELOPE [40], MCNP [43], and Geant4 [38] that can be used to carry out dose calculation in macroscopic scale. Treatment planning system using Monte Carlo simulation as a dose calculation engine is therefore developed for human [15] and small animal [44, 45]. Monte Carlo nanodosimetry focuses on the spatial and temporal distribution of energy in the nanometer scale of the cellular and molecular level and deals with the stochastics of energy deposition. To investigate the DNA damage due to secondary electrons emitted by gold nanoparticles, Monte Carlo nanodosimetry can track particle interaction as random event. There are Monte Carlo codes such as PARTRAC, KURBUC, CAP100, NOREC, PITS, RITRACKs, PENELOPE, FLUKA, and Geant4, which can predict the energy deposition or dose enhancement due to gold nanoparticle addition under irradiation [13]. These codes have a thorough low-energy physics model, usually based on a liquid water model, to process the particle transports with matter. For example, the Geant4-DNA code considers particle interactions of elastic scattering, excitation, charge change of proton and ionization, and Auger interaction in its low-energy physics library [46]. On the other hand, Li et al. developed the Nanodosimetry Monte Carlo Simulation Code (NASIC) with the physical, pre-chemical, chemical, geometric, and DNA damage module [47]. NASIC can simulate physical low-energy electron tracks in water event by event. For the DNA modelling, the code includes a geometric module which can build an atomic model up to 46 chromatin fibers in a spherical nucleus of human lymphocyte. The DNA damage module can predict both the direct and indirect damages induced by the radiolytic chemical species. However, there is no DNA repair module in the code. The computing time in Monte Carlo nanodosimetry is much longer than macrodosimetry because the simulation needs to track the particle continuously up to

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a very low energy in eV. This is different from macrodosimetry, in which the simulation process can be stopped at a particle energy of about 0.7 MeV. It is because the voxel size of dose calculation is in millimeter scale [13].

Recent Progress in Monte Carlo Simulation Monte Carlo simulation on gold nanoparticle needs to track the radiation interaction down to a very low energy level in the cellular environment. Some interactions such as Auger effect, which is not significant in the macrodosimetry, are significant in the nanometer scale. For Monte Carlo simulation in biomedical nanomaterials, Montenegro et al. [48] improved the general purpose Geant4 code to account for the resonant atomic and molecular transitions of heavy-atom nanoparticles under radiation beam irradiation. They found that there was energy deposition enhancement at the gold nanoparticle layer, when they considered the Auger decays in the simulation. He et al. [49] performed Monte Carlo simulation based on energy properties of bulk gold to predict self-consistent coordination-averaged energies for gold atoms. They found that the energy barrier of gold film was about 0.2 eV for atomic diffusion of gold on the (111) surface which has undergone a late transition state. Martinov et al. [50] constructed a Monte Carlo heterogeneous multiscale model for energy deposition and dose enhancement due to gold nanoparticle addition. They proved that their model could be used accurately in both macroscopic and microscopic effects in gold nanoparticle-enhanced radiotherapy. However, since their macroscopic model was based on condensed history approach with electron energies larger than few hundred electron volts, the model was not suitable for nanodosimetry which required event-by-event particle tracking with electron energy down to 10 eV. For the Geant4-DNA Monte Carlo code, Sakata et al. [51] compared the new physics models of the code with the Geant4 PENELOPE and LIVERMORE condensed history models. They found that in submicron-sized volumes, only the new physics model of the Geant4-DNA code could predict the high backscattering that should be present around the gold nanoparticle in nanometer scale. They therefore concluded that the Geant4-DNA was good at particle transport based on its physics model in the simulation of irradiated gold nanoparticles. He et al. [8] used the Geant4-DNA Monte Carlo code to evaluate the DNA damage due to the dose enhancement in gold nanoparticle-enhanced radiotherapy. They found that there was a strong dependency of the dose enhancement on the nanoparticle size, distance to the DNA, and photon beam energy. Zabihzadeh et al. [52] used the MCNP-4C Monte Carlo code to study the distribution of gold nanoparticles using the 35–95 keV photon beams. They found that the 55 keV beam had the highest dose enhancement in the photon energy range. They also found that a heterogeneous model was better than homogeneous as the former was closer to the real environment of a tumour for nanoparticle addition study. On the other hand, Brivio et al. [53] investigated the potential benefit of using gold nanoparticles in treating the neovascular age-related macular degeneration in stereotactic radiosurgery using Monte Carlo simulation. Brivio et al. [53] found that

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a dose enhancement of 1.97 was found when using gold nanoparticles of 20 nm in the treatment. They also found that prescribed dose could be reduced to half for the macular endothelial cells when using gold nanoparticles in radiotherapy. Zheng et al. [25] used Monte Carlo simulation to compare the dose enhancement due to different types of nanoparticles, namely, gold, platinum, silver, iodine, and iron oxide, in electron and kilovoltage skin therapy. They found that the dose enhancement depended significantly on the nanoparticle material, concentration, skin target thickness, and beam energy.

Dose and Imaging Contrast Enhancement in Gold Nanoparticle-Enhanced Radiotherapy Due to the recent progress of Monte Carlo simulation on nanodosimetry, predictions of dose and imaging contrast in gold nanoparticle-enhanced radiotherapy have become more accurate [13]. In this chapter, we explore some recent progress of Monte Carlo simulation results regarding the dose and imaging contrast enhancement with gold nanoparticle addition. Since photoelectric interaction enhancement is the main reason of dose enhancement in gold nanoparticle-enhanced radiotherapy, it can be seen that gold nanoparticles provide a very high dose enhancement, when interacting with the kilovoltage photon beams. This means that dose enhancement is possible in kilovoltage photon therapy such as skin cancer treatment. We compare dose enhancements in skin therapy between kilovoltage photon and megavoltage electron beam, which is also a popular skin cancer treatment option [25]. For imaging contrast enhancement, we review Monte Carlo simulation results using kilovoltage photon beams (100–140 kVp) from computed tomography [7]. It is because the computed tomography image set is essential in the target contouring and dose calculation in treatment planning. In radiotherapy using the megavoltage photon beams, it is found that gold nanoparticle-enhanced radiotherapy would benefit some deep-seated tumour (e.g., prostate) treatment, as more low-energy electrons, which are sensitive to the photoelectric interaction, are generated along the central beam axis from the patient surface. In addition, flattening-filter-free photon beams are recently introduced in intensity modulated radiotherapy [16, 17, 54]. When the flattening filter in the treatment head of the linear accelerator is removed, the photon beam has a much higher output and more low-energy photons, which would benefit the heavy-atom nanoparticle-enhanced radiotherapy. We compare Monte Carlo simulation results of dose enhancements in prostate radiotherapy using flattening-filter-free and flattening-filter photon beams. For the imaging contrast enhancement of megavoltage portal imaging in gold nanoparticle-enhanced radiotherapy, we explore results of comparison between the flattening-filter-free and flattening-filter photon beams using Monte Carlo simulation.

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Dose Enhancement Using Kilovoltage Photon Beams In skin therapy using kilovoltage photon beams, nanoparticles are added to the tumour, and dose enhancement is determined using Monte Carlo simulation under the macroscopic approach. The dose enhancement or dose enhancement ratio (DER) is defined as: DER ¼

Dose with nanoparticle addition in the target Dose without nanoparticle addition in the target

ð1Þ

Using a simple single-beam geometry with kilovoltage photon beams (105 and 200 kVp) based on the Gulmay D3225 orthovoltage treatment machine, Monte Carlo simulations using the EGSnrc code were carried out to determine the DER with variations of nanoparticle material, concentration, photon beam energy, and skin target thickness. Figure 2 shows the relationship between the DER and the target thickness with variations of the photon beam energy and gold (Au) nanoparticle concentration. It is seen in Fig. 2 that the lower-energy photon beam of 105 kVp had higher DER compared to the higher-energy beam of 220 kVp. This is because the photoelectric interaction is inversely proportional to the energy of the incident photon. Moreover, it is found that the higher the nanoparticle concentration, 7 6 5 AuNP Concentration (mg/ml) 105 kVp

DER

4

3 7 18 30 40 220 kVp 3 7 18 30 40

3 2 1 0 0

1

2

3

4

5

6

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Thickness (mm) Fig. 3 Relationship between the DER and skin target thickness with nanoparticle concentration of 7 mg/ml using the gold (Au), platinum (Pt), iodine (I), silver (Ag), and iron oxide (Fe2O3) nanoparticles for the 105 and 220 kVp photon beams [25]

the higher was the dose enhancement. For the skin target thickness, there was no significant change to the thickness with variation of gold nanoparticle concentration using the 220 kVp photon beam but the 105 kVp. When the nanoparticle concentration was set to 7 mg/ml according to the smallanimal experiments carried out by Hainfeld et al. [11], relationship between the DER and target thickness with variations of the photon beam energy and nanoparticle material is shown in Fig. 3. For different nanoparticle materials, namely, gold (Au), platinum (Pt), iodine (I), silver (Ag), and iron oxide (Fe2O3), it is seen that gold nanoparticles had the highest DER for all target thicknesses of tumour and photon beam energies. The nanoparticle material to produce the second highest DER was platinum and then silver, iodine, and iron oxide. It can be seen that the DER for gold nanoparticle-added tumour was close to 2 when the 105 kVp photon beam was used. This means that the dose at the tumour was increased to almost double when the tumour target thickness was equal to or smaller than 1 mm. For gold nanoparticles producing the highest DER, Fig. 4 shows the relationship between the DER and nanoparticle concentration with variation of photon beam energy and skin target thickness. In Fig. 4, it is seen that the DER increased with an increase of nanoparticle concentration and decreased with an increase of target thickness. Moreover, lower photon energy of 105 kVp had a higher DER compared to the 220 kVp. These results agreed with those in Figs. 2 and 3. From the Monte Carlo simulation results, it is found that gold nanoparticles produced the highest dose enhancement compared to other materials such as

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platinum and silver. Moreover, higher dose enhancement could be achieved by using lower-energy photon beam and higher nanoparticle concentration in skin radiotherapy. The DER was also found higher for the thinner tumour layer. This is because of the depth dose characteristic of the kilovoltage photon beam with a large dose gradient at the target volume.

Dose Enhancement Using Megavoltage Electron Beams Since megavoltage electron beam is also very popular in skin therapy, Monte Carlo simulation was carried out to determine the DER in electron skin therapy using nanoparticles. Instead of using the kilovoltage photon beam from an orthovoltage treatment unit, clinical electron beams produced by a medical linear accelerator were used in the simulation. As skin lesion is not a deep-seated tumour, only 4 and 6 MeV electron beams were simulated in the study [55]. Figure 5 shows the relationship of DER and skin target thickness with variations of gold nanoparticle concentration and electron beam energy. In Fig. 5, the DER is found increasing with the target thickness of the tumour. This is because of the depth dose characteristic of the electron beams in the target volume. The DER for the 4 MeV electron beam was higher than that of the 6 MeV. However, it is seen that the DERs for the electron beams were only slightly larger than 1 but lower than 1.1. These DER values were very small compared to the

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kilovoltage photon beams having DERs equal to or more than 6 as shown in Fig. 2. It should be noted that such high DER for the kilovoltage photon beam is due to the photoelectric interaction enhancement in the energy range of kilovoltage photon beam, while such enhancement is missing for the megavoltage electron beams [56]. Therefore, for the nanoparticle addition in skin radiotherapy, kilovoltage photon beam is better than megavoltage electron beam regarding cancer cell kill. Figure 6 shows the variation of DER with the nanoparticle concentration for different target thicknesses using the 6 MeV electron beam. It is seen that the higher the DER, the higher the gold nanoparticle concentration. This agreed with results of kilovoltage photon beams as shown in Fig. 4. However, it is found that some DER values were lower than one when the target thickness was smaller than 2 mm. This shows that the efficiency of adding gold nanoparticles in the skin tumour was not high in electron therapy, when the tumour thickness was smaller than 2 mm. Figure 7 shows the relationship between the DER and nanoparticle concentration when the target thickness was equal to 2 mm for different nanoparticle materials. In Fig. 7, it is seen that the DER increased with the nanoparticle concentration when the tumour target thickness equal to 2 mm, using the 4 MeV electron beam. Moreover, gold nanoparticle had the highest dose enhancement compared with other nanoparticle materials such as platinum, iodine, silver, and iron oxide. These results agreed well with those using the kilovoltage photon beams (Fig. 3).

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Fig. 6 Relationship between the DER and gold (Au) nanoparticle concentration with variation of skin target thickness using the 6 MeV electron beams [25]

Fig. 7 Relationship between the DER and nanoparticle concentrations of gold(Au), platinum(Pt), iodine (I), silver (Ag), and iron oxide (Fe2O3) using the 4 MeV electron beams. The target thickness is equal to 2 mm [25]

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Imaging Contrast Enhancement Using Kilovoltage Photon Beams Adding gold nanoparticles to the tumour increases its contrast in medical imaging such as computed tomography [26]. This is important in radiation treatment planning as a high imaging contrast of tumour makes the contouring of target more accurate, leading to an accurate radiation dose delivery. In this section, we review some recent Monte Carlo simulation results of imaging contrast enhancement from computed tomography. The imaging contrast ratio is defined as Contrast Ratio ¼

Imaging contrast with nanoparticle addition in the target Imaging contrast without nanoparticle addition in the target ð2Þ

In Eq. (2), the imaging contrast is calculated by Imaging Contrast ¼

It
Ib , Ib

ð3Þ

where It is the transmitted photon beam intensity and Ib is the transmitted background intensity. These two intensities were calculated by Monte Carlo simulation using the Beer-Lambert Law [57]: It = Io e
μx, where Io is the incident intensity and x is the thickness of the medium. The mass attenuation coefficient (μ) is the sum of three interactions (photoelectric, Compton, and pair production) between photons and medium in a proper energy range. In Monte Carlo simulation, photon beam energies in the range of 100–140 kVp were used. This is the typical photon beam energy range for a computed tomography scanner. Figure 8 shows the relationship of

Fig. 8 Relationship between contrast ratio and nanoparticle concentration with different nanoparticle materials using the 100 kVp photon beam [7]

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the contrast ratio and nanoparticle concentration varying with different nanoparticle materials using the 100 kVp photon beam [7]. In Fig. 8, the imaging contrast enhancement in terms of contrast ratio increased with the nanoparticles concentration. Moreover, gold nanoparticles had the highest contrast ratio among all nanoparticle materials such as platinum, silver, iodine, and iron oxide. This means gold nanoparticles were the best nanomaterial for imaging contrast enhancement in radiotherapy. The contrast ratio of gold nanoparticle was higher than 4 when the nanoparticle concentration is equal to 40 mg/ml. When the nanoparticle concentration was set to 7 mg/ml as per the concentration used by Hainfeld et al. [11] in their small-animal experiment, the relationship between the contrast ratio and photon beam energy is shown in Fig. 9. It is seen that the contrast enhancement increased with a decrease of photon beam energy from 140 to 100 kVp. Again, gold nanoparticles had the highest contrast ratio for all photon beam energies. For gold nanoparticles, the relationship between the contrast ratio and target volume thickness is shown in Fig. 10. For a photon beam energy of 120 kVp, it is found in Fig. 10 that the contrast ratio increased with a decrease of target thickness. Moreover, for each target thickness, the contrast ratio increased with the gold nanoparticle concentration. From the Monte Carlo simulation results, it is concluded that adding gold nanoparticles increased the imaging contrast of tumour target. High imaging contrast enhancement could be obtained by using heavy-atom nanoparticles (e.g., gold), low photon beam energy (e.g., 100 kVp), high nanoparticle concentration (e.g., 40 mg/ml), and a target layer with small thickness (0.5 cm) [7].

Fig. 9 Relationship between contrast ratio and photon beam energy with different nanoparticle materials. The nanoparticle concentration is equal to 7 mg/ml [7]

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Fig. 10 Relationship between the contrast ratio and target volume thickness with different gold nanoparticle concentrations using the 120 kVp photon beam [7]

Dose Enhancement Using Megavoltage Flattening-Filter-Free and Flattening-Filter Photon Beams In conformal radiotherapy, flattening filter is used to produce a uniform dose profile of photon beam for a homogeneous dose coverage at the tumour volume. However, when intensity modulated radiotherapy is developed to replace conformal 3D radiotherapy, multileaf collimator has been invented to produce beam intensity modulation in dose delivery. This means that the flattening filter can be removed from the head of linear accelerator [58]. The advantage of removing the flattening filter is that it increases the photon beam output from 2 to 4 times. This highly shortens the treatment time and increases the patient throughput in the cancer center. However, removing the flattening filter also increases the low-energy photons in the beam and decreases the head scatter from the treatment head [17]. As gold nanoparticle-enhanced radiotherapy is sensitive to low-energy photon due to the photoelectric interaction enhancement, it is worthwhile to investigate the dose enhancement between the flattening-filter-free and flattening-filter photon beam using Monte Carlo simulation. Using a homogeneous phantom mimicking the patient’s pelvis, Monte Carlos simulation was carried out to determine the dose enhancement in a prostate volumetric modulated arc therapy(VMAT) plan using the flattening-filter-free and flattening-filter photon beams of 6 MV [84]. In Monte Carlo simulations, 6 MV photon beams were used based on the prostate VMAT technique with different sizes of prostate targets (2.5–5.5 cm diameter) and phantoms (20–30 cm). Flattening-filter-free (FFF) and flattening-filter (FF) photon beams produced by a TrueBeam Varian linear accelerator were used in the simulation. The dose enhancement ratio was determined with and without adding gold nanoparticles to the target (prostate) according to Eq. (1).

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Figure 11 shows the relationship between the DER and nanoparticle concentration for different nanoparticle materials (gold (Au), platinum (Pt), iodine (I), silver (Ag), and iron oxide (Fe2O3)) using the 6 MV FFF and FF beams. In Fig. 11, it is found that the dose enhancement increased with the heavy-atom nanoparticle concentration, namely, Ag, I, Pt, and Au. The gold nanoparticles had the highest dose enhancement compared to other nanoparticles for both the 6 MV FFF and FF beams. For gold nanoparticles, it is seen that the FFF beam produced higher dose enhancement than the FF beam. This is because the FFF beam contained more low-energy photons and therefore more photoelectric interactions at the target compared to the FF beam. Similar results of DER comparison between the FFF and FF photon beam can be found in other nanoparticles. For different prostate sizes with gold nanoparticle addition, Fig. 12 shows the relationship between the dose enhancement and nanoparticle concentration using the 6 MV FFF and FF photon beams. Apart from higher dose enhancement observed at higher nanoparticle concentration and the FFF photon beams as shown in Fig. 11, it is seen that dose enhancement increased with a decrease of prostate size of the patient. This shows that gold nanoparticle-enhanced radiotherapy is more effective for patient with small prostate. Moreover, higher gold nanoparticle concentration and FFF photon beam should be used to maximize the dose enhancement and cancer cell kill.

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Concentration mg/mL Fig. 12 Relationship between the DER and gold nanoparticle concentration using the 6 MV FFF and FF photon beams with varying prostate sizes [59]

Figure 13 shows the variation of dose enhancement with gold nanoparticle concentration when the Monte Carlo phantom of different sizes was used. The prostate size was kept in 3.5 cm diameter in the simulation. It is seen in Fig. 13 that dose enhancement increased with a decrease of the phantom (or patient) size. This shows that though a bigger phantom size produced more attenuation of photon beam and hence more low-energy photon, the depth dose of the beam decreased at the same time. This led to a reduction of gold nanoparticle radiosensitization at the target as shown in Fig. 13. Such effect was also found by Chow et al. [54] when considering the change of photon energy distribution along the depth of the central beam axis. Since 10 MV photon beam can also be used in prostate VMAT, Monte Carlo simulations were carried out to compare the dose enhancement of prostate target with gold nanoparticle addition using the FFF and FF photon beams. The relationship between the DER and gold nanoparticle concentration with variations of the 6 and 10 MV beams is shown in Fig. 14. It is seen from Fig. 14 that both the 6 MV FFF and FF photon beams had higher dose enhancement than those of the 10 MV. This is due to the 6 MV photon beam containing more low-energy photons compared to the 10 MV. Focusing on the 10 MV FFF and FF beams, higher DER was found in the FFF beams with the flattening filter removed. This resulted in more low-energy photons in the beam. Similar results could be found in the 6 MV FFF and FF photon beams.

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From the Monte Carlo simulation results of the FFF and FF photon beams in prostate VMAT nanoparticle-enhanced radiotherapy, it is found that gold nanoparticles achieved the highest dose enhancement among other nanoparticle materials. Moreover, higher dose enhancement could be found in lower photon beam energy of 6 MV, FFF photon beam, smaller patient, and smaller prostate size based on simulations.

Imaging Contrast Enhancement Using Megavoltage FlatteningFilter-Free and Flattening-Filter Photon Beams Monte Carlo simulations were carried out to determine the imaging contrast enhancement in terms of fractional contrast enhancement in the megavoltage portal imaging in nanoparticle-enhanced radiotherapy. Different photon beams, namely, 6 MV FFF, 6 MV FF, 10 MV FFF, and 10 MV FF, were used in this study with different nanoparticle materials and concentrations [60]. In Fig. 15, it is seen that high imaging contrast enhancement could be found in the lower photon energy beam of the 6 MV compared to 10 MV. For the same beam energy, however, the FFF beam had a higher fractional contrast enhancement than the FF. This is because the FFF beam had more low-energy photon leading to more photoelectric interaction and absorption at the tumour target. For any photon beams

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Fig. 15 Relationship between the fractional contrast enhancement and photon beams using different nanoparticle materials with concentration equal to 18 mg/ml [60]

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Fig. 16 Relationship between the fractional contrast enhancement and nanoparticle concentration using the 6 MV FFF photon beams [60]

in Fig. 15, it is found that gold nanoparticles produced the highest contrast enhancement among other nanoparticle materials. Platinum nanoparticles had the second highest enhancement, while iron oxide nanoparticles had the lowest. Considering the photon beam (6 MV FFF) which produced the highest imaging contrast enhancement as shown in Figs. 15 and 16 shows the relationship between the fractional contrast enhancement and nanoparticle concentration with different nanoparticle materials. In Fig. 16, it is seen that the higher the concentration, the higher the contrast enhancement. Moreover, gold nanoparticles produced the highest fractional contrast enhancement compared to other nanoparticle materials. When flattening filter is used for the photon beam, the relationship of the fractional contrast enhancement and different nanoparticle materials is shown in Fig. 17 using the 6 and 10 MV beam. In Fig. 17, it is found that the 6 MV photon beam produced a higher contrast enhancement compared to the 10 MV. This is understood as the 6 MV photon beam contained more low-energy photon than the 10 MV beam. Similar results were found in Fig. 18 showing contrast enhancement of different nanoparticles using the FFF photon beams. With the flattening filter removed from the head of the linear accelerator (i.e., FFF), the 6 MV photon beam produced better imaging contrast enhancement than the 10 MV beam. Moreover, high imaging contrast enhancement was found in gold and platinum nanoparticles. From the Monte Carlo simulation results, high megavoltage portal imaging contrast could be obtained when using gold nanoparticles, high nanoparticle concentration, low photon beam energy (6 MV), and the FFF photon beam in nanoparticle-enhanced radiotherapy.

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Fig. 17 Relationship between the fractional contrast enhancement and nanoparticles with concentration equal to 18 mg/ml using the 6 MV FF and 10 MV FF photon beams [60]

Fig. 18 Relationship between the fractional contrast enhancement and nanoparticles with concentration equal to 18 mg/ml using the 6 MV FFF and 10 MV FFF photon beams [60]

Conclusions and the Further Outlook Development and design of the next generation radiosensitizer of heavy-atom nanomaterials require a thorough understanding of the physical and geometric characteristics of the material such as gold nanoparticles. These characteristics include dependences of dose and imaging contrast enhancement on the nanoparticle

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material, size, shape, concentration, distribution pattern, radiation type, and energy. With the recent progress of Monte Carlo simulation regarding the low-energy physics development, nanodosimetry model can be constructed accurately using some advanced computing codes. Using Monte Carlo simulation, relationships between dose/imaging contrast enhancement of nanoparticles and their characteristics can be investigated in different treatment sites such as the skin and prostate, using kilovoltage and megavoltage photon beams and megavoltage electron beams. With the recent Monte Carlo simulation results, we found that gold nanoparticles achieved the highest dose and imaging contrast enhancement compared to other nanoparticles. Moreover, dose and imaging contrast enhancement were found increased by using kilovoltage photon beam, high nanoparticle concentration, and flattening-filter-free photon beam. For further improvement of Monte Carlo nanodosimetry, more experimental work on nanoparticles (e.g., in form of thin film) irradiated by radiation are needed for the verification of the low-energy physics library. In addition, more studies concerning the biological effect of cancer cell kill linked to the physical effect of energy deposition should be carried out. With a complete Monte Carlo nanodosimetry model for nanoparticle-enhanced radiotherapy, more preclinical and clinical trials can be conducted pointing at the unique characteristic of heavy-atom radiosensitizer in radiotherapy.

References 1. Her S, Jaffray DA, Allen C (2017) Gold nanoparticles for applications in cancer radiotherapy: mechanisms and recent advancements. Adv Drug Deliv Rev 109:84–101 2. Chow JCL (2018) Monte Carlo nanodosimetry in gold nanoparticle-enhanced radiotherapy. In: Chan MF (ed) Recent advancements and applications in dosimetry, Chapter 2. Nova Science Publishers, New York 3. Chow JCL (2017) Dose enhancement effect in radiotherapy: adding gold nanoparticle to tumour in cancer treatment. Nanostruct Cancer Ther 2017:383–400 4. Chow JCL (2017) Application of nanoparticle materials in radiation therapy. In: Martinez LMT, Kharissova OV, Kharisov BI (eds) Handbook of ecomaterials. Springer Nature, Cham 5. Chow JCL (2016) Photon and electron interactions with gold nanoparticles: a Monte Carlo study on gold nanoparticle-enhanced radiotherapy. Nanobiomater Med Imaging 8:45–70 6. Chow JCL (2015) Characteristics of secondary electrons from irradiated gold nanoparticle in radiotherapy, Chapter 10. In: Aliofkhazraei M (ed) Handbook of nanoparticles. Springer, Cham, pp 41–65 7. Albayedh F, Chow JCL (2018) Monte Carlo simulation on the imaging contrast enhancement in nanoparticle-enhanced radiotherapy. J Med Phys 43:195–199 8. He C, Chow JCL (2016) Gold nanoparticle DNA damage in radiotherapy: a Monte Carlo study. AIMS Bioeng 3:352–361 9. Zhang XD, Guo ML, Wu HY, Sun YM, Ding YQ, Feng X, Zhang LA (2009) Irradiation stability and cytotoxicity of gold nanoparticles for radiotherapy. Int J Nanomedicine 4:165 10. Berbeco RI, Korideck H, Ngwa W, Kumar R, Patel J, Sridhar S, Johnson S, Price BD, Kimmelman A, Makrigiorgos GM (2012) DNA damage enhancement from gold nanoparticles for clinical MV photon beams. Radiat Res 178(6):604–608 11. Hainfeld JF, Slatkin DN, Smilowitz HM (2004) The use of gold nanoparticles to enhance radiotherapy in mice. Phys Med Biol 49(18):N309

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12. Leung MK, Chow JC, Chithrani BD, Lee MJ, Oms B, Jaffray DA (2011) Irradiation of gold nanoparticles by x-rays: Monte Carlo simulation of dose enhancements and the spatial properties of the secondary electrons production. Med Phys 38(2):624–631 13. Chow JCL (2018) Recent progress in Monte Carlo simulation on gold nanoparticle radiosensitization. AIMS Biophys 5(4):231–244 14. Andreo P (1991) Monte Carlo techniques in medical radiation physics. Phys Med Biol 36(7):861 15. Rogers DW (2006) Fifty years of Monte Carlo simulations for medical physics. Phys Med Biol 51(13):R287 16. Chow JC, Owrangi AM (2014) Dosimetric dependences of bone heterogeneity and beam angle on the unflattened and flattened photon beams: a Monte Carlo comparison. Radiat Phys Chem 101:46–52 17. Chow JC, Owrangi AM (2016) A surface energy spectral study on the bone heterogeneity and beam obliquity using the flattened and unflattened photon beams. Report Pract Oncol Radiother 21(1):63–70 18. Zubizarreta E, Van DJ, Lievens Y (2016) Analysis of global radiotherapy needs and costs by geographic region and income level. Clin Oncol 29:84–92 19. Medintz IL, Uyeda HT, Goldman ER, Mattoussi H (2005) Quantum dot bioconjugates for imaging, labelling and sensing. Nat Mater 4(6):435 20. Kimling J, Maier M, Okenve B, Kotaidis V, Ballot H, Plech A (2006) Turkevich method for gold nanoparticle synthesis revisited. J Phys Chem B 110(32):15700–15707 21. Eastoe J, Hollamby MJ, Hudson L (2006) Recent advances in nanoparticle synthesis with reversed micelles. Adv Colloid Interf Sci 128:5–15 22. Zhao P, Li N, Astruc D (2013) State of the art in gold nanoparticle synthesis. Coord Chem Rev 257(3–4):638–665 23. Chithrani BD, Chan WC (2007) Elucidating the mechanism of cellular uptake and removal of protein-coated gold nanoparticles of different sizes and shapes. Nano Lett 7(6):1542–1550 24. Chow JC, Leung MK, Fahey S, Chithrani DB, Jaffray DA (2012) Monte Carlo simulation on low-energy electrons from gold nanoparticle in radiotherapy. J Phys Conf Ser 341(1):012012. IOP Publishing 25. Zheng XJ, Chow JCL (2017) Radiation dose enhancement in skin therapy with nanoparticle addition: a Monte Carlo study on kilovoltage photon and megavoltage electron beams. World J Radiol 9:63–71 26. Hainfeld JF, Slatkin DN, Focella TM, Smilowitz HM (2006) Gold nanoparticles: a new X-ray contrast agent. Br J Radiol 79(939):248–253 27. Cho SH (2005) Estimation of tumour dose enhancement due to gold nanoparticles during typical radiation treatments: a preliminary Monte Carlo study. Phys Med Biol 50(15):N163 28. Rahman WN, Bishara N, Ackerly T, He CF, Jackson P, Wong C, Davidson R, Geso M (2009) Enhancement of radiation effects by gold nanoparticles for superficial radiation therapy. Nanomedicine 5(2):136–142 29. Berbeco RI, Ngwa W, Makrigiorgos GM (2011) Localized dose enhancement to tumor blood vessel endothelial cells via megavoltage X-rays and targeted gold nanoparticles: new potential for external beam radiotherapy. Int J Radiat Oncol Biol Phys 81(1):270–276 30. Herold M, Das IJ, Stobbe CC, Iyer RV, Chapman JD (2000) Gold microspheres: a selective technique for producing biologically effective dose enhancement. Int J Radiat Biol 76(10):1357–1364 31. Boudaıffa B, Cloutier P, Hunting D, Huels MA, Sanche L (2000) Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons. Science 287(5458):1658–1660 32. Zheng Y, Wagner JR, Sanche L (2006) DNA damage induced by low-energy electrons: electron transfer and diffraction. Phys Rev Lett 96(20):208101 33. Barrios R, Skurski P, Simons J (2002) Mechanism for damage to DNA by low-energy electrons. J Phys Chem B 106(33):7991–7994

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55. Chow JC, Jiang R (2012) Bone and mucosal dosimetry in skin radiation therapy: a Monte Carlo study using kilovoltage photon and megavoltage electron beams. Phys Med Biol 57(12):3885 56. Chow JC, Leung MK, Jaffray DA (2012) Monte Carlo simulation on a gold nanoparticle irradiated by electron beams. Phys Med Biol 57(11):3323 57. Swinehart DF (1962) The Beer-Lambert law. J Chem Educ 39(7):333 58. Georg D, Knöös T, McClean B (2011) Current status and future perspective of flattening filter free photon beams. Med Phys 38(3):1280–1293 59. Martelli S, Chow JCL (2019) Dose enhancement in gold nanoparticle-enhanced Prostate VMAT: a Monte Carlo Phantom study. Med Phys 46:e490 60. Abdulle A, Chow JCL (2019) Contrast enhancement of MV portal imaging in nanoparticleenhanced radiotherapy: a Monte Carlo Phantom study. Med Phys 46:e437

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Contents Introduction: Shape Memory Materials as Smart Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Brief Overview of some Shape Memory Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Shape Memory Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SMA Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NiTi SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cu Based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fe-Based SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functionality of Shape Memory Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Techniques for Enhancing the Usability of SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shape Memory Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanostructured Shape Memory Alloys: SPD Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanostructured Shape Memory Alloys: Other Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoscale Shape Memory Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping in Shape Memory Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping Mechanisms in SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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E. Okotete (*) · A. Osundare Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria J. L. Olajide Department of Mechanical and Automation Engineering, Tshwane University of Technology, Pretoria, South Africa D. Desai Department of Mechanical Engineering, Mechatronics and Industrial Design, Tshwane University of Technology, Pretoria, South Africa E. R. Sadiku Department of Chemical, Metallurgical and Materials Engineering, Institute of NanoEngineering Research (INER), Tshwane University of Technology (TUT), Pretoria West Campus, Pretoria, South Africa Department of Mechanical Engineering, Maharashtra Institute of Technology, Pune, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_165

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Damping in NiTi/TiNi SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping in Low Cost SMAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping in Shape Memory Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Smart materials are currently the most sought-after material systems, today. This is because of their peculiarities in response and functionality when compared to conventional materials. Shape memory materials are one of the most relevant and evolving smart materials because of their associated anthropomorphic properties, which rightly complement their inherent engineering properties. The functionality of shape memory systems has been long discovered to be able to scale with the grain sizes in the materials. In this regard, processing of ultrafine/nanostructured shape memory materials has become expedient for the development of “new age” shape memory materials with outstanding functional properties. This new subclass of shape memory materials are sometimes referred to as shape memory nanomaterials. This chapter discusses the shape memory potentials of nanomaterials, as viable materials for addressing mechanical vibrations in structural systems. The first section of this chapter, briefly, discusses the different classes of shape materials in an attempt to skip the details available in several reviews. The grain size effect on the engineering properties of shape memory materials is discussed in the second section. In this section, special attention is placed on shape memory nanomaterials and how they differ from other sub classes of shape memory materials. The third section focuses on the nano structuring of shape memory alloys into shape memory nanomaterials, thereby presenting their comparative performance. The fourth section extensively covers damping in shape memory nanomaterials and possible applications of these materials. The chapter concludes by summarizing this vast topic, highlighting the exciting future possibilities. Keywords

Shape memory materials · SPD · Nanomaterials · Mechanical damping · Vibration

Introduction: Shape Memory Materials as Smart Materials Material scientists and engineers in the last few decades use the collocation “Smart Materials” to describe material systems which possess “exceptional” properties in addition to conventional engineering properties. Hence, such materials have enhanced performance and are sought after for specific applications. Stimulus Responsive Materials (SRMs) is a subclass of smart materials that has sparked interest in the material science community [1]. The humanlike ability to respond to

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different stimuli makes SRMs interesting materials for specific functions in the world today. Some known stimuli include heat, stress/pressure, current/voltage, magnetic field, moisture, and light [2]. Several materials and composites have been grouped under SRMs; shape memory materials are an example worth discussing from the group. Shape Memory Materials (SMMs) respond to stimuli like other SRMs and as a result display shape change and recovery under right conditions. This property which has been described as “rubber-like” was only associated with engineering metallic materials after the early Nineties when it was observed in Au-Cd alloys. SMMs were seen to recover their predeformed shapes when exposed to certain stimuli (temperature and stress). The transformations which occur as a result of the stimuli were named “Shape Memory Effect” (sensitivity to temperature) “Pseudoelasticity” (sensitivity to stress) [3]. Shape memory effect or thermoelastic martensitic transformation is the transformation which occurs when a material deformed in the low temperature phase (martensite phase) regains its predeformed shape after heating. When external stress is applied the material deforms in the martensite phase, the presence of heat causes a reverse transformation to the austenite phase (the high temperature phase) and subsequent cooling restores the initial shape of the material [4–6]. Figure 1 shows the transformation cycle, the detwinned martensite is the deformed phase, and the twinned martensite is the initial low temperature. After heating, the material always returns to the initial state. On the other hand, the pseudoelastic effect is a shape change with constant temperature. The deformation takes place in the high temperature phase when an external load is applied. The applied load induces a martensite phase (stress induced martensite) and as soon as the load is removed, the original shape in the austenite phase is restored [3, 7]. Metallic and nonmetallic materials and composite materials can undergo these transformations. SMMs are therefore currently categorized into shape memory alloys, shape memory ceramics, shape memory polymers, shape memory composites, and shape memory hybrids [8, 9]. Shape memory alloys and shape memory polymers are the two systems extensively studied and engineered for potential use in different sectors. Some important details of these two systems are presented in the next section.

Fig. 1 Shape Memory transformation; thermoelastic transformation cycle (Curled with permission from [10])

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A Brief Overview of some Shape Memory Systems Shape Memory Alloys Metallic alloys which display shape transformation properties like the shape memory effect and pseudoelasticity are known as shape memory alloys (SMAs). The “rubberlike” effect later known as shape memory transformation was first discovered in metals by Swedish scientist Ölander during a study on Au-Cd alloys [11, 12]. Subsequent other studies revealed similar transformations in other metallic materials. However, the notable point in the studies on shape memory transformation occurred when Büehler and his colleagues reported shape recovery in Ni-Ti alloys two decades after Ölander’s study. The study by Büehler and co showed that strain recovery in SMAs can be deployed for engineering applications [13]. This stirred up interest in SMAs and the shape memory transformation in general ( [14–16]. Thus far shape memory effect has been observed in different alloys including Ag–Cd, Au–Cd, Cu–Al–Ni, Cu–Sn, Cu– Zn, Cu–Zn–X (X ¼ Si, Sn, Ga, Al), In–Ti, Ni–Al, Ni–Ti, Mn–Cu, Fe–Pt, Fe–Mn–Si, and others [6, 7]. However, Ni-Ti alloys otherwise known as Nitinol remains the most used for a broad range of applications because of their high recoverable strains (8%), stable transformation temperatures, and other engineering properties (high strength, corrosion resistance, and others) [17, 18]. Aside from Ni-Ti SMAs, moderate strain recovery has also been observed in Cu-based and Fe-based SMAs (3–4%). Shape Memory Polymers Polymers also exhibit shape memory properties, but the mechanism of the shape recovery is a bit different from what happens in metallic alloys. The shape deformation and subsequent recovery in shape memory polymers (SMPs) is majorly linked to the intrinsic nature of the polymeric network in addition to stimuli. SMPs have two phases, a fixed/permanent phase and a reversible or switching phase which undergoes temporary changes. The transformation takes after an SMP deformed at temperatures above its glass transition temperature recovers its original shape when subjected to a heat stimulus [19–21]. Transformation in SMPs is associated with the cross-link nature of polymers. Conventional SMP systems studied by researchers include epoxy-based polymers, styrene-based polymers, segmented polyurethane (PU), polynorbornene, cross-linked polycyclooctene, thermosetting PU, and so many others [20, 22]. Interest in SMPs has increased over the years because of the presence of several stimuli (light, magnetic field, chemical, and electricity) besides temperature, their biocompatibility and biodegradability, tunable properties, and lightweight. Despite having these advantages, there is limited application of SMPs because of low stress and strain recovery [23].

Application of Shape Memory Materials Materials that exhibit shape memory properties have several applications today either as whole systems or as part of bigger component assembly. The most prominent area of application being biomedical with NiTi and SMPs being the

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obvious candidates. Excellent properties such as bio-compatibility [25, 26], nonmagnetic [26], high corrosion resistance [13, 24, 27], unique physical properties (the replicative ability of human tissues and bones) [28], and possibility of responding to change in human body temperature [29] made them suitable. SMAs are used in medical equipment and devices for diverse medicals fields which include orthopedics, neurology, cardiology, and interventional radiology [28]. These applications include stents [30], medical tweezers, anchors for attaching tendon to bone, implants [31, 32], aneurism treatment [33], eyeglass frame [34], guide wires [35], and endodontics [36]. Peculiar properties of SMPs such as the ability to respond to stimuli such as pH values [37], solvents [38], moisture/water [39], and even enzymes [40] have resulted in increased medical applications. This includes self-tightening sutures, laser or magnetically activated devices for cardiovascular stents [41–44], aneurysm coils for the treatment of intracranial aneurysms [45], biodegradable intelligent surgery sutures [46], orthodontic appliances [47], self-deployable neuronal electrodes [48], dialysis needle adapters [49], intelligent electrodes, drug-controlrelease devices [50, 51], restorative dental materials [52] for anticaries, as well as composites with antibacterial activity [53]. Automotive and aerospace application of SMAs has increased tremendously in recent years. Due to increasing demand for safer and more comfortable vehicles with better performance, the number of sensors and actuators in modern vehicles has greatly increased. This has resulted in increased opportunity for application of SMA actuators [12, 54, 55]. Other existing or potential applications of SMAs in a vehicle include wipers, headlights/lamps, radiator, sunroof/sunshade [12]. A few examples of aerospace applications are actuators [56, 57], structural connectors, vibration dampers, sealers, release or deployment mechanisms [58–62], inflatable structures [63, 64], manipulators [65, 66], and the pathfinder application [67, 68].

SMA Systems In this section, a summary of the properties and peculiarities of the major SMA systems as recorded in literature is presented as a precursor to subsequent sections in the chapter.

NiTi SMAs NiTi SMAs are metallic alloys with near-equiatomic composition of Ni and Ti. Variations in the composition usually alter the shape transformation characteristics of the alloy and by extension the performance of the alloy [69–71]. Conventionally, NiTi SMAs are produced by arc or induction melting followed by hot working but problems of inhomogeneity of melt and contamination from the fabrication routes have led to the use of powder metallurgy approaches instead [70, 72]. The thermoelastic shape memory transformation (temperature stimulated shape change) in NiTi SMAs proceeds from a B2 austenite phase (high temperature phase)

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to a B190 martensite phase [73]. An intermediate R-phase is also known to accompany shape memory transformation in NiTi alloys. Beside core shape memory properties, NiTi SMAs exhibit high damping capacity of the martensite phase which is linked to the mobility of twinned interfaces [74, 75]. Additionally, the superior thermomechanical and thermoelectrical characteristics combined with other engineering properties makes NiTi (SMAs) stand out as the most scientifically and technologically important SMA [76, 77]. Some examples of applications where these SMAs have been used are electrical connectors, fasteners, orthodontic wires, headbands of headphones, eyeglass frames, actuators,and sensors in a variety of different devices, bone plates, active endoscopes, air conditioning vents, and guide wires [69, 76].

Cu Based Cu-based alloys stand out among the three known SMAs as a low-cost alternative to NiTi SMAs which are relatively expensive due to the high cost of constituent metals (Ni and Ti) and the complicated fabrication routes. These SMAs are often preferred to Fe-based SMA (the other low cost SMA) because of their relatively superior shape recovery properties [78, 79]. One of the reasons for the low-cost advantage of Cubased SMA is the easy fabrication route adopted. Traditional liquid metallurgy approaches are used for producing polycrystalline Cu SMA systems, while the bridgemann technique is used for single crystal fabrication [6, 80, 81]. Thermoelastic transformation in Cu-based SMAs moves from a high temperature β phase (austenite/parent phase) to low temperatures α and γ2 (martensite phase) [82, 83]. Peculiar properties of the Cu-based systems include high thermal stability in Cu-Al-Ni systems (ability to maintain transformation characteristics). This property in addition to good resistance to functional fatigue makes Cu-Al-Ni SMAs the only available high temperature SMA (HTSMA) [84–86]. A broad range of compositions for martensitic transformation in Cu-Zn-Al system also makes Cu-based SMAs of interest [87]. However, the polycrystalline Cu-based SMAs such as Cu–Al–Ni and Cu–Zn–Al are too brittle to be sufficiently cold-worked due to the high degree of order and high elastic anisotropy in the β-parent phase. Attempts have been made to improve the ductility of these polycrystalline Cu-based SMAs, mainly by grain refining, with limited success [88, 89]. Age hardening is a feature of Cu-based SMAs that leads to an increase in mechanical strength [90]; however, ageing, in general, has been reported to lead to martensite stabilization, memory loss, and changing transformation temperatures in Cu-based SMAs [91–93].

Fe-Based SMAs Fe-based SMAs also sometimes called shape memory steels have strain recoveries of about 4 percent after prior deformation and are best suited for application where low shape recovery is desired. The term shape memory steels is suited for this alloys because

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conventional steel production processes are used for alloy production and no additional cost is incurred. Shape memory transformation in Fe-based alloys progresses from an fcc parent phase to an hcp martensite when stress is applied [94, 95]. Stress-induced transformation in these alloys results in four different martensite variants in the low temperature phase. Fe-Mn-Si systems are currently the major focus of research in this class of SMAs because of their good workability, machinability, and weldability. A major drawback of this system is the large thermal hysteresis [96].

Functionality of Shape Memory Materials This chapter has in addition to existing literature highlighted the peculiarity SMAs and their importance. The outstanding shape memory effect and pseudoelastic effect are the properties which distinguish these class of materials from other functional materials which are of technological relevance. However, the applicability of SMAs is often affected by phase transformation temperatures (hysteresis, martensite stabilization) and mechanical properties [97, 98]. Techniques proposed and explored in the past to enhance the functionality of SMAs include mechanical cold-working, thermal processing, alloying, and severe plastic deformation [71].

Techniques for Enhancing the Usability of SMAs Studies have shown that the projected range of applications inherent in SMAs can be adequately harnessed if some of the current limitations are properly addressed. This section discusses some of the popular techniques which have been successfully used to eliminate the setbacks of SMAs.

Thermal Processing Thermal treatment of NiTi SMAs was conducted by Frick and his co-workers [99] using hot rolling and subsequent straightening. Solutionizing treatment given to this SMA increased the formation and growth of Ti3Ni4 precipitates which in turn affected the final properties of the SMA. The size, coherency, and distribution of the formed precipitate phase affected the internal stress present in the alloys which consequently affected the stability of the low temperature martensite phase. Such changes are expected to affect pseudoelastic, shape memory, and mechanical properties of the treated SMA. Similar experiments conducted on as-cold-drawn NiTi SMAs by the same group revealed that the heat treatment process annihilates dislocation and encourages precipitate growth. Thermomechanical treatments of Cu-Al-Mn SMAs were done by Sutou et al. [100] to control the texture of the alloys and improve the shape memory and superelastic properties. Alloys were processed through hot rolling, annealing, coldrolling, and solutionizing then final quenching to stabilize the martensite transformation temperatures. Recrystallized texture was present after the treatment in with Cu-Al-MnSMAs. Furthermore, orientation dependence was seen in the alloys linked

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to the change in shape memory and superelastic strains in different directions. At 45° from the rolling direction, a 7% superelastic strain was observed which is three times more than what had been recorded in Cu-based SMA at the time. In hot rolled powder metallurgy produced Cu-Al-Ni SMAs, Rodriguez et al. [101] observed microstructure characterized by low angle grain boundaries and the presence of subgrains. The new microstructure facilitated stress distributions at the grain boundaries reducing the brittleness of the alloy and promoted plastic deformation ahead of fracture in the Cu-Al-Ni SMA studied. In addition to improved ductility, shape memory transformation was not altered after the heat treatment because the microstructure did not inhibit the movement of martensite plates.

Alloying Ternary alloy additions have traditionally been used for grain refinement in materials design and engineering. This technique has also been explored to improve the properties of SMAs. Some of the reports on alloying in SMAs include the work by Liang et al. [98] on NiTi-based SMA produced with additions of Nb and Ta. Postexperimental results showed that the microstructure of the unalloyed and alloyed SMAs had equiaxed grains, not dendrites. However, the precipitate phases present in addition to the B2 austenite was different in the alloyed SMAs. The presence of multiple precipitates in the alloyed SMAs led to increased yield strength at a cost of reduced ductility. Additionally, the introduction of the elements had a certain effect on transformation temperature the path of phase transformation in the SMA. Other works on alloying additions in NiTi SMAs include reports from Es-Souni et al. [102] and Mehrabi et al. [103]. Es-Souni and his group demonstrated that alloyed NiTi SMA has different transformation behavior (direct austenitic to martensitic transformation and no R-phase formation) and superior mechanical properties (lower superelastic hysteresis stress). The Mehrabi group alloyed the same SMA with W and annealed the alloys before subsequent analysis. The annealing affected both the mechanical and functional properties of the alloyed NiTi SMA. W-rich dispersoids were observed and they enhanced the hardness and strength of the alloys but not the shape memory property. These works show the extent of progress achieved using alloying to improve properties of NiTi SMA. Like the reports and investigations on NiTi SMA, some studies have been done on alloying and microalloying of Cu-based SMAs to improve the mechanical and functional properties of the SMA. One of such studies is the work by Sampath [80] on Cu-Al-Ni SMA alloyed with Mn, Ti, and Zr. From experiments and analysis, the author observed 60% grain size reduction from the 1.5 mm average grain diameter in the unalloyed composition. The grain refinement observed was nearly invariant to the weight percent of the elements, and there was no recorded adverse effect on the functional property of the Cu-Al-Ni SMA. In terms of shape memory transformation, the grain-refined alloys showed higher transformation start and finish temperature for the martensite phase. This was attributed to the smaller grain size and the free energy of the refined alloys. A marginal increase in hardness and softer martensite was also observed in the grain refined SMA. Quaternary alloying of a Cu-Al-Mn SMA was carried out by Mallik and Sampath [104].

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These few accounts and many others have shown researchers that both functional and conventional properties of different SMAs can be enhanced and engineering using the right processing route and/or alloying elements. Hence, the research interest in SMAs remained till today.

Shape Memory Nanomaterials From the beginning of this chapter, we have discussed different aspects of shape memory materials and emphasized shape memory alloys. At this point, we would go on to discuss shape memory materials. This subject can either be discussed as a subset of nanomaterials or shape memory materials. However, in this chapter, we would discuss shape memory nanomaterials as a subset of shape memory materials since we spent the three previous sections laying a background on shape memory materials and we have talked briefly about grain refinement in this material system.

Nanomaterials Nanomaterials have attracted attention because of their unique physical, chemical, and mechanical properties that differ from those of bulk solids and molecules. Nanomaterials exhibit distinct size-dependent properties in the 1–100 nm range where quantum phenomena are involved [105].

Nanostructured Shape Memory Alloys: SPD Techniques Nanostructures in materials can be achieved through several methods. One prevalent method for producing ultrafine and nanosized grains in SMAs is the use of severe plastic deformation (SPD) techniques. The term SPD is used to refer to a broad group of deformation processes where high strains are imposed to materials (bulk, large scale) to cause a significant reduction/refinement in grain size [106, 107]. Another attribute of the SPD deformation process is the shape restriction part of the technique where special geometries are used to constrain free material flow thereby producing hydrostatic pressure which induces high strains and lattice defects necessary for grain refinement [106]. Some of the established SPD techniques used for grain refinement include equal channel angular pressing/extrusion (ECAP/ECAE), high pressure torsion (HPT), accumulative roll-bonding (ARB), twist extrusion (TE), multidirectional forging (MDF), and a few others. These techniques have been used to develop materials with grain sizes ranging from submicrometer to nanometer ranges. Materials with grain size 0.1. The damping loss factor increased up to h >0.2 as the pillars reduced into the nanometer range and this damping was stable even after thousands of cycles, indicating that the damping property is retained after long term use. Also noteworthy from this work was the super-fast (103 S for full evolution of strain plateau) superelastic response with ultra-high damping maintained at test frequencies up to 14 Hz.

Conclusion The records have shown that shape memory nanomaterials are an interesting class of future smart materials which should be given considerable attention. Shape memory functionality is dependent on the martensitic transformation which is equally sensitive to microstructure and length scales. Hence, we looked at transformation evolution at different length scales and saw the changes in material behavior. Therefore, the present chapter has shown that in considering shape memory nanomaterials for damping applications, the functionality changes should be addressed.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Pollutants in Waste Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis and Characterization of Nanophotocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol–Gel Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrothermal Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coprecipitation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2/Fly Ash Photocatalytic Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Pollutant from Wastewaters Using Nanophotocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Photocatalytic materials are currently intensively studied because of their high potential in UV and solar-light water purification. The photocatalysis is based on charge carrier generation and mobility of electron/hole pairs. Usually, a large amount of photoexcited charge it is lost by recombination, there are necessary coupled materials, highly active photocatalysts, and a very good electron acceptor semiconductor – that means composite material. There are several catalysts reported in the literature. Among the metal oxides such as TiO2, ZnO, SnO2, and CeO2, CuO is extensively used in heterogeneous photocatalysis for degrada-

L. Favier Univ Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, Rennes, France M. Harja (*) Department of Chemical Engineering, Faculty of Chemical Engineering and Environmental Protection, “Gheorghe Asachi” Technical University of Iasi, Iasi, Romania e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_11

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tion of organic compounds degradation. Its activity can be enhanced by several methods such as doping, surface modification, and shape tailoring. These triggered a large volume of publications, and studies demonstrated that enhancing the dominance of reactive facets can enhance oxidation/reduction processes or even tune the adsorption of pollutants. On the other hand, photocatalyst nanoparticles immobilized on fly ashes improved their ability to destroy the organic compounds in the contaminated water. The literature indicated that nanoparticles can be uniformly dispersed on the surface of fly ash particles. The composite catalysts can be fabricated by different methods (hydrothermal, physical blending, sol–gel method, etc.). The effects of synthesis methods, precursors, promoters, calcination temperature, photocatalyst dosage, initial concentration, and irradiation time on the photodegradation of pollutants will be presented; these information will shed light to select the best photocatalyst. Keywords

Fly ash · Nanocomposite · Photodegradation · Organic pollutant

Introduction Water resources are limited so there is a need to impose efficient purification methods to remove different organic compounds. More than 650 pollutants have been reported in water; due to these organic pollutants, a serious concern arises for the environment and human health [1–3]. For wastewater treatment, several methods such as coagulation, flocculation, adsorption, ion exchange, ultrafiltration, reverse osmosis, chemical precipitation, electrochemical, catalysis, and photocatalysis can be used [4, 5]. Each method has advantages and disadvantages based on the type of pollutant. Advanced technologies utilize oxidation methods such as catalytic or photocatalytic oxidation and electrocoagulation [6]. For organic pollutants, photocatalysis is more studied. Advantages include low investment, environmentally friendly, safety, advanced destroying without recurrent polluting, and effective for persistent pollutant [7–9]. Synthetic dyes are widely used in leather and textile industries, paper colorizing, cosmetics, food, drugs, and toys [10–12]. Over 7
105 t of dyes is produced annually, and inevitably large amounts are discarded in water effluents, causing considerable environmental pollution [12–14]. Different photocatalytic materials are nowadays intensively investigated due to their high use in organic wastewater treatment [15–17]. Titanium dioxide (TiO2), or titania, is the most commonly used photocatalyzer because it is highly efficient, nontoxic, inexpensive, UV stable, biologically and chemically inert, and easily available, etc. [18]. Titania facilitates the production of reactive hydroxyl (•OH) and superoxide (O2•) radical species that transform organic compounds into nontoxic anionic byproducts [19]. An essential advantage of TiO2 is that in specific

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conditions organic compounds are completely mineralized, and this process has high rates even at ambient temperature. Titanium dioxide mostly exists in three crystallographic phases, but two of the most common forms are anatase and rutile crystal phases, with band gap energies of 3.2 and 3.02 eV, respectively [20]. The anatase TiO2 has better photoactivity, and its wider band gap and crystallographic orientation allow faster charge migration on the surface and formation of radical species that destroy organic molecules by oxidation [19, 21, 22]. Titania activity can be enhanced by several methods such as doping, surface modification, or shape tailoring. The shape tailoring strategy focuses on obtaining those crystal geometries (in anatase or rutile crystals) where the oxidation and reduction favoring facets are in balance (e.g., {001} and {101}). This will favor the dual functionality of the material as holes will be spatially separated from the electrons, creating high local oxidation/reduction potentials, as it was demonstrated in other semiconductors [21]. One of the objectives of photocatalysis processes is to find materials that can be activated by the solar light (3% UV and 97% visible radiation). The metal doping and heterojunction composites (TiO2/Me) allow the utilization of a large part of the solar spectrum. Noble metals (Pt, Pd Ag, Au) and non-noble metals (Ce, Zn, Mo, Ta, Rh, etc.) had been mostly investigated in this respect [23, 24]. Of all metals, silver is mostly investigated due to its remarkable catalytic activity and antibacterial activity [25]. Other materials have band-gap energies sufficient to promote photocatalytic activities, for example, WO3 (2.7 eV), α-Fe2O3 (2.2 eV), ZnO (3.2 eV), ZnS (3.6 eV), and CdS (2.4 eV) [26, 27]. Synthesis methods, such as chemical vapor deposition, reactive sputtering, sol– gel process,, and synthesis conditions, play a decisive role in the development of nanostructures. TiO2 containing different nanoparticles as dopants, with increased homogeneity and controlled particle size, are relatively easy to obtain by the sol–gel method [25]. Chemical synthesis methods have advantages such as simplicity, low cost, easy operation, and easy to control in experimental conditions. They also produce nanoparticles of uniform size and high purity [24]. Unlike other methods, the sol–gel synthesis method has many advantages: the process is shorter, is carried out at a low temperature, and requires simple and inexpensive synthesis equipment. The precursor and type solvent, pH stabilizer, solution viscosity, water content, different dopants and their drying conditions, and calcination temperature affect the properties of synthesized materials. The calcination process (time and temperature) influences the phase composition and size of the photocatalyzer. Different substrates like activated carbon, zeolites, silica, or fly ash cenospheres can be coated with a TiO2 film [27, 28]. Fly ashes are used as construction materials in the form of ceramics and brick, but only about 40% of the ash is capitalized; alternative solutions are looked after for maximum utilization of fly ashes [28–30]. Ashes (bottom or fly ash) were studied as adsorbents for pollutant removal from wastewaters [31]. To increase efficiency, modified ashes with zeolite-type structures were necessary for larger specific surface [32]. Other methods were also reported to make fly ash a more reproducible substrate and to increase its surface area and charge [29].

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A step forward was done by simultaneous adsorption, and photocatalytic processes were occurring on the same substrate. Fly ash and modified fly ash can be successfully used as a substrate for TiO2 [28, 30]. These abundant waste materials produced in power plants or various industrial processes are considered suitable starting materials for the preparation of zeolites or porous geopolymer spheres. To increase the surface area, silica fume, zeolites, geopolymers, or alkali activated fly ash materials can be used as a substrate for photocatalytic material deposit. Geopolymers can be synthesized using direct activation method [33], utilizing a synergistically mixed system of abundant waste products – fly ash – and alkali solution. The resulting geopolymer with a porous surface provides greater surface area for in situ TiO2 growth via the minimal-energy, low-waste, and nontoxic sol–gel coating technique [28]. TiO2 nanobelts and nanotube were grown on the geopolymer surface via vapor growth, where the stability of geopolymer spheres to TiO2 deposition has been established [34]. The green chemistry approach qualifies the sol–gel process as a sustainable coating technique for the in situ formation of anatase TiO2 on the geopolymer surface. Furthermore, the metal components of the precursor waste materials will serve as dopants to anatase TiO2 that will make the visible-light-active photocatalyst system e [21]. Figure 1 shows the synthesis process starting with the geopolymer synthesis employing rice hull ash (RHA) as the activator and fly ash (FA) as the aluminosilicate source. FA is a waste material that is generated by coal power plants and typically consists of silicon dioxide (SiO2), aluminum oxide (Al2O3), ferric oxide (Fe2O3), and calcium oxide (CaO) [35]. The heterogeneous photocatalysis is a promising alternative for wastewater treatment. Several research demonstrated the possibility of the photocatalyst synthesis directly on fly ash grains/cenospheres for removing persistent organic pollutants.

TiO2 Rice hull ash Geopolymer

Photocatalytic degradation UV or Visible Light

Organic pollutant

Fly ash Recycled

Non-toxic products

Fig. 1 Shows the synthesis of geopolymer-based fly ash–TiO2 for the photodegradation of organic dye in waste water [21]

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Organic Pollutants in Waste Waters Organic compounds, reported as pollutants, include wastes from food processing, dyes, drugs, phenols, detergents, disinfection byproducts, pesticides, and polyaromatic hydrocarbons (https://ec.europa.eu/environment/archives/waste/sludge/ pdf/sludge_pollutants3.pdf) [36]. These refractory organic pollutants are not destroyed naturally, persist in water bodies, can bioaccumulate in human and animal tissue, and modify food chains with significant impacts on human health and the environment [37]. In the last decades, water contamination with dyes and pharmaceutical compounds has become an emerging environmental problem due to their continuous persistence into the aquatic ecosystem [12]. Organic compounds discharged in the environment may impose toxicity on every rank of the biological hierarchy. Apart from toxic effects, some compounds, even at low concentrations, may cause irreversible changes to the microbial genome and make them resistant Dyes, pesticides or their precursors, chlorinated organic compounds, phenolic and phthalic compounds, surfactants, and halogenated anhydrides are very dangerous. In the past few years, dyes and pharmaceutical residues have been discovered on every continent in all environmental factors: surface water, groundwater, wastewater treatment plant, effluent, and sludge [38]. There is limited knowledge about the effect of pharmaceutical contaminants over flora and fauna, and even less about their potential long-term effects at environmental concentrations on human health [39]. More than 3500 pharmaceuticals are registered only in UE [40], but their numbers are increasing everyday [38]. Wastewater treatment plants were not designed for completely removing pharmaceuticals. The efficiency of traditional treatment efficiencies is less than 10% if pharmaceuticals such as carbamazepine, atenolol, acetylsalicylic acid, diclofenac, mefenamic acid, propranolol, atenolol, clofibric acid, and lincomycin are present in wastewaters [4]. However, pharmaceuticals can be active at very low concentrations (ng/Lμg/L) like micropollutants [18]. The worldwide consumption of antibiotics is over 200,000 tons per annum, and this quantity is increasing daily. The total consumption of important pharmaceuticals around the world is represented in Fig. 2 [38]. The consumption of pharmaceuticals can be influenced by socioeconomic conditions. The crises can cause striking changes in pharmaceutical consumption patterns, particularly in the antiviral and antithermic drug usage. Pharmaceutical consumption patterns also differ on the basis of the country, economic development, and traditional practices. The hazardous potential of pharmaceutical compounds on ecosystems was relatively recently established [41]. The advanced analytical techniques have permitted the determination that some environmental impacts of pharmaceuticals can be established in the μg/L and ng/L concentration ranges [18]. The “low concentration” and diversity of pollutants (micropollutants) not only complicate the associated detection and analysis procedures, but also create challenges for water and wastewater treatment processes. Micropollutants are commonly present in waters at trace concentrations, ranging from a few ng/L to several μg/L. The

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Metformin 20%

Gabapentin 1%

Active pharmaceutical ingredients with a consumption less than 80 t per year 30% Ibuprofen 12%

Diclofenac 1% Valsartan 1% Levetiracetam 1%

Metamizole 8%

Valporic acid 1% Mesalazine 1% Acetylcysteine 1% Amoxicillin 2%

Acetaminophen Metoprolol Allopurinol 6% 2% lomeprol 2% 3%

Acetylsalicylic acid 7%

Fig. 2 Consumption pattern of different pharmaceuticals around the world [38]

micropollutants in the aquatic environment have become a worldwide issue of increasing environmental concern. Sources of micropollutants in the environment are diverse, and many of these originate from mass-produced materials and commodities. Table 1 presents the sources and categories of micropollutants in the aquatic environment [42]. Table 1 shows that micropollutants are emerging contaminants and consist of a vast and expanding array of anthropogenic substances. These include pharmaceuticals, personal care products, steroid hormones, industrial chemicals, pesticides, and many other emerging compounds. Physicochemical processes such as adsorption, electroflocculation, flocculation, coagulation, reverse osmosis, and ultrafiltration only transfer pollutants from one phase to another. Expensive posttreatment and hence regeneration are needed for their complete elimination. The most efficient technology in treatment of aqueous solutions for pharmaceuticals degradation is advanced oxidation processes (AOPs) [15]. In the last few decades, AOPs have been applied for many organic compounds as they have different advanced oxidation processes such as ozonation, Fenton, photo-Fenton, ultrasound waves, sonochemical, photo-sonochemical processes, ultraviolet irradiation, and sulfate radicalbased oxidation [43]. They are based on highly reactive transitory species such as H2O2, OH•, O2•, and O3 for the complete mineralization of organic pollutes, pathogens, and disinfection byproducts.

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Table 1 Sources of micropollutants in the aqueous environment [42] Category Pharmaceuticals

Personal care products Steroid hormones Surfactants Industrial chemicals Pesticides

Subclasses Anticonvulsants, lipid regulator, β-blockers, antibiotics, stimulants Disinfectants; UV filters Estrogens Nonionic surfactants Plasticizers, fire retardants Insecticides, herbicides, and fungicides

Major sources Distinct Domestic waste water; hospital effluents; Runoff and aquaculture

Domestic wastewater Domestic wastewater; Runoff and aquaculture Industrial wastewater; domestic wastewater Domestic wastewater (by leaching out of the material) Domestic wastewater (runoff improper cleaning, from gardens, lawns, and roadways); agricultural runoff

Nonexclusive Sources that are not exclusive to individual categories include: Industrial wastewater (product manufacturing discharges)Landfill leachate (from improper disposal of used, defective, or expired items)

Synthesis and Characterization of Nanophotocatalysts Several studies have been carried out for developing new and efficient materials as a sustainable photocatalyst to degrade pollutants in wastewaters [27–30, 43, 44]. Considering the concept of circular economy and sustainable development, advanced capitalization of fly ash is mandatory for obtaining high-added-value materials. Fly ashes were byproducts, extensively studied as absorbent, with two benefits: reusable and low-cost material [3, 13]. Main methods for TiO2 nanoparticles synthesis are sol–gel, sonochemical, coprecipitation, hydrothermal synthesis, inert gas condensation, ion sputtering scattering, microemulsion, microwave, pulse laser ablation, spark discharge, template assistant synthesis, and biological synthesis [45]. For synthesis of nanocomposite, photocatalyzer can be used: sol–gel, spray pyrolysis, liquid infiltration, the rapid solidification process, high-energy ball milling, chemical or physical vapor deposition, and chemical processes, etc. [46–48 ]. Figure 3 summarizes the production methods of nanomaterials [48].

Sol–Gel Method The sol–gel method is the most investigated technique applied to obtain nanomaterials. Advantages include being simple, reproducible, adaptable for industrial scale, and easily controllable.

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Fig. 3 Scheme of the most relevant production methods of nanomaterials [48]

This method has following steps: preparation of the precursor solution; hydrolysis of the precursor with a proper surfactant; condensation reaction; removal of the solvent for generation of the gel; and calcination for the complete removal of organic compounds, such as surfactant, solvent, and unreacted precursor (Fig. 4). In recent years, evaporation-induced self-assembly (EISA) has been applied for the synthesis of metal oxide nanoparticles. EISA method uses templating agent as a surfactant. Template agents, such as poly(ethylene glycol), poly(propylene glycol), acid izopropilic carboxylic acid, and sodium dodecyl sulphate, are recognized as the most promising surfactants used for sol–gel method [7, 45–47]. The surfactant selection represents one of the most important parameters of EISA, because it influences the textural properties of the resulting material that can be deposited as a thin film on a suitable substrate. The first step involves the preparation of an ethanol solution containing the metal precursor (titanium butoxide, titanium tetrachloride, or titanium-izopropoxid for TiO2) and the templating agent. The mixture is kept at low temperature usually for 24 h to induce the coordination bonds between the metal ions and oxygen-containing group of template agent. Drying at 100 C promotes the formation of xerogel or aerogel. The final calcination at 400–600 C removes organic molecules and results in the formation of metal oxides [7]. A typical sol–gel EISA synthesis of TiO2 utilizes titanium butoxide to produce nanoparticles 5–10 nm in size with a surface area of 145.59 m2/g [49]. The main goal in TiO2 synthesis for environmental photocatalytic applications is to increase the optical response in the visible light. Other sol-gel method, for TiO2 synthesis, having titanium butoxide as precursor, consists in using of lauryl lactyl lactate as biodegradable and inexpensive additive, to control the size of the large inorganic cluster [49], using titanium butoxide as precursor, ethanol, and hydrochloric acid.

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Fig. 4 The sol–gel method for the production of photocatalyzer

Fig. 5 Micrographs of samples obtained by sol–gel method, before (a) and after calcination (b) [25]

In recent studies [25], TiO2 nanoparticles were synthesized using a precursor titanium tetra-isopropoxide, ethanol, nitric acid used as hydrolysis-condensation ratio controlling agent, and ammonia solution as neutralization agent. The solid was recovered by filtration, dried in the oven at 110 C, and calcined at 650 C to generate impose anatase/rutile ratio. The sample morphology determined by scanning electron microscopy (SEM) is presented in Fig. 5. Room temperature ionic liquid method has advantages such as low cost, the control over phase composition and morphology, colloidal stability, and possibility to scale-up. In a typical synthesis, 1-butyl-3-methylimidazoliumtetrafluoroborate is used as solvent and TiCl4 as TiO2 precursor. The reactive mixture was stirred at room temperature, resulting turbidity indicates hydrolysis.

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This solution is again stirred at 80C for 12 h, and the resulting product is separated by centrifugation. The method permits synthesis of anatase phase with 10–50 nm in size [49].

Hydrothermal Method Hydrothermal process is carried out in a Teflon autoclave under controlled temperature and/or pressure when a solvent (usually water – H2O) at a moderate temperature and high pressure determines the production of nanoparticles. This method can be carried out either in distilled water or in the presence of mineralizing species such as hydroxides, chlorides, and fluorides of alkali metals at different pH values. Hydrothermal process is extremely attractive by being environmentally friendly. Therefore, the use of aqueous solutions is not required for post-calcination treatment and the recovery of photocatalyst after can be facile. However, synthesized materials have high crystallinity, uniform size distribution, nanosize diameters, rapid dispensability in polar or nonpolar solvents [22], and proper interfacial properties that permit easy fabrication of high-quality coatings on several supporting materials, including fly ash. In hydrothermal processing, mostly a lamellar phase is formed in which the anions are “surfactant” to stabilize the layered organization of metal cation species. By this method, one-dimensional nanostructures (nanotubes, wires, polyhedrons, and rods) are usually obtained. By using suitable parameters, such as precursors, pressure, temperature and treatment time, and structural phases, shape and size distribution of synthesized materials can be precisely controlled. Numerous morphologies such as cuboid shape, wires, tubes, and belts, have been effectively obtained by hydrothermal method (Fig. 6) [22]. The hollow TiO2-based core-shell structures show a high photocatalytic activity, and their unique morphology permits the reflection of UV light within the inner cavity [50]. This morphology can be obtained by facile one-step hydrothermal process [50] using polyethylene glycol (PEG) as a templating agent for the generation of hollow structures. Ye et al. have developed an interesting strategy for the synthesis of hallow microspheres by using a hydrothermal treatment followed by a calcination step [51]. As the titanium precursor has used potassium titanium oxalate, the hydrothermal process occurred at 150 C for 4 h followed by calcination at 500 C showing the highest photocatalytic activity.

Fig. 6 Hydrothermal synthesis of TiO2: (a1) nanowires, (a2 and a5) nanotubes, (a3) polyhedrons, and (a4) nanobelts [22]

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Usually, the titanium precursors are expensive, but were determined that, for obtaininig TiO2 nanoparticles, can be used TiO2 – P25 (Degussa). By using of a concentrated NaOH solution as a dispersing agent for TiO2 – P25 and the subsequent hydrothermal treatment, was obtained titanate tubular structures, while washing with HCl determined convert tubular titanate in TiO2 nanotubes [52]. The TiO2–P25 powder is introduced into the NaOH aqueous solution and maintained at 115 C for 24 h. Later the mixture was cooled, separated, and washed. Finally, TiO2 nanoparticles were obtained with a specific surface area of 200.38 m2/g and a wall thickness of 2.5 nm [52]. The hydrothermal method synthesizes proper three-dimensional TiO2 structures [53]. These structures show enhanced specific surface area, improved charge separation, and transfer of the e/h+ pairs with remarkable photocatalytic activity [49].

Coprecipitation method Coprecipitation chemical strategy is based on the condition of supersaturation from aqueous solutions followed by thermal decomposition. It is also known as the wet chemical solution method. In this method, nanostructures are greatly influenced by reaction conditions, pH value, and parameters, such as temperature and time. Due to the variations of these parameters, different high-quality nanoparticles and other nanostructures can be obtained. Various shapes of multiphase TiO2 nanostructures, including nanorods and nanoparticles, have been synthesized (Fig. 7) [22]. Preparation of TiO2 nanoparticles via coprecipitation is very easy and simple. Through this method, nanostructures are obtained in a short-reaction time using hazardless precursors, which are inexpensive to feasible scale-up.

TiO2/Fly Ash Photocatalytic Nanocomposites For obtaining a novel, sustainable, and low-cost substrate, fly ash (FA) was analyzed for synthesis of a new class of nanocomposites materials TiO2/FA. Nanocomposites with fly ash can be obtained by two methods based on precursors: (a) using TiO2 – P25 or (b) synthesis of TiO2 nanoparticles in situ on fly ash–modified particles. The SEM images of commercial TiO2 – P25 are presented in Fig. 8. The first step in the synthesis of TiO2/fly ash nanocomposite is the modification of fly ash with hydrochloric acid (HCl) to get higher specific surface (Fig. 9) [54]. After activation, the sol–gel, hydrothermal, or coprecipitation method can be used for synthesis of nanocomposite materials. Gilja et al. [54] used the sol–gel method using a precursor tetra-n-butyl titanate in ethanol and a template dodecyl dimethyl ammonium chloride in ethanol. The mixture was heated at 85 C for 24 h, and the transparent sol was obtained. The obtained pure TiO2 (TiB) sample was washed, dried at 100 C, and calcined at 400 C for 3 h [54]. For synthesis of FA/TiO2 nanocomposite photocatalyst, in situ synthesis of TiO2 can be used. After fly ash modification with HCl solution, this is added to precursor

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Fig. 7 SEM images for TiO2 nanostructures synthesized by coprecipitation method: (a1) nanorods, (a2 and a3) rutile nanoparticles, and (a4) anatase nanoparticles

and template alcoholic solutions. Different compositions of photocatalyst can be obtained, by modify of the precursor quantities. The commercial TiO2 – P25 can be mixed with modified fly ash and obtained composites with 16, respectively 20% TiO2 (namely FA/16TiO2 and FA/20TiO2). These composites can be treated by solgel method when new phases are grown on initial materials (FA/20TiO2 –TiB). The SEM images for the photocatalytic nanocomposites are presented in Fig. 10 [54]. Duta and Visa [30] mixed TiO2–P25 Degusa with FA in the ratio FA:TiO2 ¼ 3:1. As the surfactant used hexa-decyl-trimethyl-ammonium bromide for surface charge control and as a potential templating agent, it was able to regulate zeolite-type structures. The

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Fig. 8 SEM micrographs of TiO2–P25 at different resolutions

Fig. 9 SEM micrographs of fly ash: (a) before modification (FA) and (b) after modification [54]

FA/TiO2 composite was obtained by stirring at 100 C for 48 h. After the reactions were completed, the suspended matter was washed with ultrapure water having constant pH, filtered, dried at 105–115 C, and treated at 235 C for removing the residual organic compounds. The nano-composite material was tested for simultaneous adsorption and photocatalyst for treatment of wastewaters that containing dyes. For improving photodecomposition property, TiO2/fly ash composites can be doped with different transitional metals such as Mo, La, and Fe [55]. The experimental results showed that the catalyst maintains the structure of anatase (TiO2), and the majority of the doped ions substituted the anatase lattice, which utilizes solar energy effectively. The photocatalytic activity of Mo-TiO2/FA sample for the degradation of methylene blue under visible light irradiation was investigated; optimum value was obtained at 0.3% Mo/Ti molar ratio.

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Fig. 10 SEM micrographs for FA/TiO2 nanocomposites: (a) pure TiO2, (b) FA/16TiO2, (c) FA/20TiO2, (d) FA/20TiO2-TiB [54]

The main results obtained for TiO2/FA with or without dopants are presented in Table 2 [56].

Removal of Pollutant from Wastewaters Using Nanophotocatalysts The photocatalytic activity of the FA/TiO2 nanocomposite photocatalysts was validated by performing the removal of different organic compounds [54].

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Table 2 A comprehensive summary of the ash-supported nanocomposites Method Solgel

TiO2/FA composite AgCI– TiO2/FA Fe3+/TiO2/ FA

TiO2–FA

Hydrothermal

Zn–TiO2/ FA FA/TiO2

Solvothermal

TiO2– zeolite (FA)

Hydrolysis precipitations

N– Co/TiO2/ FA FA/TiO2

Electrospinning

Impregnation

Physical coating Direct activation

TiO2zeolite (FA) C–TiO2/ FA TiO2/FA FA/Perlite

Synthesis methodology Prepared by mixing tetrabutyl titanate and alcohol, AgNO3 solution, subsequent by the addition of FA, and vacuum dried at 70  C The acid-treated FA was added into titanium sol containing Fe(NO3)3and polyethylene glycol (PEG) for 24 h, followed by filtration, drying, and calcination at 450  C for 2 h Titanium tetraisopropoxide dissolved with acetic acid at 50  C for 2 h, followed by the addition of FA aqueous suspension, washing, drying, and calcination at 500  C Impregnation of 10 g of Zn-FA with titanium solution for 24 h, followed by drying at 100  C for 24 h Synthesized by mixing acid-modified FA with TiO2 and hexadecyl-trimethylammonium bromide at 100  C for 48 h, followed by washing, filtration, drying, and heating at 235  C for 24 h Synthesized by mixing FA with alkaline solution and adding titanium solution at 120  C for 6 h, followed by filtration, washing, drying, and calcination at 480  C for 3h Prepared from FA, dropwise addition of TiCl4 with Co ions and urea, followed by filtration, washing, and drying at 75  C, and calcination at 500  C for 2 h Prepared by mixing titanium tetraisopropoxide and acetic acid for 10 min, with the addition of PVP, ethyl alcohol, and FA, subsequently by electrospun, vacuum drying at 60  C for 12 h, and calcination at 600  C for 3 h Prepared by mixing zeolite (synthesized from FA) with titanium precursor solution, subsequent by aging, drying at 80  C for 48 h, and calcination at 480  C for 3 h FA is added into tetrabutyl titanate (in ethanol), dried at 90  C for 12 h, and calcination at 300  C for 2 h under N2 flow Coating TiO2 slurry (8% of TiO2) on the surface of FA, followed by drying in air for 1 day Sodium silicate and NaOH solution (12 M) were mixed with FA and heated at 60  C for 24 h

Ref. [57]

[58]

[59]

[60] [28]

[61]

[62]

[63]

[61]

[64]

[65] [66]

The efficiency of photocatalysis is influenced by several operating parameters such as catalyst type, pollutant concentration, pH value, irradiation time, lamp type, and temperature. From the environmental and economic perspectives, natural pH is more desirable; several studies were carried out with solution without pH adjustment (in generally neutral). All photocatalytic experiments showed adsorption/desorption equilibrium (by contact materials with solution for 30 min in the dark).Because of adsorption

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c1/c0

Fig. 11 Photodegradation of RR45 dye in the presence of pure TiB and FA/TiB nanocomposite catalysts under UV-A irradiation; 1-TiB, 2-FA/16TiO2, 3-FA/ 20TiO2, 4-FA/20TiO2-TiB

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1

0.6 0.5

4

0.4 0.3

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–15

0

15

30

45

60

75

90

t [min]

of pollutant into the photocatalyst, there is an excessive concentration of organic compounds molecules which lead to the saturation of surface and make the photocatalytic process less efficient [54]. The photocatalytic efficiency of materials shown in Fig. 10 is also presented in Fig. 11 for an azo dye (RR45). The composite materials obtained in situ synthesis having 20% TiO2–P25 removed 90% of the dye at the end of the process, while TiO2 synthesized by sol– gel method exhibited the same performance in a shorter time (after 90 min). Studies developed by Gilja et al. demonstrated that nanocomposite photocatalyst containing fly ash is more effective under UV-A irradiation. The nanocomposites obtained in situ produced more electron hole pairs, which result in efficient photocatalytic activity; it means that both TiO2 and fly ash absorb the photons at their interface, and the charge separation occurs at the interface. The synergistic effect between TiO2 and fly ash for the degradation of RR45 dye was demonstrated [54]. TiO2/geopolymer composite materials were studied for their photodegradation activity using organic molecule model methylene blue (MB) solution (5 ppm) with irradiation time 0–8 h. Figure 12 presents the results about degradation efficiency; the maximum value was registered at 48.18% [21]. Foura et al. performed photocatalysis tests for 3–15 wt.% TiO2-loaded HY zeolite [67] in order to establish the influence of the amount of TiO2 over degradation of MB under UV irradiation (Fig. 13). Results revealed that the HY zeolite has a very high influence on the degradation rate. TiO2 particles at the surface of HY zeolite enhance the electron transfer from the semiconductor to the HY zeolite, and consequently, the zeolite prevents the electron-hole recombination [67, 68]. The presence of the zeolite phase improves the photodegradation efficiency through synergistic factors such as highspecific surface area, adsorption capacities, and its ability to keep TiO2 as a dispersed phase. Authors found the optimum TiO2 content at 10 wt.%, but beyond this concentration the excited particles may not be in close proximity to the zeolite surface [69].

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Fig. 12 Photocatalysis efficiency of TiO2-coated geopolymer for MB [21]

C/C0

Fig. 13 Photocatalytic degradation of MB over TiO2 and TiO2/zeolite under UV irradiation [67]

1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Photolysis

TiO2 TiO2 /zeolite

0

20

40

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80

100

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irradiation time (min)

Other studies were performed for Rhodamine 6G [25] using TiO2 synthesized by the solgel method. The higher activity of the synthesized photocatalyst compared with the TiO2–P25 can be explained by the proper ratio between anatase and rutile generated by calcination and a high surface area, (Fig. 14). Other TiO2/fly ash composites were investigated in the degradation of Rhodamine 6G (R6G) and Congo Red (CR) dyes. Therefore, several R6G concentrations between 15 and 65 ppm were used to test their performance. The conversion values for different initial dye concentrations and irradiation time values are shown in Fig. 15. The results indicate that the dye decolorization is higher in the case of lower initial pollutant concentration values. Over 65% after 60 min of irradiation recommended these materials for micropolutant degradation.

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Fig. 14 Conversion of Rhodamine 6G by photocatalytic degradation on TiO2-based samples [25]

Fig. 15 Conversion degrees on 6G function of initial concentration

70

Conversion, %

60 50

15 ppm 25 ppm 35 ppm 65 ppm

40 30 20 10 0 0

30

60

90

120

Irradiation time, minutes

TiO2/geopolymer composite was obtained by adding 5 g TiO2 (10 mass%) in the foamed geopolymer paste in single step (GFA1) [68]. By dissolving TiO2 in HNO3, adding deionized water and stirring for 1 h at 70 C, mixing with geopolymer and stirring for 1 h, and separation, washing, and calcination at 500 C for 2 h, TiO2/geopolymer was obtained in two steps (GFA2) [70]. The photocatalytic efficiency for the degradation of MB was analyzed (Fig. 16).

TiO2/Fly Ash Nanocomposite for Photodegradation of Organic Pollutant

Fig. 16 MB concentration versus the irradiation time of geopolymer and geopolymer-TiO2 composites: 1 – geopolymers; 2 – GFA1; 2 – GFA2 [70]

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40

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Figure 16 shows the composite synthesized by one-step method exhibited better photocatalytic activity for MB degradation after 20 min of irradiation.

Conclusion and Outlook In the last few years, the energy generation by coal burning had increased, resulting in large quantities of fly ash. Only a small quantity of fly ash is reused, whereas remaining high quantities of byproducts are still disposed. Fly ash (FA) proved to be useful for various environmental applications as catalysts for air or water purification, adsorbents for wastewater treatment, etc. Therefore, effective recycling of fly ashes is mandatory. Also, the synthesis of new photocatalyzer, geopolymers, or adsorbents permits the optimum capitalization of ashes leading to reduced environmental pollution and reduced treatment cost by replacing expensive commercial materials. In this chapter, various methods for obtaining new TiO2/FA composite materials were examined by considering the synthesis method in order to identify the influence over photocatalytic activity. The properties of the materials synthesized confirmed their ability to degrade organic compounds with commercial photocatalyzer. According to the literature, ash from different sources has been used as potential low-cost and viable supporting materials for the preparation the TiO2/FA photocatalytic nanocomposite. Fly ashes have a fine consistence with spherical particles, ranging from nanometer to micrometer diameters, which permit the synthesis of different composite materials; using as support materials various types of ashes is an advanced recycling method. TiO2/FA composites have been prepared by different techniques and exhibited different morphologies and chemical and physical properties.

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Morphological studies show that TiO2 coated the spherical particles of the fly ash, and these titania nanoparticles were homogeneously distributed on the ash surface. The deposition of TiO2 on the ash support did not destroy the ash structure. The morphology of ash, especially modified ash by acidic treatment, illustrated the good availability of the surface binding site and active sites. The composites on the base of TiO2/FA demonstrated photocatalytic ability, especially for dyes, high values for degradation were obtained when by thermal treatment was obtained anatase. The degradation degree was increased with the increase of catalyst loading, and increasing the initial concentration demonstrated the decreases of the degradation efficiency. The TiO2/FA composite materials synthesized by sol–gel method archived comparable performance with TiO2-P25. The nanocomposites produced more electron hole pairs, which result in efficient photocatalytic activity; it means that both TiO2 and fly ash absorb the photons at their interface, and the charge separation occurs at the interface. Another mechanism observed was that the zeolite phase prevents the electron–hole recombination leading to increased photodegradation efficiency. On the other hand, acidic-treated fly ash and alkali-activated fly ash (zeolites and geopolymers) through the high-specific surface area have the potential to keep TiO2 as dispersed phase on the its surfaces. Conclusively, the utilization of fly ash as substrate of TiO2/FA nanocomposites in the field of photocatalysis represents a viable and powerful method for sustainable water pollution control with respect to the concept of zero waste. Industrial wastewaters are usually loaded with more pollutants, which can be involved in concurrent or parallel processes when removed via adsorption or photocatalysis processes. Zeolites coated with TiO2 is the better solution for simultaneous treatment. The photocatalytic activities of TiO2/FA nanocomposites must be analyzed for the different industrial effluents, in addition to synthetic dye testing solution. In this chapter, the interdisciplinary knowledge between material science, chemical, and nanotechnology engineering are presented to provide new approaches regarding the synthesis and application of TiO2/FA nanocomposites for water pollutant treatment. Analyzing presented data, ashes can be used as the precursor for the preparation of low-cost composite photocatalyst. Further investigations are needed to expand this research area and to improve the performance of TiO2/FA nanocomposites and scale-up its application for complex polluted waters.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals of the HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Principle of Hydrogen Evolution Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters and Descriptors for HER Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection Criteria for HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-Dimensional Transition Metal Chalcogenides (2D-TMCs) for HER . . . . . . . . . . . . . . . . . . . . Structures and Properties of 2D-TMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2D-TMCs as HER Catalysts (The Intrinsic Activity of 2D-TMCs for HER) . . . . . . . . . . . . . Modification Strategies for Boosting the HER Activity of TMCs . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Sustainable water electrolysis represents a promising approach to establish new hydrogen-based energy system. As the central component in water electrolysis, electrocatalysts play a vital role in the energy conversion efficiency. Numerous efforts have been devoted to developing inexpensive but efficient catalysts to replace the precious platinum catalysts, and various catalysts have been fabricated for hydrogen evolution catalysis in the past several decades. In this chapter, we will summarize the recent process using two-dimensional metal chalcogenides for hydrogen generation via water electrolysis. We will introduce the commonly used two-dimensional metal chalcogenides for hydrogen evolution catalysis, their performance limitations, and the strategies to improve the catalytic performance. S. Niu · G. Wang (*) Hefei National Laboratory for Physical Science at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, P. R. China e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_42

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Finally, we will discuss the key challenges and the future opportunities in the twodimensional metal chalcogenides for hydrogen generation. Keywords

Two-dimensional transition metal chalcogenides · Hydrogen evolution reaction · Heteroatom doping · Interface engineering · Phase transition · Basal plane · Edge sites

Introduction Over the past century, global development and growth in population have resulted in ever-growing demand for energy. The heavy dependence on conventional energy sources (such as coal, petroleum oil, and natural gas) has inevitably caused the detrimental environmental issues and global warming. Therefore, it is urgent to seek sustainable alternatives that can resolve the current issues and sustain long-term development [1]. Molecular hydrogen gas (H 2) with its high gravimetric energy density and pollution-free combustion product (water) has been regarded as an excellent energy carrier and a potential candidate for future low-carbon energy systems. Water electrolysis has been deemed a green and sustainable manner to obtain hydrogen gas, compared with traditional steam methane reforming and coal gasification [2, 3]. However, only 4% of the hydrogen is produced by water electrolysis, and the practical widespread application of this technique is constrained by its high cost. As one of the central components of electrolyzers, electrocatalysts have decisive impacts on the energy conversion efficiencies and the total cost of H2 production. Platinum (Pt) and Pt-based materials are the state-of-the-art hydrogen evolution reaction (HER) catalysts, but their scarcity and high cost substantially limit their widespread use. In this regard, it is highly imperative to develop Earth-abundant and cost-effective but efficient HER catalysts to replace precious Pt. Examples of these non-precious electrocatalysts include transition metal chalcogenides [4– 6], carbides [7, 8], nitrides [9–11], and phosphides [12–14]. Among these available HER electrocatalyst candidates, two-dimensional transition metal chalcogenides (2D-TMCs), such as MoS 2, WS2, MoSe2, and WSe2, have attracted substantial interest due to their low cost, large specific surface area, high atomic exposure, tunable electronic structure, and high intrinsic per-site HER activity [4, 15–18 ]. However, these materials are significantly limited by the main three factors: the poor conductivity and the low density of active sites which are concentrated at the layer edges [15, 19–22]. Accordingly, significant strategies have been developed to increase the charge-transfer dynamics, expose additional active edge sites, and activate the inert basal plane to enhance the overall performance [18, 23–25]. In this chapter, we first summarize the fundamentals of HER and theoretical calculations related to the reaction process. In the subsequent sections, the structure

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properties of 2D-TMCs are simply introduced. Additionally, the recent process using two-dimensional metal chalcogenides for hydrogen generation via water electrolysis is also reviewed, and the commonly used two-dimensional metal chalcogenides for hydrogen evolution catalysis are detailed introduced, such as their performance limitations and the strategies to improve their catalytic performance. Finally, we will discuss the key challenges and the future opportunities in the 2D-TMCs for hydrogen generation.

Fundamentals of the HER The Principle of Hydrogen Evolution Reaction Typically, an electrolyzer has three component parts: an electrolyte (i.e., H2O), a cathode, and an anode as shown in Fig. 1. To trigger the water-splitting reaction, the hydrogen evolution catalyst and the oxygen evolution catalyst are coated on the cathode and anode, respectively. Also some self-supporting catalysts are directly placed on the corresponding electrode to carry out the whole reaction. When driven by an external voltage applied to the electrodes, water molecules were reduced into hydrogen gas (H2) on cathode and oxidized into oxygen gas (O2) on anode. The H2 and O2 are separately stored for further use. The water-splitting reaction can be divided into two half-reactions: the water oxidation reaction (or oxygen evolution reaction (OER)) and the water reduction reaction (or HER). According to different media (acidic, neutral, and alkaline media) in which water splitting takes place, the redox reactions occur on each electrode in different ways (see below).

Fig. 1 Schematic diagram of an electrolyzer. (Reproduced with the permission of Ref. [2]. Copy right 2015, The Royal Society of Chemistry)

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1 Total reaction H2 O ! H2 þ O2 2

ð1Þ

Cathode 2Hþ þ 2e ! H2

ð2Þ

1 Anode H2 O ! 2Hþ þ O2 þ 2e 2

ð3Þ

In acidic media:

In neutral and alkaline media: Cathode H2 O þ 2 e ! H2 þ 2OH

ð4Þ

1 Anode 2OH ! H2 O þ O2 þ 2e 2

ð5Þ

The thermodynamic voltage of water splitting is 1.23 V at 25 °C and 1 atm. However, to conduct water splitting at a practical rate, we must apply voltages higher than the thermodynamic potential value due to the excess potential (also known as overpotential ƞ), which is mainly used to overcome the intrinsic activation barriers present on both anode (ƞa) and cathode (ƞc), as well as some other resistances (ƞother), such as solution resistance and contact resistance. Thus, the practical operational voltage (Eop) for water splitting can be described by the following equation: Eop ¼ 1:23 V þ ηa þ ηc þ ηother ½26

ð6Þ

It is clearly seen from this equation that reduction of these overpotentials by suitable methods is the central issue to make the water-splitting reaction more energy-efficient. Indeed, ƞother can be reduced by optimizing the design of the electrolytic cell, whereas the magnitude of ƞa and ƞc can be minimized by highly active oxygen evolution and hydrogen evolution catalysts, respectively. In this context, a central challenge for chemistry is to develop improved catalysts to make the hydrogen production by electrochemical water splitting more practical. Generally, the detailed mechanisms of HER in acidic and basic electrolyte contain two successive electrochemical steps. In the Volmer step, the external circuit provides an electron coupling with a proton adsorbed on the surface of the electrocatalyst to yield an adsorbed hydrogen atom (Hads) intermediate. Volmer reaction is then followed by either a combination of two Hads (Tafel reaction) or the combination of a Hads with a proton and an electron (Heyrovsky reaction). From the perspective of HER mechanism, the source of adsorbed protons varies with the electrolytes. In acidic media (e.g., H2SO4), protons exist in the form of hydronium ions (H3O+), while water molecules (H2O) act as the proton source in alkaline (e.g., KOH) or neutral media (e.g., phosphate buffer electrolyte) (Table 1). In comparison to the acidic conditions where inimitably water is electrochemically dissociated into adsorbed hydrogen atom in the Volmer step, in neutral and alkaline media, the

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Table 1 Mechanism of HER electrocatalysis in acidic, neutral, and alkaline media Steps Volmer step Heyrovsky step Tafel step

Acidic media H3O+ + e ! H2O + Hads Hads + H3O+ + e ! H2O + H2 Hads + Hads ! H2

Neutral/alkaline media H2O + e ! Hads + OH Hads + H2O + e ! OH + H2 Hads + Hads ! H2

catalyst requires to break the stronger covalent H-O-H bond prior to Hads, which makes the HER in alkaline solution needs extra energy to produce the protons, and thus, the HER is comparatively harder to achieve at low overpotentials in alkaline medium [27].

Parameters and Descriptors for HER Process To elucidate the catalytic activity of a given HER electrocatalyst, there are some important parameters that are required to be measured/calculated carefully. They mainly include onset potential, overpotential (ƞ), electrochemical surface areas (ECSA), Tafel plot, stability, as well as Faradic efficiency (FE) in experiment. To better reveal the intrinsic HER activity, thermodynamic and kinetic energy barriers should be considered, such as the Gibbs free energy of hydrogen bonding (ΔGH) for HER in acidic media and the adsorption and dissociation of water for HER in neutral and alkaline media.

Onset Potential and Overpotential (ƞ) The standard electrode potential of HER is zero under standard conditions. The absolute value of difference between zero and the onset potential to initiate HER using an electrocatalyst is the corresponding overpotential. Generally, there are three kinds of overpotentials, activation overpotential, concentration overpotential, and resistance overpotential. The former can be greatly reduced by using suitable electrocatalysts. To evaluate the total catalytic activity of a material, linear sweep voltammetry (LSV) method is firstly performed. To get accurate overpotential of electrocatalysts, resistance overpotential should be excluded by IR compensation according to the following formula: Ecorrected ¼ Euncorrected  I  R

ð7Þ

where E is the potential, I is the current flowing through the system, and R is the resistance (across surfaces and interfaces) which is tested by electrochemical impedance spectroscopy (EIS). The obtained currents are usually normalized to the superficial geometric electrode area, and the total overpotential is obtained by the polarization curves. To compare the activities among the samples, two special overpotentials in HER are often provided deliberately. One is onset potential at which the reaction starts. The onset overpotential is actually a poorly defined term. If one wants to use this term,

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a suitable current density value (0.5–2 mA cm2) should be clearly shown. The other widely used overpotential is that the electrocatalyst needs to yield a current density of 10 mA cm2 (which is the current density expected for a 12.3% efficient solar water-splitting device).

Electrochemical Surface Areas The ECSA of the electrode is particularly important for electrochemical applications of nanomaterials. ECSA is the amount of surface exposed to the electrolyte, which is much related to the electrical current measured when sweeping the electrode potential. Nanoengineering of catalysts can increase the surface area so that the electrochemically surface area is substantially different from the geometrical area of the flat electrode [28]. Therefore, accurate estimation of the ECSA is exceptionally important for evaluating the activity of the catalyst. The ECSA of the catalysts were estimated using the electrochemical double-layer capacitance (Cdl) obtained by cyclic voltammetry in the non-Faradaic regions (i.e., at potentials where no charge-transfer reactions occur but absorption and desorption processes can take place) [29]. When cycling the electrodes at different scan rates (ν), the evolution of non-Faradaic current density (j) should scale linearly with the scan rate so that the slope gives the Cdl. The ECSA of the catalyst can be  calculated by dividing Cdl by  the specific capacitance of the sample ECSA ¼ CCdls . Thus, the current density
J is more accurate. normalized by the ECSA JECSA ¼ ECSA Tafel Slope Tafel plot depicts the dependence of steady-state current densities on a variety of overpotentials. By replotting the polarization curves (j vs. ƞ) into Tafel plots (ƞ vs. log|j|), the Tafel slope can be determined by fitting the linear regions of Tafel plots to the Tafel equation, ƞ ¼ b(logj/j0), where j0 is the exchange current density and b is the Tafel slope. The small b value means a fast reaction kinetics. Importantly, b is generally related to the catalytic mechanism of the electrode reaction [30], when the value of b is close to 120 mV dec1, Volmer reaction is a rate-determining step, and the electrochemical adsorption reaction of hydrogen atoms on the electrocatalyst surface is sluggish in acid media. Also, the process of H2O adsorption and dissociation to produce adsorption hinders the total reaction in alkaline solution. If the value of Tafel slope approaches to 40 mV dec1, Heyrovsky reaction is the rate-determining step, and the generation of H2 is mainly governed by electrochemical desorption. If the value of Tafel slope is about 30 mV dec1, Tafel step is the slowest kinetic process, and the combination of two adsorbed hydrogen atoms and desorption of H2 will be the limiting steps. Based on the above discussion, Tafel slope can give a reference to infer the hydrogen generation pathway and is an important bridge between theory and experiment. In addition, j0, which is obtained when ƞ is assumed to be zero, profoundly depends on the reaction activation energy at the surface of the electrocatalyst [31]. j0 describes the intrinsic catalytic activity of the electrode material under equilibrium conditions. A good catalytic material should have a high j0 and a small Tafel slope.

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Faradic Efficiency Faradic efficiency, also called columbic efficiency, describes the efficiency by which the electrons provided by external circuit are transferred to desired reaction. For the electrochemical HER, Faradic efficiency is defined as the ratio of the experimentally detected H2 amount versus the theoretical H2 amount. The theoretical hydrogen production can be calculated from galvanostatic or potentiostatic electrolysis by integration. And the practical hydrogen production can be measured by gas chromatography (GC). The FE of an ideal HER catalyst is expected to be ~100%. Stability Stability is another important parameter for HER electrocatalyst evaluation. The stability can be divided into electrochemistry performance and structural stabilities during the HER process. For electrocatalytic stability, there are two techniques for the measurements: cyclic voltammetry (CV) and galvanostatic (V-t curve) or potentiostatic (I-t) electrolysis. For I-t, it would be better to set a stationary overpotential larger than 10 mV for a long period of time (>10 h). Additionally, the V-t measurement would be better to set a current density larger than 10 mA cm2 for at least 10 h continuous test and the potential without obvious change. The other method is to conduct the recycling experiment by performing CV or LSV, and the potential cycles are repeated in the region including the onset HER potential. The number of cycles should be larger than 1000 times to elucidate the stability of a material. After electrocatalytic stability test, the transmission electron microscope (TEM), scanning electron microscope (SEM) for morphological characterization, Xray diffraction (XRD) for phase composition, and X-ray photoelectron spectroscopy (XPS) for chemical state characterization are also necessary. Descriptors Hydrogen evolution activity is strongly correlated with the ΔGH to the catalyst surface in acid media. Plotting the exchange current density of various catalysts in acids vs. Gibbs free energy of hydrogen adsorption can obtain the “Volcano plot” (Fig. 2) [15]. An optimal HER catalyst should provide catalytic surfaces that exhibit

Fig. 2 Volcano plot of the exchange current density as a function of the density functional theory (DFT)calculated ΔGH for nanoparticulate MoS2 and the pure metals. (Reproduced with permission from Ref. [15]. Copyright 2007, American Association for the Advancement of Science)

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Fig. 3 The schematic illustration of energy barrier for water dissociation in alkaline HER

a ΔGH value close to zero, which means the balance between the rate of proton reduction and the ease of removal of adsorbed hydrogen from the surface [31, 32]. The electrocatalyst that lies at the top of the curve exhibits better HER activity. Accordingly, one can also easily understand why Pt is a most efficient catalyst for the HER in acids. Importantly, by comparison of the exchange current densities of the common metals at their corresponding free energy of absorbed H, ΔG0H, the value for MoS2 just lies below those of the noble Pt group metals, suggesting the great potential of MoS2-based materials as an alternative to Pt for the HER [15]. However, water dissociation is an additional energy barrier required that needs to be overcome by the catalyst to continue the electrocatalytic hydrogen production (Fig. 3) [27]. This is the reason why the alkaline catalytic activities of Pt are approximately two orders of magnitude lower than those in the acids. Thus, theΔGH is not enough to describe HER in alkaline media.

Selection Criteria for HER Catalysts When one chose and evaluate the HER catalyst, these parameters discussed above should be considered comprehensively (Fig. 4). An ideal HER catalysts should be a material with small onset potential and overpotential, small Tafel slope, large ECSA, high FE, superior stability, moderate H adsorption energy, and low water dissociation energy barrier.

Two-Dimensional Transition Metal Chalcogenides (2D-TMCs) for HER Despite the significant efforts over the past decades to develop new HER materials for hydrogen production via electrochemical water splitting, there are only few 2DTMCs that can meet the requirements for commercially hydrogen conversion. The

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Fig. 4 The summary of parameters and descriptors for evaluating HER catalysts

long-standing bottleneck for 2D-TMCs is their poor conductivity, activity, and stability, which limited their large-scale industrial application. Here, we review the recent development using 2D-TMCs materials for efficient and stable water splitting for hydrogen production.

Structures and Properties of 2D-TMCs Geometry Structures The layered 2D-TMCs which have the general chemical formula MX2, where M is a transition metal (e.g., Mo, W, Ta, and Nb) and X is a chalcogen atom, generally S, Se, or Te, exhibit the most favorable electrochemical properties. In this chapter, we focus on our attention on MoS2, WS2, MoSe2, and WSe2 which are the most researched TMC materials for HER. A typical 2D-TMC consists of one layer of M-site atoms sandwiched between two layers of X site atoms. Each repeated structural unit of TMCs exposes the prismatic edges and basal planes, while the edge termination by either metals or chalcogen atoms. The “missing” coordination at the edges of MX2 gives rise to metallic edge states, which express important implications for catalysis [33]. The atomic layers are bonded to each other by the relatively weak van der Waals forces [34]. There are three commonly known phases for TMCs, namely, 1 T, 2H, and 3R, where the digit indicates the number of layers in the crystallographic unit cell and the latter indicates the type of symmetry with T standing for tetragonal (D3d group), H for hexagonal (D3h group), and R for rhombohedral (C53ν group) [35]. The stacking methods for both 2H and 3R types are the “A-B-A” method, and the M atoms occupy the center of triangular prisms. The 3R type is easily converted to 2H by heating treatment, meaning that the 2H type

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Fig. 5 The typical 1 T, 2H, and 3R structures of 2D-TMCs (top and side view). (Reproduced with permission from Ref. [33]. Copyright 2020, Royal Society of Chemistry)

is more stable than the 3R type. The 1 T phase is defined as one where the transition metal atom coordination is octahedral (Fig. 5). The 2H phase has semiconducting property, whereas the 1 T phase is metallic. So the transformation of 2H to 1 T shows a positive effect for enhancing HER performance.

Band Structures and Density of State Analysis The band structures of materials can be calculated from DFT to gain further insight in their electronic structures. The band structures and partial density of states (PDOS) of pristine monolayer MoS2, MoSe2, WS2, and WSe2 calculated by Perdew-Burke-Ernzerhof functional (PBE) are shown in Fig. 6. All of the four TMCs are direct band gap semiconductors [36]. The values of their band gaps are summarized in Table 2. Due to the characteristics of semiconductors of these TMCs, the efficient strategies by tuning band structures to improve the electronic conductivity is very important for boosting the HER performance. By further analyzing the PDOS (Fig. 6), it can be seen that the electronic states near the (conduction band maximum) CBM and (valence band maximum) VBM contributed mainly from dz2 (blue), d x2 y2 (red), and dxy (red) orbitals of the M atom (Mo and W) and p (black) orbitals of the X atom (S and Se). The strong coupling between p orbitals of the X atom and dxz + dyz orbitals of the M atom leads to a large splitting between their bonding and antibonding states. Thus, dxz and dyz orbitals do not directly contribute to states near CBM and VBM as shown by the green PDOS in Fig. 6.

2D-TMCs as HER Catalysts (The Intrinsic Activity of 2D-TMCs for HER) A deeper insight into the HER performance of TMCs is gained by examining their intrinsic HER catalytic activity, which is determined in terms of ΔGH and the

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Fig. 6 The band structures and partial density of states of different molecular orbitals, under PBE, are presented. (Reproduced by permission from Ref. [36]. Copyright 2013, American Physical Society)

Table 2 The band gaps in eV of monolayer MoS2, MoSe2, WS2, and WSe2 calculated by PBE. (Reproduced by permission from Ref. [36]) Materials Eg (eV)

WS2 1.819

MoS2 1.679

MoSe2 1.444

WSe2 1.548

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coverage of H (θH) at each metal or chalcogen edge and also the basal plane chalcogen atoms. θH is the fraction of chalcogen atoms on the edge or basal plane that enables hydrogen binding. The inherent catalytic activity of the TMCs was studied by DFT calculations which is reported in 2014 by Tsai et al. As illustrated in Fig. 7, the ΔGH for the basal planes is ~2 eV across TMCs, which indicates that the basal plane is inert regardless of the type of transition metal or chalcogen. All edges are noted to have ΔGH close to zero except for the W edge in WSe2 and S edge in MoS2. Briefly, the primary active site for MoS2 is the Mo edge; for WS2 and MoSe2, both the metal and the chalcogen edges are active; for WSe2, the Se edge is active. According to the above analysis, the main limitations of TMCs are their poor conductivity, lack of active sites, less-efficient edge active site, and inert large basal plane. Therefore, we think that there are four main strategies for the modification of pristine TMCs to improve their HER performance: (1) improving the intrinsic activity of sites by appropriately tuning the electronic structure of the edge or activating the inherently inert basal plane, (2) increasing the number of edge active sites by creating holes inside the TMCs layers, (3) improving the conductivity by introducing new electronic states near the Fermi level and narrowing the band gap, and (4) interface engineering to make a synergic catalyst by introducing a cocatalyst. Along these strategies, various approaches have been developed to modify 2D-TMCs for the HER performance enhancement, and the details will be described below.

Modification Strategies for Boosting the HER Activity of TMCs Increasing Edge Sites and Optimizing Edge Activity Both experimental [15] and computational [20, 21] studies have shown that the edge sites of TMC layers are active sites for HER catalysis. Motivated by the higher catalytic activity along the edges of TMCs, many forms of nanostructured MoS2, such as nanoparticles, vertical nanoflakes, nanowires, and mesoporous structures, have been actively pursued to maximize the exposed energetic active sites [37–39]. Kong et al. [37] presented a synthesis process for growing MoS2 and MoSe2 thin films with vertically aligned layers. The molybdenum chalcogenide films are converted from eBeam evaporated, ultrathin Mo films by a rapid sulfurization/ selenization process in a horizontal tube furnace, where elemental sulfur/selenium powders are used as the precursors. The prepared molybdenum chalcogenide films provide maximum exposure of the edges on the film surface and correlated the HER catalytic activity directly with the density of the exposed edge sites (Fig. 8). Kibsgaard and co-workers created a highly ordered double gyroid mesoporous MoS2 network structure, first, by electrodepositing Mo onto a silica template and, second, by sulfidization with H2S (Fig. 9) [17]. This double gyroid MoS2 structure exhibited a high surface curvature preferentially exposing a significant portion of edge sites, which leads to excellent activity for electrocatalytic hydrogen evolution. Although the edge sites are active for HER and the same strategies are developed to maximize these sites, the performance is far from practical application. To

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Fig. 7 The structures and hydrogen adsorption free energies for MoSe2, WSe2, MoS2, and WS2. All values of ΔGH are shown for the final adsorbed hydrogen at the corresponding θH. (Reproduced with permission from Ref. [21]. Copyright 2014, the PCCP Owner Societies)

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Fig. 8 (a) Schematic of the synthesis setup in a horizontal tube furnace. (b) Digital photos of a pristine oxidized silicon (300 nm SiO2/Si) substrate (left), a MoS2 film on oxidized silicon (middle), and a MoSe2 film on oxidized silicon (right). (c) Volume-rendered reconstructed TEM tomogram of a MoS2 film grown by rapid sulfurization, which resembles the ideal edge-terminated structure. (d) Statistical distribution of tilt angles (θ) of layers in individual grains in edge-terminated MoS2 film, where the tilt angle is defined in the inset. (e) Transmission electron microscopy (TEM) image of a MoS2 film produced by rapid sulfurization. (f) Idealized structure of edge-terminated molybdenum chalcogenide films with the layers aligned perpendicular to the substrate, maximally exposing the edges of the layers. (Reproduced with permission from Ref. [37] Copyright 2013, American Chemical Society)

optimize the activity of edge sites, heteroatom doping to optimize the electronic structure and interface engineering are also developed. Although some previous research have shown the optimized H adsorption at edge site for TMCs, the sluggish water dissociation limits their application for HER in KOH solution. Zhang et al. [37]

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Fig. 9 Synthesis procedure and structural model for mesoporous MoS2 with a double gyroid (DG) morphology. (Reprinted with permission from Ref. [17]. Copyright © 2012, Springer Nature)

reported a novel strategy to improve the kinetics of water dissociation process of MoS2 via doping nickel atoms in MoS2 nanosheets. The DFT calculation results have shown that the kinetic energy barrier of the initial water dissociation step of MoS2 in Mo site is 1.17 eV, which is significantly reduced by Ni doping (0.66 eV), and the adsorption interaction of OH is substantially reduced on Ni-doped MoS2 catalysts (Ni-MoS2) (Fig. 10a). The resultant Ni0.13Mo0.87S2 nanosheets exhibit an excellent electrochemical HER activity in 1 M KOH aqueous solution with an extremely low overpotential of ~98 mV at a current density of 10 mA cm2 (Fig. 10b) [40]. Chemical doping can change the intrinsic activity of the active site, whereas the interface engineering can yield electrocatalytic properties distinct from those of their individual components by synergistic effect. This synergistic effect can effectively integrate the multiple advantages of each component, resulting in a much improved catalytic performance. Some studies have proven that oxides/hydroxides can serve as effective promoters for the breaking of the O-H bond, further improving the HER performance. Thus, introducing a water dissociation promoter into TMCs can enhance the HER performance of TMCbased catalytic system [41–46]. For instance, Ni(OH)2 nanoparticles were electrodeposited on the surface of MoS2 nanosheets that were previously vertically grown on conductive carbon cloth [47]. This hybrid nanostructure exhibited an onset overpotential of 20 mV and a low overpotential of 80 mV at 10 mA cm2 in 1.0 M KOH electrolyte, both of which surpassed those of the unitary counterparts. Further theoretical calculations demonstrate the synergistic effect of Ni(OH)2/MoS2 interface: Ni(OH)2 provides the active sites for hydroxyl adsorption, and MoS2 facilitates adsorption of hydrogen intermediates and H2 generation. In another similar research, Hu et al. demonstrate a dramatic enhancement of HER kinetics in base by judiciously

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Fig. 10 (a) The results of the DFT calculations and the corresponding mechanisms of the electrocatalytic HER on the surfaces of different catalysts under alkaline conditions. ΔG(H2O) and ΔG(H) are the related kinetic energy barriers for the Volmer and Tafel steps on the catalysts, respectively. ΔG (OH) is the Gibbs free energy of the adsorbed -OH on the surfaces of catalysts. E (eV) in the diagram represents the free energies of the different reactive stages. The yellow, blue, and red spheres represent S, Mo, and Ni, respectively. (b) Polarization curves of the MoS2, NiMoS2, Co-MoS2, Fe-MoS2, and commercial Pt/C catalysts. (Reproduced with permission from Ref. [40]. Copyright 2016, The Royal Society of Chemistry). (c) Polarization curves of the CFP substrate, bare NiCo-LDH, MoS2, and MoS2/NiCo-LDH composite catalysts in 1 M KOH solution.

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hybridizing vertical MoS2 sheets with layered double hydroxide (LDH). The resultant MoS2/NiCo-LDH hybrid exhibits an enhanced HER performance with extremely low overpotential of 78 mV at 10 mA/cm2 and a low Tafel slope of 76.6 mV/dec in 1 M KOH solution (Fig. 10c). In this system, the presence of NiCo-LDH accelerates the water dissociation steps and thus reduces the activation energy of the Heyrovsky and Volmer steps in the reactions over the MoS2/NiCo-LDH hybridization catalyst (Fig. 10d). And MoS2 work for H2 desorption (Fig. 10e).

Increasing Defect Density on the Basal Plane Even though the fraction of edge sites was dramatically increased in this nanostructure, the catalytically inert basal plane sites still have a large proportion of the layered TMCs. Thus, developing efficient strategies to activate the inert basal planes is paramount to increase the atomic utilization and enhance the HER performance for TMCs. Both experiment and computational studies proved that the vacancy of X atoms (S and Se) and chemical doping and phase conversion of TMCs from 2H to 1 T are efficient means for tuning the electronic state and catalytic activity [16, 19, 24, 25, 48–52]. The strategy of manipulating S vacancy can be achieved by hydrogen treatment, chemical etching strategy, and Ar plasma treatment, and S vacancy can work as an intrinsic active site for HER and concentration. Recently, Wang et al. developed a facile and mild H2O2 chemical etching strategy to introduce homogeneously distributed single S vacancies onto the MoS2 nanosheet surface (Fig. 11a) [50]. The single S vacancy concentration was precisely tuned by systematic tuning of the etching duration, etching temperature, and etching solution concentration. The spherical aberration corrected scanning transmission electron microscopy (STEM) imaged together with the line profiles proved the homogeneously distributed single-atom type S vacancies in the basal plane of MoS2 (Fig. 11b, c). The optimal HER performance reaches a Tafel slope of 48 mV dec1 and an overpotential of 131 mV at a current density of 10 mA cm2, indicating the superiority of single S vacancies over agglomerate S vacancies (Fig. 11d). This is ascribed to the more effective surface electronic structure engineering as well as the boosted electrical transport properties, which is shown in Fig. 11e, f. Additionally, the S vacancy concentration dependence HER performance is also studied. Li et al. applied strain and Ar plasma treatment on ultra-thin MoS2 to introduce strained S vacancies [19]. By proper tuning of the strain and plasma conditioning, S vacancy concentrations were tuned, where 12.5% has been reported as an optimal to lead to the highest HER catalytic activity (Fig. 12a–d). The vacancy engineering to enhance the HER performance can be applied to other TMCs. Gao et al. synthesized MoSe2 nanosheets by ä Fig. 10 (continued) (d) Free energy diagram of the dominant Volmer-Heyrovsky pathway for HER in alkaline electrolyte for bare MoS2 (blue) and MoS2/NiCo-LDH composite (red) catalysts. (e) TEM images of MoS2/NiCo-LDH composite; enlarged image is the schematic illustration of the HER at MoS2/LDH interface in alkaline environment. Synergistic chemisorption of H (on MoS2) and OH (on LDH) for enhancing the water dissociation step. (Reproduced with permission from Ref. [42]. Copyright 2017, Cell Press)

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Fig. 11 (a) Schematic of the chemical etching process to introduce single S vacancies. (b and c) STEM image together with the line profiles extracted from the areas marked with purple rectangles

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chemical vapor deposition (CVD). The prepared 2D MoSe2 shown the most amount of dual-native vacancy, and the computational study demonstrated that both Se and Mo vacancies can activate the electrocatalytic sites in the basal plane. Experimentally, 2D MoSe2 nanosheet arrays with a large amount of dual-native vacancies possess an overpotential of 126 mV at a current density of 100 mV cm2, a Tafel slope of 38 mV dec1, and an excellent long-term durability [53]. The M-site modification with other metal elements or X-site doping with other nonmetallic elements are effective method to tune the electronic properties of TMCs and thus enhance the HER activity [24, 25, 40, 54–58]. For examples, the single Ptdoped MoS2 was synthesized through a one-pot chemical method. In this system, single-atom metals replace “Mo” atoms in MoS2 to trigger the HER activity of inert MoS2 surface. The doped metal atoms can tune the adsorption behavior of H atoms on the neighboring S sites, resulting in a significantly enhanced HER activity on the MoS2 surface (Fig. 13a, b) [25]. O and C elements can replace the X site to activate the inert basal plane. The pioneering work of Xie’s group [59] has demonstrated that the presence of O can significantly contribute to the charge density for both valence band and conduction band, thus effectively enhancing the conductivity of MoS2. And also, O incorporation with controllable disorder engineering optimized the intrinsic active sites for H adsorption. Most recently, Zang et al. demonstrated that the HER inert of the basal plane originates from the unfavorable orbital orientation [24]. They developed C-doped MoS2 nanosheets for HER catalysis. Carbon is in situ incorporated into MoS2 basal plane through a unique incomplete sulfurization of Mo2C. The obtained C-MoS2 with sulfur vacancies have shown a reduced energy barrier of water dissociation due to the strong coupling of O atom in H2O with 2pz orbital of C atom which is perpendicular to the basal plane (Fig. 13c–g). In experiment, the optimized C-MoS2 have shown an exceptional HER activity with a low overpotential of ƞ10 ¼ 45 mV and low Tafel slope of 46 mV dec1, which represented the best alkaline HER activity among ever-reported MoS2-based material (Fig. 13h, i).

Phase Transition Engineering Since the first report of Manish Chhowalla and colleagues [60] on the 1 T phase of WS2 showing over five times higher conductivity and enhanced HER activity, vast research efforts have been made in the modulation of 1 T/2H phases in TMCs. Generally, 2H-TMCs exhibit good electrocatalytic activities, due to the available active sites and stability. However, the low electrical conductivity can hinder their overall performance. 1 T phase is metallic, and thus the phase transition from 2H to 1 T is very important because it completely alters the electronic properties of the ä Fig. 11 (continued) of a CVD-grown monolayer MoS2 flake film after etching. The yellow dotted circles represent the S vacancies. (d) HER polarization curves. (e) Top-view and side-view electron density difference maps and (f) the projected electronic density of states of the d-band for the Mo atoms of P-MoS2, MoS2-FGJK, and MoS2-EGMO. The horizontal dashed lines in part b indicate the calculated d-band center. (Reproduced with permission from Ref. [50]. Copyright © 2020, Springer Nature)

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Fig. 12 (a) Schematic of the top (upper panel) and side (lower panel) views of MoS2 with strained S vacancies on the basal plane. (b) Free energy versus the reaction coordinate of HER for the S vacancy range of 0–25%. (c) ΔGH versus %x strain for various % S vacancy. (d) Colored contour

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materials. However, the 1 T phase is a metastable state and can be easily converted into 2H phase; how to stabilize the 1 T phase is a challenge. There are several promising reports for modulating the phase transition engineering to realize HER performance. It would be of interest to properly combine the 1 T and 2H phases, where both the enhanced conductivity and stability could lead to much improved HER catalytic performance. Qu et al. [61] adopted plasma-assisted selenization process to produce Mo/(1 T-2H)-MoSe2 core/shell arrays with Tafel slope of ≈35 mV dec1. Deng et al. developed N-induced phase transition from 1 T to 2H-MoSe2. The 1 TMoSe2/TiC-C arrays were synthesized by hydrothermal method and phase modulation of MoSe2 from 1 T phase to 2H phase and mixed 1 T-2H phase via a facile hydrothermal plus annealing process, followed by N doping. The DFT calculation demonstrated that N doping not only decreases the band gap of MoSe2 but also reduces the energy barrier promoting easier phase transition from 2H to1T. The fabricated N-(1 T-2H)-MoSe2/TiC-C arrays exhibit the best HER performance both at routine current density of 10 mA cm2 and large current density of 100 mA cm2, with the lowest overpotentials and smallest Tafel slopes, better than other 1 T-MoSe2 and 2H-MoSe2 counterparts in their study [52]. In another study using the nickelcobalt complexes to stabilize the 1 T MoS2 [62], the author presented a facile strategy to synthesize porous hybrid nanostructures combining amorphous Ni-Co complexes with 1 T phase MoS2 (denoted as PHNCMs) through hydrazine-inducing (Fig. 14a). In this synthesis process, hydrazine hydrate (HZH) concentration can regulate the crystallization of Ni-Co-based compounds and the phase of MoS2. They propose that the phase transformation of MoS2 is attributed to the enrichment of amorphous Ni-Co complexes with electron-donor ability of hydrazine because of the stabilization effects of the complexes on 1 T phase MoS2. The optimal material has shown enhanced HER performance and better stability (Fig. 14b, c).

Conclusions and Future Outlook Conclusions This chapter has summarized various modulation strategies for improving the electrocatalytic HER performance of TMCs, which are divided into three broad groups of parameters, i.e., increase in the conductivity, increase the number of edge active sites and optimize those sites for better HER performance, ä Fig. 12 (continued) plot of surface energy per unit cell γ (with respect to the bulk MoS2) as a function of S vacancy and uniaxial strain. (Reproduced with permission from ref. [19]. Copyright © 2016, Springer Nature). The calculated HER free energy results and ΔGH values with Mo site and Se site in the case of (e) basal plane Se vacancy, (f) basal plane Mo vacancy, (g) Se edge with Se vacancy, and (h) Mo edge with Mo vacancy. Insets show the optimized MoSe2 structure with both basal plane and edge-related Se and Mo vacancies with H adsorption. (Reproduced with permission from Ref. [53]. Copyright © 2018, John Wiley & Sons, Inc)

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Fig. 13 (continued)

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introduction of additional active sites by several pathways to activate the inert basal plane. These pathways include the using of a phase transition engineering, chemical doping, defect creation, construction of heterostructures, and their combinations. In particular, the site engineering approach by dopant introduction on either M-site or X-site or both sites is of interest and effective in several cases. Indeed, this approach has attracted much more considerable research efforts than others, where computational calculations have played a critically important role. Several types of doping elements have been well studied for TMCs, for their efforts in changing the conductivity and the overall electrocatalytic performance. It is also true that some of the TMCs have been studied in more detail than others, for example, MoS2. Due to their structure similarity, the modulation strategies are also available to other TMCs, such as WS2, MoSe2, and WSe2.

Future Outlook Based on the successes that have been achieved so far, there will be extensive and continuing efforts in tuning TMCs for their electrocatalytic HER performance. Studies will be extended to various co-dopings, other types of TMCs, and several other doping/modification elements that have yet done properly. The studies of other TMCs beyond MoS2 should be systematically done in the future. As computational studies will continue to be ahead of the experimental investigations, they will definitely continue to play vital roles. Although ΔGH obtained from DFT calculation is an ideal descriptor for HER, the energy barrier of water dissociation should be considered. ä Fig. 13 (a) The relation between currents (log(i0)) and ΔGH0 presents a volcano curve. The left and right sides of the volcano plot adopt two sets of scales for better visibility. The inserted graphs point to different configurations of doped MoS2 as coordinated with four (left) and six (right) S atoms. The adsorption sites for H atoms are marked by red dashed circles. The studied metal atoms are located in the periodic table as shown by the inset at the bottom. Green balls, Mo; yellow balls, S; blue and purple balls, doped metal atoms. (b) Magnified domain of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pt-MoS2 with red dashed rectangle showing a honeycomb arrangement of MoS2 and the single Pt atoms occupying the exact positions of the Mo atoms (marked by red arrows). The bottom inset shows the simulated configuration of Pt-MoS2. The green, yellow, and blue balls represent Mo, S, and Pt, respectively. (Reproduced with permission from Ref. [25]. Copyright 2015, The Royal Society of Chemistry). The top-view and side-view sp2 hybrid orbitals (highlighted by red dash circle) at the top of valence band (c) and the empty 2p orbitals (highlighted by red dash circle) perpendicular to the basal plane at the bottom of conduction band (d) of C-MoS2. The top-view electrostatic potential of water adsorbed on the basal plane of C-MoS2 (e) and MoS2 (f) and the corresponding side-view bonding and nonbonding orbitals. (g) The relative energy diagram along the reaction coordinate, including the first (left panel) and second (right panel) water dissociation process on the basal plane of MoS2 and C-MoS2, respectively. R reactant, RC reactant complex, TS transition state, IM intermediate. (h) The LSV curves of CC, Mo2C, C-MoS2, MoS2, and Pt/C with IR correction. (i) The corresponding Tafel slopes. (Reproduced with permission from Ref. [24]. Copyright © 2019, Springer Nature)

Fig. 14 (a) Schematic representation of the formation of PHNCMs. (b) The LSV curves of studied material (no HZH, 0.05 ml of HZH, 1 ml of HZH, and 2.5 ml of HZH denoted as 0H-PHNCMs, 0.05H-PHNCMs, 1H-PHNCMs, and 2.5H-PHNCMs, respectively). (c) Chronoamperometric responses (I-t) recorded on 2.5H-PHNCMs for 24 h at a constant applied potential of 0.13 V versus RHE for HER. (Reproduced with permission from Ref. [62]. Copyright © 2019, Springer Nature)

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43. Qin C et al (2019) Interface engineering: few-layer MoS2 coupled to a NiCo-sulfide nanosheet heterostructure as a bifunctional electrocatalyst for overall water splitting. J Mater Chem A 7 (48):27594–27602. https://doi.org/10.1039/c9ta10547f 44. He Z et al (2019) NiSx@MoS2 heterostructure prepared by atomic layer deposition as highperformance hydrogen evolution reaction electrocatalysts in alkaline media. J Mater Res 35 (7):822–830. https://doi.org/10.1557/jmr.2019.325 45. Ouyang C et al (2017) Three-dimensional hierarchical MoS2/CoS2 heterostructure arrays for highly efficient electrocatalytic hydrogen evolution. Green Energy Environ 2(2):134–141. https://doi.org/10.1016/j.gee.2017.01.004 46. Chen W et al (2020) Achieving rich and active alkaline hydrogen evolution heterostructures via interface engineering on 2D 1T-MoS2 quantum sheets. Adv Funct Mater 30:2000551. https:// doi.org/10.1002/adfm.202000551 47. Zhang B et al (2017) Interface engineering: the Ni(OH)2/MoS2 heterostructure for highly efficient alkaline hydrogen evolution. Nano Energy 37:74–80. https://doi.org/10.1016/j.nanoen.2017.05.011 48. Lin S, Kuo J et al (2015) Activating and tuning basal planes of MoO2, MoS2, and MoSe2 for hydrogen evolution reaction. Phys Chem Chem Phys 17(43):29305–29310. https://doi.org/ 10.1039/c5cp04760a 49. Zhou Y et al (2020) Enhanced performance of in-plane transition metal dichalcogenides monolayers by configuring local atomic structures. Nat Commun 11:2253. https://doi.org/10.1038/ s41467-020-16111-0 50. Wang X et al (2020) Single-atom vacancy defect to trigger high-efficiency hydrogen evolution of MoS2. J Am Chem Soc 142(9):4298–4308. https://doi.org/10.1021/jacs.9b12113 51. Lukowski MA et al (2013) Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J Am Chem Soc 135(28):10274–10277. https://doi.org/10.1021/ ja404523s 52. Deng S et al (2018) Phase modulation of (1T-2H)-MoSe2/TiC-C shell/core arrays via nitrogen doping for highly efficient hydrogen evolution reaction. Adv Mater 34(30):e1802223. https:// doi.org/10.1002/adma.201802223 53. Gao D et al (2018) Dual-native vacancy activated basal plane and conductivity of MoSe2 with high-efficiency hydrogen evolution reaction. Small 14(14):e1704150. https://doi.org/10.1002/ smll.201704150 54. Sun X et al (2014) Semimetallic molybdenum disulfide ultrathin nanosheets as an efficient electrocatalyst for hydrogen evolution. Nanoscale 6(14):8359–8367. https://doi.org/10.1039/c4nr01894j 55. Sun K et al (2019) Design of basal plane active MoS2 through one-step nitrogen and phosphorus co-doping as an efficient pH-universal electrocatalyst for hydrogen evolution. Nano Energy 58:862–869. https://doi.org/10.1016/j.nanoen.2019.02.006 56. Liu P et al (2017) P dopants triggered new basal plane active sites and enlarged interlayer spacing in MoS2 nanosheets toward electrocatalytic hydrogen evolution. ACS Energy Lett 2 (4):745–752. https://doi.org/10.1021/acsenergylett.7b00111 57. Gao D et al (2018) Activation of the MoSe2 basal plane and Se-edge by B doping for enhanced hydrogen evolution. J Mater Chem 6(2):510–515. https://doi.org/10.1039/c7ta09982g 58. Li Y et al (2016) Carbon doped molybdenum disulfide nanosheets stabilized on graphene for the hydrogen evolution reaction with high electrocatalytic ability. Nanoscale 8(3):1676–1683. https://doi.org/10.1039/c5nr07370g 59. Xie J et al (2013) Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution. J Am Chem Soc 135(47):17881–17888. https:// doi.org/10.1021/ja408329q 60. Voiry D (2013) Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution. Nat Mater 12:850–855. https://doi.org/10.1038/NMAT3700 61. Qu Y et al Wafer scale phase-engineered 1T- and 2H-MoSe2/Mo core–shell 3D-hierarchical nanostructures toward effcient electrocatalytic hydrogen evolution reaction. Adv Mater 44(28). https://doi.org/10.1002/adma.201602697 62. Li H et al (2017) Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting. Nat Commun 8:15377. https://doi.org/10.1038/ncomms15377

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M. L. Dotaniya, C. K. Dotaniya, Kuldeep Kumar, R. K. Doutaniya, H. M. Meena, A. O. Shirale, M. D. Meena, V. D. Meena, Rakesh Kumar, B. P. Meena, Narendra Kumawat, Roshan Lal, Manju Lata, Mahendra Singh, Udal Singh, A. L. Meena, B. R. Kuri, and P. K. Rai

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inorganic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Pollution on Soil Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect on Water Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect on Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollutant Removal Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges of During Analysis of Pollutant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M. L. Dotaniya (*) · M. D. Meena · P. K. Rai ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur, India C. K. Dotaniya College of Agriculture, SKRAU, Bikaner, India K. Kumar ICAR Indian Institute of Soil and Water Conservation, Dehradun, RS Kota, India R. K. Doutaniya OPJS University, Churu, India H. M. Meena ICAR-Central Arid Zone Research Institute, Jodhpur, India A. O. Shirale · V. D. Meena · B. P. Meena ICAR-Indian Institute of Soil Science, Bhopal, India R. Kumar Division of Crop Research, ICAR-RCER, Patna, Bihar, India © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_65

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Abstract

Soil pollution is everywhere and poses a serious threat to human and ecological health. Growing population pressure on natural resources beyond the recycling capacity of soil is creating soil pollution. Different organic and inorganic pollutants are dumped on healthy soil/water bodies and reach to human body via food chain contamination. It causes different carcinogenic effects like organ failure (kidney, pancreas, liver), suppression of immunity system, imbalance of endocrine hormone, and failure of reproductive system. With pace of scientific development, pollutant detection capacity, identification of pollutant routes origin to final disposal, interaction with soil particles, and impact on soil health, degradation time and uptake by crop plant are well studied. Soil is acted as a sink for pollutant. Longterm application of polluted water in agricultural production system poses health hazards effect and leads to cancer. Soil biodiversity also reduces and degradation of pollutant is slowed down, which is responsible for longer persistence of chemicals in an ecosystem. Use of tradition and modern tool and techniques for combating soil pollution are advocating. Spread awareness among the grassroot peoples by different agencies to identify the pollutant toxicity prior to dispose. Healthy soil produces healthy crop yield and provides better eco-services for human welfare. Keywords

Crop growth · Heavy metal · Phytoremediation · Pollutant estimation methods

Introduction Increasing population pressure on limited natural resources promotes pollution in different ecosystems. India holds second largest population after china only on 2.4 percent land. This chunk of land is also ranged as chemically polluted, deserted land, N. Kumawat College of Agriculture, Indore, Madhya Pradesh, India R. Lal Shri Bhawani Niketan Law College, Jaipur, India M. Lata Barkatullah University, Bhopal, India M. Singh Bihar Agricultural University, Sabour, India U. Singh College of Agriculture, Lalsot, India A. L. Meena ICAR-Indian Institute of Farming Systems Research, Modipuram, India B. R. Kuri School of Agriculture, Suresh Gyan Vihar University, Jaipur, India

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forest land, water bodies, infrastructure and household land, etc.; however, only 264.5 million hectare (Mha) is available for agricultural production. If we are comparing per capita agricultural land in Indian condition is only 0.12 ha, whereas global scale is 0.29 ha. This situation is beyond the carrying capacity of agricultural land for food production and leads to deteriorating soil health. As per estimation, India needs 333 million tons (Mt) of food grain production to feed growing population by the year 2050 [1]. Effluents generated from different industrial units are discharged after primary treatment or as a raw on the surface of land and in water bodies, creating health hazards to living creatures. It contains different type of pollutant, heavy metals, organic matter, and meager amount of plant nutrients [2]. Composition and characteristics of pollutant depend on type of industrial unit, procedure of treatment, discharge pattern and also climatic factors. Long-term application of poor quality water for irrigation during crop production accumulated significant quantities of pollutant in agricultural fields [3]. Most of the high populated developing countries have moderate development of scientific technology and infrastructure development, and also lower level of awareness among the peoples has enhanced soil pollution. Soil is a complex natural body and meeting point of lithosphere, atmosphere, and hydrosphere. It is an important component of biosphere and responsible for providing food material to living creatures. Shelter for human, animals, birds, and thousands of microbes, it does maintain food chain and food web for interpersonal sustainability of different ecosystems [4, 5]. Microbial diversity and population both are important constituents for mineralization, degradation of organic matter, plant nutrient dynamics, secretion of organic acids, release of plant growth hormone, rhizosphere activities, and detoxification of pollutants [6]. Alarming rate of population increasing with modern life style also promotes waste generation. Anthropogenic activities enhance pollutant discharge rate over a period, and overpressure of liquid and solid waste declines environmental health and sustainability [7, 8]. Rate of pollution enhancement is directly proportional to growing rate of populations in developing countries. In developed countries this rate is little bit affected by technological management and scientific disposal of waste [9]. On an average per day discharge rate of urban waste is 33,900 million liters per day (MLD) and industrial units are also not behind that their contribution 23,500 MLD in India as per pollution statistics of 2009 [10]. India is having more number of small- and medium-scale industries and contributing more polluting effects due to poor organization setup and direct release of pollutants on healthy soils or in sewage channels [11, 12]. Whereas big industries are having sufficient currency flow and improve scientific technology that lead to reduced pollutant concentration in final disposal effluent. Some of the industries are based on zero discharge patterns and strictly follow norms of environmental health and human health issues. India has a central agency known as Central Pollution Control Board (CPCB), which identifies pollutant sites based on different scientific parameters and comes out with the Comprehensive Environmental Pollution Index (CEPI) rating. On the basis of index, many Indian cities are categorized under polluted zone (Table 1). At present more than 43 zones were identified in 16 states and most of the locations are in the states of Gujarat, Uttar Pradesh, Maharashtra, and Tamil Nadu [13, 14].

3106 Table 1 Critically polluted industrial area located in Indian cities. (Modified from [14])

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State Andhra Pradesh Chhattisgarh Delhi Gujarat

Haryana Jharkhand Karnataka Kerala Madhya Pradesh Maharashtra

Orissa

Punjab Rajasthan

Tamil Nadu

Uttar Pradesh

West Bengal

Located city Vishakhapatnam Patancheru – Bollaram Kobra Nazafgarh drain basin Ankeshwar Vapi Ahmedabad Vatva Bhavnagar Junagarh Faridabad Panipat Dhanbad Mangalore Bhadravati Greater Cochin Indore Chandrapur Dombivalli Aurangabad Navi Mumbai Tarapur Angul Talcer Ib Valley Jharsuguda Ludhiana Mandi Gobind Garh Bhiwadi Jodhpur Pali Vellore (North Arcot) Cuddalore Manali Coimbatore Ghaziabad Singrauli Noida Kanpur Agra Varansi-Mirzapur Haldia Howrah Asansol

CEPI rating 70.82 70.07 83.00 79.54 88.50 88.09 75.28 74.77 70.99 70.82 77.07 71.91 78.63 73.68 72.33 75.08 71.26 83.38 78.41 77.44 73.77 72.01 82.09 74.00 73.34 81.66 75.08 82.91 75.19 73.73 81.79 77.45 76.32 72.38 87.37 81.73 78.90 78.09 76.48 73.79 75.43 74.84 70.20

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Different types of soil pollutants adversely affect soil ecosystem functions. It is a need of hours that assessment of soil health parameters at an interval to compute dynamic changes in soil properties. Use of organic and inorganic soil amendment to immobilize/remove trace metals from soil system is also advocating. Application of farm yard manure (FYM) reduced Cr uptake pattern in spinach and reduced chances of human health. Hazard quotient (HQ) should be identified for most of the consumptive crops grown in the polluted areas. Scientific methods and procedures practiced to identify pollutant levels should be standardized at an interval for precision results. Long-term dumping of waste leads to the formation of different types of pollutant derivatives, which are difficult to estimate, and development of remediation procedure is a challenging task for researchers and policy makers. In this chapter, we will describe the type of pollution and their characteristics, major pollutants, method of estimation, and major challenges during analysis.

Type of Pollutants On the basis of composition it is categorized into mainly two groups, the carbon containing pollutant known as organic pollutants (OPs) and inorganic pollutants (IPs). Both type of pollutants of are geogenic and anthropogenic origin. Many researchers have also categorized pollutants into three groups by adding biological pollutants. Organic pollutants are more resistant to biodegradation and easily soluble in organic solvents creating lot of ecosystem problems. The path of organic pollutant contamination is very complex due to formation of complex with soil constituents. Inorganic pollutant are of acute toxicity in nature and widely known as potential toxic elements. These pollutants are degradable by soil microbial biomass and also change toxic to nontoxic form of chemicals in nature. Soil properties are extensively affected by biodegradation kinetics of pollutants and mediate as a source and sink of pollution.

Organic Pollutants Organic pollutants mostly originate naturally by means of volcanic eruption, combustion of fossils fuels, and forest fire accidents. Some man-made organic pollutants are more toxic and persist in nature for long time. Across the globe, toxicity of dichloro-diphenyl-trichloroethane (DDT) is well documented and much worst effect was observed in developing countries. These pollutants have high solubility in organic solvent, fat solvent, and water. Volatilization and conversion of one form to another form by climatic or soil properties are also observed. In last few decades OPs have attracted the mindset of researchers and policy makers to remediate it from different ecosystems due to long persistence. Soil organic matter plays a lipophilic effect to make a complex with organic pollutant and enhances half-life in ecosystems. Volatilization nature of pollutant also travels long distance and makes spatial distribution from origin sources. Most of the organic pollutants have complex

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structure with wide diversity due to conversion as a by-product or transformation during industrial activities. These pollutants are widely discharged in environment during their use as an agrochemical. Some of the well-known chemicals [like DDT and their derivatives, chlordane, dioxins, furans, polycyclic aromatic hydrocarbons (PAHs)] act as potential pollutants for ecosystem services. PAHs are well-known priority pollutants classified by United States Environmental Protection Agency (US EPA) and also by European Union (EU) with respect to health hazards. Organic pollutants are carcinogenic in nature and their persistence and degradation period is too long [15].

Polycyclic Aromatic Hydrocarbons (PAHs) PAHs contribute significantly to pollution in organic pollutant category. More than 300 organic compounds are made up of PAHs by containing benzene rings. It is also categorized under carcinogenic pollutant as per US EPA. Increasing molecular weight of PAHs proportionally enhances the potential of carcinogenic toxicity. There are low molecular weight (2–3 rings) and high molecular weight (>4 ring structure) PAHs. Some of the PAHs are phenanthrene, pyrene, chrysene, anthracene, and benzo-anthracene. A range of PAHs pollutant sources in nature are biosynthesis by microbial communities and geochemical reactions related to volcanic, erosion, fossil fuel, and production of mineral materials. Its toxicity hampers smooth function of lung, kidney, skin, and pancreas, and also reduces the immune system of body by endocrine disruptors. Longer exposure of peoples in the field of cleaning, road construction, rubber and steel industries, and roofers make them vulnerable to PAHs toxicity effect and face different ill effect in body [15]. Soil acts as a sink for PAHs pollutant and transfers significant portion to food chain by different pathways. Accumulation of PAHs in soil adversely affects the soil microbial population by reducing diversity and decomposition rate of organic matter. Agricultural Chemicals It is widely used in agricultural production system for achieving higher crop yield. It comprises herbicides, insecticides, fungicides, rodenticides, molluscicides, and nematocides to prevent, destroy, or control any pest, and these are widely used mainly in agriculture and in domestic sectors. Across the global market different pesticides are marketed without knowing their ecological impact assessment. Farmers are centric only on higher crop yield and careless for soil health mostly in developing countries. In the world, more than 75 percent of pesticides are used in European countries and the USA; and less than 25 percent are consumed by rest of the countries. Among the Asian countries pesticide use is found to be highest in China and followed by Korea, Japan, and India. Chunk of applied pesticide reaches water bodies during heavy rainfall as a non-point source of pollution. It causes different diseases in human being and degrades the soil health. Among the different pesticides used across the globe, organochlorine pesticides pose serious health concerns. These pollutants are responsible for chronic and acute toxicological effects and cause failure of liver, kidney, and reproductive system; different types of cancer; and weaker immune system.

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Biological Pollution Researchers have also categorized excessive biological load in an environment as a biological pollutant. There are microorganisms, that is, bacteria, fungus, virus, molds, and pollens and other harmful biological matter, that adversely affect the ecosystem services. Transmission from sources to the new place totally depends on spreading medium.

Inorganic Pollutants Mostly, inorganic pollutants are geogenic origin and altered into a complex form of chemical by interacting environmental factors. Anthropogenic activities promote inorganic pollution by accelerating diverse industries across the globe (Table 2). A range of industries (like metallurgical, tannery smelting, battery, aluminum, copper electric industries, etc.) release toxic pollutants into soil and water ecosystems. These industrial units discharge heavy metals into environment that reach human body through food chain contamination and cause different metabolic irregularities, in extreme conditions living organisms may die. These trace metals are well known for their carcinogenic effects in human beings. The inorganic pollutants form new compounds under different climatic conditions and induce modifications in species and its labile form [16]. These pollutants are known as potentially toxic elements (PTEs). It is a group of metals and metalloids and categorized broadly as heavy metals. Atomic density of these pollutants is more than 4 g/cm3 which is highly carcinogenic in nature. The PTEs are ready to form different compounds as (1) compound with ions or organic matter, (2) complex reactions to colloidal particles, (3) adsorbed on inorganic element species, (4) fixed into oxide clay minerals, (5) incorporated in supergenic oxide phase or on salt ions.

Arsenic Pollution Arsenic contamination is of geogenic origin and is mostly in groundwater. It is concentrated in western USA, Mexico, Chile, Argentina, Hungary, Romania, Magnolia, Nepal, Taiwan, Vietnam, Thailand, and West Bengal of India (Fig. 1). Its concentration is more in deeper layer of well. Overuse of groundwater enhances As concentration in drinking water due to oxidation of As containing minerals. Arsenic acid (H3AsO4) and arsenous acid (H3AsO3) play crucial role in As toxicity. In 2007, a study was conducted to monitor As distribution and its toxicity impact on ecosystem. The study revealed astounding finding that 137 million people are suffering in more than 70 countries by As toxicity. It is a huge figure and most of the Asian countries are big sufferer of this problem. Bangladesh is most affected by consumption of As contaminated drinking water. Different government agencies and international organizations (UNICEF) have installed different types of groundwater pumping instrument technologies and modifications of digging well for safe drinking water. The periodical impact assessment of installed technology showed As toxicity related diseases in human beings. Over the perspective more than 20 percent

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Table 2 Anthropogenic sources of potentially toxic elements categorized by US EPA [17] Industrial process Alloys Batteries and electro/ chemical cells Biocides (agriculture, antifouling) Ceramics and glass Chemicals, pharmaceuticals, dental Coatings (anticorrosives) Electrical equipment and apparatus Fertilizers Fossil fuel combustion (electricity) Mining, smelting, metallurgy Nuclear reactor (moderator, absorber) Paints and pigments Petroleum refining Pipes, sheets, machinery Plastics Pulp and paper Rubber Semiconductors, superconductors Tanning and textiles Wood preservative treatment

As *

Be

Cd *

Cr *

Cu *

*

*

*

*

*

*

*

* * *

* *

Ni * *

Pb * *

*

* *

Sb *

Se * *

Ti

* *

*

*

*

* * *

*

*

*

*

*

*

*

* *

* * *

*

*

*

*

*

*

*

*

* * *

*

* *

*

*

*

*

*

*

*

* *

* * *

*

*

* *

*

*

*

* * *

* *

*

* *

* *

* * *

Zn * *

*

* *

*

Hg

* *

*

of total death was classified under As-related disease. In India As toxicity is seen in the Indo-Gangetic plain and in Bihar and West Bengal part. As per WHO the limit for As in drinking water is 0.01 mg/L, which has been modified in Bengal up to fivefold. A study found that the collected groundwater samples from West Bengal locations were reported to have As concentration of 0.05 mg/L, which is responsible for different types of metabolic disorders [18]. Arsenic is a heavy metal. Long-term application of As contaminated water used for irrigation accumulates significant amount of As in soil. It reaches human body via food chain contamination. It causes vomiting, stomach pain, diarrhea, blood

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Fig. 1 Areas of the world with high naturally occurring arsenic levels in the groundwater

cancer, skin itching, cardiovascular diseases, and in extreme intake leads to cancer or death [19]. Arsenic uptake by crop plants shows different types of toxicity symptoms like reduction in germination percent, root and shoot growth retardation, yellowing of leaf, reduction in formation of chlorophyll, and death of plant tissues, which are common under cultivation with As contaminated soils (Table 3). Arsenic toxicity initially affects the root growth of the plant and reduces the water and nutrient uptake mechanisms. Interconversion of different forms of As is controlled by the genetic potential of a species. Different enzymes act as electron donor during the As uptake process (Fig. 2). These contaminated parts of plant consumed by human being impose different metabolic disorders. If a plant acts as a hyperaccumulator for As, then its Bio Concentration Factor (BCF) will be higher than one. Management of As Toxicity Arsenic toxicity is associated with groundwater exploitation. Minimize the use of As contaminated water for crop production by enhancing water use efficiency, development of improved varieties, addition of biosorption materials, and minimum withdrawal of groundwater. Soil fertility management is also prime need for managing As contaminated soils for sustainable crop production. Addition of organic matter also reduces As concentration in crop rhizosphere [33]. Addition of phosphorus, iron, silicon, and sulfur containing fertilizers alleviates As harmful effect in crop plant by modifying As uptake pattern from root to shoot [32, 34, 35]. It is also advocating that periodical collection and analysis of soil and water samples from As contaminated areas is important for planning and management strategies. As mitigation is highly affected by climatic factors, hydrological cycle, soil properties, and the level of As in soil [36].

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Table 3 Arsenic phytotoxicity effect on crop plants Crop Rice

Maize

Barley Mustard

Potato Soybean Cotton

As content 2 mg L 1 soil solution 0.5 mg L 1 soil solution 0.5 mg L 1 soil solution 10 mg kg 1 soil

Harmful effect Inhibition of germination by 10% Inhibition of root growth by 20% Reduction of shoot height by 30% More than 45% grain yield reduced

25 mg kg

1

soil

Dry matter reduction by 50%

30 mg kg

1

soil

Both chlorophyll-a and chlorophyll-b contents in rice leaf decreased significantly No rice plant survived up to maturity stage

60 and 90 mg kg 1 soil 12.5 and 25 mg kg 1 soil 50 and 100 mg kg 1 soil 20 mg kg 1 tissue 150 μM in hydroponic solution 290 mg kg 1 soil >1 mg kg tissue >4 mg kg tissue

Reference Abedin and Meharg [20]

Jahiruddin et al. [21] Onken and Hossner [22] Rahman et al. [23]

Promoted maize growth and the nutritional quality of the grain Toxic effects for the crop

Ci et al. [24]

Growth inhibition

Davis et al. [25] Praveen et al. [26]

Inhibition in shoot length, chlorophyll, carotenoid, and protein No growth inhibition

1

Yield reduction

1

Yield reduction

Codling et al. [27] Deuel and Swoboda [28]

Cadmium Pollution It is a heavy metal with carcinogenic properties and found in sedimentary rocks. Its atomic number is 48 and represented as Cd. It is mainly used in nickel-lead automobile batteries, pigments, stabilizers, plating, etc. Cultivation of food crops with the help of Cd contaminated effluent accumulates significant amount in soil and reaches the human body via food chain contamination process. In Japan, Cd effluents used for cultivation of paddy caused serious health hazard which is known as itai-itai disease across the globe. Longer use of cadmium-contaminated drinking water may lead to osteomyelitis. In human being, it accumulates mainly in kidney. It disturbs calcium balance in body, causing hypercalciuria as a result of which stones are formed in the kidney. Cadmium pollution reduces ecosystems health and adversely affects the plant nutrient dynamics and soil organic carbon mineralization rate. Intake of Cd through smoking is dangerous for health, and it is absorbed by lungs and stomach tissues. In agricultural crop production system, significant amount of Cd is contributed during the use of P fertilizers (Table 4).

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Fig. 2 Pictorial diagram of arsenic uptake by plant roots from soil. GSH glutathione, AR arsenate reductase, GSSG oxidized glutathione, PCs phytochelatins. (Adopted and modified from Ma [29]; Ali et al. [30]; Zhao et al. [31]; Mondal et al. [32])

Widely Cd levels range in phosphate rock with the lowest concentration (0.2 mg Cd kg 1 P) in igneous ores to over 150 mg Cd kg 1 P in sedimentary rock [37]. In recent decades, particular concern has been focused on Cd in fertilizers because of the relative ease of transfer of Cd from soil to plant and the perceived health risk [38]. Management of Cadmium Pollution Cadmium is a cation, having Cd2+ elemental form. It is reacting with other element present in environmental and form stable compounds. Application of organic matter reduced Cd uptake by spinach crop [40–42]. Increasing the concentration of other cationic ions in soil solution reduces the uptake pattern of Cd by plants. It is also important to treat effluent prior to discharge Cd contaminated water on open field or water bodies. Vegetable and fish cultivation with Cd contaminated water should be

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Table 4 Cadmium concentration in rock phosphate reserve [39] Country South Africa Russia USA Jordan Morocco Israel Tunisia Senegal Togo Other countries (Algeria, Syria, Finland, Sweden)

Cd content (mg/kg P) 0.04–4 0.1–2 3–165 5–12 6–73 7–55 41 71–148 72–79 0.1–28

Type of deposit Igneous Igneous Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary/ Igneous

avoided in view of health hazards. Periodical sampling of Cd contaminated soil, irrigation water, and crop samples should be analyzed and critically monitored.

Chromium Pollution Chromium is the 21st most abundant element in the earth’s crust [43]. It occurs in nature in bound forms that constitute 0.1–0.3 mg/kg of the earth’s crust. It has several oxidation states ranging from Cr (-II) to Cr (+VI). It exists predominantly in the Cr+3 and Cr+6 oxidation states. The most stable oxidation state of Cr is +III, and under most prevailing environmental conditions Cr (VI) is rapidly reduced to Cr (III). The intermediate states of +IV and +V are metastable and rarely encountered [44]. Most of the soils have Cr concentration in the range of 15 to 100 μg/g of soil which increases with the proportion of clay [45]. The total Cr concentration levels in igneous and sedimentary rocks are usually up to 100 μg/g. Soil Cr is largely unavailable to plants because it occurs in relatively insoluble compounds such as chromite (FeCr2O4) in mixed oxides of Cr, Al, and Fe, or in silicate lattice. In addition Cr3+ binds tenaciously to negatively charged sites on clays and organic matter. Due to this reason the translocation of Cr from soil to plants is generally insignificant [46]. Chromates (hexavalent Cr) in soils are relatively rare and stable only in alkaline oxidizing conditions. It is reported that Cr3+ and CrO42 are taken up by two different mechanisms. The uptake of CrO42 is depressed by SO42 [47]. Chromium is used in metal alloys and pigments for paints, cement, paper, rubber, and other materials. Low-level exposure can irritate the skin and cause ulceration. Long-term exposure can cause kidney and liver damage, and damage to circulatory and nerve tissues. Chromium often accumulates in aquatic life, adding to the danger of eating fish that may have been exposed to high levels of chromium. Excessive Cr accumulation in contaminated soils results in decreased soil microbial activities, soil fertility, and overall soil quality, and thus reduction in yield and also the entry of toxic materials into food chain causing many diseases in human [48–50]. Very high levels of Cr (VI) contamination (14,600 mg/kg in groundwater and 25,900 mg/kg in soil) were reported at the United Chrome Products site in

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Corvallis, Oregon [51]. Chromium critical concentration in plant varies according to species but generally varies from 1 to 2 mg/kg and maximum allowable limit in soil is 100 mg/kg [52]. Symptoms of Cr phytotoxicity include inhibition of seed germination, early seedling development, reduction of root growth, leaf chlorosis, and depressed biomass [53, 54]. It significantly affects the metabolism of plants such as barley, citrullus, cauliflower, wheat, and maize. The subcellular localization of Cr as found by electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) suggested that Cr is accumulated mainly in the cell wall and vacuoles.

Lead Pollution It is also a heavy metal. It is of geogenic origin and found as compounds with oxide and sulfur element. It has been identified for human use since the start of the industrial revolutions across the globe [55]. It is purified with chemical refining process. Lead is directly released into environment by combustion of petrol and diesel, burning of fossils fuel, mining, battery industries, smelting wire and soldering, etc. It is gray in color with Pb2+ ions in pure form, but rarely found. Atmospheric oxygen reacts with elemental Pb species and forms PbO compound. The outer layer of oxygen protects Pb from further degradation. Long-term application of Pb contaminated industrial effluent as an irrigation purpose or discharge on open field accumulated significant concentration of Pb [56]. Higher Pb content in soil modified the microbial population and diversity adversely. Lead is slowly degraded by soil microbes and its long persistence creates huge loss to soil biodiversity [57, 58]. It also reaches living organisms via food chain or web. In plant initially uptake of Pb inhibits roots from soil. Root and shoot growth is adversely affected by Pb toxicity. In human being, Pb toxicity causes retardation of metal development in children, retardation of growth, imbalance of enzymes like δ-aminolevulinic acid dehydratase in blood and urine, skin diseases, and cardiovascular disease, and longer exposure may lead to cancer [59]. Pb in human body interacts with other essential metal ions and alters homeostasis and their essential function. Lead toxicity is also governed by its species, for example, tetraethyl Pb causes 100 times more mortality than inorganic Pb in living system [55]. Management of Lead Toxicity Lead is carcinogenic in nature. Its harmful effect is more in children and shows different type of irregularity in metabolism. Regular testing of blood samples from peoples of Pb polluted areas should be a regular practice. Refinement of scientific method of Pb estimation and toxicity limits in different crops with respect to temporal and special must be evaluated. Effluent treatment plant should be installed with strong policy, regulation, and supervision which will restrict Pb entry into environment [60]. Application of Ni plant nutrient in soil reduced Pb uptake by spinach [61, 62]. Promote use of lead free technology mostly in soldering, ceramic industries, glass manufacturing, and also in development of hydrometallurgical items [63]. Application of organic matter in Pb contaminated soil reduces Pb concentration in rhizosphere by formation of Pb-humus complex [64]. Phytoremediation methods are also helpful to reduce Pb concentration from Pb contaminated ecosystems [65].

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Mercury Toxicity It is a natural occurring element and exists mostly in sulfide form in the earth’s crust. Its concentration generally found is 0.5 mg/kg. Major sources of mercury contamination in nature are the volcanic eruptions, outgassing from mineral rock, coal burning, amalgam, use in medical instruments etc. Anthropogenic activities accelerate Hg pollution in atmosphere and affect the ecological services. Mercury pollution was initially reported at Minamata Bay of Japan. Across the world its toxicity clicks as Minamata disease in mind. It is a carcinogenic metal and contributes toxicity by both organic and inorganic forms. Mercury exists as mercurous (Hg2+), vapor mercury (HgO), and organic mercury mostly in complex bond with other carbonaceous materials. During discharge of Hg contaminated water into water bodies Hg settles down into lower profile of water and changes its properties as organic Hg by microbial activities. Toxicity highly depends on forms of Hg, level, and period of exposure. Research results show that methyl mercury is less toxic than elemental Hg which is easily absorbed by human tissues and reduces changes to reach across blood-brain barrier [66]. Mercury vapor forms complex with sulfhydryl group or sulfur containing amino acids in living bodies [67]. Some of the Hg containing salts are also available in nature and have poor adsorption kinetics, stable, and less soluble. Long-term use of Hg contaminated waste/effluent for growing food production or longer exposure to vapor form of Hg also leads to carcinogenic effect in human being. It affects the metabolic functions of every organs but major damage is caused in gut lining and kidney [67]. Excretion of mercuric Hg is through urine and stool, and a little amount also by tears, saliva, and milk. Polluted water bodies are the greatest chance of organic Hg intake by aquatic animals and reach to human body. It is mainly in methyl mercury form and sometimes replaced with ethyl mercury without changing the acute mode of toxicity. Mercury salt like HgCl2 widely affects the functions of gastrointestinal tract [67, 68]. Management of Mercury Toxicity Mercury toxicity is also managed with the help of organic and inorganic soil amendments and use of biotechnological techniques. Use of plant for extraction of Hg concentration from soil solution is well documented. In this line, Cyrtomium macrophyllum is identified as a potential plant for phytoremediation. Different microbes also reduce the Hg toxicity in soil system. Addition of FYM, biochar, and other organic matter substrates immobilize formation of methyl mercury (MeHg). Use of different nanoparticles (ZrP, S, CoS, ZnS, Mn and Fe) for removal of Hg from ecosystem are also advocating. Polymers are used for absorption of Hg from polluted sites due to huge surface area, potential chemical stability, tunable pore, and chemical stability. Apart from these different minerals, pyrite (FeS2) is also used for decontamination of Hg toxicity [69].

Nitrogen Pollution Nitrogen (N) is essential for plant growth. It is a main constituent of amino acid and formation of protein. Its deficiency in plant reduces the photosynthesis rate in plant. Its

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deficiency symptoms appear on lower leafs as a yellow color. Different crops have different rate of N requirement, and its efficiency is also governed by physicochemical properties of soils, genetic potential of cultivars, and climatic factors. In general nitrogen use efficiency (NUE) ranges from 40% to 50% under optimum crop and soil management practices. It is a mobile nutrient in soil as well as in plant. Applications of FYM or crop residues enhance NUE and crop yield. Excessive applications of chemical N fertilizers leach down into lower profile under heavy rain. Leaching rate is more under sandy soil as compared to clay soils. Increasing organic matter content in sandy soil reduces the N leaching rate and improves NUE. Nitrate leaching is a common process in intensive cropping systems during excessive application of N. It causes groundwater pollution and different type of metabolic diseases. It causes methemoglobinemia, in which red blood cells are affected and reduce the metabolic activities. As per the US EPA fixed maximum permissible level of nitrate in drinking water is 10 mg/L [70]. The toxicity of nitrate concentration in human body is much affected by body weight and acceptable daily intake (ADI) rate, FAO/WHO fixed 0– 3.7 mg (kg body weight) 1 day 1 [71]. Apart from this ammonia (NH4) volatilization also affects the environmental quality. Under high pH excessive application of N fertilizer on soil with high temperature and humidity leads to volatilization loss of N. It affects air quality and causes acid rain in polluted areas. Management of N Pollution Nitrogen pollution is indispensable and based on agricultural activities. Application of N fertilizers in right amount, at right time, and at right place with appropriate calibration of crop requirement is important. Management of saline and alkaline soils with suitable soil amendment reduces N losses and improves NUE. It is also suggested that application of FYM during crop cultivation reduced the N leaching loss. Split application of N fertilizers at critical crop growth stages is also advocated by researchers [72].

Phosphorus Pollution Phosphorus (P) is one of the most important plant nutrients. Its deficiency in crop reduces the crop yield drastically. The phosphorus use efficiency (PUE) also lowers (˂20%), which is a challenge for the scientific community and policy makers. Nowadays, food production has become highly dependent on the use of P fertilizers whereas the reuse of alternative P sources receives much less attention. This can be considered an unsuitable development because the rock deposits, from which most P fertilizers originate, are finite [73]. Increase in the world population creates more challenges to produce sustainable food for the burgeoning population. More emphasis should be placed on sustainable use of P fertilizers and recycle/reuse of P containing alternatives for food and other uses. Raise public awareness by government agencies and private organizations on scarcity of rock phosphate, while presenting policy options and measures showing a way out of the problem. On the one hand P fertilizers are useful in crop production but on the other hand they create environmental pollution due to mismanagement [74]. Only about one-fifth of the world mined P is eventually eaten. Each stage between mine and fork is associated with losses. The amount of P

3118 Table 5 Sources of phosphorus to waters in Europe [73]

M. L. Dotaniya et al.

Sources Point source (urban contribution) Agriculture Natural loading

Contribution (%) 50–75 20–40 5–15

entering a spatial entity (field, country, and region) exceeds the amount of P exported from the entity, so-called P surplus. This surplus leads to the imbalance in country economics and ecological system (Table 5). A significant amount of P enters freshwater systems in four main ways: (i) atmospheric inputs, including rain and dust; (ii) point sources, including sewage treatment plants and industrial effluents [75, 76]; (iii) non-point sources, including storm water, agricultural, and land clearing runoff [77]; and (iv) non-point sources from within the water system, including washout from riverbanks and resuspension from sediments (internal loading). It leads to P accumulation in the agricultural field, in the waste sector, emission in environment in various forms, and most importantly in water bodies as eutrophication. Phosphate fertilizer is often applied carelessly, which leads to waste and pollution. It is more precisely hypertrophication, which is the ecosystem response to the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system. It accelerates the growth of bloom or greatly increases phytoplankton in a water body as a response to increased levels of nutrients. Negative environmental effects include hypoxia, the depletion of oxygen in the water, which induces reductions in specific fish and other animal populations. It reduced the biodiversity and new species invasion, causes foul smell, and reduced water transparency. Pathways of P loss and the character of environmental differ considerably country wise and region wise and also depend on the food chain. The typology of losses is important because it can help to develop specific response strategies.

Effect of Pollution on Soil Health Soil is a complex structure and nonrenewable resource. It is a base unit of all creatures. During crop production farmers apply imbalanced and excessive amount of chemical fertilizers. For the insect–pest and weeds control, a range of chemicals are also practiced across the global world. These chemicals have long persistence module in nature. Accumulated pollutant in soil is either taken up by plants, degraded by soil microorganism, or converted from toxic to nontoxic form by different reaction in soil. Over a period toxic materials formed new derivatives and slowed down the degradation process. Heavy metals in soil hamper the plant nutrient dynamics and reduce uptake pattern [78]. For example, excessive chromium concentration in soil solution damages spinach root as brown color and reduces essential plant mechanism; plant shows starvation of nutrient and over a period die (Fig. 3). The increase in Cr concentration in soil reduces the soil organic carbon mineralization rate and plant nutrient concentration in soil solution. Organic pollutant concentration also limits the degradation rate of plant nutrient compounds and

Type of Soil Pollutant and Their Degradation: Methods and Challenges

Fig. 3 Chromium toxicity in spinach [79]

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reduces soil physical properties like infiltration rate, bulk density, soil aggregates, etc. Sometimes complex formations of organic and inorganic pollutants are tough to degrade by soil microbial population. Many pathways deliver these pollutants to food web/food chain and ultimately reach to the living system. Long-term application of contaminated water in agricultural crop production system accumulates significant amount of organic and inorganic pollutants which deteriorate soil health by mediating microbial population and diversity.

Effect on Water Bodies Water is a prime need for industrial and agricultural activities, which generate significant volume of contaminated water. Poor quality water is being discharged on open field or in water bodies. It contains organic and inorganic pollutants which are toxic to aquatic animals. Organic pollutant enhances growth of aquatic plants and lowers down the oxygen concentration in water bodies. Water ecosystem is totally disturbed due to modification in biological habitats and breakdown mechanism of pollutant by aquatic animals. Arsenic toxicity in human being is a well-known example which is caused by consumption of fish from polluted water.

Effect on Air Air pollution includes dust and particulate matters (PMs) in environment. Both the groups are accelerated due to fast industrialization as well as population growth of a country. Most of the countries are debating on PM2.5 and PM10, which are mainly contributed by anthropogenic and natural incidences (storm, volcanic events, weathering of rock, during soil erosion process). These fine particles are harmful to health and cause respiratory, skin, and eye infections, cardiovascular diseases, premature death, etc.

Pollutant Removal Methods Heavy metal removal from environment is a big task. It requires technological advancement with huge infrastructure cost. Many methods are used by industrialist and policy planner to reduce the pollutant from different ecosystems. In broad categories are physical, chemical, and biological methods. Each and every method has its own merits and limitations.

Physicochemical Methods In this category use of chemicals for immobilization/removal of heavy metal from ecosystem is in practice. It is a good method and needs higher cost for infrastructure and chemicals. The chemical compounds formed in soil of water bodies sometimes

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create greater problem for microorganism during degradation process. Some of the chemical processes are as follows. Precipitations In precipitation process, acid or alkaline material is used for neutralizing polluted soil or water bodies. For example, acid mines are neutralized with the help of calcium carbonate, quicklime, magnesium hydroxide, and other materials that show alkaline properties. In vice versa, addition of gypsum neutralizes the higher pH pollutant. Added organic and inorganic materials make complex with heavy metals and reduce labile concentration of a metal in soil solution. Acidic properties of pollutant are much affected by the metal and its species present in ecosystem (Table 6). Most of the soil amendments act based on precipitation mechanisms. Adsorption Kinetics In absorption kinetics pollutants are adsorbed onto the surface of an adsorbent. Application of soil organic matter in contaminated soil leads to secretion of different types of low molecular organic acids, and act as an adsorbent surface for metal species [81]. It is affected by metal ion, type of soil amendments, soil properties, climatic factors, etc. [82]. In adsorption sometimes toxic pollutants are adsorbed for a shorter period and again release metal ions in the presence of other metal ions. This process is more suitable for a dilution purpose and it is labor intensive. Ion Exchange It is a method in which one metal ion exchange with other ions in solution. The soil solution maintains equilibrium conditions by releasing or adsorption of metal ions from solid surface to solution phases. Reclamation of alkaline and acidic soils is also based on ion exchange from solution to solid phases. This process is governed by a particular temperature and soil reaction [83, 84]. Table 6 Soil reaction potential of metals [80]

Metal species Fe3+ Al3+ Cr3+ Cu2+ Fe2+ Pb2+ Cd2+ Na+ Zn2+ Hg2+ Mn2+

pH 3.5 4.1 5.3 5.3 5.5 6.0 6.7 7.0 7.0 7.3 8.5

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Biosorption This process involves use of biological material for removal of pollutant from any system. It has different types of processes including adsorption kinetics, ion exchange, and also precipitation mechanisms. It is easy to operate with lower cost. It is more suitable for removal of heavy metals from liquid system [85].

Biological Techniques Heavy metal contamination is steeply increasing in the environment. It can be remediated by physical, chemical, and biological approaches. In biological approach it can be separated by phytoremediation as well as bioremediation (microorganism). Among the all remediation technologies, phytoremediation is cheaper and eco-friendly in nature. Microbial Method for Pollutant Removal With the help of different microbes soil pollutants can be removed. In this process, bacteria, fungi, actinomycetes, and algae are used for removal of heavy metals. These organisms degrade the toxic pollutants and convert them into nontoxic substances. These are important for degradation of azo-dye, trace metal degradation, and petroleum pollutants. These microbes are specific in nature and need to be particular for a unique pollutant [86]. Skilled person with scientific knowledge is more required to handle this type of pollutant removal. Phytoremediation It is also referred to as botanical bioremediation which involves the use of green plants to decontaminate soils, water, and air. It is an emerging technology that can be applied to both organic and inorganic pollutants present in the soil, water, or air. It employs plants and their associated root-bound microbial community to remove, degrade, or render harmless environmental contaminants. At present there are about 400 species of known terrestrial plants that hyperaccumulate one or more of several metal(loid)s. It helps in reducing heavy metal pollution. It has the advantage of relatively low cost and wide public acceptance. It can be less than a quarter of the cost of excavation or in situ fixation. Phytoremediation has the disadvantage of taking longer time to accomplish than other treatments. There are different categories of phytoremediation, including phytoextraction, phytofiltration, phytostabilization, and phytodegradation, depending on the mechanisms of remediation [54]. Phytoextraction

It is the name given to the process where plant roots take up the metal contaminant from the soil and translocate them to their aboveground plant tissues. Once the plants have grown and absorbed the metal pollutants, they are harvested and disposed of safely. This process is repeated several times to reduce contamination to acceptable levels. Hyperaccumulator plant species are used on many sites due to their tolerance of relatively extreme levels of pollution. Phytoextraction offers an efficient, costeffective, and environmentally friendly way to clean up heavy metal contamination.

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There are two types of phytoextraction. They are (1) natural phytoextraction and (2) induced phytoextraction. (i) Natural phytoextraction: It is usually conducted by planting selected plant species in the contaminated soil. These plants are grown under normal farming conditions until they reach their maximum size. The aboveground parts of the plants containing the contaminants are then harvested and disposed of appropriately. (ii) Induced phytoextraction: In non-hyperaccumulators plants, factors limiting their potential for phytoextraction include small root uptake and little rootshoot translocation of metals. Throughout the growth period, amendments are added to the soil to increase availability of metals to the plants. The most commonly used agents for induced phytoextraction are: EDTA, DTPA, CDTA, citric acid, etc. Phytodegradation

Phytodegradation is the degradation or breakdown of organic contaminants by internal and external metabolic processes driven by the plant. Some contaminants can be absorbed by the plant and are then broken down by plant enzymes. These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues. Plant enzymes have been identified that break down ammunition wastes, chlorinated solvents such as TCE (Trichloroethane). Rhizofiltration

Rhizofiltration is similar in concept to phytoextraction but is concerned with the remediation of contaminated groundwater rather than the remediation of polluted soils. The contaminants are either adsorbed onto the root surface or are absorbed by the plant roots. Plants used for rhizofiltration are not planted directly in situ but are acclimated to the pollutant first. Plants are hydroponically grown in clean water, rather than soil, until a large root system has developed. Once a large root system is in place the water supply is substituted for a polluted water supply to acclimatize the plant. After the plants become acclimatized they are planted in the polluted area where the roots uptake the polluted water and the contaminants along with it. As the roots become saturated they are harvested and disposed of safely. Rhizodegradation

Rhizodegradation (also called enhanced rhizosphere biodegradation, phytostimulation, and plant assisted bioremediation) is the breakdown of organic contaminants in the soil by soil dwelling microbes which are enhanced by the rhizosphere’s presence. Plant root exudates such as sugars, alcohols, and organic acids act as carbohydrate sources for the soil microflora and enhance microbial growth and activity. The plant roots also loosen the soil and transport water to the rhizosphere thus additionally enhancing microbial activity.

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Phytostabilization

Phytostabilization is the process in which plants are used to immobilize soil and water contaminants. This technique can also be used to reestablish a plant community on sites that have been denuded due to the high levels of metal contamination. Once a community of tolerant species has been established the potential for wind erosion (and thus spread of the pollutant) is reduced and leaching of the soil contaminants is also reduced. Phytostabilization involves three processes which include humification, lignification, and irreversible binding.

Method of Analysis Different methods and associated instruments are involved to identify the contamination level of a pollutant (Tables 7 and 8). It will give right feedback to act for remediation as well as management of a pollutant. Some of the methods are easy and running cost is also less. However, running cost of biological pollutant identification is little bit higher than inorganic pollutant. Trained labor is the prime need for all the pollutant analysis. Interpretation from result is equally important to analysis, otherwise misinterpretation leads to extension of pollutant at the place of reduction. Time is needed to calibrate each method prior to estimate of an element.

Organic Pollutant Analysis Different types of organic pollutant are estimated with the help of standard method. These pollutants are complex in nature and have lot of interference during estimation. Different methods used to determine organic compounds (organohalogen, hydrocarbons, pesticides, PAHs) in water include: direct aqueous injection or sorption on solid sorbent (XAD-4) and extraction with pentane followed by a gas chromatography-electron capture detection (GC-ECD) for determination of volatile Table 7 List of common techniques/methods for measurement of impact assessment related to natural resource management Method pH (soil:water 1:2.5) EC (soil:water 1:2.5) Texture Organic carbon Available phosphorus Available nitrogen Available potassium DTPA extractable micronutrient Ca + Mg DHA Alkaline and acid phosphatases FDA

Reference Jackson [87] Jackson [87] Bouyoucos hydrometer [88] Walkley and Black [89] Olsen et al. [90]; Brays and Kurtz [91] Subbiah and Asija [92] Jackson [87] Lindsay and Norvell [93] Singh et al. [94] Casida et al. [95] Tabatai and Bremner [96] Adam and Duncan [97]

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Table 8 List of common instruments for measurement of soil pollution impact assessment related to natural resource management [98] Instrument pH meter EC meter TOC HPLC CHNS GC GM counter AAS/ ICP-OES Flame photometer Flow injection N analyzer Spectrophotometer MIR spectrophotometer TXRF Pressure plate apparatus Cone penetrometer Yoder apparatus/Permeameter IR gas analyzer Mridaparikshak COD meter BOD N auto-analyzer GPS Munsell color chart Boucous hydrometer Tensiometer G:C ratio, DNA hybridization, Ribotyping, RISA, RAPD, Metagenomics

Used in environmental impact assessment activities Soil reaction (acidity, alkalinity) Conductivity in soil Organic carbon in soil Organic compounds in the ecosystem Carbon, nitrogen, sulfur, and hydrogen in soil and plant Gaseous composition of atmosphere Radioactivity in system Metal concentration in system Analysis of K, Ca, Na For N analysis Phosphorus determination Soil fertility evaluation Soil fertility evaluation Soil water holding capacity Soil hardness Soil aggregates Respiration rate in plants Soil fertility assessment Chemical oxygen demand Biological oxygen demand N content in system Identification of location Color of soil Soil texture Measurement of moisture in soil For identification of soil biodiversity

organohalogen compounds; purge and trap with a gas chromatography-flame ionization detection for determination of volatile hydrocarbons; sorption on solid sorbent XAD-4 or C18, elution with organic solvent, and GC-ECD for determination of pesticides; sorption on solid sorbent C18, elution with organic solvent, and a gas chromatography-mass spectrometry for determination of PAHs and sorption on solid sorbent C18, elution with organic solvent, and high-performance liquid chromatography-ultraviolet determination of phenols [99].

Biological Pollutant Analysis Biological pollutant also plays a crucial role in deteriorating soil health and reducing crop yield potential. Most of the organic pollutant intensity is affected by contamination of organic and inorganic pollutants. Higher amount of organic material

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sometimes act as a food source for soil bacteria, fungi, and actinomycetes. These microorganisms secrete different types of soil enzymes/organic acids in soil [100]. Measurement of these acids and microorganisms’ population and diversity are performed as per the method described in Chhonkar et al. [101]. Inorganic Pollutant Different methods are used for identification of heavy metal, phosphorus, and nitrate pollution in soil and water samples as described below: Heavy Metal Analysis

Heavy metal analysis from soil, effluent, and plant part is a two-step process. In the first step digestion of material and in later step measurements of concentration. Known amount of soil and plant is taken in a conical flask, then add 5 ml of nitric acid and keep overnight for predigestion. In case of effluent sample predigestion period will be 1–2 h. Add 10 ml of di-acid in predigested sample and heat the conical flask on hot plate till a clear supernatant does not appear. After that add doubledistilled water, and filter with the help of Whatman no. 42 filter paper. Make final volume of extract and measure concentration with the help of atomic absorption spectrophotometer (AAS) or inductively coupled plasma-optical emission spectrometry (ICP-OES). It gives value in mg/kg and multiplies with dilution factor. This procedure can analyze trace metals like Cd, Cr, Pb, Ni, Zn, and Fe. Arsenic heavy metal is also analyzed with the abovementioned procedure, except it is coupled with Hydride Generator for precious results. We can also analyze the Cr concentration from soil with the help of diphenylcarbazide (DPC) method. Phosphorus Analysis Method

In soil, phosphorus exists in the form of various types of orthrophosphates. A very small fraction of these is available to plants at a given time. Available phosphorus content of soil consists mainly of Ca-, Al-, and Fe-P. In the neutral and alkaline soil, Ca-P is the dominant fraction. Organic-P fraction is also in considerable amount, but is usually not included in the determination of available phosphorus. A large number of extraction reagents varying from Dyer’s 1% citric acid to some of the multinutrient extractants, buffer solutions, acids, and chelating agents have been suggested for available phosphorus from time to time. However, no single extractant appears to be suitable for all types of soils. Two types of extraction methods are more popularly adopted. Under acidic soil conditions, Bray’s P-1 (or Bray no. 1), which involves soil extraction with a solution consisting of 0.03 N NH4F and 0.025 N HCl, is widely followed. The fluoride complexes with Al and Fe in soil, thus releasing some bound P besides the easily acid-soluble P (largely Ca-P). Available phosphorus in alkaline soils is determined with the help of Olsen et al. [90]. This extractant is suitable for soils containing less than 2% calcite or dolomite; because in calcareous soils, carbonates quickly neutralize the acid, resulting in less extraction of P. The most widely used extractant is 0.5 M NaHCO3 solution at pH 8.5. The reagent is mostly suitable for neutral to alkaline soils and is designed to control the ionic activity of calcium through solubility product of CaCO3, thus extracting the most

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reactive forms of P from Al-, Fe-, and Ca-P. Phosphorus in the extract can be determined using a suitable method of color development and measuring the color intensity at an appropriate wavelength. Available phosphorus in acid soils is determined by the method by Bray and Kurtz [91]. In this method, 0.03 M NH3F and 0.025 M HCl are taken as an extracting solvent. For plant sample analysis digestion of plant samples is as depicted in heavy metal analysis method. After this follow the procedure as described in Olsen and Bray methods. Phosphorus determination is necessary for crop production and environmental aspects. In soil P is determined to know the available concentration of P for plant growth and in water for ecological purposes. A list of P determination methods is identified, but mostly Olsen method is preferred for neutral to alkaline pH soils (Table 9). This method estimates the relative bioavailability of ortho-phosphate (PO4P) using 0.5 N NaHCO3 adjusted to pH 8.5 (Olsen et al. [90]). The Bray’s method is used for P determination in acidic range of soils (Bray and Kurtz [91]). Both methods are important for soil testing purpose in relation to sustainable crop production. Sometimes different P fractions are also analyzed in soil and water by sequential extraction methods. It is a critical, analytical step for separation of the different P forms which, after conversion into orthophosphates, may be determined by a multitude of various techniques. Spectrophotometric methods are often preferred for routine analysis. Several rapid automatic methods for the separation and determination of orthophosphate, linear polyphosphates, cyclic condensed phosphates, and lower oxidation state anions of P, which may exist in nature and waste waters, have been developed. Nitrate Pollution Determination

Nitrate concentration in water or soil may be identified by the method described in Singh et al. [94]. In this procedure a known amount of contaminated water is taken in a conical flask to which a pinch of CuSO4 is added for removal of chloride interference. Nitrate concentration is measured colorimetrically. Similar method is also advocated for estimation of nitrate contamination in soils. Known amount of soil sample is extracted with distilled water (1:5 soil:water ratio) and add 0.2 g Ca (OH)2 and 0.5 g of MgCO3 and shake for 10 min on a shaker. After that filter with Whatman no. 42 filter paper and measure nitrate concentration by colorimetrically at 470 nm with blue filter.

Challenges of During Analysis of Pollutant • Both the Olsen and Bray methods are reliable and easy to handle; however, these have some precautions during measurement and preparation of reagent. Sampling of soil and plant samples are key component for precise results. Addition of acid during extraction should be calibrated; otherwise P extraction may be over- or underestimated. • During analysis of heavy metal calibration of instrument and preparation of standard are needed special precautions.

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Table 9 Detection limits for phosphorus in the analysis of water samples (Spivakov et al. [102]) Technique Spectrophotometry (SP)

Flame photometry

1–100 0.03a 0.006a 30

Voltammetry

1–3

Amperometric titration Potentiometric titration Inductively coupled plasma atomic emission spectrometry (ICP-AES)

2000 40 1–20 0.5–0.005a

Inductively coupled plasma mass spectrometry (ICP-MS) Electrothermal vaporization ICP-MS Molecular fluorimetry

40 0.3 2–20

Ion chromatography

10

Liquid chromatography-SP Flow-injection analysis (FIA)-SP

10 0.1–12

X-ray fluorescence analysis (XRFA)

a

Detection limit (μg/L) 1–100 2–5 0.005a 0.1–400

Sample River water Tap water Sea and lake waters Natural and tap waters River water River water Natural waters Tap, river, and lake waters Natural and waste waters Natural waters Natural waters Natural waters River and sea waters Natural water Natural waters Natural and tap waters Natural and tap waters Natural waters Natural waters

After preconcentration

• Sewage irrigated polluted soil; analysis of trace metals is little bit tedious. To overcome organic matter hindrances add 10 ml of NHO3 acid during predigestion and add 2–3 times more di-acid during digestion. • In higher organic matter soil produce frothing during digestion, to minimize bumping keep initial lower temperature of hot plate. • Safe disposal of removed heavy metals is a big problem and interconversion of one ecosystem to another is observed.

Conclusions Soil pollution is a necessary evil with respect to industrial development. Population growth put extra pressure every year on natural resources. Generation of municipal solid waste, effluent from industrial units, sewage, and anthropogenic activities are

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polluting the fresh natural resources limiting the ecosystem sustainability and soil biodiversity. Different types of adverse effects were observed due to organic and inorganic contamination in soil. Decline in microbial population and diversity is a prime matter of concern for pollutant remediation over a period. Different methods of soil pollution identification are in practice for sustainable management of polluted soils. Some methods are easy to operate and cheaper, but others are costlier and time consuming. Use of traditional and scientific method of pollution remediation is a need of hour. Creation of awareness among the peoples is to reduce the soil pollution with the help of government and nongovernment organization. Cultivation of nonfood crops in polluted land is also advocating to farmers for enhancing farm income.

Further Outlook • More research work should be done in the field of soil pollution with respect to rhizospheric change. • Effect of climate change on pollutant degradation under contaminated soil and their effect on plant nutrient dynamics. • Temporal and spatial studies to identify the changes over a period in polluted soils. • Determination of soil critical limits in different food crops with respect to soil properties and climatic conditions.

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Jaison Jeevanandam, Saikumar Manchala, and Michael K. Danquah

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outline of Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic Principle of Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosynthesized Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biosynthesized Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Properties of Biosynthesized Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Metal and Metal Oxide Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Carbon-Based and Polymer Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Nanoparticles in Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Decomposition of Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Detoxification of Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Decomposition of Other Harmful Organic Compounds . . . . . . . . . . . . . . . . . . . Photocatalytic Antimicrobial Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Organic and toxic pollutants released by industries into water bodies are a major cause of water pollution. Organic pollutants contaminate water resources and affect aquatic lives, including phytoplanktons and zooplanktons. Conventional J. Jeevanandam (*) CQM - Centro de Quimica da Madeira, MMRG, Universidade da Madeira, Campus da Penteada, Funchal, Portugal S. Manchala Department of Chemistry, National Institute of Technology, Warangal, Telengana, India M. K. Danquah (*) Chemical Engineering Department, University of Tennessee, Chattanooga, TN, USA e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_137

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wastewater treatment methods such as disinfection via chemicals, UV treatments and filters may be costly, toxic to aquatic lives, and/or unable to routinely deliver the acceptable wastewater regulatory standards for discharge. Advancements in the field of nanotechnology provides opportunity to utilize semiconductor nanomaterials as photocatalysts capable of converting organic pollutants into carbon dioxide and water. However, the potential toxicity of these nanomaterials to water bodies becomes a major concern, especially for large-scale applications. Biosynthesized nanoparticles are seen as a more environmentally benign alternative to conventional chemically synthesized semiconductor nanoparticles. This chapter provides an overview of biosynthetic approaches to nanoparticle generation and their photocatalytic properties. Also, the performance and the mechanism of photocatalytic nanoparticles for organic pollutant removal in wastewater treatment are discussed. Keywords

Nanoparticles · Photocatalysts · Wastewater treatments · Water purification · Biosynthesis

Introduction The escalating advancements in the field of nanotechnology have yielded several unique nanoparticles and nanocomposites with exclusive properties to be beneficial as sensors, catalysts, drug carriers, and stand-alone medicines [1]. Among these benefits, nanosized materials are widely employed as catalysts to swift the rate of reaction without participating in the reaction [2]. In particular, it is possible to engineer the band gap or bandwidth of the semiconductor nanomaterials via size reduction or doping process, which increases their electron-hole pair recombination process via light irradiation called photocatalysis [3]. The photocatalytic property of nanomaterials is extensively employed in environmental applications, such as dye degradation, wastewater treatment, contaminated soil remediation, air purification, antimicrobial activity, water splitting, and hydrogen storage [4]. Generally, nanomaterials are fabricated via physical and chemical methods, which supports largescale synthesis to be useful for industrial applications [5]. Although, these synthesis approaches are useful in producing stable and monodispersed nanoparticles, some of them may exhibit toxic reactions towards humans and other beneficial microbes, which limits their usage in environmental applications [6]. Thus, there is a need for an alternative synthesis approach that can yield stable and uniform nanoparticles without any adverse toxic effects. Biosynthesis approach to fabricate nanoparticles are recently gaining significant attention among researchers to mitigate the adverse toxic effect towards several living organisms, compared to physical or chemical synthesized nanoparticles [7]. Bacteria, fungi, algae, and plant are the most noticeable biological entities that are utilized for the fabrication of essential nanoparticles [8]. These biological organisms

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possess biomolecules, such as enzymes and phytochemicals, which exhibits an ability to act as a potential reducing and stabilizing agent for the fabrication of nanoparticles [9]. Further, these biomolecules bind with the surface of the nanoparticles as functional groups, which drastically reduces their ability to cause adverse toxic effects [10]. It can be noted that these biosynthesized nanoparticles will be highly beneficial in exhibiting photocatalytic activity without causing adverse toxic effects to the living organisms and will be useful for environmental remediation application [11]. Furthermore, organic and toxic pollutants released by industries into water bodies are a major cause of water pollution [12]. Organic pollutants contaminate water resources and affect aquatic lives, including phytoplanktons and zooplanktons [13]. Conventional wastewater treatment methods such as disinfection via chemicals, UV treatments, and filters may be costly, toxic to aquatic lives, and/or unable to routinely deliver the acceptable wastewater regulatory standards for discharge [14]. Thus, this chapter provides an overview of biosynthetic approaches to nanoparticle generation and their effective photocatalytic properties. In addition, the performance and the mechanism of biosynthesized photocatalytic nanoparticles for organic pollutant removal in wastewater treatment are also discussed.

Outline of Photocatalysis Photocatalysis is a process of utilizing sunlight as an energy source for several applications, especially pollutant degradation, to overcome the current day energy crisis and challenges related water pollution. The advantage of solar energy lies in its ecological purity, which offers the possibility of accomplishing energy cycles without environmental pollution and complimenting to global warming complications [15]. Further, it can mimic photosynthesis process of plants to directly transform solar energy into chemical energy. Basically, it refers to increased rate of chemical reactions (oxidation or reduction) via catalyst, which will be a semiconductor material activated by visible or ultraviolet (UV) light. Thus, photocatalytic degradation of organic pollutants can be simply defined as an artificial photosynthesis. In photosynthesis, plants use chlorophyll and light to generate starch and oxygen from carbon dioxide and water, whereas, a photocatalyst can decompose organic matter into carbon dioxide and water using oxygen and water via irradiation of visible light from the sun.

Basic Principle of Photocatalysis In general, a photocatalytic cycle of a semiconductor material comprises three steps. Initially, a photocatalyst material is irradiated by the light (photons) with energy equal to (or higher than) band gap energy, which leads to the transfer of valence band electrons (e
) to the conduction band (CB), leaving a hole (h+) in the valence band (VB) (step I). In the second step, the excited electron-hole pairs can recombine, releasing the input energy as heat or emitted light, with no chemical effect. Finally,

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Fig. 1 Photocatalytic mechanism of nanoparticles or nanocomposites

the travelled excited electrons and holes without recombination will lead to redox (oxidation or reduction) reactions at the surface of the material with adsorbed species (step III) [16]. The main principle involved in photocatalysis is displayed in Fig. 1. þ D þ hþ VB ! D ðOxidationreactionÞ ! ð3Þ

It is noteworthy that the O2•
, •OH, and holes are the main reactive species in photocatalytic decomposition of environmental pollutants [17]. However, the VB and CB of a semiconductor should be designed according to the thermodynamic requirement for yielding the reduction potential of superoxide radicals and the oxidation potential of the hydroxyl radicals within the band gap for actual photodegradation [18]. In other words, the redox potential of the holes must be positive to produce •OH, while the electrons must be negative to produce O2•
for efficient photocatalysis. In recent years, the heterogeneous photocatalytic nanoparticles have been under extensive research, due to their potential application in wastewater treatment, including the decomposition of dyes, warfare agents, pesticides, heavy metals, and other harmful organic substances. However, nanoparticles fabricated via chemical approaches are reported to be toxic towards humans and the environment, which cannot be suggested for wastewater treatment and other environmental remediation applications [19]. Thus, nanoparticles synthesized via biological entities are recommended for these applications as they are non or less toxic towards humans and the environment [20].

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Biosynthesized Nanoparticles In general, nanoparticles are fabricated via three approaches such as physical, chemical and biological methods. Ball milling, laser ablation and sputtering are some of the physical approaches that are used for the synthesis of nanoparticles [21]. However, the high cost of equipment and tedious fabrication process are the limitations to utilize these approaches for large-scale nanoparticle production [22]. Similarly, sol-gel, co-precipitation, poly-ol, and hydrothermal are the chemical methods that are used for nanoparticle fabrication [23]. Even though, this approach helps in yielding stable nanoparticles via less-expensive methods, the chemicals used for nanoparticle formation are toxic to the environment and humans, which restricts their usage in biomedical and environmental applications [6]. Thus, biological approaches using the extracts of bacteria, fungi, algae, and plants are recently employed in the fabrication of nanoparticles to yield non/less toxic nanosized particles with enhanced biocompatibility, bioactivity and bioavailability [8].

Types of Biosynthesized Nanoparticles Biosynthesized nanoparticles are classified into four major types, depending on the origin of biomolecules that are used as reducing and stabilizing agent for the formation of nanoparticles [24]. The biomolecules from bacteria, fungi, algae, and plants are extensively used in recent times for the synthesis of nanoparticles [25].

Bacterial Synthesis It has been proven in several previous literatures that nanoparticles can be formed using the biomolecules that are extracted from bacteria [26]. These biomolecules serve as a potential reducing and stabilizing agents for nanoparticle synthesis to yield non/less toxic nanosized particles with desired size and morphology [27]. It is also worthy to note that bacterial synthesis can be subclassified into intracellular and extracellular methods, depending on the fabrication of nanoparticle using bacterial biomolecules that are secreted either interior or exterior of the bacterial cells [28]. The precursor for the nanoparticle fabrication will be absorbed by the bacterial cells, where the internal biomolecules act a reducing and stabilizing agent to form nanosized particles in the intracellular bacterial synthesis approach [29]. Primarily, metal nanoparticles such as gold, silver, palladium, and platinum are extensively synthesized using intracellular extracts of various bacterial strains. Gold nanoparticles are the most common metal nanoparticles that are synthesized using the intracellular biomolecular extracts of Lactobacillus kimchicus DCY51T, Rhodococcus species and mesophilic anaerobic Shewanella algae bacteria [30]. Further, silver nanoparticles using autochthonous Proteus mirabilis strain and marine Vibrio alginolyticus [31], platinum nanoparticles via Acinetobacter calcoaceticus PUCM 1011 [32], selenium nanoparticles using Bacillus licheniformis [33], palladium nanoparticles using Desulfovibrio desulfuricans, Bacillus benzeovorans [34], and marine bacterial Shewanella loihica PV-4 [35] are the other nanosized metal

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particles that are fabricated via intracellular bacterial synthesis approach. Furthermore, metal oxide nanoparticles such as nanosized zinc oxide via Lactobacillus plantarum VITES07 [36] and Lactobacillus plantarum TA4 [37] and copper oxide nanoparticles using gram-negative bacteria of genus Serratia isolated from the midgut of Stibara beetles species [29] are the other nanoparticles that are synthesized using intracellular approach. However, it is difficult to extract the synthesized nanoparticles as it will be bound with the bacterial cell organelles and the vigorous extraction process will lead to bacterial cell disruption or damage in the nanoparticle morphology [22]. Thus, extracellular synthesis approaches are widely used for the fabrication of nanoparticles using bacterial strains. The extracellular bacterial synthesis approach has been employed for the fabrication of almost all the types of nanoparticles. Metal nanoparticles such as gold from Paracoccus haeundaensis BC74171T, Lactobacillus casei, and Bacillus marisflavi [38], as well as silver nanoparticles using Bacillus cereus, Sporosarcina koreensis DC4, and Pseudomonas species THG-LS1.4 [39] are the most common nanoparticles fabricated via extracellular bacterial synthesis approach. Further, palladium nanoparticles using Geobacter sulfurreducens, Desulfovibrio desulfuricans, and Escherichia coli [40]; platinum nanoparticles via Pseudomonas aeruginosa SM1 [41]; and selenium nanoparticles using Bacillus safensis JG-B5T and Enterococcus faecalis [42] are the other metal nanoparticles that are fabricated using extracellular metabolites of bacterial strains. Similarly, metal oxide nanoparticles such as zinc, copper, titanium, iron, and silver are prepared via extracellular bacterial extracts [43]. Moreover, celluloses extracted from bacteria are combined with montmorillonite, copper oxide, zinc oxide, bioactive glass, and graphene oxide to form nanocomposites for effective biomedical applications [44]. It is also possible to modify the morphology and surface charge of the resultant nanoparticles by altering reaction parameters such as pH and temperature [45]. Thus, the extracellular approach of nanoparticles fabrication using bacteria is highly beneficial than the intracellular methods.

Fungal and Algal Synthesis Apart from bacteria, microbes such as fungi and algae were also used for the fabrication of nanoparticles. Fungal extract acts as an excellent source for nanoparticle synthesis as they contain a wide variety of biomolecules and intracellular enzymes that can act as reducing and stabilizing agent to yield large quantities of nanosized particles [46]. Similar to bacteria, intracellular and extracellular approaches are available for the formation of nanoparticles using fungi [47]. However, extracellular fungal synthesis is widely used for biomedical purposes, due to its rapid and simplicity in the synthesis process [48], compared to intracellular approach, which requires additional purification process [49]. Gold is the most common nanoparticle that are fabricated via fungal extracts that are obtained from 29 thermophilic filamentous [47], 21 mesophilic filamentous fungal extracts [50], Cladosporium cladosporioides, Cladosporium oxysporum AJP03, and Mariannaea sp. HJ [51]. Likewise, nanosized silver was also extensively synthesized using fungal extracts from Trametes ljubarskyi, Ganoderma enigmaticum [52], Phoma

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exigua var. exigua, Beauveria bassiana, and Raphanus sativus [53]. Further, platinum, palladium, and copper are the other metal nanoparticles that are synthesized using fungal extracts [7]. Similarly, metal oxide nanoparticles such as zinc, cobalt, copper, iron, magnesium, sulfur, and aluminum oxide are fabricated using biomolecules extracted from fungal strains. Likewise, carbon-based nanoparticles such as two-dimensional graphene, one-dimensional carbon nanotubes and zero-dimensional carbon dots are also synthesized using fungal extracts [54]. Moreover, polymeric nanoparticles, namely, chitosan [55] and quantum dots, such as cadmium selenide [56], were also synthesized using specific fungal strains. Additionally, nanocomposites such as Penicillium species spores-iron oxide nanoparticles, fungal asparaginase-silver, black Aspergillus spores-titanate nanotubes, fungal matrix-gold, and core-shell chitosan functionalized maghemite-silver nanoparticles were also recently reported to be synthesized using fungal species, either intracellular approach or extracellular extracts [57]. Algae is another significant biological entity that is used to fabricate several types of nanoparticles. In recent times, algal extracts from brown Cystoseira baccata, Sargassum tenerrimum, Turbinaria conoides, Calothrix species, and Gelidiella acerosa are widely utilized for the synthesis of gold nanoparticles [58]. Similarly, fabrication of other metal nanoparticles such as silver using Caulerpa serrulata, Caulerpa racemose, and Polysiphonia; platinum via Padina gymnospora and Chlamydomonas reinhardtii; palladium using Spirulina platensis, Chlorella vulgaris, and Dictyota indica, copper using Botryococcus braunii; selenium via Sargassum latifolium; and ruthenium using Dictyota dichotoma algal extracts were also reported [59]. Likewise, metal oxide nanoparticles such as zinc via Sargassum muticum and S. wightii, copper using Sargassum polycystum and Anabaena cylindrica, iron via Colpomenia sinuosa and Pterocladia capillacea, magnesium oxide using S. wightii, aluminium via S. ilicifolium, and cerium using Chlamydomonas reinhardtii were also fabricated using algal extracts [60]. Further, carbon-based nanoparticles, namely, graphene oxide, carbon dots, as well as polymeric nanoparticles and quantum dots were synthesized using extracellular metabolites of algae [7]. Furthermore, nanocomposites such as algal biochar-lanthanum-copper-zirconium trimetallic particles, zinc oxide-copper oxide in polyethylene oxide polymer, and silica nanoparticles embedded on two-dimensional carbon nanostructures were also reported to be synthesized using algal extracts [61]. Although, fungi- and algal-mediated synthesis of nanoparticles are beneficial in yielding large quantities of nanoparticles, the time-consuming and tedious downstreaming process are the major challenges, which limits their application in largescale biomedical applications [62].

Plant-Mediated Synthesis Plants are the most significant biological organism for the fabrication of nanoparticles, as they are enormously available throughout the world [63]. Further, the phytochemicals and phytocompounds present in the extracts of plants are beneficial as reducing and stabilizing agent for nanoparticle formation [64]. In addition, the morphology, size and surface charge of the nanoparticles formed via plant extracts

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can be modified according to the desired application, by altering synthesis parameters such as pH, temperature, and extract volume [65]. The phytochemical extracts can also be obtained from different parts of the plants, such as stem, root, bark, leaves, flower, and fruits, which depending on the quantity are beneficial for nanoparticle fabrication [66]. Metal nanoparticles such as gold from Sphaeranthus indicus leaves, Elettaria cardamomum seed, and Cucurbita pepo leaves; silver via Azadirachta indica leaves, Ocimum sanctum leaves, and Cleome viscosa; copper using Eclipta prostate leaves, Eucalyptus globules leaves, and Plantago asiatica leaves are effectively synthesized in recent times. Further, other metal nanoparticles, including platinum, palladium, selenium and ruthenium were also fabricated using phytochemical extracts from plants, especially from leaves [67]. Furthermore, metal oxide nanoparticles such as zinc, copper, titanium, iron, silver, cobalt, magnesium, aluminum, and cerium are extensively fabricated using plant extracts [68]. Moreover, carbon-based nanoparticles such as graphene using cloves, white mulberry, black cumin seed, blackthorn, dark grape, and rosehip [69]; carbon nanotubes via Cynodon dactylon, Rosa, Azadirachta indica, Juglans regia, and tea plant waste; carbon dots using banana stem and Thymus vulgaris essential oils are synthesized using phytochemicals from plants [7]. Likewise, polymeric nanoparticles such as chitosan, cellulose, and lignin as well as quantum dots [70] were also formed using phytochemicals of plants and are utilized as nanoformulation agents. Nanocomposites such as porphyrins-silver, copper nanoparticles-iron oxide-chitosan, palladium-manganese dioxide, graphene-silver/gold, and magnesium oxide-perlite were synthesized using Sesbania sesban, Euphorbia falcata, Solanum melongena, Xanthium strumarium, and Melissa officinalis plant extracts, respectively [7]. However, it is noteworthy that the achievement of monodispersity in the fabricated nanoparticles is tedious, while using phytochemicals as reducing and stabilizing agents [71]. Thus, the broad-spectrum variety of nanoparticles that can be fabricated via biosynthesis approach has placed them as a primary synthesis method for several biomedical and environmental applications, especially for photocatalytic purposes.

Photocatalytic Properties of Biosynthesized Nanoparticles The bandwidth or bandgap in the electronic configuration of materials is the prime factor to exhibit photocatalytic property. It is possible to alter these bandgaps via size reduction in nanoparticles, which is significant in exhibiting enhanced photocatalytic property. Thus, it is possible to engineer nanoparticles, such as metal, metal oxides, carbon-based, polymer nanoparticles, and nanocomposites, to possess desired bandgap to be beneficial as an enhanced photocatalysts for specific applications.

Photocatalytic Metal and Metal Oxide Nanoparticles There are several metal and metal oxide nanoparticles that are widely fabricated via a biosynthesis approach to exhibit enhanced photocatalytic activity. Khan et al. [72]

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biosynthesized gold nanoparticles using Longan fruit juice to yield 25 nm sized particles to exhibit photocatalytic activity. The results revealed that these nanoparticles possess ability to photocatalytically degrade 76% of methylene blue dye in 55 min. Further, these nanoparticles possess antibacterial efficacy against gramnegative Escherichia coli and gram-positive Bacillus subtilis and Staphylococcus aureus. Similarly, Kumari et al. [73] fabricated nanosized silver particles via leaf extracts of Cordia dichotoma to yield uniformly distributed, spherical ~20 nm sized particles with antibacterial and photocatalytic properties. These nanoparticles are reported to be highly beneficial in degrading toxic dyes such as Congo red and methylene blue via photocatalytic effects. In addition, these nanoparticles possess effective antibacterial activity against E. coli and Pseudomonas aeruginosa. Likewise, Karthik et al. [74] synthesized nanosized silver particles using leaf extract of Camellia japonica with 12–25 nm sized and spherical morphology, without any significant agglomeration. These nanoparticles possess enhanced photocatalytic and electrocatalytic activity to degrade toxic nitrobenzene, even in the existence of common metal ions as well as nitroaromatic substances and 97% of Eosin-Y dye after visible light irradiation for 60 min. Further, Samuel et al. [75] recently reported that spherical, 20–40 nm sized silver particles can be biosynthesized via Bacillus amyloliquefaciens to exhibit enhanced photocatalytic activity. The results emphasized that these nanoparticles highly useful in the chemocatalytic degradation of toxic 4-nitrophenol dye into nontoxic 4-aminophenol with 98% of efficiency, within 15 min under solar light irradiation. Additionally, the cytotoxic assay proved that these nanoparticles are highly beneficial in inhibiting A549 lung cancer cells. Moreover, Tripathi et al. [76] fabricated 45–95 nm sized polycrystalline selenium nanoparticles using leaf extracts of Ficus benghalensis to exhibit photocatalytic property. The study showed that these nanoparticles possess fluorescence properties with efficacy to photocatalytically degrade toxic methylene blue dye for about 57.63% within 40 min in the aquatic environment with 0.021 S
1 of rate constant. Saraswathi et al. [77] reported that zinc oxide nanoparticles synthesized via leaf extracts of Lagerstroemia speciose possess enhanced photocatalytic property. The study demonstrated that the nanoparticles are stable with hexagonal morphology in 40 nm in size and possess ability to effectively degrade up to 93.5% of methyl orange under sunlight exposure within 2 h. Further, the study also showed that the nanoparticle possess ability to reduce 5600 mg/l of chemical oxygen demand to 374 mg/l after sunlight exposure for 100 min with less hemolytic property against human erythrocytes. Likewise, Osuntokun et al. [78] demonstrated that the fresh extracts of Brassica oleracea can be used to fabricate tin dioxide nanoparticles with 3.6–6.3 nm sized quasi-spherical and spherical morphological structure. These nanoparticles are proven to be beneficial as an excellent photocatalytic agent to degrade toxic methylene blue dye with an efficiency of 88–92% under the exposure towards ultraviolet rays after 180 min of radiation. Similarly, Vasantharaj et al. [79] synthesized iron oxide nanoparticles using leaf extract of Ruellia tuberosa to exhibit enhanced photocatalytic activity. The study revealed that the nanoparticles are 52.7 nm in size and hexagonal in morphology with effective antibacterial activity against E. coli, Klebsiella pneumoniae and S. aureus, while coated in cotton fabrics.

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Additionally, these nanoparticles possess photocatalytic ability to degrade 80% of toxic crystal violet dye under irradiation of solar light. Further, Sorbiun et al. [80] compared the photocatalytic activity of silver, zinc oxide, and bimetallic silver-zinc oxide alloy nanoparticles that are synthesized from the aqueous oak fruit hull extract. The silver nanoparticles are 34 nm in size with spherical morphology and zinc oxide nanoparticles are spherical ~57 nm in size, whereas the bimetallic alloys were spherical and less than 21 nm in size. The study revealed that the alloys possess enhanced photocatalytic activity to degrade basic violet 3 dye under visible light exposure in less than 30 min, compared to nanosized silver and zinc oxide particles.

Photocatalytic Carbon-Based and Polymer Nanoparticles Recently, carbon-based nanoparticles are gaining much attention as photocatalytic agents for the degradation of toxic dyes and other environmental applications. Mahajan et al. [81] reported that carbon nanodots with 38.5% of quantum yield can be fabricated using musk melon extract with photocatalytic property. The study showed that the nanodots are 5–10 nm in size range with fluorescent property under irradiation of 365 nm wavelength that corresponds to ultraviolet rays. These nanoparticles exhibited enhanced photocatalytic activity to effectively degrade toxic methylene blue dye in 60 min under ultraviolet exposure with 37.08% of efficiency and 0.0032 min
1 of reaction rate. Further, Song and Shi [82] synthesized 10 nm sized silver nanoparticles using Shewanella oneidensis bacterium strain MR-1 and deposited them on the surface of few layer thin graphene oxide to demonstrate their effective catalytic activity. This novel nanoparticle exhibited excellent antibacterial activity against E. coli by inhibiting their growth in 15 min. In addition, these nanoparticles along with S. oneidensis synergistically exhibit enhanced catalytic degradation of p-nitrophenol with 98.2% of efficiency within 10 min, which will be beneficial in the bioremediation of contaminated water bodies. Similarly, Li et al. [83] recently fabricated copper nanoparticles and embedded them on the surface of carbon nanotubes using S. oneidensis MR-1 bacterial strain to perform catalytic degradation of toxic 4-nitrophenol dye. The results revealed that the copper nanoparticles are highly crystalline with 4–10 nm in size and are embedded on the surface of carbon nanotubes without any agglomeration. These nanoparticles exhibited excellent catalytic activity to degrade 99.5% of 4-nitrophenol within 80 min at 45 °C of temperature and at alkaline pH of 10 with a rate constant of 0.0532 min
1. Further, it can be noted that these nanoparticles exhibit 95.2% of high catalytic stability for up to six reaction cycles. Polymeric nanoparticles are generally used to protect the catalytic nanoparticles from environmental degradation and exhibit photocatalytic activity, which restricts their usage in direct photocatalytic applications. Wahid et al. [84] recently fabricated cellulose pellicle from Gluconacetobacter xylinus bacteria and are embedded with zinc oxide nanoparticles to form a composite film for enhanced photocatalytic and antibacterial activity. The results revealed that these natural polymer nanocomposite films possess 91% of efficiency in degrading toxic methyl orange dye within 2 h under ultraviolet light

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exposure. Further, these nanocomposites showed improved ultraviolet ray blocking and antibacterial activity against S. aureus, B. subtilis, and E. coli, which will be beneficial in eliminating microbial infections and toxic dyes in wastewaters. Likewise, chitosan from shrimp shell were extracted to form cross-linked films and are embedded with cadmium selenide quantum dots to form a photocatalytic composite. These composites are reported to possess enhanced visible light-mediated photocatalytic effect to degrade and decolorize toxic methyl orange solution, to eliminate organic pollutant from the environment [85]. Later, biosynthesized quantum dots were demonstrated to possess enhanced photocatalytic activity. Cao et al. [86] recently fabricated 3.2 nm sized cadmium selenide quantum dots using an aerobic yeast extract of Rhodotorula mucilaginosa PA-1. These nanoparticles were reported to exhibit superior photocatalytic activity to degrade ~86–95% of toxic malachite green dye, under the influence of visible and ultraviolet radiation for 60 min. Likewise, Fatimah et al. [87] synthesized tin dioxide quantum dots using flower extract of Clitoria ternatea to exhibit photocatalytic property. These 4–10 nm sized quantum dots exhibited excellent photocatalytic activity to degrade toxic Rhodamine B dye, depending on parameters such as pH, dosage of catalyst, and concentration of hydrogen peroxide, while exposing ultraviolet-B light. Moreover, Jacob et al. [88] showed that a spherical, 6.3 nm sized zinc selenide quantum dots fabricated from Aspergillus species possess enhanced photocatalytic and biosensing properties. These quantum dots exhibited enhanced antibacterial activity against gram positive S. aureus, B. subtilis and gram-negative E. coli and Klebsiella pneumoniae as well as degrade methylene blue, Congo red and crystal violet under sunlight irradiation. Further, these electronically confined quantum dots are beneficial in detecting metal ions via active surface groups, which will be useful for environmental monitoring applications.

Photocatalytic Nanocomposites Nanocomposites are recently gaining much attention among researchers in the photocatalytic applications, as it is possible to incorporate the bandwidth of two significant materials, which leads to transition regions for the formation of enhanced electron-hole pairs and exhibit exclusive photocatalytic property. Song et al. [89] fabricated palladium nanoparticles using Shewanella oneidensis MR-1 and immobilized them on the surface of titanium dioxide nanotubes to form nanocomposites. These novel nanocomposites are demonstrated to possess enhanced photocatalytic property to degrade toxic methylene blue dyes under the exposure of sunlight for 10 min with an efficiency of 98.7%. Likewise, Homarmand et al. [90] recently reported that 18 nm sized tin dioxidebentonite nanocomposites synthesized via Jujube fruit extract possess enhanced photocatalytic property. The results from the study showed that the nanocomposites possess ability to photocatalytically degrade organic methylene blue and eriochrome black-T dyes under the influence of solar radiation. Further, the study also revealed that the photocatalytic activity was retained for up to three times without any activity loss. Similarly, Ghasemi et al. [91] stated that plasmonic silver-silver chloride-titanium dioxide nanocomposites prepared from an aqueous leaf extract from mangrove

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Avicennia marina tree possess photocatalytic activity with the help of visible light. These nanocomposites are proven to be highly beneficial in photocatalytically degrade toxic eosin Y dye with 99.8% of efficiency under visible light irradiation. Furthermore, leaves of Camellia sinensis (tea plant) were used to fabricate ~5.2 nm sized zinc oxide nanoparticles as well as silver-zinc oxide nanocomposites to exhibit improved photocatalytic property to degrade environmental pollutants such as para-nitro phenol, methylene blue, and paracetamol drug. It can be noted that the visible light-mediated photocatalytic activity of the nanocomposite is better than the zinc oxide nanoparticles, due to the presence of photodegrading phytochemicals from tea extract as well as transition regions in the nanosized composites [91]. Moreover, iron oxide-silicon dioxide nanocomposites are prepared using the peel extracts of Musa balbisiana to degrade toxic methylene red dye via their photocatalytic ability. Besides, these nanocomposites are highly stable to exhibit photocatalytic degradation of toxic organic dye for up to five times [92]. Thus, biosynthesized nanoparticles and nanocomposites are widely utilized as photocatalytic agents for environmental remediation applications, to mitigate the toxic effects caused by chemical-based nanoparticles.

Photocatalytic Nanoparticles in Wastewater Treatment The textile, petrochemical, food, and chemical industries develop and produces products in large quantities to meet the global needs. Most of these industries use toxic chemicals in producing their products, and their resultant toxic byproducts are released into nearby water bodies. These released toxic chemicals are the prime reason for the contamination of water resources, which eventually lead to water pollution and scarcity. Pollution of water resources, that are used for consumption and other everyday activities, with hazardous organic compound has been known as the prime causative factor for water pollution throughout the world [93]. The majority of these water pollutants are nonbiodegradable and accumulate in the aquatic environment, which shows adverse effects on the aquatic life, humans, and the environment. In fact, these pollutants are responsible for the deterioration of the earth’s environment via acid rain, Chlorofluorocarbon (CFC), global warming, and severe water pollution. Thus, development of alternative pollution-free technologies and remediation of these contaminated water is essential for the sustainable growth of human society. Recently, utilization of biosynthesized semiconductor NPs in the wastewater treatment through photocatalysis has attracted great attention of researchers as shown in Fig. 2, since its discovery for water splitting application to produce hydrogen by Fujishima and Honda, due to its low cost, high stability, great efficiency, simple separation, and easy operation [94].

Photocatalytic Decomposition of Dyes Exploitation of industries such as leather, food, paint, plastic, textile, cosmetics, paper, rubber, and pharmaceuticals results in the usage of various toxic dyes, which

Fig. 2 Photocatalytic activities of nanoparticles to convert contaminated water to clean water

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eventually leads to contamination of water resources, that is hazardous to human health and aquatic plants. Thus, the removal of dyes from contaminated water is essential, as even a low quantity of dye in wastewater is highly toxic to the environment. Recently, photocatalysts are employed for the removal of dyes from waste water [95]. For instance, Srivastava and Mukhopadhyay [96] developed tin dioxide nanoparticles with the help of Erwinia herbicola bacterium and evaluated its photocatalytic ability to degrade dyes, such as erichrome black T (EBT), methyl orange (MO), and methylene blue (MB). The results revealed that approximately 97.8, 94, and 93.3% of EBT, MO, and MB was degraded by these biosynthesized nanoparticles under UV-light exposure. Recently, Weldegebrieal [97] fabricated spherical copper oxide nanoparticles using leaf extracts of Verbascum thapsus and evaluated its photocatalytic activity to degrade MB dye under sunlight irradiation. Further, Pugazhendhi et al. [98] reported the preparation of efficient magnesium oxide nanoparticles using Sargassum wightii algae for the photocatalytic decomposition of MB under both UV and solar light irradiation. Furthermore, Vasantharaj et al. [79] demonstrated the photocatalytic degradation performance of iron oxide nanoparticles, that are fabricated via Ruellia tuberose leaf extract. The results showed that these magnetic nanoparticles are effective in degrading crystal violet (CV) dye with 80% of degradation efficiency. Similarly, Muthuvel et al. [99] reported that the biosynthesized zinc oxide nanoparticles using leaf extract of Solanum nigrum possess enhanced photocatalytic activity towards the decomposition of MB under solar light exposure.

Photocatalytic Detoxification of Heavy Metals Water contaminated with heavy metals such as cadmium, chromium, copper, lead, mercury, nickel, and zinc are highly toxic to the plants, beneficial microbes, animals, and humans, as they may lead to hazardous effects on their growth rate and metabolic functions, even at low concentration. Further, it can be noted that the effect of toxicity depends on the concentration of bio amplification. Generally, metal ions such as arsenic (III), cadmium (II), lead (II), mercury (II), and silver (I) interact with biomolecules in humans and generate hazardous compounds, which initially affects kidney filtration. Among several methods, such as chemical precipitation, electrochemical treatment, coagulation-flocculation, membrane filtration, adsorption, and ion exchange approach for the detoxification of heavy metals present in wastewater, photocatalysis has received worldwide attention due to its low cost, easy operation, and eco-friendly nature [100]. Recently, Goutam et al. [101] developed titanium dioxide nanoparticles via aqueous leaf extracts of Jatropha curcas L. and utilized them for the photocatalytic removal of chromium (VI) ions from tannery water by detoxifying them into chromium (III). The study also showed that the biosynthesized spherical-shaped titanium dioxide nanoparticles possess enhanced ability to remove 76.48% of chromium (VI). Likewise, Padhi et al. [102] synthesized iron oxide (Fe3O4) nanoparticles using Averrhoa carambola leaf extract that are decorated on the surface of reduced graphene oxide (rGO) to fabricate

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nanocomposites and employed them for the photocatalytic detoxification of chromium (VI) ions. The leaf extract utilized in this study played a significant role in improving the electronic, optical, and structural characteristics of the magnetic Fe3O4 nanoparticles. In addition, the study emphasized that the biosynthesized nanocomposite possess ability to reduce 97% of chromium (VI) ions via a photocatalytic reduction process.

Photocatalytic Decomposition of Other Harmful Organic Compounds Phenolic and nitro substances such as phenol, 2, 4-dichloro phenol (2, 4-DCP), 3nitrophenol (3-NP), 2, 6-dichloro-4-nitro aniline, 2-nitro phenol (2-NP), 2nitroresorcinol (2-NR), 2-nitro aniline (2-NA), and 4-nitro phenol (4-NP) are highly toxic towards the environment, humans, and animal health. Particularly, 4-NP readily exists in industrial wastes due to its high solubility in water. Further, these substances can damage mitochondria and alter the metabolic activities in animals and humans, which may be lethal in severe cases [103]. Samuel et al. [75] developed silver nanoparticles via Bacillus amyloliquefaciens bacterial metabolites and evaluated its photocatalytic activity for the reduction of 4-NP to 4-amino phenol (4-AP). The study revealed that the biosynthesized nano-silver is less toxic and possess enhanced ability to degrade the phenolic and nitro compounds within 15 min. Likewise, Khoshnamvand et al. [104] reported the synthesis of gold nanoparticles using stem and leaf extracts of Apium graveolens and employed them for the photo degradation of 4-NP to 4-AP. Similarly, Sun et al. [105] synthesized gold nanoparticles using Sargentgloryvine stem extract and evaluated their photo degradation ability to reduce toxic compounds such as 2-NP, 3-NP, 4-NP, and 2-NR. Further, Liu et al. [106] reported the synthesis of palladium nanoparticles using bacterial metabolites from Pantoea species that are decorated on the surface of nitrogen-doped carbon. The resultant nanocomposites are revealed to possess excellent photodecomposition activity to reduce toxic chemical compounds, including 4-NP, 2-NA, 4-NA, and 2, 6-dichloro-4-nitro aniline. In addition, the study emphasized that the decorated palladium nanoparticles played a significant role to improve the photocatalytic activity of the nanocomposite.

Photocatalytic Antimicrobial Property In recent times, light sensitive, photocatalytic nanoparticles are employed as a lightbased disinfectant for the inhibition of infectious and pathogenic microbes, which will be highly beneficial in hospitals and surgical areas to protect patients from infections. Recently, Hu et al. [107] fabricated multifunctional nanoparticles by combining photosensitizer chlorin e6 and quaternary ammonium chitosan to be responsive to light for photocatalytic inhibition of pathogenic bacteria. The study showed these nanoparticles releases reactive oxygen species upon visible laser light irradiation to photocatalytically inhibit pathogenic E. coli and S. aureus. Further, Li et al. [108] fabricated iron-doped titanium dioxide thin films using bamboo extracts

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to exhibit visible light-mediated antifungal efficacy. The 9.7 nm sized iron doped nanoparticles help to reduce the band gap of titanium dioxide from 3.2 eV to exhibit enhanced photocatalytic activity under sunlight irradiation to inhibit the growth of mould fungi on the natural environment. Furthermore, it is noteworthy that biodegradable chitosan nanoparticles along with methylene blue exhibited photocatalytic ability to inhibit bacterial biofilms formed by P. aeruginosa and S. aureus after exposure towards 650 nm of visible light from a diode laser [109]. In addition, several biosynthesized nanoparticles such as silver from cyanobacteria [110]; zinc sulfide nanoparticles using Tridax procumbens, Syzygium aromaticum, and Phyllanthus niruri [111]; zinc oxide nanoparticles from Capparis zeylanica [112]; gold and silver nanoparticles from Paederia foetida Linn [113]. are reported to possess both antimicrobial and photocatalytic property, which will be beneficial in eliminating toxic dyes and infectious microbes from the wastewater.

Conclusion and Future Outlook This chapter is an overview of various nanoparticles and nanocomposites that can be fabricated using microbes such as bacteria, algae, and fungi as well as macroorganisms such as plants and their enzyme and phytochemical extracts. Further, the chapter also emphasized on the photocatalytic activity of various biosynthesized nanoparticles for dye degradation applications. In addition, the chapter also discussed on the photocatalytic effect of biosynthesized nanoparticles to be beneficial in the environmental remediation applications, especially for wastewater treatment. Even though, biosynthesized nanoparticles are recommended for photocatalytic wastewater treatment applications, due to their less toxic nature, there exists certain challenges, which limits their large-scale applications. These photocatalytic nanoparticles enhance the regeneration of electron-hole pair, which eventually forms hydrogen peroxide as a byproduct, which is an essential process to exhibit photocatalytic antimicrobial activity. However, a high concentration of hydrogen peroxide release may also affect other organisms and cause adverse toxic effects. Moreover, most of the photocatalytic nanoparticles are fabricated using a metal-based precursor, which lacks stability in real-time environment, especially in water and leads to agglomeration. The agglomerated particles will eventually form into micro-sized metal particles, which will increase the heavy metal content in the water body and affect the growth of aquatic organisms. Thus, it is necessary to overcome these challenges via photocatalytic nanocomposites in the future for effectively using these particles in the purification of contaminated water bodies. Likewise, plant wastes are also recommended to extract phytochemicals for the synthesis of photocatalytic nanoparticles to remediate contaminated environmental sites, instead of fresh plants, to avoid afforestation or biopiracy. Since, the methodology of using biosynthesized photocatalytic nanoparticles for environmental applications is in its infancy stage, their limitations can be regulated by the suggestions mentioned in this chapter to increase their application to the largescale level in the future.

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Water Treatment and Desalination Using the Eco-materials n-Fe0 (ZVI), n-Fe3O4, n-FexOyHz[mH2O], and n-Fex[Cation]nOyHz[Anion]m [rH2O]

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part 1: Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Morphology of n-Fe0 Aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZVI Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollutant Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollutants Removed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZVI Water Treatment Markets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part 2: ZVI Water Treatment Plants and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fixed Bed Commercial Municipal Water Treatment Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anderson Moving Bed Commercial Water Treatment Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roeske Moving Bed Commercial Water Treatment Process (RP) . . . . . . . . . . . . . . . . . . . . . . . . . Fixed Bed Reactor Advances since 1908 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Moving Bed Reactor Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion Reactor Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part 3: Manufacture of n-ZVI for Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manufacture of Nano-Zero Valent Iron for Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part 4: Emerging n-ZVI Water Treatment Processes Which Produce a Commercial Product . . . Bioremediation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Commercial Future of n-ZVI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationship Between ZVI Composition and Product Water Composition . . . . . . . . . . . . . . . Relationship Between Water Remediation and Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary of the Principal Advances Since 1857 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZVI as an Eco-material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Further Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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D. D. J. Antia (*) DCA Consultants Ltd, Falkirk, UK e-mail: [email protected] © Springer Nature Switzerland AG 2021 O. V. Kharissova et al. (eds.), Handbook of Nanomaterials and Nanocomposites for Energy and Environmental Applications, https://doi.org/10.1007/978-3-030-36268-3_66

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Abstract

ZVI (zero valent iron, Fe0, n-ZVI, m-ZVI) and the associated corrosion products n-FeO, n-Fe3O4, n-FexOyHz [mH2O], and n-Fex[Cation]nOyHz[Anion]m [rH2O] are eco-materials. They operate by reaction, redox equilibrium shift, adsorption, adsorption/desorption, and catalysis. They have been used commercially to treat (i) river water to municipal potable water (100,000 m3 d
1 in the 1890s, before reducing to 10% FeO. PRB Remediation: Replacement of Fe with Fe3O4 or Fe Pillared Clay (US Patent 5,750,036) Halogenated compounds are removed from contaminated groundwater using Fen+ ions. The process replaces the ZVI, as a source of Fen+ ions, with either n-Fe pillared clay or n-Fe charged Fe3O4. An example of Fe pillared clay is provided in Fig. 15a. Figure 15b illustrates the structure that develops in a PRB containing pillared clay and flowing water. PRB Remediation: Construction of a PRB Using n-Fe/Fe (US Patent 5,975,798) ZVI is delivered into the subsurface (by injection) using a liquid slurry containing ZVI combined with a pressurizing gas. PRB Remediation: Reducing the ZVI Permeability Loss (US Patent 7,347,647) A PRB is constructed using 5–20% weight Fe0/n-Fe0; 5–80% weight fibrous organic matter;