Advances in Metal Oxides and Their Composites for Emerging Applications 9780323857055

Table of contents :
Cover
Half Title
Metal Oxides Series
Advances in Metal Oxides and Their Composites for Emerging Applications
Copyright
Contents
List of contributors
Series editor biography
About the editor
Foreword
Preface to the series
Preface
Acknowledgment
Part I : Introduction to metal oxide-based composites
1. Metal oxide engineering
1.1 Human development and metal oxides nexus
1.2 Metal oxide engineering: strategies and significances
1.2.1 Bulk versus nanoscale
1.2.2 Undoped versus doped
1.2.3 Phase diversity
1.2.4 Composite formation
1.2.5 Morphology engineering
1.2.6 Porosity generations
1.2.7 Surface modifications
1.2.8 Thin-film formations
1.3 Application of engineered metal oxides
1.3.1 Energy technologies
1.3.1.1 Solar cells
1.3.1.2 Water splitting
1.3.1.3 Energy storage system
1.3.2 Biomedical application
1.3.2.1 Biosensing studies
1.3.2.2 Cancer treatments
1.3.2.3 Antimicrobial study
1.3.3 Catalytic applications
1.3.3.1 Wastewater treatment
1.3.3.2 Catalytic organic transformations
1.4 Concluding remarks
1.5 Futuristic outlooks
References
2. Metal oxide-based composites: synthesis and characterization
2.1 Introduction
2.1.1 Metal oxides
2.2 Synthetic approaches
2.2.1 Top-down approaches
2.2.1.1 Mechanical milling
2.2.1.2 Electrospinning
2.2.1.3 Lithography
2.2.1.4 Sputtering
2.2.1.5 Laser ablation
2.2.2 Bottom-up approaches
2.2.2.1 Sol-gel technique
2.2.2.2 Solvothermal technique
2.2.2.3 Microwave synthesis
2.2.2.4 Combustion synthesis
2.2.2.5 Electrodeposition
2.3 Characterization of metal oxide-based composite nanostructures
2.3.1 X-ray Diffraction
2.3.2 Scanning electron microscopy
2.3.3 Transmission electron microscopy
2.3.4 UV Vis spectroscopy
2.3.5 Fourier transform infrared spectroscopy
2.3.6 Temperature-programmed reduction
2.3.7 X-ray photoelectron spectroscopy
2.3.8 Electrochemical characterization
2.4 Summary and outlook
References
Part II : Metal oxides-based composites in energy technologies
3. Metal oxides as photoanodes for photoelectrochemical water splitting: synergy of oxygen vacancy
3.1 Introduction
3.2 Role of metal oxides in photoelectrochemical hydrogen/oxygen evolution
3.3 Oxygen vacancy engineering in metal oxides for photoelectrochemical water splitting
3.3.1 TiO2
3.3.2 WO3
3.3.2.1 ZnO
3.3.2.2 In2O3
3.3.2.3 SrTiO3
3.4 Scope of improvement in the field
3.4.1 Quality and cost-effective materials
3.4.1.1 Stability of metal oxides
3.5 Conclusion
References
4. Transition metal oxide conducting polymer nanocomposites and metal-organic framework-based composites for supercapacitor application
4.1 Introduction
4.2 Energy storage device evolution
4.2.1 Supercapacitor evolution
4.3 Market scenario
4.3.1 Market size
4.3.2 Companies with supercapacitor production
4.3.3 Global supercapacitor market end-users
4.4 Types of supercapacitors
4.4.1 Electric double layer capacitor
4.4.2 Pseudocapacitor
4.4.2.1 Conducting polymers-based supercapacitors
4.4.2.2 Metal oxides-based supercapacitors
4.4.3 Hybrid supercapacitors
4.4.3.1 Asymmetric supercapacitor
4.4.3.2 Rechargeable battery type supercapacitor
4.4.3.3 Composite hybrid supercapacitors
4.5 Electrical properties studies of energy storage devices
4.5.1 Operating voltage
4.5.2 Self-discharge
4.5.3 Polarity
4.5.4 Internal resistance
4.5.5 Dependency of device capacitance and resistance on operating voltage and temperature
4.5.6 Current load and cycle stability
4.5.6.1 Swelling induced degradation
4.5.6.2 Overoxidation induced degradation
4.5.7 Energy density
4.5.8 Power density
4.5.9 Capacitance
4.6 Metal oxide-conducting polymer composites for supercapacitor
4.6.1 Composite of polyaniline with the representative metal oxides
4.6.2 Composite of polypyyrole with the representative metal oxides
4.6.3 Composite of poly 3,4-ethylene dioxythiophene and polythiophene with the reprentative metal oxides
4.7 Metal oxide-metal-organic frameworks and metal organic frameworks derived material for supercapacitor
4.8 Conclusions and future outlooks
References
5. Metal oxide-based nanocomposites for supercapacitive applications
5.1 Introduction
5.2 Charge storage mechanism
5.2.1 Non-faradic mechanism
5.2.2 Redox mechanism
5.2.2.1 Redox reactions at the surface
5.2.2.2 Intercalation type reactions inside the pores of electrode material
5.2.3 Battery type charge storage
5.3 Carbon-based materials as an electrode
5.4 Metal oxides/metal oxide composites as an electrode in supercapacitors
5.4.1 Ruthenium oxide
5.4.2 Manganese dioxide
5.4.3 Nickel oxide
5.4.4 Cobalt tetraoxide
5.4.5 Other metal oxide/metal oxide composites
5.4.6 Performance of negative electrode
5.5 Mixed transition metal oxides
5.5.1 Nickel cobaltate (NiCo2O4)
5.5.2 Ferrites
5.6 Flexible supercapacitors
5.7 Futuristic scope
5.8 Conclusions
References
6. Nanostructured WO3-x based advanced supercapacitors for sustainable energy applications
6.1 Introduction
6.2 Crystallographic characteristics of WO3
6.2.1 Role of ion intercalation in WO3 and electrochemical charge storage
6.3 Designing nanostructured WO3 for supercapacitor application
6.4 Recent developments in WO3 composites for supercapacitor application
6.5 Conclusions
6.6 Future prospects
References
7. Metal oxide nanomaterials for organic photovoltaic applications
7.1 Introduction
7.2 Organic photovoltaic: principle, designing and mechanism
7.2.1 Mechanism
7.2.1.1 Absorption of light and exciton generation
7.2.1.2 Exciton diffusion
7.2.1.3 Exciton dissociation
7.2.1.4 Types of organic photovoltaics
7.2.2 Commonly used organic sensitizers in organic photovoltaics
7.3 Metal oxide nanomaterials
7.4 Properties of nanomaterials
7.5 Representative metal oxides used in organic photovoltaics
7.6 Metal oxides based organic photovoltaic studies
7.6.1 Photovoltaic devices applications of nanomaterials
7.6.1.1 Organic photovoltaics
7.6.2 Titanium dioxide
7.6.3 Zinc oxide
7.6.4 Molybdenum oxide
7.6.5 Tin oxide
7.6.6 Tungsten oxide
7.6.7 Vanadium pentaoxide
7.7 Concluding summary and future prospective
References
8. Nanocrystalline metal oxide-based hybrids for third-generation solar cell technologies
8.1 Introduction
8.2 Modifications of metal oxides
8.2.1 Doped MxOy
8.2.2 Metal-supported MxOy
8.2.3 Metal oxide metal oxide hybrids (MxOy AmOn)
8.2.4 Other additives or Supportive materials
8.2.4.1 Graphene metal oxide hybrids
8.2.4.2 Carbon nanotube 2 metal oxide nanocomposites
8.2.4.3 Polymer 2 metal oxide hybrids
8.2.4.4 Chalcogenides 2 metal oxide hybrids
8.3 Emerging strategies of third-generation solar cell technologies
8.3.1 Dye-sensitized solar cells
8.3.2 Quantum dot-sensitized solar cells
8.3.3 Organic solar cells
8.3.4 Tandem solar cells
8.3.5 Perovskite solar cells
8.4 Present state of art in emerging photovoltaic devices
8.5 Conclusion and future outlooks
References
9. Role of metal oxides as photoelectrodes in dye-sensitized solar cells
9.1 Introduction
9.2 The operational principle of dye-sensitized photo electrochemical cells
9.3 Photo-physics of dye-sensitized photo electrochemical cells
9.3.1 Energy levels of components
9.3.2 Charge separation
9.3.3 Recombination rate
9.3.4 Charge transfer rate
9.4 Metal oxide photoanode in dye-sensitized photo electrochemical cell
9.4.1 Influence of morphology in performance
9.4.1.1 Nanorods/wires/tubes metal oxide
9.4.1.2 Carbon-based metal oxide nanostructure
9.4.1.3 Hierarchical hollow spheres and beads
9.4.1.4 Nanospindles
9.4.2 Influence of interfacial engineering
9.4.2.1 Influence of the compact blocking layer
9.4.2.2 Influence of light-scattering layer
9.5 Metal oxide cathode in dye-sensitized photo electrochemical cells
9.5.1 Role of metal oxide cathode in dye-sensitized photo electrochemi
9.5.2 Variable to evaluating the catalytic activity of metal oxide ca
9.5.2.1 Active sites
9.5.2.2 Conductivity
9.5.3 Recent progress on metal oxide-based cathode
9.5.3.1 Metal oxide/carbon composites
9.6 Conclusion and perspectives
References
10. Nanostructured inorganic metal oxide/metal organic framework based electrodes for energy technologies
10.1 Introduction
10.2 Metal oxides for solar energy studies
10.3 Metal organic frameworks for solar energy studies
10.3.1 Metal organic frameworks as sensitizers
10.3.2 Guest@ metal organic frameworks system
10.4 Metal oxides/metal organic frameworks nanocomposite: pros and cons
10.5 Metal oxide/metal organic frameworks: present state of the art
10.6 Electrode designing and its features studies for energy technologies
10.7 Metal oxides/metal organic frameworks nanocomposites for solar energy harvesting
10.7.1 TiO2/ZIF-8
10.7.2 TiO2/Cu-BTC
10.7.3 TiO2/Co-DAPV
10.7.4 ZnO/ZIF-8
10.7.5 TiO2/MIL-125
10.7.6 ZnO/PPF-11
10.8 Metal oxide/metal organic frameworks nanocomposites for water splitting
10.8.1 α-Fe2O3/imidazole-based metal organic frameworks
10.8.2 BiVO4/MIL-101(Fe)
10.8.3 TiO2/MIL-125
10.8.4 ZnO/ZIF-8
10.9 Conclusion and future perspectives
References
Part III : Other applications of metal oxide-based composites
11. Metal oxide nanocomposite-based electrochemical biosensing studies
11.1 Introduction
11.2 Present scenario of biosensor market
11.3 Nonenzymatic electrochemical biosensors
11.4 Functional nanocomposites in electrochemical biosensor
11.4.1 Metallic nanoparticle-based composites
11.4.2 Metal oxide nanomaterial’s-based composites
11.5 Conclusions
11.6 Challenges and future perspectives
References
12. Functionalized magnetic iron oxide-based composites as adsorbents for the removal of heavy metals from wastewater
12.1 Introduction
12.2 Water pollution by heavy metals and its removal
12.2.1 Methods for the removal of heavy metal ions
12.2.2 Adsorption process for the removal of heavy metal ions
12.3 Magnetic nanoparticles as nanoadsorbents
12.3.1 Functionalization of magnetic nanoparticles for heavy metal ions
12.3.1.1 Surface functionalization by organic materials
12.3.1.2 Surface functionalization by inorganic materials
12.4 Batch adsorption experiment
12.4.1 Factors affecting the adsorption of heavy metal ions
12.4.1.1 Effect of solution pH
12.4.1.2 Effect of contact time
12.4.1.3 Effect of adsorbent dose
12.4.1.4 Effect of initial metal ion concentration
12.4.2 Adsorption kinetics
12.4.3 Adsorption isotherms
12.5 Removal of heavy metal ions by magnetic nanoparticles
12.5.1 Removal of a single type of heavy metal ions
12.5.2 Simultaneous removal of multiple heavy metal ions
12.6 Conclusions and future perspectives
References
13. Mixed metal oxide nanocomposites for environmental remediation
13.1 Introduction: environmental remediation principles and applications
13.2 Types of environmental remediation
13.2.1 Soil remediation
13.2.2 Groundwater and surface water remediation
13.2.3 Sediment remediation
13.3 Semiconducting metal oxides
13.4 Environmental remediation: need of the hour
13.5 Different composites in metal oxide
13.6 Mixed metal oxide NCS and environmental remediation: present state of the art
13.6.1 TiO2-based nanocomposites
13.6.2 Fe2O3-based nanocomposites
13.6.3 ZnO-based nanocomposites
13.6.4 Al2O3-based nanocomposites
13.6.5 WO3-based nanocomposites
13.6.6 SnO2-based nanocomposites
13.6.7 Graphene oxide-based nanocomposites
13.6.8 Rare earth oxides-based nanocomposites
13.7 Advanced oxidation processes or degradation processes
13.8 Synthesis of metal oxide nanocomposites
13.9 Tailoring properties of metal oxide nanocomposites
13.9.1 Doping
13.9.2 Modeling phase structure
13.9.3 Stoichiometry controlling
13.9.4 Microstructure forming
13.9.5 Heterostructure forming
13.9.6 Controlling crystal growth
13.9.7 Impact of heat treatments
13.10 Protocols of mixed metal oxides used in environmental remediation
13.10.1 Adsorbent studies
13.10.2 Catalytic studies
13.10.3 Membrane studies
13.10.4 Biological studies
13.11 Monitoring of pollutants during environmental remediation
13.11.1 Monitoring of air pollutants
13.11.2 Monitoring of soil pollutants
13.11.3 Monitoring of water pollutants
13.12 Concluding remarks and future perspectives
References
14. Metal oxide nanocomposites in water and wastewater treatment
14.1 Water: the key to life on the earth
14.2 Present scenario of water pollution
14.3 Water treatment
14.4 Waste water treatment
14.5 Challenges
14.6 Nanotechnology in water and wastewater treatment
14.6.1 Nanosorbents
14.6.2 Nanocatalysts
14.6.3 Nanostructured membrane
14.6.4 Nanobiocides
14.7 Use of metal-oxide nanocomposites in water and wastewater treatment
14.8 Features of metal oxide nanocomposite in water/ wastewater treatment
14.9 Future prospects
14.10 Conclusions
References
15. Self-cleaning photoactive metal oxide-based concrete surfaces for environmental remediation
15.1 Introduction
15.2 Photocatalytic mechanism of self-cleaning concretes
15.3 Preparation of photoactive concrete surface
15.3.1 Method (i)
15.3.2 Method (ii)
15.3.3 Method (iii)
15.4 Properties of photoactive self-cleaning concretes
15.5 Photocatalytic activity testing methods
15.5.1 Self-cleaning test
15.5.2 Depollution testing
15.6 Advantages and disadvantages of self-cleaning concretes
15.7 Self-cleaning photoactive concrete in real-world applications
15.8 Market status of photoactive materials
15.9 Summary and conclusions
15.10 Future prospects
References
Further reading
16. Metal oxide nanocomposites: design and use in antimicrobial coatings
16.1 Introduction
16.2 Microbes and microbial infectious diseases
16.3 Antimicrobial coatings: market scenario
16.4 Metal oxide nanocomposites as potential antimicrobial agents
16.4.1 Composites of metal oxide with inorganic moieties
16.4.1.1 Metal/metal oxide composites
16.4.1.2 Metal oxide/metal oxide (mixed metal oxide) composites
16.4.1.3 Metal oxide/carbon nanostructures composites
16.4.2 Composites of metal oxide with organic moieties
16.4.2.1 Metal oxide/metal-organic framework composites
16.4.2.2 Metal oxide/polymer composites
16.4.2.3 Metal oxide/organic molecule composites
16.5 Plausible mechanisms for nanocomposites-based microbes inactivation
16.6 Synthesis strategies for designing metal oxide nanocomposite
16.7 Metal oxide nanocomposites based on antimicrobial coatings in different fields
16.7.1 Hospital sector
16.7.2 Textile sector
16.7.3 Food sector
16.7.4 Polymer sector
16.7.5 Paint sector
16.7.6 Leather sector
16.8 Conclusions
16.9 Future outlooks
Acknowledgment
References
17. Metal oxide composites in organic transformations
17.1 Introduction
17.2 Design and characterization of nanocomposites
17.3 Applications of metal oxide composites for organic transformations
17.3.1 Synthesis of bis (pyrazol-5-ol) and dihydropyrano[2,3-c] pyrazole analogs
17.3.2 Synthesis of pyrimido benzazoles
17.3.3 Synthesis of pyridine-3-carboxamides
17.3.4 Synthesis of benzimidazolo[2,3-b]quinazolinone derivatives
17.3.5 Synthesis of dihydroquinazolinones
17.3.6 Synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones
17.3.7 Synthesis of chromene derivatives
17.3.7.1 Synthesis of aminochromenes
17.3.7.2 Synthesis of 2-amino-benzochromenes
17.3.7.3 Synthesis of pyrano[3,2- c]quinolones and pyrano[3,2-c] chromen
17.3.7.4 Synthesis of novel 4H-chromene-3-carbonitriles
17.3.8 Synthesis of 1,4-disubstituted-1,2,3-triazoles
17.3.9 Synthesis of pyran derivatives
17.3.10 Synthesis of thieno[2,3-d]pyrimidin-4(3H)-one Derivative
17.3.11 Synthesis of α-chloro aryl ketones
17.3.12 C H arylation reactions through aniline activation
17.3.13 Synthesis of unsymmetrical ureas
17.3.14 Synthesis of Betti bases and bisamides
17.3.15 Synthesis of 3-aryl-2-[(aryl)(arylamino)]methyl-4H-furo [3,2-c]chromen-4-one derivatives
17.3.16 Synthesis of benzo[4,5]thiazolo[3,2-a]chromeno [4,3-d] pyrimidin-6-one derivatives
17.3.17 Synthesis of substituted pyrazolones
17.3.18 Synthesis of 7-aryl-benzo[h]tetrazolo[5,1-b]quinazoline- 5,6-dione
17.3.19 Reduction of nitrobenzene and p-nitrophenol
17.4 Concluding remarks
References
18. Metal oxide-based composites as photocatalysts
18.1 Introduction
18.1.1 Principles of metal oxide-based composites as photocatalysts
18.1.2 Mechanism of photocatalytic reactions
18.2 Unitary metal oxides versus composite-based metal oxide photocatalysts
18.3 Applications of metal oxide-based photocatalysts
18.3.1 Photoelectrocatalysis for energy conversion
18.3.2 Hydrogen production
18.3.3 Water treatment and environment
18.3.4 CO2 reduction (hydrocarbon generation)
18.3.5 Antibacterial, anticancer, and biomedical applications
18.3.6 Layered double hydroxides/metal-organic frameworks
18.3.7 Polymeric nanophotocatalysts
18.3.8 Food safety
18.4 Future perspectives of metal oxide-based composites as photocataly
References
19. Metal oxide-based composites for magnetic hyperthermia applications
19.1 Introduction
19.2 Present cancer treatment: pros and cons
19.3 Hyperthermia
19.3.1 Classification of hyperthermia
19.3.1.1 Local hyperthermia
19.3.1.2 Regional hyperthermia
19.3.1.3 Whole-body hyperthermia
19.3.2 Magnetic hyperthermia
19.4 Representative nanomaterials for magnetic hyperthermia
19.5 Magnetic metal oxide nanomaterials-based composites for magnetic hyperthermia application
19.6 Iron oxide nanoparticles and surface functionalization
19.7 Methods for measuring the magnetism of the magnetic materials
19.7.1 Superconducting quantum interference device magnetometry
19.7.2 Zero-field cooling and field cooling measurements
19.7.3 Vibrating-sample magnetometer
19.7.4 Heating capacity: induction heating system
19.8 Conclusions
19.9 Challenges and future perspectives
References
Index
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Advances in Metal Oxides and Their Composites for Emerging Applications

The Metal Oxides Book Series Edited by Ghenadii Korotcenkov Forthcoming titles Palladium Oxides Material Properties, Synthesis and Processing Methods, and Applications, Alexander M. Samoylov, Vasily N. Popov, 9780128192238 Metal Oxides for Non-volatile Memory, Panagiotis Dimitrakis, Ilia Valov, Stefan Tappertzhofen, 9780128146293 Metal Oxide Nanostructured Phosphors, H. Nagabhushana, Daruka Prasad, S.C. Sharma, 9780128118528 Nanostructured Zinc Oxide, Kamlendra Awasthi, 9780128189009 Multifunctional Piezoelectric Oxide Nanostructures, Sang-Jae Kim, Nagamalleswara Rao Alluri, Yuvasree Purusothaman, 9780128193327 Transparent Conductive Oxides, Mirela Petruta Suchea, Petronela Pascariu, Emmanouel Koudoumas, 9780128206317 Metal oxide-based nanofibers and their applications, Vincenzo Esposito, Debora Marani, 9780128206294 Metal-oxides for Biomedical and Biosensor Applications, Kunal Mondal, 9780128230336 Metal Oxide-Carbon Hybrid Materials, Muhammad Akram, Rafaqat Hussain, Faheem K Butt, 9780128226940 Metal Oxide-based heterostructures, Naveen Kumar, Bernabe Mari Soucase, 9780323852418 Metal Oxides And Related Solids For Electrocatalytic Water Splitting, Junlei Qi, 9780323857352 Advances in Metal Oxides and Their Composites for Emerging Applications, Sagar D. Delekar, 9780323857055 Metallic glasses and their oxidation, Xinyun Wang, Mao Zhang, 9780323909976 Solution methods for metal oxide nanostructures, Rajaram S. Mane, Vijaykumar Jadhav, Abdullah M. AlEnizi, 9780128243534 Metal oxide defects, Vijay Kumar, Sudipta Som, Vishal Sharma, Hendrik Swart, 9780323855884 Renewable Polymers and polymer-metal oxide composites, Sajjad Haider, Adnan Haider, 9780323851558 Metal oxides for optoelectronics and optics-based medical applications, Suresh Sagadevan, Jiban Podder, Faruq Mohammad, 9780323858243 Graphene oxide-metal oxide and other graphene oxide-based composites in photocatalysis and electrocatalysis, Jiaguo Yu, Liuyang Zhang, Panyong Kuang, 9780128245262 G

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Published titles Metal oxides in nanocomposite-based electrochemical sensors for toxic chemicals, Alagarsamy Pandikumar, Perumal Rameshkumar, 9780128207277 Metal oxide-based nanostructured electrocatalysts for fuel cells, electrolyzers, and metal-air batteries, Teko Napporn, Yaovi Holade, 9780128184967 Titanium Dioxide (TiO2) and Its Applications, Leonardo Palmisano, Francesco Parrino, 9780128199602 Solution Processed Metal Oxide Thin Films for Electronic Applications, Zheng Cui, 9780128149300 Metal oxide powder technologies, Yarub Al-Douri, 9780128175057 Colloidal metal oxide nanoparticles, Sabu Thomas, Anu Tresa Sunny, Prajitha V, 9780128133576 Cerium Oxide, Salvatore Scire, Leonardo Palmisano, 9780128156612 Tin Oxide Materials, Marcelo Ornaghi Orlandi, 9780128159248 Metal oxide glass nanocomposites, Sanjib Bhattacharya, 9780128174586 Gas sensors based on conducting metal oxides, Nicolae Barsan, Klaus Schierbaum, 9780128112243 Metal oxides in energy technologies, Yuping Wu, 9780128111673 Metal oxide nanostructures, Daniela Nunes, Lidia Santos, Ana Pimentel, Pedro Barquinha, Luis Pereira, Elvira Fortunato, Rodrigo Martins, 9780128115121 Gallium Oxide, Stephen Pearton, Fan Ren, Michael Mastro, 9780128145210 Metal oxide-based photocatalysis, Adriana Zaleska-Medynska, 9780128116340 Metal oxides in heterogeneous catalysis, Jacques C. Vedrine, 9780128116319 Magnetic, ferroelectric, and multiferroic metal oxides, Biljana Stojanovic, 9780128111802 Iron Oxide Nanoparticles for Biomedical Applications, Sophie Laurent, Morteza Mahmoudi, 9780081019252 The future of semiconductor oxides in next-generation solar cells, Monica Lira-Cantu, 9780128111659 Metal oxide-based thin film structures, Nini Pryds, Vincenzo Esposito, 9780128111666 Metal Oxides in Supercapacitors, Deepak Dubal, Pedro Gomez-Romero, 9780128111697 Transition metal oxide thin film-based chromogenics and devices, Pandurang Ashrit, 9780081018996 G

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Metal Oxides Series

Advances in Metal Oxides and Their Composites for Emerging Applications Edited by

Sagar D. Delekar Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India

Series Editor

Ghenadii Korotcenkov Moldova State University, Chisinau, Moldova

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-85705-5 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Kayla Dos Santos Editorial Project Manager: Clodagh Holland-Borosh Production Project Manager: Surya Narayanan Jayachandran Cover Designer: Miles Hitchen Typeset by MPS Limited, Chennai, India

Contents

List of contributors Series editor biography About the editor Foreword Preface to the series Preface Acknowledgment

xvii xxi xxiii xxv xxvii xxxi xxxiii

Part I Introduction to metal oxide-based composites

1

1

3

2

Metal oxide engineering Pramod A. Koyale, Dillip K. Panda and Sagar D. Delekar 1.1 Human development and metal oxides nexus 1.2 Metal oxide engineering: strategies and significances 1.2.1 Bulk versus nanoscale 1.2.2 Undoped versus doped 1.2.3 Phase diversity 1.2.4 Composite formation 1.2.5 Morphology engineering 1.2.6 Porosity generations 1.2.7 Surface modifications 1.2.8 Thin-film formations 1.3 Application of engineered metal oxides 1.3.1 Energy technologies 1.3.2 Biomedical application 1.3.3 Catalytic applications 1.4 Concluding remarks 1.5 Futuristic outlooks References Metal oxide-based composites: synthesis and characterization H.M. Yadav, S.K. Shinde, D-Y. Kim, T.P. Chavan, N.D. Thorat, S. Ramesh and C.D. Bathula 2.1 Introduction 2.1.1 Metal oxides 2.2 Synthetic approaches

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2.2.1 Top-down approaches 2.2.2 Bottom-up approaches 2.3 Characterization of metal oxide-based composite nanostructures 2.3.1 X-ray Diffraction 2.3.2 Scanning electron microscopy 2.3.3 Transmission electron microscopy 2.3.4 UV Vis spectroscopy 2.3.5 Fourier transform infrared spectroscopy 2.3.6 Temperature-programmed reduction 2.3.7 X-ray photoelectron spectroscopy 2.3.8 Electrochemical characterization 2.4 Summary and outlook References

Part II Metal oxides-based composites in energy technologies 3

4

Metal oxides as photoanodes for photoelectrochemical water splitting: synergy of oxygen vacancy Keval K. Sonigara, Jayraj V. Vaghasiya and Saurabh S. Soni 3.1 Introduction 3.2 Role of metal oxides in photoelectrochemical hydrogen/oxygen evolution 3.3 Oxygen vacancy engineering in metal oxides for photoelectrochemical water splitting 3.3.1 TiO2 3.3.2 WO3 3.4 Scope of improvement in the field 3.4.1 Quality and cost-effective materials 3.5 Conclusion References Transition metal oxide conducting polymer nanocomposites and metal-organic framework-based composites for supercapacitor application Swapnajit V. Mulik, Sushilkumar A. Jadhav, Pramod S. Patil and Sagar D. Delekar 4.1 Introduction 4.2 Energy storage device evolution 4.2.1 Supercapacitor evolution 4.3 Market scenario 4.3.1 Market size 4.3.2 Companies with supercapacitor production 4.3.3 Global supercapacitor market end-users

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5

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Types of supercapacitors 4.4.1 Electric double layer capacitor 4.4.2 Pseudocapacitor 4.4.3 Hybrid supercapacitors 4.5 Electrical properties studies of energy storage devices 4.5.1 Operating voltage 4.5.2 Self-discharge 4.5.3 Polarity 4.5.4 Internal resistance 4.5.5 Dependency of device capacitance and resistance on operating voltage and temperature 4.5.6 Current load and cycle stability 4.5.7 Energy density 4.5.8 Power density 4.5.9 Capacitance 4.6 Metal oxide-conducting polymer composites for supercapacitor 4.6.1 Composite of polyaniline with the representative metal oxides 4.6.2 Composite of polypyyrole with the representative metal oxides 4.6.3 Composite of poly 3,4-ethylene dioxythiophene and polythiophene with the representative metal oxides 4.7 Metal oxide-metal-organic frameworks and metal-organic frameworks derived material for supercapacitor 4.8 Conclusions and future outlooks References

143 143 145 146 149 150 150 152 154

Metal oxide-based nanocomposites for supercapacitive applications Sarita Patil, Nanasaheb D. Thorat, Joanna Bauer and Syed A.M. Tofail 5.1 Introduction 5.2 Charge storage mechanism 5.2.1 Non-faradic mechanism 5.2.2 Redox mechanism 5.2.3 Battery type charge storage 5.3 Carbon-based materials as an electrode 5.4 Metal oxides/metal oxide composites as an electrode in supercapacitors 5.4.1 Ruthenium oxide 5.4.2 Manganese dioxide 5.4.3 Nickel oxide 5.4.4 Cobalt tetraoxide 5.4.5 Other metal oxide/metal oxide composites 5.4.6 Performance of negative electrode 5.5 Mixed transition metal oxides

187

154 154 156 158 159 159 159 164 165 167 171 177

187 189 189 190 192 193 193 194 194 195 196 197 197 198

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Contents

5.5.1 Nickel cobaltate (NiCo2O4) 5.5.2 Ferrites 5.6 Flexible supercapacitors 5.7 Futuristic scope 5.8 Conclusions References

6

7

Nanostructured WO32x based advanced supercapacitors for sustainable energy applications Akshay V. Salkar, Sheshanath V. Bhosale and Pranay P. Morajkar 6.1 Introduction 6.2 Crystallographic characteristics of WO3 6.2.1 Role of ion intercalation in WO3 and electrochemical charge storage 6.3 Designing nanostructured WO3 for supercapacitor application 6.4 Recent developments in WO3 composites for supercapacitor application 6.5 Conclusions 6.6 Future prospects References

Metal oxide nanomaterials for organic photovoltaic applications Harshad A. Mirgane, Dinesh N. Nadimetla, Dipak J. Garole and Sheshanath V. Bhosale 7.1 Introduction 7.2 Organic photovoltaic: principle, designing and mechanism 7.2.1 Mechanism 7.2.2 Commonly used organic sensitizers in organic photovoltaics 7.3 Metal oxide nanomaterials 7.4 Properties of nanomaterials 7.5 Representative metal oxides used in organic photovoltaics 7.6 Metal oxides based organic photovoltaic studies 7.6.1 Photovoltaic devices applications of nanomaterials 7.6.2 Titanium dioxide 7.6.3 Zinc oxide 7.6.4 Molybdenum oxide 7.6.5 Tin oxide 7.6.6 Tungsten oxide 7.6.7 Vanadium pentaoxide 7.7 Concluding summary and future prospective References

199 200 201 201 202 203

213 213 214 217 218 224 231 232 232

239

239 242 243 245 245 246 249 250 250 251 252 252 252 253 253 255 255

Contents

8

9

Nanocrystalline metal oxide-based hybrids for third-generation solar cell technologies Prakash S. Pawar, Pramod A. Koyale, Ananta G. Dhodamani and Sagar D. Delekar 8.1 Introduction 8.2 Modifications of metal oxides 8.2.1 Doped MxOy 8.2.2 Metal-supported MxOy 8.2.3 Metal oxide metal oxide hybrids (MxOy AmOn) 8.2.4 Other additives or Supportive materials 8.3 Emerging strategies of third-generation solar cell technologies 8.3.1 Dye-sensitized solar cells 8.3.2 Quantum dot-sensitized solar cells 8.3.3 Organic solar cells 8.3.4 Tandem solar cells 8.3.5 Perovskite solar cells 8.4 Present state of art in emerging photovoltaic devices 8.5 Conclusion and future outlooks References Role of metal oxides as photoelectrodes in dye-sensitized solar cells Jayraj V. Vaghasiya, Keval K. Sonigara and Saurabh S. Soni 9.1 Introduction 9.2 The operational principle of dye-sensitized photo-electrochemical cells 9.3 Photo-physics of dye-sensitized photo-electrochemical cells 9.3.1 Energy levels of components 9.3.2 Charge separation 9.3.3 Recombination rate 9.3.4 Charge transfer rate 9.4 Metal oxide photoanode in dye-sensitized photo-electrochemical cells 9.4.1 Influence of morphology in performance 9.4.2 Influence of interfacial engineering 9.5 Metal oxide cathode in dye-sensitized photo-electrochemical cells 9.5.1 Role of metal oxide cathode in dye-sensitized photoelectrochemical cells 9.5.2 Variable to evaluating the catalytic activity of metal oxide cathode 9.5.3 Recent progress on metal oxide-based cathode 9.6 Conclusion and perspectives References

ix

263

263 265 265 266 267 267 269 269 271 272 273 274 275 279 281

287 287 289 290 290 291 293 293 294 294 309 314 314 316 318 324 325

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10

Contents

Nanostructured inorganic metal oxide/metal organic framework-based electrodes for energy technologies Pramod A. Koyale, Dillip K. Panda and Sagar D. Delekar 10.1 Introduction 10.2 Metal oxides for solar energy studies 10.3 Metal organic frameworks for solar energy studies 10.3.1 Metal organic frameworks as sensitizers 10.3.2 Guest@ metal organic frameworks system 10.4 Metal oxides/metal organic frameworks nanocomposite: pros and cons 10.5 Metal oxide/metal organic frameworks: present state of the art 10.6 Electrode designing and its features studies for energy technologies 10.7 Metal oxides/metal organic frameworks nanocomposites for solar energy harvesting 10.7.1 TiO2/ZIF-8 10.7.2 TiO2/Cu-BTC 10.7.3 TiO2/Co-DAPV 10.7.4 ZnO/ZIF-8 10.7.5 TiO2/MIL-125 10.7.6 ZnO/PPF-11 10.8 Metal oxide/metal organic frameworks nanocomposites for water splitting 10.8.1 α-Fe2O3/imidazole-based metal organic frameworks 10.8.2 BiVO4/MIL-101(Fe) 10.8.3 TiO2/MIL-125 10.8.4 ZnO/ZIF-8 10.9 Conclusion and future perspectives References

Part III Other applications of metal oxide-based composites 11

Metal oxide nanocomposite-based electrochemical biosensing studies Ankita K. Dhukate, Sajid B. Mullani, Lynn Dennany and Sagar D. Delekar 11.1 Introduction 11.2 Present scenario of biosensor market 11.3 Nonenzymatic electrochemical biosensors 11.4 Functional nanocomposites in electrochemical biosensor 11.4.1 Metallic nanoparticle-based composites 11.4.2 Metal oxide nanomaterial’s-based composites

339 339 342 343 343 348 350 352 352 355 355 356 357 358 358 358 361 361 361 361 362 365 366

377 379

379 381 382 383 385 389

Contents

11.5 Conclusions 11.6 Challenges and future perspectives References 12

13

Functionalized magnetic iron oxide-based composites as adsorbents for the removal of heavy metals from wastewater Prashant B. Patil and Vijay P. Kothavale 12.1 Introduction 12.2 Water pollution by heavy metals and its removal 12.2.1 Methods for the removal of heavy metal ions 12.2.2 Adsorption process for the removal of heavy metal ions 12.3 Magnetic nanoparticles as nanoadsorbents 12.3.1 Functionalization of magnetic nanoparticles for heavy metal ions removal 12.4 Batch adsorption experiment 12.4.1 Factors affecting the adsorption of heavy metal ions 12.4.2 Adsorption kinetics 12.4.3 Adsorption isotherms 12.5 Removal of heavy metal ions by magnetic nanoparticles 12.5.1 Removal of a single type of heavy metal ions 12.5.2 Simultaneous removal of multiple heavy metal ions 12.6 Conclusions and future perspectives References Mixed metal oxide nanocomposites for environmental remediation S.M. Patil, S.A. Vanalakar and Sagar D. Delekar 13.1 Introduction: environmental remediation principles and applications 13.2 Types of environmental remediation 13.2.1 Soil remediation 13.2.2 Groundwater and surface water remediation 13.2.3 Sediment remediation 13.3 Semiconducting metal oxides 13.4 Environmental remediation: need of the hour 13.5 Different composites in metal oxide 13.6 Mixed metal oxide NCS and environmental remediation: present state of the art 13.6.1 TiO2-based nanocomposites 13.6.2 Fe2O3-based nanocomposites 13.6.3 ZnO-based nanocomposites 13.6.4 Al2O3-based nanocomposites 13.6.5 WO3-based nanocomposites 13.6.6 SnO2-based nanocomposites 13.6.7 Graphene oxide-based nanocomposites 13.6.8 Rare earth oxides-based nanocomposites

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392 392 393

401 401 402 402 403 404 405 409 409 410 411 411 412 412 417 418 425

425 427 427 427 428 428 429 431 432 436 437 438 440 441 441 442 443

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Contents

13.7 13.8 13.9

14

Advanced oxidation processes or degradation processes Synthesis of metal oxide nanocomposites Tailoring properties of metal oxide nanocomposites 13.9.1 Doping 13.9.2 Modeling phase structure 13.9.3 Stoichiometry controlling 13.9.4 Microstructure forming 13.9.5 Heterostructure forming 13.9.6 Controlling crystal growth 13.9.7 Impact of heat treatments 13.10 Protocols of mixed metal oxides used in environmental remediation 13.10.1 Adsorbent studies 13.10.2 Catalytic studies 13.10.3 Membrane studies 13.10.4 Biological studies 13.11 Monitoring of pollutants during environmental remediation 13.11.1 Monitoring of air pollutants 13.11.2 Monitoring of soil pollutants 13.11.3 Monitoring of water pollutants 13.12 Concluding remarks and future perspectives References

444 447 450 452 453 453 454 454 455 455

Metal oxide nanocomposites in water and wastewater treatment Shubhangi D. Shirsat, Rajaram S. Mane, Joanna Bauer and Nanasaheb D. Thorat 14.1 Water: the key to life on the earth 14.2 Present scenario of water pollution 14.3 Water treatment 14.4 Waste water treatment 14.5 Challenges 14.6 Nanotechnology in water and wastewater treatment 14.6.1 Nanosorbents 14.6.2 Nanocatalysts 14.6.3 Nanostructured membrane 14.6.4 Nanobiocides 14.7 Use of metal-oxide nanocomposites in water and wastewater treatment 14.8 Features of metal oxide nanocomposite in water/wastewater treatment 14.9 Future prospects 14.10 Conclusions References

479

455 455 457 458 459 460 461 461 462 463 466

479 481 482 484 485 486 488 493 494 496 499 505 508 508 509

Contents

15

16

Self-cleaning photoactive metal oxide-based concrete surfaces for environmental remediation Valmiki B. Koli and Shyue-Chu Ke 15.1 Introduction 15.2 Photocatalytic mechanism of self-cleaning concretes 15.3 Preparation of photoactive concrete surface 15.3.1 Method (i) 15.3.2 Method (ii) 15.3.3 Method (iii) 15.4 Properties of photoactive self-cleaning concretes 15.5 Photocatalytic activity testing methods 15.5.1 Self-cleaning test 15.5.2 Depollution testing 15.6 Advantages and disadvantages of self-cleaning concretes 15.7 Self-cleaning photoactive concrete in real-world applications 15.8 Market status of photoactive materials 15.9 Summary and conclusions 15.10 Future prospects References Further reading Metal oxide nanocomposites: design and use in antimicrobial coatings Vijay S. Ghodake, Shamkumar P. Deshmukh and Sagar D. Delekar 16.1 Introduction 16.2 Microbes and microbial infectious diseases 16.3 Antimicrobial coatings: market scenario 16.4 Metal oxide nanocomposites as potential antimicrobial agents 16.4.1 Composites of metal oxide with inorganic moieties 16.4.2 Composites of metal oxide with organic moieties 16.5 Plausible mechanisms for nanocomposites-based microbes inactivation 16.6 Synthesis strategies for designing metal oxide nanocomposite 16.7 Metal oxide nanocomposites based on antimicrobial coatings in different fields 16.7.1 Hospital sector 16.7.2 Textile sector 16.7.3 Food sector 16.7.4 Polymer sector 16.7.5 Paint sector 16.7.6 Leather sector 16.8 Conclusions 16.9 Future outlooks Acknowledgment References

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523 523 526 530 530 531 531 531 535 535 536 537 538 541 542 542 543 547

549 549 550 555 557 558 569 570 572 574 575 578 580 583 584 586 589 589 590 590

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17

18

Contents

Metal oxide composites in organic transformations Meghshyam K. Patil, Sambhaji T. Dhumal and Vijay H. Masand 17.1 Introduction 17.2 Design and characterization of nanocomposites 17.3 Applications of metal oxide composites for organic transformations 17.3.1 Synthesis of bis (pyrazol-5-ol) and dihydropyrano[2,3-c]pyrazole analogs 17.3.2 Synthesis of pyrimido benzazoles 17.3.3 Synthesis of pyridine-3-carboxamides 17.3.4 Synthesis of benzimidazolo[2,3-b]quinazolinone derivatives 17.3.5 Synthesis of dihydroquinazolinones 17.3.6 Synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones 17.3.7 Synthesis of chromene derivatives 17.3.8 Synthesis of 1,4-disubstituted-1,2,3-triazoles 17.3.9 Synthesis of pyran derivatives 17.3.10 Synthesis of thieno[2,3-d]pyrimidin-4(3H)-one Derivative 17.3.11 Synthesis of α-chloro aryl ketones 17.3.12 C H arylation reactions through aniline activation 17.3.13 Synthesis of unsymmetrical ureas 17.3.14 Synthesis of Betti bases and bisamides 17.3.15 Synthesis of 3-aryl-2-[(aryl)(arylamino)]methyl-4Hfuro[3,2-c]chromen-4-one derivatives 17.3.16 Synthesis of benzo[4,5]thiazolo[3,2-a]chromeno [4,3-d]pyrimidin-6-one derivatives 17.3.17 Synthesis of substituted pyrazolones 17.3.18 Synthesis of 7-aryl-benzo[h]tetrazolo[5,1-b]quinazoline5,6-dione 17.3.19 Reduction of nitrobenzene and p-nitrophenol 17.4 Concluding remarks References Metal oxide-based composites as photocatalysts Sandeep R. Patil 18.1 Introduction 18.1.1 Principles of metal oxide-based composites as photocatalysts 18.1.2 Mechanism of photocatalytic reactions 18.2 Unitary metal oxides versus composite-based metal oxide photocatalysts 18.3 Applications of metal oxide-based photocatalysts

601 601 602 605 605 607 607 608 609 610 611 614 615 616 616 617 617 618 619 620 620 621 622 623 623 633 633 633 633 634 641

Contents

18.3.1 Photoelectrocatalysis for energy conversion 18.3.2 Hydrogen production 18.3.3 Water treatment and environment 18.3.4 CO2 reduction (hydrocarbon generation) 18.3.5 Antibacterial, anticancer, and biomedical applications 18.3.6 Layered double hydroxides/metal-organic frameworks 18.3.7 Polymeric nanophotocatalysts 18.3.8 Food safety 18.4 Future perspectives of metal oxide-based composites as photocatalysts References 19

Metal oxide-based composites for magnetic hyperthermia applications Amol B. Pandhare, Rajendra P. Patil and Sagar D. Delekar 19.1 Introduction 19.2 Present cancer treatment: pros and cons 19.3 Hyperthermia 19.3.1 Classification of hyperthermia 19.3.2 Magnetic hyperthermia 19.4 Representative nanomaterials for magnetic hyperthermia 19.5 Magnetic metal oxide nanomaterials-based composites for magnetic hyperthermia application 19.6 Iron oxide nanoparticles and surface functionalization 19.7 Methods for measuring the magnetism of the magnetic materials 19.7.1 Superconducting quantum interference device magnetometry 19.7.2 Zero-field cooling and field cooling measurements 19.7.3 Vibrating-sample magnetometer 19.7.4 Heating capacity: induction heating system 19.8 Conclusions 19.9 Challenges and future perspectives References

Index

xv

641 644 647 650 652 655 657 659 660 661

673 673 675 676 676 679 679 680 683 684 684 685 686 687 689 689 690 697

List of contributors

C.D. Bathula Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, South Korea Joanna Bauer Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wrocław, Poland Sheshanath V. Bhosale School of Chemical Sciences, Goa University, Taleigao, Goa, India T.P. Chavan D. Y. Patil College of Engineering & Technology, Kolhapur, Maharashtra, India Sagar D. Delekar Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Lynn Dennany Department of Pure and Applied Chemistry, University of Strathclyde, Technology and Innovation Centre, Glasgow, United Kingdom Shamkumar P. Deshmukh Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India; Department of Chemistry, D.B.F. Dayanand College of Arts and Science, Solapur, Maharashtra, India Ananta G. Dhodamani Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India; Department of Chemistry, Rajarshi Chhatrapati Shahu College, Kolhapur, Maharashtra, India Ankita K. Dhukate Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Sambhaji T. Dhumal Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Sub-Campus Osmanabad, Maharashtra, India; Department of Chemistry, Ramkrishna Paramhansa Mahavidyalaya, Osmanabad, Maharashtra, India

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List of contributors

Dipak J. Garole Directorate of Geology and Mining, Government of Maharashtra, Nagpur, Maharashtra, India Vijay S. Ghodake Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Sushilkumar A. Jadhav School of Nanoscience and Technology, Shivaji University, Kolhapur, Maharashtra, India Shyue-Chu Ke Department of Physics, National Dong Hwa University Shou-Feng, Hualien, Taiwan D-Y. Kim Department of Biological and Environmental Science, College of Life Science and Biotechnology, Dongguk University Biomedical Campus, Gyeonggido, South Korea Valmiki B. Koli Department of Physics, National Dong Hwa University ShouFeng, Hualien, Taiwan Vijay P. Kothavale Department of Physics, Bhogawati Mahavidyalaya Kurukali, Shivaji University, Kolhapur, Maharashtra, India Pramod A. Koyale Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Rajaram S. Mane School of Physical Sciences, SRTMU, Nanded, Maharashtra, India Vijay H. Masand Department of Chemistry, Vidya Bharati Mahavidyalaya, Amravati, Maharashtra, India Harshad A. Mirgane School of Chemical Sciences, Goa University, Taleigao, Goa, India Pranay P. Morajkar School of Chemical Sciences, Goa University, Taleigao, Goa, India Swapnajit V. Mulik Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Sajid B. Mullani Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India Dinesh N. Nadimetla School of Chemical Sciences, Goa University, Taleigao, Goa, India

List of contributors

xix

Dillip K. Panda Department of Chemistry, Clemson University, Clemson, SC, United States Amol B. Pandhare Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India; Department of Chemistry, M.H. Shinde Mahavidyalaya, Kolhapur, Maharashtra, India Meghshyam K. Patil Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Sub-Campus Osmanabad, Maharashtra, India Pramod S. Patil Department Maharashtra, India

of

Physics,

Shivaji

University,

Kolhapur,

Prashant B. Patil Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, India Rajendra P. Patil Department of Chemistry, M.H. Shinde Mahavidyalaya, Kolhapur, Maharashtra, India S.M. Patil Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India; Department of Chemistry, Karmaveer Hire College, Gargoti, Kolhapur, Maharashtra, India Sandeep R. Patil School of Science, Navrachana University Vadodara, Vadodara, Gujarat, India Sarita Patil Department of Physics, Sanjay Ghodawat University, Kolhapur, Maharashtra, India Prakash S. Pawar Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India; Department of Chemistry, Shri Yashwantaro Patil Science College Solankur, Kolhapur, Maharashtra, India S. Ramesh Department of Mechanical, Robotics and Energy Engineering, Dongguk University-Seoul, Seoul, South Korea Akshay V. Salkar School of Chemical Sciences, Goa University, Taleigao, Goa, India S.K. Shinde Department of Biological and Environmental Science, College of Life Science and Biotechnology, Dongguk University Biomedical Campus, Gyeonggido, South Korea Shubhangi D. Shirsat Department of Biotechnology, SRTMU New Model Degree College, Hingoli, Maharashtra, India

xx

List of contributors

Saurabh S. Soni Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India Keval K. Sonigara Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India; Oxford Suzhou Centre for Advanced Research (OSCAR), University of Oxford, Suzhou Industrial Park, Jiangsu, P.R. China N.D. Thorat Medical Science Division, Nuffield Department of Women’s & Reproductive Health, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom Nanasaheb D. Thorat Nuffield Department of Women’s and Reproductive Health, Medical Science Division, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom; Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wrocław, Poland Syed A.M. Tofail Modelling Simulation and Innovative Characterisation (MOSAIC), Department of Physics and Bernal Institute, University of Limerick, Limerick, Ireland Jayraj V. Vaghasiya Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India S.A. Vanalakar Department of Physics, Karmaveer Hire College, Gargoti, Kolhapur, Maharashtra, India H.M. Yadav Department of Biological and Environmental Science, College of Life Science and Biotechnology, Dongguk University Biomedical Campus, Gyeonggi-do, South Korea; School of Nanoscience and Biotechnology, Shivaji University, Kolhapur, Maharashtra, India

Series editor biography

Ghenadii Korotcenkov received his PhD in physics and technology of semiconductor materials and devices in 1976 and his Doc. Habil. degree in physics of semiconductors and dielectrics in 1990. He has more than 50 years of experience as a teacher and scientific researcher. He has been a leader of gas sensor group and manager of various national and international scientific and engineering projects at the Laboratory of Micro- and Optoelectronics, Technical University of Moldova, Chisinau, Moldova. From 2007 to 2008, he served as an invited scientist at the Korea Institute of Energy Research (Daejeon). Until 2017, Dr. G. Korotcenkov worked as a research Professor at the School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Korea. Currently, G. Korotcenkov is working as a chief scientific researcher at Moldova State University, Chisinau, Moldova. His scientific interests include material science, especially metal oxide film deposition and characterization, surface science, thermoelectric conversion, and design of physical and chemical sensors, including thin film gas sensors. G. Korotcenkov is the author and editor of 45 books and special issues, including 11-volume “Chemical Sensors” series published by Momentum Press, 15-volume “Chemical Sensors” series published by Harbin Institute of Technology Press, China, 3-volume “Porous Silicon: From Formation to Application” issue published by CRC Press, 2 volumes “Handbook of Gas Sensor Materials” published by Springer, and 3-volume “Handbook of Humidity Measurements” published by CRC Press. Currently, he serves as the series editor of “Metal Oxides” book series published by Elsevier. G. Korotcenkov is the author and coauthor of more than 650 scientific publications including 33 review papers, 38 book chapters, more than 200 peer-reviewed articles published in numerous scientific journals [h-factor = 42 (Web of Science), h = 43 (Scopus), and h = 59 (Google scholar citation)]. He is a holder of 17 patents. He has presented more than 250 reports on the National and International conferences, including 17 invited talks. G. Korotcenkov, as the cochair or as the member of program, scientific and steering committees, participated in the organization of more than 30 international scientific conferences. Dr. G. Korotcenkov is an editorial board member in five scientific international journals. His name and activities have been listed by many biographical publications including Who’s Who. His research activities are honored by a Honorary Diploma of the Government of the Republic of Moldova (2020), an Award of the Academy of Sciences of Moldova (2019), an

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Series editor biography

Award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004); the Prize of the Presidents of the Ukrainian, Belarus, and Moldovan Academies of Sciences (2003); Senior Research Excellence Award of Technical University of Moldova (2001; 2003; 2005), the National Youth Prize of the Republic of Moldova in the field of science and technology (1980), among others. G. Korotcenkov also received a fellowship from the International Research Exchange Board (IREX, USA, 1998), Brain Korea 21 Program (2008 12), and BrainPool Program (Korea, 2015 17).

About the editor

Dr. Sagar D. Delekar is presently working as a professor in the Department of Chemistry at Shivaji University, Kolhapur, India. Prof. Delekar has over 20 years of teaching and research experiences in the field of Chemical Sciences. Prof. Delekar obtained his Master of Science and PhD degree in chemistry from Shivaji University, Kolhapur. He has completed a summer research fellowship at the reputed Indian Institute of Science, Bangalore, India. He is also the recipient of fast track proposals for Young Scientists as well as the prestigious Raman Fellowship sponsored by the Indian Government. Under the Raman fellowship, he worked as a postdoctoral fellow with the Nobel Laureate Sir Harold Kroto as well as Prof. Naresh Dalal at the Department of Chemistry and Biochemistry, Florida State University, Tallahassee, USA. Prof. Delekar has broad research interests in the field of inorganic chemistry, solid state chemistry, and functional nanocomposites for various applications, such as energy studies, catalytic studies, and biomedical fields. So far, Prof. Delekar has published more than 80 research articles of international repute with the completion of major research projects funded by DST, UGC, RGSTC, etc., and filed four Indian patents. Prof. Delekar is actively engaged in the academics, research, and extension activities of the University.

Foreword

According to the famous quote of Scientist Dmitri Mendeleev (Father of Periodic Table), “Without the material, the plan alone is but castle in the air—a mere possibility whilst the material without a plan is useless matter.” This statement has been true forever, and hence various materials have been developed continuously for the different purposes of mankind from ancient era onward. Therefore these are the ubiquitous components of our daily life. As like our basic needs, the material is one of the most common dire need, and hence the layman as well as the scientists have made their efforts to discover, search, invent, and innovate different materials for various potential applications. In addition, materials not only fulfill the requirements of individuals but also help to gear up the socioeconomic growth of the society. Among various materials, metal oxides are the most well-known materials used in all the facets of our life. Many metal oxides are found in nature, while others are made artificially. For example, sand is one of the natural metal oxides used for various applications such as construction, decoration, glass and manufacturing. Similarly, lime is one of metal oxides used in water purification, making different cores, etc. Semiconducting metal oxides such as tin oxide (SnO2), indium oxide (In2O3), zinc oxide (ZnO), and titanium dioxide (TiO2) have been used in various electronic devices. In Indian Ayurveda system, various bhasmas belong to the metal oxide origin (Loah bhasma contains iron oxide, Rajata bhasma contains silver oxide, Jasada bhasma contains zinc oxide, Tamra bhasma contains cupric oxide, etc.) are used for the treatment of different diseases. In addition, many metal oxides have been used as catalysts in different chemical transformations. Therefore metal oxides are commonly used in different sectors such as energy, medical, catalysis, and environment. Nowadays, further developments in bare metal oxides have been ongoing exponentially in all the sectors, and hence metal oxide-based materials are playing an eminent role in the blossoming futuristic technologies such as solar-driven water splitting, solar energy harvesting, energy storage devices like Li-ion batteries and supercapacitors. Along with this, many research endeavors of metal oxide-based materials have been fruitful to achieve effectual and excellent biomedical applications such as biosensing studies, hyperthermia treatment, and antibacterial coatings. Also, the studies in concern with environmental remediation is worthwhile for achieving advanced effective tools for the treatment of wastewater and analysis of air pollutants. In addition to this, catalytic studies for industrial organic transformations are also covered by progressive research in the field of of metal oxide-based materials. Owing to the burgeoning materials field, various organic, inorganic, as well as their hybrids of metal oxides have been used to build up materials having excellent properties for efficient tools or devices. In addition, nanoscale dimensions of metal oxides or their hybrids also result

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in the overall improvement in properties for the desired applications. Therefore metal oxides and their composites have been established as effective candidates for various applications due to their excellent characteristic properties, and various editors and contributing authors have contributed to the present state of the art in metal oxide-based composites in various potential applications. Hence, this book entitled “Advances in Metal Oxides and Their Composites for Emerging Applications” highlights important topics in a definite manner and their aforementioned emerging applications. This book emphasizes various representative metal oxides, and their composites along with their synthesizing protocols, characterization techniques, theoretical aspects, and their importance are elucidated and clarified in a simple and lucid manner. The main emphasis of this book is in concern with the studies of metal oxides in energy technologies, environmental remediation, catalytic transformations, biomedical applications, etc. In connection to energy technologies, the representative portion of the book deals with the detailed study of water splitting system for hydrogen generation, solar energy harvesting for electricity generation, and energy storage devices. The widely explored metal oxide-based materials and their importance in energy technologies are emphasized in depth. In case of solar photovoltaic studies, the current trends of the third-generation solar cells such as organic, perovskite, dye-sensitized, and quantum dot-sensitized solar cells are explained elaborately. Various metal oxides for water splitting with respective mechanism are summarized in the few respective chapters. Energy storage devices, particularly supercapacitors with brief introduction to its evolution, are elucidated. Also, some representative chapters from this book reveal the successive biomedical applications of various metal oxides and their composites for antibacterial coatings in numerous fields, hyperthermia study for cancer treatment, as well as biosensing studies. In addition, both theoretical and practical aspects of metal oxide-based composites for environmental remediations are highlighted, and different facets such as photodegradation studies, removal of heavy metals, self-cleaning materials, are discussed in detail. Also, few chapters are reserved to highlight the importance of metal oxidebased composites as catalysts for heterogeneous transformations as well as photocatalytic reactions. Each chapter of this book provides futuristic scientific vision in a particular area in terms of future perspectives, limitations associated with the present state of the art, and various strategies to overcome these laggings. The sound expertise and great knowledge of contributing authors are reflected through the contents of topic/s included in this book. As all the topics in this book are elaborated thoroughly and effectively by representative authors, this may bring an excellent platform for all scientists, students, teachers, industrial experts, etc. who are working in field of science and technology. This book is admirably edited by Prof. Dr. Sagar D. Delekar, an expert who is working in the field of material science for more than twenty years. He has taken prompt efforts to complete this book. I wish this book will get wide exposure and appreciation worldwide. Prof. Apparao M Rao

Preface to the series

The field of synthesis, study, and application of metal oxides is one of the most rapidly progressing areas of science and technology. Metal oxides are one of the most ubiquitous compound groups on earth, which have a large variety of chemical compositions, atomic structures, and crystalline shapes. In addition, metal oxides possess unique functionalities that are absent or inferior in other solid materials. In particular, metal oxides represent an assorted and appealing class of materials and exhibit a full spectrum of electronic properties ranging from insulating to semiconducting, metallic, and superconducting. Moreover, almost all the known effects including superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism can be observed in metal oxides. Therefore metal oxides have been emerged as an important class of multifunctional materials with significant properties, which have a great potential for numerous device applications. Specific properties of the metal oxides, such as the wide variety of materials with different electrophysical, optical, and chemical characteristics, their high thermal and temporal stabilities, and their ability to function in harsh environments, make metal oxides very suitable materials for designing transparent electrodes, highmobility transistors, gas sensors, actuators, acoustical transducers, photovoltaic and photonic devices, photo- and heterogeneous catalysts, solid-state coolers, highfrequency and micromechanical devices, energy harvesting and storage devices, nonvolatile memories, and many others in the electronics, energy, and health sectors. In these devices metal oxides can be successfully used as sensing or active layers, substrates, electrodes, promoters, structure modifiers, membranes, and fibers, that is, can be used as active and passive components. The other advantages of metal oxides are the low fabrication cost and robustness in practical applications. Furthermore, metal oxides can be prepared in various forms such as ceramics, thick films, and thin films. Thin film deposition can be used for deposition techniques that are compatible with standard microelectronic technology. Last factor is very important for large-scale production, because the microelectronic approach promotes low cost for mass production, offers the possibility of manufacturing devices on a chip, and guarantees good reproducibility. Various metal oxide nanostructures, including nanowires, nanotubes, nanofibers, core-shell structures, and hollow nanostructures also can be synthesized. The field of metal oxide nanostructured morphologies (e.g., nanowires, nanorods, and nanotubes) has become one of the most active research areas within the nanoscience community. The ability to create a variety of metal oxide-based composites as well as to synthesize various multicomponent compounds significantly expand the range of

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Preface to the series

properties that metal oxide-based materials can have, making metal oxides by a truly versatile multifunctional material for widespread use. Small changes in their chemical composition and atomic structure can be accompanied by the spectacular variation in properties and behavior of metal oxides. Even now, advances in synthesizing and characterizing techniques have demonstrated the numerous new functions of metal oxides. Considering the importance of metal oxides for progress in microelectronics, optoelectronics, photonics, energy conversion, sensor and catalysis, various books devoted to this class of materials have been published. However, some books from this list are too general, some books are collections of various original works without any generalizations, and other ones were published many years ago. But, during the past decade, a great progress has been made on the synthesis as well as on the structural, physical, chemical characterization, and application of metal oxides in various devices. Several papers have been published on metal oxides. In addition, till now, many important topics related to metal oxides study and application have not been discussed. To address the situation, we decided to generalize and systematize the results of research in this direction and to publish a series of books devoted to metal oxides. The proposed book series “Metal Oxides” is the first one devoted to only metal oxides. We believe that combining books on metal oxides in a series could help readers in searching required information on the subject. In particular, we plan that the books from our series, which have a clear specialization by its content, will provide interdisciplinary discussion for various oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing and then to device fabrications and applications. This book series is prepared by a team of highly qualified experts, which guarantee its high quality. I hope that our books will beneficial and user friendly, and the readers will consider this “Metal Oxides” book series like an encyclopedia of metal oxides that enables them to understand the present status of metal oxides, to estimate the role of multifunctional metal oxides in the design of advanced devices, and then based on observed knowledge to formulate new goals for the further research. The intended audience of this book series is scientists and researchers, working or planning to work in the field of materials related to metal oxides, that is scientists and researchers whose activities are related to electronics, optoelectronics, energy, catalysis, sensors, electrical engineering, ceramics, biomedical designs, etc. This “Metal Oxides” book series will also be interesting for practicing engineers or project managers in industries and national laboratories, who would like to design metal oxide-based devices, but don’t know how to do it, and how to select optimal metal oxide for specific applications. With many references to the vast resource of recently published literature on the subject, this book series will be serving as a significant and insightful source of valuable information, providing scientists and engineers with new insights for understanding and improving existing metal oxidebased devices and for designing new metal oxide-based materials with new and unexpected properties.

Preface to the series

xxix

This “Metal Oxides” book series would be beneficial for university students, post docs, and professors. The structure of these books offers a basis for courses in the field of material sciences, chemical engineering, electronics, electrical engineering, optoelectronics, energy technologies, environmental control, and many others. Graduate students could also find the book series to be very useful in their research and understanding the features of metal oxides synthesis, study, and application of this multifunctional material in various devices. This series would be beneficial for each group in terms of finding adequate information. Finally, I thank all contributing authors and book editors who have been involved in the production of this books. I am thankful that they agreed to participate in this project and deeply appreciate their efforts in the preparation of this book. Without their participation, this project would have not been possible. I also express my gratitude to Elsevier for giving us the opportunity to publish this series. I especially thank the Elsevier editorial team for their patience during the development of this project and for encouraging us during the various stages of preparation. Ghenadii Korotcenkov

Preface

Various materials have played vital roles in human development since ancient history. Each material has a crucial impact on human lives throughout the globe. As the world population is steadily increasing, the utilization of various materials has been heightened for numerous potential applications in energy, medical, industrial, food sectors, etc., which may bring an excellent utility in human lives. Metal oxides have drastically changed our daily life for their potential applications, especially in the human development area. Various metal oxides such as bare and their hybridbased materials with organic and inorganic moieties have been utilized throughout the globe, which play a crucial role from the social, scientific, as well as economical point of views. Among the various materials, nanocrystalline metal oxides have gained a vital importance throughout the various emerging applications such as solar energy harvesting, photocatalysis, energy storage devices, biomedical applications, environmental remediations, and organic transformations. This is due to their advantages such as physicochemical, optical, and electrical properties, easy synthetic protocols, nontoxicity, thermal stability, and cost-effectiveness. Also, efforts were made to create novel and enhanced properties for improving the performance of nanocrystalline bare metal oxides. Hence, it is necessary to study such phenomenal concepts of metal oxides and their composites for various emerging applications, which include their synthetic techniques, fabrication, and utilization in specific areas with their consequences. In connection with this, the present book entitled “Advances in Metal Oxides and Their Composites for Emerging Applications” displays various important topics, which include theoretical overview as well as practical aspects of metal oxide-based composites for emerging applications. This includes synthetic strategies for metal oxides and their composites, recent trends involved in research activities, corresponding present state of the arts and also covers the representative metal oxide-based composites used in energy technologies (such as water splitting studies, supercapacitor studies, organic photovoltaics, and third-generation solar devices), catalytic transformations (such as photocatalysis and organic synthesis), biomedical applications (such as biosensing studies, antimicrobial coatings, and hyperthermia studies), and environmental remediations (such as photodegradation studies, wastewater treatments, and CO2 reductions). It also highlights the future perspectives and challenges regarding each applicatory portion, which provides the platform for the futuristic research endeavors. This book describes the desired contents in three subsections with a total of nineteen chapters. The first section briefly provides a description of the metal oxides and their properties, which further include the necessity of metal oxide composites

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with almost all synthetic strategies and characterization techniques. The second section describes the applications of metal oxide composites for energy technologies. This section comprises the recent trends and strategies in the field of solar cells, energy storage devices, organic photovoltaics, and water splitting studies. The third and final section demonstrates metal oxide composites-based applications in the field of biosensing, photocatalysis, antimicrobial coating studies, environmental remediation, catalytic transformations, and hyperthermia studies. All the contributing authors have provided a phenomenal input for designing appropriate platform to witness the current trends as well as futuristic research scopes to overcome the existing challenges. This book will be a convenient reference material for the material scientists, chemists, physicists, industrial experts, as well as environmental scientists working in the areas of functional composites for the emerging applications in energy sectors, medical fields, environmental sectors, chemical transformations, etc. In addition, this book will also be beneficial to teachers, undergraduate and graduate students to understand the theoretical, experimental, as well as practical aspects of metal oxide-based composites in the different applied areas of science and technology. As the editor, I sincerely appreciate all the contributing authors for their imperative contributions to complete this book. I also express special gratitude to Prof. Ghenadii Korotcenkov (Series Editor, Metal Oxides Series, Elsevier Science & Technology Book), Chiara Giglio and Clodagh Holland-Borosh (Editorial Project Managers at Elsevier), Indhumathi Mani (Copyright Coordinator, Elsevier), and Unni Ramu (Payment Coordinator, Elsevier) for their constant support in successfully completing this project. I am thankful to Elsevier and the entire team who have played a significant role in contributing to this book. Sagar D. Delekar

Acknowledgment

As the editor, I would like to express my deepest appreciation to all the contributing authors and colleagues for their authoritative support for successfully completing this book. Also, I would like to thank Honorable Prof. P.S. Patil (Pro Vice Chancellor, Shivaji University, Kolhapur, India), Dr. Lynn Dennany (University of Strathclyde, Glasgow, United Kingdom), Dr. Dillip Kumar Panda (Clemson University, Clemson, USA), Dr. Nanaso Thorat (Oxford University, United Kingdom), Prof. Sheshanath Bhosale (Goa University, Panjim, India), Prof. Saurabh Soni (Sardar Patel University, Anand, India), Prof. Rajaram Mane (Swami Ramanand Teerth Marathwada University, Nanded, India), Dr. Meghshyam Patil (Dr. Babasaheb Ambedkar Marathwada University, Sub-campus Osmanabad, India), Dr. Sandeep Patil (Navrachana University, Vadodara, India), Dr. Hemraj Yadav (Dongguk University, South Korea), Dr. Sushilkumar Jadhav (Shivaji University, Kolhapur, India), Dr. Pranay Morajkar (Goa University, Panjim, India), Dr. Prashant Patil (The New College, Kolhapur, India), Mr. Vijay Kothavale (Bhogawati Mahavidyala, Kurukali-Kolhapur, India), Dr. Valmiki Koli (National Dong Hwa University Shou-Feng, Taiwan), and Dr. Rajendra Patil (Department of Chemistry, M.H. Shinde Mahavidyalaya, Tisangi, Gaganbavda, Kolhapur, India) for their significant contributions for the respective book chapters so as to complete this book. I would also like to acknowledge my research scholars including Dr. Shamkumar Deshmukh (Dayanand College of Arts and Science, Solapur, India), Dr. Satish Patil (Department of Chemistry, Karmaveer Hire College, Gargoti, Kolhapur, India.), Dr. Anant Dhodamani (Department of Chemistry, Rajarshi Chhatrapati Shahu College, Kolhapur, India), Dr. Sajid Mullani (Department of Chemistry, Shivaji University, Kolhapur, India), Mr. Prakash Pawar (Shree Yashwantrao Patil Science Collage, Solankur, India), Mr. Pramod Koyale, Mr. Swapnajit Mulik, Mr. Vijay Ghodake, Miss. Ankita Dhukate, and Mr. Amol Pandhare (Department of Chemistry, Shivaji University, Kolhapur) for their sincere and dedicated efforts in completing their assigned task in the stipulated time frame. I am so much thankful to all who directly and indirectly helped through their distinctive assistance for the completion and writing of this book. I express my special gratitude to Prof. Ghenadii Korotcenkov (Series Editor, Metal Oxides Series, Elsevier Science & Technology Book), Chiara Giglio and Clodagh Holland-Borosh (Editorial Project Managers at Elsevier), Indhumathi Mani (Copyright Coordinator, Elsevier), and Unni Ramu (Payment Coordinator, Elsevier) for their constant support and pledge to this endeavor. Finally, I am grateful to the entire team of Elsevier for their generous support at every stage during the production of this book. Sagar D. Delekar Department of Chemistry, Shivaji University, Kolhapur, India

Part I : Introduction to metal oxide-based composites

Metal oxide engineering

1

Pramod A. Koyale1, Dillip K. Panda2 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, Clemson University, Clemson, SC, United States

1.1

Human development and metal oxides nexus

Materials are so prevalent and play a significant role in human life. Housing, clothing, food manufacturing, transportation, and almost every portion of our daily endeavor are influenced by the extensive use of materials. Such arena of the materials is very enormous and mixed, where history started from the necessity of human beings itself and these materials furnished name to the ages of human civilization such as the Stone age, the Bronze age, the Iron age, the Steel age, and the Space age (Materials & Man’s Needs: Materials Science and Engineering, 1974). Each age was marked by the emergence of certain materials (Fig. 1.1). Human civilization started with the Stone age (from 3 million BCE up to 3300 BCE) where people used natural materials like stone, clay, wood, etc. for making weapons, shelters, tools, etc. Further evolution from the Stone age to the Bronze age was due to the necessity of better-quality tools for human lives, where people found copper which started at 3300 BCE and ended in 1200 BCE. This age was further followed by the Iron age (between 1200 BCE to 1800 CE), where this brought tools and utensils, that is, domestic materials. Iron was abundant and its use affected every aspect of human civilization. The Steel age (1800 CE onwards) is the next key change in human civilization, which brought the cheap process to make steel. Owing to these, the Steel age established railroads, instruments, and the beginning of the Industrial Revolution to date. In addition, another age in which we are present is the Space age, which is marked by many technological developments toward light and stronger materials for electronic gadgets, semiconductors, high-temperature ceramics, biomaterials, etc. (Ashby & Jones, 1996; Callister, 2004). The present age is also called the age of science and technology, where many scientific discoveries, as well as technologies, came into the globe using different materials. In this age, the technologies such as nanotechnology, biotechnology, information technology as well as artificial intelligence, data analysis, machine learning, etc. are the hot cakes in the scientific community and would have the potential to fulfill the dire needs of human beings. In concern with the rapid growth in population and industrialization, the developments of new technologies, as well as materials for the devices, are one of the demanding research endeavors in science to the present scientific community. Especially, the materials include metals (Kawawaki et al., 2020), metal oxides Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00004-X © 2022 Elsevier Inc. All rights reserved.

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 1.1 Different ages of human civilization based on materials, where BCE:before the common era; BC:before christ; CE:common era and AD:anno domini.

(Medhi et al., 2020), polymers (Guo & Facchetti, 2020), organic compounds (Zhao et al., 2020), carbon nanostructures (Luo et al., 2020), organic inorganic compounds (Ali et al., 2021), etc. which have been exploiting to great extent over the various applications. Among them, the metal oxides have received significant consideration for use in various purposes due to their remarkable physicochemical properties, optoelectrical properties, ferromagnetic, ferroelectric, and photoluminescence nature, etc. (Ghosh et al., 2021; Guo et al., 2015). The importance and prevalence of the metal oxides can be known by sight at the research activities in various sectors, which are executing in terms of research articles and patents published. To date, nearly 1.8 million research articles related to metal oxides have been published, which demonstrates that the dominance of metal oxides over every segment in the research community. The following bar diagram (Fig. 1.2) demonstrates the chronological order of year-wise publications related to metal oxides. In addition, the emergence of nanoscale dimensions after the 1990s playing significant aspects in engineering the metal oxides as well, and hence nanodimensions metal oxides have an emerging interest in the present state of the art for the remarkable applications in various fields such as energy, biomedical, electronics, catalysis, etc. as shown in Fig. 1.3 (Ray & Pal, 2017). From a scientific as well as technological point of view, the metal oxide nanostructures frolicked a significant role due to having rewards of easy synthesis, non-toxicity, cost-effectiveness, crystalline size/shape, having diverse morphologies, and many more. Particularly, transition and inner transition metal oxides which include TiO2 (Sharma et al., 2020), ZnO (Ong et al., 2018), Fe3O4 (Liu et al., 2020), Fe2O3 (Lv et al., 2015), CuO (Gawande et al., 2016), NiO (Song et al., 2020), WO3 (Song et al., 2019), BiVO4, (Chen, Jiang, et al., 2020), SrTiO3 (Call et al., 2016), CeO2 (Xu & Qu, 2014), etc. that have been commonly deployed in various applications with the intent of

Metal oxide engineering

5

Figure 1.2 Year-wise number of publications related to the utilization of metal oxides from 2011 onward. Source: Data is based on the number of articles published on the Elsevier platform using the keyword “metal oxides” (total number of publication-12,59,996) dated 1 April 2021.

Figure 1.3 Representative applications of metal oxides in various sectors. Source: Reprinted with permission of Gopiraman, M., Karvembu, R., & Kim, I. S. (2014). Highly active, selective, and reusable RuO2/SWCNT catalyst for heck olefination of aryl halides. ACS Catalysis, 4(7), 2118 2129; Hoekstra, D., Nickmans, K., Lub, J., Debije, M. G., & Schenning, A. P. H. J. (2019). Air-curable, high-resolution patternable oxetane-based liquid crystalline photonic films via flexographic printing. ACS Application Materials Interfaces, 11(7), 7423 7430 and Maity, C. K., Hatui, G., Tiwari, S. K., Udayabhanu, G., Pathak, D. D., Chandra Nayak, G., & Verma, K. (2019). One pot solvothermal synthesis of novel solid state N-Doped TiO2/n-Gr for efficient energy storage devices. Vacuum, 164, 88 97.

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Advances in Metal Oxides and Their Composites for Emerging Applications

optimistic usage. Thus, it is conceivable to say that metal oxides are ubiquitous materials used to cover necessity as advanced, future, and modern materials for the well beings of human lives. Though having several positive points of metal oxides over various applications, however, these materials have been further engineered to overcome their constraints (Fig. 1.4). The common constraints have been observed such as being able to absorb light in only the ultraviolet (UV) region, having limited active sites, the poor charge separation which leads to the possibility of charge recombination and poor rate of electron transfer, poor electronic conductivity, etc. (Zhu et al., 2020). So, in concern with the improvisation of performance of metal oxides, many research groups have been continuously making efforts by means of engineering metal oxides till the date. In the present state of the art, such semiconducting metal oxides playing a vital role in various sectors which provide numerous positive insights. In connection with these, this book chapter highlights the significances of metal oxides, their engineering by various strategies for better performance as well as their significances. Further, this chapter also demonstrates the applications of engineered metal oxides. Hence, it is necessary to deal with the properties, synthesis of such materials as well as the importance of their modification, for a better outlook toward emerging research activities. With these motivations, detailed study of representative metal oxides, their properties, phase, and composition-controlled engineering of metal oxides, and their applications are the prime objectives of this book chapter.

1.2

Metal oxide engineering: strategies and significances

As already discussed, metal oxides are the prime candidates over various potential applications nearly in every sector. The various bare semiconducting metal oxides

Figure 1.4 Advantages, disadvantages, and types of metal oxides.

Metal oxide engineering

7

have been demonstrated as active materials in wide potential applications. But in most of the studies, such bare metal oxides have been added restraints which resulted in their futile performance. So, in concern with the motives of enhanced properties for metal oxides, continuous breakthroughs, and significant progress have been employed to design engineered metal oxides as promising candidates. Hence, various strategies such as doping, composite formation, crystallinity engineering, surface reconstruction, morphology engineering, nanodimensions engineering, etc. (Medhi et al., 2020; Zhu et al., 2020) have been adopted for the aforementioned purpose for high activity toward potential applications. Let’s highlight the various strategies used for tuning the properties of metal oxides which is represented in following Fig. 1.5.

Figure 1.5 Various strategies deployed for engineering the metal oxides with properties.

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Advances in Metal Oxides and Their Composites for Emerging Applications

1.2.1 Bulk versus nanoscale In the current era of science and technology, the formation of nanoscale materials is the most exciting area of research where the properties of these materials significantly different from that of their bulk counterparts. The carbon nanotubes (CNTs) were the first synthesized nanomaterials; which were invented by Sumio Iijima in 1991 at the National Research Laboratory, New Jersey using an arc discharge method in an effort to assess carbon structures comprised of needle-like tubes (Patel et al., 2020). These nanotubes show some fascinating properties such as having high elastic modulus, high strength (several dozen times stronger than steel), extraordinary electrical and thermal conductivity, superior thermal stability up to 2800 C, etc. which differ from that of their other allotropes such as diamond, graphite, and fullerenes (Aguzzi et al., 2019; Norizan et al., 2020; Yu et al., 2018). So, in concern with such strengths CNTs have fabulous latent for respective applications in many scientific as well as technological areas. Similarly, the nanocrystalline metal oxides are highly focused in the different research endeavors due to their remarkable tuning of optoelectrical properties to that of their bulk forms. For example, ZnO or TiO2 is used commonly in sunscreen lotions as UV-blockers. The use of such materials in bulk form is easily recognized as white cream when the lotion is applied to the face, but nanodimensions ZnO- or TiO2-based lotions are totally transparent and hence are not recognized through the naked eye. Therefore, nanodimensional metal oxides became transparent to that of their bulk counterparts. Similar observations were also noted when we deposited these metal oxides in thin-film forms as well. Therefore, it is imperative to start the basics of nanoscience and technology for learning how the properties of metal oxides to be engineered in nanodimensions. Nanoscience is one of the emerging sciences since 1990 onwards, which relates to design, construct, and synthesizing materials having sizes within the range of 1 to 100 nm (Weiss et al., 2020). Such materials with nanoscale dimensions display different assets such as electrical, optical, magnetic, mechanical, and chemical properties than that of bulk counterparts (Mallakpour & Madani, 2015; Mourdikoudis et al., 2018). For example, the sizeoriented properties of gold (Au) nanoparticles (NPs) have been described properly from the last few decades. At the nanoscale, Au exhibited purple color which was different from bulk one (Fig. 1.6A). This was attributed to either surface plasmon resonance or confinement effect resulting in the change in band model (continuous to discrete) as shown in Fig. 1.6B (Jose Varghese et al., 2019; Son et al., 2014a). In addition, the nano Au was observed to be melting at a lower temperature than bulk Au (1336K) because nano Au having more surface atoms required less thermal energy to overcome the atomic energy barriers in comparison to bulk Au. Similarly, bulk Au is the bad catalyst and hence not used in catalytic transformations; however nano Au has been established as excellent catalysts in oxidation and reduction reactions. This is due to having the high surface-area-to-volume ratio, which drastically improve the surface area and result in the more active sites at the nanoscale level for the respective reactions (Mourdikoudis et al., 2018). Similarly, the metal oxides with nanoscale dimensions also result in the higher surface area, wider optical bandgap resulting in the separation of energy levels between

Metal oxide engineering

9

Figure 1.6 (A) Size & color dependence of Au NPs and (B) schematic of electron confinement & change in bandgap with size. Source: From Jose Varghese, R., Sakho, E. hadji M., Parani, S., Thomas, S., Oluwafemi, O. S., Wu, J., Thomas, S., Sakho, E. H. M., Kalarikkal, N., Oluwafemi, S. O., & Wu, J. (2014). (pp. 75 95). Elsevier. https://doi.org/10.1016/B978-0-12-813337-8.00003-5.

valence bands and conduction bands, multiple exciton generations through absorption of the different quanta of incident photons, formations of more charge carriers, etc. for uplifting their aforesaid properties. The comparison of bulk counterparts vs. nanomaterials is also demonstrated by the quantum confinement effect, which is one of the basic reasons for explaining the properties changes that occurred in the materials. One of the research groups demonstrated the GeO2 based lithium battery, where it was observed that the theoretical reversible lithium storage was about 1126 mAh g21, showing fast capacity disappearance during cycling which caused particle cracking and reduction, that is, unstable. (Son et al., 2014a) Further, attempts were made to develop nanosized GeO2 material to diminish the significances of stress and instability in an electrochemical study of batteries. Rasmussen et al., reported the theoretical study of bulk vs. nanoscale NiO, CuO, and TiO2 for their dissolution performance in cell culture and water channel. (Avramescu et al., 2020) In this assessment, the observed surface area for nanoscale dimensions metal oxides was high, almost 5 10 times greater than their bulky counterparts. (Table 1.1) So, the use of metal oxide nanostructures has been achieved substantial interest in the present state of the art for the different applications. These nanoscale materials have been classified according to their dimensions such as zero-dimensional (quantum dots, nanoparticles, etc.), one dimensional (nanorods, nanowires, nanotubes, etc.), two dimensional (nanosheets, nanolayers, nanofilms, etc.), and three-dimensional (framework type, polyclusters, etc.) nanomaterials. The pictorial

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Advances in Metal Oxides and Their Composites for Emerging Applications

Table 1.1 Characteristics of bulk and nanoscale dimension metal oxides in terms of particle size and surface area. Metal oxides

Particle size (nm)

Specific surface area (m2 g21)

1500 5000 5000

B5 8 5 20 4 6

19 15 35 28

55.55 50 100 33

Bulk TiO2 NiO CuO Nano TiO2 (anatase) NiO CuO

Source: From Avramescu, M.-L., Che´nier, M., Palaniyandi, S., & Rasmussen, P. E. (2020). Dissolution behavior of metal oxide nanomaterials in cell culture medium vs distilled water. Journal of Nanoparticle Research, 22(8), 222.

Figure 1.7 Bulk to nanoscaled materials transformation and types of nanomaterials based on dimensions.

presentation of this transformation of bulky to nanoscale materials with their types is given in Fig. 1.7.

1.2.2 Undoped versus doped Metal oxides doped with a suitable metal or non-metal-ions influenced the change in properties as compared to that of their bare or undoped counterpart. The main goal of doping is to enhance the desirable properties while in others it is to eliminate the

Metal oxide engineering

11

undesired effects as well. Therefore, doping has been demonstrated as an effective and easy way not only to introduce metal or non-metal-ions but also to tune the performance of metal oxides. Doping usually leads to the generation of defects in the host lattice; which makes it possible to lift essential catalytic activity by tempering electronic structures, ionic or electronic conductivity, optical properties, and active sites for transformations, etc. The heating method is one of the oldest methods for defect formation; which is also capable to change structure or morphology (Deng et al., 2020). Similarly doping is usually deployed as effectual paths to generate oxygen vacancies in stoichiometric oxides without change in structural or morphological properties (Zhu et al., 2020). In addition, the changes in optical, as well as electronic structure, can also be induced by doping, which further causes to change in bandgap value. As earlier discussed, both metal- and non-metal-ions have been utilized widely as dopants for doping purposes. Ions of nitrogen, carbon, boron, halogens, phosphorous, sulfur, etc. are the commonly deployed non-metal dopants (Medhi et al., 2020). Owing to doping, additional dopant states can be created between the original bandgap, which triggered new excitations related to visible light facets. As different elements such as earth metals, transition and inner transition metals and non-metals have been utilized as dopants, the use of transition metals found to be widely and effectively (Singh et al., 2019). One of the research groups demonstrated the use of chromium (III) as a dopant in ZnO, which revealed that optical absorption of ZnO enhanced from ultraviolet (UV) to visible region after the incorporation of Cr (III) (Khan et al., 2020). Herein, the concentration of charge carriers was enhanced by decreasing electrical resistivity. In concern with non-metallic doping, Batalovi´c, et. al., reported nitrogen-doped TiO2 prepared via the solvothermal method (Batalovi´c et al., 2017). In this present investigation, efforts were taken to enhance the absorption of TiO2 toward the visible region (up to 450 nm) which further expected to enhance photocatalytic activity. In addition, the desired detailed investigation was done for vanadium, chromium, manganese, iron, cobalt and nickel doped TiO2 nanomaterials in one of the reviews (Coropceanu et al., 2007). With the help of density functional theory, the electronic structures for doped TiO2 were calculated (Fig. 1.8A & B). It is conceivable to say that after doping, occupied levels were generated along with the localization of electrons around each dopant. And hence it was demonstrated that there was the shifting of localized levels to lower energy, as the atomic number of dopants enhanced (Coropceanu et al., 2007; Umebayashi et al., 2002). So, it reveals that because of doping there is an alteration in optical as well as electrical assets, which parallelly affects the respective recital of the material. In the present research communal, the efforts are also established to design a multidopant system to achieve desired assets for enhanced performances in various studies. In the case of such a multidopant system, if the concentration or composition of either one dopant is changed, then there is a change in the properties of materials, such as optical, electrical, mechanical, etc. In connection with this, one of the reviews displayed the study of the multidopant system for lithium-ion batteries (Wang et al., 2020). It was observed that after a change in the composition of solid electrolytes, the mechanical properties were altered, which can be seen through Table 1.2.

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 1.8 (A) Bonding diagram of TiO2, and (B) density of states (DOS) of the M-doped TiO2 (M: V, Cr, Mn, Fe, Co, or Ni). Gray solid lines: total DOS. Black solid lines: dopant’s DOS. Source: From Umebayashi, T., Yamaki, T., Itoh, H., & Asai, K. (2002). Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations. Journal of Physics and Chemistry of Solids, 63(10), 1909 1920. https://doi.org/10.1016/S0022-3697(02)00177-4. Table 1.2 The mechanical properties over the different compositional solid electrolytes. Composition

Young’s modulus, E (GPa)

Hardness, H (GPa)

Li6.24La3Zr2Al0.24O11.98 Li6.91La3Zr1.98Al0.13O12 Li6.17Al0.28La3Zr2O12 Li0.33La0.57TiO3

149.8 6 0.4 145.6 6 7.3 162.6 (0 K) 200 6 3

6.4 6 0.4 8.5 6 0.4 9.2 6 0.2

Source: From Wang, C., Fu, K., Kammampata, S. P., McOwen, D. W., Samson, A. J., Zhang, L., Hitz, G. T., Nolan, A. M., Wachsman, E. D., Mo, Y., Thangadurai, V., & Hu, L. (2020). Garnet-type solid-state electrolytes: Materials, interfaces, and batteries. Chemical Reviews, 120(10), 4257 4300.

The different doped metal oxides and their applications are listed in Table 1.3.

1.2.3 Phase diversity Among the existing structural parameters such as morphology, size, dimension, composition, etc., the phase change is one of the emerged parameters and hence revealing the different properties for the different phases of the same materials. It is able to determine the properties as well as the functionality of materials by their phase studies (Chen, Lai, et al., 2020). So, in addition to doping and oxygen vacancy creation, phase structure engineering gained striking attention including the synthesis of metal oxides of a particular

Table 1.3 Doped metal oxides for various applications. Metal oxide

Dopant

Synthesis method

Application

Properties

References

ZnO

Cr

Aerosol assisted chemical vapor deposition (AACVD) method

Water splitting

Khan et al. (2020)

TiO2

N

Solvothermal

Photocatalytic study

Nanosheets; enhanced light absorption; band gap reduced; stability enhanced Enhanced optical properties

TiO2

Yb

Sonochemical

Dye degradation

TiO2

Co

Biosynthesis

Anode for Li-Battery

SrTiO3

Ag

Solvothermal

Degradation of NO

ZnO

Ni

Sol-gel

Uric acid sensing

TiO2

Cr

Sol-gel

Photovoltaic

ZnO

Co

Sol-gel

Photovoltaic

TiO2

Fe

Sol-gel

Antibacterial study

α-Fe2O3

Ti

Spin coating followed by annealing

Water oxidation

Spherical morphology; bandgap reduced; enhanced light absorption Bandgap narrowing—1.7 eV broad absorption peak until 800 nm Broad absorption peak at 450 nm until 800 nm; nanocubes Nanorods; enhanced surface area; decreased charge transfer resistance Red shift; decreased charge transfer resistance; efficiency enhanced Red shift; efficiency enhanced; irregular morphology Enhanced light absorption; active sites; high surface area Inhibited charge recombination; enhanced photocurrent density

Batalovi´c et al. (2017) Jiang et al. (2017) Kashale et al. (2019) Zhang et al. (2016) Mullani et al. (2020) Dhodamani et al. (2020a) Kumbhar et al. (2020) Koli et al. (2016) Zhang et al. (2020)

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phase or transformation of phase. All these efforts are made to uplift the characteristic properties of metal oxides which can be further convenient to various applications. Li et al. demonstrated one strategy to design an effective metal oxide-based hydrogen evolution reaction (HER) electrocatalyst by appropriate phase structure engineering (Li et al., 2019). Ti2O3 has been occurred in three polymorphs, trigonal (α-Ti2O3), orthorhombic (o-Ti2O3), and cubic (γ-Ti2O3). Among these, the γ-Ti2O3 phase proved to be effectual as it revealed the highest activity toward HER utilizing a small overpotential of 271 mV at 10 mA cm22 in 0.5 M H2SO4. This activity of metal oxides strongly correlated to their electronic structures and hence this work was able to highlight the significance of electronic structures toward the application. TiO2 has also different crystal phases, having common phases named rutile and anatase. In connection to the study of photocatalytic decomposition of methylene blue, one of the research groups demonstrated the phase transformation from rutile TiO2 to anatase TiO2 (Medvids et al., 2020). In this investigation, the observed decomposition rate enhanced after the successful phase structure engineering of TiO2. Also, it is able to be diverse in the phases in the case of the multi compositional system. In concern with this, one of the research groups demonstrated the structural study for solid electrolyte Li7La3Zr2O12 in a lithium-ion battery (Murugan et al., 2007; Wang et al., 2020). It was observed that as a phase of the material changed, the properties such as conductivity, activation energy, etc. were altered. So, in connection with this, the highest conductivity was observed for the cubic phase of Li7La3Zr2O12 than that of their other phases. This was due to the disordered distribution of lithium ions in the system. For the tetragonal phase of Li7La3Zr2O12, all lithium sites were completely filled and hence lithium-ion gained ordered distribution resulting in lower conductivity than cubic one (Awaka et al., 2009; Murugan et al., 2007). Along with phase engineering of bare metal oxides, it is also possible to obtain the phase transformations for the composites system. Hidalgo-Jimenez et al., performed the formation of high-pressure phases for TiO2 ZnO composite where optical and photocatalytic properties were changed with transformation (Fig. 1.9) (Hidalgo-Jimenez et al., 2020).

1.2.4 Composite formation Semiconducting metal oxides belong to one of the categories which widely applicable in various fields. But there is a possibility to lead the deprived performance in connection with constraints over the use of bare semiconducting bare metal oxides. So along with various strategies to enhance their performance, the metal oxidebased composite formation has been played a vital role to date. Owing to the utilization of such a strategy, it is possible to obtain a system with changes in properties such as surface area, conductivity, structural phases, optical, and mechanical properties, etc. (Rani et al., 2020). In composite formations, nanoscale composites have been gained attractive attention and provide an alternative way to overcome the laggings of bulk composites. The major advantages behind the use of nanocomposites are having a high surface area to volume ratio, high mechanical properties, further improvement in optical characteristics, etc. Hence, many scientists made their emphasis on designing cost-effective and well-organized metal oxide-based composite structures that can be utilized in various fields.

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Figure 1.9 (A) Optical properties, (B) photocatalytic properties, and (C) molar fraction of various phases and their crystallite size in TiO2 ZnO composites before and after processing with N 5 3 and N 5 15 High-Pressure Torsion turns. Source: From Hidalgo-Jimenez, J., Wang, Q., Edalati, K., Cubero-Sesı´n, J. M., RazaviKhosroshahi, H., Ikoma, Y., Gutie´rrez-Fallas, D., Dittel-Meza, F. A., Rodrı´guez-Rufino, J. C., Fuji, M., & Horita, Z. (2020). Phase transformations, vacancy formation and variations of optical and photocatalytic properties in TiO2 ZnO composites by high-pressure torsion. International Journal of Plasticity, 124, 170 185. https://doi.org/10.1016/j.ijplas.2019.08.010.

The many efforts made to synthesize composite of metal oxides with other metal oxides, organic conductors or semiconductors, metal nitrides, or polymers by research communal. Lim et al. reported the hydrothermal method to design TiO2/ SnO2 hierarchical nanocomposite architectures (Lim et al., 2016). The designed TiO2/SnO2 nanocomposites demonstrated as active photoelectrode in dye-sensitized solar cell (DSSCs), which displayed improved photovoltaic performance. The observed outputs for the composite system were excellent as compared to bare SnO2 photoelectrode (Fig. 1.10). Along with the application in energy technologies, metal oxide-based composites also have been deployed effectively in biomedical applications. Since having remarkable physicochemical properties as well as desired electrochemical features, ZnO was utilized in serotonin sensing (Mullani et al., 2020). During the electrochemical measurement, bare ZnO displayed a high charge transfer resistance and moderate stability. These difficulties were overcome by making a composite of ZnO with carbon nanostructures (CNs) such as rGO and MWCNTs. Further, it was found that after successful composite formation actual improvement in electrochemical measurements was displayed showing efficient charge transport, stability, and reproducible system (Fig. 1.11).

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 1.10 (A) Morphological study, (B) photocurrent density voltage (J V) characteristic of dye-sensitized solar cell (DSSC), (C) Nyquist plot of DSSCs with SnO2 nanofibers and hierarchical TiO2 SnO2 heterostructures as photoelectrode under simulated sunlight illumination and (D) Summary of the observed results for SnO2 nanofibers and hierarchical TiO2 SnO2 heterostructures based photoelectrodes for DSSCs. Source: From Lim, C. K., Wang, Y., & Zhang, L. (2016). Facile formation of a hierarchical TiO2 SnO2 nanocomposite architecture for efficient dye-sensitized solar cells. RSC Advances, 6(30), 25114 25122. https://doi.org/10.1039/C5RA25772G.

Various metal oxide-based composites with other metal oxides, organic conductors and semiconductors, metal nitrides, etc., and their applications are listed in Table 1.4.

1.2.5 Morphology engineering In connection with the improved performance of metal oxides, morphology engineering has been playing an effective role in the current era. As compared to structural parameters such as surface area and size of metal oxides, the parameter of morphology has been considered more significant (Yong Li & Shen, 2013). So, different strategies such as CVD, sol-gel, hydrothermal, solvothermal methods, etc. have been deployed by varying synthetic parameters such as temperature, pressure, reaction time, etc. to date. Such efforts have performing to design metal oxides with controllable morphologies which brought innovative direction toward an efficient system for various applications. One of the research groups demonstrated the

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Figure 1.11 (A) The morphological study, (B) X-ray diffraction pattern, (C) current response, and (D) charge transfer resistances of bare ZnO and ZnO/CNs nanocomposites. Source: From Mullani, S. B., Dhodamani, A. G., Shellikeri, A., Mullani, N. B., Tawade, A. K., Tayade, S. N., Biscay, J., Dennany, L., & Delekar, S. D. (2020). Structural refinement and electrochemical properties of one dimensional (ZnO NRs)1 2 x(CNs)x functional hybrids for serotonin sensing studies. Scientific Reports, 10(1), 15955. https://doi.org/10.1038/ s41598-020-72756-3.

several morphologies for ZnO such as nanopowders, nanovolcanoes, nanorods, nanotubes, and nanoflowers by a hydrothermal method which were time-dependent (Fig. 1.12) (Tong et al., 2006). The optical properties observed were altered by the change in the shape of the ZnO. As the change in heads of cones for ZnO nanostructures took place, there was alternation in optical properties, that is, enhanced fluorescence background and hence diminished UV emission observed as well as an increase in green emission in PL spectra was showing overall squalor of optical properties. In the case of gas sensing selectivity, sensitivity as well as working temperature, the metal oxides with favorable microstructures and morphologies play a vital role. Li et al. reported diverse morphologies such as octahedron, sponge-like octahedron, and sphere for the CuO in triethylamine (TEA) sensing application (Y.-P. Wu et al., 2017). In this investigation, thermal decomposition of Cu-BTC metal organic frameworks (MOFs), that is, HKUST-1 was carried out at different temperatures to get different morphologies (Fig. 1.13A). Among all morphologies, the obtained CuO at 400 C with spherical shape revealed better sensing performance than other CuO morphologies due to the special structural factor having numerous open active sites ( Fig. 1.13E).

Table 1.4 Metal oxides-based composites for various applications. Metal oxide-based composite

Synthesis method

Application

Properties

References

TiO2/Cellulose nanofibril-derived carbon TiO2/SnO2

Hydrothermal

Good conductivity; well charge transfer; luminescence property; red shift

Li et al. (2020)

Solvothermal

TiO2/MWCNTs

Sol-gel

Photocatalytic study (Rhodamine degradation and Cr (VI) reduction) Dye-sensitized solar cells (DSSCs) DSSCs

Qureshi et al. (2021) Dhodamani et al. (2020a)

ZnO/rGO

Sol-gel

Biosensing

TiO2@CuFe2O4

Ex-situ sol-gel

Antibacterial study

WO3/MIL-100(Fe)

Mechanochemical (ball milling)

Photodegradation

BiVO4/CuS

Hydrothermal

Photoelectrochemical sensing

TaON/BiVO4

Doctor blade method Chemical vapor deposition

Water oxidation

Enhanced photocurrent density; improved charge transfer & dye loading Inhibited charge transfer resistance; improved light absorption; enhanced performance Hexagonal nanorods; high surface area; less charge transfer resistanc>e Spherical nanoparticles; visible active; supermagnetic behavior displayed good recyclability and reusability; excellent photodegradation; visible active material Peanut like morphology; red shift; low electron recombination Improved activity & stability; high porosity; low charge transfer resistance Improved electrochemical performance; high reversible specific capacity

NiO/CNTs

Lithium-ion batteries

Mullani et al. (2020) Shevale et al. (2020) Wang et al. (2021)

Yang et al. (2020) Wei et al. (2021) Zhang et al. (2021)

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Figure 1.12 Various ZnO nanostructures (A,B) on ITO substrate with 3 hours, (D,E) on Si substrate with 3 hours, (G,H) on Si substrate with 6 hours, (C) powder for 30 minutes, (F) powder for 3 hours, and (I) powder for 3 hours 1 4 days. Source: From Tong, Y., Liu, Y., Dong, L., Zhao, D., Zhang, J., Lu, Y., Shen, D., & Fan, X. (2006). Growth of ZnO nanostructures with different morphologies by using hydrothermal technique. Journal of Physical Chemistry B, 110(41), 20263 20267. https://doi.org/10.1021/ jp063312i.

So, it is conceivable to say that morphology of the metal oxides has been playing a vital role and their engineering for better performance has been gained attractive attention in the present state of art.

1.2.6 Porosity generations From a scientific as well as technological point of view, metal oxides with porous nature have gained considerable attention. Such materials own a high surface area, faster mass charge transfer, higher volume storage and always tend to interact with other molecules, ions, and atoms. Henceforth the utilization of such porous metal oxides partaking high surface area have been observed to be expansively enhanced as the heterogeneous catalysts (Wang, Arandiyan, et al., 2017). There are mainly three types of porous metal oxide materials that depend on pore size namely: microporous (pore diameter ,2 nm); mesoporous ( pore diameter 5 2 50 nm ); and macroporous (pore diameter .50 nm) (Recommendations, 1982). Many research activities have been executed to get suitable porous nature of metal oxides to exploit in various applications such as catalysis, energies, supercapacitors, gas sensing, etc. (Xiao et al., 2018). Habibi et al. reported the fabrication of mesoporous nanocrystalline MgO/Al2O3 having different MgO:Al2O3 molar ratios (Habibi et al., 2016). It was observed that as the molar ratio of MgO:Al2O3

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 1.13 (A) Formation of CuO products via thermal decomposition of HKUST-1 template, (B) SEM images of HKUST-1 precursors, (C,D) SEM images of CuO-300 C, (E) SEM images of CuO-350 C, (F) SEM images of CuO-400 C and (G) three-dimensional graph of the CuO nanostructures showing their concentration-dependent sensitivity. Source: From Wu, Y.-P., Zhou, W., Dong, W.-W., Zhao, J., Qiao, X.-Q., Hou, D.-F., Li, D.S., Zhang, Q., & Feng, P. (2017). Temperature-controlled synthesis of porous CuO particles with different morphologies for highly sensitive detection of triethylamine. Crystal Growth & Design, 17(4), 2158 2165. https://doi.org/10.1021/acs.cgd.7b00102.

increased, the porosity and surface area enhanced which revealed the high efficiency in CH4 conversion (Fig. 1.14). The porous metal oxides are usually fabricated by template-assisted processes, which are time-consuming, also limited with low crystallinity temperature, slow solvent evaporation, and multiple impregnation process. So, to overwhelm these issues, Dai et al. designed various metal oxides such as ZrO2, Fe2O3, CeO2, CuOx CeOy catalyst, and CuOx CoOy CeOz catalysts, and for which the mechanochemical nanocasting method deployed (Xiao et al., 2018). The surface area was observed to be enhanced as metal oxides converted into porous form. It is conceivable to comprehend the outcomes and concepts which were observed in this research investigation with help of Fig. 1.15.

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Figure 1.14 (A) Mesoporous nanocrystalline MgO/Al2O3 having different MgO:Al2O3 molar ratios for CH4 conversion, and (B) The observed structural assets with different MgO/ Al2O3 molar ratios. Source: From Habibi, N., Arandiyan, H., & Rezaei, M. (2016). Mesoporous MgO
Al2O3 nanopowder-supported meso macroporous nickel catalysts: A new path to high-performance biogas reforming for syngas. RSC Advances, 6(35), 29576 29585. https://doi.org/10.1039/ C6RA01656A.

Figure 1.15 (A) Synthesis of porous metal oxides via mechanochemical nano casting approach and (B) pore parameters observed for a series of metal oxides. Source: From Xiao, W., Yang, S., Zhang, P., Li, P., Wu, P., Li, M., Chen, N., Jie, K., Huang, C., Zhang, N., & Dai, S. (2018). Facile synthesis of highly porous metal oxides by mechanochemical nanocasting. Chemistry of Materials, 30(9), 2924 2929. https://doi.org/ 10.1021/acs.chemmater.7b05405.

Therefore, getting effectual porosity has been considering one of the emergent approaches for the engineering of metal oxide. In this connection, the various applications of porous metal oxides are to be focusing on the present state of the art.

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Advances in Metal Oxides and Their Composites for Emerging Applications

1.2.7 Surface modifications For the numerous applications such as antibiosis, sensing, catalysis, luminescence, etc., the various metal oxides have been established significant impact with characteristic physicochemical properties. Owing to these, surface atomic structure is a critical factor that affects the surface’s assets (Kuang et al., 2014). So, by modifying the surface of metal oxides, it is conceivable to achieve better activities via the use of modified metal oxides. Surface modification through voids is generally introduced method known as bulk defects (3D macroscopic defects, i.e., volume defects) (Raizada et al., 2020). These 3D defects can enhance the metal oxide’s photo-response to accelerate various surface redox reactions. This is due to having low density, high surface-tovolume ratio, excellent optical as well as electronic properties, which are observed because of 3D defects in metal oxides. 3D voids are shown in the photocatalytic MOs, which are present on the surface for example, ZrO2, SnO2, etc. (Basahel et al., 2015; Zhao et al., 2020). Liu et al., fabricated novel visible light-responsive yok-shell nanosystem, that is, Pd/N-Cs@SnO2 for photocatalytic reduction of 4-nitrophenol. This system of Pd/N-Cs@SnO2 exhibited exceptional activity and higher reusability than core-shell Pd@SnO2. This was observed because of the existence of interlayer void which was responsible for amended light absorption by multiple scattering. Along with these, it was favored the more active sites for contact with reactants in photocatalytic reactions (Fig. 1.16) (Zhao et al., 2020).

Figure 1.16 (A) The proposed formation procedures, (B) UV-vis absorption spectra, (C) PL spectra and (D) yolk shell Pd/NCs@SnO2 nanoreactor with improved photocatalytic activity. Source: From Zhao, X., Liu, X., Yi, C., Li, J., Su, Y., & Guo, M. (2020). Palladium nanoparticles embedded in yolk shell N-doped carbon nanosphere@void@SnO2 composite nanoparticles for the photocatalytic reduction of 4-nitrophenol. ACS Application Nano Materials, 3(7), 6574 6583. https://doi.org/10.1021/acsanm.0c01038.

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1.2.8 Thin-film formations Among the various strategies for modification of metal oxides, thin-film formation is one of the embryonic techniques to develop a coating that employed to modify and enhance the functionality of bulk materials. Thickness can be settled into the region of ,1000 nm during such strategy, which may bring alternation in optical, electrical as well as mechanical properties of bulk materials. Various methods like chemical vapor deposition (CVD), electrochemical deposition, dipcoating, spray pyrolysis, pulsed laser deposition (PLD), etc. have been deployed to advance well-organized thin films of metal oxides (Aguir et al., 2020). Many research groups have been demonstrating the fabrication of thin films in representative applications. Let us consider one of the metal oxides, SnO2 which has been consistently employed for various applications to date. In concern with this metal oxide, for the thick film the problems raised regarding stability as well as long recovery rate (Aguir et al., 2020). Hence, by designing thin-film metal oxides, it is possible to obtain proper characteristic properties for respective applications. Murugan et al. described the synthesis of garnet-type Li7La3Zr2O12 having predominant ionic conductivity, where efforts were made to measure the bulk and total conductivity into thick as well as thin pellets of Li7La3Zr2O12 (Murugan et al., 2007). It was observed that in the case of thin pellets the detected ionic conductivity was slightly higher than that of thick pellets. This study can be illustrated by remarking at impedance measurement for the said garnet-type Li7La3Zr2O12. At room temperature, for thick pellet, the bulk and total conductivity observed were about 4.67 3 1024 and 2.44 3 1024, respectively. And these values increased to 5.11 3 1024 and 7.74 3 1024 for bulk and total conductivity, respectively in the case of thin pellet. Fig. 1.17 shows the different possibilities over the fabrication of thin films for different investigation and via different strategies with their conductivities for solid-state electrolyte batteries. Hence, all these aforementioned strategies such as structural, morphological, compositional, etc. play a vital role to design the effectual engineering of metal oxides. And therefore, these can be further employed for various applications, which are discussed in the following sections.

1.3

Application of engineered metal oxides

In the present state of the art, instead of bare metal oxides, the use of modified metal oxides gained vital importance for various applications including energy sectors (solar cells, water splitting, energy storage devices, etc.) (Bhargava et al., 2020; Lin et al., 2021), medical sector (biosensing, antibacterial, etc.) (Deshmukh et al., 2020; Mullani et al., 2020), catalytic studies (Deshmukh et al., 2020), etc.

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Figure 1.17 Thin-film development at different years and via different methods with their conductivities for Li7La3Zr2O12 electrolyte Li-ion batteries. Where, S-G, sol-gel, ALD, atomic layer deposition; PLD, pulsed layer deposition; CVD, chemical vapor deposition; RFMs, radio frequency magnetron sputtering; FSP-TC, flame spray pyrolysis 1 tape casting. Source: From Wang, C., Fu, K., Kammampata, S. P., McOwen, D. W., Samson, A. J., Zhang, L., Hitz, G. T., Nolan, A. M., Wachsman, E. D., Mo, Y., Thangadurai, V., & Hu, L. (2020). Garnet-type solid-state electrolytes: Materials, interfaces, and batteries. Chemical Review, 120(10), 4257 4300. https://doi.org/10.1021/acs.chemrev.9b00427.

1.3.1 Energy technologies 1.3.1.1 Solar cells Solar energy has been gained a striking path as an alternative way to inhibit the use of non-renewable energy sources. In connection to solar energy harvesting, the characteristics properties such as optical absorption coverage, optical band gap, electrical conductivity, structural, morphological properties, charge separation and transportation, etc. are focused highly for modified metal oxides to that of their bare metal oxides. And hence many attempts are made to establish efficient protocols for utilizing solar energy effectively. Once the synthesis and characterization of the modified metal oxides were completed, then designing and assembly of solar devices are the next steps and hence it is ended with the different measurements of the fabricated solar devices. Usually, bare metal oxides, as well as their modified forms, are utilized

Metal oxide engineering

25

widely over the different structures of solar devices such as dye-sensitized, quantum dot sensitized, perovskite, organic, and hybrid solar cells, etc. Recently, our research group reported the efficient solar energy conversions for TiO2/MWCNTs nanocomposites prepared via the in-situ sol-gel method, which is utilized for designing wellorganized DSSCs (Delekar et al., 2018). In this investigation, the modified metal oxide, that is, TiO2/MWCNTs displayed the right alignments of energy levels for well charge separation and transportation, high electron mobility as well as effective dye loading and hence better light absorption toward the visible region (Fig. 1.18A). In addition, the solar energy conversion performance was further improved by the incorporation of Cr(III) into the said TiO2/MWCNTs system via the in-situ sol-gel method (Fig. 1.18B) (Dhodamani et al., 2020a). One of the research groups reported the ZnO-based DSSCs with I3 2/I2 electrolyte, where TiO2 NPs were coated on ZnO nanorods by layer-by-layer technique to form photoanode (Manthina et al., 2012). In this investigation, after successful coating of TiO2 NPs on ZnO nanorods, the surface area and electron transport were observed to be enhanced and resulted in a superior surface for effectual dye loading as compared to bare ZnO. The observed outcomes were deliberated with help of the following Table 1.5. Along with DSSCs, such heterostructures can be also deployed for designing efficient perovskite solar cells with high electron mobility and well energy levels alignment and thereby achieving efficient charge transport. Khan et al. reported low-temperature processed aluminum-doped ZnO nanoparticles (Khan & Kim,

Figure 1.18 (A) Charge transfer mechanism observed in TiO2/MWCNTs based dyesensitized solar cell (DSSC), (B) charge transfer mechanism observed in Cr-doped TiO2/ MWCNTs based DSSCs. Source: From Reprinted with permission of (A) Delekar, S. D., Dhodamani, A. G., More, K. V., Dongale, T. D., Kamat, R. K., Acquah, S. F. A., Dalal, N. S., & Panda, D. K. (2018). Structural and optical properties of nanocrystalline TiO2 with multiwalled carbon nanotubes and its photovoltaic studies using Ru(II) sensitizers. ACS Omega, 3(3), 2743 2756.https:// doi.org/10.1021/acsomega.7b01316, and (B) Dhodamani, A. G., More, K. V., Patil, S. M., Shelke, A. R., Shinde, S. K., Kim, D.-Y., & Delekar, S. D. (2020). Synergistics of Cr(III) doping in TiO2/MWCNTs nanocomposites: Their enhanced physicochemical properties in relation to photovoltaic studies. Solar Energy, 201, 398 408. https://doi.org/10.1016/j. solener.2020.03.001.

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Table 1.5 The observed value of dye loading and photocurrent densities for ZnO/TiO2based dye-sensitized solar cell. Photoanode ZnO ZnO/TiO2 (3 layers) ZnO/TiO2 (30 layers)

Dye loading 22

3.5 nmol cm 13.9 nmol/cm22 B14.5 nmol cm22

Photocurrent density B0.35 mA cm22 B1.8 mA cm22 B3.4 mA cm22

Source: From Manthina, V., Correa Baena, J. P., Liu, G., & Agrios, A. G. (2012). ZnO TiO2 Nanocomposite films for high light harvesting efficiency and fast electron transport in dye-sensitized solar cells. Journal of Physical Chemistry C, 116(45), 23864 23870.

Figure 1.19 (A) XRD spectra, (B) J-V characteristics, and (C) TRPL spectra of the MAPbI3 films deposited on bare glass and on AZO NPs with various degrees of Al doping. Source: From Khan, F., & Hyun Kim, J. (2019). Enhanced charge-transportation properties of low-temperature processed Al-doped ZnO and its impact on PV cell parameters of organic-inorganic perovskite solar cells. Solid-State Electronics, 164, 107714.

2019). Owing to the Al-doing in ZnO structure, charge recombination rate suppressed at electron transporting layer/perovskite interface as well as enhanced the overall performance of the device (Fig. 1.19). Also, for quantum dot (QDs)-sensitized solar cells (QDSSCs), numerous metal oxides have been deployed effectively. It is possible to extend the absorption region for the metal oxides, by modifying the metal oxides with QDs to form QDSSCs (Tian & Cao, 2015). In the case of QDSSCs, there is the existence of charge

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Figure 1.20 (A) UV-visible spectra, (B) representation of the solar device, and (C) energy band diagram of ZnO/TiO2 and bare TiO2 for CdS-based QDSSCs. Source: From Tyagi, J., Gupta, H., & Purohit, L. P. (2021). Mesoporous ZnO/TiO2 photoanodes for quantum dot sensitized solar cell. Optical Materials, 115, 111014. https:// doi.org/10.1016/j.optmat.2021.111014.

recombination due to insufficient charge separation and transportation. So, the engineering of metal oxides revealed an effective approach for diminishing the constraints observed. Recently the successive ionic layer adsorption and reaction (SILAR) method was deployed to design ZnO/TiO2 system by one of the research groups for CdS-based QDSSCs (Tyagi et al., 2021). In this research investigation, it was observed that the efficiency was enhanced by nearly 70% for ZnO/TiO2 composite than that of bare TiO2. This was due to the generated energy barrier at photoanode resulting in the decreased rate of recombination of electrons and holes (Fig. 1.20). Such engineered metal oxides also have been fascinated at a significant position to design effective organic solar cells (OSCs). This is due to the constraints observed in the case of most of the bare metal oxides deployed as electron transport layers (ETLs) in OSCs. Recently Zhang et al. reported the zirconium-doped ZnO for constructing well-organized OSCs (Song et al., 2021). In the case of Zr-doped ZnO, the optoelectrical as well as morphological properties of ZnO ETLs were observed to be tailored than that of pristine ZnO ETLs. There was an improvement in light absorption as well as charge extraction properties recognized due to such

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engineering of ZnO ETLs. And hence after modification, the efficiency enhanced from 15.7% for pristine ZnO ETLs to 17.2% for Zr-doped ZnO ETLs. Along with OSCs, these strategies of engineering for metal oxides also have been utilized over the construction of effective hybrid solar cells. Hence literature revealed that the metal oxide-based heterostructures (modified metal oxides) are highly recognized materials for efficient solar-to-electrical conversions and also their devices are competent to the first-generation as well as second-generation solar devices.

1.3.1.2 Water splitting Water splitting is the promising energy technology that demonstrated where water is to be split to produce clean and pure hydrogen fuel and hence which is one of the green fuels to be useful for various purposes. In concern to this area, the different materials have been continuously used so that this technology is feasible commercially. Among the different materials, the metal oxides have been exhibiting an emergent role to date. But with sighted at constraints existed such as low light absorption in the visible region, insufficient charge separation and transportation, having moderate stability in aqueous media, etc., the use of bare metal oxides was restricted up to a certain limit. To overcome these, the engineered metal oxides have been playing a vital role to improve water splitting performance. In connection with this study, Atkinson et al. developed a photoanode of Co-doped ZnO nanorods which further coated with ZIF-8 MOFs for efficient water splitting (Gala´nGonza´lez et al., 2020). In this investigation, the laggings of bare ZnO overcame after its engineering, which further revealed the redshift observation, efficient charge separation, and transportation. The incident photon to current efficiency (IPCE) observed in the case of these modified ZnO (75% at 350 nm) was almost double of bare ZnO (B35% at nm) with enhanced the overall efficiency in water splitting (Fig. 1.21). As already discussed, metal oxides with a wide bandgap displayed low light absorption in the visible region, that is, mostly in the UV region that representing only 3% 5% of total solar energy (Yang et al., 2017). Wang et al. was reported ion implantation method for designing N-doped TiO2 nanowires (Wang et al., 2015). Where these engineered TiO2 structures displayed bright yellow color showing altered optical properties of TiO2. The IPCE observed in this investigation was about 17% at 450 nm. The outcomes of this investigation are further demonstrated here with help of Fig. 1.22. Fattakhova-Rohlfing et al. demonstrated the NiO engineered with Fe doping for effectual water splitting (Fominykh et al., 2015). This research endeavor revealed heightened turnover frequency (TOF) with a photocurrent density of 10 mA cm22 at an overpotential of 297 mV. It is conceivable to say that such efforts have been accomplished to improve the reactivity of metal oxides with reduced oxygen evolution reaction (OER) overpotential for the sake of better significances. As morphology engineering in metal oxides has been playing an attractive role to study water splitting applications effectively, some of the research groups reported the

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Figure 1.21 (A) ZnO@MOFs core-shell nanorods growth and assembly of photoelectrochemical cell (B) SEM micrograph of as-grown ZnO:Co NRs, (C) SEM micrograph of the core 2 shell ZnO:Co@ZIF-8 NRs, (D) IPCE measurements and (E) The electronic band diagram exhibiting the flow of charge carriers within Co-doped ZnO and MOFs. Source: From Gala´n-Gonza´lez, A., Sivan, A. K., Herna´ndez-Ferrer, J., Bowen, L., Di Mario, L., Martelli, F., Benito, A. M., Maser, W. K., Chaudhry, M. U., Gallant, A., Zeze, D. A., & Atkinson, D. (2020). Cobalt-doped ZnO nanorods coated with nanoscale metal organic framework shells for water-splitting photoanodes. ACS Application Nano Materials, 3(8), 7781 7788. https://doi.org/10.1021/acsanm.0c01325.

morphology engineered metal oxides for this desired application. Haque et al. reported the hydrogen evolution reaction (HER) catalyst based on 2D MoO3 having ordered hexagonal pores (Haque et al., 2019). This displayed the photocurrent of 10 mA cm22 in 0.1 M KOH at 138 mV. Owing to these ordered pores, the material itself is capable to migrate water molecules and gaseous species smoothly through it (Fig. 1.23). Therefore the efficient solar-to-fuel conversion for water splitting reaction is observed using the modified metal oxides-based photoelectrodes where the oxygen evolution observed usually. Hence these metal oxide-based materials are considered to be suitable catalysts for the water oxidation reaction; which is the bottleneck step in the water splitting studies.

1.3.1.3 Energy storage system Energy storage devices are devices that are proficient in storing energy. This energy can be stored in several forms such as kinetic, electrochemical, thermal, etc. by the

Figure 1.22 Morphological analysis with SEM images of bare TiO2 (A), N-doped TiO2 (B & C), optical studies of bare TiO2 and N-doped TiO2 (D), I-V characterization of bare TiO2 and prepared N-doped TiO2 (E), Stability measurement of bare TiO2 and N-doped TiO2 under chopped light illumination at 0.5 V versus Ag/AgCl (F) and IPCE spectra of bare TiO2 and N-doped TiO2 at 0.5 V versus Ag/AgCl (G). Source: From Wang, G., Xiao, X., Li, W., Lin, Z., Zhao, Z., Chen, C., Wang, C., Li, Y., Huang, X., Miao, L., Jiang, C., Huang, Y., & Duan, X. (2015). Significantly enhanced visible light photoelectrochemical activity in TiO2 nanowire arrays by nitrogen implantation. Nano Letters, 15(7), 4692 4698. https://doi.org/10.1021/acs.nanolett.5b01547.

Figure 1.23 Schematic of ion and molecule diffusion through the 1D hexagonal intercrystalline pores in the 2D lateral domain. Source: From Haque, F., Zavabeti, A., Zhang, B. Y., Datta, R. S., Yin, Y., Yi, Z., Wang, Y., Mahmood, N., Pillai, N., Syed, N., Khan, H., Jannat, A., Wang, N., Medhekar, N., Kalantarzadeh, K., & Ou, J. Z. (2019). Ordered intracrystalline pores in planar molybdenum oxide for enhanced alkaline hydrogen evolution. Journal of Materials Chemistry A, 7(1), 257 268. https://doi.org/10.1039/C8TA08330D.

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utilization of fuel cells, batteries, capacitors, supercapacitors, etc. Such energy storage devices are effective in small gadgets to large-scale applications to diminish the present energy crises (Maity et al., 2019; Wu et al., 2019). As earlier discussed, batteries, capacitors, etc. are being considered as auspicious choices for energy storage. To construct the competent devices, a number of materials including carbon-based structures, metal oxides, conducting polymers, organic components, organic inorganic hybrids, etc. have been borrowed to date. Among these, metal oxides have been gained attractive interest by research communal. This is due to characteristic features of metal oxides such as high theoretical capacitance, ecological features, abundant nature, cost-effectiveness, diverse structural properties, etc. (Mohd Abdah et al., 2019). But in concern with the constraints of metal oxides such as low electrochemical activity and inadequate conductivity, as well as to improve the surface area, porosity, etc.; engineering of the metal oxides plays a crucial role. In this connection, Nayak et al., reported the solvothermal synthesized N-doped TiO2/N-doped graphene (n-TiO2/n-Gr) composite for supercapacitor application (Maity et al., 2019). In this research investigation, the electrochemical activity of the overall system was observed to be enhanced exhibiting a higher specific capacitance of 937.5 F g21 at a current density of 0.6 A g21 in aqueous electrolyte than that of bare TiO2 (580 F g21), which was due to the composite formation and further with doping effect of nitrogen leading to facile electron allocation. This engineered system reported the 87% retaining of specific capacitance even after charge/discharge cycles of about 5000. Fig. 1.24 demonstrates the morphological, electrochemical as well as impedance data for this research investigation.

Figure 1.24 Morphological study of (A to B) bare TiO2, (C to D) N-doped TiO2, (E to F) NTiO2/N-Gr composite, (G) schematic illustration for supercapacitor device, and (H) cyclic voltammetry for bare to engineered metal oxide composite in electrochemical measurement. Source: From Maity, C. K., Hatui, G., Tiwari, S. K., Udayabhanu, G., Pathak, D. D., Chandra Nayak, G., & Verma, K. (2019). One pot solvothermal synthesis of novel solid state N-Doped TiO2/n-Gr for efficient energy storage devices. Vacuum, 164, 88 97. https://doi. org/10.1016/j.vacuum.2019.02.002.

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Likewise, to construct the rechargeable batteries also, the engineered metal oxides have fascinated cumulative considerations. In concern with this, one of the reviews displayed the various metal oxides such lithium-based mixed metal oxides, TiO2, V2O5, MnO2, NiO, CoO, FeO, etc. as positive or negative electrodes and the strategies for their modification to obtain high performance in lithium-ion batteries (Wang et al., 2018). In this research investigation, it was deliberated that the various strategies counting doping, surface coating, morphology controlling, composite formations, etc. can be exploited to enhance the structural, mechanical as well as electrochemical properties which provide high reusability and recyclability as well. Liu et al. described the nickel-based oxide, for example, Li1U2Ni0U3Ti0U3Nb0U2O2 with carbon coating for improved specific capacity as well as rate performance in high energy Li-ion batteries (Yu et al., 2021). The discharge capacity of 268.2 mAhg21 owning high stability with retention of about 90% after 50 cycles were observed for modified Ni-based oxide, which was almost 27% higher than that of bare oxide material. Along with such composite formation, structural and morphology engineering are also one of the effectively deployed approaches for efficient energy storage devices. In the case of Cu2O based Li-ion battery, the electrochemical performance was observed to be tailored for different surface structures namely cubic, octahedron, and 26-facet polyhedra. The cubic structure with {100} surface displayed the highest capacity, and it was lower for 26-facet polyhedral. This was due to the highest surface area accomplished by the cubic structure of Cu2O polyhedra. Similarly, Wong et al. described the morphology tuning of TiO2 for Li-ion battery, in which they possessed more toward anatase phase due to the high surface area of TiO2 than other phases named rutile and brookite. And hence more efforts were made to tune the morphology of TiO2 for getting high performance in the battery (Wang, Yue, et al., 2017). Along with phase tuning, the dimensional behavior of TiO2 displayed vital performance in the specific capacity of the system. Hence, the case of anatase TiO2 with 3D nanostructures displayed the highest capacity with noticeable stability than 0D, 1D as well as 2D structures (Fig. 1.25) (Wang, Yue, et al., 2017). Hence, it is conceivable to say that such strategies for modification of the metal oxides have been considered as an effective tactic to enhance the performance of different energy storage devices.

1.3.2 Biomedical application Among all the trending studies in the present state of the art, biomedical studies have been considered the important path to bring a magical task for the sake of a better environment around human lives. This biomedical applications include the development of biosensors, hyperthermia study, antibacterial tools, enzyme encapsulations, etc. For developing attractive and efficient biomedical tools or processes or formulations, modified metal oxides have been playing the dominant one in improving the concerned properties. In addition, the engineering of metal oxides resulting in a favorable biocompatible system with alternation in cytotoxicity, as well as other physicochemical, magnetic properties such as saturation magnetization

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Figure 1.25 The schematic representation of TiO2-based Li-ion battery, with enhancement strategies including morphological as well as chemical engineering of metal oxides. Source: From Wang, L., Yue, S., Zhang, Q., Zhang, Y., Li, Y. R., Lewis, C. S., Takeuchi, K. J., Marschilok, A. C., Takeuchi, E. S., & Wong, S. S. (2017). Morphological and chemical tuning of high-energy-density metal oxides for lithium ion battery electrode applications. ACS Energy Letters, 2(6), 1465 1478. https://doi.org/10.1021/acsenergylett.7b00222.

and magnetic susceptibility, etc. (McNamara & Tofail, 2017). In this present investigation, the various biomedical applications of engineered metal oxides are demonstrated as follows:

1.3.2.1 Biosensing studies In the present state of the art, many research activities have been focusing on developing the sensor with high sensitivity, reliability, response, better selectivity, and also with low-cost monitoring. In concern with bare metal oxides, it may lead to inefficient biosensing application due to moderate stability, moderate electrochemical properties, etc. So, the strategies deployed for engineering the metal oxides play a crucial role to improve the overall performance of biosensors. In connection with this, the morphology engineering of metal oxide can alter the characteristic properties of metal oxides for getting the desired performance. Randhawa et al. recognized the research investigation on catalytic nonenzymatic glucose sensing by fabricating the CuO nanostructures with different morphologies (Chawla et al., 2017). In this investigation, the morphology of CuO was altered by a variation into precursor components (i.e., various precursors namely copper acetate, copper nitrate, and copper sulfate

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Advances in Metal Oxides and Their Composites for Emerging Applications

utilized and deployed to synthesis CuO nanostructures via precipitation technique). It was observed that high glucose sensitivity was about 1830 mA Mm21 cm22 with a detection limit of 8 mM showed by flow shape CuO prepared by using copper sulfate precursor. Recently our research group reported the Ni-doped ZnO which is further coated with conducting carbon nanostructures (MWCNTs) for the electrochemical sensing of nonenzymatic uric acid (UA) (Mullani et al., 2020). The easy and low-cost sol-gel approach was deployed to design this nanocomposite assembly with desired morphology. The enhanced sensing activity was observed with the engineering of ZnO NPs with Ni doping and composite formation with MWCNTs (Fig. 1.26). MnO2 nanoparticles (NPs) are one of the widely deployed metal oxides in biosensing studies having high surface area, high ability to be visible active, as well as efficient fluorescent quenchers (Yan et al., 2016). Still, it is possible to engineer the MnO2 NPs for better sensing with high detection limit via structural changes. One of the research groups demonstrated the structural modification as well as composite formation for MnO2 nanospheres (NS) to carry out detection of Glutathione (GSH)

Figure 1.26 (A) XRD patterns of bare ZnO, Ni-doped ZnO and nanocomposites with varying content of MWCNTs (0.01 to 0.1 wt.%), (B) cyclic voltagram of bare ZnO, Nidoped ZnO and nanocomposites with 25 3 10 2 4 M uric acid in 0.2 M phosphate buffer solution (pH 7.4) at 50 mV S 2 1, FESEM images of (C) bare ZnO, (D) doped ZnO and (E) Ni-doped ZnO/MWCNTs NCs, (F) table with charge transfer resistance, error and surface area values for bare as well as a composite system. Source: From Mullani, S. B., Tawade, A. K., Tayade, S. N., Sharma, K. K. K., Deshmukh, S. P., Mullani, N. B., Mali, S. S., Hong, C. K., Swamy, B. E. K., & Delekar, S. D. (2020). Synthesis of Ni21 ion doped ZnO MWCNTs nanocomposites using an in situ sol gel method: An ultra sensitive non-enzymatic uric acid sensing electrode material. RSC Advances, 10(61), 36949 36961.

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Figure 1.27 (A) Synthesis protocol of C@MnO2 nanospheres (NS), (B) UV-Visible spectra, (C) curve showing fluorescence responses [(F 2 F0)/F0] of the C@L-MnO2 NS system verses the concentration of GSH, and (D) table which illustrates size, limit of detection as well as the efficiency of C@MnO2 NS for different sizes. Source: From Sohal, N., Maity, B., & Basu, S. (2020). Carbon Dot MnO2 nanosphere composite sensors for selective detection of glutathione. ACS Application Nano Materials, 3 (6), 5955 5964. https://doi.org/10.1021/acsanm.0c01088.

(Sohal et al., 2020). This investigation processed the synthesis of MnO2 NPs with different sizes (small, medium, and large) and further coating of carbon dots on these nanoparticles (C@MnO2) via microwave irradiation followed by magnetic stirring. The limit of detection (LOD) observed for large nanosphere was minimum as compared to small and medium one. So as size increased, the efficiency over the GSH detection, observed to be enhanced up to 22.5% from 7.7% (C@MnO2, small). Fig. 1.27 illustrates the detailed investigation for carbon-coated MnO2 NS for the detection of GSH. So, one can easily comment on the importance of engineering for metal oxides over wide application in biosensing studies including morphological, structural, compositional variations.

1.3.2.2 Cancer treatments The morbidity and mortality in human lives are accountable to cause nearly 70% of deaths in low and middle-income countries, and hence the upsurge in the morbidity and mortality rate over the world is one of the crucial issues to human lives. Biopsy, surgery, chemotherapy, radiations, etc. are the conventional tactics to treat

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cancer. Among these, chemotherapy has been exploited with high demand but having shortcomings of side effects such as metastasis, nausea, anemia, etc. leads to making dire need of effective treatments. Based on metal oxides materials, the different protocols such as nanomedicine, hyperthermia, photodynamic therapy are the evolving ways to the conventional methods for cancer treatment (Verma & Kumar, 2019). In the nanomedicine approach, the metal oxides and their modified forms have been singing a vital role to treat cancerous cells effectually. The composite formation of metal oxides such as MgO, NiO, and ZnO resulted in the manipulation of their anticancer and antibacterial properties, considered as active candidates for biomedical applications (Kannan et al., 2020). The engineered metal oxides can facilitate both in vitro and in vivo cell precise targeting and are also able to uplift the intercellular concentration of the drugs in cancer cells while avoiding cruel toxicity toward healthy cells. One of the research groups demonstrated the surface defects engineering in CeO2 since the surface defects in nanomaterial lead to produce bends, racks, and point defects which act as high energy active sites for the chemical interface (Seal et al., 2020). Nano form of CeO2 exploited as an active material for anticancer studies, which acts as enzyme memetic. CeO2 was performed by capturing and releasing oxygen from its surface to modulate the reactive oxygen concentration in the cellular environment. But this nano form of CeO2 was found to be metastable, so further enhancement into stability and performance was observed due to the engineering of CeO2 nanostructures. One of the research activities demonstrated the Zn12 doped CeO2 to create the surface defects which directly improved its anticancer properties over Neuroblastoma treatment (Seal et al., 2020). Along with these, various magnetic metal oxides also have been focused on the hyperthermia study which is a highly investigated method for cancer treatment (Muela et al., 2016). In concern with the enhancement of stability as well as biocompatibility, the engineering of metal oxides has been gained attractive attention for magnetic hyperthermia. Fe3O4 is one of the materials deployed effectively for hyperthermia application, (Kowalik et al., 2020) andit has been found to be an effective and suitable tool due to its characteristic magnetic and physicochemical properties. But still, there is a better way to develop and design efficient tools over this application. According to this, Ognjaovic et al. reported the surface modification of Fe3O4 NPs by utilizing the ligands such as citric acid (CA), dextran (DEX) as well as (3-aminopropyl)triethoxysilane (APTES) for the said application (Ognjanovi´c et al., 2021). Because of the intrinsic assets of both Fe3O4 and APTES, it was observed that the composite system of Fe3O4@APTES displayed the highest specific absorption rate than that for the bare Fe3O4 NPs. And hence the same system was exploited for the biosensing studies. Fig. 1.28 illustrates the hyperthermia as well as biosensing characteristics of surface-modified F3O4 NPs. In addition to this, photodynamic therapy (PDT) is one of the techniques exploited over cancer treatment which comprises the light phenomena and photoactive materials (Zhao et al., 2021). As most of the metal oxides are being used as photoactive candidates. And hence the same aforementioned strategies for metal oxides have been deployed to enlarge their performance. Among numerous metal

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Figure 1.28 Illustration of electrochemical sensitivity as well as magnetic hyperthermia application of surface-modified Fe3O4 nanoparticles. ˇ K., Kosovi´c-Perutovi´c, M., Source: From Ognjanovi´c, M., Stankovi´c, D. M., Ja´cimovi´c, Z. Dojˇcinovi´c, B., & Anti´c, B. (2021). The effect of surface-modifier of magnetite nanoparticles on electrochemical detection of dopamine and heating efficiency in magnetic hyperthermia. Journal of Alloys and Compounds, 884, 161075. https://doi.org/10.1016/j. jallcom.2021.161075.

oxides, TiO2 is one of the highly deployed metal oxides for PDT due to their characteristic physicochemical, optoelectrical properties, non-toxic as well as costeffective nature. Still, the use of this material has been limited by its wide bandgap, high recombination rate as well as low light absorption from the solar spectrum (Yang et al., 2018). In concern with this, one of the research investigations described the good performance of carbon dots (C-dots) coated TiO2 nanotubes (NTs) for PDT (Yang et al., 2018). Due to this composite formation, there was good band alignment which exhibited the proper charge separation and transport, which diminished the electron-hole recombination rate, providing the stability, as well as fine biocompatibility, good solubility, and photoluminescence properties due to C-dots. C-dots/TiO2 NTs exhibited light absorption up to 650 nm, almost nearly 65% higher than bare TiO2 (Dhodamani et al., 2020b). Also, this composite assembly displayed the moral in vivo combability results as well as exhibited the excellent photodynamic effect based on tumor volume variations. The anticancer mechanism is displayed here with help of the following Fig. 1.29A.

1.3.2.3 Antimicrobial study The world is facing various types of infections such as COVID-19, cold, pneumonia, meningitis, food poisoning, and HIV due to innumerable pathogenic bacteria,

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Figure 1.29 Schematic of (A) anticancer and (B) antibacterial activity mechanism of engineered metal oxides. Source: From Kannan, K., Radhika, D., Sadasivuni, K. K., Reddy, K. R., & Raghu, A. V. (2020). Nanostructured metal oxides and its hybrids for photocatalytic and biomedical applications. Advances in Colloid and Interface Science, 281, 102178. https://doi.org/ 10.1016/j.cis.2020.102178.

viruses, and fungus. Infection is the frontier issue in the current era because of the different outbreaks of infection-causing diseases due to a variety of pathogenic microorganisms, viruses, fungus, etc. Because of the excessive use of antibiotics and mutation of microbes, the resistance has been befallen; hence to combat resistant microorganisms is the current challenge to the human being. So, to overcome such issues many techniques have been developed and among these, an antimicrobial study is one of the important medical trends which is essential to fight against infections by killing microorganisms. For this purpose, antibacterial agents have been utilized effectually. For the sake of better performance of antimicrobial agents, metal oxide nanocomposites have been considered as emergent tools having advantages of non-toxicity, biocompatibility, and ease to synthesis, etc. Metal oxides-based material releases its ions on the surface leads to enhancing its biocidal properties due to ion exchange and the continuous process of release for

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longer periods. Also, these are leading to advance the antimicrobial activity of materials. Henceforth, the engineering of metal oxides nanocomposites is playing a crucial role and such engineered metal oxides nanocomposites lead to target effectually. In addition, to disinfect the surface, our research group reported ultrasonochemically treated Ag@TiO2 nanocomposites as a photoactive agent for antibacterial paint formulation. In this research endeavor, attempts were made including numerous contents of Ag in TiO2 for the antibacterial study. Such a study is important to use in healthcare for reducing hospital-acquired infections. These were observed that as compared to bare TiO2, engineered Ag@TiO2 demonstrated showed better antimicrobial activity (Deshmukh et al., 2021). Along with TiO2, other metal oxides such as ZnO, ZrO2, V2O5, MgO, NiO, etc. have been demonstrated as an antibacterial agent. Further enhancement in the antibacterial performance of these aforesaid metal oxides was observed after the suitable and effective engineering. The antibacterial mechanism is displayed here with help of the Fig. 1.29B. One of the research groups was displayed the ZnO and polymer-based nanocomposite assembly, which was utilized to carry out the antibacterial activity for Gram-positive Staphylococcus aureus as well as Gram-negative Escherichia coli, (Lefatshe et al., 2017) Where ZnO/cellulose nanocomposite system was equipped via solution-based casting method, which was displayed better activity than bare ZnO material. This was due to the smaller size of ZnO (decreased to 14 from 30 nm of bare ZnO) with higher surface area arrived because of the cellulose polymer. Fig. 1.30 illustrates the structural, morphological as well as antibacterial study of ZnO/cellulose nanocomposite. Therefore, the aforementioned biomedical applications such as anticancer, antibacterial as well as biosensing studies using modified metal oxides have been signing great consequences in current era of research which revealing vital role from scientific as well as social point of view.

1.3.3 Catalytic applications 1.3.3.1 Wastewater treatment Clean water is one of the most vital components of all living things. Due to rapid growth in population and industrialization, most of the water is contaminated with many impurities and pollutants. To remove soluble, insoluble moieties present in water, that is, contaminants, water treatment has been focused on the main aim in the same way. Such treatment of wastewater is always useful to protect the health of various ecosystems and can be utilized for various purposes. Many methods have been deployed for such treatment with inoffensive end products such as solvent extraction, ultrafiltration, evaporation, reverse osmosis, UV treatment, etc. And these methods have been incessantly using numerous types of materials for water treatment. Recently, the advanced oxidation process (AOP) is one of the

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Figure 1.30 Scanning electron microscopy micrographs for (A) ZnO, (B) cellulose, (C) ZnO/cellulose nanocomposite, (D) XRD pattern, (E) FT-IR spectra for bare ZnO as well as ZnO/cellulose, and antibacterial activity of ZnO/cellulose nanocomposite against (F) Staphylococcus aureus and (G) Escherichia coli. Source: From Lefatshe, K., Muiva, C. M., & Kebaabetswe, L. P. (2017). Extraction of nanocellulose and in-situ casting of ZnO/cellulose nanocomposite with enhanced photocatalytic and antibacterial activity. Carbohydrate Polymers, 164, 301 308. https://doi. org/10.1016/j.carbpol.2017.02.020.

methods deployed, where organic as well as inorganic moieties can be removed by the oxidation reaction. Also, the various photoactive metal oxide materials can be used to overcome the problems present in wastewater treatment to provide an environmentally friendly and effective route. In concern to AOP, many research efforts have been made in concern with the development of nanoscaled metal oxide-based wastewater treatment. The most commonly deployed metal oxides for water treatment are TiO2, ZnO, Fe3O4, etc. (Oyewo et al., 2020). In the case of bare metal oxides, there may be the existence of instability that causes leaching or contamination and directly cause ineffective mechanisms in wastewater treatment. So, the guest insertion for the modification of metal oxides gained vital importance in research communal from a social as well as a scientific point of view. In connection with this, the metal oxides/polymers nanocomposite system improved the dispersion as well as provided stability in presence of solution. It was observed that toxic metals were greatly affected by modified metal oxides than bare metal oxides (Oyewo et al., 2020). So, it is conceivable to say that composites provide stable nature and retention ability to prevent contamination. Mondal et al. demonstrated

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Figure 1.31 Wastewater treatment using (A) bare metal oxides, (B) metal metal oxide, and (C) metal oxide-metal oxide nanostructures. Source: From Mondal, K. (2020). Recent Advances in the Synthesis of Metal Oxide Nanofibers and Their Environmental Remediation Applications, Inventions, 2(2). https://doi. org/10.3390/inventions2020009.

the mechanism of photocatalytic water treatment with the help of the following Fig. 1.31. It is easy to understand the better performance of modified metal oxides (Fig. 1.31B and C) than bare metal oxide (Fig. 1.31A). There are numerous pollutant contents produced through industrial wastewater, where azo dyes are harmful contents in the aquatic environment. Hence the degradation of these dyes is playing a vital role in wastewater treatment. Congo red and methyl red (CR and Mr) dyes are the commonly exploited azo dyes, and hence the variety of materials have been deployed to degrade such dyes to date. Metal oxides are playing a crucial role in this concept due to their characteristic properties and can be employed effectively for photocatalytic degradation also. In this connection, our research group demonstrated the TiO2-based photocatalytic degradation of CR and Mr dyes (Patil et al., 2019). The results observed for bare TiO2 were found to be limited due to their inadequate optical properties, sluggish charge separation, and transfer. This issue was overawed by modification of TiO2 with WO3 metal oxide by ultrasonic-assisted sol-gel method to form nanocomposite assembly. The further system was functionalized by the sulfonate group SO3H. These sulfated-TiO2/WO3 (sulfated-TW) nanocomposites displayed the enhanced surface area as well as optical property by reducing bandgap to 2.4 from 3.1 eV (bare TiO 2) resulting in well-organized charge transfer and hence revealing the high efficiency over degradation. Fig. 1.32 demonstrates the structural, optical, photocatalytic properties as well as reusability and recyclability of sulfated-TW nanocomposites for CR and Mr dye degradation. Therefore, it is possible to organize engineered metal oxides for wastewater treatment via heterogeneous catalysis as well as photocatalysis.

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Figure 1.32 (A) XRD pattern for bare TiO2, bare WO3, and sulfated-TW, (B) UV-visible Drs spectra for bare TiO2, bare WO3, and sulfated and non-sulfated-TW, (C and D) illustrates the photocatalytic properties for CR and Mr dye degradation, respectively, (E and F) displayed the reusability as well as the recyclability of sulfated-TW for CR and Mr dye degradation, respectively. Source: From Patil, S. M., Deshmukh, S. P., More, K. V., Shevale, V. B., Mullani, S. B., Dhodamani, A. G., & Delekar, S. D. (2019). Sulfated TiO2/WO3 nanocomposite: An efficient photocatalyst for degradation of Congo red and methyl red dyes under visible light irradiation. Materials Chemistry and Physics, 225, 247 255. https://doi.org/10.1016/j. matchemphys.2018.12.041.

1.3.3.2 Catalytic organic transformations n the world of chemistry, organic transformations are observed as dominant ones for a variety of applications in numerous sectors such as medical, chemical industries, polymer industries, environmental remedies, etc. And hence from the scientific, social as well as economic point of view these organic transformations playing a vital role. Various materials including, metals NPs, metal oxides, organic compounds, polymers, organometallic compounds, etc. have been deployed as catalysts to carry out organic reactions effectively. Among these, in the current era of research, most of the scientific communal focuses on the utilization of metal oxides as catalysts due to their proper physicochemical, optoelectrical as well as structural properties. Till to date various metal oxides contributed widely as an effective catalyst, which constituted effectively in heterogeneous catalyst also. With the proper utilization of metal oxides, it is possible to carry out variety of organic reaction such as oxidation, reduction, dehydration, isomerization, etc. successfully. In

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Table 1.6 Summary of observed parameters and catalytic activity by HNSs@C for reduction of 4-nitro phenol. Sample

Catalyst Conc. (mg mL21)

Average size (nm)

BET surface area (m2/g)

Catalytic activity (min21)

CuO NiO NiO/ CuO

0.0017 0.0017 0.0017

87.3 33.6 18.2

3.90 8.09 38.81

0.5984 0.4118 1.5032

Source: From Wu, G., Liang, X., Zhang, L., Tang, Z., Al-Mamun, M., Zhao, H., & Su, X. (2017). Fabrication of highly stable metal oxide hollow nanospheres and their catalytic activity toward 4-nitrophenol reduction. ACS Application Materials Interfaces, 9(21), 18207 18214.

concern with bare metal oxides, organic reaction performed with high activation energy as well as more reaction time with a less inadequate yield of the reaction. In addition, the ability to recycling and reusability are observed to be unfortunate. To overcome such constraints and hence to improve the performance of catalytic reaction, the use of modified metal oxides can be effective for increasing active acidic/ basic sites, surface area, the major yield of reaction with reduced reaction time, lower activation energy, etc. (Chatterjee et al., 2021; Gawande et al., 2012). Let us consider the reaction of reduction of the aromatic nitro compound using LaFeO3, SrFeO3, and La0.8Sr0.2FeO3. It was observed that in the case of mixed metal oxide La0.8Sr0.2FeO3 reduction was observed to be well with an activation energy of 73.9 kJ, which was lower than that of metal oxides LaFeO3 and SrFeO3 with an activation energy of 88.9 and 84.4 kJ, respectively (Gawande et al., 2012; Kulkarni & Jayaram, 2004). Also, it is possible to design a composite system with a variety of morphology with a coating of other components to improve the catalytic activity. In this connection, Su et al. reported the NiO/CuO hollow nanospheres (HNSs) having a coating of porous carbon (i.e., NiO/CuO@C) for the reduction of 4-nitro phenol (Hoekstra et al., 2019). It was observed that from bare CuO and NiO HNSs to NiO/CuO HNSs, average size decreased which led to enhancing surface area (Table 1.6). So, with maximum surface area, it was possible to generate mesopores into the NiO/CuO HNSs@C, which helped to insert several small molecules into pours to access inner space and created efficient catalytic property. Fig. 1.33, demonstrated the synthesis, mechanism, reusability along with activation energy plot for the assembly. Among many catalytic reactions for constructing a C C bond, the Heck olefination reaction has been found to be an effective one. For this reaction, various metalbased NPs have been employed for better catalytic activities. Ru-based NPs, that is, RuO2 is one of the deployed materials for such reaction having high catalytic activity as well as being found to be reusable. But in the case of bromo- as well as chloro-arenes, these RuO2 NPs are showing less reactivity with less TOF/TON values. In connection to this, Kim et al. reported the decoration of RuO2 NPs on single-walled carbon nanotubes (SWCNTs) through the dry synthetic protocol,

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Figure 1.33 (A) synthetic illustration for HNSs@Carbon, (B) plot of ln(C/C0) versus reaction time, (C) reusability illustration for NiO/CuO HNSs@C and (D) mechanism for reduction of 4-nitro phenol. Source: From Wu, G., Liang, X., Zhang, L., Tang, Z., Al-Mamun, M., Zhao, H., & Su, X. (2017). Fabrication of highly stable metal oxide hollow nanospheres and their catalytic activity toward 4-nitrophenol reduction. ACS Application Materials Interfaces, 9(21), 18207 18214.

(Gopiraman et al., 2014) which was found to effective catalyst with high catalytic activity, reusable nature, showing regio- as well as chemo-selectivity, proper reusability, high yield, etc. This was due to the strong support of SWCNTs to RuO2, which provide a high surface area, chemical inertness as well as interface between the metal cluster and carbon vacancies. Following Fig. 1.34 displays the synthetic strategy, the reaction schemes, mechanism as well as reusability of nanocatalyst. Also, to design biological active molecules as well as functional polymers, Sonogashira cross-coupling reactions have been employed effectually, and hence for these, various catalysts were investigated to date. In this connection, Veisi et al. reported the Fe3O4 based heterogeneous catalyst, where efforts were made to utilize the amidoxime (AO) on Fe3O4 and further deposition on Pd NPs to form Fe3O4/amidoxime (AO)/ Pd nanocatalyst (Veisi et al., 2015). In this research investigation, the observed results were shown that the catalyst to be reusable and found active with a fast reaction rate. These modified metal oxide-based catalysts are used in heterogenous catalytic systems usually and hence having various advantages such as ease separation, costeffectiveness, fast reaction rate, high surface area, resulting in numbers of active surface sites, etc. over the homogenous catalyst.

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Figure 1.34 (A) Dry synthesis method for RuO2/SWCNTs, (B) general reaction depiction for Heck olefination of aryl halides, (C) olefination of iodobenzene with styrene in presence of RuO2/SWCNTs nanocatalyst, and (D) reusability (recyclability) test for RuO2/SWCNTs nanocatalyst. Source: From Gopiraman, M., Karvembu, R., & Kim, I. S. (2014). Highly active, selective, and reusable RuO2/SWCNT catalyst for heck olefination of aryl halides. ACS Catalysis, 4 (7), 2118 2129. https://doi.org/10.1021/cs500460m.

1.4

Concluding remarks

The rapid growth of the population, combined with industrialization, has resulted in an increased need for effective technologies. Diverse metal oxide-based structures have been developing novel routes to enhance various technologies, including medicine, catalysis, and energy. The chapter reviews strategies for modification and their use for potential applications, despite the restraints of bare metal oxides. Here is a summary of this chapter. Various bare metal oxides are used in numerous applications. Owing to their properties of non-toxicity, high crystallinity, tunable opto-electrics, ease of synthesis, flexible tunability, diverse morphology, and structure, etc., these materials are preferred. Metal oxide-based technologies have been the focus of extensive research to date. In contrast, bare metal oxides are restricted due to their low light absorption from the solar spectrum, a limited number of active sites on the surface, high rate of electronhole recombination due to inefficient charge separation and transportation, moderate chemical stability, and poor electrical conductivity. As a result of these restraints of

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bare metal oxides, the engineering of metal oxide materials is becoming increasingly important in the current era of research. Various strategies have been used for this purpose, such as designing materials into nanodimensions, doping, forming composites, making materials with high porosity, surface modification, and morphology engineering. It has been observed that the mechanical, electrical, optical, and structural characteristics of the system are improved because of the engineered metal oxides. Engineered metal oxides have also been demonstrated effectively in various applications in the following fields: solar devices, hydrogen generation, energy storage units, biosensors, antibacterial studies, cancer treatment, wastewater treatment, organic transformations, etc.

1.5

Futuristic outlooks

The strategies and importance of engineered metal oxides need to be improved; yet, these materials have gained the attention of many potential applications. Research efforts for developing metal oxide-based technologies are focusing most of the time on the present state of the art. These can be viewed in light of future outlooks. For the design of modified metal oxides, a less sophisticated, more cost-effective approach can be adopted, that is, easier synthetic methods with better outcomes can be implemented. The formation of ternary/quaternary systems may result in complex mechanisms as well as synthetic protocols during metal oxide modification. The formation of binary metal oxides and surface modification will provide a practical route. In ternary and quaternary systems, interconnections will always play a vital role in their development and stability. A greater effort can be made to provide durable stability in materials by enhancing their chemical connectivity. Various applications can change dramatically with the advent of multidopant systems. With a multitude of compositions, it will yield diverse paths to generate efficient systems. To gain thermal, chemical, as well as aqueous stability for metal oxide systems, simple and easy materials should be employed rather than expensive supporting materials. A growing number of events are focusing on the design of heterogeneous metal oxide systems at the present time and hence it’s engineered properties are always associated with dominant concerns.

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Mullani, S. B., Tawade, A. K., Tayade, S. N., Sharma, K. K. K., Deshmukh, S. P., Mullani, N. B., Mali, S. S., Hong, C. K., Swamy, B. E. K., & Delekar, S. D. (2020). Synthesis of Ni21 ion doped ZnO MWCNTs nanocomposites using an in situ sol gel method: An ultra sensitive non-enzymatic uric acid sensing electrode material. RSC Advances, 10 (61), 36949 36961. Available from https://doi.org/10.1039/D0RA06290A. Murugan, R., Thangadurai, V., & Weppner, W. (2007). Fast lithium ion conduction in garnettype Li7La3Zr2O12. Angewandte Chemie - International Edition, 46(41), 7778 7781. Available from https://doi.org/10.1002/anie.200701144. Norizan, M. N., Moklis, M. H., Ngah Demon, S. Z., Halim, N. A., Samsuri, A., Mohamad, I. S., Knight, V. F., & Abdullah, N. (2020). Carbon nanotubes: Functionalisation and their application in chemical sensors. RSC Advances, 10(71), 43704 43732. Available from https://doi.org/10.1039/D0RA09438B. ˇ K., Kosovi´c-Perutovi´c, M., Dojˇcinovi´c, B., Ognjanovi´c, M., Stankovi´c, D. M., Ja´cimovi´c, Z. & Anti´c, B. (2021). The effect of surface-modifier of magnetite nanoparticles on electrochemical detection of dopamine and heating efficiency in magnetic hyperthermia. Journal of Alloys and Compounds, 884161075. Available from https://doi.org/10.1016/j. jallcom.2021.161075. Ong, C. B., Ng, L. Y., & Mohammad, A. W. (2018). A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications. Renewable and Sustainable Energy Reviews, 81, 536 551. Available from https://doi.org/10.1016/j.rser.2017.08.020. Oyewo, O. A., Elemike, E. E., Onwudiwe, D. C., & Onyango, M. S. (2020). Metal oxidecellulose nanocomposites for the removal of toxic metals and dyes from wastewater. International Journal of Biological Macromolecules, 164, 2477 2496. Available from https://doi.org/10.1016/j.ijbiomac.2020.08.074. Patel, D. K., Kim, H.-B., Dutta, S. D., Ganguly, K., & Lim, K.-T. (2020). Carbon nanotubesbased nanomaterials and their agricultural and biotechnological applications. In Materials, 13(7). Available from https://doi.org/10.3390/ma13071679. Patil, S. M., Deshmukh, S. P., More, K. V., Shevale, V. B., Mullani, S. B., Dhodamani, A. G., & Delekar, S. D. (2019). Sulfated TiO2/WO3 nanocomposite: An efficient photocatalyst for degradation of Congo red and methyl red dyes under visible light irradiation. Materials Chemistry and Physics, 225, 247 255. Available from https://doi.org/ 10.1016/j.matchemphys.2018.12.041. Qureshi, A. A., Javed, S., Javed, H. M. A., Akram, A., Mustafa, M. S., Ali, U., & Nisar, M. Z. (2021). Facile formation of SnO2 TiO2 based photoanode and Fe3O4@rGO based counter electrode for efficient dye-sensitized solar cells. Materials Science in Semiconductor Processing, 123105545. Available from https://doi.org/10.1016/j. mssp.2020.105545. Raizada, P., Soni, V., Kumar, A., Singh, P., Khan, A. A., Asiri, A. M., Thakur, V. K., Nguyen, V.-H., & Alqurashi, G. (2020). Surface defect engineering of metal oxides photocatalyst for energy application and water treatment. Journal of Materiomics, 7. Available from https://doi.org/10.1016/j.jmat.2020.10.009. Rani, E., Talebi, P., Cao, W., Huttula, M., & Singh, H. (2020). Harnessing photo/electro-catalytic activity via nano-junctions in ternary nanocomposites for clean energy. Nanoscale, 12(46), 23461 23479. Available from https://doi.org/10.1039/D0NR05782G. Ray, C., & Pal, T. (2017). Retracted article: Recent advances of metal metal oxide nanocomposites and their tailored nanostructures in numerous catalytic applications. Journal of Materials Chemistry A, 5(20), 9465 9487. Available from https://doi.org/10.1039/ C7TA02116J.

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Wu, Y.-P., Zhou, W., Dong, W.-W., Zhao, J., Qiao, X.-Q., Hou, D.-F., Li, D.-S., Zhang, Q., & Feng, P. (2017). Temperature-controlled synthesis of porous CuO particles with different morphologies for highly sensitive detection of triethylamine. Crystal Growth & Design, 17(4), 2158 2165. Available from https://doi.org/10.1021/acs.cgd.7b00102. Xiao, W., Yang, S., Zhang, P., Li, P., Wu, P., Li, M., Chen, N., Jie, K., Huang, C., Zhang, N., & Dai, S. (2018). Facile synthesis of highly porous metal oxides by mechanochemical nanocasting. Chemistry of Materials: A Publication of the American Chemical Society, 30 (9), 2924 2929. Available from https://doi.org/10.1021/acs.chemmater.7b05405. Xu, C., & Qu, X. (2014). Cerium oxide nanoparticle: A remarkably versatile rare earth nanomaterial for biological applications. NPG Asia Materials, 6(3), e90. Available from https://doi.org/10.1038/am.2013.88, e90. Yan, X., Song, Y., Zhu, C., Song, J., Du, D., Su, X., & Lin, Y. (2016). Graphene quantum Dot MnO2 nanosheet based optical sensing platform: A sensitive fluorescence “turn off on” nanosensor for glutathione detection and intracellular imaging. ACS Application Materials Interfaces, 8(34), 21990 21996. Available from https://doi.org/ 10.1021/acsami.6b05465. Yang, D., Yang, G., Sun, Q., Gai, S., He, F., Dai, Y., Zhong, C., & Yang, P. (2018). CarbonDot-decorated TiO2 nanotubes toward photodynamic therapy based on water-splitting mechanism. Advanced Healthcare Materials, 7(10)1800042. Available from https://doi. org/10.1002/adhm.201800042. Yang, Y., Liang, J., Jin, W., Li, Y., Xuan, M., Wang, S., Sun, X., Chen, C., & Zhang, J. (2020). The design and growth of peanut-like CuS/BiVO4 composites for photoelectrochemical sensing. RSC Advances, 10(25), 14670 14678. Available from https://doi.org/ 10.1039/D0RA01307B. Yang, Y., Niu, S., Han, D., Liu, T., Wang, G., & Li, Y. (2017). Progress in developing metal oxide nanomaterials for photoelectrochemical water splitting. Advanced Energy Materials, 7(19)1700555. Available from https://doi.org/10.1002/aenm.201700555. Yu, J., Li, L., Qian, Y., Lou, H., Yang, D., & Qiu, X. (2018). Facile and green preparation of high UV-blocking lignin/titanium dioxide nanocomposites for developing natural sunscreens. Industrial & Engineering Chemistry Research, 57(46), 15740 15748. Available from https://doi.org/10.1021/acs.iecr.8b04101. Yu, Z., Qu, X., Dou, A., Zhou, Y., Su, M., & Liu, Y. (2021). Carbon-coated cation-disordered rocksalt-type transition metal oxide composites for high energy Li-ion batteries. Ceramics International, 47(2), 1758 1765. Available from https://doi.org/10.1016/j. ceramint.2020.09.001. Zhang, J., Tahmasebi, A., Omoriyekomwan, J. E., & Yu, J. (2021). Microwave-assisted synthesis of biochar-carbon-nanotube-NiO composite as high-performance anode materials for lithium-ion batteries. Fuel Processing Technology, 213106714. Available from https://doi.org/10.1016/j.fuproc.2020.106714. Zhang, Q., Huang, Y., Xu, L., Cao, J., Ho, W., & Lee, S. C. (2016). Visible-light-active plasmonic Ag SrTiO3 nanocomposites for the degradation of NO in Air with high selectivity. ACS Application Materials Interfaces, 8(6), 4165 4174. Available from https://doi. org/10.1021/acsami.5b11887. Zhang, S., Zhang, Z., & Leng, W. (2020). Understanding the enhanced photoelectrochemical water oxidation over Ti-doped α-Fe2O3 electrodes by electrochemical reduction pretreatment. Physical Chemistry Chemical Physics: PCCP, 22(15), 7835 7843. Available from https://doi.org/10.1039/C9CP06138J.

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Metal oxide-based composites: synthesis and characterization

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H.M. Yadav1,2, S.K. Shinde1, D-Y. Kim1, T.P. Chavan3, N.D. Thorat4, S. Ramesh5 and C.D. Bathula6 1 Department of Biological and Environmental Science, College of Life Science and Biotechnology, Dongguk University Biomedical Campus, Gyeonggi-do, South Korea, 2 School of Nanoscience and Biotechnology, Shivaji University, Kolhapur, Maharashtra, India, 3D. Y. Patil College of Engineering & Technology, Kolhapur, Maharashtra, India, 4 Medical Science Division, Nuffield Department of Women’s & Reproductive Health, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom, 5Department of Mechanical, Robotics and Energy Engineering, Dongguk University-Seoul, Seoul, South Korea, 6Division of Electronics and Electrical Engineering, Dongguk UniversitySeoul, Seoul, South Korea

2.1

Introduction

Nanomaterial research has been well matured and well documented (Akram et al., 2018). Metal oxide-based composite nanostructures (MOCN) are considered as a key component of technological applications. For the past several decades, MOCN has been utilized in science and technology, as their physicochemical properties are determined by metal oxidation states (Balamurugan et al., 2018). In the academic, research, and industrial fields, oxide nanostructures have gained importance. So, the basic question is that what are these nanostructured metal oxides (MOs), and why they are an extraordinary class of nanomaterials compared with the solitary MO? Among different MOs, spinel-type transition-metal oxides with stoichiometric and non-stoichiometric configurations are studied mostly by researchers. Benefiting from their remarkable physicochemical properties, these MOCN have been utilized in eco-friendly, cost-effective energy storage technologies (Yuan et al., 2014). This chapter is mainly designed to explain the basics of MOCN-its various preparation techniques, tools for the characterization, and structural, morphological, optical, functional, thermal, and compositional properties. Generally, various chemical synthetic approaches are involved in the development of MOCNs includes, including sol-gel, solvothermal, microwave, combustion, electrodeposition, etc. These synthesis approaches offered access to MOCNs with a wide variety of compositions, structures, morphologies, and extraordinary physiochemical properties. The synthesis of MOCNs with controlling size, shape, and composition is a special feature of

Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00010-5 © 2022 Elsevier Inc. All rights reserved.

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Advances in Metal Oxides and Their Composites for Emerging Applications

the chemical synthetic route. Among various nanomaterials, MOCNs have gained interest in scientific and technological ways. MOCNs are the distinct class of materials with unique characteristics and properties possessing most of the aspects of materials science. Along with the development of synthetic approaches, the development of sophisticated analytical tools offers the characterization of nanostructures. The basic tool for the structural characterization of MOCNs is X-ray diffraction (XRD) along with high-level tools such as transmission electron microscopy (TEM). XRD provides crystallographic information while TEM detects structure as well as defects. Additional techniques used for studying optical, morphological, functional, and elemental compositions are UV
Vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, respectively. These techniques are discussed with their examples in this chapter.

2.1.1 Metal oxides Generally, MOCN contains a different number of metal cations and they are classified as binary, ternary, quaternary, etc. based on the number of metal cations that exist in the mixed metal oxides. The amalgamation of different metal oxides forms composite or hybrid metal oxides. MOCNs play a key role in various industrial applications in catalysis, sensors, biomedical and environmental remediation. The physiochemical properties of materials can be tuned by altering the composition of two or more metals in a stoichiometric or non-stoichiometric way. Interestingly, several MOCNs displayed superior catalytic properties compared to component oxides due to a large number of active acidic or basic sites as well as a high specific surface area. This ultimately increases the yield of the reaction and decreases the time of reaction (Gawande et al., 2012). Fig. 2.1 represents different properties and applications of MOCN. In the case of MOCN, it is crucial to characterize the distribution of each component to know how the interfaces or heterojunctions in the components affect the performance of the material. The choice of synthesis method, type of dopants, and amount of dopant alters the properties of the final MOCN (Miller et al., 2014).

2.2

Synthetic approaches

Numerous physical, chemical, and mechanical methods have been utilized for the preparation of a variety of MOCN. The most important parameter for the development of functional materials is the choice of preparation method to perform multifarious tasks. In different potential applications, the MOCN must contain multifunctional metal catalysts with textural properties such as morphology, crystalline nature, surface area, porosity, and defects (Balamurugan et al., 2018). There are various synthetic strategies are found in a diverse class of science including physical and chemical techniques. There are two main

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Figure 2.1 Schematic of the properties and applications of metal oxide-based composite nanostructures.

techniques for the preparation of nanomaterials, the bottom-up and top-down approaches (Fig. 2.2). Different lithographic methods have been used in the top-down approaches for synthesizing nanostructures are lithography (optical, e-beam, soft, nanoimprint, block copolymer, and scanning probe), mechanical milling, electrospinning, sputtering (RF, DC, magnetron), arc discharge method, laser ablation, etc. Self-organizing nanostructures are derived from the bottom-up approaches such as atomic layer deposition, sol-gel method, molecular self-assembly, solvothermal, combustion, electrochemical, soft and hard templating, revers micelle, physical and chemical vapor deposition, etc. In this chapter, we have focused on the preparation of MOCNs by chemical methods. Chemical methods have been used widely for structuring materials at the nanoscale and it has several advantages over physical methods such as versatility in developing novel materials, simplicity, chemical homogeneity, low cost, and being environmentally friendly. The summary of MOCNs synthesized by different approaches is depicted in Table 2.1. There are

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Figure 2.2 Synthesis of approaches for nanostructures.

several chemical synthetic approaches used for the development of MOCNs, which are discussed in detail below.

2.2.1 Top-down approaches 2.2.1.1 Mechanical milling Mechanical milling is a simple top-down approach performed in the presence or in absence of solid-state chemical reactions (Virji & Stefaniak, 2014). A powder charge or element blend is blended using a high-energy ball milling under a milling medium to decrease the particle size (Koch & Whittenberger, 1996) and to change structure and microstructure. Benjamin (1970) developed a ball milling process to get alloys of complex oxides (Benjamin, 1970). Mechanical milling techniques such as tumbler ball mills, vibratory mills, rod mills, planetary mills, and attritor mills (Szegvari attritor) are used to develop complex composites (Szcze˛´sniak et al., 2021; Yadav et al., 2012). During mechanical milling, particles collision occurs in between balls and powder undergoes deformation. Automatic ball mills are used widely for the large-scale preparation of several composites are shown in Fig. 2.3. Mechanochemical preparation of TiO2 and CuO with particle size below 20 nm has been reported by Anuradha and Ranganathan (2007). The mechanochemicthe al synthesis of several metal oxides such as ZrO2, Fe2O3, CeO2, CuOx
CeOy, CuOx
CoO
CeOz has been studied by Xiao et al. (2018).

Table 2.1 Summary of metal oxide-based composite nanostructures synthesized using different synthetic approaches. Synthesis method

MOC

Precursor

Reaction conditions (Temp.  C, pH)

Particle size (nm)

Characterization

References

Thermal decomposition Precipitation

MnxCo3y¨xO4 (x 5 0, 0.25, 0.5, 0.75, 1) MnO2
RuO2
GO

Mn(NO3)2.4H2O and Co (NO3)2.6H2O RuCl3  6H2O, MnCl2  4H2O

400




XRD

80




Precipitation

NiO
CdO

NiCl2.6H2O, Cd (CH3COO)2.2H2O

pH 5 10, 200/1 h




Combustion

TiO2
CuO

Anatase TiO2 powder, Copper acetate

400/4 h




Raman, XRD, SEM, TEM, XPS, EDS, etc. TGA, XRD, XPS, FTIR, SEM, XPS, UV
Vis, PL, etc. XRD, Raman, SEM, EDS, etc.

Hydrothermal

a-Fe2O3/Cu2O

a-Fe2O3, Cu(CH3COO)2  H2O

200/12 h, 60/24 h




Sol-gel

MnxNi1-xCo2O4

450/5 h

11.7
22.1

Hydrothermal

ZnFe2O4/ZrO2

180/24, 60/6 h

17
30

Electrodeposition

Co
Ni
Cu ternary oxide

Co(CH3COO)2.4H2O, Ni (CH3COO)2.4H2O and Mn (CH3COO)2.4H2O Zn(CH3COO)2.2H2O, FeCl3.6H2O sol-gel prepared ZrO2 Co(NO3)2  7H2O, Ni (NO3)2  7H2O, Cu (NO3)2  3H2O

Rios et al. (1998) Ahuja et al. (2018) Anitha et al. (2018) Kumar et al. (2018) Lakhera et al. (2018) Dolla et al. (2018)

current density of 200 A m22 at RT, 300/2 h

crystallite size of 100
200 nm

XRD, SEM, FTIR, XPS, UV
Vis, etc. XRD, TGA, FTIR, XPS, SEM, EDS, TEM, VSM, etc. XRD, SEM, UV
Vis, FTIR, VSM, BET, etc. XRD, FESEM, FTIR, AAS, etc.

Mataji et al. (2018) Biswal et al. (2020)

(Continued)

Table 2.1 (Continued) Synthesis method

MOC

Precursor

Reaction conditions (Temp.  C, pH)

Particle size (nm)

Characterization

References

Combustion

CoCuBi oxide

250




XRD, FESEM, XPS,

Sesu et al. (2020)

Coprecipitation

Tia
ZrbOx

Cu(NO3)2.3H2O, Co (NO3)2.6H2O, (Bi (NO3)3.5H2O), TiOSO4 and ZrOCl2

pH 5 7, 300/3 h




XRD, Raman, FTIR

Hydrothermal

ZrO2:CuOx(180, 24)

180/24 h, 350/1 h

400
600 μm

XRD, XPS, TEM.

Coprecipitation

CoNiCu oxides

450/4 h




Co-precipitation

NiO
CYO-CSO [NiO
Ce0.9Y0.1O2δ
Ce0.9Sm0.1O2- δ ] g-C3N4/Cu2O
FeO

Cu(CH3COO)2.4H2O and ZrOCl2 H2O Cu(CH3COO)2.H2O, Ni (CH3COO)2.4H2O), Co (CH3COO)2.4H2O Ni(NO3)2, Ce(NO3)3.6H2O, Y (NO3)3, Sm(NO3)3

300,450,600 and 750

65
100 nm

XRD, FTIR, BET, TEM, UV
Vis, etc. XRD, TGA, FTIR, SEM, EDS, Impedance, etc.

Liu et al. (2020) Xu et al. (2021) Emara et al. (2021) Kannan et al. (2021)

200/12 h.

10 nm

Hydrothermal

Dicyandiamide, (Cu (CH3COO)2 H2O, 99%), iron (III) acetylacetonate (Fe (C5H7O2)3), G

TEM, SEM, XPS, FTIR, GC

Woyessa et al. (2021)

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Figure 2.3 Overview of the equipment for mechanochemical synthesis. (A). Planetary mills (vessels and equipment). (B) Mixer mill (vessels and equipment). (C) Grinder mortar. (D) Drum-mill. (E) Cryo-mill. (F) High energy mill (Emax). Source: Reprinted with permission from ACS Sustainable Chem. Eng. 2018, 6, 8, 9530
9544. Copyright (2018) American Chemical Society.

2.2.1.2 Electrospinning Electrospinning is an easy top-down approach to fabricating nanofibers from different inorganic materials and polymers. The typical electrospinning setup is shown in Fig. 2.4 which consists of a glass syringe with the polymer solution, metallic needle, collector, and electric power supply (Haider et al., 2018). When the electric field is applied to the metallic needle containing polymer solution then the electrospinning process occurs and ultrafine nanofibers of different materials are deposited on the collector. There are several parameters that can be controlled during the electrospinning process are applied voltage, the distance between the needle and collector, the flow rate of solution, size of needle and type of polymer solution, viscosity, concentration, temperature, humidity, etc.(Haider et al., 2018). MONCs give rise to a significant class of nanofibers with fascinating properties. Several transition metal and metal oxides are utilized in the fabrication of nanofibers (Arumugam & Moodley, 2019).

2.2.1.3 Lithography Lithography is used to develop nanostructures via a physical or chemical top-down approach. Chemical lithography is carried out using acid or bases or by using a heating process while physical lithography is performed with the help of photons, electrons, and ions (Yu et al., 2013). Electron beam lithography is utilized to overcome the limitations of light diffraction to get nanostructures. Colloidal lithography has gained increasing attention in recent years. The combination of photoactive

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Figure 2.4 Schematic depicting electrospinning setup and the phenomenon of electrospinning. (A) The basic electrospinning setup. (B) The flow of solution in the direction of the electric field. Source: Reprinted with permission from Elsevier. Copyright (2018). Haider, A., Haider, S., & Kang, I.K. (2018). A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology. Arabian Journal of Chemistry 11(8) 1165
1188, ISSN 1878-5352, https://doi.org/10.1016/j. arabjc.2015.11.015.

diazo-2-naphthol-4-sulfonic acid and oxide materials (ZrO2, TiO2, HfO2, and ITO) has been demonstrated by Pan et al. under irradiation with 405 nm light (J.-A. Pan et al., 2021). Integrated circuits are fabricated using the nanolithography approach. Lithographic processes such as photolithography, soft lithography, and nanoimprint lithography are performed using masks to develop patterns. Electron beam lithography focused ion beam lithography and scanning probe lithography was performed without the use of masks (Jose Varghese et al., 2019).

2.2.1.4 Sputtering Wet chemical deposition approaches have limitations of homogeneous and monodisperse deposition of metals and metal oxides on the different surfaces for technological applications (Ayyub et al., 2001). To overcome these limitations physical deposition techniques such as sputtering or laser deposition are superior. In the sputtering process, thin films are developed on the anode substrate with the help of plasma or gas and electric fields. Atoms are expelled from the cathode target and deposited on the anode substrate depending on the applied energy (Shaat, 2018). The magnetic field is applied in the magnetron sputtering process which accelerates

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the deposition rate and the efficiency of the gas ion formation (Shaat, 2018). The sputtering process depends on the distance between two electrodes, deposition rate, and pressure inside the chamber. A typical process of magnetron sputtering is shown in Fig. 2.5. ARC discharge method is also similar to the sputtering process, a concentrated electric discharge or ARC is used for the evaporation of materials. ARC discharge is having higher ionization power than the sputtering process and is used to prepare metal, metal oxides, and mostly carbon nanotubes, graphene, and fullerenes (Virji & Stefaniak, 2014).

2.2.1.5 Laser ablation A high-intensity laser beam is used to deposit metals, metal oxides, and composites. The target material is removed from the source material due to the high-intensity laser beam (Voon et al., 2020). Pulsed-laser deposition (PLD) is a greener approach for the development of complex-oxide heterostructures. In PLD the material is removed from a solid target rapidly and a developed energetic plasma plume is condensed onto a substrate. In laser ablation, photons are transformed into electronic excitations and ultimately generate thermal, chemical, and mechanical energy (Christen & Eres, 2008). PLD of Zn target in water to prepare high crystalline ZnO is shown in Fig. 2.6 (Huang et al., 2019).

Figure 2.5 Configuration of a DC-powered magnetron sputtering. Source: From Huang, H., Lai, J., Lu, J., & Li, Z. (2019). Pulsed laser ablation of bulk target and particle products in liquid for nanomaterial fabrication. AIP Advances 9. https://doi.org/ 10.1063/1.5082695.

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Figure 2.6 Schematic of PLD process. Source: From Huang, H., Lai, J., Lu, J., & Li, Z. (2019). Pulsed laser ablation of bulk target and particle products in liquid for nanomaterial fabrication. AIP Advances, 9(1), p.015307. https://doi.org/10.1063/1.5082695; From licensed under a Creative Commons Attribution (CC BY) license.

2.2.2 Bottom-up approaches 2.2.2.1 Sol-gel technique The sol-gel method has gained significant attention to prepare MOCNs. This is a versatile technique with advantages such as low cost, low temperature, high phase purity, uniformness, and size can be tunable (Pinna et al., 2009). Since the discovery of silica gel by Ebelmen (Ebelmen, 1846) and Graham (Graham, n.d.) during the 19th century the sol-gel technique has been used for the synthesis of metal oxides and particularly SiO2. The sol-gel technique comprises two phases of sol (colloidal suspension in liquid) and gel (inorganic integrated network). The typical process of the sol-gel technique is shown in Fig. 2.7. Metal alkoxides are used as precursors in the sol-gel process, which are hydrolyzed in acidic or alkali followed by the condensation of the hydrolysis product creating a solvent molecule surrounded network (Soyta¸s et al., 2018). The gel forms a dense “xerogel” via the collapse of the porous network due to the removal of solvent (aerogel in supercritical drying). Finally, the stable MOCNs were obtained by calcination (Esposito & Mater, 2019). However, the large-scale applicability is limited due to a long post-heat treatment, lengthier processing time, and formation of impure phases. Nevertheless, there are many MOCN that have been synthesized using this technique (Clapsaddle et al., 2004). Sol-gel prepared zirconia with titania nanostructures shows enhanced optoelectronic properties (Xianzhi et al., 1996). The complete photocatalytic oxidation of ethylene was achieved by a sol-gel derived MOCN of silica or zirconia with titania and this mixture showed significantly higher photocatalytic activity than pure titania. Mg
Ni MOCN photocathodes for

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Figure 2.7 Typical sol-gel process.

Figure 2.8 Schematic of solvothermal synthesis.

dye-sensitized solar cells have been prepared by a sol-gel technique using NiCl2 and MgCl2 precursors (Zannotti et al., 2015). A recent report by Dolla et al., studied structural, electronic, and magnetic properties of spinel-type oxides of MnxNi1-xCo2O4 (x 5 0, 0.3, 0.5, 0.7, 1) prepared by a sol-gel route using metal acetates, citric acid, and ethylene glycol (Dolla et al., 2018).

2.2.2.2 Solvothermal technique Several MOCNs have been prepared by hydrothermal or solvothermal methods. A tight container was used for this synthesis technique and which was maintained at high pressure and the temperature ranges from 100 C
200 C (Fig. 2.8). The high purity product with different morphologies can be obtained under low reaction kinetics (Akram et al., 2018). The most powerful and extensively used methods to synthesize MOCNs because of the advantages such as high control of the size, shape, adjustable reaction parameters, reproducibility, simple procedure, and the easy scale-up. However, it has disadvantages as it requires expensive autoclaves, safety issues, and unable to observe the reaction process. Recently, Lakhera et al. prepared α-Fe2O3/Cu2O photocatalysts by a one-step hydrothermal method with a heterojunction between α-Fe2O3 and Cu2O particles (Lakhera et al., 2018). Similarly, Chen et al. followed a two-step hydrothermal route

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to get porous TiO2 nanobelts decorated with Sn3O4 nanosheets core-shell composites for Li-ion batteries (Chen et al., 2018). Uniform ZnMn2O4@ZnFe2O4 microspheres were grown on Ni foam and tested for supercapacitor application by Reddy et al. (Reddy, Anitha, Muralee Gopi, et al., 2018). Mataji et al. studied structural, optical, and magnetic feathers of ZnFe2O4 microwave-assisted-ZrO2 nanocomposite prepared with different ratios by a hydrothermal method (Mataji et al., 2018).

2.2.2.3 Microwave synthesis Recently, the microwave-based synthesis approach was followed by several research groups to develop a variety of MOCNs. The advantages of this method are short time, milder reaction conditions, low energy requirements, better selectivity, higher reaction rates, better yielding of the end products, powerful and uniform heating ability. This is an alternative to classical energy by microwave radiation for chemical synthesis (Schu¨tz et al., 2018). A microwave oven is used instead of conventional furnaces for the synthesis of MOCNs (Fig. 2.9). Microwaves can diffuse into the precursor material and thereby the microwave energy is utilized for the required reaction (Manikandan et al., 2016). Manikandan et al. (Manikandan et al., 2016) synthesized Co and Mgdoped ZnO by microwave-assisted method at 350 W within 5 minutes. They found that disk and needle-shaped Co-doped ZnO own higher ferromagnetic behavior and

Figure 2.9 Schematic diagram of microwave-assisted synthesis.

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photocatalytic antibacterial properties than spherical-shaped Mg-doped ZnO. Karthik and coworkers prepared CdO
NiO
ZnO as well as CdO
ZnO
MgO MOCN nanocomposite by microwave heating and evaluated photocatalytic and antibacterial properties of these composites (Karthik et al., 2018; Revathi & Karthik, 2018).

2.2.2.4 Combustion synthesis The solution combustion method is commonly used for the preparation of MOCNs. It provides exothermic heat of different redox reactions during the synthesis of MOCN. Generally, MOCNs are prepared by quick heating the metal precursor solutions ( Fig. 2.10) (Aruna & Mukasyan, 2008; Varma et al., 2016). Solution combustion requires less time and energy than other methods and complex MOCNs can be produced on a large scale (F. Li et al., 2015). However, combustion synthesis has a few drawbacks such as agglomeration, uncontrollable morphologies, and the existence of impurities due to incomplete combustion. The solution combustion synthesis route was utilized for the preparation of a series of ceria-incorporated zirconia (Ce12xZrxO2,x 5 0 to 1) solid solutions by Ranga Rao and Sahu (2001) MOCNs of CoO 2 NiO was developed using the chemical looping combustion method by Jin et al. (Okamoto & Ishida, 1998). In the same way, Ryden et al. prepared LaxSr12xFeyCo12yO32δ perovskites and MOCNs of NiO, Fe2O3, and Mn3O4. A solid oxygen carrier was used for the oxidation of fuel in the chemical-looping combustion, and this also facilitates CO2 capture as well as H2 production (Ryde´n et al., 2008). Several transition MOCNs with the different classes such as spinel, corundum, and perovskite have been prepared and characterized by Busca and coworkers (Busca et al., 1997). The authors tried to enhance the morphological properties and stability of these MOCNs.

Figure 2.10 Schematic of solution combustion synthesis.

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 2.11 Schematic of electrochemical deposition setup.

2.2.2.5 Electrodeposition Electrochemical deposition is a versatile and eco-friendly method, which has been applied for the preparation of different nanomaterials. The electrochemical analysis deals with chemical reactions of samples or solutions with the applied electrical potential. The rate of redox reactions is analyzed and controlled by a potentiostat, connected to electrodes submerged in an electrolyte (Fig. 2.11). Generally, the solution of metal salt or metal ion complexes is electrolyzed to deposit metal on the substrate and these substrates are further oxidized to get desired metal oxides. The direct precipitation of the metal oxide can be also achieved by adjusting electrochemical parameters. Biswal et al. (Biswal et al., 2020) studied the role of organic additives on the electrodeposition of ternary MO of cobalt 2 nickel 2 copper. They prepared these oxides by electrodeposition followed by calcination at 300 C. They used metal nitrate precursors for the electrochemical deposition of MOCN (Fig. 2.12).

2.3

Characterization of metal oxide-based composite nanostructures

Metal oxide-based composite nanostructures are characterized using different techniques to get insight into the structural, morphological, optical, compositional, thermal, magnetic, and electrical properties.

2.3.1 X-ray Diffraction X-ray diffraction is mostly used to determine crystallite size, phase, and structural parameters of the MOCN’s. The comparison of preparation methods and the effect of various MO species can be studied using the XRD technique. Researchers used this technique to determine the impurities present in the samples. The shape of the diffraction peak suggests the crystallinity of the material. For example, the narrow and sharp peak suggests a highly crystalline nature while the broad peak suggests the amorphous nature of the material. Similarly, the preferential growth of a crystal can be correlated with the intensity of a diffraction peak.

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Figure 2.12 Schematic diagram of the general mechanism of electrochemical precipitation/ deposition of TH from nitrate bath. Source: From Biswal, A., Panda, P. K., Acharya, A. N., Mohapatra, S., Swain, N., Tripathy, B. C., Jiang, Z. T., & Minakshi Sundaram, M. (2020). Role of additives in electrochemical deposition of ternary metal oxide microspheres for supercapacitor applications. ACS Omega, 5(7), 3405
3417. https://doi.org/10.1021/acsomega.9b03657.

XRD system consisting of several components like source of X-ray generator, detector, and sample holding stage as shown in Fig. 2.13. The X
rays are produced from target metal (Cu or Mo) with high-energy electrons and filtered to produce monochromatic radiations. Photon counter detectors (Geiger Muller tube, proportional counter, scintillation counter, etc.) detect diffracted X-rays from the sample, and a diffraction pattern is generated from this data. X-rays are diffracted if the wavelength of the wave motion is of similar to the repeat distance between scattering points. X-rays interact with samples to produce diffraction from the atomic planes. The distance between inter atomic planes can be estimated from the Bragg’s equation (nl 5 2dsinθÞ. Where, n is an integer, l is wavelength of Xray, d is distance between planes, q 5 angle of diffraction. Powder diffraction data is mostly used to get information of crystal structures of materials by Rietveld refinement process. Hugo M. Rietveld refined structures by diffraction technique (David, 2004; Hammond, 2009). Refinement is the second step after a model has been established (Will, 2006). During refinement, experimental powder diffraction data and calculated profile is matched by changing the positions of the atoms, the occupation rates and the thermal agitation factors (Pecharsky & Zavalij, 2004). Recently, Lakhera et al. (2018) demonstrated photocatalytic activity of α-Fe2O3 loaded Cu2O MOCN photocatalyst towards demineralization of methyl orange.

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Figure 2.13 Schematic of X-ray diffractometer.

Figure 2.14 XRD Patterns of α-Fe2O3/Cu2O (CF) heterojunction catalysts with different α-Fe2O3 loadings; (A) bare Cu2O, (B) CF-1, (C) CF-3, (D) CF-5, (E) CF-8, (F) CF-10, (G) bare α-Fe2O3 and (Inset
H), characteristic peaks of α-Fe2O3 in CF-10. Source: From Lakhera, S. K., Watts, A., Hafeez, H. Y., & Neppolian, B. (2018). Interparticle double charge transfer mechanism of heterojunction α-Fe2O3/Cu2O mixed oxide catalysts and its visible light photocatalytic activity, Catalysis Today. 300, 58
70. https://doi.org/ 10.1016/J.CATTOD.2017.03.020.

The XRD pattern of bare α-Fe2O3, Cu2O, and α-Fe2O3/Cu2O photocatalysts with varying amounts of
Fe2O3 loading is shown in Fig. 2.14. The diffraction peaks of α-Fe2O3 were relatively very weak in the
Fe2O3/Cu2O photocatalysts due to their small amount (Fig. 2.14). Also, the photo corrosion and phase transformation of material can be detected using the XRD technique (Fig. 2.15).

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Figure 2.15 X-ray diffraction patterns of (A) as-prepared catalyst CF-5, (B) after 2 cycles of photocatalytic degradation of MO, and (C) after 4 cycles of photocatalytic degradation of MO under visible light irradiation. Source: From Lakhera, S. K., Watts, A., Hafeez, H. Y., & Neppolian, B. (2018). Interparticle double charge transfer mechanism of heterojunction α-Fe2O3/Cu2O mixed oxide catalysts and its visible light photocatalytic activity. Catalysis Today, 300, 58
70. https://doi.org/ 10.1016/J.CATTOD.2017.03.020.

The Rietveld refinement technique was developed by Hugo Rietveld for matching a theoretical line profile with the measured profile. Observed and calculated XRD patterns are shown in Fig. 2.16 (Dolla et al., 2018). Delekar et al. also performed Rietveld refinement for various iron-doped TiO 2 composites to get structural parameters (Delekar et al., 2012).

2.3.2 Scanning electron microscopy Scanning Electron Microscopy (SEM) is a powerful tool to observe the surface morphology of several materials including MOCNs. SEM is consisting of an electron source, electromagnetic lenses, detectors, vacuum chamber, and specimen stage (Fig. 2.17). Secondary electrons with energy smaller than 50 eV are generated during the interaction of electron beam and sample. The interaction of the electron with the specimen affects the image resolution and image resolution up to 2 nm can be obtained. The emission efficiency is proportional to the geometrical and chemical characteristics of a material (Wang, 2001). The probing of MOCNs with high resolution is possible and crystallographic data can be obtained by SEM analysis. Backscattered electrons, X-rays, and secondary electrons are formed due to the interaction of the primary electron beam with the sample. Generally, backscattered

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Figure 2.16 Rietveld refined X-ray diffraction patter of MnxNi1-xCo2O4 (x 5 0.5). The blue, red, and gray lines represent experimental (observed) patterns, calculated patterns, and difference curves, respectively. Source: From Dolla, T. H., Pruessner, K., Billing, D. G., Sheppard, C., Prinsloo, A., Carleschi, E., Doyle, B., & Ndungu, P. (2018). Sol-gel synthesis of MnxNi1-xCo2O4 spinel phase materials: Structural, electronic, and magnetic properties. Journal of Alloys and Compounds, 742, 78
89. https://doi.org/10.1016/j.jallcom.2018.01.139.

electrons are forming images in the SEM analysis. X-ray detector attached to SEM helps to detect X-rays and provides the elemental composition of the sample. This technique is known as energy dispersive X-ray spectroscopy (EDAX or EDS) Generally, the surface of the samples should be electrically conductive while for non-conducting samples the thin layer of Pt, Au, or carbon is coated. The effect of doping or mixing of different MO on the matrix agglomeration, shape, size, etc. was visualized. The use of surfactants alters the nucleation as well as the crystal structure of the oxide material deposited by the electrochemical method (Biswal et al., 2020). Cetyltrimethylanium bromide and dodecyl trimethyl ammonium bromide (DTAB) have been applied to enhance the morphological and structural characteristics of Co
Ni
Cu oxide. The SEM image of ternary oxide with ternary oxides (TO) cetyltrimethylammonium bromide (CB) critical micelle concentration (CMC) (Fig. 2.18) showed a flowerlike porous structure with an ununiform agglomeration of microspheres and the crystallite size in the range of 200 to 300 nm. Similarly, DTAB labeled TODBL CMC and TODB CMC sample showed nanosheet-like morphology and with an average crystallite size of 100 nm or less in the case of TODBCMC (Fig. 2.19). Heterogeneous catalysis requires a large amount of catalytic active sites as it’s a surface adsorption process. Nanostructured MOCNs owning higher catalytic activity owning high surface-to-volume ratio and intrinsic surface defects. Engineered materials with nanosize have been widely applied for a variety of chemical reactions as a catalyst or supporting materials. MOCNs have a vital function in the research and manufacturing field due to their acid-base, reduction-oxidation, and catalytic nature. A variety of MOCNs comprising alkali, alkaline, rare earth, and noble metals, and their applications are well reported (Gawande et al., 2012). MOCNs facilitate

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Figure 2.17 Schematic of an scanning electron microscopy microscope. Source: From Inkson, B. J. (2016). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization. Materials characterization using nondestructive evaluation (NDE) methods, 17
43. https://doi.org/10.1016/B978-0-08100040-3.00002-X.

selective reduction of C 5 O in a b-unsaturated carbonyls through catalytic H2 transfer reaction (Sonavane & Jayaram, 2004), analysis of chemical properties and performance of perovskite oxides (Pen˜a & Fierro, 2001), epoxidation on MoO3/ TiO2, analysis of phosphate ions on the textural and catalytic activity of MOCNs of TiO2
SiO2 (Kanai & Ikeda, 2003). Yang et al. prepared nanoporous electrocatalyst

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Figure 2.18 FESEM, Field emission scanning electron microscopy images of the modified ternary metal oxide TOCBCMC (different magnifications). Source: From Biswal, A., Panda, P. K., Acharya, A. N., Mohapatra, S., Swain, N., Tripathy, B. C., Jiang, Z. T., & Minakshi Sundaram, M. (2020). Role of additives in electrochemical deposition of ternary metal oxide microspheres for supercapacitor applications. ACS Omega, 5(7), 3405
3417. https://doi.org/10.1021/acsomega.9b03657.

of Ni
Co binary oxides for oxidation of water using electrochemical deposition method (Y. Yang et al., 2014). Similarly, Rios et al. studied surface structure and oxygen reduction reaction (ORR) and evolution reaction (OER) using Co
Mn spinel-type oxides and found that Mn inhibits OER while accelerating ORR (Rios et al., 1998). MOCNs are able to sense a variety of gas molecules and have higher compatibility for the microelectronic fabrication process. Another advantage of MOCNs is cost-effectiveness and high stability. The gas sensing is based on the mechanism of change in resistivity of MOCNs in presence of gas molecules. The reaction between gas molecules and MOCN’s occurs when these gas molecules come in contact with the surface of MOCNs. So, the structure of MOCNs is very important for sensing gas molecules, and structures like hollow, porous, and hierarchical demonstrated excellent sensing performance. Most of the metal oxides suffer from poor selectivity towards different gas molecules (Srivastava, 2014). This selectivity was found to be enhanced in the case of binary and ternary MOCN nanostructures even at low

Figure 2.19 FESEM images of the modified ternary metal oxides (A,B) TODBLMC (C,D) TODBCMC, and (E
H) TODBHCMC (different magnifications). Source: From Biswal, A., Panda, P. K., Acharya, A. N., Mohapatra, S., Swain, N., Tripathy, B. C., Jiang, Z. T., & Minakshi Sundaram, M. (2020). Role of additives in electrochemical deposition of ternary metal oxide microspheres for supercapacitor applications. ACS Omega, 5(7), 3405
3417. https://doi.org/10.1021/acsomega.9b03657.

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operating temperatures (Balamurugan et al., 2018). Also, perovskite-type (ABO3) metal oxides of the rare earth elements with MOCNs have shown specific selectivity in harsh conditions and can be an alternative to precious noble metals. Along with gas sensing characteristics, MOCNs have shown applicability in electrochemical sensing also. Miura et al. performed electrochemical sensing of NO and NO2 potentiometrically and amperometerically at high temperature using zirconia and MOCNs. CdCr2O4 showed superb NOx sensing performance compared to the other perovskite-type and spinel-type oxides tested (Miura et al., 1998). Subramanian et al. fabricated polyaniline-MOCNs (TiO2/SnO2) by in situ method and coated them on a printed circuit board. This hybrid sensor showed excellent performance under ambient air than that of the polyaniline-SnO2, which worked only under a nitrogen environment (Subramanian et al., 2018). MOCNs can sense chemical species such as catechol and hydroquinone also. Recently, a voltammetric sensor based on ZnO
Al2O3 ceramic nanofibres and Au nanoparticles has been displayed simultaneous detection of catechol and hydroquinone. Ceramic nanofibers were electrodeposited onto the mixture of graphene oxide and chitosan simultaneously with gold nanoparticle electrodeposition (Nazari et al., 2018). Khan et al. noted that the impedance of the sensor developed using SnO2
Mn3O4 between copper electrodes fixed on a glass substrate is strongly dependent on relative humidity (RH) and mechanical pressure. The impedance of the material decreases by a factor of 54.7 with increasing RH in the range of 10%
90% in pristine SnO2
Mn3O4 nanorods, but 29.6 times in SnO2
Mn3O4 nanocomposites. Miller and coworkers compared hydrogen sensing performance for TiO2-doped SnO2 synthesized via coprecipitation and mechanical mixing. The authors found that the optimum composition for H2 sensing was 10 mol.% for co-precipitation and 20 mol.% TiO2 for mechanical mixing. Tin
copper MOCN nanowires were prepared by thermally oxidizing electrodeposited metallic nanowires sensors that exhibited higher sensitivity than that of pure tin oxide nanowire sensors (Li et al., 2011). This proved the sensors made up of MOCNs showed superior performance than single metal oxide.

2.3.3 Transmission electron microscopy Transmission electron microscopy is a revolutionary tool for structural and morphological analysis of MOCNs with a very high resolution. Fig. 2.20 shows schematically for a TEM consisting of an electron source, electromagnetic lenses, and detectors. Electron source emits electrons which are passed through a vacuum chamber. These electrons are propelled through a sample and transmitted electrons are detected by a detector and the image is magnified on a fluorescent screen. There are different magnification modes are available such as low, high, convergent beam electron diffraction, and selected area diffraction. The magnifications up to 0.1 nm can be achieved by TEM. TEM can provide information on atomic distribution and chemical constituents of the sample (Ayache et al., 2010). TEM requires thin samples which are able to pass electron beams through it. Materials that have dimensions small can provide in-depth information of the sample. The size, shape, lattice spacing, etc. can be determined using TEM.

Figure 2.20 A schematic diagram of the transmission electron microscopy. Source: From Inkson, B. J. (2016). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization. Materials characterization using nondestructive evaluation (NDE) methods 17
43. https://doi.org/10.1016/B978-0-08100040-3.00002-X.

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Ahuja et al. observed by TEM that the reduced graphene oxide nanoribbon (GNR) surface was uniformly decorated with MnO2
RuO2 nanoflakes with the size of about 10 nm (Ahuja et al., 2018). Xu et al. (2021) developed capsule-like ZrO2:CuOx hybrid catalysts by hydrothermal method and studied its morphology by TEM. The CuOx sheets are surrounded by ZrO2 leaves, where ZrO2 is gradually embedded into the CuOx. The authors calculated lattice spacing from the HRTEM image as shown in Fig. 2.21. The lattice spacing of ZrO2 was 0.369, 0.261, and 0.187 nm, corresponding to (110), (020), and (022) facets, respectively while for CuOx it was 0.248 nm, corresponding to the (111) plane.

2.3.4 UV
Vis spectroscopy Ultraviolet-Visible Spectroscopy (UV
Vis) is a mostly used absorption or reflectance mode in the ultraviolet-visible range. Metal oxides having characteristics UV
Vis absorption bands with the electronic transition providing information of the electronic structure of the samples. UV
Vis spectroscopy is also useful for determining the optical bandgap of the materials from the Tauc plot (Srivastava, 2014). In UV
Vis, spectrophotometer light is passed through a sample and the transmitted light is detected by a suitable detector as shown in Fig. 2.22. UV
Visible

Figure 2.21 (A) Transmission electron microscopy (TEM) image of ZrO2: CuO(180,24), (B) HRTEM image of ZrO2:CuOx(180,24), and (C) TEM image and element mapping of ZrO2: CuOx(180,24). Source: From Xu, N., Coco, C. A., Wang, Y., Su, T., Wang, Y., Peng, L., Zhang, Y., Liu, Y., Qiao, J., & Zhou, X. D. (2021). Applied Catalysis B: Environmental 282. https://doi.org/ 10.1016/j.apcatb.2020.119572.

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Figure 2.22 Schematic of UV
Vis spectrophotometer. Source: From Rocha, F. S., Gomes, A. J., Lunardi, C. N., Kaliaguine, S., & Patience, G. S. (2018). Experimental methods in chemical engineering: Ultraviolet visible spectroscopy— UV-Vis. Canadian Journal of Chemical Engineering 96(12), 2512
2517. https://doi.org/ 10.1002/CJCE.23344.

spectroscopy is applicable for soluble materials while UV
Visible diffuse reflectance spectroscopy (UV
Vis Drs) is applicable for solid samples. The d
d charge transfer transitions of MOCNs can be studied by both techniques (Jose Varghese et al., 2019). Drs spectroscopy is based on reflection mode and provides data for the outer shell electrons of a material (Kortum, 1969). The doping of different metals or formation of composites of oxides improves the optical structure of the resulting hybrid metal oxides and this can be confirmed with the help of UV
Vis spectroscopy. The optical absorbance spectra of pure NiO, CdO, and NiO
CdO nanocomposites show absorption edges at 355, 518, and 458 nm, while band gap energy values 3.51, 2.45, and 2.7 eV, respectively () The solid-solid formation can be confirmed with the intermediate value of bandgap energy of NiO
CdO composite (Anitha et al., 2018). Lakhera et al. (Lakhera et al., 2018) found that the absorption range of the Cu2O catalysts is significantly broadened in the higher wavelength region when loaded with different amounts of α-Fe2O3 and the formation of a p-n heterojunction between p-type Cu2O and n-type α-Fe2O3 (Fig. 2.23). Li et al. (2011) estimated the amount of amorphous MgAl2O4 in different samples using photoabsorption spectra. They found that in the case of amorphous MgAl2O4 sample Al
O bond length was shorter than that in crystalline MgAl2O4. Zhang et al. (2014) detected redshift in the UV
Vis absorption with a change in calcination temperature from 600 C to 700 C of ZnAl2O4 (Fig. 2.24). Emara et al. (2021) studied absorption spectra of single and mixed metal oxides of Co3O4, NiO, and CuO nanocomposites (Fig. 2.25). The absorption spectra of all samples, except the Ni sample, covered the whole visible region and extend into the near-infrared, to 800 nm or longer wavelength. The bandgap energies were calculated using the Tauc equation, and its values range from 1.24 to 1.46 eV for all

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Figure 2.23 (A) UV
Vis diffuse reflectance spectra of α-Fe2O3/Cu2O catalyst with different loading of α-Fe2O3 and (B) plots between (αhν)2(eV) versus hν (eV) for α-Fe2O3/ Cu2O. Source: From Lakhera, S. K., Watts, A., Hafeez, H. Y. & Neppolian, B. (2018). Interparticle double charge transfer mechanism of heterojunction α-Fe2O3/Cu2O mixed oxide catalysts and its visible light photocatalytic activity. Catalysis Today 300: 58
70. https://doi.org/ 10.1016/J.CATTOD.2017.03.020.

Figure 2.24 The UV
vis diffuses reflectance spectra of ZA-500, ZA-600, and ZA-700. Source: From Zhang, L., Liu, J., Xiao, H., Liu, D., Qin, Y., Wu, H., Li, H., Du, N., & Hou, W. (2014). Chemical Engineering Journal 250. https://doi.org/10.1016/j.cej.2014.03.098.

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Figure 2.25 Absorption spectra of the MMO NPs collected via Drs (A) Tauc equation of the optical band gap (B) where the red lines intersect the x-axis at Eg versus metal composition (C). Source: From Zhang, L., Liu, J., Xiao, H., Liu, D., Qin, Y., Wu, H., Li, H., Du, N., & Hou, W. (2014). Chemical Engineering Journal, 250. https://doi.org/10.1016/J.CEJ.2014.03.098.

samples while NiO showed a bandgap value of 2.60 eV even though it’s a wide bandgap material (B4 eV). CoNiCu mixed metal oxide exhibited two-band gap values (1.24 and 1.82 eV), which might be due to the existence of multiple phases. Several stages are involved in the photocatalytic process of semiconducting materials. When semiconducting materials are exposed to light radiation, photons are absorbed and electrons are transferred to the conduction band and holes are created in the valance band. These photogenerated carriers participate in the demineralization of chemicals. The rate of photocatalysis depends on many factors including band gap value, size, crystallinity, the intensity of light, etc. (Yadav et al., 2016). Recently, Lakhera et al. demonstrated photocatalytic activity of α-Fe2O3 loaded Cu2O MOCN photocatalyst towards demineralization of methyl orange. The bandgap of Cu2O and α-Fe2O3 is tunned and resulted in the absorption of visible light and charge carrier separation (Lakhera et al., 2018). Ternary NiCoFe MOCNs with different metal ratios were obtained by a hydrotalcite-like precursor pathway by coprecipitation, followed by thermal treatment. These ternary MOCNs demonstrated 96.8% mineralization of methylene blue dye under visible light irradiation. The photocatalytic activity of these ternary MOCNs was higher than that of commercial P25 TiO2, binary NiFe oxides, and pure Fe2O3, CoO, and NiO particles with similar conditions (Pan et al., 2018). Yadav et al. reported the preparation of TiO2
SiO2 thin films for photocatalytic and self-cleaning applications. TiO2 was uniformly coated over the SiO2 layer and films were transparent and anti-reflecting in nature (Kim et al., 2016; Yadav & Kim, 2016). Semiconducting MOCNs photovoltaic system converts light into electric current upon exposer to light irradiations. These systems are also known as dye-sensitized

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solar cells (DSSC) (Gr¨atzel, 2004). MOCNs have been explored as a photoanode in DSSC for efficient energy conversion. Zannotti et al. utilized MgO/NiO MOCNs photocathodes in DSSC and found that the concentrations of MgO and Mg21 plays an important role in controlling photovoltage and photocurrent of DSSC. Tennakone and coworkers (Tennakone et al., 1999) applied MOCNs of ZnO
SnO2 as photoanode to achieve high photoconversion efficiency (8%) by overcoming limitations of single photoanode as ZnO. DSSC photoanode was fabricated with layer-by-layer deposition of ZnO and SnO2to utilize beneficial properties of each oxide (Milan et al., 2015). In another study by Liu et al. developed Ti and Zn- based MOCN photoanode by urea method for DSSC to improve conversion (J. Liu et al., 2013). Researchers have tried diverse types of materials like MOCNs for biomedical applications. MOCNs owning special electrical, optical, and magnetic properties are promising for the healthcare and medical field. Surface functionalized magnetic mixed oxides are widely explored in biomedical research due to their special magnetic, structural, and electrical properties and non-toxic nature. The dynamic behavior of oxide materials in the biological conditions is mostly affected by composition, porosity, calcination temperature, and surface functionalization (Bass et al., 2007). Recently, Karthik et al. established the preparation of ternary MOCNs of CdO
NiO
ZnO by microwave-assisted method and studied its antibacterial activity in-vitro against coMOCNn pathogenic gram-positive and gram-negative bacteria. In another report by Karthik and coworkers, they studied the antibacterial effect of CdO
ZnO
MgO nanocomposite in vitro against Escherichia coli, Pseudomonas aeruginosa, Vibrio cholera, Klebsiella pneumoniae, Proteus vulgaris, Salmonella typhi, and Bacillus subtilis bacteria species (Revathi & Karthik, 2018). Similarly, MOCNs of NiO
CdO showed superb antibacterial activity towards S. aureus and K. pneumonia bacteria strains (Anitha et al., 2018). Kumar et al. utilized TiO2
CuO nanocomposite as nanofillers for epoxy coatings to protect the surface of steel against corrosion and bacterial growth (Kumar et al., 2018). Zhao et al. prepared MOCN ZnAl2O4 at different calcination temperatures. They confirmed by UV 2 visible spectroscopy that the ZnO/ZnAl2O4nanocomposite prepared at 800 C has superior UV-blocking properties to both commercial ZnO and a physical mixture of ZnO and ZnAl2O4 (Zhao et al., 2010).

2.3.5 Fourier transform infrared spectroscopy Infrared spectroscopy is a well-known technique for the identification of functional moieties present in chemical substances. It is also useful in quantitative analysis as the extent of IR energy absorbed by a material is proportional to its concentration. The peaks of the IR spectrum represent the excitation of the vibrational mode of the molecules and are related to the chemical bonds as well as a functional group present in the sample. Thus, the infrared spectrum can be used as a fingerprint for identification, in support of the X-ray diffraction technique for the purpose of characterization. IR spectrum involving phonon modes and vibrational modes for solid nanoparticles and the surface molecules / molecular ions, respectively (S. Yang & Gao,

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2005). IR spectroscopy provides details of the atomic arrangement and interatomic forces in the crystal lattice. The interaction between IR vibrational frequencies and atomic forces of semiconducting materials can be studied. In attenuated total reflectance (ATR) mode, ATR crystal is used in contact with the sample and the incident angle of IR beam; the refractive index of the crystal and sample affects the spectra (Ramer & Lendl, 2013). An infrared spectrometer basically consists of a source of continuous infrared radiation, a means for resolving the infrared radiation into its component wavelengths, and a detector to record the energy of the IR radiations (Fig. 2.26). The optical principle of the Michelson interferometer, which is employed in most FTIR spectrometers. A beam splitter is used to divide the beam of radiation from the infrared source into two parts, one part being reflected in a fixed mirror and another part being transmitted to a rotating mirror. The reflected beams are recombined at beamsplitter to give a constructive/ destructive interference pattern because of the difference between paths traveled by both the beams (Pare & Belanger, 1997). FTIR spectroscopy is an absorption spectroscopic technique for detecting the vibrational transitions in molecules generated by various stretching and bending motions, the presence of surface functional groups, hydroxyl ions, and water molecules of oxides. The nature of acidic sites of oxides can be estimated from the pyridine-FTIR (H. Liu et al., 2020). FTIR peaks at 1446 and 1608 cm21 attributed due to the characteristic of Lewis acid sites of Ti 1
Zr 1O x catalyst. The effect of Mn content in NiCo2O4 oxide was studied by FTIR spectroscopy (Dolla et al., 2018). The FTIR spectra of the Mn
Ni
Co citrate gel and calcined

Figure 2.26 Optical diagram of a simple Fourier transform infrared spectrometer.

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Figure 2.27 Fourier transform infrared spectra of (A) the dried Mn
Ni
Co gel (x 5 0.5) before calcination and Mn
Ni
Co oxide (x 5 0.5) obtained after calcining at 450 C respectively. (B) MnxNi1-xCo2O4 (x 5 0,0.3, 0.5, 0.7, 1) spinel oxides obtained after calcination at 450 C. Source: From Dolla, T. H., Pruessner, K., Billing, D. G., Sheppard, C., Prinsloo, A., Carleschi, E., Doyle, B., & Ndungu, P. (2018). Sol-gel synthesis of MnxNi1-xCo2O4 spinel phase materials: Structural, electronic, and magnetic properties. Journal of Alloys and Compounds, 742, 78
89. https://doi.org/10.1016/j.jallcom.2018.01.139.

Mn
Ni
Co oxide (x 5 0.5) and MnxNi1-xCo2O4 (x 5 0, 0.3, 0.5, 0.7, 1) oxides are shown in Fig. 2.27. Fig. 2.27A shows two FTIR bands in the range of 400
700 cm21 for metal oxide bonding in the calcined sample only. The M
O vibration frequencies of Ni, Mn, or Co were detected at B560
563 cm21 and B653
655 cm21 for the octahedral and tetrahedral sites, respectively (Fig. 2.27B). Fig. 2.28 represents the FTIR spectra of the single metal oxide and mixed metal oxides of Cu, Ni, and Co (Emara et al., 2021). The peak for metal oxide, O
H stretching, O
C
O bending, and O
H bending are denoted as (A), (B), (C) and (D), respectively. Similarly, FTIR analysis was utilized to determine the quality of different metal oxides and exitance of metal oxide bond of oxides such as CdO
ZnO
MgO (Revathi & Karthik, 2018), CdO
NiO
ZnO (Karthik et al., 2018), CoOx
Al2O3 (Azurdia et al., 2006), and ZnO
CuO (Saravanakkumar et al., 2018).

2.3.6 Temperature-programmed reduction Temperature-programmed reduction (TPR) is generally used for the characterization of solid heterogeneous catalysts to estimate the efficient reduction conditions. In this process, a reducing gas is passed over the oxidized catalyst precursor with a controlled increase in temperature. H. Liu et al. (2020) performed CO2/NH3
temperature-programmed desorption (TPD) of Tia
Zrb
Ox mixed-metal oxides and found that the amount and concentration of acidic sites are increased for Ti1
Zr1
Ox and TiO2 1 ZrO2, compared with the corresponding single metal oxide (Fig. 2.29). This revealed that the increased number of acidic sites is might

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Figure 2.28 Fourier transform infrared spectra of single and mixed metal oxides of Cu, Ni, and Co. Source: From Emara, M. M., Ali, S. H., Hassan, A. A., Kassem, T. S. E., & Patten, P. G. V. (2021). How does photocatalytic activity depend on adsorption, composition, and other key factors in mixed metal oxide nanocomposites? Colloids Interface Science Communication 40. https://doi.org/10.1016/j.colcom.2020.100341.

be due to the mixed metal oxide and the high dispersion single metal oxide component in the mixed oxide with a high specific surface area (H. Liu et al., 2020).

2.3.7 X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) is one of the most commonly used non-destructive techniques for the determination of chemical bonding, electronic structure, and density of the electronic states of various materials (Greczynski & Hultman, 2020; Srivastava, 2014). The basic principle of XPS is based on the photoelectric effect that is the emission of the electrons upon light illumination. During illumination by a photon of known energy, electrons from the surface of the sample are ejected into the vacuum and analyzed by a spectrometer to measure their energy. The plot of intensity versus binding energy was obtained which provides the information of chemical states existing in the sample. XPS instrument consists of a source of X-rays, an ultra-high vacuum chamber with magnetic shielding, an electron collection lens, an electron

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Figure 2.29 NH3/CO2
TPD and pyridine-Fourier transform infrared profile of TiO2, ZrO2, Ti1
Zr1
Ox and TiO2 1 ZrO2 catalyst calcined at 300  C. Source: From Liu, H., Li, Z., Wang, J., Lu, S., Wang, M., Liu, Y., & Li, C. (2020). Carboxylation of toluene with CO2-derived dimethyl Carbonate over Amorphous Ti 2 Zr mixed-metal oxide catalysts. ChemCatChem, 12(1), 95
99. https://doi.org/10.1002/ cctc.201901276.

energy analyzer (hemispherical electron analyzer), an electron detector, a sample stage, etc. (Fig. 2.30) (Greczynski & Hultman, 2020). XPS has been used widely to analyze several materials and more than 9000 papers are published annually based on XPS results (Greczynski & Hultman, 2020). For example, Lakhera et al. analyzed
Fe2O3/Cu2O by XPS and confirmed the existence of Cu11 as a main phase in the catalysts (Lakhera et al., 2018). Similarly, the presence of Mn21, Mn31, Ni21, Ni31, Co21, and Co31 species on the surface of MnxNi1-xCo2O4 was investigated by Dolla et al. (2018). Several MOCN’s have been characterized to determine chemical environment using XPS techniques are listed in Table 2.1.

2.3.8 Electrochemical characterization Compact electronic devices, as well as electric vehicles, contain rechargeable Li-ion batteries as the main component. Scientists are engaged in replacing conventional graphite used as anode with alternative materials such as MOCNs. The carbon anode possesses a low gravimetric capacity of B370 mAh g21. Recently, Ma et al. developed (Ma et al., 2018) macro/mesoporous MOCNs of Fe
Ni deposited on Ni foam as anode for Li-ion batteries. They developed a quasi-gel-state tri-copolymer, that is, F127
resorcinol
melamine, as the N-doped carbon source to regulate the interfacial

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Figure 2.30 The schematic view of the X-ray photoelectron spectrometer. Source: From Greczynski, G. & Hultman, L. (2020). X-ray photoelectron spectroscopy: Towards reliable binding energy referencing. Progress in Materials Science 107. https://doi. org/10.1016/J.PMATSCI.2019.100591.

chemistry of the MOCN electrodes. The electrochemical performance of this anode in both half-cell and full-cell systems solved the problem of irreversible capacity loss and exhibited cyclability at high rates. The MOCNs of Sn3O4 and TiO2 as an anode showed initial discharge capacity was 1513.7 mAh g21 and the reversible capacity was 659 mAh g21 after 50 cycles (Chen et al., 2018). However, applications of MOCNs in Li-ion batteries are limited due to several parameters such as poor conductivity, low solid Li-ion diffusion rate, etc. Numerous MOCNs have been recognized as potential candidates for electrochemical energy storage applications. The demand for highly stable and efficient energy storage systems has been raised due to the increasing production of compact and wearable electronics. Conventional batteries are failed due to low cycling stability and are unable to charge-discharge quickly. Electrochemical capacitors are an alternative state-of-the-art charge storage process, also known as supercapacitors. Electrochemical supercapacitors owning higher power density, fast charging/discharging rate, long cycling life than batteries, and higher energy density. Metal oxides demonstrate faradic behavior and redox reactions. These reactions are similar to battery-type materials and having less cycling stability as well as low power density. MOCNs show a pseudocapacitive nature and the charge storage mechanism of pseudocapacitors involves reversible and fast Faradaic reactions on its surface. Several transition metal oxides have been used to fabricate hybrid supercapacitors made up of electrical double-layer capacitors like carbon materials and pseudocapacitors (Karuppasamy et al., 2020; Kathalingam et al., 2020; Yadav et al., 2020).

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Reddy, Anitha, Gopi, et al. (2018) reported a facile two-step fabrication of honeycomb-like NiMoO 4@NiWO4 MOCNs with a specific capacitance of 1290 F g21 at a current density of 2 A g21. Ahuja and coworkers found that the interaction of 3d-4d transition MOCNs of MnO2
RuO2 nanoflakes and reduced graphene oxide nanoribbon as simultaneous existence of M(3d) and M(4d) dramatically enhanced supercapacitor properties (Ahuja et al., 2018). Recently, Reddy et al. achieved a specific capacitance of 1024.66 F g21 at 10 mA cm22 forZnMn2O4@ZnFe 2O4 microspheres. These MOCNs showed low internal resistance and high cycling stability (95.8%) at 3000 charge-discharge cycles (Reddy, Anitha, Muralee Gopi, et al., 2018). Shinde et al. (Yadav et al., 2019) prepared CuO doped Mn2O3 thin films by a successive ionic layer adsorption and reaction (SILAR) method and studied the effect of CuO doping on Mn2O3 towards morphological and electrochemical performance. The pure manganese oxide thin film showed rod-like structures, 1% and 3% CuO-doped Mn 2O3 showed spherical nanoparticles and flakes-like morphology while pristine manganese oxide showed rod-like morphology. The higher specific capacitance of 500 F cm22 at 5 mV s21 was achieved for 3% CuO-doped Mn2O3. Similarly, Ramesh et al. (Ramesh et al., 2019) reported a thermal reduction process for the preparation of highly electrochemical stable and conductive composites of SnO2@NiCo2O4/N-doped multiwalled carbon nanotubes.

2.4

Summary and outlook

In this chapter, we have discussed the recent progress in the field of preparation and characterizations of MOCNs. Also, we have highlighted their promising applications in photocatalysis, catalysis, sensor, Li-ion batteries, supercapacitor, and biomedical fields. The composition and the synergic effects of multiple metal oxide species in the composite give rise to their remarkable physiochemical properties. For the synthesis of nanostructures, top-down and bottom-up approaches have been explored by researchers. The bottom-up approaches are found to be simple and cost-effective compared to the top-down approaches. Characterizations tools offer a promising way to understand the physicochemical properties of MOCNs at nanoscale, and synthesis parameters can be altered to get desired MOCNs with novel properties. Along with the development and characterization of MOCNs, there are still new fascinating opportunities for material chemists. Advancement in the synthesis and characterization helped to understand the physicochemical properties of MOCNs in great detail. However, some aspects of the development of composite metal oxides are not fully understood and will require continuous efforts to gain more insight into their development and properties. There are many challenges ahead, and it is expected that MOCNs will be the platform soon for chemical and biochemical technologies. There is a need for an indepth understanding of the mechanisms of the MOCNs and their optimization for energy, catalysis, and healthcare applications.

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Part II : Metal oxides-based composites in energy technologies

Metal oxides as photoanodes for photoelectrochemical water splitting: synergy of oxygen vacancy

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Keval K. Sonigara1,2,*, Jayraj V. Vaghasiya1,* and Saurabh S. Soni1 1 Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India, 2 Oxford Suzhou Centre for Advanced Research (OSCAR), University of Oxford, Suzhou Industrial Park, Jiangsu, P.R. China

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Introduction

Green, sustainable, and renewable energy resources become a necessity for the environment-friendly development of society. Hence, the development of these energy resources is an argent and big challenge nowadays (Nandakumar et al., 2020). Hydrogen diatom (H2) is accepted as a key alternative resource due to its promise as a sustainable fuel, which is regarded as an economically and environmentally feasible renewable energy resource (Turner, 2004; Zhu, Hu, et al., 2020). Further, a revolution in automotive vehicle technology investigate the future in hydrogen fuel-based engines due to the wide abundance of H2 in the universe (Louis, 2009). Meanwhile, H2 is the lightest gas with a density of 1/14 of air, which is 0.0899 g L21 at standard atmospheric pressure, at 0 C, and is termed as a potential energy holder with gravimetric energy density 143 MJ kg21. The technologies already developed to store and transport the hydrogen and aimed to be used in the fuel cells with water as an economical combustion product without any pollution. Hence, hydrogen economy growth, hydrogen production, and knowledge in energy alternatives are very argent. The community targeted to overpast the hydrogen production methods which are leading to CO2 emission and required high costs like natural gas, coal, petroleum, or water electrolysis assisted industries. Then, making efforts to produce it from renewable or natural resources and technologies for a better future of technology (Sharma et al., 2019; Zhao et al., 2019; Zhu, Lin, et al., 2020) Hence, the economic development of methods for H2 generation is very argent, which can boost the H2 economy and reduce CO2 emission. One suitable device for this purpose is photoelectrochemical cell (PEC) (Gr¨atzel, 2001; Roger et al., 2017), which induces water splitting induced hydrogen production through solar energy as cost-benefited, environmentally friendly, and 

First two authors contributed equally

Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00017-8 © 2022 Elsevier Inc. All rights reserved.

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sustainable way. Hence, the sun provides a resource flux of B3 3 1024 J per year energy, which is much higher than the current energy need of the world. Therefore, solar energy can act as a sustainable alternative energy source in the future of hydrogen production. Hence, photoelectrochemical water splitting (PECWS) (Xiao et al., n.d.; Zhu, Lin, et al., 2020) emerged as fast-growing technology where a semiconductor material in film or particle form generates electron hole by absorbing photons from the sunlight according to the bandgap; these charges are transported to the semiconductor/electrolyte interface, at which electrochemical conversions take place, which split water into hydrogen and oxygen diatoms by sustainable way (Walter et al., 2010). The most popular semiconductor part is governed by the metal oxides, which promise to generate the required Gibbs free energy (228.71 kJ mol21) for the water splitting reaction (Fujishima & Honda, 1972). Regarding this technology, huge work has been carried out during the last decade for the development of efficient metal oxide-based photoanode and photocatalyst materials to achieve higher hydrogen/oxygen evolution efficiency in the PECs in the presence of light, which succeeds to achieve above 10% solar-tohydrogen (STH) conversion and photocathodes showed photocurrent as maximum as 10 mA cm22. (McKone et al., 2013; Paracchino et al., 2011). The n-type metal oxide photoanodes, like titanium oxide (TiO2) (Asahi et al., 2001; Barreca et al., 2015; Davide et al., 2016; Mascaretti et al., 2017), tungsten oxide (WO3)(Kong et al., 2017; Zhao et al., 2016), Fe2O3(Iron oxide), and (Bismuth vanadium oxide) BiVO4, have been explored and studied deeply. Simultaneously, photocathodes have also been utilized, but their search is still limited due to a lack of a suitable material library of p-type metal oxides (Jiao et al., 2021; Lin et al., 2021; Scardamaglia et al., 2021). However, the water splitting process is just more than the straightforward redox reaction as it happens at the electrode electrolyte surface. Hence, hole electron separation and charge diffusion in the bulk of metal oxide and at the surface is critical. Hence, the core knowledge about metal oxide properties like bandgap, photoresponsivity, oxygen vacancy (Vo), phase, and morphological engineering features have been developed through rigorous research (Sharma et al., 2019; Xiao et al., n.d.; Zhu, Lin, et al., 2020). To develop high-quality metal oxide, Vo engineering is a key tool that responds to many beneficial properties with main parameters (Jiao et al., 2021). This review chapter describes the basic occurrence of Vo and its importance in the metal oxide properties for PECWS reaction particularly developed since the year 2015 20. This will focus on collecting the knowledge that how to tune Vo in various types of popular metal oxides and how it improves the water splitting reaction. First, we have explained the concept of PECWS, the role of metal oxides in PECWS, a general plot of strategies used for improving metal oxides, and then summarized the recent development of Vo engineering in metal oxide in photoanode and photocatalyst. The discussion mainly focused on Vo generation through the metal oxide, synthesis, alkali metal doping, transition metal doping, modulating facet-control, crystal phase manipulation, etc. In concluding remarks, challenges, future perspective, and research trend prediction of this topic.

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Role of metal oxides in photoelectrochemical hydrogen/oxygen evolution

The PECWS mechanism is the reaction of water molecules splitting into H2 (gas) and O2(gas) in the presence of light illumination. Fig. 3.1 shows a mechanistic diagram of the hydrogen evaluation process occurring in the electrochemical cell described by early-stage research of Fujishima and Honda (1972). In this type of cell, the positive and negative electrodes are connected with the water medium electrolyte. While the water splitting process, light irradiation on the photoanode generates the electron from the valance band to the conduction band by absorbed photon energy from light and liberated holes at the valance band and oxidized the water molecule into oxygen, and shows oxygen evolution. Same time, the cathode back electrode collects the electron and reduces a water molecule to resulting hydrogen evolution by donating to electrolyte from Fermi level. The chemical redox reaction of the H2 and O2 evolution is also provided in Fig. 3.1, which shows the 1.23 V required potential to split the water molecule. Hence, the photoelectrochemical cell should generate more than 1.23 eV energy to trigger the redox reaction in the presence of light.

Figure 3.1 Schematic presentation of photoelectrochemical water splitting in the presence of light. Source: From Jeong, S. Y., Song, J., & Lee, S. (2018). Photoelectrochemical Device Designs toward Practical Solar Water Splitting: A Review on the Recent Progress of BiVO4 and BiFeO3 Photoanodes. Applies Sciences, 8(8), 1388 https://doi.org/10.3390/app8081388.

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Typically, this process can be summarized in three steps, as mentioned in Fig. 3.1. 1. photon absorption from solar light in the photoelectrode to generate charge species 2. separation of the charge species, electron/hole and 3. water oxidation and/or reduction at the surface of the electrodes

Looking at the above three steps, the role of metal oxides is significant, which can be played as photoanode or photocathode, or photocatalyst with electrolyte. Hence, the role of various properties of metal oxides is finally responsible for the performance of PECWS. Fig. 3.2A described the flow chart of roles that can be controlled or monitored through spectral responsivity, bandgap, electronic conductivity, charge diffusion, the morphology of the metal oxide nanomaterials, and the method used to prepare the metal oxide and electrodes. The origin of energy in the PEC is the total light absorbed by the photoelectrode. Hence, it is directly proportional to the bandgap of metal oxides. Fig. 3.2B shows the various simple metal

Figure 3.2 (A) Various metal oxide families improved through Vo engineering to achieve suitable qualities for photocatalytic hydrogen evolution. (B) Valance band, conduction band, and bandgap of various classes of metal oxides and their water oxidation reduction potential at pH 5 0. Source: From Tamirat, A. G., Rick, J., Dubale, A. A., Su, W. N., & Hwang, B. J. (2016). Using hematite for photoelectrochemical water splitting: A review of current progress and challenges. Nanoscale Horizons, 1(4), 243 267. https://doi.org/10.1039/c5nh00098j.

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oxides with a suitable bandgap, valance band, and conduction band to create the 1.23 eV potential in the PECWS device with light-harvesting (Hua et al., 2018). Further, the next role is to transport the generated holes/electrons to the electrolyte, which is part of the bulk conductivity and surface activity of metal oxides. Hence, the electronic/hole conductivity and the various morphological nanostructures play an important role in succeeding in the maximum STH efficiency. Further, at the mechanism kinetics, the electron hole pair recombination within the metal oxide is the crucial parameter that is responsible for the efficiency loss in the mismatched OR poor band alignment of the metal oxides. Hence to mitigate this problem, the feasibility and applicability of bandgap (BG) engineering, the flexibility of valance band (VB), and conduction band (CB) features are very important. Many strategies have been developed to improve the metal oxide-based photoanode performance by implementing doping strategies through metal, non-metal, and multi-component blending. The morphology tuning explored from nanoparticles, nanorods, nanowires, nanodots, etc., to improve the surface activity by surface area and hence the improvement in the charge transfer processes. Tremendous work also did to engineer the crystal phases of the metal oxides that show promise of improved activity based on the suitability of crystal phases. This knowledge also extended to the facet cutting, tuning, and fining to target the specific improvement of charge separation at the electrode electrolyte interfaces. Apart from the above strategies, there is a very efficient way to achieve multiple beneficial properties by tuning the oxygen vacancies (Vos) in the metal oxides intrinsically available in all photoanode types, cathode, and catalysts (Hua et al., 2018; Huang, Gao, et al., 2020; Jia et al., 2019; Ren et al., 2020; Sun et al., 2020; Wang et al., 2017; Ye et al., 2020; Zhang, Ning, et al., 2018). The increase and decrease in the Vo directly made changes in the electronic, optical, and charge transport properties at a time that can tune band gape, movement of VB and CB, and the emergence of charge separation centers. The metal oxides also play an external role in the PECWS as a key volumetric part of PEC. Hence, the metal oxide materials cost is the cost decider of the water splitting technology. So far, many methods have been developed to produce low-cost and efficient photoelectrode and photocatalyst materials. Hence, synthetic protocols play an essential role in the development of metal oxide nanomaterials in the PECWS (Concina et al., 2017; Rafique et al., 2020). Looking to the current scenario, researchers are improving the PECWS technology majorly through enhancing the knowledge and advancement in the metal oxides. However, metal oxide nanomaterials still suffer from the ideal properties in one material. Hence, the low photoresponsivity, catalytic activity, and the cost of the materials are key issues to be solved yet.

3.3

Oxygen vacancy engineering in metal oxides for photoelectrochemical water splitting

In general, vacancy defects are the absent centers of atoms or ions in the lattice structure of metal oxides. These defects can be anionic or cationic based on the nature of

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the missing center, and it is positively or negatively charged. Usually, they are associated with the energy level of metal oxides by localizing positions near the VB and CB. The positive defects localized with VB energy levels and negative defects localized with CB energy levels. Hence, these energy localizations made an impact on the charge conduction, charge diffusion, localized energy levels, charge diffusion, optical absorption, and emission properties. The Vo creates the two-electron charge deficiency at the missing site due to the loss of the O2- atom from the lattice during the synthesis process (Nowotny et al., 2014). The mechanism of formation of Vos in metal oxide during the annealing (in chemical equilibrium reaction) and can be written with Kro¨ger notation (Kro¨ger & Vink, 1956). based equations (1 3) (Ferna´ndez-Climent et al., 2020) (In this chapter, the vacancy defect equation represented by this notation): 1 X OX o 2Vo 1 O2 2

(3.1)

where Oox is the saturated oxygen and Vox is the saturated oxygen vacancy with two associated electrons considering along with present donor level in the gap. During increasing the heating, the saturated vacancies start to ionize related to its activation energy Ea and eliminate free electrons in the conduction band. This process can be written as: 21 2 VX o 2Vo 1 2e

(3.2)

where Vo21 is an ionized Vo. So, the full reaction of vacancy formation induce charge carrier generation can be written as, 1 21 2 OX o 2Vo 1 2e 1 O2 2

(3.3)

The benefit of Vo formation is that they require minimum energy for formation compared to lattice engineering. It is easy to implement and an easier way to modify the metal oxide properties. Janusz et al. explained detailed vacancy formation kinetic by taking the example of TiO2 and provided equilibrium constant for each step and heat changes (Nowotny et al., 2014). In PECWS, all the metal oxides associated with the oxygen atom in the crystal lattice and hence, Vo are common types of vacancy generated during the various synthesis and post-treatments (Wang et al., 2017). Due to the diverse role of Vo in the metal oxides, it directly shows an effect on the performance of water splitting, which may be associated with many aspects which are propagated by the effect of Vo generation. In general, the portion of Vo generation in the bulk and the photoanode surface is always different, reflecting in the bulk charge carriers and the interfacial charge carrier separation (Xiao et al., n.d.). These charge carriers are prominent components for hole and electron separation at the electrode/ electrolyte surface, which is the key factor of PEC efficiency. Therefore, the understanding and knowledge of Vo formation and its effect on various metal oxides are essential in the PEC energy conversion field.

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The search and development of Vo-deficient metal oxides nanomaterials continuously grow from experimental to theoretical tools to promote their promise in various application(G. Wang et al., 2017). The knowledge development of the role of Vo in the improvement of PECWS inspired the utilizing such approach in other advanced devices like metal electrode base dye-solar cells, (Barpuzary et al., 2014; Qureshi et al., 2015; Vaghasiya et al., 2016) energy storage devices, (Hua et al., 2018; Huang, Li, et al., 2020; Ye et al., 2020) fuel cells, (Ren et al., 2020; Sun et al., 2020) and catalytic synthesis (Pandey et al., 2018) methods. The majority of literature and studied available related to Vo are more focused on the TiO2, WO3, and BiVO3 based metal oxides for the photoanodes. Moreover, the techniques to introduce the Vo and their identification and characterization are also welldeveloped utilizing various metal oxide nanomaterials (Pandey et al., 2018; Sarkar & Khan, 2019; Wang et al., 2017; Wang & Wang, n.d.; Zhao, Chen et al., 2019) In general, annealing in the vacuum with an oxygen-deficient environment induces the Vo (Nakajima et al., 2014; Yang et al., 2014), while in the air, Vo will be healed again. The Vo also introduced by reduction treatment on the metal oxide with like flam reduction treatment (Zuo et al., 2014), electrochemical treatment (Wang et al., 2016), chemical treatment in the presence of hydrogen (Mehta et al., 2016; Rioult et al., 2016), nitrogen (Sasinska et al., 2017), ammonia (Liu et al., 2019), hydrazine (Mao et al., 2014), NaBH4 (Fang et al., 2014; Tan, Zhao, Niu, et al., 2014), carbonyl (Lin et al., 2014), and aluminum (Cui et al., 2014). However, other metal oxides like ZnO, In2O3, SrTiO3 are fairly discussed at one platform for their promise of Vo engineering. This chapter has provided contemporary art of Vo engineering in TiO2, WO3, brief, while ZnO, In2O3, and SrTiO3 are discussed with detailed case studies.

3.3.1 TiO2 Understanding the impact of Vo is well studied in the TiO2 based photoanodes in the PECWS, as TiO2 is the most explored nanomaterial due to its highest photostability and suitable energy alignment. Sarkar et al. described the detailed polymorphs with various types of Vo formation, the effect of Vo formation and the methods to create Vo, and the techniques to study the Vo in the TiO2 based metal oxides (Sarkar & Khan, 2019). Typically, foreign metal ion doping from the lanthanide, actinide, and transition metal series, (Nguyen et al., 2021; Sarkar et al., 2016; Song et al., 2021; Wang & Gong, 2021) non-metals like N, C, S, F, P, etc. (Gonza´lez-Torres et al., 2018; Ji et al., 2020), wet and solid-state chemical routes by reduction and oxidation, different types of annealing, electron irradiation, (Scheiber et al., 2012) argon irradiation, (Barman et al., 2017) different types of plasma-assisted treatments (Sasinska et al., 2017; Wu et al., 2013), and so on. Above all, methods dominate the Vo formation at the bulk of TiO2 or the surface of TiO2 nanostructures, but the vacancies at the surface are higher the most due to less energy at the surfaces (Kim et al., 2016). However, these methods play a similar role in inducing the vacancy in TiO2 lattice, but understanding the effect on the PECWS is critical. Thus, regardless of the

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discussion of the method to introduce Vo, we have discussed the impact of vacancy amount on the charge transfer process between the electrode and electrolyte. Huang et al. used electrochemical impedance spectroscopy to study this role and found that the appropriate amount of Vo is suitable rather than higher or lower (Huang, Gao, et al., 2020). The TiO2 nanowires were prepared from the hydrothermal method and were annealed at an interval of 100oC from 300 C 600 C ( Fig. 3.3A) to generate the higher to lower Vo in TiO2 and then evaluated the effect on bulk charge diffusion and the surface charge conduction for resultant charge separation and recombination in PCEWS. The XPS (X-ray photon spectroscopy) method is the common tool in metal oxide characterization, providing the proportion of Vo generated. In the hydrothermal method, the produced TiO2 shows the indicative peak of oxygen atoms O22 in there in the lattice for bulk vacancy and peak for the adsorbed hydroxy group as the surface vacancy (Fang et al., 2018). Hence, the authors have identified the proportion of vacancies appropriate by -OH peaks. It was found that the area proportion of OH is, and for TiO2 300 (28.6%,), TiO2 400, (24.5%), TiO2 500(21.2%,) and TiO2 600(19.0%), which indicated that the Vo generation observed initially which is decreased with increase in temperature on nanowires. These distinct samples of various Vo concentrations reflect an exciting trend while PCE performance. The TiO2 400 with intermediate Vo concentration exhibits the highest photocurrent density valued at nearly 1.23 mA cm22 V versus RHE. However, the photoresponse was improved in TiO2 400 and TiO2 500 samples, along with the improved bias potential compared to the TiO2 300 (Fig. 3.3B). A similar performance trend was observed when ON/OFF cycles were measured (Fig. 3.3C). Another effect of Vo was observed in the onset potential, which was noticed lowest for the TiO2 300. Herewith lowest and highest Vo-based photocathode show lower photocurrent due to trapping of photo-generated holes in the defect points in the lattice and at the interface later. This can be observed in Fig. 3.3C. The lower Vobased system becomes steady-state very faster than the higher Vo in TiO2 300 during the illumination. This indicates that the Vo is forced to be steady-state faster due to the hole trapping during the charge transfer process and shows lower PEC performance. The impact of Vo on the recombination and separation kinetic the Electrochemical impedance spectroscopy (EIS) is the powerful tool that can reflect its effect from the identified resistance profile at photoanode/electrode interface (Klahr et al., 2012) (The detailed technique can be found in chapter Metal oxide for DSSC). An electrical equivalent circuit model (Fig. 3.3E inset) with resistors (R) and capacitors (C) was used to fit the PEC water splitting process: Fig. 3.3E described the Nyquist plot measured with different photoanode and extracted the resistance profiles using the equivalent circuit provided in inset. Based on this technique, the resistance provides information on the charge transfer, while capacitance provides charge diffusion information. The 400 oC annealed sample showed small resistance (R1; 30 Ω, and R2; 85 Ω) than high temperature annealed samples indicate good interface characteristics, but high capacitance values C1 (0.35 3 10 4 F) and C2 (3.5 3 10 4 F) indicates huge bulk charge diffusion. The importance of the

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Figure 3.3 (A) Schematic of thermal treatment induced Vo formation in nanowires; (B) currentvoltage curves of photoelectrochemical cell; (C) current-time profile switched at 1.23 V versus reversible hydrogen electrode; (D) the voltage output profile biased at 0.2 mA cm22; and (E) Electrochemical impedance spectroscopy measured under illumination with biased voltage. Source: From Huang, X., Gao, X., Xue, Q., Wang, C., Zhang, R., Gao, Y., & Han, Z. (2020). Impact of oxygen vacancies on TiO2 charge carrier transfer for photoelectrochemical water splitting. Dalton Transactions, 49(7), 2184 2189. https://doi.org/10.1039/c9dt04374h.

Vo effect can be understood from the comparing resistance of 300 and 500 annealed samples where higher Vo-based samples show a small R1 than a lower Vo-based sample and lower Vo samples show high R2 than a high Vo sample.

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Thus, high Vo favors the internal charge diffusion, but it is not suitable for surface charge transfer. Many attempts have been reported recently to study the Vo with other methods summarized in Table 3.1 shows the PEC performance of engineered TiO2 metal oxide photoanodes, including the engineering method and strategies. Further, Vo is also associated with the foreign element doping strategies for improvement in the TiO2 properties; hence, the Vo present in the lattice makes the interaction with dopant materials and improves or mitigates the charge transport process. Sasinska et al. studied the chemical vapor deposited TiO2 photoanode with nitrogen and hydrogen plasma treatment individually and together (Sasinska et al., 2017). In this case, H2 plasma-treated samples show higher trapping numbers at Vo sites, while the N2 plasma-treated samples improved the charge carrier by alleviating charges from the Vo sites. Hence, N2 treatment with Vo shows better charge separation and hence the pronounced PEC performance. Some reports adopted the utilization of photo-generated Vo in the TiO2 in the presence of Ni/Cu doping, where the photo-thermal chemical effect on Vo improves the 36.0-time higher hydrogen production compared to bare TiO2 photoanode (Wu et al., 2021). Ultimately, Vo engineering is a powerful tool in TiO2 photoanode improvement, which has broad opportunities.

3.3.2 WO3 WO3 is a popular partial semiconductor having a thermodynamically positive VB (3.0 V vs NHE) position to inject the holes in the water due to intrinsic n-type characteristic and shows a good photocurrent onset for water oxidation, particularly in an acidic condition (Sarnowska et al., 2016) WO3 possesses suitable large BG (2.3 2.7) eV and having a theoretical limit of photocurrent density of 4 mA cm22 and STH efficiency up to 6% under 1 sun illumination of solar light, which utilized B12% photons (G. Wang et al., 2012). This electrode suffers from very high internal charge recombination at the practical approach due to its use as a thicker film for harvest maximum light in partial semiconducting properties. This inherent limitation induces sluggish photogeneration and propagates in considerable transport barriers that slow down the charge separation at the photoanode/electrolyte surface and limit the PECWS performance (Feng et al., 2016; Osterloh, 2013). Hence, many efforts have been carried out so far to maximize the performance of WO3 by resolving these drawbacks, which include surface modifications, doping strategies, synthetic approaches, and so on (Kong et al., 2017; Zhao et al., 2016). Zheng et al. provided brief art of WO3 photoelectrode nonbacterial synthesis, morphology, and synergic strategies (Zheng et al., 2019). WO3 is known for pertaining to inherent Vo defects in the lattices, including the nature of high doping density. Hence, it is often reported with excellent PEC behavior is assigned to directly dependent on intrinsic Vo defects and the reduced W51 states neighboring these lattice sites (Smith et al., 2014) shows the recent work of WO3 photoanode related to Vo induced performance improvement in PECWS integrated with various approaches. These studies suggested that Vos are intrinsic dopants that have a highly relevant role in WO3 photoanodes by introducing donor levels inside the bandgap and being

Table 3.1 Summary of photoelectrochemical water splitting performance of metal oxides improved with oxygen vacancy engineered with various aspects. (RHE: reversible hydrogen electrode). Photoanode

Oxygen vacancy method

Dopant/phase/morphology engineering

Photocurrent density

References

TiO2

Nanotube arrays with anatase-rutile mixture Ni/Cu doping

2.74 mA cm22 at 0.6 V versus Ag/AgCl 36 times higher H2 evolution (Exception)

Liu et al. (2018)

TiO2

Electrochemical hydrogenation Photo-thermal chemical coupling induces vacancies NaBH4 reduction,

Ti Si alloy nanotubes

TiO2

Thermal annealing

black Ti Si O nanotubes

TiO2

1D/3D nanorod

TiO2

Hydrogen plasma treated Electrochemical reduction Ar1 ion irradiation

Nano films

Dong et al. (2018) Dong et al. (2018) Madhavi et al. (2020) Dong et al. (2021) Wu et al. (2018)

TiO2

Thermal treatment

Cr doped nanowires

TiO2

Ni/Si-doped nanostructure

TiO2

Electrochemical reduction Hydrogenation treatment Thermal annealing

TiO2

Thermal annealing

1.39 mA cm22 at 0 V versus Ag/AgCl 1.78 mA cm22 at 0 V versus Ag/AgCl 0.369 mA cm22 at 1 V versus Ag/AgCl 1.63 mA cm22 at 0 V versus Ag/AgCl 0.56 mA cm22 at 1.23 V versus RHE 0.53 mA cm22 at 1.8 V versus RHE 2.41 mA cm22 at 0 V versus Ag/AgCl B3.0 mA cm22 at 1.23 versus RHE 1.23 mA cm22 at 1.23 V versus RHE 1.66 mA cm22 at 1.23 V versus RHE

TiO2

TiO2

TiO2

Si doped nanotubes

Carbon quantum dots-doped nanorods Nanowires Cobalt-doped TiO2 Nanowire arrays coated with NiFe

Wu et al. (2021)

Song et al. (2021) Xiao et al. (2020) Liang et al. (2019) Huang, Gao, et al. (2020) Liu et al. (2020)

(Continued)

Table 3.1 (Continued) Photoanode

Oxygen vacancy method

Dopant/phase/morphology engineering

Photocurrent density

References

TiO2

NaBH4 reduction

TiO2

Ar-plasma

0D/1D graphitic carbon nitride (g-C3N4)/ TiO2 heterostructures Co3O4/TiO2

0.72 mA cm22 at 1.23 V versus RHE 2.5 mA cm22 (1.23 vs RHE)

WO3

WO3/BiVO4 nanojunction

WO3

Annealing in the presence of O2 Atomic vapor deposition Ozone treatment

WO3

Thermal annealing

WO3

Thermal annealing

Tungsten vacancy with oxygen vacancy

WO3

Thermal annealing

Ti-doped nanoporous

WO3

Ar plasma treatment

WO3 nanoplate arrays

In2O3

Thermal annealing

In2O3

Thermal annealing

In2O3

Thermal annealing in Ar and Air Thermal annealing

Oxygen vacancy engineering on facets of nanowires Heterojunction of In2O3 nanorods and black Ti Ni O nanotubes Heterojunction of In2O3 nanorods and black Ti Ni O nanotubes In2S3/In2O3 nanocomposite

5.5 mA cm22 at 1.23 V versus RHE 2.96 mA cm22 1.23 V versus RHE 2.25 mA cm22 at 1.23 V versus RHE 1.35 mA cm22 at 1.23 V versus RHE 4.12 mA cm22 at 1.6 V versus Ag/AgCl 1.139 mA cm22 (at 1.23 vs RHE) 1.32 mA cm22 1.23 V versus RHE 1 mA cm22 at 0.22 V versus Ag/AgC 5.32 mA cm22 at 0 V versus Ag/AgCl 2.80 mA cm22 at 0 V versus Ag/AgCl 0.83 mA cm22 under an applied potential of 1.18 V versus the RHE

Xiao et al. (2019) Dong et al. (2018) Zhou et al. (2017) Ma et al. (2019)

WO3

In2O3

WO3/BiVO4/ZnO Two-dimensional WO3 nanoflakes

Zhang, Chang, et al. (2018) Zhang, Ning, et al. (2018) Soltani et al. (2019) Kalanur et al. (2017) Liu et al. (2021) Meng et al. (2020) Liu et al. (2016) Dong et al. (2019) Sharma et al. (2020)

ZnO

2.10 mA cm22 at 1.23 V versus RHE -0.67 mA cm22 at a bias potential of -0.6 V versus RHE 4.5 mA cm22 at 0.4 V versus Ag/AgCl 0.67 mA cm22 At 1.23 V (vs RHE) 1.517 mA cm22 at 1.45 V versus RHE 1.3 mA cm22 at 1 V versus Ag/ AgCl 4.75 mA cm22 at 1.5 V (vs Ag/ AgCl)

Long et al. (2018) Wang et al. (2019)

Charge-Trapping Zn(OH)2 Annihilation on Nanorod arrays

600 μA cm22 at 1.23 versus RHE

Baek et al. (2017)

ZnO as passivation layer with ternary WO3/BiVO4/ZnOPhotoanode TiO2/SrTiO3 heterojunction

2.96 mA cm22 at 1.23 V versus RHE 0.32 mA cm22 at 1.23 V versus RHE 1.71 mA cm22 at 1.23 V versus RHE

Ma et al. (2019)

ZnO

Solvothermal reduction method Thermal annealing

CoOx co catalyst doping in ZnO nanorod array p-n homojunction of ZnO nanoparticles

ZnO

Thermal annealing

ZnO

Atomic layer deposition

carbon-quantum-dot-sensitized ZnO@HZnO1-x multijunction Heterojunction with TiO2 nanotubes

ZnO

Redox reaction with graphene Thermal annealing

Graphene oxide/ZnO hybrid structures (nanorod and triangle shaped) Nano pencil arrays ZnO NWs grown on GaN

ZnO

Metal 2 organic chemical vapor deposition Oxygen Vacancy Generation by vacuum Annealing Atomic layer deposition

SrTiO3

Reduction-Induced

SrTiO3

NaBH4 reduction

ZnO ZnO

ZnO

Nonmetallic materials with localized surface plasmon resonance in the perovskite structure

Lin et al. (2021) Wang et al. (2019) Chandrasekaran et al. (2016) Lv et al. (2014) Hassan et al. (2018)

Tieping et al. (2011) Shi et al. (2018)

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responsible for the n-doping, improving the PCE performance by improving its semiconducting properties. A case of Yb-doped WO3 photocatalysts by the co-sputtering method of WO3 and Yb. They achieved charge transfer and increased n-type characteristic due to the presence of Vo (Liew et al., 2014). When the nitrogen-doped WO3 was demonstrated with the Vo planted with control in WO3 nanoporous electrodes in ammonia and nitrogen gas, a similar type of improvement of donor density was observed (Liu et al., 2012). The attempts were also found by a combination of thermal treatment with and without hydrogen in the atmosphere during the fabrication for the Vo introduction. The implanting gold and silver doped assisted Vo generation increased the charge carrier density, including high optical response results to improved PECWS. The use of acid treatment in the synthesis also participates in controlling or introducing the Vo in the WO3 bulk as proposed by Chai et al. They concluded that pure WO3 formation is possible in acid. In contrast, a lower amount of acid can generate the Vos in the WO3. However, some reports explain the benefits of the decrement of surface Vo when the excess volume controls the recombination of hole-electrons. Ozone exposure is especially used to oxidize the W51 ions on the electrode surface, limiting the volume of unnecessary Vos, which reported up to 150 mV cathodic overpotential shifting (Zhang, Chang, et al., 2018). As discussed in the TiO2 section, the appropriate number of vacancies has a positive impact on the charge transfer. But the amount of Vo also made a huge impact on the charge separation kinetic at the VB and CB. Corby et al. studied that the bulk vacancies from 5.8%, 2.3%, 2.0%, and 0.5%, the 2% Vo supports the charge separation kinetic for better PEC performance, which is analyzed by femtosecond (Corby et al., 2020). Fig. 3.4 shows the respective Vo concentration between VB and CB and respective electron conduction toward the electrode and hole injection. The higher concentration of Vo induced the trapping sites and pushed the fast recombination charge recombination and cannot detect the electron kinetic. High concentration (2.3%) Vo also slows down the electron transport due to hole trapping in bulk and cannot reach for charge separation. The lower Vo again, the charge separation is limited due to fast bimolecular recombination. An appropriate amount of Vo balanced the highest degree of charge separation and showed facile electron kinetic results better PEC performance. Hence, bulk Vo has a significant impact on the trap mediation kinetic in the WO3 photoelectrodes. The partial and precise layer by layer Vo formation on WO3 electrodes generates the heterojunction between Vo reach and the poor part of the electrode due to energy level difference generation. Zhang et al. introduced this concept and achieved higher PEC performance in such heterojunction compare to bare WO3 electrodes due to forming built-in electronic fields, which reduces the charge recombination (Zhang, Ning, et al., 2018).

3.3.2.1 ZnO ZnO is also an n-type semiconductor with a wide bandgap of about B3.3 eV (Liu et al., 2013). It is most abundant, environmentally friendly, and pertaining to

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Figure 3.4 The role of oxygen vacancies in hole electron separation and electron transport toward FTO in the WO3 bulk of the electrode. Source: From Corby, S., Franca`s, L., Kafizas, A., & Durrant, J. R. (2020). Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO3 for water oxidation. Chemical Science, 11(11), 2907 2914. https://doi.org/10.1039/c9sc06325k.

low cost, including economic synthesis, which emits the use as photoanode in various photo harvesting devices. Owing to the large bandgap, it can absorb UV light, but its right VB edges make it important for PECWS with a theoretical limit of 4% solar spectrum harvesting (Yang et al., 2009). But the inherent hurdle is difficult to charge separation, which requires high energy for the transfer of an electron from VB to CB and induces recombination issues. The majority of ZnO photoanodes synthesized from the hydrothermal methods induce the formation of unnecessary species along the ZnO crystal lattices and are also responsible for behaving as trap centers and limits the STH efficiency of ZnO (Lu et al., 2012). Thus, the PECWS community developed various strategies to improve the PEC performance of ZnO nanomaterials photoanodes, which is essential in overcoming large bandgap limitations to improving light-harvesting and reducing charge trap sites during synthesis. Like other metal oxides, dopant engineering is the most explored method here, which manipulates the donor and acceptor densities and benefits electric conductivity and charge recombination by modulating BG (Jian et al., 2020; Korir et al., 2021; Maity et al., 2020; Patil et al., 2021). Vo engineering has been explored as a powerful tool to creates the heterojunction structures within the single-phase, with foreign materials doping and positive effects of Vo to mitigate the trap sites (Janotti & Van De Walle, 2005; Korir et al., 2016; Vanheusden et al., 1996). Wang et al. Used Vo to make homogenous p-n junction in ZnO nanoparticles by Vo creation as n-type and metal vacancy for the p-type to achieve better charge separation, which induces the better charge separation during the PECWS as shown in Fig. 3.5A (Wang et al., 2019).

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Figure 3.5 (A) Schematic of CB-VB of homogenous ZnO nanoparticles with p-n junction induced by Vo and metal vacancy. Source: From Wang, S., Huang, C. Y., Pan, L., Chen, Y., Zhang, X., Fazal-e-Aleem, & Zou, J.J. (2019). Controllable fabrication of homogeneous ZnO p-n junction with enhanced charge separation for efficient photocatalysis. Catalysis Today, 335, 151 159. https://doi.org/10.1016/j.cattod.2018.10.059. (B) The sequential hydrothermal method of synthesis of Ov-CoOx/ZnO NRAs. Source: From Long, X., Li, F., Gao, L., Hu, Y., Hu, H., Jin, J., & Ma, J. (2018). Heterojunction and oxygen vacancy modification of ZnO nanorod array photoanode for enhanced photoelectrochemical water splitting. ChemSusChem, 11(23), 4094 4101. https://doi.org/10.1002/cssc.201801828. (C D) I-V and IPCE performance of ZnO NWAs, as-growth to 700 C annealing. (E F) PL measurement and band alignment description of ZnO NWAs. Source: From Baek, M., Kim, D., & Yong, K. (2017). Simple but effective way to enhance photoelectrochemical solar-water splitting performance of zno nanorod arrays: Chargetrapping zn(oh)2 annihilation and oxygen vacancy generation by vacuum annealing. ACS Applied Materials and Interfaces, 9(3), 2317 2325. https://doi.org/10.1021/acsami.6b12555.

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By creating a p n junction, they observed that the emerged differences in the fermi levels improved the electric interface field and pushed the fast flow of electrons toward the CB. At the same time, VB becomes wider, contributing to higher hole mobility, thus achieving pronounced charge operation. Long et al. studied Vo emergence in the heterogeneous p n junction with the foreign CoO3 metal oxide, and ZnO Nano rode array photoanodes (Ov-CoOx/ZnO NRAs) (Long et al., 2018). Fig. 3.5B describes the sequential method for hydrothermal synthesis of ZnO NRA followed by deposition of CoOx to create the heterojunction. Further, combined annealing induces the common Vo and results in Ov-CoOx/ZnO NRAs. Hence, this structure improved the migration and trapping of holes around the Vo and benefited by low BG of CoOx and 76.7% IPCE at 350 nm. Baek et al. correlated Vo knowledge to suppress the trap site annihilation in ZnO nanowire arrays (ZnO NRAs) (Baek et al., 2017). The hydrothermal synthesis of ZnO NRAs evident in the Zn (OH)2 growth and NRA formation known for charge trapping centers (Djuriˇsi´c & Leung, 2006). It is important to control the concertation of such sites for better PEC performance. The PEC performance is shown in Fig. 3.5C described that the poor photocurrent in the as-grown sample was due to significant trapping sites, while annealed samples perform well. Hence, the presence of these sites induces the trapping of photoexcited holes in as-grown materials and cannot transfer to the water oxidation reaction which lowers the PEC results. While annealing, the Vo formation happened along with annihilation of the trap center, and this combined process improves the PEC performance. Here, IPCE results also proved that the 700 C vacuum annealed sample shows the highest harvesting without change the BG of the sample. This suggests that the current enhancement is contributed by the higher charge separation supported by faster hole transport. The hole scavenging method is popular to study the hole injection kinetics in PECWS when the charge separation is the dominant factor for the potential performance. The present case of ZnO NRAs with the Vo induced hole injection performance was also studied by authors with adding external scavenger during the PEC measurement. This can be traced by manipulating the water oxidation process based on the method discussed by M. Zhou et al. (2012) in the equation of calculation of photocurrent density (JPEC), JPEC 5 Jabs 3 Psep 3 Pinj

(3.4)

Based on the above equation, the total current output is corresponding to the rate of light absorption (Jabs), rate of successively separated holes that didn’t face any recombination (Psep), and successively reacted holes with the electrolyte at the electrode surface (Pinj). The hole separation kinetic can be understood by measuring the photocurrent generated in the present and absence of hole scavengers like sodium sulfate. Here, 700 C annealed sample achieved the Pinj over 50% even at low bias and reached nearly 90% over 0.2 VSCE due to the excellent hole transfer property. Photoluminescence spectroscopy is broadly adopted to analyze the annealing and Vo formation process in metal oxides. Fig. 3.5E described the annihilation of Zn (OH)2 trap centers and increased the crystalline ZnO proportion.

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Further, at 700 C annealing, green emission was observed as confirmation of Vo formation in the sample. It is well studied before that this emission in PL is subjected to monovalent or bivalent vacancies, where electron recombines with full Vo or single valent Vo with a hole in the valance band and emits the green emission (Vanheusden et al., 1996; Wu et al., 2001). Hence, the Vo formation improves the hole conduction, and finally the charge separation kinetic also improves, which yields higher PEC performance. Other than hydrothermal and annealing methods, the Vo generation was also reported with atomic layer and chemical vapor deposition. This method has the promise to minimize the trapped centers and better control of Vo. Usually, using this method, Vo doped ZnO is reported as a passivation layer and active components in the metal oxide heterostructure photoanodes (Su et al., 2018; Tam et al., 2006) shows ZnO photoanode’s enhanced performance by Vo engineering with various methods.

3.3.2.2 In2O3 Indium oxide (In2O3) takes huge attention of PECWS scientific society to have a broad absorption spectrum and get more interest to harvest some visible light, including the UV light spectrum (Sun et al., 2013). The first principle DFT studies of R facets of In2O3 evident that the facet band generation aligns near the Fermi energy level and is responsible for recombination, which still requires cutting these facets (Meng et al., 2014). Hence, facet cutting strategies have been popular to tune the In2O3 properties and are limited due to engineering method boundaries. Recently, Vo engineering in porous and crystal faceted In2O3 nanomaterials was also studied and proven to improved PEC conversion with higher Vo (Meng et al., 2020). Meng et al. have integrated Vo engineering with the facet cutting of (001) faceted In2O3 nanowires (NWs) and proved the new way to increase the PCE performance (Meng et al., 2020). Fig. 3.6(A D) shows the morphology of the faceted NW with SEM and HRTEM images, and it shows the clear square geometry of wire with (001) faceted In2O3. Further, Vo was introduced with different concentrations during chemical vapor deposition. During the PCEWS, the appropriate Vobased sample resulted in 3.3 times higher photocurrent compared to the bare sample. The metal oxide PECWS performance can be studied and be modeled by the first principle DFT calculation. The reaction that can be considered for the study is as given below, and respective results the Density of states and free energy of the PECWS with and without Vo are provided in Fig. 3.6(A D). Fig. 3.7 stand for the PECWS reaction happen at the surface of the photoanode, and each step free energy (ΔG) can be calculated. Fig. 3.6D shows the higher free energy for the Vo-based sample, and it belongs to the increased density of state at the VB of the sample due to placed Vo in the facet by increasing the charge separation. The In2O3 was also studied with heterostructures and other methods to tune the Vo, summarized in, including the performance of PECWS. Liu et al. studied the heterostructure of SnO2-x/In2O3-y as a photocatalyst about Vo formation and its

Figure 3.6 (A) High-magnification FE-SEM images of the In2O3 nanowires. (B) Magnified image of single In2O3 nanowire showing square cross-section. (C) HRTEM image of the magnified single nanowire. (D) Free energy vs the reaction coordinate of the oxygen evolution activity for In2O3 (001) facets with and without O-vacancies and corresponding free energy changes, the inset in (E) shows the optimized atomic models for intermediates adsorption on In2O3 with (001) facets and O-vacancies, and schematic illustration of PEC water splitting on the surface of In2O3 nanowire-based photoanode with active (001) facets and O-vacancies. Source: From Meng, M., Yang, L., Wu, X., Gan, Z., Pan, W., Liu, K., Li, C., Qin, N., & Li, J. (2020). Boosted photoelectrochemical performance of In2O3 nanowires via modulating oxygen vacancies on crystal facets. Journal of Alloys and Compounds, 845. https://doi.org/ 10.1016/j.jallcom.2020.156311.

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Figure 3.7 Reaction scheme steps are considered for free energy calculation. Source: From Meng, M., Yang, L., Wu, X., Gan, Z., Pan, W., Liu, K., Li, C., Qin, N., & Li, J. (2020). Boosted photoelectrochemical performance of In2O3 nanowires via modulating oxygen vacancies on crystal facets. Journal of Alloys and Compounds, 845. https://doi.org/ 10.1016/j.jallcom.2020.156311. (A)

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Figure 3.8 O1s X-ray photon spectroscopy spectra of (A) SnO2 NW, (B) SnO2-x/In2O3-y heterostructures formed in furnace annealing, (C) SnO2-x/In2O3-y heterostructures formed in ultrahigh vacuum. Source: From Liu, W. T., Wu, B. H., Lai, Y. T., Tai, N. H., Perng, T. P., & Chen, L. J. (2018). Enhancement of water splitting by controlling the amount of vacancies with varying vacuum level in the synthesis system of SnO2-x/In2O3-y heterostructure as photocatalyst. Nano Energy, 47, 18 25. https://doi.org/10.1016/j.nanoen.2018.02.037.

impact on the material and PEC performance properties (Liu et al., 2018). The traditional vacuum annealing technique was used to introduce the Vo. Fig. 3.8A C shows XPS of O1s orbital for the materials and Vo growth in the sample during annealing of sample in the furnace and under ultra-high vacuum. The blue line on the graph is indicative of metal bind oxygen. While a red line with intermediate energy is assigned to Vo proportion in the sample. At last, the high-energy green line belongs to oxygen adsorption. In this study, the heterostructure with Vo improves the lifetime of hole and electron and shows higher H2 evolution than other samples.

3.3.2.3 SrTiO3 SrTiO3 is a well-known perovskite structure metal oxide nanomaterial with a gap of B3.2 eV and appropriate VB (2.8 eV) for water oxidation and (0.4 eV) CB potential to ensure the high hydrogen evolution, which is found promising for as one of

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overall water splitting photocatalyst (Amano et al., 2016; Crespillo et al., 2018). SrTiO3 is often used in powder form as a photocatalyst and heterojunction catalyst. The structural perovskite polymorphs and the morphological details are well described by Shilpa et al. (Patial et al., 2020). The SrTiO3 perovskite geometry is built by TiO6 and SrO12 counterparts arranged with titanium atoms at the corner of octahedron and Sr arranged in space of eight octahedron groups in the form of SrO12 Fig. 3.9A. Recently, SrTiO3 get huge attention due to its higher stability compared to other metal oxides (Grabowska, 2016; Kanhere & Chen, 2014). Still, wide bandgap, absorption output in selected energy, law active area, and intrinsic defect of backward reactions limit its promise (Han et al., 2017). The doping of SrTiO3 with the cation of the element from alkali, transition, actinide, and lanthanide series (Li, K, Na, Cs, Rb, Mg, Al, In,and Ga) led to significant promise in STH efficiency (Ham et al., 2016; Ma et al., 2018; Yang et al., 2018; Zhou et al., 2011). Al1 was proved an efficient dopant and resulted in an apparent quantum yield (AQY) of 56% (at 365 nm) and 0.4% PECWS efficient (Goto et al., 2018). The Vo engineering was also studied in the heterojunction construction of SrTiO3 with CZS (Cd0.5Zn0.5S) (D. Sun et al., 2018). While studying XPS analysis a strong interaction was observed within SrTiO3, Vo, and CZS. Particularly, from the SrTiO3 (110) plane, sulfur replaced oxygen site into vacancy and binds to neighboring titanium sites forming, strong interaction between the two groups, this cause helps to better charge separation. The effect of Al13 doping and correlation of Vo was recently studied by Zhao et al. where the B300 improvements were observed after the doping which is dependent on the place of Vo and Al13 including the distance of Vo (Zhao et al., 2019). The understanding and plantation knowledge of Vo along with dopant in such metal oxide is very important to achieve the theoretical limit of 2.5% STH evolution performance. The metal doping on Vo can be understood by taking an example of the Al13 studied by Zhao et al. The Vo formation and its dependence and the positions can be explained at the equilibrium of SrTiO3 that are expressed in Fig. 3.10. Step (1) is showing the general process to produce Vo in non-doped SrTiO3 by thermal annealing. Step (2) describes the changes that happen at the Ti sites during the same procedure which is plays to a role in the neutralization of formed Vo by the formation of two Ti31 ions. At the time of Vo formation in step (3), Al31 ions occupy the sites of Ti31 and decrease the Ti13 concentration, and lose sone n-type characteristic. Here, some Ti41 ions also occupy by Al31 and increase the Vo formation. Further, if the sample is exposed to the air, it would become p-type by uptake of oxygen by the formation of holes as noted in step (4). Hence, the Vo formation and metal Vo complexes manipulate the n-p characteristic of the sample which changes the light-harvesting and charge-separation. To better understand how the electronic structure of SrTiO3 is modified by Al31 incorporation, DFT calculations were conducted on SrTiO3 in the absence and presence of Vo and Al31 dopants. The systematic calculation of density of states of pure SrTiO2, Vo doped SrTiO3, Al-doped SrTiO3 with Vo is studied by DFT study including changing the Al dopant position corresponding to Vo position (Fig. 3.9A E). In such type of metal oxides doped with Al, as described in

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Figure 3.9 The density of states of (A) SrTiO3, (B) Vo -(Sr8Ti8O23), (C E) Al-doped SrTiO3 with VO (Sr8Ti6Al2O23). (C D) Al31 is near to Vo and Al31 is near to the VO site while the other is away. For (E), both Al31 are away to VO. (F) Cyclic voltammetry in dark condition. (G) Schematic energy diagrams for SrTiO3 and Al:SrTiO3, Vo on Al31 and Ti31 sites and on electron/hole recombination. Source: From Zhao, Z., Goncalves, R. V., Barman, S. K., Willard, E. J., Byle, E., Perry, R., Wu, Z., Huda, M. N., Moule´, A. J., & Osterloh, F. E. (2019). Electronic structure basis for enhanced overall water splitting photocatalysis with aluminum-doped SrTiO3 in natural sunlight. Energy and Environmental Science, 12(4), 1385 1395. https://doi.org/10.1039/ c9ee00310j.

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Figure 3.10 Vo formation in aluminum-doped SrTiO3. Source: From Zhao, Z., Goncalves, R. V., Barman, S. K., Willard, E. J., Byle, E., Perry, R., Wu, Z., Huda, M. N., Moule´, A. J., & Osterloh, F. E. (2019). Electronic structure basis for enhanced overall water splitting photocatalysis with aluminum-doped SrTiO3 in natural sunlight. Energy and Environmental Science, 12(4), 1385 1395. https://doi.org/10.1039/ c9ee00310j.

Fig. 3.9C, if Vo is just near to both Ti31 sites and it will not induce the change in band alignment. But, once the distance increase of Vo from the Ti31 site, it shows a decrement in the bandgap. Thus, Vo distance in the doped metal oxide lattices and make a huge influence in the doping strategies which plays an important role in performance improvement or decrement. While dark cyclic voltammetry of both materials shows Ti31 to Al31 replacement with reduction peak in bare SrTiO3 and Al-doped SrTiO3 show anodic current shift due to improved hole transport due to proper position of Vo(Fig. 3.9F). Fig. 3.9G shows the PECWS mechanism in this system. Vo generation can be induced by NaBH4 reduction treatment on the SrTiO3 by time thermal input by time and temperature-dependent, which can control the concentration of Vo. Tan et al. studied the Vo effect on core-shell type nanocrystallineamorphous structure using this method and the atomic representation provided in Fig. 3.11A (Tan, Zhao, Zhu, et al., 2014). The bottom part described as crystalline SrTiO3 and top amorphous cell containing the Vo at the surface. While the heating treatment it observed that the lightharvesting properties is improved little, while the hydrogen evolution only improved in mid temperature range of 325 C for 60 minutes. Similar PEC performance observed in the photo-current performance during illumination between bare and Vo introduced sample. Here it is well concluded that the Vo has minimal impact on light-harvesting improvement, but the PEC performance is improved due to better charge separation. The techniques also used like pulse laser deposition of Nb dopant insertion in SrTiO3 to control the Vo formation by controlling oxygen pressure (Cen et al., 2017). The Vo engineering is not much explored except few attempts and identification experimentally is quite difficult in this type of metal oxides. Usually, lattice expansion creation by Vo and alteration of dopant and titanium sight is tough to identify by XRD measurement. The dependency is just not the concentration of Vo, but distance and dopant also take part in material property changes (Tokuda et al., 2011).

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Figure 3.11 (A) Atomic cluster of crystalline core and amorphous shell structure of SrTiO3 with surface Vo. Bare SrTiO3 and reduction treated SrTiO3 at different temperatures. (B) UV-vis spectra and the insets are photographs samples. (C) H2 evolution rate at the initial time. (D) PEC current performance of samples. Source: From Tan, H., Zhao, Z., Zhu, W. B., Coker, E. N., Li, B., Zheng, M., Yu, W., Fan, H., & Sun, Z. (2014). Oxygen vacancy enhanced photocatalytic activity of pervoskite SrTiO3. ACS Applied Materials and Interfaces, 6(21), 19184 19190. https://doi.org/ 10.1021/am5051907.

3.4

Scope of improvement in the field

3.4.1 Quality and cost-effective materials However, the metal oxides made a great beneficial effect in the PECWS technology by offering a range of materials, but there are still no efficient new materials in these fields. The major scope is the development of new materials, which could promise to achieve the theoretical limits of the oxygen and hydrogen evolution. In particular, the photoabsorption capabilities of photoanodes are still limited, while in the cathode materials they are far away from the reference materials like platinum. The low electronic conductivity of catalytic materials in the field of scope for the improvement in the technology. Further, the metal oxides engineering methods are solely dependent on sophisticated methods, high-temperature treatments, and inert atmosphere facilities for synthesis and creating morphology as well as oxygen

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defects. Thus, the cost of materials has increased a lot, which increases the cost of the hydrogen economy. So, one of the parts of improvement in the technology is the implementation or development of the new cost-effective synthetic processes and the post-treatment methods. This can lead to the availability of cheaper materials. In short, there is wide scope for new cost-effective materials with significant qualities.

3.4.1.1 Stability of metal oxides Any technology can be regarded as efficient if it lasts for longer. In the PECWS technology, the reports are often without demonstration of a lifetime of devices, their stability performances, or the materials stability performance during the application. However, the metal oxides are regarded as the most stable form of the metals, but during the PECWS application, it participates in the charge transfer processes with the liquid media. Further, the photocatalysts are usually dispersed in the liquid electrolyte medium. The stability in the acidic and basic medium is diverse with various materials. During the photoconversion reactions, there are chances of degradation of metal oxides in the active form. Usually, the static stability profile measurement is in practice with techniques chronoamperometry/chronopotentiometry. These techniques are not integrated with long-term tests and are even not that confident. It is of immense scope to study the metal oxide dissolution or the degradation by active means through analyzing the mass of electrodes before and after the PEC operations. There is a requirement to develop such technologies to measure the actual stability of metal oxides. The stability improvement can be also introduced by making heterostructures or using stabilizing blends, and these parts of engineering still needed to improve.

3.5

Conclusion

This chapter is focused on addressing the general role of metal oxide photoanodes on the PECWS and the importance of the role of Vo in four highlighted water splitting photoanode materials: TiO2, WO3, ZnO, In2O3, and SrTiO3. Vo is a generic part of all these metal oxides, and engineering of these defects tunes the various properties of the oxides. There is a huge variation of mechanisms for the introduction of the vacancies based on the metal oxide structure, crystal phases, nano morphology, and the presence of dopant with the parent metal oxide. As observed in the method variation, it can be tuned easily based on our requirements. Further, the surface vacancies play an important role in the improvement of the PECWS efficiency, whether the bulk vacancies have negligible or diverse effects on the bulk conductivity in some cases. Using the proper knowledge of material design, it can be possible to introduce selective parts with vacancies in the metal oxide nanostructure, as explained in the ln2O3 vacancy faceted electrodes. In the case of metaldoped metal oxides, the distance and location of dopant, parent, and Vo centers plays an important role in charge separation kinetics. In conclusion, knowledge of

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Vo engineering is a powerful tool for the ideal design and production of quality metal oxides for PECWS.

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separation for efficient photocatalysis. Catalysis Today, 335, 151 159. Available from https://doi.org/10.1016/j.cattod.2018.10.059. Wang, Z., & Wang, L. (n.d.). Role of oxygen vacancy in metal oxide based photoelectrochemical water splitting. Wu, H., Wang, Z., Jin, S., Cao, X., Ren, F., Wu, L., Xing, Z., Wang, X., Cai, G., & Jiang, C. (2018). Enhanced photoelectrochemical performance of TiO2 through controlled Ar 1 ion irradiation: A combined experimental and theoretical study. International Journal of Hydrogen Energy, 43(14), 6936 6944. Available from https://doi.org/10.1016/j. ijhydene.2018.02.061. Wu, H., Xu, C., Xu, J., Lu, L., Fan, Z., Chen, X., Song, Y., & Li, D. (2013). Enhanced supercapacitance in anodic TiO2 nanotube films by hydrogen plasma treatment. Nanotechnology, 24(45). Available from https://doi.org/10.1088/0957-4484/24/45/ 455401. Wu, Q., Li, Z., Zhang, X., Huang, W., Ni, M., Cen, K., & Zhang, Y. (2021). Enhanced defect-water hydrogen evolution method for efficient solar utilization: Photo-thermal chemical coupling on oxygen vacancy. Chemical Engineering Journal, 408. Available from https://doi.org/10.1016/j.cej.2020.127248. Wu, X. L., Siu, G. G., Fu, C. L., & Ong, H. C. (2001). Photouminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films. Applied Physics Letters, 78(16), 2285 2287. Available from https://doi.org/10.1063/1.1361288. Xiao, L., Liu, T., Zhang, M., Li, Q., & Yang, J. (2019). Interfacial construction of zerodimensional/one-dimensional g-C3 N4 nanoparticles/TiO2 nanotube arrays with Zscheme heterostructure for improved photoelectrochemical water splitting. ACS Sustainable Chemistry and Engineering, 7(2), 2483 2491. Available from https://doi. org/10.1021/acssuschemeng.8b05392. Xiao, L., Zhu, H., Zhang, M., Yang, X., Li, Q., & Yang, J. (2020). Enhanced photoelectrochemical performance of g-C3N4/TiO2 heterostructure by the cooperation of oxygen vacancy and protonation treatment. Journal of the Electrochemical Society, 167(6). Available from https://doi.org/10.1149/1945-7111/ab84f7. Xiao, M., Luo, B., Wang, Z., Wang, S., & Wang, L. (n.d.). Recent advances of metal-oxide photoanodes: Engineering of charge separation and transportation toward efficient solar water splitting. Yang, D., Zou, X., Sun, Y., Tong, Z., & Jiang, Z. (2018). Fabrication of three-dimensional porous La-doped SrTiO3 microspheres with enhanced visible light catalytic activity for Cr(VI) reduction. Frontiers of Chemical Science and Engineering, 12(3), 440 449. Available from https://doi.org/10.1007/s11705-018-1700-4. Yang, X., Wolcott, A., Wang, G., Sobo, A., Fitzmorris, R. C., Qian, F., Zhang, J. Z., & Li, Y. (2009). Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting. Nano Letters, 9(6), 2331 2336. Available from https://doi.org/10.1021/nl900772q. Yang, Y., Zhang, T., Le, L., Ruan, X., Fang, P., Pan, C., Xiong, R., Shi, J., & Wei, J. (2014). Quick and facile preparation of visible light-driven TiO2 photocatalyst with high absorption and photocatalytic activity. Scientific Reports, 4, 7045. Available from https://doi. org/10.1038/srep07045. Ye, L., Zhou, Y., Zhao, Y., Feng, L., Wen, Z., Zhao, L., & Jiang, Q. (2020). Engineering oxygen vacancy on iron oxides/hollow carbon cloth electrode toward stable lithium-ion batteries. Chemical Engineering Journal, 388. Available from https://doi.org/10.1016/j. cej.2020.124229. Zhang, J., Chang, X., Li, C., Li, A., Liu, S., Wang, T., & Gong, J. (2018). WO3 photoanodes with controllable bulk and surface oxygen vacancies for photoelectrochemical water

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oxidation. Journal of Materials Chemistry A, 6(8), 3350 3354. Available from https:// doi.org/10.1039/c7ta10056f. Zhang, R., Ning, F., Xu, S., Zhou, L., Shao, M., & Wei, M. (2018). Oxygen vacancy engineering of WO3 toward largely enhanced photoelectrochemical water splitting. Electrochimica Acta, 274, 217 223. Available from https://doi.org/10.1016/j. electacta.2018.04.109. Zhao, S., Chen, J., Liu, Y., Jiang, Y., Jiang, C., Yin, Z., Xiao, Y., & Cao, S. (2019). Silver nanoparticles confined in shell-in-shell hollow TiO2 manifesting efficiently photocatalytic activity and stability. Chemical Engineering Journal, 367, 249 259. Available from https://doi.org/10.1016/j.cej.2019.02.123. Zhao, Z., Butburee, T., Lyv, M., Peerakiatkhajohn, P., Wang, S., Wang, L., & Zheng, H. (2016). Etching treatment of vertical WO3 nanoplates as a photoanode for enhanced photoelectrochemical performance. RSC Advances, 6(72), 68204 68210. Available from https://doi.org/10.1039/c6ra11750c. Zhao, Z., Goncalves, R. V., Barman, S. K., Willard, E. J., Byle, E., Perry, R., Wu, Z., Huda, M. N., Moule´, A. J., & Osterloh, F. E. (2019). Electronic structure basis for enhanced overall water splitting photocatalysis with aluminum doped SrTiO3 in natural sunlight. Energy and Environmental Science, 12(4), 1385 1395. Available from https://doi.org/ 10.1039/c9ee00310j. Zheng, G., Wang, J., Liu, H., Murugadoss, V., Zu, G., Che, H., Lai, C., Li, H., Ding, T., Gao, Q., & Guo, Z. (2019). Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting. Nanoscale, 11(41), 18968 18994. Available from https://doi.org/10.1039/c9nr03474a. Zhou, M., Bao, J., Bi, W., Zeng, Y., Zhu, R., Tao, M., & Xie, Y. (2012). Efficient water splitting via a heteroepitaxial BiVO4 photoelectrode decorated with Co-Pi catalysts. ChemSusChem, 5(8), 1420 1425. Available from https://doi.org/10.1002/ cssc.201200287. Zhou, X., Shi, J., & Li, C. (2011). Effect of metal doping on electronic structure and visible light absorption of SrTiO3 and NaTaO3 (Metal 5 Mn, Fe, and Co). Journal of Physical Chemistry C, 115(16), 8305 8311. Available from https://doi.org/10.1021/jp200022x. Zhou, Y., Zhang, L., Lin, L., Wygant, B. R., Liu, Y., Zhu, Y., Zheng, Y., Mullins, C. B., Zhao, Y., Zhang, X., & Yu, G. (2017). Highly efficient photoelectrochemical water splitting from hierarchical WO3/BiVO4 nanoporous sphere arrays. Nano Letters, 17(12), 8012 8017. Available from https://doi.org/10.1021/acs.nanolett.7b04626. Zhu, J., Hu, L., Zhao, P., Lee, L. Y. S., & Wong, K. Y. (2020). Recent advances in electrocatalytic hydrogen evolution using nanoparticles. Chemical Reviews, 120(2), 851 918. Available from https://doi.org/10.1021/acs.chemrev.9b00248. Zhu, Y., Lin, Q., Zhong, Y., Tahini, H. A., Shao, Z., & Wang, H. (2020). Metal oxide-based materials as an emerging family of hydrogen evolution electrocatalysts. Energy and Environmental Science, 13(10), 3361 3392. Available from https://doi.org/10.1039/ d0ee02485f. Zuo, F., Wang, L., & Feng, P. (2014). Self-doped Ti3 1 @TiO2 visible light photocatalyst: Influence of synthetic parameters on the H2 production activity. International Journal of Hydrogen Energy, 39(2), 711 717. Available from https://doi.org/10.1016/j. ijhydene.2013.10.120.

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Swapnajit V. Mulik1, Sushilkumar A. Jadhav2, Pramod S. Patil3 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2School of Nanoscience and Technology, Shivaji University, Kolhapur, Maharashtra, India, 3Department of Physics, Shivaji University, Kolhapur, Maharashtra, India

4.1

Introduction

Owing to the increase in population and industrialization, the energy crisis is one of the major constraints in front of governments and policymakers. Especially, sustainable energy technologies such as solar energy, wind energy, biomass energy, hydro energy are the hot cakes in energy generation. On the other end, energy storage is also playing a dominant role in the energy sector (Wang, Lin, et al., 2016). In connection to energy storage, batteries and capacitors are two important energy storage systems. The batteries have high energy storing capacities but low power outputs, while capacitors have high power outputs with low energy density. Batteries are used where energy to be delivered for a long time, and capacitors are used to deliver energy at very high power in a very short time (Joo et al., 2019). When high energy density, as well as high power density, are required simultaneously, batteries and capacitors are insufficient for this application (Divyashree & Hegde, 2015). This requirement leads to a new opportunity for an intensive investigation for new energy-storing devices known as supercapacitors or electrochemical capacitors or ultracapacitors. Compared to batteries, the energy density of supercapacitors is much lower, but the power density is higher (less than capacitors). Capacitance depends on the characteristics of an electrode material such as surface area, composition, morphology, transport properties, etc as well as electrolyte properties that can be used for electrolyte ion accumulation (Chen et al., 2017). The electrostatic double-layer capacitance or electrochemical pseudocapacitance resulting from reversible reduction and oxidation (redox) reactions define the capacitance of the supercapacitor (Fig. 4.1A and B) (Repp et al., 2018). The life cycle, fast charging
discharging rate (seconds), high power density, no maintenance, and Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00006-3 © 2022 Elsevier Inc. All rights reserved.

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Figure 4.1 Schematic representation of (A) electric double layer, (B) pseudocapacitor, and (C) hybrid supercapacitor. Source: Reproduced with permission from ref. (Chen, X., Paul, R., & Dai, L. (2017). Carbon-based supercapacitors for efficient energy storage. National Science Review, 4(3), 453
489) https://doi.org/10.1093/nsr/nwx009.

increased safety compared with batteries are the merits of supercapacitors (Najib & Erdem, 2019). In particular, the electrode features play a major role in the efficient energy storage capacity of supercapacitors. Among the different materials used for electrodes, transition metal oxides (TMOs) are promising due to their high specific energy densities due to the availability of more redox-active sites (Delbari et al., 2021). However, the theoretical capacitance of these TMOs in bare form is practically hindered by poor electrical conductivity, slow ion diffusion rate, low surface area, improper electrode formation, poor cyclability, etc. (Dhodamani et al., 2019; Tomboc et al., 2020). Therefore, it is a dire need to explore TMO-based hybrid materials for better electrochemical performance in supercapacitor applications. In connection to improved electrochemical properties, the TMOs are to be amalgamated with various materials to mitigate their lagging (Mullani, Tawade, et al., 2020; Wu et al., 2020). In particular, conducting polymers (CP) and metal-organic frameworks (MOFs) are promising candidates to improve the conducting properties of

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host TMO-based materials. Moreover, the nanocomposite of TMOs with MOF or CPs would result in higher surface area as well as more porous structures for ease infiltration of the electrodes, full contact between the active materials and the electrolyte, the faster ion transportation in the electrolytes leads to rates of high charging-discharging rate (Han & Dai, 2019). In connection to composites, the combinations of CP with TMOs usually uplift the conducting properties and charge carrier, etc. (Mohd Abdah et al., 2020a). Basically, CPs are organic polymers with conjugated double bonds and hence are a promising material in supercapacitor applications. In addition, the other overriding features such as fast reversible electrochemical ability, low cost, convenient manufacture, excellent flexibility, lightweight, and durability are also considered further for uplifting the performance of the energy storage devices (Simotwo et al., 2016). Among the different CPs, polyaniline (PANI), polypyrrole (PPY), poly 3,4-ethylene dioxythiophene (PEDOT) are commonly used, and hence these CPs are usually amalgamated with TMOs for supercapacitor applications (Movassagh-Alanagh et al., 2019). Similar to CPs, the MOF is also a supportive material to TMOs in relation to the improvement of energy storage performance. MOFs have highly symmetrical as well as well-ordered structures, accommodates the different metal sites, a high surface area, and highly porous materials (Dume´e et al., 2013). Hence, these materials are also useful for better intercalation/de-intercalation of ions in electrolytes for tackling the shortcomings of bare TMOs as electrode material in electrochemical energy storage devices (Xiong et al., 2020). MOFs, due to their tunable properties, like pore size, and desired specific structure, textural characteristics also 1-D, 2-D, and 3-D dimensional structures can meet the needs of desired applications (Hwang et al., 2020). So integrating TMOs with MOFs can provide the best solutions to overcome the lagging of separate components. In addition, well-controlled porous architectures, tunable pore volumes, more surface areas are to be obtained for TMOs through MOF-derived templates (Tang et al., 2015). In the preparation of TMOs derived from the MOF template, the respective MOFs are heated at high temperatures directly in the air for decomposing into the corresponding TMOs (Vilian et al., 2018). During this protocol, certain parameters like annealing temperature, time, and other factors need to be optimized. Considering the advantages of TMO-based composites, the present book chapter focuses on the present state of the art of TMOs nanocomposite with MOFs or CPs for supercapacitor applications. Initially, basic information about supercapacitor and their types is highlighted. Thereafter, the importance, advantages, and constraints of the proper solutions of the different supercapacitors are also discussed. Additionally, the significance of two main materials, namely TMOs-CP composites and MOF-derived materials or TMOs-MOF composites for supercapacitor application are highlighted along with scope in the future.

4.2

Energy storage device evolution

Energy production heavily relies on fossil fuels and it plays a vital role in the world economy and has an adverse effect on environmental ecology. So, along with

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energy production, energy storage is also crucial. In that sense, efforts are made to develop energy storage systems. Consumer requirements and industrial demands are the key points for the development of energy storage devices. Electrical storage technologies such as batteries and capacitors need to be developed according to the demands and need. The developments in energy storage devices are illustrated in Fig. 4.2 (Zhao et al., 2011).

4.2.1 Supercapacitor evolution In 1879, Helmholtz discovered the double layer capacitance, but its practical applications in energy storage have been studied recently (Huang et al., 2019). In 1957, Bcker first devised the use of smaller capacitors having good energy density in energy storage devices. Later in 1960, Standard oil company Sohio first filed a patent for capacitors made from high surface area carbon material following a double layer mechanism. This patent technology was further flourished by using it in the engine starting system of vehicles in 1979 by Nippon Electric Company (NEC) (Kim et al., 2015). Outstanding applications of supercapacitors experienced industrialization on a large scale, leading to the race among countries to have better technology. This race leads to more and more investment in research and commercialization of supercapacitor technology, which started early in the United States, Japan, Russia, Switzerland, South Korea, France, and other European and American countries as shown in Fig. 4.3. Maxwell of the United States, Japan’s NEC, Panasonic, Tokin, and Russian Econd are the companies with advanced research, which occupy most of the global market [Lucintel.Net.].

Figure 4.2 Evolution of energy storage devices with years [Pinterest.Net.], (Zhao et al., 2011).

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Figure 4.3 Evolution of the supercapacitors in different countries. 9 8 7 6 5 4 3 2 1 0

2014

2015

2016

2017

2018

2019

2020

2021

2022

2023

2024

2025

Figure 4.4 Global supercapacitor market from 2014 to 2025 with compound annual growth rate growth rate.

4.3

Market scenario

4.3.1 Market size According to the IDTechEX report, by 2025, the expected global supercapacitor market is 8.3 US$ billion. As the market for supercapacitors is projected to grow at a 30 % compound annual growth rate (CAGR) through 2025 as shown in Fig. 4.4. It is analyzed that, by 2025, the highest income-generating segments would be electronic and automotive for supercapacitors. Therefore, the blossoming supercapacitor market has opened emerging growth opportunities for investors in the future. As per TechSci Research, the supercapacitor in India is not fruitful yet but is expected to growing, with a 16 % CAGR value during 2017
22 because of an increase in

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demand for consumer electronics. It is assumed that the energy storage market will be at its peak in the future. Along with these considerations, reducing the dependency of people on non-renewable energy sources is the top priority, due to the hike in fuel prices in India. In addition, supercapacitor technology has not yet been commercialized fully in developing countries like India, Brazil, etc., as supercapacitors are not commonly in use. A very few companies and research laboratories are dedicated to the supercapacitor. However, considering the importance of the growing concerns of environmental issues such as global warming, air pollution, etc., the huge Indian market will be a supercapacitor hub and will be a major supplier of supercapacitors globally for the various sectors [businesswire.Net.].

4.3.2 Companies with supercapacitor production 1. Cellergy: Cellergy is among the top-class leader which produces and markets low voltage, high current,end-users and high capacity designs of supercapacitors and ultracapacitors to the industrial, consumer, mobile, and electronic markets Table 4.1 shows the estimated financing and revenues of top companies having supercapacitor productions. Cellergy is mainly devoted to carbon-based materials for supercapacitor, having operating voltage 5.5 WVDC Max, Low electrochemical series resistance, operating life is 10 years (5,00,000 charge-discharge cycles), operating temperature ranges from 240 C to 165 C, and is with compatible size. 2. Loxus: Loxus design’s lightweight and durable double layer ultracapacitors with the highest performance have the lowest resistance possible. Loxus ultracapacitors are used to drastically increase the system’s peak power, which is generally combined with other high energy sources (batteries), to prolong the life span. These ultracapacitors can also act as a stand-alone energy source and also have no harmful effects on the environment. Loxus ultracapacitors are also said to be the epitome of new green technology in the developing world [Loxus.Net.]. 3. Maxwell Technologies: Maxwell’s current commercial ultracapacitors, such as durablue, have a much lower energy density of 8
10 Wh kg21 (around 5% that of lithium-ion batteries), yet a much higher power density of 12
14 kW kg21 (around 45 3 that of lithium-ion). In the mid-2018 San Diego Business Journal, Maxwell reported having already sold 6.1 million ultracapacitors to automakers [Maxwell.Net.]. 4. Nanoramic Laboratories: Nanoramic Laboratories is the exclusive designer, manufacturer, and licenser of Neocarbonix electrodes, Fastcap. Neocarbonix utilizes pure carbon-based (EDLCs) electrodes, that is, without any polymer binders for ultracapacitors. Neocarbonix uses their registered low processing cost 3-D nanoscopic carbon binding structure. Such ultracapacitors are stable up to 85 C with ACN electrolytes as well as the resulting product has greater power, energy density, and good performance in an extreme environment [Nanoramic.Lab.Net.]. 5. Panasonic: Panasonic is involved in the manufacture of EDLCs (wound type) for heavy power supply applications. It contains a series of EDLCs with outstanding parameters suitable for diverse applications in different sectors. Features include maximum operating voltage 2.3
2.7 V, Capacitance 2.5
100 F, internal resistance 0.01
0.3 Ω, category range 240 C to 70 C [Panasonic.Net.]. 6. Paper Battery Company (PBC): PBC is offering ultrathin 1 Farad EDLCs with high energy and power density. This PBC tech manufactures an ultrathin supercapacitor and

Table 4.1 Top global supercapacitor manufacturing companies report [Thomas.Net.]. Sr.No.

Company

Country

Founded

Estimated financing

Revenue

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Cellergy Loxus Maxwell Technologies Murata Manufacturing Nanoramic Laboratories Nec Tokin Nippon Chemi-Con Panasonic Paper-battery Company Skeleton Technologies Yunasko ZapGo

United states of America United states of America United states of America Japan United states of America Japan Japan Japan United states of America Estonia United Kingdom United Kingdom

2002 2007 1965 1944 2008 1938 1931
2008 2009 2010 2013

NA $160.1 million NA NA $ 9 million NA NA NA $ 5.7 million $53.8 million NA $18.2 million

NA NA $130 million NA NA $24.0 Billion $1.02 Billion $71.8 Billion NA NA NA NA

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aims for the replacement of lithium batteries. The company targets various sectors like consumer electronics, wearable, and wireless sensor markets and also serves transportation. The company targets automotive, industrial, and renewable energy markets [Thomas. Net.]. 7. Yunasko: Yunasko develops EDLCs and (Lithium-ion capacitors) LICs with prismatic designs. This special prismatic encasement is made up of multi-layered aluminum-coated foil. The module handling voltage ranges from 2.7 V (single-cell) up to 750 V (large assemblies of cells), which is based on the number of cells connected in series [Yunasko.Net.]. 8. ZapGo: ZapGo produces supercapacitors and modules using novel nanocarbon materials and ionic electrolytes. ZapGo modules are ultrafast, safe, recyclable charging modules that charge in minutes. 100 times faster-charging technology than the existing electric vehicles can charge, that is in just 35 seconds, which has already been demonstrated on autonomous vehicles by ZapGo [ZapGO.Net.].

4.3.3 Global supercapacitor market end-users Developing economies are concerned about non-renewable energy sources. Hence, government policymakers are very devoted to the conversion and storage of energy efficiently. The world is very much concerned about having efficient as well as compatible energy storage devices for end-user segments. End users broadly include electric vehicles, consumer electronics, and power utilities as shown in Fig. 4.5. Heavy power supply needs can be addressed by supercapacitor vendors. Technavio’s analysts categorize the global supercapacitor market into five major segments by end-user. End users are the Consumer Electronics Sector, Automotive Sector, Industrial Sector, Energy Sector and Others [Businesswire.Net.]. 1. Automotive Sector: The incomparable stability of supercapacitors in warm as well as cold conditions increased its demand in engine-cranking applications in the industrial, military, and automotive sectors. Whereas, lead-acid batteries are way behind when compared with the versatility of supercapacitors.

Figure 4.5 Global supercapacitor markets according to end users.

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2. Industrial Sector: Looking at the growing interest of industrial sectors in supercapacitors, rapid growth is to be expected in this segment in the forecast period. The above results are due to the incomparable benefits of supercapacitors. Initially, the high investment requirement of supercapacitors is expected to be compensated by the productivity, consistency, and durability of these devices. The peak power cycle requirement of industries during the manufacturing process can be served by a supercapacitor by acting as a supplement for existing industrial equipment. 3. Consumer Electronics Sector: The global supercapacitor market has been experiencing steady growth over the last couple of years. Technological optimization has created the demand for portable, intelligent, and sustainable devices. Supercapacitors used in the consumer electronics sector have fast-charging capability through a primary power supply. The use of supercapacitors eliminates the hazardous effect of battery disposal on the environment as they do not contain any toxic chemicals. Another reason for using supercapacitors in the consumer electronics sector is the requirement for small and light energy storage solutions that can supply the same level of power.

4.4

Types of supercapacitors

A wide range of materials is used to manufacture supercapacitor electrodes and electrolytes, thereby determining the energy storage mechanisms occurring in the supercapacitor. On the basis of electrodes used, supercapacitors are classified into three types shown in Fig. 4.6. Their properties are also compared with respect to each other, and the electrical performance of each type is described on the basis of their cyclic voltammetry and galvanostatic charge-discharge (Kate et al., 2018).

4.4.1 Electric double layer capacitor Electric double layer capacitor (EDLCs) are one of the supercapacitors, also called ultra-capacitors, where electrical charge is stored through the formation of an electric double layer consisting of ions and electrodes. Fig. 4.1A shows the schematic of the EDLC, which consists of two separated electric layers at the electrodeelectrolyte interface (Chen et al., 2017). Because of its higher power density (larger than 1000 W kg21), these are proposed as the alternative power source for hybrid electric vehicles. The working principle of EDLCs in energy storage is based on the charge-discharge process through end-users separation between electrode and electrolyte. In supercapacitors, the electrode-electrolyte double layers have proved a pronounced impact on energy storage, where the charge is stored through a nonfaradic process i.e. charge is accumulated at the electrode-electrolyte interface (Najib & Erdem, 2019). The opposite charges from the electrolyte are layered at the electrode-electrolyte interface with separation of atomic distance. This mechanism makes it more superior for energy storage, which can be proved for all capacitors through an elementary equation given as follows: C 5 ðεo 3 εr 3 AÞ=d

(4.1)

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Figure 4.6 Classification of supercapacitors depending on their energy storage mechanisms.

where A is the surface area of the electrode; εo permittivity of free space; εr is the relative permittivity of the dielectric constant; and d is the thickness of the double layer with surface area A (Raza et al., 2018). As per Eq. (4.1), the capacitance of the materials mainly depends on the increase in the dielectric constant as well as the surface area of the material and also a decrease in the interplanar thickness between the electric double layers (Zhang & Zhao, 2009a). The EDLCs usually utilize carbonaceous materials like activated carbon, graphene, carbon nanotubes, carbon foams or aerogels, etc., due to their highly tunable surface area, excellent electrical conductivity, outstanding chemical stability, and incomparable mechanical behaviors, etc. Generally, charging and discharging cycles in EDLCs are highly reversible due to their non-faradaic electrical mechanism, which results in an extremely stable galvanostatic charge-discharge cycling ability of up to 106 cycles or more (Dubey & Guruviah, 2019). However, the selection of electrode materials having extraordinary features such as reasonable cost, higher power density, higher surface area, higher dielectric constant, higher electrical conductivity, etc. is the most important step for getting efficient

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performance in energy storage systems (Mullani, Dhodamani, et al., 2020; Scibioh & Viswanathan, 2020). In addition, energy density with high internal resistance of the materials is the other constraint to be resolved for better performance in supercapacitor applications.

4.4.2 Pseudocapacitor A pseudocapacitor is one of the supercapacitors, which creates a capacitance together with an electric double-layer (EDL) undergoing the Faradic process. It stores electrical energy faradically by fast electron charge transfer between electrode-electrolyte and hence it resemblances with battery behavior (Wang et al., 2017). In pseudocapacitor, there is charge transfer across a double-layer resulting in the capacitance, which has a magnitude in accordance to the relationship between the amount of charge accepted and varying potential. When compared with EDLCs, pseudocapacitors have a better energy density, but lag in power density, life cycle, surface area as well as stability (Jiang & Liu, 2019). In addition, the energy density of the pseudocapacitors is to be enriched at the battery level through the different strategies conducted such as compositing, element doping, and intentional creation of oxygen vacancies (An et al., 2019). Based on the materials used, pseudocapacitors are classified as conducting polymer-based or metal oxides-based supercapacitors (Fig. 4.6).

4.4.2.1 Conducting polymers-based supercapacitors Nowadays, CPs are extensively used in various devices due to their basic structure containing alternate single and double bonds (in conjugation) for better charge transport properties. CPs are also studied well in supercapacitor applications due to their low cost, high energy storage capacity, good electronic conductivity, high intrinsic flexibility, relatively ease synthesis, excellent redox characteristics, and advantageous delocalization of π-electrons over the entire polymer backbone (Deshmukh et al., 2020; Guo & Facchetti, 2020). Moreover, CPs backbone can be doped with a good conducting material to improve the conducting behavior further and maintain charge neutrality during intercalation/de-intercalation. Flexible electronic devices can be prepared by the use of inherent elastic properties of CPs. In the past few years, various CPs such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) have been investigated for supercapacitor applications. The working voltage of CPs is approximately 21.0 to 11.0 V; which makes their feasibility for positive as well as negative electrodes. However, the major problems with CPs are swelling and shrinking during the charging/discharging process, leading to mechanical degradation, which reduces the cycle life of the CPs-based supercapacitors (Abdelhamid & Snook, 2018a). Therefore, CP-based composites with optimized porous microstructure and Physico-chemical properties are focused on efficient supercapacitor devices.

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4.4.2.2 Metal oxides-based supercapacitors TMO are well known for their MOs-based supercapacitor applications. In this connection, the commonly used TMOs are Co3O4, NiO, RuO2, MnO2, etc, due to their high theoretical specific capacitance between 2 and 3.6 k F g21 (Wang et al., 2015). In addition, valence states of metals, surface area, good capacitance, copious reserves, environmental friendliness, easy approachability, tuneable morphologies, excellent reversibility (surface properties), high energy density, and other intriguing properties and morphology of TMOs are the key aspects for better pseudocapacitor performance (Li et al., 2019). For example, Co3O4 nanowires showed a higher specific capacitance of 2815 F g21 at 1 A g21 to that of its other morphologies. This is also an indication of the importance of the morphology of material used for supercapacitor application (Xu et al., 2018a). As pseudocapacitor type materials, the theoretical specific capacitance of metal oxides can be calculated by Eq. (4.2), Csp 5 ðn 3 F Þ=ðM 3 V Þ

(4.2)

where n is the number of electrons transferred in redox reactions, F is Faraday’s constant, M is the molar mass of the metal oxides, V is the operating voltage window. From Eq. (4.2), it is clear that metal oxides with lower molar mass and multivalence metal oxide will have better specific capacitance. For example, RuO2 is multivalence due to which shows good electrical conductivity, is naturally stable and corrosion-resistant. RuO2 has higher theoretical capacitance (1400
2000 F g21) that makes it a highly explored material for supercapacitor applications. The versatility of the RuO2 can be produced using a diverse range of methods, such as sol-gel, chemical bath deposition, and cathodic electrodeposition methods (Majumdar et al., 2019). In addition to capacitance, energy density and power density of the devices also play a vital role, so their analysis is also crucial and hence these can be obtained with the following Eqs. (4.3) and (4.4), E 5 CV 2 =2

(4.3)

P 5 V 2 =4R

(4.4)

where C is the capacitance, V is the nominal voltage and R is the equivalent series resistance. In this category, literature reports reveal that the composites of TMOs with suitable materials like carbonaceous materials and others that is forming hybrid materials are now focused highly on obtaining highly efficient supercapacitor devices (Jeon et al., 2020).

4.4.3 Hybrid supercapacitors The high energy and power density of electrochemical energy storage devices follow different energy-storing principles. Recent studies are oriented to

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meeting good energy and power density outcomes. To meet both requirements in single carbon-based or pseudocapacitor materials is a challenging task (Vlad et al., 2014). Hybrid supercapacitors are combined versions of pseudo and EDLCs as shown in Fig. 4.1C. This idea of hybrid materials was put forth to overcome the energy density-related drawbacks of EDLCs and cycle life as well as the power density issues of pseudocapacitor. In this type of supercapacitor, the composites of MOs or CPs with carbon-based materials like reduced graphene oxide (rGO), carbide-derived carbon, activated carbon, biomass-derived carbon, etc are used commonly. Nowadays, MOF-derived hybrid materials of TMOs and carbon after annealing are under current progress as well (Wang, Li, et al., 2016). The enhanced capacitance values of TMOs composites are the results of high porosity (MOF derived) as well as charge transport properties, which allow more sites to be explored by electrolyte ions without any sluggishness. Usually, the performance of capacitance and operating potential in hybrid supercapacitors are superior in comparison to EDLCs and pseudocapacitors (Muzaffar et al., 2019). Literature reveals that the present research endeavor in supercapacitors is highly concerned with hybrid materials as they have the potential of overcoming the lagging of supercapacitors. For example, (NiCo2O4, ZnCo2O4, CoMn2O4)/rGO composite delivered high specific capacitance as high as 1414.7, 1075.4, and 992.9 F g21 at 1 A g21 respectively and almost retention of (90.2%, 89.3%, and 91.2%) (Yan et al., 2021). Classification of a hybrid supercapacitor is partly based on Fig. 4.6 and is elaborated in the following sections.

4.4.3.1 Asymmetric supercapacitor This type of supercapacitor uses two dissimilar electrodes (with respect to the charge storage mechanism), so the name is an asymmetric supercapacitor. It is generally accepted that an “asymmetric capacitor” describes the case where the two electrodes have two different charge-storage mechanisms: one non-Faradic (EDLCs) and another battery-type Faradaic. To meet both higher energy density and power density outcomes, non-Faradic and Faradic type electrodes are combined in asymmetric supercapacitors (Wang, Song, et al., 2016). The self-discharge problem of supercapacitors can be lowered or tackled by incorporating such asymmetric electrodes. The working voltage is another challenge with supercapacitors, so that can be increased if the negative electrode is activated carbon-based (Nirosha et al., 2020). Cell voltage can also be increased in the case of phosphorus-doped carbon as electro-oxidation is enhanced by the prevention of carbon-based electrodes activated by phosphorus-containing functional groups. This also ultimately increases the energy density of supercapacitors (Govindarasu et al., 2021). Mostly, carbonderived materials serve as the negative electrode and the metal oxide electrode serves as the anode. An increase in energy densities is seen due to metal electrodes having a high intrinsic volumetric capacity and these types of combined versions have a long cycle life, a high energy density compared to symmetric ones (Dai et al., 2017).

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4.4.3.2 Rechargeable battery type supercapacitor The fabrication of such a device includes one battery-type electrode and another capacitive carbon or pseudocapacitive-based electrode as shown in Fig. 4.7. This battery type of electrode is currently in focus due to its high energy densities and power densities (Zuo et al., 2017). This combination enhances the potential window and capacitance (which also depends on the electrolyte used). Energy densities are enhanced due to the intrinsic charge-storage mechanism at the cathode electrode

Figure 4.7 From battery
supercapacitor hybrid devices: recent progress and future prospects. (2017). Advanced science, 4, 1
21. https://doi.org/10.1002/advs.201600539 general storage mechanism of battery type supercapacitors. Source: Reproduced with permission from Zuo, W., Li, R., Zhou, C., Li, Y., Xia, J., & Liu, J. (2017). Battery-supercapacitor hybrid devices: Recent progress and future prospects. Advanced Science, 4(7), 1600539.

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(pseudocapacitive battery-type materials) and porous anode electrode (carbon-based materials) (Muralee Gopi et al., 2020). Supercapacitor device like AC//Li4Ti15O12, where Li4Ti5O12 no structural changes are seen during Li-ion insertion/extraction reversibility, that is during the charge-discharge cycle. Recently, such types of materials have been proved to be promising hybrid materials (Wang, Song, et al., 2016). During charging-discharging in such electrodes, bulk redox reactions occur at the battery type electrode and faster adsorption
desorption of ions takes place at the capacitive type electrode, which ultimately leads to the proper flow of electrons across the external circuit.

4.4.3.3 Composite hybrid supercapacitors The composite hybrid supercapacitor combines both the advantages of EDLCs and pseudocapacitor material-based materials in a single electrode. Such hybrid designs are designed with a definite goal of having synergistic outcomes related to specific capacitance, cycle stability, high conductivity, and higher power density. As seen in EDLCs above, these have a high surface area, excellent electrical conductivity, outstanding chemical stability, and incomparable mechanical behaviors. However, carbon itself suffers from poor energy density when compared to metal oxide materials. To overcome the lagging of bare materials, merging these bare materials is the key principle for composite or hybrid formation for putting the merits of both materials under the same roof. Composite hybrids show uplifted outcomes of both carbon and MO-based materials in terms of specific capacitance, high conductivity, and cycle life (Veerakumar et al., 2020). This is attributed because the carbon provides a better ion transport path (channel), while MO stores charge efficiently via redox reactions, contributing to high specific capacitance and energy density. In addition, the conductivity of the composite can also be tuned by changing or manipulating the pore size of the carbon materials (microporous, mesoporous, macroporous). For instance, NiAl layered double hydroxide/CNT composite material showed a capacitance value of 2447 F g21 at 2 A g21, which is the result of synergistic of both the materials (Luo et al., 2020). Owing to the importance of composites or hybrids, the composites of metal oxide with either conducting polymer or MOF are focused herewith.

4.5

Electrical properties studies of energy storage devices

Following parameters are studied for getting the overall performance of Energy Storage Devices and hence these are discussed as follows: Operating Voltage Self-Discharge Polarity

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Dependency of device capacitance and resistance on operating voltage and temperature Current load and cycle stability Internal resistance Energy density Power density Capacitance

4.5.1 Operating voltage Operating voltages are the internal voltages needed to power systems or subsystems. Energy storage devices like supercapacitors are operated at lower voltages. That’s why safe operations of supercapacitors require that voltage remains within the specified limits. When capacitors are continuously subjected or operated above specified limits of operation then it will definitely lower down its lifetime. The capacitance and internal resistance can also be stabilized by following the rated voltage range. Operating above rated voltage affects the stability of the electrode materials, as well as higher operation voltages, which may lead to reach the breakdown voltages of the electrolytes (Salanne, 2017). The breakdown voltage decomposes electrolyte molecules in the Helmholtz double-layer, for example, aqueous electrolytes like potassium hydroxide will split into K1 and OH
. Further electrolyte molecules cannot separate the electrical charges from each other. In addition, higher voltages than rated voltage lead to the gas formation or short circuit (breakdown voltages). Standard supercapacitors with operational voltage limits are summarized in Table 4.2. Single cells or supercapacitors higher voltage applications are possible by connecting them in series. The difference in the capacitance value and ESR (Electrochemical series resistance) values of supercapacitors can be actively or passively balanced by stabilizing applied voltage as shown in Fig. 4.8. Passive balancing strategy employs resistors in parallel with the cell or supercapacitors. Where active balancing may include electronic voltage above a threshold that varies the current. Active and passive balancing have been employed to uplift the charge
discharge life cycle of the supercapacitor (Operating voltage studies).

4.5.2 Self-discharge Self-discharge is nothing but the drop in the open-circuit voltage of the capacitors when the capacitors are not connected to a charging circuit or an electrical load. Self-discharge and leakage currents are essentially the same thing measured in different ways. The actual cause for the self-discharge is observed due to the higher energy states of supercapacitors undercharged conditions as compared to that of the discharged state and hence the energy state difference thermodynamically forces or drives the current leading to the self-discharge. Ohmic leakage, Faradic reactions and charge redistributions are mainly responsible for self-discharge. In ohmic resistance, discharge is due to the short circuit between positive and negative electrodes.

Table 4.2 Comparsion of the various electrolytes for supercapacitor applications. Electrolyte

Operation window

Ionic conductivity

Cost

Toxicity

Size of the ions

Aqueous Organic

#1 2.5
2.7

H L

L M/H

L M/H

Ionic electrolytes

3
6

VL

VH

L

KOH (aqueous) K1 5 138 nm and OH
5 0.153 nm. Et4N1  7ACN 5 1.30 nm (solvated) (0.67 nm bare cation) BF4  9ACN 5 1.16 nm (solvated) (0.48 nm bare anion) EMI150.76 3 0.43 nm TFSI 5 0.8 3 0.3 nm

H, higher; L, lower; H, medium; VL, very low; VH, very high.

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Figure 4.8 (A) Actively or passively balanced cells, (B) Unbalanced cells.

Further Faradic self-discharge arises due to the presence of redox-active impurities or due to overcharged cells; while, charge redistribution arises due to the improper adsorption
desorption of ions depending on the nature of electrodes porosity (Chen et al., 2014). So as discussed earlier it is clear that along with a choice of electrode material’s morphology its porosity should also be analyzed wisely in concern to the self-discharge issues. Black et al. studied the relationship between pore geometry of electrodes materials and its effect on charge redistribution. According to their studies, it was analyzed the various pores geometries like cone, inverted cone, and cylindrically shaped pores. The largest number of charge accumulations were observed for cone-shaped pores during charging. Cone geometry showed an increase in resistance within the pore and a lower open circuit potential loss rate. In addition to the pore geometry, its diameter also has a significant impact on the selfdischarge (Black & Andreas, 2010). Wei Zhang et al. studied the impact of the pore diameter of MWCNTs, which is considered to be one of the important classes of an electrode material having a wider range of diameters. In the current study, MWCNT with different pore diameters (20, 30 and 50 nm) was employed as the electrode material to foster the understanding of supercapacitors self-discharge behaviors with varying diameters of the pore. It was observed that pore diameter and self-discharges are directly related to each other. The supercapacitor with an Initial voltage of 2.5 V dropped to 0.75, 0.46, and 0.21 V in 24 hours for MWCNTs with pore diameters 20, 30 and 50 nm, respectively, which can be clearly observed in Fig. 4.9. This study clarified the effect of various microstructures of the electrode material particularly on the self-discharge processes (Zhang et al., 2020).

4.5.3 Polarity Polarity refers to the electrical conditions which govern the direction of the current flow with respect to the electrodes. In supercapacitor, device fabrication may be symmetric or asymmetric. In the case of the symmetric supercapacitor, both positrode and negatrode (or simply positive and negative electrodes, respectively)

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Figure 4.9 Open circuit voltage decays of CNT20, CNT30, CNT50 supercapacitors charged to different voltages of (A) 1.5 V, (B) 2.0 V, and (C) 2.5 V. Source: Reproduced with permission from Zhang, W., Yang, W., Zhou, H., Zhang, Z., Zhao, M., Liu, Q., Yang, J., & Lu, X. (2020). Self-discharge of supercapacitors based on carbon nanotubes with different diameters. Electrochimica Acta, 357, 136855. https://doi.org/ 10.1016/j.electacta.2020.136855.

Figure 4.10 A negative bar on the insulating sleeve indicates the cathode terminal of the capacitor.

consist of the same material. Theoretically, in such cases, supercapacitors have no true polarity. So, the polarity claimed at the time of the fabrication should be maintained during charging for the long life of the supercapacitor as shown in Fig. 4.10. Otherwise, random charging may lower its capacity as well as stability. Whereas asymmetric supercapacitors are inherently polar. Electrochemical charge properties of pseudocapacitors and hybrid supercapacitors may not be operated with reverse polarity, impeding their use in AC operation. However, such limitations are not observed in the case of EDLC supercapacitors. On the other hand, in most instances, the terms “anode” and “cathode” are used in place of the negative electrode and the positive electrode. Using terms anode and cathode to describe the supercapacitor polarity can lead to confusion in

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understanding polarity. Components polarity changes depending on whether it is a generator or consumer of current. Particularly, in the case of supercapacitors based on EDLCs, there are no oxidation nor reduction reactions on any of the two electrodes. In electrochemistry, cathode and anode are related to reduction and oxidation reactions, respectively. So its use in supercapacitor may lead to catastrophic failure. Therefore, the concepts of cathode and anode do not apply [Polarity (Maxwell technologies)].

4.5.4 Internal resistance Adsorption
desorption of the electrolyte ions over the porous structure of the electrodes is responsible for the movement of charges across the circuit or the device. Hurdles for the adsorption
desorption of ions ultimately lead to the resistance termed internal resistance. It is generally observed that with an increase in the pore depth resistance of the electrode also gets increased. The resistance can be calculated from the observed voltage drop ΔVd observed during discharging of the capacitor with respect to the starting constant current Idischarge (Wiki.Internal resistance). The internal resistance can be calculated from the formula: Ri 5

ΔV2 Idischarge

4.5.5 Dependency of device capacitance and resistance on operating voltage and temperature Depending on the operating voltage and temperature the capacitance, initial resistance, as well as steady-state resistance of the supercapacitor vary. The capacitance varies linearly with the operating voltages with a notable change in the capacitance values. Conversely, initial resistance, as well as steady-state resistance of supercapacitor are inversely related to operating voltages (Tomiyasu et al., 2017). In addition, device performance is also reliant on temperature. Because as the device temperature changes during operation the internal properties such as capacitance and resistance will vary as well. Device capacitance is seen to be increased with an increase in the operating temperature [Supercapacitor application guidelines (eaton. com)].

4.5.6 Current load and cycle stability Supercapacitors stores energy without undergoing any type of chemical bonds therefore, current loads, including charge-discharge and peak currents are not limited by any kind of reaction constraints. Therefore, the current load and chargedischarge cycle stability of supercapacitors are much higher than for rechargeable

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batteries. In supercapacitors, internal resistance can have a substantially lower effect on current loads than that in the case of batteries. Due to these internal resistances Ri during the charge-discharge cycles or the peak current I and a certain amount of heat may get generated or termed as internal heat loss Ploss. The relationship between internal resistance Ri, peak current I and internal heat loss Ploss can be given as: Ploss 5 RiUI 2 To have enhanced cycle stability the heat generated during charging and discharging must be released and the device should operate at ambient temperature below a specified temperature. When the device is operated above the specified maximum temperature. There may be serious electrode materials degradation issues with the capacitors. The temperature and operating voltage are the key parameters that decide the lifetime of the capacitors. Also, the lifetime of electrode material depends on the combination of electrode porosity, pore size, and electrolyte (Fong et al., 2017) (cycle stability boostcap).

4.5.6.1 Swelling induced degradation Long-term cyclic stability is one of the major issues faced by pseudocapacitors. In contrast to EDLCs, which can retain their initial capacitance even after 100,000 charge-discharge cycles. So, these long-term stability issues are generally observed due to mechanical fatigue accompanied due to volumetric changes due to continuous faradic reactions (Brousse et al., 2007; Zhang & Zhao, 2009). Tianyu Liu et al. focused on the structural instability resulting from repeated volumetric swelling and shrinking during charge/discharge. By deposition of a thin carbonaceous coating over the surface of conductive polymer electrodes, this work offered a simple and universal technique for significantly improving cycling stability. After 10,000 cycles, carbonaceous shell-coated polyaniline and polypyrrole electrodes retained outstanding capacitance retentions of 95% and 85%, respectively (Liu et al., 2014). For this purpose, carbonaceous material coating is one of the widely explored strategies for uplifting the stability of materials, which can be clearly seen in Fig. 4.11.

4.5.6.2 Overoxidation induced degradation In addition to the degradation of the electrode materials, over-oxidation of the pseudocapacitive material is caused by operating outside an appropriate potential window, which limits its long-term applications. For example, operating PANI-based materials above rated operating potential window will lead to its degradation to yield products such as hydroquinone and p-aminophenol (Arsov et al., 1998; Gao et al., 2011). The overoxidation issues faced by the supercapacitors while operating at higher operating voltages can be minimized by availing an asymmetric device configuration. Generally, two different electrode materials have been employed

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Figure 4.11 Cycle stability of (A) PANI and PANI@C composite electrodes and (B) PPy and PPy@C composite electrodes, collected at a scan rate of 100 mV s21 in 1 M H2SO4 electrolyte. Insets show CV curves collected for the bare polymer electrodes (black) and carbonaceous shell-coated polymer electrodes (blue) at a scan rate of 20 mV s21 in 1 M H2SO4. Source: Reproduced with permission from Liu, T., Finn, L., Yu, M., Wang, H., Zhai, T., Lu, X., Tong, Y., & Li, Y. (2014). Polyaniline and polypyrrole pseudocapacitor electrodes with excellent cycling stability. Nano Letters, 14(5), 2522
2527. https://doi.org/10.1021/ nl500255v.

with different stable potential windows. In this case, it is possible for the maximum voltage window of a device to reach the stability limit of the electrolyte (Zhong et al., 2015).

4.5.7 Energy density Energy density is the amount of energy stored by the material per unit volume. The energy density of the system can be expressed in two ways volumetric capacitance and gravimetric capacitance. Where volumetric energy density is expressed as energy per unit volume and gravimetric energy density can be expressed as energy stored per unit mass. Typically, volumetric capacitance and gravimetric capacitance can be expressed as watt-hours per liter (Wh L21) and watt-hours per kilogram (Wh kg21) respectively (https://energyeducation.ca). The higher the energy density of the system or material, the greater is the ability of the system to store energy. Supercapacitors are devices particularly designed for bridging the gap between electrolytic capacitors and batteries (NIPPON CHEMI). For comparison, an aluminum electrolytic capacitor stores typically 0.01 to 0.3 Wh kg21, while a conventional lead-acid battery stores typically 30 to 40 Wh kg21 and modern lithium-ion batteries 100 to 265 Wh kg21. Supercapacitors can store almost one to two magnitude more energy than electrolytic capacitors but only one-tenth of energy is stored when compared to that of the batteries (Ed supercapacitor). Table 4.3 shows the comparison of various energy storage devices. The energy density (W) can be measured with respect to total capacitance (Ctotal) and operating window (V). W 5 1/2 Ctotal. V2 loaded

Table 4.3 Comparison of the supercapacitor performance with electrolytic capacitors and Li-ion batteries. Energy storage performance Parameters

Electrolytic capacitors

Supercapacitors

Li-ion batteries

Double layer capacitor

Pseudcapacitors

Hybrid (Li-ion) 300 2 3300 220 C to 170 C 20k 2 100k


220 C to 160 C 0.5k 2 10k

2.2 2 3.8 V 3 2 14 10 2 15 Long 90% 5
10 y

2.5 2 4.2 V 0.3 2 1.5 100 2 265 Long 90% 3
5 y

Capacitance (BF) Optimum temperature

# 2.7 240 C to 1125 C

2470 240 C to 170 C

Continuous charge-discharge cycles Maximum chargeVolts (V) Specific power (W/kg) Specific energy (Wh/kg) Self-discharge Columbic efficiency Life span

,Unlimited

100k 2 1000k

100 2 1200 220 C to 170 C 100k 2 1000k

4
630 V 100 0.01 2 0.03 Short 99% .20 y

1.2
3.3 V 2 2 10 1.5 2 3.9 Medium 95% 5
10 y

2.2 2 3.3 V 3 2 10 429 Medium 95% 5
10 y

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4.5.8 Power density The rate at which energy is delivered per unit volume of the system is termed power density. Term power density is not commonly used in batteries as like energy density but it is commonly used in the case of capacitors (https://wiki/ Power_density). A system having higher power density is capable of delivering large amounts of energy based on its volume like large batteries. Typically, Supercapacitors specific power is almost 10 to 100 times greater than that of the batteries and can reach peak power up to 15 kW kg21. As in supercapacitors energy is stored without undergoing any chemical reactions or bonding and hence adsorption
desorption takes place very rapidly with higher power density values. This property makes it useful for conversations about energy systems (often for portable applications like transportation). A tiny capacitor has approximately the same power densities compared to large batteries as these are very small having the ability to rapidly release energy as well as recharge quickly. The importance of having a small energy storage device with high energy output can be understood from the following example: Use of energy storage devices like capacitors in lighting camera flash. It can easily fit into the camera having enough power to light up the subject of your photo and recharges quickly in between the photos. So capacitors are chosen over higher energy density batteries (energy education). Ragone plotting clears the position of the various energy storage devices in terms of energy storage and power density (Aravindan et al., 2014); (Fig. 4.12)

Figure 4.12 Ragone plotting of electrochemical energy storage devices. Source: Reproduced with permission from Aravindan, V., Gnanaraj, J., Lee, Y.-S., & Madhavi, S. (2014). Insertion-type electrodes for nonaqueous Li-Ion capacitors. Chemical Reviews, 114(23), 11619
11635. https://doi.org/10.1021/cr5000915.

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4.5.9 Capacitance Capacitance is the ability of a component or circuit to collect and store energy in the form of an electrical charge. This is the actual value for which capacitors are designed. The typical capacitance values of supercapacitors are five to six orders of magnitude higher than electrolytic capacitors (Fleischmann et al., 2020). We have already discussed capacitance under types of supercapacitors.

4.6

Metal oxide-conducting polymer composites for supercapacitor

The composites of metal oxide with CP are supposed to mitigate the aforementioned drawbacks of the individuals (Mohd Abdah et al., 2020b). Many reports reveal that CPs are one of the promising supportive materials for TMOs. Further, doping of such polymers leads to a dramatic increase in the conductivity of the composites. The redox reactions occur throughout the system without any transformation in the individual materials of the composites (Abdelhamid & Snook, 2018b). The combination of redox-active components like TMOs and CPs shows a more synergistic effect on electrochemical behavior. Also, TMOs should act as templates for structuring the morphology of the polymer during composite formation. In addition, TMOs are worthy materials for charge storage applications as well as for mechanical support to the polymer. CPs perform their conceivable tasks like acting as dissolution inhibitors for materials like metal oxide and, should also enhance the overall conductance. The charge storage capability of the CP is also taken into consideration. Most of the composites are made of the most frequently used CPs, such as PEDOT, PANI and PPy. The Csp values obtained using CP-based materials in recent works range from 100 to 600 F g21. The values of these CP-based materials can be further improved by making their binary or ternary composites with other interesting materials. The charge storage mechanisms in such materials involve intercalation-de-intercalation or also reversible electronic or charge transfer processes which may lead to their degradation or instability. The addition of TMOs in a polymer matrix is found to increase their mechanical stability and assist in the charge storage mechanisms. The most frequently used TMOs as seen in the table are NiCo2O4, CeO2, BiVO4, CoFe2O4, V2O5, Co3O4, etc. These materials, due to their conducting properties, enhance the capacitance. The enhanced properties can be explained with a few examples mentioned herewith and also in Table 4.4.

4.6.1 Composite of polyaniline with the representative metal oxides 1. TiO2/PANI Gottam et al. (Gottam & Srinivasan, 2015) synthesized TiO2/PANI by oxidative polymerization, where peroxotitanic acid was used to oxidize aniline. SEM images (not given

Table 4.4 Latest reports from the literature about transition metal oxides-conducting polymers composites for supercapacitor applications. Composite

Synthetic protocol for composite

Properties

Device

References

Materials

Parameters

Electrolyte 1 M H2SO4, Electrode used Stainless steel mesh

Csp-162 F g21 Rc- 52
38,000 Ed-
Pd-


Ramesh et al. (2018)

Electrolyte used PVA/H2SO4 gel, polyethylene terephthalate substrate.

Csp-162 F g21 Rc-52
38,000 Ed-
Pd-


Xu et al. (2018b)

Electrolyte 1 M NaSO4, CFC substrate

Csp-472.5 F g21 Rc-
Ed-25.2 Pd-620.7 Gc 2 147 F g21 Rc -
Ed-32.3 Pd- 581

Song et al. (2019)

Representative composites of PEDOT:PSS with metal oxides TiO2@PEDOT: PSS

PEDOT:PSS/30% V2O5

CoFe2O4/PEDOT: PSS/Carbon fiber cloth rGO/MnFe2O4/ PPy

Facile one-pot method.PSS 1 DI water in 100 mL beaker 1 20 g of PTiA 1 Stirring for 15 min 1 0.5 mL EDOT 1 24 h of stirring 1 blue precipitate filtered and washed with water and acetone 1 drying 1 calcination. Direct injection method.PEDOT:PSS, 11 mg mL21 with 5 vol.% DMSO 1 Mixed with V2O5 separately 1 2 h bath sonication 1 2 h magnetic stirring at R.T 1 concentrated at 45 C 1 Injected into capillary 1 dried at 90 C for 1 h 1 100 C for 1 h 1 fiber pushed out from capillary 1 filtered 1 washings with DI 1 Immersed in EG for 1 h 1 dried at vacuum at 150 C for 1 h. Hydrothermal method followed by spray coating.Cotton fabric cloth 1 hydrothermally deposition of CoFe2O4 1 Spraying the PEDOT:PSS. Oxidative polymerization method.DI 1 Measured amount of rGO/MnFe2O4 1 FeCl3. 6H2O (0.06 mole) 1 Heated to 32 C 1 drop wise addition of Py (0.02 mole) 1 centrifuged 1 washed 1 dried at 80 C.

TiO2 was efficiently incorporated in PEDOT: PSS showing good electrochemical performance. Stable electrode material. Enhanced electrochemical performance. Good flexibility, conductivity and outstanding cyclic stability.

Showed well electrochemical activity, no toxic for wearable applications. Increased conductivity, high rate capability, ideal capacitive behavior.

Electrolyte 1 M H2SO4, gold electrodes.

Ishaq et al. (n.d.)

Co3O4@ PPy/ MWCNT

ZnV2O6/PPy

NiCo2S4@ Ni (OH)2@ PPy

MoO3/ PPy/ PANI

Oxidative polymerization method.In DI 1 0.5 g of PPy
MWCNT 1 7.25 g of CoCl2 1 vigorous stirring at 90 C for 3 h 1 dropwise 25% NH3 (pH 9) 1 12 h of stirring 1 filtered 1 dried in vacuum 1 Calcination 180 C for 12 h. in situ Oxidative polymerization technique. (a) In DI 1 0.321 g ZnV2O6 1 bathsonication 1 (b) 0.3 mL Py monomer in 10 mL water under Ice cold condition 1 Mixing both (a) and (b) solutions under constant stirring 1 APS solution in 20 mL chilled DI 1 precipitate 1 drying at 60 C for 48 h under vacuum. (a) pTSA was dissolved in anhydrous ethanol 1 5 μL pyrrole monomer 1 Ultrasonication. (b) APS 1 ultra pure water piece of NiCo2S4@Ni(OH)2 covered nickel foam was placed on watch glass first 1 (a) 1 (b) 1 left in dark 1 washings 1 drying in vacuum at 60 C for 24 h. In-situ chemical oxidative polymerization MoO3/PPy (100 mg) was suspended in an 80 mL of 0.5 M H2SO4 solution 1 5 mL of aniline 1 at 0 C 6 2 C ice cold condition 1 dropwise addition of 0.5 M H2SO4 solution containing (ammonium persulfate) APS 1 stirring for 12 h at 0 C 6 2 C ice cold condition 1 filtered the precipitate 1 washings with water 1 drying in vacuum at 60 C for 12 h.

Thermally stable, controllable morphology. High surface area for adsorption/desorption of electrolyte ions.

Electrolyte used 6 M KOH, Current collector Ni foam.

Csp- 609 F g21 Rc-97.1
5000 Ed-84.6 Pd-1500

Ramesh et al. (2017)

1D architecture, interconnecting network morphology, higher surface area and active charge density, increased electrochemical behavior, reduces the diffusion resistance of the electrolyte. Superior electronic conductivity of ppy, high pseudocapacity of Ni (OH)2, short ion transport pathway of NiCo2S4.

1 M aqueous KOH, A piece of SS fabric

Csp- 109.2 F g21 Rc- 93
3000 Ed- 34 Pd- 748.7

Halder et al. (2020)

Electrolyte 2 M KOH solution, Electrode NiFoam (Ni-foam).

Csp-9.113 F cm22 Rc-98.87
30,000 Ed- 34.7 Pd-120.13

Liang et al. (2018)

Fast transport of ions and electrons among the electrodes. Setaria viridis-inspired nanostructures for a promising electrode material with both high specific capacitance and good cycling stability

Electrolyte- 0.5 M H2SO4 aqueous solution, graphite paper current collector

Csp-1315 F g21 Rc- 86
20,000 Ed-63 Pd-


Liu et al. (2018)

(Continued)

Table 4.4 (Continued) Composite

Synthetic protocol for composite

rGO/MnO2PANI

One step hydrothermal synthesis.Graphene oxide 1 carbonic acid 1 KMnO4 1 Aniline 1 precipitation 1 washings 1 drying.

MnO2 (7
35 wt. %)- PANI

Chemical oxidative polymerization method. In beaker MnSO4.H2O 1 0.1 M aniline 1 0.2 M H2SO4 1 Ice bath condition under constant stirring 1 KMnO4 added dropwise at 0 C
5 C 1 stirring for 15 h at 25 C 1 collected precipitate 1 washings with D/W 1 oven drying at 80 C. Electrophoretic deposition (EPD) method. rGO, NiCO2O4, PANI 1:10:1 1 sonication in acetone (I2 added) for 2 h 1 deposition on Ni-Foam using potentiostat (DC supply) 1 vacuum drying 1 annealing under N2 flow at 400 C for 2 h. CeO2 dispersed in water 1 sonication for 30 min 1 appropriate amount of PANI 1 sonication for 2 h 1 mixed both solutions under sonication 1 precipitate filtered 1 dried 70 C for 12 h.

rGO/ NiCO2O4/ PANI

CeO2/ (5%
15%) PANI

Properties

Excellent electrochemical performance expanded the electrical conductance of composite, enhanced the interaction range according to the electrolyte and electrode. The nature and strength of interactions vary depending on the loading of MnO2 in the composites. Thermal stability increases and shows ideal capacitive behavior. Fast reaction kinetics, Effective decreases the ionic transport resistance resulting in good conductivity.

Increase in electronic conductivity and 10% PANI showed best results. Excellent redox behavior resulting in higher capacitance value.

Device

References

Materials

Parameters




Csp-592 F g21 Rc-
Ed-66.6 Pd-1800

Wadekar et al. (2020)

Electrolyte 0.5 M aqueous Na2SO4, electrodes polished graphite

Csp-99
242 F g21 Rc- Stable upto 1000 Ed-
Pd-


Roy et al. (2020)

Electrolyte 6 M KOH, substrate Ni-foam

Csp-262 F g21 Rc-78
35,00 Ed- Pd- -

Rashti et al. (2020)

Electrolyte polymer gel (PVA-HCl), Graphite sheet substrate.

Csp-1492 F g21 RcEd-75 Pd-566

Nallappan and Gopalan (n.d.)

BiVO4/ PANI

PANI @Fe3O4 @carbon fibers (CFs)

Fe3O4/ PANI

In -situ polymerization method.Aniline monomer 1 2 M H2SO4 1 BiVO4 1 Double D/W in ice cold condition 1 stirring for 2 h 1 0.8 M Potassium Peroxodisulphate for 1 h drop by drop in an ice bath 1 6 h stirring 1 24 h R.T 1 filtration of precipitate 1 washings 1 Dried. In situ oxidative polymerization method. Monomer of aniline and APS dissolved individually in 0.5 M of HCL 1 Fe3O4 NPs@CFs sub-merged into the monomer solution 1 Stirred for 30 min at 0 C vigorously 1 dropwise APS solution addition 1 vigorously stirred for 3 h 1 Fabricated PANI@ Fe3O4 NPs@CFs 1 washings with DI and 0.5 M HCL 1 dried at 80 C for 6 h. In situ oxidative polymerization.Aniline 1 amino-Fe3O4 microspheres 1 0.010 M HCl solution 1 ultrasonication for 30 min 1 solution transferred to flask 1 0.50 T plane magnet was placed to the side of the vessel 1 0.01 M HCL solution and APS solution was added to the flask 1 polymerization in ice cold condition 1 washings with ethanol 1 magnetic separation process.

Low equivalent series resistance (ESR), enhanced the active electrode—electrolyte interfaces, Less diffusion effect (High accessible surface area for electrolyte ions). Simple diffusion of electrolyte ions leading to faster intercalation/ deintercalation of ions, high active surface area, Energy stored can be accessed below 45 Hz.

Fe3O4 acts as strain buffer source, and Polyaniline as the electrochemically active part. Good charge storing capacity, PANI with distinct morphologies can be produced.

Electrolyte aqueous 1 M KOH, Nickel foil electrode Electrolyte

Csp-701 F g21 Rc-95.4
5000 Ed-
Pd-


Srinivasan et al. (2020)

Electrolyte 1 M H2SO4, stainless steel wire as current collector

Csp-245.5 F g21 Rc-82.44
1000 Ed-78.6 Pd-1047.5

MovassaghAlanaghat al. (2019)

Electrolyte 1.0 M H2SO4, Stainless steel mesh

Csp-620 F g21 Rc-85
2,000 Ed-
Pd-


Ma et al. (n.d.)

PEDOT, PSS-Poly(3,4-ethylene dioxythiophene) polystyrene sulfonate; Csp, specific capacitance; Ed, energy density; Pd, power density; PANI, polyaniline; PPy, polypyyrole; Gc, gravimetric capacitance; PVA, polyvinyl alcohol; DI, distilled water.

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Advances in Metal Oxides and Their Composites for Emerging Applications

here) revealed flake-like morphology which is better for ion transport. Further cyclic voltammetry showed pseudocapacitive behavior of the material. Specific capacity (Csp), coulombic efficiency (CE) and internal resistance (ESR) values represented the performance of the material. Capacitance loss of only 83% after 30,000 cycles at high current load was observed. Wang, Ma, et al. (2016) loaded urchin-like mesoporous TiO2 spheres with PANI by chemical oxidation and graphene oxide was added to a dispersion of this material and reduced with hydrazine; the resulting electrode showed 86.5% initial capacitance retention after the 2000 cycles. 2. PANI/MnO2 Rabbani et al. (2020) electrochemically deposited PANI/MnO2 on Fe substrate. SEM images revealed the granular and coral-like particles deposited on the Fe surface for PANI and PANI/MnO2, respectively, as shown in Fig. 4.13. CV studies divulge good capacitive behavior of the composite. A decrease in the specific capacitance with an increase in the scan rate was observed, which is attributed to the electrolyte and film resistance. He et al. (2016) in his protocol MnO2/polyaniline (PANI) hybrid nanostructures were deposited onto the carbon cloth (CC) in which MnO 2 was first deposited onto CC, and finally, a coating of PANI was applied. Excellent electrochemical behavior is shown by this composite, which is due to the Faradic process and 3D conductive CC backbone.

4.6.2 Composite of polypyyrole with the representative metal oxides 1. PPy/TiO2 Jiang et al. (2015) prepared free-standing TiO2/graphene/PPy composite films by direct mixing and dying strategy as shown in Fig. 4.14. The composite materials material’s 76.5% of initial capacitance retention after 100 continuous charge-discharge cycles was seen and without PPy almost no loss was registered. The authors ascribed this effect due to the poor stability of PPy, which is due to the shrinking and weak backbone and

Figure 4.13 SEM images of (A) bare Fe electrode, (B) deposited PANI from 0.1 M aniline in 0.1 M oxalic acid solution, and (C) PANI/MnO2 from aqueous 0.1 M oxalic solution. Source: Reproduced with permission Rabbani, M., Mollah, M., Susan, M., & Islam, M. (2020). In situ electrodeposition of conducting polymer/metal oxide composites on iron electrode for energy storage applications. Materials Today: Proceedings, 29, 1192
1198. https://doi.org/10.1016/j.matpr.2020.05.430.

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Figure 4.14 The cross-section of TiO2 2 Graphene 2 PPy. The PPy coating was marked by red arrows. Source: Reproduced with permission from Jiang, L., Lu, X., Xie, C., Wan, G., Zhang, H., & Youhong, T. (2015). Flexible, free-standing TiO2
graphene
polypyrrole composite films as electrodes for supercapacitors. The Journal of Physical Chemistry C, 119(8), 3903
3910. https://doi.org/10.1021/jp511022z.

structure of PPy. The capacity fading effect of the CP appears to be highly unusual. The CV studies depicted the pseudocapacitive behavior of materials as shown in Fig. 4.15. The hike in the capacitance values is seen in the addition of pseudocapacitive material and the highest peak values are seen at 14.6% of TiO2. 2. MnO2/PPy Bahloul et al. (2013) hydrothermally synthesized γ-MnO2 particles and they were later coated with PPy. The improved performance of the metal oxide was observed due to the electronic conductivity of CP and increase in surface area from 64 m2 g21 of pristine MnO2 to 125 m2 g21 after coating was observed. 50% initial capacity retention was observed after 500 continuous charge-discharge cycles which also articulates the stability of the composite. Sharma et al. (2008) synthesized MnO2
/PPy composite by embedding MnO2 in PPy by nucleation on the polymer or polymerization of pyrrole on MnO2 particles. The MnO2/PPy showed good stability of almost 90% retention of initial capacitance after 1000 continuous charge-discharge cycles. The PPy was used as a support for growing high surface area material. Inturns PPy contributed composite with improving the chain structure, conductivity, and stability.

4.6.3 Composite of poly 3,4-ethylene dioxythiophene and polythiophene with the representative metal oxides 1. TiO2/PTh Ambade et al. (2013) infiltrated polythiophene into TiO2 by controlled electropolymerization route. The TiO2/PTh composite exhibited excellent electrochemical performance and long cycle stability. The uniquely designed composite had a highly ordered co-axial nanotube structure, which provides a higher surface area and tubular channels that contribute to a shorter ion diffusion path, effective electron transfer path, and high conductivity. Ambade et al. (2017) developed 1D polythiophene (PTh) nanofibers in hollow TiO2

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 4.15 (A) CV curves of G and TiO2 2 G composite electrodes at the scan rate of 5 mV s21; (B) CV curves of G and G 2 PPy at the scan rate of 5 mV s21; (C) comparison of CV curves of four electrodes at the scan rate of 5 mV s21; (D) cycle stability performance of four electrodes at a current of 100 mA g21. Source: Reproduced with permission from Jiang, L., Lu, X., Xie, C., Wan, G., Zhang, H., & Youhong, T. (2015). Flexible, free-standing TiO2
graphene
polypyrrole composite films as electrodes for supercapacitors. The Journal of Physical Chemistry C, 119(8), 3903
3910. https://doi.org/10.1021/jp511022z. nanotube arrays (TNTs) by controlled nucleation and growth during the electropolymerization of the thiophene monomer. With the increase in the conjugation of the polythiophene, an increase in conductivity was observed. 2. PEDOT/MnO2 Su et al. (2013) prepared electrode material with high areal capacitance as well as excellent mechanical robustness via co-electro-deposition strategy. The mechanical property of the MnO2 was uplifted by the PEDOT: PSS, it acts as the polymer binder and it significantly enhances the integrity of the composite. Excellent rate capability, high energy density, high power density, flexible, robust, ultrathin, and high-performance supercapacitor were the advantages of this work over many other reported data. Moussa et al. (2017) deposited graphene/PEDOT/MnO2 onto the sponge pore surface by a simple dipping and drying process, followed by in situ polymerizations of 3,4-ethylene dioxythiophene as shown in Fig. 4.16. The graphene/PEDOT/MnO2 composite showed not only high specific capacitance but a high stretchability up to 400%. The capacitance behavior can be clearly be divulged in CV’s studies, as in Fig. 4.17.

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Figure 4.16 Schematic for the preparation of a GnP/PEDOT/MnO2 electrode. Source: Reproduced with permission Moussa, M., Shi, G., Wu, H., Zhao, Z., Voelcker, N. H., Losic, D., & Ma, J. (2017). Development of flexible supercapacitors using an inexpensive graphene/PEDOT/MnO2 sponge composite. Materials and Design, 1
10. https://doi.org/ DOI:101016/jmatdes201703075.

4.7

Metal oxide-metal-organic frameworks and metalorganic frameworks derived material for supercapacitor

Metal oxide
MOF and MOF-derived materials are the promising composites used in supercapacitor applications as well. Basically, MOFs can be synthesized by using metal ions and suitable organic linkers under different conditions (Baumann et al., 2019). MOFs were first discovered by Yaghi and Li in the late 1990s MetalOrganic Metal-OrganicMetal-Organic (Introduction to Metal
Organic Frameworks, 2012). These materials are also studied in energy storage applications due to their excellent features such as porous structure, high surface area, faster ion diffusion rate due to adequate pore size (Sundriyal et al., 2018). Despite such outstanding properties, the lack of conductivity and low mechanical stability (less stable in

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Advances in Metal Oxides and Their Composites for Emerging Applications

Figure 4.17 Electrochemical performance of sponge electrodes (A) CVs at 20 mV s21 and (B) GCD at 0.5 A g21. Source: Reproduced with permission from Moussa, M., Shi, G., Wu, H., Zhao, Z., Voelcker, N. H., Losic, D., & Ma, J. (2017). Development of flexible supercapacitors using an inexpensive graphene/PEDOT/MnO2 sponge composite. Materials and Design, 1
10. https:// doi.org/DOI:101016/jmatdes201703075.

alkaline conditions) are the main shortcomings of MOFs and hence the use of bare form results in lower performances in supercapacitor studies (Zheng et al., 2020). These constraints of the MOFs can be overcome by hybridizing with another functional moiety; which can give synergistic outcomes for enhancing the performance of supercapacitor studies. Among the various functional materials, TMOs are the best materials for providing higher redox-active sites, good mechanical stability, etc with certain laggings too (Veerakumar et al., 2020). Therefore, the combination of TMOs with MOFs can be the unique class of materials for supercapacitor applications (Lu et al., 2021). The latest reports of TMOs with MOFs and MOF-derived materials are shown in Table 4.3. Zheng et al. (2020) prepared a composite between Co3O4 nanocubes and leaflike Co-MOF (Fig. 4.18) using a 1:1 stoichiometric ratio of the metal precursor with an organic linker at 1:6 stoichiometric ratio of the organic linker with NaOH (Fig. 4.19). Keeping the same stoichiometric ratio of a metal precursor with an organic linker, the different compositions were obtained at the different stoichiometric ratios of the organic linker with NaOH (such as 1:2, 1:4, 1:6). The stability studies revealed that the composites were stable for 15 days in a 3.0 M KOH medium. Easy synthesis with extraordinary properties of the resultant composites is the unique credential of this investigation. In addition, the enhanced energy density and specific capacitance of the electrode materials are due to the redox behavior observed between metal ions present in MOFs and metal oxide during chargingdischarging (Zhu et al., 2018). Zhu et al. (2018) fabricated hierarchically synthesized ZnO@MOF @polyaniline (ZnO@MOF@PANI) core-shell nanorod arrays on the CC by combining

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169

Figure 4.18 (A, B) SEM images, (C) schematic morphology and (D) TEM image of Co3O4@Co-MOF. (E, F) High-resolution transmission electron microscope (HRTEM) images of Co3O4 with SAED pattern shown in inset (F). Source: Reproduced with permission from Zheng, S., Li, Q., Xue, H., Pang, H., & Xu, Q. (2020). A highly alkaline-stable metal oxide@metal
organic framework composite for highperformance electrochemical energy storage. National Science Review, 7(2), 305
314. https://doi.org/10.1093/nsr/nwz137.

Figure 4.19 Schematic of one-pot hydrothermal synthesis of Co3O4@Co-MOF composite. Source: Reproduced with permission from Zheng, S., Li, Q., Xue, H., Pang, H., & Xu, Q. (2020). A highly alkaline-stable metal oxide@metal
organic framework composite for highperformance electrochemical energy storage. National Science Review, 7(2), 305
314. https://doi.org/10.1093/nsr/nwz137.

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electrodeposition and hydrothermal methods. These well-ordered ZnO nanorods were grown on a cloth to act as a staging for the MOF/PANI shell but also a Zn source for the formation of the MOF. Along with these, materials also provide short ion diffusion pathways, rapid ion/electron transfer and high utilization of active materials, etc as extraordinary properties lead to excellent electrochemical performance. Xiong et al. (2020) synthesized a well-aligned porous NiO@NiMOF/NF cylindrical cage-shape structure by using a new strategy of using NiO as a self-sacrificed template and precursor. These unique combinations improved the conductivity of the electrode, which also endowed electron transfer properties for Ni-MOF, enhanced cycle life, and promoted ion transport in the electrochemical energy storage process. Li, Zhou, et al. (2020) successfully prepared a hollow and hierarchical Co
MOF@CoCr2O4 microplate array on nickel foam via a simple one-step in situ conversion strategy (hydrothermal route), in which Co
MOF served both as a self-supporting etching template and as a selfsacrificing ion-exchange template. The excellent electrochemical performance of the Co
MOF@CoCr2O4 composite is ascribed to the following reasons. (1) Increased electroactive sites due to the hierarchically porous architecture of the composite material (2) As the material is directly grown on the conductive Nifoam, it forms better contact between both active material and current collector boosting electron transport. Similarly, TMOs when combined with various MOFs derived from porous materials, showed an increase in the energy density of the devices. Smart carbonization treatments to MOFs give high-quality carbon embedded with metal oxides derived from the metal ions present in the MOF structure. This can be done by initial carbonization of the MOF (linker part) in an inert atmosphere followed by final oxidation of the metal ions in the air/oxygen atmosphere to obtain the metal oxides. Although MOF-based and MOF-derived materials have shown great promise in supercapacitor applications, their large-scale synthesis, optimization of synthetic parameters, and more importantly their stability are the main obstacles. These issues are being solved presently with systematic research on MOFs. The MOF-derived materials inherit the shape and porosity of parent MOFs under the optimized reaction conditions. Zhang et al. (2018) synthesized composite RuO2/porous carbons using ZiF-8 derived materials by thermolysis in a heating furnace at 500 C (5 C min21) under nitrogen atmosphere for 30 minutes, which showed enhanced capacitance. These multidimensional features were attributed to the MOF-derived PCs such as high porosity in the mesoporous range, no structural change with respect to parent MOFs, etc. Along with providing more bonding sites for MOFs, TMOs also provide an electron transfer path during redox reactions. Mostly, MOFs or MOF-derived materials are loaded onto the current collector with the help of binders which reduces the conducting nature of the material. Hence, binder-free deposition gives better contact between the electrode material and the current collector (Yang Li, Park, et al., 2020). In connection to such, protocol, Prasad Ojha et al. (2020) synthe-

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171

Figure 4.20 Schematic of the synthesis of Zn/Co-MOF-derived nanoporous carbons and bimetallic oxides via the “One-for-All” strategy. Source: Reproduced with permission from He, D., Gao, Y., Yao, Y., Wu, L., Zhang, J., Huang, Z.-H., & Wang, M.-X. (2020). Asymmetric supercapacitors based on hierarchically nanoporous carbon and ZnCo2O4 from a single biometallic metal-organic frameworks (Zn/Co-MOF). Frontiers in Chemistry, 8, 719. https://www.frontiersin.org/article/10.3389/ fchem.2020.00719.

sized composites of CuO NWs with CoS2 (ZiF-67 derived) and thereafter these are deposited on Cu-foam; which enhanced the capacitance in the energy storage applications. The use of the sacrificial template is another strategy to overcome the constraints of the bare MOFs during synthesizing bare Metal oxides, bare porous carbon, or their composites. He et al. (2020) by changing annealing conditions under different controlled (nitrogen and air) atmospheres, different compositions such as Zn/Co-MOF NPC, C@ZnCo2O4, and ZnCo2O4 were obtained as shown in Fig. 4.20. The common examples such as MOF-5 (1,4-benzodicarboxylate coordinated to Zinc center), Zeolitic imidazolate frameworks (ZIF-8, ZIF-67), UiO (Zr-terephthalic MOF), or their derived moieties are explored for supercapacitor applications (Kaneti et al., 2017). The substantial performing composites of Metal oxides with MOFs or device MOFs used for supercapacitor applications are listed in Table 4.5.

4.8

Conclusions and future outlooks

To address the energy needs and promote industrial development, a feasible plan for supercapacitor is of prime importance. For the next-generation performance of

Table 4.5 Latest reports from the literature about transition metal oxides-conducting polymers composites for supercapacitor applications. Composite

Synthetic protocol for composite

Properties

Device

References

Materials

parameters

Exceptional alkaline stability, unique structure, and larger surface area, excellent mechanical flexibility and environmental stability, CoMOF can provide an appropriate space for the electrochemical reaction and intercalation/deintercalation of cations during the energy storage process.

Electrolyte 3.0 M KOH.

Csp-1020 F g21 Rc-96.7
5,000 Ed- 21.6 Pd-


Zheng et al. (2020)

Short ion diffusion pathway, rapid ion/electron transfer and high utilization of active materials, excellent electrochemical performance, facilitate ionic conduction and ensure a full ionic access in favor of large specific capacitance and rate performance. NiO endows Ni-MOF with enhanced conductivity (as binder free electrode), promotes the transport of electrons and also has low impedance. The well aligned MOF generated more electrochemical reaction active sites and boosted the energy storage performance.

electrolyte 3.0 M KCl, carbon cloth (CC) substrate.

Csp-340.7 F g-1 Rc-82.5
5,000 Ed-
Pd-


Zhu et al. (2018)

Electrolyte 3 M KOH, Ni-foam substrate.

Csp-144 F g21 Rc-94
3000 Ed- 39.2 Pd- 7000

Xiong et al. (2020)

Representative composites of MOF with metal oxides Co3O4@ Co-ptcda

ZnO@ Co-2-MIM @PANI

NiO@ Ni-BTC

Solvothermal method.ptcda (C24H8O6) 1 cobalt acetate tetrahydrate(Co (CH3COO)2  4H2O) 1 (C24H8O6: Co (CH3COO)2  4H2O 5 1: 1) in water at 100 C for 12 h 1 C24H8O6: NaOH ratio to 1: 6 results in the formation of a composite of Co3O4 nanocubes and Co-MOF. Hydrothermal method.Co (NO3)2  6H2O 1 2-MIM were mixed in a solvent of DMF/H2O 1 immersed the CC covered by ZnO nanorods 1 Transfer to autoclave 1 60 C for 2 h 1 cooling 1 washings 1 drying 1 electrodeposition PANI. Facile hydrothermal method. 1, 3, 5-trimesic acid 1 dissolved in DMF 1 transferred to teflonlined stainless steel autoclave with a piece of NiO/NF 1 120 C for 8 h, 12 h, and 14 h respectively 1 Washings with DMF under sonication 1 dried in vacuum at 60 C overnight.

CoCr2O4@ CoBDC

Hydrothermal method.50 mL K2Cr2O7 aqueous solution 1 Co
MOF/NF was submersed in a Teflon-lined stainless-steel autoclave 1 autoclave worked for 5 h at 150 C 1 cooling at ambient temperature 1 washings with DI and ethanol 1 drying overnight at 60 C.

Electron transport at high rates between the active material and current collector (direct growth of CoCr2O4@Co-BDC on NF). The hollow, hierarchical and porous structural characteristics could provide enough electroactive sites for redox reaction and facilitate ion diffusion rate.

Electrolyte 3 M KOH, Ni-foam substrate.

Csp-596.8 C g-1 Rc-85.1
5,000 Ed- 34.36 Pd- 201.03

Li, Zhou, et al. (2020)

Better anchoring of RuO2 on PCs leads to good electrical conductivity, better ion diffusion leading to better electrochemical performance. The Composite exhibited high specific capacitance and excellent rate capability. The optimum content of RuO2 exhibits excellent capacitive performance and ultrahigh rate capability. CNF film was utilized as both an efficient substrate with high electron transferring capability and a stable backbone of a selfstanding electrode, improved electrical conductivity.

Electrolyte1 mol L21 H2SO4, carbon electrode

Csp-539.6 F g21 Rc-81.5 Ed- 23.38 Pd- 600

Zhang et al. (2018)

Electrolyte3 M KOH, CNF selfstanding electrode

Csp-742.2 F g21 Rc-
Ed-58.43 Pd-1,947

Shin and Shin (2021)

MOF DERIVED METAL OXIDES Representative MOFs derived materials RuO2/Zn-2-MIM derived carbon

PCs dispersed in DI 1 2 h ultrasonication 1 dropwise RuCl3 solution 1 magnetic stirring 1 adjusting pH at 7 of the mixture by using NaOH 1 heated in an oil bath at 120 C for 6 h with vigorous stirring 1 filtrated 1 washed with DI 1 drying at 70 C 1 annealing at 150 C for 6 h in air atmosphere.

NiO/C @ carbon nanofiber (CNF)

Heat treatment method Hydrothermally synthesized NiMOF@CNF 1 400 C (ramping time and temperature not mentioned for heating) for 1 h under N2.

(Continued)

Table 4.5 (Continued) Composite

Synthetic protocol for composite

NiO/C

Controlled annealing Hydrothermally synthesized NiMOF 1 annealed at 450 C (ramping time and temperature not mentioned for heating) for 4 h in air.

C@ ZnCo2O4

Controlled annealing Zn/Co-MOF 1 annealed at 450 C for 1 h at a heating rate of 5 C min21 under an N2 atmosphere 1 heating for 1 h in air by switching off the nitrogen flow. Separation methodMn3O4@C powder 1 8 mL of 5 wt.% GO 1 4 mL DI addition 1 extensive stirring 500 rpm for 24 h 1 sonication 1 h 1 Buchner funnel for vacuum filtration 1 filter paper was placed in the bottom of the funnel before the filtration 1 immersing in ethanol 1 h 1 separation of Mn3O4@C/rGO.

Mn3O4@ C/rGO

α-Fe2O3

Controlled annealing Hydrothermally synthesized FeMOF 1 calcination in air/ nitrogen atmosphere 1 500 C for 2 h.

Properties

NiO hollow spheres unveiled a better supercapacitive performance, enhanced surface area, abundant active sites and a large number of transport channels for electroactive species Rich redox reactions deriving from improved charge transfer between different metal ions, hierarchical porous structures, excellent rate performance, wide working potential window The fast electron transfer between rGO sheets and the Mn3O4@C particles enabled by the layered structure of Mn3O4@C. Good exposure of the metal oxide to the electrolyte, lower series resistance. High surface area and improved electrical conductivity, as a supercapacitor electrode material, When compared to pristine Mn3O4. Higher oxygen vacancies, faster charge storage kinetics. Promote faster charge storage kinetics, the α-Fe2O3 structure to be retained during the Li-ions.

Device

References

Materials

parameters

Electrolyte1 M KOH solution. Ni foam substrate.

Csp-1058 F g21 Rc-93%-5000 Ed-35.75 Pd-780

Reddy et al. (2020)

Electrolyte6 M KOH, Ni-foam substrate.

Csp- 94.4 Fg-1 Rc-87.2
5,000 Ed- 28.6 Pd- 100

He et al. (2020)

Electrolyte 0.5 M Na2SO4, Nifoam electrode.

Csp-328.4 F cm23 RC- 100
5000 Ed- 10.6 mW h cm-3 Pd- 0.17 W cm-3

Wang et al. (2020a)

Electrolyte 1 mol L21 LiOH solution, substrate Nifoam

Csp-250.2 mAh g21 Rc- 86.83
6000 Ed-63 Pd-900

Xiong et al. (2020)

v-Co3O4/ CC

NiCoO @ NiCo
OH

MnNi2O4

NiO/ NiCo2O4

DMF, dimethylformamide.

v-Co3O4/CC 1 NaBH4 ethanol solution 1 Under N2 atmosphere for (1,2,3 or 4 h) 1 washings with etahanol and water 1 drying 60 C vacuum. .Immersing NiCoO@Co-MOF (synthesized via dip coating) in an aqueous solution of Ni (NO3)2  6H2O at.R.T 1 stewed for 3 h 1 final product rinsed with a large amount of DI and ethanol before drying 1 drying at 70 C for 12 h. Controlled anneling Hydrothermally prepared Mn/ Ni-MOFs 1 annealing at 450 C in air 1 Cooling.

Controlled annealing (MOF-74 synthesized in organic solventfree solutions) MOF-74 1 Calcination in air 1 from R.T to 350 C with heating rate of 2 C min21 1 cooled to R.T.

Improved the electron and ions transfer on the interface between Co3O4 and electrolyte by oxygen vacancies.

electrolyte PVA/ LiOH gel, CC substrate.

Csp-414 C g21 RC-73.9
15,000 Ed- 45.3 Pd-


Dai et al. (2019)

Unique hierarchical nanostructure endowed the material with a high surface area, rich active sites, fast electron transport, and efficient electrolyte ion diffusion. Composite can act as a reaction model for other metal oxide@hydroxide hybrid. High specific surface area, rich active sites, unique microstructures, elemental composition and synergism of flower-like MnNi2O4 leads to superior electrochemical performance. A hollow structure capable of stress inducing, surface defects caused by lattice mismatch between NiO and NiCo2O4, appropriate mesoporous pore size 5 nm, lowest intrinsic resistance leading to fastest electron transport.

Electrolyte 2 M KOH, Ni-foam substrate.

Csp-134.5 F g-1 RC- 91.8
10,000 Ed- 57.2 Pd- 17.6 kw k g21

Wang et al. (2020b)

Electrolyte6 M KOH, Ni-foam substrate.

Csp- 2848 F g21 RC- 93.25
5000 Ed-142.8 Pd- 800

Lan et al. (2020)

Electrolyte 6 M KOH, Graphite paper substrate.

Csp- 732.0 C g21 RCEd- 46.9 Pd-425.3

Yu et al. (2018)

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supercapacitors, investigators should be focused on solving the constraints of electrode materials, electrode/electrolyte interface as well as current electrolytes for proper shuttling of ions, which are the bottleneck steps under supercapacitor studies. Also, active materials with stable physico-chemical properties should be obtained through controlled interfacial interactions. The main ultimate objectives of next-generation supercapacitor development should be the following: Synthesizing the hierarchically porous materials with outstanding stability and hollow structures with reduction of the ion diffusion resistance leads to faster charge
discharge capability as well as a larger charge storing capacity, resulting in higher Ed and Pd. Nanoscale engineering elegantly applied electrodes and electrolytes to overcome the formidable challenges as well as environmental pollution in the present state of the art in energy storage devices. Along with bare electrodes or electrolytes, the hybrids between them such as catholyte would gain in terms of performance, durability, safety, cost, etc. Multicomponent electrodes or electrolytes need to be considered for improvement in the characteristics properties such as operating temperature range, selfdischarge rate, cyclic stability, separators, packing, cell performance, etc. Storage devices having high-performance Ed as well as Pd could be achieved with novel modification strategies in materials. Along with the aforementioned horizons, the investigators should also be focused on having an excellent performing supercapacitor with low cost and capable to be applied in wearable devices. In addition, achieving flexibility in the supercapacitor device is also the need of the hour. Construction of the hybrid materials-based supercapacitors is under current focus to have high stability, flexible and having high energy density along with high power output in a single device. In conclusion, this chapter begins by highlighting the evolution of energy storage devices, particularly supercapacitors. The importance of supercapacitorbased materials and their wide applications in the developing world are highlighted on the basis of market economic size, enlist of top companies with supercapacitor productions, global supercapacitor market by end users. Further, all the types of supercapacitors are elucidated in terms of materials used as well as charge storage processes (faradic or non-faradic) followed by them. The working principles (faradic, non-faradic) of all the types of supercapacitors are considered to elaborate on their connections with energy density and power density outputs. Further, the chapter deliberately focused on MO-CP-based composites, MO-MOF, and MOF-derived materials for supercapacitor applications. These materials are elaborated in terms of synthetic protocols, properties that are responsible for their better outcomes.

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Sarita Patil1, Nanasaheb D. Thorat2, Joanna Bauer2 and Syed A.M. Tofail3 1 Department of Physics, Sanjay Ghodawat University, Kolhapur, Maharashtra, India, 2 Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wrocław, Poland, 3Modelling Simulation and Innovative Characterisation (MOSAIC), Department of Physics and Bernal Institute, University of Limerick, Limerick, Ireland

5.1

Introduction

The depletion in fuel resources has raisedconcern for the generation of energy from both sustainable and renewable energy resources. The ever-growing consumption of electric energy by humans is unavoidable due to increase in handhold gadgets and other household electronic devices. These devices have become a part of the life of humans. The increased industrialization has amplified the use of electric energy. So the entire energy scenario points toward the generation and storage of energy. Solar energy is one of the largest renewable energy resources, and a large amount of energy can be produced using solar energy. But its storage for a longer period is quite difficult due to the shortfall of efficient energy storage devices. So the study and development of such energy storage devices is of paramount importance. Among different energy storage devices, batteries and supercapacitors that work with electrochemical principles are important as energy storage devices. Despite the use of batteries and supercapacitors as energy storage devices, batteries have to compromise with the power densities and supercapacitors have to compromise with the energy densities. Although supercapacitors have been used in applications like electric vehicles, light rail, etc., its energy density (B5 Wh kg21) is less than that of the battery (B200 Wh kg21) and fuel cell (B 350 Wh kg21) (Chodankar et al., 2020; Kim et al., 2015). Basically, supercapacitors are of two designs depending on the charge storage mechanisms: Electric double layer charge (EDLC) and pseudocapacitive. These types of supercapacitors exhibit improved electrochemical performance than a traditional capacitor. In EDLC, charges are stored statically; however, in pseudocapacitive electrodes, charges are stored by reversible redox reactions. Researchers across the globe have studied the electrochemical performance of both batteries and supercapacitors. Various studies have revealed that the batteries have poor power densities; however, Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00013-0 © 2022 Elsevier Inc. All rights reserved.

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their energy densities are higher. It is found exactly opposite in the capacitors; capacitors have high power densities while their energy densities are low. So to have a perfect energy storage device, there is a need for an assembly that would demonstrate both good amounts of power and energy densities. In respect of these requirements, new terminology has been defined for supercapacitors that is hybrid supercapacitors. Basically, the word hybrid is used to denote the use of two different electrodes. The two electrodes can be selected in three different ways. Hence, there are three types of hybrid supercapacitors: (1) Asymmetric supercapacitors (ASC) (2) Composite type symmetric supercapacitors (3) battery type supercapacitors. Essentially the terms asymmetric and hybrid supercapacitor is used for the entire assembly for the cell and not for anyone electrode (Brousse et al., 2015). So the cell design of ASC consists of two different electrodes. Electrodes may be both EDLC or both pseudocapacitive or one EDLC and one pseudocapacitive. For example, the cell with a combination of activated carbon as a negative electrode and MnO2 as a positive electrode can serve as an asymmetric supercapacitor. It means both electrodes are capacitive type. So the electrochemical signature of asymmetric supercapacitor capacitor is a rectangular type CV curve. The composite type symmetric supercapacitor consists of the combination of two materials for making an electrode. The materials are so chosen to exhibit an appreciable energy density as well as power density. For example; highly conductive carbonaceous material provides high power density and metal oxides can provide high specific capacitance and therefore high energy density. So, the electrode made up of a composite of these two materials that is carbon and metal oxide will serve as an electrode with optimum power and energy density. In this type of supercapacitor, both electrodes are made up of the same composite material. So it is called a symmetric supercapacitor. However, the battery type supercapacitor is entirely different because of the combination of electrodes. The cell design of battery type supercapacitor comprises one capacitive type electrode and another battery type electrode. The battery type electrode follows charge storage by Faradaic process and the capacitive type electrode follows charge storage by adsorption of ions at the electrode surface. Therefore the electrochemical behavior of both battery type and capacitive type electrodes is different (Chodankar et al., 2020; Sinha et al., n.d.). The electrochemical behavior of hybrid supercapacitor cells is estimated from the values of the energy and power densities. The energy density (E) and power density (P) values for SCs cells are calculated by using the following formula;
 0:5 3 CðF=gÞ 3 V 2 ðVÞ E Wh=kg 5 3:6

(5.1)


 EðWh=kgÞ 3 3600 P W=kg 5 tðsÞ

(5.2)

where t is the discharging time, and the V is the resultant voltage of the SCs cell. It must be noted that the energy and power densities can only be calculated for the full device and not for the single electrode (conventional three-electrode cell).

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The design of hybrid supercapacitors is helpful in improving energy densities. But, the crucial part of these designs is the selection of proper positive and negative electrode materials. All the attempts made to improve the electrochemical performance of supercapacitor are for increasing the operating voltage window which increases the energy density. So, the combination of electrode material should be in a way to increase the potential window (Chodankar et al., 2020).

5.2

Charge storage mechanism

In supercapacitors, the storage of charges takes place at the electrolyteelectrode interface. There are three ways of charge storage in supercapacitors; EDLC, pseudocapacitance and Faradaic charge storage. This section takes the review of the charge storage mechanism in each type of capacitors mentioned earlier.

5.2.1 Non-faradic mechanism A capacitor consists of two electrodes that are separated by a separator and are electrically connected via electrolytes in between. EDLC is an electrochemical capacitor that stores energy by charge separation. The electrolyte breaks down into anion and cations. During the charging cycle, anions move toward the positive electrode, and cations move toward the negative electrode. These electrolyte ions get adsorbed at the electrodeelectrolyte interface (Long et al., 2011). The flow of ions in the electrolyte induces the oppositely directed electrons in an external circuit. During the discharge cycle, the direction of flow of ions in the electrolyte and flow of electrons in the outer circuit changes to opposite directions. The capacitance of the capacitor depends on the electrolyte ion adsorption and this adsorption, in turn, depends on the characteristics of the electrode surface that act as an interface between electrode and electrolyte. The formation of a double layer at the electrodeelectrolyte interface is well explained by the Helmholtz model. The Helmholtz double layer is formed at the interface of electrode and electrolyte as shown in Fig. 5.1. When voltage is applied across the electrodes of the capacitor, the formation of two layers of ions takes place at the electrode. The inner layer of ions is formed at the electrodeelectrolyte interface. This layer is called as inner Helmholtz plane (IHP). The second layer of oppositely charged ions is formed near the inner layer and is formed from diffused ions in the electrolyte. This second layer is called as outer Helmholtz plane (OHP). The IHP and OHP are separated by a layer of electrolyte molecules. Capacitance of EDLC can be calculated by using simple equation of parallel plate capacitor C5

εA d

(5.3)

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Figure 5.1 Charge storage mechanism in electric double layer capacitor (EDLC) through double layer formation. Source: From EDLC-simplified-principle.png. (2013). https://commons.wikimedia.org/wiki/ File:EDLC-simplified-principle.png. 9 April 2013, permissions: CC0 1.0.

where, C, double layer capacitance, ε, permittivity of the dielectic, A, surface area of the electrode and d, distance between the electrode and eleectrolyte ions. To get appreciable capacitance “C ”, the electrode with high active surface area and high conductivity should be used. Usually, carbon-based electrodes like activated carbon (Cougnon et al., 2015; Fan et al., 2011; Ma et al., 2019), carbon nanotubes (Choi et al., 2014; Dubal et al., 2016; Fan et al., 2015), graphene (Nagar et al., 2018; Wu et al., 2013; Zhang, Zhao et al., 2012) etc., have been used as an electrode. The reactions take places in EDLC are purely non-faradic in nature. The cyclic voltammetry (CV) curves are found to be rectangular box-type in shape and galvanostatic charge/discharge (GCD) curve is a triangular pattern as shown in Fig. 5.1B and C respectively. Despite the high capacitance of EDLC than electrolytic capacitors, EDLC exhibits low energy densities. Therefore, the use of EDLC in practical applications is limited.

5.2.2 Redox mechanism Conway et al. (Conway & Gileadi, 1962) first described the pseudocapacitance. In pseudocapacitor, the reactions are faradic in nature, and energy is stored by fast and reversible redox reactions at the surface of an electrode. So, the battery-like redox reactions are observed in pseudocapacitors. However, the electrochemical response of the electrode is capacitive because charge storage occurs due to the charge transfer during redox reactions that is process is faradaic in behavior, but the capacitance arises due to the linear relation existing between change in potential and storage of

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Figure 5.2 (A,B,D,E,G,H) Schematic cyclic voltammograms and (C,F,I) corresponding galvanostatic discharge curves for various kinds of energy-storage materials. A pseudocapacitive material will generally have the electrochemical characteristics of one, or a combination, of the following categories: (B) surface redox materials (e.g., MnO2 in neutral, aqueous media), (D) intercalation-type materials (e.g., lithium insertion in Nb2O5 in organic electrolytes), or (E) intercalation-type materials showing broad but electrochemically reversible redox peaks (e.g., Ti3C2 in acidic, aqueous electrolytes). Electrochemical responses in (GI) correspond to battery-like materials. Source: Reprinted with the permission from Gogotsi, Y., & Penner, R.M. (2018). Energy storage in nanomaterials—Capacitive, pseudocapacitive, or battery-like? ACS Nano, 12(3), 20812083. https://doi.org/10.1021/acsnano.8b01914. Copyright 2018 American Chemical Society.

a charge. In pseudocapacitor, CV curve has a quasi-rectangular shape and the galvanometric (GCD) curve has a quasi triangular shape as seen in Fig. 5.2. Pseudocapacitor stores charge through two different ways: Redox reactions at electrode surface Intercalation type reactions inside the pores of the electrode material.

5.2.2.1 Redox reactions at the surface It is also called intrinsic pseudocapacitance. The redox reactions take place at the electrodeelectrolyte interface. The quasi-rectangular and quasi-triangular shapes of CV and GCD curves Fig. 5.2DF, respectively, show the similarity of the pseudocapacitor behavior with the EDLC. But in pseudocapacitor, charge storage takes place through the faradic process as well as a double-layer mechanism (Gogotsi & Penner, 2018).

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Apart from carbonaceous materials, transition metal oxides like RuO2, MnO2, etc., have been found to show such electrochemical behavior. The fast and reversible redox reactions observed in intrinsic pseudocapacitive material are attributed to the multiple valence states of metal cation in metal oxide (Huang, Li et al., 2015). Owing to fast charge transfer, pseudocapacitor stores high energy at high rates.

5.2.2.2 Intercalation type reactions inside the pores of electrode material For pseudocapacitance, the term intercalation is introduced by Dunn and Simon (Augustyn et al., 2014; Simon, 2014) which means the insertion of electrolyte ions into the electrode material without altering the phase of the electrode material (Dall’Agnese et al., 2014; Feng et al., 2018; Mefford et al., 2014; Mitchell et al., 2019). The materials like TiO2, perovskite oxides, WO3, etc., exhibits such kind of pseudocapacitance. The rate at which charge is stored and restored is quick compared to the redox surface reactions (Augustyn et al., 2013; Brezesinski et al., 2010). Importantly, due to intercalation, the inner active sites of the electrode materials are also utilized for reaction. In this type of pseudocapacitance, the current increases linearly, with the scanning rate. However, peak potential remains constant irrespective of the change of the scanning rate, and the charge storage capacity also remains the same even if the charging time is changed.

5.2.3 Battery type charge storage The electrode material used for battery-type charge storage involves the change in phase of the material. In faradic or battery type reactions, the CV profile consists of welldefined redox peaks as seen in Fig. 5.1K and consist of GCD similar to a battery (Brisse et al., 2018; Brousse et al., 2015). This type of charge storage process is exhibited by oxides, selenides, phosphides, sulfides, etc., of Ni, Cd, Co, and Cu. When the electrode material is battery type then specific capacity is considered instead of the only capacity. The following formula is used to calculate the specific capacity from CV curves:
 Specific capacity Ah=g 5

Ð

iðVÞdV ðA:VÞ mðgÞ 3 ϑðV=sÞ 3 3600 ðsÞ

(5.4)

where the integration of current over voltage window will give the total voltammetric charge (which needs to be divided by 2) in A.V (ampere volts); ϑ is scan rate and m is the mass loading. Similarly, following formula is used to calculate the specific capacity form GCD curves:




Specific capacity Ah=g 5

Ð

iðAÞdtðsÞ 3600 3 m ðgÞ

(5.5)

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where i is the applied current density (A), t is discharge time (s), and m is the mass loading of the active material (g) on electrode. The specific capacity calculated from CV and GCD curves, can be converted to specific capacitance by using following conversion equation
 Specific capacity ðAh=gÞ 3 3600 Specific capacitance F=g 5 ΔVðVÞ

(5.6)

where V is the voltage window of the solitary electrode configuration (V). As we are well aware that, reducing the size of the material to nanometer changes their properties drastically. Especially, the surface area to volume ratio of the material enhances tremendously. Such improvement in the surface area proves beneficial for reducing the ion diffusion length due to fast charge transfer. It has been observed that the material that exhibits battery type of behavior in its bulk form tends to exhibit capacitive behavior in its nanometer size. This is confirmed from the changed electrochemical signatures to quasi- rectangular CV and quasitriangular GCD curves of pseudocapacitors. An important example to accept the changes is LiCoO2 which delivers battery-like behavior in its bulk form and turns to capacitive behavior in nanostructured form. This change is coined as extrinsic pseudocapacitance by Dunn and Simon (Augustyn et al., 2014; Simon, 2014).

5.3

Carbon-based materials as an electrode

Carbon-based nanomaterials, such as activated carbon, carbon aerogels, CNTs, graphene are used in electrode material in EDLC, because of their high electrical conductivity, high cyclic stability, high rate capability, high power densities, and large surface area (Pandolfo & Hollenkamp, 2006). However, carbon materials exhibit low specific capacitance and low energy densities (Alonso et al., 2006; Davies & Yu, 2011; Frackowiak & Be´guin, 2001; Huang et al., 2008; Wang et al., 2013). In pseudocapacitor, fast reversible faradic redox reaction takes place on the surface of electrodes. Metal oxide and conducting polymers are used in pseudocapacitors (Yang et al., 2018). In comparison to carbon materials, conducting polymers show higher specific capacitance of nearly 530 F g21 along with good conductivity; however, their stability is very poor (Snook & Chen, 2008; Snook et al., 2011).

5.4

Metal oxides/metal oxide composites as an electrode in supercapacitors

To remove drawbacks of carbaneous materials and polymers, the transition metal oxides such as RuO2, MnO2, Co3O4, NiO have been introduced in supercapacitor applications (Liu et al., 2014; Wen et al., 2013). Among all metal oxides, RuO2

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exhibits large specific capacitance, better electrical conductivity and fast chargedischarge rate (Hu & Chen, 2004). However, the cost of noble metal limits its use on large scale (Cottineau et al., 2006; Deng et al., 2011). The transition metal oxides having bandgaps ranging from 3 to 4 eV, possesses low intrinsic electronic conductivities and therefore cannot contribute efficiently to the electrochemical performance until combined with other good conductive materials (Anisimov et al., 1990; Memming, 1983; Yuan et al., 2013). So the different attempts made by various researchers to use metal oxide effectively in supercapacitor application are discussed in this section. Since it has cleared from the earlier discussion that the use of dissimilar electrodes is beneficial for improving the potential window and hence the energy density of supercapacitor. So, further discussions are made considering dissimilar electrodes in a device that is asymmetric or hybrid supercapacitors.

5.4.1 Ruthenium oxide Ruthenium oxide (RuO2) is an ideal positive pseudocapacitive electrode material that has been extensively used in ASC (Chang et al., 2002; Shao et al., 2018; Zheng, 1999). The RuO2 shows low internal resistance of 45 mΩ that is it shows high electrical conductivity. Additionally, RuO2 shows high specific capacitance. The theoretical value of specific capacitance for RuO2 is 14002000 F g21. The fast charge transport leads to high specific capacitance. But the high cost of ruthenium and its toxicity restricts its use at a large scale. So it is only used for limited applications like military and defense applications. Nevertheless, researchers have attempted to reduce the content of RuO2 by making its composite with some pseudocapacitive or carbon materials. For example, Gao et al. fabricated a nanosized RuO2/polyaniline/carbon double-shelled hollow spheres composite by an in situ electro-polymerization method (Zhao et al., 2012).

5.4.2 Manganese dioxide The next important material to RuO2 is MnO2. MnO2 is a low-cost, abundantly available and less toxic pseudocapacitive metal oxide that possess a theoretical specific capacitance of 1370 F g21 (Yan et al., 2010). MnO2 has been used as a positive electrode in combination with a carbon negative electrode in an asymmetric supercapacitor (Hong et al., 2002; Khomenko et al., 2006; Ou et al., 2015; Qu et al., 2009). Preferentially MnO2 is used in neutral aqueous electrolytes such as K2SO4, Na2SO4 and Li2SO4 (Qu et al., 2009). The exact combination of electrolyte and electrodes is very important in supercapacitors because ions in the electrolyte should have access to the porous surface of electrode materials. Among K2SO4, Na2SO4 and Li2SO4 neutral aqueous electrolytes, K1 ions have a small ionic radius and highest conductivity therefore they can have easy and fast access to the inner surface of δ-type MnO2 compared to Na1 and Li1 ions (as their atomic radii are greater). So K2SO4 was used as an electrolyte in combination with MnO2 as a positive electrode and carbon as a negative electrode in an asymmetric supercapacitor (Khomenko et al., 2006). MnO2 is effective in ASC because fast and reversible

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redox reactions take place between Mn31 and Mn41 that give rise to a pseudocapacitance. During reversible reactions of Mn41, it gets oxidized to Mn71 and reduced to Mn21. So the performance of MnO2 in an asymmetric supercapacitor is better than a conventional symmetric supercapacitor. Similarly, the porous nanostructure of MnO2 (nanotubes) can deliver an effective contact between electrolyte and electrode material which alternately provides a short ion diffusion path and enhances the transfer of charges (Huang, Zhang et al., 2015). Despite different morphologies like nanoflowers, nanoflakes, nanosheets (Jiang et al., 2012; Shi et al., 2013) obtained for MnO2 by different researchers; the poor conductivity has been a limit for the use of MnO2 as a pseudocapacitive material. So the conductivity of MnO2 can either be increased by depositing it directly on a conductive substrate (Gao et al., 2012; Lang et al., 2011) or by preparing its hybrid with conductive carbon or polymer (Su et al., 2013; Yu et al., 2011). As an example, MnO2 is deposited on nickel foam that consists of 3D interconnected pore which helps in achieving an energy density of 23.2 Wh kg21 (Gao et al., 2012). The composite of CNT with MnO2 has been reported by Guo et al. for supercapacitor applications (Guo et al., 2014). The CNTs act as a bridge between MnO2 spheres. The composite against the CNT as a negative electrode has delivered an energy density of the order of 11.6 Wh kg21. Additionally, graphene/MnO2 has been used to get rid of the conductivity issue of MnO2 (Shao et al., 2013).

5.4.3 Nickel oxide Low-cost NiO exhibits pseudocapacitive behavior with a high theoretical specific capacitance of 2573 F g21 (Hussain et al., 2018; Raghavendra, 2020; Zhang et al., 2016). Ni is found with two redox transition states Ni21 and Ni31 for achieving a high charge transfer rate. Different nanostructures of NiO provide the porous structure with a high surface area that ultimately enhances the electrolyte adsorption and desorption on the electrode surface. NiO has been used effectively in hybrid supercapacitors. Luan et al., have used NiO nanoflake array as the positive electrode and a reduced graphene hydrogel film as the negative electrode in a hybrid supercapacitor (Luan et al., 2013). The concern behind demonstrating a hybrid supercapacitor is to enhance the potential window. With the combination of NiO nanoflake array and graphene hydrogel, the potential window of hybrid supercapacitor could be extended to 1.7 V. In spite of the benefits of NiO in respect of cost and high theoretical specific capacitance, it still shows low specific capacitance due to poor electrical conductivity. Therefore attempts have been made to enhance the electrical conductivity. Khan et al. have reported the synthesis of Fe@NiO nanocomposite by one-pot flash combustion method. The electrochemical studies have shown the specific capacitance of 297.67 F g21 for the optimum incorporation of 5% of Fe into NiO (Khan et al., 2021). Bi-metal transition metal oxides possess numerous oxidation states which help in fast charge transfer during redox reactions leading to an increase in electrical conductivity. So the synthesis of nanocomposites of such bi metal oxides is preferred to overcome shortfalls of individual metal oxides. The synthesis of NiO@Ni(OH)2-α-MoO3 by simple hydrothermal route is reported by

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Manibalan et al. (Manibalan et al., 2021). The two electrode symmetric design is established in this device. The composite shows a high specific capacitance of 172 F g21 at 0.5 A g21, excellent rate capability, and good cycling stability. Also, as a working electrode NiO@Ni(OH)2-αMoO3 showed excellent cycling stability (capacity retention of 98% after 5000 cycles). Carbon strongly interacts with metal oxide and helps in improving the conductivity of the electrode made up of metal oxide-carbon composite. A novel combination of nickel oxide-carbon composite, NiO/C@CNF is proposed by Shin et al. (Shin & Shin, 2021) as a self-standing supercapacitor electrode. The MOF-derived NiO is uniformly distributed on the carbon matrix is used. The NiO/C@CNF composite exhibited specific capacitance of 671.1 F g21. An assembly of supercapacitor made up of asymmetric electrodes that is NiO/C@CNF as a positive electrode and the activated carbon as a negative electrode exhibits an excellent specific energy density of 58.43 Wh kg21 and a power density of 1947 W kg21.

5.4.4 Cobalt tetraoxide Among metal oxides, Co3O4 is effectively introduced in supercapacitve applications due to its 3D ion diffusion pathways. In an aqueous electrolyte, Co3O4 can form an oxyhydroxide phase. During this transition, four electrons are transferred via a twostep reversible redox reaction. In addition to this developing a nanostructure enhances the area available for ion adsorption and desorption. Vidyadharan et al. synthesized ultrathin Co3O4 nanowires and used them as the positive electrode for a hybrid capacitor (Vidyadharan et al., 2014). The Co3O4 nanowire electrode exhibited a capacity of 555 C g21 at a current density of 1 A g21. Like other metal oxides, Co3O4 also suffers electrical conductivity problems. Additionally, Co3O4 suffers a problem of an agglomeration of Co3O4 nanoparticles or volume expansion and contraction of Co3O4 electrodes during charging and discharging cycles. This concern is addressed in two different ways; (1) the use of carbaneous materials in various ways (2) introducing other metal oxides in Co3O4. Zhao et. al have followed a cost-effective method of preparing a 3d interconnected carbon network from bacterial cellulose (BC) that acts as biomass material. This carbon network is further decorated with Co3O4 nanoparticles by the facile soak-adsorption method. So the electrode formed fulfills both requirements of increasing electrical conductivity and reducing the agglomeration of Co3O4 nanoparticles (Zhaoa et al., 2019). Alternately, Wang et. al have reported the preparation of CeO2/Co3O4/rGO (CCGN) nanoparticles on nickel foam by a simple hydrothermal method. These nanoparticles are grown in a form of clusters on a nickel foam. These clusters are so separated that the volume available for expansion during charging cycles is available. The electrode made up of CeO2/Co3O4/rGO nanoparticles exhibits the specific capacitance of 1606.6 F g21 at the current density of 1 A g21. The asymmetric assembly of CCGN/activated carbon shows power density and energy density (8000 W kg21 at 47.6 Wh kg g21) when the voltage window was 1.6 V (Xu, 2020). But the use of nickel foam is not cost-effective; the use of conductive and cost-effective substrate is significant. The electrochemical deposition of Co3O4 on

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conductive and cost-effective stainless steel substrate is reported by Maile et. Al. Pesudocapacitive thin films of Co3O4 on stainless steel substrate showed the specific capacitance of 131 F g21 (Maile et al., 2020).

5.4.5 Other metal oxide/metal oxide composites As discussed in an earlier section, when the size of the material is reduced to nanosize, then its surface to volume ratio increases tremendously. The improved surface area provides more active sites on the surface of the material that can actively participate in redox reactions (Patil & Burungale, 2020). As a result, more ion adsorption and desorption sites can be available on an electrode surface by developing nanostructures/hierarchical nanostructures of electrode material. So developing nanostructured electrode material with improved surface area as well as high porosity plays a vital role in boosting the material’s electrochemical performance. The synthesis of mixed metal oxides with hierarchical nanostructures has been found to be more effective than single metal oxides. For example, Gao et al. have reported the hierarchical CoO@NiO composite coated on flexible activated carbon textiles (ACTs) (Gao, Song, et al., 2015). In this composite, urchin-like CoO acts as a base for the coating of NiO flakes. So both nanostructures can provide high surface area altogether. This hybrid CoO@NiO electrode allows fast charge and ion transport. Additionally, to take care of the electrical conductivity, the ACT is coated with graphene and hence the HSSC assembly is made. So the synergic effect is observed in the form of an extended voltage range (1.6 V) and as a result the substantial energy density of 52.26 Wh kg21. On the other hand, the electrical conductivity has been reported to be improved with the use of stainless steel as a substrate for deposition of NiOCo3O4 composite by a single step electrochemical synthesis for HSSC (Wen et al., 2018). The coating of polypyrrol on stainless steel yarn has been used as the negative electrode which exhibits the pseudocapacitive behavior. So the HSSC assembly comprising of nanostructures of Ni/Co composite and Ppy coated stainless steel yarn had achieved capacitance of 14.69 F cm23 with energy and power outputs of 3.83 mWh cm23 and 18.75 mW cm23, respectively. The retention is 90% of the original capacitance after 6000 cycles.

5.4.6 Performance of negative electrode Earlier sections elaborate more about the assembly of dissimilar electrodes; however, the performance of both electrodes should be considered because the total capacitance of the device depends on the capacitance of individual positive and negative electrodes. Various attempts have been already made for the development of positive electrodes and capacitance has been found to be raised. But the issue of the lower capacitance delivered by the negative electrode which is mostly an activated carbon or carbaneous material remains unaddressed. Let us consider CT as the total capacitance of the device which can be calculated from the capacitance of positive electrode (Cp) and capacitance of negative electrode (Cn). So the value of CT is low due to lower Cn. To overcome the limitations of the negative electrodes,

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carbaneous materials can either be replaced by pseudocapacitance material or it can be combined with pseudocapacitive material and then used as a negative electrode (Chodankar et al., 2016; Gund et al., 2015; Hou et al., 2019). One such study has been performed by Yang et al. (2014). In this study, MnO2 is used as a positive electrode and Fe2O3 is used as a negative electrode. The assembly of these electrodes with the polymer gel-type electrolyte leads to the enhancement of the potential window to 1.6 V. The cell has exhibited the specific capacitance of 91.3 F g21 at a current density of 2 mA cm22. In spite of the good electrochemical behavior, Fe2O3 being metal oxide and semiconducting in nature, show poor conductivity (Lu et al., 2012). Therefore it is necessary to improve its conductivity. In this context, a thin coating of carbon is placed on Fe2O3 nanorods so as to form a carbon shell (Fe2O3@C). So the carbon-coated Fe2O3 were used as a negative electrode in combination with the MnO2 nanowires as a positive electrode. The carbon coating helps to improve the conductivity of Fe2O3. In this cell, the positive electrode is MnO2 was coated on CuO NWs (Yu et al., 2015). The cell assembly modified with Fe2O3 and MnO2 in a wire-type form had delivered 2.46 F cm23 of capacitance along with excellent rate-performance (95.4%) and high energy density of 0.85 mWh cm23. Different carbon-based materials like carbon nanotubes, graphene, graphene oxide, etc., can also be used to improve the conductivity. The use of carbon nanotubes along with positive electrode (MnO2) and negative wrinkled Fe2O3 electrode, has been found to extend the potential window to 2 V and achieve an energy density of 45.8 Wh kg21 (Gu & Wei, 2016). The cell performance is checked for cycling over 10,000 cycles. Thus, several reports have been made for the use of a composite of carbaneous materials with a metal oxide that would act as a negative electrode against pseudocapacitive positive electrode.

5.5

Mixed transition metal oxides

So far in earlier sections, it is revealed that the use of transition metal oxides as an electrode helps in improving the specific capacitance. However, there is a problem with the cost of the metal oxide. In addition to cost, the conductivity of metal oxides is less. Many methods such as combining it with carbaneous material, etc., are addressed to avoid conductivity issues (Anisimov et al., 1990; Cottineau et al., 2006; Deng et al., 2011; Hu & Chen, 2004; Liu et al., 2014; Memming, 1983; Nandi et al., 2020; Self supported hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercarpacitor capacitance, 2011; Wen et al., 2013; Yuan et al., 2013). So as an alternative, mixed transition metal oxides (MTMOs) have been studied. MTMOs are ternary metal oxides with the formula of AxB3-xO4, in which A and B are two different transition metals like Fe, Co, Ni, Zn. The combination of two different metal actions improves redox reactions and enhances electrical conductivity which proves beneficial for supercapacitor application (Dubal et al., 2015; Lu et al., 2011; Meher et al., 2011; Wang et al., 2014; Zhou et al., 2012). MTMOs have been

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used in various applications like catalysts, supercapacitors, semiconductors (Augustine et al., 1999; Tajik et al., 2017; Yuan et al., 2014). The mixed metal oxides, having a combination of various transition metal-ion species have emerged as promising electrode materials owing to their higher electrochemical activities and relatively high electronic conductivities.

5.5.1 Nickel cobaltate (NiCo2O4) Amongst different MTMOs, nickel cobaltite shows excellent specific capacitance and higher electrical conductivity as compared to individual NiO and Co3O4. The redox reaction of NiCo2O4 is higher than the redox reaction of nickel oxide and cobalt oxide alone (Hu et al., 2012). Nickel cobaltite has attracted attention due to its features like low cost, low toxicity, natural abundance and electrochemical performance equivalent to RuO2 (Garg et al., 2014). Yu et al. developed an asymmetric supercapacitor with NiCo2O4 as a positive and carbonaceous material (like activated carbon, graphene, etc.) as a negative electrode. The positive electrode provides superior working potential window, energy density and power density in an aqueous system. However, negative electrode provides high surface area. As it is a highly conducting material, it provides an efficient electrostatic charge storage mechanism at the electrolyte/electrode interface (Yu et al., 2016). In the same context, the asymmetric supercapacitor (ASC) with PGP/ NiCo2O4/CC, where porous graphene paper (PGP) acts as a negative electrode and NiCo2O4/CC as a positive electrode has been fabricated by Gao et al. which exhibited specific energy and power density of 60.9 Wh kg21and 11.36 Kw kg21. The porous graphene paper helps in providing a large surface area (Gao, Yang, et al., 2015). Wang et al. (2012) synthesized graphene-nickel cobaltite (GNCC) nanocomposite as a positive electrode and activated carbon as a negative electrode. Especially AC/GNCC has a high energy density of 19.5 Wh kg21 with operational voltage 1.4 V. At a high sweep rate, the energy density of 7.6 Wh kg21 at power density 5600 W kg 21. The addition of graphene to nickel cobaltite allows an increase in electrical conductivity of the positive electrode so that the charge-discharge take place at a scan rate equivalent to commercial AC negative electrode. Recently, Yang et. al. (Yang & Park, 2020) fabricated ASC by nanoflower- like NiCo2O4 grown on 3D gelatin-based carbon nickel foam (GCNF) by facile hydrothermal method. It is found in this study that the resistance between the electrode and the current collector is decreased due to the use of GCNF. GCNF also acts as a current collector without the need for any binder or conducting agent. The NiCo2O4/GCNF composite electrode works with a potential window of 01.5 V and provides the energy densities of 48.6 Wh kg21and power densities of 749 W kg21 in an aqueous KOH system. Besides, Wang et al. (2018) reported the fabrication of flexible asymmetric supercapacitor by depositing mesoporous NiCo2O4 on the surface of ultrafine nickel wire as a positive electrode and Fe3O4 coated on ultrafine nickel wire as a negative electrode. Fe3O4/NiCo2O4 electrode shows a specific energy density of 32.6 Wh kg21 at a high power density of 35,000 W kg21 in an aqueous KOH system.

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NiCo2O4 have been grown on conducting substrates such as Ni-foam, low-cost carbon fiber paper (CFP), flexible carbon fabric, etc. Such conducting substrates are attracting the attention of researchers because of their good electrical conductivity, high porosity, large surface area, etc. These have been found to be promising current collectors for transition metal oxide (Luo et al., 2012). Chen et al. (2013) have used the hydrothermal method to deposit NiCo2O4 nanowires on nickel foam. These nanowires form an interconnected structure deposited on nickel foam and have exhibited specific capacitance of 2132 and 1161 F g21 at a current density of 10 and 80 A g21 respectively along with 93.8% cycles retention after 10,000 cycles. Yang et al. (2019) have used cationic surfactant CTAB for the synthesis of NiCo2O4 nanoflakes. This surfactant plays important role in reducing the agglomeration and increasing the surface area (Zhang, Wang, et al., 2012). In this work, the nanocomposite is formed by adding graphene oxide (GO) and multiwalled carbon nanotubes (MWCNT) to nickel cobaltite (NCO). This nanocomposite electrode material exhibits a high capacitance of 1525 F g21. The maximum power density of 5105 W kg21 and maximum current density of 25.2 W kg21 is obtained. After 7000 cycles, 99.6% maintenance of the initial specific capacitance shows the stability of cycles. Deng et al. (2014) synthesized hierarchical NiCo2O4 nanowires and nanosheet on CFP by hydrothermal method. Hexamethylenetetramine and urea are used as alkali sources and capping agents of nanosheets and nanowires. Both morphologies showed high specific capacitance. The NiCo2O4 nanowires are highly porous with a pore size of 24 nm and BET surface area of 97 m2 g21. The nanosheets have a pore size of around 5 nm and BET surface area of 231 m2 g21. The NiCo2O4 nanowires capacitance is 471 F g21 at a current density of 1 A g21, the specific capacitance of the nanosheet is 799 F g21 at the same current density. In nanosheet, B12.9% loss in capacitance after 2400 cycles at current density 10 A g21, similarly nanowires capacitance loss is B19.6% at same test condition.

5.5.2 Ferrites Corrosion-resistant ferrite has the general formula AB2O4, where, A and B are metal cations and oxygen ions are anions. More precisely ferrites are represented by MFe2O4, where M can be replaced by atom-like Zn, Ni, Mn, Mg, Cd, Co, etc (Patil & Jagadale, n.d.). The different oxidation states of the metal cations help in fast charge transfer and therefore MFe2O4 has high electrochemical activities. The different ferrites used as electrode material in supercapacitors are discussed in this section. The nanocomposite of ZnO and CoFe2O4 in an aqueous solution of 3 M KOH has shown remarkable capacitance of 4050 F g21 along with the energy density of 77 Wh kg21 (Reddy et al., 2018). A novel way of making a composite of CoFe2O4 with carbon material has been found out by Elseman et al. (2020). A simple solvothermal method is used to synthesis CoFe2O4/carbon spheres nanocomposite. Carbon spherical morphology is obtained by combining glucose into CoFe2O4 during synthesis only. The nanocomposite has shown enhanced capacitance of 600 F g21 with an energy density of 27.08 Wh kg21 and a power density of 750 W kg21. Ecofriendly and cost-effective ZnFe2O4 has theoretical specific

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capacitance to be 2600 F g21 (Zhang et al., 2018). The nanocomposite of ZnFe2O4/ graphene nanoplatelets shows a specific capacitance of 314 F g21. The network formed by graphene nanoplatelets enhances the charge transport across electrodes and leads to better electrochemical performance. MnFe2O4 ferrite shows high electrochemical activity and has been used as an electrode in different batteries. The preparation of nanostructures of MnFe2O4 solves the issue of ion insertion and desorption from the electrode. MnFe2O4 synthesized by solvothermal and coprecipitation methods has exhibited specific capacitance of 517 and 173 F g21 respectively (Su et al., 2018; Vignesh et al., 2018).

5.6

Flexible supercapacitors

Metal oxides play important role in flexible supercapacitors. The study of microsupercapacitors has come to the surface especially for powering the different electronic systems (Delbari et al., 2020). The cost-effective printed electronic techniques like inkjet printing, screen printing, 3D printing, etc., can be effectively used to manufacture flexible micro-supercapacitors (Zeng et al., 2020; Zhang et al., 2019). For flexible supercapacitors, materials used for an electrolyte and substrates are important. Most preferable the solid-state electrolyte or gel electrolyte is used in flexible supercapacitors. There can be a variety of gel electrolytes used in flexible supercapacitors. However, the selection of substrate material is crucial because it should possess mechanical flexibility as well as high heat resistance capacity (as the material needs to undergo heat treatments during the printing process), which polymer-based substrate may not tolerate. Various materials like metal foils, carbon nanofibers, carbon cloth, PET, polyimide (PI), polydimethylsiloxane (PDMS), paper, etc., have been used as common flexible substrates for supercapacitors. Certainly, there are advantages along with few disadvantages (Wang et al., 2019). But compared to conventional carbon materials, transition metal oxides have been found to provide more energy density and they also show better electrochemical stability than polymer materials. Therefore, metal oxides like Fe2O3, MnO2, ZnO, NiCo LDH, CoAl LDH, Co3O4 are vastly used as an electrode material in supercapacitors (Jing, Dong, et al., 2020; Jing, Song, et al., 2020; Jing et al., 2021; Li et al., 2017). These metal oxides play a crucial role in bettering the capacitance of electrodes through fast faradaic pseudocapacitance effects (Wiston & Ashok, 2019).

5.7

Futuristic scope

Despite the encouraging electrochemical performances of metal oxide as an electrode in a supercapacitor, there are few current encounters and future perceptions to be undertaken for practical use of metal oxides in supercapacitors. So far, the studies have revealed that the energy density can also be increased by improving the charge transfer and ion diffusion across the electrode-electrolyte

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interface. The charge storing capacity of electrode material can be improved by obtaining various nanostructures. But to avoid the agglomeration of nanoparticles/ nanostructures during heat treatments, facile/simple methods must be explored to achieve hollow core-shell nanostructures. To optimize the porous structure for optimum electrical conductivity, porous materials (but with maintaining electrical conductivity) should be synthesized using these methods. Because a highly porous structure decreases the electrical conductivities. It has been observed that the electrical conductance of metal oxides has limitations and can be improved by making composites. So, metal oxide composites should be formed with various nitrides, carbides, etc., to improve electrical conductivities. One of the promising ways to improve the energy density is increasing the potential window of the assembly used for supercapacitors. The current assembly works with the maximum potential window of 0.5 V in an alkaline electrolyte. Interestingly, hybrid supercapacitors made up of the assembly of two different electrodes (one pseudocapacitive and another battery type electrode) that store charge with different mechanisms can improve the potential window. However, the optimization of the electrode structure and electrolyte is necessary to utilize the assembly of a hybrid supercapacitor for enhancing potential windows. One more facet needs to be focused on the elimination of misperceptions in the terminologies used to analyze electrochemical performances like specific capacitance or specific capacity. Similarly, the electrochemical behavior of metal oxidebased electrodes needs to be clarified in respect of capacitive type or battery type. The entire depiction of various studies/ research concerning supercapacitors shows that the operability in terms of design/assembly of supercapacitors should be addressed along with the modifications done in the properties of electrode material and electrolyte.

5.8

Conclusions

The review of a variety of studies of supercapacitor shows that several parameters need to be considered when any material has to be proposed for the electrode material in a supercapacitor. A very important aspect that should not be underestimated is the cost. So a supercapacitor cell should be comprised of low-cost electrode material with a high surface area that can provide fast charge transport and a short ion diffusion path. The electrode material should show optimum ion adsorption and desorption during the intercalation process. Such low-cost electrode material should be deposited on the low-cost substrate to reduce the overall cost of the device, and the substrate should provide a low resistance direct conducting path for charges. The conductivity problem of metal oxide and the low specific capacitance of carbon materials can be solved by making their composites. Additionally, the increase in working potential window can enhance the energy density of supercapacitor that can happen if two dissimilar electrodes are used in the assembly. Therefore, the

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ultimate aim of improving the energy density of the supercapacitor at par with the batteries can be achieved by depositing a low-cost nanocomposite material with a high surface area on a low-cost conducting substrate in the form of a hybrid type of arrangement of electrodes. The future prospects for supercapacitors are to look for different aspects in different types of hybrid supercapacitors so as make the commercial use of supercapacitors equivalent to batteries.

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Nanostructured WO32x based advanced supercapacitors for sustainable energy applications

6

Akshay V. Salkar, Sheshanath V. Bhosale and Pranay P. Morajkar School of Chemical Sciences, Goa University, Taleigao, Goa, India

6.1

Introduction

The rapid advancements in portable electronics and electric vehicles have created a demand for highly efficient electrochemical energy storage devices (Gao et al., 2013; Lokhande et al., 2019). Owing to their fast charging/discharging rates and superior energy densities, supercapacitors are regarded as the emerging highefficiency energy storage systems with great potential for commercial application. Supercapacitors can store energy using two mechanisms, that is, (1) electrical double-layer capacitors (EDLCs) and (2) pseudocapacitors (Naik et al., 2020). EDLC involves reversible adsorption of ions at the electrode
electrolyte interface and produces low specific capacitance, such as in the case of carbon-based EDLCs (You et al., 2018). The second charge storage mechanism is by Faradaic charge transfer process involving reversible redox reactions and can be classified as pseudocapacitors (Fleischmann et al., 2020). Transition metal oxides exhibit pseudocapacitive behavior. For instance, due to its high theoretical and practically achievable capacitance, ruthenium oxide is the most widely explored pseudocapacitive material. However, its toxicity, along with its high cost, prevents its widespread practical applicability (Shinde & Jun, 2020). Many alternative transition metal oxides and conducting polymers such as manganese oxide, vanadium oxide, iron oxides, cobalt oxide, nickel oxide, tungsten oxides, polypyrrole, polyaniline, etc. (S et al., 2020) have been investigated for their charge storage properties. Among these, tungsten oxide is relatively promising due to its ability to switch between the oxidation state from 16 to 15 during electrochemical reactions, low cost, and environmentally friendly nature (Salkar et al., 2019). The chemistry of tungsten and its investigation dates back to the late 1700, that is more than two centuries ago, when Carl Wilhelm Scheele discovered that tungstic acid could be made from scheelite (Smeaton, 1986). Later in 1783, Juan Jose´ Elhuyar and Fausto Elhuyar found an acid produced from wolframite and successfully isolated tungsten from it and have been credited for its discovery (Schufle, 1975). Since then, much research has been focused on tungsten and its compounds. WO3 is also recognized as a candidate for several other applications such as photocatalysis, gas sensing, electrochromic, dye-sensitized solar cells, electrocatalysis, Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00001-4 © 2022 Elsevier Inc. All rights reserved.

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etc. (Jelinska et al., 2018; Li et al., 2018; Staerz et al., 2020; Zheng et al., 2010). Although WO3 has gained much attention as a promising candidate for many applications, its low electrical conductivity remains a significant shortcoming. For WO3 to be an ideal supercapacitor, it should have high conductivity and be capable of storing and delivering a large amount of charge (Salkar et al., 2021). Several characteristic properties of WO3 can be tuned not only by electronic and structural modifications but also by the structure’s defect engineering. Among the many defects found in transition metal oxides, the oxygen vacancies are the most pervasive point defects (Naik et al., 2021; Salkar et al., 2019). Several theoretical and experimental studies have revealed that oxygen vacancies in WO32x structures can significantly improve their conductivity while serving as the key factor for electrode reactions (Chatten et al., 2005; Gerosa et al., 2016). The presence of redox oxidation states (W51/W61) and oxygen deficiencies have been reported to enhance the chargestorage performance by improving the electrochemical activity and introducing more active sites that greatly facilitate the charge transfer dynamics between the electrolyte and the WO3 matrix (Salkar et al., 2019, 2021). Thus, it is most desirable to introduce defects such as oxygen vacancies in WO3 to improve its chargestorage performance (Fig. 6.1).

6.2

Crystallographic characteristics of WO3

WO3 is particularly known for its rich structural diversity. Temperature-dependent phase transitions and structural polymorphism occur in stoichiometric WO3. The ideal WO3 crystal structure consists of a ReO3-like structure. The different polymorphs of WO3 are formed due to the distortion from the cubic ReO3 structure (Fig. 6.2) (Bandi & Srivastav, 2021; Migas, Shaposhnikov, Rodin et al., 2010). This structure consists of cations, which are surrounded by oxygen atoms forming an octahedral arrangement, which appears as a 3D network of corner-sharing WO6 octahedra, and their arrangement results in a simple cubic symmetry. The WO6 octahedron consists of tungsten atoms at the center of the octahedron and oxygen atoms at the corners. The crystal network is formed by O and WO2 planes alternating disposition in the main crystallographic direction. The symmetry of WO3 is less than the ideal ReO3 like structure due to tilting of the WO6 octahedra and subsequent translation of the W atom from the center of octahedra. Moreover, the different crystal structures of WO3 are temperature-dependent. As the temperature changes, the WO6 octahedra goes through the central W atoms’ displacement, which then exhibits different polymorphic forms. Monoclinic I (γ-WO3) is the most common room temperature stable phase of WO3. This phase is stable in the temperature regime of 17 C to 330 C. Two other crystallographic phases of WO3 exist at below room temperatures: monoclinic II (ε-WO3) at temperatures below 243 C and triclinic (δ-WO3) at temperatures between 243 C to 17 C. If γ-WO3 is subjected to temperatures above 330 C, it gets converted to orthorhombic (β-WO3), which is stable up to 740 C. Above 740 C, the orthorhombic phase is converted to

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Figure 6.1 Schematics depicting the stoichiometry dependent classification of tungsten oxides for supercapacitor application.

tetragonal (α-WO3). Both the orthorhombic and tetragonal phases of WO3 are stable only at high temperatures, which upon cooling tend to revert to the stable monoclinic phase (γ-WO3) (Mardare & Hassel, 2019). The tilting angle of the WO6 octahedra is the key parameter for this phase transformation, while the interatomic distances and angles remain nearly unchanged (Pirker et al., 2020). Some of these crystal structures have been shown in Fig. 6.2A. The nonstoichiometric WO32x (i.e., 2 # x , 3) is formed as a result of oxygen vacancies induced in the crystal structure during the process of crystallization, as shown in Fig. 6.2B. The nonstoichiometric tungsten oxide was first reported by Glemser and Sauer (Cong et al., 2016; Glember & Saurr, 1943), where they account that the pure WO3 phase structure can be changed to WO2.9 through the introduction of oxygen vacancies. The nonstoichiometric tungsten with oxygen deficiency WOx (x 5 2.63
2.91) can exist as several well-defined sub-oxides with varying W to O ratios such as W32O84 (WO2.625), W3O8 (WO2.667), W18O49 (WO2.72), W17O47 (WO2.765), W5O14 (WO2.8), W20O58 (WO2.9), W2O5, WO2, and W25O73 (WO2.92)

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Figure 6.2 Classification of WO3 based on (A) crystal phases (Migas, Shaposhnikov, Rodin et al., 2010) and (B) stoichiometry (Migas, Shaposhnikov, Rodin et al., 2010). Source: (A and B) Reprinted with permission from Migas, D. B., Shaposhnikov, V. L., Rodin, V. N., & Borisenko, V. E. (2010). Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3. Journal of Applied Physics, 108(9), 093713. https://doi.org/10.1063/1.3505688, Copyright (2010), AIP Publishing.

and they are known as Magne´li phases (Fig. 6.2B) (Migas, Shaposhnikov, & Borisenko, 2010). The crystal structures are orthorhombic for WO2.625 and WO2.667, monoclinic for WO2.72, WO2.765, WO2.9, and WO2.92, and tetragonal for WO2.8. The reduction of monoclinic WO3 can also lead to the formation of Magne´li phases. During this reduction process, as the number of oxygen vacancies increases, there is a change in the crystal structure position from the corner-sharing to edge-sharing of the WO6 octahedra, split by the crystallographic shear plane (Al Mohammad & Gillet, 2002). Further, the edge-sharing WO6 octahedra with channels form pentagonal columns and hexagonal tunnels in these sub-oxides. The lattice structure of WOx can uphold substantial oxygen deficiency with partially reduced W51 species. Once they hold sufficient oxygen vacancies, these sub-oxides become highly conductive and metallic (Imran et al., 2021). Moreover, the introduction of oxygen vacancies into the WOx lattices also influences the energy gaps, Fermi level, and the number of free electrons of WOx. The enhancement in these properties with the increase in the degree of non-stoichiometry is observed in several recent studies (Vasilopoulou et al., 2012; Wang et al., 2016). Along with defect engineering, nanostructuring is another critical feature for improving energy storage in tungsten oxide. The nonstructural design of WO3

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endows several key features such as generation of kink sites, exposing electrochemically active facets, heterostructure formation, and porosity generation (Salkar et al., 2018). Various nanostructural morphologies of WO3 with dimensionality ranging from 0D to 3D, such as spherical nanoparticles, nanoclusters, nanorods, nanotubes, nanobowls, nanosheets, sponge-like, wedge-like, snowflake-like, etc. (Ali et al., 2021; Salkar et al., 2021; Zheng et al., 2011) have been reported till date. Each of these synthesized structures requires unique experimental conditions and methods for their formation. Therefore, an overview of some of the methods to produce nanostructured WO32x and their implications for energy storage applications is presented and discussed in the subsequent section.

6.2.1 Role of ion intercalation in WO3 and electrochemical charge storage Over the past couple of decades, there has been significant progress in innovative technologies such as hybrid vehicles, electronics, and memory storage systems. Tungsten oxides, in particular, exhibit good charge-storage properties and electrochemical stabilities compared to other traditional electrode materials. Additionally, their unique physico
chemical properties, which are based on their crystal structure modifications, as discussed earlier, make them highly effective electrode materials for diverse applications. During an electrochemical process, reversible redox reactions occur at the surface of electrode materials, wherein the oxidation state of W changes between W51 and W61. The charge transfer and storage process which gives rise to supercapacitor property in WO3 electrode can be understood from the following reaction: (Mardare & Hassel, 2019; Salkar et al., 2019) WO3 1 xM1 1 xe2 2Mx WO3 ð1Þ

(6.1)

where M represents cations (M 5 H1, Na1, Li1, and K1) and x represents the number of charged species. This intercalation
deintercalation process induces chromaticity in WO3. The color change associated with WO3 has extensive applications in the development of electrochromic supercapacitors. An example of this application has been demonstrated by Bi and coworkers (Bi et al., 2017), who developed a large-scale multifunctional electrochromic-energy storage device based on WO3 nanosheets. The color-modulated charge storage device was based on Li intercalation and had great color efficiency, ultrafast response time, and long cycle life, as shown in Fig. 6.3. The specific capacitance and rate of charge-discharge in SCs depend on several factors of the active electrode system. Some of these factors are; (1) the e2 transport mechanism between the current collector and the electroactive material, (2) charge transfer resistance of M1 ions across the WO3-electrolyte interface, and (3) the insertion mechanism of M1 ions into the WO3 matrix, leading to alteration in the oxidation state of W. Additional factors that could influence the charge storage properties of WO3 to include its morphological features, crystallinity, electroactive surface area (ESA) and conductivity (Shinde & Jun, 2020).

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Figure 6.3 A large-scale electrochromic-energy storage device based on WO3 (A) deintercalated state, (B) intercalated state, (C) transmission spectra, (D) in situ transmittance response at 650 nm, and (E) galvanostatic charge-discharge (GCD) curves at various current densities (Bi et al., 2017). Source: Adapted with permission from Bi, Z., Li, X., Chen, Y., He, X., Xu, X., & Gao, X. (2017). Large-scale multifunctional electrochromic-energy storage device based on tungsten trioxide monohydrate nanosheets and Prussian white. ACS Application Materials & Interfaces, 9(35), 29872
29880. https://doi.org/10.1021/acsami.7b08656, Copyright (2017), American Chemical Society.

6.3

Designing nanostructured WO3 for supercapacitor application

The unique physical and chemical properties of WO3 make it an excellent material for energy storage systems. The potential of nanostructured WO3 electrodes to enhance the charge storage properties of supercapacitors has made them unique for energy research. It is well known that an electrode system’s charge storage potential is associated with its chemical composition and surface microstructure. Thus, it is of paramount importance to tune the nanostructure of electrode materials to unravel their maximum potential. There are several reports on nanostructured WO3 synthesized using different synthetic methodologies. Fig. 6.4 showcases some of the reported 1D (Fig. 6.4A
C), 2D (Fig. 6.4E
G), and 3D (Fig. 6.4I
K) WO3 morphologies reported in the literature (Huang et al., 2013; Nayak & Pradhan, 2018; Paik et al., 2018; Salkar et al., 2019, 2021; Tong et al., 2016; Wang et al., 2014; Zhou et al., 2017). This section aims to describe some of the unique

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Figure 6.4 (A) SEM image of WO3 nanorods along with its charge-storage performance demonstrated via cyclic voltammetry, galvanostatic charge-discharge studies, and electrochemical impedance spectroscopy in D, H, and L, respectively. (B, C) SEM, TEM images of WO3 nanowires (Salkar et al., 2019; Paik et al., 2018). (E, F) SEM image of WO3 nanoplatelets, WO3 flakes, and (G) SEM image with TEM inset of WO3 nanoplates (Wang et al., 2014; Zhou et al., 2017; Huang et al., 2013). (I) SEM image of WO3 3D-nanoporous network (Salkar et al., 2018). (J) SEM image with TEM in the inset of WO3 microspheres (Tong et al., 2016) and (K) SEM image with TEM inset of W18O49 urchin-like nanostructures. (B and C) Adapted with permission from Salkar, A. V., Fernandes, R. X., Bhosale, S. V., & Morajkar, P. P. (2019). NH- and CH-substituted ureas as self-assembly directing motifs for facile synthesis and electrocapacitive applications of advanced WO3
x one-dimensional nanorods. ACS Application. Energy Materials, 2(12), 8724
8736. https:// doi.org/10.1021/acsaem.9b01704; Paik, T., Cargnello, M., Gordon, T. R., Zhang, S., Yun, H., Lee, J. D., Woo, H. Y., Oh, S. J., Kagan, C. R., Fornasiero, P., & Murray, C. B. (2018). Photocatalytic hydrogen evolution from substoichiometric colloidal WO3
x nanowires. ACS Energy Letters, 3(8), 1904
1910. https://doi.org/10.1021/acsenergylett.8b00925, Copyright (2019) and (2018), American Chemical Society, respectively; (E
F) Adapted with permission from Wang, X., Zhang, H., Liu, L., Li, W., & Cao, P. (2014). Controlled morphologies and growth direction of WO3 nanostructures hydrothermally synthesized with citric acid. Materials Letters, 130, 248
251. https://doi.org/10.1016/j.matlet.2014.05.138; Zhou, J., Lin, S., Chen, Y., & Gaskov, A. M. (2017). Facile morphology control of WO3 nanostructure arrays with enhanced photoelectrochemical performance. Applied Surface Science, 403, 274
281. https://doi.org/10.1016/j.apsusc.2017.01.209; Huang, J., Xiao, L., & Yang, X. (2013). WO3 nanoplates, hierarchical flower-like assemblies and their photocatalytic properties. Materials Research Bulletin, 48(8), 2782
2785. https://doi.org/ 10.1016/j.materresbull.2013.04.022, Copyright (2014), (2017) and (2013) Elsevier, respectively; (I) Adapted with permission from Salkar, A. V., Naik, A. P., Joshi, V. S., Haram, S. K., & Morajkar, P. P. (2018). Designing a 3D nanoporous network via selfassembly of WO3 nanorods for improved electrocapacitive performance. CrystEngComm, 20 (Continued)

L

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(42), 6683
6694. https://doi.org/10.1039/C8CE01257A, Copyright (2018), Royal Society of Chemistry; (J) Adapted with permission from Tong, H., Xu, Y., Cheng, X., Zhang, X., Gao, S., Zhao, H., & Huo, L. (2016). One-pot solvothermal synthesis of hierarchical WO3 hollow microspheres with superior lithium ion battery anode performance. Electrochimica Acta, 210, 147
154. https://doi.org/10.1016/j.electacta.2016.05.154, Copyright (2016), Elsevier; (K) Adapted with permission from Nayak, A.K., & Pradhan, D. (2018). Microwave-assisted greener synthesis of defect-rich tungsten oxide nanowires with enhanced photocatalytic and photoelectrochemical performance. The Journal of Physical Chemistry C, 122 (6), 3183
3193. https://doi.org/10.1021/acs.jpcc.7b09479, Copyright (2018), American Chemical Society.

nanostructures of WO3 and their consequences on their charge-storage performance (Table 6.1). The sol
gel method is a widely reported method for synthesizing WO3 nanostructures. The chemistry of tungsten to form the tungstic acid sol and its subsequent conversion into the gel phase can be explored for producing different WO3 morphologies for supercapacitors. For example, Salkar and coworkers (Salkar et al., 2018, 2019) reported a method for producing WO3 nanorods using a modified sol
gel method using additives such as urea and carbohydrazide. The synthesized nanorods demonstrated remarkable capacitance of 132 mF cm22 at a current density of 1 mA cm22. The high capacitance was attributed to the large ESA (0.19 cm22) of the nanorods. Susanti and coworkers (Susanti et al., 2013) also synthesized WO3 nanoparticles using the sol
gel route. The WO3 obtained using the sol
gel method was spin-coated onto a graphite substrate used for supercapacitor studies and could yield a specific capacitance of 233.6 F g21 at a scan rate of 2 mV s21. The wet chemical method is another popular strategy for producing WO3 nanostructures (mostly hydrated WO3). Gupta and coworkers (Gupta, Nishad, Chakane et al., 2020) produced WO3 nanoplates using a post-annealing method. The WO3 produced at the post-annealing temperature of 200 C demonstrated a remarkable specific capacitance of 606 F g21 at a current density of 1 A g21. The same group also produced hierarchical flowers of hydrated WO3 that could yield a specific capacitance of 457 F g21 at a scan rate of 2 mV s21 via the wet-chemical method (Gupta et al., 2021b). They also synthesized hydrated WO3 3D slabs that could give specific capacitance of 386 F g21 at a scan rate of 2 mV s21 (Gupta, Nishad, Patil et al., 2020). Shinde and coworkers (Shinde et al., 2016) designed thin films comprising of the disk-like WO3 using the chemical bath deposition technique. The thin films demonstrated a high specific capacitance of 725 F g21 at a current density of 7 mA cm22. Yoon and coworkers (Yoon et al., 2011) designed an ordered mesoporous WO3 material using KIT-6 as a template. The ordered WO3 demonstrated high rate capability and excellent capacitance of 199 F g21 at a current density of 1 mA cm22. The comparatively higher specific capacitance values obtained using the wet chemical method were attributed to the presence of confined water into the crystal structure of WO3. Such confinement favors van-der-Waals gap creation in the layered crystal structure, facilitating facile insertion of ions leading to enhanced charge storage.

Table 6.1 A comparative table highlighting synthesis method-dependent morphological modifications of nanostructured WO3 and the performance of their supercapacitor. WO3 nanostructure

Synthesis method

Synthesis condition

Characterization (particle size/ ESA/SA)

Specific capacitance, Current density/Scan rate

Capacitance retention, cycle number

Device configuration

Energy density (Wh kg21)

Power Density (W kg21)

References

WO3 nanorods

Sol
gel

Na2WO4  2H2O, urea, 550 C

ESA of 0.191 cm2

80%, 2000 cycles










Salkar et al. (2019)

WO3 nanoparticles

Sol
gel

WCl6, Peptization, Triton X-100

5.8 m2 g21

132 mF cm22 at 1 mA cm22 233.6 F g21 at 2 mV s21













WO3.H2O nanoplates

Wet chemical method

Na2WO4  2H2O, H2SO4, 110 C

124.7 m2 g21

606 F g21 at 1 A g221

89%, 4000 cycles










Hydrated WO3 hierarchical flowers Hydrated WO3 slabs

Wet chemical method

Na2WO4  2H2O, H2SO4, 80 C

146.8 m2 g21

457 F g21 at 2 mV s21

91%, 2000 cycles

WO3//rGO

31

758

Wet chemical method

Na2WO4  2H2O, H2SO4, 110 C

68.8 m2 g21

386 F g21 at 2 mV s21

96%, 3000 cycles










Disk like WO3

Chemical bath deposition

W powder, H2O2, 80 C

725 F g21 at 7 mA cm22

81%, 1000 cycles




25.18

1166

Ordered mesoporous WO3 WO3 nanoflowers

Template method

H3PW12O40, KIT-6 template, 550 C

100
200 nm WO3 particles decorated on disks 54.3 m2 g21

Susanti et al. (2013) Gupta, Nishad, Chakane et al. (2020) (Gupta et al., 2021b) Gupta, Nishad, Patil et al. (2020) Shinde et al. (2016)

199 F g21 at 1 mA cm22

100%, 1200 cycles










Yoon et al. (2011)

Electrodeposition

Na2WO4  2H2O, H2SO4, H2O2




196 F g21 at 10 mV s21

85%, 5000 cycles

WO3//TiO2 @C@PPy

0.53 mW h cm23

229.3 mW cm23

Qiu et al. (2016)

(Continued)

Table 6.1 (Continued) WO3 nanostructure

Synthesis method

Synthesis condition

Characterization (particle size/ ESA/SA)

Specific capacitance, Current density/Scan rate

Capacitance retention, cycle number

Device configuration

Energy density (Wh kg21)

Power Density (W kg21)

References

WO3 2D film

Atomic layer deposition

0.7 nm thick films

650.3 F g21 at 1.5 A g21

65.8%, 2000 cycles










Hai et al. (2017)

WO3 nanorod bundles WO3 nanorods

Hydrothermal

SiO2/Si Wafer, bis(tert-butylimino)bis (dimethylamino) tungsten(VI) Na2WO4  2H2O, NaCl, HCl, 180 C Na2WO4  2H2O, H2SO4, autoclaved at 180 C




254 F g21 at 0.5 A g21 539 F g21 at 2 mV s21

88%, 1000 cycles 92%, 2000 cycles

CNF//WO3

35.3

314

WO3//rGO

43.7

800

WO3 nanorods

Hydrothermal

(NH4)6H2W12O40  xH2O, autoclaved at 200 C

19 m2 g21

474 F g21 at 0.1 A g21

92%, 10000 cycles

7.6 mWh cm23

18.9 mW cm23

WO3 microspheres

Hydrothermal




797.1 F g21 at 0.5 A g21

100%, 2000 cycles

97.6

28

Xu et al. (2015)

WO3 nanorods

Hydrothermal

19 m2 g21

496 F g21 at 5 mV s21

100%, 50000 cycles










Chen et al. (2015)

Pancake like WO3

Hydrothermal

K2WO4  H2O, HCl, H2O2, autoclaved at 160 C (NH4)10W12O41  5H2O, H2SO4, (NH4)2SO4, autoclaved at 120 C Na2WO4  2H2O, HNO3, autoclaved at 180 C

Porous carbon cloth//WO3/ Porous carbon cloth WO3//AC

Peng et al. (2014) (Gupta et al., 2021a) Chen et al. (2018)

32.4 m2 g21

100%, 4000 cycles










Jia et al. (2018)

WO3 nanopillars

Hydrothermal

605.5 F g21 at 0.37 A g21 421.8 F g21 at 0.5 A g21

100%, 1000 cycles




10

1000

Zhu et al. (2014)

Hydrothermal

HCl, Na2WO4  2H2O, NaCl, autoclaved at 180 C

2

21

214.6 m g

26.4 m2 g21

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Furthermore, Qiu and coworkers (Qiu et al., 2016) designed WO3 nanoflowers using the electrodeposition technique on Ti foil. The Ti foil worked as an excellent scaffold and substrate for the WO3 electrodeposition. The designed nanoflowers showed capacitance retention of 196 F g21 at a scan rate of 10 mV s21. When assembled into an asymmetric device (NFL-WO3//TiO2@C@PPy), it exhibited an excellent energy density of 0.53 mW h cm23 and a power density of 229.3 mW cm23. Subsequently, Hai and coworkers (Hai et al., 2017) using the atomic layer deposition technique, designed 2D WO3 electrodes. They demonstrated that 0.7 nm thick films could yield a specific capacitance as high as 650.3 F g21 at a current density of 1.5 A g21, although their capacitance retention was limited (65.8%) only up to 2000 cycles. Another largely utilized method for synthesizing WO3 nanostructures is the hydro/solvothermal method. In these methods, high pressure and temperature conditions are utilized to promote nanostructured designs under closed systems. There are several reports on nanostructured WO3 electrodes produced using hydrothermal techniques. For example, Peng and coworkers (Peng et al., 2014) synthesized WO3 nanorod bundles which, when used as a negative electrode in an asymmetric device, resulted in high energy and power densities of 35.3 Wh kg21 and 314 W kg21. The device performance was attributed to the material conductivity and greater diffusion rate of ions. Subsequently, Gupta and coworkers (Gupta et al., 2021a) synthesized mixed-phase hydrated WO3 nanorods having well-defined hexagonal tunnels along the (020) plane and exhibited a high specific capacitance of 539 F g21 at a scan rate of 2 mV s21. When assembled into an asymmetric supercapacitor device, it could yield energy and power density of 43.7 Wh kg21 and 800 W kg21, respectively. The excellent electrochemical performance was attributed to the WO3 nanorod morphology, improved wettability, and effective diffusion of protons into the nano-tunnels. Chen and coworkers (Chen et al., 2018) conducted an interesting study on crystallinity-dependent supercapacitor performance of WO3. They synthesized lowcrystalline (LC) WO3 nanorods using the hydrothermal technique. The supercapacitor performance of LC WO3 was compared with that of highly-crystalline WO3. Using electrochemical kinetic analysis, it was confirmed that the capacitivedominant charge storage mechanism played a major role in LC-WO3 which, endowed it with improved rate capabilities. Thus the LC-WO3 demonstrated superior supercapacitor performance with a specific capacitance of 474 F g21 at a current density of 0.1 A g21. Xu and coworkers (Xu et al., 2015) synthesized mesoporous WO3 microspheres composed of self-assembly of nanofibers. The WO3 microspheres could yield a specific capacitance of 797 F g21 at the current density of 0.5 A g21. When assembled into an asymmetric supercapacitor, the device demonstrated energy and power densities of 97.61 Wh kg21 and 28.01 kW kg21. Chen and coworkers (Chen et al., 2015) developed a mixed protonic
electronic conductor based on hydrous hexagonal WO3. They could obtain a high specific capacitance of 496 F g21 and excellent capacitance retention of 100% even after 50000 cycles. The high capacitance retention was attributed to the highly reversible proton insertion/desertion mechanism at the electrode. The process was also investigated

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using in situ XRD, which revealed the dominance of (002) and (200) reflections during the charge and discharge processes, respectively. The h-WO3 did not show any phase transformations while maintaining its “breathable” nature. This intern minimized the mechanical and chemical stresses onto the material, giving it an edge over the previously reported WO3 nanostructures in terms of material stability. Furthermore, Jia and coworkers (Jia et al., 2018) produced a self-assembled pancake-like WO3 using the hydrothermal method. The pancake-like WO3 exhibited a specific capacitance of 605.5 F g21 at a current density of 0.37 A g21 along with capacitance retention of 100% up to 4000 cycles. Zhu and coworkers (Zhu et al., 2014) synthesized hexagonal WO3 nanopillars that demonstrated a specific capacitance of 421.8 F g21 at a current density of 0.5 A g21. Overall, the WO3 electrodes synthesized via hydrothermal method demonstrated facile morphological control, superior specific capacitance as well as improved electrochemical stabilities in comparison to conventional sol
gel, wet-chemical, and electrodepositions techniques. However, further improvement in terms of electrical conductivity, electrochemical stability, and surface area (SA) is needed to augment the performance of WO3 nanostructures for large-scale commercialization of supercapacitor devices. To address these inherent limitations and challenges of WO3, many researchers have devoted a great deal of research focus on preparing composites of WO3 and have tested their supercapacitor performance. Some of the recent advancement in this is presented in the next section.

6.4

Recent developments in WO3 composites for supercapacitor application

Producing nanomaterial composites is an efficient method to overcome the inherent limitations of pristine metal oxides such as their poor electrical conductivity, chemical instability, poor rate capabilities, etc. WO3-based electrodes show several merits, such as high charge storage capacity, ability to exist in multiple oxidation states, which make them significant in supercapacitors, as discussed earlier. The demerits of WO3, such as its low electroactive area and conductivity, in particular, can be overcome by designing efficient composites of WO3. Several researchers have worked on synthesizing composites of WO3 with conducting carbon. The conducting carbon materials, including porous carbon, carbon nanotubes (SW/MWCNT), polymeric carbons, graphene oxide, etc., have been widely used both independently as well for the purpose of preparing composites (Biradar et al., 2021a, 2021b; Li et al., 2020). The below table summarizes some of the recent composites of WO3 (Table 6.2). For instance, Jo and coworkers (Jo et al., 2013) developed a mesoporous WO3/C nanocomposite using a block copolymer method. The resulting hybrid composite demonstrated an average specific capacitance of 103 F g21. Xu and coworkers (Xu et al., 2019) developed hierarchical carbon layer-anchored WO32x/C ultra-long nanowires using a solvent-thermal treatment and subsequent rapid carbonization

Table 6.2 Comparative table highlighting various synthesis methods used to design nanostructured WO3 composites and their supercapacitors performance. WO3 Composites

Synthesis method

Synthesis Condition

Characterization (particle size/ESA/ SA)

Specific capacitance, current density/ Scan rate

Capacitance retention, cycle number

Device configuration

Energy density (Wh kg21)

Power density (W kg21)

Reference

Mesoporous WO3/Carbon

Block copolymer

123 m2 g21

Average Csp 103 F g21







0.04 Wh cm23

0.4 W cm23

Jo et al. (2013)

WO3/C nanowires

Solvent-thermal treatment, carbonization




1032.1 F g21 at 1 A g21

94.3%, 5000 cycles

WO3/C// WO3/C







Xu et al. (2019)

Carbon encapsulate WO32x nanorods WO3 in carbon aerogel

Sol
gel method

WCl6, PS-b-PEO (Copolymer), 700 C WO3, ethylenediamine, autoclaved at 180 C, pyrolysis at 600 C Na2WO4  2H2O, HCl, Citric acid, 450 C

ESA of 1.154 cm2

401 mF cm22 at 2 mA cm22

94% for 5000 cycles

WO32x/C//AC

15.4

498

(Salkar et al., 2021)

559 m2 g21

609 F g21 at 5 mV s21

98%, 1000 cycles










Liu et al. (2018)

W18O49-PANI

Solvothermalelectrodeposition




440 F g21 at 2 A g21













Tian et al. (2014)

WO3/PANI

Electrodeposition




WO3/PANI// PANI

9.72

53

Zou et al. (2011)

Hydrothermalelectrodeposition

168 F g21 at 1.28 mA cm22 33.8 mF cm22 at 0.2 mA cm22

60%, 1000 cycles

poly(indole-6carboxylic acid)/WO3

87%, 5000 cycles

P6ICA/WO3// PEDOT







Li et al. (2020)

WO3-poly (5-cyanoindole) hybrid

Electrochemical polymerization

Sodium carbonate, resorcinol, 60 C, WCl6, 400 C W18O49, FTO, autoclaved at 200 C H2WO4, H2SO4, H2O2, Aniline, 100 CV scans Na2WO4  2H2O, HCl, autoclaved at 200 C, TBATFB, 6ICA


89.2 mF cm22 at 0.2 mA cm22

91%, 3000 cycles

P5ICN/WO3// PEDOT

1.94 3 1023 mWh cm22

0.233 mW cm22

Zhao et al. (2021)

Pyrolysis technique







(Continued)

Table 6.2 (Continued) WO3 Composites

Synthesis method

Synthesis Condition

Characterization (particle size/ESA/ SA)

Specific capacitance, current density/ Scan rate

Capacitance retention, cycle number

Device configuration

Energy density (Wh kg21)

Power density (W kg21)

Reference

WO3/Graphene

Solvothermal method

186 m2 g21

465 F g21 at 1 A g21

97.7%, 2000 cycles

GrapheneWO3//AC

26.7

600

Nayak et al. (2017)

WO3 nanorods/rGO

Hydrothermal method




343 F g21 at 0.2 A g21

92.7%, 1000 cycles










Guan et al. (2017)

WO3/rGO

Hydrothermal

38.6 m2 g21

801.6 F g21 at 4 A g21

75.7%, 5000 cycles.










Samal et al. (2019)

Freestanding CNT-WO3

Physical vapor deposition

WCl6, graphene, ethanol, autoclaved at 200 C. GO, NaHSO4  H2O, Na2WO4  2H2O, Autoclaved at 180 C Na2WO4  2H2O, H2SO4, GO, Ultrasonication for 10 min, autoclaved at 200 C. CNT film, WO3 film




2.6 F cm23 at 10 mV s21

CNT-WO3// CNT

0.59 mW h cm23

30.6 mW cm23

Sun et al. (2015)

WO3@W18 O49-CNF

Electrospinning




333.9 F g21 at 0.5 A g21

75.8%, 50000 cycles 98.1%, 5000 cycles










(Sun et al., 2020)

WO3/CdS

Microwave method




650 F g21 at 1 A g21

133%, 5000 cycles










Periasamy et al. (2020)

WO3-WS2 core
shell nanowire

Chemical vapor deposition




32.5 mF cm22 at 5 mV s21

70%, 10000 cycles

rGO//WO3/ WS2







Kumar et al. (2020)

PAN, DMF, Ammonium metatungstate, 17 kV, carbonization Na2WO4  2H2O, HCl, CdCl2, Na2SO3, microwave, 200 C rGO, WO3, WS2, CVD

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process. The composite demonstrated excellent specific capacitance of 1032.1 F g21 at a current density of 1 A g21. The high specific capacitance was attributed to a larger number of electroactive sites and improved conductivity due to abundant oxygen vacancies and inner anchored carbon layers. Liu and coworkers (Liu et al., 2018) incorporated WO3 nanoparticles and nanowires into a carbon aerogel template. The resulting composite demonstrated a specific capacitance of 609 F g21 at a scan rate of 5 mV s21. The high specific capacitance was attributed to the presence of numerous mesopores in the carbon aerogel that could facilitate facile diffusion of ions and the possibility of bridging WO3 nanoparticles and nanowires, which could further support the charge transport and storage process. In a major breakthrough, Salkar, and coworkers (Salkar et al., 2021) developed unique double-helical DNA-like superstructures of carbon encapsulated WO32x nanorods. This development opens up a doorway to mimic biological systems using solid-state materials for energy storage and other diverse applications. These double-helical fibers were extensively characterized using Raman, TEM, XPS analysis, etc., and tested for their supercapacitor performance (Fig. 6.5). The effective carbon encapsulation over WO32x nanorods was proved by comparing the intensity

Figure 6.5 (A) SEM image and (B) Optical micrograph of DNA-like WO3 along with its supercapacitor performance demonstrated using CV, GCD, and EIS studies as presented in (C
E) and (F) Schematic elaborating the structure of DNA-like WO3/C (Salkar et al., 2021). Source: Adapted with permission from Salkar, A. V., Naik, A. P., Bhosale, S. V., & Morajkar, P. P. (2021). Designing a rare DNA-like double helical microfiber superstructure via self-assembly of in situ carbon fiber-encapsulated WO3
x nanorods as an advanced supercapacitor material. ACS Applied Materials & Interfaces, 13(1), 1288
1300. https://doi. org/10.1021/acsami.0c21105, Copyright (2021), American Chemical Society.

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ratios of D (disordered) and G (graphitized) bands obtained through Raman analysis (Claramunt et al., 2015; Morajkar et al., 2019, 2020). These fibers demonstrated a high electroactive area of 1.54 cm2, leading to an improved specific capacitance of 401 mF cm22 at a current density of 2 mA cm22. When tested by constructing an asymmetric supercapacitor prototype, the WO3/C fibers showcased energy and power density values of 15.4 Wh kg21 and 498 W kg21. Conducting polymer is another class of compounds that can be combined with WO3 to form hybrid composites with superior supercapacitor performance. Tian and coworkers (Tian et al., 2014) demonstrated an innovative supercapacitor electrode that displayed changes in color as a response to varying levels of stored energy (Fig. 6.6). The electrode was engraved with a pattern consisting of W18O49 on a polyaniline (PANI) background. The integration of the two materials enabled the electrode to work in a widened potential window of 1.3 V. The device displayed a specific capacitance of 440 F g21 at a current density of 2 A g21. Zou and coworkers (Zou et al., 2011) prepared composite films of PANI and WO3 using an electrodeposition technique. The composite films showed a specific capacitance of 168 F g21 at a current density of 1.28 mA cm22. When assembled into an

Figure 6.6 (A and B) SEM and TEM images, (C and D) XRD and Raman analysis of W18O49 nanowires. Schematic of Supercapacitor electrode fabrication using W18O49 and PANI and its subsequent supercapacitor performance demonstrated using electrochemical studies as shown in (F
I) (Tian et al., 2014). Source: Adapted with permission from Tian, Y., Cong, S., Su, W., Chen, H., Li, Q., Geng, F., & Zhao, Z. (2014). Synergy of W18O49 and polyaniline for smart supercapacitor electrode integrated with energy level indicating functionality. Nano Letters, 14(4), 2150
2156. https://doi.org/10.1021/nl5004448, Copyright (2014), American Chemical Society.

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asymmetric device (WO3/PANI//PANI), the films displayed energy and power densities of 9.72 Wh kg21 and 53 W kg21, respectively. Li and coworkers (Li et al., 2020) prepared a Poly(indole-6-carboxylic acid) (P6ICA)/WO3 nanocomposite with core
shell nanorod structure using a hydrothermal followed by electrodeposition technique. The composite displayed multi-color changes along with a specific capacitance of 33.8 mF cm22 at a current density of 0.2 mA cm22. The same group prepared a hybrid composite of WO3-poly(5-cyanoindole) (P5ICN/WO3) using a hydrothermal and electropolymerization technique (Zhao et al., 2021). The hybrid composite displayed a specific capacitance of 89.2 mF cm22 at a current density of 0.2 mA cm22, which was attributed to the oblate geometry and the large specific surface area of the electrode. When constructed into an asymmetric supercapacitor device (P5ICN/WO3//PANI) showed energy and power densities of 1.94 3 1023 mW h cm22 and 0.233 mW cm22. The comparison of the above supercapacitor performance of conducting polymers reveals that WO3-PANI composites suffer from poor electrochemical stability, which is due to PANI film decomposition as a result of continuous cycling. Thus using other polymeric materials such as poly(indole-6-carboxylic acid) and poly(5-cyanoindole) in combination with PANI could address its limitation of electrochemical stability. Nayak and coworkers (Nayak et al., 2017) reported graphene-supported WO3 nanowires using a solvothermal technique (Fig. 6.7). Morphological characterization confirmed the random distribution of WO3 nanowires on graphene sheets resulted in an appreciable specific capacitance of 465 F g21 at a current density of 1 A g21. When constructed into an asymmetric supercapacitor device, the energy and power density values of 26.7 Wh kg21 and 600 W kg21 were obtained, respectively. Guan and coworkers (Guan et al., 2017) synthesized WO3 nanorods/ reduced-graphene oxide composites by electrostatic adsorptive hydrothermal method. The composite demonstrated a specific capacitance of 343 F g21 at a current density of 0.2 A g21. Samal and coworkers (Samal et al., 2019) synthesized WO3-rGO hybrids using a hydrothermal synthesis route (Fig. 6.8). The hybrid exhibited a pseudocapacitive behavior with a high specific capacitance of 801.6 F g21 at a current density of 4 A g21 and capacitance retention of 75.7% for up to 5000 cycles. Sun and coworkers (Sun et al., 2015) synthesized freestanding CNT-WO3 hybrid films using a physical vapor deposition technique. The films displayed a specific capacitance of 2.6 F cm23 at a scan rate of 10 mV s21. When constructed into an ASC device with CNT as the positive electrode, the device demonstrated energy and power density values of 0.59 mW h cm23 and 30.6 mW cm23. Another methodology for preparing advanced WO3 composites is by utilizing a metal oxide or sulfide to form either a binary or a ternary system. Sun and coworkers (Sun et al., 2020) synthesized a WO3@W18O49-CNF composite. The composite was prepared by doping WO3@W18O49 in carbon nanofibers using an electrospinning and carbonization method. The carbonization of electrospun fibers helped to increase the degree of graphitization, thus increasing the conductivity of the composite. The composite showed a specific capacitance of 333.9 F g21 at a current density of 0.5 A g21 with capacitance retention of 98.1% for up to 5000

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Figure 6.7 (A and B) SEM images of WO3 nanowires and 10-graphene-WO3 nanowire composite, (C) Surface area analysis, (D
F) Supercapacitor charge storage performance, and (G) Schematic along with the supercapacitor device prototype (Nayak et al., 2017). Source: Adapted with permission from Nayak, A. K., Das, A. K., & Pradhan, D. (2017). High performance solid-state asymmetric supercapacitor using green synthesized graphene— WO3 nanowires nanocomposite. ACS Sustainable Chemical Engineering, 5(11), 10128
10138. https://doi.org/10.1021/acssuschemeng.7b02135, Copyright (2017), American Chemical Society.

cycles. Periasamy and coworkers (Periasamy et al., 2020) synthesized WO3-CdS nanocomposites using a microwave method. The composite yielded a specific capacitance of 650 F g21 at a current density of 1 A g21. Kumar and coworkers (Kumar et al., 2020) developed a binder-free flexible all-solid-state asymmetric supercapacitor comprised of rGO and core/shell WO3/WS2 nanowires using electrodeposition and chemical vapor deposition techniques. The composite demonstrated a specific capacitance of 32.5 mF cm22 at a scan rate of 5 mV s21 and capacitance retention of 70% for up to 10000 cycles. The integration of rGO with core/shell WO3/WS2 played a major role in the flexible supercapacitor device by accommodating the strain caused in the supercapacitor device with negligible effect on its charge storage performance. Although the nanostructures of WO3 and its composites demonstrate promising performance, the commercial prospect of the material is still limited due to its complex synthesis route and high cost. Furthermore, several such research works emphasizing the role of nanostructuring in WO3 and its composites in improving electrochemical performance are available in the present literature, and readers are directed to refer for further reading (Mardare & Hassel, 2019; Shinde & Jun, 2020; Zheng et al., 2019).

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Figure 6.8 (A and B) and (C and D) SEM images of WO3 and WO3-rGO composites. (E and F) TEM images of WO3, (G) CV studies for determining charge storage properties, and (H
K) XPS analysis of WO3-rGO composite (Samal et al., 2019). Source: Adapted with permission from Samal, R., Chakraborty, B., Saxena, M., Late, D. J., & Rout, C. S. (2019). Facile production of mesoporous WO3-rGO hybrids for highperformance supercapacitor electrodes: An experimental and computational study. ACS Sustainable Chemical Engineering, 7(2), 2350
2359. https://doi.org/10.1021/ acssuschemeng.8b05132, Copyright (2019), American Chemical Society.

6.5

Conclusions

In summary, this chapter highlights the fundamentals of the crystal chemistry of WO32x, its nanostructures, and its influence on supercapacitive performance for device scale applications. Several nanostructures (0D-3D) of WO32x have been successfully synthesized using several methods such as wet-chemical, sol
gel, hydrothermal/solvothermal method, etc. Each technique presents its unique synthesis conditions, which then reflect upon the special morphological features of the synthesized WO32x. The wet-chemical method is an effective strategy to create vander-Waals gaps in the crystal structure of WO32x by confinement of water molecules. Additionally, hydrothermal/solvothermal techniques have proven their effectiveness in optimizing the crystallinity and phase purity of WO32x, which also has a major role in augmenting its charge-storage performance. Moreover, the inherent demerits of WO3, such as its low electrical conductivity and electrochemical stability, have been addressed via defect engineering strategies and by designing nanostructured composites with different materials such as conducting carbon, carbon aerogels, conducting polymers, metal oxides, and metal sulfides, etc. This strategy has been extremely successful in improving charge storage characteristics as well

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as the stability performance of the WO32x composites. The present advancements in WO32x material design and their efficacy in energy-storage device performance along with environmental benignity provide encouraging alternative energy solutions for addressing the energy challenges in the near future. Therefore, the actual commercialization of WO32x based advanced systems will start soon for sustainable energy storage technology applications and societal development.

6.6

Future prospects

The practical supercapacitor performance of WO32x is limited due to its inherent drawbacks of conductivity and electrochemical stability, which can be overcome through nanostructuring. Nanostructuring is an essential strategy in improving the properties of WO32x such as its conductivity, surface area, porosity, etc. This chapter brings forth the brief journey of WO32x nanostructures, from its first reported articles to some of the latest developments. Various synthesis strategies are suitable for designing nanostructures of WO32x; however, the cost-effectiveness of the methodology is crucial for its practical device scale applications. In reality, the supercapacitors and other scale-up stand-alone devices based on WO32x nanostructures are still in their incipient stages; thus, herein, we identify a few prospects, which can aid in the practical applicability of WO32x as listed below; (1) Although the pseudocapacitive performance of WO32x based on the intercalation/deintercalation processes is well known and has been explored by several research groups, attention is still required to identify the intermediates of WO32x during the electrodynamic charge storage processes, which can provide a better picture of the underlying charge-storage mechanism. (2) A majority of WO32x nanostructures reported in the literature have a very low surface area of 50 m2 g21. Therefore, enhancement in the surface area along with greater population distribution of electroactive sites is a major challenge that needs to be addressed for boosting the supercapacitive performance. (3) Along with the focus on improving the nanostructure of WO32x to expand the material-bank, the interaction of the nanostructure on the passive components of the supercapacitor device such as the separator, current collector, and even the electrolyte must be prioritized. (4) Finally, the cost consideration, which is the most critical aspect for commercializing a supercapacitor device, should be taken into account. This can be achieved by prioritizing the synthesis of WO32x nanostructures through cost-effective means, avoiding the use of expensive synthesis equipment and templating agents.

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Metal oxide nanomaterials for organic photovoltaic applications

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Harshad A. Mirgane1, Dinesh N. Nadimetla1, Dipak J. Garole2 and Sheshanath V. Bhosale1 1 School of Chemical Sciences, Goa University, Taleigao, Goa, India, 2Directorate of Geology and Mining, Government of Maharashtra, Nagpur, Maharashtra, India

7.1

Introduction

The continuous increase in world’s population, together with the substantial development of industry, has brought about imperious demand for increased energy consumption. At present, most of the energy production is made from the combustion of fuels, such as oil, natural gas, and coal. However, the depletion of fossil resources, the commensurate increase in noxious gas emissions, and the other associated environmental pollutions have put forward an urgent demand for developing sustainable energy technologies. Among all of the renewable energy technologies, including hydro, solar, wind, geothermal heat, and biomass, photovoltaic (PV) technology that converts solar energy into electricity is the most promising strategy for sustainable energy supply (Bai & Zhou, 2014). Organic-based photovoltaics (OPVs) have attracted increasing attention in recent years, and efficiencies exceeding 8% have recently been confirmed. These lowcost, lightweight, and mechanically flexible devices offer unique advantages and opportunities currently unavailable with crystalline silicon technology. Progress in the field of OPV has been achieved in part due to the incorporation of transition metal oxides. These offer a wide range of optical and electronic properties, making them applicable in OPV in many capacities. Transparent electrodes can be made from doped metal oxides. The high intrinsic charge carrier mobility of many undoped metal oxides makes them attractive as active materials and charge collectors. Metal oxides can increase the charge selectivity of the electrodes due to the energetic positioning of their valence and conduction bands (VBs and CBs) (Gershon, 2011). A promising and rapidly developing low-cost PV system is based on organic semiconducting polymers. These can be dissolved and coated onto many different surfaces via low-temperature techniques such as roll-to-roll processing semiconducting materials (Sun & Sariciftci, 2017). These have excellent charge transport properties and can be tuned in various ways through the introduction of dopants, the generation of nanostructures, or modification of their surfaces. Owing to the wide range of properties that these offer both optically and electronically, transition metal oxides can play many different roles within a “hybrid” organic/ inorganic PV device. Organic molecular and polymeric semiconductors can form Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00007-5 © 2022 Elsevier Inc. All rights reserved.

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films with complex morphologies and varying degrees of order and packing modes through the interplay of a variety of non-covalent interactions. Their molecular structure consistently presents a backbone along which the carbon (or nitrogen, oxygen, sulfur) atoms are sp2-hybridized and thus possess a p-atomic orbital. The conjugation (overlap) of these p-orbitals along the backbone results in the formation of a delocalized p-molecular orbital’s, which define the frontier [Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)] electronic levels and determine the optical and electrical properties of the (macro) molecules (Kippelen & Bre´das, 2009). Tremendous progress has been made in the field of OPV materials over the last 20 years (S. Li et al., 2016). These improvements are the result of both a better understanding of the working principles of OPV devices and the development of new, more suitable organic materials (Gu¨nes et al., 2007) (Fig. 7.1). Trap states and localized electronic states in the forbidden gap of semiconductors caused by material imperfections are some of the main limiting factors to highperformance optoelectronic devices such as solar cells, ultrafast photodetectors,

Figure 7.1 Structures of organic molecules used in organic solar cells.

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field-effect transistors, and light-emitting diodes, among others. Thus suppressing these traps states through a simple and efficient approach is critical to the enhancement of optoelectronic device performance. Metal oxides, as a typical class of transparent ionic semiconductors, have been widely used for efficient PV applications as selective contacts, such as WOx, V2Ox, TiOx, SnOx, and ZnO (Liu et al., 2019; Wang et al., 2019; Yao et al., 2020). Numerous researches have been conducted on the investigation of PV technologies, to converse the energy from solar light to electricity. However, in comparison with the traditional energy sources, the high costs of PV devices still limit their wide applications. The great elevation of the photo-generated carrier transport can be achieved by the interface engineering with the insertion of the intermediate layer. The interface engineering has been demonstrated by the improved device efficiency. For example, metal oxides (Cu2O, ZnO) (Laurenti et al., 2020; Yu et al., 2016). Various metal oxide-based nanomaterials and their polymer nanocomposites have been reported in the decades. Owing to their outstanding properties, such as electrical, magnetic, mechanical, optical, catalytic, etc., metal-oxide based nanomaterials play an important role in a wide range of applications including gas sensors, fuel cells, advanced ceramics, chemical sensors, biosensors, batteries, solar cells, pyroelectric, supercapacitors, catalysts, anticorrosion coatings, etc. (Dar, 2010; Siva Prasanna et al., 2018). A series of metal oxide nanoparticles have been synthesized including TiO2, SiO2, iron oxide, zinc oxide (ZnO), gallium oxide (Ga2O3), nickel oxide (NiO), copper oxide (CuO), etc. They have different morphologies such as spherical, triangular, star, nanowires (NWs), nanotubes, nanorods (NRs), etc. Owing to their high density and limited size, metal oxide nanoparticles showed exciting results in terms of physical and chemical properties; therefore it is highly desirable to understand their various aspects in terms of synthesis, properties, and applications. To prepare polymer-metal oxide nanocomposites with a nanophaseseparated structure, the homogeneous dispersion of metal oxide nanoparticles including a reduction in the size of the polymer-metal oxide interface is very important as it essentially alters the physical property of the nanocomposites. Metal oxide nanostructures have attracted substantial research interest, mainly because of their unique characteristics at nano dimensions compared to those of bulk or singleparticle species. Generally, the distinctive electronic structure defines the specific metallic, semiconductor, or insulator characteristics of metal oxide nanomaterials (Ashik et al., 2018). The semiconducting materials at the nanoscale are widely used for the manufacturing of various electronic, electrical, and PV devices owing to their unique band structure, optical properties, good charge mobility, and ability to absorb photons from light. Among various PV devices, a regenerative photoelectrochemical solar cell called Dye-sensitized Solar Cell (DSSC) is a promising PV device for achieving reasonably high conversion efficiency as compared to the conventional silicon solar cells. Various metal oxide semiconducting nanomaterials such as ZnO, TiO2, SnO2, Nb2O5, and CeO2-based thin film electrodes and their nanocomposites have also shown comparably good conversion efficiency of DSSC due to their good optical and electronic properties.

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The different sizes and shapes of metal oxide semiconductors like NRs, NWs (Karunakaran & Senthilvelan, 2005; Ray & Biswas, 2000; Su & Kuramoto, 2000; Zhao et al., 2020) (Fig. 7.2).

7.2

Organic photovoltaic: principle, designing and mechanism

Typically most of the organic compounds are inert for electrical conductivity due to the presence of strong covalent bonds. But this general perception was altered by the discovery of conducting polymers by Shirakawa et al. (1977). In their experiment, trans-polyacetylene was exposed to chlorine, bromine, or iodine vapor, which increased the conductivity of polyacetylene films. This discovery opened up a new and wide range of applications including organic displays, organic LEDs, organic and micro-electronics, and organic photovoltaics. Whatever the organic semiconductors (OSCs) are, such as macromolecule dyes, dendrimers, oligomers, polymers, etc. all are based on a conjugated π electron system. A conjugated system is an alteration between the single and double bonds. The important property related to this conjugation is that the π electrons have more mobility than σ electrons. Therefore by absorption of energy as in the case of organic solar cells or by absorbing electrical energy in the case of organic LEDs or displays, the π bonds system breaks creating excitons or free charges or emission of light. Molecular π
π orbitals correspond to the HOMO and LUMO. In a crystalline semiconductor, these correspond to the CB and VB (Bru¨tting, 2006; Chidichimo & Filippelli, 2010). However, there are considerable differences in the basic physics of HOMO
LUMO as there exist strong van der Waals forces, which are no longer considered in CB/VB formation. Another major difference arises in the transport process which is mainly by the hopping process between localized states rather than

Figure 7.2 TEM images of (A) ZnO nanowire, (B) polyaniline ZnO nanoparticles (Su & Kuramoto, 2000; Zhao et al., 2020). Source: From Zhao, X., Nagashima, K., Zhang, G., Hosomi, T., Yoshida, H., Akihiro, Y., Kanai, M., Mizukami, W., Zhu, Z., Takahashi, T., Suzuki, M., Samransuksamer, B., Meng, G., Yasui, T., Aoki, Y., Baba, Y., & Yanagida, T. (2020). Synthesis of monodispersedly sized ZnO nanowires from randomly sized seeds. Nano Letters, 20(1), 599
605. https://doi. org/10.1021/acs.nanolett.9b04367.

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transport within the band. In the case of polymers and oligomers, there is hopping along a conjugated chain and intermolecular charge transport between adjacent polymer chains or molecules. However, the mobility in case of a later process is smaller. Thus mobility of thin films can be improved by improving order, purification, high vacuum deposition, and no oxygen contamination (Cowan et al., 2011; Heimel et al., 2011).

7.2.1 Mechanism The operating mechanism of organic solar cells is one of the most researched and debated fields. In general, all the main differences in mechanism in the case of an organic solar cell arises due to the generation of electrostatically bound electron
hole pair in organic solar cells instead of free charges. Further, this concept is explained in detail. The proposed mechanism of the OPV devices is in Fig. 7.3.

7.2.1.1 Absorption of light and exciton generation The presence of a conjugated π-electron system in organic compounds results in all interesting optical and electrical properties. The bandgap or bond energy in OSCs is tuned with the energy of the solar spectrum causing the absorption of photons producing electrostatically coupled electron
hole pairs called excitons. In the case of inorganic silicon semiconductors, produces free charges instead of excitons. This major change drives all the differences in the mechanism of electricity generation in inorganic and organic solar PV devices.

7.2.1.2 Exciton diffusion The photo-generated excitons are characterized by a very small lifetime of few picoseconds limiting the mobility of excitons to a few polymer units or molecules. The exciton moves within the chain causing chain deformation to reduce the extra

Figure 7.3 Schematic of an organic photovoltaic device. Source: From Chiechi, R. C., Havenith, R. W. A., Hummelen, J. C., Koster, L. J. A., & Loi, M. A. (2013). Modern plastic solar cells: Materials, mechanisms and modeling. Materials Today, 16(7
8), 281
289. https://doi.org/10.1016/j.mattod.2013.07.003.

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unstable energy, which is altogether called polaron. However, intermolecular transition of excitons also happens, which is termed as a hopping process. Altogether the overall mobility of excitons is limited to a range of 10 nm, which is called the exciton diffusion length. As the excitons are to be dissociated within the range of this length, exciton diffusion length plays a critical role in the design and performance of organic solar cells.

7.2.1.3 Exciton dissociation Exciton dissociation refers to the process of splitting the electrostatically coupled electron
hole pair into free charges. The dissociation of excitons occurs at the donor-acceptor interfaces or junctions. The donor and acceptor materials are designed such that there exists a difference in LUMO levels of the materials, which drives the exciton dissociation. For efficient dissociation, the difference in the energy level of LUMO of donor and acceptor should be higher than that of exciton binding energy. Typically the difference is around 0.2
0.3 eV. In general, to achieve efficient charge separation Δ (LUMOD-LUMOA) . Exciton binding energy 3.4 Charge Transport Once the free charges are produced, they travel through specific materials to get collected at the electrodes. From there they are connected to the external circuit. The efficiency of charge transport is determined by the electrical conductivity and impedance of the organic materials (Chiechi et al., 2013; Wetzelaer & Blom, 2014; Zhu et al., 2009).

7.2.1.4 Types of organic photovoltaics An OPV cell is a type of solar cell where the absorbing layer is based on OSC— typically either polymers or small molecules. For organic materials to become conducting or semiconducting, a high level of conjugation (alternating single and double bonds) is required. OPV cells are categorized into two classes: Small-molecule OPV cells and Polymer-based OPV cells. Small-molecule OPV cells use molecules with broad absorption in the visible and near-infrared (IR) portions of the electromagnetic spectrum. Highly conjugated systems are typically used for the electron-donating system such as phthalocyanines, polyacenes, and squarenes. Perylene dyes and fullerenes are often used as electron-accepting systems. These devices are most commonly generated via vacuum deposition to create bilayer and tandem architectures. Recently, solutionprocessed small-molecule systems have been developed. Polymer-based OPV cells use long-chained molecular systems for the electrondonating material (e.g., P3HT, MDMO-PPV), along with derivatized fullerenes as the electron-accepting system (e.g., PC60BM, PC70BM). Like small-molecule OPV cells, these systems have small exciton diffusion lengths. However, this limitation is circumvented by a high interface surface area within the active device (Gao et al., 2008; Lijima, 1991).

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7.2.2 Commonly used organic sensitizers in organic photovoltaics In the past two decades, the DSSC has attracted remarkable attention as one of the most promising technologies for cost-effective solar energy exploitation. Recently, much research has been directed toward the application of metal-free organic dyes in DSSCs due to low cost and facile dye synthesis (Wu et al., 2010). Many organic dyes, inclusive of indoline (Hosseinnezhad et al., 2014; Hosseinnezhad, 2016; Ito et al., 2008), polyene (Hara et al., 2003), benzothiazolium (Wang et al., 2000), carbazole (Z.S. Wang et al., 2008), based and dyes (Khazraji et al., 1999) (Fig. 7.4).

7.3

Metal oxide nanomaterials

Metal oxides, because of their electronic structure differences, exhibit metallic, semiconductor or insulator characteristics. The preparation of these materials through the novel synthesis procedures can be described as physical and chemical methods. In general, two approaches have been used for the synthesis of these metal oxide nanostructures, top-down and bottom-up fabrication techniques. These approaches involve liquid-solid or gas-solid transformations. Metal oxide

Figure 7.4 Structures of dyes, which are commonly used in organic-based photovoltaics system.

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nanostructures among the most versatile groups of semiconductor nanostructures stand out as one of the most common, most diverse, and most probably the richest class of materials due to their extensive structural, physical, and chemical properties and functionalities. In recent times metal oxides have been at the heart of many dramatic advances in materials science. These materials display the most fascinating and widest range of properties. The unique and tuneable properties of these metal oxides such as optical, optoelectronic, magnetic, electrical, mechanical, thermal, catalytic, photochemical, etc. made them excellent candidates for various high-level technological applications. For instance, fuel cells, secondary battery materials, ceramics, chemical sensors, gas sensors, and biosensors, solar cells, alkaline, and lithium-ion batteries, pyroelectric, piezoelectric, ferroelectric, magnetic, actuator, supercapacitors, optical devices, lasers, waveguides, IR and solar absorbers, gate dielectric, dielectrics in dynamics random access memories, High TC superconductivity, decoupling capacitors, magneto-resistance and so on (Maduraiveeran et al., 2019; Rao et al., 2003). Hence metal oxide nanostructure materials have been actively studied in a broader perspective by the researchers. Therefore it is essential to explore its understanding in great detail in terms of its synthesis, properties, and applications (Khan et al., 2015) (Fig. 7.5).

7.4

Properties of nanomaterials

Materials Science is a multidisciplinary field that connects material properties to the material’s chemical composition, microstructure, and crystal structure. Moreover, it is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. Materials are often organized into groups based on their physical, chemical, and mechanical properties.

Figure 7.5 Various applications of metal oxides. Source: From Khan, M. M., Adil, S. F., & Al-Mayouf, A. (2015). Metal oxides as photocatalysts. Journal of Saudi Chemical Society, 19(5), 462
464. https://doi.org/10.1016/j. jscs.2015.04.003.

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They are classified as metals, plastics, and ceramics and typically have completely different properties, and the technologies involved in their production are fundamentally different. Materials technology is a constantly evolving discipline and new materials with interesting properties lead to new applications. For example, the combination of different materials into composites gives rise to entirely new material properties. Materials Science is closely related to materials technology. The basis of materials science involves studying the structure of materials and relating them to their properties. It evolved from the early 1960s, as it was recognized to create, discover and design new materials and had to approach it in an integrated fashion. Thus, materials science and engineering emerged at the intersection of various fields such as chemical engineering, mechanical engineering, electrical engineering, metallurgy, solid-state physics, and chemistry. The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level but also on a clear understanding of the properties of materials and the fundamental science behind these properties. In this process, researchers are searching for newer materials of greater strength, lightness, safety, reliability, cheapness, corrosion, heat resistance, etc. Semiconductor and metallic nanomaterials and nanocomposites possess interesting linear absorption, photoluminescence emission, and nonlinear optical properties. Nanomaterials having small particle sizes exhibit enhanced optical emission as well as nonlinear optical properties due to the quantum confinement effect. Synthesis, characterization, and measurement of optical properties of nanomaterials with different anisotropic shapes have also drawn significant attention. Recently, a lot of research focuses have been given on the preparation of polymer semiconductors and other nanocomposite materials having potential applications in different optoelectronics and photonics devices (Kumbhakar et al., 2014). The transparent conductive electrode (TCE) is one of the most important elements in modern optoelectronic devices, from various kinds of displays to solar cells. ITO is usually used as the TCE in these applications. However, the fragile nature of ITO limits its application in some conditions. ITO is also limited by scarcity and thus increasing the cost of indium. Therefore there has been a strong quest for replacing ITO with other flexible and cost-effective materials. CNTs and graphene are both intensively pursued TCE applications (Sima et al., 2010). As a relative of CNTs, graphene has the advantages of low cost and easy processing for thin-film electrode fabrication. The increasing demand for clean energy makes PV conversion an intensively studied field. Graphene can play several different roles in PV devices, including the above-mentioned transparent electrodes, active materials in solar cells (charge separator and transporter in bulk junction polymer solar cells), and DSSCs, and counter electrodes replacing platinum in DSSCs. Some opto-electrically improved Yb:CdO films also be used for fabrication (Desai, 2018; X. Wang et al., 2008). Zinc oxide has attracted significant research interest due to its enormous potential for application in a variety of optoelectronic and electronic devices. The main advantages of ZnO for optoelectronic applications are its large exciton binding energy (60 meV), wide bandgap energy of 3.2 eV at room temperature, and the existence of well-developed bulk and epitaxial growth processes (Wang, 2004).

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Nanoscience and nanotechnology have created various nanostructures in forms such as nanotubes (Baughman et al., 2002; Hirsch-Kreinsen, 2008), NWs (Wang, 2008; Wang et al., 2018), nanobelts (Pan, 2001; Wang, 2008), nanoparticles (Jezequel et al., 1994; Panigrahi et al., 2004), and assembling thereof like oxide aggregates (Zhou & Antonietti, 2003). A predominant feature of the nanostructures is that the size of their basic units is on a nanometer scale (10 2 9 m). That is A nanostructure is any structure with one or more dimensions measuring in the nanometer range. A nanostructure should have a characteristic dimension lying between 1 and 100 nm. The structural, optical, and electronic properties of intrinsic bulk semiconductors are not expected to change with mere changes in size or shape. that is In a bulk crystal, the properties of the material are size-independent and are only chemical composition dependent. Nanostructured materials/films have received broad attention due to their distinguished performance in electronics, optics, and photonics due to their peculiar size-dependent properties. Novel properties are inherent to nanosized/nanostructured systems or films due to the reduction in dimensionality or when the size of nanoparticles decreases below intrinsic length scales such as the Fermi wavelength of electrons, the ferromagnetic exchange length, etc. (Suganthi & Rajan, 2017). When the particle size of the film material decreases, the volume fraction of atoms at surfaces or in interfaces largely increases. Owing to the enhanced surface-to-volume ratio in nanostructures, their properties may depend sensitively on their surface conditions and geometrical configurations. Structural and geometric factors play an important role in determining the various attributes of nanostructures and lead to a modification of the thermal, chemical, electrical, optical, magnetic properties, etc., compared to their coarsegrained counterparts. Specific physical and chemical properties of structured materials include size, shape, surface area, surface porosity, roughness, morphology, crystalline, solubility, chemical composition, surface chemistry, reactivity, etc. Nanostructures are of both basic and practical interest since their physicochemical properties can be tailored by controlling their size and shape at the nanoscale, leading to improved and/or novel applications. Films constructed with nanostructures are highly porous. The films comprised of nanoparticles can give a high specific surface area. Nanostructured films with well-aligned NRs or NWs or nanotubes may exhibit much larger surface areas than films prepared from randomly oriented nanoparticles. These NWs, NRs, and nanotubes can provide direct pathways for electron transport. Moreover, when these NRs are packed very densely, they enable fast and effective transport of electrons. Three-dimensional nanostructures such as metal oxide aggregates assembled with nanoparticles or other nanomaterials possess a highly porous structure and are able to generate effective scattering to light in the solar spectrum. Imperfections, size dispersion, shape dispersion, defects, residual stresses, impurities, etc., in the material are known to influence the properties of nanostructured materials. The chemical properties of nanostructured films are generally surface-dominated. The surface of nanostructured materials plays an important role in the fundamental properties. The surface atoms are chemically more active than that the bulk crystal. This is because they usually have fewer adjacent coordinate atoms and unsaturated sites. For example, oxygen vacancies on metal-

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oxide surfaces are chemically and electrically active. These vacancies function as n-type donors; often significantly increase the conductivity of oxide. Upon adsorption of charge accepting molecules at the vacancy sites such as NO2 and O2, electrons are Effectively depleted from the CB leading to a reduced conductivity of the n-type oxide (Sahana et al., 2012). On the other hand, molecules, such as CO and H2 would react with surface adsorbed oxygen and consequently remove it, leading to an increase of conductivity. The nanometer feature sizes of nanomaterials also have a spatial confinement effect on the materials, which bring the quantum confinement effects. The electronic structure of the material is altered from continuous electronic bands to discrete or quantized electronic levels. Accordingly, the continuous optical transition between the electronic bands becomes discrete. Quantum confinement varies the optical properties of the materials. Quantum confinement effects in semiconducting nanomaterials increase the bandgap energy relative to bulk materials. For films associated with larger nanoparticles, the electron excitation across the bandgap depends only on the energy difference between the VB and CB. The quantum confinement effect begins to influence the excitation energy across the bandgap especially when the size is of the order of the Bohr radius of the material (Y. Li et al., 2016; Zhu et al., 2006).

7.5

Representative metal oxides used in organic photovoltaics

Inorganic materials and in particular metal oxides are becoming an increasingly critical constituent of OPV devices (Chang et al., 2017). There are several distinct functions of metal oxides in OPV devices. The first and most common application exploits highly doped wide bandgap materials, which result in transparent conducting oxides (TCO) for transparent electrodes in OPV and other PV technologies. The second is the use of oxide as a selective hole/electron transport layer (HTL/ETL). This application is compelling for improved charge extraction and/or device lifetime in bulk heterojunction OPV devices (Irwin et al., 2008). In this case, using inorganic HTLs may also enable the use of high-performance TCO materials that might otherwise be chemically incompatible with the PEDOT:PSS or other organic HTL device architecture. The third function uses intrinsic or lightly doped metal oxide materials that serve as the acceptor in h-OPV devices. This paper discusses efforts to exploit metal oxides in both the second (HTL/ETL) and third (h-OPV) categories to understand/improve organic-based PV device performance (Fig. 7.6). The stability and efficiency of an organic solar cell are important aspects of the device to perform at its best. Hence, it is important to have a compatible energy level of the interfacial layer and successful morphological modification. The efficiency of the cell, the hole/electron mobility, and its resistance to humidity must be aligned to slow down the process of photodegradation as to obtain a long-term stable device. The use of poly(3-hexylthiophene) (P3HT) in various studies has been reported, owing to its cost-effectiveness and high hole mobility

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Figure 7.6 (A) TEM image of ZnO nanoparticles on a carbon-coated copper grid (20 nm), (B) Structure of MDMO-PPV conjugated polymer. Source: From Schulz, P., Edri, E., Kirmayer, S., Hodes, G., Cahen, D., & Kahn, A. (2014). Interface energetics in organo-metal halide perovskite-based photovoltaic cells. Energy and Environmental Science, 7(4), 1377
1381. https://doi.org/10.1039/c4ee00168k.

notwithstanding its HOMO energy level, which hinders the maximum effectiveness of the device (Beek et al., 2004, 2005; Maake et al., 2020; Olson et al., 2006; Schulz et al., 2014). Jeong et al. substantiated the stability problem of solar cells using the deep HOMO energy level of fluorinated polythiophene derivative (FEH) that displays additional effective charge-extraction capability at the interface of the perovskite/HTL in comparison to P3HT (Jeong et al., 2019; Kadem et al., 2018; Reeja-Jayan & Manthiram, 2010; Soultati et al., 2019; Yang et al., 2019) (Fig. 7.7).

7.6

Metal oxides based organic photovoltaic studies

7.6.1 Photovoltaic devices applications of nanomaterials Owing to increased energy demands and depleting fossil fuels, renewable energy approaches such as solar cells have become increasingly popular. Briefly, the solar cell is an electrical device that can convert electromagnetic radiation into electric energy. This device contains semiconducting material and works based on the PV effect. The classes of solar cells are OPVs, DSSCs, quantum-dot solar cells, oxidebased solar cells (OSCs), and perovskite solar cells (PSC). Since many metal oxides contain excellent semiconducting properties, they have been used extensively in this field (Shaikh et al., 2018). The typical all-oxide solar cell configurations are depicted in an earlier study (Pe´rez-Toma´s et al., 2018).

7.6.1.1 Organic photovoltaics OPV cells are currently attracting a great deal of scientific and economic interest and are playing a crucial role as one of the leading emergent PV technologies for low-cost power production. Recent researches have unanimously proven the promise of this young PV technology in terms of their lightweight, mechanically

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Figure 7.7 (A) Compound structures and (B) Illustrated energy level diagrams of the hole transport layer. Source: From Jeong, I., Jo, J. W., Bae, S., Son, H. J., & Ko, M. J. (2019). A fluorinated polythiophene hole-transport material for efficient and stable perovskite solar cells. Dyes and Pigments, 164, 1
6. https://doi.org/10.1016/j.dyepig.2019.01.002.

flexibility, ease of processing and low cost. In terms of performance, organic solar cells have witnessed a rapid increase in power conversion efficiency (PCE)-driven primarily by materials development, physical understanding, and device optimization. Although the highest reported PCEs have not yet reached those of their inorganic counterparts, the perspective of cheap production, as well as the perpetual improvements in device stability, drives the development of organic solar cells further in a dynamic way (Arbouch et al., 2014). OPV technology is still a relatively new type of thin-film solar cell in the PV industry, in terms of performance, stability, and maturity. It involves photoactive organic layers, comprised of chains and bucky-balls of p-type and n-type semiconducting polymers and oligomer materials. In such polymeric OPV materials, the energy levels of the HOMOs and LUMOs are analogous to the respective VBs and CBs of conventional inorganic semiconductors. OSCs are thus selected, characterized, and tailored (through different chemical syntheses) based on the energy gaps between LUMO and HOMO levels. The representative examples of the commonly used organic donors (p-type polymers) include poly(p-phenylenevinylene) (PPV), poly(2-methoxy-5-(2-ethylhexyloxy)-1
4-phenylenevinylene) (MEH-PPV), and poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)]-p-1
4-phenylenevinylene (MDMOPPV). Others are poly(p-phenylene benzobisthiazole) and poly(3-hexylthiophene) (P3HT) (Rwenyagila, 2017).

7.6.2 Titanium dioxide Organic solar cells (OSCs) with inverted structures have attracted much attention in recent years because of their improved device air stability due to the use of stable materials for electrodes and interface layers. TiO2 is widely used in PV fields like DSSCs, quantum dot-sensitized solar cells, and so on because of its proper forbidden bandwidth, good optical and chemical stability, nontoxicity, corrosion

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resistance, and simple manufacturing process, etc. Recently, inorganic-organic hybrid perovskite solar cell has attracted great attention as a new class of PV devices and its rapid development has led the PCE up to 20%. TiO2 nanomaterial appears to be a good candidate to be applied in it, which is usually used as a compact layer or skeleton layer in PSC. As an important part, its crystalline phase, particle size, morphology, preparation methods, film thickness, and coverage have a great influence on the performance of solar cells (Que et al., 2016).

7.6.3 Zinc oxide The rapidly increasing demand for energy together with more and more concerns about the environment forces us to seek sustainable energy resources and clean energy technologies. As a result, photovoltaics have received increased interest in recent years. To further lower the cost of solar cells, organic solar cells based on fully OSCs and hybrid solar cells based on a combination of organic and inorganic semiconductors have been intensively investigated as promising approaches to lowcost photovoltaics. Zinc oxide (ZnO) has received exceptional attention as a promising material for solar cell applications due to its nontoxicity, good stability, good electrical and optical properties, and low cost (Goo et al., 2018).

7.6.4 Molybdenum oxide In polymer PV cells, a poly-3,4-ethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) interlayer has been demonstrated to be effective to enhance both the short-circuit current as well as the open-circuit voltage (Zhang et al., 2002). In the organic light-emitting diodes (OLEDs), the insertion of a poly9,9-dioctylfluoreneco-N-4-butylphenyl-diphenylamine TFB interlayer between a PEDOT:PSS and the light-emitting polymer layer improves the device efficiency and stability (Kim et al., 2005). Recently, the molybdenum trioxide MoO3 interlayer between the ITO anode and the hole transporting layer has been used for small molecule OLEDs and improved hole injection and operational stability have been reported (F. Wang et al., 2008). More recently, transition metal oxide interlayer has also been used in OPV cells (Irwin et al., 2008).

7.6.5 Tin oxide Tin oxide (SnO2) is one of these oxides and last years, a growing interest has been devoted to tin oxide thin films. Owing to its high chemical stability, as well as optical magnetic and electrical properties, SnO2 has a wide range of applications such as films for electrochromic devices, OLEDs, n
p junction electrodes in DSSCs. As shown recently SnO2 appears to be one of the more promising anodic electrochromic materials (Mohammed-Krarroubi et al., 2012). Morphology of SnO2 was reported as the particle in this paper appears (Fig. 7.8).

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Figure 7.8 SnO2 particle morphology appearing on films (A) small magnification and (B) large magnification. Source: From Mohammed-Krarroubi, A., Dahou, F. Z., Barkat, L., Bernede, J. C., & Khelil, A. (2012). Realization of tin oxide like anode for the manufacture of the organic solar cells. EPJ Web of Conferences, 29. https://doi.org/10.1051/epjconf/20122900029.

7.6.6 Tungsten oxide Lee et al. investigated the effect of tungsten oxide (WO3) interlayer as a whole collection layer on the performance of OPV cells according to the thickness and temperature of the interlayer. The characteristics of OPV cells such as fill factor, current density, and open-circuit voltage are continuously improved by increasing the temperature of the WO3 interlayer. The surface of a treated WO3 film promotes the crystallization of P3HT because a treated WO3 film is more h09ydrophobic than a pristine WO3 film. Furthermore, the energy barrier between P3HT and the WO3 interlayer is minimized since the work function of the WO3 film after annealing progressively increases until a hole can be smoothly transferred. Therefore OPV cells using an interlayer of treated WO3 film have higher hole mobility and better efficiency (Lee et al., 2013). This study revealed that WO3 based cells have significant shunt resistance which contributes to a high fill factor and open-circuit voltage. This demonstrates potential for using WO3 in poly(stylenesulfonate) doped poly(3,4-ethylenedioxythiophene):(PEDOT:PSS)-free systems (Oh et al., 2013). The current density of the tungsten oxide is shown in Fig. 7.9.

7.6.7 Vanadium pentaoxide Vanadium pentoxide V2O5 films were fabricated using electrochemical deposition technique for application as hole transport buffer layer in organic solar cells. Thin and uniform V2O5 films were successfully deposited on indium tin oxide-coated glass substrate. The characterization of surface morphology and optical properties of the deposition suggests that the films are suitable for PV application. Organic solar cells fabricated using V2O5 as hole transport buffer layer showed better

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Figure 7.9 J
V characteristics of tungsten oxide. Source: From Oh, I. S., Kim, G. M., Han, S. H., & Oh, S. Y. (2013). PEDOT:PSS-free organic photovoltaic cells using tungsten oxides as buffer layer on anodes. Electronic Materials Letters, 9(4), 375
379. https://doi.org/10.1007/s13391-013-0003-7.

Figure 7.10 Schematic of (A) an energy level of organic solar cells-based V2O5 HTL and (B) device structure of oxide-based solar cells. Source: From Arbab, E. A. A., & Mola, G. T. (2016). V2O5 thin film deposition for application in organic solar cells. Applied Physics A: Materials Science and Processing, 122 (4). https://doi.org/10.1007/s00339-016-9966-1.

devices performance and environmental stability than those devices fabricated with PEDOT:PSS (Fig. 7.10). In an ambient device preparation condition, the PCE increases by nearly 80% compared with PEDOT:PSS-based devices. The device’s lifetime using the V2O5 buffer layer has improved by a factor of 10 over those devices with PEDOT:PSS (Arbab & Mola, 2016).

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255

Concluding summary and future prospective

In this chapter, we described various inorganic, organic, and composite organic
inorganic complexes. Further, we explored their PV fabrications and PCEs. We also outlined the crystalline nature of other derivatives as compared with silicon, which makes these devices too costly to compete with inexpensive fossil fuelbased electricity. Nevertheless, various organic and polymer photovoltaics have been outlined for the generation power sources. We also explain, how these lowcost, lightweight, and mechanically flexible devices offer unique advantages and opportunities currently unavailable with crystalline silicon technology. Further, organic hybrid PV materials consisting of a conjugated polymer as an electron donor and a nanocrystalline metal oxide as the electron acceptor showed to be the attractive class of semiconducting materials. Finally, we explored a wide range of properties that offer both optically and electronically, the many different roles that transition metal oxides can play within a “hybrid” OPV device, and applications of metal oxide
OPV devices. In the future, not only inorganic or organic materials play a major role in PV fabrications to achieve good PCE but the combination of both will play a major role to enhance PCE with good field factor along with the absorption spectrum and their intensity of the incident sunlight as well as the temperature of the solar cell. The scientists need to develop a system having a large bandgap between HOMO and LUMO to have better charge separation for the development of excellent PCE in future work.

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Prakash S. Pawar1,2, Pramod A. Koyale1, Ananta G. Dhodamani1,3 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, Shri Yashwantaro Patil Science College Solankur, Kolhapur, Maharashtra, India, 3Department of Chemistry, Rajarshi Chhatrapati Shahu College, Kolhapur, Maharashtra, India

8.1

Introduction

Among the various renewable sources, solar energy has the potential to replace the existing nonrenewable sources as well as to resolve the energy crisis so as to meet the energy needs of humans. This is due to its various features such as giant availability, environmental benignness, cost effectiveness, and easy availability (Gielen et al., 2019), etc. With invention of first solar cell by American Scientist Charles Fritts in 1883 onwards, investigators continuously progressing in terms of designing the different materials, device architectures and assembly so as to reach the nextgeneration performance, as well as low-cost devices (Sharma et al., 2021). For the last few decades, the solar cell technology has grown as an extraordinary source of renewable energy to meet the global energy needs. Generally, the solar cells are categorized into three different generations, depending on the type of materials deployed for their fabrication. The first generation is highly matured solar cell technology and hence it is dominated in the market. The first-generation photovoltaic devices are fabricated either by crystalline silicon (c-Si) or Gallium Arsenide (GaAs) wafers. The c-Si-based photovoltaic devices are commercialized well than that of GaAs (Płaczek-Popko, 2017), which is due to higher efficiency and toxic nature of gallium and arsenic. However, the fabrication process for making silicon wafers is tedious, highly expensive, and hence higher overall cost of such devices, which limit the excessive use of silicon-based solar devices. To overcome the laggings of first-generation solar devices, scientists have been focusing on the development of the materials under second-generation solar devices (Sharma & Chaujar, 2020). Secondgeneration that is thin-film solar devices, fabricated by depositing one or more thin layers of the materials such as copper indium gallium diselenide (CIGS), cadmium telluride (CdTe), and amorphous thin-film silicon (a-Si), etc. on the substrates viz. Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00018-X © 2022 Elsevier Inc. All rights reserved.

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glass, plastic, and metals, etc. (Enayati et al., 2018). The advantages of this second-generation device are easy thin-film deposition, cheap starting materials, lower amount of the materials, thickness dependent properties controls, etc.; however, the efficiency is somewhat lower than the first-generation solar devices. The reported laboratory efficiency of CIGS and CdTe-based devices is around 20% (Lee & Ebong, 2017). Though the devices under second-generation are cheaper, but less efficient than conventional c-Si as well as having less film lifetime (below 20 years), are the major constraints and hence their commercialization share is less than 20% worldwide in the last three decades (Kim, Lee, et al., 2018). In addition, according to Schokley-Quiseer limit, the another major constraint for these first-generation and second-generation solar cells (single-junction) is in terms of their theoretical efficiency; which would be reached up to 33.7% under the standard conditions (unconcentrated, AM 1.5 solar spectrum, 1.34 eV optical band, 1000 W m22) (Ru¨hle, 2016). To overcome these constraints of first and second-generation solar cells, researchers have been focused toward the new that is third generation which is also known as emerging photovoltaic or hybrid solar cells. These devices bands the possessions such as intermediate band positions, photon up-conversion, hot electron capture, multiple exciton generation, thermal photon up-conversion, easy fabrication, etc. (Kumar et al., 2017). Fig. 8.1 demonstrates the comparison between all generation solar cells with respect to their cost as well as efficiency which displays the performance over Shockley-Queisser limit. Therefore third-generation solar cells have been elevated as effective technology for solar to electrical conversion. These emerging devices further comprise dye-sensitized solar cells (DSSCs), quantum dot-sensitized solar cells (QDSSCs), dye-quantum dot-sensitized, dyadsensitized, pervoskite as well as organic solar cells (Yan & Saunders, 2014). Since

Figure 8.1 Illustration of efficiency versus cost for first-, second-, and third-generation solar cells (Ojaj¨arvi et al., 2011).

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the research related to the third-generation solar cells among research community enhanced attractively, which reflects the dominance over the development of third-generation solar cells. In connection with these facts, the present chapter includes the nanocrystalline metal oxide-based hybrid systems with the strategies for tailoring their properties for photovoltaic applications. Also, the efforts are to be made for demonstrating the device fabrication and detailed plasiuble mechanism for DSSCs, QDSSCs, pervoskite as well as organic solar cells. Further this chapter also demostarts the present state of the art of development of such emerging photovoltaic devices.

8.2

Modifications of metal oxides

Semiconducting metal oxides (MxOy) are the largest group of materials used in photovoltaic devices. However, the bare metal oxide-based solar devices have limited performance in energy harvesting. This is due to having wider optical bandgap lead coverage of UV region of solar spectrum as well as fast electron
hole charge recombination rate under photoirradiation. Hence, literature studies reveal that there is need tune the optoelectrical, structural, morphological properties of bare metal oxides using the different strategies (Moia & Maier, 2021). Doping, metal-supported, composites with other materials are the most viable protocols for modifying metal oxides to enhance the solar device performance further. All these different approaches are categorized into doped MxOy, metal-supported MxOy, MxOy-metal oxide hybrids, and MxOy based composites as shown in Fig. 8.2.

8.2.1 Doped MxOy Doping is a unique and effective strategy to modify the properties of metal oxides, where the dopant replaces the sites (few or completely) of cations or anions of host

Figure 8.2 Different strategies for modifying the metal oxide hybrids.

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metal oxides lattice. Therefore, the doping may result the changes in cation or anion vacancies, structural defects, tuning of optical properties, electrical properties, etc. In doping process, usually metal or non-metal ions are used as dopant in the metal oxide host lattice. The metal ions belong to transition metal, alkali metals, alkaline earth metals and other metals aluminum, gallium, indium, thallium, tin, lead, bismuth, polonium, etc. are used commonly. In this connection, lithium doped mesoporous TiO2 was used as electron transfer layer in the perovskite solar cells with remarkable improvement in electrical conductivity, electron mobility, etc. and hence substantial enhancement in parameters such as short circuit density (Isc), open-circuit voltage (VOC), fill factor (FF), efficiency of pervoskite solar cells observed (Peter Amalathas et al., 2019). The transition metals as dopant are widely used in doping to the metal oxide host lattice due to their more resemblance in the physical as well as chemical properties suitable for doping; which further modulating the band structure alignment, light absorption property, stability and efficiency (Latini et al., 2013). For example, the scandium (0.0 to 1.0 at.%) doped in anatase TiO2 reported the efficiency of 9.6% at 0.2 at.% dopant concentration; which was higher to that of anatase TiO2 (6.7%). Similarly, gallium doped copper oxide hybrids (CuGaO2) p-type transparent conducting oxides were used as hole transporting layer for organic photovoltaics showing the power conversion efficiency of 3.2% to that of bare CuO and bare GaOx metal oxides (Wang et al., 2015). In case of non-metal doped, investigators reported the narrowing of the bandgap, generating impurity energy levels or density of states between valence band and conduction band, and shifting of absorption edge of doped metal oxides than the bare metal oxides. For example, the nanocrystalline nitrogen 2p states hybrid doped TiO2 electrodes were investigated by XPS and reported the overall power conversion efficiency up to 8% for doped materials than undoped materials (Ma et al., 2005).

8.2.2 Metal-supported MxOy In this strategy, the desired metal is supported on the surfaces of metal oxides which is different than that of doping. Especially, the nanodimensions metal oxides are highly energetic due to their high surface energy and hence which may have lower stability. Therefore, through this strategy, one can not only improve stability but also tune other physicochemical properties of supported metal oxides as well. In this category, noble metals (such as Ag, Au, Pt Pd, etc.) are commonly used with the representative metal oxides (such as TiO2, ZnO, Cu2O, etc.) in solar photovoltaics. The overall architecture of nanocomposites is one of the most important factors for getting the desired physical properties such as structure, composition, particle size and other optoelectrical properties such as energy bandgap level, electrical conductivity, charge carrier concentration and transport, etc. For making the metal-supported metal oxides materials, broadly ex-situ and in-situ approaches are used. The commonly used method to synthesize noble metal nanoparticles is chemical reduction method. Metal-supported metal oxides in nanoscale dimension are divided into five categories according to their geometrical configuration such as: (1) noble metal-decorated metal oxide nanoarrays, (2) noble metal-decorated metal

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oxide nanoparticles; (3) noble metal/metal oxide core/shell nanostructures, (4) Janus noble metal-metal oxide nanostructures (5) noble metal/metal oxide yolk/shell nanostructures.

8.2.3 Metal oxide
metal oxide hybrids (MxOy
AmOn) To overcome the laggings of the bare MxOy, hybrids of metal oxide (MxOy) with another metal oxides (AmOn) have been prepared by the investigators, which are also called as mixed metal oxides. Due to the proper band alignments, high surface area, effective light scattering abilities, fast charge collection, electron transportation abilities, etc., MxOy
AmOn materials have been deployed as efficient photoanodes in the photovoltaic devices. One of the research groups discussed the study of ZnO/SnO2 composite as a double electron transport layer in perovskite solar cell; which was exhibited the high open-circuit voltage (VOC) of 1.15 V with the power conversion efficiencies (PCE) of 19.1% (Wang et al., 2018). It was observed that, the PCE for ZnO/SnO2 composite showed the higher performance than that of bare SnO2-based devices [open-circuit voltage (VOC) of 1.07 V and PCE of 18.0%] for perovskite solar cell. Furthermore, Gao et al., established SnO2/TiO2 composites for the improvement in power conversion efficiency of DSSCs. In this research investigation, it was perceived 3.53% power conversion efficiency of fabricated DSSCs with higher electron recombination lifetime than TiO2 nanotubes and ZnO nanowires (Low & Lai, 2018).

8.2.4 Other additives or Supportive materials To improve the optoelectronic properties of bare metal oxides, various additives such as carbon nanostructures (graphene, carbon nanotubes), polymers, chalcogenides, etc. have been used.

8.2.4.1 Graphene
metal oxide hybrids Composites of metal oxide with graphene-based additives are promising hybrids in solar energy harvesting due to highly conducting nature of graphene. In connection to graphene, it consists of honeycomb layer/s of hexagonal ring carbon atoms, and having high electron mobility than silicon (Pallecchi et al., 2014), as well as the electrical conductivity 13 times better than copper. It absorbs only 2.3% reflecting light, have the maximum surface-area-to-volume ratio, tunable bandgap, and fluorescence quenching capability by electron or energy allocation. Furthermore, due to the synergistic effects between the metal oxides and graphene, graphene decorated metal oxides materials exhibited remarkable properties. The growth of graphenebased metal oxide nanocomposite, as well as graphene decorated metal oxides nanoparticles, representing a significant role for the application of energy conversion devices in the third generation. In connection with this, the ZnO/graphene nanocomposite used in Quantum dot solid-state solar cells for enhancing 75% open-circuit voltage (0.8 V) than the ZnO nanowires in bare ZnO (0.1 V)

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(Dutta et al., 2012). In case of pervoskite solar cell, TiO2/rGO nanocomposite was demonstrated by one of the research groups to acquire the power conversion efficiency up to 15.3% in an average compared to 13.8% with a pure TiO2 electrode in pervoskite solar cell (Belchi et al., 2019). In connection with this, many research endvevours are focusing on graphene
metal oxides composites to get heightened recital in solar energy harvesting.

8.2.4.2 Carbon nanotube 2 metal oxide nanocomposites Carbon nanotubes (CNTs) are one of the most suitable nanomaterials for making the composites of metal oxides. CNTs are graphene nanosheets that rolled up and capped with a Buckyball hemisphere. Sumio Iijima first synthesiszed carbon nanotubes in 1991 while researching fullerene synthesis using an electric arc discharge technique. These are usually in 1D dimension and having extrao-ordinary properties making them peculiar, impressive materials. These are existed in the form of single-walled or multi-walled CNTs (Koli et al., 2017). The synergistics between CNTs and metal oxides result the new class of composites having properties differ to that of individuals. Hence, CNTs have been utilized for many application in energy, medical sectors (Delekar et al., 2020; Koli et al., 2016; Schnorr & Swager, 2011). And also in similar manner, the CNTs are singing vital role in the metal oxide-based composites for the third-generation solar cells to enhance the photovoltaic performance of the device. In connection with this, our research group demonstrated TiO2/MWCNTS composites for DSSCs and reported the photovoltaic efficiency of 6.21%; which is higher than that of bare TiO2 nanoparticles. This is attributed to decrease in optical bandgap from 3.2 to 2.85 eV as well as increase in optical absorption toward the red shift region in the electromagnetic spectrum at the optimum composition of TiO2/MWCNTS composites (Delekar et al., 2018).

8.2.4.3 Polymer 2 metal oxide hybrids Polymers are also used as one of the important component in the metal oxide-based composites needed for device fabrication due to their diversified properties. Such composites of metal oxides with polymers have been utilized in the different solar energy technologies. Among these, in DSSCs, polymers (bare or mixed) are used as flexible substrates, while composites of polymers with semiconducting metal oxides are actively used as photoanode materials due to having high porosity, uniformity, stability and film-adherence as well. In addition, their composites are also used as platinum-free counter electrodes, as well as the frameworks of quasi-solid-state electrolytes. In case of perovskite solar devices, polymers are used as the additives to adjust the nucleation and crystallization processes in perovskite films. The polymers with metal oxides hybrids are used as electron transfer materials, or hole transfer materials, and interface layer to enhance the carrier separation efficiency for reducing the recombination. In organic solar devices, polymers are often used as donor layers, buffer layers, in binary or ternary systems to enhance device performances. Some examples of polymer such as polypyrrole (PPy), polyaniline (PANI),

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polythiophene (PTh), polypyrrolidine, etc. are composed with metal oxide for the better performance in energy technologies (Hou et al., 2019). The hybrid photovoltaic device based on conjugated polymer paired with metal oxides nanoparticle or films have attained the external quantum efficiencies of over 40% (Ye et al., 2015); which was higher to that of bare metal oxide-based devices.

8.2.4.4 Chalcogenides 2 metal oxide hybrids The 16th group elements are known as chalcogen (like oxygen, sulfur, selenium, tellurium and polonium). The term chalcogenides is a chemical compound consisting of at least one chalcogen anion except for oxygen and at least one electropositive element (metals). ZnS, CdS, CdSe, CdTe, ZnSe, MoSe2, WSe2, etc. are the commonly examples of metal chalcogenides (Dhodamani et al., 2018). These materials remarkable properties such as, high thermal, aqueous stability as well as optoelectronic properties (Swarnkar et al., 2019). Due to these, many researchers have been focused on the chalcogenides as one of materials for making the composites with metal oxides. Our research group have reported the TiO2/Cds composite based QDSSC, was achieved the power conversion efficiency up to 2.37% with Open-circuit voltage (VOC 5 0.550 V), short current density (JSC 5 10.31 mAcm22) and fill factor (FF 5 42).

8.3

Emerging strategies of third-generation solar cell technologies

Various emerging strategies such DSSCs, QDSSCs, organic, polymer, tandem and perovskite solar cells have been deployed in the third-generation solar cell technologies for boosting the power conversion efficiency. All these technologies are highlighted as below:

8.3.1 Dye-sensitized solar cells Dye-sensitized solar cells (DSSCs) are promising, low-cost photovoltaic device which was first proposed by O’Regan and Gr¨atzel in 1991. These scientists were fabricated the DSSCs device by using the semiconducting nanocrystalline TiO2 as a photoanode sensitized with trimeric ruthenium complex RuL2 [(μ-CN)Ru((CN)L2)2 (L 5 2,20 - bipyridine-4,40 -dicarboxylic acid; L0 5 2,20 -bipyridine)] and it was coupled with platinum coated FTO (Pt-FTO) as a counter electrode with I2/I32 as redox electrolyte. Initially, the fabricated device showed the power conversion efficiency of B7.0%. After the Gr¨atzel’s endeavors of DSSCs, it was realized that DSSCs were fabricated mainly with four major components viz. semiconducting photoanode (coated on substrate), sensitizers (dyes), counter electrode and redox electrolytes. Due to reasonably amazing initial efficiency of DSSCs, worldwide investigators have been focused more about the different aspects of DSSCs. Among

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these four DSSCs components, semiconducting photoanodes play pivotal role for enhancing the different parameters of the photovoltaic devices with power conversion efficiency. Basically, optical bandgap, optical coverage, stability in presence of photon/temperature, dye interaction with the photoelectrode materials and charge carrier formations with separations are main aspects of photoelectrodes in DSSCs. TiO2, ZnO, SnO2, Nb2O5, etc. based-materials are showing the excellent power conversion efficiency in such strategy to that of other semiconducting materials (Yeoh & Chan, 2017). Our research group demonstrated the photovoltaic properties of the TiO2@RGO nanocomposites with varying the content of RGO from 0.1 to 1.0 wt.% with Ru(II) based dyes viz. N719 and N3. Among the different nanocomposites, photovoltaic performance of the TiO2/RGO (with 0.75 wt.% of RGO) showed the highest power conversion efficiency of 5.98% for N719 dye. The highest conversion efficiency of TiO2/RGO directly correlated with higher surface area, lower optical energy bandgap, more average electron life time, lower charge transfer resistance, lesser charge recombination rate to that of host TiO2 and other nanocomposites (Dhodamani et al., 2020). Furthermore, our group demonstrated the optoelectronic modifications of the TiO2 host lattice through insertion of Cr(III) (0.5
3.0 mol.%) as a dopant and thereafter its composites with MWCNTs with earlier optimized 0.1 wt.% of MWCNTs for making the ternary nanostructures for Ru(II) based DSSCs. The charge transportation properties of the desired binary (TiO2/MWCNTs) and ternary (Cr@TiO2/MWCNTs) hybrids are shown in Fig. 8.3. Among the different ternary hybrids, Cr@TiO2/MWCNTs with 1.0 mol.% of Cr dopant with TiO2/MWCNTs based DSSCs showed highest photovoltaic conversion efficiency up to η 5 7.69% which is 20% (η 5 6.18%) higher to that of undoped

Figure 8.3 Schematic of the plausible charge transfer mechanism of the (A) binary hybrids of TiO2/MWCNTs and (B) ternary hybrids of Cr@TiO2/MWCNTs sensitized with N719 dye under irradiation of light (Delekar et al., 2018).

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TiO2/MWCNTs based DSSCs. The observed power conversion efficiency well supported with the photophysicochemical properties of the hybrids studied through different spectroscopic measurements (Shinde et al., 2020). By considering such vigorous research events, it is conceivable to develop an well-organized solar to electrical conversion using DSSCs.

8.3.2 Quantum dot-sensitized solar cells Quantum dot-sensitized solar cells have been emerging as promising devices in the third-generation solar photovoltaics, due to the distinct physical, optoelectronic properties of quantum dots viz. tunable absorption range, multiple exciton generation, high absorption coefficient, etc. In addition, the quantum dots as sensitizers are prepared easily through facile solution protocol and their further designing in other components is easier one. Therefore, presently investigators have been working on the quantum dots as a light harvesting materials in the photovoltaic devices and reached up to 13.0% efficiency; which is comparable to other kinds of leading emerging photovoltaic devices. The exploration of new quantum dots as a light harvesting materials and their interconnectivity plays pivotal role for fast improvement of the photovoltaic devices and have a great potential for the development of next generation photovoltaic devices (Dhodamani et al., 2018; Li et al., 2014; Lin et al., 2014; Niu et al., 2014). In the recent years, the anchoring of the quantum dots on the surface of semiconducting metal oxide-based hybrids have tremendous interest due to their tunable optoelectronic properties are highlighted as here. Shen et al. studied the synergistic effect of inorganic ZnS layer and N3 dye in the TiO2
CdS
ZnS
N3 hybrids for QDSSCs and recommended the enhancement of the photovoltaic performance due to suppression of the electron
holerecombination rate. Also, employed the cobalt complex based solid-state electrolytes rather than the conventional iodine based electrolytes for the stability of the devices. The Raman and Photoluminescence (PL) studies revealed that in the TiO2
CdS
ZnS
N3 hybrids not only the CdSQDs passivated by the inorganic ZnS layer and N3 dye molecule but also N3 dye acts as an efficient hole scavenger for the CdS-QDs due to the energetic alignment between the two sensitizers. The intermediary role of N3 dye as an extraction of hole from the CdS-QDs to the electrolyte showed significant improvement of the power conversion efficiency of CdS sensitized devices and thereafter ZnS deposition and co-sensitization of N3 dye. The observed power conversion efficiency of the TiO2
CdS
ZnS
N3 is higher to that of the sum of the single CdS and N3 sensitized photovoltaic devices. Cao et al. fabricated a high-efficiency QDSCs based on Ag nanoparticles decorated on TiO2/ZnO nanorod arrays (NAs). The enhancement of the power conversion efficiency of the devices not only due to the incorporation of Ag nanoparticles to the TiO2/ZnO NAs, but also due to the decrease surface charge recombination and prolong electron lifetime collectively contribute to improving the JSC of the CdS/CdSe QDs co-sensitized solar cells. The enhancement of the VOC due to the direct contact of Ag NPs with TiO2 NPs is undergoing Fermi level alignment, and it trigger an upward shift of more negative

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Figure 8.4 Schematic of the QDs/TiO2/Ag/ZnO NAs photoelectrode structure, charge separation and transportation in QDSSCs based on TiO2/Ag/ZnO NAs under light irradiation (Zhao, Huang, et al., 2016).

potential. The highest achieved power conversion efficiency of the QDSSCs with Ag NPs decorated TiO2/ZnO NAs photoelectrode reached up to ƞ 5 5.92%, which is about 22% higher efficiency for the solar cells fabricated without Ag NPs (ƞ 5 4.80%) (Zhao, Huang, et al., 2016). (See Fig. 8.4) Hence, the utilization of QDSSCs for solar to electrical conversion is providing effective path in solar energy harvesting throughout research community.

8.3.3 Organic solar cells Organic solar cells (OSCs) are the emerging photovoltaic devices in the thirdgeneration solar cell technologies and utilized the conductive organic polymers or small organic molecules for absorption of light in the broad region of the solar spectrum and for charge transportation purpose. It has attracted enormous attention due to their easy fabrication strategies, large-area production, flexibility, light weight and highly abounded row materials (Um et al., 2017; Zacher et al., 2013). The active layers of the cell are the organic polymers, small molecules, and its hybrids, which have recently proved greatest improvement in the power conversion efficiency relative to other photovoltaic devices. The highest power conversion efficiency was reported up to 11.7% from hydrocarbon solvents-based organic molecules with environmental benign fabrication strategies (Zhao, Li, et al., 2016). Zhao et al. studied the design and use of inorganic transparent metal oxides having wide bandgaps energies as well as electron and hole transport layers for highly efficient and stable organic solar cells. Among the metal oxides suggested SnO2 is the most favorable electron transport materials, and hence which has been utilized in the highly efficient organic-inorganic hybrid perovskite solar cells but it is rarely seen in the organic bulk heterojunction solar cells. Hence, author designed planar organic bulk heterojunction photovoltaic devices with device structure as ITO/SnO2/PeNWs/PBDB-TSF:IT-4F/MoO3/Ag. In which the incorporation of PeNWs at the interface of SnO2 nanoparticles and PBDB-TSF:IT-4F and it may effectively resolve the incompatibility between two different materials, without

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Figure 8.5 Schematic of the device structure of SnO2-based organic solar cells ITO/SnO2/ PeNWs/PBDB-TSF:IT-4F/MoO3/Ag and their J-V measurement characteristic curve reprinted with permission of ref. no. Hu et al. (2020).

changing the optoelectronic properties of the SnO2 nanoparticles. Optical and electrical studies revealed that the combination of PeNWs and PBDB-TSF:IT-4F with SnO2 gave improvements of electron
hole recombination, charge extraction, and its absorption abilities in the UV 2 visible region. The experimental results suggested the lifetime of the photovoltaic devices can sustain more than 80% power conversion efficiency after 20 days (Hu et al., 2020). The device design and their J-V measurement curves are shown in Fig. 8.5. Hence, organic solar cells provide the excellent path for solar to electrical conversion like DSSCs and QDSSCs.

8.3.4 Tandem solar cells For the coverage of the solar spectrum in the broad region or absorbance of light with different wavelengths, multi-junction solar cells or tandem solar cells (TSCs) have been developed by the investigators. The TSCs devices are fabricated through the incorporation of different semiconductor materials with multiple light absorbers of different bandgaps and each junction converts light into electrical energy in response to a different wavelength of light (Adebanjo et al., 2014). The incorporation of different semiconducting materials allows the absorbance of light in the UV-visible and NIR of the solar spectrum and have great potential for breaking the Shockley 2 Queisser (S-Q) efficiency limit of single-junction solar cells by absorbing light in a broader range of the solar spectrum. The traditional single-junction solar cells have an S-Q efficiency limit of 33.16% and experimental power conversion efficiency of the single-junction silicon (c-Si) solar cells was observed between 20%
25% (Ameri et al., 2013; Peters et al., 2016). For the further improvement of the theoretical as well as experimental power conversion efficiency investigators used multi-junction solar cells and observed the optimized lab-level conversion efficiency of above 46% and having theoretical conversion efficiency limit of 86.8% for the illumination of highly concentrated sunlight. Hence, recently, the commercial tandem photovoltaic devices have shown a conversion

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Figure 8.6 (A) Structure of PbS quantum dots, polymer donor PTB7, and fullerene acceptor PC61BM. (B) Absorption spectra of the active layers of PbS CQD and the PTB7:PC61BM. (C) Tandem A: PbS CQD active layer as the front cell and organic active layer as the back cell; (D) Tandem B: PbS CQD active layer as the back cell and organic active layer as the front cell (Kim, Firdaus, et al., 2018).

efficiency of 30% under one sun illumination, and it has improved through the illumination of concentrated sunlight and reached the highest conversion efficiency up to 40% (Kim, Firdaus, et al., 2018) as indicated in Fig. 8.6. Kolay et al. developed a novel strategy to achieve the high-efficiency tandem photovoltaic device through the incorporation of p-type nickel oxide sensitized with nickel phthalocyanine (NiPcTs) as a photocathode supported over carbon fabric and n-type titanium oxide bound with conducting core-shell copper@carbon dots anchored with cadmium sulfide. Initially measured the power conversion efficiency of the individual n-type half-cell photovoltaic device (TiO2/CdS/Cu@C-dots-nS22/ Sn22-C fabric) under optimized conditions and reported the conversion efficiency up to 6.82%, which was enhanced up to 9.76% by co-assembling p- and n-type half solar cells devices (Kolay et al., 2020).

8.3.5 Perovskite solar cells Perovskite solar cells are the type of photovoltaic device in which the perovskite structured compounds of lead/tin halide materials acts as a light harvesting active layer with the other components. Since the perovskite name is included in the photovoltaic scientific community by the perovskite mineral as calcium titanate (CaTiO3), and hence the chosen materials for the designing of the perovskite solar

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cells having the same crystal structure as CaTiO3. The general chemical formula of perovskite is ABX3, where A-site is the cation and coordinated it to the 12 sites of anions and forms the cuboctahedron; while the sixfold coordinated B-site is the cation and has octahedral geometry (Goetz et al., 2021). It is well known that the perovskite is the unique structure of close-packed oxides and it can accommodate large numbers of cations. The organic-inorganic hybrids of the perovskite formed through the hybridization of small organic cations which are larger than the inorganic cations. The most commonly utilized organic-inorganic hybrids based on perovskite are formamidinium lead halide (FAPbI3), methylammonium lead halide (MAPbI3) having crystal structure same to that of the CaTiO3 (Chen et al., 2015). These hybrids were utilized in the photovoltaic devices and initially achieved the power conversion efficiency of 9.7% with pure MAPbI3 in 2012 and optimized lab-level power conversion was reached up to 25.2% in 2019 using mixed cation or mixed anion composition based perovskite (Kim et al., 2020; Salim et al., 2015). For further improvement viz. coverage of the solar spectrum in the visible-NIR region, interconnectivity between the materials, charge carrier efficiency investigators used the optoelectronically modified form of the organic-inorganic hybrids based perovskite for the photovoltaic devices are highlighted here. Yin et al. studied perovskite of organometallic halide as a light-absorbing material and recommended it have great potential due to their observed higher power conversion efficiency and low-cost fabrication strategies. Fig. 8.7 illustrate the device designing with band diagram and functioning the principle of perovskite solar cells. Generally, TiO2 and ZnO compact layers are commonly utilized as electron transport layers in perovskite solar devices. Hence, the layers of titanium
zinc oxides were deposited on the conducting glass substrate via spray pyrolysis and studied its optical bandgap, valence band maxima and conduction band minima concerning the Ti to Zn ratio in a nonmonotonic method. Ternary oxides with the ZnO
TiO2 system for the new electron transport layer reported power conversion efficiency to 15% for Zn—rich ternary metal oxides comparable to that of the TiO2 ETL of the device.

8.4

Present state of art in emerging photovoltaic devices

In the present state of art, investigators are mainly focusing to boost the photovoltaic performance of solar devices at the reasonable cost through overall processing of materials, easy device architecture through the different components, device stability, and installation technologies (Zhang, Zhu, et al., 2021). Among the different generation of solar cells, third-generation solar cells are competent to the first and second-generation solar cells in terms easy designing, efficiency, cost, and many more. One can remark on the prominence of the third-generation solar cells via the recent solar efficiency chart published by National Renewable Energy Laboratory (NREL) (Fig. 8.8).

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Figure 8.7 (A) Band diagram and operational principle, (B) schematic view of perovskite solar cell, and (C) band diagram and the functioning principle of perovskite solar cell (Hussain et al., 2018).

Figure 8.8 Year-wise solar efficiency chart for different types of solar cells by NREL (National Renewable Energy Laboratory) up to 2020 (https://www.nrel.gov/pv/cellefficiency.html).

By the last 40 years, there has been unceasing improvement in the solar efficiency for the different generational devices, and this revolution can be observed by the NREL. For the same tenure of last 40 years, the first and second-generation solar cells exhibited the efficiency improvement from nearly 13% to 27.6% and 1% to 23.4%, respectively. For the third-generation solar cell, the efficiency enhanced from nearly 6% to 29.8% in the last 25 years. This scenario revealing that the third

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generation that is emergent devices exhibiting the drastic evolution for the solar efficiency in short life span, and this signifying the dominance of the emergent solar devices over the first and second-generation solar cells. Among the third-generation solar devices, the perovskite-based solar cells such as perovskites, perovskite/Si tandem, and perovskite/CIGS TSCs have been exhibited the prodigious impact on the solar efficiency, which revealed 25.6%, 29.8% and 24.2% efficiency, respectively. The highest efficiency observed for the perovskite-based perovskite/Si TSC was 29.8%, which was demonstrated by Helmholtz Center Berlin (HZB) in 2020 (Ko¨hnen, 2021). Researchers from South Korea’s Ulsan National Institute of Science and Technology (UNIST), developed perovskite-based solar cell which resulted into 25.6% efficiency and it considered as second highest achieved efficiency for perovskites solar cells (Jeong et al., 2021). In this research investigation, semiconducting formamidinium lead triiodide (α-FAPbI3) was utilized to establish efficient perovskite solar cells. Along with perovskite-based solar devices, other emerging solar cells such as organic, quantum dot and dye-sensitized solar cells demonstrated the significant efficiency till date. In case of organic solar cells, the highest efficiency observed according to NREL was about 18.2%, which was reported by the scientists of Shanghai Jiao Tong University. But later on, Sun et al., described the ternary organic solar cell having efficiency of 18.6% (Cai et al., 2021). In this investigation, the research efforts were made to design operative ternary system containing non-fullerene acceptor L8-BO-F incorporated in PM6:BTP-eC9 mixture for the better photostability, high photocurrent density, efficient charge transfer and reduced charge recombination rate. In connection with QDSSCs, the scientists from South Korea’s Ulsan National Institute of Science and Technology exhibited the high efficiency about 18.1%. Also, recently, some researchers have further explored the performance of the QDSSCs by accumulating various materials and methods. Hu et al., reported the indium incorporated ZnO as electron transport layer for lead sulfide (PbS) colloidal QDSSCs to obtain regimented results (Bashir et al., 2021). Likewise, Kim et al., described the synthetic approach of well-purified monodispersed perovskite colloidal QDSSCs that is CsPbI3Pe-CQDs with high photoluminescence quantum yield and efficiency of 15.3% (Lim et al., 2021). However, this investigation demonstrated the less efficiency than detected highest one, still it accomplished great impact in the present scenario due to its new practices and consequences. Subsequently, in concern with the DSSCs researchers of Swiss Federal Institute of Technology in Lausanne (EPFL) have been conquered high efficiency about 13% (Zhang, Stojanovic, et al., 2021). For this Zhang, D., Stojanovic, M., Ren, Y. et al. reported the Cu(II/I) electrolyte based DSSCs comprising the engineered organic photosensitizers, namely bulky donor N-(20 ,40 -bis(dodecyloxy)-[1,10 biphenyl]-4-yl)-20 ,40 -bis(dodecyloxy)-N-phenyl-[1,10 -biphenyl] -4-amine and electron acceptor 4-(benzo[c] (Enayati et al., 2018; Gielen et al., 2019; Sharma et al., 2021) thiadiazol-4-yl)benzoic acid. And this present research endeavor expanded the high PCE of 34.5% as well as ideality factor of 1.08. So, all emergent solar devices enlarged the striking attentiveness for an efficient solar to electrical conversion in recent era of solar energy harvesting. In connection with the present state of the art, many research efforts are focusing on the operative metal oxide-based third-generation solar cells to generate high efficiency. In case of perovskite solar cells, various metal oxides are

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employing as the charge selective and transport layers (Chen et al., 2021; Yildirim et al., 2021). Along with these, organic solar have received significant attention due to the use of hybrids of metal oxides, where several acceptors as well as donor organic linkers with wide range of metal oxides are consumed to advance the performance of the organic solar cells (C¸etinkaya et al., 2021). In case of metal oxide-based DSSCs, topical research events are enticing toward the synthesis of extremely porous metal oxide nanostructures, amendment in organic linkers of the dyes as well as constructing progressive designs such as mechanically contacted liquid junctions or solvent free solid-state automaton DSSCs (Deshmukh et al., 2021; Hashmi et al., 2021). As earlier deliberated, the QDSSCs have been considered as one of the imperative emergent solar devices, where many researchers are focusing on the development of such solar cells via architecting interfacial charge transfer, heterojunction as well as flexible cell formation (Tyagi et al., 2021). The various aforesaid strategies of metal oxide-based hybrid solar cells need to be emphasized using Fig. 8.9.

Figure 8.9 Schematic representation of various trending strategies of third-generation solar cells in the present state of the art based on metal oxide hybrids (Cao et al., 2018; Choudhury et al., 2021; Geleta & Imae, 2021). Source: Reprinted with the permission from Cao, B., Zhou, H., Qiu, Z., Xu, Z., Li, N., Zhou, N., Chen, Y., Wan, X., Liu, J., Li, N., Hao, X., Bi, P., Chen, Q., (2018). Monolithic perovskite/Si tandem solar cells exceeding 22% efficiency via optimizing top cell absorber. Nano Energy 53, 798
807. https://doi.org/10.1016/j.nanoen.2018.09.052. Choudhury, B. D., Lin, C., Shawon, S. M. A. Z., Soliz-Martinez, J., Huq, H., Uddin, M. J., (2021). A photoanode with hierarchical nanoforest TiO2 structure and silver plasmonic nanoparticles for flexible dye-sensitized solar cell. Scientific Reports 11, 1
11. https://doi.org/10.1038/ s41598-021-87123-z. Geleta, T. A., & Imae, T., (2021). Nanocomposite photoanodes consisting of p-NiO/n-ZnO heterojunction and carbon quantum dot additive for dyesensitized solar cells. ACS Applied Nano Materials, 4, 236
249. https://doi.org/10.1021/ acsanm.0c02547.

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Figure 8.10 Various commercial applications of emergent solar cells in the present state of the art.

Likewise, some of the emergent solar devices are exploiting for the commercial claims in connection with human as well as envrionemtal progress Fig. 8.10. Among the commercialization of emergnet photovoltaic devices, some of the strategies have been employed successfully in market. In connection with this, recently, DSSCs reached a milestone as these devices assembled in Science Tower Austria by the SFL techologies. It is considered as the largest installation of DSSCs till date. Also, the pervoksite based solar devices are exploiting for the manufacturing of transperant glass windows by the Viridian Glass in Australia. For this scenario, researchers of ARC Center of Excellence in Exciton Science (Exciton Science) and Monash University, have been deployed the next generation pervoskite solar cell which possesing the conversion efficeincy of 17%. Similarly, manufacturer Ubiquitous Quantum Dots (UbiQD), New Mexico installed the quantum dot-tined glass luminescent solar concentrators in various commercial buildings in 2021. Also, the flexible solar powered portable generators have been exploited commercially by the researchers. Subsequently, by considering above proceedings of metal oxide-based hybrid solar cells such as the formation of heterojunction, flexible devices as well as conception of charge selective and transport layers, amendment of various organic linkers for the dyes as well as organic solar cells, etc., one can remark on the current research benefits throughout research communal.

8.5

Conclusion and future outlooks

Energy has a decisive role in social lives, which is an utmost need for everyone. Among the various energy harvesting ways, solar cell technology has gained

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attractive attention due to its key role in global energy transformations. For this purpose, various metal oxide-based hybrids have been utilized in the development of third-generation solar cells over the first and second-generation solar cells. In connection with the same, this chapter is summarized as follows: G

G

G

G

G

G

The third-generation solar cells that is emergent solar cells such as perovskite, organic, dye-sensitized as well as quantum dot-sensitized solar cells have significant role toward the commercial solar cells due to their remarkable efficiency, easy fabrication, cost effectiveness, use of wide range of materials, etc. Because of having easy synthesis, reasonable cost, appropriate physicochemical properties, feasible optoelectrical properties, etc., the various semiconducting metal oxides and their hybrids have been utilized for the developing aforementioned emergent solar cells. Further this chapter comprised the various tailoring approaches such as doping, supporting, as well as composites formation mentioned to advance the recital of bare metal oxides. Also, the strategies and working mechanism for the emergent DSSCs, QDSSCs, tandem, organic as well as perovskite solar cells, are briefly deliberated with the help of recent research investigations. Afterward, the chapter is focused toward the recent trends of emergent solar devices and their consequences. Also, the commercial claims of these emergent solar devices are revealed with the help of recent events. The measurement techniques such as current-voltage, capacitance
voltage profiling, electrochemical impedance spectroscopy, incident photon to current efficiency, etc., are described briefly for knowing the photovoltaic performance.

In the present scenario, researchers are mainly concentrating to increase the photovoltaic performance to diminish the current research challenges by fabricating the efficient devices having high durability, easy fabrication, reasonable cost, etc. There is more scope to ease the enterprise of flexible heterojunction solar cells having cost effective nature as well as to augment device lifetime with efficient charge transfer through the assembly. In case of perovskite solar cells, the device fronting the issue with instability in water as well as incompetent performance in humid environment. For quantum dot solar cell, the quantum dots can be easily de-stabilized in the presence of photons, which diminishes the overall performance under ambient air. So, there will be striking space for developing the stable metal oxide-based perovskite as well as quantum dot solar devices in commercial settings. In concern with dye-sensitized solar cells, the future research events will have chance to design a scheme with high dye loading capability, discovery of new dyes, to maintain the long-term stability of dyes, etc. The use of linkers for all these emerging devices is the dire need of hour for proper charge transport between the moieties of the hybrids; which helpful for enhancing the performance. The metal oxide-based smart and advanced solar panels are also the key factors in futuristic scopes, and hence various floating photovoltaics, solar trees, as well as bifacial solar cells will afford the well-organized route to commercial applications in society. In concern with the same, this chapter provides all the aspects for the practical as well as commercial claims throughout the globe.

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Role of metal oxides as photoelectrodes in dye-sensitized solar cells

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Jayraj V. Vaghasiya1,*, Keval K. Sonigara1,2,* and Saurabh S. Soni1 1 Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India, 2 Oxford Suzhou Centre for Advanced Research (OSCAR), University of Oxford, Suzhou Industrial Park, Jiangsu, P.R. China

9.1

Introduction

Energy consumption is increasing dramatically with the rise of urban society. Fossil fuels are the dominant energy sources utilized to meet our everyday needs. Nevertheless, these sources are unrenewable, and their rising utilization rate is adding to fossil fuels depletion. The continued use of fossil fuels has been attributed to significant environmental challenges, including air pollution, global warming and so forth (Perera, 2017; Wuebbles & Jain, 2001). Therefore, researchers have concentrated on finding renewable, plentiful and clean energy sources as an alternative to conventional fossil fuels. Among all renewable energy sources, sunlight is the most efficient alternatives to fossil fuels. Sunlight is the clean and limitless source; only B0.17% of the sun is types procedures utilized by photovoltaic technologies to transform electricity on the earth’s surface area, which is enough energy to satisfy our present demands (Shang et al., 2015). Therefore, photovoltaic development is essential for future energy generation. Si-based photovoltaic (PV) devices are pricey because of their complex manufacturing process, which restricts the commercial applications of this technology (O’Regan & Gr¨atzel, 1991). Fortunately, many thin-film organic solar cells (TFOSCs) have recently been discovered that seem to have a crucial edge in terms of flexibility, less expensive fabrication, lightweight over traditional Si-PV (Green, 2007). In terms of cost-saving, maximum performance, and facile assembly process; the dye-sensitized photoelectrochemical cells (DSPECs), a subsection of TFOSCs, have proven to be among the most viable options Si-PV (Janne, 2002). DSPECs are usually made up of various components, including semiconductor photoanode, light-absorbing dyes, redox electrolyte, conductive substrate and counter electrode (CE) (Fig. 9.1) (Soni et al., 2015). The DSPECs contains each component draws attention toward the power conversion efficiency (PCE) of the device, and better architecture of DSPEC is always the optimizing and exchange off between various components.


First two authors contributed equally.

Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00009-9 © 2022 Elsevier Inc. All rights reserved.

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Figure 9.1 Schematic of dye-sensitized photo-electrochemical cells with various types of metal oxides for photoanode and cathode.

The essential components of the DSPECs are the selection of anode and cathode materials that defines the current-voltage (I-V) characteristics; for example, current density (Jsc, mA/cm2), dye absorption and regeneration properties, open-circuit voltage (Voc, V), and fill factor (FF, %). Nowadays, a key priority in the DSPECs field is the design of photoanode and cathode that could harvest light efficiently. Enhanced charge recombination, light transmission ability, charge transportability and dye absorption are the variables to engineer high PCE. To date, the overall PCE of Ru dye-sensitized TiO2 photoanode exhibited more than 12% (Graetzel et al., 2012), whereas the theoretical expected maximum PCE of DSPECs is B32% (Rani et al., 2008). Similarly, metal-free dye-sensitized TiO2 photoanode-based DSPECs achieved the highest PCE is 13% (Mathew et al., 2014). Other most extensively investigated photoanode material is ZnO has a comparable conduction band (CB) and work feature as TiO2 with better charge transport mobility than TiO2, which enables it suitable material, but ZnO degradation in the acidic media and dye aggregate decrease its PCE, which restricting its usage as photoanode in DSPECs. The most commercial DSPECs utilize TiO2 that possess a specific surface area between 50 to 100 m2 g21. Fortunately, these TiO2 photoanode exhibit poor light scattering ability because of tinier particle size relative to the wavelength of illumination light. Based on these criteria, researchers have developed many other metal oxides-based photoanode materials such as SnO2, Zn2SnO4, Fe2O3, ZrO2, SrTiO3, Nb2O5, carbon nanomaterials, CeO2 and so forth. On the other hand, the most critical component in DSPECs is the CE liable for the redox couple reaction, which defines in part the cost and efficiency of DSPECs. A high-performance DSPEC may be recognized if the electrolyte redox reaction at the CE is rapid enough to efficiently inhibit the charge recombination at the photoanode. Therefore, the appropriate CE material must have excellent electrocatalytic activity for the redox electrolyte, adequate conductivity and corrosion resistance. Platinum is the typical CE material owing to its outstanding conductivity and electrocatalytic activity toward redox couple electrolytes. However, the realistic implementation of platinum in DSPECs is seriously limited by its disadvantages, including rarity and high cost.

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Nowadays, the quest for a low price and excellent active electrocatalysts to substitute platinum without any operation limitation is one of the critical challenges. Numerous earth-rich and low-price substitutes such as metal oxide, polymers, carbon materials, metal carbides and nitrides were mainly formed CEs in DSPECs (Meng et al., 2018). Many types of carbon materials, such as graphene, active carbon, carbon nanotubes (CNTs) and carbon black, have been utilized as CEs for DSPECs because of their excellent specific surface areas and high conductivity. However, their catalytic activities are still incapable of matching that of platinum (Wang, Sun et al., 2013). Whereas metal oxides are of rising popularity as CEs in DSPECs because of their excellent catalytic activity, inexpensive, excellent chemical stability, and endless supply (Shahpari et al., 2015). Several outstanding review articles have been published concentrating on restrictions on the development of CEs based on alloy, carbon materials, conducting polymer and metal oxide for DSPECs (Yun et al., 2015). Nevertheless, there is still a shortage of chapters on the subject of metal oxide-based CEs for DSPECs. The latest advancements in the development of facile and complex metal oxidebased photoanodes and cathodes in DSPECs have been described in this chapter. Morphological design approaches of the metal oxides materials have also been illustrated. The issue of metal oxide photoanode and cathode and their solutions are addressed in subsequent sections. This chapter has been divided into three topics, for example, (1) photo-physics of DSPECs; which involved the role of photoanode and cathode in charge separation, rate of electron injection, recombination rate, electron transportation (diffusion of electrons) and so forth (2) metal oxide photoanode research and development; which focuses on the effect of morphology on performance, interfacial engineering and light scattering capability, and (3) metal oxide cathode research and development; which dedicated to the role and requirement of metal oxide cathodes, interfacial properties and recent progress on hybrid metal oxide cathode. Finally, conclusions and perspectives are provided to illustrate the possibilities and obstacles in this complex and pioneer research field.

9.2

The operational principle of dye-sensitized photoelectrochemical cells

The operating principle of energy production in DSPECs utilizes similar electrochemical principles by generating power upon irradiation of sunlight. It is comparable to photosynthesis in plants, and therefore it is often called artificial photosynthesis. Ruthenium (Ru) metal-based dye absorbing on metal oxide photoanode which not only assist for dye sensitizer but also acts as electron acceptor and conductor (Fan et al., 2017; Roy-Mayhew & Aksay, 2014). Fig. 9.1, shows a schematic of DSPECs that indicates the electron transfer mechanism in the cell. Upon shining light on the surface of DSPECs, dye absorbs visible light and is transformed into an excited state [Photon excitation, D-TiO2 1 hv!D
-TiO2, (Eq. 1)]. In this process, excitation of electrons occurs from the highest occupied molecular orbital (HOMO) in the ground state to the Lowest Unoccupied Molecular

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Orbital (LUMO) in the excited state of the dye. The electrons from photoexcited dye are injected into the CB of metal oxide photoanode [Electron injection, D
TiO2 1 hv!D1-TiO2 1 e2 injected in CB (Eq. 2)]. The hole and electrons are divided; at this stage; the holes are placed in the LUMO level of oxidized dye, and electrons are placed in the CB of the photoanode. Afterward, the electrons inserted into the CB are then transferred via metal oxide photoanode through diffusion to the conductive substrate - fluorine-doped tin oxide (FTO), and then to the outer circuit energy transfer. The above procedure is superfast and establishes the transformation of light into electrical energy in DSPECs. While, the oxidized dye is regenerated [D
-TiO2 1 RE!D-TiO2 1 OX (Eq. 3)] to its neutral state by obtaining ground-state electrons from iodide/triiodide redox electrolyte and moves the positive charges to electrically linked CE [I32 1 2e2 (Cathode)!3I2 (cathode) (Eq. 4)]. Upon illumination with implemented external load, the DSPEC then produces electricity that has the ability to regenerate and steady. Nevertheless, there are some unwanted recombination reactions, for example, free-electron catches through the excited dye and electrolyte from TiO2 before reaching into the external circuit, which influences the PCE of DSPEC [Recombination by D
dark reaction; D1-TiO2 1 e- injected in CB!D-TiO2 (Eq. 5) and Electrolyte reduction reaction; I32 1 2e2(TiO2)!3I2 (Eq. 6)].

9.3

Photo-physics of dye-sensitized photoelectrochemical cells

To reach high PCE in DSPEC, an effective electron transportation process is required. Identifying the rate of electron transport in DSPECs is important because the difference between the rates of excited-state decay, electron insertion via an interface, and recombination regulates the quantum yield of electrons transferred into the metal oxide.

9.3.1 Energy levels of components Fig. 9.2A shows the energy level diagram of the metal oxide photoanodes, electrolyte, sensitizers, and CE in DSPECs. This figure illustrates an electron transportation kinetics between the photosensitive dye, the bandgap of several commonly used metal oxides and redox mediator into the electrolyte. A diagram of the charge separation at the FTO/metal oxide photoanode interface and band bending at the FTO/metal oxide photoanode/electrolyte interface is shown in Fig. 9.2B and C. Some undesirable recombination reactions occur at the metal oxide anode/electrolyte interface because of the existence of trap states in the metal oxide anode. Fig. 9.2D demonstrates the mechanism of hole/electron recombination and a depiction of the sensitizer-metal oxide anode interface. The solid-state physics family has accepted the electron energy in a vacuum as a reference, whereas scientists have typically utilized the “Normal Hydrogen Electrode” (NHE) as an energy reference. NHE is at 24.5 eV concerning the

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Figure 9.2 Photo-physics of DSPECs: (A) Energy level diagram showing the energy transfer pathway and BG of various metal oxides based photoanodes. Interfacial study; (B) charge separation at fluorine-doped tin oxide (FTO)/metal oxide (TiO2) photoanode interface (C) and band shifting at FTO/metal oxide (TiO2) photoanode/electrolyte interface, and (D) electron/hole recombination at dye loading metal oxide (TiO2) photoanode interface. Source: (B-C) From Kron, G., Egerter, T, Werner, J. H., & Rau, U. (2003). Electronic transport in dye-sensitized nanoporous TiO2 solar cells comparison of electrolyte and solidstate devices. The Journal of Physical Chemistry B, 107(15) 35563564. https://doi.org/ 10.1021/jp0222144. (D) From Bisquert, J., Zaban, A., & Salvador, P. (2002). Analysis of the mechanisms of electron recombination in nanoporous TiO2 dye-sensitized solar cells. Nonequilibrium steady-state statistics and interfacial electron transfer via surface states. Journal of Physical Chemistry B, 106(34), 87748782. https://doi.org/10.1021/jp026058c.

vacuum level. The relationship between the Fermi Level (EF) and the redox potential (Eredox, determine with NHE) can be stated as; EF,redox [eV] 5 24.5 eV -eEF, redox [V]. Where V and e are represented volts unit and elementary charge, respectively. The excited state of the sensitizer (LUMO) seems to have a greater negative energy state relative to the CB of the metal oxide anode. Therefore, these capture states may obtain the electrons from the CB of the photoanode or the injection of electrons from the excited sensitizer.

9.3.2 Charge separation In DSPECs, charge separation is caused by an electric field formed from the creation of a Helmholtz double layer (HDL) at the metal oxide photoanode/sensitizer

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interface by a diffusion process. The HDL is generated by electrolytes and cations of the sensitizer and protons (sensitizer cations and protons created by deprotonation of the acidic sensitizer) (Fig. 9.3A and B). These distinct holes/electrons are inserted and transferred into energy transport material, redox electrolyte, and CE. Evaluating the kinetics, such as the lifetime and diffusion coefficient of both charges before they deliver their relative contacts, is crucial since it significantly impacts the PCE of DSPECs. In metal oxide photoanodes (e.g., TiO2), charge transportation may occur through diffusion and migration. However, there is a lack of substantial potential differentiation due to the nanostructure of metal oxide, which consists of only the characteristics of diffusion electron transportation. During redox

Figure 9.3 (A and B) Schematic view of charge separation at the dye-sensitized metal oxide (TiO2) and electrolyte interface under “OFF” and “ON” lighting. (C) Schematic view of charge transportation at the dye-sensitized metal oxide (TiO2) and electrolyte interface. Schematic view of the energy levels of FTO/metal oxide (TiO2) and electrolyte/cathode interfaces upon (D) dark environment, (E) lightning environment with applied bias voltage and (F) lightning environment with open-circuit voltage. Source: (A and B) From Cahen, D., Hodes, G., Gr¨atzel, M., Guillemoles, J. F., & Riess, I. (2000). Nature of photovoltaic action in dye-sensitized solar cells. Journal of Physical Chemistry B, 104(9), 20532059. https://doi.org/10.1021/jp993187t. (C-F) From Andrade, L., Sousa, J., Ribeiro, H. A., & Mendes, A. (2011). Phenomenological modelling of dyesensitized solar cells under transient conditions. Sol. Energy, 85, 781793. https://doi.org/ 10.1016/j.solener.2011.01.014.

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reactions, the electrolyte contains components iodide and triiodide moves toward the metal oxide anode and the CE, respectively (Fig. 9.3C) (Bisquert, 2003). Further, energy level diagram of the metal oxide photoanode/FTO and CE/electrolyte interface upon variable circumstances; that is under dark environments (Fig. 9.3D), in lighting but applied bias voltage lower than the open-circuit voltage (Fig. 9.3E) and under light exposure under open circuit condition(Fig. 9.3F), respectively. In the dark state, the EF of the metal oxide photoanode is comparable to the electrolyte Eredox (EF 5 Eredox). The EF within the photoanode changes from the equilibrium value with the corresponding applied bias voltage (ΔVin 5 EF 2 Eredox/ q). This potential disparity approaches its highest value when there is no overpotential at the CE.

9.3.3 Recombination rate Recombination is the kinetics that negatively influences the Jsc and Voc by sacrificing electrons by recombination and capturing with holes (Vaghasiya, Mayorga-Martinez, et al., 2020). In metal oxides, various capturing thresholds are found below the CB. A few inserted electrons from the dye are captured through these energy levels, which causes the decrease of electrons. Some inserted electrons can also be recombined with holes into the HOMO level, which means the dye molecules are oxidized before they can be regenerated. To obtain a high Jsc, the re-reduction of the oxidized dye by electrolyte redox species should be rapid than the recombination reactions.

9.3.4 Charge transfer rate As described DSPEC operation principle in Fig. 9.2A, the excited dye energy level (LUMO) is higher than the CB of metal oxide. The variation between ELUMO (Dye)ECB (metal oxide) gives an electrostatic driving force for electron transfer at the CB of metal oxide (B10 2 12 seconds) (Vekariya et al., 2016). Likewise, the ground state energy level (HOMO) of dye is lower than the redox potential of electrolytes. This energy gap would deliver a driving force for transporting a hole and inserting it into the electrolyte. It means, when the dye is excited with visible light, electrons are transferred from HOMO to LUMO levels through remaining a hole in the HOMO level (Hagfeldt et al., 2010). Besides, there is also the probability of some electrons in the excited level of dye further reabsorb with holes in the HOMO level of the dye (around B10 2 8 seconds), it is called recombination (Tractz et al., 2019). A greater Jsc emerges only when electron mobility is quicker than the recombination procedure. Generally, electron insertion of metal (Ru) based and metal-free dyes were obtained around 100 femtoseconds (fs) and 100 picoseconds (ps), respectively, which suggest that the rate of electron injection relies on the types of dyes (Sunahara et al., 2011). The electron insertion happens in ps, and the yield depends on the metal oxide surface properties, the number of cations and redox species in the electrolyte. Afterwards, the inserted electrons are moved to the FTO. However, few electrons synchronize with the triiodide in electrolyte or dye cations.

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Metal oxide photoanode in dye-sensitized photoelectrochemical cells

Photoanode is an essential element in DSPECs. It serves as a porous network for dye loading and works as a charge transport layer to acquire and transfer electrons from dye molecules to the FTO. To obtain maximum dye loading and rapid electron transportation without inducing recombination, a photoanode could have many features as described by literature, such as greater transparency to minimize the loss of incident light, and high electrical conductivity allows electron transport, broad specific surface area increasing absorption of dye molecules and light-harvesting ability, not reacting with electrolytes to decreases the rate of recombination and so forth (Jadhav et al., 2018; Vaghasiya, Sonigara, Beuvier, et al., 2017). Several traditional semiconductors materials were used as photoanodes, that is poly/mono silicon, CdTe/CdS and gallium arsenide. Fortunately, these compounds suffer from photodegradation in DSPEC because they react with an electrolyte that significantly decreases the device’s lifespan (Ali et al., 2016). Therefore, as photoanode, researchers rely more on metal oxide semiconductors. Based on this statement, we have addressed the abovementioned photoanode features and their influence on photovoltaic performance in the following sections.

9.4.1 Influence of morphology in performance Dye-sensitized photo-electrochemical cell photoanode thin film is typically made up of mesoporous metal oxide (i.e. TiO2, ZnO NPs) with a wide surface area for high loading of photosensitizers molecules (Jadhav et al., 2019). These metal oxides thin films provide adequate surface area for the adhesion of dye and serve as an electron pathway to transfer the excited electron from dye molecule (LUMO level) to the conductive FTO substrate through the CB of metal oxide (Raju et al., 2020). Thus, the energy levels between the dye and the metal oxide should meet the synergetic criterion. The light flow should also be deemed when the incident light shines through the metal oxide thin film; in this scenario, the metal oxide structure should be constructed coherently to minimize illuminating light losses. Besides this, the electron passed to the metal oxide thin film can be consolidated with holes, crystal boundaries and defects; consequently, the recombination mechanism (Eq. 3) is shown in Fig. 9.2A. Also, the crystal structure and morphology of the metal oxides significantly impact the output of DSPECs. Enhancing the metal oxide properties is the easiest and potentially the most powerful way to boost the efficiency of DSPECs. Provided that TiO2 and ZnO are the most widely used metal oxides as photoanode for DSPECs, this section mainly focuses on TiO2 and ZnO (and their composites) micro/nanomaterials expect as mentioned otherwise. Several researchers have described enhancing the metal oxide of photoanodes in DSPECs. The influence of metal oxide morphology on their efficiency as photoanode for DSPECs application was mention in the ongoing discussion.

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9.4.1.1 Nanorods/wires/tubes metal oxide Generally, metal oxide (TiO2 or ZnO) NPs with a diameter of B15 6 5 nm are used to create a photoanode for DSPECs. Hence, such metal oxide NPs cause the electron transfer via an arbitrary pathway; thus, several defects, traps and grain boundaries in the photoanode are observed (Fig. 9.4A). As a result, electron recombination during this step happens to a drastic extent and reduces the efficiency of DSPECs. Therefore, several studies are focused on one-dimensional (1D) metal oxide structures, such as nanorods (NRs), nanowires (NWs) and nanotubes (NTs), with a broad surface area. In general, 1D metal oxide materials have better diffusion coefficients than spherical NPs and yield a longer electron diffusion length than photoanode film thickness and greater light-harvesting ability (Ku & Wu, 2007). Unlike conventional NPs, well-ordered 1D array nanostructures can provide a direct transfer route for electron transfer from the metal oxide layer to the FTO substrate, resulting in less recombination and increased photoelectric performance of DSPECs (Fig. 9.5) (Law et al., 2005). based on the above phenomena, a special composite of TiO2 NPs and NRs was utilized to assemble a photoanode to develop DSPEC with better photosensitivity (Hong et al., 2014). The SEM images of synthesized TiO2 NRs and TiO2 NPs@NRs composites were depicted in Fig. 9.4B. The highest PCE received from the TiO2 NPs/NRs photoanode was up to 6.1% (Jsc 5 16.1 mA cm22, Voc 5 0.68 V, and FF 5 0.52%) under simulated sunlight. The findings show that the composite nanostructure can benefit both the high surface area of the NPs and the fast electron transport of the NRs. Hafez et al. (2010) prepared TiO2 NRs/NPs bilayer electrode by hydrothermal process. The acquired TiO2 NRs have a randomly organized structure (Fig. 9.4C). However, the DSPECs performance with TiO2 NRs photoanode only revealed PCE of 4.4% (Jsc 5 8.8 mA cm22, Voc 5 0.739 V and FF 5 0.67%), which was lower than TiO2 NPs (PCE 5 5.8%, Jsc 5 11.8 mA cm22, Voc 5 0.738 V and FF 5 0.67%). It was discussed by the narrow surface area of the NRs, which contributed to the inadequate loading of dye. When TiO2 NR/NPs were merged in the photoanode, DSPECs exhibited a PCE as high as 7.1% (Jsc 5 14.4 mA cm22, Voc 5 0.756 V and FF 5 0.65%). Uniform arranged metal oxide NRs (TiO2) were anticipated to be enhanced DSPECs performance because charges can be delivered straight to the FTO electrode, eliminate the transport between metal oxide NRs. Uniform aligned metal oxide NRs were cultivated directly on conductive FTO substrates by hydrothermal process (Liu & Aydil, 2009). SEM image of uniformed arranged TiO2 NRs on FTO shown in Fig. 9.4D. As fabricated DSPECs were found only PCE of 3%, owing to the small length (4 μm) and narrow surface area of the NRs. Very recently, a microwave-assisted hydrothermal process was reported to prepare single-crystalline TiO2 NRs (Bertorelle et al., 2014). In titanium tetrachloride treated coated NRs, more dye was loaded on the TiO2 anode, and the PCE increased to 3.7% for a 2.5 μm thick TiO2 film. Further, Lai et al. (2015) developed ZnO NRs on AZO coated substrate via a low-temperature hydrothermal technique and investigated the efficiency as photoanode for DSPECs by adjusting metal oxide NR formation times such as 9, 18 and

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Figure 9.4 (A) Electron transportation track in commercial metal oxide NPs (arbitrary drive, left side) and 1D metal oxide NRs (straight drive, right side) in the photoanode of dyesensitized photo-electrochemical cells. (B) SEM images of (a) pristine TiO2 NRs and (b) TiO2 NPs@NRs composites. (C) TEM images (a) low and (b) high magnification of TiO2 NRs. (D) FESEM images (a) top and (b) cross-view of TiO2 NRs film grown on FTO. Source: (A and B) From Hong, C. K., Jung, Y. H., Kim, H. J., & Park, K. H. (2014). Electrochemical properties of TiO2 nanoparticle/nanorod composite photoanode for dyesensitized solar cells. Current Applied Physics, 14(3), 294299. https://doi.org/10.1016/j. cap.2013.12.003. (C) From Hafez, H., Lan, Z., Li, Q., & Wu, J. (2010). High-efficient dyesensitized solar cell based on novel TiO2 nanorod/nanoparticle bilayer electrode. Nanotechnology Science and Applications, 3, 4551. https://doi.org/10.2147/NSA.S11350. (D) From Liu, B., & Aydil, E. S. (2009). Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells. Journal of the American Chemical Society, 131, 11, 39853990. https://doi.org/10.1021/ja8078972.

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Figure 9.5 SEM images of (AC) top view (DF) cross-view of ZnO NRs with grown at 9, 18 and 27 h, respectively. Source: From Lai, F. I., Yang, J. F., & Kuo, S. Y. (2015). Efficiency enhancement of dyesensitized solar cells’ performance with ZnO nanorods grown by low-temperature hydrothermal reaction. Materials, 8(12), 88608867. https://doi.org/10.3390/ma8125499.

27 hours. ZnO NRs were grown hexagonal wurtzite structure and the length of the NRs expanded from 76100 nm after 9 hours to 47.9 μm after 27 hours. Fig. 9.5 showcases the surface and cross-section SEM images of the hydrothermally produced ZnO NRs on AZO substrate for 9, 18 and 27 hours. The DSPEC made of ZnO NRs formed after a prolonged hydrothermal time of 27 hours was reported to have PCE twofold higher than 9 hours. Similarly, our group (Barpuzary et al., 2014) reported hydrothermally grown ZnO NRs on FTO substrate. The DSPECs had ZnO NRs based photoanode and metal-free carbazole photosensitizer exhibited overall PCE of 5.7%, Jsc of 12.0 mA cm22, Voc of 0.719 V and FF of 0.64%. L. Y. Lin and co-workers achieved PCE of 2.17% (Jsc 5 7.06 mA cm22, Voc 5 0.59 V, and FF 5 0.52%) by

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integrating ZnO NRs across ZnO film (Lin et al., 2013). The ZnO NRs offered a 1D electron transfer pathway and increased the charge collection efficiency via reducing electron collection spacing. Fan et al. explored the impact of heating environments on the performance of DSPECs based on hydrothermally synthesized ZnO NRs (Fan et al., 2013). The High PCE occurred in the Ar atmosphere (PCE 5 1.62%) than the open-air atmosphere (PCE 5 1.26%) because of more excellent electrical conductivities in the Ar atmosphere. Many approaches have been utilized to developed metal oxide NRs. The PCE of these metal oxide NRs based DSPECs are listed in Table 9.1. Three-dimensional hierarchical nanostructures that incorporate two or more metal oxide nanostructures to get the benefits of each structure. The most widely utilized hierarchical structure for DSPECs is the mixture of one-dimensional (1D) metal oxide NRs and zero-dimensional (0D) metal oxide NPs. The 0D NPs improve a surface area, whereas 1D NRs enhance the electron transfer pathway. An integrating the rapid electron transfer of 1D NRs and wide surface area of 0D NPs enhanced the DSPECs efficiency (Sun et al., 2008). Photoanodes composed of 1D and 0D metal oxide materials were prepared by mixing TiO2 NPs with ZnO NRs or NWs. In this case, the interaction between the 0D NRs and the 1D NPs metal oxides are substantially enhanced (Yang et al., 2017). SEM images of 0D NRs@1D NPs composite photoanode are shown in Fig. 9.6A. A composite layer has been developed by combining TiO2 NPs and ZnO NRs, followed by transforming the nanocrystals into ZnO NRs via the hydrothermal method. ZnO NRs play crucial roles in enhancing light absorption capability and reducing electron transfer pathways (see schematic in Fig. 9.6B). This ZnO NRs@TiO2 NPs composite photoanode based dye-sensitized photo-electrochemical cell exhibited an overall PCE of 8.4% with Jsc of 16.08 mA cm22, Voc of 0.763 V and FF of 68.8% (Bai et al., 2012). Zhang et al. (2020) reported TiO2 coated ZnO NRs as an efficient photoelectrode (Fig. 9.6C). The PCE of in DSPEC based on ZnO@TiO2 core-shell NRs (PCE 5 2.68%, Jsc 5 6.73 mA cm22, Voc 5 0.63 V and FF 5 0.63%) was lowered than as-deposited ZnO NRs (PCE 5 1.31%, Jsc 5 5.01 mA cm22, Voc 5 0.60 V and FF 5 0.43%) due to TiO2 core-shell improved the surface to volume ratio of the ZnO NRs, which increased their light-harvesting property. Other 1D nanostructures of metal oxide NTs are widely used in DSPECs due to their special and distinct morphology. Self-aligned TiO2 NTs clusters are developed by simple electrochemical anodic oxidation of titanium sheet. These vertically organized TiO2 NTs can be load by dyes of both inner and outer surfaces; this function reveals that the TiO2 NTs cluster can provide greater surface area than other 1D nanostructures materials such as TiO2 NWs, NRs and so forth. Besides, TiO2 NTs can be synthesized by an easy and inexpensive electrochemical approach and demonstrate greater electron diffusion length and controllable diameter and length; these features encouraged the application of these metal oxides NTs in DSPECs (Jennings et al., 2008) [46]. In 2005, (Maca´k et al., 2005) DSPEC was first fabricated using TiO2 NTs (diameter x length; 100 nm 3 2.5 μm), however, the overall PCE was quite lower (PCE 5 0.036%) than the device based on TiO2 NPs. Until now, many researchers have been used TiO2 NTs as photoanode in DSPECs. For

Table 9.1 Photovoltaic performance of various metal oxide photoanode for dye-sensitized photo-electrochemical cells. Photoanode

Jsc (mA cm22)

Voc (V)

Fill factor (%)

Power conversion efficiency (%)

References

TiO2 NRs@NPs TiO2 NRs@NWs TiO2 NTs TiO2 NRs TiO2 NRs@NPs TiO2 NRs TiO2 NWs TiO2 NWs ZnO NRs ZnO NWs ZnO NWs ZnO NRs ZnO NRs ZnO NRs ZnO NWs@TiO2 NPs

18.7 7.3 7.19 20.4 14.4 8.80 16.5 13.9 2.8 3.58 8.78 12.0 7.06 13.5 16.0

0.82 0.78 0.67 0.70 0.75 0.73 0.78 0.82 0.58 0.60 0.68 0.71 0.59 0.61 0.76

0.67 0.68 0.61 0.54 0.65 0.67 0.55 0.64 0.40 0.62 0.53 0.64 0.52 0.62 0.68

10.3 3.90 2.90 7.91 7.10 4.40 7.11 7.34 0.66 1.34 2.63 5.7 2.17 5.20 8.44

Yan et al. (2011) Zhang et al. (2010) Xie et al. (2008) Lv et al. (2013) Hafez et al. (2010) Hafez et al. (2010) Bakhshayesh et al. (2013) Wu et al. (2013) Kim et al. (2007) Zhang et al. (2009) Lin et al. (2012) Barpuzary et al. (2014) Lin et al. (2013) Wu et al. (2013) Huang et al. (2012)

13.5 11.1 17.2 16.9 15.4 14.9 18.6 5.68

0.63 0.61 0.79 0.79 0.75 0.71 0.72 0.66

0.59 0.61 0.72 0.77 0.62 0.64 0.66 0.72

4.97 4.14 9.79 10.2 7.15 6.94 9.05 2.70

Lee et al. (2007)

Composites TiO2CNTs0.1 TiO2CNTs0.2 TiO2MWCNT0.02 TiO2MWCNT0.025 TiO2MWCNT0.06 TiO2MWCNT0.03 TiO2MWCNT0.3 ZnOMWCNT0.01

Sawatsuk et al. (2009) Younas et al. (2019) Chan et al. (2013) Chang et al. (2012) (Continued)

Table 9.1 (Continued) Photoanode

Jsc (mA cm22)

Voc (V)

Fill factor (%)

Power conversion efficiency (%)

References

ZnOCNT ZnOCNT ZnOCNT ZnOCNT TiO2 NPS/Graphene TiO2 NPS/Graphene TiO2 NPS/Graphene/CNT

8.94 6.5 16.1 5.68 16.8 13.9 11.2

0.55 0.70 0.71 0.66 0.60 0.70 0.78

0.53 0.44 0.45 0.72 0.56 0.73 0.70

2.50 5.06 5.50 2.70 5.77 7.10 6.11

Aseena et al. (2020) Mohamed et al. (2019) Hwang et al. (2014) Chang et al. (2012) Fan et al. (2012) Chen et al. (2013) Yen et al. (2011)

18.8 2.53 6.90 9.11 10.6 14.6 11.9 14.4 8.0

0.76 0.71 0.75 0.71 0.75 0.75 0.76 0.77 0.73

0.76 0.66 0.61 0.63 0.72 0.69 0.55 0.52 0.65

11.0 1.20 3.16 4.33 5.78 7.66 5.11 5.84 3.79

Li et al. (2015) Liao et al. (2014) He et al. (2010)

Hierarchical hollow spheres (HHS) DLS-18 1 YS-TiO2(4.5 1 7.1 μm) MHS-TiO2 ZnO HHSs (4.4 μm) ZnO HHSs (9.1 μm) Double-shelled ZnO Triple-shelled ZnO THS-400 THS-600 anatase TiO2 HHSs .

Xia et al. (2016) Zhao et al. (2016) Liu et al. (2012)

Figure 9.6 (A) SEM images (a) top and (b) side view of TiO2 NPs@ ZnO NRs. (B) Schematic view of (a) OD NPs@ 1D NR composite photoanode and (b) electron transportation in OD NPs@ 1D NRs photoanode. (C) SEM top and cross-view of (a-1 and a-2) pristine ZnO NRs , (b-1 and b-2) TiO2 decorated ZnO NRs and (c) TEM image of single TiO2 decorated ZnO NRs. Source: (A) From Yang, M., Dong, B., Yang, X., Xiang, W., Ye, Z., Wang, E., Wan, L., Zhao, L., & Wang, S. (2017). TiO2 nanoparticle/nanofiberZnO photoanode for the enhancement of the efficiency of dyesensitized solar cells. RSC Advances, 7, 4173841744. https://doi.org/10.1039/c7ra07644d. (B) From Bai, Y., Yu, H., Li, Z., Amal, R., Lu, G. Q., & Wang, L. (2012). In situ growth of a ZnO nanowire network within a TiO2 nanoparticle film for enhanced dye-sensitized solar cell performance. Advanced Materials, 24, 58505856. https://doi.org/10.1002/adma.201201992. (C) From Zhang, Q., Hou, S., & Li, C. (2020). Titanium Dioxide-Coated Zinc Oxide Nanorods as an Efficient Photoelectrode in DyeSensitized Solar Cells. Nanomaterials, 10, 1598. https://doi.org/10.3390/nano10081598.

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instance; The fabrication of DSPECs with electrospinning TiO2 NTs as photoanode was stated by Wang, He et al. (2013). The PCE of DSPECs based on TiO2 NTs (PCE 5 3.3%, Jsc 5 6.01 mA cm22, Voc 5 8.0 V and FF 5 0.68%) was smaller than TiO2 NPs (PCE 5 5.98%, Jsc 5 11.5 mA cm22, Voc 5 7.42 V and FF 5 0.69%) due to poor dye loading, which significantly reduced the Jsc. On the other hand, Ranjusha et al. (2011) have reported a 1.01% PCE of the DSPEC based on ZnO NTs clusters with an average thickness of B7 μm (see SEM image in Fig. 9.7A). Similarly, Han, Cho et al. (2010) and Han, Fan et al. (2010) developed high density vertically arranged ZnO NTs for application as photoanode in DSPECs and exhibited only 1.18% PCE under simulated sunlight. Further expanding the surface area of ZnO NTs requires generating longer NTs clusters with a greater aspect ratio. Sadly, when ultralong ZnO NTs were utilized to fabricate DSPECs a relatively poor PCE of 1.6% (Jsc 5 3.3 mA cm22, Voc 5 0.73 V and

Figure 9.7 (A) SEM top view of ZnO NTs. (B and C) SEM cross-section of TiO2 NTs with anodization of 1.5 and 5 h at 5.6 mA cm22 current density. (D) Schematic of dye-sensitized photo-electrochemical cells front and rear light irradiation and their loss of light absorption. (E and F) FE-SEM image of TiO2@ZnO NTs and their energy levels with charge separation. Source: (A) From Ranjusha, R., Lekha, P., Subramanian, K. R. V., NairShantikumar, V., & Balakrishnan, A. (2011). Photoanode activity of ZnO nanotube based dye-sensitized solar cells. Journal of Materials Science & Technology, 27(11), 961966. https://doi.org/10.1016/ S1005-0302(11)60170-9. (B and C) From Li, L. L., Tsai, C. Y., Wu, H. P. Chen, C. C., & Diau, E. W. G. (2010). Fabrication of long TiO2 nanotube arrays in a short time using a hybrid anodic method for highly efficient dye-sensitized solar cells. Journal of Materials Chemistry, 20, 27532758. https://doi.org/10.1039/B922003H. (E and F) Xie, Y. L., Li, Z. X., Xu, Z. G., & Zhang, HL. (2011). Preparation of coaxial TiO2/ZnO nanotube arrays for high-efficiency photo-energy conversion applications. Electrochem Commun, 13, 788791. https://doi.org/10.1016/j.elecom.2011.05.003.

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FF 5 0.64%) was obtained (Martinson et al., 2007). When L. L. Li and co-workers reported, PCE of TiO2 NTs based DSPECs can be strengthened by extending the length of the NTs. In this research, the developed TiO2 NTs had an adjustable length from 1.5 to 5.7 cm (Fig. 9.7B and C). Moreover, the related PCE of DSPECs improved from 4.5% (Jsc 5 8.6 mA cm22, Voc 5 0.78 V and FF 5 0.67%) to 6.1% (Jsc 5 13.6 mA cm22, Voc 5 0.70 V and FF 5 0.64%) due to the extreme wide surface area and excellent light scattering influence of the extended TiO2 NTs. The upcoming light on TiO2 NT-based DSPECs could only shine from the transparent CE side due to the opacity of the titanium substrate usually utilized to develop TiO2 NT clusters; this method is termed as back illumination (Fig. 9.7D). By comparison, for a typical DSPEC based on FTO substrate, the light exposes from the FTO side, this method is defined as front illumination. The key downside of back illumination is the failure light-absorption ability of the TiO2 NT photoanodes; this effect is attributed to the absorption and dispersion of light when the upcoming light travels through the CE and electrolyte before contacting the TiO2 NTs photoanode. This restriction impedes the high PCE of DSPECs based on TiO2 NTs with titanium metal substrates (Zheng et al., 2011). For instance, Ito et al. (2006) reported PCE of DSPEC with front side illumination were PCE 5 9.9% (Jsc 5 16.9 mA cm22, Voc 5 0.79 V and FF 5 0.74%). Those of backside illumination PCE was of 7.2% (Jsc 5 13.6 mA cm22, Voc 5 0.78 V and FF 5 0.68%). Therefore, the PCE could be vastly enhanced by rendering front illumination possible for TiO2 NT-based DSPECs. Xie et al. (2011) and his research team developed a new type of coaxial TiO2@ZnO NTs for high-performance DSPECs (see Fig. 9.7E and F). As-fabricated DSPEC exhibit a fair PCE of 2.8% (Jsc 5 7.28 mA cm22, Voc 5 0.65 V and FF 5 0.60%) because of the increased charge separation for such appropriate structure alignment. Similarly, Ren et al. (2014) planned to boost the PCE of DSPEC by using the same type of TiO2/ZnO NTs by the facile immersion approach. The method reduces the rate of recombination and improves light-harvesting ability to display excellent PCE (PCE 5 3.98%, Jsc 5 8.20 mA cm22, Voc 5 0.759 V and FF 5 0.63%).

9.4.1.2 Carbon-based metal oxide nanostructure As discussed earlier, various types of 1D nanostructured metal oxide semiconductors have been used to increase the electron transport properties and decrease the charge recombination in DSPEC (Adachi et al., 2003). Metal oxide nanostructurebased photoanodes developed by different materials in diverse nanostructure morphologies are composite-based another strategy to improve electron collection capability. Carbon-based nanomaterials such as graphene, CNT and multiwalled CNT (MWCNT) can enhance the overall PCE of the DSPECs via good electron transport characteristics induced by well-defined band gaps between contiguous films (Kongkanand et al., 2007). MWCNTs with outstanding electrical conductivity and large surface area are a suitable candidate for enhancing the transport charges and hence the photo-generated current in the photoanode of DSPECs (Kiran et al.,

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2018). MWCNTs were commonly utilized in various layers of DSPECs, including in photoanode with a combination of metal oxides, in CE and solid-state electrode for varied uses (Kilic et al., 2016). However, it was demonstrated that integrating MWCNTs into photoanode is the most efficient path to increase the PCE of DSPECs thanks to greater charge transport characteristics (Dembele et al., 2013). The DSPECs based on TiO2@MWCNT photoanode display the better interfacial contact between TiO2 nanostructures and CNT and contribute to the concurrence of double pores that possess a high surface area is required for the dye loading (Chung et al., 2012). For instance, Kilic et al. (2016) have developed MWCNT@TiO2 based photoanode and examine the impact of CNT on the DSPECs performance and the improved ability of A-CNT@TiO2 photoanode on PCE in DSPECs is compared with a traditional TiO2 photoanode. The obtained outcomes indicate that PCE is enhanced from 6.51% (Jsc 5 15.6 mA cm22, Voc 5 0.77 V and FF 5 0.54%) to 7.00% (Jsc 5 15.9 mA cm22, Voc 5 0.77 V and FF 5 0.57%) because of the increased light harvesting capability and electron transport within the TiO2 based thin films (Fig. 9.8A). Further, the MWCNTs are introduced into the TiO2 NRs by L. Yang and co-workers (Yang & Leung, 2013) (Fig. 9.8B). The integration of MWCNT into the TiO2 NRs could be improved the rate of charge transport and increase the PCE of the DSPECs. A high PCE of 10.24% has been achieved with a high Jsc of 18.53 mA cm22 and FF of 74%. By utilizing the CNT@ZnO NWs in DSPECs as photoanodes, it has been revealed that the composite nanostructure is a viable substitute in typical DSPECs due to the ZnO and CNTs demonstrate decent work-function coordination, excellent photophysical properties and wide surface area (Hwang et al., 2014). Using this hybrid materials photoanode in DSPECs exhibit a PCE of 5.55% (Jsc 5 16.1 mA cm22, Voc 5 0.71 V and FF 5 0.45%), whereas without CNT layer photoanode exhibited only 5.05% PCE (Jsc 5 14.4 mA cm22, Voc 5 0.71 V and FF 5 0.49%). Hu et al. (2015) demonstrated a triple-layer photoanode structure consisting of ZnO and ZnO@CNT nanostructured thin films for DSPECs. The ZnO@CNT based DSPECs exhibited PCE of 6.25%, which is 35.57% higher than DSPECs assembled without CNTs (PCE 5 4.61%, Jsc 5 12.4 mA cm22, Voc 5 0.67 V and FF 5 0.55%). Many different facile approaches have been used to prepare carbon-metal oxide composite for DSPECs photoanodes (see Table 9.1).

9.4.1.3 Hierarchical hollow spheres and beads In the photoanode of DSPECs, different metal oxides hierarchical hollow spheres (HHSs) and beads were utilized as substitutes to metal oxide NPs (Liao et al., 2011). A couple of reported TiO2 and ZnO hierarchical spheres for the application of DSPECs illustrates in Fig. 9.9AC (Lei et al., 2014; Yang et al., 2011). The DSPECs based on these hierarchical TiO2 spheres photoanodes show a PCE of more than 10% (i.e. Liao et al., 2011) reported PCE 5 10.34% corresponds Jsc of 18.7 mA cm22, Voc of 0.827 V and FF of 67%. The significant enhancements in PCE (also in Jsc) for the hierarchical TiO2 spheres compared to typical P25 NPs of TiO2 are mostly due to a higher light scattering ability, lower recombination rate,

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Figure 9.8 (A) Schematic of the electron transfer pathway (left side) and energy level diagrams with prospective electron loss mechanisms in TiO2@CNTs composite based dyesensitized photo-electrochemical cell. (B) Low magnification SEM image of TiO2@MWCNTs hybrids electrode (a), TEM image of pristine MWCNTs (b), low and high magnified TEM images of TiO2@MWCNTs hybrids electrode (cd). Source: (A) From Kilic, B., Turkdogan, S., Astam, A., Ozer, O. C., Asgin, M., Cebeci, H., Urk, D. & Mucur, S. P., (2016). Preparation of carbon nanotube/TiO2 mesoporous hybrid photoanode with iron pyrite (FeS2) thin films counter electrodes for dye-sensitized solar cell. Scientific Reports, 6, 27052. https://doi.org/10.1038/srep27052. (B) Yang, L., & Leung, W. W. F. (2013). Electrospun TiO2 Nanorods with Carbon Nanotubes for Efficient Electron Collection in Dye-Sensitized Solar Cells. Advanced Materials, 25, 17921795. https://doi. org/10.1002/adma.201204256.

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Figure 9.9 SEM and TEM images of (A) TiO2 HHSs prepared by autoclave. (B) TiO2 HHSs prepared by an acid thermal method. (C) TiO2 NSs prepared by solvothermal method. (D) The solvothermal way designed ZnO NPs aggregation spheres, TEM image (a) low and (b) high magnification of ZnO NPs, (c) schematic view of ZnO NPs aggregation spheres. Source: (A) From Lei, B. X., Zeng, L. L., Zhang, P., Qiao, H. K., & Sun, Z. F. (2014). Sugar apple-shaped TiO2 hierarchical spheres for highly efficient dye-sensitized solar cells. Journal of Power Sources, 253, 269275. https://doi.org/10.1016/j.jpowsour.2013.12.035. (A) Reproduced with permission 94. © Elsevier 2014; (B) From Liao, J. Y., Lei, B. X., Kuang, D. B., & Su, C. Y. (2011). Tri-functional hierarchical TiO2 spheres consisting of anatase nanorods and nanoparticles for high efficiency dye-sensitized solar cells. Energy & Environmental Science, 4, 40794085. https://doi.org/10.1039/C1EE01574E. (C) From Yang, W., Li, J., Wang, Y., Zhu, F., Shi, W., Wan, F., & Xu, D. (2011). A facile synthesis of anatase TiO2 nanosheets-based hierarchical spheres with over 90% {001} facets for dyesensitized solar cells. Chemical Communications, 47, 18091811. https://doi.org/10.1039/ C0CC03312J. (D) Jung, M. H. (2017). High efficiency dye-sensitized solar cells based on the ZnO nanoparticle aggregation sphere. Materials Chemistry and Physics, 202, 234244. https://doi.org/10.1016/j.matchemphys.2017.09.034.

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high surface area, and faster electron transfer rate for the former. Jung, 2017 synthesized sub-micrometer-sized ZnO spheres aggregated by ZnO NPs with 700 nm diameter (Fig. 9.9D). In DSPECs, the utilize of ZnO NPs aggregation spheres as photoanode reveals overall PCE of 4.02% (Jsc 5 8.16 mA cm22, Voc 5 0.56 V and FF 5 0.50%) which is 30% better than those of colloidal ZnO spheres. Besides, other hierarchical spheres materials like nanobelts, nanosheets and nanoribbons could be used as photoanode materials in DSPECs; the assembled DSPECs displayed greater PCE compared to those based on NPs (Lin et al., 2014). These results may be attributed to improved light absorption and electron transfer rate across the photoanode without losing the availability of surface area for dye sensitization on the metal oxide anodes. Furthermore, the HHSs with sole or many shells were established as innovative materials for enhancing the PCE of DSPEC. Such advanced materials show rapid electron transfer, large surface area and excellent light dispersion impact due to dramatically improved light absorption by reflective light from the shell (see inset of Fig. 9.10A) (Hwang et al., 2014). Whenever HHSs are used in DSPECs, the shell layers are critical for enhancing the PCE of cells (Dong et al., 2012).

Figure 9.10 (A) Photovoltaic characteristics of TiO2SnO2 MHSs as photoanode in dyesensitized photo-electrochemical cells (inset represent the various reflectance and scattered light in the MHSs). (BC) SEM and TEM images of TiO2SnO2 MHSs. (DF) TEM images of single, double and multiple shell TiO2 HHS. Source: (A-C) From Qian, J., Liu, P., Xiao, Y., Jiang, Y., Cao, Y., Ai, X. & Yang, H. (2009). From TiO2-coated multilayered SnO2 hollow microspheres for dye-sensitized solar cells. Advanced Materials, 21, 36633667. https://doi.org/10.1002/adma.200900525. (DF) From Hwang, S. H., Yun, J., & Jang, J. (2014). Multi-Shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye-sensitized Solar Cells. Advanced Functional Materials, 24, 76197626. https://doi.org/10.1002/adfm.201401915.

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For instance; the used of TiO2 coated with multilayered SnO2 hollow microspheres (TiO2SnO2 MHSs) as a bifunctional photoanode in DSPECs was reported by Qian et al. (2009). The high magnification of SEM and TEM images of TiO2SnO2 MHSs shows in Fig. 9.10B and C. The J-V characteristics of photoanodes (Fig. 9.10A), one with TiO2SnO2 MHSs DSPECs is exhibit overall PCE of 5.65% (Jsc 5 14.6 mA cm22, Voc 5 0.664 V and FF 5 0.58%), relatively higher than those of TiO2 single layered SnO2 (PCE 5 4.22%, Jsc 5 10.3 mA cm22, Voc 5 0.665 V and FF 5 0.61%). Similarly, Koo et al. (2008) have stated nanoembossed TiO2 HHSs (NeTiO2 HHSs) as a bifunctional photoanode for high performance of DSPECs. The insertion of NeTiO2 HHSs into DSPECs as light scattered layer caused in an increment of the PCE from 7.79% (Jsc 5 12.5 mA cm22, Voc 5 0.857 V and FF 5 0.72%). to 9.73% (Jsc 5 15.8 mA cm22, Voc 5 0.836 V and FF 5 0.71%). As the shell layers of TiO2 HHSs are increased, leading to boost PCE of DSPECs due to the increased multi reflectance effect (Fig. 9.10D-F); the acquired PCE could be as high as 9.4% (Jsc 5 16.5 mA cm22, Voc 5 0.77 V and FF 5 0.73%) than single layer TiO2 based DSPECs (PCE 5 8.0%, Jsc 5 14.2 mA cm22, Voc 5 0.77 V and FF 5 0.74%) (Hwang et al., 2014). Z. H. Dong et al. have synthesized multi shelled ZnO MHSs with various nanostructures via a modified serial templating process and implemented them as photoanode in DSPECs (Dong et al., 2012). It has been shown that expanding shell number and adjusting the distance of the inter-shell will significantly improve the light absorption ability resulting in a considerable PCE enhance. As the shells number rising, the PCE will be more increased from 3.7% (Jsc 5 8.9 mA cm22, Voc 5 0.60 V and FF 5 0.68%) to 4.5% (Jsc 5 12.3 mA cm22, Voc 5 0.60 V and FF 5 0.61%). Comparison with ZnO NPs and TiO2 NPs, HHSs reveal apparent dominance and therefore show a capability to enhance DSPECs efficiency. Further, Qureshi et al. have physically blended 2D SnO2 nanosheets decorated 3D mesoporous MgO NPs (MgO@SnO2) as photoanode in DSPEC (Qureshi et al., 2015). They showed porous MgO@SnO2 hybrids’ influence on the device performance and exhibited a PCE of 3.71%, whereas pristine SnO2 achieved PCE of only 0.98%. This result indicates that the MgO increases the PCE of the DSPECs by reducing the leakage of captured electrons in the CB of the SnO2-electrolyte interface and offers a high specific surface area for the loading of dye.

9.4.1.4 Nanospindles As we have addressed, metal oxides hollow microspheres (HMSs) fabricated by directed connected nanostructures were shown to strengthen interparticle interaction and then increase the charge transport rate as well as light-harvesting ability in the photoanode of DSPECs. While the HMSs with specially built subunits may increase the electron transition inside the sole HMSs, the limited are of the nearby HMSs can minimize the conductivity of the photoanode and affect the electron transport through photoanode (Adachi et al., 2003). To enhance the thin film connectivity, metal oxide nanocrystalline have been purposely inserted into the HMSs to occupy the space present in the photoanode (Xi et al., 2011). Nevertheless, he inserted

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metal oxide nanocrystalline could expand the number of grain boundaries and encourage the recombination rate. You et al. (2010) have proposed a dual-layered film structure with two distinct sizes of TiO2 nanospindles for DSPEC photoanode. The DSPECs based on TiO2 nanospindles exhibited overall PCE of 8.32% (Jsc 5 16.4 mA cm22, Voc 5 0.760 V and FF 5 0.66%) which is far greater than device fabricated with typical TiO2 NPs photoanodes. This double-layered photoanode constructed from various sized TiO2 nanospindles (diameter from 6 to 45 nm and length from 20 to 300 nm) offers a large surface area and exhibits greater aggregation-induced light dispersion in the visible spectrum than those produced from conventional TiO2 NPs. Next, TiO2 nanospindles were synthesized through the hydrothermal process was reported by D. Wu and co-workers (Fig. 9.11A) (Hu et al., 2013). This specific structure is constructed from well-crystallized nanospindles with a diameter ranging from 400 to 1000 nm and lengths ranging from 10 to 200 nm by simply changing hydrothermal time intervals and ammonia concentration in the reaction mixture (Fig. 9.11B). The DSPECs showed a high PCE of 8.10% (Jsc 5 17.4 mA cm22, Voc 5 0.726 V and FF 5 0.63%) due to the adjusting strength of dye sensitizing and scattering effect of the photoanode. Further, TiO2 nanospindles with high surface area (88 m2 g21) and uniform diameters (B450 nm) were developed by the same group (Wu, Gao et al., 2013; Wu, Wang, 2013). The DSPEC based on nanospindles photoanode reported a high PCE of 8.5% (Jsc 5 16.3 mA cm22, Voc 5 0.766 V and FF 5 0.68%), which showed 30% increment relative to microcrystalline MHSs based device (PCE 5 6.5%, Jsc 5 14.8 mA cm22, Voc 5 0.732 V and FF 5 0.59%). Xue et al. (2014) have established anatase TiO2 NPs covered nanospindles on FTO substrate via hydrothermal process. The J-V characteristics of these TiO2 nanopindles as a photoanode showed an overall PCE of 6.40% corresponding to Jsc of 11.4 mA cm22, Voc of 0.764 V and FF of 0.73%. Cheng et al. (2016) have hydrothermally synthesized TiO2@ZnO composite HNSs, which consisting of 1D nanospindles and TiO2 NPs. The DSPECs based on the TiO2@ZnO composite photoanode displayed far greater optoelectronic characteristics and the PCE was 8.78% (Jsc 5 15.9 mA cm22, Voc 5 0.766 V and FF 5 0.72%), which revealed a B30% rise in the PCE than commercial P25 NPs electrode (PCE 5 6.79%, Jsc 5 14.6 mA cm22, Voc 5 0.749 V and FF 5 0.62%). The great enhancement acquired in photoelectric properties and PCE for TiO2@ZnO composite-based DSPEC was primarily due to the outstanding electronic transmission features.

9.4.2 Influence of interfacial engineering The DSPECs performance relies on back electron recombination, which usually arises at dye-sensitized metal oxide photoanode/electrolyte interface and FTO/ electrolyte interface (Fan et al., 2017; Vaghasiya et al., 2018). Back electron recombination causes a reduction of Jsc and Voc, and adversely impacts the PCE of DSPECs. The insertion of a compact layer (or blocking layer) at the FTO/metal oxide interface is efficient for preventing electron recombination, which is typically coated on FTO for passivation (Fig. 9.12A) (Kavan et al., 2014). The electron

Figure 9.11 (A) The synthesis process of amorphous TiO2 nanospindles. (B) SEM and TEM images of TiO2 nanospindles as function of time; (ac) 30 min, (df) 240 min, (gi) 480 min and (jl) 960 min. Source: From Wu, D., Zhu, F., Li, J., Dong, H., Li, Q., Jiang, K., & Xu, D. (2012). Monodisperse TiO2 hierarchical hollow spheres assembled by nanospindles for dyesensitized solar cells. Journal of Materials Chemistry, 22, 11665. https://doi.org/10.1039/ c2jm30786c.

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Figure 9.12 Typical structure of metal oxide photoanodes in dye-sensitized photoelectrochemical cells having (A) blocking layer on FTO and (B) covering layer on metal oxide for minimizing back electron recombination, (C) double light scattering layer and (D) mixture scattering layer.

recombination at the interface between metal oxide (i.e. TiO2 or ZnO) and electrolyte could be minimized by depositing the thin layer of broad BG semiconductors with more negative CB than that of metal oxide (Fig. 9.12B). The surface boundary is intended to decrease the rate of back electron recombination, hence boosting the DSPECs performance (Elbohy et al., 2015). Besides the compact blocking layer, the DSPECs performance could be enhanced by coating a light-scattering layer (LSL) on the metal oxide film to improve the light-harvesting ability of photoanode (Zhang & Cao, 2011). Typically, the bilayer structure comprises metal oxide NPs as an underlying layer and metal oxide nanocrystalline with a wide surface area including NTs, NWs and NRs as LSL (see Fig. 9.4A). Besides enhancing the light-harvesting capability, the LSL may minimize the grain barriers and increase the dye loading because of the broad specific surface area (Nakayama et al., 2008). In the following section, the effects of compact block layer and LSL on the DSPECs performance are addressed in depth based on the existing literature.

9.4.2.1 Influence of the compact blocking layer The blocking layer of mesoporous metal oxides can be deposited on FTO by various techniques such as sol-gel, atomic layer deposition (ALD), sputtering, pyrolysis,

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electrochemical deposition and so forth. The impact of these various approaches on the DSPECs performance was explored by Kavan et al. (2014). They stated that the blocking layer function of ADL and electrodeposited films were substantially decreased upon annealing at 500 C. On the other hand, the blocking behavior of spray-pyrolyzed films was the least effective but also less resistant to annealing. Whereas titania that thermally oxidized was very well blocked and insensitive to annealing. Choi et al. (2012) have TiO2 blocking layer deposited by various approaches, among them hydrolysis of TiCl4 is easy and affordable. The PCE gain from 4.15% (Jsc 5 8.12 mA cm22, Voc 5 0.732 V and FF 5 0.69%), to 5.16% (Jsc 5 9.42 mA cm22, Voc 5 0.764 V and FF 5 0.71%), was found for tailored thickness at 25 nm, that was consistent with impaired electron recombination at FTOredox mediator interface. To minimize the back-electron recombination at TiO2/ electrolyte interface (Fig. 9.12B), various large BG metal oxide layers have been deposited on TiO2 NPs photoanode of DSPECs. For instance, the large BG of the metal oxide with CBs more negative than that of TiO2, the depositing layers of magnesium oxide (MgO) on TiO2 can limit the surface states of the TiO2 electrode and significantly inhibit the back electron recombination from TiO2 to redox couple (I32) in the electrolyte (Tennakone et al., 2001). This mechanism leads to decreased interfacial electron recombination and increases DSPECs performance. Several other metal oxides were used as a compact blocking layer for the assembly of DSPECs, including Nb2O5, ZrO2, SnO2, Ga2O3, Al2O3, SiO2 and Ta2O5. These materials will obstruct the back electron recombination from TiO2 to the redox electrolyte, contributing to enhanced PCE (also Voc) of DSPECs. D. H. Kim and co-workers reported the coating of a thin TiO2 block layer (510 nm) via the ALD method (Kim et al., 2013). Voc delay and enhance electron lifetime (τ) were spotted, indicating the reduction of electron recombination because of the involvement of the blocking layer. S. Noh and co-workers have also mentioned the reduced interfacial resistance and back electron recombination by fluorine-doped TiO2 blocking layer (Noh et al., 2013). Further, the effect of the niobium pentoxide (Nb2O5) blocking layer in the performance of DSPECs was studied by Cho et al. (2013). Optimum coating of Nb2O5 in the photoanode of DSPEC resulted in improved PCE from 4.43% to 6.19%. It has been observed that the gel developing conditions and annealing treatment have a substantial effect on DSPECs performance. Similarly, Nb2O5 blocking layer was reported by Chun et al. via the sol-gel process and acquired B14% improvement in PCE of DSPECs (Chun & Kim, 2014). Nevertheless, a recent finding has shown that these metal oxides’ insulating behavior is not adequate to prevent the undesirable back reaction. Other features, including CB location and oxidation state, must be regarded as the essential requirements. Such specifications suggest that the TiO2/insulating layer’s interfacial modification needs to be carefully engineered and adjusted to assemble DSPECs. Other reported blocking layer deposition approaches for various metal oxides and their influence on DSPECs performance are illustrated in Table 9.2.

Table 9.2 Influence of blocking layer and scattering layer on the performance of dye-sensitized photo-electrochemical cells. Materials

Coating techniques

Jsc, mA cm22

Voc, V

Fill factor, %

Power conversion efficiency, %

References

Atomic layer deposition (ALD) ALD sputtering Hydrolysis of TiCl4 Hydrolysis of TiCl4 Sol-gel Sol-gel

 8.5 12.2 12.9 13.8 11.4 13.8 16.34

 0.840 0.73 0.714 0.67 0.664 0.762 0.769

 61 0.65 73 0.57 59 52 67

8.50 4.3 5.81 6.76 5.24 4.7 5.49 8.42

Spin coating Electrodeposited Sol-gel Chemical method Spin coating

6.90 20 13.2 11.7 15.02

0.515 1.0 0.67 0.62 0.690

63 68 0.69 0.52 0.65

2.23 13.6 6.19 4.51 6.70

Kim et al. (2013) Chandiran et al. (2013) Go´es et al. (2012) Sung (2013) Hwang et al. (2014) Lee et al. (2012) Hart et al. (2006) Han, Cho et al. (2010) and Han, Fan et al. (2010) Sangiorgi et al. (2014) Su et al. (2015) Cho et al. (2013) Roh et al. (2006) Liu et al. (2011)

15.9 18.1 15.59 19.4 15.8

0.826 0.712 0.8 0.743 0.836

0.71 0.59 0.72 0.57 0.71

9.37 7.72 9.04 8.27 9.43

Su et al. (2015) Gao et al. (2013) Sangiorgi et al. (2014) Wu, Lu et al. (2011) Koo et al. (2008)

Blocking layer TiO2

Nb2O5 ZnO ZnO

Scattering layer TiO2 HHSs TiO2 HHSs TiO2 HHSs TiO2 HHSs TiO2 HHSs

    

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9.4.2.2 Influence of light-scattering layer Insertion of light-scattering materials (LSMs) on the metal oxide photoanode is an effective and widespread technique utilized to improve the light-harvesting ability of the photoanode and enhance the overall PCE of DSPECs. LSMs are comparatively broader particles that could be dispersed or reflect incident light when it falls through the photoanode, leading to the sustained light path and increased light-harvesting abilities. Traditionally, mixture and double-layer structures are utilized to adhere LSMs in photoanodes (Fig. 9.12C and D). In the blend (or mixture) structure, giant scattering NPs are trapped into a metal oxide thin film, which effectively loading of the dyes. The big NPs trapped into the metal oxide thin film serve as light dispersions to produce various degrees of light scattering and expand the light transport direction across the photoanode; this process helps to improve the likelihood of photons being consumed by the dye loaded metal oxide electrode (Zhang et al., 2012). Similar to LSMs utilized in dual-layer structures, different nanomaterials, including NTs, NWs and HHSs, could be used as light scatterers into the photoanode of DSPECs. In the dual-layer structure, an additional thin layer is composed of big NPs mounted on the top surface of the active metal oxide film, which typically includes of NPs to fabricate the basic harnessing layer of the DSPECs. In contrast with the size of NPs (usually .50 nm) utilized to provide adequate surface area for dye absorption, the size of scattering particles in the LSL is greater and often equivalent to the light spectrum (i.e. B250500 nm). Such function causes the reflection of incident light that travels through the nanocrystalline thin film, thereby minimizing light loss by transmission. Consequently, the size of the scattering particles in the LSL is crucial for light scattering and reflection impacts. For instance, DSPECs in the context of dual-layer structure photoanode containing over and under-layer TiO2 crystal structure was reported by A. M. Bakshayesh and co-workers (Bakhshayesh et al., 2012). They stated that dual-layer DSPECs exhibit enhanced PCE (PCE 5 6.34%, Jsc 5 14.3 mA cm22, Voc 5 0.810 V and FF 5 0.62%) than monolayer device (PCE 5 1.36%, Jsc 5 2.74 mA cm22, Voc 5 0.790 V and FF 5 0.62%), because of the high amount of dye loading, electron lifetime and excellent light scattering. Similarly, the reason for improving PCE has been stated by Z. H. Liu and co-workers by utilizing TiO2 NRs aggregates as LSL (Liu et al., 2012). The influence of various blocking layers and LSL on their photovoltaic performance is tabulated in Table 9.2.

9.5

Metal oxide cathode in dye-sensitized photoelectrochemical cells

9.5.1 Role of metal oxide cathode in dye-sensitized photoelectrochemical cells Cathode (or CE) is one of the most significant elements in the DSPECs and shows many important functions such as electrocatalyst by reducing redox species in an

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Figure 9.13 The equivalent circuit used for the interfacial study of dye-sensitized photoelectrochemical cells.

electrolyte and the accumulation of electrons from was the external circuit as a current collector. Much of the DSPECs study focuses on growing the overall PCE by improving Jsc, Voc and FF. Usually, platinum-coated FTO substrate is employed as a CE. By enhancing the cathode materials, the FF of the DSPECs increases, which is mostly affected by the series resistance (Rs) of the device. There are several resistances in the charge transfer from the metal oxide photoanode to the cathode in DSPECs, such as the Rs including the sheet and the interface resistance of DSPEC, the FTO/metal oxide interfacial resistance; the charge transfer resistance at the photoanode/electrolyte interface (RCT); RCT at the cathode/electrolyte interface [RCT (CE)] and the attention at metal oxide photoanode/electrolyte [RCT (TiO2)]; Warburg parameter describing the Nernst diffusion of triiodide species in the electrolyte (Wsol) (Vaghasiya, Sonigara, Prasad, et al., 2017). As mentioned above, RCT(CE) is the most dominant resistance amongst different charge carrier resistance in DSPECs and hence the RCT is always attributed to RCT(CE). In addition to RCT and Rs is also directly associated with the cathode. Fig. 9.13, illustrates the equivalent circuit utilized to describe interfaces in DSPECs. A smaller RCT value indicates immense electron transfer from the cathode to the electrolyte to reduce the triiodide to the iodide at the electrocatalytic interfaces of the cathode. A superficial electron transfer at the cathode/electrolyte interface diminished the Rs, thereby providing a high FF, leading to a high PCE. The electrocatalytic activity of the cathode could be described in terms of the Jsc, which is estimated from the RCT [RCT 5 RT/nF Jsc, where F is the Faraday constant, R is the gas constant, T is the absolute temperature, and n is the total number of electron transfer in the catalytic reaction (n 5 2)]. The cathode reactions mainly depend on the electrolyte-containing redox species utilized to transfer electrons between the anode and the cathode (Vaghasiya et al., 2019) [138]. Generally, iodide/triiodide is employed as the redox mediator in DSPECs. The triiodide formed because of the regeneration of oxidized dye is reduced at the cathode by the reaction of I32 12e2 3I2 (Vaghasiya, Sonigara, et al., 2020). The cathode material is picked based on the specific operation of DSPECs. A cathode material in DSPECs should have superior catalytic activity, and high electrical conductivity and stability against the redox mediator utilized in the device. The typical preference of material for the cathode in DSPECs, platinum is chosen because of its desirable properties, including excellent activity and durability against the iodide redox couples. When the catalytic activities of the cathodes are unable to maintain the necessary currents, the effective exchange current should be increased by expanding the surface area of the

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cathode. A wider surface area of the cathode may contribute to more active sites for the redox reaction in an electrolyte that enhances oxidized dye regeneration. Thus, the formation of porosity of the catalysts is crucial to increase the electroactivity. Since platinum metal is costly and decreasing availability, many other types of substitute resources like carbon materials including CNT, carbon black, MWCNT, graphene and acetylene black, were used as a cathode for DSPECs. However, their catalytic activity is still incapable of matching that of platinum. Morphology regulation, functionalization, and doping were used as effective strategies to increase carbon components’ catalytic activity. Fortunately, the complicated synthesis method and the low reliability of the functional groups can be hindered their practical uses. Besides, transition metal carbides have been demonstrated suitable catalytic activities, but their low electrical conductivity, chemical stability and comparatively complicated synthesis process have mainly limited their practical uses. However, metal oxides increase attention as cathode materials in DSPECs, because of their supreme catalytic activity, inexpensive, chemical stability, and easy availability. In the forthcoming section, various types of metal oxides and their composites (with carbon materials) as CEs are examined in depth based on the studies from the literature.

9.5.2 Variable to evaluating the catalytic activity of metal oxide cathode In DSPECs, the most relevant cathode pre-requirement is excellent catalytic activity. Typically, the activity of metal oxide cathodes is determined by four major factors such as electrical conductivity, active site, interfacial engineering and morphology (surface area).

9.5.2.1 Active sites The integration of platinum NPs with metal oxides to establish hybrids is a helpful approach to increase active sites for the reduction reaction of redox species in the electrolyte. For instance, Wang et al. reported a ternary composite of TiO2/WO2/Pt as a cathode in DSPECs (Wang et al., 2013). The intense bonding could be seen when platinum is distributed on metal oxides, establishing an attractive attraction to improve the catalytic activity. The photovoltaic performance with ternary composite as cathode exhibited high PCE of 7.23% (Jsc 5 12.5 mA cm22, Voc 5 0.83 V and FF 5 0.70%), which is higher than individual cathodes. It was stated that oxygen deficiencies might serve as poor electron donors to facilitate the electron transfer rate. Also, oxygen vacancies can act as active sites for the reduction of triiodide (Elbohy et al., 2018). Nevertheless, it produces oxygen deficiencies utilizing the facile approach, which is a critical concern, and the architecture-efficiency correlation is not yet properly known. Therefore, Zhou et al. synthesized oxygen vacancy-rich WO2.72 with significant catalytic activity to reduce redox species (Zhou et al., 2013). Likewise, L. Cheng and co-workers have electrochemically passive consumer WO3 converted into effective cathode materials through a simple hydrogen treatment and

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Figure 9.14 Schematic view of I32 reduction reaction on surface commercial WO3 (A), hydrogen-treated WO3 (B), srRuO3 (C) and srRuO3@GQDs composite (D) cathodes. Source: (A and B) From Cheng, L., Hou, Y., Zhang, B., Yang, S., Guo, J. W., Wu, L., & Yang, H. G. (2011). Hydrogen-treated commercial WO3 as an efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells. Chemical Communication, 49, 59455947. https://doi.org/10.1039/c3cc42206b. (C and D) From Liu, T., Yu, K., Gao, L., Chen, H., Wang, N., Hao, L., Li, T., He, H., & Guo, Z. (2017). A graphene quantum dot decorated srRuO3 mesoporous film as an efficient counter electrode for high-performance dyesensitized solar cells. Journal of Materials Chemistry A, 5, 1784817855. https://doi.org/ 10.1039/C7TA05123A.

associated reaction principle, as shown in Fig. 9.14A and B (Cheng et al., 2013). Such a hydrogen process improved both the conductivity and triiodide reduction reaction of WO3, and the PCE of DSPECs using this cathode dramatically enhanced over 850% than the individual cathode. This treatment is a helpful technique to develop a highly active cathode by forming oxygen vacancies, which offers some assistance for the further engineering of active platinum-free cathode based on metal oxides for DSPECs.

9.5.2.2 Conductivity As described above, a suitable cathode must show high electrical conductivity for efficient charge transfer in DSPECs to obtain high PCE. Transition metal oxides,

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NiCo2O and ZnCO2O4 were revealed to be viable cathodes in DSPECs, because of their redox stability and adjustable valance state. Fortunately, the electroactivity of these metal oxides is restricted by their poor electrical conductivity. Composite of carbon materials (i.e. CNT, RGO, graphene, etc.) with metal oxides is a beneficial way to maximize the electrical conductivity of metal oxides. The hydrothermally synthesized RGO doped NiCo2O4 (NiCo2O2@RGO) as cathode material was reported by Du et al. (2016). The DSPEC based on NiCo2O4@RGO hybrids as cathode exhibited an overall PCE of 6.17%, while pristine NiCo2O4 achieved only 5.20%, indicating the improved conductivity by inserting RGO is crucial for a high electrolyte redox reaction. Likewise, hydrothermally prepared ZnCo2O4@RGO hybrids used as a cathode in DSPEC and showed a considerable electrocatalytic activity toward the reduction of I32. Further, graphene quantum dots (GQDs) is doped in strontium RuO3 NPs (srRuO3) by a simple dipping process and used as a cathode in DSPECs by Liu et al. (2017) The improved reduction reaction of I32 of srRuO3@GQD composite could be attributed to the enhanced available active sites, rapid electron transfer and ions diffusion compared with the pristine srRuO3. The electron transfer mechanism of GQD doped and undoped srRuO3 cathodes for reduction of I32 is displayed in Fig. 9.14C and D. Photovoltaic performance of srRuO3@GQD cathode showed higher PCE (PCE 5 8.0%, Jsc 5 15.6 mA cm22, Voc 5 0.758 V and FF 5 0.68%) than individual srRuO3 cathode (PCE 5 7.16%, Jsc 5 14.9 mA cm22, Voc 5 0.771 V and FF 5 0.62%).

9.5.3 Recent progress on metal oxide-based cathode As stated above, metal oxides are possible replacements for platinum CEs owing to their excellent thermal stability, low cost and perfect catalytic activity. In 2011, DSPEC was first fabricated using WO2 and WO3 as CEs. Catalytic activity for WO2 as CEs proved to be excellent in reducing I32. The WO2 based DSPEC exhibits a high PCE of 7.25%, which is very near the platinum-based device. Next, Zhou et al. (2013) [142] have prepared oxygen vacancy-rich WO2.72 NR bundles with better catalytic activity toward thiolate and I32 reduction by a solvothermal approach. DSPEC with WO2.72 based CE displayed an overall PCE of 8.03%, corresponding Jsc of 14.09 mA cm22, Voc of 0.77 V and FF of 0.70%. Likewise, the photovoltaic performance of WO2 based DSPEC with disulfide redox electrolyte showed better (PCE 5 7.76%) than conventional platinum CE (PCE 5 7.55%) (Wu, Lin et al., 2011; Wu, Wang et al., 2011). The necessary observation for the high catalytic activity of WO2 is still unexplained and more research needed. Afterward, Nb2O5, TiO2, Cr2O3, V2O3, Fe3O4, CuO, RuO2, ZrO2, TaOx, and MoO2, were progressively implemented as cathode catalysts into DSPECs. The photovoltaic parameters of the above metal oxide cathodes were tabulated in Table 9.3. For instance, as fabricated DSPECs based on TaOX, Nb2O5, RuO2, and V2O3 exhibited promising PCE of 4.8%, 6.8%, 7.87% and 5.4%, respectively (Seok et al., 2015; Wu et al., 2012; Yun et al., 2013). In comparison, the MoO2, Cr2O3, ZrO2, and TiO2, displayed minimal catalytic activity to reduce redox species (I32). Mutta et al. fabricated DSPEC using commercial and hydrothermal treated V2O5 as

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Table 9.3 Photovoltaic performance of various metal oxide cathodes. Cathodes

Jsc, mA cm22

Voc, V

Fill factor, %

Power conversion efficiency, %

References

MoS2 Fe3O4 TiO2 V2O3 ZrO2 TaOx SnO2N2

13.8 16.6 3.10 10.9 11.8 13.1 15.7

0.76 0.69 0.74 0.78 0.78 0.76 0.73

0.73 0.63 0.33 0.63 0.28 0.68 0.53

7.5 7.65 0.76 5.4 2.6 6.7 6.0

Wu, Lu et al. (2011)

SnO2Air

9.81

0.53

0.36

1.8

NiO

9.38

0.71

0.63

4.20

In2O3

11.1

0.33

0.26

0.96

RuO2 TiO2@CNFs CoCr2O@CNFs Mn3O4@RGO RuO2@RGO TiO2@MWCNTs MnO2@RGO Nb2O5@Carbon W18O49@RGO

10 13.6 18.9 15.2 16.1 20.1 15.9 15.6 17.1

0.76 0.84 0.75 0.63 0.76 0.66 0.70 0.86 0.70

0.30 0.63 0.59 0.61 0.67 0.60 50 0.73 0.60

2.36 7.25 8.40 5.90 8.32 7.95 5.77 9.86 7.23

Yun et al. (2013) Wu, Lin et al. (2013) and Wu, Lei et al. (2013) Wu, Lin et al. (2013) and Wu, Lei et al. (2013) Okumura et al. (2013) Zhang, Li et al. (2014) and Zhang, Liu et al. (2014) Dao et al. (2015) Sigdel et al. (2014) Guo et al. (2014) Zhang et al. (2014) Dao et al. (2015) Yang et al. (2016) Jin et al. (2016) Li et al. (2016) Sui et al. (2017)

low-cost cathode materials. They note that the PCE of DSPECs with hydrothermal V2O5 exhibited a 25% (PCE 5 1.6%) improvement than commercial V2O5 (PCE 5 1.2%) (Bhat, 2016). Similarly, in solid-state DSPEC with V2O5 cathode electrode achieved PCE of 2.0% (Jsc 5 8.0 mA cm22, Voc 5 0.730 V and FF 5 34%), which is very near to the PCE of the DSPECs using silver-based cathode (Xia et al., 2010). RuO2 is an appealing cathode material in DSPECs, owning to high catalytic activity, conductivity and electrochemical stability over a broad range of potential. G. H. An and co-workers have developed rough surface Ru nanofibers with nanosized grains, a well-aligned nanostructure shown in Fig. 9.15A (An et al., 2016). Three critical factors suggested by the authors can boost the performance of DSPEC; (1) metallic nature of Ru offers excellent catalytic activity and conductivity, (2) as prepared Ru nanofibers made of nano-sized grains enhance active sites, and (3) the unique structural configuration enables fast electron transfer and

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Figure 9.15 (A) SEM images of and inserted FESEM images of (a) Ru NPs decorated CNFs, (b) RuO2/Ru nanofiber hybrids, (c) pristine Ru nanofibers. (d) Schematic view of the Ru nanofibers. (B) The synthesis process of NbO2, Nb2O5 and NbO2@Nb2O5 hybrids. (C) Photovoltaic characteristics of dye-sensitized photo-electrochemical cells with Fe3O4 cathodes. (D) structural architecture of W18O49 (ab), fast electron transport and regeneration electrolyte on 1D W18O49, (c) and cathode based on W18O49 nanofiber network (d). Source: (A) From An, G., An, H., & Ahn, H. (2016). Ruthenium nanofibers as efficient counter electrodes for dye-sensitized solar cells. Journal of Electroanalytical Chemistry, 775, 280285. https://doi.org/10.1016/j.jelechem.2016.06.014. (B) From Yun, S., Si, Y., Shi, J., Zhang, T., Hou, Y., Liu, H., Meng, S., & Hagfeldt, A. (2020). Electronic Structures and Catalytic Activities of Niobium Oxides as Electrocatalysts in Liquid-Junction Photovoltaic Devices. Sol. RRL, 4, 1900430. https://doi.org/10.1002/solr.201900430. (C) From Wang, L., Shi, Y., Zhang, H., Bai, X., Wang, Y., & Ma, T. (2014). Iron oxide nanostructures as highly efficient heterogeneous catalysts for mesoscopic photovoltaics. Journal of Materials Chemistry, A 2, 1527915283. https://doi.org/10.1039/C4TA03727H. (D) From Zhou, H., Shi, Y., Dong, Q., Wang, Y., Zhu, C., Wang, L., Wang, N., Wei, Y., Tao, S., & Ma, T. (2014). Interlaced W18O49 nanofibers as a superior catalyst for the counter electrode of highly efficient dye-sensitized solar cells. Journal of Materials Chemistry A, 2, 43474354. https://doi.org/10.1039/C3TA14345G.

electrolyte diffusion (An et al., 2016). The Ru nanofibers-based cathode in DSPECs exhibited low charge transfer resistance along with high PCE (6.2%), which is higher than commercial platinum-based DSPEC. Further, Seok et al. reported a simple method to utilize RuO2 as an efficient cathode in place of using platinum for DSPECs. They have prepared RuO2 thin

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film on FTO by heating treatment in an air and exhibited overall PCE of 6.77% (Jsc 5 13.3 mA cm22, Voc 5 0.75 V and FF 5 0.68%). Hou et al. reported a two-step synthesis process of RuO2 nanocrystals, and it has been shown that RuO2 catalysts possess impressive catalytic activity against a reduction of I32, which results in comparable PCE to that of the platinum cathode (Hou et al., 2014). More specifically, they investigated the catalytic mechanism of electrocatalysis for reducing I32 on different cathodes, and it observed that the electrochemical reduction reaction of I32 on RuO2 surfaces may be increased greatly due to the better conductivity and electrocatalytic activity. Based on these, obtained an overall PCE of 7.22%, which beats the DSPEC based on the platinum cathode (PCE 5 7.17%). The authors stated that the high electrocatalytic activity was owing to the perfect combination of electrical conductivity and catalytic activity, and this finding also demonstrated our conception. Very recently, the nanosized Nb2O5 and their hybrids (i.e. NbO2, Nb2O5 and M-NbO2) were synthesized by A. Hagfeldt and co-workers (Fig. 9.15B) (Yun et al., 2020). The DSPECs using NbO2 based cathode exhibited PCE of 6.06%, corresponding Jsc value of 15.77 mA cm22, Voc value of 0.72 V and FF value of 0.54%. The lonepair 4d1 electrons of Nb41 in NbO2 improve the niobium-iodide interaction and facilitate electron transfer from the NbO2 cathode to iodide ions, leads to better catalytic activity in NbO2-based DSPECs. Lin et al. synthesized NbO2 as a cathode in DSPECs achieved a high PCE of 7.88% (Lin et al., 2011). As stated earlier, it is possible to use Fe3O4 as cathode materials in DSPECs. Wang et al. have hydrothermally synthesized micro-sized Fe3O4 (flowers shapes) composed of nanosheets and utilized as a cathode in DSPECs (Wang et al., 2014). The formation of flower-like Fe3O4 provides more reaction sites and demonstrates a notable PCE of 7.65%, which is much higher than a sputtered platinum-based device (Fig. 9.15C). The photovoltaic parameters of the Fe3O4 based DSPECs are shown in Table 9.2. Next, H. Zhou and co-workers have synthesized 1D W18O49 nanofibers (Fig. 9.15D) through a simple and templet-free solvothermal method (Zhou et al., 2014). The DSPEC using W18O49 nanofibers-based cathode showed a high PCE 8.58% (Jsc 5 17.3 mA cm22, Voc 5 0.73 V and FF 5 0.68%) which equivalent to that of 8.78% using platinum as the cathode. The authors suggested that the synthesized W18O19 displays exceptional catalytic activity due to the; (1) 1D nanofibers being more useful for electron transport, (2) Huge oxygen occupations provide ample active site to a reduction of triiodide, and (3) large pore structure can be extensive interaction with electrolyte.

9.5.3.1 Metal oxide/carbon composites The metal oxide integrated with other conductive catalysts can boost active site and conductivity as well as activity, which is referred to as the synergistic effect. The composites of metal oxide/carbon (i.e. graphene, CNT, etc.) provide a range of additional special interfacial and physical/electrochemical properties and functions that are both exceptionally attractive and surprisingly beneficial for electrolyte reduction when compared to the use of pristine material. Recently, Yue et al. (2020)

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Figure 9.16 (A) SEM images of MoIn2S4 (a) and MoIn2S4@CNTs composites (b). (B) Synthesis route of RuO2NPs@rGO composite by dry plasma reduction. (C) Possible I32 reduction reaction on different metal oxide catalyst. (D) Synthesis of CuO NRs and their hybridization with acetylene black. Source: (A) From Yue, G., Cheng, R., Gao, X., Fan, L., Mao, Y., Gao, Y. & Tan, F. (2020). Synthesis of MoIn2S4@CNTs composite counter electrode for dye-sensitized solar cells. Nanoscale Research Letters, 15, 179. https://doi.org/10.1186/s11671-020-03410-0. (B) From Dao, V. D., Larina, L. L., Lee, J. K., Jung, K. D., Huy, B. T., & Choi, H. S. (2015). Graphene-based RuO2 nanohybrid as a highly efficient catalyst for triiodide reduction in dyesensitized solar cells. Carbon, 81, 710719. https://doi.org/10.1016/j.carbon.2014.10.012. (C) From Yuan, H., Jiao, Q., Zhang, S., Zhao, Y., Wu, Q., & Li, H. (2016). In situ chemical vapor deposition growth of carbon nanotubes on hollow CoFe2O4 as an efficient and low cost counter electrode for dye-sensitized solar cells. J. Power Sources, 325, 417426. https:// doi.org/10.1016/j.jpowsour.2016.06.052. (D) From Ahmada, W., Chua, L., Al-bahrania, M. R., Yang, Z., Wang, S., Li, L., & Gao, Y. (2015). Formation of short three dimensional porous assemblies of super hydrophobic acetylene black intertwined by copper oxide nanorods for a robust counter electrode of DSSCs. RSC Adv., 5, 3563535642. https://doi. org/10.1039/C5RA02730F.

have synthesized hydrothermally ternary and Moln2S4@CNT composite CE with a hedgehog ball structure. The SEM image of pristine Moln2S4 and Moln2S4@CNT composite is shown in Fig. 9.16A. The MoIn2S4@CNTs hybrid film has a high surface area (SBET 5 66.8 m2 g21), which is beneficial to adsorbing more electrolytes and offers a wider active contact area for the CE. Besides, the MoIn2S4@CNTs hybrid CE shown low RCT and excellent electrocatalytic activity. The DSPECs based on the MoIn2S4@CNTs hybrid CE exhibits an outstanding PCE of 8.3% (Jsc 5 17.7 mA cm22, Voc 5 0.745 V and FF 5 0.65%), which significantly beats that of the DSPECs with the pristine MoIn2S4 and the conventional platinum electrode. Next, Bagavathi et al. (2016) have mechanically blended the different amounts of Fe3O4 and carbon black were examined as a substitute cathode to conventional

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platinum in DSPECs. In DSPECs, the optimum ratio of Fe3O3@carbon black yielded the highest PCE of 6.1% (Jsc 5 14.4 mA cm22, Voc 5 0.665 V and FF 5 0.51%), which was far higher than those of pristine Fe3O4 (2.1%) and CB (2.2%). Similarly, Ahmad et al. (2015) have synthesized CuO@acetylene black composite for CE by a fast solvent technique. They claimed that the hybrid CE exhibited a high DSPEC performance because of the formation of dimension assemblies, a rapid reduction rate of triiodide and improved hole cascade relative to pristine acetylene black CE. While the process of physical blended has been extensively utilized to make the metal oxide@ carbon composite CEs for DSPECs. However, the cathode materials’ recycling performance remains challenging or has not been discussed so far. Therefore, in situ growing methods and chemical routes have been utilized to prepare highly efficient metal oxide@carbon composites as CEs for DSPECs. Li et al. (2016) have used a facile in situ method for the synthesis of Nb2O5@carbon composite. The catalytic activity of Nb2O5@carbon composites for the regeneration of redox species (T2/T2 and Co31/Co21) was significantly increased because of the decreased particle aggregation and expanded mesoporous network to offer a large surface area with a diffusion channel for the electrolyte and to increase the charge transfer process. More importantly, Nb2O5@carbon composite was batter to the platinum catalyst the regeneration of electrolyte, resulting PCE of DSPEC exhibited as high as 9.86% (Jsc 5 15.6 mA cm22, Voc 5 0.857 V and FF 5 0.70%). Nevertheless, the reliability of these materials and electrolytes could be further regarded in future studies. To achieve high performance of DSPECs, Dao et al. synthesized graphene-based RuO2 composite materials (Fig. 9.16B) for the cathode in a device and achieved the highest PCE of 8.32% (Jsc 5 16.13 mA cm22, Voc 5 0.766 V and FF 5 0.67%), which is analogous to the value of platinum-based DSPECs (Dao et al., 2015). The obtained RGO@RuO2 composite with a shallow concentration of Ru NPs showcases lower diffusion impedance and charge transfer resistance as well as excellent long-term stability. Next, the metal oxide cathode electrode, hollow CoFe2O4/CNTs hybrids, was prepared via in situ chemical vapor deposition. The resultant CoFe2O4@ CNT composite insertion with polypyrrole NPs may further enhance electrical conductivity and catalytic performance for the reduction of I32. The photovoltaic performance of CoFe2O4/CNTs/PPy composite as cathode exhibited overall PCE of 6.55%, corresponding Jsc, Voc and FF values of 12.6 mA cm22, 0.69 V and 0.70%, respectively, which is quite comparable to that of the platinum-based cathode (PCE 5 6.61%). Because of CoFe2O4/CNT/PPy composite’s high inherent conductivity than individual materials (CoFe2O4, CNT, and PPy), the reduction of I32 only exists between the CoFe2O4/CNT/PPy and FTO interface. It might be liable for the outcomes that Jsc and PCE’s values for CoFe2O4/CNT/PPy were much higher than other cathode materials. The schematic mechanism of different cathodes toward reduction reaction of triiodide is shown in Fig. 9.16C. W. Ahmed and co-workers reported fast solvation approach for synthesis CuONRs@acetylene black hybrids (Ahmad et al., 2015). The synthesis detail process was illustrated in Fig. 9.16D. Thanks to the excellent conductivity of acetylene

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black and better catalytic activity of CuO NRs, the CuO NRs@acetylene black as cathode in DSPECs displayed overall PCE of 8.05% (Jsc 5 15.9 mA cm22, Voc 5 0.77 V and FF 5 0.65%), in comparison to pristine acetylene black (PCE 5 6.5%) and platinum (PCE 5 6.9%) cathodes based DSPECs.

9.6

Conclusion and perspectives

In the coming years, DSPECs will become a viable technology because of their inexpensive and environmental-friendly nature, the easy processing technology needed, and the continuing progress made by developing various nanostructure photoanodes, sensitizers and cathodes. This chapter on metal oxides materials to enhance photoanodes and cathodes offers a framework for scale-up and practical applications of DSPECs. The metal oxide photoanode engineering requires improving the dye absorption ability, efficient electron transportation features, and light scattering process. The above-described properties of metal oxide photoanodes have been achieved by manipulating semiconductor materials with hierarchical, mesoporous, NWs, NRs, NTs, and so forth. For instance, 1D metal oxides (TiO2 or ZnO) NTs or NRs would be collaborating with comparatively large-sized scattering particles; the latter helps to move an electron from photoanode to FTO and minimize the rate of electron recombination, and the former contribute to improved overall PCE of DSPECs. In certain cases, specific material with a unique nanostructure could be demonstrated multi-function to improve the device’s efficiency. A few of the most extraordinary instances are the hierarchical metal oxides, such as TiO2 or ZnO HHS and NTs, which possess wide surface area, easy electron transfer, and efficient light scattering concurrently, which made them viable candidates to traditional NPs. Recently, a new approach has been utilized to increase the performance of DSPECs by using a combination of light-harvesting cum redox couple solid-state organic ionic conductors, like a metal oxide photoanode sensitized with a metalfree dye. Nevertheless, some problems should be marked in this research field, considering the enormous technological and measurement variation laboratories, a rational analogy between the abovementioned DSPECs is almost unlikely. A cautionary decision needs to be made when efficient and higher PCE is reported. Secondly, a standard protocol is immediately required; recently said of PCE for DSPECs that employ conventional TiO2 NPs with porphyrin dye and cobalt (Co31/Co21) redox mediator exhibited B13% (uncertified), surely still outweigh those of the DSPECs assembled using the above materials and procedures (Meng et al., 2018). Therefore, if these hierarchical metal oxides with unique and elaborately designed morphologies will automatically surpass their NPs counterparts inherently in the device remains uncertain. Besides, commercially available NPs (P25) are commonly utilized to compare synthesized metal oxide materials to demonstrate the benefits of the former in DSPECs. The problematic aspect is that P25 is not an ideal material for use in DSPECs due to the non-optimal particle size and combination of rutile

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and anatase phases, which decrease the dye adsorption and light-harvesting ability. Therefore, it is not straightforward to announce that materials that exceeded P25 is superior to other mesoporous metal oxides materials for DSPECs. Nonetheless, as made in several studies, this assertion could be viewed as considered misguided. One of the critical-component cathodes plays a crucial role in photoexcited dye regeneration and circuit completion of the DSPECs. The significant advances in the development of metal oxide cathodes are outlined in the seond-half of this chapter. Many primary aspects are discussed that decide the electrocatalytic behavior of metal oxide-based cathodes, including electrical conductivity, active sites, morphologies and catalytic activity. There are also logical design techniques that include specific recommendations for developing and designing highly effective cathodes for DSPECs. As shown, the morphology and number of active sites play vital roles in obtaining more significant electrocatalytic activity. For instance, oxygen vacancy rich WO2.72 was greater than pristine WO2.72 for reducing redox species (Wang et al., 2014). Based on this chapter, there are a few obstacles that need to be addressed in the future. Firstly, new cathodes should be aligned appropriately with electrolyte redox couples and sensitizers to optimize the efficiency of DSPECs. Although the use of cobalt (Co31/Co21) electrolytes could be improved the overall PCEs of DSPECs, the platinum-based cathode is not ideal and appropriate toward cobalt-based electrolytes. Therefore, more focus should be devoted to the component matching in the development of platinum-free cathodes, and relevant research must be systematically performed in future studies. In the device, platinum-free cathodes may demonstrate excellent activity; nevertheless, the unidentified durability of DSPECs with these new metal oxide cathodes minimizes their potential use in real applications. Thus, long-term stability assessments are essential for designing new cathodes to substitute platinum in DSPECs. Even so, long-term stability problems have not yet been discussed in most of the research studies. Whether environment effect (i.e. heat, light, humidity, etc.) is the most crucial element that decides the stability of metal oxide cathodes in DSPECs remains uncertain.

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An, G.-H., An, H., & Ahn, H.-J. (2016). Ruthenium nanofibers as efficient counter electrodes for dye-sensitized solar cells. Journal of Electroanalytical Chemistry, 775, 280285. Available from https://doi.org/10.1016/j.jelechem.2016.06.014. An, G.-H., Sohn, J. I., & Ahn, H.-J. (2016). Hierarchical architecture of hybrid carbonencapsulated hollow manganese oxide nanotubes with a porous-wall structure for highperformance electrochemical energy storage. Journal of Materials Chemistry A, 4(6), 20492054. Available from https://doi.org/10.1039/C5TA10067D. Aseena, S., Abraham, N., & Babu, V. S. (2020). Synthesis of CNT based nanocomposites and their application as photoanode material for improved efficiency in DSSC. Ceramics International, 46(18), 2835528362. Available from https://doi.org/10.1016/j. ceramint.2020.07.339. Bagavathi, M., Ramar, A., & Saraswathi, R. (2016). Fe3O4carbon black nanocomposite as a highly efficient counter electrode material for dye-sensitized solar cell. Ceramics International, 42(11), 1319013198. Available from https://doi.org/10.1016/j. ceramint.2016.05.111. Bai, Y., Yu, H., Li, Z., Amal, R., Lu, G. Q., (Max)., & Wang, L. (2012). In situ growth of a ZnO nanowire network within a TiO2 nanoparticle film for enhanced dye-sensitized solar cell performance. Advanced Materials, 24(43), 58505856. Available from https:// doi.org/10.1002/adma.201201992. Bakhshayesh, A. M., Mohammadi, M. R., Dadar, H., & Fray, D. J. (2013). Improved efficiency of dye-sensitized solar cells aided by corn-like TiO2 nanowires as the light scattering layer. Electrochimica Acta, 90, 302308. Available from https://doi.org/10.1016/ j.electacta.2012.12.065. Bakhshayesh, A. M., Mohammadi, M. R., & Fray, D. J. (2012). Controlling electron transport rate and recombination process of TiO2 dye-sensitized solar cells by design of doublelayer films with different arrangement modes. Electrochimica Acta, 78, 384391. Available from https://doi.org/10.1016/j.electacta.2012.06.087. Barpuzary, D., Patra, A. S., Vaghasiya, J. V., Solanki, B. G., Soni, S. S., & Qureshi, M. (2014). Highly efficient one-dimensional ZnO nanowire-based dye-sensitized solar cell using a metal-free, D 2 π 2 A-type, carbazole derivative with more than 5% power conversion. ACS Applied Materials & Interfaces, 6(15), 1262912639. Available from https://doi.org/10.1021/am5026193. Bertorelle, F., Ceccarello, M., Pinto, M., Fracasso, G., Badocco, D., Amendola, V., Pastore, P., Colombatti, M., & Meneghetti, M. (2014). Efficient AuFeOx nanoclusters of laserablated nanoparticles in water for cells guiding and surface-enhanced resonance raman scattering imaging. The Journal of Physical Chemistry C, 118(26), 1453414541. Available from https://doi.org/10.1021/jp503725w. Bisquert, J. (2003). Comment on “diffusion impedance and space charge capacitance in the nanoporous dye-sensitized electrochemical solar cell” and “electronic transport in dyesensitized nanoporous TiO2 solar cells comparison of electrolyte and solid-state devices. The Journal of Physical Chemistry B, 107(48), 1354113543. Available from https:// doi.org/10.1021/jp034793y. Chan, Y. F., Wang, C. C., Chen, B. H., & Chen, C. Y. (2013). Dye-sensitized TiO2 solar cells based on nanocomposite photoanode containing plasma-modified multi-walled carbon nanotubes. Progress in Photovoltaics: Research and Applications, 21(1), 4757. Available from https://doi.org/10.1002/pip.2174. Chandiran, A. K., Yella, A., Stefik, M., Heiniger, L. P., Comte, P., Nazeeruddin, M. K., & Gr¨atzel, M. (2013). Low-temperature crystalline titanium dioxide by atomic layer

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Pramod A. Koyale1, Dillip K. Panda2 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, Clemson University, Clemson, SC, United States

10.1

Introduction

Energy is the 4th basic requirement for every humans not only for survival and sustainable life but also for the growth of industrialization and urbanization. The global energy demand is around 18.58 TW,(World-Energy-Balances-and-Statistics, 2020) among this about 86.5% of energy needs to be fulfilled with the excessive use of nonrenewable energy sources like coal (5.2 TW), oil (5.85 TW), natural gases (4.08 TW), and nuclear sources (0.92 TW); while the remaining amount of energy is fulfilled by renewable energy sources (2.50 TW) (Hu et al., 2019; World-Energy-Balances-and-Statistics, 2020; World-Energy-Balances-and-Statistics, 2020). The schematic representation of this data is demonstrated in the following pie chart (Fig. 10.1). India is ranked third after China and United States, having an energy demand of around 1.07 TW considering the 5.8% global share of energy (IEA, 2020). Among this, about 78% of the energy is consumed with the utilization of nonrenewable sources, while 22% of energy is fulfilled with the use of renewables such as biofuels, hydro, winds, etc. (Fig. 10.2). And hence traditional nonrenewable energy sources are used exhaustively for producing electricity throughout the globe. However, these nonrenewable sources have been continuously depleted as well as introducing the various hazardous effects related to the environment and human beings too (Khatibi et al., 2019). Due to excessive use of such traditional sources, nearly 33.1 gigatonne (Gt) CO2 emission has been released over the world (Global CO2 Emissions in 2019, 2020). In the case of India, the energy-related CO2 emission holds the global share of 6.64%, with an amount of 2.20 gigatonne (Gt) per year, which is very alarming (Global Energy & CO2 Status Report, 2019; Global Energy & CO2 Status Report, 2019; India 2020, 2020). Due to projected growth in world population, such global energy demands are growing exponentially and by 2030, annual energy consumption predicted to increase by 12% (World Energy Outlook 2020, IEA, Paris, 2020). Therefore it is conceivable to say that, economically as well as environmentally, Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00012-9 © 2022 Elsevier Inc. All rights reserved.

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Figure 10.1 Total global energy consumption (18.58 TW) by fuel sources, 2019.

Figure 10.2 Total primary energy consumption (1.07 TW) by fuel sources in India 2019.

the currently existing energy trends are problematical. Hence, with the use of clean and renewable energy sources, effectual attempts are signifying to diminish the use of nonrenewable sources. Particularly, renewable energy technology has the potential to contribute significantly to overcoming the issue of present high energy demand and also leading to reduce carbon emissions as well. In these contexts, solar energy harvesting and water splitting for hydrogen generations are the front runners, and hence the scientists with their research endeavors focusing more on these energy studies.

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organic framework-based electrodes

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Among the solar-driven energy harvesting, solar photovoltaic (PV) (Dhodamani et al., 2020; Pang et al., 2021) and solar water splitting technology (Kuyuldar et al., 2019) are game-changer investigations and hence significant research endeavors have been continuously going on so as to get greater conversion efficiency. Sunlight is one of the renewable sources of energy; which is exempt from pollution and noise, so one can practically convert the sunlight into electrical energy with the use of a solar PV device (Sharma et al., 2015; Sudhakar Babu et al., 2016). In concern with the present state of the art, third-generation solar cells have been considered as able technology amongst the generations of solar cells; which has comparable solar-to-electrical energy conversion performance as well as reasonable costs (Yan & Saunders, 2014). Along with this, to decompose the water for hydrogen production, solar-driven water-splitting technology has been deployed by using solar energy effectively. However, water splitting for hydrogen generation is not established well for applied point of view and hence this protocol for hydrogen generation is yet obscure (Kuyuldar et al., 2019). The performance of these solar PV, as well as water splitting technologies, mainly depends upon the use of suitable, efficient photoelectrodes used. The special features of the electrode materials such as having lower cost, easy to synthesize, higher surface area, wide coverage of optical solar spectrum, more charge carrier concentrations, and proper charge transport throughout a system, etc., are considered. Among the various photoelectrodes materials, a large class of inorganic solids including transition/innertransition metal oxides, have been received significant interest as an important family of functional materials for energy studies (Zhu et al., 2020). This is due to their compensations such as structural diversity, flexible tunability, low cost, ease synthetic protocol, environmental friendliness, etc. On the other side, such bare inorganic materials have the constraints of poor electrical conductivity, sluggish charge transfer, low absorption in the solar spectrum, etc., which lead to hinder the concert over energy application (Lina et al., 2020; Yang, Niu, et al., 2017). So, it is necessary to deal with modification of such inorganic materials by means of composite formation, doping, formation of mixed metal oxides, etc., to enlarge the presentation of materials for energy studies. In addition, tuning of bare materials into the nanoscale dimensions would result in the higher surface, multiple exciton generations, formations of more charge carriers, etc., for uplifting their properties as well (More et al., 2022; Mullani et al., 2020). In this connection, to overcome the constraints of bare metal oxides, different organic moieties have been organized for combining with nanocrystalline metal oxides. the combinations of bare nanocrystalline metal oxides with different organic moieties have been deployed to overcome the constraints of bare metal oxides. In recent years, in the field of energy studies, metal
organic frameworks (MOFs) have gained striking attention due to their various topologies, high porosity, good chemical stability, high surface area, number of catalytic active sites, etc. (Majewski et al., 2018). Consequently, by considering these advantages, as regards inorganic metal oxides and MOFs as well, it is of great interest to study such materials and metal oxide/ MOFs hybrid structures as active, suitable and efficient materials for photoelectrodes in solar energy studies including PV and water-splitting technology (Ahn et al., 2017; Chen et al., 2020; Gordillo et al., 2019; Spoerke et al., 2017).

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In concern with the current state of the art, this book chapter highlights the latest significances of the research endeavors related to the metal oxides nanostructures and their composites with MOFs for energy technologies. Further, this chapter also includes the high points of such inorganic metal oxides/MOFs hybrid system counting their properties, synthetic protocols, and applicative measurements for desired studies. Due to the prevailing features such as environmentally friendly, cheap, follow ease preparative route, inorganic metal oxides, and their composites are fulfilling the necessary requirements for the fabrication of the efficient device and therefore these materials lead to form a solid platform for renewable energy conversions. With these motivations, inorganic metal oxides-MOFs systems used for not only PV studies but also solar-driven water splitting are of prime importance in this chapter.

10.2

Metal oxides for solar energy studies

To carry out the study of energy technologies, various materials including inorganic metal oxides, organic semiconductors, conducting polymers, etc., have been exploited. This is due to their significant features such as easy to synthesize, reasonable cost, high stability, and durability, etc. (Guo & Facchetti, 2020; Li, 2020; Liras et al., 2019). The different types of metal oxides demonstrated herein with help of Fig. 10.3. Among these, metal oxides have been commonly used and hence extensive research has been focusing on the use of the active inorganic metal oxides for solar cells as well as water splitting. Particularly, metal oxides such as TiO2 (Cai et al., 2019; Delekar et al., 2018b), ZnO (Chen et al., 2012; Kumbhar et al., 2021), WO3 (Wang et al., 2021), BiVO4 (Ta et al., 2020), Fe2O3 (Cesar et al., 2006; Forster et al., 2015), NiO (Kim et al., 2020), Cu2O (Paracchino et al., 2011), IrO2 (Li et al., 2020a),

Figure 10.3 Different types of metal oxides based on the position of metallic elements.

Nanostructured inorganic metal oxide/metal
organic framework-based electrodes

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RuO2 (Galani et al., 2020), CeO2 (Sun et al., 2020) etc., have been gained attractive attention in the application of solar energy technology due to having numerous remarkable physicochemical properties. The various metal oxide semiconductors deployed for energy studies are mentioned herein using Tables 10.1 and 10.2. Among the various potential candidates for solar energy conversions, TiO2 and ZnO are the ubiquitous materials, while WO3 and BiVO4 are widely explored as photoelectrodes in solar-driven water splitting. Though having numerous advantages, still need to focus on the proper designing for their specific structural and morphological properties, chemical bonding through the linkers, large surface area and more porosity, optical bandgap engineering, and tuning electrical conductivity for efficient charge transfer. The metal oxides similar to those applied in solar cells are also good candidates for solardriven water splitting. Though these studies have different operating principles, both require common functional features in the materials such as fast charge transport, charge lifetime, high recombination resistance, etc. (Concina et al., 2017). As TiO2, ZnO, and SrTiO3 are wide band-gap (3.0
3.4 eV) metal oxides, having limited light absorption, long hole diffusion length increases recombination possibility and photoelectrochemical instability (Yang, Niu, et al., 2017). On the other side, metal oxides with small band-gap such as WO3, Fe2O3, BiVO4, etc., pose a misaligned band edges position with water electrolysis potentials (Lina et al., 2020; Mi et al., 2013; Wang et al., 2012; Yang, Niu, et al., 2017). So, it limited to efficient charge separation, sluggish photogenerated carrier transport for photoanode in water splitting. Due to these reasons, the importance of composites, mixed metal oxides, and/or doping increased which resulted in attractive enrichment in the overall performance of the materials.

10.3

Metal
organic frameworks for solar energy studies

In recent years, numerous materials have been inspected for the improvement of power conversion efficiency (PCE), and MOF plays a significant role for increasing the efficiency. MOFs are organic 2 inorganic hybrids, also called porous coordination polymers (PCPs) formed by metal ions known as secondary building units (SBUs) and organic ligands with highly porous network structures having high surface areas, high porosity, various topologies, and good chemical stability (Fig. 10.4) (Majewski et al., 2018). These are some of the most exciting advances in recent porous materials science that symbolize the beauty of PCPs with different 75,600 structures in the Cambridge Structural Database (Tian et al., 2018). MOFs are conservatively employed as porous materials for gas storage,(Ma & Zhou, 2010) separation,(Ting et al., 2020) catalysis, (Yang & Gates, 2019) and PV devices (Kaur et al., 2016). These are thermally stable above 300 C and sometimes also above 500 C (Zhang & Kitagawa, 2008).

10.3.1 Metal
organic frameworks as sensitizers Various MOFs are deployed as a light sensitizer component since hybrid organicinorganic structures of MOFs deliver several opportunities for light-harvesting as

Table 10.1 Various metal oxides deployed for the solar cell application. Metal oxides for solar cells Metal oxides

Synthesis methods

Properties

Outputs

References

TiO2

Sol-gel, doctor blade technique

Tetragonal anatase crystal; 3.2 eV bandgap; spherical nanostructures

Delekar et al. (2018a)

In-situ sol-gel method; doctor blade technique Atomic layer deposition (ALD), spin coating Chemical method/ precipitation, Spin coating

Anatase TiO2 nanoparticles; crystallite size of 12
15 nm; surface area: 81.98 m2 g21

Spray pyrolysis followed by thermal treatment Solid-state method, doctor blade technique Sol-gel method

Excellent bulk electron mobility; nanocrystals; hierarchical aggregates

Dye sensitized solar cells (DSSCs), efficiency: 1.04%; photocurrent density of 07.37 mA cm22 DSSCs, efficiency: 1.35%; photocurrent density of 6.571 mA cm22; charge transfer resistance (Rct): 224.8 Ω Perovskite, Efficiency: 16.12%; photocurrent density of 22.81 6 0.83 mA cm22 Organic solar cells, efficiency: 0.71%; photocurrent density of 4.95 mA cm22; shunt resistance: 0.80 3 103 Ω cm22 DSSCs, efficiency: 7.5%; photocurrent density of 19.8 mA cm22 DSSCs, efficiency: 5.08%; photocurrent density of 13.90 mA cm22; Rct: 9.84 Ω Perovskite, efficiency: 13.2%

Cheng et al. (2018)

ZnO

ALD Chemical method

Chemically stable; ease in fabrication; bandgap- 3.32 eV Anatase nanorods; rapid recombination of photo-generated electron-hole pairs

Particle size of 60 nm; moderate charge transfer Suppressed charge recombination; less charge transfer resistance High electron conductivity; high photosensitivity due to surface defects; Plasmonic nanoparticles; active throughout visible region

Organic solar cells, efficiency: 13.3% Organic solar cells, 9.33%; photocurrent density of 16.37 mA cm22

Delekar et al. (2019) You et al. (2020) Loiudice et al. (2012)

Nafiseh et al. (2011)

Xu et al. (2015) Chang et al. (2021) Ji et al. (2019)

α-Fe2O3

Sol-gel

2.21 eV Band gap

DSSCs, efficiency: 0.01%
0.03%

n-type indirect semiconductor; flower shaped morphology Maintain 60% of the original PCE after UV aging for 220 min (highly stable); rhombohedral crystal phase

DSSCs, efficiency: 1.24%; photocurrent density of 4.07 mA cm22 Perovskite, efficiency: 9.2%; photocurrent density of 13.4 mA cm22; charge transfer resistance: 11.10 Ω cm22 Perovskite, efficiency: 0.015%

SnO2

Solvothermal reaction Spin coating followed by sintering at 500 C for 1 h in air Hydrothermal method Precipitation method

Microwave-assisted technique; atomic layer deposition Spin coating method

CeO2

SrTiO3

BaTiO3

Nanorod morphology; good cyclic stability Electron mobility of up to 240 cm2 Vs21; 3.6 eV band gap; existence of charge recombination Electron transport material; more stable under UV light illumination; inept overall stability Electron transporter; densely packed crystalline grains; durable system

Sol-gel followed by thermal reaction; microwave irradiation Chemical method; Spin coating

High surface area (83.5 m2 g21) and pore size (29.3 nm); crystallite size of 39 nm

Precipitation method

Perovskite semiconductor; flower shaped nanostructure, band gap of 3.2
3.5 eV; recombination effect and low dye adsorption

Long term stability; 3.2 eV band gap

DSSCs, efficiency: 0.005%; photocurrent density of 0.79 mA cm22 Perovskite, efficiency: 14.2%; photocurrent density about B22 mA cm22 Organic solar cells, efficiency: 10.72%; photocurrent density of 23.92 mA cm22; series resistance: 47 Ω DSSCs, efficiency: 4.48%; photocurrent density of 8.61

Perovskite, efficiency: 20.58%; photocurrent density of 25.75 mA cm22 DSSCs, efficiency: 0.012%; photocurrent density of 0.130 mA cm22

Cavas et al. (2013) Niu et al. (2013) Guo et al. (2017)

Zhang et al. (2017) Snaith and Ducati (2010) Liu et al. (2019) Zhao et al. (2020)

Gayathri et al. (2020)

Mahmoudi et al. (2020) Rajamanickam et al. (2017)

Table 10.2 Various metal oxides deployed for the solar-driven water splitting. Metal oxides for water splitting Metal oxides ZnO

Synthesis methods

Properties

Aerosol assisted CVD method

Wide bandgap of 3.4 eV, only active in UV region; flower like morphology; shows the current of 0.028 mA cm22 over 900 seconds

Metal-organic CVD technique

Nanowire morphology; hexagonal phase wurtzite (WZ) structure

TiO2

Anodization Process

BiVO4

Electrodeposition method

Insufficient oxygen vacancies; sluggish charge transfer; 1D TiO2 nanotube arrays; less number of active sites over surface; rapid recombination of photo-generated electron-hole pairs; resulting in poor PEC activity Bandgap of B2.4 eV; poor water oxidation kinetics

Electrodeposition method

Sluggish charge transfer; charge recombination

Performance

References

Components of system

Results

0.1 M Na2SO4 solution; Platinum (Pt) as counter electrode (CE); Ag/ AgCl as reference electrode (RE) 0.5 M Na2SO4;Pt as CE; Ag/AgCl as RE

Photocurrent density of 0.25 mA cm22

Khan et al. (2020a)

Power conversion efficiency (PCE): 0.03%; photocurrent density of 0.278 mA cm22 at 2.0 V versus RHE PCE:0.13%; photocurrent density of 0.38 mA cm22; charge transfer resistance: 14.44 Ω

Hassan et al. (2020)

PCE: B0.83%; photocurrent density of B2.23 mA cm22 at 1.23 V versus RHE Photocurrent density of 3.2 mA cm22; PCE: 2.67%

Huang et al. (2020)

0.5 M Na2SO4 aqueous solution; Pt sheet as CE and Ag/AgCl (saturated KCl) electrode was the RE 0.5 M Na2SO3; Pt as CE and Ag/AgCl as RE

0.4 M KI and 0.04 M Bi (NO3)3; Pt mesh as CE and a saturated Ag/ AgCl electrode as RE

Zheng et al. (2020)

Ye (2019b)

WO3

Hydrothermal method

Hydrothermal synthesis

Band gap of 2.5
2.8 eV; moderate length of hole diffusion; H2 cannot be generated without the assistant of external bias potential; nanorods morphology Nanoplate morphology; hole diffusion range (B150 nm); good electron mobility (B12 cm2 V22 s21); formation of peroxo-species; slow charge transfer; sluggish kinetics of photogenerated holes; rapid recombination of hole-electron pairs; photo corrosive p-type nanoclusters; optically transparent due to wide band gap of 3.6 eV; insufficient chemical stability;

NiO

Anodization Process

RuO2

Sonochemical method; precipitation method

Insufficient durability and activity of RuO2; less number of active sites

Commercial one

Good electrocatalytic activity toward HER and OER; Insufficient durability; less electron conductivity

CBD Method

p-type oxide; direct bandgap of 2.0 eV; most use as photocathode

Cu2O

0.5 M Na2SO4; Pt as CE; Ag/AgCl as RE

Photocurrent density of 2.26 mA cm22 1.23 V versus RHE

Kalanur et al. (2013)

0.1 M H2SO4 electrolyte; Pt as CE; Ag/AgCl as RE

Photocurrent: 3.7 mA cm22 at 1.23 V versus RHE; IPCE: 67% at 350 nm at 1.23 V versus RHE

Zheng et al. (2019a); Wang et al. (2016)

Electrolyte-0.1 M NaClO4 aqueous solution saturated with N2 gas; Pt as CE; Ag/AgCl as RE 0.5 mol L21 H2SO4; reversible hydrogen electrode (RHE) as RE; glassy carbon slab as CE 1 M KOH electrolyte

Charge transfer resistance: 104.8 k Ω; turnover frequency: 1023 s21

Hu et al. (2014)




Audichon et al. (2016)

Required overpotential of 450 mV to reach photocurrent density of 10 mA cm22 Theoretical photocurrent density of 14.7 mA cm22 and a STH efficiency of 18%

Fan et al. (2020)

Saturated calomel electrode (SCE); Pt as CE; aqueous solution of 0.5 M NaOH

Hou et al. (2014)

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Figure 10.4 Showing the framework structure of metal and organic linker.

well as tunable energy carriage (Allendorf, 2017). The following factors should be preferred by respective MOFs for desirable sensitizer: (1) it should be active in the broad visible region (400
800 nm), (2) it should retain the electron-donating groups attached to the metal in MOFs, to hold the better metal to ligand charge transfer and hence easy linkage with oxide film surface, (3) to establish suitable sensitization, the energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of MOFs should be plausible with that of the metal oxide used in the cell, also (4) MOF should comprise the metal which has partly filled orbital (e.g., d7, d9 case) than filled orbital (d10 case of Zn12). This is due to the fact that MOFs with full shell-configured central metal ion as well as an organic linker with low conjugation organic grasp relatively large bandgap Eg, which cannot absorb light effectually in the solar spectrum (Chao-Wei et al., 2020; Chueh et al., 2019; Hendrickx et al., 2015). The most commonly deployed MOFs for energy studies are listed in Table 10.3.

10.3.2 Guest@ metal
organic frameworks system Though MOFs have numerous overriding properties, to get more charge-transfer complex with tunable electronic conductivity, the derivatization of MOFs played an appealing role. Fluorescence resonance energy transfer signifies the study for energy transport in guest@MOFs system. In the case of MOFs, the band gaps can be tuned by bringing the alteration in the cluster size of the secondary building units (SBUs) and/or modification in the conjugation of the organic linkers. Also the functionalization of the organic ligand with electron-donating groups (
OH,
NO2,
NH2) leads to reduce the bandgap (Eg) (Lin et al., 2012). Generally, the light-harvesting properties and the range of charge transformation are improved by the functionalization of MOFs with either acceptor or donor guests (Fig. 10.5). In the case of guest@MOF-177, DH6T (donor) and PCBM (acceptor) moiety get functionalized in MOF-177 for the long-range charge transformation from donor to the acceptor and to reduce for example, As we go from MOF-177, PCBM@MOF-177 to (DH6T 1 PCBM)@MOF-177, bandgap reduced from 3.35,

Table 10.3 The different metal
organic frameworks utilized for solar energy studies. Metal
organic Frameworks

Metals

Linkers

Synthesis methods

HOMO eV (vs vacuum)

LUMO eV (vs vacuum)

Eg (eV)

Visible region (nm)

References

PPF-11 MOF-177 Cu- BTC/ MOF199

Zinc Zinc Copper

TCPP BTB BTC

2 5.00 2 6.15 2 5.37

2 3.1 2 2.80 2 3.82

1.9 3.35 B1.64

400
650 450
500 400
700

Gordillo et al. (2019) Foster (2012) Deng et al., (2019); Tian et al. (2019)

Co-DAPV

Cobalt

DAPV

2 5.40

2 3.76

B1.64

527
755

Ahn et al. (2017)

ZIF-67

Cobalt

2-MeIm

2 5.8

2 3.81

B2.0

450
700

Sun et al. (2019); Yang, Niu, et al. (2017)

ZIF-8

Zinc

2-MeIm

Solvothermal Sonochemical Solvothermal; deep coating; sonochemical Layer by layer (LBL) deposition Solvothermal; Precipitation followed by thermal treatment Solvothermal; Precipitation

2 6.4

2 0.9

B5.5

230
250

UiO-66 UiO-67

Zirconium Zirconium

BPDC BPDC

2 6.95 2 7.9

2 3.9 2 4.1

3.05 B3.6





Co-NDC

Cobalt

NDC

Solvothermal Solvo/Hydrothermal Solvothermal; LBL

2 6.22

2 3.95

2.27

Chin et al. (2018); Papurello et al. (2019) He et al. (2014) Cao et al. (2020); Chavan et al. (2012) Hu et al. (2014)

MIL-125

Titanium

BDC

Single step hydrothermal

2 6.9

2 3.1

3.8

Absorption edge at 548 nm B340 nm absorption band edge

Hendon et al. (2013); Vinogradov et al. (2014); Yuan et al. (2015)

TCPP, tetrakiscarboxyphenylporphyrin; BTB, 1,3,5-tris(4-carboxyphenyl)benzene; BTC, benzene-1,3,5-tricarboxylic acid; DAPV, di(3- diaminopropyl)-viologendibromide; BPDC, biphenyl-4,4’dicarboxylate; NDC, 1,4-naphthalenedicarboxylic acid; BDC, 1,4-benzenedicarboxylic acid; 2-MeIm, 2-methyl imidazole.

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Figure 10.5 Guest insertion in metal
organic frameworks.

1.50 to 0.92 eV respectively (Foster, 2012). MIL-125 MOFs with 1,4-benzenedicarboxylate (bdc) linker, has an optical bandgap in the UV region (3.6 eV/345 nm). The functionalization of bdc linker with
NH2 group in MIL-125 MOFs demonstrated the reduced bandgap (from 3.6 eV/345 nm to 2.6 eV/475 nm), which exhibited the absorption in the visible region (Hendon et al., 2013). By considering the all above facts, these MOFs can be utilized for hybrid systems with desired metal oxides. This is due to their prevailing properties such as controllable architecture, adjustable pore size, high surface areas, and stability as well for application in solar energy studies as photoelectrodes with enhanced performance.

10.4

Metal oxides/metal
organic frameworks nanocomposite: pros and cons

Metal oxide nanostructures have been continuously using in various applications due to their common properties of controllable size, shape, morphology, crystallinity, tunable optoelectrical, etc. The further uplifting of such properties of bare metal oxides can be done by making the composites of bare metal oxide with MOFs (Zhu & Xu, 2014). As already deliberated, MOFs are able to add various functionality into semiconducting metal oxides due to their porous nature, the number of active sites, as well as having a variety of constituents in their structure. Some of the common advantages due to metal oxide/MOFs composite are highlighted here (Chueh et al., 2019; Li et al., 2018a). 1. 2. 3. 4. 5.

Improved light absorption to the visible region (i.e., redshift) Efficient separation and transfer of charges Fortunate thermal stability Generation of number of active sites through a system Improved film quality and many more (Fig. 10.6).

Figure 10.6 Pros and cons of bare metal oxide, bare metal
organic frameworks (MOFs) and pros of MO/MOFs composites.

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Advances in Metal Oxides and Their Composites for Emerging Applications

Metal oxide/metal
organic frameworks: present state of the art

Metal oxide nanocomposites including homogenous as well as heterogeneous systems have gained considerable interest in various fields globally. In the present state of the art, a heterogeneous system such as metal oxide/MOFs composite for energy studies is an emergent field as acknowledged by most of the research community. In the current situation, these nanostructured metal oxide/MOFs composites are singing an imperative role from both scientific as well as technological approaches. In recent years, such systems utilized over gas separation, catalysis, gas storage, biosensors, solar cells, hydrogen generation, etc. (Stock & Biswas, 2012). In concern with updated research communal, most of the research focused to use low cost, ease and reproducible manufacturing process for nanostructured metal oxide/ MOFs system. In concern with the same, one of the research groups demonstrated the ZnO-based photoanode integrated with Zirconium-MOF in a solar cell with efficient consequences (Bhardwaj et al., 2018). In this investigation, the mechanical synthetic approach has been devolved to form a hybrid system showing the ability toward efficient charge separation and their well transportation. Also, such heterogeneous composite systems are to be scrutinized as photoanode in DSSCs to improve the performance of DSSCs in the present state of the art (Moloto, 2021). In the field of perovskite solar cells, extensive research is going to utilize such heterostructured assemblies as active material in the device, electron transporting layer, or interface layers. This includes enhancement in the quality of films as well as stability of device which improve the efficiency (Ahmadian-Yazdi et al., 2020). Along with solar harvesting technology, the present state of the art has another emerging area so-called solar-driven water splitting, where the use of the said metal oxide/MOFs systems gained dire interest in PEC water splitting for hydrogen generation (Yang & Bright, Kasani, et al., 2019). Many research groups demonstrated the use of such systems as a well-organized photoanode with durable stability, the high surface area having the number of active sites as well as charge transfer (da Trindade et al., 2021; Yoon et al., 2021). The present state of the art related to metal oxide/MOFs for energy studies, is represented with the help of Fig. 10.7. So in overall, it is easy to say that, in the present state of the art most of the research endeavors demonstrating high performance in a solar study by designing heterogeneous composites such as metal oxide/MOFs system.

10.6

Electrode designing and its features studies for energy technologies

In connection to solar energy harvesting as well as water splitting, throughout the overall performance, the photoelectrodes are playing a significant role. The

Figure 10.7 Metal oxide/metal
organic frameworks composites with their applications and outcomes in the present state of the art. Source: Reprinted with permission of references:(Gordillo, M. A., Panda, D. K., & Saha, S. (2019). Efficient MOF-sensitized solar cells featuring solvothermally grown [100]-oriented pillared porphyrin framework-11 films on ZnO/FTO surfaces. ACS Applied Materials and Interfaces, 11(3), 3196
3206.; da Trindade, L.G., et al., 2021; Moloto, W. (2021). Stabilizing effects of zinc (II)-benzene-1, 3, 5-tricarboxylate metal organic frameworks (MOF) on the performance of TiO2 photoanodes for use in dye-sensitized solar cells. Journal of Photochemistry and Photobiology A: Chemistry, 407. & Ahmadian-Yazdi, M. R., Gholampour, N., & Eslamian, M. (2020). Interface engineering by employing zeolitic imidazolate framework-8 (ZIF-8) as the only scaffold in the architecture of perovskite solar cells. ACS Applied Energy Materials, 3(4), 3134
3143).

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various efforts have been utilized by the research community over the fabrication of efficient photoelectrodes and for this purpose number of materials have been established. Among them, the metal oxide-MOFs composite-based photoelectrode for solar cells and solar-driven water splitting, have enlarged striking attentiveness with a number of optimistic properties (Gala´n-Gonza´lez et al., 2020; Gordillo et al., 2019). For the effectual photoelectrodes, the material should accomplish the features such as uniformity, well adherence, binder-free deposition, high surface area, high active sites, porous nature, durable stability under photoirradiation and aqueous media, cost-effective as well as having low resistance to the electrolyte. For designing the well-organized photoelectrodes, various strategies have been developed such as chemical deposition, physical deposition, electrochemical deposition, solvent-based deposition, etc. (Becker et al., 2017; Cheng-an et al., 2020; He et al., 2019; Xue et al., 2019). All such methods tend toward designing effectual photoelectrode showing efficient performance. Physical vapor deposition (PVD) includes sputtering, electron beam evaporation, pulse laser deposition, thermal evaporation, etc., having versatility and reproducibility (He et al., 2019), whereas, solvent-based deposition comprises a deep coating, LBL method, spin coating, etc., having a low cost of utilization and adequate growth rate. Among these methods, chemical method and solution-based method exploited utmost having prevailing advantages such as wide range of materials, cost of fabrication, strong chemical interconnectivity achieved which deliver stability to the electrodes, etc. The various strategies deployed for designing thin-film electrodes were demonstrated with the help of Fig. 10.8. In concern with the solution-based method, the doctor blade method is one of the emerged techniques for thin-film development. It is cost-effective, simple to perform, possible to adjust the thickness of the film as well as forming uniform layer. Spin coating and drop-casting are the techniques which gained importance to develop well-organized thin films (Cha et al., 2017; Rashid et al., 2020). Though these methods are cost-effective, the spin coating has over advantage of covering the large surface area with appropriate thickness. Successive ionic layer adsorption and reaction method, which partakes its advantage of thin-film scheming which comprise low-priced, modest, and expedient large area deposition (Jia et al., 2020). Along with these solution-based methods, printing technique has been utilized among the research communal especially to develop lightweight, low cost, flexible and stretchable thin films in solar device fabrications (Nam et al., 2016). On the other side, chemical vapor deposition (CVD) and electrochemical deposition are the techniques that emerged as attractive techniques for direct deposition study (He et al., 2019). In the case of CVD, possible to develop film with a precisely controlled growth rate but, having the use of sophisticated instruments that is high cost and low growth rate are the cons of this method. Also, the electrochemical deposition technique is a low-cost method with applications to a wide range of materials to design uniform and well-organized thin films.

Nanostructured inorganic metal oxide/metal
organic framework-based electrodes

355

Figure 10.8 Various strategies for electrode designing.

10.7

Metal oxides/metal
organic frameworks nanocomposites for solar energy harvesting

10.7.1 TiO2/ZIF-8 In 2011, for the first time, ZIF-8 MOFs used to coat TiO2 nanostructures via deep coating to construct efficient photoanode for the dye-sensitized solar cell (Li et al., 2011). In this research work, efficiency obtained was about 5.34% for TiO2/ZIF-8 based DSSCs which was somewhat greater than that of TiO2 based device; 5.11%. But it was found that with increasing ZIF-8 growth time, the short-circuit photocurrent (Jsc) and PCE (η) standards diminished severely. The group of same researchers performed the posttreatment of ZIF-8 MOFs at interfaces in TiO2 based DSSCs, where it was observed that conversion efficiency observed to 14.4% (Yafeng et al., 2014). On the other hand, it is conceivable to say that the large bandgap of 5.1 eV for ZIF-8 MOFs, may lead to

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improper performance over the charge transfer in the device (Ahmadian-Yazdi et al., 2020). So, further by tuning the band structures as well as optical properties of the MOFs, it will be deployed for more efficiency in solar energy harvesting. The MOFs sensitized solar cells (MSSCs) is represented as given in the following Fig. 10.9.

10.7.2 TiO2/Cu-BTC For the efficient charge transfer to obtain well performance over the device, the materials should carry suitable band alignments, less probability of electron-hole recombination, and so on. The MOFs should own an appropriate bandgap as well as well-matched band configurations with TiO2 for deploying in composite formation with TiO2. Among various MOFs, Cu-BTC has significant characteristics. In concern with this fact, Cu-MOFs having suitable band alignments synthesized using LBL technique to sensitize doctorbladed TiO2 NPs on a fluorine-doped tin oxide (FTO) substrate (Lee, Shin, et al., 2014). After iodine doping, the cell performance was observed about 0.26% by reducing the charge transfer resistance at TiO2/Cu-BTC interface. Though having required characteristics over the better performance of the device, still efficiency was less and hence need to study regarding the modification over photoelectrodes materials. To overcome this issue, one of the research work demonstrated the use of CNT within Cu-based MSSCs with the motive of accelerating electron transfer rate and thereby enhancing PCE (Lee et al., 2014). In this research work, TiO2-MWCNT composite was synthesized through the

Figure 10.9 Representation of TiO2 based metal
organic frameworks sensitized solar cell.

Nanostructured inorganic metal oxide/metal
organic framework-based electrodes

357

hydrothermal method and deposited over FTO via doctor blade technique. The efficiency observed in this system was about 0.47% which is nearly 60% greater than that of the device previously reported (Lee, Shin, et al., 2014).

10.7.3 TiO2/Co-DAPV Along with various MOFs, cobalt-based MOFs named Co-DAPV [di(3- diaminopropyl)viologendibromide] can be adequately used in solar harvesting as a sensitizer. In the case of a solid-state PV device, iodine provoked conductivity was not ample to passage charge carriers in the device (Ahn et al., 2017; Lee et al., 2015). So further use of intrinsic p-type hole conductive MOF such as Co-DAPV in the solid-state device was applicable (Hyung et al., 2005). In the same way, one of the research work demonstrated the use of cobalt-based MOFs as a sensitizer in solid-state PV device with a considerable PCE of 2.1% (Ahn et al., 2017). The energy levels of Co-DAPV were well matched to be suitably employed for sensitizing TiO2. However, the LBL method was performed to sensitize spin-coated TiO2 nanostructures. It was observed that the number of cycles of sensitization increased the efficiency up to 2.1%; however, a further increase in cycle’s number decreased efficiency. Following Fig. 10.10 demonstrates the various synthetic methods deployed metal oxide/metal
organic frameworks composites.

Figure 10.10 Various synthetic methods deployed metal oxide/metal
organic frameworks composites.

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10.7.4 ZnO/ZIF-8 In the case of quantum dot-sensitized solar cells (QDSSCs), ZIF-8 was deployed to enhance the functional sorts of semiconducting ZnO structures which were exploited as photoanode material. After the composite formation of ZnO with ZIF-8, light trapping ability as well as fine charge transfer observed in material, which directly helps to enhance the solar to electric conversion efficiency. The solar to electric conversion efficiency for bare ZnO was observed to be 0.88%, and that for the ZnO/ZIF-8 composite system was 1.75% which was almost twice of bare ZnO (Li et al., 2020b). It is conceivable to improve the efficiency of this material by tuning the structure of MOFs and also prerequisite to focus on the stability of the system.

10.7.5 TiO2/MIL-125 MIL-125@TiO2 composite, synthesized via the single-step hydrothermal method for the first time to produce a highly photoactive material with virtuous prospects for constructing quasi-bulk depleted monolithic perovskite/ MOF@TiO2heterojunction solar cells (Vinogradov et al., 2014). In this work, it was revealed that materials possessed high stability over time (up to 30 days), with an efficiency of 6.4%. It is also possible to make this material more visible active up to 550 nm by insertion of some guest functional moieties such as amino group (
NH2) (Ahmadpour et al., 2020). In such a case, bang gap values decreased from 3.68 to 2.68 eV.

10.7.6 ZnO/PPF-11 Among the metalloligands, porphyrin linkers enlarged high curiosity because these linkers can lodge various elements including from transition to main group elements in the periodic table (Burnett et al., 2012). Meanwhile, the proper band alignments, optical properties, porous nature and also having numerous choices of SBU units in porphyrin-based MOFs, are deployed in solar cell technologies. By exploiting the synergetic performance of both ZnO and PPF-11, work demonstrated solvothermally grown PPF-11 on ZnO/FTO substrate to form efficient photoanode in MSSCs (Gordillo et al., 2019). The observed photocurrent was about 4.65 mA cm22 with an efficiency of about 0.86%, which was due to proficient well-agreed band alignments between ZnO and PPF-11. The LUMO of PPF was well agreed with that of ZnO, which lead to fluent electron transfer and having suitable HOMO of PPF-11 with I2/I32 redox couple (24.8 eV) for hole transportation. The efficiency was found to be less though having sufficient properties over the application. So this work has some future chance for the researchers to establish good results with the same system. Such various demonstrated heterostructured systems of metal oxide/ MOFs are listed in Table 10.4.

Table 10.4 Various metal oxide/MOFs composites utilized in solar energy harvesting. Metal oxide/ metal
organic frameworks composite

Synthesis methods

Properties

Outputs

Reference

TiO2/ZIF-8

Deep coating method, polymerization followed by reflux Post treatment strategy

Jsc and η values decreased with increasing ZIF-8 growth; core shell structure

Dye sensitized solar cells (DSSCs), efficiency: 5.34%; photocurrent density: 10.28 mA cm22

Li et al. (2011)

Absorption enhanced to visible region; MOF used as interface between dye and TiO2

DSSCs, efficiency: 14.4%; photocurrent density: B12 mA cm22; charge recombination resistance: 48.2 Ω MSSCs, efficiency: 0.26%; photocurrent density: 1.25 mA cm22

Li et al. (2011)

MSSCs, efficiency: 0.47%; Charge transfer resistance: 528.0 Ω; photocurrent density: B1.95 mA cm22 MSSCs, efficiency: 2.1%; IPCE: B25% at 540 nm; photocurrent density: 4.92 mA cm22

Lee, Shin, et al. (2014)

TiO2/Cu-BTC

TiO2/Co-DAPV

Doctor blade technique, LBL method Hydrothermal method, doctor blade technique LBL, Spin coating

I2 doping for reduction of electron resistance at interface Use of MWCNTs with Cu-BTC; electron transfer enhanced, maximum absorption at 680 nm Use of hole conductive MOFs; well aligned energy levels of MOF with TiO2

Lee et al. (2014)

Ahn et al. (2017) (Continued)

Table 10.4 (Continued) Metal oxide/ metal
organic frameworks composite

Synthesis methods

Properties

Outputs

Reference

TiO2/MIL-125

Single step hydrothermal method Sol-gel method

High stability endured 30 days; visible active up to 550 nm

Perovskite heterojunction solar cells, efficiency: 6.4%; short circuit current: 10.9 mA Quasi solid state DSSCs, efficiency: 2.34% with photocurrent density of 6.22 mA cm22 DSSCs, efficiency: 4.8% with 0.036 mA cm22 photocurrent density MSSCs, efficiency: 0.86% with photocurrent of 4.65 mA cm22

Vinogradov et al. (2014) Lopez et al. (2011)

TiO2/Zn-MOF

TiO2/Al2(BDC)3

Spin coating technique

ZnO/PPF-11

Solvothermal, drop casting

ZnO/ZIF-8

Self-assembled opal template method

Aerogel form of TiO2; enrichment in dye absorption; visible light active; Durable stability; short lifetime of charge separation Well-agreed band alignments; efficient charge separation and transportation Moderate light trapping ability; 3D inverse opal structure

QDSSCs, efficiency: 1.75%;

Lopez et al. (2011) Gordillo et al. (2019) Li et al. (2020a)

Nanostructured inorganic metal oxide/metal
organic framework-based electrodes

10.8

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Metal oxide/metal
organic frameworks nanocomposites for water splitting

10.8.1 α-Fe2O3/imidazole-based metal
organic frameworks α-Fe2O3 gained attractive attention as photoanode material in concern with the study of water splitting for hydrogen generation. With the required conditions for efficient water splitting, α-Fe2O3 partakes the properties such as a bandgap of 2.1 eV, absorption of visible light up to 40% of the solar spectrum, also suitable energy levels to efficient charge transfer, etc. (Seki, 2018). In consideration of disadvantage over α-Fe2O3, which possessed sluggish water oxidation kinetics. To overcome such issue, many efforts have been accomplished. One of the research groups demonstrated the use of imidazole-based MOF prepared by using cobalt as SBU to modify α-Fe2O3 nanorods (Zhang et al., 2018). This α-Fe2O3/Co-MOF resulted in photocurrent density of 2.0 mA cm22 at 1.23 V versus RHE which was higher than that of bare α-Fe2O3 nanorods (0.7 mA cm22 at 1.23 V vs RHE). This observed enhancement in photocurrent density was due to the insertion of Co-MOF synthesized via the ion-exchange method. Though this system showing efficient performance over the application of water splitting, still the stability remained unspecified.

10.8.2 BiVO4/MIL-101(Fe) BiVO4 has excellent stability with abundant nature, low cost, nontoxicity and so this material is utilized widely over the solar-driven water splitting studies (Qi et al., 2019). But having the shortcoming of poor electron conductivity and sluggish oxidation kinetics leads to unfortunate mechanisms in water splitting (Tang et al., 2013). In connection with this, to overcome such an issue, BiVO4 was utilized by coupling with MIL-101 (Fe) MOFs by hydrothermal route as a photoanode (Liu et al., 2020). As compared with pristine BiVO4 (1.00 mA cm22), the observed photocurrent was 4 times greater for the composite system (4.01 mA cm22). Though having better outcomes of this system, the pieces of evidence related to stability were ambiguous. In concern with the fabrication of a water-splitting device, a tentative H-shaped device for solar-driven water splitting to generate hydrogen is represented using Fig. 10.11.

10.8.3 TiO2/MIL-125 As nanostructured TiO2 has been consumed widely over solar-driven water splitting studies, many efforts have been accomplished to get better results and high STH conversion efficiency. In presence of MOFs, these TiO2 nanostructures revealed better performance with high stability and enhanced light absorption. In connection with such a case, TiO2/MIL-125 MOFs composite system designed via two-step solution-phase hydrothermal method was deployed as photoelectrode in PEC water splitting (Zhang, 2016). Though MIL-25 MOFs was only UV-active, their aminated derivatives were utilized for the sake of good efficiency. So further use of MIL-125(NH2) with TiO2 nanostructures enhanced photocurrent density up to 0.025 mA cm22 at 1.5 V versus

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Figure 10.11 Representation of H-shaped PEC water splitting device.

RHE which was almost double that of pristine TiO2. In the case of TiO2/MIL-125 (NH2), stability was further enhanced by incorporating gold (Au) nanoparticles (NPs) into this system, where photocurrent was observed about 0.035 mA cm22 at 1.5 V versus RHE. By considering the use of Au NPs in this study, the overall process perceived to be cost ineffectual and stability mentioned was up to 200 seconds. So for the longterm durable system, more efforts should be carried out.

10.8.4 ZnO/ZIF-8 Among various metal oxides which were utilized for energy studies, ZnO is one of the most suitable metal oxides. But by considering the laggings for bare ZnO, Alejandro, et al. established Cobalt (Co)-doped ZnO nanorods (NRs)-based photoanode for water splitting, where this Co doping contrived the enhancement in the light absorption of ZnO toward the visible region (Gala´n-Gonza´lez et al., 2020). Further use of ZIF-8 MOFs on Co-doped ZnO, noticeably amended the stability as well as charge separation and transfer properties of the NRs. The observed incident photon to current efficiency was about 75% at 350 nm which was almost double that of the bare ZnO. The chemical bath deposition (CBD) method was deployed to grow ZnO NRs which was further followed by spin coating for ZIF-8 loading on NRs. With the use of Pt as CE, HgO/Hg as RE, and

Table 10.5 Various metal oxide/metal
organic frameworks composites utilized in solar-driven water splitting studies. Metal oxide/ metal
organic frameworks composite

Synthesis methods

Properties

α-Fe2O3/Co-MOF (imidazole-based MOFs)

Hydrothermal, spin coating Ion exchange method

Nanorods; unspecified aqueous stability

BiVO4/MIL-101 (Fe)

Hydrothermal method

TiO2/MIL-125

Two step solution phase hydrothermal method

Molybdenum doping for enhancing conductivity with efficient charge carries transmission; red shift observed Visible light absorptive; cost ineffectual; stability mentioned was up to 200 s; use of aminated MOF

Co3O4/Co-MOFs

Hydrothermal synthesis

Nanocube morphology; number of active sites for HER & OER; stable over 20,000 s

Co3O4/NH2-MOF-5

Solution-based coating method

NiO/NH2-MOF-5

Solution-based coating method

Improper band alignments; insufficient visible light absorption; efficient toward OER than HER; Band alignment is appropriate only for OER; UV active

Performance

References

Components of device

Results

Ag/AgCl as reference electrode (RE); Pt as counter electrode (CE) and 1.0 M NaOH Ag/AgCl as RE; Pt as CE and 1.0 M Na2SO4 as electrolyte Electrolyte: 0.5 M Na2SO4 solution (pH 5 6.5); Ag/ AgCl as RE; Pt as CE Nickel foam (NF) as CE

Photocurrent density of 2.0 mA cm22 at 1.23 V versus reversible hydrogen electrode (RHE) Photocurrent density of 4.5 mA cm22 at 1.45 V versus RHE

Qi et al. (2019)

photocurrent density up to 0.025 mA cm22 at 1.5 V versus RHE

Zheng et al. (2019a, b)

Photocurrent density of 10 mA cm22 at 1.637 V versus RHE

Nickel foam (NF) as CE

Photocurrent density of 10 mA cm22 at overpotential 223 mV

Zheng et al. (2019a, b) Fiaz et al. (2021)

Nickel foam (NF) as CE

Photocurrent density near about 7 mA cm22 at 1.5 V versus RHE

Zhang (2016)

Fiaz et al. (2021)

(Continued)

Table 10.5 (Continued) Metal oxide/ metal
organic frameworks composite

Synthesis methods

ZnO/ZIF-8

Chemical bath deposition, spin coating Solvothermal method, ultrasonication

Solvo and hydrothermal, spin coating Fe2O3/NH2-MIL101(Fe)

Solvothermal method

ZnO/ZIF-67

Hydrothermal method, spin coating, deep coating

Fe2O3/MIL88B@ZIF-67

Hydrothermal method, spin coating

Properties

Performance

References

Components of device

Results

Stable system; well charge transfer; nanorod morphology

Pt as CE, HgO/Hg as RE and electrolyte

Xue et al. (2019)

Use of Ni(OH)2 as co-catalyst; nanorod morphology; durable stability

Pt as CE; Saturated calomel electrode (SCE) as RE and 0.1 M KOH

Tedious synthetic protocol; nanotube morphology; red shift observed; abstruse aqueous stability Core/shell nanorod; light absorption in visible region up to 590 nm; fast charge transfer; short hole diffusion length

Ag/AgCl as RE; Pt as CE, and 0.5 M Na2SO4 (pH 6.8)

Photocurrent density of B0.15 mA cm22 at 1.23 V versus RHE Photocurrent density of 2.0 mA cm22 and IPCE of 40.05% at 1.4 V versus RHE 1.95 mA cm22 Photocurrent density of B0.4 mA cm22 at 0.4 V versus RHE

Ag/AgCl as RE; Pt as CE and 1.0 M NaOH as electrolyte

Photocurrent density of 2.27 mA cm22 at 1.23 V versus Ag/ AgCl

Nanoarrays; durable stability (for 1000 s); charge separation-near about 80% at 0.8 V versus SCE; visible active within 450
700 nm range Durable stability with only 5% decay (up to 20 h); number of layers of dual MOFs for better efficiency still obscure

Pt as CE; SCE as RE and 0.5 M Na2SO4 (pH 6.8)

Photocurrent density of 1.25 mA cm22 at 1.0 V versus SCE

Liu et al. (2020) Dong et al. (2018) Dou et al. (2017)

Pt as CE; Ag/AgCl as RE and 1.0 M NaOH as electrolyte

Photocurrent density of 2.52 mA cm22 at 1.23 V versus RHE

Xue et al. (2019)

Han et al. (2019)

Hong et al. (2020)

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organic framework-based electrodes

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electrolyte, a three-electrode system utilized for the experimental results. The photocurrent displayed in this work was about 0.16 mA cm22 with moderate stability. With help of Table 10.5, it is conceivable to overlook on numbers of metal oxide/MOFs composites for water splitting application.

10.9

Conclusion and future perspectives

The utilization of solar energy to overcome the excessive use of nonrenewable energy sources is of prodigious interest to a forthcoming society. PV devices and solar-driven water splitting both are excellent routes to utilize solar energy efficiently. In concern with such energy studies, the step of developing efficient and stable photoelectrodes for this purpose is a critical one. Among various active materials, metal oxides and their composites with MOFs have shown remarkable existence in these studies. Such a system of metal oxide/MOFs composites is very effective in avoiding each other’s limitations. This chapter summarizes numerous metal oxide-MOFs heterostructured assemblies including their synthetic protocols, properties, development of their stable and efficient thin-film electrodes as well as outcomes of studies. It can be summarized as follows. Various synthetic protocols including the techniques such as hydro/solvothermal, microwave, sonochemical, mechanochemical, etc., can be deployed to synthesize metal oxides, MOFs, and metal oxide/MOFs composites. These efforts are made to develop materials with diverse structures having in-situ/ex-situ growth of metal oxides on MOFs or MOFs on metal oxides. The formation of metal oxide/MOFs composite assemblies may lead to inhibiting the restraints of each other. This imparts benefits of both metal oxides (having unique optical, electrical, mechanical properties, etc.) and MOFs (having high porosity, surface area, diverse morphologies, fortunate thermal stability, etc.) to the energy studies. Various strategies can be utilized to design and fabricate efficient, stable, uniform, and well-organized photoelectrodes of metal oxide/MOFs composites. This encompasses chemical, physical, electrochemical, as well as solution-based methods for the cost-effective advance of electrodes in energy studies. The aforementioned composite systems can deliver superior properties toward regimented energy studies. The observed outcomes of studies including power conversion efficiencies, photocurrent density values, stability of materials, optical, electrical properties, etc., found to be enhanced using such heterostructured assemblies. In concern with future standpoints, there will be room for stable systems to develop ideal heterostructures to solve energy as well as environmental issues. It is clear that surface and structure modification of MOFs for composite formation will gain a striking path for enhancing prerequisite assets in energy studies. Also, the offered consequences of this chapter can provide some new outlooks for designing cost-effective and simplistic fabrication of metal oxide/MOFs systems and their commercial production toward practical application. This chapter provides the

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future perspective of designing in a more water-stable heterostructured system by means of various modifying strategies. However, more research needs to be done for encouraging renewable sources and their use for potential energy studies.

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Part III : Other applications of metal oxide-based composites

Metal oxide nanocomposite-based electrochemical biosensing studies

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Ankita K. Dhukate1, Sajid B. Mullani1, Lynn Dennany2 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Pure and Applied Chemistry, University of Strathclyde, Technology and Innovation Centre, Glasgow, United Kingdom

11.1

Introduction

The growing world population is facing various challenges such as energy crisis, environmental sustainability, global public health problems, etc. Among these, the resolution of public health issue is of prime importance for the people to live happily and enjoy each moment of their life. However, diseases such as diabetes mellitus, HIV, cancer, Alzheimer’s disease, stroke, and other neurological diseases have no permanent cure yet. New life-threatening diseases and viruses like COVID-19, Zika, Ebola, SARS, MERS and H1N1 etc. are affecting humans, which lead to loss of life. Early diagnosis, proper medical treatment and accessible health monitoring systems can be both cost and life saving for the patients. Particularly, biosensing protocols in connection to early diagnosis as well as health monitoring have placed a significant importance in biomedical sector. In the current context where population and bio-testing capacity have a remarkable gap, and the effect of prompt and specific detection of target molecules is also important from the point of view of biosensor field (Huang et al., 2017). Hence, the development of biomedical sensors having the features such as ease to handle, cost effective, short time span for analysis, multianalyte sensing, sophisticated digital display, etc. are the need of hour. For example, glucometer is a well-known example of a biosensor device; glucometer measures the sugar content present in blood sample. Similarly, various sensors such as glucowatch, tooth enamel biosensor, colorimetric sweat biosensor, etc. have wide application in biosensing for the analysis of different biomolecules. In addition to these, the nanomaterial-based biosensor has gained much attention among the scientists (Zhu et al., 2015). This leads opportunities regarding new materials for achieving specific functionality and selectivity. Therefore, it is imperative to start with the basics of biosensors so the readers can understand the present aspects in depth. A biosensor is analytical devices that sense an analyte (biomolecule usually) via interaction with a recognition element (receptor—biological and non-biological); which process through transducer system so as to get the electrical signals in relation to the analyte concentrations. Therefore, the analyte, receptor, transducer, and output signal are the basic components of the biosensing device (Fig. 11.1). Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00015-4 © 2022 Elsevier Inc. All rights reserved.

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Figure 11.1 General schematic of electrochemical biosensor.

Numerous approaches including optical, thermal, electrochemical, and physical biosensor platforms have been explored in the field of biosensor technology (Shavanova et al., 2016). Compared with other detection methods, an electrochemical approach offers a much less expensive, more facile, and highly sensitive detection method, which enables the monitoring of the different analytes, fast response recovery times, and very low detection limits. Also, electrochemical detection is not affected by sample components such as chromophores, fluorophores, and particles that often interfere with spectrophotometric detection (Ronkainen et al., 2010). Based on the receptor, the electrochemical sensor has two types; one is enzymatic-based and other is nonenzymaticbased one. Especially, enzyme-based biosensors are of most interest from last many years, since sufficient research has been going-on but still there are various constraints in these biosensors (Baghayeri, 2015; Jain et al., 2020). The various constrains are instability, short shelf life, high cost of enzymes, difficult immobilization technique and critical operating procedure, etc. Therefore, considerable attentions have been focused to overcome these limitations and hence investigators trying continuously for the excellent electrochemical biosensors. Non-enzymatic sensor is the one of the sensing devices; which has a potential to overcome the limitations of enzymatic sensors. With the introduction of nanoscale materials, researchers are highly focusing on the development of novel nanomaterials as electrode material in non-enzymatic sensing substrates. This is because of their advantages such as improved catalytic reaction, more efficient electron transfer (Dhodamani et al., 2020), increased surface area, good biocompatibility and fine control over electrode microenvironment, etc. (Zhu et al., 2015). Nanostructured materials viz. metals, metal oxides, conducting polymers (CPs), carbon nanostructures have opened new ways to improve the efficiency of non-enzymatic electrochemical biosensors (Cho et al., 2020). Among the various nanostructured platforms, metal oxides nanostructures possess overriding advantages such as high stability (Yadav et al., 2014), ease synthesis, easily engineered for desired size, shape, and porosity, no swelling variations, easy incorporation into hydrophobic and hydrophilic systems and ease functionalization, etc. that make them a promising tool for biomedical applications (Ahmad et al., 2010). Various metals and their oxides including gold (Au), copper (Cu), cupric oxide (Cu2O), copper oxide (CuO), cobalt oxide (CoO), manganese dioxide (MnO2), nickel (Ni), nickel oxide (NiO), palladium (Pd), platinum (Pt), tin oxide (SnO2), titanium dioxide (TiO2), etc. have shown attractive properties such as

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tunable structural-electrical properties, efficient biocompatibility, non-toxicity, and catalytic ability, which is beneficial in non-enzymatic biosensing determination (Ahmad et al., 2021). These nanostructured materials display fast electron transfer rates and easy adsorption properties results in enhanced biosensing activity. To modulate desired properties of bare metal oxides, the functional nanocomposites of metal oxide with various materials such as CPs, carbon nanostructures, noble metal nanoparticles, metal organic frameworks (MOF) are also promising materials for efficient non-enzymatic biosensing applications. Therefore, this chapter focuses on recent advances in nanomaterial-based electrochemical sensors for biosensing applications. It also provides the reader with a clear and concise view of new advanced modification strategies for electrochemical signal amplification and novel electrochemical approaches used in the miniaturization and integration of the sensors.

11.2

Present scenario of biosensor market

The global biosensor market was valued at $17,500.0 million in 2018, and is expected to reach $38,600.2 million by 2026, registering a CAGR of 10.4% from 2019 to 2026 (Fig. 11.2). New communicable, non-communicable diseases and increasing number of diabetes as well as cardiovascular patients demands the advanced and reliable biosensors. Increasing applications of biosensors in healthcare sector, industries, research and point of care testing drives the growth of global electrochemical biosensor market. However, factors such as strict regulatory requirements, and reimbursement policy issues in healthcare system are further hampering the market. Along with healthcare, applications of biosensors are increasing in various industries such as

40000 35000 30000 25000 20000 15000 10000 5000 0

2018

2020

2022

Figure 11.2 Global biosensor market in millions.

2024

2026

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food, environment, drug discovery, and security, etc. Biosensors are also used for detection and identification of diseases in crops and for measuring the level of pesticides, herbicides, and heavy metals in soil and ground water. Similarly, increasing presence of toxins, chemicals, and pathogens in food is a major concern. Biosensors are used to ensure quality and safety of products and detection of microbial pathogens and toxins in food, and are cost-efficient. Increasing number of harmful pollutants in the environment has also created a need for analytical and monitoring devices. Biosensors are also used in bio-defense for detection of harmful chemical and biological warfare agents, forensic identification for crime detection. Therefore, increasing applications of biosensors would boost the growth of the market.

11.3

Nonenzymatic electrochemical biosensors

The non-enzymatic electrochemical sensors proved as an inexpensive and effective analytical method for the quantitative detection of various biomolecules. After the discovery of amperometric glucose sensor by Clark, many developments are done in biosensor field. Clark’s amperometric glucose sensor was categorized in first-generation biosensor, where glucose is oxidized to gluconic acid in presence of oxygen and glucose oxidase immobilized on platinum electrode. As the instrumentation of Clarks, sensor is very complex and inconvenient because of free oxygen involvement, which restricts the application area of first-generation biosensors. To solve problem related to use of free oxygen, second-generation biosensors have been introduced, where redox mediators used instead of direct oxygen. Commonly used mediators like ferrocene derivatives (Bhalinge et al., 2016) possess essential aspects such as low molecular weight, insoluble nature, etc. These aspects of mediators provide a reversible or quasi reversible process, effective diffusion throughout system, a suitably lowering redox potential to avoid oxidation of interfering species, a high stability and resistance to forming side compounds, and a low toxicity. Although mediators have fascinating properties, still problems may arise due to small and diffusive nature of mediator molecules resulting in the difficulties to maintain concentration of mediator near electrode surface (Toghill & Compton, 2010). The constraints of second-generation biosensors are to be resolved with designing of third-generation biosensors. The third-generation biosensors are mediator free and based on direct electron transfer from enzyme to electrode. Although third-generation biosensors have many advantages and are successfully commercialized, few aspects again restrict third-generation biosensor applications. Along with stability and cost-effectiveness, a major barrier lies in electron transfer between biological element and transducer system as only few proteins show direct electron transfer with electrode. Therefore, the limitations of third-generation biosensor are the real motivations to construct the label-free (non-enzymatic) biosensors. These label-free biosensors potentially detect many biomolecules without using any biorecognition element and hence these form 4th generation biosensors. The need of fragile enzymes is eliminated by non-enzymatic approach. Nonenzymatic biosensors can be defined as

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Figure 11.3 Evolution of electrochemical biosensors: (A) first-generation biosensor, (B) secondgeneration biosensor, (C) third-generation biosensor, and (D) 4th generation biosensor.

“Electrochemical sensors contain non-biologically active elements, improving the sensitivity and selectivity of the sensors in analyte detection.” Non-enzymatic sensors generally detect chemical or biological species through their redox activity. However, metal oxide (MO)-based electrochemical sensors are perfect for the electro analysis of biomolecules because of their simplicity, inexpensive, quick response, and good portability. Hence, enzyme-free electrochemical sensors have been widely used for determining the presence of hydrogen peroxide, glucose, dopamine, uric acid, etc. Chen et al. (2014) (Fig. 11.3) However, the analytical performance of enzyme-free electrodes has some drawbacks like slow electrode kinetics and high over-potential, etc. Moreover, the poor measurement stability caused by surface poisoning from the intermediate products adsorbed or the effect of co-existing electro-active species is still a serious problem in the application of these electrodes. Therefore, current efforts have mainly focused on discovering new materials with high electrocatalytic activity and good stability to construct enzyme-free sensors.

11.4

Functional nanocomposites in electrochemical biosensor

Nanocomposite is a multi-phase solid material where at least one of the combining phases/components having dimensions of less than 100 nanometers (nm). Functional nanocomposites are the nanocomposites where same host materials

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(usually nanocomposites) used for the different applications through their functionalization’s or modification’s. The functional nanocomposites have the promising properties through the additional surface modification; which are to be modified via chemical and physical routes. Bare materials possess some limitations which impede their usage in the applications. For instance, the electrical as well as ionic conductivity of bare metal oxide is poor which limits their applications in electronics and electrochemical devices. Additionally, most of the bare metal oxides exhibit a wide band gap, due to which these are only responsive to ultraviolet light and cannot absorb visible light. Thus, additional surface modification is highly indispensable to improve the different physical and chemical properties. Further functionalization is also able to enhance their sensitivity as a diagnostic tool and imbue metal oxides with additional capabilities such as drug delivery and biomolecule sensing. Ascribed to the large surface-to-volume ratio and tunable electronic band structure of planar MOs, functionalization of MOs with various approaches is relatively straightforward, such as surface modification via chemical bonding and physical adsorption and inducing oxygen vacancies and doping (Deshmukh et al., 2021; Ren et al., 2020). Along with surface functionalization, the morphology as well as covalent/non-covalent interactions of MOs with others is also playing dominant role in alteration of the properties of MOs. The creation of covalent bonds or coordinate bonds with atoms on the surface of 2D MOs occurs when organic molecules modify the surface via a chemical mechanism. Such a strong chemical bond is necessary to firmly anchor functional groups on MO surfaces as well as to modify properties of MOs that are not easily altered by the physiological environment. Polymers or smaller organic molecules are typical organic moieties covalently or non-covalently bonded to MOs for biosensing and therapeutic purposes (Xu et al., 2017). The physisorption of desired biomolecules or target analyte onto the relatively broad basal surface of the MOs, allowing for maximum loading as well as interaction with the surrounding environment. Typically, physical adsorption is non-covalent and achieved via hydrophobic interactions, electrostatic attraction and van der Waals forces. The large surface area of metal oxides provides large amounts of anchoring sites for guest agents, hence metal oxides nanocomposites can strongly adsorb diverse guest agent, which make metal oxides excellent for transducer systems (Zhou et al., 2019). The electronic functionalities and catalytic properties of MOs are strongly influenced by defects, doping (Koli et al., 2020), and the cation oxidation state. Therefore, introducing oxygen vacancies and doping in MOs are important modification strategies for surface functionalization of 2D MOs to modify the electronic band structure and to adjust the Fermi level. For example, the existence of various oxygen vacancies endows molybdenum oxides with diverse interesting characteristics. Typically, fully stoichiometric MoO3 is regarded as catalytically inert, while sub-stoichiometric MoO3 introduces a large number of active sites due to the increment of oxygen vacancies. Furthermore, molybdenum oxides with different oxygen vacancies typically exhibit controllable band gaps of 2.8 3.6 eV. The change of stoichiometry of MOs can be achieved chemically or electro-chemically, by removing oxygen via. chemical reduction reactions with proper reductants (Wang et al., 2018) (Fig. 11.4).

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Figure 11.4 Schematic of the role of metal oxide nanoparticles in electrochemical determination of analyte.

As per the literature, the functional nanocomposites are broadly classified into the following types; which are used in non-enzymatic electrochemical biosensing: (Table 11.1)

11.4.1 Metallic nanoparticle-based composites Nanoparticles of various metals such as palladium, copper, platinum, nickel, gold, silver, etc. are comprehensively used in electrode preparation for detection of analytes like Glucose, Ascorbic acid (AA), Uric acid (UA), Dopamine (DA), Creatinine, hydrogen peroxide(H2O2), etc. A novel electrochemical sensor was fabricated by Demirkan et al. using palladium nanoparticles supported on polypyrrole/ reduced graphene oxide (rGO/Pd@PPy NPs) composites. These composites were deposited onto glassy carbon electrode (GCE) for the simultaneous detection of AA, DA and UA. rGO/Pd@PPy nanocomposite-based electrodes were highly selective and sensitive toward the specific analytes. This is attributed to their higher conductivity as well as superior catalytic activity in the reactions between analyte and composite materials for the detection of specific analytes with higher current as well as oxidation peak intensities (Demirkan et al., 2020). Similarly, palladium nanoparticles decorated on the surface of magnetic graphene oxide functionalized with amine-terminated poly(amidoamine) dendrimer (GO-Fe3O4-PAMAM-Pd) was utilized for non-enzymatic electrochemical determination of H2O2 by Baghayeri et al. (2019) Under optimum conditions, the designed sensor showed good electrochemical performance toward H2O2 reduction, offering extended linearity of 0.05 to 160 μM and a low detection limit of 0.01 μM. Raveendran et al. fabricated a disposable copper modified non-enzymatic sensor for creatinine. The estimation of

Table 11.1 Transition metal-nanocomposite based Electrochemical biosensors: Materials

Analyte

LOD

Linear range

References

Glucose H2O2 Ascorbic acid Dopamine Uric acid Glucose Acetylcholine Creatinine Glutamate H2O2 Ascorbic acid Dopamine Uric acid L- Glutamate Glucose Ascorbic acid Glucose H2O2 Glucose H2O2 Dopamine H2O2

10 μM 0.01 μM 4.9 3 1028 M 5.6 3 1028 M 4.7 3 1028 M 1.39 μM 39 μM 0.0746 μM 83 μM 1.2 μM 0.43 μM 0.07 μM 0.63 μM 0.1 μM 4.1 μM 0.5 μM 50 μM 1 μM 0.79 μM 0.15 μM 1.26 nM 0.53 μM

0.05 10 mM 0.05 160 μM 1 3 1023 1.5 3 1022 M 1 3 1023 1.5 3 1022 M 1 3 1023 1.5 3 1022 M 0.02 2.3 mM 0.12 2.68 6 378 μM 0.5 8.0 mM 1.0 8.0 mM 10 1800 μM 0.5 211.5 μM 9.5 1187 μM 0.004 0.9 mM 0.2 9 mM 1.0 3 1025 M 1.8 3 1023 M 0.1 30 mM 5 μM 11.5 mM 50 0.1 μM 0.02 20 mM 0.01 1609 μM 0.001 2 5 mM

Shen et al. (2015) Baghayeri et al. (2019) Demirkan et al. (2020)

Glucose Dopamine Uric acid H2O2

11 μM 0.18 μM 0.08 μM 0.04 μM

50 μM 1 mM 0.3 1.4 μM 0.4 400.0 μM 0.2 2000 μM

Ahmad et al. (2021) Kudur Jayaprakash et al. (2017) Karimi-Maleh and Arotiba (2020) Han et al. (2019)

Metals Palladium

Copper

Platinum

Nickel Pd Au bimetallic cluster AuPd@GR Silver Molybdenum Iron

Wang et al. (2019) Shadlaghani et al. (2019) Raveendran et al. (2017) Shadlaghani et al. (2019) Bian et al. (2010) Zhang et al. (2016)

Barman et al. (2018) Chen et al. (2014) Zhang et al. (2013) Shen et al. (2015) Thanh et al. (2016) Ahmad et al. (2021) Sheng et al. (2020) Keerthi et al. (2019) Sheng et al. (2020)

Metal oxides Copper oxide

Zinc oxide

Iron Oxide

Titanium Dioxide

Nickel Oxide

Glucose Dopamine Uric acid Epinephrine Norepinephrine Levadopa Urea Serotonin Glucose Dopamine Xanthine Uric acid H2O2 Glucose Dopamine H2O2 NADH Uric acid Guanine Hemoglobin L-Cyteine Ascorbic acid Cholesterol Glucose H2O2 Acetylcholine Dopamine

0.1 μM. 0.039 nM 5.72 nM 0.0093 μM 0.2 μM 0.08 μM 0.011 mg dL21 0.66 μM 234 μM 0.035 μM 0.092 μM 0.106 μM 1.0 μM 8 μM 0.03 μM 0.7 μM 0.2 μM 70 nM 50 nM 0.3 μM 250 μM 1.13 μM 0.13 μM 0.16 μM 0.2 μM 26.7 μM 1.038 μM

0.25 110 mM 0.12 nM-152 μM 6.25 625 nM 0.02 216 μM 0.5 30 μM 0.6 100.0 μM 0.1 250 mg dL21 7.5 300 μM 1 16 mM 0.3 210 μM 0.5 177 μM 0.5 265 5.0 4495.0 μM 0.03 14 mM 0.049 30 μM 10 200 μM 10 240 μM 0.1 500 μM 0.1 40 μM 1 1170 μM 500 6000 μM 2.5 to 100.0 μM 2 40 μM 1 110 μM 3 700 μM 0.25 5.88 mM 2 100 μM

Imran et al. (2021) Song et al. (2019) Mullani, Tawade, et al. (2020) Zhu et al. (2020) Wang et al. (2015) Karimi-Maleh et al. (2018) Migliorini et al. (2018) Mullani, Dhodamani, et al. (2020) Wang et al. (2018) Yasmin et al. (2015) Ganesan et al. (2021) Wang et al. (2014) Gao et al. (2016) Mahshid et al. (2011) Fan et al. (2012) Chang et al. (2019) Sun et al. (2011) Hussain et al. (2021) Jahani & Beitollahi (2016) Rengaraj et al. (2015) Gao et al. (2016) Lata et al. (2012) Shadlaghani et al. (2019) Roychoudhury et al. (2016)

(Continued)

Table 11.1 (Continued) Materials

Analyte

LOD

Linear range

References

H2O2 Glucose Urea Lactic Acid L-glutamic acid Uric acid Urea Glucose Dopamine H2O2

0.015 μM 0.04 μM0.14 μM 5.0 μM 0.006 mM 10.0 pM 60.0 pM 14.693 μM 1.8 μM 0.027 μM 0.55 μM

0.05 400 μM450 1250 μM 5 60 μM0.2 3.0 mM 0.06 0.30 mM 0.05 3 mM 1 nM 0.1 M 0.1 nM 0.1 M 5 100 μM 5 μM 2 mM 0.1 μM 0.08 mM0.08 041 mM 40 10230 μM

Kogularasu et al. (2017) Mondal et al. (2017) Nguyen et al. (2016) Chang et al. (2019) Hussain et al. (2016)

Metals Cobalt Oxide

Manganese Dioxide

Ramasami Sundhar Baabu et al. (2020) Han et al. (2016) Li et al. (2020) Bohlooli et al. (2021)

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creatinine was based on the formation of soluble copper-creatinine complex with linear range of 6 378 μM (Raveendran et al., 2017). Platinum nanoparticles are favorable candidates due to their surface electrical effect and catalytic properties used widely in electrochemical biosensors. Zhang et al. prepared Pt NSs/C60/GCE electrode exhibited three well-resolved voltammetric peaks in the differential pulse voltammetry (DPV) measurements, allowing a simultaneous detection of these biomolecules. The limits of detection (LOD) (S/ N 5 3) are down to 0.43, 0.07 and 0.63 μM for AA, DA and UA, respectively. Bian et al. prepared Pt/polypyrrole (PPy) hybrid hollow microspheres by wet chemical method for determination hydrogen peroxide. The composite showed the linear range in between 1.0 and 8.0 mM with a relatively low detection limit of 1.2 μM as well as high sensitivity of 80.4 mAM21 cm22 (Shen et al., 2015). Another metal nanoparticles popularly used in electrochemical biosensor is gold. Nanosized gold nanoparticles exhibit extraordinary catalytic and biosensing, properties due to large surface-to-volume ratio and the interface-dominated properties, and these can decrease overpotentials of many analytically important electrochemical reactions. Shen et al. synthesized bimetallic Pd Au cluster through direct chemical reduction method for non-enzymatic sensing of glucose. The glucose contents detected by the electrode were in good agreement with those from the hospital when the electrode was employed to detect glucose in blood samples (Shen et al., 2015). The amperometric sensor developed with enzyme-free silver nanoparticles-based electrodes for the glucose detection. The excellent electrocatalytic activity toward non-enzymatic glucose sensors was reported through the excellent sensitivity (2.7664 mA mM21 cm22), a wide linear range of detection (50 0.1 mM), LOD (0.79 mM) and rapid response time (Deshmukh et al., 2020). Guan et al. studied electrochemical performance of Ag/NCNFs-based sensor Ag/NCNFs-based sensor reported superior reproducibility and excellent stability and used for H2O2 detection in milk samples (Guan et al., 2018).

11.4.2 Metal oxide nanomaterial’s-based composites Metal oxides have gained practical as well as theoretical importance in the biological science, environmental science and analytical chemistry. This is due to their extraordinary properties such as low cost, easy to synthesize, high surface area-to-volume ratio, high chemical stability in physiological environment, and electrochemical activity. The metal oxides are widely used in electrochemical biosensors for non-enzymatic detection of biological species like proteins, sugars etc. due to their very well-known catalytic properties. Literature studies revealed that the various nanoparticles like gold nanoparticles (AuNPs) (Comotti et al., 2004) and Fe3O4 (Perez, 2007) showed enzyme like activity in biosensing studies. Non-enzymatic detection comes into light. Similarly, nanoparticles of Pt, Pd, NiO, CuO etc. with types of morphologies and dimensions are used for the development of the fourth generation of electrochemical biosensors. Therefore, numerous researchers have been focused on the various aspects of electrochemical biosensors so as to make extremely interesting technology in the field of biomedical applications. The modification of the working electrode using metal oxide

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nanoparticles conjugated with carbon matrix like graphene, graphene oxide, reduced graphene oxide, carbon nanotubes (CNTs), multiwalled carbon nanotubes (MWCNTs), CPs, biopolymers, silanes, antibodies, etc. are conferred in detail. In addition, doping of transition metal oxide nanoparticles with various elements like nitrogen, noble metals and enzymes are also considered for modifying the properties of bare metal oxides. Reddy et al. synthesized CuO nanoparticles of various morphologies by coprecipitation method, and used to prepare modified carbon paste electrode for electrochemical determination of DA. Owing to properties like surface area of the electrode, the heterogeneous rate constant (ks) and the lower detection limit (5.5 3 1028 M), this sensor showed good results in electrochemical sensing of DA (Kudur Jayaprakash et al., 2017). The copper/cuprous oxide (Cu/Cu2O) nanocomposites were electrodeposited on a fluorine doped tin oxide (FTO) glass substrate for sensitive determination of H2O2 by Han et al. The sensor exhibited a wide linear range of 0.2 2000 μM for the determination of H2O2 with a detection limit of 0.04 μM (Han et al., 2019). Hassan et al. synthesized CuO-rGR/M3OIDTFB/CPE electrode for simultaneous detection of cholesterol (CL), AA, UA. The sensor displayed linear response in the concentration ranges 0.04 300.0, 0.04 240.0 and 0.4 400.0 μM for CL, AA and UA with the detection limits 9.0 nM, 9.0 nM and 0.08 μM, respectively (Karimi-Maleh & Arotiba, 2020). Imran et al. developed a non-enzymatic biosensor for glucose detection using graphitic carbon nitride (g-C3N4) modified with platinum and zinc oxide. The electrochemical glucose sensing at the ZnO-Pt-g-C3N4 occurred at low applied potential of 10.20 V (vs Ag/ AgCl) with high sensitivity 3.34 μA mM21 cm22 and fast response (5 s) time. This sensor exhibited a wide linear range 0.25 110 mM with lower limit of detection of 0.1 mM. The ZnO existence with Pt providing more hydroxyl ions and also promoting the electrocatalysis in neutral physiological buffer solution (Imran et al., 2021). Song et al. studied DA-imprinted chitosan (CS) film/ZnO nanoparticles (NPs)@carbon (C)/ three-dimensional kenaf stem-derived macroporous carbon (3D-KSC). This is a fast, efficient and sensitive method for detecting DA. The sensor has the detection limit of 0.039 nM, and the sensitivity of 757 μA mM21 cm22 (Song et al., 2019). Nickel (Ni21) ion doped zinc oxide-multi-wall carbon nanotubes (NZC) composites were used as electrode materials with glassy carbon electrodes (GCEs) for electrochemical detection of UA by Mullani et al. The limit of detection (LOD) and limit of quantification (LOQ) for the NZC 0.1/GCE were reported to 5.72 and 19.00 nM (S/N 5 3) respectively, which is the lowest compared to the literature values reported so far for enzymatic and nonenzymatic detection techniques (Mullani, Tawade, et al., 2020). Zhu et al. synthesized epinephrine sensor based on the hierarchical flower-like zinc oxide nanosheets (flowerlike ZnO) embedded into three-dimensional ferrocene-functionalized graphene framework. The flower-like ZnO/3D graphene@Fc exhibited excellent sensing performance for epinephrine with a wide linear range of 0.02 to 216 μM, a low detection limit of 0.0093 μM, high anti-interference and good cyclic stability. The sensor has a linear range of 0.192 to 527 μM and a detection limit of 0.1 μM for oxidized derivative of epinephrine (Zhu et al., 2020). Mullani et al. conducted efficient 5-HT sensing studies using the (ZnO NRs)12x (CNs)x nanocomposites. The sensor has wide linear response range (7.5 300 μM); lower limit of detection (0.66 μM), excellent LOQ (2.19 μM) and good reproducibility (Mullani, Dhodamani, et al., 2020).

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One-step electrode position method of Polypyrrole-Chitosan-Iron oxide (Ppy-CSFe3O4) nanocomposite films developed by Abdul Amir Al-Mokaram for the fabrication of advanced composite coatings for biosensors applications. The fabricated electrode PpyCS-Fe3O4 NP/ITO showed a fast amperometric response with good selectivity to detect glucose non-enzymatically with improved linearity (1 16 mM) and the detection limit of (234 μM) at a signal-to-noise ratio (S/N 5 3.0) (Wang et al., 2018). Nitrogen and sulfur dual doped graphene supported Fe2O3 (NSG-Fe2O3) have been prepared by hydrothermal methods and subsequently utilized for the electrochemical determination of DA in presence of AA by Yasmin et al. The NSG-Fe2O3 has shown sensitivity (29.1 μA mM21), long linear detection range (0.3 210 μM) and detection limit (0.035 μM) (Yasmin et al., 2015). Ganesan et al. synthesize a nitrogen-doped carbon quantum dots (N-CQD) decorated iron oxide (N-CQD@Fe2O3) for simultaneous electrochemical determination of structurally similar analyte of anticancer drug 5-fluorouracil (5-FU) and inflammatory agents UA and xanthine (XA). The modified electrode, N-CQD@Fe2O3/MWCNT/GCE, showed excellent sensitivity for the detection of UA, XA, and 5-FU with the detection limit of 0.106, 0.092, and 0.019 μM (Ganesan et al., 2021). Gao et al. prepared a electroactive Ni(OH)2 and protective TiO2 composite film on NiTi alloy(Ni(OH)2/TiO2/NiTi) for glucose sensing. The sensor has sensitivity of 192 A μM21 cm22, short response time of less than 1 second, and detection limit of 8 μM (Gao et al., 2016). Biswas et al. synthesized a titanium dioxide nanoparticle (NPs) by a sol-gel method. NPs were used to modify a graphite paste electrode for simultaneous determination of UA and guanine. The sensor has linear range of 0.1 500 and 0.1 40 μM for UA and GU, respectively and detection limit is 70 nM for UA and 50 nM for GU (Chang et al., 2019). Nickel oxide is promising candidate for enzyme-free electrochemical detection of biomolecules, due to the existence of the redox couple of Ni(OH)2/NiOOH formed on the electrode surface in alkaline medium. Mu et al. developed a fast and sensitive sensor for glucose determination based on NiO modified carbon paste electrode. The sensor has a sensitivity of 43.9 nA mM21, and detection limit of the electrode is 0.16 μM (Mu et al., 2011). Lata et al. immobilized cytochrome-c onto nickel oxide nanoparticles/carboxylated multiwalled carbon nanotubes/polyaniline modified gold electrode for H2O2 detection. The sensor has linear range of 3 700 mM and detection limit of 0.2 M with high sensitivity of 3.3 mA mM21 cm22 (Lata et al., 2012). Sattarahmady et al. synthesized Lichen-like nickel oxide nanostructure and then applied to modify a carbon paste electrode for the fabrication of the electrocatalytic oxidation of acetylcholine (ACh). A sensitive and timesaving hydrodynamic amperometry method was developed for the determination of ACh. ACh was determined with a sensitivity of 392.4 mA M21 cm22 and a limit of detection of 26.7 μM (Sattarahmady et al., 2013). Roychoudhury et al. developed a DA biosensor using nickel oxide nanoparticles (NPs). The electrochemical response studies of the fabricated electrode showed improved sensitivity (0.0602 mA mM21) for dopamine detection in wide linear detection range (2 100 mM) with fast response time of 45 seconds due to the presence of stable and highly catalytic NiO NPs that facilitates fast electron transport from biorecognition element to the underlying ITO substrate (Roychoudhury et al., 2016).

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Sakthivel et prepared polyhedrons-structured cobalt oxide (Co3O4 PHs) and three-dimensional graphene oxide-encapsulated cobalt oxide polyhedrons (3D GO Co3O4 PHs) by hydrothermal route for the determination of H2O2. The sensor has wide working range (0.05 400 and 450 1250 μM) and low detection limit (15 nM) (Kogularasu et al., 2017). Mondal et al. prepared a selective and sensitive non-enzymatic electrochemical glucose sensor by using cobalt oxide nanoflowers (NF). The developed amperometric glucose sensor exhibited excellent antiinterfering property and two wide linear ranges of 5 to 60 μM and 0.2 to 3.0 mM, with high sensitivities of 693.02 mA mM21 cm22 and 228.03 μA mM21 cm22 and detection limits (LOD) as low as 0.04 and 0.14 μM, respectively (Mondal et al., 2017). Nguyen et al. synthesized a NiCo2O4 bimetallic electro-catalyst on threedimensional graphene (3D graphene) for the non-enzymatic detection of urea. The NiCo2O4/3D graphene/ITO sensor showed high sensitivity of 166 μA mM21 cm22, detection limit of 5.0 μM and linear range of 0.06 0.30 mM (Nguyen et al., 2016).

11.5

Conclusions

Inherent sensitivity, simplicity, speed, and cost benefits continue to be strong driving forces for the development of electrochemical biosensors. Metal oxides are considered versatile materials that can be successfully integrated into biosensor technology. Based on features such as chemical stability, environmentally benign, non-toxic, electrocatalytic activity with or without light irradiation, and high surface area-to-volume ratio, these materials are highly competitive in the biosensors market. This chapter has outlined the basics of biosensors with their different types followed by details about the metal oxidebased composites used in non-enzymatic electrochemical biosensors. The overall performance of biosensor depends on the various parameters such as, properties of materials used, evaluation procedure or protocol designed, and chemistry between materials and analyte during the biosensing process. Several functional metal oxide-based nanocomposites have provided as an innovative solid substrate for a highly sensitive on-site analysis via signal amplification in electrochemical biosensors. In this connection the various types of functional nanomaterials (carbon nanotubes, graphene, metallic, silica nanoparticles, nanowire, indium tin oxide, and organic polymers), have been included in this chapter. For the functionalization, the electrode surfaces can be coated with various organic groups (silanes, thiols, and CPs) for effective connectivity or interactions with analytes for efficient responses.

11.6

Challenges and future perspectives

The use of nanocomposite in electrochemical biosensor accompanied by various challenges, like stability of materials used at physiological conditions, good performance of biosensing device at different temperatures, must tolerate high ionic strength buffers, etc. In addition, the other desired properties such as less use of

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toxic materials and preparation cost of material should also be considered. The interaction between analyte and material plays major role in the outcome of biosensor, and many researchers did not study the interaction between analyte and material very well. Although tremendous work has been done yet, many publications lack the information regarding about operation conditions as well as experimental demonstrations. Although the electrochemical biosensors have been shown to be suitable for high-performance analysis in diverse field applications, the matrix interference influencing the bio molecular interaction from real samples (blood, food, etc.) still remains the most critical issues that need to be solved for improving the analytical performances. The various constraints in the present state of art of biosensors have been resolved with the following research endeavors: (1) synthesize the functional nanomaterials for real-world applications of biosensors, (2) development of multianalyte sensor for the analysis of different biomolecules with single device effectively and selectively remains a challenge, (3) non-specific binding in non-enzymatic electrochemical biosensor is due to high surface-to-volume ratio of metal oxide. The high surface area of metal oxide nanomaterial’s causes surface fouling. The materials with antifouling properties are the future of biosensor materials. Polymers and polymer nano brushes show excellent antifouling properties. (4) Selectivity is the issue in application of nonenzymatic-based electrochemical biosensor. New strategies like molecular-imprinted polymers, enzyme-mimicking materials like MOFs, etc. can be used to improve selectivity. (5) The application in physiological condition and reliability of results is another important factor. Hence, in concern with this, stability study of materials at various parameters like pH, temperature, and interactions with other moieties is necessary for the implementation of biosensors.

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sensitive non-enzymatic uric acid sensing electrode material. RSC Advances, 10, 36949 36961. Available from https://doi.org/10.1039/d0ra06290a. Nguyen, N. S., Das, G., & Yoon, H. H. (2016). Nickel/cobalt oxide-decorated 3D graphene nanocomposite electrode for enhanced electrochemical detection of urea. Biosensors and Bioelectronics, 77, 372 377. Available from https://doi.org/10.1016/j. bios.2015.09.046. Perez, J. M. (2007). Iron oxide nanoparticles: Hidden talent. Nature Nanotechnology, 2, 535 536. Available from https://doi.org/10.1038/nnano.2007.282. Ramasami Sundhar Baabu, P., Gumpu, M. B., Nesakumar, N., Rayappan, J. B. B., & Kulandaisamy, A. J. (2020). Electroactive manganese oxide reduced graphene oxide interfaced electrochemical detection of urea. Water, Air, and Soil Pollution, 231. Available from https://doi.org/10.1007/s11270-020-04899-y. Raveendran, J., Resmi, P. E., Ramachandran, T., Nair, B. G., & Satheesh Babu, T. G. (2017). Fabrication of a disposable non-enzymatic electrochemical creatinine sensor. Sensors and Actuators, B: Chemical, 243, 589 595. Available from https://doi.org/10.1016/j. snb.2016.11.158. Ren, B., Wang, Y., & Ou, J. Z. (2020). Engineering two-dimensional metal oxides: Via surface functionalization for biological applications. Journal of Materials Chemistry B, 8, 1108 1127. Available from https://doi.org/10.1039/c9tb02423a. Rengaraj, A., Haldorai, Y., Kwak, C. H., Ahn, S., Jeon, K. J., Park, S. H., Han, Y. K., & Huh, Y. S. (2015). Electrodeposition of flower-like nickel oxide on CVD-grown graphene to develop an electrochemical non-enzymatic biosensor. Journal of Materials Chemistry B, 3, 6301 6309. Available from https://doi.org/10.1039/c5tb00908a. Ronkainen, N. J., Halsall, H. B., & Heineman, W. R. (2010). Electrochemical biosensors. Chemical Society Reviews, 39, 1747 1763. Available from https://doi.org/10.1039/ b714449k. Roychoudhury, A., Basu, S., & Jha, S. K. (2016). Dopamine biosensor based on surface functionalized nanostructured nickel oxide platform. Biosensors and Bioelectronics, 84, 72 81. Available from https://doi.org/10.1016/j.bios.2015.11.061. Sattarahmady, N., Heli, H., & Vais, R. D. (2013). An electrochemical acetylcholine sensor based on lichen-like nickel oxide nanostructure. Biosensors and Bioelectronics, 48, 197 202. Available from https://doi.org/10.1016/j.bios.2013.04.001. Shadlaghani, A., Farzaneh, M., Kinser, D., & Reid, R. C. (2019). Direct electrochemical detection of glutamate, acetylcholine, choline, and adenosine using non-enzymatic electrodes. Sensors (Switzerland), 19. Available from https://doi.org/10.3390/s19030447. Shavanova, K., Bakakina, Y., Burkova, I., Shtepliuk, I., Viter, R., Ubelis, A., Beni, V., Starodub, N., Yakimova, R., & Khranovskyy, V. (2016). Application of 2D nongraphene materials and 2D oxide nanostructures for biosensing technology. Sensors (Switzerland), 16, 1 23. Available from https://doi.org/10.3390/s16020223. Shen, C., Su, J., Li, X., Luo, J., & Yang, M. (2015). Electrochemical sensing platform based on Pd-Au bimetallic cluster for non-enzymatic detection of glucose. Sensors and Actuators, B: Chemical, 209, 695 700. Available from https://doi.org/10.1016/j. snb.2014.12.044. Sheng, Z. M., Gan, Z. Z., Huang, H., Niu, R. L., Han, Z. W., & Jia, R. P. (2020). M-Nx (M 5 Fe, Co, Ni, Cu) doped graphitic nanocages with high specific surface area for nonenzymatic electrochemical detection of H2O2. Sensors and Actuators, B: Chemical, 305, 127550. Available from https://doi.org/10.1016/j.snb.2019.127550. Song, Y., Han, J., Xu, L., Miao, L., Peng, C., & Wang, L. (2019). A dopamine-imprinted chitosan film/porous ZnO NPs@carbon nanospheres/macroporous carbon for

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electrochemical sensing dopamine. Sensors and Actuators, B: Chemical, 298, 126949. Available from https://doi.org/10.1016/j.snb.2019.126949. Sun, J. Y., Huang, K. J., Zhao, S. F., Fan, Y., & Wu, Z. W. (2011). Direct electrochemistry and electrocatalysis of hemoglobin on chitosan-room temperature ionic liquid-TiO2-graphene nanocomposite film modified electrode. Bioelectrochemistry (Amsterdam, Netherlands), 82, 125 130. Available from https://doi.org/10.1016/j.bioelechem.2011.06.007. Thanh, T. D., Balamurugan, J., Lee, S. H., Kim, N. H., & Lee, J. H. (2016). Novel porous gold-palladium nanoalloy network-supported graphene as an advanced catalyst for nonenzymatic hydrogen peroxide sensing. Biosensors and Bioelectronics, 85, 669 678. Available from https://doi.org/10.1016/j.bios.2016.05.075. Toghill, K. E., & Compton, R. G. (2010). Electrochemical non-enzymatic glucose sensors: A perspective and an evaluation. International Journal of Electrochemical Science, 5, 1246 1301. Wang, G., Morrin, A., Li, M., Liu, N., & Luo, X. (2018). Nanomaterial-doped conducting polymers for electrochemical sensors and biosensors. Journal of Materials Chemistry B, 6, 4173 4190. Available from https://doi.org/10.1039/c8tb00817e. Wang, M. Y., Shen, T., Wang, M., Zhang, D. E., Tong, Z. W., & Chen, J. (2014). One-pot synthesis of α-Fe2O3 nanoparticles-decorated reduced graphene oxide for efficient nonenzymatic H2O2 biosensor. Sensors and Actuators, B: Chemical, 190, 645 650. Available from https://doi.org/10.1016/j.snb.2013.08.091. Wang, S., Zhao, L., Xu, R., Ma, Y., & Ma, L. (2019). Facile fabrication of biosensors based on Cu nanoparticles modified as-grown CVD graphene for non-enzymatic glucose sensing. Journal of Electroanalytical Chemistry, 853, 113527. Available from https://doi. org/10.1016/j.jelechem.2019.113527. Wang, Y., Wang, S., Tao, L., Min, Q., Xiang, J., Wang, Q., Xie, J., Yue, Y., Wu, S., Li, X., & Ding, H. (2015). A disposable electrochemical sensor for simultaneous determination of norepinephrine and serotonin in rat cerebrospinal fluid based on MWNTs-ZnO/chitosan composites modified screen-printed electrode. Biosensors and Bioelectronics, 65, 31 38. Available from https://doi.org/10.1016/j.bios.2014.09.099. Xu, J., Wang, Y., & Hu, S. (2017). Nanocomposites of graphene and graphene oxides: Synthesis, molecular functionalization and application in electrochemical sensors and biosensors. A review. Microchimica Acta, 184, 1 44. Available from https://doi.org/ 10.1007/s00604-016-2007-0. Yadav, H. M., Otari, S. v, Koli, V. B., Mali, S. S., Hong, C. K., Pawar, S. H., & Delekar, S. D. (2014). Preparation and characterization of copper-doped anatase TiO2 nanoparticles with visible light photocatalytic antibacterial activity. Journal of Photochemistry and Photobiology A: Chemistry, 280, 32 38. Available from https://doi.org/10.1016/j.jphotochem.2014.02.006. Yasmin, S., Ahmed, M. S., & Jeon, S. (2015). Determination of dopamine by dual doped graphene-Fe2O3 in presence of ascorbic acid. Journal of the Electrochemical Society, 162, B363 B369. Available from https://doi.org/10.1149/2.0751514jes. Zhang, X., Cao, Y., Yu, S., Yang, F., & Xi, P. (2013). An electrochemical biosensor for ascorbic acid based on carbon-supported PdNi nanoparticles. Biosensors and Bioelectronics, 44, 183 190. Available from https://doi.org/10.1016/j.bios.2013.01.020. Zhang, X., Zhang, Y. C., & Ma, L. X. (2016). One-pot facile fabrication of graphene-zinc oxide composite and its enhanced sensitivity for simultaneous electrochemical detection of ascorbic acid, dopamine and uric acid. Sensors and Actuators, B: Chemical, 227, 488 496. Available from https://doi.org/10.1016/j.snb.2015.12.073. Zhou, Y., Fang, Y., & Ramasamy, R. P. (2019). Non-covalent functionalization of carbon nanotubes for electrochemical biosensor development. Sensors (Switzerland), 19. Available from https://doi.org/10.3390/s19020392.

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Functionalized magnetic iron oxide-based composites as adsorbents for the removal of heavy metals from wastewater

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Prashant B. Patil1 and Vijay P. Kothavale2 1 Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Physics, Bhogawati Mahavidyalaya Kurukali, Shivaji University, Kolhapur, Maharashtra, India

12.1

Introduction

Water is a vital natural resource for humans and other living systems. The earth’s surface covers 71% water. Out of this, 97% of the water is saltwater in the sea, and only 3% is fresh water. Two-thirds of the freshwater is in frozen form in glaciers and polar ice. Only 1% of freshwater is present in lakes, rivers, and reservoirs suitable for human consumption. There is an exponential growth of the population and economies in the world. Still, the total quantity of freshwater remains the same. During this century, 2.2 billion people in more than 62 countries face insufficient and polluted water supply. Annually, about five million people lose their life due to water-related diseases or poor sanitation (Cervantes et al., 2006; Wolf et al., 2001). Water is required for various purposes such as agricultural, industrial, household, and environmental activities [Pimentel et al., 2007; World Business Council for Sustainable Development (WBCSD), 2009]. These activities discharge many pollutants like toxic heavy metal ions, dyes, inorganic particles, pharmaceuticals, and pesticides that seriously affect human health (Ali & Gupta, 2007). These pollutants can change the color, taste, and odor of the water. Therefore, such water is unsafe for human consumption. The heavy metal ions and dyes, even in low concentrations, affect human health and the living system. The heavy metal ions are nonbiodegradable and can affect the functioning of the vital body organs (Carolin et al., 2017; Singh et al., 2011). Hence, removing toxic heavy metal ions from contaminated water is essential to supplying clean water for the world population. For this, several conventional methods have been developed, such as membrane separation, electrochemical treatment, coagulation and flocculation, adsorption, chemical precipitation, photocatalysis, ion exchange, and flotation (Aderhold et al., 1996; Ahmed & Ahmaruzzaman, 2016; Gautam et al., 2014; Rubio et al., 2002). Among these, adsorption is an effective method because of cost-effectiveness, efficiency, and eco-friendliness. Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00016-6 © 2022 Elsevier Inc. All rights reserved.

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Several novel nanoadsorbents have been explored for the removal of heavy metal ions such as carbon nanotubes, chitosan, iron oxide, graphene oxide, etc. Out of these, iron oxide magnetic nanoparticles (MNPs) are promising due to their high surface-to-volume ratio, colloidal stability, and biocompatibility. Moreover, due to its magnetization, MNPs can be rapidly separated from the water for reuse after a suitable desorption process. However, the bare iron oxide MNPs can quickly oxidize and also agglomerate. The surface modification prevents the MNPs from oxidization. Furthermore, surface functionalization provides adsorption sites such as
COOH,
OH,
SH,
NH2, etc., which improves the adsorption capacity of the adsorbent (Kothavale et al., 2020). In this chapter, we discuss various aspects of the application of MNPs as a nano-adsorbent for the removal of a single type of heavy metal ions as well as simultaneous removal of coexisting heavy metal ions.

12.2

Water pollution by heavy metals and its removal

The elements with an atomic weight between 63.5 and 200.6 and a specific gravity of more than 5.0 are referred to as heavy metals (Srivastava & Majumder, 2008). Particular amounts of heavy metals are essential for the living system for metabolic activities. Still, beyond certain limits, it is toxic and harmful to human health (Uddin, 2017). The heavy metals include lead, cadmium, copper, nickel, zinc, cobalt, mercury, arsenic, chromium, manganese, etc. (Fu & Wang, 2011; Hashemzadeh et al., 2019). The industries like electroplating, tanneries, metal plating, dyeing, battery, printing, metallurgical engineering, mining operations, pigment, nuclear power operations, electric appliances manufacturing, semiconductor, cosmetics, paper industries, pesticides, industries, etc., discharge heavy metal waste into the environment and also in the water sources (Ali & Gupta, 2007; Gupta et al., 2009). Heavy metal ions are accumulative and nonbiodegradable. The heavy metal ions discharged into the water affect aquatic life, leading to oxygen insufficiency and rapid growth of algal blooms (Uddin, 2017). The permissible limits of heavy metals according to US EPA (the United States Environmental Protection Agency) (Engwa et al., 2019), Indian Standard (Bureau of Indian standard, 2012), and World Health Organization (WHO) (Krishna & Bhattacharyya, 1999) are presented in Table 12.1

12.2.1 Methods for the removal of heavy metal ions Many research groups worldwide are actively working to develop efficient techniques to remove toxic heavy metal ions from water. During the last few years, several chemical and physical methods have been developed to remove heavy metal ions. The overview of such techniques, along with advantages and disadvantages, is presented in Fig. 12.1 (Aderhold et al., 1996; Ahmed & Ahmaruzzaman, 2016; Barakat, 2011; Gautam et al., 2014; Kurniawan et al., 2006; Rubio et al., 2002). These conventional methods have limitations like low removal efficiency, complex operating conditions, high operational cost, and high production of secondary sludge, which

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Table 12.1 Permissible limits of heavy metal according to US EPA (Kumar & Puri, 2012), (Azeh Engwa et al., 2019) Indian Standard (BIS, 2012), and WHO. Metal

Lead Cadmium Copper Nickel Zinc Mercury Arsenic Chromium Manganese

Permissible limits US EPA (mg L21)

Indian standard (mg L21)

WHO (mg L21)

0.015 0.005 1.3 0.1 5 0.002 0.01 0.1 -

0.10 0.01 0.05 0.02 5 0.001 0.05 0.05 0.3

0.05 0.005 1 0.02 5 0.001 0.05 0.05 0.1

Figure 12.1 Overview of the techniques for the removal of heavy metal ions along with their advantages and disadvantages.

requires additional treatment (Burakov et al., 2018). On the other hand, adsorption has many advantages over other methods, like high efficiency, low initial cost, easy operation, eco-friendly in nature, non-toxic by-products, strong affinity, high metal binding capacities, feasibility in terms of production of high-quality products, and effectiveness in treating pollutants at low concentrations. Also, adsorption can be reversible and the adsorbent can be reused after desorption (Ali & Gupta, 2007).

12.2.2 Adsorption process for the removal of heavy metal ions Adsorption is the surface phenomenon of mass transfer of ions from liquid to the solid phase. The adsorbed substance is known as adsorbate. The adsorbent is the solid surface on which adsorption takes place. The adsorption occurs due to the unbalanced

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force of attraction on the surface of the solid. During the adsorption process, three main steps occur; (1) the transportation of the toxic heavy metal ions from the water to the adsorbent surface, (2) adsorption on the surface of the adsorbent, and (3) diffusion of the heavy metal ions inside the adsorbents. The adsorption process can be divided into physisorption (or physical adsorption) and chemisorption (or chemical adsorption) (Fig. 12.2). In physisorption, the physical bond between adsorbent and adsorbate is weak, e.g., Van der Waals forces of attraction. Physisorption is weakly specific, reversible, and has a minimal thermal effect. Chemisorption occurs due to the chemical bonding between adsorbent and adsorbate molecules, for example, covalent or ionic bonds. Chemisorption is selective, irreversible, and the thermal effect is high (Burakov et al., 2018; Tripathi & Rawat Ranjan, 2015).

12.3

Magnetic nanoparticles as nanoadsorbents

Fig. 12.3 represents different types of adsorbents used for the removal of heavy metal ions (Ahmed & Ahmaruzzaman, 2016; Alekseeva et al., 2016; Apiratikul & Pavasant, 2008; Cao et al., 2014; Dave & Chopda, 2014; Di Natale et al., 2013; Moazeni et al., 2020; Peng et al., 2017; Ugochukwu et al., 2014). Among them, MNPs have a high surface-to-volume ratio which plays an important role in the adsorption process for different heavy metal ions in aqueous solutions. The MNPs below a critical size exhibit superparamagnetic properties, that is zero remanent magnetization and zero coercivity at room temperature but respond to the external magnetic field (Kanase et al., 2020). MNPs exhibiting superparamagnetism offer good colloidal stability and magnetization for easy and rapid separation of adsorbents from the solution. The combination of adsorption and magnetic separation is convenient for operational flexibility, recovery of heavy metals, and reusability of adsorbent MNPs.

Figure 12.2 Physical and chemical adsorption of heavy metal ions on the adsorbent.

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Figure 12.3 Different adsorbents used for the removal of heavy metal ions from water.

The iron oxide nanoparticles are found in various phases such as magnetite (Fe3O4), hematite (α-Fe2O3), maghemite (α-Fe2O3), wustite (FeO), β-Fe2O3, and ε-Fe2O3 (Wu et al., 2008). Magnetite and maghemite phases are magnetic and hence are most commonly used. The superparamagnetic iron oxide nanoparticles are approved by Food and Drug Administration. It should also be noted that magnetite particles have been explored more than maghemite because it has relatively high saturation magnetization and fast magnetic response to the external magnetic field (Hajba & Guttman, 2016; Yang et al., 2017).

12.3.1 Functionalization of magnetic nanoparticles for heavy metal ions removal The iron oxide MNPs has a large surface-to-volume ratio. Consequently, it possesses high surface energy. To minimize the surface energy, bare MNPs tend to agglomerate. Also, the bare MNPs are susceptible to oxidization in the air, resulting in loss of magnetism, adsorbing properties, and dispersibility. The MNPs can be stabilized by surface modification. Surface modification of MNPs can be done by functionalizing or grafting it with organic molecules, polymers or coating it with an inorganic layer, such as silica (Ojemaye et al., 2017). Besides stabilizing the MNPs, surface modification provides the adsorption sites for heavy metal ions or can be used for further functionalization. Thus, the proper surface functionalization of MNPs improves their chemical stability, prevents oxidation, and provides functional groups for the adsorption of heavy metal ions from polluted water. MNPs can be functionalized by different linkers such as meso-2,3-dimercaptosuccinic acid (DMSA) (Chavan et al., 2019;

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Kothavale et al., 2020), (3-aminopropyl) triethoxysilane (APTES) (Karade et al., 2021; Kothavale et al., 2018; Patil, Karade, Waifalkar, & Patil, 2017; Patil, Karade, Waifalkar, Sahoo, et al., 2017), chitosan (Dhavale et al., 2021; Waifalkar et al., 2016), reduced graphene oxide (Patil et al., 2021), and APTESglutaraldehyde (GA) (Kothavale et al., 2019). The functionalized MNPs provide active adsorption groups such as carboxylic (
COOH), hydroxyl (
OH), thiol (
SH), amine (
NH2), etc., that helps to improve the adsorption capacity and adsorption-desorption kinetics(Neyaz et al., 2014; Ojemaye et al., 2017; Wu et al., 2008). For the functionalization of MNPs, two different approaches are generally used: ligand addition and ligand exchange (Sharma et al., 2021). In ligand addition, the surface of MNPs is modified by the addition of ligand without eliminating any preexisting ligands. In the ligand exchange process, the bifunctional ligand is used for surface modification (Ojemaye et al., 2017). The original ligand is substituted by one of the functional groups of bifunctional ligand and binds the MNPs surface. The remaining ligand is used for further functionalization or as adsorption sites to remove heavy metal ions. Recent developments in surface functionalization of MNPs by organic and inorganic materials for heavy metal ion removal are discussed in the following subsections.

12.3.1.1 Surface functionalization by organic materials Polymers are widely used for the surface modification of MNPs to improve the adsorption capacity for the removal of heavy metal ions from contaminated water. The polymer coating of MNPs provides functional groups like
COOH,
OH, and
NH2, which can be used for metal ion adsorption or can be used for further functionalization. Polymer-modified MNPs are prepared either by direct reaction with molecules such as silanes and by grafting polymeric molecules via covalent interaction to the OH groups on MNPs surface (Ojemaye et al., 2017). N. Mirrezaie et al. prepared the polymer polypyrrole (PPy) modified MNPs for the removal of Cr(VI) from an aqueous solution through in situ polymerization, which provided the amine group as an adsorption site (Mirrezaie et al., 2014). I. Larraza et al. prepared magnetite
Polyethylenimine
Montmorillonite (Fe3O4
PEIx
MMT) composite adsorbent (Larraza et al., 2012). The polymer PEI was introduced in the MMT layers and used to coat the Fe3O4 MNPs, offering amine groups as adsorption sites. The obtained magnetic composite Fe3O4
PEI
MMT was used for the removal of Cr(VI) from an aqueous solution. X. Wei et al. prepared polymer poly(acrylic acid) coated Fe3O4 through the ligand exchange protocol for the removal of Pb(II) and Ni(II) heavy metal ions from wastewater (Wei et al., 2017). The magnetic adsorbent PAA, polyacrylic acid polymer-coated Fe3O4 was effectively reused after desorption. Fig. 12.4 shows the schematic of different steps in the removal process: adsorption of heavy metal, separation of magnetic adsorbent, recovery, regeneration, and reuse. Wang et al. fabricated an efficient adsorbent using Fe3O4 functionalized by hyperbranched polymer (HBPA) and then modified by amino-salicylic acid (Wang

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Figure 12.4 Schematic representation of the typical process of the removal of heavy metal ions using magnetic adsorbent (A) adsorption of heavy metal using the PAA-coated Fe3O4, (B) separation of magnetic adsorbent using the magnetic filter, (C) recovery and regeneration of the magnetic adsorbent, and (D) reuse. Source: From Wei, X., Sugumaran, P. J., Peng, E., Liu, X. L. & Ding, J. (2017). Low-field dynamic magnetic separation by self-fabricated magnetic meshes for efficient heavy metal removal. ACS Applied Materials and Interfaces, 9(42), 36772
36782. https://doi.org/ 10.1021/acsami.7b10549.

et al., 2020). The prepared adsorbent Fe3O4-HBPA-ASA provided high density and multiple adsorption sites of amine, carboxyl, and hydroxyl and was used for the individual and simultaneous removal of Cu(II), Pb(II), and Cd(II). C. Chou et al. developed the MNPs functionalized by amine-terminated poly (amidoamine) dendrimers and employed them for the adsorption study of Zn(II) from aqueous solutions (Chou & Lien, 2011). Shen et al. prepared the cysteine functionalized Fe3O4 MNPs (Cys-Fe3O4 MNPs) adsorbent, which offers three functional groups (
SH,
NH2,
COOH) as adsorption sites (Shen et al., 2014). These Cys-Fe3O4 MNPs adsorbents were successfully employed to remove Hg(II) from aqueous solutions. Kothavale et al. prepared the MNPs functionalized with (3-aminopropyl) triethoxysilane (APTES), which provides the amine group as adsorption sites for the removal of Cu(II) metal cations from aqueous solution (Kothavale et al., 2018).

12.3.1.2 Surface functionalization by inorganic materials Silica is the most common inorganic surface modifier of the MNPs (Liu et al., 2020; Ojemaye et al., 2017). The coating of silica offers stability as well as avoids dipolar interactions, and prevents agglomeration of MNPs. Silica forms an external shielding layer that protects the internal MNPs structure. The silica-coated MNPs possess good biocompatibility, hydrophilicity and offer an opportunity for further functionalization. The surface of silica-coated MNPs has silanol (Si-OH) as a terminal group suitable for the further functionalization of MNPs with phenyl, aliphatic

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hydrocarbons, vinyl, and amine groups (Akbari et al., 2020). Wang et al. developed the SiO2 coated MNPs modified by chitosan and triethylenetetramine (TETA) (Wang et al., 2017). The prepared composite Fe3O4@SiO2@CS-TETA was anchored on the graphene oxide (GO) surface by an amidation reaction between amino groups on TETA and carboxyl groups on GO. The adsorbent was used for the adsorption of Cu (II) ions and methylene blue (MB) from aqueous solutions. The schematic of the synthesis of Fe3O4@SiO2@CS-TETA-GO, along with the adsorption and desorption process, is shown in Fig. 12.5. Zhang et al. developed the nanostructured adsorbents in which Fe3O4/carbon composite layer was grown on the snowflake-shaped ZnO coated SiO2 layer ZnO@SiO2 by a solvothermal method (Zhang et al., 2014). The resulting adsorbent ZnO@SiO2@Fe3O4/C was successfully employed for the removal of Pb(II) and As(V) from water. Gold has also been used to modify the surface of MNPs, which provides the thiol group for many applications. L. Maia et al. prepared the adsorbent by modifying the surface of MNPs by gold to efficiently remove Hg(II) (Maia et al., 2020).

Figure 12.5 Schematic representation of the synthesis of Fe3O4@SiO2@CS-TETA-GO nanoadsorbents. Source: From Wang, F., Zhang, L., Wang, Y., Liu, X., Rohani, S. & Lu, J. (2017). Fe3O4@SiO2@CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II). Applied Surface Science, 420, 970
981. https://doi.org/10.1016/j. apsusc.2017.05.179

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12.4

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Batch adsorption experiment

The batch adsorption experiment is usually performed to investigate the adsorption behavior of heavy metal ions on MNPs in an aqueous solution. A known amount of MNPs and specified aqueous solution of heavy metal ions are added in an Erlenmeyer flask at room temperature in the adsorption experiment. The mixture is shaken in an incubator shaker to attain the adsorption equilibrium. Then the MNPs are separated from aqueous solutions by magnetic decantation. In the batch adsorption experiment, the parameters like pH, adsorbent dose, contact time, and initial concentration of metal ions are optimized. The removal efficiency (R) and the adsorption capacity (qe) of metal ions are expressed by the following equations, Rð%Þ 5 qe 5

Co 2 Ce Co

3 100

ðC0 2 Ce Þ V m

where, C0 5 Initial concentration of metal ions (mg L21), Ce 5 Final equilibrium concentration of metal ions (mg L21), V, Total volume of metal ions solution (mL), and m, Mass of the MNPs used (mg).

12.4.1 Factors affecting the adsorption of heavy metal ions Parameters that affect the adsorption process while removing heavy metal ions from the water are discussed below (Burakov et al., 2018).

12.4.1.1 Effect of solution pH The pH of water is a significant factor that affects the adsorption of metal ions on MNPs. For positively charged metal ions, adsorption increases with the increase in pH and attains equilibrium at higher pH. The adsorption behavior at different pH can be explained using the isoelectric point (IEP) of the MNPs (Ahmed & Ahmaruzzaman, 2016). IEP corresponds to the pH of the solution at which the surface charge of the MNPs is zero. For pH less than IEP, the surface is positively charged and consequently, adsorption capacity is less for positive metal ions. On the other hand, for pH greater than IEP, MNPs surface is negatively charged and is favorable for adsorption.

12.4.1.2 Effect of contact time The contact time between heavy metal ions and MNPs is an important parameter to achieve maximum adsorption capacity. With an increase in contact time, the adsorption capacity of most heavy metal ions increases initially and attains equilibrium. Initially, a sufficient number of adsorption sites were available for the

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adsorption. However, with the increase in time, ions occupy adsorption sites, and then the available adsorption sites decrease. This leads to a slow adsorptive process until adsorption capacity reaches equilibrium (Kothavale et al., 2019).

12.4.1.3 Effect of adsorbent dose The adsorbent dose (amount of MNPs) is a crucial factor in efficiently removing the metal ions from water. Initially, the removal efficiency increases, and adsorption capacity decreases with increasing adsorbent dosage. This is owing to the availability of adsorption sites on the MNPs surface. Further increase in adsorbent dose has no significant effect on removal efficiency. However, adsorption sites can not be fully exploited with a higher adsorbent dose due to the lack of metal ions, and consequently, adsorption capacity decreases (Tang et al., 2018). Also, at a high adsorbent dose, the aggregation and overlapping of available active sites of MNPs occur. This can lead to a reduction in the surface area of the MNPs, affecting the removal efficiency and adsorption capacity (Izanloo et al., 2019).

12.4.1.4 Effect of initial metal ion concentration The adsorption capacity of MNPs strongly depends on the initial metal ion concentration in water. At lower metal ion concentrations, the adsorption capacity is lower due to the scarcity of ions. With the increase in metal ion concentration, adsorption capacity increases due to the availability of both the adsorbate ions and the adsorbent sites. However, adsorption capacity saturates when all the adsorbent sites are occupied (Chavan et al., 2019). The maximum adsorption capacity can be achieved by optimizing all the above parameters. The evaluation of adsorption isotherms and kinetic models of the experimental adsorption data is needed to find out the adsorption mechanism.

12.4.2 Adsorption kinetics The kinetic models are applied to the experimental data to interpret the adsorption behavior of heavy metal ions on MNPs. The pseudo-first-order and pseudo-secondorder kinetic models are generally used to analyze the adsorption kinetics of heavy metal ions. The nonlinear form of the pseudo-first-order kinetic model is expressed as (Ho, 2004), qt 5 qe

2

qe

e2k1 t

The nonlinear form of the pseudo-second-order kinetic model is expressed as (Ho & McKay, 1999), qt 5

k2 q2e t 1 1 k 2 qe t

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where qe (mg g21) and qt (mg g21) are the adsorption capacities at equilibrium and time t (min), respectively. k1 (min21) and k2 (g mg21 min) are the rate constants for pseudo-first-order and pseudo-second-order adsorption models respectively.

12.4.3 Adsorption isotherms The adsorption processes depend on solid
liquid equilibrium and the rate of mass transfer (Inyinbor et al., 2016). Equilibrium behavior is characterized by expressing the number of metal ions adsorbed on MNPs as a metal ion concentration at a constant temperature. The isotherm models are used to analyze the experimental data to elucidate the surface properties of MNPs and the adsorption mechanism. The interaction of metal ions with MNPs and the theoretical uptake capacity of the MNPs can be examined using the adsorption isotherm study. The Langmuir and Freundlich isotherm models are most commonly used for this. Irving Langmuir developed the Langmuir isotherm model. This model is based on the assumption of monolayer adsorption of metal ions on the homogeneous adsorbent surface (Tang et al., 2018). The nonlinear form of the Langmuir isotherm model is expressed as (Langmiur, 1918), qe 5

KL qm Ce 1 1 KL Ce

where qe is the adsorption capacity (mg g21), KL is the Langmuir adsorption constant (L mg21), qm is the maximum adsorption capacity (mg g21), and Ce is the equilibrium concentration of heavy metal ions (mg L21). The Freundlich isotherm model developed by Herbert F. Freundlich assumes the adsorption of metal ions on the multilayer and heterogeneous adsorbent surface (Tang et al., 2018). The nonlinear form of the Freundlich isotherm model is expressed as (Chen et al., 2009), qe 5 KF

1

Cen

where qe is the adsorption capacity (mg g21) and Ce is the equilibrium concentration of heavy metal ions (mg L21). KF and n are Freundlich constants related to adsorption capacity (L g21) and heterogeneity factor (mg g21), respectively.

12.5

Removal of heavy metal ions by magnetic nanoparticles

In the following subsections, the removal of single heavy metal ions and the simultaneous removal of coexisting heavy metal ions using MNPs are briefly reviewed.

412

Advances in Metal Oxides and Their Composites for Emerging Applications

12.5.1 Removal of a single type of heavy metal ions Chavan et al. reported the MNPs functionalized by DMSA using ligand exchange protocol for the adsorption of Cu(II) (Chavan et al., 2019). MNPDMSA nano adsorbent exhibited a maximum adsorption capacity of 25.44 mg g21 for Cu(II) at pH 6 and adsorbent dose 8 g L21. The kinetics study revealed that the adsorption data follows the pseudo-first-order model, which implies the physisorption process. The experimental data followed the Langmuir adsorption isotherm model, which shows that monolayer adsorption took place on the homogeneous surface of the nano adsorbent. The functionalization protocol and removal process of Cu(II) by MNP-DMSA are represented in Fig. 12.6. MNP-DMSA nano adsorbent exhibited a maximum adsorption capacity of 25.44 mg g21 for Cu(II). The application of MNPs as nanoadsorbents to remove heavy metal ions in a single metal system is reviewed in terms of adsorption capacity, isotherm model, kinetic model, etc., and presented in Table 12.2.

12.5.2 Simultaneous removal of multiple heavy metal ions It is seen that water is usually contaminated by coexisting multiple heavy metal ions. The coexistence of different heavy metals is more toxic to human health through intermolecular interaction or complexation. Also, coexisting heavy metal ions show different adsorption behavior and the adsorption process is complex compared to the single metal system (Bing et al., 2019; Maheshwari et al., 2015). It is seen that MNPs have the ability to remove the coexisting heavy metal ions. However, the presence of multiple metal ions may inhibit the adsorption capacity or removal efficiency (Liu et al., 2015; Wang et al., 2014, 2020). The competition between the coexisting metal ions for binding the surface of MNPs affects the

Figure 12.6 Schematic of the ligand exchange protocol for the functionalization of magnetic nanoparticles with meso-2,3-dimercaptosuccinic acid and adsorption of Cu(II). Source: From Chavan, V. D., Kothavale, V. P., Sahoo, S. C., Kollu, P., Dongale, T. D., Patil, P. S. & Patil, P. B. (2019). Adsorption and kinetic behavior of Cu(II) ions from aqueous solution on DMSA functionalized magnetic nanoparticles. Physica B: Condensed Matter, 571, 273
279. https://doi.org/10.1016/j.physb.2019.07.026.

Table 12.2 The review of the application of magnetic nanoparticles as nanoadsorbents for the removal of heavy metal ions in a single metal system. Sr. no.

Nanoadsorbent

Heavy metal ions

Concentration/ adsorbent dose

Equilibrium contact time/pH

Adsorption capacity

Isotherm model

Kinetic model

References

1

APTES-GA modified magnetic iron oxide nanoparticles Amino-functionalized Fe3O4 magnetic nanoparticles Asparagine functionalized Fe3O4 Amino-functionalized PAA-coated Fe3O4 nanoparticles

Cu(II)

50
300 mg L21, 10 g L21

105 min, 6

19.26 mg g21

Langmuir

Pb(II)

1
50 mg L21, 1 g L21

60 min, 5

40.1 mg g21

Langmuir

Kothavale et al. (2019) Tan et al. (2012)

Ni(II)

20
80 mg L21, 0.15 g L21

70 min, 6

87.18 mg g21

Langmuir

Cu(II)

0
600 mg L21, 20.64 g L21 0
400 mg L21, 20.64 g L21 5
100 mg L21, 0.2 g L21

60 min, 5

12.43 mg g21

Langmuir

Pseudofirstorder Pseudosecondorder Pseudosecondorder -

60 min, 2

11.24 mg g21

120 min, 7

369 mg g21

Langmuir

Xin et al. (2012)

Cd(II)

5
100 mg L21, 0.2 g L21

120 min, 7

446.4 mg g21

Langmuir

Cu(II)

5
100 mg L21, 0.2 g L21

120 min, 7

523.6 mg g21

Langmuir

Pseudosecondorder Pseudosecondorder Pseudosecondorder

2

3

4

5

Amine-functionalized mesoporous MNPs (AF- Fe3O4)

Cr(VI) Pb(II)

Singh et al. (2016) Huang and Chen (2009)

(Continued)

Table 12.2 (Continued) Sr. no.

Nanoadsorbent

Heavy metal ions

Concentration/ adsorbent dose

Equilibrium contact time/pH

Adsorption capacity

Isotherm model

Kinetic model

References

6

Fe3O4
SiO2-GSH

Pb(II)

10
500 mg L21, 1 g L21

120 min, 5.5

357.37 mg g21

Freundlich

Xu et al. (2017)

7

L-arginine functionalized MNPs (MNP-L)

Zn(II)

150 mg L21, 0.5 g L21

20 min, 6.0

150.4 mg g21

Langmuir

Cd(II)

120 mg L21, 0.5 g L21

20 min, 6.0

120.2 mg g21

Langmuir

Ni(II)

25
100 mg L21, 0.2 g L21

35 min, 8

362.31 mg g21

Freundlich

As(V)

100 mg L21, 1 g L21

1 h, 7

45 mg g21

Pseudosecondorder Pseudosecondorder Pseudosecondorder Pseudosecondorder

Freundlich

Hg(II)

0
100 mg L21, 0.5 g L21

60 min, 9

79.59 mg g21

Langmuir

8

9

10

Superparamagnetic Fe3O4 nanoparticles γ-Fe2O3 nanoparticles Gold- functionalized Fe3O4 nanoparticles

Pseudosecondorder

Guo et al. (2017)

Gautam et al. (2015) Kilianova´ et al. (2013) Maia et al. (2020)

Table 12.3 The review of the application of magnetic nanoparticles as nanoadsorbents for the simultaneous removal of coexisting multiple heavy metal ions. Sr. no.

Nanoadsorbent

Coexisting heavy metal ions

Concentration/ adsorbent dose

Equilibrium contact time/pH

Adsorption capacity/ removal efficiency

Isotherm model

Kinetic model

References

1

Fe3O4@SiO2@NH2@SH

Pb(II)

5
50 mg L21, 1.2 g L21 5
50 mg L21, 1.2 g L21 10
100 mg L21, 4 g L21

40 min, 6

23.92 mg g21

Langmuir

40 min, 6

30.21 mg g21

Pseudosecondorder

Izanloo et al. (2019)

3h-

Freundlich

Pseudosecondorder

Wang et al. (2014)

Cr(III)

10
100 mg L21, 4 g L21

3h-

Pb21

10
150 mg L21, 1 g L21

180 min, 6.5

Langmuir

Pseudosecondorder

Fato et al., (2019)

50
500 mg L21, 2 g L21 5
100 mg L21, 2 g L21

240 min, 5

80.56%— individual 41.41%— coexisting 42.37%— individual 38.48%— coexisting 98%—single 86%— mixed 87%—single 80%— mixed 90%—single 84%— mixed 78%—single 54%— mixed 50.25 mg g21

Langmuir

480 min, 5

181.81 mg g21

Pseudosecondorder

Zendehdel et al., (2019)

2,4-D 2

3

Magnetite nanoparticles

Ultrafine mesoporous magnetite nanoparticles

Pb(II)

Cd21

Cu21

Ni21

4

Amino-functionalized Fe3O4/NaP zeolite nanocomposite

Pb (II) Cd (II)

(Continued)

Table 12.3 (Continued) Sr. no.

Nanoadsorbent

Coexisting heavy metal ions

Concentration/ adsorbent dose

Equilibrium contact time/pH

Adsorption capacity/ removal efficiency

Isotherm model

Kinetic model

References

5

EDTA functionalized Fe3O4 nanoparticles

10 mg L21, 1 g L21

10 min, 7.9




Ghasemi et al., (2017)

Montmorillonite/Fe3O4/ humic acid (MFH) nanocomposites

0
40 mg L21, 0.1 g L21

40 min, 3

$ 112 mg g21 for simultan eously removal of all target ions 374.19 mg g21




6

Ag(I), Hg (II), Mn (II), Zn (II), Pb (II), Cd (II) Cr(VI)

Langmuir

Lu et al., (2018)

Aniline

0
100 mg L21, 0.1 g L21

50 min, 3

393.53 mg g21

Freundlich

Cu(II)

50
2000 mg L21, 5 g L21

30 min, 3

50
2000 mg L21, 5 g L21

Langmuir and Freun dlich

Pb(II)

163.1 mg g21 for single, 133 mg g21 for binary 101.6 mg g21 for single, 70 mg g21 for binary

Pseudofirstorder Pseudosecondorder Pseudosecondorder

7

Fe3O4@MoS2

Zolgharnein & Rast gordani, (2018)

Functionalized magnetic iron oxide-based composites as adsorbents

417

adsorption capacity and removal efficiency (Su et al., 2014). There is a limited number of reports in the literature on the simultaneous removal of coexisting multiple heavy metal ions compared to removing a single type of metal ions. Wang et al. prepared amino-hyperbranched MNPs for the simultaneous removal of Cu(II), Pb(II), and Cd(II) heavy metal ions from an aqueous solution (Wang et al., 2020). The adsorption capacities were 136.66, 88.36, and 165.46 mg g21 for individual Cu(II), Cd(II), and Pb(II) metal ions, respectively, at 120 minutes, 20 mg dose, and pH 7. However, for the simultaneous removal of Cu(II), Cd(II), and Pb (II), the adsorption capacities were reduced to 45.35, 35.18, and 52.05 mg g21, receptively. The adsorption capacities were reduced due to the strong competition between the different heavy metal ions for the adsorption on the vacant adsorption sites. The experimental data were well fitted to the Langmuir isotherm model, and the kinetic adsorption behavior was well explained by a pseudo-second-order equation. The application of MNPs as nanoadsorbents for the simultaneous removal of coexisting multiple heavy metal ions is reviewed in terms of adsorption capacity, isotherm model, kinetic model, etc., and presented in Table 12.3.

12.6

Conclusions and future perspectives

Toxic heavy metal ion pollution of water is a severe environmental issue. There are continuous efforts to develop new processes to remove the heavy metal ions from the contaminated water. The use of MNPs as an adsorbent is attractive to remove heavy metal ions from water because of their unique physicochemical properties, viz. larger surface area, the ability of functionalization, high efficiency, fast kinetics, strong affinity to various metal ions, and reusability. The surface functionalization of MNPs provides active adsorption sites and improves the adsorption capacity of adsorbents. For reuse, the MNPs can be easily separated from water by applying a magnetic field. The removal efficiency and adsorption capacity of MNPs for the adsorption of heavy metal ions depend on the experimental conditions viz. the solution pH, adsorbent dose, contact time, and initial metal ion concentration. The adsorption behavior and mechanism of MNPs can be explained in terms of different kinetic and isotherm models. The simultaneous removal of coexisting multiple heavy metal ions is more challenging than a single type of heavy metal ion. For practical applications, low-cost methods of synthesis of MNPs should be developed for large-scale production. The novel functionalization strategies need to be designed so that the surface-modified MNPs have higher binding sites for heavy metal ions. The use of MNPs in wastewater treatment is still limited to laboratory experiments. Various approaches such as continuous, fixed, and packed bed reactor systems should be explored to use the MNPs in large-scale industrial wastewater treatment applications.

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S.M. Patil1,2, S.A. Vanalakar3 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, Karmaveer Hire College, Gargoti, Kolhapur, Maharashtra, India, 3Department of Physics, Karmaveer Hire College, Gargoti, Kolhapur, Maharashtra, India

13.1

Introduction: environmental remediation principles and applications

To address the frontline problem, environmental pollution, various novel methods have been used to eliminate hazardous contaminants such as pesticides, herbicides, oil spills, heavy metals, toxic gases, sewage, organic compounds, etc., present in air, soil, and water (Guerra et al., 2018). An effective remediation strategy to remove or manage contaminated air, soil, or water is a challenging task. To set basic principles that are practicable and feasible for the execution of any remediation strategy is of the utmost importance. The following are some of such principles that need to be implemented. The fundamental principle of any remediation method is the protection of humans and the environment. The method should consider both environmental and socio-economic factors. This includes energy inputs, risk management of workers and surrounding community, time factors, operation and maintenance requirements. Data management and careful documentation that clearly records assumptions and the remediation decision process leads to the final strategy. For any successful and sustainable remediation strategy, it is required to have stakeholder involvement and awareness. Nanoscale metal oxides with fascinating properties such as high dispersivity, a higher surface-to-volume ratio, and high thermal/photostability attracted significant research in the field of environmental remediation. Further modification of metal oxides by making their composites, anchoring active functional moieties on the surface makes them more versatile and selective for the removal of specific contaminants from the sample (Goswami et al., 2018; Mohamed et al., 2012). Moreover, the deliberate efforts made to tune various physical properties (size, morphology, porosity, and chemical composition) of metal oxides and their composites are detrimental in environmental remediation studies. Remediation methods employing Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00014-2 © 2022 Elsevier Inc. All rights reserved.

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single nanoscale metal oxides are potentially less effective compared to the methods utilizing a combination of different active materials like hybrid or composite materials (Lu & Astruc, 2020). The components in such a nanoscale combination proved complementary to each other, furnishing several adventurous properties that phenomenally increase their selectivity toward specific target contaminant molecules and thus removal of contaminants. Environmental remediation in which contaminants are degraded by using such nanodimensional hybrid/composite materials is generally termed as nanoremediation crane (Crane & Scott, 2012). To treat soil or groundwater, nanoscale materials are interacted with the contaminant bodies either by in situ injection or a pump and treat method. When these materials come in contact with contaminants, various redox reactions cause degradation of contaminants, or contaminants get adsorbed onto the surface of nanomaterials and can be immobilized. Organic contaminants are removed by redox reactions whereas heavy metal ions like Hg, Pb, As, etc., can be removed by later adsorption techniques (Gil-Dı´az et al., 2017). The applicability of metal oxide NCs in environmental remediation is depicted in Fig. 13.1. Renewable energy and environmental remediation are two important concepts of recent research activities. These two concepts are balanced by using mixed metal oxide (MMO) nanodimentional photocatalysts, as they offer a clean environment and renewable fuel sources (Chuaicham et al., 2020; Luo et al., 2019). The sun is a promising, long-lasting energy nanodimensional source that can supply energy that mankind needs. Current research activities are aimed at obtaining semiconducting

Figure 13.1 Applicability of metal oxide nanocomposites in environmental remediation.

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photocatalysts capable of harvesting this solar energy and utilizing it to address environmental issues and to produce renewable energy. Environmental remediation studies are mainly focused on air pollution and wastewater treatment. The first type of study is focused on the removal of air pollutants like NOx and volatile organic compounds (VOCs) from fresh air. Whereas, later studies focused on either decomposing or absorbing hazardous organic and inorganic moieties present in the water bodies. In this review, we focus comprehensively on a variety of studies used to synthesize and characterize mixed metal oxide nanomaterials used as catalysts in environmental remediation. Moreover, efforts are also taken to encompass application studies on those materials in environmental remediation point-of-view. Systematically, the effect of synthetic strategies to tailor the Physico-chemical properties of photocatalysts by crystal lattice reformation, functionalization of surfaces and interfaces, etc., has been elaborated. An improvement in the catalytic performance due to enhancement in the light absorption, development of the surface-active site, and enhanced separation of electron
hole pairs can also be discussed in detail.

13.2

Types of environmental remediation

The process of removal of contaminants from environmental resources polluted by different industrial, manufacturing, mining, and commercial activities is called environmental remediation. This remediation process includes various steps like detection, investigation, assessment, determination of remedial measures, actual cleanup, and site redevelopment. The following are different common types of remediation implemented in many polluted sites.

13.2.1 Soil remediation The quality of soil depends on many factors. All the soil remediation techniques and processes are focused on the removal of contaminants in soil. The direct contact, ingestion, or introduction of soil contaminants such as hydrocarbons, pesticides, and radioactive materials in the food supply chain poses plenty of hazards to human and ecological health. A variety of physical, chemical, thermal, or biological methods can be employed for soil remediation depending upon the nature of the contaminant that is being treated. The areas or environments where soil remediation is employed are urban environments, brownfield redevelopment, urban mining, and raw material extraction areas, opencast coal mines, mining heaps, mining terrains that have subsided.

13.2.2 Groundwater and surface water remediation The survival and sustainability of living systems depend on the most vital and essential natural resource, which is water. To keep water desired for its application,

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the quality of water sources is required to be managed and maintained. Environmental remediation of industrial and drinkable grade water has become an advancing thrust in enforcement with the increase of industrial effluent discharge. Surface water remediation is very similar to groundwater remediation, except it is far easier and thus more vulnerable to contamination as compared to groundwater remediation. Surface water is exposed to many factors. Water is a natural breeding ground for insects, bacteria, and other things which are generally harmful to humans. Surface water remediation is important because humans are more likely to come in contact with surface water than they are with groundwater.

13.2.3 Sediment remediation Sediment remediation is a combination of soil and water remediation. According to the EPA (USA), the definition of contaminated sediment is given as soil, sand, clay, organic matter, hydrated oxides or other material containing toxic materials at high levels gathered at the bottom of a water body. Different in-situ and ex-situ physical, chemical, biological, and thermal techniques are used for sediment remediation (Song et al., 2017). Fig. 13.2 shows some important in situ remediation methods for soil and sediments.

13.3

Semiconducting metal oxides

Semiconducting metal oxides in their nanodimension are pivotal in various applications like pollutant sensing, energy harvest, conversion, storage and most important among all is environmental remediation. The functional nature of metal oxides

Figure 13.2 In situ methods of soil and sediment remediation.

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strongly depends on their structural, morphological, compositional, optoelectrical properties. All these properties of metal oxides are possible to tailor by adapting specific synthetic protocols or modifying those protocols in certain specific ways. Metal oxide-based materials furnish versatile heterocatalytic functionalities due to their size and dimension-based property, which is the optical bandgap. Both chemicals, as well as physical methods of synthesis, offer structural diversity to these materials. Thus the careful selection of synthetic methods and proper control of process parameters are key factors in the development of good metal oxide nanomaterials for environmental remediation studies (Krishnamurthy et al., 2018). In short, methods should be developed which offer combinations of various materials (composites/hybrids) to seek specific desired properties from every component in the combination. Such combinations, like MMO or, heterometal oxides, are extra potential, more efficient, more stable and highly selective as compared to their single counterparts. Furthermore, functionalizing these mixed metal nanomaterials with particular chemical functionalities (acidic/basic) leads to targeting molecules of contaminants more efficiently and selectively (Guerra et al., 2017). If a material possesses some specific or advanced properties such as target-specific reactivity, abundancy, ease of synthesis, greener nature, low toxicity, environmental benign behavior, recyclability, reusability, and cost-effectiveness, then such material is considered ideal for a large number of applications. As semiconducting metal oxides possess all these properties, they are the most focused class of materials used by researchers for various applications, including remediation studies (Guerra et al., 2018). Fig. 13.3 shows some important applications of metal oxide NCs. As multiapplicative materials, metal oxide NCs are widely studied in different fields. Crumpled nanosheets of single metal oxides such as SnO2, ZnO, Co3O4 as well as their MMO such as SnO2/ZnO and SnO2/Co3O4 are prepared by the spray pyrolysis method. Because of a crumpled nature with a high surface-to-volume ratio, small grain size, high mesoporosity, etc., these two-dimensional nanosheets showed improved gas sensing performance toward VOC’s like formaldehyde and acetone with high selectivity and response rate. In this study, it has been well revealed that multicomponent metal oxide NCs show enhanced sensing performance than single-component metal oxides (Kim et al., 2019). The synthesis of heterogeneous metal oxide NCs with desirable composition and morphology is not an easy task. Nanotechnology has been recognized as one of the most advanced processes for the synthesis of such desirable nanomaterials. A simple synthetic protocol is always needed to obtain MMO NCs with high surface area and porosity (Liu, Li, et al., 2019).

13.4

Environmental remediation: need of the hour

For sustainable life on the planet earth, environmental remediation of hazardous pollutants from vital resources like air, water and soil has become very crucial in the 21st century. In the scarcity of existing remediation methods, the essentiality of new methods is

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Figure 13.3 Applications of mixed metal oxide nanocomposites.

of prime importance. Right from the evolution period when humans learned to handle fire and molten metals, anthropogenic pollution put an extra burden on the surrounding environment. But, it is industrialization issues after which environmental pollution issues come to a focus. Manufactures of different chemicals that are not naturally available are now discharged in large quantities into the environment, thereby causing serious problems to the living organisms in its vicinity. An increasing population worldwide demands a wide variety of chemicals, different products, and commodities to fulfill its day-by-day requirements (Fernando et al., 2019). A vast industrial supply chain is continuously engaged in the production of these demandable products and also in the discharge of hazardous chemicals into the air, water, sediments, soil, ground, etc. These discharged products in the environment, due to their long persistence, nonbiodegradability, toxicity, can cause serious health issues for human beings and other ecosystems. The pollution of the environment not only has an adverse effect on the living ecosystem but also puts an economic burden on society. A simple strategy used in the earlier periods was to bury all

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hazardous products causing pollution underground. But this will not destroy pollutants completely and they remain underneath for the long period as a potential problem (Alharbi et al., 2018). shows various anthropogenic activities by which hazardous contaminants are incorporated into the environment (soil, water and sediment). Many workers have developed numerous strategies to meet environmental remediation. All these include various methods to either completely remove pollutants from the environment or to transform hazardous contaminants in the environment into environmentally friendly, harmless end products. Some important remediation strategies employed by using metal oxide NCs are discussed in Fig. 13.4. Besides, the efficiency of all such strategies depends on many factors such as concentration and nature of the contaminant, the time required for the treatment, and consumption of space and energy. However, in the presence of all such deciding factors, there are plenty of remediation methods available which are found effective for in-situ and ex-situ removal of environmental pollutants/contaminants (Thangadurai et al., 2020). The high persistence period of refractory pollutant compounds in the environment leads to harmful effects on the surrounding living ecosystem and, unfortunately, many of the currently available methods are inadequate for the removal of those compounds from the natural environment. Thus novel strategies for the removal of pollutants from the environment have become the need of time.

13.5

Different composites in metal oxide

Broadly, the compounds formed by the combination of metal and oxygen in the air, where the oxidation state of oxygen is 12, are called metal oxides. They are the particular class of compounds having a broad range of applications in various sectors such as catalysis, gas sensing, fuel cells, biomedical appliances, supercapacitors and other microelectronics (Li et al., 2011; Wang et al., 2010). This is because of

Figure 13.4 Important remediation strategies employed by using metal oxide nanocomposites.

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their high surface area, mesoporosity, better magnetic, tunable optical, electronic and catalytic properties. Therefore bare or composite forms of metal oxides are promising candidates in various potential applications. An environmental remediation by using metal oxide NCs largely depends on their nature, composition and crystallo-chemical characteristics. Different types of metal oxide composites can be employed in the ecological direction (sorbent and photocatalysts). Mostly, morphological and dispersion properties of metal oxide particles are focused intentionally as these are detrimental to controlling physicochemical properties (Palchoudhury et al., 2020). The applicability of different metal oxide composites with respect to their synthetic protocol, morphology, and important physico-chemical properties has been shown in Table 13.1. Because of fast kinetics and high adsorption power, metal and metal oxide NCs are usually used for environmental remediation. In addition, they are highly flexible toward both in situ and ex-situ applications in aqueous systems (Guerra et al., 2018). An ideal photocatalytic nanomaterial derived by artificial means must possess some basic properties, such as (1) formation of photogenerated charges having a long lifetime, (2) tunable optical band gap, (3) wide absorption range of solar wavelength, and (4) cost-effectiveness, high efficiency, and high thermal and photostability (Luo et al., 2019). In earlier research, most of the strategies focused on morphological and other structural modification of catalysts which cannot satisfy the current demand for efficient photocatalyst and remediation methods. Therefore modern researchers have attempted to develop photocatalysts with some surface modifications, like anchoring of abundant active sites and tuning optoelectronic properties.

13.6

Mixed metal oxide NCS and environmental remediation: present state of the art

It is apparent from the modern research activities that the customized or tailormade MMO-based nanocomposite can serve as capable candidates for the remediation of different classes of chemicals in the form of pollutants. The most vital aspects of heterostructured UV/visible light active metal oxide-based semiconductor NCs are controllable growth mechanisms, which facilitate unique surface morphology, tunable optoelectronic properties, high photo and thermal stability, ability to absorb a large quantum of solar radiation (UV as well as visible), etc. Several organic moieties are released into the earth’s environment by overgoing agricultural and industrial activities (Ahmad et al., 2019). The use of MMO heteronanostructured semiconductors for the removal of pollutants from wastewater is an efficient process. So the development of newer metal oxide-based photocatalysts to solve the major issues related to environmental remediation is important for researchers. Fig. 13.5 demonstrates the systematic process of environmental remediation using metal oxide photocatalysis.

Table 13.1 The applicability of different metal oxide composites with respect to their synthetic protocol, morphology, important physico-chemical properties. Sr. no.

Metal oxide nanocomposite

Synthetic method used

Morphology and structural properties

Application

Reference

1.

Binary TiO2/SiO2

Sol
gel

Self-cleaning fabric designing

Qi et al. (2007)

2.

Binary TiO2/SnO2

Sol
gel wet impregnation

Rhodamine B decomposition

Chen et al. (2014)

3.

Binary Fe2O3/TiO2

Precipitation-assistedpyrolysis method

Anatase TiO2 nanoparticles onto the surface of spherical SiO2 NPs (powder) nanocrystalline TiO2 layers on top of inverse opal SnO2 photonic crystals films Fe2O3/TiO2 NPs on porous biochar

Chen et al. (2020)

4.

Functionalized binary Ag@ZnO/TiO2

Combination of simple electrospinning method and subsequent hydrothermal reaction and photodeposition process

Ag nanoparticles are evenly decorated on ZnO/TiO2 nanocomposite

5.

Functionalized binary sulfated TiO2/WO3

Ultrasonic assisted sol
gel wet impregnation

Sulfonic groups anchored onto the surface of TiO2/WO3 NCs

6.

Ternary Mn2O3/ Al2O3/Fe2O3

Coprecipitation

XRD analysis confirms the average crystallite size for this sample was 40 nm

Rhodamine B, Methyl red, Methyl orange decomposition Exhibits superior photocatalytic efficiency toward degradation of tetracycline hydrochloride within 1 h, and also shows outstanding antibacterial activity against Escherichia coli after 1 h simulated solar light exposure. Efficient visible active photocatalyst for azo dyes (like Congo red, Methyl red) degradation Malachite green dye degradation

Song et al. (2020)

Patil et al. (2019)

Habib et al. (2018) (Continued)

Table 13.1 (Continued) Sr. no.

Metal oxide nanocomposite

Synthetic method used

Morphology and structural properties

Application

Reference

7.

Ternary zeolitesupported TiO2 and ZnO

Well intermixed spherical ternary composite

Photocatalytic decomposition of acetaminophen and codeine medicines in presence of UV and sunlight irradiation

Behravesh et al. (2020)

8.

Ternary ZnO/ CuO@rGO

Zeolite was produced from waste materials of stone cutting industries using alkali hydrothermal synthesis method and TiO2 ZnO nanoparticles immobilized into various zeolite samples Solid-state method

Visible light driven degradation of RhB dye and

Kumaresan et al. (2020)

9.

Ternary TiO2/ SnO2/WO3

Sol
gel and hydrothermal method.

Degradation of 1,2dichlorobenzene under visible light

Nadarajan et al. (2016)

10.

Trenary SnO2/CuO/ TiO2

Sol
gel wet impregnation method

Formation of well-developed flowers like morphology of (ZnO/CuO) nanoparticles on rGO sheets Comparatively larger NPs of TiO2 and WO3 closely attached with each other and smaller SnO2 partially adsorbed on the exposed facets of TiO2 and WO3 NPs SnO2 and CuO NPs randomly distributed on anatase TiO2 NPs

Degradation of 2,4-dichloro phenol under UV-light

Golestanbagh et al. (2018)

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Figure 13.5 Systematic of environmental remediation by metal oxide photocatalysis.

Metal oxides are usually well-built and stable systems with specific crystallographic structures. However, the increasing significance of surface activity and change in properties with decreasing particle size should be considered. Again, metal oxides in nanosize exhibit diverse chemical and physical properties as a result of their high density and controlled size of corners and edges on the surface sites (Ganachari et al., 2019). Several functional metal oxides like TiO2, ZnO, Fe2O3, SnO2, WO3, ZrO2, SiO2, Al2O3, graphene oxide (GO), etc., are studied in a large extent due to their special characteristic properties such as abundancy, nontoxocity, biocompatibility, costeffectiveness, tunable optoelectronic properties and eco-friendly nature. Recent research activities are aimed at using heterometal or MMO instead of single metal oxides as a combination of two or more materials in nanodimensions give rise to incredibly advanced physicochemical properties and therefore catalytic performance in remediation studies. The forecasts for the synthesis of metal oxide NCs and their utilization for environmental applications are illustrious and show the way to a new class of materials and new environmental methods. The blend of two or more metal oxides in nanodimensions led to exciting advancements in physicochemical properties. Mainly, these nanodimensional MMOs are used in photocatalysis and gas sensing applications. They are also used as active and passive materials in many electronic devices due to their exceptional electronic and magnetic properties (Kapoor et al., 2004b). The use of MMOs in

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catalysis and gas sensing was promising practice due to their increased surface acidity and improved band structures. These two factors are crucial in enabling MMONCs as efficient materials for remediation studies (Guo et al., 2014; Wen & Tian-mo, 2010). There is a very strong connection between surface acidity and catalytic activity. It has been proved that the surface adsorbed hydroxyl groups can be responsible for surface acidity. These hydroxyl groups can absorb photogenerated holes to form electron deficient hydroxyl radicals. Subsequently, these hydroxyl radicals can cause the oxidation of surface adsorbed molecules and thus their removal (Xianzhi et al., 1996). Along with the trapping of the hole, surface hydroxyl groups also act as good adsorption sites for other molecules. The adsorption of gases or vapors on the surface of metal oxides has proved to be significant in the field of gas sensing or bio-sensing (Chaudhari et al., 2006). It has been well known that the resistance of semiconducting metal oxide surfaces is strongly influenced by the presence of surface adsorbed oxidizing or reducing gases (Yamazoe & Miura, 1992). This change in resistance response is possible to monitor by using highly sensitive microelectronic devices and, therefore MMO films are mostly used as sensors in environmental monitoring and biomedical applications. The following are some important MMO NCs used as functional materials for environmental remediation studies.

13.6.1 TiO2-based nanocomposites TiO2 is one of the ubiquitous materials extensively used in various fields, such as photocatalysis, heterogeneous catalysis, energy harvesting, biomedical applications, gas and biosensing applications, etc. Because of its interesting bandgap structure, it is used in photocatalysis. With respect to a normal hydrogen electrode, the conduction band potential of TiO2 is more negative (B 20.48 eV) as compared to the potential of water reduction reaction (B0 eV). In contrast, its valence band potential is more positive (B2.8 eV) than the potential of water oxidation reaction (B1.25 eV) (Gr¨atzel, 2001). Thus TiO2 can be applied as a superior redox material in photodegradation studies. The semiconductor TiO2-based metal oxides can be promising candidates for sensing pollutant gases, vapors, as well as VOC including hydrocarbons like benzene, xylene, and toluene due to their many advantages such as simple synthetic protocols, environmentally benign nature, low cost, fast response and recovery time (Wen & Tian-mo, 2010). But, the major problem of TiO2 is, it is UV active only and unable to absorb light in the visible region of the electromagnetic spectrum, which limits its use as a single photocatalyst (Dong et al., 2015). The inability of visible absorptivity of TiO2 may be attributed to its wide optical band gap (B3.2 eV). In addition to that, there is fast recombination of photogenerated electron-hole pairs. The TiO2 material needs to be modified using different strategies and then the material’s modified TiO2 materials are found effective in a large number of applications including photodegradation, photocatalytic or heterogeneous catalytic organic transformations, etc. Doping of TiO2 with metals or nonmetals and forming its composite in nanodimensions with other materials, particularly, metal oxide semiconductor/s to form binary or ternary MMONCs are the two most used strategies (Park et al., 2013). Binary TiO2/ZnO NCs find multiple applicability in photocatalysis and gas sensing

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(Moradi et al., 2016; Park et al., 2009). Other MMOs NC, rGO/TiO2 found to be active in photocatalytic degradation of dyes as well as for gas sensing with high responserecovery time (Li et al., 2016; Wang et al., 2012). In catalytic and gas sensing applications, the surface characteristics of NCs are more important (Park et al., 2013). Therefore surface modification of MMOs NCs by anchoring different acidic groups on their surface is another technique to improve their performance in various applications. The sulfated TiO2 loaded on RuO2 is used for thiophene oxidation reaction and proved to be an efficient photocatalyst with increased surface acidity due to surface adsorbed sulfate groups (Lin et al., 2016). Sulfated TiO2/mordenite binary NCs are prepared as solid acid catalysts and utilized as efficient heterogeneous catalysts for the conversion of inulin to various furan derivatives at ambient reaction conditions (Shen et al., 2012). Selective catalytic reduction (SCR) of NO with NH3 has been carried out using sulfated Mn, Co, and Ce metals supported on binary TiO2/SiO2 composites (Qiu et al., 2016). Here, the catalyst can cause SCR of NO with good recyclability. Photocatalytic degradation of heptane at ambient temperature can be achieved by using sulfated TiO2/SiO2 as an efficient photocatalyst. It is concluded that the surface adsorbed sulfate groups favor the separation of photogenerated charge carriers and thus increase the catalytic activity of sulfated binary NCs in comparison to nonsulfated NCs (Xie et al., 2006). Additionally, the modification of TiO2 is done by forming ternary MMOs NCs. Among various methods, the addition of third component metal oxide NPs into the lattice matrix of binary metal oxide-metal oxide NCs is a simple and promising method, as it leads to forming pure metal oxide heteronanostructures with improved physicochemical properties. Ternary CoO/TiO2/rGONCs prepared by a simple sol
gel method rapidly degraded 2-chlorophenol in the presence of visible light (Sharma & Lee, 2016). A cost-effective, reusable, and stable ternary photocatalyst of graphene/TiO2/Fe3O4 has been prepared for the removal of a mixture of various organic dyes just under sunlight. This ternary photocatalyst showed enhanced degradation activity than bare TiO2 as well as binary TiO2/ Fe3O4 NCs (Lin et al., 2012). Similarly, TiO2/NiO loaded on rGO support showed high visible light photocatalytic activity toward the removal of o-chlorophenol. Here, these ternary MMO NCs were recycled and retained their activity even after four successive repeat cycles (Sharma & Lee, 2016). Ternary MMOs are also explored for gas and volatile organic vapor sensing studies. The GO/SnO2/TiO2 a ternary NCs are used to detect the very low concentrations of acetone (1 ppm) exhaled from the breath of a diabetic patient within a fast response time (10 seconds). Ternary TiO2/GO/PANI nanocomposite is prepared and tested for sensing of NH3 gas. It is revealed that these temporary MMOs NCs show good selectivity, sensitivity and stability toward NH3 gas with low to a high concentration ranging from 5 to 300 ppm (Tian et al., 2016).

13.6.2 Fe2O3-based nanocomposites Iron is one of the abundant elements in the earth’s crust. Nanodimensional iron oxides rendered a variety of remediation applications. This is due to the eco-friendly nature of these materials. They can be directly put into contaminated pieces of stuff without having chances of secondary contamination (Hua et al., 2012). Mainly iron oxide-based NCs are studied as substantial materials to treat heavy metal polluted

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environments by various mechanisms including adsorption, redox reactions and precipitation. Varieties of stoichiometrically different crystal structures are possible in Fe2O3, among them α-Fe2O3 and γ-Fe2O3 phase structures are comparatively more stable and extensively used for remediation studies. Fe2O3 has several advantages over other functional materials used for remediation processes due to its small optical band gap (2.2 eV), low cost, ample availability, nontoxic nature, high efficiency of visible light utilization, excellent magnetic ability, abundant active sites for adsorption/degradation and high chemical stability. Along with these fascinating physico-chemical properties, Fe2O3 has some limitations such as high recombination rates of charge carriers, low conductivity and low diffusion lengths of holes (2
4 nm) (Theerthagiri et al., 2018). Researchers have been continuously engaged to overcome these limitations by using several strategies like using heterostructure of NCs to lower recombination rate, doping material with conducting polymers or metals to improve conductivity and charge transferability. Because of the versatility of Fe2O3 materials as efficient redox catalyst and adsorbent, its applicability for different remediation processes are studied under specific aspects such as (1) magnetically separable and recyclable adsorbents, (2) oxidation and thus removal of contaminants as photo-Fenton catalysts, (3) Redox photocatalysis in the form of the composite heterostructure, and (4) Fe2O3-based self-propelled micro/nanomotors for decontamination of wastewater (Tao et al., 2020). Chang et al. (2018) fabricated Binary α-Fe2O3/Bi12O17Cl2 composites by in-situ deposition route and used as efficient visible light photo-Fenton catalyst for the degradation of methyl orange (MO) and colorless 2,4-dichlorophenol (2,4-DCP). Facile α-Fe2O3/GO composites prepared by hydrothermal method and used as adsorbent for removing two hazardous dyes Congo red (CR) and methyl violet (MV) from aqueous solutions (Bulin et al., 2020). A Fe2O3/bentonit composite synthesized by mechanical milling was tested as UV active photocatalyst for the degradation of indigo carmine (IC) dye (Lubis et al., 2018). Efficient visible light active photocalyst Ag3PO4/α-Fe2O3 prepared by solvothermal process and utilized for rapid disinfection of environment form pathogenic microorganisms like Staphylococcus aureus and Escherichia coli under visible light irradiation within just 15 min. (Su et al., 2020). Pentachlorophenol (PCP) is one of the persistent organic pollutants in the environment and its remediation by photodegradation process was carried out by using α-Fe2O3/ZnO composites under UV-visible light (Xie et al., 2015). The crystalline particles with large and open facets of α-Fe2O3 in α-Fe2O3/SiO2 composites facilitate adsorption of NOx pollutants on its surface and cause its removal by photooxidation process under UV-visible irradiation (Balbuena et al., 2018). In short, there is a number of several composites based on Fe2O3 material which can be used for environmental remediation processes.

13.6.3 ZnO-based nanocomposites Zinc oxide has emerged as one of the efficient alternative functional materials for titanium oxide in recent years. TiO2 and ZnO are the most extensively studied photocatalysts for remediation studies due to their abundance, nontoxicity, costeffectiveness and high stability. Among these two, ZnO exhibit various astounding

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advantages over TiO2, such as fast charge carriers transport, easy crystallization and anisotropic crystal growth (Kumar et al., 2020). Some intrinsic impurities in the crystal lattice of ZnO lead to improve its electron mobility (200
300 cm2 V21 s21), which is nearly double in magnitude than TiO2. This increased electron mobility facilitates the quick transfer of photogenerated charges to the surface of ZnO during the photocatalytic reaction (Li et al., 2010). Moreover, ZnO due to its low refractive index (2.0) compared to other conventional photocatalysts, is highly transparent and excellent light-absorbing material with minimum scattering. The photocatalytic reactions of ZnO-based photocatalysts display the greatest performance in ambient conditions, such as neutral pH, a low temperature which is also its advantage over other catalysts (Chandiran et al., 2014). With a large number of aforementioned advantages, bare ZnO or ZnO based materials have several drawbacks, such as fast recombination rate of photogenerated charge carriers, only UV active absorption of solar radiation which contributes only 5% of the total solar spectrum, rapid agglomerate formation during the reaction, low recyclability as high-temperature reactions lead to cause structural deformation, and photo corrosion as holes can photocorrode an oxide catalyst, ZnO 1 2 h1! Zn21 1 1/2 O2 (Kumar et al., 2020). Remarkable efforts have been devoted to resolving the abovementioned drawbacks which include, bandgap modification of ZnO, the introduction of optimal defects in the crystal lattice, formation of heterojunction by coupling with other semiconductor metal oxides, loading over carbonaceous materials, noble metal loading, etc. (Ong et al., 2018). All these efforts mainly consequence in the efficient separation and transfer of photogenerated charge carriers with prolonged lifetime and enhanced photostability of catalyst. This in turn improves its photocatalytic performance. Thus ZnO has emerged as one of the most captivating materials in the field of environmental remediation. The novel Csx WO3/ZnO nanocomposite achieved multifunctions and proved to be promising material not only for energy saving (by insulation of heat) but also for environmental cleanup (by photodecomposition of NOx gases and blocking of harmful UV radiations) (Wu et al., 2015). Multifunctional ZnO/Fe3O4 nanocomposite prepared by one-pot synthesis method and found to be capable of degradation of harmful methylene blue (MB) dye under UV light, adsorption of inorganic metal ions like Pb21 and Cu21 at optimum pH 5.5 and antimicrobial activity against E. coli and S. aureus bacteria (Goyal et al., 2018). Ternary ZnO
TiO2
Carbon nanofibres prepared through electrospinning technique followed by a hydrothermal process showed enhanced photocatalytic performance for the removal of MB dye under UV illumination. The catalyst was recyclable and reusable after three consecutive cycles (Pant et al., 1960). Sayadi et al. (Sayadi et al., 2019) reported superparamagnetic GO@Fe3O4/ZnO/SnO2 nanocomposite and utilized this material for the treatment of pharmaceutical pollution of azithromycin. About 90.06% of 30 mg L21 azithromycin were efficiently degraded by UV irradiation under optimal conditions of pH 5 3, time 120 min with 1 g L21 amount of GO@Fe3O4/ZnO/SnO2. The ternary flexible Ag@ZnO/ TiO2 fibrous membranes with hierarchical nanostructures were synthesized by combining a simple electrospinning method, hydrothermal reaction and

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photodeposition process. Herein, ZnO nanorods anchored onto the surface of TiO2 nanofibres and both these oxides were evenly decorated by Ag nanoparticles. Such modified ternary composite with improved light absorption, large surface area, and effective charge separation showed excellent degradation performance toward organic pollutant tetracycline hydrochloride within 1 hour and at the same time exhibited prominent antibacterial activity with a 6.5 log inactivation of E. coli after 1 hour simulated solar light exposure without losing structural integrity and mechanical flexibility after utilization (Song et al., 2020).

13.6.4 Al2O3-based nanocomposites Most of the new generation nanomaterials with versatile applications, morphologies comprise incorporation of nanodimentional particles with aluminum matrix leads to produce excellent mechanical and physical properties. Such Al2O3-based NCs find utility in various demanding fields of science and technology like automobiles, defense, chemistry, aerospace engineering, electronics, biotechnology, consumer products, energy and the environment. γ-Al2O3/anatase TiO2 mesoporous nanocomposite prepared by precipitation method show enhanced photocatalytic performance in the removal of reactive black 5 (RB5) dye. UV-Vis analysis of this decolorization process clearly showed incomplete degradation of RB5 dye under natural indoor light concurrently with its adsorption on the surface (Deng et al., 2019). A bimetallic heterojunctionbased material Al2O3/ZnO prepared by hydrothermal method. This composite with micro flower-like morphology was used as bifunctional material for electrochemical sensing of harmful environmental pollutant hydroquinone and catalytic removal of RhB dye (Renganathan et al., 2019). Al2O3@ZnO core-shell nanocomposite with a three-dimensional hierarchical structure was designed and fabricated by a combined techniques initial hydrothermal reaction with a subsequent chemical bath deposition process and final calcination (Zheng et al., 2019). This porous microfibrous composite showed high adsorption capacity for hazardous CR dye and thus its removal from aqueous solution. Zhang et al. (2018) synthesized hybrid chitosan-Al2O3@SiO2 composite for the effective removal of carcinogenic hexavalent chromium Cr(VI) from aqueous systems. The material furnished high performance of adsorption rate and capacity and good reusability as a potential adsorbent for wastewater treatment. Nickel-supported Zirconia-alumina porous nanofibres composite (Ni/Al2O3@ZrO2)revealed high catalytic performance in CO methanation. This core-shell nanocomposite was prepared by the impregnation method and it was observed that with an increase in zirconia content in the catalyst rate CO methanation increases (Yang et al., 2013). The ternary metal oxide composite Fe2O3/Al2O3/ZrO2 prepared by co-precipitation method and tested under different optimized conditions like pH, catalytic amount, temperature, contact time, etc., for the adsorption of various heavy metal ions, such as Pb (II), Cd(II), and Cr(VI). Under all optimized conditions, this nanocomposite revealed very good adsorption efficiency of 96.65%, 96.55% and 97.2% for Cd (II), Cr(VI) and Pb(II), respectively (Tsegaye et al., 2020).

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13.6.5 WO3-based nanocomposites In recent environmental remediation studies, more importance has been given to design visible active superior metal oxide photocatalysts. Among various metal oxide catalysts, tungsten oxide (WO3) is a promising candidate because of its high absorption of visible light (up to 480 nm), tunable bandgap, and outstanding optical properties. Other captivating properties of WO3 include nonhazardous nature, costeffectiveness, and excellent stability in oxidative and acidic conditions. Further enhancement in the photocatalytic performance of WO3 was carried out by various workers by using different strategies like fabrication of catalytic material with versatile morphologies, size, and composition. Making composite of WO3 with other functional metal oxide semiconductors in nanodimention is one of such prominent strategies utilized by researchers. Such efforts lead to improve the catalytic performance to distinctive degrees (Dutta et al., 2021). WO3/TiO2 and modified Au-WO3/TiO2 NCs were prepared and utilized for wastewater treatment. With optimized 5 mole % nonmodified WO3/TiO2 NCs, a high degradation efficiency was observed for organic pollutants phenol and MB dyes in aqueous solutions under visible light. Further, surface modification of this catalyst by uniformly anchoring Au nanoparticles onto its surface showed phenomenally enhanced photocatalytic activity for visible-light-driven degradation of both these pollutants (Rhaman et al., 2020). In the same manner, MO-loaded WO3/TiO2 NCs synthesized by coupling two methods sol
gel and hydrothermal furnished excellent photocatalytic performance for the removal of RhB and p-chlorophenol in aqueous media. Here, also it was observed that Mo ion doping significantly enhanced visible light activity of WO3/TiO2 composite (Pirzada et al., 2018). Priyadharsan et al. (2019) introduced ternary hybrid WO3
Fe2O3
rGO (WFG) NCs by simple hydrothermal route. These advanced NCs revealed multiple applications in photocatalysis, heavy metal ion removal by adsorption and microbial disinfection. Heterostructured WO3@Co3O4 with better separation of photogenerated charge carriers and increasing light absorption ability exhibited high photoelectrochemical performance. The photocurrent generated by this coupled material during the water oxidation reaction was 20 times higher than that of pure WO3 (Markhabayeva et al., 2020). In our own work, we have developed acid-functionalized binary TiO2/WO3 NCs by ultrasonicassisted sol
gel method. Cholorosulfonic acid was used as a source of sulfate group and these sulfate group functionalities were anchored uniformly onto the surface of binary TiO2/WO3 NCs. As prepared sulfated TiO2/WO3 NCs performed degradation of CR and methyl red dyes under visible light illumination (Patil et al., 2019). This work was extended further and ternary TiO2/SnO2/WO3 hybrid nanostructures were prepared by using the same protocol mentioned above followed by wet impregnation method. Finally, this functional material was investigated for NH3 gas sensing applications (Patil, Vanalakar, et al., 2018).

13.6.6 SnO2-based nanocomposites As mentioned earlier, there are many semiconducting metal oxides like TiO2, WO3, ZnO, Fe2O3, Al2O3, and SiO2. But, among these oxides, SnO2 is one of the

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promising oxides for the removal of hazardous pollutants by oxidative degradation method (Parale et al., 2019). SnO2 metal oxide with optical band gap (Eg 5 3.4 eV to 3.8 eV) may be coupled with other metal oxides having comparable bandgap to form intermixed electronic density states. Such electronic density states play a crucial role in trapping photogenerated charge carriers thereby facilitating the degradation of organic pollutants as well as improved capacity to sense various gases. In addition, SnO2 has potential applications in different fields such as catalysis, gas sensing, transparent conducting electrodes, rechargeable lithium batteries, and optoelectronics. The preparative methods and materials required for SnO2 are abundant and less expensive. For example, there are so many similarities in the structure of ˚ and Sn41 is TiO2 and SnO2 as well as the ionic radii of the cations (Ti41 is 0.605 A ˚ ). Such similarities in the properties of these materials allow them to form a 0.69 A heterojunction hybrid between the two oxides (Patil, Dhodamani, et al., 2018). Zhao et al. (2019) introduced an evaporation-induced oriented co-assembly strategy to incorporate SnO2 nanocrystals (NCs) into a three-dimensional branched mesoporous TiO2 framework by using poly(ethylene oxide)-block-polystyrene (PEO-b-PS) as a template. This binary mesoporous material was used as highly efficient ethanol sensing. The diclofenac is one of the dangerous chemicals and is very difficult to degrade due to its complex structure. TiO2/SnO2 nanocomposite with molar ratio (20:1) was found effective in the photocatalytic degradation of these hazardous pollutants from aquatic systems (Mugunthan et al., 2019). Various conventional methods used in synthetic organic chemistry lead to produce environmental pollutant waste. Such methods are often energy and time consuming, expensive and proceed with low atom efficiency. To overcome problems associated with these nongreen conventional synthetic protocols, we have developed sulfate group anchored binary TiO2/SnO2 nanocomposite as a heterogeneous green catalyst for the direct amidation reaction between a series of amine derivatives and acetic acid. The reaction gives excellent product yield within 120 minutes, and at a relatively moderate temperature of B115 C. The catalyst was recycled and reused for a large number of times and fulfilled maximum conditions of green chemistry (Patil et al., 2021). Ternary TiO2/ZrO2/SnO2 nanocomposite prepared by simple templateassisted sol
gel method attributed relatively regular large pores and therefore high surface to volume ratio. Herein, it was noteworthy that the incorporation of SnO2 into binary TiO2/ZrO2 significantly enhances photodegradation of RhB dye and the material can be used several successive times without loss of photocatalytic activity (Lu et al., 2013).

13.6.7 Graphene oxide-based nanocomposites Graphene is a two-dimensional allotrope of carbon discovered in 2004 by A. K. Geim and K. S. Novoselov through mechanical exfoliation of highly oriented pyrolytic graphite (Geim & Novoselov, 2010). As dynamic functional material graphene has a high surface area, high mechanical strength and high charge carrier mobility. It has attracted the attention of researchers due to its high potential in nanotechnology and environmental remediation. In remediation studies, it has been extensively

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used as an adsorbent for heavy metal ions and photocatalyst for cleaning harmful organics in water bodies. Common graphene materials include reduced graphene oxide (rGO) and GO. The GO is isostructural to that of graphene, but at the same time it has distinguished properties from graphene. The conversion of graphene to GO resulted in the introduction of numerous oxygen-containing functional groups, such as carbonyl, hydroxyl and carboxyl groups. These functional groups act as active sites to attach to other functional groups or dope elements. This is attributed to the optimization of properties of GO. Owing to its edge hydrophilicity and planar hydrophobicity GO has good dispersion in aqueous media. So GO and its NCs can interact with organic pollutants in aqueous media thereby degrading these organics. GO is easily reducible into rGO which is again modified from having strong chemical and thermal stability (Pan et al., 2020). The GO/TiO2 nanocomposite was used for the removal of hexavalent chromium from water using a photocatalytic process. Here in the presence of catalyst Cr (VI) was thermodynamically reduced to Cr (III) under UV light illumination (Hu et al., 2018). Rajput et al. (Rajput et al., 2018)fabricated Ag-loaded GO/TiO2 nanotube photoelectrode by in-situ anodic oxidation method and employed this novel material as photoelectrocatalyst (PEC), photocatalyst (PC) and electrocatalyst (EC) against pentachlorophenol (PCP). This study demonstrates that the combination of semiconductor TiO2 and GO decorated with Ag nanoparticles can act as highly efficient and cost-effective photoelectrodes for removing biorecalcitrant compounds. Metallic Ag NPs dispersed on GO/TiO2 mesocrystals synthesized via photoreduction deposition method. This modified nanocomposite with high surface area, mesoporous nature and high crystallinity rendered superior performance in photocatalytic degradation of Rhodamine B (RhB) and dinitro butyl-phenol (DNBP) under visible light irradiation (Qi et al., 2019). Ternary GO/TiO2/NiO composite was developed and subjected for dual functional electrocatalytic oxygen evolution reaction in water splitting and photocatalytic removal of bromophenol blue (BPB) (Noor et al., 2020). rGO/TiO2 prepared by electrophoretic deposition method and used for photocatalytic degradation of 4-chlorophenol in an aqueous solution (Zouzelka et al., 2019). In another study, Ag incorporated rGO/TiO2 was utilized as photoelectrochemical and photocatalytically active material for hydrogen evolution reaction (HER) and methyl orange dye degradation has been reported (Saquib et al., 2020). Ternary RGO-TiO2/ BiVO4 heterostructure photocatalysts prepared by simple wet-impregnation method and employed for wastewater treatment. More particularly synthetic wastewater containing MB dye was degraded with 100% efficiency by using 1.0 wt.% RGO-TiO2/BiVO4 under solar light irradiation (Samsudin et al., 2018).

13.6.8 Rare earth oxides-based nanocomposites The group of 17 metallic nature elements having similarities in their chemical properties including lanthanides, scandium, and yttrium are termed as rare earth metal elements. On the basis of electronic configuration, there are two categories of these elements from atomic number 57 to 71, (1) the light rare earth elements also called as cerium group from La 5 57 to Gd 5 64 and (2) the heavy rare earth elements from Tb 5 65 to Lu 5 71. The first mineral “Ytterbite” containing rare earth was

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discovered in 1787 and since then rare earth has become more familiar to the research fraternity (Yu et al., 2018). The rare-earth-based MMO NCs have turn into unique and be likely to display versatile applications in all kinds of fields, such as photocatalytic, electrochemical, and biological. In recent years coupling of rare earth oxides with other metals, oxides have become a popular practice in research arenas. This is because metal oxides like TiO2, ZnO, WO3, Al2O3, SnO2, etc., have advanced properties such as mechanical, optical, thermal, and anticorrosive. TiO2 modified by doping trivalent lanthanide ions (e.g., Tb31, Eu31, and Sm31) showed enhanced photocatalytic activity toward degradation of textile dye compared to bare TiO2 (Saif et al., 2014). Rare-earth oxide-doped titania NCs (RE31/TiO2, where RE 5 Eu31, Pr31, Gd31, Nd31, and Y31) synthesized by a one-step sol
gel-solvothermal method performed enhanced activity toward the photocatalytic degradation of partially hydrolysis polyacrylamide (Li et al., 2009). Zhang et al. (2020) prepared CeO2/ ZnO nanomaterials with rich oxygen vacancy via a facile sol
gel method. The photocatalytic performance of this material was evaluated in the degradation process of RhB dye under visible light. It was found that enhanced oxygen vacancy concentration contributed to the absorption efficiency of visible light of photocatalyst and its activity during degradation mechanism. A similar result was also obtained by Lv et al. (2016) in the degradation of MB under UV irradiation. Heterostructured CeO2/Al2O3 NCs synthesized via chemical coprecipitation method furnished excellent bifunctional applications in visible light-driven photocatalysis and antimicrobial studies (Farraj et al., 2021).

13.7

Advanced oxidation processes or degradation processes

Advanced oxidation processes (AOPs) Fig. 13.6 is commonly utilized to remove mainly the organic compounds present in the wastewater by oxidation via reactions to produce the hydroxyl radicals (
OH) (Glaze et al., 1987). These
OH be active with high effectiveness to destroy the organic compounds. The AOPs are a set of chemical procedures and many times used to treat inorganic materials as well. The AOPs combines ozone (O3), ultraviolet (UV), hydrogen peroxide (H2O2) and/or catalyst to the wastewater treatment. However, the AOPs are effective than single oxidation processes. The beauty of this method is the decomposition of hazardous organic and/or inorganic materials without creating hazardous by-products or sludge which requires further handling. The rapid reaction rates, potential to remove toxicants with no footprint, ability to mineralize the organic materials into salt CO2, Ease in controlled automation with low labor inputs are the merits of the AOPs. However, intensive capital, designing of complex chemistry and requirement of excess peroxide quenching for some applications are the drawbacks of these systems. The AOP consist of a number of systems that mainly have a strong oxidant as the major component such as ozone, transition metal ions, irradiation like UV, ultrasound (US), or electron beam and hydrogen peroxide. Therefore there are mainly the ozonation, sonolysis, wet air oxidation, Fenton, photo-Fenton and UV

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Figure 13.6 Advanced oxidation processes or degradation processes in environmental remediation.

based AOPs systems are developed and now available for the purification of wastewater and environmental remediation. However, the selection of particular AOPs systems is application-dependent. The ozonation AOPs is the process in which the ozone is applied for the oxidation with hydroxyl radicals. In addition, to O3 there may be the use of UV radiation, hydrogen peroxide, activated carbon, catalysts, ultrasound for water purification. The decomposition of ozone gives rise to hydroxyl radicals as secondary oxidants. However, such secondary oxidants are much stronger oxidizing agents than the ozone (Legube & Karpel Vel Leitner, 1999). The O3 decomposition rate will enhance the concentration of the OH2. However, the radical concentration and the decomposition rate depends on the temperature, pH, type of toxicants, etc., the chemical reactions observed in the ozonation H2O2 reactions are as follows, O3 1 OH2 ! OH radicals 3 O3 1 hv ! 2 OH radicals H2 O2 1 hv ! 2 OH radicals H2 O2 1 O3 ! 2 OH radicals H2 O2 1 O3 1 hv ! OH radicals Sonolysis is the special type of AOPs having an amalgamation of ultrasonic sound waves and radiation along with the semiconductor photocatalyst material.

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The ultrasonic wave and catalytic material in aqueous systems will be used to improve the chemical reaction by the formation of free radicals. This method is commonly utilized to purify the water relatively fast and without excessive reagents. In this process, ultrasonic sound waves having a frequency of about 20 to 1000 kHz are used. Such ultrasonic wave is transmitted through an aqueous solution to generate acoustic cavitations and subsequent regions for the chemical reactions and the formation of the free radicals (Joseph et al., 2009). The reaction involved in the sonolysis reactions is given below, H2 O 1 ultrasonic waves ! OH radical 1 OH2 H2 O 1 ultrasonic waves ! 1=2H2 1 1=2H2 O2 Wet air oxidation is one more AOPs containing the hydrothermal treatment of wastewater, where oxygen is used as the oxidizer. In this system, the oxidation reactions take place in superheated water. In general, the temperature of the water is kept between 100 C to 374 C at a pressure in the range of 1.5 to 10 MPa. This is a large-scale process used for the last 60 years to treat the wastewater by reducing chemical oxygen demand from 10,000
150,000 ppm to acceptable values. The chemical reactions that occur during wet oxidation reactions are as follows (Mishra et al., 1995), H 1 O2 ! R radical 1 HO2 radical RH 1 HO2 radical ! R radical 1 H2 O2 H2 O2 1 M ! 2OH radical RH 1 OH radical ! R radical 1 H2 O R radical 1 O2 ! ROO radical ROO radical 1 RH ! ROOH 1 R radical The Fenton oxidation reaction was discovered by H. J. Fenton in 1894 to enhance the oxidative potential of hydrogen peroxide with iron as a catalyst under acidic conditions. However, this reaction was applied in 1960 to the destruct the toxicants from the wastewater. The Fenton oxidation reaction requires specific valued parameters such as temperature, pH, H2O2, and catalytic concentrations to accomplish a maximum efficiency to the reduction of organic matter. Nowadays, this method is applied to treat a variety of industrial wastes comprising of a variety of toxicants such as rubber and plastic chemicals, phenols, formaldehyde, etc. Meanwhile, The chemical reaction during this process is as follows (Parmar, 2014), Fe21 1 H2 O2 ! Fe31 1 OH radical 1 OH2

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In the photon-Fenton oxidation system, light is used in association with the catalyst. This method can produce biodegradable intermediates with a reduction of pollutants. However, the cost is the main hurdle for this system as compared to its counterparts. Meanwhile, researchers are working to minimize cost on account of the improvement of photo-Fenton efficiency by using heterogeneous catalysts and/ or chelating agents (Pouran et al., 2015). In the context of drinking water treatment, UV radiation-based AOP is the popular technology used nowadays. The UV-based AOP system involves the use of UV radiation to produce hydroxyl radicals (_OH) from hydrogen peroxide (H2O2) (Khan et al., 2018). The incorporation of UV and H2O2 in the AOP system was the result of The Orange County Water Factory 21 project (California, USA) completed in 2001. Recently, the interest in potable reuse projects based on UV/Cl2 AOPs has emerged. In addition, the heterogeneous catalytic processes such as the use of TiO2 for the production of hydroxyl radicals also attracted the attention of the scientific community.

13.8

Synthesis of metal oxide nanocomposites

Although the metal oxides NCs have notable property synergies for various environmental remediations, the main challenge is their fabrication with desirable physicochemical properties. In this direction, the protocol used for the synthesis of NCs to get desired shape and size is very important because the nanoscale dimensions with desired structure have overriding advantages over bulk counterparts. For the synthesis of nanomaterials, two common approaches bottom-up and top-down are generally used. The bottom-up approach includes the preparation of nanomaterial from atomic or molecular precursors by using chemical or physical forces, which permit the particles to assemble in the desired nanodimension. The theme of this approach is to arrange smaller components into more complex assemblies. In contrast, the top-down approach is based on the principle of breaking bulk material into small pieces by using different types of forces such as chemical, physical, and mechanical (Biswas et al., 2012). In the synthesis of NCs, it is very important to choose synthetic methods which is being suitable and appropriately satisfy green principles and environmental points of view. These green principles aim to the complete removal or at least less generation of waste during the synthesis process (Jagadeesh et al., 2014). Though it is difficult to find a synthetic method fulfilling all 12 principles of green chemistry, one should select the method which can comprehensively address most of these principles (Hosseinpour-Mashkani & Sobhani-Nasab, 2017). The best synthetic method is thus which consumes nontoxic precursors and chemicals, ecofriendly solvents, ease workup at ambient temperature and pressure, and having a high atom economy with less production of byproduct waste. In emerging areas of nanoscience and technology, metal oxides have numerous applications. Various physical parameters of metal oxide nanomaterials like size, shape, surface and morphology play

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significant roles in determining their physiochemical, optoelectronic, and catalytic properties (Lebaschi et al., 2017). Among the variety of top-down synthetic methods, very few methods guarantee the formation of material with desirable shape and size. The top-down protocols, generally, physical methods utilizing physical forces like a mechanical force for cutting down bulky material into the micro to nanodimensions. The problem of such type of protocol is, it can produce nanostructures with nonuniform shape and size with more defects, nonuniform chemical composition, and asymmetrical ordering of constituents. Therefore the nanostructures obtained from physical top-down methods are not so active for general use in the fields like catalysis, gas sensing, biosensing, and energy harvesting. All these shortcomings of physical methods are well addressed by bottom-up synthetic methods using chemical forces for the synthesis of nanomaterials. Synthesis of scalable nanomaterials is possible by the bottom-up approach of synthesis. Here, in this method staking of atoms layer by layer onto the surface of each other can be achieved. Such ordered deposition of constituent atoms results in the formation of crystal planes. These crystal planes are again staked on each other by chemical forces and finally give ordered, uniform sized and shaped nanostructures with unified composition and thus wide potential applicability in various fields (Wang et al., 2017). Therefore for the fabrication of scalable and active nanostructures, it is quite obvious to use the bottom-up chemical approach of synthesis. The most popular route which gives large-scale production of nanomaterial with desirable properties is a sol
gel method. This method has some overriding advantages like proper control on structure, incorporation of one material into the lattice of another material. For example, formation of metal or nonmetal doped metal oxides or coupling of two or more metal oxides with each other to form NCs of desired compositions and even compositions that are not observed naturally (Abbas et al., 2016). As it mixes materials homogeneously at the molecular or atomic level the resulting material is of high purity. This method is workable at ambient temperature conditions as network formation of atomic or molecular building blocks is possible at relatively low temperatures just by means of constant stirring (mechanochemical route) (Hankhuntod et al., 2017). The modified sol
gel protocols are yet another better approaches to synthesize nanostructures under obedient experimental conditions. The ultrasonic-assisted sol
gel method is one such approach. In this method, the preparation of nanomaterials can be achieved by using ultrasonic waves of appropriate energy to affect the better separation of particles from each other and retarding possible agglomeration to yield mesoporous nanostructures having potentiality as heterogeneous catalysts for water treatment and organic transformations (Mirmasoomi et al., 2017). The flow diagram of various methods used for the synthesis of nanomaterials is represented in Fig. 13.7. There are mainly two methods for the synthesis of MMO NCs; these are chemical and physical methods. In chemical methods, the chemical reactions between the precursors and substrate give rise to the desired metal oxide composite as a product in nanocrystalline form or in thin-film form. The chemical reactions may take place in the presence of an inert gas atmosphere or at normal atmospheric conditions. There is a number of chemical methods to synthesize MMO as shown in Fig. 13.8.

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Figure 13.7 Synthesis methods for nanomaterials.

Figure 13.8 Chemical methods for the synthesis of metal oxide nanocomposites.

The chemical method can be selected by taking care of the properties of the material required. The majority of the physical methods used for the synthesis rely on the continuous breaking of bulk materials into nanodimentional materials. Such methods follow a topdown approach, in which large or bulky materials are breaks down into nanosized materials. These methods utilize physical or chemical forces to break down the bulk material

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into small sizes. For example, mechanical methods like ball milling and melt mixing use physical forces (crushing, grinding, mixing, polishing, etc.). On the other hand, other physical methods like physical vapor deposition, laser ablation, sputter deposition, chemical vapor deposition, spray pyrolysis, etc., use chemical forces (bond breaking and bond making), under the influence of high pressure and temperature (Kulkarni, 2015). Fig. 13.9 shows a flow diagram of representative physical methods of NCs synthesis. To obtain advanced functional and operational NCs, it is required to bring together ordered arrays of nanostructures by using various physical and chemical methods. In the present technology, the synthesis of NCs can be achieved by following maximum protocols of green chemistry. Thus diverse methods having close interplay between physics, chemistry, and biology required to obtain metal oxide NCs of captivating properties.

13.9

Tailoring properties of metal oxide nanocomposites

MMO due to their crystallographically ordered alignment become superstructures furnishing interesting properties owing to their high surface area, mesoporous nature, good electronic conductivity and thermal stability. All these properties are possible to induce deliberately into the MMO right from the beginning such as by designing synthetic protocols, using pure precursors to control stoichiometry, doping of functional metals and nonmetal and coupling metal oxides with each other to form heterojunctions (Tachikawa & Majima, 2014). Such hierarchical architectures of MMO have potentially tunable electronic, optical and magnetic properties, which promise various applications ranging from catalysis to optoelectronics.

Figure 13.9 Physical methods for the synthesis of metal oxide nanocomposites.

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The shapes of metal oxide nanomaterials are diversified and can be named as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and threedimensional (3D), as summarized in Fig. 13.10. The 0D nanomaterials are generally spherical in shape with size ranging from 1 to 100 nm. 1D nanomaterials include nanowires, nanotubes, etc., and have fascinating applications in piezoelectricity, chemical sensing, and photoelectric detection. The 2D and 3D high performance nanomaterials due to their high mechanical strength, surface to volume ratio are useful in various applications like catalysts, photovoltaics, sensors, etc. The recent research activities in nanotechnology mainly focused on synthesis of the shape controlled metal oxide nanomaterials. The shape of nanomaterial determines their physicochemical properties. Nanomaterials with different shapes furnishes different catalytic activities, which is largely due to the differences in crystal structure, geometries and electronic state of the nanomaterials related to diverse crystal facets. A large number of corner and edge atoms are required to show the high activity for any metal oxide catalysts. This is because tiny nanoparticles with such atoms adsorb and activate reactants on its surface (Chen, Li, et al., 2020; Chen, Liu, et al., 2020). Some important strategies utilized for tailoring properties of MMO include doping, phase structure modeling, stoichiometry controlling, forming a microstructure, forming a heterostructure, crystal growth control, densification and heat treatments

Figure 13.10 Different shapes of metal oxide nanomaterials (0D, 1D, 2D, 3D).

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like annealing, sintering, and calcinations. All such important strategies have been discussed with their impact on improvising activity of MMO NCs.

13.9.1 Doping This is one of the best practices of researchers in which the activity of metal oxides was improved just by substituting a small fraction of the cation/s of a “host oxide” with a different cation/s. The substitution disrupts chemical bonding at the surface of the host oxide, and optimists hope that this will modify favorably its activity. Doped system’s activity is due to the two reasons, the oxygen atoms near the dopant or the dopant itself. Addition of dopant in the crystal lattice of oxide is like a “creative disturbance” which can activate surface properties of that oxide. Though the doped metal oxides have been long time practiced by workers, its exact mechanism of activity was unknown. It was after Paravano who first time intentionally prepared doped metal oxides to improve performance of oxide catalyst. Later on it was proved by various workers that most industrial catalysts have little amounts of “additives” that avoid coarsening, offer mechanical stability, stop the evaporation of the catalyst, increase conversion, selectivity, or “poison” an unwanted reaction (one way of achieving selectivity). It was evidenced that the doping changes the electronic properties of oxides (mainly the conductivity and the adsorption spectrum). The idea was that an additive that changed conductivity was likely to affect catalytic chemistry, especially for reactions in which electron transfer might be involved (McFarland & Metiu, 2013). Doping strategy mainly consists of two ways cation doping and anion doping. Most of the time cation doping also called as cation substitution is more practiced way in which oxygen atoms are placed under unusual bonding situations thereby improving its ability as a good oxidizing agent. Sujatha et al. (2019) developed surfactant assisted Zn doped SnO2 nanoparticles and compared its photocatalytic ability for the degradation of MB dye under UV light with bare SnO2 and only Zn doped SnO2 NPs. The surfactants assisted Zn doped SnO2 NPs showed a great shift in band gaps from 3.292 to 3.695 eV. Triton assisted Zn doped SnO2 nanoparticles were found to have high photocatalytic activity (80%) and better optical property among the synthesized NPs. Sonochemically prepared Ir doped ZnO photocatalysts revealed visible light photocatalytic activity against malachite green dye aqueous solution (Babajani & Jamshidi, 2019). The cobalt doped TiO2 and ZnO oxides were used to degrade ketoprofen under UV light illumination present in real wastewater sample. Here, it was observed that low amount of Co into the TiO2 and ZnO matrices improve their surface properties and make them more efficient catalyst for ketoprofen mineralization than that of bare materials (Gonc¸alves et al., 2019). In other way anion substitution is also better in which replacement of some anionic sites of one element with other element has been done to induce desirable properties among the materials. Anion doping proved to be effective in improving optical properties of photocatalyst. The oxide lattice can be doped with anions either by replacing the lattice oxygen or by occupying the interstitial sites between different layers in the lattice. The nonoxygen anions like F2, Cl2, S2 22, N3 2, Br2

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can be inserted into a MMO lattice typically by four pathways, (1) one of the O22 anions in the lattice is replaced by one nonoxygen anion, leading to a smaller lattice in the metal oxide; (2) two nonoxygen anions are substituted in place of one O22 anion in the lattice, resulting in either a reducing lattice or an oxidizing lattice in the metal oxide; (3) the introduction of nonoxygen anions into interstitial sites along with the generation of an oxidizing lattice in the metal oxide; (4) the oxygen and nonoxygen anion site preferences may rearrange the structure of the metal oxide. The various doping sites obtained critically influence the catalytic, electrocatalytic and photocatalytic ability of the oxide materials. The doping of anions into the metal oxide crystal expands the interstitial sites, take the place of oxygen vacancies, and also take the place of some of the O22 anions (Liu, Wang, et al., 2019).

13.9.2 Modeling phase structure The structural factor of metal oxides is more complicated formation, which include not only the size, but also a crystallite shape, microscopic structure, and crystallographic orientation of crystallite planes resulting into the formation of functional complex matrix. It has been well established that, even the crystals with specific crystallographic symmetry can have different external forms depending on a size. The phase structure modification is a smart engineering approach used to obtain multiapplicative metal oxide nanomaterials. Such phase structure modification is supposed to take place through numerous ways. For example, the process of growth of grains which includes the change in size or the external shape of crystallites, the external planes of nanocrystals provide active surface area for the large number of molecules to adsorb and react thereby increasing its catalytic activity, in addition every crystal with specific phase structure has its own combination of crystallographic planes, framing the nanocrystal. Such every specific crystallographic plane of particular phase structure showed its own total combination of surface electron parameters. It includes surface state density; energetic position of the levels, induced by adsorbed species; adsorption/desorption energies of interacted molecules; concentration of adsorption surface states; position of surface Fermi level; activation energy of native point defects; and so on. It means that the chemisorption characteristics change noticeably from the crystal surface of one orientation to another, indicating a striking variation of chemical bonding of adsorbed particles with surface in dependence on its atomic structure. Thus the phase structure modification has become one of the standard approach for the synthesis MMO with different applications (Korotcenkov, 2005). Ong et al. (2014) reviewed enhanced photochemical activity of anatase TiO2 , 001. facets on the micro- and nanoscale. The review encompasses the basic concepts to augment the photocatalytic activity by using highly reactive ,001. faceted TiO2-based composites.

13.9.3 Stoichiometry controlling The relative acidic and basic natures the atoms present on the surface of metal oxides determine their catalytic activity. Such acidic or basic behavior of metal atoms

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depends on their coordination with oxygen anions in the vicinity. In the case of binary metal oxides, the accessible property regimes in this respect are quite restricted. Whereas, in the case of ternary metal oxides, there are two acidic sites and one basic site; hence, the multitude of their interactions renders wider property windows for the catalytic performance (Jain et al., 2018). Controlling stoichiometry of MMO NCs during their preparation improve band edge alignments and thus solar absorbance. Different works demonstrate that stoichiometry control can be used effectively tune photocatalytic performance.

13.9.4 Microstructure forming The microstructured composite of metal oxides leads to an extreme increase in both the mechanical stability and the electrochemical performance. This is due to the migration of metal atoms into the resulting composite lattice. The exclusive physicochemical properties of metal oxide microstructures lead to its use in a enormous multiplicity of applications including the removal of heavy metals, poisonous gas sensing, textile coating for wearable electronic devices, biomedical applications, and photocatalytic degradation of organic contaminants (Danish et al., 2021). Adsorption is a physicochemical surface interaction between the adsorbate and the adsorbent. For a speedy and effective removal of heavy metals in wastewater, adsorbents should have a high surface area, desirable textural and surface features, and good mechanical stability. Nanosized metal oxides are used for adsorption processes due to the fact that they exhibit remarkable surface characteristics, microstructural features, and high surface area. The active sites and high surface area facilitate adsorption events. The surface energy of adsorbent materials is increased by reducing the size and more active sites on their surface are available for organic molecules to interact, and in this case, be adsorbed on to the surface of the adsorbent thus nanomaterials show better adsorption capacity compared to the bulkier counterparts.

13.9.5 Heterostructure forming Heterogeneous photocatalysis is a multipurpose, less-cost and environment friendly for the removal of organic dye pollutants in wastewater. The advancement of heterostructured visible-light active metal oxide based semiconductor photocatalysts have received an enormous consideration in the field of environmental remediation due to their excellent growth with unique surface morphology, tunable properties, suitable for absorbing more visible region in the solar radiation. The several categories of organic pollutants are released into the earth by an expanding insurgency in agricultural and industrial areas. The solar-light driven photocatalytic degradation is an efficient process for the elimination of toxic pollutants from wastewaters. In this manner, it is important to develop newer metal oxide-based photocatalytic materials to solve the majority of basic issues associated with environment. The practical uses of the transition metal oxide nanomaterials as photocatalysts have been prohibited because of the quick recombination of charge carriers and the

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electrons at CB cannot be proficiently trapped by O2 to yield superoxide radicals. To overcome this issue, researchers are modifying the band gap energy of the bare materials by forming heterostructures by any one of the methods like doping or making composite with other metal oxide materials to diminish the recombination of charge carriers and thus enhancing its photocatalytic performance (Theerthagiri et al., 2018).

13.9.6 Controlling crystal growth Molten-salt reactions can be used to prepare single-crystal metal-oxide particles with morphologies and sizes that can be varied from the nanoscale to the microscale, subsequently enabling a growing number of novel investigations into their photocatalytic activities. Crystal growth using flux-mediated methods facilitates finer synthetic manipulation over particle characteristics. The synthetic flexibility that flux synthesis affords for the growth of metal-oxides has led to the stabilization of phases with limited stability, the discovery of new compositions, and access to alternate crystal morphologies and sizes that exhibit significant changes in photocatalytic activities at their surfaces, such as for the reduction of water to hydrogen in aqueous solutions. This approach has significantly impacted the current understanding of the optical and photocatalytic properties of metal-oxides, such as the dependence of band gap energies on the structure and chemical composition (i.e., obtained from flux-mediated ion-exchange reactions). Thus flux preparations of metal-oxide photocatalysts assist in the growth and optimization of their particles to understand and tune the photocatalytic reaction rates at their surfaces (Boltersdorf et al., 2015).

13.9.7 Impact of heat treatments Heat treatment like annealing, sintering and calcination played important role in the improvement of particle size and specific surface area. Various characterization methods indicated that the particle size and crystallinity were higher for the calcined than the hydrothermally treated TiO2 (William et al., 2008). This is believed to help explain the higher photocatalytic activity of the calcined photocatalyst.

13.10

Protocols of mixed metal oxides used in environmental remediation

The remediation of aqueous environments can be done by using five common approaches as shown in Fig. 13.11 by means of nanomaterials.

13.10.1 Adsorbent studies An enormous discharge of dangerous substances especially heavy metal ions and organic dyes to the air, water and soil has been foremost concerns due to peoples

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Figure 13.11 Five common approaches for remediation of water, wastewater, and groundwater remediation.

ignore the proper protocols required to be implemented in the waste management. The water bodies are in jeopardy and massive discharge of harmful pollutants result from a range of man-made activities may create a great threat to the living organisms and harmfully affect the ecosystem stability. The current research activities involving utilization of MMO NCs for removal of heavy metal ions and dyes focused on various parameters like controlling size of NCs and their dispersion in aqueous media. The other unique features of the synthesized NCs such as acidicbasic properties, morphology, specific surface area, nontoxicity, ease separation, and cost effectiveness will be also investigated. The advantages and limitations of the various NCs will be highlighted to determine their adsorption ability. The effect of various parameters such as pH, contaminants concentration, adsorbent dosage, contact time and temperature will be summarized to identify the best condition for effective pollutants removal (Abdullah et al., 2019). Saravanan et al. (2012) synthesized Gum kondagogu grafted magnetic nanoparticles by aqueous precipitation technique. These magnetically separable composite with magnetization value of B60 emu g21 and composed of spherical iron oxide nanoparticles of size 8
15 nm proved to be effective in the removal of various toxic metals in the descending order Cd21 . Cu21 . Pb21 . Ni21 . Zn21 . Hg21 at a pH of 5.0 6 0.1 and at a temperature of 30.0 C 6 1.0 C. A series of bentonitebased geopolymers was prepared and coupled with magnetic Fe3O4 NPs to form efficient geopolymer/Fe3O4 adsorbent for heavy metal ions removal. The prepared magnetic geopolymer/Fe3O4 NCs showed 99%, 99%, 92%, 96% and 92% removal efficiency for the sorption of copper, lead, nickel, cadmium, and mercury ions from industrial effluents. This protocol includes diverse advantages such as environmentally-friendly, magnetic separation, inexpensive raw materials, easy and simple conditions and high yields as same as short adsorption times (Maleki et al., 2019). In another protocol grinding method was employed to obtain Fe/Mn MMO

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NCs with superior size and surface area. Various instrumental techniques revealed formation of composites with size 3
5 nm and high surface area 268 m2 g21. These NCs showed good adsorption capacity, stability, and regeneration activity for Cr (VI) removal (Weilong & Xiaobo, 2013). Many researchers have used different metal oxide NCs as adsorbents to remove water polluting dyes as well as harmful heavy metals including Cr, Pb, Hg, Zn, Mn and Cu. The main focus of every study is to design proper protocol for effective heavy metal and dye adsorption process. The different parameters, pH, contact time, etc., which affect the adsorption process required to be properly optimized. To find out low cost of adsorbent on waste material, polymer, lesser contact time and its regeneration in low energy consumption are major challenges in front of researchers.

13.10.2 Catalytic studies Most of the photocatalytic systems involving bare metal NPs or metal oxide NPs showed limitations such as little capability to transfer the photogenerated electrons and holes, fast charge recombination, and shortage in use of visible-light restricts its industrial application. To overcome these limitations various elements, metal oxides, transition metals, rare earth metals, polymers are doped or coupled with these bare counterparts. Utilization of various protocols right from the synthesis to functionalization provides a wide range of nanoplatforms vital for the catalytic studies. Strategies are focused on their potentiality as excellent catalytic materials and their use in various environmental remediations such as hydrogen production, wastewater treatment, and air purification. A wide range of reactions involved in photocatalytic protocol using mixed metal oxide NCs. These reactions range from mineralization of organic pollutants to refined organic processes. The main advantage of this protocol is the conversion of light energy into the chemical energy thereby reducing energy use or offering a sustainable, cost effective solution for pollution (El-Naggar & Shoueir, 2020). Catalytic reduction of water pollutant, nitrophenol (4-NP) was carried out by using size controlled (15 nm) Pd-NiO NCs synthesized using metallosurfactants. This material showed excellent catalytic performance for the reduction of 4-NP (k 5 0.29 min21) and showed good recyclability as catalyst over 5 cycles. The protocol exhibited high atom economy with minimum organic waste (Kaur et al., 2019). Fe2O3/TiO2 nanofibers were made by combining electrospinning with the calcination process and successfully utilized for rhodamine B removal under solar light. The catalytic performance of composite material was best as compared to pristine TiO2 and further advantage is catalyst is recovered easily just by magnetic separation (Liu, Zhang, et al., 2018). Ternary MMO ZnO.La2O3.CeO2 (ZLC) are synthesized using hydrothermal and solution combustion approach. The large surface area, presence of active sites and the regular morphology drastically increase the lifetime of the charge carriers. The ternary NC sample showed 94.99% degradation of RhB on irradiation of light emitted by 125 W mercury vapor lamp for about 160 minutes in basic medium (Kumari et al., 2019).

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Thus designing of new semiconductor materials or enhance the potential of existing materials for the removal of the environmental pollutants is growing area of scientific interest.

13.10.3 Membrane studies Heavy metal contamination in eco-system especially in water bodies is one of the global threats. This is due to the toxicity and carcinogenicity of heavy metals toward living bodies. Among available removal techniques, adsorptive removal by nanosized materials such as metal oxide, metal organic frameworks, zeolite and carbon-based materials has attracted much attention due to the large active surface area, large number of functional groups, high chemical and thermal stability which led to outstanding adsorption performance. However, the use of nanosized materials is restricted by the difficulty in separating the spent adsorbent from aqueous solution. The shift toward the use of adsorptive composite membrane for heavy metal ions removal has attracted much attention due to the synergistic properties of adsorption and filtration approaches in a same chamber. Exposed population is in great threat of infections caused due to viruses in the environment entered through solutions and aerosols. To trap viruses before entering environment needs to prepare nanoscale membranes with controlled surface properties and chemical functionalities. Such ultra-thin membranes can be prepared on large scale to provide commercial water remediation strategies for low-energy requirement separation systems for the simultaneous capture and degradation of pathogens, and to self-cleaning materials. There are numerous works reported ultra-thin membranes containing polymeric nanoscale composites with high surface area and surface charge which is phenomenal in high adsorption capacity and selectivity toward targeted heavy metals and pathogens (Ojha, 2020; Wadhawan et al., 2020). The hybrid membranes also furnished good photocatalytic activity and therefore allowed for continuous flow operation as continuous membrane reactors. There are numerous challenges and obstacles in the manufacture and use of different adsoptive nanocomposite membranes for environmental remediation. Researchers are striving to overcome all such hurdles by various modification strategies to improve the compatibility and dispersion of nanoadsorbents membranes. Bandara et al. (2019) employed response surface methodology to prepare and optimize the composition of a chitosan
polyethyleneimine
GO nanocomposite membrane coating to more effectively remove Cr(VI) and Cu(II) from water. The results indicated that the multifunctional coatings were able to remove both heavy metal ions from water, suggesting its ability to remove positively and negatively charged ions. The two-dimentional nanosheets of graphene like TiO2@C were synthesized by simple solvothermal method. These nanosheets with unique structure, active functionalities and high surface area proved to be effective in the ultrafast removal of MB and Pb21 ion from aqueous solution (Gan et al., 2019). In another study, cellulose acetate/chitosan/SWCNT/Fe3O4/TiO2 composite nanofibers were fabricated for the removal of Cr(VI), As(V), MB and CR from aqueous solutions

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(ZabihiSahebi et al., 2019). Recently, bimetallic Au0.1Ag0.9/TiO2/Cellulose acetate (mass ratio of Au or Ag to TiO2) NCs membrane has been developed for enhanced photocatalytic degradation of tetracycline an antibiotics and killing E. coli a waterborne pathogenic bacteria under visible light (Li et al., 2019). The activity of Bi2O3/nylon multilayered nanocomposite membrane was studied in a continuous mode reactor. The composite showed excellent photocatalytic performance in the degradation of separate and mixed solutions of anionic (indigo carmine) and cationic (rhodamine B) organic dyes. The composite membrane also furnished bactericidal behavior toward E. coli bacteria (Gadhi et al., 2021).

13.10.4 Biological studies The biological application of metal oxide-based nanomaterials is a recent area of interest. This is due to their extraordinary characteristics such as nontoxic nature, large specific surface area, useful optical bandgap, and high biological activity. Some functional metal oxide-based NCs have been used as advanced materials for photocatalytic anticancer and antibacterial studies. Such anticancer and antibacterial properties were induced in metal oxides by coupling them with each other and making their composites most preferably in nanodimensions (Kannan et al., 2020). For example, Hauksdo´ttir and Webster (2018) developed selenium/iron oxide NCs as efficient anticancer agent for the removal of breast cancer cells by magnetically targeted mechanism. In a green approach, Alchornea cordifolia leaf extract was used to prepare Cu2O/CuO NPs, ZnO NPs and Cu2O/CuO
ZnO NCs. In vitro cytotoxicity tests carried by using MTT assay method of the nanomaterials on cervical Hela cell lines (cancer cells) revealed efficiency in the order: Cu2O/CuO
ZnO . ZnO NPs . Cu2O/CuO (Elemike et al., 2020). Generally, antibacterial performance of nanomaterials dependent upon various factors such as generation of reactive oxygen species (ROS), release of heavy metal ions (cation release), biomolecule damages, ATP depletion, and membrane interaction in the vicinity of bacterial cell wall. To tackle global challenge of antimicrobial resistance, advanced new generation catalyst are needed. Kumari et al. (2021) have introduced extremely competent magneto-catalytic platforms of mixed iron cobalt oxide-based surface textured nanostructures (MTex). These materials are capable to produce defensive ROS over an extensive pH range and can well diffuse into the biofilm and eradicate the embedded bacteria. The systematic scheme of in situ synthesis of Fe-Co MMO nanocrystals and their bactericidal action can be depicted in Fig. 13.12. MMO NCs with high surface-to-volume ratio are capable to differentiate bacterial cells from mammalian cells and can provide long-term antibacterial and biofilm prevention. There are certain NCs which generate ROS via various ways like Fenton reaction and cause destruction of DNA, oxidation of lipid proteins ultimately killing the bacteria without causing any harm to the nonbacterial cells. A bare ZnO and its NCs with Ag NPs in different wt.% like 0.5%, 1.0% and 2.0% composition were prepared by green method using Ocimum tenuiflorum (Tulsi) plant seed extract (PSE). In this, the 1% Ag/ZnO NCs attributed superior

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Figure 13.12 Synthetic scheme and bactericidal action of surface textured mixedmetal
oxide nanocrystals.

antimicrobial as well as photocatlytic activity. This was due to ROS formation on to the surface of NCs thereby supporting electron transfer for photocatalysis and bacterial inhibition (Panchal et al., 2020).

13.11

Monitoring of pollutants during environmental remediation

Environmental monitoring includes the processes and actions that required for characterizing and monitoring the quality of the environment. It has been employed in the preparation of environmental impact assessments, that is, risks caused by various human activities to the living organisms in the natural environment. In every monitoring strategy there should be proper reasoning, reviews and statistical procedures to establish current status of environment and also various parameters newly arising to analyze the environmental status. Functional MMO NCs with their versatile properties are not only applicable in pollution mitigation strategies but also effective in the environmental monitoring and measurements. These materials are effectively useful in the detection of contaminants in the environment and thus their remediation. Metal oxide-based NCs proved to be portable, rapid, sensitive, and cost-effective materials for monitoring the level of pollutants in the environment. There are numerous traditional pollutant monitoring and measurements techniques are available like mass spectrometry, NMR spectroscopy, Raman spectroscopy, HPLC, etc., but all these techniques are relatively expensive, time consuming and have elaborate operating and maintenance procedures that does not make them user friendly. Consequently, metal oxide NCs have received great attention from researchers due to their properties such as magnetism, appropriate surface area, relative ease of their synthesis and potential use for varied applications including environmental monitoring

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and remediation processes (Meteku et al., 2020). Now-a day, hazardous pollutants such as carbon monoxide, VOC, hydrocarbons, nitrogen oxides and chlorofluorocarbons are continuously contaminating the air quality. Soil and water bodies are contaminated with arsenic, heavy metals, and chlorinated compounds. Sewage water, industrial effluents, arbitrary use of pesticides, fertilizers, and oil spills are a few of the major reasons for water and soil degradation. Thus these contaminating pollutants need to be monitored, measured and finally removed with effective nanotechnology-based methods. Such methods are able to design and manipulate materials at both atomic and molecular levels in such a way that they can recognize a particular contaminant in a mixture selectively.

13.11.1 Monitoring of air pollutants Metal oxide NCs-based sensors are one of the rapid and cost-effective tools for the detection of air pollutants. SnO2/GO and CuO/GO NCs sensor arrays were used to detect indoor air pollutants like ammonia and formaldehyde. These nanoarrays were found capable to recognize and quantitative prediction of both the substances in their mixture at room temperature (Zhang et al., 2017). Ternary rGO/TiO2/Au NCs thin films were used to detect very low concentration of NH3 at low temperature with high sensitivity (Zhou et al., 2019). Liu, Xu, et al. (2018) developed ternary Ru/Al2O3/ZnO micro-sensor to monitor SO2. The developed micro-sensor furnished a selective response to SO2 with a very good linear relationship in the range of 5 to 115 ppm concentration of SO2 gases. Therefore this type of metal oxide-based sensing composite materials exploit the advantage of each element, and makes it possible the trace and selective detection of pollutant gases.

13.11.2 Monitoring of soil pollutants Because of continuous anthropogenic activities, soil pollution is becoming ever expanding problem in all over the world. The living eco-system like plants, animals, humans, etc., can be vulnerable to the discharge of harmful organics and heavy metal ions in the soil. Especially, the heavy metal ions are nonbiodegradable and have long persistence in the environment. Such heavy metal ions are responsible for the noncarcinogenic and carcinogenic health problems such as kidney failure, osteoporosis, and even different types of cancers to the human being. Long time exposure to the heavy metal ions leads to cause muscular, physical and neurological degenerative processes such as Parkinson’s disease, multiple sclerosis, muscular dystrophy and Alzheimer’s disease. Thus the timely monitoring of heavy metal ion concentrations in soil is of great importance for the early detection of soil contamination (Xu et al., 2020). Xu et al. (2020) used Au/MnO2 NCs for the development of colorimetric and fluorometric double-channel responsive assay method for acid phosphatase (ACP) for the detection of Cd in the soil. This method takes advantage of the competitive redox reaction of NCs between the dye IC and the enzymatic product L-ascorbic acid (AA). The sharp increase in ACP activity at a suitable time confirmed as an indicator for cadmium (Cd) contamination in soil, which is of great importance for evaluation of soil quality as well as ecological risk assessment. The Cd(II), Cu(II) and Pb(II) ions were monitored in

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very low limits of detection (LODs) 0.19, 0.24 and 0.35 ng mL21 respectively using Fe3O4@GO NCs modified via 2-mercaptobenzothiazole as a novel and efficient nanosorbent. In another study monitoring of heavy metal ions Cd(II), Pb(II), Cu(II), and Hg(II) was carried out by SnO2/rGO NCs via electrochemical detection method (Wei et al., 2012). Ternary Ag-CoFe2O4/rGO NCs prepared by microwave assisted method used as efficient surface enhanced Raman spectroscopy substrates for detection of mercury ions (Guo et al., 2018). In conclusion, there are many simple, rapid, novel, and highly efficient monitoring techniques that are possible to establish making use of different nanomaterials.

13.11.3 Monitoring of water pollutants The widely used agrochemicals including pesticides, herbicides, fertilizers and heavy metal ions coming from agricultural, domestic, and industrial wastes are major sources of water pollutants. Fig. 13.13 shows the various sources of emerging (micro)-pollutants and there life-cycle distribution to potential receptors.

Figure 13.13 Life-cycle distribution of emerging (micro)-pollutants from sources to receptors viewpoint.

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A precision farming promising higher crop yields and lower input costs of agrochemicals as well as industrial processes minimizing discharge of specific nitrogen and phosphorus compounds, heavy metals, chlorinated compounds needs simple, cheap and precise monitoring systems. Metal oxide NCs has played vital role in designing of such efficient monitoring systems and finally resulted in to minimizing water pollution. The majority of the pollutants are soluble in water because of some hydrophilic groups in their chemical structures and thus cause serious threat for the living ecosystem through the water cycle (drinking water, groundwater, wastewater and surface water). The detection and assessment of these pollutants in various waters are considered to be a key scientific task, which requires very highly sensitive analytical techniques for the detection of pollutants at nanograms per liter (ng L21) scales. Consequently, the designing of fast, sensitive analytical techniques for effective monitoring and determination of extensive pollutants is imperative (Rasheed et al., 2019). Aptamer and GO/Cu NCs were electrodeposited on screen-printed carbon electrode surface to fabricate sensor for detection of traces of various organophosphorus pesticides (Fu et al., 2019). Ternary Fe3O4-TiO2/rGO as an excellent peroxidase-like activity was established as nanozyme for selective detection and photodegradation of atrazine pesticide (Boruah & Das, 2020). Radhakrishnan et al. (2015) fabricated electrochemical sensing platform by using ternary polyaniline/Fe2O3/rGO for hydroquinone determination. The sensor exhibited excellent sensitivity, stability and reproducibility. The synthesis of novel K-MnO2
rGO NCs carried out by simple one-pot route and it was successfully employed for the removal of water-soluble pollutant 4- nitroaniline present in real water samples collected from river and pond (Yamuna et al., 2021). In that way, metal oxide NCs are applicable in numerous water treatment processes. The main advantage of metal oxide-based NCs in remediation studies is their recyclability after use. In addition, these materials are portable and user friendly and hence are suitable for employ in monitoring pollutants in difficult geographic terrains.

13.12

Concluding remarks and future perspectives

The use of heterometal oxides or MMO nanostructures instead of using single metal oxide has become a very trendy practice. The combination of two materials in nanodimensions led to interesting advancements in physicochemical properties. Such advent nanodimensional MMOs are used in environmental remediation, catalysis and gas sensing applications. They are also been used as active and passive materials in many electronic devices due to their exceptional electronic and magnetic properties (Kapoor et al., 2004a). The use of MMOs in various applications was promising practice due to their increased surface acidity and improved band structures. These two factors are fundamental in enabling MMONCs as efficient materials for remediation studies. The surface acidity and catalytic activity are interrelated with each other. It has been well established that the surface adsorbed

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hydroxyl groups can be responsible for surface acidity. These hydroxyl groups can absorb photogenerated holes to form electron deficient hydroxyl radicals. Subsequently, these highly reactive hydroxyl radicals cause oxidation of surface adsorbed molecules. In addition to trapping the holes, surface hydroxyl groups also act as an excellent adsorption sites for pollutant molecules. The adsorption of gases or vapors on the surface of metal oxides proved to be significant in the field of gas sensing or bio-sensing sensing. The resistance of semiconducting metal oxide surfaces is strongly influenced by the presence of surface adsorbed oxidizing or reducing gases. This change in resistance response is monitored by highly sensitive microelectronic devices and therefore MMO films are mostly used as sensors in environmental monitoring and biomedical applications (Patil, Vanalakar, et al., 2018). Versatile nanomaterials like metals NPs, nanomembranes, nanopowders, etc., are utilized for the detection and removal of heavy metals and persistent organic pollutants in the environment with minimal side effects and with less cost as given in Fig. 13.14. Moreover, there is need to widen the scope of application of metal oxide NCs beyond the remediation of polluted lands (soil), water bodies, and air to the designing of efficient nanoscale pollution monitoring techniques. Issues of nanotoxicity should beconsidered to ensure the safe use of NCs without side effects or repercussions on the environment in the long term. Environmental pollutants not only caused by chemical species but also microbes can constitute large portion of environmental pollution. An emerging area in the study of metal oxide NCs is their applications as disinfectants. The common methods of treating microbes involve the use of various antibiotic drugs. However, microbes tend to develop resistance to antibiotics and consequently, antibiotics potent for eliminating pathogens are becoming ineffective. The recent

Figure 13.14 Schematics of challenges in environmental remediation and scope of nanotechnology in environmental remediation.

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developments in nanotechnology bring in some respite. Metal oxide NCs incorporated with metal NPS such as Ag, Au, and Cu are known to exhibit antimicrobial properties. Similarly, some NCs have been used to facilitate the capture of some strains of bacteria. This leads to exciting frontiers in biological pollution remediation (Datta et al., 2020). In addition, metal and metal oxide nanomaterials can be used to remove organic pollutants and metals by reduction or oxidation of nanomaterials, and degree of removal can be enhanced through functionalization with chemical groups that can capture selectively target pollutants in water and air media. This method is effective, promising, and can be used in the treatment of water and air improvements. Nanomembranes have found applications in the purification of potable water, water reclamation, the removal of metals, dyes, natural organic matter and the removal of pesticides from contaminated water. Further improvements must be made in the application of environmental remediation to selectively remove materials, have a greater resistance to changes in pH and the concentrations of chemicals present in the contaminated water, greater stability for a longer period of time and cost optimization. A high flux could be produced via nanofibrous prefilters with even higher loading capacities. Such pre-filters can be used in various applications, such as the removal of microparticles from waste water and with ultrafiltration or nanofiltration membranes to prolong the life of these membranes. On-going investigations are under way to develop engineered nanomaterials of various fiber diameters and morphologies to identify their effects on the performance of nanofibres. The environmental applications of polymer supported NCs in photocatalytic/chemical catalysis degradation, the adsorption of pollutants and pollutant sensing and detection result in a greener environment. However, the study of the interaction between the host polymers and the encapsulated NPs and its effect on the dispersion in polluted air and water is necessary. In addition, the large scale production of polymersupported NCs and more practical applications remain open. The extensive application of sorbents in environmental remediation have shown the capability of adsorbing metals and organic pollutants from contaminated water and air. Iron-based nanomaterials, TiO2 nanomaterials and polymeric adsorbents have shown high adsorption capacities and selectivities. The surface modifications of sorbents are being studied for process optimization. Enhancing the reusability of sorbents and the extension of their lifespan ust be explored to reduce the cost in environmental remediation. Sensors have been developed for sensing gases, chemicals and VOC and the detection and identification of bacteria. Further development is necessary in the functional properties of nanomaterials to meet the need for trace detection and the treatment of pollutants in water and air and important fundamental and mechanistic studies are required to fully explore their real potentials. One-dimensional CNTs with single and multiple layers have shown superior adsorption capacities in the removal of diverse range of biological and chemical contaminants due to their fibrous shape with high aspect ratio and provision of large external surface area. Dendritic nanopolymers have been developed for low pressure filtration processes to remove perchlorate and uranium from contaminated water and recover metal ions (e.g., copper, silver, nickel and zinc) from industrial waste water. The long-term efficiencies of dendritic nanopolymercomposites as an important practical aspect have not been reported and should be addressed in the future (Bardos et al., 2018).

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65. Available from https://doi.org/10.1016/j.snb.2016.08.085. Zhang, Q., Zhao, X., Duan, L., Shen, H., & Liu, R. (2020). Journal of Photochemistry and Photobiology A: Chemistry. 392. Zhang, W., Zhang, S., Wang, J., Wang, M., He, Q., Song, J., Wang, H., & Zhou, J. (2018). Hybrid functionalized chitosan-Al2O3@SiO2 composite for enhanced Cr(VI) adsorption. Chemosphere, 55(2), 188
198. Available from https://doi.org/10.1016/j. chemosphere.2018.03.188. Zhao, T., Qiu, P., Fan, Y., Yang, J., Jiang, W., Wang, L., Deng, Y., & Luo, W. (2019). Advanced science. 6. Zheng, Y., Liu, J., Cheng, B., You, W., Ho, W., & Tang, H. (2019). Hierarchical porous Al2O3 @ZnO core-shell microfibres with excellent adsorption affinity for Congo red molecule. Applied Surface Science, 473, 251
260. Available from https://doi.org/ 10.1016/j.apsusc.2018.12.106.

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Zhou, Y., Li, X., Wang, Y., Tai, H., & Guo, Y. (2019). UV illumination-enhanced molecular ammonia detection based on a ternary-reduced graphene oxide
titanium dioxide
Au composite film at room temperature. Analytical Chemistry, 29(10), 3311
3318. Available from https://doi.org/10.1021/acs.analchem.8b04347. Zouzelka, R., Remzova, M., Plsek, J., Brabec, L., & Rathousky, J. (2019). Catalysts. 9.

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Shubhangi D. Shirsat1, Rajaram S. Mane2, Joanna Bauer3 and Nanasaheb D. Thorat4 1 Department of Biotechnology, SRTMU New Model Degree College, Hingoli, Maharashtra, India, 2School of Physical Sciences, SRTMU, Nanded, Maharashtra, India, 3 Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wrocław, Poland, 4Nuffield Department of Women’s and Reproductive Health, Medical Science Division, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom

14.1

Water: the key to life on the earth

Water, a simple molecule having two atoms of hydrogen (H) and one of oxygen (O) with the general formula H2O, is the most important need for living and nonliving organisms on the Earth. Many physical and biological properties of cells, macro-molecules, mainly proteins, and nucleic acids, originate from their interaction with water molecules of the surrounding medium. Water plays a profound and determinative role in biological evolution. All lives on the Earth depend on water (Henry, 2005). The physical and chemical properties of water support the environment, including living and non-living forms. For example, the formation of a thin layer of ice on the top of a lake protects all lives forms in the lake by avoiding the freezing process of the entire water in places where the temperature of the atmosphere falls below 0 C. The water under the thin ice layer remains at 4 C due to the unique behavior of water density. Generally, the density of the material increases with a decrease in temperature. However, water density has a maximum value of around 4 C and starts to decrease at temperatures below or above this value. Thus, the ice is less dense than the liquid water in the lake, which leads to the formation of a thin solid layer on top of the water content. Solubility of gases like oxygen (O2) and carbon dioxide (CO2) in water is essential for sea life as well. This is highly significant for human life as our species also depend on food from the sea (Ball, 2001). Approximately 70% of the human body comprises water. The heart, one of the most vital organs, contains 73.69% water, while the lungs contain 83.74% which is one of the largest contents. 31.81% water is found in bones. Fig. 14.1 gives an illustration of the human body and the water percentages present in these organs Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00003-8 © 2022 Elsevier Inc. All rights reserved.

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Figure 14.1 Illustration of water percentage of vital organs of human body (35 years of age, 70.55 kg in weight, and 183 cm tall). Source: Reproduced with permission from Noronha, V. T., Aquino, Y. M. L. O., Maia, M. T., & Freire, R. M. (2019). Sensing of water contaminants: From traditional to modern strategies based on nanotechnology. In Nanomaterials applications for environmental matrices: Water, soil and air (pp. 109 150). Elsevier.

(Noronha et al., 2019). Water plays a major role in the human body, large number of its functions, including as a solvent, reactant, product, catalyst, chaperone, messenger, and controller, etc. (Brini et al., 2017). The attractive forces between water molecules and the slight tendency of water to ionize are of importance to the structure and function of biomolecules. Therefore drinking water is an essential nutrient for the survival of living and nonliving lives on the Earth. The value of percentage is related to the water content found in various parts of the human body including the brain, spinal cord, and nerve trunks, etc. Water content in the human body decreases over age (Fomon et al., 1982; Schoeller, 1989). According to the World Health Organization (WHO), a person should drink an amount of water equal to their body losses and enough to avoid adverse effects, such as dehydration. Minimum three liters per day of water should be drunk by the 70 kg person in a temperate zone to maintain the water balance and stay healthy. This amount changes with age and gender. The WHO reports that each person requires 50 100 L of water per day to accomplish his/her most basic needs (“World Health Organization,” 2011).

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Present scenario of water pollution

Water pollution is mainly categorized into two classes; domestic water pollution, and industrial water pollution. 1. Domestic water pollution Residential wastewater discharged from public residences is known as sewage. It is composed of 99.90% of water and 0.10% suspended solids. The suspended solid contains organic particles such as food waste, body waste, toilet paper, etc., and inorganic solids are mainly composed of sediments, salts, and metals (Fig. 14.2). Organic materials are generally biodegradable and are primarily carbon-containing compounds, like proteins, carbohydrates, and fats, which can be converted to CO2 biologically. The sewage water also contains nutrients, such as nitrogen and phosphorus, which should be removed to avoid ecological as well as human toxicity concerns (temleton & Butler, 2011). 2. Industrial water pollution Industrial wastewater is mainly discharged from industries, commercial activities (e.g., hospitals, shops, restaurants, etc.), and agricultural fields. There are several kinds of industrial wastewaters; each industry produces a different type of contaminant. Industrial wastewater is categorized into inorganic and organic wastewater. Inorganic industrial wastewater is mainly released from the coal and steel industry, the nonmetallic minerals industry, and commercial projects for metals surface processing (iron pickling works and

Figure 14.2 Sewage water composition distribution.

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electroplating plants). Wastewater coming from these activities contains extremely toxic solutes like blast-furnace gas-washing wastewater, inclusive of cyanides; metal processing industry wastewater contains nonferrous metals, and cyanides or chromates. All metal processed plants release their wastewater into a municipal wastewater system and their effluents must be treated before liberation, according to local regulations (Savant et al., 2006; Sinha et al., 2014). Organic industrial wastewater released from chemical industries and chemicals that work on a large scale mostly contains harmful substances. The wastewater includes organic materials with variable properties and origins (Buzzini & Pires, 2007). The following industries and plants release the majority of organic industrial wastewater; 1. Factories manufacture cosmetics, pharmaceuticals, glue and adhesives, synthetic detergents, organic dyestuff, herbicides, and pesticides 2. Leather and tanneries factories 3. Textile factories 4. Paper and cellulose manufacturing plants 5. Factories related to the oil-refining industry and 6. Metal processing industry.

Additionally, wastewater produced from various industries and agricultural activities releases enormous amount of harmful chemical and biological pollutants. Heavy metal ions are another part of wastewater from some industries, which are toxic to living organisms. Rainwater also carries organic and inorganic pollutants from streets in addition to the pesticides and fertilizers from agricultural fields (Beychok, 1971; Tchobanoglous et al., 2003). Table 14.1 presents different water pollutants released from the industrial sector.

14.3

Water treatment

High quality and safe drinking water is essentially required for every human being to prevent water-borne infections, and the water industry is aware of this. Researchers are trying to develop cost-effective and stable materials and methods to address the challenges of providing safe potable water in adequate quantities. Rarities of fresh water coupled with its rapid depletion have forced scientists to find methods to purify the water so it can be reused. Various treatment processes are being carried out on raw water before it reaches our tap. At the beginning of the 20th century, several water treatment methods used today were already established (Hazen, 1914). These methods include coagulation, flocculation, sedimentation, filtration, and chlorine disinfection, etc. (Fig. 14.3) (Fuller, 1933). In the coagulation process, aluminum sulfate (alum) is added into the water with the objective of destabilizing natural fine physical particulate matter suspended in water. These impurities found in water suspensions consist of charged colloids ranging in size from 5 nm to 1 μm and particulates greater than 0.5 mm. Most of these particles float on the surface of water due to their charge and also because of

Table 14.1 Major water pollutants released from industrial sector. Industrial process

Released pollutants in water

Acetate rayon, beet- root manufacturing Chem. manufactures, mines, textiles manufacture Cotton and straw kiering, wool scouring Plating Plating, tanning, alum anodizing Copper plating, copper pickling Gas manufacture, plating, metal cleaning Scrubbing of flue gases, glass etching Petrochemical and rubber factories

Acetic acid Acids

Plating Gas and coke manufacture, chemicals plants Food processing, textile industries Textile industries, tanneries, gas manufacture Dyeing, leather, chemicals manufacture Galvanizing zinc plating, rubber process Paper mills, textile bleaching Iron and steel industry Pulp and paper industry Nonferrous metals Microelectronics Mining industries

Alkalis Cadmium Chromium Copper Cyanides Fluorides Hydrocarbons, mineral oils, phenols, and chromium Nickel Phenols, heavy metals, and cyanides Starch Sulfides, sulfates, and chromium Tartaric acid Zinc Free chlorine Oil, metals, acids, phenols, and cyanides Chlorinated organic compounds Fluorine Organic chemicals Metals, acids, and salts

Figure 14.3 Water purification processes. Source: Reproduced with permission from Suthar, R. G., & Gao, B. (2017). Nanotechnology for drinking water purification (pp. 75 118). Elsevier BV.

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less density than water. Flocculocides (floc) is formed after the addition of alum, causing clumping of these particles to form heavier particles that easily settle down. After flocculation, the water and floc move slowly through large basins, where it settles down and the process is known as sedimentation. Coagulation typically occurs in less than 10 seconds, whereas flocculation occurs over a period of 20 45 minutes (Crittenden et al., 2012). Floc along with other materials, such as clay and silt-based turbidity, and natural organic matter are removed in the sedimentation process. The impurities include synthetic organic chemicals, microbial contaminants, toxic metals, and humic substances. Humic substances come from the soil and are produced within natural water and sediments by chemical and biological processes, such as the decay of vegetation (McCarthy & Zachara, 1989). Removal of humic substances from drinking water is essential since they form disinfection by-products when chlorine is added to the water. The last step in purifying the water is accomplished by filtration through a bed of sand and gravel. It removes the remaining particles of suspended matter. Water is often pretreated with chlorine: chlorine kills biological contaminants, such as bacteria, protozoa, and viruses that may cause illness in humans. Chlorine also prevents the growth of algae in the treatment plant that may hinder the treatment of water and cause taste and odor problems. Enough chlorine is added to maintain a residual of one part per million or residual chlorine in water after filtration to prevent the regrowth of micro-organisms in the distribution systems, pipes, and home plumbing. Fluoride is added into the filtered water to reduce tooth decay. Treatment plants maintain fluoride levels between 0.7 and 1.2 parts per million in the treated water (Suthar & Gao, 2017). Finally, lime (or calcium oxide) is added into the treated water before the water is added to the distribution system. This raises the pH of the water to eight standard units, reducing the ability of the water to corrode water mains and home plumbing materials, such as copper, lead, and brass.

14.4

Waste water treatment

Wastewater treatment is a process where the contaminants or pollutants are separated from the aqueous phase using a number of physical and chemical processes, before its environmental release. There are many ways of treating residentially and nonresidentially used wastewater. Most commonly, wastewater is discharged to a municipal sewage treatment plant. In a properly designed and functional plant, a major portion of the pollutants can be removed. Sewage treatment plants operate mainly in three stages; preliminary, primary, and secondary stages (temleton & Butler, 2011). Fig. 14.4 schematically represents the complete process of typical wastewater treatment. The preliminary treatment process involves the removal of large and/or heavy debris from the sewage. Screening and grit removal steps typically include and

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Figure 14.4 Schematic showing a typical wastewater treatment process.

removing large floating debris, such as rags (B60%), paper (B25%), and plastics (B5%). The primary treatment separates the major portion of the suspended solids from the wastewater using a sedimentation process. The effluent is allowed to flow through sedimentation tanks, where the majority of the solid material gets settled at the bottom of the tanks, called sludge. The primary treatment generally removes up to about 40% of the biochemical oxygen demand (BOD), around 80% to 90% of suspended solids, and up to 55% of fecal coliforms bacteria in water. Secondary treatment involves biological processes that can reduce the BOD level and most of the organic matters, as well as a low amount of suspended solid materials. In this process, the sewage undergoes strong aeration to encourage the growth of aerobic bacteria and other micro-organisms that oxidize the dissolved organic matter to carbon dioxide and water. In addition to the removal of organic matter and BOD from sewage, some nutrients present in the sewage, such as nitrogen and phosphorus, are also removed in the secondary treatment stage using processes like nitrification and luxury cell uptake. Some treatment plants introduce a tertiary treatment stage to remove remaining organic and inorganic matter and microorganisms from the secondary stage effluent using physical and chemical processes (Tchobanoglous et al., 2003).

14.5

Challenges

Existing technologies for water purification endow many limitations, like high energy consumption, sludge disposal, sludge treatment, product quantity etc.

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High energy consumption: In the treatment of municipal wastewater, biological treatment consumes the major part of the energy, nearly 50% 60% of the plant’s total energy consumption. Reduction in energy consumption is one of the major challenges faced by researchers with presently available technologies (Sid et al., 2017). Sludge disposal: Contaminated with a variety of pollutants such as heavy metals, chemicals, and pathogens, thereby its safe disposal is a matter of great concern (Raheem et al., 2018). Sludge treatment: Activation of sludge reduces the load of pollutants, but it is a highly challenging task due to the very high footprint demand. Sludge activation plants are costly to construct and, in addition, occupy a large land area for the setting up of large settling tanks and aeration basins. Increasing population: With the constantly increasing population density, there is a great burden on water treatment plants, as they have to expand their capacity.

14.6

Nanotechnology in water and wastewater treatment

Water is a precious resource; however, urbanization and increasing population rates create drastic climate changes. Water scarcity is a serious predicament. Various technological developments are employed on the various bases for the purification of water. Researchers are trying to find a new method for water/wastewater treatment through nanotechnology. Advances in nanoscale research have made it possible to invent economically feasible and environmentally stable technologies for effective treatment of water/wastewater. Nanotechnology has provided the opportunities to meet the freshwater demands of future generations of humans by using different types of nanoparticles and/or nanofibers (Savage & Diallo, 2005). Nanotechnology uses materials of particle size smaller than 100 nm in at least one dimension and these materials have already demonstrated applications in medicine, industry, engineering, and materials science (Eijkel & van den Berg, 2005; Masciangioli & Zhang, 2003). At the nanoscale, materials possess novel and significant changes in physical, chemical, and biological properties to their bulk forms (Rickerby & Morrison, 2007; Vaseashta et al., 2007). The wide applications including wastewater treatment of nanoparticles have already been reported (H. Ma et al., 2011). Smaller sizes of nanoparticles allow them deeper penetration and thus can treat water/wastewater, which is generally not possible by conventional technologies (Prachi et al., 2013a). Larger surface area of nanomaterials provides higher reactivity toward the contaminants. Nanotechnology has the potential to provide both water quality and quantity in the long run through the use of, for example, membranes enabling water reuse and desalination (Riu et al., 2006; Theron et al., 2008). Nanoparticles, having high absorption, interaction, and reaction capabilities can behave as colloids by mixing with aqueous suspensions as they display quantum size effects (Alivisatos, 1996a,b; Gaponenko, 1998; Rosenthal, 2001; J. Z. Zhang, 1997). Nanomaterials have effectively contributed to the development of

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more efficient and cost-effective water filtration processes as membrane technology is considered as one of the advanced water/wastewater treatment processes in the past (Allabashi et al., 2007; Arkas et al., 2006; Bhattacharyya et al., 1998; Chatterjee et al., 2005; DeFriend et al., 2003; Dotzauer et al., 2006; Hollman & Bhattacharyya, 2004; Ritchie et al., 2001, 2001, 1999; Stanton et al., 2003; Xu et al., 2005). Nanoparticles have frequently been used in the manufacturing of membranes, allowing permeability control and fouling resistance in various structures and relevant functionalities (Cortalezzi et al., 2003; J. H. Li et al., 2009). Both polymeric and inorganic membranes are manufactured by either assembling nanoparticles into porous membranes or a blending process (Bottino et al., 2002; Kim et al., 2003; Li et al., 2009). The use of different types of nanomaterials in catalytic membranes is possible to disinfect diseases causing microbes, removing toxic metals from water/wastewater. The nanoparticles are designed for water treatment and are highly porous and absorb water, like a sponge, repelling dissolved salts and other impurities. Hydrophilic nanoparticles embedded in the membrane repel organic compounds and bacteria that are more likely to obstruct conventional membranes over time. Diverse types of nanoparticles are used in wastewater treatment. Fig. 14.5 displays some commonly used approaches to treat wastewater (Naseem & Durrani, 2021).

Figure 14.5 Waste water treatment processing by various ways using nanoparticles. Source: Reproduced with permission from Naseem, T., & Durrani, T. (2021). The role of some important metal oxide nanoparticles for wastewater and antibacterial applications: A review. Environmental Chemistry and Ecotoxicology, 3, 59 75.

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14.6.1 Nanosorbents Pollutants in the water/wastewater are generally adsorbed over nanoparticle surfaces and these nanoparticles can be separated or removed from the water. Nanosorbents endow a benefit over the conventional adsorbent materials (on the basis of mass) alongside their exclusive structure and electronic characteristics (Zamboulis et al., 2011). The nanomaterials used as nanorobots in water treatment must satisfy the following criteria (Nowack, 2008); 1. They should be nontoxic 2. They should have high sorption capacity and selectivity to the low concentration of pollutants 3. Adsorbed pollutants should be easily removed from the nanosorbent surface and 4. They should be recyclable.

Carbon and metal-based nanoparticles are most commonly used for adsorptionbased water treatment. New generation porous carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene demonstrate great potential as absorbent materials for organic wastewater treatment (Q. Ma et al., 2016). Graphene is the basic structural unit of all the carbon allotropes. The stacking, sealing and rolling of graphene units in some way results in the formation of traditional graphite, fullerenes, and CNTs (F. Yu et al., 2016). CNTs and graphene are the types of pure carbon allotropes (Fig. 14.6A) in which three of the four valance electrons are covalently shared in a 2D plane to form a sigma bond, a relatively strong force, and the last valance electron is delocalized among all atoms, forming a weak pi-bond in the third dimension (Apul & Karanfil, 2015) (Ajayan, 1999). Graphene is a 2D flat material with an open surface, which can be considered to be a nanoplatelet of graphite that permits adsorption in both directions. CNTs are 1D tubular materials of an external surface and a hollow-core for adsorption. Stacking them individually or mixing results in the formation of pore structure in 3D space. Therefore, the agglomeration of CNTs and graphene provides a sufficient number of micropores and macropores. Specifically, stacking the pore structures of CNTs or graphene agglomerates can significantly enlarge their adsorption capacity. Thus, permitting the adsorption of several tens or even hundreds of times the number of pollutants than of the adsorbents. In addition, capillary filling promotes adsorption because it combines the effects of the surface area, curvature and pore structure (Anjum et al., 2019). CNTs and graphene are possible for physical interactions, such as van der Waals forces, pi-bonds, or electrostatic forces with pollutants on them without any significant change in their chemical or physical properties (Teow & Mohammad, 2019). Chemical bonding of pollutants and CNT/graphene is also possible via the dangling bonds or functional groups on the basal surface, such as carboxyl or hydroxyl groups (Fig. 14.6B). Chemical bonds are apparently stronger than physical adsorption, but the ratio of these interactions depends on the number of functional groups and the surface contributing to the adsorption. Pure sp2 hybridized carbon is hydrophobic and favors the high-efficiency adsorption of hydrocarbons with π π

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Figure 14.6 Chemosphere. (A) Carbon allotropes based on graphene sheet and (B) Adsorption mechanisms between graphene and different kinds of contaminants. Source: Reproduced with permission from (A) Yu, F., Li, Y., Han, S., & Ma, J. (2016). Chemosphere. 153, and (B) Kim, S., Park, C. M., Jang, M., Son, A., Her, N., Yu, M., Snyder, S., Kim, D. H. & Yoon, Y. (1104).

interactions. Diversified modification of CNTs or graphene makes them partly hydrophilic or entirely hydrophilic and effective at absorbing pollutants with both hydrophilic and hydrophobic properties. The π π interactions are strongest in CNTs and graphene even in the presence of functional groups. For CNTs and graphene with nearly perfect structures, most of the adsorbates physically adhere to them while very few are absorbed by chemical interactions (G. C. Chen et al., 2009) (Table 14.2). Functionalization of carbon nanomaterials affects adsorption process. For example, the hydrophilic nature of the carbon nanomaterial determines the amount of interacting active sites with water as well as with pollutants. For nonpolar and hydrophobic pollutants, the hydrophobic interaction can be one of the most significant adsorption mechanisms for graphene and reduced graphene oxide (rGO) (Ersan et al., 2017). Extensive efforts are being made to study the hydrophilic nature and other properties of CNTs and graphene along with their adsorption mechanism. Table 14.3 summarizes the types of functionalized CNTs and graphene fabricated for different wastewater treatment processes.

Table 14.2 The adsorption mechanisms of carbon nanotube and graphene-based adsorbents on several chemicals. Sr. no.

Adsorbent

Pollutant

Dominant interaction

References

1

SWCNTs/ MWCNTs SWCNTs

p-Xylene; benzene toluene ethylbenzene and xylene Sulfamethoxazole; TC; tylosin

Chin et al. (2007)

SWCNTs/ MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs CNTs Graphite Graphene/ Go Graphene

lysozyme; PNT; TCE Xylene Cd [II]; Zn (II) Hg (II) Pb (II) MB Phenanthrene lysozyme; Rhodamine B

graphene/ GO/rGO GO GO

PNT; TCE; 17β-estradiol; 17αethynyl estradiol. MB MV; Zn (II)

π π electron donor acceptor interactions Mesopores adsorption, π π electron donor acceptor interaction Van der Waals forces; electrostatic attractions Electrostatic dispersions Metal interaction with π-electrons Hydrophobic surface Micropores; the large surface area Ion exchange; electrostatic interactions π π interactions Van der Waals forces; electrostatic attractions Surface or pores adsorption; intraparticle diffusion Hydrophobic interactions; π π bonding

2 3 4 5 6 7 8 9 10 11 12 13

Emulsified oil

π π stacking; anion cation interaction Electrostatic interactions

Ji et al. (2010) Ersan et al. (2016) Bina et al. (2014) Cho et al. (2010) Vikrant et al. (2019) Li et al. (2005) Yao et al. (2010) Zhao et al. (2014) Kumar et al. (2017) Chacra et al. (2018) Ersan et al. (2016), Zhao et al. (2014), Jiang, et al. (2017) Liu et al. (2012) Liu et al. (2012), Ramezanzadeh et al. (2018)

Table 14.3 The structural modifications and recombinations of the carbon nanotubes/graphene-based adsorbents. Sr. no.

Modification

Example

Binding groups

Reference

1

Oxidation

O-MWCNT oxidized by H2SO4/ HNO3mixed acids O-MWCNT oxidized by nitric acid and citric acid GO made by the Hummers method

Oxygen containing functional groups

Zhou et al., (2019) Lee et al. (2018) Chin et al. (2007)

2 3

4

Biologicalization

5

6

7

DESs as a functionalization agent

MWCNTs/Gly/β-CD prepared by decorating glycine and β-cyclodextrin on functionalized CNTs m-MWCNT-rTl-Cyn prepared by amino-functionalizing as-made magnetic MWCNTs (m-MWCNTs) and then immobilized a recombinant cyanate hydratase (rTl-Cyn) onto them Macroporous poly (acrylic acid)-CNT composites fabricated where DESs acted as a monomer as well as the solvent Functionalized MWCNTs treated by two DESs systems: methyltriphenylphosphonium bromide (MTPB) and benzyltriphenylphosphonium chloride (BTPC) as salts and glycerol (Gly) as hydrogen bond donor

Carboxylic groups Bonded hydroxyl of phenol group, carboxylic group, C O C bonds associated with epoxy groups Amino groups, hydroxyl groups, α-1,4glucose bonds Carboxyl and hydroxyl groups, amino groups

Mohammadi and Veisi (2018) Ranjan et al. (2019)

Double bonds

Mota-Morales et al. (2013)

Carboxyl and hydroxyl groups, carbanyl groups

AlOmar et al. (2016)

(Continued)

Table 14.3 (Continued) Sr. no.

Modification

Example

8

Composite adsorbents

Graphene oxide-polyaniline composite

9

10

Oleophobic-superhydrophilic composite membranes prepared by a mixture of aqueous polymer solutions and nanoparticles and graphene CNTs/FeS Fenton-like adsorbent prepared by the deposition of Fe iron nanoparticles on MWCNTs

Binding groups C 5 N bond, Ar N bond, C 5 C CO OH bond

Reference Ramezanzadeh et al. (2018) Yoon et al. (2014)

Ma et al. (2015)

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Minimum environmental impact, low solubility, and less secondary pollution make metal and metal oxide nanomaterials superior to be used as nanosorbents to remove heavy metals. Ferric oxide (L. Feng et al., 2012), silver (Fabrega et al., 2011), titanium oxide (Luo et al., 2010), manganese oxide (K. Gupta et al., 2011), magnesium oxide (C. Gao et al., 2008), cerium oxide (C. Y. Cao et al., 2010; C. Gao et al., 2008) and copper oxide (Goswami et al., 2012) etc., are frequently used nanomaterials in water purification treatment. Packed bed columns prepared from metal oxide nanomaterials demonstrate a high arsenate removal ability (Hristovski et al., 2007) and titanate nanoflowers remove highly toxic Cd21 (J. Huang et al., 2012) similarly Mg(OH)2 nanotubes show higher removal efficiency of nickel ions from wastewater (S. Zhang et al., 2006). However, the major disadvantage of metal and metal oxide-based nanomaterials is the difficulty in separating them from waste water due to their high surface energy and dimensions. Iron is one of the most abundant metals available on the Earth. Oxides of iron are easy to synthesize and modify. Their high surface-to-volume ratio and super magnetism make them very good adsorbents. They are less toxic, chemically inert, and biocompatible in nature. Various forms of iron oxides which have been researched intensely for their ability to act as nano-adsorbents include maghemite (γ-Fe2O3), hematite (α-Fe2O3), and magnetite (Fe3O4), goethite (α-FeOOH), iron oxide (FexOy), etc. (Badruddoza et al., 2011; Y. H. Chen & Li, 2010; Fan et al., 2005; Grossl et al., 1994; Guan et al., 2007; Hu et al., 2005; Huang & Chen, 2009; Liu et al., 2008; Macdonald & Veinot, 2008; Mahdavian & Mirrahimi, 2010; Wang et al., 2010). These are used in the separation of heavy metals such as Cr61 and Pb21. Here, the protonation or deprotonation of the magnetite surface hydroxyl (Fe OH) group followed by water loss causes Cr61 and Pb21 adsorption. Similarly, various kinds of nanostructured wastewater metal adsorbents are being identified with different properties, such as ZrO2, TiO2, CuO, and MgO, etc. (Jiao et al., 2017; Mohammadi et al., 2017; Sponza & Oztekin, 2016). Magnetic nanomaterials involved in wastewater treatment can be easily removed and regenerated using a high magnetic gradient separation (Nowack, 2008). Maghemite in wastewater treatment combines the adsorption ability of nanoparticles and magnetic separation. The majority of the wastewater pollutants are nonmagnetic in nature (Prachi et al., 2013a); so, the magnetic filtration is costeffective, simple, and environmental-friendly because the adsorption-desorption cycles help to regenerate the original magnetic nanomaterials (Shit & Sharma, 2012).

14.6.2 Nanocatalysts Nanocatalysts boost the surface catalytic activity due to their unique characteristics including a greater surface area that enhances the reactivity and degradation of contaminants (Prachi et al., 2013a). Despite the evidence in the applications of nanotechnology for improved waste water treatment performance, challenges associated with human and ecological toxicity caused by ineffective wastewater treatment still exist (Jr, 2008).

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Figure 14.7 Fabrication of photocatalytic membranes. Source: Reproduced with permission from Yin, J., & Deng, B. (2015). Polymer-matrix nanocomposite membranes for water treatment. Journal of Membrane Science, 479, 256 275.

Nanocatalysts assist in oxidative elimination of pollutants such as dyes, antibiotics, organics, microbes, etc. (Quek et al., 2018; Wang et al., 2018a; Zhang et al., 2018). For example, heterostructured BiVO4/CH3COO (BiO) nanocomposite degrades the refractory organic contaminants such as sulfamethoxazole, bisphenol A, 4-aminoantipyrine, and ibuprofen (Zhang et al., 2019). Similarly, photodegradation of dyes like rhodamine B and methyl orange was observed by iodine and carbon co-doped TiO2 nanocatalyst. Here, the co-doping of iodine and carbon has enhanced the separation of photo-induced carriers and also extended the range of light absorption of TiO2 (Wang et al., 2018a). Photocatalytic membranes are based on membrane separation technology and the photocatalytic activity of the nanocatalysts. These membranes offer several advantages, such as elevated regularity of catalytic sites, the capacity of optimization, limiting catalyst contact time, and simplicity on the industrial scale as shown in Fig. 14.7.

14.6.3 Nanostructured membrane Membrane filtration shows high positivity in water and wastewater treatment as they do not require chemical additives, thermal inputs, and media regeneration. The use of

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nanostructured materials for membrane filters is gaining considerable importance due to the superior properties of nanomaterials for organic and biological contaminant removal, resistance to fouling, metal selectivity, durability, and cost-effectiveness, ease of operation, high efficiency, and small space requirement (Saliby et al., 2008). Furthermore, the filtration capacity of the nanomembranes can be enhanced by incorporating appropriate chemicals or nanoparticles or their combinations (Zhao et al., 2016). The diverse nature of water pollutants requires more specialized membranes to purify the polluted water, so, based on the application, nanomembranes can be produced with improved properties such as anti-fouling, antimicrobial, enhanced permeability, selectivity, photocatalytic, etc. (Munnawar et al., 2017). Aquaporins are vertically aligned nanotubes where isoporous block copolymers are important membranes. The protein channels that help in controlling water flux across biological membranes are called aquaporin membranes (Agre et al., 1993). Aquaporin channels with lipid bilayers that are incorporated into water treatment membranes hold higher selective permeability than many commercial reverse osmosis membranes (Kaufman et al., 2010). Aquaporin combined membranes stimulate water permeation and high selectivity (Kumar et al., 2007; Zhu et al., 2004). The osmotic pressure gradient causes the transportation of water and minerals across biological membranes (Kaufman et al., 2010). Until now, aquaporin-based membranes are under research and scale-up process, as well as simplifying the fabrication process will place this membrane material in the water purification market. Biosystems inspired by multifunctional solar-driven paper-based rGO composites have been synthesized for clean water generation. These rGO composites immediately generate localized heating under solar irradiation. They generate clean water through distillation and also enhance upward vapor flow-assisted adsorption removal performance. The composites have the capability of being photocatalysts with combined adsorption, photodegradation, and interfacial heat-assisted distillation mechanisms to transform contaminated water into a clean water (Fig. 14.8) (Lou et al., 2016). Recently, a biobased nanofiber membrane with the ability to eradicate pollutants such as water-soluble dyes, oils, and organic pollutants has been developed. This membrane has been modified according to the application. Electrospun poly-(l-lactic acid) is used as the scaffold nanofiber while fabrication and surface have been modified with polydopamine (PDA) and β-cyclodextrins (β-CDs). Poly-(l-lactic acid) is nontoxic and easily degradable in nature into CO2 and water; hydrophilic surface modification of PDA provides the rough surface and negative charges. The negative charges due to N- or Ocontaining groups support the adsorption of positively charged soluble organic pollutants. Then the surface wettability was modified using β-CDs, which help to exhibit properties such as superhydrophilicity and high underwater oleophobicity. The resultant nanofibers are capable of simultaneous removal of oil-inwater emulsion and positively charged water-soluble organic pollutants such as methylene blue (MB) and toluene emulsion. This membrane contains entirely biobased, biodegradable, and environmentally friendly PLA, dopamine, and

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Figure 14.8 Schematic showing clean water generation by floating air-laid paper-based reduced graphene oxide composite under solar irradiation. Source: Reproduced with permission from Lou, J., Liu, Y., Wang, Z., Zhao, D., Song, C., Wu, J., Dasgupta, N., Zhang, W., Zhang, D., Tao, P., Shang, W. & Deng, T. (2016). Bioinspired Multifunctional Paper-Based rGO Composites for Solar-Driven Clean Water Generation. ACS Applied Materials and Interfaces, 8(23), 14628 14636.

β-CD which can be easily reused after washing with a small amount of cleaning solvents (Kang et al., 2018).

14.6.4 Nanobiocides Nanomaterials having antimicrobial properties are known as nanobiocides. Nanofibers containing polymer materials, such as poly-(vinyl pyrrolidone), PVA, and PEO act as nanobiocides, and also the antimicrobial properties of these nanofibers can be enhanced by adding NPs with antimicrobial properties (Li & Huang, 2006). Metal-based NPs of silver, zinc, copper, titania, carbon, fullerenes (Deguchi et al., 2001), CNTs (G. Cao, 2004), and polyethylene mine (Park et al., 1998) and natural materials, such as peptides (Gazit, 2007) and chitosan (Eby et al., 2008) demonstrate strong antibacterial activity against a wide spectrum of bacteria. Zinc oxide nanoparticles are used in pharmaceutical products, such as lotions, antibacterial creams, ointments, and surface coatings to prevent biofilm

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formation by microorganisms (Jain & Pradeep, 2005a). Zinc oxide nanoparticles kill bacteria either by disrupting the cell membrane by penetrating into the cell envelope or through photo-catalytically generated hydrogen peroxide (Jo et al., 2016a). Copper oxide nanoparticles (Ren et al., 2009), copper-hydrotalcite (Sunayama et al., 2002), copper nanoparticles incorporated with an acrylic group (Anyaogu et al., 2008) demonstrate antimicrobial potential. The main advantage of this material is economical as compared to silver. TiO2 is a semiconductor photocatalyst (Ahmad et al., 2015) that shows the potential for inactivating Gram-positive, Gram-negative bacteria and viruses (Zan et al., 2007). Immobilized TiO2 nanoparticle films, TiO2 nanoparticles incorporated in isotactic polypropylene polymeric matrix, and nanocomposite water membranes with TiO2 NPs are used in water treatment application (Kubacka et al., 2009). The advantages of TiO2 nanoparticles are high stability in water, cost-effectiveness, and the ability to incorporate them into thin films or membrane filters for water filtration (Kwak et al., 2001a). Self-immobilized, stable antimicrobial peptide nanoparticles prepared by biomineralization process have demonstrated high antibacterial and antifungal activity at minimum inhibitory concentration against pathogenic microorganisms viz Escherichia coli, Staphylococcus epidermidis, S. aureus, and Candida albicans (Brogden, 2005; Eby et al., 2008; Etienne et al., 2004; Gregory & Mello, 2005). Nisin A is a broad-spectrum antibiotic with good antibacterial activity against gram-positive bacteria. When Nisin A is incorporated into poly-L-lactide nanoparticles its antibacterial properties got enhanced and it is notable that the antimicrobial activity of Nisin A against Lactobacillus delbrueckeii subsp. bulgaricus was increased up to 45 days (Salmaso et al., 2004). Natural antimicrobial materials incorporated in nanoformulation are in high demand to improve their antimicrobial properties. Chitosan is a natural polysaccharide with high antimicrobial activity against Gram-positive bacteria and is also effective against fungi and viruses (Qi et al., 2004). The nanoparticulate form of chitosan (Ye et al., 2006) is used as antimicrobial agents in sponges, membranes, and surface coatings of water storage tanks and is highly impressive in drinking water disinfection applications. Other chitosan derivatives, such as chitosan-Ag1 complexes (Chen et al., 2005), the chitosan-Zn21 complex (Wang et al., 2004), and chitosan triphosphate nanoparticles loaded with metal ions (Mu¨ller et al., 2001) are used in wastewater treatment and to kill harmful microbes in water (Du et al., 2009). The major downsides of all these nanobiocides are their loss of antimicrobial activity due to membrane leaching, the high cost involved, and their adverse impact on human health and the environment (Wiesner et al., 2006). Extensive research has been directed to solving these drawbacks to produce innovative nanobiocides that can be used in water treatment applications. Table 14.4 Summarizes the properties, applications, and advantages of various types of nanomaterials envisaged in water and wastewater treatment (Cheriyamundath & Vavilala, 2021).

Table 14.4 Properties, applications and advantages of various nanomaterials envisaged in waste water treatment. Nanomaterial types Nanoadsorbent a) Metals and metal oxides

b) Carbon nanotubes

Nanomembranes

Nanocatalyst

Nanobiocides

Properties

Applications

Advantages

High surface to volume ratio increases the chemical interaction between the adsorbent and adsorbate. pH of the solution modifies the adsorptive properties Surface modification, addition of functional group, metal/metal oxide helps to increase its adsorptive capacity

Mainly using for removing heavy metals from water

Low cost and high adsorptive capacity, easy to synthesize and high chemical stability.

Remove wide varieties of pollutants such as heavy metals, mycotoxin, dyes, antibiotics, uranium etc. MWCNTs are also works as selective sensors.

Low cost, high efficiency, ease of operation, flexibility, applicable for large range of water pollutants. Adsorption capacity and specify can be increased according to the type of pollutant. Ease of operation, high efficiency and low space Requirement.

Contaminants can be removed depending on the pore size and molecule size. Filtration capacity can be enhanced by using appropriate chemicals or by nanoparticles or their combinations. Nanoparticles use radiation from light to oxidize a range of contaminants in the water These are mainly the metal/metal oxides, photocatalytic nanoparticles or carbon nanotubes etc. with antimicrobial properties. They kill the microbes by degrading or damaging its cell membrane proteins or DNA.

Remove various pollutants, includes oils, organic pollutants, microbes, dyes etc.

Remove various pollutants such as organic, microbes, dyes, antibiotics, etc. Helps to remove microbial contamination in water

Efficient, ease of operation and the activation just require light to oxidize and degrade the pollutant. Efficient and ease of operation. combinations of metal/metal oxides, carbon nanotubes, membrane, photocatalytic nanoparticle etc. to improve the antimicrobial property

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Use of metal-oxide nanocomposites in water and wastewater treatment

The search for improving the properties of materials for better performance led to the synthesis of nanocomposite materials. Nanocomposites are multiphasic materials, in which at least one phase adduces dimensions in the nanorange, that is 10 100 nm with morphology such as nanotubes, lamellar nanostructure, or nanoparticles (Friedrich et al., 2005). These properties of nanocomposites are influenced by the individual components, morphology, volume, and shape of filler material as well as the nature of the interphase between them (Osman et al., 2005). The enhancement of these properties can be accomplished when there is suitable interaction and good dispersion between the matrix and the nanoparticles (Ratna, 2007). Metal oxides are revealed as a potential materials for the synthesis of membranes used in water and wastewater treatment. Multimetal oxide nanoparticles or nanocomposites exist as combinations of two or more metal oxides, for example, ZnxMg12xO, Ta-doped ZnO, Ag/Fe3O4, Zn CuO, ZnTiO3, or ZnO TiO2, etc. These multimetal oxides or polymetal oxides seem to be more promising in applications than pure metal oxide nanoparticles. They address problems associated with pure metal oxides such as, but not limited to, toxicity and agglomeration of nanoparticles. The method of preparation determines the possible size, crystallinity, morphology and other physicochemical properties of these composite nanostructures (Cao et al., 2013). Because of their structures, nanocomposites have specific, physical, and chemical properties can be applied in a wide variety of fields, including engineering, medicine, and ecology. Mesoporous oxides of titanium and zirconium are characterized by high values of exchange capacity and chemical and thermal resistance, and carbon materials exhibit mechanical strength, resistance to aggressive environments, and a welldeveloped porous structure. It is possible to obtain fundamentally new materials that will have all of these properties by combining these phases (Gao et al., 2013). 1. In chemical treatment of wastewater Ozonation and oxidation are the most popular chemical methods for wastewater treatment where chemicals such as hydroxides, carbonates, and sulfides combine with the pollutants in the wastewater to form insoluble precipitates (Tu¨nay et al., 1996). However, during these processes, highly toxic and unstable metabolites are generated, which may cause adverse effects on human health and aquatic life (Reddy et al., 2003). High operating cost and short life span are the two major disadvantages associated with the ozonation process. The chlorination process, which provides protection against the regrowth of pathogens and bacteria, results in undesirable odors and tastes as well as in the formation of disinfection by-products (Gopal et al., 2007). Nanocomposites of metal oxides help in cutting costs while at the same time achieving effective degradation of pollutants. For example, polyaniline/hexanoic acid/ TiO2/Fe3O4 nanocomposite with different ratios of Fe3O4 and TiO2 fabricated using in-situ chemical polymerization via template-free approach was used in wastewater treatment (Koh et al., 2013). ZnO/cellulose nanocomposites fabricated by advanced technology have

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documented better efficiency in the degradation of MB dye as compared to the ZnO nanoparticles alone (Lefatshe et al., 2017). 2. In disinfection of biological contaminants Biological contaminants such as micro-organisms, biological toxins, and natural organic matter are predominantly present in wastewater (Dugan & Williams, 2006). The presence of bacteria is the main indication of water contamination, and nearly 80% of the diseases are noted due to bacterial contamination in drinking water. WHO recommended that drinking water should be free from fecal coliform. Conventionally, biological processes such as microbiological, biodegradation, and enzymatic decomposition have also been used for dye removal from wastewater. However, most of these biological treatment techniques are unable to eradicate a wide range of pollutants, and most of these pollutants remain soluble in the effluent (Savage & Diallo, 2005). Recently, many scientific studies showed that nanoparticulate forms of zinc (Zn), silver (Ag), and Titanium (Ti) have antimicrobial properties against several waterborne diseasecausing microbes. The mechanism of the bacterial cell damage caused by the incorporation of Zn/Ag NPs is presented in Fig. 14.9 (Hajipour et al., 2012). Nanocomposites synthesized using these metals demonstrate promising applications in water and wastewater treatment (Jain & Pradeep, 2005b). A polyaniline/ TiO2/graphene nanocomposites prepared by polymerization of TiO2 and graphene nanoparticles have revealed a high photocatalytic and antibacterial activity against Enterobacter ludwigii and E. coli. The enhanced activity of this nanocomposite is due to the low recombination of the graphene electron scavenging property and the sensitizing effect of polyaniline. Similarly, the nanocomposite ultrafiltration membrane of poly(1-vinylpyrrolidone-co-acrylonitrile)-g-ZnO and poly (ether sulfone)-gZnO possesses an improved water flux, high antibacterial activities, and antifouling

Figure 14.9 Mechanisms of toxicity of nanoparticles against bacteria. Source: Reproduced with permission from Hajipour, M. J., Fromm, K. M., Akbar Ashkarran, A., Jimenez de Aberasturi, D., Larramendi, I. R. d., Rojo, T., Serpooshan, V., Parak, W. J., & Mahmoudi, M. (2012). Antibacterial properties of nanoparticles. Trends in Biotechnology, 30(10), 499 511.

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characteristics (Jo et al., 2016a). The hybrid reverse osmosis membranes with aromatic polyamide thin films and TiO2 particles can be prepared through a self-assembly route (Kwak et al., 2001a). The nanocomposite under irradiated with UV light adduces enhanced photocatalytic bactericidal efficiency. 3. In adsorptive technologies There are several methods used in the literature for the purification of water, such as desalination, filtration, adsorption, sedimentation, osmosis, and disinfection; however, adsorption holds several benefits over the other methods (Dubey et al., 2009; Shannon et al., 2008). Adsorption is a surface occurrence phenomenon where adsorbate is concentrated on the adsorbent surface and the process can be chemisorption or physisorption based on the mechanism of adsorption (Gupta & Suhas, 2009). In this process, pollutants get adsorbed on adsorbent surfaces via various forces such as hydrogen bonding, electrostatic, and van der Waal interaction (Ahmad et al., 2015). Generally, adsorbents demonstrate porous structures to increase the surface area and allow faster flow of water. Adsorption is an economical and simple method for pollutant elimination from water as it; is free from the additional chemicals, has a low energy requirement, and is applicable for large quantity water (V. Kumar et al., 2011). Recently, several inorganic and organic adsorbents, such as activated carbon, zeolites, clay minerals, biosorbents, montmorillonite, polymer-based adsorbent, trivalent, and tetravalent metal phosphates, etc., are being employed as an adsorbent in the adsorption process (Mahmud et al., 2014; Muhammad Ekramul Mahmud et al., 2016). Among them, polymer/metal oxide nanocomposites containing polymers, such as polypyrrole, polyfuran, polythiophene, and polyethyleneimine etc., have a strong affinity for cations owing to the electrostatic interaction between the positively charged ions of the metal oxide and the lone pairs of the polymers (Abdi et al., 2009). In addition, the presence of positively charged nitrogen atoms in polypyrrole offers a potential application in the adsorption process as an adsorbent (Mahmud et al., 2014). A polyaniline-modified TiO2 nanocomposite has showed excellent regeneration and adsorption behavior with a maximum adsorption capacity of 454.55 mg g21 with an adsorption equilibrium time of 5 minutes (Wang et al., 2015). The acetate/polypyrrole/TiO2, succinic-polypyrrole/TiO2, tartaric/polypyrrole/TiO2 and citric/ polypyrrole/TiO2 nanocomposites are fabricated and used for water purification. All nanocomposites have displayed an excellent adsorption capacity within 30 minutes and were reused without any reduction in capacity at least four times. However, the variation of adsorptive capacity and physicochemical properties among these four nanocomposites was due to the presence of the hydroxyl group (Feng, Chen, et al., 2016). Similarly, hybrid ternary rGO/ZnFe2O4/polyaniline nanocomposite fabricated from the ZnFe2O4 and rGO, has exhibited a superior adsorption performance in the sewage purification process (J. Feng, Hou, et al., 2016). Novel nanocomposite adsorbent, graphene oxide modified with magnetite nanoparticles, and Lauric acid-containing ethylenediaminetetraacetic acid (GFLE) synthesized by co-precipitation method has been used for elimination of copper (Cu21) ions from water (Fig. 14.10). The study shows that GFLE nanocomposite efficiently removes Cu21 ions due to the superparamagnetic properties of adsorbent and the highest adsorption occurred at the time of 105 minutes, 40 C temperature, initial concentration of 280 mg per liter, and pH 5 1. GFLE nanocomposite is a low-cost adsorbent for the deletion of Cu21 ions due to quick kinetics, great adsorption capacity, and high regeneration capabilities even after three adsorption-desorption cycles (Danesh et al., 2021).

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Figure 14.10 Multistep processes are involved in GFLE nanocomposite preparation and Cu (II) ion absorption process. Source: Reproduced with permission from Danesh, N., Ghorbani, M., & Marjani, A. (2021). Separation of copper ions by nanocomposites using adsorption process. Scientific Reports, 11(1). 4. In membrane technologies Membrane technologies are more efficient in water and wastewater treatment as they effectively remove pollutants without creating harmful by-products. In membrane technology, a semipermeable membrane is used to eliminate particles, gases and solutes. For the effective separation of pollutants from water reservoirs, the membrane must be water permeable as well as less permeable to particles or solutes. Water treatment by membrane technologies can effectively remove the pollutants due to their feasibility, environmental friendliness, and costeffectiveness (Owen et al., 1995). Polymer materials such as polysulfone, cellulose nitrate and acetate, polytetrafluoroethylene, polypropylene, polyethersulfone, polyacrylonitrile, polyimide, polyvinylidene fluoride (PVDF) and polyvinyl alcohol, etc., are nowadays the most extensively used organic membrane materials. These materials are well-known for their mechanical stability, chemical resistance, and selective permeability (Mulder, 1994). The immobilization of metal oxide nanoparticles in polymer membranes has been effective for the photodegradation of contaminants in water treatment (Xu et al., 2014). Antifouling membrane using poly(hexafluorobutyl acrylate) (PHFBA) and TiO2 nanoparticles were synthesized for water treatment (X. Zhao et al., 2015). To achieve significant morphological changes and a multidefense mechanism in the membrane, PHFBA and TiO2 concentrations are effectively customized. Modified membrane when subjected to any kind of irradiation such as UV, visible light, etc., TiO2 generates energy due to the excitation of

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electrons in the photo-catalyst (Romanos et al., 2013). This leads to the creation of a couple of positively charged electron holes and negatively charged free electrons which react with the atmospheric oxygen and water, forming reactive oxygen species (OH radicals and hydrogen peroxide) (Choi et al., 2007). These hydroxyl radicals effectively react with biomolecules such as bacteria, thereby exhibiting antibiofouling activity in their respective nanocomposite membranes. Another important advantage of TiO2 in the membrane is the prolonged lifetime due to the unaltered nature of TiO2 during the photo-degradation process involving micro-organisms and organic pollutants (Liou & Chang, 2012). Poly-(ether sulfone) (PES), membrane incorporated with poly (2-dimethylamino ethyl methacrylate) (PDMAEMA) grafted SiO2 NPs (PSBMA) were synthesized and further used in water treatment. The resultant membrane is reported to have high antifouling properties due to the addition of PSBMA addition (Zhu et al., 2014). The zwitter-ionic sulfobetaine groups present in PSBMA reveal a tendency to form a hydration layer through electrostatic interaction, thereby reducing the risk of membrane surface fouling. The incorporation of functionalized SiO2 in nanocomposite membranes reveals enhanced hydrophilicity and better water-absorbing properties. Phosphoryl silica nanotubes possess much more active groups, which are highly compatible with the PVDF polymer chains. The dispersive properties of the silica nanotubes could be improved by functionalization. There is no possibility for particle agglomeration which greatly enhances the membrane performance in terms of permeation and foulant rejection (Yu et al., 2013). 5. In-ion exchange technologies Contamination of water with heavy metals has become a global environmental issue. Wastewater from industries releases heavy metals such as Pb, Zn, Cu, Hg, etc., into water bodies. Heavy metals can also have adverse effects on the environment as well as on living lives. They cannot be degraded by microorganisms. Once they are released into the environment, they will accumulate through the food chain. Heavy metals are highly toxic to human most of which are even reported to be carcinogenic (Agdi et al., 2000; Lewis & Gattie, 2002). The ion-exchange technique is beneficial for removing heavy metals and other charged contaminants from water. It successfully eliminates pollutants from aqueous solutions via a strong interaction between the functional groups on the charged contaminants and the ion-exchange resins (Moghadam et al., 2015). Ion-exchange membranes are of two types; cation and anion exchangers, depending upon the ionic group attached to the membrane medium. The most commonly used anion exchange is weak base-type, which are type I ion exchange resins (-N(CH3)3), and type II ion exchange resins ( N(CH3)2C2H4OH), while cation exchangers are the weak acidic carboxylate groups ( COO ) d strong acidtype groups ( SO3) (Rivas et al., 2016). These ion-exchange membranes have their advantages and disadvantages, so modification of existing ion exchangers and the design of appropriate ion exchanger membranes with biocide and catalytic performance have been of great interest (Chesnut et al., 1999). The combination of polymer materials with metal oxides offers a new form of ion exchanger that has high stability, high ion exchange capacity, excellent reproducibility, and mechanical stability (Alberti et al., 1973; Niwas et al., 1999). Excellent metal oxide/ polymer ion exchange nanocomposite membranes fabricated and effectively used in water purification processes in the past. For instance, nanocomposite materials formed by the immobilization of metal acid salts into conducting polymers, such as polypyrrole, polyaniline, or polythiophene, offer a hybrid ion exchange membrane with high reproducibility, stability, granulometric, and mechanical properties as well as excellent selectivity for

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heavy metals and ion-exchange capacity. The hybrid organic-inorganic ion exchangers were highly selective toward Cd (II) (Khan & Mezbaul Alam, 2003). A novel polymeric-inorganic cation-exchanger nanocomposite fabricated through a sol-gel route by immobilizing polyaniline into zirconium titanium phosphate showed a high ion exchange capacity (4.52 meq g21), good thermal, chemical stability compared to the bulk parental materials and highly selectivity to Pb(II) and Hg(II) (Khan & Paquiza, 2011). The nanocomposite of spherical carbon (SC) synthesized from waste carbon and iron nanoparticles, is found to be effective in the removal of Cr(VI) from water (Fig. 14.11). The SC was prepared from waste carbon by hydrothermal route. Nanoparticles of zero-valent iron (Fe) are prepared by chemical reduction of FeSO4U7H2O in the presence of SC. Porous SC particles act as a carrier for dispersion of Fe nanoparticles and prevent their aggregation. Fe nanoparticles play a key role in the removal of Cr(VI). Cr(VI) gets adsorbed on the nanocomposite, where Fe nanoparticles are reduced from Cr(VI) to Cr(III) and itself gets oxidized to iron oxide. After the removal of Cr(VI), the nanocomposite can be separated by magnetic separation (Han et al., 2018). Fig. 14.8 Schematic representation of the synthesis of nanocomposite of SC loaded with zero valent iron (ZVI) nanoparticles and its application in removal of Cr(VI) from water (104). 6. In photocatalytic degradation of pollutants The photodegradation of pollutants in water and wastewater using a photocatalysis process is more effective than conventional methods. Photocatalysis degrades highly toxic

Figure 14.11 Schematic representation of the synthesis of nanocomposite of spherical carbon loaded with zero valent iron (ZVI) nanoparticles and its application in removal of Cr (VI) from water. Source: Reproduced with permission from Han, J., Zhang, G., Zhou, L., Zhan, F., Cai, D., & Wu, Z. (2018). Waste carton-derived nanocomposites for efficient removal of hexavalent chromium. Langmuir, 34(21), 5955 5963

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pollutants as well as eliminates microbial pathogens and micropollutants without consumption of high energy and generation of secondary poorly biodegradable products (Friedmann et al., 2010; Malato et al., 2009). Photocatalysis process uses semiconductors such as metal oxides, nitrides, and sulfides which are normally photoactivated by the incoming photon from the light source. Natural sunlight has been the ideal source of energy for the activation process as it is an abundant, clean, renewable and safe energy source (Balachandran et al., 2014). Numerous metal oxide based photocatalysts have attracted moderate attention in the degradation of highly toxic and nonbiodegradable compounds because of their exceptional optical properties, low cost, nontoxicity, and high stability toward photo and chemical corrosion activites (Chen et al., 2015; Qu et al., 2013). Fast recombination of the charge carriers and wide bandgap limitation in harvesting a wider portion of the solar energy is the major limitations in bulk metal oxide materials (Kanakaraju et al., 2014). In recent times, nanocomposites of conductive polymers and metal oxide nanoparticles have been synthesized with an improved photocatalytic activity in the visible region. The incorporation of metal oxide into a polymeric material with appropriate energy levels enhances the charge migration between the inorganic metal oxide and polymer, hence reducing the recombination of the charge carriers (Huang et al., 2013). Cellulose, polyacrylonitrile, polyvinylpyrrolidone, polytetrafluoroethylene, and polyvinyl acetate are considered excellent catalyst supports owing to their high porosity, high permeability, large surface area and good flexibility (Kang et al., 2016). Table 14.5 displays polymer/metal oxide nanocomposites with their preparation methods and photodegradation activities (Cao et al., 2015; Cheng et al., 2014; Ding et al., 2013; Gao et al., 2016; Ma et al., 2015; Macedo et al., 2015; Ong et al., 2013; Pruna et al., 2017; Sun et al., 2013; Zhang et al., 2016). The calcium silicate hydrate (CSH) nanocomposite prepared from steel slag by a simple alkaline activation process without using any hazardous chemicals is used in water treatment (Shao et al., 2018) (Fig. 14.12). CSH nanocomposite is placed in an aqueous solution containing heavy metal ions; Cu(II), Pb(II), and Zn(II). All the heavy metals get adsorbed on CSH nanocomposite. In an aqueous solution, CSH nanocomposite releases OH and Ca21 and heavy metal ions bind to OH forming hydroxides, and gets coated over the surface of CSH (Guan & Zhao, 2016). The heavy metalcontaining CSH is then separated from the aqueous phase by a simple filtration process and the resultant heavy metal-CSH nanocomposite is converted into a photocatalyst by simple heating at low temperature. The newly converted photocatalyst is used for the degradation of MB. The CSH nanocomposite is reused after the adsorption of heavy metal ions.

14.8

Features of metal oxide nanocomposite in water/ wastewater treatment

With the fast development of nanomaterials and nanotechnology, environmental nanotechnology has attracted increasing concerns in recent years. Nanomaterials have demonstrated great potential in improving the performance and efficiency of water decontamination processes as well as providing a sustainable way to secure water supply. Initially, nanoparticles and nanotubes were frequently used in water

Table 14.5 Polymer-metal oxide nanocomposites with photocatalytic activity. Nanocomposite

Method of preparation

Pollutant

Reference

Au/polyaniline/TiO2 PVDF/(Ag, Pt)/rGO/TiO2 MoO3/polyimide

One-step chemical redox Phase inversion process One-pot homopolymerization Electrospinning and direct ion-exchange process Reverse microemulsion polymerization in situ Fractal growth Wet chemical simple and fast electrochemical In situ polymerization

Rhodamine B Methyl orange Methyl orange

Ong et al. (2013) Ma et al. (2015) Ding, et al. (2013)

Methylene blue

Sun et al. (2013)

Methyl orange Rhodamine B

Methyl orange and orange II Methylene blue

Gao et al. (2016); Macedo et al. (2015); Cheng et al. (2015) Pruna et al. (2017) Cao et al. (2015)

Rhodamine B

Shao et al. (2018)

Polyimide/ZnO Polypyrrole/TiO2

Fe3O4/polypyrrole/silver ZnO/polypyrrole/rGO Polypyrrole/polyvinyl alcohol/TiO2

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Figure 14.12 Schematic illustration of the synthesis of calcium silicate hydrate nanocomposite from steel slag, adsorption of Cu(II), Zn(II) & Pb(II) from water and reuse of composite. Source: From Shao, N., Tang, S., Liu, Z., Li, L., Yan, F., Liu, F., Li, S., & Zhang, Z. (2018). Hierarchically structured calcium silicate hydrate-based nanocomposites derived from steel slag for highly efficient heavy metal removal from wastewater. ACS Sustainable Chemistry and Engineering, 6(11), 14926 14935.

and wastewater treatments. Large-scale applications in water treatment, nanoparticles have to face some integral technical bottlenecks such as aggregation, leakage into the contact water, difficult separation, as well as probable adverse effects imposed on the ecosystem and living systems. Nanocomposites are multiphase solid materials, consisting of porous media, gels, colloids, and copolymers in a broad sense. Nanocomposite materials take advantage of both porous medium and impregnated nanoparticles. Compared with free nanomaterials, the performance and usability of nanocomposites could significantly increase. Nanocomposite encourages the diffusion of pollutants, thus enhancing interfacial interaction, nanoparticle dispersion, stability and recyclability. Hence, nanocomposite materials bridge the gap between nanoscopic and mesoscopic scales. They mitigate the release of nanoparticles into the environment by improving the compatibility of nanotechnology with existing infrastructures.

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Future prospects

The rapid growth of the world population and global climate change are closely associated with a growing demand for freshwater. Continuous progress in technologies for water treatment lay a foundation for the use of nanomaterials in water remediation. Emerging nanomaterials provide unprecedented possibilities. They have unique size-dependent properties and allow the development of novel hightech materials for efficient water and wastewater treatment processes, viz. membranes, nanocatalysts adsorption materials, functionalized surfaces, reagents and coatings. Nanoparticles are the most promising candidates for the development of next-generation water treatment technology but there are still many obstacles to overcome before their engineering applications. Van der Waals forces and other interactions in nanomaterials help in aggregation, thus making them unstable for long-term use. It is still challenging to recycle nanomaterials from purified water except for magnetic nanoparticles, thus making the commercial application of nanomaterials uneconomic or impractical. Also, the long-term fate, transformation and toxicity of nanoparticles are mainly unclear. One promising approach to forwarding the application of nanomaterials is to develop nanocomposite materials that take advantage of both the hosts and the impregnated functional nanoparticles. Hosts like polymers, minerals, biopolymers, activated carbons or membranes could facilitate the dispersion and stability of the loaded nanoparticles. Materials functionalized with nanoparticles incorporated or deposited on their surface have risk potential since nanoparticles may release and emit to the environment where they can accumulate for long periods of time. Nanocomposites also effectively mitigate the release of nanoparticles into water. Nanocomposites improve the compatibility of nanotechnology with existing infrastructure like fixed-bed columns or fluidized beds. The potential of nanocomposites in various sectors of research and application is promising and attracts increasing investment from governments and businesses in many parts of the world. Multimetallic nanocomposites are futuristic nanocomposites for potential antimicrobial, absorbent, sensing, and photocatalytic properties. Although these ideas can pave the way for sustainable tomorrow, in reality, a detailed technical study is required to bridge the gap between laboratory-scale and industrial applications. Future work should be a focus on understanding interactions between immobilized nanoparticles and hosts, developing procedures for host structure and nanoparticle manipulation, and guiding the design of more rational nanomaterials of multifunctionality.

14.10

Conclusions

Water is a precious resource. It acts as a universal solvent and allows many physiological reactions to happen. All lives on the Earth are dependent on water. Water pollution is a serious environmental challenge and is majorly caused by the discharge of industrial effluents into water. The consequences of these

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untreated effluents on human health, marine organisms, plants, and the entire ecosystem are very disastrous. Scientists and researchers are being actively engaged to vanquish the need for clean water worldwide. Remediation techniques for various water contaminants are established in various industries, excluding membrane technologies, include adsorption, filtration, coagulation, flocculation, precipitation, electrodialysis, ion exchange, etc. Wastewater treatment plants are also operated by the Municipal Corporation to treat industrial and domestic wastewater. All these methods adduce their advantages and disadvantages. Upgradation in technology is one such step toward the same. Water purification techniques need to become more environmentally friendly and cost-effective. Nanomaterials possess several unique physicochemical features, which make them attractive candidates for wastewater treatment. The high surface area provides a large surface for surface-catalyzing reactions, the capability of being functionalized with diverse chemical groups increases their affinity toward the specific compound and high selectivity of recyclable regions for detrimental elements or ions in effluents. Nanomaterials based on metal oxides are a diverse class of materials in terms of electronic structure and physical, chemical, and electromagnetic properties. The creation of metal oxide composite nanomaterials of better sorption and catalytic properties than their individual metal oxide nanomaterials due to the synergistic effects and special structural and adsorption characteristics may be the most promising future direction. This will allow obtaining a fundamentally new class of nanomaterials for environmental purposes. Based on the analysis of modern literary sources, the use of metal oxide nanocomposites for environmental applications can lead to the development of novel environmentally effective and economically feasible technologies in the future.

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Yu, H., Zhang, X., Zhang, Y., Liu, J., & Zhang, H. (2013). Development of a hydrophilic PES ultrafiltration membrane containing SiO2@N-Halamine nanoparticles with both organic antifouling and antibacterial properties. Desalination, 326, 69 76. Available from https://doi.org/10.1016/j.desal.2013.07.018. Zamboulis, D., Peleka, E. N., Lazaridis, N. K., & Matis, K. A. (2011). Metal ion separation and recovery from environmental sources using various flotation and sorption techniques. Journal of Chemical Technology and Biotechnology, 86(3), 335 344. Available from https://doi.org/10.1002/jctb.2552. Zan, L., Fa, W., Peng, T., & Gong, Z. k (2007). Photocatalysis effect of nanometer TiO2 and TiO2-coated ceramic plate on Hepatitis B virus. Journal of Photochemistry and Photobiology B: Biology, 86(2), 165 169. Available from https://doi.org/10.1016/j. jphotobiol.2006.09.002. Zhang, H., Tao, Z., Tang, Y., Yang, M., & Wang, G. (2016). One-step modified method for a highly efficient Au-PANI@TiO2 visible-light photocatalyst. New Journal of Chemistry, 40(10), 8587 8592. Available from https://doi.org/10.1039/c6nj02408d. Zhang, J. Z. (1997). Ultrafast studies of electron dynamics in semiconductor and metal colloidal nanoparticles: Effects of size and surface. Accounts of Chemical Research, 30 (10), 423 429. Available from https://doi.org/10.1021/ar960178j. Zhang, L., Chen, B., Ghaffar, A., & Zhu, X. (2018). Nanocomposite membrane with polyethylenimine-grafted graphene oxide as a novel additive to enhance pollutant filtration performance. Environmental Science and Technology, 52(10), 5920 5930. Available from https://doi.org/10.1021/acs.est.8b00524. Zhang, S., Cheng, F., Tao, Z., Gao, F., & Chen, J. (2006). Removal of nickel ions from wastewater by Mg(OH)2/MgO nanostructures embedded in Al2O3 membranes. Journal of Alloys and Compounds, 426(1 2), 281 285. Available from https://doi.org/10.1016/ j.jallcom.2006.01.095. Zhang, X., Ma, Y., Xi, L., Zhu, G., Li, X., Shi, D., & Fan, J. (2019). Highly efficient photocatalytic removal of multiple refractory organic pollutants by BiVO4/CH3COO(BiO) heterostructured nanocomposite. Science of the Total Environment, 647, 245 254. Available from https://doi.org/10.1016/j.scitotenv.2018.07.450. Zhao, J., Wang, Z., Zhao, Q., & Xing, B. (2014). Environmental science & technology. 48. Zhao, M., Xu, Y., Zhang, C., Rong, H., & Zeng, G. (2016). New trends in removing heavy metals from wastewater. Applied Microbiology and Biotechnology, 100(15), 6509 6518. Available from https://doi.org/10.1007/s00253-016-7646-x. Zhao, X., Su, Y., Dai, H., Li, Y., Zhang, R., & Jiang, Z. (2015). Coordination-enabled synergistic surface segregation for fabrication of multi-defense mechanism membranes. Journal of Materials Chemistry A, 3(7), 3325 3331. Available from https://doi.org/ 10.1039/c4ta06179a. Zhou, Y., He, Y., Xiang, Y., Meng, S., Liu, X., Yu, J., Yang, J., Zhang, J., Qin, P., & Luo, L. (2019). The science of the total environment, 646. Zhu, F., Tajkhorshid, E., & Schulten, K. (2004). Theory and simulation of water permeation in aquaporin-1. Biophysical Journal, 86(1 I), 50 57. Available from https://doi.org/ 10.1016/S0006-3495(04)74082-5. Zhu, L. J., Zhu, L. P., Zhao, Y. F., Zhu, B. K., & Xu, Y. Y. (2014). Anti-fouling and antibacterial polyethersulfone membranes quaternized from the additive of poly(2-dimethylamino ethyl methacrylate) grafted SiO2 nanoparticles. Journal of Materials Chemistry A, 2(37), 15566 15574. Available from https://doi.org/10.1039/c4ta03199g.

Self-cleaning photoactive metal oxide-based concrete surfaces for environmental remediation

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Valmiki B. Koli and Shyue-Chu Ke Department of Physics, National Dong Hwa University Shou-Feng, Hualien, Taiwan

15.1

Introduction

Globalization has a solid economic impact; human material life is abundant and valuable. Increased productivity, however, comes at the cost of reduced living space, depletion of natural resources, and contamination of the ecological ecosystem. As a result, the human future is endangered (Figge et al., 2017). Owing to such unforeseeable circumstances, the researcher intends environmentally sustainable products to reduce total environmental pollution. Concrete is the most widely used substance in our world today, and it is inextricably linked to our lives. Environmentally friendly concrete applications are in growing demand. Botanist Wilhelm Barthlott discovered self-cleaning plant cells in 1973 (Gressler et al., 2010). Because of the hydrophilic surface of the substance produced in the self-cleaning reaction with TiO2 nanoparticles (NPs), the products formed in the self-cleaning reaction can easily be isolated by rain or simply rinsing, as shown in Fig. 15.1 (Banerjee et al., 2015). Selfcleaning concrete oxidizes organic compounds using ultraviolet light. It has the normal process of pollutant decomposition. Self-cleaning concrete has two advantages: it keeps surfaces clean and reduces pollution. Solar-powered photocatalytic materials assist in the removal of airborne pollutants. This material has the potential to absorb toxic ultraviolet radiation and produce reactive oxygen species. They will collaborate with toxins, like nitrous oxide, to create less toxic substances (Opc¸u & Akkan, 2020). This chapter describes the fundamentals of photocatalytic concrete, including the concepts, recent developments, and potential applications. A self-cleaning surface can clean itself without the help of humans. The most often used method is the application of a hydrophilic or hydrophobic coating dirt removal from surfaces with hydrophobic properties. Dirt is washed away due to the deposition of water droplets on surfaces that roll away with dirt when washed away by hydrophilic surfaces as shown in Fig. 15.1 (i). In recent years, photocatalysis has also been used to decontaminate buildings using light to photodegrade contaminants on the surface. The leaves of the lotus plant (Li et al., 2004), rice plants, butterfly wings (Bixler & Bhushan, 2012), fish scales (Hay, 1996), and other plants exhibit self-cleaning behavior. Lotus leaves have a waxy surface and microscopic structures that give them a highly hydrophobic appearance (Zhang et al., 2012). Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00002-6 © 2022 Elsevier Inc. All rights reserved.

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Figure 15.1 (i) Photocatalytic reactions on the self-cleaning surface (ii) (a) a superhydrophilic and (b) a superhydrophobic surface. Source: (i) Banerjee, S., Dionysiou, D. D., & Pillai, S. C. (2015). Self-cleaning applications of TiO2 by photo-induced hydrophilicity and photocatalysis. Applied Catalysis B: Environmental, 176
177, 396
428, ( ii) Nishimoto, S., & Bhushan, B. (2013). Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity. RSC Advances, 3(3), 671
690.

The dust/dirt particles are regularly wiped away as water droplets roll off the trees. This is commonly known as the “Lotus effect.” There are two kinds of selfcleaning surfaces: (1) hydrophilic surfaces and (2) hydrophobic surfaces. Water drops scatter over the surface of hydrophilic materials, forming a layer of water. The pollutants on the soil are swept away during the spreading process. Because of the surface’s water repellent and poor adhesive properties, water droplets glide off hydrophobic surfaces fast, eliminating impurities on the surface Fig. 15.1 (ii). Significant work has been devoted in recent years to the development of selfcleaning biomimetic surfaces with anti-fouling and anti-reflective characteristics (Zhao et al., 2014). Several studies on the self-cleaning operation of TiO2 have been published in recent years, with the majority of them concentrating on the photocatalytic behavior of TiO2 (Fujishima et al., 2008; Ragesh et al., 2014), but there are few papers that deal with the preparation, testing, photocatalytic activity, impact of photoactive materials on concrete properties, and commercial applications. Self-cleaning technology for indoor and outdoor use has been commercialized. Self-cleaning technology is used in a number of applications, including paints, glass, textiles, and tiles. Photocatalysis cementitious materials have been investigated to reduce environmental pollution by using construction materials containing photocatalyst materials. Additionally, maintaining concrete (cementitious material) aesthetic quality was a crucial pillar in developing photoactive concrete. Fig. 15.2 shows the evolution of photoactive concrete. As a result, in recent years, research teams have concentrated on investigating building materials or paints containing photocatalytic chemicals that degrade pollutants. Photocatalysis, as defined by the International Union of Pure and Applied Chemistry, is a process in which the rate or start of a reaction changes as a result of UV, visible, or infrared light. The presence of a photocatalyst, which absorbs light

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Figure 15.2 History of photoactive concretes (cementitious materials). Note: PICADA project: European project as photocatalytic innovative coverings applications for depollution assessment. Source: From Hamidi, F., & Aslani, F. (2019). Additive manufacturing of cementitious composites: Materials, methods, potentials, and challenges. Construction and Building Materials, 218, 582
609.

and participates in the chemical transformation of reactants, is required. Photocatalysis is considered as one of the “advanced oxidation process groups.” The semiconductor made up of valence, band, and conduction band is stimulated by absorbing a specific radiation wavelength. The ejection of an electron in the valence band (VB) results in the formation of holes (h1), which are responsible for the extremely hydrophilic conversion (Hamidi & Aslani, 2019; Wu et al., 2019) Photogenerated holes diffuse to the surface and become entrapped in oxygen-rich regions of the network. Following that, a mechanism happens that leads to the formation of hydroxyl radicals. The resultant products, hydrogen peroxide and hydroxyl radicals are potent oxidants that can decompose organic molecules into carbon dioxide and water when they combine with them. Among numerous materials or compounds investigated for use in photocatalysis processes, such as ZnO, CeO2, SnO2, ZrO2, CdS, ZnS, WSe2, Fe2O3, SrTiO2, WO3, TiO2 is the best owing to its high photocatalytic activity, physical and chemical stability in light and dark, and low cost (Weon et al., 2019). As a result, TiO2, also known as a white pigment, is most often used as an additive to construction materials. The TiO2 NPs are used in building materials, especially concrete, to improve selfcleaning properties. The incorporation of TiO2 NPs into concrete would aid in the photocatalytic process allowed by air purification and the self-cleaning surface under UV light irradiation. Thus by degrading airborne contaminants, the building can maintain its esthetics while also improving air quality (Guo et al., 2009). Anatase TiO2

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coated with perfluorooctylo triethoxysilan ethanol suspension (C14H19F13O3Si) for self-cleaning study in a recent report and TiO2 is used in paint to create self-purifying surfaces (Banerjee et al., 2015; Zailan et al., 2016). Several TiO2 NPs coating reports have discovered that they improve mechanical properties and have a high stain removal of bacteria (Zailan et al., 2016). Titania is used as a photocatalyst in concrete, focusing on self cleaning and atmospheric purification (Chen et al., 2011). Photocatalytic reaction included self-cleaning concrete, which eliminated nitrous oxide, volatile organic compounds (VOCs) such as formaldehyde, benzene toluene, and other organic contaminants items. Photocatalytic removal of VOCs with TiO2 NPs in sunlight splits dirt into fundamental oxygen, water, CO2, nitrate, and sulfate (Gopalan et al., 2020). The self-cleaning concrete surface with UV light depollution eliminates contaminants and impurities from the natural environment.

15.2

Photocatalytic mechanism of self-cleaning concretes

The beginning of the photocatalysis era started in 1972 when Fujishima and Honda published the first paper on photocatalytic water splitting using TiO2 NPs (Fujishima & Honda, 1972). Ohtani (2011) define “photocatalysis” or “photocatalytic reaction” as a chemical process produced by photoabsorption of solid material, or “photocatalyst” that stays chemically unaltered during and after the reaction. The idea of photocatalysis is frequently presented using an example such as Fig. 15.3. Photoirradiation causes an electron (e2) to be excited from the VB to an empty conduction band (ECB) (CB). After excitation, a positive hole (h1) forms in the VB; these electrons and positive holes participate in the reduction and oxidation processes of compounds adsorbed on the surface of a photocatalyst. Eqs. (15.1) and

Figure 15.3 Photocatalytic reaction mechanism over semiconductor material. ˇ Source: From Perovi´c, K., dela Rosa, F. M., Kovaˇci´c, M., Kuˇsi´c, H., Stangar, U. L., Fresno, F., Dionysiou, D. D., & Loncaric Bozic, A. (2020). Recent achievements in development of TiO2-based composite photocatalytic materials for solar driven water purification and water splitting. Materials, 1338. https://doi.org/10.3390/ma13061338.

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(15.12) outline the photocatalytic mechanism. Second, in the conduction and VB, electrons (e2) and holes (h1) are produced [Eq. (15.1)]. Photogenerated holes [Eqs. (15.3) and (15.4)] reach the TiO2 surface and react with surface adsorbed hydroxyl groups or water to form trapped holes [Eq. (15.2)]. The stuck hole (TiO∙) is known as a surface-bound or adsorbed OH∙ radical OH ads∙. OH∙ is generated at the surface of the semiconductor and then departs the surface to create free OH∙ (OH free∙), according to Eq. (15.7). As indicated by Eqs. (15.5)
(15.7), electron transfer can occur if electron donors (Redorg) are present at the TiO2 NPs surface Eqs. (15.8). In aerated systems, oxidative species such as O2 and H2O2 are produced at the reduction site (Janus and Zaja˛c, 2016; Chen et al., 2005): G

TiO2 1 hv ! h1 1 e2

(15.1)

OH ! TiðIVÞ OH∙1 ! fTiðIVÞ OH∙ g1 ! TiðIVÞ O∙ 1 H1

(15.2)

h1 1 e2 ! heat

(15.3)

e2 1  TiðIVÞ O∙ 1 H1 ! TiðIVÞ OH

(15.4)

h1 1 RedðorgÞ ! OxðorgÞ

(15.5)

 TiðIVÞ O∙ 1 RedðorgÞ ! OxðorgÞ

(15.6)

h1 H2 O ! H2 O∙1 ! H1 1 OH∙

(15.7)

OH∙ 1 RedðOrgÞ ! Ox∙ðorgÞ

(15.8)

2

e2 1 O2ðadsÞ ! O∙2


2 O∙2 1 e2 1 2H1 ! H2 O2 2

(15.9) (15.10)

O∙2 1 H2 O2 ! OH∙ 1 OH2 1 O2

(15.11)

H2 O2 1 hv ! 2OH∙

(15.12)

Photocatalysts have the ability to degrade a mixture of organic compound pollutants that are harmful to both human health and the environment. The controlling degradation process includes the generation of radicals as a result of light irradiation on the photocatalyst material, which is subsequently converted from hazardous organic molecules to nontoxic products (Ballari et al., 2010; Zhong & Haghighat, 2015). Eqs. (15.13)
(15.15) provide the recognized reaction mechanism for the photocatalytic conversion of NOx compounds (Chen & Poon, 2009; Hu¨sken et al., 2009; Sikkema et al., 2014). In small amounts, NO3 is harmless and can be wiped away by water droplets (Yang et al., 2018). Fig. 15.4 (A) depicts the photocatalytic

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Figure 15.4 Schematic of NOx removal by photocatalysis concrete pavement and NOx removal pattern by photocatalytic pavement blocks obtained from laboratory test methods. Source: (A) Boonen, E., & Beeldens, A. (2014). Recent photocatalytic applications for air purification in Belgium. Coatings, 4(3), 553
573., (B) Chen, J., & Poon, C. s. (2009). Photocatalytic construction and building materials: From fundamentals to applications. Building and Environment, 44(9), 1899
1906.

reaction used to remove NOx contaminants from concrete pavements using photocatalysis. H∙2 ! NO2 1 OH∙

(15.13)

NO 1 OH∙ ! NO2 1 H1

(15.14)

1 NO2 1 OH∙ ! NO2 3 1H

(15.15)

As one of the most widely used technologies for NOx restoration (Hoekman & Robbins, 2012), photocatalytic NOx degradation has been emerged as a viable choice in recent decades, as evidenced by considerable research efforts (Ichiura et al., 2003; Zhao & Yang, 2003) and the steady growth of commercial products in the market, especially TiO2-containing cement and paints (Cucitore et al., 2011).

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Fig. 15.4 (B) depicts a typical NOx reduction by photocatalytic paving blocks based on laboratory evaluations. Photocatalysts like TiO2 are easily integrated into cementitious materials and paints, resulting in air-purifying surfaces in a variety of systems and infrastructures, like pavement blocks, filters, and membranes for indoor/ outdoor air purification, etc. (Sikkema, 2013). It is essential to state that the photocatalysis process is dependent on the following factors: photocatalyst type and concentration, kind of removed contamination, energy, and light intensity. All of these factors must be considered to assess the best possible conditions for performing the photocatalytic operation. Most studies are based on air purification in modified concrete. Nitrogen oxide reduction, VOCs, and self-cleaning characteristics, in particular, were investigated. As the concrete matrix is porous and absorbs toluene gas on the top, TiO2 and the concrete sample play critical roles. For improving the adherence and photocatalytic activity of TiO2-coated concrete, SiO2 is used as a binder between concretes and TiO2 NPs. In VOCs, toluene gas degradation by modified concrete as follows, when UV light was shone on the TiO2 and N-TiO2-SiO2 coated samples, OH radicals were generated, which interacted with the phenyl ring of toluene and numerous intermediates have been produced during the reaction, such as phenol, benzaldehyde, or benzoic acid. They were finally reduced to CO2 and H2O∙ (Eskandarloo et al., 2014; Koli et al., 2019), as shown in Fig. 15.5. G

Figure 15.5 The probable mechanism of the N-TiO2/SiO2 coated concrete sample for photodegradation of toluene gas. Source: From Koli, V. B., Mavengere, S., & Kim, J. S. (2019). An efficient one-pot N doped TiO2SiO2 synthesis and its application for photocatalytic concrete. Applied Surface Science, 491, 60
66.

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15.3

Advances in Metal Oxides and Their Composites for Emerging Applications

Preparation of photoactive concrete surface

Concrete processing strategies are categorized into three groups: Fig. 15.6. The first method includes applying a thin coating of TiO2 compounds, such as paints or TiO2 suspensions, to the concrete Fig. 15.6 (i). The second method is to apply a thick coating of photoactive concrete to the concrete Fig. 15.6 (ii). The third method is to utilize concretes with varying amounts of TiO2 in the mass (substituted cement) (Fig. 15.6 (iii)]. Fig. 15.7 illustrates a potential process for substantial alteration using photocatalysts.

15.3.1 Method (i) In a previous study, we reported the coating of a thin layer of N doped TiO2/SiO2 on the concrete surface for photocatalytic degradation of Methylene blue dye and toluene gas (Koli et al., 2019). N doped TiO2/SiO2 sol spray-coated on the concrete surface and calcined at two different temperatures, 180 C and 450 C. Compared to

Figure 15.6 Scheme of possible methods for concrete modification by photocatalysts. Source: Data from Janus, M., & Zaja˛c, K. (2016). Concretes with photocatalytic activity, High Performance Concrete Technology and Applications (pp. 141
160).

Figure 15.7 TiO2-based photocatalytic material on roadway surfaces to reduce air pollution (A) application of the coating; and (B) finished roadway, with the coated surface showing a lighter color. Source: From Fujishima, A., Zhang, X., & Tryk, D. A. (2008). TiO2 photocatalysis and related surface phenomena. Surface Science Reports, 63(12), 515
582; Fujita Road Construction Co., Ltd.

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the uncoated concrete sample and the 180 C calcinated sample, the sample calcinated at 450 C has the highest photodegradation efficiency.

15.3.2 Method (ii) This concrete is split into two layers: the bottom layer is unmodified concrete, and the top layer is cement containing TiO2. For instance, the amount of TiO2 utilized in the top layer varies. Folli et al. (2015) used concrete with a density of approximately 40 kg m23. Monthly average NO concentrations around the photocatalytic zone were roughly 22% lower than the references during optimal environmental and irradiation circumstances, that is summer months. Fiore et al. (2013) reported that at a thickness of 3 and 5 mm, concrete covered with photocatalytic cement mortar was examined. The experimental results show that employing photocatalytic surface layers reduces the depth of concrete carbonation substantially. The findings also demonstrate that using titanium dioxide to modify cementitious materials on the outer surface of reinforced concrete components improves the corrosion efficiency of reinforcing bars in the presence of concrete carbonation.

15.3.3 Method (iii) Concretes of varying weight percentages of TiO2 in the mass (TiO2 substituted cement) Titanium dioxide modified concrete in mass replacement cement is used in these examples. Typically, commercial titanium dioxide, such as P25 (Evonic) or PC-105, is utilized (Millennium). The percentages of photocatalysts were 0.5%, 1%, 2.5%, 5%, and 10% by weight

15.4

Properties of photoactive self-cleaning concretes

The addition of TiO2 NPs to concrete specimens will contribute to the self-cleaning properties of concrete. Pollutants from factories and automobiles (e.g., VOCs, CO, NOx, aldehydes, and chlorophenols) may be photo catalytically decomposed in concrete containing these NP. This influence, however, becomes less effective with aging due to carbonatation (Zhang et al., 2015; Chen, 2018). Fig. 15.7 TiO2-based photocatalytic material on roadway surfaces to reduce air pollution. This is owing to the existence of a thin coating of TiO2 on the concrete floor, which may deliver active oxygen when exposed to UV radiation from the sun. As a result, it catalyzes organic matter oxidation on the TiO2 NPs-coated concrete surface (Singh et al., 2017). Rainwater is used to scrub the concrete floor, which helps to avoid dirt accumulation. Another significant feature of TiO2 NPs is their chemical stability and low cost as compared to other materials. Furthermore, TiO2 NPs will improve the resistance of cement-based structures to water permeability (Silvestre et al., 2016). Fig. 15.8 (A
D) depicts a scanning electron microscopy image of the surface

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Figure 15.8 SEM image of the fracture surfaces considering: (A) control cementitious composites (1200 3 ); (B) TiO2 nanoparticles (NPs) (50 nm size) modified cementitious composites (1200 3 ); (C) TiO2 NPs (500 nm size) modified cementitious composites (1200 3 ); (D) aggregations of TiO2 NPs (50 nm size) in cementitious composites (1200 3 ). Source: (A
D) Li, Z., Han, B., Yu, X., Zheng, Q., & Wang, Y. (2019). Comparison of the mechanical property and microstructures of cementitious composites with nano- and microrutile phase TiO2. Archives of Civil and Mechanical Engineering, 19(3), 615
626. (E
H) Wang, L., Zhang, H., & Gao, Y. (2018). Effect of TiO2 nanoparticles on physical and mechanical properties of cement at low temperatures. Advances in Materials Science and Engineering, 2018.

defects of cementitious composites containing two sizes of TiO2NPs (Li et al., 2019) studied cement mortar specimens’ mechanical and physical properties with varying TiO2 NP contents at curing temperatures of 0 C, 5 C, 10 C, and 20 C. They utilized natural river sand, Portland cement, and TiO2 NPs with a diameter of 15 nm. TiO2 NPs doses of 1%, 2%, 3%, 4%, and 5% by cement weight were employed in the experiments, respectively. The TiO2 NPs were dispersed in water during the specimen fabrication process using ultrasonication. Following that, cement and sand are combined for 1 minute. The well-dispersed TiO2 NPs were then applied and stirred for 60 seconds after the water was integrated. The mortars are then collocated into molds and cured at various temperatures in the following step. A water to binder ratio of 0.5 was used for the specimens. Fig. 15.8 (E
H) shows SEM. photos of cement pastes containing 2wt.% TiO2 NPs after 28 days of curing at various temperatures. The compressive strength characterization is performed in accordance with ASTM C109 (C109
93, 2007), A hydraulic measurement machine and a balanced load of 1350 N s21 was used. The flexural stress test was carried out in accordance with ASTM C293 standards (C293/C293M
10, 2007). At healing ages of 3, 7, 28, and 56 days, this characterization is calculated. The compressive and flexural strength responses of cement mortar samples are shown in Fig. 15.9 (A and B). Both compressive and flexural strength decreased at low curing temperatures. In contrast, the flexural and compressive strength of

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Figure 15.9 (A) Compressive strength of cement mortar samples incorporating different TiO2 NPs dosages (B) Flexural strength of cement mortar samples containing different TiO2 NPs dosages. Source: From Wang, L., Zhang, H., & Gao, Y. (2018). Effect of TiO2 nanoparticles on physical and mechanical properties of cement at low temperatures. Advances in Materials Science and Engineering, 2018.

mortar specimens containing TiO2 NPs increased fast when compared to conventional mortar until the TiO2 NPs content reached up to 2 wt.%. This increase slowed for TiO2 NPs doses greater than 2wt.%. TiO2 NPs facilitated cement hydration and filled holes in C
S
H gels, resulting in enhanced mortar sample power. This nanoparticle has a high surface-area-to-volume ratio, allowing for a larger surface area to precipitate hydration materials. TiO2 NPs also establish a connection with C
S
H gel, which improves their strength. (Feng et al., 2013) the microstructures of TiO2 NP-containing concrete matrices, as well as the mechanical characteristics of cement paste, were investigated. Fig. 15.10 depicts a TEM picture of TiO2 NPs with electron diffraction in one area. After 28 days, the addition of TiO2 NPs (0.1%, 0.5%, 1.0%, and 1.5% by cement weight) to cement paste with a water-cement ratio of 0.4 enhanced flexural strength (4.52%, 8.00%, 8.26%, and 6.71%). Jalal et al. (2013) the characteristics of high resistance self-compacting concrete incorporating fly ash and TiO2 NPs were investigated. They utilized Portland cement to replace up to 15% of the weight of waste ash and up to 5% of the weight of TiO2 NPs. The addition of TiO2 NPs to the concrete improved its quality and reduced the likelihood of segregation. TiO2 NPs dramatically decrease water absorption and capillarity. Weight losses in concrete samples were driven by the fast production of hydrated products. The self-compacting concrete containing TiO2 NPs exhibited a more refined microstructure, which enhanced its mechanical resistance. Yu et al. (2018) reported TiO2 NPs were used to increase the mechanical strength of concrete microstructures. TiO2 NP accelerate the decomposition of hazardous gases in the environment. Furthermore, the TiO2 NP-enhanced concrete exhibited a 7% greater total compressive strength than the bare concrete. Furthermore, Yu et al. (2018) studied temperature variations that might produce cracks and accelerate the hydration process. TiO2 NP were used to test the toughness of ultra-high-performance concrete. The addition of 1% TiO2 NPs to concrete improved its mechanical characteristics. They examined dry shrinkage, carbonation

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Figure 15.10 TEM image of the morphology of the TiO2 nanoparticles and their selected area electron diffraction (SAED). Source: From Feng, D., Xie, N., Gong, C., Leng, Z., Xiao, H., Li, H., & Shi, X. (2013). Portland cement paste modified by TiO2 nanoparticles: A microstructure perspective. Industrial and Engineering Chemistry Research, 52(33), 11575
11582.

resistance, freeze-thaw resistance, and chloride ingress resistance. When TiO2 is added to concrete, it may self clean and function as a photocatalyst. Furthermore, TiO2 NPs in ordinary concrete can minimize capillary porosity (Chunping et al., 2018). Previous investigations on TiO2-modified cement material mechanical characteristics have revealed that adding TiO2 NPs to cement increases the mechanical properties of cement mortar (Khataee et al., 2013). Compressive strength, on the other hand, increases by 1% as the amount of TiO2 NPs increases. This is attributed to TiO2 NPs’ pozzolanic activity in hydrated cement, which resulted in the modified concrete’s enhanced compressive strength. According to a study on the effect of nanomaterials as cement substitutes on the physical properties of concrete, the compressive strength of concrete and cement was decreased by utilizing a variety of nanoproducts that have been widely used as additives and admixtures in published studies, as shown in Table 15.1. According to published studies on mechanical properties, TiO2 NPs can improve compressive strength by up to 1% when replacing cement. The key explanation for the rise in compressive strength is that voids in the mortar were filled with small TiO2 NPs, which enhanced the strength. Furthermore, the cement particles are protected by an excess of TiO2 NPs, disrupting the water-cement reaction, resulting in a drop-in strength with further increment. As seen in Fig. 15.11, the compressive

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Table 15.1 Compressive strength test of cement cube (MPa). Day

Normal concrete morter

Nanoalumina morter

Nanotitania morter

Nanosilica morter

7 14

22.069 32.102

16.050 20.731

12.049 18.725

7.356 16.719

Figure 15.11 Comparison of compressive strength with the replacement of cement by titanium dioxide (Days 28). Source: Data from Zailan, S. N., Mahmed, N., Abdullah, M. M. A. B., Sandu, A. V., & Shahedan, N. F. (2017). MATEC Web of Conferences, 97.

strength was compared to that of cement substituted with titanium dioxide (1%, 2%, or 3%) that cured in 28 days. The author proposes to extend this research to other concrete characteristics by changing the titanium dioxide particle size and the concrete flexural strength grade. Flexural strength is used to evaluate the tensile strength of concrete that can survive bending failure (Khataee et al., 2013). It has been stated that raising the volume of TiO2 NPs by up to 1% improves flexural strength because the reaction between TiO2 NPs and calcium hydroxide is quick. These processes happen during the hydration of the cement, resulting in a denser foundation. According to published research on the compressive, tensile, and flexural characteristics of cement mortar incorporating NP, Al2O3 NPs (1% and 3%) have stronger mechanical properties than regular cement mortar (Rao et al., 2015). Flexural strength results are weaker than compressive strength results. However, the implications of adding photocatalytic materials (TiO2 NPs) are less critical.

15.5

Photocatalytic activity testing methods

15.5.1 Self-cleaning test A self-cleaning test was used to assess the efficiency of photoactive concrete samples. Previous studies had described the photodegradation of methylene blue (MB) solution to evaluate the photocatalytic capabilities of self cleaning on the surface of

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Figure 15.12 (A) Photocatalytic effect of the self-cleaning concrete on the M.B. solution with the exposing time, (B) Photodegradation of MB dye under UV light for the N-TiO2-SiO2 CS. Source: (A) Shen, W., Zhang, C., Li, Q., Zhang, W., Cao, L., & Ye, J. (2015). Preparation of titanium dioxide nano particle modified photocatalytic self-cleaning concrete. Journal of Cleaner Production, 87(1), 762
765. (B) Koli, V. B., Mavengere, S., & Kim, J. S. (2019). An efficient one-pot N doped TiO2-SiO2 synthesis and its application for photocatalytic concrete. Applied Surface Science, 491, 60
66.

concrete when exposed to UV light. As seen in Fig. 15.12 (A), the color of the solution indicates that self-cleaning concrete has a photocatalytic impact on the degradation of MB blue over time. As the duration of light irradiation increases, the rate of degradation increases. This demonstrates TiO2 NP’s promise as a photocatalyst for building materials (Shen et al., 2015). In earlier work, we covered a concrete surface with N doped TiO2/SiO2 and examined the degradation of MB dye under visible light irradiation, as shown in Fig. 15.12 (B). Apart from M.B. degradation, another published study has established the efficacy of self-cleaning mortars by monitoring the discoloration of organic dyes Rhodamine B (RhB).

15.5.2 Depollution testing In earlier work, we coated concrete samples with TiO2 NP and TiO2/SiO2 and investigated the photocatalytic degradation of toluene gas in a 2 L Teflon bag exposed to UV light. The photocatalytic performance of TiO2 and TiO2
SiO2 coated concrete samples were evaluated, with photodegradation efficiencies of 57% and 69% after 90 minutes of UV light irradiation, respectively. Additionally, when N doped TiO2/ SiO2 coated concrete samples were evaluated for photodegradation of toluene gas, they exhibited good photodegradation behavior when exposed to UV light, as seen in Fig. 15.13 (A) and kinetics study shown in Fig.15.13 (B). Krishnan et al. (2013) reported the photocatalytic degradation of organic pollutants under light irradiation with photoactive Cementous substrate. Yang et al. (2018) discovered that supported TiO2 shows enhanced NOx (De-NOx) degradation at a rate approximately 9 times

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Figure 15.13 N-TiO2
SiO2 coated concrete sample irradiated under UV light (A) Photocatalytic degradation of toluene gas (B) Pseudo-first-order kinetics of toluene gas degradation. Source: From Koli, V. B., Mavengere, S., & Kim, J. S. (2019). An efficient one-pot N doped TiO2-SiO2 synthesis and its application for photocatalytic concrete. Applied Surface Science, 491, 60
66.

that of TiO2 powder scattered in the mortar and approximately 150 times that of TiO2 in a conventional photocatalytic mortar (with 5% loading).

15.6

Advantages and disadvantages of self-cleaning concretes

As the temperature rises in densely populated cities, pollution results from air emissions and chemical reactions. The objective of self-cleaning concrete is to use materials that are cool in the sun and have a solar reflection index (SRI) of at least 29. The SRI value of ordinary Portland cement is approximately 35, whereas the SRI value of new white cement concrete is around 86. It has been observed that concrete containing TiO2 has a higher SRI value over a much longer period of time ´ ci´c et al., 2017). Building energy simulations utilizing computer mod(Topliˇci´c-Curˇ els will illustrate the benefits of TiO2 cement concrete by comparing the dynamic light reflection and emission rates of TiO2 cement concrete to concrete manufac´ ci´c et al., 2017). Outstanding accomtured using standard procedures (Topliˇci´c-Curˇ plishments in terms of reducing air pollution. Organic and inorganic pollutants that contribute to air pollution are removed by utilizing TiO2-containing cement in concrete. Longevity without the application of protective coating materials, TiO2-containing cement-treated concrete retains its durability for an extended length of time. Titanium dioxide adds to the development of white streaks on self-cleaning concrete surfaces. Because chemical reactions require light, it is not suitable for indoor ´ ci´c et al., 2017). use (Topliˇci´c-Curˇ

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Advances in Metal Oxides and Their Composites for Emerging Applications

Self-cleaning photoactive concrete in real-world applications

Implementation of laboratory-obtained goods onto the real-world market is based on several parameters. Geometrical conditions, traffic speed, wind speed and direction, temperature, humidity, etc affect the final reduction in the rate of contaminants. The gaseous exhaust emissions must remain in contact with the photoactive surface for a set amount of time. Furthermore, the real-world environments necessitate additional dimensions of the new products related to their various applications. For example, in the case of concrete paving bricks, TiO2 is applied over the entire thickness of the paver’s wearing sheet. This ensures that even though the surface is abraded by traffic, to maintain photocatalytic activity, additional TiO2 would be present. Another possibility is to utilize a double-layered concrete with TiO2 added to the bulk and dispersed over the surface (Jimenez-Relinque et al., 2015). The first real-world application of self-cleaning cementitious materials through TiO2 occurred in 1996, during the construction of Rome’s church Dives in Misericordia. The project was completed in 2003 by Italcementi SpA, an Italian cement manufacturer (Richard Meier, architect). Fig. 15.14 Misericordia shows the project. After six years of monitoring, it was discovered that there was only a slight difference between the white exterior and the inside walls. In Belgium, attempts were made to incorporate science community realizations (Biolzi et al., 2013). On the sidewalls and top of Brussels’ Leopold II tunnel, photocatalytic cementitious materials were employed. Fig. 15.16 illustrates the object’s state prior to and during photocatalytic restoration. The photocatalytic materials were employed up to a length of 100 m. Within the tube, the effect on air pollution

Figure 15.14 “Dives in Missericordia” Church-Rome (1996). Source: From Church of 2000/Richard Meier & Partners. (2005). https://www.archdaily.com/ 20105/church-of-2000-richard-meier.

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(NOx, VOCs, CO2, O3, etc.) was detected. Within the tube, a specialized UV illumination system was built that could be adjusted (on/off) to view the photocatalytic walls in detail Fig. 15.15 (A and B). Folli et al. (2015) reported on the outcomes of a field research study in Denmark using photocatalytic pavement components to minimize NOx emissions. The laboratory trials came first, followed by the large-scale studies. The research facility was located on a key Copenhagen street near the Central Railway Station. On both sides of the roadway, there were 200 meters of sidewalk pavers. A hundred meters are built with regular concrete blocks, and another hundred meters are built using titanium dioxide as a photocatalyst. Over the course of the year, the regular 154 High-Performance Concrete Technology and Applications average NO concentration in the region paved with TiO2-containing concrete components was kept to very low levels (below 40 ppb). The discovery of seasonal fluctuation is significant. Because water and NO compete for catalytic sites, NO conversion is reduced as relative humidity increases. Meanwhile, NO conversion increased as temperature

Figure 15.15 Inside view of test site within Leopold II tunnel in Brussels (A) before renovation, (B) after renovation with using photocatalytic walls. Source: From Boonen, E., & Beeldens, A. (2013). Photocatalytic roads: From lab tests to real scale applications. European Transport Research Review, 5(2), 79
89.

Figure 15.16 Cite´ de la Musique et des Beaux- Arts in Chambe´ry. Source: Data from (A) https://www.lofficiel.net/cite-des-arts_8_7409.aspx. (B) https://i.pinimg. com/originals/ba/82/95/ba82952ba1b4abd898813cb8d6e333eb.png.

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increased due to increased gaseous pollutant diffusivity against the photocatalytic surface. Ballari and Brouwers (2013) have presented their findings from a full-scale demonstration of air purification pavement. Concrete pavement containing TiO2 was constructed throughout the whole width of a 150-m section of the lane in Hengelo, the Netherlands. The NOx concentrations in the experimental and control streets, as well as meteorological conditions, were determined. The findings were corroborated by the weather circumstances. Additionally, the NOx concentration was 19% lower than that of the reference system. When just afternoons or periods of high radiation and low relative humidity were considered, the value was found to be 28% or 45% lower than in the reference scenario, respectively. The suggested techniques are promising for lowering a wide range of air pollutants, particularly at high-pollution areas such as heavily frequented canyon streets or road tunnels. In white concrete systems, self-cleaning components are commonly employed. The Cite´ de la Musique et des Beaux-Arts in Chambe´ry, seen in Fig. 15.16, is another example of a photocatalytic cementitious material-built building. Approximately five years of monitoring indicated that the primary hue of the Chambe´ry City Hall stayed nearly unaltered in various facade positions (on West, North, East and South). By 2003, self-cleaning TiO2-based tiles had been deployed in over 5000 buildings in Japan, according to Fujishima and Zhang (2006). The Maru Building, located in Tokyo’s major commercial sector, is the most well-known. As seen in Fig. 15.17, there are numerous additional examples in the world.

Figure 15.17 (A) MSV Arena Football Stadium-Germany, 2004 (B) Bienvenue a Ciments du Maroc’-Morocco, 2005, (C) Manuel de Gonzalez Hospital-Mexico, (D) Tupras Refinery -Kocaeli-Turkey, 2014. Source: (A) MSV Arena Football Stadium-Germany, 2004 in. https://www.alamy.com/stockphoto/msv-arena-stadium.html (B) Bienvenue a Ciments du Maroc’-Morocco, 2005, in. https://www.cimentsdumaroc.com/fr, (C) Manuel de Gonzalez Hospital-Mexico, in. https://www. centreforpublicimpact.org/case-study/mexico-citys-manuel-gea-gonzalez-hospital-facade (D) Tupras Refinery -Kocaeli-Turkey, 2014, in. https://www.tupras.com.tr/uploads/cr_en/1.pdf.

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541

Market status of photoactive materials

The global photocatalyst market was valued at almost US$ 2.5 billion at the end of 2018. The photocatalyst market is anticipated to reach US$ 6 billion by the end of the forecast period, 2029, with an annual growth rate of over 10%. Rising demand for low VOCs coatings in the architecture and construction industries would boost demand for photocatalysts with excellent properties such as organic compound decomposition. As a result of the strengthening economy, Europe is spending a lot of money on the construction industry, which is projected to drive market growth over the forecast period (Photocatalyst Market Size, 2018). Due to rising concerns about building upkeep and cleanliness, the self-cleaning application is anticipated to develop at a 12.0% compound annual growth rate (CAGR) through 2025. This application is now widely used in the building and construction sector, and its use is projected to increase as the industry’s need grows. As a consequence, as shown in Fig. 15.18 A and B, the market for photocatalysts is likely to increase throughout the forecast years. Titanium dioxide and zinc oxide are both photocatalysts that can help guard against air pollutants such as formaldehyde, VOCs, bacteria, and ammonia. Additionally, these photocatalysts can be utilized in air filters, air conditioning systems, and ventilation systems. As a result of growing pollution concerns, demand is anticipated to increase. At the moment, the sector is tiny, with the bulk of producers headquartered in Japan. Increased knowledge of the technology’s benefits, as well as the development of novel photocatalytic materials, are expected to attract new market participants, therefore increasing demand across geographies. The primary market participants are TOTO Corporation, JSR Corporation, KRONOS Worldwide, Inc., CRISTAL, TiPE, and OSAKA Titanium Technologies

Figure 15.18 (A) Japan photocatalyst market revenue by material from 2014 to 2025. (B) photocatalyst market revenue by application. Source: Photocatalyst market size, share & trends analysis report by material (titanium dioxide, zinc oxide), by application (self-cleaning, air purification, water purification), by region, and segment forecasts, 2018
2025. (c.2018). (2018).https://www.grandviewresearch. com/industry-analysis/photocatalyst-marke.

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Co., Ltd. These firms compete on the basis of price and geographic reach. Owing to the country’s huge concentration of manufacturers, competition in Japan is far more intense than in other areas (Photocatalyst Market Size, 2018).

15.9

Summary and conclusions

The most recent advances in TiO2 NP-based concrete have been thoroughly evaluated. The fundamental mechanism of photocatalytic oxidation semiconductor photocatalyst materials is explored. Furthermore, the self-cleaning and depolluting properties of photocatalysis materials are thoroughly investigated. The three fundamental processes for producing TiO2 NP-based concrete are extensively described. The influence of TiO2 concentration on different concrete qualities, including mechanical and chemical properties, has been thoroughly studied. The photocatalytic behavior of self-cleaning and depollution testing has been extensively researched utilizing a variety of situations such as MB dye degradation, RhB dye degradation, NOx oxidation, toluene gas degradation, and so on. The benefits and drawbacks of photoactive concrete based on TiO2 are also highlighted. Finally, there are numerous examples of self-cleaning concrete utilized in the construction of several distinctive worldwide buildings. When studying the literature on self-cleaning concretes that have been emerged as a result of advances in concrete production, it is apparent that the technologies have greatly improved. According to current understanding, adding photocatalysts TiO2 NPs to cementitious materials as a filler or as a partial replacement for the cement enhances self-cleaning behavior. Photocatalysts may also be utilized to eliminate smells and improve interior air quality, making communities more secure and appealing. As a result of the deposition of organic waste and chemicals in the air, air pollution is now causing the exterior deterioration of buildings. Environmentally friendly building materials will help to reduce air pollution, which is one of the most significant issues. Self-cleaning concrete has the potential to keep communities safe by eliminating pollutants. Self-cleaning concrete has been shown to lower air pollution by 30%
40%.

15.10

Future prospects

In terms of economic impact and environmental protection, one of the most critical aspects of photocatalytic research is the development of usable structural materials with outstanding photocatalytic activity. The development of visible light-responsive photocatalysts would be a significant technological improvement in photocatalysis structural materials (Vittoriadiamanti & Pedeferri, 2013). Metal and nonmetal doping reduce the bandgap in TiO2. It promotes photocatalytic process activation in the presence of visible light. Furthermore, to expand the practical uses of photocatalysis building materials, their efficiency and workability must be improved. One potential strategy in this respect is to increase the specific surface area of the

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photocatalysts (Zhong & Haghighat, 2015). Long-term photocatalytic structural material efficiency remains a challenge, necessitating additional groundbreaking research to (1) reduce electron–hole recombination, (2) increase active sites on the photocatalyst surface, (3) control TiO2 dispersion within the cement matrix to achieve optimal cluster size and porosity and maximize photocatalytic activity, (4) create more efficient photocatalysts by mixing anatase and rutile, and (5) enhance cement pore structure optimization. More research is required to evaluate the breakdown performance of air pollutants and the durability of the photocatalyst. It is critical to examine the byproducts of the photocatalytic process as well as any potential detrimental health consequences. The presence of a photocatalyst affects the microstructure of the cement composite, and the long-term durability of concrete buildings is a major study subject. Undoubtedly, the use of energy will have an impact on the future of construction materials. Furthermore, for commercialization, the photocatalysis cement technique must be combined with other novel cement technologies. In one case, researchers are attempting to include functional cementitious materials into 3D printing processes and to enhance process parameters as a result of the incorporation of these multifunctional cementitious materials (Hamidi & Aslani, 2019). The efficacy of incorporating TiO2 into other environmentally acceptable cementitious materials, such as magnesium phosphate cement (Haque & Chen, 2019), is also investigated, and this will be considered as an environmental friendly option for the building sector. Undoubtedly, the commercialization of multifunctional structural materials is very likely in the near future.

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90. Available from https://doi.org/10.1016/S1672-6529(11) 60100-5. Zhao, N., Wang, Z., Cai, C., Shen, H., Liang, F., Wang, D., Wang, C., Zhu, T., Guo, J., Wang, Y., Liu, X., Duan, C., Wang, H., Mao, Y., Jia, X., Dong, H., Zhang, X., & Xu, J. (2014). Bioinspired materials: From low to high dimensional structure. Advanced Materials, 26(41), 6994
7017. Available from https://doi.org/10.1002/adma.201401718. Zhong, L., & Haghighat, F. (2015). Photocatalytic air cleaners and materials technologies Abilities and limitations. Building and Environment, 91, 191
203. Available from https://doi.org/10.1016/j.buildenv.2015.01.033.

Further reading Chen, Y. (2018). A review on the effects of nanoparticles on properties of self-compacting concrete. In IOP Conference Series: Materials Science and Engineering (Vol. 452, Issue 2). Institute of Physics Publishing. https://doi.org/10.1088/1757-899X/452/2/ 022134 Chen, Y., Yang, S., Wang, K., & Lou, L. (2005). Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of acid orange 7. Journal of Photochemistry and Photobiology A: Chemistry, 172(1), 47
54. Available from https:// doi.org/10.1016/j.jphotochem.2004.11.006. Ichiura, H., Kitaoka, T., & Tanaka, H. (2003). Photocatalytic oxidation of NOx using composite sheets containing TiO2 and a metal compound. Chemosphere, 51(9), 855
860. Available from https://doi.org/10.1016/S0045-6535(03)00049-3. Zhang, R., Cheng, X., Hou, P., & Ye, Z. (2015). Influences of nano-TiO2 on the properties of cement-based materials: Hydration and drying shrinkage. Construction and Building Materials, 81, 35
41. Available from https://doi.org/10.1016/j.conbuildmat.2015. 02.003. Zhao, J., & Yang, X. (2003). Photocatalytic oxidation for indoor air purification: A literature review. Building and Environment, 38(5), 645
654. Available from https://doi.org/ 10.1016/S0360-1323(02)00212-3.

Metal oxide nanocomposites: design and use in antimicrobial coatings

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Vijay S. Ghodake1, Shamkumar P. Deshmukh1,2 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, D.B.F. Dayanand College of Arts and Science, Solapur, Maharashtra, India

16.1

Introduction

According to WHO, infectious disease is one of the leading causes of different pandemics and epidemic disease outbreaks like COVID-19, Ebola virus, Chikungunya, Yellow fever, Cholera, etc., because of a variety of pathogenic microorganisms. As an example of COVID-19 infections, more than nine crore people are suffering from coronavirus infections, and hence around two crores of people have been dead in a very short period. As with the COVID-19 infections, the other microbial infections are the most common threats globally, which also would be an ineffective strategy to become a superpower among other countries. Though such microbial infections occur naturally or artificially, it is a question of debate whether ordinary people want to control the microbial infections for an appropriate sustainable life (Mallakpour et al., 2021). In addition, superbugs and mutation of microbes are the challenges for controlling these microbial infections (Geisinger & Isberg, 2017). The center for disease control and prevention reports that around two million people per year are suffering from superbug infections in the USA, and out of which, more than twenty-three thousand patients die annually due to resistant microorganisms (McGuire, 2013). In connection to these challenges, the World Health Assembly and policymakers are inspired to develop the proper solutions for controlling infection-causing microbes. Among the various approaches, the use of efficient antimicrobial agents or their different formulations is the promising one for bacteriostatic or bactericidal strategies. Antimicrobial agents are chemical agents which kill or inhibit the growth of microbes. Based on their composition, these agents are broadly classified as inorganic and organic agents. Basically, organic agents have been utilized as effective materials to inhibit bacteria (Jing et al., 2011). Concerning these agents, β-lactam (like penicillin), cephalosporin or carbapenem, aminoglycosides, and sulfonamides are the commonly used antibiotics for controlling infectious diseases. In the USA, infectious diseases are controlled with limited use of antibiotics, while in developing countries like India, excessive use of antibiotics is the common solution for controlling microbial infections. In addition, organic antiseptic agents like chloroxylenol, triclosan, chlorhexidine, etc. as well as organic disinfectants Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00011-7 © 2022 Elsevier Inc. All rights reserved.

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like benzalkonium chloride, phenolic compounds are usually used to kill or inhibit bacteria or fungi or sometimes a few viruses (Amin et al., 2020). Also, organic agents like polymer-based materials (chitosan, heparin, ε-polylysine, polyacrylamides, polysiloxanes, etc.) and peptides are used as antimicrobial agents. However, on the contrary, superbugs, thermal stability, toxicity, and lower life shell are major shortcomings of various organic antimicrobial agents. Similar to organic-based agents, elemental metal and its precursors have been used as antimicrobial agents since ancient times. Metals such as silver (Ag), copper (Cu), Zinc (Zn), gold (Au), etc. and metal oxides titania (TiO2), zinc oxide (ZnO), and magnesium oxide (MgO) have been used commonly as an inorganic antimicrobial agent (Deshmukh et al., 2019; Gharpure et al., 2020). Owing to the introduction of the nano-dimensional concept, these inorganic agents have received more attention as effective antimicrobial agents due to their overriding characteristics features. However, the bare metal or metal oxides in bulk or nanoscale have certain limitations in antimicrobial activity as well. To boost the properties further, the formation of nanocomposites is one of the promising ways, and hence synergetic of the components would uplift the antimicrobial activity against the microbes. The antimicrobial properties of the nanocomposites rely on the size, dimension, morphology, functionalities present on the surfaces, and release of ions of the combined components (Kro´l et al., 2017). In addition, the antimicrobial activity also depends upon other preparative parameters such as pH of the solution, culture medium, hydrophilicity, composition and amount of nanocomposites, illumination time, photons used, etc. (Fontecha-Uman˜a et al., 2020). After the proper testing of antimicrobial activity of nanocomposites, these would be further used in coatings for various applications. Among different uses, antimicrobial coating emerges as the new and efficient strategy in the various industrial sectors, such as food, biomedical, healthcare, textile, paint, etc. Therefore, various aspects related to antimicrobial coatings are discussed in this chapter. In addition, the state-of-the-art of functional nanocomposites-based antimicrobial coatings in the various industrial sectors such as the hospital, textile, paint, food, and polymer sectors, as well as future perspectives have been highlighted as well.

16.2

Microbes and microbial infectious diseases

This chapter focuses on metal oxides-based nanocomposites as antimicrobial coatings for use in different sectors. Therefore, it is necessary to learn about microbes and the various infectious diseases caused by them. A class of living microorganisms is called microbes; they may exist in a unicellular and multicellular state. It consists of viruses, bacteria, fungi, archaea, algae, and protozoa (Ehrlich, 1997; Gulrajani et al., 2008). The family of fungi includes yeasts (unicellular), smuts, rusts, mildews, molds, and mushrooms (multicellular). These microbes co-exist in the immediate surroundings of human beings as well as inside the bodies of living organisms with harmony (Gerba et al., 1996). Although it is conceived that the majority of microbes are pathogenic and exert harmful ill-effects on health. But, in reality, the majority of them are completely harmless (Hayat et al., 2010). For example, some microbes even help us in

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body functions like digestion (Escherichia coli) and preparation of nutrient-containing food such as bread (Saccharomyces cerevisiae) (Corsetti et al., 1998), citric acid (Aspergillus niger) (Eikmeier & Rehm, 1984), riboflavin (Eremothecium sp.) and even has medicinal uses such as antibiotic isolation (Adeleye et al., 2004). Even some bacteria do not cause fatal or high-risk diseases but are associated with unpleasant smells and overall bad hygiene (Ara et al., 2006). The human skin itself is home to a microbiota of diverse species ranging from bacteria and viruses to yeasts, prokaryotes, and fungi, etc. in their acceptable values. Beyond the acceptable values, these may cause different infections as well as infectious diseases. These microbes usually enter our bodies through the mouth, nose, eyes, or urogenital openings, or wounds, or bites that breach the skin barrier and hence may result in different infectious diseases. Table 16.1 shows the different types of microbes with their respective diseases. Table 16.1 Types of microbes with their respective disease. Sr. No.

Microbes

Infectious disease

Symptoms

1

Coronavirus

COVID-19

2

Influenza virus

Flu

3

Human immunodeficiency virus

4

Varicella-zoster virus

Acquired immunodeficiency syndrome [Contagious disease] Chickenpox

5

Respiratory syncytial virus

Shortness of breath, chest pain or pressure, loss of speech or movement, fever, dry cough, and tiredness Shortness of breath, chest pain or pressure, loss of speech or movement, fever, dry cough, and tiredness Fever, chills, rash, night sweats, muscle aches, sore throat, and fatigue Fever, feeling tired, headache, and stomachache Cough, fever, shortness of breath

Virus

Viral pneumonia

Bacteria 1

Streptococcus pneumoniae (pneumococcus)

Bacterial pneumonia

Cough, shortness of breath, rapid, shallow breathing, loss of appetite, low energy, and fatigue (Continued)

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Table 16.1 (Continued) Sr. No.

Microbes

Infectious disease

Symptoms

2

Vibrio cholerae bacteria

Cholera

3

Mycobacterium leprae

Leprosy

4

Bacterium Neisseria gonorrhoeae

Gonorrhea

5

E. coli

Urinary tract infections [bladder infection]

Vomiting, thirst, leg cramps, restlessness, or irritability Thick, dry skin, Loss of eyebrows, and growths (nodules) on the skin Greater frequency of urination, swelling or pain in the testicles, and swelling or redness at the opening of the penis A strong, persistent urge to urinate, burning sensation when urinating, strong-smelling urine Chills, yellowing of the skin and eyes (jaundice), nausea and vomiting, claycolored stools, and dark urine An urgent need to defecate, abdominal cramps, nausea, vomiting, and fever Fever and chills, very low body temperature, peeing less than usual, fast heartbeat, nausea and vomiting, and diarrhea Swelling, tenderness, pain, warmth, fever, and red spots Furuncles start as red, tender lumps, These fill with pus, grow, then rupture, and drain

Cholangitis

Traveler’s diarrhea

6

Staphylococcus aureus

Bloodstream infections

Cellulitis

Furuncles

(Continued)

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Table 16.1 (Continued) Sr. No.

Microbes

Infectious disease

Symptoms

7

Yersinia pestis

Plague

Headache, chills, extreme weakness, abdominal pain, and painful lymph nodes

One-sided facial swelling, headache, nasal congestion, black lesions on nasal bridge, and fever Chest pain, cough, coughing up blood, shortness of breath, and fever

Fungus 1

Mucormycetes

Mucormycosis

2

Aspergillus

Aspergillosis

3

Blastomyces

Blastomycosis

4

Candida albicans

Candida auris

5

Coccidioides

Coccidioidomycosis

1

Domoic acid (marine biotoxin)

Amnesic shellfish poisoning (ASP)

2

Gambierdiscus toxicus

Ciguatera fish poisoning

3

Dinoflagellate Dinophysis

Diarrhetic shellfish poisoning

Fever, cough, night sweats, muscle aches, weight loss, chest pain, and fatigue Chills, fever, bloodstream infections, and wound infections Fatigue, cough, fever, shortness of breath, headache, and night sweats

Algae Headache, dizziness, confusion, disorientation, and short-term memory loss Diarrhea, weakness, vomiting, numbness, itchiness, sensitivity to hot and cold, and dizziness Vomiting, nausea, abdominal pain, and diarrhea (Continued)

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Table 16.1 (Continued) Sr. No.

Microbes

Infectious disease

Symptoms

4

Karenia Brevis

Neurotoxic shellfish poisoning

5

Gonyaulacoid dinoflagellates

Paralytic shellfish poisoning

Nausea and vomiting, paraesthesias of the mouth, lips, and tongue as well as distal paraesthesias, ataxia, slurred speech, and dizziness. Vomiting, Diarrhea, nausea, abdominal pain, tingling or burning lips, gums, tongue, face, neck, arms, legs, and toes.

Protozoa 1

Plasmodium parasite

Malaria

2

Giardia duodenalis

Diarrhea

3

Phlebotomine sandflies

Leishmaniasis

4

Zooflagellate protozoans

Trypanosomes

5

Trichomonas vaginalis

Trich moniasis

High temperature, headaches, vomiting, muscle pains, diarrhea, and feeling unwell Frequent loose, watery stools, abdominal cramps, abdominal pain, fever, and bleeding Weight loss, weakness, fever that lasts for weeks or months, enlarged spleen, and enlarged liver Aching muscles, fever, severe headaches, irritability, extreme fatigue, and swollen lymph nodes Itching, burning, redness or soreness of the genitals, and discomfort with urination

Metal oxide nanocomposites: design and use in antimicrobial coatings

16.3

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Antimicrobial coatings: market scenario

As per the market reviews, the size of US antimicrobial coating present in the market size, by product, is around USD 7.9 billion in 2019 and expected to reach around USD 20.55 billion up to the end of 2027 by the CAGR (Compound Annual Growth Rate) of around 12.7% in comparison to that of last year (Fig. 16.1). [Antimicrobial Coating Market Globally]. With the emergence of COVID-19, the use of antimicrobial coatings is growing enormously at a faster pace with substantial growth rates over the last year. Most of the market available coatings have been using organic or inorganic antimicrobial additives. The inorganic agents led the market and accounted for the majority share of the global revenue in 2019 than organic-type agents. In inorganic-based agents, silver-, copper-, and zinc-based additives are highly demanded, while nano-dimensional composites would lead the market due to their extraordinary properties as well as efficient antimicrobial activity. In addition, inorganic-based additives are used commonly as antimicrobial agents owing to their properties, such as higher antibacterial activity against a wide range of microbes, non-toxicity, continual performance for a long duration, easily prepared, and high stability, etc. Particularly, copper-based agents are useful in preservative and sterilized applications, with underlying substrates or layers being paints, coatings, and polymers. Organic agents provide long-lasting protection against stain and odor-causing microbes as well as biodegradation. Major aspects to spirited

Figure 16.1 Bar diagram of antimicrobial coating market.

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the growth of inorganic antibacterial coating market size raising. researchers have been reported so far 23 commercially accessible inorganic antimicrobial coating agents and constituents against E. coli, Staphylococcus aureus, Acinetobacter baumannii, Mycobacterium tuberculosis pathogenic bacteria using a standard protocol of the International Organization for Standardization (ISO) test ISO 22196 (Molling et al., 2014). The global antibacterial market is segmented into various fields, like the paint industry, the automobile industry, the food industry, the hospital industry, building & construction, food & beverages, and textile industries, etc. Within the different endusers of the antimicrobial coating market, Fig. 16.2 shows the medical industry will remain the prominent segment due to the increasing demand for antimicrobial coatings for controlling medical issues like hospital-acquired infections, non-sterilized hospital tools, infection to patients through communicable ways, etc. Antimicrobial surface coating can reduce various communicable diseases. In the market, the various multinational companies such as AkzoNobel, BASF SE, PPG Industries Inc., Royal DSM, RPM International Inc., Dow Chemical Company, Sherwin-Williams Company, Diamond Vogel, etc. have been developed antimicrobial coatings. In addition, other producers such as Arch Lonza, AK Coatings Inc., DuPont, Nippon Paint Co. Ltd. have a key share of antimicrobial coatings in the market. Among these multinational companies (Fig. 16.3), Dow Chemical Company and Sherwin-Williams Companies having the highest share, are about 17% globally. These agents are mainly available in powder form to be useful for surface coating, which

Figure 16.2 Pie diagram of antimicrobial coating market by application.

Metal oxide nanocomposites: design and use in antimicrobial coatings

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Figure 16.3 Pie diagram of antimicrobial coating market by companies.

plays a key role in avoiding bacterial infection. The coating industry was produced 131.4 kilotons of powder for coating purposes and 154.3 kilotons for other forms of surface coating (Research, 2014). In the commercial market, the silver share is higher than that of other constituents such as copper, zeolite, titanium, and silicon.

16.4

Metal oxide nanocomposites as potential antimicrobial agents

Metal Oxide-based nanocomposites have driven effective antimicrobial agents since the last couple of years (Kanmani & Rhim, 2014). The key physicochemical properties such as stability, ease of interaction and release of ions, or the formation of reactive oxygen species (ROS), etc. of these metal oxide-based nanocomposites are superior to that of bare Metal Oxide for effective antimicrobial activity (Patil et al., 2017). In addition, the choice of these nanocomposites as an efficient antimicrobial agent also depends on the parameters, such as the ability to kill pathogens over a broad spectrum in a short time, compatibility with wide surfaces, green and nontoxic composition, etc. (Patil et al., 2017). Antimicrobial coatings of these nanocomposites have been a contemporary field with new pathways with substitutes for old-fashioned trends in the respective fields. In antimicrobial coatings, the various

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functionalities can enhance the activity with the respective surrounding (Patil et al., 2018). Researchers have developed numerous nanocomposites-based antimicrobial coatings for the various pathways. Therefore, the highlights of the representative metal oxide as a host material to be used in antimicrobial coatings are shown in the following Table 16.2. Therefore, it is realized that transition metal oxide is commonly used in antimicrobial applications due to its excellent properties. However, these bare materials are suffering from various constraints, and hence their composites with supportive materials or doping lead to the improvement of overall properties as well as the activity in the antimicrobial coatings. In addition, the tuning of these composites into nano dimensions also results in higher surface area and would enhance the antimicrobial properties as well. Taking into consideration the advantages, the representative metal oxide-based nanocomposites are highlighted in Table 16.3. Therefore, the proper surface functionality, composition, size, and morphology of the metal oxide-based nanocomposites pave the way toward the potential ability for antimicrobial activity. The key aspects of the composites such as mechanical, physical, chemical, and electrical properties are tuned in accordance with the synergetics of the materials for efficient antimicrobial activity (Kango et al., 2013). It also reveals that the lifetime of charge carriers increases in the composites to that of bare metal oxides (Bonnet et al., 2015). Recently, nanocomposites consisting of metal oxides with polymers have also been a new strategy toward implanting the few functional groups in the nanocomposites or alteration of surface properties by the fibers to ease interaction with microbes (Tamboli et al., 2013). Based on the compositions, the metal oxides based nanocomposites are broadly classified as per the following Fig. 16.4.

16.4.1 Composites of metal oxide with inorganic moieties In this class, the metal oxide combines with the different inorganic moieties like metal, or metal oxide, or carbon nanostructures, and hence this category is further divided into the following sub-types:

16.4.1.1 Metal/metal oxide composites This class of composite is formed by combining a metal with a metal oxide. The composites such as Au/TiO2, Au/ZnO, Pt/SnO2, Ag/ZnO, and Pd/CeO2, etc. are the best-known examples of metal-metal oxide composites. In the case of nanocomposites, different ways are possible for similar composite formation, which is based on the arrangement/structure of the metal oxides-metal composites. These include (1) metal-decorated metal oxides nanoarrays, such as nanosheets, nanotubes, nanowires, and nanorods (e.g., Au/ZnO, Ag/CuO, Ag/TiO2), (2) metal/metal oxide yolk/ shell nanostructures (e.g., Au/TiO2, Au/SiO2, Au/ZrO2), (3) metal/metal oxide nanostructures (e.g., Cu/CuO, Au/ZnO, Au/TiO2), (4) metal/metal oxide core/shell nanostructures (e.g., Pt/Fe2O3, Ag/Cu2O, Pd/CeO2) (Manuscript, 2017).

Table 16.2 Bare Metal oxides for antibacterial study. Metal oxide synthesis

Properties

Antimicrobial outcomes

References

Highly crystalline CuO monoclinic structure and formation of a lamellar structure, a sponge like porous structure with visible pore aperture of 1 2 2 μm.

Strongly reduced biofilm formation against the Gram (1) bacteria and also more inhibition against S. epidermidis and E. coli.

Vieillard et al. (2019)

Specific surface area of synthesized material is about 6.3 g cm3 and particle size is 30 nm. Luminol reacts with reactive oxygen species to produce a luminophore with an emission peak at B 425 nm.

Cell viability was determined by measuring the absorbance at OD 570. Toxicity of CuO against (E. coli and S. aureus). E. coli is more susceptible to CuO than S. aureus to antibacterial treatment.

Applerot et al. (2012)

The average size of CuO nanoparticles (NPs) is also necessary for their potential antimicrobial activity, as smaller nanoparticles have higher portability and ability to potentially penetrate between the bacterial cells, Crystal size is 13 nm.

Effective antimicrobial action against bacterial Strains such as Salmonella typhimurium, S.aureus K. pneumonia, Pseudomonas aeruginosa, E.coli, Enterococcus faecalis, Shigella flexneri, Proteus vulgaris, and E. faecalis.

Halbus et al. (2019)

CuO Wet Chemical Method: 0.2 M copper (II) sulfate anhydrate under vigorous stirring at 50 C 1 15 mL of NaOH (1 M) under continuous stirring at 75 C for 4 h 1 black precipitate cooled, washed with deionized water, filtered, and then dried at 80 C Sonochemical method: copper (II) acetate dehydrated 1 100 mL of 10% v/ v water N,N-dimethylformamide 1 Irradiated with a high intensity ultrasonic horn 1 flow of argon at room temperature for 3 h 1 Products obtained 1 washed with doubly distilled water 1 absolute ethanol 1 Inert glove box (O2 , 1 ppm) 1 dried in vacuum Precipitation method: Copper (II) chloride dissolved in 160 mL of ethanol 1 Sodium hydroxide dissolved in 50 mL of ethanol 1 then mixed together 1 constant stirring at room temperature 1 color turned from green to greenish blue and finally to black 1 black precipitate (copper hydroxide) 1 centrifuged 1 washed 1 dried at 60 C

(Continued)

Table 16.2 (Continued) Metal oxide synthesis

Properties

Antimicrobial outcomes

References

Co-precipitation method: Cu (NO3)2
3H2O (aq.) 1 2 g Na12[(AlO2)12 (SiO2)12]
27H2O (Zeolites) 1 refluxed at 80 C for 5 h 1 pH 5 11 12 using NaOH solution 1 filtered, washed 1 dried overnight in an oven at 90 C

The average crystallite size is around 6.5 nm, Spherical in shapes, they have stable physical and chemical properties.

CuO with inhibition zone (8 and 4.5 mm) against Salmonella choleraesuis and Bacillus subtilis microbes.

Alswat et al. (2017)

Square in shape, and particle size is in the range of 40 50 nm. TEM images lie in the range of about 15 35 nm.

Maximum mean activity (21.8 6 0.7) found for B. subtilis and minimum mean activity (14.0 6 0.6) was observed for E. coli.

Hafeez et al. (2020)

TiO2 in its anatase form, Shape is spherical, particle size 12 18 nm, generate electron-hole pairs

Good antibacterial effect (60% 100% killing efficacy), against E. coli and B. megaterium.

Fu et al. (2005)

average size of the TiO2 nanoparticles is 77.4 nm and he size distribution range was between 70 85 nm, It has a triangular and spherical shape.

Antimycobacterial activity of TiO2 nanoparticles (10 22 nm size) was evaluated against M. tuberculosis, M. bovis and Mycobacterium species.

Ramalingam et al. (2019)

Co3O4 Biosynthesis method: Co (NO3)2.6H2O 1 Plant leaves extract (Populus ciliata (safaida)) (20 mL) 1 heated at 80 C for three hours 1 color changed from light brown to dark brown 1 formation of Co3O4-NPs TiO2 Sol gel method: Titanium isopropoxide 1 1-propanol 1 deionized water 1 Adjust pH 1.5 with 6 M HClO4 and HNO31stirring about 1 1.5 h 1 TiO2 NPs Sol gel method: 0.1 M titanium (IV) oxysulfate 1 100 mL DW 1 stirring 1 0.2 wt.% polyvinylpyrrolidone (PVP) capping agent, followed by the drop-wise addition of aqueousammonia at room temperature. A white gelatinous precipitate

ZnO Hydrothermal method: Zn(CH3COO)2. 2H2O 1 0.5 M NaOH 1 5 wt.% polyvinylpyrrolidone (PVP) at 60 C 1 Stirring at 400 rpm 1 asprepared dispersion was thermally treated in a two-liter Parr stainless steel stirred reactor up to 120 C 1 Constant heating rate 2 C min21 1 Stirring at 400 rpm Microwave decomposition: Zinc acetate dihydrate 1 50 mL DW 1 Solid NaOH 1 Stirring 1 [bmim][NTf2] 1 3 mL above solution 1 Microwave Oven (2.45 GHz, 850 W) in air 1 30% 1 white precipitate 1 centrifuge 1 washed with deionized water 1 dried in vacuum oven at 40 C for 10 h. Solvothermal method: Zn(CH3COO)2. 2H2O 1 50 mL DW 1 Stirring at 60 C 1 0.5 M NaOH (Maintain pH 5 9) 1 mixture was then transferred into a Teflon-lined autoclave 1 Maintained at 170 C for 18 h 1 White precipitate 1 filtered, washed with ethanol and distilled water 1 dried at 80 C

ZnO particles are ellipsoid in shape with a length of 500 600 nm and a diameter is about 100 nm, Crystal structures is Hexagonal wurtzite, Type of ZnO is Hexagonal prismatic rod.

Potassium hydrogen phosphate buffer solution (pH 7.0) used as a liquid medium, S. aureus and E. coli in inoculums were 2.1 3 107 CFU mL21 and 4.1 3 107 CFU mL21, respectively.

Jalal et al. (2010)

Sphere shapes, Mean particle sizes for the crystallographic planes (101), (002), and (100) were 41.30, 47.52, and 37.15 nm respt. Antibacterial activity increases with increasing conc. of H2O2.

Antibacterial activity of ZnO nanofluids was tested against E. coli. Antibacterial activity increases with increasing nanoparticles concentration.

Talebian et al. (2013)

Morphology is spherical as well as flower-like, Average crystallite size is 76, 65 and 45 nm, Higher wavelength was observed with increasing solvent polarity.

ZnO flower-like showed significantly higher photocatalytic inactivation than ZnO rod- and sphere-like against E. coli compared with S. aureus microbes.

Ma et al. (2013)

(Continued)

Table 16.2 (Continued) Metal oxide synthesis

Properties

Antimicrobial outcomes

References

Microwave hydrothermal method: 0.02 mol L21 Zn(NO3)2. 6H2O 1 DW/ Ethyl alcohol 1 0.075 mol triethanolamine (TEA) 1 Stirring 1 Sealed in a Teflon-lined autoclave 1 heated to 180 C 1 15 min in Microwave Digestion Instrument, power of 600 W 1 White precipitate 1 filtered 1 washing 1 dried at 60 C for 4 h.

ZnO particles has uniform mulberry-like shape. The diameter of ZnO was about 150 nm with high crystallinity, High crystallinity of the ZnO spheres and gave a lattice fringe of about 0.245 nm,

Mulberry like ZnO particles showed stronger antibacterial property on C. albicans than sheet like and flower like ZnO, Inhibition rates of ZnO 90% (mulberry), 85% (sheet), 50% (flower).

Rufus et al. (2017)

Size of synthesized nanoparticles is approximately, B43 and B29 nm. Crystal system of prepared nanoparticle is rhombohedral.

In vitro antibacterial activity of the α-Fe2O3 nanoparticles against E. coli and S. aureus is investigated by agarwell diffusion method.

Pallela et al. (2019)

Crystallite size of hematite nanoparticles of plane (104) about 18 nm and particle size of hematite nanoparticles was 10 22 nm, morphology is spherical nano clusters.

The zone of inhibition of green synthesized α-Fe2O3 NPs against S. aureus, E. coli, and K. pneumonia were 13.67 6 0.58, 11.33 6 0.58 and 12.00 6 1.00 mm respectively.

Giannousi et al. (2014)

Fe2O3 Biosynthesis: 0.01 M FeCl3 1 boiled for 2 min 1 A. occidentaleleaf extract 1 stirring 1 brownish precipitate 1 centrifuging at 10,000 rpm 1 washed 1 Dried precipitate 1 annealing at 600 C for 3 h 1 red powder (hematite nanoparticles) Biosynthesis: 0.01 M iron nitrate 1 double-distilled water 1 stirring 30 min 1 Sida cordifolia extract 1 Stirring and boiled at 60 C 1 Solution turns deep brown 1 Precipitate 1 Centrifuged at 10,000 rpm 1 dried 1 annealing at 300 C for 8 h 1 deep red colored α-Fe2O3 nanoparticles.

Table 16.3 Metal oxides-based nanocomposites for antibacterial study. Metal oxide nanocomposite synthesis

Properties

Antimicrobial outcomes

References

The excitation wavelength for samples was 455 nm giving an emission maximum at approximately 515 nm., the Crystalline sizes of Cu2O 5 33 and 15 nm, spherical in shape, and Good biocompatible in nature.

The cultivation medium used was minimal medium salts broth, high antifungal activity of the composite with 3.73 mg mL21 IC50 viability compared to the pristine Cu2O nanoparticles and it was effective against Saccharomyces cerevisiae cells.

Giannousi et al. (2014)

CuO directly depends on the ability to generate electron hole pairs, Bulk cubic and hexagonal phase for Ag and bulk orthorohmbic for CuO nanoparticles. Particle size is approximately 7 nm with circular morphology.

In comparision with Ag nanoparticles, Ag/CuO NCs shows higher antibacterial property against Serratia sp., E. coli, diameter of zone of antibacterial activities is 13 6 2 and 24 6 2 mm for CuO and Ag/CuO disks.

Ghasemi et al. (2017)

Nanocomposites has uniform shape and size with very high agglomeration. CuO is a nanoflakes type structure and ZnO has a hexagonal rod-type structure.

Agar medium was prepared by 9.8 gm nutrient agar, liquid broth was prepared by 1.3 gm of nutrient broth, better performance showed against E. coli, Pseudomonas aeruginosa.

Bandekar et al. (2020)

Composites of CuO/Cu2O Cu/Cu2O Solvothermal method: 4.13 mmol Cu (NO3)2
3H2O 1 DW 1 4.13 mmol N2H4
H2O 1 58 mmol TEG 1 brownish mixture transferred into 23 mL Teflon-lined stainless-steel autoclave 1 Crystallization under pressure at 120 C for 2 h 1 centrifuge 1 precipitate 1 Washed with ethanol 1 dry Ag/CuO Biosynthesis: AgNO3.2H2O 1 CuSO4. H2O 1 Morganella morganii 1 24 h in a shaking incubator at a temperature of 37 C and 150 rpm 1 change in the color of the growth medium 1 double distilled water and ethanol 1 centrifuge (at 12,000 rpm) for 15 min 1 dry 1 Ag/CuO composite. CuO/ZnO Sol gel method: Zinc acetate dehydrate 1 Citric acid 1 DW 1 stirring at 60 C for 60 min 1 add copper sulfate pentahydrate at interval of 1 h 1 ZnO-CuO nanocomposite.

(Continued)

Table 16.3 (Continued) Metal oxide nanocomposite synthesis

Properties

Antimicrobial outcomes

References

CuO/ZnO/Eggshell

Diameter of CuO and ZnO is 50 2 100 nm, crystallite structure is monoclinic and hexagonal, the pores and pits are regularly distributed and the size was roughly measured to be 100 2 400 nm

Antibacterial activity is for destruction of E. coli and S. aureus. at the concentration of 108 CFU were uniformly attached onto the agar culture medium.

Zhang et al. (2019)

Different stoichiometric composition, of cobalt sulfides such as urchin-like Co9S8, flower-like CoS, hollow spheral CoS2, TEM reveals that regular tube-like morphology and also hollow structure and an average diameter is B166.7 nm.

108 colony forming units (CFU) of Staphylococcus sciuri are cultured on 2216E agar plates under changed condition, Bactericidal performance of Co4S3/Co3O4 NTs Showed against E. coli and S. sciuri. Microbes.

Wang et al. (2020)

Size of bare TiO2 is 71 nm, particle size of CS/TiO2 nanocomposite is 99 nm, CSTiO2 shows tetragonal

Tested against two Gram-positive bacteria (Staphylococcus aureus, Streptococcus pneumonia), two Gram-negative

Karthikeyan et al. (2017)

Deposition-calcination method: 2 g of ES powder 1 100 mL solution of Cu(NO3)2
3H2O (0.2 M) and Zn(NO3)2
6H2O (0.2 M) 1 Stirring 24 h 1 Centrifuge 1 Washed with DW 1 dry at 60 C for 10 h 1 Calcinated at 600 C 1 product Composites of CO3O4 Co4S3/Co3O4 Hydrothermal method: CoCl2
6H2O 1 CO (NH2)2 1 DW 1 Stirring 1 transferred into a 100 mL Teflon-lined stainless steel autoclave 1 Heated at 100 C for 12 h 1 drying at 60 C 1 prepared precursor solution 1 Na2S 1 DW 1 Stirring 1 transferred into a 100 mL Teflon-lined stainless steel autoclave 1 sealed and maintained at 160 C for 24 h Composites of TiO2 CS/TiO2 Hydrolysis method: Titanium tetra isopropoxide 1 Isopropanol 1 DW 1 Stirring for 6 h 1 precipitate 1 centrifuge 1 washed 1 dry at

80 C 1 1% chitosan in (0.1 M) CH3COOH 1 (0.2 M) NaCl 1 Stirr continuously overnight 1 TiO21Chitosan 1 stirring 1 24 h at 700 rpm 1 filter 1 washed 1 dry at 80 C TiO2/polymer Sol gel method: Titanium (IV) isopropoxide 1 PVA 1 PEG powder 1 NH3 1 DW 1 Sonication 1 Stirring for 3 h 1 dry at 40 C in oven 1 PVA-PEG/ TiO2 composites TiO2/ZnO Sol gel method: Zn(NO3)2
6H2O 1 EtOH/ H2O 1 Stirring 1 ZnO Sol 1 Titanium butoxide (TBOT) 1 EtOH 1 TiO2 Sol 1 Mix ZnO Sol and TiO2 Sol 1 stirring 1 aging for 1 day 1 calcined at 450 C TiO2/cotton Sol gel method: Bidistilled water 1 HNO3 1 heated at 70 C 1 isopropyl alcohol solution of TTIP 1 heated upto 8 h for 80 C 1 sol formation 1 raw cotton boiled in 200 mL of DW solution 1 Na2CO3 at 100 C for 2 h 1 washed 1 dry in air 1 modified fibers washed with DDW and dry at 60 С for 2 h

crystalline structure, maximum absorbance at 494 nm, which is the λ-max for CR (Congo red) dye.

bacteria (P. aeruginosa, Proteus vulgaris), and one fungal (Candida albicans).

Spherical in shape, Diameter of TiO2 is 15 nm, diffraction planes (101), (004), (200), (105), (211), (204) showed that the anatase phase of TiO2.

Time-dependent kill curve method used, Outstanding antibacterial inhibition against E. coli bacteria. antibacterial activity of composites is PEG/TiO2 .PVA/TiO2 . TiO2

Tekin et al. (2020)

Band gap of TiO2 is high (3.2 eV), specific surface area is as high as 180 m2 g21, TiO2 with anatase structure, interplane distance is 0.23 and 0.35 nm are of (103) and (101) plane of TiO2.

Composite aerogels have the maximum inhibition zone of 23 and 19.5 nm for S. aureus and E. coli, and the minimum inhibitory concentration for S. aureus and E. coli was 100 ppm.

Suo et al. (2019)

Crystallite size of pure titania is 8.9 nm, Crystallite size of the CF/TiO2 sample was 9.8 nm, Pure titania anatase and brookite phases, 2θ 5 12.2 degrees, 24.7 degrees and 2θ 5 14.3 degrees, 22 degrees, Anatase peak of CF/ TiO2 are observed at 2θ 5 12.2 degrees, 17.6 degrees and brookite peak at 2θ 5 22 degrees

Bacteriostatic effect against gramnegative E. coli bacteria and decrease bacteria survival by 70%. cultivation in the liquid phase of E. coli DH10B, having optical density at 600 nm (OD600) equal to 0.9 was diluted 100 times by LB100 medium.

Galkina et al. (2014)

(Continued)

Table 16.3 (Continued) Metal oxide nanocomposite synthesis

Properties

Antimicrobial outcomes

References

Fine size distribution of around 30 40 nm of quasi spherical ZnO particles on graphene sheet. Graphene has excellent mobility of charge carriers and high surface area, Interlayer spacing of GO was 0.961 nm.

FLG/ZnO composite showed substantial antimicrobial activity against E. coli and S. typhi. Increased the doping concentration is directly proportional to antibacterial zones.

Bykkam et al. (2015)

Crystallite size of the nanoparticles (NPs) is nearly 36 nm. ZnO is a hexagonal wurtzite structure. FESEM revealed the typical size of the NPs is in the range 35 6 10 nm.

Better performance show against P. aeruginosa, Proteus mirabilis, E. coli bacterias, disk diffusion method, the inoculums for the experiment were prepared in a fresh nutrient broth,

Saravanakkumar et al. (2018)

Spherical in shape and size is about 10 12 nm, structure is hexagonal wurtzite, g-C3N4 appropriatebandgap (B2.7 eV),

Antibacterial efficacy of the samples ordered as g-C3N4 , ZnO , Cr-ZnO , g-C3N4/ ZnO , g-C3N4/CrZnO, Showed against Bacillus subtilis, Streptococcus salivarius.

Qamar et al. (2020)

Composites of ZnO FLG/ZnO Hydrothermal method: graphene oxide 1 ethylene glycol 1 zinc acetate 1 NaOH 1 DW 1 stirring for 60 min 1 transferred to 250 mL glass bottle and tightly sealed with Teflon cap 1 hot air oven for 10 h at 100 C 1 centrifuge 1 washing 1 dried at 700 C ZnO/CuO Modified perfume spray pyrolysis method: Zn (CH3COO)2
2H2O 1 Cu (CO2CH3)2
2H2O 1 Stirring for 50 min at 50 C 1 Transferred into a well cleaned perfume spray bottle at 500 C 1 calcined at 250 C for 1 h g-C3N4/Cr-ZnO Co-precipitation method: 1.0 M Zn(SO4)2. 7H2O 1 Cr2(SO4)2. 6H2O 1 gC3N4 1 DW 1 2 M NaOH 1 Stirring for 1 h 1 pale yellow precipitates 1 filtered 1 washed

Composites of Fe3O4/Fe2O3 Fe2O3 ZrO2/BC Co-precipitation method: 0.1 M ZrOCl2 1 0.1 M FeCl3 1 8 M NaOH 1 Stirring at 60 C for 1 h 1 brown precipitates 1 Stirring for half an hour 1 Cool at RT 1 Fe2O3 ZrO2/ BC 1 filter 1 washed 1 dry upto 24 h at RT Fe3O4/RGO Hydrothermal method: FeSO4
7H2O 1 DW 1 Stirring 1 Leaf extract 1 Stirring 1 NH4OH 1 Transferred to a Teflon-lined autoclave for 180 C for 6 h 1 Precipitate 1 GO 1 dispersion in DW 1 obtained exfoliated graphene oxide 1 centrifuge 1 washing 1 Dry at 100 C 1 g-Fe3O4/RGO Fe3O4/Zn Sol gel method: FeCl2
4H2O (10 mmol/5 mL 0.01 N HCl) 1 ZnCl2 (10 mmol/5 mL 0.01 NHCl) 1 80 mL containing 240 mg porcine gelatin 1 86 mL NaNO3 (6 mmol/5 mL H2O) solution 1 1 N NaOH

Dried seed powder (Black cumin seed) of grain size 60 200 mesh, average diameter of the particles was observed in the range 17 22 nm, uniform deposition of irregular shaped.

Antibiotic sensitivity, toward E. coli, studied by disk diffusion antibiotic sensitivity test, minimum inhibitory concentration (MIC) of Fe2O3 ZrO2/BC for E. coli was 0.030 mg mL21

Siddiqui and Chaudhry (2019)

Diffraction of the (110), (130), (111), and (210) planes of gFe3O4 nanoparticles indicating its polycrystalline in nature, the average particle size of bare Fe3O4 was observed to be 46 6 2 nm. and Fe3O4/2RGO nanocomposite was observed to be 22 6 2 nm.

Nanocomposite showed better bactericidal properties against Gram-positive bacteria (Staphylococuss aureus, Bacilus subtilis) in contrast to gramnegative bacteria (E. coli), with the zone of inhibition ranging from 7.0 to 16.0 mm.

Padhi et al. (2017)

Antibacterial activity depend on the weight ratio, the effect of the weight ratio depend on their properties such as, composition, size, magnetic behavior and colloidal stability, size is 17 nm.

Antibacterial inhibition against S. aureus was better, in comparison to E. coli, and it was dependent on the stoichiometric weights between Zn and Fe.

Gordon et al. (2011)

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Figure 16.4 Classification of metal oxides based on nanocomposites.

16.4.1.2 Metal oxide/metal oxide (mixed metal oxide) composites The combination of two or more different metal oxides produces mixed metal oxide composites having distinctive properties to those of individual counterparts. The produced composites are superior for various applications, including antibacterial activity, catalytic transformations, and energy studies, etc. The composites such as Co3O4/ZnO, TiO2/Fe2O3, ZnO/CuO, ZnO/Fe3O4, ZnO/NiO, and Al2O3/ZrO2, etc. are the best examples under this category and hence their continuous evolution for the various applications is in progress.

16.4.1.3 Metal oxide/carbon nanostructures composites With the discovery of carbon nanostructures, a new class of metal oxide composites came into existence; named metal oxide—carbon nanostructures composites, and hence, according to the name, it is formed by mixing metal oxide with carbon nanostructures for getting excellent properties as well. The various composites such as NiO/CNTs, ZnO/CuO/CNTs, CeO2/CuO/CNTs, α-Fe2O3/CNTs, and Co3O4/CNTs, etc. are the best examples under this category (Koli et al., 2016). These composites have been extensively used as antimicrobial agents with extreme toxicity to Gram-positive and Gram-negative bacteria.

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16.4.2 Composites of metal oxide with organic moieties These composites are formed by mixing metal oxide with organic moieties and hence resulting in inorganic-organic heterostructures. The organic moieties such as metal-organic framework (MOF) such as CuBTC, Cu3 (BTC) 2, etc., and organic polymers such as polystyrene (PS), polyethylene (HDPE), polypropylene (PP), etc. are extensively used as an additive in this class.

16.4.2.1 Metal oxide/metal-organic framework composites These composites are formed by mixing metal oxide with MOFs and hence resulting in the metal oxide/MOF heterostructures. MOFs are one of the most potential materials of the last decade due to their extraordinary properties such as their facile synthesis, porosity, surface functionalities, structural features, more active sites, etc. Most of the MOFs have a large pore size, which permits the storage of guest species, and their ability to release metal ions or ligands. In recent years, an increasing number of composites of MOF with other different materials are being actively investigated. Particularly, metal oxide/MOFs (such as CuO/UiO-66, ZnO/ZIF-67, and NiO/PPF-11, etc.) are used as antimicrobial agents due to superior antimicrobial and other physicochemical properties to that of the individuals (Shen et al., 2020; Xu & Yan, 2017).

16.4.2.2 Metal oxide/polymer composites The metal oxide/polymer nanocomposites result in more tunable antimicrobial activities than the bare metal oxides. In most cases, there is weak interaction between polymer and metal oxide nanoparticles (NPs) that interfere with the properties of composites, but these interactions between metal oxides and polymers are tuned by modifying the surface of composites. Some metal oxide/polymer composites have been synthesized for use in various sectors of life, such as, Chitosan/ZnO used as an antibacterial agent, polyaniline (PANI)/ZnO, which was used in the antimicrobial coating in paint formulation, poly(butylene succinate)/TiO2, that was used in the decomposition of organic compounds, Chitosan/Fe3O4 that was used as a potentiometric urea biosensor.

16.4.2.3 Metal oxide/organic molecule composites Composites of metal oxides with organic molecules such as carbohydrates, lipids, proteins, and nucleic acids are also formed for boosting their properties and hence to be effectively used in various applications as well. Numerous ways have been applied for making the composites of inorganic metal oxides and organic moieties, such as (1) combination of inorganic metal oxides with organic compounds through weak Vander Waal interactions, or (2) surface modification using functional groups between both. The composites of metal oxide with organic sensitizers have been used in third-generation dye-sensitized solar cells. The representative composites are also used in organic light-emitting diodes, biosensing applications, and catalytic studies (Kaushik & Moores, 2016; Varaprasad et al., 2017).

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Advances in Metal Oxides and Their Composites for Emerging Applications

Plausible mechanisms for nanocomposites-based microbes inactivation

The literature reports revealed the different plausible ways to inactive the microbes in the presence of nanocomposites (Fig. 16.5). These include surface contact, ease passing into the cell, destruction of biomolecules present in the cell, protein malfunction, restriction of protein and nucleic acid synthesis, metabolic function disruption, cell disruption, and formation of ROS in presence of a photon, etc. (Singha et al., 2017). Among the various above paths to inhibit or kill the pathogenic microbes, the antimicrobial mechanism of metal oxide-based nanocomposites solely depends upon the key processes, which include a contact of nanocomposites with microbial cells, the release of ions from the cells in the presence of nanocomposites, ease cell insertion leads the damage of cell wall as well as a disturbance in biological processes, ROS formation with nanocomposites in presence of photo-irradiation, damage of DNA, etc. (Sotiriou & Pratsinis, 2011). Antimicrobial action of metal oxide nanocomposites against microbes is possible through the following plausible mechanisms: 1. Microbial cell disturbances and leakages 2. Interferences in metabolic processes in microbial cells

Figure 16.5 Schematic of stepwise mechanism of action of antimicrobial agents.

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3. Reactive oxidative species generations 1. Microbial cell disturbances and leakages Nanoscale materials are highly energetic due to their very small size and hence may easily bind to the surfaces of microbes. In addition, these small particles may enter into the microbes through cell wall rupture. This may lead to a disturbance to the cell wall membrane resulting in the death of microbes. Particularly, bacterial cells have a net negative surface charge due to the presence of acidic phospholipids on their outer membrane with the extent of charge varying from strain to strain. Therefore, cationic-based composites are attracted to negatively charged bacteria electrostatically. Then, the metal ions are permitted to enter through ion channels as these ions belong to a class of essential nutrients. These entered metal ions (e.g., Cu) can also cause membrane damage through lipid peroxidation, attacking the integrity of the cell (Yadav et al., 2014). Similarly, silver metal ions are known to cause leakage of protons through the cell by unbalancing the chemiosmotic force across the membrane. Other metal cations can bind with transporters, inhibiting the complete assimilation of ions. Therefore, if the cell membrane breaks or ruptures through metal-ion-based composites, then the cell will not be able to interchange materials for the biological processes from its surroundings by diffusion or osmosis processes, which result in the death of microbial cells. 2. Interferences in metabolic processes in microbial cells Metabolism is a corresponding process involving two kinds of activities that is Energy stores (i.e. anabolism) and building up tissues when breaking down energy stores and tissues to get more fuel for body functions (i.e. Catabolism). Once the metal ions enter into the microbial cell, they interfere with the various metabolic processes such as cell wall synthesis, protein synthesis, DNA or RNA synthesis, etc. (Fig. 16.6). Most pathogenic microbes have a cell wall, which provides tensile strength, and also maintains internal osmotic pressure and it contains peptidoglycan. Inhibiting the growth of cell walls by antimicrobial agents would inhibit the growth of cells. Protein synthesis inhibitors prevent bacterial protein synthesis by blocking the function of ribosomes and elongation factor-2 (EF-2). Inhibition of protein synthesis stops or reduces the growth or proliferation of cells by rupturing processes that avoid directly the generation of new microbial cells. Inhibition of DNA or RNA synthesis in bacterial cells, then it causes the communication from DNA in the nucleus to the ribosomes in the cytoplasm. Transfer RNA transmits amino acids to the ribosome during translation to help build an amino acid chain. So inhibition of DNA or RNA synthesis continues and other biological processes then result in bacterial cell death. 3. Reactive oxidative species generations Metal oxides may also kill the microbes using ROS to be formed in the presence of photon irradiation through photo-oxidative processes (Deshmukh et al., 2018). Silver loaded TiO2 showed antibacterial activity in the presence of UV light or visible light radiation due to the formation of different ROS such as superoxide ions ( O22) and hydroxide ion (OH ). Photocatalytic activity of TiO2 nanotubes was executed by transfer of an electron from the valence band to conduction band that reduced oxygen to form free and hydroxyl radicals. Along with the silver ion itself as an antimicrobial agent, its presence in the composites also helped toward the more generation of ROS through separation of charge carriers to be formed in the conduction band of TiO2. The linear correlation was observed with the concentration of metal ions with the formation of ROS, which was reflected through an increase in cell retardation rate as well. These species are mainly responsible for antibacterial activity through alteration of the cell such as DNA breaking or destruction. The antibacterial mechanism of photoactive metal-based compounds G

G

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Figure 16.6 Schematic of plausible ways of action of antimicrobial agents. Refs. (Protein synthesis inhibitor, Disruption of cytoplasmic membrane, Inhibition of DNA or RNA synthesis, Inhibition of cell wall synthesis). depends upon the size, shape, dimension as well as nature of bacteria. The antibacterial activity of the nanocomposites is similar to that of the photocatalytic mechanism used in various transformations and hence to understand the formation of the different species with AgTs (TiO2 Nanotubes) under UV light (A) and visible light (B) in the catalytic reaction (Fig. 16.7).

Therefore, reactive oxidation species such as free radicals, electron-hole pairs, and scavengers are also the key causes of microbial inhibitions (Viet et al., 2018).

16.6

Synthesis strategies for designing metal oxide nanocomposite

Metal oxide nanocomposites have been commonly used as effective antibacterial agents for those of their bulk counterparts as well as individual components. For synthesizing metal oxide nanocomposites, the various methods for the individuals have been reported and, thereafter, two ways, such as in situ, and ex situ, are

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Figure 16.7 Photocatalytic mechanism of Ag/TNTs under UV light (A) and visible light (B). Source: From Silver nanoparticle loaded TiO2 nanotubes with high photocatalytic and antibacterial activity synthesized by photoreduction method. (2017). Journal of Photochemistry and Photobiology A: Chemistry. https://doi.org/10.1016/j.jphotochem.2017.10.051

considered for making the composites (Fig. 16.8). In the in situ route, the supportive or other material is to be added to the running synthetic protocol of metal oxides followed by calcination so as to get the desired metal oxide composites in bulk or nano-dimensional forms. The advantage of this method is to avoid particle agglomeration for maintaining the spatial distribution in the entire matrix as well as to connect the different components electrostatically through the functional linkers. For example, an in situ route for Ag/TiO2 nanocomposites is adopted where Ag suspensions are added during the conversion of titanium salt into its hydroxide precursor followed by its calcination at a higher temperature. However, in the case of the ex-situ route, the individual materials of the composites need to be prepared separately, and then these materials should be dispersed in the solvent medium using a sonicator followed by filtration, drying, and calcination at a higher temperature. In this route, the physical mixture of the combined components is to be formed. For example, the ex-situ route for Ag/TiO2 nanocomposites is adopted by homogeneous mixing of the Ag suspensions with TiO2 suspension followed by drying at low temperature (Stepanov, 2005).

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Figure 16.8 Synthetic strategies for designing metal oxide nanocomposites.

Fig. 16.8 shows in situ as well as ex-situ routes for synthesizing TiO2-MWCNTs nanocomposites.

16.7

Metal oxide nanocomposites based on antimicrobial coatings in different fields

The increase of infectious diseases is at the front among the different concerns regarding human health. On a global scale with extremely high social and economic costs, like the pandemic COVID-19 constraints, the world has been facing several infections or communicable diseases (Mantecca et al., 2017). It is all due to the numerous pathogenic viruses, fungi, bacteria, which are threats to living organisms, and hence there are many challenges in front of all people, scientists, and governments as well. It is our responsibility to be aware of the prevention of diseases due to a wide range of pathogenic microbes (Brophy & Nolan, 2015). In addition, there is a dire need to develop different biostatic or biocidal materials in powder or coatings form for inhibiting disease-causing microbes. Among the different forms, the antimicrobial coating is a promising strategy not only to protect the different surfaces from microbes but also to kill as well as to suppress the growth of microbes, which results in the complete prevention of the microbes infections (Geuli et al., 2019; Yadav et al., 2014). Antimicrobial coatings are typically useful to all for domestic apparatuses, hospital tools, office tools, etc., like machines, devices, counters, door handles, walls, ceramic materials, etc., and other high-touch zones, and many other public surfaces. In some cases, antimicrobial infection is to be occurred because of the textile materials, masks, gloves, carpeting, and high-touch areas.

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Among the different materials used in antimicrobial coatings, nanocomposites can effectively prevent microbial proliferation in public places as well as all types of infected surfaces (Kaushik & Moores, 2016; Murariu et al., 2011; Yuan et al., 2020). In this connection, the entire use of metal oxide nanocomposites-based antimicrobial coatings used in the different sectors or industries such as hospitals, textile, food, polymer, and paint formulations have been discussed herewith.

16.7.1 Hospital sector Nowadays, the healthcare industry is one of the largest consumers of antimicrobial coatings (Choudhary & Das, 2019). A variety of antimicrobial coatings have been used for different purposes in this sector; which include self-killing surfaces for inactivating, controlling the microbes, avoiding the transmission of microbes between the various objects for reducing hospital-acquired infections, surgical tool coatings, medical implant coatings, wound healing coatings, etc. The use of antimicrobial coatings in the health and medical sectors has been shown in Fig. 16.9. The thin films of polymer/zinc oxide nanocomposites via the co-deposition method were carried out and thereafter their antimicrobial performance with other surface characteristics were systematically investigated against the representative microbes. As compared to pristine polymer, the designed nanocomposites showed significant antimicrobial activity against microbes. From the present endeavors, it is revealed that the present nanocomposites could be used as effective coatings to protect the surfaces of medical kits from microbial infection, adhesion, and colonization (Eco-friendly

Figure 16.9 Metal oxide nanocomposites antimicrobial coating in hospital kit.

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nanocomposites derived from geranium oil and zinc oxide in one step approach, 2019). The biocompatible hyperbranched epoxy/silver RGO curcumin nanocomposites were tested against the microbes. The curcumin was immobilized onto epoxy-silver-reduced graphene oxide nanohybrids by the ultra-sonication method. Along with the higher antimicrobial activity, the designed nanocomposites have high mechanical properties, high tensile strength, and elongation. The epoxy/silver RGO curcumin nanocomposite inhibited the growth of S. aureus and Candida albicans; the microbes found in precise infection places, with minimum inhibitory concentrations (MICs) of 38 and 41 μg mL21, respectively, at 3% loading of the immobilized nanohybrids. In addition, the synthesized nanocomposites also inhibit the growth of green microalgae Chlorella. The designed nanocomposite was efficient as an antimicrobial agent against S. aureus, Candida albicans, Microalgae chlorella (Barua et al., 2014). The combination of Hydroxyapatite (HAp) and zinc hydroxide (ZnO) has resulted in a synergic improvement in the bioactivity property against the bacteria. The different compositions of ZnO-HAp coatings were fabricated via electrophoretic deposition by changing the concentrations of each constituent in the dispersion. The fabricated coatings have not only good adhesion properties but also the ability to coat metallic supports with different geometries useful for medical devices. This simple and economical method offered a pivotal strategy for tailoring the configuration of multicomponent nanomaterial-based coatings. Chitosan-iron oxides coated with graphene oxide nanocomposites were formed using the co-precipitation method and then these composites were tested as antibacterial agents against the S. aureus (gram-positive) and E. coli (gram-negative) bacteria. The prepared nanocomposite films showed the non-cytotoxic nature of the films (Konwar et al., 2016). Ag/ZnO core-shell nanocomposites were synthesized using Andrographis paniculata leaf extract. Designed nanocomposites have shown great antimicrobial performance against Candida krusei; which is an adaptable microbe wellknown to cause various diseases like candidemia. Investigators reported that the surface of biogenic silver (Ag) nanoparticles coated by zinc oxide nanoparticles has significant antimicrobial efficacy against Candida krusei compared to the bare materials (Deshmukh et al., 2018). Therefore, such designed nanocomposites were widely used in biomedical sites (Understanding the Antifungal Mechanism of Ag@ZnO Core-shell Nanocomposites against Candida krusei, 2016). Polyurethane/ZnO nanocomposites have been used for different biomedical applications such as hospital bedding, wound dressing, tubing, injection equipment, surgical drapes, and implants. Along with killing or controlling properties against microbes, antimicrobial coatings of bare metal oxides or their nanocomposites have been extensively used in medical implants as well (Geuli et al., 2019). Javidi et al. reported ZnO nanoparticles for antibacterial root canal sealers and also measured their microleakage up to 90 days at different intervals for knowing their sealing and stability properties. The higher microleakage of the ZnO sealer was observed for the bigger nanoparticles synthesized at higher calcination temperature (from 500 C to 700 C), which results in a decrease in the effective surface area. In addition, the different sealers prepared using ZnO nanoparticles exhibited less microleakage compared to that of bulk ZnO sealers (Javidi et al., 2014). Zebarjad synthesized porous and spongier ZnO/MgO nanocomposites for polycarboxylate dental cement used in clinical dentistry for luting agents, orthodontic attachments, cavity lining, and

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bases and restoration for teeth. In contrast to conventional zinc polycarboxylate cement, the coatings of ZnO/MgO nanocomposite cement have increased mechanical strength and a comparable setting time for preparation of the dentin cement to those obtained by commercial samples (Panchal et al., 2019). Li et al. deposited ceria nanoparticles on the surfaces of metallic titanium and evaluated the in vitro and in vivo biological response with their mechanisms of novel bone formation. It was concluded that the mixed-valence cerium oxide nanoparticles had the potential to induce bone regeneration without the need for any exogenous osteogenic inducer. Even in the absence of osteogenic supplements, nanocomposite scaffolds of glass foams containing ceria nanoparticles demonstrated the enhancement of collagen production and osteoblastic differentiation of human mesenchymal stem cells compared to scaffolds without ceria nanoparticles (Li et al., 2018) (Fig. 16.10). Fig. 16.10 represents the coatings for dental implants using a silica-titania hybrid for improving dental implant bioactivities. The porous titania hybrid coatings were shown to provide the necessary surface morphology, mechanical properties, and bioactivity. Recent scenarios reveal that nanocomposites-based wound healing coatings have been emerged as an effective protocol against wound healing, causing bacteria like S. aureus (initial phase), Pseudomonas aeruginosa, and E. coli (deep layer of skin) approach. In this regard, the potential nanocomposites comprising silver, copper, chitosan, zinc, titanium dioxide, etc. were reported for the wound healing approach. Composites of carbon-functionalized (Fe3O4@C16) with limonene and eugenol showed worthy antibacterial and anti-adherence properties against the representative bacteria, which are crucial for wound healing restoration (Vasile et al., 2014). Similarly, the composites of gentamicin-loaded ZnO nanoparticles with chitosan in gel

Figure 16.10 Metal oxide nanocomposites used in dental implant (Dental Implant).

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form exhibited the synergistic antibacterial activity with superior growth-inhibition ability against S. aureus and P. aeruginosa compared to the individual control (Lu et al., 2017) Microporous Ag/ZnO nanoparticles loaded with chitosan demonstrated significantly accelerating the initial stage of the wound healing process tested with in vivo mice protocol. The composite coated dressings reported long-time moisture retention, enhanced blood clotting capability, high antibacterial activity against pathogenic bacteria, and denser collagen deposition properties compared with either pure chitosan-based dressing or ZnO-based ointment. TiO2-impregnated with biodegradable zein-polydopamine-based nano-fibrous scaffolds demonstrated good adhesion, proliferation, and relocation of cells during in vitro wound healing tests (Khalid et al., 2017). BC-TiO2 nanocomposite coatings promoted suitable healing through fibroblast migration and suitable growth of epithelial cells along with blood supply restoration forming new blood vessels. Therefore, these results indicated that the different nanocomposite coatings have been beneficial for wound-healing applications for pathogens killing or controlling at the wound sites.

16.7.2 Textile sector Currently, antimicrobial textile fibers and fabrics are increasingly being used from hospitals to daily household items such as medical, protective, technical, sportswear, and decorative textiles, etc., and hence revolutionary progress has resulted in the textile industry (Mantecca et al., 2017). Basically, textiles provide suitable conditions for growing the microbes and proliferating into the surrounding environment (Petkova et al., 2014). Natural textile fibers (e.g., pure cotton, wool, linen) are readily attacked by microbes and hence cause the loss in quality of fabrics in terms of strength, color, appearance, etc., but synthetic fibers (e.g., polyester and nylon) are hardly attacked by microbes. In addition, the exponential multiplication of microbes is also assisted by internal factors of the human body (sweat, dead cells, etc.). As a result, imparting antimicrobial textile fiber or fabric is one of the significant solutions to control the microbes present on the textile and hence to control their further consequences in terms of avoiding the loss in quality of textile (strength, color, appearance) as well as microbial transmission and their respective diseases. In this connection, the production of effective antimicrobial agents anchored on the fiber is the actual need so that textile fibers can have either a bacteriostatic or bactericidal nature. Conventionally, textiles are treated with organic-based antimicrobial formulations such as quaternary ammonium salts, polybiguanides, halogenated phenols like triclosan, N-halamines, biopolymers, etc. With the development of nanomaterials, the different inorganic-based (either bulk or nanoscale) antimicrobials such as TiO2, Ag2O3, ZnO, Cu2O3, Fe2O3, SnO2, etc. have been used in the textile industry (Khalid et al., 2017). The antibacterial activities of the MgO/Al2O3 fabric nanocomposite were tested against E. coli (Gramnegative) and E. coli (Gram-positive) cultures. MgO-coated cotton exhibited complete growth inhibition of E. coli. While Al2O3 coated cotton, other microbial infections are the most common threats globally started with about a 23% growth inhibition of E. coli. Nanocomposites of ZnO NPs with chitosan were developed by sonication process; thereafter, these were coated on the cotton samples and tested for their

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antibacterial activity (Petkova et al., 2014). It was revealed that nanocomposites coated on cotton inhibited 48% and 17% bacterial growth against S. aureus, E. coli, respectively (Mantecca et al., 2017). The different binary mixtures between various metal oxides (ZrO2, MgO, and TiO2) were prepared by the sol gel process and further utilized to impregnate the cotton fabrics. Along with antibacterial activity, the impregnated cotton fabrics were tested for thermal stability as well as flame retardancy properties and they reported a significant enhancement in all these properties of the fabrics. In connection to antibacterial activity, the best performance of coated metal oxide nanocomposites cotton fibers against microbes was observed in the order of TiO2/SiO2 . MgO/SiO2 . SiO2 . ZrO2/SiO2 (Rajendran et al., 2014; Yadav et al., 2014). The fabrics coated by graphene oxides-based antibacterial nanocomposites were tested with the representative microbes such as S. aureus and E. coli and hence, coated fabrics showed significantly inhibiting the bacterial growth (Hu et al., 2019). Functionalization of ZnO NPs was carried on cotton fibers with Ag nanoparticles as well as curcumin, resulting in the formation of ZnO Ag/cotton and ZnO-curcumin/ cotton nanocomposites. The synthesized ZnO Ag composites coated on cotton resulted in higher antimicrobial activity than ZnO/cotton materials. Similarly, the antimicrobial activity of ZnO-curcumin/cotton nanocomposites showed higher activity against S. aureus (Gram-positive) and E. coli (Gram-negative) microbes than that of ZnO/cotton materials (El-Nahhal et al., 2020). Synthesis of the TiO2/Fe3O4/Ag nanocomposite was reported to result in enhanced antimicrobial activity in contradiction of S. aureus and E. coli bacteria (Harifi & Montazer, 2014) (Fig. 16.11). Nanocomposites between iron oxide (Fe2O3) and titanium oxide (TiO2) nanoparticles with carbon nanostructures were coated on biomedical textiles to test the antibacterial activity against gram-positive and gram-negative bacteria using the agar diffusion plate method (Rajan et al., 2019). The nanocomposites fabrics showed a perfect zone of inhibition against bacteria, representing excellent antibacterial activity. The nanocomposite of polyamide-6 with ZnO nanoparticles was synthesized using a melt intercalation route with varying content of ZnO nanoparticles (Dural Erem et al., 2011). At the optimum compositions of the nanocomposite fibers, around 99% reduction in S. aureus and Klebsiella pneumoniae bacterial colonies were observed. Composites of flower-shaped ZnO nanoparticles and chalcone (E)1-(3-hydroxyphenyl)-3-(4 methoxyphenyl) prop-2-en-1-one were coated on cotton fabrics using padding mangle and gum acacia as binders (Sivakumar et al., 2010). The synergistic effect of chalcone and ZnO NP-based composites showed the complete inhibition of growth of different bacteria such as E. coli, P. aeruginosa, S. aureus. In recent times, the development of superhydrophobic coatings on cotton fabric has shown great interest because of their self-cleaning, antibacterial, antifungal, anti-stain, and other properties (Chen et al., 2015). Superhydrophobicity is a phenomenon where water dewdrops make a contact angle greater than 150 6 1 degrees, and it can be seen in naturally found materials such as rose petals, lotus leaves, and wings of a butterfly 2020 (Tudu et al., 2020). Therefore, antimicrobial coatings on textile fibers as well as fabrics are used to retain the quality of textile materials, to kill as well as to avoid the transfer of microbes, and to make superhydrophobic textile surfaces.

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Figure 16.11 Metal oxide nanocomposites antimicrobial coating on textile.

16.7.3 Food sector The metal oxides-based nanocomposites are having a considerable interest in all aspects of the food industry and hence these may be useful for sensing technology to know about the microbes, contaminants (e.g., adulterants, heavy metal or pesticide residues), nutrients, etc. present in food as well as for food packaging technology (smart, active, biodegradable) (Zare et al., 2019). Particularly, metal oxidesbased nanocomposites as antimicrobial coatings are commonly used in food packaging (e.g., active, smart, and biodegradable). Food packaging is a very important aspect for protecting food items from living or non-living moieties, preserving the nutritional value, enabling proper handling, and facilitating proper storage and transport through the supply chain. Mostly, microbial contamination is one of the extreme challenges of the food industry, which could cause different diseases to human beings and animals and also reduce the quality and life of food products and hence, such microbial contamination is also protected by using the proper antimicrobial coated food packaging. In this connection, investigators have been studying the different metal oxides nanocomposites based antimicrobial coatings for packaging the various food materials. Zinc oxide-silver nanocomposites (ZnO Ag nanocomposites) were fabricated using Thymus vulgaris (T. vulgaris) leaf extraction through the facile, green hydrothermal decomposition reaction (Zare et al., 2019). The designed nanocomposite was combined with poly (3-hydroxybutyrateco-3hydroxy valerate)-Chitosan (PHBV-CS) by the ultra-sonication method to get a novel biodegradable polymer-based nanocomposite. The prepared biodegradable

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nanocomposites showed an increase in mechanical properties as well as antimicrobial activity against S. aureus and E. coli microbes than that of bare materials. ElSayed et al. prepared chitosan/guar gum/Roselle calyx extract/zinc oxide (CS/GG/ RE-ZnO) bio-nano composites and studied the antimicrobial activity on Ras cheese substrate for food packaging applications. Different Re-ZnO contents of 1, 3, and 5 wt.% were added to CS/CG blends. The results showed that 3 wt.% Re-ZnO exhibited the best performance against bacterial and fungal species, especially E. coli, Listeria monoytogenes, and Aspergillus terries, which was attributed to the combined antibacterial properties of RE and ZnO extract. Also, Re-ZnO helps in forming a barrier against water vapor permeability (Youssef et al., 2019). Currently, there is an increase in research endeavors with the use of degradable bionanocomposites-based antimicrobial coatings having higher mechanical properties, serving as a barrier for microbes, gases (O2 and CO2), and moisture. These biodegradable nanocomposites are a great substitute for conventional non-biodegradable materials like glass, plastics, metals, polymers, etc., which cause harmful effects on not only human beings but also the environment. Basically, these biodegradable nanocomposites are prepared by mixing the nanocomposites with biodegradable polymers such as polysaccharides (e.g., chitosan, starch, cellulose,), proteins, and bio-derived polymers conjugated with synthetic polymers [e.g., polyhydroxy butyrate (PHB), polylactic acid (PLA)] (Lambert & Wagner, 2017). Polyethylene (PE) films coated with ZnO/chitosan or chitosan nanocomposites were prepared. The formation of spherical zinc oxide nanoparticles in the coating was shown as a chemical interaction between zinc oxide and chitosan. Chitosan-based nanocomposite showed effective antimicrobial defense against E. coli and S. aureus microbes. Chitosan/ ZnO nanocomposites fully suppressed the growth of pathogens and microbes and hence the prepared nanocomposite coatings were effectively used as food packaging materials for increasing the shelf life of food products (Al-Naamani et al., 2016). Biodegradable nanocomposites were prepared by mixing zinc oxide nanoparticles with bacterial polyester poly (3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) by solution casting technique. The prepared PHBV/ZnO nanocomposites coatings reported significantly higher antimicrobial activity against humanoid pathogen bacteria. These sustainable nanocomposites with antibacterial functions were used as bowls for beverages and food products also (Dı´ez-Pascual & Dı´ez-Vicente, 2014). Active packaging films for scrambled eggs were developed using zinc oxide and clove essential oil into the polylactide/polyethylene glycol/polycaprolactone matrix using the solution cast technique. The mechanical rigidity of films was lowered due to the catalytic degradation effect. Both zinc oxide and clove essential oil improved the mechanical strength and structural properties of the nanocomposites films. The composite film activity was verified against S. aureus, and E. coli injected in scrambled egg, and results indicated that the PLA/PEG/PCL/ZnO/CEO film showed the highest antimicrobial activity during 21 days storing at 4 C. The Weibull model was working to fit the kinetics of test organisms, and it was initiated that the model was tailored for the growth of food packaging materials (Ahmed et al., 2019). Superior antimicrobial poly (vinyl alcohol) (PVA)/pluronic (PLUR)/ zinc oxide (ZnO) nanocomposite films were developed as active packaging materials for food

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materials. Mixing of ZnO into the poly (vinyl alcohol)/pluronic matrix showed an improvement in the thermal and barrier properties of nanocomposite films. The PVA/PLUR/ZnO nanocomposites also demonstrated broad-spectrum antimicrobial activity verified against Gram-positive, Gram-negative bacterial pathogens, fungi and also reported the increase in antimicrobial activity with increasing concentrations of ZnO nanoparticles in the composites (de Chiara et al., 2015) (Fig. 16.12). ZnO-chitosan nanocomposites were coated on the pouches and then the antimicrobial activity of the prepared nanocomposites was tested against the representative microbes. It was observed that the antimicrobial efficacy was linearly related to the content of ZnO NPs. The antimicrobial coated pouches were used as a packaging material for raw meat and the results showed increased antimicrobial action as well as an extended shelf-life to that of conventional PE materials. In addition, metal oxide-based composites are also useful to scavenge the gases emitted (e.g., ethylene and oxygen) by foods or fruits. Excessive release of ethylene gas boosts the ripening, then deterioration rate of the food materials, which is reflected through the decrease in shelf-life of those food products. Similarly, the presence of oxygen gas also degrades the food materials due to the rapid oxidation of nutrients like fats, carbohydrates, proteins, etc. present in it. Therefore, the different scavengers (e.g., natural clays, catalytic oxidizers like potassium permanganate and silver nitrate, sorbents like propylene glycol, electron-deficient dienes or trienes having electronwithdrawing substitutes like tetrazines and adsorbents like activated carbons,

Figure 16.12 Metal oxide nanocomposites antimicrobial coating on food.

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zeolites or silica, etc. for ethylene degradation and metallic (e.g., iron-based), organic or natural reducing agents like ascorbic acid, natural O2 scavengers like alpha-tocopherol, enzyme catalysts or photosensitive dyes impregnated onto polymeric coatings, etc. for oxygen scavengers) are useful to degrade these gases evolved by the food materials (Dey & Neogi, 2019). However, the nanocompositesbased scavengers have overriding advantages over these bulk scavengers. The bananas were covered with a layer of nanofiber TiO2-polypropylene film and reported composite with 5 wt% TiO2 exhibited the highest photocatalytic activity for the ethylene degradation than that of polymer film only (Zhu et al., 2019). Similarly, mixed materials of TiO2 with SiO2 showed excellent ethylene degradation capabilities under UV light irradiation and also reported an enhancement in tomato shelf-life (de Chiara et al., 2015).

16.7.4 Polymer sector In our everyday life, polymers are used as an advanced material that is found in nearly every material. The importance of polymers has been much more because of their basic uses in the wide range in the medical field as biopolymers and therapeutic polymers. Polymers are used to make a variety of electronic components; sunglass lenses, paint, plastic bottles, DVDs, etc. For instance, poly(vinyl chloride) is a strong, corrosion-resistant polymer commonly used in plumbing applications, whereas polyurethane and polyethylene is an example of flexible polymer found in plastic products (Tai et al., 2017). The combination of polymers with the metal oxides nanomaterials resulted in the novel composites having higher antimicrobial activity usually than that of bare polymer only. Numerous natural polymer-based ZnO nanocomposite coatings such as chitosan/ZnO, polylactide/ZnO, polypyrrole/ ZnO, and rubber/ZnO have been studied for effective antimicrobial coatings in recent years. As an environmentally benign material, ZnO is biocompatible and non-toxic for various applications such as biomedical applications, food additives, and textile industries, etc. The in situ polymerization method was used for making a malleable hybrid caprolactam-casein/ZnO nanocomposite. The effect of the ZnO dose was studied on the appearance, hand feeling, mechanical properties as well as antibacterial activity of as-prepared composite films. The designed films showed predictable mechanical properties and outstanding antibacterial activities against S. aureus and E. coli. The possible pathway for manufacturing natural polymer-based nanocomposite as an antibacterial coating would have excessive potential use in numerous fields, such as hospital, textile, leather, packaging, paper building, and interior wall coating (Wang et al., 2017). The composites of silane with ZnO Nanoparticles showed good mechanical performance, improved thermal stability, and antibacterial activity. Similarly, PLA/ZnO nanocomposites were demonstrated in film and fiber coating forms and also reported their antibacterial activity against the representative pathogens (Deshmukh et al., 2020; Murariu et al., 2011). In addition, surface modification is another efficient way to improve the antimicrobial activity in a coating through surface adhesion and binding as well. The functional groups of the various agents have been employed as capping or stabilizing agents to

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stabilize metal or metal oxide nanoparticles; thereafter, formed hybrid metal nanocomposites have resulted in enhanced antimicrobial properties. In this progression, the positive and negative charged functional groups on the surfaces of nanomaterials are capable of reducing the surface energy through stabilizing metal nanoparticles by electrostatic interactions (Hoeng et al., 2015). Functionalized silver-decorated grapheneoxide nanosheets were prepared with an increase in super hydrophilicity as well as antibacterial properties, which are also suitable for making polymer-based composites (Soroush et al., 2015).

16.7.5 Paint sector Ten million deaths because of infectious diseases have been reported worldwide; and nearly 80% of these infections occur from contaminated surfaces (Hoque et al., 2015). In daily life, paint is used for painting various objects like the house, toys, automobiles, metallic products, etc., and hence paints are the most commonly used materials (Bajc et al., 2015). With the addition of certain antimicrobial agents, the paint would have the ability to resist or kill the microbes present on the surfaces of the different objects. Hence, antimicrobial paint coated on objects can resist the growth of bacteria, fungi, and viruses. Basically, organic and inorganic materials are used as antimicrobial agents in different paints. However, inorganic materials as antimicrobial agents have overriding advantages such as higher activity due to higher surface area, ease of interaction, zero microbe resistance, etc. Recently, our group have investigated the different nanocomposites as antibacterial agents against the representative bacteria. In addition, Ag@TiO2 nanocomposites as antimicrobial agents were used for visible active antibacterial paint formed through the industrial process. The prepared paint showed enhancement in the antibacterial activity against E. coli and S. aureus microbes in the presence of visible light irradiation (Deshmukh et al., 2021) (Fig. 16.13). Silica nanospheres containing immobilized silver or copper nanoparticles were used effectively as antibacterial or antifungal additives for architectural paints (Zielecka et al., 2011). These composites were synthesized using an in situ route in which a desired metal precursor solution with a reducing agent was added into the running synthetic route of silica nanospheres followed by drying in an oven at the temperature of 90 C for 1.5 h and then at 250 C for 2 h. Thereafter, silicone acrylic-based emulsion paint containing silica nanospheres immobilized with silver or copper nanoparticles was prepared using a laboratory mixer. Antimicrobial activity of paint formulations was performed according to standard PN-EN ISO 846 against the microbes such as A. niger, Paecilomyces varioti, Penicillium funiculosum, Chaetomium globosum mixture, and bacterial strains P. aeruginosa and then reported the excellent inhibition of the representative microbes using these composites (Zielecka et al., 2011). Similarly, CuxO/TiO2 nanocomposites were reported as antimicrobial agents against microbes. Basically, TiO2 was not a bactericide in the dark but remarkably, however, devised hybrid CuxO/TiO2 nanocomposites where CuxO species conferred antimicrobial properties to TiO2 under dark circumstances as well. The antimicrobial properties of CuxO were independent of light. However, the antimicrobial action was mainly due

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Figure 16.13 Schematic representation of the formulation and antibacterial activity testing strategy of the visible active antibacterial paint. Source: From Deshmukh, S. P., Koli, V. B., Dhodamani, A. G., Patil, S. M., Ghodake, V. S., & Delekar, S. D. (2021). Ultrasonochemically modified Ag@TiO2 nanocomposites as potent antibacterial agent in the paint formulation for surface disinfection. ChemistrySelect, 6(1), 113 122. https://doi.org/10.1002/slct.202002903

to Cu21 ions released upon oxidation of copper in the bare or hybrid nanocomposites in the dark. In comparison to elemental Cu or Cu21 species, the Cu1 species was performed much better activity against bacteria and pathogenic viruses also. Hence, Cu 1 species present in TiO2 showed higher antimicrobial properties than others (Ganguli & Chaudhuri, 2020). Composites of ZnO have been equally reported as a paint additive for residential buildings as well as for shielding stone monuments in the form of a coating. Go´mez-Ortı´z et al. examined the antimicrobial activities of the surfaces equipped using paint suspension containing Ca(OH)2 particles mixed with metal oxides such as ZnO or TiO2 NPs. Ca(OH)2 was widely used as an agglomerating material in the protection and renewal of stone heritage monuments. The coatings containing Ca(OH)2. ZnO systems were superior to those containing TiO2 and also the antibacterial efficiency of the Ca (OH)2. ZnO systems increased with an increase in the amount of ZnO addition. Therefore, the reported composites in the form of a coating effectively protect stone monuments (De et al., 2019). Composites of metal oxide

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with polymer are promising antimicrobial agents in the paint sector. The important features of these composites include stability, less toxicity, solubility in weak acids, biocompatibility, pH-sensitivity, non-antigenic, film-forming properties, higher antimicrobial activity, and low cost. The composites such as TiO2/chitosan, ZnO/polylactide, and NiO/polypyrrole, etc. are used commonly in this class of materials. Among the various composites of this category, TiO2/chitosan nanocomposites were used as additives for making antibacterial paint, and developed paint was studied on various surfaces of metal, glass, wall, and wood, etc., (Kumar et al., 2019). Therefore, metal oxide-based composites have been commonly used as antimicrobial additives in paint, ink or lacquer, coating, etc. during their manufacturing process so as to make a microbial resistant suspension or powder as well, and hence such cost-effective paints provide not only long-lasting protection against microbes observed on the surfaces of objects but also the high durability of the objects.

16.7.6 Leather sector Since ancient times, leather products have been used extensively in our daily lives. The leather industry is extremely important in the world economy, with an estimated global trade value of around US$100 billion per year leather market. In India, due to the steady export revenues in international trade, the leather sector has been established as the prevailing section in the economy. India is the world’s second-largest exporter of leather garments, the second-largest producer of footwear, the fifth-largest exporter of leather goods, and the third-largest exporter of saddlery and harness items. Leather is a biobased material created from animal hides or skins that have been utilized in our daily life for centuries due to its outstanding comfort, breathability, and durability. In addition, proteins and fats are abundant in leather products, providing a suitable environment for the growth and proliferation of microbes, essential elements for microbial development and colonization, etc. Similarly, leather particularly in footwear can absorb sweat-carrying metabolites and sebum from the sweat/sebaceous glands of the skin, potentially providing nutrient sources for bacteria growth. This leads to a variety of undesired issues such as discoloration, foul odor, decreased performance and hence it must be treated with antibacterial agents to avoid the microbes attack. To prevent microbial attack, the promising way is to cover the surfaces of these leather materials with solid antimicrobial coatings. The different solid antimicrobial coatings such as polymer-based materials (such as synthetically sourced polymers: polyurethane, polyacrylates; and naturally sourced polymers: casein, nitrocellulose, chitosan, etc.), inorganic based materials (such as Ag nanosuspensions, ZnO nanoparticles, TiO2 nanoparticles, nickel antimony titanate (NiTiSb)O2, cobalt chromite (CoCr2O4), cobalt titanate (Co2TiO4), cobalt aluminate (CoAl2O4), zinc ferrite (ZnFe)Fe2O4, etc.), organic biocides (such as quaternary ammonium salts, pentachlorophenol, polyhalogenated phenolic compounds, dimethylfumarate, etc.) are used. Among them, inorganic biocides have overriding advantages in antimicrobial properties and hence this point is reserved for the use of the different inorganic moieties as antimicrobial coatings in leather industries.

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Silver-modified silica-based volcanic ash was used as an antibacterial agent for leather coatings. The highest inhibition zones of S. aureus were reported to vegetable tanned leather@SiO2/Ag composites (24.80 6 1.64 mm) as compared to leather@SiO2 (21.25 6 0.50 mm) and bare leather (11.40 6 0.55 mm). Similarly, the sol gel method was used to design the TiO2 SiO2 nanocomposites from tetraethoxysilane and titanium n-butoxide catalyzed with acid at room temperature for leather coatings. These nanocomposites were sprayed in the form of uniform, well dispersed, homogenous films on leather. After spraying the nanocomposites, the performance of leathers in terms of dry and wet rubbing fastness, fastness to perspiration and water, finishing adhesion and UV light fastness, grain cracking, and bursting resistance values, etc. were improved drastically (Kaygusuz et al., 2017). PEGylated chitosanprotected silver nanoparticles (PEG-g-CS@AgNPs) with a core-shell configuration were formed using an in situ method for forming highly stable colloids. Antimicrobial activity of such silver nanoparticles against Gram-negative E. coli and Gram-positive S. aureus (S. aureus) was studied and hence reported a substantially lower MIC than chitosan or PEG-g-CS. This PEG-g-CS@AgNP was found to be a good water-borne and environmentally acceptable antibacterial coating for leather surfaces. Because of the synergistic impact of bacteria, resistance by PEGylation and bactericidal property based on chitosan moiety and Ag1 release, leather samples coated with PEG-g-CS@AgNPs had significantly greater antibacterial efficiency than CS and PEG-g-CS coating (Liu et al., 2017) (Fig. 16.14). In various organic solvents, ZnO microstructures were prepared in different morphologies such as microrod, hollow fusiform, and hollow columnar-like morphology. After that, physical mixing was made between polyacrylate with ZnO microstructures for forming polyacrylate/ZnO composite emulsions. Because of the higher surface area of hollow columnar-like ZnO, the polyacrylate/hollow columnar ZnO

Figure 16.14 Schematic of PEG-g-CS@AgNPs coating onto the leather surface and its multi-function antibacterial mechanism. Source: From Liu, G., Li, K., Luo, Q., Wang, H., & Zhang, Z. (2017). PEGylated chitosan protected silver nanoparticles as water-borne coating for leather with antibacterial property. Journal of Colloid and Interface Science, 490, 642 651.https://doi.org/10.1016/j.jcis.2016.11.103

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composite films reported a superior overall performance than others. The inhibition zone diameters of the polyacrylate emulsion-finished leather matrix and the polyacrylate/hollow columnar-like ZnO composite emulsion-finished leather matrix were 39.7 and 45.3 mm, respectively. The hygienic and antibacterial performance of the polyacrylate/hollow columnar-like ZnO composite emulsion for a leather matrix was promising material than pure polyacrylate emulsion (Bao et al., 2017). Gallic acidmodified silver nanoparticles (GA@AgNPs) with a core-shell structure were first produced in situ to fabricate such functional leather. The GA@AgNPs showed excellent stability and dispersibility across a wide pH range from 3 to 12, as well as effective antibacterial activity (Because of its nanoscale size and high carboxyl group surface density) with a MIC of roughly 10 μg mL21, due to hydrophilic surface of gallic acid (Liu et al., 2018). Composites of Ag TiO2 and TiO2 nanoparticles with anatase phase were prepared using a new, easy, and reliable synthesis approach. These nanoparticles were disseminated in a variety of leather substrates with various surface finishes. Although huge agglomerates were formed, it was proved that nanoparticles remained on the surface following water washing. In contrast to samples covered with TiO2 nanoparticles, all leather samples covered with Ag TiO2 nanoparticles demonstrated antibacterial activity. As a result, silver has been recognized as the most important antibacterial agent. Furthermore, cytotoxicity experiments revealed that Ag TiO2 nanoparticles have low toxicity, with a dead cell percentage of less than 30% (Carvalho et al., 2018). The photocatalytic antibacterial activity of dendritic fibrous nanosilica (DFNS) nanoparticles loaded with carbon quantum dots (CQDs) and MoS2 QDs (DFNS@CQDs-MoS2) nanoparticles were successfully demonstrated. Antibacterial activity of the DFNS@CQDs-MoS2 nanoparticles against S. aureus, E. coli, and A. niger was 95.35%, 99.98%, and 99.99%, respectively. HWPUA (hydrophobic waterborne polyurethane acrylate) with improved bacterial adhesion resistance was developed. The leftover bacterial cells were totally inactivated by the HWPUA/DFNS@CQDs-MoS2 coating (Ma et al., 2022). Using trisodium citrate (TSC), in situ deposition of silver nanoparticles (Ag NPs) onto the leather surface resulted in the multifunctional leather surface. TSC has the ability to reduce as well as stabilize. Silver ions were reduced to silver nanoparticles using TSC, and citrate ions were employed to stabilize silver nanoparticles on the leather surface. For leather/Ag NPs (300 ppm) and leather/Ag NPs (1000 ppm), the particle size distribution on the leather surface was in the range of 55 6 17 and 130 6 16 nm, respectively. The leather/Ag NPs samples showed remarkable antibacterial activity against five pathogenic microorganisms such as two Gram-positive bacteria, B. subtilis and S. aureus, two Gram-negative bacteria, E. coli and P. aeruginosa, and one fungus, C. albicans (El-sayed et al., 2021). Antibacterial properties of leather treated with ZnO nanoparticles as a substitute for volatile organic biocide compounds. In comparison to sol gel approaches, hydrothermal synthesis of ZnO nanoparticles was described as an efficient and easy to control process. Traditional approaches based on sprayed layers uniformly spread the ZnO nanoparticles across the leather surface, making them available for direct contact with bacteria and fungi. The treated leather surfaces were examined using customized diffusion standard procedures and showed bacterial sensitivity against S. aureus (ATCC 25923) and E. coli (ATCC 25922). The

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photocatalytic formation of ROS linked to antimicrobial efficacy was demonstrated by the alteration of the dynamic contact angle of water on the leather surface exposed to UV and visible light irradiation (Gaidau et al., 2018).

16.8

Conclusions

Metal oxide nanocomposites in coatings contain a promising material not only to kill the microbes effectively but also to inhibit the microbe growth observed on different surfaces, and hence the development of biostatic or biocidal coatings has been progressing dramatically for controlling such microbes. Among the different uses of antimicrobial agents, surface coating is one of the promising strategies to disinfect the surfaces of various objects or tools. Particularly, in this chapter, a review on the metal oxide nanocomposites as antimicrobial agents has been provided due to their overriding advantages such as ease of synthesis, higher surface area, excellent stability, non-toxicity, higher activity toward a wide range of microbe’s inactivation, etc. The nanocomposites are formed by the combination of various metal oxides with other materials such as carbon nanostructures, reduced graphene oxide, polymer, etc., for obtaining enhanced chemical, physical, and antimicrobial properties of the materials. Different protocols for preparing the metal oxide-based nanocomposites have been also included. In addition, this chapter also reviews the metal oxide nanocomposites-based antimicrobial coatings used in different sectors, such as the hospital sector, the paint sector, the polymer sector, and the textile sector. Therefore, the metal oxide nanocomposites-based antimicrobial coatings can be used effectively to control the microbial proliferation of various objects and devices used in common places of high-risk infections.

16.9

Future outlooks

Though metal oxide-based nanocomposites are effective antimicrobial coatings, further endeavors need to be considered to address the lagging of the present state of the art in this area: 1. Owing to the toxicity as well as cytotoxicity of the nanomaterials, their composites have been used in a limited scope for paint formulations, and hence more attention should be focused on the safety concerns of living organisms. 2. Exterior nanocomposites like antimicrobial additives in paint coatings have superior antimicrobial activity against the microbes than that of its interior present in paint formulations. Further protocols need to be designed so that either the internal or external presence of nanocomposites would not decline the antimicrobial activity of the nanocompositebased antimicrobial paints. 3. In the case of light-activated paints, the photoactive nanomaterials as antimicrobial agents may degrade the other additives in the presence of photons, and hence these may hamper the quality as well as activity of the paints. Therefore, strategies need to be developed for photoactive antimicrobial agents so that this problem can be solved.

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4. Further research endeavors in antimicrobial paints will be focused on the studies of antimicrobial activity of the nanomaterials-based paints concerning the different environmental conditions of weather. 5. The stability of the nanocomposites coated on the various objects as well as the toxicity of eventually released components from paints need to be studied very well so that objects have permanent antimicrobial coatings without harmful effects.

Acknowledgment The author VSG and SDD thank Rajiv Gandhi Science and Technology, Mumbai Government of Maharashtra for financial support [RGST/SUK/JPJ/PAC-07/2019 20].

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Understanding the Antifungal Mechanism of Ag@ZnO Core-shell Nanocomposites against Candida krusei. (2016). Scientific Reports, 6, 1 12. https://doi.org/10.1038/srep36403 (Original work published 2016). Varaprasad, K., Pariguana, M., Raghavendra, G. M., Jayaramudu, T., & Sadiku, E. R. (2017). Development of biodegradable metaloxide/polymer nanocomposite films based on polyε-caprolactone and terephthalic acid. Materials Science and Engineering C, 70, 85 93. Available from https://doi.org/10.1016/j.msec.2016.08.053. Vasile, B. S., Oprea, O., Voicu, G., Ficai, A., Andronescu, E., Teodorescu, A., & Holban, A. (2014). Synthesis and characterization of a novel controlled release zinc oxide/gentamicin-chitosan composite with potential applications in wounds care. International Journal of Pharmaceutics, 463(2), 161 169. Available from https://doi.org/10.1016/j. ijpharm.2013.11.035. Vieillard, J., Bouazizi, N., Morshed, M. N., Clamens, T., Desriac, F., Bargougui, R., Thebault, P., Lesouhaitier, O., Le Derf, F., & Azzouz, A. (2019). CuO nanosheets modified with amine and thiol grafting for high catalytic and antibacterial activities. Industrial and Engineering Chemistry Research, 58(24), 10179 10189. Available from https://doi.org/10.1021/acs.iecr.9b00609. Viet, P. V., Phan, B. T., Mott, D., Maenosono, S., Sang, T. T., Thi, C. M., & Hieu, L. V. (2018). Silver nanoparticle loaded TiO2 nanotubes with high photocatalytic and antibacterial activity synthesized by photoreduction method. Journal of Photochemistry and Photobiology A: Chemistry, 352, 106 112. Available from https://doi.org/10.1016/j. jphotochem.2017.10.051. Wang, J., Wang, Y., Zhang, D., Xu, C., & Xing, R. (2020). Dual response mimetic enzyme of novel Co4S3/Co3O4 composite nanotube for antibacterial application. Journal of Hazardous Materials, 392122278. Available from https://doi.org/10.1016/j. jhazmat.2020.122278. Wang, Y., Ma, J., Xu, Q., & Zhang, J. (2017). Fabrication of antibacterial casein-based ZnO nanocomposite for flexible coatings. Materials and Design, 113, 240 245. Available from https://doi.org/10.1016/j.matdes.2016.09.082. Xu, X. Y., & Yan, B. (2017). Eu(iii)-functionalized ZnO@MOF heterostructures: Integration of pre-concentration and efficient charge transfer for the fabrication of a ppb-level sensing platform for volatile aldehyde gases in vehicles. Journal of Materials Chemistry A, 5(5), 2215 2223. Available from https://doi.org/10.1039/C6TA10019H. Yadav, H. M., Otari, S. V., Bohara, R. A., Mali, S. S., Pawar, S. H., & Delekar, S. D. (2014). Synthesis and visible light photocatalytic antibacterial activity of nickel-doped TiO2 nanoparticles against Gram-positive and Gram-negative bacteria. Journal of Photochemistry and Photobiology A: Chemistry, 294, 130 136. doi:10.1016/j. jphotochem.2014.07.024. Yadav, H. M., Otari, S. V., Koli, V. B., Mali, S. S., Hong, C. K., Pawar, S. H., & Delekar, S. D. (2014). Preparation and characterization of copper-doped anatase TiO2 nanoparticles with visible light photocatalytic antibacterial activity. Journal of Photochemistry and Photobiology A: Chemistry, 280, 32 38. Available from https://doi.org/10.1016/j. jphotochem.2014.02.006. Youssef, A. M., Assem, F. M., Abdel-Aziz, M. E., Elaaser, M., Ibrahim, O. A., Mahmoud, M., & Abd El-Salam, M. H. (2019). Development of bionanocomposite materials and its use in coating of Ras cheese. Food Chemistry, 270, 467 475. Available from https:// doi.org/10.1016/j.foodchem.2018.07.114. Yuan, H., Chen, L., & Hong, F. F. (2020). A biodegradable antibacterial nanocomposite based on oxidized bacterial nanocellulose for rapid hemostasis and wound healing. ACS

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Applied Materials and Interfaces, 12(3), 3382 3392. Available from https://doi.org/ 10.1021/acsami.9b17732. Zare, M., Namratha, K., Alghamdi, S., Mohammad, Y. H. E., Hezam, A., Zare, M., Drmosh, Q. A., Byrappa, K., Chandrashekar, B. N., Ramakrishna, S., & Zhang, X. (2019). Novel green biomimetic approach for synthesis of ZnO-Ag nanocomposite: Antimicrobial activity against food-borne pathogen, biocompatibility and solar photocatalysis. Scientific Reports. Available from https://doi.org/10.1038/s41598-019-44309-w. Zhang, X., He, X., Kang, Z., Cui, M., Yang, D. P., & Luque, R. (2019). Waste eggshellderived dual-functional CuO/ZnO/eggshell nanocomposites: (Photo) catalytic reduction and bacterial inactivation. ACS Sustainable Chemistry and Engineering, 7(18), 15762 15771. Available from https://doi.org/10.1021/acssuschemeng.9b04083. Zhu, Z., Zhang, Y., Shang, Y., & Wen, Y. (2019). Electrospun nanofibers containing TiO2 for the photocatalytic degradation of ethylene and delaying postharvest ripening of bananas. Food and Bioprocess Technology, 12(2), 281 287. Available from https://doi. org/10.1007/s11947-018-2207-1. Zielecka, M., Bujnowska, E., Ke¸pska, B., Wenda, M., & Piotrowska, M. (2011). Antimicrobial additives for architectural paints and impregnates. Progress in Organic Coatings, 72 (1 2), 193 201. Available from https://doi.org/10.1016/j.porgcoat.2011.01.012.

Metal oxide composites in organic transformations

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Meghshyam K. Patil1, Sambhaji T. Dhumal1,2 and Vijay H. Masand3 1 Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Sub-Campus Osmanabad, Maharashtra, India, 2Department of Chemistry, Ramkrishna Paramhansa Mahavidyalaya, Osmanabad, Maharashtra, India, 3Department of Chemistry, Vidya Bharati Mahavidyalaya, Amravati, Maharashtra, India

17.1

Introduction

Composite material has a grouping of two materials with dissimilar physical and chemical properties. When they are combined, they create a material that is specialized to do a certain job, for instance, to become stronger, lighter, or resistant to electricity. They can also improve strength and stiffness (Mishnaevsky et al., 2017). Several examples of composites are known, which are used in daily routine, including reinforced concrete and masonry, ceramic matrix composites, metal composites, fiber-reinforced polymers, fiberglass, and so on (Masuelli, 2013). Composite material gives surprising or better properties than its individual counterparts, for example, carbon fiber-reinforced composite is five times stronger than 1020 grade steel while having only one-fifth of the weight. In addition, the composite material of graphene and copper is 500 times stronger than copper (Jafarian et al., 2021; Song, 2017). Similarly, a composite of graphene and nickel has 180 times greater strength than nickel (Arciniegas Jaimes et al., 2020; Gu¨ler & Ba˘gcı, 2020). Composites have several different applications in chemistry and material science. In the last few decades, composites of metal oxides with other materials are gaining a lot of interest. In the literature, composites of metal oxides are found with other metal oxides, inorganic materials like g-C3N4, RGO, organic compounds, polymers, and so on. Composites of metal oxides have different applications in material science and chemistry, which includes photocatalytic degradation (Al-Rawashdeh et al., 2020; Sutar et al., 2020, 2021), solar cells (Xuhui et al., 2014), drug delivery (Park et al., 2016), water splitting (Zhu et al., 2020), organic transformations, and so on (Chaudhary et al., 2020). Among these applications, composites of metal oxides are relatively less explored for organic transformations. Generally, homogeneous catalysts or liquid reagents have been used to perform organic transformations. Homogeneous catalysts include several catalysts such as Bronsted acids, Lewis acids, superacids (such as H2SO4, HCl, HNO3, HF, HClO4, H3PO4, AlCl3, SbF5), hydrogenation catalysts (such as palladium chloride, Wilkinsons catalyst), metal complexes, and so on (Denisov et al., 2003; Leeuwen et al., 2003; Patil et al., 2011; Reddy & Patil, 2008; Regan, 1995). In homogeneous catalysis, degree of interaction between catalyst and reactants is very Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00008-7 © 2022 Elsevier Inc. All rights reserved.

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high as all are in the same phase. This offers faster reactions and in some cases, the reaction takes place in milder conditions. However, removal of the catalysts at end of the reaction or recovery of the product at the end of the reaction requires a tedious work-up procedure. Further, one cannot reuse the homogeneous catalyst. In addition, during the work-up procedure, a lot of waste has been generated, which is directly or indirectly responsible for water pollution. Therefore, increased research is going on to replace these homogeneous catalysts with heterogeneous catalysts, which can offer several advantages over the traditional homogeneous catalysts. Heterogeneous catalyst is easy to recover and reuse, further, it offers an easy work-up procedure (Li et al., 2020; Liu & Corma, 2018; Patil et al., 2011; Reddy & Patil, 2009; Wang, Li, et al., 2018). Heterogeneous catalysts offer the processes, which would be chemically, economically, and environmentally more favorable (Roma´n-Martı´nez & SalinasMartı´nez de Lecea, 2013). Predominantly, heterogeneous catalysts are nanomaterials, which provide better interfacial contact between reactants and active sites of the catalysts (Astruc, 2020; Lo et al., 2021) In heterogeneous catalysts, composite materials are eye-catching as the formation of composites provides some extra and promising properties along with properties of individual counterparts (Sudarsanam et al., 2018; Xie et al., 2010; Yuan et al., 2020). The composite of metal oxide as catalysts for organic transformations is a surprise package, as in most of the cases composite gives higher activity than their individual counterparts. For example, PANI gC3N4 TiO2 has shown higher catalytic efficiency than g-C3N4 TiO2, g-C3N4, and TiO2 (Wang, Shen, et al., 2018). Furthermore, composites can be designed as per the need of the reactions. Composite of metal oxides used for organic transformations are generally binary composites but sometimes ternary composites are also used. Further, some of the nanocomposites have magnetic properties. These magnetic nanocomposites can be easily recycled after the completion of the reaction. These catalysts can catalyze the organic transformations or act as effective support for the active catalysts (Chng et al., 2013; Lee et al., 2012; Maleki et al., 2019; Yi et al., 2006; Zhou et al., 2010). Different composites of metal oxides are mentioned in the literature, which is well explored for organic transformations. Some notable examples are Ag2O ZrO2, Au ZrO2, Bi2O3 ZnO, CeO2 MWCNT, CoFe2O4@B2O3 SiO2, Bi2O3 ZnO, Sulphated mesoporous La2O3 ZrO2, ZnO@zeolite-Y, MgO ZrO2, Fe3O4@cellulose, BNC PS-ZnO, Fe3O4@chitosan, Ag2O/GO/TiO2, sulfonated carbon@titania, polyaniline g-C3N4 TiO2, SiO2/Fe2O3, CuO CeO2, nanoFe3O4@TiO2/Cu2O and so on. Different metal oxide-based composites have been used for several organic transformations such as click reactions, coupling reactions, MCRs, photocatalytic reactions, and so on.

17.2

Design and characterization of nanocomposites

Several different types of metal oxide-based nanocomposites have been reported in the literature. These composites include metal-metal oxide, metal oxide-metal oxide, metal oxide-polymer, and metal oxide composite with inorganic materials like GO, rGO, or

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g-C3N4 and so on (Butt et al., 2020; El-Shafai et al., 2018; Veith et al., 1998; Wu et al., 2017). As there are several varieties of metal oxide-based composites, different synthetic methods have been employed for the design of the composites. Some of the notable methods are physical vapor deposition, chemical vapor deposition (CVD), sol gel, solvothermal, hydrothermal, coprecipitation, microemulsion, microwaveassisted synthesis, and so on (Akram et al., 2018; Hu et al., 2013; Malik et al., 2012). Molaei and Javanshir prepared the lignocellulose-based bio nanocomposite via in situ hydrothermal synthesis (Molaei & Javanshir, 2018). CoFe2O4@B2O3 SiO2 magnetic nanocomposite has been reported by ultrasonic treatment using the glass-ceramic method (Maleki et al., 2017). Albadi et al. prepared CuO CeO2 nanocomposite by using the coprecipitation method using precursors cerium nitrate and copper nitrate and KOH as precipitating agents (Albadi et al., 2014). To understand the physicochemical properties of the prepared metal oxides, nanocomposites should be studied using sophisticated characterization techniques such as X-ray diffraction (XRD), energy-dispersive X-ray (EDX), Scanning Electron Microscopy (SEM), High-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, Raman Spectroscopy, thermal gravimetric analysis (TGA)/differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS) and N2 adsorption-desorption by Brunauer Emmett Teller (BET) analysis. These characterization results tell us about the properties, structural features, and surface morphology of the catalysts. Apart from the regular information, characterization results also help to confirm the formation of composites. Other than these techniques, UV-Visible diffuse reflectance spectroscopy (UV Vis DRS) has been used for the characterization of photocatalysts. Magnetic properties have been studied for magnetic nanocomposites. In this section, characterization results for some catalysts have been discussed. Akonadi et al. have prepared Ce/SiO2 nanocomposites by sol gel method and characterized by using XRD, XPS, UV-DRS, SEM, TEM, and surface area analysis (Akondi et al., 2016). XRD result confirms the presence of cubic CeO2 in the composites (fresh and after reuse also) from the presence of peaks at 2Θ values 28.6 degrees, 32.8 degrees, 47.5 degrees, 56.3 degrees, and 76.7 degrees (Fig. 17.1). Further, SEM, TEM analysis also tells about nano-sized particles and the morphology of the catalyst. Zhao et al. prepared sulfated mesoporous La2O3 ZrO2 composite oxide solid acid catalyst by surfactant-assisted coprecipitation/hydrothermal crystallization with subsequent impregnation method (SACPHC-IM) (Zhao & Ran, 2015). Acidic sites on the surface of the catalysts are responsible for the acidic properties of the catalysts, which are responsible for the catalytic performance of the solid acid catalyst. FTIR spectra and NH3 temperature-programmed desorption (NH3-TPD) have provided valuable information regarding the acidic sites of the catalyst (Chen et al., 2012; Reddy & Patil, 2009). Fig. 17.2 shows the FTIR spectra of mesoLa2O3 ZrO2 before and after sulfate impregnation. The presence of four new bands at 1225, 1137, 1043, and 995 cm21 in the FTIR of SO422/meso-La2O3 ZrO2 sample suggests chelating bidentate SO42 a structure which is responsible for the strong Lewis acid sites and partial Lewis acid site changes to the Bronsted acid site when it absorbs water (Chen et al., 2012; Reddy & Patil, 2009).

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Figure 17.1 X-ray diffraction analysis of (a) fresh and (b) used Ce-SiO2 nanocomposite. Source: From Akondi, A. M., Kantam, M. L., Trivedi, R., Bharatam, J., Vemulapalli, S. P. B., Bhargava, S. K., Buddana, S. K., & Prakasham, R. S., (2016). Ce/SiO2 composite as an efficient catalyst for the multicomponent one-pot synthesis of substituted pyrazolones in aqueous media and their antimicrobial activities. Journal of Molecular Catalysis A: Chemical, 411, 325 336. https://doi.org/10.1016/j.molcata.2015.11.004.

Gold nanoparticles deposited on MnO2 nanorod-modified graphene oxide composite (Au MnO2 GO) have been prepared by Nayak et al. (2019). Composite Au MnO2 GO has been characterized by different techniques such as XRD, FTIR, Raman spectroscopy, FE-SEM, TEM, HRTEM, XPS, N2 adsorptiondesorption BET isotherm, and Inductively coupled plasma—optical emission spectrometry (ICP-OES) (Tang et al., 2010). Composite Au MnO2 GO has been characterized by different techniques such as XRD, FTIR, Raman spectroscopy, FESEM, TEM, HRTEM, XPS, N2 adsorption-desorption BET isotherm, and ICP-OES. Fig. 17.3 shows the TEM and HRTEM images of GO MnO2 Au of ternary composite. TEM images revealed that the MnO2 has well-defined rod-like morphology. TEM (Fig. 17.3A D) and HRTEM (Fig. 17.3E and F) reveal that MnO2 is in the form of nanorods having high crystallinity. Further, gold nanoparticles are spherical in nature and have particle sizes 7 6 1.9 nm. From Fig. 17.3D, lattice spacing is to be 0.30 nm, confirming the presence of tetragonal β-MnO2 crystal, which is consistent with the d-spacing of the (200) plane (Tang et al., 2010). Further, XPS, Raman spectroscopy, SEM, EDAX, TGA are used for the study of metal oxide-based composites. From TGA analysis, Kour et al. (2016) in their study confirmed that the Lewis acid grafted sulfonated carbon@titania composite is thermally stable up to 240 C, and it could be safely used for organic reactions at 100 C. Harikrishna et al. studied the surface morphology of the CeO2/MWCNT nanocomposites by using SEM analysis (Harikrishna et al., 2020). Liu et al.

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Figure 17.2 Fourier transform infrared spectroscopy spectra of meso-La2O3 ZrO2 and SO42-/meso-La2O3 ZrO2 solid acid catalysts. Source: From Zhao, Z., & Ran, J., (2015). Sulphated mesoporous La2O3-ZrO2 composite oxide as an efficient and reusable solid acid catalyst for alkenylation of p-xylene with phenylacetylene. Applied Catalysis A: General, 503, 77 83. https://doi.org/10.1016/j. apcata.2015.01.023.

examined the PANI-g-C3N4 TiO2 composites by XPS, which confirms the same chemical element in the catalyst as their formula (Liu et al., 2017).

17.3

Applications of metal oxide composites for organic transformations

MCRs and other organic transformations are gaining much more attention (Gore & Rajput, 2013; Reddy & Patil, 2008; Reddy et al., 2008). These organic transformations give a suitable route for the synthesis of bio-active compounds and other valueadded organic compounds (Patil et al., 2008, 2011; Reddy & Patil, 2009). Selected organic transformations performed by using metal oxide-based composites have been discussed in this section. Also, comment on some relative information such as the importance of particular composite and organic reaction has been mentioned.

17.3.1 Synthesis of bis (pyrazol-5-ol) and dihydropyrano[2,3-c] pyrazole analogs Fatahpour et al., have prepared the Ag/TiO2 nanocomposites films on glass substrate by employing the spray pyrolysis technique (Fatahpour et al., 2017). For the

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Figure 17.3 Transmission electron microscopy (A D) and high-resolution transmission electron microscopy (E F) images of GO MnO2 Au of ternary composite. Source: From Nayak, P. S., Barik, B., Achary, L. S. K., Kumar, A., & Dash, P., (2019). Gold nanoparticles deposited on MnO2 nanorods modified graphene oxide composite: A potential ternary nanocatalyst for efficient synthesis of Betti bases and bisamides. Molecular Catalysis, 474, 110415. https://doi.org/10.1016/j.mcat.2019.110415.

preparation of these thin films, titanium isopropoxide and silver nitrate were applied as precursors of Ti and Ag respectively. These thin films have been efficiently used as catalysts for the MCR for the synthesis of bis (pyrazol-5-ol) and dihydropyrano [2,3-c]pyrazole analogs. 4-(Arylmethylene)bis (1H-pyrazol-5-ol) derivatives have been synthesized by the MCR between ethyl acetoacetate (2.0 mmol), hydrazine hydrate (2.0 mmol), and aromatic aldehydes (1 mmol) (Fig. 17.4), however, dihydropyrano[2,3-c] pyrazole derivatives have been prepared by MCR between hydrazine hydrate (1.0 mmol), ethyl acetoacetate (1.0 mmol), aromatic aldehyde (1 mmol) and malononitrile (1.0 mmol) (Fig. 17.5). Both of these MCRs have been performed at 70 C using water: ethanol (2:1) as solvent. These MCRs associated with high yield of products using simple starting materials, eco-friendliness, and heterogeneity, and reusability of catalyst. Pyrazoles and pyranopyrazoles are important classes of five-membered N-fused heterocycles, which have received much attention. As they are part of numerous drug molecules possessing and varied biological activities such as analgesic, antipyretic, antifungal, antimicrobial, antiinflammatory, antidepressant, antitumor, anti-HIV, etc. (Ismail et al., 2007; Mariappan et al., 2011).

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Figure 17.4 Synthesis of 4-(arylmethylene)bis (1H-pyrazol-5-ol) derivatives.

Figure 17.5 Synthesis of dihydropyrano[2,3-c] pyrazole derivatives.

17.3.2 Synthesis of pyrimido benzazoles Molaei and Javanshir (2018) have prepared a new lignocellulose-based bio nanocomposite with ZnO. The lignocellulosic waste peanut shells and zinc acetate have been used for the synthesis of this new lignocellulose-based bio nanocomposite PS/ZnO, which was prepared by employing in situ hydrothermal process. PS/ZnO has been successfully applied as a catalyst in water as a solvent and under microwave irradiation for the synthesis of pyrimido [1, 2-b] benzazole derivatives. By using this composite catalyst, Molaei and Javanshir have performed the synthesis of pyrimido [1, 2-b] benzazole derivatives by using four different sets of one-pot three-component reactions, involving condensation of 2-aminobenzimidazole, 2-aminobenzothiazole, or guanidinium chloride, with aldehydes and C H acidic compounds such as 2-hydroxy-1,4-naphtoquinone, dimedone, malononitrile and ethyl acetoacetate (Fig. 17.6). This new prepared composite has shown excellent catalytic activity for synthesis of pyrimido [1, 2-b] benzazole derivatives as compared to their individual counterpart. By employing this strategy, eighteen different compounds have been synthesized. This type of polycyclic fused-ring heterocycles with nitrogen and sulfur atom having importance in pharmaceutical due to associated diverse biological properties such as antimicrobial, antifungal, antibacterial, antihypertensive, antiviral, anticancer activities, and so on (Sahu et al., 2012; Yildiz-Oren et al., 2004; Youssef & Noaman, 2007).

17.3.3 Synthesis of pyridine-3-carboxamides Harikrishna et al. have synthesized ceria doped multi-walled carbon nanotubes nanocomposites that is 1% CeO2/MWCNT, 2.5% CeO2/MWCNT, and 5% CeO2/ MWCNT by employing a simple method (Harikrishna et al., 2020). These composites have been employed for click reaction between acetoacetanilide, ammonium

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Figure 17.6 Synthesis of pyrimido [1, 2-b] benzazole derivatives by using different sets of one-pot three-component reactions.

acetate, substituted aromatic aldehyde, and ethyl cyanoacetate to produce pyridine3-carboxamides at room temperature and employing ethanol as a solvent (Fig. 17.7). Among these different catalysts including individual counterparts of composites that is CeO2 and MWCNT, the 2.5% CeO2/MWCNT composite have shown better activity for this MCR. Further, CeO2/MWCNT nanocomposite catalyst has been recycled several times (six times) with very little loss in activity. Pyridine containing scaffold is a key component of various clinically used or FDA-approved drugs such as antiallergic agent Ibudilast, platelet aggregation inhibitor KC-764, dopamine D4 antagonist FAUC213, antitubercular agents- isoniazid, ethionamide, and prothionamide (Tang et al., 2015).

17.3.4 Synthesis of benzimidazolo[2,3-b]quinazolinone derivatives Maleki et al. employed Fe3O4@chitosan composite for the MCR between 2aminobenzimidazole or 2-aminobenzothiazole, dimedone, and various aldehydes

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Figure 17.7 Multicomponent reaction for the synthesis of pyridine-3-carboxamides.

Figure 17.8 Synthesis of benzimidazolo[2,3-b]quinazolinones.

for the synthesis of benzimidazolo[2,3-b]quinazolinone derivatives (Maleki et al., 2015). Fe3O4@chitosan is an environmentally benign, reusable nanocomposite catalyst and offered good to excellent yields for this MCR (Fig. 17.8). Among different catalysts (such as I2, Amberlyst-15, H6P2W18O62
18H2O, Fe3O4@chitosan, Fe3O4, Nano-Fe3O4), composite Fe3O4@chitosan has shown the best activity. Most interestingly, Fe3O4@Chitosan showed better activity than Fe3O4 and Nano-Fe3O4, which shows the effect of composite formation on the reactivity of the catalyst. Further, this catalyst has been recycled many times with minute loss in activity (for five cycles, loss in activity is from 90% to 89%). Benzimidazoloquinazolines are an important class of heterocycles as they incorporate both biodynamic heterocyclic rings benzimidazole and quinazoline, which have shown significant anticancer activities (Grasso et al., 2000). Further, benzimidazoquinazolines are found to be cyclin-dependent kinase and glycogen syntheses kinase-3 (GSK-3) inhibitors (Testard et al., 2005). Also, the same reaction has been studied by the Fe3O4@clay core-shell magnetic nanocomposite having advantages such as the easy recovery of the catalyst due to magnetic property, high yield, easy work-up procedure, and so on (Maleki & Aghaei, 2017).

17.3.5 Synthesis of dihydroquinazolinones Ali Maleki et al., have prepared the hybrid magnetic nanocomposite CoFe2O4@B2O3 SiO2 via ultrasonic treatment by glass-ceramic method (Maleki et al., 2017). The prepared catalyst has been characterized by using different characterization techniques such as field-emission scanning electron microscopy (FESEM), XRD, vibrating sample magnetometer (VSM), EDX, element distribution image (EDX mapping), TGA/DTA and N2 adsorption-desorption by BET analysis.

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Figure 17.9 Sonochemical and CoFe2O4@B2O3 SiO2 catalyzed synthesis of 2-phenyl-3(phenylamino)-dihydroquinazolin-4(1H)-ones.

Further, this catalyst has been efficiently applied for the synthesis of 2-substituted3-(phenylamino)-dihydroquinazolin-4(1H)-ones by MCR between isatoic anhydride, phenylhydrazine, and different aldehydes in deep eutectic solvent (DES) (Fig. 17.9). The advantages associated with this protocol include high yield of products, use of recyclable non-hazardous solvent, short reaction times, easy work-up procedure, simple recovery of the catalyst due to its magnetic property, and recyclability of the catalyst (at least five times). The literature study revealed that dihydroquinazolinones is an active scaffold in medicinal chemistry because it is part of several marketed drugs and drugs in clinical trials. Evodiamine, quinethazone, fenquizone, and metolazone are examples of dihydroquinazolinone containing drugs. In addition to this, DPC083, DPC961, and DPC963 are acting as non-nucleoside reverse transcriptase inhibitors and used in HIV treatment (Hemalatha & Madhumitha, 2016).

17.3.6 Synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones A series of Lewis acid grafted sulfonated carbon@titania composites have been synthesized by Kour et al. (2016). In this series, different Lewis acids have been used such as AlCl3, FeCl3, SbCl3, SnCl2, Cu(OAc)2, and Bi(NO3)3. This series of catalysts were synthesized by sulfonation of carbon@titania composites, which was further treated with different Lewis acids. These composite catalysts have been explored for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones (Kour et al., 2016). Among these designed catalysts, carbon-titania composite C/TiO2 SO3 SbCl2 was found to be the best candidate. Different 4H-pyrimido [2,1-b]benzothiazoles have been synthesized by the MCR between ethyl acetoacetate, aldehyde, and 2-aminobenzothiazole under the solvent-free condition at 90 C (Fig. 17.10). Further, benzoxanthenone derivatives have been synthesized by the MCR between α- or β-naphthol, aldehyde, and dimedone under the solvent-free condition at 100 C (Fig. 17.11). Both the multicomponent methodologies are associated with easy work-up procedure, high yield of the product, and easy recovery of the catalyst. The catalyst was dried at 90 C and further reused. Researchers paid much attention to the synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones due to their varied pharmacological activities, use in dyes and fluorescent materials (Kour et al., 2016).

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Figure 17.10 The multicomponent reaction between dimedone or ethyl acetoacetate, aldehyde, and 2-aminobenzothiazole.

Figure 17.11 The multicomponent reaction between α- or β-naphthol, aldehyde, and dimedone.

17.3.7 Synthesis of chromene derivatives Chromene derivatives are an important group of heterocycles having diverse biological properties and therapeutic applications (Abrunhosa et al., 2007; Pratap & Ram, 2014; Robert et al., 2008). Further, these heterocycles are widely distributed among many plants. These natural compounds show valuable bio-activities. Also, fused chromenes are vital constituents of pharmacologically active compounds. Different MCRs are reported in the literature for the synthesis of chromene derivatives (Akondi et al., 2016; Pratap & Ram, 2014). Several verities of catalysts have been used for these MCRs. Further, metal oxide composites are also used as catalysts for the synthesis of different series of chromene derivatives. These syntheses are discussed in this section.

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17.3.7.1 Synthesis of aminochromenes Albadi et al. have prepared CuO CeO2 nanocomposite by coprecipitation method using nitrate precursors of cerium and copper (Albadi et al., 2013). The pH of the solution has been maintained by the dropwise addition of KOH as a precipitating agent. The resulting precipitate was filtered, washed, and finally calcined to obtain the nanocomposite. To check the physicochemical properties of the catalyst, characterization has been carried out by using XRD, FESEM, and EDS, which gives information about the phases present, structural features, and elemental composition. This prepared catalyst has been used for the MCR between aldehyde, malononitrile and resorcinol at 80 C under solvent-free conditions for the synthesis of aminochromenes (Fig. 17.12). The advantages associated with this protocol are a simple recovery of catalyst by filtration, simple work-up, short reaction time, clean procedure, high yield of products, and recyclability of the catalyst. Aminochromenes are an important class of compounds, which received significant attention due to a variety of biological and pharmaceutical properties, includes antisterility and anticancer agents (Kidwai et al., 2005). Also, the main constituent of many natural products is aminochromenes. Further, these compounds are used as cosmetics, pigments, and potential biodegradable agrochemicals (Dekamin et al., 2013).

17.3.7.2 Synthesis of 2-amino-benzochromenes Compounds containing benzochromene moiety emerged as an attractive scaffold in drug development due to a wide range of pharmacological activities such as antimicrobial, anticancer, hypolipidemic, antioxidant, and analgesic (Zhang et al., 2020). Kalhor et al., have prepared ZnO@zeolite-Y mesoporous composite by the hydrothermal method by using ZnCl2.2H2O and zeolite-Y as precursors, whereas ammonia solution has been used to maintain a pH at 11. The prepared composite ZnO@zeolite-Y has been characterized by using XRD, FTIR, EDX, FE-SEM, and BET surface area analysis. Composite showed an increase in surface area as compared to that of zeolite-Y. This composite has been successfully utilized for the synthesis of 2-amino-4H-chromenes by MCR (Kalhor et al., 2020). Initially, to optimize the reaction condition multicomponent condensation reaction between benzaldehyde, malononitrile, and β-naphthol has been studied by varying catalysts (i.e., by using ZnO, zeolite-Y, and ZnO@zeolite-Y), catalyst amount, and reaction temperature. ZnO@ zeolite-Y is found to be the best candidate for this MCR. Accordingly, the reaction between different aromatic aldehydes, malononitrile, and

Figure 17.12 The multicomponent reaction for the synthesis of aminochromenes.

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Figure 17.13 The multicomponent reaction for synthesis of 2-amino-4H-chromenes.

Figure 17.14 The multicomponent reaction for the synthesis of pyrano[3,2-c]quinolones and pyrano[3,2-c]chromene derivatives.

β-naphthol at 110 C under solvent-free conditions has been studied for the synthesis of different 2-amino-benzochromenes (Fig. 17.13). The advantages associated with this protocol involve short reaction time (10 minutes to 40 minutes), solvent-free reaction, easy work-up procedure, recyclability of the catalyst.

17.3.7.3 Synthesis of pyrano[3,2- c]quinolones and pyrano[3,2-c] chromene derivatives Balivand and Ghashang prepared NiO SnO2 nanocomposite by using the sol gel method (Balivand & Ghashang, 2018). Prepared composite is characterized by XRD and FE-SEM. XRD confirmed the formation of NiO and SnO2 with rhombohedral and tetragonal phases respectively. Further, nanocomposite having spherical morphology having a particle size less than 100 nm. This nanocomposite has been employed for the MCR between 4-hydroxy-6-methylquinolin-2(1H)-one derivatives/4-hydroxycoumarin, aldehydes, and malononitrile for the synthesis of pyrano [3,2- c]quinolones and pyrano[3,2-c]chromene derivatives (Fig. 17.14). This methodology has been performed under reflux conditions using ethanol as a solvent and reaction has been accomplished in 2 hour to 7 hour with good to better yield. Further, NiO SnO2 nanocomposite has been successfully recycled for this methodology. This type of organic compounds has been successfully used for a wide range of pharmacological activities which include anti-inflammatory, antibacterial, antitubercular, antiproliferative, and anti-tubulin activities, selective σ1-receptor ligands, and mitotic kinesin-5 inhibitors (Dı´az et al., 2013; Kantevari et al., 2011; Nadaraj et al., 2012; Rad-Moghadam et al., 2013; Schiemann et al., 2010).

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17.3.7.4 Synthesis of novel 4H-chromene-3-carbonitriles Ghashang has prepared ZnAl2O4 Bi2O3 composite nano-powder by a simple precipitation-complexation method (Ghashang, 2016). Corresponding nitrate precursors of Zn, Al, and Bi has been used in stoichiometric amount for the synthesis of ZnAl2O4 Bi2O3 composite. This prepared nanocomposite has been utilized for the synthesis of 4H-chromene-3-carbonitriles by MCR between five different substituted phenols, aromatic aldehyde, and malononitrile (Fig. 17.15). This reaction has been performed by using EtOH/H2O as solvent under reflux conditions. 4Hchromene and its derivatives are found to possess noteworthy biological activities viz; anticancer, anticonvulsant, antimicrobial, antitubercular, and antidiabetic (Raj & Lee, 2020).

17.3.8 Synthesis of 1,4-disubstituted-1,2,3-triazoles Among the various 1,2,3-triazoles, 1,4-disubstituted-1,2,3-triazole derivatives have special importance as it is a bioisostere of peptide linkage. It shows a broad scale of applications in different field areas such as chemistry, biochemistry, medicine, industry, supramolecular chemistry, bioconjugation, polymers, pesticides, pharmaceuticals, and surface science. It also displays various biological activities, specifically, antifungal, antibacterial, antitubercular, anticancer, antioxidant, antidiabetic, anti-HIV, antiviral, and antileishmanial activity (Dhumal et al., 2019). The catalyst CuO CeO2 has been earlier discussed in a synthesis of aminochromenes. Further, it has been used for another MCR, which involves the click synthesis of 1,4-disubstituted-1,2,3-triazoles (Albadi et al., 2014). However, in this click reaction instead of sodium azide (which is generally used), amberlite-supported azide has been used. This amberlite-supported azide has been prepared as shown in Fig. 17.16. The MCR between α-haloketones or benzyl halides and terminal alkynes in the presence of amberlite-supported azide and CuO CeO2 nanocomposite has been studied for regioselective click synthesis of 1,4-disubstituted-1,2,3-triazoles (Fig. 17.17). In another study, Nemati et al. have prepared the magnetic nano-Fe3O4@TiO2/ Cu2O core-shell composite and efficiently utilized it for click reaction between benzyl bromides/phenacyl bromide, phenylacetylene, and sodium azide under reflux condition and water as a solvent (Fig. 17.18) (Nemati et al., 2015).

Figure 17.15 Preparation of 4H-chromene-3-carbonitriles by the multicomponent reaction.

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Figure 17.16 Preparation of amberlite-supported azide.

Figure 17.17 CuO CeO2 composite catalyzed synthesis of 1,4-disubstituted-1,2,3-triazoles.

Figure 17.18 Nano-Fe3O4@TiO2/Cu2O catalyzed synthesis of 1,4-disubstituted-1,2,3triazoles.

17.3.9 Synthesis of pyran derivatives Poly ethyleneoxide (i.e., PEO)-based magnetic nanocomposites Fe3O4/PEO and Fe3O4/PEO/SO3H were synthesized by using FeCl3 and FeCl2 as a precursor for Fe (III) and Fe(II), whereas PEO-900000 as a precursor for PEO, ammonium hydroxide is used to maintain the pH and chlorosulfonic acid is used for the sulfonation. Initially, the MCR between 4-chlorobenzaldehyde, malononitrile, and ethyl acetoacetate was studied by using different catalysts such as PEO, Fe3O4, Fe3O4@PEO, and Fe3O4@PEO-SO3H by using different solvents for the synthesis of pyran derivatives (Maleki et al., 2018). It has been found that the Fe3O4@PEO-SO3H is the best catalyst among the tested catalyst and the optimum condition for the reaction is room temperature and ethanol as a solvent. By taking these conditions in hand, the MCR between different aromatic aldehydes, malononitrile, and ethyl acetoacetate or methyl acetoacetate has been studied by using Fe3O4@PEO-SO3H catalyst

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Figure 17.19 Fe3O4/PEO/SO3H catalyzed multicomponent reaction for the synthesis of pyran derivatives.

at ambient temperature and ethanol as solvent (Fig. 17.19). Different pyran derivatives have been synthesized by using this protocol in good to better yield (83% 95% yield). The advantages associated with this protocol include easy recovery of the catalyst due to magnetic property, high yield of product, recyclability of the catalyst. The pyran bearing structural framework commonly possesses a wide range of biological activities, such as antiviral, anticancer, antimicrobial, antiproliferative, antitubercular, anti-parasitic, antiangiogenesis, and anti-inflammatory (Shehab & Ghoneim, 2016).

17.3.10 Synthesis of thieno[2,3-d]pyrimidin-4(3H)-one Derivative ZnO CeO2 nanocomposite has been used for the synthesis of thieno[2,3-d]pyrimidin-4(3H)-one derivative by condensation reactions. ZnO CeO2 composite has been prepared by the coprecipitation method (Ghayour et al., 2018). Characterization of this catalyst has shown that CeO2 is present in the cubic phase as the dominant phase. Further, zinc and cerium oxides are homogeneously distributed in the sample, and nanocomposite have particle size at about 58 nm. ZnO CeO2 nanocomposite has been successfully employed for the reaction between different aromatic aldehydes and 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxamide at 140 C under solvent-free conditions (Fig. 17.20). The nanocomposite has been recycled for 10 cycles with very little loss in activity. The thieno[2,3-d]pyrimidine scaffold is considered to be pharmacophore as it has some similarities with the biogenic pyrimidines. Therefore, most of the researchers show their interest in the synthesis of thieno[2,3-d]pyrimidine derivatives and also have been investigated for their biological activity. Many of them display better antitumor, antioxidant, cytotoxic, antiproliferative activities as well as tend to inhibit PI3K p110α isoforms, protein kinases, thymidylate synthase, and dihydrofolate reductase (Mavrova et al., 2014).

17.3.11 Synthesis of α-chloro aryl ketones Liu et al. have prepared different PANI (polyaniline) g-C3N4 TiO2 composites having different percentages of polyaniline (PANI) from 20% to 80%. In this study, g-C3N4 has been prepared by using melamine, TiCl4 is used as a precursor of TiO2,

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Figure 17.20 ZnO CeO2 nanocomposite catalyzed synthesis of thieno[2,3-d]pyrimidin-4 (3H)-one derivatives.

Figure 17.21 PANI g-C3N4 TiO2 catalyzed synthesis of α-chloro aryl ketones.

and aniline is used for polyaniline. Ternary PANI g-C3N4 TiO2 composite has been employed for the photocatalytic synthesis of a-chloro aryl ketones (Liu et al., 2017). Initially, different ternary PANI g-C3N4 TiO2 composites, binary PANI TiO2 composites, g-C3N4 TiO2, g-C3N4, and anatase TiO2 have been tested for optimization of reaction conditions for the synthesis of α-chloro aryl ketones. Among all the prepared catalysts PANI(40%) g-C3N4 TiO2 composite is found to be the best catalyst for the reaction of aryl diazonium tetrafluoroborate and aryl alkynes. Reactions were carried out under visible light irradiation and by using sodium chloride as a source of chloride (Fig. 17.21). Different α-chloro aryl ketones have been prepared by employing this strategy in good to better yields (up to 78%). Catalyst has been successfully recycled for eight cycles with some loss in activity. Literature survey shows that α-haloketones are the promising synthons used in the synthesis of various functionalized carbo- and heterocyclic compounds, which confirms the high synthetic potential of α-haloketones (Erian et al., 2003).

17.3.12 C H arylation reactions through aniline activation Previously mentioned ternary PANI g-C3N4 TiO2 composites (Liu et al., 2017) have been utilized for C H arylation reactions through aniline activation under visible light irradiation (Wang, Shen, et al., 2018). In this study also, PANI(40%) gC3N4 TiO2 catalyst was found to be the best working catalyst. Reactions of different anilines with furan or thiophene under visible light using ternary composite have been successfully carried out (Fig. 17.22).

17.3.13 Synthesis of unsymmetrical ureas Unsymmetrical urea compounds have been explored a significant and broad spectrum of biological applications found in agriculture, industry, medicine, pharmaceuticals, and petrochemicals. Most of the unsymmetrical urea compounds are acts as

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Figure 17.22 PANI g-C3N4 TiO2 catalyzed C H arylation reaction.

Figure 17.23 PANI g-C3N4 TiO2 catalyzed synthesis of unsymmetrical ureas.

potent receptor tyrosine kinase inhibitor, antimicrobial, herbicidal, antifungal, antibacterial as well as antileukemic (Janakiramudu et al., 2017). PANI g-C3N4 TiO2 composites have been used for C H arylation reaction and for the synthesis of α-chloro aryl ketones under visible light irradiation (Liu et al., 2017; Wang, Shen, et al., 2018). Also, this ternary composite has shown photocatalytic activity for the synthesis of unsymmetrical ureas. Unsymmetrical ureas have been synthesized by reaction of oxidative decarboxylation of oxamic acids with different primary amines by using ternary composite under visible light and BiOAc as oxidant (Fig. 17.23) (Wang et al., 2020).

17.3.14 Synthesis of Betti bases and bisamides The Betti reaction produces racemic and non-racemic aminobenzylnaphthol ligands. It shows several synthetic applications in asymmetric synthesis such as chiral ligands and chiral auxiliaries. Some of these compounds possess biological activities like antifungal, antibacterial, anti-yeast, and anticancer activity (Olyaei & Sadeghpour, 2019). Gold nanoparticles deposited on MnO2 nanorods modified graphene oxide composite (Au MnO2 GO) have been synthesized by hydrothermal method. This ternary nanocomposite has been used for the synthesis of Betti bases and bisamides (Nayak et al., 2019). Au MnO2 GO is found to be more active than MnO2 GO, MnO2, and GO. Synthesis of Betti base has been performed by MCR between aromatic aldehydes, amines (aniline, pyrrolidine, piperidine, and nbutylamine), and β-naphthol at ambient temperature and water as solvent (Fig. 17.24). Also, the synthesis of bisamides has been carried out by reaction of different aromatic aldehydes and amides (in 1:2 molar proportion) by using Au MnO2 GO as heterogeneous catalyst under the solvent-free condition at 80 C (Fig. 17.25).

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Figure 17.24 Au MnO2 GO catalyzed synthesis of Betti bases by reaction between aldehyde, amine [(A) aniline, (B) pyrrolidine, (C) piperidine] and β-naphthol.

17.3.15 Synthesis of 3-aryl-2-[(aryl)(arylamino)]methyl-4H-furo [3,2-c]chromen-4-one derivatives Furo[3,2-c]chromen-4-one derivatives are biologically active and reported for several activities such as antimicrobial, anti-inflammatory, antiviral, and antibacterial activities and DNA intercalating agents (Al-Sehemi & El-Gogary, 2012; Kale et al., 2016). Abbasi-Dehnavi and Ghashang (2018) have used ZnO ZnAl2O4 nanocomposite for the synthesis of furo[3,2-c]chromen-4-one derivatives by MCR between 4-hydroxycoumarin, aromatic amines, and a,b-epoxy ketones. Heterogeneous catalyst ZnO ZnAl2O4 nanocomposite is prepared by coprecipitation method by using a stoichiometric amount of zinc acetate and aluminum nitrate. ZnO ZnAl2O4 composite has been characterized by using XRD, FE-SEM, and dynamic light scattering techniques. By using this catalyst MCR between 4-hydroxycoumarin, aromatic amines, and a,b-epoxy ketones have been performed for the synthesis of 3-aryl-2[(aryl)(arylamino)]methyl-4H-furo[3,2-c]chromen-4-one derivatives (Fig. 17.26). This methodology has been successfully performed under solvent-free conditions at 120 C.

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Figure 17.25 Au MnO2 GO catalyzed synthesis of bisamide.

Figure 17.26 ZnO ZnAl2O4 catalyzed synthesis of furo[3,2-c]chromen-4-one derivatives.

17.3.16 Synthesis of benzo[4,5]thiazolo[3,2-a]chromeno [4,3-d] pyrimidin-6-one derivatives Fused thiazolo and chromenopyrimidines have shown significant bioactivity which involves anti-inflammatory, antiparkinsonian, anti-HIV, anti-HSV-1, and cardiovascular agents (Amr et al., 2008; Mohamed et al., 2010; Selvam et al., 2012). Also, these classes of compounds have been reported for the antioxidant and antimicrobial, and antibacterial activities (against pathogenic strains Staphylococcus aureus ATCC 6538 and Pseudomonas aeruginosa ATCC 9027) (Afradi et al., 2017; Youssef & Amin, 2012). Mehdi Khalaj has prepared MgO MgAl2O4 nanocomposite and successfully implemented it as a catalyst for the synthesis of benzo[4,5] thiazolo[3,2-a]chromeno [4,3-d]pyrimidin-6-one derivatives (Khalaj, 2020). MgO MgAl2O4 nanocomposite has been prepared by simple coprecipitation technique using magnesium nitrate and aluminum nitrate as precursors and ammonia as a precipitating agent. Fused thiazolo and chromenopyrimidines have been prepared by MCR between 4-hydroxycoumarin, aromatic aldehyde, and 2aminobenzothiazole in stoichiometric amount. This reaction has been successfully studied by using MgO MgAl2O4 nanocomposite by using three different experimental conditions (Fig. 17.27). This catalyst has shown good recyclability for this proposed reaction.

17.3.17 Synthesis of substituted pyrazolones Akondi et al. have prepared Ce/SiO2 composite by sol gel method and characterized by XRD, XPS, UV-DRS, SEM, TEM, and surface area analysis (Akondi et al., 2016). Further, this catalyst has been tested for the MCR between 2-naphthol, aldehydes, phenylhydrazine, and ethyl acetoacetate by using water as a solvent for the synthesis of pyrazolone derivatives (Fig. 17.28). Further, some newly synthesized

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Figure 17.27 MgO MgAl2O4 catalyzed synthesis of fused thiazolo and chromenopyrimidines.

Figure 17.28 Synthesis of pyrazolone derivatives by the multicomponent reaction.

pyrazolone derivatives have been tested for antimicrobial activity. Advantages associated with this methodology include easy recovery of catalyst, good activity after recycling, use of water as a solvent, moderate to a good yield of product. The pyrazolone moieties have gained a lot of research interest in the last several years and shown increasing attention due to their diverse biological activities such as analgesic, antitubercular, antifungal, antibacterial, anti-inflammatory, antioxidant, and antitumor activities (Al-Haiza et al., 2001; Badawey & El-Ashmawey, 1998; Castagnolo et al., 2009; Gu¨rsoy et al., 2000; Himly et al., 2003; Manojkumar et al., 2009; Moreau et al., 2008).

17.3.18 Synthesis of 7-aryl-benzo[h]tetrazolo[5,1-b]quinazoline5,6-dione Tetrazoloquinazoline derivatives mostly show a wide range of pharmacological activities such as antifungal, antibacterial, anticancer, and antimicrobial activity (Khidre et al., 2020). Maleki et al. prepared a novel magnetic polymeric nanocomposite having Bronsted acid-functionalization, Ba0.5Sr0.5Fe12O19@PU-SO3H. Prepared heterogeneous catalyst has been characterized by FE-SEM, XRD, VSM, FTIR spectroscopy, energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and TGA techniques. Further, this magnetic polymeric nanocomposite has been used as catalyst for the synthesis of 7-aryl-benzo[h]tetrazolo[5,1-b] quinazoline-5,6-diones in a DES (Fig. 17.29) (Maleki et al., 2019). DES is an eco-friendly and recyclable media that is based on choline chloride and urea. The magnetic property of the catalyst allows easy and readily recovered from the

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Figure 17.29 Synthesis of 7-aryl-benzo[h]tetrazolo[5,1-b] quinazoline-5,6-diones.

Figure 17.30 Reduction of p-nitrobenzene to aniline.

Figure 17.31 Reduction of p-nitrophenol to p-aminophenol.

reaction mixture with the help of an external magnet and could be reused 6 times without significant loss in activity.

17.3.19 Reduction of nitrobenzene and p-nitrophenol Aromatic amines are essential intermediates as well as the final products in the many chemical and polymer industries (Krogul-Sobczak et al., 2019; Ragaini, 2009; Tafesh & Weiguny, 1996). For example, aniline is an important chemical product having production of more than 2.3106 tons per year and out of which 2/3rd is utilized for the synthesis of polyurethane. The reduction of nitroarenes is the most convenient approach for the synthesis of aromatic amines. Yi et al. have prepared nanocomposite of Pd nanoclusters supported on silica-coated Fe2O3 nanoparticles and used for the hydrogenation of nitrobenzene to aniline with excellent reactivity and reusability (Fig. 17.30) (Yi et al., 2006). In another study, Zhou et al. have prepared magnetic nanocatalysts Fe3O4/SiO2 Pt/Au/Pd with multifunctional hyperbranched polyglycerol amplifiers and employed for the reduction of p-nitrophenol to p-aminophenol (Fig. 17.31) (Zhou et al., 2010). Apart from this, selective hydrogenolysis of glycerol to propylene glycol has been carried out using Cu ZnO composite catalysts (Wang et al., 2010). PPymodified ZrO2 composite has been used as support in selective hydrogenation of cinnamaldehyde (Zong et al., 2015). Furthermore, sulfated mesoporous La2O3 ZrO2 composite has been employed for the alkenylation of p-xylene with phenylacetylene (Zhao & Ran, 2015). Fe3O4/MIL-101(Fe) nanocomposite has been used for the Strecker reaction (Mostafavi & Movahedi, 2018).

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Concluding remarks

Composites of metal oxides have several advantages as compared to homogeneous catalysts and other heterogeneous catalysts for organic transformation reactions. In general, composites show enhanced activity for organic transformations as compared to their single counterparts. By using the metal oxides-based composites as catalysts, several pharmaceutically important compounds and value-added products have been reported which include pyrimido benzazoles, pyridine-3-carboxamides, dihydroquinazolinones, benzoxanthenones, different chromene derivatives, 1,2,3triazoles, pyran derivatives, and so on. Also, synthesis of chloro aryl ketones, C H arylation reaction, and synthesis of unsymmetrical ureas have been performed by using ternary PANI g-C3N4 TiO2 composite as photocatalyst. Further, recyclability, easy work-up procedure, and excellent yield are some of the advantages associated with these catalysts. In the future, there is significant scope for the use of these catalysts for organic transformations due to enhanced activity and associated advantages.

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supported azide. Journal of Chemical Sciences, 126(1), 147 150. Available from https://doi.org/10.1007/s12039-013-0537-0. Al-Haiza, M. A., El-Assiery, S. A., & Sayed, G. H. (2001). Synthesis and potential antimicrobial activity of some new compounds containing the pyrazol-3-one moiety. Acta Pharmaceutica, 51(4), 251 261. Al-Rawashdeh, N. A. F., Allabadi, O., & Aljarrah, M. T. (2020). Photocatalytic activity of graphene oxide/zinc oxide nanocomposites with embedded metal nanoparticles for the degradation of organic dyes. ACS Omega, 5(43), 28046 28055. Available from https:// doi.org/10.1021/acsomega.0c03608. Al-Sehemi, A. G., & El-Gogary, S. R. (2012). Synthesis and photooxygenation of furo[3,2-c] coumarin derivatives as antibacterial and DNA intercalating agent. Chinese Journal of Chemistry, 30(2), 316 320. Available from https://doi.org/10.1002/cjoc.201180483. Amr, A. E. G. E., Maigali, S. S., & Abdulla, M. M. (2008). Synthesis, and analgesic and antiparkinsonian activities of thiopyrimidine, pyrane, pyrazoline, and thiazolopyrimidine derivatives from 2-chloro-6-ethoxy-4-acetylpyridine. Monatshefte Fur Chemie, 139(11), 1409 1415. Available from https://doi.org/10.1007/s00706-008-0937-x. Arciniegas Jaimes, D. M., Ma´rquez, P., Ovalle, A., Escrig, J., Linarez Pe´rez, O., & Bajales, N. (2020). Permalloy nanowires/graphene oxide composite with enhanced conductive properties. Scientific Reports, 10(1). Available from https://doi.org/10.1038/s41598-02070512-1. Astruc, D. (2020). Introduction: Nanoparticles in catalysis. Chemical Reviews, 120(2), 461 463. Available from https://doi.org/10.1021/acs.chemrev.8b00696. Badawey, E. S. A. M., & El-Ashmawey, I. M. (1998). Nonsteroidal antiinflammatory agents - Part 1: Antiinflammatory, analgesic and antipyretic activity of some new 1-(pyrimidin2-yl)-3- pyrazolin-5-ones and 2-(pyrimidin-2-yl)-1,2,4,5,6,7-hexahydro-3H-indazol-3ones. European Journal of Medicinal Chemistry, 33(5), 349 361. Available from https://doi.org/10.1016/S0223-5234(98)80002-0. Balivand, Z., & Ghashang, M. (2018). NiO-SnO2 nano-composite efficient catalyst for the preparation of substituted pyrano[3,2-c]quinolones and pyrano[3,2-c]chromene derivatives. Biointerface Research in Applied Chemistry, 8, 3739 3743. Butt, F., Ullah, S., Ahmad, J., Rehman, S., & Tariq, Z. (2020). Graphitic carbon nitride/metal oxides nanocomposites and their applications in engineering. Composite Materials: Applications in Engineering, Biomedicine and Food Science, 231 265. Available from https://doi.org/10.1007/978-3-030-45489-0_10. Castagnolo, D., Manetti, F., Radi, M., Bechi, B., Pagano, M., De Logu, A., Meleddu, R., Saddi, M., & Botta, M. (2009). Synthesis, biological evaluation, and SAR study of novel pyrazole analogues as inhibitors of Mycobacterium tuberculosis: Part 2. Synthesis of rigid pyrazolones. Bioorganic and Medicinal Chemistry, 17(15), 5716 5721. Available from https://doi.org/10.1016/j.bmc.2009.05.058. Chaudhary, R., Mondal, A., Aziz, T., Potbhare, A., Lambat, T., Mondal, S., & Abdala, A. (2020). Applications of metal/metal oxides nanoparticles in organic transformations. In Metal Oxides and Composites Part II, (pp. 134 156). Material Research Forum LLC. Available from https://doi.org/10.21741/9781644900970-6. Chen, P., Du, M., Lei, H., Wang, Y., Zhang, G., Zhang, F., & Fan, X. (2012). SO42 2 / ZrO2 titania nanotubes as efficient solid superacid catalysts for selective mononitration of toluene. Catalysis Communications, 18, 47 50. Available from https://doi.org/ 10.1016/j.catcom.2011.10.035.

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Tang, N., Tian, X., Yang, C., Pi, Z., & Han, Q. (2010). Facile synthesis of α-MnO2 nanorods for high-performance alkaline batteries. Journal of Physics and Chemistry of Solids, 71 (3), 258 262. Available from https://doi.org/10.1016/j.jpcs.2009.11.016. Testard, A., Picot, L., Lozach, O., Blairvacq, M., Meijer, L., Murillo, L., Piot, J. M., Thie´ry, V., & Besson, T. (2005). Synthesis and evaluation of the antiproliferative activity of novel thiazoloquinazolinone kinases inhibitors. Journal of Enzyme Inhibition and Medicinal Chemistry, 20(6), 557 568. Available from https://doi.org/10.1080/ 14756360500212399. Veith, M., Kneip, S., Faber, S., & Fritscher, E. (1998). Metal/ metal oxide composites by single source precursor CVD. Materials Science Forum, 269 272, 303 306. Available from https://doi.org/10.4028/http://www.scientific.net/MSF.269-272.303. Wang, A., Li, J., & Zhang, T. (2018). Heterogeneous single-atom catalysis. Nature Reviews Chemistry, 2(6), 65 81. Available from https://doi.org/10.1038/s41570-018-0010-1. Wang, L., Shen, J., Yang, S., Liu, W., Chen, Q., & He, M. (2018). C-H arylation reactions through aniline activation catalysed by a PANI-g-C3N4-TiO2 composite under visible light in aqueous medium. Green Chemistry, 20(6), 1290 1296. Available from https:// doi.org/10.1039/c8gc00012c. Wang, L., Wang, H., Wang, Y., Shen, M., & Li, S. (2020). Photocatalyzed synthesis of unsymmetrical ureas via the oxidative decarboxylation of oxamic acids with PANI-gC3N4-TiO2 composite under visible light. Tetrahedron Letters, 61(23). Available from https://doi.org/10.1016/j.tetlet.2020.151962. Wang, S., Zhang, Y., & Liu, H. (2010). Selective hydrogenolysis of glycerol to propylene glycol on Cu-Zno composite catalysts: Structural requirements and reaction mechanism. Chemistry—An Asian Journal, 5(5), 1100 1111. Available from https://doi.org/10.1002/ asia.200900668. Wu, X., Xing, Y., Pierce, D., & Zhao, J. X. (2017). One-pot synthesis of reduced graphene oxide/metal (oxide) composites. ACS Applied. Materials Interfaces, 9(43), 37962 37971. Available from https://doi.org/10.1021/acsami.7b12539. Xie, Z., Liu, Z., Wang, Y., Yang, Q., Xu, L., & Ding, W. (2010). An overview of recent development in composite catalysts from porous materials for various reactions and processes. International Journal of Molecular Sciences, 11(5), 2152 2187. Available from https://doi.org/10.3390/ijms11052152. Xuhui, S., Xinglan, C., Wanquan, T., Dong, W., & Kefei, L. (2014). Performance comparison of dye-sensitized solar cells by using different metal oxide- coated TiO2 as the photoanode. AIP Advances, 4(3). Available from https://doi.org/10.1063/1.4863295. Yi, D. K., Lee, S. S., & Ying, J. Y. (2006). Synthesis and applications of magnetic nanocomposite catalysts. Chemistry of Materials, 18(10), 2459 2461. Available from https://doi. org/10.1021/cm052885p. Yildiz-Oren, I., Yalcin, I., Aki-Sener, E., & Ucarturk, N. (2004). Synthesis and structureactivity relationships of new antimicrobial active multisubstituted benzazole derivatives. European Journal of Medicinal Chemistry, 39(3), 291 298. Available from https://doi. org/10.1016/j.ejmech.2003.11.014. Youssef, A. M., & Noaman, E. (2007). Synthesis and evaluation of some novel benzothiazole derivatives as potential anticancer and antimicrobial agents. Arzneimittel-Forschung/ Drug Research, 57(8), 547 553. Available from https://doi.org/10.1055/s-00311296647. Youssef, M. M., & Amin, M. A. (2012). Microwave assisted synthesis of some new thiazolopyrimidine, thiazolodipyrimidine and thiazolopyrimidothiazolopyrimidine derivatives

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with potential antioxidant and antimicrobial activity. Molecules (Basel, Switzerland), 17 (8), 9652 9667. Available from https://doi.org/10.3390/molecules17089652. Yuan, S., Wang, M., Liu, J., & Guo, B. (2020). Recent advances of SBA-15-based composites as the heterogeneous catalysts in water decontamination: A mini-review. Journal of Environmental Management, 254109787. Available from https://doi.org/10.1016/j. jenvman.2019.109787. Zhang, Z., Zheng, C., & Yuan, A. (2020). One-pot multicomponent synthesis of 2-amino-4aryl-4H-benzo[h]chromene derivatives. Null, 40(5), 1397 1405. Available from https:// doi.org/10.1080/10406638.2018.1553196. Zhao, Z., & Ran, J. (2015). Sulphated mesoporous La2O3-ZrO2 composite oxide as an efficient and reusable solid acid catalyst for alkenylation of aromatics with phenylacetylene. Applied Catalysis A: General, 503, 77 83. Available from https://doi.org/10.1016/j. apcata.2015.01.023. Zhou, L., Gao, C., & Xu, W. (2010). Robust Fe3O4/SiO2-Pt/Au/Pd magnetic nanocatalysts with multifunctional hyperbranched polyglycerol amplifiers. Langmuir: The ACS Journal of Surfaces and Colloids, 26(13), 11217 11225. Available from https://doi.org/ 10.1021/la100556p. Zhu, Y., Lin, Q., Zhong, Y., Tahini, H. A., Shao, Z., & Wang, H. (2020). Metal oxide-based materials as an emerging family of hydrogen evolution electrocatalysts. Energy and Environmental Science, 13(10), 3361 3392. Available from https://doi.org/10.1039/ d0ee02485f. Zong, L., Tian, Z., Li, Y., & Lu, F. (2015). Promotional effects of PPy-modified ZrO2 composite as support and microemulsion medium for selective hydrogenation of cinnamaldehyde over supported Ru catalyst. Indian Journal of Chemistry - Section A Inorganic, Physical, Theoretical and Analytical Chemistry, 54A(3), 309 315. Available from http://nopr.niscair.res.in/bitstream/123456789/30961/1/IJCA%2054A%283%29%20309315.pdf.

Metal oxide-based composites as photocatalysts

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Sandeep R. Patil School of Science, Navrachana University Vadodara, Vadodara, Gujarat, India

18.1

Introduction

18.1.1 Principles of metal oxide-based composites as photocatalysts Photocatalysis is the science of employing catalysts to speed up chemical reactions that require or engage light (Khan et al., 2015; Zaleska-Medynska, 2018). A photocatalyst is defined as a material that is capable of absorbing light, producing electron hole pairs that enable chemical transformations of the contaminants repeatedly coming into its contact into greener products and regenerating its chemical composition after each cycle of such interactions (Liu, Li, et al., 2018). There are many catalysts reported in the literature. Among these, metal oxides and metal oxide nanocomposites (Magdalane et al., 2016; Ong et al., 2014), have also been extensively used as photocatalysts, particularly as heterogeneous photocatalysts (Liu, Liang, et al., 2018; Xu et al., 2019) for several decades. Nanocomposite, a multiphase solid material that has the phases of one, two, or three dimensions less than 100 nanometers (Arenas et al., 2013; Hu et al., 2013; Koltsov et al., 2019; Ponnamma & Sadasivuni, 2015). Nano-size photocatalysts (Kannan et al., 2020; Rao et al., 2019) exhibit multifunctional properties that have opened the door for improved efficacy in energy (Camarillo et al., 2017; Dong et al., 2018; Herna´ndez-Callejo et al., 2019), environment (Brunetti et al., 2019; Dontsova et al., 2019; Seema, 2019; Zhao et al., 2018; Zhao, Tian, et al., 2020), and health care applications. Metal oxide nanocomposites have proven their importance in the present time with a wide variety of applications (Zhang, Liu, et al., 2020) in every industrial sector. The controlled morphological and textural features (Lu et al., 2020), variable surface chemistry, high surface area, specific crystalline nature, and abundant availability make the nanostructured metal oxides and their composites (Zhang et al., 2019) as the most sought-after materials for several critical applications in water treatment, healthcare (Yemmireddy & Hung, 2017), and energy storage (Sun et al., 2019; Yang et al., 2018; Zhao & Yang, 2016), etc.

18.1.2 Mechanism of photocatalytic reactions Metals possess a continuum of electronic states, whereas semiconductors bear an empty region wherein there is no accessible energy level that can promote the Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00005-1 © 2022 Elsevier Inc. All rights reserved.

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recombination of an electron and a hole produced by the photoactivation of a solid. This empty region, which extends from the top of the filled valence band (VB) to the bottom of the vacant conduction band (CB), is called the bandgap. Hence, the unique electronic structure of semiconductors is characterized by a filled VB and an empty CB. When a semiconductor is irradiated by a photon of energy hv that equals or surpasses the bandgap energy, an excited electron moves from the VB to the CB, leaving a hole behind. The excited-state CB electrons and the VB holes can (1) recombine and be deactivated, (2) be trapped in metastable surface states, or (3) react with surrounding species. In the absence of a suitable scavenger or a surface defect state, photogenerated charge carriers can recombine and be deactivated by emission of radiation (emission of light) or radiationless transition (release of heat). If a relevant scavenger or a surface defect state is available to trap the electron or hole, recombination is avoided and electron hole pairs react with electron donors and electron acceptors adsorbed on the surface of the semiconductor or within the circumfluent electrical double layer of charged particles. The VB holes are powerful oxidizers (11.0 to 13.5 V vs SHE [standard hydrogen electrode], depending on the semiconductor and pH) and the CB electrons are good reducers (10.5 to 21.5 V vs SHE). For TiO2 (the most used metal oxide in heterogeneous photocatalysis), the redox potential for photogenerated holes is 12.53 V versus SHE (at pH 7), while the redox potential for CB electrons is 20.52 V. The ability of a semiconductor to undergo the transfer of photoinduced electrons to adsorbed species on its surface is a function of the positions of the CB and VB edges and the redox potential of the adsorbate. The potential level of the acceptor molecules or species thermodynamically has to be placed below (i.e., more positive than) the lower edge of the semiconductor CB. The potential level of the donor has to be located higher than (i.e., more negative than) the upper edge of the semiconductor VB to be able to provide an electron to the vacant hole (Hoffmann et al., 1995; Zaleska-Medynska, 2018).

18.2

Unitary metal oxides versus composite-based metal oxide photocatalysts

Metal oxide-based heterogeneous photocatalysts have attracted significant interest among researchers owing to their potential use in several areas of applications in environmental technologies, viz. degradation of environmentally harmful pollutants. Unitary metal oxides have been extensively studied and well demonstrated as effective photocatalysts for the degradation of harmful pollutants in the environment. One of the most extensively studied photocatalysts is titanium dioxide (TiO2), as it is highly stable, low cost, biocompatible, and chemically inert to the environment. Titanium dioxide (TiO2) nanoparticles can be explored efficiently for vital applications, such as photovoltaic cells, gas sensors, pigments, and photocatalysis because they possess high specific surface area, high pore volume, and large pore size,

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which increase the size of the accessible surface area and the rate of mass transfer for organic pollutant adsorption (Li et al., 2007; Liu et al., 2010; Zheng et al., 2010). Furthermore, these structural attributes significantly enhance the lightharvesting capabilities of these materials as they allow the maximum amount of light to access the interior. However, TiO2 suffers from several disadvantages too, which limit its use in the field of photocatalysis. These are (1) wider bandgap energy (B3.2 eV), that is it can work only under the ultraviolet region (,390 nm), which is only 4% of incoming sunlight. For practical applications, the TiO2 absorption band should be tuned to the visible range and (2) rapid recombination rate of electron hole pairs. Consequently, most of the research efforts have therefore focused on developing modified titania to maximize the utilization of solar radiation. The doping technique has been found to be the most common approach for improving the photoresponse of TiO2 photocatalysts in both the UV and visible regions. These methods include metal and nonmetal doping, co-dopThe doping (with metal-metal, metal on metal, and nonmetal-nonmetal), and doping with various elements restricted to the tridoping system. When doping with metallic or nonmetallic elements, the doped ions are either incorporated into the bulk TiO2 or highly dispersed on the surface as clusters of mononuclear complexes (Zhang et al., 2010). Doping leads to redshift in the band position to visible light, active photocatalytic material. When TiO2 is doped with metal ions, such as Au, Ag, Pt, Pd, Rh, Cu, Mn, Ce, Al, Co, and Eu, it leads to the creation of shallow or deep-level states without altering the bandgap of the host material and also able to absorb the light visible region. However, these intra-band states created in metal doping can act as a recombination center, thereby subsequently reducing the photocatalytic efficiency. In the case of nonmetal doping, such as N, S, Cl, and C, impregnation of dopant atoms leads to the engineering of bandgap by forming the local defect states and also extends the absorption region of TiO2 toward the visible range. Doping TiO2 with transition metals, such as Cr, Cu, Mn, W, V, Ni, and Fe, enhances the visible light absorption and reduces the recombination rate of photogenerated electrons and holes (Rauf et al., 2011). TiO2 photocatalysts activated by rare-earth elements, which have shown tremendous potential as dopants, have not only red-shifted absorption but also improved photocatalytic activity and increased anatase-to-rutile transformation temperature. Additionally, materials (such as TiO2) modified by RE31 ions usually possess luminescent properties. Thus, besides classical UVexcited emission, these materials can also display upconversion luminescence. Moreover, because the f-orbitals of the lanthanide ions can form complexes with various Lewis bases, the substrates are concentrated onto the TiO2 surface. Recently, tri-doped TiO2 has been found to further improve the visible light photocatalytic activity. Moreover, tungsten trioxide (WO3), zinc oxide (ZnO), iron (III) oxide (Fe2O3), tantalum pentoxide (Ta2O5), copper oxide (CuO), nickel oxide (NiO), chromium oxide (Cr2O3), cerium oxide (CeO2), ruthenium oxide (RuO2), perovskite oxides (CaTiO3, SrTiO3, BaTiO3, NiTiO3, NaTaO3, KTaO3, NaNbO3, and KNbO3), etc. are the other metal oxides, which have been extensively studied with an emphasis on the correlation among preparation methods, synthesis conditions, crystal

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structures, electronic properties or surface properties, and photocatalytic activity under UV or visible light irradiation in several typical reactions. The following section provides a brief account of the advantages as well as disadvantages of some of the unitary metal oxide photocatalysts mentioned above (WO3, ZnO, Fe2O3, Ta2O5, CuO, NiO, Cr2O3, CeO2, RuO2, etc.). The advantages and disadvantages of some of the prominent unitary metal oxide photocatalysts have also been summarized in Table 18.1 (Zaleska-Medynska, 2018). Tungsten oxide has become one of the most investigated functional metal oxides in many fields including photocatalysis, electrochemistry, and phototherapy, due to its strong solar spectrum absorption compared with TiO2 (Yan et al., 2015). Tungsten oxide (WO3) has a CB edge positioned slightly more positive (vs normal hydrogen electrode) than the H2/H2O reduction potential, and a VB edge much more positive than the H2O/O2 oxidation potential (Zheng et al., 2011). Thus, WO3 is capable of efficiently photo-oxidizing a wide range of organic compounds, dyes, and bacterial pollutants. Moreover, compared to TiO2, WO3 can be irradiated by the blue region of the visible solar spectrum, as its bandgap energy is in the range of 2.6 3.2 eV. WO3 also has remarkable stability in acidic environments, making it a promising candidate for treating water contaminated by organic acids. Zinc oxide (ZnO) is one of the most important II VI semiconductors because of its interesting and unique characteristics, including a wide bandgap (3.37 eV), a large exciton binding energy (60 meV), physical and chemical stability, biocompatibility, nontoxicity, high photosensitivity, and piezoelectric and pyroelectric properties. These unique properties have rendered ZnO indispensable in applications, such as solar cells, thin-film transistors, laser diodes, piezoelectric and optoelectronic applications in surface acoustic wave devices, transparent conductive contacts, and ultraviolet lasers (Janotti & Van De Walle, 2009). However, the wide bandgap of ZnO greatly limits light absorption in the ultraviolet (λ , 380 nm) and substantially limits the photocatalytic efficiencies. Moreover, ZnO normally suffers from the intrinsic drawback of photo corrosion, which greatly restricts its use as an efficient photocatalyst in wastewater treatment. ZnO-based photocatalysis suffers from the following drawbacks: (1) ZnO does not absorb the visible portion of the solar spectrum, while UV light is expensive for bandgap excitation; (2) rapid recombination of the charge carriers slow down the degradation reactions at the semiconductor-liquid interface; (3) after the reaction, it can be difficult to recover ZnO powder from the suspension by conventional filtration; (4) the tendency to aggregate during catalytic reactions and the susceptibility to corrosion under UV light (Kumar & Rao, 2015). Fe2O3 is a transition metal oxide that has different stoichiometric and crystalline structures, among which α-Fe2O3 is the most thermodynamically stable crystal phase iron oxide. Owing to its suitable Eg value (2.0 2.2 eV) and high chemical stability, Fe2O3 has become an excellent photoelectrode material for electrochemical water splitting (Pu et al., 2014), an anode material in lithium-ion batteries (Abazari et al., 2014; Lin et al., 2011; Lucas-Granados et al., 2016), and a photocatalyst for the degradation of organic pollutants (Cao et al., 2010; Zhou et al., 2011). Substantial progress has been achieved in the degradation of organic dyes and other

Table 18.1 Unitary metal oxide photocatalysts (advantages and disadvantages). Unitary metal oxide photocatalysts

Advantages

Disadvantages

References

Titanium oxide (TiO2)

Moderate band gap, nontoxicity, high surface area, low cost, recyclability, high photoactivity, wide range of processing procedures, and its excellent chemical and photochemical stability.

Li et al. (2007), Liu et al. (2010), Zheng et al. (2010)

Tungsten trioxide (WO3)

Capable of efficiently photo-oxidizing a wide range of organic compounds, dyes, and bacterial pollutants. It can be irradiated by the blue region of the visible solar spectrum, as its bandgap energy is in the range of 2.6 3.2 eV. WO3 also has remarkable stability in acidic environments. Wide bandgap (3.37 eV), a large exciton binding energy (60 meV), physical and chemical stability, biocompatibility, nontoxicity, high photosensitivity, and piezoelectric and pyroelectric properties.

Wider bandgap energy (B3.2 eV), i.e., it can work only under ultraviolet region (,390 nm), which is only 4% of incoming sunlight, for practical applications. Rapid recombination rate of electron hole pairs. Facile method for the large-scale synthesis of WO3 nanomaterials with high photocatalytic activity is still a challenge. Pure WO3 nanomaterials are usually not efficient photocatalysts because their low charge mobility results in a high electron hole recombination rate. Does not absorb the visible portion of the solar spectrum. Rapid recombination of the charge. Difficult to recover ZnO powder from the suspension by conventional filtration after reaction. Tendency to aggregate during catalytic reactions and the susceptibility to photo corrosion.

Zinc oxide (ZnO)

Yan et al. (2015), Zheng et al. (2011)

Janotti and Van De Walle (2009), Kumar and Rao (2015), Lin et al. (2011), Zhou et al. (2011)

(Continued)

Table 18.1 (Continued) Unitary metal oxide photocatalysts

Advantages

Disadvantages

References

Iron (III) oxide (Fe2O3)

An excellent photoelectrode material for electrochemical water splitting, an anode material in lithium-ion batteries, and an efficient photocatalyst for the degradation of organic pollutants.

Cao et al. (2010), Lin et al. (2011), Lucas-Granados et al. (2016), Pu et al. (2014), Zhou et al. (2011)

Tantalum pentoxide (Ta2O5)

High dielectric and refractive coefficients and excellent photoelectric performance. Low cost, nontoxic, ease of availability, relatively high efficiency, and structural stability. Superhydrophobic properties of CuO nanostructure are ideal for cleaning coatings (antibiofouling), surface protection, textiles, water movement, microfluidics, and oil-water separation.

Suffer from activity decline because of the electron hole charge recombination on the oxide surface. Difficult separation of catalyst materials after the treatment process, and low quantum yield of the treatment process, which restricts the kinetics and efficiency. Owing to wide bandgap, Ta2O5 photocatalysts can only absorb UV light, which accounts for only 4% of the total sunlight, thereby greatly restricting their practical applications. A systematic study on the one-pot synthesis of CuO nanostructures with various morphologies is urgently needed and CuO nanostructures for wastewater treatment are still lacking. Moreover, the growth mechanisms of CuO nanostructures are still not fully understood.

Copper oxide (CuO)

Kominami et al. (2001), Zhang, Zhang, & Gong (2014), Zhu et al. (2005)

Liu et al. (2012), Mumm et al. (2009), Yang et al. (2015, 2016), Zhang, Zhang, Xu, et al. (2014)

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organic pollutants in water by using α-Fe2O3 powders. However, they frequently suffer from activity decline because of the electron hole charge recombination on the oxide surface, which can occur within nanoseconds. Some other obstacles that can also hinder the wide application of iron oxide nanoparticles for the photocatalysis of toxic compounds: (1) difficult separation of catalyst materials after the treatment process, and (2) low quantum yield of the treatment process, which restricts the kinetics and efficiency. Considerable efforts have been made to enhance the photocatalytic activity, such as by decreasing the photocatalyst size to increase surface area. Tantalum oxide (Ta2O5) is one of the most important transition metal oxides because of its extraordinary physical and chemical properties, including high dielectric and refractive coefficients and excellent photoelectric performance. Moreover, Ta2O5 has certain advantages over other materials due to its low cost, nontoxicity, availability, relatively high efficiency, and structural stability. It is utilized in applications, such as photovoltaic devices, electronics, and antireflective layer material (Zhang, Zhang, & Gong, 2014). Ta2O5 is a semiconducting material that exhibits photocatalytic activities too (Kominami et al., 2001; Zhu et al., 2005). However, due to the wide bandgap, Ta2O5 photocatalysts can only absorb UV light, which accounts for only 4% of the total sunlight, thereby greatly restricting their practical applications. Copper oxide (CuO) is an important p-type semiconducting metal oxide with a narrow bandgap of 1.2 1.5 eV, which has been explored for various applications, including energetic materials (EMs), bio-sensors, magnetic storage media, gas sensors, electronics, solar cells, and so on, owing to its outstanding photoconductive and photochemical properties (Zhang, Zhang, Xu, et al., 2014). Copper oxide has also been widely used as a heterogeneous catalyst in many important chemical processes, such as the degradation of nitrous oxide, selective catalytic reduction of nitric oxide with ammonia, and oxidation of carbon monoxide, hydrocarbon, and phenol in supercritical water (Liu et al., 2012). The superhydrophobic properties of CuO nanostructures make these materials promising candidates in Lotus effect self-cleaning coatings (antibiofouling), surface protection, textiles, water movement, microfluidics, and oil-water separation (Mumm et al., 2009). Very recently, this metal oxide has been explored as a new class of materials for photocatalytic processes (Yang et al., 2015, 2016). NiO is a p-type semiconducting oxide with a wide bandgap (3.6 4.0 eV). It has aroused considerable interest due to its applications in various fields, such as gas sensors, electrochromic devices, catalysis, battery cathodes, magnetic materials, fuel cell electrodes, and also dye-sensitized solar cells. The CB and VB of NiO (20.5 and 13.1 V, respectively) are the attributes that make them suitable for water splitting and photocatalytic processes. CeO2 and titania have common features, viz. wide bandgap, nontoxicity, and high stability. Cerium oxide has been investigated for multiple applications, such as photocatalysis, electrolyte material for solid oxide fuel cells, a material with a high refractive index, ceramic materials, oxygen gas sensors, cosmetics, and as an insulating layer on silicon substrates (Zhai et al., 2007). Nanocrystalline CeO2 has been prepared by sol gel process, sonochemical synthesis, gas condensation, solvothermal and hydrothermal synthesis, microwave, combustion synthesis, and template-assisted precipitation

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(Phonthammachai et al., 2004; Tambat et al., 2016). Unfortunately, these methods tend to be complex and require expensive raw materials, so the cost of production is very high, and industrial production is difficult to realize. Ruthenium dioxide (RuO2), a transition metal oxide with a rutile-like structure, has been investigated for its unique properties, such as high chemical stability, electrical conductivity, and excellent diffusion barrier properties. The CB and VB positions of RuO2 are both lower than those of TiO2, so they are expected to have band overlap (Uddin et al., 2013). Moreover, RuO2 is a very powerful oxidation catalyst and is well-known for its application in heterogeneous catalysis. Molecules, such as CO, O2, NH3, NO, methylene, and methane, adsorb well on the RuO2 (110) surface via the under coordinated Ru atoms, which is a prerequisite for catalytic reactions. Additionally, the smaller RuO2 nanoparticles have higher activity due to the numerous active sites with increased specific surface area (Xiang et al., 2012). However, small RuO2 nanoparticles tend to become deactivated due to the occurrence of particle sintering during the catalytic process at elevated temperatures. Perovskite-type oxides, originating from CaTiO3, are a family of oxides having the general formula ABO3 (where A is a rare or alkaline earth metal and B is a first-row transition metal), wherein cations with a large ionic radius coordinate to 12 oxygen atoms and occupy A-sites and cations with a smaller ionic radius are six-coordinate and occupy B-sites (Tanaka & Misono, 2001). The perovskite crystal structure provides a good framework in which to tune the bandgap values to enable visible-light absorption and the band edge potentials to suit the needs of specific photocatalytic reactions (Grabowska, 2016). Titanium oxide (TiO2), tungsten trioxide (WO3), zinc oxide (ZnO), iron (III) oxide (Fe2O3), tantalum pentoxide (Ta2O5), copper oxide (CuO), Nickel oxide (NiO), Chromium oxide (Cr2O3), Cerium oxide (CeO2) Ruthenium oxide (RuO2), perovskite oxides (CaTiO3, SrTiO3, BaTiO3, NiTiO3, NaTaO3, KTaO3, NaNbO3, and KNbO3), etc. exhibit excellent photocatalytic activity owing to their bandgap and distinct electronic structure. However, one of the main shortcomings of these photocatalysts is the recombination of photogenerated charge carriers, which leads to decreases in photo efficiency. Thus, metal/nonmetal doping and decoration of the semiconductor with plasmonic metallic nanoparticles, nanocomposites offer an efficient strategy for high-activity visible-light-driven photocatalysis, wherein, for the practical visible light applications, photocatalysts are modified either by narrowing the bandgap or by inhibiting the recombination of charge carriers via the formation of heterojunction nanocomposites. Coupling a photocatalyst material or catalyst support material with another metal, a semiconductor with a narrower bandgap or a polymer forming nanocomposite material may result in the transfer of electrons from the excited semiconductor with a small bandgap into the other material when the positions of the CB are properly aligned. This favors the separation of photogenerated charge carriers and consequently improves the photocatalytic efficiency in visible light irradiation. These nanocomposites, such as visible-light-induced, photocatalyst may include mixed metal oxide nanocomposites, nanoporous nanocomposite materials, polymeric nanocomposites, and carbon-based nanocomposites. The wider bandgap semiconductors (e.g., TiO2, WO3, ZnO, and

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SnO2) could be used to obtain heterojunctions with visible photoresponse using a material with narrower bandgaps. What is more, these multi semiconductor devices can absorb a larger fraction of the solar spectrum, which helps with the excitation of the semiconductor and thus the photoinduced generation of electrons and holes. It is pertinent to synthesize various metal oxide composites, investigate their enhanced properties, and explore their potential for various applications to overcome the limitations of unitary metal oxides, viz. inefficient light absorption and poor optoelectronic properties (short electron hole lifetimes and low carrier mobility).

18.3

Applications of metal oxide-based photocatalysts

Owing to the growing demand for sustainable green technologies to reduce the impact of modern industrial processes on the ecology and environment as well as to develop cleaner energy sources, metal oxide-based composites for photocatalysis have gained prominence. Metal oxide-based composites as photocatalysts are viable and effective as the most attractive technologies for applications, including degradation of a wide range of pollutants, water splitting for H2 production, the transformation of volatile organic compounds into biodegradable molecules, CO2 reduction (hydrocarbon generation), photoelectrocatalysis, food safety, construction, textiles, antimicrobial coatings, photocatalytic ozonation, etc. A detailed account of some of the prominent applications of metal oxide-based composites as photocatalysts has been presented in the subsequent sections herein.

18.3.1 Photoelectrocatalysis for energy conversion The versatility of semiconductor materials and the possibility of the generation of electrons, holes, hydroxyl radicals, and/or superoxide radicals have significantly enhanced the applicability of photoelectrocatalysis in the contemporary world. Photoelectrocatalysis (integration of electrochemistry and photocatalytic technology) involves a photocatalytic system to which an external positive bias is applied, which can significantly increase the rates of photocatalytic reactions by driving the photogenerated electron hole pairs in opposite directions, reducing their recombination rates (Lianos, 2017). This configuration allows more effectiveness in the separation of photogenerated charges due to light irradiation with energy being higher compared to that of the bandgap energy of the semiconductor, which thereby leads to an increase in the lifetime of the electron hole pairs. Photoelectrocatalysis is used for solar energy conversion (An et al., 2018) into usable forms of energy including electricity or as a clean energy carrier such as hydrogen, and can be categorized as: 1. Regenerative PEC solar cells, in which direct conversion of solar to electrical energy takes place 2. Photoelectrosynthetic cells, in which conversion of solar-to-chemical energy takes place 3. Photoelectrolytic cells (photoelectrocatalytic hydrogen generation).

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Moreover, photoelectrocatalysis (PEC) has attracted more and more attention to oxidizing organic pollutants. It has exhibited greater potential as compared to individual photocatalysis and electrooxidation processes. The synergistic interaction between photocatalysis and electrocatalysis has been attributed as a factor that contributes to the high efficiency in the photoelectrocatalytic oxidation of organic pollutants. The basic principles and mechanisms of photoelectrocatalysis in oxidizing organic contaminants were discussed by the authors. It has been inferred that with the increase of the applied potential, different synergistic mechanisms enhance the degradation of organic contaminants. The development of ultraviolet response and visible lightresponse photoanode materials were also reviewed and the application of photoelectrocatalysis in the removal of organics such as dyes, pharmaceuticals, and personal care products and endocrine-disrupting chemicals were summarized (Cao et al., 2017). Furthermore, photoelectrocatalytic oxidation was found to be a powerful technique for water disinfection. Photoelectrocatalysis promotes faster inactivation of various microorganisms at a short required time of treatment. The different groups of microorganisms vary in their susceptibility to treatment, but PEC seemed to be an excellent alternative providing greater efficiency for the disinfection of water contaminated by Escherichia coli, mycobacteria, and fungus (Bessegato et al., 2015). Photoelectrocatalysis is also appropriate for inorganic anion reduction. The preliminary studies summarized by the authors indicate that the subject deserves attention as it could lead to the development of an effective and economical method for the conversion of CO2 into value-added products. The possibility of simultaneous wastewater treatment and hydrogen production is a promising prospect for the exploitation of photoelectrocatalysis, bringing a meaningful contribution to our societies in both the economic and environmental domains. A novel three-dimensional (3D) Fe2O3 nanorods (NRs)/TiO2 nanosheets (NSs) heterostructure was fabricated by employing the TiO2 NSs with dominant highenergy {0 0 1} facets as a synthetic template via facile hydrothermal and chemical bath deposition methods. All Fe2O3/TiO2 composites exhibit superior photocatalytic and photoelectrochemical (PEC) performances compared to pure TiO2 NSs. 3D Fe2O3/TiO2 heterostructures were observed to be not only beneficial in improving the photocatalytic degradation efficiency, but also beneficial to enhance the PEC performance of materials. which is ascribed to the larger surface area, better optical absorption, and efficient electron hole separation of 3D Fe2O3/TiO2 heterostructure (Zhang et al., 2019). The optimization mechanism of photocatalytic and PEC performance was also discussed and it was inferred that the 3D heterostructure between Fe2O3/TiO2 can effectively improve the visible-light utilization and absorption ability, which will ultimately be advantageous for enhancing the visiblelight photocatalytic and PEC performances. Owing to the small size, high aspect ratio, and larger surface area, carbon nanostructures (CNs) can be used as templates or anchors for the fabrication of various types of nanostructured materials and potentially be applied to PEC cells. Taking these attributes into consideration, a facile hydrothermal approach was adapted for synthesizing hybrid single-crystalline cubic hematite decorated carbon nanostructures (α-Fe2O3/CNs) nanocomposites for PEC application. The highly exposed

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nature of CNs was harnessed and used as a basal structure to anchor α-Fe2O3 semiconductor materials for enhanced light trapping to produce better PEC performance. Improved PEC performance of the samples as evidenced by several indications obtained from LSV curves. The PEC analysis shows that clustered caterpillar-like surfaces exhibit the highest PEC performance owing to the highly exposed sites available, better light absorption, and low crystallite size (Li et al., 2020). A partially covered Fe2O3 modified BiVO4 heterostructure was constructed using a simple solution dry-out method, in which MIL-53(Fe) was utilized as the precursor of Fe2O3. Fe2O3 spindles act as a hole-transport channel in the photocatalytic process. Consequently, BiVO4/PC-Fe2O3 exhibited higher PEC performance than other BiVO4/Fe2O3 nanocomposites (Li et al., 2020). The charge transfer mechanism of Mo, W:BVO/PC-Fe2O3 has been schematically illustrated in Fig. 18.1. It has been reported that the bandgap of Fe2O3 is about 2.0 eV, which is smaller than that of BiVO4 (2.4 eV). Moreover, the CB and VB potential of Fe2O3 is near 0.4

Figure 18.1 Schematic illustration of the charge transfer mechanism of Mo, W:BVO/PCFe2O3 heterostructures. Source: From Li, J., Li, J., Yuan, H., Zhang, W., Jiao, Z., & Song Zhao, X. (2020). Modification of BiVO4 with partially covered α-Fe2O3 spindles serving as hole-transport channels for significantly improved photoelectrochemical performance. Chemical Engineering Journal, 398, 125662. https://doi.org/10.1016/j.cej.2020.125662.

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and 2.4 eV, respectively, which are both embodied in the bandgap of BiVO4. Hence, a type-I band alignment is formed when Fe2O3 spindles are formed in BiVO4 films. The photogenerated electrons could transfer from the CB of BiVO4 to that of Fe2O3 and then migrate to the fluorine-doped tin oxide substrate. Whereas, the photoinspired holes coming from the VB of BiVO4 could take part in the oxidation reaction through the exposed part of Fe2O3 spindles, and thus avoid the recombination with the electrons. Therefore the exposed part of Fe2O3 spindles acted as a holetransport channel during the charge transfer process (Li et al., 2020). A thorough review to seek insights into the key optoelectronic properties of the semiconductors, such as absorption coefficient, dielectric constant, effective masses, exciton binding energy, and band position with an aim of utilizing these materials for photoelectrocatalysis was carried out. The combined assessment from theoretical and experimental points aided a thorough understanding of the semiconductors and the ways to reliably characterize them, and in turn to develop improved photoelectrocatalytic materials (Le Bahers & Takanabe, 2019).

18.3.2 Hydrogen production Photocatalytic hydrogen production via water splitting is one of the favorable technologies for solar energy conversion to renewable and sustainable energy to meet the growing energy demand without compromising the future energy demand. Hydrogen energy generation by photocatalysis is recognized to be a cost-effective and sustainable energy generation technique because of its zero environmental impact and it can be generated easily by nonreplenished sources. Photoelectrochemical cells (PEC) are at a development stage but they exhibit great potential in exploring a broader range of the solar spectrum in an efficient way. In the PEC system, hydrogen generation is a result of integrated solar energy conversion and water electrolysis in a single photocell. The main components of a PEC cell are presented in Fig. 18.2 and comprise working and counter electrodes, one or both being photoactive (photoelectrode) (Choudhary et al., 2012). The PEC system also consists of a reference electrode to observe half-reactions in the cell. This electrode system is mainly immersed in an aqueous electrolyte (Na2SO4). The reactor is either transparent to light or comprises an optical window that allows irradiation to reach the photoelectrode. An n- or p-type semiconductor is generally used as a working electrode with a platinum (Pt) counter electrode. Electron hole pairs are generated on the working electrode by photon absorption with an energy level that is equal or higher than the bandgap (Eg) of the photoanode semiconductor (Acar & Dincer, 2016). If an n-type semiconductor is used, electrons are collected in the photoanode and are subsequently transported to the counter electrode through an external circuit (Choudhary et al., 2012; Minggu et al., 2010). The photogenerated electrons are consumed to reduce H1 into H2 at the cathode, while holes take part in the oxidation of water into O2 and H1 at the anode. Conversely, if a p-type semiconductor is employed as the working electrode, photogenerated electrons are used to reduce H1 into H2, while in the counter electrode, water is oxidized into O2 and H1. In summary, n-type semiconductors produce an

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Figure 18.2 Construction and mechanism of hydrogen generation in a photoelectrocatalytic (PEC) cell. Source: From Zaleska-Medynska, A. (2018). Metal oxide-based photocatalysis fundamentals and prospects for application. Cambridge University Press. https://doi.org/10.1017/ CBO9781107415324.004.

anodic photocurrent in which holes are transferred toward the electrolyte, while ptype semiconductors generate a cathodic photocurrent by transmitting electrons toward the electrolyte. Under normal operating conditions, such as room temperature (25 C) and atmospheric pressure (1013.25 hPa), water splitting is not a spontaneous reaction, and thus a suitable potential must be applied (Choudhary et al., 2012). Hence, the selection of a semiconductor, as a working electrode, with optimum bandgaps and energy levels for both conduction and VBs are necessary for spontaneous water splitting. Recombination of photogenerated electron hole pairs decreases the water splitting efficiency. Therefore the generated charge needs to be quickly separated and transferred to the counter electrode through an external circuit. To limit recombination of e 2/h1 pairs, the diffusion length of the charge carriers should be similar or larger than the thickness of the semiconductor film so that charges are collected before recombination, thereby increasing the charge lifetime (Acar & Dincer, 2016). Therefore, to fabricate a suitable photoelectrode for PEC, a material with appropriate visible light absorption (bandgap range, 1.8 2.2 eV), fast transport, efficient separation e 2/ h1 pairs to limit recombination, and favorable CB and VB positions for a water redox potential, is required (Choudhary et al., 2012). Additionally, materials used in PECs should be characterized by high chemical stability in the electrolytic solution, noncorrosive nature, and low cost. In general, effective photocatalytic composites for visible light water splitting should possess the following attributes: (1) proper bandgap energy and band potentials,

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(2) photostability in aqueous solution (3) high crystallinity, (4) high specific photoactivity (.104 mmoles of H 2 /h.g) (Clarizia et al., 2017). CuOx/TiO2 photocatalysts with the same copper loading were prepared by incipient wetness impregnation method employing TiO2 nanocrystals with different morphologies. TiO2 morphology-dependent CuOx TiO2 interaction, compositions and structures, and photocatalytic performance were identified. CuOx TiO2 -{0 0 1} with the strongest CuOx TiO2 interaction, the highest Cu2O dispersion, and the CuOx TiO2 heterojunction population exhibit the highest photocatalytic H2 production (Liu, Ye, et al., 2019). Very recently, a detailed review on an overview of perovskites, the mechanism for hydrogen production, thermodynamics analysis, and different approaches to improve the efficiency of perovskite for sustainable hydrogen production was carried out. The authors inferred that (1) introduction of metals forming an appropriate Schottky barrier and noble metals showing SPR effect on perovskite can lead to an increase in activity for hydrogen production, (2) composite formation by combining perovskite with other semiconductors, that is TiO2 to construct heterojunction promotes charge carrier separation toward boosting H2 evolution (Tasleem & Tahir, 2020). Sunlight-driven photocatalytic water splitting for hydrogen (H2) evolution is restricted by insufficient light-harvesting and high photogenerated electron hole recombination rates of TiO2-based photocatalysts. To overcome this limitation, a graphene-modified WO3/TiO2 S-scheme heterojunction photocatalyst (WTG) was prepared by a facile one-step hydrothermal approach. The WTG composite exhibited superior photocatalytic activity for H2 production through water splitting, and it was 3.fivefold higher than that of bare TiO2 under the same conditions. It was also inferred that the positive cooperative effect between the S-scheme heterojunction formed between WO3 and TiO2 and the Schottky heterojunction formed between TiO2 and graphene sheets could effectively suppress the recombination of useful carriers, enhance the light harvest, and increase the number of active sites for the reduction reaction (He et al., 2020). The importance of quantum dots (QDs) was highlighted as an efficient photocatalyst to scale up the efficiency of photocatalysts for practical applications, viz. hydrogen production in a recent review. The authors inferred that QDs-based photocatalysts show some remarkable activities. However, these catalysts must be improved further in terms of efficiency and scale-up efficiency in hydrogen production. Further, band energies can be controlled by the size change in quantum dots, which gives new ways to control the response and efficiency in a multi-excitons generation as a rainbow photocatalyst for efficient hydrogen production (Rao et al., 2019). Z-scheme g-C3N4/Au/BiVO4 photocatalysts were constructed to achieve excellent photocatalytic activities and optimal stability toward H2 production without any cocatalyst under vis-light irradiation through regulating the interface charge migration pathway. It was inferred that the Z-scheme g-C3N4/Au/BiVO4 photocatalysts could greatly improve the charge separation efficiency and redox driving force by using Au nanoparticles as electron shuttles, which benefits from photocatalytic H2 evolution. Moreover, the SPR effect of the Au nanoparticle is beneficial to

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improve the visible light response and then also promote photocatalytic H2 production capability (Song et al., 2019). A facile inserting-removing strategy was employed to prepare a unique N-deficient carbon nitride homojunction featuring the N-deficient carbon nitride microdomains implanted into porous N defects modified carbon nitride with two kinds of N defects (N vacancies and cyano groups), generating a highly-efficient photocatalyst with a high hydrogen evolution rate (HER) of 3882.5 μmol h21 g21 and a notable quantum yield of 8.6% at 420 nm. The markedly improved H2 production of the developed g-C3N4 photocatalyst is attributed to efficient suppression of recombination of photoinduced charge carriers as well as the promoted light absorption capacity and the enlarged reduction potential led by the unique N-efficient homojunction structure. A key challenge for future advances in hydrogen generation efficiency relies on the combination of material engineering with the design of proper reactor configurations. In particular, the adoption of specific solar collectors could allow employing the beneficial effect of higher operating temperatures on hydrogen evolution. For implementing photocatalytic hydrogen production in real applications, cost reduction and toxicity assessment of the photocatalytic composites willplay a vital role (Clarizia et al., 2017).

18.3.3 Water treatment and environment It is imperative to design and develop a technology that would completely remove pollutants from contaminated waters due to agricultural, industrial, and domestic uses. Sustainable nanotechnology has made substantial contributions to providing contaminant-free water to humanity. Several examples of emerging nanomaterials that could be translated or have already translated into nanotechnologies for clean water are tabulated in Table 18.2 (Nagar & Pradeep, 2020). Table 18.2 Examples of emerging nanomaterials that could be translated or have already translated into nanotechnologies for clean water. Material

Application (Nagar & Pradeep, 2020)

Metal 2 organic framework

Atmospheric waterharvesting Water purification

Organic-templated nanometal oxyhydroxide impregnated with silver nanoparticles Cationic and anionic membranes Metal oxide nanocomposite heterostructure powder Layer-by-layer assembly of graphene oxide membranes Aromatic diimide chromophores

Capacitive deionization Sensing of hydrogen sulfide Water purification Sensing of volatile organic compounds

Source: From Nagar, A. & Pradeep, T. (2020). Clean water through nanotechnology: Needs, gaps, and fulfillment. ACS Nano, 14(6), 6420 6435. https://doi.org/10.1021/acsnano.9b01730.

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The controlled morphological and textural features, variable surface chemistry, high surface area, specific crystalline nature, and abundant availability make the nanostructured metal oxides and their composites highly selective materials for efficient water treatment and environment. Various applications of titanium oxide photocatalysts in the fields of environment, energy, and water treatment are depicted in Fig. 18.3 (Lee & Park, 2013). Apart from TiO2, a wide range of metal oxides like iron oxides, magnesium oxide, zinc oxides, tantalum oxides, tungsten oxides, copper oxides, metal oxide composites, and graphene-metal oxide composites possessing variable structural, crystalline, and morphological features have been utilized for adsorptive removal and photocatalytic degradation of organic pollutants, namely, dyes, pesticides, phenolic compounds (Gusain et al., 2019). The efficiency of pollutant degradation by a photocatalyst depends on the following factors: (1) photocatalyst type and properties (2) pollutant type and concentration (3) photocatalyst loading pH (4) temperature (5) irradiation spectrum and intensity (6) dissolved oxygen concentration (7) presence of additional substances (suspended matter, scavengers, etc.).

Figure 18.3 Applications of TiO2 photocatalysts in environment and energy fields. Source: From Lee, S. Y. & Park, S. J. (2013). TiO2 photocatalyst for water treatment applications. Journal of Industrial and Engineering Chemistry, 19(6), 1761 1769. https:// doi.org/10.1016/j.jiec.2013.07.012.

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A wide range of metal oxides like iron oxides, magnesium oxide, titanium oxides, zinc oxides, tantalum oxides, tungsten oxides, copper oxides, metal oxide composites, and graphene-metal oxides composites having variable structural, crystalline, and morphological features have been utilized for adsorptive removal and photocatalytic degradation of organic pollutants viz. dyes, pesticides, phenolic compounds, etc. (Gusain et al., 2019). The efficiency of pollutant degradation depends on the following factors: (1) photocatalyst type and properties (2) pollutant type and concentration (3) photocatalyst loading pH (4) temperature (5) irradiation spectrum and intensity (6) dissolved oxygen concentration (7) presence of additional substances (suspended matter, scavengers, etc.). A 2D/2D Wg-C3N4/g-C3N4 isotype composite was utilized for adsorption-induced photodegradation of AV-7 dye. The composite exhibited better adsorption than bare g-C3N4 and higher photodegradation activity than Wg-C3N4. The coupling of protonated Wg-C3N4 with g-C3N4 has a significant effect on the adsorption and photodegradation of organic molecules. The enhancement in the rate of dye decomposition was attributed to sufficient adsorption of pollutants to adsorptive sites followed by surface diffusion to photoactive domains for concomitant photo-oxidation. Moreover, the AV-7 photodegradation activity of the composite is comparable or higher than the commercial TiO2, ZnO, and CdS photocatalysts (Vidyasagar et al., 2019). Graphene incorporated composite nanofibers (TZB-Gr) (Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) with a bandgap of (2.5 eV) were found to effectively activate organic dyes under visible-light and UV-light irradiation. The produced radicals are powerful oxidizing species to degrade most of the organic pollutants to become CO2 and H2O. TZB-Gr demonstrated a higher activity than TZB (Titanium dioxide-Zinc Oxide-Bismuth Oxide) and P25 (commercial TiO2 nanoparticles), therefore providing a novel strategy for environmental remediation (Kanjwal et al., 2019). Several researchers have successfully utilized metal oxide nanocomposites to substantially enhance degradation activity for remediation of organic compounds as well as carcinogenic pollutants (Akbari et al., 2019; Asif et al., 2014; Nekouei et al., 2020; Rehman et al., 2019; Zhao, Wang, et al., 2020). 3D/2D/2D BiVO4/FeVO4@rGO heterojunction composite catalysts were synthesized by a simple one-step hydrothermal method, facilitating the separation efficiency of photogenerated charges for high photocatalytic degradation efficiency of tetracycline and hexavalent chromium ions in water (Yang et al., 2020). MoS2 nanosheets based on p n heterostructures were constructed for the purposes of achieving excellent NO2 detection at room temperature. Upon functionalization with tin oxide (SnO2) nanoparticles, the optimal MoS2/ SnO2 heterostructure-based gas sensor exhibited an outstanding selectivity toward nitrogen dioxide (NO2) as compared to other gases, and excellent long-term stability for 4 weeks (Han, Ma, et al., 2019). A series of Ag3PO4/g-C3N4/PVA composite films were prepared by facile ion-exchange and solvent blending method and utilized in degrading airborne toluene under visible light irradiation. It was inferred that the core-shell structure might be more suitable for Ag3PO4/g-C3N4 heterojunction with gC3N4 as the shell to protect the Ag3PO4/g-C3N4/PVA photocatalysts from photo corrosion and further improve the photocatalytic ability and stability in degrading pollutants (Cheng et al., 2019).

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18.3.4 CO2 reduction (hydrocarbon generation) Photocatalytic reduction of CO2 is an environmentally friendly and sustainable source of renewable fuels and chemicals. Metal oxide semiconductors play a significant role in photocatalytic reduction of CO2, owing to their photo-electrochemical stability, low cost, favorable band edge positions, and bandgaps. Photocatalytic CO2 conversion is a promising and viable option for low cost, clean, and environmentally friendly production of fuels by solar energy under relatively mild conditions with lower energy input. Photocatalytic conversion of CO2 into a variety of useful hydrocarbons, including CO, CH3OH, HCHO, CH4, and HCOOH, can be realized in the liquid or gas phase (Nikokavoura & Trapalis, 2017). However, at present, the prominent limitations of photocatalytic conversion of CO2 are its low conversion efficiency due to (1) low solubility of CO2 in H2O, (2) competition with water reduction to hydrogen, (3) mismatching between the absorption ability of the photocatalysts and the solar spectrum, and (4) back reactions during CO2 reduction (Li et al., 2014). Hence, attaining high selectivity and satisfactory production yields is still a challenging prospect (Sohn et al., 2017). The formulation of metal oxide nanocomposites is an efficient strategy to design a material platform consisting of a unique set of structural, optoelectronic, and chemical features for accomplishing the efficient photocatalytic reduction of CO2, wherein the shortcomings of single-phase materials can be overcome and also allow synergistic effects to come into play. The ability to absorb efficiently over the whole wavelength range of the solar spectrum is a prerequisite of an efficient photocatalyst for photocatalytic CO2 reduction. A comprehensive review that focused specifically on the fundamentals of CO2 reduction has been reported recently, wherein, all the recent advances in the photoselective protocols and engineering strategies for CO2 reduction were outlined, and the three most challenging factors of those strategies were also discussed in detail. Various strategies employed for improving photocatalytic CO2 reduction are illustrated in Fig. 18.4 (Matavos-Aramyan et al., 2020). Several metal oxide-based composite photocatalysts, viz. AgBr-TiO2 (Asi et al., 2011), Cu2O/SiC (Li et al., 2011), Ag3PO4/g-C3N4 (He et al., 2015), C3N4-TiO2 (Brunetti et al., 2019), Pd/TiO2 (Camarillo et al., 2017), MoS2/Bi2WO6 (Dai et al., 2017), rGO/TiO2 (Shehzad et al., 2018), and Fe-MMT/TiO2 (Tahir, 2018) have been synthesized and have exhibited enhanced carbon dioxide photoreduction compared to their single components. The photocatalytic activity of rGO/TiO2 photocatalyst (Shehzad et al., 2018) was further compared with various rGO/TiO2 photocatalysts synthesized through different methods reported by several researchers in literature. The details have been enlisted in Table 18.3. Liang et al. (2012) reported the RGO/TiO2 photocatalysts for CO2 reduction into CH4 prepared by the film deposition technique. The activity of RGO/TiO2 was even lower than that of TiO2 owing to the absence of chemical bonding between RGO and TiO2. However, most of the studies reported higher activity of rGO/TiO2 than bare TiO2 due to interfacial bonding achieved via the solvothermal or hydrothermal process. The activity of rGO/TiO2 photocatalyst synthesized by Shehzad et al. (2018) was significantly better than TiO2 and other reported rGO/TiO2

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Figure 18.4 Strategies for enhancement of photocatalytic CO2 production. Source: From Matavos-Aramyan, S., Soukhakian, S., Jazebizadeh, M. H., Moussavi, M., & Hojjati, M. R. (2020). On engineering strategies for photoselective CO2 reduction—A thorough review. Applied Materials Today, 18. https://doi.org/10.1016/j.apmt.2019.100499.

Table 18.3 Comparison of activity of different rGO/TiO2 systems. Photocatalyst

Method

Total product (max)

References

TiO2/rGO

Reflux and vacuum thermal treatment Film deposition Solvothermal Rapid thermal treatment Solvothermal Wet impregnation Microwave irradiation technique Hydrothermal Solvothermal Reflux method Vacuum activation and ultraphonic method

30.30 μmol g21 h

Shehzad et al. (2018)

1.70 μmol m22 h 1.40 μmol g21 7.00 μmol m22 h 0.13 μmol g21 h 0.33 μmol g21 h 10.00 μmol g21 h

Liang et al. (2012) Ong et al. (2014) Sim et al. (2015) Tan et al. (2013) Tan et al. (2015a) Tu et al. (2012)

27.00 μmol g21 h 0.65 μmol g21 2.20 μmol g21 h 1.30 μmol g21

Lin et al. (2017) Tan et al. (2015b) Liu et al. (2016) Xing et al. (2014)

SRGO/TiO2 TiO2/Gr TNT/rGO TiO2/rGO GO-OTiO2 TiO2G TiO2/rGO TiO2/rGO TiO2/rGO P25/GR

Source: From Shehzad, N., Tahir, M., Johari, K., Murugesan, T., & Hussain, M. (2018). Improved interfacial bonding of graphene-TiO2 with enhanced photocatalytic reduction of CO2 into solar fuel. Journal of Environmental Chemical Engineering, 6(6), 6947 6957. https://doi.org/10.1016/j.jece.2018.10.065.

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photocatalysts. The improved photoactivity of rGO/TiO2 photocatalysts has been attributed to interfacial bonding among rGO and TiO2 that efficiently transferred the electrons from TiO2 to graphene sheets. However, such a generalization for comparing the activity of different photocatalysts is not viable because system efficiency depends on several factors. Titanium dioxide/carbon nitride nanosheet nanocomposites were utilized for gasphase CO2 photoreduction under UV-visible irradiation. The study demonstrated that the photocatalytic reduction of CO2 could be improved greatly by forming composite materials with increased CO2 adsorption capacity and which can suppress electron hole recombination by facilitation of charge transfer (Brunetti et al., 2019). Investigations concerning the development of graphene-based photocatalysts were reviewed and it was inferred that in addition to overcoming inherent flaws of photocatalysts, graphene/graphene derivatives also favor the selectivity toward the generation of solar fuels like methane and ethane, a highly demanded energy source (Low et al., 2015). Currently, despite numerous efforts by researchers across the globe, the efficiency of solar-to-chemical energy conversion is too low for commercial application. The development of novel photocatalysts that exhibit high reaction selectivity, high chemical and physical stability, significant activity, and wide spectral response under sunlight irradiation is crucial for large-scale utilization.

18.3.5 Antibacterial, anticancer, and biomedical applications The antibacterial coating is widely used in health care, food service, and hospitals for disinfection and microbial control (Han, Wu, et al., 2019). Photocatalytic disinfection is an efficient method for the treatment of microbial polluted waters because metal oxide-based nanocomposite materials are able to distinguish mammalian cells from bacterial cells and thus provide long-term antibacterial growth inhibition. Taking this into consideration, chitosan/metal oxide polymeric nanocomposites and metal oxide nanoparticles were synthesized and characterized (Mizwari et al., 2021). Antibacterial activity of the prepared samples was confirmed by the disk diffusion method against E. coli (gram 2 ve) and Staphylococcus aureus (gram 1 ve) bacteria strains. The characterization results evidenced the formation of NiO, MgO, CS NiO, and CS MgO. The antibacterial results confirmed that all the samples exhibited antibacterial activities against the bacteria strains. The CS MO nanocomposites exhibited superior antibacterial efficacy compared to chitosan or the metal oxides alone. Noticeably, the CS NiO proved to be the most efficient antibacterial agent as it reduced the S. aureus and E. coli viabilities to 2% 8% after 12 hours incubation. According to the probable antibacterial mechanism of chitosan-coated metal oxide (CS MO) nanocomposites, the authors assumed that protonated amino groups on the Chitosan (CS) bind to the anionic groups (carboxyl and phosphate) on the bacterial surface and consequently disorganize the structure of the outer cell membrane, permitting metal ions (MO) to penetrate into the cell and thereby inducing antimicrobial activity. It was also inferred that the noticeably enhanced activity

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Figure 18.5 The plausible mechanism of antimicrobial activity in the presence of nanocomposite. Source: From Raut, A. V., Yadav, H. M., Gnanamani, A., Pushpavanam, S., & Pawar, S. H. (2016). Synthesis and characterization of chitosan-TiO2:Cu nanocomposite and their enhanced antimicrobial activity with visible light. Colloids and Surfaces B: Biointerfaces, 148, 566 575. https://doi.org/10.1016/j.colsurfb.2016.09.028.

of the CS MO in comparison with CS is due to an increase in chelation of CS MO on the surface of the bacterial cell and subsequent release of Mg12 and Ni12 ions, which increased antimicrobial effects regardless of the composition of the bacteria’s phospholipid or peptidoglycan layer (Mizwari et al., 2021). Similarly, Raut et al. (2016) reported the fabrication of chitosan-TiO2:Cu nanocomposite for biomedical applications. The resulting composite exhibited a 200% enhanced inactivation of bacterial strains of E. coli and S. aureus, compared to chitosan-only inactivation samples. The plausible mechanism of the disinfection process is illustrated in Fig. 18.5 (Raut et al., 2016). The photocatalytic disinfection properties of graphene oxide/V2O5/Pt [GOVPt (1%)] nanocomposite as well as its anticancer activity using Salmonella typhimurium as a model system were evaluated in a recent study. Total organic carbon analysis confirmed the deterioration of the bacterial cell wall, leading to disinfection. In real effluents, a major decrease (98%) in the total coliform colony forming units (CFU) was observed after disinfection. The results of MTT

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(3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide) assay, cell staining assay, and apoptosis assay confirmed that the composite has anticancer activity but no cytotoxic activity (Mohan et al., 2020). In a recent investigation, a novel multi-metal oxide nanocomposite, Ag2O.SrO.CaO, was synthesized by a facile co-precipitation method followed by calcination. The composite exhibited enhanced photocatalytic activity under visible light irradiation and excellent antibacterial performance against both Gram-positive and Gram-negative bacteria. In the presence of light, the antibacterial activities were found to be boosted remarkably because of the formation of reactive oxygen species. The minimal inhibitory concentration and minimum bactericidal concentration value of the Ag2O.SrO.CaO nanocomposites were also determined against these pathogenic bacteria, which indicated the minimum dose for effective antibacterial functions (Subhan et al., 2020). The green synthesis of nontoxic, eco-friendly, economical, and stable Cu2O/C3N4 nanocomposites by using lemon leaf extract was successfully accomplished. The biosynthesized nanocomposite exhibited good antibacterial activity with the maximum zone of inhibition (ZOI) of 22 6 1.67, 15 6 1.08, 11 6 1.22, 6 6 0.09 mm to B. subtilis E. coli, S. aureus and P. aeruginosa at 100 μg mL21 under visible light (Induja et al., 2019). In pursuit of the efforts for preparing photocatalysts suitable for antimicrobial applications, researchers across the world have reported studies on the construction of Z-scheme Ag-AgBr/BiVO4/graphene nanocomposite (Lin et al., 2020), Cu2O nanospheres decorated graphitic carbonitride (g-C3N4) (Liu, Wu, et al., 2019), CeO2/CdO nanocomposites (Magdalane et al., 2016) and silver-graphene oxide/ poly(vinyl alcohol) antibacterial nanocomposites (Cobos et al., 2019). Agar well diffusion method was employed for the evaluation of the antimicrobial activity of the CeO2 CdO binary metal oxide nanocomposites (Magdalane et al., 2016) against gram-positive bacteria (S. aureus MTCC-96 and Streptococcus pyogenes MTCC-1926) gram-negative bacteria (P. aeruginosa MTCC-4673 and K. pneumoniae MTCC-109) and fungi (C. albicans MTCC-183, F. oxysporum NAIMCC-F-00886, A. niger MTCC-282 and A. candidus MTCC-2202). Streptomycin (200 μg mL21) and Clotrimazole (200 μg mL21) were used as the positive control. The antimicrobial activities were evaluated by measuring the ZOI (mm), which appears as a clear area around the wells (Devi & Bhimba, 2014). Bacterial cell suspensions of 108 CFU mL21 and a fungal suspension containing 105 spore mL21 were prepared and evenly spread on the surface of Mueller-Hinton agar and Sabouraud dextrose agar medium using sterile swabs. Wells of 8 mm diameter were made using a sterile corn borer and the nanocomposites (200 μg) were suspended into the wells. The plates were incubated at 37 C for 24 hour to evaluate the ZOI. The antimicrobial activity of CeO2 CdO nanocomposites thus determined by agar well diffusion assay has been presented in Table 18.4. The authors inferred that the synthesized nanocomposites can be used as antibacterial and antifungal agents. Furthermore, several research endeavors have also been dedicated to the field of biomedical science for the synthesis of TiO2/FeOx/POM composite for accelerating the photocatalytic removal of the emerging endocrine disruptor: 2,4-dichlorophenol

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Table 18.4 Antimicrobial activity of CeO2-CdO nanocomposites determined by agar well diffusion assay. Bacteria

S. aureus (MTCC 96) S. pyogenes (MTCC 26) P. aeruginosa (MTCC 73) K. pneumoniae (MTCC 09)

Antibacterial zone of inhibition (mm) Precipitation (200 µg mL21)

Hydrothermal (200 µg mL21)

Streptomycin (200 µg mL21)

12 mm 20 mm 30 mm 8 mm

14 mm 16 mm 35 mm 13 mm

22 mm 21 mm 20 mm 22 mm

Fungi

C. albicans (MTCC 83) F. oxys (MTCC-F86) A. niger (MTCC 82) A. candidus (MTCC 22)

Antifungal zone of inhibition (mm) Precipitation (200 µg mL21)

Hydrothermal (200 µg mL21)

Clotrimazole (200 µg mL21)

17 mm 8 mm 8 mm 8 mm

18 mm 8 mm 8 mm 8 mm

24 mm 22 mm 20 mm 20 mm

Source: From Magdalane, C. M., Kaviyarasu, K., Vijaya, J. J., Siddhardha, B., & Jeyaraj, B. (2016). Photocatalytic activity of binary metal oxide nanocomposites of CeO2/CdO nanospheres: Investigation of optical and antimicrobial activity. Journal of Photochemistry and Photobiology B: Biology, 163, 77 86.https://doi.org/10.1016/j.jphotobiol. 2016.08.013.

(Yu et al., 2019), preparation of (3D/2D/2D) BiVO4/FeVO4@rGO heterojunction composite photocatalyst for the removal of tetracycline and hexavalent chromium ions in water (Yang et al., 2020), solar light promoted photodegradation of metronidazole over ZnO ZnAl2O4 heterojunction (Ghribi et al., 2020), sphere-like Ni3S4/ NiS2/MoOx composite modified glassy carbon electrode for the electrocatalytic determination of D-penicillamine (Kumar et al., 2020) and superparamagnetic MnFe2O4 dispersed over graphitic carbon sand composite and bentonite as magnetically recoverable photocatalyst for antibiotic mineralization (Gautam et al., 2017).

18.3.6 Layered double hydroxides/metal-organic frameworks Photocatalysis driven by functionalized metal-organic frameworks (MOFs) is a promising direction for the development of renewable energy conversion and environmental pollution rehabilitation by direct utilization of solar energy. Recent research progress on the photocatalytic applications of Ti-MOF-based photocatalysts and their derived porous materials in photocatalytic applications, including applications in water splitting, CO2 reduction, organic synthesis, and degradation of organic pollutants, was reviewed. It was inferred that the connection of the Tioxo cluster and organic ligand produces efficient active catalysis sites, providing Ti-MOFs with substantial photocatalytic activity (Chen et al., 2020). ZnFe-LDH

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Figure 18.6 Proposed photocatalytic mechanism of DZF hybrid composites for methylene blue and MG degradation. Source: From Zhao, G., Liu, L., Li, C., Zhang, T., Yan, T., Yu, J., Jiang, X., & Jiao, F. (2018). Construction of diatomite/ZnFe layered double hydroxides hybrid composites for enhanced photocatalytic degradation of organic pollutants. Journal of Photochemistry and Photobiology A: Chemistry, 367, 302 311. https://doi.org/10.1016/j.jphotochem.2018.08.048.

derived mixed metal oxides (ZnO/ZnFe2O4, ZnFeMMO) were innovatively adopted to modulate the g-C3N4 photocatalytic performance for the enhanced degradation of ibuprofen (IBF) and sulfadiazine (SDZ) as targeted pollutants. Additionally, degradation pathways for IBF and SDZ were established and an attempt to gain a new perspective for developing rationally architectural g-C3N4 based photocatalysts for the decontamination of water polluted by pharmaceuticals was made (Di et al., 2019). Construction of diatomite/ZnFe layered double hydroxides hybrid composites (DZF hybrid composites) for enhanced photocatalytic degradation of organic pollutants has been reported in the literature, wherein the higher photodegradation activity of the hybrid was attributed to the synergistic effect of the rapid separation rates of charges caused by the formation of the heterostructures between diatomite and pure ZnFe-LDH, strong visible light absorption, large surface area, and pore volume. The probable mechanism for degradation of organic pollutants has also been proposed, and this is illustrated in Fig. 18.6 (Zhao et al., 2018). As per the proposed mechanism, the catalysts are activated by visible light irradiation, and then the photocatalytic process gets initiated by radiation with higher energy than that of the bandgap of the hybrids. The DZF hybrid composites adsorb a photon with energy equal to or larger than its bandgap, then, the electrons at the surface of DZF hybrid composites can react with the molecular oxygen to generate the reaction active O22 species, while the holes in the VB of

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ZnFe-LDHs can react with water molecules to generate active OH species. The h1, O22 and OH are typically strong and nonselectively reducing or oxidizing agents of organic pollutants. Layered double hydroxides (LDH) photocatalysts have attracted a lot of attention from researchers for photodecolorization of organic dyes. Graphitic-C3N4 modified ZnAl-layered double hydroxides were utilized for enhanced photocatalytic removal of organic dye. It was inferred that the photocatalytic activities of C3N4/ZnAl-LDH composites toward decolorization of methylene blue (MB) highly depended on the loading of g- C3N4. The enhanced activity of C3N4/ZnAl-LDH composites was ascribed to the synergistic effect between two components of the composites (Yuan & Li, 2017). An effort for the utilization of a metal-organic framework (MOF) for efficient visible-light photocatalysis is reported in the literature, wherein bicomponent MOF/transition metal oxide heterojunction was prepared and kinetic as well as mechanistic studies were carried out. Moreover, the authors were successful in almost complete degradation of para-nitrophenol within 3.0 hours (Haroon & Majid, 2020). In line with the efforts for utilizing mixed metal oxide derived from layered double hydroxide for water remediation, ZnAl-layered double hydroxide ZnO/ZnAl2O4 (ZnAl-MMO) nanocomposite was used for photocatalytic Cr(VI) reduction. The ZnO/ZnAl2O4 (ZnAl-MMO) nanocomposite exhibited high photocatalytic Cr(VI) reduction activity under UV light irradiation and the rate constant for the process was substantially high (Yuan, & Jing, Chen, et al., 2017). The most recent advances in visible-light-driven photodegradation of organic pollutants using LDHs-based materials with emphasis on the manipulation of their morphological, compositional, and electronic properties and the mechanistic understandings of the photocatalytic processes were reviewed recently. It was concluded that despite significant progress made in photocatalyzed degradation of organic pollutants by LDHs, challenges remain pertaining to catalysis on a laboratory scale, mechanistic understandings, and practical application of LDHs-based photocatalysts Layered double hydroxides-based photocatalysts and visible-light-driven photodegradation of organic pollutants (Zhang, Zhang, et al., 2020).

18.3.7 Polymeric nanophotocatalysts Polymers and transition metal oxides have gained great interest as photocatalysts for environmental remediation, and highly-efficient solar energy conversion as they could be modified with each other to improve their activity. Moreover, semiconducting polymers have application potential in the fields of corrosion protection, antimicrobial coatings, etc. Polypyrrole nanofiber/Zn-Fe layered double hydroxide (Ppy NF/Zn-Fe LDH) was synthesized as a nanocomposite with enhanced adsorption and photocatalytic properties for the removal of safranin dye from water. It was inferred that the ability of this composite for the removal of dye from raw water samples opens up possibilities for the purification of tap water, groundwater, and sewage water. The photocatalytic degradation process was considered to be controlled by the created hydroxyl radicals and formed photogenerated holes as the dominant active oxidizing radicals. The photocatalytic degradation mechanism of

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Figure 18.7 Photocatalytic degradation mechanism of safranin dye using Ppy NF/Zn-Fe LDH nanocomposite. Source: From Mohamed, F., Abukhadra, M. R., & Shaban, M. (2018). Removal of safranin dye from water using polypyrrole nanofiber/Zn-Fe layered double hydroxide nanocomposite (Ppy NF/Zn-Fe LDH) of enhanced adsorption and photocatalytic properties. Science of the Total Environment, 640 641, 352 363. https://doi.org/10.1016/j.scitotenv.2018.05.316.

safranin dye using Ppy NF/Zn-Fe LDH nanocomposite has been illustrated in Fig. 18.7 (Mohamed et al., 2018). Upon irradiation of incident photons of visible light illumination on Ppy NF/Zn-Fe LDH composite, an electron can be excited from the VB to the conducting band which produces electron hole pairs and the associated photogenerated oxidizing radicals (hydroxyl radicals, superoxide radicals, and positive photogenerated holes). Such generated radicals cause photocatalytic degradation of the safranin dye (Mohamed et al., 2018). Similarly, a sunlight-responsive hierarchically structured ternary composite of nickel oxide, polyaniline, and reduced graphene oxide (NiO@PANI/RGO) was synthesized and employed as a catalyst for dye [methylene blue (MB)] degradation. It was reported that the efficiency of 98% methylene blue MB degradation within 11 minutes at a degradation rate constant 0.086 min21 for NiO@PANI/RGO is better than any other reports on metal oxide/graphene-based ternary composites (Ahuja et al., 2018). Polymeric carbon nitride hybridized by CuInS2 quantum dots was utilized for photocatalytic hydrogen evolution. Owing to the enhanced electron hole separation efficiency and improved visible light-harvesting ability, the HER of the CuInS2/CN hybrid was observed to be two folds higher than that of pristine carbon nitride (CN), thus opening up new avenues for the creation of highly-efficient photocatalysts for solar energy conversion (Zheng et al., 2019). Moreover, silver-graphene oxide/poly(vinyl alcohol) antibacterial nanocomposites are synthesized via a single-step eco-friendly method. The PVA/AgNPs-GO nanocomposites showed antibacterial activity against Gram-negative bacteria Escherichia coli and against Gram-positive bacteria Staphylococcus aureus. However, PVA/GO films showed no activity against both bacteria over the GO loading range investigated.

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These findings imply the tremendous potential of PVA/AgNPs-GO nanocomposites as wound dressings (Cobos et al., 2019). In a recent study, electrospun polyacrylonitrile nanofibers coated with TiO2 hollow spheres (TiO2@PAN nanofibers) were utilized for highly synergistic photo-conversion of Cr(VI) and As(III) using visible light. It was inferred that whether a single reaction or a synergistic catalytic reaction, the TiO2@PAN-3 photo-conversion rate increased relative to PAN and other samples. TiO2@PAN-3 composite efficiently converts heavy metals while exhibiting good stability (Cai & Li, 2020). Furthermore, conjugated polymer poly(3-hexylthiophene-2,5-diyl)-P3HT coupled with hierarchical ZnO (H-ZnO) photocatalysts were fabricated by a two-step solution method by assembling P3HT nanoparticles on the surface of H-ZnO micro-flowers. The as-prepared P3HT@H-ZnO composite exhibited higher photocatalytic activity than H-ZnO for photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The enhanced photocatalytic activity was attributed to the strengthened visible light absorption and the closely contacted interface between P3HT and H-ZnO. This research endeavor provides new insights for designing Z-scheme photocatalysts with enhanced photocatalytic performance via metal-free polymer modification (Liu, Li, et al., 2018).

18.3.8 Food safety Cross-contamination of foods with pathogenic microorganisms such as bacteria, viruses, and parasites may occur at any point in the farm to fork continuum. Food contact and nonfood contact surfaces are the most frequent source of microbial cross-contamination. In the wake of new and emerging food safety challenges, including antibiotic-resistant human pathogens, conventional sanitation, and disinfection practices may not be sufficient to ensure safe food processing, proper preparation, and also not be environmentally friendly. Mixed metal oxide nanocomposites that enable novel food safety interventions have a great potential to mitigate the risk of microbial cross-contamination in the food chain. Metal oxide photocatalysts that have been effectively utilized for prevention of microbial crosscontamination include various oxide semiconducting materials (TiO2, ZnO, CuO, MgO, SnO2, WO3, SiO2, Fe2O3, and Nb2O3, etc.), their metal hybrid nanocomposites (CdS and ZnS) and doped structures (such as Ag/TiO2, TiO2/CuO, TiO2/Pt, Au/ TiO2, Fe2O3/TiO2, and N-, C-, S doped TiO2) (Fu et al., 2005; Sawai, 2003). Although metal oxide photocatalysts exhibit significant potential for the prevention of microbial cross-contamination, the development of robust coatings with strong antimicrobial properties to withstand harsh food processing conditions is a challenging prospect. Application of this technology in the food chain also requires addressing the stability of these coatings and migration/release of nanoparticles into food systems. This novel technology, if used appropriately, can provide an additional layer of protection for the safety of fresh and processed foods during processing, preparation, and storage. Other standard GMPs and sanitation standard operating procedures are still needed to be in place to ensure food safety and prevent cross-contamination (Yemmireddy & Hung, 2017).

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Apart from the eight applications described earlier, mixed metal oxide nanocomposites can also be utilized for applications in the fields of air depollution and removal of volatile organic compounds (Banisharif et al., 2015; Jiang et al., 2015; Jiao et al., 2014; Pham & Lee, 2015; Zhu et al., 2015), the photochemical transformation of specific compounds (Ciambelli et al., 2005; Di Paola et al., 2015; Yamauchi et al., 2011; Zhang et al., 2011) and photodynamic therapy (Dou et al., 2015; Hou et al., 2015).

18.4

Future perspectives of metal oxide-based composites as photocatalysts

The year 2030 is the deadline for achieving the United Nations Sustainable Development Goals (SDGs). Undoubtedly, the fall in economic activity, the increase in debt, and the decrease in demand will be major obstacles for governments and businesses to invest the attention, ambition, and resources needed to achieve the SDGs. Chemistry has much to contribute to the 2030 agenda (GarciaMartinez, 2021). Sustainability, innovation, diversity, and education are four pillars that should define our efforts in the next decade to achieve the SDGs. Dedicating our chemistry endeavors toward the reuse of atoms, molecules, monomers, polymers, etc. willplay a critical role in mitigating the challenge of climate change. Such an endeavor represents an opportunity to place chemistry at the center of the new circular economy. A good example in this direction is the enormous advances in (photo)conversion of CO2 to produce not only solar fuels but also a great variety of platform molecules that can serve as raw materials for new solar chemistry. Exciting discoveries in material science have led to a revolution in illumination, catalyzed by LED-based lighting technologies, energy storage, which greatly benefit from Li-ion batteries, and renewable energy, which is taking advantage of novel solutions, such as perovskite-based photovoltaic (PV) cells. These and other important advances have allowed us to more than double our economic output per unit of energy used in the last 25 years (Garcia-Martinez, 2021). Despite numerous efforts by researchers across the globe, the efficiency of solarto-chemical energy conversion is still too low for commercial application. The development of novel photocatalysts that exhibit high reaction selectivity, high chemical and physical stability, significant activity, and wide spectral response under sunlight irradiation willplay a crucial role in large-scale utilization. Metal oxide-based composites have been utilized successfully for wastewater treatment. However, it is imperative to utilize a photocatalytic reactor that is energy efficient, simple, able to handle high wastewater volumes, and inexpensive to build and operate for overcoming the challenges encountered at an industrial scale. The efficiency of the photoelectrodes under visible light, as well as stability, must also be improved. Moreover, the cost of these materials should be taken into consideration. The creation of novel, more efficient, photoelectrocatalytic (PEC) systems is

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expected in the future. Photoelectrocatalytic hydrogen production with simultaneous oxidation of organic pollutants is also promising. The possibility of simultaneous wastewater treatment and hydrogen production will contribute toward efficient and versatile exploitation of photoelectrocatalysis. Additionally, solar-driven or lowpowered UV lamp-irradiated (LEDs) photoreactors are crucial for the broader-scale application of photocatalytic processes for the degradation of pollutants. The LEDs are low-power and low-cost irradiation sources that reduce power consumption and the cost of photocatalytic processes. Hydrogen generated from water splitting is an innovative and potential route to produce green fuel using solar light. However, the low quantum efficiency of the reaction is a major limitation for the practical application of this process. Hence, the development of photocatalysts with remarkable activity and visible-lightresponsive stable materials is a must. Novel strategies for efficient separation of photogenerated electrons and holes in powder semiconductors, the scaling-up process, and separation of powder photocatalysts after the process are highly desirable attributes (Tasleem & Tahir, 2020). Photocatalytic CO2 conversion is a promising and viable option for low cost, clean, and environmentally friendly production of fuels by solar energy under relatively mild conditions with lower energy input. However, at present, the prominent limitations of photocatalytic conversion of CO2 are its low conversion efficiency due to (1) low solubility of CO2 in H2O, (2) competition with water reduction to hydrogen, (3) mismatching between the absorption ability of the photocatalysts and the solar spectrum, and (4) back reactions during CO2 reduction (Li et al., 2014). The ability to absorb efficiently over the whole wavelength range of the solar spectrum is a prerequisite of an efficient photocatalyst for photocatalytic CO2 reduction. Several strategies such as doping, morphology control, defect introduction, surface modification, and heterojunction construction, etc. can be employed for improving photocatalytic CO2 reduction.

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Metal oxide-based composites for magnetic hyperthermia applications

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Amol B. Pandhare1,2, Rajendra P. Patil2 and Sagar D. Delekar1 1 Nanoscience Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India, 2Department of Chemistry, M.H. Shinde Mahavidyalaya, Kolhapur, Maharashtra, India

19.1

Introduction

Magnetic hyperthermia treatment is one of the emerging treatments for elevating the temperature of the specific body part under magnetic exposure in the presence of magnetic particles (Moise et al., 2018). Though this protocol is under the early stage of the investigation, it will be highly beneficial for the treatment of cancer in humans and hence exhaustive research endeavors are going at reputed research institutes worldwide. Cancer is one of the awful diseases, and hence it is the second leading cause of death of people globally (Sun et al., 2019). The International Agency for Research on Cancer (WHO agency) reported that around 19.3 million people per year are suffering from cancer disease and out of which 10 million patients have died [WHO data]. Furthermore, about 70% of cancer fatalities occur in low- and middle-income nations, making it a major public health concern in the 21st century (Sakthikumar et al., 2019). Cancer is a generic term for a large group of diseases that can affect any part of the body. Other terms used are malignant tumors and neoplasms. One of the cancer risk factors is the rapid emergence of abnormal cells that grow beyond their normal boundaries and spread to other parts of the body, a process known as metastasis. Metastasis is the leading cause of cancer-related death (Jiang et al., 2021). The old cells do not die as a result of cancer; instead, they expand out of control, generating new, aberrant cells. Some cancers cause cells to grow and divide quickly, while others cause cells to grow and divide slowly (Guan, 2015). Certain forms of cancer result in visible growths (solid tumors cancer) like tumors, breast, prostate, lung, etc., while others in invisible form (Hematologic cancer), such as leukemia, lymphoma, etc. Most of the body’s cells have specific functions and fixed life spans. While it may sound like a bad thing, cell death is part of a natural and beneficial phenomenon called apoptosis (Nickells, 1999). A cell receives instructions to die so that the body can replace it with a newer cell that functions better. Cancerous cells lack the components that instruct them to stop dividing and to die. As a result, they build up in the body, using oxygen and nutrients that would usually nourish other cells. Cancerous cells can form tumors, impair the immune system and cause other changes that prevent the body from functioning regularly (Gowsalya et al., 2021). Cancerous cells can start in one place and spread throughout the body via lymph nodes. These are clusters of Advances in Metal Oxides and Their Composites for Emerging Applications. DOI: https://doi.org/10.1016/B978-0-323-85705-5.00019-1 © 2022 Elsevier Inc. All rights reserved.

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immunological cells present throughout the body. Tumors come in two types: benign and malignant. Tumors are lumps or growths on the body. Although some lumps are cancerous, many are not. Benign lumps are those that are not cancerous, while malignant lumps are those that are cancerous (Baroud et al., 2021). Cancer cells may have the ability to move away from the place of origin. These cells can spread throughout the body and end up in other organs, as well as interfering with normal activity. Cancer cells develop because of multiple changes in their genes (Jones & Baylin, 2007). These changes can have many possible causes. Lifestyle habits, genes you get from your parents, and being exposed to cancer-causing agents in the environment can all play a role. Many times, there is no obvious cause. When cancer is found, tests are done to see how big the cancer is and whether it has spread from where it started. This is called the cancer’s stage. A lower stage (such as stage 1 or 2) means that cancer has not spread very much. A higher number (such as a stage 3 or 4) means it has spread more. The highest stage is 4. The stage of a person’s cancer is critical in determining the optimal treatment. Cancer can spread to different places of the body from where it started (the original site). When cancer cells break free from a tumor, they can spread to other parts of the body via the bloodstream or the lymph system. Cancer cells can migrate through the bloodstream to reach distant organs (Sen et al., 2020). Cancer cells may end up in lymph nodes if they pass through the lymphatic system. In either case, the majority of the escaping cancer cells die or are eliminated before they can spread. However, one or two of them may settle in a new location, develop, and form new tumors. Metastasis is the process through which cancer spreads to a new part of the body (Zhang et al., 2015). Cancer cells must go through various alterations to move to new regions of the body. They must first be able to separate from the original tumor and connect to the exterior wall of a lymph vessel or blood vessel. Then they must get through the vessel wall to join the blood or lymph node. The predicted new cases of cancer in both sexes among all ages in 2020 are summarized in Fig. 19.1. Breast cancer is one of every four malignancies diagnosed in women worldwide, according to the IARC (International Agency for Research on Cancer). In this desperate situation, nanoscience and nanotechnology have a leading edge for cancer treatment, which involves the use of nanosized materials for improved diagnosis, treatment, and prevention from disease and traumatic injury (Gharatape et al., 2016). Many new methods and techniques have been developed using nano-based composites to improve the diagnosis and treatment of cancer, often promising in the beginning, but with limited results during their applications. Among different types of nanomaterials (Deshmukh et al., 2019), superparamagnetic metal oxide-based composites for magnetic hyperthermia are promising candidates for cancer diagnosis and therapy (Moise et al., 2018). Taking into consideration the motivations, the present chapter focuses on the detailed about the use of the different metal oxide-based composites for hyperthermia treatment. This chapter also includes the different cancer treatments available with their pros and cons, basics of magnetic hyperthermia with its types, synthetic methods for magnetic metal oxide nanomaterials, an overview of the metal oxide-based composites for hyperthermia studies, etc. This chapter ends with a concluding summary and future outlook of the metal oxide-based composites for hyperthermia studies.

Metal oxide-based composites for magnetic hyperthermia applications

Colorectum

Breast Liver Prostate 7%

Lung

Prostate

Stomach

Breast 12% Colorectum 10%

Cervix uteri

Other cancer

Other cancer 46%

Lung 11% Liver 5%

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Other 55% Cervix uteri 3% Stomach 6%

Figure 19.1 Estimated number of new cases in 2020 worldwide for both sexes and all ages. Source: Global cancer data.

19.2

Present cancer treatment: pros and cons

There are many cancer treatments. The type of cancer to choose depends on its type and location. Compared to others, each treatment has its pros and cons. 1. Chemotherapy: Chemotherapy is a common cancer treatment. Chemotherapy is a common cancer treatment method that involves utilizing specific chemical medications to stop tumor cells from growing. Basically, in this protocol, various drugs are used to control or cure cancer either at a specific target or throughout systems (Nabavinia & Beltran-huarac, 2020). It has certain pros like the easiest way for controlling cancer, cancerous cells killed fastly, etc. However, there are certain cons like extra pains due to shrinking of tumors, also kills/damage normal or healthy cells, hampers the immune system leads the hair loss, skin infections, mouth sores, digestive problems, sluggishness, etc. (Huang et al., 2021). 2. Radiation therapy: Radiotherapy is an important part of cancer treatment. Radiation therapy is also used for controlling cancerous tumors and hence in this therapy, high radiation doses are used to control/kill cancer cells (Goswami et al., 2017). Radiotherapy is a highly effective treatment when it is used in early-stage cancer located near or nearby body surfaces (Ni et al., 2020). In addition, it is a very expensive treatment as well as may also cause effects such as hard to eat, throat problems, skin blackening, etc. 3. Surgery: Surgery is also another treatment protocol commonly used for tumor types of cancer (Sharmiladevi et al., 2021). During surgery treatment, the proper protocol is to be adopted; otherwise, it may lead the various health issues like alarming fast growth of cancer, high pains to the patients, even up to the death of the patient. 4. Hormone therapy: Hormone therapy is used to retard the growth of cancer that uses hormones to grow (Torres-Garcı´a & Domenech, 2017). Such treatment is used for the treatment of breast and prostate cancer as well. However, it may cause weight, loss of interest in sex, also mood changes, etc. 5. Immunotherapy: Immunotherapy is one of the most advanced treatments available for improving the immune system’s ability to fight cancer (Nabavinia & Beltran-huarac, 2020). Immunotherapy is one of the biological treatments which utilize white blood cells

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and living organisms or tissue. But immunotherapy is not still widely used as compared to other treatments like chemo or radiation therapy (Ozik et al., 2019). 6. Stem cell treatment: The versatility of this treatment allows stem cells to be used to regenerate or repair diseased tissue and organs of cancer patients. Stem cell transplant helps us to restore blood-forming stem cells in the human body (Chan et al., 2017). Blood form stem cell well, but meanwhile white blood cell which is more beneficial against infection, and red blood cell helps to carry oxygen throughout the body. Stem cell-based transplant is very expensive; it is a complicated process. But stem cell transplant is used for leukemia blood cancer and this transplant for other cancer is under clinical trial.

The aforementioned treatments have various laggings such as long-term duration, hampering patient’s health regularly, frequent health check-ups, costlier, trialerror protocols, etc. These leggings of the present cancer treatments would have been resolved with the use of magnetic hyperthermia treatments for cancer treatment.

19.3

Hyperthermia

Hyperthermia is derived from two Greek words, “hyper” and “therme,” which mean “rise” and “heat,” respectively (Hedayatnasab et al., 2017). Hyperthermia is a therapeutic technique that acts to heat the target cells with sufficient temperature to destroy them without harming adjacent cells or tissue (Ban˜obre-Lo´pez et al., 2013). This approach is commonly utilized for cancer treatment as an alternative to conventional chemotherapy and radiotherapy because it has been proved to be safer with fewer side effects, and most crucially, tumor cells are more sensitive to heat than normal cells, making this technique easier to apply. There is no agreement on what temperature is the safest or most beneficial for the entire body. During therapy, the body temperature rises from 39.5 C to 40.5 C. Others, on the other hand, describe hyperthermia as a temperature of 41.8 C to 42 C, with a target temperature of 43 C 44 C. Heating regimens are classified based on temperature raised in the area of the tumor tissues as well as the length of the treatment. Cancer treatments can be intended to generate local or systemic warmth between 42 C and 45 C (hyperthermia) or over 45 C (thermal ablation) (Deatsch & Evans, 2014). The distinctions between the two procedures are focused on the patient’s safety, as systemic treatments expose the entire body to high temperatures. Instead, with local treatment, heat is directed at the tumor to eliminate it while avoiding injury to other tissues. In some clinical protocols, hyperthermia is being employed as adjuvant therapy for cancer treatment, displaying a synergic effect with radiation and chemotherapy, boosting their cytotoxic effects (Clavel et al., 2015).

19.3.1 Classification of hyperthermia The National Cancer Institute (NCI) identifies the three hyperthermia systems. Based on the size of the cancer area being treated, hyperthermia can be classified into the

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following three types (Mahmoudi et al., 2018): Hyperthermia of the whole body, regionally, and locally, which are shown in Fig. 19.2 (Kumar & Mohammad, 2011).

19.3.1.1 Local hyperthermia Local hyperthermia heats a relatively small region and is commonly used to treat tumors that are close to or on the skin, or tumors that are close to natural holes in the body as shown in Fig. 19.3 (e.g., the mouth) (Kumar & Mohammad, 2011). In other cases, the goal is to heat the tumor to death without causing any other damage. Microwave, radiofrequency, ultrasonic radiation, or magnetic hyperthermia can all be used to generate heat (Laurent et al., 2011). Heat may be administered to the surface of the body, inside normal body cavities, or deep in tissue via needles or probes, depending on the location of the tumor. It should not be confused with tiny tumor ablation, which uses higher temperatures ( . 55 C) to kill tumor cells. Local heating is also effective for treating dense tumors (Suleman et al., 2020).

Figure 19.2 Types of hyperthermia.

Figure 19.3 Local hyperthermia.

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19.3.1.2 Regional hyperthermia Regional hyperthermia is the heating of a greater area of the body, such as an organ or limb as shown in Fig. 19.4. The goal is usually to weaken cancer cells so that radiation and chemotherapy drugs can destroy them. Regional hyperthermia is most commonly utilized to treat disorders that affect the deeper tissues or when a vast area needs to be treated. Regional hyperthermia is used to deliver heat to advancedstage tumors (Hervault & Thanh, 2014).

19.3.1.3 Whole-body hyperthermia Metastatic cancer that has spread throughout the body is treated with whole-body hyperthermia (Hervault & Thanh, 2014). Whole-body hyperthermia is commonly used to treat cancer that has gone beyond its primary site to other organs; this type of disease is known as metastatic cancer as shown in Fig. 19.5. It is commonly used to treat cancer that has spread to other places of the body (cancer that spreads to many parts of the body). Heating blankets and warm-water immersion can help to boost body temperature (Suleman et al., 2020). Infrared hyperthermia domes, which cover the entire body or just the head, putting the patient in a very hot room/ chamber, or wrapping the patient in hot, wet blankets or a water tubing suit are also options. This treatment frequently results in complications affecting the heart, blood vessels, and other vital organs (Deb et al., 2019).

Figure 19.4 Regional deep hyperthermia application on the deep-seated tumor.

Figure 19.5 Whole-body hyperthermia.

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19.3.2 Magnetic hyperthermia Magnetic hyperthermia is a revolutionary cancer treatment that uses magnetic nanoparticles in an external magnetic field to heat only the tumoral area without harming the surrounding healthy tissue (Nemati et al., 2016). In this method, the challenging task is to design uniform hyperthermia (heating) system working in the temperature range of 42 C 46 C throughout the deeply seated tumors without harming healthy cells or tissues (Phong et al., 2017). The heating capacity or efficiency of the magnetic nanoparticles depends on their magnetic as well as physicochemical properties. Greeks, Egyptians, and Romans were the first to use hyperthermia many years ago (Hervault & Thanh, 2014). Magnetic nanomaterials have biocompatibility, low cytotoxicity, surface functionalization, and a high specific absorption rate (SAR) value, making them the most attractive candidate for magnetic hyperthermia. This is owing to the material’s magnetic hysteresis when subjected to an alternating magnetic field. Steam, water, and radiation are all options for heating (i.e., infrared, electromagnetic, microwaves, and ultrasound). Magnetic nanoparticles (MNPs) generate heat in the presence of an alternating magnetic field, that damages cancerous tumor cells (Ban˜obre-Lo´pez et al., 2013). Superparamagnetic MNPs are prominently used in magnetic hyperthermia because these materials can be magnetized and demagnetized quickly without hysteresis losses when exposed to an external magnetic field (Sa´nchez-Cabezas et al., 2019). In hyperthermia therapy, particle size is an important determinant of energy efficacy (Myrovali et al., 2020). Nanoparticles have a set of properties that are very well suited for the magnetic hyperthermia treatment of cancerous tumor cells. The efficiency of magnetic hyperthermia treatment can be improved by optimizing nanoparticle size, particle distribution, and the frequency of alternating magnetic fields. Magnetic hyperthermia can be used to replace existing cancer treatments that have costly, complex, and fatal adverse effects (Laurent et al., 2011). The monitoring of the temperature of a tumoral area using magnetic materials there is still a more challenging task in hyperthermia treatment. Although magnetic hyperthermia treatment for brain tumors is legal in Europe, it is rarely used in clinical practice.

19.4

Representative nanomaterials for magnetic hyperthermia

Literature studies reveal that the different nanomaterials such as metal oxides-based, metal-based, and their composites or hybrids, etc. have been used for magnetic hyperthermia. Particularly, the nanomaterials made up of iron-oxide phases and other magnetic ferrites with spinel structures (doped/undoped) are the most widely employed in clinical applications. The use of iron oxide nanoparticles in magnetic hyperthermia resulted in innovative, efficient treatment methods that almost cure cancer (Suleman et al., 2020). This is due to its superior properties including biocompatibility, low toxicity, surface functionalization, ease of production, etc.

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Figure 19.6 Application of iron oxides nanoparticles in the biomedical field.

Iron oxide-based nanoparticles have been emerged as major candidates in the biomedical area among the nanomaterials examined, as illustrated in Fig. 19.6. Palanisamy and Wang (2019) Because of their nontoxicity and biocompatibility, iron oxide-based nanoparticles are ideal for biomedical applications like magnetic hyperthermia treatment, Drug delivery, MRI (Magnetic resonance imaging), Chemotherapy, Biosensor, Photodynamic therapy, Tumor targeting, Detection of bacteria, Protein therapy, Photothermal treatment (Zhou et al., 2017), etc. Previously, maghemite and magnetite nanoparticles are easily manufactured using a variety of preparation processes, including sol gel, coprecipitation, and hydrothermal. Nanoparticles developed for biomedicine are coated with both organic and inorganic chemicals to improve their biocompatibility. Furthermore, these particles can be functionalized with a variety of supports to improve their biocompatibility and stability in biological contexts, as well as provide functionality such as targeted drug delivery (Bohara et al., 2016). Magnetic nanoparticles for biomedical applications can be synthesized using a variety of ways. Wet chemical methods such as Co-precipitation, Polyol, Microemulsion, Hydrothermal, Sonochemical, Sol gel (Delekar et al., 2012), Microwave, Solvothermal, and Precipitation method (Balducci et al., 2017), etc.; which are summarized in Fig. 19.7.

19.5

Magnetic metal oxide nanomaterials-based composites for magnetic hyperthermia application

Composite nanomaterials, which are made up of many functional components, have piqued materials scientists’ interest due to their unique physicochemical properties and wide range of potential applications in electronics, photocatalytic activity (Delekar et al., 2012), catalysis (Deshmukh et al., 2020), biology, and

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Figure 19.7 Wet chemical methods.

nanotechnology (Chen et al., 2020). The nanocomposites often include a core/shell or binary nanostructure that can be changed on the surface with various charges, reactive groups, or functional moieties for improved stability and compatibility (Tee & Ye, 2021). Magnetic metal oxide nanoparticles are more attractive due to they provide various properties high saturation magnetization, high magnetic susceptibility, low cytotoxicity, superparamagnetism, high magnetic permeability, colloidal stability, and physicochemical properties (Hedayatnasab et al., 2017). Magnetic nanoparticles have been explored for medical applications due to their unique physical as well as magnetic properties. Host-guest nanomaterials are fascinating because they can have a combination of contrasting properties and offer new multifunctionality. After all, the host and guest may have different material compositions in a single particle (Kumar & Mohammad, 2011). Host-guest structured nanocomposites are very interesting in the biomedical field. Host-guest structured nanocomposites consisting of magnetic iron oxide host other metal oxide guests have an attraction in particular unique magnetic properties, low cytotoxicity, and also provide high surface area. These properties are more beneficial in the medical field. Mixed metal oxides (MMO) nanocomposites have developed a reputation for excellent performance in a variety of sectors, including electrochemistry photocatalysis (Patil et al., 2018), Sensing, Antibacterial (Deshmukh et al., 2021; Yadav et al., 2014), and biological fields (e.g., hyperthermia application) are shown in Table 19.1. Ke et al. looked at core shell structured particles, particularly magnetic materials, which have attracted attention due to their distinctive magnetic responsivity and chemically responsive surface, and which offer new nanostructured materials with a wide range of uses (Ke et al., 2012). The magnetic properties of magnetic spinal ferrites CoFe2O4 and MnFe2O4 nanocrystals with core shell nanocrystals have been researched by Qing Song et al. and found them to be highly specific. The encapsulation of the nanoparticles with biocompatible oxides such as (e.g., SiO2, TiO2) is one of the strategies usually implemented to decrease the toxicity of magnetic oxides. Shells are used to protect sensitive core material, and having a biocompatible shell around poisonous host material helps to lessen core material toxicity.

Table 19.1 Metal oxides-based composites for magnetic hyperthermia applications. Composite

Synthesis method

Cell culture

Specific absorption rate (SAR) (W/g)

Heating ability

References

MgFe2O4 coated dextran

Combustion method

Mice fibroblast L929 cells

SAR of about 85.57 W g21 at 26.7 kA m21 (265 kHz)

Khot et al. (2013)

SPIONs@Au

Sol gel method

H9c2 cardiomyoblasts

Ms 5 58 emu g21, f 5 44 Hz at 465 Oe magnetic fields

Hyperthermia temperature range near 50.25 C and 73.32 C (at 5 and 10 mg mL21 ) Hyperthermia temperature range 42 C

Fe3O4@SiO2

Modified hydrothermal method Surfactant templating approach

HeLa cells (cervical cancer cell line)

ZnCoFe2O4@Fe3O4

Coprecipitation method

La0.7Sr0.3MnO3 with dextran

Combustion method

Vitro culture of murine NIH-3T3 fibroblasts HeLa and L929 cell lines

mFe3O4@dm-SiO2 show saturation magnetization Ms 5 52 emu g21 SPIONs coated with m-SiO2 show saturation magnetization Ms 5 41.0 emu g21 ZnCoFe2O4@Fe3O4 show saturation magnetization Ms 5 58.89 emu g21 La0.7Sr0.3MnO3 show saturation magnetization Ms 5 29 emu g21 SAR 5 51 W g21

SPIONs coated with m-SiO2

Human breast adenocarcinoma, MCF-7 cells

Hyperthermia temperature range 41 C 43 C

Mohammad et al. (2010) Guo et al. (2014)

Hyperthermia temperature range 41 C

Lu et al. (2017)

Hyperthermia temperature range near 48 C

Darwish et al. (2020) Thorat et al. (2013)

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Among the different metal oxides, the iron oxides-based composites have been studied in hyperthermia applications majorly and hence it is mandatory to learn the details about these composites used in hyperthermia applications.

19.6

Iron oxide nanoparticles and surface functionalization

Surface functionalization is one of the strategies for surface covering of host nanomaterials for further endeavors. This is helpful to promote biocompatibility, as well as to reduce the cytotoxicity of host nanoparticles and many more. The biocompatibility of magnetic nanoparticles usually depends on the host material as well as its surface coverings (Darwish et al., 2020). In the biomedical field, multifunctional nanomaterials offer significant potential over the bare one. The surface of iron oxide is commonly functionalized using different moieties such as inorganic coatings, polymers, bioactive molecules, organic surfactants, etc. which are shown in Fig. 19.8. 1. Synthetic and natural polymer: The synthetic polymers such as PVA (polyvinyl alcohol), PVP (polyvinylpyrrolidone), PEG (polyethylene glycol), etc., are identified commonly for functionalization of host iron oxide; which have also biocompatible (Parveen et al., 2012). In addition, natural polymers such as dextran and chitosan are also widely used as surface functional agents due to their low cost, nontoxicity, and biocompatibility (Bohara et al., 2016). The majority of polymers used in iron oxide nanoparticles are used to prevent particle aggregation. 2. Organic surfactants: Organic surfactants such as oleic acid, lauric acid, and dodecylphosphonic acid, among others, are frequently used to make surface functionalization of iron oxide nanoparticles (Parveen et al., 2012). The aggregation of iron oxide nanoparticles has been reduced by using oleic acid. 3. Inorganic coating: Inorganic coatings in the form of core shell structure, mixed metal oxides are used in magnetic applications (Li et al., 2021). Therefore the various metal

Figure 19.8 Representation sketch of the chief guest for functionalization of iron oxide nanoparticles. The spheres signify the host of iron oxide nanoparticles.

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oxides such as SiO2, TiO2, ZnO, etc. are utilized commonly for iron oxide functionalization due to their being biocompatible, nontoxic, and inexpensive. The purpose of the inorganic coating is to improve the host’s characteristics and performance. 4. Bioactive molecules: Bioactive molecules such as peptides and proteins, lipids, are commonly utilized for the functionalization of iron oxide nanoparticles (Nagamune, 2017).

19.7

Methods for measuring the magnetism of the magnetic materials

Magnetization of the materials are studied using the different tools including superconducting quantum interference device magnetometry (SQUID), zero-field cooling (ZFC) and field cooling (FC) measurements, vibrating-sample magnetometer (VSM), heating capacity: induction heating system as well.

19.7.1 Superconducting quantum interference device magnetometry Superconducting quantum interference device magnetometry is a technique used for the magnetic measurement of materials that display different properties (Mourdikoudis & Pallares, 2018). SQUIDS is a sensitive magnetometer capable of detecting a magnetic field of 10218 T. Nuclear magnetic resonance detectors and magnetic field measurements in the brain, heart, and stomach are among the areas for which SQUID technology is presently used (Evans et al., 2009). A SQUID is a device that can notice very weak signals, such as changes in the electromagnetic energy field of the human body. For all types of AC and DC magnetic measurements, it is amazingly sensitive. In SQUID analysis the various properties such as blocking temperature (TB), magnetization remanence (Mr), and magnetization saturation (Ms) are to be investigated well. Apart from NPs, the magnetic response of individual molecules can also be measured by SQUID (Mourdikoudis & Pallares, 2018). The MPMS-XL (Magnetic property measurement system) provides solutions for a unique class of sensitive magnetic measurements in key areas such as biochemistry, high-temperature superconductivity, and magnetic recording media. SQUID technology is used in MPMS sample magnetometers as well as in patented modifications. It is also used to achieve previously impossible levels of measurement sensitivity, dynamic range, and repeatability (Milosevic et al., 2014). A SQUID detection system, an accurate temperature control unit built into a highfield superconducting magnet, and a state-of-the-art computer operating system are all integrated into the MPMS design (Yamamoto et al., 2013). Proprietary software, running within an in-built Ms Windows environment, allows full automation of all system parameters while controlling measurements, making data collection and analysis easy and quick. SQUIDs have been used for a variety of testing purposes that demand extreme sensitivity, including medical, engineering, and geological field (Forstner et al.,

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Figure 19.9 Nanosized material progresses from multidomain to finally superparamagnetic.

2012). A Josephson junction is mounted on a superconducting ring in radiofrequency (RF SQUID). An oscillating current is applied to an external circuit, whose voltage changes as an effect of the interaction between it and the ring. The magnetic flux is then measured. A far more sensitive direct current (DC) SQUID consists of two Josephson junctions connected in series so that electrons tunneling across the junctions exhibit quantum interference, which is dependent on the strength of the magnetic field within the loop (Walbrecker et al., 2014). DC SQUIDs show resistance to even small changes in a magnetic field, demonstrating the capability to detect such small changes. Magnetic materials contain three types of domains such as multidomain, single domain, and superparamagnetic domain. These domain structures change with chemical composition and thermal treatment. Nanosized material progresses from multidomain to single domain and finally to superparamagnetic status as shown in Fig. 19.9. SQUID magnetometry with ferromagnetic resonance was used for measuring the static and dynamic aspects of biocompatible Fe3O4 NPs based ferrofluids (Mourdikoudis & Pallares, 2018). Roberto Russo et al. studied well magnetization measurements of Fe3O4 nanoparticles by using a nanosized SQUID. The nanoSQUIDs made it possible to investigate individual magnetic nanoparticles or surface magnetic states at formerly incredible sensitivities (Mourdikoudis & Pallares, 2018). An in-depth investigation using a superconducting quantum interference device (SQUID) from Niccol Silvestri et al. found a hysteresis loop at 5K and 298K in a magnetic field of 4800 Ka m21.

19.7.2 Zero-field cooling and field cooling measurements When a magnetic sample is cooled to its critical temperature in the absence of a magnetic field, the magnetization is recorded. When the sample is warmed in the presence of a magnetic field, ZFC magnetization is achieved (Bhatt et al., 2013). This method of magnetization measurement differs from FC magnetization measurements as shown in Fig. 19.10. The sample is cooled to the critical temperature

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Figure 19.10 A typical curve of field-cooled and zero field-cooled magnetization as a function of temperature under applied magnetic field.

while a magnetic field is supplied in the field-cooled case, and the magnetization is measured during cooling or after the cooling process (Joy & Date, 2000). Polycrystalline Fe3O4 and the CoO layer were carefully researched by Fanghua Tian et al. They found that at 10K, the magnetic-moment differential (DM) between zero-field and field cooling curves increases and then reduces as the field changes, peaking at 20 kOe, indicating that the interfacial spins can be tuned by the cooling field (Tian et al., 2021).

19.7.3 Vibrating-sample magnetometer A VSM is a laboratory tool used to determine a material’s magnetic properties. The VSM is used to measure the magnetic properties of solids and liquids. The magnetic moment is obtained as a function of the magnetic field applied to the magnetometer. Faraday’s law of induction, which states that changing magnetic fields create electric fields, is the previous principle of the VSM (Nowrot, 2021). The magnetic properties of solids are extremely important and their study has revealed a great deal about the basic structure of many solids, both metallic and nonmetallic. The VSM is the instrument used to measure the magnetic moment, the most fundamental quantity in magnetism, of solid samples (Laokul et al., 2011). The instrument displays the magnetic moment in e.m.u. units (Patil et al., 2016). VSM is used to record the M H loops for magnetic nanomaterials (Patil et al., 2013) and obtain parameters such as Ms and Mr as shown in Fig. 19.11. The magnetic properties of NPs are studied as a function of the magnetic field, temperature, and time (Mourdikoudis & Pallares, 2018).

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Figure 19.11 A superparamagnetic material’s typical curve.

The material to be investigated is first placed in a steady magnetic field by a VSM. If the sample is magnetic, the magnetic domains, or individual magnetic spins, will align with the field, causing the sample to become magnetized. The magnetization will be larger if the constant field is stronger. The sample’s magnetic dipole moment creates a magnetic field surrounding it, which is known as the magnetic stray field (Dvoretskaya et al., 2021). This magnetic stray field changes as the sample move up and down, and it can be detected by a series of pick-up coils. According to Faraday’s Law of Induction, the alternating magnetic field will create an electric field in the pick-up coils (Nowrot, 2021). The current can be proportional to the magnetization pattern. As the magnetization increases, the induced electric current increases. Induction current is also amplified using a trans-impedance amplifier with a lock-in amplifier. All components are connected via a computer interface. This device can tell you how much the sample is magnetized and how it changes according to the strength of the static magnetic field using controlled and monitoring software. In comparison to SQUID, VSM is a somewhat less sensitive technique of magnetic measurements (Pradeep et al., 2008). Materials magnetic properties are investigated concerning a magnetic field, temperature, and time. Such a series of FeCo, FeCo@SnO2, and FeCo@SnO2@graphene@PA composites were characterized by using VSM (Mourdikoudis & Pallares, 2018). Alamolhoda et al. studied well the magnetic behavior of OA Fe3O4, Fe3O4@mSiO2@PCBMA, and OA Fe3O4 mSiO2 by using VSM analysis. Hassan Keshavarj et al. reported the saturation magnetization of bulk Fe3O4 was reported to be 91 emu g21, which decreases to 34.4 emu g21 when magnetite nanoparticles are coated with silica and up to 15.5 emu g21 when magnetite mesoporous silica nanoparticles (Keshavarz et al., 2020).

19.7.4 Heating capacity: induction heating system Induction heating has been used to test the magnetic hyperthermia performance of many materials. Induction heating is a process in which current is used to heat the

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metal workpiece. It works on electromagnetic induction, due to induction, eddy current is generated and this eddy current is utilized for heating. The coil is wound around the material, and the material to be heated is known as the workpiece as shown in Fig. 19.12 (Heidari et al., 2018). The instrument produces a strong magnetic field inside a round heating coil. Synthesized nanoparticles are placed in a test tube inside the heating coil and the rate of temperature response is measured. One object is to raise the temperature of the nanoparticle quickly and at the lowest concentration level inside living tissue. When subjected to AMF, any magnetic material can generate heat due to loss of hysteresis. The heating capacity depends upon the properties of the magnetic material and the AMF parameters (Mahmoudi et al., 2018). Typically, those materials having large heating power generation per particle unit mass are mostly applicable for hyperthermia. Therefore various types of MNPs are developed and used as magnetic mediators. The most successful type which has been widely investigated consists of SPION (Baroud et al., 2021). Along with this various iron oxide-based, NPs such as MFe2O4 (M 5 Mn, Zn, Co, Mg, Ni, etc.) were investigated for hyperthermia (Chaibakhsh & Moradi-Shoeili, 2019). The heating efficiency of magnetic nanoparticles is largely determined by their size (Myrovali et al., 2020). Compared to spherical parts of their same size and composition, Hassan A. Albarki and others, it is claimed that nanoparticles (e.g., cubical, hexagonal) show high thermal efficiency in the nonspherical form. According to the literature, nanoparticles of hexagonal shape demonstrated an enhancement in heating performance due to their optimal surface magnetic anisotropy. To further enhance their heating efficiency, hexagonal IONP were doped with cobalt (Co) and manganese (Mn). Incorporating certain metals ions into iron oxide nanoparticles at a specific ratio were enhancing the magnetic properties and improved the heating performance of these nanoparticles (Bauer et al., 2016).

Figure 19.12 Schematic representation of induction heating system.

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689

Conclusions

The different magnetic composites for hyperthermia therapy as well as the advancement of nanotechnology in the field of cancer treatment were highlighted. Initially, the basic information of hyperthermia, types of hyperthermia, metal oxide nanoparticles, and their use in cancer treatment have been stated. Among the various treatments, magnetic hyperthermia is the most promising treatment not only for cancer but also for pain, wounds, arthritis, and so on. Along with physicochemical properties, the other important features such as biocompatibility, minimum toxicity, ease surface functionalization, and high SAR value have been considered as the main characteristic features during the selection of materials in magnetic hyperthermia. The efficiency of magnetic hyperthermia treatment can be improved by optimizing nanoparticle size, particle distribution, the frequency of alternating magnetic fields, type of magnetizations, etc. The various measurements for studying the magnetic properties of materials were discussed. Finally, it is concluded that magnetic hyperthermia would have the overriding advantages in terms of reasonable cost, easy protocol, less complication, and minimum side effects compared with the currently available cancer treatments.

19.9

Challenges and future perspectives

The outstanding hyperthermia performance of magnetic iron oxide nanoparticles when incorporated into tumor cells has been studied in recent works. Hyperthermia is an efficient tool for controlling cancer, but it is largely an experimental technique at this time. Further progress is the need of the hour so that it can be feasible for killing the cancerous cells without other constraints. Also, it requires special costlier equipment with a specialized doctor and other team members who are skilled in using it. In addition, it is not offered in all cancer treatments due to various constraints. Many clinical experiments are being conducted to better understand and improve hyperthermia. Hyperthermia is still being studied to see how it might be used in conjunction with other cancer treatments to improve outcomes. Techniques to reach deeper organs and other regions that are difficult to treat with hyperthermia are also being investigated in studies. Although hyperthermia has been extensively researched, its clinical applicability has yet to be related to research and testing. The goal of recent technological advances is to reduce operator dependence for hyperthermia applications so that more reproductive treatments become available and how hyperthermia can be more easily used. The current study is examining how it could be used to treat several malignancies, including those listed below: Melanoma, Bladder Cancer, Head and Neck Cancer, Leukemia, Lung Cancer, Neuroblastoma. The findings of investigations on laboratory animal models and early human clinical trials have been very positive, indicating that hyperthermia could play a key role in cancer treatment in the future. Hyperthermia therapy is challenged by two key technological challenges: the capacity to produce a uniform

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temperature in a tumor and the ability to properly monitor the temperatures of the tumor and surrounding tissue. Advances in devices to deliver uniform levels of the precise amount of heat desired, and devices to measure the total dose of heat received, are hoping for. Chemists, materials scientists, biologists, engineers, and doctors all need to be involved and collaborate well in this sector.

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Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Adsorbate, 403 404 Advanced oxidation processes in environmental remediation, 445f Fenton reaction, 446 447 oxidant, 444 445 ozonation, 445 photon-Fenton system, 447 Age of science and technology, 3 Antimicrobial coatings bar diagram of, 555f organic or inorganic additives, 555 556 pie diagram, 556f, 557f Antimicrobial study, 37 39 Apoptosis, 673 674 Application of engineered metal oxides biomedical, 32 39 anticancer and antibacterial activity mechanism of, 38f antimicrobial study, 37 39 biosensing study, 33 35 cancer treatments, 35 37 cyclic voltagram of bare ZnO, 34f protocol of C@MnO2 nanospheres (NS), 35f scanning electron microscopy micrographs, 40f surface-modified Fe3O4 nanoparticles, 37f XRD patterns of bare ZnO, 34f ZnO/MWCNTs NCs, 34f catalytic, 39 44 dry synthesis method for RuO2/ SWCNTs, 45f HNSs@Carbon, 44f NiO/CuOHNSs@C, 44f organic transformations, 42 44 parameters and activity by HNSs@C, 43t

wastewater treatment, 39 41, 41f XRD pattern for bare TiO2 and WO3, 42f energy technology, 24 32 band diagram of ZnO/TiO2, 27f 1D hexagonal intercrystalline pores in the 2D lateral domain, 30f J-V characteristics, 26f MAPbI3 films, 26f morphological study of TiO2, 31f organic-inorganic perovskite solar cells, 26f solar cells, 24 28 solar device, 27f storage system, 29 32 TiO2 and N-doped TiO2, 30f TiO2-based Li-ion battery, 33f TiO2/MWCNTs based dye-sensitized solar cell (DSSC), 25f UV-visible spectra, 27f water splitting, 28 29 XRD spectra, 26f ZnO@MOFs core-shell nanorods growth, 29f ZnO/TiO2-based dye-sensitized solar cell, 26t Applications of metal oxide-based photocatalysts advances in hydrogen generation, 647 Agar well diffusion method, 654 antibacterial, anticancer, and biomedical, 652 655, 653f antimicrobial activity of CeO2-CdO, 655t charge transfer mechanism, 643f CO2 reduction (hydrocarbon generation), 650 652 DZF hybrid composites, 656f for energy conversion, 641 644

698

Applications of metal oxide-based photocatalysts (Continued) food safety, 659 660 hydrogen production, 644 647, 645f layered double hydroxides/frameworks, 655 657 nanotechnology for clean water, 647t PEC system, 644 645 photocatalytic CO2 production, 651f photoelectrocatalysis, 641 642 polymeric nanophotocatalysts, 657 659 Ppy NF/Zn-Fe LDH nanocomposite, 658f rGO/TiO2 systems, 651t sunlight-driven water splitting, 646 TiO2 in environment and energy fields, 648f water treatment and environment, 647 649 Applications of metal oxide composites for organic transformations amberlite-supported azide, 615f 2-amino-4H-chromenes, 613f 3-aryl-2-[(aryl)(arylamino)]methyl-4Hfuro[3,2-c]chromen-4-one derivatives, 619 7-aryl-benzo[h]tetrazolo[5,1-b] quinazoline-5,6-diones, 621 622, 622f 4-(arylmethylene)bis (1H-pyrazol-5-ol) derivatives, 607f Au-MnO2-GO, 619f, 620f benzimidazolo[2,3-b]quinazolinone derivatives, 608 609 benzo[4,5]thiazolo[3,2-a]chromeno [4,3-d] pyrimidin-6-one derivatives, 620 Betti bases and bisamides, 618 bis (pyrazol-5-ol) and dihydropyrano[2,3c]pyrazole analogs, 605 606 α- β- naphthol, aldehyde, and dimedone, 611f C-H arylation reactions through aniline activation, 617 α-chloro aryl ketones, 616 617 chromene derivatives, 611 614 2-amino-benzochromenes, 612 613 aminochromenes, 612, 612f CuO-CeO2 composite catalyzed synthesis, 615f dihydropyrano[2,3-c] pyrazole derivatives, 607f

Index

dihydroquinazolinones, 609 610 1,4-disubstituted-1,2,3-triazoles, 614 enzimidazolo[2,3-b]quinazolinones, 609f ethyl acetoacetate, aldehyde, and 2aminobenzothiazole, 611f Fe3O4/PEO/SO3H, 616f 4H-chromene-3-carbonitriles by the multicomponent reaction, 614f 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones, 610 MgO-MgAl2O4, 621f nano-Fe3O4@TiO2/Cu2O, 615f nitrobenzene and p-nitrophenol, 622 novel 4H-chromene-3-carbonitriles, 614 PANI-g-C3N4-TiO2, 618f 2-phenyl-3-(phenylamino)dihydroquinazolin-4(1H)-ones, 610f p-nitrobenzene to aniline, 622f p-nitrophenol to p-aminophenol, 622f pyran, 615 616 pyrano[3,2-c]quinolones and pyrano[3,2c]chromene derivatives, 613, 613f pyrazolone derivatives by the multicomponent reaction, 621f pyridine-3-carboxamides, 607 608, 609f pyrimido [1,2-b] benzazole, 608f pyrimido benzazoles, 607 substituted pyrazolones, 620 621 thieno[2,3-d]pyrimidin-4(3H)-one Derivative, 616 unsymmetrical ureas, 617 618 ZnO-CeO2 nanocomposite catalyzed, 617f ZnO-ZnAl2O4 catalyzed synthesis, 620f Aquaporin membranes, 495 Artificial photosynthesis, 289 Asymmetric supercapacitor, 147 B Bandgap, 633 634 Batch adsorption experiment factors affecting, 409 410 contact time effect, 409 410 dose effect, 410 effect of solution pH, 409 initial metal ion concentration effect, 410 isotherms, 411 Freundlich isotherm model, 411

Index

Biomedical application, 32 39 Biosensor market scenario applications of biosensors, 381 382 global biosensor market in millions, 381f quality and safety of products, 382 Bulk versus nanoscale, 8 10 C Cancer treatment chemotherapy, 675 hormone therapy, 675 immunotherapy, 675 676 radiation therapy, 675 stem cell treatment, 676 surgery, 675 Cancer treatments, 35 37 Capacitance, 159 Carbon-based materials as an electrode, 193 Carbon nanotube metal oxide nanocomposites, 268 Catalytic applications, 39 44 Catalytic organic transformations, 42 44 Cation substitution, 452 Cerium group, 443 444 Chalcogen, 269 Chalcogenides-metal oxide hybrids, 269 Characterization of metal oxide-based composite nanostructures of Cu, Ni, and Co, 87f electrochemical, 88 90 Fe2O3/CuO (CF), 72f Fourier transform infrared spectra, 84 86, 85f, 86f MMO NPs collected via Drs, 83f Rietveld refined X-ray diffraction, 74f scanning electron microscopy, 73 78 gas molecules, 76 78 schematic of, 75f surfactants, 74 ternary metal oxide TOCBCMC, 76f, 77f temperature-programmed reduction, 86 87 NH3/CO2- TPD and pyridine-Fourier transform infrared, 88f transmission electron microscopy, 78 80, 79f, 80f UV-Vis diffuse reflectance spectra, 82f

699

UV-Vis spectroscopy, 80 84, 81f X-ray diffraction, 70 73, 72f, 73f X-ray photoelectron spectroscopy, 87 88, 89f ZA-500 ZA-600 and ZA-700, 82f Charge storage mechanism battery type charge storage, 192 193 electric double layer capacitor (EDLC), 190f non-faradic, 189 190 redox, 190 192 intercalation type reactions, 192 redox reactions at surface, 191 192 Cobalt tetraoxide, 196 197 Combustion synthesis, 69 Composite-based metal oxide photocatalysts copper oxide (CuO), 639 perovskite-type oxides, 640 ruthenium dioxide (RuO2), 640 stoichiometric and crystalline structures, 636 639 tungsten oxide, 636 unitary photocatalysts, 637t unitary versus, 634 641 zinc oxide (ZnO), 636 Composite formation, 14 16 Composite hybrid supercapacitors, 149 Composites in metal oxide fast kinetics and high adsorption power, 432 synthetic protocol, morphology, important physico-chemical properties, 433t Conducting polymers-based supercapacitors, 145 metal oxides-based, 146 Crystallographic characteristics of WO3 crystal phases, 216f and electrochemical charge storage, 217 large-scale electrochromic-energy storage device, 218f stoichiometry, 216f structural polymorphism, 214 215 temperature-dependent phase transitions, 214 215 Current load and cycle stability, 154 156

700

D Design and characterization of nanocomposites Fourier transform infrared spectroscopy spectra, 605f physicochemical properties of, 603 sol-gel method, 603 transmission electron microscopy, 606f X-ray diffraction analysis, 604f Designing metal oxide nanocomposite, 572 574, 574f Designing nanostructured WO3 for supercapacitor application charge-storage performance demonstrated via cyclic voltammetry, 219f sol-gel method, 220 synthesis method-dependent morphological, 221t wet chemical method, 220 Developments in WO3 composites for supercapacitor application to design nanostructured, 225t nanowires and 10-graphene-WO3 nanowire composite, 230f optical micrograph of DNA-like, 227f SEM and TEM images, 228f WO3 and WO3 -rGO, 231f Dye-sensitized photo-electrochemical cells, 289 290 Dye-sensitized solar cells (DSSC), 83 84, 241, 269 271 E Electrical properties study of energy storage devices actively or passively balanced cells, 152f capacitance, 159 current load and cycle stability, 154 156 overoxidation induced degradation, 155 156 PANI and PANI@C composite electrodes, 156f swelling induced degradation, 155 internal resistance, 154 open circuit voltage decays, 153f operating voltage, 150 polarity, 152 154 cathode terminal of the capacitor, 153f

Index

power density, 158 electrochemical energy storage devices, 158f resistance on operating voltage and temperature, 154 self-discharge, 150 152 unbalanced cells, 152f Electric double layer capacitor, 143 145 Electrochemical characterization, 88 90 Electrode designing and its features study doctor blade method, 354 for electrode designing, 353f for energy technology, 352 354 well-organized photoelectrodes, 354 Electrodeposition, 70 Electrode system’s charge storage, 218 220 Emerging photovoltaic, 264 Emerging strategy of third-generation solar cell technology dye-sensitized, 269 271 binary hybrids of TiO2/MWCNTs, 270f organic, 272 273 ITO/SnO2/PeNWs/PBDB-TSF, 273f perovskite solar cells, 274 275 band diagram and operational principle, 276f perovskite solar cell, 276f principle of, 276f quantum dot-sensitized, 271 272 QDs/TiO2/Ag/ZnO NAs photoelectrode structure, 272f tandem, 273 274 PbS quantum dots, 274f Energy dispersive X-ray spectroscopy (EDAX or EDS), 73 74 Energy storage device evolution evolution of, 138f supercapacitor, 138 in different country, 139f Energy storage system, 29 32 Energy technology, 24 32 Environmental remediation principles and applications applicability of, 426, 426f properties, 425 426 renewable energy and, 426 427 and socio-economic factors, 425

Index

701

Environmental remediation types groundwater and surface water, 427 428 sediment, 428 soil and, 428f soil, 427 Exciton diffusion length, 243 244 Excitons, 243

Iron oxide nanoparticles and surface functionalization bioactive molecules, 684 functionalization of, 683f inorganic coating, 683 684 organic surfactants, 683 synthetic and natural polymer, 683

F Ferrites, 200 201 Flexible supercapacitors, 201 Fourier transform infrared spectroscopy, 84 86 Functional nanocomposites in electrochemical biosensor metallic nanoparticle-based composites, 385 389 metal oxide nanomaterial’s-based composites, 389 392 electrode position method, 391 enzyme-free electrochemical detection, 391 non-enzymatic biosensor for glucose detection, 390 synthesized CuO nanoparticles, 390 multi-phase solid material, 383 384 role of metal oxide nanoparticles in, 385f surface functionalization, 384

L Laser ablation, 65 Lithography, 63 64 Lotus effect, 524

G Global supercapacitor market end-users, 142 143 Graphene-metal oxide hybrids, 267 268 H High energy consumption, 486 Hybrid solar cells, 264 Hybrid supercapacitors, 146 149 Hyperthermia classification of, 676 678 local, 677, 677f magnetic, 679 regional, 678, 678f types of, 677f whole-body, 678, 678f I Inner Helmholtz plane (IHP), 189 Intrinsic pseudocapacitance, 191 192

M Magne´li phases, 215 216 Magnetic hyperthermia application magnetic metal oxide, 680 683 metal oxides-based composites for, 682t Magnetic iron oxide-based composites batch adsorption experiment, 409 411 nanoparticles as nanoadsorbents, 404 408 removal of heavy metal ions by, 411 417 water pollution by heavy metals and its removal, 402 404 Magnetic nanoparticles as nanoadsorbents functionalization of, 405 408 Fe3O4@SiO2@CS-TETA-GO, 408f inorganic materials, 407 408 surface, 406 407 using magnetic adsorbent, 407f removal of heavy metal ions from water, 405f Manganese dioxide, 194 195 Market scenario company with supercapacitor production, 140 142 cellergy, 140 global supercapacitor manufacturing company, 141t loxus, 140 Maxwell Technology, 140 nanoramic laboratory, 140 Panasonic, 140 Paper Battery Company (PBC), 140 142 Yunasko, 142 ZapGo, 142 global supercapacitor, 139f end-users, 142 143, 142f size, 139 140

702

Market status of photoactive materials global market, 541 Japan photocatalyst revenue, 541f Measuring the magnetism of the magnetic materials field-cooled and zero field-cooled magnetization, 686f heating capacity, 687 688 induction heating system, 687 688, 688f multidomain into finally superparamagnetic, 685f superconducting quantum interference device magnetometry, 684 685 superparamagnetic material’s typical curve, 687f vibrating-sample magnetometer, 686 687 zero-field cooling and field cooling measurements, 685 686 Mechanical milling, 60 62 Metal-metal oxide composites, 558 Metal-organic frameworks for solar energy study guest insertion in, 350f guest@ metal, 348 350 linker, 348f organic utilized, 349t as sensitizers, 343 348 Metal oxide-based composites characterization of, 70 90 metal oxides, 58 as photocatalysts applications of, 641 660 future perspectives of, 660 661 mechanism of, 633 634 as photocatalysts, 633 unitary versus composite, 634 641 properties and applications of, 59f synthetic approaches, 58 70 Metal oxide-based nanocomposites for supercapacitive applications carbon-based materials as an electrode, 193 charge storage mechanism, 189 193 flexible, 201 metal oxides/metal oxide composites as an electrode, 193 198 mixed transition metal oxides, 198 201 Metal oxide cathode in dye-sensitized photoelectrochemical cells based cathode, 318 324

Index

facile in situ method, 323 metal oxide/carbon composites, 321 324 MoIn2S4@CNTs, 322f photo-electrochemical cells with Fe3O4 cathodes, 320f photovoltaic performance of, 319t Ru NPs decorated CNFs, 320f catalytic activity of, 316 318 active sites, 316 317 conductivity, 317 318 hydrogen-treated WO3, 317f role of, 314 316 equivalent circuit, 315f Metal oxide composites in organic transformations applications of, 605 622 design and characterization of, 602 605 Metal oxide-conducting polymer composites for supercapacitor composite of polyaniline, 159 164 bare Fe electrode, 164f PANI/MnO2, 164f electrochemical performance of sponge electrodes, 168f literature about transition, 160t poly 3,4-ethylene dioxythiophene, 165 166 GnP/PEDOT/MnO2, 167f polypyyrole with the, 164 165 CV curves of G, 166f TiO2- Graphene-PPy, 165f Metal oxide engineering application of, 23 44 human development and nexus, 3 6 strategy and significances, 6 23 bonding diagram of TiO2, 12f bulk and nanoscale dimension, 10t bulk versus nanoscale, 8 10 composite formation, 14 16 composites for various applications, 18t compositional solid electrolytes, 12t CuO products via thermal decomposition of HKUST-1, 20f density of states (DOS), 12f doped metal oxides, 13t electron confinement & change in bandgap with size, 9f Li7LaZr2O12 electrolyte Li-ion battery, 24f

Index

mesoporous nanocrystalline MgO/ Al2O3, 21f molar fraction of phases, 15f morphological study, 16f, 17f morphology engineering, 16 19 with nanoscale dimensions, 8 9 nanoscaled materials transformation, 10f Nyquist plot of DSSCs with SnO2, 16f optical properties, 15f phase diversity, 12 14 photocatalytic properties, 15f photocurrent density-voltage (J-V), 16f PL spectra, 22f porosity generations, 19 21 porous via mechanochemical nano casting, 21f size & color dependence of Au NPs, 9f strategies deployed for engineering the metal oxides with properties, 7f surface modifications, 22 thin-film formations, 23 undoped versus doped, 10 11 UV-vis absorption spectra, 22f ZnO nanostructures, 19f Metal oxide-metal-organic frameworks composites for supercapacitor applications, 172t mechanical synthetic approach, 352 and metal organic frameworks derived material for supercapacitor, 167 171 nanocomposites BiVO4/MIL-101(Fe), 361 imidazole-based, 361 in solar-driven water splitting study, 363t TiO2/MIL-125, 361 362 for water splitting, 361 365 ZnO/ZIF-8, 362 365 one-pot hydrothermal synthesis, 169f present state of the art, 353f Zn/Co-MOF-derived nanoporous carbons, 171f Metal oxide nanocomposites antimicrobial coatings, 555 557 -based electrochemical biosensing study of biosensor, 380f challenges and future perspectives, 392 393

703

functional nanocomposites in, 383 392 nanostructured materials, 380 381 nonenzymatic, 382 383 present scenario of biosensor market, 381 382 based on antimicrobial coatings, 574 589, 575f antibacterial activity testing strategy, 585f in dental implant, 577f on food, 582f food sector, 580 583 hospital sector, 575 578 leather sector, 586 589 paint sector, 584 586 PEG-g-CS@AgNPs coating, 587f polymer sector, 583 584 polymer/zinc oxide nanocomposites, 575 577 textile sector, 578 579, 580f with inorganic moieties composites, 568 nanostructures, 568 of metal oxide with inorganic moieties oxide composites, 558 567 microbes and microbial infectious diseases, 550 554 nanocomposites for antibacterial study, 563t with organic moieties, 569 composites, 569 framework composites, 569 molecule, 569 plausible mechanisms for, 570 572 as potential antimicrobial agents, 557 569 bare for antibacterial study, 559t based on nanocomposites, 568f composites of metal oxide with inorganic moieties, 558 568 synthesis strategy for designing, 572 574 in water and wastewater treatment challenges, 485 486 features, 505 507 key to life on the earth, 479 480 nanotechnology in water and, 486 498 percentage of vital organs of human body, 480f present scenario of pollution, 481 482

704

Metal oxide nanocomposites (Continued) use of metal-oxide nanocomposites, 499 505 waste water treatment, 484 485 water treatment, 482 484 Metal oxide nanomaterials applications of, 246f for organic photovoltaic applications metal oxide nanomaterials, 245 246 metal oxides based study, 250 254 organic photovoltaic, 242 245 polyaniline ZnO nanoparticles, 242f polymer nanocomposites, 241 properties of nanomaterials, 246 249 structures of organic molecules, 240f used in organic photovoltaics, 249 250 ZnO nanowire, 242f Metal oxide photoanode in dye-sensitized photo-electrochemical cells influence of interfacial engineering, 309 314 blocking layer and scattering layer, 313t compact blocking layer, 311 313 in dye-sensitized, 311f light-scattering layer, 314 morphology in performance, 294 309 amorphous TiO2 nanospindles, 310f carbon-based metal oxide nanostructure, 303 304 for dye-sensitized photoelectrochemical cells, 299t electron transfer pathway, 305f electron transportation in OD NPs@ 1D NRs, 301f electron transportation track, 296f hierarchical hollow spheres and beads, 304 308 hierarchical spheres, 307 nanorods/wires/tubes metal oxide, 295 303 nanospindles, 308 309 NRs/NPs bilayer electrode, 295 pristine TiO2 NRs and TiO2, 296f sugar apple-shaped TiO2, 306f three-dimensional hierarchical nanostructures, 298 TiO2 HSSs by acid thermal method, 306f TiO2 NSs by solvothermal method, 306f

Index

TiO2 -SnO2 MHSs as photoanode, 307f of ZnO NRs, 297f ZnO NTs, 302f Metal oxides as photoanodes evolution, 101 103 scope of improvement in the field, 122 123 vacancy engineering, 103 121 Metal oxides as photoelectrodes in dyesensitized solar cells metal oxide cathode in DSPECs, 314 324 metal oxide photoanode in, 294 314 operational principle of, 289 290 photo-physics of DSPEC, 290 293 Metal oxides-based composites for magnetic hyperthermia applications hyperthermia, 676 679 iron oxide nanoparticles and surface functionalization, 683 684 magnetic, 680 683 methods for measuring the magnetism, 684 688 nanomaterials for, 679 680 present cancer treatment, 675 676 treatment, 673 674 Metal oxides based organic photovoltaic study devices applications of nanomaterials, 250 251 organic, 250 251 molybdenum oxide, 252 SnO2 particle morphology, 253f tin oxide, 252 titanium dioxide, 251 252 tungsten oxide, 253 J-V characteristics of, 254f vanadium pentaoxide, 253 254 device structure of oxide-based solar cells, 254f organic solar cells-based V2O5, 254f Metal oxides for solar energy study cell application, 344t position of metallic elements, 342f solar-driven water splitting, 346t Metal oxides in photoelectrochemical hydrogen/oxygen evolution quality for photocatalytic, 102f water splitting in the presence of light, 101f

Index

Metal oxides/metal-organic frameworks nanocomposite pros and cons, 350 351, 351f for solar energy harvesting, 355 360 synthetic methods deployed, 357f TiO2/Co-DAPV, 357 TiO2/Cu-BTC, 356 357 TiO2/MIL-125, 358 TiO2/ZIF-8, 355 356 utilized in solar energy harvesting, 359t ZnO/PPF-11, 358 360 ZnO/ZIF-8, 358 Metal oxides/metal-organic frameworks nanocomposites sensitized solar cell, 356f Metal oxides/metal oxide composites as electrode in supercapacitors cobalt tetraoxide, 196 197 manganese dioxide, 194 195 nickel oxide, 195 196 performance of negative electrode, 197 198 ruthenium oxide, 194 Metal oxides nexus advantages, disadvantages, and types of, 6f ages of human civilization, 4f human development and, 3 6 rapid growth in population and industrialization, 3 4 utilization of, 5f in various sectors, 5f Metal oxides used in organic photovoltaics compound structures, 251f hole transport layer, 251f MDMO-PPV conjugated polymer, 250f stability and efficiency of, 249 250 ZnO nanoparticles on a carbon-coated copper grid, 250f Metal oxide synthetic approaches bottom-up, 66 70 combustion, 69, 69f electrochemical deposition setup, 70f of electrochemical precipitation/ deposition of TH, 71f electrodeposition, 70 microwave, 68 69, 68f sol-gel technique, 66 67 solvothermal synthesis, 67f

705

solvothermal technique, 67 68 typical sol-gel process, 67f for nanostructures, 60f top-down approaches, 60 65 DC-powered magnetron sputtering, 65f electrospinning, 63, 64f equipment for mechanochemical synthesis, 63f laser ablation, 65 mechanical milling, 60 62 PLD process, 66f sputtering, 64 65 using different synthetic approaches, 61t Microbes and microbial infectious diseases, 550 554 Microwave synthesis, 68 69 Mixed metal oxide nanocomposites for environmental remediation advanced oxidation processes, 444 447 different composites in, 431 432 monitoring of pollutants during, 460 463 need of hour, 429 431 present state of art, 432 444 principles and applications, 425 427 protocols of mixed, 455 460 semiconducting metal oxides, 428 429 synthesis of, 447 450 tailoring properties of, 450 455 types of, 427 428 Mixed metal oxide NCS Al2O3- based nanocomposites, 440 and environmental remediation, 432 444 metal oxide photocatalysis, 435f Fe2O3-based nanocomposites, 437 438 functional, 435 hydroxyl groups, 435 436 photocatalytic degradation of heptane, 436 437 rare earth oxides-based nanocomposites, 443 444 SnO2-based nanocomposites, 441 442 TiO2-based nanocomposites, 436 437 WO3-based nanocomposites, 441 ZnO-based nanocomposites, 438 440 Mixed metal oxides, 267 Mixed transition metal oxides ferrites, 200 201 nickel cobaltate, 199 200

706

Modifications of metal oxides additives or supportive materials, 267 269 carbon nanotube, 268 chalcogenides, 269 graphene-, 267 268 polymer, 268 269 based solar devices, 265 doped MxOy, 265 266 metal-supported MxOy, 266 267 strategy for modifying, 265 Monitoring of pollutants during environmental remediation of air, 461 life-cycle distribution, 462f of soil, 461 462 of water, 462 463 Morphology engineering, 16 19 MOs-based supercapacitor applications, 146 N Nanobiocides, 496 Nanocrystalline metal oxide-based hybrids efficiency versus cost for first-, second-, and third-generation, 264f emerging strategy of, 269 275 first and second-generation, 264 matured technology, 263 264 modifications of, 265 269 photovoltaic devices, 275 279 Nanoscience and nanotechnology, 674 Nanostructured inorganic metal oxide electrode designing and its features, 352 354 frameworks nanocomposite, 350 351 nanocomposites for water splitting, 361 365 present state of the art, 352 solar-driven energy harvesting, 341 for solar energy harvesting, 355 360 for solar energy study, 342 350 total global energy consumption, 340f total primary energy consumption, 340f traditional nonrenewable energy sources, 339 340 Nanostructured WO3-x based advanced supercapacitors for sustainable energy applications crystallographic characteristics of WO3, 214 217

Index

designing for, 218 224 portable electronics and electric vehicles, 213 recent developments in, 224 230 tungsten oxides for supercapacitor application, 215f Nanotechnology in water air-laid paper-based reduced graphene oxide, 496f application of nanoparticles, 486 487 carbon and metal-based nanoparticles, 488 carbon nanotubes/graphene-based adsorbents, 491t chemosphere, 489f CNTs and graphene, 488 graphene-based adsorbents, 490t magnetic nanomaterials, 493 nanobiocides, 496 498 nanocatalysts, 493 494 fabrication of photocatalytic membranes, 494f nanosorbents, 488 493 nanostructured membrane, 494 496 property, applications and advantages, 498t using nanoparticles, 487f and wastewater treatment, 486 498 Nickel cobaltate, 199 200 Nickel oxide, 195 196 Nonenzymatic electrochemical biosensors analytical method for, 382 evolution of, 383f third-generation biosensor, 382 383 Non-faradic mechanism, 189 190 O Operating voltage, 150 Organic photovoltaic mechanism, 243 244 absorption of light and exciton generation, 243 device, 243f exciton diffusion, 243 244 exciton dissociation, 244 types of organic photovoltaics, 244 principle, designing and mechanism, 242 245 sensitizers in, 245 structures of dyes, 245f

Index

Organic solar cells, 272 273 Outer Helmholtz plane (OHP), 189 Oxygen vacancy engineering in metal oxides for photoelectrochemical water splitting, 103 121 photoelectrochemical water splitting performance of, 109t search and development of, 105 thermal treatment induced Vo formation in nanowires, 107f TiO2, 105 108 WO3, 108 121 atomic cluster of crystalline core, 122f CB-VB of homogenous, 114f density of states, 120f electron separation and electron transport, 113f for free energy calculation, 118f high-magnification FE-SEM images, 117f hydrothermal method of synthesis, 114f In2O3, 116 118 O1s X-ray photon spectroscopy spectra, 118f SrTiO3, 118 121 UV-vis spectra and the insets, 122f Vo formation in aluminum-doped SrTiO3, 121f ZnO, 112 116 X-ray photon spectroscopy, 106 P Perovskite solar cells, 274 275 Phase diversity, 12 14 Photoactive concrete surface preparation method (i), 530 531 method (iii), 531 possible methods for, 530 TiO2-based photocatalytic material, 530f Photocatalytic activity testing methods degradation of toluene gas, 537f depollution testing, 536 537 self-cleaning, 535 536 photodegradation of MB dye under UV light, 536f Photocatalytic mechanism of self-cleaning concretes NOx removal by concrete pavement and NOx removal, 528f

707

NOx restoration, 528 529 of organic compound pollutants, 527 528 over semiconductor material, 526f photo-degradation of toluene gas, 529f process, 529 Photo-physics of dye-sensitized photoelectrochemical cells charge separation, 291 293 at dye-sensitized, 292f electrolyte/cathode interfaces, 292f transportation at, 292f charge transfer rate, 293 energy levels of components, 290 291 energy transfer pathway and BG, 291f recombination rate, 293 Plausible mechanisms for nanocompositesbased microbes inactivation action of antimicrobial agents, 570 572, 570f Ag/TNTs under UV light, 573f microbial cell disturbances and leakages, 570 plausible ways of action of antimicrobial agents, 572f reactive oxidative species generations, 571 visible light, 573f Polaron, 243 244 Polymer-metal oxide hybrids, 268 269 Porosity generations, 19 21 Porous coordination polymers (PCP), 343 Power density, 158 Present state of art in emerging photovoltaic devices commercial applications of, 279f third-generation solar cells, 278f third-generation solar devices, 277 278 year-wise solar efficiency chart, 276f Properties of nanomaterials materials science, 246 247 nanoscience and nanotechnology, 248 249 structural and geometric factors, 248 249 Properties of photoactive self-cleaning concretes cement by titanium dioxide, 535f chemical stability and low cost, 531 532 compressive strength of cement mortar samples, 532 533, 533f fracture surfaces, 532f

708

Properties of photoactive self-cleaning concretes (Continued) morphology of the TiO2 nanoparticles, 534f strength test of cement cube, 535t Protocols of mixed metal oxides adsorbent study, 455 457 approaches for, 456f biological study, 459 460 catalytic study, 457 458 in environmental remediation, 455 460 membrane study, 458 459 synthetic scheme and bactericidal action, 460f Pseudocapacitor, 145 146 Q Quantum dot-sensitized solar cells (QDSSCs), 271 272 R Rechargeable battery type supercapacitor, 148 149 Redox mechanism, 190 192 Removal of heavy metal ions by magnetic nanoparticles simultaneous removal of multiple, 412 417, 415t single type of, 412 meso-2,3-dimercaptosuccinic acid and adsorption of Cu(II), 412f in single metal system, 413t Representative nanomaterials for magnetic hyperthermia application of iron oxides nanoparticles in biomedical field, 680f wet chemical methods, 681f Ruthenium oxide, 194 S Scanning electron microscopy (SEM), 73 74 Scope of improvement in field quality and cost-effective materials, 122 123 stability of metal oxides, 123 Secondary building units (SBUs), 343 Sedimentation, 482 484

Index

Self-cleaning photoactive concrete in realworld applications Cite´ de la Musique et des Beaux- Arts in Chambe´ry, 539f Dives in Missericordia, 538f Leopold II tunnel in Brussels, 539f MSV Arena Football Stadium-Germany, 540f renovation with using photocatalytic walls, 539f TiO2-based tiles, 540 Self-cleaning photoactive metal oxide-based activity testing methods, 535 537 advantages and disadvantages of, 537 market status of, 541 542 mechanism of, 526 529 preparation of, 530 531 properties of, 531 535 in real-world applications, 538 540 Self-discharge, 150 152 Semiconducting metal oxides, 428 429 advanced properties, 428 429 applications of mixed, 430f Sewage, 481 Sludge disposal, 486 Sludge treatment, 486 Solar cells, 24 28 Solar-driven water splitting, 352 Sol-gel technique, 66 67 Solid tumors cancer, 673 674 Sputtering, 64 65 Supercapacitors, 89 90, 135 136 evolution, 138 types electric double layer capacitor, 143 145 energy storage mechanisms, 144f hybrid, 146 149 asymmetric, 147 battery devices, 148f composite, 149 rechargeable battery type, 148 149 pseudocapacitor, 145 146 metal oxides-based, 146 polymers-based, 145 Symmetric supercapacitor, 187 188 Synthesis of metal oxide nanocomposites chemical methods for, 449f methods for nanomaterials, 449f physical methods for, 450f

Index

sol-gel protocols, 447 448 synthetic methods, 447 448 top-down synthetic methods, 447 448 T Tailoring properties of metal oxide nanocomposites controlling crystal growth, 455 doping, 452 453 heterostructure forming, 454 455 impact of heat treatments, 455 modeling phase structure, 453 shapes of, 451, 451f stoichiometry controlling, 453 454 Tandem solar cells, 273 274 Thin-film formations, 23 Transition metal oxide conducting polymer composites for supercapacitor, 159 166 electrical properties study of energy storage devices, 149 159 electric double layer, 136f energy storage device evolution, 137 138 hybrid supercapacitor, 136f market scenario, 139 143 metal-organic frameworks derived material for supercapacitor, 167 171 organic frameworks, 167 171 pseudocapacitor, 136f supercapacitors types, 143 149 Transmission electron microscopy, 78 80 U Ultra-capacitors, 143 145 Undoped versus doped, 10 11 Use of metal-oxide nanocomposites in water and wastewater treatment, 499 505 in adsorptive technology, 501 biological contaminants, 500 501 calcium silicate hydrate (CSH), 504 505 nanocomposite, 507f chemical of, 499 500 Cu (II) ion absorption process, 502f

709

disinfection of biological contaminants, 500 501 GFLE preparation, 502f ion exchange technique, 503 504 in ion exchange technology, 503 504 in membrane technology, 502 503 in photocatalytic degradation of pollutants, 504 505 polyaniline-modified TiO2, 501 polymeric-inorganic cation-exchanger, 503 504 polymer-metal oxide nanocomposites, 506t purification of, 501 spherical carbon loaded with zero valent iron (ZVI), 504f toxicity of nanoparticles against bacteria, 500f UV-Vis spectroscopy, 80 84 W Wastewater treatment, 39 41 preliminary, 484 485 typical, 485f Water pollution domestic, 481 industrial, 481 482 industry and agricultural activity, 482 organic industrial, 481 482 pollutants released from, 483t sewage composition distribution, 481f Water pollution by heavy metals and its removal adsorption process for, 403 404 physical and chemical adsorption, 404f removal methods, 402 403 advantages and disadvantages, 403f Water splitting, 28 29 Water treatment coagulation process, 482 484 pretreated with chlorine, 484 purification process, 483f X X-ray diffraction, 70 73 X-ray photoelectron spectroscopy, 87 88