Handbook of Green and Sustainable Nanotechnology: Fundamentals, Developments and Applications 3031161009, 9783031161001

Table of contents :
Preface
Acknowledgments
Contents
About the Editors
Contributors
Part I: Synthesis of Nanomaterials
1 Biomimetic Route-Assisted Synthesis of Nanomaterials: Characterizations and Their Applications
Introduction
The Benefits of Using Biological Entities as Medium for Nanomaterial Preparations
Specific Modes of Synthesis for Bionanomaterials
Probable Mechanisms Involved in the Synthesis Processes
Unique Characterization of Bionanomaterials Synthesized via Biomimetic Routes
Applications of Nanomaterials in the Field of Modern Sciences and the Key Challenges Involved
Conclusions and Future Scope: Add Some Future Scope and Make Depth in Conclusion Required
References
2 Fabrication of Green Nanomaterials: Biomedical Applications and Ecotoxicology
Introduction
Green Nanotechnology
Green Synthetic Route
Radiolytic Reduction Method
Metallic Nanoparticles (Gold Nanoparticles)
Polymeric Nanomaterials
Hydrogel and Nanogels
Protein-Based Nanoparticles
Albumin Nanoparticles
Proteolytic Enzymes
Papain Nanoparticle
Plant-Mediated Synthesis
Phytochemical-Based Nanoparticles
Clinical Translation
Nanoradionutrapharmaceuticals
Nanoimmunotherapy
Ecotoxicology
Conclusion
References
3 Green Synthesis of Metallic Nanoparticles and Their Biomedical Applications
Introduction
Types of Metallic Nanomaterial
Methods of Synthesis of Metal Nanoparticles
Physical and Chemical Synthesis
Green Biosynthesis of Metal Nanoparticles
Biosynthesis of Metallic Nanoparticles by Bacteria
Biosynthesis of Metallic Nanoparticle by Fungi and Yeast
Biosynthesis of Metallic Nanoparticles by Cyanobacteria
Biosynthesis of Metallic Nanoparticle Using Algae
Metallic Nanoparticle Synthesis Using Viruses
Metallic Nanoparticle Synthesis Using Plants
Factors Affecting Biosynthesis by Plant Extracts
pH of Reaction
Concentrations of Plant Active Principles
Reaction Time
Reaction Temperature
Characterization of Metal Nanoparticles
Biomedical Applications of Metal Nanoparticles and Nanocomposites in Biomedicine
Antimicrobial and Antiparasitic Activities
Mycotoxins and Bacterial Toxin Control
Diseases Diagnosis and Therapy
Cancer Detection and Therapy
Bioimaging
Drugs and Vaccine Delivery
Livestock Production, Reproduction, and Nutrition
Food Packing and Preservation
Mechanism of Action of Metallic Nanoparticles and Their Composites
Mechanism of Cytotoxicity Risk of Metallic Nanoparticles and Their Composites
Conclusions and Future Perspectives
References
4 Chitosan: Postharvest Ecofriendly Nanotechnology, Control of Decay, and Quality in Tropical and Subtropical Fruits
Fruit Consumption Benefits
Diseases of Fruits
Factors Related to Infection Development
Environmental Conditions
Temperature
Humidity
Maturity
Wounds
Postharvest Pathogens
In Vitro Assessments
In Vivo Assessments
Conclusions
References
5 Green Synthesis and Applications of Metal-Organic Frameworks
Introduction
Synthesis of MOFs via Green Methods
Green Solvent-Based Synthesis
Sustainable Precursor-Based Synthesis
Efficient Energy-Based Synthesis
Green MOF-Based Applications
Catalysis
Therapeutics
Adsorption
Sensing
Challenges Associated with Green MOFs
Conclusion and Future Scope
References
6 Nanotechnology for the Obtention of Natural Origin Materials and Environmentally Friendly Synthesis Applied to Tissue Engine...
Introduction
Advantages of Eco-Friendly Materials
Novel Naturally Derived Biomaterials
Plants
Fungi
Bacteria
Animals
Algae
Recycled Materials
Advantages of Eco-Friendly Synthesis Techniques
Engineered Living Materials (ELMs)
Decellularization
Biofactories
Solvent-Free Synthesis of Nanoparticles/Nanomaterials.
Clean
Non-solvent-Induced Phase Separation for Membrane Obtention
Electrospinning
Centrifugal Electrospin
Solid Free-Form Fabrication
Conclusion
References
7 Green Synthesis of Curcuminoid Nanostructure for White Light Emission Application
Introduction
Solid-State Lighting
White Organic LEDs
Concepts of Light Down-Conversion
Organic Dyes
Band Gap of Organic Materials
Electronic Excitation in Organic Molecules
Types of Absorbing Electrons
Fluorescent Layer Preparation
Material and Polymer Composite Technique
Electrospinning
Stress Test
White Light Generated by Curcuminoid Nanofibers Using Electrospinning
Preparation of Polymer PMMA
Extracting Curcuminoids Dye Using a Centrifuge
Cleaning of Glass Substrates
Electrospinning Setup
Annealing Processes
Colorimeter Measurement Setup
Conclusion
References
8 Green Synthesis of Zinc Oxide Nanoparticles Using Salvia officinalis Extract
Introduction
Importance of Nanoparticle Synthesis
Polymers Effective on ZnO Nanoparticles
Hydrothermal Process
Synthesis Approaches of Metal Nanoparticles
Application of Nanoparticles Used in Various Fields
Zinc Oxide Synthesis and Applications
Green Synthesis of ZnO NPs Using Salvia officinalis Extract
Methodology
Nanoparticle Formation
ZnO NP Characterization
Conclusion
References
9 Top-Down Production of Nanocellulose from Environmentally Friendly Processes
Introduction
Green Fractionation Processes
Gamma-Valerolactone (GVL)
Supercritical Fluid (SF)-Based Processes
Subcritical Water (SBW)
Green Processes to Produce Nanocellulose
Enzymatic Pretreatment
Deep Eutectic Solvents (DES) and Ionic Liquids (IL)
Oxidative Processes
Conclusion
References
10 Green Synthesis of Metal Oxide Nanoparticles and Gamma Rays for Water Remediation
Introduction
Green and Sustainable Methods for Metal Nanoparticle Synthesis
Iron Oxide Nanoparticles Used for Pollutant Removal on the Water or Wastewater
Gamma Rays as Novel Eco-friendly Alternative for Nanoparticle Synthesis
Trends, Challenges, and Risk Management of the Green Nanoparticles as Promote of a Healthier World
Conclusions
Recommendations
References
11 Recent Progress on Doped ZnO Nanostructures and Its Photocatalytic Applications
Introduction
ZnO and Its Photocatalytic Mechanism
Improvement of ZnO´s Photocatalytic Activity via Doping Engineering
Doping with Nonmetal
Doping with Metal
Alkali and Alkaline Metal Doping
Transition Metal Doping
Rare Earth Metal Doping
Transition Metal-Doped ZnO Nanostructures and Their Photocatalytic Applications
Mn-Doped ZnO Nanostructures
Fe-Doped ZnO Nanostructures
Co-Doped ZnO Nanostructures
Ni-Doped ZnO Nanostructures
Cu-Doped ZnO Nanostructures
Ag-Doped ZnO Nanostructures
Other Transition Metal-Doped ZnO Nanostructures
Conclusion
References
12 Role of Green Nanomaterials for 3-Chloropropane-1,2-diol Ester (3-MCPDE) Reduction
Introduction
Green Nanomaterials
Approaches for Green Nanomaterials Synthesis
Green Synthesis Using Plant Extracts
Enzyme Immobilization on Green Nanomaterials
3-Chloropropane-1,2-diol Ester (3-MCPDE)
Toxicity of 3-Chloropropane-1,2-diol Ester (3-MCPDE)
Approaches for 3-Chloropropane-1,2-diol (3-MCPD) Reduction
Parameters for 3-Chloropropane-1,2-diol (3-MCPD) Reduction
Scientific Endeavor in the Reduction of 3-MCPD via Green Immobilization
Conclusion and Future Prospects
References
13 Fabrications from Renewable Sources and Agricultural Wastes and Characterization Strategies of Green Nanomaterials
Introduction
Synthesis and Characterization of Various Bio-based Nanomaterials
Introduction to Different Biomass and the Nanomaterials
Cellulose and Nanocellulose Based
Carbon Nanomaterials
Silica
Generalized Synthesis Techniques of Nanomaterials
Characterization Methods of Nanoparticles
Conclusion and Future Scope
References
14 Algal Extract-Biosynthesized Silver Nanoparticles: Biomedical Applications
Introduction
Biosynthesis of AgNPs Using Algae
Specific Properties of Algal-Produced AgNP
Biomedicinal Applications of AgNPs
Antibacterial Properties
Antifungal Activity
Antiviral Activity
Miscellaneous Activities
Conclusion and Future Recommendations
References
15 Green Synthesis of Metal Oxide Nanoparticles
Introduction
Metal Oxide Nanoparticles and Their Green Synthesis Techniques
Metal Oxide Nanoparticles
CuO Nanoparticles
TiO2 Nanoparticles
ZnO Nanoparticles
Green Mediated Synthesis for the Growth of Metal Oxide Nanoparticles
Plant-Based Method
Microorganism-Mediated Method
Microwave-Assisted Method
Mild Reducing Agent-Based Technique
Ultrasound-Assisted Method
Solar Energy-Mediated Method
Bacteria-Based Method
Fungus-Based Method
Factors Influencing the Synthesis of Various NPs
Temperature
pH
Reaction Time
Synthesis of Metal Oxide Nanoparticles from Plant Extract
CuO Nanoparticle Synthesis by Plant Leaf Extract
ZnO Nanoparticle Synthesis by Plant Leaf Extract
TiO2 Nanoparticle Synthesis by Plant Leaf Extract
Characterization Techniques of Nanoparticles Prepared from Plant Extract
Mechanism of Nanoparticle Evolution with Plant Extracts
Advantages of Green Synthesis of NPs
Antimicrobial Activities
Textile Industry
Wastewater Treatment
Food Industry
Agriculture
Conclusion
References
16 Green Synthesis of Carbon Dot-Based Materials for Toxic Metal Detection and Environmental Remediation
Introduction
Green Synthesis of CDs
Optical Properties of CDs
Application of CD-Based Materials as Fluorescent Sensors
Sensing of Hg2+
Sensing of Pb2+
Sensing of Cd2+
Sensing of Cr (VI)
Sensing of Cu2+
Mechanism of Fluorescence Sensing
Static Quenching
Dynamic Quenching
Resonance Energy Transfer (RET)
Photoinduced Charge Transfer (PCT)
IFE
Application of CD-Based Materials as Photocatalysts
Organic Dye Degradation
Photocatalytic Heavy Metal Reduction
Mechanism of Photocatalytic Degradation
Conclusion
References
17 Green Synthesis of Metal Oxide Nanomaterials and Photocatalytic Degradation of Toxic Dyes
Introduction
Principles of Green Chemistry
Advantages of Green Synthesis over Traditional Methods
Green Methods of Preparation of Metal Oxide Nanoparticles
Plant-Based Synthesis
Microbe-Mediated Synthesis
Ionic Liquid-Mediated Synthesis
Applications of Metal Oxide Nanoparticles in Photocatalytic Degradation of Toxic Dyes
Future Perspective
Conclusions
References
18 Green Synthesis of Hybrid Nanostructure for Wastewater Remediation by Photocatalytic Degradation
Introduction
Plant-Mediated Green Synthesis of Nanoparticles
TiO2-Based Materials for Photodegradation of Pollutants
CuO-Based Materials for Photodegradation of Pollutants
ZnO-Based Materials for Photodegradation of Pollutants
ZrO2-Based Materials for Photodegradation of Pollutants
Conclusions
References
19 Natural Polymer-Based Nanocomposite Hydrogels as Environmental Remediation Devices
Nanotechnology: Origin and Definitions
Hydrogels
Transformation of Hydrogels to Nanocomposite Hydrogels
Classification of Nanocomposite Hydrogels
Nanocomposite Hydrogels Based on Carbon
Nanocomposite Hydrogels Based on Carbon Nanotubes
Nanocomposite Hydrogels Based on Graphene
Nanocomposite Hydrogels Based on Carbon Quantum Dots
Nanocomposite Hydrogels Based on Crown-ethers
Nanocomposite Hydrogels Based on Polymer
Natural Polymer-Based Nanocomposite Hydrogels
Synthetic Polymer-Based Nanocomposite Hydrogels
Based on Metal and Metal Oxide
Methods of Synthesis of Nanocomposite Hydrogels
Synthesis of Nanocomposite Hydrogels Using Sol-Gel Method
Radiation-Assisted Synthesis of Nanocomposite Hydrogels
Microwave-Assisted Synthesis
Gamma-Ray Irradiation Synthesis
Ultrasonic-Assisted Synthesis
Free-Radical Co-polymerization
Grafting
Characterization of Nanocomposite Hydrogels
Applications of Natural Polymer-Based Nanocomposite Hydrogels in Environmental Remediation
Adsorption
Degradation
Ion Exchange
Soil Conditioning
Conclusion and Future Outlook
References
20 Biogenic Metallic Nanoparticles: Synthesis and Applications Using Medicinal Plants
Introduction
Biogenic Synthesis of Nanoparticles
Synthesis of Nanoparticles Using Algae
Synthesis of Nanoparticles Using Fungus
Synthesis of Nanoparticles Using Bacteria
Synthesis of Nanoparticles Using Plants
Applications of Green Nanoparticles
Biomedical Applications of Nanoparticles
Role of NPs in Diagnostics and Drug Delivery
Application of NPs in Food Industry
Applications of Nanoparticles in Agriculture
Nanoparticles as Fungicides
Nanoparticles as Fertilizers
Nanoparticles as Pesticides
Applications of Nanoparticles in Bioremediation
Future Perspectives and Conclusions
References
21 Synthetic Nanoparticle-Based Remediation of Soils Contaminated with Polycyclic Aromatic Hydrocarbons
Introduction
Role of Nanomaterials in PAH Removal from Soil
Nanoscale Zero-Valent Iron (nZVI)
Metal Oxides
Other Synthesized Materials
Materials Based on Carbon
Polymer-Based Nanomaterials
Mechanism of PAH Degradation Using Synthetic NPs and By-Products Identification
Conclusions
References
22 Multifunctional Nanoprobes for the Surveillance of Amyloid Aggregation
Introduction
Types of Amyloidosis
Cross-beta Structure
Diseases Associated with Amyloid Deposits
Alzheimer´s Disease and Type 2 Diabetes
The Connection Between Alzheimer´s and Type 2 Diabetics
Amyloid Aggregation Mechanism in Alzheimer´s Disease and Type 2 Diabetes
Impacts of Alzheimer´s and Type 2 Diabetes
In Vivo Diagnosis for Amyloid Aggregation
In Vitro Diagnostics for Amyloid Aggregation
Nanomaterials for the Treatment of Type 2 Diabetes and Alzheimer´s Disease
Quantum Dots for the Treatment of Alzheimer´s Disease and Type 2 Diabetes
Nanocarriers for Alzheimer´s Disease Treatment
Conclusion
References
23 Generation of Nanoparticles from Waste via Solvent Extraction Method
Introduction
An Overview on the Problem of Waste
Waste Management Strategies
An Overview on Nanoparticles and Its Applications
Recovery of Metals/Metal Oxide (NPs)
Liquid-Liquid Extraction (Solvent Extraction) Procedure
Solvent Extraction Procedure and Measurements
Literature Review on the Preparation of Metal Nanoparticles/Metal Oxides from Waste via Solvent Extraction Method
Characterization of End-Products
Conclusions
References
24 Plant-Mediated Synthesis of Nanoscale Hydroxyapatite: Morphology Variability and Biomedical Applications
Introduction
Morphology Control: General Aspects
Surfactants
Surfactant Free
Nanoscale Synthesis
Synthesis Methods for Nanoparticles
Green Synthesis of Hydroxyapatite Nanoparticles - Morphological Variability
Biomedical Applications
Conclusions and Remarks
Reference
25 Green and Sustainable Technology for Clean Energy Production: Applications
Introduction
Anthropogenic Emissions from Conventional Power Plants and Their Impact on Environment
Green and Sustainable Nanotechnology for the Reduction of Anthropogenic Pollution
Solar Energy
Wind Energy
Hydropower
Geothermal Energy
Biomass
Thermoelectric Energy
Hydrogen Energy
Conclusions
References
26 Generation of Nanomaterials from Wastes
Introduction
Classification of Wastes
Biodegradable Wastes
Non-biodegradable Waste
Synthesis of Different Nanoparticles from Wastes
Nanoparticles from Biodegradable Wastes
Carbon Dots from Wastes
Carbon Nanotubes (CNTs) from Wastes
Silver-Based Nanoparticles from Wastes
Other Nanoparticles from Wastes
Nanoparticles from Non-biodegradable Wastes
Nanoparticles from Industrial Waste
Nanoparticles from Spent Batteries
Nanoparticles from Plastics
Waste-Derived NM Applications
Catalytic Activity
In Energy Storage
In Biomedical Field
Other Applications
Conclusions
References
27 Photoelectrochemical CO2 Reduction: Perspective and Challenges
Introduction
Method for CO2 Reduction
Thermocatalytic CO2 Reduction
Photothermal CO2 Reduction
Photocatalytic CO2 Reduction
Electrochemical CO2 Reduction
Photoelectrochemical CO2 Reduction
Designing Photoelectrochemical CO2 Reduction System
Photoelectrochemical Cell
Design of Photoelectrode Materials and Type of PEC Cell
Electrolyte
Design of Photoelectrocatalysts
Thermodynamic and Kinetic Parameters
Conclusion and Future Direction
References
28 Nanomaterial´s Safety Regulations in Food and Drug Industry
Introduction
Awareness with Respect to Nanomaterials in Food and Drug Products
Government-Sponsored Regulatory Science and Other Research Activities in Foods and Drug Products
The Food Sector
The Medical Products Sector
Some Efforts from Regulatory Agencies
The United States
The European Union (EU)
Japan
China
Australia and New Zealand
Canada
Necessity of Regulatory Development
Conclusion
References
Part II: Environmental Remediation
29 Role of Engineered Nanomaterials for Eradication of Endocrine Disrupting Phenols
Introduction
Sources of Phenolic-Based Compounds in Water
Toxic Effects of Phenolic Compounds on Living Beings
Removal of Endocrine Phenols Using Nanomaterials
Adsorption
Photocatalysis
Miscellaneous
Environmental Impact of Nanomaterials
Conclusion and Future Scope
References
30 Technologies for Treatment of Emerging Contaminants
Introduction
Physical Treatments
Membrane Technology
Adsorption
Physical Pretreatment Processes
Biological Treatments
Conventional Treatments
Bioremediation
Activated Sludge Process
Biological Nitrification and Denitrification
Nonconventional Treatment Technologies
Membrane Bioreactor
Constructed Wetlands
Biosorption
Chemical Treatments
Conventional Oxidation Processes
Ozonation Method
Photolysis
Fenton Oxidation Processes
Advanced Oxidation Approaches
Photo-Fenton Process
Photocatalysis
Ferrate
Electro-Fenton Method
Hybrid Treatments
Membrane Filtrations and Membrane Bioreactors
Membrane Filtrations, Adsorbents, and the Process of Activated Sludge
Miscellaneous Systems
Applications of Nanotechnology in the Treatment of Emerging Contaminants
Conclusions
References
31 Photocatalytic and Adsorptive Remediation of Hazardous Organic Chemical Pollutants from Waste Water
Introduction
Assessment of Publication
Adsorption
Activated Carbon
Biosorbents
Carbon Nanotubes
Clay
Zeolites
Photocatalytic Remediation
Z-Scheme Heterojunction Photocatalysts
Type II Heterojunction Photocatalysts
S-Scheme Heterojunction Photocatalysts
Tandem Heterojunction Photocatalysts
Remediation of Organic Pollutants by Green and Sustainable Nanomaterials
Conclusion and Perspective
References
32 Occurrence, Distribution, and Removal of Phthalates by Nanomaterials
Introduction
Background of Phthalates
Distribution, Application, and Physiochemical Properties of Phthalates
Occurrence of Phthalates in the Environment
Occurrence of Phthalates in the Atmosphere/Air
Occurrence of Phthalates in Fresh Water and Sediments
Occurrence in Soils
Occurrence in Landfills
Distribution, Exposure, and Transmission of Phthalates
Removal of Phthalates from Wastewater by Conventional Methods
Phthalate Degradation by Nanomaterials
Single Metal Oxide Nanomaterials (SMOs)
Doped and Composite Photocatalysts
Composite Photocatalysts
Conclusion
References
33 Functionalized Nanomaterials for Environmental Remediation
Introduction
Methods of Surface Functionalization
Various Approaches to Surface Functionalization
In Situ Functionalization
Post-Synthesis Functionalization or Grafting
Functionalization Chemistry
Covalent Functionalization
Noncovalent Functionalization
Various Environmental Remediation Techniques
Nano-Adsorption
Nano-photocatalysis
Membranes and Membrane Processes
Nanosensing
Application of Functionalized Nanomaterials for Environmental Remediation
Organic Dyes
Heavy Metal Ions
Pesticides
Virus
Pharmaceutical Drugs
Gases
Conclusion and Future Recommendations
References
34 Photocatalytic Degradation of Drugs
Introduction
Toxicity and Ecotoxicological Effects of Drugs on Environment
Methods for Removal of Drugs from Wastewater
Adsorption Process
Advanced Oxidation Process (AOPs)
Ozonation Method
Fenton and Photo-Fenton Method
Photolysis Method
Electrochemical Method
Photoelectrocatalysis (PEC) Method
Photocatalysis Method
Factors Affecting the Photocatalytic Removal of Drugs
Effect of pH
Effect of Amount of Catalyst
Effect of Initial Concentration of Drug
Effect of Operating and Calcination Temperature
Conclusions and Future Perspective
References
35 Environmental Remediation Through Metal Green Nanomaterials
Introduction
Synthetic Approaches of Metal Green Nanomaterials
Metal Green Nanomaterials from Microorganism
Metal Green Nanomaterials from Plant Extract
Metal Green Nanomaterials from Biomolecules
Factors Affecting the Synthesis of Metal Green Nanomaterials
Environmental Remediation Approaches Through Metal Green Nanomaterials
Antimicrobial Activity
Dye Eradication
Heavy Metal Ion Removal
Wastewater Treatment and Organic Pollutant Removal
Ongoing Challenges and Future Prospects
Conclusion
References
36 Implications of Green Nanomaterials for Environmental Remediation
Introduction
Green Synthesis of Nanomaterials
Adsorption Potentials of Green Nanomaterials for Air, Water, and Soil Remediation
Green Nanomaterials as Adsorbents
Metal Nanoparticles
Silver and Gold Nanoparticles
Metal Oxide Nanoparticles
Iron Oxide Nanoparticles
Titanium Dioxide Nanoparticles (TiO2NPs)
Zinc Oxide Nanoparticles
Cerium Oxide-Based Nanoparticles
Aluminum Oxide-Based Nanoparticles
Magnesium Oxide-Based Nanoparticles
Nanocellulose
Chitosan-Based Nanomaterials
Dendrimers
Conclusions and Future Scope
References
37 Eradication of Emerging Contaminants like Brominated Flame Retardants by Green Nanomaterials
Introduction
Classification of FRs
PBDEs
TBBPA
HBCD
PBBs
NBFRs
BTBPE
DBDPE
BEH-TEBP
TEBP-Anh
PBEB
PBT
HBB
TBP-AE
TBP
BB-209
TBBPA-BDBPE
Chlorinated Flame Retardants
Environmental Concern of Halogenated Flame Retardants
Remediation Techniques for BFRs
Conventional Techniques
Adsorption
Thermal Treatment
Photocatalytic Degradation
Direct Photolysis
Anaerobic Degradation
Removal Technique for TDCPP and TCEP
Advanced Techniques for Degradation of Halogenated Flame Retardants
Nanoparticles (NPs)
Copper Oxide (CuO) NPs
Titanium Oxide (TiO2)
Iron-Based NPs
Conclusion and Future Scope
References
38 Nanocatalysts for Advanced Oxidation Processes in Heterogeneous Systems
Introduction
Advanced Oxidative Processes: Heterogeneous Catalysis
Nanocatalysts
Green Synthesis of Nanocatalysts
Nanocatalysts Based on Natural Polymers
Synthesized Nanocatalysts with the Stabilization of Plant Extracts
Synthesis by Microorganisms
Conclusion and Future Perspective
References
39 Sustainable Nanomaterials for Environmental Remediation
Introduction
Novel Methods of Synthesis of Sustainable/Green Nanomaterials
Green Synthesis of Inorganic (Metal, Metal Oxide, and Metal Sulfide) Nanomaterials
Green Synthesis of Silica Nanomaterials
Green Synthesis of Carbon-Based Nanomaterials
Green Synthesis of Polymer-Based Nanomaterials
Water Pollution
Organic Contaminants in Water
Inorganic Contaminants in Water
Toxic Metal Ions in Water
Radionuclide´s Contaminants
Biological Contaminants in Water
Green Nanomaterials for Water Treatment
Removal of Organic Pollutant from Water
Metal Ion Removal from Water
Removal of the Biological Contaminant from Water
Removal of Inorganic and Radionuclide Impurities
Inactivation of Microorganism
Soil Pollution
Modes of Soil Pollution
Natural Soil Pollution
Anthropogenic Soil Pollution
Inorganic Pollutants in Soil
Heavy Metals in Soil
Organic Pollutants in Soil
Pesticides in Soil
Green Nanotechnology for Soil´s Contamination Remediation
Heavy Metal Removal from Soil
The Mechanism for Cd Removal
Conclusions and Future Recommendations
References
40 Green Nanomaterials as Photocatalyst/Catalyst: Exploration of Properties
An Introduction to Green Nanotechnology
Why Green Nanotechnology Is Needed?
Green Synthesis of Nanomaterials
Catalyst
Metal and Metal Oxide Nanoparticles
Fundamental of Photocatalyst
Important Factors in Photocatalytic Activity
Structure/Surface Area
Oxygen Vacancies
Sacrificial Reagent
Band Gap Energy
Light Intensity
Temperature
pH
Types of Photocatalysts
TiO2
BiVO4
Carbon Dot
WO3
SnS
ZnO
SnO2
Zeolites
Metal Selenide
Metal Sulfide
g-C3N4
Polymeric Nanomaterials
Metal- and Metal Oxide-Incorporated Biopolymer Nanomaterials
Advantage and Disadvantage
Conclusion and Future
References
41 Photocatalytic Properties of Metal-Based Nanoparticles
Introduction
Semiconductors
Mechanism of Photocatalysis
Photocatalytic Reactions
The Requirements for an Ideal Photocatalyst
Synthesis of Semiconductors
Nanomaterial-Based Photocatalysis
Successful Examples of Photocatalysts
Bimetallic Structures: The Mixture of Plasmonic and Catalytic Elements for Photocatalysis
Applications of Metal Nanoparticles as Photocatalysts
Self-Cleaning Surfaces
Air-Water Remediation
Air Cleaning
Sulfur Family Pollutant
CO2 Photoreduction
Disinfection/Antimicrobial Surfaces
Hydrogen Production
Photoelectrochemical Conversion
Solar-Water Splitting
Lithography
Outlook and Prospectives
Conclusions
References
42 Green Nanomaterials for Environmental Remediation
Introduction
Green Metals and Metal Oxides Nanoparticles
Removal of Toxic Metals
Removal of Organic Dyes
Bactericidal Activity
Flocculant Agents
Green Carbon-Based Nanomaterials
Green Nanoporous Carbons
Green Carbon Quantum Dots
Green Carbon Nanotubes, Fullerene, and Reduced Graphene Oxide
Green Carbon Nanofibers
Green Chitin and Chitosan-Based Nanomaterials
Green Chitin-Based Nanomaterials
Green Chitosan-Based Nanomaterials
Green Cellulose-Based Nanomaterials
Green NC Produced by TEMPO-Mediated Oxidation
Green NC Produced by Alternative Green Methods
Conclusions and Future Recommendations
References
43 Sustainable Doped Nanomaterials for Environmental Applications
Introduction
Photocatalysis Process with Nanomaterials
Mechanism of Metal-Doped Nanomaterials
Mechanism of Nonmetal-Doped Nanomaterials
Environmental Applications of TiO2- and ZnO-Based Nanomaterials
Removal of Pesticides
Organic Dye Removal
Removal of Pharmaceuticals, Personal Care Products (PPCPs), Endocrine-Disrupting Compounds (EDCs), and Other Toxic Aromatic Co...
Heavy Metal Ion Removal
Conclusion and Future Perspective
References
44 Graphene-Based Nanomaterials for Water Remediation Applications
Introduction
Green Synthesis of 3D-GBNs: Hydrothermal Route
Adsorption and Photocatalytic Degradation Properties of GBNs
Synthetic Dye Molecules
Heavy Metal Ions
Emerging Pollutants
Oil
Antibacterial Disinfection Properties of GBNs
Conclusion and Prospects
References
45 Metal Oxide-Based Nanocomposites for Elimination of Hazardous Pesticides
Introduction
Classification of Pesticides
Type A
Insecticide
Herbicide
Fungicide
Antimicrobials
Acaricide
Rotenticides
Type B
Organochlorine Pesticides
Organophosphate Pesticides
Carbamate Pesticide
Substituted Ureas
Techniques for Remediation of Pesticides
Different Kinds of Metal Oxides Synthesized via Greener Approach
Titanium-Based Engineered Nanomaterials
Zinc Oxide Nanoparticles
Iron Oxide Nanoparticles
Silver Oxide Nanoparticles
Miscellaneous Metal Oxide Nanocomposite
Environmental and Human Risks of Pesticides
Banned Pesticides in India
Photocatalytic Degradation of Pesticides
Conclusion and Future Scope
References
46 Nanophotocatalysis for Degradation of Organic Contaminants
Introduction
The Nanophotocatalysts
Metallic and Non-metallic Nanophotocatalysts
Ti- and Zn-Based Nanophotocatalysts
Nanophotocatalysts Based on Carbon Nitride
Use of Sunlight and Sustainability
Additives for Enhancing Photocatalysis Efficiency
Effect of the Experimental Variables on Photocatalysis
Morphology of the Catalyst
pH
Catalyst Dosage
Photocatalytic Degradation Rate and Initial Concentration
Light Wavelength and Intensity
Temperature
Conclusion and Future Work
References
47 Remediation of Chromium Heavy Metal Ion by Green Synthesized Nanocomposites
Introduction
Environmental Concern of Heavy Metals
Synthetic Methods for Green Composite Nanostructures
Carbothermal
Microwave
Solvothermal
Coprecipitation
Hydrothermal
Sol-Gel
Reduction
Impregnation Method (Incipient Wetness, Capillary, or Dry)
Micelle and Reverse Micelle
Miscellaneous
Heavy Metal Treatment Methods
Removal of Cr
Conventional Technologies
Green Adsorbent
Cr Adsorption Mechanism
Mechanism I
Mechanism II
Conclusion
References
48 Abatement of PAHs by Engineered Nanomaterials
Introduction
EPA Priority PAHs
Degradation Techniques for PAHs
Photocatalysis
Photocatalysis-Based Degradation of PAHs by Engineered Nanomaterials
TiO2-Based Photocatalysts
ZnO-Based Photocatalysts
Green Synthesized Nanomaterials in Degradation of PAHs
Conclusion
References
49 Quantum Dots: Applications in Environmental Remediation
Introduction
Prolonged Library of 2D-QDs: Diversity in 2D-QDs
2D-QDs Composed of a Single Element
Carbon Dots (CDs)
Phosphorene (P) Dots
Other Quasi-2D-QDs Composed of a Single Element (Si, B, Ge, Pb, and Sn Dots)
2D-QDs Containing Two Elements
Silicon Carbide (SiC) Dots
Carbon Nitride (C3N4) Dots
Boron-Nitride (BN) Dots
Transition Metal Dichalcogenides (TMDs): Quantum Dots
Oxides of Transition Metals: Quantum Dots
Carbon Quantum Dots
Green Synthesis of Carbon Quantum Dots
Environmental Applications
Photocatalysis
Super Capacitors
Adsorption
Environmental Concern
Conclusion and Future Scope
References
50 Removal of Organic Dyes by Functionalized Nanomaterials
Introduction
Detail Discussion of Different Hazardous Dyes
Azo Dyes
Anthraquinone Dyes
Triarylmethane Dyes
Details of Dyes Based on their Source and Complex Structure
Cationic Dyes
Malachite Green (MG)
Methylene Blue (MB)
Methyl Violet (MV)
Rhodamine B (RB)
Brilliant Blue R (BBR)
Brilliant Green (BG)
Crystal Violet (CV)
Basic Fuchsin (BF)
Anionic Dyes
Indigo Carmine (IC)
Eriochrome Black T (EBT)
Rose Bengal (RB)
Acid Orange II (AO)
Eosin Y (EY)
Alizarin Red S (ARS)
Congo Red (CR)
Methyl Orange (MO)
The Environmental Concern of Hazardous Dyes
Methods of Nanomaterials´ Functionalization
Direct Functionalization
Covalent Functionalization
Noncovalent Functionalization
Inorganic Functionalization
Functionalization by Heteroatom Doping
Functionalization by Immobilization
Indirect or Post-Synthetic Functionalization (Grafting)
Polymer Coating
The Necessity of Functionalization of Nanomaterials for Remediation of Organic Contaminants
Working Mechanism of Functionalization of Nanomaterials
Degradation of Dyes by Functionalized Materials
Titanium-Based Functionalization of Nanomaterials
Zinc Oxide-Based Functionalization of Nanomaterials
Carbon-Based Functionalized Nanomaterials
Iron-Based Functionalization of Nanomaterials
Miscellaneous Functionalization of Nanomaterial-Based Degradation Studies
Application of Green Synthesized Functionalization of Nanomaterials for Dye Removal
Toxicity and Functionalized Nanoparticles
Conclusions and Future Perspectives
References
51 Synthesis of Silver Nanoparticles with Environmental Applications
Introduction
Synthesis of Silver Nanoparticles
Chemical Method
Physical Method
Biological Technique
Role of Coupling/Doping
Characterization Techniques
Importance of Silver Nanoparticles
Environmental Concern and Assessment
Future Scope and Viewpoints
Conclusions
References
52 Polymer Nanocomposites in Wastewater Treatment
Introduction of Polymer Nanocomposites
Polymer Nanocomposites
Methods for the Synthesis of Polymer Nanocomposites
Type of Nanofillers
Carbon Nanomaterials
Graphene Oxide
Carbon Nanotubes
Activated Carbon/Charcoal
Nanoclays
Nanometal Oxides
Natural Polymer Nanocomposites
Chitosan Polymer Nanocomposite
β-Cyclodextrin (β-CD) Polymer Nanocomposite
Alginate Polymer Nanocomposite
Cellulose
Starch
Polyacrylic Acid
Conclusions
References
53 Zinc-Based Nanomaterials for Sustainable Environmental Remediation
Introduction
Synthesis Methodologies for Zn-Based Nanomaterials
Environmental Concerns of Pollutants
Application of Zn-Based Nanoparticles
Utilization of Green Synthesized Zn-Based Nanomaterials
Conclusion and Future Scope
References
54 Applications of Green Nanomaterials in Environmental Remediation
Introduction
Synthesis of Nanomaterials
Green Synthesis of Nanoparticles
Green Synthesis of Nanoparticle by Microorganism
Green Synthesis of Nanoparticles by Plant
Use of Green Solvent/Green Methodology
Green Synthesis of Nanomaterials from Waste
Characterization of Nanoparticles
UV-Visible Spectroscopy
FTIR Spectroscopy
Powder X-Ray Diffraction
Electron Microscopic Techniques
Energy Dispersive X-ray Spectroscopy
Properties of Nanomaterial
Physical and Mechanical Properties
Optical Properties
Electrical and Magnetic Properties
Water Remediation by NMs Synthesized by Green Route
Removal of Heavy Metals from Water
Removal of Organic Compounds from Water
Removal of Dyes from Water
Removal of Pharmaceuticals from Water
Photocatalytic Degradation of Pollutants in Water by Green NM
Nanotechnology for Adsorption of Toxic Gases
Metal Oxide Semiconductor Gas Sensors
Graphene Gas Sensors
Graphene Metal Oxide Gas Sensors
Adsorption of Gases on Multiwalled Carbon Nanotubes
Nanomaterials as Sensors and Detectors for Water Remediation
Nanomaterial-Based Electronic Sensors for Water Pollution
Heavy Metal Detection
Microorganism Detection
Nanomaterial-Based Optical Sensors
Nanomaterial-Based Electrochemical Sensors
Nanotechnology for Pollution Prevention
Carbon Nanotubes in the Removal of Organic Pollutants
Metal Oxide-Based Nanomaterials
Silica-Based Nanomaterials
Chitosan-Based Nanomaterials
Conclusions
References
55 Green Nanomaterials for Remediation of Environmental Air Pollution
Introduction
Sources of Air Pollution
Particulate Matter
Black Carbon
Surface Ozone
Nitrogen Dioxide
Sulfur Dioxide
Carbon Monoxide
Air Pollution Control with the Help of Green Nanotechnology
Detection of Air Pollutants
Remediation/Treatment of Air Pollutants
Green Nanomaterials as Adsorbents
Filtration/Separation of Air Pollutants
Filtration/Separation by Green Nanofilters
Filtration by Nanofiber Net
Degradation (Photocatalyst) of Air Pollutants
Transformation of Air Pollutants Via Chemical Reactions
Conclusions
References
56 Polymer-Based Nanocomposites for the Removal of Personal Care Products
Introduction
Classification of Polymeric Nanoparticles
Natural Polymer-Based Nanoparticles
Biosynthesized Polymer Nanoparticles
Chemically Synthesized Polymer Nanoparticles
Surface Modification
CNT/Polymer Functionalization
Metal-Polymer Nanoparticles
Metal Oxide/Polymer Functionalization
Silica Grafted Polymer
Pharmaceuticals and Personal Care Products
Toxicity of Pharmaceutical and Personal Care Products
Application of Polymer-Coated Surface
Environmental Concern
Conclusion
References
57 Metal Oxides-Based Nanomaterials: Green Synthesis Methodologies and Sustainable Environmental Applications
Introduction
Classification of Metal Oxides Nanoparticles
(I) Monometallic Oxides
(II) Bimetallic Oxides
Trimetallic Oxides
Synthesis of Metal Oxides Nanoparticles Using Green Methodology
Using Plant Extracts
Using Microorganisms
Removal of Traditional Pollutants Using Metal Oxides-Based Nanoparticles
Conclusion
References
58 Environmental Occurrence and Degradation of Hexabromocyclododecanes
Introduction
Types of HBCD Isomers and Physicochemical Properties
Production and Application of HBCDs
Plastic
Textile
Possible Emission Sources and Environmental Concerns of HBCDs
Release During Production
Release During Use
Release at the Time of Disposal
Recycling
Risk Management and Evaluation
Biodegradation of HBCDs
Biodegradation of HBCDs Using a Microorganism
Degradation of HBCDs Using Nanoparticles
Conclusion and Upcoming Challenges
References
59 Sustainable Green-Doped Nanomaterials for Emerging Contaminants Removal
Introduction
Leaves Based
Fruit, Seed, and Root Based
Microbes-Mediated Methods
Algae
Bacteria
Fungi
Biochar
Green Polymeric Nanomaterials
Carbon-Based
Graphene
CNTs
Doping of Nanomaterials and Their Significance
Pollutants
Classification of Emerging Pollutants
Discuss Here Typical Mechanism of Degradation by Doped Nanomaterials
Removal of Pharmaceuticals Pollutants
Removal of Plastic Additives
Future Scope and Perspectives
Conclusion
References
60 Nanotechnology in Veterinary Sector
Introduction
Types of Nanoparticles
Polymeric Nanoparticles
Liposomes
Solid Lipid NPs
Carbon-Based Nanomaterials
Nanoshells
Polymeric Micelles
Dendrimers
Metallic Nanoparticles
Magnetic Iron Oxide Nanoparticles
Ceramic Nanomaterials
Quantum Dots
Nanoemulsion
Nanobubbles
Respirocytes, Microbivores, and Clottocyte
Nano-/Microrobots
Aluminosilicate NPs
Use in Veterinary Sector
Nanotechnology-Based Feed Additives
Nanotechnology for Feed Safety
Nanotechnology for Disease Diagnosis
Nanotechnology for Animal Treatment
Nanotechnology for Nanovaccines
Nanotechnology for Breeding and Reproduction
Challenges and Limitations
Nanotoxicity and Stability of Nanoparticles
Spermatotoxic Effects and Immunosuppression
Feed Safety Concerns, Economics, and Environmental Effects
Inadequate Literature, Research Bias, and Risk Assessment
Conclusion and Future Prospects
References
61 Consumer Nanoproducts for the Remediation of Environmental Problems
Introduction
Benefits of Environmental Nanotechnology
Classification of Nanomaterials
Environmental Risk Management
Health Risk Assessment (Theodore et al. 2008; Theodore and Hoboken, 2006)
Hazard Risk Evaluation Process (Mnestudies.com/Disaster Management, Online)
Nanotechnology for Sterilization of Water
Pollutant Removal Using Different Nanomaterials
Nanotechnology for Decontamination of Polluted Air and Soil
Conclusion
References
62 Properties of Green Nanomaterials as Catalysts and Photocatalysts
Introduction
Green Approaches for the Synthesis of Nanomaterials
Conventional Synthetic Route
Green Synthetic Route
Catalytic Applications of Green Nanomaterials
Photocatalytic Applications of Green Nanomaterials
Conclusion
References
Part III: Coatings Applications
63 Applications of Green Nanomaterials as Surfaces and Coatings
Introduction
Antimicrobial Coatings
Self-Cleaning: Dirt and Water Repellent Coatings
Photocatalytic Self-Cleaning Coatings
Anticorrosive Coatings
Self-Healing (or Scratch-Resistant) Coatings
Other Coating with Green Nanoparticles
Current State of Research and Development
Conclusions and Future Recommendations
References
64 Green Coatings: Materials, Deposition Processes, and Applications
Introduction
Materials
Waterborne and Oil-Based Coatings
Chitosan
Cellulose and Lignin
Re-use of Polymers in Coatings
Deposition Methods
Sol-Gel
Electrophoretic Deposition
Casting
Layer-By-Layer Assembly (LBL)
Applications
Anticorrosion Coatings
Superhydrophobic Coatings
Coatings for Food
Coatings in Energy Applications
Other Applications
Conclusions
References
65 Green Nanomaterials as Surfaces and Coatings
Introduction
Green Nanomaterials
Application of Green Nanomaterial as Surfaces and Coatings
Ultraviolet (UV) Light Protective Coatings
Anticorrosion Coatings, Water Beading, and Water Sheeting Coatings
Building Coatings
Antifogging Coatings and Antifouling Coatings
Nanocoatings for Energy Storage and Conversion: Supercapacitors, Solar Cells, and Fuel Cells
Self-Cleaning Coatings
Nanocoatings in Textile Manufacturing
Nanocoatings in Drug Delivery
Nanocoatings in Sensor and Space Exploration
Conclusion and Future Perspective
References
Part IV: Biomedical and Biological Applications
66 Biomedical Applications of Green Nanomaterials
Introduction
Physicochemical Properties of Green Nanomaterials
Anticancer Applications of Green Nanomaterials
Microbicidal Applications of Green Nanomaterials
Drug Delivery Applications of Green Nanomaterials
Conclusion and Future Perspectives
References
67 Biomedical Applications of Nanomaterials
Introduction
Carbon Dots (CDs): Synthesis and Toxicity Evaluation
Reduced Graphene Oxide (rGO): Synthesis and Toxicity Evaluation
Iron Oxide Nanoparticles (IONPs): Synthesis and Toxicity Evaluation
Conclusion and Future Work
References
68 Emerging Roles of Carbon Nanohorns as Sustainable Nanomaterials in Sensor, Catalyst, and Biomedical Applications
Introduction
Properties of CNHs
Functionalization
Covalent Functionalization
Noncovalent Functionalization
Biomedical Applications
CNHs As Drug Delivery Systems
Carbon Nanohorns As Anticancer Drug Carriers
CNHs in Cancer Imaging
Carbon Nanohorns As Gene Delivery Vectors
Biosensing Application
Other Applications
Nanohorns As Catalyst
Gas Storage Media
Toxicity to the Cells
Conclusion
References
69 Green Synthesis and Fabrication of Nanomaterials: Unique Scaffolds for Biomedical Applications
Introduction
Green Synthesis of Nanomaterials
Green Synthesis and Characterization of Nanomaterials Using Plant Extracts
Gold Nanoparticles
Silver Nanoparticles
Iron and Iron Oxide Nanoparticles
Palladium and Platinum Nanoparticles
Copper and Copper Oxide Nanoparticles
Tin Oxide, Zinc Oxide, and Titanium Oxide Nanoparticles
Green Synthesis and Characterization of Nanomaterials Using Microorganisms
Using Bacteria
Using Fungi
Nanomaterials for Biomedical Applications
Nanomaterials for Drug Delivery
Nanomaterials for Biosensing Applications
Nanomaterials for Magnetic Resonance Imaging (MRI)
Nanomaterials Used Antibacterial Agents
Nanomaterials for Cancer Treatment
Conclusion
References
70 Natural Polymer-Based Nanocomposite Hydrogels for Biomedical Applications
Introduction: Development of Nanocomposite Hydrogels
Biomedical Importance of Natural Polymers Used for Synthesis of Hydrogels
Alginate
Gelatin/Collagen
Carrageenan
Cellulose
Chitin/Chitosan
Dextrin
Fibrin
Gellan Gum
Guaran
Hyaluronan
Karaya Gum
Pectin
Starch
Xanthan Gum
Synthesis of Nanocomposite Hydrogels
Biomedical Properties of Different Types of Nanocomposite Hydrogels
Carbon Nanoparticle-Based Nanocomposite Hydrogels
Nanocomposite Hydrogels Based upon Metallic Nanoparticles
Nanocomposite Hydrogels Based upon Polymeric Nanoparticles
Silica and Calcium Nanoparticle-Based Nanocomposite Hydrogels
Biomedical Applications of Nanocomposite Hydrogels
Wound Dressing
Tissue Engineering
Drug Delivery
Antibacterial Activity
Status of Compatibility and Toxicity of Nanoparticles, Biopolymeric Hydrogels, and Nanocomposite Hydrogels
Conclusions and Future Remarks
References
71 Application of Nanoclusters in Environmental and Biological Fields
Introduction
Application of Metal NC in Environmental Monitoring
pH Sensing
Metal Ion Sensing
Copper Ion Detection
Mercury Ions
Anion Sensing
Explosive Sensing
Application of Metal Nanocluster in Biological Fields
Other (Cu/Pt) Nanoclusters
Theoretical Studies on Nanoclusters
Conclusions and Future Prospective
References
72 Amylose-Based Green Nanoparticles as Carriers in Drug Delivery and Controlled Release Applications
Introduction
Green Nanocarriers for Drug Delivery for Biomedical and Health Applications
Nanomaterials: Sustainability and Toxicity
Development of Nanocarriers from Renewable and Green Materials such as Natural Polymers
The Use of Nanomaterials from Polysaccharides
Nanomaterials Based on Starch
Amylose-Based Nanocarriers
Mechanism of Inclusion Complexes Formation in Amylose
Administration Route of Drug Inclusion Complexes of Amylose
Limitations Associated with Amylose-Based Nanocarriers Production Such as Low Water Solubility and Retrograde
Modification of Amylose by Acetylation and Investigation of Inclusion Capacity Behavior
Preparation and Characterization of Amylose Inclusion Complexes with Rhodamine B
Synthesis of the Acetylated Amylose
Preparation of Amylose-Rhodamine B (AM-RB) and Acetylated Amylose-Rhodamine B (AMA-RB) Inclusion Complexes
Characterization of Inclusion Complexes
Conclusion
References
73 Biopolymeric Nanohydrogels as Devices for Controlled and Targeted Delivery of Drugs
Introduction: History, Definition, and Applications of Nanotechnology
The Journey from Hydrogels to Nanohydrogels
Classification of Nanogels
Based on Their Behavior to an External Stimuli
Based on the Structure
Methods of Synthesis of Nanohydrogels
Direct Polymerization Method
Precursor Polymerization
Physical Self-Assembly of Interactive Polymers
Fabrication Methods for the Synthesis of Biopolymeric Nanogel
Photolithographic Technique
Characterization of Nanogels
Applications of Nanogels in Various Routes of Drug Administration
Oral Route of Drug Delivery
Nasal Route of Drug Delivery
Topical Route of Drug Delivery
Transdermal Route of Drug Delivery
Parenteral Route of Drug Delivery
Drug Release Mechanism and Bioelimination of Nanogels
Clinical Approach
Patent Status
Conclusion and Future Scope
References
74 Advances in Medical Applications: The Quest of Green Nanomaterials
Introduction
Type of Nanomaterials
Green Synthesis of Nanomaterials
Use of Green Nanomaterials in the Biomedical Field
Green Nanomaterials in Biomedicine
Biomedical Imaging
Implants
Tissue Engineering
Biosensors
Drug Delivery
Conclusion and Future Perspectives
References
75 Metrics for the Sustainability Analysis of Nano-synthesis in the Green Chemistry Approach
Introduction
Nanotechnology Under the Green Chemistry Approach
Green Chemistry Metrics Applied to Nanotechnology
Metrics Based on Mass
Atom Economy and Reaction Mass Efficiency
E-Factor and Process Mass Intensity
Metrics Based on Energy
EQZ-Factor
Conclusions
References
76 Modulation of the Bioactivity of Inorganic Nanomaterials by Controlling Nanobiointerface
Introduction: The Interface as Interaction Key Point Between Nanomaterials and Biosystems
Nanobiointerface
Manufacturing Strategies
Strategies of Control
Characterization Techniques
Fourier-Transform Infrared Spectroscopy
Thermogravimetric Analysis
Nuclear Magnetic Resonance (NMR)
Raman Spectroscopy
Conclusions and Remarks
References
77 Role of Nanomaterials in Combating COVID-19
Introduction
Diagnostic Tests and Equipment During COVID-19
Nanomaterials and COVID-19
Environmental Impact of COVID-19
Conclusion and Future Outlook
References
Part V: Sensing Applications
78 Sustainable Nanotorus for Biosensing and Therapeutical Applications
Introduction
Synthesis and Characteristics of Carbon Nanotorus
Properties of Carbon Nanotorus
Electronic Properties
Magnetic Properties
Mechanical Properties
Chemical Modification of Carbon Nanotorus
Toxicity of CNT-based Nanostructures
Biomedical Applications of CNT-based Nanostructures
Controlled Drug Delivery Applications
Targeted Drug Delivery Applications
Biosensor Applications
Biomedical Imaging Applications
Conclusion
References
79 Nanogels and Nanocomposite Hydrogels for Sensing Applications
Introduction: Need of Nanotechnology for Sensing Applications
Types of Nanomaterials Used for Designing Nanocomposite Hydrogel/Nanogel Based Sensors
Zero-Dimensional Nanomaterials
Metal Nanoparticles
Fluorescent Nanoparticles and Nanocrystals
One-Dimensional Nanomaterials
Nanofibers, Nanorods, and Nanowires
Carbon Nanotubes
Two-Dimensional Nanomaterials
Graphene Oxide Nanosheets
MXene Nanosheets
Three-Dimensional Nanomaterials
Bionanomaterials
Polymeric Nanomaterials
Fabrication Techniques for Designing Nanocomposite Hydrogels and Nanogels
Status of Natural and Synthetic Polymer Based Sensors
Mechanism of Response in Nanocomposite Hydrogels and Nanogels
Mechanism of Response in Temperature Sensitive Sensors
Mechanism of Response in pH Sensitive Sensors
Mechanism of Response in Light Sensitive Sensors
Mechanism of Response in Fluorescent Sensitive Sensors
Mechanism of Response in Chemical, Drug, and Enzyme Sensitive Sensors
Sensing Applications of Nanocomposite Hydrogels and Nanogels
Biological Sensing
Chemical Sensing
Physical Sensing
Conclusions and Future Outlook
References
80 Two-Dimensional (2D) Nanostructures for Hazardous Gas Sensing Applications
Introduction
Motivation of Gas Sensor
Mechanism of Gas Sensor
Adsorption Mechanism
Charge Transfer Mechanism
Sensor Parameters
The Key Factors Influencing the Sensor Performance
What Are 2D Materials?
Why 2D Materials?
2D Materials Used in Gas Sensing
Classification of 2D Materials
Graphene
Graphene Oxide (GO)
Reduced Graphene Oxide (rGO)
Hexagonal Boron Nitride (hBN)
Transition Metal Dichalcogenides (TMDs)
Molybdenum Disulfide (MoS2)
Tungsten Disulfide (WS2)
Phospherene (Black Phosphorus)
Tin Sulfide (SnS2)
Rhenium Disulfide (ReS2) and Rhenium Diselenide (ReSe2)
Metal Oxide
Applications of Gas Sensors
Safety Applications
Indoor Air Quality
Medical Applications
Industrial Applications
Agriculture Applications
Aerospace Applications
Transportation Applications
Food Industry
Other Applications
Environmental Monitoring
Conclusion and Future Perspective
References
81 Electrochemical Sensing and Biomedical Applications of Green Nanomaterials
Introduction
Approaches for the Synthesis of Green Nanomaterials
Various Biological Components in the Synthesis of Green Nanomaterials
Plant Extracts-Based Green Nanomaterials
Microorganism-Based Green Nanomaterials
Bacteria
Fungi
Yeasts
Algae
Mechanism of Green Synthesis
Applications of Green Nanomaterials in the Fabrication of Electrochemical Sensors
Green Nanomaterials in Biomedical Applications
Conclusions and Future Aspects
References
Part VI: Energy Applications
82 Metal- and Carbon-Based Nano-frameworks as Catalysts for Supercapacitance and Fuel Industry
Introduction
Clean Energy Production
Photocatalytic Splitting of Water
Materials for Water Splitting Applications
Covalent Organic Frameworks (COFs)
Metal-Organic Frameworks (MOFs)
Clean Energy Storage
Supercapacitors
Materials for Supercapacitor Applications
Covalent Organic Frameworks
Metal-Organic Frameworks
Mixed-Metal Metal-Organic Frameworks (M-MOFs)
Conclusion
References
83 Microencapsulated Phase Change Materials and Their Applications for Passive Cooling in Buildings
Introduction
Classification of Phase Change Materials
Organic Phase Change Materials
Inorganic Phase Change Materials
Eutectic Phase Change Materials
Comparison Between Different Phase Change Materials
Microencapsulated Phase Change Materials
Organic Shells
Inorganic Shells
Organic-Inorganic Hybrid Shells
Synthesis Techniques of Microencapsulated Phase Change Materials
Physical Methods
Spray Drying
Solvent Evaporation
Chemical Methods
In Situ Polymerization
Interfacial Polymerization
Suspension Polymerization
Emulsion Polymerization
Physical-Chemical Methods
Coacervation
Solgel Method
Conclusions and Future Scopes
References
84 Porous Nanomaterials for CO2 Remediation for a Sustainable Environment
Introduction
Sources of Carbondioxide (Azarkamand et al. 2020; Hermawan et al. 2015)
Greenhouse Gas Emissions
Industry
Heat and Electricity Production
Transportation
Agriculture, Forestry, and Other Land Use
Effects of Carbon Dioxide
Effects of Enhanced Greenhouse Effect
Effects on Human Health
Advance Diseases and Pandemics
Shortage of Food
Effects on Environment and Living Conditions
Flooding of Seaside Cities and Islands
Desertification of Fertile Land
Migration of Species
Influence on Agriculture
Destructive Cyclones
Thawing of Glaciers
Treatment Technologies
Carbon Capture and Storage (CCS) Techniques
Pre-combustion Carbon Dioxide Capture
Oxy-Fuel Combustion Technique
Post-combustion Carbon Dioxide Capture
Absorption Technique
Membrane Technique
Pressure Swing Adsorption (PSA) Technique
Mineral Carbonation Technique
Materials for CO2 Adsorption
Zeolites
Metal-Organic Frameworks (MOFs)
Porous Organic Polymers (POPs)
Covalent Organic Frameworks (COFs)
Adsorbents Based on Carbon
Carbon Nanotubes (CNTs)
Conclusions and Future Outlook
References
85 Nanocatalysts for Environmental Benign Biofuel Production
Introduction
Homogeneous Base Catalysts
Homogeneous Acid Catalysts
Heterogeneous Base Catalysts
Heterogeneous Acid Catalyst
Biocatalysts (Enzyme) for Transesterification
Nanocatalysts Used for Transesterification
Metal Oxide-Based Nanocatalysts
Magnetic Nanocatalysts
Layered Double Hydroxides Nanocatalysts or Nanohydrotalcites
Nanozeolites as Catalyst
Conclusion and Future Scope
References
86 Heterojunction Photocatalysts for Solar Energy Conversion
Introduction
Type II Heterojunction Photocatalysts
Oxide-Based Heterojunction
Sulfide-Based Heterojunction
g-C3N4-Based Heterojunction
Others
The Disadvantages of Type II Heterojunction
Z-Scheme Heterojunction Photocatalysts
Oxide-Based Heterojunction
Sulfide-Based Heterojunction
g-C3N4-Based Heterojunction
Others
The Disadvantages of Z-Scheme Heterojunction
S-Scheme Heterojunction Photocatalysts
Oxide-Based Heterojunction
Sulfide-Based Heterojunction
g-C3N4-Based Heterojunction
Others
The Disadvantages of S-scheme Heterojunction
Tandem Heterojunction Photocatalysts
Oxide-Based Heterojunction
Sulfide-Based Heterojunction
g-C3N4-Based Heterojunction
Others
The Disadvantages of Tandem Heterojunction
Conclusion and Perspective
References
87 Pool Boiling Heat Transfer Enhancement Using Nanoparticle Coating on Copper Substrate
Introduction
Pool Boiling Heat Transfer
Overview
Pool Boiling Curve
Critical Heat Flux and Heat Transfer Coefficient
Formula to Calculate Critical Heat Flux (q) and Heat Transfer Coefficient (h)
Porosity and Wettability
Surface Coating for Pool Boiling Heat Transfer Enhancement
Metal Oxide Nanoparticle Coating
Nano-porous Metal Coating
Wettability: Hydrophilic and Hydrophobic Coatings
Carbon-Based Nanoparticle Coating
Graphene Coating
Carbon Nanotube (CNT) and Multiwalled Carbon Nanotube (MWCNT) Coatings
Nanowire Coating
Microscale Surface Modification
Microporous Metallic Coating
Microstructured (Microgrooves/Micro-Fin) Surface
Flow Boiling
Overview
Surface Coating for Flow Boiling Heat Transfer Enhancement
Conclusion
References
88 Sustainable Clean Energy Production from the Bio-electrochemical Process Using Cathode as Nanocatalyst
Introduction
Necessity for Clean Energy
Energy and Environment
Economy
World Renewable Energy Prospects
Renewable Energy Production Technologies
Solar Energy
Wind Energy
Hydropower
Geothermal Energy
Electrochemical Energy
Photo-electrochemical Energy
Bioenergy
Employment of Nanotechnology for Engendering Clean and Sustainable Energy
Use of Nanofluids in Energy Applications
Thermal Energy Storage (TES)
Nanocrystals
Applications of Nanomaterials in the Clean Energy Generation from Waste and Wastewater
Use of Nanomaterials in the Feedstock Pretreatment for the Aerobic Digestion
Nanomaterials with Algae Membrane-Based Bioreactor
Use of Nanotechnology in the Bio-electrochemical System for the Hydrogen Production
Conclusion
References
89 Renewable Resource-Based Green Nanomaterials for Supercapacitor Applications
Introduction
Nanomaterials for Supercapacitor Applications
Traditional Methods and Feedstocks
Sustainable Platform for Nanomaterials
Biosynthesized Metal and Metal-Based Nanomaterials
Metal and Metal Chalcogenides
Metal Oxide Nanostructures
Biomass-Derived Carbon Nanostructures
Zero-Dimensional
One-Dimensional
Two-Dimensional
Three-Dimensional
Nanocomposites Fabricated Using Renewable Resources
Metal-, Metal Hydroxide-, and Metal Oxide-Based Nanocomposites
Renewable Carbon-Based Nanocomposites
Cellulose Nanofiber-Based Nanocomposites
Future Directions
Conclusions
References
90 An Investigation on Mechanical Characteristics of Carbon Nanomaterials Used in Cementitious Composites
Introduction
Nanomaterials Typically Used in Cement-Based Materials
Carbon-Based Nanomaterials in Cementitious Composites
Graphene
Graphene Oxide
Mechanical Strength
Reduced Graphene Oxide
Mechanical Strength
Graphene Nanoplatelets
Mechanical Strength
Carbon Nanotubes
Mechanical Strength
Carbon Nanofibers
Mechanical Strength
Effects of CNMs as Cement Nanoreinforcement
Conclusions and Future Scope
References
91 Zinc Batteries: Basics, Materials Functions, and Applications
Introduction
Operation of Zinc Battery
Zinc-Ion Battery
Zinc-Air Battery
Aqueous Zinc Battery
Performance Controlling Factors
Zinc-Ion Battery
Zinc-Air Battery
Advantages of Zinc-Based Batteries
Challenges Associated with Zinc-Based Batteries
Zinc-Ion Battery System
Low Conductivity
Reversible Stripping
Irreversible Consumption and Corrosion of Zinc
Zinc-Air Battery System
Limited Discharge Capacity of Zinc Anode
Formation of Carbonates
Water Loss
Failure of Zinc Electrode
Electrode Materials for Zinc-Based Batteries
Anode Materials
Cathode Materials
Manganese (Mn) Oxide-Based Cathodes
Tunnel Structured MnO2
Spinel Structured MnO2 (λ-MnO2)
Layered Structured MnO2 (δ-MnO2)
Vanadium Oxide-Based Cathodes
Prussian Blue Analogs (PBAs)
Applications of Zinc-Based Batteries
Conclusion and Future Perspectives
References
Part VII: Industrial Applications
92 Importance of Waste to Wealth and Renewable Energy Toward Sustainable Development
Introduction
Literature Review and Discussion
Bioenergy
Wind Energy
Solar Energy
Issues, Recommendations, and Conclusion
References
93 Nanotechnology for Green and Clean Technology: Recent Developments
Introduction: Nanotechnology- A Door to Revolution
Milestones of Nanotechnology
Synthesis of Nanoparticles
Silver Nanoparticles
Physical Approach
Chemical Approach
Bio-stimulated Nanoparticles
Novel Techniques of Synthesis of Nanoparticles Using Green Technology
Microwave-Assisted Irradiations
Sonochemical-Induced Reduction
Benefits of Green Nanotechnology
Complications to Green Nanotechnology Adoption
Conclusion
References
94 Nanobiochar-Based Formulations for Sustained Release of Agrochemicals in Precision Agriculture Practices
Introduction
Biochar-Based Nanoformulations
Synthesis of Biochar-Based Nanoformulation
Characteristics of Biochar-Based Nanoformulation
Characterization of Biochar-Based Nanoformulations
Nanobiochar as Carrier for Sustained Delivery of Agrochemicals
Neat nanobiochar versus engineered nanobiochar
Biochar-Based Nanocomposites
Nanobiochar as Slow-Release Fertilizer
Other Applications of Biochar-Based Nanoformulation
Biocompatibility of Biochar-Based Nanoformulation
Other Applications of Nanoformulations
Plant Growth Improvement
Heavy Metal Immobilization in Soil
Soil Remediation
Future Scope
Conclusion
References
95 Nitrate and Phosphate Recovery from Contaminated Waters Using Nanocellulose and Its Composites
Introduction
Types of Nanocellulose
Nanocrystalline Cellulose
Nanofibrillated Cellulose
Bacterial Nanocellulose
Preparation of Nanocellulose
Chemical Methods
Physical Methods
Biological Method
Removal of Nitrate and Phosphate by Nanocellulose and Its Composites
Mechanism of Removal
Application of phosphorous (P)/nitrogen (N)-Laden Nanocellulose as Slow Release Fertilizer
Conclusion
References
96 Mediation of Nanotechnology and Biotechnology: An Emerging Pathway for the Treatment of Environmental Pollution
Introduction to Environmental Pollution and Its Key Negative Impacts
Water Pollution: A Major Culprit for the Deoxygenation of Water Bodies
Key Functions of Nanotechnology in Treating Water and Wastewater Pollution
Photocatalytic Degradation and Treatment of Toxic Water and Wastewater
Mechanism of Photocatalysis
Photocatalytic Degradation and Rapid Treatment of Wastewater
Nanoadsorbents and Membranes for the Treatment of Wastewater
Optimum Microbial Diversity in Water and Wastewater Treatment Processes
Role of Biotechnology in Water and Wastewater Treatment
Nitrification and Denitrification: A Definite Biotechnology for WWTPs
Moving Bed Biofilm Reactors: An Emerging Biotechnology for WWTPs
Anaerobic Ammonium Oxidation (Anammox): A Great Innovation for WWTPs
Partial Nitrification-Anammox Process: Technology for Nitrogenous Pollution
Design and Development of Nanobiotechnology for Wastewater Treatment
Conclusions and Future Perspectives
References
97 Application of Nanomaterials for Precious Metals Recovery
Introduction
Types of Nanomaterials
Metallic Nanomaterials
Nonmetallic Nanomaterials
Silica-Based Nanomaterials
Carbon-Based Nanomaterials
Organic Nanomaterials
Metal Organic Frameworks
Recovery of Precious Metals
Methods
Mechanisms
Physical Adsorption
Chemical Adsorption
Gold
Silver
Platinum Group Metals
Reusability of Nanomaterials
Conclusion and Future Outlook
References
Part VIII: Safety, Toxicity, and Legal Aspects of Nanomaterials
98 Toxicity Aspects of Nanomaterials
Introduction
Environmental Applications of Green Nanomaterials
Interaction of NPs with Plants
Use of Nanomaterials in the Remediation of Water
Use of Nanomaterials in the Detection of Environmental Pollution
Safety and Health Applications of Green Nanomaterials
Antimicrobial Activity of Nanomaterials
Wound-Healing Effects of Nanomaterials
Absorption and Toxicity of Nanomaterials
Ecotoxicity of Nanomaterials
Mammalian Toxicity of Nanomaterials
Conclusion and Future Scope
References
99 Health Issues and Risk Assessment of Nanomaterials
Introduction
Source of Contamination
Exposure in Air
Exposure in Water
Exposure in Food
Human Exposure Assessment
Respiratory Exposure
Gastrointestinal Exposure
Dermal Exposure
Bio-Uptake and Bioaccumulation in Humans
Physicochemical Properties Inducing Biological Activity
Human´s Biomarker
Nanomaterial Toxicity
Respiratory System Damage
Gastrointestinal System Damage
Skin Damage
Liver Damage
Heart Damage
Kidney Damage
Toxicity Testing Methods
In Vitro
In Vivo
Mechanisms of Toxicity
Factors Affecting Nanomaterial Toxicity
Strategies to Mitigate the Toxicity of Nanomaterials
Risk Assessment of Nanomaterial Toxicity
Risk-Assessing Parameters: Bio-Persistence and Bio-Durability
Mass Concentration
Appropriate Route of Exposure and Duration of Tests
Acute to Chronic Extrapolation and Partition Coefficient
Risk Characterization
Risk Management
Case Studies
Health Surveillance
Current Status of Regulatory Guidelines
Conclusions and Future Scope
References
100 Toxicological Effects of Nanomaterials in Terrestrial and Aquatic Insects
Introduction
Environmental Behavior and Uptake Pathway of Nanomaterials
Mode of Action of Nanomaterials
Conclusion
References
101 Environmental Risk Assessment of Plastics and Its Additives
Introduction
Plastic Additives
UV Stabilizers
Flame Retardants
Plasticizers
Dyes and Organic Pigments/Colorant
Leaching Process of Plastic Additives
Environment Risk Assessment of Plastics and Their Additives
Importance of Analysis of Plastic
Degradation of Plastics and Their Additives
Photooxidative Degradation
Ozone-Induced Degradation
Mechanochemical Degradation
Degradation of Plastics with Engineered Nanomaterials (ENMs)
Conclusion and Future Scope
References
102 Toxicity of Nanomaterials
Introduction
Classification of Nanomaterials
Synthetic Approaches
Physical Approaches
Lithography
Photolithography
Electron Beam Lithography
Scanning Probe Lithography
Nanoimprint Lithography
Nanoparticle Lithography
Colloidal Lithography
Vapour-Liquid-Solid (VLS) and Solution-Liquid-Solid (SLS)
Chemical Approaches
Green Synthesis Approaches
Plant-Based Nanomaterials
Microorganism-Based Nanomaterials
Bacteria-Based
Fungi-Based
Actinomycetes-Based
Yeast-Based
Algae-Based
Factors Affecting Nanoparticles Synthesis
Applications of Nanomaterials
Medicine
Agriculture
Sports
Toxicity of Nanomaterials in Organisms
Toxicity Assessment
Conclusion and Future Remarks
References
103 Impacts of Metal Nanoparticles on Fish
Introduction
Metal-Based Nanomaterials
Impacts of Metal Nanoparticles on Fish
Tissue Accumulation and Histopathology
Blood Biochemical Parameters
Antioxidant Enzymes
Impacts of Various NPs in Different Fish Species
Conclusions and Future Recommendations
References
104 Toxicological Evaluation of TiO2 Engineered Nanoparticles in Soil Invertebrates: A Cue for Revisiting Standard Toxicity Te...
Introduction
Detection and Characterization of TiO2 Nanoparticles in the Environment
Toxicity Assessment of Engineered Nanoparticles
Toxicity Evaluation of TiO2 Nanoparticles in Invertebrates as Sentinels/Indicators
The Earthworm as a Sentinel for TiO2 Ecotoxicity Assessment
Conclusion
References
105 Toxicity, Legal, and Health Aspects of Nanomaterials
Introduction
Nanomaterials
Historical Aspects
Classification of the Nanomaterials
Dimensions-Based Classification
Zero-Dimensional Nanomaterials
One-Dimensional Nanomaterials
Two-Dimensional Nanomaterials
Three-Dimensional Nanomaterials
Classification on the Basis of Composition
Inorganic-Based Nanomaterials
Ceramic Nanomaterials
Metallic Nanomaterials
Quantum Dots
Carbon-Based Nanomaterials
Graphene
Fullerenes/Buckyballs
Nanotubes
Nanowires
Nano-Cones
Organic-Based Nanomaterials
Dendrimers
Liposomes
Micelles
Composite-Based Nanomaterials
Properties of Nanomaterials
Physical Properties
Mechanical Properties
Optical Properties
Spectrum and Color of Nanomaterials
Chemical Properties
Synthesis of Nanomaterials
Top-Down Approach
Ball Milling
Planetary Milling
Attrition Milling
Horizontal Milling
Rotatory Milling
Thermal Evaporation
Laser Ablation
Sputtering
Chemical Vapor Deposition
Bottom-Up Approach
Hydrothermal Method
Coprecipitation Method
Sol-Gel Method
Hydrolysis
Condensation
Growth and Agglomeration
Microemulsion
Green Synthesis of Nanomaterials
Toxicity of Nanomaterials
In Vitro Toxicity of the Nanomaterials
In Vivo Toxicity of Nanomaterials
Cytotoxicity of the Nanomaterials
Neurotoxicity of the Nanomaterials
Hepatotoxicity of Nanomaterials
Cardiovascular Toxicity of the Nanomaterials
Nephrotoxicity of Nanomaterials
Genotoxicity of Nanomaterials
Dermal Toxicity of Nanomaterials
Legal Aspects of Nanomaterials (Rules and Regulation)
Health Aspects of Nanomaterials
Inorganic Nanoparticles
Gold Nanoparticles
Nonporous Silica Nanoparticles
Magnetic Nanoparticles
Nano-Patches
Organic Nanoparticles
Liposomes
Polymeric Nanoparticles
Dendrimers
Micelles
Detection of the Genetic Disorders
Future Aspects
Conclusion
References
Index

Citation preview

Uma Shanker Chaudhery Mustansar Hussain Manviri Rani Editors

Handbook of Green and Sustainable Nanotechnology Fundamentals, Developments and Applications

Handbook of Green and Sustainable Nanotechnology

Uma Shanker • Chaudhery Mustansar Hussain • Manviri Rani Editors

Handbook of Green and Sustainable Nanotechnology Fundamentals, Developments and Applications

With 667 Figures and 243 Tables

Editors Uma Shanker Department of Chemistry Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Punjab, India

Chaudhery Mustansar Hussain Chemistry and EVSC New Jersey Institute of Technology Newark, NJ, USA

Manviri Rani Malaviya National Institute of Technology Jaipur, Rajasthan, India

ISBN 978-3-031-16100-1 ISBN 978-3-031-16101-8 (eBook) https://doi.org/10.1007/978-3-031-16101-8 © Springer Nature Switzerland AG 2023 The translation was done with the help of an artificial intelligence machine translation tool. A subsequent human revision was done primarily in terms of content. This work is subject to copyright. All rights are solely and exclusively licensed 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 have shown their excellent potential for numerous applications due to their enhanced properties. Numerous types of nanomaterials have been synthesized via different approaches like top down and bottom up, that includes sol-gel, chemical vapor deposition, ball milling, micro-emulsion, hydrothermal, and many more. Very soon, it was realized that such methods are utilizing elevated energy physical procedures, harmful regent, and hazardous solvents, which might be causing environmental concern. Moreover, the health risks of exposure to nanoparticles are probably poorly understood and that requires to be addressed seriously. Their fabrication and usage are practically unregulated around the globe. However, the economic and environmental sustainability of green solutions involving nanotechnology is unclear in many cases and some novel solutions bring with them environmental, health, and safety (EHS) risks (e.g., high energy manufacturing processes and processes which may rely on toxic materials). In view of this, the involvement of green chemistry principles into this fast developing field of nanomaterials fabrication is the need of hour. Green chemistry is an approach to chemical synthesis that considers life cycle factors such as waste, safety, energy use, and toxicity in the earliest stages of molecular design and production, in order to mitigate environmental impacts and enhance the safety and efficiency associated with chemical production, use, and disposal. It takes a lifecycle approach to minimizing undesirable impacts that can be associated with chemicals and their production. In this regard, green nanotechnology is indeed a multidisciplinary field that has come into sight as a swiftly developing research area serving as a significant technique that is emphasizing on making the procedure which are clean, safe, and, in particular, environmental friendly, in gap with currently employed methods such as chemical and physical methods for nanosynthesis. These risks must be mitigated in advancing green nanotechnology solutions. Green nanotechnology – objectives for products and processes which are environmental friendly, economically sustainable, safe, energy-efficient, decrease waste, and diminish greenhouse gas emissions. Such products and processes are based on renewable materials and/or have a low net impact on the environment. Green nanotechnology is increasingly being referred in connection with green chemistry and sustainable engineering or manufacturing. This book will showcase recent trends for production, fabrication, and characterization of various nanomaterials; their multiple roles and v

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impacts across the whole value chain of a product. Efforts to ensure the responsible development of nanotechnology is being made through a growing number of national and international initiatives looking at environmental health and safety (EHS) and ethical and social issues. Sustainable materials with characteristics of multiple reusability and low cost have been comprehensively presented. The updates offered in this book will beneficial for large scale productions of nanomaterial’s. Moreover, green approaches for nanomaterials synthesis will have advantages over conventional techniques in terms of biocompatibility, safer use, and sustainability. The aim of green manufacturing with newly developed industrial processes and materials will replace the hazardous processes and constituents causing waste generation. Detailed information about industrial viability of nanomaterials has also been offered. Environmental concerns with nanotechnology with their safety concerns are also an interesting addition in this book. This book is also offering many biomedical applications of green and sustainable nanomaterials. In the recent past, several water contaminants are causing serious threat to the environment, and the response of nanomaterials in mitigating such pollutants has been comprehensively discussed in this book. Eradication of such contaminants utilizing recent technology based on photocatalysis is one of the best parts of the book. This section offers recent updates on several photocatalysts utilized for the removal of pollutants. Some chapters offer quality information about coatings as well as sensing applications of green and sustainable nanomaterials. Comprehensive details about energybased applications of advanced nanomaterials have been provided. Finally, the book concludes with discussing toxicity aspects of nanomaterials along with regulations and laws related to handling of nanomaterials. On behalf of Springer, we are very grateful to the authors of all chapters for their outstanding and passionate efforts in the making of this handbook. Special thanks to Miss Anita Lekhwani (Senior Editor) and Miss Swetha Varadharajan (Ms.), Project Coordinator (Books), and the Springer Reference Editorial Team for their enthusiastic support and help during this project. In the end, all thanks to Springer for publishing the handbook. Jalandhar, India Newark, USA Jaipur, India March 2023

Uma Shanker Chaudhery Mustansar Hussain Manviri Rani

Acknowledgments

A journey is easier when you travel together. Interdependence is certainly more valuable than independence. The Handbook of Green and Sustainable Nanotechnology, compromising more than 100 chapters, is based on updates on research on sustainable and green technologies. During this book, I have been accompanied and supported by many people. It is a pleasant aspect that now I have the opportunity to express my gratitude to all of them. At the outset, I express my deepest gratitude and reverence to my teachers, Prof. Bina Gupta, Prof. S. N. Tandon, Prof. Kamaluddin, and Prof. S. M. Sondhi. I thank my fellow editors for supporting me while making the book proposal, inviting authors, and evaluating chapters. I thank the editorial and publishing team of Springer for their constant support. I thank all the authors for contributing quality chapters to the book. I want to thank my PhD students, Jyoti Yadav, Keshu, Meenu, Sudha Choudhary, Vipin, Rishabh, and Gauri Shukla, for their generous support and kind cooperation. The book could not have been completed without the endless love and blessings from my parents, who, apart from providing me with the best education, have also encouraged me in all my endeavors. Finally, and most importantly, I will dedicate this book to Kanha Ji, Vaishnavi (my daughter), and God, the almighty, for it is under his grace that we live, learn, and flourish. Uma Shanker Manviri Rani

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Contents

Volume 1 Part I 1

2

3

4

5

Synthesis of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomimetic Route-Assisted Synthesis of Nanomaterials: Characterizations and Their Applications . . . . . . . . . . . . . . . . . . Vinars Dawane, Satish Piplode, Man Mohan Prakash, and Bhawana Pathak Fabrication of Green Nanomaterials: Biomedical Applications and Ecotoxicology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Velaphi C. Thipe, Lucas F. Freitas, Caroline S. A. Lima, Jorge G. S. Batista, Aryel H. Ferreira, Justine P. Ramos de Oliveira, Tatiana S. Balogh, Slawomir Kadlubowski, Ademar B. Lugão, and Kattesh V. Katti Green Synthesis of Metallic Nanoparticles and Their Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atef A. Hassan, Rasha M. H. Sayed-ElAhl, Ahmed M. El Hamaky, Mogda K. Mansour, Noha H. Oraby, and Mahmoud H. Barakat Chitosan: Postharvest Ecofriendly Nanotechnology, Control of Decay, and Quality in Tropical and Subtropical Fruits . . . . . . Ramsés Ramón González-Estrada, Francisco Javier Blancas-Benitez, Francisco Javier Hernández-Béjar, Tomás Rivas-García, Cristina Moreno-Hernández, Lizet Aguirre-Güitrón, Surelys Ramos-Bell, and Porfirio Gutierrez-Martinez Green Synthesis and Applications of Metal-Organic Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arushi Gupta, Shalini Singh, Amit L. Sharma, and Akash Deep

1

3

23

47

73

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6

7

8

9

10

11

12

13

14

Nanotechnology for the Obtention of Natural Origin Materials and Environmentally Friendly Synthesis Applied to Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noelia L. D’Elía, Javier Sartuqui, Pablo D. Postemsky, and Paula V. Messina

111

Green Synthesis of Curcuminoid Nanostructure for White Light Emission Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Al Shafouri and Naser M. Ahmed

141

Green Synthesis of Zinc Oxide Nanoparticles Using Salvia officinalis Extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adnan. H. Alrajhi and Naser M. Ahmed

163

Top-Down Production of Nanocellulose from Environmentally Friendly Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanci Ehman, María Evangelina Vallejos, and María Cristina Area

185

Green Synthesis of Metal Oxide Nanoparticles and Gamma Rays for Water Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cristina A. DeLeón-Condés, Gonzalo Martínez-Barrera, Gabriela Roa-Morales, Patricia Balderas-Hernández, and Fernando Ureña-Núñez Recent Progress on Doped ZnO Nanostructures and Its Photocatalytic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samriti, Ashish Upadhyay, Rajeev Gupta, Olim Ruzimuradov, and Jai Prakash Role of Green Nanomaterials for 3-Chloropropane-1,2-diol Ester (3-MCPDE) Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sharifah Shahira Syed Putra, Wan Jefrey Basirun, Adeeb Hayyan, and Amal A. M. Elgharbawy

203

221

251

Fabrications from Renewable Sources and Agricultural Wastes and Characterization Strategies of Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Krutarth H. Pandit, Pranit B. Patil, Abhijeet D. Goswami, and Dipak V. Pinjari

271

Algal Extract-Biosynthesized Silver Nanoparticles: Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vinita Khandegar and Perminder Jit Kaur

287

15

Green Synthesis of Metal Oxide Nanoparticles Sharmi Ganguly and Joydip Sengupta

..............

303

16

Green Synthesis of Carbon Dot-Based Materials for Toxic Metal Detection and Environmental Remediation . . . . . . . . . . . . Samarjit Pattnayak, Ugrabadi Sahoo, and Garudadhwaj Hota

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17

18

19

20

21

22

23

24

25

xi

Green Synthesis of Metal Oxide Nanomaterials and Photocatalytic Degradation of Toxic Dyes . . . . . . . . . . . . . . . . . . Baishali Bhattacharjee and Md. Ahmaruzzaman Green Synthesis of Hybrid Nanostructure for Wastewater Remediation by Photocatalytic Degradation . . . . . . . . . . . . . . . . Shubhalaxmi Choudhury, Pragnyashree Aparajita, and Garudadhwaj Hota Natural Polymer-Based Nanocomposite Hydrogels as Environmental Remediation Devices . . . . . . . . . . . . . . . . . . . . . . Sapna Sethi, Anjali Singh, Medha, Swati Thakur, Balbir Singh Kaith, and Sadhika Khullar

355

377

407

Biogenic Metallic Nanoparticles: Synthesis and Applications Using Medicinal Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amanpreet Kaur, Himanshu Gupta, and Soniya Dhiman

443

Synthetic Nanoparticle-Based Remediation of Soils Contaminated with Polycyclic Aromatic Hydrocarbons Himanshu Gupta and Soniya Dhiman

467

.......

Multifunctional Nanoprobes for the Surveillance of Amyloid Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thanojan Jeyachandran, Suraj Loomba, Asma Khalid, and Nasir Mahmood Generation of Nanoparticles from Waste via Solvent Extraction Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rashmi Singh Plant-Mediated Synthesis of Nanoscale Hydroxyapatite: Morphology Variability and Biomedical Applications . . . . . . . . . Ana Paula Fagundes, Afonso Henrique da Silva Júnior, Domingos Lusitâneo Pier Macuvele, Humberto Gracher Riella, Natan Padoin, and Cíntia Soares Green and Sustainable Technology for Clean Energy Production: Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Beer Pal Singh, Kavita Sharma, Shrestha Tyagi, Durvesh Gautam, Manika Chaudhary, Ashwani Kumar, Sagar Vikal, and Yogendra K. Gautam

26

Generation of Nanomaterials from Wastes . . . . . . . . . . . . . . . . . . Manviri Rani, Meera, and Uma Shanker

27

Photoelectrochemical CO2 Reduction: Perspective and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pankaj Kumar Singh, Ravinder Kaushik, and Aditi Halder

489

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537

563

587

613

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Contents

Nanomaterial’s Safety Regulations in Food and Drug Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Md Abdus Subhan and Tahrima Subhan

641

Volume 2 Part II 29

Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . .

Role of Engineered Nanomaterials for Eradication of Endocrine Disrupting Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . Manviri Rani, Keshu, and Uma Shanker

659

661

30

Technologies for Treatment of Emerging Contaminants . . . . . . . Berileena Hazarika and Md. Ahmaruzzaman

31

Photocatalytic and Adsorptive Remediation of Hazardous Organic Chemical Pollutants from Waste Water . . . . . . . . . . . . . Manviri Rani, Sudha Choudhary, Jyoti Yadav, and Uma Shanker

703

Occurrence, Distribution, and Removal of Phthalates by Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meenu, Manviri Rani, and Uma Shanker

729

Functionalized Nanomaterials for Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pratibha, Atul Kapoor, and Jaspreet Kaur Rajput

763

32

33

34

Photocatalytic Degradation of Drugs . . . . . . . . . . . . . . . . . . . . . . Babita Kaushik, Gyaneshwar Rao, and Dipti Vaya

35

Environmental Remediation Through Metal Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ruchi Gaur, Parashuram Kallem, Dipankar Sutradhar, and Fawzi Banat

36

37

38

681

797

827

Implications of Green Nanomaterials for Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Luqmon Azeez, Idris Adekale, and Olalekan A. Olabode

863

Eradication of Emerging Contaminants like Brominated Flame Retardants by Green Nanomaterials . . . . . . . . . . . . . . . . . Manviri Rani, Vikas Sharma, and Uma Shanker

881

Nanocatalysts for Advanced Oxidation Processes in Heterogeneous Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kevin Jhon Fernández-Andrade, Alex Ariel Fernández-Andrade, Braulio Agusto Ávila-Toro, Luis Ángel Zambrano-Intriago, Ricardo José Baquerizo-Crespo, and Joan Manuel Rodríguez-Díaz

913

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39

Sustainable Nanomaterials for Environmental Remediation . . . . Kavita Sharma, Shrestha Tyagi, Sagar Vikal, Arti Devi, Yogendra K. Gautam, and Beer Pal Singh

40

Green Nanomaterials as Photocatalyst/Catalyst: Exploration of Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hossein Bazgir, Zahra Issaabadi, and Hassan Arabi

xiii

933

973

41

Photocatalytic Properties of Metal-Based Nanoparticles . . . . . . . 1005 Sona Ayadi Hassan and Parinaz Ghadam

42

Green Nanomaterials for Environmental Remediation . . . . . . . . 1031 Patrícia Prediger, Tauany de Figueiredo Neves, Natália Gabriele Camparotto, and Everton Augusto Rodrigues

43

Sustainable Doped Nanomaterials for Environmental Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065 Uma Shanker, Vipin, and Manviri Rani

44

Graphene-Based Nanomaterials for Water Remediation Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1097 Alvin Lim Teik Zheng, Che Azurahanim Che Abdullah, and Yoshito Andou

45

Metal Oxide-Based Nanocomposites for Elimination of Hazardous Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1123 Manviri Rani, Sudha Choudhary, Jyoti Yadav, Keshu, and Uma Shanker

46

Nanophotocatalysis for Degradation of Organic Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1149 Florencia San Roman Napoli, Damián Uriarte, Mariano Garrido, Claudia Domini, and Carolina Acebal

47

Remediation of Chromium Heavy Metal Ion by Green Synthesized Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1193 Manviri Rani and Uma Shanker

48

Abatement of PAHs by Engineered Nanomaterials . . . . . . . . . . . 1223 Manviri Rani and Uma Shanker

49

Quantum Dots: Applications in Environmental Remediation . . . 1245 Manviri Rani, Jyoti Yadav, and Uma Shanker

50

Removal of Organic Dyes by Functionalized Nanomaterials . . . . 1267 Manviri Rani and Uma Shanker

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51

Synthesis of Silver Nanoparticles with Environmental Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1299 Jyoti Yadav, Manviri Rani, and Uma Shanker

52

Polymer Nanocomposites in Wastewater Treatment Ruksana Sirach and Pragnesh N. Dave

53

Zinc-Based Nanomaterials for Sustainable Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355 Manviri Rani and Uma Shanker

54

Applications of Green Nanomaterials in Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1375 N. B. Singh, Anindita De, Mridula Guin, and Richa Tomar

55

Green Nanomaterials for Remediation of Environmental Air Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 Kavita Sharma, Shrestha Tyagi, Sagar Vikal, Arti Devi, Yogendra K. Gautam, and Beer Pal Singh

56

Polymer-Based Nanocomposites for the Removal of Personal Care Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1437 Manviri Rani, Jyoti Yadav, and Uma Shanker

57

Metal Oxides–Based Nanomaterials: Green Synthesis Methodologies and Sustainable Environmental Applications Uma Shanker, Vipin, and Manviri Rani

. . . . . . . . . . 1323

. . . 1459

58

Environmental Occurrence and Degradation of Hexabromocyclododecanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1487 Manviri Rani, Meenu, and Uma Shanker

59

Sustainable Green-Doped Nanomaterials for Emerging Contaminants Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1511 Manviri Rani, Ankit, Jyoti Yadav, and Uma Shanker

60

Nanotechnology in Veterinary Sector . . . . . . . . . . . . . . . . . . . . . . 1541 P. Ravi Kanth Reddy, D. Yasaswini, P. Pandu Ranga Reddy, D. Srinivasa Kumar, Mona M. M. Y. Elghandour, and A. Z. M. Salem

61

Consumer Nanoproducts for the Remediation of Environmental Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1569 Vaneet Kumar, Saruchi, H. Kumar, and Diksha Bhatt

62

Properties of Green Nanomaterials as Catalysts and Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1587 Jinu Mathew and Sanjay Pratihar

Contents

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Volume 3 Part III

Coatings Applications

.............................

1603

63

Applications of Green Nanomaterials as Surfaces and Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1605 Sulaxna Sharma, A. Ansari, Kuldeep Kumar, Arvind Kumar, and Awanish Kumar Sharma

64

Green Coatings: Materials, Deposition Processes, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1627 M. Federica De Riccardis

65

Green Nanomaterials as Surfaces and Coatings . . . . . . . . . . . . . . 1655 Pranit B. Patil, Chandrakant R. Holkar, and Dipak V. Pinjari

Part IV

Biomedical and Biological Applications . . . . . . . . . . . . . . .

1675

. . . . . . . . . . . . 1677

66

Biomedical Applications of Green Nanomaterials Parteek Prasher and Mousmee Sharma

67

Biomedical Applications of Nanomaterials . . . . . . . . . . . . . . . . . . 1699 Ashreen Norman, Emmellie Laura Albert, Dharshini Perumal, and Che Azurahanim Che Abdullah

68

Emerging Roles of Carbon Nanohorns as Sustainable Nanomaterials in Sensor, Catalyst, and Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1721 Jayamanti Pandit, Md. Sabir Alam, Md. Noushad Javed, Aafrin Waziri, and Syed Sarim Imam

69

Green Synthesis and Fabrication of Nanomaterials: Unique Scaffolds for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . 1749 Ankita Garg and Aman Bhalla

70

Natural Polymer-Based Nanocomposite Hydrogels for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1777 Sapna Sethi, Medha, Swati Thakur, Anjali Singh, and Balbir Singh Kaith

71

Application of Nanoclusters in Environmental and Biological Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1811 Dipankar Sutradhar, Sourav Roy, and Ruchi Gaur

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Contents

72

Amylose-Based Green Nanoparticles as Carriers in Drug Delivery and Controlled Release Applications . . . . . . . . . . . . . . . 1833 Andresa da Costa Ribeiro, Nádya Pesce da Silveira, and Luís Joaquim Pina da Fonseca

73

Biopolymeric Nanohydrogels as Devices for Controlled and Targeted Delivery of Drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1857 Sapna Sethi, Medha, Swati Thakur, Anjali Singh, Balbir Singh Kaith, and Sadhika Khullar

74

Advances in Medical Applications: The Quest of Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1889 Nor Azrini Nadiha Azmi and Amal A. M. Elgharbawy

75

Metrics for the Sustainability Analysis of Nano-synthesis in the Green Chemistry Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 1911 Manuel Palencia, Angelica García-Quintero, and Víctor J. Palencia Luna

76

Modulation of the Bioactivity of Inorganic Nanomaterials by Controlling Nanobiointerface . . . . . . . . . . . . . . . . . . . . . . . . . . 1937 Manuel Palencia, Jhoban Meneses Rengifo, and Tulio A. Lerma

77

Role of Nanomaterials in Combating COVID-19 . . . . . . . . . . . . . 1961 Manviri Rani, Keshu, and Uma Shanker

Part V

Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1983

78

Sustainable Nanotorus for Biosensing and Therapeutical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1985 Md. Farhan Naseh, Jamilur R. Ansari, Md. Sabir Alam, and Md. Noushad Javed

79

Nanogels and Nanocomposite Hydrogels for Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2007 Sapna Sethi, Medha, Swati Thakur, Anjali Singh, and Balbir Singh Kaith

80

Two-Dimensional (2D) Nanostructures for Hazardous Gas Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033 Vinay Kumar, Arvind Kumar, Priyanka, Smriti Sihag, and Anushree Jatrana

81

Electrochemical Sensing and Biomedical Applications of Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2055 Ankit Kumar Singh, Ravindra Kumar Gautam, Shreanshi Agrahari, and Ida Tiwari

Contents

xvii

Volume 4 Part VI

Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2079

82

Metal- and Carbon-Based Nano-frameworks as Catalysts for Supercapacitance and Fuel Industry . . . . . . . . . . . . . . . . . . . . . . 2081 Ritika Jaryal, Rakesh Kumar, and Sadhika Khullar

83

Microencapsulated Phase Change Materials and Their Applications for Passive Cooling in Buildings . . . . . . . . . . . . . . . 2111 Kwok Wei Shah, Teng Xiong, and Charry Chang Yuan Jin

84

Porous Nanomaterials for CO2 Remediation for a Sustainable Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2133 Sakshi and Sadhika Khullar

85

Nanocatalysts for Environmental Benign Biofuel Production . . . 2161 Subhalaxmi Pradhan, Chandreyee Saha, Soumya Parida, and Sushma

86

Heterojunction Photocatalysts for Solar Energy Conversion . . . . 2181 Zhenzi Li, Decai Yang, and Wei Zhou

87

Pool Boiling Heat Transfer Enhancement Using Nanoparticle Coating on Copper Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2223 Sudhir Kumar Singh and Deepak Sharma

88

Sustainable Clean Energy Production from the Bio-electrochemical Process Using Cathode as Nanocatalyst . . . . 2247 Himanshu Kachroo, A. K. Chaurasia, Shailesh Kumar Chaurasia, and Vinod Kumar Yadav

89

Renewable Resource-Based Green Nanomaterials for Supercapacitor Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2277 Sivashunmugam Sankaranarayanan, Maria Michael Christy Priya, Dhileepan Priyadharshini, and Singaravelu Vivekanandhan

90

An Investigation on Mechanical Characteristics of Carbon Nanomaterials Used in Cementitious Composites . . . . . . . . . . . . 2309 Kanchna Bhatrola, Sameer Kumar Maurya, Bharti Budhalakoti, and N. C. Kothiyal

91

Zinc Batteries: Basics, Materials Functions, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2331 Sharafadeen Gbadamasi, Suraj Loomba, Muhammad Waqas Khan, Babar Shabbir, and Nasir Mahmood

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Contents

Part VII

Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2369

92

Importance of Waste to Wealth and Renewable Energy Toward Sustainable Development . . . . . . . . . . . . . . . . . . . . . . . . . 2371 Arpita Ghosh

93

Nanotechnology for Green and Clean Technology: Recent Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2389 Surbhi Sharma, Vaneet Kumar, and Saruchi

94

Nanobiochar-Based Formulations for Sustained Release of Agrochemicals in Precision Agriculture Practices . . . . . . . . . . . . 2413 Mansi Sheokand, Karuna Jain, Vineeta Rana, Sarita Dhaka, Anuj Rana, Krishna Pal Singh, and Rahul Kumar Dhaka

95

Nitrate and Phosphate Recovery from Contaminated Waters Using Nanocellulose and Its Composites . . . . . . . . . . . . . . . . . . . . 2439 Pooja Rani, Sarita Dhaka, Sachin Kumar Godara, Krishna Pal Singh, Anuj Rana, and Rahul Kumar Dhaka

96

Mediation of Nanotechnology and Biotechnology: An Emerging Pathway for the Treatment of Environmental Pollution . . . . . . . 2457 Muhammad Ahmad, Maryam Yousaf, Ijaz Ahmad Bhatti, Wajiha Umer Farooq, Muhammad Mohsin, Abeer Mazher, and Nasir Mahmood

97

Application of Nanomaterials for Precious Metals Recovery . . . . 2501 James McNeice and Harshit Mahandra

Part VIII

Safety, Toxicity, and Legal Aspects of Nanomaterials . . .

2533

98

Toxicity Aspects of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . 2535 Balasubramanian Malaikozhundan, Jayaraj Vinodhini, Subramanian Palanisamy, and Natarajan Manivannan

99

Health Issues and Risk Assessment of Nanomaterials . . . . . . . . . 2553 Pramendra Kumar Saini, Nitish Kumar, Keshu, and Uma Shanker

100

Toxicological Effects of Nanomaterials in Terrestrial and Aquatic Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2581 Benay Tuncsoy and Mustafa Tuncsoy

101

Environmental Risk Assessment of Plastics and Its Additives . . . 2597 Manviri Rani, Meenu, and Uma Shanker

102

Toxicity of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2623 Nur Khalida Rahayu Zainon, Che Azurahanim Che Abdullah, and Nazzatush Shimar Jamaludin

Contents

xix

103

Impacts of Metal Nanoparticles on Fish . . . . . . . . . . . . . . . . . . . . 2645 Mustafa Tunçsoy

104

Toxicological Evaluation of TiO2 Engineered Nanoparticles in Soil Invertebrates: A Cue for Revisiting Standard Toxicity Testing for Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2663 B. Siva Prasad, J. Usha Rani, and P. Sankar Ganesh

105

Toxicity, Legal, and Health Aspects of Nanomaterials . . . . . . . . . 2685 Shubhangi Mishra, Vibhuti Sharma, and Reena Gupta

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2725

About the Editors

Uma Shanker, PhD, is Associate Professor in the Department of Chemistry, Dr. B R Ambedkar National Institute of Technology Jalandhar, Punjab, India. His research focuses on green synthesis of various types of nanomaterials and their applications in environmental remediation. He is interested in environmental nanotechnology, green chemistry, and environmental analytical chemistry.

Chaudhery Mustansar Hussain, PhD, is Adjunct Professor, Academic Advisor, and Lab Director in the Department of Chemistry and Environmental Sciences at the New Jersey Institute of Technology (NJIT), Newark, New Jersey, USA. His research is focused on application of nanotechnology and advanced materials, environment and analytical chemistry.

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About the Editors

Manviri Rani, PhD, is Assistant Professor in the Department of Chemistry, Malaviya National Institute of Technology Jaipur, Rajasthan, India. Dr. Rani completed her PhD from Indian Institute of Technology Roorkee, India. Her research focuses on environmental nanotechnology, green chemistry, and environmental analytical chemistry. Dr. Rani is an expert of analytical method developments for various persistent organic pollutants.

Contributors

Carolina Acebal INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Argentina Idris Adekale Department of Biochemistry, Osun State University, Osogbo, Nigeria Shreanshi Agrahari Department of Chemistry (Centre of Advanced Study), Institute of Science, Banaras Hindu University, Varanasi, India Lizet Aguirre-Güitrón Universidad Politecnica del Estado de Nayarit, Xalisco, Nayarit, Mexico Muhammad Ahmad Department of Structures and Environmental Engineering, University of Agriculture Faisalabad, Faisalabad, Pakistan Department of Environmental Engineering, Key Laboratory of Water and Sediment Sciences, Ministry of Education, Peking University, Beijing, China Md. Ahmaruzzaman Department of Chemistry, National Institute of Technology, Silchar, Assam, India Naser M. Ahmed School of Physics, Universiti Sains Malaysia, Penang, Malaysia Md. Sabir Alam SGT College of Pharmacy, SGT University, Gurugram, Haryana, India Emmellie Laura Albert Biophysics Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Nanomaterial Synthesis and Characterization Laboratory, Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Adnan. H. Alrajhi School of Physics, Universiti Sains Malaysia, Penang, Malaysia Yoshito Andou Department of Life Science and Systems Engineering, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Fukuoka, Japan Collaborative Research Centre for Green Materials on Environmental Technology, Kyushu Institute of Technology, Fukuoka, Japan xxiii

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Contributors

Ankit Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India A. Ansari Department of Physics, Graphic Era Deemed to Be University, Dehradun, India Jamilur R. Ansari University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi, India Department of Applied Science & Humanities, Dronacharya College of Engineering, Gurugram, Haryana, India Anushree Jatrana Department of Chemistry, COBS&H, CCS Haryana Agricultural University, Hisar, India Pragnyashree Aparajita Department of Chemistry, NIT Rourkela, Odisha, India Hassan Arabi Department of Polymerization Engineering, Iran Polymer and Petrochemical Institute (IPPI), Tehran, Iran María Cristina Area Programa de Celulosa y Papel (PROCYP), IMAM, UNaM, CONICET, FCEQYN, Posadas, Argentina Braulio Agusto Ávila-Toro Departamento de Procesos Químicos, Facultad de Ciencias Matemáticas, Físicas y Químicas, Universidad Técnica de Manabí, Portoviejo, Ecuador Luqmon Azeez Department of Pure and Applied Chemistry, Osun State University, Osogbo, Nigeria Nor Azrini Nadiha Azmi International Institute for Halal Research and Training (INHART), International Islamic University Malaysia, Kuala Lumpur, Malaysia Patricia Balderas-Hernández Centro Conjunto de Investigación en Química Sustentable, Universidad Autónoma del Estado de México – Universidad Nacional Autónoma de México (UAEM-UNAM), Toluca, Atlacomulco, Mexico Tatiana S. Balogh Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil Fawzi Banat Center for Membranes and Advanced Water Technology (CMAT), Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates Ricardo José Baquerizo-Crespo Departamento de Procesos Químicos, Facultad de Ciencias Matemáticas, Físicas y Químicas, Universidad Técnica de Manabí, Portoviejo, Ecuador Mahmoud H. Barakat Faculty of Medicine, Cairo University, Cairo, Egypt Wan Jefrey Basirun Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia

Contributors

xxv

Jorge G. S. Batista Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil Hossein Bazgir Department of Polymerization Engineering, Iran Polymer and Petrochemical Institute (IPPI), Tehran, Iran Aman Bhalla Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh, India Kanchna Bhatrola Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India Diksha Bhatt School of Natural Science, CT University, Ludhiana, Punjab, India Baishali Bhattacharjee Department of Chemistry, National Institute of Technology, Silchar, Assam, India Ijaz Ahmad Bhatti Department of Chemistry, University of Agriculture Faisalabad, Faisalabad, Pakistan Francisco Javier Blancas-Benitez Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico Bharti Budhalakoti Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India Natália Gabriele Camparotto School of Technology, University of Campinas – UNICAMP, Limeira, São Paulo, Brazil Manika Chaudhary Department of Physics, Ch. Charan Singh University Meerut, Meerut, India A. K. Chaurasia Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India Shailesh Kumar Chaurasia Department of Mechanical Engineering, I.E.T, MJ P R University, Bareilly, Uttar Pradesh, India Che Azurahanim Che Abdullah Biophysics Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Nanomaterial Synthesis and Characterization Laboratory, Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia UPM-MAKNA Cancer Research Laboratory, Institute of Bioscience, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Material Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Sudha Choudhary Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India Shubhalaxmi Choudhury Department of Chemistry, NIT Rourkela, Odisha, India

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Contributors

Noelia L. D’Elía Department of Chemistry, Universidad Nacional del Sur, INQUISUR-CONICET, Bahía Blanca, Argentina Andresa da Costa Ribeiro Department of Physics, Universidade Estadual do Centro Oeste – Unicentro, Guarapuava, PR, Brazil Luís Joaquim Pina da Fonseca Institute for Bioengineering and Biosciences, Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal Afonso Henrique da Silva Júnior Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Nádya Pesce da Silveira Chemistry Institute, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil Pragnesh N. Dave Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India Vinars Dawane School of Environment and Sustainable Development, Central University of Gujarat, Gandhinagar, Gujarat, India Anindita De Department of Chemistry and Biochemistry, Sharda University, Greater Noida, India Tauany de Figueiredo Neves School of Technology, University of Campinas – UNICAMP, Limeira, São Paulo, Brazil M. Federica De Riccardis ENEA- Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Territorial and Production Systems Sustainability Department, Research Center of Brindisi, Brindisi, Italy Akash Deep CSIR-Central Scientific Instrument Organisation (CSIR-CSIO), Chandigarh, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India Cristina A. DeLeón-Condés División de Ingeniería Industrial, Tecnológico de Estudios Superiores de Tianguistenco, Santiago Tianguistenco, Mexico Arti Devi Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Rahul Kumar Dhaka Department of Chemistry, College of Basic Sciences & Humanities, Chaudhary Charan Singh Haryana Agricultural University, Hisar, India Centre for Bio-Nanotechnology, Chaudhary Charan Singh Haryana Agricultural University, Hisar, India Sarita Dhaka Department of Chemistry, Sanatan Dharm (PG) College (Muzaffarnagar), Maa Shakumbhari University, Saharanpur, Uttar Pradesh, India

Contributors

xxvii

Soniya Dhiman Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology Delhi, New Delhi, India Claudia Domini INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Argentina Nanci Ehman Programa de Celulosa y Papel (PROCYP), IMAM, UNaM, CONICET, FCEQYN, Posadas, Argentina Mona M. M. Y. Elghandour Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma del Estado de México, Toluca, Mexico Amal A. M. Elgharbawy International Institute for Halal Research and Training (INHART), International Islamic University Malaysia, Kuala Lumpur, Malaysia Bioenvironmental Engineering Research Centre (BERC), Department of Biotechnology Engineering, Faculty of Engineering, International Islamic University Malaysia (IIUM), Gombak, Kuala Lumpur, Malaysia Ana Paula Fagundes Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Wajiha Umer Farooq Department of Chemistry, University of Agriculture Faisalabad, Faisalabad, Pakistan Alex Ariel Fernández-Andrade Departamento de Procesos Químicos, Facultad de Ciencias Matemáticas, Físicas y Químicas, Universidad Técnica de Manabí, Portoviejo, Ecuador Kevin Jhon Fernández-Andrade Programa de Posgrado en Ingeniería Química, Facultad de Posgrado, Universidad Técnica de Manabí, Portoviejo, Ecuador Aryel H. Ferreira MackGraphe, São Paulo, Brazil Lucas F. Freitas Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil P. Sankar Ganesh Department of Biological Sciences, BITS Pilani, Hyderabad Campus, Hyderabad, Telangana, India Sharmi Ganguly Electronics and Communication Engineering, Meghnad Saha Institute of Technology, Kolkata, India Angelica García-Quintero Research Group in Science with Technological Applications (GI-CAT), Department of Chemistry, Universidad del Valle, Cali, Colombia Mindtech Research Group (Mindtech-RG), Mindtech s.a.s, Cali/Barranquilla, Colombia Ankita Garg Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh, India

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Contributors

Mariano Garrido INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Argentina Ruchi Gaur Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India Durvesh Gautam Department of Physics, Ch. Charan Singh University Meerut, Meerut, India Ravindra Kumar Gautam Department of Chemistry (Centre of Advanced Study), Institute of Science, Banaras Hindu University, Varanasi, India Yogendra K. Gautam Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Sharafadeen Gbadamasi School of Engineering, RMIT University, Melbourne, VIC, Australia Parinaz Ghadam Department of Biotechnology, Faculty of Biological Sciences, Alzahra University, Tehran, Iran Arpita Ghosh Centre for Sustainability and Environmental Management, Indian Institute of Management (IIM) Sirmaur, Paonta Sahib, Himachal Pradesh, India Sachin Kumar Godara Department of Apparel and Textile Technology, Guru Nanak Dev University, Amritsar, Punjab, India Ramsés Ramón González-Estrada Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico Abhijeet D. Goswami Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India Mridula Guin Department of Chemistry and Biochemistry, Sharda University, Greater Noida, India Arushi Gupta CSIR-Central Scientific Instrument Organisation (CSIR-CSIO), Chandigarh, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India Himanshu Gupta Department of Chemistry, School of Sciences, IFTM University, Moradabad, Uttar Pradesh, India Rajeev Gupta Department of Physics, School of Engineering, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, India Reena Gupta Department of Biotechnology, Himachal Pradesh University, Summerhill, Shimla, India Porfirio Gutierrez-Martinez Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico

Contributors

xxix

Aditi Halder School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, HP, India Ahmed M. El Hamaky Animal Health Research Institute (AHRI), Agriculture Research Center (ARC), Giza, Egypt Atef A. Hassan Animal Health Research Institute (AHRI), Agriculture Research Center (ARC), Giza, Egypt Sona Ayadi Hassan Department of Biotechnology, Faculty of Biological Sciences, Alzahra University, Tehran, Iran Adeeb Hayyan Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia Berileena Hazarika Department of Chemistry, National Institute of Technology, Silchar, Assam, India Francisco Javier Hernández-Béjar Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico Chandrakant R. Holkar Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India Garudadhwaj Hota Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India Syed Sarim Imam Department of Pharmaceutics, King Saud University, Riyadh, Saudi Arabia Zahra Issaabadi Department of Polymerization Engineering, Iran Polymer and Petrochemical Institute (IPPI), Tehran, Iran Karuna Jain Department of Chemistry, College of Basic Sciences & Humanities, CCS Haryana Agricultural University, Hisar, India Nazzatush Shimar Jamaludin Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia Ritika Jaryal Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Md. Noushad Javed School of Medical and Allied Sciences, K. R. Mangalam University, Gurugram, Haryana, India Center for Nano Technology, Mechanical Engineering Department, The University of Texas Rio Grande Valley, TX, Edinburg, USA Thanojan Jeyachandran School of Engineering, RMIT University, Melbourne, VIC, Australia

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Contributors

Charry Chang Yuan Jin School of Electrical and Electronic Engineering, Advanced Materials and Printed Electronics Center, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, Tianjin, China Himanshu Kachroo Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India Slawomir Kadlubowski Institute of Applied Radiation Chemistry (IARC), Lodz University of Technology, Lodz, Poland Balbir Singh Kaith Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Parashuram Kallem Center for Membranes and Advanced Water Technology (CMAT), Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates Atul Kapoor Department of Chemistry, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India Kattesh V. Katti Institute of Green Nanotechnology, Department of Radiology, School of Medicine, University of Missouri Columbia, Columbia, MO, USA Amanpreet Kaur Department of Chemistry, School of Sciences, IFTM University, Moradabad, Uttar Pradesh, India Perminder Jit Kaur Centre for Policy Research, Department of Science and Technology, Indian Institute of Science (IISC), Bangalore, India Jaspreet Kaur Rajput Department of Chemistry, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India Babita Kaushik Department of Chemistry, Amity School of Applied Sciences, Amity University, Gurgaon, Haryana, India Ravinder Kaushik School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, HP, India Keshu Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India Asma Khalid School of Sciences, RMIT University, Melbourne, VIC, Australia Muhammad Waqas Khan School of Engineering, RMIT University, Melbourne, VIC, Australia Vinita Khandegar University School of Chemical Technology, Guru Gobind Singh Indraprastha University, New Delhi, India

Contributors

xxxi

Sadhika Khullar Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India N. C. Kothiyal Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India Arvind Kumar Department of Physics, Graphic Era Deemed to Be University, Dehradun, India Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi, India Ashwani Kumar Nanoscience Laboratory, Institute Instrumentation Centre, Roorkee, India D. Srinivasa Kumar Department of Animal Nutrition, NTR College of Veterinary Science, Sri Venkateswara Veterinary University, Gannavaram, India H. Kumar Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, India Kuldeep Kumar Department of Physics, JV Jain College, CCS University Meerut, Meerut, India Nitish Kumar Gurukula Kangri Vishwavidayalaya, Haridwar, India Rakesh Kumar Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Vaneet Kumar Department of Applied Sciences, CT Group of Institutions, Jalandhar, India Research and Innovation, CT Group of Institutions, Jalandhar, Punjab, India Vinay Kumar Department of Physics, COBS&H, CCS Haryana Agricultural University, Hisar, India Tulio A. Lerma Research Group in Science with Technological Applications (GI-CAT), Department of Chemistry, Universidad del Valle, Cali, Colombia Mindtech Research Group (Mindtech-RG), Mindtech s.a.s, Cali, Colombia Zhenzi Li Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China Caroline S. A. Lima Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil Suraj Loomba School of Engineering, RMIT University, Melbourne, VIC, Australia Ademar B. Lugão Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil

xxxii

Contributors

Domingos Lusitâneo Pier Macuvele Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Center for Studies in Science and Technology (NECET), Department of Science, Engineering, Technology and Mathematics, University of Rovuma-Extension of Niassa, Lichinga, Mozambique Harshit Mahandra The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada Nasir Mahmood School of Engineering, RMIT University, Melbourne, VIC, Australia School of Science, RMIT University, Melbourne, VIC, Australia Balasubramanian Malaikozhundan Department of Biology, The Gandhigram Rural Institute (Deemed to be University), Gandhigram, Dindigul, India Natarajan Manivannan Department of Biology, The Gandhigram Rural Institute (Deemed to be University), Gandhigram, Dindigul, India Mogda K. Mansour Animal Health Research Institute (AHRI), Agriculture Research Center (ARC), Giza, Egypt Gonzalo Martínez-Barrera Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química, Universidad Autónoma del Estado de México, San Cayetano, Mexico Jinu Mathew Department of Chemistry and Centre for Research, Baselius College, Kottayam, Kerala, India Sameer Kumar Maurya Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India Abeer Mazher Deep Earth Imaging-Future Science Platform, CSIRO, Kensington, WA, Australia James McNeice The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada Medha Department of Chemistry, DAV University, Jalandhar, Punjab, India Meenu Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India Meera Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India Paula V. Messina Department of Chemistry, Universidad Nacional del Sur, INQUISUR-CONICET, Bahía Blanca, Argentina Shubhangi Mishra Department of Microbiology, Himachal Pradesh University, Summerhill, Shimla, India

Contributors

xxxiii

Muhammad Mohsin Department of Chemistry, University of Agriculture Faisalabad, Faisalabad, Pakistan Cristina Moreno-Hernández Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico Florencia San Roman Napoli INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Argentina Md. Farhan Naseh University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi, India Ashreen Norman Biophysics Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Nanomaterial Synthesis and Characterization Laboratory, Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Olalekan A. Olabode Department of Pure and Applied Chemistry, Osun State University, Osogbo, Nigeria Noha H. Oraby Animal Health Research Institute (AHRI), Agriculture Research Center (ARC), Giza, Egypt Natan Padoin Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Subramanian Palanisamy East Coast Life Sciences Institute, Gangneung-Wonju National University, Gangneung-si, Gangwon-do, Republic of Korea Manuel Palencia Research Group in Science with Technological Applications (GI-CAT), Department of Chemistry, Universidad del Valle, Cali, Colombia Víctor J. Palencia Luna Mindtech Research Group (Mindtech-RG), Mindtech s.a.s, Cali/Barranquilla, Colombia Research Group in Quimio-, Bioanalytic and Data Engineering (GIQBID), Institute of Analytical Science and Technology Golden-Hammer, Montería/Cali, Colombia Jayamanti Pandit Department of Pharmaceutics, School of Pharmaceutical Education & Research, Jamia Hamdard, New Delhi, India Krutarth H. Pandit Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India Soumya Parida G.L.Bajaj Institute of Technology and Management, Greater Noida, UP, India Bhawana Pathak School of Environment and Sustainable Development, Central University of Gujarat, Gandhinagar, Gujarat, India Pranit B. Patil Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India

xxxiv

Contributors

Samarjit Pattnayak Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India Dharshini Perumal Biophysics Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Nanomaterial Synthesis and Characterization Laboratory, Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Dipak V. Pinjari Department of Fibers and Textile Processing, Institute of Chemical Technology, Mumbai, India Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India Satish Piplode Department of Chemistry, S. B. S. Govt. P. G. College Pipariya, Hoshangabad, Madhya Pradesh, India Pablo D. Postemsky Laboratorio de Biotecnología de Hongos Comestibles y Medicinales, CERZOS-UNS/CONICET, Bahía Blanca, Buenos Aires, Argentina Subhalaxmi Pradhan Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, UP, India Man Mohan Prakash Department of Zoology, Govt. Holkar Science College, Indore, Madhya Pradesh, India Dr. B. R. Ambedkar University of Social Sciences, Mhow (Indore), Madhya Pradesh, India Jai Prakash Department of Chemistry, National Institute of Technology Hamirpur, Hamirpur, HP, India B. Siva Prasad Department of Biological Sciences, BITS Pilani, Hyderabad Campus, Hyderabad, Telangana, India Parteek Prasher Department of Chemistry, University of Petroleum & Energy Studies, Energy Acres, Dehradun, India Pratibha Department of Chemistry, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India Sanjay Pratihar Inorganic Material and catalysis Division, CSIR-Central Salt and Marine Chemical Research Institute (CSMCRI), Bhavnagar, Gujarat, India Patrícia Prediger School of Technology, University of Campinas – UNICAMP, Limeira, São Paulo, Brazil Maria Michael Christy Priya Sustainable Materials and Nanotechnology Lab (SMNL), Department of Physics, V.H.N.S.N. College (An Autonomous Institution, Affiliated to Madurai Kamaraj University), Virudhunagar, Tamil Nadu, India

Contributors

xxxv

Dhileepan Priyadharshini Sustainable Materials and Nanotechnology Lab (SMNL), Department of Physics, V.H.N.S.N. College (An Autonomous Institution, Affiliated to Madurai Kamaraj University), Virudhunagar, Tamil Nadu, India Priyanka Department of Physics, COBS&H, CCS Haryana Agricultural University, Hisar, India Sharifah Shahira Syed Putra Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Surelys Ramos-Bell Tecnológico Nacional de México/I. T. Tepic, División de Estudios de Posgrado e Investigación, Tepic, Nayarit, Mexico Justine P. Ramos de Oliveira Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil Anuj Rana Department of Microbiology, College of Basic Sciences & Humanities, Chaudhary Charan Singh Haryana Agricultural University, Hisar, India Centre for Bio-Nanotechnology, Chaudhary Charan Singh Haryana Agricultural University, Hisar, India Vineeta Rana Department of Botany and Plant Physiology (Environmental Science), CCS Haryana Agricultural University, Hisar, India J. Usha Rani Department of Microbiology, Little Flower Degree College, Hyderabad, Telangana, India Manviri Rani Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India Pooja Rani Department of Chemistry, College of Basic Sciences & Humanities, Chaudhary Charan Singh Haryana Agricultural University, Hisar, India Gyaneshwar Rao Department of Chemistry, Amity School of Applied Sciences, Amity University, Gurgaon, Haryana, India P. Ravi Kanth Reddy Animal Husbandry Department, Veterinary Dispensary, Taticherla, Andhra Pradesh, India P. Pandu Ranga Reddy Animal Genetics and Breeding, College of Veterinary Science, Sri Venkateswara Veterinary University, Proddatur, India Jhoban Meneses Rengifo Research Group in Science with Technological Applications (GI-CAT), Department of Chemistry, Universidad del Valle, Cali, Colombia Humberto Gracher Riella Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Tomás Rivas-García CONACYT-Universidad Autónoma Chapingo, Texcoco, Estado de México, Mexico

xxxvi

Contributors

Gabriela Roa-Morales Centro Conjunto de Investigación en Química Sustentable, Universidad Autónoma del Estado de México – Universidad Nacional Autónoma de México (UAEM-UNAM), Toluca, Atlacomulco, Mexico Joan Manuel Rodríguez-Díaz Departamento de Procesos Químicos, Facultad de Ciencias Matemáticas, Físicas y Químicas, Universidad Técnica de Manabí, Portoviejo, Ecuador Laboratorio de Análisis Químicos y Biotecnológicos, Instituto de Investigación, Universidad Técnica de Manabí, Portoviejo, Ecuador Programa de Pós-graduação em Engenharia Química, Universidade Federal da Paraíba, João Pessoa, Brazil Everton Augusto Rodrigues School of Technology, University of Campinas – UNICAMP, Limeira, São Paulo, Brazil Sourav Roy Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India Olim Ruzimuradov Department of Natural and Mathematic Sciences, Turin Polytechnic University in Tashkent, Tashkent, Uzbekistan Chandreyee Saha Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, UP, India Ugrabadi Sahoo Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India Pramendra Kumar Saini Environmental Science and Technology Division, CSIR-Central Building Research Institute, Roorkee, India Sakshi Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India A. Z. M. Salem Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma del Estado de México, Toluca, Mexico Samriti Department of Chemistry, National Institute of Technology Hamirpur, Hamirpur, HP, India Sivashunmugam Sankaranarayanan Sustainable Materials and Nanotechnology Lab (SMNL), Department of Physics, V.H.N.S.N. College (An Autonomous Institution, Affiliated to Madurai Kamaraj University), Virudhunagar, Tamil Nadu, India Javier Sartuqui Department of Chemistry, Universidad Nacional del Sur, INQUISUR-CONICET, Bahía Blanca, Argentina Saruchi Department of Biotechnology, CTIPS, C.T. Institute of Engineering, Management and Technology, Shahpur, Jalandhar, Punjab, India Rasha M. H. Sayed-ElAhl Animal Health Research Institute (AHRI), Agriculture Research Center (ARC), Giza, Egypt

Contributors

xxxvii

Joydip Sengupta Department of Electronic Science, Jogesh Chandra Chaudhuri College, Kolkata, India Sapna Sethi Department of Chemistry, DAV University Jalandhar, Jalandhar, Punjab, India Babar Shabbir Department of Material Science and Engineering, Monash University, Clayton, VIC, Australia M. Al Shafouri School of Physics, Universiti Sains Malaysia, Penang, Malaysia Kwok Wei Shah Department of the Built Environment, College of Design and Engineering, National University of Singapore, Singapore, Singapore Uma Shanker Polymer and Nanomaterials Synthesis Laboratory, Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, India Amit L. Sharma CSIR-Central Scientific Instrument Organisation (CSIR-CSIO), Chandigarh, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India Awanish Kumar Sharma Department of Physics, Graphic Era Deemed to Be University, Dehradun, India Deepak Sharma Department of Mechanical Engineering, National Institute of Technology, Hamirpur, India Kavita Sharma Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Mousmee Sharma Department of Chemistry, Uttaranchal University, Arcadia Grant, Dehradun, India Sulaxna Sharma THDC-Institute of Hydropower Engineering and Technology, Bhagirathipuram, New Tehri, India Surbhi Sharma Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India Vibhuti Sharma Department of Biotechnology, Himachal Pradesh University, Summerhill, Shimla, India Vikas Sharma Malaviya National Institute of Technology JLN Marg, Jaipur, Rajasthan, India Mansi Sheokand Department of Chemistry, College of Basic Sciences & Humanities, CCS Haryana Agricultural University, Hisar, India Smriti Sihag Department of Physics, COBS&H, CCS Haryana Agricultural University, Hisar, India Ankit Kumar Singh Department of Chemistry (Centre of Advanced Study), Institute of Science, Banaras Hindu University, Varanasi, India

xxxviii

Contributors

Anjali Singh Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Beer Pal Singh Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Krishna Pal Singh Biophysics Unit, College of Basic Sciences & Humanities, G.B. Pant University of Agriculture & Technology, Pantnagar, Uttarakhand, India Vice Chancellor’s Secretariat, Mahatma Jyotiba Phule Rohilkhand University, Bareilly, Uttar Pradesh, India N. B. Singh Department of Chemistry and Biochemistry, Sharda University, Greater Noida, India Pankaj Kumar Singh School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, HP, India Rashmi Singh Department of Chemistry, School of Basic and Applied Sciences, Lingaya’s Vidyapeeth, Faridabad, Haryana, India Ruksana Sirach Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India Shalini Singh CSIR-Central Scientific Instrument Organisation (CSIR-CSIO), Chandigarh, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India Sudhir Kumar Singh Department of Mechanical Engineering, National Institute of Technology, Hamirpur, India Cíntia Soares Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Md Abdus Subhan Department of Chemistry, SUST, Sylhet, Bangladesh Tahrima Subhan The Sylhet Khajanchibari International School and College, Sylhet, Bangladesh Department of Social Work, SUST, Sylhet, Bangladesh Sushma Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, UP, India Dipankar Sutradhar Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India Swati Thakur Department of Chemistry, DAV University Jalandhar, Jalandhar, Punjab, India

Contributors

xxxix

Velaphi C. Thipe Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/CNEN-SP, São Paulo, SP, Brazil Institute of Green Nanotechnology, Department of Radiology, School of Medicine, University of Missouri Columbia, Columbia, MO, USA Ida Tiwari Department of Chemistry (Centre of Advanced Study), Institute of Science, Banaras Hindu University, Varanasi, India Richa Tomar Department of Chemistry and Biochemistry, Sharda University, Greater Noida, India Mustafa Tunçsoy Biology Department, Faculty of Science and Letters, Çukurova University, Adana, Turkey Benay Tuncsoy Faculty of Engineering, Department of Bioengineering, Adana Alparslan Turkes Science and Technology University, Adana, Turkey Mustafa Tuncsoy Faculty of Science and Technology, Department of Biology, Cukurova University, Adana, Turkey Shrestha Tyagi Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Ashish Upadhyay Department of Chemistry, National Institute of Technology Hamirpur, Hamirpur, HP, India Fernando Ureña-Núñez Instituto Nacional de Investigaciones Nucleares, La Marquesa, Ocoyoacac, Mexico Damián Uriarte INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Argentina María Evangelina Vallejos Programa de Celulosa y Papel (PROCYP), IMAM, UNaM, CONICET, FCEQYN, Posadas, Argentina Dipti Vaya Department of Chemistry, Amity School of Applied Sciences, Amity University, Gurgaon, Haryana, India Sagar Vikal Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India Jayaraj Vinodhini Department of Biotechnology, Dr. Umayal Ramanathan College for Women, Karaikudi, India Vipin Department of Chemistry, Dr B R Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Singaravelu Vivekanandhan Sustainable Materials and Nanotechnology Lab (SMNL), Department of Physics, V.H.N.S.N. College (An Autonomous Institution, Affiliated to Madurai Kamaraj University), Virudhunagar, Tamil Nadu, India

xl

Contributors

Aafrin Waziri Cell and Molecular Biology Lab University School of Biotechnology GGS Indraprastha University, New Delhi, India Teng Xiong Department of the Built Environment, College of Design and Engineering, National University of Singapore, Singapore, Singapore Jyoti Yadav Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India Vinod Kumar Yadav Department of Chemical Engineering, Government Polytechnic, Gorakhpur, Uttar Pradesh, India Decai Yang Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China D. Yasaswini Department of Veterinary Medicine, College of Veterinary Science, Sri Venkateswara Veterinary University, Tirupati, India Maryam Yousaf Department of Chemistry, University of Agriculture Faisalabad, Faisalabad, Pakistan Nur Khalida Rahayu Zainon Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Luis Ángel Zambrano-Intriago Laboratorio de Análisis Químicos y Biotecnológicos, Instituto de Investigación, Universidad Técnica de Manabí, Portoviejo, Ecuador LAQV-REQUIMTE/Departamento de Ciências Químicas, Faculdade de Farmácia, Universidade do Porto, Porto, Portugal Alvin Lim Teik Zheng Department of Science and Technology, Faculty of Humanities, Management and Science, Universiti Putra Malaysia Bintulu Campus, Bintulu, Sarawak, Malaysia Wei Zhou Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China

Part I Synthesis of Nanomaterials

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Biomimetic Route-Assisted Synthesis of Nanomaterials: Characterizations and Their Applications Vinars Dawane, Satish Piplode, Man Mohan Prakash, and Bhawana Pathak

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Benefits of Using Biological Entities as Medium for Nanomaterial Preparations . . . . . . . . . Specific Modes of Synthesis for Bionanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Probable Mechanisms Involved in the Synthesis Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unique Characterization of Bionanomaterials Synthesized via Biomimetic Routes . . . . . . . . . . . . Applications of Nanomaterials in the Field of Modern Sciences and the Key Challenges Involved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Future Scope: Add Some Future Scope and Make Depth in Conclusion Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Biomimetic route of nanoparticle synthesis means the use of biological entities such as cells of microorganisms, plants, and animals as a medium to generate materials having nano size ranges or at least one-dimensional range in nanoscale (between 1 and 100 nm). Biomimetic route of synthesis is now becoming a preferential choice over other synthesis processes because of the biocompatibility of process, use of nontoxic chemicals, application of environmentally friendly V. Dawane · B. Pathak (*) School of Environment and Sustainable Development, Central University of Gujarat, Gandhinagar, Gujarat, India e-mail: [email protected] S. Piplode Department of Chemistry, S. B. S. Govt. P. G. College Pipariya, Hoshangabad, Madhya Pradesh, India M. M. Prakash (*) Department of Zoology, Govt. Holkar Science College, Indore, Madhya Pradesh, India Dr. B R Ambedkar University of Social Sciences, Mhow (Indore), Madhya Pradesh, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_1

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solvents, affordability and quickness of the process, flexible control over the synthesis process, and easy purification of product. The present book chapter explains the roles of various cells of bacteria, viruses, fungi, actinomycetes, plants, and animals in the biofabrication of nanomaterials in details. The case studies on the synthesis of selected nanomaterials by the use of various biological cells as biomediums have been discussed briefly along with the probable synthesis mechanisms involved in the processes. The specific characterization methods and instrumentation involved in the analysis of shape, size, and structures of generated nanobiomaterials have been included. Finally, this chapter highlights very recent selected applications of nanobiomaterials including environmental, biomedical, agricultural, and interdisciplinary. Thus, this chapter provides a latest update in the field of biomimetic route assisted nanomaterials synthesis along with mechanism insides, recent trends in their characterization, and cutting-edge applications in the allied sectors of science and technology. Keywords

Biomimetics · Nanomaterials · Synthesis and characterization · Biomedical · Agricultural and environmental applications

Introduction Biomimetics or more specifically biomimetic route assisted/biological synthesis of materials/nanomaterials means the use of biological mediums or cells to prepare materials/nanomaterials (Prathna et al. 2010). Thus, biomimetics is a branch of science that deals with the bio-inspired technological advancements and applications toward the science and technology or more precisely toward the material sciences or nanotechnology (Dadashpour et al. 2018). As the use of biological mediums or cells to create tiny scale materials in the range of 1–100 nm is a relatively new concept in nanotechnology, it is a burning field of cutting-edge research. It is increasing day by day and becoming a preferential choice among the recent material sciences research and development as well as in the domain of new class of nanomaterial preparations for selected applications (Sharma et al. 2020). The low making cost, environmental friendliness, comparatively low toxicities, no use of toxic solvents, and exceptional bioactivities or applications made the biomimetic route assisted nanomaterials an eye-catching material for intentional scientific applications (Chandra and Singh 2018). For these reasons, many biological mediums are applicable to synthesis nanomaterials such as bacteria (Mandal et al. 2006), fungi (Sastry et al. 2003), viruses (bacterial, plant or animals viruses) (Lee et al. 2012), and actinomycetes (soilborne or soil-sediment-borne) (Golinska et al. 2014). The other attractive mediums included plants (terrestrial, coastal, and aquatic) (Makarov et al. 2014), diatoms, cyanobacteria, as well as animal cells (Das et al. 2017). This book chapter highlights the potentials of biological mediums or cells to engineer fine nanomaterials specifically the metal nanomaterials. The importance of

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Biomimetic Route-Assisted Synthesis of Nanomaterials:. . .

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these biomimetic route assisted preparation of materials has been reviewed with respect to their specific synthesis, potential mechanisms involved in the fabrication, specific characterization, and remarkable selected applications.

The Benefits of Using Biological Entities as Medium for Nanomaterial Preparations The biomimetic route of nanomaterial fabrication is a far better alternative over other synthesis methods with the respect of safety, cost-effectiveness, and environmentally expressed synthesis procedure. There are some very important comparative advantages of biological route synthesis or biomimetic inspired synthesis of nanomaterials over other physical or chemical or combined methods (Aliofkhazraei 2016). The available physical methods such as grinding, arc discharging, thermal deposition, laser alternatives, etching, or kind of milling processes are having an exceptional time advantage over biological synthesis methods of nanomaterials. Thus, these processes can give large amount of bulk synthesis in a very short time, but the major problem has been associated with the surface morphological imperfections in the generated nanomaterials along with the expensiveness. These things become very critical when the product is intended for specific work such as in the terms of surface chemistry and surface to volume-dependent physical properties as applications (Thakkar et al. 2010). The biological synthesis procedures have far better control over these issues and can easily generate nonhomogeneous and polydisperse nanomaterials by using top to down approach (Chandra and Singh 2018). The biological methods have several advantages over physical methods by using the bottom to top approach and as a result the biological methods provide less disarrangements in atom to atom nucleation during synthesis thus provide mono disperse and homogeneous tiny materials (Syafiuddin et al. 2017). When the safety and toxicity of the products are on the line, the biological methods become far better options over chemical methods. As most of chemical methods also use the bottom-up approach, the problem of mono disparity as well as homogeneity already becomes less critical, but the chemical methods often use the various chemicals such as hazardous stabilizing or reducing agents as well as other toxic solvents in the fabrication of nanomaterials. These chemicals become a critical concern when the intended applications related to the specific areas such as clinical or biomedical as well as environmental. So with regard to toxicity and safety concern, the biological methods have the advantages over physical methods (Jamkhande et al. 2019). Many combined physicochemical methods are often useful nowadays, but they also use demanding conditions in the synthesis such as specific temperature, pressure, presence of any catalytic agent, and other conditions that directly linked with the cost of the overall fabrication. Thus, biological fabrication gives a far better advantage over cost issues because no specific conditions are required for the synthesis and one can easily synthesize the nanomaterials at room temperature (Iravani et al. 2014).

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Thus, the intentional necessity to create a cost-effective, safe, and eco-friendly method for the synthesis of multifunctional as well as multitalented/applicable nanomaterials has attracted the attention of interdisciplinary scientific community to explore the exceptional potentials of biomimetic routes of nanomaterial synthesis (Singh et al. 2016).

Specific Modes of Synthesis for Bionanomaterials The fabrication of nanomaterials with important characteristics such as very fine, uniform, disperse, well-functional attributes as well as multipliable under simple/ vary common conditions have been still remain a great challenge for research hand technology. Various biological entities such as plants, bacteria, diatoms, cyanobacteria, viruses, yeasts, other fungi, algae, as well as actinomycetes have been already tested as a potential medium for nanomaterial synthesis. As a synthesis medium, these biological entities provide various essential factors and agents that are directly important for the making of fine nanomaterials. The plants are rich in phytochemicals or secondary metabolites such as polyphenols, alkaloids, tannins, saponins, triterpenoids, etc. that can act as various agents such as reducing agents and capping and/or stabilizing agent in the biofabrication method of nanomaterials (Fig. 1). These various phytochemicals help in the synthesis of nanomaterials with defined size and shapes. Plants as biomimetic mediums are having preferential positions over other biomediums such as microorganisms because of the extra need of specific handling, culture conditions, difficulties in maintenance, and safety. Table 1 is showing the

Fig. 1 Plants as biomimetic mediums

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Biomimetic Route-Assisted Synthesis of Nanomaterials:. . .

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Table 1 A representative list of plant used for the synthesis of nanoparticles. The table also highlights the potential of selected plant part along with the specific generated nanomaterial Plant Punica granatum Sasa borealis Trifolium pratense

Parts of plant Peel Leaves Flower

Nanomaterial Cu Au ZnO

Camellia japonica

Leaves

CuO

Cocos nucifera

Leaves

Lead

Catharanthus roseus Artocarpus gomezianus Various mangroves, saltmarshes, and coastal plants

Leaves Fruit Leafs and other parts

Palladium Zinc Ag

References Kaur et al. (2016) Patil et al. (2018) Dobrucka and Długaszewska (2016) Aminuzzaman et al. (2017) Elango and Roopan (2015) Kalaiselvi et al. (2015) Suresh et al. (2015) Kathiresan et al. (2012)

Table 2 A representative list of algae used for the synthesis of nanoparticles Algae Sargassum muticum Gelidium amansii Sargassum crassifolium Cystoseira trinodis Turbinaria conoides Chlorella vulgaris Padina tetrastromatica and Turbinaria conoides Spirogyra varians Sargassum muticum

Nanomaterial ZnO Ag Au CuO Au Palladium ZnO

References Sanaeimehr et al. (2018) Ovais et al. (2017) Maceda et al. (2018) Gu et al. (2018) Vijayaraghavan et al. (2011) da Silva Ferreira et al. (2017) Rajeshkumar (2018)

Silver ZnO

Salari et al. (2016) Azizi et al. (2014)

potential of various types of plants such as terrestrial plants, coastal plants, as well as marine plants in the synthesis of various nanomaterials. Various microorganisms are also outstanding biological resources for the generation of nanomaterials that can provide fine, versatile, eco-friendly, as well as costeffective synthesis method. The attractive advantages of microbial cell assistance in the nanomaterial synthesis are present in the industrial applicability in the term of easily scaling up processes or in the form of improved biocompatibilities. Many times the microbial synthesis can give far better results as compared to the plant-mediated synthesis because the plants use phytochemicals as reducing/capping agents; thus, an issue regarding the polydispersity becomes always a comparative subject. To make metal ions into their elemental forms and fabricate the nanomaterials, the microbes use their enzymes (intracellular and/or extracellular) and not use any kind of other capping or stabilizing or reducing agents in the synthesis; thus, they are becoming an attractive candidate for green synthesis of nanomaterial synthesis. In the following table, a very broad range of microorganism has been

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Table 3 A representative list of bacteria used for the synthesis of nanoparticles Bacteria Desulfovibrio desulfuricans NCIMB8307 (Beijerinck) Kluyver and van Niel Streptomyces sp. HBUM171191

Nanomaterial Palladium

Shewanella loihica Shewanella loihica

Cu Pt, Pd, Au

Kocuria flava

Copper

Shewanella loihica PV-4 Ochrobactrum sp. MPV1

Palladium and platinum Tellurium

Aquaspirillum magnetotacticum

Fe3O4

Klebsiella pneumoniae

CdS

MnSO4, ZnSO4

References Yong et al. (2002) Waghmare et al. (2011) Lv et al. (2018) Ahmed et al. (2018) Kaur et al. (2015) Ahmed et al. (2018) Zonaro et al. (2017) Kharissova et al. (2013) Mittal et al. (2013)

Table 4 A representative list of actinomycetes used for the synthesis of nanoparticles Actinomycetes Thermomonospora sp. Rhodococcus sp. Streptacidiphilus Durhamensis Streptomyces griseoruber Streptomyces capillispiralis Ca-1

Nanomaterial Gold Gold Silver Gold Copper

References Ahmad et al. (2003a) Ahmad et al. (2003b) Buszewski et al. (2018) Ranjitha and Rai (2017) Hassan et al. (2018)

targeted for the synthesis of different types of nanomaterials. Table 2 is showing the use of various algae cells (microalgae and others). Similar to the algae cells, other microorganisms are also capable to make different nanomaterials. The bacteria show impressive potential in the making of nanomaterials. The synthesis process can be optimized with very minor monitoring, and that can give a fine, clean, fast, controlled size, and desired morphological output in the nanomaterial product. Table 3 is a fine piece of collective data showing the use of various types of bacteria (aerobic, anaerobic, facultative, and others) in the making of different nanomaterials. Similar to the most of bacteria, the specific type of microorganisms called actinomycetes is also capable to make nanomaterials. They are unique microbes having a transitional conjugational characteristic between bacteria and fungi. Table 4 is showing the potentials of actinomycetes in the synthesis of nanomaterials. The other precious microbes such as various types of yeasts and fungi that are also having exceptional industrial or biotechnological applicability are capable to make nanomaterials (Fig. 2). Some selected viruses are also tested for nanomaterial

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synthesis, and they showed promising results (Fig. 3). Tables 5, 6, and 7 explore the brilliant abilities of fungi, yeasts, and viruses, respectively, in the bionano fabrication and nanomaterial preparations.

Fig. 2 The biomimetic routes of various fungi in nanomaterial generation

Fig. 3 Virus as biomimetic medium for nanomaterial generation

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Table 5 A representative list of fungi used for the synthesis of nanoparticles Fungi Rhizopus nigricans Ehrenberg Fusarium oxysporum F. oxysporum F. oxysporum F. oxysporum F. oxysporum Penicillium chrysogenum Macrophomina phaseolina Aspergillus nidulans Colletotrichum sp. Fusarium oxysporum Fusarium oxysporum

Nanomaterial Silver

References Ravindra and Rajasab (2014)

CdS Silica Titanium Magnetite CdSe Pt

Ahmad et al. (2002) Bansal et al. (2005) Bansal et al. (2005)

Ag/AgCl

Spagnoletti et al. (2019)

CoO

Vijayanandan and Balakrishnan (2018) Suryavanshi et al. (2017) Ahmad et al. (2002) Bansal et al. (2005)

Kumar et al. (2007) Subramaniyan et al. (2018)

Aluminum oxide CdS Silica and titanium particles (SiF62_ and TiF62_)

Table 6 A representative list of yeast used for the synthesis of nanoparticles Yeast Yarrowia lipolytica 3589 (Wick., Kurtzman, and Herman) Van der Walt and Arx Saccharomyces cerevisiae Meyen ex E.C. Hansen

Nanomaterial Gold

Candida glabrata

Amorphous iron phosphate CdS

Yeast strain MKY3

Ag

Schizosaccharomyces pombe

CdS

Torulopsis sp.

PbS

References Agnihotri et al. (2009) He et al. (2009) Dameron et al. (1989) Kowshik et al. (2003) Kowshik et al. (2002a) Kowshik et al. (2002b)

Table 7 A representative list of virus used for the synthesis of nanoparticles Virus Potato virus X Tobacco mosaic virus (TMV) M13 virus Potato virus X Hepatitis E virus

Nanomaterial Nanocarriers Palladium Titanium dioxide Nanoconjugates Nanoconjugates

References Le et al. (2017) Yang et al. (2013) Chen et al. (2015) Esfandiari et al. (2016) Chen et al. (2018)

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Viruses are noble biomimetic media for nanomaterial synthesis. Due to their very tiny size and mono dispersity along with the availability of various chemical groups that can bring the modifications in the medium, they are able to develop fine and stable scaffolds from the meso-micro-molecular level assemblies into nano level engineered materials (Velusamy et al. 2016) (Fig. 3).

Probable Mechanisms Involved in the Synthesis Processes The understanding of mechanistic phenomenon associated with the biofabrication process is very important because this is directly associated with the desired shape, size, and intentional nanomaterial product. As mentioned earlier, the plants use their unique phytochemicals as reducing, capping, as well as stabilizing agents to generate nanomaterials, while the microorganisms use their secretary enzymes (intra and/or extracellular) as actual drivers to initial reduction of precursor on macro /microscale to convert into the nanoscale. Thus, the synthesis mechanism in the case of microorganisms is relatively simple as compared with plant-mediated synthesis. The healthy and growing cells of microorganisms act as a manufacturing factory for nanomaterial generation because they already have an evolutionary as well as natural tendency to chemically process the precursors such as metal ions (reducing or oxidizing) into their tiny forms such as their metallic or metal oxide nanoparticle forms. In a more mechanistic way to explain the phenomenon, the microbial cells in the medium act as an entity that is having a high negative electrokinetic potential. This electrokinetic potential works as a trigger and elongation switch that enable the cations (metallic ions) in the medium to interact and attract with this potential, and as a result, the biosynthesis and fabrication process occurs in the medium (Raliya 2012). The plants use relatively complex phenomenon because of the presence of complex phytochemicals, so it is relatively difficult to identify the specific reducing or stabilizing agent in the biofabrication process. Plants use three sequential specific steps as probable working mechanism of the whole process for nanofabrication such as initiation step, elongation step, and termination step. In the initiation step, the phytochemicals use their reducing potential to recover the metal ions from the metallic precursor (in the case of metal nanoparticle synthesis process). This reduction potential drives the metallic ions to convert into zero valent state forms from their high oxidation states, and this triggers the nucleation of the atoms in the fabrication medium. After this step, the elongation or tailoring stage occurs in which segregated metal atoms form the associations and merging, and as a result, the formation of nanomaterial occurs in this stage. As parallel various phytochemical inspired fabrication processes also make alterations and modifications simultaneously in this stage, this step also decides various morphological shape formations of fabricated nanomaterial (Princy 2019) (Fig. 4).

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Fig. 4 The significant insides in the probable mechanisms involved in the formation of materials via biomimetic routes

Unique Characterization of Bionanomaterials Synthesized via Biomimetic Routes The characterization of bionanomaterials belongs to the understanding of their accurate sizes, precise shapes, specific formations or structures, nucleation and material generation process, purity and crystal information, fringe size and lattice formation, surface dynamisms, and other properties linked with their descriptions. In this regard, the particle size analysis or dynamic light scattering evaluations, the spectroscopic analysis, and the microscopic estimations are the most significant to describe the synthesized bionanomaterials. Table 8 is highlighting various advanced instrumentation and techniques in the characterization of bionanomaterials.

Applications of Nanomaterials in the Field of Modern Sciences and the Key Challenges Involved In this section of the book chapter, the very recent, specific, and selected applications of various nanomaterials have been merged to provide a deep as well as review data oriented understanding of various biomimetic route synthesized nanomaterials to encourage the future research and development curiosities among the readers. Table 9 is an outstanding specimen of cutting-edge references as well as applications on biomimetic route synthesized nanomaterials in the diverse fields. The costeffectiveness, less toxicities, biocompatibilities, easy preparation, and multifarious applicable in nature and several other qualities made biomimetic route assisted nanomaterials a remarkable choice in diverse field of allied and advanced fields of sciences (Fig. 5).

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Table 8 Various methods and their applications used in the characterization of nanoparticles (Mourdikoudis et al. 2018) Technique Powdered X-ray diffraction (PXRD)

X-ray absorption spectroscopy (XAS) ASay abso (edge X-ray absorption fine structure (EXAFS)) and X-ray absorption near edge structure (XANES)

Small angle X-ray scattering (SAXS)

X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA)

Fourier-transform infrared spectroscopy (FTIR) and nanoFTIR

Nuclear magnetic resonance (NMR) and nanoscale NMR

Characterization information obtained Evaluation of crystal structure, analysis of the composition, crystalline grain size determination Local geometric evaluation, determination of electronic structure of samples (liquid, gas, or solid matter), more sensitive as compared with XRD for local structure analysis, multiple scattering resonance evaluations, X-ray absorption coe resonancesh XRD for local structures M; double-check the information distances, Debye-Waller factors, also for noncrystalline NPs Structure evaluation at nano-mesomicrolevels, particle size, size distribution, growth kinetics, morphological analysis, identification of phases including quasicrystalline and Frank-Kasper phases, conformational diversity analysis of macromolecules, crystalline samples not required for the characterization Evaluation of overall electronic structure and density analysis of the electronic states, elemental composition, oxidation states, ligand binding (surface-sensitive), quantitative surface information, study of surface layers or thin film structures FTIR is a more sensitive version of dispersive spectrometry, surface composition, ligand binding, spectroscopic chemical identification, understanding the biofabrication process of nanomaterials, investigation of various structures of proteins Noninvasive evaluation of various nuclear species, identification of compounds, structure analysis, atomic composition, evaluation of dynamics and reaction state of any chemical process and study of chemical environment. Study of (continued)

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Table 8 (continued) Technique

Brunauer-Emmett-Teller (BET) spectroscopy

Thermogravimetric analyzer (TGA)

Low energy ion scattering (LEIS) spectroscopy

UV-Vis spectroscopy

Photoluminescence (PL) spectroscopy

Dynamic light scattering (DLS)

Nanoparticle tracking analysis (NTA)

Transmission electron microscopy (TEM)

Characterization information obtained wetted surface area and pore size distribution. Nano NMR can give nanoscale spatial resolution of samples, emulsion droplet size of nanomaterials in liquid Evaluation of surface area are pore size distribution of various dry powdered catalysts and nanomaterials, study of absorption/ desorption phenomenon Thermal stability evaluations, study the mass of samples with temperature and time dynamics, ligand analysis, temperaturedependent stability or decomposition, mass and composition of stabilizers Surface analysis, structure, thickness and chemical composition evaluation Optical properties and surface plasmon responses, nanomaterial size, concentration, agglomeration state, information about bandgap using Tauc plot Optical property evaluation with the respect to structure characteristics such as deformities, size, and composition Hydrodynamic volumes and size distribution evaluation, analysis of homogeneity, detection of agglomerates and bulks, light scattering evaluations of liquid samples Light scattering and Brownian motion determination of liquid samples, visualization and study of the nanomaterials in liquid, NP size, and understanding the size distribution profiles of the sample Evaluation of ultrathin films and their size, dispersity analysis, study of shape and aggregation states, understanding the nucleation patterns and internal morphology of nano assemblies, study growth kinetics (continued)

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Table 8 (continued) Technique HRTEM

Liquid TEM

Cryo-TEM

Electron dilectron analysis

Scanning transmission electron microscopy (STEM)

Aberration-corrected (STEM, TEM)

Electron energy loss spectroscopy (EELS)

Characterization information obtained Evaluation of the single crystal structure of single particle, quantitative purpose studies, polycrystalline analysis of samples, study the amorphous nanomaterials, nanoparticle crystalline defect analysis Provides the ability to study the TEM evaluation on liquid samples and overcomes the limitation of TEM for physical changeability of states of samples under vacuum, nanomaterial growth kinetics and mechanisms in real time, single particle movement study, evaluation of superlattice formation Evaluation of samples at cryogenic temperatures specifically at liquid nitrogen temperatures, study complex growth mechanisms, aggregation pattern understandings, useful for colloidal chemistry and other branches of biological sciences where presence of artifacts or destroyed samples is a critical thing Study the crystal structure, lattice related characteristics and parameters analysis, evaluation of order and disorder change phenomenon Dynamic evaluation of topography by switching the operations of STEM, combined with high-angle annular dark field (HAADF) to obtain annular imaging results, analysis of crystal structure and elemental composition, evaluation of the atomic structure of heterogeneous interfaces Atomic structure of the clusters such as bimetallic nanoparticle samples, evaluation of alloy homogeneity, phase segregation phenomenon Detection of the elemental components, evaluation of type and quantity of atoms and their (continued)

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Table 8 (continued) Characterization information obtained

Technique

Electron tomography

Scanning probe microscopy (SPM), types – atomic force microscopy (AFM) and scanning tunneling microscopy (STM)

chemical states, understanding of collective interactions of atoms with neighboring atoms and environment, bulk plasmon resonance analysis Realistic three-dimensional particle evaluation and visualization, quantitative evaluation down to the atomic level scale Surface imaging at atomic levels, evaluation without the use of vacuum or unstandard temperatures, high resolution and sensitive evaluation free from diffraction limitations

Table 9 The selected applications of various bionanomaterials, their biological mediums used, the nanoparticles formed, and selected applications Plant/microbes Padina tetrastromatica and Turbinaria conoides Spirogyra varians

Nanoparticles ZnO

Applications Antibacterial activity

Silver

Antibacterial activity

Shewanella loihica PV-4

Palladium and platinum Tellurium

Degradation of methyl orange dye Reduction of toxic compound Antibacterial and anticancer activity Degradation of methylene blue Antimicrobial activity

Ochrobactrum sp. MPV1 Streptacidiphilus durhamensis Streptomyces griseoruber

Silver

Streptomyces capillispiralis Ca-1 Colletotrichum sp.

Copper

Potato virus X Tobacco mosaic virus (TMV) M13 virus

Gold

Aluminum oxide Nanocarriers

Antimicrobial activity

Palladium

Photo-electrochemical properties Herceptin drug delivery in breast cancer therapy Doxorubicin delivery in cancer therapy Cancer therapy

Potato virus X

Titanium dioxide Nanoconjugates

Hepatitis E virus

Nanoconjugates

Doxorubicin delivery

References Rajeshkumar (2018) Salari et al. (2016) Ahmed et al. (2018) Zonaro et al. (2017) Buszewski et al. (2018) Ranjitha and Rai (2017) Hassan et al. (2018) Suryavanshi et al. (2017) Le et al. (2017) Yang et al. (2013) Chen et al. (2015) Esfandiari et al. (2016) Chen et al. (2018)

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Fig. 5 Various applications of nanomaterials generated via biomimetic routes

Applications of biomimetic route based materials and technologies for unique or diverse applicability are still in their budding stage and are facing huge challenges due to the various aspects of toxicity, safety, performances, inherent features, and diverse characteristics of biomimetic mediums (Fig. 1). Recently, incorporation of different biomediums for nanomaterial brings new tactics for improving their applicability and analytical performances. While many of such nanomaterials have been developed and tested till date against a wide range of applications at the lab-scale, only a few are currently actively being used. Thus, it is clear that more advancement and efforts are needed in joining improvement that will play very vital role in the development of efficient, automated, real-time biomimetic route assisted materials with high throughput analysis and applications.

Conclusions and Future Scope: Add Some Future Scope and Make Depth in Conclusion Required The world is facing many challenges such as unmet medical needs, food-waterenergy security, environmental pollution, as well as issues related toward the sustainable development, and the nanotechnology is giving a strong hope to solve them. In this regard, biomimetics is showing a new era of technological advancements as the biological media or cells (various microorganism, plants, or animals) are capable to synthesize well-defined nanomaterials toward marvelous applications along with attractive features such as easy to preparation, less toxic methods, comparative affordability, environment friendliness, and capacity to generate fine, homogenous,

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and active nanomaterials. The understating about synthesis and mechanisms involved as well as characterization process is a key step to understand and make better materials for wider applications. The biomimetic synthesis is still a not fully explored field; thus, very genuine affords are needed to make more reliable, reproducible, and precise synthesis methods to generate attractive bionanomaterials. The understandings toward the synthesis mechanisms are also a much unexplored field and very important to know the processes involved in the making of these nanomaterials. The current scenario is observing a remarkable and dynamic progress in the field of bionanomaterial characterizations, and thus the understanding of the potentials of various analytical tools and their combinations and hyphenations will surely provide a better understanding of this class of nanomaterials along with better control over new applications. Thus, special attentions and directions as well as more researches are required toward the biomimetics or biomimetic rout assisted preparation of nanomaterial synthesis and characterizations to understand this benign field and future aspects of various appliances to achieve the exceptional potentials present in this technology.

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Ravindra BK, Rajasab AH (2014) A comparative study on biosynthesis of silver nanoparticles using four different fungal species. Int J Pharm Pharm Sci 6(1):372–376 Salari Z, Danafar F, Dabaghi S, Ataei SA (2016) Sustainable synthesis of silver nanoparticles using macroalgae Spirogyra varians and analysis of their antibacterial activity. J Saudi Chem Soc 20(4):459–464 Sanaeimehr Z, Javadi I, Namvar F (2018) Antiangiogenic and antiapoptotic effects of greensynthesized zinc oxide nanoparticles using Sargassum muticum algae extraction. Cancer Nanotechnol 9(1):1–16 Sastry M, Ahmad A, Khan MI, Kumar R (2003) Biosynthesis of metal nanoparticles using fungi and actinomycete. Curr Sci 85(2):162–170 Sharma D, Sharma R, Chaudhary A (2020) Microbial cell factories in nanotechnology. In: Microbial diversity, interventions and scope. Springer, Singapore, pp 99–108 Singh P, Kim YJ, Zhang D, Yang DC (2016) Biological synthesis of nanoparticles from plants and microorganisms. Trends Biotechnol 34(7):588–599 Spagnoletti FN, Spedalieri C, Kronberg F, Giacometti R (2019) Extracellular biosynthesis of bactericidal Ag/AgCl nanoparticles for crop protection using the fungus Macrophomina phaseolina. J Environ Manag 231:457–466 Subramaniyan SA, Sheet S, Vinothkannan M, Yoo DJ, Lee YS, Belal SA, Shim KS (2018) One-pot facile synthesis of Pt nanoparticles using cultural filtrate of microgravity simulated grown P. chrysogenum and their activity on bacteria and cancer cells. J Nanosci Nanotechnol 18(5): 3110–3125 Suresh D, Shobharani RM, Netravathi PC, Pavan Kumar MA, Nagabhushana H, Sharma SC (2015) Artocarpus gomezianus aided green synthesis of ZnO nanoparticles: luminescence, photocatalytic and antioxidant properties. Spectrochim Acta A Mol Biomol Spectrosc 141:128–134 Suryavanshi P, Pandit R, Gade A, Derita M, Zachino S, Rai M (2017) Colletotrichum Sp.-mediated synthesis of sulphur and aluminium oxide nanoparticles and its in vitro activity against selected food-borne pathogens. LWT Food Sci Technol 81:188–194 Syafiuddin A, Salim MR, Beng Hong Kueh A, Hadibarata T, Nur H (2017) A review of silver nanoparticles: research trends, global consumption, synthesis, properties, and future challenges. J Chin Chem Soc 64(7):732–756 Thakkar KN, Mhatre SS, Parikh RY (2010) Biological synthesis of metallic nanoparticles. Nanomedicine 6(2):257–262 Velusamy P, Kumar GV, Jeyanthi V, Das J, Pachaiappan R (2016) Bio-inspired green nanoparticles: synthesis, mechanism, and antibacterial application. Toxicol Res 32(2):95–102 Vijayanandan AS, Balakrishnan RM (2018) Biosynthesis of cobalt oxide nanoparticles using endophytic fungus Aspergillus nidulans. J Environ Manag 218:442–450 Vijayaraghavan K, Mahadevan A, Sathishkumar M, Pavagadhi S, Balasubramanian R (2011) Biosynthesis of Au (0) from Au (III) via biosorption and bioreduction using brown marine alga Turbinaria conoides. Chem Eng J 167(1):223–227 Waghmare SS, Deshmukh AM, Kulkarni SW, Oswaldo LA (2011) Biosynthesis and characterization of manganese and zinc nanoparticles. Univ J Environ Res Technol 1(1):15 Yang F, Li Y, Liu T, Xu K, Zhang L, Xu C, Gao J (2013) Plasma synthesis of platinum nanoparticles and their deposition on the active fibres in one microreactor cycle. Chem Eng J 226:46–51 Yong P, Rowsen NA, Farr JPG, Harris IR, Macaskie LE (2002) Bioreduction and biocrystallization of palladium by Desulfovibrio desulfuricans. Biotechnol Bioeng 80(4):369–379 Zonaro E, Piacenza E, Presentato A, Monti F, Dell’Anna R, Lampis S, Vallini G (2017) Orchobactrum sp. MPV1 from a dump of roasted pyrites can be exploited as bacterial catalyst for the biogenesis of selenium and tellurium nanoparticles. Microb Cell Fact 16(1):1–17

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Fabrication of Green Nanomaterials: Biomedical Applications and Ecotoxicology Velaphi C. Thipe, Lucas F. Freitas, Caroline S. A. Lima, Jorge G. S. Batista, Aryel H. Ferreira, Justine P. Ramos de Oliveira, Tatiana S. Balogh, Slawomir Kadlubowski, Ademar B. Luga˜o, and Kattesh V. Katti

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Nanotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Synthetic Route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiolytic Reduction Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metallic Nanoparticles (Gold Nanoparticles) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymeric Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogel and Nanogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Protein-Based Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Albumin Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proteolytic Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Papain Nanoparticle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant-Mediated Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phytochemical-Based Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Clinical Translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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V. C. Thipe (*) Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/ CNEN-SP, São Paulo, SP, Brazil Institute of Green Nanotechnology, Department of Radiology, School of Medicine, University of Missouri Columbia, Columbia, MO, USA e-mail: [email protected] L. F. Freitas · C. S. A. Lima · J. G. S. Batista · J. P. Ramos de Oliveira · T. S. Balogh · A. B. Lugão Energy and Nuclear Research Institute (IPEN) – National Nuclear Energy Commission – IPEN/ CNEN-SP, São Paulo, SP, Brazil A. H. Ferreira MackGraphe, São Paulo, Brazil S. Kadlubowski Institute of Applied Radiation Chemistry (IARC), Lodz University of Technology, Lodz, Poland K. V. Katti Institute of Green Nanotechnology, Department of Radiology, School of Medicine, University of Missouri Columbia, Columbia, MO, USA © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_2

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Nanoradionutrapharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoimmunotherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ecotoxicology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The purpose of this chapter is to discuss the production of biocompatible green nanomaterials for biomedical applications using green nanotechnology. To enhance drug loading and delivery, these nanomaterials are engineered with immunomodulatory ligands such as phytochemicals (Epigallocatechin gallate, Mangiferin, Resveratrol), proteins (albumin and papain), crosslinked hydrogels, and nanogels. The nanomaterials described herein are synthesized via redox potential of electron-dense phytochemicals that reduce metallic precursors to their stable corresponding nanoparticles and via water radiolysis with ionizing radiation as a green approach (due to the absence of any reducing agent) for use as radiosensitizers (albumin and papain nanoparticles) in nuclear medicine – theranostics applications. The phytochemicals facilitate the delivery of nanoparticles through receptor mediated endocytosis, while the proteins such as papain, due to their proteolytic action enhances the permeation of nanoparticles into tumor tissue, and albumin increase the pharmacokinetic efficiency of these nanoparticles. The nanoparticles developed have shown effectiveness against a variety of human cancers while posing no toxicity to normal tissue. Additionally, a pilot human clinical combing Ayurvedic medicine with green nanomedicine is given as a novel approach for treating breast cancer and other related illnesses. Finally, the importance of ecotoxicology for nanomaterials is discussed in order to provide safety data in relevant multiple species (fish, daphnia, algae, rodents, etc.) with paratope/epitope distributions for evaluating tissue cross-reactivity profiles in human tissues and to provide critical information on in vivo toxicity in order to predict the possible adverse effects of nanomaterials on human and environmental health as an effort to establish regulatory limits and ISO standards for nanomaterials. Keywords

Green nanomaterials · Immunomodulatory · Combinational therapy · Ecotoxicology

Introduction Nanotechnology has provided extraordinary benefits in a broad range of applications for the advancement of human civilization and will continue to provide new innovative frontiers in nuclear medicine, radiopharmacy, and oncology for many millennia, equivalent to the Biotechnology revolution. Nanotechnology continues

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to receive attention due to the properties associated with the availability of surface atoms that serve as multifunctional platforms for the incorporation of target-specific biomarkers (peptides, proteins/antibodies) or therapeutic probes – all aimed at achieving unprecedented multiplexity for achieving and amplifying therapeutic outcomes using both metallic and non-metallic nanoparticles. Over the next few years, this technology is expected to become an obligatory specialty in medicine encompassing all facets of medicine, including diagnostics, treatments, theranostics, and regenerative medicine. Numerous non-metallic and/or hybrid polymer-based nanomaterials are being developed as carriers for encapsulating pharmaceutical drugs in order to allow effective tumor-targeted and specific treatment. The architectural design and development of nanoparticles enables their functionalization with a variety of signaling and targeting biomolecules for clinical translational medicine, enabling previously unfathomable approaches in molecular imaging and therapy of breast, head and neck, lung, and prostate cancers, as well as a variety of other debilitating diseases. Advances in nanotechnology for single-cell-specific delivery of diagnostic or therapeutic payloads will undoubtedly result in significant medical breakthroughs because they will provide realistic clinical translational hope for early detection and subsequent therapy – all guided with unprecedented clinical precision and exhibiting no to low systemic toxicity, improved prognosis, and therapeutic efficacy.

Green Nanotechnology Numerous biomimetic synthetic approaches for the fabrication of various metallic and non-metallic nanoparticles have been published in the recent years. This includes microbiological, plant extracts, biopolymers stabilizer templates, and radiation synthesis – all of which are regarded to be benign, environmentally friendly methods for producing nanoparticles. Several researchers have reported on the use of green nanotechnology in sophisticated biological applications such as drug delivery systems, imaging, and therapy (Al-Yasiri et al. 2019; Katti et al. 2018; Khoobchandani et al. 2020; Santos et al. 2018; Seniwal et al. 2021; Thipe et al. 2019). Due to their size and availability of surface atoms, immunomodulatory functionalized nanoparticles have a strong affinity for overexpressed tumor cell receptors, making them an attractive tool for detecting, imaging, and treating tumors/cancers at cellular and molecular levels (Khoobchandani et al. 2021). Against the backdrop of the current status quo in terms of climate changes and disease outbreaks, this chapter focuses on green nanotechnology advancements in our collaborative research groups in Brazil, Poland, and the United States, which are backed up by tremendous strength in terms of interdisciplinary research attributes, synergy, and a cohesive thematic of cellular/subcellular theranostic applications in oncology. Due to their biocompatibility, simplicity of synthesis, multifunctionality with target-specific moieties/ligands, and ability to retain an adequate biological half-life, gold nanoparticles and albumin nanoparticles are ideal probes for a broad range of biomedical imaging and treatment applications. As seen in Fig. 1, we have

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Fig. 1 Green generational nanosystems used in imaging and targeted delivery of therapeutic biomolecules/drugs for biomedical applications

classified the presently investigated and well-known nanosystems, such as therapy, imaging, and targeted delivery of biomolecules, into three “generation” groups (Santos et al. 2018). Our overarching goal is to develop green nanomedicines using a multilateral niche-specific approach that leverages beneficial characteristics of Nano-Ayurvedic medicine and radiopharmaceuticals to develop targeted “Nanoradio-Nutra-Pharmaceutical” (NNP) pharma with emphasis on immunomodulation for improved safer cancer diagnosis and treatment. This is done via the development of novel multifunctional nanosized delivery systems aimed at optimizing diagnostic and therapeutic outcomes. The physicochemical properties of these nanocarriers dictate particle-cell crosstalk interactions, optical properties, cellular trafficking processes, pharmacodynamics/pharmacokinetics, biodistribution, and overall therapeutic outcome and response. On the non-metallic nanoparticles front, researchers have investigated the use of a range of nanoparticles consisting of hydrogels/nanogels, albumins, papain, and a variety of biopolymers and biocompatible polymeric matrix as effective drug delivery vehicles have been explored (de Lima et al. 2020; Fazolin et al. 2020; Freitas et al. 2020). Green nanotechnology has proactively steered the development of nanostructures in such a way that they are both safe and effective modalities without compromising the environment or human/animal health. The following section discusses (i) polymeric nanomaterials (hydrogels and nanogels), (ii) protein- and protease-based nanoparticles (albumin and papain), (iii) metallic nano-architectures based on phytochemicals (epigallocatechin gallate (EGCG), mangiferin (MGF), and resveratrol (RESV)) and their immunomodulatory effects, (iv) nanoradiopharmaceuticals for nuclear nanomedicine, and (v) the ecotoxicity of nanomaterials in order to provide safety data for predicting the potential adverse effects of said nanomaterials on human and environmental health in order to establish regulatory limits

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and ISO standards for nanomaterials in the context of green chemistry towards a “zero carbon emission” innovations.

Green Synthetic Route Nanomaterials may be synthesized in a variety of ecologically friendly ways. These include plant-mediated synthesis (e.g., leaves, roots, flowers, and fruits extracts) based on the availability of effective phytochemicals and their redox potential, microbial-mediated synthesis (e.g., bacteria, algae, and fungal cells), and irradiation (e.g., gamma radiolysis, electron beam bombardment, microwave, ultrasonic, ultraviolet photolysis, X-rays) utilizing a benign solvent and taking into account various reaction parameters such as solvent, temperature, pressure, and pH conditions (acidic, basic, or neutral). The techniques for producing nanomaterials are governed by electron density, which contributes electrons for reduction/crosslinking throughout the nanomaterials manufacturing process. Through green nanotechnology, green methods must comply to the twelve principles of green chemistry. In this chapter, we explain how to synthesize nanomaterials (metallic and non-metallic nanoparticles) using ionizing radiation (gamma and high-energy electron irradiation) and phytochemistry, as well as their biological applications.

Radiolytic Reduction Method The use of ionizing radiation, such as gamma-irradiation, to synthesize metallic and non-metallic nanoparticles is a potentially environmentally friendly technology that generates significant amounts of reactive species. In aquatic conditions, reactive species have a short half-life of roughly 150 s to 5 min after ionizing radiation from gamma rays and high-energy electrons. H2O absorbs most of the energy, generating

Fig. 2 Schematic of mechanisms of water radiolysis for the generation of radicals as a function of time. *Represents macroradicals

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hydroxyl radicals (•OH), hydrogen atoms (H), and hydrated electrons (e
(aq)). In solutions saturated with N2O, hydrated electrons – inert to water-soluble aliphatic polymers may be converted to additional hydroxyl radicals (Ma et al. 2018). As seen in Fig. 2, such radicals and hydrogen atoms may remove hydrogen atoms from polymer chains, forming free radicals centered on carbon throughout the chain, resulting in macroradicals (ROO•). These macroradicals can be recombined either through intermolecular crosslinking, which results in higher molar mass species that do not contribute to the three-dimensional architectural design of the nanomaterials, or through intramolecular crosslinking, also known as cyclization, which results in three-dimensional network structures without increasing molar mass (Kadlubowski 2014; Matusiak et al. 2018; Neamtu et al. 2017). Radiolysis provides several advantages for synthesis, including its robustness, the nontoxic nature of the irradiation energy, and the ability to execute controlled synthesis without using an excessive amount of reducing agent or generating undesirable oxidation products. At some point, the solvated electrons reduce the metal ions and the metal atoms unite to form aggregates (Piroonpan et al. 2020; Tangthong et al. 2021). Additionally, a biopolymer stabilizer template (chitosan, silk fibroin or albumin) may be introduced simultaneously to the radiolysis synthesis. Due to the multivalent properties of nanoparticles resulting from the availability of surface atoms, they can be functionalized with specific and selective moieties/ ligands for targeted delivery to tumors as an effective strategy for theranostics (imaging and targeted therapy in a single entity) with the goal of minimizing non-specific targeting to adjacent normal tissues. As mentioned earlier, green nanotechnology permits the production of metallic and non-metallic nanoparticles. The next sections address the production of gold nanoparticles, nanogels, albumin, and papain nanoparticles, as well as their biological applications.

Metallic Nanoparticles (Gold Nanoparticles) Gold nanoparticles (AuNPs) are one of the most extensively studied nanoparticles as nanocarriers for imaging, diagnostics, and photothermal therapy-based cancer treatment. AuNPs are seen as promising agents to produce new molecules with potential uses in cancer therapy, as well as the treatment of chronic inflammation, diseases, degenerative, and autoimmune disorders. To synthesize AuNPs, a reducing agent with high redox potential that is also stable is required. Due to the highly negative redox potential of metal atoms, certain moderate reducing agents are unable to initiate the reduction of these metals without adsorption on surfaces or already produced nanoparticles. Therefore, chemical reduction often necessitates the use of powerful and often toxic reducing agents that are difficult to regulate, resulting in particles with a wide size distribution. By contrast, lysis of a solution containing metal atoms produces radicals with a high reducing potential such as hydrogen atoms, solvated electrons, and radicals generated from organic solvents, such as 2-propanol. These radicals can reduce metal atoms in their free state, hence initiating the nucleation of metal aggregates and nanoparticles (Fig. 3).

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Fig. 3 Radiolytic synthesis of gold nanoparticles where (1) gold cations in solution (water and a co-solvent, i.e., isopropanol and/or acetone) are irradiated with high-energy radiation; (2) water radiolysis leads to hydroxyl radicals (oxidative potential), hydrogen radicals, and solvated electrons (both with reducing potential); (3) the co-solvent scavenges hydroxyl radicals, becoming other reducing radicals themselves; (4) all the reducing agents contribute to the reduction of gold cation into metal gold; (5) the nucleation of the nanoparticles takes place and (6) growth of the nanoparticles

The radiation impacts may be amplified via the use of nanoparticles and metal aggregates. In principle, the presence of radiation-absorbing elements in tissue, especially high-Z elements, amplifies the impact of many forms of radiation by generating secondary electronic effects (Ni et al. 2020). For example, it is well recognized that photons with energies between 100 and 150 kV interacting with metallic gold may generate photoelectrons with high energy (about 80 keV) capable of enhancing the energy dosage delivered to the target tissue. After interaction with high-energy radiation, metallic golf may also release extremely low-energy electrons, leading to the increased absorbed dose in the irradiated tissue. These low-energy electrons are referred to as Auger electrons. They are characterized by their capacity to generate numerous ionizations with high linear energy transfer (LET) (between 4 and 26 keV/μm), which enables them to travel very short distances, often less than 500 nm, which is crucial for localized therapies, as shown in Fig. 4. If the freshly formed electronic vacancy is not immediately filled by an electron from a more outer layer, a second Auger event may occur, resulting in an Auger cascade. The amount of Auger electrons released in this event is proportional to the atomic number of the target element, which has already been determined for many elements. Due to the fact that the release of a single electron from an interior layer may result in the cascade release of several Auger electrons, incoming radiation can be significantly amplified in the presence of certain high-Z elements such as In, I, Au, Pt, and Pd. Globally, the number of research and preclinical procedures in this sector is large and expanding, due to the therapeutic potential it offers. Given the complexities of the dosimetry in these settings as a result of the accumulation of direct and indirect effects of radiation, specific computational approaches based on MonteCarlo simulations are necessary to offer quantitative information about the effective dosage and cellular damage (Seniwal et al. 2021). Gold and palladium are biocompatible high-Z metals that have the potential to be used as radio-sensitizing

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Fig. 4 Gold nanoparticles produced by ionizing radiation and their activity against tumor cells facilitated by high linear energy transfer generation from secondary electronic effects

nanomaterials (Miller et al. 2017). Gold is well-suited for a wide variety of applications in a plethora of areas. One recent examples is the potentiation of proton radiation in an in vitro technique utilizing a 160 MeV proton beam. Kim et al. (2012) also similarly demonstrated proton radiotherapy potentiation in vivo, using tumor-bearing mice and irradiation with a 40 MeV proton beam in the presence of 100–300 mg/kg gold nanoparticles in the tumor tissue, among other comparable and positive findings.

Polymeric Nanomaterials Polymeric-based nanoparticles have gained enormous popularity in recent years, due to their superior biocompatibility, biodistribution, and biodegradability, as well as their novel properties (such as low toxicity, absorbability, and self-assembly properties, among others) that make them suitable for a wide variety of biomedical applications. The development of protein-based nanoparticles for application in oncology and nuclear medicine as carriers for therapeutic compounds and radionuclides demonstrates a commitment to clinical translation. This is due to abundance of proteins derived from natural sources, their biocompatibility, biodegradability, and

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the ease with which synthetic automation processes may be automated for industrialization (Voci et al. 2020).

Hydrogel and Nanogels Hydrogels and nanogels have sparked significant interest in the realms of biomedical and pharmaceutical industries. They have shown their adaptability and feasibility as platforms for continuous drug delivery, release, and tissue engineering. These materials are composed of either synthetic polymers (e.g., poly(lactic acid), poly(lactic-coglycolic) copolymers, polyacrylates, polymethacrylates, poly(N-vinylpyrrolidone), poly-(vinyl alcohol), poly-(methyl vinyl ether), and poly(
ε-caprolactone) among others) or natural polymers (such as collagen, albumin, fibrin, chitosan, hyaluronic acid, heparin, chondroitin sulfate, agarose, alginate, gellan gum, zein, soy, camelina protein, and wheat gluten). Hydrogels with an adequate amount of hydrogen bond-forming groups (such as -OH and -COOH), ionic charges, flexible chains, and/or adequate surface tension exhibit increased mucoadhesiveness, a property that can be used to extend the residence time of encapsulated drugs and improve the specificity of pharmaceutics delivery to the desired location. Hydrogels may be used as scaffolds in tissue engineering to restore and heal wounded tissues or organs, as well as to fabricate artificial organs (e.g., kidney, skin, etc.). Hydrogels are already commercially available, with several products approved by federal health authorities such as the United States Food and Drug Administration (FDA), including Apligraf, TranCyte, PuraPly, and Moraxen. The nano-regime method of manufacturing hydrogels leads in nanogels that have decrease swelling ability but increased mechanical stability, functionality, stimuli-responsiveness, and therapeutic administration of various biological substances by intravenous injection. Crosslinking hydrogels and nanogels chemically crosslinked gels with covalent bonds for greater stability and physically crosslinked gels with weak, non-covalent bonds that permit sol-gel phase transitions in response to environmental stimuli (de Lima et al. 2020; Kadlubowski 2014; Matusiak et al. 2018; Neamtu et al. 2017). Radiation-induced inter- and intramolecular crosslinking have attracted substantial attention as environmentally friendly nanotechnology approaches to produce hydrogels and nanogels, respectively. High polymer concentrations result in the development of hydrogels, while low polymer concentrations allow intramolecular crosslinking and the formation of nanogels as shown in Fig. 5. Crosslinking can be accomplished by bombarding the sample with fast electron beam and gamma ionizing radiation from a cobalt-60 source. This is a green synthetic approach since it utilizes additive-free hydrogel and nanogel processes that are nontoxic and biocompatible. Radiation has numerous applications in synthesis, allowing for simultaneous polymeric crosslinking and sterilization (Tipnis and Burgess 2018) in a single step as a rapid procedure requiring just a single dose between 15–45 kGy, resulting in ease of application. Apart from being economically feasible, the

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Fig. 5 Green synthetic route for hydrogel and nanogel production utilizing gamma-irradiation to facilitate crosslinking and sterilization for safe and effective biomedical applications such as wound-healing dressing and filler for dental or bone implants. *Recommended dose to achieve crosslinking and sterilization

technique is unquestionably environmentally benign and a zero-carbon emission process. Nanogels may be modified for targeted controlled delivery of therapeutic agents and used in gene therapy and tissue engineering by conjugating their surface with binders, antibodies, and biological macromolecules such as peptides, proteins, polysaccharides, oligonucleotides, and enzymes. Numerous methods may be used to assist the incorporation of bioactive agents into nanogels: (i) covalent conjugation into preformed nanogels or during the synthesis of nanogels, (ii) physical entrapment via carrier-drug physicochemical characteristics, and (iii) controlled self-assembly, which is facilitated by diffusion and specific association of molecules via non-covalent intermolecular interactions, including electrostatic interactions and hydrophobic associations (Neamtu et al. 2017). Additionally, despite their promise for application as theranostic agents, research on nanogels as imaging agents remains in its infancy, with the great majority of investigations undertaken in vitro. To be employed as in vivo imaging agents in oncology, neuropsychiatry, and cardiology, nanogels must satisfy the following criteria: structural stability, in vivo contrast capacity, disease-specific accumulation, minimal toxicity, and background signal (Zhou et al. 2020). Cho et al. (2020) developed a fucoidan-based theranostic nanogel (CFN-gel) consisting of a fucoidan backbone, redox-responsive cleavable linker, and a photosensitizer for active nearinfrared fluorescence imaging of tumor sites in combination with enhanced photodynamic therapy (PDT) to induce cancer cell eradication. Nanogels are gaining popularity in a variety of biomedical applications as components of biosensors, nanoreactors, carriers and vehicles for controlled release

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of therapeutic agents, and biomimetic mechanical devices such as artificial muscles, in addition to their significant potential for use in gene therapy, tissue engineering, chemotherapy, release of bioactive substances, diagnosis of diseases and vaccines, cell culture systems, biocatalysis, and the development of bioactive scaffolds in regenerative medicine (Neamtu et al. 2017).

Protein-Based Nanoparticles Protein-based nanoparticles including both water-soluble proteins (e.g., bovine and human serum albumin) and insoluble proteins (e.g., zein and gliadin), gelatin, legumin, elastin, soy protein, and milk protein have attracted substantial interest. Nanoparticulate systems may be supplied orally, intravenously, or subcutaneously or inhaled. Protein-based nanoparticles have been employed as an alternative to enhance pharmacokinetics/pharmacodynamic characteristics and biodistribution of several drugs and radiopharmaceuticals. Due to the inherent properties of proteins, such as their abundance of hydroxyl, amino, carboxyl, thiols, and phenols groups, the internal and external surface reactivity of protein nanoparticles can be functionalized with a variety of different moieties/ligands (peptides, proteins, carbohydrates, antibodies, drugs, metals, among others) for targeted theranostics with low systemic toxicity (Varca et al. 2016). These ligands may be introduced by covalent bonding or intermolecular interactions, hence modifying the uses of nanoparticle (Varca et al. 2016). Nanostructured protein systems have been used in biomedical applications for drug and gene distribution, vaccine development, bioimaging, catalysis, and material production. In comparison to alternative systems based on metals and other inorganic and synthetic materials, proteins are attractive starting materials for a multitude of reasons, the most notable of which being their greater biocompatibility. However, in comparison to these materials, they exhibit a lack of stability (Paredes et al. 2019). Currently, protein-based nanoparticles are classified into the following nanomedicine classes: (i) Drugs conjugated to protein nanocarriers (ii) Active therapeutic proteins (iii) Hybrid platforms reliant on protein for targeted therapeutic delivery In nanomedicine, these biogenic systems are now a reality. Several of them have been authorized by FDA, including the following: • Abraxane – Paclitaxel encapsulated in albumin nanoparticles (130 nm) containing 100 mg of paclitaxel and approximately 900 mg of human albumin were approved in January 2005 for the treatment of metastatic breast cancer, lung cancer (2012), and metastatic pancreatic adenocarcinoma (2013).

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• Ontak – It is a recombinant protein-derived diphtheria toxin, called denileukin diftitox, fused to human interleukin-2, that is indicated for the treatment of relapsed or refractory cutaneous T-cell lymphoma. • Rebinyn – It is a drug derived from pegylated human recombinant Factor IX that is indicated for the treatment of hemophilia. Nanoparticle production strategies aimed at developing smart delivery systems with increased bioavailability, tissue specificity, and prolonged systemic circulation duration are a continuous goal of nanomedicine research for precision/personalized medicine. Plasma protein nanoparticles have been found to be superior to synthetic nanoparticles in terms of efficacy. This is largely due to their properties (low toxicity, biodegradability, prolonged circulation time, and a general absence of cytokine storms) which makes them more suitable for in vivo applications. Blood plasma is a good source of biomaterials due to its high concentration of serum albumin (35–50 mg/mL), transferrin (2.5–3.5 mg/mL), globulins (2.0–2.5 mg/mL), and fibrinogen (0.3–0.45 mg/mL) (Pinals et al. 2020). Numerous synthetic nanoparticles are recognized by the body through serum proteins and other biomolecules adsorbed on their surfaces, forming a protein corona. This protein corona alters and influences the in vivo pharmacokinetics/pharmacodynamics and biodistribution of the nanoparticles, ultimately restricting their targeting ability by preventing them from adhering to and internalizing their targets.

Albumin Nanoparticles Albumin (molecular weight 66.5 kDa) is the most prevalent plasma protein, constituting 55% of total human serum proteins. It is stable between physiological pH values of 4–9 and is heat resistant (at least 10 h at 60  C). Additionally, it has a serum half-life (t1/2) of ~21 days (Kenanova et al. 2010). It is produced in the liver from 584 amino acid chains, yielding a polypeptide that contributes for about 12% to 20% of hepatic capacity (Queiroz et al. 2016). Albumin regulates blood pH; maintains plasma volume in circulation; modulates fluid distribution throughout body compartments, responsible for 80% of oncotic pressure; and transports endogenous and exogenous compounds (fat-soluble molecules of fatty acids, hormones, metal ions, peptides, proteins, and drugs) into biological fluids, increasing their bioavailability and stability. Albumin is currently accessible in a consistent and reproducible supply using recombinant DNA technology in bacterial systems. In oncology, albumin is a naturally occurring effective transport protein with multiple ligand binding sites for active transport mediated by sialoglycoprotein (GP60) receptors present in vascular endothelium cells and caveolin-1 (transcytosis), as well as functional binding of tumor albumin-drug complexes via secreted protein acidic and rich in cysteine (SPARC) proteins, all of which result in proteolysis protection. Therefore, albumin is an ideal carrier for therapeutic, diagnostic, and theranostic drugs.

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SPARC proteins are released into the extracellular milieu and regulate tumor proliferation and invasion, as well as angiogenesis (Chong et al. 2012). SPARC proteins are overexpressed in a number of cancers (breast, prostate, esophageal, colorectal, liver, and lung) and may serve as a target for albumin nanoparticles by suppressing carcinogenesis (Parodi et al. 2019). Numerous processes are available for the development of protein nanoparticles, including desolvation, emulsification, thermal gelation, solvent evaporation, and spray drying. A new approach, the radiolytic route, was recently revealed. Chemical modifications of protein nanoparticles are often necessary during or after production to improve their stability and reduce their degradation rate. In this instance, highly hazardous synthetic crosslinking chemicals (such as formaldehyde or glutaraldehyde) were used. On the other hand, natural crosslinkers such as transglutaminase and genipin are less hazardous than their synthetic equivalents. However, multiple washing procedures are required to effectively remove these chemicals. As an alternative, as previously indicated, irradiation to induce crosslinking allows for size control without the use of crosslinking agents and promotes medium sterilization without generating hazardous byproducts (Kadlubowski 2014; Varca et al. 2016). The synthesis of nanoparticle produced by radiation in an aqueous solution is principally mediated by free radicals formed during water radiolysis. When these species interact with protein, conformational changes occur, exposing the protein to oxidative stress conditions. This results in the formation of dityrosine bonds, which are a major kind of the primary linkages involved in protein crosslinking (Fazolin et al. 2020). Additionally, irradiation offers complete control over the dose of radiation, resulting in reproducible and reliable nanoparticles shape and distribution. This technique can be used to synthesize organic nanoparticles, hybrid nanoparticles, nanocomposites, and nanogels using a gamma-radiation source, electron or ion beam, photons, or X-rays. The fundamental concern while producing protein nanoparticles is ensuring that their native protein structure and primary function are preserved. The difficulty is that the synthesis technique has no influence on these characteristics since it might result in the loss of functionality, such as binding affinity at desirable sites for effective release of the active compound. Despite these concerns, the radiolytic pathway used to produce protein nanoparticle may have a detrimental effect on the biological properties of the materials in relation to the architectural protein nanoparticle design – preservation of the protein’s original molecular properties, primary function, and binding efficiency for hydrophobic drugs, as well as pharmacodynamics and pharmacokinetics of drugs for improved release (Siri et al. 2019). Optimizing the synthesis of nanomaterials has emerged into a thriving topic of study in the design and development of radio- and nutra-pharmaceuticals. The production of radiolytic protein nanoparticle synthesis should be considered a green nanotechnology strategy, since it does not include the use of hazardous chemicals, which limits its use in clinical translational medicine, and requires very little energy. Additionally, radiolytic synthesis of protein nanoparticles provides enormous potential for industrial-scale production. Plasma protein-based nanoparticles have generated considerable attention in recent years due to their

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hemocompatibility and relatively long elimination half-life (t1/2). Blood is a great supply of material for nanostructured systems used in therapeutic and diagnostic approaches, including albumin, transferrin, lipoproteins, and ferritin (Iqbal et al. 2020). These may be used to increase the drug’s cellular absorption and biodistribution. Albumin has been widely explored and is used in a range of pharmacological applications. It has been used in three distinct ways as a green nanotechnological approach to zero-carbon emission: • Model of encapsulated pharmaceuticals used as the main polymeric constitute to encapsulate other drugs • Polymeric coating agent for a variety of formulations including metallic nanoparticles • Radiopharmaceutical used to detect sentinel lymph nodes and lymphoscintigraphy (Lamichhane and Lee 2020) The most common protein in the blood (it accounts for 52–60% of all proteins) is human serum albumin, which is generated by liver cells. It is crucial for regulating the plasma volume and osmotic pressure of circulating blood, as well as transporting a range of hormones, enzymes, fatty acids, bilirubin, metal ions, and drugs (Levitt and Levitt 2016). Numerous advantages have been reported for the use of albumin nanoparticles as drug delivery carriers in clinical oncology; these include passive accumulation in tumor sites due to fenestrated vasculature, which results in increased permeability and retention effect (Golombek et al. 2018) and decreased toxicity of encapsulated drugs when compared to free drugs and allows for optimal drug biodistribution, thereby eliminating the need for specialized excipients. In nuclear medicine, albumin nanoparticles complexed with a radioactive element with a short half-life (such as 99mTc) allow for non-invasive visualization/imaging of physiological processes when used in conjugation with the appropriate imaging equipment. Currently, this is the gold standard for defining the lymphatic system and detecting sentinel lymph node (SLN) in breast cancer and melanoma in the European Union (EU). Ionizing radiation crosslinking of albumin nanoparticles offers precise size control while maintaining bioactivity and successfully conserving the three-dimensional structural characteristics of the particles for biomedical applications. This low-cost technique has been improved to ensure specific properties and eliminates the need for hazardous agents or chemical crosslinkers that are often used in the production of protein nanoparticles. The radiolytic synthesis of albumin nanoparticles has shown significant clinical translation promise. There are products already on the market including nuclear imaging agent Nanocoll by GE Healthcare, USA – a 99mTc nano-sized albumin colloid radiopharmaceutical (5–80 nm) that is used in multimodal imaging of SLN via single photon emission computed tomography (SPECT) or computed tomography (CT), as well as near-infrared (NIR) imaging; ##Nanotop/NanoHSA by ROTOP Pharmaka GmbH, Germany is a 99mTc nano-sized albumin colloid radiopharmaceutical composed of 0.5 mg human albumin, colloidal nanoparticles (80 nm), stannous chloride, dihydrate, glucose, poloxamer 238, disodium phosphate dihydrate,

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and sodium phytate used for scintigraphic sentinel node mapping imaging and assessment of SLN in tumor diseases (i.e., malignant melanoma and breast cancer) and the integrity of the lymphatic system and differentiation of venous from lymphatic obstruction (Persico et al. 2020). Due to the well-defined particle size distribution, quality assurance and control criteria such as qualitative and quantitative composition are comparable to those used in Nanocoll, the EU’s gold standard treatment.

Proteolytic Enzymes Plant proteases provide a therapeutic modality characterized by their ability to catalyze a variety of physiological processes, including protein catabolism, blood coagulation, inflammation, tumor proliferation, and migration. Proteolytic enzymes have been investigated in oncology as a systemic enzyme therapy, including the use of plant proteases such as papain, bromelain, ficine, and cumisin.

Papain Nanoparticle Papain is a papaya protease with a molecular weight of 23.4 kDa. It contains 212 amino acids within the pocket of the enzyme’s active site, including cysteine (Cis-25), histidine (His-159), and asparagine (Asn-175). Papain has a high thermal stability and a high solubility in organic solvents and is a denaturing agents with maximal activity at pH 6.0–7.0 and 60  C (Amri and Mamboya 2012). Additionally, papain contains a variety of potentially beneficial pharmacological effects for medicinal applications, including antibacterial, antifungal, and antioxidant capabilities, as well as anticancer, antiproliferative, and antimetastatic properties (BudamaKilinc et al. 2018). Papain nanoparticles may be produced via a synthetic route that utilizes radiationinduced free radicals in an aqueous solution generated during water radiolysis. Papain undergoes structural changes in the presence of these substances, resulting in oxidative stress. The production of dityrosine bonds is one of the key bonds involved in crosslinking (Fazolin et al. 2020). This approach is preferable to traditional methods in that it is not constrained by the presence or absence of monomers throughout the reaction. Simultaneously with the nanoparticle creation, the reaction flask’s inside is sterilized. The lack of crosslinking agents ensures low residual toxicity and reduces the number of possible monomer purification stages (Queiroz et al. 2016). While protein-based nanoparticles allow for a variety of surface modifications including covalent attachment of drugs and targeting ligands, which enables systemic delivery of anticancer agents, it is widely accepted that small particles (5, and as pH increased, particle size increased (15–20 nm) at pH Eg’ as shown in Fig. 3. This process causes a redshift in the UV absorption spectra. So overall, the reduction of the bandgap and the rate of rejoining of e /h+ pair is evident (Chand and Singh 2020; Kumar et al. 2021). Additional reports also suggest that rare earth metals form the oxide coating in ZnO, as rare earth metals are significantly bigger than those in ZnO. Due to this the dopant forms a layer on semiconductor surface. However, such a layer of oxide is composed of rare earth metals in ZnO surface and works in the same way. The oxide layer has a defect that also holds the electrons produced due to radiation by a light source. The oxide layer restricts the e /h+ pair reassembling and improves the photocatalytic activity. This chapter mainly deals with doping engineering of ZnO nanostructures with transition metals for the enhanced photocatalytic activities. When a metal oxide (MO)n+ is doped with some dopant (x), it creates defects in the crystal lattice of the metal oxide with generation of sub-energy levels in between the VB and CB. These energy levels act as electron trapping centers and also reduce the energy bandgap for excited electron from VB. In this process, the metal dopants are oxidized and form x+a where a depends upon the metal. The x+a actually traps the electron which is created in irradiation by sunlight. This capture center is responsible for preventing the rejoining of the electron hole and thus improves the delivery of electrons to the CB involved in photocatalytic activity. The actual reactions that take place are shown in Scheme 2.

Transition Metal-Doped ZnO Nanostructures and Their Photocatalytic Applications Apart from all classes of metal-doped ZnO nanostructure discussed in section “Doping with Metal,” transition metals have the vast option of doping elements such as Mn, Fe, Cu, Co, Ni, Ag, Cr, Ru, etc. In literature transition metal-doped ZnO nanostructures were studied extensively to improve the photocatalytic activity because of their comparable size and similar chemical nature as of Zn. Due to which they can easily show substitutional or interstitial doping and create surface defects and trapping centers as discussed in section “Transition Metal Doping.”

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Scheme 2 Chemical reaction in doped metal oxide photocatalysis

Doping with transition metal also decreases the crystallite size and recombination rate of e /h+ pair and increases the surface area; all these tuned properties further enhance the photocatalytic activity of the doped ZnO sample. In this section, we discuss briefly the recent transitional metal-doped ZnO nanostructures and their effect on structural, optical, and electric properties. Various transition metal-doped ZnO nanostructures with improved photocatalytic activity were listed in Table 1.

Mn-Doped ZnO Nanostructures Mn was mostly used to tune the optical and electronic properties of ZnO and further enhance the photocatalytic activity (Shahmoradi et al. 2011). The ionic radii of Mn2+ is similar to Zn2+, in that it can easily install Zn2+ in ZnO lattice. Mn doping creates a broad absorption band due to d-d transitions. The sub-energy levels formed by intrinsic defects and Mn-induced states act as an intermediate and facilitate the electron excitation from VB to CB by absorbing visible light (Voicu et al. 2013). Mn doping induces abundant defect levels and strain in ZnO lattice, due to which the crystallite size of ZnO nanostructures decreases and surface area increases leading to the improved photocatalytic activity (Mahmood et al. 2011). Raskar et al. (2020) studied the improved photocatalytic activity of Mn-doped ZnO nanoflowers. They observed that Mn doping creates abundant oxygen vacancies in crystal lattice which traps the electron and reduces the recombination rate of the charge carriers showing excellent photocatalytic activity. Doping effect was studied with XPS as shown in Fig. 4a. The FWHM of O 1 s peak decreased in the case of 4% Mn-doped ZnO (2.40) as compared to pure ZnO (2.70), which indicates that for Mn-doped ZnO nanoflowers have higher surface defects. SEM image in Fig. 4c confirmed the nanoflower-like morphology formed by cluster of nanorods. Also, in binding energy at 531.5 eV peak of oxygen vacancies, change in intensity of

Morphology Nanoflower

Spherical flakes

Spherical shape

Nanoparticles

Rice like

MWCNT

Nanoparticles

Transition metal dopant Mn

Mn

Mn

Mn

Mn

Fe

Fe

Coprecipitation method

Solgel method

Coprecipitation

Solgel method

Coprecipitation method

Synthesis method Hydrothermal method Green synthesis

Redshift with bandgap of 3.82 eV

UV lamp

UV lamp 365 nm

UV light

UV light 256 nm

– PL spectra show defect states and bandgap is 3.1 eV UV spectra show broad absorption so bandgap is 3.31 eV from PL spectra

UV light 253.7 nm

UV light

Bandgap 2.45 eV

Bandgap 2.75 eV

Optical properties Bandgap 2.99 eV

Irradiation sources for photocatalytic reactions Visible light

Table 1 List of transition metal-doped ZnO nanostructures showing photocatalytic activity

Doped sample shows both ferromagnetic, low recombination rate of electronhole pairs, and higher photocatalytic activity 0.075% Fe-doped sample shows 68% degradation efficiency at pH 6

Photocatalytic mechanism, efficiency (yield), and results Doped sample shows 81% photocatalytic efficiency Under 3 h irradiation time, doped sample without H2O2 shows efficiency of 78% and 81% with H2O2 Mn-doped sample shows rate constant 0.0145 min 1 and BET surface area 9.8 m2/g Improved photocatalytic activity Superior photocatalytic activity

(Kumar and Arunagiri 2021)

(Popa et al. 2021)

(Otadi et al. 2021) (Das et al. 2020)

(Biswas et al. 2021)

References (Raskar et al. 2020) (Dhivya and Yadav 2021)

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Spherical nanoparticle

Nanofibers

Sea urchin like

Nano ellipsoidal

Nanowires

Nanorods and nanoparticles

Hairlike

Nanoparticles

Spindle shape

Fe

Fe

Fe

Fe

Co

Co, Ni

Co

Co

Ni

Coprecipitation

Biologicalmediated method Green synthesis

Solgel method

Lowtemperature process Solvothermal method

Precipitation method

Electrospinning Method

Coprecipitation method

3.59 eV

2.95 eV bandgap

Bandgap narrowing

Decrease in bandgap energy (3.62–2.40 eV)

Low recombination rate of e /h+ pair

3.3 eV bandgap

Bandgap 3.19 eV and low recombination rate of e /h+ pair 2.96 eV bandgap

Bandgap 2.93 eV

0.05% doped sample shows 72% degradation rate against MO Co-doped sample shows better photocatalytic activity as compared to Ni doped Photodecoloration was achieved within 1–30 min for MB, MO, and RhB Photocatalytic efficiency of 99.6% of MB was observed on 8th day of Co accumulation 6% doped sample shows 77% degradation rate with photocatalyst dosage 0.1 g/L, 15 ppm of dye concentration, and pH 9

Visible light

UV lamp 254 nm

Visible light

UV lamp 365 nm

UV lamp

UV lamp

5% Fe-doped sample shows degradation efficiency of 97.70% Excellent photocatalytic activity

Photocatalytic activity decreases with increase in dopant concentration 1.5% doped sample shows excellent photocatalytic activity

UV lamp

UV light

UV lamp 254 nm

(continued)

(Devi et al. 2021)

(Zelekew et al. 2021)

(Dhandapani et al. 2020)

(Kumar and Arunagiri 2021) (Ali et al. 2020)

(Kumar et al. 2018)

(Hui et al. 2017)

(Liu et al. 2018)

(Roguai and Djelloul 2021)

11 Recent Progress on Doped ZnO Nanostructures and Its Photocatalytic Applications 231

Spherical nanoparticles

Nanoplate

Nanoparticle

Thin films

Nanorods

Nanoparticles

Nanoparticle

Cu

Cu

Cu

Ag

Ag

Ag

Morphology Hexagonal rod and sheetlike nanostructures Nanoparticles

Cu

Ni

Transition metal dopant Ni

Table 1 (continued)

Coprecipitation method Coprecipitation method Green synthesis

Hydrothermal method Solgel method

Flash combustion method Coprecipitation method

Synthesis method Ultrasonicassisted coprecipitation Solgel method

Blueshift in absorption band

Bandgap narrowing

Bandgap narrowing

Narrowing of bandgap 3.11 eV Increase in bandgap

Bandgap narrowing (3.19 eV) and generation of more defects Bandgap of 2.995 eV reduced e /h+ pair recombination Blueshift in absorption peak

Optical properties Reduced bandgap 3.02 eV

Photocatalytic action of Cu-ZnO samples was 3.5 times greater to neat ZnO Degradation efficiency for Cu-doped ZnO becomes much higher than pure ZnO Enhanced photocatalytic activity Enhanced photocatalytic activity Ag-doped sample shows 95% photodegradation efficiency 2% Ag-doped sample shows excellent photocatalytic activity Higher photodegradation rate of doped sample

Visible irradiation 425–460 nm Visible light, a lamp with 25 watt Visible light

UV lamp

Fluorescent tubes 15 W 500 W halogen lamp UV lamp

Doped sample shows excellent photocatalytic activity

Photocatalytic mechanism, efficiency (yield), and results 99% dye degradation rate

UV lamp

Irradiation sources for photocatalytic reactions UV lamp

(Gaurav et al. 2019) (Jongnavakit et al. 2012) (Shelar et al. 2020) (Fadhil et al. 2020) (Saravanadevi et al. 2020)

(Chatterjee and Kar 2020)

(Chandekar et al. 2020)

(Prerna et al. 2020)

References (Gnanamozhi et al. 2020)

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Thin film

Nanoparticles

Nanorods and nanoparticles

Nanorods

Nanoparticles

Nanoparticles

Ag

Ag

Ag and Ni

Cd

Cr

Zr

Solgel method

Solgel method

Hydrothermal method

Solgel method

Solgel and doctor blade technique Green synthesis

Narrowed bandgap and reduced recombination rate of e /h+ pairs

Reduced bandgap of 3.09 eV

UV lamp

Reduced e /h+ pair recombination rate and bandgap Ni doping shows increase in bandgap

UV lamp

Visible light (OSRAM lamp 58 IM/W)

UV lamp

UV lamp 352 nm

UV lamp

Narrowed bandgap 2.95 eV

5% doped sample shows degradation rate is 20.5 times higher than unmodified ZnO Ag-doped ZnO shows 96% and 94% photodegradation rate of Congo red and MB Ag doped shows better photodegradation rate of 99.93% as compared to Ni doped Showed that at pH 7, Cd-ZnO dosage of 3 g/l, and phenol concentration of 20 mg/l under irradiation of 120 min have highest photodegradation High surface area, the Cr-doped ZnO (500  C) shows the highest photocatalytic activity with efficiency of 94% within 60 min under visible light 4% Zr-doped ZnO shows excellent photocatalytic activity at neutral pH under irradiation for 40 min (Christy et al. 2021)

(Elamin et al. 2021)

(Shahmoradi et al. 2019)

(Azfar et al. 2020)

(Chauhan et al. 2020)

(Vallejo et al. 2020)

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Fig. 4 (a) XPS spectra of O 1 s of 4% Mn-doped ZnO nanoflowers (Raskar et al. 2020). (Reproduced with permission from Raskar et al. 2020). (b) HRTEM image of 0.005 M Mn-doped ZnO (Dhivya and Yadav 2021). (Reproduced with permission from Dhivya and Yadav 2021). (c) SEM image of 4% Mn-doped ZnO nanoflowers (Raskar et al. 2020). (Reproduced with permission from Raskar et al. 2020). (d) Photocatalytic mechanism of ZnO and Mn-doped ZnO (Dhivya and Yadav 2021). (Reproduced with permission from Dhivya and Yadav 2021)

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this peak indicates variation in the number of oxygen defects. It was observed that after doping the intensity of this peak increases due to substitution of Mn2+ in ZnO lattice. Dhivya et al. (Dhivya and Yadav 2021) reported on green synthesis of ZnO and Mn-doped ZnO nanostructures for their photocatalytic degradation applications against methylene blue (MB) dye molecules. Slight shift in XRD spectra in the case of doped sample indicates the incorporation of Mn in ZnO lattice. At higher dopant concentration (0.005 M), spherical flake-like morphology appeared as shown in Fig. 4b. The deterioration of MB mainly depends on the physical properties and the material gap. The Mn-doped ZnO bandgap has been found to be reduced by increased dopant concentration due to the proliferation of d-d orbitals which are very helpful in improving MB photodegradation as shown in Fig. 4d. Biswas et al. (2021) synthesize Mn-doped ZnO using the coprecipitation method and continued to study malachite green (MG) degradation. They calculate the reaction rate and surface area by using Langmuir-Hinshelwood model and Brunauer-Emmett-Teller (BET) method, respectively. Concentration of Mn dopant was optimized to achieve high degradation rate. UV spectra showed that higher dopant concentration leads to reduce the bandgap energy. Also, an increase in Mn2+ concentration decreased the BET surface area. It was reported that optimum Zn/Mn molar ratio (1:25) showed excellent photocatalytic activity with degradation rate and BET surface area of 0.0145 min 1 and 9.8 m2/g, respectively. Otadi et al. (2021) carried out theoretical study of degradation of pyridine molecule by Mn-doped ZnO and found that 6% Mn-doped ZnO sample showed high degradation rate under visible light in a batch reactor.

Fe-Doped ZnO Nanostructures Like other transition elements, Fe doping was also used to improve ZnO nanostructure photocatalytic performance. The interaction of d orbital of metal with sp-d orbital of Zn is responsible for the formation of sub-energy levels due to which bandgap gets reduced (Abd Aziz et al. 2014). Fe doping also induces defect of mainly oxygen vacancies in ZnO lattice which acts as trap centers and reduces the e /h+ pair recombination rate. It has been observed that Fe doping influences the morphology, crystallite size, and surface area of the ZnO nanostructures which in turn affects the photocatalytic activity. Kumar et al. (Kumar and Arunagiri 2021) studied the effect of Fe dopant concentration on the morphology of ZnO nanostructures and the effect of various parameters such as pH and the concentration of dye on the photodegradation function of doped ZnO. It was seen with an increase in dopant concentration that the crystallite size and bandgap were found to be decreased leading to the visible light absorption. Also, the morphology changed from nanoparticles to needlelike structures at the higher dopant concentration. The photodegradation of MB and MO

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was studied under different pH (2–6) of solution; it was observed that pH 6 showed excellent degradation of dyes. The optimum dosage concentration of dye molecules was found to be 10 mg, and optimum Fe doping concentration was 0.075% for the best photocatalytic activity with irradiation time of 150 min at pH 6 as shown in Fig. 5a, b. Similar results of change in morphology with Fe doping were observed by Hui et al. (Hui et al. 2017). It was reported that undoped ZnO exhibited flowerlike morphology which changed to sea urchin-shaped after Fe doping as shown in Fig. 5c. Also, doping had great effect on the surface area of the ZnO nanostructure, and it was found that 5% Fe-doped sample showed highest specific surface area (24.8 m2 g 1), i.e., more adsorption sites. This also resulted in redshift in UV spectra indicating the interaction of orbitals of Fe dopant and host lattice leading to the decreased bandgap Fig. 5e. Photodegradation studies showed that 5% Fe-doped ZnO nanostructures exhibited high degradation rate as 97.70%. Liu et al. (2018) reported the photocatalysis property of Fe-doped ZnO nanofibers produced via electrospinning method. No change was observed in the morphology of ZnO nanofibers after Fe doping. However, size of the crystallite decreased as the doping concentration increased. The reduced crystallite size and the bandgap helped to reduce the frequency of e /h+ pair recombination and also strengthened the photocatalytic function. Similarly, Bousslama et al. (2017) reported that Fe doping decreased the crystallite size and bandgap of ZnO. It was observed that there was formation of (FeZn–VZn) complexes in 3% Fe-doped ZnO sample which showed the dominance of Zn vacancies (VZn). It indicated that Fe occupied the Zn site and formed sub-energy levels below CB acting as trap centers and facilitated the photodegradation process. Kumar et al. (2018) synthesized nano-ellipsoidal Fe-doped ZnO (Fig. 5d) at low temperature and studied their photocatalytic and sensing applications. The doped sample showed high degradation rate for dyes such as MB, MO, and RhB shown in Fig. 5f.

Co-Doped ZnO Nanostructures The ionic radii of Co2+ is smaller than Zn2+ due to which it can easily substitute the Zn2+ in ZnO lattice without any deterioration. Similar to other transition metals, the overlapping of d orbital and sp-d orbitals of Co and ZnO leads to formation of new levels below the CB which acts as acceptor levels and reduces the bandgap of ZnO. Doping of Co restricted the particle growth and increases the surface area and oxygen vacancies in ZnO matrix. It has also been reported that higher Co concentration leads to abundant oxygen vacancies which can trap the photogenerated electrons and enhance the photocatalytic activity (He et al. 2012). Sutka et al. (2016) studied the photocatalytic function of Co-doped ZnO nanowires synthesized via solvothermal method. At higher Co concentration, short and nonuniform ZnO nanorods were obtained. They observed that Co-doped nanowires showed higher degradation rate (4.2  10 3 min 1) as compared to Co-doped ZnO nanoparticles (1.9  10 3 min 1) at same Co doping concentration following the same synthesis process. The enhanced photodegradation results from synergetic

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Fig. 5 (a) UV absorption spectra of MB dye degradation of 0.075 Fe-doped ZnO (b) influence on the photodegradation of MB (Kumar and Arunagiri 2021). (Reproduced with permission from Kumar and Arunagiri 2021). (c) FESEM 5% Fe-doped ZnO (Hui et al. 2017). (Reproduced with

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effect of both absorption of visible light and reduced recombination rate of electron hole pair. Similar results were seen by Kuriakose et al. (2014) in Co ZnO nanorods and nanodisk-like structures Fig. 6a, b. They observed 91% degradation rate of dye molecules in just 8 min. It was attributed to the narrow bandgap and more oxygen vacancies which lowered the recombination rate of photogenerated charge carriers. Figure 6e shows the NBE at 3.26 eV, and the emission at 2.38 eV attributed to the oxygen vacancies. Below the CB at 2.28 eV, the oxygen interstitial level was located shown in Fig. 6e. All these defects were responsible for enhanced photocatalytic activity. Zelekew et al. (2021) reported a novel green method to synthesize Co-doped ZnO by accumulating the Co ion on Eichhornia crassipes plant and then combined with ZnO precursor. The photocatalytic activity was studied against MB dye, and 99.6% degradation efficiency was observed on the 8th day of Co accumulation under sunlight placed for 45 min as shown in Fig. 6c, d. Highly porous catalyst with narrow bandgap (2.95 eV) was formed with excellent photodegradation properties. Ali et al. (2020) synthesized Co- and Ni-doped ZnO nanoparticles via solgel method. The crystallinity of doped sample increased with dopant concentration. The interaction between d orbital of transition metals and sp-d orbitals of Zn leads to narrowing of the bandgap. They studied the photocatalytic activity using the reducing agent NaBH4, and it was observed that Co- and Ni-doped ZnO showed better photocatalytic activity than the undoped ZnO. Also, among Co and Ni, Co-doped ZnO showed improved photodegradation rate of MB under visible light irradiation.

Ni-Doped ZnO Nanostructures The size of Ni2+ (0.69 Å) is smaller than Zn2+ (0.74 Å), and it can be easily doped in ZnO by replacing Zn2+ ion without any significant change in the crystal lattice. Devi et al. (2021) synthesized Ni-doped ZnO and studied the photodegradation of MG with various parameters affecting the degradation process. SEM image Fig. 7a, b shows that with increasing dopant concentration, the morphology was changed from spindle-shaped to spherical-shaped nanoparticles. The grain size firstly decreased after doping up to an optimum value then started increasing. Further, the effect of catalyst dose, pH, and concentration of dye on degradation was studied, and it was found that first increase in catalyst dose (0.05 g/L, 0.10 g/L, 0.15 g/L) enhanced the degradation rate of dye up to 77%. However, after an optimum value (0.10 g/L), further increase in catalyst dose lowered the rate due to agglomeration of the nanoparticles. Similarly, with increase in dye concentration, the degradation rate ä Fig. 5 (continued) permission from Hui et al. 2017). (d) TEM image of Fe-doped ZnO nanoellipsoids (Kumar et al. 2018). (Reproduced with permission from Kumar et al. 2018). (e) Schematic showing the photocatalytic mechanism of Fe-doped ZnO NPs (Hui et al. 2017). (Reproduced with permission from Hui et al. 2017). (f) Photodegradation versus time interval for MO, RhB, and MB dyes on Fe-doped ZnO (Kumar et al. 2018). (Reproduced with permission from Kumar et al. 2018)

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Fig. 6 (a and b) SEM images of pure and Co-doped ZnO nanostructures (Kuriakose et al. 2014). (Reproduced with permission from Kuriakose et al. 2014). (c and d) Co-doped ZnO absorption spectrum (Zelekew et al. 2021). (e) Schematic illustrating the band diagram in Co-doped ZnO nanowires (Kuriakose et al. 2014). (Reproduced with permission from Kuriakose et al. 2014)

increased to a maximum value than decreased because more dye molecules hindered the catalyst surface inhibiting the light to reach. It was also observed that in basic medium (pH 9), greater dye degradation was observed. It was concluded that catalyst dose of 0.10 g/L into 15 ppm dye solution at pH 9 showed excellent photodegradation ability. Similarly, Gnanamozhi et al. (2020) synthesize Ni-doped (3% and 5%) ZnO nanoparticles via ultrasonic-assisted coprecipitation method. Wurtzite hexagonal structure was confirmed as studied by XRD pattern, and also narrowing of bandgap

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Fig. 7 (a and b) SEM images of undoped ZnO and doped ZnO (Devi et al. 2021). (Reproduced with permission from Devi et al. 2021). (c and d) Photocatalysis mechanism and mechanism and bandgap energy of pure and Ni-doped ZnO nanoparticles (Gnanamozhi et al. 2020). (Reproduced with permission from Gnanamozhi et al. 2020)

with doping was achieved as shown in Fig. 7c. Nanorods and nanosheets like morphologies were obtained for 3% and 5% Ni-doped samples, respectively. The 5% Ni-doped sample showed excellent photodegradation rate as compared to 3% due to reduced bandgap energy Fig. 7d.

Cu-Doped ZnO Nanostructures Cu is the most extensively used transition metal for doping in ZnO nanostructures for improving their properties and making them a promising material for photocatalysis. Cu2+ has ionic radius of 0.73 Å which is well matched with the ionic radii with Zn2+ that is 0.74 Å. Due to their comparable size, Cu2+ shows substitutional doping in ZnO lattice. Chandekar et al. (2020) reported the effect of Cu doping in ZnO nanoparticles on photocatalysis applications. They observed that crystallite size was decreased as dopant concentration was increased. At higher dopant

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concentration (5%), there was also a formation of oxide layer (CuO) because of unreacted Cu ions. The wurtzite phase of ZnO become weakened with doping because of generation of defects by doping which caused lattice distortion as studied by XRD. Incorporation of Cu2+ in ZnO lattice was further confirmed by redshift in UV spectra, caused by the strain generated in lattice which decreases the bandgap. Lowering in bandgap was attributed to overlapping of band electrons with d orbital electron of Cu (p-d orbitals). This overlapping created impurity levels in ZnO lattice which decreased the optical bandgap (Eg). Further, in PL spectra Fig. 8a, NBE peak (407–411 nm) showed redshift which confirmed the formation of impurity levels. On increasing Cu concentration, green emission was shown by ZnO which corresponds to the formation of oxygen vacancy (VO) and intrinsic defects in ZnO lattice also discussed in section “Introduction.” The photocatalytic activity was evaluated under visible light (λ > 425 nm) by degradation of methylene green (MG) dye. 3.5 times higher photocatalytic activity was observed in Cu-doped ZnO than pure ZnO, and also the activity was increased with increase in dopant concentration as shown in Fig. 8b. High photocatalytic activity of 5% doped sample attributed to the formation of CuO layer over ZnO nanoparticles which acted as electron trap center and reduced the electron hole pair recombination rate. In some cases, it was observed that the Cu-doping concentration also affects the morphology and optical properties of the ZnO nanostructures. The morphology of the Cu-doped ZnO nanostructures and their optical properties have been studied by Chatterjee et al. (Chatterjee and Kar 2020). Figure 8c, d shows that platelike structure was observed for pristine ZnO, whereas Cu-doped ZnO showed layered structure. UV spectra showed blueshift in absorption edge after Cu doping, and a broad peak was observed in visible region (around 600 nm) attributed to SPR as shown in Fig. 8e. PL spectra showed broad visible region emission peak indicating the presence of various defects generated by Cu doping. It was seen that Cu-doped sample showed 25% higher efficiency of degradation of MO dye under visible light irradiation due to the formation of more defects which trapped the electrons and enhanced the photodegradation process.

Ag-Doped ZnO Nanostructures Ag is another widely used transition metal for doping in ZnO lattice which acts as a potential dopant for tuning the optoelectronic properties of the ZnO nanostructures. As the ionic radii of Ag2+ is larger than Zn2+, it can hardly substitute the Zn2+ ion in ZnO crystal lattice. Ag doping creates acceptor levels below the CB and increases the separation of electrons and photogenerated holes also narrowing the bandgap energy helpful in improving the photocatalytic activity. Vallejo et al. (2020) studied the photodegradation of MB by Ag-doped ZnO thin film prepared by combining solgel and doctor blade technique. They observed decrease in the grain size after doping and also the optical properties got enhanced. The bandgap energy of Ag-doped ZnO was nearly 2.95 eV due to which the absorption shifted in visible range. Photocatalytic studies show that Ag-doped

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Fig. 8 (a) Photoluminescence spectra of pure and Cu-doped ZnO NPs, (b) first-order kinetic profiles (Chandekar et al. 2020). (Reproduced with permission from Chandekar et al. 2020). (c and d) SEM images of pure and Cu-doped ZnO NPs and (e) absorbance spectra of pure and Cu-doped ZnO NPs (Chatterjee and Kar 2020). (Reproduced with permission from Chatterjee and Kar 2020)

sample exhibited better photodegradation ability as compared to bare ZnO due to the presence of intraband transitions and reduction in grain size which facilitated the ROS formation. Saravanadevi et al. (2020) reported green synthesis of Ag-doped ZnO by using Vitis vinifera leaf extract, an economical route of synthesis. UV spectra showed blueshift in the absorption peak with an additional peak at 498 nm which corresponds to the presence of Ag as shown in Fig. 9a. Photocatalytic activity of Ag-doped ZnO was excellent as compared to undoped ZnO. Similarly, Chauhan et al. (2020) also performed green synthesis of Ag-doped ZnO by using Cannabis

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Fig. 9 (a) UV absorption spectra of Ag-doped ZnO (Saravanadevi et al. 2020). (Reproduced with permission from Saravanadevi et al. 2020). (b) Mechanism of photocatalysis of Ag-ZnO nanoparticles (Chauhan et al. 2020). (c and d) SEM image of undoped, Ag-, and Ni-doped ZnO. (f and g) Optical bandgap of Ag- and Ni-doped ZnO. (h and i) Photocatalytic efficiency of Ag- and Ni-doped ZnO (Azfar et al. 2020)

sativa leaf extract and studied the antimicrobial and photocatalytic activities. It was seen that Ag-doped ZnO and pristine ZnO nanoparticles photodegraded up to 96% and 38% of Congo red and 94% and 35% of methyl orange dyes, respectively, under

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visible light in 80 min. Mechanism of photocatalysis of Ag-doped ZnO was shown in Fig. 9b. Similarly, Noukelag et al. (2021) reported the photodegradation of industrial textile effluent (TE) up to 63% under visible light for 100 min using Ag-doped ZnO nanoparticles. Shelar et al. (2020) studied the effect of various doping parameters on the photodegradation ability of Ag-doped ZnO. They found that varying the pH (2–12) with fixed concentration of dye and catalyst, the photodegradation ability showed a maximum value at pH 8; after that it was decreased. This indicates that at pH 8 MB dye molecules get attracted to the catalyst and show high adsorption and photocatalytic degradation. In more basic medium, there is a certain type of columbic force which restrict the adsorption. Initial dye concentration also play an important role; it was found that degradation efficiency has inverse relation with dye concentration. Higher dye concentration reduces the formation of ROS which is the primary radical of photodegradation process. Also, as the dopant concentration increases, the dye degradation rate also increases. A comparative study of photocatalytic activity of Ag/Ni-doped ZnO was carried out by Azfar et al. (2020). There was no change in XRD peak suggesting that Ag ions did not replace Zn2+ lattice but squeeze into ZnO grain limits because of the size difference. It was seen that Ag doping did not alter the morphology, i.e., nanorods, but Ni doping above 3% showed mixture of nanoparticles and nanorods Fig. 9c–e. The study exhibited that Ag ions did not enter in the ZnO lattice but remained on the surface; on the other hand, the bandgap of Ni-doped ZnO increased with increasing dopant concentration (Fig. 9f–i). Ni-doped ZnO showed highest specific surface area followed by undoped ZnO and then Ag-doped ZnO. Because of high surface area, it was assumed that Ni-doped ZnO would show excellent photocatalytic activity which was not true. Photocatalytic studies showed that Ag-doped ZnO exhibited higher photocatalytic activity than Ni-doped ZnO with same dopant concentration. To understand the mechanism, active site measurement was carried out by using temperature-programmed desorption of carbon dioxide (TPD-CO2) method. The calculation showed that Ag-doped sample had higher number of active sites than Ni doped which made it a promising photocatalyst.

Other Transition Metal-Doped ZnO Nanostructures Apart from the above-discussed transition metal-doped ZnO nanostructures, there are some other transition metal-doped ZnO nanostructures in which the study is very limited, for example, Cd-, Ru-, Nb-, Zr-, and Ta-doped ZnO nanostructures. Dumrongrojthanath et al. (2019) study the photocatalytic activity of Cd-doped (0–5%) ZnO nanostructures synthesized by precipitation method. It was seen that the crystallite and particle size increase with increase in the dopant concentration confirmed by XRD and TEM data. The PL spectra Fig. 10a show that the intensity of peak decreases with increase in dopant concentration which indicates that the recombination rate of e /h+ pair also decreases. Three percent Cd-doped ZnO shows lowest e /h+ pair recombination rate and also highest photocatalytic activity

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Fig. 10 (a) PL emission of Cd (0.3 and 5 mol%)-doped ZnO samples (Dumrongrojthanath et al. 2019). (Reproduced with permission from Dumrongrojthanath et al. 2019). (b) pH effect of 1% Cd-ZnO nanorod on photocatalytic degradation of phenol. (c) SEM image of Cd-doped ZnO

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against MB dye. 3 mol% Cd-doped ZnO is the most suitable material for wastewater treatment. Shahmoradi et al. (2019) reported the solar light photodegradation of phenol by Cd-doped ZnO nanostructure. Highly uniform, dispersed nanorods without agglomeration were formed via hydrothermal method Fig. 10c. They also studied the effect of various parameters such as pH, catalyst amount, analyte concentration, etc. on the photodegradation activity of Cd-doped ZnO. It was seen that at pH 7 higher degradation of phenol was recorded because of the positively charged surface of ZnO and the more phenol will adsorb on surface and lead to higher degradation rate Fig. 10b. Optimum concentration of Cd-doped ZnO and analyte was needed for higher degradation rate; higher concentration of Cd-doped ZnO increases light scattering and lowers the degradation rate. Also, at higher analyte concentration, the absorbed light is not enough to initiate the degradation of phenol. The result showed that at pH 7, Cd-ZnO dosage of 3 g/l and phenol concentration of 20 mg/l under irradiation of 120 min have highest photodegradation rate. Elamin et al. (2021) prepared Cr-doped ZnO in the presence of starch and varied the annealing temperature via solgel method. Hexagonal wurtzite ZnO was formed, and the particle size varies with annealing temperature such as at 500  C and 700  C; the particle size was 26, 22, and 31 nm for pure ZnO and Cr-doped ZnO, respectively. SEM image shows that irregular nanorod-like morphology was obtained shown in Fig. 10d. UV spectra and BET analysis show that Cr-doped sample (500  C) shows lowest bandgap energy value (3.09 eV) Fig. 10e and higher specific surface area 9.5 m2/g, respectively. Due to lower bandgap, crystallite size, and high surface area, the Cr-doped ZnO (500  C) shows the highest photocatalytic activity with efficiency of 94% within 60 min under visible light. Doping of transition metals of 4d series in ZnO also shows the improved photocatalytic activity due to their unfulfilled d orbital subshells. Also, the radii are comparable which make the process of doping easy and improve the stability, surface area, particle size, etc. Stobel Christy et al. (2021) synthesized different concentration (1–4%) Zr-doped ZnO nanocomposite via solgel method and studied the photodegradation of RR 141, RO 84, and RY 105 dye molecules. It was seen that 4% Zr-doped ZnO shows excellent photocatalytic activity at neutral pH under irradiation for 40 min. Zr doping creates acceptor level which traps the photoexcited electron and reduces the e /h+ pair recombination rate which further enhances the photodegradation activity Fig. 10f, g.

ä Fig. 10 (continued) (Shahmoradi et al. 2019). (d) SEM image of Cr-doped ZnO. (e) Bandgap energy of Cr-doped ZnO (Elamin et al. 2021). (Reproduced with permission from Elamin et al. 2021). (f) Comparison of ZnO and Zr-doped ZnO photodegradation percentage at various concentrations. (g) Schematic representation of Zr(IV)-doped ZnO nanocomposite degradation of selected dyes (Christy et al. 2021)

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Conclusion Doped ZnO nanostructures have widely been used under visible light illumination for the photocatalytic activity. This doping strategy used sunlight as a photocatalytic source of clean energy. Transition metal doping was used to alter the ZnO structure and also displays a shift in the absorption. In transition metal doping, the Zn2+ ion was replaced by the transition metal dopant because of their comparable size and generates acceptor levels below the CB. The creation of acceptor levels decreases the bandgap energy and alters the absorption spectrum toward the visible region. Apart from it, there were various reaction parameters which affect the photocatalytic activity of doped ZnO nanostructure such as type of dopant, their concentrations, synthesis route, and various properties like structure of crystal, surface, pores, etc. In this book chapter, detailed studies of transition metal-doped ZnO nanostructures in addition to the effects of different parameters on their photocatalytic activity were studied. Even though there is great progress in this field in the last decade, there is scope to explore doped ZnO nanomaterials in the field of photocatalysis for other applications such as photocatalytic water splitting, CO2 reduction, etc. There is a great interest of these nanomaterials in the field of energy and environment including solar cell, water splitting, CO2 reduction, etc. The photocatalytic activities of such doped ZnO nanomaterials can be enhanced by coupling with other functional nanomaterials for different applications.

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Role of Green Nanomaterials for 3-Chloropropane-1,2-diol Ester (3-MCPDE) Reduction

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Sharifah Shahira Syed Putra, Wan Jefrey Basirun, Adeeb Hayyan, and Amal A. M. Elgharbawy

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approaches for Green Nanomaterials Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Synthesis Using Plant Extracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzyme Immobilization on Green Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-Chloropropane-1,2-diol Ester (3-MCPDE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toxicity of 3-Chloropropane-1,2-diol Ester (3-MCPDE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approaches for 3-Chloropropane-1,2-diol (3-MCPD) Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters for 3-Chloropropane-1,2-diol (3-MCPD) Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Scientific Endeavor in the Reduction of 3-MCPD via Green Immobilization . . . . . . . . . . . . . Conclusion and Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter discusses the promising green nanomaterials synthesis for 3-chloropropane-1,2-diol ester (3-MCPDE) reduction. Toxic chemicals such as 3-MCPDE are classified as potential carcinogen and have been shown to affect kidney function and male fertility. It was reported to be found in food such as soy sauce, bread and baby milk formula. More recently, this compound has been highly found in refined edible oils. One of the techniques to breakdown 3-MCPDE is enzymatic hydrolysis. However, enzymes possess poor reusability S. S. S. Putra · W. J. Basirun Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia A. Hayyan Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia A. A. M. Elgharbawy (*) International Institute for Halal Research and Training (INHART), International Islamic University Malaysia, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_70

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and recovery which are often prone to denaturation. Thus, immobilization of enzyme using green nanomaterials is the best technique to enhance the enzymatic activity for reducing 3-MCPDE. Green nanomaterial and green chemistry techniques can play a crucial role in the enzymatic application. This method paves the way for future research to learn more about the effects of green immobilization and their interactions for 3-MCPD reduction which can impact the food industry and hold a great promise for edible oil products with a better quality that is safe to consume by society. Keywords

3-chloropropane-1,2-diol Ester · Enzyme · Immobilization · Nanomaterials

Introduction 3-chloropropane-1,2-diol esters (3-MCPDE) is a food processing contaminant commonly found in a variety of food products as well as refined vegetable oils and fat. The formation of contaminants induced by processing is linked to the precursors’ content from previous agriculture and refining practices. The composition of oils and fats containing 3-MCPD depends on the intensity of the industrial process. One of these chemical contaminants belonging to the group of chloropropanol, which is 3-MCPD (Kupska et al. 2015). In 2006, the Expert Committee on Food Additives (JECFA) of the Joint Food and Agriculture Organization/World Health Organization (WHO) stated that 3-MCPDE were observed to be found in food (EPSA 2012). The contaminants may be found in food as a free form and in esterified form when in higher concentrations (Kamikata et al. 2018). Refined vegetable oils such as palm oil (2.91 ppm), olive oil (1.46 ppm) and soybean oil (1.23 ppm) have the highest content of 3-MCPDE, in contrast to the average of animal, vegetable fats and oils (1.03 ppm) (EFSA 2016). Unfortunately, it can also be found in other foods, such as margarine, bread, infant formula and soy sauce, as well as cheese, fish and meat, as a result of domestic or industrial processing (EFSA 2016). Wong et al. (2019) reported that 3-MCPDE is a heat-induced food contaminant that has the potential to cause cancer. Moreover, based on toxicological animal studies, 3-MCPDE can cause reproductive organ and kidney failure (Arisseto et al. 2017). Study by Liu et al. (2016) states that the Swiss mice’s thymus and lungs were discovered to be 3-MCPD targeted organs for the first time. Recent study by Liu et al. (2021) also found the toxicity of 3-MCPD in the kidney, lung, testis and heart of C57BL/6 male mice when exposed to 3-MCPD and glycidol for 28 days. Hence, German Federal Institute for Risk Assessment (BfR) and COMTAM panel of European Food Safety Authority suggested the highest free 3-MCPD daily intake is 2 μg/kg body weight (Kyselka et al. 2018). Therefore, the utilization of green nanomaterial to remove or reduce 3-MCPD is strongly recommended as a safe approach. These can significantly improve

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operational and process efficiency by implementing a beneficial use of resources, thus improving economics (Asmat and Husain 2018). Green nanomaterials are currently the subject of extensive study by the researcher. There is a critical need to apply green nanomaterials that have high efficiency/activity, eco-friendly, and easy to handle (Nasrollahzadeh et al. 2021). There are numerous and diverse sources of green nanomaterials such as nanoclays, nanocelluloses, and chitins. Moreover, it has a broad application domain due to the absence of toxic chemicals during its synthesis, as evidenced by the number of sustainable development goals impacted by their use (Bartolucci et al. 2020). In addition, green properties such as biodegradability and biocompatibility are also expected from natural produced raw materials, which will be far better compared to synthetic organic and inorganic nanomaterials (Tsuzuki et al. 2010). Since the last decade, plant extract-based green synthesis of various nanoparticles has been widely researched (Chowdhury et al. 2020; Nabi et al. 2020; Awwad and Amer 2021). Recently, using nanomaterials as support for enzyme immobilization has been suggested to eliminate or reduce agglomeration between the particles to provide significant surface area and improve enzyme binding to the matrix (Ansari and Al-Shaeri 2019). Moreover, application of novel green process or eco-friendly pathways that use benign processing conditions can contributes in enzyme-catalyzed systems (Hartmann and Kostrov 2013). This is mainly due to increasing environmental regulations and concerns among society nowadays. Lipases (triacylglycerol ester hydrolases, EC 3.1.1.3) are broadly used as versatile biocatalysts to catalyze a varied range of reactions, making them unique in the chemical, pharmaceutical, biotechnology, food and agrochemical industries owing to their outstanding enantioselectivity and stereoselectivity (Asmat and Husain 2018). However, the enzyme’s molecular structure may be unstable and vulnerable to severe temperatures and pHs (Ashkan et al. 2021). Therefore, enzyme immobilization on various supports material of varying shapes and sizes is the most efficient and successful way to enhance enzymatic activity (Reis et al. 2019). Even though there is limited research currently regarding green immobilization to reduce 3-MCPD, scientific effort in this study has been demonstrated. In this chapter, we focus on the immobilization of green nanomaterials synthesis for reducing 3-MCPDE content in vegetables oils. Besides that, we highlight the approach for green nanomaterials synthesis especially using plant extract from leaf, root, flowers, pod, and fruits. This chapter also discusses a recent work on enzyme immobilization using green nanomaterials. The 3-MCPDE toxicity, approaches and parameters for reducing 3-MCPDE have been discussed in this study.

Green Nanomaterials Nanoparticles are the building blocks of nanotechnology, defined as a nanoscale particle in the range 9 until 10 with a dimensional size of 1–100 nm (Sharma et al. 2018). Nanoparticle-derived materials have a wide range of technological and

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scientific applications in chemistry, pharmaceuticals, energy generation, environmental monitoring, electronic sensors and other field of biosciences (Singh et al. 2017; Tiwari et al. 2017; Tripathi et al. 2017). The great contributions of two legends, Paul T. Anastas and Richard Phillips Feynman to the conceptual beginnings of “Nanotechnology” and “Green Chemistry” have converged into the zone of interception (Kumar Das et al. 2015). The “clean and green” principle of nanomaterials define the potential of nanomaterials in moving toward sustainable technological development. Despite its high demand, there is growing concern that researchers working on engineered nanomaterials have given unsatisfactory attention to the negative aspects of these technologies in terms of human health and environmental impacts (Pallas et al. 2018). According to Zhai et al. (2016), nanomaterial manufacturing techniques may have a significant human health and environmental impact that stems not from the nanomaterials themselves, but from related processes involved in its manufacturing, as well as the related material and energy outputs and inputs. Surface toxicity has been reported to be resolved with the use of suitable matrix or capping agents. However, the toxicity caused by the synthesis technique must be addressed through the careful selection groups of precursors (Jose Varghese et al. 2020). Suitable natural resources and solvent systems, such as organic systems, are necessary to overcome these problems. The key merit of the green synthesis process is illustrated in Fig. 1.

Environmental friendly approach, non-toxic solvent used

External experimental conditions (e.g., high pressure and energy are not required), leads to energy saving process

Green Synthesis

Biological component act as reducing and capping agent, hence reduce overall cost for synthesis process

Fig. 1 Key-merit for green synthesis method. (Singh et al. 2018)

Can be improve in larger scale

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Approaches for Green Nanomaterials Synthesis Proteins, plant extracts, peptides, biopolymers, sugars, and vitamins are just a few of the chemical compounds that act as reducing agents found in nature, making green nanomaterials abundant and versatile (Palit and Hussain 2020). Different approaches are identified among the recently reported green strategies for the synthesis of nanomaterials. Green raw materials, such as natural polymers from plant extracts and chitosan or green synthesis approaches using microwave-assisted and ultrasonic irradiation techniques, or a combination of techniques, are commonly used (Khalaj et al. 2020). Different approaches for nanoparticle synthesis are shown in Fig. 2. The first method begins with a solid mass, which is then broken down into smaller nanosized particles using any mechanical method like mechanical grinding before being stabilized to the desired size (Devatha and Thalla 2018). Despite that, the second process begins with atomic-scale material, which is then fabricated to the desired nanoscale using chemical methods such as sol-gel, thermolysis, hydrothermal, hydrolysis, and gas phase. Nevertheless, it is difficult to reach the desired small size with first technique, while it is challenging to manage the size, nanoparticles structure, and surface chemistry using second technique. As a result, biologically synthesized green nanomaterials such as plant and their extracts, microorganisms,

Fig. 2 Synthesis approach for nanomaterials/nanoparticles (top-down and bottom-up)

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algae, enzymes, and biomolecules becoming more attractive compare to the first and second techniques. Generally, green synthesis of nanomaterials can help to improve their eco-friendly as it was produced through control, regulation, remediation and cleanup. According to Singh et al. (2018), several components, such as waste minimization or prevention, reduction of pollution, non-toxic solvents and renewable feedstock, can indeed explain the basic principles of green synthesis.

Green Synthesis Using Plant Extracts In contrast to fungal or bacteria-mediated synthesis, plant extracts are a comparatively easy and simple technique for generating nanoparticles at a large scale among the existing green ways to synthesize nanoparticles/nanomaterials (Ahmad et al. 2019). Furthermore, the use of plant extracts in synthesis decreases the possibility of additional contamination by reducing the reaction time and keeping the structure of the cell intact (Ajitha et al. 2015). Recently, green synthesis has gained exceptional importance in all disciplines focused on a green environment as a result of these advantages. The advantages of plant extracts are shown in Fig. 3. It has become a key emphasis which inspires researchers to develop green methods utilizing various plant components, such as the peel, leaf, fruit, flower and root. Moreover, plants are generally considered renewable resources; hence the process is also considered sustainable (Zhu et al. 2019). Besides, many chemicals found in plant extracts such as ascorbic acids, polyphenols, proteins, terpenoids, and flavonoids play an important roles in the reduction of salt precursors, capping agents,

Fig. 3 Advantages of green synthesis using plant extracts

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Table 1 Comparison of different sources and their green synthesis nanomaterials Extract parts Leaf

Size (nm) 13

Leaf

40

Pod

4–32

Root

500 30–70

Lantana Camara

Leaf and root Flower

25–40

Fritillaria

Flower

10

Lemon

Peel

80–140

Orange

Peel

Citrus limon

Fruit

21.61 & 17.30 5–28

Syzygium Alternifolium

Fruit

2–69

Plant name Jatropha curcas L. Rosemary (Rosmarinus officinalis) Cocoa pod husk Codonopsis Lanceolata Berberis Vulgaris

Applications Photocatalytic treatment of wastewater Nematicidal activity against Meloidogyne javanica

References Goutam et al. (2018) Banna et al. (2020)

Antimicrobial, larvicidal, and antioxidant activities Photocatalyst

Lateef et al. (2016) Lu et al. (2019)

Antibacterial effect on Escherichia coli and Staphylococcus aureus Act as catalyst system to various substituted substrates Antibacterial activity against human pathogens Photocatalytic and optical properties Antibacterial, humidity sensor, and cytotoxicity

Behravan et al. (2019)

Antibacterial activity Antiviral activity against Newcastle Disease Virus (NDV)

Chowdhury et al. (2020) Hemmati et al. (2019) Nabi et al. (2020) Mobeen Amanulla and Sundaram (2019) Awwad and Amer (2021) Yugandhar et al. (2018)

and uptake of metal ions (Garibo et al. 2020). The green nanomaterials synthesis using plant extract is summarized in Table 1.

Enzyme Immobilization on Green Nanomaterials Free enzymes are unstable and difficult to recover which resulting in low productivity and expensive. Due to its drawbacks, the immobilization method has been applied to handle these problems. When contrast to chemical processes, enzymatic catalysis is a promising method for dealing with green nanomaterials because it occurs under moderate circumstances and consumes less energy (Venezia et al. 2020). Moreover, enzymatic catalysis can enhance the recyclability of enzymes, operation stability, and continuous operations for industrial applications. In terms of catalytic activity, lipases have two distinct states. In the closed or inactive form, the enzyme active site is isolated from the reaction environment, whereas in the open form, its active site is readily accessible to the reaction environment (Ashkan et al. 2021). Therefore, the presence of nanomaterials or support can improve the lipase activity.

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A recent study by Nouri and khodaiyan (2020) shows green synthesis of chitosan to immobilized pectinase enzyme from Aspergillus japonicus. They found the stability of immobilized enzymes was improved at high temperatures. The recyclability of immobilized pectinase from Aspergillus Japonicus Pectolyase (EC 3.2.1.15) was observed up to 10 cycles whereas free and immobilized pectinase activity is 60.23% and 70.02%, respectively. Similarly, Xiang et al. (2018) synthesized chitosan with hybrid nanomaterials (mesoporous silica SBA-15). Results reveal that the immobilization yield was increasing and shows better enzymology properties. Moradi et al. (2019) investigated the covalently immobilized β-glucosidase (BGL) enzyme onto modified amino-tannic acid with magnetic iron oxide (Fe3O4) nanoparticles. In contrast to free enzymes, the immobilized BGL can enhance the pH value and withstand high temperatures. Furthermore, the immobilized activity was maintained at 83% even after ten cycles which highlights its excellent durability and stability for industrial use. Wan et al. (2019) synthesized a novel approach of nanoemulsion co-assembly of hollow magnetic mesoporous multifunctional magnetic polydopamine (PDA) nanoflowers (HM-MPDA-NFs) to immobilize with lipase on its pore wall for biodiesel preparation. The conversion of biodiesel was observed up to 87.9% under ideal esterification conditions and after six cycles of recycling, it still exceeds 71.3%. In addition, Xie and Huang (2018) also developed magnetic Fe3O4 nanoparticles incorporated in graphene oxides (GO) for the production of biodiesel after immobilization with Candida Rugosa lipase. The authors found 85.5% efficiency of enzyme immobilization, 64.9% of activity recovery and 92.8% of biodiesel production was obtained. This study demonstrated that immobilized lipase might be simply recovered by utilizing an external magnetic field, allowing the biocatalyst to be recycled five times without losing its substantial activity.

3-Chloropropane-1,2-diol Ester (3-MCPDE) The 3-monochloropropane-1,2-diol ester (3-MCPDE) is a food contaminant which belongs to the chemical group of chloropropanols (Arris et al. 2020). It consists of a group of alcohols that have a 3-carbon backbone substituted by one or two chlorine atoms (Fig. 4). Generally, during the deodorization of refined oils, 3-MCPDE is generated. Its concentration in refined oils mainly varies depending on the composition of fatty acid in oils. The concentrations of bound and esterified 3-MCPD in refined oils and fats range from less than 0.03 ppm to more than 19 ppm (Graziani OCOR

OH Cl

OH

OCOR

3-MCPD

Fig. 4 Chemical structure of 3-MCPD and 3-MCPD diester

3-MCPD diester

Cl

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et al. 2017). According to a comprehensive survey conducted by the Food and Drug Administration (FDA), their di-esters and mono are normally present in less quantities than 0.5 ppm but occasionally can exceed more than 3 ppm (Leigh and MacMahon 2017). The 3-MCPDE tolerable daily intake (TDI) recommended by EFSA (2018) is 2 g/kg bodyweight/day, while the Provisional Maximum Tolerable Daily Intake (PMTDI) suggested by JECFA (2017)is 4 g/kg bodyweight/day.

Toxicity of 3-Chloropropane-1,2-diol Ester (3-MCPDE) The increasing number of findings pointing to 3-MCPD toxicity in food products emphasizes the need to prevent the production of this process byproduct in the edible oil industry (Oey and Fogliano 2019). The presence of 3-MCPDE in food could be a serious concern because these esters are hydrolyzed by enzymes in the gastrointestinal tract, releasing their free forms (3-MCPD), which could be dangerous when consumed (Arisseto et al. 2017). Furthermore, in vivo studies of adult rodent models exposed to free and ester-bound 3-MCPD have found a wide range of negative health effects (Fig. 5), including kidney disease, neurological weakness, cardiotoxicity, testicular damage and cancer development, which outcomes normally in sub-chronic and acute exposure studies (Schultrich et al. 2017). However, without

Fig. 5 Negative effect of 3-MCPD consumptions

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sufficient human clinical data or in vitro data, it is questionable whether and to what extent rodent animal model findings can be applied to human systems, especially considering the prevalence of species-related variations throughout early development (Masjosthusmann et al. 2018). As a result, Mossoba et al. (2020) used an in vitro cellular system (human proximal tubule HK-2 cells) to investigate the human nephrotoxic potential of 3-MCPD and its common fatty acid esters present in toddler and newborn formula. At the highest treatment concentrations examined, they discovered minor yet statistically significant evidence of cytotoxicity, which was also linked to mitochondrial malfunction and temporary changes in cellular metabolism. Despite that, Araujo et al. (2020) used immortalized human colorectal adenocarcinoma cells line (Caco2) to study 3-MCPD metabolism, absorption and theirs monoesters (1-linoleoyl (1-Li), 1-palmitoyl (1-Pa) and 1-oleoyl (1-Ol)). They discovered that 1-Li generated 3–4 times more 3-MCPD, with a high permeability (30.36  1.31 cm/s 10 6), indicating that it is highly soluble and well digested by intestinal epithelial cells. Moreover, a study by Nazari et al. (2020) used a multiparametric oxidative stress assay to examine into the possible toxicity pathways of 3-MCPD in the HEK-293 cell line (immortalized cell line generate from human kidney embryonic cells or an aborted fetus), isolated mitochondria and cells isolated from the rat kidney and liver. They discovered that 3-MCPD can increase reactive oxygen species (ROS), hinder mitochondrial electron transfer in isolated cells, and mitochondrial membrane potential (MMP) failure to release cytochrome from the mitochondria to the cytosol which ultimately activate cell death signal. Besides that, in vivo studies on mice are widely studied by researchers nowadays. Lee et al. (2020) recently demonstrated 3-MCPD muscle toxicity in vitro using C2C12 myoblast cells. When the concentration of 3-MCPD was increased, a substantial decrease in muscle regulatory factors (MRFs) related to muscle differentiation, was observed. They also discovered a significant reduction in protein expression (p70S6 and mTOR kinase) in the downstream synthesis pathway. According to a previous study, oxidative stress may play a significant role in 3-MCPD toxicity. Therefore, Schultrich et al. (2019) used transgenic reporter mice that carry a lacZ (encodes β-galactosidase) reporter under the control of the heme oxygenase 1 (Hmox1) promoter to explore full impact of 3-MCPD toward organ damage. In the mouse organs, a dose-dependent increase in blue stain was found in the renal cortex as well as in brain regions (pons, midbrain and cerebellum) but not in the renal medulla.

Approaches for 3-Chloropropane-1,2-diol (3-MCPD) Reduction The need for edible oil is currently increasing due to the growing global population. In order to meet the rising demand, the refining process such as degumming, deodorization and bleaching will inevitably become most crucial phase to produce edible oil with better appearance, storability and quality. Many food industry and research groups emphasize implementing various mitigation measures at various

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stages of production of vegetable oil and fat to reduce the 3-MCPDE formation. So far, the solutions investigated have concentrated on various aspects of oil production, ranging from palm plantations to revolutionary oil refining procedures to specially designed methods to break down 3-MCPDE after the refining process. One of the proposed mechanisms for removing 3-MCPD is the chlorine elimination in the glycerol backbone of di- and monoacylglycerols using an acyloxonium intermediate formation (Fig. 6). Owing to transformation of triacylglycerols into monoacylglycerols and diacylglycerols via chemical conversion (under basic or acidic conditions), thermal decomposition, or enzymatic conversion; they are known to be indirectly related in the synthesis of 3-MCPDE (Oey and Fogliano 2019). The degumming process is one of the techniques used to reduce the 3-MCPDE formation in the refining process. During the first step of the refining process, crude palm oil (CPO) is treated with salts, water, caustic soda, enzymes, or dilute acids to remove waxes, phosphatides, pro-oxidants, and other contaminants from gums that are insoluble in oil. Water degumming is used to remove water-soluble gums from oils with high gum concentrations, such as palm oils (Oey and Fogliano 2019). Notably, edible oils are commonly degummed with citric or phosphoric acid. In terms of decreasing the concentrations of 3-MCPDE, dry degumming is less effective than water degumming. Water degumming could be considered as a washing phase in which chlorine-containing 3-MCPDE polar precursors are removed from the oil using water. Besides the degumming process, bleaching is used to remove colorful pigments (e.g., carotenes) from oil due to thermal degradation. According to Oey and Fogliano (2019), the bleaching phase alone is unable to remove color pigments due to its thermal stability entirely. Therefore, heat treatment and bleaching are combined in order to remove the pigments through degradation during the deodorization step. Li et al. (2016) reported the 3-MCPDE formation in refined palm oil during deodorization is due to the crude palm oil’s natural structure. They found that eliminating similar precursors before the deodorization phase can reduce the formation of 3-MCPDE.

Fig. 6 Mechanism for the 3-MCPD mono- and diesters inhibition from TAG or DAG. (Adapted from Zhang et al. 2016)

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Sim et al. (2018) used D-optimal design to demonstrate the relationships between bleaching and degumming processes, as well as their effects on the 3-MCPDE formation in palm oil. They found the amount of 3-MCPDE was reduced up to 50% after water degumming. Acid-activated bleaching earth has a better effect on lowering 3-MCPDE than natural bleaching earth and acid-activated earth with neutral pH. This suggests that the performance capacities and adsorption of bleaching earth are the most important elements in the esters removal. Besides that, Matthäus et al. (2011) study the strategies for 3-MCPDE reduction in palm oil. They reported a reduction of 3-MCPDE is up to 20% from 2.8 to 2.1 mg/kg when the raw material was washed with 75% ethanol or water before the refining process. Ramli et al. (2011) also investigated the degumming and bleaching effect on the 3-MCPDE formation using six types of bleaching clays (acid and natural activated clays) during physical refining. At a fixed dose (1.0%) of the bleaching process, deodorization of bleached oils was achieved for 90 min at 260  C. As a result, 3-MCPDE was not discovered in crude palm oil (CPO), but the maximum quantities (3.89 ppm) of 3-MCPDE were formed by phosphoric acid degumming with acid activated clays. Noteworthy, the data show that reducing acid dosage can limit the formation of the ester. The contaminants were at their lowest levels (0.25 ppm) when the oil was water bleached and degummed using natural bleaching clay. Moreover, a significant relationship was achieved when the esters concentration was plotted against the bleaching earth’s acidity for the respective water (0.9351) and acid (0.9759) degumming processes. Apart from the refining process (degumming, bleaching and deodorization) to reduce 3-MCPD contents, Ahn et al. (2020) have demonstrated great deprotonation of 3-MCPD absorption on carboxylated iron (Fe) based MIL-88 s (Fig. 7). After surface modification, 3-MCPD showed a significant increase in efficiency. Moreover, they discovered esterification of 3-MCPD with carboxyl groups can improved the adsorption performance dramatically. In 2010, Bornscheuer and Hesseler (2010) had introduced an enzymatic approach (Arthrobacter sp. AD2 halohydrin dehalogenase and Agrobacterium radiobacter AD1 epoxide hydrolase) to remove 3-MCPD by converting it to harmless glycerol in an aqueous environment. In the presence of edible oil, lipase from Candida

Fig. 7 Adsorption mechanism of 3-MCPD and glycidol using Fe-MIL-88. (Adapted from Ahn et al. 2020)

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Fig. 8 Mechanism of nickel catalysts action on the removal of 3-MCPD-d5-1,2-dipalmitate. (Adapted from Kyselka et al. 2018)

antarctica has converted free 3-MCPD and the equivalent fatty acid in a biphasic system for 3-MCPD removal. Kyselka et al. (2018) study the removal of 3-MCPDEs during palm oil hydrogenation and wet fractionation. They discovered that fractionated palm stearin reduced the concentration of 3-MCPDEs by 71–74%, and 22% of Raney nickel catalysts was the best way to remove the 3-MCPDEs. The mechanism of nickel catalysts action on the removal of 3-MCPD-d5-1,2-dipalmitate is shown in Fig. 8. Meanwhile, under high-temperature and low-moisture circumstances, Zhao et al. (2016) investigated the reduction of 3-MCPD monoesters by thermal degradation with ferric ion (Fe3+). The findings revealed that Fe3+ could act as the best catalyst for 3-MCPD reduction.

Parameters for 3-Chloropropane-1,2-diol (3-MCPD) Reduction A study by Wong et al. (2017) shows the influence of frying temperature (160  C and 180  C), duration (100 min/day for five consecutive day) and sodium chloride (NaCl) concentration (0%, 1%, 3% and 5%) on the 3-MCPDE formation during deep frying potato chips. It’s worth noting that the trend for 3-MCPDE decreased as the frying time increased, but the trend rose as the NaCl concentration and frying temperature increased. Hence, it is important to consider the parameter conditions for 3-MCPDE reduction. Pudel et al. (2016) focused on short path distillation to generate low 3-MCPD in edible oils with equal quality to conventional deodorized oils. They study the effect of different process parameters such as evaporator temperature, condenser temperature, pump frequency and mixing speed on quality parameters of the oil. They observed the optimum oil quality parameters were better at pump frequency of nearly 20 Hz, mixing speed at 100 rpm, evaporator and condenser temperature at 170  C and 60  C, respectively. Other parameters such as dosage of bleaching earth and phosphoric acid, degumming and deodorization temperature to reduce 3-MCPDE were accomplished by Sim et al. (2020). The authors found the optimized processing conditions were performed at 3% bleaching earth dosage, 50  C degumming, 0.31% phosphoric acid dosage and 240  C of deodorization temperature. The optimized conditions achieved 80% of 3-MCPDE reduction levels with a range of color and free fatty acid (FFA)

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contents is approved by Palm Oil Refiners Association of Malaysia. Notably, this study has shown a higher dosage of phosphoric acid that can leads to a greater reduction in the adsorption of acylglycerols. In addition, a greater 3-MCPDE reduction was detected when the right bleaching earth amount are used, demonstrating the precursors’ activation may be occurring by adsorption. Another study by Zulkurnain et al. (2013) also determined these process parameters. They found 87.2% (2.9 mg/kg to 0.4 mg/kg) of 3-MCPDE reduction was evaluated with oil stability up to 14.3 hours and index color at 2.4 R. In addition, Li et al. (2016) also evaluated the formation of 3-MCPDE via physical lab-scale refining processes. They discovered that at a specific deodorizing temperature (220–260  C), 3-MCPDE was formed at its highest levels in one to 1.5 hours. To some extent, the level of 3-MCPDE was reduced when the precursors were partially removed by washing bleached oil with an ethanol solution before deodorization. Moreover, they discovered that diacetin efficiently decreased the amount of 3-MCPDE. Hew et al. (2020) investigated the physical refining of palm oil (bleaching and degumming processes) at different doses of bleaching earth (0.5%, 1.0% and 1.5%) in order to evaluate their effects on esters formation. The authors are advised that bleaching earth with high acidity should be avoided. However, mildly acidic bleaching earth at pH 5.0 was proven to reduce 3-MCPDE effectively when compared to neutral and natural activated bleaching earth. Moreover, a study by Tivanello et al. (2020) demonstrated the palm oil deodorization in different conditions of temperature (210, 230, 250, and 270) C and time (30, 60, 90, and 120) min. At the mildest studied condition (30 min, 210  C), levels of 3-MCPDE were observed to vary from 1.91 to 2.70 mg/kg. There was no possible correlation between 3-MCPDE and physicochemical changes were observed in this study. The optimized parameter conditions for 3-MCPDE reduction were summarized in Table 2.

Scientific Endeavor in the Reduction of 3-MCPD via Green Immobilization Enzymatic methods have been shown as a promising mitigating strategy for reducing 3-MCPD during extraction of palm oil as a substitute for mechanical extraction because it is considered a green immobilization technique. Compared to mechanical extraction, the procedure for enzymatic extraction can run with mild conditions (50  C) to reduce 3-MCPD formation and produce oils with higher quality (Arris et al. 2020). For example, Silvamany and Jahim (2015) study the effect of a different combination of enzymes such as Cellic HTec2, Cellic CTec2, and Pectinex Ultra SP-L on palm oil recovery. As a result, highest oil recovery (88%) was obtained at 50%w/v substrate loading, 30 mgprotein/10 gsubstrate of enzyme at pH 4.8 for 2 hours in 50  C. Lipases used in the enzymatic production of structural lipids can be designed to be selective toward specific properties of the starting materials like glycerol backbone site and fatty acids, thus tailoring the fatty acid rearrangement in the final product. Kleiner and Akoh (2018) reported that the reaction medium for these processes may or may not contain solvents. The enzyme may be recovered for

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Table 2 The optimized conditions for 3-MCPDE reduction Optimized Conditions Condenser temperature (60  C), evaporator temperature (170  C), mixing speed (100 rpm) and pump frequency (20 Hz) Phosphoric acid dosage (0.31%), degumming temperature (50  C), bleaching earth dosage (3%) and deodorization temperature (240  C) Deodorization temperature (250  C), Deodorization time (1.5) h, Steam speed at 0.1 g/min Deodorization temperature (250  C), Deodorization time (90 min), vacuum (0.5–1.0 mbar) Deodorization temperature (210  C), Deodorization time (30 min) Water dosage (3.5%), acid dosage (0.08%), degumming temperature (60  C), bleaching clay dosage (0.3%), deodorization temperature (260  C)

3-MCPDE Reduction Not detected (LOD >0.1 mg/kg)

References Pudel et al. (2016)

80% (1.10 mg/kg)

Sim et al. (2020)

0.436 mg/kg – 0.622 mg/kg 1.37  0.19 mg/kg

Li et al. (2016) Hew et al. (2020) Tivanello et al. (2020) Zulkurnain et al. (2013)

1.91 mg/kg to 2.70 mg/kg 87.2% from 2.9 mg/ kg to 0.4 mg/kg

subsequent use, resulting in a green chemistry method. However, there are limited studies regarding the reduction of 3-MCPD via green immobilization. A study by Yuan et al. (2019) developed a biosensor for evaluating 3-MCPD based on the electrochemically catalytic reduction via hemoglobin (Hb). Magnetic molecularly imprinted nanoparticles (MMIPs-NPs) was immobilized with Hb to constitute the Hb-enzyme sensor. They found that this biosensor can catalyze the reduction of 3-MCPD electrochemically. The reduction peak current shows a linear relationship with concentrations of 3-MCPD ranging from 1.0 mg/L to 160 mg/L, with a detection limit of 0.25 mg/L. One of the 3-MCPD components diacylglycerol was studied by Von Der Haar et al. (2015). They optimized its content after immobilized with Rhizomucor miehei lipase. The authors observed 23% of maximum diacylglycerol content was achieved after optimization. The reusability of immobilized lipase can be up to 14 cycles if washed with hexane and iso-propanol. In addition, Zhang et al. (2016) used additives such as lipophilic tea polyphenol and rosemary extract to study the mechanism of free radical scavengers on 3-MCPDE mitigation in palm oil. During deodorization, lipophilic tea polyphenol and rosemary extract can reduce 3-MCPDE by 75% and 82%, respectively, from its initial concentration (100%).

Conclusion and Future Prospects Recently, the synthesis of nanomaterials/nanoparticles using the green chemistry principle has gained a lot of attention. Green synthesis using plant extract seems superior to other nanomaterials synthesis as it is easier and more cost-effective to be immobilized with enzymes. In this chapter, synthesis of green nanomaterials as well as previous study of enzyme immobilization with green nanomaterials have been reviewed. Despite the necessity of obtaining monodispersed nanomaterials for

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research and other uses, current research demonstrates that there is still a lack of information during these synthesis methods. Furthermore, the consumption of 3-MCPD contaminants in food products has been an issue nowadays. Rising awareness toward healthy lifestyles has encouraged society to study the food products before being consumed. Therefore, this chapter provides insight into the approaches and parameters for 3-MCPD reduction based on previous studies. However, there is limited study of green immobilization for reduction of 3-MCPD. Due to the mechanism of 3-MCPD reduction is not clearly elaborated at the moment, future study should focus on the mechanism in order to advance the food products quality. Hence, knowing the mechanisms involved in the green immobilization technique is a useful practice for future advancement of green synthesis toward 3-MCPD reduction.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis and Characterization of Various Bio-based Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . Introduction to Different Biomass and the Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose and Nanocellulose Based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generalized Synthesis Techniques of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization Methods of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Plants are the most important source of a variety of phytochemicals having a wide range of biomedical applications. Nanotechnology is a fast-developing field that has applications in nearly every branch of modern science, medicine, and technology. The plant materials used for nanoparticle production, evaluation, and applications are summarized in this chapter. The production of several metallic nanoparticles uses plant materials such as leaves, fruits, seeds, roots, stems, flowers, barks, and fruit peels. Plants with minimal costs and a high level of eco-friendliness are very advanced and beneficial for human applications. Nanoparticles such as silver, gold, zinc oxide, tin oxide, titanium oxide, copper,

K. H. Pandit · P. B. Patil · A. D. Goswami Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India D. V. Pinjari (*) Department of Fibers and Textile Processing, Institute of Chemical Technology, Mumbai, India Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_74

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palladium, platinum cadmium sulfide, cadmium oxide, zinc sulfide, and others were synthesized with the help of various plants, and their various parts are described briefly along with characterization strategies in this book chapter. Keywords

Green nanomaterials · Sustainable · Agrowaste · Waste valorization · Carbon nanomaterials · Silica nanoparticles

Introduction The branch of nanotechnology has been growing since the last century. A variety of research conducted today is connected directly or indirectly toward nanotechnology. Nanotechnology is basically the characterization, synthesis, and applications of devices and materials by modifying them on the nanoscale. The key and fundamental elements of nanotechnology are known as nanomaterials. These are materials having a size of less than 100 nm in any particular direction. Nanomaterials are shown to have different physiochemical properties than the bulk materials, the properties of which depend on the shape and size. Nanomaterials have a diverse range of applications ranging in various fields like engineering, food, textile, healthcare, and electronics. Metallic nanocomposites, nanoparticles, nanofilms, polymer nanocomposites, nanorods, and nanotubes are the different forms of nanomaterials. Zero-dimensional nanomaterials are called nanoparticles, those having one dimension are called nanotubes or nanorods, those having two dimensions are called nanofilms, and those having three dimensions are called nanocomposites. Nanomembranes and nanoabsorbents have shown important applications in the field of wastewater treatment. The field of biotechnology has also seen a huge involvement of nanomaterials and nanotechnology. Additionally, in the international market, there has been a growing need for engineered nanomaterials. The applications of nanomaterials can be seen in Fig. 1 (Sangeetha et al. 2017; Prasad et al. 2016). Life cycle assessment and analysis of nanomaterials have shown that the production of these materials is very energy intensive and needs a variety of natural resources than conventional technologies (Hertwich et al. 2015). Possible utilization of agricultural waste materials for production of nanomaterials can prove to be promising since these waste materials are readily available in India at a cheap cost. Also, the synthesis of nanomaterials can be an innovative method for reuse, recycling and treatment of waste. Recycling and valorization of selected waste materials for the synthesis of nanomaterials can prove to be very beneficial for the society. The primary difference between green and green synthesized nanomaterials is the raw materials used for the production of these nanomaterials. Additionally, the utilization of bio-based raw materials for production of nanomaterials can make these nanomaterials green as well as green synthesized. All in all, green nanomaterials as compared to synthetic nanomaterials are nontoxic, environmentally friendly, simple, faster production, and less time-consuming production processes (Abd-Elsalam et al. 2022). The effective utilization of biowastes thus can reduce the

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Fig. 1 Applications of nanomaterials

environmental burden and prove to be a viable solution by reducing the negative impact of waste on the environment (Samaddar et al. 2018). The present chapter discusses the fabrication and characterization techniques of nanomaterials using agricultural biomass wastes. The introduction section focuses on the wide scope of nanomaterials followed by the different types of nanomaterials. An insight into why there is a need for green nanomaterials from biomass and agricultural waste has been presented to highlight the importance of the same. The synthesis and characterization section will focus on a variety of commonly seen nanomaterials. Also, different chemicals inside the biomass and how these chemicals will transform into bionanomaterials will be focused on. The nanomaterials covered in this chapter will be cellulose and nanocellulose based, graphene oxide based, silica nanoparticles, and carbon nanomaterials as well. A table highlighting the manufacturing of other nanomaterials will also be given for details into a variety of nanomaterials. The future scope section will focus on the future research areas which will prove to be value addition in the field. The conclusion section will give generalized conclusions based on the suggestions in the chapter.

Synthesis and Characterization of Various Bio-based Nanomaterials Introduction to Different Biomass and the Nanomaterials Biomass and agricultural waste contain different primary and secondary metabolites like terpenoids, phenolic acids, flavonoids, and alkaloids (Kuppusamy et al. 2016; Thangadurai et al. 2021; Nasaruddin et al. 2021; Gupta and Mao 2021). The role of

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Fig. 2 Schematic of nanoparticle production from agricultural waste

primary metabolites in plants is for growth, reproduction, and overall development. Meanwhile, secondary metabolites are found in a limited number of plant groups and have a role of defense mechanism against herbivores and interspecies. The production of green nanoscale particles is done by redox reaction of the metabolites, an example being zerovalent metallic nanoparticle production (Pasinszki and Krebsz 2020). The terpenoids also play a big role in the conversion of silver ions into nanoparticles by using geranium leaf extracts. Other terpenoids like eugenol found in cinnamon are seen in the bioreduction of AgNO3 and AuCl4 nanoparticles. Meanwhile, flavonoids like flavanols, flavones, isoflavonoids, and flavanones can chelate and reduce the metallic ions into nanoparticles. Nanomaterials are produced due to the release of hydrogen atoms by tautomeric transformations to the keto form from the enol form. The fruit residues in biomass waste contain different flavonoids that can be used for nanomaterial production. A variety of biomolecules like proteins, amino acids, enzyme, sugars, and trace metals are present in plants. Various cations like gold, silver, platinum, and copper are reduced to be metallic zerovalent nanoparticles. An example for the same is production of silver nanoparticles from methanolic extract of the stem of beautyberry plant (Shameli et al. 2012) and production of gold nanoparticles from Artocarpus heterophyllus and Azadirachta indica extracts (Manik et al. 2020). Nanomaterials are synthesized from agricultural waste in the following manner. First, the agricultural waste is sorted and cleaned after which the extraction of chemicals present in the waste is done. The extracted phytochemicals from the agricultural waste along with the precursor materials are then made to undergo bioreduction, followed by which a nanomaterial solution is obtained. From this nanomaterial solution, the nanomaterials are separated by filtration or other separation techniques. Figure 2 shows schematic of nanoparticle production from agricultural waste.

Cellulose and Nanocellulose Based Nanocellulose has important cellulose properties like hydrophilicity, capability of chemical modification, and formation of semicrystalline fiber morphologies (Chu et al. 2020). Nanocellulose can be produced by a variety of biomass-based lignocellulosic sources, namely, agricultural wastes and forest wood. This can be done by three different methods which are chemical, physical, and bacterial. Common carbon

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compounds generally constitute into what we know as lignocellulose, and these are mainly found in decaying plant cell walls. These plant cell walls are rich in a variety of organic compounds. Similarly, nonedible agricultural waste contains biomass that consists of a similar variety of organic compounds. Immobilization of metal particles as well as nanoparticles can be done successfully by cellulose since cellulose has six hydroxyl groups per cellobiose. Additionally, cellulose has important properties like high mechanical strength, low density, nontoxicity, biodegradability, and renewability. For production of nanocellulose from cellulose, acid hydrolysis method is used. In this method, negative ions are introduced into the cellulose which hydrolyses the amorphous to nanofibers. Nanocellulose is classified based on function, dimension, and isolation method and also depends on source of cellulose and process conditions (Charreau et al. 2013; Kardam et al. 2014; Mariño et al. 2015; Shahi et al. 2021; Adel et al. 2021). Nanocellulose can have direct applications in wastewater treatment due to the presence of numerous reactive groups, high specific surface areas, and excellent absorption performance by modifying the specific nanocelluloses. This can prove to be beneficial as compared to conventional materials like ion exchange resins, activated carbon, zeolites, etc. (Kalia et al. 2014; Pagliaro et al. 2021; Ma et al. 2022). Nanocellulose can be surface modified with some specific groups like carboxyl, ammonium, amine, and xanthate groups. An example for the same will be the use of oxolane-2,5-dione-modified cellulose nanofibers for absorption of Cd and Pb from wastewater. The waste of the orange juice industry can also prove to be very effective for the production for orange nanofibrils. This will also aid the development of sustainable textile and proves to be very cost-effective. The by-products of citrus-based materials can also be cost-effective as well as eco-friendly since these materials have better oil and water holding capacity and less calorie content as well (Awan et al. 2013; Shahi et al. 2021; Adel et al. 2021; Pagliaro et al. 2021).

Carbon Nanomaterials Carbon, a p-block nonmetallic element, is the sixth most abundant element in the entire universe and is also the second most present element by mass in the human body. Almost all of our carbon is derived from coal deposits. The presence of carbon determines the nature of molecule to be either labeled as inorganic or organic. Diamond, graphite, and C-60 also known as buckminsterfullerene are the three commonly found and used forms of carbon (Antonyraj et al. 2013; Peng et al. 2020; Kour et al. 2020). Activated carbon is a type of carbon which is crystalline, has a large pore structure, and favors the carbon to absorb more. The carbon undergoes the process of activation under which it goes through very high temperatures. This forces the carbon to undergo an activation protocol where maximum possible surface pore area is exposed which increases the adsorption rate. Surface area of activated carbon is around 500 m2/g. Due to these properties, activated carbon has a variety of

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applications where there is a need of adsorption and desorption. Carbon is present in all organic molecules, specially in the plant kingdom. This also refers to the agricultural waste like rice husk, coconut shells, wheat straw, sugarcane bagasse, and many more. There are various methods of production of activated carbon, one of which is known as carbonization where wood, coal, and nutshells are used as raw materials and heated up to 600–900  C under the presence of inert gasses like Ar and N2. Another production method is known as oxidation where the carbonized mass is heated to 600–1200  C under O2, CO2, or steam. Additionally, carbon nanoparticles, a form of activated carbon, are known to have tremendous applications. The property of carbon where it can form low dimension be it two-, one- or zero-dimension allotropes are known to be as carbon nanomaterials. Some applications included use in high-performance electrode substance in batteries, photoluminescent materials, and supercapacitors. Also, carbon nanomaterials are used in the medical field for cancer treatment. However, the production of carbon nanomaterials is a highly expensive process which needs utilization of high-cost feedstocks and catalysts along with a high energy requirement. Chemical vapor deposition is a common industrial method used for carbon nanomaterial synthesis. This process is known to use high quantities of ethylene, carbon monoxide, and hydrogen (H2) (Wen et al. 2014; Srivastava et al. 2015). Agricultural waste like rice husk, coconut shell, bagasse, groundnut shell, wheat straw, and many more can be used as sources for the production of carbon nanoparticles. Graphitic carbon nanostructures were prepared by treating cellulose with hydrothermal treatment, followed by thermal treatment at 900  C with nickel salt and hydrothermal char (Fathy et al. 2019). Another study showed how sugarcane bagasse and corn grains can be converted into carbon nanomaterials by pyrolysis at 600  C to 1000  C. Multiwalled carbon nanotubes were produced from this sugarcane bagasse with tubes of length of 0.05 mm and diameter of 20–50 nm. The use of bamboo charcoal as a catalyst while using ethanol vapor as carbon source has also been used using chemical vapor deposition at a high temperature of 1200  C to 1400  C. Also, utilization of rice straw with hydrothermal treatment and with chemical vapor deposition has been studied. However, low yield of 44% with surface area of approximately 30 m2/g was observed. Rice husk was also made to undergo microwave plasma irradiation for production of graphenated carbon nanotubes (Fathy et al. 2019). Graphene is another type of carbon nanomaterial and a new type of one-atomthick and a two-dimensional structure made from carbon. The molecules in graphene are organized in hexagonal shape as seen in graphite. The primary advantage of graphene is that it is very lightweight with 0.77 mg weight for 1 m2 size sheet. Since the discovery of graphene in 2004, it has come a long way and is used in a large variety of applications. Some significant advantages of graphene are potential electronic properties, mechanical strength, thermal conductivity, and great scattering execution. These are some primary reasons for graphene having a wide scope of applications. However, the primary disadvantage when it comes to graphene is the cost. Graphene is expensive to produce, and many attempts have been made to make it inexpensive so

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that it can have a wider scope for applications (Novoselov et al. 2012; Yadav et al. 2014; Xiao et al. 2014; Tatrari et al. 2021; Rajakumari et al. 2022; Huang et al. 2022). One such material related to graphene is graphene oxide which is formed by oxidation of graphite. Graphene oxide is a single atomic layer compound and has applications in various interdisciplinary fields like material and biological sciences as well as pharmaceuticals (Mao et al. 2013). Standard methods for synthesis of graphene oxide are Hoffmann’s method (HO), Staudenmaier’s method (ST), and Hummers’ method (HU). The early techniques to synthesize graphene oxide included inclusion of KClO3 to a slurry of graphite in raging HNO3. However, a better synthesis angle graphene oxide can be produced by oxidation of graphite from HU and then reducing it to obtain reduced graphene oxide. Additionally, there are different commercial methods of silicon production like chemical vapor deposition, exfoliation procedures, chemical origin methods, and epitaxial growth on SiC. Highquality graphene oxide at a large scale can be made with the mentioned techniques. These synthesis methods have some major drawbacks like high temperature and energy requirement. The chemical vapor deposition method is a complicated process, while the epitaxial process gives a product of wafer-scale graphene along with temperature of 1500  C. Also, the cost of SiC is a major factor along with the use of hazardous chemicals and emissions of NOx gasses. All these factors make the graphene oxide manufacturing processes expensive (Chung et al. 2013; Somanathan et al. 2015). A novel method of producing graphene oxide from agricultural waste, specifically sugarcane bagasse, was researched (Li et al. 2018; Trung et al. 2020; Tatrari et al. 2021). The sugarcane bagasse was oxidized under muffled atmosphere. This process was known to be cost-effective and eco-friendly (Tatrari et al. 2021). Another team (Rajakumari et al. 2022) manufactured graphene oxide sheets from different green wastes including animal waste, vegetation wastes, agricultural waste, and wood and fruit wastes as well (Li et al. 2018; Rajakumari et al. 2022; Huang et al. 2022). Furthermore, another graphene oxide synthesis method using risk husk by calcination followed by chemical activation. Rapid, scalable, cost-effective, and reliable production of graphene oxide can be done by using rice husk ash as the starting material (Akhavan et al. 2014; Muramatsu et al. 2014; Kumar et al. 2016).

Silica The second most common element found in the soil is silicon. Silica is a polymer made from silicic acid having a general formula SiO2. The two common forms of silica are crystalline and amorphous form. Crystalline silica is found in natural state and is known to have toxic effects. Amorphous silica on the other hand has been known to a diverse set of applications in the electronic industry due to the fact that it acts as a good semiconductor. Consequently, there are various silicon production methods of synthesizing amorphous silica, one of which is the sol-gel method, thermal decomposition method, and the Stober method (Gu et al. 2015). Silicate

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reagents or solution is used for the synthesis of amorphous silica. From the past 20 years, it has been established that silicon directly enhances plant growth and also has a positive impact on crop production under various environmentally stressful conditions. Research has shown that silicon nanoparticles can have a promising application in the agricultural sector. The yield of crop can be enhanced by reduction in the use of fertilizers and pesticides. The importantance of amorphous silica are physiochemical properties, large surface area, and nontoxicity. Highly toxic and expensive chemicals like triethoxysilane and tertamethoxysilane (Karimi and Mohsenzadeh 2016; Sun et al. 2016; El-Gazzar et al. 2021). Different synthesis methods using agrowastes like rice husk, corn hub, and plant sources have been used. Also, wheat straw ash contains high amount of silica about 75% and is a promising candidate for extraction and synthesis of silica. The process involves acid leaching and combustion of wheat straw. Rice husk is a second major type of crop waste seen worldwide. Traditionally, rice husk was used as fertilizer additives and fuel; however, rick husk contains 90–97% silica. Hence, it can also act as a promising source of silica nanoparticles. It was reported that silica nanoparticles were produced from rice husk using an environmentally friendly method with 97% pure silicon as the product upon HCl pretreatment (Ghorbani et al. 2015). After rice, the major crop produced is corn. The agrowaste produced by corn is greater than rice, and the corn cob ash has reported to have 48% of silica. Synthesis of silica nanoparticles from corn cob ash is around 97% in purity (Qadri et al. 2015; Okoronkwo et al. 2016). Bagasse ash is another major agricultural waste and generally burnt for electricity generation. However, the leftover ash of bagasse comprises of large amounts of silica and can act as a source for synthesis of biogenic silica. Laser ablation mediated synthesis-based production of silica nanoparticles using sugar beet bagasse ash. These produced nanoparticles were lesser in size as compared chemically synthesized nanoparticles with a size of 38–190 nm (Birla et al. 2013). It was also seen that the chemically synthesized nanoparticles being synthetic had a negative effect on algae growth, while the naturally derived nanoparticles did not cause any harm to the aquatic ecosystem. It was also studied that vermicomposting of sugarcane bagasse, rice husk, coffee husk, and various agrowastes could lead to the synthesis of silica nanoparticles (Bose et al. 2018; Usgodaarachchi et al. 2021; El-Gazzar et al. 2021). Other than the abovementioned nanoparticles, there has been research into synthesis of different varieties of nanomaterials from agricultural and biomass waste. Table 1 gives a brief information about the same.

Generalized Synthesis Techniques of Nanomaterials The synthesis of any kind of nanoparticles initially starts with sample preparation. This is an essential step where samples are prepared depending on type and composition of waste. This step is followed by the pretreatment step which is classified into three categories, namely, physical, chemical, and combined methods. Any kind of treatment that does not use chemicals is known as physical pretreatment. Some

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Table 1 Synthesis of nanoparticles from agricultural waste Source Leaf extract Sugarcane bagasse

Method of synthesis Ferrous sulfate heptahydrate as precursor added to leaf extract Chemical activation

Plant waste Orange peel waste Potato peel waste Hemp waste Pomelo peel waste

Activation and carbonization Solvothermal process Solvothermal process Hydrothermal carbonization and KOH activation Solvothermal process

Product Iron nanoparticles Nanoscale activated carbon Carbon nanosheets TiO2 Cellulose nanocrystal Carbon nanosheets Carbon nanoparticles

Particle size (nm) 300–500 2–5

0.52–2.19 100–1000 410 1–6 2–4

References (Devatha et al. 2016) (Jain and Tripathi 2015) (Chen et al. 2016) (Kandregula et al. 2015) (Chen et al. 2012) (Wang et al. 2014) (Lu et al. 2012)

common examples are grinding and milling. The chemical pretreatment aims to solubilize and remove any kind of hazardous chemicals present in the waste. A commonly used chemical method is acid hydrolysis where nitric acid, sulfuric acid, or hydrochloric acids can be used. However, there are problems with the corrosive nature of acids and the need for neutralization and also the recovery. A combination of both these methods can also be used to obtain a better overall result and to mask some disadvantages of both the methods. After the pretreatment step, the actual synthesis methods come to the picture (Samaddar et al. 2018). The first method is chemical or thermal activation method. In this method, the pretreatment waste material is mixed with a chemical reagent in a particular quantity and concentration. Some commonly used reagents included potassium hydroxide, iron chloride, potassium chloride, calcium chloride, and phosphoric acid (Jain and Tripathi 2015). The chemicals or activating agents are mixed with the raw material to carry out the reaction and get the desired product. The temperature, pressure, and residence time followed by cooling or drying is also done to obtain the exact desired product. Another method is the electric arc discharge method. This method is used when high temperatures >1700  C are needed and is generally used for production of carbon nanotubes. It is known that by the use of this method, the nanotubes produced have less structural defects (Eatemadi et al. 2014). The main advantage of this method is that it can produce on a larger scale. However, a disadvantage is there is less control over the alignment of the generated carbon nanotubes. Vacuum evaporation is another method for synthesis for nanomaterials. In this process, a solid is deposited via a chemical reaction from gas or vapor phase on the heated surface. The process is applied at very low pressures under vacuum, and the evaporation temperature is achieved after heating. The atoms or molecules of the solid start to evaporate and migrate to another surface coming in contact with

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Fig. 3 Synthesis of different types of nanomaterials

it. Since temperature at the surface is lower, the gas molecules condense, and since vapor pressure at new surface is high than initial, the molecules will not re-evaporate and will stick to the new surface. Synthesis of metallic nanoparticles has been done by this method. Metallic nanoparticles are also produced by sodium borohydride reduction. NaBH4 is then added rapidly for the waste materials for production of nanoparticles via reduction. Another method for production of metallic nanoparticles is the solvent thermal method or sol-thermal method (Dubin et al. 2010). In this process, a solvent is used and heated to a critical temperature. Generally, autoclave reactor is used for this process where pressure regulation is done as well. This process can also be called as hydrothermal treatment as the solvent used is water. A schematic of the synthesis process can be seen in Fig. 3.

Characterization Methods of Nanoparticles The important properties that are needed for the characterization of nanoparticles are particle shape, particle size and distribution, particle topography and roughness, surface area and properties, stability, swelling, dispersion and agglomeration, purity, reactivity, hydrophobicity, and more properties if needed. The characterization of nanomaterials is done on the structural basis and on the morphological basis. X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray fluorescence (XRF). For morphological analysis, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) (Mourdikoudis et al. 2018). The XRD technique can provide information about the nature of phase, crystalline structure, crystal grain size, and lattice parameters of the nanomaterial sample which have been obtained from waste materials. The role of FTIR is to analyze the precursors and the product and to characterize their vibrational modes. FTIR is also done for understanding the functional groups present in the nanomaterials and also for analysis of purity. For insights into the content of synthesized nanomaterials,

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Fig. 4 Synthesis of different types of nanomaterials

XRF can be directly used. For manufactured nanomaterials, knowledge and estimation of particle size is an important aspect and a crucial parameter. SEM characterizes the internal dispersion of the synthesized nanoparticles, while EDS can be done along with SEM to estimate the actual distribution pattern of the nanoparticles. Particle size or pore size can also be obtained through TEM images, while the size of nanoparticles can be manipulated by adjusting the temperature. For estimation of thermal properties, techniques like DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis) can be used. To analyze the impurities in the synthesized nanoparticles, techniques like ESR (electron spin resonance), NMR (nuclear magnetic resonance), EPMA (electron probe microanalysis), XRF, AES (atomic absorption spectroscopy), and XPS (X-ray photoelectron spectroscopy) (Salame et al. 2018; Sharma et al. 2018) can be used. Figure 4 summarizes the different commonly used characterization of nanomaterials.

Conclusion and Future Scope A great deal of research attention has been focused on sustainable production of nanomaterials and production of nanomaterials from biomass and agricultural waste sources. Nanomaterials also referred to as nanobiomaterials from agricultural waste are considered to have tremendous potential for a variety of applications. The key properties of these nanomaterials like enhanced surface area, specific capacity, or absorptive activity are better as compared to the virgin or synthetic nanomaterials. Nanocellulose can be used in a variety of applications specifically related to environmental field, thus making nanocellulose as an important player in pollution management. Additionally, effective technique for extraction along its application of nanocellulose in the field of biological science is expected in upcoming future. Graphene oxide has high surface area, and abundance of functional groups with many applications makes graphene oxide a promising carbon compound. Graphene oxide synthesis from rice husk and sugarcane can be successfully employed for

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large-scale production of graphene oxide. However, processes of production of graphene oxide from other agricultural waste sources are still at rudimentary stages. Silica nanoparticles have a very important and relevant application in enhancing agricultural yield, and hence more work needs to be done for production of these nanoparticles from bio-based sources. The application of carbon- and carbon-based materials has also been explored for its application in the medical field and industrial purposes. Insights into nanomaterial generation from agricultural waste sources have been provided. The synthesis and characterization methods for the production of nanomaterials have been highlighted as well. However, it has to be noted that increase in the development of nanomaterials can cause some harmful threat to the environment if the waste management is not done properly. In spite of all the different advantages that bionanomaterials possess, their commercial use seems to be mostly ignored. The advancements in research mainly seem to be used by small businesses or some academic sectors. Hence, there should be considerations at an administrative level for rapid commercialization of such agriculturally derived bionanomaterials at a large scale.

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Algal Extract-Biosynthesized Silver Nanoparticles: Biomedical Applications

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Vinita Khandegar and Perminder Jit Kaur

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosynthesis of AgNPs Using Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specific Properties of Algal-Produced AgNP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedicinal Applications of AgNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antibacterial Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antifungal Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antiviral Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miscellaneous Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The green approach to prepare nanoparticles (NP) is becoming widely popular due to its eco-friendliness, reliability, and economic feasibility. Biosynthesis exploits biological agents such as bacteria, fungi, yeast, plant, and algal extracts to synthesize target molecules or materials of interest. The present chapter focuses on one of the widely and abundantly present sustainable resources: algae for the biosynthesis of silver nanoparticles (AgNPs) and their specific properties. Further, the noble AgNPs strive towards the edge-level utilities in every aspect of science and technology, including the medical fields. Thus, the applications of AgNPs in biomedicines have been discussed in detail as compared with the other NPs. V. Khandegar University School of Chemical Technology, Guru Gobind Singh Indraprastha University, New Delhi, India e-mail: [email protected] P. J. Kaur (*) Centre for Policy Research, Department of Science and Technology, Indian Institute of Science (IISC), Bangalore, India e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_82

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Keywords

Algal synthesis · Silver · Nanoparticles · Eco-friendly · Biomedical · Applications

Introduction A wide variety of chemical, mechanical, electrochemical, and photochemical methods are used to synthesize metal NPs (Abdel-Raouf et al. 2019). These methods are low in cost for a high volume of NP synthesis. However, their significant drawbacks include infection from precursor chemicals, fatal solvents, and cogeneration of risky products (Sharma et al. 2019). There is a need for replacing toxic ingredients with an environmentally safe method for synthesizing NPs. To overcome this, researchers are converging on using the biological method for the synthesis of NPs. They are generally cost-effective, harmless, and biodegradable. Biosynthesis of NPs is superior to chemical processes because it forms monodisperse particles with no use (energy, toxic chemicals, high temperature, and pressure). Further, the biosynthesis process is environmentally friendly and safe. Among different biological sources (fungi, bacteria, plant extract), various algae forms are now being used for NP synthesis. Xu et al. (2020) reported that algae contain active organic content like carbohydrates, polysaccharides, enzymes, proteins, vitamins, pigments, and secondary metabolites. These organic matters are responsible for shape-controlled AgNPs. Also, biomolecules (amino acids, proteins and sulfated polysaccharides) in algae extracts act as stabilizers or capping agents in the biosynthesis of AgNPs (Aziz et al. 2015). Additionally, algae are single or multicellular organisms and are found in various places such as freshwater, marine water, or damp rock surfaces. Further, algae have tremendous abilities such as high metal uptake capacity and low cost and rapid synthesis and are very feasible. Also, algae can make various metal oxide NPs and have the ability to catalyse specific reactions (Patel et al. 2015). However, the biosynthesis of safer AgNPs depends on multiple parameters such as green chemistry, solvent medium, reducing agent, and a non-hazardous stabilizing agent. So far, biological methods of NP synthesis using extract, bacteria, fungi, enzymes, and algae have been recommended as substitutes to chemical and physical methods. An emerging trend of synthesizing NPs using algae is developing in recent years.

Biosynthesis of AgNPs Using Algae Algae are an economically and ecologically important group of photosynthetic organisms. They are unicellular or multicellular organisms dwelling in different environments such as freshwater, marine water, or the surface of moist rocks (Shu et al. 2020). Algae are characterized as micro and macro. Algae are a valuable source for the production of natural dyes and biofuels. A different group of algae such as

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Chlorophyceae, Phaeophyceae, Cyanophyceae, Rhodophyceae, and others (diatoms and euglenoids) has been used for the biosynthesis of NPs. Algae can accumulate metals and reduce metal ions. Moreover, algae are fairly suitable and easy to handle, with numerous advantages such as synthesis at low temperature, better energy efficiency, and no risk to the environment and toxicity. Live and dead algae biomasses can be used for the synthesis of NPs. Several algae such as L. majuscule, S. platensis, and C. vulgaris were used as a cost-effective method for silver nanoparticles synthesis. The synthesis of AgNPs using U. fasciata extract as a reducing agent and these nanoparticles inhibited the growth of Xanthomonas campestris pv. malvacearum. In addition to seaweeds, microalgae such as diatoms (N. atomus and D. gallica) can synthesize AuNPs and SiNPs. The ease of obtainability of algae makes their right choice for the synthesis of NPs. According to Lewis Oscar et al. (LewisOscar et al. 2016), three-step biosyntheses of NPs using algae are (i) preparation of algal extract in an organic solvent/water by heating/boiling at a particular period, (ii) preparation of molar solutions of ionic metallic complexes, and (iii) development of algal solutions and molar solutions of ionic metallic complexes followed either by continuous stirring or without stirring for a given time under controlled environments. The synthesis of NPs is dosage-dependent on the type of algae used. Further, different biomolecules are accountable for reducing metals, including polysaccharides, peptides, and pigments. Stabilizing and capping the metal nanoparticles in aqueous solutions is done by proteins through amino groups or cysteine residues and sulfated polysaccharides (Sharma et al. 2016; Abdel-Raouf et al. 2019). Synthesis of AgNPs by algae takes relatively less time as compared to other biosynthesizing methods. In a similar line, brown alga P. pavonica for synthesizing AgNPs submits numerous benefits for pharmaceutical and biomedical applications as they do not use toxicity. Marine macroalgae have various chemicals such as flavonoids, alkaloids, steroids, phenols, polysaccharides, saponins, hydroxyl, carboxyl, and amino functional groups, which can serve as effective metal-reducing and capping agents to provide a robust coating on the metal nanoparticles in a single step (Abdel-Raouf et al. 2019). Among the lower organisms, microalgae have a remarkable role in the bioremediation of toxic and precious metals. S. platensis, blue-green microalgae (cyanobacteria), is a free-floating filamentous cyanobacterium characterized by cylindrical, multicellular trichomes in an open, left-hand helix. They occur naturally in tropical and subtropical lakes with high pH and high carbonate concentrations and bicarbonate. Functional groups and enzymes exist in the cell walls of algae, where the metabolites excreted by the algal culture cause the reduction and deposition of Ag or Ag oxide NPs at ambient conditions cause the removal of Ag ions (CrookesGoodson et al. 2008; Sharma et al. 2019). Mahdieh et al. (2012) tested the ability of Spirulina platensis to produce AgNPs in an aqueous system by taking 5 g of a thoroughly washed S. platensis biomass from an exponential growth phase in a 250 ml Erlenmeyer flask with 100 ml of 1 mM aqueous AgNO3 solution (pH 7) for 24 h. The entire process was carried out at 25  C. Abdel-Raouf et al. (2019) synthesized 49.58–86.37 nm size of AgNPs by

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reducing aqueous solutions of AgNO3 with solvent extracts of Padina pavonica (brown algae). Prepared AgNPs revealed high stability and fast biogenic process formation (2 min to 3 h). For the synthesis of AgNPs, flasks containing 88 ml AgNO3 solution (1 mM) were separately reacted with 12 ml fresh and dry C. capitatum extracts. These reactions were incubated in the dark to minimize the photoactivation of AgNO3 at room temperature under static conditions. Similarly, Kannan et al. (2013) investigated the environmentally friendly AgNPs using fresh and dried extracts of the green seaweed C. capitatum. The amine, peptide, and sulfate groups present in the C. capitatum extract are involved in the bioreduction and stabilization of AgNP. The prepared AgNPs can be used in biomedical and agricultural applications (Ram Naraian and Abhishek 2020). Kingslin and Ravikumar (2018) biosynthesized AgNPs by mixing 50 ml aqueous seaweed extract with 50 ml of 1 mM AgNO3 solution, stirred well for 1 minute, kept in a water bath 60  C for 1 hr., and then incubated in the dark at room temperature under static condition. A control setup was also maintained without seaweed extract. The bioreduction of AgNO3 into AgNPs can be confirmed visually by the change in color from yellow to brown. A systematic synthesis method of AgNPs via the reduction of silver nitrate (aqueous solutions) and extract of algae is shown in Fig. 1. Synthesized AgNPs using algal extract showed high stability, fast formation, and small size. Furthermore, studies reported in Table 1 support algae is a promising bioresource for the synthesis of AgNPs with various shapes and sizes. In conclusion, the biosynthesis of AgNPs by algae extract offers a facile, sustainable, and eco-friendly process. Different algae can be considered in the biosynthesis of AgNPs due to their exceptional properties of rapid growth, high metal adsorption ability, and great organic content present in the structure of algae.

Fig. 1 Biosynthesis of AgNPs using silver nitrate and algal extract

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Table 1 Unique physiochemical properties and biofunctional features of AgNPs Algae Chlorococcum humicola

Size (nm) 4 and 6

Caulerpa racemose

14

Amphora-46

20–25

Oscillatoria sp.

0.000 nm and 558.1 nm 14.9

Nostoc sp. Bahar M

Activity Antibacterial activity against E. coli Antibacterial activity against S. aureus and P. mirabilis Antimicrobial activity against gram-positive and gram-negative bacteria Antibacterial, antibiofilm potential, and low cytotoxicity Antitumor activity

Reference Jena et al. (2013) Kathiraven et al. (2015) Jena et al. (2015) AdebayoTayo et al. (2019) Bin-Meferij and Hamida (2019) El-Rafie et al. (2013)

Pterocladia capillacea, Jania rubins, Ulva fasciata, and Colpmenia sinuosa Amphiroa rigida

7–20

Antibacterial against S. aurous and E. coli

25

Gopu et al. (2021)

Sargassum incisifolium

3–21

Aphanothece sp., Oscillatoria sp., Microcoleus sp., Aphanocapsa sp., Phormidium sp., Lyngbya sp., Gloeocapsa sp., Synechococcus sp., and spirulina Alternaria alternata

40–80

Antibacterial, low cytotoxicity, and larvicidal efficiency against Staphylococcus aureus and Pseudomonas aeruginosa Antimicrobial and anticancer activity against two gram-negative bacteria, two gram-positive bacteria, and one yeast strain Antibacterial activity against pathogenic bacteria

Pithophora oedogonia (Mont.) Wittrock Laurencia aldingensis and Laurenciella sp. Caulerpa serrulata

34.03

Salari et al. (2016) Sinha et al. (2015) Vieira et al. (2016) Aboelfetoh et al. (2017)

Neodesmus pupukensis

52–179

Antibacterial against various pathogenic bacteria Antibacterial against pathogenic bacteria Cytotoxicity against sarcoma tumor cells. Antimicrobial against Shigella sp., S. aureus, E. coli, P. aeruginosa, and salmonella typhi Antibacterial potential against various strains of bacteria (pseudomonas sp., E. coli, K. pneumoniae, S. marcescens)

17.6

7–10 12

Mmola et al. (2016)

Sudha et al. (2013)

Omomowo et al. (2020)

(continued)

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Table 1 (continued) Algae Sargassum longifolium

Size (nm) –

Gelidiella acerosa aqueous

–

Green algae Ulva lactuca and red algae Hypnea musciformis S. vulgares

–

10

S. myriocystum

22

Seaweed, Gracilaria corticata Noctiluca scintillans

4.13

Saccorhiza polyschides

–

Seaweed C. hornemannii

–

Activity Antifungal against pathogenic fungal strains, including A. fumigatus, fusarium sp., and C. albicans Antifungal activity against fusarium dimerum, Mucor indicus, Humicola insolens, and Trichoderma reesei Antifungal activity against A. niger, C. albicans, and Candida parapsilosis fungal strain Anticancerous activity against HeLa cells and human myeloblastic leukemic cells HL60 Cytotoxic abilities against the HeLa cell line Antimicrobial and anticancerous activity against human hepatic carcinoma (HepG2) cell line Antimicrobial, anticancer, colorimetric sensing Immunostimulant and antiproliferative activity on immune and tumor cells Cytotoxic effects in U937 cell lines and antioxidant activity, differential expression of chemokine, cytokines, and hemolytic activity

Reference Rajeshkumar et al. (2014)

Vivek et al. (2011)

Dhavale et al. (2020) LewisOscar et al. (2016) Balaraman et al. (2020) Supraja et al. (2016) Elgamouz et al. (2020) Ansari et al. (2021) Govindan et al. (2021)

Specific Properties of Algal-Produced AgNP The different types of NPs include carbon-based, ceramic, metal, semiconductor, polymeric, and liquid-based NPs. Among the nanoparticles, AgNPs have recognized substantial attention due to their attractive physicochemical properties (Kannan et al. 2013). AgNPs have been widely explored in biosensors, catalysis, antibacterial water filters, antibacterial activity, and controlling plant pathogens, textiles, cosmetics, etc. (see Fig. 2a, b). Ag has an inhibitory effect on many bacterial strains and microorganisms and anti-inflammatory, antiplatelet, and antiangiogenesis potentials, thus making their application in the medical field paramount (Aboelfetoh et al. 2017), which includes topical ointments and creams containing Ag to prevent the infection of burns and open wounds. AgNps are nontoxic to humans and most efficient against bacteria, viruses, and other eukaryotic microorganisms at low concentrations without side effects (Jeong et al. 2005). Additionally, numerous

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Fig. 2 Applications of biosynthesized AgNPs using algal extract

salts of Ag and their derivatives are industrial produce for antimicrobial agents (Dwivedi 2013). AgNPs are commonly used in many biomedical products due to their excellent properties, especially antimicrobial activity and biochemical detection. Ag is a nontoxic, safe inorganic antibacterial agent capable of killing about 600 types of diseases causing microorganisms. Further, AgNPs possess unique electrical, optical, and biological properties, and especially antimicrobial activity is very advantageous in reducing acute toxicity, lowering cost, and overcoming resistance compared with other prevalent antibiotics (Prakash and Vedanayaki 2020). The use of different species of algae in the synthesis of AgNPs has gained widespread attention due to their attractive physicochemical properties. Further, the AgNPs prepared from algae extract have good optical properties, and metabolites (flavonoids and terpenoids) present in the extract were found to be effective capping and stabilizing agents and resulted in the formation of NPs with an average size of 30 nm having potential applications in the field of medicine (Kannan et al. 2013). Biomolecules such as polysaccharides present in algal species also play an important role in controlling the size and desired shape of AgNPs. Marine macroalgae are good sources for synthesis if NPs have antioxidant activities (Ibraheem et al. 2016a). Furthermore, several metabolites were detected as antioxidant compounds from brown algae. Oxidative stress is the most popular reason for the pathogenesis of chronic diseases. Dietary antioxidants have a positive role in controlling degenerative disorders such as cardiovascular disease, neurological disorders, diabetes, Alzheimer’s disease, and gastric ulcers (Ibraheem et al. 2016b; Abdel-Raouf et al. 2018). The different types of NPs include carbon-based, ceramic, metal, semiconductor, polymeric, and liquid-based NPs. Titanium oxide (TiO2) is found to be effective

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against both gram-positive/gram-negative bacteria, parasitic, and viral infections. ZnO nanoparticles (ZnONPs) are another broad-spectrum antibacterial agent, based on the concentration and size of the NPs, and they are operative against methicillinsensitive S. aureus, methicillin-resistant S. aureus, and S. epidermis (LewisOscar et al. 2016). They are of low cost and inhibit the growth of a wide range of pathogenic bacteria with less toxicity to human cells. Their UV blocking and antibiofilm activity make them a suitable coating material for medical and other devices. It is approved by the Food and Drug Administration to treat disease and ingredients in food additives. Iron oxide nanoparticles are usually inactive in their bulk form. Reducing their size to nanoscale makes them a potential antimicrobial agent. Iron oxide nanoparticlescoated surfaces prevent gram-positive and gram-negative bacteria (LewisOscar et al. 2016). As compared to Ag, Au nanoparticles are less effective and lack antimicrobial properties when used alone but are effective when used in combination with antibiotics such as ampicillin, lysozyme, and vancomycin. The AuNPs can be used with nonantibiotic particles such as amino-substituted pyrimidines and citrate, hence used in cancer therapy. Despite copper oxide nanoparticles being used as antibacterial agents, they are less effective than Ag and ZnO NPs. Nonetheless, some bacteria are more vulnerable to CuO than Ag. CuO NPs exhibit antibacterial activity by membrane disruption. Magnesium oxide nanoparticles (MgONPs) are efficient antimicrobial agents showing bactericidal activity against gram-positive and gram-negative bacteria, viruses, and spores. Nitric oxide nanoparticles (NONPs) are a highly reactive antibacterial agent. Like other NPs, the activity of NO is also dependent on size. They are effective against various biofilmforming bacterial classes. Aluminum oxide nanoparticles are a minor antibacterial agent and effective only on higher concentrations.

Biomedicinal Applications of AgNPs AgNPs synthesised by algae play a key role in medical, pharmaceutical, agriculture, aquaculture, cosmetics applications (Fig. 2). Silver and its compounds have been used as an antibacterial agent since ancient times. Its therapeutic properties make it applicable for a multitude of medicinal applications. There is evidence of the use of silverware to preserve food products. Silver nitrate was used for wound and burnt healing applications and disinfectant for medical instruments. The concentration, pH, contact time, and temperature of extract affect the overall process effectiveness. Several researchers have highlighted the properties of NP to inhibit bacterial adhesion of their surface. They also stop the formation of biofilm on their surface. Vijayan et al. (2014) reported T. conoides-based AgNPs could inhibit biofilm formation in E. coli, Salmonella sp., S. liquefaciens, and A. hydrophila. The identical particles were found to be toxic to brine shrimp Artemia salina (LC50 value of 88.914 μL/mL), indicating its anti-microfouling properties (Vijayan et al. 2014). Sargassum ilicifolium-based AgNP was found to be cytotoxic against Artemia salina (Kumar et al. 2013).

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In the past few decades, nanotechnology has grabbed the attention of researchers for various reasons. Most of the research on nanomaterials is focused on biomedicine and catalysis. AgNPs find the maximum application in the healthcare sector (about 30% to total) with expected market size of $2.45 billion by 2022. It is expected that AgNPs will gain further importance due to the increase in infectious diseases in the present scenario (Marassi et al. 2018). Various global agencies have issued new guidelines and safety protocols regarding Ag NPs in medical devices, equipment, and textiles (Bos et al. 2015). AgNPs in the medical field vary from usage in diagnosis, drug-delivery, and actuation to nano-robot purposes (Neelakandan and Thomas 2018). Other widely used applications are medical devices and implants prepared with Ag-impregnated polymers (Jung et al. 2008). In addition, Ag-containing consumer products, such as colloidal Ag-gel and Ag-embedded fabrics, are now used in sporting equipment. AgNPs have high potential activities against various strains of bacteria, virus, and cells (see Fig. 3).

Antibacterial Properties The ability of an agent to either kill or reduce bacterial growth without causing harm to the surrounding cells is known as antibacterial activity. Many antibacterial drugs are available commercially. However, the multidrug-resistant bacteria prompted researchers to look at alternatives. Silver ions released by AgNPs in an aqueous solution get attached to sulfur and hydrogen groups present in the various bacterial proteins. Silver nanoparticles showed strong bacterial resistance properties, especially for gram-negative bacterial strains. The two types of bacteria strain exhibit

Fig. 3 Potential activities of AgNPs

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different cell wall thickness, and silver nanoparticles can penetrate the walls of gramnegative bacterial strain more effectively. The presence of silver nanoparticles leads to the formation of pits on the cell walls of the bacterial community, which allows them to impregnate the periplasm of bacteria. The cell envelope of gram-positive bacteria like S. aureus includes a thick peptidoglycan layer (30–100 nm) to protect cells from harmful reagents. The cell wall of these types of bacteria is also wide enough to restrict the penetration by silver nanoparticles. Conversely, the cell envelope of gram-positive bacteria like E. coli consists of a peptidoglycan layer (3–4 nm) and cell membrane, thin enough to allow penetration by silver nanoparticles (Chatterjee et al. 2015). C. serrulata algal extract was used to prepare AgNP. Agar well diffusion technique was used for gram-negative bacteria (E. coli, S. aureus, Shigella sp., S. typhi) and gram-positive bacteria (P. aeruginosa). The ability of AgNP to disrupt the cell wall of gram-negative bacteria is depicted from the observed high growth inhibition for E. coli for 21 mm at 75 μL. Conversely, the inhibitory zone for S. typhi (grampositive) value was 10 mm for 50 μL (Aboelfetoh et al. 2017). Concerning the efficacy of biologically and chemically synthesized silver nanoparticles, experimental investigations have been performed. The inhibition zone of chemically synthesized silver nanoparticles was significantly lower than biologically synthesized particles. Biological agents are reported to act as stabilizing agents as well, providing them with improved bactericidal activity. The use of A. niger fungal strains to produce silver nanoparticles resulted in complete disruption of the cell walls of E. coli. Likewise, the growth inhibition zone against S. typhimurium was reported to be 12, 13, 13 mm for bacterial, mucilage-supported, and fungal-extracted AgNPs (Ramírez Aguirre et al. 2020). P. hornemannii was tested against fish pathogenic bacteria V. harveyi, V. parahaemolyticus, V. vulnificus, and V. anguillarum. Intense antibacterial activity was observed against all these bacteria. The close width of the zone (CWZ) of inhibition against V. harveyi was found to be 16 and 19 mm. For V. anguillarum, CWZ was 23 and 28 mm. CWZ against V. parahaemolyticus was around 20 mm (Fatima et al. 2020). The size and shape of silver nanoparticles can be optimized concerning the treatment time to obtain maximum efficiency. Dimensions in the range less than 10 nm are reported to be the most effective size for silver nanoparticles. The efficacy of antibacterial treatment increases with a decrease in the size of particles and increases in treatment time. Studies were performed on shape effect on bactericidal activity. It was found that the minimum inhibitory concentrations of nanocubes, nanosphere, and nanowires were 37.5, 75, and 100 μg/mL, respectively. The study highlighted that facet exposure and shape are critical factors (Hong et al. 2016).

Antifungal Activity Silver nitrate solution was used to prepare silver NP using chemical reagents like trisodium citrate dehydrate and polyvinyl pyrrolidone. Tests were done against

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fungi, i.e., A. citri, P. digitatum, and A. alternate. The percentage rate of inhibition showed as about 83%, 78%, and 81% in A. alternate, P. digitatum, and A. citri., respectively, comparable to commercially used toxic fungicide (Salaheldin 2016). Using sodium citrate solution in silver nitrate solution as a reducing and stabilizing agent, silver NPs were produced and tested against R. solani, a plant pathogenic fungus. Most of the produced particles showed a high fungicidal effect at the concentration of 0.002 mmol/L (Elgorban et al. 2016). Plant extracts of A. absinthium were used to prepare AgNPs, which showed antifungal activity against Candida genus fungi. At AgNPs to extract ratio of 6:4, the minimum inhibitory concentration (MIC) was 0.325 μg/ mL, and the minimum fungicidal concentration (MFC) was 1.3 μg/ mL showing its potential as a future eco-friendly fungicidal agent (Rodríguez-Torres et al. 2019). Similarly, when A. sydowii fungal strain interacted with silver nitrate, resultant silver NPs exhibited excellent biological properties. Antifungal assays were performed against a broad range of fungi like Candida (C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis) and Aspergillus (A. fumigatus, A. flavus, and A. terreus). Mechanism study had shown that for NPs as the diameter is minimal, their penetration into the cell wall to the cell membrane is easier. They can enter into the cells of pathogen without effort and can bind quickly with enzyme protein sulfhydryl. They can make necessary enzymes of pathogens inactive. AgNPs can denature the fungi by reacting with the protein molecules available on the fungi’s surface. In addition to this, it has been found that AgNP can further deactivate pathogens by crosslinking with their DNA bases in pathogenic fungi, which leads to losing the ability to duplicate (Wang et al. 2021).

Antiviral Activity Researchers are trying to find suitable medicines against viral diseases like influenza, hepatitis, herpes simplex virus (HSV), and human immunodeficiency virus (HIV), which can lead to life-threatening diseases (Murphy et al. 2015). There is still no effective vaccine or medicine that can target specific viruses and maintain cell viability. AgNPs have shown activity against. Thus, it becomes even more essential to develop an effective medicine with a broad range of viral strains, including hepatitis B virus (HBV), human parainfluenza virus (HPIV), herpes simplex virus (HSV), and influenza A (H1N1) virus (Sinclair). Researchers have shown AgNP’s ability to block the entry of the virus into the human cells and thus prevent viral diseases. The particles were able to bind to cellular heparin sulfate through sulfonate end groups of the virus. Studies have demonstrated antiviral activity of AgNPs against human immunodeficiency virus (HIV), monkeypox virus, and hepatitis B virus (Elechiguerra et al. 2005; Lu et al. 2007; Rogers et al. 2008). The binding affinity of these particles in size range of around 10 nm for HBV dsDNA and extracellular virions is very high, inhibiting the production of both HBV RNA and extracellular virions (Lu et al. 2007). Most of these AgNPs are chemically synthesized. The efficacy of algal-produced AgNPs needs detailed investigations.

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Miscellaneous Activities Researchers are finding effective agents to cure cancer, highly target-specific without damaging the surrounding cells. Silver NP produced from A. sydowii fungal strain inoculated on silver nitrate provides the good antiproliferative activity. They can retard the growth of HeLa cells and MCF-7 tumor cells at a concentration of more than 30 μg/mL or 10 μg/mL (Wang et al. 2021). Silver oxide NP was prepared using microalgae Oscillatoria sp. and S. platensis. Tests for their efficacy against human colon cancer (CaCo-2), cervical cancer (HeLa), and normal cells (WISH) were performed. To understand the effectiveness of the drug, half-maximal inhibitory concentration (IC50) was evaluated. The low concentration of AgNP was 3.9 and 15.6 μg/ml against HeLa and CaCo-2 cells (El-Sheekh et al. 2021). Diabetes mellitus, a chronic metabolic disorder, was inhibited using silver nanoparticles, which could otherwise damage the nerves and blood vessels. In Sargassum wightii algae, mediated silver nanoparticles were tested for their α-amylase and α-glucosidase inhibitory activity. The lC50 value of algae AgNP against α-amylase was 55.87 μg  ml 1, considerably higher than control acarbose of 27.41 μg  ml 1. Similarly, α-glucosidase inhibitory activity of these particles was evident from its IC50 values of 84.51 μg  ml 1, more increased than control acarbose of 31.75 μg  ml 1 (Deepak et al. 2018).

Conclusion and Future Recommendations AgNPs can be produced using chemical as well as biological methods. However, greener methods for the synthesis of AgNPs are the most suitable ones. Further, biosynthesized NPs have high stability and longevity as compared to NPs prepared by other methods. AgNPs using algal stains are still promising. Comparison in the efficiency of AgNPs could provide more insight into these particles. Some associated issues like maintenance of aseptic culture environment, production time, cost, and yield need to be addressed before moving towards large-scale algal-based AgNP production. AgNPs have high potential as antifungal, antiviral, and cancer treatments. The studies on the medicinal properties of algal-based AgNPs need detailed investigations. However, there were no reports on “pilot plant” synthesis of AgNPs using algae. AgNPs have significant applications in the biomedical field; therefore, the use of algae is a developing and exhilarating area, and more research is needed. There is still more work required to improve the optical properties of AgNPs to reduce aggregation. It is necessary to utilize surface-active agents to achieve a good effect. Determine the exact origin of the cytotoxicity of AgNPs to make the appropriate modification to obtain safer products and technologies. The potential harm of AgNPs to organs and systems in the body has been gradually observed, which may influence the biomedical application of AgNPs. Therefore, it is necessary to review the dynamics of AgNPs in vivo. Additionally, computational techniques in the synthesis of AgNPs will be broadened and could be used in medicine as “nano drugs” shortly.

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Green Synthesis of Metal Oxide Nanoparticles

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Sharmi Ganguly and Joydip Sengupta

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide Nanoparticles and Their Green Synthesis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Mediated Synthesis for the Growth of Metal Oxide Nanoparticles . . . . . . . . . . . . . . . . . Factors Influencing the Synthesis of Various NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Metal Oxide Nanoparticles from Plant Extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CuO Nanoparticle Synthesis by Plant Leaf Extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO Nanoparticle Synthesis by Plant Leaf Extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2 Nanoparticle Synthesis by Plant Leaf Extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization Techniques of Nanoparticles Prepared from Plant Extract . . . . . . . . . . . . . . . Mechanism of Nanoparticle Evolution with Plant Extracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of Green Synthesis of NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antimicrobial Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Metal oxide nanoparticles (MONPs) have a wide range of uses in numerous disciplines of science, industry, biomedicals, etc. Recent developments in science and technology, notably nanotechnology, have made this possible. Providing trustworthy and environmentally sustainable solutions for the development of S. Ganguly Electronics and Communication Engineering, Meghnad Saha Institute of Technology, Kolkata, India J. Sengupta (*) Department of Electronic Science, Jogesh Chandra Chaudhuri College, Kolkata, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_91

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nanoparticles (NPs) is one of the current goals of today’s nanotechnology. NPs have a significant surface area-to-volume ratio and consequently have very novel properties because of their nanoscale dimension. For more than a decade, NPs have been studied and used in a variety of industrial applications. Initially, a high volume of hazardous compounds was utilized in typical chemical and physical synthesis procedures of MONPs resulting in harsh environments. However, for the sustainable development of MONPs, green manufacturing approaches have been recently developed. Green NP production is simple, inexpensive, and environmentally beneficial, and it involves a less hazardous process employing mainly biological extracts, and their antioxidant or reducing characteristics are usually responsible for the reduction of metal compounds into their respective NPs. The complexity of biological extracts, however, offers a barrier to the understanding of the reactions and mechanisms of formation that occur throughout the synthesis, thus making large-scale manufacturing employing green synthesis approaches difficult. This chapter will describe the basic principles or methods of green synthesis with an emphasis on three MONPs, namely, zinc oxide (ZnO), titanium dioxide (TiO2), and copper oxide (CuO). The reason behind opting for these three NPs is their wide range of potential applications in various sectors like biomedical, food storage, cosmetics, etc. Keywords

Metal oxide · Nanoparticle · Green synthesis · Zinc oxide · Copper oxide · Titanium dioxide

Introduction Science is credited with ushering in the “nano era” by achieving miniaturization in every sector of commerce and industry. The initial idea and concept of nanoscience were born on December 29, 1959, when an article describing “There is Plenty of Room at the Bottom” was presented by physicist Prof. Richard Feynman (Bhattacharyya et al. 2009). Nanotechnology arose from the control of atoms and molecules, as well as their subsequent modification. Nanotechnology has transformed science by allowing scientists to manipulate materials at the nanoscale. The science of nanotechnology involves the study and control of matter with dimensions of 1–100 nm. This field of science has undergone considerable progress in the recent decade, resulting in improvements over existing or conventional procedures. Various applications of nanotechnology are shown in Fig. 1. NPs are one of the fascinating products of nanotechnology. Considering their large surface area-to-volume ratio, NPs have augmented catalytic activity, thermal conductivity, nonlinear optical performance, and chemical stability (Shams Tabrez Khan et al. 2016), which alters both physical and chemical properties, as compared to the bulk chemical compositions (Bogunia-Kubik and Sugisaka 2002; Vladimir

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Fig. 1 Various applications of nanotechnology. (Reproduced with permission from Elsevier)

P. Zharov et al. 2005). Top-down and bottom-up approaches are the prominent methods which can be used to synthesize NPs with optimized shape, size, and usefulness (Jagpreet Singh et al. 2018), as shown in Fig. 2. The former entails synthesizing nanomaterials/NPs utilizing a variety of synthetic techniques, such as ball milling, lithographic techniques, etching, and sputtering. Strong reducing agents, as well as a capping agent and volatile solvents like chloroform and toluene, are commonly employed in the bottom-up strategy to fabricate NPs. Although these approaches are productive in synthesizing distinct and unadulterated metallic NPs, the high price of manufacturing is still a major barrier (Panagiotis Grammatikopoulos et al. 2016). As a result, a cost-effective and ecologically acceptable alternative is needed, allowing for the employment of an environmentally friendly reductant, environmentally adaptable solvents, and nonhazardous capping agents for the synthesis of NPs. All of these parameters are the fundamental requirements for the production of green NPs. Physical, chemical, and green processes are used to synthesize NPs. The physical process necessitates the use of expensive apparatus, ambient temperatures and pressures, and a big area for machine setup. The chemical process entails the use of hazardous chemicals that can be detrimental to nature as well as to the operating person. According to the literature, some of the harmful compounds used in physical and chemical processes may end up in the NPs in embedded form, posing a risk in the realm of medical use (Dhandapani et al. 2014). For NP synthesis, a process is required that is both environmentally benign and cost-effective. In this regard, chemical procedures comprise chemical microemulsion, wet chemical, spray pyrolysis, electrodeposition, direct precipitation, and microwave-assisted combustion.

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Fig. 2 Fundamental approaches for the synthesis of NPs. (Reproduced with permission from the Royal Society of Chemistry)

Physical processes incorporate pulsed laser deposition, MBE (molecular beam epitaxy), thermal evaporation, and a few others. Because of worries about climate change, water pollution, limited natural resources, human health, and other issues, the creation of environmentally favorable products and processes has achieved a lot of attention in the last few years. As a result, scientists are working on ways to use greener technologies to synthesize MONPs. The utilization of biological substrates for the ecologically friendly production of MONPs has been intensively researched as a potential replacement for chemical and physical processes commonly employed in the industry. Although a significant amount of research has been published on this topic, the mechanism of green synthesis NP creation has yet to be characterized and understood due to the great complexity of biological extracts. Green NP synthesis is a process developed from nano-biotechnology, and green nanomaterials have become the focus of nanotechnology research. Green NP synthesis is emerging as a nontoxic, ecologically friendly, clean, less expensive, and almost novel approach that can be carried out at ambient temperature. Synthesizing biocompatible NPs, which is the prominent contemporary feasible method of integrating material science with biotechnology, can be regarded as a green alternative for NP synthesis. As a result, the controlled shape and size of NPs produced by green mediated synthesis using genetic

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engineering approaches, molecular cloning, plant extracts, and other biological procedures will be a significant step forward in nano-biotechnology. Various parts of the plant body have also been used to synthesize NPs because a plant is enriched in phytochemicals that constitute both reducing and stabilizing agents.

Metal Oxide Nanoparticles and Their Green Synthesis Techniques The cost-effectiveness and environmental friendliness of green NP or composites synthesis techniques make them a preferred candidate among other physical, chemical, and other methods for synthesizing NPs. For pharmaceutical and biomedical applications, the utilization of living organisms is also an environmentally friendly process which is explored extensively nowadays. Different metal oxides and green synthesis method techniques, in general, are discussed below.

Metal Oxide Nanoparticles CuO Nanoparticles Biogenically produced CuO NPs are cost-effective and environmentally friendly, and depending on the plant source, they come in a variety of sizes. The UV absorption peaks of green CuO NPs are between 265 and 285 nm and around 670 nm, respectively, and correlate to the Cu metal inter-band transition of core electrons and CuO band edge transition. When generated with aloe vera, X-ray diffraction (XRD) investigations revealed a monoclinic structure, while transmission electron microscopy (TEM) revealed well-dispersed, flexible, and spherical NPs with 0.23 nm inter-planar spacing and a size range of 20 to 30 nm. CuO vibrations are responsible for a prominent band at 1100 cm
1 and peaks at 529 and 350 cm
1 in these phyto-mediated CuO NPs (G. Sharmila et al. 2017). TiO2 Nanoparticles TiO2 NPs are one of the most extensively generated MONPs, owing to their adjustable properties derived from physical, chemical, optical, and electrical capabilities. It comes in three different mineral forms, anatase, brookite, and rutile; however, TiO2 is more abundant in the former due to increased photocatalytic activity. ZnO Nanoparticles Due to its fast synthesis rate, nontoxicity, high biocompatibility, antibacterial activity, and photocatalytic activity, ZnO NPs have recently been discovered to be another of the most advantageous MONPs synthesized by green produced. In the synthesis of ZnO NPs, crude extraction from the plant is employed as a biological reducing and stabilizing agent. ZnO NPs have also been made using fungi and other microbes. They’ve been used in a variety of science and engineering sectors, including electrochemistry, catalysis, medical devices, cleaning agents, and the textile industry

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(Shakeel Ahmed et al. 2017). Elavarasan et al. demonstrated a nonhazardous route to synthesize ZnO NPs by using Sechium edule leaf extract in a temperature-dependent manner, and spherical shape NPs have formed at 400  C temperature revealing an intense absorbance peak at a wavelength of 362 nm. When compared to traditionally generated ZnO NPs, green-route ZnO NPs are found to be extremely efficient in photocatalytic, antibacterial, and anticancer properties (Elavarasan et al. 2017).

Green Mediated Synthesis for the Growth of Metal Oxide Nanoparticles NPs can be synthesized using a variety of techniques as shown in Fig. 3. However, the process could be changed with good reason to fulfil the goal of green synthesis. The most important advantages of green synthesis are shown in Fig. 4. The following information describes a handful of the green synthesis pathways.

Plant-Based Method The use of phytochemicals of plant extracted juice, polysaccharides, and biomolecules as reducing agents for the synthesis of NP is a typical approach to green nanotechnology. Plant juices are proven to be more useful than other biological

Fig. 3 Different nanomaterials synthesis methods. (Reproduced with permission from Springer)

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Fig. 4 Advantages of green synthesis. (Reproduced with permission from Springer Nature)

Fig. 5 General schematic diagram for the formation of NPs from different sources. (Reproduced with permission from Elsevier)

resources among all common/well-known bio-reductants. A general schematic diagram for the formation of NPs from a different source is shown in Fig. 5. Plant-based NP synthesis is a straightforward technique that involves mixing a metal salt with the extraction from the plant and allowing the chemical reaction to complete in less time at ambient temperature. The particle size and the growth rate can also be regulated

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using plant extracts by adjusting the synthesis parameters such as pH, reductant concentration, temperature, and reactant mixing ratio. The plant-based technique is a cost-effective alternative to microbial mediated NP manufacturing since it uses plant extracts in an aqueous solution rather than using solvents.

Microorganism-Mediated Method Microbes have been an excellent choice because of their genetic variability, easy availability, easy cultivation, and potential for improving the structural and functional characteristics of NPs. Because bacteria have a variety of biocatalysts, the process has become more selective and suitable. A wide range of bacteria can produce extracellular and intracellular NPs. The metallic salts are reduced to their ionic state by a collection of bacteria associated with different biogeochemical cycles, usually through diverse enzymatic activity. Extracellular NP creation occurs during enzymatic action, while internal NP synthesis occurs when bacteria are attacked by macrophages and reduced to a nano-form, which is then expelled or stored in vacuoles for later use or disposal. Because of its mycelial meshwork, exacting growth properties, and ease of handling and manufacture, fungal NP production is more beneficial. Microwave-Assisted Method Microwaves are a type of electromagnetic wave made up of pure energy, radiated in the form of a wave that travels at the speed of light. Microwave propagation in condensed matter occurs at slower speeds than in air or vacuum. The power and time required for microwave production of NPs must be regulated. Hydrothermal synthesis devices can also benefit from microwaves. The most often employed microwave frequencies in NP synthesis are between 2 and 45 GHz, with all dielectric parameters being highly influenced by temperature. The transfer of energy between microwaves and matter is difficult. Microwave-endorsed synthesis methods have been widely utilized to create oxide, hydroxide, and sulfide NPs. They are easy, are clean, and have a temperature gradient effect. Microwave irradiation has an advantage over traditional biological synthesis. It improves the kinetic rate of the reaction due to the quick heating and penetration involved, which can result in a narrow particle size distribution. Mild Reducing Agent-Based Technique For MONP synthesis, the concentration of reductant and stabilizing precursors has a significant impact. This method of NP production is essentially a hydrothermal or solvothermal process. The presence of active functional groups in the stabilizing agents has a significant impact on NP morphology. When reducing sugar (glucose and fructose) are utilized as reducing agents, for example (V. V. Makarov et al. 2014), variations in morphology and dispersity can be observed. The reason for this is due to changes in the reaction processes of the active functional groups, which result in metal ion reduction for NP production. The synthesis process is made less hazardous and ecologically friendly by using delicate reducing conditions such as

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ambient temperature, pH, solvent concentration, and mild surfactants as reducing agents and stabilizers.

Ultrasound-Assisted Method Ultrasound is a secure and clean method for making NPs, and it is one of the most popular nonbiological methods for making green NPs. Ultrasound is an excellent energy carrier for disintegrating bulk precursors which has frequencies as high as 20,000 Hz, and it is generating NPs with precise morphologies for specific purposes. Ultrasound has been intensively studied for material synthesis over many years and is regarded as one of the most powerful methods. Sonochemistry and ultrasonic spray pyrolysis, two of the most extensively used ultrasound technologies, are widely used as the green approach for NP fabrication. Ultrasonication is a very reliable method for controlling NP synthesis that uses sound waves to create a clear path (Bang and Suslick 2010). Solar Energy-Mediated Method Sunlight, the world’s most abundant renewable energy source, can also be used to mediate the NP synthesis, which is a nonbiological path to green synthesis in general. Because of its abundance and environmentally friendly character, sunlight is an excellent choice for green synthesis. Bacteria-Based Method Living pathogens such as bacteria have been used in economical biotechnological processes for bioleaching (metal solubilization by microorganisms) and bioremediation for decades. MONPs were synthesized using a variety of bacterial species. Bacteria are preferred for the production of NPs due to the minimal growth conditions required with high yield. Bacteria have remarkable abilities to decrease heavy metal ions and could be used to generate NPs. Bacteria can be utilized as a biocatalyst for the synthesis of inorganic materials and a bio-scaffold for the synthesis of NPs. Metal oxides can have a diverse variety of physicochemical properties and structural geometries, depending on whether they have a metallic, semiconductor, or insulator electronic structure. Common chemical methods, viz., coprecipitation, sol-gel, microemulsion (Uskoković and Drofenik 2005), and solvothermal, are widely accepted to compose MONPs. Even though these techniques have been successfully producing oxide NPs, the main drawback of these methods is the involvement of toxic and expensive chemicals. As a consequence, developing environmentally acceptable and long-term ways to produce MONPs is of great interest. In recent trends, bacterial strains are the promising green alternative to the synthesis of several oxide NPs, especially CuO, TiO2, and ZnO. Even though the biological synthesis is well accepted to yield minor harmful MONPs, it can need meticulous cell culture and complex size distribution control (Narayanan and Sakthivel 2010). Because chemical synthesis dominates the manufacture of MONPs, still a paucity of information on biological synthesis has to be standardized. Various researchers adopted bacteria-mediated green protocols for the synthesis of

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NPs to reduce certain chemicals. The main advantage of using bacteria to generate NPs over chemical synthesis is that it allows for extensive synthesis with minimum usage of hazardous and costly chemicals.

Fungus-Based Method Fungi and yeast both synthesize NPs with great efficiencies. Fungi have recently been added to the list of microorganisms utilized to make NPs. Fungi may produce MONPs in a variety of shapes, including meso- and nanostructures, using an extracellular or intracellular reducing enzyme and a biomimetic mineralization process. Due to the wide range of fungi, this scientific word serves as a bridge between “mycology” and “nanotechnology” and has tremendous promise. Fungi-mediated NP biosynthesis is a convenient method because fungi have extremely high intracellular metal absorption capabilities (Volesky and Holan 1995) and can synthesize a large number of enzymes per unit of biomass. Reductases, extracellular enzymes produced by fungi, have aided the synthesis of MONPs with various chemical compositions. Cultivation of fungus on the surface of inorganic substrates, on the other hand, resulted in effective metal distribution as a catalyst.

Factors Influencing the Synthesis of Various NPs Temperature Temperature regulation over NPs is the focus of a large number of research projects worldwide. Temperature is one of the most critical parameters affecting the size and shape (morphology) of NPs, along with their rate of synthesis, as shown in Fig. 6. The pace of reaction and the generation of nucleation centers increases as the temperature rises (Krishnamurthy Sneha et al. 2010).

Fig. 6 Factors influencing the synthesis of various NPs. (Reproduced from Hindawi Publishing Corporation)

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pH In the synthesis of NPs, the pH of the reaction plays a very decisive role in determining the morphology and size of NPs. pH affects the generation of nucleation centers, similar to temperature. As the pH rises, the number of nucleation centers increases, resulting in increased production of MONPs. It was observed that as pH rises, the size and aspect ratio falls (Kenji Okitsu et al. 2009). Reaction Time Together with temperature and pH, reaction time was revealed to be one of the most essential parameters influencing the morphology of the synthesized NP. Karade et al. (2018) presented a significant study on “the effect of reaction time” on magnetic NPs. Green tea extract was used to synthesize Fe NPs from ferric nitrate solution. The magnetic and structural properties of magnetic NPs were shown to be influenced by response time. NPs have been extensively investigated due to their intriguing qualities, which are influenced by their structured shape, and many studies have created NPs using chemical and physical approaches. The characteristics of the NPs differ significantly from those of their bulk counterparts (Imtiyaz Hussain et al. 2016). Due to their exclusive properties, they have been found to have a lot of potential for use in biomedical, biosensor, and antimicrobial applications. Most chemical approaches use hazardous reducing agents, but these result in poor control over particle size. The use of additional capping or stabilizing agents is necessary to assure steric stability and prevent NP aggregation (Talib and Hui-Fen-Wu 2016). The reducing agent adsorbed on the NP surface is then removed with a high heat treatment, which may act as a barrier at the active sites of the particles, resulting in a lower surface area and impaired reactivity of the products (Zhufang Liu et al. 2007). Furthermore, to get good crystalline samples, annealing is required. This synthetic approach is of high cost, and potentially hazardous features may limit its use on a broad scale with a low product yield. The syntheses of NPs employing nontoxic chemicals, renewable materials, and finally degradable waste products are all part of the green synthesis approach to NPs (Virkutyte and Varma 2011). Plant-based materials have received a lot of attention and are widely considered to be the greatest material due to their biodegradability, simplicity of use, and abundance. MONPs have been manufactured using green technologies based on plant or plant extracts and microorganism extracts. The synthesis of MONPs by green-derived routes, such as microwave-aided technique with plant extract, has recently been discovered and is of great interest to researchers. Green synthesis techniques for additional metal oxides utilizing plant extracts have been reported in response to rising demand. Because of their wide range of applications in industry, most investigations have focused on the synthesis of CuO, ZnO, and TiO2 NPs. MONPs are unstable compounds that require high-temperature treatment, to remove the oxygen molecules and generate crystalline NPs. Among the various synthesis techniques, plant-mediated techniques are very popular because biosynthetic approaches for synthesizing NPs have sparked a lot

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of attention due to their environmental friendliness, simplicity, economy, and cleanliness; they don’t require dangerous chemicals and produce no impurities or wastes. Thus, the green synthesis of MONPs using the plant-mediated technique will be the main focus hereafter.

Synthesis of Metal Oxide Nanoparticles from Plant Extract Biochemical and yield-specific sources for NP production are abundant in plant biodiversity. Plants are indispensable suppliers of metal ions because they include biomolecules like coenzyme and vitamin-based intermediates that can convert metal ions to NPs (Parth Malik et al. 2014). Other biosynthetic processes are less useful than plant extracts as reducing or stabilizing components for NP synthesis. This occurs because there is no requirement for cell culture or cell upkeep in this procedure (Antariksh Saxena et al. 2012). Furthermore, plant-based NP synthesis is more stable, allowing for the manufacture of NPs of various shapes and sizes, and is less expensive (Siddiqui et al. 2015). The general synthesis method using plant extract is shown in Fig. 7. Plant extracts, in complement to bacteria and fungi, are a suitable source for the production of MONPs. Plant-derived iron oxide NPs, for example, have been widely manufactured (Mihir Herlekar et al. 2014). Spherical and irregular cluster-shaped iron oxide NPs were synthesized using extracts from Camellia sinensis (green tea extracts). Kuang et al. (2013) employed tea extracts from three different species (green tea, oolong tea, and black tea) to make irregularly shaped iron oxide NPs with a size of 20 to 40 nm. Leaf extracts of Eucalyptus globulus (Zhiqiang Wang 2013), pomegranate (Ashit Rao et al. 2013), and aqueous extract from the dry fruit pericarp

Fig. 7 General green synthesis method using plant extract. (Reproduced from Catalysts)

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of Terminalia chebula (Mohan Kumar et al. 2013) have been used to synthesize ZnO NPs and have been made using Calotropis gigantea, Camellia sinensis (Senthilkumar and Thirumal 2014), and aloe leaf broth extract (Sangeetha Gunalan et al. 2012a) [38]. Jatropha curcas latex (Manish Hudlikar et al. 2012) and neem leaf extract (Hiremath et al. 2016) have also been used to synthesize TiO2 NPs. Leaf extracts of Aloe barbadensis miller (Sangeetha Gunalan et al. 2012b) yielded CuO NPs. Leaf extracts are a suitable source for NP biosynthesis since they are high in these bioactive chemicals. Plant cultures, on the other hand, can only synthesize NPs under particular conditions. The key influencing parameters in a plant extractmediated synthesis are temperature, pH, and salt concentrations. Plant biodiversity provides a large biochemical and yield-specific supply. Plant leaf derives are readily available, are nonhazardous, and include a variety of metabolites that act as a reductant in the formation of NPs. In plant leaf extract-mediated NP production, the extract is blended at room temperature with a metal precursor solution. The metal ion reduction takes slower in fungi and bacteria, although watersoluble phytochemicals can decrease metal ions in a fraction of the time. As a result of the existence of various phytochemicals that may be easily removed, plant leaf extract is an effective source for MONP formation. Thus, leaf extracts act as both reducing and stabilizing agents in the formation of NPs. The NP formation is also influenced by the type of the leaf extract, as various extractions contain varied quantities of biochemical reducing agents. Terpenoids, flavones, ketones, aldehydes, amides, and carboxylic acids have been identified as the primary phytochemicals responsible for NP production. Photocatalysis (Tariq Khalafi et al. 2019), catalysis (Vara et al. 2019), sensors (George et al. 2018), and heavy metal removal (Panji et al. 2016) have all benefited from MONPs because of their excellent electrical, optical, magnetic, and catalytic properties.

CuO Nanoparticle Synthesis by Plant Leaf Extract Sankar et al. (2014) used Carica papaya leaf extract to make CuO NPs. The bioactive compounds contained in the plant leaf extraction reduced the precursor and produced CuO NPs, according to Fourier transform infrared spectroscopy (FTIR) analyses. XRD and scanning electron microscopy (SEM) studies indicated that the green-synthesized CuO NPs were rod-shaped crystalline with an average particle size of 140 nm. Raja Naika et al. (2015) used the leaf extract of Gloriosa splendid L. to make CuO NPs. SEM and TEM investigations indicated that the produced CuO NPs were spherical, agglomeration-free, and in the size range of 5–10 nm. Nagajyothi et al. (2017) used the black bean aqueous extract to synthesize CuO NPs. This extract contains many phytochemicals that convert copper sulfate to copper NPs, according to ultraviolet-visible spectroscopy (UV), TEM, and XRD examinations shown in Fig. 8. The produced NPs were predominantly spherical, hexagonal, and irregular, and the average size of the NPs was calculated to be 26.6 nm. CuO NPs were produced using Gundelia tournefortii aqueous extract by Nasrollahzadeh et al. (2015). The presence of polyphenols in the plant extract may

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Fig. 8 CuO NPs: (a) UV-visible spectra, (b) XRD pattern, (c) SEM images, and (d) TEM images. (Reproduced with permission from Springer Nature)

Fig. 9 (i) Original image of the Gundelia tournefortii: (a) SEM image, (b) TEM images of CuO NPs. (Reproduced with permission from Taylor and Francis)

be responsible for the formation of NPs by reduction of metal ions, according to FTIR experiments. SEM and TEM studies verified that the synthesized NPs were spherical as shown in Fig. 9.

ZnO Nanoparticle Synthesis by Plant Leaf Extract Sharmila Devi and Gayathri (2014) used Hibiscus rosa-sinensis leaf extract to synthesize ZnO NPs. Individual NPs, as well as several aggregates, were seen in

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Fig. 10 (a). (i) Original image of Punica granatum peel, SEM images of (a) flower and (b) plateletshaped synthesized ZnO NPs (Ghidan et al. 2017) (b). (i) Original image of Artocarpus gomezianus fruit; (a) SEM, (b) TEM image of ZnO NPs synthesized using water extract of Artocarpus gomezianus fruit (Suresh et al. 2015). (Reproduced with permission from Taylor and Francis)

SEM images. The particles were spongy in shape and ranged in size from 30 to 35 nm. Sangeetha et al. (2011) used Aloe barbadensis miller leaf extract to make green ZnO NPs. The creation of ZnO NPs could be attributed to phenolic chemicals, flavonoids, proteins, and other functional groups found in the leaf extract, according to FTIR analyses. TEM investigation indicated that the produced ZnO NPs were polydispersed and hexagonal, with a size range of 25–40 nm and an average size of 34 nm. The green production of ZnO NPs utilizing Polygala tenuifolia root extract was described by Nagajyothi et al. (2014). FTIR confirmed that phenols, alkanes, alkenes, aliphatic amines, etc. are present in the root extract. The presence of these compounds played an important role in the production and stabilization of ZnO NPs. Furthermore, TEM revealed that the produced ZnO NPs were spherical and had diameters ranging from 33.03 to 73.48 nm. Ghidan et al. (2017) used Punica granatum peel extract for the green synthesis of ZnO NPs as shown in Fig. 10. SEM examination verified that the produced ZnO NPs were nanoplatelets with an average size of 40 nm and a thickness of about 8 nm. Suresh et al. (2015) used Artocarpus gomezianus fruit extract as a solution combustion approach to synthesize ZnO NPs. Sekar Vijayakumar et al. (2016) used the coprecipitation method for the green synthesis of zinc oxide NPs using the aqueous leaf extract of Laurus nobilis (Ln). The flowerlike hexagonal wurtzite structure was calculated to obtain a mean particle size of 47.27 nm showing the antibacterial activity of Ln-ZnO NPs against Gram-positive (Staphylococcus aureus) bacteria than Gram-negative (Pseudomonas aeruginosa) bacteria. Ultimately, the morphological changes in the Ln-ZnO NPs were treated for lung cancer cells and were observed under a phase-contrast microscope. Sadiq Hamad et al. (2021) utilized the leaf extract of Syzygium cumini, which

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was used for the synthesis of NPs for the removal of toxicity of MB dye with various properties like eco-friendly, cheap, nontoxic, and low time-consuming properties. The aqueous leaf extract was put into the ZnO solution resulting in the change of color from yellow to reddish-brown which indicates the reduction of zinc ions in the solution. They showed the synthesis of NPs by the biogenic method and its utilization in the removal of harmful dyes. Again, Viswanathan Vinotha et al. (2019) synthesized ZnO NPs using leaf extract of Costus igneus. The synthesized NPs were prominent against antidiabetic and antioxidant activities. Apart from that, it showed antibacterial and antibiofilm activity. Apart from biological activities, leaf extracts also affect electronic measurements. Khan Mohammad Mansoob et al. (2019) synthesized bandgap ZnO NPs using unboiled and boiled leaf extracts of Costus woodsonii. The study confirmed the reduction of bandgap significantly to (Eg ¼ ~2.68 eV to ~2.77 eV) of the as-synthesized ZnO NPs compared to the ZnO-C (Eg ¼ 3.18 eV).

TiO2 Nanoparticle Synthesis by Plant Leaf Extract Santhosh Kumar et al. (2014) used an aqueous solution of Psidium guajava leaf extract to synthesize TiO2 NPs. The leaf extraction contained numerous functional groups that were responsible for the formation of NPs, including alcohols, alkenes, and aromatics. XRD and SEM analysis indicated that the synthesized NPs were clustered with an average size of 32.58 nm. Valli and Geetha (2015) reported the synthesis of TiO2 NPs from Cassia auriculata leaf extract. The presence of O-H bonds in the leaf extract was revealed by FTIR analysis, indicating that the oxygen atoms in the O-H group bind to the TiO2 in these NPs. Rajakumar et al. (2012) used a leaf extract of Eclipta prostrata to make TiO2 NPs. XRD and SEM confirmed the green synthesized spherical NPs with an average size of 49.5 nm (Fig. 11a). Hudlikar et al. (2012) used an aqueous extract of Jatropha curcas L. latex to synthesize TiO2 NPs. As confirmed by TEM and XRD, the synthesized NPs were generally spherical (25–50 nm) and uneven, with a size range of 25–100 nm (Fig. 11b). Sreekanth et al. (2017) used Coptidis rhizome as a reducing and capping agent to make TiO2 NPs. TEM and XRD confirmed that the synthesized rectangular and hexagonal NPs with a size range of 24–32 nm are shown in Fig. 11c. Ahmad Waseem et al. (2020) investigated the antibacterial and antifungal activity of selected microorganisms by the green synthesis of TiO2 NPs using leaf extract of Mentha arvensis, titanium tetraisopropoxide which acts as the precursor and leaf extract which acts as the reducing agent. Thakur et al. (2019) synthesized TiO2 in a green method by using Azadirachta indica (neem) leaf extract. Synthesized NPs showed antibacterial activity over a broad spectrum. The lowest MIC value was reported from this group for Salmonella typhi and E. coli. Again, the flowerlike TiO2 NPs have been identified as potential electrode material for efficient next-generation electrochemical energy storage devices by Reddy et al. (2019). They synthesized TiO2 NPs using medicinal

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Fig. 11 (a) (i) Original Eclipta prostrata leaf, (ii) and (iii) FE-SEM images of the TiO2 NPs. (b) (i) Original Jatropha curcas L.; (ii and iii) TEM images of TiO2 NPs. (c). (i) Original Coptidis rhizome; (a–d), TEM images of TiO2 NPs and their corresponding magnified view are shown in (f– j) and line profiles in (e–i). (Reproduced with Permission from Rajakumar et al. 2012; Hudlikar et al. 2012; Sreekanth et al. 2017) (Reproduced with permission from Taylor and Francis)

plant leaf extracts, namely, Ocimum tenuiflorum plant and Calotropis gigantea plant. The electrochemical investigations of the sample exhibited a high specific capacitance of 224 F g
1 at 0.5 A g
1 with 71% of capacitive retention after 5000 cycles.

Characterization Techniques of Nanoparticles Prepared from Plant Extract The characterization of synthesized physicochemical NPs is a crucial stage that should be carefully studied. The size, surface area, uniformity, etc. provide useful knowledge of NP production control for commercial applications. Color change testing, UV-visible spectrometry, FTIR, electron microscopy (TEM and SEM), energy-dispersive spectroscopy (EDX), XRD, vibrating sample magnetometer (VAM), thermogravimetric analysis (TGA), and other instruments are employed for this purpose (Mohamed et al. 2019).

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Fig. 12 Possible mechanism of NP formation via green synthesis. (Reproduced with permission from Elsevier)

Mechanism of Nanoparticle Evolution with Plant Extracts Plant extracts mainly contain carbohydrates and proteins. They act as reducing agents, enabling metal ions to be reduced and form MONPs. Plant extracts contain proteins with functional amino groups, which may help to reduce metal ions (Shikuo Li et al. 2007). Kesharwani et al. (2009) hypothesized that the reduction of metal ions was due to the presence of quinones molecules in the leaf extract. Plants’ extracellular synthesis of MONPs was thus attributed to biomolecules and heterocyclic chemicals. The process underlying the synthesis of MONPs from plant extracts, however, is still unknown. For the synthesis of MONPs, plant phytochemicals play a vital role. In a nutshell, phytochemicals reduce metal precursors to metal ions, which is the first step in the formation of NPs by plants. Phytochemicals are then used to help this metal ion grow and stabilize. Metal oxide synthesis, on the other hand, occurs when oxygen from the atmosphere attaches to the metal ion before the development and stabilization stages. Fig. 12 depicts a summary mechanism for the creation of MONPs based on multiple mechanisms documented in the literature (Mohd Sayeed Akhtar et al. 2013). It was also observed that flavonoids play a major role in the synthesis of MONPs. The most basic structure of flavonoids is aglyconic. The presence of a condensed benzene ring classifies flavonols, flavanones, and their dihydro derivatives. Flavonoids also contain a variety of functional groups that can create NPs. The transition of flavonoids from the enol to the keto form has been proposed to release reactive

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hydrogen atoms that can decrease metal ions to produce NPs. When compared to other phytochemicals, most plant leaf extracts employed for the manufacture of MONPs have higher flavonoid content (Raduan et al. 2013). As a result, scientists believe that the flavonoid concentration in plant extracts will play a role in the formation of MONPs. Flavonoid stabilizes NPs, which is helpful to reduce nano-formulation toxicity. The generation of reactive oxygen species (ROS) produced by metal oxides can be decreased since flavonoids have antioxidant capabilities (Priyanka Uddandarao et al. 2019). Flavonoids also have antibacterial (Shashank Kumar et al. 2013), anti-inflammatory (Manthey 2000), and antiviral (Critchfield et al. 1996) effects.

Advantages of Green Synthesis of NPs The benefits of nanotechnology are rapidly expanding in several industries. New technologies, such as UV-protected creams and cosmetics and filtration systems for water-repellent garments, farming, and pharmaceuticals (Noelia GonzálezBallesteros et al. 2019), are emerging rapidly. Some of the recent and important biotechnological applications of NPs are discussed below.

Antimicrobial Activities The emergence of new drug-resistant microorganisms encapsulates the fundamental issues that medicinal practitioners confront. As a result, fresh drug research is required to combat a wide range of disorders. Antibacterial and immunoassay capabilities, nanotechnology, and the methods utilized to synthesize nanocomposites/NPs have revolutionized biomedicine (Le et al. 2017; Razavi et al. 2019). Different researchers have produced many types of MONPs for use in medical applications, including metals such as ZnO, TiO2, CaO, CuO, and SiO2. Plant- and microbe-mediated NP biosynthesis is a promising alternative for a potential antibacterial NP manufacturing process (Amr Fouda et al. 2020; Prakash Bhuyar et al. 2020).

Textile Industry In recent years, the inclusion of NPs into textiles during manufacturing has risen, because NPs improve the performance of finished fabrics. ZnO NPs are also used in textiles to improve UV locking and antibacterial characteristics (Amr Fouda et al. 2018). UV blockers made from inorganic NPs are preferred over UV blockers made from organic NPs in the textile sector (Ascensión Riva et al. 2006). In reality, TiO2 and ZnO are the most commonly used NPs because of their sturdy components and are harmless when exposed to extreme conditions.

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Wastewater Treatment The most important element in our lives is water. Due to overpopulation, a shortage of aquatic sustainable resources and pollution is currently the most pressing issues confronting day-to-day needs. Nanotechnology offers a novel approach to addressing most concerns related to water scarcity and quality. Nanotechnologybased wastewater treatment has recently been shown to produce high-performance treated water with fewer pollutants and hazardous compounds, as well as the removal of heavy metals (Zonaro et al. 2017). CuO, ZnO, and TiO2 are examples of nanoscale metal oxides that have a lot of potential for wastewater and water disinfection. Nanotechnology is regarded as a unique technique for addressing the difficulties of energy conservation and water resources from an economic standpoint.

Food Industry The safety and security of packaged food are examples of NP applications in the food business. Two main areas of food processing using nanotechnology are food packaging and food ingredients (Osama M. Sharaf et al. 2019). The researchers proposed utilizing ZnO NPs in the production of nutritional coverings and containers, as they had antibacterial qualities (Usha Rajamanickam et al. 2012; Prasad et al. 2014). As a result, using nanomaterial-treated packaging containers is an excellent approach for storing food fresh for a long period, while avoiding contamination.

Agriculture Agriculture is the backbone of human and animal feed. Nanotechnology has a positive impact on various agricultural sectors. Controlling plant pathogenic bacteria is one of these benefits, and NPs can be utilized as nano-pesticides, nano-insecticides, and nano-fertilizers.

Conclusion The development and applications of green synthesis employing extraction from plants to synthesize MONPs are described in this chapter. These NPs produced by the green approach have sparked engrossment in a variety of fields. Herb-derived NPs have the prospect to be widely used in the medical disciplines, chemotherapeutic and antibacterial factors, as well as in pharmaceuticals and cosmetic products. As a result, this approach based on herb extract NPs offers a unique scope for advanced research in the design and development of innovative techniques for producing

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MONPs with desired properties for various applications, viz., biosensor, electrochemical, antimicrobial, etc. Following are the primary challenges that were experienced during the green synthesis of NPs: • The green process of synthesis of a given size and form necessitates more intensive investigation. Moreover, the synthesis of NPs with certain physicochemical properties necessitates additional research, particularly for biomedical applications. • The mechanistic component of using green technologies to fabricate NPs deserves more research. • To identify the role of each molecule in NP bio-fabrication, the metabolites present in the biological biomass filtrate should be carefully investigated. • Another challenge in commercializing NPs is scaling up production using environmentally friendly ways. • The stability of high-yielding NPs was connected to parameters like pH, reaction time, and temperature. These variables vary depending on the biological entities used. However, for extended applications to be conceivable, the cost of reduction must be given first attention.

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Green Synthesis of Carbon Dot-Based Materials for Toxic Metal Detection and Environmental Remediation

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Samarjit Pattnayak, Ugrabadi Sahoo, and Garudadhwaj Hota

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Synthesis of CDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Properties of CDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of CD-Based Materials as Fluorescent Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Hg2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Pb2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Cd2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Cr (VI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing of Cu2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Fluorescence Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Static Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resonance Energy Transfer (RET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoinduced Charge Transfer (PCT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of CD-Based Materials as Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Dye Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Heavy Metal Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Photocatalytic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Carbon dots (CDs), a unique family of multifunctional carbon nanomaterials, have attracted enormous consideration due to their diverse physicochemical properties like good biocompatibility, unique optical characteristics, low cost, eco-friendliness, abundant functional groups, etc., thereby assisting in the field of heavy metal detection and photocatalytic environmental remediation. Recently, S. Pattnayak · U. Sahoo · G. Hota (*) Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India e-mail: [email protected]; [email protected]; [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_97

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green method of synthesis has received a lot of attention as a reliable, sustainable, and environmentally friendly protocol for synthesizing a wide variety of nanomaterials. As a result, green synthesis has been regarded as an important tool for reducing the harmful effects associated with traditional methods of synthesis of CDs. Environmental contamination has become a severe concern as a result of population growth and rapid industrial expansion, for which the detection and remediation of toxic metals is highly apprehended. The low-cost, high photostability, and nontoxicity of CDs empower them as suitable fluorescent probes compared to traditional toxic inorganic quantum dots (QDs). Further, CDs are organic semiconductors that can form photoinduced electron–hole pairs when exposed to sunshine. They also function as electron donors and acceptors in solution or suspension. These properties make CD-based compounds suitable to be used as photocatalysts in wastewater treatment like pollutant degradation and heavy metal reduction. Briefly, the novel optical properties of CDs meet all the criteria to be employed as fluorescent probe for heavy metal detection and photocatalytic environmental remediation. Keywords

Carbon dots · Green synthesis · Toxic metals · Fluorescent sensor · Photocatalyst · Photodegradation

Introduction Carbon dots (CDs), in the last few years, have remarkably broadened the research horizon because of their fascinating optical and electronic properties. Generally, CDs are quasi-spherical particles with a diameter of less than 10 nm. The cores of the CDs usually comprise sp2 hybridized carbon atoms mingled with a few sp3 carbons, containing various functional groups on their surface (Lim et al. 2015). The origin of CDs could be dated back to 2004 when Xu et al. serendipitously discovered this new member of the carbon family while refining single-walled carbon nanotube solution by an arc-discharge method (Xu et al. 2004). Since then the enthralling aspects of CDs have revolutionized the new scientific era significantly. The tunable fluorescence emission and exceptional optical properties of CDs can be attributed to their fluorescence emission from conjugated domain bandgap transitions and surface imperfections (Shen et al. 2012). This novel family of fluorescent carbon nanomaterials is gradually replacing the age-old semiconductor quantum dots (QDs) which have been severely criticized due to their intrinsic toxic heavy metals and subsequent health hazards. Additionally, the low toxicity, water solubility, biocompatibility, cell permeability, and chemical inertness of fluorescent CDs have been proven as a great substitute to traditional heavy metal-containing semiconductor QDs like CdSe, CdTe, PbS, etc. Not only this, the high quantum yield, upconversion, and excitation and pH-dependent emission properties of CDs make them promising candidates in the field of sensing and photocatalysis. In recent decades, the number

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of papers based on novel synthesis approaches and widespread applications of CDs has boomed dramatically. Since their discovery in 2004, only 528 papers on CDs were published till 2008, while at present more than 5000 research articles have been reported. Environmental contamination due to toxic metals such as lead, cadmium, mercury, etc., is a worldwide concern and needs to be tackled comprehensively. The presence of these metals in environmental bodies even at low concentrations is highly toxic to human life and to all biotic and abiotic components (Rasheed et al. 2018). Industrial effluents and agricultural wastes are being continuously discharged to soil and freshwater resulting in exposure of toxic metals to living organisms through consumption of food and water. The gradual accumulation of these toxic metals in human bodies causes adverse health effects like renal failure, neurotoxicity, gastrointestinal dysfunction, vascular damage, hyperkeratosis, lung cancer, and so on. Therefore, the US Environmental Protection Agency (USEPA) and World Health Organization (WHO) have declared acceptable restrictions of such metals based on their toxicity in potable water. Further, the enrichment of the pollutants due to the agricultural, nonagricultural, and industrial pollution is assuredly one of the major concerns (Mohaghegh et al. 2015). Many organic dyes (RhB, MB, TCH, etc.) and heavy metals (Cr(VI)) are among the various contaminants that pose a major threat to humanity. To address this issue, semiconductor-based photocatalysts which harness unlimited solar energy have received a lot of interest. A variety of semiconductor-based photocatalysts, such as TiO2, g-C3N4, and MOFs, have been used in environmental remediation. However, due to low visible light absorption capacity, poor charge transfer, and a small number of surface active sites, the efficiency of these photocatalysts is restricted. To solve this, QD-based heterostructures in order to achieve a high-quality photocatalyst is widely anticipated. Most crucially, while synthesizing a heterojunction with a QD, the bandgap energy value of a semiconductor may be changed, making it a prominent visible light photocatalyst. Traditional analytical methods for the detection of toxic metals include atomic absorption/emission spectroscopy, inductively coupled plasma mass spectrometry, Auger-electron spectroscopy, polarography, etc. These techniques measure metal ion concentrations precisely even at low concentrations. However, these methods are limited by expensive instrumentations, complex procedures, tedious sample preparation, and high operating cost which make them incompatible for real-time detection of toxic metals. Meanwhile, fluorescent sensors have opened up a new way to overcome these issues in detection of toxic metal ions due to their excellent sensitivity and simplicity of operation. Thus, detection and remediation of toxic metals from the environmental system is an alarming global requisite. Herein, the green synthetic approach, fluorescence sensing, and photocatalytic efficacy of CDs and related materials in environmental remediation applications are highlighted. Many biomass-derived CDs using diverse synthesis techniques have been developed for the detection of toxic metal ions. Further, the precise band position alignment of these materials is thoroughly addressed so that photocatalytic applications such as dye degradation and heavy metal reduction can be thoroughly

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understood. This chapter is expected to assist readers in gaining a better grasp of the remarkable aspects of CDs, their green synthetic approach, and application in fluorescence sensing and photocatalysis for environmental remediation.

Green Synthesis of CDs Green chemistry is concerned with the concepts of design, production, and application that decrease or abolish the use or synthesis of hazardous chemical compounds. CDs generated from natural and biowaste have become a new topic in green synthesis with a wide range of uses and economic benefits. For a simple, economical, and environmentally friendly synthesis of CDs, carbon sources might be easily accessible from natural bio-resources. The synthesis procedures of CDs are broadly divided into two categories; namely, top-down and bottom-up. However, bottom-up approach is more convenient, environmentally friendly, faster, and easier to produce in large scale with costeffectiveness. Because of these aspects of green chemistry in recent years, CDs made from sustainable natural biomass materials have received a lot of attention. These natural sources make the manufactured CDs harmless and biocompatible, making them a superior choice for biosensing and photocatalytic remediation. CDs produced from biomass have been reported by Liu et al. for chemical and biological investigation (Liu et al. 2019b). Feng and Qian described the production of CDs from several sources to be used as chemosensors and biosensors (Feng and Qian 2018). Recent improvements in CD-based heavy metal detection in water were also highlighted by Devi et al. (Devi et al. 2019). Depending on the carbon precursor, CDs may contain a variety of heteroatoms such as nitrogen and sulfur. Nitrogendoped CDs could be easily made from a nitrogen-rich precursor with amino functional groups. The integration of organic compounds containing nitrogen in natural sources would result in N-doping and improved characteristics of CDs. Milk, for example, is strong in proteins and carbohydrates, both of which act as sources of nitrogen and carbon in the fabrication of N-doped CDs (Su et al. 2018). The 3D folded proteins in milk solution were denaturized, which resulted in separation of molecules into individual amino acids after hydrolysis. Thus, CDs were formed from the interaction between the amine and carboxyl groups in the solution. In comparison to pristine CDs, N-doped CDs had a higher quantum yield. The higher quantum yield in hetero atom-doped CDs may be ascribed to generation of new energy levels and hence radiative combination. Not only nitrogen but also other heteroatoms such as sulfur and phosphorous improve the optical characteristics of CDs remarkably. Recently, Das et al. synthesized N- and S-codoped CDs (Das et al. 2019). Amino acids and ethylenediamine (EDA) were commonly employed as additives, with EDA being particularly popular for making N-containing groups, while l-cysteine (l-Cys) and glutathione (GSH) were utilized for S-containing groups. Picard et al. made CDs out of miscanthus grass, which contains a lot of oxygen-containing functional groups (Picard et al. 2019). Sharma et al. presented N- and S-doped CDs produced from rose petals using EDA and l-Cys as N and S sources, respectively (Sharma et al.

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Fig. 1 (a) Schematic illustration for the synthesis of multifunctional N,S-doped CDs from rose petals (Sharma et al. 2018) (© with permission of Elsevier) and (b) synthesis of magnetofluorescent CDs from crab shell by microwave-assisted hydrothermal approach (Yao et al. 2017) (© with permission of American Chemical Society)

2018) (Fig. 1a). Furthermore, N- and P-doped CDs were made by extracting N and P sources from miscanthus grass using EDA and orthophosphoric acid, respectively. These heteroatom-doped CDs possessed exceptional optical characteristics while being less hazardous. Few transition metals, such as Co, Mn, Cu, Au, and Ag, were also employed to dope CDs in order to improve their properties. The hydrothermal approach was used to dope CDs with Co based on 1-(2-pyridylazo)-2-naphthol and cobalt chloride (Zhang et al. 2017a). Manganese-doped CDs (Mn/CDs) were synthesized using sodium citrate, citric acid, and manganese carbonate by hydrothermal method

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(Xu et al. 2018). Because of metal charge transfer, these Mn/CDs showed a high quantum yield (QY) of 54%. The valence electrons in Mn improve electron or charge transfer, leading to an increase in electron and hole radiative recombination and hence an increase in QY. Additionally, CDs were combined with metal nanoparticles, particularly silver nanoparticles (Ag NPs), to create a nanocomposite to be used as chemosensors. Wang et al. pyrolyzed tris(hydroxymethyl)aminomethane to make N-doped CDs and then hybridized Ag NPs and CDs to make a composite of CDs–Ag NPs (Wang et al. 2018). According to Amjadi et al., CDs were used as a reducing and stabilizing agent in the preparation of Ag NPs to generate the CDs–Ag NP nanocomposite (Amjadi et al. 2017). Chemical reduction of AgNO3 with glucose-derived CDs was used to generate the nanocomposite. Not only transition metals but also transition metal ions were incorporated into CDs to improve their optical properties. Figure 1b shows microwave-assisted synthesis of CDs from waste crab shell in a mixture of various metal ions like Gd3+, Eu3+, and Mn2+ as dopant was reported by Yao et al. (Yao et al. 2017). Another important functionalization technique is the encapsulation of CDs in host materials to provide a novel fluorescent sensor with distinctive features. Metal–organic frameworks (MOFs) are inorganic–organic hybrid materials with a large surface area, homogeneous pore size, and excellent encapsulation capacity. The reduced quantum yield of MOFs, on the other hand, may limit the use of sensors due to their poor selectivity and sensitivity. However, the drawbacks can be mitigated by incorporating luminous guest elements into the MOF to create a hybrid composite. The encapsulation strategy is expected to combine the advantages of both host and guest materials to boost sensing and photocatalytic activity. Recently, CDs have been encapsulated within MOFs, which resulted in the production of a CDs@MOFs hybrid composite with properties of both MOF and CDs. The porous structure of MOFs provides enormous surface area, which increases the area of contact between the sensor and the analyte, while the bright fluorescence of CDs can improve the optical properties. Wang et al. used the hydrothermal process to make europium-based MOFs (Eu-MOFs) with CDs for sensing application (Wang et al. 2019c). The synthesized hybrid material exhibited good water stability and dual emission, which correspond to CDs and Eu-MOFs, respectively. CDs embedded in mesoporous silica, like CDs@MOFs, have recently been used as a novel fluorescence hybrid material in sensing application. Mesoporous silica has a porous architecture with a large surface area and pore volume, making it an attractive host material for adsorbing guest molecules. Furthermore, the pore size of mesoporous silica ranges from 2 to 50 nm, making it compatible with encapsulated QDs and nanoparticles. CDs having a diameter of 2–5 nm can be implanted in mesoporous silica to make a nanocomposite in this way. Wang et al. described the hydrothermal synthesis of CDs@MCM-41 composite (Fig. 2) (Wang et al. 2019b). The CDs dispersed in MCM-41 own both the properties of CDs and MCM-41. However, the mesoporous structure of MCM-41 impacted the optical characteristics of CDs, causing the absorption and fluorescence emission maxima of CDs@MCM-41 to be redshifted compared to pristine CDs.

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Fig. 2 Schematic illustration of hydrothermal synthesis of MCM-41 (Wang et al. 2019b) (© with permission of Elsevier)

Optical Properties of CDs CDs absorb light mostly in the near ultraviolet, as well as visible and near-infrared regions. The tunable emission wavelength and fluorescence intensity of CDs can be attributed to a wide size range, distinct surface defects, or varying degrees of π-conjugation. On tuning the size of CDs, the number of sp2 hybridized sites, as well as the bandgap resulting from the quantum confinement effect, would change. Despite the fact that the real origins of CD fluorescence emissions are unclear, two types of fluorescence emission mechanisms have been proposed. The first form of fluorescence emission process involves conjugated π-domain bandgap transitions, and the second type involves sources that are more sophisticated, such as surface defects in CDs. According to Xie’s group, emission from PEG1500N-passivated CDs spans the visible light spectrum from purple to red and extends into the near infrared, with the excitation wavelength steadily increasing from 400 nm with 20 nm increments (Fig. 3) (Sun et al. 2006). Furthermore, pH-dependent photoluminescence emission is also observed by the distinct surface states of CDs resulting from H-bonding by -NH and -OH functional groups. Deprotonation or protonation of functional groups can cause surface states and fluorescence to change when pH varies. In addition to excitation and pH-dependent fluorescence, solvent polarity may influence the fluorescence emission of CDs as well. When switching the solvent from water to ethylene glycol, the fluorescence emission intensity is increased. This might be related to the changes in surface state and reduction in non-irradiative transitions. In addition to regular fluorescence, recent research has discovered a new and intriguing phenomenon

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Fig. 3 (a) PEG1500Npassivated CDs in aqueous solution with increasing excitation wavelength from 400 nm on the left with 20 nm increments; (b) absorption and emission spectra of PEG1500N-passivated CDs; inset: emission intensities adjusted to quantum yields (Sun et al. 2006) (© with permission of American Chemical Society)

known as upconversion fluorescence emission. When the fluorescence emission wavelength is shorter than the used excitation wavelength, the optical phenomenon of upconversion fluorescence emission occurs.

Application of CD-Based Materials as Fluorescent Sensors Sensing of heavy or toxic metal ions is a pivotal task for monitoring and controlling environmental contamination worldwide. Heavy metals are harmful at trace amounts in water resources, causing serious health problems. CDs have a lot of oxygencontaining surface groups including carboxylic and hydroxyl groups, which makes them more hydrophilic and offers them more surface active sites. As a result of surface bonding, metal ions can efficiently interact with CDs, allowing for the tuning

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of CD characteristics. Furthermore, metal ion sensing capabilities can be improved by element doping and/or surface modification.

Sensing of Hg2+ CDs produced from citric acid and tris(hydroxymethyl)methyl amine displayed both down- and upconversion fluorescence characteristics (Bai et al. 2018). Based on static and dynamic quenching mechanisms, the CDs were employed to sense Hg2+ with concentration ranging from 0 to 40 μM. CDs made from citric acid, edible mushrooms, and cysteine were also used to sense Hg2+, with LODs of 20 nM, 4.13 nM, and 14 nM, respectively (Gao et al. 2016; Venkateswarlu et al. 2018; Liu et al. 2019a) (Fig. 4). Previously, nanosensors toward detection of Hg2+ were produced using CDs generated from orange juice, Coccinia indica, and barley (Li et al. 2017; Radhakrishnan et al. 2019; Xie et al. 2019). The functional groups on the surface of CDs interacted with Hg2+ ions to reduce the fluorescence intensity of CDs.

Fig. 4 (a) PL emission spectra of the CDs upon the addition of Hg2+ (0–100 nM); (b) linear range of fluorescence intensity with respect to Hg2+ concentration; (c) selectivity of CDs toward Hg2+ ions over various other metal ions; (d) photographs of CD solution under daylight and UV light (365 nm) (Venkateswarlu et al. 2018) (© with permission of Elsevier)

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Fluorescence quenching could be produced by changes in surface states of CDs caused by Hg2+ binding. Wang et al. employed citric acid carbonization to make N-doped CDs for Hg2+ sensing (Wang et al. 2019a). The fluorescence of CDs was quenched by Hg2+ via a static quenching mechanism where a nonfluorescent complex was generated by Hg2+ with the functional groups of CDs. Han et al. found that sulfhydryl-functionalized CDs (CDs/SH) had even better sensitivity (4.2 pM) when mediated with Ag+ (Han et al. 2019). The fluorescence on–off–on mechanism was used to detect Hg2+ in this sensor device. The high fluorescence of CDs/SH was suppressed when Ag+ was added to create CDs/SH/Ag agglomerates. The fluorescence of CDs was regained after injecting Hg2+ into the CDs/SH/Ag complex due to the fast conversion from CDs/SH/Ag to CDs/SH/ Hg. The substantial attraction of SH groups for Hg2+ improves the performance of the designed sensor. In a separate study, Silva et al. proposed a reversible optical sensor for Hg2+ detection based on CDs immobilized on silica optical fibers (Gonçalves et al. 2010). Kang et al. introduced a CD-based ultrasensitive sensing device to detect Hg2+ with a LOD of 1 fM (Kong et al. 2014). Zhou et al. recently introduced N-doped CDs as a fluorescent and colorimetric Hg2+ assessment (Wang et al. 2016).

Sensing of Pb2+ Kumar et al. and Bandi et al. synthesized biomass-derived amine functional groups containing CDs from tulsi leaves and berries, respectively (Kumar et al. 2017; Bandi et al. 2018). Both tulsi and berries-derived CD sensors displayed exceptional sensitivity toward Pb2+ with LODs of 0.59 and 9.64 nM, respectively (Fig. 5). Similarly, Radhakrishnan et al. described the use of an amine-functionalized CDs to detect Pb2+ with a LOD of 0.21 μM (Radhakrishnan et al. 2019). The addition of Pb2+ efficiently quenched the fluorescence of CDs since CDs in the excited state are capable of transferring electrons from the amine group to unoccupied d-orbitals of Pb2+. The oxygen and amino functional groups of CDs have a strong binding affinity for Pb2+ ions, causing them to be in close contact for effective electron transfer. Zhu et al. stated N-doped CDs exhibiting blue emission for sensing Pb2+ (Jiang et al. 2015). The quenching process was primarily caused by static interactions with a detection limit of 15 nM. Nandi et al. demonstrated a lead ion paper strip sensor made by immobilizing CDs on cellulose-based filter paper (Gupta et al. 2016). The fluorescence of CDs could be quenched by the addition of Pb2+ because of the development of a complex between Pb2+ and CDs and the electron transfer events. Surprisingly, this sensor can detect Pb2+ and has a detection limit of 110 pM. As a result of electron transfer, Wee et al. observed selective fluorescence quenching of CDs by Pb2+ ions (Wee et al. 2013). The linear equation in this investigation was y ¼ 606 x þ 1.0145, and the LOD was measured to be 5.05 mM using 3σ/s method. In a similar way, Li et al. demonstrated a “turn-off” CD sensor with a LOD of 2 nM for the detection of Pb2+ (Wang et al. 2015).

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Fig. 5 PL emission intensity of CDs (a) in the presence of several metal ions, (b) with increasing concentration of Pb2+; (c) the linear relationship between relative fluorescence intensity (F0/F) of CDs and the concentration of Pb2 +; (d) fluorescence decay curves of CDs with and without Pb2+ (Bandi et al. 2018) (© with permission of Elsevier)

Sensing of Cd2+ Gu et al. used N,S-codoped CDs made from scallion to detect Cd2+ ions via a fluorescence quenching technique with a LOD of 15 nM (Gu et al. 2018) (Fig. 6). CDs generated from camphor were also used by Raju et al. for Cd2+ sensing, as they can efficiently suppress the green fluorescence of CDs (Gaddam et al. 2014). For the ratiometric measurement of Cd2+, Zhang et al. described a CDs/Au cluster nanohybrid with a dumbbell form (Niu et al. 2016). The great selectivity in this study came from the static quenching and inner filter action of Cd2+ on CDs/Au NC, with a detection limit of 32.5 nM. Pandey et al. used hydrothermal method to make CDs from curry leaves (Murraya koenigii) for the turn-off sensing of Cd2+ ions with a LOD of 0.29 nM. The quenching was attributed to ligand-to-metal charge transfer (LMCT), and dynamic quenching was followed from the lifetime decay analysis (Pandey et al. 2020). Sochor et al. exploited the widely occurring potentially hazardous bacteria Staphylococcus aureus to determine markers in order to detect cadmium(II) ions (Sochor et al. 2011).

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Fig. 6 (a) PL emission spectra of CDs upon addition of various concentrations of Cd2+. Inset: Plot of the fluorescent intensity against Cd2+ in the range of 0–50 μM; (b) sensing of the CD-based sensor for Cd2+; the linear relationship of F/F0 against the Cd2+ concentration over the range of (c) 0.1–3.0 μM and (d) 5.0–30.0 μM (Gu et al. 2018) (© with permission of Elsevier)

Sensing of Cr (VI) Cr(VI) exhibits a strong absorption band at 350 nm with a shoulder that extends to 500 nm that coincides with its emission band. This case meets the requirements of inner filter effect (IFE). Based on this, Sun et al. proposed a practical and costeffective system for detecting Cr (VI) (Zheng et al. 2013). Wang et al. employed CDs to quantify Cr(VI) in a range of 0.05 to 200 mM based on the same effect (Zhang et al. 2016). According to Dong et al., the fluorescence of N,S-codoped CDs was effectively suppressed by Cr(VI) (Dong et al. 2013). The detection limit was calculated to be 20 nM, and the linear range was determined to be 0.5–125 mM in this study. Polyethyleneimine-capped CDs were described by Chen et al. to detect trace Cr(VI) in the surroundings (Liu et al. 2015). In contrast to the inner filter action described above, Cr(VI) would induce a signal rise. This can be explained by the phenomenon that positive surface of CD interacts extensively with Cr(VI), and this interaction can improve atomization efficiency, resulting in the creation of aerosol

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particles and an increase in motion resistance. Furthermore, Chowdhury et al. employed CDs to detect Cr(VI) via UV–vis absorption and color change (Gogoi et al. 2015). Cr(VI) caused the CDs containing strip to change yellow, and the detection limit was calculated to be 1 pM. Further, the CDs produced from tulsi leaves and groundnuts were used for the detection of Cr(VI) with LOD of 1.6 and 1.9 μM, respectively (Bhatt et al. 2018; Roshni et al. 2019) (Fig. 7). Higher Cr(VI) sensitivity was attained with N-doped CDs produced with citric acid and glutamic acid, which had a LOD of 5 nM (Zhang et al. 2018). N/S-codoped CDs and N/P-codoped CDs, on the other hand, showed effective Cr(VI) detection with LODs of 110 and 23 nM, respectively (Gong et al. 2017; Yang et al. 2018).

Sensing of Cu2+ The IFE mechanism, like Cr(VI), is based on the exceptionally rapid detection of Cu2+ by lemon extract- and l-arginine-derived CDs. The amino groups on the surface of CDs produce a cupric ammine complex with Cu2+ ions, which results in fluorescence quenching through IFE. Besides, FRET- and aggregation-induced mechanisms were found to be involved in the detection of Cu2+ (Beiraghi and Najibi-Gehraz 2017; Zhang et al. 2019) (Fig. 8). A ratiometric fluorescence sensor based on CDs@Eu-MOFs was also created to detect Cu2+ with great sensitivity (Hao et al. 2017). The IFE was used by SalinasCastillo et al. to create a CD-based sensor for Cu2+ detection (Salinas-Castillo et al. 2013). Downconversion and upconversion fluorescence were seen in the produced CDs, and both of these phenomena can be exploited in the sensitive detection of copper ions. Jiang’s group has published a cyclam-functionalized CD sensor (cyclam:1,4,8,11-tetraazacyclotetradecane) for Cu2+ detection (Chen et al. 2016). The mechanism was attributed to FRET mechanism between the cyclam–Cu2+ complex and CDs. Wu et al. also presented a form of CD as a Cu2+ probe, with a LOD of 5 nM (Gedda et al. 2016).

Mechanism of Fluorescence Sensing Any change in fluorescence intensity, anisotropy, or lifetime by addition of certain concentration of distinct analytes can be employed as a sensor. CDs, which have been widely applied in sensing technologies, display fluorescence changes due to a number of factors.

Static Quenching Static quenching occurs by the interaction of CDs and the quencher, resulting in a nonfluorescent ground-state complex. The complex quickly recovers to its ground state after being exposed to light without producing a photon. For static quenching,

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Fig. 7 (a) PL emission spectra of CDs with different anions, (b) with different concentrations of Cr (VI); (c) interference of different anions with Cr(VI); (d) fluorescence quenching of CDs with Cr (VI) concentration; (e) the plot of F0/F against Cr(VI) concentration; and (f) lifetime decay curve of CDs (blue) and with Cr(VI) (pink) (Bhatt et al. 2018) (© with permission of Elsevier)

τ0/τ ¼ 1 (where τ0 and τ are lifetime of CDs without and with a quencher, respectively). The development of the ground-state complex can affect the absorption spectra of the CDs which is often evident by perturbation in the UV–visible absorption spectra. An increase in temperature weakens the stability of the groundstate complex, thereby reducing the extent of static quenching.

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Fig. 8 (a) Fluorescence spectra of the sensor against different Cu2+ concentrations (inset: corresponding color under UV radiation); (b) fluorescent responses to the various concentrations of Cu2+, with a linear range of 0.08–10 μM (inset); (c) UV–vis absorption spectra of the sensing system to increasing Cu2+ concentrations (inset: corresponding color changes under daylight); (d) relative absorbance within 0–40 μM. The linear range for Cu2+ is 0.08–25 μM (inset) (Zhang et al. 2019) (© with permission of Elsevier)

Dynamic Quenching The excited state of CDs returns to the ground state by colliding with the quencher using the energy transfer or charge transfer mechanism, and this procedure may be expressed by a simple equation: A þ Q ! A þ Q

ð1Þ

where A stands for CDs, Q for quencher, and * for excited state. The Stern–Volmer relationship governs the kinetics of this process: F0 =F ¼ 1 þ kq τo ½Q

ð2Þ

where F0 and F are the fluorescence intensities without and with the quencher, kq is the quencher rate coefficient, τo is the lifetime of the excited state of CDs without the quencher, and [Q] is the quencher concentration.

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When related to static quenching, there are a few differences: • The lifetime of CDs varies depending on whether or not the quencher is present. • Because dynamic quenching exclusively impacts the excited states of CDs, there will be no changes in their absorption spectra. • A rise in temperature can increase the effect of dynamic quenching.

Resonance Energy Transfer (RET) RET occurs when the emission spectra of CDs coincide with the absorption spectrum of the quencher. Fluorescence resonance energy transfer (FRET) is due to the interaction between CDs in the excited state and quencher in the ground state. In FRET, an excited molecule (donor) returns to the ground state, while the transferred energy sends an electron to the excited state on the acceptor. The following conditions are met during FRET mechanism: • The absorbance spectrum of the quencher and the fluorescence spectrum of the CDs must overlap. • The lifetime of CDs reduces. • The distance between CDs and the quencher must range from 10 to 100 Å.

Photoinduced Charge Transfer (PCT) This mechanism promotes fluorescence by transferring an electron between electron donor and acceptor. In a conjugated system, PCT sensors induce partial charge transfer, and the complexation of the donor and acceptor induces a modification in electron energy levels, which results in a change in fluorescence signals. In PCT sensors, the integrated receptor and fluorophore are visible, but, in PET sensors, the electron donor moiety is detached from the fluorophore by a spacer.

IFE IFE occurs when the absorption spectrum of the quencher coincides with the excitation or emission spectra of CDs. IFE is often referred to as apparent quenching; however, it is caused by absorption of emitted radiation when excess concentration of CDs or the quencher is present in solution. Although this impact causes a drop in intensity (but not decay time), it is not referred as quenching. Rather, a second absorber simply filters out the emission of a particle. This can also happen if the distance between the emitter and the re-absorber is more than 10 nm. The absorption peaks of the CDs do not move because IFE does not correspond to either the static or dynamic quenching processes, suggesting that no new complex is produced. As a result, lifetime of CDs remains unchanged.

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Application of CD-Based Materials as Photocatalysts Organic Dye Degradation Liu et al. developed highly effective CDs/ms/tz-BiVO4 heterostructure. They discovered that this compound can remove 98% of RhB in 140 min under visible light (Liu et al. 2021). To produce sorption desorption equilibria, the RhB was first treated with catalyst for 1 h in the absence of light before being irradiated with visible light. Within 140 min of visible light irradiation, the degrading impact of pristine ms-BiVO4 on RhB is not noticeable. The photocatalytic activity of the ms/tz-BiVO4 composite was improved over the ms-BiVO4 (88% degradation in 140 min). The photoactivity of the ms/tz-BiVO4 was further improved after loading CNQDs and NCQDs, and the degradation efficacy of the CNQDs-ms/tz-BiVO4 and NCQDs-ms/ tz-BiVO4 composite were about 90% and 98%, respectively. The probable mechanism and RhB photodegradation plot was depicted in Figs. 9 and 10. Plasmonic photocatalysis, in addition to heterostructure production using metal oxides, has lately developed as a viable tool for generating effective photocatalysts. The surface plasmon resonance (SPR) of noble metal nanoparticles has gained interest due to its ability to speed up the separation of photoinduced charge under visible light. Several studies demonstrate that silver nanoparticles (AgNPs) placed on semiconductors exhibit effective plasmon resonance. It was discovered that linking CDs with different noble metal nanoparticles like Ag, Au, and Pt increases the photocatalytic efficiency (Liu et al. 2014). Sabet et al. published an article in 2019 describing the synthesis of nitrogen-doped CDs from grass using a hydrothermal approach that was used to degrade six dyes in aqueous medium (Sabet and Mahdavi 2019). The same composite was used in adsorption of heavy metals like Cd2+ and Pb2+ as well. The results demonstrate that after 30 min of visible light irradiation, each of the organic dyes had almost completely deteriorated due to the massive amount of reactive oxide species radicals (ROR) produced in the aqueous solution. In 2016, Tyagi et al. published a green method for synthesizing CDs from

Fig. 9 Probable photocatalytic mechanism for RhB degradation for CNQDs-ms/tz-BiVO4 and NCQDs-ms/tz-BiVO4 composites (Liu et al. 2021) (© with permission of Elsevier)

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Fig. 10 (a) The RhB degradation plot for different samples; (b, c) cycling experiment for the photodegradation of ms/tz-BiVO4, CNQDs-ms/tz-BiVO4, NCQDs-ms/tz-BiVO4, and ms-BiVO4 (Liu et al. 2021) (© with permission of Elsevier)

lemon peel waste decorated on TiO2 nanofiber surface for photocatalytic degradation of methylene blue (MB) and heavy metal Cr(VI) detection (Tyagi et al. 2016). The round-shaped CDs were well dispersed on the superficial part of TiO2 nanofiber, and it was discovered that the composite catalyst had 2.5 times degradation efficiency than pure TiO2 nanofiber. Bozetine et al. in 2021 described a simple and environmentally friendly method for synthesizing ZnO/CDs/AgNPs ternary heterostructure for photocatalytic MB dye degradation (Bozetine et al. 2021). The results showed that within 50 min of irradiation, around 95% of the MB was degraded. To test the degrading efficiency, different percentages of AgNPs were used while keeping the amount of CDs constant. The achieved outcomes revealed that the photodegradation efficacy surges with increasing AgNP content up to 4% and then declines as the amount of AgNPs increases. This shows that AgNPs in the ZnO/CDs nanocomposite act as charge separation centers below 4% and electron–hole recombination centers beyond 4%. Zhang et al. stated similar findings (Zhang et al. 2017b) where CDs have been recognized as a suitable guest constituent for building CD-based functional heterostructures, as they have amazing features such as strong chemical stability, enhanced charge carrier mobility, low photogenerated electron and hole recombination rate, and electrochemical stability. Due to its favorable properties, CDs, among all carbon-based nanomaterials, open up new opportunities for photocatalytic

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carriers or promoters. As a result, the synthesis of CDs/semiconductors has gained utmost importance as CDs can help in stabilizing semiconductors while also acting as a synergist. It can enhance charge separation, reduce charge recombination, and enhance electron capture-storage-mobility properties of the composites. Hence, mixing CDs with a variety of metal oxides can greatly boost the effectiveness of the composite material. Au–CDs and TiO2–CDs nanocomposites, for example, have been discovered to have excellent photocatalytic dye degradation in visible light. Bozetine and colleagues manufactured ZnO/CDs/AgNPs nanocomposite using a facile one-step hydrothermal technique preventing the use of any additional reducing agent (Bozetine et al. 2021). When the photoactivity of the synthesized nanocomposite was examined, it was found that ZnO/CDs/AgNPs heterostructure had improved activity toward the decontamination of MB dye under the irradiation of simulated sunlight. Recently, a small number of literatures have been addressed in which CDs have been hybridized with various semiconductors, such as TiO2, SiO2, Fe2O3, Cu2O, Ag3PO4, and CdS. The photocatalytic efficiency of the synthesized semiconductor/CDs was found to be superior than that of pure semiconducting materials. Bozetine et al., for example, used a one-pot approach to make ZnO/CDs nanocomposites. They claimed that because the manufacturing of ZnO/CDs occurs in a water medium, the technique follows “green chemistry” principles. When the effectiveness of the synthesized heterostructure was measured in Rhodamine B (RhB) degradation, the results revealed that the nanocomposite was visible light active and its efficacy was significantly improved than that of ZnO synthesized under alike experimental conditions (Bozetine et al. 2016).

Photocatalytic Heavy Metal Reduction Cr(VI) is a familiar heavy metal contaminant in wastewater, is harmful to aquatic species, and causes cancer, perforation of the nasal septum, and kidney mutilation in humans. Therefore, reduction of Cr(VI) is regarded as an effective approach since less hazardous Cr(III) rapidly precipitates as Cr(OH)3 in aqueous solution. The photoinduced electrons can convert Cr(VI) to Cr(III) if the conduction band (CB) of the photocatalyst is more negative than the Cr(VI)/Cr(III) potential (0.51 V vs. NHE). The photo harvesting capabilities of CDs boost the efficiency with which CD-based composites harness solar light, thereby improving their catalytic activities. In 2019, Xu et al. presented the facile production of CDs/NTiO2-x nanocomposite for photoreduction of hexavalent chromium. They further stated that after 60 min of visible light irradiation, the above composite displays a 94% of photoreduction of Cr (VI) (Xu et al. 2019). Similarly, in 2021, Li et al. reported a simple hydrothermal method for synthesizing polyvinyl pyrilodone (PVP)-assisted CD-anchored hollow microspheres of Bi2WO6 nanocomposite for photocatalytic Cr(VI) reduction. PVP has a dual role in this study, influencing hollow microsphere morphologies while also acting as a precursor for CDs. The photoreduction efficiency in the above composite was found to be 83% (Li et al.

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Table 1 CD-based photocatalysts for Cr(VI) reduction Serial No. 1 2 3 4 5 6 7

Materials CDs/ReS2 CDs/CdS g-C3N4 QD/WO3 CDs/MIL-53 CDs-modified N-TiO2-x CDs/Bi2WO6 CDs/NaBiO3

Synthetic approach Solvothermal Solvothermal Hydrothermal Dispersion Hydrothermal

Cr(VI) photoreduction efficiency 95.6% 94% 100% 99% 97%

References Zhou et al. (2022) Zhang et al. (2020) Zhou et al. (2021) Lin et al. (2018) Xu et al. (2019)

Hydrothermal Hydrothermal

83% 97.7%

Li et al. (2021) Wu et al. (2021)

2021). Bhati et al. have described the photoreduction of Cr(VI) to Cr(III) in contaminated water using nitrogen- and phosphorus-codoped CDs. The N,P-CDs were proved to be a robust photocatalyst, evidenced by its reusability up to six consecutive cycles, with 98% efficacy for 400 ppm Cr(VI) (Bhati et al. 2019). Table 1 summarizes a number of reports on CD-based photocatalysts for Cr (VI) reduction by various research groups.

Mechanism of Photocatalytic Degradation With the existence of CDs, photocatalytic degradation of organic contaminants has a significant influence on environmental decontamination. Broadly, the photocatalytic dye degradation mechanism is classified as direct and indirect mechanism (Hong et al. 2016). Indirect dye decontamination includes a sequence of redox processes in semiconductor CB and VB, in which reactive species such as holes, electrons, .OH, . O2, and H2O2 are formed. But the direct method is caused by the capacity of dye to absorb a little amount of visible light. When a dye is excited from its ground state to triplet excited state, the reactions begin. Hydroxyl radical (.OH) is the major reactive species in the oxidation of organic molecules. In 2019, Jiang et al. synthesized CD-SnNb2O6/BiOCl Z-scheme system for the decontamination of benzocaine (Phang and Tan 2019). The photocatalytic mechanism was predicted in three different pathways. The CB and valence band (VB) positions of BiOCl and SnNb2O6 are 0.79/2.45 eV and 1.03/1.52 eV, respectively. The photoinduced electrons in the CB of SnNb2O6 move to the CB of BiOCl, as shown in Fig. 11a. In addition, photoinduced holes in BiOCl shift from its VB to the VB of SnNb2O6. In contrast to the outcomes of active species scavenger tests and ESR spectra, SnNb2O6 cannot create superoxide radical (.O2) since its CB (1.52 eV) is lower than the redox potential of OH/OH (2.3 eV vs. NHE). In Fig. 11b and c, a Z-scheme photocatalytic reaction mechanism is proposed for the SNO/BOC and CD-SNO/BOC composites. The superoxide radical (.O2) created by photoinduced electrons in the VB of BiOCl and the .OH produced by photoinduced holes in the CB of SnNb2O6 can degrade benzocaine. Further, the recombination of

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Fig. 11 Probable photocatalytic mechanisms for (a) conformist SNO/BOC heterojunction, (b) Z-scheme SNO/BOC system, and (c) Z-Scheme CD-SNO/BOC system (Phang and Tan 2019) (© with permission of Elsevier)

electrons in the CB of BiOCl and holes in the VB of SnNb2O6 produces additional . O2 and .OH. The migration of the charges is further increased after the efficient active sites of the CDs are introduced (Fig. 11c) due to the recombination of more electrons and holes. Finally, the upconversion effects of CDs can boost the light absorption capabilities of the catalyst. In general, CDs play a similar function in direct and indirect photocatalytic dye decontamination and often act as a co-catalyst with other 2D nanomaterials. CDs play a major role in the dye degradation by reducing the recombination rates of electron and hole pairs. Di et al. used a simple hydrothermal approach to make 0D CDs/2D Bi2WO6 nanocomposites (Jiang et al. 2019). The catalytic efficacy of hybrid composite was assessed for the decontamination of a variety of organic contaminants, including RhB, ciprofloxacin (CIP), tetracycline hydrochloride (TC-HCl), and the hormone upsetting chemical bisphenol A (BPA). The photoactivity of the 2 wt% of CDs/Bi2WO6 hybrid material for the breakdown of RhB in the presence of visible light for 120 min is about 1.8 times greater than pristine Bi2WO6 nanosheets.

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Conclusion The efficiency of CDs has been demonstrated in sensing and catalysis applications for monitoring and further removing contaminants from wastewater. This emerging material possesses a number of unique characteristics, such as low cost, ease of extraction from natural/biomass sources, low toxicity, and ease of functionalization which endows it the potential to be used in numerous applications. Notably, natural and biomass waste materials could be transformed into CDs in a sustainable and efficient manner when compared to other nanomaterials. Furthermore, in terms of synthetic approach, the large-scale production of CDs for commercial uses should necessitate greater work. It should be noted that CD-based hybrid/composite materials have demonstrated outstanding catalytic performance in the degradation of environmental contaminants; nevertheless, the characteristics of CDs may be altered by the formation of hybrid/composite materials. When CDs are combined with a semiconductor or other nanomaterial, their toxicity level is likely to increase, potentially limiting their application. To further construct nontoxic CD-based hybrid/composite materials, detailed research on the toxicity effect of CDs is necessary. We believe that the long-term application research on CD-based materials will always be crucial in solving environmental-related problems.

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Green Synthesis of Metal Oxide Nanomaterials and Photocatalytic Degradation of Toxic Dyes

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Baishali Bhattacharjee and Md. Ahmaruzzaman

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of Green Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of Green Synthesis over Traditional Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Methods of Preparation of Metal Oxide Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant-Based Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbe-Mediated Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionic Liquid-Mediated Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Metal Oxide Nanoparticles in Photocatalytic Degradation of Toxic Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the recent years, nanomaterials are widely used in various fields, such as environmental remediation processes, energy production, industries, and medicines, but the synthesis of nanomaterials using the conventional chemical methods has various adverse effects such as high energy consumption, environmental pollution, and health problems. To address these challenges, green synthetic methods can be applied in which plant extracts are used rather than the toxic chemical substances. Compared to the traditional chemical methods, green synthesis is more beneficial as it is environment-friendly, cost-effective, and biologically safe. In this review, we have discussed about the adverse effect of chemical synthetic methods on the environment. To protect the environment from the harsh chemicals, green synthetic methods can be applied for the synthesis of metal

B. Bhattacharjee · M. Ahmaruzzaman (*) Department of Chemistry, National Institute of Technology, Silchar, Assam, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_98

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oxide nanoparticles. Here, we have mainly emphasized on the green synthesis of some metal oxide nanoparticles such as SnO2, ZnO, and CuO. In this procedure, extracts of various plant parts (such as fruits, leaf, root), microbes, and ionic liquids can be used as precursors. These “green” metal oxide nanoparticles exhibit high biocompatibility and low toxicity, which enables them for their wide applications in various fields such as environmental remediation processes and removal of various organic pollutants. This chapter also discussed the applications of green synthesized metal oxide nanoparticles in the photocatalytic degradation of various toxic dyes from water bodies. In the end, we have also discussed about the future perspective of green synthetic methods and photocatalytic applications of metal oxide nanoparticles. Keywords

Green synthesis · SnO2 · ZnO · CuO · Photocatalysis · Dyes

Introduction Metal and metal oxide nanoparticles are very important because of their unique characteristics and wide range of applications. Size, structure, and surface morphology are some of the physical factors of nanostructured materials that govern the physical, chemical, electrical, and optical capabilities of nanomaterials. Innovative synthesis strategies for nanomaterials (e.g., carbon nanotubes (CNTs), metal nanoparticles, graphene, quantum dots, and their composites) have become a hot topic in nanoscience and nanotechnology during the last decade. Two different fundamental concepts of synthesis, i.e., bottom-up and top-down techniques, are used to create nanomaterials of required dimensions, shapes, and properties (Oskam 2006). In bottom-up approaches, simple molecules are used as precursor for the preparation of nanoparticles using various methods such as sol-gel method, chemical vapor deposition, microemulsion, and atomic or molecular condensation. In top-down approach, nanoparticles are synthesized in the former using a variety of synthesis processes such as ball milling, lithographic methods, sputtering, and etching (Cao 2004). Structural properties of nanoparticles (shape and size) may be modified by altering chemical concentrations and reaction conditions, e.g., pH and temperature. However, when these synthesized nanomaterials are put to use in real-world applications, they may face the following limitations or obstacles: (i) stability in harsh environment, (ii) poor understanding in basic mechanisms and modeling aspects, (iii) biomagnification or toxicology features, (iv) high-cost analytical necessities, (v) device assembly and constructional problems, and (vi) recycling, reuse, and regeneration. In the real world, the characteristics, functionality, and variety of nanomaterials should be enhanced to fulfil the aforementioned requirements. These constraints are presenting new and exciting opportunities in this rapidly developing field of study. To overcome these obstacles, a new age of “green synthesis” techniques is growing rapidly in contemporary material science and technology research.

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“Green synthesis” methods prevent the generation of undesirable or dangerous by-products through the development of safe, efficient, and environment-friendly synthesis technique. To attain this goal, use of natural resources (such as plants, bacteria, fungi, algae) and ideal solvent (ionic liquids) system is very essential. Among the various green synthesis methods of metal or metal oxide nanoparticles, use of plant extracts is a relatively easy and simple procedure for the production of nanoparticles at large scale comparative to other microbe (bacteria, fungi, algae)mediated synthesis. In green synthesis procedures using biological precursors, different reaction parameters such as pH, pressure, temperature, and solvent play a vital role. Plants are widely used for the production of metal or metal oxide nanoparticles because various plant extracts contain effective phytochemicals such as amides, aldehydes, ketones, carboxylic acids, terpenes, phenolic compounds, amides, and flavones (Devi and Ahmaruzzaman 2016). These phytochemicals can reduce metal salts into the metal nanoparticles. The fundamental properties of such nanomaterials were studied for applications in biomedical investigations, antimicrobials, catalysis, molecular sensors, optical coherence tomography, environmental remediation, and biological system labeling. Here, in this chapter we have discussed the principles of green chemistry and advantages of green methods over traditional methods. Various green syntheses such as plant-mediated, microbe-mediated, and ionic liquid-mediated methods for the preparation of metal oxide nanoparticles are discussed. Further, applications of green synthesized metal oxide nanoparticles (SnO2, ZnO, and CuO) in the removal of various toxic organic dyes are also overviewed in this chapter.

Principles of Green Chemistry The ideology of green synthesis depends on a set of rules published by P.T. Anastas and J.C. Warner as “Principles of Green Chemistry” in 1998 (Table 1). These ideas were created from a conceptual framework for replacing dangerous chemicals with potentially safer substitutes.

Advantages of Green Synthesis over Traditional Methods The traditional methods comprise of physical methods such as chemical vapor deposition (CVD), thermal deposition, electro-spinning, etc. (Abdullah et al. 2018; Rehman et al. 2019) and the chemical methods such as sol-gel, template-based precipitation, solvothermal, hydrothermal, etc. (Chakravarty et al. 2016; Abdelkader et al. 2016). The sol-gel process is widely used by scientists, but it requires very high temperature for the synthesis of metal oxide nanoparticles. In chemical synthesis of nanoparticles, toxic reducing agents and various harmful organic and inorganic solvents are required. To reduce the metal ions, various reducing agents such as sodium borohydride, tollens reagent, DMF, sodium citrate, etc. are used. In chemical method stabilizers are also required along with the reducing agents. Chemical method uses harmful chemical reagents, toxic solvents, as well as very high

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Table 1 Twelve principles of “Green Chemistry” Sl. no. 1 2 3 4 5 6

7 8 9

10

11 12

Statement It is preferable to reduce waste than to remove or clean up waste that has already occurred Synthetic procedures should be developed to incorporate all components utilized in the process as much as possible into the final outcome Synthetic techniques should be designed to employ and manufacture compounds that are low or nontoxic to human health and environment whenever possible Active ingredients should be designed to maintain function efficacy while lowering toxicity Auxiliary compounds should be avoided if possible and utilized only when absolutely essential Energy demand should be considered in terms of its environmental and economic consequences and decreased. Synthetic procedures should be carried out at room temperature and pressure The raw materials should be taken from the renewable sources rather than the nonrenewable sources (e.g., petrochemical sources) Use of protecting groups should be as seldom as possible In reactions, catalytic reagents should be used instead of stoichiometric reagents. Catalysts should be selected to improve the selectivity, reduce waste, and shorten reaction durations and energy requirements Chemicals should be designed in such a way that they can be degraded and conveniently discarded. It has to be kept in mind that both the chemicals and their breakdown products are nontoxic and nonpersistent in the environment To avoid the production and discharge of any potentially dangerous or polluting compounds, chemical reactions should be monitored in real time as they occur Chemical substances utilized in a chemical reaction should be selected to reduce the risk of chemical accidents such as leaks, fires, and explosions

temperature and thus creates a serious threat to the environment. Although compared to chemical methods, physical methods are quite easy and cost-efficient for the preparation of metal oxide nanoparticles and their nanocomposites. This approach is highly appreciated by researchers since it does not involve capping agents, high temperatures, or high pressure and produces pure products (Mishra and Ahmaruzzaman 2022). However, there were several drawbacks to this process, such as the requirement for complex apparatus, a trained worker, and very high energy. As a result, researchers develop the idea of green synthesis process to avoid such situations. “Green synthesis” method has gained a lot of research interest among the scientists. The use of the green segments of various plants and algae as reducing agents in the synthesis of nanoparticles has been described in a number of ways. Green methods are extremely beneficial over the conventional chemical methods because this method reduces the use of harmful chemicals which causes environmental pollution and various health issues. This is an environmentally beneficial method that uses waste products as precursor in the synthesis method. All the raw materials are renewable in green synthesis methods (Fig. 1).

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Fig. 1 Advantages of green synthesis methods

Green Methods of Preparation of Metal Oxide Nanoparticles In green synthesis metal oxide nanoparticles can be synthesized from plants, microbes, biomolecules, and ionic liquids. Plant-mediated synthesis is easy and simple compared to microbe- or biomolecule-mediated synthesis. Plant parts contain various metabolites such as alkaloids, terpenoids, phenols, proteins, steroids, and tannins which can reduce the metal ions into metal nanoparticles. Here we have discussed the plant-mediated, microbe-mediated, and ionic liquid-mediated green synthetic procedures for the preparation of metal and metal oxide nanoparticles.

Plant-Based Synthesis Plant-mediated synthesis is used by scientists all around the world since it is nontoxic and contains natural reducing agents. This procedure produces pure metal oxide nanoparticles, and it was simple and environmentally friendly. This approach does not necessitate high temperatures, pressures, or expensive equipment;

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Fig. 2 Synthesis of metal oxide nanoparticles using plant extract (Kumar et al. 2021). (Reprinted from Kumar et al., Copyright (2021), with permission from Elsevier)

as a result, it is relatively inexpensive and generates stable nanoparticles on a huge scale (Mohanta and Ahmaruzzaman 2020). The synthesis technique is comparatively easy, the salt of the required nanoparticle is mixed with the plant extract, and then the solution is centrifuged. The pellets are then subjected to heat treatment to produce the required product. The product is also subjected to a variety of analytical procedures in order to determine its purity, structure, and functionality. The mechanism for the synthesis of metal oxide nanoparticles is shown in Fig. 2. Gomathi et al. (2021) synthesized SnO2 nanoparticle using kiwi (Actinidia deliciosa) peel extract. FTIR spectroscopy was used to investigate how phytochemicals available in kiwi peel extract operate as a reducing and capping agent. A strong band is observed at 2800 to 3000 cm 1 which indicates the existence of H-bond from the adsorbed H2O molecules. The amino acids, amides, and polyphenols found in kiwi peel extract have vibrational intensities ranging from 1200 to 1500 cm 1 as shown in Fig. 3. The produced SnO2 nanoparticles also showed the band of kiwi peel extract but with a lowered intensity, which confirms that the phytochemicals present in the Actinidia deliciosa (kiwi) peel extract behave as reducing and capping agent for the preparation of SnO2 nanoparticles. The Actinidia deliciosa (kiwi) peel extract is a rich source of various phytochemicals such as flavonoids, carotenoids, ascorbic acid, polyphenols, and isoflavones. To suggest a plausible process for the production of SnO2, isoflavones were chosen as a typical phytochemical. Oxygen atom and “OH” group present in isoflavones form the p-track conjugation. The “OH” group

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Fig. 3 FTIR spectra of (a) kiwi peel extract, (b) biologically synthesized SnO2 NPs (Gomathi et al. 2021). (Reprinted from Gomathi et al., Copyright (2021), with permission from Elsevier)

binds with metal ions through chelating effect and form metal phenolate complex which on hydrolysis gives the desired metal oxide nanoparticles. Luque et al. (2021) synthesized SnO2 NPs through green synthesis, by Citrus  paradisi extract as a capping agent. The synthesized SnO2 NPs were characterized by various tools such as XRD (X-ray diffraction), ATR-IR (attenuated total reflectance infrared spectroscopy), TEM-SAED (transmission electron microscopy), SEM (scanning electron microscopy), UV-vis spectroscopy, etc., and the result obtained exhibited that the Citrus  paradisi extract can be an effective medium for the production of SnO2 nanoparticles. The synthesized SnO2 nanoparticles possess quasi-spherical structure having particle size 4–8 nm, with rutile phase crystalline structure and bandwidth of 2.69 at 3.28 eV (Luque et al. 2021). Buniyamin et al. (2022) synthesized SnO2 NPs by using the leaves extract of Chromolaena odorata. They have synthesized the SnO2 NPs at room temperature, and then it was calcinated at five different temperatures such as 500  C, 600  C, 700  C, 800  C, and 900  C. They have determined SnO2 nanoparticles energy properties by using the KubelkaMunk (KM). The primary findings of this study can be classified into two categories. Firstly, at slow calcination process, the synthesized SnO2 NPs showed largest energy bandgap value of 3.2 eV at 900  C with a reflectance rate of 53%. Secondly, in the in situ thermal calcination process, the SnO2 nanoparticles showed the largest energy bandgap of 3.3 eV at 800  C and a reflectance rate of 49% (Buniyamin et al. 2022). Ma et al. (2020) used the aqueous leaf extract of Limonia acidissima and synthesized tin oxide nanoparticles. To characterize the synthesized SnO2 NPs, they have used FTIR, XRD, and TEM. They have found that the morphology of the synthesized

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SnO2 NPs has quasi-spherical shape. Synthesized SnO2 NPs could be used in conjunction with current treatments to create a novel pharmacological strategy for the treatment of cervical cancer (Ma et al. 2020). Further, scientists have also synthesized zinc oxide nanoparticles using various plant parts. Gur et al. (2022) proposed a green method of synthesis of zinc oxide nanoparticles (ZnO NPs) by using Thymbra spicata L. plant. The size of the synthesized ZnO NPs was found to be between 6.5 nm and 7.5 nm. Imade et al. (2022) proposed a simple and environment-friendly method of preparation of ZnO nanoparticles from plantain peel extract. The hexagonal wurtzite structure was confirmed by X-ray diffraction analysis, and the shape of the synthesized ZnO NPs is spherical and is 20 nm in size which was confirmed by transmission electron microscopy (TEM) (Imade et al. 2022). Also, for the synthesis of CuO nanoparticles, scientists have used Calotropis procera (Shah et al. 2022). The synthesized CuO nanoparticles possess spherical shape and size is in between 20 and 80 nm. Scientists have used various plant parts such as prickly pear, Eucalyptus globulus, Calotropis procera, Mussaenda frondosa L., pomegranate leaf, Punica granatum, spinach leaves, etc. for the synthesis of CuO nanoparticles (Badri et al. 2021; Alhalili 2022, Shah et al. 2022; Manasa et al. 2021; Vidovix et al. 2019; Siddiqui et al. 2021; Altuwirqi et al. 2020). Table 2 shows plant-mediated synthesis of some metal oxide nanoparticles.

Microbe-Mediated Synthesis Most microorganisms are nontoxic and eco-friendly, so they can be utilized for the production of metal oxide nanoparticles. The microbes such as fungus, bacteria, and yeast use intracellular and extracellular processes for the production of metal oxide nanoparticles. Hazardous chemicals and high energy are not required for this procedure. In the intracellular mechanism, the metal ion gets absorbed inside the microbial cell, and the enzymes and coenzymes present inside the microbial cell help in the formation of nanoparticles. But in case of extracellular mechanism, the metal ion attaches to the surface of the microbe, and then the enzymes and proteins present on the microbe’s surface reduce the metal ions and produce stable nanoparticles (Li et al. 2011). According to the study, the extracellular mechanism is particularly effective, because it creates nanoparticles in bulk and avoids some phases in the recovery process (Yusof et al. 2019). Various bacterial species such as Lactobacillus sporogenes, Rhizopus oryzae, Magnetospirillum gryphiswaldense, Salmonella typhimurium, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Zoogloea ramigera, and Bacillus licheniformis (Das et al. 2012; Moisescu et al. 2008; Srivastava and Mukhopadhyay 2013; Mishra et al. 2017; Tripathi et al. 2014) are used for the synthesis of various metallic nanoparticles. However, only one bacterialassisted production of SnO2 nanoparticles using Erwinia herbicola has been reported so far (Srivastava and Mukhopadhyay 2014). Clean Erwinia herbicola cells were added in the aqueous solution of SnCl2.2H2O salt. Then they have incubated the mixture for 48 h at 30  C, 120 rpm orbital shaking incubator. After

ZnO ZnO ZnO ZnO ZnO

ZnO

Metal oxide nanoparticles SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2

Plantain peel Cucumis melo Eryngium foetidum L. Cayratia pedata Camellia sinensis

Plant used Actinidia deliciosa Citrus  paradisi Chromolaena odorata Limonia acidissima Ceropegia jainii Ziziphus jujuba fruit Lycopersicon esculentum Psidium guajava Persea americana Parkia speciosa Hassk pods Thymbra spicata L. ZnC4H6O4 Zn (NO3)2 Zinc acetate dihydrate Zn (NO3)2.6H2O (Zn(O2CCH3)2(H2O)2)

Zinc acetate dihydrate

Metal source used SnCl4
5H2O SnCl2
2H2O SnCl4
5H2O SnCl2
2H2O Tin chloride SnCl2
2H2O SnCl2
2H2O SnCl4 SnCl2 SnCl4.5H2O

Table 2 Plant-mediated synthesis of some metal oxide nanoparticles

Hexagonal wurtzite Spherical Hexagonal Spherical Horizontal Wurtzite

Shape Spherical Quasi-spherical Spherical Quasi-spherical Spherical Tetragonal Quasi-spherical Tetragonal Fine flakes Spherical

20 nm 12.8 nm 8 nm 52.24 nm 25 nm

6.5–7.5 nm

Size 5–10 nm 4.7–9.1 nm 28–37 nm 5.56 nm 100–150 nm 18 nm 4.01–5.56 nm 8.44 nm 4 nm 1.9 nm

(continued)

(Imade et al. 2022) (Archana et al. 2022) (Begum et al. 2018) (Jayachandran et al. 2021) (Rao et al. 2021)

References (Gomathi et al. 2021) (Luque et al. 2021) (Buniyamin et al. 2022) (Ma et al. 2020) (Nayak et al. 2021) (Honarmand et al. 2019) (Galvez et al. 2019) (Kumar et al. 2018) (Elango et al. 2015) (Begum and Ahmaruzzaman 2018a) (Gur et al. 2022)

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Metal oxide nanoparticles ZnO CuO CuO CuO CuO CuO CuO CuO

Table 2 (continued)

Plant used Ziziphus jujuba Prickly pear Eucalyptus globulus Calotropis procera Mussaenda frondosa L. Pomegranate leaf Punica granatum Spinach leaves

Metal source used Zinc acetate CuSO4
5H2O Copper sulfate CuSO4
5H2O Cu (NO3)2
3H2O CuSO4
5H2O Cu (CH3COO)2
H2O Pure copper powder

Shape Hexagonal Spherical Spherical Spherical Spherical Spherical shape Spherical Spherical

Size 21.5–26.40 nm – 88 nm 20–80 nm 7.625 nm 20.33 nm 12.5 nm 1–12 nm

References (Alharthi and Salam 2021) (Badri et al. 2021) (Alhalili 2022) (Shah et al. 2022) (Manasa et al. 2021) (Vidovix et al. 2019) (Siddiqui et al. 2021) (Altuwirqi et al. 2020)

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completion of reaction, they have centrifuged the sample for 10 min to isolate the produced nanoparticles. The synthesized SnO2 nanoparticles possess spherical shape and have the size range of 3–18 nm. Singh et al. (2014) used Pseudomonas aeruginosa rhamnolipids bacteria for the synthesis of zinc oxide nanoparticles. The shape of the nanoparticles was found to be spherical and the particle size is from 35 to 80 nm. Copper nanoparticle was synthesized using Salmonella typhimurium by extracellular method (Ghorbani et al. 2015). They have found the size range of copper nanoparticle was in between 40 and 60 nm.

Ionic Liquid-Mediated Synthesis Ionic liquids (ILs) have received a lot of attention in the recent years. Ionic liquids (ILs) possess low surface tension, so they have a rapid nucleation rate, which results in smaller particles, while electronic and steric stabilizer delays the growth of particle. In aqueous solutions, metal NPs tend to bind or agglomerate and are very unstable (Wang et al. 2011). As a result, a stabilizer must be added to metallic NPs to avoid agglomeration. In this aspect, ionic liquids have been regarded as outstanding stabilizers. Ionic liquids are molten (organic/inorganic) salts having melting point lower than 100  C. The low melting point of ionic liquid is due to the fact that they have low lattice energy and the latter is because of weak bonds between anions and cations. Although ionic liquids have some disadvantages such as it requires purification by distillation process, they have very low vapor pressure, which makes them very good solvents, and these are nontoxic and environment-friendly (Fabre and Murshed 2021). So, ionic liquids can be used as green solvents in the preparation of metal oxide nanoparticle eliminating the toxic organic solvents. Ionic liquids can also stabilize the metal without the use of additional stabilizing agents, surfactants, or capping agents. Ionic liquids follow two types of mechanisms for the stabilization of nanoparticles: one is electrostatic and the other is steric stabilization. In the electrostatic stabilization, ionic liquid creates an ionic double layer outside the nanoparticle and produces electrostatic repulsive force among the nanoparticles. Thus, it prevents the agglomeration phenomenon. In case of steric stabilization, when two metal nanoparticles are coming close to each other by polymers or any other surfactants, these layers get compressed, which creates strong repulsion or steric stabilization with the help of entropic and osmotic portions (Avirdi et al. 2022). Rademacher et al. (2022) synthesized tin nanoparticles (Sn-NPs) embedded in Ketjen Black carbon (KB) using various imidazolium-based ionic liquids. They have used various ionic liquids such as [BMIm][NTf2], [BMIm][PF6], [BMIm] [BF4], and [HO-EMIm] [BF4]. The size of the synthesized nanoparticles is found to be in the range of 49  25 to 96  49 nm depending on the type of the ionic liquid used (Rademacher et al. 2022). Goharshadi et al. (2011) used an ionic liquid “trihexyltetradecylphosphonium bis{(trifluoromethyl)sulfonyl}-imide, [P66614] [NTf2]” for the synthesis of zinc oxide nanoparticles. This “trihexyltetradecylphosphonium bis{(trifluoromethyl)sulfonyl}-imide” ionic liquid has low surface tension which increases the nucleation rate and, thus, creates smaller

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ZnO nanoparticles. The synthesized ZnO nanoparticles have hexagonal wurtzite structure. “1-Hexyl-3-methylimidazolium hexafluorophosphate” was also used for the synthesis of ZnO nanoparticles (Amade et al. 2015). From the TEM analysis, the average size of the synthesized nanoparticles was found to be 30.41  4.01 nm. Ionic liquids have been also used for the synthesis of copper nanoparticles. Wang et al. (2021) used surfactant free poly-ionic liquid for the synthesis of copper nanoparticles. They have successfully introduced copper nanoparticles on poly-ionic liquids without oxidation, and the synthesized nanoparticle showed enhanced adsorption capacity for dispersed red dye in water. Shukla et al. (2022) synthesized copper (I) oxide (Cu2O) nanoparticles using “1-butyl-3-methylimidazolium tetrafluoroborate” ionic liquid (IL). They have studied the photocatalytic degradation efficiency of the synthesized nanoparticle and found that the ionic liquid functionalized Cu2O nanoparticle showed enhanced photocatalytic degradation efficiency toward the methyl orange dye under solar light.

Applications of Metal Oxide Nanoparticles in Photocatalytic Degradation of Toxic Dyes Metal oxide nanoparticles can be used as a potential agent for the degradation of various organic contaminants such as dyes, pesticides, pharmaceuticals, etc. Green synthesis of metal oxide nanoparticles is quite popular among scientists for the photocatalytic degradation of harmful dyes. Diallo et al. (2016) used Aspalathus linearis leaf extract to make SnO2 nanoparticles and applied it for the photodegradation of methylene blue from aqueous system. On exposure to UV light for 12 min, the green synthesized SnO2 photocatalyst exhibited 100% efficiency. The photocatalyst also exhibited photocatalytic degradation efficiency of 100% for Congo red and Eosin Y dyes on irradiation of UV light for 20 and 18 min, respectively. When the SnO2 nanoparticles were produced from Calotropis betacea plant extracts, however, photodegradation of methylene blue needed more than 1-hour irradiation of UV light (Elango and Roopan 2016). Apart from UV light exposure, Kumar et al. (2018) synthesized the SnO2 nanoparticles from the leaf extract of Psidium guajava and used it for the photodegradation of RY-186 dye with the photodegradation efficiency of 90% under the irradiation of visible light for 3 h. Fu et al. (2015) synthesized SnO2 nanoparticles from Plectranthus amboinicus leaf extract and used it for the degradation of Rhodamine B dye. On exposure to visible light (having wavelength 420 nm) for about 2 h, the green synthesized nanoparticle exhibited 95% photocatalytic degradation efficiency. Furthermore, scientists have used extract of Vitex agnus-castus fruit for the synthesis of SnO2 nanoparticles and used it to degrade the Rhodamine B dye (Ebrahimian et al. 2020). The size of the SnO2 nanoparticles was found to be 4–13 nm. It exhibited 91.7% photodegradation efficiency of Rhodamine B dye in around 3 h at 298 K. Also, Ziziphus fruit extract is used to synthesize SnO2 nanoparticles for the photodegradation of methylene blue and Eriochrome Black T. The synthesized photocatalyst exhibited photocatalytic

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degradation efficiency of 83% and 90% for Eriochrome Black T and methylene blue, respectively (Honarmand et al. 2019). The synthesized SnO2 nanoparticles were found to be very stable and can be used up to 4 times under the irradiation of visible light. Haritha et al. (2016) studied the degradation of diazo dye (Congo red dye) using the green synthesized SnO2 nanoparticles. They have synthesized SnO2 nanoparticles using extract of C. spinosa. The shape of the nanoparticles is spherical and the size is 47  2 nm. This synthesized photocatalyst exhibited photocatalytic degradation efficiency of 92% in about 20 min with a degradation rate of 0.0952  10 3 min 1 following pseudo first-order kinetics. Najjar et al. (2021) used the mercury vapor lamp (500 W) for the degradation of Eriochrome Black T dye using the green synthesized SnO2 nanoparticles. They have synthesized SnO2 nanoparticles using green sol-gel method involving the usage of chitosan. SEM analysis exhibited that the shape is spherical and the size of the nanoparticle was found to be 25.6 nm. The synthesized SnO2 nanoparticles showed photocatalytic degradation efficiency of 77% on exposure to mercury vapor lamp for about 270 min. Begum and Ahmaruzzaman (2018b) used Phaseolus lunatus L. leaves SnO2 nanoparticles loaded on activated carbon. They have used this SnO2-NP-AC for the degradation of toxic dye Alizarin Red S (ARS) from water bodies. This photocatalyst showed an excellent photocatalytic degradation efficiency of 99% within 45 min of exposure to direct sunlight. Figure 4 depicts the mechanism of photodegradation of dyes using the SnO2 nanoparticles (Bhattacharjee and Ahmaruzzaman 2015a). Furthermore, ZnO nanoparticles synthesized by green methods also exhibited very good photocatalytic efficiency in the degradation of toxic dyes. Rao et al. (2021) synthesized ZnO nanoparticles using Camellia sinensis extract and applied it in the photocatalytic degradation of MO dye. The synthesized photocatalyst exhibited 80% efficiency in the photodegradation of MO dye in about 180 min. Rambabu et al. (2021) used Phoenix dactylifera (date pulp) waste for the synthesis of ZnO nanoparticles. The synthesized ZnO photocatalyst showed photodegradation efficiency of 90% for hazardous eosin yellow and methylene blue dyes. Leaf extract of Artocarpus heterophyllus (jackfruit) was also used for the preparation of the ZnO nanoparticle (Vidya et al. 2016). The size of the nanoparticle was found to be 15–25 nm and it shows hexagonal wurtzite structure. The synthesized ZnO nanoparticles showed photodegradation efficiency of more than 80% of harmful Rose Bengal dye in about 1 h. Abelmoschus esculentus (okra) mucilage was also used for the preparation of ZnO nanoparticles (Prasad et al. 2019). They have studied the degradation of RhB, MB, CR, and MO using the green synthesized ZnO nanoparticles under the irradiation of UV light. Their result found out that 95% of the MB dye and 100% of RhB dye can be removed by the ZnO nanoparticles in about 60 min and 50 min, respectively. The rate constants for the degradation of MB and RhB dyes were found to be 0.0536 min 1 and 0.0643 min 1. Park et al. (2021) used Gynostemma plant extract for the synthesis of ZnO nanoparticles applying coprecipitation method. The synthesized nanoparticles possess hexagonal shape with an average size of 35.41 nm. The synthesized photocatalyst on exposure to UV light for about 180 min showed the degradation efficiency of 89% for the toxic

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Fig. 4 Photocatalytic degradation of dyes by green synthesized SnO2 NPs (Bhattacharjee and Ahmaruzzaman 2015a). (Reprinted from Bhattacharjee and Ahmaruzzaman Copyright (2015), with permission from Elsevier)

MG (malachite green) dye. The hexagonal structure of the nanoparticle produces more active site for the interaction with the toxic dye molecule and makes the reaction fast. Their study found that this green synthesized ZnO nanoparticle can be reused up to five times with proper activity.

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Copper oxide nanoparticles (CuO) synthesized by green methods are also studied for the degradation of various harmful organic dyes. Selvam et al. (2022) synthesized CuO nanoparticles using Canthium coromandelicum leaves. They have characterized the synthesized CuO nanoparticles using various methods such as UV-Vis, FTIR, SEM, TEM, XRD, etc. From the XRD study, the size of the synthesized nanoparticles was found to be 33 nm. The synthesized nanoparticle exhibited photodegradation efficiency of 89.35% and 91.32% for methyl orange (MO) and methylene blue (MB) dyes, respectively. Carica papaya leaves extract was also used for the preparation of CuO nanoparticles (Sankar et al. 2012). Under sunlight, the synthesized copper oxide NPs efficiently degrade the Coomassie brilliant blue R-250 dye. Kaur et al. (2022) synthesized CuO nanoparticles using Punica granatum (pomegranate). It showed flowerlike structure and size is ranging from 8 to 16 nm. They have studied the effect of pH on the photocatalytic activity against the anionic and cationic dyes. The results exhibited that the synthesized CuO nanoparticles showed the maximum degradation efficiency of 91% and 97.8% for anionic dyes such as RO-4 and RY-86 dyes, respectively, at lower pH value of 3, whereas, for cationic dye, i.e., MB, the synthesized CuO nanoparticles exhibited maximum degradation efficiency of 93% at higher pH value of 11. Rafique et al. (2020) used Citrofortunella macrocarpa leaves extract for the synthesis of CuO nanoparticles and studied the photocatalytic degradation of Rhodamine B dye under UV light. The synthesized nanoparticles possess spherical shape and the size was found to be 54–68 nm. The CuO nanoparticle showed an excellent photocatalytic degradation efficiency of 98% for the Rhodamine B dye on exposure to UV light. The synthesized photocatalyst can be reused up to five times and exhibited very high stability of 93.88% after five cycles. Ruellia tuberosa was also used for the synthesis of CuO nanoparticles (Vasantharaj et al. 2019). TEM analysis showed that the CuO nanoparticles exhibited rodlike structure having the size ranging from 20 to 100 nm. They have studied the photocatalytic activity of the CuO nanoparticles using the crystal violet dye under direct sunlight. The synthesized CuO nanoparticle degraded the crystal violet dye completely in just 2 h of direct sunlight exposure. Table 3 indicates the degradation of various toxic dyes using green synthesized metal oxide nanoparticles.

Future Perspective In the near future, the use of nanoparticles for numerous applications is expected to grow at a faster rate. Despite the development of next-generation nanoparticles or composites, metal and metal oxide nanoparticles should remain a popular choice. Apart from the traditional or chemical methods of nanoparticle synthesis, green synthesis method is expected to grow in popularity, providing nontoxic synthesis and utilization of environmentally friendly nanoparticles. Contributions from variety of fields are likely to result in novel green nanoparticle synthesis methods. Despite the fact that there are a number of obstacles to overcome, the field has enormous potential. Understanding the concept through thorough study and data validation can pave the way for the long-term development of green nanotechnology.

UV light Visible light Visible light Visible light Visible light Visible light Visible light

Mercury vapor lamp UV light Visible light

Calotropis betacea Psidium guajava Plectranthus amboinicus Vitex agnus-castus

Ziziphus fruit

C. spinosa Aspartic acid and glutamic acid

Chitosan

Persia americana Phaseolus lunatus L. leaves

Arginine

Piper longum

Camellia sinensis Phoenix dactylifera (date pulp)

SnO2 SnO2 SnO2 SnO2

SnO2

SnO2 SnO2

SnO2

SnO2 SnO2

SnO2

ZnO

ZnO ZnO

Visible light UV light

UV light

Visible light

Light source UV light

Green precursor Aspalathus linearis

Metal oxide nanoparticle SnO2

MG MB MO Methyl orange Eosin yellow Methylene blue

Methylene blue

Phenolsulfonphthalein Alizarin Red S

Eriochrome Black T

Methylene blue Eriochrome Black T Congo red Rose Bengal Eosin Y

Dye degraded Methylene blue Congo red Eosin Y Methylene blue RY-186 Rhodamine B Rhodamine B

Table 3 Degradation of toxic dyes using green synthesized metal oxide nanoparticles

96.4%, 240 min 96% 69% 48% 80%, 180 min 90%, 180 min

100%, 120 min 99%, 45 min

Efficiency and time 100%, 12 min 100%, 20 min 100%, 18 min 100%, 60 min 90%, 180 min 95%, 120 min 91.7%, 180 min 83% 90% 92%, 20 min 99.1%, 180 min 99.3%, 90 min 77%, 270 min

(Rao et al. 2021) (Rambabu et al. 2021)

(Elango et al. 2015) (Begum and Ahmaruzzaman 2018b) (Bhattacharjee and Ahmaruzzaman 2015b) (Asha et al. 2022)

(Najjar et al. 2021)

(Haritha et al. 2016) (Bhattacharjee and Ahmaruzzaman 2015a)

(Honarmand et al. 2019)

(Elango and Roopan 2016) (Kumar et al. 2018) (Fu et al. 2015) (Ebrahimian et al. 2020)

References (Diallo et al. 2016)

370 B. Bhattacharjee and M. Ahmaruzzaman

Carica papaya leaves

Punica granatum (pomegranate)

Citrofortunella macrocarpa Ruellia tuberosa Glutamic acid

Punica granatum Cordia sebestena

ZnO CuO

CuO

CuO

CuO CuO CuO

CuO CuO

ZnO

Artocarpus heterophyllus (jackfruit) Abelmoschus esculentus (okra) Gynostemma Canthium coromandelicum

ZnO

– Visible light

UV light Visible light Visible light

Visible light

Visible light

UV light –

UV light

– Methylene blue Rhodamine B Malachite green Methyl orange Methylene blue Coomassie brilliant blue R-250 Reactive Orange-4 Reactive Yellow-86 Methylene blue Rhodamine B Crystal violet Rose Bengal Methyl violet 6B Methylene blue Bromothymol blue

Rose Bengal

96.91% 100%, 180 min

91%, 90 min 97.8%, 90 min 93%, 180 min 98% 100%, 120 min –

95%, 60 min 100%, 50 min 89%, 180 min 89.35% 91.32% 100%, 120 min

80%, 60 min

(Rafique et al. 2020) (Vasantharaj et al. 2019) (Bhattacharjee and Ahmaruzzaman 2018) (Vidovix et al. 2019) (Prakash et al. 2018)

(Kaur et al. 2022)

(Sankar et al. 2012)

(Park et al. 2021) (Selvam et al. 2022)

(Prasad et al. 2019)

(Vidya et al. 2016)

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Conclusions In this chapter, we have discussed various green methods such as plant-mediated, microbe-mediated, and ionic liquid-mediated methods for the preparation of some metal oxide nanoparticles such as tin oxide nanoparticles, zinc oxide nanoparticles, and copper oxide nanoparticles. Also, their applications in the photocatalytic degradation of various harmful dyes such as methyl orange, methylene blue, eosin yellow, Eriochrome Black T, Rhodamine B, crystal violet, RY-186, etc. are discussed. Green synthesis methods are highly advantageous over the traditional (chemical or physical) methods of nanoparticle synthesis, as it is cost-effective, uses waste materials as precursors, and eliminates harmful chemical substances. Metal and metal oxide NPs have made huge impact in variety of sectors, and their potential applications are versatile. Controlling the size and shape of the nanoparticles and achieving monodispersity in the solution are two important problems in the green synthesis of nanoparticles. In addition to that, little knowledge is available about the mechanical features of particle synthesis, which is essential for the rational and cost-effective development of green synthesized nanoparticle. Another significant and noteworthy difficulty is the scalability of production-level processing. Therefore, such significant technical obstacles and issues must be resolved before this green synthesis approach can become a promising option of nanoparticle synthesis at the industrial level.

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Green Synthesis of Hybrid Nanostructure for Wastewater Remediation by Photocatalytic Degradation

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Shubhalaxmi Choudhury, Pragnyashree Aparajita, and Garudadhwaj Hota

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant-Mediated Green Synthesis of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2-Based Materials for Photodegradation of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CuO-Based Materials for Photodegradation of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO-Based Materials for Photodegradation of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZrO2-Based Materials for Photodegradation of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

378 380 380 389 393 396 400 402

Abstract

With the advent of civilization and industrialization, the contamination of natural resources like hydrosphere, atmosphere, and lithosphere has been inevitable over the last few decades. Hence, the development of “green” methods in order to minimize the hazardous aftereffects of the chemically synthesized products on the environment has been the primary goal of researchers. This environmentally benign approach in the synthesis of nanocomposite materials has been widely speculated to reduce or degrade the harmful pollutants persisting in the environment, thereby consuming lesser energy and minimizing the formation of side products. Utilization of biomass wastes, natural plant extracts, microbiological materials, etc., has also been reported for effective synthesis of hybrid nanocomposite materials. It has been found that the basic phytochemicals extracted from various parts of the plants could serve as reducing, capping, and stabilizing agents and have gained a wide range of interest so far. This chapter focuses to S. Choudhury · P. Aparajita Department of Chemistry, NIT Rourkela, Odisha, India e-mail: [email protected]; [email protected] G. Hota (*) Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_99

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provide comprehensive detail about the recent development of nanomaterials by a plant-mediated greener approach. Furthermore, the physicochemical behavior and the applicability of the hybrid nanomaterial toward wastewater remediation will be interpreted. The effective degradation of the organic pollutants utilizing the hybrid nanocomposite shall serve as the primary theme of the corresponding chapter. The challenges, limitations, and future prospects of green synthesis of hybrid nanocomposites will also be highlighted. Keywords

Green synthesis · Plant extracts · Nanomaterials · Degradation · Water pollution · Metal oxide

Introduction Environmental pollution is a global menace whose magnitude is growing day by day as a result of urbanization, intensive industrialization, and people’s changing lifestyles. As a result, providing people with clean air, water, and a healthy environment is a difficult endeavor (Tesh and Scott 2014). Among different types of pollutants, organic pollutants such as cationic and anionic dyes are widely employed in a variety of applications. In the printing, paper, plastic, textile, culinary, leather, and pharmaceutical industries, organic dyes are in high demand. The dyes are used during the coloring process for various materials and discarded and dumped into the hydrosphere during the fabric process, and because of their recalcitrance, they are a significant cause of pollution. These manufacturing operations pollutants are the most significant contributors to environmental contamination. They cause unwanted turbidity in the water, which reduces sunlight penetration, reducing photochemical synthesis. As a result, one of the most difficult tasks in environmental chemistry is the treatment of effluents containing dyes. Given this, the utilization of semiconductor nanoparticles for oxidizing harmful contaminants has gained a lot of attention in recent material research. Photodegradation is one of the most extensively utilized environmental remediation techniques. The nanomaterials are synthesized through a diverse range of synthesis approaches like ball milling, sol-gel processes, laser pyrolysis, etching, hydrothermal, chemical vapor deposition, spray pyrolysis, sputtering, etc. (Jadoun et al. 2021). But these physical and chemical synthesis approaches can suffer from various limitations like stability in a hostile environment, bioaccumulation or toxicity, expansive analysis needs, and regeneration. Therefore, the properties, types, and behavior of nanomaterials should all be improved to satisfy the aforementioned goals (Singh et al. 2018). To address these limitations, current research and development are focusing on “green synthesis” technologies. Green nanomaterial is produced through regulation, control, cleanup, and remediation, and some of its basic concepts are waste minimization, pollution reduction, the use of nontoxic solvents, and the renewable feedstock, which will help to increase their environmental friendliness (Saratale et al. 2018). Long-term, environmentally friendly

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synthesis procedures must use green synthesis to avoid the production of dangerous by-products. Appropriate solvent systems and natural resources are needed to accomplish this goal. Green synthesis of nanoparticles has been used in various biological materials like plant part extracts, bacteria, algae, and fungi (Khairul Hanif Mohd Nazri and Sapawe 2020). When compared to bacterial- and fungal-mediated synthesis, plant component extracts are a comparatively easy and straightforward process for synthesizing nanoparticles on a large scale. Because of the availability of beneficial phytochemicals in diverse plant component extracts, notably in leaves, such as ketones, amides, phenols, aldehydes, carboxylic acids, terpenoids, flavones, and ascorbic acids, plant biodiversity has been widely studied for the synthesis of nanoparticles. These phytochemicals present in plant part extracts have an astonishing ability to reduce metal salts into metal nanoparticles in a considerably shorter time. Also, the functional groups found in the phytochemicals such as –C¼O–, –C–O–C–, –NH2, – C¼C–, and –C–O– can help in the formation of nanoparticles (Singh et al. 2018). For plant-mediated nanoparticle synthesis, plant component extracts are combined with precursor solutions of metal under various reaction conditions. To assist nanoparticle formation, plant leaf extract acts as both a reducing and stabilizing agent in the nanoparticle creation process (Aarthye and Sureshkumar 2021). The capping ligand’s primary function is to stabilize the nanoparticles and prevent them from further growth and agglomeration. The composition of the plant leaf extract plays a vital role in nanoparticle formation; for example, various plants have variable phytochemical concentration levels. As a result, plant component extracts are thought to be a good and safe source for metal and metal oxide nanoparticle production. The semiconductor nanostructures outperform bulk materials in terms of photocatalytic activity. For the photocatalytic activity of synthetic dyes, metal oxide semiconductor nanoparticles such as titanium oxide (TiO2), zinc oxide (ZnO), tin oxide (SnO2), copper oxide (CuO), zirconium oxide (ZrO2), and tungsten oxide (WO3) have been used primarily (Singh et al. 2018). The advantages of these nanophotocatalysts come from their high surface area-to-mass ratio which indicates that a huge number of surface reactive sites on nanoparticle surfaces raise the surface energy of the nanoparticles, enhancing organic pollutant adsorption. This results in a faster rate of low concentrated contaminant elimination. As a result, in comparison to bulk material, fewer nanocatalysts will be needed to clean up polluted water (Nabi et al. 2018). Here, in this book chapter, we presented a comprehensive understanding of the green synthesis of metal oxide nanoparticles such as TiO2, ZnO, ZrO2, and CuO as well as its advantages over chemical synthesis approaches (Fig. 1). Because of its simplicity, costeffectiveness, efficiency, and feasibility, we focused on the synthesis of nanomaterials using biosynthesis techniques employing only plant part extracts. For nanoparticle synthesis, the biosynthesis approach is an excellent alternative to the traditional preparation methods. The synthesized nanomaterials are being applied toward photocatalytic degradation of diverse organic contaminants from the aquatic resource. Overall, the goal of this book chapter is to describe a plant-mediated green synthetic approach and its related nanocomponents toward photodegradation of organic contaminants from water in order to benefit readers to gain a better understanding of the factor and obstacles involved in the green synthesis and photocatalytic application.

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Fig. 1 Schematic illustration of green synthesis of nanomaterials for photocatalytic degradation

Plant-Mediated Green Synthesis of Nanoparticles TiO2-Based Materials for Photodegradation of Pollutants Due to excellent photocatalytic capabilities, titanium dioxide (TiO2) has been widely explored as an excellent photocatalyst among a vast variety of metal oxide-based semiconductors. The photocatalytic decontamination of hazardous pollutants by using TiO2-based nano-photocatalyst is an extremely efficient approach for wastewater treatment methods. In general, the fabrication of TiO2-based nanocomposite materials necessitates the use of highly expensive, poisonous, and dangerous chemical compounds, which cause substantial ecological damage when discharged into the environment. As a result, the green synthetic root is used to prevent the drawbacks of the traditional chemical synthetic approach. Green synthesis is the most simple, cost-effective, less hazardous, and environmentally friendly method for

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Fig. 2 The synthesis method of nanoparticles from plant extracts

the fabrication of nanoparticles out of biodegradable components such as plant extracts (leaves, stems, roots, blossoms, and fruits), enzymes, and microorganisms. However, microbe-mediated synthesis methods, in particular, include a number of difficult and time-consuming steps. Plant-medicated nanoparticle production offers various advantages over other biological approaches, including being a fast method for extracellular nanoparticle creation on a large scale. Therefore, employing plant part extracts to synthesize nanoparticles has advantages over others in terms of largescale production, minimizing reaction time, and reducing the risk of contamination. The presence of several phytochemicals in the leaf extract suggested that they may be responsible for the reduction of metallic ions and act as capping agents during the green synthesis of nanoparticles, thus stabilizing the nanoparticles. In this book chapter, we exclusively concentrated on the synthesis of plant-mediated nanoparticles and their photodegradation application for the removal of organic pollutants from wastewater. The use of plant extract makes the synthesis approach reliable, green, and rapid. Many researchers have synthesized TiO2 nanoparticles utilizing many plant parts as reducing and stabilizing agents. They used plant leaves, blossoms, stems, and roots to prepare the plant part extracts. Figure 2 depicts the synthesis approach for the creation of nanoparticles from plant component extracts. In most cases, the extraction procedure was carried out at a temperature of no more than 100  C. Depending on the phytochemicals being sought, water or organic solvents had been utilized for the extraction process. For example, Nithya et al. synthesized TiO2 nanoparticles by using Aloe vera gel extracts using the green synthesis approach. They discovered that the green produced TiO2 nanoparticles have good photocatalytic activity for the Rhodamine B dye decolorization compared to TiO2, which was about 41% for TiO2 nanoparticles

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Fig. 3 Possible reaction mechanism for the formation of TiO2 NPs in the presence of hydroxyl group(–OH) of leaf extract of Jatropha curcas L. as capping agent [10]. (© with permission of Elsevier)

and 24% for TiO2 under visible light irradiation (Nithya et al. 2013). Similarly, Sankar et al. synthesized TiO2 nanoparticles from the Azadirachta indica aqueous leaf extracts under temperature- and pH-dependent conditions using the green synthetic approach. They authorized that different bioactive components were present in the leaf extract which might have played a role as a capping and reducing agent. Further, they revealed that under the solar light, green-generated colloidal TiO2 nanoparticles efficiently degrade toxic methyl red dye (Sankar et al. 2015). Goutam et al. described a simple green synthetic approach for synthesizing green TiO2 nanoparticles by using the biodiesel plant Jatropha curcas L. leaf extract. Furthermore, they tested the photocatalytic performance of green synthesized TiO2 nanoparticles for removing chemical oxygen demand and chromium from the secondary treated tannery effluent. They found that during the photocatalytic treatment of tannery wastewater, the green synthesized TiO2 nanoparticles demonstrated outstanding activity which was about 82.26% chemical oxygen demand removal and 76.48% chromium removal under sunlight exposure (Goutam et al. 2018). Figure 3 depicts a possible reaction pathway for the production of TiO2 nanoparticles in the presence of hydroxyl groups from Jatropha curcas L. leaf extract as a capping agent. Santhilkumar et al. used Diospyros ebenum leaf extract as a reducing agent to demonstrate an eco-friendly, simple, and green manufacturing technique for

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crystalline anatase TiO2 nanoparticles. They investigated the performance of green synthesized TiO2 nanoparticles to photodegrade crystal violet dye when exposed to UV light. Also, they studied the effect of temperature on structural characteristics, photocatalytic efficiency, and antibacterial activity of TiO2 nanoparticles. When they compared the TiO2 nanoparticles synthesized at 600  C to other synthesized TiO2 nanoparticles, they discovered that the nanoparticles produced at 600  C had superior photocatalytic efficiency, antibacterial activity, and dye adsorption capacity (Senthilkumar et al. 2018). Manikandan et al. used a biosynthetic green chemistry strategy to synthesize TiO2 nanoparticles from Prunus x yedoensis leaf extract and then evaluated their applicability for phosphate removal as well as their antibacterial effectiveness against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria strains. When they employed TiO2 nanoparticles (10 mg/L) for removal of phosphate under sunlight, the greatest rate constant of y ¼ 0.068 min1 (R2 ¼ 0.994) was obtained. They got the best rate constant of y ¼ 0.068 min1 (R2 ¼ 0.994) when they employed TiO2 nanoparticles (10 mg/L) for phosphate removal under sunshine. The TiO2 nanoparticle higher rate constant value suggests that they had good photocatalytic activity when exposed to sunshine (Manikandan et al. 2018). Again, Ganesan et al. used the microwave irradiation method to generate green synthesized TiO2 nanoparticles utilizing Ageratina altissima (white snakeroot) medicinal plant aqueous leaf extracts. The green produced TiO2 nanoparticles were discovered to be the best photocatalyst due to their extended thermodynamic stability and great oxidizing activity. They discovered that under solar light, the produced TiO2 nanoparticles had increased photocatalytic activity in decoloring textile dyes. They discovered the discoloration efficiency of methylene blue, alizarin red, crystal violet, and methyl orange was 86.79%, 76.32%, 77.59%, and 69.06% (Ganesan et al. 2016). Arabi et al. used Alcea and thyme plant extracts (Fig. 4) to establish a simple and environmentally friendly method for the manufacture of stabilized TiO2 nanoparticles for photocatalytic applications. They investigated its photocatalytic activity in the photodegradation of methylene blue when exposed to UV light. They discovered that as the UV irradiation period is increased, the absorption rate declines, and the color changes after 90 min of irradiation (Arabi et al. 2020). Sethy et al. synthesized green TiO2 nanoparticles using Syzygium cumini leaf extract aqueous solution as a capping agent. They also tested the photocatalytic removal of lead from industrial effluent utilizing green synthesized TiO2 nanoparticles in a self-designed photoreactor. They determined the concentration of lead by using the inductive coupled plasma spectroscopy. Chemical oxygen demand was removed by 75.5%, and lead (Pb2+) was removed by 82.53%, according to the data (Sethy et al. 2020). Nabi et al. presented the synthesis of TiO2 nanoparticles by a green synthesis approach from the extract of lemon peel. They chose the lemon peel extract for synthesizing because of its amazing properties and having multiple necessary characteristic alkaloids as well as a variety of phytochemicals, including citrus acid, minerals, vitamin C, ascorbic acid, and flavonoids. The hesperidin flavonoid, found in the hydrolyzed extract of lemon peel, acts as a capping and

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Fig. 4 Alcea (a) and thyme (b) plants and prepared aqueous extracts [14]. (© with permission of Elsevier)

reducing agent by releasing a glycone. They analyzed the photocatalytic response of the prepared TiO2 nanoparticles for Rhodamine B degradation. They discovered that the produced nanoparticles have a photocatalytic activity of more than 70%, which is significantly higher than commercial TiO2 particles (Nabi et al. 2022). Similarly, Sonker et al. demonstrated the photocatalytic decolorization of Rhodamine B using a green TiO2 nanoparticle synthesized from Aloe vera leaves. The result revealed that after 50 min of UV light irradiation, 58% of Rhodamine B dye was photodegraded (Sonker et al. 2020). Figure 5 depicts the photocatalytic reaction mechanism. Pushpamalini et al. demonstrated the green synthesis of TiO2 nanoparticles utilizing four distinct leaf extracts of Piper betel, Ocimum tenuiflorum, Moringa oleifera, and Coriandrum sativum as reducing agents. They studied how the photocatalytic performance of produced TiO2 nanoparticles differed in the degradation of malachite green dye. When comparing the four TiO2 nanomaterials, they discovered that TiO2 manufactured using Moringa oleifera leaf extract had higher efficiency than the other three leaf extracts (Pushpamalini et al. 2020). For the first time, Kaur et al. introduced green and simple synthesis approach for TiO2 nanoparticles using Carica papaya leaf extract, which has potential application as a photocatalyst. TiO2 nanoparticles showed remarkable photocatalytic efficiency of 91.19% for photodegradation of reactive orange-4 dye at the optimum dosage of 25 mg after 180 min of UV exposure (Kaur et al. 2019). Aravind et al. successfully synthesized TiO2 nanoparticles utilizing both chemical and green synthesis routes. Here, they employed jasmine flower extract as a reducing and stabilizing agent in the green synthesis of TiO2 nanoparticles because it includes various phytochemicals. They discovered that TiO2 nanoparticle properties were identical in both chemical and green synthesis approaches. They performed the photodegradation of methylene blue dye to investigate its efficiency under UV–vis irradiation. The findings revealed that the maximum degradation efficiency of 92% was achieved after 120 min of irradiation. They also studied the antibacterial activity of prepared TiO2

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Fig. 5 Mechanism of photocatalytic reaction [17]. (© with permission of Elsevier)

nanoparticles using Gram-positive and Gram-negative strains.The result revealed that the biosynthesized TiO2 nanoparticles exhibit a higher photodegradation efficiency as well as biological activities as compared to chemically synthesized TiO2 nanoparticles (Aravind et al. 2021). Ansari et al. introduced a new biogenic source, as well as a capping and reducing agent, for the fabrication of TiO2 utilizing Acorus calamus plant leaf extract (Ansari et al. 2022). They evaluated the photocatalytic activity of biosynthesized TiO2 nanoparticles in an aqueous solution of Rhodamine B dye, as well as their antimicrobial efficacy using the disc diffusion approach. In comparison to bare TiO2, biosynthesized TiO2 nanoparticles showed strong photocatalytic activity, degrading 96.59% of the Rhodamine B dye under visible light irradiation. They also demonstrated superior antimicrobial action against Grampositive pathogenic bacteria (B. subtilis, S. aureus) than Gram-negative pathogenic bacteria (P. aeruginosa, E. coli). However, the broad usage of TiO2-based nanoparticles has been limited due to some inherent limitations. On the one hand, because of its large bandgap, TiO2 only accounts for 3–5% of the whole solar spectrum. However, the rapid recombination of photogenerated electron-hole pairs reduces the photocatalytic activity efficiency. Therefore, in recent years, nonmetal modification has been recognized as an ideal and cost-effective way of lowering the bandgap of TiO2 and enhancing the visiblelight-harvesting capacity of synthetic hybrid TiO2 nanocomposites. For example, Helmy et al. demonstrated a comparative chemical and green synthetic approach for S-doped TiO2 production via a sol-gel process. For green synthetic S-doped TiO2,

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Fig. 6 Synthesis of Pt/TiO2 nanocomposite [23]. (© with permission of Elsevier)

they used an aqueous extract of Malva parviflora plant as a green and adaptable medium with outstanding reducing and capping capabilities. However, they used isopropanol as the solvent in the chemical synthesis technique. Then, in the presence of visible light, they evaluated their photocatalytic abilities by degrading methyl orange dye. The green synthetic S-doped TiO2 exhibited higher photocatalytic activities in methyl orange bleaching and chemical oxygen demand reduction as compared to chemical synthesis S-doped TiO2. Further, they evaluated the antimicrobial and antioxidant capabilities of S-doped TiO2 which showed that the greener technique had higher activities than the chemical approach (Helmy et al. 2021). Apart from nonmetal modification, doping TiO2 with noble metal as well as metal elements boosts the light-harvesting capability of synthesized TiO2 nanocomposites, which is another effective way to tackle TiO2 limitations as a photocatalyst. In the photocatalytic process, the noble metal serves a dual purpose – first, it may be employed to harvest visible light via noble metal-induced SPR, and, second, the creation of a Schottky barrier between the semiconductor and the noble metal is advantageous for electron-hole pair separation. There are several kinds of literature available on noble metal-doped and metal-doped TiO2 produced from plant part extracts. For example, Kumar et al. developed a green synthetic route for the synthesis of Pt-doped TiO2 using the lead extract of Mentha arvensis (mint) as a reducing agent (Fig. 6) (Yogesh Kumar et al. 2021). For the first time, they studied photocatalytic degradation of doxorubicin, an anticancer drug, by using Pt-doped TiO2 nanocomposite. They revealed that 88% of doxorubicin was photodegraded after 100 min of visible light irradiation, indicating improved photocatalytic efficiency. They also used MCF-7 (human breast), HCT 116 (colon cancer), and Hep G2 (hepatocellular carcinoma) cell lines to examine TiO2 and Pt-doped TiO2 materials for anticancer efficacy. Ramzan et al. revealed that Cedrus deodara extract, which is freely available and high in phenolic content, was used to successfully synthesize Cu@TiO2 nanoparticles in an appropriate fashion. They picked this synthesis approach because it saves time, is chemical-free, and has no negative environmental effects. They employed a copper dopant in TiO2 to narrow the energy bandgap and reduce the rate of photogenerated exciton recombination. They studied the potential application

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Fig. 7 Graphical illustration of Ag/TiO2 nanocomposites using B. vulgaris peel extract [25]. (© with permission of Elsevier)

of green synthesized Cu@TiO2 nanoparticles for photocatalytic methylene blue dye degradation as well as antibacterial activity. They discovered that biosynthesized Cu@TiO2 nanoparticles show 95% of methylene blue dye degradation under sunlight irradiation; however, when exposed to visible light irradiation, only 73% of dye was photodegraded (Ramzan et al. 2021). To date, numerous methods for synthesizing the Ag/TiO2 heterojunction have been devised, including sol-gel, hydrothermal, chemical reduction, and UV irradiation techniques, however these procedures often require either a stringent synthetic condition or a toxic reducer. In the Ag/TiO2 heterojunction the produced Ag nanoparticles are rarely crystallinity and size uniform. As a result, Jayapriya and Arulmozhi have described a simple and flexible process for synthesizing the Ag/TiO2 heterojunction with well defined Ag nanocrystals using beta vulgaris peel extract. They have used a one step synthetic process for biological production of Ag/TiO2 nanocomposite (Jayapriya and Arulmozhi 2021) (Fig. 7). In the peel extract, the phytoconstituents serve as a reducing and capping mediator in this study, as determined by Fourier transform infrared spectroscopy. Further, they studied the effectiveness of the fabricated Ag/TiO2 nanocomposite for catalytic degradation of anthropogenic pollutants such as methyl orange, methylene blue, and Congo red, as well as antibacterial activity against both Gram-positive and Gram-negative pathogens. The photocatalytic activity result revealed that the synthesized Ag/TiO2 nanocomposite has a better degradation efficiency of 92% in 9 min for methylene blue, 84% in 20 min for Congo red, and 88% in 10 min for methyl orange. Similarly, without the use of any stabilizer or surfactant, Atarod et al. demonstrated a facile and environmentally friendly production of Ag/TiO2 nanocomposite from leaf extract of Euphorbia heterophylla (Atarod et al. 2016). The flavonoids in

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E. heterophylla leaf extract function as both reducing and capping or stabilizing agents. In the presence of NaBH4 in water at room temperature, they employed the Ag/TiO2 nanocomposite to effectively reduce different dyes such as methylene blue, Congo red, 4-nitrophenol, and methyl orange. They used UV–vis spectroscopy to monitor the catalytic reactions and discovered that they followed a pseudo-first-order rate equation. Without losing much of its catalytic activity, the produced catalyst may be retrieved and reused numerous times. Coupling GO with Ag/TiO2 to create heterojunction is a viable approach for achieving good absorption, conduction, and visible-light-harvesting capacity in the composite. The presence of GO in the nanocomposite material acts as an electron transport carrier and restrains the rate of electron-hole recombination by improving the photocatalytic activity, which could permit effective pollutant degradation. Using Euphorbia helioscopia L. leaf extract as a stabilizing and reducing agent, Nasrollahzadeh et al. proposed an environmentally benign, simple, cost-effective, surfactant-free, and green technique for the synthesis of Ag/reduced graphene oxide (RGO)/TiO2 nanocomposite. They used E. helioscopia leaf extract for the reduction of Ag+ ions to Ag nanoparticles and GO to RGO, respectively. They discovered that at room temperature, this nanocomposite was extremely active in reducing Congo red, 4-nitrophenol, and methylene blue. Centrifugation was utilized to easily separate and recover the as-prepared nanocomposite from the reaction mixture, and it could be reused for numerous cycles without losing catalytic activity (Nasrollahzadeh et al. 2016). After photocatalytic reactions, recycling powdered TiO2 photocatalysts from an aqueous solution is not economically feasible. The separation efficiency can be improved by immobilizing nanocrystalline TiO2 on supporting materials such as ceramic, glass, sand, fiber, etc. The nanocrystal deposition is often nonuniform and rapidly detaches from the substrate, resulting in a significant reduction in photocatalytic activity and selectivity. Because of the large content of inactive supports, light-harvesting is ineffective. As a result, designing a photoactive and recyclable multifunctional photocatalyst is a considerable problem. For large-scale practical applications, immobilization of nanostructured TiO2 on appropriate substrates endowed with reduced bandgap and increased pollutant adsorption capability is needed. Recently, Rusman et al. effectively produced ZnO/TiO2 nanocomposite utilizing Calopogonium mucunoides leaf extract using a green synthesis technique. For photocatalyst application, they synthesized the composite at varied calcination temperatures and TiO2 concentrations. They discovered that the best photodegradation performance was 98.26% (in only 10 min) for ZnO/TiO2 (5 g) with a calcination temperature of 800  C. This could be attributed to the greatest distance between two optical phonon modes and the lowest attenuating and propagating constant, according to the researchers. They revealed that the composite ZnO/TiO2 had a strong photodegradation potential for Congo red dye, as well as high recyclability (five cycles, >95%) (Rusman et al. 2021). Erim et al. synthesized TiO2/GO/ chitosan nanocomposite using Olea europaea leaf extract as a phenolic solvent. They investigated the photocatalytic activity of synthesized nanomaterial for cefixime trihydrate degradation under UV-A irradiation. They used TiO2 as the primary photocatalyst compound, which was then impregnated with doped graphene

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oxide and chitosan to narrow the bandgap. They discovered that under ideal conditions, TiO2/GO/chitosan had a degradation efficiency of 95.34%. According to the recycling tests, the photocatalyst developed has exceptional durability and stability when exposed to UV-A irradiation (Erim et al. 2021). In the presence of pomegranate juice, Shabani et al. successfully produced Fe3O4 and TiO2 nanoparticles. Using the pad–dry–cure method, they created multifunctional cotton fabrics coated with Fe3O4/TiO2 nanocomposite. Pomegranate (Punica granatum) juice was discovered to be a good green capping agent for the synthesis of Fe3O4–TiO2 nanocomposites. Under ultraviolet light irradiation, they studied the photocatalytic behavior of Fe3O4/ TiO2 nanocomposites by degradation of acid black, Congo red, and acid brown dyes. The result revealed that the azo dyes were photodegraded 90–95% within 60 min of light irradiation and decomposed to water, carbon dioxide, and various harmless residuals (Shabani et al. 2016).

CuO-Based Materials for Photodegradation of Pollutants Cupric oxide (CuO) has been widely used in various applications as a p-type metal oxide semiconductor with a narrow bandgap (1.2–2.2 eV), due to its favorable physical and chemical properties. The quantum confinements have a strong influence on the properties of CuO nanoparticles. CuO is a form of visible light active photocatalyst that has been extensively researched for water purification and the removal of numerous organic and hazardous chemicals from wastewater. In recent years, many types of copper-based photocatalysis nanomaterials have been developed, due to their narrow bandgap, and they could be driven by visible light. The CuO nanoparticles are synthesized via various preparation methods, with the green methodology being the most popular due to its clean, nontoxic, affordable, and reliable approach. Here, in this book chapter, we only explored the synthesis of plant-mediated CuO nanoparticles and their application to the decontamination of various organic contaminants from water resources. For example, Saruchi et al. used Aloe barbadensis miller leaf extract to demonstrate a green technique for the bioproduction of copper oxide-Aloe vera (CuO-A)-based nanoparticles. They employed the CuO-A nanoparticles they made to remove the methylene blue dye. With an initial concentration of 100 mg/L at alkaline pH and a shaking speed of 150 rpm, they discovered that the maximum dye removal was 98.89% in 210 min. They also revealed that CuO-A nanoparticles exhibited potent antibacterial properties against a wide range of microorganisms. They discovered that as the size of the particles shrinks and the surface area grows, it becomes an excellent adsorbent. The adsorption was found to be endothermic and spontaneous in a thermodynamic investigation. They discovered that the Langmuir results are more consistent than the Freundlich model, which has a consistency of 95.5 mg/g (Saruchi and Kumar 2019). Similarly, Sukumar et al. explored the environmentally friendly green manufacturing of CuO nanoparticles utilizing Annona muricata leaf extract and tested their photocatalytic performance and cytotoxicity on human breast cancer

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cell lines. They tested CuO nanoparticle photocatalytic activity by photodegrading reactive red 120 and methyl orange in the presence of sunlight. At 60 min of irradiation, the degradation efficiency was approximately 90% and 95%, respectively. According to the findings, green-synthesized CuO nanoparticles could be employed as an environmentally friendly photocatalyst for textile dye decomposition (Kayalvizhi et al. 2020). Rafique et al. used a cost-effective, environmentally friendly, and harmless way to synthesize CuO nanoparticles utilizing a biological synthesis approach based on Citrus aurantifolia leaf extract (key lime). They applied this as a photocatalyst and antibacterial agent to purify wastewater from the industrial and residential areas. They investigated the photocatalytic efficiency of CuO nanoparticles for Rhodamine B industrial dye and discovered that 91% of the dye was removed. Additionally, the antibacterial activity was also assessed, and it was found to be efficient against the Gram-positive bacteria Staphylococcus aureus (S. aureus) and the Gram-negative bacteria Escherichia coli (E. coli) (Rafique et al. 2020). Kerous et al. used Aloe vera extract as solvent and copper sulfate as the precursor to the synthesis of cuprous oxide (Cu2O) nanoparticles with octahedral- and spherical-like geometries utilizing an eco-friendly, easy, and cost-effective method (Kerour et al. 2018). Different characterization approaches were used to investigate the effect of Aloe vera extract concentration on the morphological, structural, and optical aspects of as produced nanoparticles. They tested the photocatalytic activities of Cu2O nanomaterial by photodegrading methylene blue and discovered that after 10 min of visible light exposure at room temperature, the methylene blue dye was completely degraded. To synthesize copper oxide nanoparticles, Nwanya et al. used Zea mays L. dry husk extract. For the first time, they were able to create red cubic Cu2O nanoparticles via this straightforward, environmentally friendly, green synthesis pathway. They investigated its photocatalytic degradation of methylene blue dye as well as its antibacterial efficacy. At 600  C, the Cu2O nanoparticles were thermally oxidized to pure monoclinic CuO nanoparticles. They discovered that under visible light irradiation, the 600  C annealed copper oxide nanoparticles degraded methylene blue dye and textile effluent by 91% and 90%, respectively. They also discovered that CuO-300 is superior for inhibiting the growth of Escherichia coli 518,133 and Staphylococcus aureus 9144, while Cu2O is better for Pseudomonas aeruginosa and Bacillus licheniformis (Nwanya et al. 2019). For photosynthesis of CuO nanoparticles, Chandrasekar et al. employed aqueous leaf extracts of Gomphrena globosa (G. globosa) and Gomphrena serrata (G. serrata). They evaluated the CuO nanoparticle photocatalytic activity against crystal violet dye and found that under the sunlight irradiation about 84–96% of dye was photodegraded. These CuO nanoparticles also showed antibacterial activity against E. coli, B. subtilis, and P. aeruginosa, as well as in vitro cytotoxicity against hepatocellular carcinoma HepG2 cells (Chandrasekar et al. 2021). Das et al. utilized Madhuca longifolia plant extract as a nontoxic reducing agent to establish an effective and eco-friendly technique for the green synthesis of CuO nanoparticles (Das et al. 2018). They discovered that nanoparticles have high photoluminescence properties depending on particle size, as well as outstanding photocatalytic

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Fig. 8 (a) and (b) pictorial and schematic representation of photocatalytic mechanism [37]. (© with permission of Elsevier)

performance in the presence of visible light irradiation toward methylene blue degradation, making them a viable wastewater treatment material. Its photocatalytic mechanism was shown in Fig. 8. Also, the antibacterial activity of the produced CuO nanoparticles was tested, and the findings were compared to ampicillin and tetracycline. Vidovix et al. presented a green synthesis technique for producing CuO nanoparticles from pomegranate leaves which they used in methylene blue adsorption experiment (Vidovix et al. 2019). The proposed adsorbent was shown to be highly efficient, removing up to 96.91% of the contaminants. Weldegebrieal et al. used Verbascum thapsus extract to biosynthesize CuO nanoparticles and investigated their antibacterial and photocatalytic activity for methylene blue dye. They discovered that by utilizing nature sunlight at the environmental temperature of 28  C, the nanoparticles photodegraded methylene blue by 34.4% in 3 h. When 1 mL of 0.5 M NaOH and 2 mL of 30% H2O2 was added into the dye solution, the degradation rate is greatly increased, reaching 94.6% and 99.3% degradation percentages in 2 h, respectively. More than 96% of the dye was photodegraded in 3 h when the same amount of alkali and oxidant is employed but without a catalyst. They found that in the absence of solar irradiation, dye degradation was hardly observed, showing the relevance of light in dye degradation. Furthermore, the as-prepared nanoparticles reduced the growth of two bacterial strains, implying that they could be used as an antibacterial agent (Weldegebrieal 2020). As a result, CuO nanoparticles play a key role in overall catalytic performance. On the other hand, CuO nanoparticles have some drawbacks, including agglomeration, failure to separate quantitatively from the reaction mixture, inability to recycle, and excessive use of Cu reagents. So to solve these difficulties, CuO nanoparticles have been supported on solid supports to construct composite catalysts. Furthermore, because photogenerated charge carriers can recombine quickly within a single nanocrystal structure, heterojunction nanocomposite structures have been constructed to effectively segregate photogenerated electron-hole pairs and boost photocatalytic performance. It would be ideal if a straightforward method for producing well-dispersed, stable CuO heterojunction nanocomposites and immobilizing them on a suitable substrate could be developed for large-scale application. In the absence of any stabilizer or surfactant, Bordbar et al. developed

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Fig. 9 The synthesis schematic of Cu-doped NiO-NPs [41]. (© with permission of Elsevier)

an environmentally safe, clean, and nontoxic process for the green production of CuO nanoparticles/clinoptilolite employing root extract of Rheum palmatum L. as a reducing and stabilizing agent (Bordbar et al. 2017). In the presence of sodium borohydride in water at ambient temperature, they looked at the catalytic activity of CuO nanoparticles/clinoptilolite nanocomposite in the reduction of 4-nitrophenol, methylene blue, and Rhodamine B. This nanocomposite was found to be a highly active catalyst in the corresponding processes and can be retrieved and regenerated several times without losing its activity. Ghazal et al. used a sol-gel method (Fig. 9) to produce copper doping nickel oxide nanoparticles (Cu-doped NiO-NPs), with okra (Abelmoschus esculentus) plant extract, and examined their photocatalytic activity as well as cytotoxicity effects. They investigated its photocatalytic activity by studying the degradation of methylene blue pigment under UV-A light and found that after 105 min of irradiation, 78% of methylene blue had degraded. They also studied the cytotoxicity of nanoparticles in suppressing cancer CT26 cells (Ghazal et al. 2021). In the absence of dangerous and toxic components, Bordbar et al. developed an effective technique for the synthesis of CuO/ZnO nanocomposite employing leaf extract of Melissa officinalis L. as a moderate, renewable, and nontoxic reducing agent and efficient stabilizer. They determined that the nanocomposites synthesized using this process are quite stable for up to a month. Furthermore, at room temperature, the CuO/ZnO nanocomposite exhibited excellent photocatalytic activity in the degradation of 4-nitrophenol and Rhodamine B in water (Bordbar et al. 2018). Pakzad et al. described a green synthesis technique for making magnetic Ni@Fe3O4 and CuO nanocomposites from Euphorbia maculata aerial part extracts (Pakzad et al. 2019). They investigated the photocatalytic activity of the synthesized nanocomposite for the degradation of several organic dye pollutants such as Rhodamine B, methylene blue, and Congo red under UV illumination. They also looked at how numerous variables like catalytic dosage, pH, initial concentration of dyes, and contact time affected the photocatalyst adsorption capability. The photocatalytic activity of the biosynthesized nanoparticles was compared, and it was discovered that CuO nanocomposites have greater catalytic activity than

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Ni@Fe3O4 nanocomposites. Furthermore, following recycling, the nanocomposites showed good photocatalytic stability in the methylene blue degradation under exposure to UV irradiation. They discovered that after four cycles the photocatalyst performance had not changed significantly, indicating outstanding photocatalytic stability.

ZnO-Based Materials for Photodegradation of Pollutants ZnO is regarded as an n-type semiconductor with a bandgap of about 3.37 eV and binding energy of about 60 mV at room temperature. Hence, it absorbs within a limited UV region which comprises an inadequate fraction of photons, thereby making it an unsuitable candidate for photocatalytic applications (Dimitrijevic et al. 2013). Because of its higher surface area-to-volume ratio and the possibility to tune its various morphologies, ZnO is widely speculated as a potent precursor for the degradation of perilous chemicals particularly in the wastewater resources. Several literature insights depict the use of physical and chemical methods in context to ZnO for treating hazardous chemicals that are costly, unyielding, and ecologically malicious. Henceforth, several green technologies for ZnO nanoparticles production have been adopted recently for detoxifying these pollutants in a cost-efficient and environmentally benign way. Plant extracts derived from a variety of plant parts would effectively serve as capping agents and stabilizing agents for the synthesis of zinc oxide nanoparticles. The synthetic routes through plants do not involve complex methodologies or complicated protocols. Furthermore, it prevents the use of complex equipment for higher-scale production of nanoparticles. Anisochilus carnosus, Plectranthus amboinicus, and Vitex negundo are examples of Lamiaceae plants that have been intensively examined (Ahmad et al. 2018). Plant parts used in the green synthesis of ZnO nanoparticles include flowers of Jacaranda mimosifolia and Anchusa italica; fruits of Artocarpus gomezianus; leaves of Plectranthus amboinicus, Tamar indusindica, Parthenium hysterophorus, Pongamia pinnata, etc.; stem bark of Boswellia ovalifoliolata; and shoots of Sedum Alfred, to name a few (Ali et al. 2018). They have been used to degrade certain toxic pollutants of the environment comprising of dyes like methyl orange and methylene blue. The plant extract secretes a few phytochemicals that act as both reduction and capping or stabilization factors; for example, in the synthesis of ZnO nanoparticles from Conyza canadensis leaf extract, phytochemicals with hydroxyl (O-H) and carbonyl side groups help to stabilize the ZnO nanoparticles (Ahmad et al. 2018). Organic dyes are a prominent water contaminant found in industrial effluents. Before being disposed of in the environment, these organic dyes must be degraded. The catalytic activity of green produced ZnO nanoparticles for the reduction of dyes like methyl orange and methylene blue was investigated. Furthermore, the reusability of the catalyst was increased up to six consecutive cycles. Raja et al. used Tabernaemontana divaricata leaf extract to synthesize sphericalshaped ZnO nanoparticles via a green approach. These synthesized ZnO

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nanoparticles were also used to assess the photocatalytic performance of methylene blue dye under sunshine. They found that in about 90 min, methylene blue was nearly completely degraded. Furthermore, the ZnO nanoparticles were made by Rajkumar et al. using dry onion peel aqueous extract (Allium cepa L.), which is a waste product (Krishnasamy Sekar et al. 2020). Several ways for synthesizing ZnO nanoparticles from plant genic or biogenic wastes have also been devised. Biowastes including sorghum bran, corn cob, wheat bran, fruit pulp, etc., could be employed as green resources for efficient fabrication of ZnO nanoparticles (Roopan et al. 2012; Vickers 2017). Such resources are enriched with resin, lignin, and phenolic compounds that readily serve as a template, capping agents, and stabilization agents, respectively. One of the most effective methods to derive such phytochemicals from parts of the plants is extraction. Herein, the phytochemicals are transferred from the green sources onto the solution. To eliminate trash and prevent future contamination, the plant’s components are extensively cleansed with water. Following this, the specific part is cut down into small fragments or ground into fine powder. Then, they are boiled in solvents like water, ethanol, etc., without inferring damage to the phytochemicals present. So, factors like the effective modulation of temperature and choosing a suitable solvent for the purpose are obligatory. To make ZnO nanoparticles, Jafarirad et al. extracted natural components from Rosa canina fruit pulp (Jafarirad et al. 2016). Such derived nanoparticles could have a number of applications and future prospects. Elavarasan et al. used Sechium edule leaf extract to make ZnO nanoparticles and studied its notable applicability toward photocatalysis of reactive blue dye, cytotoxicity, and antibacterial behavior (Elavarasan et al. 2017). Similarly, ZnO nanoparticles synthesized from leaves of Azadirachta indica by Ramamoorthy et al. reportedly showed better photodegradation ability than the pristine ZnO nanoparticles. They had also illustrated the greater corrosion efficiency rate of ZnO-coated plates rather than the non-coated ones. Other methods of extraction of ZnO NPs from plant parts like Agathosma betulina (Thema et al. 2015), Punica granatum leaf extract (Singh et al. 2019), aqueous extract of Phoenix roebelenii palm leaves (Aldeen et al. 2022), etc., were also reported showing greater efficacy in photocatalytic degradation of the noxious dyes from the wastewater systems. They have also been used for the degradation of dyes like Coomassie brilliant blue and methylene blue with a higher efficacy rate. Reports of synthesis of ZnO nanoparticles from the stem extract of Dalbergia parviflora (Chankhanittha et al. 2022) have also shown considerable enhancement of the degradation efficiency of RR141 dye. The nanocatalyst shows photoactivity until three consecutive cycles. Moreover, after increasing the temperature to 850  C, the produced ZnO photocatalyst remained stable. The photocatalytic degradation of organic contaminants over the natural-product-capped ZnO photocatalyst was displayed in Fig. 10. The ability of a catalyst to generate photogenerated electron-hole pairs determines its photoactivity. The fast recombination rate of photogenerated electron-hole pairs which perturbs the photodegradation activity is the fundamental restriction of ZnO as a photocatalyst. It has also been reported that the optical absorption ability of ZnO which is linked to its huge bandgap energy affects its solar energy conversion

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Fig. 10 A possible photocatalytic degradation mechanism of organic pollutants over the naturalproduct-capped ZnO photocatalyst [55]. (© with permission of Elsevier)

performance. As a result, considerable work has gone into improving the optical characteristics of ZnO by lowering the bandgap energy and preventing photogenerated charge species from recombination by interfacing semiconductor nanoparticles with various materials like noble metals, carbanion materials, and other semiconductor nanoparticles (Boon et al. 2018). For example, using a microwaveassisted green method, Hassanpur et al. synthesized Co3O4/ZnO nanocomposite that shows 86% of methylene blue and 91% of Rhodamine blue dye removal under UV light (Hassanpour et al. 2017). Through the demonstration of literature by researchers, it has been proved that bandgap engineering and heterojunction formation improved photocatalytic activity of the formed nanocomposites under UV and visible light. However, regarding the biosynthesis of heterostructure photocatalyst, a lesser number of literatures are available. Sohrabnezhad and colleagues reported employing Urtica dioica leaf extract as a reducing agent to synthesize Ag/ZnO in montmorillonite. Using the green synthetic approach, Ag/ZnO nanoparticles were successfully integrated into the montmorillonite (MMT) matrix, as shown by SEM and TEM images. They also demonstrated that by combining the unique features of MMT and silver nanoparticles, the photocatalytic activity of ZnO for the photodegradation of methylene blue may be significantly increased. Furthermore, the characterization studies of the synthesized nanocomposite also proved that Ag/ZnO was successfully incorporated onto the MMT surface. The photocatalytic degradation capability has been notably enhanced from 37.7% to 82.5% after the formation of the Ag/ZnO nanocomposite (Sohrabnezhad and Seifi 2016). Despite

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such notable works, the method for the incorporation of such noble metal elements on the surface of ZnO nanoparticles surface may have some dreadful consequences like uneven distribution or development of undefined morphology, etc. The fabrication of noble metal nanoparticles functionalized with bifunctional ligands using pre-synthesized noble metal nanoparticles capped with bifunctional ligands ZnO, on the other hand, has several benefits, including the capacity to independently control the size of noble metal nanoparticles and the ability to achieve a very narrow size distribution using well-established synthetic processes. Meanwhile, the extra processes for removing excess noble nanoparticles and contaminants have been removed. The hybridization of carbon materials with inorganic components has long been recognized as a dependable method for generating novel functionalities and improving features. Several base materials for the loading of the green synthesized nanoparticles have been discovered so far among which RGO is regarded as one of the most relevant since its lamellar structure possesses some remarkable properties like high specific surface area, electrochemical stability, and high mobility of charge carriers. Hence, it could be deduced as a potent photocatalytic promoter or carrier. Apart from the above strategies, the synthesis of ZnO composite nanoparticles through a green synthetic approach still remains a major challenge. Furthermore, the small-sized nanoparticles get agglomerated easily, and recovery from the reaction system becomes too tough. Furthermore, the process for dye degradation in nanocomposites shows that both nanoparticles and the base reagent are good electron acceptors, enabling photogenerated electron-hole pair separation as well as particle recovery from the reaction system during the photodegradation process. However, the insight into adopting the green method still remains a long way, and efforts are still done to achieve the feat of photocatalytic degradation of organic pollutants using ZnO heterostructure nanoparticles.

ZrO2-Based Materials for Photodegradation of Pollutants Zirconium dioxide (ZrO2) is one of the highly stabilized oxides formed from enforcing thermal conditions on various zirconium compounds. ZrO2, also referred to as zirconia, is considered one of the most promising catalysts widely utilized for the deformation, dehydrogenation, and isomerization of organic molecules. Because of its extreme stability and low toxicity, it’s a fascinating substance with a wide range of applications. The wide bandgap and high negative conduction band energy of ZrO2 nanocomposites simply create oxygen holes as the catalyst’s carrier, making connections with the active components. Possessing a high bandgap in a range of 5–5.5 eV, ZrO2 is a p-type semiconductor possessing oxygen vacancies (Gurushantha et al. 2017), thereby creating various holes on the surface facilitating its photocatalytic behavior. Henceforth, ZrO2 is widely considered a potent reagent for numerous applications. Regardless, due to the harmful impacts of synthetic substances on the ecosystem and the growing interest in environmentally friendly

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nanoparticles, there is a compelling need to develop new technologies for large scale production of sustainable and cost effective metal oxide nanoparticles. ZrO2 is the only metal oxide to possess both reducibility and oxidizability characteristics owing to its amphoteric nature (Gurushantha et al. 2017). The physical method involves the use of instruments and a large amount of heat wherein the chemical method implements the use of toxic chemicals, thereby making both of the methods not appertaining to the green synthetic protocols (Bolade et al. 2020). Plants are the most common, readily available, and costeffective source of ZrO2 nanoparticles for biosynthesis. Owing to various benefits of plant resources, they often serve the purpose of biotemplates. Through calcination, the various phytochemicals present easily reduced the functionalized groups like carbonyl, amine, alkyl, etc., to simpler molecules like CO2, H2O, and N2. Moreover, the presence of the phytochemicals inhibits the agglomeration in nanoparticles and thus improvises the surface defects and properties conducive to their optical, catalytic, and adsorption behavior. In addition to this, their presence could effectively reduce the hazardous chemical moieties and could ultimately cut the number of cycles pertaining to the complicated synthetic steps. The green synthetic approach to extracting phytochemicals is through the use of simple solvents like water, alcohol, hexane, etc. The polar solvents dissolve the aqueous soluble phytochemicals easily and could be incorporated onto the surface of ZrO2 nanoparticles through complexation or thermal calcination methods. The former involves any precursor moiety of zirconium which forms nanoparticles under regulated temperature and pressure conditions. For example, by hydrolyzing (ZrF6)2 in the presence of Curcuma longa tuber extract, Sathishkumar et al. (Sathishkumar et al. 2009) created ZrO2 nano-chains. Gowri et al. used aqueously soluble carbohydrates isolated from Nyctanthes arbortristis flowers to successfully biosynthesize ZrO2 nanoflakes by hydrolysis of ZrOCl2 8H2O. Many plant extracts, such as Lagerstroemia speciosa (Sai Saraswathi and Santhakumar 2017) and Salvia rosmarinus (Davar et al. 2018), can produce semispherical and oval ZrO2. Similarly, Nabil Al-Zaqri et al. have synthesized ZrO2 nanoparticles using Wrightia tinctoria leaf extract as an alternative to chemical synthetic system. For the RY 160 dye, biosynthesized ZrO2 nanoparticles show degradation at an enhanced rate of 94%. Furthermore, they depicted its antibacterial and photocatalytic applications. ZrO2 NPs have lately been synthesized using plant leaf extracts from Moringa oleifera (Annu et al. 2020), Acalypha indica (Shanthi Tharani 2016), Nyctanthes, Lycopersicon esculentum (Shinde et al. 2018), Azadirachta indica (Nimare and Koser 2016), etc. Gurushantha et al. described the photocatalytic, structural, and photoluminescent capabilities of green produced zirconia nanoparticles using Phyllanthus acidus leaves (Fig. 11) as fuel (Gurushantha et al. 2015). The synthesized nanoparticles exhibited higher degradation rate of acid orange at 98%. This higher activity was attributed to the generation of hydroxyl radicals, dopant concentration, narrow bandgap, textural properties, and effective crystallize size.Under UV light irradiation, the photocatalytic effectiveness of ZrO2 nanoparticles is assessed for the photodegradation of methylene blue and

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Washed with water Dried in sunlight and grinded

Phyllanthusacidius

ZrO(NO3)2 + Fe(NO3)3

Fine powder

400°C

Cubic Fe–doped ZrO2

Fig. 11 Synthesis of zirconia nanoparticles through an extract of Phyllanthus acidus as fuel [69]. (© with permission of Elsevier)

methyl orange dyes as target pollutants. Various parameters, such as catalyst loading quantity and pH, have also been investigated. The photodegradation of methylene blue and methyl orange by using ZrO2 catalyst was found to be up to 91% and 69% within 240 min. Furthermore, for the creation of ZrO2 nanoparticles, this green synthesis approach is a viable alternative to traditional multistep processes. It was notified that owing to the large bandgap of zirconia, its nanoparticles alone show poor photocatalytic behavior. However, numerous bandgap modification strategies like bandgap engineering, heterojunctions, etc., could assist in the lowering of the conduction band and uplifting valence band of zirconia, thus facilitating charge transfer. Modifications of the structure and energy levels by the addition of new compounds through doping could help in reducing the recombination of the charge carriers and thus enhance the photoadsorption behavior. Henceforth, various works had reported the chemical synthesis of hybrid heterojunctions of zirconia with various moieties possessing high photocatalytic behavior. For instance, the multifunctional nature of ZnO-ZrO2 synthesized using Daphne alpina (D. alpina) leaf extract has been evaluated, and its antioxidant activities and photocatalytic behavior against Rhodamine 6G were verified by Rasheed et al. (2020). Another efficient catalyst comprising Ag/ZrO2 heterostructure showing high thermal stability up to 1000  C was reported by Maham et al. which was made from the extract of Ageratum conyzoides that facilitated the reduction of Congo red, Nigrosine, and 4-nitrophenol (Maham et al. 2020). Apart from the pairing of Ag forming hetero composites, other metal oxides like Cu, Ni, Sm, V, and Ce show considerable

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photocatalytic degradation of water pollutants as reported from the previous literature survey. Using the leaf extract of Daphne alpina, the surface area of the as-prepared photocatalyst V2O5/ZrO2 shows considerable enhancement at 214 m2/ g and bandgap at 3.93 eV. The objective of the nanocomposite was greatly achieved by Rasheed et al. at the degradation of methyl orange and picloram for 75 min (Rasheed et al. 2020). Moreover, the thermal stability was tested, and 11.79% weight loss was observed. A degradation rate of 79% ad 86% was observed for methyl orange and picloram respectively. These results further clarifies that the nanocomposites obtained from the plant extract possess no hindrance to the competence of the photocatalyst, rather denunciates the use of harmful chemicals. 7-Hydroxy-40 methoxy-isoflavone phytochemical obtained from the extract of Commelina diffusa serves as a reducing and stabilizing agent that inhibits the agglomeration of CuO/ZrO2 with the size of nanoparticles formed ranging from 18 to 25 nm (Hamad et al. 2019). The composite facilitates the catalytic reduction of numerous organic dyes and persistent pollutants like 2,4-dinitrophenyl hydrazine, Nigrosine, Congo red, and methyl orange in the presence of NaBH4 at room temperature. Furthermore, the ZrO2 nanoparticles are derived from the leaf extract of Ficus benghalensis. In a microwave oven at 900 W for 15 min (20 s on–40 s off cycle), the resulting solution was evaporated to dryness. To make a fine powder, the substance was pounded for 15 min with a mortar and pestle. The powder was then calcined for 3 h at 500  C in a temperature-controlled muffle furnace. Within 240 min, the degradation efficiency of methylene blue and methyl orange dyes was 7.36% and 3.73%, respectively. The Aloe vera extracts have also been found to be a potent source for the formation of Mg-doped ZrO2 nanocomposites that are in form of hollow microspheres (Renuka et al. 2016). Furthermore, after 60 min of reaction, a total organic carbon test revealed a mineralization rate of 79%. The oxygen vacancies present contributed to the controllability of the reaction conditions and enhanced catalytic behavior. Silva et al. used a response surface approach to improve the tetracycline removal effectiveness of ZrO2 nanoparticles biosynthesized from Euclea natalensis roots. Thus, green production of ZrO2-based nanocomposites improves several of the intrinsic ZrO2 nanoparticle shortcomings, such as bandgap energy, surface area, surface chemistry, and stability. The synthesis of ZrO2-based nanocomposites uses microbial or botanical sources to achieve a green and sustainable approach. Microbial or botanical sources play a significant role in the biostabilizing, biochelating, biocapping, and bioreduction process for the transformation into composites. Doping of various metals onto the ZrO2 surface aids in the recyclability and photocatalytic and thermal stability of the as-formed nanocomposite. Apart from all these advantages, the recovering of the catalyst and their separation post-reaction becomes a tedious job. For these reasons, the idea of incorporating magnetic components was put into force so that the particle separation would be facile and less time-consuming. Inspired by this concept, Vartooni et al. effectively biofabricated Ag/Fe3O4/ZrO2 nanocomposites from Centaurea cyanus with average particle sizes ranging from 30 to 90 nm (Rostami-Vartooni et al. 2019). Because the nanocomposite’s saturation

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magnetization value was 10 emu/g, it was easy to separate it from the reaction solutions. The authors also found that catalytic reduction of 4-nitrophenol and degradation of methyl orange green Ag/Fe3O4/ZrO2 with a reaction time of 7.5–8 min produced good results. As a result, this advantage may encourage magnetic ZrO2 nanocomposites to undergo recyclability tests. The treatment of reused ZrO2 nanoparticles has not been addressed, unlike the treatment of contaminants such as antibiotics and textile dyes. This action may result in secondary contamination, necessitating the resolution of posttreatment issues. Furthermore, the majority of studies on the application of green ZrO2 nanoparticles only reported their findings in laboratory settings, resulting in a disparity between simulated and real-world results. As a result, if the testing is conducted under realistic settings, this flaw could be overcome. The potential of ZrO2 nanocomposites is substantially exploited in many sectors like bioremediation, photocatalysis, detection of toxic gasses and pollutants, and so on. It could be utilized on a large scale as an essential component in environmental degradation. However, controlling the interactions of the green extract with dopants in ZrO2-based nanocomposites is often problematic. To avoid these interactions, the biomass proportion should be optimized. Even though some published works have improved our understanding of the formation mechanism and applications of green ZrO2-based nanocomposites, the number of studies is still limited. The entire understanding of the interactions between ZrO2 nanoparticles and the environment is still developing. To minimize future issues, thorough assessments of the eco-toxicity of green ZrO2 nanoparticles should be undertaken on a regular basis (Van Tran et al. 2022). Herein, Table 1 gives the overall illustration of the plant-mediated green synthesis of TiO2-, CuO-, ZnO-, and ZrO2-based hybrid nanomaterials for photodegradation of organic pollutants.

Conclusions Nanoparticles have radically different properties than bulk particles due to their ultrasmall size and so present a new potential in nanoscience and nanotechnology. Green synthetic root has recently gained a lot of scientific attention since it is an ecologically friendly, inexpensive, and easily scaled-up approach. Nanoparticles can be generated from various natural substances such as plant extract, fungi, bacteria, and yeast; however, the nanoparticles synthesized from plant part extracts are relatively easy and straightforward as compared to bio-organism-mediated synthesis. Furthermore, it has been demonstrated that the green synthetic technique allows for greater control over the structure, size, morphology, and other specific aspects of nanocomposites. The “state-of-the-art” research on plant-mediated green nanocomposites and their application in the photodegradation of organic contaminants is covered in this book chapter. To address the existing issues in green synthesis and to extend laboratory-based research to an industrial scale by tackling existing

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Table 1 Plant-mediated green synthesis of hybrid nanomaterials for photodegradation of organic pollutants SL. NO. Plant species 1 Malva parviflora 2 Mentha arvensis 3 Cedrus deodara 4 Beta vulgaris

Heterostructure synthesized S-doped TiO2

Degradation of pollutant Methyl orange

Pt-doped TiO2

Doxorubicin

Cu@TiO2

Methylene blue dye

Ag/TiO2

Methyl orange, methylene blue, and Congo red

Ag/TiO2

Methylene blue, Congo red, 4-nitrophenol, and methyl orange Congo red, 4-nitrophenol, and methylene blue Congo red

5

Euphorbia heterophylla

6

Euphorbia Ag/RGO/TiO2 helioscopia L. Calopogonium ZnO/TiO2 mucunoides Olea europaea TiO2/GO/chitosan Cefixime trihydrate

7 8 9 10

11 12 13

14 15

Punica granatum Rheum palmatum

Fe3O4/TiO2

Acid black, Congo red, and acid brown 4-Nitrophenol, methylene blue, and Rhodamine B

CuO nanoparticles/ clinoptilolite Abelmoschus Cu-doped Methylene blue esculentus NiO-nanoparticles Melissa CuO/ZnO 4-Nitrophenol and officinalis L. Rhodamine B Euphorbia Ni@Fe3O4 and Rhodamine B, methylene maculata CuO blue, and Congo red nanocomposites Urtica dioica Ag/ZnO in Methylene blue montmorillonite Rhodamine 6G Daphne alpina ZnO-ZrO2

16

Centaurea cyanus

17

Ageratum Ag/ZrO2 conyzoides Daphne alpina V2O5/ ZrO2

Congo red, Nigrosine, and 4-nitrophenol Methyl orange and picloram

19

Commelina diffusa

CuO/ZrO2

20

Aloe vera

Mg-doped ZrO2

2,4-Finitrophenyl hydrazine, Nigrosine, Congo red, and methyl orange Rhodamine B

18

Ag/Fe3O4/ZrO2

Methyl orange and 4-nitrophenol

References Helmy et al. (2021) Yogesh Kumar et al. (2021). Ramzan et al. (2021) Jayapriya and Arulmozhi (2021) Atarod et al. (2016) Nasrollahzadeh et al. (2016) Rusman et al. (2021) Erim et al. (2021) Shabani et al. (2016) Bordbar et al. (2017) Ghazal et al. (2021) Bordbar et al. (2018) Pakzad et al. (2019) Sohrabnezhad and Seifi (2016) Rasheed et al. (2020) RostamiVartooni et al. 2019 Maham et al. (2020) Rasheed et al. (2020) Hamad et al. (2019) Renuka et al. (2016)

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environmental concerns, an updated literature review based on the mechanism of diverse nanocomposites synthesis and their photocatalytic applicability has been demonstrated. Acknowledgments The authors would like to acknowledge NIT Rourkela, India (Odisha), for providing the research facility and funding to carry out this work.

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Natural Polymer-Based Nanocomposite Hydrogels as Environmental Remediation Devices

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Contents Nanotechnology: Origin and Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transformation of Hydrogels to Nanocomposite Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Nanocomposite Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Hydrogels Based on Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Hydrogels Based on Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Based on Metal and Metal Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of Synthesis of Nanocomposite Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanocomposite Hydrogels Using Sol-Gel Method . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiation-Assisted Synthesis of Nanocomposite Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Free-Radical Co-polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of Nanocomposite Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Natural Polymer-Based Nanocomposite Hydrogels in Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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S. Sethi (*) · Medha · S. Thakur Department of Chemistry, DAV University Jalandhar, Jalandhar, Punjab, India e-mail: [email protected] A. Singh · B. S. Kaith · S. Khullar (*) Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_100

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Abstract

The exceptional physical and chemical characteristics of nanoparticles (enhanced reactivity, high surface to volume ratio, unique surface chemistry) have been providing innovatory solutions to an array of environmental issues. The traditional hydrogels of biopolymers (starch, cellulose, chitosan, alginate, gums, dextran, pectin, carrageenan, etc.) provide hydrophilic materials with threedimensional network and porosity. The combination of hydrogel and nanoparticles through a particular synthesis method could enhance the characteristics (physical, chemical, biological) of the resulting material which are not feasible to achieve individually. Nanocomposite hydrogels are nanoparticles- incorporated polymeric networks formed by inclusion of nanoparticles into the hydrogels and exhibit higher strength as well as flexibility. The different types of nanoparticles (inorganic, carbon based, polymeric) have been assimilated into several hydrogels originated from natural polymers. Further, the applications of nanocomposites for environmental remediation (as chemical sensors, adsorbents, and photo catalysts) depend upon the nature of nanoparticles integrated into the hydrogel matrix. The size, morphology, chemical composition, and functionalization of nanoparticles could be easily tuned to improve their selectivity and performance. The eco-friendly nanocomposite hydrogels could provide potential scavengers for the adsorption of heavy metals and synthetic organic compounds including dyes, halogenated compounds, herbicides, and insecticides. Further, they have application prospects as soil conditioners and ion exchangers. The chapter addresses the synthesis, characteristics of biopolymer- based nanocomposite hydrogels, and their promising applications as environmental remediation devices Keywords

Nanocomposite · Chemical sensors · Porosity · Eco-friendly · Environmental remediation Abbreviations

Ag-NPs APT Au-NPs BCAm BET CLSM CNTs CQDs CRG

Silver nanoparticles Atomically precise technology Gold nanoparticles Benzo-18-crown-6-acrylamide Brunauer-Emmett-Teller analysis Confocal laser scanning microscopy Carbon nanotubes Carbon quantum dots κ-Carrageenan

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CS Cu-NPs CVD DETA DLS DNA DOX DSC FTIR GC-MS GO HPLC IBM IPN LCST LDA MWNTs NCHs NCs NMR NMs PAA PAAm PCs PCS PNIPAM PVP RAFT RNA SA-CMBC SAPs SEM SH SPS SRGs STM SWNTs TEM TGA UCST USB UV XRD

Chitosan Copper nanoparticles Chemical vapor deposition Diethylenetriamine Dynamic light scattering Deoxyribonucleic acid Doxorubicin Differential scanning calorimetry Fourier transform infrared Gas chromatography – mass spectrometry Graphene oxide High-performance liquid chromatography International business machines corporation Interpenetrating network Lower critical solution temperature Laser Doppler anemometry Multi-walled nanotubes Nanocomposite hydrogels Nanocomposites Nuclear magnetic resonance Nanomaterials Polyacrylic acid Polyacrylamide Personal computers Photon correlation spectroscopy Poly (N-isopropyl acrylamide) Polyvinyl pyrrolidone Reversible addition fragmentation chain transfer Ribonucleic acid Sodium alginate/carboxymethyl bacterial cellulose hydrogels Superabsorbent polymers Scanning electron microscope Starch based hydrogels Spark plasma sintering Surface relief gratings Scanning tunneling microscope Single-walled nanotubes Transmission electron microscope Thermogravimetric analysis Upper critical solution temperature Universal serial bus Ultraviolet X-ray powder diffraction

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Nanotechnology: Origin and Definitions Nanotechnology has given vast perspectives to the researchers worldwide due to its various positive attributes in field of science and technology. This era in which we are living is being declared as “An Era of Nanotechnology.” The chronological series of this technological revolution has become evident which gives an idea about its origin. This technique has been utilized by many human civilizations a long time back. Firstly, nanomaterials were discovered 4500 years ago as asbestos nanofibers for reinforcement of ceramic mixtures. Nanomaterials efficacies were observed by Egyptians over 4000 years ago as they used it for dying hairs and decorative purposes (Li et al. 2017). Thereafter, came Bronze Age where Cu NPs were utilized to make celtic red enamels for decoration. Starting from fourth century up to mid-nineteenth century, Cu-, Ag-, and Au-based nanoparticles were castoff for metallic lustre decorations and ceramic pottery painting. The major commercialization of nanomaterials and nanoparticles occurred in early twenty-first century with the trade names like Silver Nano™. Nanotechnology has grabbed huge attention from the research community due to its recent technological advancements and hence named as tiny science. Earlier, this technique has also been named as “atomically precise technology” (APT). US National Nanotechnology Initiative proposed that “the quintessence of nanotechnology is the ability to work at the molecular level, atom-by-atom, to generate large structures with essentially new molecular association and it is linked with materials and systems whose structures and components exhibit novel and significantly improved physical, chemical, and biological properties, phenomena, and processes due to their nanorange.” Finally, a comprehensive definition of nanotechnology has successfully been given “the implementation of scientific knowledge to measure, create, pattern, manipulate, utilize or incorporate materials and components in the nanoscale.” This technology encompasses other technologies from various fields. Modern nanotechnology truly began in 1981 where scanning tunneling microscope (STM) was introduced to observe individual atoms as the study of these particles was largely hampered due to the paucity of a good microscope that is efficient enough to study these minute particles. The fathers of modern nanotechnology are eminent scientists Heinrich Rohrer and Gerd Binnig for inventing the scanning tunneling microscope (STM) who also got the Nobel Prize in Physics (1986). Sequentially, many Nobel laureates have given their remarkable contribution in this technology. The word “nanotechnology” was itself coined by its foremost father Dr. Norio Taniguchi who was Japanese scientist in 1974 to describe the ultimate limit of atomic bit machining and work on land subsidence in Tokyo. Nanotechnology deals with the study of particles that lie under the nano-range of 1–100 nm with at least one external dimension. These have different physicochemical properties with same composition as known materials in bulk. Therefore, their range can be compared with an example of human hair, that is, approximately considered as 80,000–100,000 nanometers wide. Recent advancements in modern technology have given scientists wide prospects to explore these nanoparticles with

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the help of microscopes and synthesis of large number of products based upon nanotechnology. The U.S. Energy Department has proposed comparison of many objects to estimate how precise a nanometer is in illustration titled “Scale of Things.” The prime factors that differentiate the traits of nanomaterials from the bulk materials are mentioned as the quantum effects with large surface area to volume ratio. These factors are responsible for improvising the properties given as reactivity-, electrical-, and strength-related physiognomies. It has been observed that the atoms that exist inside are less as compared to their presence on surface with large proportion if the particle starts decreasing in size. It is being easily illustrated with an example of a particle size 20 nm that contains 5% of its atoms on its surface, whereas 5 nm contains 50% of its atoms on the surface. Hence, NPs hold a higher surface area per unit volume when compared with bulky sized molecules. The progressive chemical processes that have taken place at surfaces lead towards their feasibility with good reactivity which is further not possible in case of bulk materials. Scrutinizing expenses done on Nanotechnology by the time now through several sectors have raised their utility in materials and manufacturing areas. Nanotechnology-related aspects include several pragmatic findings by emphasizing their fabrication methods. Moreover, many research ideas have been proposed to move the focus from their fabrication methods towards their effective utilization as robust materials, biosensors, and actuators from the past few years. Nanomaterials are generally leading towards the miniaturizations of materials and particles to furbish their properties further. They frequently entail various synthesis methods. Meanwhile, there are numerous pathways to create nanomaterials of different sizes, categorized into two approaches named as “bottom up” and “top down.” These can be prepared from top down method that yields minute structure from bulk constituents by using mechanical grinding. The second method to prepare them is through bottom up method, with self-assembly where the molecules and atoms assemble themselves at molecular and atomic level because of their natural traits and can be attained by colloidal dispersion. Nanotechnology is implemented or utilized one way or the other in almost every field and interest in nanocomposite materials has grown exponentially as shown in Fig. 1. Nanomaterials with tailored properties open a mode for its investigation with their modifications in hydrogels into smart nanocomposites due to their pH- and temperature-sensitive behavior. The common example includes the fabrication of robust and recyclable NCHs that are efficiently scavenging toxic dyes from water hence frequently utilized for environmental remediation. The property of absorbing the toxic dyes has come from their increased pore size without any contortion present in their matrix when synthesized on natural polysaccharides such as cellulose, alginate, and gum xanthan. Over a few decades, the industrial effluents have been increased dramatically due to the commercialization of textile products where the extensive usage of dyes and harmful chemicals has been observed. Earlier, many purification methods such as ion-exchange, adsorption, and membrane separation techniques have been taken into account to rectify the aforementioned problem. Recent scenario presented many evidences where the problem of water contaminants has been addressed to an appropriated level. The main focus is being given on

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Fig. 1 A graphic representation of increasing interest in nanocomposite hydrogels from 2000 to 2020 (Data collected with reference taken from research articles on nanocomposite hydrogels in Science Direct; 2000–2022)

the efficient sorbents with desired properties. Hence, hydrogels with imbibition of nanocomposites such as clay have proved themselves as most suited biosorbents for proficient elimination of pollutants from aquatic land (Ojea-Jimenez et al. 2012). These NCHs have been curtailed from traditional methods in such a way that they can be modified with desired functionalities which further targets the concerned pollutants for environmental remediation. However, from significant modification in the physical properties of the NMs, for instance, variation in permeability, surface morphology, and chemical composition, the desired traits could be attained to enhance these material performances (Campbell et al. 2015).

Hydrogels Hydrogels are the 3D cross-linked polymeric network with a capacity to imbibe water or biological fluids with higher volume within their polymeric matrix (Brannon-peppas and Peppas 1991). Their polymeric networks are basically composed of homopolymers and copolymers. Hydrogels are itself insoluble in water due to the presence of highly cross-linked entanglements lies within their matrix (Sethi et al. 2021). These aquagels with their water retention properties make them swell up to 97% of their original volume. The occurrence of hydrophilic assemblies in their cross-linked chain leads towards the formation of highly porous structures without shackling their structural integrity (Flory and Rehner 1943). Earlier these hydrogels were considered as cross-linked polymers which firstly be used in making of soft

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lens for biomedical use reported in 1960 (Wichterle and Lím 1960). In last few years, modern developments in hydrogel have spurred a lot of improvement through the modification of natural polymers to make tunable hydrogels that are more efficient and biodegradable in nature (Sethi et al. 2022). These hydrogels with enhanced properties have shown remarkable implications in pharmaceutical and biomedical field (Sethi et al. 2020a; Sethi et al. 2019a). Their property of mimicking the living tissues than any other class of synthetic biomaterials makes them biocompatible by nature. Their mystic properties like environment-responsive, pH-responsive, temperature-responsive behavior along with biocompatibility make them the smart, intelligent, and robust materials that can be altered and modified to infix desired properties (Liu et al. 2008). The available literature reports the evidences of evolution of hydrogels to superabsorbents (SAPs) and biomimetic materials (El-husseiny et al. 2022). Hydrogels due to their unique properties have been used in tissue engineering, wound care, cartilage regeneration, controlled release of drugs and agrochemicals, effective removal of industrial effluents (Sethi et al. 2020b), and toxic dyes (Sethi et al. 2019b; Ma et al. 2020; Guo et al. 2021; Fang et al. 2022; Miao et al. 2022). Hydrogels are categorized according to their mechanical structures as affine or phantom networks. The presence of various types of functional groups on hydrogels classifies them as neutral and ionic (cationic and anionic). Moreover, based on the methods of preparation, these can be further categorized as homo-polymer and copolymer. Eventually, the interactions that are present in the networks further can be labeled as super molecular and hydrogen bonded structures with crystallinity present in networks as amorphous and semi-crystalline in nature. These hydrogels are also classified on the basis of their stimuli responsive behavior towards the external environment that ultimately affects their swelling behavior with respect to change in temperature as LCST and UCST, pH, ionic strength, and electromagnetic radiation. Based on their source, these are classified as natural and synthetic-based hydrogels. Based on their degradation, these are further classified as biodegradable and nonbiodegradable (Sharma and Tiwari 2020). Based on the methods of preparation, the covalent crosslinking could be processed via free radical polymerization technique either with the help of cross-linker or by crosslinking of previously present polymer through various paths such as from exposure of irradiation and simple heat treatment. The hydrogels that are interacting via physical forces are usually considered as amorphous networks. These networks are associated together through noncovalent interactions, such as hydrogen bonding and Van der Waals forces. The hydrogels formed with strong chemical interactions show swelling without itself getting dissolved in water, whereas the gels with physical interactions get dissolved in water and even melt down when heated. These hydrogels with thermo-responsive and pH-responsive behavior prompted their successful utilization in biomimetic actuators, immobilized biocatalysts, and targeted drug delivery hence given name as “Smart Materials” by Galaev in 1999 (Galaev and Mattiasson 1999). These hydrogels with smart properties such as pH-responsiveness can change their degree of swelling. Their biodegradable nature has significant importance in pharmaceutical applications along with their

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reasonable mechanical strength. Even for environmental remediation applications, the hydrogel must maintain its structural integrity until unless the system is immersed in water. The mechanical strength basically dependent on the type of crosslinking present in the hydrogels as the slight modifications in their pattern might attain desired mechanical property in the hydrogels. Hydrogels due to their multiresponsive behavior with their extraordinary contributions as actuators, smart sensors, targeted drug delivery, adsorbents, and catalysts have shown certain developments. Hydrogels can be characterized based upon the source of the materials used. Natural polymer-based hydrogels has found more efficient over synthetic polymer due to their good biodegradability and nontoxicity.

Transformation of Hydrogels to Nanocomposite Hydrogels Despite these versatile properties, hydrogels still possess some limitations like having less thermal stability and deprived tensile strength with low strain that further restricts them to be used in various applications. Therefore, many trials have been made to overcome their shortcomings by designing new nanocomposite hydrogels with specific and desired properties. New technologies have been reported in literature to achieve tunable hydrogels in collaboration with nanotechnology. Nanotechnology has shown its applications in almost every field so as the hydrogels. However, the modern researches on hydrogels have produced many instances where nanoparticles are encapsulated in their polymeric matrix to induce excellent functionalities that could further be explored for many applications, for instance, in environmental remediation, biosensing, and successful delivery of drugs. In recent years, the combination of hydrogels with nanotechnology has become a hot-spot for many researchers. Nanocomposite hydrogels (NC gels) are hydrated nanomaterial-incorporated polymeric networks that possess relatively good mechanical strength with higher flexibility than that of traditional gels. A varied series of synthetic and natural originbased nanocomposite hydrogels have been synthesized. The interactions of polymer chains with encapsulated nanoparticles must be controlled to design the physical, chemical, and biological contrived materials. Hence, these nanocomposite hydrogels possess large volume of water and distinctive physical, chemical, and biological characteristics. These nanocomposite hydrogels are assumed as a blessing in environmental remediation due to their bio-sorptivity and reusability. The efficacious incorporation of nanocomposites in polymeric matrix generates the probability of being successfully detoxifying the contaminated water. Nanocomposite hydrogels that are prepared from natural sources have provided an intelligent podium for removal of heavy metal ions and toxic dyes in more eco-friendly and cost-effective manner. The chitosan- and chitin-derivatives-based biosorbents are best known for their effective removal of metal ions such as Pb2+, Cd2+, and Cu2+. These metal ions are very toxic in nature and when gets a chance to accumulate in food chain can destroy many organisms and possess a great danger to human beings. The uptake of these metal ions from the water could only be achieved by synthesizing natural

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origin-based nanocomposites which act as bio-sorbents and more-economical in nature. Earlier, the hydrogels were known for its swelling capacity, but nowadays, more focus has been given to their nanocomposite forms as these are more capable to be coated on the ore and getting cross-linked with metal-complexes that further helps them in effective sorption of metal anions (Bhatnagar and Sillanpää 2009). Many factors have been mentioned in the literature where it has been noticed that the effectiveness of the hydrogels can only be enhanced by modifying their surface with various functionality, which prevents biofouling, and have progressive implications on aquatic life. The CNT-based hydrogels have become the hot topic in environmental technology as many advances have taken place for the removal of chlorine containing compounds and organic stock from aquatic realm due to the high sorption capability and good adsorbate–adsorbent characteristics. It is a kind of symbiosis relationship between hydrogels and nanocomposites as both of them dependent on each other for giving their best results. Their blends make them a perfect candidate for the removal of pollutants and carcinogenic repercussions on commercial scale (Upadhyayula and Gadhamshetty 2010).

Classification of Nanocomposite Hydrogels Nanocomposite Hydrogels Based on Carbon These nanocomposite hydrogels include carbon-based NMs (nanomaterials), that is, graphene, buckminsterfullerene (C60), and carbon nanotubes, investigated and incorporated within the polymeric matrix. These encapsulations further enhance the mechanical strength, mechanical toughness, electrical conductivity, and optical properties of the hydrogels. These mentioned properties make them prominent candidate to be successfully utilized in removal of contaminated dyes from water (Sayed et al. 2022).

Nanocomposite Hydrogels Based on Carbon Nanotubes Carbon nanotubes (CNTs) are basically the unique tubular structures that are fabricated by rolling the graphene sheets which are further embedded in polymeric hydrogel matrix. These nanotubes are used to synthesize hydrogels with their excellent intrinsic properties such as good mechanical and tensile strength. These nanotubes are further of two types: single walled (SWNTs) and multi-walled (MWNTs). These nanotubes within nanocomposite network are modified with the incorporation of hydrophilic groups such as hydroxyl(–OH), carboxylic (-COOH), and amine (-NH2). There are many methods of synthesis of these nanocomposite hydrogels; one of the mostly used methods is by grafting the polymeric chains onto the nanotubes surface or by covalent association of CNTs to the hydrogel matrix to acquaint with multireceptiveness in hydrogels. Many organic pollutants are highly responsible for the environmental contamination by polluting water through the dumping of industrial wastes such as organic dyes. Therefore, the surface modified with polymers-based MWCNTs is being fabricated to deal with these carcinogenic

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effluents. For example, xanthan gum-based MWCNTs were synthesized where the CNTs got trapped in the hydrogel matrices for removal of methylene blue dye. The surface of hydrogel networks is comprised of many hydrophilic groups such as hydroxyl and carboxylic groups, which results in the electrostatic interactions with methylene blue dye. MWCNT used in the polymeric networks has enhanced the surface area with increment in sorption areas. However, the synthesized hydrogels are made up of xanthan gum-based polymeric backbone that is considered an ecofriendly approach (Makhado et al. 2017).

Nanocomposite Hydrogels Based on Graphene Another carbon allotrope is Graphene comprised of a monolayer of atoms organized in honeycomb framework with 2D form. From 2008 onwards, this carbon allotrope and its nanomaterials have largely gain attention for their miraculous use in scavenging toxic metal ions. Graphene’s exclusive structure and its exceptional attributes make it most suited for the environmental remediation. For instance, keratin-based graphene oxide was synthesized due to its robust nature with reusability and costeffectiveness. It is being denoted as smart nanocomposite gels and implied in efficient removal of ciprofloxacin due to its pH-responsive behavior (Ramos et al. 2018). Ciprofloxacin recently has been accounted as toxic pharmaceutical wastes in water discharges. Traditional water management won’t help for the amputation of these pollutants therefore to overcome this problem, PAM-GO-Ag hydrogel composites were fabricated and investigated from gamma ray irradiation method against toxic dyes such as methylene blue (MB) and rhodamine-B (Sivaselvam et al. 2021). Graphene reinforced hydrogels with its oxide have been synthesized from spark plasma sintering (SPS) method (Khoshghadam-Pireyousefan et al. 2020). Consecutively, Abdollahi et al. synthesized GO/PAA-g-amylose nanocomposite hydrogel films from direct solution technique (Barati et al. 2014). These prepared films own a good thermal stability and improved mechanical properties that could further be used as a good adsorbent to remove the effluents. Nanocomposite Hydrogels Based on Carbon Quantum Dots Carbon quantum dots (CQDs) are also categorized as novel carbon nanomaterials made up of distinct and carbon nanoparticles with sizes below 10 nm, given by (Xu et al. 2004) through purification of single-walled carbon nanotubes through preparative electrophoresis. These carbon quantum dots are well-efficient, nontoxic, eco-friendly in nature and can be prepared from different synthetic methods (Sharma et al. 2017d). These CDs are modernized to be utilized in viable economy when combined with hydrogel matrices that endows their implications in many potential applications. Industrialization and population explosion are considered the main cause for the severe water pollution globally which in turns affected the human health and our ecosystem to a larger extent. In concern with this, the CDs are explored in polymeric matrices for their good biocompatibility and effective pollutant detection properties. Their main purpose is to detect the contaminants present in aquatic regions due to

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Polypyrrole ring Ion DNA/CNT

DNA/PPY/CNT hybrid fiber

Polymeric networks Graphene oxide Crosslinker

Polymeric networks Carbon Dots

Graphene oxide interaction with polymeric matrix

Types of Carbon based Nanocomposite hydrogels

CQDs based nanocomposite hydrogels

Crown ether based hydrogels

Fig. 2 Nanocomposite hydrogels based on carbon

their fluorescent quenching properties. This is only possible when there are successful interactions between the contaminant molecules with the functional groups present on the surface of CDs that can further alter the energy transfer pathways. Besides the mentioned implications, these CDs also possess dual applications, for example, CDs when crosslinked with polymeric hydrogels (Fig. 2) like polyvinyl pyrrolidone are used in the removal of toxic dyes as well as their adsorption (Feng et al. 2021).

Nanocomposite Hydrogels Based on Crown-ethers Crown ethers are cyclic chemical compounds that comprise of a ring containing numerous ether groups. Crown ethers intensely bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. Many novel crown ether-based nanocomposite hydrogels were fabricated such as poly (N-isopropyl acrylamide) incorporated with Dibenzo-18-crown-6, 40 -allyldibenzo18-crown-6, CE a crown ether derivative incorporated in poly(N-isopropyl acrylamide) (PNIPA), benzo-18-crown-6-acrylamide (BCAm) into the thermosresponsive poly(N-isopropyl acrylamide) (PNIPAM) hydrogel, novel poly(acrylic acid-co-benzo-18-crown-6-acrylamide) (poly(AAc-co-B18C6Am)) hydrogels through free radical copolymerization. The aforementioned crown ether-based sandwiched compounds were successfully entailed with exceptional adsorption of Cs+ through electrostatic interactions in ratio 2:1, hence, contributing towards detoxification of environment (Sharma et al. 2017a).

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Nanocomposite Hydrogels Based on Polymer Nanocomposite hydrogels are prepared by imbibition of polymers and nanoparticles through chemical and physical interfaces. The polymers involved in fabrication of these hydrogels are copolymers, homopolymers, hybrid polymers, and cross-linked polymers. These nanocomposite hydrogels show dramatic changes in their properties with good flexibility and improved mechanical strength. Literature survey reported many instances where natural polymer-based nanocomposite hydrogels were prioritized over synthetic one due to the toxicity level of the latter and to ensure the biocompatibility, biodegradability, easy synthesis methodology, and more flexible derivatization. Natural polymer-based hydrogels when combined with CDs via electrostatic interactions have emerged out as an interesting research field due to their wide scope with tunable properties such as enormous surface area with abundant groups present in their networks. Agarose-based CDs when fabricated show competent and specific amputation of Pb2+ ions from polluted water which is further considered as a hazardous metal in water, as the Pb2+ concentration has decreased up to 2.39 nmol/L from 100 nmol/L. The same CDs-incorporated NCHs have also be used as a good biosensor due to its capacity of effectively sensing single metal ions such as nitrite, laccase, and reactive oxygen species (Sui et al. 2019). Starch-based hydrogels are also dominating nowadays because of their costeffectiveness and eco-friendly nature. Their structural modifications have opened up a new horizon for the synthesis of NCHs.

Natural Polymer-Based Nanocomposite Hydrogels Natural polymer-based nanocomposite hydrogels are those hydrogels, which are procured from natural sources such as plant extract. These have grabbed substantial attention from the researcher community worldwide for their potential applications in environmental remediation such as removal of toxic dyes and acid from contaminated water (Zinatloo-ajabshir et al. 2022). Polysaccharide-Based Nanocomposite Hydrogels Nowadays, polysaccharide-based nanocomposite hydrogels are highly in demand among all natural polymers. Although it is readily available and can be easily fabricated, this makes them an ideal candidate for the preparation of nanocomposite hydrogels. Sharma et al. (2017c) reported an in situ eco-friendly novel synthesis of gum tragacanth-based hydrogel nanocomposite, which was proved to be cheapest natural backbone among all polysaccharides and was found very effective in MG dye removal. GumT-cl-HEMA/TiO2 hydrogel composite has proved efficient sustainable materials, which maintain adsorption capability in each reusable cycle. The estimated adsorption % for GumT-cl-HEMA/TiO2 hydrogel composite was 99.3% and 94.8% in first and third cycle, respectively. In sequestration, another preparation of silverincorporated nanocomposite hydrogels based on gum acacia-poly (acrylamide-IPNacrylic acid) was reported by Sharma et al. (2020) to study their antimicrobial applications. They also mentioned chitosan, alginate, konjac glucomannan-based hydrogels surveyed for successful adsorption properties with enhanced surface areas.

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Starch-Based Nanocomposite Hydrogels These hydrogels emerged out as a “rising star” for environmental remediation. Literature survey reported many future prospects that are multidirectional for the starch-based hydrogels synthesis after seeing its remarkable properties. MahmoodiBabolan et al. (2018) synthesized starch-g-(acrylic acid-co-acrylamide) superabsorbent via solution polymerization method for the effective removal of methylene blue. These starches can be modified to enhance its mechanical strength and viscosity from various modification practices that further involve physical methods as annealing, ultrasonication, mechanical milling, heat moisture treatment, and chemical methods as hydropropylation, esterification, and acid hydrolysis (Singla et al. 2020).

Synthetic Polymer-Based Nanocomposite Hydrogels Synthetic polymers such as poly (ethylene glycol), poly(vinyl alcohol), poly (acrylamide), poly(hydroxyl ethyl methacrylate)-based hydrogels were synthesized by photo polymerization and copolymerization (Fig. 4) as these possess good chemical and swelling properties but lacking the sorption properties due to surface ratio and sorption properties (Li et al. 2018; Su and Chen 2018). To deal with these limitations, the synthetic polymers have been used as blends/hybrids with natural backbones such as cellulose, starch, and gums. However, various nanocomposites, for instance, silicate clay, attapulgite, mica, kaolinite, laponite, and hydroxyapatite, were incorporated inside hydrogel matrices to improve its reusability and biocompatibility. These polymer blends attain very good properties along with their cost effective and facile synthesis techniques (Khan and Khan 2021).

Based on Metal and Metal Oxide Recent trends in the synthesis of metal-incorporated nanocomposite hydrogels idealized them as the best materials to be used in environment remediation with enhanced optical, catalytic, and electrical properties. Mahmoud et al. (2021) synthesized starchgelatin hydrogels doped with ferrite@biochar@molybdenum oxide for effective removal of Pb2+ ions, which makes it highly proficient nanocomposite. The most important aspect is to develop mechanically strong nanocomposite hydrogels with the inculcation of metal frameworks within their matrix along with the furbished physicochemical properties (Fig. 3). These hydrogels are synthesized by various synthetic and greener approaches. For example, transition metal dichalcogenide/graphene nanocomposites were fabricated from solvothermal, hydrothermal, microwave-assisted, and CVD methods (Wu et al. 2016). Green synthesis of the core-shell hydrogels were also reported such as 3D graphene hydrogels that were co-doped with sulfur and nitrogen and utilized for efficient adsorption of toxic dyes such as malachite green, crystal violet and methylene blue (Li et al. 2012). Their structurally diverse characteristics make them useful in biomedical applications. The incorporation of various metals like silver, gold, copper, and their respective oxides in their polymeric networks

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Fig. 3 Nanocomposite hydrogels based on organic/inorganic frameworks

is also widely explored. These metals are majorly governed by the properties such as antimicrobial, anti-inflammatory, and nontoxicity. Gold-based NCHs prepared by various synthetic methods have been used for the addition of Au-NPs into the hydrogel matrix. Au-NPs-based composite hydrogels display high electrochemical and thermos-responsive properties and can be developed as a photo-responsive hydrogel for effective photocatalytic degradation of toxic dyes. Similarly, Ag-NPs-based composite hydrogels are also considered vital due to their nontoxicity and easy production methods. For example, poly(N-vinyl-2pyrrolidone) (PVP)/Ag nanocomposite hydrogel established improvised mechanical properties and high elasticity with high sorption properties. Hence, studies have deliberated gold- and silver-based nanocomposite hydrogels, although there are numerous other metals such as copper, nickel, and cobalt-based nanocomposite hydrogels. Copper nanoparticles (Cu-NPs) have good electrical and thermal conductivity with easy availability as compared to gold and silver. Recent researches have presented many instances where inorganic nanoparticles-based hydrogels are fabricated due to their chemical and mechanical stabilities with good porous networks specifically the clays (Lin et al. 2018). In spite of the information that the superabsorbent hydrogels were sufficient with their blending combination to attain good properties, still some inadequacies have been observed that limit these hydrogels to certain applications. In order to enhance the mechanical performance of these hydrogels, inorganic frameworks have been synthesized. MnO2 is introduced in starch network to explore its applications in textile engineering and water treatment (Sarah Faheem et al. 2021). Silica-based nanoparticles were fabricated by free radical polymerization method after mixing it with polymers such as chitosan and polyvinyl alcohol (da Costa Neto et al. 2014; Li et al. 2010).

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Methods of Synthesis of Nanocomposite Hydrogels Mostly, nanocomposite hydrogels can be prepared from either synthetic polymers or natural polymers (Fig. 4). The synthetic polymers when incorporated with mineral powders display good attributes with lower production cost and low toxicity. The NCHs were also fabricated from HEMA, 2-acrylamido-2-methylpropane sulfonic acid monomers, and montmorillonite. These formed NCHs show their contribution

Fig. 4 Methods of synthesis of Nanocomposite hydrogels (a) Sol – gel method (b) Grafting method (c) Radiation assisted synthesis

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in removal of water contaminants that accumulated from food and leather industries, textile, paper, and plastic wastes. Nanocomposite hydrogels can be prepared by cross-linking of polymer chains using gamma-radiations, microwave-assisted, electrostatics, and crystalline formation. The major four components for the synthesis of nanocomposite hydrogels are monomer, cross-linker, polymer, and initiator. Nanocomposite hydrogels can be synthesized by the combination of these natural and synthetic monomers in the form of hybrid and blends for the removal of environment contaminants. Generally, the synthesis of encapsulated nanocomposite hydrogels has been started a decade ago through solvothermal method. Earlier, silica nanoparticles were being incorporated in polymeric matrix by sol-gel synthesis (Bortolin et al. 2013). This synthesis in recent advancements has introduced various innovative methods for their fabrication. Natural polymers such as polysaccharides and alginate were used as a template for their synthesis with the sol-gel process by using novel precursors (Hernández-gonzález et al. 2020). Hydrogel-clay composites were synthesized via direct co-polymerization. Nanocomposite basically can be created from organoclays and clay by adopting various methodologies that include latex, solution, in situ polymerization, and most frequently melt-processing method. This method is cost-effective and suppler for preparation and involves compounding and production facilities for commercial practice (Flory and Rehner 1943). Weian et al. (2005) prepared novel polymeric hydrogel nanocomposites which are made up of poly (acrylic acid) (PAA) and organophillic clay in which clay is considered as a crosslinking agent. In situ freeradical polymerization of acrylamides with diffusion of clay platelets dispersed uniformly in aqueous medium was also reviewed by Haraguchi (2007) that has been reflected among major breakthroughs for the creation of NCHs. Recently, many facile strategies to synthesize these nanocomposite hydrogels have become the center of attraction due to their versatile properties. Among them the popular one is free-radical copolymerization and the incorporation of various nanoparticles onto their conventional networks (Thomas et al. 2007). The various approaches for the synthesis of these NCHs are discussed in the following sections.

Synthesis of Nanocomposite Hydrogels Using Sol-Gel Method Sol-gel method is the well-known synthetic method to formulate superior nanocomposites on polymeric networks. This method has brilliant control over the surface properties of the materials and also has its own significances, for instance, its low processing temperature, homogeneity of the acquired material, and creation of the complex structures or composite materials that make this process more diverse and efficient than their traditional synthesis methods. Overall, sol-gel method can be attained in five main steps: hydrolysis, poly-condensation, aging, drying, and thermal decomposition. Recently, Parashar et al. (2020) reported the synthesis of various metal oxides-incorporated nanoparticles via sol-gel method. They synthesized zinc, tungsten, tin, and titanium oxide in order to attain desired traits. This method includes different structural modifications such as core-shell structure, sandwiched

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complexes, and intercalated forms. Composites in the form of core-shell agglomerates have also been prepared in the presence of amylose by adding small amount of precursor via sol-gel method (Wu et al. 2022). Chibac et al. (2012) fabricated photocrosslinked sol-gel hybrid composites containing silver nanoparticles in urethaneacrylic matrixes via one-step synthesis followed by sol-gel method. Sequentially, Fang et al. (2022) synthesized injectable sodium alginate/ carboxymethyl bacterial cellulose hydrogels (SA-CMBC) via physical cross-linking under regular conditions. They have formed dual-network structure with carboxymethyl cellulose and nanoparticles of hydroxyapatite and calcium carbonate with improved mechanical strength followed by the regulation of sol-gel transition that further facilitates these gels’ degradation behavior. In order to formulate good mechanical strength and tunable properties, hybrid hydrogels are also used in this method, for example, in situ synthesis of silica-alginate hybrid hydrogels by a sol-gel route with the addition of catalyst/water/inorganic precursor.

Radiation-Assisted Synthesis of Nanocomposite Hydrogels Radiation technology has extensively been established as a simple, rapid, green, and sustainable technology with macroscale applications in healthcare, industry, and environment. Ionizing radiation has beneficial effects for the synthesis and modification of structure and properties of nanocomposites.

Microwave-Assisted Synthesis Many facile syntheses were reported based on microwave-assisted irradiation where silver nanoparticles were incorporated for the removal of contaminants and toxic dyes on natural backbones. Bardajee et al. (2017) prepared silver nanocomposite hydrogels with the help of microwave irradiation for the removal of methyl red. Graphene oxide combined with diethylenetriamine (GO-DETA) and the hybrid of graphene oxide-diethylenetriamine and polyacrylamide (GO-DETA/PAM) hydrogel with maximum adsorption of methylene blue were prepared using microwaveassisted methods (Viana et al. 2020). Greener approaches, such as kappaCarrageenan (κ-Carrageenan/CRG), were adopted and were used to articulate CRG-silver nanocomposites from a microwave-assisted method (Goel et al. 2019). Gamma-Ray Irradiation Synthesis Gamma-ray irradiation method has now become a very prevalent method in the synthesis of nanocomposite hydrogels. One-pot rapid synthesis of PAM-GO-Ag nanocomposite hydrogel was reported from irradiation of gamma-ray for remediation of environment pollutants (Sivaselvam et al. 2021). Gamma-irradiation is mainly used on hybrid/blends to enhance hydrogel properties for its further utilization in biological applications, for instance, polyvinyl alcohol/chitosan/AgNO3/ vitamin E membranes are also being utilized (Nasef et al. 2019). Magnetic NVP/CS nanocomposite hydrogels synthesized using gamma radiations are found to be useful in effective removal of toxic dyes. These radiations are also used on dual

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network alginate/polyacrylamide-based hydrogels to furbish their mechanical strength (Lin et al. 2018). There should be appropriate dose of gamma irradiation to achieve homogeneous nanocomposite hydrogels.

Ultrasonic-Assisted Synthesis Ultrasonic-assisted synthesis is used to make the molecules undergo chemical reactions with powerful ultrasound radiation (20 kHz–10 MHz), which causes the modification of nanostructures in the sonochemical process. For example, polyacrylamide/bentonite hydrogel nanocomposite was prepared by using ultrasonic radiation which is being further used for confiscation of lead and cadmium from aqueous phase (Khan and Khan 2021). They have found clay-hydrogel nanocomposite as an effective material for extenuating the pollution and environmental impact due to their good adsorption capacity. The synthesis of these nanocomposite hydrogels was assisted by ultrasound, and nanobentonite was successfully incorporated as a crosslinker and filler into polyacrylamide framework. The preparation of nanocomposite hydrogels was also done with the combination of nanocomposite suspension in hydrogel matrices followed by the addition of cross linker and initiator via UV irradiation (Wu et al. 2016).

Free-Radical Co-polymerization It is a method of polymerization, by which a polymer forms by the successive addition of free-radical building blocks that are generated in chemical reaction. Further, it includes three major steps: initiation, propagation, and termination. Nowadays, greener synthesis is in vogue due to its eco-friendly approach with easy fabrication methods. Initiation includes the generation of free radicals via photochemical and thermal decomposition of the starting compounds. Propagation includes the progress of the polymer series by consecutive addition of monomers. This chain will keep on growing until the monomers used are not consumed fully. Therefore, the termination process will terminate the chain due to either disproportionation or combination. Many novel semi-IPNs and IPNs were synthesized recently based on doublenetwork hydrogel through free-radical polymerization and ion-crosslinking process (Shen et al. 2020). This polymerization method is usually considered very economical and traditional methods among all the given methods for the synthesis of nanocomposite hydrogels and smartly utilized by the research community.

Grafting Graft polymerization is a process in which monomers are covalently bonded and polymerized as side chains onto the main polymer chain (the backbone). Grafting is a striking method to impart a variety of functional groups to a polymeric matrix by using two or more monomers in the same polymer that further strengthens the

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network and modifies the surface. In case of nanocomposite hydrogels, these are basically prepared by simple grafting on the natural backbone. These hydrogels can be synthesized in membrane form also, for example; PVA/starch membrane is prepared by mixing their aqueous solution with plasticizer (glycerine), and this membrane is then allowed to get mixed with AgNPs with different concentrations and sonicated for 15 minutes (Batool et al. 2019).

Characterization of Nanocomposite Hydrogels There are different techniques used to determine the surface morphology and the chemical structure of natural polymer-based nanocomposite hydrogels (Table 1). The formation of cross-linked structures in hydrogels is determined by nuclear magnetic resonance (NMR), Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM), etc.

Applications of Natural Polymer-Based Nanocomposite Hydrogels in Environmental Remediation Due to industrialization, the release of pollutants into the environment has become a global issue. These pollutants cannot degrade by themselves and cause toxicity to the natural bodies including soil, water, and air. Based on their chemical nature, the environmental pollutants are classified as organic and inorganic. Organic pollutants are generally organic compounds originated from industrial waste including textile dyes, pesticides, poly-aromatic hydrocarbons, halogen compounds, while inorganic pollutants comprise mainly of heavy metals (Guclu et al. 2010). Various chemical and biological treatments have been utilized for the removal of these pollutants from the water bodies. Among all the techniques used for remediation (precipitation, adsorption, membrane filtration, coagulation, flocculation, nanotechnology treatment, and electrochemical processes), the adsorption and photocatalytic degradation are the most common methods for purification of water. Generally, the inorganic pollutants are removed via adsorption, whereas the organic pollutants due to their large size are purified by catalytic degradation (Özkahraman et al. 2011). Nanocomposite hydrogels synthesized using different natural polymers and nanoparticles could play a major role in environmental remediation via adsorption, degradation, and ion exchange due to their advanced characteristics. Table 2 describes the natural polymer and nanoparticles used for the synthesis of nanocomposite hydrogels along with their synthesis technique and application.

Adsorption Adsorption is the most common technique for effluent purification due to its high efficiency and low cost. Thus, several adsorbents have been used for water

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Table 1 Characterization techniques of nanocomposite hydrogels and their uses Sr. No. 1.

Technique Scanning electron microscopy (SEM)

2.

Fourier transform infrared spectroscopy (FTIR)

3.

Thermo gravimetric analysis (TGA)

4.

X-ray diffraction (XRD)

5.

Transmission electron microscopy (TEM-SAD)

6.

Raman spectroscopy

7.

Brunauer–Emmett–Teller analysis (BET)

8.

Ultraviolet-visible spectroscopy (UV-Visible)

9.

Nuclear magnetic resonance (NMR)

10.

Confocal laser scanning microscopy (CLSM)

11.

Laser Doppler anemometry (LDA)

12.

Zeta Potential analysis

13.

Optical microscopy

Purpose of technique The technique is used to analyze the surface topography as well as performances of the synthesized nanocomposite hydrogels. The technique is used to analyze functional groups present in nanocomposites. The technique is used to measure the thermal properties of sample through various degradation stages with the glass transition temperature. X-ray diffraction studies are there to depict the crystallinity and amorphous nature of the sample. It is used to study the particle size, shape, and morphology of nanocomposite hydrogels. It is used to identify the amorphous or crystalline regions and defects present in the nanocomposite hydrogels. It is also used to identify the size, phases, and phase transitions in hydrogels. The surface area and porosity of nanocomposite hydrogels is measured with BET analysis. Ultraviolet-visible spectroscopy is used to study the concentration of loaded drugs released from the hydrogels and it also used for analyzing the stability nanocomposite hydrogels. The structure of nanocomposite hydrogels is analyzed by using nuclear magnetic resonance technique. Confocal laser scanning microscopy technique is used for analyzing uptake and release of loaded substances by the nanocomposite hydrogel Laser Doppler anemometry technique is used for measuring the fluid velocities of nanocomposite hydrogels. It is used for evaluating the surface charge of nanocomposite hydrogels. The size determination of nanocomposite hydrogels along with nanometer-scale sensitivity is analyzed by optical microscopy.

References Naushad et al. (2019)

Hajikarimi and Sadeghi (2020) Fang et al. (2022)

Fosso-Kankeu et al. (2016) Zhai et al. (2018) Makhado et al. (2017)

Mallakpour et al. (2018) Allahbakhsh and Bahramian (2018) Krakovský et al. (2020) Yang et al. (2022)

Fischer et al. (2010) Bozoğlan et al. (2020) Rückmann et al. (2022)

(continued)

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Table 1 (continued) Sr. No. 14.

15.

Technique Fluorescence spectrometry

Dynamic light scattering (DLS)

Purpose of technique The technique is used to measure the concentration of loaded substance (e.g., drug), uptake and release from the hydrogels. The technique is used to calculate the particle size, particle molecular weight, size distribution, and phase behavior of nanocomposite hydrogels.

References Bhatt et al. (2016)

Ali et al. (2022)

purification. Among all the adsorbents, the nanocomposite hydrogels gained more interest for the treatment of heavy metal ions and dyes from industrial waste. Hydrogels can swell in water due to their porous structure; hence, they can easily achieve good adsorption capacity (Fig. 5). The nanoparticles increase the mechanical strength of the hydrogel and improve their adsorption capacity. The presence of nanoparticles provides a large surface area for maximum adsorption and hence shows higher efficiency towards the removal of waste (Mahdavinia et al. 2012). Various natural polymers such as sodium alginate, starch, chitosan, dextran, cellulose, and gums have been used for the synthesis of nanocomposite hydrogels because of their multifunctionality, abundance, and sustainable approach. Also, the adsorption capacities of natural polymer-based hydrogels can be altered up to certain limits via suitable functionalization (Sethi et al. 2019b). The various factors including electrostatic interaction, hydrogen bonding, surface functionality, and Van der Walls forces are responsible for the adsorption of dyes and heavy metals on the nanocomposite hydrogels. Due to the presence of various cationic (-NH3+) and anionic (-COO, -SO42) functional groups, the hydrogel shows improved adsorption selectivity toward dyes and heavy metal ions (Wang et al. 2013).

Degradation The degradation process is also highly useful for the removal of industrial waste. Due to mild reaction conditions and low energy consumption, photocatalytic degradation is considered as a highly efficient method. Various catalysts of the nanometric range are widely used for degradation purposes due to their reduction ability. But these nanoparticles have some limitations such as small size, tough handling, and aggregation. To overcome these difficulties, nanocomposite hydrogels gained more interest in wastewater treatment. The polymeric matrix of hydrogel supports the nanoparticles and acts as a promising stabilizer for nanoparticles. The presence of polar groups including -OH, -COOH, and –NH2 on hydrogel enhances photocatalytic activity. It provides a platform for catalytic centers for the conversion of solar energy (Sharma et al. 2017b). The UV rays and solar radiation help in the generation of electron hole pairs onto nanocomposite hydrogels and are responsible

Kappacarrageenan Tamarind Kappacarrageenan Cellulose

3.

14.

13.

12.

11.

10.

9.

8.

7.

6.

Kappacarrageenan Xanthan gum Kappacarrageenan Starch

Sodium alginate Wheat bran

Sodium alginate Starch

Cellulose

2.

4. 5.

Natural polymer Starch

Sr. No. 1.

Fe2O3

Fe3O4

Silica

Montmorillonite

Clinoptilolite

Graphene oxide

SiO2 coated Fe3O4

Illite/smectite

Montmorillonite

Silica Sepiolite

Laponite

Montmorillonite

Nano particles Montmorillonite

In situ copolymerization Copolymerization

Free radical polymerization In situ intercalated polymerization Solution copolymerization Graft polymerization

Graft polymerization

Free radical polymerization Graft polymerization

Solution polymerization Graft polymerization Graft polymerization

Graft polymerization

Synthesis methods Graft polymerization

Removal of Pb(II), Ni(II), and Cu(II) metal ions from industrial wastewater

Removal of methylene blue and methyl violet dyes Adsorption of cationic dye crystal violet

Adsorption of methylene blue

Removal of heavy metal cations

Removal of crystal violet dye from aqueous solutions Dye adsorption

Adsorption of methylene blue

Slow release of fertilizers

Adsorption of methylene blue dye Adsorption of cationic dye

Purpose Removal of Cu2+ and Pb2+ ions from aqueous solutions Removal of Cu2+ and Pb2+ ions from aqueous solutions Removal of cationic dye

Table 2 Various natural polymer-based nanocomposite hydrogels for environmental remediation

Mahdavinia et al. (2014b) Alimardan and Darabi (2015)

Mahdavinia et al. (2014a) Ghorai et al. (2014)

Barati et al. (2014)

Pourjavadi et al. (2013) Fan et al. (2013)

Özkahraman et al. (2011) Mahdavinia et al. (2012) Pal et al. (2012) Mahdavinia and Asgari (2013) Bortolin et al. (2013) Wang et al. (2013)

References Guclu et al. (2010)

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Gum karaya Xanthan gum Starch

Tragacanth gum Sodium alginate Cellulose

Gelatin

Gum arabic

Sodium alginate Sodium alginate Pullulan

Chitosan

Starch

Cellulose

16.

19.

21.

22.

23.

24.

26.

27.

28.

29.

25.

20.

18.

17.

Starch

15.

Clinoptilolite

NiFe2O4 and TiO2

TiO2

Montmorillonite

Montmorillonite

Montmorillonite

Ni(OH)2 and FeOOH

Silver

Cloisite

Attapulgite

Calcium carbonate

Fe/Zn

Fe3O4

Fe3O4

SiO2-coated Fe3O4

Graft copolymerization Copolymerization

Free radical crosslink polymerization Chemical polymerization Chemical polymerization Copolymerization

Free radical polymerization Free radical polymerization Free radical polymerization Copolymerization

Copolymerization

Free radical graft copolymerization Crosslink polymerization Polymerization

Graft polymerization

Adsorption and photodegradation of Cr(VI) and Congo red dye Desorption of Diquat Herbicide

Removal of methylene blue dye

Removal of crystal violet dye

Sustained release of pesticides

Sustained release of cyhalothrin (pesticide)

Photodegradation of methylene blue

Adsorptive removal of auramine-O from wastewater Removal of Cu(II) metal ions

Adsorption of metal lead ions

Adsorption of methyl violet from aqueous solution Photocatalytic degradation of a mixture of malachite green and fast green dye Removal of Pb2+ heavy metal

Adsorption of crystal violet from aqueous solution Removal of heavy metal ions from mine effluent

Natural Polymer-Based Nanocomposite Hydrogels as Environmental. . . (continued)

Mahmoud et al. (2020) El-saied and El-Fawal (2021) Tanaka et al. (2021)

Su et al. (2019)

He et al. (2019)

Abdolhosseinzadeh et al. (2018) Dil and Sadeghi (2018) Naushad et al. (2019) Wang et al. (2019)

Sharma et al. (2017b) Mallakpour et al. (2018) Mao et al. (2018)

Pourjavadi et al. (2015) Fosso-Kankeu et al. (2016) Mittal et al. (2016)

19 429

Chitosan

Sodium alginate

32.

33.

31.

Natural polymer Sodium alginate Chitosan

Sr. No. 30.

Table 2 (continued)

Biochar

Graphene oxide

Nano particles Polyaniline-polypyrrole modified graphene oxide Fe3O4 Chemical polymerization Free radical polymerization

Synthesis methods In situ copolymerization Copolymerization

Abou Taleb et al. (2021) Sarah Faheem et al. (2021) Wu et al. (2022)

Removal of methyl orange from wastewater

Water treatment

Removal of Cu2+, Zn2+, and Ni2+ ions

References Zhang et al. (2021)

Purpose Removal of Cr(VI) and Cu(II) from water

430 S. Sethi et al.

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Fig. 5 Mechanism of adsorption of dye

for the degradation of various types of dyes and removal of heavy metal ions. On irradiation, the free electrons of nanoparticles excite from valence band to conduction band and help in generation of electron hole pairs onto the nanocomposite hydrogel. The overall oscillations of electrons result in the generation of free radicals. These generated free radicals on nanocomposite hydrogel act as an oxidizing agent and help in the degradation of dyes and production of nonhazardous side products (Fig. 6). As mentioned above, the organic pollutants are difficult to remove via adsorption. The nanocomposite hydrogels also help in the removal of organic pollutants via catalytic degradation and convert them into carbon dioxide and water (El-saied and El-Fawal 2021).

Ion Exchange The ion exchange method is the easiest and effectual technique for the treatment of industrial effluents containing heavy metal ions that are toxic in nature. A large number of organic and inorganic ion exchangers are available, but they have many limitations. The organic ion exchangers show poor stability, whereas the handling of inorganic ion exchangers is tough so both kinds of ion exchangers cannot be used on large scale. Hence, a new class of nanocomposite hydrogel ion exchangers has come into existence. Nanocomposite ion exchangers possess various functional groups that are capable of binding ions of opposite charge (Barati et al. 2014). In acidic

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Fig. 6 Mechanism of photocatalytic degradation of dye

conditions, the primary amine groups of the ion exchanger become positively charged due to protonation and converted into –NH3+ ions. It helps in the formation of ionic bonds between sulfonate groups of anionic reactive dyes and the nanocomposite. Similarly, in basic conditions, the anionic groups of ion exchanger being negatively charged help in the removal of cationic dyes (Fig. 7). These are selective in nature and provide specific functionalities and their large scale production is easy. They possess excellent ability to remove colored impurity from industrial waste, reducing the number of organics up to permissible levels and helping in metal recovery and water softening (Alimardan and Darabi 2015).

Soil Conditioning Due to water scarcity, the whole agricultural land in the world suffers a lot. The change in climate and uncivilized human activities are responsible for arid, semiarid, and dry land. These difficulties can be overcome with utilization of polymeric nanocomposite hydrogels. These provide good water retention capacity and help in increasing the productivity of crops. The nutrients are loaded onto the nanocomposite hydrogel and are released into the soil (Fig. 8). The nanoparticles increase the mechanical strength of the hydrogel and provide a large surface area for

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Fig. 7 Types of ion exchanging hydrogel (a) anion exchange hydrogel (b) cation exchange hydrogel

adsorption of nutrients and hence show higher efficiency towards crop production. The major essential nutrients for agricultural soil are boron, nitrogen, phosphorous, sulfur, potassium, calcium, iron, and copper. Most of the nitrogen and potassium are lost by leaching; hence, those remain unabsorbed by the roots of plant (Bortolin et al. 2013). To overcome this problem, the nanocomposite hydrogels have been widely used as controlled release devices in soils. Nanocomposite hydrogels help in improving the efficiency of nutrients, fungicides, herbicides, and insecticides as well as help in lowering toxicity, cost, and pollution. The release process from

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Fig. 8 Mechanism of release of nutrients from hydrogel

hydrogels is mainly divided into three concentration gradient steps: (i) initial stage without any nutrient release because during this stage penetration of water molecule via porous structure of hydrogel (diffusion) takes place, (ii) the controlled release from hydrogel via convective flow, and (iii) gradual decay of release happens due to macromolecular chain relaxation via swelling/deswelling process of polymeric hydrogels (Wang et al. 2019).

Conclusion and Future Outlook Nanocomposite hydrogels based on natural backbone have achieved a consumable attention from the research community to address many environmental issues. The synergistic traits of these nanocomposites with hydrogels acquired many innovative and facile methods for their synthesis. Therefore, the structural modification in conventional hydrogels with nanocomposites has opened up a new spectrum for various applications. Their stimuli-responsive behavior makes them smart and intelligent materials to be used in widespread applications. However, the prime importance of this chapter is that it includes the classification, methods of fabrication, characterization techniques, and their potential utility as good soil conditioners, ion-exchanger, and in photocatalytic degradation of dye. This chapter also presents the most recent advancements in polymer interfaces with nanocomposite hydrogels to appreciate its inherent performance. The applications are not only limited to

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pharmaceutical areas but also be explored in agriculture, in electronics as supercapacitors, in bio-sensing areas, and in removal of industrial effluents. Basically, addressing the environmental issues, their structural modifications lead to inculcate desired traits in them. Accordingly, these can be customized and utilized in many potential applications. But in concern with the toxicity level, the various optimization methods for their degradation and fabrication techniques could be explored as a future aspect for further research. Natural polymer-based nanocomposites are preferred over synthetic ones due to toxicity level and hence attenuate many pollutionrelated problems. These natural backbones-based nanocomposites are well known for their biodegradability due to the degradation achieved by the large number of microorganisms present in them. Their sustainable and tuneable photochemical properties make them the versatile candidate in water treatment and removal of organic pollutants from wastewater with its reusability and biodegradability. A vast spectrum of areas has already been explored for their successful usage by introducing new variety of materials after their modifications. These areas include automobile industry, aeronautics, packaging, electrical appliances, and domestic goods. Moreover, after alteration in their respective properties, blend nanocomposites are still a matter of concern due to their laboratory scale preparations. Therefore, more emphasis is necessary to reconnoitre and endorse the progress of these nanocomposites in many areas.

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Biogenic Metallic Nanoparticles: Synthesis and Applications Using Medicinal Plants

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Amanpreet Kaur, Himanshu Gupta, and Soniya Dhiman

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biogenic Synthesis of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles Using Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles Using Fungus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles Using Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles Using Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Green Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of NPs in Diagnostics and Drug Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of NPs in Food Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nanoparticles in Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles as Fungicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles as Fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles as Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nanoparticles in Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the last decade, the synthesis of nanoparticles is one of the most concerned fields in research due to their high applicability in various segments of science and technology, ranging from material science to biotechnology. The environmentalists have grown interests in the preparation of nanoparticles from the point of view of biological and environmental safety. As the physicochemical production of nanoparticles requires an extreme environments and toxic chemicals in a A. Kaur · H. Gupta (*) Department of Chemistry, School of Sciences, IFTM University, Moradabad, Uttar Pradesh, India S. Dhiman Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology Delhi, New Delhi, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_101

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large quantity, green methods of nanoparticle synthesis employing biological sources are in vogue. Green fabrications of metallic nanoparticles are growing rapidly due to their eco-friendly nature and low cost. In the past decade, the field of nanobiotechnology has been employed rigorously for biosynthesis of metallic nanoparticles. Due to their spectacular biophysical properties and enrichment in biocompatibility, the metallic nanoparticles have significant impact on the food processing, biomedical, environmental, agricultural, and industrial areas. Due to availability of various biologically active compounds such as phenolic acids, saponins, tannins, terpenoids, flavonoids, and alkaloids, plants have promising medicinal significance. Medicinal plant extracts were widely utilized for green synthesis of metallic nanoparticles as stabilizing agents. In order to synthesize biomolecule-encapsulated metallic nanoparticles, metallic ions are reduced by phytochemicals present in plant extracts. The growth of multiple drug-resistant bacteria may be inhibited by biogenic metallic nanoparticles. The present chapter provides information on highly stable, biocompatible, environment friendly, and cost-effective approach for metallic nanoparticles synthesis using diverse medicinal plants and their applications as a powerful nanomedicine against multidrugresistant pathogens. Keywords

Nanomaterials · Green synthesis · Plant extract · Biogenicnanoparticles · Nanomedicine

Introduction Nanoscience and nanotechnology research and development have progressed at an unparalleled rate in recent years. Nanotechnology is a branch of study that has only been around for a 100 years. Since Nobel Laureate Richard P. Feynman represented “nanotechnology” during the famous 1959 lecture “There’s Plenty of Room at the Bottom,” there have been numerous current breakthroughs in the field of nanotechnology. There is increasing anticipation that nanotechnology, when used in medicine, will lead to significant improvements in the disease detection and treatment. At the nanoscale, nanotechnology created a variety of extensive materials which possess at least one dimension hematite > maghemite > lepidocrite. First-order rate kinetics were observed for the degradation of pyrene. It was also observed that iron oxides along with oxalic acid may initiate photo-Fenton-like mechanism under UV irradiation in the absence of additional H2O2. The degradation was also studied in red soil contaminated with 09 h PAHs using α-FeOOH along with oxalic acid for a

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Table 2 Metal oxides for decontamination of PAHs-contaminated soils Nonmaterial TiO2

Target pollutants PYR

TiO2

PHE, PYR, and B[a]P

Fe3O4

PYR

Rutile TiO2 and anatase TiO2

PHE and PYR

Iron oxides

B[a]P

Iron oxide

PYR

Akaganeite

PHE

Fe3O4 and α-FeOOH

ANT

Iron nano-oxides

PYR

Photo-Fenton catalyst (α-FeOOH)

PAHs

Remarks Occurrence of hydroxylation, ketolysis, and ring-open reactions for the formation of intermediates. The solution pH had little effect on photodegradation rate of pyrene Phenanthrene, pyrene, and B[a]P were degraded on soil surfaces in the presence of TiO2 under UV irradiation The rate of photodegradation of pyrene is in the order: goethite > hematite > maghemite > lepidocrite. First-order rate kinetics was observed for the degradation of pyrene. It was also observed that iron oxides along with oxalic acid may set up photo-Fenton-like mechanism under UV irradiation in the absence of additional H2O2 In both cases, photodegradation rate followed pseudo-first-order rate kinetics and the rate increased with the increasing concentration of H2O2 and humic acid and intensity of light The results suggested that degradation of BaP was enhanced due to goethite in all soils. The addition of oxalic acid further increased the decay rate due to involvement of Fenton-like mechanism without H2O2 The results suggested that the PAH pyrene in soils was successfully decayed in the presence of goethite NPs into smaller fragments, which are nontoxic in nature The rate constant for phenanthrene degradation was 6  10 3 h 1 with nano-rods of akaganeite (4%). The possible decay profiles for phenanthrene degradation in soils were also demonstrated The degradation of anthracene was faster using α-FeOOH compared to Fe3O4 After 6 h of Fenton’s process, 93% pyrene decay was reported After irradiation of 480 μW cm 2 UV light for 12 h, various PAHs degradation was in the range 12–22%

References Wen et al. (2003)

Zhang et al. (2008) Wang et al. (2009)

Dong et al. (2010a, b)

Gupta and Gupta (2015a)

Gupta and Gupta (2015b)

Gupta (2016)

Gupta et al. (2017) Jorfi et al. (2013) Wang et al. (2009) (continued)

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Table 2 (continued) Nonmaterial g-C3N4/Fe3O4 as visible-light-driven photocatalyst Fe2O3

Target pollutants PHE

16 PAHs

Potassium zinc hexacyanoferrate nanocubes

ANT, PHE, FLU, CHR, and BaP

Iron hexacyanoferrate TiO2-based zinc hexacyanoferrate

BaP, CHR, FLU, PHE and ANT PHE, ANT, and FLU

TiO2

12 PAHs

Remarks 92% photodegradation of phenanthrene was achieved by g-C3N4/Fe3O4

References Wang et al. (2019)

The degradation of PAH involves photocatalysis as well as photolysis. Quick volatilization influences medium and low molecular weight PAHs BaP photodegradation was higher (71%) in the presence of Fe2O3 than in the absence of Fe2O3 (50%) Under sunlight, at neutral pH, maximum degradation of PAHs was observed. The rate of degradation becomes slower in soil than in water due to soil organic content. The degradation of smaller PAH, PHE, and ANT was in the range 80–93%, whereas the degradation of high molecular weight PAH, BaP, CHR, and FLU was in the range 70–80% The prepared NPs work as photocatalyst cum adsorbent in the order: dark < UV light < sunlight After 24 h irradiation of sunlight, FLU, PHE, and ANT were undergone 82, 84, and 86% degradation, respectively The study suggested that the removal efficiency in industrial soil was increased due to presence of metal oxide, TiO2

Marquès et al. (2020)

Shanker et al. (2017a)

Shanker et al. (2017b) Rachna et al. (2019) Eker and Hatipoglu (2019)

time interval of 12 h. The degradation of PAHs was in the range of 12.2–21.8% (Table 2). Dong et al. investigated degradation of phenanthrene and pyrene on soil surfaces using rutile TiO2 (Dong et al. 2010a) and anatase TiO2 (Dong et al. 2010b) as photocatalysts under UV irradiation. In both cases, photodegradation rate followed pseudo-first-order rate kinetics and the rate increased with the increasing concentration of H2O2 and humic acid and intensity of light. Barzegar et al. (2017) achieved the degradation of pyrene by using nanoscale Fe3O4 through sonolysis-assisted Fenton-like process. The removal of pyrene was 98% with 0.066 min 1 as pseudofirst-order rate constant under optimum conditions (Table 2). Along with the spiked soil samples, real samples containing chrysene (CHR), phenanthrene (PHE), acenaphthene (ACE), fluoranthene (FLU), and pyrene were also investigated, achieving a percentage removal in the range of 37.7–85.2%. In the spiked soil, the removal of pyrene was also examined using a mixture of Fe2O3 and Fe3O4 (nanoform). The efficiency of pyrene removal at neutral pH was found to be >90% on application of Fenton-like reaction (Jorfi et al. 2013).

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The doping of metal oxide NPs with other photocatalysts was also found to be popular among the environmentalists throughout the world. Photocatalytic degradation of phenanthrene was performed using TiO2 nanostructure doped with iron. The electron trap ability, band gap, and specific surface area were observed to be improved due to doping. Under alkaline conditions, the degradation efficiency of catalyst for PHE was greater than neutral and acidic conditions. The percentage degradation was >80%, which was attributed to the generation of (Fe3+-ligand) complex and ●OH under strong oxidizing conditions (Table 2). The amount of phenanthrene was diminished to 40 and 20 mg kg 1 without and with photocatalyst, respectively, from an initial amount of 60 mg kg 1 after a period of 2 h (Theerakarunwong and Phanichphant 2016). The phenanthrene-contaminated soil was treated with the photocatalyst g-C3N4/ Fe3O4 under visible light. At a pH near 7, the degradation of phenanthrene (200 mg kg 1) under visible light irradiation of 2 h was >90% (Wang et al. 2019). The degradation of phenanthrene was also examined under UV light using iron oxide nano-rods as photocatalyst by Gupta (2016). The rate constant for phenanthrene degradation was 6  10 3 h 1 with nano-rods of akaganeite (4%). The possible decay profiles for phenanthrene degradation in soils were also demonstrated. The study of intermediates and their persistence clearly indicated the complete degradation as well as removal of phenanthrene from contaminated soils. The photocatalyst Fe2O3 was employed to investigate decay of 16 PAHs in soil for a period of 28 days under optimum conditions. The presence of photocatalyst Fe2O3 significantly enhanced the degradation of BaP, phenanthrene, and fluoranthrene, whereas the degradation of dibenzo(a,h)anthracene (DBahA) and benz(a)anthracene (BaA) was reduced with the presence of photocatalyst (Marquès et al. 2020) (Table 2). Chien et al. (2011) indicated that total organic carbon as well as soil texture may affect the degradation of pyrene. The researchers utilized 100–392 nm TiO2 particles in red, alluvial soil, and quartz sand under sunlight to achieve 32, 23, and 78% pyrene degradation in respective soils (Table 2). Shanker et al. (2017a) used nanocubes of potassium zinc hexacyanoferrate to degrade BaP, CHR, FLU, PHE, and ANT along with irradiation of sunlight. Green method of synthesis using Sapindus Mukorossi was employed to prepare the photocatalyst. The degradation of smaller PAH, PHE, and ANT was in the range of 80–93%, whereas the degradation of high molecular weight PAH, BaP, CHR, and FLU was in the range of 70–80%. Compared to aqueous conditions, the PAHs diffusion on organic carbon was slow, which makes the decay process more difficult. The same group of authors, Shanker et al. (2017b), demonstrated the synthesis of green nanoparticles of iron hexacyanoferrate to degrade BaP, CHR, FLU, PHE, and ANT. The prepared NPs work as photocatalyst cum adsorbent in the order: dark < UV light < sunlight. Rachna et al. (2019) established the framework of TiO2-zinc hexacyanoferrate for PAH degradation under visible light. The results reveal that nanocomposite’s positive surface and PAHs were interacted due to cation-π complexes. After 24 h irradiation of sunlight, FLU, PHE, and ANT were undergone 82, 84, and 86% degradation, respectively.

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Gupta and Gupta (2015a) studied the degradation of BaP in the presence of different forms of synthesized iron oxides and reported fastest degradation with goethite NPs. Goethite NPs were applied for degradation of BaP in soils of varying pH under UV irradiation. The results suggested that degradation of BaP was enhanced due to goethite in all soils. The oxalic acid addition further increased the decay rate by involvement of Fenton-like mechanism without H2O2. The study also demonstrated the degradation pathways and persistence of intermediate degradation processes. A smaller PAH pyrene was also observed as intermediate during the degradation of BaP. In another study, Gupta and Gupta (2015b) demonstrated the decay of PAH pyrene in soils with goethite under UV irradiation. The results suggested that the PAH pyrene in soils was successfully decayed in the presence of goethite NPs into smaller fragments, which are nontoxic in nature. Gupta et al. (2017) synthesized Fe3O4 and α-FeOOH NPs and applied for degradation of anthracene. It was observed that the degradation of anthracene was faster using α-FeOOH compared to Fe3O4. The metal oxide nanomaterials combined with plant materials form a more efficient photocatalyst for remediation of PAHs. Włóka et al. (2019) used Brassica napus L. and Phalaris arundinacea L. in combination with SiO2 NPs to remediate PAHs. The reduction in PAH toxicity was reported due to the presence of SiO2 NPs. Eker and Hatipoglu (2019) studied the photodegradation applications for remediation of PAHs in industrial soils. The study suggested that the removal efficiency was increased due to presence of metal oxide, TiO2. The application of UV radiations reduces 86–90% PAH content from polluted soils. In the light of above discussion, it is well established that highly reactive radicals are generated by persulfate and Fenton’s process for mineralization of PAHs in soils. Moreover, the pH of soil plays an important role during degradation of PAHs.

Other Synthesized Materials Materials Based on Carbon Carbon-based nanomaterials are one of the most promising materials for remediation of various environmental contaminants. Due to surface functionality, excellent thermal conductivity, and greater surface area, graphene is applied for decay of organic pollutants globally (Huang et al. 2012). Due to occurrence of strong π-π interactions, high surface area, and dispersibility, nano-sulfonated graphene have prominent affinity toward various PAHs. The removal of PAH decreases with the increase in ring size under optimum conditions (Gan et al. 2017). Alhendal et al. (2022) removed PAHs using hybrid sol–gel sorbent, aramid-wrapped CNT. In soil samples, the affinity of sorbent was increased due to hydrophobic and hydrophilic interactions as well as π–π stacking. In case of silt loam and sandy loam, multiwalled carbon nanotubes (MWCNTs) showed sorptive characteristics for naphthalene, fluorene, and phenanthrene. The study reported three times higher sorption of studied PAHs onto MWCNTs as compared to silt loam and sandy loam (Li et al. 2013a). In another study, the same

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researchers reported smaller mobilities of studied PAHs due to occurrence of higher sorption capability of carbon nanotubes. Retention in mobility of PAHs was observed during their leaching in presence of carbon nanotubes in column mode. The hydrophobic properties, micropore volume, and surface area are main factors, governing PAH mobility in column mode (Li et al. 2013b). A few of the carbonbased nanomaterials for remediation of PAHs are summarized in Table 3.

Polymer-Based Nanomaterials The remediation of PAHs has also been demonstrated using polymer-based nanomaterials. The urethane acrylate nonionic chain was employed through the chemical cross-linking route to fabricate amphiphilic nanoscale polyurethane (APU) particles. The results reveal that APU particles behave similar to Triton X-100 reagent, but it is better due to its cross-linked structure, leading to lower adsorption tendency onto aquifer sand (Kim et al. 2003). Rani et al. (2020) degraded carcinogenic PAHs using nanocomposites based on chitosan and metal oxides. The natural surfactant Azadirachta indica was used to synthesize nanocomposite via green method. Table 3 Carbon- and polymer-based nanomaterials for PAHs-contaminated soils Nonmaterials used Aramid-wrapped CNT

Multiwalled carbon nanotubes (MWNTs) for sorption Carbon nanotubes (CNTs)

Nanocomposites based on chitosan and metal oxides

Target pollutants PAHs

Single PAH NAP, FLU, PHE, and PYR PHE and ANT

APU particles

PHE

Fabricated fibrous membranes carrying laccase

B[a]P, FLU, PHE, and BaA

Cu2O/Polylactic acid composite

FLU

Remarks The affinity of sorbent was increased due to hydrophobic and hydrophilic interactions as well as π–π stacking Three times higher sorption of studied PAHs onto MWCNTs as compared to silt loam and sandy loam The hydrophobic properties, micropore volume, and surface area are main factors, governing PAH mobility in column mode Higher degradation of anthracene than phenanthrene was due to lower stability of anthracene. GC-MS analysis suggested the formation of by-products which are relatively safe as well as smaller than parent compounds APU particles behave similar to Triton X-100 reagent, but it is better due to its cross-linked structure, leading to lower adsorption tendency onto aquifer sand The effectiveness of remediation for benzo[a]pyrene, benz[a]anthracene, fluoranthene, and phenanthrene were 72.5%, 79.1%, 93.2%, and 95.1%, respectively Under visible light irradiation, >65% of PAH, fluoranthene was found to be degraded

References Alhendal et al. (2022) Li et al. (2013a) Li et al. (2013b)

Rani and Rachna (2020)

Kim et al. (2003)

Dai et al. (2011)

Xu et al. (2020)

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Under direct sunlight, phenanthrene and anthracene were decayed exponentially (>90%) with photocatalyst in 12 h. Higher degradation of anthracene than phenanthrene was due to lower stability of anthracene. GC-MS analysis suggested the formation of by-products which are relatively safe as well as smaller than parent compounds. Dai et al. (2011) employed different fabricated fibrous membranes carrying laccase to investigate the removal of shoal soil contaminated with PAHs. The core of the structure was composed of laccase, which is responsible for degradation of PAH and nanofibers cause PAH adsorption, whereas mass transfer was performed by pores on the shell. The effectiveness of remediation for benzo[a] pyrene, benz[a]anthracene, fluoranthene, and phenanthrene were 72.5%, 79.1%, 93.2%, and 95.1%, respectively. The half-lives for PAHs were 1.25–12.50 h (membrane adsorption) or 17.9–67.9 h (free laccase), while using the fibrous structures these become much shorter, i.e., 0.003–1.52 h. In the overall reaction, the ratelimiting steps were internal diffusion and superficial adsorption, as suggested by third-order reaction kinetics. The triple-phase distribution and kinetics propose synergistic effect between degradation and adsorption. In a recent study, Xu et al. (2020) fabricated nanofibers of Cu2O/polylactic acid, consisting of Cu2O NPs and biodegradable polymer, polylactic acid. Under visible light irradiation, >65% of PAH, fluoranthene was found to be degraded using Cu2O/polylactic acid nanofibers (Table 3).

Mechanism of PAH Degradation Using Synthetic NPs and By-Products Identification The NPs generally have a band gap between the conduction band and the valence band. Upon irradiation of synthesized NPs with radiation of suitable wavelength, an electron-hole pair is generated due to transfer of an electron from valence band to conduction band. The electron combines with oxygen (O2) to produce O2●¯, whereas the hole combines with water molecule to form OH● radical and H+ by splitting. The H+ combines with O2●¯ to produce H2O2, the produced H2O2 combines with iron oxide to demonstrate Fenton-like mechanism (Shanker et al. 2017a; Gupta and Gupta 2015a; Wang et al. 2009). The generated H2O2 molecule also produces OH● radical, which is responsible for the initiation of degradation mechanism. The OH● radical combines with the PAH molecule and generates another radical. It is well established that radicals are short lived and undergo fragmentation or combination to form different entities. Generally, the molecule undergoes fragmentation to smaller molecules, which further converted to even smaller counterparts, leading to mineralization of environmental contaminant. The degradation process and generation of OH● radical is summarized in Fig. 3. Several photocatalytic studies also reported the by-products generated after degradation of PAHs in the presence of photocatalysts. Shanker et al. (2017a) proposed the production of malealdehyde, o-xylene, 2-methylhex-3-enoic acid, 2,4a-dihydro-1H-cyclopenta [cd]indene, 2-hydroxycyclohexa-1,3-dienecarboxylic acid, 2,3-dimethylbenzoic

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Fig. 3 Photocatalytic degradation process in presence of NPs

acid, and 2-hydroxy-1-naphthoic acid using iron hexacyanoferrate NPs and potassium zinc hexacyanoferrate nanocubes under visible light (Fig. 4). Similarly, Gupta and Gupta (2015a) proposed the degradation of BaP in different soils contaminated with PAHs. The degradation of BaP in neutral soil produces benzo[a]pyrene-4,5-dione, cyclopenta[d,e,f]chrysene-4-one, 5-hydroxybenzo[a] pyrene, and 1,2-epoxypyrene. In case of basic soils, the degradation of BaP produces 4,5-dihydroxybenzo[a]pyrene, cyclopenta[d,e,f]chrysene-4-ol, benzo[a]pyrene-4,5ketol-7,8-dihydrodiol-9,10-epoxide, and 1,2-dihydroepoxypyrene. In acidic soils, along with several other degradation products, smaller PAH pyrene was observed. Gupta and Gupta (2015b) identified different degradation products of pyrene in different soils. The intermediate generated in neutral soils are 1-hydroxypyrene, 3,4-dihydroxy-3,4-dihydrophenanthrene-4-carboxylic acid, 3,4-dihydrophenanthrene4-carboxylic acid, 3-(1-hydroxy-2-naphthalenyl)-2-propenal, and 1,2-naphthalenedione. In basic soil, intermediate phenenanthrene-4-carboxylic acid was observed. In acidic soils, the intermediates 6-hydroxy-4H-cyclopenta[def]phenanthrene-4-one, 2-hydroxypyrene-4,5-dione, 4Hcyclopenta[def] phenanthrene-4-ol, and phenanthrene-4-carboxylic were examined. Due to appearance of different chemical environment, the degradation products were of different persistence as well as different nature and size. The studies on the identification of metabolites of benzo [a]pyrene and pyrene reveal that the photodegradation of the PAHs occurs through oxidative mechanism. The metabolic pathways seem to be complex and different metabolites were observed in soils with different pH. During photodegradation of benzo[a]pyrene, metabolite benzo[a]pyrene-[4,5]-dione (m/z 283) was observed in neutral and acidic soil due to full oxidation of benzo[a]pyrene, while in case of basic soil metabolite 4,5-dihydroxybenzo[a]pyrene (m/z 285) was observed due to partial oxidation of benzo[a]pyrene. The metabolite 9-hydoxybenzo[a]pyrene-7,8-dione was observed in neutral soil only and in acidic or alkaline soils, it oxidized further to benzo[a]

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Fig. 4 Degradation of B[a]P using iron hexacyanoferrate NPs and potassium zinc hexacyanoferrate nanocubes. (Reprinted from original article by Shanker et al. (2017a) after permission through RightsLink)

pyrene-4,5-ketol-7,8-dione-9,10-epoxide (m/z 331). In acidic and basic soils, metabolite 5-hydroxybenzo[a]pyrene was persistent after 120 h, but was not observed in neutral soil after 120 h. In acidic soils, benzo[a]pyrene was converted to smaller PAH pyrene.

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In case of pyrene, metabolites corresponding to naphthalene and its derivatives are persistent in basic and neutral soil, but in acidic soil, hydrogenated phenanthrene was persistent. The degradation pathways thus indicate that the mutagen pyrene is degraded efficiently in presence of goethite into smaller nonmutagenic/noncarcinogenic hydrocarbons. Photodegradation studies further reveal that the metabolites of benzo[a]pyrene and pyrene, reported to be toxic, such as diones, diols, and epoxides, disappear in all the three soils after 120 h. Thus, the study provides an efficient method for the remediation of PAH-contaminated soil surfaces. Theerakarunwong and Phanichphant (2016) reported phenanthrene degradation under visible light in presence of Fe-doped TiO2. The degradation of phenanthrene was initiated by attack of generated OH● radical to the K-region phenanthrene molecule. The main by-products formed during the decay process were 1,4 Napthoquinone and heptadecane. Wang et al. (2019) demonstrated on the basis of ESR analysis that not only OH● radical but also O2●¯ causes the degradation of PAH. The two types of radicals were generated by solar light irradiation of g-C3N4/Fe3O4 in their study. Gupta (2016) described the proposed mechanisms for the decay of phenanthrene in acidic, basic, and neutral soils using akaganeite nano-rods under UV irradiation. In case of acidic soils, nontoxic and simpler intermediates such as phenylmethanol, phthalic acid, pyrocatechol, salicylic acid, and open-chain compounds were reported at the later part of reaction. In neutral soils, simpler intermediates were 9H-fluoren-9-ol and benzoic acid. In basic soils, the simpler intermediates were pyrocatechol, 2,5-dihydroxybenzoic acid, 2-hydroxybenzaldehyde, or benzoic acid. The simpler intermediates were also reported to disappear at the time of completion of reaction. The degradation in the three types of soils occurs through loss of CO group, loss of oxygen, oxidative ring opening, and oxidation cleavage. The study concluded that phenanthrene degradation with akaganeite nano-rods depends on soil characteristics. It was also suggested that the identified intermediates may serve as data bank to different toxicological studies.

Conclusions Polycyclic aromatic hydrocarbons are globally distributed and persistent environmental contaminants. The extent and seriousness of the potential hazards due to these pollutants have already been defined by various environmental agencies such as USEPA and EC by categorizing them as priority pollutants. Among different technological innovations for their remediation, the application of synthetic NPs for remediation of PAHs in the contaminated soils is one of the most suitable techniques due to their superiority in the capacity to degrade the PAHs efficiently. Their high degradation capacity creates nanoparticles an ideal material to decontaminate soil from PAH pollution. The current chapter reports the application of various NPs based on carbon, polymer, metal oxides, and nanoscale zero-valent iron. It was observed that different types of nanoparticles degrade the PAHs efficiently in varying soils. Among different iron oxides, the rate of photodegradation of pyrene was in the order: goethite > hematite > maghemite > lepidocrite. In the spiked soil,

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the removal of pyrene was also examined using a mixture of Fe2O3 and Fe3O4 (nanoform). The removal efficiency of pyrene, at neutral pH, was found to be >90% on application of Fenton-like reaction. The nanocubes of potassium zinc hexacyanoferrate degrade smaller PAH, PHE, and ANT up to an extent of 80–93%, whereas the same photocatalyst degrades high molecular weight PAH, BaP, CHR, and FLU up to an extent of 70–80%. The mechanism of degradation suggests that the decay process occurs through oxidative mechanism. The major species responsible for degradation was found to be OH●, whereas based on ESR analysis it was also concluded that O2●¯ can also cause the degradation of PAH. The pH of the soil plays a significant role in the degradation process. A large number of intermediate and by-products have been proposed by diverse researchers, which may serve as a data bank for various toxicological studies in future.

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Multifunctional Nanoprobes for the Surveillance of Amyloid Aggregation

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Thanojan Jeyachandran, Suraj Loomba, Asma Khalid, and Nasir Mahmood

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Amyloidosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-beta Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diseases Associated with Amyloid Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alzheimer’s Disease and Type 2 Diabetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Connection Between Alzheimer’s and Type 2 Diabetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amyloid Aggregation Mechanism in Alzheimer’s Disease and Type 2 Diabetes . . . . . . . . . Impacts of Alzheimer’s and Type 2 Diabetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Vivo Diagnosis for Amyloid Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Vitro Diagnostics for Amyloid Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials for the Treatment of Type 2 Diabetes and Alzheimer’s Disease . . . . . . . . . . . Quantum Dots for the Treatment of Alzheimer’s Disease and Type 2 Diabetes . . . . . . . . . . . Nanocarriers for Alzheimer’s Disease Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Protein accumulation is a biological process in which mis-folded proteins aggregate and clump together either intra- or extracellularly. The lack of reliable sensors and the complex nature of these peptide aggregate make it challenging to detect them in the early stages of formation/growth. Rapid advances and ongoing research in the field of nanomaterials can provide practical solutions Thanojan Jeyachandran and Suraj Loomba contributed equally with all other contributors. T. Jeyachandran · S. Loomba · N. Mahmood (*) School of Engineering, RMIT University, Melbourne, VIC, Australia e-mail: [email protected] A. Khalid (*) School of Sciences, RMIT University, Melbourne, VIC, Australia e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_105

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for the early monitoring and detection of amyloid aggregation. This chapter focuses on using quantum dot multimodal probes for amyloid detection. The fluorescent probes enable the in vitro monitoring of insulin, human islet amyloid polypeptide (hIAPP), and amyloid (Aβ 42) (Aβ42) oligomers and monomers during the fibrillogenesis dynamic. Moreover, research has shown that the quantum dot probe demonstrates 10 times greater signals when it comes to real-time detection of amyloid intermediates and fibrils compared to the traditionally used thioflavin dye. A negative ΔG (standard free energy change for the reaction) value( 36.21 kJ/mol) for quantum dot probes indicates spontaneous interaction of the probe with the peptides. Thermodynamic measurements show that these interactions involve hydrogen bonding as well as hydrophobic (in an aqueous solution, nonpolar materials tend to accumulate and omit molecules of water) surface interactions. These probes monitor the in vitro fibrillation kinetics of various amyloid proteins having high specificity and sensitivity compared to thioflavin dye, as well as the existence of a 19F center (unique spectral signature in proteins). These properties make quantum dots effective probes for nonradiative and noninvasive in vivo detection of amyloid plaques. Keywords

Amyloidosis · Alzheimer · Type 2 diabetes · In vivo · In vitro · Cross-beta structure

Introduction Amyloidosis is a very rare underlying disease caused by an unwanted/abnormal protein buildup in organs, which eventually interferes with the organ’s normal functions. This abnormal protein is called Amyloid, which translates to “Starch alike” (Kyle and Bayed 1975). Amyloid proteins are insoluble in water and possess an unusually stable chemical structure, but an interesting factor is amyloid does not normally exist inside the human body like other proteins. Amyloid is only formed when a certain kind of protein folds in an abnormal way. This abnormal folding can be heredity or acquired, and in some cases, it can only be limited to a certain organ or a small area of the body or can circulate with the blood to several organs in other cases, leading to multiorgan failure. Some amyloidosis foldings are also associated with cancer. These complex folding phenomena can be pathologically explained via the amyloid state (Husby et al. 1982), as discussed in the following section.

Types of Amyloidosis With reference to the pattern of amyloid fibril deposition, clinical signals, and biochemistry of amyloid proteins, we can ably divide the amyloidosis process into two types – systematic (Yazaki and Higuchi 2014), primary or localized (Symmers

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1956). Systemic amyloidosis: In this process, deposition of amyloid fibrils can or might develop in any human organ. Primary amyloidosis: On the other hand, primary amyloidosis occurs because of excess aggregation of immunoglobulin light chain amyloid fibrils. Around 20 types of amyloid-forming proteins have been medically identified. The protein folding results in the formation of elongated fibers identified as the “Amyloid-state.” Amyloid formation can be identified via extracellular, unbranched fibers with the ability to bind with Congo Red dye to produce green fluorescence (Turnell and Finch 1992). Likewise, another interesting characteristic of amyloid fiber aggregation is cross-beta polypeptide (cross-β) confirmation.

Cross-beta Structure The amyloid spine atomic structure is commonly called the “Cross-β” structure. This structure demonstrates that the most powerful repeating characteristic of the amyloid fibril is an array of β-sheets which can be parallel to the fibril axis, with perpendicular strands. The diffraction pattern of cross-β is realized when X-rays are focused on amyloid fibers (Fig. 1) (Eisenberg and Jucker 2012). Understanding the cross-β diffraction and its molecular and structural characteristics has been a complicated and slow process because of the limited crystallographic techniques in the past. Nuclear magnetic resonance (NMR) and electron microscopy (EM) methods can

Fig. 1 Cross-β fiber diffraction pattern. The characteristic diffraction pattern of cross-β is realized when X-rays are focused on amyloid fibers. The diffused reflection at 4.8 A spacing along the meridian (vertical) demonstrates prolonged protein chains moving approximately at right angle to the fibril and placed 4.8 A apart. A larger diffused reflection at 10 A spacing along the equator (horizontal) demonstrates that the prolonged chains are arranged into sheets placed 10 A apart. (Reproduced with permission (Eisenberg and Jucker 2012), copyrights reserved to Elsevier Inc. 2012)

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hardly isolate the amyloid-affected tissues from healthy ones and limited order of fibrils. Recent advances in computer modeling-constrained X-ray fibers methods, spin labeling techniques, solid-state NMR studies, and single crystal X-ray diffraction have enabled a better understanding of structure and properties of the amyloid. The amyloid fibers’ structure displays a cross-β diffraction pattern when analyzed via X-ray diffraction (XRD) imaging, as demonstrated in Fig. 1. The formation of the cross-β structure appears to be a dead end to the protein folding which also leads to amyloid-based diseases.

Diseases Associated with Amyloid Deposits These excess amyloid deposits most commonly cause cardiac or renal dysfunction. Also, on some occasions, amyloidosis affects one organ, which might include aggregation in the pancreatic, which leads to the amyloid polypeptide in the type 2 diabetes (Saiki et al. 2005). In several cases, these amyloid deposits replicate and colonize in extra neural secondary lymphoid organs prior to spreading to central nervous system (CNS). Amyloid fibrils are produced by normally soluble proteins, which gather to produce insoluble fibers that are resistant to degradation. These amyloidosis fibrils undergo hydrophobic and hydrophilic intermolecular interactions, and as a result, “Amyloid plagues” are formed (Yazaki and Higuchi 2014). Various types of protein aggregation patterns lead to different kinds of diseases, such as Alzheimer’s, Type 2 diabetes, and Sponge form disease. However, these amyloidosis diseases share the same structural (β sheet rich structure) and morphological features. Proteins that cause the actual amyloidosis fibril process are derived and called precursor proteins. Many of these protein aggregation (mostly systematic) problems affect the central nervous system (CNS). Certain protein aggregations are exclusively toxic to the CNS, even though they are expressed ubiquitously (Symmers 1956). Neurodegenerative diseases (caused by degeneration of the nervous system) include disorders where pathological proteins might aggregate within the nucleus, causing either (i) diseases like polyglutamine expansion (spinocerebellar ataxias and Huntington’s disease), (ii) cytoplasmic inclusions characterized disorders (like α-synuclein inclusion leading to Parkinson’s disease), (iii) disorders where pathological proteins aggregate extracellularly (as in prion diseases), or (iv) both extracellularly and intracellularly (amyloid-β (Aβ) and tau in Alzheimer’s). Among all the diseases caused by protein aggregation, type 2 diabetes and Alzheimer’s are often addressed as twenty-first-century plagues.

Alzheimer’s Disease and Type 2 Diabetes Alzheimer’s disease (Ad), commonly known as dementia, is the most studied neurodegenerative disease to date. Statistics reveal that more than 24 million people get infected with this disease annually, causing a huge burden on the economy

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(Abedini and Schmidt 2013). Studies show that there are two distinctive features found in patients suffering from Alzheimer’s: (1) extracellular amyloid plaques – these plaques are produced by the amyloid precursor protein’s (APP) proteolytic product, a receptor of cell surface prepared by ϓ-secretase and β-secretase (β-amyloid cleavage enzyme 1, BACE1); (2) neurofibrillary tangles (NFTs) – these NFTs are produced by the neuronal microtubule-associated protein tau’s hyperphosphorylated forms (Aguzzi and O’Connor 2010). The Alzheimer’s patients undergo rigorous and progressive amyloid β deposition surrounding the brain’s cytopathology and neurotic regions which are responsible for cognition and memory. However, recent experiments show that all dominant mutation causing the early onset of Alzheimer’s is caused by amyloid protein or a new protein called presenilin (Aguzzi and O’Connor 2010). A well-known fact is that Presenilin 1/2 is the catalytic subunit of ϓ-secretase, and it undergoes missense mutation, which increases the production of amyloid-β 42/43 peptides (Aβ 42/43). The self-aggregation of Aβ 42/43 is the leading cause of excess Aβ deposition in midlife (45–64 years) (Selkoe and Hardy 2016). Hence apart from Presenilin, Aβ and Tau proteins are identified as the two hallmark proteins for Alzheimer’s (Ittner and Gotz 2011). Although in Alzheimer’s the neuronal toxicity is connected to tau and Aβ, it is still uncertain which molecular interaction occurs among these proteins (Querfurth and LaFerla 2010). Frontotemporal dementia (FTD) which occurs before the age of 65 is known as the second most prevalent form of dementia. One of the subset characteristics of FTD is the overt absence of amyloid-beta pathology in the Tau pathology (Cairns et al. 2007). NFT progression throughout the brain is called spreading, which along with loss of synapses are the earliest events of the Alzheimer’s progression, eventually leading to functional impairment. Recent studies have also suggested that people with metabolic disorders are at significantly increased risk of Alzheimer’s and vascular dementia development (Jayaraman and Pike 2014). Evidence and extended research suggest that neural dysfunction caused by type 2 diabetes is also one of the major reasons for Alzheimer’s (Chatterjee and Mudher 2018). In the year 2030, the disease will reach up to 4.4% of the entire population from an initial percentage of 2.8% in the year 2000. As a result, 366 million people will be affected worldwide in 2030. Unfortunately, the modern lifestyle with limited outdoor activities, less exercise due to motorized transport, and spending time indoors for television entertainment are the root causes for type 2 diabetes. Our low-energy expenditure and high intake of calories constitute a nonhealthy lifestyle which is the leading cause of type 2 diabetes epidemic (Webb et al. 2009). Type 2 diabetes is a noncurable disease, and insulin is the major player in the pathology of type 2 diabetes. Insulin, a peptide hormone, is released by the pancreatic islets’ β cells of Langerhans and preserves normal levels of blood glucose by aiding the uptake of cellular glucose, regulating lipid, protein, and carbohydrate metabolism, and aiding growth and cell division through its mitogenic effects. Pathology of type 2 diabetes is simple; it either resists insulin or does not produce enough insulin resulting in

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pancreatic beta-cell failure in both cases. The most common symptoms of the disease are feeling of hunger, frequent urination, and fatigue; however, in some cases, there may be no symptoms present (Jack et al. 2004). However, the technological advancement in insulin radioimmunoassay enables a better understanding of type 2 diabetes (Webb et al. 2009). Recent research shows that the rapid increase in type 2 diabetes patients is one of the major reasons for Alzheimer’s. In fact, type 2 diabetes is a chronic metabolic disorder, which also enhances the possibility of dementia and cerebrovascular disease. Increased threat for such diseases evolves years before the start of clinically apparent diabetes and is greater in people with insulin resistance and obesity. Amylin (islet amyloid polypeptide) is a hormone that is consecrated with insulin from the pancreatic β-cell. Insulin-resistive patients have increased concentrations of amylin in their blood (Kahn et al. 1990). Hyperamylasemia can be described as an excess of the pancreatic enzyme, aka amylin, in the blood. Recent experiments show that excess amylin deposition induces toxicity in the peripheral organs such as kidneys and heart found in diabetic patients. The peripheral aggregation of amylin is related to tissue and vascular damage to kidneys and heart. A comparable procedure can also appear in the brain and cerebrovascular structure of diabetic patient’s eventually leading to Alzheimer’s (Despa and Decarli 2013). Therefore, it is crucial to prevent the aggregation of insulin to control type 2 diabetes which otherwise might result in the development of Alzheimer’s.

The Connection Between Alzheimer’s and Type 2 Diabetics Compelling evidence has undiscovered type 2 diabetes mellitus as a risk factor for Alzheimer’s disease. Significant similarities were identified while looking at the risk factors. Putative pathophysiological methods and comorbidities of mellitus of type 2 diabetes are two of the most critical and undelaying epidemics of our time. There is a lot to learn and unveil about the biology of these two neurophysiological disease conditions. However, it is still not known if Alzheimer’s disease and type 2 diabetes are similar events arising from related backgrounds in synergistic diseases and aging or are linked by vicious pathophysiological cycles. Insulin resistance and amyloid aggregation are the most important features of type 2 diabetes and are also emerging as potentially core features of Alzheimer’s disease (Sebastião et al. 2014). The molecular signaling route via which insulin uses its actions in the body also moderates its part in synaptic glial metabolism, neurotransmission, and neuronal as well as in the neuroinflammatory response of the brain (Craft 2005). Central modulation of metabolism of the body, emotional functions, cognitive and improvement, or adjustment of memory is controlled by insulin in healthy individuals inside the brain. Resistance to insulin is a primary function of type 2 diabetes and aids to the hyperglycemia that defines diabetes mellitus in addition to the hyperlipidemia, atherosclerosis, inflammation, and oxidative stress that accompanies it (Kim et al. 2004). Brain insulin resistance is known as the failing of brain cells to reply to

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insulin as they generally could, leading to impairments in immune, synaptic, and metabolic reaction features. Type 2 diabetes is related to brain insulin resistance, and research shows that brain insulin resistance is a function of Alzheimer’s disease; however, whether or not the two conditions are mechanistically related or display unconnected circumstances in getting old is uncertain (Su et al. 2017). On the other hand, Amyloid aggregation is the most important and dangerous feature in any given neurodegenerative disease. To cure the disease, the aggregation should be halted or stopped permanently. Amyloid aggregation also plays a major role in connecting the dots between these two diseases.

Amyloid Aggregation Mechanism in Alzheimer’s Disease and Type 2 Diabetes Senile amyloid plaques are the most common inhabitant in the Alzheimer’s disease patients’ brains which are made of Amyloid β peptide (Aβ) (Masters et al. 1985), whereas, in type 2 diabetes patients, Langerhans (small cluster of cells scattered in the pancreas, aka islet of Langerhans) is observed. Pancreatic islet amyloids are produced by the accumulation of human islet amyloid poly peptides (hIAPP) (Westermark et al. 1987). Cross-beta spine is the commonly adopted similar structure found in both patients. These cross-beta spines are formed by the amino acids which were required for self-assembly in that region. In accordance with the amino acid sequence, 25% of homology was shared by Aβ and hIAPP (Toyama and Weissman 2011). APP (Amyloid Precursor Protein) is the precursor protein that produces both hIAPP and Aβ. There are several cellular mechanisms available to prevent hIAPP accumulation. Preventing hIAPP accumulation via its degradation is one of them. The degradation pathway of hIAPP and Aβ was enabled by an insulin-degrading enzyme (IDE) (Kurochkin and Goto 1994). Numerous antioxidants with catechol moiety like acteoside, di- and tri-O-caffeoylquinic acids, hispidin, clovamide, and associated compounds have helped in preventing accumulation of Aβ. Research indicates that the aggregation process for Aβ and hIAPP is potentially blocked by glucuronosylated favonoids with a catechol moiety (Kidachi et al. 2016). Considering the resemblance in the aggregation route of hIAPP and Aβ (illustrated in Fig. 2), antioxidants with identical structures could be effective compounds to prevent the accumulation of both hIAPP and Aβ. The amyloid fibril (Aβ) deposits mainly consist of 40 (Aβ40) and 42-mer(Aβ42) Aβ-proteins; both are also produced by APP precursor proteins (Ben Hmidene et al. 2017). While looking at the pathogenesis of Aβ40 and Aβ42, Aβ42 plays a major role because of its neurotoxicity and its stronger aggregative ability (Masters et al. 1985). However, recent advances in technology helped the researchers to discover three underlying microscopic steps in the Aβ aggregation which leads to neurotoxicity, which are (i) primary nucleation of monomers (A monomer is a molecule which reacts with other monomer molecules to create a three-dimensional network or long

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Fig. 2 hIAPP aggregation mechanism, an overview. During the progression of TYPE 2 DIABETES M, hIAPP aggregation and islet amyloid formation can be triggered by several factors. The soluble hIAPP oligomers and monomers can pass the blood-brain barrier (BBB) and constitute cerebral hIAPP deposits resulting in AD pathology via different routes: separately exert toxic consequences, integrate and have interaction with Ab, or lose the physiological function of soluble IAPP in the brain. (Reproduced with permission (Zhang and Song 2017), copyrights reserved to Elsevier Inc. 2017)

polymer chain in a process known as polymerization), (ii) monomers’ secondary nucleation on fibril surface, and (iii) monomer addition leading to fibrils elongation (Linse 2019). Primary nucleation can happen on the surface of the Aβ fibrils or on a foreign surface; even though primary nucleation mainly comprises of monomers, it may take place in bulk as well. Secondary nucleation takes place when a parent seed aggregate has the same types of monomers. It is also called “Monomer-dependent.” The same type of building blocks of monomers aggregates and forms of the nucleus is called the secondary nucleation process (Linse 2019). Both primary and secondary nucleation processes speed up the aggregation processes. The third and final step, elongation, occurs by adding a monomer to the end of the fibril. However, hIAPP aggregation follows a different pathway. The aggregation mechanism, the structure of early oligomers (Oligomers are low-molecular weight polymers consisting of a small number of repeat units having physical properties significantly dependent on the chain length), and β-sheet formation of hIAPP are poorly understood for this 37-residue peptide (Luca et al. 2007).

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To understand the aggregation process, we must first have a rigid understanding of the complicated structure of hIAPP. IMS-MS (Ion mobility combined with mass spectrometry) is one of the methods used to understand the structure of hIAPP. Spectrographic results discovered a “mutation region”; only six amino acids were mainly located between residues 20–29 (Bernstein et al. 2009). MD simulation (molecular dynamics) was also used to identify the extended β-hairpin family and helix-coil family within the mutation region. IMS-MS and MD modeling provide insight into the isolated and unmodified dimers of hIAPP. This study provides new insight into the amyloid aggregation mechanism. To understand the fibril formation, NCC (nucleated conformational conversion) and CS (conformational section) were used. NCC mechanism is the widely adapted mechanism so far. It states that, in ligand binding systems, monomers first collapse through hydrophilic or nonspecific hydrophilic interactions and further rearrange from β-ordered proto fibril (Nguyen and Hall 2004). NMR experiments further support this NCC mechanism adaptation for hIAPP. On the other hand, the CS mechanism suggests that selectively collapsing happens in monomers containing preexisting β-structures and further grows to form fibrils (Miller et al. 2010).

Impacts of Alzheimer’s and Type 2 Diabetes In general, Alzheimer’s disease is associated with chronic inflammation, increased oxidative stress, metabolic disturbances, reduced glucose metabolism, impaired cerebral energy metabolism, and cognitive deficits (Albert et al. 2011). On the other hand, type 2 diabetes will result in decreased production/availability of insulin, hyperglycemia (high blood sugar), and insulin resistance (Spranger et al. 2003). As we all know, one of the common impacts of Alzheimer’s disease is the development of the reduced cognitive function, also known as dementia. Some research shows that risk factors for dementia were initiated by the impact of type 2 diabetes, obesity, and excess intake of saturated fat (Wynne et al. 2005). Some of the pathological mechanisms were shared between Alzheimer’s disease and type 2 diabetes. Among the shared mechanisms, one of the most important events is the discharge of proinflammatory cytokines by the inflammatory process. These proinflammatory cytokines can pass the blood-brain barrier from microglia in the central nervous system in Alzheimer’s disease patients and can also be from macrophages in the periphery in type 2 diabetes (Hirosumi et al. 2002). This inflammatory process activates cellular stress pathways, which eventually leads to insulin resistance in both patients. Insulin resistance is common in both kinds of patients (older adults-65 years and older). As a result, insulin resistance will lead to a brain insulin-deficient state. This insulin-deficient state is achieved by downregulation of insulin transport to the brain, also known as peripheral hyperinsulinemia (Stein et al. 1987). Subtle declarative memory impairment was additionally triggered by insulin resistance in older adults

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and middle-aged people who are affected by type 2 diabetes. Insulin resistance also often results in the elevation of inflammatory cytokines as well as free fatty acids (Milanski et al. 2009). Inflammation is related to the formation of Aβ plaques. Astrocytes and microglia cells are usually a part of normal neuroinflammatory responses; however, their sustained and uncontrolled activation by Aβ plaques in Alzheimer’s disease patients can enhance proinflammatory cytokines such as interleukins (ILs), tumor necrosis factor-alpha (TNF-α), and increase oxidative stress reactive oxygen species, and these will eventually cause secondary neuronal injury or may lead to death (Heneka and O’Banion 2007). In summary, even though Alzheimer’s disease and type 2 diabetes are two distinct diseases, it should be well noted that many of these two diseases’ pathological characteristics overlap. Tau neurofibrillary tangles and amyloid β plaques are the most important hallmark feature of Alzheimer’s disease, which are absent in type 2 diabetes. Additionally, the main reasons are different. Obesity and resistance to insulin are the main causes of type 2 diabetes, while the primary reason for Alzheimer’s disease is still not known. However, the amyloid-beta hypothesis is the long and best hypothesis to date for Alzheimer’s disease (Walker and Harrison 2015). Early diagnosis is vital for controlling both diseases. We can divide the diagnostics tools into two, in vivo and in vitro.

In Vivo Diagnosis for Amyloid Aggregation In vivo stands in Latin for “within the living,” and biomarkers play a major role when it comes to in vivo (illustrated in Fig. 3). Some important imaging techniques are voxel-based morphometry (VBM), positron emission tomography (PET), singlephoton emission computed tomography (SPECT), functional magnetic resonance imaging (f-MRI), magnetic resonance imaging (MRI), and magnetic resonance perfusion imaging (MRPI). F-labeled fluorodeoxyglucose (FGD) is used to differentiate other forms of dementia patients from Ad (Leuzy et al. 2018). In the past couple of decades, many promising studies have been published evaluating the value of PET in Alzheimer’s disease. PET is based on picomolar selective uptake on a biological substance labeled and identified using positron emitters like carbon-11, nitrogen-13, oxygen-15, and fluorine-18. Research indicates that the best regions for diagnosing Alzheimer’s disease using a PET scanner in the brain are the middle frontal gyrus and hippocampus. Studies using PET indicate that various radiotracers showing response to Aβ in vitro may also be utilized in vivo to identify amyloid aggregation. On the other hand, voxel-based techniques use analyzing the diseases associated and pathological functional changes in the brain to determine the differences, for example, by comparing gray matter density (Ziegler 2005). However, a widely available method for brain evaluation is SPECT imaging technique. SPECT imaging technique uses a rotating gamma camera and depends on the brain’s uptake of technetium 99 m-based lipid-solvable radionuclides like hexamethyl propylene amine oxime or cysteinate dimer. Both SPECT and PET are

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Fig. 3 A schematic overview of established and candidate imaging and fluid-based Alzheimer’s disease biomarkers. An imaging biomarker is a biological feature that can be easily detectable via medical imaging systems such as PET, MRI, etc. CSF biomarkers are beneficial as supplementary information and as screening tools to diagnostic analysis but are inadequate as diagnostic equipment in seclusion. For Alzheimer’s disease, traditional markers are Aβ42, Total tau, and Phospho tau. Blood biomarkers can be distinctive molecules or biological properties which can be measured and detected in body parts’ tissue or blood. They may demonstrate either normal or diseased processes in the body. Blood biomarkers may be genes, specific cells, gene products, molecules, hormones, or enzymes. (Reproduced with permission (Leuzy et al. 2018), copyright reserved to The Yale Journal of Biology and Medicine)

noninvasive techniques; similarly, they use radiolabeled probes with the ability to diagnose the presymptomatic cerebral β-amyloidosis in the brain (Pickut et al. 1999). However, SPECT probes tend to emit γ -radiation which can be measured directly, while PET probes tend to emit positrons which destroy electrons, resulting in the emission of two γ -photons in opposite directions. These emissions are coincidentally detected by the PET scanner to administer images of higher resolution compared to SPECT. SPECT scans are relatively cheaper than PET as longer-lived radioisotopes are utilized by SPECT. Therefore, when it comes to routine diagnostic

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utilization, SPECT would be more suitable than PET (Tiwari et al. 2015). Another most important imaging technique in vivo is fMRI. fMRI measures regional brain activity because of local variations in the concentration of deoxyhemoglobin to respond to numerous tasks and stimuli. In summary, the results of fMRI studies show statistically considerable dissimilarities in mean infusibility between the control group and the Alzheimer’s disease group. Pathological variations in hippocampus are embodied by heightened apparent diffusion coefficient (ADC) in these regions (illustrated in Fig. 4) (Courtney et al. 1997). Apart from these techniques, diffusion and perfusion magnetic resonance imaging, diffusion-weighted magnetic resonance imaging, and magnetic resonance spectroscopy are also used to detect amyloid aggregation in vivo techniques. In vitro diagnostics also play a vital role when it comes to amyloid aggregation detection.

In Vitro Diagnostics for Amyloid Aggregation In Latin, in vitro stands for “within the glass.” In vitro detection of Aβ (amyloid-beta) plaques at the nucleation stage can support incredibly in diagnosing and treating diseases like Alzheimer’s and type 2 diabetes at an early stage (Wang et al. 2017). In the recent era, in vitro, several techniques have been implemented in the field of spectroscopic and microscopic to study and understand the function of amyloid fibrillation, such as fluorescence spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), circular dichroism (CD), and NMR. The most conventional method for monitoring amyloid fibrillation in real time is spectroscopy and fluorescence imaging (Pradhan et al. 2015). In some micrography techniques, when analyzing the biological samples, negative staining techniques (A well-established microscopy technique, background containing the sample, is stained and leaving the specimen untouched) were also used to determine the amyloid morphology in microscopic techniques such as TEM. However, TEM images are two-dimensional (2D); therefore, TEM images cannot report three-dimensional (3D) morphology of the amyloid fibers. To get information in the 3D morphology on TEM, Pt shadowing technique can be applied for known grid orientations. As a result, TEM images display that the amyloid fibers are helical even at the lowest hierarchy. On the other hand, we can also use other microscopy techniques which can directly provide us with information in 3D, such as SEM, AFM, and STM (Scanning Tunneling Microscope). We can visualize ~100 fibers from at least three different fiber batches of each peptide using SEM without any fixation and staining. As a result, SEM micrographs display that the handedness of the amyloid fibers is stable through all the hierarchy level. CD spectrographs were used to study the kinetics of secondary changes in amyloid aggregation. NMR studies are used to understand the β structure of the amyloid formation. Amyloid fibrillation’s monitoring in real time is crucial. Most conventional and simple methods are spectroscopy and fluorescence imaging.

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Fig. 4 FMRI scan, yellow and red show regions with considerable positive interaction with either memory task, visual stimulation, or both. Regions demonstrated in green and blue have considerable negative interactions. Percent variations in signal with time, averaged overall voxels in

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In both spectrographic and fluorescence approaches involved with these two techniques, aggregated amyloid is to be stained with one of the following Congo red, thioflavin T fluorophores, a naphthalene derivative, a curcumin derivative, or a conjugated oligothiophene. Both processes require molecular probes such as 9-(dicyanovinyl)-julolidine (DCVJ), 1-anilinonaphthalene-8-sulfonate (ANS), and bis-ANS, which are sensitive to hydrophobic environments (Hawe et al. 2008). The emission from these molecular probes creates a strong binding with a hydrophobic β-sheet which is situated in the region of an amyloid fibril. Although generated binding seems powerful, different fluorophores can aggregate differently in hydrophobic, which can lead to poor signal reproducibility with prompt emission. This cause occurs due to the marking of aromatic rings. Hence, the detection of oligomeric intermediates could result in poor sensitivity, shifting small stokes, lower, and false-positive responses (Celej et al. 2008). Therefore, it is necessary to develop tools that can immensely support detecting early-stage events in amyloid oligomers. Later, new findings have been developed to identify amyloid fibrillation at the early stage. They have used two combined different methods, including using fluorescent protein fragments, which can channel through fluorescence complementation as well as using aggregation-induced emission (Hötzer et al. 2012). Moreover, in recent time, an MRI probe has been developed to detect amyloid fibrillation. This probe is specially made with magnetic nanostructure, which contains an antibody that leads to detect the amyloid β-brain oligomers (Lin et al. 2001). Thus, it leads to detecting the disease at an early stage. Moreover, to overcome the issues that were faced previously in identifying fibril formation, the invention of fluorescence anisotropy is a major support as it increased when fluorescence yield lowered in the process of fibril formation. It was stated in the previous studies that the negatively charged lipid surface stimulated the fibrillation. However, the fluorescence method used to identify fibrillation demonstrated that it occurred when the hydrophobic fatty acyl chain interacted. In this method, it is also possible to detect different fluorescence events separately without depending on each other by using two excitation sources – fluorophores (Xu and Webb 1996). On the other hand, luminescent-conjugated polymer technology is available to analyze amyloid. It plays an important role as it has been developed using high-resolution spectral imaging techniques, which can perform well because of its multiphoton excitation capabilities (Lindgren and Hammarström 2010). Furthermore, there are several intermediate factors that are involved in the process of amyloidogenic that are monomers, oligomers, and protofibrils. This kind of product binds and creates fibrils that significantly form cytotoxicity. The ä Fig. 4 (continued) activated area, are presented by the black thick line to the right of each MRI image: a: Fusiform gyri. and Posterior lingual; b: mid-to-anterior fusiform gyrus; c: posterior midand inferior frontal gyri; d: anterior insula and inferior frontal gyrus; and e: anterior midfrontal gyrus. (Reproduced with permission (Courtney et al. 1997), copyright reserved to Nature Publishing Group 1997)

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major role in amyloid aggregation is conducted by the products of monomers, protofibrils, and oligomers (Kodali and Wetzel 2007). During the most recent 50 years, the use of amyloid assays has a wide range despite limitations. Very limited methods have been used to detect as detection is difficult to distinguish legitimately. In the amyloidogenic process, when in log phase, it is difficult to diagnose the true toxic nature that causes the disease by aggregation (Gilbert 2014). However, new techniques have helped to identify such cases in recent times. Alzheimer’s diseasescreening techniques are currently being used and are deficiently sensitive for preventive measures. The drugs used to treat Alzheimer’s disease and type 2 diabetes do not offer complete recovery (Yiannopoulou and Papageorgiou 2013). It provides only temporary symptomatic relief. Besides, these medications have restricted viability on account of issues, including delivery of these drugs barriers and reduced potency. Therefore, the rising field of nanotechnology brings several diagnostics tools and techniques to beat these challenges.

Nanomaterials for the Treatment of Type 2 Diabetes and Alzheimer’s Disease Technological advancement in the past couple of decades opened various new pathways to understand and study the amyloid aggregation pathogenesis and exploit therapy methods, even though, sadly, complicated puzzles regarding the amyloid aggregation process remain unsolved. Among the new various therapeutic possibilities, nanomaterials are gaining special interest. In recent years, the interaction between biomolecules and nanomaterials turned out to be a very interesting and extensively studied topic. The large surface area of the nanomaterials enables great absorption capabilities and, most importantly, is able to cross the blood-brain barrier (Linse et al. 2007). Therapeutic drugs for Alzheimer’s disease and type 2 diabetes to be penetrated and to be delivered to the brain have been blocked by the physiological and physical barrier called blood-brain barrier in the brain, arguably the most tightly regulated interface in the human body, as it is permeable to weight molecules that could be lipophilicity below 600 Da (Pardridge 2012). Therefore, it will not allow normal therapeutic drug particles to move through. To overcome the challenges faced by the normal therapeutic drugs, nanoscale carriers are being developed to target drug delivery and protect drug molecules that are ineffective on their own. Hence, the study of nanoscale particles is potentially required in medical applications. Usually, nanoscale particles can be characterized as a combination of one kind of chemical, electronic, and physical properties that is biocompatible. Nanoscale properties are the chemistry-based surface that can be tunable, and it has a high surface area. Thus, it remarkably plays an important role in a range of applications like drug gene transfection, cell-imaging photothermal therapy, delivery, and so on (Patra et al. 2018). Recent research shows that whenever a nanoparticle (NP) surfaces, protein confirmation is often altered. This could affect the protein aggregation mechanism and clearly shows that it is capable enough to influence the protein folding. Several

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inorganic NPs show encouraging therapeutic applications: magnetic NPs, Au NPs, quantum dots, etc. Au NPs are inert, biocompatible, and can be easily identified using microscopic and spectroscopic techniques. In recent times, Au NPs have been utilized to observe protein-protein interaction. As a result of Au NP experiments (Illustrated in Fig. 5), the following observations were made: Au NP-Amyloid complex is in an unfavorable traditional amyloid formation; meanwhile, Au NPs covered with glutathione (GSH) can interrupt amyloid aggregation. Peptide Au NPs conjugates redissolve the deposits and restrict the accumulation. Peptide Au NPs help clearing the amyloid by activating microglial (Zhang et al. 2009). On the other hand, magnetic iron oxide NPs are gaining attention because of their magnetic properties.

Fig. 5 (a) Upon adsorption on Au NP, native globular proteins are subjected to a change in configuration; (b) proteins which are partly unfolded on the surfaces of Au NP seed further accumulation of protein; and (c) coalescence of Au NPs that are protein coated. Aggregates of dislodged protein or unfolded proteins in solution nucleate additional accumulation of free-standing aggregate structures of protein. (Reproduced with permission (Zhang et al. 2009), copyright reserved to American Chemical Society 2009)

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Like Au NPs, Magnetic NPs also have similar properties, such as being biocompatible, biodegradable, and relatively nontoxic. Rapid detection, imaging of amyloid plagues, and quantification of target proteins in low abundance, including amyloid in vivo specific biomarkers, that are of great significance in the early diagnosis MRI are used for these applications (Moraes et al. 2012). Insulin fibrils were selectively binding with Meghemite (Υ-Fe2O3) NPs. As a result of magnetic NPs, studies following observations were made. Magnetic NPs selectively adhere to and effectively remove amyloid fibrils. Super magnetic iron oxide NPs (SPIONs) have a surface area-dependent “dual” impact on amyloid fibrillization. Fluorinated magnetic core-shell NPs restrict the formation of amyloid fibril (Skaat et al. 2009). Polymeric particles are one of the important types of NPs used widely. Biologically modified polymeric particles can be used mostly as nanoscale particles in the range of 10–1000 nm in drug delivery, including deoxyribonucleic acid/ ribonucleic acid probes, some antibodies, and peptides (Mudshinge et al. 2011). NPs used to treat Ad and type 2 diabetes can modulate intracellular tight junctions, and it could defeat BBB by assembling an analog in protein that targets the cerebrovascular amyloids. Also, NPs prevent the aggregation from being temporarily reversed which also could prevent fibrillogenesis. NP is also used to detect and study protein aggregation. Dendrimers used for Poly(propyleneimine) eliminate prion and repair the infected cells, poly(amidoamine) demonstrates potential against the formation of amyloid, gallic acid–triethylene glycol dendrimer helps in reducing the toxicity accompanied by accelerated fibril formation process, and sialic acid-conjugated dendrimers contest for cell surface binding with Aβ (Klajnert et al. 2006). Bimolecular aggregates used for the amino acid residues 16–20 (KLVFF) scaffold prevent aggregation of Aβ into amyloid fibrils, lipid-based NPs decrease aggregation of amyloid and prevent conformation transformation, cholesterol-bearing pullulan nanogels prevent the formation of amyloid fibrils and contain toxicity (Mourtas et al. 2011), carbon nano tubes inhibit amyloid fibrillization by destabilizing structure of the β -sheet and evanishing natural susceptibility of proteins to collapse, graphene oxides (GO) used for graphene promote conformation transition and favored amyloid adsorption, fullerenes are used for fullerenes, and derivatives prevent the aggregation of amyloid and evade the cytotoxic effects (Kim and Lee 2003). Among these various nanomaterials, carbon-based nanomaterials got special and undivided attention when it comes to protein aggregation. GO nanosheets are one of the most promising nanomaterials for biomedical research. Similarly, zero-dimensional graphene quantum dots (GQD’s) are considered a potential antiamyloidogenic therapeutic agent.

Quantum Dots for the Treatment of Alzheimer’s Disease and Type 2 Diabetes Amyloid-inhibiting factors in carbon nanomaterials on different dimensions, such as two-dimensional GO, one-dimensional carbon nanotubes, and zero-dimensional carbon dots, have been studied to a great extent because of their structure and

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Fig. 6 Schematic representation of quenching mechanism of BSA@FGQDs in the presence of various peptides. (Reproduced with permission (Yousaf et al. 2019), copyright reserved to American Chemical Society 2019)

properties. Hydrophobic interaction between fluorine-doped Graphene Quantum Dots (FGQD’s), any amyloid peptide (illustrated in Fig. 6), biologically friendly nature, and possibly π π stacking attracts special interest in graphene-based materials known as GQDs. Also, we are able to modify the surface chemistry of the GQDs with heteroatoms doping (Zhang et al. 2013). Generally, QDs have some amazing properties among the nanomaterials, which make them unique such as narrow and symmetric emission spectrum, high photostability, broad excitation band with excellent photoluminescence, biocompatibility, and water solubility. QDs are often used as a probe for amyloid aggregation detection and tracking the fibrillation process (Auer et al. 2009). Capping agents allow us to do modifications on the QD surfaces; as a result, modified QDs are shown as either inhibitory or acceleration on the amyloid aggregation mechanism. Also, capping agents can be used to create model proteins to study the fibrillation process; QDs capped into the human serum albumin system (HSA) with Dihydrolipoic acid (DHLA) led to HAS accumulation and generation of fibrils of protein by being an existing nucleus (Xiao et al. 2010). The larger surface areas of the NPs cater for locally larger concentrations of monomers of Aβ on particles’ surface, thus reducing the nucleation process and aiding fibrillization of Aβ (Zhou et al. 2018). As a result of cerium’s variable valence, cerium oxide NPs (CNPs) were suggested to possess a defensive response against Aβ -mediated neurodegeneration and oxidative stress by altering signal transduction routes involved in neuroprotection. With anti-Aβ antibody conjugation and polyethylene glycol (PEG)-coating, the CNPs can be delivered selectively to the Aβ plaques accompanied by an increment in neuronal survival (Paradise et al. 2008). QDs are

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good for solving the issues related to other carbon materials-induced cytotoxicity and make it possible for clinical trials. Furthermore, the GQDs have an excellent inhibiting affectivity. This study has the potential to significantly bolster the advancement of the therapeutic drug for Alzheimer’s disease.

Nanocarriers for Alzheimer’s Disease Treatment Penetrating the blood-brain barrier is one of the greatest difficulties when it comes to the drug trial for this irreversible brain degeneration. BBB inhibits drugs as well as any toxic substances into the brain for protection. Diffusion and active transport are the two main transport mechanisms across the BBB. Different approaches are being tested for the successful delivery of neurotherapeutic drugs across the blood-brain barrier; however, it is still a big puzzle waiting to be solved (Bhaskar et al. 2010). On the other hand, the NPs show a promising track record when it comes to targeted delivery, disease diagnostics, and therapy in the last couple of decades (Liu et al. 2016). Multifunctional nanocarriers can penetrate the protective neuron shields and successfully deliver the neurotrophic factors into the targeted sites (Hu et al. 2015). Different drug-loading models are able to control the interaction between drugs. Ligand modifications can be used to improve the accumulation in targeted sites. To sustain long-term therapeutic effects, polymeric materials with high degradability and biocompatibility could be applied with nanocarriers (Yang et al. 2016). Optimizing the drug efficiency and further exploring and understanding the pathology are the two most unfavorable conditions when it comes to finding a cure for Alzheimer’s disease. However, theranostic agents that combine targeted cell therapy as well as diagnostics could overcome the disadvantages in the current approaches. The following conditions need to be satisfied when it comes to theranostic systems for Alzheimer’s disease: (1) It should have diagnostic probes that can image amyloid plagues in vivo, and (2) theranostic system should be capable of targeted delivery and controlled release of therapeutic agents (Prades et al. 2012). A fluorescent chelator (BTTA) is a potential theranostic agent for fluorescence and attenuating imaging of Alzheimer’s disease brain (Zhang and Song 2017). Moreover, BTTA is also capable of visually detecting the Aβ aggregation via fluorescence techniques.

Conclusion In summary, research indicates that the deposition of aggregates is made up of a misfolded form of the amyloid beta-peptide (amyloid aggregation) which is the central event in the disease pathogenesis of Alzheimer’s disease and type 2 diabetes. It is very challenging to detect the intermediates of peptide aggregate in early stages due to their complex and variable nature and the dearth of reliable sensors for diagnosis. However, damages done by the Alzheimer’s disease are irreversible as

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well as uncontrollable. Early detection and continuous monitoring are vital to control both Alzheimer’s disease and type 2 diabetes. Nanotheranostics is more suitable for neurodegeneration. Multimodal nano probes, which are capable of detecting the monomers/oligomers of amyloid peptide, implement the investigation of dynamics of monomers of amyloid into amyloid fibrillation in real time, and in vivo label amyloid β aggregates, which will be very useful in early detection of Alzheimer’s disease. As a new fluorescent probe, the QDs can monitor fibrillation of amyloid protein with more sensitivity as compared to the traditional dye method. Targeted drug delivery will be key in Alzheimer’s disease, without any doubt. The QDs probe is capable of crossing the BBB, leading to the amyloid β plaques’ detection in the brain of Alzheimer’s disease patients, and can be easily detected with higher contrast by MRI imaging as compared to a traditional benchmark. Imaging agents that are capable of detecting in vitro and in vivo Aβ monomers, like developed QDs probes, are considered critical for the prevention of disease, its diagnosis, and the medical treatment monitoring and are therefore immensely required. However, developing QDs with multifunctionality and high biosafety is the critical challenge needed to be tackled in the near future to save not only economic burden but also human life and maintain healthier society. These QD-based probes should be capable of sensing the imitating proteins and mapping their paths toward abnormalities which cause Alzheimer’s disease. Acknowledgments The authors would like to acknowledge the Vice-Chancellor fellowship scheme at RMIT University and the School of Engineering for financial support and RMIT University for the RMIT Research Stipend Scholarship (RRSS).

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Generation of Nanoparticles from Waste via Solvent Extraction Method

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Overview on the Problem of Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Waste Management Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Overview on Nanoparticles and Its Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recovery of Metals/Metal Oxide (NPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature Review on the Preparation of Metal Nanoparticles/Metal Oxides from Waste via Solvent Extraction Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of End-Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Global population growth, urbanization, and industrialization have all contributed to a sharp rise in the production of solid waste. Solid waste management (SWM) is a major problem for society since it causes regional problems that have an impact worldwide. Solid waste management techniques that recycle garbage are thus potential procedures that advance sustainable goals. Due to their high population density and unstable economies, developing nations confront various difficulties, including sorting and processing municipal solid waste (MSW). This poor management could exacerbate the development of detrimental environmental and economic issues. In light of this, it is crucial to manage hazardous wastes, including their disposal, in an economical and sustainable manner, and suggestions are provided for improving conventional systems. This chapter indicates the waste management strategies along with the advantages of hydrometallurgical method over the other management strategies. Solvent extraction method or R. Singh (*) Department of Chemistry, School of Basic and Applied Sciences, Lingaya’s Vidyapeeth, Faridabad, Haryana, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_113

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solvent extraction technique has been extensively used due to its flexible handling, versatility, simple operation, environment friendly nature, relatively low operational cost, high selectivity, and recovery of high-purity product. Solvent extraction process can be easily extended from bench level to plant scale. In recent decades, nanotechnology has attained prominent attentiveness and extensive usage in diversified fields. Solvent extraction method is a recycling-based methodology that is applied for the recycling or reduction of end-of-life (EOL) product. The method proposed for the recovery of metal oxides or nanoparticles from spent materials is not constrained by factors like extractant loss during combustion, the release of hazardous gases during pyrolysis, or the integration of contaminants (in direct precipitation from leach liquor). The synthesized nanoparticles can be used in electronic devices, battery electrodes, smart windows, lubricants, display devices, and gas sensors used as catalyst for degradation of dyes and surfaces with tunable emittance for temperature control of space vehicles. Keywords

Spent material · Solvent extraction · Nanoparticles · Environmental remediation · Recovery

Introduction An Overview on the Problem of Waste In recent decades, waste generation has increased tremendously around the world, and there are no signals to decelerate the waste. Waste is a material that is wasted without expecting compensation for its intrinsic value. When these wastes are improperly handled, stored, transported, disposed of, or managed, wastes could constitute a risk to human health or the environment (soil, air, or water). For developing countries, industrialization is must and still this activity excessively demands to uplift nation’s economy. However, rapid industrialization, urbanization, exploitation of non-renewable resources, and improved living standards of people have resulted in adverse effects on the quality of the environment. The generation of solid wastes is expected to exceed 3.40 billion metric tons (MT) annually by 2050 as the global population continues to rise. By 2050, it is anticipated that the amount of waste produced in low-income countries (LIC) will have increased by more than three times. At the moment, Asia produces one-third of the world’s solid waste, including India (0.50–0.9) kg per-capita and China (0.44–4.3) kg per-capita per day (Khan et al. 2022). India is a typical example of developing country receiving considerable pollution from various industrialization processes. The effect of mining and refining of metal minerals on environmental quality is portrayed. Mines produce a lot of waste because the metal is just a little part of the complete volume of the mined material. In the metal business, creation of Cu, Pb, and Zn causes the most

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noteworthy debasement of the climate. Copper mining results in extensive mine squanders and tailings, and Copper purifying discharges approximately 0.11 Mg of S per Mg of Cu delivered in the United State of America. Pb and Zn smelters discharge enormous amounts of Cd and Pb into the atmosphere. Metal purifying and refining produces vaporous (carbon and sulfur dioxide, NOx, etc.) and particulate matter outflows, solid wastes, and sewage waters. Soil tainting with follow metals is considered a big problem connected with purifying; be that as it may, mining and refining are not main well springs of worldwide metal contribution to the earth soils. Smelters are the primary source of environmental emissions of toxic metals on a global scale. A quantitative assessment of the ecological well-being impacts of mining and smelting is troublesome in view of the intricacy of elements involved and absence of reliable strategy. Day by day, the case studies demonstrate that harmful impacts could emerge from lead mining and refining. Risk appraisal uncovered that pecking order contamination by Cd from soils polluted by refining (Ali et al. 2017). Along with the mining waste, electronic waste (e-waste) has been examined as the fast growing part of solid waste in the world. The electrical and electronic industries have experienced a global transformation that has made these products a need in everyone’s daily lives. Numerous household items, such as refrigerators, televisions, washing machines, computers, copiers, and iPads, are among the electronic appliances. As the population uses these goods more frequently, waste linked with them will inevitably be produced. Electronic waste (or “e-waste”) is any abandoned or broken electronics that are no longer useful. Household appliances make up around 42% of the e-waste, followed by communication devices (34%), electronics (14%), and accessories (10%). Worldwide, it was estimated that more than 40 million tons of e-waste were disposed out each year (Baldé et al. 2017). The exact composition of WEEE is unpredictable and also evolves when new innovations are introduced. The two main categories of WEEE are metallic and nonmetallic. Important metal components that can be considered as being in WEEE include valuable metals like Silver, Gold, and Platinum group metals, as well as other valuable transition metals that can have detrimental effects on the climate. Additionally, the ultra nonmetallic components identified in WEEE material include polybrominated diphenyl ethers (PBDEs), which include fire retardants, polychlorinated biphenyls (PCBs), phosphorus (P), Cr (hexavalent), and ozone-exhausting chemicals. When a metal component is plated, a significant amount of bath solution sticks to it as it exits the plating tank, making plating operations another source of valuable metals. This results in the “drag-out” of precious components into the next cascade rinse tanks. These polluted rinse solutions, also known as electroplating wastewater, include significant quantities of acids, sulfate, chloride ions, and other chemicals that are persistent, bioaccumulative, and toxic. Wastewater also contains varying concentrations of iron, copper, zinc, chromium, and nickel. Before wastewater is released into the environment, these toxins must be eliminated due to the considerable damage they pose to ecological systems and human health (Kul and Oskay 2015). Dyes are widely used in a variety of industries, including textile, printing, dyeing, and food, as consumer interest in product appearance on the market grows. There are numerous hazardous and

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non-biodegradable colored dyes in use today. The waste produced during the dying process is extremely hazardous and has an impact on the environment, natural processes like eutrophication, and the metabolism of living things. Due to the release of potentially harmful and poisonous substances into the receiving water body, dye effluent released by industries has the potential to seriously harm the environment. The highly colored materials prevent light from passing through and create harmful compounds when dye is hydrolyzed in water, endangering aquatic life. Aesthetic and environmental issues make color removal from textile dyeing and finishing industry wastewater significant (Sarayu and Kanmani 2003). As a result, recycling waste is crucial for preventing environmental damage from improper disposal or treatment, increasing resource usage efficiency, and lowering environmental impacts (Zeng et al. 2018).

Waste Management Strategies The “fix and reuse” rule ought to be applied for powerful administration of waste emerging to empower both decrease and reusing of waste material. To carry out this head, government ought to likewise energize renovation model where customers can be engaged with temptations, for example, lesser duty rates to buy revamped articles. Notwithstanding energize the assets reusability and reusing, makers ought to be bound to regulations to adhere to the EPR standard and rules should incorporate for least utilization of risky substances and virgin crude constituent. There is likewise a need to build up the current regulations and strategies through reliable evaluations and revisions. Industrial waste and e-waste are often managed using a combination of collection, transport, landfilling, incineration, and recycling techniques. Some nations have set up garbage collection systems with the intention of enhancing the environment. However, because consumers find it challenging to distinguish between different types of waste, implementing a separate collecting system for each type of waste is inconvenient (Xará et al. 2015). Waste is disposed of through landfilling or incinerated when there is no established means of collection and transport. There are three different types of landfills: open dumps, regulated dumping, and secure landfilling (Chandrappa and Das 2012). Long-term confinement waste disposal is one of these secure landfilling processes. Before disposal, pre-treatment procedures such as hazardous property reduction or elimination, volume reduction, and waste stabilization are carried out. When you landfill something, rainwater leaches the components and infiltration happens. The toxicity of metal elements in surface and groundwater streams is increased by leaching and infiltration (Karnchanawong and Limpiteeprakan 2009). The cost of safely disposing of industrial waste and e-waste is rising because there aren’t enough landfill deposition sites and there’s a lot of it being produced. The waste is collected throughout the incinerating process, and then the waste is burned in large quantities. In nations where there is a limited amount of land, this approach is common. The material is converted by the incinerator into ash, flue, and

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heat. Additionally, burning used batteries results in environmental hazards since hazardous metals including Cd, Hg, and dioxins are released into the atmosphere. Eutrophication of the ocean is brought on by the disposal of fly and bottom ash leachate. Environmental regulations have recently imposed restrictions on landfilling and incineration. Recycling, combined with the simultaneous preservation of primary raw materials and the recovery of precious or valuable metals, is the most efficient strategy to manage trash and minimize environmental damage. The two basic activities in recycling-based management are pyrometallurgical and hydrometallurgical processes. The primary consideration is the metal’s purity. Since they are found in low concentrations in secondary sources such as metal scraps and industrial effluents, the approach should be such that they can be readily collected in high purity. Pyrometallurgical and hydrometallurgical procedures are the two main activities used in recycling-based handling of used batteries. The general answer to recovering valuable components from industrial or metal-loaded wastes is the pyrometallurgical process. Pyrometallurgical processes have ended up being more effective for the extraction of metals, like Ti, Zr, Nb, Ta, Mo, and so on. Pyrometallurgy uses thermal treatment to cause physical and chemical changes in the materials such that valuable metals can be recovered. Metal recovery uses a variety of techniques, including melting, roasting, converting, and refining. Because it is not as critical as it is for chemical treatments, this process can be completed quickly. The principal drawbacks of the pyrometallurgical process also include high energy consumption, gas evolution, dust emission into the environment, and metal loss from the scrap during combustion. One of the most efficient methods for recovering metals and their oxides is hydrometallurgy. This process includes breaking down the waste material (solid waste), washing it in water to remove alkali or alkaline salts, etc. Different approaches, such as leaching with acid (H2SO4 and HCl) and an alkaline medium (NaOH/NH3), have been investigated in a solution for desired metals. The concentration of alkali or base, the solution’s pH, the metal’s immersion solvability, and temperature are only a few of the variables that might affect how productively metals are extracted. Additionally, the effect of reductants (such as citrus extract, ascorbic acid, and oxalic acid) and oxidants (such as hydrogen peroxide) on the specific precipitation of particles has been studied. Processes used in hydrometallurgy are typically more productive and cost-effective. Less air pollution, less energy use, ease of scaling up, and great selectivity are the benefits. In order to recover valuable metals from industrial and electronic waste, a number of hydrometallurgical techniques have been documented (Gunarathne et al. 2022; Ashiq et al. 2019; Chen et al. 2015a; Jha et al. 2016). Figure 1 depicts the closed loop for materials or a product.

An Overview on Nanoparticles and Its Applications In recent decades, nanotechnology has attained prominent attentiveness and extensive usage in diversified fields. The nanoparticles are the foundation of nanotechnology. Between 1 and 100 nanometers in size, nanoparticles can be composed of

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Fig. 1 Closed loop for product cycle

New Product Production Use of product Recovery of valuable Metal or product

End of Life/Function

Recycle

Disposal Waste

carbon, metal, metal oxides, or organic material. When compared to their counterpart particles at higher scales, nanoparticles have unique physical, chemical, and biological features. This effect is caused by factors such as significantly higher surface area to volume ratio, better chemical reactivity or stability, improved mechanical strength, less temperature change, adjustable pore size, closer interparticle diffusion distance, and numerous related adsorption sites. These characteristics of nanoparticles have led to their use in a variety of applications with the creation of novel materials that make it possible to identify and remove a variety of targets from a range of media. Chemicals are found in natural waterways, wastewaters, and air, furthermore to organic contaminants, polluting gases (SO2, CO, NOx, etc.), and biological substrates (such as bacteria, viruses, and antibiotics), among others (Thirunavukkarasu et al. 2020). According to their characteristics, forms, or sizes, nanoparticles can be categorized into a number of types. The diverse group includes fullerenes, metal nanoparticles, burned nanoparticles, and polymeric nanoparticles. Due to their extensive surface area and small size, NPs exhibit amazing physical and chemical characteristics. Absolute metal precursors are used to make metal nanoparticles (NPs). These NPs possess distinctive optoelectrical features because of their remarkable localized surface plasmon resonance (LSPR) attributes. The visible region of the electromagnetic sun spectrum has a wide absorption band for NPs of alkali and noble metals including Cu, Ag, and Au. It is important today to prepare metal nanoparticles in a regulated manner for their aspect, size, and shape. Metal NPs find use in numerous scientific fields due to their superior optical characteristics. For instance, Gold NPs are frequently used for SEM examination in order to enhance the electronic stream and facilitate the acquisition of high-quality SEM images. In order to effectively remove different heavy metals, organic contaminants, and other inorganic pollutants from aqueous solutions, metal nanoparticles have been utilized as adsorbents, reductants, oxidants, and catalysts. They can also be used to clean up soil that has been contaminated with heavy metals and organic pollutants. However, because of their increased surface energy, metal nanoparticles are more likely to aggregate to create large-scale particles. By customizing their uptake mechanism, NMs can be created with great selectivity against target contaminants. Iron, gold, titanium oxide, iron oxide, and zinc oxide are a few of the popular nanoscale metals and metal oxides

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used for such purposes. The form, size, and structure of the nanoparticles vary. Its shape can be spherical, cylindrical, tubular, conical, hollow core, flat, etc., or irregular, and its size can range from 1 nm to 100 nm. The surface may be uniform, asymmetrical, and have surface differences. Some nanoparticles are crystalline or amorphous, containing one or more loosely dispersed crystals or aggregates of crystals. The capacity of NPs to medical assistance within the ideal dosage range has attracted growing interest from many areas of medicine. This frequently results in better therapeutic effectiveness of the medications, improvements in patient compliance, and less negative effects. Iron oxide particles like magnetite (Fe3O4) or its oxidized equivalent maghemite (Fe2O3) are most frequently used in biological applications (Ali et al. 2016). Because heavy metals have harmful impacts on the environment and human health, such as mercury, lead, thallium arsenic, and cadmium, their removal from natural water has attracted a lot of interest. For this hazardous soft substance, superparamagnetic iron oxide NPs work well as sorbent materials. For instance, gold nanoparticles are useful for purifying water of impurities like mercury, pesticides, and chlorinated organic compounds. Carbon nanotube clusters are frequently used to remove heavy metals and absorb microbes from water. Many high- and low-level firms have documented the potential advantages of nanotechnology, and industries like microelectronics, aircraft, and pharmaceuticals are currently massproducing commercial products. Health and fitness products currently make up the largest category of consumer goods utilizing nanotechnology. These are followed by electronic and computer devices, as well as home and garden products. Numerous industries, especially those involved in packing and food processing, have praised nanotechnology as the upcoming revolution. Many of the researchers and scientists found that NPs are the best substitute because of their high surface area, optical properties, and catalytic nature, to produce renewable energies. NPs are commonly used to generate energy from photoelectrochemical (PEC) and electrochemical water splitting, especially in photocatalytic applications. NPs can have various uses in mechanical industries, particularly in coating, lubricants, and adhesive applications, as shown by their exceptional young modulus, stress, and strain properties. Additionally, this feature can be used to create nanodevices that are mechanically stronger for a variety of applications. By encapsulating nanoparticles (NPs) in the metal and polymer matrix to boost their mechanical strengths, tribological properties can be adjusted at the nanoscale level. This is because of the possibility of very low friction and wear from NPs rolling in a lubricated contact area. Additionally, the lubricating effect of NPs is increased by their good sliding and delamination capabilities, which could result in minimal friction and wear (Mobasser and Firoozi 2016). Pictorial presentation of applications of nanoparticles in various fields is shown in Fig. 2.

Recovery of Metals/Metal Oxide (NPs) In the past two decades, processing or recovery of metals from electronic scrap, secondary sources, and industrial waste by hydrometallurgical techniques has

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Fig. 2 Pictorial presentation of applications of nanoparticles in various fields (Kumar et al. 2019)

become a well-established research area. Hydrometallurgical processes are more precise, predictable, and manageable as compared to pyrometallurgical processing and use low temperature and better control of the recovery of by-products. Pyrometallurgical processes involve high energy consumption, production of harmful greenhouse gases, and large capital investment. Hydrometallurgical techniques such as solvent extraction (SX), adsorption, ion exchange, precipitation, and supported liquid membranes have been extensively used in the separation and recovery of metals from various solutions. Among these, solvent extraction technique has been extensively used due to its flexible handling, versatility, simple operation, environment friendly nature, relatively low operational cost, high selectivity, and recovery of high-purity product. Solvent extraction process can be easily extended from bench level to plant scale.

Liquid–Liquid Extraction (Solvent Extraction) Procedure Solvent extraction has led to the development of innumerable separation procedures that have been successfully applied in organic, inorganic, and analytical chemistry fields, pharmaceuticals, and food and petrochemical industries. Its economic viability and integrated concentration step, which offers a viable commercial method for the beneficiation of low-grade metal sources and recovery of compounds from complicated matrices, can be attributed to the widespread use of liquid–liquid extraction for metal recovery. Additionally, solvent extraction is a useful method for researching the principles of kinetics and equilibrium in complicated formation processes. The solvent extraction process mainly consists of three basic steps as shown in Fig. 3.

Stripping aqueous phase

Loaded organic phase

Fig. 3 Solvent extraction scheme applied for metal recovery

Loaded aqueous phase

Scrubbing

Feed aqueous phase + fresh organic phase

Stripping

Extraction Recycled organic phase

Loaded aqueous phase

Raffinate

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• An extraction step, for selective extraction of a given metal • Scrubbing step, to remove the co-extracted metals • Stripping step, to separate the desired metal from the loaded organic phase For the purpose of producing tons of uranium through the selective extraction of uranyl nitrate with ether, the first large-scale industrial solvent extraction plant was constructed in 1942. Solvent extraction was first used as a separation and purification technique in a significant range of chemical and metallurgical sectors in the 1950s and early 1960s (Rydberg 2004).

Solvent Extraction Procedure and Measurements By contacting the solubilized form of the metal in an organic solvent, which is typically used to extract metals from aqueous solutions, the dissolved metal ions in the aqueous phase are effectively transferred to the organic phase (selectively at specific Ph values or acidic conditions), where they are then selectively recovered by transferring back to the aqueous phase, one at a time (stripping process). The concentration of metal ions in the organic phase was calculated by mass balance as follows ½Metalorganic ¼ ½Metalaqueous before extraction  ½Metalaqueous after extraction

ð1Þ

The following equations were used to determine the distribution ratio (D), extraction efficiency (%E), stripping efficiency (%S), and separation factor (β). D¼ %E ¼

½Metalorganic ½Metalaqueous

ð2Þ

½MetalB:E  ½MetalA:E  100 ½MetalB:E

ð3Þ

½Metalaqueous  100 ½Metalorganic

ð4Þ

%S ¼

β¼

D1 D2

ð5Þ

[Metal]B.E and [Metal]A.E are concentration of metal ion before and after extraction. D1 and D2 are the distribution ratio of metal ion 1 and 2. The standard setup that has been frequently used in research and is typically used in industrial applications is counter-current extraction. The two phases are handled in inverse directions at the end of each bank of contactors in a counter-current extraction system. The amount of hypothetical stages predicted to extract a metal (M) in a counter-current extraction framework has been thoroughly estimated using the

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McCabe-Thiele graph. This graph takes into account an isotherm extraction that displays the required metal’s (M) concentration in equilibrium in both phases and an operational line with the A/O proportion used throughout as its slope. According to Ritcey and Ashbrook (1984), McCabe-Thiele charts are constructed for the purposes of extraction and stripping. Counter-current occurs when the number of hypothetical steps is obtained. Since then the interest of researchers for the synthesis of novel extractants for the separation and recovery of metal ions has rapidly increased. A perusal of literature reveals that various extractants such as carboxylic acids chelating agent, organophosphorus compounds, and high-molecular-weight amine have gained attention for the extraction, separation, and recovery of different metal ions (Kim et al. 2015; Nusen et al. 2015; Fu et al. 2011). It is known that use of carboxylic acids and chelating extractants for metal recovery has been declined due to some of their drawbacks like water solubility and emulsion formation in aqueous phase. Neutral organophosphorous extractants possess similar drawback of solubility in the aqueous phase. The release of H+ ions into the aqueous phase during the extraction of metal ions using acidic extractants has a negative impact on the extraction efficiency. Neutralization of the discharged acid and saponification of the extractant are needed to prevent this issue (Kumari et al. 2016). When used with organic diluents, high-molecular-weight amine extractants frequently experience the issue of colloidal aggregation, which leads to emulsion formation and poor selectivity (Moore 1960). Therefore, it is worthwhile to research novel reagents to see whether there is a way to avoid the aforementioned disadvantages. Over the past few decades, from an environmental and technological perspective, ionic liquids (ILs) have become a new class of substances and have attracted much more interest from the scientific and industrial communities. The number of articles investigating ionic liquids has grown exponentially in the literature. Ionic liquids exhibit unique properties such as negligible vapor pressure (1011– 1010 mbar), high thermal stabilities (generally thermal decomposition temperature range is 250–450  C), ability to solubilize a wide range of inorganic, organic, and polymeric materials, non-flammability, wide electrochemical window, and good extraction power. Some other advantages of ionic liquids are their strong hydrophobicity and are liquid over a wide range of temperature. A remarkable advantage of ILs is possibility of anion substitution that enables designing of compounds with the attributes required for a specific task. Owing to these properties, applications of ILs are being extensively studied in very diverse areas, such as organic synthesis, catalysis, polymer science, separation technology, analytical chemistry, electrochemistry, nano-chemistry and functional fluids (e.g., lubricants, heat transfer fluids, and corrosion inhibitors) has become the preferred method to extract different metals (Rios et al. 2010; Palacio and Bhushan 2010; Kumar and Malhotra 2009; Larsson and Binnemans 2014). The

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Spent catalyst/ Industrial waste/EOL product

Leaching/Digetion

Filtration Residue Leach liquor (various type of metals such as Zn, Cd, Al, Fe etc.)

Extraction with Ionic liquid

Raffinate

Desired metal Loaded organic phase

Stripping with solution

Stripped solution

Evaporation

Precipitation with suitable agent

Nanoparticles

Ionic Liquid Washing with water

Ionic Liquid

Washing with water

Ready to reuse

Ready to reuse Thermal decomposition at 400˚ C for 2 hours

Nanoparticles Fig. 4 Flow sheet for the extraction and recovery of metals and synthesis of nanoparticles from waste via solvent extraction method

overall process in the form of flow sheet for the extraction and recovery of metals and synthesis of nanoparticles from waste via solvent extraction method is shown in Fig. 4.

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Literature Review on the Preparation of Metal Nanoparticles/Metal Oxides from Waste via Solvent Extraction Method To cite examples for recovery of metal oxides (nanoparticles) from waste via SX followed by precipitation or hydrothermal method has been provided here in accordance to year wise. Guo et al. (2009) synthesized rare earth metal nanoparticles, that is, NdF3 and TbF3 nanoparticles via solvent extraction method by Cyanex 923. The metal-loaded organic phases were mixed with solution of NH4F and Triton X-100. The final product was then subjected to a 20minute ultrasonic treatment in room air. It produced white precipitation characterized by SEM and TEM in the range of 50–80 nm. Barakat et al. (2009) recovered nano-palladium (Pd) from spent Pd/Al2O3 catalyst. Leaching agent was implemented as a mixture of HCl and AlCl3. Palladium that was deposited was observed to be in the nano-size range (34.5–70.5 nm). Chen et al. (2015b) reported the separation and recovery of metal values from mixed-type used lithium-ion battery leaching fluid. Lithium remaining in the leaching fluid was progressively precipitated as highly pure metal oxide Ni(OH)2 and Li3PO4 by NaOH and Na3PO4 solutions. Suh et al. (2015) employed low-grade iron ore to make magnetite nanoparticles (MNPs) and a Mg-rich solution, which were used as coagulants and nano-adsorbents for water treatment, respectively. Trin-butyl phosphate was used as an extractant to remove the impurities from the ore after it had been leached with aqueous hydrochloric acid. Pagnanelli et al. (2016) reported a process to recover Co from portable lithiumion batteries (LIB). Finally, the improved process stages were carried out in order to produce cobalt carbonate. Products with varying degrees of purity were produced based on the purification stages that were carried out (precipitation with or without solvent extraction). By using the procedure, which included solvent extraction phases using D2EHPA and Cyanex 272, 95% purity was attained. Deep et al. (2016) reported the recovery of the ZnO nanoparticles with Cyanex 923 from spent batteries based on the effects of the solid/leaching solution (w/v) ratio, the solid/extractant ratio (w/v), and the reaction temperature, and the desired product ZnO nanoparticles were obtained quantitatively (>95%) recovered by liquid–liquid extraction followed by precipitation with 70% ethanol. Morphology and structural studies of recovered ZnO were done by TEM analysis. With the help of EDX and XRD analyses, the phase purity of the recovered product is evaluated. The recovered nanoparticles were found to be in the 5 nm or less size range. Sinha et al. (2016a) recovered Zn and Cu in the form of their oxide from spent brass pickle liquor. Authors have selectively extracted Cu and Zn from other metal ions such as Fe, Ni, and Cr with 30% LIX 984 N via solvent extraction method. Stripped solutions have been used for the synthesis of ZnO and Cu metal powder via precipitation method. Sinha et al. (2016b) proposed a method for recovering zinc and iron as added-value materials from the Cyanex 923 waste chloride solution. To create the high-purity (99.9% pure) zinc oxide, the zinc oxalate was calcined. After further reducing the ferric oxalate solution with Fe-powder to obtain ferrous oxalate, which was then

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calcined to produce pure hematite, chemical analysis, XRD, and SEM examinations were used to characterize the oxalates and oxides in order to determine the purity and shape of the as-synthesized products. Singh et al. (2017a) studied separation and recovery of zinc and cadmium from spent batteries and prepared ZnO and CdO nanoparticles from E-waste. For this objective, Zn and Cd were extracted and recovered from Zn-C and Ni-Cd battery leach liquid using Cyphos IL 102 that had been diluted in toluene. Using the loaded organic phase via precipitation method, ZnO and CdO were also synthesized, and their properties were examined using XRD, FE-SEM, and EDX methods. The zincite and monteponite peaks on the X-ray diffractogram for ZnO and CdO, respectively. For ZnO and CdO, the average particle sizes were 27.0 nm and 37.0 nm, respectively. ZnO and CdO exhibit a nearly 1:1 atomic proportion according to the EDX study. Further authors have used Cyphos IL 102 as a new reagent for the extraction and recovery of Cadmium(II) and Zinc(II) from a solution containing hydrochloric acid. A separation strategy was developed based on the extraction data and used a synthetic mixture as well as used zinc plating mud to recover Zn (II). With a purity of greater than 99.00%, Zn(II) was quantitatively recovered (96.64%) (Singh et al. 2017b). The research group Singh et al. (2018b) earliest exploration For the extraction, separation, and recovery of Mo(VI) from a hydrochloric acid medium in the form of MoO3 particles from used waste, Cyphos IL 102 was used as an organic phase. The project is focused on recovering metals from hazardous solid waste called wasted catalyst. The recovered metal can also be used in a variety of other fields in the form of metal oxide. Due to its economic viability and integrated concentration stage, solvent extraction for metal recovery has become a widely used commercial technology for the beneficiation of low-grade metal sources and recovery of compounds from intricate matrices. On Mo(VI) extraction, loading, and recycling capacity of the extractant, the impact of fundamental extraction factors has also been assessed. MoO3, produced by stripping the solution by thermal decomposition XRD, FE-SEM, and EDX methods were used to characterize. High loading capacity and reusability of extractant help the economic and environmental aspects. Singh et al. (2018) separated and recovered V(V) from sulfate medium from spent catalyst. On the V(V) extraction, the effects of many fundamental extraction factors, including acid and extractant concentration, equilibration duration, and temperature, have been assessed. The back extraction of vanadium has been investigated using several stripping agents. The extractant’s saturation and recycling capabilities have both been tested. For the extraction and recovery of V(V) from used V2O5 catalyst leach liquor, optimized parameters have been used. Vanadium oxide was also created through thermal degradation and examined using XRD, FE-SEM, and EDX methods. Mahandra et al. (2018b) investigate recovery of molybdenum as MoO3 from the stripped solution obtained after solvent extraction with Cyphos IL 104 diluted in toluene from spent hydrodesulphurization catalyst. The impact of different parameters has been calculated. Both loading capacity and recyclability have been evaluated. The process’s endothermic character was confirmed by the influence of temperature. Finally, molybdenum is obtained from a strip solution and is then

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examined using XRD, FE-SEM, and EDX techniques. Assefi et al. (2018) created a practical and environmentally friendly approach for synthesizing porous NiO nanocuboids using a discarded Ni-Cd battery as a precursor. When a Ni-Cd battery reaches the end of its useful life, this technique can recycle the nickel from the cathodic electrode by first leaching it in hydrochloric acid and then reprocessing it via hydrothermal synthesis. Mahandra et al. (2018a) recommended employing a novel extractant to recover zinc and cadmium from wasted battery leachates via a hydrometallurgical process. In order to create zinc and cadmium oxides from the loaded organic phase, precipitation and thermal breakdown of precursors at 400  C were used. XRD, FE-SEM, and EDX methods were used to characterize synthesized materials. Hexagonal zincite and cubic monteponite forms, respectively, zinc and cadmium oxide particles were obtained. Zinc and cadmium oxides have typical particle sizes of 43 nm and 55 nm, respectively. Zinc and cadmium oxide particles are thought to have a globular shape with particle aggregation. Nanusha et al. (2019) reported that by combining solvent extraction with the extractant N,N0 -dimethyl-N,N0 dicyclohexylthiodiglycolamide (DMDCHTDGA) in toluene with biological methods based on the employment of sulfate-reducing bacteria, Pd and Fe were separated and recovered as nano-sized metal sulfides. Dhiman and Gupta (2019) addressed the recycling of valuable metals from spent Li-ion battery waste. Researchers quantitatively extracted the Co metal with Cyphos IL 102 via solvent extraction route, and loaded organic phase was used for the preparation of Cobalt oxide nanoparticles and MnO2 and lithium oxide were also synthesized and characterized by XRD, FE-SEM, and EDX. Rezazadeh et al. (2019) successfully produced the magnetite nanoparticles from the raffinate of an industrial copper solvent extraction unit in Iran. For hydrometallurgical copper recovery to be successful, iron must be removed from the leach solution. The nearly spherical nanomagnetite particles have a diameter of 30 nm obtained. Swain et al. (2020) described a hydrometallurgical route (liquid–liquid extraction) for the separation and recovery of samarium and cobalt from waste magnetic solution using Tri-n-Octyl Phosphine Oxide (TOPO). Quantitative extraction of samarium was observed with TOPO at PH of 3.0. Stripping of Sm from loaded organic phase was done with 0.02 M HCl, and this stripped solution and raffinate solution undergo the formation of Sm2O3 and Co3O4 nanoparticles and characterized by X-ray diffraction, FE-SEM, and EDX studies. Zhang et al. (2020) provided novel approach for the recovery of lithium from spent lithium battery using solvent extraction method with β-diketone system, and stripped solution was used in the preparation of metal oxide Li2CO3. The synthesized product was analyzed by XRD and SEM techniques. Dhiman and Gupta (2020b) created a method for recovering germanium using solvent extraction from the leach fluid of used Zener diodes. A phosphonium ionic liquid called Cyphos IL 104 is used as an extractant throughout the extraction process. By carefully removing these metal ions, germanium has been separated from copper, mercury, and iron. Using NaOH as a precipitating agent, germanium is recovered from the strip solution as germanium oxide. Dhiman and Gupta. (2020a) reported effective separation and recovery of In, Zn, and Sn from discarded LCD screen with Cyphos IL 104 via

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solvent extraction method which has both environmental and economical advantage. Indium, Zinc, and Tin were eventually recovered as nanoparticles with average sizes of 42.4 nm for In2O3, 41.1 nm for ZnO, and 68.8 nm for SnS, respectively, from their respective strip solutions. Mahandra et al. (2020) addressed the extraction and recovery of vanadium (V) from synthetic and real leach solution of spent V2O5 catalyst using Cyphos IL 104. The extractant has a high loading capacity and is recyclable for multiple cycles. With good separation factors, vanadium (V) was isolated from aluminum (III), titanium (IV), chromium (III), manganese (II), iron (II)/(III), molybdenum (VI), and tungsten (VI). Alkaline leaching was used to extract metals from a used catalyst. Rodriguez et al. (2020) extracted Nb and Ta via liquid– liquid extraction using the Cyanex 923 extractant, from the Sn-Ta-Nb mining tailings. They were then separated from the equivalent aqueous solution and calcined, producing tantalum salt with a purity of 97.3% after calcination and Nb2O5 with a purity of 98.5%. Kalpakli et al. (2021) used leaching, solvent extraction, precipitation, and thermal decomposition routes to recover Zn in the form of ZnO from waste materials used in the production of steel. Leach solution of spent material was employed for solvent extraction with D2EHPA solution (20%, vv). Oxalic acid solution at pH 4 was added to strip solution obtained from solvent extraction, and ZnC2O4∙2H2O powder was precipitated. Zhou et al. (2021) synthesized nanomaterial and the recovery of gold from used circuit boards for mobile phones. By using the emulsion liquid membrane approach with MIBK as a carrier, Au(III) was successfully recovered from WMPCBs and synthesized into nanomaterials in a single step and characterized by SEM and EDX. The same authors have prepared cobalt oxide nanoparticles (Co3O4) waste Li-ion batteries and of particle size about 47 nm for nanoparticles. The methyl blue dye (MYB) was degraded using the produced particles under various light sources. They addressed the numerous experimental conditions, including catalyst loading, methyl blue starting concentration, solution pH, and light source type. Results of the study observed that pseudo first-order kinetics was followed for photodegradation of dye and exhibited >86% efficiency for dye removal (Dhiman and Gupta 2021). Huang et al. (2021) proposed a method using an alkaline glycine solution to separate zinc and cobalt from harmful zinc–cobalt slag. By precipitating with an oxalic acid solution, zinc and cadmium were recovered from the glycine leachate with precipitation ratios of 97.7% and 99.0%, respectively. Nanda et al. (2022) used to remove hematite (α-Fe2O3) using a hydrometallurgical process from Red-Mud following solvent extraction with 1 M TBP, the leach liquid is precipitated with sodium hydroxide to yield iron hydroxide. At 400  C, this iron hydroxide is calcined, and X-ray diffraction confirms that the calcined result is α-Fe2O3. To create a hetero structure (CuO/-Fe2O3), different weight percentages of CuO are combined with the α-Fe2O3. Malachite green (MG) is broken down utilizing the photo-Fenton reaction and the α-Fe2O3-CuO catalyst. By using XRD, FTIR, UV-vis DRS, PL, and an electrochemical analysis, all the composites are identified. Alcaraz et al. (2022) investigated the extraction method for lanthanum oxide from various spent fluid catalytic cracking catalysts using Cyanex 923 solvent extraction and nitric acid leaching. The corresponding lanthanum oxalates were

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obtained by quantitatively stripping lanthanum using oxalic acid, as shown by X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTA), and Fourier transform infrared (FTIR) methods. These solids produced lanthanum oxide after 2 hours of thermal processing at 1200  C. Nobahar et al. (2022) investigated prior separation of ferric iron by solvent extraction using AliCy and/or alkalinization for zinc recovery from an extreme copper-free acid mine drainage (AMD). Extreme AMD can be converted into ZnS nanoparticles by separating Cu and Fe3+ by AliCY’s solvent extraction, followed by alkalinization at pH 3.25 to 3.5. Paramount care has been taken in citing the relevant literature. However, if any important contributions are omitted, they are unwittingly or due to unavailability of the information or error in judgment. Hence, the author would like to apologize for any such omission/oversight.

Characterization of End-Products Portrayal of the prepared items is a basic step of the generation/synthesis. In this unique circumstance, the significant issues that should be considered are the virtue and morphology of the recuperated nano-sized item and the rate yields. The utilization of the suitable portrayal apparatuses not just guarantees that the created items coordinate with the basic organization, molecule size, and morphology of NPs, yet additionally decides their novel surface qualities, for example, explicit surface region and surface reactivity, which are valuable for modern applications. Characterization helps us to assess the procedure’s efficacy as well as the information’s content and structure. Numerous characterization methods have been investigated for analysis of the materials, including scanning electron microscope (SEM) analysis, energy dispersive X-ray analysis (EDX), transmission electron microscope (TEM) analysis, scanning tunneling microscopy (STM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectrometry (UV-vis), X-ray photoelectron spectroscopy (XPS), and dynamic light scattering. High-energy electron beams from the SEM scan the sample’s surface. SEMs use light waves to produce a magnified image, which makes them different from traditional light microscopes. The electron beam in an SEM interacts with the specimen surface when it hits the surface. Three different types of electrons – backscattered (or primary), secondary, and Auger electrons – as well as X-rays are released when the input electron beam strikes the sample. The primary, or backscattered, and secondary electrons are used in SEM. Using secondary electrons, the SEM creates high-resolution images that reveal details as small as 1 to 5 nm. The EDX method makes use of distinctive X-rays to determine elemental compositions. In this method, the image is likewise formed using the backscattered electrons. By examining elements that are close to the surface and estimating the amount of each element at various places, this method provides an overall mapping of the sample. In EDX, when an electron beam with an energy of 10–20 keV strikes the surface of a conducting sample, the energy of the emitted X-rays varies on the

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material being investigated. By moving the electron beam across the sample, each element can be seen clearly. Transmittance electron microscopy (TEM) provides morphological features and particle size of prepared nanoparticles. TEM also provides the information about the bulk material from very low to higher magnification with two or more layers. In TEM analysis, an electron beam interacts with the sample as it passes through it, and the transmitted electrons are then magnified and focused by an objective lens to create the image. Scanning tunneling microscopy (STM) converts surface scans with atomically fine lateral resolution. With the assist of a piezoelectric crystal, a fine probe with a tip scans the surface of the conducting sample, and the resulting tunneling current is detected. The main premise of STM is quantum tunneling. Due to the bias, electrons can tunnel through the vacuum when the sample’s surface makes contact with the conducting tip. At low voltages, the sample’s local Fermi level density of states determines the tunneling current. STM is considered to have good resolution at depth resolutions of 0.01 nm and lateral resolutions of 0.1 nm. Atomic force microscopy (AFM) is an effective and extensible type of microscopy that is used to examine samples at the nanoscale. Engineers and scientists can use it to capture an image of a three-dimensional topography and get a variety of surface metrics. AFM can create images with atomic resolution and information about height while requiring minimal sample preparation. It can be used to determine the surface roughness and see the surface texture on various types of materials in polymer nanocomposites. It also has a high three-dimensional spatial resolution and is nondestructive. By scanning the materials using infrared light, FTIR analysis is used to determine organic, inorganic, and polymeric components. Changes to the regular pattern of absorption bands clearly demonstrate a change in the material composition. FTIR can be used to determine breakdown and oxidation, identify additives, find impurities in a material, and identify and classify unidentified components. The sample is exposed to infrared radiation at a wavelength of 10,000–100 cm1, part of which is absorbed and some of which passes through. The radiation’s absorbed energy is converted by the sample into vibrational or rotational energy. The final signal generated by the detector, which corresponds to the molecular fingerprint of the samples, has a spectrum that normally falls between 4000 and 400 cm1. Since every molecule has a unique fingerprint, FTIR is an essential method for classifying compounds. UV-Visible spectroscopy (UV-Vis) analyses the extinction of light passing through a substance (scatter plus absorption). Due to the distinctive optical properties of nanoparticles, which depend on their size, shape, concentration, level of aggregation state, and refractive index close to the surface, UV-Vis is a useful tool for recognizing, describing, and researching nanomaterials. In addition, most laboratories have UV-vis spectrometers, analysis doesn’t change the material, and spectrum registration only takes a short while.

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Using X-ray photoelectron spectroscopy (XPS) can determine the nature of NP surfaces as well as the composition and thickness of coatings on NPs and bonding structure of elements in prepared NPs. It can be used in various ways to extract significant information on the composition of the NPs, the presence of contamination, the consistency of the functionalization, the amount of adsorbates on surfaces, and the thicknesses of layers and coatings. The hydrodynamic diameter of nanoparticles in solution is measured using dynamic light scattering (DLS), which also provides information on the aggregation condition of the particles in solution. DLS measures the amount of laser light that is scattered as it passes through the colloidal solution. We can determine the particle size from the analysis of the time-dependent modulation of the scattered light’s intensity. XRD analysis is used to determine the sample’s crystallinity. It can be used to provide additional information in addition to serving as identification. If the sample is a combination, XRD patterns can be utilized to calculate the elemental ratios. The data analysis can also be used to determine a given element’s structural condition, degree of crystallinity, and departure from its ideal composition. Only a portion of the X-ray beam is transmitted as a result of its interaction with the atomic planes; the remainder is absorbed, refracted, dispersed, and diffracted by the sample. X-rays are diffracted differently by each element based on the type of atoms and their atomic arrangement. In addition to this, solubility of NPs (in water, etc.) is examined using inductively coupled plasma-mass spectrometry (ICP-MS) techniques. The optical absorption, transmittance, and reflectance are measured using the UV/vis-diffuse reflectance spectrometer instrument, which is typically employed to determine the bandgaps of NPs. Some images and spectra of XRD, FE-SEM, and EDX of the prepared ZnO and CdO from spent batteries via solvent extraction method are shown in Fig. 5 (Singh et al. 2018a).

Conclusions Starting from the commencement of NPs over 50 years prior, researchers are consistently investigating progressed novel techniques for NPs with the ideal size and morphology that would be valuable for different disciplines. To design and manufacture an optimal size and morphology of NPs, contingent on their application/use, various scope of physical, substance, or natural techniques are now accessible. There is no question with respect to the significance of reusing EoL commercial items and recuperating the value-added products, for example, nanosized metal oxides and different materials. In this survey, we gave a complete outline for the preparation of nanoparticles via solvent extraction method from end of life of the product (spent material). The present study suggests that solvent extraction is a simple and efficient method to produce high-quality metal oxide product from a multi-element leach solution of spent materials. The approach suggested for recovering metal oxides from used materials is not constrained by problems like extractant

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Fig. 5 a X-ray diffraction (XRD) pattern of (a) ZnO and (b) CdO nanoparticles. b FE-SEM images of synthesized (a) ZnO and (b) CdO. c EDX spectra of synthesized (a) ZnO and (b) CdO

loss during burning, the genesis of dangerous gases during pyrolysis, or the integration of contaminants (in direct precipitation from leach liquor). The synthesized nanoparticles can be used in electronic devices, in gas sensors, display devices, lubricants, smart windows, optical filters, electrodes of batteries, and surfaces with

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tunable emittance for temperature control of space vehicles. All of the generated metal oxide particles can be utilized as adsorbents to clean up pollutants and as catalysts to break down organic contaminants. The planning of this chapter is such that it primarily focuses on metal separation chemistry using commercial extractant. If after working out the desired parameters the proposed bench-level chemistry is favorably scaled up to plant scale, it can be very useful economically by recovery of valuable metals and environmentally by taking care of the problem of metal pollution.

References Alcaraz L, Largo OR, Alguacil FJ, Montes MÁ, Baudín C, López FA (2022) Extraction of lanthanum oxide from different spent fluid catalytic cracking catalysts by nitric acid leaching and Cyanex 923 solvent extraction methods. Metals 12(3):378 Ali A, Zafar H, Zia M, Haq I u, Phull AR, Ali JS, Hussain A (2016) Synthesis, characterization, applications, and challenges of iron oxide nanoparticles. Nanotechnol Sci Appl 9:49 Ali SH, Giurco D, Arndt N, Nickless E, Brown G, Demetriades A et al (2017) Mineral supply for sustainable development requires resource governance. Nature 543(7645):367–372 Ashiq A, Kulkarni J, Vithanage M (2019) Hydrometallurgical recovery of metals from E-waste. In: Electronic waste management and treatment technology. Butterworth-Heinemann, Amsterdam, pp 225–246 Baldé CP, Forti V, Gray V, Kuehr R, Stegmann P (2017) The global e-waste monitor 2017: quantities, flows and resources. United Nations University, International Telecommunication Union, and International Solid Waste Association Barakat MA, EL-Mahdy G, Hegazy M, Zahran F (2009) Hydrometallurgical recovery of nanopalladium from spent catalyst. Open Miner Proces J 2(1):31–36 Chandrappa R, Das DB (2012) Waste from electrical and electronic equipment. In: Solid waste management. Springer, Berlin, Heidelberg, pp 197–216 Chen X, Chen Y, Zhou T, Liu D, Hu H, Fan S (2015a) Hydrometallurgical recovery of metal values from sulfuric acid leaching liquor of spent lithium-ion batteries. Waste Manag 38:349–356 Chen X, Xu B, Zhou T, Liu D, Hu H, Fan S (2015b) Separation and recovery of metal values from leaching liquor of mixed-type of spent lithium-ion batteries. Sep Purif Technol 144:197–205 Deep A, Sharma AL, Mohanta GC, Kumar P, Kim KH (2016) A facile chemical route for recovery of high quality zinc oxide nanoparticles from spent alkaline batteries. Waste Manag 51:190–195 Dhiman S, Gupta B (2019) Partition studies on cobalt and recycling of valuable metals from waste Li-ion batteries via solvent extraction and chemical precipitation. J Clean Prod 225:820–832 Dhiman S, Gupta B (2020a) Cyphos IL 104 assisted extraction of indium and recycling of indium, tin and zinc from discarded LCD screen. Sep Purif Technol 237:116407 Dhiman S, Gupta B (2020b) Recovery of pure germanium oxide from Zener diodes using a recyclable ionic liquid Cyphos IL 104. J Environ Manag 276:111218 Dhiman S, Gupta B (2021) Co3O4 nanoparticles synthesized from waste Li-ion batteries as photocatalyst for degradation of methyl blue dye. Environ Technol Innov 23:101765 Fu W, Chen Q, Hu H, Niu C, Zhu Q (2011) Solvent extraction of copper from ammoniacal chloride solutions by sterically hindered β-diketone extractants. Sep Purif Technol 80(1):52–58 Gunarathne V, Rajapaksha AU, Vithanage M, Alessi DS, Selvasembian R, Naushad M, Ok YS (2022) Hydrometallurgical processes for heavy metals recovery from industrial sludges. Crit Rev Environ Sci Technol 52(6):1022–1062 Guo F, Li H, Zhang Z, Meng S, Li D (2009) Synthesis of REF3 (RE¼ Nd, Tb) nanoparticles via a solvent extraction route. Mater Res Bull 44(7):1565–1568

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Pagnanelli F, Moscardini E, Altimari P, Atia TA, Toro L (2016) Cobalt products from real waste fractions of end of life lithium ion batteries. Waste Manag 51:214–221 Palacio M, Bhushan B (2010) A review of ionic liquids for green molecular lubrication in nanotechnology. Tribol Lett 40(2):247–268 Rezazadeh L, Sharafi S, Schaffie M, Ranjbar M (2019) Synthesis and characterization of magnetic nanoparticles from raffinate of industrial copper solvent extraction plants. Mater Chem Phys 229:372–379 Ritcey GM, Ashbrook AW (1984) Solvent extraction: Principles and applications to process metallurgy. Part I, New York: Elsevier Scientific Pub. Co. Rodríguez O, Alguacil FJ, Baquero EE, García-Díaz I, Fernández P, Sotillo B, López FA (2020) Recovery of niobium and tantalum by solvent extraction from Sn–Ta–Nb mining tailings. RSC Adv 10(36):21406–21412 Rydberg J (ed) (2004) Solvent extraction principles and practice, revised and expanded. Boca Raton, CRC Press Sarayu G, Kanmani S (2003) Treatment of textile dyeing wastewater using UV/solar photofenton oxidation processes. Indian J Environ Health 45(2):113–120 Singh R, Mahandra H, Gupta B (2017a) Recovery of zinc and cadmium from spent batteries using Cyphos IL 102 via solvent extraction route and synthesis of Zn and Cd oxide nanoparticles. Waste Manag 67:240–252 Singh R, Mahandra H, Gupta B (2017b) Solvent extraction studies on cadmium and zinc using Cyphos IL 102 and recovery of zinc from zinc-plating mud. Hydrometallurgy 172:11–18 Singh R, Mahandra H, Gupta B (2018a) Cyphos IL 102 assisted liquid-liquid extraction studies and recovery of vanadium from spent catalyst. Miner Eng 128:324–333 Singh R, Mahandra H, Gupta B (2018b) Optimization of a solvent extraction route for the recovery of Mo from petroleum refinery spent catalyst using Cyphos IL 102. Solvent Extr Ion Exch 36(4): 401–419 Sinha MK, Pramanik S, Sahu SK, Prasad LB, Jha MK, Pandey BD (2016b) Development of an efficient process for the recovery of zinc and iron as value added products from the waste chloride solution. Sep Purif Technol 167:37–44 Sinha MK, Sahu SK, Pramanik S, Prasad LB, Pandey BD (2016a) Recovery of high value copper and zinc oxide powder from waste brass pickle liquor by solvent extraction. Hydrometallurgy 165:182–190 Suh YJ, Do TM, Kil DS, Jang HD, Cho K (2015) Production of high-purity magnetite nanoparticles from a low-grade iron ore via solvent extraction. Korean Chem Eng Res 53(1):39–45 Swain N, Mishra S, Acharya MR (2020) Hydrometallurgical route for recovery and separation of samarium (III) and cobalt (II) from simulated waste solution using tri-n-octyl phosphine oxide–a novel pathway for synthesis of samarium and cobalt oxides nanoparticles. J Alloys Compd 815: 152423 Thirunavukkarasu A, Nithya R, Sivashankar R (2020) A review on the role of nanomaterials in the removal of organic pollutants from wastewater. Rev Environ Sci Biotechnol 19(4):751–778 Xará S, Almeida MF, Costa C (2015) Life cycle assessment of three different management options for spent alkaline batteries. Waste Manag 43:460–484 Zeng Y, Lai Z, Han Y, Zhang H, Xie S, Lu X (2018) Oxygen-vacancy and surface modulation of ultrathin nickel cobaltite nanosheets as a high-energy cathode for advanced Zn-ion batteries. Adv Mater 30(33):1802396 Zhang L, Li L, Rui H, Shi D, Peng X, Ji L, Song X (2020) Lithium recovery from effluent of spent lithium battery recycling process using solvent extraction. J Hazard Mater 398:122840 Zhou W, Liang H, Xu H (2021) Recovery of gold from waste mobile phone circuit boards and synthesis of nanomaterials using emulsion liquid membrane. J Hazard Mater 411:125011

Plant-Mediated Synthesis of Nanoscale Hydroxyapatite: Morphology Variability and Biomedical Applications

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Ana Paula Fagundes, Afonso Henrique da Silva Ju´nior, Domingos Lusitaˆneo Pier Macuvele, Humberto Gracher Riella, Natan Padoin, and Cíntia Soares

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Morphology Control: General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surfactant Free . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoscale Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis Methods for Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Synthesis of Hydroxyapatite Nanoparticles – Morphological Variability . . . . . . . . . . . Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Hydroxyapatite nanoparticles are the most versatile materials. This mineral comprises phosphorus and calcium, a natural constituent of bones and teeth. Its chemical, textural, morphological, and toxicological characteristics present potential applications in various fields. It is applied in bone scaffolding, drug delivery, coating metallic implants, and others in the biomedical area. Due to environmental concerns related to the application of nonenvironmentally friendly templates, the obtention of hydroxyapatite nanoparticles was revisited in the last A. P. Fagundes · A. H. da Silva Júnior · H. G. Riella · N. Padoin · C. Soares (*) Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil e-mail: [email protected]; [email protected]; [email protected] D. L. P. Macuvele Laboratory of Materials and Scientific Computing (LabMAC), Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil Center for Studies in Science and Technology (NECET), Department of Science, Engineering, Technology and Mathematics, University of Rovuma-Extension of Niassa, Lichinga, Mozambique © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_96

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20 years by applying a green synthesis approach. This approach includes the use of plants extracts as templates, biowastes as a source of calcium or phosphorus, the free-template method, and others. Hydroxyapatite green synthesis using plants as a template was achieved, and the results are promising. Nonetheless, size and shape control remains a challenge. Hydroxyapatite with various sizes (less than 100 nm) and shapes (rods, flower-like, spheres, peanuts-like, irregular shapes, and others) is obtained depending on the plant species. The properties and applications of nanoparticles are size and shape dependent. Understanding the morphology control in plant-mediated hydroxyapatite nanoparticles is extremely important. Therefore, this chapter discusses the green synthesis of hydroxyapatite nanoparticles mediated by plant extracts. Emphasis is directed to size and shape variations and biomedical applications. Keywords

Nanotechnology · Green synthesis · Green nanomaterials · Biomedical applications · Bone tissue engineering

Introduction Hydroxyapatite nanoparticles represent a biomaterial with a plethora of interest in the biomedical field. These materials can form a real bond with bones because these are a major component of the mineral part of bones of several vertebrates. Additionally, hydroxyapatite is biocompatible and nontoxic and presents outstanding chemical, textural, and morphological characteristics. Consequently, it is a promising material for various fields such as biomedical, environmental, and many other fields (Melgar et al. 2021a). In the last years, nanotechnology researchers and scientists have been worried to turn nanoparticles synthesis greener and more eco-friendly (Gopi et al. 2015a, b; Kalaiselvi et al. 2018; Melgar et al. 2021a; Citradewi et al. 2021). Turning the process ecofriendly includes but not limited to substitution or reduction of surfactants to obtain nanoscale hydroxyapatite with desired shape and size. For these reasons, the synthesis of hydroxyapatite nanoparticles was revisited by including the green chemistry protocols (Melgar et al. 2021a). There are several strategies to turn the hydroxyapatite nanoparticles eco-friendly. This includes but is not limited to the use of biomass as a source of calcium or phosphorus, reduction of surfactants templates, the use of plant extracts as green templates, etc (Mathina et al. 2022; Saravanakumar et al. 2022). However, the use of plants extracts in the hydroxyapatite nanoparticles synthesis presents several reports, due to the larger variability and availability of plants. Green synthesis of hydroxyapatite of nanoparticles assisted by plants extracts is promising. Consequently, there are several reports that obtained hydroxyapatite with several sizes and shapes. However, the optimization of synthesis process to obtain hydroxyapatite nanoparticles with certain morphology remains a challenge.

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Depending on the type of the plant used in hydroxyapatite synthesis, various particles with different shapes and sizes are obtained. Using Euclea natalensis root extracts in green synthesis, spherical like hydroxyapatite nanoparticles was obtained (Melgar et al. 2021a). Nanorods hydroxyapatite were also obtained using licorice root extract (LE) as a green organic template (Ali et al. 2021). Other shapes such as plates were also obtained using Moringa oleifera flower extract (Sundrarajan et al. 2015). Therefore, the present chapter provides a general outlook on morphology variability of hydroxyapatite nanoparticles obtained in the presence of plant extracts. Firstly, general aspects regarding the morphology control of nanoparticles will be addressed, with special emphasis on conventional techniques to control the morphology of hydroxyapatite nanoparticles. In this section, synthetic surfactants are highlighted as the main approach to obtaining the hydroxyapatites nanoparticles with controlled morphology. Secondly, the main issue of this chapter is stated, starting with general aspects of nanoparticle synthesis. Thirdly, biomedical applications of hydroxyapatite-based materials are presented. Finally, in the concluding remarks, the main insights are addressed, and the main gaps are also highlighted.

Morphology Control: General Aspects In nanotechnology, the morphology control represents an important issue, because the properties and applications of the nanoparticles are influenced by morphological aspects (Bakshi 2016). Morphology in nanoscience includes size, shape, surface properties, etc. However, in this chapter, the emphasis will strictly be directed to the shape and size of the nanoparticles. Morphology control is dependent on kinetics and thermodynamics aspects. Kinetics refers to the stabilized face that led to the most favorable pathway toward final products. Thermodynamics refers to stabilized faces leading to the most stable final product, with desired shape and size. Generally, the methods to control the particle morphology are related to the synthesis approach. In nanoparticle synthesis, bottom-up and top-down are the main group of methods that are applied. Bottom-up is the most applied approach to obtaining nanoparticles with desired size and shape. This trend is possible because, in this approach, the nanoparticles are obtained from atoms or ions, leading to control parameters during the synthesis. Different frombottom-up, top-down methods start with large particles to nanoparticles. This approach’s main challenge is the difficulty to control the characteristics of obtained particles. Several factors affect the morphology of nanoparticles, for example, pH, concentration of precursors, temperature, solvents, surfactants, etc. In bottom-up method, the surfactant is a most studied factor and it directly interferes in the shape of the nanoparticle (Viswanath and Ravishankar 2008; Bakshi 2016; Hajimirzaee et al. 2019). Basically, the surfactants interfere on crystal growth via diffusion and selective adsorption. Additionally, surfactants can act as capping agents, oxidation

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etching agents, structure-directing agents, reducing etch agents, and ion exchange agents.

Surfactants Surfactants are most applied in the synthesis of nanoparticles with desired size and shape (Mary et al. 2016). The main advantage is related to the easy reproducibility of the synthetic approach. Surfactants present an active surface, leading to their easy preferential adsorption on the surface of the crystal. Several types of surfactants can be used in shape-directed synthesis. However, ionic surfactants resulted in an evident effect on shape directing (Bakshi 2016). Surfactants were reported in hydroxyapatite nanoparticles synthesis and the results are promising. More recently, spherical like hydroxyapatite nanoparticles were synthesized via a microemulsion method. The microemulsion system was constituted of polyvinyl alcohol (PVA), dimethyl sulfoxide, and hexane. The hydroxyapatite nanoparticle obtained presented a pure phase and uniformity in size and shape (Prakash et al. 2021). A plate-like hydroxyapatite nanoparticle was obtained hydrothermally in the presence of anionic surfactant and sodium dodecyl sulfate (Nathanael et al. 2020). Charge distribution on the hydroxyapatite surface revealed that the anionic surfactant hindered the crystal growth in the c-axis, favoring the growth in the a-axis (Nathanael et al. 2020). In the same perspective, microwave-assisted hydrothermal synthesis was applied in the obtention of hydroxyapatite nanoparticles with tunable nanoscale characteristics for biomedical applications. Oxalic and sodium dodecyl sulfate was used to direct the size and the shape. Interestingly, in the absence of oxalic acid and sodium dodecyl sulfate, the hydroxyapatite nanoparticles presented a rod-like shape with agglomeration. However, in the presence of surfactant, the shape remained the same, but with reduction of agglomeration (Karunakaran et al. 2019). No-ionic surfactant (polyethylene glycol, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), polyvinyl alcohol, and polyoxyethylene sorbitan monolaurate) and cationic surfactants (hexadecyl-trimethyl-ammonium bromide, hexadecylamine, polyacrylamide) were applied in the synthesis of hydroxyapatite nanoparticles. The evident effects of the presence of surfactants are the reduction of particle agglomeration. However, the shape of nanoparticles is not clear enough, because the authors present SEM micrographs with a resolution that does not allow for observing the shape of nanoparticles (Hajimirzaee et al. 2019). Similarly, cetyl-trimethylammonium-bromide (CTAB) induced the morphological change of hydroxyapatite nanoparticles from nanorods to nanowires in a hydrothermal process (Nathanael et al. 2016). The pristine hydroxyapatite nanoparticles presented a rod-like shape, after the addition of CTAB hydroxyapatite with a wirelike shape was observed. Additionally, this shape modification was also attended with a slight reduction of diameter and an increase in nanoparticle length. The formation of hydroxyapatite nanowires was ascribed to the anisotropic charge distribution on the different faces of the HA (Nathanael et al. 2016).

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Surfactant Free Beside surfactants, other factors such as pH, the concentration of precursors, Schiff base can control the morphology of nanoparticles. An innovative approach for obtaining hydroxyapatite nanoparticles with diverse shapes was developed by the addition of Schiff bases (Mohandes and Salavati-niasari 2014). Nanorods and nanobundles were obtained in the presence of Schiff bases. Furthermore, the mechanism of formation of these shapes is complex, but the authors suggested that it involves the coordination of Ca2+ and Schiff base molecules forming the [CaNO]2+ complexes in the reaction solution (Mohandes and Salavati-niasari 2014). Another approach to control the morphology of hydroxyapatite nanoparticles consists of adjusting the reactant addition rate. In a typical hydroxyapatite synthesis, ammonium phosphate was added at different rates to calcium nitrate. When adding between 0.80 mL∙min 1 and 14 mL∙min 1 of ammonium phosphate, rod-like shaped hydroxyapatite nanoparticles were obtained. From 23.3 mL∙min 1 to 140 mL∙min 1, hydroxyapatite nanoparticles with spherical shapes were obtained (Kramer et al. 2014). Supersaturation can reduce the crystal size and increases the chance to increase nucleation rather than crystal growth (Kramer et al. 2014). However, the mechanism behind the shape tailoring is not clear, from rod to spherical. Uniform hydroxyapatite nanobelts were obtained hydrothermally adjusting reactional variables (An et al. 2016). The results showed that the formation of hydroxyapatite nanobelts is reaction time-dependent. Before hydrothermal reaction, the precursors are composed of nanorods and nanosheets. In the first 30 min, small amounts of nanorod and nanosheet are converted into nanobelts. Between 2 and 4 h, all nanorods and nanosheets are transformed into nanobelts (An et al. 2016). The mechanism of the transformation of nanosheets and nanorods into nanobelts involves several steps. Briefly, due to the poor crystallinity and stability of precursors at high temperatures, are dissolved, and the nucleation occurs again, favoring the most stable shape (nanobelts) (An et al. 2016). Recently, hydroxyapatite with plates-like shape was obtained in the presence L-histidine. This work is most interesting because the synthetic approach was able to produce the hydroxyapatite nanoparticles with the same shape as naturally occurring hydroxyapatite in the bones of vertebrates. Hydroxyapatite nanoparticles were obtained from the mineralization approach at physiological pH and temperature. All conditions tested in the presence of L-histidine resulted in hydroxyapatite nanoparticles of plate-like shape, while in the presence of L-glutamine, the morphology was dependent on reactional variables (Chauhan and Singh 2021). More recently, an innovative approach to control the morphology of hydroxyapatite nanoparticles was presented. This strategy consists of the addition of citric acid to the reactional system. Interestingly, the presence of citric acid induced the formation of spherical-like shapes and negatively charged hydroxyapatite nanoparticles (Liu et al. 2022). In summary, several approaches can be adapted to control the morphology of nanoparticles, for example, adjusting of reactional variables, such as pH, the concentration of reactants, rate of addition, surfactants, and others. However, the

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surfactants are most reported due to their intrinsic mechanism to direct a certain shape. Additionally, the surfactants not only control the morphology, but can also serve as dispersants, avoiding the agglomeration of nanoparticles. In the case of hydroxyapatite nanoparticles, several approaches were found in the literature to control the morphology, including the adjusting of parameters, the presence of surfactants, Schiff bases, and others. Despite these advances, most approaches to obtain a specific morphology were done in a batch system, suggesting that still necessary additional works are needed to produce in-flow hydroxyapatite nanoparticles with desired morphology.

Nanoscale Synthesis Nanoscience and technology provide a basis for the development of materials with specific and adjustable properties depending on the desired application. Nanoparticles involve numerous classes of structures that can be classified as nanorods, nanospheres, nanotubes, nanocubes, nanofibers, nanosheets, and nanoplates, with size equal or less to 100 nm. However, in some cases, some particles with size > 100 nm are considered nanoparticles, when its properties are significantly enhanced compared to bulk materials. (These nanostructured materials can be synthesized by various methods such as co-precipitation (Guesmi et al. 2018), hydrothermal (Bensalah et al. 2018), microwave irradiation (Mishra et al. 2014), ultrasonic (Deng et al. 2019), and reverse microemulsion (Ma et al. 2016). However, most of these methodologies require more complex synthesis conditions, such as the use of toxic solvents and high energy consumption; in addition, they generate problems for the postprocess, with the generation of dangerous by-products. To overcome the disadvantages, environmentally friendly (clean and nontoxic) procedures for the synthesis of nanoparticles are under increasing research and development. Given this scenario, green nanotechnology approaches have been gaining prominence, as an environmentally friendly and cheap process, using plant substrates. Depending on the synthesis conditions, it is possible to control the morphology of the nanomaterials (e.g., size, proportion, geometry, and porosity). Thus, nanoparticles synthesized using green models can act as an excellent material for biomedical applications, being able to face the obstacles that these technologies imply.

Synthesis Methods for Nanoparticles Synthesis procedures can be defined in two ways: top-down methods (bead milling, thermal evaporation, laser ablation, sputtering) and bottom-up methods (chemical vapor deposition, hydrothermal, co-precipitation, sol-gel). Briefly, in the top-down proposal, crystals with dimensions in the nanometer range are obtained through chemical and physical processes (high-pressure environment, high temperatures, processing equipment, etc.). The disadvantages of this route are related to large

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particle size variation, surface defects, and high process cost. In the bottom-up approach, nanomaterials are built from atomic or molecular precursors that react by increasing the particle size or agglomerate to form structures. The great advantage of this method is the well-defined control of shape, size, and chemical composition. In addition, they feature lower surface defects and good atomic packing, providing greater reliability and further expanding the applications of nanoparticles (Jia et al. 2019; Abid et al. 2022). To ensure that the synthesized nanostructures have a defined size and morphology and agglomerate control, the use of specific reagents is adopted, providing satisfactory results (Viswanath and Ravishankar 2008). This point is the major bottleneck of the synthesis process because depending on the morphology of the nanoparticles, it is the most recommended type of application; in addition, it can make the application result even more favored. Some examples of morphological structures obtained by the synthesis methods are nanorods (Gopi et al. 2013; Mohandes et al. 2014; Kalaiselvi et al. 2018; Subramanian et al. 2019), nanosheet (Qi et al. 2012; Yahia et al. 2017; Cheng et al. 2021), ball-like structure (Siriphannon et al. 2002; Gopi et al. 2015a), cluster-like/granular-like (Gopi et al. 2015b; Mathina et al. 2020), bead-like structure (Rusu et al. 2005), cubic-like (Farzadi et al. 2011; Kumar Vemulapalli et al. 2020), capsule-like (Mishra et al. 2014; Kumar Mishra et al. 2016), flake-like (Santana Vázquez et al. 2016; Pang et al. 2018; Ibraheem et al. 2019), needle-like (Wijesinghe et al. 2014; Klinkaewnarong and Utara 2018), crystalline plate-like structure (Neira et al. 2009; Sundrarajan et al. 2015), hexagonal structure (Neira et al. 2008; Chen et al. 2011), rectangular and elongated (Wakamura et al. 1997, 2003; Nayar and Guha 2009), and spherical (Song et al. 2021; Fu et al. 2008; Ma et al. 2016). In the top-down approach, the simplest, most efficient, and used mechanical process is ball milling, producing nanomaterials by friction. This methodology consists of grinding the microparticles to nanoparticles using equipment (e.g., planetary mill, high energy mill, friction mill, horizontal mill, vibratory mill, and rotary mill). The definition of the materials obtained depends on process parameters such as rotation, time, temperature, atmosphere, and ball/powder mass ratio (Kang et al. 2021; Selmani et al. 2022). In bottom-up processes, chemical vapor deposition is one of the most diversified, attractive, and applied methods, showing good results for the creation of nanostructures. The methodology consists of the deposition of a thin layer of gaseous reagents on a substrate. When heated, a chemical reaction takes place, and as a result, a thin layer is formed on the surface of the substrate. This technique has many advantages, highlighting the production of extremely hard, robust, homogeneous, and pure nanostructures. It is also possible to regulate the crystal structure, the surface morphology, and the orientations of the nanostructures, through the control of the process parameters (Zhong et al. 2021; Abid et al. 2022). Overall, both have advantages and disadvantages, but the choice of the appropriate synthesis method must comprise several points of interest, based on the available primary materials, the facilities, and equipment, the final application, in addition to economic and environmental issues. With a deeper understanding, it is also possible

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to perform the integration of different nanofabrication methods, providing a better combination of nanomaterial-based synthesis tools. However, most of these methods employ organic solvents that are dangerous for the environment and for living things. To overcome the challenges mentioned above, many researchers are studying the synthesis of nanoparticles from green materials that are naturally abundant (Zhou et al. 2012; Melgar et al. 2021b; Ali et al. 2021; Bee et al. 2022). These green routes make use of biological materials such as extracts derived from plants, algae, bacteria, and fungi. It is classified as an environmentally friendly route compared to the methods mentioned above as the materials are nontoxic, readily available, and inexpensive. The variety of materials available is numerous; however, the application of agricultural plant residues can be even more attractive due to the aggregation of value to a residual material.

Green Synthesis of Hydroxyapatite Nanoparticles – Morphological Variability The synthesis of nanomaterials using the green route has been receiving great attention nowadays, establishing itself as a technology for the production of nanostructures that minimizes the toxic effects resulting from conventional procedures (Naikoo et al. 2021; Kumar et al. 2021). This method, in addition to being sustainable, is economically interesting; thus, it is considered a viable and reliable tool in the synthesis of hydroxyapatite nanoparticles (nHAP). One of the most fascinating factors in nanoscience is the wide variety of nanomaterials. And in search of this environmentally friendly alternative to produce nanostructures, the use of plant extracts as a precursor or surfactant is studied, replacing chemical reagents and avoiding the agglomeration of nanoparticles. These plant extracts contain phytochemicals, which have reducing and antioxidant effects. Providing shape and size below and above the micellar concentration induces the growth and stability of nanoparticles. In this way, this material can be used as a template/surfactant for the nucleation and controlled growth of nanoparticles, controlling the morphology, stabilizing the size, and varying adjustable properties in order to prepare nanoparticles for specific applications (Elbasuney 2020; Melgar et al. 2021a). In recent years, the production of nanoparticles with controlled morphology has been extensively studied, mainly of hydroxyapatite nanoparticles that use different reducing agents and antioxidants under different experimental conditions. Ibraheem et al. (2019) used pectin derived from Parkia biglobosa pulp for the production of nHAP; according to the morphological analysis, the nanoparticles were slightly porous and in flakes with a lower degree of agglomeration, being suitable for application in biomedicine. At 0.5% pectin, the morphology of nHAP appeared to be spherical and clustered compared to nHAP synthesized using 0.1% and 1% pectin. This result suggests that pectin not only served as a template for the synthesis but also influenced the nature of the particles formed. In addition to this work, there

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were already reports in the literature that polysaccharides influenced the physical properties of nanomaterials when they act as models (Gopi et al. 2014). In a publication by Bee et al. (2022), nHAP was prepared via the green route, using an aqueous extract of Indian curry leaves (Murraya koengii) as a reducing and stabilizing agent. By varying the concentration of AgNO3 precursor used, the amount and size of spherical silver nanoparticles (AgNPs) deposited on the surface of the nHAP can be adjusted. Figure 1 shows the transmission electron microscopy

Fig. 1 HRTEM and lattice fringe for pure nHAP (a, b), nHAP-AgNPs-1 (c, d), nHAP-AgNPs-5 (e, f) and nHAP-AgNPs-7 (g, h) nanocomposites. (Copyright 2022, Elsevier; Reproduced with permission) (Bee et al. 2022)

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(TEM) images of the materials. The chicken bone-derived nHAP showed an irregular structure with interplanar spacing, corresponding to the hexagonal crystal structure (Fig. 1a, b). The micrographs of the nanocomposites confirmed the presence of spherical nanoparticles on the surface of the nHAP. The biosynthesis methodology favored the deposition and growth of AgNPs on the surface of the nHAP rather than in the reaction solution. In Fig. 1c, e, g, it can be seen that the amount of AgNPs on the surface of nHAP increased as the concentration of AgNO3 was also increased. AgNPs of uniform size and without noticeable agglomeration were evenly distributed along the surface of the nHAP (Fig. 1e, f). However, when the concentration used was 7 mM (Fig. 1g, h), larger clusters formed, modifying the size of the AgNPs deposited. In summary, the high content of silver ion precursors may have contributed to the formation of inhomogeneous sizes of AgNPs due to aggregation, resulting in a wider distribution of nanoparticle size. Melgar et al. (2021b) applied Euclea natalensis roots as a green extract for the first time. The green template induced a change in the shape of the nanohydroxyapatite. The nanoparticles synthesized through a chemical precipitation method showed a rod-shaped structure (Fig. 2a). The material resulting from the same previous procedure, but more diluted, also has a similar shape to the previous one, but with a larger particle size (Fig. 2b). Nanogrooves on the surface of nHAP were observed for the methodologies that did not use the green extract (Fig. 2d, e). However, the morphology of the sample that used the plant extract as a mold showed

Fig. 2 TEM micrographs of the sample via chemical synthesis (a), sample via dilute chemical synthesis (b), and green synthesis (c). Magnification of TEM micrographs to show the difference of nanogrooves: chemical synthesis (d) and dilute chemical synthesis (e). (Copyright 2022, Elsevier; Reproduced with permission) (Melgar et al. 2021b)

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a different shape (Fig. 2c), in this case, as spherical agglomerates. In addition, they presented a smooth surface in relation to chemical syntheses. Citradewi et al. (2021) prepared hydroxyapatite doped with silver nanoparticles mediated by Clitorea ternatea derived from cockles bark. The in situ incorporation of AgNPs into the nHAP structure was performed by the hydrothermal method. By TEM analysis, the samples showed particle sizes ranging from 4 to 9 nm with irregular spherical shapes. This size distribution was excellent, being smaller than the size obtained by previous works. In addition, the material was tested as antibacterial, being efficient against E. coli, K. pneumoniae, S. aureus, and S. pyogenes. There are also reports of obtaining hydroxyapatite nanorods synthesized in the presence of licorice root extract (Glycyrrhiza glabra), as a green organic template through a microwave hydrothermal synthesis route. The morphology of the sample from the licorice root showed aggregates of well-crystallized, uniform nanorods with similar size and morphology (Fig. 3a, b), while the control sample (without green extract) presents aggregates of nonuniform rods to stick-like nanoparticles of variable dimensions, shape, and orientation (Fig. 3c, d). Therefore, the green mold improved the crystallinity of the product, the size, and the shape of the nHAP (Ali et al. 2021). Hydroxyapatite nanorods were prepared by the green synthesis process using leaf extract of Azadirachta indica and Coccinia grandis. The results indicated that the samples obtained from nHAP have a hexagonal crystal structure (nanorods),

Fig. 3 SEM (a, c) and TEM (b, d) morphographs of nHAP samples with licorice root (a, b) and nHAP control (c, d). (Copyright 2022, Elsevier; Reproduced with permission) (Ali et al. 2021)

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whereas the control sample (no green template) consists of aggregated spherical particles. Therefore, when the plant extract was used as a solvent, the formation of nHAP with reduced particle size occurred due to the strong interaction between the carboxyl and nHAP groups, controlling the growth of the nanoparticles. FTIR spectra indicated that biomolecules such as flavonoids, terpenoids, and protein compounds are present with nHAPs, confirming their chemical heterogeneity. Finally, the application test confirmed that the material produced using plant extract as a solvent has antibacterial activity against Escherichia coli and Staphylococcus aureus (Kumar et al. 2017). In addition to extracts produced from roots and leaves, it is also possible to extract the green template from fruits. Buitrago-Vásquez and Ossa-Orozco (BuitragoVásquez and Ossa-Orozco 2018) used ripe fruits grown in Colombia (mango, grapes, and tamarind) to produce extracts for green synthesis. In the images obtained through SEM analysis, it was observed for all protocols that the morphology is nanorods on a nanometric scale. However, the nanorods that were synthesized with grape extract are smaller and shorter. Possibly the fiber content affected the particle size in each extract obtained. Thus, different characteristics of the fruit extracts affect the nanoparticles obtained, being able to act as inhibitors of particle growth during hydrothermal synthesis. In a comparative study (Nayar and Guha 2009), nHAP nanocomposites were synthesized using different waste materials, such as eggshell, orange and potato peel, papaya leaf, and calendula flower. The eggshell extract has calcium, amino acids, proteins, and glycoproteins; these compounds exerted control over the synthesized nHAP. The enoic acids present in the potato peel extract have a strong affinity for calcium ions and also exert morphological control in the synthesis. Beta-carotene and other vitamins present in papaya leaf extract and limonene content in orange peel oil also played a role in strict control over the size and shape of the nanoparticles. The carotenoids present in the calendula resulted in an elongated morphology. In addition, the formation of agglomerates was observed when the potato peel extract was used. Finally, nHAP synthesized without additives showed no control over the size and shape of the synthesized particles. Rod-like hydroxyapatite nanoparticles with reduced length and diameter in the presence of saponin from plants were obtained (Balakrishnan et al. 2019). Hydrothermally obtained hydroxyapatite in the absence of saponins has rod-like shape with larger particles between 50 and 300 nm. In the presence of saponin, the length and diameter of nanoparticles reduced evidently. This phenomenon was ascribed to increase the nucleation sites and inhibition of random growth in the presence of saponin (Balakrishnan et al. 2019). Sustainable manufacturing of hydroxyapatite nanoparticles is critical for biomedical applications due to their excellent biocompatibility with the body. It can be applied in biofunctional scaffolds, drug delivery systems, bone replacement, and implant coatings. But to ensure good results in the application, nHAPs need to be synthesized with quality through consistent parameter control, obtaining nanoparticles in different morphologies, including nanorods, nanospheres, nanotubes, nanocubes, nanofibers, nanosheets, and nanoplates. These surface properties can be

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controlled using different plant extracts as surfactants and precursors. This approach may offer greater compatibility between synthesized hydroxyapatite nanoparticles with human and animal bodies.

Biomedical Applications The rise of biocompatible materials has significantly contributed to the progress of science (Khan et al. 2017). Hydroxyapatite (Hap), a calcium phosphate-based material, has drawn the attention of biomedicine due to its numerous advantages, such as similarity to the mineral phase of bone tissue, biocompatibility, bioactivity (Ghosh et al. 2022). In the past, the lack of biocompatible materials with the mineral phase of bone tissue made treatments ineffective due to rejection by the body, high toxicity, and low resistance (Yilmaz et al. 2019). The use of hydroxyapatite as a drug delivery system, bone replacement, coating of metallic prostheses, and artificial enamel on teeth has revolutionized several dental and biomedical treatments (Panda et al. 2021). Table 1 shows the numerous applications using Hap in the biomedical area. The properties of hydroxyapatite can be optimized through alternative routes of synthesis, doping with other elements, etc. (Arokiasamy et al. 2022). The enhancement of the properties of Hap can increase the application possibilities of this Table 1 Use of hydroxyapatite in biomedical applications Application Antibacterial

Dental

Bone healing process

Material Palladium/vanadate hydroxyapatite and fibrous phase preparation in different Pd(II) ions contents. Dental flosses coated by hydroxyapatite nanoparticles. Hydroxyapatite and the herbal extract ellagic acid

Cartilage regeneration

Chitosan-alginatehydroxyapatite hybrid scaffolds

Bone implant

Chlorinated hydroxyapatite

Highlights Composite showed the most significant inhibition zones against E. coli and S. aureus.

Ref. El-Morsy et al. (2022)

Potential use in tooth remineralization.

Babayevska et al. (2021)

Promoted fibroblast growth factor 2, vascular endothelial growth factor, and alkali phosphatase expression as angiogenesis markers in the bone defect model. Nanohydroxyapatite particles improved scaffolds’ elastic modulus and thermal stability behavior. Material as novel synthetic bone substitute with negative zeta potential.

Nirwana et al. (2022)

Sadeghianmaryan et al. (2022)

Naqshbandi and Rahman (2022)

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material. There are numerous strategies to improve the characteristics (chemical and physical) of Hap, such as doping of metallic ions in its structure, alternative synthesis routes, and blending of precursors (Sathiyavimal et al. 2020). Silver-doped hydroxyapatite nanoparticles showed excellent antibacterial, antifungal, and anti-free radical properties (Sebastiammal et al. 2022). In addition, Hap nanoparticles with silver showed high performance as an anticancer agent. The production of a biocomposite of hydroxyapatite and natural residue showed antibacterial properties. The synthesis of Hap from Gastropoda shells and sap from flowers of Musa paradisiaca presented a possible alternative and economically viable route (Saravanakumar et al. 2022). The combination of precursors gave the hybrid an increase in its bioactivity against Escherichia coli and Staphylococcus aureus. The presence of floral sap may be one of the reasons for the tremendous antibacterial potential (gram-positive and gramnegative). The antibacterial property was evaluated with different volume concentrations of the biocomposite (25, 50, and 75 μL). All concentrations showed satisfactory performance for both microorganisms. Figure 4 shows the antibacterial assays performed with the biocomposite. Also, the in vitro cell viability through the MTT assay showed excellent biocompatibility of the material.

(a) 4.5

Inhibition Zone (mm)

Inhibition Zone (mm)

(b) 12

Staphylococcus aureus

4 3.5 3 2.5 2 1.5 1 0.5 0

Control

25mL

50mL

Concentration (mL)

75mL

Escherichia coli

10 8 6 4 2 0

Control

25mL

50mL

75mL

Concentration (mL)

Fig. 4 Antibacterial activity of the Hap/Musa paradiasica hybrid: (a) Staphylococcus aureus and (b) Escherichia coli. (Copyright 2022, Elsevier; Reproduced with permission) (Saravanakumar et al. 2022)

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The application of Hap in the production of drug delivery systems is a hot topic in engineering (Mushtaq et al. 2021). The synthesis of a magnesium-coated hydroxyapatite-titania cement presented a potential nanomotor in orthopedic implants for targeted drug delivery in the treatment of infectious nonunion (Duan et al. 2022). Incorporating the coating as a nanomotor presented a great proposal, as it can provide kinetic movement through the formation of bubbles in an aqueous biological medium, thus, improving propulsion efficiency in a drug-loaded implant. The composite coating was generated in Ti6Al4V by electrophoretic deposition, followed by physical vapor deposition of the magnesium coating. The microstructure of the biomaterial showed a distinct bimodal particle size distribution of small spherical-shaped particles (90  30 nm) and large particles (700  250 nm). Furthermore, the crystallite size for the complex structure through Rietveld refinement was 42.4 nm. Therefore, the application of Hap in the fabrication of titania-based hybrids to produce nanomotors can be an excellent strategy for progress in orthopedic implants. Cobalt-doped Hap (Co-Hap) was investigated for biomedical and therapeutic imaging applications (Doan et al. 2022). The replacement of calcium ions by cobalt in the hydroxyapatite structure was carried out via co-precipitation. In addition, drug release in nanoparticles was evaluated in vitro at different pHs for doxorubicin (DOX). The Hap and Co-Hap nanoparticles showed a rice shape. However, the spherical shape became evident as the cobalt concentration in the hydroxyapatite increased. The adsorption of DOX molecules by the cobalt biomaterial was higher than in the raw material. The explanation for this increase in the DOX adsorption by Co-Hap is the difference in the size of calcium ion and cobalt. The ideal concentration of cobalt for the biomaterial was 2.0 mol%, in which the absorption spectrum of the drug showed lower intensity. The material with maximum doxorubicin loading capacity (2.0 mol%) was used to verify the drug release performance at five different pHs (5.5; 6.0; 6.5; 7.2; 9.0). Co-Hap at pH 7.2 and 9.0 released approximately 19% and 28% of the drug present, respectively. The highest amounts of DOX released were at pH 5.5 and 6.0, with approximately 61.2% and 51.3%, respectively. In addition, it is observed that in the application of Co-Hap to obtain fluorescence images (loaded with FITC dye), the hydroxyapatite nanoparticles showed lower intensity when compared to Co-Hap. The production of silver-doped hydroxyapatite by the hydrothermal method was recently reported (Chatterjee et al. 2022). The microstructural, compositional, electrical, and biocompatibility characteristics for application in drug delivery systems were evaluated. The incorporation of silver ions instead of calcium ions in the hydroxyapatite caused the lattice parameters to increase in the material. Consequently, the average crystallite size decreased. Ion substitution affected the electrostatic interaction of the material, changing the interatomic distances within the Hap lattice. The material’s structural and electrical characteristics demonstrated promising possibilities for its use as electret-mediated nanocarriers in transdermal drug delivery. Furthermore, cytotoxicity assays of the silver-doped nanoparticles demonstrated a significant cell viability rate for human kidney cells. The fabrication of a hydroxyapatite-based composite supported by chitosan and alginate for orthopedic applications was developed via casein (Cs) micelle-assisted synthesis (Jariya et al.

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2021). In addition, the production of material with two different fluorine contents (2% and 5%) was analyzed for their presence in the composite (FHap). Ciprofloxacin (CIP) drug release and antimicrobial activity were evaluated to verify the effectiveness of the proposed system. The nanoparticles in most of the conditions analyzed showed a typically spherical shape and monodisperse. Figure 5 shows the FESEM images for all composites. The composite produced from casein and 5% fluorine (Cs-5% FHap) was indicated as the most suitable for orthopedic applications of bone tissue growth and replacement. The composite was stable and efficient regarding crystallinity, morphology, bioactivity, and CIP drug-controlled release behavior. In addition, porosity (88.78%), water absorption (65%), water retention (15%), biodegradability (67.5% in 21 days), and CIP controlled-release capacity (60% in 11 days) of Cs-5% FHap were the best values observed among the composites produced. The antibacterial (Escherichia coli and Staphylococcus aureus) and antifungal (Candida albicans) activity for the composite with the best CIP release performance was effective against these microorganisms. Most biomaterials available for hard tissue replacement and regeneration therapies are synthetic osteoconductive bone graft materials (Liu 2015). Hydroxyapatite is a material with osteogenic, osteoconductive, and osteoinductive properties (Kapoor et al. 2022). Hap allows adapting its structure to expand applications in

Fig. 5 FESEM images: (a) Hap, (b) 2% FHap, (c) 5% FHap, (d) Cs-2% FHap, and (e) Cs-5% FHap. (Copyright 2021, Elsevier; Reproduced with permission) (Jariya et al. 2021)

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tissue engineering. The inorganic part of human hard tissues is mainly composed of biological hydroxyapatite (Liu et al. 2007). The application of three-dimensional desktop printers (D3DPs) using hydroxyapatite to produce customized implants in medical clinics is a trend for bone tissue engineering. The performance of an experiment for surgical implantation of the anterior cruciate ligament printed in D3DP using a rabbit as a model demonstrated the applicability of this strategy (Liu et al. 2016). A well-defined, orthogonal, and porous polylactic acid (PLA)-based screw-shaped support was printed and coated with hydroxyapatite to improve its osteoconductive property. In addition, an internal fixation, an ideal cell delivery system, and the osteogenic scaffold loaded with mesenchymal stem cells (MSCs) were evaluated through both in vitro and in vivo tests to observe bone-ligament healing via cell therapy. MSCs suspended in Pluronic F-127 hydrogel on the printed support showed the highest cell proliferation and osteogenesis in vitro. In vivo evaluation of rabbit anterior cruciate ligament models for 4 and 12 weeks showed that the developed scaffold loaded with MSCs suspended in Pluronic F-127 hydrogel exhibited significant bone growth and bone-graft interface formation within the bone tunnel. Therefore, the application of Hap for coating printed supports in 3D printers is a promising alternative, in which it allows effective and economically viable treatments. Figure 6 shows 3D reconstructed images of a 3 mm circular region of interest from within the bone tunnel of the rabbit knee joint. A new hybrid electrospinning scaffold system was proposed using biodegradable polyhydroxybutyrate and hydroxyapatite for in vivo bone regeneration (SadatShojai et al. 2016). In addition, the hybrid was combined with a protein-based hydrogel into a single three-layer scaffold. The proposed strategy was for the hydrogel to act as a suitable environment for cell encapsulation and the embedded fibers to play the role of a high-strength backbone. Thus, the mechanical properties of the scaffold were increased. The mechanical properties of the hybrid material were much superior to traditional hydrogels. Furthermore, bone cells within the

Fig. 6 Micro-computed tomography (micro-CT) assessments: (a) Vertical plane of rabbit knee joint (macroscopic); (b) femoral bone tunnel in a sagittal view of the micro-CT image – the region of interest was shown with the new bone inside the bone tunnel (white rectangle) and the tendon graft (white triangle); (c) the region of interest with the new bone within the bone tunnel (white circle); and (d) the external opening of the femoral bone tunnel (black star), where bone growth can be seen. (Reprinted under the terms of the Creative Commons CC BY license) (Liu et al. 2016)

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scaffold were highly viable and infiltrated after 14 days of encapsulation. The use of electrospinning for the synthesis of hybrid materials based on Hap can be an option with numerous advantages to in vivo bone treatments, such as high strength and the ability to encapsulate cells. Coating with ultrathin films of hydroxyapatite of titanium dental implants can increase osteogenesis of teeth. The fabrication of ultrathin films of hydroxyapatite on a pure titanium substrate by pulsed laser deposition was evaluated for the effects of these surfaces on rat bone marrow cells (Hashimoto et al. 2008). Film thicknesses were 500, 2000, and 5000 Å. Cell proliferation was not affected by the ultrathin films of hydroxyapatite. However, the 2000 Å film showed the highest expression of osteoblastic markers in mouse cells. The deposition of a 300-nm-thick thin film of hydroxyapatite on artificial articular cartilage (poly(vinyl alcohol) hydrogel) increased cell adhesion (Hayami et al. 2007). The cell adhesion ability of the material without Hap coating is lower. Therefore, the adhesion values of the proposed material were promising. In addition, the thin film deposition of hydroxyapatite proved to be an excellent option for application in specific areas. Histological assessment of osteoconductivity of three types of implants was performed by coating different types of hydroxyapatite and conventional materials (after implantation for 4–24 weeks) (Hayami et al. 2011). A commercial titanium screw implant was coated with stoichiometric hydroxyapatite (50 nm thick) and then with bovine hydroxyapatite (300 nm thick) using the pulsed laser deposition technique. The control materials were a commercial implant coated with Hap (20 μm thick) by the flame spray method (sprayed implant) and a simple titanium implant (basic implant). Hydroxyapatite desquamation from the pulverized implant was observed in the fourth postoperative week. Also, the basic implant and the pulverized implant were surrounded by a gap containing connective tissue and the bone adhered to the Hap coating, respectively. Hydroxyapatite implants with two types of design (radial and orthogonal) were applied in vivo to assess bone regeneration (Fig. 7) (Chu et al. 2002). Twenty-four Hap implants with the two designs were implanted in the mandibles of four Yucatan

Fig. 7 Orthogonal and radial hydroxyapatite implants. (Copyright 2002, Elsevier; Reproduced with permission) (Chu et al. 2002)

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minipigs for 5 and 9 weeks. Normal bone regeneration occurred in both groups. Bone penetrated 1.4 mm in both designs at 9 weeks. Bone growth in the penetration zone was more significant in the orthogonal canal design, but the low number of samples was not considered statistically different. However, the general shape of the regenerated bone tissue was significantly different. In the orthogonal implant, bone and Hap formed an interpenetrating matrix. In the radial implant, the regenerated bone formed an entire piece in the center of the material. Therefore, the control of the general geometry of the regenerated bone tissue can be performed through different formats of hydroxyapatite-based materials. Fabrication of a multifunctional biocomposite composed of hydroxyapatite derived from crab shell, aloe vera, strontium oxide, and polyhydroxybutyrate for improved antibacterial, mechanical, and biocompatible properties was recently reported (Mathina et al. 2022). The presence of aloe vera, strontium oxide, and polyhydroxybutyrate in the hydroxyapatite derived from crab shells improved the antibacterial, mechanical, and biocompatible properties of the composite. In addition, the fabricated composite was electrophoretically deposited on 316 L stainless steel (316 L SS) to enable orthopedic applications. The incorporation of hydroxyapatite and Fe3O4 nanoparticles was carried out by the in situ crystallization technique and lyophilization in the organic matrix of chitosan (CS) and collagen (Col) (Zhao et al. 2019). The composite’s biocompatibility and physicochemical analyzes of the material showed excellent structural and mechanical performance, cell proliferation, and osteogenic differentiation in adhesion. In addition, the composite showed significant bioactivity and good biomimetic mineralization in situ in the experiments. The in vivo model of cranial defects in rats observed that the hybrid scaffold showed better tissue compatibility and greater bone regeneration capacity when implanted in cranial defects than the control group. Figure 8 shows the micro-CT, histological sections of the implanted defect regions, bone mineral density (BMD), and morphometric analysis of the composite in rats. Therefore, once again, the use of hydroxyapatite to produce hybrids in bone regeneration is a promising material for biomedical applications. The application of hydroxyapatite in the biomedical area has been widely seen in recent decades and it remains a promising material and is widely applied in several areas (environmental, dental, pharmaceutical remediation, etc.) (Hontsu and Yoshikawa 2015). Hap has several advantages, such as the capacity link with bone tissue, biocompatibility, biostimulator, etc. Furthermore, the production of hydroxyapatite hybrids or doping with metallic/polymeric particles can give the material new properties or improve the characteristics of Hap (mechanical, chemical, etc.). Also, the different synthesis routes allow obtaining Hap or hybrids with different shapes, nanorods, spherical, etc. Thus, this biomaterial’s different shapes and structures can help in applications. For example, hydroxyapatite or composite using this biomaterial with amorphous nature presents the best performance for applications in bone regeneration. Also, a trend in biomedical applications using Hap is the replacement of functional groups of the particle by fluorine due to the biological importance of the element to living organisms. Therefore, numerous strategies and alternatives can be utilized in synthesizing hydroxyapatite-based materials that make this area of research promising for biomedical applications.

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Fig. 8 (a–f) Micro-CT and (g–l) hematoxylin and eosin-stained histological sections of rat calvarial defect regions implanted with the blank control, the CS/Col/Hap scaffold and the CS/Col/Fe3O4/Hap scaffold after 12 weeks. Representative coronal (a, c, e) and sagittal (b, d, f) images of calvarial bone defects after 12 weeks postimplantation (red circles and rectangle: defect area). (h, j, l) Higher magnification of the framed area (g, i, k). S, scaffold; HB, host bone (black arrows); NB, new bone (triangle); MB, bone marrow (black circles). (m) Local BMD analysis and (n) morphometric analysis (bone volume/tissue volume – BV/TV) of new bone formation for three groups in the defect site. (Copyright 2019, Elsevier; Reproduced with permission) (Zhao et al. 2019)

Conclusions and Remarks Hydroxyapatite nanoparticles obtained from plant-mediated synthesis present diverse sizes and shapes. Depending on the type of plant and part of the plant (flower, leaf, fruit, and roots), the size and shapes vary. However, the most

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predominant shape reported in the literature, in the presence of plant extracts, is rods. Furthermore, the biomedical applications of hydroxyapatite nanoparticles are upand-coming; however, some points still need to be well defined through in vivo and in vitro assays, exploring the full potential of this material. These studies are necessary because, so far: (i) Until now the mechanism that governs the formation of a certain shape in the presence of plant extracts is not clear enough. (ii) More work is still necessary to understand the role of the size and shape of hydroxyapatite nanoparticles in interactions with some cells, such as osteoblasts. (iii) Identification and isolation of compounds from plants to understand the role of individual or in combination into shape of hydroxyapatite nanoparticles. (iv) Theoretical calculations aim to understand the crystal growth and evolution using plants extracts. Acknowledgements This work was supported by the National Council for the Coordination for the Improvement of Higher Education Personnel (CAPES, Brazil)- a foundation linked to the Ministry of education and culture (MEC) and The National Council for Scientific and Technological Development (CNPq) is a foundation linked to the Ministry of Science and Technology (MCT).

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Beer Pal Singh, Kavita Sharma, Shrestha Tyagi, Durvesh Gautam, Manika Chaudhary, Ashwani Kumar, Sagar Vikal, and Yogendra K. Gautam

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anthropogenic Emissions from Conventional Power Plants and Their Impact on Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green and Sustainable Nanotechnology for the Reduction of Anthropogenic Pollution . . . . . . Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydropower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geothermal Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermoelectric Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In this chapter, we present a comprehensive study on green and sustainable technological solutions for the production of clean energy for the accomplishment of global energy requirements. The production of energy from the most widely used renewable energy sources, such as solar, wind, wave, biomass, etc. via green and sustainable nanotechnology has been discussed in the chapter. In addition, other looming green nanotechnology such as hydrogen energy and thermoelectric technology are also presented in the chapter. It is observed that solar energy, wind energy, geothermal, bio-energy, hydropower, thermoelectric, and hydrogen B. P. Singh · K. Sharma (*) · S. Tyagi · S. Vikal · Y. K. Gautam (*) Smart Materials and Sensor Laboratory, Department of Physics, Ch. Charan Singh University, Meerut, UP, India D. Gautam · M. Chaudhary Department of Physics, Ch. Charan Singh University Meerut, Meerut, India A. Kumar Nanoscience Laboratory, Institute Instrumentation Centre, Roorkee, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_64

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energy are clean sources or carbon-free energies. The chapter also highlights the various challenges that prevent the adoption and development of renewable energy technologies. Keywords

Anthropogenic environmental pollution · Green energy, Sustainable energy · Renewable energy technologies

Introduction Clean energy is used to describe sources of energy that are renewable and considered to be environment-friendly. These sources of energy facilitate to negate the effects of growing global problems such as climate change, environmental pollution, population growth, and inefficient use and reduction of natural resources (Khan 2020). These problems have raised a worldwide concern to employ technologies and approaches that are less harmful to the worldwide environment and also preserve natural resources. Sustainable and green nanotechnology is such innovations which drastically reduce above-said pollution-based problems. The emerging world population and swift economic growth are putting forth demands on auxiliary energy production. It is expected from green energy technologies that they should play a considerable role in the sustainable energy development in the near future in conjunction with the successful implementation of green and sustainable energy technologies (Khan 2020). The clean and sustainable nanotechnologies include the manufacture, characterization, and use of nanomaterials and nanodevices in energy production, conversion, storage, and utilization. The technology based on nanomaterials consist various components/devices and processes such as supercapacitors, batteries, photovoltaics, hydrogen production, detection and storage, fuel cells, photo- and electrocatalysts for energy conversion and storage, thermoelectric materials and devices, optoelectronic devices, and flexible, self-powered, and/or integrated energy devices/systems. Among all the energy storage devices, photovoltaic device is one of the most important renewable energy technologies in which sunlight directly converts into electric power. In the fabrication of solar cell devices, nanotechnology has played a crucial role as electrode material. Moreover, it is worth mentioning that nanotechnology also shows impressive impact in fuel cell devices in the conversion of chemical energy directly into electricity. Fuel cells employed with polymeric electrolyte membrane can be used as effective energy conversion devices, especially in automotive applications. Nano-porous metals (multi (bi-tri)-metallic Pt alloys) with high surface area, rich surface chemistry, and low specific densities are highly efficient electro-catalysts for the critical electrode oxidation/reduction reactions in fuel cells. Recently, CNTs and graphene are mostly studied as metal-free catalysts in fuel cells due to their excellent electrical conductivity, high surface area, mesoporosity, stronger mechanical strength, light weight, and superb corrosion-resistance for decreasing precious-metal loading, enhancing catalyst activity and durability.

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Hydrogen has a great potential to be a future energy option. Nano-structured carbons, metal-organic framework (MOF), polymers and metal hydrides (Mg-based) and related complex hydrides have been studied for hydrogen storage and transportation for high hydrogen capacity and smallest deterioration during hydrogenation. Almost 80% of the world’s energy comes from fossil fuels. There is continuous harmful emission of carbon monoxide (CO) and carbon dioxide (CO2) gases, etc. Hence, it is essential to reduce the released CO and CO2 into the atmosphere (Leung et al. 2014). In this regard, nanotechnology can provide this alternate in a very efficient, inexpensive, and eco-friendly way because nanotechnology includes the engineering of any system at the atomic or molecular scale. Norio Taniguchi, in 1974, coined the term nanotechnology for the very first time (Prasad 2014). Moreover, the concepts of nanotechnology can be applied in various fields such as healthcare, fabrics, environment, energy storage devices such as solar, fuel cells, and nanocoatings (Theron et al. 2008; Sinha et al. 2007). At the nanoscale, the reactivity and the mobility of materials are very high. In some cases, this could be risky and sometimes lifethreatening. This is known as nanopollution as this is very harmful to the environment. Green technology can be described as the technology used to produce environment-friendly nanomaterials. This is an essential technology as it can directly counter harmful processes and contaminations. The advancement of any technology is directly proportional to sustainability (Watlington 2005). This technology can work as a critical tool for the benefits of the environment and conservation of natural resources. The green technology is able to minimize the use of non-renewable energy resources. Apart from this, it can lead to a pollution-free healthy environment. The main aim of green technology is to fulfill society’s needs without damaging the sustainable natural resources (Sinha et al. 2007). The production of nanomaterials without causing harm to human health and nature is referred to as Green Nanotechnology. In a nutshell, green technology refers to the fabrication of nanomaterials/nanoparticles using specialized techniques and biological procedures that involve biological tools such as viruses, plants, microorganisms, etc. Green technology involves inexpensive biological or plant-based raw materials for the synthesis of nanoparticles (Theron et al. 2008). The by-products generated by these techniques are environmentally friendly. Moreover, the main aim of green nanotechnology is to reduce carbon footprints and make nanomaterials using environmentally friendly and less toxic techniques (Patra and Baek 2014). The other possible reason for using biological synthesis techniques is that it can produce nanomaterials in huge numbers with well-defined morphologies, shapes, and sizes. These techniques are based on natural renewable resources. Apart from this, since the waste and by-products are non-toxic, they can be managed, i.e., recycled and disposed of quickly. Also, this technology can reduce the emission of greenhouse gases to a greater extent. Owing to the aforementioned advantages, green nanotechnology finds application in various fields ranging from automobiles, nanoelectronics to energy conversion and storage (Basiuk and Basiuk 2015). Hence, it is basic need to use sustainable, environmentally friendly green methods for the synthesis of nanomaterials. This will possibly counter the risks associated

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with the use of nanotechnology, and it will increase the productivity of nanomaterials, and will reduce the carbon footprints. In a nutshell, it is of utmost importance to adapt green and sustainable nanotechnology concepts for energy conversion and storage applications.

Anthropogenic Emissions from Conventional Power Plants and Their Impact on Environment Most of the power generation systems are known for impacting the environment harshly. Fossil fuels-based energy systems even do more damage to the environment as compared to other energy sources. Energy-related environmental problems have now become the global concern during the past decades. Environmental problems are specifically evident in developing nations, where energy expenditure growth rates are enormously soaring and management of environment has not been included completely so far into the infrastructure. Figure 1 depicts a conventional fossil fuel power generation system and, in the process, expelling a number of pollutants such as greenhouse gases (GHGs), NOx, SO2, aerosols VOCs, PM, CO, etc. in the ambient air. In addition, such system might be a reason for mishaps, risks, filth of ecosystem through polluting air and water, vitiating animal, anthropogenic GHGs emissions, leakage of CO, and depletion of ozone in stratosphere (Hake and Eich 2017). The effluvium of power generation systems which entered into the environment can be separated majorly into two categories: GHGs and aerosols. The emitted GHGs voyage all the way into the atmosphere and arrive at the troposphere. GHGs soak up a significant amount of infrared radiation there released by the surface

Greenhouse Effect

GHGs, NOx, SO2

Earth Climate

VOCs, PM, CO etc.

Global Warming

Acid Formation Acid Precipitation

Sea Level Rise

Precipitation Change

Extreme Events Occurrence

Acidification of Water and Soil

Fig. 1 Schematic view of emission of pollutants from the conventional power plants and the emitted pollutants impact on environment

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Table 1 The major pollutants, expelled by power generating units into the atmosphere Atmospheric pollutants 1. Greenhouse gases (GHGs)

2. Nitrogen and its oxides (NOX)

3. Sulfur dioxide (SO2)

4. Volatile organic compounds (VOCs)

5. Particulate matter (PM)

Impact on environment These are the gases that entrap heat in the atmosphere and produce greenhouse effect. The main GHGs are CO2, CH4, N2O, and fluorinated gases. Greenhouse effect is considered as the main cause of global warming (Bartos 2009) Nitrogen oxides (NOx) are very imperative class of air polluting chemical compounds. It is generated by the burning of fossil fuels. It can cause respiratory problems to humans and animals. In addition, it can form acid rain in high altitude atmosphere. Acid rain, along with cloud and dry deposition, severely affects certain ecosystems (EPA 1999) Fossil fuels-based power generation systems and other industrial activities are the main cause of SO2 in the atmosphere. Even short period exposure of SO2 harms the respiratory system of human. SO2 and its oxides also adding up to acid rain which in turn impair the receptive ecosystems The VOCs facade damaging effects on the atmosphere and hinder the formation of stratospheric ozone which is essential for preventing the harmful ultraviolet radiation to enter into the lower atmosphere of earth A variety of particles from dust, filth, metals, fly ash, sea salt, liquid droplets, etc. enter in the air through various natural and anthropogenic sources. The industrial factories, power plants, vehicles, etc. are the major man-made sources that formed PMs in the atmosphere by condensation or chemical alteration of the gases emission. The environmental effects of PMs are acid precipitation, damage to plant life and human structures, haziness, noxious or mutagenic effects on humans, and probably the contingent deaths

of the earth (Venterea 2009). While an outcome, the surface temperature of earth is likely to rise, and known as the greenhouse effect (Venterea 2009). However, effluent aerosols condense in upper layers of the atmosphere and reflect back a part of incident solar radiation into the space. In consequence, the earth’s temperature reduces and the process is well-known as the albedo effect. The equilibrium between these two effects sets up the habitable temperature of earth and regularizes climate of the earth. Nonetheless, the industrial revolution has ignited the enormous anthropogenic emission of GHGs in environment from many sectors mainly energy, transportation, and industry. Its impact on the climate has become apparent in terms of global warming (Oberschelp et al. 2019). Furthermore, an additional impact of energy systems on environment is resulting via acidic precipitation. The fizzy expellant in the atmosphere can ultimately be converted into acids which come back again to the earth and acidifying the soil and seas. The main pollutants of the atmosphere expelled by the power generating units are listed in Table 1, and subsequently their impact on it is also mentioned underneath (Power and Systems 2014). Therefore, the continuous emission of air pollutants may cause significant environmental issues like global warming and climate change which further lead to rising

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sea levels, dangerous storms, floods, droughts, change in rain patterns, shifting of wildlife species, and ozone layer depletion and severely affect human life. According to IPCC report, due to continuous increase in greenhouse gas emission, it is predicted that global temperature will rise at 0.2
C [10]. In addition to this, IPCC also predicts the water shortage in mid latitudes and excess of water in high latitude regions which results in consistent years of droughts and heavy floods. Extreme weather conditions give rise to typhoons, dangerous storms, hurricanes, and intense tropical cyclones. Serious efforts are required on a global scale to combat the challenges for a cleaner environment which include minimal use of fossil fuel, changes in farming patterns, protecting forest, proper treatment of animal manure, energy harvesting, and maximum use of clean and renewable energy.

Green and Sustainable Nanotechnology for the Reduction of Anthropogenic Pollution Clean and renewable energy is well known to overcome the worst effects of climate change due to increasing concentration of air pollutants because clean and renewable energy sources produce power without impacting the environment negatively. Figure 2 depicts the possible clean energy solutions such as solar energy, wind energy, hydropower, geothermal, biomass, thermoelectric, and hydrogen energies. In the year 2018, 28% of electricity was produced from renewable energy sources throughout the world, out of which 96% was generated from wind, hydropower, and solar technologies. US Energy Information Administration (EIA) predicted that the

Fig. 2 Future remedies to combat the challenges posed by the increasing concentration of air pollutants in earth’s environment

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world’s power production will be increased to 49% by the contribution of these renewable sources by 2050 (Khan 2020). Green and sustainable nanotechnology refers to the nanoscience and nanotechnology keeping its focus to the health of the environment, via its fabrication, applications, and the final disposal. Hence, it basically meant the production of sustainable and harmless nanomaterials (Khan 2020). The sustainability frame includes the choice of the reagents, the synthetic process in terms of mass and energy, the particulars of the probable purpose, and their clearance at the end while taking care of environmental impacts during the procedure (Gilbertson et al. 2015). With increasing demand for sustainable energy, resource, and environment fortification, green materials-based technologies are continuously growing during the last decades. Specifically, nanomaterials are progressively a key component of the clean energy production either by increasing the efficiency of the energy storage and conversion processes or by improving the device design and performance (Chen et al. 2015).

Solar Energy Solar energy is the best choice for the world owing to numerous reasons among various renewable energy technologies. The very first reason is that solar energy is found to be the most abundant energy and the sun produces energy at the rate of 3.81023 kW, out of which nearly 1.81014 kW is acquired by the earth. The second reason is that, it is a potential energy source in the whole world that is not exhaustible, providing solid and growing output efficiencies in comparison to other energy sources. Third reason is consumption and tracking of solar energy poses no damaging impact on ecosystem and kept natural balance for the benefit of living organisms. The inexhaustible free solar energy can directly harness from the sun. Nowadays electricity is generated using novel technologies from harvested solar energy. To reduce global greenhouse emissions which are major environmental challenge in recent years, many researchers have been focused on solar energy because sun provides abundant and clean energy source (Energy Agency). It is quite obvious that power plants use fossil fuel to generate electricity and are a major source of GHG emission giving rise to nearly 25% of all anthropogenic emissions (Jerez et al. 2015). The carbon dioxide emission ratio from coal, natural gas, and solar energy is estimated to be 18:9.5:1, respectively, which again approves the environmental friendliness of the solar power in comparison to others. Photovoltaics (PV) commonly known as solar cells are a kind of electronic devices utilized for the conversion of sunlight into electricity. The group of solar cells known as solar panels are commonly seen installed on houses and in calculators. Currently PV technology is rapidly growing in comparison to other renewable energy technologies and is a promising technology for future electricity generation. Photovoltaic devices generally use semiconductors such as silicon for electricity generation. The basic principle of the device is that electrons after taking additional energy from sunlight activate from lower energy state to higher, thus creating number of electron and

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holes pairs in semiconductor which in turn generate electricity. Additionally, some other technologies are also used for conversion of solar energy into electricity such as concentrating solar thermal power (CSP) and concentrated photovoltaic technology (CVT)(Raboaca et al. 2019). The large-scale electricity production is possible by solar farms where large numbers of solar panels or concentrating solar systems are used for harvesting solar energy. Similar to solar farms power plant they are also known as solar power stations or solar parks. The Sekdoorn floating solar farm situated in the Netherlands has annual energy yield of 13.330 MWh, saving nearly 6,500 tons of CO2 emissions a year along with providing power to nearly 4,000 households (Karimirad et al. 2021). Floating solar farms are the solar panels, and these are installed on a structure that floats on water with the benefit of minimizing the land requirements with installation in water reservoirs and small lakes. Thus, solar power will become the most feasible solution for decreasing the concentration of GHGs in environment. Since output of the solar panels depends on the total solar energy received by it, this indicates that an improved energy source exists at the locations only where the intensity is comparatively high. Another issue to solar energy is photovoltaic efficiency. Utmost commercial solar panels have efficiency ratings of around 25 percent. Higher efficiency of panel leads to high production cost. Besides this, the manufactures of solar panels involve some environmental unfriendly substances, e.g., nitrogen trifluoride, a greenhouse gas that is 17,000 times more effective than carbon dioxide (Bajagain et al. 2020). In addition, several solar cells contain little quantity of the toxic metal cadmium and other heavy metals. As solar technology improves, these must be manufactured with environment-friendly substances. Nanotechnology is the one that can be used to address the present efficiency hurdles and significantly enhance the generation and storage of solar energy. The application of nanotechnology in solar cells has paved the way towards the progress in novel high-performance products. Variety of innovations has been explored in the field of solar cell generation, spectrum modulation, thermo-photoelectric cells, multi-generation, hot carrier, and several other techniques (Ghasemzadeh and Esmaeili Shayan 2020). Nanostructured materials have been shown not only high absorption of light but also increase the conversion of light to energy with enhanced thermal storage and transport. Nanomaterials have been used in variety of devices such as solar collector, fuel cell, photocatalysis, and solar photovoltaic to enhance the efficiency of that particular device. Silicon is the most widely explored material used in both bulk and thin film forms. However, there are several other possibilities such as copper indium gallium selenide (CIGS), CdTe, dye-sensitized solar cells (DSSCs), organic solar cells, and perovskite solar cells. Nowadays several green methods are also used for the fulfillment of the need for clean energy. Mesoporous anatase TiO2 nanomaterials (MATN) prepared by green approach was used in dye-sensitized solar cells (DSSCs) and hole-conductor-free perovskite solar cells (HPSCs). The MATN-based DSSCs demonstrated high-power conversion efficiency (PCE) up to 7.78% owing to its high dye-adsorption capacity and long chargetransfer channels, whereas the PCE based on the P25 photoelectrodes is 6.61%

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Photon

ZnO FTO

Redox

CuS Counter Electrode

CdS TiO2 Polysulfide electrolyte

e-

ee-

e-

e-

Fig. 3 ZnO/TiO2/CdS quantum dot sensitized solar cell (QDSSC) (Tyagi et al. 2020)

(Chu et al. 2018). Zinc oxide nanoflakes (ZnO-Nfs) having size ~100 nm prepared via hydrothermal process were utilized as photo anode for the DSSC applications. The ZnO-Nfs displays good power conversion efficiency of 3.39% with large short circuit current (JSC) of 8.84 mA/cm2 and open circuit voltage (VOC) of 0.706 V along with the fill factor (FF) of 0.54 (Han et al. 2018). ZnO/graphene quantum dots (GQDs) photoelectrode was prepared by green synthesis and utilized as electron transport layer for perovskite solar cell, exhibiting the power conversion efficiency of up to 17% on fluorine tin oxide (FTO) under AM1.5G illumination (Ahmed et al. 2020). For the improvement in the electrons transport layer, ZnO nanowires/nanorods and TiO2 nanotubes were found to be useful in polymer solar cells and dye-sensitized solar cells (Sharma et al. 2018). Cascade structured ZnO/TiO2/CdS quantum dot sensitized solar cell (QDSSC) is shown in Fig. 3. Nanostructures are used as active layer to reduce the optical losses and enhance the optical absorption. Iron oxide, titanium dioxide, and alumina are proven effective and cost-effective adsorbents for heavy metals like arsenic, lead, mercury, copper, cadmium, chromium, and nickel using solar energy (Thekkudan et al. 2017). A good photocatalytic activity in presence of sunlight is also one of the main advantages of nanoparticles. Nanotechnology in solar energy is now applicable for various systems as given below: • Nanomaterials-based PV-systems for building integration. • Nanocatalysts are new window into improving efficiency.

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• Self-cleaning and anti-reflective nanocoatings protect and increase efficiency of solar devices. • Nanofluid is used to move power and raise the output of solar devices. • Nanomaterials-based solar pumps are used for irrigation of crops. • Nanomembranes for desalination. • Nanoadsorbants for waste water treatment. Solar energy is used for heating and cooling of buildings, heat generation in industries, food refrigeration, distillation, drying, power generation, and so many other processes (Henning and Döll 2012). Undoubtedly, solar energy is the emerging field to fulfill the world’s energy demand; however, it also faces several hurdles on its way towards advancement. The very first barrier is the requirement of advance manufacturing technology and expenses. In addition to this, several factors such as sunshine, intensity, and clouds affect the performance of the solar panels. Also, solar cells are typically composed of chemicals that have toxic effect on environment, and their direct disposal into environment may cause serious harm to the environment. So, it is a key challenge for the researchers to eliminate such barriers for the betterment in the production of solar energy with enhanced efficiencies.

Wind Energy Wind energy is better substitute as it is renewable energy and has very less effects on earth’s environment. Wind energy can be considered as world’s one of the cheapest energy sources. The onshore and offshore wind-generation capacity has been increased globally by a factor of almost 75 in the past two decades, increasing from 7.5 GW in 1997 to some 564 GW by 2018, according to IRENA’s latest data (International Renewable Energy Agency (IRENA) 2019). The wind electricity produced doubled between 2009 and 2013, and in 2016 wind energy accounted for 16% of the electricity generated by renewable sources. Wind energy is produced from wind turbines which are mostly located in open land and offshore of oceans. These wind turbines required at least wind speed 13 miles per hour for utility scale turbines and 9 miles per hour for small turbines; the group of these wind turbines is termed as wind farms. Wind energy does not contribute to any carbon dioxide emissions and air pollution responsible for climate change (Sayed et al. 2021). The cycle lifetime of wind turbine blades can be enhanced by using nanocoatings and nanopaints, whereas nano-based prepregs can be used for the reduction in weight. Also, the efficiency can be improved by using nanolubricants, nanofluids, and nanoenabled wires and cables. Non-destructive testing of composites can be performed via nanosensors. The energy obtained by wind turbines is proportional to the square of its blade length. Nanotechnology can play a significant role to attain maximum efficiency and utilization of wind energy. Owing to their high strength and stiffness, nanocomposite materials enable the construction of longer and stronger blades. Nanotechnology enables high efficiency for wind turbine by the reduction in several

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ControlSystem

Generator Outerbox

PowerInterface

Turbine

Wind Power

Mechanical Power

Electric Power

Fig. 4 Typical control system of wind turbine generator

energy losses due to high gravitation forces, high external loading, and high stress causing micro pitting, scuffing, and spalling in gear boxes. These losses can be minimized by using nanolubricants and low friction coatings (Rohrig et al. 2019). Nanocomposites composed of graphene and multi-walls carbon nanotubes (MWCNTs) provide the protection against harmful impact occurring from lightning strikes that causes 10–15% of the failures of wind generators. Nanocoatings of ZnO, TiO2, ZrO2, SiO2, Al2O3, and CeO2 nanoparticles and carbon nanomaterials (CNTs and graphene) prevent the accumulation of airborne particles, UV degradation, and corrosion. (Oke et al. 2017). The nano-colloidal boron nitride works as commercial gear oil and reacts with broad surface enhancing the mechanical properties thus forming a wear protective film. Soft metals, boron-based materials, carbon-based materials, metal oxides, and organic/polymer materials have been used as additives in lubricating oils used for proper functioning of gear boxes (Deepika 2020). The typical control system of wind turbine generator (WTG) is shown in Fig. 4. Nanofluids are of great interest due to their demand as coolants for wind energy industry because wind turbines dissipate a large amount of heat during operation which led damaged electric generator and mechanical part of turbine. Cu nanoparticles have been used to develop nanofluids due to their high thermal conductivity at room temperature which is 700 times greater than that of water and about 3000 times greater as compared to engine oil (Hussein 2015). In recent years, there is substantial increase in power generation from wind energy globally, the Horse Hollow Wind Energy Center (Texas), which is one of the largest wind farms consisting 430 turbines within 4700 acres and has capacity of 735 megawatts electricity generation. But some major challenges identified before the wind energy generation are the better utilization of wind currents as well as structural and system dynamics of wind turbines along with grid reliability of wind power. In addition to this, various environmental influences must also be

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taken into consideration when exploring maximum utilization of wind energy (Veers et al. 2019).

Hydropower Hydropower is also renewable energy which generates from captured water through the flowing water sources as water constantly moves through a cycle, evaporating from lakes and oceans, forming clouds, raining, and then flowing back to the ocean. This energy from sun-driven water cycle captured and turned into power is hence termed as hydropower or hydroelectric power (Shiji et al. 2021). Moreover, hydropower is considered as renewable energy because water cycle is continuous and endless cycle; also it does not contribute to any GHG emission which causes climate change and global warming. Water is used as fuel in hydropower which makes it as a clean source of energy having no pollution like in power plants by burning fossil fuels such as coal or natural gas. Turbines and generators are used for the conversion of kinetic energy of flowing water into electricity (Betti et al. 2021). International Renewable Energy Agency (IRENA) provides data about renewable sources, for example, 99% of electricity in Norway comes from hydropower. Three Gorges Dam, which is the world’s largest hydropower plant, is situated in China having a capacity of 22.5 GW. It produces 80 to 100 terawatt-hours per year, which is enough for consumption of 70–80 million households (Lcoe 2014). Hydropower is a sustainable renewable energy source as well as provides so many benefits like flood control, irrigation, and water supply. But there are some environmental issues which would have been imposed by the hydropower plants such as diverting a river affects the nature of the countryside and does not lend itself to use on a large scale. Permanent complete or partial blockage of a river for energy conversion is adversely affected by variations in flow. Building large-scale hydro power plants can be polluting and damaging to surrounding ecosystems. Changing the course of waterways can also have a detrimental effect on human communities, agriculture, and ecosystems further downstream. Hydro projects can also be unreliable during prolonged droughts and dry seasons when rivers dry up or reduce in volume (Mishra et al. 2015). An alternative form of hydropower is based on harnessing osmotic energy by using pressure-retarded osmosis or reverse electrodialysis. Their commercial development still constrains due to relatively high costs and low efficiency. Nevertheless, last few years, new nanomembranes have been found to be capable of generating voltage effectively when an electrolyte solution flows through narrow channels driven by a pressure gradient (2018). For example, Siria et al. reported membranes made of boron nitride nanotubes that can produce power with a density of several kilowatts per square meter (Siria et al.) and Feng et al. reported a power density as high as 103 kW m2 when using MoS2 nanomembranes (Feng et al. 2016). Zhang et al. explore a range of phenomena, which they refer to as “emerging hydro voltaic effects,” in which the electricity is generated by the direct interaction between the materials and water (Zhang et al. 2018).

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Fig. 5 Schematic of geothermal power plant

Geothermal Energy Geothermal energy is thermal energy which lies in the rock and fluids beneath the earth’s crust, and some countries have used it as energy source for heating and cooking. In simple words it is energy derived from internal heat of earth. Geothermal energy is future energy which is available for all 365 days a year (Neves et al. 2021). It does not generate power by burning of fossil fuels, and hence emission level of GHGs from it is negligible. Geothermal power plants emit 97% less sulfur compounds and nearly 99% less carbon dioxide than conventional power plants driven by fossil fuels. Moreover, these power plants insert back the used geothermal steam and water into the earth and thus make geothermal energy as renewable energy source (Cousse et al. 2021). A schematic of geothermal power plant is shown in Fig. 5. Geothermal power plants are commonly built at site of geothermal reservoirs under the earth’s surface. The United States generates the world’s largest geothermal electricity, and The Geysers north of San Francisco in California is the largest geothermal development across the globe (Stephens and Jiusto 2010). The future of geothermal energy will strongly depend on to the number of geothermal power plant deployment by the government or public sector. Accelerating enhanced geothermal systems (EGS) development could establish a breakthrough, provided that a strong financial involvement can take place.

Biomass Biomass energy or bioenergy is renewable source of energy and originates from organic material that comes from plants and animals. In photosynthesis plants absorb

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energy from sun and biomass stored that energy of sun within it. Burning of biomass causes the change of chemical energy in biomass into heat (Nandimandalam et al. 2022). Biomass energy use is categorized as traditional and modern. Traditional use comprises direct burning of wood, animal wastage, and garbage while modern use includes several technologies such as biogas plants, liquid biofuels generated from bagasse and plants, bio-refineries, and wood pellet heating technology (Mukhopadhyay et al. 2018). Biogas mainly consists of methane which is major GHGs, but many facilities that generate biogas capture methane and burning of methane produces electricity. However, burning methane also produces carbon dioxide but as methane is stronger GHG the overall greenhouse effect is reduced. According to IRENA, biomass energy accounts for 1.4% of the world’s electricity generation and will be potential energy source for nations having large populations with increasing power demands. Brazil leads in area of liquid biofuels and contains large number of flexible fuel vehicles that use bioethanol and biodiesel (Zimbres et al. 2021). Chen et al. have developed Cu-Co bimetallic NPs coated with carbon layer via direct heating treatment of bimetallic oxide precursors (Chen et al. 2017). Polyethylene glycol was used over the precursor which acts as the carbon sources and the reductant for metals. For protection, these Cu-Co bimetallic NPs were covered by carbon layers. It was found that Cu-Co@Carbon catalyst shows excellent performance in selective hydrogenolysis of HMF to DMF. The catalyst showed good recoverability and recyclability in six-run recycling test in which selectivity and activity of the catalyst were kept constant (Chen et al. 2017). Salama et al. observed that 2 ppm concentration of nickel oxide nanoparticles plays a crucial role for enhancing the biogas production. Substances like P. pectinatus and P. stratiotes were used as substrates. It was found that biogas production was more 87.91% in case of NiONPs with using P. stratiotes, while 82.38 % was noticed in NiONPs with P. pectinatus (Salama et al. 2020). Zaidi et al. synthesized the metal (Ni, Co) and metal oxide (Fe3O4, MgO) NPs via green method for increasing the yield of biogas. It was noticed that among all, Fe3O4 and Ni nanoparticles achieved the highest amount of chemical oxygen demand of 14,760 mg/L and 14,745 mg/L, respectively. This study revealed that nanoparticles moderately have positive influence in biogas production (Zaidi et al. 2018).

Thermoelectric Energy The thermoelectric technology (TE), one of the many green technologies, have been applied to different areas in order to design simple, compact, and environmentfriendly systems. The applied areas comprise of kerosene lamp, aerospace applications, transportation tools, industrial utilities, medical services, electronic devices, and temperature detecting and measuring facilities. Thermoelectric materials can also be used for power generation. Construction of thermoelectric materials involves arrays of N and P-type semiconductors, in which a heat source is one side and a cooler-sink to the other side; electric power is generated

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Fig. 6 A schematic of mechanism of thermoelectric power generator

and vice versa, as shown in Fig. 6. The electric power can be converted to cooling or heating by changing the current direction. Despite of the low conversion efficiency (around 10%) when used as power generators, they are more beneficial due to no moving parts. This made them both more reliable and everlasting compared to conventional energy technologies. Apart from that, they are scalable without releasing any pollutant to the environment during the operations. Hence, they would be ideal for applications in many fields at various scales replacing the traditional cooling and power generation methods. Although the extensive applications of thermoelectric materials have been limited to some specific fields due to their low efficiency, therefore the reliability rather than the cost is a major consideration. The development of new thermoelectric materials with high efficiency is one of the key factors for expanding the range of thermoelectric applications to the medium/large scale (Zheng et al. 2014). Various nanomaterials and their thermoelectric properties are given in Table 2. The performance parameters such as voltage, power, and current profile of Nano-Enhanced D-Mannitol based thermoelectric generator (TEG) have been studied by Karthick et al. (Karthick and Suresh 2021). Ali et al. prepared Cu2ZnSnS4 thin films by the chemical solution method (sol-gel method) and studied the thermoelectric properties by varying the concentration of sulfur atoms from 2 to 8 mM. It was found that as the molar concentration increases, thermoelectrical properties also enhance with it. It was found that Seebeck coefficient from 148 to 393 μV/
C and power factor increased from 4.05105 to 1.17103 W/m K2 (Ali et al. 2021). Chen et al. have prepared p-type transparent copper iodide films by successive ionic layer adsorption and reaction method. In this investigation, Seebeck coefficient was found very less (70μV*K1) (Chen et al. 2021). To enhance the thermoelectric properties of materials, Kimura et al. prepared n-type Bi2(SexTe1x)3 nanoplates

–

130.0 (μV/K) 56 (μV/K)

Bi2(SexTe1x)3

Cu3(HHTP)2

Integrated nanocomposite film adding SWCNTs (SWCNT)ethanol solution: 9 mL) Al-doped ZnO

3.

4.

5.

6.

81.8 S/cm

128.6 (μV/K) 121.4 (μV/K)

Cu2ZnSnS4

2.

–

–

–

2.28  103 S/cm

~300K

~300

~301

–

~373

– –

Temperature (K) ~300

Thermal conductivity (Wm1 K1) 0.07–0.50

317 S/cm

163 S/cm

Materials name MoS2

Electrical conductivity 2320 (Ω1 m1)

S. no. 1.

Seebeck coefficient 466.34 (μV/ K) 810 (μV/
C)

Table 2 Various nanomaterials and their thermoelectric properties

–

1.38 μW/ (cm K2

0.0107 (W/mK2) 4.1 μW/ (cmK2) 3.15  103 μW/mK2

Power factor –

Ambedkar et al. (2020)

Ref. Kogo et al. (2020) Ali et al. (2021) Kimura et al. (2020) GonzalezJuarez et al. (2020) Yabuki et al. (2020)

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via solvothermal method by changing the composition of selenium. It was observed that at the atomic composition of x ¼ 0.75, Bi2(SexTe1x)3 nanoplates exhibited larger power factor of the order of 4.1 μW/(cm∙K2). It was noticed in this study that optimized composition of Se atoms enhanced the thermoelectric properties of the synthesized material (Kimura et al. 2020).

Hydrogen Energy Today’s transport sector accounts for global CO2 emissions, and a major part of this is coming from road transport. There would be 20% of the projected increase in both the sectors, demand of global energy and greenhouse gas emissions until 2030 due to transport (Ball and Wietschel 2009). Hydrogen is recently recognized as alternative, efficient, green source of energy across the world due to its great energy capacity and zero emission during combustion. Hydrogen provides several advantages as a clean energy carrier and receiving a great attention for policy makers of energy sector. Hydrogen provides clean fuel which is one of the best options for sustainable future. Various energy sources such as nuclear power, biomass, and different renewable sources generate hydrogen and make it a sustainable fuel option for various applications. Hydrogen fuel can be used in variety of applications such as electricity generation, transportation, portable power, etc. (Singh et al. 2020). Solar energy and wind energy cannot generate energy all the time, but they produce hydrogen which can be stored and will be used when needed and can also be transported to the required locations. When hydrogen fuel is used in fuel cells, it produces water, i.e., use of hydrogen in fuel cell is free from any emission and this keeps away from the transport-induced emissions of both CO2 and air pollutants. Among the various energy sources options, hydrogen energy offers effective remedies to both energy supply and control of GHG emissions and pollution. There are three hydrogen grand challenge areas (Borgschulte 2016): (i) High conversion efficiencies of hydrogen production: The conversion of photon energy (solar light) into an electrochemical potential and electro-catalytic conversion of water into oxygen, electrons, and protons (ii) Hydrogen storage: Development of hydrogen storage system using suitable materials and technology like metal hydrides, etc. that fulfill the DOE’s demand (iii) Hydrogen use: High efficiency of fuel cells and combustion The rapid development of nanomaterials has opened up new paths for the conversion and utilization of renewable energy. The nanomaterials play an important role in the production, separation, storage, and utilization of hydrogen as a separate fuel. The technologies for nanomaterial design are important for future research into renewable energy. Hydrogen being a secondary energy carrier can be produced from a diversity of both renewable and non-renewable sources. Currently, non-renewable sources are

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mainly used for hydrogen production. However, for long-term sustainable hydrogen production, it is important that renewable energy sources such as solar, wind, or biomass must primarily be used, for example, nanomaterial design for high efficiency photocatalytic/photoelectrochemical solar hydrogen conversion (Fărcaş and Dobra 2014). Hydrogen storage also remains as one of the key challenges to realize hydrogen economy. The conventional hydrogen storage by pressure or cryogenics system suffers from low hydrogen capacity, high cost, and safety issues. Hydrogen storage (solid-state) materials can safely store the higher density of hydrogen compared to the gaseous and liquid hydrogen storage systems. Hydrogen storage systems based on nanomaterials are highly attractive alternatives, which overcome these difficulties at a certain level. Hydrogen physically or chemically stored into nanomaterials in the solid state is a desirable prospect for effective large-scale hydrogen storage, which exhibits great potentials in reversible energy storage applications. These nanomaterial absorbents show the high hydrogen content absorption and the easyhandling desorption. Nanomaterials such as an increase in the surface area of materials for absorption/desorption giving functional sites to improve the storage condition are current efforts for large-scale hydrogen storage (Frattini et al. 2021). The important properties of the hydrogen storage materials to be evaluated for real-time applications are (i) light weight, (ii) cost and availability, (iii) high volumetric and gravimetric density of hydrogen, (iv) fast kinetics, (v) ease of activation, (vi) low temperature of dissociation or decomposition, (vii) proper thermodynamic properties, (viii) long-term cycling stability, and (ix) high degree of reversibility. Various hydrogen storage systems, such as metal hydrides, complex hydrides, chemical hydrides, adsorbents and nanomaterials (nanotubes, nanofibers, nanohorns, nanospheres, and nanoparticles), polymer nanocomposites and metal organic frameworks, etc., have been explored for onboard hydrogen storage applications (Niemann et al. 2008). Many approaches have been applied for a simple and effective strategy for the production of hydrogen. Therefore, photoanode can be improved in photoelectrochemical water splitting by fabricated heterostructures of p-type BiOI nanoplates decorated on n-type ZnO nanorod arrays (Kuang et al. 2015b), the highly efficient double-shelled CdS- and CdSe-Co sensitized ZnO porous nanotube arrays (Kuang et al. 2015a), the BiOBr nanoplate-wrapped ZnO nanorod arrays (Liu et al. 2016), the vertically aligned ZnO/PDMcT core/shell nanorod arrays (Su et al. 2013), and the highly ordered ZnO/CdS/Au nanotube (Wei et al. 2017), well-aligned ZnO NT/Ag arrays (Rasouli et al. 2019). Hydrothermal synthesized photocatalyst with different wt. % of SnS2/TiO2 nanocomposite have effectively utilized for hydrogen production. The SnS2/TiO2 nanocomposite with 10 wt. % shows an optimum rate of hydrogen production of 195.55 μmolg1(Shanmugaratnam et al. 2021). To construct a novel cross-linked highly selective growth of vertically oriented PtO nanowires on ZnO nanoarrays, arrays revealed that the PtO nanowire could significantly improve the PEC performances for water splitting (Fu et al. 2018). Hydrothermally synthesized WO3 nanocubes with controlled shapes and sizes can be used as an efficient photocatalyst in PEC solar water-splitting (Rani et al. 2019).

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The higher H2 adsorption capacity of 3.67wt% for PANI-BF3/SWNT composite was achieved at low temperature (196
C) under high pressure (100 bar) (Karatepe et al. 2013). Reversibly and massively storing H2 into solid-state media like Mg-based hydrides are capable and theoretically reasonable for online H2 supplying towards practical application. The strategically and technical development of the Mg-based hydrogen storage materials, most of which come from the several modifying concepts, shifting from traditional bulk materials to nanostructures via specific techniques such as functionalized nanoconfinement provides a prospective trend to achieve light-weighted hydrides with prominent hydrogen storage properties (Bellosta von Colbe et al. 2019). For MgH2@X (X¼CSC, CNT, G, and AC) composites, hydrogen storage property is significantly influenced by the structure of the carbon materials (Bellosta von Colbe et al. 2019). The layered structure of carbon material is contributed to the high surface area and provide the superior de/hydrogenation kinetics (Zhang et al. 2020). The MgH2 nanoparticle-graphene nanosheet composites exhibited improved hydrogen storage. The 10 wt %-graphene nanosheet MgH2 nanoparticle composites hydrogen desorption temperature is 318
C, which is 62
C lower than the pure MgH2 (Zhang et al. 2019). Mesoporous Zn2V2O7 nanostructures with different morphologies revealed the high capacity of these nanostructures for optimizing hydrogen storage of more than 2899 mAh/g after twenty cycles (Ashrafi et al. 2020). Samantaray et al. present the transition metal alloy decorated di-atom frustule-graphene nanomaterials showed an enhanced hydrogen storage capacity of ~3.2 wt% (Pd3CoD(50)-G) and ~4.83 wt% (Pd3CoD(100)-G) at 25
C and ~20 bar H2 equilibrium pressure, which is close to the US DOE target of ~5.5 wt % (Samantaray et al. 2020). Super alkali NLi4 clusters decorated graphene can be promising hydrogen storage material with high storage capacities. When two and six NLi4 units are decorated on graphene supercell, 20 H2 and 60 H2 molecules can be adsorbed with a hydrogen storage capacity 8.55 wt% and 10.75 wt%, respectively (Qi et al. 2021). Recently N-doped penta-graphene (PG) with Li decoration have been suggested significantly a promising material for hydrogen storage with gravimetric densities 7.88 wt% (Hao et al. 2021). A fuel cell uses the chemical energy of hydrogen or other fuels to cleanly and efficiently produce electricity. If hydrogen is the fuel, the only products are electricity, water, and heat. Fuel cells operate best on pure hydrogen (Fărcaş and Dobra 2014). H2 þ 1=2 O2 ! H2 O þ Electricity þ Heat Bimetallic palladium alloy nanoparticles attached with reduced graphene oxide (rGO) like PdFe/rGO, PdAg/rGO, and PdAu/rGO, support for both cathode and anode reactions of the direct borohydride fuel cell. PdAu/rGO reveals high activity and the highest number of exchanged electrons for both oxygen reduction reaction (ORR) and borohydride oxidation reaction (BOR) (Martins et al. 2017). A schematic diagram of hydrogen energy production, solid-state storage, and electricity produced by fuel cell is shown in Fig. 7. For the supply of energy to people is a basic need to

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Fig. 7 Schematic diagram of hydrogen energy production, solid-state storage, and electricity produced by fuel cell

every person, the hydrogen grand challenge has also economic, ecological, and social confines.

Conclusions Nanotechnology builds an enormous revolution in renewable energy devices utilized for energy conversion and storage and environmental monitoring, also as green manufacturing of environmental pleasant materials. Green and sustainable nanotechnology plays a major role in the reduction of environmental impact of burning fossil fuels by the production of a cheap and clean energy for global use. Nanomaterials are widely developed to improve the efficiency of solar cells, fuel cells and wind turbines, thermoelectric generator, etc. Nanotechnology has ability to decrease the cost of high-priced tools/devices used in hydrogen production, storage, sensors solar cells, and hydropower plants. Nanotechnology has been measured as a key solution for significantly enhancing the biofuels production by utilizing nanopraticles, such as metal-oxide (MnO2 & TiO2, etc.), carbon nanotubes, and magnetic nanoparticles (Fe3O4, etc.). Wind turbine performance can be significantly enhanced by using composite blades reinforced by metal-oxide nanoparticles. This also improves the resistance to wear, fatigue failure, and rigorous operating conditions and provide long life to wind turbines. There are major challenges such as inadequate technological information, the lack of infrastructure, inadequate servicing and maintenance of equipment, higher cost, and hence hinder their adoption. However, more efforts are required to improve the efficiency and sustainability of all the clean and sustainable energy technologies and to overcome the challenges for achieving the worldwide demand of renewable energy.

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Manviri Rani, Meera, and Uma Shanker

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-biodegradable Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Different Nanoparticles from Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles from Biodegradable Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles from Non-biodegradable Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Waste-Derived NM Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Biomedical Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the modern era, waste management becomes a global issue. A variety of problems are caused by these wastes to humans, animals, ecosystem, and plants. Every year a large quantity of waste is produced, so an efficient and cost-effective treatment is required. Nanoparticles have high advantages such as simplicity, economic, and environmental friendliness so their synthesis has achieved great

M. Rani (*) · Meera Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India e-mail: [email protected] U. Shanker Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, Punjab, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_78

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interest from researchers. In this chapter we discuss the synthesis of nanoparticles derived from different wastes, i.e., bio-wastes, industrial wastes and their further application. The main objective of this chapter is to summarize the data for the synthesis of nanoparticles from wastes. Also, the applications of these derived nanoparticles in various fields including energy storage, bio-imaging, environmental remediation, and sensing by exploring their fluorescence and catalytic activities are considered. In addition, the remaining challenges and future perspectives are also highlighted. Keywords

Waste · Nanomaterials · Photocatalyst · Environmental remediation

Introduction Currently, due to different types of waste (bio-wastes and industrial wastes) are generated in the environment so our environment is facing lots of challenges that ultimately harm biota and abiota on Earth. Every year 7.6 billion tons of industrial wastes are produced and disposed of (Li et al. 2015). Toxic, corrosive, infectious, flammable, highly reactive, and radioactive behaviors produce a negative impact on the environment. For the sake of environmental safety, it is very important to recycle these wastes or convert them into value-added products. Mechanical recycling and energy recovery are the two main approaches applied to manage waste materials. In mechanical recycling, waste material can be reprocessed into secondary raw material without damaging its basic structure whereas in energy recovery wastes are burned. From the environmental safety perspective, mechanical recycling is more appropriate than energy recovery because energy recovery as incineration releases harmful chemicals. However, mechanical recycling is highly expensive, so it is non-profitable. Therefore, in order to have a sustainable and better life, a better management processor is required. On behalf of these facts many new methods and pathways are explored to minimize the negative environmental impact and the cost of recycling process. In the past few years, waste-derived nanomaterials (NMs) are highly focused on. These wastes act as the initial raw material for nanoparticle synthesis. Nanomaterials synthesis from various wastes is a much better way of recycling these wastes. Recently, researchers developed a variety of NMs such as carbon nanotubes/nanosheets, nanofibers, metallic/carbon nanoparticles, and nanoactivated carbon from a variety of waste materials. Agricultural wastes are potentially cost-effective when it is employed as initial raw material for NMs. It is remarkable that different parts of plants have been widely utilized to manufacture beneficial goods including paper, wooden furniture, and pencils (Martinez-Alier 2009). However, these materials will finally lead to the production of waste after consumption. So, the potential consumption of waste materials for nanoparticle synthesis is easily available and inexpensive and has become an innovative pathway for recycling waste. Similarly, industrial wastes such as tires, batteries, and polymers also act as raw materials for nanoparticles. For example, every year China alone produces 50–60

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million lead battery wastes (Zhou et al. 2017). These batteries include lead sulfate, lead oxide, and lead dioxide which are very hard to recycle. Similarly, other batteries are also released in massive amounts so recycling these batteries is an ultimate option to manage these wastes. For instance, lead nanopowder was prepared from the spent lead paste by the pyrolytic route (Zhou et al. 2017). Submicron-sized lead oxide particles are synthesized through the thermal decomposition process with the help of poly(Nvinyl-2-pyrrolidone) (PVP). PVP helps in reducing the size of particles from the micron to submicron range. Metallic lead, α-PbO, and β-PbO mixture was produced by calcination at 350
C for 3 h. In the case of spent Zn-Mn batteries a variety of nanomaterials are generated such as nanofibers, zinc nanoparticles, and flaky nanoparticles (Xiang et al. 2015). Xiang et al. (2015) used zinc cathodes to prepare Zn nanoparticles. A major portion of the battery waste stream is covered by lithium-based batteries, and a better recycling process should be adopted for their recovery (Ra and Han 2006; Richa et al. 2017). The large-scale use of synthetic polymer (non-biodegradable) has resulted in one of the largest categories of waste materials in the world. These synthetic polymers have been used to produce bags, bottles, and utensils which can act as a major source of solid waste. NMs can also achieved by this pathway, For instance, multi-walled carbon nanotubes (MWCNT) and nanochanneled ultrafine nanotubes were produced from polyethylene terephthalate waste through a catalyst-free and solvent-free arc discharge method (Joseph Berkmans et al. 2015). CNTs of different shapes including non-branched/branched ultra-fine nanochanneled carbon tubes, solid carbon spheres, and multi-walled carbon nanotubes (MWCNTs) are prepared through an arc discharge method. On the other hand, novel methods for recycling rubber tires are highly focused. In waste tires, zinc is present in higher quantities so zinc nanoparticles are synthesized from waste tires (Moghaddasi et al. 2013). Green synthesized zinc nanoparticles are produced from waste rubber tires via ball milling for 5 h. The size of rubber ash particles is converted from 500 to 50 nm after milling. This rubber ash can be used in root dry matter production, equivalent to the commercial ZnSO4 fertilizer. Hence, in view of the above utilization of wastes, the present chapter deals exclusively with the synthesis of nanoparticles from various wastes. The different pathways for synthesis and the size of nanoparticles are discussed. In addition, the applications of derived nanoparticles are also highlighted.

Classification of Wastes Different types of wastes such as printed circuit boards, batteries, and plastics are released from different sources. It is very difficult to study these wastes on the basis of sources, from there, they are released. So, for our interest and easiness, wastes are broadly divided into two categories such as biodegradable and non-biodegradable wastes.

Biodegradable Wastes Biodegradable waste may be defined as waste that can degrade by itself after some time and convert into manure. These wastes include cotton waste, old newspapers,

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orange peels, garden waste, kitchen waste, and husk. Mainly biodegradable wastes are wet in nature, so they are degraded easily. Its degradation time depends upon the material.

Non-biodegradable Waste Non-biodegradable waste may be defined as waste that cannot degrade without manpower. These wastes include industrial waste (batteries, scraps), broken glass pieces, plastics, ink, rubber, and tires. These types of wastes are dry, so they are hard to degrade by nature. Due to their non-degradable behavior these wastes act as major pollutants.

Synthesis of Different Nanoparticles from Wastes Nanoparticles from Biodegradable Wastes Biodegradable wastes consist of husk, peels, waste tea, waste coffee, and rice straw. In literature a variety of nanoparticles are derived from these biodegradable wastes. In this section different types of nanoparticles derived from different types of waste are discussed. A summary of different types of nanoparticles from biodegradable wastes along with their synthesis process is shown in Table 1.

Carbon Dots from Wastes Due to the unique properties of carbon dots such as biocompatibility, eco-friendly, high photostability and strong fluorescence, they are most focused nanostructure for various applications (Athinarayanan et al. 2021). These properties of carbon nanodots make them more important for a variety of applications such as bio-imaging, photo-catalysis, optical sensing, and photovoltaic (Athinarayanan et al. 2020; Yang et al. 2017; Liu et al. 2018; Rani and Shanker 2020). In the last few decades many researchers produced carbon nanodots from solid wastes. For instance the formation of photocatalyst carbon dots from bitter apple peels was done via a single-step carbonization method (Aggarwal et al. 2020). Its morphology and interplanar arrangement are detected with the help of transmission electron microscopy. Results show the carbon dots are spherically shaped, and HRTEM shows the presence of graphitic fringes. These carbon dots are further used for the degradation of crystal violet dye. Similarly, Athinarayanan et al. (2020) fabricated the fluorescent carbon nanodots from the laboratory paper wastes through thermal decomposition process (Athinarayanan et al. 2021). Its morphological properties are analyzed with the help of atomic force microscopy which indicated that it is spherical in shape with a size of 3–10 nm. These carbon nanodots are efficiently quenched in the presence of Fe3+ ions. Gunjal et al. (2019) synthesized water-soluble carbon nanodots from kitchen wastes such as tea residue and found highly selective fluorometric recognition of free chlorine in acidic water (Gunjal et al. 2019). It can be done by the one-step

Laboratory paper waste

Waste tea

Olive solid wastes

Waste chicken fat

Plant waste (Thypa orientalis) Waste eggshell

Waste coffee

Waste Nypa fruticans

Onion peels Rosa canina waste seed extract Cassava periderm, maize cop, maize stalk

2.

3.

4.

5.

6.

8.

9.

10. 11.

12.

7.

Type of waste Bitter apple peels

Serial no. 1.

Silicon nanoparticles

Silver nanoparticles Silver nanoparticles

Carbon quantum dotcarbon nanotube Nanoporous carbon incorporated nanotubes Silver nanoparticles

Carbon nanotubes

Carbon nanotubes

Carbon nanodots

Carbon nanodots

Carbon nanodots

Nanoparticle derived Carbon nanodots

Sol-gel method

Green synthesis Biosynthesis

Chemical and thermal deposition Hydrothermal and calcinations Chemical vapor deposition method Ball milling followed by doctor blade technique Green synthesis

Carbonization method

Thermal decomposition method Carbonization method

Synthesis method Carbonization methods

Table 1 Nanoparticles derived from biodegradable waste along with synthesis method

7.85–33.98 nm, 17.20–32.88 nm, and 14.85–31.20 nm, respectively

25–100 nm 150 nm

10–15 nm

86% within 45 min under solar light irradiation. The schema for the preparation of cobalt oxide is shown in Fig. 5.

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Fig. 5 Schema for the synthesis of cobalt nanoparticles and degradation of methylene blue dye under sunlight

In direct photolysis, the dye solution showed 2.3% degradation. In recent years, lithium-ion battery cathode is made up of manganese oxide with lithium which made it cheaper than the cobalt used battery. Researchers also prepared MnO4 particles from waste lithium ion-manganese batteries. These nanoparticles were found to be very useful for the oxidation of volatile organic compounds (VOCs) (Min et al. 2021). Here, manganese was efficiently and selectively recovered from the waste lithium-ion battery by the advanced oxidation pathway with the help of ozone, potassium permanganate, and the transition metal-doped α-MnO2 and β-MnO2 prepared in a one-step approach for catalytic oxidation of VOCs. Here, the recovery rate of manganese was found to be about 100%. Toluene and formaldehyde were used as volatile organic compounds.

Nanoparticles from Plastics Large quantities of waste plastic are made up of polyolefin system (¼>85.7 wt % carbon content) which is created from olefin monomer (CnH2n) (Gong et al. 2012). Polyolefin polyethylene terephthalate (PE) and polypropylene (PP) are used for the synthesis of carbon material. Polyethylene terephthalate (PET) and polyvinyl alcohol (PVA) are generally used for the preparation of carbon nanotubes (CNTs) (Deng et al. 2016). The synthesis CNTs from plastic waste is carried out in diverse systems such as muffle furnaces, crucibles quartz tubes, and autoclaves (Bazargan and McKay 2012). Based on this Gong et al. (2012) produced a new layer-by-layer

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assembling mechanism. Here, PP wastes were catalyzed using Ni2O3 and activated carbon in a quartz tube reactor. PP wastes were broken into small hydrocarbons with the help of activated carbon (AC). Other than fragmenting of PP, activated carbon is also helpful in the formation of different aromatic groups because it provides high catalytic conversion with nickel by encouraging aromatization and dehydrogenation. The growth of CNTs was based on benzene rings. When the raw materials were used in the proportion of PP:10Ni2O3:8 AC (wt%) the carbon yield was found to be 50 wt % at 820
C temperature (Gong et al. 2012). Similarly Zhou et al. published another method for PE decomposition (Zhuo et al. 2010). In this method the generation of light hydrocarbon is completely in situ. The light hydrocarbon was generated by an exothermic process. A stainless-steel wire mesh acts as a substrate and catalyst. For the elimination of the soot a ceramic filter was used before the stainless-steel wire mesh. This was because soot may be deactivated on the catalyst surface of stainlesssteel wire mesh. The yield of CNTs was found to be >10% (Zhuo et al. 2010). Bajad et al. derived multi-walled carbon nanotubes (MWNTs) from the PP waste with a yield of 45.8% (Bajad et al. 2015). This preparation is catalyzed by Nickel/Molybdenum/MgO with the help of the combustion technique (Bajad et al. 2015). A mixture of finely powdered catalyst and PP waste were heated at 800
C in a muffle furnace. Here silicon covered crucible was used for the muffle furnace. The size and yield of nanotubes are highly affected by the presence of Ni/MO. This result was calculated with the help of HRTEM images. It was found that lowering the content of Mo produced short radius CNTs with higher yield and vice versa (Table 2).

Table 2 Nanoparticles derived from non-biodegradable waste along with synthesis method Serial no. 1. 2.

Type of waste Rubber tire

7.

Cathode ray-funnel glass Printed circuit board Zn-Mn battery Lithium battery Lithium battery Waste plastic

8.

Waste plastic

9.

Waste plastic

3. 4. 5. 6.

Nanoparticle derived Zinc nanoparticles Lead nanoparticles Cu nanoparticles Zinc nanoparticles Co3O4 nanoparticles Mn-based nanoparticles Carbon nanotubes Carbon nanotubes Carbon nanotubes

Synthesis method Ball milling Vacuum carbon-thermal reduction and inert-gas consolidation Micro emulsion processes

Chemical synthesis Chemical method Combustion method Combustion method Combustion method

References Moghaddasi et al. 2013 Xing and Zhang 2011 Mdlovu et al. 2018 Xiang et al. 2015 Dhiman and Gupta 2021 Lu et al. 2020 Gong et al. 2012 Zhang and Zhang 2012 Bajad et al. 2015

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Waste-Derived NM Applications Many researchers used the NMs for the degradation of hazardous chemicals, organic containment removal, and heavy metals removal from environmental effluents (Buruga et al. 2018; Sarkar et al. 2018). For these applications a variety of nanoparticles are derived from the waste materials. Highly efficient, stable, and biocompatible nanoparticles are required for this purpose. Recently different types of waste-derived nanoparticles are used in the treatment of wastewater. The catalytic properties of the nanoparticles play an important role in the removal of containments. In the last few years, the rice husk synthesized nanoparticles attracted worldwide attention and provides a highly useful method for the management of agricultural wastes. Other than agricultural waste synthesized nanoparticles, wastegenerated carbon-based NMs are on the top priority of the researchers. This is because waste-generated carbon nanoparticles are used in different fields like optical probes, bioimaging probes, and medical diagnosis. In this section, various applications of waste-derived nanoparticles are discussed.

Catalytic Activity In liquids and gases the chemical reaction is accelerated with the help of heterogeneous catalysts (solid catalysts). This is because, in a chemical reaction, the collision probability is increased by the catalyst (by decreasing activation energy) which is directly proportional to the rate of reaction. So, as collision frequency or probability increases, the rate also increases. The catalytic performance of an NM is determined by its surface area to volume ratio. The NMs have more catalytic performance than the conventional catalyst because NMs have a very small size than the conventional catalyst. On the other hand, nanocatalysts have unique and different properties which are not observed at macroscopic levels. A nanocatalyst is generated from the waste polymer (Alonso-Fagúndez et al. 2014). A solid-SiO2 polystyrene sulfonic acid and a soluble polystyrene sulfonic acid were prepared from tetraethyl orthosilicate with the help of sol-gel process. These waste-derived sulfonic acid catalytic activities were employed on biomass valorization reactions which include oxidation of furfural to maleic and succinic acids, biodiesel synthesis, and xylose dehydration to furfural. With the help of centrifugation followed by filtration the SiO2 polystyrene sulfonic acid nanocomposite was separated from the reaction mixture. So, in the case of biodiesel reaction the nanoparticles are completely degraded due to the deposition of by-products. In liquid phase oxidation with the help of H2O2 reactions and furfural dehydration, SiO2 nanocomposite was deactivated during the first runs but the catalyst remained intact in consecutive runs. The activity of these catalysts was the same up to 5 runs. The deactivation is due to the leaching of polymer. Other NMs are also made for this purpose such as metal oxides, ZrO2, and TiO2 due to their acidbase properties (De Crozals et al. 2016; Watanabe and Kasuya 2005). In another report solid base ZrO2 along with solid acid SO2/TiO–SiO yielded 48% of

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5-hydroxymethylfurfural from glucose at a solid acid-to-solid base ratio of 3:4 (Yang et al. 2011). These do not tell about the recyclability of catalyst NMs. Another catalyst was prepared from the blast furnace flue dust which was a fine powder containing zinc oxide, iron, copper, and calcium. From this powder an ironbased nanoparticle is developed which acts as a catalyst. These catalysts degrade 69% of the dye content in wastewater through Fenton-like oxidation in the presence of H2O2 (Amorim et al. 2013). These waste-derived nanoparticles have high decolorization rate than the Fe2+ ions in solution under identical reaction conditions. The reproducibility of these catalysts was up to three cycles. The efficiency of this catalyst was greater than 85%. By following this information, the flue dust wastederived catalyst is highly useful in the treatment of wastewater. Silica nanoparticles are synthesized from stainless steel slags waste by hydrothermal treatment with the help of acid digestion. This produced 9–20 nm sized silica nanoparticles which are further impregnated with platinum (1 wt%). These SiO2 catalysts are used for the oxidation of carbon monoxide and VOCs (such as toluene). The oxidation takes place under 21 volumes % of O2 with 95% conversion of VOCs at a temperature range of 200
C to 500
C. Its catalytic performance for CO oxidation is relative to the commercial Pt/SiO2. The oxidation of VOCs is weaker because the dispersion of platinum was very low at the waste support (Domínguez et al. 2008). Similarly, Cu2O/TiO2 catalyst was prepared from the WPCBs waste printed circuit boards (Xiu and Zhang 2009). The catalytic activity of these nanoparticles was evaluated by the degradation of toxic organic dye such as methylene blue (Xiu and Zhang 2009). For this purpose Cu-doped TiO2 nanoparticles were found to be more active than undoped TiO2. This is because the presence of Cu2O on the surface of TiO2 increases the rate of electron transfer to oxygen and produced a large number of holes for the degradation of dye (Xiu and Zhang 2009). A 10 ppm methylene blue was completely degraded in 1 h with the help of 4.5 wt% Cu-doped TiO2 nanoparticles. Similarly, the photocatalyst activity of pure CuO nanofibers had been investigated for degradation of methyl orange (Sahay et al. 2012). The methyl orange dye was degraded up to 60% under UV light within 1 h.

In Energy Storage Rice husk is an abundant and organic agricultural waste. Generally, the quantity of SiO2 present in rice husk waste is in the range of 8.7% to 12.1% (Ding et al. 2005). Sankar et al. (2016) synthesized silica nanoparticles from rice husk powder with a uniform particle size and morphology distributions (Sankar et al. 2018). Silica nanoparticles synthesized from sticky rice husk were found to be 50 nm and from brown rice husk 10 nm. Due to the uniform size and morphology distribution SiO2 nanoparticles are abundantly used for energy storage and drug delivery applications (Sankar et al. 2016). For the fabrication of battery materials pure and highly crystalline silica is needed but this is highly expensive than the conventional process. So, many researchers have focused on developing a commercial, large volume, and low-priced process for the synthesis of high purity silica for electrical applications.

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The synthesis of silica from rice husk satisfies all these terms such as low cost, commercial, and large volume. In this direction, silica nanoparticles are derived from rice husk through a magnesiothermic reduction process (Wong et al. 2014). In the lithium-ion battery the anode material composite was made using these silicon nanoparticles (silicon/graphene composite). The initial capacity of silica was about 1000 mA h g1 at a current density of 1000 mA g1 and its reproducibility was maintained up to 30 cycles (Wong et al. 2014). Similarly, by electrospinning and carbonization Si/C nanocomposites are synthesized for energy storage applications (Li et al. 2015). The storage capacity of silica NPs derived from rice husk (1000 mA h g1) was comparable to that of Si/C nanocomposite (1300 mA h g1 (10 wt% Si)) (Wong et al. 2014). In the same way, SiO2 nanoparticles are synthesized through microwave-assisted procedure followed by magnesiothermic reduction for 30 min at 650
C from agricultural residues such as rice husk, bamboo culm, and sugar cane waste in the absence of reducing gas (Praneetha and Murugan 2015). This reduction produces Si-C passive heating element plates through absorption of microwave radiation. These waste-derived materials are highly used in the field of energy storage applications (Praneetha and Murugan 2015). But these rice husk synthesized Si nanoparticles possess a very limited life cycle and low coulombic efficiency anode materials because of their high surface reactions and low thermodynamic stability. For this reason, researchers developed silicon nanoparticles along with carbon nanotubes which are considered a competent matrix in view of their robust structure with high electrical conductivity (Candelaria et al. 2012). There is a cross-linked network present in carbon nanotubes which is helpful in electron transport among active particles. Due to these facts, carbon nanotubes are acts as a novel anode material (Xiao et al. 2014). Rice synthesized silicon nanoparticles (50 nm) are embedded with carbon nanotubes and N2-doped carbon (Zhang et al. 2016). This Si/N2-doped carbon/CNT exhibits robust structure, good surface/interface stability, and high electronic conductivity. It also has high cycling stability (1031 mA h g1 at 0.5 A g1 after 100 cycles) with exceptional rate competence for lithium-ion batteries. Many other biomass wastes (including firewood, coffee endocarp, pistachio shell, and rubber wood saw dust) are helpful in the synthesis of activated carbon through physical activation which is used in the supercapacitor carbon electrodes. These capacitors are more demanding due to its low cost and abundant availability (Gao et al. 2017). In this field, Li et al. (2015) synthesized activated carbon from fallen leaves by the chemical activation method using K2CO3 and KOH. The synthesized activated carbon from leaves show good cycling stability up to 2000 cycles without capacitance diminution and high specific capacitance values up to 242 F g1.

In Biomedical Field Carbon nanodots have unique properties due to their exceptional features including multiwalled CNTs emission, electrochemical oxidation of graphite, size-dependent fluorescence pyrolysis of saccharide, resistance to photobleaching, and strong

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luminescence. The conventional synthesis methods of these carbon nanodots are highly costly. This is because of low yield and costly starting material. For instance Park et al. (2014) utilized food waste for the production of green nanodots by ultrasound irradiation at room temperature. Carbon nanodots of size 2–4 nm were prepared from food waste. On average 120 g of carbon nanodots are prepared from per 100 kg of food waste. For calculation of its advantage in living system, human hepatocellular liver carcinoma (HepG2) cells were incubated with 0–2 mg mL1 nanodots in a culture medium for 3 h at 30
C. The advantage of carbon nanodots was shown by its photoluminescence imaging. After incubation, the HepG2 cells remained attached to the incubation green nanodots, and a sharp red-colored photoluminescence in the nuclei inferred the penetration of nanodots into the cells. On the other cell, there was no interaction with carbon nanodots and displayed no red photoluminescence. These calculations show that food waste-derived nanodots are very useful in bioimaging applications (Park et al. 2014).

Other Applications Recently another important application of waste-derived NMs has been published (Akter Jahan et al. 2017). Here Jahn et al. (2017) synthesized the nanohydroxyapatite (nano-HAP) from eggshell-derived Ca-precursor with the help of reverse microemulsions at ambient temperature. The average size of nano-HAP particles was near about 5–20 nm. Further these nano-HAP particles were also calcinated at 600
C. These nano-HAP nanoparticles are highly useful in the as (V) removal. It had been calculated that at different pH the removal tendency of As(V) from aqueous solution showed a significant increase for non-calcinated nanoHAP (38.27%) as compared to the calcinated hydroxyapatite (0.97%) (Akter Jahan et al. 2017).

Conclusions This chapter covered the application of waste-derived nanoparticles and the synthesis of waste-derived nanoparticles along with their shape and size. These nanoparticles are considered to have great potential for diverse fields of application. Such materials can be generated in an easy and economical way. Moreover, their specific capacity (for energy storage application) and properties (e.g., surface area for catalytic or adsorptive activity) are often found to be greatly improved relative to virgin NMs. The utility of waste-derived NMs in the field of waste and wastewater treatment was comprehensively analyzed with respect to their catalytic activities. New age applications of waste-derived NMs from bioimaging to energy storage were explored for enhancing the circular economy. However, the rapid increase in the development of nanotechnology poses uncertain toxicological threats due to the release of NMs into the environment. This review may be an important documentation for future researchers.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method for CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermocatalytic CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photothermal CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoelectrochemical CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Designing Photoelectrochemical CO2 Reduction System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoelectrochemical Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design of Photoelectrode Materials and Type of PEC Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design of Photoelectrocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermodynamic and Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The modern technology and device-driven society needs a large uninterrupted source of energy and currently most energy production is fossil fuel dependent. This leads to excessive emission of CO2, causing serious concern all over the world by altering the carbon cycle and global warming. Large number of research works is currently underway by various research groups to mitigate this problem. One of these approaches is a light-driven reaction, a promising energy-efficient strategy to reduce the CO2 into valuable products. Both photoelectrochemical and photocatalytic techniques are explored to convert anthropogenic CO2 to valueadded products. Photoelectrochemical (PEC) techniques play a key role by utilizing abundant solar energy and using it to address an environmentally P. K. Singh · R. Kaushik · A. Halder (*) School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, HP, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_89

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challenging issue. However, most of these light-driven catalysts are suffering from selectivity toward targeted products. Therefore, targeting valuable products such as methanol, ethanol, formic acid, acetate, carbon monoxide, methane, ethane, etc. is still challenging. Conversion of CO2 to valuable products also depends on various parameters such as the advancement of the design of photoelectrode materials as a catalyst, thermodynamic and kinetic parameters, reactor design, selection of proper electrolyte, etc. This book chapter extensively focused on the technique as well as dependent and independent parameters of CO2 reduction. This also includes the discussion on the efficiency of the CO2 to fuel conversion, as well as solar to fuel conversion, and how that can be upgraded by playing with other factors of nanotechnology. Keywords

Photoelectrochemistry · CO2 reduction · Photocatalysis · Morphology · Heterojunction materials

Introduction The usage of nonrenewable fossil fuels for energy productions creates serious threats toward the production of greenhouse gases. The major contribution of CO2 emission is associated with the energy demand using nonrenewable resources. The major part of energy demand is accounted for by power generation and transportation; nevertheless, two-thirds of total emission has been recorded up to 2019, and the remaining emission is covered by the industrial and other sectors (Fig. 1). Greenhouse gases mainly consist of carbon dioxide, methane, nitrous oxide, and fluorinated gases.

Fig. 1 CO2 emissions from electricity and heat generation by energy sources. The figure is plotted according to the data provided by IEA (2021), Greenhouse Gas Emissions from Energy Data Explorer, IEA, Paris (left) and methods of CO2 reduction to value-added products (right)

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Among these anthropogenic gases, CO2 is the one of main cause of global warming. Starting from the industrial revolution, the level of CO2 in the atmosphere has been increased by 50%. In fact it is alarming to note that despite the COVID-19 pandemic, the global average of CO2 had reached to 412.5 parts per million with the record high in 2020 (https://www.climate.gov/news-features/understanding-climate/climatechange-atmospheric-carbon-dioxide). Although, CO2 can absorb less heat per molecule than methane or nitrous oxide, however being the most abundant it can stay longer in the atmosphere than other greenhouse gases. Thus, CO2 is mainly responsible for about two-third of the total energy imbalance which causes global warming. The effects of global warming and greenhouse gases simultaneously can be observed in the rise in the sea level, associated with rain fall change. The continuous increase in CO2 level and other associated changes in the atmosphere have resulted in the rise of the earth’s temperature. The average global temperature in 2021 was about 1.11 (
0.13)  C above the preindustrial (1850–1900) levels and the dataset of World Meteorological Organization shows that 2021 is the seventh consecutive year (2015–2021) where global temperature has been over 1  C above preindustrial levels (https://public.wmo.int/en/media/press-release/2021-one-of-seven-warmest-yearsrecord-wmo-consolidated-data-shows). The global warming has severe effect on the climate change and ultimately affecting the natural and managed ecosystems with loss or increase in growth, biomass, or diversity at the level of species populations and ecosystem. Another factor which also causes the matter of concern is the dissolution of ocean. It reacts with water molecules, producing carbonic acid and resulting the sea water to be acidic (low pH). It is interesting to note that industrial revolution also caused the ocean acidification by dropping pH of the sea water. Thus, it is very crucial and need of the hour to take steps to reduce the amount of CO2 in air. Carbon dioxide has many industrial usages and is the primary source of various valuable products. The most derived product from the CO2 is the fuel which includes methane, methanol, ethanol, gasoline, and aviation fuels. In addition to this, derived fuels from CO2 provide an imminent need to commercialize carbon-based products. The presented data by IEA represents that the derived liquid products render the alternative resources for the coming generation and facilitate slackening the environmental issue as well as effectuate the energy demand. The conversion of anthropogenic CO2 must be channelized into proper way to accelerate the utilization as well as reduction of CO2 to mitigate associated problem like global warming. Given figure tabulates the data based on global demand (IEA 2019).

Method for CO2 Reduction Multiple techniques have been adapted to convert CO2 into value-added products. In this regard, the major techniques developed are: (i) thermocatalytic, (ii) photothermal, (iii) photocatalytic, (iv) electrochemical, and (v) photoelectrochemical techniques. All these techniques have its own advantages as well as challenges. In the following sections, we will be seeing how effectively CO2 could be utilized into value-added products by using these techniques.

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Thermocatalytic CO2 Reduction In this process, CO2 molecules are activated and convert to value-added products by supplying thermal energy as energy source. Thermocatalytic transformation of CO2 predominately involve relatively low-temperature (523 K) hydrogenation reaction, where various hydrocarbons can produce from CO2 reduction. To scale up such technology, high temperatures (200–1000  C) and pressure are required with the heterogenous catalysis to perform the CO2 reduction reaction. It is highly recommendable for large-scale production in industries. In this process, H2O is employed as proton source for the hydrogenation of CO2 to fuel (Tackett et al. 2019). Baddour et al. illustrated the thermocatalytic CO2 hydrogenation by using solution-phase molybdenum carbide nanoparticles as a catalyst which substantially improved the selectivity toward the CO2 to C2 products (Baddour et al. 2020). Barroso-Bogeat et al. reported how nickel-loaded ceria nanocatalysts have the ability to thermocatalytically convert CO2 to methane with the help of H2 molecules. Nickel catalyst completely dispersed over the ceria could lead to the adsorption of CO2, in presence of H2 it resulted in the production of CH4 (Barroso-Bogeat et al. 2021). Guzmán et al. investigated temperature-dependent CO2 conversion over Cu/ZnO catalyst and found that specifically methanol is forming between 200  C and 300  C (Guzmán et al. 2021). Conclusively, thermocatalytic CO2 conversion largely depends on catalyst’s nature and working temperature. The dependency over hydrogen is one of the major drawbacks of the thermocatalytic conversion of CO2. In fact, the generation of hydrogen from coal-based technology and need of high temperature make this method nonsustainable in many ways. The other drawbacks of this method are in suitable reactor design and involvement of nonrenewable energy source like oil, gas, coal, etc., along with the additional cost of a heating source (Samanta and Srivastava 2020).

Photothermal CO2 Reduction Photothermal CO2 reduction incorporates the coupling of solar energy with external heat energy. The augmentation of the external heating source with the solar energy leads to the term “photothermal effect” for the conversion of CO2 into solar fuels. The simultaneous utilization of solar energy and a thermal energy could be done by using a setup termed as the solar-thermal reactor where principles of thermocatalysis and photocatalysis could be applied. The large thermal energy input is required for the breaking of stable C¼O bond and formation of a new bond. This high thermal energy can be provided by the solar energy harvested through solar thermal technologies that concentrate the solar rays by localized heating approach with the help of plasmonic nanomaterials (He and Janáky 2020). There are four kinds of photothermal processes are available: (1) thermal-assisted photocatalysis, (2) photo-assisted thermocatalysis, (3) photothermal cocatalysis, and (4) photodriven thermocatalysis. Thermal-assisted photocatalysis shows a remarkable photothermal effect is discovered in the optimized TiO2-graphene composite and

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demonstrated to have great impact on the photocatalytic CO2 conversion. Like earlier, this technique also relies on reactor design, catalyst’s nature and morphology, and working temperature. For example, Zhang et al. reported the microflower-like ruthenium nanoparticles supported on Mg (OH)2. They observed that the selectivity to CO and CH4 varies by varying light intensity. Improvements in the selectivity and efficiency are also associated with the strong absorption capability of catalyst and photothermal effect (F. Zhang et al. 2021a). The vertically arranged silicon nanowires are one of the promising substrates for photothermal CO2 reduction. The low bandgap of silicon (1.1 eV) has the ability to absorb over a broad spectral range. It was reported that silicon nanowires with ruthenium black support as composite materials could act as efficient light harvesters and could effectively convert CO2 to CH4. Zhao et al. synthesized FeO-CeO2 nanocomposite as an efficient and highly selective catalyst (with 99.87% selectivity) for the formation of CO by photothermal CO2 reduction (Zhao et al. 2020). Porous materials are also very efficient for this process as controlling thermal transport properties within the catalyst could be achievable due to their ability to reduce thermal conductivity both by decreasing the phonon mean free path. On the other hand, photothermal technique has limitations such as efficient photoreactor design, low solar energy utilization, and often it is challenging to deconvolute photothermal heating from photoinduced charge transfer effect.

Photocatalytic CO2 Reduction In this technique solar energy is used as renewable source of energy for the CO2 reduction. Thus, solar energy is transformed and stored in the form of chemical bonds when CO2 molecules convert to hydrocarbons such as formic acid, methanol, and ethanol-like value-added products. As solar energy is the only source of energy used in these types of reactions and thus photocatalytic CO2 reduction is a sustainable method (Alkhatib et al. 2020). For example, Debnath et al. studied the potassium-doped and low nitrogen–rich gC3N5 for CO2 to CH4 conversion under visible light illumination. It was observed that modified catalyst showed 100% selectivity for CH14 whereas pure gC3N4 showed only 21% (Debnath et al. 2022). Wang et al. used the butterfly/gC3N4/Au and found that the 10 AuCN-B photocatalyst exhibits the highest photocatalytic efficiency compared to gC3N4. The formation rate of CO and CH4 are 331.57 and 39.71 μmolg/h achieved which is 36 and 88 times higher than the pure gC3N4. The enhancement in the photocatalytic performance is attributed to the periodic arrangement of a hierarchical structure of the wing and noble metal/semiconductor junctions (Qingtong Wang et al. 2021a). Multijunction materials are found to be the most suitable one for the photocatalytic conversion as they have the capability to facilitate the interfacial electron-hole separation (thus reduction in charge-carrier recombination) and also helps in migration of charge. This also improves the separation efficiency, leading the catalytic conversion efficiency. Although, photocatalysis has shown promising performance in CO2 reduction, but still suffers from the drawback associated with poor light

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absorption, uncontrollable selectivity, instability, low yields, and photocorrosion (Kovačič et al. 2020).

Electrochemical CO2 Reduction In this technique electrical energy is used as the driving energy source for CO2 reduction. It is important to find out an alternative pathway – a pathway which will be useful to convert this anthropogenic CO2 into useful products. To reduce atmospheric CO2 into useful products has a broad area of research and electrochemical reduction is only one of them. Electrochemical reduction of carbon dioxide is a process by which electrochemically carbon dioxide can be reduced to many other chemical species. But the major challenges lie on the fact that electrochemical reduction of carbon dioxide results in multiple products with very high overpotential. An efficient catalyst should control the multielectron and proton transfer pathways for CO2 without compromising with overpotential and selectively reduce CO2 to a particular product. The main challenge for the advancement of electrochemical reduction of carbon dioxide is the relatively high overpotential. The limiting step in the CO2 reduction process is the formation of formate (CO2.-) radical anion intermediate. This anion radical intermediate requires a standard potential of 1.9 V versus SHE for formation and the main reason for the high overpotentials. Carbon dioxide is a linear molecule with a bond distance of 1.16 Å, and activation of CO2 involves the formation of bend CO2 associated with a decrease in CO bond order. The one electron transfer of CO2 for conversion of CO2 thus occurs at very negative potential due to the large amount of energy required for the structural rearrangement from linear CO2 to the bent CO2. This is one of the reasons why the electrochemical reduction of carbon dioxide involves a large overpotential. There are several approaches to reduce this high overpotential by stabilizing the CO2 by several means, e.g., using pyridine as a soluble electrolyte. In those cases, one electron electrocatalysts like pyridinium and its substituted derivatives help in the electrochemical reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. However, the presence of electrolyte as cocatalyst adds up the complexity of the system. The complexity of the system having catalysts, electrolyte (cocatalysts), and reaction intermediate together generates a transition state which is very hard and difficult to understand. Thus, the usage of ionic liquids, organic compounds, and cationic polymer might not be the best solution for longterm operations to convert CO2 to value-added feedstock via electrocatalysis (Garg et al. 2020). Sun et al. revealed the role of interfacial interaction among the metals and support by using the Au3+ and CeO2. Fabricated Au-CeO2 dramatically improved the selectivity and activity in the reduction of CO2 with the Faradaic efficiency of 95% under the potential of 0.7 to 1.0 V (Sun et al. 2022). Similarly, Feng et al. prepared Bi2O3/BiO2 nano-heterostructure and showed that the coupled heterojunction has about 95% selectivity of format formation in the potential range of 0.9 V to 1.3 V versus RHE (Feng et al. 2022). In addition to the excellent performance, electrochemical CO2 reduction still suffers from multiple obstacles

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such as poor selectivity, retard kinetic reaction, high overpotentials, etc. (P. Wang et al. 2018; Z. Wang et al. 2021c). One of the other approaches is the use of metal electrocatalysts for the efficient reduction of carbon dioxide. In solid oxide electrolytic cell, it is possible to convert carbon dioxide to carbon monoxide in the presence of platinum, palladium, and nickel catalysts. Due to higher operational temperature of 700–1000  C, the cost-effective conversion of carbon monoxide faces lots of challenges. Thus, the development of low-temperature conversion of carbon dioxide by electrochemical method has become a very popular research area.

Photoelectrochemical CO2 Reduction Among the several technologies for CO2 reduction (thermochemical processes [i.e., hydrogenation and reforming], mineralization, electrochemical reduction, and photo/photoelectrochemical reduction), photoelectrochemical processes have their own environmental and economic benefits. In brief, thermolysis process breaks CO2 at extremely high temperature, then reduced it by reacting with H2O molecules, thus suffers from the issue of high temperature, low carbonization rate, high cost, and energy requirements limiting the large-scale applicability of thermolysis process. However, electrochemical processes with the aid of electricity are capable to reduce CO2 at ambient condition to valuable products and draw glob attention because of its economic and environmental benefits (Pawar et al. 2019). The photoelectrochemical technology for CO2 reduction on the other hand provides augmented advantages of both electrochemical method as well as photocatalysis. This method provides an alternative route to store intermittent energy from renewable sources in the form of chemical conversion of anthropogenic CO2 gas into value-added products. Introduction of photons also reduces the loses in transportation of electrons between electrodes, thus increases the efficiency. The additional use of renewable energy can reduce the energy consumption and system capital cost in electrochemical processes if direct photons from the renewable energy sources (solar light) are illuminated over working electrode. Thus, combination of electrocatalysis with photons, “photoelectrochemical process,” has intensively attracted the researches in past few years (Kumaravel et al. 2020). There are many factors which can control the efficiency and performance of the photoelectrochemical CO2 reduction which are listed below.

Designing Photoelectrochemical CO2 Reduction System The photoelectrochemical CO2 reduction is much similar to natural photosynthesis where CO2 reduced to form carbohydrates with the help of solar energy through a series of enzymatic process. The difference majorly lies on the type of materials we are using along with the applied potential. In brief, the goal is to use excited electrons that are generated by excitation of light energy and use its redox abilities to produce desired chemicals. In the case of PEC, the endothermic CO2 conversion reaction is driven by combination of both electricity and light irradiation. However, in the CO2

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reduction case, PEC systems are in the initial phase and has a huge potential for future application. It should be keep in mind that basic working principle or concept of a PEC cell and a conventional electrochemical cell is comparable, except in PEC cell, intermittent source of light is used to partially fulfill the energy demand that is necessary to cause the redox reactions. The light is absorbed by a semiconductor electrode and utilized to excite electrons from its valance band to a higher energy level. These excited electrons can be used to carrying out reduction reactions and corresponding generated holes can used to perform oxidation reactions at a semiconductor/liquid interface (S. S. Liu et al. 2019). For real-life applicability, photoelectrode must satisfy several characteristics; efficiently absorb the photons from solar spectrum, semiconductors bandgap should lie in between 1.6 and 2.7 eV, the band positions must be in stagger manner with the redox potential of water electrolysis, and most important the photoelectrode material should have stability and be resistant to corrosion in the working electrolyte. In below sections we will discuss PEC cell more elaborately.

Photoelectrochemical Cell The concept of a photoelectrochemical cell is comparable to a conventional electrochemical cell, except that energy necessary to cause the redox reactions are partially provided by light. A semiconductor material absorbs the light at the working electrode, exciting electrons to a higher energy level that, together with the corresponding holes generated, are able of carrying out reduction and oxidation reactions at a semiconductor/liquid interface. In practice, several characteristics of the photoelectrodes must be satisfied simultaneously; for the efficient photon collection from the solar spectrum the bandgap should be 1.23 eV or typically at least 1.6–1.7 eV, the band edges must stagger with the water electrolysis redox potentials, and most important the photoelectrode material should have stability and be resistant to corrosion in the working electrolyte. Now coming back to the PEC cells, it comprises two electrodes connected through external wiring and working as anode and cathode to perform oxidation and reduction reactions. Both the electrodes are immersed in a suitable electrolyte to perform the required reactions. It is highly important to take special care when choosing electrode material and supporting electrolytes. The efficiency and the type of product formation largely depend upon the type of electrolyte used. A photoelectrochemical cell can be further divided into three types, viz. anode/photocathode or photoanode/cathode or photocathode/photoanode system depending on the photoactive nature of the electrode (Fig. 2). However, during laboratory testing, PEC cell consists of one more electrode to observe the exact reduction potential value at the working electrode during half-reaction in a cell. In general, p-type semiconducting materials are employed as photocathode and n-type semiconducting materials are employed as photoanode. In the following sections, laboratory-based PEC systems are described in much details.

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Fig. 2 Schematic diagrams of all the three kinds of PEC reactors for photoelectrochemical CO2 reduction. (a) Photoanode/Cathode-based system, (b) photoanode/cathode-based system, and (c) photoanode/photocathode-based system. (Adapted with permission from (Pawar et al. 2019). Copyright (2016) Royal society of Chemistry)

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Design of Photoelectrode Materials and Type of PEC Cell In the following sections, laboratory-based PEC systems are described in details. (a) Photoanode/Cathode-based system: The major PEC cell components are: working electrode, reference electrode, counter electrode, and supporting electrolyte. In this PEC system, the reference and working electrode should be kept near to each other or in the same anodic compartment whereas the cathodic compartment contains counter electrode. General examples of reference electrodes are Ag/AgCl and standard calomel electrode whereas counter electrodes are carbon rod and Pt metal electrodes. The material selection for photoanode is very crucial in this case, as semiconducting material should have high solar light absorption range, high-charge diffusion path length, and suitable band positions. As photoanode and cathode are connecting though external wire, external energy can be supplied by applying external basing with respect to the reference electrode. In brief, illumination generates electron-hole pair at photoanode. Anodic electrons move toward the cathode through the external wire, leaving holes behind at the anode itself. These accumulated electrons at cathode further participate in several reduction reactions like CO2 reduction and holes at the anode are utilized to oxidize water. Many n-type semiconducting materials like TiO2, BiVO4, ZnO, Fe2O3, and WO3 are reported in literature to enhance the PEC CORR performance when used as photons absorber. (b) Photoanode/Cathode-based system: Major PEC components are same as described above, but here photocathode is photoactive material and used as working electrode. Same like previous, Ag/AgCl and standard calomel electrodes can be used as reference electrodes while metal/metal alloy electrode can be used as counter electrode depending on the reaction condition. In this system, electron-hole pairs generate over photocathode after illumination to suitable solar light and these generated electrons participate in CORR and oxidation takes place through the anodic holes. In case of photocathode/anode system electron-hole pair generates at photocathode and incoming electrons from anode are collectively utilized for the reduction reaction. As previous case, holes that remain at anode were consumed by oxidizing water molecules. Generally, p-type materials are used as photocathode like CuO. But some n-type materials like TiO2 and WO3 are employed both as photoanode and photocathode equally. Modifications are generally employed to improve the activity of photoelectrodes by any means: by improving light absorption coefficient/range, by improving charge separation efficiency, and by improving active sites at the surface. (c) Photoanode-photocathode-based PEC system. In this system, both anode and cathode are made up of photoactive materials, i.e., semiconducting materials. In this case, both the electrode illuminates under solar light. Photoexcited electrons from anode and cathode at cathode are used in reduction reaction, at the same time holes generated at photoanode compartment are used in an oxidation reaction. Depending on the Fermi levels of anode and cathode, this system may work without any external biasing and should be considered an ideal

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system. However, the majority of anode-cathode combinations required external biasing to work (P. Wang et al. 2018). Although many factors play a crucial role in the selective reduction of CO2, valence band and conduction band positions along with the additional energy in form of external bias are most important. Different combinations of electrode materials significantly permute the reaction selectivity and product yield. As another important component, electrolyte controls the solubility of CO2 and proton diffusion from anode to cathode chamber, thus directly affecting the CORR efficiency for different fuel products.

Electrolyte In the CO2 reduction reaction, electrolyte plays a crucial role in order to provide the proton source for further reduction of CO2 into chemicals and fuels. A low-cost environmentally friendly catalyst is essential and advantageous in PEC CO2 reduction reaction. Parallelly, the pH, ionic strength, and concentration of electrolyte directly affect the long-term stability of photocathode. Thus, selection of electrolyte that will favor the higher hydrocarbon products formation and noncorrosive is crucial. Nature of electrolyte plays very crucial role for controlling the type of product formation. Electrolyte is majorly divided into three types as discussed below. (i) Aqueous electrolyte: Electrochemical CO2 reduction is usually performed in weakly acidic or alkaline CO2-saturated aqueous electrolytes. Sodium or potassium bicarbonate solution is usually used. The cationic species present in the electrolyte participate in the formation of double-layer structure over the electrode surface and can control the competitive hydrogen evolution reaction. Large cations favor the formation of HCOOH on a Hg electrode, C2H4 on a Cu electrode, and CO on an Ag electrode. Large cations are also good to suppress H2 evolution (den Bossche et al. 2021). Bicarbonate is mostly favored electrolyte in aqueous solution, which can improve the CO2 reduction reaction due to the formation of complex bicarbonate CO2, in which carbon is utilized during the CO2 reduction reaction. Bicarbonate concentration in aqueous solution is mostly preferred in the range of 0.1–0.5 M (Kumaravel et al. 2020). However, low solubility and limited diffusion rate in water make the use of aqueous electrolytes as the critical challenges in PEC CO2 reduction reaction. A recent study revealed that different halogen anions in electrolytes resulted in distinct product distributions for ECR on a Cu electrode. Addition of both Cl and Br increased CO selectivity. (ii) Nonaqueous electrolyte: In contrast to aqueous electrolyte, nonaqueous electrolyte or organic solvent is employed to increase the solubility of CO2 as well as suppressing the hydrogen evolution reaction and have positive impact for the enhancement in the overall performance of CO2 reduction reaction in PEC. Solubility of CO2 in DMF is 20 time larger than the water, whereas in DMSO and ACN nearly 4 times greater than the water. Its solubility in CH3OH and

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propylene carbonate is about 5 and 8 times higher (Lu et al. 2021). By manipulating the solubility of water in organic solvent leads to control in the selectivity in CO2 reduction reaction and improvement in Faradaic efficiency. However, this nonaqueous solvent suffers from expense, environmental, and safety concern and also results in light ohmic losses and reaction overpotential, limiting the use for CO2 reduction. (iii) Ionic liquid electrolyte (IL): Ionic liquid gains a lot of attention due to its ability to lower CO2 reduction overpotential by complexation CO2. – intermediate, and also having the advantage of inhibiting the hydrogen evolution reaction (HER), thus improving the selectivity for CO formation. Ionic liquid featuring the high solubility of CO2 has a strong adsorption capacity, promising conductivity, and capable to supress the HER and is favorable for producing the C1 products. Most common ionic liquids preferred are imidazolium cations based such as 1-ethyl-3-methylimidazolium cation, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), etc. which have been used in PEC CO2 reduction reaction. Nonetheless, expense of ionic liquid limits the use over the wide application (Lu et al. 2021). There are certain efforts going on to find the optimum ratio of ionic liquid and water as an electrolyte affecting the pH and viscosity of the electrolyte.

Design of Photoelectrocatalysts The CO2 molecule has linear structure in terms of geometry. Carbon atom is bonded with double oxygen (C¼O) atom, which makes it highly stable in terms of thermodynamics having bond energy 750 kJ/mol. On the other hand, the bond energies of C-H (441 kJ/mol), C-O (327 kJ/mol), and C-C (336 kJ/mol) bonds are much lower than the C¼O. Due to the high bond energy between C¼O, it is very difficult to break the bond and activate it to generate the value-added products by creating new C-H and C-C bonds. Thus, it is essential to think before and design the photoelectrodes in such way which can increase the efficiency of PEC cell at low overpotential with the selective production formation. Designing the photocathode materials for CO2 reduction is exploring a suitable platform that follows few basic criteria: (1) the maximum harvesting of solar irradiation, i.e., catalyst should absorb wide solar spectra, (2) should have high photostability and should not undergo photocorrosion, (3) capable to utilize efficiently the photogenerated charges, and (4) should have suitable band edge position. Additionally, it should work at low overpotential, i.e., with minimum external energy supply to drive the CO2 conversion reaction to achieve the respective products. There are many ways to design a suitable photoelectrode. (a) Use of plasmonic material: The light absorption process can be extended by introducing plasmonic material as the cocatalyst, coupling it with the semiconductor in a heterostructure, with controlled morphology.

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(i) Localized Surface Plasmon Resonances (LSPR) The introduction of plasmonic metal with semiconductors provides a promising route for the enhancement of the performance of photoelectrochemical CO2 reduction. Localized surface plasmon defines as the collective oscillation of free electrons in the plasmonic metal under the irradiation of light (Vu et al. 2020). LSPR phenomena are strongly dependent on the shape, size, and composition of the plasmonic NPs to tune the properties. When the plasmon metal is coupled with photoelectrode, plasmon can harmonize the effect by i) providing hot charge carriers, ii) enhancing the light absorption, and iii) plasmonic heating effects at the nanoscale. These effects improve visible light harvesting, accelerated surface reaction kinetics, and charge separation of the photoelectrode. These all-attractive properties are associated due to the LSPR phenomena. The main aim here is to utilize the hot electron generated by plasmonic excitation. Light excitation of the catalyst/adsorbate system may accelerate the movement of charge carriers and transfer them near the surfaces as well as control the rate of reaction by lowering kinetic barriers. This additional photoexcitation has profound effect on changing the reaction kinetic and on the type of product formation which may be significantly different from the product of the electrochemical reaction. Plasmon-based photoexcitation process differs from the interband excitation of metals as resonance is a multielectron excitation process with modulating electric field concentrated on the surface of the metal nanoparticles and generating hot electrons. These hot electrons can raise the Fermi energy and lowers the activation energy of an electron transfer process. For example, Yao et al. reported the Au-decorated ZrO2 catalyst for CO2 reduction where plasmonic Au could impart stable photoactivity leading to the reduction of CO2 into CO and CH4 as shown in Fig. 3. The improved quantum efficiency is observed due to the gold nanoparticles acting as a plasmonic catalyst in the ZrO2. (Gu et al. 2021). Kim et al. studied the CO2 hydrogenation over silica-supported metal catalysts like Ru, Rh, Pt, Ni, and Cu prepared by a wet impregnation method. The photochemically inert silica support

Fig. 3 (a) LSPR-induced photocatalytic CO2 reduction mechanism using Au/ZrO2-based systems and (b) calculated quantum efficiencies for all Au/ZrO2 catalysts. (Adapted with permission from ref. (Gu et al. 2021, Dalton Trans)

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Fig. 4 Light enhancement on metal catalysts. CO2 conversion for CO2 hydrogenation on a Ru, b Rh, and c Pt, Ni, and Cu catalysts deposited onto silica with and without light irradiation. 0.5 vol% CO2/N2 (50 sccm) and H2 (1.5 sccm) was fed into the reactor through a static mixer with or without light irradiation. The error bar indicates the deviation among three independent measurements. (Adapted under the terms of the Creative Commons CC by license from Kim et al. 2018)

had been coated with metal nanoparticles which can provide hot electrons to the system. Their research report shows a nice demonstration that how the nature of the deposited metal can direct that type of product obtained. Ru and Rh produced CH4 only, whereas Pt and Cu produced CO only. On the other hand, Ni could produce only 89% CO and 11% CH4. There is no change in product formation observed in case of Pt, Ni, and Cu. However, on light irradiation Ru shows a remarkable enhancement in performance at low temperature which was inactive in absence of light (Kim et al. 2018). In some cases, the use of the cocatalyst to facilitate the plasmonic interaction has been also taken to improve the physicochemical properties and photoactivity of the semiconducting materials (Fig. 4). For examples, Xiao et al. had used brookite TiO2 nanoparticles with Ag/MnOx as dual cocatalysts to boost photocatalytic performance of CO2 reduction reaction. In comparison to the single cocatalyst, the dual cocatalyst has significant effect on the CO2 reduction process. As per the mechanism to CO2 reduction, the photoexcited electrons can reduce the CO2 molecules to produce the desired products like CO/CH4 and water molecules capture the photoinduced holes to produce oxygen. These processes are strongly dependent upon the availability of active sites for adsorption/activation process on the surfaces

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of photocatalyst. In dual cocatalysts containing composites, the proton-transfercoupled photoreaction creates a remarkable difference in performance than singlecocatalyst-decorated composites. In this case, Ag/MnOX provides electron/hole sinks to promote the photoinduced charge separation and active sites for adsorption/activation behaviors of CO2/H2O molecules. Metal nanoparticles (Ag) can act as electron sink and give better charge separation, whereas oxide materials control the surface chemistry. Thus, overall photocatalytic performance gets enhanced as shown in Fig. 5. In this example, Ag and MnOX as a cocatalyst composite with Brookite TiO2 leads to the CO/CH4 yields of 31.70/129.98 μmol g1 with an overall photoactivity of 1103.28 μmol g1 h1, 11.98 times higher than that of the titania nanoparticles alone (Xiao et al. 2021). Duchene et al. constructed a Cu-based plasmonic nanoparticle with a p-NiO called Cu/p-NiO photocathode for the selectivity of CO2 reduction in aqueous electrolyte (Fig. 6). In the constructed photocathode, the Cu nanoparticle completely disperses on the p-type NiO surface with the mean diameter d of 8
2 nm which plays a significant role in optical properties and completely activates in the visible range displaying a broad peak of 600–800 nm, whereas bare NiO exists nearly transparent in the visible spectrum. Due to the introduction of Cu as a plasmonic nanoparticles, it can harvest the holes on the surface of NiO and allows to accumulate the hot electron on the metal surface. These hot electrons help for further reduction of CO2 to CO and HCOO reduction (DuChene et al. 2020). Landaeta et al. fabricated copper oxide coated with Ag nanodendrite to study the role of LSPR for the photoelectrochemical CO2 reduction at low overpotential (Fig. 7). Photoelectrode performance was studied and found that Ag/copper interaction dendrite

Fig. 5 Effects of (a) Ag and (b) Mn loading amounts on the photocatalytic CO2RR activity of the obtained BT nanoparticles. (c) Effect of Mn loading amount on the photocatalytic CO2RR activity of the 0.5Ag-BT composite. (d) Photostability for the photocatalytic CO2RR of the 0.5AgBT-0.5Mn composite and (e) transient photocurrent curves of the pristine BT and its typical composites. (Adapted with the permission Xiao et al. 2021, Langmuir)

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Fig. 6 (a) SEM image with corresponding size distribution histogram of Cu nanoparticles (mean diameter, d ¼ 8
2 nm) on a 60 nm thick p-NiO film supported on FTO glass. (b) Absorption spectra of the plasmonic Cu/p-NiO photocathode before (yellow curve) and after (red curve) electrochemical reduction via three successive cyclic voltammetry scans. The spectrum of the bare p-NiO film (blue curve) is also shown for comparison. (Adapted with the permission DuChene et al. 2020, Nano Lett. 2020)

Fig. 7 (a) Photocurrent transient at 0.2 V in a 0.1 M Na2SO4 solution saturated with CO2 for Ag and Ag/Cu2O/CuO electrodes with electrodeposition times from 1 to 5 min. (b) Schematic of Ag nanodendrites for CO2 acetate formation. (Adapted with the permission Landaeta et al. 2020, ACS Appl. Nano Mater)

reduces the CO2 to acetate at low overpotential with a Faradaic efficiency of 54%. The impressive performance was observed due to the Ag as a plasmonic nanoparticle with copper (Landaeta et al. 2020). Kamal et al. proposed the synergetic surface effect of Au plasmonic nanoparticle on TiO2 decorated with N-doped graphene heterostructure for CO2 to methane formation in Fig. 8. They have used N doping graphene oxide as N doping facilitates the formation of activated regions and promotes host guest reactive species due to the high charge and spin densities of N atoms. These N sites also provide adsorption centers to anchor and activate CO2 molecules. Prepared catalysts showed the improved charge mobility, and separation of photogenerated e/h+ thus minimizing

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the charge recombination rate. The catalyst showed the maximum CO2 uptake of 12.74 mg/g. LSPR of Au nanoparticle with TiO2 was responsible for excellent electronic transport and high CO2 conversion (Kamal et al. 2022). (b) Controlling the band position using heterojunction materials: Use of heterojunction materials have many advantages for the photochemical reduction of CO2. The use of graphene oxide with other oxide materials for preparing the heterojunction is a well-adapted method. Karim et al. made a composite of CuFe2O4 with graphene oxide to use a wide range of solar spectrum. The hybrid catalyst showed improved catalytic activity leading to the high methanol yield. Here graphene oxide helps to provide fast charge transport through the interfacial layers by inhibiting the charge recombination (e/h+ pairs) and ensuring the accessibility of free charge carriers to support the catalytic activity (Fig. 9) (Rezaul Karim et al. 2019). A proper semiconductor photocathode should give a negative enough conduction band (CB) edge superior than the reduction potential of CO2. However, the many

Fig. 8 (a) Steady-state PL spectra with an excitation wavelength of 300 nm. (b) Transient photocurrent responses measured under visible light illumination and (c) EIS spectra of NG, ANGT2, and AT2. (d) Solar fuel production rate for ANGT0, ANGT1, ANGT2, and AT. (Adapted with the permission Kamal et al. 2022, Applied Catalysis)

Fig. 9 (a) UV–vis spectra of CuFe2O4 and 5% GOCuFe2O4 hybrid catalyst. (b) PL emission spectra of CuFe2O4 and 5% GOCuFe2O4 at excitation wavelength 565 nm. (c) Products of GO and CuFe2O4 with differing wt % of GO in CuFe2O4 at 0.4 V in 0.1 M NaHCO3 solution. (Adapted with the permission, Karim et al. 2019, Ind. Eng. Chem. Res.)

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competitive reduction reaction takes place at this band position and hinders the selectivity toward CO2 reduction. One of the competitive at the photocathodes is the preferential H2 production. In these cases, designing a proper heterojunction material is a very good approach. For instance, Cai et al. fabricated the porous ZnO@ZnSe nanosheets array for photoelectrochemical reduction of CO2 (Cai et al. 2018). Zinc selenide (ZnSe) is a material with a direct band gap of 2.67 eV and the conduction band edge of ZnSe is negative enough to fulfill the driving force for CO2 reduction kinetics absorbance spectra that show the red shift after the introduction of ZnSe over ZnO. The addition of ZnSe in ZnO composite shows high CORR activity and selectivity for CO2 reduction against H2 evolution reaction at a negative voltage. The ZnO alone shows higher production of hydrogen and low Faradaic efficiency, however with ZnSe the Faradaic efficiency for photoelectrochemical CO2 reduction to CO reached 52.9% at 0.4 V (vs. RHE). (c) Morphology of photoelectrocatalyst: The morphological study was found to be a very appealing research route to modify the catalyst surface, active facets as well as to boost solar light absorption. The architecture of different morphologies can be modified at different length scales starting from the microscale to the nanoscale. The surface kinetic for the photoreduction of CO2 can be tuned by controlling the morphology of a nanostructure catalyst. This can be achieved by many ways such as exposing reactive facets, creating more corner sites and edge sites, changing the atomic coordination numbers, etc. Attaining these structures and morphology will remarkably boost the overall CO2 reduction performance. Here we cite an example where a composite material was made of a few layers of MoS2 on a hierarchical porous structure of mesoporous TiO2 and microporous 3D graphene aerogel. In this work, it was shown how a hierarchical structure contributes to the high photocatalytic catalyst performance by controlling the morphologies of the mesopores and macropores (Fig. 10) (Jung et al. 2018). Hailili et al. studied the morphological controlled CeTiO4 with molten salt for photocatalytic enhancement of CO2 reduction. The different morphologies of CeTiO4 in the form of nanorod, polyhedron, and cubic were prepared for test. Among all the morphologies CeTiO4 nanorod possesses the best photoactive catalyst for CO2 to CO and CH4 with quantum efficiencies of 0.36% and 0.065%, respectively (Hailili et al. 2018). Dong al. constructed a p-n heterojunction using CuS and GaN nanowire on Si as a photocathode for the CO2 reduction reaction. These photocathodes possess excellent chemical stability during the whole reaction in the electrolyte. It has been observed that the reaction selectivity of this bifunctional catalyst significantly improved due to the partial coverage of CuS on GaN nanowire. Photogenerated electron mostly populated the more active sites of CuO rather than GaN nanowire which was readily responsible for the formation of HCOOH with the Faradaic efficiency of 74.2% (Dong et al. 2021). Recently the growth of morphology-controlled synthesis of 2D heterostructures (WO3/Ag) nanocomposites was studied for enhanced PEC CO2 reduction reaction (Paul et al. 2020). The study shows that optimum loading of silver (1.5%) dispersed on WO3 NRs that led to the

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Fig. 10 Photocatalytic activities of the four different composites: mesoporous TiO2 and microporous 3D graphene aerogel (TGM), TiO2–3D graphene (TG), TiO2–MoS2 (TM), and bare TiO2. (a) PL spectroscopy. (b) Photocurrent density measured at 300 W Xe lamp illumination for 10 s and covered for 10 s, periodically. Each graph represents TGM, TG, TM, and TiO2 as a red, blue, green, and black line, respectively, and (c) CO formation rate as a function of MoS2:TiO2. Inset is TEM image of TGM with 2:1 ratio. (Adapted with the permission Jung et al. 2018, ACS Sustainable Chem. Eng.)

outstanding performance toward CO2 to formic acid formation. The morphology and the surface plasmon resonance are associated with Ag-NP result in the synergistic effect (both morphology as well as hot electrons generated through SPR) and responsible for such a profound activity. Higher loading Ag will lead to agglomeration. In another work, the assembly of Au-TiO2 over an InP nanopillar array are good enough to manipulate the intermediate formed in the reaction and hence can control PEC performance CO2 reduction reaction. The morphology of InP photocathode with Au-TiO2 is responsible for increased light absorption as well as prolonged charge carrier concentration and resulted in the improved selectivity of CO (G. Liu et al. 2021). Thus, optimum defect size and concentration at the interface are important factor for enhanced CO2 reduction. (d) Surface modification: The selectivity of products and the reaction pathway of CO2 are substantially influenced by the catalyst surface. The kinetic barrier can be drastically reduced by appropriate modification of the catalyst surface. The surface of the catalyst can be tuned by loading the proper catalyst, masking the surface of the catalyst, increasing the local pH, and introducing plasmonic metal nanoparticles (NPs) (e.g., Au, Ag, and Cu). By modifying the surface of the catalyst, surface properties can be tuned to improve the catalytic activity, separation of the charge carriers, stability, light absorption capacity, etc. Wu et al. proposed the various model of CO2 adsorption on the photocatalyst surface by DFT as shown in Fig. 11 to demonstrate the micromechanistic study of CO2 adsorption on TiO2 photocatalyst surface. They propose that (a) first a linear binding of CO2 molecule adsorbed (η1) on the surface via the Oa atom, or (b) CO2 molecule is absorbed via the C atom to generate a monodentate carbonate (η1) species. In the third, (c) a bidentate carbonate (η2) species is generated through the interaction of a CO2 molecule with the surface via both the Oa and C atoms. There is also (d) a fourth structure to generate a bridged

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Fig. 11 Adsorption configuration of CO2 on TiO2 surface: (a) linear chemisorption (η1); (b) monodentate carbonate (η1); (c) bidentate carbonate (η2); (d) bridged carbonate (μ3  η3)) Adapted with the permission F. Wu et al. 2022, ACS Omega

Fig. 12 (a) Partial current density for CO formation and (b) partial current density for HER. (Adapted with the permission Goyal et al. 2020, J. Am. Chem. Soc)

carbonate (μ3  η3) geometry with the C atom of CO2 pointing downward, forming a C  O bond. It was proposed that the two O atoms of CO2 bind with two metal atoms to form a Ti  O bond with the Ti atom on the surface (F. Wu et al. 2022). Goyal et al. performed the competitive reaction of CO2 reduction and HER evolution on the Au electrode surface using a rotating disk at a different rpm (Fig. 12). He found that the Au electrode improves the water reduction in a high alkaline pH resulting in the decrease in local alkalinity near the electrode surface which enhances the mass transport and suppressed the HER, due to water reduction. As the rpm of the rotating disk increases, CO2 to CO improves (Goyal et al. 2020). Y. Wu et al. examined the CO2 to CO conversion as a major product by varying the composition of the Sn-doped Cu to suppress the hydrogen evolution reaction. It was found that when the surface composition is with Sn 92% FeHCC >91% NiHCC under sunlight 85% NP removal in 4 h under UV irradiation

Reference Kuila et al. 2021 Zhao et al. 2019 Yu et al. 2019 Wang et al. 2021 Mengting et al. 2020 Xiao et al. 2012 Zhou et al. 2018 Cao et al. 2020 Li et al. 2019a Jaseela et al. 2020 Subagio et al. 2010 Liu et al. 2010 Li et al. 2018 Substances USAFT 1995 Rani et al. 2020 Wang et al. 2018 Rani et al. 2020 Dzinun et al., 2015

Nonyl phenol

90% under neutral conditions after 120 min UV illumination

Xin et al. 2014

14

Fe3O4@SiO2@TiO2/ rGO

2,4-DNP

15

ZnHCC, NiHCC, and FeHCC g-C3N4/TiO2

3-AP

ZnHCC, NiHCC, and FeHCC Polyvinylidene fluoride/titanium dioxide WO3/TiO2 nanotube

Phenol

16 17 18

19

Phenol

92% ZnHCC >91% FeHCC >90% NiHCC under sunlight 98.40% in 150 min under solar light

over cost; nonetheless, nanofiltration is frequently seen as a more acceptable membrane technology for treating phenols because to its smaller long-term over cost (Vieira et al. 2020). Electrocoagulation is a cutting-edge technique that

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integrates the functions of traditional coagulation, flotation, and electrochemistry in a single device. Electrocoagulation is a three-step process that involves (a) the formation of a coagulant by dissolving metal ions from the anode electrode, (b) the destabilization of pollutants and suspended particles, and (c) the de-emulsification and aggregation of insoluble phases, as well as the formation of flocs. Because of floc creation and floc removal through sedimentation, the electrocoagulation method minimizes pollutants and suspended particles (Prica et al., 2015).

Environmental Impact of Nanomaterials The discharge of industrial waste into the ecosystem contains various types of nanoparticles. The direct release of nanomaterials into the air, water, and soil results in widespread environment pollution. The health of aquatic life may be harmed as a result of wastewater effluents, either directly or indirectly. Nanomaterials dissolving properties released hazardous chemicals into the environment. However, nanoparticles can often conglomerate with other nanomaterials or be mixed with other organic or natural compounds, causing substantial interactions with biota and environmental toxicity. Heteroaggregation is a phenomenon that has been introduced into aquatic nanomaterials that collect in bottom sediments (Chen et al. 2018). By altering features such as suspension stabilization and steric repulsion between various metal nanoparticles and organisms, various organic materials were enhanced to reduce the toxicity of various nanomaterials. The hazardous nanoparticles adsorb on the surface of aquatic life’s cells, causing membrane disruption (Batley et al. 2013). Smaller quantities of nanomaterials disperse easily in the air over a large area, resulting in high human exposure and ecotoxicological consequences on living things. Swallowing or skin contact allows nanoparticles to reach the human body. When nanomaterials diffused throughout the ecosystem, they underwent several alterations or conversions. Several modifications in the toxicity of green nanomaterials were caused by specific features of nanomaterials, such as aggregation and dissolution properties alterations (Lowry et al. 2012). Nanoparticles that penetrate into soil from various fertilizers, sludges, and wastewater treatment products have a direct impact on soil quality. However, poisoning has a negative impact on plant development and fertility. Nanoparticles have been shown to have a negative impact on nitrogen and other biogeochemical cycles in several studies (Moghaddasi et al. 2017). In a recent study, in vitro produced iron nanoparticles with a size of 18 nm were utilized to investigate the toxicity level of zebra fish at various doses. The zebra fish embryo was found to have developmental problems and DNA damage. By increasing the amount of nanomaterial, the mortality rate rises with a slower rate of hatching. The larvae deformity and aquaculture were influenced by a higher amount of maghemite nanoparticle.

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Conclusion and Future Scope EDCs are a type of developing contaminant that can accumulate in aquatic environments since they are not naturally degradable and soluble in water. Because these pollutants have a significant impact on the body’s endocrine system, they are classified as harmful to living beings. Among the various types of EDCs, phenols and substituted phenols provide significant issues in wastewater cleanup because they are highly persistent in the environment and have a negative impact on human health. Phenols present in modest amounts (ngL
1) might increase their presence for extended periods of time. Because of their toxicity and the necessity for fresh water resources, their removal from wastewaters and water resources has become a hot topic in recent years. Following the photodegradation procedure, various metal oxides should be utilized to remove toxic phenols. We need to expand our research area in this framework because this phenomenon is cost-effective, efficient, and technically viable. Nanomaterials adsorb phenols as well as breakdown them into nontoxic by-products. Because it is low cost, readily available, nontoxic, and very efficient, nanomaterials are widely used for the removal of phenols. The majority of nanoadsorbents can destroy phenols and some can even degrade the pollutant completely in a short period of time. Nanomaterials-mediated research on a variety of classic and emerging phenols is required in order to ban or restrict their use. Finally, the discussion suggests that nanomaterials could be used as cheaper and more effective catalysts in the future. Biopolymer-based nanobiocomposites should be created for long-term development for the removal of organic pollutants. Finally, long-term research in water systems across the treatment pipeline is needed to better understand the interacting effects of phenols in combination with other selective pressures on native microorganisms, such as antibiotics and physiochemical water qualities. Acknowledgments One of the authors Dr. Manviri Rani is grateful for the funding from DST-SERB, New Delhi (Sanction order no. SRG/2019/000114) and TEQIP-III MNIT Jaipur, India. Dr. Uma Shanker wishes to thank TEQIP-III, NIT Jalandhar for partial funding. Ms. Keshu is thankful to the Ministry of Education, New Delhi, India for providing funding.

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Technologies for Treatment of Emerging Contaminants

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membrane Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Pretreatment Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conventional Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonconventional Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conventional Oxidation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advanced Oxidation Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hybrid Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membrane Filtrations and Membrane Bioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membrane Filtrations, Adsorbents, and the Process of Activated Sludge . . . . . . . . . . . . . . . . . Miscellaneous Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nanotechnology in the Treatment of Emerging Contaminants . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The issue of rising levels of “emerging contaminants” is causing great concern because these synthetic or naturally occurring chemicals are not usually monitored, but they enter the environment and have negative effects on both marine ecosystems and human health. Since ECs are persistent in the environment and have the ability to disrupt the physiology of target receptors, they are regarded as contaminants of emerging environmental concerns in recent years. The emerging contaminants mainly include pharmaceuticals, pesticides, personal care products (PCP), chemicals from industries, surfactants, and plasticizers which are found in B. Hazarika · M. Ahmaruzzaman (*) Department of Chemistry, National Institute of Technology, Silchar, Assam, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_114

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different water bodies and food sources. Various treatment processes, such as physical, biological, and chemical methods, have been developed to remove different ECs. Regardless of the absence of substantial studies, nanoscience and engineering appears to be an effective method, since nanostructured materials have demonstrated capability in eliminating various toxins from effluents. This chapter aims at presenting the different treatment technologies for the removal of emerging contaminants from wastewater. Furthermore, the majority of the proposed new treatment methods have yet to be assessed for large-scale feasibility. Keywords

Emerging contaminants · Adsorption · Bioremediation · Advanced oxidation process · Nanotechnology

Introduction One of the most valuable resources in the world is water. Globally, water is consumed twice the amount as it did a several decades earlier (Van Vliet et al. 2017). Water is increasingly becoming a limited resource due to its extensive usage in various fields like agriculture, household, industrial sectors, and also in transportation, as well as difficulties with climate change (Yap et al. 2019). As a consequence, development and research in water treatment technology is expanding to facilitate water reuse, with one of the key areas of focus being effluent water quality improvement. The primary factors of water quality are the quantity of substances and particles in it, such as heavy metals, nutrients, bacteria, and prioritized pollutants. Furthermore, emerging contaminants are a major class of organic pollutants which have recently captured the public’s attention and caused worry. They have been discovered to severely worsen the quality of the water, but also provide significant difficulties for the current water treatment processes in terms of how well they are removed (Rathi et al. 2021). Any man-made or natural substance or microorganism that is not properly managed in the ecology has the potential to have harmful effects on the environment and/or human health (Mofijur et al. 2021). Emerging contaminants include pharmaceuticals, personal care products (PCPs), pesticides, dioxins, hormones, polycyclic aromatic hydrocarbons, surfactants, nanomaterials, alkyl phenolic compounds, and perflourinated substances. Analgesics, antibiotics, lipid regulators, nonsteroid anti-inflammatory drugs (NSAIDs), diuretics, antiseptics, stimulant drugs, beta-blockers, cosmetics, antimicrobials, sun screen agents, food supplements, fragrances, and their metabolites and transformation products are examples of pharmaceutical organic contaminants and PCPs. They can have an impact on water quality, as well as drinking water sources, ecosystems, and human health (Naidu et al. 2016) (Fig. 1). Because of their hydrophobic qualities, ECs can bioaccumulate in lipid-rich tissues of organisms, causing damage to human and animal endocrine systems and spreading antimicrobial resistance (Rodriguez-Narvaez et al. 2017). Endocrine-disrupting chemicals (EDCs) have been linked to prostate, endometriosis,

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Fig. 1 Life cycle distribution of emerging contaminants (Rasheed et al. 2019)

testicular, and also breast cancers, as well as severe complications in human and animal reproductive health, in which they were seen to weaken aquatic wildlife’s immune systems, cause the creation of fragile embryos, and lower sperm counts in humans (Lauretta et al. 2019). PPCPs, which include analgesics, antibiotics, lipid regulators, stimulant drugs, nonsteroid anti-inflammatory drugs (NSAIDs), diuretics, sunscreen agents, antiseptics, beta-blockers, antimicrobials, cosmetics, fragrances, and food supplements and their metabolites, are associated to increased antimicrobial resistance and decreased reproductive health (Ahmed et al. 2017). Furthermore, surfactants can disrupt human endocrine function by altering hormone formulations and the stability of human growth. There are various treatment technologies for the elimination of these emerging contaminants which comprise of both conventional and advanced treatment approaches (Wen et al. 2021). Natural attenuation mechanisms, such as dilution, photolysis, sorption, biodegradation, and also volatization, are generally easy and cost-effective, yet they can be inefficient and ineffective (Barbosa et al. 2016). Despite this, adsorption using activated carbon and biochar has been the primary focus of EC removal technology study and development. In recent decades, conventional and innovative therapeutic procedures have attracted more attention

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(Chen et al. 2021). These procedures, however, require a lot of energy and are expensive to operate and maintain. Due to this, hybrid treatment methods have been investigated as alternatives to capitalize on the comparative benefits of various methodologies, and several combinations are very beneficial. Conventional techniques have also been found to be successful, including membrane filtration techniques, Fenton oxidation reactions, activated carbon-assisted adsorption, photocatalytic membrane processes, photocatalysis, ultrafiltration, and ozonation (Arzate et al. 2017). In certain nations, advanced treatment procedures are also presently researched for their ability to eliminate ECs. The process of advanced oxidation, which can eliminate significant amounts of EC from municipal wastewater, is one of the most prominent advanced treatment systems that has been intensively investigated (Norra and Radjenovic 2021). Various nanomaterials like metal oxide nanomaterials, carbon-based nanomaterials, and zero-valent nanomaterials are also effective for the removal of emerging contaminants (Zhang et al. 2016; Zhao et al. 2018). Thus, the purpose of this chapter is to investigate the most recent developments in the industry for the elimination of new contaminants. This chapter also provides an analysis of different conventional and advanced methods that are now available for the removal of pollutants.

Physical Treatments Physical wastewater treatment involves the elimination of emerging contaminants without modifying the biochemical characteristics of the pollutants, as these kind of treatment processes ignore the impact of any biological or chemical agents. Sedimentation, screening, skimming, heat treatment, membrane-based techniques, and adsorption are a few of the frequently used physical remediation technologies. The physical approach relies upon the mass transfer strategy (Samsami et al. 2020). The primary benefit of adopting physical treatments is that they are technologically basic and versatile since they require minimal equipment and can be adapted to a variety of therapy forms (Crini and Lichtfouse 2019).

Membrane Technology Membrane technology is a physical separation method which filters impurities across a membrane depending on size and qualities of the pollutants. The hydrostatic pressure around the membrane is the primary driving factor for filtration via the membrane (Ricceri et al. 2021). Based on the size of the holes in the membrane, this method of membrane technique or filtration might be of many varieties. Microfiltration (MF), for example, has pore sizes ranging from 0.001–0.1 m, ultrafiltration (UF) seems to have pore size below 0.001 m, nanofiltration (NF) has a pore size of 1–10 nm, and so on (Anis et al. 2019).

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Adsorption Adsorption is the process through which an ion or molecule in the bulk phase of a liquid or gas stays on the surface of a solid. Adsorbates are molecules or ions, whereas adsorbents are solids used for adsorption. Liquids are sometimes employed as adsorbents. Adsorption is a surface phenomenon where the surface of the adsorbent is concerned. One of the most efficient methods for reducing dangerous pollutants in effluents is adsorption, which is used by industry as a part of advanced wastewater treatment. Organic materials may be removed from waste effluents using the adsorption technique. Adsorbents made of activated carbons are frequently employed because of their great ability to remove organic contaminants. Activated carbons have significant adsorption capabilities because of their pore volume, large surface area, and porosity (Ahmaruzzaman 2008). The most extensively used adsorbent is activated carbon, which is made by carbonizing organic materials. Emerging toxins pose a growing threat to human health and the environment. Most ECs are made up of organic compounds, including medicines, fertilizers, personal care products like polymer and hormones, antiseptics, surfactants, wood preservatives, food additives, cleaning products, and fire retardants chemicals. Even at low concentrations, many emerging pollutants were harmful to humans and aquatic life, and there were no conventional standards to govern them. In the conventional sense, primary and secondary treatment plants cannot adequately remove or eliminate these dangerous poisons, necessitating tertiary treatment. Wastewater is extracted from ECs using composite adsorbents, nanoadsorbents, and other adsorbents. Acetaminophen, androstenedione, atrazine, bisphenol-A, caffeine, carbamazepine, dilantin, diclofenac, estrone, estriol, ethynylestradiol, fluoxetine, hydrocodone, ibuprofen, triclosan, naproxen, ketoprofen, salicylic acid, and others are some of the growing contaminants (Katsigiannis et al. 2015; Rossner et al. 2009). A dye is a chemical substance that may be used to provide color to a material or structure. Textiles, food, paper, rubber, tannery, clothing, cosmetics, printers, and the dyeing industry are just a few of the numerous sectors that routinely employ dyes. Dye discharge into the environment would result in an overly colored water body, reducing sunlight penetration and harming aquatic life. The dyes removal may be accomplished using various techniques, including chemical oxidation, coagulation, adsorption, membrane separation, electrochemical, and microbiological degradation; each has its drawbacks (Al-Ghouti et al. 2003). Adsorption provides the fewest disadvantages of all of these methods, as well as distinct advantages such as minimal adsorbent loss, the removal of all kinds of dyes, and relatively low cost using low-cost adsorbents. Heavy metal pollution is also one of the most pressing environmental issues in our day-to-day life. Water is polluted with hazardous metals from a variety of industries. As a result, they may pose a severe threat to human health and the health of the marine environment. Adsorption of heavy metals also depends upon the pH, temperature, adsorbent dosage, and other factors (Table 1).

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Table 1 List of emerging contaminants that can be removed by adsorption and their sources Types and sources Industrial wastes Pharmaceuticals Personal care industries Petroleum refineries Power plants

Agricultural wastes Pesticides Herbicides Hormones Waste from domestic use Sewage wastes Farm animal wastes

Examples Sulfamethazine, diclofenac, bisphenol A, metalaxyl, datrizoate, caffeine, atenolol, ibuprofen, metronidazole, clofibric acid, iopamidol, carbamazepine, and bentazon (Lin et al. 2015) Estrone, methyl paraben, propyl paraben, estradiol, testosterone, ethyl paraben and progesterone (Lima et al. 2019) Acetone, phenol, benzene, nitrogen, and petroleum products Particulate matter, furans, formaldehyde, oxides of nitrogen, carbon, sulfur, chromium, cadmium, lead, mercury, and copper (Vysokomornaya et al. 2015) Triclosan, atrazine, bisphenol A, carbendazim, estrone, chlorpyrifos, clomazone, ametryn, difenoconazole, diclofenac, testosterone, estriol, progesterone, and caffeine (Petrovi et al. 2003) Atrazine, bromacil, desisopropyl atrazine, dalapon, flumeturon, and diuron (Birch et al. 2015) 4-androstene-3,17-dione, estriol, progesterone, testosterone, 17,20dihydroxyprogesterone, 17-beta-estradiol (Blair et al. 2013) Paraxanthine, mercury, cadmium, zinc, arsenic, acetaminophen, caffeine, and lead (Wells et al. 2009) Dioxins, polychlorinated dibenzodioxins, biphenyls, polychlorinated dibenzofurans, lead, cadmium, and zinc (MacLachlan 2011)

Physical Pretreatment Processes Under this category, the methods included are sedimentation, skimming, and screening. Among these methods, the earliest one is screening, which refers to the elimination of solid objects that are bigger and can float, thereby clogging subsequent treatment procedure. Screening is required in treatment of effluents to assure the proper operation of various processes by preventing possible harm, obstruction, and process disruption. Screening is of two forms, depending upon the nature of impurities removed: coarse screening and fine screening. Coarse screening eliminates bigger particles, waste, and rags above 6 mm, while solid objects that are suspended and smaller than 6 mm are removed by fine screening (Bhargava 2016). The sedimentation tank and a grit chamber tank aid in the separation of floating solid materials. The grit chamber ensures that the bigger residual solid particles are sedimented such that they do not block the equipment. Grit chambers are of several varieties based on their features, size, and subject of concern. Horizontal flow, aeration chambers, and vortex flow are all types of grit chambers (Esfahani et al. 2018). These several grit chambers start sedimentation in various ways. Together with the grit chambers, sedimentation tanks are placed to ensure that the wastewater is thoroughly cleaned. These kinds of sedimentation tanks and biological processes are combined together in which heavier sludge containing biological entities are segregated out. Gravitational force is used in sedimentation tanks to settle heavier particles.

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Fig. 2 Removal of emerging contaminants by physical treatment method (Nathanson and Ambulkar 2021)

Sedimentation tanks are classified into different categories depending on the configuration: (i) inclined surface, where splitting of depth into shallow divisions takes place for rapid establishment; (ii) solid contact, where movement of liquid is in upward direction while the solid remains in sludge; (iii) and the horizontal flow, where the flow as well as the velocity is distributed uniformly (Pickering and Hiscott 2015). The velocity of the sewage and the period of settling are two elements that impact the rate of sedimentation in all of these distinct sedimentation techniques (Ramin et al. 2014) (Fig. 2). Skimming is another significant physical pretreatment method. It relates to the methods of removing oil, grease, fat, or oil-like compounds. A skimmer chamber in several large-scale waste water treatment plants (WWTPs) removes oil and oil-like substances floating above wastewater. Depending on the number and kind of probable oil-like impurities, skimmer types may vary (brush, wire, disc, and so on) (Mintenig et al. 2017).

Biological Treatments Biological treatment techniques have been widely used to remove ECs, primarily through the biodegradation process. One such mechanism is biodegradation where the ECs of high molecular weight undergo degradation into tiny molecules using

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microorganisms such as bacteria, algae, and fungus. Traditionally, biodegradation technologies have been utilized in wastewater treatment systems to remove ECs. They are classified as aerobic or anaerobic processes (Garcia-Rodríguez et al. 2014). The methods used to treat waste water may be generally divided into conventional as well as advanced methods, each of which is explained in the following section.

Conventional Treatments Conventional biological treatment refers to classic biological processes namely biofilm reactors, microorganism-based therapy, biofilters, bioremediation, aerobic or anaerobic treatment, and biological nitrification and denitrification.

Bioremediation Biological remediation is a type of traditional biological therapy that isolates ECs using organisms that are biological such as plants, fungus, and unicellular algal species. Because of its rapid breakdown and adsorption capacity, this approach is proven to be particularly efficient in case of emerging contaminants removal. Large levels of ECs in industrial effluent can be handled using appropriate bioremediation procedures. This is the most appropriate and cost-effective method of treating pharmaceutical industrial effluent. For example, bioremediation based on biosorption of various microorganisms, such as Chlorella vulgaris, is found to be a potential method of EC removal in wastewater from textiles. Plants, in addition to microbes, can be employed as an efficient way of bioremediation, more especially phytoremediation. Thelypteris palustris and Typha latifolia phytoremediation characteristics are recognized as effective because they may bioaccumulate heavy metals produced by animals. In many circumstances, these bioremediation technologies can provide additional benefits, motivating numerous stakeholders to embrace them: For example, the phytoremediation characteristics of flowering shrubs can increase soil health, plant development, and sustainable development; this also aids in the avoidance of soil contamination and mycoremediation in agricultural regions improves pesticide function. Bioremediation is frequently carried out in biofilters, oxidation ponds, and wetlands. This type of remediation includes a number of elements such as biotransformation, biodegradation, bioaccumulation, and several others. Thus, bioremediation is a potential method of biological wastewater treatment (Chandanshive et al. 2018). Activated Sludge Process Activated sludge process is one of the most widely used treatment process for most of the ECs. This can be defined as a method in which biomass is formed in sewage water in the presence of dissolved oxygen by microbe growth in aeration basins. Activated sludge process is mostly applicable for the removal of various personal care products (PCPs). Beta-blockers such as atenolol, metrolol, and metoprolol and polar herbicides like diuron, triclosan, and atrazine were observed to be poorly

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removed after the treatment of activated sludge that was caused by adsorption against suspended particles instead of biodegradation. Pharmaceutical ECs were removed by activated sludge systems in the following order: stimulant drugs > metabolites > analgesics > antibiotics > anti-inflammatory > lipid regulators > NSAIDs > other medicines (fluoxetine, iopromide, omeprazole, ranitidine, and tamoxifen). Further, to enhance the emerging contaminants removal percentage, the method of activated sludge can be combined to ozonation process (Buttiglieri and Knepper 2008).

Biological Nitrification and Denitrification The processes biological nitrification and denitrification are significant in the removal of ECs because they remove numerous forms of EC as a by-product. In this context, nitrification refers to the oxidative process of converting ammonia to nitrate or nitrite molecules, while denitrification refers to the reductive process of converting nitrite and nitrate to gaseous forms of nitrogen. Both these processes have different emerging contaminants extraction efficiency for every one of the contaminants where biological organism–induced denitrification and nitrification are frequently promoted as a wastewater management technique. Furthermore, the nitrification and denitrification processes substantially eliminate salinity features in various situations, indicating the prospective of both the mechanisms in extracting ionic emerging contaminants. Employing nitrification as well as denitrification in the specific context of wastewater is extremely appropriate to the removal of contaminants (Silva et al. 2018).

Nonconventional Treatment Technologies Nontraditional therapy employing natural techniques is indeed a developing topic of study that is always being investigated for future advancement. Some of these major treatment processes include biosorption, constructed wetland, and membrane bioreactor.

Membrane Bioreactor Because of the outstanding quality of effluent attained in relation to various emerging contaminants, membrane bioreactor is commonly regarded as a cutting-edge technology for the purpose of industrial and municipal wastewater treatment. This technique is capable of successfully removing a broad spectrum of contaminants, mainly those compounds that are opposed to activated sludge processing, and building wetland. It is made possible by the preservation of sludge on the surface of the membrane that can encourage bacterial decay and physical maintenance of all the molecules bigger than that of the membrane’s molecular mass. The factors affecting removal of contaminants in such systems include: (i) sludge aging and concentration, (ii) availability of hypoxia and anaerobic sections, (iii) wastewater production, and (iv) temperature, conductivity, and pH. Some PCPs like propyl parabene and salicylic acid can also be removed by MBR. In addition,

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pharmaceuticals like analgesics, antibiotics, stimulants, and anti-inflammatory drugs are removed by this technology. Thus, MBR process of removing contaminants is a very efficient mechanism because of its high EDC value (Luo et al. 2014).

Constructed Wetlands A constructed wetland can be defined as a method that is based on biological means, which is mainly planned and built to replicate the changes that occur in natural wetland in a better controlled setting. Constructed wetlands can be classified into horizontal flow, vertical flow, and subsurface/surface flow. Removing pesticides and beta-blockers like triclosan and mecoprop can be attained by the process of surface flow constructed wetlands. Also, all the constructed wetland methods have a good removal tendency for various pharmaceuticals and PCPs. Analgesics like ibuprofen, naproxen, and diclofenac are some of the examples which can be removed by constructed wetlands (Matamoros et al. 2008). Biosorption The process of immobilizing microorganisms on an adsorbent is referred to as biosorption, where microorganisms are used either in a harvested or cultivated form. Biosorption can extract a wide range of emerging contaminants from wastewater, and can remove lipids, heavier metals, and tiny ions as well as bigger macromolecules. During this method, immobilization of microorganisms against an adsorbent is carried out resulting in bio-oxidation and sorption. Pollutants now can gradually concentrate as well as attach to specific biomass with cellular structures. Biosorption contributes to a rise in the factor that binds the pollutants and microorganisms, resulting in removal of EC from wastewater owing to their tight connection. As a result, biosorption is recognized as a very diversified and significant mechanism for EC elimination (Daneshvar et al. 2018).

Chemical Treatments Chemical treatment is the practice of disinfecting sewage water by way of a sequence of chemical processes. The dissolved pollutants in wastewater are driven to separate in chemical wastewater treatment processes by adding particularly targeted chemicals. In some circumstances, biological and mechanical wastewater treatment procedures are insufficient to enable purified water to penetrate sources of water, and it is at this point that chemical wastewater management techniques are helpful. Techniques such as ion exchange, ultraviolet light, disinfecting by chlorination, precipitation, and neutralization can be categorized under chemical treatment processes (Collivignarelli et al. 2018). The method of chemical oxidation has evolved as a novel method among chemical treatment approaches, in which chemical oxidizing agents are employed to transform contaminants into a safe as well as a controlled form. Another chemical measure known as chemical oxygen demand is widely used to treat wastewater with a low COD level. During the process of chemical oxidation, oxidizing agents are

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used to remove impurities, and also contaminants are converted to harmless by-products (e.g., water and carbon dioxide). Conventional oxidation processes and advanced oxidation processes are two main classes of chemical oxidation treatment (Marcinowski et al. 2020).

Conventional Oxidation Processes Ozonation Method One of the most powerful oxidants that may be employed in a variety of reactions with both inorganic and organic compounds is ozone. For the past ten years, this molecule has been extremely essential in wastewater treatment since no toxic by-products are created in ozone interactions. Ozonation is a complicated oxidation method that involves the inclusion of ozone to wastewater and thus is notable for greatly enhancing its biodegradability. The elimination of pharmaceuticals and personal care items using ozonation is particularly effective since the majority of them may be removed. However, no one can disagree that because of its short life span, ozone is regarded as a hazard if the concentration rises over a particular point, which is around 23% (Shen et al. 2019). Photolysis Photolysis is a mechanism in which EC molecules decompose as a result of absorption of radiation or light. Despite the introduction of many light sources, UV disinfection of water remains a popular procedure. There are two forms of photolysis: direct photolysis, in which photons are directly absorbed and degrade contaminants, and the other is indirect photolysis, that requires photosensitizers like hydrogen peroxide or any other photosensitizer. Some medications, including diclofenac, iopami-dol, mefanamic acid, tetracycline, ketoprofen, and oxytetracycline, are fully eliminated by UV photolysis (Nguyen et al. 2013). Furthermore, gamma radiations are also effectively used in the treatment of ECs from wastewater. Carbazepine, ketoprofen, metoprolol, diclofenac, clofibric acid, mefenamic acid, and chloramphenicol are just a few of the contaminants that gamma radiation–based oxidation may effectively remove (Slegers and Tilquin 2006). Fenton Oxidation Processes Fenton can be defined as an oxidation process in which hydrogen peroxide reacts with iron to form hydroxyl radicals. Fenton reactions are a potential alternative for wastewater treatment as iron is plentiful and nonhazardous. The oxidation of H2O2 to OH radical increases its oxidation power, and the Fenton chemistry chain reactions are shown below as: Fe2þ þ H2 O2 ! Fe3þ þ OH • þ OH

ð1Þ

Fe3þ þ H2 O2 ! Fe2þ þ HO2 • þ Hþ

ð2Þ

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The regeneration of ferrous ion from Fe (III) can be done from the above reactions. However, the second reaction is substantially slower than the first. And that is why Fe (III) builds up in the solution and precipitation of Fe(OH)3 sludge is formed. As a result, removing Fe from solution might reduce process efficiency. For the treatment of emerging contaminants in wastewater, the concentrations of iron employed, which are typically added as ferrous sulfate, are typically in the 10–50 mg /L range. Since H2O2 is used in this reaction, the quantity of this reagent that must be added largely depends on the quantity of organic substance along with the severity of treatment employed. The Fenton method is also commonly employed in wastewater treatment to decompose and eliminate persistent organic contaminants. Furthermore, the Fenton approach has been regarded as an environmentally acceptable procedure for filtrating, dewatering, and treating wastewater (de Luna et al. 2012).

Advanced Oxidation Approaches Wastewater containing rigid organic compounds may now be treated with the AOP, which has developed as a hopeful technique (Ghime and Ghosh 2020). That is because this method uses hydroxyl reactive radical species. Each advanced oxidation method has a unique reactive radical source. The conventional or simple oxidation process does not always degrade organic molecules in wastewater. Therefore, it has developed a sophisticated oxidation strategy that uses very hydroxyl reactive species molecules like free radicals to perform many contemporaneous oxidation activities. It is common practice in wastewater treatment to employ advanced oxidation methods, including solar-driven processes, photocatalysis, and Fenton processes (Gutierrez-Mata et al. 2017).

Photo-Fenton Process The photo-Fenton method utilizes UV radiation and Fenton processes (BarreraSalgado et al. 2016). Hydroxyl radical formation limits the creation of secondary chlorinated compounds, which is why this water purification technique is considered so adequate. According to studies, this wastewater treatment approach is technically straightforward, cost-effective, and has high contaminants abatement effectiveness, especially for antibiotics. In contrast to the standard Fenton method, photo-Fenton requires less hydrogen peroxide because UV light accelerates the formation of hydroxyl radicals (Ahmed et al. 2020). Photo-Fenton studies are typically conducted in acidic or near-neutral conditions, which are ideal for aquatic solutions devoid of organic matter. In acidic solutions, Fe3+ forms hydroxyl complexes such as Fe (OH)2+ and Fe(OH)24+, which absorb UV/visible light and undergo photoreduction to produce OH• and Fe2+ as shown in Eq. 3: ½FeðOHÞ2þ þ hv ! Fe2þ þ OH •

ð3Þ

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Fig. 3 Mechanism of photocatalysis (Rauf and Ashraf 2009)

As a result, the entire mechanism improves as more OH• are produced and for the reaction with H2O2, Fe2+ can be recycled at a faster rate. Therefore, Fe2+ may react with hydrogen peroxide to form OH•, and thus the oxidized ligand can be used in novel processes for the degradation of micropollutants. As a result, the effluent must be acidified to achieve this value, and then neutralized before discharge (De la Cruz et al. 2012) (Fig. 3).

Photocatalysis The removal of pollutants in the presence of light is termed photocatalysis. It is an advanced oxidation process that utilizes catalysts to promote the energy transfer from photon to water molecule (Johnson et al. 2019). Compared to other photocatalysts, UV photocatalysis requires more than three times the energy to reduce pollutants to form total organic carbon (TOC). In the most sophisticated studies to date, photocatalysis employing TiO2 as a catalyst may remove contaminants and other microorganisms from sewage water in the most sophisticated studies. The primary benefits of this AOP include the ability to reuse the catalyst, lower costs, operation at ambient pressure and temperature, and the potential of using sunlight for catalysts. The mineralization of numerous compounds is also a result of this process. However, the photocatalysis technique has several noticeable drawbacks, such as uniformly generating radiation on the whole surface of catalyst and high separation cost after use (Cuerda-Correa et al. 2019). The mechanism of photocatalysis can be explained as given below: Direct Mechanism Visible light (wavelength more than 400 nm) is used for the photocatalytic degradation of dyes because several dyes can undergo adsorption in this particular range. In direct mechanism, excitation of the ground state of a dye takes place to the triplet

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state through a visible light source, mainly proton. The excited dye is then transformed into a cation that is semioxidized by injection of electron into the conduction band of any metal oxide nanoparticle. The electrons in the conduction band can react with the system’s dissolved oxygen resulting in the formation of anionic superoxide radical. This radical anion is then changed into a hydroxyl radical that takes part in the oxidation process. Indirect Mechanism (a) Generation of electron hole: Some dyes cannot attain the excited state when exposed to ultraviolet range (wavelength less than 400 nm); in such cases, the semiconductor initiates the process. The semiconductor has a larger bandgap and the ultraviolet light has an energy much higher than the bandgap. The valence band of the semiconductor is filled with electrons and hence transfer of these electrons takes place to the vacant conduction band, thereby generating a hole in the valance band. As a result, the process of excitation produces a hole and an electron pair. I. Water ionization: The holes generated at the valence band by the light source photon now reacts with water and forms a hydroxyl radical (OH) upon the surface of the semiconductor. It then combines with the organic compounds present close to the surface of the catalyst, thus resulting in mineralization to a particular extent based on their composition and level of stability. II. Sorption of oxygen ion: Here, superoxide radical anion (O2) is generated by the reaction of oxygen and the electron present in conduction band. This O2 ion undergoes oxidation and maintains the electron neutrality of the metal oxide nanoparticle by preventing the electron hole to recombine. (b) Superoxide protonation: Superoxide radicals may undergo protonation which further generate hydroperoxyl radical. The produced hydroperoxyl radical (HO2) is capable of producing hydrogen peroxide that dissociates further into extremely reactive OH radicals. Dyes react with these OH- radicals to produce carbondioxide and water (Rauf and Ashraf 2009). Dyes + hole pair (h+) ! oxidation product Dyes + electron pair (e) ! reduction product Dyes + OH ! CO2 + H2O

Ferrate Ferrate (FeO42) is an efficient oxidizing agent with strong disinfectant properties. It has the ability to produce a gel of the type Fe(OH)3 that eliminates other ions by precipitation. The strong oxidizing power of ferrate has piqued the interest of researchers over the last decade due to its biological, industrial, and environmental significance. It is also used to remove arsenic, contaminants like estrogens, medicines, and personal care products. The key mechanisms concerned in EC therapy are Fe6+ oxidation/disinfection and Fe3+ coagulation/flocculation. At ng/L concentrations, certain organic micropollutants can be oxidized by ferrate up to 90%. It is also

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found to be more effective in cleansing coliforms in wastewater treatment plants (Ghernaout and Naceur 2011).

Electro-Fenton Method The Electro-Fenton method was designed to compensate the shortcomings of the traditional Fenton method and also to improve pollutant removal effectiveness. H2O2 is electrochemically created in situ and regulated in this method. The reaction is equated in Eq. 4: O2 þ 2Hþ þ 2e ! H2 O2

ð4Þ

With the in situ formation of the Fenton, electro-Fenton mechanisms seems to be both environment conscious and effective. This avoids the price of chemicals and risks associated with their storage and transport, the sludge formation, and the side reactions caused because of small concentration levels of reagents maintained in the medium. Some pesticides, including atendol, meto-prolol, propranolol, triclosan, and triclocarban, as well as antibiotics, including sulfamthazine, cephalexin, sulfamethaxazole, acetaminophen, and tetracycline have improved removal abilities through this process. The solar photoelectroFenton technique is also used to remove beta-blockers like metoprolol tartrate, propranolol hydrochloride, and atendol (Ganzenko et al. 2014).

Hybrid Treatments The hybrid treatment of wastewater combines two or more EC removal treatment strategies. This approach is long lasting and reliable in terms of energy savings and treatment efficiencies. Some of the hybrid treatment approaches are discussed below.

Membrane Filtrations and Membrane Bioreactors Multiple methods are employed in this hybrid system to remove pollutants in a single phase. As an illustration, tiny to moderate natural organic materials are extracted by absorbents, big hydrophobic NOM are flocculated with coagulants, and sludge including chemicals and other compounds is segregated by the physical border created by the membrane. Membrane technologies, including different membrane filtering systems, have, on the other hand, appeared as a preferred alternative in order to recover water from multiple sources for wastewater. Methods of membrane filtration such as ultrafiltration, nanofiltration, reverse osmosis, and microfiltration need hydraulic pressure to achieve pollutant separation while treating wastewater, with pore size being another removal criterion. Because of the high efficiency of reverse osmosis, smaller particles such as bacteria and even some ions that are monovalent can also be separated (Jalilnejad et al. 2020).

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Membrane Filtrations, Adsorbents, and the Process of Activated Sludge By using hybrid biological absorbents, several hazardous metals from industrial wastes may be removed, including zinc. A more practical alternative has been devised by combining membrane filtration and the process of activated sludge due to the limited capacity for degradation of pollutants like anionic surfactants, pharmaceuticals, and persisting organic compounds. Some of these filtration technologies such as microfiltration, nanofiltration, and ultrafiltration and others are used in conjunction with the process of activated sludge for the treatment of wastewater or recover resources from it. Thus, as compared to typical activated sludge method, this combination of hybrid wastewater treatment approach offers benefits such as lower sludge generation and energy consumption, a smaller reactor capacity, and greater effluent quality (Jagaba et al. 2020).

Miscellaneous Systems Osmotic membrane bioreactors (OMBR), one of the sophisticated configuration technologies, were created to meet the precise requirements of the refining industries. The key advantages of this technique over a typical MBR are lower fouling propensity since there is no requirement of hydraulic pressure for water penetration, higher fouling reversibility, and superior water quality. Common additional processes such as reverse osmosis, membrane distillation, and electrodialysis are coupled with OMBR to create a combination of systems and produce safe water. Another unique hybrid system could be combination of microfiltration with osmotic membrane bioreactorsmembrane distillation (MD) approach to decrease wastewater salinity as well as restore phosphorus levels using a magnesium-based solution (Molinari et al. 2020).

Applications of Nanotechnology in the Treatment of Emerging Contaminants Nanotechnology garnered much attention in the field of adsorption because adsorption is highly reliant on the high surface area, and nanoparticles with high surface area show high adsorption efficiency. Nanomaterials are efficient adsorbents because of their changeable particle size. Metal oxide–based nanomaterials, carbon-based nanomaterials, chitosan-based nanoparticles, and silica-based nanoparticles are among the four categories of nanoadsorbents for the removal of organic contaminants (Nithya et al. 2018). Metal oxide’s exceptional quality and low cost in eliminating contaminants have drawn increasing attention. Manganese oxides, ferric oxides, titanium oxides, and aluminum oxides are examples of metal oxide nanomaterials. Several investigations have shown that metal oxides have the excellent ability as well as selectivity in eliminating contaminants like uranium, phosphate, organics, and arsenic by positive

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sorption. For example, titanium oxide has great photostability, excellent photocatalytic activity, and is also cost-effective. Titanium oxide nanoparticles have poor selectivity and may thus degrade a wide range of contaminants such as polycyclic aromatic hydrocarbon, organic chlorine, phenols, dyes, arsenic, pesticides, and heavy metals. Zinc oxide nanomaterial is efficient for wastewater treatment due to its good performance abilities, such as its powerful oxidizing capability, broad wavelength area, and great photocatalytic properties. They are ecologically beneficial since they are biocompatible, making them perfect for treating wastewater. More light energy can be captured by zinc oxide nanoparticles than by several other semiconductor metal oxides. On the other hand, iron oxide nanoparticles are identified as a possible contender for wastewater treatment. Iron oxides are rapidly being utilized to remove heavy metals from contaminated water due to their flexibility and availability. Heavy metals such as chromium, arsenic, selenium, lead, copper, and nickel have been adsorbing from natural water systems and synthetic solutions by maghemite and magnetite (Ahmed et al. 2011). Carbon nanostructures have distinct structural and electrical features that make them intriguing for simple and sophisticated applications, notably in sorption techniques. Their advantages include a high ability to adsorb a broad range of contaminants, fast kinetics, aromatic selectivity, and broad surface area (Nasrollahzadeh et al. 2021). Carbon nanostructures, such as carbon beads, carbon quantum dots, carbon fibers, graphite carbon nitrite, and biochar, are plentiful. Aside from their large surface area and variety of porous materials, as well as their excellent efficacy in adsorption of different pollutants such as ethylbenzene, dichlorobenzene, cadmium, lead ions, zinc and copper ions, and dyes, they also exhibit an amazing adsorption capacity (Saleem and Zaidi 2020).

Conclusions ECs are synthetic materials that are discharged into wastewater. ECs are found in trace amounts in water, yet their presence raises concerns for human and environmental health. The treatment of these ECs by a single approach is challenging. An integrated approach may look promising for effectively removing various ECs from the environment. Recently, various treatment technologies have been very effective in treating these ECs. An adsorbent-based technology for treating hazardous wastewater would surely be practical because of the low cost and accessibility of the adsorbent. Other technologies like chemical treatment, advanced oxidation processes, photodegradation, biosorption, nanotechnology, and the Fenton process are effective in treating ECs. However, no entire removal of emerging contaminants has so far been recorded. Therefore, the following future perspectives are recommended: • The detailed research on reaction kinetics, the impacts of functional factors, and reactor designs along with processes for emerging contaminants degradation should be done.

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• In order to choose the appropriate treatment strategy, there has to be proper examination of the treatment methods as well as their operational measures. Some of the factors that should be kept in mind while selecting the treatment methods are: adaptability, utility capacities, durability, environmental friendliness, quality of water resources, and the cost of maintenance of the selected method. • It is important to continue developing hybrid techniques that combine both biological and chemical methods. Some of these may include photo-Fenton accompanied by membrane bioreactors, ozonation in presence of H2O2 accompanied by membrane bioreactors, and UV photolysis accompanied by membrane bioreactors. • Finding “greener” approaches toward the development of ecologically safe and benign technologies for the synthesis of nanoparticle is much needed in order to minimize harmful environmental impacts. The synthesis of nanoparticles through biological procedures has improved so far, since they avoid highly energetic expenditures as they are less expensive, and can synthesize relatively tiny nanoparticles. These are also ecologically benign and so requirement of harmful chemicals is avoided. • Solar irradiation must be investigated as a potential substitute for AOP methodology in order to make major commercial operations cost-effective.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Assessment of Publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Z-Scheme Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type II Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-Scheme Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tandem Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Remediation of Organic Pollutants by Green and Sustainable Nanomaterials . . . . . . . . . . . . . . . . Conclusion and Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nanotechnology is an advanced method for the synthesis of nanoparticles having unique physicochemical properties. Adsorptive removal and photocatalytic degradation of organic pollutants have emerged as energy- and cost-effective technologies. Both have attracted considerable attention to treat the world’s wastewater treatment. As a class of recently developed hybrid, nanoparticles have shown huge potential and bright perspective in adsorptive removal and M. Rani (*) · S. Choudhary · J. Yadav Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India e-mail: [email protected] U. Shanker Department of Chemistry, Dr. B R Ambedkar National Institute of Technology, Jalandhar, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_112

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photocatalytic degradation of organic pollutant. This chapter summarizes the current research progress on adsorptive and photocatalytic removal of organic pollutant. Possible interactions between the pollutants and active sites on the nanoparticles are discussed to understand the adsorption/photocatalysis mechanism. The outlooks on adsorptive removal and photocatalytic degradation of pollutant using nanoparticles are given, albeit often with barriers as addressed. The integration of adsorption and photocatalysis using nanoparticles is discussed. Keywords

Nanoparticles · Organic pollutants · Water treatment · Photocatalysis

Introduction Water is the most precious source of life on earth. Approximately, 71% of earth is covered by water (oceans, rivers, ponds, glaciers, etc.) from which 97% of earth water is non potable or unhealthy. From the last decade, these water sources get polluted by various kinds of toxic and hazardous chemicals which has raised global concern. Emerging pollutants include pharmaceutical active compounds, pesticides, personal care products, and industrial effluents which are new kinds of compounds introduced in the environment (Rani et al. 2021a, b, c, d). A total of 3, 50,000 kinds of chemicals were used in parent or modified form for various purposes and approximately 1.8 billion kg waste release per day pollutes water resources (Wang et al. 2021). Trace concentration (ng/L to μg/L) of these effluents led toxic effect for both humans and aquatic organisms (Sanganyado et al. 2021). Availability of clean or treated water in developing countries is a major challenge stated by United Nation Sustainable Development Goals (UNSDG 2018). Therefore, numerous suitable, cost-effective, and eco-friendly methods have been developed for remediation of toxic and highly persisting emerging pollutants. It may include adsorption, advanced oxidation process, phytoremediation, biological treatment, coagulation, and membrane treatment (Adeola and Forbes 2019). Most of these techniques have high maintenance cost, toxic by-product formation, complex operation, and production of sludge (Iqbal et al. 2022). Nowadays, advanced oxidation process received great attention due to its novel characteristics, i.e., low cost, safe to use, and reliable (Rachna et al. 2020; Rani and Shanker 2020b, c; Rani et al. 2021b). This process involves semiconductor as a catalyst to accelerate the rate of photocatalytic degradation of emerging pollutants under ultraviolet or visible light irradiation (Shanker et al. 2017). Synthesis of semiconducting nanoparticles via green chemistry is easy to handle, and there is less production of secondary pollutants, more reusable materials, development of less hazardous pollutants, and prevention of pollution. Table 1 summarizes some novel nanoparticles fabricated by plant parts as extract. Furthermore, in this method volcanic ashes and sludge waste can easily be converted into valuable materials and used for elimination of pollutants from environment. The decision to explore carbon-based materials for water treatment

NiFe2O4/GCE (iron acetate, nickel acetate) Au NPs (HAuCl4)

Iron nanoparticles

CoFe@Zn-Ce MNC

TO-CoNPs (CoSO4)

Fe/Co bimetallic (FeSO4)

CoFe2O4 (Fe(NO3)3.9H2O),(Co(NO3) 2.6H2O)

2.

4.

5.

6.

7.

8.

3.

Nanoparticle AuNPs (HAuCl4)

S. No. 1.

EM, EDS, SEM, BET, FTIR, XRD, and XPS

Ginkgo biloba

UV-Vis, XRD, TEM, and FTIR

UV–Vis, SEM, TEM, and FTIR

Taraxacum officinale

Sesame seeds

FT-IR, FESEM, TEM, DRS, VSM, EDS, TGA, and XRD

FTIR, XRD, SEM, TEM, XPS, and EDS

FTIR, XRD, SEM, TEM, XPS, and EDS

XRD, TEM, and EDS

Characterization techniques TEM, FTIR, DLS, Zeta, and UV-Vis

C. microphylla

Jatropha leaf

Rosa canina

Ixora coccinea

Plant species Turkey Berry

Table 1 Green synthesis of nanoparticles using plant extract

Highly crystalline material, average size between 20 and 30 nm, and degradation of organic dyes (MB, RhB) Particle size 3 eV), concentration 50 mg L
1), catalyst dose 40 mg, pH 7, surface area (35.7 m2g
1), Langmuir adsorption, half-life (t1/2) of 3-AP (0.9–1.7 h), and EBT (0.6–0.8 h)

Rani and Shanker 2021

Ismail et al. 2018

Rajaboopathi and Thambidurai 2017 Kumar et al. 2019

Jassal et al. 2016

Reference Shanker et al. 2017a

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Fig. 1 The no. of publication and type of publication on adsorption and photocatalyst keywords resulted by scopus

was driven by the concept of sustainable chemistry, and the need to address waste management challenges; the conversion of “waste to wealth” is a cost-effective path to proper waste management. Therefore, green chemistry techniques can be employed as a viable and eco-friendly alternative for water purification. This chapter summarized the structural and morphological properties of nanoparticles synthesized by green approach, photocatalytic mechanism.

Assessment of Publication The literature scope of this review is based on SCOPUS and Web of Science published articles and books, while the search keywords are adsorbents and photocatalytic remediation Fig. 1. On the base of this, a search was performed in the Scopus database (the year 2016–2021) on the application of nanomaterials for the degradation of organic pollutants. However, maximum research articles are available in this direction, then 14.35% of review, 7.6% of book chapter, and 3.03% of short communication.

Adsorption Adsorption is an economically designed method which works effectively and a highquality purified product. Various kinds of adsorbents such as clays, activated carbon, astragalus, saw dust, biosorbents, carbon nanotubes, and zeolites are used to adsorb toxic organic compounds (Fig. 2).

Activated Carbon Long time ago, in 3750 BCE waste wood biochar was used by the Egyptians and Sumerians to manufacture bronze from different metal ores. During World War I, gas

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M. Rani et al. Activated Carbon Zeolite Others Alumina Clay Cellulose Manganese oxide

Fig. 2 Different types of adsorbents used to hybrid the properties of nanoparticles

masks fitted with canisters full of activated carbon were as protector from poisonous gases by German army. Activated carbon showed high adsorption capacity due to large surface area, high number of pores, and cost-effectiveness and showed electrostatic interaction between toxic pollutants, and up to 90% it may be constituted from carbon (Gopinath and Kadirvelu 2018). Many functional groups contain its carbon structure such as carbonyl, lactone, carboxyl, and quinone that are responsible for absorbing contaminants. In the activated carbon structure also present hydrogen, sulfur, oxygen, and nitrogen in the form of chemical atoms and functional groups. It can be synthesized by agricultural waste, rice husk, rice straw, oat hulls, corn stoves, oil palm fiber, and coconut shell. The preparation of activated carbon used raw material should be cheap, abundant, and safe (Jolly et al. 2006). Two basic steps were involved for their preparation: (i) carbonization step of carbanaceous raw materials that is usually conducted at temperature below 800  C without oxygen; and (ii) activation step of the char generated from the carbonization step. Mainly, temperature, particle size, and heating rate influence the rate or activity of activated carbon. Therefore, they have wide spectrum applications in waste water treatment, removal of toxic gases from environment, and behave as catalyst and catalyst carrier Table 2. Activated carbon modified with Fe-based MOF was synthesized for the reduction of Cr (VI). SEM, FT-IR, PXRD, and BET were used to identify the characteristics of adsorbent. Activated carbon was fabricated by cinnamon sticks which reduces the water solubility and increases adsorption capacity (100 mg g
1). Mechanism follows Langmuir adsorption model with first-order kinetics (Abuzalat et al. 2022). Anionic azo dyes can be removed by modification of activated carbon by cetyltrimethylammonium bromide (CTAB). CTAB is an cationic surfactant and increases surface charge of activated carbon. Activated carbon obtained from ashitaba biomass can significantly activate ZnCl2 and remove methylene blue dye. FT-IR, XRD, SEM, and EDS mapping were used to characterize the sample. Granulated active carbon modified with 10Gy

M-MD (coprecipitation)

MAF-SCMNPs (coprecipitation methods) Fe2O3 (wet impregnation method) ZnO-BC (chemical solution preparation) Nano-Fe/Cu–zeolite

Na2CO3

CuNPs- MMT

2.

3.

6.

7.

8.

5.

4.

Nanoparticle CFe and CarFe (sol-gel method)

S. No. 1.

Clay

Clay

Zeolite

TEM, XRD, and FTIR

SEM, EDS, and FTIR

XRD and SEM-EDS

XRD, SEM, and XPS

XRD, TEM, and UV-Vis

Zeolite

Biochar

XRD, TEM

XRF, XRD, SEM, EDX, FT-IR, VSM, and BET

Characterization SEM,TEM, XRD, XPS, and BET

Silica

Marble dust

Adsorbent Carbon (pineapple)

Table 2 Summarizes the type of adsorbents for catalytic applications

Concentration of MB was 160 mg g
1, MB removal efficiency up to 95.19% Batch sorption experiments, aximum adsorption capacity of nFe/Cu–Z (77.51 mg g
1) Concentration of MB is 10 mg L
1, dosage range of 0.25 to 1 g L
1, pseudo-secondorder kinetics, and removal efficiency of 99% of MB Diameters of 90% after 12 h

Degradation Pseudo-second-order model (R2 ¼ 0.99), 50 mg of CaFe and 80 mg of CFe, and 100% of the As(V) removal MB removal amount of 10 mgL
1, a pH ¼ 9, an adsorbent dosage of 1200 mg L
1, and 90 min, with the proposed model achieving 40.932 Maximum adsorptions 10–12, pH 5–7, maximum adsorption capacities 355–292, for Hg(II) and Pb(II) above 93% adsorption Particles in size of 30–40 nm, 90% of phenol removal

(continued)

Bagchi et al. 2013

Mundkur et al. 2022

Eljamal et al. 2019

Yu et al. 2021

Yang et al. 2019

Bao et al. 2017

Reference GutierrezMuñiz et al. 2013 Ahmed et al. 2022

31 Photocatalytic and Adsorptive Remediation of Hazardous Organic. . . 711

XRD, FT-IR, SEM, TEM, EDX, and BET XRD, FTIR, SEM, and UV-Vis

Clay

Graphite oxide

Graphene oxide

Graphite oxide

ZnO-TiO2

NH3GO (solvothermal)

Fe3O4/GO

GOαγ-Fe2O3Co3O4 (ultrasonication process)

12.

13.

14.

15.

Montmorillonite

TiO2

11.

SEM, EDS, FTIR, and XRD

XRD, WDXRF, UV–Vis, XPS, N2 adsorption, SEM, and TEM

FTIR, TG–DTA, BET, XRD, and SEM–EDX

UV-DRS, FT-IR, XPS, and PL

Montmorillonite

TiO2 NPs

10.

Characterization TGA, DTA, SEM, and EDX

Adsorbent Kaolin clay

Nanoparticle TiO2NPs

S. No. 9.

Table 2 (continued) Degradation Particle size of 10 nm, smooth and homogeneous inner surface, pH ¼ 9 and dye concentration of 150 ppm, and alizarin red dye as color 99% removal Bandgap 2.79 eV, particle size (11 nm), and 0.15 g of photocatalyst was added to 100 mL of Rh-B solution (10 mg L
1) Anatase crystallite size of about 15–20 nm, cation exchange capacity of 96.5 mmol 100 g
1, and 0.08 g of the photocatalyst and 500 mL of dye solution at 10
4 mol L
1 were stirred under irradiation for 6 h, 90–30% organic dye removal Average size of 25–30 nm, e decay upon solar irradiation with the catalysts tested: [Ant]0 ¼ 5 mg L
1; [cat] ¼ 250 mg L
1; W ¼ 450 W m
2; T ¼ 38  1  C, antipyrine 90% removal Co ¼ 20 mg L
1, V ¼ 100 mL, mBB41 ¼ 10 mg, mMO 30 mg, m42 NP ¼ 30 mg, k2 value of 0.002, R value (0.985), and removal of organic pollutants Freundlich isotherm model (R2 ¼ 0.99) and pseudo second-order kinetics model, enhanced % adsorption capacity (>95.7%) Pseudo-second-order model, maximum adsorption capacity of 28.94 mg g
1, 80% caffeine removal

Andrade et al. 2022

Khan et al. 2022

Verma et al. 2022

Belver et al. 2017

Djellabi et al. 2014

Tao et al. 2021

Reference Oun et al. 2017

712 M. Rani et al.

Polymer

Biochar

ZnO@UF and ZnS@UF

CaWO4-BC (coprecipitation method) CSWP (one-step pyrolysis process)

18.

19.

Biochar

Biochar

P-TiO2/ BC (Pyrolytic method)

17.

20.

Carbon

MWCNT

16.

SEM, FTIR, and XPS

XRD, EDX, and SEM

FESEM, EDS, FTIR and XRD, BET, and PL

XRD, FTIR, XPS, SEM, UV–Vis DRS, and N2 adsorption–desorption

SEM, FTIR, and BET

Pseudo-first order, R square 0.9993, concentration of 4 ppm, adsorption capacity of MWCNT obtained as 1.7446 mgg
1, and removal of ciprofloxacin hydrochloride of more than 88% 100 mL of dye solution, biochar (60 mg), Langmuir, and Freundlich and Redlich– Peterson models Dose (25 mg), concentration (2 mg L
1) and under neutral pH, first-order kinetics, and chlorpyrifos (89–92%) Porous structure with a 2.64 cm3, surface area 139.02 m2/g, and removal capacity improved by 5% and 17% for RhB and MO 50 mg CSWP with 10 mL atrazine solution (2 mg L
1), 96% atrazine degradation Suo et al. 2019

Zhang et al. 2019

Yadav et al. 2022

Song et al. 2022

Avcı et al. 2020

31 Photocatalytic and Adsorptive Remediation of Hazardous Organic. . . 713

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dose gamma irradiation was used to increase the ability of air purification. FT-IR, Raman, SEM, XRD, EDX, and BET were used to characterize the sample and further used for elimination of chlorpyrifos pesticide.

Biosorbents Biosorption is an alternative technology for the degradation of a board spectrum of pollutants from the waste water. The biosorption technology has emerged worldwide and is extensively used as it is very economical in the processing of biosorbents (Gupta et al. 2015). Biosorbents are collected from food waste or agricultural waste which mainly contains three main components, i.e., hemicelluloses, cellulose, and lignin (Ince and Ince 2017) which provide the stability of nanocompounds. These biosorbents are eco-friendly, inexpensive, high efficiency, and reusable, and have high surface area and easy separation. Biosorbents are derived from biomass material and combined with metal or metal oxides nanoparticles by enhancing magnetic properties. In developing countries, the application of biosorption for the treatment of waste water is attracted for three reasons: (i) Large amounts of biomaterials are used as a biosorption; (ii) compared to other advanced methods, biosorption is relatively cheap and eco-friendly; and (iii) there is a lack of advanced waste water treatment system. Biochar is one of the most significant biosorbents which is influenced by pyrolysis temperature, duration, and kind of plant residue. Calcium alginate nanoparticles synthesized by honey which acts as biosorbent reduced 94% Cr (VI) under acidic conditions within 180 min (Geetha et al. 2016). Carbon-modified sugarcane bagasse biosorbent is used for copper reduction. It has polymeric structure, hemicelluloses, rich in cellulose and lignin cause reduction and stabilization (Carvalho et al. 2021). Several studies have investigated the use of biosorbents for the removal of heavy metals and dyes (Chaukura et al. 2017); there are relatively few studies on biosorption of organics such as emerging contaminants like PPCPs.

Carbon Nanotubes Carbon nanotubes are synthesized by advanced technology that provides unique morphology, MWCNT’s adsorption, and have superior adsorbing capability and effective desorption than activated carbon (Ince and Ince 2017). Carbon nanotubes loaded N, S co doped TiO2 ternary heterojunction generated Schottky barrier and increases the recombination rate. The incorporation of CNT provides a high number of adsorption sites, facilitating the effective separation of photo-generated carriers (Shabir et al. 2022). CNTs act as a barrier to trap the electron thus hinder the recombination of electron hole pairs (Shabir et al. 2022). Musawi et al. synthesized MWCNTs/ CoFe2O4 nanoparticles (Solvothermal method) for the degradation of acid blue 113 dye. Optimized conditions were 0.4 g L
1 catalyst dose, acidic pH, 25 mg L
1 pollutant concentration, and 40-min reaction time with 36-W intensity (Musawi et al. 2022). Nowadays, CNTs draw attention due to the abovementioned

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beneficial properties, which can be exploited for many different fields of environmental analysis. Due to repaid increasing amount of organic pollutants such as pesticides, dyes, heavy metals, polychlorinated biphenyls [PCBs], polycyclic aromatic hydrocarbons [PAHs], and new emerging in the environment, it is very important to develop new, rapid, and low-cost techniques for sample preparation prior to analysis.

Clay Clay minerals are naturally occurring materials and have very attractive properties which widen their application area. Clay have unique composition and structural features. Clay showed high cation exchange capacity, large surface area, having layered structure, chemical and mechanical stability, and sorption capacity. Turan and Ozgonenel (2013) illustrated that Montmorillonite acts as potential ionic exchanger for heavy metals because of its easy manipulation, high abundance, eco-friendly behavior, and harmlessness to the environment (Ince and Ince 2017). It is significantly used for environmental remediation, catalysis, storage, sensing devices, and energy production. ZnO clay minerals synthesized via ultrasonic approach used for the degradation of ciprofloxacin under visible irradiation. It provides a suitable support for dispersion of high photocatalytic activity Zn/Cu metal ion-modified natural polygorskite clay- TiO2 for indoor and outdoor air purification (Marvikos et al. 2022).

Zeolites Metal oxide (TiO2, ZnO, Bi2O3, WO3, and SnO2) nanoparticles have aggregating and high recombination behavior of charge carriers. This limitation was removed by paring of nanoparticles with various biogolical stable adsorbents. Zeolites are nontoxic, cheap, and chemically and thermally stable. Zeolites acted as ion exchange materials due to their structural characteristics and valuable properties. Easily, ion exchanges with metal cations; large specific surface area makes them a suitable candidate for adsorption (Ince and Ince 2017). Haounati et al. (2022) synthesized Zeolite@Ag2O nanocompounds. Individual Ag2O molecules have low stability in visible light and high recombination rate of charge carriers. Aggregation of silver nanoparticles can reduce their surface area which slows down the photocatalytic rate. In other study, ZnO functionalized fly ash-based zeolite via hydrothermal method for the degradation of ciprofloxacin antibiotic (Amariei et al. 2022). Ag ion exchanged zeolite/TiO2 nanocomposite that can significantly be used for antibacterial purpose and photocatalytic removal of antibiotics. XRD, SEM, EDS, and FT-IR characterization were used to define the characteristic of nanocompounds (Torkian et al. 2022). V2O5/WO3 decorated over zeolite is used for the degradation of bisphenol A. Complete degradation of bisphenol was observed within 50 min under acidic conditions, 0.75 g L
1 catalyst dose and 100 mg L
1 pollutant concentration ZnO-zeolite imidazole hybrid nanoparticles were used for the degradation of methylene blue.

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Photocatalytic Remediation In recent years, the rapid development of the society has not only brought people a comfortable life, but has also produced various environmental problems. Energy crisis and environmental pollution have become the two major themes in the field of science and technology. Therefore, it is urgent to solve the current environmental pollution problem and develop new energy sources. The development and utilization of the new technology has become the top priority of current research. Among various options, photocatalysis has been favored by people because of its high efficiency, low energy consumption, simple operation, and no secondary pollution. It can directly use solar energy to generate new energy sources, such as hydrogen energy and hydroxide compounds, and can also degrade or convert pollutants and fix nitrogen. Of course, it has become the most effective new method to solve the current increasingly serious environmental and energy problems, and it will have good application prospects in the near future. Different studies were summarized in Table 3. Since Fujishima and Honda discovered that TiO2 photoelectric phenomenon splits water to produce hydrogen in 1972, photocatalysis has begun to attract people’s attention. The photocatalytic process is mainly composed of three parts, namely, photoexcitation, charge migration, and redox reaction. First, under the light irradiation, when the light energy is greater than or equal to the band gap of semiconductor, the electrons in the valence band absorb energy and transition to the conduction band, forming photogenerated electrons (e
) on the conduction band. At this time, the holes (h+) in the valence band are formed, which form photo-generated electron-hole pairs. Then, the electrons and holes migrate to the surface of the semiconductor oxide. Finally, the electrons and holes complete the redox reaction with the absorbed electron acceptor and electron donor, respectively, and finally complete the photocatalytic process. But in the second step, photo-generated electrons and holes are prone to recombine, which limits the efficiency of photocatalysis. Therefore, the hot topic in the current research on the field of photocatalysis is how to suppress the recombination process. In addition, the low utilization of solar energy by a single catalyst (about 5%) and the low oxidation-reduction capacity of a single photocatalyst result in low overall photocatalytic efficiency. It is also a great challenge. In recent years, in order to solve the above problems, people have made great effort, such as doping (B, S, N, etc.), combining metals (Pt, Ag, Au, etc.), and forming heterojunctions. A large amount of data show that the design of heterostructures is a new and most effective strategy to inhibit photogenerated charge carrier recombination and promote efficient charge separation in photocatalysis.

Z-Scheme Heterojunction Photocatalysts Z-scheme heterojunction has similar band arrangement as Type-II heterojunction, but the electron transfer path is different. The electron migration path between

RhB

RB

SnO/ZnO

Fe2O3–Kaolin

GNPs/ZrV2O7 and ZrV2O7

CdS/BiOBr/ Bi2O2CO3

ZnO@UF and ZnS@UF

Zn(cur)O

6.

7.

8.

9.

10.

Chrysene

FTIR, SEM, XRD, TGA, UV, and PL

XRD, SEM, TEM, XPS, Band gap, and PL

XRD, FE-SEM, UV–vis, TEM, FT-IR, and PL

Atrazine

Chlorpyrifos

XRD, TEM, FE-SEM, TGA, PL and XPS, and Raman spectroscopy

XRD, TEM, SEM XPS, and DRS

XRD, TEM, SEM XPS, and DRS

PXRD, FE-SEM, and FTIR

Chlorpyrifos

EBT

RhB

5.

4.

3. XRD, DRS, FTIR, and UV

XRD, SEM, and FTIR

RhB

Ag/FeWO4 /g-C3N4 Ag-doped CdS-WO2 ZnO-CuHCF

2.

Characterization FTIR, SEM, and EDS

Nanoparticle PANI-Ag/ZnS

S. No. 1.

Organic pollutants Methylene blue Degradation Catalysts dose: 30 mgL
1; pH: 7; oxidant dose: 3 mM; time: 60 min; under UV-254 nm; and 95% degradation Catalyst dose: 50 mg/100 ml; dose: 9 mM; pH: 8; and irradiation time:120 min: RhB conc.: 50 ppm Hexagonal structure; degradation with 82.5, 93.2, 95.0, and 96.5% 90 min irradiation Size: 50–100 nm; surface area: 95 m2 g
1; zeta potential:
40.4 mV; concentration: 25 mg L
1; dose: 15 mg; and pH: 7 Diameter: 2.0–2.2 μm; flower-like structure; Conc.: 5 ppm: catalyst: 150 mg; and 95% degradation Concentration: 15 mg L
1; catalyst: 1 g L
1; pH: 7; and 98% degradation At neutral pH; flow rate 5 mL/min; reaction time 90 min; under visible light source; follows Hinshelwood model; photo-degradation; and 97% and 85% Catalyst: 20 mg; pH: 5.2; visible light source; pseudo-first kinetics order; kapp: 0.122 min
1; and 95% degradation Particle size: 50 nm; concentration: 2 mg L
1; dose: 25 mg; and first-order kinetics, 89–92%: degradation Hexagonal structure; size:10–20 nm; and 100% removal in 2.2 h

Table 3 Summarizes the degradation of organic pollutants by photocatalysis mechanism

Photocatalytic and Adsorptive Remediation of Hazardous Organic. . . (continued)

Moussawi et al. 2016

Yadav et al. 2022

Bo et al. 2020

Chen et al. 2020 Zhou et al. 2010 Jacob et al. 2020

Saher et al. 2021 Roshini et al. 2022 Rachna et al. 2020

Reference Mazhar et al. 2022

31 717

Nanoparticle ZnO@FeHCF

Zn-TiO2

CuO/Ag

CNT-COOH/ MnO2/Fe3O4

MnO2

EGnZVI

Ni doped ZnO

Gd-doped ZnS QDs/g-C3N4

Ag-AgCl

ZnVFeO4

S. No. 11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

Table 3 (continued)

MG dye

MB dye

Azo dye and bis-phenol

MB dye

Azo dye

Parathion

Methyl orange Drugs

Phenol

Organic pollutants Phenol

XRD, FTIR, SEM, DRS, and PL

XRD, TEM, SEM, and FTIR,

XRD, SEM, EDAX, dot-mapping, TEM, BET, PL, EIS, and DRS

XRD, SEM, TEM, and EDS

XRD, FTIR, Raman spectroscopy, SEM, and TEM/EDS

XRD, XPS, BET, SEM, and TEM

XRD, SEM, TEM, XPS, and BET

XRD, SEM, FTIR, and TEM

XRD, SEM, TEM, FTIR, and BET

Characterization XRD,SEM, TEM, FTIR, BET, and TEM

Degradation Zeta potential:
29.8 mV; surface area: 80 m2 g
1; catalytic dosage: 5–25 mg; pH: 3–11; and time: 0–24 h Flow rate: 1.0 mL/min; volume: 20 μL; concentration: 10 mg/L; pH: 3.5; and 70% degradation Monoclinic structure; size: 250 nm; bandgap~1.5 eV; and 100% photocatalyst Surface area: 114.2 m2 g
1, concentration: 40 mg L
1; pH: 2; dose:1 g L
1, temperature:20  C; and time: 20 min Surface area:184.32 m2 g
1; thicknesses: 2 nm; and 90% degradation Particle size: 50–500 nm; temperature: 15–35  C; concentration: 10–250 mg L
1; pH: 2.5–8.5; dose: 0.5–5 mmol L
1 and EGnZVI: 0.2–10 mg L
1; and UV-light Crystallite size: 36–60 nm; lattice spacing 2.6005 Å; and degradation: 94% Experimental condition: AR14 0 ¼ 20 mg/L; photocatalyst: 0.2 g/L; under UV and visible: efficiency: 97.1% for AR14; 67.4% for BA; TOC: 67.1% for AR14; and 59.2% for BA Average crystalline: 28 nm; particle size: 20–30 nm; and degradation efficiency: 98% Crystalline size: 30–65 nm; conc.: 50 mL; and dose: 1 gL
1 90% photocatalytic degradation

Wanjari et al. 2022 Mostafa et al. 2022

Shkir et al. 2022 Ghadim et al. 2022

Khalid et al. 2017 Puiatti et al. 2022

Kumar et al. 2019 Lung et al. 2021

Yang et al. 2019

Reference Rachna et al. 2020

718 M. Rani et al.

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semiconductors is similar to the English letter “Z,” so it is called Z-scheme photocatalytic system. Z-scheme heterojunction band arrangement and charge transfer mechanism are as shown in Fig. 3, and the electrons in CB of A and the holes in VB of B recombine and annihilate. The remaining electrons mainly exist in CB of B, and the holes mainly exist in VB of A, which could realize the spatial separation of electrons and holes. This unique electron migration pathway results in the Z-scheme heterojunction maintaining high REDOX capacity while increasing the separation efficiency of photogenerated electron-hole pairs.

Type II Heterojunction Photocatalysts Among various heterojunction photocatalysts, the research on type II heterojunction has always been dominant. In 1984, Serpone et al. first proposed the use of electronic transfer to avoid recombination. Since then, much research has been done to use “transfer” strategies to inhibit recombination. As shown in Fig. 4, electrons from VB

Fig. 3 The photocatalytic mechanism of Z scheme heterojunction photocatalyst

Fig. 4 The photocatalytic mechanism of Type II heterojunction photocatalyst

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transition to the CB, leaving holes on VB position and these holes get transferred from catalyst A to B. The electrons are going to be transferred from B, which is higher in the valence band to A, and they are going to be reduced; similarly, the holes are going to be transferred from A, which is higher in the valence band to B, and they are going to be oxidized. The type II heterojunction can effectively separate the photogenerated charge carriers in heterostructured composites, so it is indeed a useful strategy to design type II heterojunction photocatalysts to improve the photocatalytic performance.

S-Scheme Heterojunction Photocatalysts Based on the understanding of traditional heterojunction, the concept of a new stepscheme (S-scheme) heterojunction-simulating photosynthesis system was first proposed by Yu’s research group in 2019. As shown in Fig. 16 (Yu et al. 2019), the heterojunction is mainly by the work function of smaller, Fermi level higher reduction type semiconductor photocatalyst (RP) and the work function is bigger, the Fermi level lower oxidation-type semiconductor photocatalyst (OP) is constructed from the staggered type way, effective electrons and holes are saved, and meaningless photoproduction carrier was back together. The electrons with strong reducing capacity and the holes with strong oxidizing capacity are retained to participate in the reduction reaction (e.g., hydrogen production) and oxidation reaction (e.g., oxygen production), respectively. The charge transfer process in the S-scheme heterojunction is shown in Fig. 5. Through three factors, such as the built-in electric field, band bending, and electrostatic interaction, the spatial separation of semiconductor photogenerated electron-hole pairs with strong redox capacity is realized.

Fig. 5 The photocatalytic mechanism of S-scheme photocatalyst

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Tandem Heterojunction Photocatalysts Inspired by photosynthesis in nature, Z-scheme and S-scheme photocatalytic systems have been constructed, which improve the absorption and utilization rate of sunlight and significantly improve the photocatalytic performance, but the quantum efficiency is reduced by half. Therefore, a reasonable heterojunction photocatalytic system should be constructed, and the photocatalytic materials that can improve the utilization of sunlight absorption and the separation and transfer efficiency of photogenerated carriers can be obtained by taking advantage of the light absorption characteristics and synergistic effect of different semiconductor materials, so as to improve the efficiency of solar photocatalytic activity. In order to realize the efficient transfer and separation of photogenerated charge carriers and the effective absorption and utilization of sunlight, Sun et al. proposed to construct tandem heterojunction to improve the solar-driven photocatalytic performance through the effective tandem between two kinds of semiconductors. According to the energy band position and energy-level structure characteristics, the research group connected two kinds of light-capturing semiconductor materials with cocatalyst MoS2 as a bridge to construct the hollow hierarchical structure black TiO2-MoS2 heterojunction solar photocatalyst, which not only expanded the absorption of visible light and NIR, but also realized the efficient separation and transfer of photogenerated charge carriers, so that it had excellent solar-driven photocatalytic hydrogen production performance. In that work, mesoporous hollow black TiO2 microspheres were used as hosts, and MoS2 was vertically coated on the surface of the hollow spheres, which did not affect the absorption of light by black TiO2. Moreover, the deposited CdS mainly grew on the edge of MoS2, which was the main active site for hydrogen production of MoS2. Black TiO2 and CdS could absorb ultraviolet light and visible light simultaneously on both sides of MoS2, thus forming an effective tandem heterojunction. Under the irradiation of simulated sunlight AM 1.5G, the tandem heterojunction showed excellent photocatalytic performance, photocatalytic H2 evolution was up to 280 μmol h
1 20 mg
1, and the performance almost remained unchanged after ten cycles, indicating high stability. This was because the formation of the tandem heterojunction significantly inhibited the photocorrosion of CdS and prolonged the lifetime of photogenerated charge carriers, showing an important application prospect in the field of energy.

Remediation of Organic Pollutants by Green and Sustainable Nanomaterials Nanomaterials particularly green nanomaterials need to be synthesized for the degradation of organic pollutants (inorganic, dyes, organic and biological pollutants) from waste water. Nanomaterials show a good achievement in environmental remediation compared to conventional method which must be very effective, green, eco-friendly, efficient, and easy to handle. In green synthesized nanoparticles are used plant extract. They contain some phytochemicals (phenolic, flavonoids

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compounds, and other biomolecules possess hydroxyl, carboxylate, groups) and act as a reduction of metal salt into nanoparticle and capping agent in terms of stabilized nanoparticles. In addition, combining a mixture of plant extract surfactants may further assist or enhance the physical, chemical, and biological reduction processes. The main benefits of these methods are on natural and biodegradable materials, and no hazardous waste is produced. Zinc and titanium oxides, ceramic, nanowire, and polymeric membranes, carbon nanotubes, and submicron particles are used in various remediation ways such as photolysis, filtration, adsorption, and oxidation. Ismail et al. (2018) synthesized silver nanoparticles by taro plant. These nanoparticles reveal high removal efficiency toward organic azo dyes like methyl red, Congo red, methyl orange, and rhodamine B. Fatimah and coworker (2020) synthesized iron oxide by using Parkia speciosa Hassk pod extract. Green synthesized nanomaterials also show their potential of removal of organic pollutants (Shanker et al. 2022a, b). These nanoparticles contained magnetic property as well as showed a great reduction rate.

Conclusion and Perspective Photocatalytic technology has good application prospects in the fields of energy and environment. The photocatalytic activity of single catalyst is low, but the heterojunction can significantly improve the photocatalytic activity. In the type-II heterojunction, two kinds of semiconductor are excited at the same time, and the photogenerated electrons and holes transfer in reverse to produce spatial isolation, which can effectively inhibit the recombination and provide more photogenerated electrons and holes. In the Z-scheme heterojunction, electrons are transferred from the valence band of a semiconductor to the conduction band of another semiconductor with higher energy level through a special interfacial phase or conductive medium, which can not only make the spatial isolation between the photogenerated electrons and holes, but also ensure that the photogenerated electrons have a strong reducing ability. In the S-scheme heterojunction, the effective electrons and holes are preserved, while the meaningless photogenerated carriers are recombined, which can effectively achieve the separation of the electron-hole pairs with strong REDOX capacity. The tandem heterojunction can greatly improve the absorption efficiency of sunlight and the efficiency of photogenerated charge separation and transfer, so as to significantly improve the photocatalytic performance of sunlight.

References Abuzalat O, Wong D, Elsayed MA (2022) Nano-porous composites of activated carbon–metal organic frameworks (Fe-BDC@ AC) for rapid removal of Cr (VI): synthesis, adsorption, mechanism, and kinetics studies. J Inorg Org Polym Mater 32:1924–1934 Adeola AO, Forbes PB (2019) Optimization of the sorption of selected polycyclic aromatic hydrocarbons by regenerable graphene wool. Water Sci Technol 80:1931–1943

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background of Phthalates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distribution, Application, and Physiochemical Properties of Phthalates . . . . . . . . . . . . . . . . . . . Occurrence of Phthalates in the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Occurrence of Phthalates in the Atmosphere/Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Occurrence of Phthalates in Fresh Water and Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Occurrence in Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Occurrence in Landfills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distribution, Exposure, and Transmission of Phthalates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Phthalates from Wastewater by Conventional Methods . . . . . . . . . . . . . . . . . . . . . . . . . . Phthalate Degradation by Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single Metal Oxide Nanomaterials (SMOs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Doped and Composite Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composite Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Phthalates are an ester category of well-known developing xenobiotic chemicals extensively utilized as plasticizers in PVC plastics. As phthalate plasticizers are not chemically bound to PVC, they can leach, migrate, or evaporate into indoor

Meenu · M. Rani (*) Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India e-mail: [email protected]; [email protected] U. Shanker Polymer and Nanomaterials Synthesis Laboratory, Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_111

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air, atmosphere, foodstuff, other materials, etc. Consumer products containing phthalates can result in human exposure through direct contact and use, indirect leaching into other products, or general environmental contamination. Therefore, due to growing worry about the danger of phthalate exposure having negative consequences on human health and the environment, it is critical to not only understand the existing state of phthalate pollution and its sources, exposure pathways, and health effects but also to discover remediation strategies for phthalate pollution mitigation. Various technologies including adsorption, electrochemical technology, photocatalysis, membrane filtration, and microbial degradation are used for the remediation of phthalate pollution. Adsorption and photocatalysis are the most widely used techniques for phthalate removal in recent years. The current chapter intends to teach its readers the ever-increasing facts on the occurrence and distribution while highlighting new advances in research to reduce phthalate contamination in the environment. This chapter lists the significant phthalates in everyday use, traces their environmental fate, and provides an in-depth understanding of the various treatment methods currently in use or being researched to reduce the risk of phthalate pollution, their challenges, and future research perspectives. Keywords

Plasticizers · Phthalate · Plastic additives · Occurrence · Nanomaterials

Introduction Plasticizers are a group of phthalate esters that improve the flexibility of materials. They are used in flooring, paints, lubricants, modeling clay, glow sticks, food packaging, medical devices, toys for children, cosmetics, medical items, plastic packaging films, electronics, and automobiles, among other things (Zhang et al. 2021; Singla et al. 2016). Phthalates are a class of compounds used to increase polymers’ durability. They are frequently referred to as plasticizers. Some phthalates are used to aid in the dissolution of other materials (Anh et al. 2021). Phthalates may be found in many goods, including vinyl flooring, lubricants, and personal care items (soaps, shampoos, hair sprays). Some phthalates can be found in polyvinyl chloride polymers, which produce items like plastic containers, garden hoses, and medical tubing (Fig. 1). From 2007 to 2017, global phthalate manufacturing climbed from 2.7 to approximately 6 million tons per year (Bajt et al. 2008). Despite their numerous uses, they are perhaps one of the most investigated and contentious compounds. Because of their widespread use in various industries, phthalates have become a common environmental pollutant. Phthalate emission or migration happens throughout the life cycle of a phthalate-containing product, from manufacture through disposal. They are continually discharged into the environment as a result of manufacturing process losses as well as weathering, leaching, or evaporation from finished goods (Bi et al. 2015; Ashworth et al. 2018; Bauer and Herrmann 1997). Once discharged, they make their

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Fig. 1 Application of plasticizer (phthalates esters a) in various areas

way into nearly every ecosystem component via various biogeochemical processes (Brodskiy et al. 2019; Chang et al. 2005). Exposure to phthalates is a severe problem for humans, domestic animals, and wildlife due to its enormous manufacturing volume, extensive usage, and pervasive presence in the environment (Chen et al. 2017; Das et al. 2014; Fromme et al. 2002). Phthalate esters have been found in significant levels in river water, wastewater, drinking water, and sediments worldwide. The US Environmental Protection Agency (US EPA) and several equivalents have designated the most prevalent phthalate esters as priority pollutants and hormone disruptors (Fudala-Ksiazek et al. 2018). Phthalate esters can adversely disrupt humans’ respiratory, reproductive, and endocrine systems. Incorporating phthalate esters into a polymer matrix lowers the polymer’s glass transition temperature. Because phthalate esters are not covalently bonded to the polymer, they can migrate to the polymer matrix’s surface and be lost by several physical processes (Fang and Zheng 2004). Despite increasing worry about their potentially detrimental effects on humans, a substantial quantity of data on these characteristics of chemicals, environmental destiny, exposure, and animal model toxicity studies has been created since they were brought into commerce around 90 years ago. Because of growing concern about its presence in significant quantities in the environment and potential toxic effects, the US Environmental Protection Agency (USEPA) has designated five phthalate congeners as priority pollutants: dimethyl phthalate (DMP), di-ethyl phthalate (DEP), di-n-butyl phthalate (DnBP), di (2-ethylhexyl) phthalate (DEHP), and di-n-octyl phthalate (Dn (Kashyap and Agarwal 2018). Furthermore, some developed countries, such as the European Union and the United States, have restricted the production and use of phthalates (CPSIA 2017; European Commission 2005, 2007), and the majority of phthalates are now produced and consumed by developing countries such as Brazil, China, and India (Gao et al. 2018). The demand for phthalates is increasing due to low manufacturing costs and a lack of low-cost

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alternatives; hence, the environmental impact associated with this category of chemicals is increasing. In this context, investigating the destiny of these chemicals in the environment not only aids in evaluating population exposure risk but also opens the door to remediation alternatives.

Background of Phthalates The invention of cellulose nitrate plastic in 1846 resulted in castor oil’s patent as the first plasticizer in 1856. Camphor was the preferred plasticizer for cellulose nitrate in 1870. In the 1920s, phthalates were introduced and swiftly replaced the volatile and odorous camphor. With the commercial availability of PVC and the invention of DEHP in 1931, the plasticizer PVC business began to thrive. The Joint Research Centre (JRC) of the European Commission presented a paper on phthalate measurement techniques in food in February 2009. Research in 2021 investigated phthalates in 64 fast food products. DnBP was discovered in 81% of the samples, whereas DEHP was detected in 70%. DEHT, the primary alternative to DEHP, was found in 86% of the samples. Further, phthalates may be classified into two groups based on molecular weight. Accordingly, low-molecular-weight phthalates (ester side-chain lengths, one to four carbons) include DMP, DEP, DBP, and DIBP, and highmolecular-weight phthalates (ester side-chain lengths, five or more carbons) include DEHP, DOP, and DINP.

Distribution, Application, and Physiochemical Properties of Phthalates Phthalate esters are the dialkyl or alkyl aryl esters of phthalic acid (also called 1,2-benzene dicarboxylic acid, not to be confused with the structurally isomeric terephthalic or isophthalic acids); the name “phthalate” derives from phthalic acid, which itself is derived from the word “naphthalene.” When added to plastics, phthalates allow the long polyvinyl molecules to slide against one another. The phthalates have a clear syrupy liquid consistency and show low water solubility, high oil solubility, and low volatility. The polar carboxyl group contributes little to the physical properties of the phthalates, except when R and R’ are very small (such as ethyl or methyl groups). Phthalates are colorless, odorless liquids produced by reacting phthalic anhydride with appropriate alcohol (usually 6- to 13-carbon) (Fig. 2). All physical and chemical properties of phthalates are discussed in Table 1. The mechanism by which phthalates and related compounds affect polar polymers’ plasticization has been a subject of intense study since the 1960s. The mechanism is one of the polar interactions between the polar centers of the phthalate molecule (the C¼O functionality) and the positively charged areas of the vinyl chain, typically residing on the carbon atom of the carbon-chlorine bond. For this to be established, the polymer must be heated in the presence of the plasticizer, first above the Tg of the polymer and then into a melt state. This enables an intimate mix

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Fig. 2 Structure and name of some phthalates (DMP ¼ dimethyl phthalate; DBP ¼ dibutyl phthalate; DAP ¼ diallyl phthalate; DPP ¼ di-n-propyl phthalate; BCP ¼ butyl cyclohexyl phthalate; DIBP ¼ diisobutyl phthalate; DNPP ¼ di-n-pentyl phthalate; DCP ¼ dicyclohexyl phthalate)

of polymer and plasticizer to be formed and for these interactions to occur. When cooled, these interactions are paramount, and the network of PVC chains cannot reform (as is present in plasticized PVC or PVC-U). The alkyl chains of the phthalate then screen the PVC chains from each other. They are blended within the plastic article as a result of the manufacturing process.

Name of compound and abbreviation Dimethyl phthalate (DMP) Diethyl phthalate (DEP) Di-n-propyl phthalate (DPP) Dibutyl phthalate (DBP) Benzyl butyl phthalate (BBP) Di(2-ethylhexyl) phthalate (DEHP) Di-n-octyl phthalate (DnOP) Di-n-hexyl phthalate (DHP) Diisoheptyl phthalate (DIHP) Diisononyl phthalate (DINP) Diisodecyl phthalate (DIDP) Diundecyl phthalate (DUP) Ditridecyl phthalate (DTDP) Diallyl phthalate (DAP) Diisobutyl phthalate (DIBP) Di(2-methoxyethyl) phthalate (DMEP) Dicyclohexyl phthalate (DCP) Diisotridecyl phthalate (DITP)

Molecular formulae C10H10O4 C12H14O4 C14H18O4 C16H22O4 C19H20O4 C24H38O4 C24H38O4 C20H30O4 C22H34O4 C26H42O4 C28H46O4 C30H50O4 C34H58O4 C14H14O4 C16H22O4 C14H18O6 C20H26O4 C28H46O4

Molecular weight (g mol1) 194.2 222.2 250.3 278.4 312.4 390.6

390.6 334.5 363 418.6 446.7 447.7 530.8 246.26 278.34 282.29

330.42 530.82

Table 1 Physical and chemical properties of phthalates

0.002 7  10–11

117-84-0 84-75-3 7188-89-6 28553-12-0 26761-40-0 3648-20-2 119-06-2 131-17-9 84-69-5 117-82-8

1). It causes worst-case scenarios in the environment. The hazard quotient of UV fillers is not significantly calculated yet. Chronic toxicity was observed at the initial hazard stage of PPCP exploitation. So, there is a requirement of efficient technique for their remediation with low cost, ease of handling, and environmental friendliness.

Conclusion This chapter summarized the polymeric nanoparticles with physicochemical properties. Furthermore, it includes the classification of polymeric nanoparticles, i.e., natural, biosynthesized, and chemical polymer nanoparticles. It also discussed the requirement of surface modification via coupling or doping of different surfaces. Change in properties of carbon nanotubes, single transition metal, metal oxides, and silica-coated polymeric surface after doping. A brief classification of pharmaceutical and personal care products with their toxicity is also concluded in the chapter. Furthermore, the remediation approach and role of the photocatalysis mechanism are also concluded in the chapter.

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Metal Oxides–Based Nanomaterials: Green Synthesis Methodologies and Sustainable Environmental Applications

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Uma Shanker, Vipin, and Manviri Rani

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Metal Oxides Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (I) Monometallic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (II) Bimetallic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Trimetallic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Metal Oxides Nanoparticles Using Green Methodology . . . . . . . . . . . . . . . . . . . . . . . . Using Plant Extracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Using Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of Traditional Pollutants Using Metal Oxides–Based Nanoparticles . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Environmental pollution is increasing day by day due to the development of various types of industries at the present time. These industries dump their untreated, contaminated waste and harmful smoke into the environment. Water and air pollution are some of the most concerning topics around the world. With the help of several types of nanomaterials, nanotechnology has found great potential in U. Shanker (*) Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar Punjab, India e-mail: [email protected] Vipin Department of Chemistry, Dr B R Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India e-mail: [email protected] M. Rani Department of Chemistry, Malaviya National Institute of Technology Jaipur, Jaipur Rajasthan, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_80

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the removal of toxic pollutants from the environment. Metal oxides and metal oxide–based nanomaterials were found to be very effective toward the removal of harmful pollutants from the environment. The photocatalytic degradation practice in bare metal oxide semiconductors such as ZnO, TiO2, NiO, Fe2O3, and others is usually initiated in the presence of UV light, and this process is very efficient and the utilization of energy is high in this UV light irradiation but is lower in natural light and visible light. Incorporation of dopant improves the photocatalytic properties of metal oxides nanomaterials such as lowers band gap energy, delays the recombination rate as well as separation of charge carriers, and enhances the surface area. Metal-coupled nanoparticles show excellent photocatalytic activity under direct solar and visible light than undoped nanomaterial. In the present chapter, some recently developed techniques for sustainable environmental applications are briefly discussed. Metal oxides and their classification as monometallic and bimetallic synthesis of metal oxides and metal oxide–based nanomaterials using green methodology are summarized below. Keywords

Nanotechnology · Nanomaterials · Metal oxides · Green methodology

Introduction Nanotechnology is a relatively new field of science and technology that studies tiny objects. The field of nanotechnology and nanoscience study has expanded in recent years, gaining the epithet “tiny science.” Nanotechnology seems to be at the center of the stage when technical developments are evaluated and has grown immense attention with time. Nanotechnology offers a wide range of important applications in science and technology. Nanoparticles are the primary building materials of nanotechnology. Nanoparticles are dimension less than 100 nm in size composed of carbonbased nanoparticles, metal, metal oxides, metal sulphides–based nanomaterials, porous substances such as zeolite, graphene oxide, chitosan-based nanomaterials, polymeric, organic substances–based nanomaterials, and many others. On comparing to their bulk counterparts, nanoparticles have higher surface area, absorption, reactivity, surface charge, sensitivity, strength, and stability (Khan et al. 2021). Nanomaterials are not only a very important footmark in the area of miniaturization, but also an important milestone as the nanosized domain lies in between atomic and quantum realm and the bulk scale (Gupta and Mao 2021). Based on different parameters such as dimension, phase composition, and manufacturing process, they can be classified into various categories (Gatoo et al. 2014). These properties of nanomaterials have led to their use for various applications, which are classified mostly based on surface properties. The development of nanotechnology provides potential for the manufacturing of materials, specifically those for environmental remediation, when traditional techniques may reach their limitations (Khan et al. 2021). In general, two approaches are employed for the fabrication of nanomaterials: top down and bottom up. In the top-down technique, the bulk material is broken down

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into nanosized structures or particles using physical processes such as chemical vapor deposition, high-energy ball milling, arc discharge, electron beam lithography, plasma synthesis, etc. The bottom-up approach involves developing a material from the bottom to the nanoscale by agglomerating atom-atom, molecule-molecule, or cluster-cluster agglomerations in the solution phase using chemical methods such as revere micelle, hydrothermal, sol-gel synthesis, coprecipitation, microwave, and many more (Dhand et al. 2015; Patwardhan et al. 2018). The physical and chemical methods produce a large number of nanoparticles. However, they are not preferred due to the use of hazardous chemicals, high costs, and energy demands. Green methods have emerged to overcome the disadvantages of physical and chemical processes (Bhavyasree and Xavier 2021). Synthesis using naturally derived materials is referred to as “green synthesis.” This goal may be achieved by employing a number of biological organisms, such as bacteria, algae, plants, actinomycetes, and fungus. Green nanomaterial synthesis methods include cost-effective, sustainable, reliable, and energy-efficient techniques. Green synthesis techniques based on plant extracts are preferred because they are easily accessible, convenient to use, nontoxic, cost-effective, and ecologically friendly (Shanker et al. 2016). Metal oxides are essential in a variety of fields including chemistry, physics, and materials research. Metal elements can combine to generate a wide range of oxide compounds. These can have a wide range of structural geometries, as well as an electrical structure that can be metallic, semiconductor, or insulator in nature (Fernández and Rodriguez 2007). Metal oxide nanoparticles have seen increased demand for numerous scientific and technical applications, especially in the fields of medical sciences, information technology, catalysis, energy storage, and sensors, due to several favorable and nontoxic features associated with oxide systems. As a result, there has been an explosion in research into developing novel metal oxide nanomaterial fabrication processes. Many wet chemical approaches for the production of nanoparticles have been reported in the literature, including hydrothermal, sol-gel, coprecipitation, microemulsion, combustion, and others. Many of these methods entail the use of hazardous and expensive chemicals, specialized equipment, and organic solvents for cleaning, among other things. Given the wide range of possible metal oxide nanostructures and the characteristics of such materials, there is still a strong focus on pushing the boundaries of existing synthetic techniques to such nanomaterials. In recent years, there has also been a move to improve novel synthetic processes for nanomaterials that are less harmful to the environment. We will look at some of the most recent advancements in the creation of metal oxide nanoparticles here. The synthesis of metal oxides and metal oxides–based nanomaterials through green methodology are more efficient and eco-friendly (Corr 2013; Gupta and Mao 2021). Various literature surveys reviewed on environmental remediation and found that metal oxides and the metal oxides–based nanomaterials are used as photocatalysts because of their exclusive photochemical properties, less toxicity, and low cost. The metal oxides mainly including TiO2, ZnO, Fe2O3, Al2O3, MgO, Fe3O4, MnO2, NiO, SiO2, CeO2, and many others are the most commonly used nanoparticles in the environmental remediation. The band gap energy of some oxides is found to be quite higher between valence band and conduction band, for example, 3.2 eV for ZnO, 3.6 eV for NiO, and 3.0 eV for TiO2 rutile and anatase state, so the low movement of

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the excited electrons of valence band electron-hole (e

h+) pairs and as a result the recombination rate of those electrons and holes will be high (Niu et al. 2014; Lu and Astruc 2020; Kiran et al. 2020). Hence, bare ZnO, NiO, and TiO2 photocatalysts are activating and work productively under UV light illumination, which makes them more energy consuming and less effective in visible light because the solar light that reaches the planet contains just around only 4–6% of UV light and approximately 45% of visible light (Rahimi et al. 2016; Daneshvar et al. 2004). These problems of high recombination rate of electron-hole pairs and more energy consumption have been solved by using a few procedures and the metal oxides–based nanomaterials take place (Kou et al. 2010; Kruth et al. 2014). One of the best method is doping of the bare nanomaterials with different metals, nonmetals, metal oxides, polymeric materials, and others mainly including Fe, Au, Ni, Cu, Cd, Ag, C, N, S, Al2O3, Fe2O3, hexacyanoferrates, chitosan, activated carbon, and zeolite (Elhalil et al. 2018; Kaur et al. 2018; Yi et al. 2019, Rachna et al. 2020a, b). In this chapter, we have explored numerous kinds of metal oxides and metal oxide–based nanomaterials. A better understanding of green nanotechnology via the use of multiple green methodology for the synthesis different forms of metal oxides nanomaterials, as well as their numerous characterization techniques, are also explored, helping readers to learn more about the size and shape of nanoparticles. The last section of this chapter discusses the use of these several synthesized metal oxide–based nanoparticles in the sustainable environmental applications, mainly including in the removal of various toxic, hazardous, and carcinogenic contaminants that present significant problems to our environment.

Classification of Metal Oxides Nanoparticles Generally, the metal oxides are classified into two categories including (i) monometallic oxides, (ii) bimetallic oxides, and (iii) trimetallic oxides; apart from these oxides some miscellaneous metal oxides nanomaterials are also present such as hexacyanoferrates, metal oxides doped with porous materials, doped metal oxides with various metallic and nonmetallic dopants, and many others. Metal oxides and metal oxides–based nanomaterials have a wide range of effective applications in various fields. Because of their small size and high density of corner or edge surface sites, metal oxides nanoparticles can demonstrate unusual physical and chemical features. In any material, particle size is predicted to have an impact on three categories of fundamental characteristics. The structural properties, such as lattice symmetry and cell parameters, are included in the first one. Bulk oxides are typically solid, stable, and have well-defined crystalline structure. However, as particle size decreases, the increasing importance of surface free energy and stress must be considered: Changes in thermodynamic stability associated with size can cause changes in cell parameters or structural transformations, and in extreme cases, the nanomaterials can disappear resulting from interactions with its surroundings and a high surface free energy. The nanoparticles should have a minimal surface free energy in order to be mechanically or structurally stable. As

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a result of this need, phases that are unstable in bulk materials can become extremely stable in nanostructures. This morphological phenomenon has been seen in metal oxides such as TiO2, ZnO, VOx, Fe2O3, Al2O3, and MoOx (Fernández and Rodriguez 2007).

(I) Monometallic Oxides Metal oxides with nanoscale dimensions exhibit a number of unique features, including high surface area, desirable band gap, and the ability to get synthesized easily. The monometallic oxides show environmental applications such as high removal capacity for heavy metal and degradation of organic pollutants and dyes. They have excellent potential as heavy metal adsorbents. ZnO, Fe2O3, CeO2, TiO2, Al2O3, NiO, Cu2O, CuO, and SnO2 are examples of monometallic oxides nanomaterials, among them ZnO, Fe2O3, and TiO2 are widely used metal oxides (Naseem and Durrani 2021). Every metal oxide has their exclusive properties, features of some monometallic oxides are discussed as follows. ZnO nanoparticles have great chemical stability and outstanding photocatalytic activity in the removal of water contaminants. Therefore, ZnO is considered a good photocatalyst. At ambient temperature, ZnO has a large band gap (3.37 eV) and a high binding energy (60 meV). Nanosheets, nanowires, nanobelts, nanorods, and complex hybrid structures are among the ZnO nanostructures that may be produced. Hollow spheres are of special significance among these nanostructures because of their large specific surface area, low density, and excellent surface permeability, as well as their high light-harvesting efficiencies and greatly improved photocatalytic activity (Baruah et al. 2012; Naseem and Durrani 2021). CuO nanoparticles have a monoclinically organized structure and are semiconductors. Superconductivity at extreme temperatures, solar energy efficiency, relative stability, cheap cost, and antimicrobial activities are only a few of the chemical and physical properties. Due to their interesting electrochemical behavior, CuO nanoparticles have a variety of technological uses, including catalysis and battery usage. Solvothermal techniques, electrochemical methods, hightemperature combustion, and novel rapid precipitation processes may all be used to synthesize nanoparticles of various sizes and forms. Because the size of the particle reduces the surface-to-volume ratio, the number of active sites on the surface rises. Therefore, using CuO nanoparticles with a narrow size distribution for these applications will further boost the nanoparticles’ photocatalytic activity (Ren et al. 2009; Sawsan et al. 2014). Ag2O nanoparticles are large-surface area oxide magnetic nanomaterial particles that are spherical or faceted. Silver oxide nanoparticles typically have a particle size of 20–80 nm and a surface area of 10–50 m2/g. High purity, ultrahigh purity, and transparent silver oxide nanoparticles are also available in covered and dispersed forms. They can also be disseminated in the AE nanofluid fabrication group. Surfactants or surface charge technologies in the solution are used to define nanofluids as suspended nanoparticles. Nanohorns, nanocomposites, nanorods,

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nanopyramids, and nanowhiskers are examples of other nanostructures. Numerous nanomaterials have been described as antibacterial agents, including silver oxide nanoparticles, which have good antibacterial action and are now being investigated and exploited in many commercial applications (Nakamura et al. 2019). Titanium oxide (TiO2) nanoparticles have been the most extensively investigated metal oxide in recent decades. TiO2 is the most outstanding photocatalyst to date, owing to its photostability, low cost, high photocatalytic activity, and biological and chemical stability. Because of the substantial bandgap energy (3.2 eV) and ultraviolet (UV) stimulation, charge separation within particles is usually caused in TiO2. Since TiO2 nanoparticles have low selectivity, they can degrade a range of pollutants, including aromatic hydrocarbons, chlorinated organic compounds, dyes, pesticides, phenolic compounds, cyanide, arsenic, and heavy metals. TiO2 nanoparticles’ photocatalytic characteristics can destroy a wide range of microorganisms, including gram-positive and gram-negative bacteria, viruses, algae, fungus, and protozoa (Guesh et al. 2015; Guo et al. 2015). Iron oxide nanoparticles have been usually employed to remove heavy metals in past few years due to its convenience of usage and accessibility. Enhanced membranes characteristics, high surface area, high durability, and tiny particle size are some of the advantages of iron oxide–based nanomaterials. Nanoadsorbents commonly include magnetic magnetite (Fe3O4), nonmagnetic hematite (α-Fe2O3), and magnetic maghemite (γ-Fe2O4). Because of the tiny size of nanosorbent materials, separation and recovery from polluted water is a significant problem for water treatment. Fe3O4 and (γ-Fe2O4) are, on the other hand, simple to separate and recover from a system. As sorbent materials, both have been utilized successfully to extract various heavy metals from wastewater (Lei et al. 2014; Nizamuddin et al. 2019).

(II) Bimetallic Oxides In comparison to monometallic nanoparticles, bimetallic nanoparticles have created an interest. Today, new bimetallic nanoparticle formulations in various forms, such as alloys, core shells, and contact aggregates, are being investigated (Kumaravel et al. 2021). Titanium dioxide (TiO2) has been used to treat water pollution; however, as TiO2 research has progressed, the photodegradation effectiveness of TiO2 photocatalysts in presence of visible light has been restricted due to significant photoelectron-hole pair recombination due to higher band gap energy. As a result, lowering the rate of photoelectron-hole pair recombination is essential (Cheng et al. 2016; Torralvo et al. 2018). Several investigators have tried to improve the photocatalytic performance of Tio2 nanoparticles by doping with metals and nonmetals, exposing highly reactive facets, and doping with metal oxide nanostructures such as CeO2, Fe2O3, Cu2O, ZnO, and other metal oxide nanostructures (Zhang and Lei 2008; Fan et al. 2016; Qin et al. 2019). ZnO is one of the most important semiconductor materials among all composite photocatalysts with TiO2. It has comparable band gap energy to TiO2 and has been used in solar cells, increased random lasing, and decorative films. Furthermore, because ZnO has a high electron mobility, TiO2-ZnO

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composite photocatalysts are projected to have a lower photoelectron-hole pair recombination rate than pure TiO2. The photocatalytic performance of synthesized photocatalysts was determined by measuring the rate constant of photodegradation of rhodamine B in aqueous solution under visible light, which increased from 0.0059 min
1 to 0.011 min
1. Lowered band gap energy, improved surface physicchemical characteristics, separation of light-stimulated electron-hole pairs, and a lower recombination rate are all factors that contribute to the improved photocatalytic capabilities of target photocatalysts. The TiO2-ZnO composite photocatalysts showed high stability for rhodamine B photodegradation in visible light, according to the reusability test. The TiO2-ZnO composite photocatalysts may be used in the field of treating with organic pollutants due to their good photocatalytic properties, low cost, environmentally friendly nature, and simple experimental technique. Additionally, the TiO2-ZnO composite nanocatalysts are expected to have applicability in photocatalytic water splitting into hydrogen and oxygen (Qin et al. 2019).

Trimetallic Oxides Frequently in recent years, it was observed that trimetallic oxide nanoparticles have superior properties as compared to monometallic and bimetallic oxide nanoparticles. Trimetallic oxides nanoparticles are formed by the combination of three different metal oxides. The incorporation of metal oxides on the matrix of metal and other metal oxides provides many advantages, including reduced leaching, proper dispersion of metal oxide nanoparticles, increased surface area, and increased number of available active sites. They indicate the averaged qualities of the metals that comprise them. The aggregation rate of trimetallic oxide nanoparticles is considerable (Sharma et al. 2019; Tariq et al. 2021). The CoMnFeO4 nanoparticles initially reported that they had remarkable electrochemical performance, outperforming several other binary oxides. Based on the abovementioned premise, trimetallic oxides including Co, Fe, and Mn metal may be a suitable catalyst for the development of a high-performance aqueous-phase oxidation system. On the one hand, the significant synergistic action of these three metals will boost PMS activation even more. The Fe oxide component, on the other hand, would give the catalyst with high magnetic properties, which will be advantageous for recycling (Permien et al. 2016). Cu/Cr/Ni trimetallic oxide nanoparticles were synthesized by using Echinops persicus plant extract through green method. Cu/Cr/Ni nanoparticles are more effective than tetracycline as a typical antibiotic against both gram-positive and gram-negative bacteria, including E. coli, S. aureus, and B. cereus. Comparative tests also revealed that the antibacterial activity of Cu/Cr/Ni nanoparticles was significantly more stronger than that of individual metal oxide nanoparticles generated in a similar manner using Echinops persicus flower extract (Mahmoudi et al. 2021). Fe-doped Co3V2O8 trimetallic oxide nanoparticles were effectively produced utilizing a simple hydrothermal approach and shown efficient catalytic activity for the oxygen evolution process in alkaline environments. The Fe-doped Co3V2O8 catalyst, with its high specific surface area and high rate of amorphization, can

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improve the exposure of the catalytic surface to electrolyte, providing additional active sites for oxygen evolution reaction (Gao et al. 2015).

Synthesis of Metal Oxides Nanoparticles Using Green Methodology According to the principles of green chemistry, a green technique is an effective and eco-friendly approach for synthesizing nanoparticles that should ideally employ sustainable energy, reduce waste discharges, and optimize energy usage (Anastas and Werner 1998). Biogenic sources for the synthesis of nanomaterials may be categorized into two, namely: (i) plant based and (ii) animal based, as shown in Fig. 1. Natural products derived from the aforementioned two sources are considerably less harmful and exhibit a wide range of pharmacological and biological activity (Shanker et al. 2016). Surprisingly, approximately half of the pharmaceutical compounds now accessible are derived from natural products. Plant-based products are potential options and may be easily used for large-scale biosynthesis of a wide range of nanoparticles (Liang and Fang 2006). Plant components such as leaves, tubers, buds, bark, and fruits/seeds are used in the synthesis of nanoparticles. These plant sections are readily accessible.

Fig. 1 Representation of green synthesis of nanoparticles (Ali et al. 2020) with permission

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Furthermore, the most important phytochemicals included in plant-based materials are “polyphenols,” and storage of such materials is relatively straightforward. The by-products created are environmentally friendly and do not harm the ecosystem. Because they are biologically obtained extracts, they are environmentally safe, and synthesis employing plant extracts gives an advantage over other approaches in terms of cost-effectiveness (Armendariz et al. 2004). Plant extracts were employed as a source of reducing agents in the early 1900s, though their specific nature was unknown to most people. However, during the last three decades, considerable usage of various types of plant extracts has been carried out for the production of nanoparticles depending on the necessity. These plant extracts have the potential to function as both reducing and stabilizing agents in the fabrication of several nanomaterials (Parsons et al. 2007). Generally, plant extracts are used to achieve bioreduction of the concerned metal salt, which eventually leads to the production of metal oxides–based nanoparticles. These extracts have a strong potential to accumulate onto the surface of nanoparticles, enhancing their stability in aqueous medium. Following the extraction of metal ions from the solution of salt, these complex biomolecules decompose at high temperatures, resulting in the creation of metal oxide nanoparticles. As a result, the same may be used to synthesize a range of monometallic, multimetallic, metal oxides, and other types of nanoparticles. Extensive research is being conducted all over the globe in the green production of nanoparticles utilizing biogenic plant extracts (Shanker et al. 2016).

Using Plant Extracts Green synthesis of nanoparticles using plant extracts or biomass is one of the most efficient, fast, clean, nontoxic, and environmentally acceptable ways. This methodology has mostly been used to synthesize nanoparticles of noble metals, metal oxides, and bimetallic alloys which have sufficiently delimited several plant biometabolites that might contribute in the synthesis of nanoparticles due to their significant role of reducing agent (Dhand et al. 2015). The plant-based synthesis is represented in Fig. 2. Furthermore, plant extracts are also employed as a capping agent in the synthesis of nanoparticles, providing a cost-effective route. Plant materials utilized in the synthesis of nanoparticles include stems, roots, flowers, fruits/seeds, leaves, calluses, and peels (Ahmad et al. 2010; Bindhu and Umadevi 2015; Yallappa et al. 2015). The source of a plant extract influences the characteristics of nanoparticles since many extracts include varying amounts and combination of natural reducing agents. Many water-soluble phytochemicals, including alkaloids, terpenoids, and phenolic chemicals, function as reducing agents. The hydroxyl and ketonic groups found in phenolic compounds can attach to metals and exhibit chelation. Nanoparticles are thought to be environmentally favorable, with superior precise size, morphologies, and stability (Shanker et al. 2016; Bai et al. 2018; Patwardhan et al. 2018).

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Fig. 2 Representation of plant-based green synthesis of nanoparticles (Dikshit et al. 2021) with permission

Manjula et al. (2019) synthesized MnO2 nanoparticles using Gardenia resinifera leaves extract with green method. UV-Visible spectroscopy, PSA, FT-IR, XRD, SEM-EDAX, and HR-TEM analyses were used to evaluate the fabricated MnO2 nanoparticles. According to the observations, the produced MnO2 nanoparticles are 17–35 nm in diameter and spherical in morphology. The findings of the FT-IR analysis reveal the presence of numerous functional groups that might be involved in diverse biological functions. The antibacterial activity of the produced MnO2 nanoparticles is strong. CuO nanoparticles with sizes ranging from 20 to 140 nm were synthesized using Aloe barbadensis and Carica papaya as reducing agents (Sankar et al. 2014). Garrafa-Galvez et al. (2019) employed Lycopersicon esculentum peel extract for the production of SnO2 nanoparticles. The nanoparticles that were developed have the following characteristics: Sn-O bond at 666 cm
1 in FTIR; crystallization growth in a purely tetragonal crystalline structure; size and shape uniformity that varies depending on the quantity of extract used; and a band gap of around 3.3 eV. Abboud et al. (2014) synthesized CuO nanoparticles of 5–45 nm from Bifurcaria bfurcata which were employed in antimicrobial investigations. The capping of CuO nanoparticles by reducing agent diterpenoids was validated by FT-IR measurement. Naz et al. (2019) synthesized Fe2O3 nanoparticles using Rhus punjabensis plant extract through green methodology. The Fe2O3 nanoparticles are more chemical and thermodynamically stable than other iron oxide nanoparticles; they are ideal for biological applications. XRD, SEM, TEM, EDS, and FFT were used in the structural, morphological, and composition examinations. Hematite (Fe2O3) nanoparticles with a rhombohedral crystal structure and a size of 41.5  5 nm were produced. The introduction of a capping agent resulted in phase-pure Fe2O3 production with regulated size, according to TEM, FTIR, and TG/DTA results. The crystalline morphology of zinc-doped Prussian blue nanocatalyst made employing A. Indica plant extract and

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utilized to remove phenols from water under the sunlight. The photocatalyst has a nanoflakes form with a surface area of 80 m2g
1, as disclosed by FE-SEM (Rachna et al. 2020a, b). The metal oxides–based nanocomposites such as CuO-Fe2O3CS, NiFe2O4-CS, Co2O3-Fe3O4-CS, FeCr2O4-CS, and ZnFe2O4-CS with different morphologies were produced in a green way by using Azadirachta indica plant leaves extract as a natural surfactant in aqueous media. Under direct sunlight, the synthesized materials demonstrated high photocatalytic efficacy in the exponential decay of ANTH and PHEN (Rani et al. 2020). Green synthesis of ZnO nanoparticles based on Allium Sativumextract (ZnO-Green) resulted in crystalline nanorods. For comparative solar photocatalytic experiments, ZnO nanoparticles were also chemically synthesized using a conventional coprecipitation method (ZnO-Chem). The photocatalytic activity of the green-synthesized ZnO was excellent (El Golli et al. 2021). Citrus limetta extract was utilized to produce TiO2 nanoparticles utilizing an environmentally friendly green synthesis approach. The extract’s citric acid served as a reducing and capping agent for nanoparticles, resulting in pure TiO2 nanoparticles. The produced nanoparticles are around 80–100 nm in size, spherical in shape, and almost uniformly distributed throughout the sample (Nabi et al. 2021). Nigella sativa seeds extract was used as a stabilizing component in the development of green-synthesized NiO nanoparticles. With the addition of NaBH4, the pH of the solution has been adjusted. XRD, FTIR, SEM, and TEM have all been used to characterize NiO nanoparticles. FTIR and TEM investigations revealed that NiO was surrounded by biological components derived from the extract (Boudiaf et al. 2021). Cu/Cr/Ni trimetallic oxide nanoparticles with strong antibacterial and catalytic activity were successfully synthesized utilizing an Echinops persicus plant extract in a simple, biocompatible, costeffective, and nontoxic approach. Cu/Cr/Ni nanoparticles with a diameter of 20 nm and spherical shape were synthesized (Mahmoudi et al. 2021). Summarization of plant extract–based synthesis of metal oxides nanomaterials is depicted as Table 1.

Using Microorganisms Bioreactors for the production of nanoparticles employ microorganism techniques, including prokaryotic bacteria, actinomycetes, fungus, algae, and yeast. Great scientific efforts went toward developing this approach for creating a wide range of nanoparticles such as metal, metal oxides, and metal oxides–based nanomaterials as shown in Fig. 3. Microorganisms capture targeted ions from their surroundings and use enzymes produced by cellular processes to convert the metal ions into the element metal. Depending on where the nanoparticles is synthesized, it might be classed as intracellular or extracellular. Bacteria of various types can be found in soil, water, plants, and animals. They can survive in a wide range of soil pH, salinity, temperature, and nutritional conditions. Bacteria may be found in normal to very salty water in the sea, as well as on ice with a freezing temperature. Some of these can arise in highly polluted or highly accumulated soils and plants (Huynh et al. 2020). Metal ions are transported inside the microbial cell to generate nanostructures in the presence of enzymes in the intracellular approach. The extracellular synthesis of NPs involves

Aloe barbadensis and Carica papaya Lycopersicon esculentum peel extract Bifurcaria bfurcata

Rhus punjabensis

Azadirachta indica

Azadirachta indica

Allium Sativum Citrus Limetta

Nigella sativa seeds

Echinops persicus

CuO

SnO2

Fe2O3

ZnO@FeHCF

ZnFe2O4-CS CuO-Fe2O3-CS NiFe2O4-CS FeCr2O4-CS Co2O3-Fe3O4-CS ZnO TiO2

NiO

Cu/Cr/Ni oxide

CuO

Plant extract used Gardenia resinifera leaves

Nanoparticles MnO2

UV-vis, FTIR, EDX, XRD, FE-SEM, DLS, and TEM

UV-visible, FT-IR, XRD, and FE-SEM UV-visible, XRD, SEM, TEM, BET, and PL XRD, FTIR, SEM, and TEM

SEM, XRD, FTIR and FFT, and TG/DTA UV-visible, FT-IR, EDX PXRD, FE-SEM, TEM, and XPS UV-visible, FT-IR, EDX PXRD, FE-SEM, and BET

Characterizations methods used UV-visible, PSA, FT-IR, XRD, SEM-EDAX, and HRTEM UV–visible, SEM, FT-IR, XRD, DLS, and zeta XRD, FTIR, UV visible, HRTEM, and SAED XRD, FTIR, UV visible, and TEM

Table 1 Plant extract–based green synthesis of metal oxide–based nanomaterials

20

Spherical

Spherical

Spherical Spherical Spherical Plates like Flower Nanorods Spherical

 50  50  50 10–30  50 30–40 80–100 15–22

Cubic

Spherical

41.5  5 60–180

Spherical

Spherical

Rod shaped

Shape of nanoparticles Spherical

5–45

4–5

20–40

Average particle size (nm) 15–30

Boudiaf et al. (2021) Mahmoudi et al. (2021)

El Golli et al. (2021) Nabi et al. (2021)

Rachna et al. (2020a, b) Rani et al. (2019, 2020)

Garrafa-Galvez et al. (2019) Abboud et al. (2014) Naz et al. (2019)

References Manjula et al. (2019) Sankar et al. (2014)

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Fig. 3 Metal-based nanoparticle synthesis through microorganisms (Huynh et al. 2020) with permission

trapping of metal ions on the surface of the cells and reducing ions in the presence of enzymes. Bacteria utilize a number of anionic functional groups, proteins and enzymes, reducing sugars, etc. in bacterial biomass to reduce interacting metal ions. Raliya et al. (Raliya and Tarafdar 2013) used the fungus Aspergillus fumigates for the biosynthesis of ZnO nanoparticles, which is a low-cost, environmentally benign method for manufacturing monodisperse ZnO nanoparticles. These biologically produced nanoparticles have been discovered to be appropriate for plant nutrition, especially cluster bean nutrition. The application of biologically synthesized ZnO nanoparticles at 10 mg L
1 concentration on 2-week-old plants significantly improved shoot–root growth, chlorophyll, total soluble leaf protein content, rhizospheric microbial population, and P nutrient–mobilizing enzymes. During Alternaria alternata’s extracellular mycosynthesis of γ-Fe2O3 nanoparticles, when the fungal biomass was subjected to an aqueous iron (III) chloride solution, very stable γ-Fe2O3 nanoparticles were formed extracellularly. The effects of these biologically synthesized γ-Fe2O3 nanoparticles on the characteristics of hydroxypropylmethyl cellulose were also studied (Sarkar et al. 2016). The immobilization of Pseudomonas aeruginosa on magnetic-multiwalled carbon nanotubes was used to develop an effective magnetic adsorbent, P.a@Fe2O3-

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MCNT (multiwalled carbon nanotubes), which was characterized by FT-IR, FE-SEM, XRD, BET, and VSM (Yousefi et al. 2020). Salvadori et al. (2015) used the fungus’ dead biomass Hypocrea lixii in an innovative, effective, and sustainable bioprocess for the generation of nickel oxide nanoparticles. In an aqueous medium, the fungus’ dead biomass was effectively employed to convert nickel ions to nickel oxide nanoparticles. Through biosorption these nanoparticles were collected on the cell wall surface both intracellularly and extracellularly. Transmission electron microscopy, high-resolution transmission electron microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were all used to determine the average size, shape, and distribution of the nanoparticles. Araújo et al. (2017) synthesized the Cu2O bacterial cellulose nanocomposite employing hydrothermal deposition of Cu derivatives nanomaterials (i.e., Cu(0) and CuxOy species) on bacterial cellulose (BC) hydrogel membranes, antibacterial cellulosic nanocomposites were produced. The influence of hydrothermal processing time on the final physicochemical characteristics of BC-Cu nanocomposites was investigated using FTIR, SEM, AFM, XRD, and TGA. The XRD results demonstrate that distinct CuxOy phases were formed depending on the heating period (3–48 h). The great effectiveness of microbiological nanocomposite “Paecilomyces lilacinus-silica nanoparticles calcium-alginate beads” (P. lilacinus-SN-Cal-Alg) for eliminating Pb(II) ions in aqueous solution was observed. FT-IR, SEM-EDS, and XPS studies were used to characterize P. lilacinus-SN-Cal-Alg beads before and after Pb (II) adsorption. In an aqueous medium, the adsorption capacity of Pb(II) by P. lilacinus-SN-Cal-Algbeads was investigated (Ruan et al. 2020). Raliya et al. (2015) developed an environmentally favorable and convenient technique for the production of TiO2 nanoparticles utilizing fungi Aspergillus flavus TFR 7, in this method no toxic chemical reagents or surfactant templates were used, allowing the bioprocess to be environmentally benign. Synthetic TiO2 nanoparticles were utilized as a plant nutrient fertilizer to improve agricultural yields. A green methodology was used to synthesize super paramagnetic Fe3O4 nanoparticles employing alga Ulva flexuosa, which offers a biocompatible, safer, less expensive, and nontoxic route to future eco-friendly nanoproduction. A wide range of conventional analyses were used to characterize the synthesis of Fe3O4 nanoparticles. The cubo-spherical and polydispersed nano-sized seaweed/Fe3O4 nanoparticles were obtained, according to the observations (Mashjoor et al. 2018). Table 2 provides information about various microorganism-based nanomaterials.

Removal of Traditional Pollutants Using Metal Oxides–Based Nanoparticles Metal oxide–based nanoparticles have very wide range of applications such as environmental applications, uses in medicine, drug delivery, detectors, beauty products, horticulture, biological sciences, catalysis, light emitters, optics, electronics, food, and other field, because of their well-known distinct features such as conductance, catalytic activity, biocompatibility, high surface area to volume ratio, and

FT-IR, XRD, FE-SEM, TEM, PSA, zeta, TGA, and VSM

12–15

HRTEM and EDX

Green macroalga

_______

Fe3O4

TiO2

SnO2

Gluconacetobacter Hansenii Paecilomyces lilacinus Aspergillus flavus

HRTEM, SEM, XRD, FTIR, and XPS FTIR, SEM, AFM, XRD, and TGA FT-IR, SEM-EDS, and XPS

12.3

25–35

1.2–3.8

________

Fungal biomass

75–650

XRD, FTIR, AFM, SEM, and EDX FT-IR, FE-SEM, XRD, BET, and VSM

Characterizations methods used DLS, HRTEM, SEM, AFM, XRD, and EDS

Average particle size (nm) 1.2–6.8

Alternaria alternate Pseudomonas aeruginosa

Cu2O

Pseudomonas aeruginosa immobilized Fe3O4-multiwalled carbon nanotubes (P.a@Fe3O4-MCNT) NiO

γ-Fe2O3

Nanoparticles ZnO

Microorganisms used Aspergillus fumigatus biomass

Table 2 Microorganism-based green synthesis of metal oxide–based nanomaterials

Cubo-spherical, polydisperse

Spherical

Nanofiber network Spherical

Spherical

Quasi-spherical and rectangular Nanotubes

Shape of nanoparticles Spherical

Salvadori et al. (2015) Araújo et al. (2017) Ruan et al. (2020) Raliya et al. (2015) Mashjoor et al. (2018)

References Raliya and Tarafdar (2013) Sarkar et al. (2016) Yousefi et al. (2020)

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density. The metal oxides synthesized through green methodology are more effective and sustainable because the purpose of green nanoparticle synthesis is to minimize waste and improving environmentally friendly solutions. A greater variety of innovative plant extracts, fruits, microorganisms, and other green constituents are estimated to be employed in the near future to accumulate, stabilize, and synthesize metallic nanoparticles. Green components that have been used for these purposes, particularly those with potential medicinal uses, include fungicides, bactericides, and green components such as sugar and starch. This includes morphological, optical, and structural characteristics in order to accomplish desired applications in sectors like as environmental applications, sensing, cancer diagnostics, imaging, and so on. In order to enhance environmental sustainability, green approaches that employ moderate reaction conditions and nontoxic precursors have been emphasized in the development of nanotechnology (Das et al. 2018). Nanoparticles are frequently employed in environmental sensors, biological diagnostics, and forensic applications. Recognition and a transduction element are the two fundamental components of sensors. The recognition element assists in target binding, whereas the transduction element aids in the analysis and signaling of the binding event. In agriculture, nanoparticles are utilized as nanofertilizers, nanoherbicides, nanocoatings, and nanopesticides (Mujbeer and Tanveer 2014). In the present chapter, we mainly focus on the sustainable environmental applications such as photocatalytic degradation of dyes, removal of organic pollutants, adsorption of heavy metal ions, antimicrobial activities, and CO2 capture from the environment. The environmental applications the metal oxides and metal oxides–based nanomaterials with some recent research papers are summarized as follows. Rani and Shanker (2018) used Azadirachta indica leaves extract to synthesize nanocomposites of ZnO doped with zinc hexacyanoferrate (ZnO@ZnHCF) employing green methodology. The ZnO@ZnHCF nanocomposite was used for the photocatalytic degradation of the endocrine disrupter and carcinogen pollutant bisphenol-A (BPA) under normal conditions of sunlight. Doped ZnO@ZnHCF nanocomposites have a band gap energy of 2.2 eV, which is smaller than ZnO (3.3 eV) and ZnHCF (2.34 eV) nanoparticles. Doping increased the charge isolation of ZnO@ZnHCF nanocomposites, allowing e
and h+ to remain in the conduction and valence bands for a longer period of time, respectively. The degradation was observed to vary depending on the pollutant concentration (BPA) of 2–10 mg L-1, photocatalyst dosage of 5–30 mg, and sample pH of 5–9. At 2 mg L
1 of BPA treated with 25 mg of nanophotocatalyst at pH ¼ 7 in the daytime, the best results were obtained. The maximum degradation was observed for ZnO@ZnHCF (97%) accompanied by ZnHCF (88%) and ZnO (75%) due to the greater surface area of the doped nanocomposite (113.491 m2g
1) than the guardians were (ZnHCF: 108.70 mm2g
1 and ZnO: 12.099 m2g
1). A cost-effective and environmentally friendly green synthesis approach was used to create pure anatase phase TiO2 nanoparticles. XRD, SEM, EDS, and UV-Vis spectroscopy were used to investigate the structural, compositional, and optical characterization of TiO2 nanoparticles, proving their effective fabrication. The fabricated nanostructures were between

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80 and 100 nanometers in size. The nanoparticles’ predicted band gap was 3.22 eV, which was validated by UV-Vis absorption and PL spectroscopy. Within 80 min, more than 90% of Rodhamine B(RhB) was degraded (Nabi et al. 2021). Ramzan et al. (2021) synthesized Cu@TiO2 nanocomposite employing Cedrus deodara extract. The synthesized nanocomposite Cu@TiO2 was used for the photocatalytic degradation of dye and antibacterial activity against bacteria. On compared to un-doped TiO2 nanoparticles, the fabricated Cu@TiO2 nanoparticles displayed excellent photocatalytic and antibacterial activity against pathogenic bacteria. Cu@TiO2 nanoparticles photocatalytically degraded methylene blue by 95% under solar light. Under visible light irradiation, however, it indicated a dye degradation of 73%. Antibacterial activity against E. Coli and S. Aureus demonstrated a considerable zone of inhibition up to 29 mm where 8% Cu@TiO2 was used. ZnO nanostructures were synthesized using a green synthesis process based on garlic bulb extract, which resulted in crystalline nanorods. For comparative solar photocatalytic experiments, ZnO nanoparticles were also chemically synthesized using a conventional coprecipitation method. When exposed to focused sunlight, the greensynthesised ZnO showed a good photocatalytic activity in colloidal solution for the breakdown of the methylene blue (MB) dye. When compared to chemically synthesized ZnO, breakdown rates are nearly identical at 94% at optimum loading conditions. Tariq et al. (2021) used Sonchus Oleraceus, a Chinese medicinal plant as a reducing agent in the fabrication of ZnO@GO nanocomposite. Various spectroscopic and microscopic methods, including as UV-Vis, XRD, FTIR, and TEM, have been used to authenticate the biomedaited ZnO@GO nanocomposite. The ZnO@GO nanocomposite that was produced is biocompatible, causing low toxicity in normal cells while being extremely active in pathogenic cells. When compared to ZnO nanoparticles alone, the ZnO@GO synergistically enhanced its potentials against MDR gram-negative bacteria, killing around 95% of E. coli (BL21 DE3) after 5 h. By breaking the cell membrane and entering into the bacterial cytoplasm, the synthesized nanomaterials kill the bacteria, resulting in cytoplasmic organelle leakage and bacterial cell shrinkage. Biswal et al. (2020) used guava leaves extract to synthesize Ag-doped-Fe2O3 nanocomposite utilizing a green synthesis approach. FESEM was used to analyze the morphology of this produced nanocomposite, which revealed an irregular shape of Fe2O3-Ag nanocomposite with a diameter of 50–90 nm. The synthesized Fe2O3-Ag nanocomposite has a surface area of 112.72 m2/g and an average pore diameter of 3.7 nm, according to BET surface area analysis. The Fe2O3-Ag nanocomposite was then utilized as an adsorbent to decontaminate aqueous media of chromium (VI) ions. The adsorption of Cr (VI) is pH dependent, with maximum adsorption occurring at pH ¼ 4. Fe2O3-Ag has a greater adsorption capacity of 71.34 mg/g, according to the findings of the adsorption experiments. Ahmad et al. (2018) used the green approach to synthesize a chitosan-iron oxide (CS-Fe2O3) nanocomposite. Various instrumental methods, such as SEM/EDX, TEM, AFM, XRD, FTIR, TGA, and BET analysis, were used to characterize the surface morphology of nanostructure. The nanomaterials were also investigated for removing hazardous elements such as Pb (II), Cd (II) in single

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systems, and Pb (II) in binary systems from aqueous solution. The removal efficiency of Pb(II) from electroplating, battery manufacturing, and medical hospital wastewater, respectively, was determined to be 72, 83, and 73%. The removal efficiency of Cd(II) from electroplating, battery manufacturing, and medical wastewater, respectively, was determined to be 79, 86, and 75%. Ruhaimi et al. (2021) used egg-shell membrane-templating method for the synthesis of CeO2 nanoparticles. With an average particle size of 30–34 nm and a tiny crystallite size, this templating procedure helped to improve the morphological features of CeO2. When compared to CeO2 formed by thermal decomposition, using this templating process gives more than double the adsorbent surface area. The CO2 adsorption was carried out using metal oxide nanoparticles that had been synthesized. The enhanced CO2 absorption by CeO2-BT (biotemplating), which was up to 12-fold greater than CeO2TD (thermal decomposition), was due to the overall textural and structural improvement. CeO2-BT (1.412 m mol/g) has good CO2 absorption as a result of these improvements. Wu et al. (2021) employed green technique to successfully manufacture B, N co-doped TiO2 nanocomposite, which was then used for CO2 photoreduction. The nanocomposite has a high specific surface area of 136.5 m2 g
1, a well-developed mesoporous structure, and a large number of surface modification sites. Optical and electrochemical tests reveal that the nanocomposite has a lower band gap energy, elevated CB band edges, and high defect levels, resulting in improved visible light adsorption, longer photogenerated charge carries durability, and rapid electron transfer from the catalyst to the surface-adopted CO2. Under simulated sunshine, photocatalytic CO2 reduction activities in the presence of H2O vapor were monitored using a gas-closed photoreactor. B, N co-doped TiO2 nanocomposite reduced CO2, and demonstrated the greatest CO output of 37.1 μmolg
1 after 4 h of simulated sunshine irradiation. Ramu et al. (2021) used green hydrothermal technique coupled with calcination for the synthesis of CuO/NiO nanoparticles. The effective reduction of hazardous nitrophenols (NP, DNP, and TNP) in aqueous medium was achieved using green produced bimetal oxides (CuO/NiO) nanocomposite. XRD, XPS, FTIR, SEM, and HR-TEM methods were used to investigate the physiochemical characteristics of the produced CuO/NiO NPs. The XRD pattern and SEM morphology of calcined CuO/NiO NPs indicate a higher crystallinity than noncalcined CuO/NiO NPs. The XPS and FTIR data, on the other hand, supported the establishment of metal oxide bonding and bimetal interaction. The HR-TEM pictures showed spherical crystals with a particle size of around 25 nm on average. The CuO/NiO nanocomposite showed excellent photocatalytic activity and performed the NP, DNP, and TNP reduction reactions in 2, 5, and 10 min, respectively. Furthermore, for NP, DNP, and TNP, CuO/NiO NPS had an outstanding kinetic rate constant k value of 1.519, 0.5102, and 0.4601 min1 for NP, DNP, and TNP, respectively. Sadhukhan et al. (2020) produced RGO/NiO nanocomposite in a single-stage in situ coprecipitation process using a green hydrothermal technique. For the synthesis of RGO, Psidium guajava leaf extract was used as green reducing agents. The photocatalytic degradation of MB dye and antibacterial properties of the produced

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RGO/NiO nanocomposites were investigated. In comparison to the primary components NiO NPs (65%) and RGO (negligible percent), the nanocomposites’ photocatalytic degradation activity demonstrates a high ability for MB dye degradation (93%) when exposed to visible light. As a result, RGO/NiO NCs may be an effective photocatalyst for the removal of organic pollutants from wastewater. Synthesized nanostructures had effective antibacterial activity against a variety of microorganisms, including S. typhimurium, L. monocytogenes, S. aureus, E. coli, and K. pneumoniae, with an overall impact that was quite similar to the usual. Bhatia and Nath (2020) used green technique to synthesized NiO/ZnO nanocomposite with various molar concentrations. The synthesised nanoparticles were utilized to adsorb Congo red (CR) dye and reduce the organic contaminant 4-nitrophenol (4-NP). Mixed morphology was seen in FE-SEM and TEM data, comprising spheres, hexagons, and rods, with particle sizes ranging from 15–45 nm. NiO/ZnO (3:2) nanocomposite was shown to be a good adsorbent for Congo red (CR) dye, removing 94.9% of the dye in under 1 h. This photocatalyst can be used up to five cycles. NiO/ZnO (1:1) nanocomposite, on the other hand, catalyzed the conversion of 4-nitrophenol (4-NP, 0.2 mM) to 4-aminophenol (4-AP) in 9 min with a conversion efficiency of 100% for up to three cycles. Mahmoudi et al. (2021) synthesized Cu/Cr/Ni trimetallic oxide nanoparticles utilizing an Echinops persicus plant extract in a simple, biocompatible, cost-effective, and nontoxic approach. Cu/Cr/Ni nanoparticles are more effective than tetracycline as a typical antibiotic against both gram-positive and gram-negative bacteria, including E. coli, S. aureus, and B. cereus. Comparative tests also revealed that the antibacterial activity of Cu/Cr/Ni nanoparticles was significantly more and stronger than that of individual metal oxide nanoparticles generated in a similar manner using Echinops persicus flower extract. At 100 ppm Cu/Cr/Ni nanoparticles, E. coli and S. aureus had growth inhibition zones of 22 and 25 mm, respectively. The MIC values for E. coli, B. cereus, and S. aureus were 0.08, 0.02, and 0.008 mg/ml, respectively, while the MIC values for conventional tetracycline were 0.001, 0.004, and 0.008 mg/ml, respectively. Sharma et al. (Sharma et al. 2019) produced a trimetallic oxide nanocomposite of La/Co/Ni@GO, the photocatalytic activity of which has been thoroughly investigated, and the results show that it may be efficiently used for the photodegradation of 2-chlorophenol. However, La/Co/Ni@GO TNC had superior photocatalytic properties, which were ascribed to its capacity to minimize electron-hole recombination and facilitate charge transfer. Degradation of 2-chlorophenol (Fig. 4) was tested under two photocatalytic performances, with providing higher photocatalytic outcomes 71% degaraded in 300 min. Tariq et al. (2021) synthesized a trimetallic nanocomposite Bi2O3-SrOFeO@SiO2. It was used to remove MB dye from an aqueous solution by utilizing the ability of the Bi2O3-SrO-FeO@SiO2 nanocomposite to behave as an effective adsorbent. The concentrations of MB dye in the solution and the adsorbent dosage have a significant impact on adsorption. The maximum adsorption capacity estimated from the slope of the linear plot of Langmuir data is 294.11 mg/g, showing that the Bi2O3-SrO-FeO@SiO2 nanocomposite has substantial adsorption capability.

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Fig. 4 Degradation mechanism of 2-chlorophenol Sharma et al. (2019) with permission

The reusability analysis demonstrated that the Bi2O3-SrO-FeO@SiO2 nanocomposite has a high adsorption rate even after six repeated cycles. Table 3 summarizes the literature related to remediation of pollutants by metal oxides–based nanomaterials.

Conclusion Many researchers have been conducted recently in order to enhance the quality of drinking water, which is attributable to human safety and health as well as the environment. Pollutants can be successfully removed using nanomaterials with precise physical and chemical characteristics. The notion of nanomaterial manufacturing has been elevated in order to prioritize implementation options. Heavy metals and organic contaminants are best absorbed by metal oxide nanoparticles, which have demonstrated promising results in a variety of applications.

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Table 3 Removal of traditional pollutants using metal oxides–based nanoparticles Type of nanomaterial Application ZnO@ZnHCF Degradation of bisphenol-A (BPA)

TiO2

Degradation of rodhamine B (RhB)

Cu@TiO2

Antibacterial activity and dye degradation

ZnO

Photocatalytic degradation of dye

Zno@GO

Antibacterial activity

Ag-dopedFe2O3 CS-Fe2O3

Adsorption of Cr(VI)

CeO2

CO2 adsorption

Removal of heavy metal ions from industrial waste

B, N co-doped Photo reduction of TiO2 CO2 CuO/NiO

Reduction of toxic nitrophenols

RGO/NiO

Photocatalytic degradation of dye and antibacterial activity

NiO/ZnO

Adsorption of Congo red (CR) dye and reduction of 4-nitrophenol (4-NP) organic pollutant

Size and shape Findings ˂100 nm; Optimum loading distorted cubic of catalyst dosage is 25 mg at neutral pH, 97% of BPA was degraded 80–100 nm; Within 80 min, spherical more than 90% of RhB was degraded ~10 nm; 95% of MB was tetragonal degraded under solar light, inhibition up to 29 mm of E. Coli and S. Aureus bacteria 30–40 nm; 94% of MB was nanorods degraded at optimum loading conditions 5 nm; trigonal Killing about 95% toxic bacteria within 5 h 50–90 nm; 71.34 mg/g of Cr (VI) irregular shape was adsorbed 1.52–4.12 nm; Maximum removal Porous and was found in battery cross-linked manufacturing network waste Pb(II) 83% and Cr (VI) 86% 30–34 nm; 1.412 mmol/g CO2 was adsorbed using Spherical CeO2 nanoparticles ~36 nm; CO2 reduced to Rectangular gives, CO output of nanosheet 37.1 μmolg
1 25 nm; NP, DNP, and TNP Spherical reduced within 2, 5, and 10 min, respectively 4–10 nm; 93% of MB dye Oval shaped was degraded and strong antibacterial activity against the selected microbes 15–45 nm; 94.4% of CR dye Spheres, adsorbed in 1 hr. hexagons and and 100% of 4-NP rods was reduced to 4-aminophenol with in 9 min

References Rani and Shanker (2018)

Nabi et al. (2021)

Ramzan et al. (2021)

El Golli et al. (2021)

Tariq et al. (2021) Biswal et al. (2020) Ahmad and Mirza (2018)

Ruhaimi and Ab Aziz (2021) Wu et al. (2021)

Ramu et al. (2021)

Sadhukhan et al. (2020)

Bhatia and Nath (2020)

(continued)

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Table 3 (continued) Type of nanomaterial Cu/Cr/Ni oxide

Application Size and shape Antibacterial activity 20 nm; against E. coli, Spherical B. cereus, and S. aureus

Removal of MB dye Bi2O3-SrOFeO@SiO2 La/co/Ni@GO Photodegradation of 2-chlorophenol

Tetragonal structure Spherical

Findings MIC values for E. coli, B. cereus, and S. aureus were 0.08, 0.02, and 0.008 mg/ml, respectively 294.11 mg/g of MB was removed 71% of 2-chlorophenol was degaraded in 300 min

References Mahmoudi et al. (2021)

Tariq et al. (2021) Sharma et al. (2019)

Their physical and chemical features have contributed to their effectiveness, although their use in wastewater treatment remains restricted. In this chapter, general introduction of metal oxides and their properties, classification of metal oxides as monometallic oxides, bimetallic oxides and trimetallic oxides, synthesis of metal oxides and metal oxides–based nanomaterials using green methodology, and finally the removal of traditional pollutants using metal oxides–based nanoparticles are summarized.

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Environmental Occurrence and Degradation of Hexabromocyclododecanes Manviri Rani, Meenu, and Uma Shanker

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of HBCD Isomers and Physicochemical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production and Application of HBCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possible Emission Sources and Environmental Concerns of HBCDs . . . . . . . . . . . . . . . . . . . . . . . . . Release During Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Release During Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Release at the Time of Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Risk Management and Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of HBCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of HBCDs Using a Microorganism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Degradation of HBCDs Using Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Upcoming Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Hexabromocyclododecanes (HBCDs) are used as a fire extinguisher additive mainly for constructing buildings composed of extruded or expanded polystyrene foam. The synthetic profile of HBCD diastereoisomers, on average, was 28%,

M. Rani (*) · Meenu Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Rajasthan, India e-mail: [email protected]; [email protected] U. Shanker Polymer and Nanomaterials Synthesis Laboratory, Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_81

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13%, and 59% for α-, β-, and γ-HBCD, respectively. Due to persistence, longdistance transportation, biodiversity, and environmental toxicity, cycloaliphatic brominated flame retardant, HBCD (1,2,5,6,9,10-hexabromocyclododecane, C12H18Br6), is a global concern and was selected as a type of persistent organic pollutant (POP) under the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2013. HBCD is a high-volume production brominated flame retardant (BFR) that has been raising environmental concerns and public health issues due to its potential environmental persistence, environmental accumulation, and toxicity. The concentration of HBCD is discharged from various ways, including during production, manufacturing, customer use, waste disposal, landfilling, incineration, and recycling. HBCD is persistent in the air and further migrates up to long-range distances such as the Arctic, where concentrations in the atmosphere and top predators are elevated. HBCD has been found in human plasma, blood, and adipose tissue and at higher trophic levels in biota. HBCD is toxic to aquatic organisms and alters their reproductivity and development and causes deterioration of the central nervous system. Various conventional techniques are used for adsorption and removal of HBCD concentration. However, bioremediation of HBCD by using microorganisms and nanomaterials is a more prominent and advanced method that degrades HBCD into safer and less toxic end products by utilizing natural energy sources. Keywords

Flame retardants · Hexabromocyclododecanes · Persistent organic pollutants · Risk assessment

Introduction Brominated flame retardants (BFRs) are widely used in consumer products for fire safety purposes. BFRs are used mainly in construction materials, plastic composites, and fabrics to shrink the threat of accidental fires (BSEF 2005). BFRs consist of several chemical groups, most commonly tetrabromobisphenol A (TBBPA), polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE), and hexabromocyclododecane (HBCD). HBCD has been produced since the 1960s and is the third most widely used brominated flame retardant (BFR) globally (Janak et al. 2005). HBCD is primarily used (above 95%) in building commerce, where it is typically incorporated at 10,000 >20,000 >202 40.5  0.5 1024 0.59 3.9–4.3 3.7–6.1 0.3–2.2 63 (aerobic, 20  C) 6.9 (anaerobic, 20  C)

Log Pow partition function, Log Kow octanol/water partition coefficient, LD50 lethal dose 50, LOD limit of detection, LOQ limit of quantification, BCF bioconcentration factor, BAF bioaccumulation factor, BMF biomagnification factor, TMF trophic magnification factor, CTD characteristic travel distance (distance at which air concentration is 1/e of initial value)

Production and Application of HBCDs The production market of HBCD has been on since the 1960s internationally (Xiang et al. 2015). Europe, China, Japan, and the United States hold the main production market share. Estimated international usage of HBCD in 1999 was at 15,900 metric tons, with approx. 56% market share by Europe only (Morris et al. 2004). According to demand and supply internationally in 2001, 9500 tons out of 16,500 tons (more than half) of production are by European industry.

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Table 2 Production, composition, and end use of HBCD worldwide

Material EPS (foam), XPS

Polymer matrices for cotton or cotton containing textile HIPS

% of total volume 85

Amount (tons/ year) 10,000

10

1000

5

600

Application Insulation material, “sandwich constructions” Mixtures Back-coating of textiles Plastics for electronic devices

Example of products Building material, plastic wrappings Upholstery, mattresses VCR covers

EPS expanded polystyrene, HIPS high-impact polystyrene, PS polystyrene, XPS extruded polystyrene

The international demand for HBCD increased by more than 28% to 21,447 tons in 2002 and slightly increased to 21,951 tons in 2003 (BSEF 2006). Several goods are manufactured and imported in the United States. HBCD was reported to have dropped from 4540 tons in 2005 to 22,900 tons (Stapleton et al. 2006). In 2006, the total number of HBCD used in the European Union was estimated at 11,580 tons. Demand for HBCD in the EU has exceeded production, and the EU’s total export capacity was estimated at 6000 tons in 2006 (Chen et al. 2011; Yi et al. 2016). According to Japanese officials, the total production and importation of HBCD were 2844 tons in 2008 and 2613 tons in 2009. Canada (100–1000 tons), Australia (95%). The flue gas is composed of water vapor and concentrated CO2, which are easily separated via cooling process as water gets condensed and rich carbon dioxide gas stream is found. This method has the ability to eliminate up to 100% carbon dioxide from the flue gas. The major disadvantage of this technique is that it causes energy penalty by separating oxygen from the air. Therefore, this method consumes an additional significant quantity of energy by giving less energy output (electricity) named as energy penalty which increases the cost of power generation. The other technique termed as chemical looping combustion which is under evolution can eliminate oxygen easily from the air by indirect method. In this, combustion of fuel occurs without coming into direct contact with air. Therefore, transfer of oxygen takes place between fuel and air by means of oxygen carrier and a stream of carbon dioxide can be produced by condensation of water vapors (see Fig. 2b). Various fuel gases (syngas, natural gas, etc.) can be employed in this method to capture carbon dioxide and replacing other carbon dioxide capturing techniques that utilize the costly solvent.

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Post-combustion Carbon Dioxide Capture This process occurs after combustion of fuel and air to produce electricity as shown in Fig. 2c. The advantage of this technique is that it can be reconstructed for existing plants without any significant modifications. It is involved in methods like chemical absorption, membrane, mineral carbonation process and pressure swing absorption (Wang et al. 2017a). Absorption Technique In this method, CO2 is absorbed chemically from flue gas into an amine solution (monoethanolamine (MEA)) which acts as an absorbent. The CO2-containing MEA is passed through the stripper for the recovery of solvent and separation of CO2 from MEA. On commercial scale, chemical absorption process is employed in most of the electricity generating plants. This process occurs kinetically faster due to the chemical bond formation between alkaline solvent and carbon dioxide gas. Chemical absorption is preferred at partial pressure < 3.5 bar while physical absorption is preferred at high pressure and at a large concentration of carbon dioxide. However, physical absorption is still under primary development although it requires lesser energy for solvent regeneration than chemical absorption because physical absorption involves weak interactions between solvent and CO2 (Leung et al. 2014). Membrane Technique This process uses a membrane based on ceramic and polymer in order to take out carbon dioxide gas from flue gas which is kept at constant temperature using a water bath (Miguez et al. 2018). The selection of membrane material and solvent plays a significant role. Mostly, membranes based on polymer promoted transport membrane, Pd membrane based on alloy, and molecular sieve membrane are employed to separate carbon dioxide. The term mass transfer coefficient plays a crucial role in membrane technology. It mainly consists of three paths: (1) diffusion of gas into the wall of membranes, (2) diffusion of liquid membrane via pores, and (3) eventual dissolution onto the liquid adsorbent. It must be noted that in this process, a composition having amino acid solution and piperazine shows better results instead of only an amino acid solution. The efficiency of carbon dioxide recovery by this process is almost 90% (Lu et al. 2009). In this process, carbon dioxide in the gaseous mixture is dispersed through the pores of a membrane into the liquid shell which is absorbed by the absorbent. However, this process involves costly equipment and requires high energy. Pressure Swing Adsorption (PSA) Technique This technique is reported in US Patent by Skarstrom in 1960. In this method, a gas mixture is delivered to the absorber and then the gas having low affinity with adsorbent crosses the bed and collected at closed end while the gas with high affinity gets readily adsorbed into the adsorbent bed. By reducing the pressure, the gas adsorbed is eliminated from adsorbent. In this process, the choice of adsorbent plays a significant role for gas adsorption. It is concluded that zeolite 13X is an effective

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adsorbent for carbon dioxide due to its good working capacity, lesser requirements for purging, and larger equilibrium selectivity over activated carbon (Chue et al. 1995). Nowadays, zeolite 13X is employed in lime and steel industry to lower the carbon dioxide from flue steam. Mineral Carbonation Technique In this technique, carbon dioxide obtained from number of sources reacts with magnesium oxide and calcium oxide to form an exceptionally stable natural carbonated solid product. Due to the stability, this product has a large storage capacity and can easily be stored at environmental suitable conditions. Overall, this technique involves the conversion of carbon dioxide into geological naturally stable carbonates. The chemical equations include in this method are illustrated below: CaO þ CO2 ! CaCO3 þ 179 kJ=mol MgO þ CO2 ! MgCO3 þ 118 kJ=mol Instead of CCS techniques, there are also some other techniques like carbon dioxide removal (CDR) technology and solar radiation management (SRM) technology that address the elimination carbon dioxide from the environment. However, these techniques are not so popular and efficient like CCS technology.

Materials for CO2 Adsorption Zeolites The conventional example of porous materials is zeolites, which act as physical adsorbents. Zeolites are crystalline, three-dimensional hydrated alkali or alkalineearth aluminosilicates having a general formula Mn + x/n[(AlO2)x(SiO2)y]x.wH2O [w ¼ 24–26] (M ¼ alkali or alkaline- earth metal). Their framework constructed from TO4 tetrahedral (T ¼ Al, Si) unit sharing the corners displays interconnected cages in which molecules of water and metal ions are encapsulated. Upon removal of such water molecules, these materials show their porosity. Table 1 summarizes the Table 1 Zeolites as adsorbent for CO2 adsorption S. No. Zeolites 1 Zeolite 13X

Surface area (m2g1) 585.5

CO2 Uptake (cm3/g) 3

Stability in moisture –

2

Zeolite A5

499

3

–

3

ZSM-5

–

5.2

–

4

Zeolite 13X-APG

–

4.3

–

References Dantas et al. (2011) Wang et al. (2012) Hefti et al. (2015) Wang et al. (2012)

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selected examples of zeolites acting as CO2 adsorbents. Zeolite 13X is widely used in the PSA technique as mentioned above (Wang et al. 2012). The other promising candidate is synthetic zeolite 5A to capture carbon dioxide from the mixture of flue gas. In order to have effective adsorption, the ratio of SiO2/Al2O3 should be low, and there must be the presence of cation that led to form strong electrostatic interactions between zeolites and carbon dioxide. For their use through the PSA technique, these require significant energy for regeneration, which is one of their disadvantages. However, these porous materials can be modified by including alkyl amines to their inside surfaces that increases gas adsorption even at low pressure. On the other hand, the amine grafted porous material shows better adsorption than non-grafted ones but their stability decreases with repeated cycles. Therefore, it is necessary to introduce an appropriate amount of grafted amine in order to capture CO2.

Metal-Organic Frameworks (MOFs) The term MOFs was first introduced by Omar Yaghi and his coworkers in 1995. These are porous hybrid materials synthesized from metal (either transition metal or lanthanides metal ions) which act as a node that connects through the organic ligand. Depending upon the coordination mode of the organic ligand and geometry of metal ion, 2D and 3D MOFs can be synthesized. The pore size of MOFs can be tuned by changing the length and functionality of a linker. Furthermore, the exceptional porosity due to their regular structure is not found in other porous materials like zeolites and carbon black. A good example of controlling pore size is ZIFs (zeolitic imidazole framework) in which zinc metal is connected to the functionalized imidazolate linker (C3N2H3 ¼ Im) varying from polar functional group (-NO2, CN) to nonpolar (-CH3). In the isoreticular series of eight ZIFs, the desired pore size is obtained just by increasing the size and functionality of ligand with similar geometries or symmetry. The pore size in these MOFs varies from 7.1 to 15.9 Å (Banerjee et al. 2009). ZIF-100 is also zinc-based MOF formulated as Zn20(cbIm)39(OH) with a pore size of 35.6 Å. It exhibits excellent adsorption due to the formation of a unique large cage. The BET surface area of this MOF is 595 m2/g with CO2 uptake capacity of 2.6 cm3/g at 273 K (Wang et al. 2008). The unique property of MOFs is their extraordinary internal surface area and fascinating network topologies. A MOF based on zinc with terephthalate (MOF-2) exhibits permanent porosity and therefore has numerous adsorption sites. This layered MOF was the first evidence of porosity by studying its CO2 and nitrogen isotherm. It exhibits 310 m2/g BET surface area with 0.086 cm3/g of pore volume at 780 torr and 196 K (Li et al. 1998). A MOF of copper and benzene-1,3,5-tricarboxylate also named as HKUST-1 exhibit a great carbon dioxide uptake capacity of 7.3 mmol/g (Zheng et al. 2018). The flexible MOFs are also referred as breathing MOFs due to their ability to show both adsorption and desorption properties. These MOFs have the ability to move their framework and show stepwise desorption for carbon dioxide and other gases. Therefore, these are excellent material for carbon capture and

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Fig. 3 3D structures of some common MOFs (Xiang et al. 2010). (Copyright 2021 reprinted with permission from the Royal Society of Chemistry)

sequestration. The flexibility can be modified by an addition of alkyl groups to the chain length. These MOFs show enhanced adsorption even at low pressure. In the last two decades, a lot of efforts have been made to develop new MOFs, which show significant adsorption of carbon dioxide. In 2005, the researchers constructed a MOF-177 with carbon dioxide adsorption capacity of 1470 mg/g at 35 bar pressure at room temperature. The research in this field gradually increased, and till 2012, there are 37,241 structures of MOFs for efficient capturing of carbon dioxide on the basis of Cambridge Structure Database, 2014. Few structures of common MOFs are shown in Fig. 3, and examples of selected MOFs for CO2 adsorption are listed in Table 2. For the utility of MOFs for applications like gas adsorption, one of the most important features, which must be considered, is their chemical (particularly towards water) and thermal stability. Many MOFs are found to be chemically and thermally stable. Their chemical stability can be determined experimentally by comparing the PXRD data of a MOF before and after introducing it into pH solution. For example, MIL-101(Cr) exhibits exceptional stability even in highly acidic medium (pH ¼ 0–4) while there is no change in the PXRD pattern after its exposure up to 2 months (Leus et al. 2016). With the introduction of an inert metal ion in the structure of a MOF, an increase in its chemical stability is observed (Mutyala et al. 2019). In order to consider the stability of MIL-53(Cr), MIL-47(V), and MIL-53 (Al), the first two MOFs are soaked into 0.07 M HCl solution where it was found that the MIL-47(V) dissolves in 6 h into the acid solution while the MIL-53(Cr) maintains its structure (Kang et al. 2011). This is corroborated with the fact that chromium is more inert than vanadium. Overall, the order of their stability is MIL-53

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Table 2 MOFs as adsorbent material to capture carbon dioxide

S No. 1

MOF Nickel3O-MOF

BET surface area (metre2/ gram) 743.5

Carbon dioxide uptake (cm3/gram) 34.6

2

Nickel-MOF-1

658.49

37.57

3

MIL-101 (Chromium)

3324

4

JLU-MOF 58

3663

2.52 millimol/ gram at 303 K 49 at 273 K

5

138

65

6

{[Zinc2(TBIB)2(HTCP B)2].9DMF.19H2O}n Europium- MOF

209.7

44.9 at 273 K

7

Binary UiO- 66/copper-BTC

693

31.81

8

838.94

30.3

9

MOF-801 incorporated PEBA MMM NKU-521a

1100

86 at 298 K

10

IRMOF series-1

2833

11.1

11

IRMOF series-3

2160

10.3

12

IRMOF series-6

2516

10.5

13

IRMOF series-11

2096

8.9

14

HKUST-1

1781

7.3

15

MOF-505

1547

0.7

16

UPG-1

410

8

17

NU-111

4932

61.8

18

Utsa-62a

2190

43.7

19

Basolite ® Cu 300

1706.42

41.9

20

Basolite ® Fe300

1716.46

24.1

Reference Yang et al. (2019) Wang et al. (2019) Mutyala et al. (2019) Yang et al. (2010) Sun et al. (2019b) Agarwal et al. (2019) Wang et al. (2017b) Liao et al. (2019) Kurisingal et al. (2019) Sun et al. (2019a) Li et al. (2019) Millward and Yaghi (2005) Millward and Yaghi (2005) Millward and Yaghi (2005) Millward and Yaghi (2005) Millward and Yaghi (2005) Millward and Yaghi (2005) Taddei et al. (2014) Peng et al. (2013) He et al. (2013) Deniz et al. (2013) Deniz et al. (2013) (continued)

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Table 2 (continued) BET surface area (metre2/ gram) 1524.8

Carbon dioxide uptake (cm3/gram) 26.9

S No. 21

MOF Basolite ® Al100

22

1980

38.1

23

Zinc2(BDC-X)2(DABCO) (BDC ¼ 1,4-benzenedi carboxylate, X ¼ functional group; DABCO ¼ diazabi-cyclooctane) Zinc2(BDC- DM1/2)2(DABCO)

1500

27.5

24

Zinc2(BDC-Br)2(DABCO)

1320

24.3

25

Zinc2(BDC-NO2)2(DABCO)

1310

32

26

Zinc2(BDC-TM1/2)2(DABCO)

1210

23.9

27

Zinc2(BDC-Tf)2(DABCO)

1210

16.2

28

Zinc2(BDC-Cl)2(DABCO)

1180

26.4

29

Zinc2(BDC-OH)2(DABCO)

1130

24.8

30

Zinc2(BDC-DM)2(DABCO)

1120

25.4

31

Zinc2(BDC-TM)2(DABCO)

1050

23.6

32

Zinc2(BDC-A)2(DABCO)

760

17.1

33

SIFSIX-3-zinc

–

8.9

34

SIFSIX-3-copper

–

9.6

35

SIFSIX-3-cobalt

223

10

36

SIFSIX-3-nickel

368

10.4

37

UTSA-49

710.5

13.6

38

ZJNU-40

2209

16.4

40

Zinc-DABCO

1870

7.2

41

Nickel-DABCO

2120

8.1

42

Copper-DABCO

1616

6.2

Reference Deniz et al. (2013) Burtch et al. (2013)

Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Burtch et al. (2013) Thallapally et al. (2015) Thallapally et al. (2015) Thallapally et al. (2015) Thallapally et al. (2015) Xiong et al. (2014) Song et al. (2014) Chaemchuen et al. (2015) Chaemchuen et al. (2015) Chaemchuen et al. (2015) (continued)

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Table 2 (continued)

S No. 43

MOF Cobalt-DABCO

BET surface area (metre2/ gram) 2022

Carbon dioxide uptake (cm3/gram) 4.1

44

ZincAcBPDC

920

11.7

45

ZincBuBPDC

850

7.6

46

1415.1

25

47

Magnesium/dobdc(dobdc ¼ 2,5dioxide-1,4-benzenedicarboxylic acid Copper/dobdc

1089.3

21.6

48

Nickel/dobdc

1017.5

20.5

49

{Ag3[Ag5(l3–3,5-Ph2tz)6](NO3)2}n

–

1.6

50

{Ag3[Ag5(l3–3,5-tBu2tz)6](BF4)2}n

–

1.6

51

CopperBTTri

1700

10.8

52

pip-CuBTTri

380

7.1

53

CPM-33a

966

12.6

54

CPM-33b

808

19.9

55

Nickel3OH(NH2bdc)3tpt

805

14.8

56

Nickel3OH(1,4-ndc)3tpt

222

4.6

57

Nickel3OH(2,6-ndc)3tpt

1002

7.9

58

Nickel3OH(bpdc)3tpt

724

5.5

59

Zinc(pyrz)2(SiF6)

–

10.8

60

Magnesium2(dobpdc)

1940

23.8

61

Nickel2(dobpdc)

1593

21.2

62

MMEN-Magnesium2(dobpdc)

15.8

Reference Chaemchuen et al. (2015) Keceli et al. 2014 Keceli et al. (2014) Li et al. (2014a) Li et al. (2014a) Li et al. (2014a) Xiang et al. (2015) Xiang et al. (2015) Das and D’Alessandro (2014) Das and D’Alessandro (2014) Zhao et al. (2014b) Zhao et al. (2014b) Zhao et al. (2014a) Zhao et al. (2014a) Zhao et al. (2014a) Zhao et al. (2014a) Mason et al. (2015) Mason et al. (2015) Mason et al. (2015) Mason et al. (2015) (continued)

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Table 2 (continued)

S No. 63

MOF mmen-Nickel2(dobpdc)

BET surface area (metre2/ gram) –

64

mmen-CopperBTTri

–

Carbon dioxide uptake (cm3/gram) 7.3 11.3

Reference Mason et al. (2015) Mason et al. (2015)

COOH

+ Metal ions

id Ac e bas or

Energy

Analogous Me-BDCs

COOH

Chemical Stability Cr >AI>V

Time

Fig. 4 Stability of MOFs under acidic/basic conditions based on metal ion inertness in Me-BDCs (Younas et al. 2020). (Copyright 2021 reprinted with permission from Elsevier)

(Cr) > MIL-53(Al) > MIL-47(V). MOFs having Rh(III) or Cr(III) metal ion are highly stable (see Fig. 4). TGA curves of MIL-101(Cr) are studied before and after introducing into 1 M hydrochloric acid solution. It is concluded that before treating with acid, the first decomposition peak shows at 250–260  C. However, after exposure to acid, the first decomposition curve at 250  C disappears which clearly illustrates that the free organic ligand gets eliminated from the pores (Leus et al. 2016). It must be noted that the carbon dioxide adsorption at low pressure is significantly enhanced by including polar functional groups. This can be explained by taking the example of UiO-66. The various functional groups in UiO-66 such as –OCH3, -NO2, and –NH2 groups are responsible to increase the carbon dioxide adsorption interactions even at low pressure (Burtch et al. 2013). The MOFs based on Zr metal ion [UiO-66 (Zr)(COOLi), UiF-66(Zr)- (COOH)2, etc.] maintain their crystallinity as well as porosity at a temperature above 500  C with good adsorption capacity (Canivet et al. 2014). MOFs are employed to adsorb carbon dioxide from flue gases and various other sources of carbon dioxide. It is also significant to desorb CO2 and further regeneration of adsorbent MOFs. Ben-Mansour and Qasem investigated the temperature swing adsorption technique to separate CO2 in four cycles (feeding, rinsing, heating, and cooling) as shown below in Fig. 5.

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Fig. 5 Schematic representation of TSA process involved in MOFs (Ben-Mansour and Qasem 2018). (Copyright 2021 reprinted with permission from Elsevier)

Fig. 6 Representation of open Mg site via solvent exchange route (Zheng et al. 2018). (Copyright 2021 reprinted under the terms of Creative Commons Attribution 3.0 license)

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At first, the mixture of gases (CO2/N2) is passed into the Mg-MOF-74 adsorbent bed at a particular temperature and pressure for a certain period of time. In the mixture of CO2 and N2, the pure carbon dioxide is included in order to rinse nitrogen from the adsorbent bed. Now, for desorption of carbon dioxide and regeneration process, heat is essential to rise the temperature of adsorbent bed; then for the next cycle of adsorption, it is necessary to decrease the temperature by cooling process (see Fig. 6). This study provides evidence that the structural modification of MOF-74(Co)-Pd and MOF-74(Ni)-Pd exhibits higher uptake of CO2 in comparison to their pristine structures in MOF-74(Co) and MOF-74(Ni) at normal conditions. Although at a high pressure of 32 bar and 298 K temperature, MOF-74(Co)-Pd and MOF-74(Ni)-Pd exhibit 11.42 and 12.24 mmol/g than 10.28 and 11.06 mmol/g in the case of MOF-74-Co and MOF-74-Ni. This increase in carbon dioxide adsorption and separation is due to the interaction between the partial positively charged Pd and partial negatively charged oxygen atom of polar carbon dioxide group. Wang et al. prepared a MOF based on nickel metal having a trinuclear nickel cluster with CO2 uptake of 37.57 cm3/g and also possessing greater selectivity towards carbon dioxide over N2 which is ~42.89 at standard temperature conditions (Wang et al. 2019). Liao et al. prepared a lanthanide-based MOF, i.e., Eu-MOF, which was employed in selective adsorption of CO2 upto 109.4 at standard temperature and pressure conditions (W-M Liao et al. 2019). The other Zr-based MOF is JLU-MOF-58 formulated as [Zr6O4(OH)8(H2O)4(TADBA)4].24DMF.45H2O (DMF¼N,N0 -dimethyl formamide, H2TADBA ¼ 4,40 -(2H-1,2,4-triazole-3,5-diyl)dibenzoic acid) which possesses two kind of cages. The one is an octahedral cage with pore size 2.76 nm and the other is a cuboctahedral cage with pore size 4.10 nm. This complex shows exceptional stability in acid as well as in base. This MOF can be treated as heterogeneous catalyst for carbon dioxide conversion with remarkable efficiency and great recyclability. Therefore, this MOF has good utilization for carbon dioxide fixation in the treatment of global climate (Sun et al. 2019a). A desolvated form of 2D tetragonal MOF {[Zn2(tbib)2(HTCPB)2].9DMF.19H2O}n (where tbib ¼ 1,3,5-tri(1H-benzo[d] imidazol-1-yl) benzene and H3TCPB ¼ 1,3,5-tris(40 -carboxyphenyl)benzene) exhibits a carbon dioxide uptake of 65 cm3g1 at low temperature (195 K) and low pressure (1 bar). This MOF also acts as a heterogeneous catalyst due to the presence of carboxyl group, keto group, and uncoordinated nitrogen atom on the pores, which proved to be a promising candidate in catalytic reactions (Agarwal et al. 2019). The presence of K+ binding sites in NKU-521a exhibits great affinity towards carbon dioxide in CO2/N2 and CO2/CH4 separation. It displays significant carbon dioxide uptake than methane and nitrogen. The uptake of carbon dioxide is nearly 7 times greater than methane at standard temperature (Li et al. 2019). The MOF UPG-1 exhibits good adsorption affinity towards carbon dioxide and n-butane. This porous compound display Langmuir and BET surface areas of 514 m2g1 and 410 m2g1, respectively. Overall, the maximum possible CO2 uptake capacity is of 5 mmolg1. This is due to its elongated morphology like a prismatic rod having size range 0.5–10 μm. The estimated pore volume of this MOF is 0.2 cm3g1 (Taddei et al. 2014).

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NU-111 exhibits very high adsorption at ~65 bars and displays the BET surface area of 4932 m2g1 with a pore volume of 2.09 ccg1 (Peng et al. 2013). The lanthanide-based MOF given as UTSA-62a have the potential for carbon dioxidemethane-hydrogen gas separation with good productivity and low cost required for regeneration (He et al. 2013). Commercial Basolite ® is one of the promising candidates for carbon dioxide capture and storage. Basolite ® C300 exhibits a significant rise in CO2 capture capacity greater than 25 bars on the basis of DFT calculations. The order of adsorption properties of these three MOFs is C300 > F300 – A100. Out of these three MOFs, Basolite ® C300 has appropriate properties for CO2 adsorption (Deniz et al. 2013). DMOF-1 [Zn2(BDC)2(DABCO)] pillared structure exhibits good CO2 adsorption selectivity over methane, CO, and N2. This illustrates that the presence of the methyl group provides a significant increase in carbon dioxide selectivity in comparison to other functional groups (Burtch et al. 2013). MOFs M/DOBDC (where M ¼ Ni, Mg, or Co) exhibit good separation efficiency of Carbon dioxide/methane as well as methane/nitrogen mixture relative to activated carbon and MIL-100(Cr). Also, the Mg/DOBDC displays excellent efficiency to separate carbon dioxide from CO2/CH4 mixture and its regeneration (Li et al. 2014a).

Porous Organic Polymers (POPs) Porous organic polymers (POPs) are one of the fascinating porous materials which are employed for carbon dioxide adsorption due to their tunable surface morphology as well as pore structure, high surface area, good physical and chemical stability, low density, and tailored structure modified with amine or polar functional moieties (Tan and Tan 2017). However, the attractive properties such as specific size of pore, high surface area, and appropriate functionalization of these materials have been extensively utilized for carbon dioxide adsorption. POPs have a specific large surface area due to their extended three-dimensional network structure of the organic building block which causes irregularity of pores. Various reactions such as Friedel-Craft reactions, Schiff-base condensation, thermal/solvothermal condensation, diazocoupling, and free-radical polymerization are employed to construct POP materials with a diverse network (Singh et al. 2020). POPs containing iron(III) (FePOPs) exhibit high carbon dioxide uptake capacity (19.0 wt%) at standard temperature and pressure (273 K and 1.0 bar) conditions (Dawson et al. 2012).

Covalent Organic Frameworks (COFs) COFs are formed by lightweight components (hydrogen, boron, carbon, oxygen, and nitrogen) connected by covalent bonds with uniform and tunable pore size. Various ligands or linkers are employed to design 2D or 3D framework structure. In these frameworks, the carbon dioxide uptake capacity can be increased by including a functional group with a nitrogen atom (azine, imine, and amine). Various chemical

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Table 3 Examples of COFs as adsorbent materials to capture CO2

S. No. 1

List of COFs IPB- 2H

2

COP (NUT-4 – NUT-1)

3

IPB- 1H

Adsorption conditions (pressure/ temp.) 1 bar/25  C (pure CO2) 1 bar/0  C

CO2 uptake capacity 0.902 mmol/g 0.57–1.87 mmol/g

5

1 bar/25  C (pure CO2) TBICOF 1 bar/77  C, also reported at 0  C and 25  C COP (COP-22 – COP-17) 1 bar/25  C

6

TRITER-1

1 bar/25  C

1 mmol/g

7

ACOF-1

1 bar/25  C

2.07 mmol/g

8

COF series-1

55 bar/25  C

230 mg/g

9

COF series-5

55 bar/25  C

870 mg/g

10

COF series-6

55 bar/25  C

310 mg/g

11

COF series-8

55 bar/25  C

630 mg/g

12

COF series-10

55 bar/25  C

1010 mg/g

13

COF series-102

55 bar/25  C

1200 mg/g

14

COF series-103

55 bar/25  C

1190 mg/g

15

CTF-1

1 bar/0  C

108 mg/g

16

CTF-1-600

1 bar/0  C

168 mg/g

17

FCTF-1

1 bar/0  C

205 mg/g

18

FCTF-1-600

1 bar/0  C

243 mg/g

4

0.767 mmol/g 377.14 cm3/g

1.30–1.70 mmol/g

References Apriliyantoa et al. (2020) Sun et al. (2015) Apriliyantoa et al. (2020) Das and Mandal (2019) Xiang et al. (2015) Gomes et al. (2015) Li et al. (2014b) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Furukawa and Yaghi (2009) Zhao et al. (2013) Zhao et al. (2013) Zhao et al. (2013) Zhao et al. (2013) (continued)

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Table 3 (continued)

S. No. 19

List of COFs CTF-CSU1

Adsorption conditions (pressure/ CO2 uptake temp.) capacity 151 mg/g 1 bar/0  C

20

CTF-CSU19

1 bar/0  C

129 mg/g

21

cCTF-400

1 bar/0  C

126 mg/g

22

cCTF- 450

1 bar/0  C

99 mg/g

23

cCTF- 500

1 bar/0  C

133 mg/g

24

TpPa-1

1 bar/0  C

153 mg/g

25

TpPa-2

1 bar/0  C

127 mg/g

26

COF- JLU6

1 bar/0  C

129 mg/g

27

COF- JLU7

1 bar/0  C

151 mg/g

28

[HO]25%-H2PCOF

1 bar/0  C

54 mg/g

29

[HO]50%-H2PCOF

1 bar/0  C

46 mg/g

30

[HO]75%-H2PCOF

1 bar/0  C

52 mg/g

31

[HO]100%-H2PCOF

1 bar/0  C

63 mg/g

32

[H2OC]25%- H2P-COF

1 bar/0  C

96 mg/g

33

[H2OC]50%- H2P-COF

1 bar/0  C

134 mg/g

34

[H2OC]75%-H2P-COF

1 bar/0  C

157 mg/g

35

[H2OC]100%-H2P-COF

1 bar/0  C

174 mg/g

36

[C  C]0%-H2P-COF

1 bar/0  C

72 mg/g

37

[C  C]25%-H2P-COF

1 bar/0  C

54 mg/g

38

[C  C]50%-H2P-COF

1 bar/0  C

48 mg/g

39

[C  C]75%-H2P-COF

1 bar/0  C

43 mg/g

References Yu et al. (2018) Yu et al. (2018) Buyukcakir et al. (2017) Buyukcakir et al. (2017) Buyukcakir et al. (2017) Kandambeth et al. (2012) Kandambeth et al. (2012) Zhi et al. (2018) Zhi et al. (2018) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015a) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) (continued)

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Table 3 (continued)

S. No. 40

List of COFs [C  C]100%-H2P-COF

Adsorption conditions (pressure/ CO2 uptake temp.) capacity 1 bar/0  C 39 mg/g

41

[Et]25%-H2PCOF

1 bar/0  C

55 mg/g

42

[Et]50%-H2PCOF

1 bar/0  C

46 mg/g

43

[Et]75%-H2PCOF

1 bar/0  C

41 mg/g

44

[Et]100%-H2PCOF

1 bar/0  C

38 mg/g

45

[MeOAc]25%-H2P-COF

1 bar/0  C

84 mg/g

46

[MeOAc]50%-H2P-COF

1 bar/0  C

88 mg/g

47

[MeOAc]75%-H2P-COF

1 bar/0  C

82 mg/g

48

1 bar/0  C

65 mg/g

49

[MeOAc]100%H2P-COF [AcOH]25%-H2P-COF

1 bar/0  C

94 mg/g

50

[AcOH]50%-H2P-COF

1 bar/0  C

117 mg/g

51

[AcOH]75%-H2P-COF

1 bar/0  C

109 mg/g

52

[AcOH]100%-H2P-COF

1 bar/0  C

96 mg/g

53

[EtOH]25%-H2P-COF

1 bar/0  C

92 mg/g

54

[EtOH]50%-H2P-COF

1 bar/0  C

124 mg/g

55

[EtOH]75%-H2P-COF

1 bar/0  C

117 mg/g

56

[EtOH]100%-H2P-COF

1 bar/0  C

84 mg/g

57

[EtNH2]25%-H2P-COF

1 bar/0  C

116 mg/g

58

[EtNH2]50%-H2P-COF

1 bar/0  C

133 mg/g

59

[EtNH2]75%-H2P-COF

1 bar/0  C

157 mg/g

60

[EtNH2]100%-H2P-COF

1 bar/0  C

97 mg/g

References Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) Huang et al. (2015b) (continued)

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Table 3 (continued)

S. No. 61

Adsorption conditions (pressure/ CO2 uptake List of COFs temp.) capacity 177 mg/g PyridineTTABFBImiCOF 1 bar/0  C

62

TPA-COF-1

1 bar/0  C

68.68 mg/g

63

TPA-COF-2

1 bar/0  C

82.42 mg/g

64

TPA-COF-3

1 bar/0  C

91.15 mg/g

65

TPT-COF-4

1 bar/0  C

54.03 mg/g

66

TPT-COF-5

1 bar/0  C

59.44 mg/g

67

TPT-COF-6

1 bar/0  C

92.38 mg/g

68

COF-TpAzo

1 bar/0  C

112 mg/g

69

CMP-1

1 bar/0  C

90.2 mg/g

70

CMP-1-(OH)2

1 bar/0  C

47 mg/g

71

CMP-1-(CH3)2

1 bar/0  C

41.3 mg/g

72

CMP-1-NH2

1 bar/0  C

63 mg/g

73

CMP-1-COOH

1 bar/0  C

63 mg/g

References Huang et al. (2017) EL-Mahdy et al. (2018) EL-Mahdy et al. (2018) EL-Mahdy et al. (2018) EL-Mahdy et al. (2018) EL-Mahdy et al. (2018) EL-Mahdy et al. (2018) Ge et al. (2016) Xiong et al. (2017) Xiong et al. (2017) Xiong et al. (2017) Xiong et al. (2017) Xiong et al. (2017)

functionalities are employed to construct COFs with a diverse topology of framework, which is responsible to successfully show gas capture and its storage applications. In comparison to zeolites and coordination polymers, COFs are prepared from low-weight element connected by covalent bond led to the formation of structures with low density. 2D COFs act as molecular sieve membrane for separation and adsorption of carbon dioxide from the mixture of gases. A list of COFs used for CO2 adsorption is listed in Table 3. A two-dimensional COF synthesized from a 1,3,5-tris-(chloromethyl)benzene moiety and linkers based on diamine (hydrazine & p-diaminobenzene) is capable of attracting CO2. The polymers with secondary amine functional group act as good adsorbent for CO2 adsorption. These amines exhibit adsorbate and adsorbent interaction to capture carbon dioxide and show high selectivity of CO2 over nitrogen and methane, and it can be easily regenerated under normal conditions (Sun et al. 2015). A nitrogen-rich COF based on triazine and benz-bis(imidazole) is reported with 377.14 cm3 g1 CO2 adsorption at 195 K (77  C) proving the high affinity of this gas with nitrogen rich walls of COF (Das and Mandal 2019). The porous covalent organic polymers (COPs) exhibit

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tunable porosity with a broad range of BET surface area having dimensions 430–3624 m2g1as well wide range (0.24–3.50 cm3g1) of pore volume (Xiang et al. 2015). A COF-1 is the covalent organic framework with azine functional group. This material has a large surface area and minimum pore size. This COF has overall CO2 uptake of 177 mgg1 with greater selectivity over nitrogen and methane (Li et al. 2014b). TRITER-1 is hexagonal COF with triazine functionality. This material exhibits a 716 m2g1 surface area and exceptional capacity of carbon dioxide uptake nearly 58.9 wt% at standard temperature and pressure of 5 bar (Gomes et al. 2015). The two-dimensional COFs named as IPB-1H and IPB-2H are capable to capture carbon dioxide and separate it from nitrogen gas. These are almost flat membranes with a little distorted site on the ligand region. These exhibit flexibility because of carbon-nitrogen (sp3 hybridized) bond which act as bridge between the ligand and building unit. Their pores are almost of similar shape but size of pores is different. IPB-1H and IPB-2H both have the pores of circular shape with pore size about ~6.35 Å and 8.37 Å respectively (Apriliyantoa et al. 2020).

Adsorbents Based on Carbon The adsorbents like coal, charcoal, and activated carbon display carbon dioxide capture at high pressure. One of the advantages to utilize these materials is its low cost as these are derived from carbon based naturally occurring materials. It has possessed insensitivity towards moisture. The activated carbon that is made by the pyrolysis reaction of biomass, fly ash, or carbon-based resins is an amorphous porous material (Ben-Mansour et al. 2016). There is one more advantage of activated carbon over zeolites is their hydrophobicity which decreases the effect of moisture and do not breakdown in hydrated conditions. Also, these materials needed a lower temperature to regenerate in comparison to zeolites. These materials exhibit selective adsorption of carbon dioxide over nitrogen at low pressure and rise in pressure reduces the selectivity. Charcoal and activated carbon shows limited adsorption selectivity for nitrogen and carbon dioxide at low pressure and the adsorption capacity decreases with rise in pressure. Carbon-based adsorbents have lesser selectivity in comparison to zeolites due to their low CO2 uptake capacity compared to zeolites at low pressure. This happens due to the uniform electric potential present on the surface of activated carbons which consequently lower the enthalpy for carbon dioxide adsorption (Radosz et al. 2008).

Carbon Nanotubes (CNTs) These materials are promising candidates for CO2 adsorption due to their high electrical and thermal conductivity and chemical and physical properties. There is also a possibility to modify the surface of these materials by the addition of functional group that gives rise to high storage capacity. CNTs are capable of eliminating the organic and inorganic pollutants and various microorganisms due

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to surface charge density. According to the prior research, CNTs with amino functional group exhibit excellent adsorption properties depending upon physical conditions, and their regeneration requires a very low energy. The CO2 adsorption capacity shown by a functionalized APTES (3-aminopropyltriethoxysilane)–CNT is 2.5 mmol/g at 293 K which is one of the good examples of CNT absorbing CO2 (Su et al. 2011). However, with the increase in temperature, the storage capacity of adsorption decreases. CNTs containing polyethylene imine moiety have adsorption capacity about 2.1 mmol/g at 300 K (Dillon et al. 2008). On the basis of literature values, CNTs with amine functional moiety are promising candidates for carbon dioxide adsorbents. Only a few research articles are available that employ CNTs in the form of membrane to capture carbon dioxide. Membranes of mixed matrix with PVA (polyvinyl alcohol) consist of MWCNTs having amine functional group diffused as fillers illustrated large stability towards gas separation at pressure 1.5 MPa (Zhao et al. 2014b).

Conclusions and Future Outlook In summary, various sources of CO2 and its effects on the lives in this world have been discussed. A majority of such sources includes transportation, industries, etc. The advantages and disadvantages of various CCS technologies (oxy-fuel and preand post-combustion), absorption technique, membrane technique, pressure swing adsorption (PSA) technique, and mineral carbonation technique are highlighted. Emerging porous materials (MOFs, COFs, and POPs) for carbon dioxide adsorption are summarized. Other than these materials, the use of zeolites, activated carbon, and CNTs is also discussed. The main aim of using these methods and application of materials is to achieve the three goals of sustainability so that the gap between intra and intergenerational equity can be minimized. The minimal gap definitely would promise a better and healthy life on the mother earth. Researchers from the entire world are working at their best to make the technologies more efficient and developing next generation materials for better adsorption and conversion of carbon dioxide. CO2 captured in zeolites and MOFs can be reduced to make compounds of industrial importance, but this area requires explorative research in the future.

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Subhalaxmi Pradhan, Chandreyee Saha, Soumya Parida, and Sushma

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Homogeneous Base Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Homogeneous Acid Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Base Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Acid Catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biocatalysts (Enzyme) for Transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocatalysts Used for Transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide-Based Nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layered Double Hydroxides Nanocatalysts or Nanohydrotalcites . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanozeolites as Catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The excessive use of petrochemical resources for energy purpose causes environmental pollution. Due to exhaustion of non-renewable petrochemical reserves and unpredictable enhancement of price of petrochemical fuels, there is tremendous need of alternative renewable fuels of biological origin to replace petrochemicals. Low carbon neutrality, inexhaustibility, and accessibility at fair prices are the prominent features of an alternative biobased fuel. Biodiesel is a sustainable and ecofriendly fuel produced by transesterification processes using different types of homogeneous, heterogeneous catalyst and in non-catalytic conditions from waste cooking oil, animal fats, and vegetable oils. Because of their low cost S. Pradhan (*) · C. Saha · Sushma Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, UP, India e-mail: [email protected] S. Parida G.L.Bajaj Institute of Technology and Management, Greater Noida, UP, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_19

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and ease of availability, homogeneous catalysts are promising materials for biodiesel synthesis in industry. However, they have a corrosion problem and are difficult to separate from the product after the reaction. To address this, many heterogeneous catalysts have been used for biodiesel production with comparable efficiency to that of the homogeneous catalyst and the catalyst can be easily isolated from the product. Recently, several heterogeneous nanocatalysts are in trend for transesterification, in view of exceptional properties such as high activity, excellent catalytic performance, easy recovery, and reusability. Among various nanocatalysts, metal oxide-based nanocatalysts, nanohydrotalcites, magnetic nanocatalysts, and nanozeolites have tremendous potential for catalyzing transesterification of various plant-based oils to produce biodiesel with high yield. In this chapter, we represented biodiesel production using different catalysts and also mentioned optimum condition for various feedstocks. This chapter also focuses on utilization of various nanocatalysts, their effects, and methodologies adopted for biodiesel production. Magnetic nanocatalyst is found to be the promising nanocatalyst for biodiesel synthesis due to high catalytic activity and multiple reusability. Keywords

Petrochemicals · Transesterification · Homogeneous and heterogeneous catalysts · Biodiesel Abbreviations

DG FAAE FAME FFA LDH MG TG WCO

Diglyceride Fatty acid alkyl ester Fatty acid methyl ester Free fatty acid Layered double hydroxides Monoglyceride Triglyceride Waste cooking oil

Introduction Energy plays a major role for development of nation. As population growth and industrialization activity increase continuously, the demand for energy is also growing. Because of the decline in fossil fuels and the clean climate, researchers have paid more attention to renewable energy worldwide. Biodiesel is the methyl and ethyl ester of long chain fatty acids generated from plant-based oils, animal fats, and algae oil via transesterification reaction. Biodiesel has many industrial as well as environmentally beneficial properties. It is described as “carbon neutral” non-toxic, sulfur and aromatic free as compared to petrochemical fuels (Nas and Berktay 2007). Generally, biodiesel is FAAE (fatty acid alkyl ester), developed through

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Fig. 1 Reaction between triglyceride with alcohol

transesterification reaction using homogeneous base catalyst. Transesterification is the foremost economical process requiring only low pressures and temperatures to produce 98% conversion (Nahar 2010; Thanh et al. 2010). Transesterification is the alcoholysis of TG (Triglyceride) with C1-C2 alcohols like ethanol or methanol to make mono alkyl esters and glycerol. In transesterification, 1 mole of TG interacts with 3 moles of alcohol within the existence of a catalyst, ordinarily a powerful alkaline like hydroxide to generated 1 mole of glycerol and 3 moles of mono alkyl ester (Fig.1). In transesterification reaction different oils react with C1-C2 alcohols such as CH3OH and C2H5OH using catalyst to produce mono alkyl ester and glycerol. It is a 3-step reaction, the catalyst first promotes the breakdown of TG to DG, then DG to MG, and finally MG to alkyl ester and glycerol. There are different varieties of alkaline, acid catalyst, enzymes, and waste derived biobased heterogeneous catalysts employed during alcoholysis of vegetable oils, microorganism oil, and WCO to generate fatty acid alkyl ester. Conventionally biodiesel is generated from various feedstocks using variety of catalysts (Fig. 2). Generally, three different categories of catalysts such as acids, alkalis, and enzymes are used during transesterification reaction for producing biodiesel (Talha and Sulaiman 2016; Changmai et al. 2020). Alkali and acid catalysts have been extensively used for biodiesel synthesis compared to biocatalysts (Ma and Hanna 1999). Generally, biocatalysts (enzymes) fetch more attention of researchers due to reusability, easy recovery, and resistance towards soap formation. But biocatalysts are not commercially viable as they are costly and require longer reaction time (Leung et al. 2010). Metallic base catalysts such as oxides of tin, zinc, magnesium, and titanium alcoholates and acid catalysts, such as hydrochloric, sulfonic, and sulfuric acid, belong to homogeneous catalysts (Talha and Sarina Sulaiman 2016). Generally, sodium and potassium hydroxides are used for FAAE production from vegetable oils due to their low cost. Heterogeneous catalysts are preferred for FAAE production because of several benefits of heterogeneous catalysts over homogeneous catalysts. In this chapter, different types of catalysts such as homogeneous catalysts, heterogeneous catalysts, and biocatalysts used for biodiesel production from different feedstocks and the optimum reaction condition are discussed. The present work also extensively reviews the use of various nanocatalysts for transesterification of several biobased oils to produce biodiesel, which is an environmental benign biofuel.

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Fig. 2 Conventional method of biodiesel production using different catalysts

Homogeneous Base Catalysts Due to high reaction rates, the homogeneous catalyst has been extensively used for the production of FAAE. Alkaline catalysts such as KOH, NaOH, KOCH3, and NaOCH3 are commonly used for biodiesel production due to their high efficiency and cheaper in cost (Lam et al. 2010; Narasimharao et al. 2007). Theses catalysts are frequently used due to various benefits like they catalyze the reaction at ambient conditions, improve yield in less time, and are cheap (Lam et al. 2010). NaOCH3 and KOCH3 are better catalyst than hydroxides of Na and K due to easy dissociation into methoxide ion, sodium, and potassium ion. Commercially alkaline catalysts are mostly used for biodiesel production as it does not produce water at the time of transesterification reaction (Sharma and Singh 2008). Table 1 shows that KOH and NaOH are the potential homogeneous alkaline catalysts used during transesterification reaction. These catalysts gave about 98% biodiesel yield from Jatropha oil at optimum condition of 1:6 oil to alcohol molar proportion, 1.0% catalyst concentration at 65 oC in 1hr time (Pradhan et al. 2010). The raw vegetable oil required for alkaline catalyzed transesterification should have FFA less than 0.05% and moisture content less than 0.03%, so that soap formation easily avoidable which creates the problem in biodiesel separation.

Homogeneous acid catalyst Hydrochloric acid (HCl) Sulfuric acid (H2SO4) Sulfuric acid (H2SO4)

NaOCH3

KOH KOH KOH

NaOH

Catalyst Homogeneous base catalyst NaOH NaOH

Sunflower oil Chlorella pyrenoidosa Vegetable oil by-product

Pongamia pinnata oil Jatropha oil Calophyllum inophyllum Sesamum indicum L. seed oil

Soybean oil Waste soybean cooking oil Rapeseed

Feed Stocks

Methyl alcohol Methyl alcohol (40:1) Methyl alcohol (6:1)

Methyl alcohol (6:1)

Methyl alcohol (475:1) Methyl alcohol (4:1) Methyl alcohol (6:1) Methyl alcohol (9:1)

Ethyl alcohol (9:1) Methyl alcohol (3:1)

Molar ratio of alcohol to oil

Table 1 Homogeneous and heterogeneous catalysts used for biodiesel production

1.85 0.5 3

0.75

0.1 0.075 1

0.1

1.3 0.5

Catalyst loading (%)

100 50

50

60 50 55

60

40 55

Temp (o C)

95.2% 92.5% >90%

87.8%

98.5% 87% 98.53%

88.8%

95% 68.5%

Biodiesel yield

(continued)

Sagiroglu et al. (2011) Cao et al. (2013) Javidialesaadi and Raeissi (2013)

Dawodu et al. (2014)

Silva et al. (2011) Hossain and Mazen (2010) Zakaria and Harvey (2012) Porwal et al. (2012) Kartika et al. (2013) Silitonga et al. (2014)

References

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Methyl alcohol (15:1) Methyl alcohol (6:1) Methyl alcohol (40:1) Methyl alcohol (9:1) Methyl alcohol (16.8:1)

Palm olein oil

Palm olein oil

Soybean oil Soybean oil

Rapeseed oil

Neem oil Waste vegetable oil

Snail shells

Oyster shells Eggshells Heterogeneous acid catalyst Titanium-doped amorphous zirconia Sulfated zirconia Carbon-based solid acid catalyst

Methyl alcohol (15:1)

Methyl alcohol (20:1) Methyl alcohol (20:1) Methyl alcohol (24:1) Methyl alcohol (6.03: 1) Methyl alcohol (20:1)

Eggshells

Molar ratio of alcohol to oil

Rapeseed oil Peanut oil Soybean oil Waste frying oil

Feed Stocks

Catalyst Heterogeneous base catalyst Bone Bone Rice husk Snail shells

Table 1 (continued)

1 0.2

11

25 3

10

10

18 4 2 18

Catalyst loading (%)

65 220

245

65 65

65

60

60 60 65 60

Temp (o C)

95% 94.8%

65%

96.5% 95%

93.2%

94.1%

96% 94% 99.5% 99.58%

Biodiesel yield

Muthu et al. (2010) Shu et al. (2010)

Brucato et al. (2010)

Viriya-empikul et al. (2012) Viriya-empikul et al. (2012) Nakatani et al. (2009) Wei et al. (2009)

Bajaj et al. (2010) Bajaj et al. (2010) Chen et al. (2013) Birla et al. (2012)

References

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Homogeneous Acid Catalysts Homogeneous acid catalysts like H2SO4, HCl, RSO3H, and ferric sulfate have been used for biodiesel production (Atadashi et al. 2013). The oil having high FFA, two step reaction is generally followed first acid catalysed esterification then alkali catalysed transesterification to prepare fatty acid alkyl ester. Generally FFA generate soap during alkali catalyzed transesterification, which prohibits the separation of glycerol and biodiesel. Two-step reaction requires alcohol in large amount and also reaction takes more time. Mineral acids are the catalysts used for biodiesel production, but due to slow reaction rate, acid catalysts have rarely used in transesterification reaction (Arzamendi et al. 2007; Helwani et al. 2009). During the mechanism of acid catalyzed reaction, oxonium ion is produced by protonation and an intermediate formed due to exchange reaction between oxonium ion and alcohol, followed by release of a proton to yield an alkyl ester (Atadashi et al. 2013). About 95.2% of biodiesel from sunflower oil was obtained using 1.85 wt% HCl as catalyst at 100  C (Sagiroglu et al. 2011). Acid-catalyzed transesterification of Chlorella pyrenoidosa with 0.5% of H2SO4 gives biodiesel yield of 92.5% (Cao et al. 2013).

Heterogeneous Base Catalysts It has been reported that base catalysts with heterogeneous nature have high basicity and non-toxicity. Some important heterogeneous catalysts used for biodiesel production are are CaO catalyst, potassium oxide and potassium aluminates, potassium carbonate, magnesium aluminide calcined hydrotalcite, Li/ZnO, calcium ethoxide, mixed oxides of Ca and Zr with varied Ca-to-Zr ratio, K-Pumice, Palm ash from empty fruit bunches, aluminum-silicon supported in potassium carbonate (Sun et al. 2014). Heterogeneous base catalyst showed 97% yield in 2 hrs with 1.5% catalyst, 10:1 methanol to oil molar proportion at 65  C. These catalysts occur in various ways such as by co-precipitation, precipitation, impregnation of amounts of base metals, and conversion to oxides by calcination (Changmai et al. 2020). Biowastebased catalysts such as oyster shells have also been used in the alcoholysis of Soyabean oil yielded 96.5% biodiesel at oil methanol molar ratio of 1:6 in 5hrs reaction time at 65  C (Nakatani et al. 2009). Biodiesel was obtained from palm olein oil where catalyst used was shells of eggs with 10 wt% catalysts loading, alcohol to oil molar proportion of 15:1 at 60  C in 2 hours yielded 94.1% biodiesel (Viriyaempikul et al. 2012). Wei et al. (2009) reported 95% biodiesel yield for soybean oil using biowaste catalyst generated from egg shell with 3 wt% catalysts loading at 65  C in 3 hours at 9:1 methanol to oil molar ratio.

Heterogeneous Acid Catalyst Sulfonated mesoporous silica, metal oxides modified with C, hetero-polyacids, porous oxides incorporated with different meal, zeolites, ion exchange resins, tungsten oxides, sulfated zirconia, sulfonated saccharides, and inorganic oxide

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have been successfully employed as acid catalysts for alcoholysis of low-quality oils to produce FAAE (Jimenez-Morales et al. 2011; Chen et al. 2014; Balakrishnan et al. 2014). Heterogeneous acid catalysts have been used to substitute strong liquid acids and eliminate corrosion problems and used to produce biodiesel from WCO (Waste Cooking oil). High FFA containing waste oils produce biodiesel via two step reactions, first is the esterification to reduce acid value and then transesterification in the presence of starch-derived catalyst. WCO having 27.8 wt% FFA yields 92% biodiesel obtained using starch-derived catalyst in 8 hrs. This type of catalysts is strong acid catalyst due to having good operational stability and also has potential for high FFA oil. Neem oil on reaction with sulfated zirconia (1% w.r.t oil) as solid acid catalyst at 9:1 molar proportion of methyl alcohol to oil yielded 95% FAME in 2hrs at 65 oC (Muthu et a1. 2010). About 94.8% of FAME yield was achieved from WCO utilizing solid acid catalysts containing carbon at alcohol to oil molar of 16.8:1 in 4.5 hrs (Shu et al. 2010). Using Ti-doped amorphous Zr catalyst, FAME yield of 65% was obtained from rapeseed at higher molar ratio of oil and alcohol of 1:40 (Brucato et al. 2010). Due to many advantages over homogeneous catalyzed transesterification reactions, heterogeneously catalyzed transesterification reactions are the favored mode of biodiesel synthesis. It is re-usable, easily segregated, and reduces the amount of waste water produced. Since heterogeneous catalysts were costly and obtained from non-renewable sources, researchers think for the green and environmental benign catalysts of biological origin.

Biocatalysts (Enzyme) for Transesterification Purification of biodiesel produced by conventional transesterification generates wastewater and other problems like recovery of glycerol, which increases the cost of FAAE production and creates huge amounts of glycerol, which will remain as glut in environment. Henceforth researchers focus on enzymatic transesterification which occurs at mild reaction condition with easy recovery of product and without generation of by-products. Pretreatment of High FFA oil is not required for enzyme catalyzed transesterification, and the catalyst can be reused several times with high efficiency (Kulkarni and Dalai 2006). Thus, biodiesel produced by enzyme catalytic process is an environment-friendly method and a prime substitute of conventional method. Whereas enzyme catalytic method can’t be executed in industrial scale as enzymes are costly, reaction rate is slow and due to deactivation of enzyme (Bajaj et al. 2010). Lipases are produced by microorganisms and are used as a heterogeneous biochemical catalyst in the biodiesel synthesis process. Lipase B is produced by the yeast Candida antarctica; however, lipases are also produced by other Candida species (Hama et al. 2018). Some lipases are also provided by filamentous fungi Rhizomucor, Thermomyces, Aspergillus, and Penicillium, which are used for alcoholysis of TG (Amoah et al. 2016). About 96.0% of biodiesel was obtained by transesterification of babassu oil using silica immobilized lipase at 12:1 molar ratio of ethanol to oil at 50 oC (Simões et al. 2015). Bergamasco et al. (2013) produced FAME from soybean oil using immobilized lipase at 1:6 molar ratio of oil to ethanol

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at 37 oC which yielded 66.3% of biodiesel, whereas Chen et al. (2013) produced biodiesel from soybean oil using lipase at 1:4 molar proportion of oil to methyl alcohol at 52 oC which improved yield of 83.3%. The drawbacks of homogeneous base catalysts are formation of soap with high FFA oil, production of large amount of by-products, product separation is difficult and generates waste water during purification. There is low reaction rate and corrosion problem with container, and catalyst separation is difficult with homogeneous acid catalysts (Lam et al. 2010; Atadashi et al. 2013). Similarly the drawbacks of heterogeneous acid catalysts are low reaction rate, unfavorable side reactions, longer reaction time, and large energy requirement (Lam et al. 2010; Atadashi et al. 2013). The disadvantages of heterogeneous base catalysts are the catalyst is sensitive to saponification, poisoning of the catalyst on ambient air, requires higher molar ratio of alcohol to oil, and diffusion limitations (Lam et al. 2010; Leung et al. 2010). To address these disadvantages, nanocatalysts are used during transesterification to minimize reaction time, catalyst use, and requires low molar ratio of alcohol to oil.

Nanocatalysts Used for Transesterification The green approach of biodiesel synthesis by transesterification is use of wastederived heterogeneous catalysts from oyster shells and egg shells (Talha and Sarina Sulaiman 2016). However, use of heterogeneous catalyst is not effective as it is concerned with mass transfer resistant, consumes more time, and has limited industrial application. To solve this current situation, researchers synthesize nanosized heterogeneous catalysts which have excellent catalytic activity and improved surface reactivity due to large surface area and greater pore size (Sudarsanam et al. 2018; Yoosuk et al. 2010; Hu et al. 2012). Since the surface area is more in metal nanoparticles, so the catalyst provides more active sites to the substrate. For the synthesis of fatty acid alkyl esters, metal oxides such as calcium oxide, stannous oxide, magnesium oxide, ferric oxide, calcium/aluminum/ferrous ferric oxide, potassium fluoride/alumina, double-layered hydroxide, and zeolite have been used (Mallesham et al. 2014; Zuliani et al. 2018). The comparison of different nanocatalysts used for the development of biodiesel at optimal reaction conditions is represented in Table 2.

Metal Oxide-Based Nanocatalysts Nanosized metal oxides-based compounds have been considered as one of the most effective out of different heterogeneous nanocatalysts, for transesterification of different feedstocks. The nanosized silicon oxide/zirconium oxide catalyst synthesized by the sol-gel method were applied for the alcoholysis of soybean oil to produce biodiesel due to its high efficiency (Faria et al. 2009). The catalyst exhibited a promising biodiesel yield of 96.2% in 3 h (Table 2), and the catalyst showed reusability up to six times during transesterification with slight dropping of the

70 60

60 60

15:1

15:1

16:1

6:1

7:1 6:1

12:1

Mn doped ZnO Fe/Sn oxide nanoparticles CuO/Mg nanocatalyst

KF/CaO-Fe3O4 magnetic nanocatalyst

65

50 200 60

60–90

6:1 to 12:1

Temperature (o C) 60 65

Solid microporous KF/CaO Graphite oxide-supported CaO CaO supported on graphite oxide SiO2/ ZrO2 Potassium bitartrate loaded ZrO2 TiO2/ZnO

Nanocatalyst CaO KF/CaO

Reaction parameters Molar ratio of alcohol to oil 15:1 12:1

4

8 0.25

2

6

2.5

3

-

Catalyst loading (%) 7 4

Table 2 Comparative study of nanocatalysts used for biodiesel production

3

0.833 1 0.5

Stillingia oil, 95%

Mahua oil, 97% Soyabean oil, 84% Sunflower oil, 82.83%

Oil of palm, 92%

Soyabean oil, 96.2% Soyabean oil, 98.3%

3 2 5

Soyabean oil, 97.8%

Jatropha oil, 99%

Biodiesel yield Palm oil, 95.7% Seed oil of Chinese tallow, 96.8% Rape seed oil, 93%

2

2

-

Reaction Time (hr) 1 2.5

Madhuvilakku and Piraman (2013) Baskar et al. (2017) Alves et al. (2014) Rintu Varghese et al. (2016) Hu et al. (2011)

Faria et al. (2009) Qiu et al. (2011)

Zu et al. (2011)

Zu et al. (2011)

Hu et al. (2012)

References Yoosuk et al. (2010) Wen et al. (2010)

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Mg/Al nanohydrotalcite Magnesium-aluminum nanohydrotalcite Cerium modified Mg-Al hydrotalcites KF/Ca-Mg-Al hydrotalcite

Ca/Fe3O4@SiO2 magnetic nanocatalyst MgO/MgFe2O4 magnetic nanocatalyst ZnO/BiFeO3 magnetic nanocatalyst KOH/Fe3O4@-Al2O3 magnetic catalyst Zn-Mg-Al hydrotalcites

65

12:1

65

10:1

67

65

16.14:1

9:1

65

15:1

45 65

110

12:1

4:1 6:1

65

15:1

5

5

1.0

7.5

6.45

4

4

-

0.166

3

1.5

4

5.43

-

4

5

Palm oil, 90%

Soybean oil, 90%

Jatropha oil, 95.2% Karanja oil, 90.8%

Neem oil, 90.5%

Canola oil, 95.43%

Canola oil, 95.43%

Sunflower oil, 91.2%

Soyabean oil, 97%

Gao et al. (2010)

Dias et al. (2012)

Chelladurai and Rajamanickam (2014) Deng et al. (2011) Obadiah et al. (2012)

Salimi and Hosseini (2019) Ghalandari et al. (2019)

Alaei et al. (2018)

Feyzi and Norouzi (2016)

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conversion rate to 84.1%. Potassium bitartrate (C4H4O6HK) loaded ZrO2 nanocatalyst of size (10–40 nm) was synthesized and utilized in alcoholysis of soybean oil and yielded 98.03% biodiesel at optimum reaction condition of 1:16 molar proportion of oil and methanol with 6% catalyst concentration at 60  C in 2 hr (Qiu et al. 2011). TiO2-ZnO nanocomposite was synthesized and employed for alcoholysis of palm oil and yielded 92% FAME using 200 mg of catalyst at 60  C in 5 hr with 1:6 proportion of oil and alcohol (Madhuvilakku and Piraman 2013). It has been investigated that catalytic activity and stability of the catalyst was improved by defect formation on the catalyst surface due to introduction of Ti on Zn lattices. Sulfated doped TiO2 solid nanocatalyst was synthesized, and these catalysts exhibited better performance than the sulfated metal oxide catalysts in production of FAME from cooking oil because of greater acidic nature of titanium oxide (TiO2) particles which can be further enhanced by SO42- group loading on TiO2 surface (Gardy et al. 2016; Gardy et al. 2017). Bimetallic catalysts adsorbed on metal oxides, i.e., copper–nickel adsorbed on titanium oxide, zirconium oxide, and selenium oxide, also used during transesterification for generating biodiesel (Ambursa et al. 2016; Ambursa et al. 2017). Calcinated Mn doped zinc oxide nanocatalyst at 600  C is used for the alcoholysis of mahua oil and yielded 97% FAME at optimized reaction condition given in Table 2 (Baskar et al. 2017). From the kinetic study it was found that the above transformation required an activation energy of 181.91 kJ/mol. In an easy and rapid nanotechnological approach, Alves et al. (2014) examined the mixture of Fe2O3/CdO and Fe2O3/SnO2 nanoparticles having certain magnetic properties to produce FAME from soybean oil. It has been observed that Fe2O3/SnO2 nanoparticles exhibited efficient catalytic performance giving 84% biodiesel yield. It was also suggested that such catalysts showed significant application towards esterification, transesterification, and hydrolysis of soybean oil. Kumar et al. (2018) reported the use of four different forms of calcium oxide nanocatalysts like neat calcium oxide, doped calcium oxide, loaded calcium oxide, and waste calcium oxide which are used effectively in the synthesis of FAME from a variety of vegetable oils. Out of them, the loaded and doped CaO were preferred over the neat CaO nanocatalyst for their capability to rapidly form hydrogen bonds with glycerol and alcohol. It was reported that about 100% FAME conversion obtained using calcium oxide nanocatalysts in the alcoholysis of poultry fat at oil to alcohol molar ratio of 3:10 at ambient temperature (Reddy et al. 2006). KF/CaO nanocatalysts (size range varying in between 30 and 100 nm) are synthesized employing impregnation technique, and their activity was evaluated for transesterification of Chinese tallow seed oil, yielded 96.8% biodiesel (Wen et al. 2010). In another study, microporous KF/CaO nanocatalyst was employed to transesterify rapeseed oil at 70–90  C taking alcohol with 6:1–12:1 molar proportion to oil, where 93% yield conversion was obtained (Hu et al. 2012). CaO nanocatalyst supported on graphite oxide has higher activity towards biodiesel synthesis, and the catalyst showed about 99% conversion rate for Jatropha oil at oil to alcohol molar proportion of 1:15 at 70  C. Soybean oil exhibited 97.8% yield using same catalyst at oil to alcohol molar proportion of 1:15 at 60  C (Zu et al. 2011). Tahvildari et al. (2015) employed two different methods sol-gel self combustion and sol-gel method to synthesize calcium

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oxide and magnesium oxide nanocatalysts and examined the efficiency of the catalysts in transesterification of cooking oils. It has been observed that CaO nanoparticles exhibited high performance in biodiesel as compared to MgO nanoparticles. Researchers also used CuO/Mg nanocatalyst to transesterify sunflower oil by ultrasonic method, and about 82.83% biodiesel yield was obtained (Rintu Varghese et al. 2016).

Magnetic Nanocatalysts Researchers focused on magnetic nanocatalysts because of their reusability and excellent magnetic properties and employed for the synthesis of fatty acid alkyl esters (Alaei et al. 2018; Alves et al. 2014; Hu et al. 2011). Novel KF/CaO-Fe3O4 magnetic catalyst was employed to transesterify stillingia oil, and about 95% FAME yield (Table 2) was achieved at optimum reaction parameters of alcohol to oil molar proportion of 12, 4% (w/w) catalyst at 65 oC in 3hr (Hu et al. 2011). This novel magnetic nanocatalyst has 14 times reusability without much loss in its activity with 90% recovery of catalyst when applied under above-cited optimum reaction parameter. Alves et al. (2014) developed magnetic mixed Fe2O3/CdO and Fe2O3/SnO2 nanocatalyst and employed for synthesis of biodiesel from soybean oil and obtained 84% yield at 200  C in 1hr. These nanocatalysts were found reusable for 4 times without significant loss in activity while Fe2O3/CdO nanocatalyst showed low activity compared to Fe2O3/SnO2. Feyzi and Norouzi (2016) developed Ca/Iron oxide@Silica magnetic nanocatalyst by following two different means sol-gel and incipient wetness impregnation technique. The catalyst showed strong magnetic properties and efficient catalytic activity at optimum conditions with 97% biodiesel yield (Table 2). MgO/MgFe2O4 magnetic nanocatalyst was developed by Alaei et al. 2018 and used for synthesis of FAME from sunflower oil at 12:1 molar ratio of alcohol to oil with 4% catalyst concentration at 110 oC, obtained 91.2% FAME in 4h. The catalyst showed five times reusability with lowest conversion rate of 82.4%. Salimi and Hosseini (2019) prepared ZnO/BiFeO3 magnetic nanocatalyst via co-precipitation method and applied for synthesis of FAME from canola oil. The FAME yield obtained is 95.43 and 95.02% in 1st and 2nd round, respectively, at methanol and oil molar ratio of 15 at reaction temperature of 65 oC and 4% catalyst concentration. It is reported that this catalyst is of low cost and easily recovered after the reaction by employing magnetic field. It is noticed that the catalyst has 5 times reusability having biodiesel yield more than 92.08%. Some researchers synthesized novel magnetic KOH/Fe3O4@-Al2O3 nanocatalyst using the support of ferric oxidealumina core-shell structure and potassium hydroxide as active component. This novel magnetic catalyst exhibited high efficiency for canola oil having yield of FAME at 97.4% under standard conditions of reactions (Ghalandari et al. 2019). Researchers synthesized magnetic nanoparticles containing acid functional group and used for alcoholysis of waste cooking oil to produce FAME. This nanocatalyst is mainly composed of H3NSO3 (sulfamic acid) and RSO3H functionalized silicacoated crystal-like iron/ferrosoferric oxide magnetized nanoparticles, and both the

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acid functionalized nanocatalysts exhibited excellent catalytic efficiency. Higher activity was observed for H3NSO3 functionalized nanocatalysts in comparison to RSO3H functionalized nanocatalysts (Wang et al. 2015).

Layered Double Hydroxides Nanocatalysts or Nanohydrotalcites LDH compounds have general formula [M2 +1 ―xM3 +x (OH) 2]x +[Ap-x/p ]x+ .mH2O, where charge balancing anion represented by Ap and divalent and trivalent cations, respectively, represented by M2+ and M3+ are called hydrotalcites (Zhang et al. 2017). Researchers have shown interest in the synthesis of nanohydrotalcites, typically negatively charged clays or layered double hydroxides of Al-Mg, due to the wide use of hydrotalcite. Hydrotalcites are anionic clays that contain two forms of metal ions mounted in +vely charged brucite-like layer with the help of a close packed hydroxide configuration (Shekoohi et al. 2017; Chelladurai and Rajamanickam 2014). Zn-Mg-Al layered hydroxides are used for the methanolysis of oil extracted from neem, and about 90.5% FAME conversion was obtained under standard conditions of reaction using molar ratio of methyl alcohol to oil at 10:1 at 65  C in 4hr (Chelladurai and Rajamanickam 2014). Mg/Al nanohydrotalcite is developed using NH2CONH2 (urea) as precipitating agent in co-precipitation technique and utilized for transesterification of Jatropha oil in ultrasonic method at 210 W power produced 95.2% FAME (Deng et al. 2011). Cerium modified magnesium-aluminum hydrotalcites were synthesized from Al/Mg molar ratio of 1:3 (Ce/Mg ratio > 0.03) and applied for transesterification of soybean oil and obtained 90% FAME at 9:1 molar ratio, 5% catalyst concentration in 3hr (Dias et al. 2012). Obadiah et al. (2012) synthesized magnesium-aluminum nanohydrotalcite and used for methanolysis of pongamia oil, obtained 90.8% FAME conversion at 6:1 methanol to oil molar ratio at 65  C temperature. Nanoengineered microporous hydrotalcites are the promising catalyst for biodiesel production and showed improved yield. Macroporous Mg-Al LDHs produced by co-precipitation method without alkali exhibited excellent catalytic efficiency in the alcoholysis of medium chain (C4–C18) triglycerides. Gao et al. (2010) synthesized potassium fluoride/calcium-magnesium-aluminum hydrotalcite with varied ratios of metal and studied the effects of ratio of ions of the nanohydrotalcite on the yield of FAME during methanolysis of palm oil at different methanol/oil molar ratios. Nanohydrotalcite at different cationic ratio exhibited high catalytic performance during transesterification reactions. The FAME yield of 90% was achieved in 10 min at oil/alcohol molar ratio of 1:12 and 5% catalyst concentration at 65  C. When the catalyst prepared from 2.2:0.8:1 molar ratio of Ca-Mg-Al (with potassium fluoride mass ratio of 100%) was used for transesterification, about 99.6% FAME was obtained in 10 min. Some researchers modified Mg-Al nanohydrotalcite by introducing Zn-SBA-15 composite and examined the catalytic efficiency towards methanolysis of soybean oil and obtained 90% biodiesel yield at ambient reaction conditions. It was observed that textual properties and basic sites increased by introducing zinc in magnesium/aluminum hydrotalcite, which enhances the catalytic

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activity of nanohydrotalcite composite (Prabu et al. 2019). In comparison to Mg/Al hydrotalcite, calcinated Li/Al hydrotalcites are effective catalyst for glycerolysis of fatty acid because of high basicity (Di Serio et al. 2007). Henceforth above study confirmed that nanohydrotalcites are promising and ecofriendly catalyst for biodiesel production.

Nanozeolites as Catalyst Minerals containing hydrated aluminosilicates of sodium, potassium, calcium, and barium are called zeolites, which is used as commercial adsorbents. Since decades, zeolites have been employed as efficient catalysts due to its high surface area, shape selectivity, microporous nature, and the presence of strong acid sites (Al Ani et al. 2018; Vasconcellos et al. 2018; Amalia et al. 2019). Currently there has been a lot of focus on the synthesis of nanozeolites for better catalytic activity. Researchers have synthesized nanozeolite–enzyme complexes as the most effective catalysts to enhance rate of transesterification since nanozeolites are hydrophobic systems with high external surface areas. Nanozeolites can be dispersed efficiently in hydrophilic and hydrophobic medium, henceforth provides better interaction between enzyme and its substrate. However, the most important advantage of using such nanozeolite– enzyme complexes is that it can be reused for several times. It is easy to recover and reuse the immobilized nanocatalysts in consecutive cycles of transesterification as the catalyst is immobilized on solid nanosupports (de Vasconcellos et al. 2018; Kim et al. 2018). Enzyme–nanozeolite complexes are used to generate fatty acid ethyl esters in microalgae oil ethanolysis. The results showed that the APTMS functionalized glutaraldehyde-linked nanozeolites had the potential to immobilize further enzyme amounts (Vasconcellos et al. 2018). Such nanozeolite-enzyme complexes exhibited enhanced enzymatic activities than free enzymes. Using a lipasenanozeolite complex, the yield of FAEEs is more than 93%, and the catalyst is reusable for five consecutive ethanolysis reaction with same yield at the end of the five cycles. It was also observed that enzyme immobilization through covalent bond formation was found to be more efficient catalyst than the complex enzyme–nanozeolite formed by physical adsorption. Thus, it was concluded that excellent biocatalyst for biodiesel could be generated by chemically modulating nanozeolite surfaces. Ion-exchanged zeolite P nanocatalyst was generated and used for transesterification reaction and exhibited excellent catalytic performance due to use of nanosized zeolite (Al-Ani et al. 2018). Some researchers synthesized KNa/ZIF-8, which is a K and Na doped zeolite imidazolate structure, and used it for methanolysis of soybean oil. More than 98% conversion was observed in case of KNa/ZIF-8 in 10: 1 proportion of methanol to oil in 3.5 h, and the catalyst can be reused for three cycles (Saeedi et al. 2016). Na and K were added to enhance the basic properties and the activity of ZIF-8 during methanolysis of oil from soyabean. Potassium hydroxide impregnated nanozeolite catalyst is used for production of FAME from castor oil at 55  C, 70% catalyst concentration in 7 h (Amalia et al. 2019). Brito et al. synthesized nanozeolite Y with varied concentration of alumina

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and used for the methanolysis of waste oil. These catalysts exhibited high efficiency between 466 and 476  C in 12.35 min and 21.99 min reaction time at oil to alcohol proportion of 1:6. Applying pelletization process zeolite-based catalyst was developed from kaolinite, which was also used to produce FAMEs from high acid value WCO. Therefore, zeolite-based catalysts have the ability to simultaneously catalyze both esterification and TG transesterification. About 46% conversion of WCO was obtained using such nanozeolite catalyst at 50–85  C in 2–10 h reaction period at methyl alcohol to WCO molar proportion of 2.6–6.0 (Hassani et al. 2014). The above studies thus confirmed the application of zeolites and nanozeolites as effective heterogeneous catalysts for transesterification to generate biodiesel.

Conclusion and Future Scope Biodiesel is a promising non-conventional energy option that is renewable and sustainable, with more attention paid to replacing non-renewable fossil-based fuels. It is primarily derived from plant-based oils, animal fats, microorganisms, and algae oil through transesterification. Various homogeneous and heterogeneous catalysts essentially catalyze the transesterification reaction. Heterogeneous catalysts are more preferred over homogeneous because it is separable, reused, recycled, and regenerated. It is highly selective, is low cost, and requires mild reaction condition and doesn’t produce any aqueous waste. It can tolerate high FFA and has less corrosive character and is more environment-friendly. Nanocatalysts, such as metal oxide-based nanocatalyst, nanozeolites, nanohydrotalcites, and magnetic nanocatalysts are the efficient catalysts for biodiesel synthesis among the arrays of heterogeneous catalysts. Nanocatalysts are found to be the more competent catalyst with high performance in comparison to other heterogeneous catalysts towards biodiesel synthesis from variety of feedstocks. Among all the nanocatalysts discussed above, magnetic nanocatalyst is the most efficient catalyst for biodiesel production due to strong magnetic properties and multiple reusability, which make biodiesel production economically viable. Nanocatalysts produced in recent years are thought to be the best catalysts for heterogeneous catalysis because they have a number of advantages, including the ability to improve the efficiency of traditional heterogeneous catalysis methods. However extensive research is needed to study the toxic effects of nanocatalysts during biodiesel production and how to overcome the toxicological effects of nanocatalysts.

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Zhenzi Li, Decai Yang, and Wei Zhou

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type II Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g-C3N4-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Disadvantages of Type II Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Z-Scheme Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g-C3N4-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Zhenzi Li and Decai Yang contributed equally with all other contributors. Z. Li Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China e-mail: [email protected] D. Yang Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China e-mail: [email protected] W. Zhou (*) Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_53

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Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Disadvantages of Z-Scheme Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-Scheme Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g-C3N4-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Disadvantages of S-scheme Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tandem Heterojunction Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfide-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g-C3N4-Based Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Disadvantages of Tandem Heterojunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Photocatalysis is a green and sustainable technology for solar energy conversion, which is beneficial for energy and environmental fields. The efficiency of solar light utilization and photoinduced charge separation is vital for improving the photocatalytic performance. However, any single photocatalysts cannot be satisfied with the above requirements simultaneously. Heterojunction photocatalysts are ideal candidates for high-efficient photocatalysis due to the wide-spectrum response and long lifetime of charge carriers. In this chapter, various heterojunction photocatalysts (e.g., type II heterojunction, Z-scheme heterojunction, S-scheme heterojunction, and tandem heterojunction) are introduced, and the photocatalytic mechanisms are also summarized. The advantages and disadvantages for different heterojunction photocatalysts are summarized, which could supply helpful insights for the wide readership of photocatalysis. In addition, the photocatalytic application of different types of photocatalysts (e.g., oxide, sulfide, g-C3N4, etc.) is summarized. Finally, the perspective of heterojunction photocatalysis is proposed for the future fields of solar energy conversion. Keywords

Photocatalysis · Heterojunction · Z-scheme · S-scheme · Tandem heterojunction · Solar energy conversion

Introduction In recent years, the rapid development of the society has brought people a comfortable life, but has also produced various environmental problems (Li et al. 2019a). Energy crisis and environmental pollution have become the two major themes in the field of science and technology (Li et al. 2019b). Therefore, it is urgent to solve the current

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Fig. 1 Diagram of photocatalytic process

environmental pollution problem and develop new energy sources. The development and utilization of the new technology has become the top priority of current research. Among various options, photocatalysis has been favored by people because of its high efficiency, low energy consumption, simple operation, and no secondary pollution (Lin et al. 2019). It can directly use solar energy to generate new energy sources, such as hydrogen energy and hydroxide compounds, and can also degrade or convert pollutants and fix nitrogen (Ma et al. 2021). Of course, it has become the most effective new method to solve the current increasingly serious environmental and energy problems, and it will have good application prospects in the near future. Since Fujishima and Honda (1972) discovered that TiO2 photoelectric phenomenon splitting water to produce hydrogen in 1972, photocatalysis has begun to attract people’s attention. As shown in Fig. 1, the photocatalytic process is mainly composed of three parts, namely, photoexcitation, charge migration, and redox reaction (Low et al. 2017). First, under the light irradiation, when the light energy is greater than or equal to the band gap of semiconductor, the electrons in the valence band absorb energy and transition to the conduction band, forming photogenerated electrons (e
) on the conduction band. At this time, the holes (h+) in the valence band are formed, which form photogenerated electron-hole pairs. Then, the electrons and holes migrate to the surface of the semiconductor oxide. Finally, the electrons and holes complete the redox reaction with the absorbed electron acceptor and electron donor, respectively, and finally complete the photocatalytic process. However, in the second step, photogenerated electrons and holes are prone to recombine, which limits the efficiency of photocatalysis (Meng et al. 2019). Therefore, the hot topic in the current research on the field of photocatalysis is how to suppress the recombination process. In addition, the low utilization of solar energy by a single catalyst (about 5%) (Fu et al. 2018) and the low oxidation-reduction capacity of a single photocatalyst result in low overall photocatalytic efficiency. It is also a great challenge. In recent years, in order to solve the above problems, people have made great effort, such as doping (B, S, N, etc.), combining metals (Pt, Ag, Au, etc.), and forming heterojunctions. A large amount of data show that the design of heterostructures is a new and most effective strategy to inhibit photogenerated charge carrier recombination and promote efficient charge separation in photocatalysis (Wang et al. 2019a).

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Fig. 2 Trend of publications and citations of heterojunction photocatalysis in 2011–2020 with keyword search as “heterojunction” and “photocatalysis”

As shown in Fig. 2, the research on photocatalyst heterojunctions has become more and more extensive in the past 10 years, showing an obvious upward trend and indicating the continuous high level of attention. As shown in Fig. 3, heterojunctions are generally divided into three types, straddling gap (type I), staggered gap (type II) and broken gap (type III). For type I (Fig. 3a), when light is excited, electrons and holes are concentrated on the catalyst with low conduction band (CB) and valence band (VB), which is not conducive to the separation of electrons and holes. For type III (Fig. 3c), the energy levels of the two catalysts are not matched. Electrons and holes cannot be transferred between the two catalysts when light is excited. Furthermore, this structure is not suitable for heterojunction photocatalysis. For type II (Fig. 3b), when the two catalysts are excited by light, electrons jump from the valence band to the conduction band, leaving holes in the valence band, and electrons and holes are transferred from the catalyst with strong reducing and oxidizing ability to the weaker catalyst, thereby forming an effective separation of electrons and holes. Therefore, both type I and type III are inferior junctions, which are not conducive to charge separation, and the photocatalytic effect does not increase but decreases. The current research on photocatalytic heterojunctions is mostly concentrated on type II, which is favorable for efficient charge separation, thus promoting the photocatalytic performance obviously. In this work, the mechanism, advantages, and disadvantages of type II heterojunction photocatalysts, Z-scheme heterojunction photocatalysts, S-scheme heterojunction photocatalysts, and tandem heterojunction photocatalysts are

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Fig. 3 The photogenerated charge transfer of type I (a), type II (b), and type III (c) heterojunction photocatalysts

Fig. 4 Outline diagram of the types of heterojunction photocatalysts

summarized, and the contribution of heterojunction photocatalysts to solar energy conversion including hydrogen production, oxygen production, CO2 reduction, and degradation of pollutants is highlighted. As shown in Fig. 4, the above issues are discussed from four aspects (oxide-based, sulfide-based, g-C3N4-based, and others). Finally, we summarize the current status of heterojunction photocatalysts, and the challenges and future development prospects in field of solar energy conversion are also proposed.

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Type II Heterojunction Photocatalysts Among various heterojunction photocatalysts, the research on type II heterojunction has always been dominant. In 1984, Serpone et al. (1984) first proposed the use of electronic transfer to avoid recombination. Since then, much research has been done to use “transfer” strategies to inhibit recombination. As shown in Fig. 5, electronic from VB transitions to the CB, leaving holes on VB position when the catalyst A and B are irradiated by light, due to the band position differences between A and B at the same time. The electrons are going to be transferred from B, which is higher in the valence band to A, and they are going to be reduced; similarly, the holes are going to be transferred from A, which is higher in the valence band to B, and they are going to be oxidized. The type II heterojunction can effectively separate the photogenerated charge carriers in heterostructured composites, so it is indeed a useful strategy to design type II heterojunction photocatalysts to improve the photocatalytic performance.

Oxide-Based Heterojunction After decades of development, many kinds of photocatalysts with different materials and different structures have been developed, and the composite photocatalysts based on oxide semiconductor materials have become a hot spot. Common oxide semiconductor titanium dioxide (TiO2), zinc oxide (ZnO), and other materials, although easy to synthesize, and stable properties and their large band gap widths make them impossible to effectively use visible light (Wang et al. 2018). Therefore, it is an important way to increase the photocatalytic efficiency by compounding the semiconductor materials to extend the photoresponse range and completely utilize the heterojunction to promote the effective separation of photogenerated charge carriers. TiO2 is one of the most frequently used materials in heterostructures. TiO2 (L) and TiO2 (S) were used to synthesize the homodirectional heterojunction B/Be (Bai et al. 2019), which greatly improved the photocatalytic hydrogen production rate. It was attributed to the built-in electric field formed by the two substances, which promoted space charge separation and inhibited charge recombination. Sotelo-Vazquez et al. (2017) successfully synthesized WO3/TiO2 heterojunction by means of vapor phase chemical deposition, greatly reducing the recombination and prolonging the lifetime and quantity of photoelectric charge. Fig. 5 The photocatalytic mechanism of type II heterojunction photocatalyst

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Zhao et al. successfully prepared CdS QDs/b-TiO2 (Zhao et al. 2018) heterojunction photocatalyst using hydrothermal chemical deposition and in situ solid state chemical reduction. In Fig. 6, it could be clearly seen that a heterostructure was

Fig. 6 SEM (a), TEM images (b, c, d) and schematic illustration of the energy band structure (e) of CdS QDs/b-TiO2 heterojunctions. (Reprinted with permission from Zhao et al. 2018)

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Fig. 7 The SEM images of M-Fe2O3/TiO2 (a) and M-Fe2O3/b-TiO2 (b), the TEM (c), HRTEM images (d, e) and EDS elemental mappings (f–i), and ultraviolet-visible absorption spectra (j) and the proposed photogenerated charge transfer mechanism (k) of M-Fe2O3/b-TiO2. (Reprinted with permission from Sun et al. 2018b)

formed between CdS quantum dots and TiO2 nanorods. Combined with the mechanism diagram shown in Fig. 6e, it could be clearly seen that the formed heterojunction was type II heterojunction, which enhanced the separation of photogenerated electron-hole pairs and greatly improved the photocatalytic hydrogen production and degradation ability. Sun et al. prepared M-Fe2O3/b-TiO2 (Sun et al. 2018b) by wet impregnation and surface hydrogenation. The successful loading of M-Fe2O3 could be clearly confirmed from Fig. 7, and the lattice fringe of M-Fe2O3 was d(311) ¼ 0.296 nm in Fig. 7d, e. The catalyst extended the light response from the ultraviolet region to the near-infrared region (Fig. 7j), which greatly improved the utilization ratio of sunlight. As shown in Fig. 7k, the CB position of b-TiO2 was higher than that of M-Fe2O3 nanoparticles, while the VB position of M-Fe2O3 nanoparticles was lower than that of b-TiO2. After illumination, electrons on CB of b-TiO2 were transferred to M-Fe2O3, and a reduction reaction occurred on the CB of M-Fe2O3, generating O2
and H2O2. On the contrary, oxidation reaction took place on VB of b-TiO2 to generate OH, which together acted to decompose the organic pollutants.

Sulfide-Based Heterojunction Sulfide is a novel semiconductor material with suitable band gap, good chemical stability, and low price, which has attracted extensive attention of scholars in recent

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years. At the same time, sulfide has a good ability to absorb visible light and nearinfrared (NIR) light, so it can be used as a promising photocatalyst for visible light. The photocatalytic performance of single sulfide is not ideal, because the photogenerated electrons and holes separated under light cannot be transferred in time, and the recombination rate is high, which greatly reduces the photocatalytic performance. In order to effectively improve the photocatalytic performance of visible light, sulfide is compounded to form heterostructures, so that the photogenerated electrons and holes are effectively separated and transferred, and the recombination of photogenerated charge carriers is reduced. This is the most effective method to solve the poor performance of single catalyst at present. ZnIn2S4 has attracted much attention in photocatalysis due to its unique structure and controllable energy band. In situ growth of Mxene (Ti3C2Tx) nanosheets (MNS) on ultrathin ZnIn2S4 nanosheets (UZNS) was performed by Zuo et al. (2020) to generate heterostructures, which effectively inhibited the recombination of photoexcited electron hole, promoted the transfer and separation of photoexcited charge, and demonstrated excellent photocatalytic hydrogen production performance. Zhang et al. (2020) successfully grew 2D ZnIn2S4 nanosheets on 1D hollow Co9S8 nanotubes, forming a type II heterojunction, which exposed a large specific surface area and generated enough active sites to promote the transfer of charge. It had excellent photocatalytic hydrogen production and CrVI reduction ability. ZnIn2S4 can also be used as a carrier in addition to a payload. Ultrathin 2D ZnIn2S4 was prepared by a highly scalable self-surface charge stripping technology and electrostatic coupling method, and then, MoSe2 composite scalable mixed layer of mixed compound was formed (Yang et al. 2017); the heterogeneous structure could improve the visible light reactivity and effective charge separation and then effectively promoted the development of the 2D layer heterojunction photocatalysts. SnO2 has been proposed as an economical and efficient method for the removal of Cr(VI). Zhang et al. (2016) prepared SnS2/SnO2 nanotubes (SNF) with good dispersibility by using electrospin technology and hydrothermal method. The heterostructure also showed a response to NIR region, showing excellent performance in photocatalytic degradation of Cr(VI). This was attributed to the staggered type II heterostructure formed by SnS2 and SnO2. Ye et al. (2019) introduced an ultrathin C layer between CeO2 nanorods and SnS2 particles as an electron transport channel and proposed a new strategy to promote charge transfer, which greatly improved the photocatalytic performance. Two-dimensional (2D) layered Bi2WO6 plays an important role in photoelectric devices and solar energy conversion systems. Adhikari and Kim (2018) designed and successfully prepared Bi2S3/Bi2WO6 heterojunction through the top phase transformation of Bi2WO6. In the test of photocurrent, it can be seen that the electron transfer was faster, which greatly promoted the photooxidation performance of oxfloxacin. Jiang et al. prepared Ag/Bi2S3/MoS2 heterojunction photocatalysts (Jiang et al. 2020). As shown in Fig. 8d, e, f, it could be clearly seen that Bi2S3 and Ag had been

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Fig. 8 SEM images of oxygen-doped flower-like MoS2 nanospheres (a) and Bi2S3 ultrathin nanobelts (b), TEM images of oxygen-doped flower-like MoS2 nanospheres (c) and Ag/Bi2S3/ MoS2 (d), and HRTEM images (e, f) and the formation procedure (g) and schematic diagram of possible electron transfer paths and photocatalytic mechanism (h) of Ag/Bi2S3/MoS2 (Reprinted with permission from Jiang et al. 2020)

successfully supported on MoS2 by hydrothermal and photoreduction method (Fig. 8g), and their lattice fringes could be clearly seen. As shown in Fig. 8h, the formation of heterojunction promoted charge separation and widened the absorption range of light. Under visible light irradiation, it exhibited excellent photocatalytic degradation and hydrogen production performance, which could be applied to the environment and energy fields.

g-C3N4-Based Heterojunction Graphite phase carbon nitride (g-C3N4), as a visible-light-responsive organic semiconductor material, has the advantages of high stability, low-cost, high structure, and property controllability. With the continuous development of green photocatalytic technology without secondary pollution, g-C3N4 photocatalyst has gradually become a research hotspot in the field of environment and energy. However, the photocatalytic efficiency of single g-C3N4 is low due to the defects of fast photoexcited electron-hole recombination and low utilization of visible light. Among various modification methods, heterocoupling is considered to be an effective method to improve the photocatalytic performance of g-C3N4. In recent years, researchers have

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combined different inorganic semiconductors, precious metals, and carbon materials by heterocoupling with g-C3N4, the transfer efficiency of photoelectrons in the photocatalytic system was improved, the absorption range of g-C3N4-based photocatalyst was extended to visible light and NIR regions, and the stability of g-C3N4based photocatalysts was also enhanced obviously. As a typical non-toxic metal oxide, Nb2O5 has attracted extensive attention due to its strong oxidizing ability. Hong et al. (2016) successfully prepared Nb2O5/g-C3N4 heterojunction with only one-step heating approach. The photocatalytic degradation performance was not only limited in neutral environment, but also in the acidic environment for the degradation of TC also showed excellent performance. At the same time, it also could photocatalytic degrade other antibiotics. The construction of heterojunctions improved the efficiency of charge separation and produced more superoxide free radicals and holes, which were the main active substances in the photocatalytic degradation process, and this work had potential applications in the field of environment. The vacancy can break up the ordered structure inside the crystal lattice, which is of great significance for charge separation. Hao et al. (2018) prepared a Zn-rich vacancy ZnS/g-C3N4 heterojunction photocatalyst, which extended the light absorption to the visible light region and significantly improved the visible light utilization. At the same time, the close contact between interfaces promoted the charge transfer and improved the separation efficiency. A g-C3N4/P25(N)-Pd type II heterojunction with significantly enhanced photocatalytic activity compared with the traditional g-C3N4/P25 (Cai et al. 2019) type II heterojunction was synthesized by using a single-pot synthesis strategy, which greatly improved the separation efficiency of electrons and holes, showed excellent hydrogen production performance, and played an important role in the field of new energy. The development of ferroelectric-photocatalytic materials with polarized electric field facilitates the separation and transfer of charge carriers, which is of great significance for photocatalytic hydrogen production. Lin et al. (2020) constructed the room temperature ferroelectric CuInP2S6 (CIPS) and formed the CIPS/g-C3N4 heterojunction photocatalysts on this basis. Due to the effect of the internal polarization electric field between the contact surfaces, the photocatalyst showed amazing charge transfer and remarkable photocatalytic hydrogen production performance. As shown in Fig. 9b, c, Su et al. (2021) successfully loaded the ultrathin microporous g-C3N4 onto the NH2-MIL-101(Fe) octahedron to form the type II heterojunction photocatalyst. The uniform introduction of g-C3N4 was conducive to the diffusion of reactants, and the presence of ferric iron could also form Fenton system and improve the photocatalytic efficiency. In addition, the heterojunction could improve the charge separation efficiency, which made the catalyst had excellent performance in the photocatalytic degradation of 2,6-dichlorophen and 2,4,5-trichlorophenol and the production of H2O2 (Fig. 9d). It made a significant contribution to the fields of energy and environment.

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Fig. 9 The SEM images of NH2-MIL-101(Fe) (a) and M101-U6 (b) and the TEM image of M101-U6 (c) and the proposed photocatalytic mechanism of photo-Fenton-like system to degrade pollutants and generate O2 simultaneously (Reprinted with permission from Su et al. 2021)

Others In addition to the above materials, other heterostructures have also been put forward, such as bismuth hydrogen halide heterostructures, MOF heterostructures, carbide heterostructures, etc., which play an important role in the development of heterojunction photocatalysts. Zhao et al. (2020b) used a two-step solvothermal method to synthesize Bi2MoO6/ MIL-88B (Fe) heterostructure (BMO/M88), which promoted spatial charge separation and broadened the visible light absorption, and showed excellent photocatalytic performance for the degradation of RhB. Xiao et al. (2020) synthesized 1D CdS nanorod/2D Ti3C2 Mxene nanosheet heterojunction using a simple solvothermal method. The 1D structure could provide a low Schottky barrier for water splitting to drive hydrogen evolution, and the heterostructure could accelerate charge separation, which made it exhibit excellent photocatalytic hydrogen evolution performance. The solar energy conversion of some type II heterojunction photocatalysts is summarized as shown in Table 1.

MO and metribuzin degradation

H2 evolution H2 evolution; Cr(VI) reduction

H2 evolution Cr(VI) photoreduction

Phenol degradation Ofloxacin degradation

2,4-dichlorophenol degradation; H2 evolution Degradation of TC

H2 evolution

M-Fe2O3/b-TiO2

MNZIS-4 Co9S8/ZnIn2S4

ZnIn2S4/1%MoSe2 SnS2@SnO2

CeO2/C/SnS2 Bi2S3/Bi2WO6

Ag/Bi2S3/MoS2

ZnS/g-C3N4

Nb2O5/g-C3N4

300 W Xe lamp (λ > 420 nm)

H2 evolution; MB degradation

CdS /b-TiO2

250 W Xe lamp with and without a 420 nm UV-cut filter 300 W Xe lamp (λ > 420 nm)

Sunlight irradiation (AM 1.5G)

300 W Xe lamp (λ > 420 nm) 150 W Xe lamp with AM 1.5 filter

Visible light irradiation 300 W Xe lamp (λ > 420 nm)

300 W Xe lamp (λ > 420 nm) 300 W Xe lamp

300 W Xe lamp

Light source 300 W Xe lamp Black light-bulb lamp

Application H2 evolution Octadecanoic acid degradation

Photocatalyst B/Be WO3/TiO2

Table 1 The solar energy conversion of type II heterojunction photocatalysts

99.2% after 210 min; 526.3 μmol h
1 g
1 76.2% after 150 min 90.1% after 60 min 713.68 μmol h
1 g
1

Rate 9000 umol h
1 g
1 17.1  10
4 molecules photon
1 4806 μmol h
1 g
1; 99% for 120 min 99.8% with 40 min 99% within 60 min 3475 umol h
1 g
1 9039 μmolh
1 g
1; 100% in 45 min 6454 μmol g
1 h
1 nearly 90% degradation rate 99% after 60 min 87% after 180 min

(continued)

Hao et al. 2018

Hong et al. 2016

Ye et al. 2019 Adhikari and Kim 2018 Jiang et al. 2020

Yang et al. 2017 Zhang et al. 2016

Zuo et al. 2020 Zhang et al. 2020

Sun et al. 2018b

Ref. Bai et al. 2019 Sotelo-Vazquez et al. 2017 Zhao et al. 2018

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H2 evolution

CM-20

BMO/M88

Application H2 evolution H2 evolution 2,6-dichlorophen and 2,4,5trichlorophenol degradation; H2O2 evolution RhB degradation

Photocatalyst g-C3N4/P25(N)-Pd CIPS/CN U-g-C3N4/NH2MIL-101(Fe)

Table 1 (continued)

350 W Xe lamp (420 nm < λ < 760 nm) 300 W Xe lamp (λ > 420 nm)

Light source 300 W Xe lamp 300 W Xe lamp (λ > 420 nm) 300 W Xe lamp

Zhao et al. 2020b Xiao et al. 2020

2407 μmol g
1 h
1

Ref. Cai et al. 2019 Lin et al. 2020 Su et al. 2021

Rate 6.33 mmol h
1 g
1 45.1 μmol h
1 0.1 g
1 98.7 and 97.3% after 180 min; 69 μM within 180 min 99.5% after 120 min

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The Disadvantages of Type II Heterojunction Although the construction of this heterojunction mode plays a positive role in improving the efficiency of charge separation, it also has the following disadvantages, which I mainly summarized as two aspects (as shown in Fig. 10): 1. The separation efficiency of photogenerated electrons and holes is at the cost of reducing the REDOX capacity of the two semiconductor photocatalysts. The oxidizing ability of the heterojunction after formation is weaker than that of semiconductor A, and the reducing ability is weaker than that of semiconductor B. 2. In the presence of electrostatic interaction, the existence of photogenerated electron-hole pairs in the original photocatalyst would inhibit the interface transfer of electrons and holes into other catalysts.

Z-Scheme Heterojunction Photocatalysts Z-scheme heterojunction has similar band arrangement as type II heterojunction, but the electron transfer path is different. The electron migration path between semiconductors is similar to the English letter “Z,” so it is called Z-scheme photocatalytic system. Z-scheme heterojunction band arrangement and charge transfer mechanism are as shown in Fig. 11, the electrons in CB of A and the holes in VB of B recombine Fig. 10 The charge transfer and disadvantages of type II heterojunction photocatalyst

Fig. 11 The photocatalytic mechanism of Z-scheme heterojunction photocatalyst

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and annihilate. The remaining electrons mainly exist in CB of B, and the holes mainly exist in VB of A, which could realize the spatial separation of electrons and holes. This unique electron migration pathway results in the Z-scheme heterojunction maintaining high REDOX capacity while increasing the separation efficiency of photogenerated electron-hole pairs.

Oxide-Based Heterojunction Black phosphorus (BP) is a 2D material, its excellent optical properties make it shine in the field of photocatalysis. However, the photocatalytic performance of single BP is limited due to the low separation efficiency of charge carriers. A simple method had been developed to synthesize a two-dimensional BP/monolayer Bi2WO6 (MBWO) (Hu et al. 2019) Z-scheme heterojunction. With BP, its hydrogen production rate was more than 9 times higher than that without BP. In addition to the effect of BP, the high charge separation efficiency of the heterostructure was also inseparable, which had a broad application prospect in fields of energy. The formation of heterojunction is an effective way to promote the charge separation than that of a single substance. For example, zinc oxide with poor photocatalytic performance and cadmium sulfide with strong photocatalytic activity formed ZnO/CdS hierarchical heterojunction (Wang et al. 2019b), which greatly improved the photocatalytic hydrogen production performance of the material. Synergism is an amazing way to improve photocatalysis. As shown in the Fig. 12, Guo et al. made use of the Fenton reaction, SPR effect, and photothermal effect in the

Fig. 12 Schematic diagram of the feasible mechanism of photocatalytic-Fenton reaction, SPR effect, photothermal effect, and multiple light reflection within hollow α-Fe2O3/defective MoS2/Ag Z-Scheme heterojunctions (Reprinted with permission from Guo et al. 2020)

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heterojunction as well as the synergistic effect of the hollow structure to prepare the Z-scheme heterojunction of the hollow flower-shaped polyhedron α-Fe2O3/deficient MoS2/Ag (Guo et al. 2020b). The hollow structure promoted the multiple reflection of light and improved the utilization rate of light. The defective MoS2 widened the corresponding range of light, and Ag made it have surface plasmonic resonance effect. The photocatalytic degradation of 2, 4-dichlorophenol and salicylic acid by the composite structure was significantly higher than that of any single structure.

Sulfide-Based Heterojunction The key to improve the photocatalytic performance is to promote the separation efficiency of electrons and holes. In some cases, the composition of different materials can give play to their respective advantages, such as the appropriate band gap and band alignment between CdS and other substances. Zhang et al. (2020) designed a Z-scheme heterojunction photocatalyst based on CoSx/CdS, which presented different photocatalytic activities by controlling different molar ratios of substances. This is the first report on the direct synthesis of CoSx/CdS Z-scheme heterojunction by one-pot solvothermal approach based on ZIF-67 template. The resulting photocatalyst exhibited high photocatalytic stability and could effectively achieve synergistic improvement of photocatalytic activity and Cr (VI) reduction. The double Z-scheme heterojunction combined with multichannel charge transfer characteristics is beneficial to further improve the spatial separation efficiency of charge carriers. The hollow structure of MoSe2@Bi2S3/CdS (Wang et al. 2021) heterojunction synthesized by solvothermal hydrothermal method could improve the utilization of light source, and the core-shell structure could greatly supply the active site and surface activity. The double Z-scheme heterojunction structure had the advantage of full spectrum absorption with unpredictable photothermal effects, which could promote the photocatalytic reaction (Fig. 13). Zhang et al. (2021a) prepared Co9S8/Cd/CdS tubular heterojunction structure using cheap transition metal Cd as an electron bridge. The synergistic effect of hollow Co9S8 nanotubes and Cd/CdS nanoparticles extended the photoabsorption to visible-light region and at the same time increased the surface active sites, which greatly inhibited the recombination of photogenerated electron-hole pairs and subsequently improved the photocatalytic performance. Similarly, in the Bi2S3/rGO/ BiVO4 Z-scheme heterojunction synthesized through hydrothermal approach (Liang et al. 2020), rGO acted as an effective electronic intermediary between Bi2S3 and BiVO4, which promoted the transfer of photoinduced electrons from CB of BiVO4 to VB of Bi2S3, thus reducing electron accumulation. The Ag-C3N4/SnS2 (Zhao et al. 2021) Z-scheme heterojunction formed by plasma resonance effect had greatly improved the photocatalytic performance of the system due to the enhanced visible light absorption by LSPR and the effective interfacial charge transfer and separation in Ag-C3N4/SnS2 Z-scheme system.

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Fig. 13 The schematic diagram of photocatalytic H2 production and pollutant degradation by MS@BS/CS dual Z-scheme heterojunction (Reprinted with permission from Wang et al. 2021)

g-C3N4-Based Heterojunction Recently, g-C3N4 has been widely used in photocatalysis, but its application as a photocatalytic material is limited by the rapid recombination of electron-hole pairs and the limited visible-light absorption. The matching of energy levels between two substances is the key to the formation of heterojunction. Di et al. (2017) used hydrothermal synthesis method to load SnS2 on the surface of g-C3N4 in one step, and g-C3N4 electrons were transferred to SnS2, resulting in the formation of an internal electric field at the interface between the two kinds of semiconductors at equilibrium. Driven by the internal electric field, the formation of g-C3N4/SnS2 Z-scheme heterojunction further promoted charge separation. The Z-scheme configuration resulted in a more efficient photocatalytic reduction of carbon dioxide. Fe MOF was grown in situ on surface of g-C3N4, MIL-101(Fe) was used as skeleton to prevent g-C3N4 from folding, and g-C3N4 could expand the absorption of light. The resulting visible light absorbing MIL-101(Fe)/g-C3N4 (Zhao et al. 2020a) greatly increased the interface carrier separation efficiency, resulting in increased photocatalytic activity of Cr(VI) reduction and BPA degradation. The key to the construction of Z-scheme photocatalytic system is to adjust the interface band bending and charge transfer. It has been found that modulated interface band bending can convert type II composite material into direct Z-scheme. Huang et al. (2017) took C3N4-W18O49 as an example, and the introduced triethanolamine adsorbed on the surface of C3N4 significantly increased the Femi level of the system, and the bending of the interface band changed, thus transforming the composite material from type II to Z-scheme.

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Others Electron transfer medium plays a vital role in the construction of semiconductor catalysts. Polydopamine (PDA) has a large amount of charge on its surface and good biocompatibility. Its π-π-conjugated structure has excellent electron transport ability. The PDA-modified g-C3N4@PDA/BiOBr (Guo et al. 2020) structure accelerated the interaction between interfaces, which was conducive to the rapid separation and efficient transfer of light-induced charges at the interfaces. Reasonable utilization of exciton effect produced by low-dimensional semiconductor photocatalytic materials is beneficial to the utilization of photocatalytic energy. Zhou et al. (2021) made full use of the role of hot carriers and excitons in the photoinduction process to prepare AgI/Bi3O4Br heterojunction photocatalytic materials, which could produce a large number of reactive oxygen species and greatly promoted the photocatalytic degradation of various organic pollutants. Plasmas play an important role in promoting charge separation. Du et al. successfully prepared Bi3O4Cl/AgCl (Du et al. 2020) Z-scheme heterojunction with double plasmas (Bi and Ag) by hydrothermal method and reduction method. The plasmas Bi and Ag were firmly fixed on the surface of Bi3O4Cl and AgCl, respectively, which was conducive to charge transfer. As shown in the Fig. 14, the SPR effect of Bi and Ag and the synergistic effect of Z-scheme heterojunction greatly improved the charge separation efficiency, which was conducive to the pyhotocatalytic degradation of pollutants. The solar energy conversion of some Z-scheme heterojunction photocatalysts is shown in Table 2.

Fig. 14 Proposed photocatalytic mechanism diagram of Z-Scheme photocatalytic Bi-BN/Ag-AgCl driven by visible light (Reprinted with permission from Du et al. 2020)

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Table 2 The solar energy conversion of Z-scheme heterojunction photocatalysts Photocatalyst BP/MBWO

Application H2 evolution; NO removal

Light source Visible light irradiation

ZnO/CdS

H2 evolution

α-Fe2O3/ defective MoS2/Ag

2,4dichlorophenol and salicylic acid degradation Cr(VI) reduction

350 W Xe lamp Visible light irradiation

CoSx/CdS MoSe2@Bi2S3/ CdS Co9S8/Cd/CdS

Bi2S3/rGO/ BiVO4 Ag-C3N4/SnS2

H2 evolution Cr(VI) and TCP removal H2 evolution

Cr(VI) reduction; bisphenol A degradation H2 evolution; TC degradation

g-C3N4 /SnS2

CO2 reduction

MIL-101(Fe)/ g-C3N4

Cr(VI) reduction; BPA degradation

C3N4/W18O49

H2 evolution

g-C3N4@PDA/ BiOBr

Sulfamethoxazole degradation

AgI/Bi3O4Br

RhB, phenol, APAP, and CIP degradation Cr(VI) reduction; ceftriaxone sodium degradation

Bi3O4Cl/AgCl

300 W Xe lamp 300 W Xe lamp (λ > 420 nm) 300 W Xe lamp with a UV filter 300 W Xe lamp (λ > 420 nm) 500 W Xe lamp with a 420 nm cut-off filter 300 W Xe light (λ  420 nm) 150 W halogen cold light source (λ > 420 nm) 300 W Xe lamp (λ > 400 nm) 300 W Xe lamp (λ > 400 nm) 300 W Xe lamp (λ > 420 nm) 300 W Xe lamp

Rate 21,042 μmol g
1 5 h
1; 67% after 30 min 4134 μmol g
1 h
1 99% after 150 min 99% after 150 min

100% within 30 min 11.84 mmol g
1 h
1; 98.7% and 99.2% after 80 min 10.42 μmol h
1 2 mg
1

Ref. Hu et al. 2019 Wang et al. 2019b Guo et al. 2020

Zhang et al. 2020 Wang et al. 2021 Zhang et al. 2021b

100% after 120 min

Liang et al. 2020

1104.5 μmol g
1 h
1; 94.9% with 150 min

Zhao et al. 2021

CH3OH yield 2.3 μmol g
1

Di et al. 2017

Reduction Cr (VI) 98.8%; 94.8%

Zhao et al. 2020a

8597 μmol h
1 g
1

Huang et al. 2017

Nearly 100% within 60 min

Guo et al. 2020

100% within 40 min; 88%, 82%, 90% within 120 min 98.3% and 98.3% within 210 min

Zhou et al. 2021 Du et al. 2020

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The Disadvantages of Z-Scheme Heterojunction The Z-scheme heterojunction can not only realize the spatial separation of electrons and holes but also ensure that the photocatalyst can keep the appropriate valence band and conduction band position, so as to maintain the strong REDOX reaction ability. However, there are also the following problems: 1. The system is mainly limited to the reaction of the solution phase. Although some solid phase is present, it is not significant obviously. 2. As shown in Fig. 15, as the potential difference between the VB of A and the CB of B is large, side reactions could also be occurred, which will interfere with the charge transfer process. 3. The pH requirement of the solution is stringent.

S-Scheme Heterojunction Photocatalysts Based on the understanding of traditional heterojunction, the concept of a new stepScheme (S-scheme) heterojunction simulating photosynthesis system was first proposed by Yu’s research group in 2019 (Fu et al. 2019). As shown in Fig. 16, the heterojunction is mainly by the work function of smaller, Fermi level higher reduction type semiconductor photocatalyst (RP), and the work function is bigger, the Fermi level lower oxidation type semiconductor photocatalyst (OP) is constructed from the staggered type way, effective electrons and holes are saved, Fig. 15 The charge transfer and disadvantages of Z-scheme heterojunction photocatalyst

Fig. 16 The photocatalytic mechanism of S-scheme heterojunction photocatalyst

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Fig. 17 The charge transfer process in the S-scheme heterojunction before and after contact and light irradiation

and meaningless photoproduction carrier were back together. The electrons with strong reducing capacity and the holes with strong oxidizing capacity are retained to participate in the reduction reaction (e.g., hydrogen production) and oxidation reaction (e.g., oxygen production), respectively. The charge transfer process in the S-scheme heterojunction is shown in Fig. 17. Through three factors, such as the built-in electric field, band bending, and electrostatic interaction, the spatial separation of semiconductor photogenerated electron-hole pairs with strong REDOX capacity is realized.

Oxide-Based Heterojunction Xu et al. reported a TiO2/CsPbBr3 (Xu et al. 2020a) heterojunction synthesized by electrostatic drive self-assembly. Theoretical calculations and other analyses showed that electrons were transferred from CsPbBr3 to the internal electric field of TiO2, thus proving that the heterojunction was S-scheme heterojunction, which greatly promoted the separation of electron-hole pairs and improved the reduction rate of carbon dioxide obviously. The same research group prepared TiO2/CdS (Ge et al. 2019) S-scheme heterojunction nanofibers with almost the same method. Various tests proved that there was an internal electric field in the nanofibers, which improved the charge separation efficiency. The hydrogen yield of the composite structure was 35 times as high as that of the original TiO2, and its quantum efficiency was up to 10.14%. This synthesis method provides an efficient strategy for the synthesis of heterojunction photocatalysts. Tungsten trioxide (WO3) is a typical oxidizing photocatalyst with a small band gap (2.4–2.8 eV) and a large work function. It is also an ideal semiconductor photocatalyst for constructing S-scheme heterostructures. A graphene-modified S-scheme WO3/TiO2 heterojunction photocatalyst was prepared (He et al. 2020). According to the S-scheme charge transfer mechanism, under light irradiation, the relatively useless electrons on CB of WO3 compound with the relatively useless holes on VB of TiO2 and the electrons with strong reducing ability on CB of TiO2 and the holes with strong oxidization ability on VB of WO3 could be retained. Thus,

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the photogenerated electron-hole pairs with strong REDOX ability could be separated at the heterogeneous interface. The hollow structure prepared by SiO2 template method has attracted much attention in the technical field due to its unique properties based on the microscopic “wrapping” effect. Meng et al. (2021) successfully prepared polydopamine-modified TiO2 hollow sphere (TP3) S-scheme heterojunction by using SiO2 template method. The introduction of polydopamine broadened the absorption of light and improved the utilization of visible light. The yield of methane was 1.50 μmol
1 g
1, 5 times higher than that of the original TiO2. A S-scheme heterojunction consisting of p-type and n-type semiconductor was reported for the first time (Deng et al. 2021). The p-type ZnMn2O4 and n-type ZnO successfully synthesized ZZM30 S-scheme heterojunction by means of electrospinning and calcination. In addition to energy band matching, multiple active sites and efficient charge separation also played an indispensable role in promoting photocatalytic performance. Porous graphite carbon nitride (PCN) has attracted wide attention because of its good stability and suitable band gap. On the basis of PCN, Li et al. (2021a) carried out S-doping treatment and then formed S-PCN/WO2.72 S-scheme heterojunction with tungsten oxide (WO2.72), which promoted the recombination of useless photoinduced carriers and retained photoinduced electrons (e
), holes with high REDOX capacity. It showed excellent photocatalytic performance.

Sulfide-Based Heterojunction The application of 2D photocatalytic materials plays an important role in the design of photocatalytic materials. Hu et al. (2020) successfully prepared a 2D photocatalytic material without precious metal co-catalyst. Ni2P cocatalysted 2D/2D SnNb2O6/CdS-diethylenetriamine heterojunction had higher surface active contact area, which reduced carrier transport distance and thus enabled the photogenerated electrons and holes to longer lifetime for the subsequent photocatalytic reaction. The electrostatic field formed by contact between materials can greatly improve the transport rate of internal photogenerated charge carriers. A novel Mn0.2Cd0.8S/ CoTiO3 (Liu et al. 2021) photocatalytic system greatly reduced the overpotential of hydrogen production due to the effect of the internal electric field, and at the same time, the response of the system to visible light was extended due to the participation of CoTiO3. Molybdenum disulfide as a star material is also widely used in the construction of semiconductor photocatalytic materials. The typical layered structure has excellent optical properties and thermal and electrocatalytic properties. The core-shell structure (Xu et al. 2020b) has excellent REDOX ability and plays a strong driving role in the degradation process of rhodamine B. N-doped MoS2 exposed a lot of edge active sites due to the substitution of S atom by N and thus had a low hydrogen production overpotential. The composite photocatalyst (Chen et al. 2021) formed by N-doped

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MoS2 and S-doped g-C3N4 broadened the absorption range of light, and the rate of hydrogen production was several times higher than that of single one. Jia et al. (2021) found that the use of different amounts of similar crystals, the synergistic effect of components, and the structure of the system could improve the photocatalytic activity. The photocatalytic performance of ZnO/CdS/MoS2 heterojunction could be up to 10,247.4 μmol g
1 h
1 by using the S-scheme heterojunction formed between ZnO and CdS and the high conductivity of MoS2.

g-C3N4-Based Heterojunction Appropriate contact area is an effective method to improve photocatalytic activity in the process of constructing heterojunction. 2D surface heterostructures have been increasingly used in photocatalytic materials, such as ultrathin 2D/2D WO3/g-C3N4 (Fu et al. 2019) heterostructures, which effectively inhibited the recombination of photogenerated electrons and holes due to its special 2D structure, and greatly improved the photocatalytic hydrogen production performance. Pan et al. (2020) further prepared carbon-doped 2D heterostructures using anionic polyacrylamide, and carbon also acted as an electronic bridge to shorten electron transport distance and accelerate electron transfer. The band matching of two kinds of semiconductor is an important basis for the construction of high-efficiency photocatalyst. Non-toxic semiconductor photocatalytic materials with high visible light catalytic activity have been more and more used in biological sterilization. Xia et al. (2019) used a simple method to prepare a highly efficient CeO2/PCN visible light photocatalyst that killed bacteria when excited by visible light. Transition metal phosphate sulfides have attracted much attention because they can provide many active sites for catalytic reactions. The S-scheme heterostructure with high catalytic activity was synthesized from supercentral copper phosphate nanocrystals and g-C3N4, which was the first transition metal phosphate sulfide used in photocatalytic reduction of carbon dioxide (Zhang et al. 2021a). Photocatalysis is a promising method for environmental remediation, so the preparation of economical and recyclable photocatalyst is one of the focuses for current research. Dai et al. (2021) prepared a highly efficient photocatalyst ZnFe2O4/ g-C3N4 that could convert soluble uranium (IV) to uranium (VI) precipitation, and the removal capacity of uranium (IV) reached 1892.4 mg g
1 under visible light irradiation.

Others The isotype heterojunction photocatalyst also has excellent photocatalytic performance. Some studies have found that g-C3N4 synthesized by different precursors had different molecular structures and valence band structures. Xu et al. (2019)

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prepared homotype g-C3N4 heterojunction MCN/UCN using melamine and urea, providing a new idea for the preparation of g-C3N4 photocatalyst with high performance. Favorable structural defects and interfacial contact modes can greatly improve the photocatalytic performance of the materials. The Ru/SrTiO3/TiO2 (Li et al. 2021b) hybrid photocatalyst formed in situ on SrTiO3 with S-scheme/Schottky junction and oxygen vacancy effectively promoted the transport of charge carriers and provided adequate active sites for photocatalytic hydrogen production. The solar energy conversion of some S-scheme heterojunction photocatalysts is shown in Table 3.

The Disadvantages of S-scheme Heterojunction Even though it has a great advantage, as long as it is heterojunction, its nature will not change; the highest efficiency will not exceed 50%. In addition, it also has the following disadvantages: 1. The application scope is limited. At present, it is mainly limited to the application of powder photocatalyst. 2. The two kinds of semiconductors that constitute the S-scheme system are mainly N-type semiconductors, which should have appropriate energy band structure and significant Fermi energy level difference. This greatly limits the choice of photocatalysts. 3. It is difficult to supply direct evidence for the formation of S-scheme heterojunction.

Tandem Heterojunction Photocatalysts Inspired by photosynthesis in nature, Z-scheme and S-scheme photocatalytic systems have been constructed, which improve the absorption and utilization rate of sunlight and significantly improve the photocatalytic performance, but the quantum efficiency is reduced by half. Therefore, a reasonable heterojunction photocatalytic system should be constructed, and the photocatalytic materials that can improve the utilization of sunlight absorption and the separation and transfer efficiency of photogenerated carriers can be obtained by taking advantage of the light absorption characteristics and synergistic effect of different semiconductor materials, so as to improve the efficiency of solar photocatalytic activity. In order to realize the efficient transfer and separation of photogenerated charge carriers and the effective absorption and utilization of sunlight, Sun et al. (2018a) proposed to construct tandem heterojunction to improve the solar-driven photocatalytic performance through the effective tandem between two kinds of semiconductors. According to the energy band position and energy level structure characteristics, the research group connected two kinds of light-capturing

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Table 3 The solar energy conversion of S-scheme heterojunction photocatalysts Photocatalyst TiO2/CsPbBr3

Application CO2 reduction

TiO2/CdS

H2 evolution

WO3/TiO2

H2 evolution

TiO2/ polydopamine ZnMn2O4/ZnO

CO2 reduction

S-pCN/WO2.72

SNO/CdS-D

CO2 reduction Degradation of TC; H2 evolution H2 evolution

Mn0.2Cd0.8S/ CoTiO3 BiVO4@MoS2

H2 evolution

NMS/SCN

H2 evolution

ZnO/CdS/MoS2

H2 evolution

WO3/g-C3N4

Tetracycline degradation

CeO2/PCN

ZnFe2O4/gC3N4 MCN/UCN

Photocatalytic inactivation of bacteria CO2 Reduction Uranium (VI) removal H2 evolution

Ru/SrTiO3/TiO2

H2 evolution

us-Cu3P|S/CN

Degradation of Rhodamine B

Light source 300 W Xe lamp 350 W Xe lamp 350 W Xe lamp 350 W Xe lamp 300 W Xe lamp 300 W Xe lamp (λ > 420 nm) 300 W Xe lamp 5 W lightemitting diode Xe lamp of 500 W (λ  420 nm). 300 W Xe lamp 300 W Xe lamp light source (λ > 400 nm) 300 W Xe lamp (λ > 400 nm) LED light (λ  420 nm)

Rate 9.02 μmol g
1 h
1

300 W Xe lamp 8 W LED light 300 W Xe lamp 300 W Xe lamp

2.32 mmol h
1 g
1 245.8 μmol g
1 h
1 1.50 μmol h
1 g
1 3.3 μmol h
1 g
1 85% after 120 min; 786 μmol g
1 h
1 11,992 μmol g
1 h
1 138.2 μmol within 5 h 100% within 20 min 658.5 μmol g
1 h
1 10,247.4 μmol g
1 h
1

Ref. Xu et al. 2020a Ge et al. 2019 He et al. 2020 Meng et al. 2021 Deng et al. 2021 Li et al. 2021a Hu et al. 2020 Liu et al. 2021 Xu et al. 2020b Chen et al. 2021 Jia et al. 2021

90.54% within 1 h

Pan et al. 2020

88.1% within 15 min

Xia et al. 2019

137 μmol g
1 h
1

Zhang et al. 2021a Dai et al. 2021 Xu et al. 2019 Li et al. 2021b

1892.4 mg g
1 29.9 μmol h
1 50 mg
1 2.33 mmol g
1 h
1

semiconductor materials with cocatalyst MoS2 as a bridge to construct the hollow hierarchical structure black TiO2-MoS2 heterojunction solar photocatalyst, which not only expanded the absorption of visible light and NIR but also realized the efficient separation and transfer of photogenerated charge carriers. So that it had

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Fig. 18 (a) SEM images of b-TiO2 microspheres and (b) b-TiO2/MoS2 microspheres. (c, f) SEM, (d) TEM, and (e) HRTEM image of b-TiO2/MoS2/CdS tandem heterojunctions microspheres. (g–k) The corresponding EDX mappings of elemental Ti, O, Mo, S, and Cd in the selected area in panel (f) (marked by square) (Reprinted with permission from Sun et al. 2018b)

excellent solar-driven photocatalytic hydrogen production performance. In that work, mesoporous hollow black TiO2 microspheres were used as hosts, and MoS2 was vertically coated on the surface of the hollow spheres (Fig. 18), which did not affect the absorption of light by black TiO2. Moreover, the deposited CdS mainly grew on the edge of MoS2, which was the main active site for hydrogen production of MoS2. Black TiO2 and CdS could absorb ultraviolet light and visible light simultaneously on both sides of MoS2, thus forming an effective tandem heterojunction. Under the irradiation of simulated sunlight AM 1.5G, the tandem heterojunction showed excellent photocatalytic performance, photocatalytic H2 evolution was up to 280 μmol h
1 20 mg
1, and the performance almost remained unchanged after 10 cycles, indicating high stability. This was because the formation of the tandem heterojunction significantly inhibited the photocorrosion of CdS (Fig. 19) and prolonged the lifetime of photogenerated charge carriers (Fig. 20), showing an important application prospect in field of energy.

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Fig. 19 (a, b) Cycling tests of photocatalytic hydrogen evolution under visible light (λ > 400 nm) of: (a) b-TiO2/MoS2/CdS, b-TiO2/MoS2, and b-TiO2, under AM 1.5 irradiation and of (b) b-TiO2/ MoS2/CdS and MoS2/CdS. (c) The comparison of photocatalytic H2 production activity of different photocatalysts under visible light irradiation. (d) The photocatalytic hydrogen evolution rates under single-wavelength light and the corresponding AQE of b-TiO2/MoS2/CdS, b-TiO2/MoS2, and b-TiO2, respectively. (Reprinted with permission from Sun et al. 2018)

Oxide-Based Heterojunction Three main forms of titanium dioxide in nature (anatase, rutile, and brookite) have attracted much attention in the field of photocatalysis due to their excellent optical properties and morphology regulation, and they have also been widely used in heterostructures. Wei et al. (2018) prepared mesoporous brookite/anatase TiO2/g-C3N4 hollow microspheres using the surface nano-coating process, in which brookite accounted for 48% of TiO2, anatase 44%, and rutile 8%. The hollow microsphere with the series heterojunction had an excellent photocatalytic performance of degrading phenol under visible light irradiation, about 5 times higher than that of single mesoporous g-C3N4. This series heterojunction improved the charge separation efficiency, indicating that the multicomponent heterojunction could enhance the photocatalytic performance in the visible light range. Interfacial effect plays a vital role in photocatalytic reaction of environmental remediation. In addition to the above, an innovative Mn3O4@ZnO/ TiO2 heterojunction (Li et al. 2020) had been prepared by atomic layer deposition.

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Fig. 20 (a) Nyquist plots of electrochemical impedance in the dark and under visible light irradiation, (b) Mott–Schottky plots, (c) SKP maps, and (d) time-resolved fluorescence spectra of b-TiO2/MoS2/CdS, b-TiO2/MoS2, and b-TiO2, respectively. (e) Schematics of the b-TiO2/MoS2/ CdS tandem heterojunctions used for solar-driven water splitting. (Reprinted with permission from Sun et al. 2018)

It had excellent N reduction properties, which had been demonstrated by free radical capture experiments that e
and CO∙2
played key roles in the reduction process. Efficient charge separation and stability are the keys to energy conversion on photocatalysts. Xue et al. (2020) constructed a WO3/CdS/WS2 tandem heterojunction consisting of 0D CdS, 2D ultrathin WO3, and 2D WS2. The electron

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channel between the interfaces promoted the charge separation and exhibited an apparent quantum efficiency of 22.96% at 435 nm.

Sulfide-Based Heterojunction Due to their large specific surface area, 2D semiconductor nanostructures have shown great potential in photocatalytic energy conversion. CdS@Ti3C2@CoOlayered tandem heterojunction was designed by Ai et al. (2020) with reference to energy band. The introduction of Ti3C2 successfully solved the incompatibility problem between CdS and CoO, and meanwhile, the three internal electric field significantly improved the charge separation efficiency of photogenerated carriers and also showed excellent photocatalytic hydrogen production performance and stability under the condition of no sacrifice agent. The Schottky barrier could induce spatial charge separation and prolong the carrier lifetime. A (1D/2D) CdIn2S4/carbon nanofiber (CNF)/Co4S3 tandem Schottky heterojunction was successfully prepared by in situ electrospin-hydrothermal method (Guo et al. 2021). The 2D sheets structure provided a rich active site for the photocatalytic reaction, and the hydrogen production rate was up to 25.87 mmol g
1 h
1. Guo et al. (2020) artificially realized the efficient transfer of photogenerated charge carriers and excellent photocatalytic performance and constructed the Cu2
xS/CdS/Bi2S3 series heterojunction using the continuous growth deposition method. Its unique hollow interlayer structure (Fig. 21) provided a large specific surface area and active site, and the tandem heterostructure enhanced the separation of photoinduced charge carriers, showing excellent photocatalytic hydrogen production (8012 μmol h
1 g
1) and degradation performance. Due to its large specific surface area and multichannel charge transfer, shelllayered structures have attracted much attention in photocatalysis. As shown in Fig. 22, Zhang et al. (2020) successfully synthesized MIL-125(Ti)/Znin2S4/CdSlayered and tandem heterojunction using hydrothermal and solvothermal methods, in which Znin2S4 acted as a bridge connecting MIL-125(Ti) and CdS. With a narrow band gap, it could effectively absorb visible light. This made it had excellent photocatalytic hydrogen production and degradation properties.

Fig. 21 Synthetic procedure for the hollow octahedral Cu2
xS/CdS/Bi2S3 superstructure. (Reprinted with permission from Guo et al. 2020)

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Fig. 22 The SEM images of TiM (a), 45% TiM@ZIS (b), and TiM@ZIS/7% CdS (c), respectively. And the TEM images of 45% TiM@ZIS (f), TiM@ZIS/7% CdS (d), the lattice fringes of ZIS, CdS (e), the HRTEM images of TiM@ZIS/CdS (g, h), respectively. (Reprinted with permission from Zhang et al. 2020)

g-C3N4-Based Heterojunction As a cocatalyst, MoS2 can replace Pt to complete the photocatalytic hydrogen production process. CeO2@MoS2/g-C3N4 (Zhu et al. 2019) series heterojunction showed excellent hydrogen production performance in the absence of Pt, up to 65.4 μmol h
1 50 mg
1, and its apparent quantum efficiency at 420 nm could reach 10.35%. The high photocatalytic performance could be attributed to the precise energy band matching and the reversible conversion of Ce3+ and Ce4+, which improved the charge separation efficiency, and was of great significance to solar fuel conversion. As shown in Fig. 23, the ternary photocatalyst of ZnS quantum dots/sea urchinlike Bi2S3 spheres/meso-g-C3N4 nanosheets (ZnS/Bi2S3/Meso-g-C3N4) (Jiang et al. 2020) was successfully fabricated. From Fig. 23c, f, it could be observed that the

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Fig. 23 SEM image of Bi2S3 spheres (a), TEM image of Bi2S3 spheres (b), HRTEM of Bi2S3 nanospheres (c), TEM images (d–f) of ZnS quantum dots/Bi2S3 spheres/Mesog-C3N4 nanosheets. (Reprinted with permission from Jiang et al. 2020)

lattice fringe of Bi2S3 was d(200) ¼ 0.56 nm, which fully indicated that Bi2S3 had been successfully loaded. In addition, the lattice fringe d(111) ¼ 0.295 nm of ZnS (Fig. 23e, f) could be clearly seen, indicating that ZnS had been successfully anchored. The composite structure expanded the light response, which increased the utilization rate of sunlight and significantly increased the carrier density. The recombination rate of electron and hole was significantly reduced. This work provides a new strategy for the construction of other quantum dot-semiconductor photocatalysts with high photocatalytic performance and solar energy conversion.

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Others Reasonable heterojunction design plays an important role in promoting photocatalysis. The ternary heterojunction 3J-2DT (Butburee et al. 2019), which was used to enhance the reduction of carbon dioxide, had been successfully prepared, and the carbon dioxide conversion rate could reach 86.9 μmol
1 g
1. The excellent performance could not be separated from their synergistic effects, among which ZIF-8 played the role of carbon dioxide capture, allowing the reactants to gather on the catalyst and increasing the concentration of the reactants. AuCu was mainly used as an electron transport intermediate to promote charge transfer to ZIF-8. A kind of efficient photocatalytic performance of the new type of double Z-scheme consisting of three meta vanadate series heterojunction was successful synthesis (Zeng et al. 2018); it kept the original mesoporous nanoscale piece at the same time and shortened the band gap obviously. It also increased the efficiency of sunlight; heterogeneous structure also improved the separation efficiency of the photogenerated charge carriers. A tandem heterojunction CdS/KPW/Meso-g-C3N4 composed of Z-scheme, and type II was successfully prepared by Qiu et al. (2021). As can be seen from Fig. 24, the interfaces among the three substances had been closely combined with each other, and their separate lattice fringes could be clearly observed. The photocatalyst exhibited excellent photocatalytic degradation and hydrogen production performance, and its energy band structure and degradation mechanism were shown in Fig. 25. CdS had more negative CB potential level, which was conducive to the generation of O2
nanoparticle radicals in the degradation process, improving the degradation ability. The electrons on Meso-g-C3N4 were transferred to CdS, and the reduction reaction occurred on CdS to produce hydrogen. The solar energy conversion of some tandem heterojunction photocatalysts is shown in Table 4.

The Disadvantages of Tandem Heterojunction Compared with other heterojunctions, the tandem heterojunction greatly improves the absorption and utilization rate of sunlight and the separation and transfer efficiency of photogenerated charge carriers. However, it is a multicomponent composite structure after all, and the contact surface is not easy to control. The preparation of tandem heterojunction photocatalysts is still very complicated. In addition, its essential problem of low efficiency has not been solved completely.

Conclusion and Perspective Photocatalytic technology has good application prospects in the fields of energy and environment. The photocatalytic activity of single catalyst is low, but the heterojunction can significantly improve the photocatalytic activity. In the type II heterojunction, two kinds of semiconductor are excited at the same time, and the

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Fig. 24 The SEM images of KPW (a), TEM image of meso-g-C3N4 (b), SEM images of CdS/KPW/meso-g-C3N4 (c), TEM image of CdS/KPW/meso-g-C3N4 (d–f), SEM image (g), and the corresponding EDX maps of C, N, O, P, S, Cd, K, and W elements (h). (Reprinted with permission from Qiu et al. 2021)

photogenerated electrons and holes transfer in reverse to produce spatial isolation, which can effectively inhibit the recombination and provide more photogenerated electrons and holes. In the Z-scheme heterojunction, electrons are transferred from the valence band of a semiconductor to the conduction band of another semiconductor with higher energy level through a special interfacial phase or conductive medium, which can not only make the spatial isolation between the photogenerated electrons and holes but also ensure that the photogenerated electrons have a strong reducing ability. In the S-scheme heterojunction, the effective electrons and holes are preserved, while the meaningless photogenerated carriers are recombined, which can effectively achieve the separation of the electron-hole pairs with strong REDOX capacity. The tandem heterojunction can greatly improve the absorption efficiency of

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Fig. 25 Schematic diagram of the photocatalytic mechanism for CdS/KPW/meso-g-C3N4 tandem heterojunction under visible light irradiation. (Reprinted with permission from Qiu et al. 2021)

sunlight and the efficiency of photogenerated charge separation and transfer, so as to significantly improve the photocatalytic performance of sunlight. At present, the research of heterojunction photocatalysts for solar energy conversion mainly faces the following challenges and development trends. 1. The photocatalytic activity is low, and there is still a large distance from the practical application. Combining computational and experimental methods, the design and preparation of photocatalytic materials that can enhance light absorption, promote the separation and transfer of charge carriers, prolong the lifetime of photogenerated electrons and holes, and improve the utilization ratio of photogenerated charge carriers is the key to improve the photocatalytic activity. Using DFT to calculate the electronic structure of semiconductor has guiding significance for designing efficient photocatalytic materials and elucidating the reaction mechanism. 2. Photocatalytic reduction of CO2 is still in the new stage; the focus of research is to improve the activity of photocatalyst and selective reduction of CO2. CO2 adsorption, cocatalyst activation of CO2, H2O adsorption effect, catalyst and the effect of intermediate, the exploration of the photocatalytic mechanism such as fast response, to promote the development of photocatalytic reduction of CO2 has important theoretical significance, therefore need to further research systematically. 3. It is important to note that the light harvesting and charge separation of Z-scheme heterojunction systems are strongly dependent on the spatial distribution, morphology, and crystal structure of their components. Therefore, adjusting the suitable geometrical structure creates a great opportunity to further improve the photocatalytic performance.

H2 evolution

H2 evolution

H2 evolution

H2 evolution; 2,4-dichlorophenol degradation H2 evolution; 2,6-dichlorophen and 2,4,5-trichlorophenol degradation H2 evolution Bisphenol A degradation; H2 evolution CO2 reduction

WO3/CdS/WS2

CdS@Ti3C2@CoO

CdIn2S4/CNFs/Co4S3

Cu2
xS/CdS/Bi2S3

Rh B, MO, Orange II and MB degradation

H2 evolution; Cr6+ removal

S-700

CdS/KPW/meso-g-C3N4

3 J-2DT

NH2-MIL-125(Ti) @ZnIn2S4/CdS CeO2@MoS2/g-C3N4 ZnS/Bi2S3/Meso-g-C3N4

N2 reduction

Application H2 evolution Phenol degradation

Photocatalyst b-TiO2/MoS2/CdS Brookite/anatase TiO2/gC3N4 Mn3O4@ZnO/TiO2

Light source AM 1.5G 500 W Hg (Xe) lamp (λ < 420 nm) LED light source (100 W, λ ¼ 416 nm) Visible light irradiation (λ > 420 nm) 300 W Xe lamp (λ ¼ 420 nm) Xe lamp with an optical filter (λ > 420 nm) 300 W Xe lamp (λ > 420 nm) 300 W Xe lamp with AM 1.5 irradiation UV-LEDs with the 420 nm 300 W Xe lamp (λ  420 nm) Xe lamp with 1.5 AM simulated sunlight 400 W metal halide lamp (95% of 420 nm visible light and 5% ultraviolet light) 300 W Xe lamp (λ  420 nm)

Table 4 The solar energy conversion of tandem heterojunction photocatalysts

Ai et al. 2020 Guo et al. 2021

134.46 μmol h
1 g
1 5.87 mmol g
1 h
1

568 mmol h
1 g
1; 97% after 150 min

88.99%, 56.86% and 83.29% within 75 min and 89.55% within 30 min

Qiu et al. 2021

Butburee et al. 2019 Zeng et al. 2018

Zhu et al. 2019 Jiang et al. 2020

Zhang et al. 2020

Guo et al. 2020

Xue et al. 2020

14.34 mmol h
1 g
1

8012 μmol h
1 g
1; 98.6% within 120 min 2.367 mmol g
1 h
1; 98.6% and 97.5% within 210 min 65.4 μmol h
1 50 mg
1 98.8% after 210 min; 663.3 μmol h
1 g
1 86.9 μmol h
1 g
1

Li et al. 2020

Ref. Sun et al. 2018 Wei et al. 2018

98.6% within 120 min

Rate 280 μmol h
1 20 mg
1 4.1  10
3 min
1

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4. As a new heterojunction system, S-scheme heterojunction has not been studied much, so it is necessary to find possible measures to enhance the photocatalytic performance of S-type heterojunction. 5. It is difficult to control the contact surface, and the control of the contact surface has a direct influence on the photocatalytic reactivity. 6. In situ technology is necessary for revealing the photocatalytic mechanism, especially the variation of heterojunction during the photocatalytic reaction. So, further in situ technology, such as in situ STEM, in situ X-ray absorption, in situ time resolved absorption spectrum, etc., is favorable for deeply understanding the photocatalytic mechanism. The study of photocatalytic heterojunction mechanism has far-reaching and comprehensive significance for photocatalytic technology. The development and design of efficient, stable, cheap, clean, green, and cyclic photocatalytic reaction system, especially in practice, can be applied on a large scale, and the realization of industrialized photocatalyst materials is still an important research direction in the near future. It may be the development of science that solving problems creates new one. Acknowledgments We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21871078, 52172206 and 51672073) and the Natural Science Foundation of Heilongjiang Province (JQ2019B001), and the Development Plan of Youth Innovation Team in Colleges and Universities of Shandong Province.

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Pool Boiling Heat Transfer Enhancement Using Nanoparticle Coating on Copper Substrate

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Sudhir Kumar Singh and Deepak Sharma

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pool Boiling Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pool Boiling Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Critical Heat Flux and Heat Transfer Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Formula to Calculate Critical Heat Flux (q00 ) and Heat Transfer Coefficient (h) . . . . . . . . . . Porosity and Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Coating for Pool Boiling Heat Transfer Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide Nanoparticle Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-porous Metal Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wettability: Hydrophilic and Hydrophobic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-Based Nanoparticle Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanowire Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microscale Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flow Boiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Coating for Flow Boiling Heat Transfer Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nanomaterial deposition is a very promising technique used in enhancing the heat transfer performance used for various application such as cooling microelectronic chip, cooling of nuclear rod in nuclear reactor, and size reduction of cooling device. The pool boiling heat transfer is enhanced by increasing the critical heat flux and heat transfer coefficient while reducing wall superheat. This chapter delivers a broad study of pool boiling heat transfer augmentation applying nanoparticle coating such as metal (aluminum, copper), metal oxide (Al2O3, S. K. Singh · D. Sharma (*) Department of Mechanical Engineering, National Institute of Technology, Hamirpur, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_57

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SiO2, TiO2), composite (Cu-Al2O3, Cu-TiO2, graphene-Cu), carbon-based nanomaterial (graphene, single and multiwalled carbon nanotubes), and nanowire (copper nanowire, silicon nanowire). Various augmentation techniques were analyzed to attain better heat transfer performance in which nanoparticle coating was obtained to be the most prominent method for different cooling applications. We analyzed that the heat transfer augmentation was influenced by various parameters like nanoparticle size, thermal conductivity of nanomaterial, nanofluid concentration, nanomaterial deposition techniques, coating layer thickness, working fluid, and heating surface characteristics. Electrochemical deposition and electron beam physical vapor deposition techniques were found to be best among various deposition techniques (atomic layer deposition, spray coating, spin coating, sputtering) due to their simple, economical, superior adhesiveness and restrain on the various surface properties like coating thickness, porosity, and wettability by simply controlling the current density. The reason behind the enhancement in pool boiling heat transfer coefficient and critical heat flux for nanomaterial-coated surface was increase in bubble release frequency, nucleation sites, porosity rise, and surface wettability while decrease in wall superheat. Increase in nanoparticle deposition thickness up to certain limit enhanced the heat transfer performance. Keywords

Nanomaterial · Surface coating · Heat transfer · Enhancement · Critical heat flux · Heat transfer coefficient

Introduction Nowadays, surface modification using nanomaterial deposition is a very promising technique used in increasing the heat transfer performance for various application such as cooling of electronic devices, microelectronic equipment cooling, cooling of nuclear reactor, and size reduction of cooling device (Das et al. 2016). Boiling is a bulk phenomenon which originated at solid-liquid interface. Boiling takes place only when wall temperature is greater than liquid saturation temperature. In other words, boiling occurs only when excess temperature (ΔTe) is positive. In boiling vapor, bubbles are formed. The heat transfer performance can be enhanced by enhancing the critical heat flux (CHF) and heat transfer coefficient (HTC) while reducing the wall superheat (Bergman et al. 2011). This chapter delivers a broad study of pool BHT performance using nanoparticle coating such as metal (aluminum, copper,), metal oxide (alumina, silica, TiO2), composite (Cu-Al2O3, Cu-TiO2, graphene-Cu), carbon-based nanomaterial (graphene, single and multiwalled carbon nanotubes), and nanowire (copper nanowire, silicon nanowire). Various augmentation techniques were analyzed to attain better heat transfer performance in which nanoparticle and nanocomposite

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coating were obtained to be most prominent method for different cooling application.

Pool Boiling Heat Transfer Overview Based on bulk fluid motion, boiling is of two types: pool and flow boiling. In pool boiling, fluid is kept inside the vessel and there is no external movement of the fluid. The fluid motion is only due to the buoyant force or natural convection (Cengel and Ghajar 2011). All the bubbles are formed, they will carry themselves upward, and space will be filled by cold fluid. Boiling of tap water in a pan on top of heating source is the most common illustration of pool boiling heat transfer. Fast breeder test reactor (FBTR) in nuclear power plant is another example of pool boiling heat transfer. In this the core is kept inside the molten metal liquid, and cooling will take place by liquid molten metal. Therefore, heat takes place through pool boiling. Figure 1 shows the block diagram of benefit of improvement in performance of heat transfer. Decrease in energy consumption and minimized energy dissipation resulted in improved heat transfer performance.

Pool Boiling Curve The pool boiling was first observed by Nukiyama, 1934. Pool boiling curve (Fig. 2) is the plot between degree of superheat ( C) on x-axis and heat flux (W/m2) on y-axis. Wall temperature minus liquid saturation temperature is called as degree of superheat. As we know, saturation temperature of water at atmospheric pressure is 100  C which is constant for a particular pressure.

Benefits of Nucleate Pool Boiling Heat Transfer Enhancement

To decrease the energy consumption in heat transfer equipment

To minimize the energy dissipation from thermal system

Fig. 1 Pool boiling heat transfer enhancement benefits

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Fig. 2 Pool boiling curve. (Reprinted with permission from Bergman et al. (2011) copyright John Wiley & Sons)

Pool boiling curve includes four regimes: natural convection, nucleate boiling, transition boiling, and film boiling (Bergman et al. 2011). In natural convection region, excess temperature difference is below 5  C. No boiling occurs in this region. Heat transfer occurs through natural convection only. In nucleate boiling region, excess temperature is greater than 5  C but less than 30  C. In this region, we observed the first instance of bubble formation termed as onset of nucleate boiling (ONB). Nucleate means bubble initiates from the heating surface. Bubble initiates from the heater surface usually from micro-cracks. Liquid trapped in these cracks vaporizes first. These bubbles grow, detach, and migrate toward the free surface. Till point B, bubbles are isolated (do not interact with each other) and will not reach free surface; it will collapse in the liquid itself. Beyond point B, bubble reaches to surface. More nucleation sites result in more bubble formation. Bubble merges and forms vapor column which gives rise to reduction of contact surface area between surface and liquid. The curvature slope decreases and reaches maximum at point C called as critical heat flux (CHF). At approximately 20  C excess temperature (called as inflection point), curvature changes its slope. After this inflection point, curvature decreases its slope. From B to inflection point, both heat flux and HTC increased. From inflection point to point C, heat flux increased but HTC decreased. In transition boiling region, excess temperature is greater than 30  C but less than 120  C. As the ΔTe increased beyond CHF, the vapor bubble generation rate from the

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nucleation sites surpasses the bubble separation rate from the heating substrate. Vapor bubbles from various nucleation sites coalesce to establish uninterrupted vapor film on top of some portion of the heating wall, significantly reducing the contact area of heating wall and the working fluid. Now the heat is transferred through this vapor film. So, heat from surface goes to the vapor and from vapor it goes to the liquid. These thick vapor layers are unstable. Although, vapor layer can move away or separate from the wall, causing the reestablishment of contact with the fluid and restarting of nucleate BHT, the boiling in current region is the combination of unstable vapor film alongside some nucleate boiling heat transfer, that is why it is called as region of transition boiling. When the wall superheat attains a particular temperature to maintain stable and thick vapor layer, the heat flux achieves its minimum value (called as Leidenfrost point) denoted by point D. In film boiling region, excess temperature is greater than 120  C. At excess temperature above point D, the fluid and the heating surface are entirely detached by a strong and immovable thick layer or film of vapor bubble. Therefore, it is best known by film boiling. The phase change in this region takes place at the fluid-vapor bubble interface in lieu of straight at the heating wall (alike nucleate boiling). Heat transfer in this region takes place through convection and radiation modes. As we can see in pool boiling curve, heat flux of point C (CHF) is above heat flux of point E. So, we get high heat flux at lower temperature difference (referred as wall superheat). So, we get maximum heat transfer rate in nucleate boiling. That is why all the engineering devices are operated in nucleate boiling region.

Critical Heat Flux and Heat Transfer Coefficient In nucleate boiling regimes, bubble merges and forms vapor column which gives rise to reduction in contact surface area between surface and liquid. The curvature slope decreases and attains maximum at point C called as critical heat flux. It is the highest heat flux achieved in nucleate boiling region (Cengel and Ghajar 2011). From the Nukiyama pool boiling curve for water, the critical heat flux is MW/m2 or 106 W/m2 at ΔTe of 30  C. As ΔTe increased beyond CHF, the vapor bubble generation rate from the nucleation sites surpasses the bubble separation rate from the heating substrate. Vapor bubbles from a various nucleation sites coalesce to establish uninterrupted vapor film on top of some portion of the heating wall, significantly reducing the heat flux. Heat transfer coefficient (denoted by h) can be calculated by dividing the input heat flux (q00) to excess temperature difference (ΔTe). h h

i Heat flux ðW=m2 Þ W ¼ 2 excess temperature ðT s  T sat Þ m K

ð1Þ

At approximately 20  C excess temperature difference (called as inflection point), curvature changes its slope. After this inflection point, decreases in curve slope is observed. From ONB to inflection point, h increased, but from inflection point to

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Table 1 Heat transfer coefficient (W/m2-K) for different phases Free convection Gas: 5–30 Liquid: 20–1000

Forced convection Gas: 20–300 Liquid: 50–20,000

Phase change Boiling: 2000–106 Condensation: 5000–106

critical heat flux point, it decreased. Table 1 shows the value of heat transfer coefficient of different phases and different mode of convection.

Formula to Calculate Critical Heat Flux (q00 ) and Heat Transfer Coefficient (h) Input heat flux to the Cu block: q} ¼

dT dx

ð2Þ

K is the thermal conductivity of test sample. The temperature gradient dT dx is obtained by applying the following (Cengel and Ghajar 2011): dT 3T 1  4T 2 þ T 3 ¼ dx 2x1

ð3Þ

T1, T2, and T3 are measured by thermocouple. The boiling HTC can be obtained by applying the following: h¼

q Ts  Tf

ð4Þ

Tbs is the boiling surface temperature and Tf is the average fluid temperature. The boiling surface or wall temperature Ts can be obtained by utilizing Eq. (2): T s ¼ T 1  q}

x1 K

ð5Þ

Where x1 is the gap between the upper heating surface and thermocouple T1.

Porosity and Wettability Porosity is the quantity of void “unoccupied” area in a material. Scanning electron microscopy (SEM) instrument is utilized to analyze the porosity. Porous surface is generated due to growth of hydrogen bubble from the coated heater surface (Gao et al. 2017). Increase in number of cavities implies that increase in irregularity on surface or increase in nucleation sites causes the increase in surface area. Since bubble arises from cavities, therefore increase in number of cavities results in enhanced bubble formation at faster rate causing improved nucleate boiling.

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Influence of Surface Modification on Heat Transfer Performance Critical heat flux enhancement Boiling heat transfer coefficient enhancement Reduce in wall superheat temperature Porosity rise Increase in nucleation site Enhance in surface wettability Generation of nano-porous/cavities Rise in bubble release frequency Fig. 3 Surface modification effect on heat transfer performance

Wettability is measured by using sessile drop method. It measures the contact angle between liquid and heater surface. Wettability is categorized into two types: hydrophilic (Hpi) and hydrophobic (Hpo). Contact angle (CA) less than 90 is referred as hydrophilic surface, while more than 90 is known as hydrophobic surface. Contact angle more than 150 is known as superhydrophobic (SHPo) surface, while less than 5 is termed as superhydrophilic (SHPi) surface. Hydrophilic surface is favorable in nature in terms of boiling heat transfer (Betz et al. 2011). In hydrophilic surface, we obtain the decrease in bubble release time which causes the increase in bubble occurrence rate. Figure 3 shows the block diagram of effect of nanomaterial coating on the performance of heat transfer. Various parameters affect the heat transfer performance. The reason behind the enhancement in pool boiling HTC and CHF for nanomaterial-coated surface was increase in bubble release frequency, nucleation sites, porosity rise, and surface wettability while decrease in wall superheat.

Surface Coating for Pool Boiling Heat Transfer Enhancement Metal Oxide Nanoparticle Coating Vassallo et al. (2004) investigated the pool boiling performance using SiO2/water nanofluid considering particle size as 16 nm, 51 nm, and 4 μm. The concentration of SiO2 nanoparticle was 0.5% by volume. A nickel-chromium (nichrome) of boiling

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surface area 0.13 cm2 was used as the test heater. The maximum CHF enhancement was obtained for 3 μm particle size. The CHF augmentation for 15 nm, 50 nm, and 3 μm was 60%, 60%, and 200%, respectively, as compared to uncoated surface. This enhancement was predominantly because of rise in nucleation sites. Kim et al. (2007) studied the heat transfer performance using Al2O3/water, SiO2/water, and ZrO2/water nanofluid having particle size 100–200 nm, 25–45 nm, and 120–260 nm, respectively. The concentration was in the range of 0.001–0.1% volume for all the three nanomaterials. A stainless steel flat plate of boiling surface area 2.25 cm2 was used as the test heater. The maximum CHF enhancement was obtained for SiO2 nanomaterial deposition. The CHF augmentation of 50%, 80%, and 75% was achieved for Al2O3, SiO2, and ZrO2 nanomaterial coating, respectively, as related to plain surface. This increase in CHF was due to the porosity rise and increase in bubble release frequency. Coursey and Kim (2008) studied the heat transfer performance using Al2O3/ethanol and Al2O3/water nanofluid of nanoparticle size 45 nm. The concentration of alumina nanoparticle was 0.001–10 g/l. Glass and copper of boiling surface area 0.9 cm2 and 2 cm2, respectively, were used as the test heater. The CHF enhancement for Al2O3/ethanol nanofluid with glass as heater was 25% at 10 g/l concentration, while that for Al2O3/water nanofluid with copper as heater was 37% at 0.525 g/l concentration. Kathiravan et al. (2010) studied the performance using Cu/water nanofluid of particle size 10 nm. The concentration of nanoparticle was 0.25–1% by weight. Stainless steel plate of surface area 0.3 cm2 was used as the boiling surface. The maximum CHF enhancement was obtained at 1% concentration. The CHF augmentation of 48% was obtained than uncoated surface. This enhancement was due to the rise in nucleation sites and decrease in wall superheat. Jung et al. (2013) performed the pool BHT performance using TiO2/water nanofluid. The particle size of TiO2 was 85 nm, and concentration of nanoparticle was 0.00001–0.1% by volume. Copper rod of 0.78 cm2 surface area was considered for the surface heater. They achieved CHF enhancement up to concentration 0.0004% volume as compared to unmodified heating surface. This enhancement was due to the rise in nucleation sites and porosity rise. Ahn et al. (2014) investigated pool BHT using GO/water nanofluid of 0.5–1 μm particle size and 0.675 nm thickness. 0.0001–0.001% by weight concentration was considered to deposit the GO on platinum wire test heater of 1.5 cm2 boiling surface area. A remarkable augmentation in CHF was observed as compared to uncoated surface. Gu et al. (2018) investigated pool BHT using Ag-CNT/water nanofluid of 50–80 nm particle diameter. Three different concentrations of 0.1%, 0.3%, and 1% by molar were considered to deposit the nanomaterial on copper test heater of 1.5 cm2 boiling surface area. They achieved 21% enhancement in thermal conductivity related to plain surface. Figure 4 shows the schematic of pool boiling apparatus. Nanoparticle is coated on copper heater surface-fluid interface of 1 cm2 surface area using chemical vapor deposition method, and input flux is supplied through cartridge heater (inserted inside copper block). Component used during experimental investigation of pool BHT are summarized in Table 2. Various components and their purpose while fabrication of experimental setup are shown. A PID-controlled autotransformer to

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Fig. 4 Schematic of pool boiling apparatus. (Reprinted with permission from Gupta and Misra (2018a) copyright Elsevier)

Table 2 Component used in pool boiling heat transfer investigations S. no 1 2 3

4 5 6 7 8 9 10 11 12

Components Copper testing block Primary auxiliary cartridge heater Secondary auxiliary cartridge heater

U-shaped condensation tube Pressure gauge PID-controlled autotransformer K-type thermocouple Data logger Viewing glass (polycarbonate sheet) Insulation material (glass wool) Water pump Rotameter

Purpose Test heater surface on which deposition is to be done Fitted to the copper block to provide the heat flux to Cu block Fitted inside vessel near the boiling surface for rapid heating of liquid. The heater rod is attached to the PID-controlled autotransformer, which maintains the constant temperature of working fluid Condensate the vapor generated due to the boiling Measures the atmospheric pressure inside the boiling vessel To input the heat flux to the Cu block and to control the saturation temperature of pool boiling liquid To measure the temperature of liquid inside the boiling vessel and heated surface temperature Record the instantaneous readings of thermocouples To observe the bubble behavior To reduce or eliminate the heat loss from the vessel wall to atmosphere To flow liquid in flow BHT investigation Measure the flow rate of cooling water

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control the input heat flux and saturation temperature of pool boiling liquid is shown. Secondary auxiliary cartridge heater was fitted inside vessel near the boiling surface for rapid heating of liquid. The heater rod is attached to the PID-controlled autotransformer, which maintains the constant temperature of working fluid.

Nano-porous Metal Coating Tang et al. (2012) used hot-dip galvanizing and dealloying method to deposit Cu-Zn alloy nanocomposite of 50–200 nm particle size. DI as the pool boiling fluid and copper of boiling surface area 1.76 cm2 were used as the test heater. They achieved remarkable enhancement in CHF and HTC than unmodified surface. The enhancement was mainly due to the formation of nano-porous surface which subsequently enhanced the nucleation sites and reduced the wall superheat. Arya et al. (2016) utilized anodic oxidation (also called as anodizing) technique for the formation of aluminum oxide nano-porous surface. Deionized water was utilized as pool boiling working fluid. Alumina nanomaterial of 30 nm particle size was deposited on aluminum microwire of boiling surface area 0.05 cm2. They achieved remarkable enhancement in CHF and HTC as compared to uncoated because of nano-porous surface formation which subsequently rose the bubble release frequency and enhanced surface wettability. Lu et al. (2016) studied electroplating, thermal alloying, and dealloying methods to deposit Cu-Zn alloy of 50–200 nm particle size. DI water as the working fluid was utilized. Cu-Zn alloy of 30–200 nm particle size was coated on copper surface of boiling surface area 4 cm2. They achieved significant enhancement in CHF and HTC as compared to uncoated surface. The highest enhancement in HTC was 1.4 times to that of uncoated surface. The enhancement was mainly due to the evolution of small nano-porous and cavities on heating surface which increased the nucleation sites and surface wettability. Gao et al. (2017) investigated the heat transfer performance using electrodeposition and heat treatment technique for the formation of nano-porous Cu surface. Distilled water as the working fluid was used. Copper nanomaterial of 50–200 nm particle size was deposited on copper substrate of boiling surface area 4 cm2. They achieved remarkable enhancement in CHF and HTC than uncoated surface. The maximum enhancement in HTC was 80%. Formation of small cavities on heated wall caused the nucleation site density rise and porosity rise. Table 3 shows the thermal conductivity of various nanomaterial/nanocomposite. As we can see, graphene and CNT or MWCNTs had maximum thermal conductivity. But relatively lower stability at larger mass flux and greater deformation of structure was observed. Nanocomposite and metal oxide coating are found to be very advantageous for maximum heat transfer.

Wettability: Hydrophilic and Hydrophobic Coatings Betz et al. (2011) developed superhydrophobic (SHPo) and superhydrophilic (SHPi) surface using silica coating for the pool BHT performance and compared with

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Table 3 Thermal conductivity of various nanomaterial S.no 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Nanomaterial/nanocomposite Graphene Diamond CNT or MWCNTs Silver Copper Cu-Al2O3 Gold Cu-TiO2 Aluminum SiC ZnO CuO Al2O3 TiO2 SiO2

Thermal conductivity (W/m-K) 5000 3300 3000 405 385 340 319 300 200 120 85 76.5 23 9 1.4

hydrophilic silica coating surface of contact angle 7 . Ion etching along with photolithography technique was used to deposit 30 nm coating thickness of SiO2 on indium tin oxide (ITO) test heater surface of 1 cm2 surface area. As compared to hydrophilic surface (7 ), the combined effect of superhydrophobic surface (150 ) and superhydrophilic surface (0 ) showed 400% enhancement in HTC, while that for superhydrophilic (0 ) along with hydrophobic (120 ) surface showed moderate enhancement in HTC. The enhancement in CHF of 80% was obtained for superhydrophilic (0 ) along with hydrophobic (120 ) pattern surface and no enhancement for combined SHPo (150 ) and SHPi (0 ). Hsu et al. (2012) investigated pool BHT performance using interlaced surface of silica-coated hydrophilic surface (55 ) and uncoated hydrophobic surface (105 ). Solgel technique is used for SiO2 (40 nm) preparation. DI water as a pool boiling fluid was used. Airbrushing technique was used to deposit silica nanomaterial on copper block of 2.25 cm2 surface area. Hydrophobic surface was obtained by coating the mixture of methyl alcohol and perfluorooctyl silane (fluoro-containing mixture) on copper substrate. The enhancement in HTC was 100% as compared to plain surface. Bourdon et al. (2015) developed hydrophilic and hydrophobic surface with contact angle in the range of 33 –102 for the pool BHT performance. Glass surface as test heat was modified using surface treatment technique. DI water as the pool boiling fluid and glass test heater of 20.25 cm2 surface area were used. For the 33 hydrophilic modified surface, a significant rise in HTC and CHF was observed related to plain surface. The HTC enhancement was mainly due to the increased nucleation site and porosity which concludes that CHF enhanced with increase in wettability. Kim et al. (2017) investigated pool boiling BHT using wetting transition from hydrophobic surface to hydrophilic surface. Hydrophobic surface was achieved by coating TiO2 on one side of silicon heater surface, while hydrophilic surface was achieved by heat treatment of TiO2-coated hydrophobic surface. DI water as a pool boiling fluid was used. For

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wetting transition investigation, one side of silicon substrate of 1 cm2 heating surface area was coated with TiO2 (200 nm thickness), while other side was coated with SiO2 (500 nm thickness) for insulation. They observed that wetting transition was influenced by both time and temperature of heat treatment. Wetting contact angle increased with decrease in time and temperature, which implies that contact angle was inversely proportional to time and temperature. Ji et al. (2018) developed different wettable surfaces of contact angle 4.7 –153 using SiO2-water nanofluid for the pool BHT performance. The concentration of SiO2-water nanofluid was 0.025% to 0.1% by mass. Hot press particle sintering technique was employed to develop hydrophobic surface, and vapor-phase deposition technique was used to develop superhydrophobic surface on copper test heater of 5 cm2 heating surface area. Hydrophilic surface reduced the HTC of silica-coated surface having high surface roughness. The increase in nanoparticle concentration from 0.025% to 0.1% also reduced the HTC. The HTC of superhydrophobic surface (153 ) was enhanced at lower heat flux (lower than 92 kW/m2) and decreased at high heat flux (greater than 93 kW/m2).

Carbon-Based Nanoparticle Coating Graphene Coating Park et al. (2015) performed the effect of multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO) on BHT performance. Spray deposition technique was used to deposit the carbon-based nanomaterial on zirconium boiling surface material of 0.9 cm2 heating surface area. DI water was used as working fluid. The particle size of MWCNTs and GO was 10–15 nm and 15 μm, respectively. CHF enhanced with decrease in contact angle for both the cases. The maximum enhancement in HTC for MWCNT-coated surface was observed at 19.8 contact angle, while that for GO was 20%, respectively, as compared to uncoated surface. An et al. (2016) studied the effect of graphene oxide (GO) nanomaterial coating on pool BHT performance. Spray coating method was used to deposit graphene on copper test heater surface. FC-72 was used as pool boiling working fluid. Graphene nanomaterial of 15 μm coating thickness was deposited on copper substrate of boiling surface area 2 cm2. The maximum enhancement in CHF and HTC was 67% and 41%, respectively, as compared to uncoated copper surface. Kamatchi and Kumaresan (2018) studied experimentally the influence of rGO/water nanofluid on the performance of the pool BHT. Hummer technique was used to deposit rGO nanomaterial on Ni-Cr substrate of 0.42 mm diameter and 80 mm length. The concentration of nanofluid was in the range of 0.01–0.3 g/l. The maximum enhancement in critical heat flux was 265% at 0.3 g/l nanofluid concentration. Sadaghiani et al. (2018) investigated chemical vapor deposition method to deposit 8–55 nm coating thickness graphene nanoparticle of 60–300 μm pore size. DI water was used as the working fluid. Deposition was done on silicon and silica substrate of boiling surface area 4 cm2. They achieved noteworthy enhancement in CHF and HTC. The maximum enhancement in HTC was 56% as compared to uncoated surface. Rishi et al. (2018) studied

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Table 4 Stability of various coated nanomaterials on surface S.no 1 2 3

Nanomaterial Oxide layer coating [CuO, ZnO, Al2O3, TiO2, SiO2] Metal porous coating [Cu, Al, silver, diamond] CNT/multiwalled CNT coating

4 5

Graphene/graphene oxide coating Metal nanowires coating [Si and Cu]

Stability Most stable High stability Low stability (at larger mass flux, the deformation of structure was greater) Low stability Low stability

the pool BHT performance using dip-coating two-step electrochemical deposition method to deposit GO-Cu nanocomposite of 5–10 μm pore size diameter. Distilled water as a working fluid was used to study pool boiling. GO-Cu nanocomposite deposition was done on copper alloy substrate of 1 cm2 boiling surface area. They achieved significant enhancement in CHF and HTC. The maximum enhancement in CHF and HTC was 76% and 180%. Table 4 shows the stability of various nanomaterial/nanocomposite-coated surfaces. Despite of the maximum thermal conductivity of graphene and CNT/ MWCNTs but relatively lower stability at larger mass flux and deformation of structure is point of concern for the industrial application. Nanocomposite, ceramic, and metal oxide coating are found to be very advantageous for deposition stability along with maximum heat transfer.

Carbon Nanotube (CNT) and Multiwalled Carbon Nanotube (MWCNT) Coatings Sathyamurthi et al. (2009) performed the effect of MWCNTs (9 μm and 25 μm height) deposition on boiling heat transfer performance. Chemical vapor deposition technique was used to deposit the carbon-based nanomaterial on silicon substrate of 1 cm2 heating surface area. PF-5060 fluorocarbon fluid was considered as working fluid. The maximum enhancement in CHF was 58%. Li et al. (2013) used ultrasonification method to deposit stearic acid-MWCNT composite. Water as a pool boiling fluid was used. The CHF and HTC were significantly enhanced as compared to bared surface. Ho et al. (2014) studied the pool BHT performance using chemical vapor deposition method to deposit CNTs of 10 nm particle diameter. FC-72 fluorocarbon fluid as a pool boiling fluid was considered. Deposition thickness of 215 μm on silicon substrate of 1 cm2 boiling surface area was prepared. They also separately investigated the influence of surface orientation (0–180 ) on the heat transfer performance. They achieved remarkable augmentation in CHF and HTC. The maximum augmentation in HTC was 42% at 90 orientation. Bertossi et al. (2015) studied the effect of CNT coating on stainless steel test heater using deionized water as pool boiling fluid for heat transfer performance. Chemical vapor deposition method was used to deposit CNTs of length 3–10 μm on stainless steel of 5 cm2 test heater size. They achieved a remarkable augmentation in CHF and HTC. The maximum enhancement for HTC was 100%. Seo et al. (2017) used layer-by-layer technique to deposit polyethyleneimine multilayer and MWCNTs (20–40 nm

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S. K. Singh and D. Sharma

diameter and 5–20 μm length). Coating was done on stainless steel test heater of 3.5 cm2 boiling surface area. DI water as a pool boiling fluid was used. They obtained 30% rise in surface roughness and 70% enhancement in surface wettability. The CHF and HTC were significantly enhanced as compared to bared surface. The maximum enhancement for CHF was 94%.

Nanowire Coating Im et al. (2010) used anodic aluminum oxide method to develop copper nanowire of 200 nm diameter, 300 nm pitch, and 1–8 μm height for pool boiling heat transfer performance. PF-5060 (fluorocarbon fluid) was used as pool boiling fluid. For insulation purpose, 0.4-μm-thick SiO2 layer, 0.05-μm-thick titanium layer, and 0.4-μm-thick platinum layer were developed on one side of silicon wafer using chemical vapor deposition method. Copper nanowire was developed on silicon surface of 1 cm2 heating surface area. A significant enhancement in CHF and HTC was achieved as compared to plain silicon surface. Yao et al. (2011) developed hydrophilic surface with nanowire. Copper nanowire (diameter, 220 nm; height, 2–20 μm) was fabricated on silicon surface (1 cm2) using electrochemical deposition method and silicon nanowire (SiNWs) (height, 20–35 μm) was fabricated using electroless chemical etching method. Deionized water was taken as pool boiling fluid. Both the modified nanowire surface was obtained to be more hydrophilic than plain copper and silicon surface. The contact angle for copper nanowires (CuNWs) was 28 and that for SiNWs was 0 . HTC was greatly influenced by nanowire height and achieved more enhancement with increase in nanowire height. At a particular wall superheat, the maximum heat flux of 135 W/cm2 was obtained for SiNWs at 35 μm height. For a particular height, CuNWs showed better heat transfer performance than SiNWs. Demir et al. (2014) studied experimentally the influence of silicon nanorod (SiNR) height on the performance of pool boiling heat transfer. SiNR (diameter, 850 nm; height, 900–3200 nm) was fabricated on silicon surface (4 cm2) using metal-assisted chemical etching method. The experiment was conducted for DI water pool boiling fluid. The enhancement in HTC for the largest and smallest nanorod was 120% and 254%, respectively. The augmentation in heat transfer performance decreased with the increase in height of nanorod. This was due to the reduction in bubble release frequency. Kumar et al. (2017) investigated the impact of CuNW diameter on the heat transfer performance. The experiment was performed with FC-72 as a pool boiling fluid. Electrodeposition method with the use of anodic aluminum oxide was used to fabricate NW of various diameter ranging from 35 nm to 200 nm on copper surface of 1 cm2 boiling surface area. A significant enhancement in CHF and HTC was achieved as compared to bared surface. The maximum enhancement in CHF for 36 nm, 71 nm, 131 nm, and 200 nm diameter were 38%, 40%, 48.5%, and 45.6%, respectively, while the maximum enhancement in HTC for 36 nm, 71 nm, 131 nm, and 200 nm diameter were 86%, 95.5%, 184%, and 132%, respectively. Wen et al. (2017) studied heat transfer performance using formation of Cu micropillar on Cu plate in the first step followed by preparation of two-level hierarchical surface having longer CuNW array (first level) and shorter

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Table 5 Instrument used for surface characterization S.no 1 2 3 4 5 6

Surface characterization instrument Scanning electron microscopy (SEM) Energy dispersive spectroscopy (EDS) X-ray diffractometer (XRD) Microscopic contact angle meter 2D profilometer or optical surface profiler Laser ellipsometer

Purpose Analyze the morphological studies of nanostructure surfaces (porosity, mean pore diameter) Compositional analysis of the structured surface (identifies the deposited components) Analyze the structural properties of the nanostructures Static contact angle measurement of fluid on coated and plain surface Estimate the surface roughness of plain and coated surface Measure the coating thickness

CuNW array (second level) in the second step. Electrodeposition method with the use of anodic alumina oxide was used to fabricate CuNWs of diameter 220 nm and length 5 μm and 30 μm on copper surface of 6 cm2 boiling surface area. The maximum enhancement in CHF and HTC was 71% and 185%, respectively, while 37% drop in ONB was achieved. Table 5 shows the various instrument used for surface characterization of nanoparticle-coated surface. Scanning electron microscopy (SEM) or transmission electron microscopy (TEM) is used to analyze the morphological studies of nanostructure surfaces (porosity, mean pore diameter). Microscopic contact angle meter or sessile drop technique is used for wettability to measure contact angle of fluid on coated and plain surface to find whether it is hydrophilic or hydrophobic surface.

Microscale Surface Modification Microporous Metallic Coating Arik et al. (2007) investigated the heat transfer performance using diamond microporous coating. The experiments were conducted in FC-72 (dielectric fluid) and 101 kPa to 304 kPa pressure range. Dip coating technique was used to deposit a 50–75 μm thickness of diamond nanoparticle (8–12 μm particle size) on silicon test heater of 0.41 cm2 boiling surface area. The maximum CHF of 47 W/cm2 was obtained at 3 atmospheric pressure which was 60% greater than uncoated silicon test heater. Byon et al. (2013) studied bi-porous deposition (BPD) of Cu on pool boiling performance and compared it with mono-porous deposition (MPD) and uncoated surface. The influence of particle size, cluster size, and deposition thickness on heat transfer performance was also investigated experimentally. Copper with particle size of 45–100 μm, cluster size of 250–675 μm, and coating thickness of 0.7–3 mm was deposited on copper test heater with boiling surface area of 0.78 cm2. Cu test heater was submerged in dielectric fluid FC-72. The particle size and cluster size influenced the CHF while the effect of deposition thickness was negligible. The CHF of mono-porous deposition was much greater than uncoated surface. The CHF of

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mono-porous deposition was approximately two times than that of uncoated surface. The CHF of bi-porous deposition was greater than mono-porous deposition and uncoated surface. The maximum enhancement in CHF of 72% was obtained for bi-porous deposition as compared to mono-porous deposition. Ji et al. (2013) developed uniform particle-coated porous surface and 2D and 3D porous-coated surface for pool boiling experiment. Cu test surface was submerged in acetone as a pool boiling fluid. The pore size with diameter of 130–170 μm was used for all the three cases. The pitch width of 1–2 mm was considered for 2D and 3D porous surface. The coating thickness of uniform particle-coated surface was 2.5 mm. All the three porous surfaces were obtained by sintering the coated layer of nanoparticle on copper block of 1.44 cm2 effective boiling area. The 2D porous-coated surface achieved the maximum HTC and CHF at 1.6 mm pitch width as compared to other 2D porous surface. The enhancement in the CHF for uniform porous-coated and 2D and 3D porous surface was 70%, 240%, and 270%, respectively, as compared to uncoated surface. Kim et al. (2015) used two techniques for developing microporous coating: (1) epoxy-based microporous coating which consist of aluminum nanoparticles, brushable ceramic epoxy, and methyl ethyl ketone (ABM) and (2) solder-based thermal conductive microporous coating (TCMC). R-123 refrigeration-based fluid and DI water were used as the pool boiling fluid. Surface modification was done on copper test heater of 1 cm2 boiling surface area. For the ABM deposition method, the enhancement in CHF and HTC was higher for R-123 fluid as compared to water. For R-123, the maximum enhancement in HTC and CHF was 270% and 40%, respectively, while for water, the maximum enhancement in HTC was 47%. CHF was not attained in water as working fluid. For TCMC deposition method, the maximum enhancement in HTC and CHF with water was 300% and 100%, respectively. Sarangi et al. (2017) studied experimentally the pool BHT performance in FC-72 dielectric fluid. A spherical and irregular sintered Cu particle of diameter 90–106 μm was deposited on Cu surface to obtain 360–424 μm deposition thickness. The effective boiling surface area of copper test heater was 6.45 cm2. The enhancement in HTC for irregular sintered Cu particle was observed to be greater than spherical sintered Cu particle at same porosity. The sintered microporous surface showed 16–80 times decrease in wall superheat as compared to uncoated surface. The maximum enhancement in HTC and CHF for sintered microporous surface was 9–26 times and 2 times related to plain surface.

Microstructured (Microgrooves/Micro-Fin) Surface Cora et al. (2010) studied sintering and compaction technique to develop microgrooves using copper powder of 100–200 μm diameter. The microgrooves with valley thickness of 120–1315 μm and height to width ratio of 0.84–1.68 were developed on copper surface of 0.5 mm thickness. The pool boiling was investigated in n-pentane liquid, pressure of 14–45 MPa and temperature of 300–450  C. The maximum CHF was enhanced by approximately 3.4 times than that of uncoated surface. The maximum CHF of 486 kW/m2 was obtained for modified surface, while 246 kW/m2 was obtained for uncoated surface. Deng et al. (2016) developed 12 cavities of Ω-shaped channel having cavity diameter of 798 μm. Solid-state sintering and micro-EDM method was used to deposit copper powder of

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75–110 μm diameter to develop channel on copper surface of 4 cm2 boiling surface area. The pool boiling was performed in ethanol and DI water liquid separately. The maximum enhancement in HTC for ethanol and water was 380% and 370%, respectively, than uncoated surface. For the modified surface, the wall superheat at the ONB was reduced by 13 . The generation of small cavities caused the rise in nucleation sites. Ho et al. (2016) used selective laser melting method to fabricate micro-cavity and micro-fin having cavity mouth diameter of 500–700 μm, cavity base diameter of 200–400 μm, cavity depth of 200 μm, fin depth of 350–500 μm, and fin height of 400–550 μm. Micro-cavity and micro-fin were fabricated from Al/Si powder of 20–63 μm diameter. The pool boiling was performed in FC-72 dielectric fluid. To investigate heat transfer performance, microstructured surface was developed on aluminum test heater of 2 cm2 heating surface area. The maximum HTC of 1.25 W/cm2-K was obtained for modified surface which was 70% enhancement than bared surface. The maximum CHF of 48 W/cm2 was obtained for modified surface which was 75% enhancement as compared to uncoated surface. Chen and Li (2019) studied experimentally the combined effect of two-tier microstructure on pool BHT performance. Copper nanowire and microgrooves were developed on Cu surface of 1.26 cm2 boiling surface area. Electrodeposition method with the use of anodic aluminum oxide was used to deposit copper nanowire of 70 nm diameter and 5–25 μm height. Wire cutting technique was used to fabricate microgrooves of 263 μm width, 519 μm depth, and 0.5 to 4 mm mesh pitch. The pool boiling was performed in deionized water at atmospheric pressure. The maximum CHF for copper nanowire, microgrooves, and combined two tier was 190 W/cm2 (at 25 μm height), 236.5 W/cm2 (at 0.5 mm mesh pitch), and 246 W/cm2 (at 25 μm height and 0.5 mm mesh pitch), respectively. The maximum enhancement in CHF for copper nanowire, microgrooves, and combined two tier was 69%, 110.5%, and 119.5%, respectively, than plain surface. Similar studies of surface modification using nanoparticle coating on copper surface are shown in Table 6. As we can conclude from table that surface modification with nanoparticle coating using different deposition method enhanced CHF and HTC. The optimum heater surface or boiling area was found to be approximately 1 cm2. It must be noted that different parameters were used for surface modification. Therefore, it is not possible to compare the CHF and HTC enhancement. The maximum enhancement in CHF and HTC was achieved for Cu-Al2O3-coated surface in table.

Flow Boiling Overview In flow boiling, some external agent such as blower and pump is used to make the fluid flow. The fluid motion is driven by buoyant force and external source. Higher heat transfer coefficient of flow boiling was achieved by all the researcher as compared to pool boiling heat transfer.

Cold spray

0.4 μm

Cu-diamond

(MacNamara et al. 2019)

Two-step ECD

38–62 μm

Cu-TiO2

13–45 μm

85 nm

45 nm

45 nm

Cu-Al2O3

CVD-dip coating Plasmaenhanced CVD One-step ECD

Gr/rGO

PVD

Gr: 3–4 layer, CNT:2–4 nm

1–3 layer

–

TiO2

Deposition method PVD

Gr/CNT 18 nm heterostructure

250–1000 nm

40 nm

Name SiO2

(Gupta and Misra 2018b)

(Udaya Kumar et al. 2018) (Gupta and Misra 2018a)

Reference (Das et al. 2016) (Das et al. 2017) (Jaikumar et al. 2017)

Coating thickness 100–300 nm

Particle size 40 nm

Nanomaterial

Table 6 Studies of surface modification on heat transfer enhancement

Cu

Cu

50 –56 DI water 5

Cu

Cu

45 –65 DI water 84

DI water

Cu

48 –65 DI water

FC-72

Cu

DI water

40

Material Cu

Substrate Working fluid DI water

Contact angle 40 80%

1.77 cm2 34% (1500 kW/m2)

0.78 cm2 86% (1988 kW/m2)

0.78 cm2 72.5% (1852 kW/m2)

0.78 cm2 40%

0.78 cm2 50% (192 W/ cm2)

0.6 cm2

Enhancement Surface area CHF 3.14 cm2 90%

42.5%

Wall superheat 36%

273% 59% (199 kW/m2K) 45% 185% (151 kW/m2K) 40% 300% (200 kW/m2K)

54% 110% (107 kW/m2 C) 155% 62%

75%

HTC 58%

2240 S. K. Singh and D. Sharma

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Surface Coating for Flow Boiling Heat Transfer Enhancement Sujith Kumar et al. (2015) developed Al2O3-TiO2 composite-coated surface with Fe doping of 0–7.2% for flow boiling heat transfer. Spray pyrolysis method was used for coating the nanocomposite on Cu test heater having boiling surface area of 6 cm2. A channel of 2 cm width and 0.4 cm of length was developed for flow boiling. The flow boiling was performed in deionized water at atmospheric pressure. They enhancement in CHF with rise in Fe doping was noticed. The increase in wettability, pore size, and porosity was observed with the rise in Fe doping percentage. This was because of increase in nucleation site density. The maximum enhancement of 53% and 45% in CHF and HTC was obtained for Fe doping of 7.3% and mass flux of 90 kg/m2-s. Zhang et al. (2015) studied flow boiling in R417A refrigeration-based liquid. Sixty-five internal grooves/micro-fin of 0.16 mm fin height and 0.099–0.173 mm fin root distance were fabricated on two copper tubes of 9.5 mm external diameter, 8.78 root diameter, and 8.46 mm top diameter. The operating range of refrigerant mass flux and heat flux was 175–345 kg/m2-s and 10–33 kW/m2. The enhancement of 1.26–2.8 times in HTC was obtained as compared to plain tube. Rise in the pressure drop of R417A liquid was observed for modified tube. The pressure drop of refrigeration-based liquid for the modified tube was 1.8–2.6 times than plain tube.

Conclusion We analyzed that the HTC augmentation outcomes were influenced by various parameter like nanoparticle material, thermal conductivity of nanomaterial, nanofluid concentration, nanomaterial deposition techniques, heater surface material, coating layer thickness, pool boiling working fluid, and heating surface characteristics. The following conclusion is summarized as follows: • Electrochemical deposition and electron beam physical vapor deposition techniques were found to be best among various deposition techniques (atomic layer deposition, spray coating, spin coating, sputtering) due to their simple, economical, superior adhesiveness and restrain on the various surface properties like coating thickness, porosity, and wettability by simply controlling the current density. • Greater enhancement in CHF and HTC was observed with rise in nanoparticle deposition thickness (up to a certain level). This was because of increased nucleation site density. But after a certain level of thickness, heat transfer coefficient tends to decline because of generation of additional thermal resistance. • Dielectric fluid (FC-72, PF-5060) and refrigeration-based fluid (R134a, R123) showed better hydrophilicity wettability than water. The contact angle for FC-72 pool boiling liquid was 5 .

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• The reason behind enhancement in pool boiling HTC and CHF for nanomaterialcoated surface was increase in bubble release frequency, rise in nucleation sites, porosity rise, and enhanced surface wettability and decrease in wall superheat. Increase in nanoparticle deposition thickness up to certain limit enhanced the heat transfer performance. Nomenclature

D h H L P q00 T ΔTe

Diameter (mm) Heat transfer coefficient (W/m2 K) Height (mm) Length (mm) Pitch (mm) Heat flux (W/m2) Temperature ( C) Wall superheat, Tw – Tsat

Subscripts

e f s sat

Excess Fluid Surface or wall Saturation

Abbreviations

BHT CA CHF CNTs CuNW CVD DI DNB ECD HPi Hpo HTC MWCNTs ONB PVD rGO SHPi SHPo SiNW

Boiling heat transfer Contact angle Critical heat flux Carbon nanotubes Copper nanowire Chemical vapor deposition Deionized Departure of nucleate boiling Electrochemical deposition Hydrophilic Hydrophobic Heat transfer coefficient Multiwalled carbon nanotubes Onset of nucleate boiling Physical vapor deposition Reduced graphene oxide Superhydrophilic Super hydrophobic Silicon nanowire

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Necessity for Clean Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . World Renewable Energy Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Renewable Energy Production Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydropower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geothermal Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photo-electrochemical Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Employment of Nanotechnology for Engendering Clean and Sustainable Energy . . . . . . . Use of Nanofluids in Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Energy Storage (TES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Nanomaterials in the Clean Energy Generation from Waste and Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of Nanomaterials in the Feedstock Pretreatment for the Aerobic Digestion . . . . . . . . . Nanomaterials with Algae Membrane-Based Bioreactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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H. Kachroo · A. K. Chaurasia (*) Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India S. K. Chaurasia Department of Mechanical Engineering, I.E.T, MJ P R University, Bareilly, Uttar Pradesh, India V. K. Yadav Department of Chemical Engineering, Government Polytechnic, Gorakhpur, Uttar Pradesh, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_58

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Use of Nanotechnology in the Bio-electrochemical System for the Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2270 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2271 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2272

Abstract

Urbanization and rapid industrialization have adverse impacts over the natural environment, while increasing waste generation and energy demands are potential challenges across the globe. Generations of clean and green energy from the waste materials to protect the environment gained profound attention in current research scenario. The applications of nanomaterials as nanocatalyst for green energy generation to reduce the emission or waste generation are of great interest in the research community. Thus, the aim of this chapter is to summarize the current state of the art on the applications of nanomaterials in various technologies for the clean and green energy generation. This chapter also presents the application of nanocatalyst as electrode for the bio-electrochemical process, water photolysis, water electrolysis, water thermolysis, and energy generation process. This chapter also covers the extensive review of cathode or anode nanocatalysts in the various biological fuel cell technologies such as microbial fuel cell for the clean energy generation and waste reduction in the environment. Further, the applications of nanotechnology in the various biological and bio-electrochemical processes for the hydrogen production along with current state of the hydrogen economy and applications have also been comprehensively discussed. Keywords

Cathode nanocatalyst · Anode nanocatalyst · Bio-electrochemical process · Hydrogen production technologies

Introduction Rising population, urbanization, and rapid industrialization degrade the natural environment through growing waste generation and energy demand becoming a major concern across the globe. A recent literature report suggests that an additional 3 billion people will become new energy consumers by the end of the twenty-first century. The global energy demand (~0.55 quadrillion MJ) is increasing day by day and expected to rise by 56% by 2040. Low et al. (2022) reported that ~18% of global energy demand come from renewables such as biomass, geothermal, solar, hydro, wind, and biofuels and are expected to rise up to 45% by 2050. The rising usage of limited fossil fuel resources and their depletion triggered the attention of the world toward the energy scenario in the upcoming future. Along with this, alarming and continuously rising global warming focused the world to immediate initiation of clean and renewable energies. In order to successfully achieve this outstanding goal, a revolution is demanded in production, storage, conversion, and distribution of energies in a clean, green, and sustainable

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manner. On the other hand, the volume of wastewater generated per year in North America and Europe are 85 cubic kilometers, whereas in Asia around 160 and in India 53 cubic kilometers in which around 80–86% wastewater are flow back in the ecosystem without any treatment (Chaurasia and Mondal 2022). That huge amount of waste generations has adverse consequences on our climate, health, and ecosystems. Mitigating these challenges of renewable energy resources such as energy production from the wastewater is getting strong interest as energy from wastewater will not run out ever while other sources of energy are finite and will someday be depleted. It is also essential that the waste to energy generations should be sustainable and economical. Hydrogen is the clean energy carrier that has high energy density (122–142 MJ/Kg). Despite hydrogen being capable to be served as a clean energy, very minimal hydrogen is generated for energy purposes. Most of the hydrogen is used as a chemical feedstock in various processing industries such as refineries, metallurgies, petrochemicals, and electronics. Around 50% of H2 is employed in NH3 synthesis, ~38% in petrochemicals and refineries, 9% in methanol processing, and 7% for other purposes (Chaurasia et al. 2021a). Hydrogen can act as a competent fuel in internal combustion engines. It is beneficial for automobiles as a fuel due to certain advantages like quick burning speed, high effectual octane number, and zero toxic emission. Various categories of catalyst are being prepared to provide good hydrogen yield. With the implementation of nanocatalysts, various successful nanocatalysts are synthesized (Joy et al. 2018). Despite the capability of providing good hydrogen yield, their synthesis and exposure are not good for the environment. In order to protect the environment, various authors synthesized nanoparticles with green methods. Nanotechnology is the synthesis of the materials at nanoscale that possess enhanced physiochemical properties. Furthermore, various researchers suggested that nanotechnology can improvise renewable energy generation in various process, such as nanomaterials, which can enhance the strength of rotator blades for wind energy, can prevent corrosion in tidal energy equipment, can help in making fatigueresistance drilling machines for geothermal energy, and many more (Hussein 2015). The elaborated applications of nanotechnology in solar, hydrogen, wind, and bioenergy are presented in this chapter. Recent advances in nanotechnology offer tremendous opportunities for wastewater treatment in bio-electrochemical process. Currently, various wastewater treatment processes are in technology advancement state and tending to be sustainable. The highly modular, efficient, and multifunctional processes in combination with nanotechnology are competent to furnish affordable wastewater treatment solutions with elevated performance. The key goal of this chapter is to derive a strong lineage among nanotechnology and a clean, green, and sustainable hydrogen generation from the waste materials through various technologies. This chapter lays emphasis on the role of nanotechnology in the various hydrogen production technologies, current technology advancement, and their applications. World hydrogen production prospect along with future of hydrogen economy. It also collects evidences about the tremendous need for renewable energy and international contributions toward the promotion of green energy. It enlightens the way how nanotechnology implementations in existing renewable

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technologies fosters production of pristine energy along with environmental remediation. It also includes the application of various nano-electrode (anode and cathode) materials for clean energy generation using bio-electrochemical system.

Necessity for Clean Energy Nowadays, climate change and corresponding global environmental out-turns are at a verge of crossing critical point. No doubt that these impacts will be catastrophic for the future generation. The worsening of the environment is observable in developing nations with considerable consequences. In most part of the world, water pollution is far beyond the levels of safety. As per the Journal of the American Medical Association, most cases of lung cancer are due to exposure to air/water pollution. According to the European Environmental Agency (EEA), polluted air is the main culprit of asthma and pulmonary diseases. The World Health Organization (WHO) declared that polluted air is responsible for high probability of stroke, heart diseases, and respiratory diseases (Nowotny et al. 2016). Apart from air quality, deterioration of the environment affects contamination of water as well. As per the United Nation reports, 1.1 billion people do not have access to clean drinking water and some 2 million (mostly children) die due to the lack of clean drinking water (Janusz Nowotny et al. 2016). Statistics show that large populations of Asia and Africa lack availability of clean, safe, and pure drinking water. According to World Health Organization (WHO) report in Bangladesh, only 40 million people drink safe water, while others have access to unsafe water (containing high amounts of arsenic). The consumption of fossil fuels in order to generate energy is the major factor playing a key role toward fabrication of undesired and detrimental consequences. Fossil fuel consumption via burning and utilization infuse the atmosphere with greenhouse gases (CO2, CH4, N2O, and many more) (Mondal et al. 2017; Shankar et al. 2017). Wasif et al. (2021) reported that Asia-Pacific Economic Cooperation (APEC) countries account for 3/5 of world’s energy demand and contribute more than half of global gross domestic product (GDP). They also stated that these nations meet most of their energy demands through fossil fuels, thereby resulting in 70% of global carbon dioxide emission. Water is used in various ways during energy extraction and production from fossil fuels. The water utilized in energy generation from non-renewable energy resources and nearby water bodies are contaminated with wide a variety of sediment and chemical pollutants along with liquid or solid waste generated during extraction processes (Rani et al. 2017; Chaurasia and Mondal 2021a). As signaled by the recent historic Paris Agreement, it is wise to minimize the use of fossil fuels in energy production. More prominence should be given toward development of alternate sources of energy generation, as energy generation is highly impactful toward climate and environment. Therefore, it becomes critical to emphasize over the consequences of energy generation. More decisive emphasis toward global environment sustainability will lead to more global prosperity in the longer run (Chaurasia et al. 2021a, b). An enthusiastic perspective is demanded to tackle the detrimental concerns of the environment, such as continuous

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melting of polar ice caps, altered weather conditions, and rising sea levels which had increased frequency of natural disasters, mainly droughts and floods, along with negative impacts to human health. Energy generation through unsustainable means possesses deadly outcomes toward the ecosystem, water resources, agriculture, sea level rise, human health, and infrastructure (Chaurasia and Mondal 2021b). The Future Earth Program established by United Nations is driving the whole world toward innovation of novel approaches to establish sustainable development. The success to thrive novel approaches is only feasible through a strong linkage between diverse disciplines (Hieminga and Patterson 2021). Energy is an extensive field encompassing various disciplines with diverse conceptual backgrounds and takes into consideration various key factors such as climate change, water management, pollution, etc. (Mondal et al. 2017). It is pivotal to understand the waste to energy conversion and the utilitarian of nanomaterials to achieve waste reduction. The growing interest toward nanoscale materials is important as they are capable of exhibiting outstanding properties, entirely disparate to bulk phase. With desired processing and better design, nano-size materials are competent enough to develop clean, green, and sustainable technology for energy generation.

Energy and Environment Increasing combustion of fossil-based fuels is aggregating in an untenable generation of carbon dioxide to the atmosphere, thereby contributing to an elevated greenhouse generation along with terrible climatic consequences. This large-scale burning and utilization of fossil fuels is potentially active for initiating catastrophic changes to the environment. The human-induced climate variation is the matter of danger for the upcoming generations. If it keeps on going, then access to clean water, air, and food will be an aspiration in the coming years (Nowotny et al. 2018). In order to protect the environment (for our sake as well), it is totally clear and urgent that human activity-induced climate change is to be reduced. As reported by various literature, the largest cause of greenhouse gas emission is burning of carbon (in fossil fuels) for energy generation (Fig. 1). Fig. 1 Projections on world population and energy consumption. (Data adapted from IEA 2019)

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Literatures reported that with increase in carbon dioxide levels, temperature will warm that will shift the geographical distribution and composition of many ecosystems. With change in earth’s radiation balance, climate changes will become more frequent. The agriculture sector will suffer too as the crop yields will decrease in some areas resulting in elevated risk of hunger and famine. As the climate will be warmer, sea level will eventually rise due to thermal expansion of ocean and snow and ice melting from polar ice caps and mountain glaciers. With the sea level rise, the risk of flood will be apparent. The Intergovernmental Panel on Climate Change (IPCC) has stated that adverse impact of global warming would be dealing with human health issues. Frequent weather and climate changes will subsequently increase the risk of diseases and death, high temperature will initiate the formation of secondary pollutant from the primary one (generated in fossil fuel combustion), and less food production will eventually cause hunger and malnutrition. Fossil fuels are predominantly utilized during electricity production in power plants, in transportation as well, and many more. The increasing awareness toward climate impact on the environment seeks development of environment-friendly systems. These systems should be competent enough to generate energy in a sustainable manner with less or no detrimental impact toward the environment (Nowotny et al. 2016). Large volumes of water are consumed and polluted in the extraction, refining, and combustion of fossil fuels. The contamination occurs via regular operations as well as through accidental releases or by any other means as well. But the thing is that very minimal consideration is given to the water quality innuendo of fossil fuel generation and utilization. Despite that quality of water is degraded at every step of it and no doubt fossil fuels themselves are the important water pollutants, chemicals employed during process and refining of these energy are a sincere threat to water bodies and the environment. Gleick (2013) reported that an average 15–18 billion m3 of freshwater resources are annually affected due to fossil fuel production. Similarly, the water gets contaminated due to fossil fuels across the world. This intense degree of contamination of water is terribly affecting ecosystems and other communities as well that depend on water for drinking and livelihood. Despite these impacts, the world is increasing production of unconventional oil and gas. Various literatures suggest that systems like ecological footprint buildings, hydrogen fueled transportation, etc. can be promising for achieving a better future. In fact, implementation of renewable-based energy sources is rising dramatically from the last decade and is constituting 3.2% of overall electricity produced around the globe and 4.2% in the USA (Nowotny et al. 2016). Despite the impressive deployment of renewable energy technologies like solar and wind, there are some shortcomings too. These clean technologies demand large-scale energy storage systems that can act as a shield against the generated energy. So, the rapid development and instant deployment of photoelectrochemical and electrochemical technologies for hydrogen energy are capable of providing awesome opportunities for large-scale energy storage. There is a tremendous necessity to replace the current hydrogen generation technology, utilizing natural, with technologies established on utilizing green resources. Introduction of sustainable and entirely clean technology for the generation of green energy like hydrogen is the sustainable way to reduce fossil fuel consumption.

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Economy From the past few years, the use of renewable energies like wind, solar, biomass, and hydro enhanced substantially globally. As reported by various literature, global wind capacity is seen to increase to 318 GW in 2013 compared to 6.1 GW in 1996, along with increment in solar photovoltaic capacity to 13 9GW from 0.7 GW for the same time period. The expanding consumption of renewable energy sources strongly promises elevated economic activity. Bunch of employments can be created in manufacturing, operating, and maintaining of renewable energy installations along with transportation of biofuels and biomass. Indirect employments can also emerge in other sectors linked with fulfilment of demands of inputs for renewable energy production such as electronics. Edler et al. (2009) reported in their study that indirect and direct employment for renewable energy in Germany, for the year 2013, calculated to 371,400 persons. If more emphasis of a nation will be on renewable energy production, then the cost for imports of fossil fuels will also get reduced. Not only this, the renewable energy production will create enormous opportunities to export. Blazejczak et al. (2014) reported that renewable energy production in Germany will reduce the cost of imports by €33 billion by 2030, and with export of renewable energy facilities and components these exports will generate €30 billion by 2030. However, renewable energy production will altogether raise the gross domestic product (GDP) of the country, directly accelerating the economy of the nation. With the expansion of the renewable energy generation, there lies a positive net effect on gross economy.

World Renewable Energy Prospects Globally, it is widely recognized that hazard and risks of energy generation through unsustainable means, affecting climate and environment, can only be taken into consideration through international cooperation. In January 2009, the International Renewable Energy Agency was established to act as a driving force in encouraging a rapid transformation toward the sustainable use of renewable energy. The United Nations (UN) initiated emission targets for greenhouse gases and introduced emission trading. In 2014, 164 countries adopted renewable energy policy. The EU set the target to meet 20 percent of its overall energy requirements through renewable and sustainable energy by 2021. An international agreement was adopted by the 2015 United Nations Climate Conference held in Paris to modify the fossil-based energy economy and to minimize the speed of global climate change. International initiative is set to implement hydrogen as the lead energy carrier. This decision is considered as most remarkable initiatives of the international community. The main crux of this initiative is to achieve global energy security along with preservation of environment. This step aimed to reduce the usage of gasoline for transportation. Around 20% of energy is utilized by transportation, and this energy is actually derived from gasoline. However, to implement hydrogen as an energy source demands rapid development of hydrogen-based technologies such as H2 production, transportation, storage, and distribution. Wasif et al. (2021) reported that Asia-

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Pacific Economic Cooperation (APEC) countries have initiated to double the renewable energy usage by 2030 via sustainable technology innovations and minimal use of fossil fuels.

Renewable Energy Production Technologies A variety of prospective solutions to the present environmental issues have been evolving such as production of hydrogen from waste materials. Hydrogen energy system emerges to be the most effectual technologies can play a key role in attaining preferable environmental sustainability. Globally, hydrogen is currently generated from coal, oil, natural gas, and water electrolysis. Chaurasia and Mondal (2021b) reported that global hydrogen demand is fulfilled by reforming (natural gas 50%, oil 30%, and coal 18%), water electrolysis (3.9%), and other sources (0.1%). Literatures reported catalytic reforming of biomass, hydrocarbon, or CH4 as the major source of hydrogen production. Literature reported that the above method of hydrogen production consists of severe drawbacks, such as low yield, high cost, and environment challenging. They have further proposed hydrogen production by water through photocatalysis, as this process is efficient, green, cheap, and easy to operate. There are some other water-splitting methods employed for hydrogen productions like photo-electrochemical, photo-biological, and thermal decomposition (Table 1). Various methods for green hydrogen energy generation are totally on driving sources and application. Some technologies are listed by various researchers are physicochemical, thermal, electrical, photonic, photo-thermal, electro-thermal, photo-electric, thermal-biochemical, bio-electrochemical, and photo-biochemical. Their selection is prominent on various fundamental criteria such as impact on environment, efficiency, cost-effectiveness, system integration option, and many more. Materials from which H2 can be derived are various, but the immense importance are wastewater, hydrogen sulfide, biomass, sea water, and fossil hydrocarbons. Green H2 generation requisites the utilization of renewable energy resources in place of fossil fuels. Few of it are promising and evident biological methods for green and clean hydrogen production. It is believed that in the upcoming future, green methods of H2 production will implement the use of hydrogen in the form of fuel, in production of fertilizers and other chemicals, in oil upgrade, and many more (Table 2). Various political, societal, and financial efforts are taken to boost world economy to H2. The International Association of Hydrogen Energy is playing a key role to influence world hydrogen movement. It is shown that without hydrogen, creating a carbon-neutral world is hard to imagine. This solution will help to combat global warming with clean technologies (Dincer 2012). Lodhi (2004) mentioned the implementation of water dissociation at high temperature, water electrolysis, thermochemical water splitting, and water photolysis for renewable hydrogen energy production from water. The author further listed some green energy sources such as sunlight, sea

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Table 1 Summary of clean energy generation technologies Technologies Reforming

Partial oxidation

Autothermal reforming Biophotolysis

Dark fermentation Photofermentation

Gasification

Pyrolysis

Thermolysis

Comments Commercialized stage but also produced CO, along with unsteady energy production Commercialized stage/ energy production with value added product recovery Commercialized stage but use of fossil-based feedstocks Utilize CO2, produced O2, under development stage, low rate of energy generation, high reactor volume, oxygen sensitivity, high capital cost High volume and slow rate of digestion, required technology improvement Technology at advanced stage, light-photon energy required, can treat almost all types of wastewaters, low yield of energy, slow rate of digestion Technology at advanced stage but low viability of commercialization, proper waste disposal, waste can be fed as feedstocks Irregular rate of energy generation Energy can be produced from waste and low value feedstocks, technology in advanced stage, irregular energy yield, secondary pollutant generation, yield depends on geography Sustainable technology, O2 as by-product, high capital and operation cost, corrosion issue, no secondary pollutant generation

Efficiency 75–85

Cost ($/kg) 2.27

Reference Zare et al. (2019)

61–74

1.48

Liu et al. (2020)

60–75

1.48

Lemus and Duart (2010)

10–11

2.13

Chaurasia and Mondal (2021b)

65–78

2.57

Chaurasia and Mondal (2021b)

0.1

2.83

Chaurasia and Mondal (2021b)

35–45

1.77–2.05

Hosseini and Wahid (2016)

40–50

1.59–170

Jaroenkhasemmeesuk and Tippayawong (2015)

25–50

7.98–8.40

Cong et al. (2016)

(continued)

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Table 1 (continued) Technologies Photolysis

Electrolysis

Comments No secondary pollutant generation and only O2 as by-product, under technology development, light photon and catalyst are required Green technology and at commercial stage but high cost, stationery process and only O2 as by-product

Efficiency 0.06

Cost ($/kg) 8–10

63–79

11–31

Reference Chaurasia and Mondal (2021b)

Energy (2015)

energy, winds, runoff water, and fissionable materials, to generate hydrogen. Lemus and Duart (2010) set forth green hydrogen production methods relied on water electrolysis. They itemized PV electrolysis, biomass gasification, wind power with electrolysis, hydropower with electrolysis, and many more. Dincer (2012) reported about biological methods, such as dark fermentation, photo-fermentation, and direct and indirect photolysis, for hydrogen generation. The author further characterized different forms of energy that are utilized for hydrogen generation from renewable sources, namely, biochemical, photonic, electrical, and thermal. The biochemical energy is present in organic matter and can be manipulated by various microbial species that release hydrogen from various substrates. There are some hybrid energy systems for generating hydrogen like electrical + thermal, photonic + biochemical, biochemical + thermal, and electrical + photonic. The energy resources have been categorized mainly into three sectors, viz., fossil fuels, renewable, and nuclear. Clean, green, and sustainable energy systems can resolve the present crucial tasks such as improving the energy supply, organic fuel economy, local energy, and water supply, enhancing living standards, generating employments, and protecting the environmental.

Solar Energy Solar energy generates electricity either through photovoltaic cells (direct) or concentrated solar power (indirect). In concentrated solar power technology, arrays of mirrors are employed to absorb the sun rays to a certain point to heat the working liquid, which is mostly employed to produce power. Concentrated solar power is effective in large areas, while photovoltaic cells are distributed on any surface exposed to sunlight. Globally, 177 GW of photovoltaic cells and 4.4 GW of concentrated solar power are installed as documented in 2014. Solar energy possesses negligible ecological impacts. However, the solar photons have the potential to produce electricity directly or accomplish hydrogen production photochemically or photoelectrochemically (splitting water molecules). The most prominently used solar cells to generate electricity are made up of silicon. Silicon-based solar cells are economically produced.

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Table 2 Current scenario of hydrogen production technologies Hydrogen production techniques Electrolysis

Thermolysis Thermocatalysis Thermochemical

Driving energy Electrical energy

Renewable resources Water

Thermal energy Thermal energy Thermal energy

Water Hydrogen sulfide Biomass, water

PV electrolysis

Photonic energy

Water

Photocatalysis

Photonic energy Photonic energy

Water

Bio-photolysis

Water

Comment O2& H2 is formed from H2O through electrochemical reactions by passing a direct current Decomposition of H2O thermally Sea-derived H2S is cracked thermo-catalytically H2 is formed via thermocatalytic biomass conversion, water molecule splitting via chemical reactions Electricity produced by PV panels in order to drive electrolyzer Photo-catalysts employed to generate H2 from H2O Cyanobacterial systems generate H2

References Energy (2015)

Cong et al. (2016) Cong et al. (2016) Idriss (2021)

Yin et al. (2013) Idriss (2021) Chaurasia and Mondal (2021b) Chaurasia and Mondal (2021b) Chaurasia and Mondal (2021b) Bendaikha and Larbi (2013)

Dark fermentation

Biochemical energy

Biomass

Anaerobic fermentation without light

High temperature electrolysis

Electrical energy and thermal energy Electrical energy and photonic energy Biochemical energy and thermal energy Photonic energy and biochemical energy Photonic energy and biochemical energy

Water

Chemical reactions for water splitting in electrolyte cells

Water

Photoelectrodes employed along with external electric source

Biomass

Biomass digestion with heating at low-grade temperature

Tyagi et al. (2014)

Biomass, water

Bacterial species employed for photo-generation of H2

Biomass

Fermentation in presence of light

Chaurasia and Mondal (2021b) Chaurasia and Mondal (2021b)

Photoelectrolysis

Thermophilic digestion

Bio-photolysis

Photofermentation

(continued)

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Table 2 (continued) Hydrogen production techniques Artificial photosynthesis

Driving energy Photonic energy and biochemical energy

Renewable resources Biomass, water

Comment Chemically stable molecules and compounds, enhanced systems for photosynthesis

References Dincer (2012)

The efficiency of PV (photovoltaics) semiconductor materials depends on the absorbing power. Crystalline silicon has maximum conversion efficiency but is difficult to produce and expensive. This can be replaced by TiO2 nanomaterials that are cheaper with higher energy conversion efficiency. Thus, nanotechnology introduced materials that have tailored adsorption properties which can absorb a large spectrum range of light energy up to 45%. This can be achieved by nanomaterials that arranged in order to absorb more solar energy or mimic approaches.

Wind Energy The use of nanotechnology in the wind energy applications is bringing methodological and technical advancements that are capable to tackle the challenges in the wind engineering. Nanotechnology has the combination of scientific and technological endeavors for the efficient energy generation of wind turbine material and architecture. The most significant contribution of nanotechnology is to enhance the efficiency and stability of wind energy system components. The power capture by wind energy system is the square of the blade length and application of nanotechnology aid to make large rotating blades as high as 100 m in diameter, rotating 90–125 m above the ground to produce five megawatts of power with stability (Tabassum et al. 2014). It also offers tight grip to the wind machine and blade. The use of nanocomposites materials offers high strength-to-weight and stiffness-to-weight ratios that facilitate the development of next-generation high-performance blades for wind energy applications. It also offers several enhanced properties to the materials that give novel function to the wind machine such as polymers having low molecular weight (di-acetylenes), long-term stability, and efficient performances. The development of nanosensors for wind energy applications creates smart grid integration and energy production decentralization. The development of nanomaterials contributes to prolong the life span of wind turbines and reduce the fatigue failures of structural components along with lowering their cost.

Hydropower Around 17% of electricity is produced globally from hydropower by running turbine. The advancement in the nanotechnology produces economical and durable

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large turbines that enhance the power generated from hydropower plants. Nanotechnology has made several advancements in the hydropower generation process such as nanocomposite materials for light and strong rotating blades, less wear and corrosion issues in the hydropower components, nano-coatings for bearings and power trains, etc. It also provides the exceptionally strong nanomaterials for lighter and more rugged rotor blades that provide wear and corrosion protection in the hydropower components. Literature reported that nanomembranes made of boron nitride or as nanotubes enhanced power generation with high density in reverse electrodialysis hydropower process. Feng et al. (2016) reported up to 103 kW m
2 power density with MoS2 nanomembranes. Nanotechnology offers novel device configuration that amplifies the power generation from hydropower plant to many folds. Due to development of nanotechnology, many novel strategies based on hydrovoltaic effect (HV) have been developed in the last decade in many countries (Wen et al. 2022). Nanotechnology has several improvements in the hydropower plant and their components such as carbon nanotubes as high-tensile construction materials, e.g., for rotor blades of turbine or as material for low loss cables/power lines, exceptional strong nanomaterials or nanocomposites for the rotor blades, and coatings for corrosion prevention and wear protection in the bearings. It also offers high conductive nanomaterials for improved lighting and energy storage capacity that are efficient and economical.

Geothermal Energy Nanotechnology has enhanced the energy efficiency, material durability, and performances of the geothermal energy generation component. Application of nanotechnology reduces the power losses of nanocomposite materials and efficient strategies for their deployment. Drawing the geothermal energy at very near to the earth surface where the temperature is very low has made possible with the help of nanotechnology. Thermal retaining and high heat capacity of the materials were also enhanced by the synthesis of novel nanomaterials. The employment of nanotechnology has made the geothermal energy generation device and their materials as energy efficient and reduced the power losses. Sheets of the nanotubes have also been used to build “thermocells” that produce electrical energy where all the sides are at varying temperature ranges. Some of these nanotubes are used at hot pipes (exhaust pipes) of vehicles that recover energy as electricity from heat, which will be wasted. The employment of nanofibers as piezoelectric materials or nanofibers that are elastic enough to be woven into clothing reduced the losses in the geothermal energy applications (Joy et al. 2018). The use of nanotechnology in the geothermal energy equipment, e.g., production/reinjection of silencers, separators, connecting/ delivery pipelines, turbines/generators, and cooling towers, has enhanced the energy efficiency, durability, and performances of geothermal power plants. Nanomaterials redefine the architectures of geothermal energy generation components and materials and their use in such a way that enhanced the energy-drawing power, efficiency, and

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transmission capacity (Hussein 2015). The use of nanotechnology boosts the power generation capacity of low-temperature hot springs very near to the earth’s surface. Literatures show that the nanostructured heat storage materials inside the special liquids (alkanes, etc.) assist in the rotating power of the turbine via evaporation. The applications of nanotechnology in the blend of various liquids potentially enhance the power generation efficiency of geothermal systems up to 35–45% along with reduction in the hazards.

Electrochemical Energy The applications of nanotechnology made several advancements in the material synthesis that enhanced the energy generation and improve the efficiency of the electrochemical process. The nanotechnology has redefined the electrochemical energy generation devices and components that are power efficient. In a broad term, the electrochemical energy process consists of electrode and electrolytic materials that possess efficient ionic and e-transport properties of nanomaterials, nanocomposites, and their system (such as catalyst and interface designs). The use of nanotechnology made the synthesis of novel materials that have multidimensional optimum electrochemical characteristics, namely, stability, suitability, coefficient thermal expansion, strength, and toughness. It also must be amenable, synthesized, or designed into the desired shapes and sizes at lower cost. It also must be tailored to meet technological forms and conditions in which it can be used for several applications such as higher temperature (above 500 C), gas waterproofing, interface stability, and compatibility of the electrochemical cells or system (Chaurasia and Mondal 2021b). There are various experimental applications of electrochemical system such as electrodes and their catalyst in the batteries, electrolysis, thermolysis, and photo-electrolysis process. Nanotechnology offers a diverse range of improved materials and flexibility in the operating conditions in most of electrochemical process although their operations remain the same in all such process. Various research groups reported that some economical and efficient nanostructured electrocatalysts such as Ru, Sn, Ni, Ni-Co, Ni-Co-P, Ni-Co-F, Co, and Mo give competitive results in the energy generation process such as hydrogen evolution reaction and alkaline water electrolysis, ion exchange membrane, and solid oxide water electrolysis (SOE) (Chaurasia et al. 2020; Li 2019). The nanotechnology innovation in the production of these electrochemical-catalyst also provides the good control over designing morphology for the enhanced electrocatalyst performance, efficiency, and stability of the process. The use of nanotechnology (Ni, Fe, cobalt phosphate, and some other nanocatalysts) has reduced the hydrogen generation cost by 100 times and even more in few processes. The nanomaterials such as α-Ni(OH)2 give the very less 0.33 V over-potential as well as improved stability (Chi and Yu 2018). Another study shows that the Co3O4/C porous nanowire has over-potential of 0.29V, and IrO2/C shows the over-potential of 0.31 V at a current density of 10 mA/cm2. Also,

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the use of transition-metal chalcogenides, carbides, complexes and metal alloys as electrode nanocatalyst such as Mo (IV)-disulfide complex, and carbon nanotubes decorated with cobalt phosphide (CoP) nanocrystals reduced the cost of the process along with enhancement in the process efficiency. There are various advancements in the nanomaterials increasing the efficiency of electrochemical energy generation processes and technologies but still need extensive research for economical energy process. Among these, few such as nanostructured Ti and Ni and various first-row transition metals are promising and have shown their capability to be commercialized in the upcoming future.

Photo-electrochemical Energy The very often stated photochemical energy system incorporates of light-derived water oxidation, leading to hydrogen production via water splitting (total oxidation) and removal of organic compounds (partial oxidation). The total oxidation of water generates oxygen at anode and hydrogen at cathode, but corrosion of materials is the biggest challenges, and it needs corrosion-resistant oxide semiconductors that work well when immersed in water. The development of nanostructured electrodes, sensor, and corrosion-resistant oxide semiconductor materials shows the potential and capability to be commercialized, and few of them are already commercialized. The most common strategy to develop oxide semiconductors, like TiO2, is to elevate light absorption via modified electronic structure. Recent published literature reports state that performance of these systems is determined by the associated functional properties and local properties of the interface layer (Ortega-Liebana et al. 2016). They reported some of the essential properties like concentration of surface-active sites, electric fields essential for charge separation, and gradient of defect concentration within the layer. The applications of nanotechnology in the photo-electrolysis offer low-cost and efficient renewable energy generation such as production of hydrogen. In photoelectrolysis the H2 generation by water break down in the presence of sunlight and catalyst (doped semiconductors material as photovoltaics). Recently developed nanomaterials and nanocomposites have enhanced capability to generate an e--hole pair and hole that enhanced the rate of hydrogen production with reduction in the operating cost of the process. The estimated cost of H2 production by photoelectrolysis is found to be $41.30 per gigajoule, and the application of nanotechnology reducing the cost of the hydrogen generation brings positive hopes in the renewable energy sector (Scott 2020). Few recent literatures demonstrate the photoelectrode materials such as Fe2O3 and TiO2, and other semiconductor and membrane materials that improved the H2 generation efficiency and have the capability of producing economical hydrogen (Wen et al. 2022; Basheer and Ali 2019). The nanoscale crystalline III–V semiconductors and thin-film copper chacopyrites show the exceptional optoelectronic properties that are necessary for high

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photocurrents and hydrogen evolution reactions. Still the major concerns are the stability and efficiency of semiconductor materials, energetics, catalysts, and their configuration as well as cost.

Bioenergy Bioenergy is referred as the renewable energy generated from plant- and animalbased matter. The sources of bioenergy are diverse including residues from forest sectors (wood), livestock waste residues from agricultural sector, manufacturing waste, food waste, domestic waste, and municipal waste. Adib (2015) reported that total energy (primary) demand from bioenergy was 58.5 EJ, and bioenergy’s share in the total worldwide primary energy consumption was 10%. Traditional bioenergy such as dung, charcoal, and wood fuel, used mainly for cooking and heating, accounts for 54–60%. Modern uses of bioenergy generating green economy include bio-heating, bio-fuels, and bio-power. They mainly focus on biodiversity and ecosystem impacts (UNEP 2014). The prominent technologies to drive modern bioenergy are classified into thermochemical conversion and biochemical conversion. Thermochemical conversion technologies, such as combustion, gasification, and pyrolysis, produce bio-heat and bio-power, while biochemical conversion technologies, like digestion and fermentation, produce liquid bio-fuel (bio-ethanol, bio-diesel) for transport, cooking, and heating. Most of the attention in renewable energy circle is attracted by biomass energy. It incorporates residues from forest, crops, wood, human sewage, solid waste, energy crops, and food processing waste. Far from other renewables, biomass is a readily obtainable source, and its ampleness makes it a suitable alternative to replace fossil fuels. In 2009, biomass provided 55% of total renewable energy supply. Various developing nations have met 35% of energy demand and 13% of its global consumption is through biomass. It is notable that agricultural, municipal, and forest wastes represent a capacity of 13 billion metric tons, i.e., ten times higher than the present global energy demand. On the other hand, biomass plays a key role in enhancing environmental quality. Wasif et al. (2021) reported that consumption of modern biomass energy sources has less environmental impacts compared to traditional ones. So, the new trend is focused on generating renewable energy through modern biomass technology like anaerobic digestion. Biomass energy is termed as important alternative energy as it can reduce foreign oil dependency and mitigate greenhouse gases emission. If generated in a sustainable manner, it shows the potential to mitigate carbon dioxide emissions. Gao and Zhang (2021) reported that traditional use of biomass is expected to decline and modern use of biomass is rising exponentially. Burgherr (2011) reported that some popular feedstocks employed in bio-heat and bio-power production comprised of poplar, willow, eucalyptus, etc. Adib (2015) stated common feedstock for bio-heat and bio-power production, such as agricultural residues like wheat straw in Europe Union and North America, sugarcane bagasse in Brazil, and maize straw in India and North America.

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Employment of Nanotechnology for Engendering Clean and Sustainable Energy Nanotechnology is termed as the field of nanoscience where the phenomena are applied on nanometer-scale level. Nanomaterials are the smallest structures, of a few nanometers in size, developed under human invention. A variety of formulations of nanoparticles have been developed including nanotubes, nanofilms, nanowires, nanoparticles, colloids, and quantum dots. Nanoscale research on lattice structures and electron pairing are expected to elevate the knowledge of superconductivity, new formulations, novel fabrication techniques, etc. Hussein (2015) reported various benefits of nanotechnology for renewable energy generation, i.e., increased efficiency of lighting and heating; increased electrical storage capacity; decrement in pollution, and many more. Any sort of material can have various properties at nanoscale. Some turn lighter, stronger, conductive, stable, reflective, and chemically active and enhanced magnetic properties. It is reported that nanotechnology can provide significant contributions to hydrogen and solar energy sectors. They further stated that nanotechnology resulted in elevated photovoltaic (PV) solar cell efficiency along with reduction in their manufacturing and electricity generation costs. They further discussed about the increased hydrogen adsorption capacity of fuel cells by nanotechnology further making them economical and efficient. Literature reported that integration of nanomaterials in solar and fuel cells can enhance the energy conversion efficiency of these devices, nanomaterials doped with metal hydrides can possess high hydrogen storage capacity, nanoparticles possess high sensitivity toward detection of environmental pollutants, and carbon nanotubes can be utilized as reinforcement materials in green nanocomposites. Daryoush and Darvish (2013) highlighted the applications of nanoparticles to reduce energy consumption. They suggested that nanoparticle implementation in the manufacturing sector can save nonrenewable fuels, distribution of nanowires in the electricity sector can reduce electricity losses, and nanomaterials employed in buildings can prevent thermal and cooling energy loss. The considerable challenge in using solar devices is their limitation in absorption properties of the conventional fluids which affect the efficacy of these devices. Recently, the limitations of solar energy devices can be easily resolved with the implementation of nanotechnology. Hussein (2015) mentioned that solar energy collection and efficiency can be enhanced through more conductivity and increased surface area to volume ratio of nanomaterials. They further reported that nanoparticles can facilitate release of more number of electrons when hit by a photon, thereby increasing the efficiency of solar devices. Taylor et al. (2011) reported about the advantages of nanotechnology in solar energy production. They stated that nanosized materials can easily pass via pumps and plumbing without resulting any adverse effects; nanomaterials are competent to absorb energy directly; they can be optically selective in nature; they can maintain uniform temperature, thereby reducing material constraints; and they can enhance heat transfer by elevated convection and thermal conductivity, and increased absorption efficiency can be achieved by turning nanoparticle size and shape to the required application. Quantum dots are

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reported to be three times more efficient than current materials employed in solar cells. Some recent literatures reported that nanofluids can be more effective than conventional fluids as a heat transfer medium in the solar water heater. Yuhas and Yang (2009) presented a solar cell design with a nanowire-based solar cell to use economical, durable, and environment-friendly semiconducting photovoltaic (PV) components. Otanicar et al. (2010) reported from their experimental observations that efficiency of solar thermal collectors can increase up to 5 percent by utilizing nanofluids made from carbon nanotubes, graphite, and silver. Liu et al. (2010) mentioned that TiO2 is the most investigated material for solar cell and solar fuel applications. They further stated that TiO2 cells could reach the optimal level of efficiency when coated with gold or silver nanoparticles. The enhancement can be via optimal scattering from the plasmonic nanoparticles which will directly increase the optical path of the thin film. Baraton (2011) presented TiO2 applications in photocatalytic water splitting and dye-sensitized solar cells. Various literatures reported that carbon nanospheres have the potential to improve the photo-thermal properties of working liquid. Nanotechnology even possesses the potential to solve the major drawbacks of fuel cells used to generate hydrogen energy, namely, high cost, operability issues, and durability issues. Various research groups reported the use of TiO2/SnO2 nanoparticles on Nafion membranes to improve proton exchange membrane fuel cells. Nanomaterials can be employed for clean, green, and economical hydrogen generation via photocatalysis. Zhu and Zäch (2009) referred that nanostructured photocatalysts, having large surface areas for optimized light absorption and minimal distance for charge carrier transport, offer alternate and sustainable energy systems to limit greenhouse gas emission. Photoelectrochemical hydrogen production from methanol/water decomposition is reported using Ag/TiO2 nanocomposites. B. Zhu et al. (2013) reported in their review about multifunctional ceria-based nanocomposites. Lee and Kjeang (2013) reported about a nanofluidic fuel cell which can utilize fluid flow through nanoporous media. They claimed that this fuel cell demonstrates reduced activation over-potential, higher surface area, faster kinetics characteristics, high overall efficiency, miniaturized power sources, and low cost. Mahmood and Hussain (2010) reported the use of nanobiotechnology in conversion of spent tea into hydrocarbon fuels such as bioethanol and biodiesel. Goh et al. (2012) reported the incorporation of iron oxide nanoparticles into single-walled carbon nanotubes. They have further recycled the immobilized enzyme for biofuel production. Wen et al. (2010) reported in their study that KF/Cao nanocatalyst enhanced the biodiesel yield to 96.8% and can be efficiently used to convert the oil with higher acid value into biodiesel. Hu et al. (2011) reported the use of KF/Cao-Fe3O4 nanocatalyst for efficient biodiesel production. Kaur and Ali (2011) prepared lithium impregnated calcium oxide nanocatalyst for biodiesel production via transesterification of karanja and jatropha oils. Chen et al. (2013) investigated role of highly efficient sulfated zirconia nanocatalyst for the production of biodiesel and bis(indoyl) methane. Ngo et al. (2013) performed green transformation of waste grease to biodiesel fuel using recyclable magnetic nano-biocatalyst aggregates with 98% yield. Hernández-Hipólito et al. (2014) employed sodium titanate nanotubes to

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Table 3 Impacts of nanotechnology in the energy production process Renewable energy Solar energy

Hydrogen Energy

Bioenergy

Wind energy

Advantages 1) Larger effective optical path for adsorption 2) Reduced recombination losses 3) Desired energy bandgap value by varying the size of nanoparticles 1) Improving performance as multifunctional materials 2) Reduce cost of components 3) Enhance mechanical strength 4) Enhance electrical conductivity 5) Amplify hydrogen storage 1) Reduced capital costs 2) Enhanced performance of bioreactors 3) Improves efficiency of biofuel production 1) Construction of longer and strong blades 2) Increase wind turbine efficiency 3) Improve wear resistance

achieve 97–100% biodiesel yields. Konwar et al. (2014) mentioned that carbonbased nanocatalysts can open doors for cost minimization and environmentally favorable biodiesel production. Nanomaterials have marvelous strength to weight and stiffness to weight ratios. Nano-lubricants can be employed to enhance wind turbine efficiency by decreasing energy losses due to wear, micro-pitting, scuffing, and spalling in gear boxes. Qu et al. (2013) reported that nanotechnology can offer opportunities for development of next-generation water supply systems. They further stated that the extraordinary features of nanomaterials, such as high surface area, photosensitivity, catalytic activity, antimicrobial activity, electrochemical properties, optical properties, and magnetic properties, can prove useful as sensors. Nanotechnology can go parallel with sustainable water management (Table 3).

Use of Nanofluids in Energy Applications The nanomaterials such as nanoparticles or nanofluids have significantly enhanced the efficiency in various disciplines of energy applications. Such materials are explored in thermal energy applications, oil, and gas refineries.

Thermal Energy Storage (TES) TES is utilized for energy storage applications such as salts, paraffins, etc., but thermal properties of these materials must be improved. This can be achieved by introducing the nanomaterials in the aqueous electrolytes and also by phase changing of doped materials such as freezing or melting. The concentrated solar power (CSP) plants are best examples for it, which lower the cost by around 1000 times.

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Table 4 Few nanomaterial electrode (cathode and anode) used in the bio-electrochemical system Study Kyazze et al. 2010

Selembo et al. 2010 Manuel et al. 2010b Escapa et al. 2012 Sangeetha et al. 2016 Mitov et al. 2017

Eapp (V) 0.6 0.85 1.0 0.7 0.9 0.9 0.9 0.8 0.75 0.8 0.8 0.6 0.6

Li et al. 2019 Jayabalan et al. 2021

0.6 0.7 1 1 1 1

(Chaurasia et al. 2021a) (Chaurasia and Mondal 2022) (Chaurasia et al. 2020)

Cathode CC/Pt CC/Pt CC/Pt CC/Pt Ni 201 Ni 400 Ni 625 Biocathode

1 0.6

Ni Cu Ni-MO/Ni foam Ni-W/Ni foam Ni foam Carbon paper (CP) Ni foam NiMoO4 Nickel molybdate NiO.rGO/ Co3O4.rGO Ni2P Ni/Ni-Co/ Ni-Co-P

YH2 (mol H2/ mol substrate) 0.52 1.1 0.92 1.63 – – –

Substrate Acetate Acetate Acetate Glucose Acetate Acetate Acetate Acetate

Q (m3/m3d) 0.1 0.2 0.3 0.34 0.34–0.42 0.31–0.51 0.79 0.03

Domestic wastewater ‘’ ‘’ Acetate

0.3

–

0.1 0.13 0.14

– – –

Acetate

0.13

–

Acetate Acetate

0.04 0.121–0.129

– –

Sugar industry wastewater “ ,,

0.12

–

0.14 0.12

– –

,,

0.11–0.13

–

,, Paper industry wastewater Sugar industry wastewater Acetate wastewater

0.29 0.7–0.32

1.1–2.4

0.8–0.38

0.9–2.5

0.12–0.42

1.0–2.8

TES typically enhanced the life of photovoltaic by 5 years. It also compensates for the intermittent nature of solar CSP and gives high power output. TES applications in batteries or fuel cell system enhanced the cooling efficiency (Paksoy and Beyhan 2015). The use of TES in solar collector for solar energy applications got wide research attention in the last decade. Various studies listed in Table 4 have shown that the use of nanofluids in place of conventional heat transfer fluids (water, water-oil mixture) in solar collector improved the efficiency of solar system along with reduction of its

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cost. The use of nanomaterials in water (Al2O3-Fe/water) in solar flat plate collector such as Al and Fe nanoparticles in water considerably improved the performances of solar system.

Nanocrystals The conventional material silicon has indirect bandgap and low adsorbing capacity that can be improved by nanocrystal. It makes the bandgap quasi-direct, which enhanced the higher light adsorption. The optical properties of the materials can also be influenced by the nanocrystal. The best examples are silicon-modified tandem cells at the upper part of nanocrystal and conventional silicon at the lower part that enhanced the current production of the solar system. Other examples are the nanocrystal memory properties that depend on the nanocrystal size, shape, and configuration. These are not formed by patterning; thus, fluctuations in properties become important such as memory cell size which is likely to reduce to 1000 nm2 by 2020 (Fernandes et al. 2018). Thus, it has become challenging to synthesize nanomaterials with uniform size and shape and assemble them into a well-ordered nanocrystal matrix. This can be overcome by high-density nanocrystals that store more charge in a memory device and mitigate the influence of fluctuations. It is considered that the metallic nanocrystals are promising for commercial application and co-currently achieve high program/erase speeds and long retention times, multilayer nanocrystals, work function engineering, and surrounding oxide passivation, but it needs the methods that strategically match the scaling limits (Baraton 2011). The development of nanocrystals offers various sustainable benefits such as low power consumption, less hazardous waste generation, and highly efficient materials. Nanocrystal has strong applications in the research area of hydrogen production, pollutant removal from wastewater, medical imaging, gene identification, drug and protein analysis, optical and infrared laser, opto-isolators, magneto-optical memory chips, and self-organized smart materials.

Applications of Nanomaterials in the Clean Energy Generation from Waste and Wastewater With continuous rise in world population, safe and clean drinking water for each and every person is a biggest challenge of this century. Despite water being one of the abundant natural resources present on earth, only 1% of that is available for human livelihood. In 2015, WHO reported that more than 1.1 billion strive for sufficient drinking water (Anjum et al. 2019). The contamination of freshwater by pollutants is a major challenge in the water supply chain across the globe. The wastewater treatment and energy generation from wastewater can help to overcome these concerns to some extent. But the main problem is that the traditional methods of wastewater treatment are not competent enough to remove the emerging contaminants and meet the strict standards of water quality. Ferroudj et al. (2013) mentioned

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the drawbacks of existing wastewater technologies such as generation of toxic sludge, incomplete pollutant removal, and requirement of high energy. Various literatures reported that biological wastewater treatment is no doubt widely implied, but these treatment technologies are slow, limited due to presence of nonbiodegradable contaminant, and sometimes toxic due to microbial cultures via toxic contaminants. While there are some physical processes of wastewater treatment such as filtration which are capable of removing contaminants by transforming one phase to another, they also possess a serious concern, i.e., production of highly concentrated toxic sludge. Various literatures have enlightened the pivotal need of development of efficient and powerful wastewater treatment technologies. Many researchers suggested that advanced nanotechnology can have potential to remediate wastewater treatment by developed nanomaterials including nano-adsorbents, nanocatalyst, and many more for treatment of wastewater. Yin et al. (2013) enlightened that combination of biological wastewater treatment process and advance nanotechnology provided efficient water purification system. Mainly three class of nanomaterials were employed in the wastewater treatment such as nano-adsorbents, nanocatalysts, and nanomembranes. Various nano-adsorption materials are reported recently that effectively treat the wastewater and are efficient in the removal of pollutants from wastewater. It can be synthesized by using the atoms of those elements which are chemically active and have high adsorption capacity on the surface of the nanomaterial such as activated carbon, silica, clay materials, metal oxides, and composites. Second is the nanocatalyst such as metal oxides and semiconductors that are experimentally proven to have the capacity of removing pollutants in the wastewater or electrocatalysts. It improves chemical oxidation of organic pollutants and few have antimicrobial resistance. The third category is the nanomembrane-based materials that treat wastewater in the pressure-driven processes. They have attractive properties such as small pore sizes, low cost, high efficiency, and high-level synthesis by nanomaterial particles, non-metal particles, and nano-carbon tubes. The oxide-based nanoparticles used in wastewater treatment are titanium oxides/dendrimer composites, zinc oxides, magnesium oxide, and ferric oxides that have high surface area and less solubility without any secondary pollutant’s generation. The iron-based nanocatalyst has simple synthesis process of ferric oxide used in the adsorption of noxious metals. The Fe2O3 nanoparticle adsorption depends on the pH, temperature, adsorbent dose, and incubation time (Basheer and Ali 2019). The adsorption properties can be enhanced by surface modification with 3-aminopropyltrimethoxysilane and its very effective in the removal of Cr3+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+, and As3+. The manganese oxide-based nanocatalysts have high adsorption ability, high surface area, and polymorphic structure and have wide applications in the removal of various heavy metals such as arsenic from wastewater (Lin et al. 2014). The zinc-based oxide micro-/nanostructure are very effective in the adsorption of heavy metals. These are nano-assemblies, nano-plates, microspheres with nano-sheets, and hierarchical ZnO nano-rods which are widely used as nanoadsorbent for the removal of heavy metals from wastewater. Carbon nanotubes (CNTs) are also used for the removal of organic pollutants, but poor dispersion is the key challenge in the separation of pollutants. On the other hand, the surface

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modification of the CNT improves the adsorption efficiency. Functionalization of CNT surface can be used to enhance its adsorption efficiency. Graphene-based nanoadsorbents such as graphene oxide having two-dimensional structure produced by the oxidation of graphite layer via chemical method are highly efficient in the pollutant’s removal in the wastewater.

Use of Nanomaterials in the Feedstock Pretreatment for the Aerobic Digestion Conventionally, treatment of municipal wastewater with biological process is less challenging in comparison to industrial wastewater as it contains more toxic and minimal biodegradable pollutant. The nanomaterials are capable of degrading organic contaminants and are reported as well for the remediation of chlorinated pollutants from ground water. Ma and Zhang (2008) reported that combination of ZVI nanoparticle treatment with aerobic digestion is highly promising for biodegradation of organic pollutants from wastewater. They further mentioned that some of the contaminants will be remediated by nanoparticles and the remaining ones with aerobic digestion. This process becomes more efficient and less complex. As reported by Anjum et al. (2019), the products formed by ZVI nanoparticles are more biodegradable for aerobic microbial species. Some researchers have also reported the treatment of azo dye and nitro-aromatic compounds through coupling of ZVI nanoparticles with biological processes. A pilot scale study was performed by Ma and Zhang (2008) where they revealed that the effect of ZVI nanoparticles on biological treatment resulted in 96.5% removal efficiency and 86% biological oxygen demand (BOD) and chemical oxygen demand (COD).

Nanomaterials with Algae Membrane-Based Bioreactor Cultivation of algae in wastewater is regarded as one of the prominent techniques for wastewater treatment along with energy generation. With the availability of micronutrients, such as trace metals and vitamins (cyanocobalamin, thiamin), and macronutrients, such as calcium, sodium and potassium, which are crucial for algal growth, many algae species can effectively grow in wastewater. Anjum et al. (2019) mentioned that chemical salts present in wastewater act as nutrient solution along with light and carbon dioxide which can create a perfect niche for algal growth. This will result in removal of nutrients from wastewater and formation of algal biomass for energy generation. Brennan and Owende (2010) mentioned about various algal biomass harvesting techniques such as sedimentation, air flotation, and centrifugation with chemical flocculation. They also stated that these techniques demand huge capital when implemented on a large scale. Among this advantage, there are other benefits of membrane technology as well; unlike other harvesting methods, there is no necessity of addition of chemicals like coagulants in the membrane filtration, thereby making easing the reuse of water after filtration and biomass separation.

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Furthermore, algal biomass recovery is better without cell damage and minimal energy requirement in comparison with conventional methods. Polysulfone (PSF), polyethersulfone (PES), and polyvinylidene fluoride (PVDF) membranes are mostly employed. Several researchers have discussed various approaches, such as plasma treatment, surface coating, and incorporation of nanoparticles, to enhance hydrophilicity and reduce membrane fouling. Mascaretti et al. (2017) mentioned that coating of TiO2 nanoparticles modified the polyvinyl reverse osmosis membrane along with minimal fouling through self-cleaning mechanism under UV radiation. With high surface area and hydrophilic properties, TiO2 nanoparticles can be employed for pollution control. Thus, with the possession of above characteristics, nanoparticles can be incorporated with membranes for reduction of fouling and hydrophobicity.

Use of Nanotechnology in the Bio-electrochemical System for the Hydrogen Production The applications of the various nanocatalyst in the anode materials, cathode materials, and membranes enhanced the scientific viability of the biological fuel cell technologies such as microbial electrolysis cells (MECs). It is considered that the MECs have potential to degrade almost of all types of waste materials and can recover up to 90% energy present in the waste materials. Microbial electrolysis cells (MECs) have attracted attention of various researchers due to their benefit for wastewater treatment along with renewable energy production by using microbes as biocatalysts. The energy produced by MECs can possibly reduce the electricity need for a conventional treatment process, while complex organic molecules like acetate are split into hydrogen and carbon dioxide by bacterial species (Chaurasia and Mondal 2021b). Overall low performance and expensive components are major barriers for commercialization of the MEC technology. The MEC electrode reaction is given in Eqs. 1, 2, 3 and 4 with acetate as model substrate as follows:
 Anode : CH3 COO
þ 4H2 O ! 2HCO3
þ 9Hþ þ 8e
E0 ¼ 0:187 V
 Cathode : 8Hþ þ 8e
! 4H2 E0 ¼ 0:000 V

ð1Þ ð2Þ

Overall : CH3 COO
þ 4H2 O ! 2HCO3
þ Hþ þ 4H2 ðΔGr ¼ 104:6 kj=molÞ ! RT CH3COO
0  ln
Ean ¼ Ean ¼
8F HCO3
2 ðH þ Þ

ð3Þ ð4Þ

The anode potential can be found as E0an ¼ 0:187 V and EH+/H2 ¼
0.414 V vs NHE (normal hydrogen electrode) at 25  C, pH 7, and PH2 1 atm; under standard condition, the anode potential is
0.279. Thus, the cell voltage (Ecell) necessary for a MEC to produce H2 at the cathode under these conditions are
0.14 V.

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Alternatively, it can also be calculated as Eeq ¼
ΔGr/nF ¼ (
104.6  103)/8  96485 ¼
0.14 V and ΔGr ¼ 104.6 kj/mol. MEC has the energy or voltage losses due to cathode over-potential, anode over-potential, and metabolic loss in the anodic biofilm. This indicates why the electrode materials are important in the biological fuel cells. When the targeted product is hydrogen, then MECs require external (>0.14 V) at the cathode. The implementation of cathode nanocatalyst would make MECs economically feasible (Chaurasia et al. 2020; Chaurasia and Mondal 2021c). Chaurasia et al. (2021a) mentioned that selection of appropriate material for performance enhancement is also pivotal. Some authors believe that nanoparticles will be biologically more active with regard to other materials as they possess high surface area and can effectively interact with bio-systems. Yuan et al. (2010) reported the use of economical nanocomposite materials like nanostructured carbon in electrodes. They further concluded that these electrodes possess better electrochemical catalytic activity, high conductivity, and large surface area. Ghasemi et al. (2011) concluded in their study that due to high surface area of carbon nanotubes, the catalytic activity of platinum enhanced. It is also reported that unique structural properties and high electrical conductivity of carbon nanotubes derived the above results. Few studies suggested that carbon nanotube/platinum can work as cathode catalyst for commercial purposes replacing platinum. Some study reported that electrodes incorporated with nanoparticles possess large surface area with more active sites for electrochemical reactions and microbial attachment. Thus, biofilm formation will take place easily which is necessary for electron transfer toward anode surface. The studies listed in Table 4 conclude the utilization of nanotechnology for the deployment of efficient electrode/cathode materials, which are intended to increase biofilm attachment, electron transfer rate, and higher surface area. Apart from these, few of them have lower ohmic losses that are possible due to the application of nanotechnology. These advancements make it a more viable practical option and have the capacity to commercialize the bio-electrochemical system for the largescale wastewater treatment technologies.

Conclusion The present chapter provides a comprehensive understanding and overview of nanotechnology in generating green, sustainable, and clean energies. Nanotechnology has made a huge revolution in the development of environmentally friendly materials and their use in the production of renewable energy from waste and renewable resources and energy recovery and storage. It becomes evident that nanotechnology plays a pivotal role in increasing the efficiency of fuel cell, solar cell, and wind turbine. Green and sustainable nanotechnology is advantageous for clean energy production in every aspect such as performance, efficiency, cost, and durability. Advancement of green and sustainable nanotechnology toward the direction of clean energy generation should increment in providing robust solutions for environmental remediation and fulfilment of clean energy demand. This chapter

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summarized the application of nanotechnology and use of nanomaterials in most of the clean energy generation from the waste and renewable resources. It includes the technology advancement and key challenges of these technologies and role of nanotechnology for its improvement. The further, extensive research and international collaboration can make these technologies feasible and are capable in making commercialized waste to energy generation technologies.

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Sivashunmugam Sankaranarayanan, Maria Michael Christy Priya, Dhileepan Priyadharshini, and Singaravelu Vivekanandhan

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials for Supercapacitor Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Traditional Methods and Feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sustainable Platform for Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosynthesized Metal and Metal-Based Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal and Metal Chalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Oxide Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass-Derived Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zero-Dimensional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . One-Dimensional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-Dimensional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-Dimensional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposites Fabricated Using Renewable Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-, Metal Hydroxide-, and Metal Oxide-Based Nanocomposites . . . . . . . . . . . . . . . . . . . . . Renewable Carbon-Based Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose Nanofiber-Based Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Emerging demand for energy storage devices stimulates the research and development activities on supercapacitors due to its unique advantages (power delivery and cycling performance) over secondary batteries. A wide

S. Sankaranarayanan · M. M. C. Priya · D. Priyadharshini · S. Vivekanandhan (*) Sustainable Materials and Nanotechnology Lab (SMNL), Department of Physics, V.H.N.S.N. College (An Autonomous Institution, Affiliated to Madurai Kamaraj University), Virudhunagar, Tamil Nadu, India e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_60

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range of materials, which include metals, metal oxides, chalcogenides, carbon allotropes, and their hybrid/composite architectures, have been extensively investigated as electrode materials for supercapacitor applications. It was found that the nanostructure of the above mentioned material showed superior electrochemical performances compared to their bulk forms. As a result, a wide range of nanomaterials have been extensively investigated as the electrode material in all segments (employing pseudo and electric double layer capacitive mechanisms in symmetric and asymmetric configurations) of supercapacitors. Recent research interest in supercapacitor materials turned into their synthesis/ fabrication by employing various sustainable/green strategies, which include the consumption of renewable resources, exploration of waste/recycled products as feedstocks, and the utilization of materials/processes with less environmental impacts. Thus, the present chapter summarizes the synthesis and their applications of various renewable resource-based green nanomaterials for supercapacitor applications. Keywords

Renewable resources · Green nanomaterials · Energy storage · Supercapacitor

Introduction Supercapacitors earned increasing importance due to not only their ability to deliver high power but also for their longer life cycle compared with rechargeable batteries (Najib and Erdem 2019). One of the key challenges for electrochemical supercapacitors is their lower energy density, which leads the researchers to explore new electrode materials (Wang et al. 2012). As a result, a wide range of metal oxides, carbon materials, chalcogenides, and MXenes (their pristine and heterostructure/composite/hybrid forms) have been explored as electrode material in supercapacitors (Dahiya et al. 2022; Hu et al. 2020; Wang et al. 2012). Based on the nature of charge storage mechanism in the electrode materials during the cycling (charge/discharge) performance, the supercapacitors can be classified into two categories such as electrochemical double layer capacitor (EDLC) and pseudocapacitor (faradaic supercapacitors) (Wang et al. 2021). EDLC deals with the electrochemically not active electrode materials, which are involved in pure physical charge accumulation at the electrode-electrolyte interface (Wang et al. 2012, 2021). On the other hand, pseudocapacitor deals with the electrochemically active electrode materials involved in reversible faradaic reactions (redox reactions) in the electrode (Kumar et al. 2021). Performance of both the electrode materials can be improved when they are made into nano dimensions, which greatly enhances their specific surface area, structural stability, and electrolyte wetting properties. Thus, extensive efforts have been made to synthesis various nanostructured materials for supercapacitor applications.

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Nanomaterials for Supercapacitor Applications Nanomaterials have been extensively explored as the potential candidate for the fabrication of various electrochemical energy storage and conversion systems, which include supercapacitor (Zhang et al. 2013). The unique size- and shapedependent physicochemical and functional properties with varying structural and morphological diversity found nanomaterials as the suitable candidate for supercapacitor applications as electrode materials in both electrochemical double layer capacitance and pseudocapacitance platforms (Jiang and Liu 2019). The ultimate aim in utilizing nanostructured materials, with a diverse structural and morphological feature, is to provide an improved energy density without comprising their superior power delivery and excellent cyclability (Zhao et al. 2011). Nanostructured electrode materials exhibit superior specific surface area and enhanced electroactive site, which, respectively, promotes the electrode-electrolyte interface (in EDLC) and redox reactions (in pseudocapacitors) and improves their capacitive performances (Wang et al. 2012). Carbon nanomaterials such as carbon nanofibers, carbon nanotubes, graphene/graphene oxide/graphitic nanostructures, nanostructured/nanosized carbon particulates, and their metal/metal oxide/metal chalcogenide/conducting polymer-based composites have been extensively used as the electrode material for electrochemical double layer capacitors (EDLC) (Wang et al. 2021). One-dimensional carbon nanostructures such as carbon nanotubes and carbon nanofibers exhibit excellent electrical conductivities, larger specific surface area, enhanced structural stability, and ability to integrate themselves with other materials, which makes them a suitable candidate for supercapacitor applications (Zhang et al. 2016). Graphene/graphitic nanostructures exhibit unique electric conductivity and mechanical strength (with a Young’s modulus of 1 TPa) due to their atomic configuration of C-C sigma covalent linkage (apart from larger specific surface area, electrochemical performances, chemical stability, and thermal conductivity), which makes them as the suitable electrode material for flexible and high-performance supercapacitors (Ni and Li 2016). Further, it was identified that the heteroatom (nitrogen, oxygen, sulfur, boron, and phosphorus) doping leads to the significant improvement on their capacitive properties by means of increasing their electronic density (improves their electron mobility) and enhancing the active sites (leads to superior electrolyte wettability) (Li et al. 2015). On the other hand, a wide range of metal oxides, metal hydroxides, metal chalcogenide, conducting polymers, and their composites which have been widely explored are pseudocapacitive electrodes (Jiang and Liu 2019). Along with the electrical conductivity and morphological properties, bandgap and charge carrier mobility are the critical factors for the pseudocapacitive materials for their superior performance. The capacitive performance of the above-discussed materials can be effectively enhanced by tuning their size, shape, porosity, surface functionality, and defect configuration, which can be achieved by employing various processes and precursors and also manipulating experimental parameters. As a result, a wide range of processes have been explored for the synthesis of various nanostructured materials with desired

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properties. With respect to carbon nanomaterials along with processes, various carbon precursors have been explored and reported in the literature.

Traditional Methods and Feedstocks Nanostructured materials can be synthesized by adopting various physical and chemical processes involving both bottom-up and top-down approaches, which include mechanical milling, sputtering, arc discharge, thermal evaporation, laser ablation, chemical reduction, sol-gel, coprecipitation, combustion, and polyol, sonochemical, solvothermal, hydrothermal, microwave/plasma-assisted, and template-mediated synthesis (Moriarty 2001). Ball milling is one of the widely explored top-down approaches for the synthesis of various nanomaterials, which deals with breaking larger particles into nano dimension involving longer duration and possible material contamination and exhibits poor homogeneity and size control. Other physical methods involved in the extreme environmental conditions (highvacuum), sophisticated equipments, challenges in morphological control of the products and the need of control over many experimental parameters. As the supercapacitor industry needs material supply in large scale, chemical processes are becoming more versatile for the synthesis of various nanostructured materials with a wide range of structural and morphological control. The chemical reduction by means of alcohol and various reducing agents (e.g., NaBH4) was extensively explored not only for the synthesis of metal nanoparticles but also to functionalize various material surfaces with metallic clusters. Sol-gel process has been effectively utilized for the synthesis of various metal oxide nanomaterials by controlling the transformation of precursor sol into gel by adjusting pH values in lower temperatures followed for high temperature calcinations. Precipitation of metal ions by using various precipitating agents (e.g., NH3OH and NaOH) employing suitable pH followed by washing and calcinations leads to the formation of metal oxide nanoparticles. In addition to that, coprecipitation was extensively explored for the synthesis of various metal hydroxides, which are used for supercapacitor applications. Combustion process deals with the rapid decomposition of desired metal ion precursors along with the organic fuels in shorter duration, leading to the enormous heat generation and also the release of more gases, which lead to the formation of nanostructured materials at relatively lower temperatures. Key features of the combustion process are (i) the selection of suitable fuel, which exhibits the ability to the chelation of metal oxides; (ii) utilization of precursor chemicals with oxidizing agents, mostly metal nitrates; and (iii) incorporation of external combustion aides such as ammonium salts and nitric acids. Along with these processes, polyol has become the most important source for the synthesis of a variety of nanomaterials including metals, metal oxides, chalcogenides, metal hydroxides, and their composites with carbon employing poly hydroxyl alcohols as reducing and capping agents. Many of the above-discussed chemical processes are energy intensive and need toxic chemicals/solvents and many stages. Considering carbon nanomaterials, along with the synthesis processes (arc discharge, laser ablation, and chemical vapor

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deposition), the feedstock (carbon source) also receives significant attention in terms of their nature, abundance, and cost (Choudhary et al. 2014). Most of the traditional carbon nanomaterials were synthesized using fossil resourced carbon rich feedstocks such as graphite, methane, benzene, naphthalene, ethanol, alcohol, carbon monoxide, hexane, cyclohexane, anthracene, and polyacrylonitrile. The emerging concerns in utilizing fossil resourced feedstocks are their non-renewability, depleting nature, uncertainty in their cost, and greenhouse gas emission during their processing, which need to be critically considered and addressed. Increasing demand for supercapacitor materials, rising concern in utilizing toxic chemicals/fossil resources, or the need for green/eco-friendly material for sustainable future leads to the need of a fundamental transformation toward sustainable platform for nanostructured materials.

Sustainable Platform for Nanomaterials The concerns involved in the utilization of nanomaterials are their unique properties at the nanoscale, the utilization of various toxic chemicals during their production, and their post consumption disposal, which can create undesirable impacts on health and the environment (Hutchison 2016). Figure 1 shows the mapping of green nanoscience/nanotechnology mapping and a typical strategy involved in the synthesis of gold nanoparticles. As a result, green nanotechnology has emerged as the sustainable platform and significantly contributes to the synthesis of nanomaterials and their products with zero (or) reduced impact to human health and the environment. Various strategies involved in the green nanotechnology are energy efficiency, reduction of waste generation, safer protocol, extensive utilization of renewable feedstocks/precursors, usage of less toxic chemicals/solvents, design of efficient catalyst, and exploration of biodegradable products (Bai et al. 2018). As a result, a wide range of green chemical approaches have been adopted for the synthesis of a wide range of nanomaterials and also the fabrication of nano-enhanced products. Many of the traditionally used synthesis processes are reinvented with few modifications in order to meet the requirement for green nanotechnology. Among them, exploration of various bio-based feedstocks/precursors for the synthesis of numerous nanomaterials became most popular in last two decades with many advantages including their renewability, diversity, abundance, eco-friendliness, and lower cost. Plant, animal, microbial, and marine systems were widely explored for the synthesis of various carbon, metal, metal oxide, chalcogenides, polymer, and their composite materials for various applications including supercapacitors. Especially, renewable biomasses were extensively used for the synthesis of various carbon nanostructures for energy storage and conversion. One of the key advantages of renewable feedstock for carbon nanostructures is their ability to form various carbon nanostructures which include carbon dots, carbon nanotubes, carbon nanofibers, graphene/graphitic nanostructures, and nanostructured foams by adopting traditional processes and protocols. From this point of view, the present chapter summarizes the current status and future opportunities of the renewable resource-based nanostructured materials for supercapacitor applications.

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Fig. 1 (a) Green nanoscience/nanotechnology mapping and (b) typical strategies involved in the synthesis of gold nanoparticles (Hutchison 2016)

Biosynthesized Metal and Metal-Based Nanomaterials Recently, a wide range of nanomaterials, which include metals, metal chalcogenides, metal oxides, carbon nanomaterials, and their composite/hybrid structures, have been extensively synthesized by exploring various biogenic processes, and the resulting materials have been considered as green nanomaterials. Due to their unique size- and shape-dependent physicochemical properties, they have been used for numerous applications such as energy storage/conversion, sensor, catalysis, biomedicine, polymer composites, environmental remediation, and electronics. Exploration of such biogenically derived nanomaterials for supercapacitor application is getting momentum, and the recent research accomplishments in this area of research are summarized in this section.

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Metal and Metal Chalcogenides Metal nanoparticles have been extensively explored for supercapacitor applications due to their superior chemical stability, higher electronic conductivity, and also unique size-/shape-dependent surface properties. The metal nanoparticles (Ag, Ni, Cu), which are widely synthesized by various chemical processes have been widely used for supercapacitor applications. However, the biogenically synthesized metal nanoparticles were not much utilized for the supercapacitor electrode fabrication. Lokhande et al. (2019) explored the kimchi cabbage aqueous extract for the synthesis of Ag nanoparticles. The synthesized Ag nanoparticles with the average particle size of 41.0
9.7 nm was deposited on the stainless steel (SS) strips by employing ionic layer adsorption and reaction (SILAR) method for the fabrication of capacitive cells. The biosynthesized Ag nanoparticles exhibited a specific capacitance of 424 F g1 with the energy and power density of 14.04 Wh kg1 and 6.41 kW kg1. In addition to that, Salve et al. (2020) reported the effective utilization of Ag nanoparticles synthesized using Tagetes erecta (marigold) dried flower extract for supercapacitor application. TEM analysis revealed the formation of Ag nanoparticles with the size range between 20 and 50 nm. The supercapacitor fabricated using the biosynthesized Ag nanoparticles showed excellent storage capacity of 367.16 mF cm2 at 1 mA cm2 with the capacitance retention of 92.8% over 1,500 cycles. Along with metal nanoparticles, metal chalcogenides also receive significant importance in supercapacitor applications due to their superior electrical conductivity, costeffectiveness, and environmental-friendliness (Dahiya et al. 2022). It was understood that sulfur (S) is less electronegative compared to oxygen (O) and hence exhibits superior electrical properties by accommodating more electrons. Hence, a wide range of binary and ternary metal chalcogenides which include NiS, CdS, CoS2, Co3S4, MoS2, CuSe, CuSe2, NiSe2, CuFeS2, and NiCo2S4 have been explored as active electrode material for the fabrication of supercapacitor (Theerthagiri et al. 2018). Chemical processes such as hot injection, hydrothermal, solvothermal, microwave, sonochemical, and electrodeposition have been extensively used for the synthesis of those metal chalcogenide nanomaterials. Though biogenic processes have been widely used for the synthesis of various metal chalcogenides, exploring their application for supercapacitor fabrication is very limited. Feng et al. (2017) reported the effective synthesis of CdS nanoparticle by employing bacterial-precipitation process using the isolated E. coli colony. The biosynthesized CdS nanoparticles showed a particle size range of 3–15 nm with a bandgap of around 2.57 eV and were explored as the active electrode material for photocharged capacitor. Along with the binary chalcogenides, their ternary systems also receive increasing importance due to their multiple oxidation capability. In this perception, Nsude et al. (2020) investigated the fabrication of CuFeS2 nanomaterials using Mimosa pudica dried leaf extract mediated biogenic process. They further investigated the effect of various annealing temperature (200, 250, and 300  C) of the obtained CuFeS2 nanomaterials on their capacitive performance. Figure 2 shows the SEM images of

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Fig. 2 SEM images of the biosynthesized CuFeS2 NPs (a) unannealed, (b) annealed at 200  C, (c) annealed at 250  C, (d) annealed at 300  C, and their (e) GCD plot along with (f) specific capacitance vs current density plot (Nsude et al. 2020)

the as synthesized and annealed CuFeS2 nanoparticles at 200, 250, and 300  C along with their respective galvanostatic charge-discharge (GCD), and specific capacitance vs current density plots. The highest specific capacitance of 501.4 F g1 at 10 mV s1 was obtained for the CuFeS2 nanoparticles annealed at 250  C.

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Metal Oxide Nanostructures Metal oxide nanoparticles receive widespread attention due to its pseudocapacitive behavior (electro sorption, redox reactions, and intercalation), which exhibits considerably higher specific capacitance compared to double layer capacitors. The capacitive performance of metal oxide nanostructures highly depends on their chemical affinity to the ions along with the structural, morphological, and dimensional features. Hence, a wide range of transition metal oxide nanoparticles, which include RuO2, NiO, MnO2, V2O5, Co3O4, etc., were synthesized by various soft chemical methods such as sol-gel, coprecipitation, combustion, sonochemical, and hydrothermal. Recent development in the metal oxide synthesis is the exploration of various biogenic processes using different bio-resources (plants, seaweeds, fungus, microbes, and animal products). Significant research advancement has been made on the biological synthesis of metal oxide nanoparticles/nanostructures for supercapacitor applications. Nisha et al. (2020) synthesized the RuO2 nanoparticles using Anacyclus pyrethrum plant root extract as reducing agent without adding any chemical agent. The obtained RuO2 nanoparticles exhibited average particle size of 13 nm. Electrochemical investigation of the RuO2 nanoparticles coated over carbon electrode exhibited a maximum specific capacitance value of 209 F g1 at 5 mV s1. Shaheen et al. (2021a) reported the biogenic synthesis of ZnO Nanoparticles sustained using the extract of Euphorbia cognate leaves. The phyto-organic functional groups present in the leaf extract reacted with zinc ions and created the metal-organic complex structure, and their thermal degradation led to the subsequent formation of ZnO nanoparticles with the size range of 40–65 nm. The obtained ZnO nanoparticles showed specific capacitance of 336.12 F g1 at 10 mV s1 by cyclic voltammetry (CV) in three-electrode system. Anuradha and Raji (2020) studied the facile fabrication of Co3O4 nanoparticles by Camellia sinensis leaf extract assisted combustion process. The final product was obtained by annealing the combustion residue at 200, 400, 600, and 800  C for 120 min. XRD analysis revealed that the increasing annealing temperature increased the grain size of Co3O4 nanoparticles from 32.53 (200  C) nm to 58.11 nm (800  C). From TEM analysis, the particle sizes of biosynthesized Co3O4 nanoparticles were found in the range between 37 and 40 nm. The green synthesized Co3O4 nanoparticles exhibited high specific capacitance of 130 F g1 at 1 mA cm2 with the columbic efficiency of 86.6%. Sobti et al. (2021) explored the olive leaf extract mediated wet chemical processes for the synthesis of Mn3O4 nanoparticles. The synthesized Mn3O4 nanoparticles exhibited the faceted morphology with two size ranges of 15–20 nm and 50–70 nm. In three-electrode system, Mn3O4 nanoparticles showed high specific capacitance of 583.7 F g1 with 87% capacity retention after 2,000 cycles at a 4 A g1. Kumar et al. (2020) demonstrated the template-free green method for the synthesis of copper oxide (CuO) nanoporous material using Piper nigrum (Indian black pepper) dried fruit extract as reducing agent by employing microwave irradiation. The synthesized CuO nanoparticles showed the specific surface area of 81.23 m2 g1 with the pore sizes of 3–8 nm. The nanoporous CuO showed specific capacitance of

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238 F g1 at 5 mV s1, which is very much higher than their commercial CuO counterpart (75 F g1). Nwanya et al. (2020) investigated the synthesis of NiO nanoparticles by maize (Zea mays L.) dry silk extract mediated green process. The synthesized NiO nanoparticles possess a diameter range of 10–20 nm and exhibited excellent supercabattery properties. It showed the capacity of 54 C g1 at 5 mV s1 with the 60% capacity retention over 2,000 cycles. Gunasekaran et al. (2021a) reported the synthesis of NiO nanoparticles using Opuntia ficus-indica leaf extract assisted by one-step process. The synthesized NiO nanoparticles were found to have a particle size range of 20–34 nm. Further, the assembled asymmetric capacitive cell (NiO nanoparticles/activated carbon) showed specific capacitance of ~280 F g1 (at ~38.8 mAh g1), with specific energy of 25 Wh kg1. Figure 3 shows a schematic diagram of the synthesis of NiO nanoparticles by Opuntia ficus-indica leaf extract assisted biological process, along with their SEM images and the CV and GCD curves of the asymmetric supercapacitor fabricated with NiO nanoparticles. Along with monometallic oxides, bimetallic oxide of CoWO4 also was also synthesized by using biogenic processes for the supercapacitor applications. Azevêdo et al. (2020) explored the green synthesis of CoWO4 powders using agar-agar from red seaweed (Rhodophyta) as a polymerizing agent employing sol-gel process. The CoWO4 powders synthesized using red seaweed-derived agar-agar results in a crystallite size of 84 nm. Further their electrochemical showed the highest specific capacitance of 77 C g1 at 1 A g1 with the 98% of capacity retention over 1,000 chargedischarge cycles.

Biomass-Derived Carbon Nanostructures Carbon nanomaterials are extensively used as a supercapacitor material, due to its superior electrical conductivity, excellent structural stability during the cycling performance, resistance to various electrolyte systems, and relative costeffectiveness. Recent advancement in the carbon nanotechnology is their production from various renewable resources, which reduces the dependency of fossil feedstocks for various carbon nanomaterials. As a result, a wide range of carbon nanomaterials, which have been classified into zero (0)- to three (3)-dimensional architecture (carbon nanodots, carbon nanofibers/nanotubes, graphene, and other particulate materials), were synthesized from various renewable feedstocks and explored as the electrode materials for supercapacitor applications.

Zero-Dimensional Carbon dots (C-dots (or) CD), which is also termed as graphene quantum dots and low dimensional carbon nanoparticles (CNPs), are coming under the category of zero-dimensional (0D) carbon nanostructured materials. Zero-dimensional carbon materials show their fascinating physicochemical properties, such as superior electrical conductivity, larger specific surface area, and structural stability, which are

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Fig. 3 (a) Schematic diagram of the synthesis of NiO nanoparticles by Opuntia ficus-indica leaf extract assisted biological process, (b–d) SEM images of the NiO nanoparticles with respective EDX results, and (e and f) CV and GCD curves of the asymmetric supercapacitor fabricated with NiO nanoparticles (Gunasekaran et al. 2021a)

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suitable for electrochemical energy storage applications. Only limited literatures are available for bio-derived C-dots and carbon nanoparticles as electrode materials for supercapacitor applications. Singh et al. (2020) reported the fabrication of graphene quantum dots using charcoal as renewable feedstock. The obtained graphene quantum dots exhibited the average size of 5 nm with the size distribution between 2 and 15 nm. The electrode fabricated using the charcoal derived graphene quantum dots exhibited specific capacitance of 257 F g1 at 3 A g1 with 96% of capacitance retention after 3000 cycles. Further, it was used to fabricate flexible symmetric supercapacitor, which showed the 17.36 W h kg1 energy density, with the power density of 191.7 W kg1 with 91% capacitance retention over 1,000 cycles. However, their composites with various other materials have been extensively explored as the electrode material for supercapacitor applications. In recent years, noticeable efforts have been taken for the preparation of bio-derived CNPs for supercapacitor applications. Bhat et al. (2020) converted Mimosa pudica (touch-me-not) biomass leaves into porous CNPs by one-step pyrolysis method at different temperatures (500–1000  C). The prepared porous CNPs were explored as electrode material in both three- and two-electrode setups. In CR2032 coin cell, a symmetrical supercapacitor (three-electrode) showed a maximum specific capacitance of 356.1 F g1, whereas the symmetric supercapacitor (two-electrode) exhibited a lower specific capacitance (126.8 F g1). Bondarde et al. (2020) reported the synthesis of sulfurdoped carbon nanoparticle (S-CNP) from cow margarine through a simple flame pyrolysis technique. In a symmetrical two-electrode cell configuration, the prepared S-CNP showed the maximum specific capacitance of 337 F g1 at 1.0 A g1 current density. Further, the S-CNP exhibited 89.6% retention of its initial capacitance after more than 20,000 repetitive charge-discharge cycles. Figure 4 shows the schematic representation for the preparation of S-CNP along with physicochemical characterizations and electrochemical characterizations. Bhat et al. (2021) reported the preparation of carbon nanoparticles by the self-activation of teak wood sawdust powder by calcination at different temperatures. Exploring 1 M potassium hydroxide (KOH) as electrolyte, the fabricated device using the prepared CNPs showed the maximum specific capacitance of 208 F g1 at 0.25 A g1 in a three-electrode configuration.

One-Dimensional Among the various carbon allotropes, one-dimensional nanostructure shows unique electrical and mechanical properties due to its atomic configuration and hence carbon nanotubes, nanowires, nanowhiskers, nanofibers, and rodlike structure receiving significant importance as energy storage devices. Moreover, the large aspect ratio of one-dimensional (1D) carbon nanostructure enables the effective surface modification for creating more electrochemically active sites for capacitive applications. Increasing research activities on the synthesis of various 1D carbon nanomaterials such as carbon nanotubes, carbon nanofibers, and carbon nanorods by using renewable resources for supercapacitor applications have been observed in recent years. Though the synthesis of carbon nanotubes from renewable resources

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Fig. 4 (a) XPS survey of S-CNP, (b) C1s XPS spectra, (c) O1s XPS spectra, and (d) S2p XPS spectra of S-CNP; (e) SEM image of S-CNP and corresponding EDX elemental mapping of (f) carbon (pink), (g) oxygen (green), and (h) sulfur (purple); electrochemical characterization of as-prepared S-CNP with symmetrical two-electrode devices; (i) CV plot of different scan rate 0.005 to 0.2 V s1; (j) GCD data of S-CNP at 1 to 5 A g1 current density; and (k) specific capacitance vs current density of the SCNP (Bondarde et al. 2020)

has increased in recent years, their exploration for supercapacitor applications is very limited. Wu et al. (2019) reported the growth of carbon nanotubes on the inner wall of carbonized wood tracheids using nickel nanoparticles as catalysts. The growth of carbon nanotubes increased their specific surface area from 365.5 to 537.9 m2 g1 and showed the specific capacitance of 215.3 F g1 with 96.2% of capacity retention over 10,000 cycles. Figure 5 shows the SEM images of the carbon nanotubes grown on the inner wall of carbonized wood tracheids along with their electrochemical capacitive performance. Along with carbon nanotubes, carbon nanofibers were widely used as the electrode materials for supercapacitor applications. Mostly, carbon nanofibers were fabricated from renewable precursors by employing electrospinning techniques by manipulating suitable processing conditions. During this process, the electrospun raw nanofibers (generally called as green fibers) from various bio-resources undergo thermo stabilization at lower temperatures followed by carbonization at elevated temperatures to form the carbon nanofibers. Among the various renewable feedstocks, lignin is the most extensively used biopolymer (on its own or blended with various other polymeric materials) for the fabrication of carbon nanofibers. Lignin is the most abundant biopolymer in nature available in cell walls of the plants, which is extracted by employing various chemical processes mostly in pulp/paper and

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Fig. 5 (a and b) SEM image of carbon nanotubes grown on the inner walls of carbonized wood tracheids and their (c–f) electrochemical capacitive performances (Wu et al. 2019)

lignocellulosic ethanol industries in huge amount as by-product. Exploring lignin for the fabrication of carbon nanofibers leads to their value addition along with the added benefit of waste reduction. Ma et al. (2018) reported the fabrication of hierarchical porous carbon nanofibers from alkali lignin as the carbon source by electrospinning using polyvinylpyrrolidone (PVP) as spinning aide and Mg (NO3)26H2O as additive. The obtained carbon nanofiber (with the Mg

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(NO3)26H2O/lignin ratio of 2:1) showed an excellent specific surface area of 1140 m2 g1, which exhibited the highest specific capacitance of 248 F g1 at 0.2 A g1. Kraft lignin was explored by Schlee et al. (2019) for the fabrication of modified carbon nanofibers by using NaNO3 as an oxidizing salt, which improved the specific capacitance from 151 F g1 (for the nanofibers without NaNO3 treatment) into 192 F g1. Further, capacitive performance of carbon nanofibers derived from hardwood and softwood kraft lignin were also compared which found superior performance for the carbon nanofibers obtained from hardwood kraft lignin (Schlee et al. 2020). It was identified that the modification of lignin with isophorone diisocyanate (IPDI) leads to increase in the molecular weight and results in the improved spinnability as well as the reduced weight loss of the precursor fibers during carbonization (Zhu et al. 2020). The obtained KOH activated carbon fiber showed the specific surface area of 2042.86 m2 g1 and the specific capacitance of 442.2 F g1. Du et al. (2021) also reported that lignin with high molecular weight (obtained by manipulating the fractionation method) leads to the formation of better carbon nanofibers, which showed the specific capacitance of 405.8 F g1. Further the capacitive performance of lignin-based carbon nanofibers was enhanced by the doping of nitrogen, sulfur, and their combinations. Apart from these, researchers have also explored the combination of lignin with other carbon rich polymer for the fabrication of carbon nanofibers. Wang et al. (2018) demonstrated the utilization of enzymatic hydrolysis lignin for the fabrication of carbon nanofiber along with Poly (acrylonitrile) (PAN) employing electrospinning process followed by air stabilization and N2 carbonization. The as- synthesized (without activation) carbon fiber showed an average diameter of 172 nm with a specific surface area of 675 m2 g1 and showed a specific capacitance of 216.8 F g1 at 1 A g1, with 88.8% capacitance retention over 2,000 cycles. In addition to that, Jayawickramage et al. (2019) also reported the fabrication of activated carbon nanofiber from polyacrylonitrile-lignin blends, which showed a specific surface area of 2370 m2 g1. The obtained nanofiber showed a specific capacitance of 128 F g1 when operated at 3.5 V employing ionic liquid as electrolyte. Along with lignin, various other biopolymers have also been explored as the precursor material for the fabrication of electrospun carbon nanofibers. Yang et al. (2018a) investigated the electrospinning of hordein and zein plant portions with Ca2+ ions for the fabrication of nitrogen rich carbon fibers. The addition of Ca2+ ions to the plant protein solution may influence the electrical conductivity and viscosity, which enhances their spinnability as well as fiber morphology. The optimized nitrogen-doped carbon fibers showed an areal specific capacitance of 64 μF cm2 at 0.5 A g1 with 98% capacity retention after 5,000 cycles at 10 A g1. Chen et al. (2021a) explored the acetylated sugarcane bagasse and the polyacrylonitrile (PAN) mixture followed by carbonization process. The obtained carbon nanofiber showed a specific capacitance of 289.5 F g1 with excellent capacitance retention (111.8%) and columbic efficiency (>99%) over 5,000 cycles. Without employing the electrospinning process, bacterial cellulose is effectively converted into carbon nanofibers through pyrolysis process. Taer et al. (2020) explored the acacia leaves (Acacia mangium wild) for the fabrication of activated carbon nanofiber by employing KOH

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mediated chemical activation process. The obtained carbon nanofibers found to have a specific surface area of 714.492 m2 g1 and showed a specific capacitance of 113 F g1. Farma et al. (2021) reported the production of carbon nanofibers from bamboo materials pyrolyzed at 700 and 800  C. The carbon fibers obtained at 800  C showed the highest specific surface area of 1137.86 m2 g1 and possess the specific capacitance of 174 F g1. Comparing with the electrospinning mediated process for the fabrication of carbon nanofibers (using biopolymers as precursor), direct pyrolysis of biomass for carbon nanofibers exhibits simple protocol but poor structural and morphological control. Further, the performance of carbon nanofibers derived from renewable feedstocks was enhanced by anchoring metal oxides, chalcogenides, and carbon nanomaterials, which are discussed in the composite section.

Two-Dimensional Two-dimensional (2D) carbon materials offer huge specific surface area, excellent structural stability, superior electrical conductivity, and in-plane defect active sites, which enhance their charge storage and lead to higher specific capacitance (Ni and Li 2016). Most of the graphene and related materials were traditionally produced from bulk graphite by employing various exfoliation methods. Apart from graphite, researchers have also explored various gaseous precursors like acetylene, methane, and ethylene as the carbon source for the synthesis of graphene-like materials. As the demand for sustainable materials increases continually, in recent years, graphene-like carbon nanosheets have been effectively prepared from various renewable resources for a wide range of applications including supercapacitors. A wide range of biomasses which include plant fibers, leaves, flower components, nutshells, and agro-industrial residues have been effectively converted into 2D carbon materials and explored as the efficient electrode material for supercapacitor fabrication. Purkait et al. (2017) investigated synthesis of one or two layered graphene-like nanosheets from peanut shells by employing KOH activation and a subsequent mechanical exfoliation. The obtained carbon nanosheets showed an excellent specific surface area of 2070 m2 g1 with the pore volume of 1.33 cm3 g1. Further, it also exhibited a specific capacitance of 186 F g1 in 1 M H2SO4 electrolyte. It also showed the highest energy density of 58.125 Wh Kg1, and it was enhanced into 68 Wh Kg1 while using organic electrolyte with the working potential of 2.5 V. Panmand and team explored the Bougainvillea flowers as the carbon source for the fabrication of mesoporous perforated graphene employing template free single step pyrolysis/carbonization process (Panmand et al. 2017). The carbon nanosheets synthesized at 800  C showed the highest specific surface area of 850 m2 g1, which leads to have high specific capacitance of 458 F g1 in aqueous 1 M Na2SO4. Figure 6a–f showed the photograph of dried, powdered, and carbonized Bougainvillea flower along with their TEM images (carbon nanosheets obtained at 800  C) as well as the CV and GCD curves. Fu et al. (2020) reported the effective fabrication of porous carbon quasi-nanosheets from lignin by employing gas exfoliation and in situ templating

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Fig. 6 (a) Photograph of dried, powdered, and carbonized Bougainvillea flower; (b–d) TEM images of the flower biomass-derived carbon nanosheets; (e and f) CV and GCD curve of the carbon nanosheets obtained at 800  C (Panmand et al. 2017); (g) schematic of leek biomass conversion into hierarchical 3D porous carbon for symmetrical supercapacitor; and (h and i) SEM and TEM images of leek biomass derived of hierarchical 3D porous carbon (Liu et al. 2019b)

method. The obtained porous carbon quasi-nanosheets were used to fabricate symmetric supercapacitor, which showed a specific capacitance of 274 F g1 at 0.5 A g1 in PVA/KOH gel electrolytes. Along with the above-stated biomasses, various other bio feedstocks such as corn cob, puffed rice, reed, pineapple leaf fiber, and inner shaddock skins have been explored for the synthesis of 2D carbon nanomaterials for supercapacitor applications. Review work reported by Ouyang et al. (2021) summarized the various methods involved in the fabrication of graphene and graphene-like carbons from biomasses. Capacitive performance of the 2D carbon materials can be improved by integrating nitrogen in its structure, and hence nitrogen-doped 2D carbon materials received great importance. In general, nitrogen-doped carbon nanosheets are prepared by adopting two strategies such as (i) exploring the nitrogen rich feedstock as the carbon source to get N self-doped carbon nanosheets and (ii) utilizing the nitrogen rich compounds such as urea and melamine along with the bio feedstocks to get externally N-doped carbon nanosheets. A wide range of nitrogen rich renewable feedstocks such as silk fiber, fresh clover stems, eucalyptus leaves, bio-oil, soybean milk, pine nutshells, peach gum, palm spathe, and agaric were effectively explored for the synthesis of N self-doped carbon nanosheets for supercapacitor application. In addition to that, researchers also explored the utilization of urea as the nitrogen source for the fabrication of 2D carbon nanostructures. Zou et al. (2018) reported the effective utilization of urea as nitrogen source for the fabrication of N-doped 2D porous carbon nanosheets

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from sugarcane bagasse by employing KOH based chemical activation process. The obtained carbon nanosheets showed the specific surface area of 2905.4 m2 g1 with the 2.63 wt.% of nitrogen content. They explored the effect of various electrolytes on their capacitive performance, and the specific capacitance were identified as 350.8, 301.9, and 259.5 F g1 while employing 1.0 A g1 discharge current, respectively, for the acidic, alkaline, and neutral electrolytes. Gunasekaran et al. (2021b) delivered synthesis of N-doped carbon nanosheets from bamboo biomass using KOH and urea, respectively, as the activation and nitrogen source. The produced N-doped carbon nanosheets showed a specific surface area of 769.71 m2 g1. Further, it exhibited a higher specific capacitance of 296 F g1 at 1 A g1 with specific energy of 42 Wh kg1 at specific power of 4500 W kg1 with 150% of capacitance retention over 10,000 charge-discharge cycles. Along with nitrogen doping, oxygen, sulfur, boron, and their co sopping were also investigated with the aim of improving the electrochemical performance of 2D carbon electrode materials. Liu et al. (2017b) reported the fabrication of carbon nanosheets using lignin as the carbon source by adopting freeze-casting (lignin aqueous dispersion) followed by carbonization process. It was identified that the resulting carbon nanosheets contain 11–16 at% of oxygen heteroatoms, which can contribute to additional pseudocapacitance. In three-electrode configuration, the optimized lignin-derived carbon nanosheets showed specific capacitance of 281 F g1 at 0.5 A g1 in H2SO4 aqueous electrolyte. Further, the concept of co-doping with carbon network has also been explored by the researchers to improve their electrochemical performance. Ling et al. (2016) reported the synthesis of B/N co-doped carbon nanosheets from gelatin using boric acid as the boron source and template forming agent, followed by pyrolysis. The supercapacitor made from the B/N co-doped carbon electrode displayed good specific capacitance of 240 F g1 at 0.1 A g1 with good stability over 15,000 cycles. Hao et al. (2017) effectively utilized ginkgo leaves for the fabrication of interconnected N/S co-doped carbon nanosheets with rich micro-/meso pores by employing hydrothermal and KOH activation processes. The obtained N and S co-doped carbon nanosheets were used as electrode materials for the fabrication of EDLCs, which showed a high specific capacitance of 364 F g1 at 0.5 A g1 with 98% capacity retention after 30,000 cycles. Further, Liu et al. (2017a) reported the synthesis of O/N co-doped porous carbon nanosheets from Perilla frutescens (PF) biomass. The PF leaf-derived carbon nanosheets, respectively, showed the O and N contents of 18.76 and 1.70% with the specific surface area of 655 m2 g1. The capacitive cell fabricated with the O and N co-doped porous carbon nanosheets showed the specific capacitance of 270 F g1 at 0.5 A g1, with the capacitive retention of 96.1% after 10,000 cycles at 2 A g1. Comparing with the pristine and N-doped, co-doped 2D carbon nanomaterials from renewable feedstocks are less explored for the supercapacitor applications. In-depth review work on the nitrogen-doped 2D carbon nanomaterials was performed by Sekhon and Park for energy technologies including supercapacitor (Sekhon and Park 2021).

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Three-Dimensional Carbon materials with three-dimensional (3D) hierarchical architecture receive special focus in recent years for electrochemical energy storage and conversion due to their unique (i) structural interconnectivities, (ii) 3 dimensionally arranged porous channels, (iii) a large specific surface area, and (iv) higher electrical conductivity. These unique features offer fast ion transfer and enhance the ion accessibility, which lead to superior electrochemical performances, especially in supercapacitors. In addition to that, the larger sized pores that exist on the wall of 3D network act as an electrolyte reservoir, which effectively reduces ion transmission distance. Nanostructured 3D carbon materials have been categorized into (i) hierarchical porous carbons, (ii) carbon aerogels, and (iii) flexible carbon films and explored for supercapacitor applications. Like other carbon nanomaterials, 3D carbon nanostructures have also been produced from various renewable feedstocks, which include but not limited to mushroom, dry elm samara, aloe peel, bamboo cellulose, rose petals, natural bark, and celery. Wang et al. (2019) fabricated aloe peel-derived 3D honeycomb-like porous carbon (AP-HC) via a straightforward method combining hydrothermal carbonization and chemical activation through KOH activation. AP-HC as working electrode in supercapacitor exhibited a high specific capacitance of 264 F g1 at 0.5 A g1 in a three-electrode system. Further, a superior cycling performance of 91% capacitance retention after 5,000 cycles was achieved. Yang et al. (2018b) reported the effective fabrication of porous nanoplatelet wrapped carbon aerogel by pyrolysis of regenerated bamboo cellulose aerogels as supercapacitor electrodes. The activated carbon aerogel exhibited a high specific capacitance of 381 F g1, which is 150% compared to inactivated sample. The withered rose petals were explored as the carbon source for the fabrication of 3D carbon nanosheets by Zhao et al. (2018) by employing KOH mediated chemical activation process. The rose-derived 3D activated carbon nanosheets exhibited a specific surface area of 1911 m2 g1 with the 0.55 g cm3 and a moderate bulk density. Further, it shows the specific capacitance of 208 F g1 with 99% of capacitance retention over 25,000 cycles in three-electrode configurations. Further, the spores of puffball (Lycoperdon sp.) fungus have also been explored for the synthesis of 3D hierarchal carbon nanomaterials as the electrode material for supercapacitor application (Hariram et al. 2021). The obtained spherical 3D hierarchal carbon nanomaterial with uniform size distribution showed the specific capacitance of 33 F g1 at 1 A g1 with the 96.2% of capacitance retention of after 1,000 cycles. Leek biomass was effectively used by Liu et al. (2019b) for the production of hierarchical 3D porous carbon with large cavities and pores using KOH as activating agent. The fabricated symmetric cell showed the specific capacitance of 191 F g1 at 0.5 A g1 with 77% capacitance retention at 10 A g1 in organic electrolyte. Figure 6g shows the schematic representation of leek biomass conversion into hierarchical 3D porous carbon along with the (h) SEM and (i) TEM images and their application into supercapacitor. Li et al. (2019) explored the effective utilization of natural bark as the carbon source for the synthesis of 3D porous carbon nanosheet

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using copper bromide (CuBr2) as activator. The natural bark derived biocarbon materials exhibited a 3D porous nanosheet structure with an ultrahigh specific surface area of 1955, 2159, 2396 m2 g1, respectively, for the carbon materials obtained at 600, 700, and 800  C. The as-prepared 3D biocarbon at 700  C showed the specific capacitance of 345 F g1 with 98.3% of capacitance retention over 10,000 cycles at 5 A g1 employing 6.0 M KOH electrolyte. Sichuan pepper was effectively explored by Zhang et al. (2019) for the synthesis of hierarchical porous carbon by pyrolysis followed by KOH activation. The obtained 3D structured carbon nanomaterials showed the specific surface area of 1823.1 m2 g1 with the 0.906 cm3 g1 of pore volume. The electrode fabricated using sichuan pepper derived hierarchical porous biocarbon showed the specific capacitance of 171 F g1 with near 100% capacitance retention over 10,000 cycles. Hsiao et al. (2020) explored the fabrication of 3D carbon framework using Tetrapanax papyrifer pith paper as the carbon source for branched carbon. First, they covered the carbon fiber bundle using Tetrapanax papyrifer pith paper, which exhibits honeycomb-like structure and is carbonized at 700, 800, and 900  C. Carbonization process stabilizes the honeycomb-like structure over the cover carbon fiber bundle and leads to the formation of 3D carbon framework. The carbon fiber with 3D carbon framework fabricated at 800  C showed superior capacitive performance with the specific length capacitance of 20.8 mF cm1 at 2 mV s1. It was clearly identified that the inherent porous structures of the biomasses effectively lead to the formation of 3D carbon nanostructures by controlling the processing parameters and also the implementation of suitable activation process. Researchers also explored the fabrication of N, P, and their co-doped 3D carbon materials to achieve the superior capacitive performances. Liu et al. (2019a) investigated the synthesis of 3D carbon nanosheets from micromorphologically regulated woody precursors employing sequential bio-swelling followed by low temperature carbonization process. They also used urea as nitrogen source for the fabrication N-doped carbon material along with KOH as activating agent. The resulting 3D carbon nanosheets showed hierarchical porosity with the superior N (8.7%) and O (20.9%) doping levels. Further, the obtained carbon material exhibited gravimetric capacitances of 508 F g1 and 360 F g1 at 1.0 A g1, respectively, in the three- and two-electrode systems employing 6 M KOH as electrolyte. Taro stem was used by He et al. (2019) as the precursor for the fabrication of 3D nitrogen self-doped porous carbon by using KOH mediated chemical co-activation process. The obtained 3D porous nanocarbon showed the nitrogen content of 4.8% and the specific surface area of 1012 m2 g1. Further, it also showed the specific capacitance of 236.4 F g1 at 0.1 A g1 with 89.3% of capacitance retention after 10,000 cycles at 20 A g1. In addition to that, the celery biomass has also been explored as the carbon source for the fabrication of N and P self-doped porous carbon materials with 3D hierarchical architecture employing KOH assisted chemical activation (Liu et al. 2021). The obtained biocarbon material showed the specific surface area of 1612 m2 g1 with a large amount of N and P heteroatoms and exhibited a specific capacitance of 1002.80 F g1 at 1 A g1 with 95.6% cycling stability after 10,000 cycles (10 A g1) in 1 M KOH aqueous electrolyte.

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Nanocomposites Fabricated Using Renewable Resources Apart from the pristine nanomaterials, nanocomposites with a wide range of combinations have been extensively explored for supercapacitor applications. The advantages of composite material for supercapacitor applications are their unique properties, in which their individual counterpart cannot be offered. In recent years, renewable resource-based nanocomposite material has been extensively utilized as electrode material for supercapacitor applications, and they are categorized as discussed in this section.

Metal-, Metal Hydroxide-, and Metal Oxide-Based Nanocomposites Metal and metal oxide nanoparticles were synthesized using various biogenic processes and have been extensively used for capacitive energy storage. Likewise, a wide range of biogenic processes have also been explored for the functionalization/ modification of various nano/bulk materials (metal oxide and carbon materials) with metal oxide nanoparticles. In many of the cases, the metal oxide-based composite materials were synthesized by adopting one pot method. Integration of metal nanoparticles along with various materials improves their electrical conductivity, which plays significant role in their capacitive performance. Kolya et al. (2019) reported the synthesis of silver nanoparticles (AgNPs) using an aqueous extract of mango (Mangifera indica) flower as stabilizing and reducing agent which were further utilized for the preparation of AgNPs/reduced graphene oxide (rGO). The electrochemical analysis of AgNPs/rGO nanocomposites exhibited specific capacitance of ~532 F g1 at a scan rate of 1.0 A g1, and increasing the scan rate resulted in decrease in capacitance due to the cause of reduced redox reactions. Further, 92.5% of SC retention after 2,000 charge-discharge cycles was observed that indicates the long-term electrochemical cyclic stability of AgNPs/rGO nanocomposites as supercapacitor electrode materials. Reddy et al. (2019) reported the preparation of graphene composite matrix with Ni(OH)2 via Moringa oleifera plant leaf extract in an ordinary coprecipitation method under pH 10 at 95  C for 2 h. In a three-electrode system, the prepared rGO/α-Ni(OH)2 composite electrode resulted in the specific capacitance of 350.7 F g1 at 0.5 A g1. Interestingly, asymmetric supercapacitor design of rGO/rGO-Ni(OH)2 in two-electrode mode resulted in the specific capacitance of 126.0 F g1 at 2 A g1. In addition to that, Shaheen et al. (2021b) reported the synthesis of ZnO@PdO/Pd nanomaterial using the leaf extract of Euphorbia cognata Boiss employing two stages. First, the ZnO nanoparticle was synthesized followed by the co synthesis of PdO/Pd counterpart using Euphorbia cognata Boiss extract. The obtained ZnO@PdO/Pd composite nanomaterial exhibited the specific capacitance of 178 F g1 at about 2 mV s1 in the presence of KOH electrolyte. It is reported that owing to the fast redox behavior of Pd, ZnO@PdO/Pd showed enhancement of the specific capacitance as compared to ZnO. The same research group reported the synthesis of ZnO:NiO-PdO-Pd nanomaterial from Euphorbia cognata Boiss, ZnO-CoMoO4 nanocomposite from the extract of E. cognata leaves,

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and ZnO-Co3O4 nanocomposite using the extract of E. cognata foliar powder which are used as electrode material for supercapacitor applications.

Renewable Carbon-Based Nanocomposites The carbon nanomaterials, which are derived from various renewable resources, have been integrated with a wide range of materials in order to fabricate the composite structures toward the enhancement of capacitive performances. Carbon dots derived from various traditional fossil feedstocks have been widely explored for the fabrication of composite materials with various materials. However, the exploration of biomass as the feedstock for the carbon dot toward composite fabrication is very limited. As discussed in the earlier section, carbon nanodots were integrated with activated carbon, rGO, and the obtained composite nanomaterials which were effectively explored as the electrode material for supercapacitor application. For example, cauliflower leaf waste derived C-dots were used to fabricate composites with rGO via a one-step hydrothermal treatment (Hoang et al. 2019). Out of different mass ratios of rGO/CD composites as electrode, 2:1 ratio exhibited higher specific capacitance (278 F g1) with maximum discharge capacitance (227 F g1) at 0.2 A g1 and 2 mV s1. Dai et al. (2019) fabricated the graphene reinforced carbon nanofiber composites co-doped with nitrogen-sulfur from lignin/polyacrylonitrile blends for supercapacitor applications. The KOH activated composite carbon fiber showed the specific surface area of 2439 m2 g1 and leads to the specific capacitance of 267.32 F g1 with the capacitance retention of 96.7% after 5000 cycles. Wu et al. (2021) reported the preparation of N-doped metallic cobalt embedded carbon nanofiber composite (N-Co/CNF) derived from ZIF@electrospun cellulose nanofiber by pyrolysis method. In detail, ZIF-67(Co) loaded CA nanofibers were prepared by electrospinning process, and the obtained nanofiber (NF) membrane was subjected to pyrolysis at different temperatures (450, 600, and 800  C) for 3 h in N2 atmosphere to produce N-Co/CNF. In three-electrode mode, the N-Co/CNF-800 as electrode resulted in the maximum specific capacitance of ~433 F g1 at 0.2 A g1 (based on GCD results) in 1 M H2SO4 electrolyte. Further, the prepared electrode exhibited 84% of capacitance retention after 3000 consecutive chargedischarge cycles. Ranjith et al. (2021) reported the fabrication of lignin-derived carbon nanofibers by employing electrospinning process functionalized with Ni-Mn sulfide nanograins as composite electrode. The obtained Ni-Mn sulfide modified carbon nanofiber showed the specific capacitance of 652.3 C g1 at 1 A g1 with the 91.3% capacity retention after 5000 cycles. In addition to that, another class of composite materials based on the renewable carbon combined with the metal oxide nanoparticles was also been explored as the electrode material for supercapacitor application. Researchers adopted two different processes for the fabrication of this type of composite materials such as (i) co-synthesis of biocarbon metal oxide nanocomposites and (ii) infusion of metal oxide nanoparticles into the pre synthesized biocarbon materials. In both the processes, the ultimate aim is to incorporate the metal oxide nanoparticles as

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pseudocapacitive material to improve their electrochemical performance. Edison et al. (2019) reported the preparation of carbon encapsulated RuO2 nanorods (RuO2 NRs/C) by thermolysis of ruthenium chloride and Punica granatum fruit peel at 450  C for 4 h under N2 atmosphere. In a three-electrode system, the prepared RuO2 NRs/C exhibited maximum specific capacitance of 151.3 F g1 at a scan rate of 5 mV s1 from cyclic voltammetry studies. Along with this process, researchers also explored the fabrication of biocarbon-metal oxide nanocomposite by infusing the metal oxide nanoparticles into the biocarbon materials. Zou et al. (2021) reported the preparation of Co3O4 anchored on meshy biomass carbon (Co3O4@MBC) derived from kelp by using solvothermal and pyrolysis methods. As observed in previous studies, Co3O4@MBC composite electrode showed better electrochemical performance than that of pristine Co3O4 NPs and MBC electrodes. In three-electrode mode, the Co3O4@MBC/NF as active electrode resulted in the highest specific capacitance of 1212.4 F g1 at 0.5 A g1 in 6 M KOH as electrolyte. Further, the electrode showed an excellent cycle stability with 98.9% retention of capacity at 2 A g1 after 2,000 cycles. Similarly, highly porous sponge like Co3O4 NPs decorated on Bauhinia vahlii dry fruit derived activated carbon composite (Co3O4@BVFC) was also explored as electrode material for supercapacitor applications.

Cellulose Nanofiber-Based Nanocomposites Cellulose nanofibers receive significant attention in materials science due to their abundance, renewable nature, biodegradability, chemical/biocompatibility, and superior mechanical strength. In general, these cellulose nanofibers have been extracted from plant and some specific bacterial resources. The composite materials fabricated using cellulose nanofibers infused with various metal oxides, hydroxides, chalcogenides, metal oxide frameworks, MXenes, and conducting polymers were effectively explored as the electrode material for supercapacitor applications. Transition metal oxides and their composite materials were extensively explored as the electrode material for the fabrication of supercapacitor due to their redox/pseudocapacitive properties. Rabani et al. (2021) investigated the fabrication of Co3O4 nanoparticles incorporated into a cellulose nanofibers by employing KOH assisted precipitation technique. The obtained Co3O4/cellulose nanofiber composite material showed the specific capacitance of 214 F g1 at 1 A g1 with 94% of capacity retention over 5,000 cycles. Among the various electroactive materials, metal chalcogenides play significant role in the electrochemical energy storage systems due to their unique metal like electronic conductivity, which creates effective pathway for electron transport. They have been widely used for the fabrication of various composite materials for supercapacitor applications. Huang et al. (2021) reported the fabrication of CuS modified cellulose nanofibers for the formation of conductive paper as the electrode material for supercapacitor application. The synthesized composite material showed the specific capacitance of 314.3 F g1 at 1 A g1. Along with oxide and chalcogenide nanoparticles, carbon nanostructures

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have also been explored to fabricate the composite materials with cellulose nanofibers as electrode material for supercapacitor applications. Interestingly, carbon nanofiber reinforced cellulose nanofibers and multiwalled carbon nanotube composite were used as the electrode material. Along with carbon nanofibers, reduced graphene oxide was also explored as the electroactive species for the fabrication of cellulose nanofiber-based composite electrodes. In recent years, metal-organic frameworks (MOFs) exhibit increasing importance in supercapacitor applications due to not only for their higher electrical conductivity but also for their highly accessible surface area. As a result, cellulose nanofiber decorated with MOFs are explored as the superior electrode material for highperformance supercapacitor applications. For example, Zhou et al. (2019) investigated the fabrication of cellulose nanofibers at conductive metal-organic framework as high-performance electrode material. The fabricated capacitive cell showed the specific capacitance of 141.5 F g1 at 0.075 A g1 with >99% capacity retention over 10,000 cycles. Figure 7 shows the schematic representation of synthesizing

Fig. 7 (a) Schematic of the formation of c-MOF modified cellulose nanofibers, (b–d) TEM and SEM images of the c-MOF modified cellulose nanofibers, and (e–g) CV, GCD studies, and specific capacitance vs charge density graph c-MOF modified cellulose nanofiber composite electrode (Zhou et al. 2019)

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c-MOF modified cellulose nanofibers and their TEM/SEM images along with the electrochemical characterizations. In addition to that, Mxene (Ti3C2Tx) was also explored to functionalize the cellulose nanofibers by Chen et al. (2021b) to form a composite structure as the electrode for flexible supercapacitors. The fabricated freestanding composite electrode showed the areal capacitance of 143 mF cm2 at 0.1 mA cm2 with the energy density of 2.4 μWh cm2. Conducting polymers such as polyaniline, polypyrrole, and polyoxometalate have been widely explored in batteries and supercapacitors. Modification of cellulose nanofibers with these conducting polymers will enhance their charge storing as well as electron transfer mechanism, which is well suitable for supercapacitor applications. Researchers also explored the hybrid reinforcement to the cellulose nanofibers to form advanced composite materials (e.g., polypyrrole/cellulose nanofibers) as the electrode for supercapacitor applications. This includes but not limited to polypyrrole (PPy)/ cobalt sulfide (CoS), Cu2O/Cu, polypyrrole/copper oxide, Ni-Mn-layered double hydroxides/PPy, and PPy@cobalt oxyhydroxide.

Future Directions In recent years, research interests are growing on the preparation of renewable resource-based green nanomaterials for supercapacitor applications due to not only their peculiar physicochemical and structural advantages but also their environmental, economic, and sustainable benefits. Only limited renewable sources were explored for the preparation of green nanomaterials that give wide opportunity for future research toward the synthesis of novel green nanomaterials for supercapacitor applications. In general, by choosing a suitable renewable source and the synthesis methodology, the properties/structures of the resulting nanomaterial can be precisely tuned as desired. As a unique attempt, co-processing of multiple renewable resources can be explored to produce entirely novel green nanomaterials. Though numerous synthesis methods are possible for the preparation of nanomaterials, only few methods are explored for the preparation of green nanomaterials, and that gives vast scope in exploring unique synthesis methodologies and/or modification approaches. Interestingly, in situ preparation of renewable resource-based green composite nanomaterials can create huge impact due to its synthesis flexibility and time saving approaches. The growing research interest of metal chalcogenides for supercapacitor applications opens the path for renewable resource derived green metal chalcogenides as better replacement electrode materials. Though the green synthesized metal oxide nanostructures are well-known, not much studies are reported to utilize them as electrode material for supercapacitor applications. Hence, green synthesis of (mixed) metal oxide and its modified forms (e.g., composites with organic/inorganic/biobased nanomaterials) will be an emerging electrode material for energy sector applications. A wide range of renewable products such as agroforestry biomass residues, food wastes, agro-industrial coproducts, animal by-products, and aquatic biomasses can also be effectively considered for the preparation of various

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biocarbon materials and thus create a circular economy market value by waste valorization. This will create more opportunity for the researchers to explore new feedstock for the fabrication of carbon nanomaterials with better control on their structure and morphology. Green synthesized zero-dimensional carbon nanomaterials such as carbon dots and carbon nanoparticles as well as their modified forms will play crucial role as electrode materials for supercapacitor studies in which efficient research focus should be needed. Fabrication of renewable resource-based carbon-carbon composite nanomaterials and/or carbon-based composite green nanomaterials (e.g., carbon-metal, carbonMXene, carbon-biopolymer) as active electrode materials will be an interesting task in near future for supercapacitor applications. Though renewable resourcebased nanocomposite materials were reported for supercapacitor studies, development of renewable resource-based multifunctional nanocomposite comprising metal/ metal oxide/metal chalcogenides with other active counterpart will create more research importance in the near future. In general, compared to cellulose nanofiber-based materials, less number of studies were explored for other biopolymer-based nanomaterials for supercapacitor application. Nitrogen and/or sulfur containing biopolymers (e.g., chitosan, carrageenan) can be considered as effective source for the preparation of doped green carbon nanomaterials for supercapacitor applications in which their capacitive performances can be improved by functionalization with metal/metal oxide/metal chalcogenides/carbon derivatives. In the case of device fabrication, most of the studies were carried out by using green nanomaterials as symmetric electrodes, whereas fabrication of devices with asymmetric electrodes will be an interesting task to improve the capacitive performances. The counterpart in the asymmetric electrode can be either green nanomaterial or commercially available electrode material based on the availability or demand.

Conclusions Green nanomaterials obtained using various renewable feedstocks/precursors as the source, catalyst, and reducing/stabilizing agents have been extensively used as the electrode material for supercapacitor applications. Research activities on this area are being accelerated due to not only the abundance, diversity, eco-friendliness, and cost-effectiveness of the renewable resources but also its ability to create a wide range of materials with different architecture and morphological features. Understanding the relationship between the synthesis of nanostructures, post modification techniques, physicochemical properties, and their capacitive performances of renewable resource-based nanostructured materials is a key factor for the achievement of high-performance supercapacitor with desired performances. Existing nanofabrication techniques for carbon materials needs to be improved to achieve final products with desired structural and morphological properties. In many of the studies, crude biomass or extracts have been explored for the synthesis of a wide range of nanomaterials. In-depth investigation is essential to understand the effect of

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various bio-constituents on their final products, which enables the researchers to tune the physicochemical properties in precise manner. Acknowledgments Dr. S. Vivekanandhan acknowledges the University Grants Commission (UGC) for the financial support through a Minor Research Project (MRP/UGC-SERO Proposal No.1593).

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An Investigation on Mechanical Characteristics of Carbon Nanomaterials Used in Cementitious Composites

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Kanchna Bhatrola, Sameer Kumar Maurya, Bharti Budhalakoti, and N. C. Kothiyal

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanomaterials Typically Used in Cement-Based Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-Based Nanomaterials in Cementitious Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanofibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of CNMs as Cement Nanoreinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Future Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Ongoing rapid development in many places worldwide raises concerns about negative CO2 emissions. An effort was made by building industries to reduce CO2 emissions by producing less clinker-based modified cement. In the construction industry, this type of cement provides adequate strength and durability. Various nanoparticles embedded in cement, on the other hand, have sparked huge interest in civil engineering as a means of achieving sustainability goals. Additionally, important yet hidden effects of various nanomaterials whether it be strength, durability, and workability of the suggested enhanced concretes/mortar were investigated. Even at very low concentrations (less than 0.08% by weight of cement), most carbon-based nanoreinforcers exhibit considerable improvements in flexural and compressive strength (up to 100%) and durability (up to 80% greater acid resistance). A denser cement matrix is the result of chemical, rather than physical, interactions between nanomaterials and C-S-H paste (C-S-H). This chapter presents a detailed overview of many widely utilized carbon nanomaterials used in cementitious composites. It discusses the common weight K. Bhatrola (*) · S. K. Maurya · B. Budhalakoti · N. C. Kothiyal Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_93

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percentages employed in mixes by numerous researchers in the field and evaluates the mechanical characteristics of these nanomaterials when applied. Keywords

Construction · Nanoparticles · Nanomaterials · Mortar · Reinforcement · Mechanical · Cementitious Abbreviations

AFM Aft Bwoc CDs CH CHM CNCs CNDs CNFs CNHs CNMs CNTs C-S-H CVD FCNCs FCNT-FCMs FT-IR GNPs GO GO-FCMs GON GONPs MLG MWCNTs OPC rGO SEM SWCNTs

Atomic force microscopy Ettringite By weight of cement Carbon dots Calcium hydroxide Cement hybrid material Cementitious nanocomposites Carbon nanodiamonds Carbon nanofibers Carbon nanohorns Carbon-based nanomaterials Carbon nanotubes Calcium-silicate-hydrate Chemical vapor deposition Fly ash blended cementitious nanocomposites Functionalized carbon nanotubes-fly ash blended cement mortars Fourier transform infrared Graphite nanoplatelets Graphene oxide Graphene oxide-fly ash blended cement mortars Graphene oxide nanoplatelets Graphene oxide nanoplatelets Multilayer graphene Multiwalled carbon nanotubes Ordinary Portland cement Reduced graphene oxide Scanning electron microscopy Single-walled carbon nanotubes

Introduction The rapid development toward construction sustainability necessitates the use of high-performance and innovative cement like materials in order to design infrastructure systems that are safer, more reliable, and more cost-effective. Plain

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mortar is fragile and has poor tensile strength. This is a major issue for structural applications. Many approaches and concepts have been researched and investigated to improve the performance and smartness of cementitious materials, but among the most commonly explored and investigated is the idea of employing additives in order to enhance cement paste and tailor its structural, chemical, and mechanical characteristics (Kosmatka et al. 2002; Helmuth 1987; Kumar 1998). As a result, researchers have focused research on nanofibers in an attempt to restrict the beginning of nanoscale cracking. However, the nanoscale size of the C-S-H gel means that micro-reinforcement is only a mitigation measure, not a preventative one. This means that the reinforcement is only used to make concrete stronger and more durable. Nanoreinforcement is better because it stops cracks from growing by bridging and inhibiting weak spots at the earliest (Zhao et al. 2017). Nanomaterials are now extensively employed to control cracking in cementitious composites. Nano-sized additives have gotten more attention in the previous decade to overcome this problem (Li et al. 2006; Said et al. 2012). It is being explored and used as an additive to strengthen the microstructure of cementitious materials with high surface-volume ratios such as carbon nanomaterials, titanium oxides, nanoalumina, and nanosilica (Metaxa et al. 2009; Hassan et al. 2010). Its extraordinary mechanical properties and good performance of novel carbon nanomaterials in cementitious composites have recently piqued the curiosity of some researchers. As a result of recent advances in nanotechnology and materials science, an outstanding nanomaterial that is graphene has emerged that has the potential to be used as an additive which is particularly nano-sized. On comparison with other nanomaterials, it is observed that graphene exhibits sp2 hybridized 2D structure which is not found in any other material (Novoselov et al. 2004). As a result of its unique structure, graphene has a lot of fantastic properties. These include a super-high specific surface area, ultrahigh tensile strength, and elastic modulus, as well as excellent thermal, electrical, and optical conductivity (Kuilla et al. 2010; Potts et al. 2011). The engineers are becoming more interested to make use of graphene in order to design smart, efficient, and high-performance materials (Le et al. 2014). CNMs, CNFs, CNTs, and GO that are carbon-based nanomaterials, carbon nanofibers, carbon nanotubes, and graphene oxide, respectively, are extremely promising candidates as cement matrix additives, owing to their abundance in nature and industrialized mass production (Lu and Zhong 2022). This chapter focuses on nanomaterials and nanotechnology and their immense potential in cement industry. It may help us to give a better insight and complete overview of these industrial by-products, their significant role as additives, and further improving the microstructures of the cement-based concretes, which further leads to sustainable development in the construction sectors. Understanding the role of carbon nanomaterials as a nanoreinforcement in cementitious composites may be valuable to engineers and researchers and provide a framework for further exploration.

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Nanomaterials Typically Used in Cement-Based Materials Given the enormous importance of different nanomaterials, sustainable cementbased concretes were included. Progress in nanotechnology and nanomaterials means that new properties can be added to the basic structure of materials, which could lead to new ideas for concrete durability problems and new ways to build new buildings that could save a lot of money on service and maintenance costs in the future. Steel and ceramics nanostructured composites are also viable to give great wear resistance and strength (Gajanan and Tijare 2018). The addition of nanoparticles to concrete increases its qualities as well as its performance. The large number of research works that examined the use of nanostructured materials in concretes found that their early mechanical strengths and overall bulk properties were significantly enhanced (Norhasri et al. 2017). Nanosystems have a considerable effect on the mechanical parameters of modified concretes, including durability, setting time, workability, durability indices, and strength characteristics. Nanotechnology has the potential to usher in a new era of construction by allowing for a better knowledge of the behavior of construction materials (Pacheco-Torgal and Jalali 2011). Enhancement of mechanical strength has a greater role in cementitious nanocomposites. The strength of the bond between fiber and matrix, as well as the fiber’s quality, determines the mechanical properties of fiber-reinforced materials. It is possible to increase the fiber-matrix bond by increasing the amount of fiber-matrix contact area. Nanocomposites produced by cutting-edge technologies must be improved (Zappalorto M et al. 2013). New generation of high-performance, multifunctional cementitious composites can be developed by reinforcing the cementitious matrix with nanostructures such as nanofibers, nanotubes, and nanoparticles like nano-SiO2 or nano-TiO2 (Metaxa et al. 2021). The data has been depicted in Fig. 1 considering that the incorporation of carbon-based nanomaterials (CNMs) in cementitious composites is approximately ~58% out of total publication, followed by ~nano-SiO2 corresponding to 34%, whereas nano-TiO2 ~ 7% out of total Fig. 1 Pie chart of nanomaterials utilized in cementitious composites

Nano TiO2 7%

Nano SiO2 34%

Cellulose 2%

CNMS 57%

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Table 1 Physical parameters of traditional fibers, graphene, CNTs, and GO materials (Qureshi and Panesar 2019) Material Steel fiber Glass fiber Carbon fiber Polymeric fiber Carbon nanotubes Graphene oxide Graphene

Tensile strength (GPa) 1.50 3.45 0.4–5 0.3–0.9

Elastic modulus (GPa) 200 72 7–400 3–5

Surface area (m2/g) 0.02 0.3 0.134 0.225

Aspect ratio 45–80 600–1500 100–1000 160–1000

11–63

950

70–400

1000–10,000

~0.13

23–42

700–1500

1500–45,000

~130

1000

2600

6000–6,00,000

publications. The data studies are mainly examining the effect of nanomaterials on the performance of cementitious composites. The challenges of intrinsic quasi-brittle behavior and crack production in highperformance cementitious composites, which have been addressed and considered for practical building applications around the world, continue to be a concern. Due to the fact that traditional reinforcing agents are only effective at the macro- or microscale, they are unable to work at the nanoscale due to their micro-dimensions. This results in fracture genesis at the nanoscale and, as a result, weak strength of the cement-based matrix, which causes it to fail when subjected to external loads (Wang et al. 2015). Increased porosity, reduced mechanical strength, and decreased durability are all factors that lead to the dissolution of the cementitious matrix as the ages increased of cementitious matrix. Table 1 presents a comparison of the physical characteristics of certain recently produced CNMs and some conventional reinforcements used in cementitious composites. In light of the aforementioned difficulties, the use of nanoparticles as reinforcing agents as an alternative solution could be considered, as it provides a great help in shaping cementitious matrix at nanoscales. CNMs with their outstanding mechanical capabilities, they have been widely used as prospective reinforcing elements in the cementitious matrix to increase the overall performance and durability of the cementitious structure (Mohsen et al. 2017).

Carbon-Based Nanomaterials in Cementitious Composites Carbon atoms, with a valency of four, are capable of forming single, double, and triple covalent bonds with other elements. In addition to forming extended chains, they are also involved in polymerization phenomenon. Carbon atom comprises of an electronic structure along with atomic size. This enables them to exhibit various physical structures with distinctive physical properties despite having the same chemical composition. Carbon atoms having a small band gap between their 2 s

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and 2p electronic shells are capable of sp., sp2, and sp3 hybridizations (Chatterjee and Chen 2012; Wanekaya 2011). Carbon has two major allotropes: firstly, sp3 hybridized diamond and secondly sp2 hybridized graphite. The nanomaterial particles are classified primarily on the basis of their geometrical structures. Particles can take the form of ellipsoids, tubes, spheres, or horns. Carbon nanotubes (CNTs) and carbon nanohorns (CNHs) are tube- or horn-shaped particles, respectively; fullerenes contain spherical or ellipsoidal nanoparticles. Some of the important allotropes of nanocarbon include graphene, fullerene, carbon nanotubes (CNTs), carbon dots (CDs), and carbon nanodiamonds (CNDs) (Acquah et al. 2017). 0D nanodiamonds, 1D nanotubes, and 2D graphene nanosheets can be used as prototypes for nanocomposites. The recent discovery of graphene having a 2D geometry is proved to be significantly more dominant when compared to CNTs and CNFs (Geim and Novoselov 2010). Kroto et al. in the year 1985 discovered buckyball which is defined as molecules having a ball-like structure that comprises of pure carbon atoms. Iijima in 1991 named carbon nanotubes after a tubular form of carbon (CNTs) (Kroto et al. 1987; Iijima 1991). These were defined as MWCNTs or multiwalled carbon nanotubes. They comprised of several tens of graphitic shells with adjacent shell separations of 0.34 nm, diameters of a few nanometers, and a high length/diameter ratio. Moreover, the oxygenating functional groups are involved in physical/chemical interaction with the cementitious composites and then increasing the overall strength of cementitious composites (Xu et al. 2019; Tung et al. 2008). With a variety of advantages, GO has been the most investigated CNMs for cement material modification over the last decade (as depicted in Fig. 2).

PERCENTAGE (%)

CNMs Cementitious Composites 40 35 30 25 20 15 10 5 0

31.4

36.2

16.5 9.7

6.2 Rgo

CNFs

Graphene

CNTs

GO

CARBON NANOMATERIALS

Fig. 2 The number of published publications of various CNMs utilized in cementitious composites. (Data from Google Scholar, 2011–2021, total of 14,206 publications)

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Portland Cement Matrix Phase Carbon Nanomaterials based Cementitious Nanocomposites

Supplementary Cementitious Materials: Fly Ash (FA) Graphene

Reinforcing Phase

Carbon Nanomaterials (CNMs)

CNTs GO /RGO CNFs

Fig. 3 A brief overview of carbon nanomaterial-based cementitious nanocomposites

The term “carbon nanomaterial-based cementitious nanocomposites” refers to the incorporation of CNMs into the matrix phases, i.e., cementitious nanocomposites (CNCs) or fly ash blended cementitious nanocomposites (FCNCs). Figure 3 shows a high-level overview of the components of cementitious nanocomposites/fly ash-based cementitious nanocomposites. The fig. 3 depicted that the Portland cement and fly ash were used in form of matrix phase, whereas the CNMs were utilized in the form of reinforcing phase.

Graphene The graphene nanomaterials are formed by graphite, which shows a distinctive 2D structure (Novoselov et al. 2005). Graphene has been calculated to have Young’s modulus of 1 TPa and an inherent bulk strength of more than 130 GPa (Lee et al. 2008). Granular graphene, which is around 200 times stronger than steel, is by far the strongest material we currently know and might be used in concrete structures as a nanoreinforcement option (Balapour et al. 2017). As well as being transparent, it possesses unparalleled electrical and thermal conductivity (making it suitable as a piezoresistive smart sensor) and is translucent, making it suitable for use in photonic devices. As the demand for their use in industry grows, the manufacturing costs of these products are lowered, making their use more economically viable. However, according to previous research on graphene, MLG that is multilayer graphene, CNT that is carbon nanotube, and CNF or carbon nanofiber, there are a number of difficulties in effectively and economically implementing these nanomaterials in concrete. One of the most significant compatibility issues is caused by the hydrophobic nature of the substance. The schematic modal of graphene structures is depicted in Fig. 4.

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Fig. 4 Schematic representation: (a) Graphene, (b) GO, (c) rGO, and (d) GNPs; permission taken (Shamsaei et al. 2018)

Graphene Oxide Prior to the discovery of graphene oxide, graphite was referred to as “graphitic oxide” or “graphitic acid” because of its abundance and low cost. GO was first synthesized in 1859 by Benjamin C. Brodie using fuming nitric acid and potassium chlorate to treat a graphite precursor (Hummers and Offeman 1958). The structure and properties of GO are determined by the synthesis methods, which include the following given methods: Brodie’s, Staudenmaier’s, degree of oxidation Hummer’s, and modified Hummer’s method (Paulchamy et al. 2015). GO is a hexagonal carbon network that has epoxide and hydroxyl functional groups in its basal plane and carboxyl and carbonyl functional groups at the sheet edges. Typically, three steps are involved in the preparation of GO: oxidation (in which functional groups containing oxygen are incorporated into graphite to form hydrophilic oxide), filtration (in which remaining ions are removed using deionized water), and exfoliation (in which GO is subjected to ultrasonication). GO is an amorphous substance because of its functional groups, yet it has hydrophilic qualities and is exceedingly dispersible and stable in water because of functional groups present on the surface of graphitic sheets. According to the results observed via incorporation of GO, a possible regulation mechanism for GO on cement hydration products can be proposed as shown in Fig. 5. Mechanical Strength The single-layered graphene sheet has a remarkable mechanical property. It shows 130.5 GPa of tensile strength, 1.0 TPa of Young’s modulus, and 42 Nm 1 of break strength. But after functionalization, Young’s modulus for GO drops from 207.6 to 23.4 GPa (for 0.75 nm thickness), which is still a very high value. GO with a high

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Fig. 5 Representation of step-by-step GO mechanism in cementitious composites; permission taken from Shamsaei et al. (2018)

carbon-to-oxygen ratio (4:1) has been found to have a mean break strength of 17.3 Nm 1 (i.e., 24.7 GPa) per monolayer. The various researches have been conducted to investigate the capability of GO as a reinforcement to enhance the mechanical properties of cementitious composites, as the results are shared in Table 2. It could be concluded that very less amount of GO significantly increases the flexural, compressive, and tensile strengths of cementitious composites. The mechanisms were proposed by the summarized data given as follows: GO can densify the microstructure and inhibit the crack propagation at early ages and also pore reduction of cementitious composites (Gong et al. 2015). The interaction of GO with cementitious matrix could enhance the load-transfer efficiency. Similarly, Gholampur et al. designed eight mixes, with different dosages of GO in cementitious mortar, and optimize the appropriate dosage of GO which was 01 wt% (Gholampour et al. 2017). More than that amount, it led to decrease in strength due to agglomeration behavior of GO. Likewise, properties were found by other researchers (Wang et al. 2016a). Sharma et al. investigated the effect of GO on cement. They found that compressive strength increases by addition of reduced in size of GO in cementitious

0.45 0.38 0.4

0.43 0.43 0.43 0.43 0.4

GO UCNTs MWCNTs

FCNT-FCMs GO-FCMs FCNT@GO-FCMs

GO suspension MWCNTs

CNT mix with GO GO Hybrid CNM GO

MWCNTs (long)

w/c 0.5 0.5 0.5 0.5 0.4 0.45 0.43 0.36

Carbon nanomaterials MWCNTs (short)

0.001 0.002 0.003 0.004 0.005 0.125 0.1 0.038 0.075 0.08 0.08 0.16 0.08 0.04

0.125%

wt% 0.048 0.08 0.048 0.08

Mechanical property improvement Compressive Tensile Module of elasticity – – 27.5% – – 25% – – 56.25% – – 5% – – 53% 35% 96% – 83.6% 52.7% – 13.4% 47% – 27.6% 59.5% – 38.9% 78.6% – 42.2% 36.6% – 47.9% 35.8% – 35% 95% – 51% – – – – 13% – – 31.54% 24.9% 39.7% – 39.8% 48.9% – 52.1% – – – – – 23.9% – –

Table 2 The synergistic effect of CNMs on the mechanical properties of cementitious composites Flexure 17.5% 62.5% 25% 25% 79% – – 51.7% 32.9% 60.7% 30.5% 30.2% – 22% 25.13% 49.89% – – – 64.3% 16.7%

Zhou et al. (2017)

Kaur and Kothiyal (2021)

Sharma et al. (2018) Lu et al. (2022) Zou et al. (2015)

Gao et al. (2018) Sharma et al. (2018) Kaur et al. (2020) Lv et al. (2013b)

Konsta-Gdoutos et al. (2010)

References

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0.33

0.43 – 0.42 0.4

0.43 0.3 0.6 0.03

0.4

0.35

MWCNTs

HCN-CNCS

GOa MLGs

GO mixed with carbon fiber

GO

MLGs

GO GO

0.4

CNFs MWCNTs

0.1 0.2 0.1 0.2 0.025 0.05 0.1 0.02 0.08 0.022 0.02 0.03 0.04 1 0.02 0.01 0.03 0.02 0.01 0.004 0.006 0.01 0.03 0.01

– – – – 6% 13% 15% 43.08% 52.20% 17.68% 3.80% 7.70% 14% 63% 54% – 5.5% 54% 10% 24.95% 23.89% 46.9% – 5.6%

– – – – – – – – – – – – – – – – – – – – – – – – 2% 23% 6% 22% – – – – – – – – – – – – – – – – – – – – Peng et al. (2019)

43.59% 21.86%

Sun et al. (2017)

(continued)

Sharma et al. (2015) Chen et al. (2018)

Zhao et al. (2016) Li et al. (2017)

Kaur and Kothiyal (2019a)

Xu et al. (2015)

Chen et al. (2018)

Abu Al-Rub et al. (2012)

55 14% 44% 33% 7.5% 15% 30% – – 22.55% – – – – – 21% – – – 20.48%

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w/c 0.3

0.43 0.43 0.4

0.35 0.40 0.38

Carbon nanomaterials GO

SP@go SP@FCNT Graphene sheets

Graphene Graphene Graphene

Table 2 (continued)

wt% 0.03 0.06 0.03 0.06 0.03 0.06 0.08 0.08 0.01 0.025 0.05 0.025 2

Mechanical property improvement Compressive Tensile Module of elasticity 20% – – 29% – – 28% – – 34% – – 34% – – 38% – – 23.2% 38.5% – 38.5% 35.8% – 13.5% – – 10% – – 3–8% 15.2 – 14.9% – – 33.3% – – Flexure 27% 30% 43% 39% 52% 52% – – – 16% 15–24% 23.6% –

Wang et al. (2016a) Liu et al. (2019) Xu and Zhang et al. (2017)

Liu et al. (2019)

Kaur and Kothiyal (2019b)

Lv et al. (2014)

References

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composites (Sharma and Kothiyal 2015). The more number of nanosheets provides more surface area that provides more nucleation sites to increase the rate of hydration (Sharma and Kothiyal 2015). According to Lv et al., the tensile, flexural, and compressive strengths of GO increased significantly when the oxygen content was increased from 12% to 25.5%, but further increases in the strengths were not observed when the oxygen content was increased to 29.33% (Lv et al. 2013a). GO has recently been shown to be capable of improving cement composites mechanical characteristics. There has been lack of research on the reinforcing effects of GO in ultrahigh-strength concrete (UHSC) with a low water-to-cement ratio, and most of what can be done now is centered around the performance of GO in cement paste or mortar with a high water-to-cement ratio. The examination of GO in structural applications is also lacking. More extensive studies should be done on the reinforcing performance of GO in concrete with extremely low water-to-cement ratio or large scale to implement GO in architecture.

Reduced Graphene Oxide GO is an oxidized version of graphite made up of carbon atoms exfoliated. By eliminating oxygen-containing groups from graphene oxide nanosheets, they can be converted to reduced graphene oxide (rGO). These rGO obtained from the thermal and chemical reduction of GO contain properties of both GO and pure graphene as it is well dispersible in aqueous solutions. Physical strength and electrical conductivity of graphene layers are partially restored in rGO. The reduction of functional groups from GO to rGO transforms the microstructure and physical properties into those of graphene, although rGO is not entirely oxide-free graphene and is partially dispersible in water (Saafi et al. 2015). Nowadays, more innovative work is being carried out in the field of civil infrastructure using rGO. Mechanical Strength In order to improve the strength and ductility of a cement-based material by incorporating rGO, initial work with respect to morphology, mechanical properties, and chemical functional groups by making use of rGO in geopolymeric composites was performed by M. Saafi et al. (2015). A little concentration of rGO, say 0.35%, was required in order to improve the mechanical properties of these geopolymer composites. Owing to its malleability, these rGO sheets were dispersed between fly ash particles in geopolymer composites so as to fill the hollow spaces. Using rGO as a nanofiller in fly ash geopolymer composites, an increase was observed in certain properties that includes 376% Young’s modulus, 134% flexural strength, and 56% toughness (Saafi et al. 2015). rGO with reduction times ranging from 5, 10, 15, 30, and 60 min was added to cement mortar at an optimal concentration of 0.1%. On comparison with specimens of plain cement mortar, rGO under 15-min reduction with a concentration of 0.1% addition improved tensile and compressive strength by 37.5% and 77.7%, respectively. The study discovered that reduction of rGO at optimal conditions improves cement hydration by linking C-S-H and rich oxygen functional groups in aqueous dispersions.

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Graphene Nanoplatelets GNPs and GONPs that are graphene nanoplatelets and graphene oxide nanoplatelets, respectively, are novel graphene-based nanoparticles (Sedaghat et al. 2014; Le et al. 2014). GONPs are defined as oxides of GNPs that comprise of functional groups formed during the oxidation and exfoliation process, as opposed to pure graphene. Both of these have a two-dimensional sheetlike structure having nanoscale thickness (less than 10 nm). GNPs and GONPs inherit many of graphene-like benefits, making them promising nanoscale additions and excellent reinforcement for smart structural materials. They are also low-cost nanoparticles, which are employed as additives in a range of scientific applications. According to the findings of Alkhateb et al., GONPs enhance the production of C-S-H gels and altered the microstructure of cement paste. As a result, both the C-S-H interfacial bond and the overall strength of the concrete were significantly improved (Alkhateb et al. 2013).

Mechanical Strength Nowdays, it was discovered that adding 0.03% GONPs to concrete boosted its tensile, flexural, and compressive strengths by 78.6%, 60.7%, and 38.9%, respectively. They suggested that the intrinsic reinforcing mechanism was that the hydration gels reacted preferentially with the functional groups on the surfaces of GONPs based on the XRD and SEM studies which were done by (Lv et al. 2013b). Even after multiple-cycle quasistatic and dynamic compressive loadings, the introduction of GNPs gives the stable repeatable piezoresistive characteristics to cementitious composites. Rehman et al. developed the incorporation of GNPs as a reinforcing agent in concrete beam (Rehman et al. 2017).

Carbon Nanotubes CNTs were the preferred nanomaterial for cementitious research prior to the incorporation of GO and graphene in cement because of their ease of functionalization. Sumio Iijima invented carbon nanotubes for the first time in 1991, using an arc-discharge method commonly used to produce fullerenes. CNTs were grown in the carbon soot of the anode by exposing a powdered carbon anode (with a catalyst) and a pure graphitic rod cathode to high current (50–100 A) in an inert gas environment. CNTs are further divided into two categories, namely, SWCNTs (single-walled carbon nanotubes) and MWCNTs (multiwalled carbon nanotubes). The SWCNTs are formed by a single sheet rolled up into a hollow tubelike structure, and MWCNTs are formed by multiple sheets of graphene rolled up into cylindrical structure (Abu Al-Rub et al. 2012; Nochaiya and Chaipanich 2011). In the high aspect ratio of CNTs, more energy is required for crack propagation compared to low aspect ratio. Furthermore, CNTs have excellent physical properties such as high strength and Young’s modulus that are 20 times and 10 times stronger than carbon fibers (Ruan et al. 2019). However, when using CNTs, researchers encountered

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issues such as fiber dispersion in cement paste and bonding between cementitious materials and CNTs.

Mechanical Strength Different environments and external loads can have an impact on the mechanical properties of nanomaterials, which are important in improving the performance of cement composites. Numerous researches have been performed in order to improve the performance of cement composites by varying the percentage of carbon nanomaterials that replace the weight of cement (wt%). For example, Rashid K. Abu Al-Rub et al. investigated the effect of MWCNTs in cement composites at concentrations of 0.1% and 0.2% by weight of dry cement, respectively, by using acrylic molds with dimensions of 6.5 6.5 160 mm3 in size. When MWCNTs were used in combination with cement composites to improve modulus of elasticity, the results revealed that the flexural strength of the cement composite decreased considerably when only 0.1 weight % of MWCNTs was used (Abu Al-Rub et al. 2012). This is because the authors used acid (sulfuric and nitric) that made sulfate ions, which caused the bonds to break in certain sites. As a result, the flexure strength of cement composites dropped by about 44%. Maria S. Konsta-Gdoutoset al. looked into how MWCNTs could perhaps affect the performance of a cement composite which they constructed. When they did their study, the authors used two different extents of MWCNTs. They incorporated 0.048% and 0.088% of MWCNTs, and they used both short and long lengths in their study. They noticed that with 0.048 wt% long lengths and 0.08 wt % short lengths of MWCNTs, Young’s modulus increased by 56.25% and 62.5%, respectively (Konsta-Gdoutos et al. 2010). Similarly, Bo Zou et al. reported that adding 0.038 wt % and 0.075 wt % of MWCNTs to cement composites can enhance flexural strength and modulus of elasticity (Zou et al. 2015). Moreover, Shilang Xu et al. investigated the compressive strength of cementitious composites by adding 0.025 wt %, 0.05 wt %, and 0.1 wt % of MWCNTs. The results showed that compressive strength improved by 6%, 13%, and 15%, respectively (Xu et al. 2015). Like other studies, Shilang Xu’s test results suggest that increasing the proportion of MWCNT in cement composites can improve their performance.

Carbon Nanofibers Carbon nanofibers (CNFs) have a high modulus of elasticity (TPa) and tensile strength (GPa), as well as unique electrical and chemical properties (Sanchez and Sobolev 2010). On the other hand, Hogancamp et al. demonstrated that CNFs have no significant effect on the rigidity of cement mortar (Hogancam and Grasley 2017).

Mechanical Strength Several studies involving CNFs found that the addition of silica fume (for 0.002–2 wt. % CNFs), surface treatment of CNFs with nitric acid (for 0.5 wt. % CNFs), and pre-dispersion in acetone (for 0.5 wt. % CNFs) (Sanchez and Ince 2009; Metaxa et al. 2010) all aided in the dispersion of individual CNFs in Portland cement

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pastes, through pockets. The presence of CNFs supplied the composite with residual load-bearing capability, even though no change in compressive or split tensile strengths was detected.

Effects of CNMs as Cement Nanoreinforcement CNMs have a high degree of reactivity because they have a considerable impact on the chemical processes and behavior that occur within cementitious nanocomposites. The addition of CNMs to cement mortar/concrete improves the characteristics of both fresh and hardened cement mortar/concrete (Paul et al. 2018). Additionally, CN/HCNs fill the spaces and pores in the cement matrix, resulting in a compact microstructure and reduced porosity of the cementitious nanocomposites. A brief summary has been given in Table 2. Various environmental and external loads can be affected by the physico-mechanical properties of carbon-based nanomaterials that have played a major role in the enhancement of the properties of cement composites. Wide numbers of researches have been studied to offer a high performance of cement composites by varying the % of carbon nanomaterials by the weight % of cement. The role of CNFs and MWCNTs in cementitious composite was concluded by Abu Al-Rub et al. The concentration of CNFs and MWCNTs was taken 0.1 and 0.2% by the weight of cement, respectively. The conclusion revealed that the strength of flexural in both cases of MWCNTs and CNFs intensely dropped in 0.1 (wt %) although the modulus of elasticity enhanced in both cases (Abu Al-Rubet al. 2012). The reason is that they utilized an acid such as nitric and sulfuric which induced ions of sulfates that directed to localized debonding. This concludes 44% reduction in flexure strength takes place in cementitious composites. MWCNT affecting cementitious composites was studied by Maria S. Konsta-Gdoutos et al. They incorporated two different percentages of MWCNTs by addition of 0.08 and 0.048 by the weight % of cement and also utilized a combination of long and short lengths of MWCNTs. They concluded the increased strength attained by 0.08 wt% short and 0.048 wt% long length of MWCNTs by 62.5% and 56.25%, respectively, in their investigation. This result suggests that a larger concentration of short MWCNTs is necessary for efficient reinforcing, but a lesser amount of long MWCNTs is required to achieve the equivalent increase in Young’s modulus (Konsta-Gdoutos et al. 2010). Correspondingly, Bo Zou et al. listed that the modulus of elasticity and flexure strength could be improved via incorporation of MWCNTs by 0.075 and 0.038 wt % of cement composites (Zou et al. 2015). The strength of compressive and flexural in cement composite by the incorporation of MWCNTs with 0.025, 0.05, and 0.1 wt % of cement was studied by Shilang Xu et al. The outcome concluded that the strength of flexural enhanced by 7.5, 15, and 30%, respectively, and the compressive strength enhanced by 6, 13, and 15%. As other investigators, followed results of Shilang Xu also revealed that increasing the percentile of MWCNTs can increase the properties of cement composites (Xu et al. 2015). Lots of investigations have been done in the addition of GO on the physico-mechanical properties of cement composites. The effect that arises by

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the graphene oxide nanosheets on the physico-mechanical properties of cement composites was held by Shenghua Lv et al. They used five various dosages of graphene oxide in cement composites. They utilized 0.001, 0.002, 0.003, 0.004, and 0.005 wt% of graphene oxide. Compressive strength was improved by 0.005 weight percent by adding GO. Furthermore, tensile and flexural strength increased by 78.6% when the GO dosage was increased to 0.003 wt percent. However, the percentage of tensile and flexural strength reduced marginally if the GO dosage was increased further (Lv et al. 2013a). According to Li Zhao et al., adding 0.022% of GO increased the compressive and flexural strengths of cement by 18 and 23%, respectively (Zhao et al. 2016). However, the result of Xiangyu Li et al. contradicts Li Zhao. They reported that adding 0.02% GO to cement paste decreased its compressive strength by 4% (Li et al. 2017). The concentration of GO less than 0.03% couldn’t increase the compressive strength of cementitious composites as per their statement. However, the combination of graphene oxide and CNTs was utilized in cement by Yuan Gao in their study. They had been incorporated CNT/GO solution with 0.4gm and 0.2gm of CNT and GO, respectively, added into dry cement and after that poured it into appropriate steel molds (20 mm  40 mm  160 mm). The study founded that the flexure strength enhanced by 79% and modulus of elasticity improved by 53% (Gao et al. 2018). The same example has been carried out by Cheng Zhou et al. the utilization of 0.02 wt% GO and 0.04 wt% MWCNTs in the cement matrix. The investigators conclude that the strength increased by 16.7% and 23.9% in this compressive and flexural strength, respectively (Zhou et al. 2017). Baoguo Han et al. added 0.01 and 0.02 wt% of the multilayer of graphene sheets (MLGs) in cementitious composites and investigated their strengths. They utilized the mold with 20 mm  20 mm  40 mm for compressive strength and 40 mm  40 mm  160 mm for flexural strength. The conclusion revealed that the flexural strength improved by 21% and compressive strength improved by 54% (Chen et al. 2018). As same manner Sun et al. examined with the similar percentile of MLGs in cementitious composites, the compressive strength was increased by 54% (Sun et al. 2017). All the information regarding incorporation of carbon-based nanomaterials in cement were framed in Table 1. Moreover, Table 1 shows that the multilayers of graphene help to improve the compressive strength by 54% which is slightly greater than graphene oxide.

Conclusions and Future Scope CNMs have shown promising efforts while improving the physical and mechanical properties of cement-like composites owing to innovations in nanotechnology. The primary scope of this chapter comprises recent research on the use of CNMs in cementitious materials. Few of the disadvantages of cementitious composites include high cracking propensity, poor deformation performance, high porosity, low tensile capacity, and accounting for almost 8% of world carbon dioxide emissions via cement manufacture. It is feasible to overcome these challenges and build

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new, innovative, and much efficient cementitious composites which are highperforming as well as multifunctional. This can be achieved by incorporating nanoparticles into cementitious composites. Existing research indicates that the properties of cementitious composites could be improved by incorporating a minimal amount of GO. Nevertheless, there is considerable disagreement among these results, due to the different chemical and physical properties of GO and the complexity of cement composites. The majority of current research is on the use of GO in cement paste or mortar; however, its application in concrete, particularly UHSC that is ultrahigh-strength concrete having a lower water-to-cement ratio, has not been adequately studied. In addition, more research is needed into how nanomaterials can improve the performance of concrete. Most of the research has been done on cement paste and mortar. Moreover, further research needs to be done targeting theoretical models which helps to predict how well the cementitious nanocomposites will work as a function of the concentration of the nanomaterials.

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Sharafadeen Gbadamasi, Suraj Loomba, Muhammad Waqas Khan, Babar Shabbir, and Nasir Mahmood

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation of Zinc Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Ion Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Air Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aqueous Zinc Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Performance Controlling Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Ion Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Air Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of Zinc-Based Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges Associated with Zinc-Based Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Ion Battery System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc-Air Battery System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrode Materials for Zinc-Based Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cathode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Zinc-Based Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Sharafadeen Gbadamasi and Suraj Loomba contributed equally with all other contributors. S. Gbadamasi · S. Loomba · M. W. Khan School of Engineering, RMIT University, Melbourne, VIC, Australia B. Shabbir Department of Material Science and Engineering, Monash University, Clayton, VIC, Australia N. Mahmood (*) School of Engineering, RMIT University, Melbourne, VIC, Australia School of Science, RMIT University, Melbourne, VIC, Australia e-mail: [email protected] © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_106

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Abstract

The intermittent nature of the demanding renewable energy sources required cheap energy storage systems; however, the currently used advanced energy storage systems mainly rely on lithium- or sodium-based chemistries. Both metals are highly reactive and expensive, hence increasing the energy storage system’s overall cost. Therefore, an alternative system is highly in demand, which should have comparable energy storage capacity at a much lower cost and high safety. In this regard, zinc-based batteries got tremendous attention as its less reactive nature makes it safe, while low cost and high energy density make it affordable. Recently, considerable work has been done on various battery chemistries by utilizing zinc as a charge storing agent. This chapter summarizes recent progress in zinc battery technologies and its possible applications. This chapter first describes the working operation of zinc-based batteries, emphasizing zinc-ion, zinc-air, and aqueous zinc batteries. Then, it addresses the factors which control the performance of zinc-based batteries. Afterward, the various advantages of zinc batteries are discussed along with the associated challenges, and their possible solutions are also listed. In progress, this chapter highlights the recent progress in the development of electrode chemistries for zinc batteries. Further, different applications of various zinc-based batteries are presented to highlight their commercial impact. In the end, a summary is provided with future perspectives to guide for a future possible solution to the associated challenges. Keywords

Zinc-based batteries · Electrodes · Energy storage

Introduction With the increasing population, the consumption of fossil fuels is increasing at an alarming rate, leading to the depletion of fossil fuel reserves and affecting the environment badly (Zamfir et al. 2013). The need of the hour is to look for alternative sources of energy. Thus, the current focus is on searching for renewable energy sources like solar, wind, tide, and hydrogen technology to tackle these concerns (Vu et al. 2012). However, the intermittent nature of these sources requires efficient storage systems, which can perform their functionalities at a low cost and meet the required device specifications. Various systems like rechargeable batteries of different chemistries and supercapacitors are being used for energy storage. Due to its ease of operation and high energy density, rechargeable batteries are preferred for electrical energy storage (Mahmood and Hou 2014). Over the years, batteries like lithium-ion, nickel-cadmium, sodium-sulfur, and lead-acid have been explored for storing energy on a large scale. However, none of these have the ideal properties or specifications that make them suitable for multiple applications (Poullikkas 2013). Besides, many other limitations are also associated

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with them. For instance, lead-acid batteries have a finite life and do not perform adequately at extreme temperatures. Nickel-cadmium batteries have a better life cycle than lead-acid but contain heavy and toxic metals that are unsuitable for humans and the environment (Nair and Garimella 2010). On the other hand, lithium-ion batteries have comparatively high power and energy density but pose safety issues. Lithium-ion batteries might catch fire or lead to an explosion at high temperatures, which raises the need to look for other alternatives (Kong et al. 2018). Moreover, their limited energy densities, poor power densities, and instability, mainly controlled by their electrode materials, restricted their applications, especially in the electrification of road market and grid-scale storage. Many electrode materials, electrolyte compositions, and mechanisms have been explored to address these challenges. All these efforts have brought excellent advancement in resolving associated challenges; however, the natural existence of critical elements, like lithium, sodium, etc., is limiting its use and posing high-cost pressure. Therefore, the search for alternative battery chemistries is in dire need. In recent times, zinc-based batteries have become the area of interest in rechargeable batteries because they are relatively inexpensive and present in large abundance in the Earth’s crust. Moreover, Zn is relatively less reactive than Li/Na, hence the ease of handling while manufacturing zinc-based batteries (Chen et al. 2019; Kundu et al. 2018). Numerous types of zinc-based batteries like nickel-zinc/aqueous zinc batteries, alkaline manganese dioxide/zinc batteries, silver-zinc batteries, zinc-air batteries, and zinc-ion batteries are now being used for various applications (Biton et al. 2017; Li et al. 2019; Ming et al. 2019; Parker et al. 2017; Yan et al. 2014). Alkaline manganese dioxide/ zinc batteries are economically feasible in manufacturing, exhibit good performances at varying temperatures, and are environmentally friendly. However, they face poor capacity retention with ongoing cycles, thus, limited life (Kordesh and Weissenbacher 1994). Also, silver-zinc batteries are widely used for energy storage because of its better performance than most available batteries, and its relatively unreactive nature brings good safety. Still, the high cost of silver restricts its applications (Yan et al. 2014). Aqueous zinc batteries also got tremendous attention as the aqueous nature of electrolyte eradicates any threats of fire and explosion, thus introducing high safety, but dissociating water at 1.23 V limits their voltage window, hence, applications (Parker et al. 2017). Besides all these, zinc-ion batteries have become the prominent choice as they have shown high safety, large potential windows, environment-friendly operations, costeffectiveness, and better ion conductivity than other batteries (Fang et al. 2018). Like zinc-ion batteries, zinc-air batteries are also gaining attention because of its large potential window and the reversible redox nature of zinc at comparatively low energy consumption compared to other lighter elements (Pan et al. 2018). Although zinc-based batteries have a high potential to replace expensive lithium technology in some applications, more efforts are needed to achieve the desired level of performance. Therefore, this chapter is inspired by the recent progress in zinc battery technology to provide comprehensive literature to young researchers to look for an innovative solution. This chapter outlines the recent developments that have been done in the field of zinc-based batteries in recent years, the chemical working of the zinc-ion batteries, advantages, and associated challenges with the zinc-based

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batteries. This chapter first describes the working operation of zinc-based batteries, particularly zinc-ion, zinc-air, and aqueous zinc batteries. Afterward, the factors that control the batteries’ performance are discussed. In progress, this chapter discusses zinc batteries’ various advantages, associated challenges, and possible solutions. Then, this chapter highlights the recent progress in developing electrode chemistries for zinc batteries suitable for various electrolytes. In the end, a summary is provided with future perspectives to guide for a future possible solution to the associated challenges.

Operation of Zinc Battery It is well-known that the basic principle of energy storage in batteries is an ionic separation in a closed system; however, the way this ionic separation happens introduces various operation procedures of batteries or even introduces new names to battery types. The operation of different zinc-based batteries is discussed in this section.

Zinc-Ion Battery Research on zinc-ion batteries are now the focal point among researchers because of its advantages over lithium-ion and other metal-ion-based battery systems. Like any other battery, zinc-ion batteries are made up of cathode and anode that are separated by a separator (ionically conductive but electronically nonconductive) and have a copious amount of suitable electrolytes. Generally, the anode comprises zinc metal, an electrolyte consisting of zinc-ions, and a cathode capable of hosting the zinc-ions. The various components and applications of zinc-ion batteries are highlighted in Fig. 1 (Ming et al. 2019). The zinc-ion batteries’ electrolytes can be either nonaqueous or aqueous, giving them a wide range to choose from. When it comes to the cathode, manganese, vanadium, and organic-based cathodes are often used, and among them, manganese-based cathodes are the most promising (Ming et al. 2019). During the discharging process, zinc from the anode moves through the electrolyte toward the cathode by passing the separator. At the same time, the electrons flow via the external circuit to provide energy to the required system. In reverse, during charging, the anode takes the electrons from the external circuit and reverses the deposition of zinc atoms (Xu et al. 2012). The reactions at anode and cathode are represented as Eqs. (1) and (2): Anode : Cathode :

Zn Ð Zn2þ þ 2e

Zn2þ þ 2e þ 2MnO2 Ð ZnMn2 O4

ð1Þ ð2Þ

A cathode is an important component in the zinc-ion battery as it acts as a host for zinc-ions. Therefore, its structure should be flexible to host the large ions without

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Fig. 1 Constituents and applications of zinc-ion battery. (Reproduce with permission from reference (Ming et al. 2019), Copyright © 2019 Elsevier)

structural disintegration and maintain high electronic conductivity to keep the working of the battery alive (Selvakumaran et al. 2019). Both aqueous and nonaqueous types of electrolytes can be used for the zinc-ion battery system. Both have unique advantages, introducing easy operation while the other brings higher energy density (Kundu et al. 2018; Ming et al. 2019).

Zinc-Air Battery Zinc-air batteries are highly in demand because of its high theoretical energy density of 1353 Whkg1 (excluding oxygen) and environment-friendly operation (Zhang et al. 2019). However, the practical energy density of the system is way less and equals 200 Whkg1 (Goldstein et al. 1999). The zinc-air battery system comprises a zinc anode, an air cathode that is generally porous to allow the free passage of air, a membrane separator, and electrolytes shown in Fig. 2a. The reactions involved are as follows: Anode :

Zn þ 4OH

ZnðOHÞ4 2

!

!

ZnðOHÞ4 2 þ

ZnO þ H2 O þ 2OH

2e

ð3Þ ð4Þ

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Fig. 2 (a) Schematic of a zinc-air battery. (Reproduced with permission from reference (Liu et al. 2019), Copyright © 2019 Elsevier). (b) Schematic of an aqueous zinc battery. (Reproduced with permission from reference (Ming et al. 2018), Copyright © 2018 American Chemical Society)

Cathode :

O2 þ 4e þ 2H2 O

Overall reaction : Parasitic reaction :

2Zn

þ O2

Zn þ 2H2 O

!

! !

2ZnO 2ZnO

ZnðOHÞ2 þ H2

ð5Þ ð6Þ ð7Þ

When the zinc-air battery system gets discharged, Zn0 s oxidation forms zincate (Zn(OH)42-), and the process continues until the electrolyte is supersaturated. Upon supersaturation, zincate gives zinc oxide and water as a product of the reaction (Li and Dai 2014). At the anode, along with the oxidation reaction, a simultaneous reaction produces hydrogen gas, which is called a parasitic reaction for the battery and should be avoided for better battery operation. While in the reverse cycle, the deposited ZnO is reduced, thus releasing oxygen for charging of the battery and preventing metal zinc and, most importantly, keeping the passage clear for further intake of oxygen. For better zinc-air battery system performance, it is preferable to use alkaline electrolytes (Li and Dai 2014). Moreover, the membrane separator should have greater ionic conductivity but an electronic nonconductive to attain the highest activity level for the zinc-air battery system (Pan et al. 2018).

Aqueous Zinc Battery Recent studies have shown that the zinc battery systems are most promising in the aqueous electrolytes, therefore, named aqueous batteries. Most importantly, aqueous battery systems have become a choice for energy storage because of its high operational safety, low cost, and environment-friendly nature. Aqueous battery systems are better than nonaqueous systems due to its excellent ionic conductivity. Aqueous electrolytes are used in zinc-ion and zinc-air batteries, and the mechanisms are similar, as shown in Fig. 2b. An aqueous zinc-ion battery consists of an anode

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(zinc), a suitable cathode to host zinc, and an aqueous electrolyte (Fang et al. 2018). Moreover, when it comes to aqueous electrolytes, it is preferable to use either mildly acidic or neutral electrolytes rather than a strong acid as it facilitates the easy formation of zinc oxide; however, using alkaline electrotypes is the best. For instance, in zinc-air batteries, alkaline solutions such as KOH and NaOH are much safer than an acidic solution because of the aggressive response of zinc when it encounters an acidic solution. Moreover, the acidic solutions also corrode the zinc metal, resulting in poor battery stability (Pan et al. 2018). Furthermore, alkaline electrolytes have a high conductivity even at low temperatures, as zinc salts are more soluble in an alkaline medium (Mainar et al. 2016). The selection of acidic, alkaline, or neutral electrolytes generally depends on the electrodes used to construct the battery. Therefore, better consistency in the chemistry of electrode and electrolyte is essential for safe and better battery operation (Ming et al. 2019).

Performance Controlling Factors Interest in zinc-based batteries has been increasing in the last few years, and they are considered an excellent alternative to other metal-based batteries. To better understand its operations, we need to identify the factors that control zinc-based batteries’ performance. In this section, we discussed the factors which play an essential role in improving and disrupting the performance of zinc-based batteries.

Zinc-Ion Battery As explained earlier, every battery system consists of an anode, cathode, and electrolyte. Since the anode of the zinc-ion battery system will always be a zinc metal, the material used for the cathode and the types of electrolyte (aqueous or nonaqueous) are the main factors determining the activity of the zinc-ion battery system, as represented in Fig. 3. The type of material used for cathode also works differently in aqueous and nonaqueous media. For instance, when layered V3O7‧ H2O was employed as a cathode, it performed better in aqueous electrolytes than nonaqueous electrolytes due to the fast charge transfer and high power (Kundu et al. 2018). So, this is an essential factor in deciding the system’s kinetics. In an aqueous electrolyte, the battery performance is highly influenced by the electrolyte nature, whether it is alkaline or mild aqueous. Alkaline medium hinders the use of zinc-ion battery system on a large scale along with heavy battery capacity loss due to corrosion and low Coulombic efficiency. Moreover, the anode reversibility also varies in alkaline and mild aqueous electrolyte. Therefore, Coulombic efficiency and reversibility of the anode influence the performance of the zinc-ion battery system (Zeng et al. 2019). Apart from these, other factors are associated with electrolytes that control the performance of the zinc-ion battery. The electrolyte concentration in the battery and its solvation capacity affect the battery’s performance (Ming et al. 2019). The zinc electrode’s shape, construction, and arrangement

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Fig. 3 Performance controlling factors of zinc-ion battery. (Reproduced with permission from reference (Fang et al. 2018), Copyright © 2018 American Chemical Society)

can also influence the activity of the zinc-ion battery system. Furthermore, the system operating temperature and the different phases of cathode material also affect the system’s performance (Ming et al. 2019). Therefore, electrodes and electrolytes must be selected carefully to have a high-performing zinc battery with long life, and their chemistries should be coherent and compatible.

Zinc-Air Battery In the zinc-air battery, the system’s performance is primarily dependent on the air electrode. The air electrode is sandwiched between the oxygen electrocatalyst and gas layers. Just like the zinc-ion battery, the construction of the cathode greatly influences the battery’s performance. On the other hand, the design and construction of the air electrode also affect the zinc-air battery’s performance (Pan et al. 2018). If the electrode is developed in the required shape, it helps attain a better contact and reduces the electrical resistance within the anode. The size and amount of the zinc particles impact the discharge amount and self-corrosion of the anode hence, affecting the battery’s performance (Li and Dai 2014). Moreover, the air electrode has the potential to carry out both oxygen reduction during discharging and oxygen evolution during charging to maintain the good Coulombic efficiency, which usually does not occur and causes voltage hysteresis and, ultimately, failure of the battery operation.

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Moreover, the type of electrolyte employed also affects the zinc-air battery’s performance. The Coulombic efficiency of the zinc-air battery system in the alkaline electrolyte was found to be less than 50%, thus reflecting the poor battery’s performance (Chen et al. 2019). The poor performance is due to the corrosion and disintegration of inactive zinc in the alkaline electrolyte, causing a change in the electrode configuration and resulting in irreversible dendrite formation and extensive side reactions. Thus, the type of electrolyte used greatly impacts the battery’s performance (Chen et al. 2019). Moreover, the zinc-air battery also needs to be hydrated in case of aqueous electrolyte, which is required to be topped up with water at regular intervals to cover up for the water loss as it works closely with the environment. The loss of water from the battery system affects the battery’s performance and sometimes leads to deterioration of the zinc-air battery system (Li and Dai 2014).

Advantages of Zinc-Based Batteries The abundance of zinc on the Earth’s surface and many other unique advantages of zinc batteries have made it the most suitable alternative to current battery technologies. Moreover, zinc is also safe to use and less reactive than other metals like Li, Na, etc. and has immense energy density. Zinc is also preferred over other metals because zinc battery systems can operate in aqueous and nonaqueous electrolytes. Unlike other metal-based battery systems, the formation of dendrites in zinc batteries can be prevented if mildly acidic or almost neutral electrolytes are used (Ming et al. 2019). For instance, zinc-ion batteries use a mild aqueous electrolyte that is neither harmful nor corrosive and is cost-effective. The aqueous electrolyte also provides high ionic conductivities much better than organic electrolytes. These factors, along with good design and architecture, can develop secure, dependable, environmentally safe, and relatively inexpensive sources of power capable of working at high power and energy density (Xu et al. 2012). Moreover, aqueous zinc battery systems are easier and cheap to manufacture than nonaqueous batteries. Also, they have higher ionic conductivity and reduced risks of incidents due to short circuit/explosion, which is a common risk in the nonaqueous metal-based battery system (Verma et al. 2019). Recently, hybrid electrolytes made up of both aqueous and nonaqueous room temperature ionic liquids are gaining attention for usage in zinc-air batteries. These hybrid electrolytes are nonvolatile with a broad electrochemical window and do not cause poisoning that might arise due to carbon dioxide. Hybrid electrolytes also prevent the formation of zinc dendrites and can provide the benefits of both aqueous and nonaqueous electrolytes (Xu et al. 2015). Data has shown that zinc-ion batteries cost around 70% less than lithium-ion batteries (Xu et al. 2015). Moreover, observations from past incidents have shown that lithium-ion batteries had caught fire during usage. In contrast, zinc batteries are secure and do not catch fire easily as lithium-ion batteries do. Also, zinc is easily accessible and can be recycled, which boosts the long-term use of zinc even at a grid scale (Xu et al. 2015).

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Challenges Associated with Zinc-Based Batteries No matter how perfect a system is, some challenges will always be associated with it. Similarly, zinc-ion battery systems (zinc-ion and zinc-air) also have some issues, which have been discussed in detail in this section, along with their possible solutions.

Zinc-Ion Battery System The challenges faced by the zinc batteries are associated with the zinc anode, cathode material used, and the type of electrolyte used, as presented in Fig. 4.

Low Conductivity The major challenge that a zinc-ion battery system faces is acquiring high conductivity of zinc-ion. The ionic conductivity depends on the electrolyte type and electrode used in the battery cell. Currently used polymer electrolytes possess poor mechanical strength that leads to poor ionic conductivity of the zinc-ion battery system. Overcoming these challenges requires the development of engineered electrodes and highly mobile electrolytes with good mechanical strength and exceptional ionic conductivity (Fang et al. 2018). Meanwhile, the electrical conductivity of used electrodes also needs to be improved by doping with highly conductive elements or making their composites with conductive carbon. However, while developing all these innovative battery components for improving conductivities, the cell structure should be kept in mind.

Fig. 4 Challenges and optimization strategies related to aqueous zinc-ion battery system. (Reproduced with permission from reference (Tang et al. 2019), Copyright © 2019, Royal Society of Chemistry)

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Reversible Stripping Several metal-ion battery systems face reversible stripping due to the corrosion of the anode. The anode is composed of metal, forming layers of inactive sites on the surface and preventing free movement between the anode and electrolyte. The zincion battery system also has poor reversible stripping, but only in the alkaline electrolyte. In an alkaline electrolyte, the anode forms zinc dendrite, as shown in Fig. 5, which reduces the active sites and thus prevents movement between anode and electrolyte. Overcoming this challenge can be achieved by using either an almost neutral or a mild aqueous electrolyte which prevents the formation of zinc dendrites, hence providing the medium for reversible stripping, or by surface treatment of anode which can withstand the alkaline environment (Ming et al. 2019). Zinc dendrite formation normally starts due to the growth of zinc nuclei. Nucleation starts in the areas where the zinc concentration is very high, after which the deposition of zinc-ions occurs where the crystals are already present. This lowers the energy of the surface, which in turn forms zinc dendrites (Fig. 5) (Tang et al. 2019; Zhao et al. 2019). When these dendrites break, they affect the battery’s performance

Fig. 5 Operando investigation of the growth of zinc dendrite, dissolution, and regrowth in 5 M KOH and 0.15 M ZnO aqueous electrolyte. (Reproduced with permission from reference (Yufit et al. 2019), Copyright © 2018 Elsevier)

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because of inactive broken zinc dendrite. Moreover, this can lead to the possibility of a short circuit, which will degrade the performance of the zinc battery (Yang et al. 2019). The zinc dendrite formation can be hindered by current pulses and by generating hydrodynamic conditions in the electrolyte (Naybour 1969; Shaigan et al. 2010). Also, limited dendrite growth is achievable by modulating the electrolyte’s concentration and the temperature and viscosity of the aqueous solution (Barton and Bockris 1962).

Irreversible Consumption and Corrosion of Zinc The corrosion of zinc electrodes greatly influences the battery’s performance. The zinc electrode is prone to corrosion due to the electrolyte’s type, and if it is alkaline, it poses a serious issue of being corroded. Due to the anode corrosion, the affected area of the electrode gets inactive, resulting in a hindrance to the reversible transfer of zinc-ions from an electrolyte to the anode and vice versa, impacting the system’s rechargeability. Sometimes, these problems also arise due to the type of source used. For example, using zinc foil as the anode eventually leads to the wastage and underutilization of zinc. Therefore, it is vital to know the quantity of zinc required to produce the desired electrode accordingly. Moreover, an electrodeposition method to design the desired electrode will limit or prevent zinc wastage (Ming et al. 2019).

Zinc-Air Battery System Similar to the zinc-ion battery systems, certain challenges are also associated with zinc-air battery systems. The challenges and possible solutions related to zinc and air electrodes are discussed below.

Limited Discharge Capacity of Zinc Anode In a zinc-air battery, the electrolyte saturation through taking zinc-ions from the electrode reaches the solubility limit as the zinc oxide starts to get precipitated on the surface of the zinc electrode. Consequently, a film is formed on the anode, preventing it from further discharge and influencing the battery’s performance. Porous zinc electrodes have been investigated as a possible solution to this challenge. However, the pore size decreases as the zinc oxide gets deposited on the electrode’s surface. Despite the decrease in pore size, the ease of Zn2+ diffusion is unhindered and thus improves the battery’s performance by enhancing the anode discharge capacity (Fu et al. 2017). Formation of Carbonates Zinc-air batteries use air to store energy through reduction at one of its electrodes; therefore, it must be an open system to have a continuous flow of air. Since the system is open, it faces the problem of carbonate formation due to the reaction with carbon dioxide. Sometimes there might be threats of the formation of crystals of carbonates on the air electrode surface that will close the electrode’s open spaces and affect its activity. These also impact the ionic conductivity of the battery. To achieve

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better battery efficiency, there is a need to prevent the formation of carbonates. This can be done by scrubbing the inlet air or using an electrolyte that reduces the formation of carbonates. The electrolytes used are generally alkaline, with KOH and NaOH being the most used electrolytes. Due to its higher solubility, KOH can somewhat mitigate the issue of carbonate formation (Li and Dai 2014).

Water Loss Carbonate formation is not the only issue caused due to the open system; this also leads to water loss from the battery system. This problem is only noticed when the electrolyte is in the aqueous phase. The open system has the electrolyte open to the surroundings. As a result, there is water loss from the system, thus affecting the battery’s performance by altering the electrolyte’s concentration and viscosity. Overcoming this challenge requires regular top-up of the electrolyte with water or covering the electrolyte with a gel that prevents water loss from the electrolyte to the surroundings (Li and Dai 2014). Failure of Zinc Electrode A porous zinc electrode is used as a cathode because it proves a high surface area and pores large enough to allow diffusion of Zn2+(Minakshi et al. 2010). The porosity can thus be provided by using the electrode as a granule, powder, or even a zinc fiber, as shown in Fig. 6a–c (Hilder et al. 2012; Pei et al. 2014; Zhang 2006). Zinc electrode failure can occur due to several reasons, including dendrite formation, change in the shape of zinc electrode, passivation, and hydrogen evolution (Fu et al. 2017). When charging the cell, metallic projections in the shape of

Fig. 6 (a–c) Zinc electrode morphology: (a) planar. (Reproduced with permission from reference (Yap et al. 2009), Copyright © 2009 Elsevier) (b) powder. (Reproduced with permission from reference (Hilder et al. 2012), Copyright © 2012 Elsevier) (c) fibers. (Reproduced with permission from reference (Zhang 2006), Copyright © 2006 Elsevier) (d) Zn dendrite growth and H2 evolution. (e) SEM image of Zn deposits (full of dendrites). (d and e are reproduced with permission from reference (Qiu et al. 2019), Copyright © 2019, The Author(s) and Springer Nature)

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needles start growing (dendrite formation; see Fig. 6d, e) and might detach from the electrode, which not only reduces the capacity of the electrode but can also break the membrane to go toward the cathode (Fu et al. 2017). Shape change occurs during the charging-discharging cycle of the cell. This means that zinc moves to the electrolyte, gets dissolved during the discharge, and moves toward the electrode when the charging begins. As a result, the shape of the electrode changes, affecting current distribution as well. Passivation, just like dendrites, affects the performance of the zinc electrodes. It is defined as the formation of a film on the electrode surface, thus preventing the free movement of discharged products and OH ions. Sometimes, to prevent passivation, porous electrodes are used; but in porous electrodes, it first decreases the porosity and then starts forming films on the electrode surface. Altering electrolytes’ flow rate is a strategy that can be adopted to reduce passivation (Alcazar et al. 1987). Owing to the standard reduction potential of zinc(1.26 V) and standard hydrogen electrodes (0.83 V) at a pH of 14, the corrosion of the anode occurs when the zinc electrode is at rest because of thermodynamic favoritism of the hydrogen evolution. This means that hydrogen evolution reaction will use some electrons which are to be consumed by zinc electrode, thus preventing 100% Coulombic efficiency. These factors combine to affect the zinc electrode and, hence, the performance of the zinc battery (Fu et al. 2017).

Electrode Materials for Zinc-Based Batteries Electrodes are the critical pillars of any electrochemical energy storage system, which not only control the performance of the systems but also influence the cost as ~25% of the system cost is comprised of electrode materials. Like other systems, enormous efforts have been put forward in developing low-cost, durable, and highperforming electrode materials both for cathode and anode to resolve the issues associated with zinc-based batteries, as shown in Fig. 7. In the subsequent section, we discussed the strategies that have been adopted in modifying zinc anode and cathode materials.

Anode Materials Safety is the critical parameter for future battery technology. It is well-known that organic electrolyte-based batteries like lithium-ion batteries are easy to catch fire, while the batteries using aqueous electrolytes are comparatively safer (Fig. 8a and b) (Wu et al. 2019). Moreover, the aqueous batteries also possess higher ionic conductivities, bringing higher power. However, the low electrochemical stability of water (1.23 V) leads to low operating cell voltage, thus limited energy densities. Among several metals that are/can be stable in water, zinc possesses higher stability, better activity, and the lowest possible operating potential. Moreover, zinc has a large abundance globally; thus, it is very cost-effective ($3.19 per kg) and possesses a high

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Fig. 7 Diagrammatic illustration of Zn-air battery along with structure of GDE, potential cathode catalysts candidates, and Zn anode configurations. (Reproduced with permission from reference (Zhang et al. 2019), Copyright © 2019 Royal Society of Chemistry)

capacity (5854 Ah/L and 820 Ah/kg). In addition, zinc enables redox reaction in ambient air and thus can also be utilized in zinc-air batteries (Wu et al. 2019). Figure 8c compares the energy densities of several battery technologies, showing that zinc-based batteries can compete with the existing ones. The use of zinc metal as an anode is a challenge in the battery’s rechargeability due to the formation of dendrites like other systems, dissolution of metal, formation of the passivation layer, and the hydrogen evolution as it is an aqueous system. Therefore, to fulfill the dream of high energy storage zinc batteries, especially to enable them for >50% of depth discharge and cycle life of >400 cycles with Coulombic efficiency of >80%, engineered zinc anode is highly desirable. The structural modification, compositional tuning, surface manipulation, and composite formation of zinc-based anode through nanoscale designing can enable zinc-based batteries to achieve the aforementioned goals, as shown in Fig. 8d (Wu et al. 2019). Undoubtedly, nanoengineering has offered exciting and novel opportunities to improve the electrochemical performances of zinc anode. It has been found that developing high surface area zinc anode can help introduce fast and reversible Zn/Zn2+ redox kinetics while mitigating the zinc dendrite formation during longterm cycles (Chao et al. 2018). For example, the creation of Zn nanoflake resulted in high performance by delivering a peak power density of ~5.1 kW/kg and an energy density of ~115 Wh/kg and controlling severe aggregation, pulverization, and dendrite growth (Chao et al. 2018). Generally, used zinc anodes are excessive, resulting in underutilization of zinc, hence, a limited energy density and creating a hurdle in their practical applications. A 3D structural transformation of zinc can resolve issues like underutilization by exposing the entire zinc to redox reaction,

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Fig. 8 Safety property investigation using the battery penetration test and (a) organic and (b) aqueous electrolytes. (c) Volumetric capacity and theoretical specific energy density of Zn and Li batteries. CoO2 and LiC6 were used to calculate the lithium-ion batteries’ energy densities. (d) Diagrammatic illustration of electrolyte design and Zn anodes in nanoscale in aqueous zinc batteries. (Reproduced with permission from reference (Wu et al. 2019), Copyright © 2019 Elsevier)

preventing structural changes, even deposition, and stripping better and faster mass flow. Recently, Kang et al. (2019) have developed a 3D zinc anode through electrochemical deposition on a chemically modified porous copper skeleton surface. A high-performing zinc battery was assembled on coupling with the as-obtained highperforming MnO2 nanosheet-based cathode, as shown in Fig. 9a. The presence of porous copper 3D skeleton not only ensures homogenous deposition of zinc but also assures a high conductivity of the resulting electrode. As a result, zinc anode exhibited stable performance with 100% Coulombic efficiency for continuous

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Fig. 9 Full cell electrochemical activity. (a) Illustration of the cathode (MnO2 nanosheet) and anode (3D Zn) full cell. (b) Specific capacities at 0.1 A/g and various electrolytes. (c) CV curve at 0.5 mV/s in 2 M ZnSO4 + 0.5 M MnSO4 electrolyte. (d) Full cell cycling performance with anode (planar Zn foil or 3D Zn) at 0.4 A/g in 2 M ZnSO4 + 0.5 M MnSO4 electrolyte. (Reproduced with permission from reference (Kang et al. 2019), Copyright © 2019 American Chemical Society)

350 h of deposition/stripping due to reduced polarization, enabling fast electrochemical kinetics. Moreover, comparing the electrochemical response of the 3D zinc anode and planner zinc strip, it was evident that the 3D design not only improves the electrochemical kinetics of the battery but also improves the performance and stability. Three-dimensional zinc anode-based battery delivered the capacity of 235 mA h g1 at 0.1 A/g (based on the mass of MnO2 cathode) and high stability over 300 cycles, while planer zinc anode lost its performance after 100 cycles (Fig. 9b–d) (Kang et al. 2019).

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The notorious formation of dendrites and electrochemical and thermodynamic instability of zinc in routine aqueous electrolytes are still a significant bottleneck in practical applications as rechargeable batteries. The suppression of dendrite is challenging as the initial nucleation is localized. Still, the newly plated crystals prefer to grow on the existing ones to reduce the surface energy, as shown in Fig. 10a (Zhao et al. 2019). Thus, the as-formed brittle dendrites result in dead zinc in the systems and rapture the separator, consequently causing the inner short circuit of the cell. Another Achilles heel in the actual reversibility is the incessant Faradaic and non-Faradaic side reaction. Moreover, according to the Pourbaix diagram, zinc metal is thermodynamically unstable and further ignited by oxygen and aqueous electrolytes, while hydrogen evolution further caused the formation of insulating products like hydroxides or zincates and inferred the deposition of zinc. Developing a 3D zinc anode cannot be the only solution to all these challenges; better design or surface protection is required to provide thermodynamic stability to prevent the dendrite formation and suppress the production of insulting by-products. Recently, it has been found that developing multifunctional polymeric interphase can enable zinc-based aqueous electrochemistry to store high energy with excellent stability and safety. For instance, Zhao et al. (2019) have utilized polyamide and zinc-trifluoromethanesulfonate (Zn(TfO)2), having the ability to form a network through hydrogen bond and coordinate with zinc-ions as shown in Fig. 10b (Zhao et al. 2019). Such artificial polymeric interphases act as a brightener to harmonize the metal-ion migration with uniform nucleation and control the Faradaic and non-Faradaic side reactions by acting as an inhibitor for water/O2. A high capacity of 10 mA h cm2 was achieved by subduing dendrite formation and stabilizing the zinc anode. At the same time, control over side reaction also results in excellent stability over 8000 cycles at 0.5 mA/cm2, as shown in Fig. 10c (Zhao et al. 2019). Equally, 88% capacity retention was observed after 1000 cycles with 99% Coulombic efficiency. Moreover, compared with zinc plate and foil, the coated zinc anode surpasses a stable electrochemical response without a barrier to ionic diffusion as performance matches the theoretical value of 85% depth of discharge (Fig. 10d–f) (Zhao et al. 2019). The excellent results demonstrate that controlled and functional surface coatings can enhance the zinc anode’s performance and stability. Undoubtedly, surface protection with polymeric materials can improve the homogenous deposition of zinc and prevent its dissolution; however, poor conductivity limits the capacity. Therefore, such electrode designs are highly required which can prevent the formation of an insulating layer through solid-solute-solid transformation, prevent the dissolution of its by-products into the electrolyte, and maintain high electrical conductivity to realize genuine rechargeable zinc-ion battery, as shown in Fig. 11a and b, respectively. In this regard, recently, Zhou et al. (2019) have prepared a composite to act as an anode for zinc battery by coating the zinc mesh with graphene oxide (Zn@GO) through a solution casting. The GO has multiple advantages as it can act like a sieve that allows only smaller ions to cross while blocking larger ions that maintain the faster movement of zinc-ions while preventing the dissolution of by-products. Furthermore, it can be slightly reduced in the alkaline solution and thus can maintain high conductivity. In addition, it can

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Fig. 10 Diagrammatic illustration for deposition of Zn. (a) The mechanism for zinc dendrite growth. (b) Coating of the PA layer leads to dendrite-free and dense morphology formation by nucleus size refinement, nucleus density increments as a result of restraining mass diffusion of the 2D material, and H2O and O2 permeation inhibition. (c) Symmetrical zinc cells’ long-term galvanostatic cycling for coated zinc plates and uncoated zinc plates at a current density of 0.5 mA cm2. (d) Symmetrical zinc cell cycling activity with zinc foil at a current density of

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entrap the as-formed ZnO inside the sheets and thus can prevent dendrite formations and introduce higher overall conductivity to the electrode. GO coating ensures the following benefits: i) Block the Zn(OH)42 inside the layers while allowing free movements of H2O and OH. ii) The zincate can make a hydrogen bond with the oxygen groups of GO and thus can firmly attach to the sheets which later on converts to ZnO; thus, ZnO remains inside the sheets of GO and does not impact the conductivity and initiates the formation of dendrites. The ex situ scanning electron images of GO coated and uncoated zinc anode before and after electrochemical testing clearly show the advantage of GO coating on the surface of zinc to enable for more extended stability, as shown in Fig. 11c–e (Zhou et al. 2019). The control over the surface chemistry of zinc through GO coating has shown its long-lasting benefits by protecting the surface of the zinc electrode during discharge/charge to ensure the zinc battery’s rechargeability. Carbon shells are typically utilized to prevent zinc’s surface side reactions and dissolution, stabilizing the alkaline electrolytes and improving the conductivity, as shown in Fig. 12a and b (Chen et al. 2018). Interestingly carbon is widely used for this purpose due to its high porosity to act as a sieve. Its inert nature will prevent reaction with the electrolyte and high conductivities. However, simple construction of core-shell structure or coating preventive layers did not reach the desired properties required for high-performing electrodes like maximum capacity close to that of theoretical values, >85% depth of discharge, and long cyclic life with high capacity retention and Coulombic efficiency. Therefore, 3D designing at the nanoscale with preventive coating, in a nutshell, needs to combine all the above possibilities into one structure. For example, Chen et al. (2018) designed a unique structure resembling pomegranate by coating individual ZnO (Zn-pome) particles in a carbon core shell. Subsequently, these core-shell seeds are connected to an external protective layer of carbon shell to keep them in united clusters with a specific distance among each particle, as shown in Fig. 12c and d (Chen et al. 2018). A simple microemulsion process was adopted to design a hierarchical structure using 1-octadecene and annealed to obtain carbon-coated zinc clusters, which were then coated with dopamine to get a pomegranate-like structure through additional annealing. This design can effectively prevent the issue of the passivation layer, which usually limits enough depth of discharge due to nanoscale feature while preventing dissolution due to protective layer on each particle. The overall coat brings high electrical conductivity and mechanical strength so that any pulverization can be handled. As a result, the pome-like zinc anode delivers excellent performance by delivering a much improved capacity of 400 mAh/g after 45 cycles. In comparison, Zn particlebased anode only delivers 186 mAh/g, thus clearly showing excellent performance

ä Fig. 10 (continued) 10 mA cm2 and a capacity of 10 mAh cm2. (e) Coulombic efficiency of plated zinc on coated Ti foil and uncoated Ti foil at different cycles. (f) Associated voltage profiles of coated Ti foil. (Reproduced with permission from reference (Zhao et al. 2019), Copyright © 2019, Royal Society of Chemistry)

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Fig. 11 Diagrammatic illustrations of Zn electrode morphological changes during electrochemical cycling. (a) Passivation of the ZnO surface results in underutilization of the zinc mesh anode. (b) The presence of GO in the complex of Zn@GO facilitates rapid movement of electrons across the ZnO and slows down Zn species dissolution. (c–e) Anode characterization before and after ten galvanostatic cycles. (c) SEM images of unmodified zinc anode before and after cycling. (d) SEM images of Zn@GO anode before and after cycling. (e) SEM images and EDS mapping of the Zn@GO anode after cycling. (Reproduced with permission from reference (Zhou et al. 2019), Copyright © 2018 Elsevier)

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Fig. 12 Diagrammatic illustrations of the zinc pomegranate design. (a) ZnO nanoparticles with a fast dissolution rate in aqueous alkaline solution. (b) Carbon coated ZnO nanoparticles. (c) Zn-pome in which the voids of ZnO clusters are filled with carbon and play an important role in structure stabilization, conductivity, and ion sieving of the electrode. (d) The number of nanoparticles in one

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of the pome due to the unique advantages of the special design (Fig. 12e and f) (Chen et al. 2018). Interestingly, the Zn-pome-based anode was very stable under the rate capacity test and outperformed the bare zinc anode (Fig. 12g and h) (Chen et al. 2018). Moreover, Chen et al. (2018) also carried out an ex situ microscopic analysis of the Zn-pome-based anode after ten charge-discharge cycles. They found that it well-maintained its morphology (Fig. 12i–k), which again highlights the key advantages of such structural and morphological designing over traditional ones to get stable and high performances for rechargeable zinc batteries (Chen et al. 2018). Besides designing innovative zinc electrodes, it is suggested that changing the composition and chemistry of electrolytes is possible; therefore, various electrolyte additives are being invented to stabilize the performance of zinc batteries. For example, Wang et al. (2018) reported a unique electrolyte based on zinc and lithium metal salts (1 m Zn(TFSI)2 + 20 m LiTFSI). They found that this enabled the dendrite-free zinc chemistry (plating/stripping) and brought 100% Coulombic efficiency with retaining water in the atmosphere, thus making hematic cell optional (Wang et al. 2018). When tested with LiMn2O4 or O2, the zinc electrode delivers 180 Wh/kg and retains 80% of its capacity over >4000 cycles. This shows another approach where excellent performance for zinc-based batteries can be realized. Moreover, it is also found that by developing the appropriate cathode chemistries, the battery’s overall performance can be improved, and challenges associated with the zinc side can also be addressed. Therefore, the next section of this chapter will discuss the recent advancement in cathode chemistry, both finding new candidates and enhancing the performances of existing ones.

Cathode Materials In a typical Zn battery, Zn-storage sites are supplied by the cathode, thus determining the voltage and capacity of the battery. But the major bottleneck hampering the practical application of Zn battery is the absence of suitable cathode materials with good cyclability, high operating voltage, reversible capacity, and a morphological structure that will limit the diffusion resistance of Zn2+ during charging/discharge (Zhang et al. 2021; Zhang et al. 2020). Different materials have been scouted as potential cathode materials for Zn batteries to address these challenges and are discussed below. For ease of understanding, these materials are grouped into manganese oxide-based, vanadium oxide-based, and Prussian blue analogs. ä Fig. 12 (continued) Zn-pome cluster and calculated specific area in contact with electrolyte vs. its diameter. The smaller the contact area with the electrolyte, the lower the capacity fading. (f) Zn-pome and ZnO nanoparticle specific capacities. (g) Voltage profiles of Zn-pome/Ni(OH)2. (h) Zn-pome/Ni(OH)2 and ZnO NPs/Ni(OH)2 specific capacities at a 5C discharge rate. (i) Cycling at 1C after resting for 24 h. (j, k, and l) SEM image of Zn-pome anode (j) before cycling, (k) after cycling, and (l) after two cycles. (Reproduced with permission from reference (Chen et al. 2018), Copyright © 2018, Royal Society of Chemistry)

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Manganese (Mn) Oxide-Based Cathodes The advantages of manganese, such as its abundance in the Earth’s crust, high redox potentials, low cost, and environmental friendliness, have facilitated extensive research on using manganese oxides as potential cathode materials for Zn batteries. Another factor that attracts the usage of manganese oxides is the presence of crystals with large channels that allows the easy diffusion of Zn2+ ions, thus limiting diffusion resistance. Among the oxides of Mn (MnO2, Mn2O3, and Mn3O4), MnO2 has attracted the most attention, and this is as a result of its relatively high specific capacity (308 mA h g1) and modest operating voltage (~1.35 V) (Chao et al. 2019; Huang et al. 2018; Pan et al. 2016; Zhang et al. 2020; Zhang et al. 2017). A typical MnO2 comprises polymorph crystals containing octahedral-shaped MnO6 linked by sharing corners/edges as the building block. These polymorphs of MnO2 are divided into α-, β-, and γ-MnO2 (tunnel-type structures), λ- MnO2 (spinel-type structures), and δ-MnO2 (layered-type structures) and have been investigated as potential cathode materials for Zn batteries (Wang and Zheng 2020).

Tunnel Structured MnO2 The tunnel-type structured MnO2 are α- (with large 2 x 2 tunnels and size of 4.6 Å), β- (with 1 x 1 tunnel and size of 1.89 Å), and γ- (with 1 x 2 tunnel and size of 2.3 Å), and their structure and size significantly influence their performances as cathode materials (Li et al. 2020). Taking advantage of its large tunnel and size, Xu et al. (2009, 2012) synthesized α-MnO2 using a self-reacting microemulsion method. They reported a capacity of 210 mA h g1 and discharge voltage of 1.3 V when used as cathode material in aqueous ZIB system. Furthermore, they reported capacity retention of ~100% at a discharge/charge rate of 6C after 100 cycles. Although the reported results are decent, MnO2 cathodes in Zn/MnO2 batteries can only be cycled a finite number of times in aqueous electrolytes as they undergo rapid capacity fading during longer cycles and mediocre activity at higher current density. The capacity fading is due to structural changes (phase transformation of MnO2 from tunneled to layered polymorphs) caused by the dissolution of Mn from α-MnO2 during the discharge process to form layered Zn-birnessite (2Mn3+ ! Mn2+ + Mn4+) (Lee et al. 2014; Pan et al. 2016; Wang and Zheng 2020; Zhang et al. 2020). To address this challenge, Pan et al. (2016) pre-added Mn2+ salt (0.1 M MnSO4) into the electrolyte (2 M ZnSO4) to help in inhibiting the dissolution reaction, thus protecting the integrity of the α-MnO2 cathode. They reported a capacity of 285 mA h g1, an operating potential of 1.44 V, and capacity retention of 92% after 5000 cycles, which is an improved performance compared to previously reported data. Among the tunnel-type structured MnO2, bulk β-MnO2 was considered unfavorable for usage as a cathode material in ZIBs because its narrow tunnel and size would limit the de/intercalation of Zn2+. However, by adapting the knowledge of synthesis of nanostructured materials, nanorods of β-MnO2 are now being synthesized and explored as a potential cathode material in ZIBs. Islam et al. (2017) synthesized β-MnO2 nanorods with exposed (101) planes and investigated the performance as a

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cathode in ZIB. They reported a capacity discharge of 270 mA h g1 at 100 mA g1 and a rate capability of 123 mA h g1 at 528 mA g1. Also, capacity retention of 75% and Coulombic efficiency of 100% at 200 mA g1 over 200 cycles were reported. However, just like the α-MnO2, the β-MnO2 nanorods undergo a structural transformation from a tunnel structure into a spinel structure during longer cycles, thus the need for pre-Mn2+ salt addition into the electrolyte. By employing a 3 M Zn (CF3SO3)2 electrolyte and Mn(CF3SO3)2 additive, Zhang et al. (2017) reported a reversible capacity of 225 mA h g1 at 0.65C and capacity retention of 94% over 2000 cycles when β-MnO2 nanorods were used as cathode in Zn/MnO2 battery. Pre-addition of Mn(CF3SO3)2 helps prevent the cathode’s dissolution by forming a protective layer of porous MnOx nanosheets around the cathode surface, thus maintaining the integrity of the electrode. Unlike β-MnO2, the γ-MnO2 has a wider tunnel and size and has shown to be a worthy cathode material in Zn batteries (Alfaruqi et al. 2015b; Kumar and Sampath 2003). Alfaruqi et al. (2015b) synthesized mesoporous γ-MnO2 with a high surface area and studied its suitability as a cathode material in rechargeable ZIB. They reported a capacity of 285 mA h g1 at 0.05 mA/cm2 with a peak at ~1.25 V vs Zn/Zn2+. Also, the structural evolution of the γ-MnO2 during the electrochemical reaction was probed using in situ XANES and in situ XRD. The results show that the synthesized tunnel-type γ-MnO2 undergoes a structural change to spinel ZnMn2O4 (Mn(III) phase) and intermediate Mn(II) phases (layered-type L-ZnyMnO2 and tunnel-type γ-ZnxMnO2), and all the phases coexisted after complete electrochemical Zn insertion (Fig. 13a). On successive zinc de/intercalation, most of the phases change back to the parent γ-MnO2. Ex situ HR-TEM analysis (Fig. 13b) of the recouped cathode after the initial discharge cycle confirms the structural transformation and coexistence of all the phases during the electrochemical reaction. Spinel Structured MnO2 (λ-MnO2) As previously mentioned, MnO2 consists of MnO6 octahedral subunits sharing vertices and edges, and the position of the Mn and O determines the structure type. In a typical spinel structured MnO2 (λ- MnO2), the Mn and O ions are located at the octahedral 16d sites and 32e sites, respectively. At 13.8 mA g1, a capacity of 442.6 mA h g1 was recorded in an aqueous ZnSO4 electrolyte when λ-MnO2 was used as a cathode for ZIB (Yuan et al. 2014). On the other hand, studies have shown that the perfect spinel-type λ-MnO2 is not a suitable cathode material for rechargeable ZIBs because of electrostatic repulsion during Zn2+ intercalation. To address this issue, the creation of vacancies has been reported for easier diffusion of Zn2+ during intercalation (Knight et al. 2015; Zhang et al. 2016). Layered Structured MnO2 (δ-MnO2) The δ-MnO2 comprises of loosely bonded layers of octahedral-shaped MnO6 linked by sharing corners/edges as the building block and is situated at the (001) crystallographic plane. The δ-MnO2 is characterized by an interlayer spacing of ~0.7 nm large enough to accommodate Zn2+ during de/intercalation (Li et al. 2020; Wang and Zheng 2020). Taking advantage of the large interlayer spacing, layered-type

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Fig. 13 (a) Diagrammatic demonstration of Zn insertion reaction pathway in the γ-MnO2 cathode. (b) HR-TEM image of the recouped cathode after initial discharge cycle completion. (Reproduced with permission from reference (Alfaruqi et al. 2015b), Copyright © 2015, American Chemical Society)

nanoflake δ-MnO2 was studied as a potential cathode material for aqueous ZIB. At a current density of 83 mA g1, a capacity of 252 mA h g1 with ~100% Coulombic efficiency over 100 cycles was recorded (Alfaruqi et al. 2015a). By probing the reaction mechanism using inductively coupled plasma (ICP) and XRD, a structural transformation of δ-MnO2 was observed. The new phases formed are Mn(III) phase (from spinel-type ZnMn2O4) and Mn(II) phase (layered-type δ-ZnxMnO2). Moreover, the absence of irreversible phases like MnOOH, Mn(OH)2, Mn3O4, Mn2O3,

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and ZnO is an indication that there were no side reactions at the cathode. Using a nonaqueous electrolyte, hydrated δ-MnO2 (K0.11MnO2.0.7H2O) was studied as a prospective cathode material for rechargeable ZIB (Sang-Don et al. 2017). A capacity of ~100 mA h g1 and reversible Coulombic efficiency of ~100% over 50+ cycles with an operating voltage of 1.2 V vs. Zn/Zn2+ were reported. Like the other types of MnO2, the δ-MnO2 exhibits capacity fading and structural transformation over long-term cycling.

Vanadium Oxide-Based Cathodes Vanadium oxides are being studied as a potential cathode material for Zn batteries because of its multivalence nature, availability, and high capacity (up to 400 mA h g1) but have a low working voltage of ~0.8 V vs. Zn2+/Zn. Furthermore, vanadium oxides are made up of polyhedral (VO4 tetrahedron, VO5 square pyramid and trigonal bipyramid, and VO6 octahedra) that are joined at the edges to form 3D layers or chain frameworks. These frameworks are large enough to allow for unhindered diffusion of ions during reversible Zn2+ de/intercalation. In addition, vanadium oxide frameworks sometimes contain interlayer cations like Na+, K+, Zn2+, etc. and water molecules, which help stabilize the crystal structure (Ming et al. 2019; Wan and Niu 2019). Taking advantage of its interlayer spacing, V2O5 was applied as a cathode material for ZIB using a 3 M ZnSO4 aqueous electrolyte. Though V2O5 interlayer spacing increases from 0.58 nm to 0.63 nm in the electrolyte, a low capacity of 224 mA h g1 at 0.1 A/g over 400 cycles at 1 A/g was recorded (Zhou et al. 2018). On the other hand, hydrated V2O5 has a larger interlayer spacing (~1.35 nm), resulting from incorporated water molecules, and can allow for easy diffusion of Zn2+. At 0.2 A g1, a capacity of 470 mA h g1 and 91.1% capacity retention over 4000 cycles at 5 A g1 were reported with hydrated V2O5 as the cathode and 3 M Zn(CF3SO3)2 as aqueous electrolyte. Though all the Zn2+ cannot be deintercalated during recharging, the leftover Zn2+ can act as interlayer pillars, thus keeping the structure of the hydrated V2O5 stable during the dis/charging process (Hu et al. 2017). The usage of Zn0.25V2O5.nH2O, which is a pillared V2O5, as cathode material for rechargeable ZIB using 1 M aqueous ZnSO4 as electrolyte was relayed by Kundu et al. (2016) The Zn0.25V2O5.nH2O exhibited a nanobelt morphology with ~10 μm length, 100–150 nm width, and 10–20 nm thickness, which facilitated a fast de/ interaction of Zn2+. A fast reversible Zn2+ de/intercalation storage activity with a capacity of ~300 mA h g1 was reported. Moreover, an energy density of ~450 Wh l1 and over 80% capacity retention after 1000 cycles at a rate of 8C were recorded for the ZIB. Structural analysis of the cathode (Zn0.25V2O5.nH2O) shows that there was an increase in interlayer distance (from 10.8 to 12.9 Å) upon immersion in the aqueous electrolyte, and the increment was ascribed to water molecule intercalation. A similar influence of water molecule intercalation was observed by Yan et al. (2018) and Sun et al. (2021), and the general conclusion is that the intercalated water molecules increase the interlayer spacing while the H2O-solvated Zn2+ significantly decreases the ionic charges, which leads to decrease

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in the electrostatic interactions within the V2O5 framework, thus reducing the diffusion resistance. Other vanadium oxide-based cathode materials investigated for ZIBs include MxVO4, MxVO2, MxV2O7, MxV3O7, and MxV3O8 (M ¼ NH4+ or metal ions). These oxides exhibit promising results, thus making them potential cathode materials for Zn battery. The presence of Zn2+ and water molecules as pillars in the vanadium oxide structures helps in modifying the layered structure while increasing the interlayer spacing and thus affects the electrode and intercalation chemistry (Wan and Niu 2019). Another strategy that has been adopted in improving the electrode performance is the introduction of conducting component (graphene, carbon nanotubes, or carbon black) since vanadium oxides are generally characterized by poor electrical conductivity (Pang et al. 2018; Yan et al. 2018). Remarkably, a composite of H2V3O8 nanowires/graphene exhibits higher Zn2+ storage performance with a high reversible capacity of 394 mA h g1 at 1/3 C, a 270 mA h g1 rate capability of 20 C, and 87% capacity retention over 2000 cycles. The exceptional performance of the cathode was attributed to the synergistic effect of H2V3O8 nanowires’ structural and graphene conductive properties (Pang et al. 2018).

Prussian Blue Analogs (PBAs) PBAs are characterized by large 3D open-framework features, ample redox-active sites, and strong structural stabilities. They have a general formula of AxM[M’ (CN)6]y.nH2O, where A refers to Na+ and K+ and M and M’ are redox-active ions of transition metal (M and M’ ¼ Cu, Ni, Co, Fe, V, Mo, Mn, Co, Cr, Zn, and Ru). A typical PBA has a face-cantered cubic (fcc) structure, in which M and M’ bond with N and C atoms forming MC6 and M’N6, respectively, at alternate corners of the structure. Subsequently, MC6 and M’N6 are linked by C  N bridges creating a 3D opened structure framework (Ma et al. 2019; Padigi et al. 2015; Trócoli and La Mantia 2015). Due to large tunnels in its 3D framework, PBAs allow for reversible electrochemical de/intercalation of different ions. Furthermore, the de/intercalation mechanism is generally influenced by the type of heteroatoms and water molecules present in the PBAs. Some of the already studied PBAs thus far as potential cathode for Zn batteries include hexacyanoferrates of copper (CuHCF), zinc (ZnHCF), nickel (NiHCF), iron (FeHCF), and cobalt (CoHCF). For example, Trócoli and La Mantia (2015) applied CuHCF as a cathode material, with the electrolyte being a 20 mM solution of ZnSO4 for ZIB. A 96.3% capacity retention over 100 cycles at 1C and a voltage of 1.73 V were reported. Though there has been some good output on the use of PBAs as a potential cathode in Zn batteries, especially with the operating potential (>1.7 V), however, the major bottleneck has been the unsatisfactory specific capacity (20% comes only with higher prices. Multiple issues concerning batteries are like shorter battery life, the large and heavy nature of the cells, the problem of disposal, and the requirement of wide storage space. Also, solar panels require rare and precious metals such as silver, tellurium, and indium. There is a shortage of both skilled manpower to establish and maintain solar panels and a lack of technical know-how by the users, especially in the rural areas. External factors such as pollution, water intrusion, dust, and algal growth reduce the efficiency of the system. Dumping and recycling of a toxic by-product called silicon tetrachloride is an additional concern. The most obvious limitations of solar energy are that it can’t be relied upon easily as it depends on the time of the day, sunlight exposures, weather, and climatic conditions. Finally, it also requires wide plots of land for the large-scale generation of solar power. China is the world’s leading solar energy generator. However, with the newly introduced energy supply systems, it is necessary to investigate and comprehend the long-term viability of such renewable energy technologies, as well as to prevent any potential environmental consequences. With life cycle assessment (LCA) and environmental impact analysis for currently available and emerging solar photovoltaic (PV) and concentrated solar power (CSP) systems, this paper tackles the environmental consequences of the technologies (Rabaia et al. 2021). Many countries like Japan, Germany, and European countries had started working on the technology to reduce the cost of using solar energy to produce electricity. They had encouraged the investors to fund the solar energy market and made schemes and policies to promote solar energy usage. After Japan, China went on to take initiatives on using solar energy, and, currently, it has the first position in solar energy consumption followed by the USA and India. According to the data, the largest solar farms in the world are in China. But China has focused only on the large-scale solar farms, not on the small- and medium-scale solar farms. The USA has also spent heavily on the development of solar energy. But only a small portion of total energy consumption is dedicated to solar energy. There is also a lack of implementation of laws and policies to promote the production of clean energy. The US policies have made the advancement of cleaner use of fossil fuels. India, a developing nation, is also a leading producer of solar energy. But India also faces the same problem as the USA. Only 2.28% of the total energy within the domestic boundary was allocated to renewable energy. Another obstacle is the minor investments in solar energy by nongovernment institutions. There are also climatic and economic barriers. One of them is the excessive dependency on climatic conditions. There are only a few places where the climatic conditions such as the angle of incidence of the Sun’s rays and daytime for the installation of solar panels for energy generation are suitable. Another problem is the costly storage of the electricity. The energy is stored in batteries before moving for the distribution which is quite expensive even after the technological advancements. So to reduce the costs incurred, solar energy is combined with other forms of energy. The cost of

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generating electricity using solar energy is significantly higher than generating using other sources of energy like wind, hydro, and thermal energy and fossil fuels. The total costs in the year 2008, it was found out that the cost to generate 1 megawatt of electricity using the solar panels is around 6.6 times the cost incurred using hydro systems to generate energy and nearly 2.5 times as much as using wind turbines to generate electricity. It is approximately 4.5 times the cost of generating electricity by burning coal (Rabaia et al. 2021). Liu et al. analyzed the policies implemented by the big countries India, China, and the USA to develop solar energy. More policy and law reforms are needed to increase solar energy usage. China holds the number 1 spot due to government support and technological developments. Then the economic barriers were analyzed which indicated the need to enhance technology to reduce costs. This source of energy may become a major source due to rising environmental concerns, government policies, and increased demand for electricity in both domestic and commercial sectors. It is highly recommended to use a hybrid of solar energy combined with other forms of energy like wind, hydro, and thermal energy.

Issues, Recommendations, and Conclusion The renewable energy technologies were ranked against each other after the evaluation of specified indicators, with each indicator given equal weight (Evans et al. 2009). Price of electricity generation, GHG emissions, water consumption, and technological limitations are major indicators. Table 2 shows different advantages and disadvantages of renewable energies. The most sustainable energy (Fig. 3) source was found to be wind, followed by hydropower, photovoltaics, and geothermal energy. The relative ranking was developed using information obtained from a variety of sources and only considers worldwide international events (Evans et al. 2009). In terms of the technology and energy sources they support, as well as the policy tools that enable them, renewable energy policies vary widely. Levenda et al. (2021) conducted research to establish a list of confirmed environmental justice Table 2 Different advantages and disadvantages of renewable energies Advantages Clean air and water Zero carbon emission Environmentally friendly Solution for huge global energy scarcity New field for employment generation and entrepreneurship

Disadvantages Higher capital cost Unreliable electricity generation Energy storage issues Affected by different weather conditions Still carbon footprints are present Maintenance cost After end life disposal and recyclability issues

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Fig. 3 Sustainability of energy

(EJ) implications associated with renewable energy technology, which have been implemented into several renewable energy policies throughout the world. Throughout this heterogeneous global environment of renewable energy policy, a few of common technologies have proven EJ effects. The purpose was to ensure that problems of justice are considered while developing and implementing renewable energy regulations. Levenda et al. (2021) conducted a thorough review of the literature on renewable energy technologies from the perspectives of EJ’s distributive, procedural, recognition, and capacity interpretations. Unfavorable economics cannot be sustained without taking into account the price of power producing units. Because some technologies or fuels may be severely resource constrained, it is important to examine their availability and restrictions. In order to make meaningful comparisons, the efficiency of energy conversion must be known. Process needs, capital, and operational expenses will all be lower in efficient processes. Processes that are less efficient may provide more possibility for technological innovation and improvement. Renewable energy technologies are sometimes alleged to compete with agriculturally arable land or to alter biodiversity; hence, land use requirements are necessary. Energies such as solar panels and wind turbines must become less expensive than fossil fuels in order to be competitive. The spreading nature of renewables presents

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this challenge. Storage solutions must become more cost-effective than fossil-fuel refineries (e.g., power plants). The intermittent nature of renewables poses this difficulty. For power generating, intermittent supply is a big challenge. Solar energy can only be used during daylight hours. The wind speed varies greatly. Along with storage technology, solutions are not yet fully matured. Commercialization is a roadblock in the face of old, mature technology. Existing subsidies and unequal tax loads cause price distortion. Market barriers include a lack of information, a lack of capital, and so on. One of the most recent biogas applications is hydrogen production utilizing a high-efficiency biogas reforming device. The use of hydrogen as a clean fuel, particularly for vehicles, is progressing rapidly. Biogas can also be used in fuel cells, a cutting-edge application. Fuel cells are now appropriate for power production and transportation due to recent developments in fuel cell technology that result in low emissions (CO2, NOx) and outstanding efficiency. Even though biomass conversion to biogas using AD has already become a reality in many countries, the considerable financial risks associated with its implementation necessitate more significant financial incentives from policymakers to encourage long-term shifts in existing technology. To get a better understanding of the key environmental and health risks associated with GM algae, as well as the laws that govern them more future studies are required. Examine the dangers associated with production that affect FGB growth, water management, and disposal. In most countries, actions involving modified microorganisms that are directly applicable to algae are subject to explicit limitations. A sustainable and renewable energy mandate has been implemented in various nations, resulting in the optimization of energy mixes to reduce carbon emissions. Renewable energy deployment requires green policy designs. This is because the success of such measures is determined by a country’s stage of development. Renewable energy use is a strategy for addressing climate change concerns, particularly in large countries like China. In terms of environmental benefits, renewable energy must be weighed against possibly damaging alternative sources. Further, there is a requirement of huge investment in the renewable energy sector. The 5Rs (refuse, reduce, reuse, repurpose, recycle) of waste management principles and circular material management, SDG 12 (responsible consumption and production), should be employed so that nothing can get wasted and disposed of to landfills, to acheive COP26 goals and zero carbon emission by 2070, there is urgent need to conduct more advance research on cost-cutting techniques for renewable energies and to bulid strong industry -academia collaboration to solve the market barriers for renewable energy. The waste of wealth also can be successful in this way.

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Surbhi Sharma, Vaneet Kumar, and Saruchi

Contents Introduction: Nanotechnology– A Door to Revolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Milestones of Nanotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silver Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-stimulated Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel Techniques of Synthesis of Nanoparticles Using Green Technology . . . . . . . . . . . . . . . . . . Microwave-Assisted Irradiations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sonochemical-Induced Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benefits of Green Nanotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complications to Green Nanotechnology Adoption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nanotechnology is a rapidly growing science of producing and utilizing nanoparticles such as nano-sized silver particles. Nanoparticles compared to bulk materials exhibit improved characteristics due to their size, distribution, and morphology and are used in various scientific fields. Now, engineers are studying ways by which it can be made beneficial to the environment. There are general perceptions that nanotechnology will have a significant impact on developing “clean” and “green” technologies with S. Sharma Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India V. Kumar (*) Research and Innovation, CT Group of Institutions, Jalandhar, Punjab, India Saruchi Department of Biotechnology, CTIPS, C.T. Institute of Engineering, Management and Technology, Shahpur, Jalandhar, Punjab, India © Springer Nature Switzerland AG 2023 U. Shanker et al. (eds.), Handbook of Green and Sustainable Nanotechnology, https://doi.org/10.1007/978-3-031-16101-8_61

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considerable environmental benefits and this has been branded as “Green Technology.” The meaning of green technology is that the technology is environmental friendly developed in such a way that it doesn’t disturb our environment and conserves natural resources. The objectives of nanotechnology are to create eco-friendly processes and products. Conflicting with this positive message is the growing body of research that raises questions about the potentially negative effects of engineered nanoparticles on human health and environment. The main aim of this review is to give an overview of green technology in association with nanotechnology and its complications and benefits. Keywords

Fossil fuels · Nanotechnology · Nanosilver particles · Biogenic synthesis · Electrochemical reduction · Microwave-assisted reduction · Sonochemicalinduced reduction Abbreviations

AFM AgNP’s CNT DLS FT synthesis ILCD LCA QD STM SWCNT

Atomic force microscope Silver nanoparticles Carbon nanotubes Dynamic light scattering Fischer-Tropsch synthesis International Reference Life Cycle Data System Life cycle assessment Quantum Dot Scanning tunneling microscope Single-walled carbon nanotube

Introduction: Nanotechnology– A Door to Revolution The unprecedented control of individual atoms and molecules has opened the door to a new discipline called Nanotechnology. Nanotechnology encompasses new emerging technologies that involve measuring, modeling, and manipulating elements at the nanometer scale. According to recommendations of the European Commission (2011), a nanomaterial contains particles either in an unbound state or as aggregate or agglomerate with at least 50% of them having the size in the range of 1–100 nm. Figure 1 shows the developmental stages of technology. A paradigm shift at a nanometer scale showcases some extraordinary and novel properties of matter enabling reformulation of new chemicals and materials which can transform existing technologies. Recent statistics reveal that the diverse nature of nanotechnology is translating into about a quarter trillion USD worldwide investment out of which China and the USA are spending more than 2 billion USD in this field (GAEU Horizon 2020).

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Fig. 1 Major phases of technological revolutions

By 1990s, fossil fuels, i.e., coal, natural gas, and oil, became major energy sources at the global level while zero-carbon resources like wind power and nuclear power are still of limited use. Now scientists and engineers in the fourth industrial phase are working on developing nanostructured materials capable of either raising the capabilities of existing technologies or building new materials for commercial use. Distinctive properties of nanomaterials have found applications in diverse fields, viz. food industry, nanoelectronics, medicines, sports, defense and security, agriculture, and cosmetics (Mousavi et al. 2016; Gruère et al. 2011) (Fig. 2). Inspired from biological systems like a tiny cell, which can store enormous information despite its minute size, scientists and engineers are developing innovative nanostructures and nanomaterials. Dramatic changes in the properties of some elements are evident as their dimension falls below 100 nm. Due to the increase in surface-to-volume ratio, a greater number of atoms participate in the reaction. The physical properties of a substance are influenced by characteristic lengths like critical length, diffusion length, and scattering length. For example, the electrical conductivity of a metal is associated with the mean free path. But if the size of particles is smaller than these characteristic lengths, then novel changes in the properties of the system can be witnessed. When only one out of three dimensions of the material is nanosized, the resultant structure is known as a quantum well; if any two of the dimensions are reduced to nano-size, quantum wire is formed whereas a quantum dot (QD) is formed when all the three dimensions of the material are nanosized. Owing to their small size, QDs can emit different colors and thereby can be used for cellular imaging and for minutely observing individual molecules in biological systems (Iqbal et al. 2012).

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Fig. 2 Applications of nanotechnology

Milestones of Nanotechnology Nanotechnology and nanoscience are exploited from ancient times even when the term itself was not coined. Democritus (470–380 BC) propounded that “To understand the very large we must understand very small.” Colorful glasses fabricated nearly in 1200–1300 BC reveal the discovery of soluble metals like gold and silver in China and Egypt and evidence of Ag nanoparticles in Lycurgus cup in Rome which can change colors relative to the position of light source during 290–325 AD are few examples. Books on colloidal gold by F. Antonii (1618) and drinkable gold by J. Von Löwenstern-Kunckel (1678) are a few initial types of research on nanoparticles. The concept of miniaturization was pointed out several years before the discovery of chips, by Noble laureate Richard P. Feynmann (1959) in his oft-cited lecture “There’s plenty of room at the bottom” at the annual meeting of the American Physical Society, California. The term Nanotechnology was first coined by Prof. Taniguchi in 1974, which was then popularized by Eric K. Drexler in the 1980s through his vision of molecular assembling in biological systems. The discovery of

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scanning tunneling microscope (STM) and atomic force microscope (AFM) in the 1980s brought revolution in the field of nanotechnology by proving tools for imaging and mapping of nanoscale materials. In 1991, J. Amstrong, Chief Scientist, IBM, regarded nanotechnology as the revolution in the field of science and technology and later George M. Whitesides (1998), Professor of Chemistry, Harvard University, emphasized the importance of nanotechnology by stating that information of 1000 CDs can be stored in something as small as a wristwatch with nanodevices (Snow 2015). Success in the fabrication of nanoscale structures, viz. fullerene, carbon nanotubes (CNT), quantum dots (QDs), graphene sheets, etc., has contributed to the rapid growth of nanosciences during the last 30 decades. The 2010 Nobel Prize in Physics was presented to Andre Geim and Konstantin Novoselov for their remarkable work in the field of two-dimensional graphene sheet. These Nobel Laureates showed that a carbon sheet of just 1 atom thick can exhibit some remarkable properties. Graphene sheets have a density greater than the density of helium gas, yet are completely transparent. This property is attractive for the applications of touch screens, solar panels, etc. High conductivity along with its light weightiness and heat resistance qualifies nanomaterials for application in airplanes, cars, and satellites. Recently, a team of three scientists, namely, JeanPierre Sauvage from the University of Strasbourg, France; Sir J. Fraser Stoddart from Northwestern University, Illinois; and Bernard L. Feringa from the University of Groningen, the Netherlands, respectively, are conferred with Nobel Prize in Chemistry (2016) for devising nano-molecular machines, world’s smallest machine which introduces a revolutionary way of constructing molecular matter at the nanodimensions.

Synthesis of Nanoparticles Either of two basic approaches is adopted for the synthesis of nanostructured materials, viz. top-down approach which involves narrowing down the size of bulk sample up to nanoscale dimensions by cutting, grinding, or evaporation; or bottom-up approach which involves molecular assembling. Nanomaterials of different shapes can be synthesized for certain applications using the following techniques (Table 1). Novel structures of nanomaterials especially quantum dots (QD), graphene sheets, and carbon nanotubes (CNT) have attracted the attention of the scientific community. Nevertheless, as-prepared samples when subjected to practical applications various limitations are confronted, viz. toxicity, problem in morphology, and instability, cannot be recycled or reused (Singh et al. 2018).

Silver Nanoparticles Owing to unique characteristics, viz. magnetic, electronic, chemical, and optical properties, metal nanoparticles are gaining attention in the scientific community. In particular, silver nanoparticles (AgNPs) are of special interest due to their wide

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Table 1 Nanosynthesis techniques Top-down approach Method Examples Physical Ball milling milling Mechano-chemical process Cryomilling

Bottom-up approach Method Examples Solid phase Chemical vapor synthesis deposition (CVD) Pulsed laser deposition

2.

Electrospinning

Liquid phase synthesis

3.

Etching

4.

Lithography

S. No 1.

Dry/wet etching Plasma etching Photo-lithography E-beam lithography X-ray lithography

Gas phase synthesis Green synthesis

Sputtering Atomic laser deposition Electrochemical reduction Sol-gel method Microemulsion synthesis Hydrothermal synthesis Sonochemical synthesis Microwave-assisted synthesis Laser ablation Laser pyrolysis Plants extracts Bacteria Fungi Yeast

applications in superconductors, cosmetic products, water treatment, textiles, electronics, and optics (Kumar et al. 2017). According to recent studies, by virtue of its small size (i.e.,