Functionalized Nanomaterials for Catalytic Application [1 ed.] 1119808979, 9781119808978

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Functionalized Nanomaterials for Catalytic Application

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Functionalized Nanomaterials for Catalytic Application

Edited by

Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla

This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2021 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 9781119808978 Cover image: Pixabay.Com Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface xvii 1 Functionalized Nanomaterial (FNM)–Based Catalytic Materials for Water Resources 1 Sreevidya S., Kirtana Sankara Subramanian, Yokraj Katre, Ajaya Kumar Singh and Jai Singh 1.1 Introduction 4 1.2 Electrocatalysts as FNMs 7 1.3 Electro-Fenton/Hetero Electro-Fenton as FNMs 8 1.4 Hetero Photo-Fenton as FNMs 13 1.4.1 Heterogenous-Fentons-Based FNMs 14 1.4.2 Photo-Fentons-Based FNMs 14 1.5 Photocatalysts as FMNs 19 1.5.1 Carbon-Based FNMs as Photocatalysts 24 1.5.1.1 CNT-Based FNMs 24 1.5.1.2 Fullerene-Based FNMs 25 1.5.1.3 Graphene (G)/Graphene Oxide (GO)–Based FNMs 26 1.5.1.4 Graphene-Carbon Nitride/Metal or Metalloid Oxide–Based FNMs 27 1.5.1.5 Graphene-Carbon Nitride/QD-Based FNMs 28 1.5.2 Polymer Composite–Based FNMs as Photocatalyst 29 1.5.3 Metal/Metal Oxide–Based FNMs as Photocatalyst 29 1.6 Nanocatalyst Antimicrobials as FNMs 30 1.7 Conclusions and Future Perspectives 31 References 33

v

vi  Contents 2 Functionalized Nanomaterial (FNM)–Based Catalytic Materials for Energy Industry Amarpreet K. Bhatia, Shippi Dewangan, Ajaya K. Singh and Sónia. A.C. Carabineiro 2.1 Introduction 2.2 Different Types of Nanomaterials 2.2.1 Zero-Dimensional (0D) Nanostructures 2.2.2 One-Dimensional (1D) Nanostructures 2.2.3 Two-Dimensional (2D) Nanostructures 2.2.4 Three-Dimensional (3D) Nanostructures 2.3 Synthesis of Functionalized Nanomaterials 2.3.1 Chemical Methods 2.3.2 Ligand Exchange Process 2.3.3 Grafting of Synthetic Polymers 2.3.4 Miscellaneous Methods 2.4 Magnetic Nanoparticles 2.4.1 Synthesis of Magnetic Nanoparticles 2.4.2 Characterization of Magnetic Nanoparticles 2.4.3 Functionalization of Magnetic Nanoparticles 2.4.3.1 Covalent Bond Formation 2.4.3.2 Ligand Exchange 2.4.3.3 Click Reaction 2.4.3.4 Maleimide Coupling 2.5 Carbon-Based Nanomaterials 2.5.1 Functionalization of Carbon Nanomaterials 2.5.2 Synthesis of Functionalized Carbon Nanotubes and Graphene 2.6 Application of Functionalized Nanomaterials in the Energy Industry Through Removal of Heavy Metals by Adsorption 2.6.1 Removal of Arsenic by Magnetic Nanoparticles 2.6.2 Removal of Cadmium by Magnetic Nanoparticles 2.6.3 Removal of Chromium by Magnetic Nanoparticles 2.6.4 Removal of Mercury by Magnetic Nanoparticles 2.7 Conclusions References 3 Bionanotechnology-Based Nanopesticide Application in Crop Protection Systems Abhisek Saha 3.1 Introduction 3.2 Few Words About Pesticide

53 54 55 55 56 56 56 56 57 58 58 58 59 59 60 63 64 64 64 65 65 65 67 67 74 75 75 76 76 77 89 90 92

Contents  vii 3.3 3.4 3.5 3.6 3.7

What About Biopesticide Demand 93 A Brief Look on Associates Responsible for Crop Loss 93 Traditional Inclination of Chemical-Based Pest Management 94 Nanotechnology in the Field of Agriculture 95 Why Nanotechnology-Based Agriculture is the Better Option With Special Reference to Nano-Based Pesticide? 95 3.8 Biological-Based Pest Management 96 3.9 Nano-Based Pest Management 96 3.10 Nanopesticides 97 3.11 Required to Qualify for Selection as Nanobiopesticides 98 3.12 Pestiferous Insect’s Management 99 3.12.1 Chemical Nanomaterials 99 3.12.2 Bionanomaterials 99 3.13 Critical Points for Nanobiopesticides 100 3.14 Other Pests 100 3.15 Post-Harvest Management and Their Consequences 101 3.16 Field Test for Nanobiopesticides for Pest Control 101 3.17 Merits and Consequences of Chemical and Bionanomaterials 102 3.18 Conclusion 103 References 104 4 Functionalized Nanomaterials (FNMs) for Environmental Applications 109 Bhavya M.B., Swarnalata Swain, Prangya Bhol, Sudesh Yadav, Ali Altaee, Manav Saxena, Pramila K. Misra and Akshaya K. Samal 4.1 Introduction 110 4.1.1 Methods for the Functionalization of Nanomaterials 110 4.1.1.1 Functionalization by Organic Moieties 111 4.1.1.2 Surface Polymerization 111 4.1.2 Nanomaterial-Functional Group Bonding Type 112 4.1.2.1 Functionalization by Covalent Bond 112 4.1.2.2 Functionalization by Noncovalent Bond 112 4.2 Functionalized Nanomaterials in Environmental Applications 114 4.2.1 Chitosan 114 4.2.2 Cellulose 117 4.2.3 Alumina 121 4.2.4 Mixed Composites 124 4.2.5 Other Nanocomposites for Environment 126

viii  Contents 4.3 Conclusion Acknowledgements References

130 130 130

5 Synthesis of Functionalized Nanomaterial (FNM)–Based Catalytic Materials 135 Swarnalata Swain, Prangya Bhol, M.B. Bhavya, Sudesh Yadav, Ali Altaee, Manav Saxena, Pramila K. Misra and Akshaya K. Samal 5.1 Introduction 136 5.2 Methods Followed for Fabrication of FNMs 137 5.2.1 Co-Precipitation Method 138 5.2.2 Impregnation 139 5.2.3 Ion Exchange 139 5.2.4 Immobilization/Encapsulation 140 5.2.5 Sol-Gel Technique 140 5.2.6 Chemical Vapor Deposition 141 5.2.7 Microemulsion 141 5.2.8 Hydrothermal 142 5.2.9 Thermal Decomposition 142 5.3 Functionalized Nanomaterials 143 5.3.1 Carbon-Based FNMs 143 5.3.1.1 Carbon-Based FNMs as Heterogeneous Catalysts 145 5.3.2 Metal and Metal Oxide–Based FNMs 147 5.3.2.1 Functionalization Technique of Metal Oxides 147 5.3.2.2 Silver-Based FNMs as Heterogeneous Catalysts 148 5.3.2.3 Platinum-Based FNMs as Heterogeneous Catalysts 150 5.3.2.4 Pd-Based FNMs as Heterogeneous Catalysts 153 5.3.2.5 Zirconia-Based FNMs as Heterogeneous Catalysts 153 5.3.3 Biomaterial-Based FNMs 154 5.3.3.1 Chitosan/Cellulose-Based FNMs as Heterogeneous Catalysts 155 5.3.4 FNMs for Various Other Applications 156 5.3.5 Comparison Table 157 5.4 Conclusion 158 Acknowledgements 159 References 159

Contents  ix 6 Functionalized Nanomaterials for Catalytic Applications— Silica and Iron Oxide Deepali Ahluwalia, Sachin Kumar, Sudhir G. Warkar and Anil Kumar 6.1 Introduction 6.2 Silicon Dioxide or Silica 6.2.1 General 6.2.2 Synthesis of Silica Nanoparticles 6.2.2.1 Sol-Gel Method 6.2.2.2 Microemulsion 6.2.3 Functionalization of Silica Nanoparticles 6.2.4 Applications 6.2.4.1 Epoxidation of Geraniol 6.2.4.2 Epoxidation of Styrene 6.3 Iron Oxide 6.3.1 General 6.3.2 Synthesis of Functionalized Fe NPs 6.3.2.1 Biopolymer-Based Synthesis 6.3.2.2 Plant Extract–Based Synthesis 6.3.3 Applications 6.3.3.1 Degradation of Dyes 6.3.3.2 Wastewater Treatment References 7 Nanotechnology for Detection and Removal of Heavy Metals From Contaminated Water Neha Rani Bhagat and Arup Giri 7.1 Introduction 7.2 History of Nanotechnology 7.3 Heavy Metal Detective Nanotechnology 7.3.1 Nanotechnology for Arsenic (Aas) Removal 7.3.2 Nanotechnology for Lead Removal from Water 7.3.3 Nanotechnology for Cadmium (Cd) Removal from Water 7.3.4 Nanotechnology for Nickel (Ni) Removal 7.4 Futuristic Research 7.5 Conclusion References

169 169 171 171 172 172 172 174 176 176 177 177 177 178 178 179 179 179 181 182 185 186 186 187 187 197 200 200 209 209 210

x  Contents 8 Nanomaterials in Animal Health and Livestock Products Devi Gopinath, Gauri Jairath and Gorakh Mal 8.1 Introduction 8.2 Nanomaterials 8.3 Nanomaterials and Animal Health 8.3.1 Role in Disease Diagnostics 8.3.2 Role in Drug Delivery Systems 8.3.3 Role in Therapeutics 8.3.4 Toxicity and Risks 8.4 Nanomaterials and Livestock Produce 8.4.1 Nanomaterials and Product Processing 8.4.1.1 Nanoencapsulation 8.4.2 Nanomaterials and Sensory Attributes 8.4.3 Nanomaterials and Packaging 8.4.3.1 Nanocomposite 8.4.3.2 Nanosensors 8.4.4 Safety and Regulations 8.5 Conclusion References

227 228 230 230 230 232 232 233 234 234 235 239 239 240 241 241 243 243

9 Restoring Quality and Sustainability Through Functionalized Nanocatalytic Processes 251 Nitika Thakur and Bindu Mangla 9.1 Introduction 252 9.1.1 Nanotechnology Toward Attaining Global Sustainability 252 9.2 Nano Approach Toward Upgrading Strategies of Water Treatment and Purification 253 9.2.1 Nanoremediation Through Engineered Nanomaterials 253 9.2.2 Electrospun-Assisted Nanosporus Membrane Utilization 254 9.2.3 Surface Makeover Related to Electrospun Nanomaterials 255 9.2.4 Restoring Energy Sources Through Nanoscience 255 9.3 Conclusion and Future Directions 256 References 256 10 Synthesis and Functionalization of Magnetic and Semiconducting Nanoparticles for Catalysis Dipti Rawat, Asha Kumari and Ragini Raj Singh 10.1 Functionalized Nanomaterials in Catalysis

261 262

Contents  xi 10.1.1 Magnetic Nanoparticles 262 10.1.1.1 Heterogeneous and Homogeneous Catalysis Using Magnetic Nanoparticles 263 10.1.1.2 Organic Synthesis by Magnetic Nanoparticles as Catalyst 264 10.1.2 Semiconducting Nanoparticles 264 10.1.2.1 Homogeneous Catalysis 267 10.1.2.2 Heterogeneous Catalysis 267 10.1.2.3 Photocatalytic Reaction Mechanism 267 10.2 Types of Nanoparticles in Catalysis 268 10.2.1 Magnetic Nanoparticles 268 10.2.1.1 Ferrites 268 10.2.1.2 Ferrites With Shell 269 10.2.1.3 Metallic 271 10.2.1.4 Metallic Nanoparticles With a Shell 271 10.2.2 Semiconducting Nanoparticles 271 10.2.2.1 Binary Semiconducting Nanoparticles in Catalysis 272 10.2.2.2 Oxide-Based Semiconducting Nanoparticles, for Example, TiO2, 272 ZrO2, and ZnO 10.2.2.3 Chalcogenide Semiconducting Nanoparticles for Catalysis 273 10.2.2.4 Nitride-Based Semiconducting Photocatalyst 274 10.2.2.5 Ternary Oxides 274 10.2.2.6 Ternary Chalcogenide Semiconductors 274 10.3 Synthesis of Nanoparticles for Catalysis 275 10.3.1 Magnetic Nanoparticles 275 10.3.1.1 Co-Precipitation Route 275 10.3.1.2 Hydrothermal Method 276 10.3.1.3 Microemulsion Method 277 10.3.1.4 Sono-Chemical Method 278 10.3.1.5 Sol-Gel Method 279 10.3.1.6 Biological Method 280 10.3.2 Semiconducting Nanoparticles 280 10.3.2.1 Tollens Method 281 10.3.2.2 Microwave Synthesis 281 10.3.2.3 Hydrothermal Synthesis 282 10.3.2.4 Gas Phase Method 282 10.3.2.5 Laser Ablation 282

xii  Contents 10.3.2.6 Wet-Chemical Approaches 283 10.3.2.7 Sol-Gel Method 283 10.4 Functionalization of Nanoparticles for Application in Catalysis 283 10.4.1 Magnetic Nanoparticles 283 10.4.2 Semiconducting Nanoparticles 285 10.4.2.1 Noble Valuable Metal Deposition 285 10.4.2.2 Functionalization by Ion Doping: Metal or Non-Metal 286 10.4.2.3 Semiconductor Composite or Coupling of Two Semiconductors 287 10.5 Application-Based Synthesis 287 10.5.1 Magnetic Nanoparticles 287 10.5.1.1 Silica-Coated Nanoparticles 287 10.5.1.2 Carbon-Coated Magnetic Nanoparticles 288 10.5.1.3 Polymer-Coated Magnetic Nanoparticles 289 10.5.1.4 Semiconductor Shell Formation Over the Magnetic Nanoparticle 290 10.5.2 Semiconducting Nanoparticles 290 10.5.2.1 Semiconductor Nanomaterials in Solar Cell 290 10.5.2.2 Batteries and Fuel Cells 291 10.5.2.3 Semiconducting Nanomaterials for Environment 292 10.5.2.4 Challenges for Water Treatment Using Nanomaterials 292 10.6 Conclusion and Outlook 293 References 294 11 Green Pathways for Palladium Nanoparticle Synthesis: Application and Future Perspectives Arnab Ghosh, Rajeev V. Hegde, Sandeep Suryabhan Gholap, Siddappa A. Patil and Ramesh B. Dateer 11.1 Introduction 11.1.1 Methods for Metal Nanoparticle Synthesis 11.1.2 Biogenic Synthesis of PdNPs 11.1.3 Phytochemicals: Constituent of Plant Extract 11.1.4 Techniques for Characterization of Metal NPs

303 304 305 306 307 308

Contents  xiii 11.2 Biosynthesis of PdNPs and Its Applications 308 11.2.1 Synthesis of PdNPs Using Black Pepper Plant Extract 308 11.2.2 Synthesis of PdNPs Using Papaya Peel 313 11.2.3 Synthesis of PdNPs Using Watermelon Rind 315 11.2.4 Synthesis of Cellulose-Supported PdNs@PA 316 11.2.5 PdNPs Synthesis by Pulicaria glutinosa Extract 318 11.2.6 Synthesis of PdNPs using Star Apple 319 11.2.7 PdNPs Synthesis Using Ocimum Sanctum Extract 321 11.2.8 PdNPs Synthesis Using Gum Olibanum Extract 322 11.3 Conclusion and Future Perspectives 323 References 324 12 Metal-Based Nanomaterials: A New Arena for Catalysis 329 Monika Vats, Gaurav Sharma, Varun Sharma, Varun Rawat, Kamalakanta Behera and Arvind Chhabra 12.1 Introduction 329 12.2 Fabrication Methods of Nanocatalysts 333 12.3 Application of Metal-Based Nanocatalysts 335 12.4 Types of Nanocatalysis 337 12.4.1 Green Nanocatalysis 338 12.4.2 Heterogeneous Nanocatalysis 339 12.4.3 Homogeneous Nanocatalysis 340 12.4.4 Multiphase Nanocatalysis 340 12.5 Different Types of Metal-Based Nanoparticles/Crystals Used in Catalysis 340 12.5.1 Transition Metal Nanoparticles 341 12.5.2 Perovskite-Type Oxides Metal Nanoparticles 342 12.5.3 Multi-Metallic/Nano-Alloys/Doped Metal Nanoparticles 343 12.6 Structure and Catalytic Properties Relationship 343 12.7 Conclusion and Future Prospects 344 Acknowledgment 345 References 345

xiv  Contents 13 Functionalized Nanomaterials for Catalytic Application: Trends and Developments 355 Meena Kumari, Badri Parshad, Jaibir Singh Yadav and Suresh Kumar 13.1 Introduction 356 13.1.1 Nanocatalysis 357 13.1.2 Factors Affecting Nanocatalysis 358 13.1.2.1 Size 359 13.1.2.2 Shape and Morphology 359 13.1.2.3 Catalytic Stability 360 13.1.2.4 Surface Modification 360 13.1.3 Characterization Techniques 361 13.1.4 Principles of Green Chemistry 362 13.1.5 Role of Functionalization 363 13.1.6 Frequently Used Support Materials 363 13.2 Different Types of Nanocatalysts 364 13.2.1 Metal Nanoparticles 364 13.2.2 Alloys and Intermetallic Compounds 365 13.2.3 Single Atom Catalysts 366 13.2.4 Magnetically Separable Nanocatalysts 367 13.2.5 Metal Organic Frameworks 368 13.2.6 Carbocatalysts 369 13.3 Catalytic Applications 370 13.3.1 Organic Transformation 370 13.3.2 Electrocatalysis 374 374 13.3.2.1 Electrocatalytic Reduction of CO2 13.3.2.2 Hydrogen Evolution Reaction 382 13.3.2.3 Fuel Cells 382 13.3.3 Photocatalysis 389 13.3.3.1 Photocatalytic Treatment of Wastewater 391 13.3.3.2 Photocatalytic Conversion of CO2 Into Fuels 391 13.3.3.3 Photocatalytic Hydrogen Evolution From Water 392 13.3.4 Conversion of Biomass Into Fuels 396 13.3.5 Other Applications 397 13.4 Conclusions 398 13.4.1 Future Outlook 398 References 398

Contents  xv 14 Carbon Dots: Emerging Green Nanoprobes and Their Diverse Applications 417 Shweta Agarwal and Sonika Bhatia 14.1 Introduction 417 14.2 Classification of Carbon Dots 419 14.3 Environmental Sustainable Synthesis of Carbon Dots 424 14.3.1 Hydrothermal Treatment 432 14.3.2 Solvothermal Treatment 433 14.3.3 Microwave-Assisted Method 434 14.3.4 Pyrolysis Treatment 435 14.3.5 Chemical Oxidation 436 14.4 Characterization of Carbon Dots 438 14.5 Optical and Photocatalytic Properties of Carbon Dots 440 14.5.1 Absorbance 441 14.5.2 Photoluminescence 441 14.5.3 Quantum Yield 443 14.5.4 Up-Conversion Photoluminescence (Anti-Stokes Emission) 444 14.5.5 Photoinduced Electron Transfer 445 14.5.6 Photocatalytic Property 446 14.6 Carbon Dots in Wastewater Treatment 449 14.6.1 Heavy Metal Removal 451 14.6.2 Removal of Dyes 452 14.6.3 Photodegradation of Antibiotics 453 14.6.4 Removal of Other Pollutants 453 14.6.5 Bacterial Inactivation 454 14.6.6 Oil Removal 454 14.7 Carbon Dots for Energy Applications and Environment Safety 454 14.7.1 Solar Light–Driven Splitting of Water 455 457 14.7.2 Photocatalytic CO2 Reduction 14.7.3 Photocatalytic Synthetic Organic Transformations 459 14.8 Biomedical Applications of Carbon Dots 460 14.8.1 Bioimaging 461 14.8.2 Carbon Dots as Biosensors, pH Sensors, and Temperature Sensors 463 14.8.3 Carbon Dots for Drug Delivery 466 14.8.4 Carbon Dots as Carriers for Neurotherapeutic Agents 468

xvi  Contents 14.9 Ethical, Legal, and Sociological Implications of Carbon Dots 14.10 Conclusion and Future Outlook References

469 471 472

Index 493

Preface With the rapid development in nanotechnology, it is now possible to modulate the physical and chemical properties of nanomaterials with molecular recognition and catalytic functional applications. Such research efforts have resulted in a huge number of catalytic platforms for a broad range of analytes ranging from metal ions, small molecules, ionic liquid and nucleic acids down to proteins. Functionalized nanomaterials (FNMs) have important applications in the environmental, energy and healthcare sectors. Strategies for the synthesis of FNMs have contributed immensely to the textile, construction, cosmetics, biomedical and environmental industries among others. These areas are considered a key platform for a wide range of applications, consequently becoming a top priority for science and technology policy development since they are already involved in the production of various modern products, especially in electronics, healthcare, chemicals, cosmetics, composites and energy. Due to their ultra-small size, nanostructured FNMs have extraordinary physical and chemical properties such as unique optical, electrical, thermal, magnetic and adsorption characteristics. The large specific surface area of FNMs can certainly improve their current application even at the industrial as well as academic scale. In addition, FNMs of various compositions and morphologies can provide powerful tools for advanced industrial techniques. Therefore, FNMs-based techniques can play a vital role in numerous industrial procedures such as, for example, by increasing sensitivity, magnifying precision and improving the production limit. Moreover, freedom to functionalize nanomaterials with various chemical groups can also increase their affinity toward respective counterpart analytes, which is very desirable for selective manufacturing of certain products. This book highlights the design of functionalized nanomaterials with respect to recent progress in the industrial arena and their respective applications. It presents an inclusive overview encapsulating FNMs and their applications to give the reader a systematic and coherent picture of nearly all relevant up-to-date advancements. Herein, functionalization xvii

xviii  Preface techniques and processes are presented to enhance nanomaterials that can substantially affect the performance of procedures already in use and can deliver exciting consumer products to match the current lifestyle of modern society. Advanced undergraduate and graduate students and industrial professionals will find this book a good source of knowledge that will act as a guideline for their studies and research. Because of the integral nature of the topics, it will also be of interest to a broad spectrum of audiences, including industrial scientists, industrial engineers, nanotechnologists, materials scientists, chemists, physicists, pharmacists, biologists, chemical engineers, and all those who are involved and interested in the future frontiers of nanomaterials. In other words, it was devised as a reference book for researchers and scientists who are searching for new advanced techniques for various catalytic applications. Among the editors and contributors to this book are prominent researchers, scientists and true professionals from academia and industry. We are very thankful to all the authors of the chapters for the wonderful and passionate effort they displayed while working on this book. And a special thanks to Martin Scrivener and the entire team at Scrivener Publishing for their dedicated support and help during the process of compiling and publishing this book. Chaudhery Mustansar Hussain, Ph.D. Sudheesh K. Shukla, Ph.D. Bindu Mangla, Ph.D.

1 Functionalized Nanomaterial (FNM)–Based Catalytic Materials for Water Resources Sreevidya S.1, Kirtana Sankara Subramanian2, Yokraj Katre1, Ajaya Kumar Singh3* and Jai Singh4 Department of Chemistry, Kalyan PG College, Bhilai Nagar, Durg, India Department of Food Science, Faculty of Veterinary and Agriculture Science, University of Melbourne, Melbourne, Australia 3 Department of Chemistry, Govt. V.Y.T. PG. Autonomous College, Durg, India 4 Department of Pure and Applied Science, Guru Ghasidas Vishwavidyalaya (A Central University), Bilaspur, India 1

2

Abstract

Water, one of the essential elements in the nature under a great threat, with pollutants treasured in it generates noxious suffocations with the raise in contaminants globally. It is time we wake up. Fabrication of innovative nanomaterials with unique models and approaches, deliver versatilities in overcoming the drawbacks installed in earlier protocols for a full-scale utilisation in the environmental pitch. Functionalization of nanoscales provides a promising note, when employed in remediation applicational functions for environmental system. Protection of as-synthesized nano scaled material by casing a suitable layer of organics/inorganics on their core surface by functionalization modules enhances the functionalities. Functionalized nanomaterials supported with nanocatalyst have been proven for their high selectivity and controlled sensitivity over the target samples in water management. Nano-adsorbents, nano-membranes and nanocatalysts are commendably employed for attacking and eliminating the pollutant from the resourceful water sectors either in surface or in sub-surface. Functionalized nanocatalysts like electrocatalyst, photocatalyst, electro-Fenton catalyst, Fenton-based catalyst, and oxidants (chemical) by versatile processes *Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (1–52) © 2021 Scrivener Publishing LLC

1

2  Functionalized Nanomaterials for Catalytic Application have revealed their potentialities in getting rid of biological, organic and/or inorganic toxicants from water bodies that might lead to painful health issues. Functionalized nanocatalytic materials for remediation of water resources will be mainly focused in the segments to come. Keywords:  Functionalization, nanomaterials, nanocatalyst, pollutants, remediation, water

Abbreviations Used: Organization WHO

World Health Organization

EPA

Environmental Protection Agency

Technical NIR

Near Infra-Red

ECMR

Electrocatalytic Membrane Reactor

PL

Photoluminescence

LUMO

Lowest unoccupied molecular orbital

CB

Conduction Band

VB

Valence Band

e

Electron

h

Holes

-

+

Functionals EC

Electrocatalyst

PC

Photocatalyst

EF

Electro-Fenton

P-EF

Photoelectro Fenton

F’bC

Fenton-based catalyst

H-EF

Hetero Electro-Fenton

HF

Heterogenous-Fentons

H-PF

Hetero Photo-Fenton

PF

Photo-Fentons

pF

Photo-Fenton–like

Terminologies FNMs

Functionalized Nanomaterials

NM

Nanomaterials

NP

Nanoparticles

NT

Nanotubes

NS

Nanosheets

NR

Nanorods

NF

Nanofibers

NW

Nanowires

TP

Target Pollutant

OP

Organic Pollutant

TOC

Total Organic Carbon

DP

Degraded Products

CNT

Carbon nanotubes

MWCNT

Multiwalled Carbon nanotubes

FNM-Based Catalytic Materials  3 CNO

Carbon nano onion

CF

C Felt/Fiber

CFP

Carbon fiber paper

CP

Carbon paper

QDs

Quantum Dots

MNPs

Magnetic nanoparticles

CQDs

Carbon QDs

CDs

Carbon dots

Materials R/r GO

Reduced GO

ACA

Activated C aerogel

GA

Graphene aerogels

GPCA

Graphene polyacrylamide carbonized aerogel

PC

Porous C

MPCMSs

Magnetic porous C micro-spheres

BDD

Boron doped diamond

Fe-HPAN

Fe-loaded hydrolyzed polyacrylonitrile

SS

Stainless steel

GMSA

Glycerol mediated self-assembly

HPC

Hollow porous C

MMTNS

Montmorillonite nanosheets

Fe-POM

Fe-polyoxometalates

PPy NTs

Polypyrole nano-tubes

CNQDs

Carbon nitride QDs

GCNQDs

Graphene-Carbon nitride QDs

TCNCs

Tubular Carbon Nitride

Nomenclature: PANI

Polyaniline

PTFE

Polytetrafluoroethylene

G

Graphene

CE

Cetyl Trimethyl Ammonium Bromide

GO

Graphene Oxide

EDTA

Ethylenediaminetetraacetic acid

PAN

Polyacrylonitrile

EDC

1-ethyl-3-(3-dimethylaminopropyl) carbodiimide

CS

Chitosan

PDVF

Polyvinylidene difluoride

PHB

Poly-hydroxyl butyrate

MUC

Melamine, Cyanuric acid, Urea

PEG

Poly-ethylene glycol

ABA

4-aminomethyl benzoic acid

O-TD

O-toluidine

TCPP

Tetrakis (4-carboxyphenyl) porphyrin

SA

Salicylic acid

L-Cys

L-Cystine

2-ME

2-mercaptoethanol

PAMAM

Poly(amidoamine)

Try

L-tryptophan

Glu

L-glutamic acid

4  Functionalized Nanomaterials for Catalytic Application PUF

Polyurethane foam Organic: Pollutant

PCA

p-Chloroaniline

TCP

2,4,6-trichlorophenol

4-NP

p-nitrophenol (PNP)

4-CP

4-chlorophenol

2,4-DCP

2,4-Dichlorophenol

DMP

dimethyl phthalate

5-TBA

5-tolylbenzotriazole

2,4,5-PCB

2,4,5-trichlorobiphenyl

BPA

Bisphenol A Dyes: Pollutant

MO

Methyl Orange

MB

Methylene Blue

MG

Malachite Green

CR

Congo Red

Rh B

Rhodamine B

CB

Chlorazol black

BG

Brilliant Green

RR-120

Reactive red 120

EB

Evans blue

X-3B

Reactive Brilliant Red

CV

Crystal Violet

VBR

Victoria blue R

RO-16

Reactive Orange-16

TBAC-L

Texbrite BAC-L

TNFW-L

Texbrite NFW-L

TBBU-L

Texbrite BBU-L

Drugs: Pollutant CIP

Ciprofloxacin

TC

Tetracycline

IBP

Ibuprofen

TCH

Tetracycline-Hydrochloride

CBZ

Carbamazepine

OTC

Oxytetracycline

DFC

Diclofenac

NPX

Naproxen

PCM

Paracetamol

KP

Ketoprofen

CEL

Cephalexin Pesticides: Pollutant

ATZ

Atrazine

IMI

Imidacloprid

ATP

Acetamiprid

TMX

Thiamethoxam

ALA

Alachor Consumer Products

TCS

Triclosan

1.1 Introduction Water, one of the essential elements in the nature, is under a great threat, with pollutants treasured in it. Noxious suffocations are generated with the raise in contaminants globally. It is time we wake up to take care of it, else

FNM-Based Catalytic Materials  5 would be forced to face the unsolicited deliverables stimulated. In total, 71% of Earth’s surface is blanketed with water, and 99.7% of it is in oceans, icecaps, soil, and other atmospheric fragments. But, it is unfortunate that the percentage of utility is only 0.3% according to the available data. Globally, formidable pressures are piling up with elevations to meet the increasing demand for clean water, as resources of freshwater are depleting, probably due to (i) urbanization, (ii) industrialization, (iii) increasing population, (iv) climatic droughts, and (v) competing necessitates of the end users [1]. According to the key fact sheet (drinking water) of WHO-June 2019, 785 million people are devoid of essential drinking-water amenities [2]. Growing populace, demographic variations, climatic deviations, and environmental pollution are major contributors as challengers and reducers of water resource segments. Sufficient supplies of potable water can be derived by using proper management of effluent water from diversified sources [3]. With restrictions in traditional methods, sophisticated novel methods supply the requisites in an eco-friendly, cost-effective way to combat the toxic pollutants efficiently for remediations. The toxic contaminants dispensed by organics like dyes from textile and printing industries, hydrocarbons from petro-chemical sectors, polymeric plastics, inorganics like poisonous gases, heavy-metal toxins from mining and other source, and devasting microbial consortiums pollute water resources either directly or indirectly, leading to unavailability of clean H2O [4]. WHO with EPA (Environmental Protection Agency) is evaluating suitable measures for utilizing decentralized treatment models for acquiring consumable water from wastewater. Fortification of wastewater and clean-water management systems, from various biological or chemical intruders are crucial concerning factors [5]. Recent momentums in nano-engineering and nano-sciences have stipulated unprecedented breakthrough in evolving cost-effective and environment friendly protocols for an adaptable water treatment solution [6]. Fabrication of innovative nanomaterials (NMs) with unique models and approaches delivers versatilities in overcoming the drawbacks installed in earlier protocols for a full-scale utilization in the environmental pitch. Metallic, carbonaceous, polymeric, zeolites, etc., are the various categories of natural/simple/complex/functionalized nanomaterials (FNMs) generally utilized for water management [7–9]. Functionalization of nanoscales provides a promising note for renovating and engineering new nano scaled materials that can be fruitfully employed in remediation applicational functions for detecting and removing the organics, heavy metal toxins, and microbial consortiums in chemical, biological, or environmental systems [10, 11]. Functionalization

6  Functionalized Nanomaterials for Catalytic Application with highly active points with sizeable surface area renders commendable attraction for withdrawal of pollutants in aqua center [12, 13]. Sometimes, nano-sized materials tend to accumulate, with variations in reaction condition, thereby reducing their remedial capacities [14, 15]. Protection of as-synthesized nano scaled material from chemical oxidation/reduction and toxicities delivered by them can be controlled by casing a suitable layer of organics/inorganics on their core surface by functionalization modules [16]. In the recent past, FNMs are of great demand for their assorted technological innovations and nano-engineering applications in comparison with the normal NMs, as they possess exceptional transitional characteristics [17–19]. FNMs supported with nanocatalyst have been proven for their high selectivity and controlled sensitivity over the target samples in water management [20, 21]. Nano-adsorbents, nano-membranes, and nanocatalysts have been commendably employed for attacking and eliminating the pollutant from the resourceful water sectors either in surface or in sub-surface [22, 23]. Functionalized nanocatalysts like electrocatalyst (EC) [24], photocatalyst (PC) [25], electro-Fenton catalyst (EFC) [26], Fenton-based catalyst (F’bC) [27], and oxidants (chemical) by versatile processes have revealed their potentialities in getting rid of biological, organic and/or inorganic toxicants from water bodies, that might lead to painful health issues [28]. Photo radiations of sufficient energy (solar/visible/UV-A) from light source, on interaction with FNMs, as PC produce intensified photocatalytic actions to protect water resources from damaging pollutants [29]. Sun

Bulb

Electro h+ + e–

FNMs (Catalyst) + hυ

es

Heavy Metals

Dyes

ut

es

e–

Ads o

Na

ts an

Pesticides

•OH

h+

no

ll Po

•O2–

ctro -Ele rials ero e Het n Mat to Fen

e–

od

cat aly st Pho to Ma -Fento teria n ls

ran

mb

Plastics

Me

Ele c

ctr

Drugs

h+

e– st aly cat oto Ph

Ele

dy Bo ter Wa

tro cat aly st

Functionalized Nano Materials

•OH

LED Light

rbe

nts

Fenton-Catalyst

FNMs (Cathodic) + E

e– + +

Figure 1.1  FMN-based nanocatalyst for water resources.

•O2–

FNM-Based Catalytic Materials  7 Similarly, FNMs as nanocatalyst formed from noble elements have competent catalytic ability in degrading organic pollutants (OPs)/inorganic contaminants by oxidation chemically [30, 31]. The loss of catalytic substances with disposals during reactions in Fenton-based catalysis is overcome when the reaction is supported by FNM component. EFC as FNMs with sizeable surface-area of EC shows enhanced capabilities as redox (reducer/ oxidiser) agent in the reaction media for remediation [32]. Versatile topography is depicted in Figure 1.1, to visualize the entire protocol offered in FMN-based nanocatalyst for water resources. Functionalized nanocatalytic materials for remediation of water resources will be mainly focused in the segments to come.

1.2 Electrocatalysts as FNMs Presently, the grave situation to be monitored is the impairment by undesirable activities of human race caused to the eco-system [33]. Industrial revolution, production, and utilization are accused for the deliveries of contaminants into the environment. Accumulation of unwanted that alters the food chain is to be checked. Alternatives are worked upon for removing the undesirables in a simple and cost-effective stable way [34]. Poor functioning of ECs at anodic end at low T oC leads to cathodic poisoning to retard the process sometimes [35]. Functionalization of materials can overcome this situation. Surface modification was achieved by using functionalized Pt/C as EC, to give a powerful potential for methanol oxidation [36]. The limitations like high-cost approach, non-availability, and restrictions in poisoning of intermediates [37, 38] while using pure Pt, narrows Pt’s attraction, and utility as EC are now masked by other suitable alternatives [39]. Conductivity of the EC is improved by surface modification, such that cathode attains a high electrocatalytic property. This can be best achieved by functionalization modules of NMs [40]. A schematic depiction of electrocatalytic degradative action in the presence of FNMs that easily removes the contaminants, to protect the water system is shown in Figure 1.2. An effective catalytic reducing ability was noticed by Qiu, L. et al., while using Au/PPy NTs as EC in gauging fuel cell’s capability for waste effluent water management. 4-NP was the targeted compound in this experiment [41]. FNM nano-TiO2/C membrane, an EC got by sol-gel technique, had an efficient removal of diesel oil (100%) from wastewater. Authors Yang, Y. et al., observed that ECMR was a key enhancer in this situation [42]. The choice of material, a primary factor in wastewater management, was focused in the reports of Bankole, M.Y. et al., where PHB/

8  Functionalized Nanomaterials for Catalytic Application

CO2 + H2O / Mineralized Product

2–

•O

Anode

O2

H2 O

2

TP

Fe2+

e–

Fe-FNMS

e–

e–

–

e

TP

Fe3+

•OH

CO2 + H2O / Mineralized Product Functionalized Cathodic Surface

Figure 1.2  Electrocatalytic degradative action to protect the water system.

CNTs and P-CNTs (P-Purified) had a worthy removal of heavy metals like [Fe (15.92% and 15.11%), Cr (98.19% and 99.80%), As (99.95% and 99.99%), Cd (99.34% and 98.68%), Pb (98.85% and 99.44%), Cu (83.08% and 82.91%), Zn (18.34% and 21.80%), and Ni (77.95% and 78.06%)] in redox fashion [43]. Nitrobenzene, a carcinogenic contaminant associated with dyes, pesticides, explosives, and pharmaceuticals, was successfully resolved and degraded to 99.8% in 5 h by FNM TiO2-NTs/SnO2-Sb/PbO2 electrocatalytically with an increased stability at a potential of 2.00 V [44]. In a similar protocol, the working trials were conducted for electrocatalytic action for degrading the organic contaminant benzoic acid. The meso-­ porous structured material TiO2-NTs/m-SnO2-Sb that was produced by electro-­deposition process had a significant removal efficacy [45]. Pd/TiO2 NTs got by electro-­chemical deposition method excellently removed 2,4,5PCB (90%) by electrocatalytic dehydro-halogenation process [46].

1.3 Electro-Fenton/Hetero Electro-Fenton as FNMs Fenton’s redox chemistry employs •OH released between the reacting species (H2O2 + Fe2+) for the decomposition of target pollutants (TPs), where electro-Fenton (EF) or photoelectro Fenton (P-EF) have prominent roles. Hetero-EF (H-EF) utilizes solid nanocatalyst as a supporter for reducing H2O2 → •OH. The disadvantage of small pH range (acidic) is overcome by solid supporters when used. The effluents released into the water system

FNM-Based Catalytic Materials  9 have a wide range of pH [47]. Micro-porous/meso-porous FNMs offer best solutions for degrading OPs in the water bodies. Research communities are focusing on this segment for protection of environmental crises using Fe/ other transition metal/metal oxides as cathodic FNMs in H-EF methods. Cathodic FNMs are got by (i) uni/multi step synthesis of low-density porous-solids (C aerogels), (ii) modified conducting FNMs with Fe, and (iii) carbonaceous solids supported with Fe or other components as FNMs [48–50]. Formation of sludge as Fe-hydroxides, as in normal Fentons, is retarded or inhibited, thus improving the efficiency and availability of catalyst for its activity. Hence, less energy utilization and a cost-­effective approach is favored. Similarly, reusability and recyclability for many trials were observed while using cathodic FNMs of Fe2O3/N-C by [51] and Fe-Cu-C aerogel [52]. However, Fe when strengthened with other metals (transition) embedded in it, results in a redox reaction with catalytic decomposition, and is favored with the increase in efficacy of the electrocatalytic system to bring about degradation of TPs [53–55]. A figurative description of the functionalized catalytic activity is shown in Figure 1.3. In a typical report of Cui, L. et al., MO decomposition by H-EF was proved to be accelerated by FNM - Fe3O4/MWCNTs, when prepared by solvothermal process. Degradability of the TP was noted to be 90.3% (3 h) with reusability to 5 runs, at pH (3). This system with two compartments of FNM membrane required no external additives, but had a potency

A

V

FNMs

DP Degraded Products Target Pollutants

Surface Modifiers Functional Base Material Intermediates on Cathodic Surfaces

DP

e– O2

e– e– e– e–

ANODE

TP

FNMs CATHODE

e–

e– TP H2O

Figure 1.3  Electro-Fenton functionalized catalytic degradative activity for water bodies.

Process | Current/ Voltage

Hydrothermal | 4.5 mA cm−2

Hydrothermal | 300–400 mA

Hydrothermal, carbonization | (16, 20, 24) mA cm−2

Ultra-sonification | 100 mA

Carbonization (PANI) | −0.6 V

Hydrothermal, carbonization | (−3.30, −4.42, −3.77) mA cm−2 | −0.6 V

FNMs as catalyst | Type | Year

BGA-GDE | EF | 2019

RGO-Ce/WO3 NS/CF | EF | 2018

ACF-HPC | EF | 2019

Fe-C/PTFE | H-EF | 2015

N-C (NF) as (c PANI/ GF2) | EF | 2019

FeOx/NHPC750 | H-EF | 2020 pH (6) | 90 min

pH (3) | 180 min

ATZ (96%) | Rh B (99%) | 2,4-DCP (99%) | Sulfamethoxazole (95%) | Phenol (99%) | 5

Mineralization (42%) | Florfenicol (99%)| Phenol (85%) | MO (100%) | 5

2,4-DCP (95%) |

Phenol (93.8%) | 5 TOC (85.7%) | 5

pH (3, 7, 9) | 40, 180 min pH (6.7) | 120 min

CIP (100%) | 5

BPA (~89.65%) | 5 TOC (~90%) | 5

Solution evolved (% degradation) | Reusable cycles

pH (3) | 1h

pH (3–9) | 60 min

Parametric expressions

Table 1.1  Electro-Fenton (EF)/Hetero-Electro-Fenton (H-EF) catalyst as FNMs.

[67]

[66]

(Continued)

Cleavage of O-O bond | Assists H2O2 | Fe2+ + O2 à Fe3+ + •O2−

Activation: H2O2 à •OH

[65]

pseudo-1st-order kinetics | promoters: H2O2, •OH | Cheap

[63]

[62]

[64]

O2−, H2O2, •OH | Ce-WO3 improved adsorption

OH | pseudo-1st-order kinetics

Ref.

Enhanced formation of H2O2, • OH | Low-cost

•

•

Remarks

10  Functionalized Nanomaterials for Catalytic Application

Process | Current/ Voltage

Hydrothermal | 40 mA cm−2

Carbonization | 40 mA

Hydrothermal, calcination | 30 mA

Electro-deposition | 21.7 mA cm−2

FNMs as catalyst | Type | Year

(Co, S, P)/MWCNTs | P-EF |2019

Mn/Fe@porous C (PC)-CP cathode | H-EF | 2019

3DG/Cu@C | H-EF | 2020

C felt/Fe-Oxides | H-EF | P-EF | 2016 pH (3) | 120 min

pH (3–9) | 150 min

pH (2–8) | 120 min, 240 min

pH (3) | 360 min

Parametric expressions

MG (98%) | 10

Rh B (100%) | CIP (100%) | 2,4-DCP (100%) | PCA (89.8%) | BPA (96.1%) | CAP (82.6%) | 5

TCS (100%) | TOC (~57%) | 6

Bronopol (100%) | TOC (77%) | 3

Solution evolved (% degradation) | Reusable cycles

Table 1.1  Electro-Fenton (EF)/Hetero-Electro-Fenton (H-EF) catalyst as FNMs. (Continued)

(Continued)

[71]

[70]

Contributors: •OH, •O2− | e- transfer: Cu2+/+

OH, BDD - activators | UVA | pseudo-1st-order kinetics |

[69]

Regeneration: Fe2+/Mn2+/3+| e- transfer: Fe2+/3+, Mn2+/3+/4+, pseudo-0-order kinetics

•

[68]

Ref.

Contributors: sunlight, •OH, BDD (•OH) |

Remarks

FNM-Based Catalytic Materials  11

Process | Current/ Voltage

Hydrothermal | variable | −0.2 V

Electrophoretic deposition | 170 mA | −0.5 V

Electrophoretic deposition | 0.18 A

Electrochemical | −1.5 V| (−0.75, −1.0, −1.5, −2.5) V

Thermal-induced | −0.8 V

Hydrothermal | [0 – (−95.5)] mA cm−2

FNMs as catalyst | Type | Year

(N-G@CNT | EF | 2108

F-rGO/SS membrane | EF | 2019

G-CNT-CE | EF | 2014

Fc-ErGO | EF | 2018

FeOCl-CNT | EF | 2020

3D GA/Ti wire | EF | 2018

[72] [73]

[74]

e- transfer-enhanced by rGO | low-cost membrane

−

Fe + H2O2 à Fe + e | pseudo-1st-order kinetics 3+

Ref.

EDTA-Ni (m73.2%) | 5

pH (2) | 120 min

π-π interaction | pseudo-1storder kinetics

[77]

[76]

Fe3+/Fe2+ | H2O2 + •OH à H2O + •OOH

TC (99.5%) |

pH (wide) |

[75]

OH | pseudo-1st-order kinetics

CIP (99%) | 5

pH (3) | 15 min | pH (7) | 120 Min

•

pseudo-1st-order kinetics | pseudo-2nd-order kinetics

2+

Remarks

Acid Red 14 (91.22%) | Acid Blue 92 (93.45%)

PCM (37%) | 5

DMP (100%) | 20 TOC (40.4%)

Solution evolved (% degradation) | Reusable cycles

pH (3) | 210 min

pH (3) |

pH (3) | 180 min

Parametric expressions

Table 1.1  Electro-Fenton (EF)/Hetero-Electro-Fenton (H-EF) catalyst as FNMs. (Continued)

12  Functionalized Nanomaterials for Catalytic Application

FNM-Based Catalytic Materials  13 in green wastewater treatment techniques [56]. Zhao, H. et al. reported that Fe3O4@Fe2O3/ACA (activated C aerogel) as cathodic in this EF routine degraded (90%) of OP-pesticide imidacloprid (30 min) and TOC (60 min) in pH range of (3–9) [57]. Haber-Weiss model inferred that Fe2+ aided the decomposition of peroxide to form •OH. •OH and •O2− contribute for the degradation of OP. Mesoporous FNMs MnCo2O4-CF (C felt) as cathodic EF with excellent porosity and large modified surface area prepared showed a powerful degrading capacity for CIP (100%) an antibiotic in 5 h and TOC (75%) in 6 h [58]. Mn2+/Mn3+, Co3+/Co2+ with e− transfers enhanced peroxide decomposition to form •OH and •OOH required for five cycles degradation. H-EF system with Fe oxides surrounded by Cu and N on HPC (hollow porous C) as cathodic FeOx/CuNxHPC was inferred to give a good degradability for phenol (100%/90 min/variable pH) and (81%/120 min/pH6). Slow redox reaction (Fe2+/Fe3+) favored e− movement, and formation of • OH, that were essentials for degradation in this ambient condition [59]. In a similar fashion a three-layered H-EF catalyst as FMN “CFP@PANI@Fe3O4” engineered using electrodeposition-solvothermal method was proven for the removal of 4-NP (100%/60 min/4 runs) at an acidic pH (3) and TOC (51.2%/7 runs) at the same pH [60]. Enrichment of electrocatalytic capacity was attributed to formation of Fe3O4 on the functionalized surface of the conducting layers. Wang, Y. et al. fabricated γ-FeOOH GPCA cathodic EF-catalyst for experimenting the degradability of the antibiotic sulfamethoxazole (~90%/5runs). Twelve degraded products by, hydroxylation, isomerization, and oxidation reactions were identified using chromatographic trials [61]. Table 1.1 depicts trials developed by some research personalities.

1.4 Hetero Photo-Fenton as FNMs Organic effluent’s water management is achieved well, in normal environmental pre-requisites of pH, where the hetero-photo Fenton has a good stability and reusability for notable cycles. Photo-Fenton (PF) catalytic redox reaction may utilize membrane percolation and magnetic purification to effectually decompose OPs into CO2 and H2O. Semiconductors, phosphates/oxides/oxyhalides/sulfides/molybdates/tungstates/vanadates of transition and other metals, g-C3N4, G, GO, QDs, and MOFs have fascinated the researchers to a larger degree, for probing a simplified and a cost-effective FNMs [78, 79]. Excellent features with required bandgap, got by photo-excitation in an environmentally friendly way, of the materials with notable lattice parametric change, safe guard water system by

14  Functionalized Nanomaterials for Catalytic Application

H2O

H+

+

DP

Degraded Products

H2O2

•O2–

DP

O2

CB e–

e–

DP

e–

e–

•OH e–

e–

TP

e–

Fe2+ e–

h+

h+

Fe2+ h+

Surface Modifiers

e–

h+

h+

h+

h+ •OH

VB H2 O

TP

Target Pollutants

Figure 1.4  Functionalized nanomaterials as photo-Fenton catalyst for water resource management.

nanocatalytic action. Figure 1.4 is a graphical representation of FNMs as PF catalyst for water resource management.

1.4.1 Heterogenous-Fentons-Based FNMs FNM PAN with EDC as catalyst was used to study the regeneration efficiency of the material in use, by the experimentalist. Further, the regenerated catalytic reaction as heterogenous Fenton (HF) was found to be a better option for degrading RO-16 [80]. Living species vulnerable to the revelation of pharmaceutical organic lteftovers in the water system causing ecological barriers was the point of attack by reporters Wan, Z. et al. [81]. Fe3O4-Mn3O4/RGO synthesized by polyol and impregnation processes worked well as a HF-like catalyst with the actively formed OH, decomposed SMT (sulfamethazine-drug) (99%/50 min) effectively in a water solvent. Authors Zhou, L. et al., forecasted that the drawbacks due to catalytic reactions can be knocked off by using MPCMSs [82]. C micro-spheres protected Fe3O4 NM from oxidation, while degrading MB. Formation of •OH supported this HF reaction by H2O2 and NH2OH.

1.4.2 Photo-Fentons-Based FNMs Hetero Photo-Fenton (H-PF), an interface between Fenton and photocatalysis, assisted by photon from solar or visible has powerful synergistic

FNM-Based Catalytic Materials  15 properties. PF utilizes the e−’s got from the reaction Fe3+/Fe2+ through oxidation to aid and activate e− transfer to •OH from H2O2 in the entire redox process. The scavenging radicals contribute to practical utilization in a big factor for protecting water bodies. Feox NPs/D3 (diamond NP) that worked well as a H-PF catalyst was effective in degrading and decomposing phenol and H2O2, respectively, under an ambient condition. Later, was proved to be a better alternative when in comparison with its analogous as per reports of Espinosa, J.C. et al., where phenol acts as h+ quencher and diamond NP as surface releaser of •OH favors the optimized reaction [83]. Upgraded new water treatment competencies are scaling up in saving the water resources. For instance, a low-cost valuable functionalized M (magnetite)/PEG/[(FeO (Iron III oxlate)/FeC (Iron III citrate)] showed high catalyst action for a quick disposal and degradation of BPA. Later, the evidenced PF catalyst when on exposing the chosen probe with UV-A light/H2O2, had a hierarchical degradation as (M/PEG/FeO) (15 min) > (M/PEG/FeC) > (M/PEG) [84]. In one of their work, a comparative study of MB degrading effect by the synthesized supports of Ni foam (NiF) and Ceramic foam (CM) was done by the authors, whose reports infer that the order of decomposing capacities were: (NiF/TiO2) > (CM/TiO2) > (NiF/ Bi2 WO6) > (CM/Bi2WO6). In the same manner, decomposition of Rh B was studied using NF/TiO2 for PF reactions [85]. In a different situation, reporters Bui, V.K.H. et al., revealed that Mg-AC (Mg amino-clay) with its versatility and unique characteristic, along with other 2D resources, have fascinated researchers [86]. Hence, Mg-AC finds its place and with a commendable performance in various fields especially in water resources. The resourceful material MgAC-Fe3O4/TiO2 works best for PF catalytic decomposition of MB (93%) (20 min), where •OH and •O2− formed are promoters for the redox-reaction. Cost-effective and potential approach with Fe-HPAN (carboxyl) functionalized beads developed by researchers was exposed for an effective PF catalytic reaction. Their results showed a better degrading capacity of 99.78% for TOC and 91.68% for p-nitrophenol removal and profitable reutilization was supported by the mesopores present in the FNM [87]. Rice-shaped starch functionalized iron (III)-oxyhydroxide got by green methods using akageneite/goethite had an improved HF and PF catalytic properties obeyed a pseudo-0-order kinetics. FNM was effective in decomposing the OP p-nitrophenol and MO into CO2 + H2O to protect the water bodies [88]. In a similar trial, sulfate-functionalized S-Fe2O3/TiO2 NT prepared by solvothermal/impregnation process was proven to be a good candidate for PF catalytic run to discharge the color of X-3B by adopting pseudo-1st-order kinetics. Notable degradation was observed at pH

Solvent-free milling

Ion-exchange

Aerosol spray

Ultrasonic

Ball milling

Thermal polymerization

Hydrothermal

TiO2/Fe2TiO5/Fe2O3 | PF | 2017

A-TiO2/R-TiO2/αFe2O3 | PF | 2020

TiO2-GO-Fe3O4 | PF | 2019

FeNx/g-C3N4 | PF | 2019

0D Fe2O3 QDs/2D g-C3N4 | PF | 2020

α-Fe2O3/g-C3N4 | PF | 2020

Process

TiO2/ Schwertmannite | PF | 2019

FNMs as catalyst | Year

Solar light

Visible light

Visible light

Visible light

UV - 365 nm

Visible light > 420 nm

Sunlight

Irradiation Source |

pH (neutral) | 90 min

pH (3–7) | 20 min

pH (neutral) |

pH (3) | 120 min

pH (8) | 5–30 min

pH (4.0/7.0) | 120 min | 60 min

pH (4) | 60 min.

Parametric expressions

Table 1.2  Photo-Fenton (PF)/Photo-Fenton–like (pF) catalyst as FNMs.

Rh B (96%) | 5

4-NP (90%) | 5

MB |MO |Rh B |Phenol | (variable %) | 4

Amoxicillin (90%) | 4

MB | TOC | 5

MO (100%) | Phenol (100%) | 10

Rh B (100%) | 4

Solution evolved (% degradation) | Reusable cycles

Binding Energy (284.8 eV) •O2−/h+. Fe3+ à Fe2+ + e−

[97]

[96]

[95]

[94]

[93]

[92]

[91]

Ref.

(Continued)

Fe3+ à Fe2+ + e− • OH+H2O2à•OOH+H2O

H2O2 + e− à 2 •OH

Fe3+ à Fe2+ + e−

O2/•O2− | low dose H2O2

OP+ •OH à CO2 + H2O

TiO2 à Sh + e− H2O2 + e− à •OH

Remarks

16  Functionalized Nanomaterials for Catalytic Application

Solar light

Microwave| Hydrothermal

Simple thermal

Sol gel (3 Step)

Hydrothermal | Photo-deposition

Vacuum-filtration

Self-assembly

Zn0.94 Fe0.04S/g-C3N4 | PF | 2020

Cu-FeOOH/ CNNS(g-C3N4) | PF | 2018

(Fe-CS/MMTNS | PF | 2020

Fe0)/MnOx/BiVO4 | PF | 2019

GO/MIL-88A(Fe) | PF | (2020)

Fe-POM/CNNSNvac | PF | 2020

Visible light < 420 nm

Visible light

Visible light

Visible light

Solar light

Process

Irradiation Source |

FNMs as catalyst | Year

- | 18 min

- | 40 min

pH (acidic) | 30 min

pH (3,6,10) | 2 h

pH (4.8-10.1) | 40 min | pH (low)

pH (6.1) | 60 min

Parametric expressions

TCH | ATZ | ALA |MO | 4-CP | (~96.5%) | 4

MB (98.81%) | BPA (97.27%) |12

2,4-di-CP (95.4%) | BPA (91.4%) | 4

MB (55.81%) | 5

MB | Rh B | MO | CR | 4-NP | TC | ~90% (OP) | 10

4-NP (96%) |TOC (55.4%) | 5

Solution evolved (% degradation) | Reusable cycles

Table 1.2  Photo-Fenton (PF)/Photo-Fenton–like (pF) catalyst as FNMs. (Continued)

[100]

OOH | •O2− | activators | n → π*| π → π* - transition

[103]

Contributors h+ | 1O2 |•OH | •O2− |

(Continued)

[102]

OH > •O2− >>h+ | Major part in degradation •

[101]

Rate of reaction • OH > h+ > •O2− | bandgap (2.10 eV)

•

[99]

[98]

CB favors e transfer Fe3+ à Fe2+ + e− H2O2 + e− à 2 •OH • 2− • O | OH (Scavengers) | pH (low) efficient

Ref. −

Remarks

FNM-Based Catalytic Materials  17

GMSA

Facile methodHummer’s

Hydrolysis

Co-precipitation

Sol-gel

Calcination

Solvothermal

3D FeO (OH)-rGA | PF | 2018

CQDs/α-FeOOH | PF |2020

Fe3O4 (MPs)/(HA) Humic acid | pF | 2020

Fe3O4@void@TiO2 | pF | 2017

FeCu@Fe2O3-g-C3N4 | pF | 2020

Fe3O4@MIL-100w | pF | 2015

Process

QDs-Fe/G | NRsFe/G | NSs-Fe/G | PF | 2015

FNMs as catalyst | Year

Visible light

Visible light

UV light

Sunlight

Visible light < 420 nm

Visible light

Visible light

Irradiation Source |

pH (3–6.5) | 120 min

pH (3–11) | 6 h

pH (3) | variable

pH ( 400 nm

Visible light

pH (5–9) | 300 min

pH (2–8) | 120 min

pH (3.82–8.99) | (variable) min.

UV light

White light

Visible light

Irradiation |

Parametric expressions

MB |HerbicideDinoseb | 95% |5

MB (50%)

BPA (99.9%) | OFX (99.8%)

OFX | (99%) | 6

TC (84%) | Cr (VI) (100%) | 5

Rh B | MB | 5

Probe | Solution evolved | Reusable cycles

Contributors •OH

VB (2.34V) | CB (-0.86V) Supporters: • OH, e−, h+, •O2−

Activators •O2−, •OH, • OH + H+ + e − à H2O Co3+/Co2+: redox mode

Reactive (•O2− |•OH), rGO (−4.42 eV) | ZnO (−4.05 eV)

Attributors •OH, •O2−| π-conjugation

(Continued)

[190]

[189]

[188]

[187]

[186]

[185]

Contributors O | π - π interaction 2−

Ref. •

Remarks

20  Functionalized Nanomaterials for Catalytic Application

Ultrasonic

UV | Visible

Solvothermal

Sol-gel

Ion exchange | Impregnation

Electrospinning | Thermal redox

Sol-gel

Sol-gel | Hydrothermal

ZnO/G/TiO2 (ZGT) | 2016

Zn-TCPP/Ag-TiO2 | 2016

Bi2S3/TiO2-Mt | 2018

3D-g-C3N4-NS/ TiO2-NF | 2018

FeNi3@SiO2@TiO2 | 2018

Co3O4/TiO2/GO/ amine | 2017 Solar | Visible light

UV-light

Visible light >420nm

Near UV– Vis-light

Sun light | Visible light

Sol-gel | WetImpregnation

Ni/TiO2 | Ru/TiO2 | 2019 |

Irradiation |

Process

FNMs as catalyst | Year | Scheme

Table 1.3  Photocatalyst (PC) as FNMs. (Continued)

- | 90 min |

pH (9) | 200 min

- | 120 min

pH (11) | 120 min

-| 30 min

pH (9) | 120 min

pH (natural)| 330 min (IMI) | 480 min (ATP, TMX)

Parametric expressions

OTC (91%) |CR (91%) |5

TC (100%) | 5

Rh B (~93%) | 3

KP (80%) | -

MB | PNP | 4

MB | TBAC-L | TBBU-L TNFW-L |4

IMI | ATP | TMX | 100% (Insecticides)

Probe | Solution evolved | Reusable cycles

Contributors e−CB, h+VB, • OH, •O2−

Contributor •OH |pseudo-1st-order |

[197]

[196]

[195]

[194]

[193]

[192]

[191]

Ref.

(Continued)

Activator •O2− | pseudo1st-order |

Langmuir-Hinshelwood | pseudo-1st-order | • 2− • O | OH

Enhancer-Porphyrin | 3.28 eV (bandgap)

Contributors: •O2−, •OH | π - π interaction

Solar (UV) > visible | Degraded products CO2 | H2O | •OH

Remarks

FNM-Based Catalytic Materials  21

Hydrothermal

Self-assembly | Reduction

Hydrothermal | Adsorptionpolymerization

Green synthesis | Reduction

Solvothermal precipitation

Hydrothermal

Hydrothermal

3D RGO-based hydrogel

CQDs/TCNCs

PM-CQD/TiO2 (M: Ag/Au)

BiVO4/N-CQDs/ Ag3PO4

ZnO/N, S-CQDs | (Capping agent: (Try/Glu))

3D CQDs/TiO2/GO - PUF

Process

CuInS2/Bi2WO6 | 2019

FNMs as catalyst | Year | Scheme

Visible light

Visible |near-IR | Sun-light

Visible light

- | 1–6 h

- | 20–180 min

- | 30 min | 90 min

- | 20 min | M: Ag 60 min | M: Au 108 min

pH (basic) | 30 min

Visible light

UV-VIS (MB) | Solar light (Drug)

pH (2/3/(acidic) | 120 min

| 120 min

Parametric expressions

UV-VIS

Visible light

Irradiation |

Table 1.3  Photocatalyst (PC) as FNMs. (Continued)

MB | RhB | 5

MB | RhB | MG | CEL | CIP (92.9%, 85.8%) |5

TC (88.9%) | TOC (59.8%) | 5

MB (~96%) | Erythromycin (~100%) | Several

TOC (22%) | CBZ (100 - 87.6%) | 4

NPX | IBP | DFC | (7080%) |

TCH (92.4%) | 4

Probe | Solution evolved | Reusable cycles

(Continued)

[204]

[203]

Activator: •O2− | pseudo-1st-order | Contributors: •O2− , •OH

[202]

[201]

[200]

[199]

O2− > h+ > •OH | pseudo1st-order | pseudo-2ndorder | Z-scheme •

Superoxide •O2−, | pseudo-1st-order

5 times > pure TCN | Fukui index (fo = 0.108) olefinic (double bond)

Langmuir | Freundlich | π-π interaction | pseudo-2nd-order

[198]

Activators: O , OH | Z-scheme 2− •

Ref. •

Remarks

22  Functionalized Nanomaterials for Catalytic Application

- | 120 min

pH (10.5) | 90 min pH (8.0) | 90 min

Visible light

UV

UV

Sonochemical | Grafting

Molecular fusion | Hydrothermal

Sono assisted | Chemical precipitation

Chemical precipitation

f-MWCNTS-CdS QDs | f-MWCNTS-Ag2S QDs | (Capping agent - PAMAM)

NH2-functionalized GQD-TiO2

M-ZnS QDs (M: Cu2+, Mn2+, Ag+)

ZnS: Fe QDs (Capping agent: 2-ME)

UV

- | 1h

11 | 90 min

UV light

Sonochemical | Chemical precipitation

ZnS QD (Capping agent: (L-Cys/2-ME))

- | 120 min

Sun light

Green Synthesis

r-Mg-N-CD

Irradiation |

Process

Parametric expressions

FNMs as catalyst | Year | Scheme

Table 1.3  Photocatalyst (PC) as FNMs. (Continued)

MG (98%) | 6

VBR (> 95%) | 6

MO (96.0%) | 4

MO | -

CV (98.5%, 97.0%) | 5

MB (99.1%) | 4

Probe | Solution evolved | Reusable cycles O2−, h+, •OH | Langmuir Hinshelwood| 1st - order

Langmuir-Hinshelwood | pseudo-1st-order

Langmuir-Hinshelwood | pseudo-1st-order

Contributors: h+ (44.7%), • 2− O (38.0%,), •OH (17.3%)

Langmuir-Hinshelwood | 1st-order rate constant

Langmuir-Hinshelwood | pseudo-1st-order

•

Remarks

[210]

[209]

[208]

[207]

[206]

[205]

Ref.

FNM-Based Catalytic Materials  23

24  Functionalized Nanomaterials for Catalytic Application Pollutants

•O2– Pollutants

O2

CB

CO2

s

e–

M

e–

FN

e–

+

H2O

Mineralization

h+

h+

VB

•OH

Photo Reduction

2–

•O •OH

H2O

H 2O

h+

h+

O2

Photo Oxidation

Figure 1.5  Photocatalytic action with plausible mechanism.

with single carbon derivative as PC can be overcome by FNMs when employed for water treatment [127]. Photocatalytic activity of NMs is deprived due to reconfiguration of photoelectron. Loss of photoelectrons can be reduced by composites and functionalization of NMs (Pd, Pt, Au, Ag, ZnO, ZnSe, ZnS, Cds, SnO2, TiO2, and Fe2O3) [128]. Other remarkable introductions to conserve the water bodies using PCs as bifurcated units have been pinned up in segments to come. Similar trials have been unfolded in Table 1.3. Photocatalytic action with plausible mechanism is portrayed in Figure 1.5.

1.5.1 Carbon-Based FNMs as Photocatalysts 1.5.1.1 CNT-Based FNMs Van der Waals forces, hydrophobicity, and a very poor solubility lead to agglomeration of the NMs. Sometimes, impurities present deliver a poor degrading efficiency of these materials. Surface alteration successfully resolves the problem faced to get better achievements. Functionalization divided into two main segments (1) covalent and (2) noncovalent alters the photocatalytic deliverables when dissimilar functional modules are associated with it. Rh B, an OP, was photocatalytically degraded using visible (>420 nm) light, by the heterostructure Au NPs @ POM-CNTs at room temperature. This tricomponent had 45% degradability [129]. In one of

FNM-Based Catalytic Materials  25 their trials, the authors Xu, Y. et al. witnessed that 0.1% of CNT/LaVO4 FNM could effectively photocatalytically degrade TC (81%) antibiotic that caused harmful effects to water segments [130]. In a similar manner, ZnO/NiO coated MWCNTs photocatalytically degraded (azo dye) MO efficiently, where a comparative study was done by the authors at different combinations of the trio component, at pH 7, for 6 h, using UV (280 nm) and visible radiation (480 nm) [131]. Exposure of hydrothermally formed TiO2-NRs/CNTs/braced with FeCo-Al2O3 catalytic agent was a potent remover of MB (97.5%), when the medium was photocatalytically degraded by natural sunlight, was later proved for its sustainability up to six trials with lesser doses in kinetic runs [132]. In a different situation, functionalized MWCNTs/TiO2 got by sol-gel method as per the reporters of El-Sayed, B.A. et al. had a versatile photocatalytic decomposition under variable conditions of sunlight, UV, and Xenon light-irradiation. Later, these were proven for their activity over the textile dye effluents Vat Green dyes and Dianix Blue dye in different trial runs [133].

1.5.1.2 Fullerene-Based FNMs FNMs of fullerene with exceptional features to modify the strength, stability, photocatalytic ability, and electron (e−) affinity have made attractions as nanocatalyst for photo-degradation of contaminants in water sources [134]. Strong absorptions were seen in the UV belt, while moderate absorptions are seen in visible, functionalization enhances the behavior of electron transference required for photocatalytic action [135]. New novel FNM (covalently bonded) Zn-Porphyrin functionalized/TiO2 synthesized was used to degrade the contaminants MB and phenol, where •O2−, •OH and •OOH may participate in the reaction kinetics. Appropriate small band difference between FNM’s LUMO and TiO2’s CB that leads to the electron transference was the cause for degradation according to authors Regulska, E. et al. [136]. In another situation, the authors Chai, B. et al. degraded Rh B by fullerene modified nanocomposites of g-C3N4 (C60/g-C3N4) photocatalytically, where the efficiency was because of charge separation of photo e− electrons and h+ holes in the FNM [137]. In the same way, poly-hydroxy fullerenes (PHF)/TiO2 as PC were found to be effective for decomposition Procion (a red-dye). The molecular interactions between the two were a simple electrostatic force that enabled the composite to undergo surface modification [138]. Fullerene FNM with silanes was proved for its effective photocatalytic activity on phenols for oxidative decomposition [139]. C70TiO2 hybridized FNMs were proved stable for its photocatalytic behavior

26  Functionalized Nanomaterials for Catalytic Application for five recycle runs while decomposing sulfathiazole (sulfonamides drug) a powerful antibiotic [140].

1.5.1.3 Graphene (G)/Graphene Oxide (GO)–Based FNMs The supply of graphene/graphene oxide–based functionalization resolves the constraints delivered by metal oxide PCs. Of late, attentions are focused on FNMs of graphene/graphene oxide–based semi-­conductor materials as functionalized PC due to their smaller size with larger specific surface area supported by high electron (e−) conductivity and high adsorption capacity [141]. Advanced research work has been augmented on MO-G/GO FMNs photocatalytic systems [142] for oxidativereduction of pollutants (BG) [143] and (MB) [144]. The unstable and aggregation tendency of the NMs are retarded by the advantages raised due to FNMs. In one of their studies, researchers Rao, G. et al. synthesized TiO2-NW/ Fe2O3-NP/GO FNM sheets by colloid-blending scheme, where the material was found to have 93% efficacy in getting rid of humic acid from water photocatalytically at a pH 6. TiO2 furnished h+ required for •OH and GO the e− needed for •O2− needed for the activity [145]. GO/MCU-C3N4/PVDF materials synthesized by vacuumized self-assembly and cross-linking process had exceptional self-cleaning property, which was proven fit for separating oil-in-water colloidal emulsions. Photocatalytic degrading capabilities were attributed to the e− transferences from CB (1.61 eV) to VB (1.18 eV), with π-π* transition giving h+ in VB. h+, •O2−, and •OH were controllers in the reaction for eradicating oil-foulants as observed by the researchers Shi, Y. et al. [146]. Scientific workers Gnanamoorthy, G. et al. synthesized AF-Bi2Sn2O7/ rGO (AF-amine functionalized) FNMs for photocatalytic degradation of organic dye MB in the visible region was 75% (20 min). Bandgap between pure (2.6 eV) and FNMs (1.6 eV) decreased. VB with h+ and CB with e− that favored the reaction were supported by the formation of radicals •O2− and • OH. Stability and reusability of FNMs were persistent up to four cycles [147]. In one of their methods, the authors Liu, H. et al. used FNMs of Bi2Sn2O7/RGO to reduce and degrade Rh B and phenol photocatalytically in the bright visible region (420 nm) and noticed that the degrading efficiencies were 95.8% (125 min) and 81.1% (200 min) for Rh B and phenol, respectively. On embedding RGO on Bi2Sn2O7 (pure), they observed that there was a decrease in the bandgap from 2.48 eV (pure) to 1.85 eV FNM which served well for degrading the contaminant, where the active radicals involved for the reaction was h+ and •OH [148].

FNM-Based Catalytic Materials  27

1.5.1.4 Graphene-Carbon Nitride/Metal or Metalloid Oxide–Based FNMs Recently, conjugation of C and N in a metal-free graphitic polymer is a hotspot that captivates the research workers to utilize the visible energy for the receptive photocatalytic zone in redemption of water pollutants [149]. Normally, hetero-junctions of g-C3N4–based PC are obtained by fusing g-C3N4 (semiconductor) PC and a co-catalyst (semiconductor). Significantly, type II hetero-junction and Z-scheme PC are predominantly employed by many co-workers for removing OPs. Z-scheme have been extensively utilized in BiOI/Pt/g-C3N4 [150], MoO3/g-C3N4 [151], g-C3N4/FeWO4 [152], g-C3N4/Ag/MoS2 [153], TiO2/g-C3N4 [154], and g-C3N4/Ag/Ag3VO4 [155]. While, straddling, staggered, and broken heterojunctions belonging to type 1, type 2, or type 3, with a small/large bandgap between CB and VB/or CB and VB with high potentials, are used in ZnO/g-C3N4 [156], Bi/Bi2WO6/g-C3N4 | Bi/Bi2MoO6/g-C3N4 [157], SmVO4/g-C3N4 [158], g-C3N4/CuWO4 [159], and BiVO4/g-C3N4 [160]. Thus, many FNMs have been used in fabrication, to name a few for the removal of organic toxics like MB, MO, Rh B, fuchsin, and X3B form water segments. Li, H. et al., fabricated WO3/Cu/g-C3N4 nanohybrids to degrade 4-­nonylphenol [161]. While, Yang, Y. et al. used Ag@AgBr/g-C3N4 FNMs as nano-composites to degrade MO [162]. Similarly, the authors Fu, J., et al., in their recent publication of CdS/g-C3N4, demonstrated a comparable output in enhancement-factor as 20.5 and 3.1 for dye-degradation of MO while using the composites of two active semiconductors g-C3N4 and CdS individually [163]. Later, in another experiment, the authors Yang, Y. et al. investigated SPR results of Ag NMs while studying the performance of Ag-coated-g-C3N4 over MO dye-degradation [164]. In another situation, researchers Ma, D. et al. revealed that g-C3N4/RGO/Bi2WO6 FNMs that fit the Z-scheme had RGO as a bridge to transfer the e− electrons between the two bands g-C3N4 and Bi2WO6. The photoelectrons formed in the CB of the later Bi2WO6 moves rapidly into the VB of the former g-C3N4 (holes) to accumulate sufficient (e−) electrons in the CB of the former and holes of VB in the later. FNMs were found effective to photocatalytically degrade and remove TCP from water [165]. In a separate work, Jiang, Z. et al. engineered TiO2/g-C3N4 by solvothermal method and proved its photocatalytic degrading properties over Rh B, MB, and CIP. H+ and superoxide •O2− had significant role over •OH radical in this reaction. Excitonic PL signals indicated that n-π* electronic shifts were involved by lone pairs e− present in N atoms of g-C3N4. Hetero

28  Functionalized Nanomaterials for Catalytic Application yolk-shell structure formed significantly promoted the charge transference efficacy [166]. In a new protocol, facile magnetic g-C3N4/Fe3O4/p-Ru NP FNMs photo-nano catalyst got by deposition-precipitation process showed excellent degradation capacity and reusability with only 5% efficacy lost detected after five cycles. Photocatalysts degraded organic matters— aromatic amines and coloring pigments—azo dyes (CR, CB, EB, and RR-120) efficiently from industrial aqueous water. Formation of photoelectron creates h+ (holes), where the reactive •OH formed induces a responsible oxidative photo-degradation and h+ (holes)/•O2− radicals have insignificant roles [167].

1.5.1.5 Graphene-Carbon Nitride/QD-Based FNMs Hydrothermally synthesized BWO fixed as ultrathin Bi2WO6 NSs embedded on g-C3N4 QDs as (CNQDs/BWO), belonging to Z-scheme, efficiently degraded organic contaminants of antibiotic TC and dye Rh B, with % efficacy of 92.51 and 87 in NIR and visible regions, in ~1 h. LangmuirHinshelwood model adopted by the authors Zhang, M. et al. later showed that the bandgap energy of 2.70 eV (BWO) and 2.60 eV (CNQDs) was sufficient to bring the change [168]. The authors Zhou, L. et al., proved that GCNQD-treasured on modified g-C3N4 had a worthy photocatalytic degrading activity against organic Rh B [169]. The experimentalists Lin, X. et al. observed that hydrothermally synthesized nano-heterostructures of CNQDs/InVO4/BiVO4 on a leaf-like material of InVO4/BiVO4 had •O2− radical as the main force behind the efficient oxidative-degradation of Rh B organic dye [170]. Similarly, heterostructure GCNQDs/Ag/Bi2MoO6 NSs had a very good 100% degrading capability of Rh B in visible region, where h+ of VB (Bi2MoO6) and e− of CB (CNQDs) worked effectively for oxidation/ reduction to cause degrading reaction to give H2O and CO2 [171]. Si NWs (silicon nano-wires) on g-C3N4 QDs as Si NWs @ g-CNQDs, photoelectrocatalytically could decompose 85.1% of 4-CP in ~ 2 h from aqueous solution, had a notable charge separation and good stability [172]. π-conjugated GCNQDs implanted on metalloid sulfide Sb2S3/supported by ultrathin-g-C3N4, with a bandgap of 2.7 eV, were proved to be good candidate for photocatalytic disposals of MO from unwanted water and had a very good electron (e−) transference [173]. In a novel approach, the co-workers Patel, J. et al. synthesized Mn:ZnS/QDs, for photo-­ degrading fluoroquinolone: Norfloxacin in an ambient condition of solarlight/UV-light, where Mn and •OH fortified the reaction to 4 reapplied cycles [174].

FNM-Based Catalytic Materials  29

1.5.2 Polymer Composite–Based FNMs as Photocatalysts Polymer TiO2/CS/glass FNMs were powerful in decomposing RR4 organic dye in visible region. h+ and •OH generated from TiO2 layer circulate to TiO2/CS boundary to cause oxidation of RR4. A total of 100% efficiency was noticed with the stability up to seven reusable cycles [175]. CdS/TiO2-PAN FNM degraded MB (66.29%) in 210 min [176]. Researchers studied the photocatalytic action and inferred a repeated utility to protect the water system. Chitosan-AgCl/Ag/TiO2 synthesized by the team Jbeli, A. et al. was reported to be cost-effective photocatalytic degrader of organic components ABA, O-TD, and SA under visible radiations [177]. Similarly, surface modified FNM TiO2/ZnO/chitosan had a powerful photo-degradability of MO (97%) when excited by solar radiations [178]. An organic/inorganic FNM as composites P3HT/PNP-Au NP got by re-precipitation method showed positive spectral line in UV region (~427 nm), had an enhanced photocatalytic decomposition of MB (90.6%), and inferred that it may be due to a strong π-π* shift [179]. A 3D honey-comb like ordered macro-­porous NM-3DOM Ag/ZrO2 had significant photocatalytic degradability over CR when stimulated by multi-modules of microwave-assisted, simulated-solar, UV, and visible radiation [180].

1.5.3 Metal/Metal Oxide-Based FNMs as Photocatalysts Metal/Metal oxide when entrapped as FNMs, on photo-irradiation leads to photo-excitation of particles that undergoes transference between the CB and VB, where the CB transfers the e− to degrade the pollutant [181]. Components like (1) semiconductor, (2) metal, and (3) metal-­supporters assist interfacial-interaction for photocatalytic degradative actions. In one of their reports, Park, S.J. et al., proved that T-ZnO-CNO FNMs with nano-onions prepared by green routes removed the challenging pollutant of water 2,4-DNP photocatalytically with an efficacy of ~92%. Conversion of O2 to •O2− and formation of •OH favored the degradation using 3D hybrid structures [182]. Degradation of phenol (63%) and MB (52%) at 420 nm (visible) by FNMs of Au/(WO3/TiO2) and WO3/ TiO2 were noticed by Rhaman, Md. et al. [183]. Electron transference is retarded but hole movement is favored due to Au embedded on the surface of WO3/TiO2. Sufficient bandgap energies cause photocatalytic excitations. Flower-like functionalized Au-ZnO NMs hydrothermally were responsible contributors for photo-decomposition of Rh B into CO2 and H2O, with h+(holes) and •OH formed by light radiations were functional

30  Functionalized Nanomaterials for Catalytic Application for the activity. A total of 99% reduction in 10 min was noticed by authors Hussain, M. et al. [184].

1.6 Nanocatalyst Antimicrobials as FNMs FNMs as nanocatalyst have been authenticated with a promising note for cleansing and sanitization treatment for a mixture of waste and normal water from different sources. With an excellent potentiality to inactivate the active disease-causing dreadful pathogenic micro-organisms like fungi, bacteria, and viruses, FNMs behold their role to safeguard the water bodies. Increase in the potential momentum for anti-microbial activity is efficaciously observed by surface alterations of NPs [211]. Functionalized photocatalytic materials are appropriate tools for curbing the unwanted horrible [212]. Disinfection rate of pseudo-1st-order rate kinetics for retarding activation of E. coli was observed to be 86% in 4 h by FNMs (C60/C3N4 | C70/C3N4) on photocatalytic treatment [213]. Similarly, 2D g-C3N4 NS/PAN hydrophilic filter membranes, demonstrated for excellent self-cleaning and anti-bactericidal action over E. coli, by photocatalytic visible light were proven to have 100% sterilization efficiency at optimized (12 h) condition with a restoration capacity to about three rescalable cycles [214]. The existence of the hindering polymeric substances on pathogens that retard the anti-microbial photocatalytic efficacy is to be negated for water disinfections, in a momentous scale [215]. Visible light illuminations are powerful in inhibiting the bactericidal action by the photo-generated peroxides from g-C3N4 NS [216]. In an anti-bacterial work, the co-workers defied the potent E. coli, by Ag/TiO2 film, using photocatalytic fluorescent light radiation for 3 h. A total of 81% deactivation was observed, due to the release of Ag+ ion, and the trapped photo-induced e−, h+, that diffuse on the catalytic surface to inhibit the activity of the target species [217]. Hence, the cell wall membrane of bacteria is disrupted by the photo-generated e− leading to reductive cell damage thereby deactivating the bacterial activity. gC3N4-BiFeO3-Cu2O FMNs by photocatalytic decomposition using visible light suppressed the activity of the bacterial stains, S. aureus (G+) and E. coli (G−) in ambient reaction condition [218]. In some reverse trials, the bacterial E. coli affecting the water bodies when functionalized with TiO2 as cocktails promoted the deactivation of the bacterial E. coli and S. epidermidis [219]. Abundant trials have made their way to upsurge the usefulness of FNMs as nanocatalyst, as anti-microbials by optimized advancement with molecular modifications with resourceful materials.

FNM-Based Catalytic Materials  31

1.7 Conclusions and Future Perspectives In this existing scenario, presently, the whole world is confronting a critical water challenge. With a greater reason, persevering and safeguarding the prevailing fresh natural water reserves and new water segments is an immediate need, to overcome this global challenge faced. Reports of WHO (water hygiene and sanitation) infer that 1,870,998 people (WHO 2019), especially below the age group of 5 [(361,000 deaths) (WHO 2014)], die every year, due to unsafe water supplies, caused by contaminations in aquatics [220]. In a broader perspective, nanotechnology affords better promises to solve the existing crisis, ensuring a good quality potable/consumable water by eliminating the undesirable harmful biological and chemical contaminants from water bodies. The salient features that make the NMs occupy a highest platform in the environmental remediation methodologies are (1) a greater ratio of surface:volume, (2) high stability and reactivity, (3) assorted morphological shapes and sizes, and (4) good reusability. Henceforth, NMs find their way to detect and eliminate the unmanageable contaminants (toxic gases, noxious heavy metals, toxin organics, and undesirable biological components) form the water bodies in an effectual way [221]. Similarly, the risks to human health caused by contaminants in food and water (pesticides, metallic debris, industrial wastes, dyes, and drugs) can be well curtailed, when detected with the help of NMs. NMs (FNMs: QDs, CQDS, CNQDs, MWCNTs, and MNPs) as sensors or biosensors, are found to be self-reliant, efficient, and cost-effective. The noteworthy exceptional thermal and chemical, mechanical, and electro-optical features render NMs to function as a trouble-free, inexpensive, and rapid tool in detecting the toxins present in food and water [222]. Analogously, various environmental methodologies (nano-photo catalysis, membrane process (nanofiltration), nano-adsorption, and nano-­ sensing) are employed to protect the water system [20]. Nanocatalyst with advantageous modes of coordination like •radical reactions, e− transfer, redox (photo), atomic/molecular interaction (π-π, electro-static, H-bond, and Van der Waals), ligand transference, and adsorption aids in supporting the nanocatalytic activities [223]. Functionalization offers an improved resolution to tackle the hitch delivered by simple NMs. FNMs as nanocatalysts like membranes, adsorbents, and electrodes, utilizing the natural resource of solar light/electrical energy, help in degrading the TP, thus, ultimately in overcoming the bottle-neck faced while using the traditional products. Thus, effective functionalization

32  Functionalized Nanomaterials for Catalytic Application can be achieved by adopting numerous options available based upon the requirement of the situation. Direct, post-synthetic, co-condensation, grafting (polymers), physisorption (non-covalent), chemisorption (covalent), surface oxidation, doping heteroatoms, alkali activation, sulfonation, polymer coating, and many more. Hence, delivering an improved shift in significant properties like chemical compatibility, wettability, texture with augmented unique surface-area/pore-volume, absorptivity, and enhanced mechanical-strength [20]. Every new technological advancement has its own significant demerits and merits for toxin elimination. The problems confronted in effluent-water or in other water management systems by NMs are significantly important. However, most of them primarily are only transient (trials conducted or financial). It is anticipated that new novel advancements in nanotechnology by cautious management evades unintended outcomes and can incessantly deliver a robust output in effluent-water or in other water management. The cost of time and money, energy consumptions, and compatibility of the subsisting substructure will govern the full-scale utilization [224]. As a wrap up, application of different types of FNMs used for remediation of TPs like organics (dyes, drugs, pesticides, health care, and endocrine disruptors) from (industrial/pharmaceutical) effluents, and agricultural debris/biological/inorganics (heavy-metals and carcinogenic components) have been discussed and summarized, in reference to optimization requirement and key role enhancers. FNMs as nanocatalyst are segmented and reviewed as EC, EFC, PF catalyst, PCs, and anti-microbials, with various informative trials delivered in the recent past for protecting the water resources. FNMs supported on NRs, NFs, NWs, NTs, NPs, and NSs, of metal/metal-oxides, MNPs, carbonaceous (G/GO/CNT/fullerenes/g-C3N4), QDs, CQDs, nitrides, and natural/ synthetic polymers (chitosan) as some essentials have delivered a non-toxic, less-energy, cost-effective, and multi reusable products, for sustainability. Economic deliverables are noticed while using functionalized membrane/ adsorbents that offer a better solution and multi-reusability to get rid of the contaminants like organismal (protozoa, bacteria, viruses, fungi), organic material, colloidal or suspended micro/macro solids, and metallic/nonmetallic present. The potential resistance of bacterial toward broadspectrum antibiotics lowers the intensity and increases the quantity of the drugs to be administered. Similarly, the useful bacteria in the aquatic system are devoid of their potency by these drugs that pollute the water system. Ongoing challenge for scientific community is to develop and produce competent components with high mechanical and electronic properties,

FNM-Based Catalytic Materials  33 economical, bio-degradable and eco-friendly, not for a single platform but, for a large-scale wider application. The cross-intermingling interaction between the base-hosts and supporters as surface modifiers is responsible for high stability and potentiality for field submissions for natural water/ waste effluent water management are some noteworthy points in developing and utilizing the FNMs. With refined intrinsic characteristics, reusability and economical profits, FNMs are found to exhibit a vivid scope in water resource management. FNMs, although economical, should be tailored in a better way with ecological and health hazard concerns. Additionally, engineering and commercialization pertaining to market potentiality, with regulations and co-ordinating policies, need to be altered for eco-friendly approaches.

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48  Functionalized Nanomaterials for Catalytic Application 179. Jana, B., Bhattacharyya, S., Patra, A., Conjugated polymer P3HT-Au hybrid nanostructures for enhancing photocatalytic activity. Phys. Chem. Chem. Phys., 17, 15392–15399, 2015. 180. Zhang, J., Li, L., Wang, S., Huang, T., Hao, Y., Qi, Y., Multi-mode photocatalytic degradation and photocatalytic hydrogen evolution of honeycomb-like three-dimensionally ordered macroporous composite Ag/ZrO2. RSC Adv., 6, 13991–14001, 2016. 181. Ray, C. and Pal, T., Recent advances of metal-metal oxide nanocomposites and their tailored nanostructures in numerous catalytic applications. J. Mater. Chem. A, 5, 9465–9487, 2017. 182. Park, S.J., Das, G.S., Schütt, F., Adelung, R., Mishra, Y.K., Tripathi, K.M., Kim, T.Y., Visible-light photocatalysis by carbon-nano-onion-functionalized ZnO tetrapods: degradation of 2,4-dinitrophenol and a plant-model-based ecological assessment. NPG Asia Materials, 11, 8, 2019. 183. Rhaman, Md. M., Ganguli, S., Bera, S., Rawal, S.B., Chakraborty, A.K., Visible-light responsive novel WO3/TiO2 and Au loaded WO3/TiO2 nanocomposite and wastewater remediation: mechanistic inside and photocatalysis pathway. J. Water Process Eng., 36, 101256, 2020. 184. Hussain, M., Sun, H., Karim, S., Nisar, A., Khan, M., Haq, A.U., Iqbal, M., Ahmad, M., Noble metal nanoparticle-functionalized ZnO nanoflowers for photocatalytic degradation of RhB dye and electrochemical sensing of hydrogen peroxide. J. Nanopart. Res., 18, 95, 2016. 185. Zhao, Y., Zhang, Y., Liu, A., Wei, Z., Liu, S., Construction of three-­ dimensional hemin-functionalized graphene hydrogel with high mechanical stability and adsorption capacity for enhancing photodegradation of methylene blue. ACS Appl. Mater. Inter., 9, 4006–4014, 2017. 186. Chen, D.-D., Yi, X.-H., Zhao, C., Fu, H., Wang, P., Wang, C.-C., Polyaniline modified MIL-100(Fe) for enhanced photocatalytic Cr (VI) reduction and tetracycline degradation under white light. Chemosphere, 245, 125659, 2020. 187. Sharma, P., Kumar, N., Chauhan, R., Singh, V., Srivastava, V.C., Bhatnagar, R., Growth of hierarchical ZnO nano flower on large functionalized rGO sheet for superior photocatalytic mineralization of antibiotic. Chem. Eng. J., 392, 123746, 2020. 188. Lv, S.-W., Liu, J.-M., Zhao, N., Li, C.-Y., Wang, Z.-H., Wang, S., Benzothiadiazole functionalized Co-doped MIL-53-NH with electron deficient units for enhanced photocatalytic degradation of bisphenol A and ofloxacin under visible light. J. Hazard. Mater., 387, 122011, 2020. 189. Jia, Z., Lyu, F., Zhang, L.C., Zeng, S., Liang, S.X., Li, Y.Y., Lu, J., Pt nanoparticles decorated heterostructured g-C3N4/Bi2MoO6 microplates with highly enhanced photocatalytic activities under visible light. Sci. Rep., 9, 7636, 2019. 190. Alam, U., Fleisch, M., Kretschmer, I., Bahnemann, D., Muneer, M., One-step hydrothermal synthesis of Bi-TiO2 nanotube/graphene composites: an efficient photocatalyst for spectacular degradation of organic pollutants under visible light irradiation. Appl. Catal. B: Environ., 218, 758–769, 2017.

FNM-Based Catalytic Materials  49 191. Joice, J.A.I., Aishwarya, S., Sivakumar, T., Nano structured Ni and Ru impregnated TiO2 photocatalysts: synthesis, characterization and photocatalytic degradation of neonicotinoid insecticides. J. Nanosci. Nanotechnol., 19, 2575–2589, 2019. 192. Nuengmatcha, P., Chanthai, S., Mahachai, R., Oh, W.-C., Sonocatalytic performance of ZnO/graphene/TiO2 nanocomposite for degradation of dye pollutants (methylene blue, texbrite BAC-L, texbrite BBU-L and texbrite NFW-L) under ultrasonic irradiation. Dyes Pigm., 134, 487–497, 2016. 193. Rabbani, M., Bathaee, H., Rahimi, R., Maleki, A., Photocatalytic degradation of p-nitrophenol and methylene blue using Zn-TCPP/Ag doped mesoporous TiO2 under UV and visible light irradiation. Desalination Water Treat., 57, 53, 1–9, 2016. 194. Djouadi, L., Khalaf, H., Boukhatem, H., Boutoumi, H., Kezzime, A., Santaballa, J.A., Canle, M., Degradation of aqueous ketoprofen by heterogeneous photocatalysis using Bi2S3/TiO2-Montmorillonite nanocomposites under simulated solar irradiation. Appl. Clay Sci., 166, 27–37, 2018. 195. Zhou, X., Shao, C., Li, X., Wang, X., Guo, X., Liu, Y., Three dimensional hierarchical heterostructures of g-C3N4 nanosheets/TiO2 nanofibers: controllable growth via gas-solid reaction and enhanced photocatalytic activity under visible light. J. Hazard. Mater., 344, 113–122, 2018. 196. Khodadadi, M., Ehrampoush, M.H., Ghaneian, M.T., Allahresani, A., Mahvi, A.H., Synthesis and characterizations of FeNi3@SiO2@TiO2 nanocomposite and its application in photo- catalytic degradation of tetracycline in simulated wastewater. J. Mol. Liq., 255, 224–232, 2018. 197. Jo, W.-K., Kumar, S., Isaacs, M.A., Lee, A.F., Karthikeyan, S., Cobalt promoted TiO2/GO for the photocatalytic degradation of oxytetracycline and Congo Red. Appl. Catal. B: Environ., 201, 159–168, 2017. 198. Lu, X., Che, W., Hu, X., Wang, Y., Zhang, A., Deng, F., Luo, S., Dionysiou, D.D., The facile fabrication of novel visible-light-driven Z-scheme CuInS2/ Bi2WO6 heterojunction with intimate interface contact by in situ hydrothermal growth strategy for extraordinary photocatalytic performance. Chem. Eng. J., 356, 819–829, 2019. 199. Umbreen, N., Sohni, S., Ahmad, I., Khattak, N.U., Gul, K., Self-assembled three-dimensional reduced graphene oxide-based hydrogel for highly efficient and facile removal of pharmaceutical compounds from aqueous solution. J. Colloid Interf. Sci., 527, 356–367, 2018. 200. Zhao, C., Liao, Z., Liu, W., Liu, F., Ye, J., Liang, J., Li, Y., Carbon quantum dots modified tubular g-C3N4 with enhanced photocatalytic activity for carbamazepine elimination: mechanisms, degradation pathway and DFT calculation. J. Hazard. Mater., 381, 120957, 2020. 201. Thakur, A., Kumar, P., Kaur, D., Devunuri, N., Sinha, R.K., Devi, P., TiO2 nanofibres decorated with green-synthesized PAu/Ag@CQDs for the efficient photocatalytic degradation of organic dyes and pharmaceutical drugs. RSC Adv., 10, 8941, 2020.

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2 Functionalized Nanomaterial (FNM)–Based Catalytic Materials for Energy Industry Amarpreet K. Bhatia1, Shippi Dewangan1, Ajaya K. Singh2* and Sónia. A.C. Carabineiro3 Department of Chemistry, Bhilai Mahila Mahavidyalaya, Bhilai, India Department of Chemistry, Govt. V. Y. T. PG. Autonomous College, Durg, India 3 LAQV-REQUIMTE, Department of Chemistry, NOVA School of Science and Technology, Universidade NOVA de Lisboa, Caparica, Portugal 1

2

Abstract

Industrial pollutants released from the energy industry are of great concern. They can be of various types: solid, liquid and gaseous. Gaseous/air pollutants include sulfur dioxide, carbon monoxide, carbon dioxide, particulates, and heavy metals mercury. Examples of liquid pollutants are heavy metals and metalloids, like arsenic, boron, mercury, and chromium, which affect the balance of the local ecosystems. Such liquid pollutants enter into our local environment and cause several problems. Thus, there is the need to remove them from wastewater. This chapter deals with functionalized nanomaterial (FNM)–based catalysts as new solutions for environmental problems. The FNMs are based on carbon, graphene, iron, gold, silver, etc. They have high surface area and a large amount of adsorption sites, which are very cost-effective for heavy metals removal through adsorption from industrial wastewater. FNM technology for remediation of water is the more suitable and useful procedure for the removal and separation of heavy metals. Keywords:  Energy industry, heavy metals, functionalized nanomaterials, adsorption, water remediation

*Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (53–88) © 2021 Scrivener Publishing LLC

53

54  Functionalized Nanomaterials for Catalytic Application

2.1 Introduction During the 20th century, several environmental problems like pollution, climate change and greenhouse effect have been faced by our world. They occur due to urbanization, industrialization and harmful contaminant gases, which affect the environment and human health [1]. Of all the industries, the energy industry is the most responsible for pollution. Energy industries are those involving the energy production and sale, including extraction, fabrication, refining and delivery of fuel. With the development of modern societies, the amount of consumption of fuels has also increased, in which the energy industry plays an important and crucial role as an infrastructure. The various energy industries are fossil fuels, such as petroleum, coal, natural gas, electrical and nuclear power, and renewable energy. Thus, energy industries are important contributors to pollution and environmental damage. In several countries, the main source of energy generation is from fossil fuels, those being major contributors to global warming and pollution [2]. With this industrialization, the topic of major concern is pollution, as large amounts of industrial solid, liquid and gaseous pollutants are released from these industries. Gaseous/air pollutants include carbon monoxide, carbon dioxide, sulfur dioxide, particulates, and heavy metals like mercury. Liquid pollutants include heavy metals and metalloids, like boron, arsenic, mercury, and chromium, which affect the balance of the local ecosystems. Because of human activities and natural processes, these heavy metals enter into surface and ground waters and pose a serious risk, due to their severe toxicity to aquatic and terrestrial life, including humans. Heavy metals are not biodegradable (as organics are) and have the tendency to gather in living organisms, being also toxic and carcinogenic [3]. Therefore, it is essential to reduce the level of these liquid pollutants in the environment. From past few years, researchers across the world have been trying to eliminate heavy metals from wastewater from energy industry. The various methods to remove heavy metals include adsorption, filtration, reduction, precipitation, ion exchange, and electrochemistry [4–10]. Adsorption is the most common of these methods and largely used for heavy metals removal. It is simple and has good pH tolerance, low cost, no undesirable formation of by-products, and high efficiency and selectivity. It does not need a large energy input and is the most efficient for wastewater treatment [11]. Nanomaterials are currently one of the advanced and innovative technologies in material science field, being used in our daily life, energy, and

FNM-Based Catalytic Materials  55 environment. Nanomaterials have sizes within the nanoscale (usually it from 1 to 100 nm). They can be in the form of nanoparticles, nanotubes, nanowires, and nanofibers [12–14]. They have larger adsorption ability for common water pollutants, compared to the bulk counterparts [15, 16]. In this chapter, we discuss different types of nanomaterials, synthetic approaches, its functionalization or surface modification to enhance its adsorption capacity, and their applications in the elimination of different heavy metals from energy industry effluents.

2.2 Different Types of Nanomaterials Nanomaterials have nanoscale size range, being synthetized by different procedures like mechanical, chemical, physical, and natural methods [17]. Based on dimension, structures of nanomaterials fall into four categories as shown in Figure 2.1 [18–20]: • • • •

Zero-dimensional, One-dimensional, Two-dimensional, and Three-dimensional nanomaterials.

2.2.1 Zero-Dimensional (0D) Nanostructures These nanomaterials show sizes below 100 nm, which comprise nano range dimensions. Spheres, cubes, nanorods, and polygons are common shapes Classification of Nanomaterials based on their Dimensions

Zero-D Nanostructures

1-D Nanostructures

2-D Nanostructures

3-D Nanostructures

Solid and hallow nanoparticles

Nanotubes, Nanofibers, Nanorods

Sheet or Layered Nanostructures

Polycrystals

Figure 2.1  A scheme of nanomaterials classification by dimension [18–20].

56  Functionalized Nanomaterials for Catalytic Application found in this category. Some examples are noble metal nanoparticles, holospheres, core-shell nanomaterials, metal/metal oxide nanoparticles, quantum dots, etc. [20].

2.2.2 One-Dimensional (1D) Nanostructures This type includes materials with two dimensions in the nanoscale and one dimension not in the nano range. They can be polymeric or ceramic, metallic, and crystalline or amorphous. Some common examples are nanorods, nanotubes, nanowires, filaments, fibers, etc. [20, 21].

2.2.3 Two-Dimensional (2D) Nanostructures This type consists materials with one dimension in the nano range and two dimensions above (>100 nm) the nano range. They can be single or multilayered, amorphous or crystalline, polymeric or ceramic, metallic, etc. Some examples are graphene, nanofibers, nanolayers, nanocoatings, nanowalls, nanostraws, nanosheets, etc. [22].

2.2.4 Three-Dimensional (3D) Nanostructures These are nanostructures with three dimensions above the nano range (100 nm). They are combinations of several nanocrystals and shaped in different directions. It includes fibers and nanotubes, foam and honeycomb, powdered nanoparticles, layered skeletons, and layered composites [18]. Carbon nanobuds, that are a combination of fullerenes and carbon nanotubes (CNTs), are another example [23].

2.3 Synthesis of Functionalized Nanomaterials Research in nanomaterials modification or functionalization is very important in the area of nanotechnology. Due to nanomaterials insolubility in several solvents and its rigid nature, their applications are limited. There is the need to modify their surface in order to achieve physical or chemical interactions with another material. Unmodified nano­particles create stable clusters given the intermolecular interactions, like dipole-dipole or van der Waals interactions, that prevent their dispersion in the solvent [24]. In the functionalization, various types of chemical species and functional groups can be created on the nanomaterials surface and produce certain active sites to fit the needs. Various characteristics like solubility,

FNM-Based Catalytic Materials  57 Different Approaches for Synthesis of Functionalized Nanomaterials

Chemical Methods

Acid Treatments Alkali Treatments Hydrogen Peroxide Treatments Treatment with Silane Coupling Agents Treatment with Metal Ethoxides/Alkoxides

Ligand Exchange Process

Grafting of Synthetic Polymers

Trioctylphosphine (TOP) Oleylamine (OAm) Oleic acid (OA) Polyvinyl alcohol (PVA) Polyvinyl pyrrolidone (PVP) Cetyl trimethyl ammonium bromide (CTAB) Ascorbic acid (AA)

Grafting-to, Grafting-from, Grafting-through

Miscellaneous Methods

In situ surface modification Adsorption of polymetric dispersants on the surface of nanomaterials

Figure 2.2  The nanomaterials functionalization processes [28].

interaction with other materials, and accumulation in solvents can be improved. Functionalized nanomaterials (FNMs) have a variety of applications in several areas given their remarkable surface interaction with other particles, compared to unmodified analogs [25, 26]. Functionalization of nanomaterials can be done by covalent or noncovalent interaction, like van der Waals, electrostatic, and H-bond interactions, through modification of the reaction temperature, solvent, and surfactants. Covalent bonds are formed by chemical reactions between the surface atoms found on the nanomaterials surface. This functionalization makes nanomaterials become hydrophilic and thus more soluble in organic solvents. For noncovalent interaction, the functionalization is made by hydrophobic interactions, π-π stacking, ionic bonds, or bondingbonds. Both electronic configuration and surface atom properties continue unchanged [27]. Functionalization can be done by ligand exchange, polymer grafting, or chemical methods, to enhance biocompatibility and solubility of nanomaterials, and add other active sites. Figure 2.2 shows the different approaches for the synthesis of FNMs [28].

2.3.1 Chemical Methods Functionalization of nanomaterials by chemical methods involves interface chemical reactions in solution. Functionalization by chemical treatment methods enhances solubility, reactivity, hydrophobic interaction, mechanical, and spectral properties compared to unmodified analogs. The most common treatments are acidic (that clean the oxides and contaminants of

58  Functionalized Nanomaterials for Catalytic Application the surface) alkaline, with hydrogen peroxide or heat [29] and with metal ethoxides/alkoxides and silane coupling agents [30].

2.3.2 Ligand Exchange Process In this process, polymers or organic molecules are utilized to stabilize the nanoparticles in solution and avoid cluster formation of nanomaterials. These organic molecules or polymers bind to the nanoparticles surface, which control their size and shape, and enhance their properties. Binding molecules are ligands, capping agents or surfactants, according to their role and composition. Examples of ligands are oleylamine, trioctylphosphine, polyvinyl pyrrolidone, polyvinyl alcohol, oleic acid, ascorbic acid, cetyl trimethyl ammonium bromide, etc. These materials coat the nanoparticles surface being an “organic armor,” repelling reactants approach to the surface. Ligands bond at the surface are interchanged with competitive interaction that extensively decorates the nanomaterials surface as needed [31].

2.3.3 Grafting of Synthetic Polymers This technique enhances the nanomaterials reactivity and varies the morphology of the surface. It can be performed in three different ways: graftingfrom, grafting-to, and grafting-through. In the first, chains of polymers are produced from a surface initiating site. In the second, a polymer functionalized at the end is fused directly on the nanomaterial surface. In the third, a monomer with low-molecular weight is copolymerized with an additional monomer at the nanomaterial surface [32]. Rong et al. [33] studied the grafting of polystyrene and polyacrylamide on Al2O3 [33]. Wang et al. [34] suggested photocatalytic based polymerization to obtain poly(methyl methacrylate)-grafted TiO2. With light, the polymer chains are directly grafted on the TiO2 nanoparticles surface in water.

2.3.4 Miscellaneous Methods Additional functionalization methods consist of nanomaterials surface modification during nanoparticle synthesis and polymeric dispersants adsorption. Other examples are organometallic complexes thermal decomposition, reverse micelle procedures, and polyol methods [30, 35, 36]. To avoid accumulation, surfactants or capping agents can be dissolved in the reaction medium. Conversely, cationic or anionic dispersants can be used to disperse hydrophilic nanoparticles in polar solvents. Dispersibility of nanoparticles improves through the repulsive steric forces produced by the

FNM-Based Catalytic Materials  59 dispersants, between the polymer chains, that improve the surface charge. Several polycarboxylic acids and respective salts can be used as anionic surfactants for dispersing diverse nanoparticles, like Al2O3, TiO2, and Fe2O3 [37–39].

2.4 Magnetic Nanoparticles Some examples are metal oxides, such as of Pd, Pt, Fe, Cu, Au, Ag, and Zn, with magnetic, semi-conductor, insulator, or metallic properties due to the electronic morphology. Appropriate and convenient methods were attempted to produce magnetic metal oxides, like Co3O4, NiO2, and Fe2O3 [40]. Some key factors influencing the magnetic properties of nanomaterials are chemical composition, size and shape of particles, degree and type of crystal lattice, and also configuration (for structures non-homogeneous). Catalytic and magnetic properties can be increased by modifying the features of nanoparticles like composition, size, structure, and shape [41, 42]. Such nanoparticles are non-toxic, efficiently recyclable, and have magnetic and catalytic properties with benefits of good separation by application of a magnetic external field [43]. Magnetic nanoparticles also have enormous potential as catalytic supports. Because of their magnetic behavior, they have been mentioned for many applications like removal of dyes and heavy metals, supercapacitors, biomedical, and anti-bacterial applications [44–46]. Nanoparticles with superparamagnetic strength comprise maghemite (Fe2O3) and magnetite (Fe3O4) [47]. Recently, magnetic nanoparticles like maghemite, magnetite, hematite, and ferrite colloids [48] have been used for water purification. Maghemite and magnetite are proficient adsorbents. Magnetite is also appropriate for heavy metals removal from water and can be removed and reused with the use of a magnet. However, bare magnetite is rusted in an acidic environment, and its magnetic strength and adsorption capability are reduced by accumulation of magnetic force. Thus, it is necessary to protect its surface to prevent accumulation. Oxides, biocompatible polymers and surfactants have been extensively used to enhance their stability [49].

2.4.1 Synthesis of Magnetic Nanoparticles Reasonably easy, cost-effective, and reproducible methods are needed for the synthesis of magnetic nanomaterials with defined shape and size

60  Functionalized Nanomaterials for Catalytic Application Iron salt (Fe2+, Fe3+)

Reducing agents Stirring Iron salts solution Stirring and magnetic separation

Iron oxide magnetic nanocomposites

Figure 2.3  Co-precipitation synthesis of magnetic iron oxide nanocomposite [17].

[39, 40]. The used methods can be physical (gas-phase deposition, electron beam lithography, aerosols, ball milling, pulsed laser ablation, and laserinduced pyrolysis), chemical (hydrothermal, solgel, aerosol and vapor phase, electrochemical, co-precipitation, sonochemical, and flow injection), and biological (microbial) [48, 50–53]. Co-precipitation has been applied for the synthesis of magnetic iron oxide nanoparticles by the mixture of ferrous and ferric ions in basic conditions. The characteristic features, like size, shape, composition, Fe2+/Fe3+ ratio, pH, reaction temperature, precursor salts, and medium ionic properties, determine the function and performance of magnetic iron oxide nanoparticles [54–56]. The basic approach for synthesis of magnetic iron oxide nanoparticles by co-precipitation is shown in Figure 2.3 [17]. Table 2.1 shows several studies of synthesis of magnetic nanoparticles by co-precipitation [57–73]. These approaches can be classified as aqueous and non-aqueous. For the first, water-soluble particles are produced, while in the second, only materials that dissolve in non-polar solvents. The aqueous method is the most appropriate in terms of sustainability and low cost [17].

2.4.2 Characterization of Magnetic Nanoparticles The different methods of magnetic nanoparticles characterization are shown in Figure 2.4. They include thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray adsorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) confocal micro X-ray fluorescence (μ-XRF) X-ray spectroscopy (EDS), Fourier transforms infrared spectroscopy (FE-SEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and adsorption of N2 at 177 K [54, 74].

8 and 18 Crystallite: 17 Crystallite:21 2,000 m2/g) carbon nanomaterial with a 2D hexagonal honeycomb sp2 carbon lattice, chemically inert, highly transparent, with large electron mobility, great elasticity, and thermal conductivity [92–95]. Because of their unique characteristics, graphene-based materials attracted attention for several applications, like transistors and sensors, in catalysis and treatment of pollutants [96–99].

2.5.1 Functionalization of Carbon Nanomaterials CNMs have good sorption capabilities, but also some limitations, as they are hydrophobic and poorly soluble in many solvents and thus cannot be used commercially for industrial wastewater effluents released from various energy industries. Surface modification allows to overcome those limitations. This is done by introduction of functional groups on the surface, which lead to the

66  Functionalized Nanomaterials for Catalytic Application formation of modified/functionalized CNMs that can act as catalysts to treat industrial wastewater [100]. Nanocomposites or nanohybrids of carbon with surface functionalities are attractive for environmental applications [101]. Some functional groups present on the carbon nanomaterials surface play an important part in adsorption. Thus, to increase the adsorptive capacities of carbon nanomaterials, work has been carried out dealing with further increasing the amount of already existing functional groups (like OH and COOH) or grafting non-existing groups (like SH and NH2) on the surface [102, 103]. The adsorption capacity can also be enhanced by coating with other nanoparticles. CNMs are usually large surface area, stable, and porous efficient adsorbents [104–106]. Thus, functionalization enhances the selectivity and solubility of CNMs, as it improves the interaction and affinity between the functional groups and the pollutants [107]. Functionalized CNMs are good pollutant adsorbents due to their hydrophilic properties. Functionalization can be done by covalent or non-covalent procedures. Figure 2.6 shows the CNTs usual functionalization methods. In the covalent method, changes on the surface depend on chemical reactions between C atoms and conjugation of surface hydrophilic molecules. Treatment of CNTs with strong oxidizing agents, like sulfuric acid, nitric acid, hydrochloric acid, and potassium permanganate generates O-containing groups, like −OH, COOH, and −C=O, at the open end and side walls of CNTs [108–110]. In non-covalent functionalization, no chemical reaction is involved while the molecules of CNTs surface and the polymer chains are wrapped via van der Waals interaction and π-π interaction. This method does not produce any change in the physical properties or surface of CNTs [111, 112].

Functionalization of Carbon Nanotubes

Covalent Method

Side Wall

Ends and Defects

Non-Covalent Method

Polymer Wrapping

Figure 2.6  Functionalization methods of carbon nanotubes.

Surface attachment of Surfactant

FNM-Based Catalytic Materials  67 Studies have been done for functionalization of graphene to improve its chemical and physical surface properties. Functionalized graphene has oxygenated groups, like carboxyl, hydroxyl, and epoxy, which can be introduced into the graphene lattice by oxidation. Those O-containing functional groups make graphene the best nanomaterial for industrial effluent treatment, as they are easily dispersed in water and in organic solvents [113].

2.5.2 Synthesis of Functionalized Carbon Nanotubes and Graphene CNTs and graphene materials are considered the best nanomaterials for the preparation of novel adsorbents with more functions, due to their high surface area and oxygen surface groups. Various selective and effective functionalization methods have been used for CNTs and graphene, allowing the creation of multiple functionalities [114]. Those methods are depicted in Table 2.2 [115–122].

2.6 Application of Functionalized Nanomaterials in the Energy Industry Through Removal of Heavy Metals by Adsorption Pollution caused by heavy metals is a very complicated environmental problem. Heavy metals have atomic weights ranging from ~63 to ~200, and specific gravity above 5.0, including metals from groups IIA, IIIB, IVB, VB, and VIB of the Periodic Table [123]. Heavy metals have many uses in industry, in the production of batteries, pigments, as catalysts in petroleum refining, protective coatings for other metals, so that they do not rust or corrode, and as stabilizers in plastics. They are used in many other applications like chemical synthesis and metallurgical processes [124]. Heavy metal pollution comes from industrial effluents. Some of the common sources of toxic heavy metals are listed in Table 2.3. Because of human activities and natural processes, such metals coming from industrial effluents, like oil and gas, battery, plastic, pharmaceutical, automobile, and agricultural industries, enter surface and ground waters [125] and pose a serious hazard due to their intense toxicity to aquatic and terrestrial life, including humans. Heavy metals containing ­wastewaters are being discharged into the environment mostly in developing countries. Heavy metals are non-biodegradable and often accumulate in living

68  Functionalized Nanomaterials for Catalytic Application Table 2.2  Preparation methods for various functionalized carbon nanomaterials [115–122]. Functionalization method

Nanomaterial

Preparation

References

Oxidation

MWCNTs

Concentrated HNO3 and H2SO4 in the ratio 1:3 (v/v) is reflux at 80°C for 24 h

[115]

Alkali activated

oMWCNTs

KOH and CNTs in the ratio 6:1 at 750°C for 1 h under flowing argon

[116]

Sulfonation

oMWCNTs

H2SO4 (98%) and MWCNT are sonication for 30 min, heating to 180°C for 24 h with a reflux condensation

[117]

Oxidation

GO

Graphite powder + Strong Oxidizing agents, i.e., NaNO3+H2SO4 at 66°C+ Shake + KMnO4 at −20°C + H2O2 Forms Graphine Oxide

[118]

Silylation process

GO-EDTA

Silylation reaction of N-(trimethoxy­ silylpropyl) ethylenediamine triaceticacid (EDTA-silane) with GO in ethanol solution

[119]

(Continued)

FNM-Based Catalytic Materials  69 Table 2.2  Preparation methods for various functionalized carbon nanomaterials [115–122]. (Continued) Functionalization method

Nanomaterial

Preparation

References

Noncovalent

G-CTAB

Ionic interactions between the carboxyl group of GO with the ammonium ion of CTAB. CTAB-GO was reduced to form G-CTAB

[120]

Inorganic functionalization

CNTs-FeS

Ethanol + ferrocene + thiophene + pyrolysis in the presence of argon + Oxidation for 1 h + air flow at 400°C forms CNTs-Fe2O3 which on further reduction 1 h + argon, 900°C + Sulfur powder + argon, 600°C

[121]

Inorganic functionalization

MWCNT/ Al2O3

Pyrolysis of aluminum nitrate on to the surface of oxidized MWCNTs

[122]

organisms, Several of them are toxic or carcinogenic. Table 2.4 highlights the common health problems caused to humans by heavy metals. The most toxic metal ions are Cr, Se, Fe, Cu, V, Co, Cd, Ni, Hg, Pb, As, Zn, etc. Functional nanomaterials have been used for the effective removal of these heavy metals from industry effluents, with great success, since they have better adsorption capacity than other adsorbents. In recent times, broad studies were conducted for the removal of heavy metals from water or wastewater by such materials [126, 127]. Such nanoadsorbents are often used in the form of nanoparticles. The efficiency and adsorption rate of nanoparticles are higher due to the adequate surface

70  Functionalized Nanomaterials for Catalytic Application Table 2.3  Common sources of some heavy metals pollution. Heavy metals

Common sources

Chromium

Chrome plating, petroleum refining, electroplating industry, leather, tanning, textile manufacturing and pulp processing units. It exists in both hexavalent and trivalent forms.

Nickel

Galvanization, paint and powder, batteries processing units, metal refining and super phosphate fertilizers, steel and non-ferrous alloys, tobacco smoke, etc.

Cadmium

Batteries, electroplating industries, phosphate fertilizers, detergents, refined petroleum products, paint pigments, pesticides, galvanized pipes, plastics, polyvinyl and copper refineries, coal combustion, water pipe, etc.

Lead

Petrol-based materials, pesticides, leaded gasoline, and mobile batteries.

Copper

Pulp and paper, utensils, chemicals, electroplating industry, plastic industry, metal refining, and industrial emissions.

Zinc

Rubber industries, paints, dyes, wood preservatives and ointments, galvanizing, alloys rayon, paper, etc.

Mercury

Electric/light bulb, wood preservatives, leather tanning, ointments, thermometers, adhesives, and paints.

Iron

From metal refining, engine parts.

Cobalt

Alloys, steel, electroplating glass, enamel, etc.

Manganese

Metal alloys, power plants, gasoline.

Arsenic

Automobile exhaust/industrial dust, wood preservatives, and dyes.

area, in comparison to other adsorbents. An ideal absorbent must have reasonable, sustainable, high adsorption capacity, and selectivity properties [128]. Carbon nanomaterials, having high surface area and large pore volume, are quite efficient. In particular, CNTs and graphene have been largely used to remove metal ions like Pb(II), Hg(II), Cd(II), Cr(VI), Co(II), As(III)/(V), and U(VI) from contaminated water. Table 2.5 shows some examples. In similar manner, magnetic nanoparticles have also been quite efficient in pollutants removal [139]. Table 2.6 shows the adsorption capacities of

Into inland surface waters Indian Standards: 2490 (1974)

0.10

3.0

2.00

0.10

Metal contaminant

Chromium

Nickel

Cadmium

Lead

1.00

1.00

3.0

2.00

Into public sewers Indian Standards: 3306 (1974)

On land for irrigation Indian Standards: 3307 (1974)

Permissible limits for industrial effluent discharge (in mg/L)

Table 2.4  Common health problems caused to humans by heavy metals.

10

03

-

50

WHO

05

05

-

100

USEPA

Permissible limits by International bodies (µg/L)

(Continued)

Suspected carcinogen, loss of appetite, anemia, muscle and joint pains, diminishing IQ, cause sterility, kidney problem, and high blood pressure

Carcinogenic, cause lung fibrosis, dyspnea, and weight loss

Causes chronic bronchitis, reduced lung function, cancer of lungs, and nasal sinus

Suspected human carcinogen, producing lung tumors, allergic dermatitis.

Health hazards

FNM-Based Catalytic Materials  71

Into inland surface waters Indian Standards: 2490 (1974)

3.00

5.00

0.01

0.20

Metal contaminant

Copper

Zinc

Mercury

Arsenic

0.20

0.01

15.00

3.00

Into public sewers Indian Standards: 3306 (1974)

0.20

On land for irrigation Indian Standards: 3307 (1974)

Permissible limits for industrial effluent discharge (in mg/L)

Table 2.4  Common health problems caused to humans by heavy metals. (Continued)

10

01

-

-

WHO

50

02

-

1300

USEPA

Permissible limits by International bodies (µg/L)

Carcinogenic, producing liver tumors, skin, and Gastrointestin effects

Corrosive to skin, eyes and muscle membrane, dermatitis, anorexia, kidney damage, and severe muscle pain

Causes short-term illness called “metal fume fever” and restlessness

Long term exposure causes irritation of nose, mouth, eyes, headache, stomachache, dizziness, diarrhea

Health hazards

72  Functionalized Nanomaterials for Catalytic Application

FNM-Based Catalytic Materials  73 Table 2.5  Functionalized carbon nanomaterials for the adsorption of heavy metals from industrial effluents [126–135].

Nanomaterial

Heavy metal

Maximum adsorption capacity Qmax (mg g−1)

2,6-Diamino pyridine–RGO

Cr(VI)

393.7

[126]

GO aerogel

Cu(II)

19.1

[127]

Chitosan-GO

Au(III)

1076.6

[128]

Chitosan-GO

Pd(II)

216

[128]

CNTs-base iron oxides

As (III)

24.05

[129]

GO-MnFe2O4

As (III)

146

[130]

CNTs-base iron oxides

As(V)

47.41

[129]

GO-MnFe2O4

As(V)

207

[130]

EDTA functionalized magnetic graphene oxide

Pb(II)

508.4

[131]

EDTA functionalized magnetic graphene oxide

Hg(II)

268.4

[131]

EDTA functionalized magnetic graphene oxide

Cu(II)

301.2

[131]

Polypyrrole decorated reduced graphene oxide-Fe3O4

Cr(VI)

293.3

[132]

MnO2/Fe3O4/GO

Cr(VI)

193.1

[133, 134]

CNTs/Fe3O4-NH2

As

75.0

[135]

CNTs/Fe3O4-MPTS

Pb(II)

65.0

[136]

References

74  Functionalized Nanomaterials for Catalytic Application Table 2.6  Magnetic iron oxide nanoadsorbents for the removal of heavy metals [84, 137–145]. Heavy metals

Adsorbent

Maximum adsorption capacity Qmax (mg g−1)

References

Arsenite (As (III))

Magnetic iron oxide nanoparticles

46.06

[137]

Magnetic nanoiron oxide

188.69

[137]

Arsenate (As(V))

Magnetic nanoiron oxide (MION-Tea)

153.8

[137]

Cadmium

Aluminium oxide Fe3O4

625

[138]

Maghemite

94.33

[139]

Magnetite

88.93

[140]

Hematite

65.00

[141]

Malic acid-Fe3O4-SiO2

81.63

[142]

Fe3O4-SiO2Graphene oxide

4.3

[143]

Amino-modifiedFe3O4

11.24

[144]

Mercaptomodified-Fe3O4

282

[145]

Rhodamine-Fe3O4SiO2

97.34%

[84]

Chromium

Mercury

several modified magnetic nanoparticle adsorbents utilized in the elimination of different heavy metals [86, 140–148].

2.6.1 Removal of Arsenic by Magnetic Nanoparticles Arsenic is the 20th more profuse element found almost everywhere, water, soil, air, and living organisms. In soil and water, it is normally present as

FNM-Based Catalytic Materials  75 organic and inorganic forms. As(III) (arsenite) and As(V) (arsenate) are inorganic forms, while dimethyl arsenic acid and monomethyl arsonic acid fall under the category of organic forms [149]. The International Agency for Research on Cancer (IARC) considers molecules with arsenic contents as group 1-carcinogens. Inorganic forms are more toxic than organic forms [54]. On the basis of previous research, oxides, alumina, activated carbon, and resins are good adsorbents for arsenic removal but they have a number of drawbacks, like small adsorption power, high cost, and formation of a difficult to separate sludge [150]. But, magnetic nanoparticles are appropriate adsorbents for As removal, mostly from soil and water, due to the strong Fe and As interaction. Furthermore, magnetic nanoparticles can be modified by cheap bio-related sources, like biomass and polysaccharides [151]. A study conducted by Lin et al. [152] found that magnetic nanoparticles, such as Fe2O3, show remarkable adsorption results, confirmed by FTIR and XPS, mostly for arsenate and arsenite. Additionally, they can be recycled more than five times by addition of 1 M NaOH solution [152].

2.6.2 Removal of Cadmium by Magnetic Nanoparticles Cadmium (non-degradable) ions show significant toxicities, arriving at the organism through food, causing biological harm. Cd comes from gradual erosion caused by rocks and soil abrasions, like volcanic eruptions. Cd pollution limits cell growth causes bone infections and lung injuries [54]. Therefore, WHO (World Health Organization) recommends that the limit of Cd in blood to be less than 0.005 mg L−1 [143]. For these reasons, elimination or removal of cadmium from environment through removal techniques is essential. Researchers synthesized numerous materials but, at the present time, the most efficient materials for Cd removal of cadmium are superparamagnetic nanoparticles. Cadmium is largely attracted to the external active sites of magnetite materials. Several magnetite surface coatings show high adsorption capability for Cd [153]. The coating of inorganic nanoparticles helps to prevent accumulation and keeps the magnetic properties. Al2O3Fe3O4 nanocomposite has an excellent adsorption capacity, being an exceptional nanosized metal oxide for cadmium removal [141].

2.6.3 Removal of Chromium by Magnetic Nanoparticles Chromium is a very toxic pollutant due to its carcinogenicity, nonbiodegradability, and mutagenicity. Cr enters into different environment

76  Functionalized Nanomaterials for Catalytic Application matrices through industries, like electroplating, metal finishing, dyeing, pigments, leather tanning, and steel manufacturing. Chromium be found in water or soil in two oxidation states, Cr(III) and Cr(VI) [151]. Cr(VI) is more toxic and harmful for human health than Cr (III). Due to its high toxicity, the Cr amounts in drinking and surface water are limited to 0.05 and 0.1 mg L−1, respectively [155]. Many previous studies established numerous techniques like chemical precipitation [156], electro-coagulation [154], and membrane separation [157] for the removal of chromium pollutants. The need of large energy, complicated operations, and high cost are shortcomings of these techniques [158]. Recent studies reveal that removal of chromium through superparamagnetic nanoparticles via adsorption is the most prominent and acceptable procedure [153]. Different experimental studies show that magnetite and maghemite nanoadsorbents have significant adsorption capacity for chromium removal. However, magnetic nanoparticle surface needs to be coated with mesoporous materials, like SiO2 and amine, to enhance the adsorption capacity and surface area. Moreover, such nanoadsorbents can easily be separated through an external magnetic field. Additionally, the superparamagnetic features of maghemite and magnetite nanoadsorbents can help to increase their re-dispersion and continuous reuse [159].

2.6.4 Removal of Mercury by Magnetic Nanoparticles Mercapto-functionalized magnetic nano-Fe3O4 polymers (SH-Fe3O4NMPs) were reported as adsorbents for removing mercury from wastewater [160]. Several techniques can be used for characterization of these magnetic nanoparticles. The adsorption capacity is directly influenced by pH. The adsorption data obtained with the optimized conditions, i.e., pH 3.0 and 308 K, could be well fitted to the Freundlich isotherm. The effects of different acids and Hg(NO3)2 and HgSO4 mercury salts were also intensely examined, and it was revealed that the amount of adsorbed Hg(II) decreased in the presence of Cl− [160].

2.7 Conclusions This chapter focused on the different FNMs used for adsorption of several heavy metal ions from water, especially Pb(II), Hg(II), Cd(II), As(III)/ (V), and Cr(VI) from industrial effluents, which include industries that produce and sell energy, like extraction, manufacturing, refining, and distribution of fuel. Examples are fossil fuel industries, coal industries, etc.,

FNM-Based Catalytic Materials  77 which are used for energy generation. Pure, oxidized, and other modified forms of carbon nanomaterials and magnetic nanoparticles were employed to adsorb heavy metals from water. Improved adsorption and larger capacity were found after functionalization. Thus, one can say that FNMs act as catalysts in the adsorption process. The preparation, characterization, and application of FNMs used in recent years in the adsorption of heavy metals are summarized. However, further research is required on the preparation of improved FNMs for cleaning industrial wastewater from pollutants.

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88  Functionalized Nanomaterials for Catalytic Application 149. Hughes, M.F., Beck, B.D., Chen, Y., Lewis, A.S., Thomas, D.J., Arsenic Exposure and Toxicology: A Historical Perspective. Toxicol. Sci., 123, 2, 305, 2011. 150. Hokkanen, S., Repo, E., Lou, S., Sillanpää, M., Removal of arsenic (V) by magnetic nanoparticle activated microfibrillated cellulose. Chem. Eng. J., 260, 886, 2015. 151. Magnacca, G., Allera, A., Montoneri, E., Celi, L., Benito, D.E., Gagliardi, L.G., Gonzalez, M.C., Mártire, D.O., Carlos, L., Novel magnetite nanoparticles coated with waste-sourced biobased substances as sustainable and renewable adsorbing materials. ACS Sustain. Chem. Eng., 2, 6, 1518, 2014. 152. Lin, S., Lu, D., Liu, Z., Removal of arsenic contaminants with magnetic γ-Fe2O3 nanoparticles. Chem. Eng. J., 211, 46, 2012. 153. Huang, Y. and Keller, A.A., EDTA functionalized magnetic nanoparticle sorbents for cadmium and lead contaminated water treatment. Water Res., 80, 159, 2015. 154. Shariati, S., Khabazipour, M., Safa, F., Synthesis and application of amine functionalized silica mesoporous magnetite nanoparticles for removal of chromium (VI) from aqueous solutions. J. Porous Mater., 24, 1, 129, 2017. 155. Gupta, V.K., Gupta., M., Sharma, S., Process development for the removal of lead and chromium from aqueous solutions using red mud—an aluminium industry waste. Water Res., 35, 5, 1125, 2001. 156. Özer, A., Altundoğan, H., Erdem, M., Tümen, F., A study on the Cr (VI) removal from aqueous solutions by steel wool. Environ. Pollut., 97, 1, 107, 1997. 157. Chakravarti, A., Chowdhury, S., Chakrabarty, S., Chakrabarty, T., Mukherjee, D., Liquid membrane multiple emulsion process of chromium (VI) separation from wastewaters. Colloids Surf. A Physicochem. Eng. Asp., 103, 1, 59, 1995. 158. Ghiaci, M., Kia, R., Abbaspur, A., Seyedeyn-Azad, F., Adsorption of chromate by surfactant-modified zeolites and MCM-41 molecular sieve. Sep. Purif. Technol., 40, 3, 285, 2004. 159. Singh, P., Tiwary, D., Sinha, I., Chromium removal from aqueous media by super paramagnetic starch functionalized maghemite nanoparticles. J. Chem. Sci., 127, 11, 1967, 2015. 160. Pan, S., Shen, H., Xu, Q., Luo, J., Hu, M., Surface mercapto engineered magnetic Fe3O4 nanoadsorbent for the removal of mercury from aqueous solutions. J. Colloid Interface Sci., 365, 204, 2012.

3 Bionanotechnology-Based Nanopesticide Application in Crop Protection Systems Abhisek Saha

*

Department of Chemistry, Tufanganj College under Coochbehar Panchanan Barma University, Coochbehar, West Bengal, India

Abstract

Pest affects plants in nature, resulting in overindulgence of mineral fertilizers and toxic pesticides and affecting the worldwide environments, plants, and animal, causes serious health hazards to farmers, and eventually affects the biodiversity scenario. At this example, worldwide scientists are on condition that fresh and secure food to all or any citizenry and livelihoods by adapting several green technologies. Nanotechnology is prepared to lend a hand to satisfy up the food security challenges, targeted discharge of pesticides. Nanoscience has a huge deal of application in agrochemical and plant protection region to manage pest. So far, nanoparticles were utilized in formulation pesticides and insecticides which are nano-based, encapsulated nanoparticles, nanoparticle-mediated gene, and DNA transmit in plants. Nano-based groundbreaking nanopesticides like Ag, Cu, SiO2, ZnO, and nanoformulations exhibit improved broad-spectrum pest security efficiency. There is also enormous anxiety regarding the nanobiopesticides material. The positive effect of nanopesticide may have some unenthusiastic effects insecticides, chemical and biogenic nanostructured selected metals offer an inexpensive and good option for control of pests. In this chapter, an analysis in field and usage of conventional solution, nanobiopesticides for applications in crop protection systems, and various research findings of usage of bio-nanomaterial for pest management are discussed. Keywords:  Nanotechnology, pest management, bionanopesticides, pest, food security

Email: [email protected]

*

Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (89–108) © 2021 Scrivener Publishing LLC

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3.1 Introduction In the current day, in light of the quick growth in populace and urbanization, the world is confronting a few issues because of which the presence of individuals has arrived at a basic stage [1]. The serious issues are consumption of ordinary and common vitality sources, ecological contamination, an unnatural weather change, nursery impact, absence of legitimate symptomatic devices in identifying the ailments, inappropriate farming practices by utilizing mass amounts of substance composts, pesticides, and herbicides, and absence of adequate food and safe house [2]. As indicated by the current examinations, the total populace tally will arrive at 9.0 billion before the finish of 2050. Because of this enormous increment in the populace and by fast urbanization, the creation and accessibility of food material have become a significant issue. To take care of the all-out populace, there is no doubt that the food production rate must be expanded by 60%–70%. This could be conceivable just through the advancement of green and feasible agribusiness by using the nanomaterials and methods created by nanotechnology [3]. To comprehend the reason for a few human infections at the sub-atomic and nuclear level, careful exploration through the advancement of apparatuses and materials required at the nanoscale could be conceivable just through nanotechnology and should transform the new thoughts into critical thinking ones [4]. As of late, nanotechnology has given a few nanoscaled materials through which determination, unhealthy site recognizable proof, and treatment of illnesses should be possible viably. Bionanotechnology has increased a great deal of significance in the current day by building up a few gadgets, materials, and procedures which help take care of the serious issues of people and nature like green horticulture, sustainable power source creation, indicative instruments for distinguishing the human and plant infections at beginning phases, nanoparticles in the treatment of illnesses, the contamination checking through nanobiosensors, bioremediation of dirtied destinations, wastewater treatment, and water cleaning  [5]. Thus, bionanotechnology has an extremely high potential in battling the present and future issues of the world [6]. This paper centers around ongoing improvements taken in bionanotechnology and its applications. Insect-pest is animal populations that occurred in each attainable surroundings with a varied variety of species. Many insects are vectors of various diseases and cause damages to crop plants. These are moving the economy and also the yield of crop plants and at the national and international markets. The crop yield losses caused by insects in agriculture,

Bionanotechnology-Based Nanopesticide  91 many chemicals are applied to regulate them. Insect-pests are one in all the leading causes that affect agricultural productivity leading to billion bucks loss once a year [7]. The two main forms, i.e., animal and adults are most deadly for many of the crop plants. This has created associate emergence for a man of science to compile the standard and advanced techniques to beat numerous blighter threats round the world. The insecthost plant interaction ends up inactivation of the plant system and there is cross speak between varied pathways for the production of each primary and secondary metabolite. The plasma membrane acts as a barrier to insects but becomes crossable for fatal insects leads to the final destruction of plants. The secondary metabolites made act as specific defensive systems that are activated upon insect interaction. Biopesticides are organic, low-risk, and environmentally friendly as compared to artificial pesticides [8]. Thus, the exploration of plants that having potential against insect blighter is the chief for dominant their population and to save lots of agriculture losses. Friedrich concludes that, in 2050, the expected population is 9.2 billion, and therefore, the world food production is concerning the seventieth. This might be achieved by adapting safe, abundant, property, and alimentary food to offer innovative techniques. A number of them area unit ancient, mobile, and vertical farming, cultivation of insect-protected and virus-resistant biotech crop varieties, designed crops that grow in an exceedingly places wherever they had not survived before, pesticides-tolerant varieties, nutritionally increased traits, following plant-specific protection measures and adapt bio-intensive integrated pest and illness management [9]. Nanotechnology has revolutionized the globe with tremendous advancements in several fields of science like engineering, biotechnology, analytical chemistry, and agriculture. Their use in crop protection is simply in its infancy. Nanomaterials live between close to one and one hundred nm. Over several decades, engineering and nanomaterials are used flourishing and safely in numerous fields like drugs, bionomics, and food processes [10]. However, the utilization of nanomaterials in agriculture, particularly for plant protection and production, it is associate under-explored analysis space. It is been used as conductors and semiconductors, medical devices, sensors, coatings, chemical action agents, and conjointly as pesticides. Numerous nations are presently being exchanging over from substance-based agribusiness to green horticulture, where the use of biopesticides and natural nanomaterials have heaps of task to carry out in bother control. Since the biogenesis of nanomaterials and their portrayal was basic and dependable, biogenic silver nanoparticles (AgNPs) for Pongamia pinnata, Azadirachta indica, Annona squamosa,

92  Functionalized Nanomaterials for Catalytic Application and Chrysanthemum were arranged and used for different natural purposes [11]. Be that as it may, their utility worth was not assessed against crop bugs. Moreover, an assortment of metal nanoparticles Silver (Ag), Gold (Au), Aluminum (Al), Silica (Si), and Zinc (Zn) and metal oxide– based polymers Zinc oxide (ZnO) and Titanium dioxide (TiO2) are being produced for crop bother the board. Nonetheless, not many examinations have been made in the field of nanomaterial and vermin the executives and much more is normal sooner rather than later. Thus, this audit wanted to give about the utilization of nanomaterials and bionanomaterials against pestiferous creepy crawlies and post-collect vermin. A variety of metal nanoparticles Ag, Au, Al, Si, and Zn and metal oxide–based polymers ZnO and TiO2 are being developed for crop pest management [12]. However, very few studies have been made in the field of nanomaterial and pest management and a lot more is expected soon. Hence, this review planned to provide about the use of nanomaterials and bionanomaterials against pest insects and also post-harvest pests. The need to formulate the biopesticide with economical insect-killing property is tried by compounding completely different secondary metabolites in varied ratios. However, the constant use of the most effective sale biopesticide has place itself at risk for resistance in pests. It is allowed scientists and researchers everywhere the planet to assemble the economical secondary metabolites against insect-pest. The opposite various hidden in nature against persecutors must open up with the present technologies [13]. The greener technique to avoid pollution and safer use with minimum toxicity for animals and plants are that the best various. The inexperienced chemistry topic that uses the inexperienced natural material as a beginning supply to formulate the nanosize particle capped with the pest-resistant secondary metabolites may well be a potential resolution to defect pests in farms. With the higher than a state of affairs, this chapter discusses the final demand for biopesticides, chemical, and biological ways that to dealt persecutor and also the latest nanobiopesticides which will be shaped victimization completely different plant extract complexed with Ag nanoparticles and their essays against pest-insect area unit communicated [14].

3.2 Few Words About Pesticide There are several varied styles of pesticides that wont treat effectively against specific pests. The word “cide” originates from the Latin word “to kill”. There are thousands of personal chemical manufacture, formulator,

Bionanotechnology-Based Nanopesticide  93 and producer with some national firms in numerous countries. Chemical merchandise typically comprises active ingredients and different ingredients. The active ingredients are the most entity that eliminates pests, in distinction, the opposite ingredients assist in many ways that like attracting the persecutor, spreading the active ingredients around, and/or reducing drift. The pesticides are classified as consistent with their targets. The pesticides are the most apprehension that destroying thousands of plant species [15].

3.3 What About Biopesticide Demand In modern times, the most focus is to style and formulate the ecofriendly chemical that created for a minimum quantity to the investment with the potency of high potential. The eco-accommodating bio pesticide’s definition and their interest as item utilization by farmers were raised. The nations of the world place constant effort into the event of economic biopesticides. Several countries deem biopesticides for tormenter management in agricultural biotechnology to supply food security so the sale of bio­pesticides has been increased as compared to chemical pesticides [16].

3.4 A Brief Look on Associates Responsible for Crop Loss Crop production loss was primarily caused by monocots, dicots, parasitic weeds, pestiferous insects, mites, mollusks, rodents, birds, mammals, bacteria, fungi, viruses of these organisms square measure classified as stand reducers (damping-off pathogens), photosynthetic rate reducers (fungi, bacteria, viruses), leaf senescence accelerators (pathogens), light-weight stealers (weeds, some pathogens), assimilate sappers (nematodes, pathogens, suction arthropods), and tissue customers (chewing animals, necrotrophic pathogens). They need to be managed by active completely different cultivation (cultivar selection, crop rotation) and mechanical weeding ways or utilizing numerous biological management agents (antagonists, predators, parasitoids, etc.) and conjointly exploitation chemicals (pesticides/insecticides/ acaricides/rodenticides/pheromones, etc.). Ancient pesticides have several limitations, moreover, as fewer efficacies to manage extremely

94  Functionalized Nanomaterials for Catalytic Application devastating pests. Exaggerated use of nanomaterials in agriculture has light-emitting diode to the necessity to check the impact of nanomaterials on the surroundings normally and insects before recommending identical for cuss management [17–19].

3.5 Traditional Inclination of Chemical-Based Pest Management The most common chemicals persecutoricides square measure out there in native pest stores, on-line sources, and their square measure many manufactures, dealer, and businessperson. In the time period, sulfur component wont create much chemical products like vitriol, phosphate fertilizers, fungicides, and pesticides. Excluding sulfur, the final active chemical ingredients square measure abamectin, cyfluthrin, fipronil, permethrin, bifenthrin, hydramethylnon, pyrethrum, and boric acid [20]. The chemical-based pests square measure is classified on the idea of their chemical structure like organophosphates, carbamates, organochlorines, pyrethroids, and neonicotinoids. Necessary chemicals are used as pesticides with their result on animals, insects, plant, and humans. They square measure several pesticides out there at the market, and a number of them square measure acephate, organic compound, bioresmethrin, carbaryl, carbaryl, dichlorvos, fenitrothion, malathion, pirimiphos-methyl, pyrethrum, and quinalphos. The management of persecutor has elevated the utilization of a spread of chemical insect powder, pesticides, and artificial insect powder, i.e., pyrethroids. The continual exposure of artificial pyrethroids has diode the insect to flee through resistance. Therefore, the standard chemicals based mostly persecutoricides square measure did not manage a pest on crop plants [21, 22]. These have associate degree emergence of the utilization of different modes for plant protection for the utilization of biological plant-derived natural compounds, phytochemicals, and secondary metabolites. The chemicals based mostly nanoparticles square measure synthesized and applied to gauge their potency against larvae. The nanoparticles of novaluron, a water-insoluble insect powder, were ready that consists of nanoparticles sized 30–100 nm that show toxicity in vivo experiments with cotton plant leafworm genus Spodoptera littoralis larvae. However, the byproducts stay when experiments were risky chemicals; therefore, the chemicals are not the selection for nanoparticle development within the agriculture chemicals sector [23, 24].

Bionanotechnology-Based Nanopesticide  95

3.6 Nanotechnology in the Field of Agriculture In farming, huge measures of synthetic pesticides, weedicides, and manures are utilized from which just little sums are utilized for picking up the ideal application, and the remainder of the immense sums is being delivered into the dirt, air, and water bodies causing a ton of compound contamination. Along these lines, there is a worry about the cleaning of the unutilized synthetic compounds and for finding a sheltered path for releasing and using all the measures of synthetic substances delivered into the fields. Nanotechnology helps in the eco-accommodating and reasonable farming through nanocomposts, nanopesticides, nanoporous zeolites, and different items [25, 26].

3.7 Why Nanotechnology-Based Agriculture is the Better Option With Special Reference to Nano-Based Pesticide? Biotechnology has thought-about a secure agricultural tool to reinforce crop protection, later turn out to supply to provide additional agricultural produce and product, improve food method, organic process worth, and higher flavor, etc. At constant, it is harmful ecological consequences like spreading genetically built genes to autochthonous plants, increasing toxicity, which can move through the organic phenomenon, disrupting nature’s system of blighter management, making new weeds or virus strains, loss of variety, and insecticidal resistance, etc. Hence, it is necessary to give birth to new innovative methods to beat the preceding issues [27]. One such novel technology is the technology that has been revolutionized in health care, textile, materials, info, and communication technology, and energy sectors, too. With the worldwide population explosion, the demand for enhanced provision of food has motivated scientists and engineers to style built nanoparticles (ENPs) to cut back pestiferous insect infestation later to extend agricultural production. Out there, literature reveals that each chemical and biological nanomaterials have additionally placed a considerable role within the crop protection as irrigation water filtration, the rectification of harmful pesticides/insecticides, preparation of recent pesticidal formulations, economical delivery of pesticides, fertilizers, and different agrochemicals, development of organic farming and disease management, etc. [28]. Since this field is within the infancy stage, by trial-and-error technique, this innovative technology will be utilized in crop protection and production functions considering their consequences.

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3.8 Biological-Based Pest Management The plant-derived phytochemicals have been used as biopesticides with the advantage of non-toxic to animals and humans. The plant’s secondary metabolites such as alkaloids, terpenoids, flavonoids, phenol, polyphenons, glycosides, and tannins have varied bioactivity against harmful insects. These secondary metabolite-based biopesticide has managed the pest Helicoverpa armigera to a certain extent [29]. Several research publications are indicating the optimistic effect of medicinal plant extracts, powder, against pests such as Mosquito and Helicoverpa armigera, etc. The large number of curative plants have a characteristic of larvicidal and pesticidal activity such as neem extract, Acorus calamus, Annona squamosa, Vitex negundo, Gnidia glauca, Toddalia asiatica, Argimone maxicana, and Calotropis procera. The work confirms the percent infestation reduction is highest in neem seed kernel extract [30].

3.9 Nano-Based Pest Management Nanotechnology has been proposed to a considerable extent and has been applied in numerous formulation to create many new products with a wide range of applications in several fields such as textile, geo sensing technology, paper, food, fertilizers, pesticides, plant protection, nutrition paint, food biofuel, biomass, biocomposites, and agrochemical industries. Thus, nano-based innovative nanopesticides such as Ag, Cu, SiO2, ZnO, and nanoformulations show better broad-spectrum pest protection efficiency, reducing water, soils, and environmental pollution in comparison with conventional pesticides. The Zn is an essential nutrient for plants and is less available in the soil this makes Zn metal as an important target for the development of Zn nanoparticles for dealing with the pest and beneficial for plants. ZnO NP is of less cost and safe. They are applicable in various fields such as anti-cancer, antidiabetic, antibacterial, antifungal, and agricultural properties, and agrochemical industries. The other potential option could be Ag. The Ag is used in the field of the biological system, living organisms, medicine, plant management, pest control, and agricultural aspects with better efficiency and activity that are the preferred target of the green method related to antibacterial, microbial, fungal, larvicidal, pesticidal, antiinflammatory, antiplatelet activity, anti-angiogenesis, and anti-viral activity. The antifungal activity of ZnO nanoparticles against plant pathogen Fusarium graminearum has been demonstrated. The metal nanoparticles

Bionanotechnology-Based Nanopesticide  97 have an advantage of stability, slow kinetics which can be scaled up for large quantity, performed at room temperature, and generation of ecoenvironmental byproducts. Thus, nanoformulation of biopesticide could be the best possible alternative for the development of pest-control weapons for harmful insect [31]. AgNP synthesis using Aloe vera extract and AgNO3 under SUN conditions and its spray application to control insectpest H. armegera. The synthesized bionanoparticles were mixed with water in definite quantity and filled in a spray bottle, application on pest-insect H. armegera. Larvicidal activity of synthesized AgNPs using an aqueous extract from Eclipta prostrata has been utilized to control the mosquito. Ficus religiosa (FR) and Ficus benghalensis (FB) for the fabrication of AgNPs to modulate the function of gut protease activity in H. armigera. They confirmed bioassay of AgNPs with FR (50% concentration) and FB (70% concentration) capable of reduction in larval weight and survival rate of H. armigera [32].

3.10 Nanopesticides To obliterate the hurtful vermin, these days, synthetic pesticides of exceptionally sweet-smelling and more dangerous are being utilized in agribusiness. These unsafe synthetic pesticides are making a great deal of harm non-focused on living beings. Individuals raised the worries and began displaying challenges the utilization of compound pesticides which are hurtful to people, different creatures, and nature. Nanotechnology has opened the conceivable outcomes of utilizing nanopesticides in a protected manner without making hurt nontargeted life forms and the earth. Metal nanoparticles can be utilized adequately for the powerful expulsion of vermin. Nanohexaconazole was described by a few methods, for example, SEM, TEM, and FT-IR, and so forth. As per the examinations they revealed that the readied nanohexaconazole was under 100 nm in size. Nanohexaconazole when attempted in fields was accounted for to be multiple times more viable in killing vermin, and nanosulfur was seen as multiple times more powerful in controlling parasites when contrasted and customary ones. Analysts assessed the impact of nanohexaconazole on a complete microbial that includes in the dirt examples, nitrifying microorganisms, blue-green growth, soil proteins, and seed germination properties. They additionally looked at different compound exercises, as basic phosphatases, acidic phosphatases, soil dehydrogenase, and bacterial check after the use of a nanomaterial [33]. They detailed that the nanonexaconazole is seen as sheltered in limiting the

98  Functionalized Nanomaterials for Catalytic Application impacts on non-focused on life forms. The above reports demonstrated nanopesticides as ecofriendly, and that nanotechnology can give great and productive options in contrast to regular pesticides in controlling the bugs in agribusiness without making hurt non-focused on creatures. Possible nanobiopesticides structures. The plant protection is being the busiest space for investigators for the formulation of active resolution to beat the artificial business pesticides. the normal strategies mixed with nanoscience with improved potency like higher solubility, slower emotional, and turning away of untimely degradation. The biological compounds act as capping and reducing agents to silver salt and formation of stable nanoparticles takes place [34].

3.11 Required to Qualify for Selection as Nanobiopesticides To be protected, simple to get ready, ease and powerful pesticides to lessened the plant bother, the significant focuses that should be considered are, first, it must be simple for readiness; it must be successful at financial rates; it could be focused against the particular irritation; it might be viable against a wide assortment of creepy crawly bugs; it must be more secure to every single living being and conditions; it must be nontoxic to ranchers; it must not contain any unsafe substance; it must be affirmed by wellbeing specialists; it must not get gathered in the natural pecking order; it must not offer ascent to unsuitable deposits; lawful deadly portion fixation must be surrounded; it must not influence the nature of food, flavor, scents, and surface; it must be adequate in the worldwide market; it must not be combustible, touchy, or destructive; it must be effectively appropriate [35]. Although there have been various investigations of the harmfulness impacts of nanoparticles on microscopic organisms, growths, and creature microbes. Little exploration has been done to examine the harmful impact of nanoparticles on the bug’s vermin. The broad subsidizing and heading here could be focused to concoct the extraordinary discoveries for plant security. The plant species containing optional metabolites that are creepy crawly anti-agents must be distinguished and buildings with silver or different nanoparticles. These naturally blended nanoparticles influence diverse nuisance creepy crawly needs to explain. This will finish up the plant species-explicit opposition for a specific bug. for particular insect [36].

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3.12 Pestiferous Insect’s Management 3.12.1 Chemical Nanomaterials Initially, Christenson and Foote compared the usefulness of colloidal Ca and nano-Ca on infestations of the oriental fruit fly. Chakravarthy et al. used inorganic nanoparticles CdS, nano-Ag, and nano-TiO2 against Spodoptera litura  Fab. control under laboratory environment. Evidence was found to prove the bioefficacy of Ag and Zn nanoparticles against Aphis nerii. Researchers suggested exploiting nanotechnology for the management of an economically significant polyphagous pest  Helicoverpa armigera. In recent times, reports found that calcium carbonate nanoparticles can boost plant nutrition and insect pest tolerance. Yasur and Ranee studied the impact of AgNPs on growth and feeding responses of two insect pests. The larvae were fed with PVP coated-AgNPs treated castor leaf at totally different concentrations and their activity was compared to it of caustic (AgNO3) treated leaf diets. Larval and immature body weights attenuate alongside the decrease within the concentrations of AgNPs and AgNO3 in each the check insects. Stadler et al., for the primary time, studied the insecticidal activity of nanostructured aluminium oxide against two insect pest’s, viz., Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Rhyzopertha dominica (F.) and according to important mortality once three days of continuous exposure to nanostructured alumina-treated wheat. Entomotoxicity of surface-functionalized silicon oxide nanoparticles (SNP) was tested against Sitophylus oryzae S. oryzae and its efficaciousness was compared with bulk-sized silicon oxide (individual particles larger than 1 μm). Nanoparticles (Al2O3 and TiO2) verified their insecticidal activity against S. oryzae below laboratory conditions. Magnetite octadecylsilane nanoparticles were synthesized and used for cuss management. The bioefficacy of β-cyfluthrin formulations synthesized poly (ethylene glycols)–based mostly amphiphilic copolymers was evaluated against Callosobruchus maculates [37].

3.12.2 Bionanomaterials Polyethylene glycol-coated nanoparticles loaded with garlic oil, against adult arthropod genus castaneum (Herbst) incontestable the insecticidal activity of the bio nanosynthetic resin glycol-coated nanoparticles. Inexperienced synthesis of AgNPs has been according to mistreatment rosid dicot genus prostrata and accustomed management the adult of

100  Functionalized Nanomaterials for Catalytic Application S. oryzae. Nanomaterials are often helpful in agricultural analysis and applications because of their size that is analogous to it of most biological molecules so that, they will diffuse through cell membranes to act on the target. AgNPs were synthesized by mistreatment liquid leaves extracts of rosid dicot genus prostrate showed insecticidal activity against adult of S. oryzae [38]. A polysaccharide spinoff (N-(2-chloro-6-fluorobenzyl-chitosan), chitosan has been found to show strong insecticidal activity in some plant pests. Researcher previously created DNA-tagged nanogold, DNA-tagged CdS, nano-TiO2, and nano-Ag and were tested against S. litura [39, 40].

3.13 Critical Points for Nanobiopesticides There is an excellent concern relating to the nanobiopesticides material that has been potential to exert unsafe effects on soil, human, and atmosphere. The positive impact of nanopesticide could have some negative effects, which require to be resolved by important analysis. The interaction of nano-based mostly pesticides with soil, and chemical interaction’s soil organic matter area unit different vital aspects. excluding nanoparticles the prime importance to acknowledge the actual plant species carrying peculiar chemical science could play a vital role in nanoparticles formations. The plant transmissible with chemical phytochemical and secondary metabolites influences the long run fashioned [41].

3.14 Other Pests Bionanomaterials were synthesized victimization plant extracts or microbe’s culture or their bioactive principles and macromolecule to enzymes. Antifeedant, larvicidal, and cytotoxic activities of synthesized AgNPs victimization binary compound leaf extract of magnoliopsid genus indica against third arthropod larvae of H. armigera. Chandra et al. confirmed that chitosan nanoparticle-coated plant life matter (CNPCFM) showed higher pesticidal activity when put next with uncoated plant life matter (UFM) and plant life Spores (FS) [42].

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3.15 Post-Harvest Management and Their Consequences Two major groups of insects such as Coleoptera (beetles) and Lepidoptera (moths and butterflies) comprise the most economically important post-harvest insect pests. Several species of Coleoptera and Lepidoptera attack crops both in the field and in the store. They cause physical harm, grain spilling, or disintegration, loss of weight and quality, force misfortune, germination decrease, lose an incentive for promoting and utilization, or planting. Fumigants and residual insecticides are commonly used to combat stored grain pests. In recent years, consumer awareness of the health hazard from residual toxicity and the growing problem of insect resistance to these conventional insecticides have led the researchers to look for alternative strategies for stored grains protection [43].

3.16 Field Test for Nanobiopesticides for Pest Control The nanobiopesticide mixed, in particular, a quantity of water and sprayed in known quantity in a field of specifically selected crop species to study the effect on plants compared with the standard. The field splashing of nanobiopesticide will require fitting taking care of and estimation of nanobiopesticides. The initial quantity of nanopesticide needs to be optimized and mixed with water or selected nontoxic solvent and sprayed in the field. The spraying could be done manually by farmer, automated motors, sometimes using planes and helicopter based on the size of the farm. The overall cost for the formulation of silver nitrate, plant collection, extract preparation, and lab facilities is very crucial, as it will decide the final cost of nanobiopesticide formulation. The final comparison of cost for biopesticide and nanobiopesticide will decide the future of nano-based pesticides. However, the test for their potential is a critical aspect for nanotechnologists to established plant protection via nanobiopesticide. This will conclude the effect of synthesized nano-based biopesticide and it is potential to use as nanobiopesticides as a modern trend [44].

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3.17 Merits and Consequences of Chemical and Bionanomaterials Before introducing the ENPs in agriculture application, significantly insect tormenter and phytopathogens management, their impacts on biological organisms dwelled on water or soil and conjointly their potentially harmful effects on living beings together with men. The literature survey chiefly emphasized the potential advantages of ENPs, though meager is thought concerning the security of nanomaterials or nanoparticles within the agriculture sector. Considering the soil pollution, terribly, recently, it was according to that nanoparticle pollution in soil continues to be within the method of development. Nanoparticles (multi-walled nanotube, Al, alumina, Zn, and ZnO) showed anti-germicidal activity except by nanoscale Zn (nano-Zn) on rye grass and nanoZnO on corn at 2,000 mg L−1. The CuO nanoparticles considerably stifled the expansion and development, reduced the uptake of nutrients, such as B, Mo, Mn, Mg, Zn, and Fe of each transgenic and traditional sorts of cotton. Armstrong et al. according to that AgNPs, like the majority nanoparticles, square measure doubtless poisonous on the far side a particular concentration as a result of the survival of the organism is compromised thanks to lots of pathophysiological abnormalities past that concentration. Functionality and charge (nature of the surface), size, and portal of entry (lungs, intestinal, or skin) of NPS place a vital role for the entry of nanoparticles into the material body; consequently, we tend to choose the particles [45]. However, the mechanism of AgNPs toxicity remains undetermined. It is instructed to check the physical, chemical, and biological properties, bio-encapsulation method, quality of carriers and behaviors, and, conjointly, mechanism of single or multiple NPs or NPs with chemical or biological or natural materials with its encompassing surroundings like soil, water, and organism inhabiting on them before recommending for agriculture functions. Moreover, formulation ways, handling, and application technologies may also be devised for the higher utilization of ENPs within the agriculture sector. Moreover, compared to commercially accessible pesticides, chemical and biogenic nanostructured chosen metals will offer an inexpensive and reliable various for management of insect tormenters and such studies might expand the frontiers for nanoparticle-based technologies in pest management [46–48].

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3.18 Conclusion Despite some debatable problems regarding the protection considerations concerning nanoparticles, by 2030, nanomaterials can get into the market with significant applications in many sectors across the globe. That the development of recent subtle nanodevices, nanomaterials, and nanotechniques got to be any inspired for promoting property agriculture and combatting alternative issues. It is conjointly vital that the developed nanomaterials, devices, and techniques ought to be investigated to create certainty that it is safe for all organisms and also for nature (Figure 3.1). There is conjointly a desire to expand the applications of engineering science to every corner of the globe. This article trots out present pest-control ancient choices and discusses the long-run approach of nanobiopesticides. The matter of pest is not restricted to geographical locations, and therefore, the current management measures area unit nephrotoxic to ecology and human health. The important challenges featured by the pest management area unit are the selection and handiness of safe, effective, and low-cost bioinsecticides area are the demand of present time. What are more, magnetic nanoparticles may be another for insect repellents with magnetic properties useful in geo-sensing technology. The plant with outstanding insecticidal ADVANTAGES OF PEST DEFENSE THROUGH NANOPESTICIDES Life of shelf storage Soil draining Site specific absorption Toxicity Solubility

Figure 3.1  Advantages of pest defense through nanopesticides.

104  Functionalized Nanomaterials for Catalytic Application activity may be complexed with a metal of atomic number 30 or silver and their effectualness may elevate to regulate many pests. This can conjointly assist to reduce the foremost dangerous chemicals used as pesticides nowadays in a very field to pests. The future possibility of nanopesticides may incorporate assessment of this kind of materials for hazardous impacts.

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4 Functionalized Nanomaterials (FNMs) for Environmental Applications Bhavya M.B.1, Swarnalata Swain1, Prangya Bhol1, Sudesh Yadav2, Ali Altaee2, Manav Saxena1, Pramila K. Misra3 and Akshaya K. Samal1* Center for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagara, Bangalore, India 2 Center for Green Technology, School of Civil and Environmental Engineering, University of Technology Sydney, Sydney, NSW, Australia 3 Center of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar, Odisha, India 1

Abstract

Nanomaterials empower the advances of innovative solutions to environmental problems. Among the broad range of nanomaterials, functionalized nanomaterials emerged as promising catalyst for environmental issues. Functionalization of nanomaterials offers a great advantage on cost effectiveness and enhances the properties in many folds. Implementation of bare nanomaterials has a few challenges as well as limitations such as instability, agglomeration and reusability. Functionalization of the nanomaterials facilitates to overcome these problems. Addition of functional groups helps in tailoring the nanomaterial surface to enrich the specific sites on the surface level. This chapter discusses the engineering of functionalized nanomaterials and focuses on environmental applications. Different materials such as cellulose, chitosan, silica, metal oxides, and other polymers are added to nanomaterials and the introduction of functional groups on the surface which results composite materials of enhanced capability. Different process of functionalization namely direct functionalization, postsynthetic functionalization, grafting-to, grafting-from, and grafting-through methods are illustrated. Nanomaterial and functional group interaction such as covalent and noncovalent bonding is described. This chapter highlights the limitations of different materials in various aspects of environmental applications. It provides a clear overview

*Corresponding author: [email protected]; [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (109–134) © 2021 Scrivener Publishing LLC

109

110  Functionalized Nanomaterials for Catalytic Application on selectivity of functionalized nanomaterials for the protection of environment using suitable real-time techniques. Keywords:  Functionalized nanomaterials, chitosan, cellulose, alumina, mixed composites

4.1 Introduction The outcome advantage of combination of nanomaterials and nanotechnology has unlimited and unfolds almost in every fields. Use of nanomaterials in the environment has a great impact in environmental remediation [1]. Environmental contamination occurs through different ways, such as usage of pesticides, dyes, heavy metal ions, release of toxic gases, and other organic pollutants [2–6] to the environment without proper pre-treatment. Environmental contamination leads to pollute water, air, and soil in a highly significant way and results adverse health effects in humans and other living entities [7]. Metal nanoparticles (MNPs) emerged tremendously in regulating the environmental pollution from earlier decades and a few limitations have been addressed for real-time applications, as the usage of nanomaterials may have negative impact on humans and ecosystem [8]. Next is the reusability issue of the materials used in catalysis that has an impact on cost-effectiveness. Other aspect of the limitation is the impact of environmental conditions like temperature and pH on nanomaterials [9]. Great innovations in the field of nanoscience and advancement in technologies have opened up a new solution to the limitations of bare nanomaterials using functionalization of the surface of nanomaterials. This improved functionalization of the nanomaterials surface offers to tune their properties to overcome the current limitation and enhance the stability, activity, and selectivity of the materials. Surface functionalization introduces functional groups (chemical groups) on the surface of nanomaterials which further provides specific surface sites and create advanced hybrid structure [10].

4.1.1 Methods for the Functionalization of Nanomaterials Functionalization process is an improved version of tailoring the nanomaterials surface which further controls the properties such as physicochemical and toxicological properties of the nanomaterials. Nanomaterials can be decorated by different functionalities comprising of organic, inorganic moieties, and surface polymerization.

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4.1.1.1 Functionalization by Organic Moieties 4.1.1.1.1 Direct Functionalization (Cocondensation and In Situ)

This method involves the modification of nanomaterials by ligand compounds during the course of synthesis. Nature of the ligand used in this method is comprised of bifunctional groups wherein one of the functional group helps in connecting the ligand and nanomaterial surface (complexing group) and the other group comprehends required functional group (modifying group) [11]. This method enables the homogeneous functionalization of the nanomaterials in one-pot synthesis and offers better control over the amount of ligand integrated into the nanomaterial and feasible to use a wide range of functional groups. However, the functionalizing moieties may not be well matched with the synthesis process as the modifying group may perhaps directly react with the surface of nanomaterials [12].

4.1.1.1.2 Postsynthetic Functionalization (Grafting)

This method allows the utilization of several varieties of functional groups. Present synthetic method is carried out by grafting or embedding ligands on the surface of nanomaterials after the synthesis. This process follows a binding group that reacts initially from the bifunctional compound and enables the conversion of a group of coupling sites, and process continues to the final functional group in the second step [9].

4.1.1.2 Surface Polymerization This process helps to enhance the interfacial interactions of the nanomaterials which provide the stability and dispersion. This method deals with combining the properties of polymer and nanomaterials to develop multifunctional material [9].

4.1.1.2.1 Grafting-To

A simple technique involves chemical bonding between polymers and nanomaterial surface active sites. Herein, polymers comprise of end group functionalities or attaching moieties along with their backbone. The downside of this technique is that it can be provided with low grafting densities due to steric hindrance of reactive sites by the formerly attached polymers [9].

4.1.1.2.2 Grafting-From

Grafting-from technique deals with the insertion of small monomers which lead to decrease the adverse effects from steric hindrance and formerly attached polymers to the active sites of nanomaterials. This type of technique

112  Functionalized Nanomaterials for Catalytic Application offers a great control over functionality, density, and thickness of the polymer covering of the nanomaterials for different applications [13].

4.1.1.2.3 Grafting-Through

This method deals with the copolymerization of the initiator (polymerizable units pulverized on top of the nanomaterial surface) and free monomers in the solution followed by polymerization which leads to form polymer chains straight away on the surface of nanomaterials [14]. Apart from grafting techniques, other methods are also used in the synthesis of polymer-nanomaterial hybrids, namely, in situ polymerization, sol-gel process, blending, emulsion, polymerization [15].

4.1.2 Nanomaterial-Functional Group Bonding Type The different types of functionalization of nanomaterials have already been discussed. It is important to know the type of bonding present in between nanomaterial and functional group. There are mainly two types of bonding that occurs between the nanomaterial and functional group [9]. a. Functionalization by covalent bond b. Functionalization by noncovalent bond

4.1.2.1 Functionalization by Covalent Bond The type of functionalization where interaction between the nanomaterials and functional group occurs through covalent bond and the process is also called as chemisorption process. This type of surface functionalization is prominent due to the formation of a covalent bond, which offers strong linkage and stable surface [9]. This kind of bonding can be observed when functional group of the ligand molecule is feasible to react with substrate materials, followed by chemisorption and results self-assembled structures. Chemisorption is seen very frequently in case of bonding in organic molecules to metal oxides and inorganic semiconductors. Choice of substrate material and ligands can be made by the nature of substrate material and functional group of the ligand. Example for chemisorption comprises of thiols, silanes, amines, and phosphates on metals and metal-based nanomaterials [16].

4.1.2.2 Functionalization by Noncovalent Bond Noncovalent type of bonding arises when ligand and nanomaterial is bonded with weak force of attraction or weak bonds, which leads to

FNMs for Environmental Applications  113 physisorption. This physisorption occurs mainly by hydrogen bonding, electrostatic interactions, hydrophobic interactions, and van der Waals forces. Addition to these, more prevalent noncovalent surface functionalization method is steric stabilization which comprises of polymers or surfactants as the capping layer [9]. Surface coverage stabilizes the individual nanoparticles and steric repulsion prevents agglomeration [17]. Noncovalent functionalization can be seen in graphene and carbon nanotubes by π-interaction which enables the attachment of organic molecules to the carbonaceous materials by maintaining the electronic properties intact and suitable for further application [18]. MNPs are frequently used as catalysts in various applications. Catalytic efficiency is directly associated to surface area of the nanoparticles. This is a well-known fact that MNPs have high surface area and might be a reason for instability of MNPs as particles are in nanoregime. Stabilizers or supporting materials are required to maintain the stability and avoid the agglomeration of MNPs [19]. To resolve the instability issue or for better performance, there is a need of supporting material on the surface of MNPs. Upon loading of supporting material to MNPs, the activity of MNPs as it embedded in the supporting matrix decreases. Due to this, catalytic activity decreases because of low/no contact between the metal catalyst and reactant [20, 21]. Hence, there is an urge for suitable supporting material to stabilize the MNPs without inhibiting the catalytic activity. Many materials are currently used to serve as support materials for MNPs such as SiO2, zeolites, nitrides, carbon-based materials [22], chitosan, cellulose, chelating resin, and polymers. Along with these support materials, metal-polymer complex has been extensively used as efficient composite materials. Polysaccharide biopolymers like alginate, chitosan, and starch are abundantly used as support materials, as they help in the growth of MNPs [23, 24]. Hence, functionalization of MNPs is the better solution to maintain the catalytic properties of MNPs. Functionalized nanomaterials have been used in diverse fields due to their enhanced properties. In this chapter, functionalized catalytic materials are utilized as the potential materials for environmental applications. Different nanomaterials functionalized by various materials, such as chitosan, cellulose, alumina, mixed composites, and other polymer-coated materials have been discussed. These functionalized nanomaterials were successfully employed for the environmental remediation in the form of detection and degradation of pesticides by FNMs. Even though use of pesticides helps in increasing the productivity and storage of crops in fulfilling the demand of huge population, they are great threat to environment and living system as it is toxic in nature and causes many types of diseases and pollution

114  Functionalized Nanomaterials for Catalytic Application [25]. Hence, detection and degradation are necessary for the benefit of the society. Removal/degradation of dye molecules using FNMs has been discussed. Due to increase in the industrialization in textile field and modernization, the use of dyes increasing rapidly. Dyes are used in textile industries, hair dyes, food coloring agents, and paints dumped into the water bodies without proper treatment [26]. The pesticides, paints, and dyes are contaminants to the water bodies and are threat to the humans, aquatics, and environment. Hence, a broad vision should be preserved for the removal, reuse, or degradation of all these contaminants. Heavy metal ions removal is one the important aspect, which cannot be neglected and focused in the application part [27]. Environment is contaminated due to the extensive use of all these hazardous materials. Functionalized materials have been implemented for the detection and degradation/removal of these materials from the environment.

4.2 Functionalized Nanomaterials in Environmental Applications 4.2.1 Chitosan Chitin is the second most abundant natural polymer, after cellulose. It mainly exists in fungi and exoskeleton of crustaceans. Chitin mainly obtained from the seafood wastage and it is de-acetylated to yield chitosan [28]. Chitin is a semi-crystalline homopolymer of β-(1→4)-linked N-acetyl-D-glucosamine. The profitable source of chitin is mostly from crab, krill shells, shrimp, and fungi and used for the production of chitosan. Production of shrimp and chitin from shellfish has been estimated around 3 lakh metric tonnes and 60,000–80,000 tonnes (on an average in 2013 and 2004 for shrimp and chitin from shellfish, respectively) produced in India [29]. Hence, there is huge raw material available to utilize for the environmental applications. Chitosan contains one primary amine and two free hydroxyl groups per monomer having a unit formula of C6H11O4N. Chitosan is structurally similar to cellulose, where the hydroxyl at carbon-2 has been replaced by acetamido or amino group. Polysaccharides typically contain carbon, hydrogen, and oxygen, but chitin and chitosan contain nitrogen as an additional element with 6.89% [30]. This unique feature of chitosan is the cause for the diverse applications. Chitosan is the only alkaline polysaccharide found in nature, but the other polysaccharides, namely, dextran, starch, agar-agar, cellulose, pectin, alginic acid, and carrageenan, are neutral or acidic in nature [31]. Chitosan is odorless, nontoxic, biocompatible, and biodegradable and

FNMs for Environmental Applications  115 that breaks down gradually to harmless products (amino sugars). These products are completely absorbed in the body. Clinically proven fact is that chitosan of 3–6 g a day has no negative effects on body [32]. a) Ali et al. successfully utilized tri-composite nanoparticles embedded in chitosan microspheres for degradation of congo red (CR) dye, which is threat to environment [33]. Bismuth cobalt selenide (BiSe-CoSe) nanoparticles synthesized through solvothermal process and sodium dodecyl sulfate (SDS) surfactant was used as stabilizer for the well dispersion of nanoparticles [33]. These synthesized nanoparticles were added to chitosan solution for the preparation of BiSe-CoSe nanoparticles chitosan microspheres (BCSN-CM). These microspheres were cross-linked with the help of glutaraldehyde and used for the degradation of CR. Chitosan acts as an excellent support and helps to avoid leaching of nanoparticles. BCSN-CM enables the easy recovery of the catalysts (BiSe-CoSe nanoparticles) and used for many consecutive cycles. BCSN-CM has been successfully employed for the photocatalytic degradation of CR dye. BCSN-CM displayed an efficient photocatalytic activity for CR with 70% degradation in first 20 min, whereas 98 % in 100 min. BCSN-CM material was effectively recovered and reused for five consecutive cycles [33]. b) Polyurethane sponge (PUS) was used as a substrate for the synthesis of different MNPs such as Ag, Co, Cu, and Ni. To ensure the substrate affinity toward the nanoparticles, chitosan (CS) was coated initially before the PUS contact with metals. Later, the CS-PUS was dipped in individual metal solutions for the proper adsorption. NaBH4, a strong reducing agent was added to the solution to form nanoparticles (zero valency) from metal ions such as Ag+, Co2+, Cu2+, and Ni2+. MNP-coated CS-PUS used for the reduction of 4-NP and required for 18, 7, 5, and 14 min for Ag, Co, Cu, and Ni nanoparticles, respectively, for complete reduction of 4-NP in aqueous solution. Here, Cu nanoparticle–coated CS-PUS performed excellent catalytic activity compared to other MNPs [34]. c) Fe0 nanoparticles (NZVI) were successfully immobilized on epichlorohydrin (ECH)/chitosan beads (ECH-CS-NZVI beads) and used for the successful reduction of heavy metal, Cr(VI) from wastewater [35]. Mechanical strength of chitosan can be improved by either physical or chemical modifications. One of the most common approaches of chemical modification can be achieved by ECH and beads which are modified uphold their strength in acidic solutions. Hexavalent chromium (Cr(VI)) solution of 20 mg/L was used as stock solution for the removal of Cr from water. Removal rates of Cr were observed till four consecutive cycles and the removal rate in first cycle is greater than 95% and 76.6%, 48.2%, and 30.5% for the next consecutive cycles [35].

116  Functionalized Nanomaterials for Catalytic Application d) Zhang et al. have synthesized a composite material composed of ­chitosan-coated octadecyl-functionalized magnetite nanoparticles (Fe3O4C18-chitosan MNPs) and used as an efficient adsorbent for the extraction of anionic pollutants, perfluorinated compounds (PFCs) from environmental water samples [36]. For the synthesis of Fe3O4-C18-chitosan MNPs composite, initially Fe3O4 MNPs were synthesized by the well-known coprecipitation method. In the second step, synthesized Fe3O4 MNPs were functionalized with octadecyltriethoxysilane. Finally, Fe3O4-C18 MNPs were dispersed in chitosan solution to yield Fe3O4-C18-chitosan MNPs. Chitosan polymer layer

(a)

Tripolyphosphate C18 groups

HO HO OH OH OH HO OH HO OH HO OH HO OH HO OH 3 4 HO OH HO OH HO OH HO OH HO HO OH OH

Fe O

Silanization

Fe3O4

Fe3O4

Ionotropic gelation Analyte molecule Macro-molecule

Magnetic Separation

ne

t

Adsorption

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ag

MNPs Adsorbent Adsorbent with analytes adsorbed

Analytes

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N2

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Transfer of elaute

Reuse

Magnet

Figure 4.1  (a) Steps of synthesis of Fe3O4-C18-Chitosan MNPs and (b) application of Fe3O4-C18-chitosan MNPs as adsorbents for analysis. (Reprinted with permission from Ref. (Zhang et al., 2010), Copyright 2010, Analytical chemistry).

FNMs for Environmental Applications  117 (a)

(b)

50nm

50nm

Figure 4.2  (a) TEM images of Fe3O4 MNPs and (b) Fe3O4-C18-chitosan MNPs. (Reprinted with permission from Ref. (Zhang et al., 2010), Copyright 2010, Analytical chemistry).

Figure 4.1a represents the steps of synthesis of Fe3O4-C18-Chitosan MNPs. The material is used for the detection of different compounds such as, perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), perfluoroundecanoic acid (PFUnDA), perfluorododecanoic acid (PFDoDA), and perfluorotetradecanoic acid (PFTA), performed for wastewater samples and achieved limit of detection of 0.24, 0.093, 0.24, 0.14, 0.075, 0.24, and 0.17 ng/L, respectively. Figure 4.1b represents the application of Fe3O4-C18Chitosan MNPs as adsorbents for analysis. TEM images of Fe3O4 MNPs and Fe3O4-C18-chitosan MNPs are shown in Figures 4.2a and b [36].

4.2.2 Cellulose a) A flexible transparent free standing platform was constructed using 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (TEMPOCNF) and noble MNPs, namely, gold nanoparticles (Au NPs) [37]. For instance, nanospheres (Au NSs) and nanorods (Au NRs) are used as substrates for the surface enhanced Raman scattering (SERS) technique for in situ chemical sensing of a real-world surface. Initially, Au NSs and Au NRs were prepared individually and followed by fabrication of TEMPO-CNFs. Final stage is the fabrication of TEMPO-CNF/AuNP nanocomposites with different Au structures, such as Au NSs and Au NRs. TEMPO-CNF/AuNP matrix were achieved by two-step vacuum filtration, wherein TEMPO-CNF dispersion was poured on cellulose ester membrane followed by introduction of Au NPs solution in wet condition of TEMPO-CNF matrix. TEMPO-CNF/AuNP substrate was peeled off from the filter membrane as active substrate to use in SERS. Thiram

118  Functionalized Nanomaterials for Catalytic Application (TRM), a pesticide and R6G, a dye were selected as analytes for the detection through SERS. TEMPO-CNF/AuNR nanocomposite exhibited enhanced SERS activity than the TEMPO-CNF/AuNS nanocomposite. Detection limit of TRM and R6G were achieved to 60 ng/cm2 and 10 nM, respectively, through TEMPO-CNF/AuNR nanocomposite. Flexible and transparent nanopaper used as SERS substrate, and detection was achieved on both sides of the substrate (front and back, as substrate is transparent) along with TRM was detected potentially in real-world surface, i.e., on apple peel. SERS substrate used here can be implemented in real-time application for environmental monitoring. Figure 4.3a shows the fabrication of TEMPO-CNF/AuNP nanocomposites through vacuum filtration [37]. Figure 4.3b and c display the high resolution transmission electron microscopy (HRTEM) images of Au NSs and Au NRs. Figure 4.3d shows (b)

(c) TEMPO-CNF/ Plasmonic NP solution

Filter vacuum-assisted filtration membrane

(f)

AuNP solution AuNR solution

TEMPO-CNF/AuNP TEMPO-CNF/AuNR

0.8 0.6 0.4 0.2 400

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100

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TEMPO-CNF TEMPO-CNF/ TEMPO-CNF/ AuNP AuNR

(a)

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(g)

80 Plasmonic layer

60 40 COO– H+/Na+

20 HO

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400

O OH O

OH

OH OH

O

TEMPO-CNF/AuNR nanocomposite

O

500 600 700 Wavelength (nm)

800

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Figure 4.3  (a) Fabrication of TEMPO-CNF/AuNP (Au NS or Au NR)–based SERS-active substrates using two-step vacuum-assisted filtration: (i) first filtration of the TEMPO-CNF solution on the membrane filter to produce the transparent TEMPO-CNF matrix; (ii) second filtration of the mixture solution of TEMPO-CNF and Au NP on the TEMPOCNF matrix to produce flexible, transparent nanopaper-based SERS substrates. (b, c) High resolution transmission electron microscopy images of Au NSs and Au NRs; scale bar = 50 nm. (d) UV-vis spectra of Au NS dispersion, Au NR suspension, TEMPO-CNF/AuNS nanocomposite, and TEMPO-CNF/AuNR nanocomposite. (e) Optical transmittance of the TEMPO-CNF matrix. Inset: photographic image of the transparent TEMPO-CNF matrix (3 cm × 3 cm) and the chemical structure of TEMPO-CNF. (f) Photographic and (g) top-view/cross-sectional FE-SEM images of the TEMPO-CNF/AuNR nanocomposite substrate (3 cm × 3 cm). (Reprinted with permission from Ref. (Kim et al., 2019), Copyright 2019, ACS Sustainable Chemistry & Engineering).

FNMs for Environmental Applications  119 the UV-vis absorption spectra of Au NS dispersion, Au NR suspension, TEMPO-CNF/AuNS nanocomposite, and TEMPO-CNF/AuNR nanocomposite. The optical transmittance of the TEMPO-CNF matrix is shown in Figure 4.3e and the inset in Figure 4.3e display the photographic image of the transparent TEMPO-CNF matrix with 3 cm × 3 cm dimension and also the chemical structure of TEMPO-CNF is shown in red colour. The photographic image of TEMPO-CNF/AuNR nanocomposite is shown in Figure 4.3f. The top-view/cross-sectional FE-SEM images of the TEMPOCNF/AuNR nanocomposite substrate is shown in Figure 4.3g. Comparison of the adhesive stability of the TEMPO-CNF/AuNP composite which is fabricated by two different approaches in the second filtration step are shown in Figure 4.4. Figure 4.4a shows the pure Au NP solution filtered through the TEMPO-CNF matrix and Figure 4.4b shows the mixture of TEMPO-CNFs and Au NP solution filtered through the TEMPO-CNF matrix. The corresponding Raman spectra for before and 5 mm

Rubbing

5 mm

Before rubbing After rubbing

Intensity (a.u.)

(a)

10k 500 nm

5 mm

Rubbing

600

5 mm

800

1000 1200 1400 Raman shift (cm–1)

1600

Before rubbing After rubbing

Intensity (a.u.)

(b)

500 nm

2.5k 500 nm

500 nm

600

800

1000 1200 1400 Raman shift (cm–1)

1600

Figure 4.4  Comparison of the adhesive stability of the TEMPO-CNF/AuNP nanocomposites fabricated by two different approaches in the second filtration step: (a) pure Au NP solution was filtered through the TEMPO-CNF matrix and (b) mixture of TEMPO-CNFs and Au NP solution was filtered through the TEMPO-CNF matrix. (Reprinted with permission from Ref. (Kim et al., 2019), Copyright 2019, ACS Sustainable Chemistry & Engineering).

120  Functionalized Nanomaterials for Catalytic Application after rubbing with cotton swab for two different fabricated Raman substrates are shown in Figure 4.4a and Figure 4.4b. b) A flexible SERS substrate was developed with nanocomposite based on CNFs coated with silver nanoparticles (Ag NPs) [38]. Initially, CNF film was synthesized, and later, Ag NPs were impregnated into the CNF film. Before the SERS measurements, Raman indicator, 4-aminothiophenol (p-ATP) was used as to conjugate Ag NPs impregnated on CNFs. Thiabendazole (TBZ) pesticide (frequently applied on apple for pest control) is used as analyte for SERS measurements on apple. TBZ is recognized as neutral molecule and has a low attraction toward Ag NPs. Electrostatic interaction between TBZ and Ag NPs was improved by decreasing the pH of TBZ solution to below the TBZ’s pKa. Thus, CNF-Ag NPs nanocomposites with p-ATP as Raman indicator served as an efficient substrate for real food sample analysis [38]. c) Nanocellulose-based materials are of great interest toward SERS as substrates. Ziyi et al. have developed CNF-based nanocomposite and modified with Au NPs as a SERS substrate [39]. Initially, citrate stabilized Au NPs were synthesized and kept for next use. CNF were cationized with ammonium ions using (2,3-epoxypropyl) trimethylammonium chloride (EPTMAC). In the next step, cationized CNF and Au NPs were mixed in 1:1 ratio and vertexed for the proper mixing. CNF/AuNP nanocomposites were then deposited on a gold side as SERS substrate. Cationized CNF and Au NPs were interacted by electrostatic attraction, and hence, form homogeneous nanocomposites. TRM pesticide was successfully detected in apple juice via SERS and achieved 52 ppb limit of detection. Comparison of substrates has examined by considering the intensity obtained from SERS, wherein CNF/AuNP composite and filter paper/AuNP substrates were considered. CNF/AuNP composite substrate provided with 20 times higher intensity compared to filter paper/ AuNP. This emphasize the use of CNF material along with Au NPs [39]. d) Nanoporous cellulose paper-based SERS platform was constructed for the detection of multiple analytes. Thiram (T1), tricyclazole (T2), and carbaryl (C) were selected as analytes for the detection through SERS [40]. Composite substrate composed of nanoporous CNF and gold nanorods (Au NRs). Au NRs were synthesized with three different aspect ratios (ARs) of 2.4, 3.1, and 3.7. CNF matrix comprise of nanoscale surface roughness which facilitates nanofiltration, this offers uniform and controlled distribution of Au NRs over the top layer of CNF matrix. CNF–Au NR composite with three different AR were examined for R6G detection and found that Au NRs of 2.4 AR with CNF exhibited enhancement of the signals compared to Au NRs of other two AR. Individual and multiple analytes were detected simultaneously. LOD achieved in simultaneous detection observed as down to 1 nM, 100 nM, and 1 µM for T1, T2, and C, respectively. Swab test was

FNMs for Environmental Applications  121 carried out for real-world surface, apple peeled for triple pesticide solution, and achieved LOD of 6, 60, and 600 ng cm−2 for T1, T2, and C, respectively. Hence, CNF–Au NR composite acts as a paper-based SERS platform as the powerful substrate for on-site SERS analysis [40].

4.2.3 Alumina a) Activated alumina supported Au and Ag NPs were successfully synthesized for the removal of two common pesticides, chlorpyrifos (CPF), and malathion (MLT) from water. Au and Ag NPs were synthesized using citrate as stabilizer [41]. Calculated amount of alumina globules of average diameter of 0.5 cm was taken and soaked in Au@citrate and Ag@citrate solution individually in hot condition. The globules were left in the nanoparticle solution for the complete adsorption (6 h), till the saturation level. These globules were washed with distilled water, dried, and represented as Al2O3@Ag and Al2O3@Au. For the removal of CPF and MLT, Ag NPs, Au NPs, activated alumina, Al2O3@Ag and Al2O3@Au were used. Ag or Au NP–loaded globules were added to calculated volume of CPF/MLT and UV-Vis spectra of the aliquots were monitored with a certain interval of time. This process is time consuming and completely removes the pesticide from solution. In order to decrease the time period of detection, practical online filter was used. Glass column was loaded with activated alumina powder which was previously fitted with silica frit at the bottom. Then, Au or Ag NPs solution was poured from the top of the column and the outcome of the column was monitored. NPs solution was kept on pouring till the outcome solution was colorless. CPF/MLT solution was poured to the same glass tube initially loaded with alumina powder with NPs and outcome solution was observed. This setup with alumina powder decreases the time for complete removal of pesticides. Control experiment was carried out wherein bare Al2O3 was unable to remove pesticides from water. This work described as potential tool for the removal of pesticide contamination in drinking water using Al2O3@Ag [41]. b) Visual detection of the pesticide is also an important factor to monitor the presence of pesticides. In this regard, Lisha et al. have made a tremendous effort for the visual detection of CPF and MLT in the naked eye [42]. Au NPs served as an excellent material for the visual detection of CPF and MLT. Au and Ag NPs were synthesized using citrate reduction method. For the detection CPF and MLT along with Au NPs, Na2SO4 salt solution was added for the naked eye detection. Reaction also carried out without the addition of Na2SO4 and observed no color change in the reaction. Na2SO4 helped in rapid detection of CPF/MLT as Na2SO4 binds the NPs to bring the NPs closer. Syringe filter was fabricated and loaded

122  Functionalized Nanomaterials for Catalytic Application with Ag NP–coated alumina as an adsorbent for the ground water sample analysis, where water was spiked by CPF. Ag NP–coated alumina showed a great adsorption capacity [42]. c) Application of nanomaterial for the environmental remediation is a great topic in the recent decades. Removal of pesticides still a challenge and degradation of pesticides to its harmless moieties is an add-on key point for the betterment of environment. Bhavya et al. studied the detection and degradation of CPF and MLT pesticides using Au NRs and achieved visual detection with ppt concentration and degradation of both pesticides. Au NRs were self-assembled such as side to side and end to end fashion after interaction with CPF and MLT, respectively [43]. Recyclability of Au NRs was monitored for four consecutive cycles with good percentage of recovery [43]. Citrate capped Ag NPs and Au NPs were synthesized and both the NPs were further added to neutral alumina as the support [44]. Ag and Au NPs with and without support were subjected for the degradation of chlorpyrifos (CP). Ag@citrate NPs exhibited maximum absorption at 424 nm in water (inset of Figure 4.5a, trace a). CP shows two absorption peak at 229 and 289 nm in water:methanol mixture of 1:1 (inset of Figure 4.5a, trace b). Three different concentrations of CP such as 1, 10, and 50 ppm were treated with Ag@citrate NPs, individually and UV-Vis absorption spectra recorded at 24 and 48 h, shown in Figures 4.5a and b, respectively. With increase in concentration, Ag plasmon peak shifted toward higher wavelength due to agglomeration of Ag NPs, on reacting with CP. After 24 h, the peak at 289 nm of CP disappeared (50 ppm) and new peak at 320 nm appeared as degraded product of CP (Figure 4.5a, trace c). After 24 h, for 10 ppm CP with Ag NPs, a small hump appeared at 320 nm (Figure 4.5a, trace b) and clearly visible at 48 h (Figure 4.5b, trace b). The degradation products were subjected electrospray ionization mass spectrometry (ESI MS) to know the fate of new peak. The molecular ion peak of CP observed at m/z 350.5 and three major fragments at m/z 323, 295, and 277, due to loss of C2H4 (m/z 28), 2C2H4 (m/z 56), and 2C2H4 + H2O (m/z 74), from m/z 350.5 (Figure 4.5c, trace a). The m/z 198, major species appeared due to protonated TCP. ESI MS of the reaction product shows the disappearance of m/z 350.5 (CP) (Figure 4.5c, trace b) confirms the degradation of CP. Peak a 198 is obtained due to TCP, in detail TCP having three Cl atoms and due to this isotopes 198, 200, and 202 peak observed, shown in Figure 4.5d [44]. CP decomposed to 3,5,6-trichloro-2-pyridinol (TCP) and diethyl thiophosphate (DETP) at room temperature in the presence of bare Ag and Au NPs as well as in supported forms. Among supported and unsupported NPs, supported NPs provided with better degradation of CP. This is because supported NPs can be reusable, and aggregation is very minimal

FNMs for Environmental Applications  123 (a)

0.3

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Figure 4.5  UV-vis absorption spectra of Ag@citrate NPs treated with 1, 10, and 50 ppm CP solutions (traces a, b, and c, respectively) after 24 and 48 h (a and b, respectively). The inset of A is UV-vis absorption spectra of Ag@citrate NPs and 1 ppm CP (a and b, respectively). (c) ESI MS of CP (trace a) and supernatant of reaction mixture of Ag@ citrate NPs and 10 ppm CP solution (trace b) after 48 h in positive mode. The inset of (c) is a comparison of calculated mass spectral positions (red trace) of protonated TCP and the experimental mass spectral region in m/z 198 (black trace). (d) MS/MS spectra of m/z 198, 200, and 202 (traces a, b, and c, respectively) in the reaction product. The chemical structure of m/z 198 is shown in (d). (Reprinted with permission from Ref. (Bootharaju et al., 2012), Copyright 2012, Langmuir).

as NPs were embedded in the supported matrix. But, bare NPs turned to aggregate after reacting with CP. Among supported Au and Ag NPs, Ag NPs were shown excellent rate of degradation than Au NPs. Other parameters like temperature and pH were also studied, where increase in both the parameters has positive impact on degradation of CP [44]. Mechanism of degradation of CP by Ag NPs was schematically represented in Figure 4.6. Sulfur and nitrogen atoms having lone pair of electrons and easily bound to NP to form a surface complex and Raman analysis confirmed the formation of Agn+←S bond. As the electron polarization occurs in surface complex, side chain link pyridine ring becomes weak. Attack of water molecule and phosphorus may result in P−O bond

124  Functionalized Nanomaterials for Catalytic Application Cl

Cl O

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H .. H O+ O O– P O N S

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CH3

O

CH3

DETP

Figure 4.6  Representation of degradation of CP on Ag NPs. Adsorbed CP molecules are shown with red stars. (Reprinted with permission from Ref. (Bootharaju et al., 2012), Copyright 2012, Langmuir).

cleavage. Then, withdrawing of electrons from Agn and electrons from sulfur results in the formation of TCP and DETP, respectively [44].

4.2.4 Mixed Composites a) MNPs templated chitosan-SiO2 (CS-SiO2) catalyst used for the reduction of 4-nitroaniline (4-NA) and dyes. Different metals such as Cu, Co, Ag, and Ni are treated with CS-SiO2 nanocomposite and utilized for further application toward environmental remediation [45]. Metal ions are easily attracted toward binding sites of CS, containing −NH2 or −OH group. Binding site of CS is less in number as compare to the nanocomposite (CS-SiO2). This is because nanocomposite fiber SiO2 helps to increase surface area along with enhancement in the binding sites because of lone pair of electrons of SiO2. This confirms that MNPs are prone to attach more toward nanocomposite fibers rather than CS. Initially, SiO2 nanomaterial is synthesized and then added to CH solution to obtain CS-SiO2 nanocomposite fibers. These fibers were dried and immersed in metal salt solution. A visual color change was observed when fiber reacted with different metals, wherein white fibers turned to blueish, reddish, brownish, and greenish upon reacting with Cu, Co, Ag, and Ni salt solutions, respectively. In order to reduce these metal

FNMs for Environmental Applications  125 ions to nanoparticles, NaBH4 was introduced as reducing agent. These MNP-loaded fibers MNPs (MNPs/CS-SiO2) were dried and further used for reduction of 4-NA and decoloration of CR dye. CS-SiO2 and MNPs/ CS-SiO2, both were subjected to reduction of 4-NA and decoloration of CR. In both the applications, MNPs/CS-SiO2 exhibited enhanced catalytic activity compared to CS-SiO2. In case of reduction of 4-NA by MNPs/ CS-SiO2, absorption peak at 380 nm was disappeared in 9, 14, 17, and 22 min in presence of NaBH4 and with Cu, Co, Ag, and Ni NPs, respectively. Time taken for the decoloration of CR in 5, 17, 13, and 19 min in presence of MNPs/CS-SiO2 loaded with Cu, Co, Ag, and Ni NPs, respectively, where absorption peak of CR at 495 nm was disappeared. Therefore, CS-SiO2 loaded with Cu demonstrated stronger catalytic activity in both cases [45]. b) Cai and co-workers proposed a composite of phosphorylated chitosan (CSP) and phosphate-decorated carboxymethyl cellulose (CMCP) were cross-linked to yield an effective water-stable CSP-CMCP composite [46]. Chitosan (CS) and carboxymethyl cellulose (CMC) were individually phosphorylated with methanesulfonic acid-phosphorus pentoxide and trisodium trimetaphosphate (STMP), respectively. Amino groups-CS molecules were present even after phosphorylation, which helps in further TEM image

1µm

1µm

O Kα1

C Kα1_2

1µm

1µm

P Kα1

N Kα1_2

1µm

U Lα1

1µm

Figure 4.7  TEM image and elemental mapping of U(VI)-adsorbed CSP-CMCP composite. (Reprinted with permission from Ref. (Cai et al., 2019), Copyright 2019, ACS Sustainable Chemistry & Engineering).

126  Functionalized Nanomaterials for Catalytic Application modification. Cross-linking of CSP and CMCP was carried out through a dehydration condensation process, which yields a water-stable CSPCMCP composite. CSP-CMCP composite was successfully used for the detection/capturing of radioactive uranium ion (U(VI)) in contaminated water system, under certain experimental conditions. U(VI) selectivity was checked in presence of trivalent Cr(III), La(III), Eu(III), and Yb(III) and also with divalent Cu(II), Cd(II), Co(II), and Sr(II). Presence of other ions have negligible interference on the selectivity and removal of U(VI). As shown in the TEM-elemental maps (Figure 4.7), phosphorus is abundant and uniformly distributed in the structure of CSP-CMCP. The abundant phosphate groups are expected to greatly promote the adsorption potential of CSP-CMCP composite for U(VI). CSP-CMCP composite was examined using TEM and elemental mapping analysis after the adsorption of U(VI) as shown in Figure 4.7. Elemental mapping confirmed that phosphorus is well coated over CSP-CMCP composite and further facilitates the adsorption of U, as seen in the mapping [46].

4.2.5 Other Nanocomposites for Environment a) Au NPs were sputtered on the surface of polydimethylsiloxane coated anodic aluminium oxide (PDMS@AAO) complex substrate which results in the formation of cauliflower-inspired 3D SERS substrate [47]. Initially, AAO array was fabricated and further PDMS@AAO complex was prepared. Trichlorooctadecylsilane (OTS) monolayer (self-assembled) was initially functionalized over the surface of AAO template in order to weaken the surface energy between PDMS and AAO template. In the next stage, mixed solution of PDMS elastomer and curing agent in the mass ratio of 10:1 was introduced onto the AAO template and kept at 85°C for 1 h for the solidification. Finally, the aluminium bases were detached by dipping the samples in a CuCl2 and HCl solution at 0°C and resulting complex was cut off into 5 mm ×5 mm sections for SERS application. Au was sputtered at different deposition time (4–14 min) to get cauliflower-inspired 3D substrate. The substrate was utilized for the detection of 4-mercaptobenzoic acid (4-MBA) and three mycotoxins, namely, aflatoxin B1 (AFB1), deoxynivalenol (DON), and zearalenone (ZON). Figure 4.8a shows the SERS spectra of 4-MBA with concentration ranging from 10−6 to 10−12 M. Figure 4.8b shows the SERS intensity of 10−5 M 4-MBA on 3D-nanocauli flower substrate stored in N2 atmosphere as well as in air for 21 days. Figure 4.8c represents SERS intensity mapping of 10−5 M 4-MBA measured for 1,580 cm−1 peak. SERS spectra collected randomly from 40 places on 4-MBA (10−5 M) on substrate shown in Figure 4.8d. Figure 4.8e is the intensity distribution

FNMs for Environmental Applications  127 (a)

(b)

10–6 M

Intensity (a.u.)

10–12 M

Intensity (a.u.)

N2 Air

22.0k 20.0k 18.0k 16.0k

800

1000 1200 1400 1600 1800 Raman Shift (cm–1)

(c)

×104

0

(d)

3

(e)

4 3

0

m

tru

750 1000 1250 1500 1750

Raman Shift (cm–1)

ec

1

Sp

20 µm

20 30

No .

10

2

Intensity (a.u.)

600

6 9 12 15 18 Storage Time (days) 25.0k

21

1580cm–1, RSD=4.57%

20.0k 15.0k 10.0k 5.0k 0.0

0

10

20

30

Spectrum No.

40

Figure 4.8  (a) SERS spectra of 4-MBA with different concentrations (10−6 to 10−12 M) using the 3D-nanocauliflower substrates. (b) SERS intensities of 4-MBA (10−5 M) enhanced by the 3D-nanocauliflower substrates stored in N2 atmosphere at times ranging from 0 to 21 days. (c) SERS intensity mapping of 4-MBA (10−5 M) measured at the 1,580 cm−1 peak across a 60 μm × 60 μm piece of the 3D-Nanocauliflower substrate. (d) SERS spectra of 4-MBA (10−5 M) collected at 40 sites randomly on 3DNanocauliflower substrates and (e) intensity distribution at 1,580 cm−1 corresponding to part (d) (the average intensity is indicated by blue line, and the light blue zones represent the ± 4.57% intensity variation). (Reprinted with permission from Ref. (Li et al., 2019), Copyright 2019, Analytical Chemistry).

plot of 1,580 cm−1 peak from 40 places and RSD value was shown to 4.57%. Excellent LOD were achieved as low as 10−12 M, 1.8 ng/ml, 47.7 ng/ml, and 24.8 ng/ml for 4-MBA, AFB1, DON, and ZON, respectively. Further, all three mycotoxins were detected simultaneously through SERS analysis as shown in Figure 4.9 [47]. b) Yu and co-workers developed a capillary based system for the detection of R6G and thiram (TRM) pesticide. Au NRs were different AR (1.8 and 3.2) were synthesized and used as SERS substrate [48]. SERS-active glass capillaries were prepared and cleaned capillaries were dipped in APTES/ethanol solution for 12 h, inner wall of the capillary was functionalized by amino groups which results in the positively charged capillaries and were dried. Functionalized capillaries were dipped in PVP-capped Au NR solution and inner wall was coated with Au NRs effectively due to capillary force. Detailed schematic representation of fabrication process of SERS-active glass capillaries is shown in Figure 4.10. Initially, inner wall of

128  Functionalized Nanomaterials for Catalytic Application 1271

Raman Intensity (a.u.)

Multi-component ZON

AFB1 DON

1451

881 456

400

1036

659

1365 1344

1200

800

1499

1600

Raman Shift (cm–1)

Figure 4.9  SERS spectra of multiple components of three mycotoxins (AFB1, ZON, and DON) in maize using the 3D-Nanocauliflower substrate. (Reprinted with permission from Ref. (Li et al., 2019), Copyright 2019, Analytical Chemistry).

NH2

APTES coating

NH2

NH2 NH 2 NH2

NH2

O Si NH2

Bare capillary

AuNR-coated capillary

O

O

AuNR coating

NH2-modified capillary

Concentrated AuNR solution

Figure 4.10  A schematic diagram of the fabrication process of SERS-active glass capillaries and the photograph of a bare capillary and a Au NR–coated capillary. (Reprinted with permission from Ref. (Yu et al., 2019), Copyright 2019, ACS Applied Nano Materials).

FNMs for Environmental Applications  129 the capillaries was functionalized by APTES, which gives NH2 functionalization. Later, NH2 functionalized capillary dipped into concentrated Au NRs solution and Au NRs were coated in the inner wall of capillary due to capillary action. These Au NR–coated capillaries were utilized for the detection of R6G and TRM [48]. Au NR–coated glass sheet and Au NR–coated capillary were used for the SERS study. Au NR–coated glass sheet was unable to provide any signal for the analyte. This experiment was conducted on apple surface which was spiked by the pesticide. Figure 4.11a shows the process of in situ extraction and detection of pesticide on apple surface. Figure 4.11b shows the SERS spectrum of thiram residues with a concentration of 2.4 μg/cm2 detected using Au NR–coated glass sheet. Figure 4.11c shows the SERS spectra of laser

(a)

Drying Scheme 1 Drop with ethanol

laser Thiram molecule AuNR-coated glass sheet

Scheme 2 AuNR-coated capillary

(b)

SERS spectrum

(c) 40k

3k

2.4µg/cm2

2k

1k

0 400 600 800 1000 1200 1400 1600 1800 2000 Raman shift (cm–1)

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2.4µg/cm–2

240ng/cm2

30k

24ng/cm2

20k

1380 1503

10k

560

0 400 600 800 1000 1200 1400 1600 1800 2000 Raman shift (cm–1)

Figure 4.11  (a) A schematic diagram showing the in situ extraction and detection of pesticide residues on the apple peel using a Au NR–coated capillary (AR-1.8) and a Au NR–coated glass sheet. (b) The SERS spectrum of thiram residues with a concentration of 2.4 μg/cm2 detected by a Au NR–coated glass sheet. (c) The SERS spectra of the thiram residues with a concentration of 2.4 μg/cm2, 240 ng/cm2, and 24 ng/cm2 detected by a Au NR–coated capillary (AR-1.8). The excitation wavelength is 633 nm and the laser power is 0.85 mW in these Raman measurements. (Reprinted with permission from Ref. (Yu et al., 2019), Copyright 2019, ACS Applied Nano Materials).

130  Functionalized Nanomaterials for Catalytic Application the thiram residues with a concentration of 2.4 μg/cm2, 240 ng/cm2, and 24 ng/cm2 detected using Au NR–coated capillary [48].

4.3 Conclusion Functionalization of nanomaterials offers a great advantage over the bare nanomaterials to overcome the limitations. Different processes of functionalization through organic moieties such as direct functionalization and postsynthetic functionalization have been discussed. Surface polymerization process with different techniques such as grafting-to, grafting-from, and grafting-through are illustrated. Nanomaterial and functional group bonding such as covalent and noncovalent are described. Different materials used for the functionalization such as chitosan, cellulose, and alumina with different nanomaterials like Au, Ag, bismuth cobalt selenide, Co, Cu, Ni, Fe, and Fe3O4 are well explained with the synthesis procedure and application in different aspects such as pesticide detection and dye detection/ removal. Mixed composite materials were used with significant results in organic pollutant removal and heavy metal detection. Other nanocomposites, like PDMS with MNPs, were studied for the multiple analytes detection, individually as well as in mixed solution system. All these composites were found to have better catalytic performance with number of cycles. FNMs show excellent performance in real-time sample analysis.

Acknowledgements BMB acknowledges JAIN University for Junior Research fellowship. AKS is grateful to the SERB, New Delhi, India for funding to conduct the research (CRG/2018/003533).

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5 Synthesis of Functionalized Nanomaterial (FNM)–Based Catalytic Materials Swarnalata Swain1, Prangya Bhol1, M.B. Bhavya1, Sudesh Yadav2, Ali Altaee2, Manav Saxena1, Pramila K. Misra3 and Akshaya K. Samal1* Center for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagara, Bangalore, India 2 Center for Green Technology, School of Civil and Environmental Engineering, University of Technology Sydney, Sydney, NSW, Australia 3 Center of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar, Odisha, India 1

Abstract

Nanomaterials (NMs) are the building units for advanced science and technology. The high surface area and porosity of the NMs excel its properties in many folds compared to the bulk counterpart. NMs allow easy functionalization of surface, tailoring the electronic properties, conductivity, catalytic property, and response to physical events. The functionalization of NMs allows the inclusion of functional groups and species over the surface, resulting to an improved structure with novel characteristics. As compared to homogeneous catalyst, heterogeneous catalyst has shown emerging performance in the various chemical reactions due to synergistic effect developed from intimate contact between different elements incorporated. In this chapter, various fabrication techniques such as co-precipitation, impregnation, ion-exchange, immobilization, sol-gel, chemical vapor deposition, hydrothermal, microemulsion, and thermal decomposition methods used to synthesize functionalized NMs have been briefly discussed. The characteristic features of carbon-based materials and their functionalization technique such as covalent and noncovalent functionalization based on the π-conjugation system or force of attraction are emphasized. This chapter also discusses various functionalized based materials such as Pd, Pt, Ag, graphene, carbon nanotubes, biomaterials, and metal oxides and their usefulness in influencing the catalytic performance with examples. *Corresponding author: [email protected]; [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (135–168) © 2021 Scrivener Publishing LLC

135

136  Functionalized Nanomaterials for Catalytic Application Keywords:  Functionalized nanomaterials, graphene, metal oxide, chitosan, catalyst, carbon nanotubes

5.1 Introduction The synthesis and functionalization of ultrafine well-organized catalytic active nanomaterials (NMs) are significantly important and demanding. Compared to the bulk element, nanocatalyst introduces huge number of active sites and reduces the amount of catalyst loading. Porosity of the NMs is another effective factor which amplifies the efficiency, selectivity, stability, and activity, as well as reactivity and strengthens the utilization of NMs toward various applications [1]. Nanoparticles (NPs) are broadly utilized in many applications such as water purification, environment, energy, and catalysis with outstanding performance. Due to the rapid development of nanotechnology and functional-based NMs, it is possible to combine them to modulate the physicochemical characteristics with molecular recognition and catalytic performance. The nanocatalyst suffers certain drawbacks when used without support or fabrication. Nanocatalyst shows remarkably high surface area. The use of bare nanocatalyst leads agglomeration after running for a few catalytic cycles and considerably reduces the reactivity [2]. This instability behavior of NPs leads agglomeration which significantly decreases the number of surface active sites and hence the decrease in the activity [3]. Implementation of bare NMs deals with high cost and reusability in many applications, mainly in industrial purpose where a high amount of material loading is required. Moreover, to avoid the dampening the efficiency of nanocatalysts and minimization of cost, various functionalization or stabilizer/ support required in order to avoid agglomeration or fabrication methods have been developed which functionalizes the surface of nanocatalysts. Functionalization of NMs accumulates the incorporation of functional groups on the activated surface sites which results a new organized NMs for advanced applications. Electrocatalyst plays considerable role in the energy conversion application as they fasten the rate, selectivity and enhanced the efficiency in the chemical reaction. However, adequate electrocatalyst with advanced performance remained a challenge. To overcome this pitfall, designing a functionalized or bi-functional catalyst contributing enhanced performance has been developed. When the NMs are modified with different species and groups, they facilitate high affinity, high enrichment factors, and adsorption capacity and accelerate the adsorption and desorption

FNM–Based Catalytic Materials  137 rate in the separation and preconcentration-based techniques. Moreover, it offers improved separation efficiency, low detection limit, easier dispersibility in the liquid phases, fast response, and selective response signal for various analytical techniques [4–6]. Several ways have been adopted to functionalize the surface of NMs for catalytic applications. In this chapter, different FNM-based catalysts and their fabrication techniques are discussed along with their catalytic performances and outcomes. The chapter begins the discussion with various synthetic approaches to design FNMs and their characteristics. Different solid support such as magnetic materials, silica, carbon nanotubes (CNTs), graphene, and polysaccharides stabilizes the NPs catalyst. The electrocatalytic performance of Pt, Ag, and carbon-based materials with examples highlighted the strategies adopted in order to develop the high-performance catalytic materials.

5.2 Methods Followed for Fabrication of FNMs More than 90% of NMs have been prepared by chemical methods followed by 8% of physical methods and 2% of biological methods. Several techniques have been proposed for the fabrication or functionalization of NMs [7–9]. The most frequently used methods in the manufacture and functionalization of NMs are mentioned in Figure 5.1. Out of a number of synthetic methods, chemical exfoliation, chemical vapor deposition (CVD), hydrothermal growth, solvothermal growth, and thermal decomposition are used to functionalize the NMs surface

11. Thermal decomposition

1. Coprecipitation

2. Impregnation 3. Ion exchange

10. Solvothermal

9. Hydrothermal

Functionalization Methods

8. Microemulsion

4. Immobilization 5. Sol-gel

7. CVD

6. Laser ablation

Figure 5.1  Schematic representation of the various methods followed for the functionalization of NMs.

138  Functionalized Nanomaterials for Catalytic Application for electrocatalysis applications [10–14]. We further discusses a few of the important functionalization methods.

5.2.1 Co-Precipitation Method Co-precipitation is an easy and suitable method for the preparation of functionalized NMs. This technique is easy, cost effective, requires less effort and used for industrial purpose in large scale [15]. The advantages of co-precipitation method require no organic solvents, neither high pressure nor temperature. It gives high purity to the NMs and undergoes through an environment friendly route [7]. For instance, the experimental setup for the fabrication of iron oxide or metal ferrite NPs by co-precipitation method where the aqueous solution of Fe (III) or Fe (II) and a d-block transition metal M (II) salts are frequently mixed in an alkaline solution [15]. After a minimum interval of time, the reaction mixture starts precipitating. The equation for chemical reaction is as follows:

Fe2+ (aq) + 2Fe3+ (aq) + 8OH− (aq) → Fe3O4(s) + 4H2O (l) M2+ (aq) + 2Fe3+ (aq) + 8OH− (aq) → MFe2O4(s) + 4H2O (l) With several advantages, there exist some obstacles which limits the co-precipitation method such as difficult to deal with the particle size, shape, composition, crystallinity, and magnetic properties [7]. The properties of the NMs are highly dependent upon these parameters, and hence, it is necessary to examine and control the reaction parameters, e.g., temperature of reaction mixture, type of transition metal cation precursor, molar ratio of cationic metal, pH value, type of alkaline mediator selected for the precipitation, and also the concentration of the alkaline mediator [16]. The NPs synthesized by this method are less stable and agglomerates faster. To prevent the agglomeration, different types of surfactants are used which stabilize the surface of NMs. Addition of surfactant gives homogenous precipitation and uniform size distribution of NPs [17]. Pereira and coworkers observed the significance of the type of alkaline mediator role in the product formation. Two different types of alkanolamines such as mono-isopropanolamine and di-isopropanolamine are used in the fabrication of magnetic NPs (MNPs). By altering the bases, the particle size reduces up to six times and the magnetic properties improved up to 1.3 times. This proved that the alkanolamines play both the role as base and as the growth controlling agents [7].

FNM–Based Catalytic Materials  139

5.2.2 Impregnation Impregnation is the standard method to prepare the metal supported catalysts. It is the easiest approach to load a prearranged porous oxide support (such as silica, zirconia, and titania) with the metal component. There are two ways for preparing impregnated catalyst; one is in the solid-state way where both components are mixed physically in the solid state and other is via wet impregnation. This involves the physical mixture of the support in solid state and the metal component dissolved in a liquid solution [18]. The wet impregnation method is the most common approach to prepare the metal supported catalysts where a certain volume of solution containing the active phase with the solid support which later in succeeding step dried or calcined to remove the excess solvent to form metal impregnated catalyst. Copper-doped titania catalysts prepared using wet impregnation method where Cu(NO3)2 is doped into TiO2 and various copper doped species (Cu0, CuI, and CuII) were used for the photocatalytic reduction of CO2 [19]. Miranda et al. prepared the hybrid structure of graphitic carbon nitride (g-C3N4) and titanium oxide (TiO2) composites using impregnation method for the photocatalytic degradation of phenol under UV irradiation. The method involves the addition of an appropriate quantity of g-C3N4 and TiO2 into methanol separately and sonicated for 30 min. Then, these two solutions were mixed and stirred at room temperature for 24 h. Methanol was evaporated under rotary evaporator at 80°C and the composite was obtained used for photocatalysis [20].

5.2.3 Ion Exchange Ion exchange is a reversible process where ions are exchanged between two electrolytes. Generally, mesoporous silica and alumina absorbents are used for this method in which the replacement or ion exchange occurs between the metal atoms subjected on the calcined porous silicates or aluminates with the charge compensating ion present in the framework. Ion exchangers can be divided into three types depending upon the type of metal ions exchanged with it. • Positive ion exchangers, • Negative ion exchangers, and • Amphoteric ion exchangers. Positive ion exchangers exchange the positive ions, and negative ion exchangers exchange the negative charged ions, whereas both the ions can

140  Functionalized Nanomaterials for Catalytic Application be exchanged by amphoteric ion exchangers. Ion exchange between metal cations and mesoporous silicates, aluminates, and Al-containing silicates can occur using solid-state ion exchange procedure. The ion exchange from aqueous solution of a metal salt where, initially, the framework of zeolite is present in their calcined form. Generally, metal cations are collected from the sources such as chlorides, iodides, or acetates [18].

5.2.4 Immobilization/Encapsulation Brunel et al. have briefly explained the immobilization technique [21]. Immo­bilization or encapsulation is a method used for the enclosure of metal complexes into micro- or mesoporous silicate pores using two general procedures such as “flexible ligand method” or the “zeolite synthesis method.” In the flexible ligand method, ligand exchanges with the metal cations to give metal complexes. This technique is known as in situ synthesis of the metal complex in the zeolite cavity. The zeolite synthesis method is the post synthesis modification process which means the zeolite formed in the occurrence of the previously formed metal complexes [22].

5.2.5 Sol-Gel Technique Sol-gel is a wet chemical technique which involves several steps for the preparation of NMs. This method can be used for the lowtemperature fabrication of NMs either from a chemical solution or from the colloidal solution. This technique is used to synthesize monodisperse oxide NPs including MNPs. In general, sol-gel process can be understood as the preparation of sol from the precursor and then subjected for gelation, an integrated network formation, and finally undergoes for removal of solvent to give the product [23]. The overall synthesis route is followed by precursor, sol, gel, and product, respectively. Preferably, the precursors used in the sol-gel techniques are the metal alkoxides and metal chlorides which then normally employed for further intermediate formation. Sol is the colloidal dispersion of particles in water/ ethanol solution and gel is the semi-solid-liquid formed after the solvent removal or formed from a chemical reaction, resulting in condensation and polymerization [15]. The synthetic mechanism of sol-gel method generally followed by four steps: (i) hydrolysis, (ii) condensation, (ii) drying, and (iv) thermal treatment. The synthetic process in sol-gel involves the preparation of sol is the mixing of metal ions either in the water, alkoxides, or in other applicable organic solvents and followed by hydrolysis, condensation, and

FNM–Based Catalytic Materials  141 polymerization to form a skeletal network gel. Gelation steps can be varied depending upon the type of solvent used. Two types of gels are observed such as polymeric gel and colloidal gel. Polymeric gels are found from the solution of metal salts and alkoxides solution and colloidal gels are found from metal salt solutions, oxides, and hydroxides. Solvent evaporation involves the preparation of semisolid gels which takes place in simple drying process and finally thermal treatment is followed to get the required product in the final step [16].

5.2.6 Chemical Vapor Deposition CVD is the most widely accepted fabrication process and coating can be done to all side of bulk materials at a time. It involves the low-­temperature operation and low cost of production technique. This method involves the flow of gaseous form of reagents and uses thermally induced chemical reaction on the surface of heated materials. This technique can be used for the fabrication or coating of metals, metal based on alloys, and ceramics. The fabrication can happen with very low porosity levels yielding high purity of materials. Song et al. have followed this method for CNT fabrication [24]. For the synthesis of CNT, either hydrocarbon gases or volatile carbon compounds allowed to flow through a tubular reactor to avoid the burning of CNTs in an inert gas surroundings.

5.2.7 Microemulsion Microemulsions are distinct as the clear or partially turbid mixture of two immiscible liquids. This involves the thermodynamically stable combination of oil, water, and surfactant which means microemulsion is a slightly turbid mixture containing hydrophilic and hydrophobic groups. Surfactants are used to form an interfacial layer between two immiscible liquids. In general, it consists of three components: • Polar phase-usually water, • Nonpolar phase-usually oil, and • Surfactant. Surfactants have both polar and nonpolar part which helps to stabilize the emulsions by forming an interfacial layer between the polar and non-polar domain [23]. Microemulsions are divided into three types such as water in oil (W/O), oil in water (O/W), and bicontinuous type [25]. Microemulsion technique has the advantage like thin particle size delivery. On the other

142  Functionalized Nanomaterials for Catalytic Application hand, it has certain drawbacks like low yields of NPs and requires large amount of solvent during the synthesis [26].

5.2.8 Hydrothermal Hydrothermal approach is one of the most frequently used synthetic processes performed under high pressure and temperature. It is a versatile one pot synthesis protocol helpful for the development of NMs in crystalline phases. In this method, the synthesis is based on the separation and phase transfer mechanism performed by putting the solution of metal salts and surfactant or stabilizing agent in Teflon-line autoclaves with high pressure and temperature where the reaction occurs at the interface of water, solution, and solid phases [27]. The particle size, shape, crystallinity, properties, and stability purely depend upon the pressure applied, reaction temperature, time period of reaction, and also the concentration, pH, and solubility of precursor within the solution. Easy preparation, less time consumption, hybrid material formation, and simplicity are the main advantages of this technique, and apart from all these, the reaction processes without using any catalysts or reducing agents. The drawbacks of hydrothermal route are requirement of extreme conditions such as high temperature and pressure [28].

5.2.9 Thermal Decomposition The thermal decomposition method is also known as thermolysis, a chemical decomposition process where the decomposition or breakdown of molecules caused by heat. It is an endothermic process where heat or temperature is given from outside to the precursor material containing high-boiling organic solvents and surfactants for stabilization of the manufactured NMs [15]. However, compare to all other methods, thermal decomposition technique has some advantages such as it permits synthesis of NPs with controllable size and morphology and monodisperse distribution of NPs throughout the sample. High purity iron oxide and ferrite NPs have been synthesized using this method [16]. Requirement of high temperature, time consumption, and utilization of high-cost organic solvents and toxic chemicals are some of the limitation for bulk synthesis in the industrial application [15]. The formation of different dimension of NPs, shape distribution, and properties along with stability of produced NPs can be changed by adjusting the ratio between the organometallic precursors, surfactants, and solvents and by changing the reaction parameters such as temperature, time, and aging period [26].

FNM–Based Catalytic Materials  143

5.3 Functionalized Nanomaterials Carbon-based materials such as graphene, graphene oxide (GO), fullerenes, and diamond and metallic or metal oxide NMs including silica, titania, zirconia, alumina, and ceria are widely used for functionalized NMs (FNMs) due to their exceptional behavior. Biological materials like cellulose and chitosan were introduced in NMs to add the functional groups on the surface. Functionalization of NMs generates functional groups on the activated surface sites which forms new organized NMs that can show excellent performance in the catalytic application.

5.3.1 Carbon-Based FNMs Carbon-based NMs have high surface–to-volume ratio, high electrical conductivity, and good porosity and exhibit high stability, environment friendly, and low toxicity material. Due to these exceptional physical and chemical characteristics, carbonaceous NMs have been used as functionalized materials in diverse research disciplines including chemical catalysis, energy storage, environmental remediation, photocatalysis, and biomedical applications [29–32]. Graphene and CNTs very significant carbonaceous NMs. (a) GO has 2D nanosheet structure and established significant consideration caused by the availability of oxygen vacancy which makes it soluble in organic solvent and enhances the mechanical and electrical properties. GO offers more efficient performance for electrocatalysis applications because of their exclusive properties like superior π-π interactions toward analytes, high surface area, and nanosheet morphology [33, 34]. Graphite nanofibers and GO nanosheets with lateral dimensions less than 100 nm have been prepared by chemical exfoliation method [35]. (b) CNTs are molecular scale structure of carbon atoms arranged by different cylindrical layers, and these layers are joined with each other through covalent bond. CNTs are considered as the intermediate form of fullerenes and graphene and produced by rolling up graphite sheets into cylindrical nanotubes. A carbon atom in CNT is bonded to three neighboring carbon atoms by sp2 hybridization and has π-π bond between them. Hence, CNTs can establish π-π and van der Waals interactions with atoms, ions, and molecules. CNTs are classified into three types including single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), and multiwalled CNTs (MWCNTs) [36–38]. Although the non-functionalized carbon allotropes described above have exceptional characteristics, yet their reusability and recyclability is still a challenge, and these difficulties can be compressed by fabricating the surface of NMs. High surface area to volume ratio and high

144  Functionalized Nanomaterials for Catalytic Application porosity of the carbon materials are the key factors for the outstanding performance in many applications [39, 40]. Functionalization of carbon-based materials can be pursued by following two routes: one is covalent functionalization, and another is non-covalent functionalization. (I) Covalent Functionalization of Carbon-Based NMs Covalent functionalization is the accumulation of chemical moieties on the surface through covalent bond. There are two ways to carry out covalent functionalization such as direct covalent sidewall functionalization and indirect covalent functionalization. Direct covalent functionalization can be performed at the end of sidewall of the molecule of interest where it associates the change in hybridization from sp2 to sp3 and a loss of π-conjugation system. Indirect covalent functionalization means the sidewall interaction of the molecule with the functional groups previously introduced on their surface [41]. The covalent functionalization reactions are classified as the conventional procedures in several catalytic reactions including oxidization, hydrogenation, halogenation, cycloaddition, nucleophilic addition, and radical addition [41–43]. In the covalent functionalization method, oxidation of carbon-based NMs is generally performed by taking strong acids, such as H2SO4 and HNO3 at high temperature. As a result of which, the functional groups like –OH, –COOH, and –C=O groups are introduced in their structure, which further participates directly or indirectly in the substitution reactions [44]. These NMs have zero net charge, and point of zero charge or isoelectricpoint is formed for these NMs after the oxidation. Hence, these oxidized NMs differ in their surface charge with the ambient pH values. There exist certain major drawbacks of covalent functionalization such as the change of hybridization which damages the well-organized carbon structure by disrupting the π electron system and cause important changes in their physical or chemical properties. It utilizes concentrated acid and strong oxidant which is toxic and not environment friendly [45]. Kettum et al. synthesized spherical carbon microspheres using hydrothermal carbonization of xylose found from agricultural waste and subjected for functionalized with boronic acid. After the successful functionalization, the material has been utilized for the removal of heavy metals like Cr (VI), Ni(II), and Cu(II) [46]. Behbahani and coworkers synthesized biocompatible sorbent-based carbon nanostructures by condensation of fructose and alteration of porphyrin. These nanostructures contain enormous surface area of 289 m2/g and an average pore diameter of 8 nm. The functionalized carbon nanostructures were used to remove heavy metals such as Fe(III), Ni(II), Cd(II), and Cu(II) [47]. MWCNTs modified with polyethyleneimine (PEI-MWCNTs) polymeric nanocomposite were synthesized

FNM–Based Catalytic Materials  145 by Sambaza et al. for the efficient removal of Cr(VI) [48]. Chang et al. followed microwave-assisted surface functionalization method to obtain diamond NPs functionalized with carboxyl and carbonyl groups on the surface [49]. To follow this fabrication process, diamond nanopowders were added into an HNO3 and H2SO4 (1:3, v/v) of oxidized acidic solution and the mixture was under the microwave radiation at temperature 100°C and 100 W power for 3 h. Then, diamond NPs were subjected for the fabrication of polyarginine-coated diamond NPs. Preliminarily, 1-ethyl-­3-(3dimethylaminopropyl)-carbodiimide (EDC)–mediated coupling reaction was performed by mixing carboxyl-modified diamond NPs to a solution having EDC (22 mg ml−1, 63.7 μl), polyarginine (4 mg ml−1, 155 μl), and H3BO3/NaOH buffer (5 mM, pH 8.5). The mixture was shaken well for 2 h at room temperature. The prepared diamond NPs coated with polyarginine were subjected for extraction of peptides [49]. (II) Noncovalent Functionalization of Carbon-Based NMs Even though the functionalized carbon NMs by covalent alteration of NPs with complexing agents or organic groups have many advantages, the fabrication of these materials is quite difficult. Hence, noncovalent modification of NPs with different functional groups such as tannic acid, polyaniline (PANI), polypyrrole, polydiphenylamine, poly(3,4-dioxythiophene), polyethyleneimine, polyvinyl-alcohol, poly(2-aminothiophenol) etc. are required. In addition to that, complexing biological materials and biomolecules such as bacteria, carbohydrates, proteins, enzymes, and DNA and surfactants such as hexylmethyl imidazolium hexafluorophosphate [HMIm][PF6] are also utilized in noncovalent functionalization [50, 51]. In noncovalent functionalization, weak van der Waals force of attraction, hydrophobic interaction, and electrostatic interactions are responsible for physical adsorption phenomena. These physical variation techniques are usually simpler, useful, cost effective, and environment friendly compare to covalent functionalization method. Noncovalent functionalization of NMs follows two steps: (1) substantial mixing of NMs in solution and (2) in situ polymerization of monomers in the presence of NMs or surfactant-assisted formation of polymers on the surface of NMs [52, 53].

5.3.1.1 Carbon-Based FNMs as Heterogeneous Catalysts Carbon has been employed as the most active supporting materials for catalytic performance. The high electronic conductivity, low toxicity, good corrosion resistance, micro/mesostructural architect, high surface area, low mass densities, and host of aspired surface properties make them potential FNMs [54, 55]. Shanmugam and coworkers synthesized

146  Functionalized Nanomaterials for Catalytic Application Ni/N-doped carbonaceous aerogel (NCA) using hydrothermal technique, chemical reduction, and lyophilizing process [56]. The compositions of Ni/ NCA composite were optimized from 45% to 75% and as-prepared pure Ni NPs and Ni/NCA composites were tested as an electrocatalysts for hydrogen evolution reaction (HER) and reduction of 4-nitrophenol. The electrochemical study that revealed the incorporation of nitrogen-doped carbon aerogel enhances the HER performance and durability of the Ni catalyst. Particularly, Ni/NCA 60% (Ni/NCA-60) showed enhanced electrocatalytic activity in the alkaline medium (1 M KOH). The Tafel slopes confirm the Ni/NCA exhibited higher electrocatalytic performance over bare Ni NPs. Figure 5.2 shows the Tafel plots of Ni-NPs and optimized Ni/NCA electrocatalysts. The unsupported Ni NPs showed a Tafel slope of 115 ± 3 mV dec−1 while Ni/NCA 60% achieved low Tafel slope of 74 ± 2 mV dec−1. This indicates that the Ni/NCA needs low over potentials to reach the same catalytic current that is a desired characteristic for an electrocatalyst. In addition to that, when Ni/NCA was employed in the reduction of 4-nitrophenol to 4-aminophenol in the presence of excess NaBH4, the same high catalytic and durability was obtained as compared to Ni NPs. Mou and coworkers reported functionalization of TiO2 NPs with N-doped graphene (NGR) to form (NGR/TiO2) composite for photocatalytic activity of hydrogen production from water and achieved higher performance as compared to the counter parts, i.e., TiO2-functionalized reduced GO (rGO) to form (rGO/TiO2) composite [57]. The doping of nitrogen provides enhanced nucleation with anchor sites for the formation of TiO2 nanocrystals on the NGR sheets that helps in the formation

Ni-NPs Ni/NCA-45

Overpotential/V

0.2

Ni/NCA-60 Ni/NCA-75

0.15 0.1 0.05 0 –2

–1.5

–1 –0.5 log(|j/mA cm–2|)

0

0.5

Figure 5.2  Tafel plots of Ni NPs and Ni/NCA electrocatalysts. (Reprinted with permission from Ref. [56], Copyright 2019, International Journal of Hydrogen.)

FNM–Based Catalytic Materials  147 of intimate interfacial contact among TiO2 NPs and NGR. In addition to that, NGR has high electrical conductivity as compared to rGO because of recovered sp2 graphite network and reduced defects that help in effective charge transfer and separation of charge in the NGR/TiO2 composite leading to better catalytic performance. Carbon support also enhances the performance towards electrocatalytic HER. Platinum (Pt) nanostructure loaded on carbon support prevents agglomeration and decreases the Pt utilization and minimizes the cost effect. Yin and coworkers introduced mesoporous nitrogen-doped carbon support loaded with Pt NPs and subjected to perform as HER catalyst which results a valuable electrocatalysis performance [50]. Nitrogendoped carbon support progress higher electronic conductivity of the catalyst. Additionally, synergistic effect developed between the interaction of carbon support and the Pt NPs acts beneficial for HER catalysis. These nitrogen-doped Pt nanocatalysts involve less Pt weight nearly equal to 7.20 wt%. The nitrogen doping significantly enhances the inherent action of the functionalized materials and achieved high surface area of the prepared material toward HER. The catalyst showed −7 mV η at 10 mA/cm2 and catalytic activity remains up to 20 h with minor loss of performance [50].

5.3.2 Metal and Metal Oxide–Based FNMs Metal oxide–based NMs have been used in large variety of applications due to the outstanding mechanical, electrical, optical, and catalytic properties. Metallic and metal oxide NMs belong to inorganic class and most of the transition metal group produced by uniting two or three metals and/ or their oxides [59]. The extraordinary and excellent properties observed in metal and metal oxide NMs are due to their high adsorption capacity, porosity, and high surface area. Employing simple and economical methods, these materials can be modified effortlessly and easily with organic and inorganic-based materials resulting improved catalytic performances. The most commonly used metal and metal oxide NPs are titanium oxide (TiO2), cerium oxide (CeO2), silicon oxide (SiO2), iron oxide (Fe3O4), zirconium oxide (ZrO2), copper oxide (CuO), manganese oxide (MnO), zinc oxide (ZnO), etc. [59–64].

5.3.2.1 Functionalization Technique of Metal Oxides There are three prominent methods to functionalize metal oxides, based on (1) interactions, (2) in situ chemical oxidation, and (3) liquid-phase deposition (LPD). (1) Functionalization of metal oxide NMs with

148  Functionalized Nanomaterials for Catalytic Application interactions involves the electrostatic interactions or divergent charge attraction between the surfaces of substrate with oppositely charged MNPs. This approach plays significant role in the analytical applications [60–62]. In this method, highly concentrated molecular cations and anions are used that acts as an electrostatic agent for interaction between surface and MNPs. The layer-by-layer deposition, assisted by weak van der Waals force to functionalize MNPs and metal-oxide NPs on different substrates, is used for various applications. (2) In situ chemical oxidation of metallic substrates is usually performed to obtain nanostructured surfaces for different purposes. As compared to other fabrication procedures, this method is easy to handle and offers high stability and durability. For instance, TiO2 NPs obtained by the oxidization of Ti wires at 100°C with hydrogen peroxide [65]. (3) LPD can be used for the production of a thin layer of metal oxide NP films (ZnO, ZrO2, TiO2, Cr2O3, MnO, NiO, CuO, etc.) [66]. The LPD technique involves hydrolysis reaction of metal complex ions and then the precipitation of metal oxide NPs by the addition of H3BO3 [66].

5.3.2.2 Silver-Based FNMs as Heterogeneous Catalysts The diverse morphological achievement of silver based materials (nanocubic, nanobar, nanosphere, etc.) makes it a promising functional material choice for the heterogeneous catalyst reaction owing to its physiochemical structure [67–69]. It has shown promising performance in the catalytic oxidation reactions, such as ethylene epoxidation [70], NOx abatement [71], the selective catalytic oxidation of ammonia [72], formaldehyde synthesis [73], partial oxidation of benzyl alcohol [74], the oxidative coupling of methane [75], the selective oxidation of ethylene glycol [76], the oxidation of styrene [77], and CO oxidation [78]. The catalytic performance of Ag NPs strongly depends on the surface structure and surface sites. These are very sensitive to the size of Ag NPs, pre-treatment or reaction conditions, and the preparation method. The completely filled d-band and the position of the d-band center relative to the Fermi level affect the dissociative chemisorption on the surface of Ag catalyst [79]. The surface and subsurface oxygen atoms are active site for Ag catalyst in the oxidation reaction, functionalization, and different pre-treatment environment that can affect the performance of Ag catalyst. Liu and coworkers reported highly dispersed Ag catalyst with mesostructured silica supports synthesized by one pot synthesis exhibited 100% CO conversion at room temperature [80]. Hu and group developed a cryptomelane type structure of Ag/OSM-2 catalyst using reflux method for the selective oxidation of CO in the hydrogen rich

FNM–Based Catalytic Materials  149 steam. A long-term stability testing was achieved with 100% CO conversion and maintained for 250 h with 90% selectivity at 120°C [81]. Ag catalyst has been reported to show good activity in soot oxidation even at low temperature. Aneggi and coworkers studied the activity of functionalizing Ag on various metal oxides and observed the addition of ZrO2 or Al2O3 to Ag results an effective catalyst while addition of CeO2 to Ag had more benefits as compared to ZrO2 and Al2O3 [82]. Considering sintering of Ag at high temperature, Yamazaki et al. synthesized a CeO2-Ag catalyst with Ag as a core and CeO2 as a shell [83]. The material delivered an excellent performance for soot oxidation in the presence of gaseous oxygen in tight and loose contact mode at a temperature below 300°C. The designed core-shell morphology increases the Ag/CeO2 interface area per unit surface area of Ag particles and slowdowns the Ag sintering process due to the presence of stable CeO2 shell as blockades. Figure 5.3 shows the possible soot oxidation mechanism in which the oxygen species accumulated over Ag surface from gaseous O2 transfer to the surface of CeO2 by interface transformation into Onx− species. The species further migrates onto soot particles where oxidation occurred. The plentiful OnX− species formed rapidly comes out to the external surface and efficiently allows soot particles. These surface migration phenomena plays a key role in the enhancement of soot oxidation performance and compactness between catalyst and soot.

gas phase CO2 Onx–

O2 (gaseous)

soot

Onx–

Onx–

Onx– CeO2 On

Onx–

x–

Onx–

Onx–

O*

O* O* O* O*

O*

Onx– * * O* O O O*

Ag

Onx– Onx–

Onx–

CO2 gas phase

Onx– Onx–

Onx– O* Onx– O* O* Onx– O* O* Onx– O*

Figure 5.3  Schematic mechanism for soot oxidation over the CeO2-Ag catalyst. (Reprinted with permission from Ref. [83], Copyright 2011, Journal of Catalysis.)

150  Functionalized Nanomaterials for Catalytic Application X

X

X

X

X

X O δ–

N

δ+ Ag O Al

O

N – O +δ–Oδ+ Ag O Al H2

N O + O– δ+ Hδ– H O Ag Al

H2O

H

N

H

OH

δ–

H2

OH δ+ H

H Ag O Al

δ–

δ+ Ag O Al

N

H2

H

N

H2O

H

δ–

Ag O Alδ+

Figure 5.4  Proposed mechanism for the Ag/Al2O3-catalyzed hydrogenation of nitro aromatic compounds. (Reprinted with permission from Ref. [86], Copyright 2010, Journal of Catalysis.)

As compared to other metal NPs catalyst (Ni, Pd, and Pt), Ag has gained less interest for hydrogenation reaction since Ag holds filled d-band configuration (4d105s1) as well as due to the position of the d-band center relative to the Fermi level and thus lacks the affinity toward H2. The theoretical calculation reveals the hydrogen could interact with exposed surface of Ag NPs with a very weaker interaction [84, 85]. The selective hydrogenation of the nitro group using H2 in presence of different reducible functional group results functionalized anilines producing a variety of fine chemicals for industrial applications. Shimizu and coworkers explored the size and support-dependent Ag cluster catalyst for the chemoselective hydrogenation of nitro aromatics [86]. An extreme chemoselective reduction activity on the nitro group was displayed using Ag clusters supported on Θ-Al2O3 for the reduction of substituted nitro aromatics. The cooperation of the acid-base pair sites over Al2O3 and coordinating unsaturated Ag sites attaching on the Ag cluster leads to the rate-limiting H2 dissociation to form an H+/H− pair at the metal/support interface, whereas the basic site on the Al2O3 acts as an adsorption site of nitro aromatics. Hence, a d10 metal-based selective hydrogenation catalyst is achieved and the mechanism for the Ag/Al2O3-catalyzed hydrogenation of a nitro aromatic compound is shown in Figure 5.4.

5.3.2.3 Platinum-Based FNMs as Heterogeneous Catalysts Noble metal Pt has been employed as an efficient catalyst in various oxidation reactions. Due to the poor stability, high fabrication cost, and low tolerance to CO makes researcher to adapt certain methods that can recover its utilization, efficiency, stability, and cost. In addition, Pt NPs can sinter on oxide supports upon high temperature, resulted large particles with

FNM–Based Catalytic Materials  151 loss of surface area, and catalytic performance [87]. Hence, it is necessary to design a catalyst that can be active at lower temperature ( R-Br > R-Cl. Further, electron withdrawing group substitution (−CN, NO2) exhibited better performance as compared to electron donating substitution. The supported Pd nanocatalyst stabilized on chitosan/cellulose composite shows superior properties as compared other

156  Functionalized Nanomaterials for Catalytic Application catalyst (cell-Pd(0), Fe3O4@SiO2/isoniazid/Pd nanocatalyst, γ-Alumina supported Pd nanocatalysts, Fe3O4@CS-Schiff base Pd catalyst, etc.), resulting in higher yields, faster rate, and good sustainability with successful recyclability of catalyst eight times [114].

5.3.4 FNMs for Various Other Applications With increase in the urbanization, the world is going through environmental pollution as well as energy crisis. Nowadays, the great challenge for the whole world is to decrease the uses of fossil fuels and finding a solution for rising environmental problems and produce energy in outsized quantity in a cleaner way. These aspects have been spotted as the demanding work to drag the attention in the field of research. Hence, it is imperative to develop cheap, efficient and eco-friendly catalyst to overcome the drawbacks [115]. Developing efficient nanocatalysts with cost effective and accomplishing the better catalytic behavior for water splitting, CO2 reduction, and urea oxidation reaction, and Zn-air battery is very essential for wide commercialization of fuel cells, water, and metal air batteries and also for diminishing the environmental issues. In order to carry out these issues, researchers are mainly focusing on the utilization of different FNMs as the nanocatalyst because of their extraordinary and outstanding result. Debata and coworkers have reported hydrothermal synthesis of coral-like NiCo2O4 bifunctional nanostructure supported on rGO sheets [115]. The composite material was characterized widely to know the structural, morphological, and electrochemical properties. The nanocomposite shows unique morphology and large surface area which efficiently increases the activity and reactivity. The synergistic effect of the metal toward the catalytic activity formulates appropriate FNMs for electrochemical water splitting. The rGO-NiCo2O4 coral like nanostructured nanocomposite reveals the high catalytic activity and high current density, lower Tafel slope, low onset potential, and longterm stability in both HER and OER [115]. Khezri et al. discussed briefly about the importance of CO2 reduction using different metal and metal hybrid nanocatalyst [116]. Zhu and coworkers have reported the synthesis of advanced catalysts for energy conversion based on urea oxidation reaction [117]. Patra and coworkers studied the synthesis of heteroatom-doped graphene foam by an advanced, green, and eco-friendly procedure, named as “Idli” [118]. The graphene-idli catalyst was first prepared subjecting to a domestic microwave oven using heteroatom-doped (boron, nitrogen, sulfur, and phosphorous) graphene and rice flour and then undergoes calcination to produce graphene foam which was then utilized as a metal-free bifunctional nanocatalyst for oxygen evolution reaction (OER) and oxygen

FNM–Based Catalytic Materials  157 reduction reaction (ORR). It was established that sulfur-doped graphene foam (SDGF) demonstrate improved performance due to characteristics high BET surface area (499 m2/g), enhanced electrochemical surface area (0.271 cm2), high roughness factor (0.690), and high porosity. As a bifunctional electrocatalyst for rechargeable Zn-air battery, SDGF acquires a lowest onset potential, lowest over potential, lowest Tafel slope (35.71 mV/dec), and highest current density (497 mA cm−2) [118].

5.3.5 Comparison Table Table 5.1 signifies synthesis of different FNMs with different morphologies have been utilized in different catalysis and electrocatalysis reactions. Table 5.1  Comparison of different FNMs synthesized by following several approaches. Material

Shape

Method

Reference

Porous Ni-CNT composite

Nanocone

Electrodeposition

[119]

Pt-Ni NPs on N-doped carbon

Sponge like morphology

Pyrolysis

[120]

CeO2-CoP-C nanostructure with MOF

Rhombo dodecahedral

Co-ionicexchange

[121]

Co6Mo6C nanocrystals on graphene oxide

Spherical

Thermal processing

[122]

Cobalt/nitrogen-doped carbon

Hierarchical pore structure

Pyrolysis

[123]

Ni2(l-x)Mo2xP

Nanowire arrays

Hydrothermal

[124]

3D MoS2-rGO@Mo

Nanohybrids

Hydrothermal

[125]

Mo2C on graphite oxide

Rod like structure

Solvothermal

[126]

Fe-MnO on reduced GO manoflakes

Nanocubes

Thermal treatment

[127]

Palladium nanocrystals on chitosan/cellulose

Spherical

Impregnation

[110] (Continued)

158  Functionalized Nanomaterials for Catalytic Application Table 5.1  Comparison of different FNMs synthesized by following several approaches. (Continued) Material

Shape

Method

Reference

MFe2O4 (M = Fe, Co, Mn) Nanoparticles

Spherical

Co-precipitation

[7]

g-C3N4/TiO2 composites

Small roundish

Impregnation

[20]

NiCo2O4/rGO

Coral like morphology

Hydrothermal

[115]

5.4 Conclusion From the above discussion, it was seen that NMs utilization and importance have increased in many fields of nanotechnology. NMs are undoubtedly one of the most scientific disciplines because of the unique properties and advantages offered toward various applications and it is noticeable that the utilization and importance of NMs gradually increasing day by day. However, the particles size, shape, dimension and stability etc. are responsible for the unusual and outstanding behavior of the NMs and all these properties clearly depends upon the synthesis protocols. Hence, it is important to follow a favorable way to manufacture the NMs. Functionalization or fabrication of NMs surface enhances the stability and properties of NPs which drags the special attention toward catalytic performance. In this chapter, several chemical methods of fabricating/coating the NMs surface such as co-precipitation, impregnation, ion-exchange, immobilization, sol-gel, CVD, hydrothermal, microemulsion, and thermal decomposition methods have been discussed. FNMs depending on their functionalized technique show exceptional behavior. Carbon-based materials, metals, metal oxides, and biological material like cellulose and chitosan were discussed along with their properties and catalytic performance as FNMs. FNMs utilized for other important applications such as water splitting, CO2 reduction, urea oxidation reaction, and Zn-air battery have been discussed. The mode of synthesis procedure chosen clearly depends on the type of materials taken for functionalization. In addition to that, role of catalyst support with few reported examples demonstrates the generation of synergistic effect with intimate interfacial contact between metal catalyst and catalyst support accelerates the catalytic performance. All in all, the NM combination, and functionalization with suitable FNM

FNM–Based Catalytic Materials  159 and other parameters such as substituent, temperature, reaction environment (like aerobic oxygen), precursor ratio, and fabrication technique presented in this chapter may be helpful for the preparation of advanced catalytic materials with high selectivity and yield. These FNMs can be useful for various applications such as sensing, photochemical catalysis, and environmental applications. Cost-effective metal anisotropic nanocrystals can be fabricated through various suitable supports for different applications including catalysis, electrocatalysis, environmental, and wastewater treatment.

Acknowledgements S.S. and A.K.S. are grateful to SERB, New Delhi, India, for funding to conduct this research (CRG/2018/003533).

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6 Functionalized Nanomaterials for Catalytic Applications—Silica and Iron Oxide Deepali Ahluwalia, Sachin Kumar, Sudhir G. Warkar* and Anil Kumar† Delhi Technological University, Delhi, India

Abstract

In the recent years, significant advancements have been made in the field of material chemistry. Over the time, wide research has been carried out on nanomaterials owing to their high surface-to-volume ratio. This chapter explores the synthesis of two such well-defined nanomaterials, silica and iron-oxide, that have been employed as heterogeneous catalysts for various organic transformations. Throughout this chapter, judicious designing of uprising nanomaterials with highly selective and active nanostructured catalysts is discussed. The methods used to enhance the activity and selectivity of these materials by simple manipulation onto the surface of the particle have been presented as the key feature of this report. The last section provides a deep insight into the utilization of these hybrid nanomaterials as a catalyst in a variety of organic reactions. The chapter will also draw a bridge between nanotechnology and green chemistry that strives to meet the scientific challenges of protecting environment. Keywords:  Nanoparticles, catalysis, silica, iron oxide, functionalization

6.1 Introduction We all are well familiar with the word nanomaterial or nanotechnology, being used every time for a new gadget, electronic appliance, scientific methods, or researches that are introduced in the market. Basically, the *Corresponding author: [email protected] † Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (169–184) © 2021 Scrivener Publishing LLC

169

170  Functionalized Nanomaterials for Catalytic Application prefix nano has been derived from the Greek word “dwarf ” which is used to describe the length of a scale of one billionth of a meter (10−9 m) [1]. These new materials possess physical and chemical properties which are different from their bulk counterparts. Such nanomaterial acts as a bridge between the technology and innovative scientific advances [2]. A unique feature of these materials is having a size range in proportion to the biomolecular and cellular systems [3]. These properties make them promising materials for utilization in catalytic, therapeutic, medicinal, biosensing, electronic, diagnostic, and many more applications. The use of nanoparticles (NPs) in catalysis appeared in 19th century with photography (Ag NPs) and decomposition of hydrogen peroxide (Pt NPs) [4]. Catalysts do not actually produce the reactions that are impossible but they simply assist already existing organic reactions [5, 6]. For instance, your teacher teaches you in regular class periods; it is like a normal organic reaction. But, the tutorial and extra classes help you to score better in exams. These tutorials and extra classes are nothing but catalysts in your education. Catalysts are basically of two types: homogeneous and heterogeneous. Both catalysts have some drawbacks associated with them; thus, to overcome the challenges, a new organic-inorganic hybrid nano-catalytic system is developed in recent times. The properties of all the three forms are discussed in Table 6.1. Decades have led to the growth of nanomaterials catalysis field especially in the fields of hydrogenation and catalysis of unsaturated substrates and oxidation reactions, redox, and photocatalysis. Nanocatalyts have emerged out as a powerful tool for transformation of raw materials into beneficial chemicals of both pharmaceutical and industrial importance [9]. The methodologies of synthesizing hybrid catalytic NPs (i.e., functionalization of nanomaterials) have been expanded and, at present, include coprecipitation, impregnation, deposition/ precipitation, gas-phase organometallic deposition, sol-gel, microemulsion, sonochemical, laser ablation, cross-liking methods, and electrochemical [10]. In the following sections, we shall explore the synthesis, functionalization, and catalytic applications of such nanomaterials namely, silica and iron. The synthesis of NPs is broadly classified into two classes: top-down and bottom-up. The top-down approach basically reflects breaking down of bulky material in order to reduce its size to nanoscale, using different methods. While the bottom-up approach refers to accumulating small building blocks to form a large structure, i.e., chemical synthesis. In the bottom-up approach, a NP is grown during the course of reaction and that is why this approach is more favorable [11].

Silica and Iron Oxide  171 Table 6.1  Comparison of properties of homogeneous, heterogeneous, and their hybrid catalysts [4, 7, 8]. Catalysts

Properties

Homogeneous

Heterogeneous

Homogeneousheterogeneous hybrid

Concentration

Low

High

Low

Active sites

Distinct

Indistinct

Distinct

Phase

Liquid/gas

Solid

Solid

Selectivity

High

Low

High

Thermal stability

Low

High

High

Product separation

Difficult

Easy

Easy

Catalyst recyclability

Difficult

Facile

Facile

Cost

Low

High

High

Reaction rate

Slow

Fast

Fast

Lifetime of catalyst

Variable

Long

Long

6.2 Silicon Dioxide or Silica 6.2.1 General O O O

O

Si O O

O

Si

R-Si ?

O O

Si

O O

O

O Si

O

O

Si

O O

Si ?

O

Silicon dioxide (SiO2), commonly known as silica, occurs in amorphous and crystalline, porous, non-porous (dense), hydroxylated, or anhydrous forms, regardless of their natural or synthetic nature (Table 6.2) [2].

172  Functionalized Nanomaterials for Catalytic Application Table 6.2  Types of silica material available. Silica Crystalline

Amorphous

α-quartz

Diatomaceous earth

β-quartz

Opal

Trydimite

Silica glass

Cristobalite Porosil (Porous crystalline silica)

Human-made products (e.g., cosmetics, drugs, printer toners, varnishes, and food)

6.2.2 Synthesis of Silica Nanoparticles 6.2.2.1 Sol-Gel Method A monodisperse and spherical silica NPs via the hydrolysis of tetraethylorthosilicate (TEOS) in a mixture of water, ammonia, and ethanol was reported first by Stöber et al. in the year 1968 [12]. In order to accelerate the hydrolysis step, ammonia has been added playing the role of a catalyst. This leads to hydrolyzing TEOS to silicic acid which afterward gives spherical amorphous NPs of silica, upon condensation (Scheme 6.1). Alcohols like propanol and methanol may also play the role of solvent in the reaction mixture, but as per the documented work, the product is in form of finest particles (spherical in shape and finely monodisperse) when ethanol is used as a solvent. The minimum size of the particle synthesized using Stöber’s methodology is approximately around 20 nm in diameter. The size can be furthered altered by varying the temperature, stirring speed and concentrations of the reactants and catalyst, i.e., water, TEOS, and ammonia. This method involves the usage of water as a solvent, thus avoiding the organic solvent, hence a greener method for the preparation of silica NPs.

6.2.2.2 Microemulsion A clear, isotropic liquid mixtures of oil, water, co-surfactant and surfactant is called as microemulsion [13]. The surfactant molecules align themselves in a such a way that a spherical aggregate is formed in the continuous phase. Reverse micelle or water-in-oil microemulsion is being used here for the preparation of ultra-thin silica NPs. In order to grow NPs, TEOS in its

TEOS

OC2H5

+

H2O HO

Silica NPs + 2H2O

Scheme 6.1  Preparation of silica NPs by Stöber’s sol-gel method [12].

Si(OH)4

OH

+

  

2C2H5OH

Silicon tetrahydroxide

OH

Si

OC2H5

Si

Step II: Polycondensation of Silicon tetrahydroxide

C2H5O

OH

OC2H5

Step I: Hydrolysis of TEOS

5µ

Silica and Iron Oxide  173

174  Functionalized Nanomaterials for Catalytic Application Non-Polar end Polar end TEOS

SiO2

Hydrolyzed TEOS

Particle-Filled Reverse Micelle Water Shell

Scheme 6.2  Synthesis of Silica NPs via microemulsion process [14].

hydrolyzed form is transferred from the reverse micelle to a particle-filled reverse micelle or it may undergo direct interaction with the unhydrolyzed TEOS species with a particle-filled reverse micelle (Scheme 6.2). It is worth noting here that the hydrolysis of TEOS in a particle-filled reverse micelle has been proved to be less significant than hydrolysis in an empty reverse micelle [14, 15].

6.2.3 Functionalization of Silica Nanoparticles As studied in Table 6.1, the heterogeneous and hybrid catalysis acts as a better reaction pathway compared to homogeneous catalysis. In this light, certain modifications are incorporated at the surface of the catalyst to develop heterogeneous or homogenized heterogeneous hybrid systems. Modifications on silica NPs can be achieved through incorporation of specific functional group in the direction to utilize the hybrid NP as a catalyst [10]. Functionalization can be done by implementation of physical and chemical treatments at the NP surfaces. Physical treatments involve altering of proportion of silanol to siloxane groups, while in chemical treatment, the chemical characteristics of the surface of the NPs are reshaped. Surfactant molecules get adsorbed onto the surface of silica NPs via electrostatic interactions between their polar groups, that further reduces the interactions among the lone silica NPs, thus prevents agglomeration [7, 16].

Silica and Iron Oxide  175 One such example of physical treatment includes Co (II) adsorbed on silica NPs that acts as a stable, optically transparent and highly efficient catalyst for the oxidation of water to molecular oxygen. The Co(OH)2/SiO2 catalyst showed no cobalt precipitation or deactivation, a very high selectivity and high activity was observed upon numerous cycling of Co ions through their higher (III and IV) oxidation states which are involved in the mechanism [17]. But, later, the drawbacks of this oxidation process were found to be the weak physical reversible forces leading to reversibility. Also, metal leaching as well is one critical issue at industrial processing level. Thus, the researchers shifted their interest toward the chemical treatment on silica NP surfaces that could lead to much stronger and non-leaching modification onto the nanosilica surfaces. The basic objective of chemical treatment is to obtain hybrid catalyst developed by an easy method for the establishing new functional groups onto the surface of silica that could minimize NP aggregation and non-specific binding by introducing an optimum balance of active and inert functional groups. Another aim includes exploring mechanism of NPs flocculation, aggregation, reduction, prevention that may be fostered by the addition of new species onto its surface. Silane agents such as aminopropy­ltriethoxysilane (APTES), p-aminophenyl trimethylsilane (APTS), and mercaptopropyltriethoxysilane (MPTES) are one of the most promisable postulant for altering the surface of MNPs directly. Such surface modification enhances the compatibility and provides rather high-density surface functional end groups

Microemulsion (TEOS, aq. NH4OH) Silica NPs organoalkoxysilane TEOS

24 hours

washed with ethanol Surface modified silica NPs

Scheme 6.3  Functionalization process on silica nanoparticles.

176  Functionalized Nanomaterials for Catalytic Application HO

HO

O

OH

HO

SiO2

HO HO

OH

OH

+

Si

OH

SiO2

HO O

OH

O O

O

Solvent

OH HO

HO

O

OH

O

O

Si

O

O

O

-OC2H5, -OCH3, -Cl H, Si NH2, Cl, SH

Scheme 6.4  Modification of silica nanoparticles [2].

which allow binding of other biomolecules, polymers or metals (Schemes 6.3 and 6.4) [16].

6.2.4 Applications The inherent properties of silica NPs such as nanometer size, excellent thermal, and mechanical stability, high surface area-to-volume ratio, and the availability of silanol groups on the surface allow the addition of wide variety of modifications onto the surface. The surface-active functional groups along with metal center acts as a highly efficient catalyst to provide direction to a number of organic reactions [1]. Among the wide range of examples present in the literature of the catalytic applications of functionalized silica nanomaterials, herein, we discuss a few of them.

6.2.4.1 Epoxidation of Geraniol 3,7-dimethylocta-trans-2,6-dien-1-ol, i.e., Geraniol, chemical formula C10H18O, is an acyclic monoterpene alcohol. Geraniol is isolated from Palmarosa oil (an essential oil), has a pleasant odor, and is clear to pale-yellow in color. It is widely used as a fragrance in cosmetics and toiletries [19]. Here, we discuss catalytic activity of silica NP supporting polyoxometalated (POMs), namely, iron(III) mono-substituted Keggin-type polyoxotungstate of formula α-[PW11FeIII(H2O)O39]4- and a sandwich type tungstophosphate with formula B-α-[(PW9O34)2FeIII4(H2O)2]6− in the epoxidation of geraniol using H2O2 as oxygen donor, CH3CN is used as a solvent (Scheme 6.5). The study of this reaction suggested that polyoxotungstate is effectively immobilized on silica NPs. It may be noted here that the results achieved by heterogeneous catalysis (using POMs/SiO2) show significant selectivity improvement than a homogeneous system using tetrabutylammonium (TBA) under same reaction conditions [18].

Silica and Iron Oxide  177

HO

POM/SiO2

H2O2

O HO

2,3- Epoxide

+

HO

6,7- Epoxide

+ O HO

Diepoxide

O

Scheme 6.5  Epoxidation of geraniol [18].

6.2.4.2 Epoxidation of Styrene The reaction aims to produce styrene oxide that utilized as a valuable intermediate for organic reactions at industrial platforms to synthesize fine chemicals and drugs. In this section, we shall study the link between mesoporous silica nanomaterials (MSNs) and MOFs, to prepare metal complex catalyst attached on MSNs for epoxidation of styrene (Scheme 6.6). Mn-MSN, Cu-MSN, and Co-MSN catalysts exhibit excellent catalytic activities in the epoxidation pathway using tert-BuOOH as oxidant [20].

6.3 Iron Oxide 6.3.1 General Iron oxide crystallites exist in various phases in nature. The most common of them are hematite (α- Fe2O3), magnetite (Fe3O4), and maghemite (γFe2O3). Out of these, majorly hematite iron oxide has been used for research

178  Functionalized Nanomaterials for Catalytic Application OH OH

+

EtO

Si(CH2)3NH2

-3 EtOH

EtO

OH

Channel wall of MSN

CHO

O

EtO

O

OH Si(CH2)3NH2

+

O

3-APTES

O O Si O

O

N N

O

M

O O

M(OAc)n M= Cu, Co, Mn

O

H Si(CH2)3N

O

C HO

O Si O

M-MSN

Sal-MSN

Scheme 6.6  Epoxidation of styrene [20].

purposes. The reason is its low cost and non-toxicity. Hematite phase has been considered the most stable n-type semiconductor possessing a band gap of 2.1–2.2 eV [21]. Iron oxide NPs have recently been utilized as a sensor for biomolecules, metabolites, and hyperthermia, toxicity, and nano-toxicity. Like silica, a number of synthetic pathways are iron oxide NPs exists, namely, sol-gel, chemical precipitation, microemulsion, electrochemical methods, sonochemical techniques, aerosol/vapor method, and many more [22]. The usage of toxic reagents in the production of metal nanomaterials has been a challenge for the researchers ever since. Many efforts have been put to develop reliable, efficient, and facile green method for the production of metal NPs that does not cause undesired malignant effect on environment. Researchers have found green synthetic methods via fruit and leaf extracts for biological studies. In the next section, we shall discuss some of these synthetic routes.

6.3.2 Synthesis of Functionalized Fe NPs 6.3.2.1 Biopolymer-Based Synthesis Starch is a water loving polymer consisting of nearly 20% amylose content. Starch has been found to play a crucial role in stabilizing Fe NPs (Table 6.3)

Silica and Iron Oxide  179 Table 6.3  Biopolymer-based Fe NPs [23, 26]. Biochemical reagent

Size

Application

Bimetallic Fe/Pd NPs

Starch

14.1 nm

Degradation of chlorinated hydrocarbon in water

Zero-valent Fe NPs

Ascorbic acid

20–75 nm

Cadmium (Cd) removal

Type of NP

[23]. Another such biopolymer is sodium alginate, a good precursor for magnetite (Fe3O4) NPs. It follows redox-based hydrothermal method using urea and ferric chloride hexahydrate (FeCl3.6H2O) as starting materials [24]. Core-shell iron and copper NPs can be prepared using aqueous ascorbic acid (Vitamin C), which reduces the transition metal salts to their respective nanostructures [25].

6.3.2.2 Plant Extract–Based Synthesis Many reports are present on synthesis of Fe NPs mainly using green tea (GT) extracts. One major reason for this is the inexpensiveness and easy availability. Hong et al. synthesized nanoscale zero-valent iron NPs (nZVI) using Camellia sinesis (GT) extract, containing a number of polyphenols [27]. Stable NPs can be obtained at room temperature using this pathway, without adding any surfactant or polymer. For the synthesis, 20 g/L GT is to be added to 0.1 M FeCl3 solution in 1:2 volume ratio, to obtain spherical NPs of diameter 5–10 nm. Many studied followed thereafter, involving some modifications in the synthetic route. For instance, Shahwan et al. mixed 0.10 M FeCl3 solution in GT in 2:3 volume ratio; further, 1.0 M NaOH was added till pH 6 is achieved. The black precipitates marked the formation of NPs. The reaction workup was done by evaporating the remaining water and NPs so obtained were 40–60 nm in diameter [28].

6.3.3 Applications 6.3.3.1 Degradation of Dyes The GT synthesized nanoscale zero-valent iron NPs (GT-nZVI) can be incorporated as a catalyst for hydrogen peroxide (H2O2) for degradation of bromothymol blue dye (an organic contaminant). On increasing the concentration of GT-nZVI, more H2O2 can be catalyzed which leads to degradation of bromothymol blue dye [27].

180  Functionalized Nanomaterials for Catalytic Application Another report by Kaung et al. utilized Fe NPs synthesized from three different extract, i.e., GT, oolong tea (OT), and black tea (BT), separately [29]. The so-formed NPs can be then used as a catalyst in Fenton-like oxidation of monochlorobenzene (MCB). The catalytic activity order is such that GT-Fe NPs > OT-Fe NPs > BT-Fe NPs, contributing 69%, 53%, and 39%, respectively. The oxidation takes place in three steps: (i) Adsorption MCB

Adsorption

+ Fe NPs

MCB Fe NPs

Corrosion

MCB/γ-Fe2O3/Fe3O4

(ii) Generation of hydroxyl radical species

Fe0 + H2O2 → Fe2+ + 2OH− Fe2O3/Fe3O4 + H+ → Fe2+/Fe3+ + H2O Fe2+ + H2O2 → Fe3+ + OH− + HO∙ The ions Fe2+ and Fe3+ in the solution react with water and yields oxyhydride, which can also absorb MCB.

Fe2+/Fe3+ + H2O → FeOOH Fe3+ + H2O2 → Fe2+ + H+ + H2O∙ (iii) Hydroxyl radicals attack on MCB/Fe NP surface MCB/Fe-NPs

+ OH•

Reaction intermediate/Fe NPs

OH• CO2 + H2O

Silica and Iron Oxide  181 The reaction results in mineralization of some quantity of MCB, leading to removal of chemical oxygen demand (COD). In reaction time of about 3 hours, the rate of degradation of dye is found to be 81% and removal of COD is 31%.

6.3.3.2 Wastewater Treatment

As discussed in the previous section, the starch mediated bimetallic Fe/Pd NPs can be employed for degradation of chlorinated hydrocarbons in water. Wang et al., in one of their reports, used biosynthetic iron NPs for treatment of eutrophic wastewater [6, 30]. The newly synthesized polydisperse eucalyptus leaf extract having varied reducing power can be employed as a good chemical reducing agent. In 21 days, percentage removal of total nitrogen, COD, and total phosphorus was found to be 71.7%, 84.5%, and 30.4%, respectively. Due to absence of precipitating agents like magnesium, calcium, or aluminium, very low phosphorus removal is observed. Listed below are some other nanocomposites that may be employed for removal of toxic ions from water. Plant/NP/Polymeric support composite

Environmental applications

Size

Morphology

Green Tea/Fe/ Polyvinylidene fluoride (PVDP) membrane

20–30 nm

Aggregate

Degradation of organic trichloroethylene (TCE) pollutant [31]

Commercially available tea/Fe/Clay (montmorillonite)

48–70 nm

Crystalline

Removal of arsenic [32]

Mentha spicata L./Fe/ chitosan

25–40 nm

Polydisperse cubic crystalline

Removal of arsenic [33]

182  Functionalized Nanomaterials for Catalytic Application

References 1. Mout, R., Moyano, D.F., Rana, S., Rotello, V.M., Surface functionalization of nanoparticles for nanomedicine. Chem. Soc. Rev., 41, 7, 2539–2544, 2012. 2. Sharma, R.K., Sharma, S., Dutta, S., Zboril, R., Gawande, M.B., Silicananosphere-based organic-inorganic hybrid nanomaterials: Synthesis, functionalization and applications in catalysis. Green Chem., 17, 6, 3207–3230, 2015. 3. Taylor-Pashow, K.M.L., Della Rocca, J., Huxford, R.C., Lin, W., Hybrid nanomaterials for biomedical applications. Chem. Commun., 46, 32, 5832–5849, 2010. 4. Copéret, C., Chabanas, M., Petroff Saint-Arroman, R., Basset, J.M., Surface organometallic chemistry: Homogeneous and heterogeneous catalysis: Bridging the gap through surface organometallic chemistry. Angew. Chem. Int. Ed., 42, 2, 156–181, 2003. 5. Shukla, S.K., Turner, A.P.F., Tiwari, A., Cholesterol oxidase functionalised polyaniline/carbon nanotube hybrids for an amperometric biosensor. J. Nanosci. Nanotechnol., 15, 5, 3373–3377, 2015. 6. Choudhary, M., Shukla, S.K., Narang, J., Kumar, V., Govender, P.P., Niv, A., Hussain, C.M., Wang, R., Mangla, B., Babu, R.S., Switchable graphene-based bioelectronics interfaces. Chemosensors, 8, 2, 45–57, 2020. 7. Pelletier, J.D.A. and Basset, J.M., Catalysis by Design: Well-Defined SingleSite Heterogeneous Catalysts. Acc. Chem. Res., 49, 4, 664–677, 2016. 8. Cornils, B. and Herrmann, W.A., Concepts in homogeneous catalysis: The industrial view. J. Catal., 216, 1–2, 23–31, 2003. 9. Li, X., Niitsoo, O., Couzis, A., Electrostatically driven adsorption of silica nanoparticles on functionalized surfaces. J. Colloid Interface Sci., 394, 1, 26–35, 2013. 10. Li, Y. and Benicewicz, B.C., Functionalization of silica nanoparticles via the combination of surface-initiated RAFT polymerization and click reactions. Macromolecules, 41, 21, 7986–7992, 2008. 11. Lockwood, D.J., Nanoparticles Building Blocks for Nanotechnology, Springer, New York, NY, 2004. 12. Stober, W., Fink, A., Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Phys. Ther. Sci., 26, 62–69, 1968. 13. Hoar, T.P. and Schulman, J.H., Transparent water-in-oil Dispersions: the Oleopathic Hydro-Micelle. Nature, 152, 102–103, 1943. 14. Osseo-Asare, K. and Arriagada, F.J., Growth kinetics of nanosize silica in a nonionic water-in-oil microemulsion: A reverse micellar pseudophase reaction model. J. Colloid Interface Sci., 218, 1, 68–76, 1999. 15. Arriagada, F. J., K.O.-A., Synthesis of Nanosize Silica in a Nonionic Waterin-Oil Microemulsion: Effects of the Water/Surfactant Molar Ratio and Ammonia Concentration. J. Colloid Interface Sci., 211, 210–220, 1999.

Silica and Iron Oxide  183 16. Bagwe, R.P., Hilliard, L.R., Tan, W., Surface modification of silica nanoparticles to reduce aggregation and nonspecific binding. Langmuir, 22, 9, 4357– 4362, 2006. 17. Zidki, T., Zhang, L., Shafirovich, V., Lymar, S.V., Water oxidation catalyzed by cobalt(II) adsorbed on silica nanoparticles. J. Am. Chem. Soc., 134, 35, 14275–14278, 2012. 18. Sousa, J.L.C., Santos, I.C.M.S., Simões, M.M.Q., Cavaleiro, J.A.S., Nogueira, H.I.S., Cavaleiro, A.M.V., Iron(III)-substituted polyoxotungstates immobilized on silica nanoparticles: Novel oxidative heterogeneous catalysts. Catal. Commun., 12, 6, 459–463, 2011. 19. Chen, W. and Viljoen, A.M., Geraniol - A review of a commercially important fragrance material. S. Afr. J. Bot., 76, 4, 643–651, 2010. 20. Tang, D., Zhang, W., Zhang, Y., Qiao, Z.A., Liu, Y., Huo, Q., Transition metal complexes on mesoporous silica nanoparticles as highly efficient catalysts for epoxidation of styrene. J. Colloid Interface Sci., 356, 1, 262–266, 2011. 21. R. M. Cornell, U.S., The Iron Oxides: Structure, Properties, Reactions, Occurences and Uses, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003. 22. Teja, A.S. and Koh, P.Y., Synthesis, properties, and applications of magnetic iron oxide nanoparticles. Prog. Cryst. Growth Charact. Mater., 55, 1–2, 22–45, 2009. 23. He, F. and Zhao, D., Preparation and characterization of a new class of starch-stabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in water. Environ. Sci. Technol., 39, 9, 3314–3320, 2005. 24. Gao, S., Shi, Y., Zhang, S., Jiang, K., Yang, S., Li, Z., Takayama-Muromachi, E., Biopolymer-assisted green synthesis of iron oxide nanoparticles and their magnetic properties. J. Phys. Chem. C, 112, 28, 10398–10401, 2008. 25. Nadagouda, M.N. and Varma, R.S., A greener synthesis of core (Fe, Cu)-shell (Au, Pt, Pd, and Ag) nanocrystals using aqueous vitamin C. Cryst. Growth Des., 7, 12, 2582–2587, 2007. 26. Savasari, M., Emadi, M., Bahmanyar, M.A., Biparva, P., Optimization of Cd (II) removal from aqueous solution by ascorbic acid-stabilized zero valent iron nanoparticles using response surface methodology. J. Ind. Eng. Chem., 21, 1403–1409, 2015. 27. Hoag, G.E., Collins, J.B., Holcomb, J.L., Hoag, J.R., Nadagouda, M.N., Varma, R.S., Degradation of bromothymol blue by “greener” nano-scale zero-valent iron synthesized using tea polyphenols. J. Mater. Chem., 19, 45, 8671–8677, 2009. 28. Shahwan, T., Abu Sirriah, S., Nairat, M., Boyaci, E., Eroĝlu, A.E., Scott, T.B., Hallam, K.R., Green synthesis of iron nanoparticles and their application as a Fenton-like catalyst for the degradation of aqueous cationic and anionic dyes. Chem. Eng. J., 172, 258–266, 2011. 29. Kuang, Y., Wang, Q., Chen, Z., Megharaj, M., Naidu, R., Heterogeneous Fenton-like oxidation of monochlorobenzene using green synthesis of iron nanoparticles. J. Colloid Interface Sci., 410, 67–73, 2013.

184  Functionalized Nanomaterials for Catalytic Application 30. Wang, T., Jin, X., Chen, Z., Megharaj, M., Naidu, R., Green synthesis of Fe nanoparticles using eucalyptus leaf extracts for treatment of eutrophic wastewater. Sci. Total Environ., 466–467, 210–213, 2014. 31. Smuleac, V., Varma, R., Sikdar, S., Bhattacharyya, D., Green synthesis of Fe and Fe/Pd bimetallic nanoparticles in membranes for reductive degradation of chlorinated organics. J. Membr. Sci., 379, 131–137, 2011. 32. Tandon, P.K., Shukla, R.C., Singh, S.B., Removal of arsenic(III) from water with clay-supported zerovalent iron nanoparticles synthesized with the help of tea liquor. Ind. Eng. Chem. Res., 52, 30, 10052–10058, 2013. 33. Prasad, K.S., Gandhi, P., Selvaraj, K., Synthesis of green nano iron particles (GnIP) and their application in adsorptive removal of As(III) and As(V) from aqueous solution. Appl. Surf. Sci., 317, 1052–1059, 2014.

7 Nanotechnology for Detection and Removal of Heavy Metals From Contaminated Water Neha Rani Bhagat and Arup Giri* DRDO-Defence Institute of High Altitude Research (DIHAR), Ladakh, India

Abstract

The health of a nation depends upon the health of the ecosystem. In the current era, we witness the insult of nature like global warming and increased rate of tropical cyclogenesis. Changing climate has a significant impact on global hydrology. Plenty of water is present on our globe, but only 3% is consumable freshwater. Domestic sewage, industrial water pollution, population growth, pesticides and fertilizers, plastics and polythene bags, urbanization, ground pollution, and agricultural water pollution increased weathering process, causing the further shrinkage of freshwater quantity and quality. The research found that globally about 780 million people do not have access to clean and safe drinking water. Pollution of water is causing heavy metal contamination like Arsenic (As), Lead (Pb), Cadmium (Cd), Nickel (Ni), Copper (Cu), etc. Through the food chain, all these metals may bio-accumulate and may exert its toxic effect. Conventional methods are being used but are ineffective for wastewater treatment and purification, and removal of the pollutants is at an incomplete level. As an emerging technology, nanotechnology is the current developing sector for removing heavy metals from the water. In this modern era, to date, several nanotechnologies like carbon nanotubes, nanoscale metal, oxide, nanofibers with core-shell structure, nano-zeolites, nano-Ag, nano-magnetite, and dye-doped silica nanoparticles developed for the heavy metals remediation from the water body. In this context, this book chapter reveals the latest nanotechnology for heavy metal detection and removal from wastewater. Keywords:  Heavy metals, nanomaterials, nanotechnology, wastewater treatment *Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (185–226) © 2021 Scrivener Publishing LLC

185

186  Functionalized Nanomaterials for Catalytic Application

7.1 Introduction Around the world, human civilization will now feel the threat of water scarcity. Despite the presence of plenty of water, drinking water availability is diminishing due to an increased rate of anthropogenic contaminants. Yet, wastewater discharge rates are another consideration as well. Owing to the household (77%) and industrial activity (33%), about two million tonnes of water are discharged each day. These practices have produced drinking water shortages for 780 million people worldwide [1–3]. Domestic sewage, industrial water pollution, population growth, pesticides and fertilizers, plastics and polythene bags, urbanization, soil pollution, and agricultural water pollution increased weathering processes, causing freshwater quantities and quality to shrink further [4]. The research found that about 780 million people worldwide do not have access to clean and safe drinking water. Water pollution causes the accumulation of heavy metals, such as Arsenic (As), lead (Pb), cadmium (Cd), Nickel (Ni), and Copper (Cu). These metals can bio-accumulate and exert their toxic effect through the food chain [5–8]. Conventional methods (distillation, biological treatment, ultraviolet treatment, ultrafiltration, chemical transformation, coagulation, and flocculation) are used but are unsuccessful in wastewater treatment and purification, and are not capable of incomplete pollutant removal [9]. Nanotechnology is the most possible water treatment technology emerging [10]. Nanotechnology is the current developing field as an emerging technology for removing heavy metals from the water. To date, several nanotechnologies, such as carbon nanotubes, nanoscale metal, oxide, core-shell nanofibers, nano-zeolites, nano-Ag, nano-magnetite, and dye-doped silica nanoparticles, have been developed in this modern era to extract heavy metals from the body of water [11–14]. This book chapter deals with nanotechnology history, revised nano-­ detection techniques for the presence of heavy metals in the water, and various nanotechnologies for removing specific heavy metals from the water resources.

7.2 History of Nanotechnology “Origin to Glory” of nanotechnology has a long history. In 1974, the term “nanotechnology” was first introduced by N. Taniguchi at a conference in Tokyo. After Mr. R. Feynman advanced the nanotechnology strategy, Drexler was developed by E. Drexler, published in 1986 in his book

Nanotechnology for Heavy Metals  187 “Vehicles of Creation: The Advent of the Nanotechnology Era.” The different nanotechnological strategy developed between the second half of the 1980s and 1990s began. In 1991, the National Scientific Fund’s nanotechnological program was initiated for the first time in the USA. After that, from the early 21st century, more and more research on multidisciplinary aspects of various subjects has been started worldwide [15].

7.3 Heavy Metal Detective Nanotechnology The nanotechnology paradigm overtakes the traditional methods of detecting heavy metals in water for more than a decade. Many traditional methods have been used for detection such as liquid chromatography, UV/ Vis spectroscopy, X-ray fluorescence spectroscopy (XFS), capillary electrophoresis (CE), microprobes (MPs), anodic stripping voltammetry (ASV), atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectroscopy (ICP-MS), and inductively coupled plasma optical emission spectroscopy (ICP-OES). However, the identification of heavy metals in water has been limited due to the instrumental investment, sample processing, detection limit, etc. [16–19]. Several nanosensors and nanomaterials (metal and metal oxide nanoparticles, polymeric nanomaterials, silicone, and carbonized nanomaterials) based on electrochemical, green nanotechnology have evolved to detect heavy metals in water bodies that have higher detection limits, durability, location detection, etc. [19–21]. Due to the inclusion of all these properties in nanotechnology, the current era is now exposed to nanotechnology to identify heavy metals in water. There were a lot of studies done and listed in Table 7.1.

7.3.1 Nanotechnology for Arsenic (Aas) Removal As, a widely diffused metalloid, is a natural component of the earth’s crust distributed all through the environment, including air, water, and land. As occurs in the pure elemental form [46, 47] and can alter its oxidation state to exist in four different forms, viz., arsine (As3), As (As0), arsenite (As+3), and arsenate (As+5) [47, 48]. However, when in in-organic form, it’s highly toxic [49]. According to recent studies, the quantity of As in groundwater and surface water has reached higher quantities worldwide due to either natural or anthropogenic consequence [47, 49]. Remarkably, consumption of such contaminated water for drinking, food preparation, or irrigation of crops poses the greatest threat to human and animal health [49] as it has been recognized as toxic and potentially carcinogenic [49–51]. Long-term

Nanomaterials

AgNPs + poly vinyl alcohol (PVA-AgNPs)

AgNPs + Tween-20 + Gelatin

AuNPs

BiNPs

Carbon nanoparticles

Carbon xerogel Bi NPs

Cerium oxide nanoparticles

Copper nanoparticle (CuNPs)

Sl. No.

1.

2.

3.

4.

5.

6.

7.

8.

Colorimetric method

5.47 ppb for Ni2+, 0.65 ppb for Pb2+, 0.81 ppb for Cd2+ 0.30 ppm for Pb2+, 0.50 ppm for Cu2+, 31 ppm for Hg2+ < 1 ppb Very low 100–600 mol/L

Ni2+, Pb2+, Cd2+

Pb2+, Cu2+, Hg2+

Pb2+, Cd2+ Hg2+, Cu2+, Cd2+, Pb2+ Ag2+

Electrochemical method

0.05 nM for Cu2+, 0.02 nM for Hg2+

Cu2+, Hg2+

Electrochemical method

Electrochemical method

Electrochemical method

Electrochemical method

Colorimetric method

0.13 mg/L

Hg2+

Colorimetric method

Method

10 ppb

Detection limit

Hg2+

Metal sensitivity

Table 7.1  Different nanotechnologies for the detection of heavy metals in water. Adapted from [17, 22].

(Continued)

[30]

[29]

[28]

[27]

[26]

[25]

[24]

[23]

Reference

188  Functionalized Nanomaterials for Catalytic Application

Cd2+, Pb2+

MnCo2O4 NPs

15.

7.02 nmol/dm3 for Cd2+, 8.06 nmol/dm3 for Pb2+

Very low

Cu2+, Ni2+, Pb2+, Cd2+, Zn2+, Cr3+, Cr4+

Iron oxide (Fe2O3)NPs

14.

Electrochemical method

Adsorption method

Electrochemical method

40–0.50 μg/L

Pb2+, Cd2+

Graphene cysteine composite (AuNPs-GN-Cys)

13.

Fluorescence method

10 nM

Hg2+

FET sensor constructed with single-walled carbon nanotubes (SWCNTs)

12.

Electrochemical method

300 μg/L for Zn2+, 0.02 μg/L for Pb2+, 0.05 μg/L for Cd2+

Facile fabricated 3D graphene-framework/ Bi NPs.

11.

Electrochemical method

Colorimetric method

Very low

Zn2+, Pb2+, Cd2+

CuNPs + reduced graphene oxide (CuNPs/RGO)

10.

Hg

Method

Detection limit

0.186 μM for Pb2+, 0.111 μM for Cu2+, 0.051 μM for Hg2+, 0.203 μM for Cd2+

Core-shell SiO@Ag nanoparticles

9. 2+

Metal sensitivity

Pb2+, Cu2+, Hg2+, Cd2+

Nanomaterials

Sl. No.

Table 7.1  Different nanotechnologies for the detection of heavy metals in water. Adapted from [17, 22]. (Continued)

(Continued)

[37]

[36]

[35]

[34]

[33]

[32]

[31]

Reference

Nanotechnology for Heavy Metals  189

Colorimetric method Fluorescence method

99.00% 3 μM 40 nM 10−7 M for Hg2+, 10−4 M for Cd2+ 10 μg/L

Pb2+ Ag2+, As2+, Cu2+, Pb2+

Hg2+ Hg2+, Cd2+ Pb2+, Cd2+

MnFe2O4 nanocrystals

Multicomponent nanoparticles (MCNPs)

Oligonucleotidefunctionalized AuNPs

QD−DNA−Au NP

Si nanowire

Uniform bismuth nanoparticles (Bi NPs)

18.

19.

20.

21.

22.

23.

Hg2+

Adsorption method

0.054 μM

Cd2+, Hg2+, Pb2+, Cu2+

MnFe2O4 nano crystal + L-cysteine

35.3–57.0 μA/μM

Electrochemical method

Fluorescence method

Electrochemical method

Electrochemical method

Electrochemical method

17.

0.0883 μM

Pb2+

Method

MnFe2O4 + graphene oxide (MnFe2O4/GO)

Detection limit

16.

Metal sensitivity

Nanomaterials

Sl. No.

Table 7.1  Different nanotechnologies for the detection of heavy metals in water. Adapted from [17, 22]. (Continued)

[45]

[44]

[43]

[42]

[41]

[40]

[39]

[38]

Reference

190  Functionalized Nanomaterials for Catalytic Application

1 g/L

0.1 g/L

0.4, 0.8, 2, 4, and 40 mg/L

100 mg/L

0.2 g/L

3. Copper (II) oxide

4. Mesoporous Ɣ-Al2O3

5. Calcium peroxide

6. Maghemite (sol-gel method)

7. Zeolitic imidazolate framework-8

1. Mixed magnetite-maghemite nanoparticles

0.4 g/L

For As(III), 0.01 to 0.15 g/L and For As(V), 0.01 to 0.04 g/L

2. Amorphous Zirconium oxide (am-ZrO2)

b) Mixed metal–based nanoparticles

0.01 g/L

Adsorbent (Nanoparticles dosage)

1. Hydrous cerium oxide (HCO)

a) Single metallic nanoparticles

Types of nanoparticles

1.5 mg/L

20 mg/L

1,000–11,000 μg/L

0.2, 0.4, 0.6, 0.8, 1, and 2 mg/L

100 μg/L of As(III)

100, 200, 500, and 1,000 μg/L

1 and 0.1 mg/L

1 to 100 mg/L

Adsorbate

96%–99% removal and 3.69mg/g adsorption for As(III) and 3.71 mg/g for As(V)

49.49 and 60.03 mg/g for As(III) and As(V)

25.0 mg/g

Up to 91% for As(III)

52.6% removal

100%

Over 83mg/g adsorption for As(III) and over 32.4 mg/g for As(V)

170 mg/g for As(III) and 107 mg/g for As(V)

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47].

(Continued)

[65]

[64]

[63]

[62]

[61]

[59, 60]

[58]

[57]

References

Nanotechnology for Heavy Metals  191

0.5 g

10 mg

0.2 g/L

2 g/L

1 g/L

0.4 g/L

2.0 g/L

3. Iron-zirconium binary oxide adsorbent

4. CeO2-ZrO2 nanospheres

5. Iron(III)-cerium (IV) mixed oxide

6. Magnetite and maghemite

7. Hematite nanorods (Fe3O4) and magnetite (α-Fe2O3)

8. Nano hydrous Fe (III)-Al(III) mixed oxide(NHIAO)

Adsorbent (Nanoparticles dosage)

2. Fe-hydrotalcite supported magnetite nanoparticles (MFeHT)

Types of nanoparticles

5.5 mg for As(III)/L

60 mg/L of As(V)

1 to 7 mg/L

4.5–6.5 mg As(V)/L And 4.8 mg As(III)/L

0.5–60 mg/L

5 to 40 mg/ml

100–1,000 μg/L

Adsorbate

90% removal

For Fe3O4 and α-Fe2O3, 80.5 mg/g, 94.9 mg/g, 74.1 mg/g, and 70.5 mg/g, at different calcination temperature

2.90 and 3.05 mg/g, for As(III) and As(V)

2.42 mg/g for As (III) and 2.11 mg/g for As (V).

27.1 and 9.2 mg/g for As(V) and As(III), adsorption capacities, and 97% arsenic removal in groundwater

46.1 and 120.0 mg/g, for As(V) and As(III)

95% removal and 1.28 and 0.12 mg/g adsorption for As (V) and As(III).

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47]. (Continued)

(Continued)

[72]

[71]

[70]

[69]

[68]

[67]

[66]

References

192  Functionalized Nanomaterials for Catalytic Application

100 μg/L As(III)-0.10 mg/L

0.1 g/L

0.1 g/L

5.67 to 157.68 mg/L

0.08 to 0.16 g

2.15 g/L

3. Cetyl trimethyl ammonium bromide-modified magnetic nanoparticles

4. Magnetic nanoparticles modified with arginine and lysine

5. Superparamagnetic ascorbic acid-coated Fe3O4

6. Magnetic Ɣ-Fe2O3 nanoparticles

7. Ultrafine Superparamagnetic iron(III) oxide

100 mg/L of As(V)

10 to 150 mg/L and from 10 to 200 mg/L for As(III) and As(V)

100 μg/L of As(V)

As(V): 0.14 to 10.68 mmol/L

0.100 g

2. Magnetic nanoparticles activated micro fibrillated cellulose (FeNP/MFC)

1 to 15 mg/L As(III)

Adsorbate

0.1 g/L

Adsorbent (Nanoparticles dosage)

1. Magnetic nanoparticles modified with Fe-Mn binary oxide (Mag-Fe-Mn)

(c) Magnetic nanoparticles

Types of nanoparticles

(Continued)

[78]

[77]

59.2–74.8 mg/g and 88.4–105.2 mg/g, for As(III) and As(V)

100% removal

[62]

[76]

[75]

[74]

[73]

References

16.56 mg/g for As(V), and 46.06 mg/g for As(III)

29.14 and 23.86 mg/g, Adsorption and Over 90% of As(V) removal

95% of As(V)

2.46 mmol/g

98%

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47]. (Continued)

Nanotechnology for Heavy Metals  193

2 mg/L

10 mg

9. Magnetic nanoscale Fe-Mn binary oxides loadedzeolite

10. Magnetic multi-walled carbon nanotube/iron oxide composites

2.0 g/L

0.3 g

50 mg, 300 mg

10 mg/20 ml As

1. Fe3O4-graphene macroscopic composites

2. Nano iron/oyster shell composites

3. Chitosan/Cu (OH)2 and chitosan/CuO composite sorbents

4. Nanocomposites of grapheme oxide-hydrated zirconium oxide

d) Nanocomposite nanoparticles

1 g/L

Adsorbent (Nanoparticles dosage)

8. Magnetic nanoparticles impregnated chitosan beads (MICB)

Types of nanoparticles

2 to 80 mg/L

10 and 100 mg/L

Initial concentration of As(III) of1.8 mg/L

100, 300, 500, 800, and 1,000 μg/L

1 to 15 mg/L

0.5 g/L

1 mg/L of As(V) and As(III)

Adsorbate

Adsorption capacity 95.15 and 84.89 mg/g for As(III), As(V) and over 95% removal

39.0 and 28.1 mg As/g for chitosan/ Cu (OH)2 and chitosan/CuO

100%

-

47.41 and 24.05 mg/g for As(V) and As(III),

99.0%

35.7 mg/g for As(V) and 35.3 mg/g for As(III) MICB retain 88.20%and 76.02% removal for As(V) and As(III)

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47]. (Continued)

(Continued)

[85]

[84]

[83]

[82]

[81]

[80]

[79]

References

194  Functionalized Nanomaterials for Catalytic Application

20–200 ppb

25 to 80 mg/L 1–20 mg/L of As(III) and As(V)

0.05 g

0.5 mg/ml

2.0 g

0.1 to 1 g/30 ml

0.5–16 g/L

100 mg/50 ml

6. Nanoaluminiumdoped manganese copper ferrite polymer (MA, VA, AA) composite

7. Magnetite Fe3O4 reduced graphite oxide MnO2 nanocomposites

8. Nano-alumina dispersed in chitosan-grafted polyacrylamide

9. Zero-valent iron impregnated Chitosancaboxy­ methylbcyclodextrin composite beads

10. Activated alumina by impregnation with alum

11. Carbonized yeast cells containing silver nanoparticles

0.5–2.0 mg/L

As(V) (1–25 mg/L)

0.01–10 mg/L

5–100 mg/L

1.7 g/L

5. Polymer Nanocomposites

Adsorbate

Adsorbent (Nanoparticles dosage)

Types of nanoparticles

0.975 mg/g and 99.52% removal

up to 40 ppb As(V)

18.51 mg/g and 13.51 mg/g for As(III) and As(V)

6.56 mg/g

14.04 mg/g and 12.22 mg/g for As(III) and As(V)

0.053 mg/g

95.3% removal and 2.83 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47]. (Continued)

(Continued)

[92]

[91]

[90]

[89]

[88]

[87]

[86]

References

Nanotechnology for Heavy Metals  195

ACZ-Fe carbons of 0.92%– 4.63%, ACP-Fe samples of 2.18%–5.27% and for F400-Fe activated carbons 0.73%–2.2%

30 mg of both nanoparticles/30 ml As a solution.

0.1 g

0.03 to 0.65 g/L

100 mg of Fe-GAC

0.1 g

13. Carbon with iron nanoparticles and iron carbide

14. Iron doped phenolic resin based activated carbon micro and nanoparticles

15. Granular activated carbon media impregnated with zirconium dioxide

16. Iron impregnated granular activated carbon (Fe-GAC)

17. Activated carbon, calcium alginate, and their compositebeads

Adsorbent (Nanoparticles dosage)

12. Iron hydro (oxide) nanoparticles onto activated carbon

Types of nanoparticles

75 mg/L of As

3 mg/L arsenate

120 μg/L

0.5 to 20 mg/L

Maximum As(V) adsorption (66.7 mg/g at 30°C)

0.6 to 1.95 mg/g adsorption capacity for Fe-GACs with iron contents from 1.48% to 12.13%

Lignite Zr-GAC removed 1.5 mg As/g Zr, while bituminous Zr-GAC removed 3.0 mg As/g of Zr

3–15 mg/g

1.8 mg/g for Carbon with iron and 1.4 mg/g for iron carbide

4.56 mg/g at 1.5 mg/L As

1–1.5mg/L

20 μg/L

Removal efficiency (%)/ Adsorption capacity (mg/g)

Adsorbate

Table 7.2  Arsenic removal from contaminated water using different nanomaterials. Adapted from [47]. (Continued)

[98]

[97]

[96]

[95]

[94]

[93]

References

196  Functionalized Nanomaterials for Catalytic Application

Nanotechnology for Heavy Metals  197 exposure to inorganic As may Pb to chronic As toxicity, cardiovascular disease, diabetes, skin lesions, and cancer [49]. At present, several research studies revealed that at least 140 million people in 50 countries have been consuming As contaminated water at levels above the standard guideline of 10 μg/L [49, 52]. Globally, millions of people worldwide are exposed to As at far higher levels than the acceptable value (100 μg/L or greater) and face serious health risks due to excess intake of As through drinking water [49, 53]. As is also considered one of the 10 significant public health chemicals [49]. In this context, currently, a key issue is the treatment of As tainted water with advanced or innovative technologies to provide clean water. Thus, various physicochemical related technologies such as oxidation, coprecipitation, lime treatment, ion exchange, adsorption to tangible media, and membrane filtration for As remediation have been developed and investigated [51, 54, 55]. However, these technologies have their drawbacks. They form toxic/carcinogenic derivatives in the coagulation phase and need extra oxidation in the oxidative process to eliminate neutral arsenite ions and other ion intervention in the adsorption process. Also, with very high operating cum capital costs, these processes have low productivity and require pre-treatment and a long time in the membrane and phytoremediation process [56]. Nevertheless, the precision, high selectivity, high reactivity, flexibility, and catalytic and oxidative ability of nanoparticles, nanotechnology plays a notable role in overcoming various defects linked to traditional As remediation technologies and in removing As [47]. Recent developments in nanotechnology provide major advantages in wastewater treatment through the introduction as adsorbents of porous nanomaterials, namely, metal oxide nanoparticles, carbon nanomaterials, and nanocomposites. Besides, non-material is the most desirable technology for reducing the As problem in the current scenario. The efficiency of As removal of various nanoparticles used up to now and published in literature was illustrated in Table 7.2.

7.3.2 Nanotechnology for Lead Removal from Water Pb is a naturally occurring element and a significant contaminant to the environment. It is used mainly in the battery, steel, wood, paper, paint, dyeing, and electroplating industries [99]. Also, it is harmful when occurring in large quantities in the human skeleton, muscles, bones, kidney, liver, and brain, causing organ damage, reproductive, and nervous systems. It can also be carcinogenic [99]. Besides, its biological half-life is considered longer in children than in adults, and early prenatal exposure on brain

3.4 to 17 mg/L -

640 mg/L

640 mg/L

640 mg/L

-

0.002 to 0.11 g

167 mg/L

Fe3O4

CeO2

TiO2 nanoparticles

TiO2 nanowire

Iron oxide/hydroxide (α,γ-FeOOH)

Polyvinylpyrrolidone-coated magnetite nanoparticles (PVP-Fe3O4)

10–80 μeq/L 30 to 200 mg

0.1 g/L

-

Graphene oxide (GO)

0.1, 1, 10, and 100 mg/L

5–50 mg/L

3.4 to 17 mg/L

Carbon nanotube (CNT)

Carbon nanomaterials

10 to 150 mg/L

1, 1.5, 2.0, 2.5, and 3.0 g/L

L-Cysteine functionalized Magnetite (Fe3O4) 3.4 to 17 mg/L

2 to 100 mg/L

Adsorbate

0.5 to 1.0 g/L

Adsorbent (Nanoparticles dosage)

Magnetic Fe3O4

Metal oxide nanoparticles

Types of nanoparticles

Table 7.3  Lead removal from contaminated water using different nanomaterials.

80.7% to 85.0%

482 μeq/g

80% removal

66% removal

97.1%

159 mg/g

189 mg/g

83 mg/g

45% to 99% removal and 18.8 mg/g

37% to 70% removal efficiency and 53.1 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

(Continued)

[107]

[106]

[105]

[104]

[103]

[102]

[101]

[100]

References

198  Functionalized Nanomaterials for Catalytic Application

30 mg

10 mg

10 mg

nZVI/rGO

Al2O3 coated MWCNTs

MHC/OMWCNT

0.125 and 0.25 wt.%

PANI@GO/PES nanoparticles

101.1 mg/g

-

6 g/L

Zeolite materials obtained from fly ash

More than 80% removal

76.28% to 94% removal and 49.72 mg/g

0.01 g/L

PMDA/TMSPEDA

5–250 mg/L

80%

20 g/L

Magnetic nano-adsorbent

98% removal in low lead concentrations @ 0.125 dosage

682 mg/g

-

25–500 mg/L

94.2% removal and 9.42 mg/g

-

-

1–50 mg/L

904 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

-

Adsorbate

Using Zeolite Nanoparticles Impregnated Polysulfone Membranes

Miscellaneous nanoparticles

0.1–1 g

Nanocellulose fibers (NCFs)

Nanocomposite

Adsorbent (Nanoparticles dosage)

Types of nanoparticles

Table 7.3  Lead removal from contaminated water using different nanomaterials. (Continued)

[116]

[115]

[114]

[113]

[112]

[111]

[110]

[109]

[108]

References

Nanotechnology for Heavy Metals  199

200  Functionalized Nanomaterials for Catalytic Application development may be associated. Conventional methods do not help to fix Pb efficiently, so nanomaterials are used to detect and remove Pb, as shown in Table 7.3.

7.3.3 Nanotechnology for Cadmium (Cd) Removal from Water Cd occurs in zinc, Pb, and Cu ores in nature, fossil fuels, and volcanic eruptions. Owing to anthropogenic activities, waste streams, and landfill leaching, these deposits, as well as massive industrial releases, serve as sources of Cd to ground and surface waters. Also, significant industries release Cd that comprises effluents such as metallurgical alloys, ceramics, batteries, plastics, metal electroplating, and pigment-related industries into nature [117]. Cd is also a harmful and non-biodegradable metal that has degrading environmental and human health consequences resulting in severe damage to the kidneys, emphysema, hypertension, cardiovascular disease, and skeletal malformation [118, 119]. WHO has thus set 0.003 mg/L of Cd as the acceptable water limit [120]. Effective removal of Cd from industrial wastewater in this sense is a matter of concern to the environmentalists. Similar to As remediation, in Cd removal, traditional approaches often hold various shortcomings. Thus, nanoparticles are currently being used for Cd removal and are shown in Table 7.4.

7.3.4 Nanotechnology for Nickel (Ni) Removal Ni is a commonly distributed element in nature, constituting 0.008% of the earth’s crust, and higher species are persistently exposed to this metal due to its abundance. Ni levels in water are somewhat low, as 0.228–0.693 μg/L in ocean water, and typically less than 2 in freshwater systems [170]. More often, due to anthropogenic activities, this metal exists in the form of oxide, sulfide, and soluble compounds in the water sources [171, 172]. Ni and its compounds have numerous industrial and commercial uses in the manufacture of stainless steel and Ni alloys, mining, electroplating, metallurgical battery dying, pesticide, and reactor materials. Besides, due to its use in many main enzyme productions, Ni is considered an essential trace element within different organisms, including mammals, microbes, and plants, so Ni has extensive research and medical applications [120]. However, it also causes toxicity depending on the dosage, the route of exposure, the solubility of its compounds, and the duration of exposure time, leading to serious health conditions such as anemia, diarrhea, encephalopathy, hepatitis, lung and kidney damage, gastrointestinal distress,

4 to 8 g/L

10. Zerconium oxide (ZrO2)

1 to 10 ppm

16 mg/L

0.2 g/L

50 ppm

9. Magnetic Nanoparticles-orange peel powder (MNP-OPP)

8 mg/16ml

6. γ-Fe2O3/MgSNTs

1.0 mg/L

100 ppm

20 mg

5. Magnetic CoFe2O4/SiO2

20 to 120 mg/L

0.5 mg/ml

0.6 g/L

4. Ni-αFe2O3

50–1,000 ppm

8. Copper oxide (CuO) nanoblades

0.5 to 2 gm

3. Cashew nut shell resin–Fe3O4 (CNSR-Fe3O4)

10 to 200 mg/L

20 mg/L

10 mg/10 ml Cd solution

2. Fe3O4-SO3H

50 mg/L

Adsorbate

7. NiO (nickel oxide)

0.6 g/100 ml of Cd solution

1. Fe3O4

a) Metal oxide nanomaterials

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

99.9% and 3.18 mg/g

82% and 76.92 mg/g

99.2% to 100% removal depending on the pH and 192.3 mg/g capacity

97 mg/g

200 mg/g

95% and 5 mg/g

98% and 90.91 mg/g

99.9% and 54.6 mg/g

94% and 80.9 mg/g

90% and 9.68 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.4  Cadmium removal from contaminated water using different nanomaterials. Adapted from [121].

(Continued)

[130]

[109]

[129]

[128]

[127]

[126]

[125]

[124]

[123]

[122]

References

Nanotechnology for Heavy Metals  201

250 mg/L

1 to 5 g/L

0.1 g/100ml of Cd solution

25 mg/25ml

10 mg/L

-

12. Zinc oxide (ZnO) nanorods

13. Polyaniline/cobalt hexacyanoferrate (PANI/CoHCF)

14. Granular activated carbonNanoscale iron oxides (GAC-NSIO)

15. Titanium carbide (Ti-C)

16. Hydroxyapatite-Titanium oxide (HA-TiO2)

16. Titanium oxide (TiO2) dandelions

500 mg/L

500 mg/L

1. Zerovalent iron (nZVI) NPs

2. FeSSi

b) Metal nanoparticles

0.05 gm/15ml

11. Samarium-Magnetic nanoparticles (SM-MNP)

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

0.1, 0.2, 0.4, and 0.6 mM Cd2+ solution

10–40 mg Cd/L

40, 60, 80, 100, 140, 180, and 200 mg/L

40 ug/L

50 mg/L

0.05 to 50 mg/L

0.1 g/L

100–400 mg/ L

1 mg/L

Adsorbate

105 mg/g

78 and 188 mg/g

80% and 396 mg/g

0.36 mg/g

31.7 mg/g

7.84 mg/g

64.15% and 27.17 mg/g

91% and 147.25 mg/g

92% and 5.18 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

(Continued)

[139]

[138]

[137]

[136]

[135]

[134]

[133]

[132]

[131]

References

Table 7.4  Cadmium removal from contaminated water using different nanomaterials. Adapted from [121]. (Continued)

202  Functionalized Nanomaterials for Catalytic Application

4 g/L

0.1, 1, 5, 10, and 30 mg/L

5. Cadmium sulphide Nanoparticles (CdS Np)

6. Silver nanoparticles (Ag NP)

25–250 mg

1 mg/10 ml

1 mg/2ml

0.01 g/10ml

0.5 mg/ml

-

-

10 mg/10 ml

1. Carbon nanotubes (CNT)

2. Multi-walled carbon nanotubes (MWCNT)

3. e-MWCNT

4. N-doped CNT

5. Oxidized N-doped MWCNT

6. MWCNT-COOH

7. CNT-PAC

8. 8 γ-Fe2O3/MWCNT

c) Carbon-based nanomaterials

6.5 ppm, 9 ppm, 11.5 ppm, 14, and 18 ppm/30 μl

3. Molybdenum Disulfide/p-toluenesulfonate 4. (MoS2/TMPyP)

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

1 to 150 mg/L

-

-

100 ppm

-

0.1 to 1 mmol/L

0.1, 0.5, 1, 3, and 5 mg/L

-

5 mg/L

5–1,000 mg/L

7.5 uM

Adsorbate

97.2% and 78.81 mg/g

98.35% and 11.16 mg/g

96% and 16.34 mg/g

9.33 mg/g

7.86 mg/g

25.7 mg/g

95.1% and 181.8 mg/g

27% and 0.65 mg/g

4.67 mg/g

141.5 mg/g

91%

Removal efficiency (%)/ Adsorption capacity (mg/g)

(Continued)

[149]

[148]

[147]

[129]

[146]

[145]

[144]

[143]

[142]

[141]

[140]

References

Table 7.4  Cadmium removal from contaminated water using different nanomaterials. Adapted from [121]. (Continued)

Nanotechnology for Heavy Metals  203

5–50 mg L−1 Cd(ii) 100 μg/L

20 mg/20 ml

10 mg

33 mg/100 ml

10 mg/100 ml

0.05 g/50 ml

5 mg

30 mg/50 ml

0.1 g/L

5–100 mg

32 mg/8ml

9. PEG-MWCNT

10. Magnetic graphene oxide (MGO)

11. aGO

12. MDFGO

13. M-Fe3O4-GO

14. SG-PySA-GO

15. TCAS-rGO

16. nZVI/rGO

17. 3D-SRGO

18. Ethylenediamine-glucose derived and functionalized-carbon nanoparticles (EDA-GCNP)

19. MT-CD/SiO2

-

5-50 mg/L

10 to 70 mg/L

50 mg/L

5 to 25 mg/L

50 mg/L

20 mg/L

10 to 1,000 mg/L

0.1, 0.5, 1, 2, 5 and 10 mg dm−3

10 mg

Types of nanoparticles

Adsorbate

Adsorbent (Nanoparticles dosage)

90% and 0.3 mg/g

99% and 18.7 mg/g for 10 mg L−1 Cd(ii)

93% and 234.8 mg/g

76.11% and 25.37 mg/g

90% and 128 mg/g

10 mg/g

125 mg/g

99.4%

156 mg/g

85% and 91.29 mg/g

97% and 77.6 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

(Continued)

[110]

[158]

[157]

[108]

[156]

[155]

[154]

[153]

[152]

[151]

[150]

References

Table 7.4  Cadmium removal from contaminated water using different nanomaterials. Adapted from [121]. (Continued)

204  Functionalized Nanomaterials for Catalytic Application

0.50 g/L

2 g/L

0.5 to 4 g/L

0.5 g

-

0.20 wt.%

2. NZP

3. P(MB-IA)-g-MNCC

4. CTS-VMT

5. Nanocellulose fibers

6. nHApC

7. Carboxycellulose nanofibers

64.07 mg/g

-

5. nHAp

-

87% and 17.22 mg/g

20 to 150 mg/L

-

4. Nano γ-Al2O3

93% and 148.32 mg/g 1,500 mg/g

2 mg/L

10 mg/10 ml

2. TPDP/SiO2

98% and 220 mg/g

84% and 2,550 mg/ml

92% and 122 mg/g

90.7% and 9.7 mg/g

97% and 58.48 mg/g

99% to 100% removal and 262.27 mg/g

214.7 mg/g

82% and 209.2 mg/g

Removal efficiency (%)/ Adsorption capacity (mg/g)

3. MgO NP

0.05 to 200 mg/L

17 mg/L

50–5,000 ppm

-

25 mg/L

25 to 300 mg/L

-

0.05 to 3.00 mmol/L

50 to 300 mg/L

Adsorbate

1. Polylysine-resorcinol alumina nanotube

e) Miscellaneous nanomaterials

30 mg/30 ml

1. MECT

d) Nanocomposites

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

[169]

[168]

[167]

[166]

[165]

[164]

[163]

[111]

[162]

[161]

[160]

[159]

References

Table 7.4  Cadmium removal from contaminated water using different nanomaterials. Adapted from [121]. (Continued)

Nanotechnology for Heavy Metals  205

-

3. α-Fe2O3 nanoparticle

1. Zero-valent iron

100 mg/10 ml

167 mg/L

2. Polyvinylpyrrolidone-coated magnetite nanoparticles (PVP-Fe3O4)

b) Metal-based nanoparticle

-

1. SiO2 nanoparticle

a) Metal oxide nanoparticle

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

25 to150 mg/ml

1 mg/L

0.1, 1, 10, and 100 mg/L of Ni (II)

-

Adsorbate

97%

56%

90% removal for 0.1 and 1 mg/L of Ni (II) and 13 to 29.86 mg/ml adsorption capacity in soft water and sea water for 10 mg/L Ni

60.01% to 70.3%

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.5  Nickel Removal from contaminated water using different nanomaterials. Adapted from [174].

(Continued)

[176]

[175]

[105]

[107]

References

206  Functionalized Nanomaterials for Catalytic Application

30 to 200 mg/L

-

-

50 mg/L of 50% AC-nZVI

2. Carbon nanotube (CNT)

3. Graphite oxide (GO)

4. Nano zero-valent iron supported on activated carbon (AC/nZVI)

10, 50, 100, and 200 mg/L Ni

-

25 to 75 mg/ml

-

0.25 g/50ml Ni solution

Adsorbate

1. Activated carbon (AC)

c) Carbon nanomaterials

2. Alumina nano particles (sol gel method)

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

99% removal and 1,190 mg/g adsorption capacity

78 to 89% removal and 250 mg/g adsorption capacity

41.9 mg/g

-

96 to 99% removal of Ni (II)

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.5  Nickel Removal from contaminated water using different nanomaterials. Adapted from [174].

(Continued)

[177]

[107]

[106]

[106]

[173]

References

Nanotechnology for Heavy Metals  207

25 mg/L

3. Nanocellulose fibers

-

500 mg/L

2. Zeolite nanoparticles impregnated polysulfone membranes

1. Nano-crystalline calcium hydroxyapatite

30, 40, 60, and 80 mg/L

0.0032 mol/L Ni (II)

0.1000 g/50 ml of Ni solution

3. Lignocellulose/ Montmorillonite Nanocomposite (LNC/ MMT)

0.4 g/500ml

-

-

2. Magnetite Dowex 50WX4 resin nanocomposite

e) Miscellaneous nanomaterials

30 to 200 mg/L

-

Adsorbate

1. Silica/GO composite

d) Nanocomposite

Types of nanoparticles

Adsorbent (Nanoparticles dosage)

8.55 mg/g

122 mg/g

46.17 mg/g

94.86 mg/g

384 mg/g

84.7% to 91.8%

Removal efficiency (%)/ Adsorption capacity (mg/g)

Table 7.5  Nickel Removal from contaminated water using different nanomaterials. Adapted from [174].

[111]

[113]

[180]

[179]

[178]

[107]

References

208  Functionalized Nanomaterials for Catalytic Application

Nanotechnology for Heavy Metals  209 respiratory problems, skin dermatitis, central nervous system dysfunction, and cancer [171, 172]. The presence of Ni in water sources is, therefore, of concern, considering its harmful and non-biodegradable nature [173]. While Ni-contaminated water treatment is a critical issue. Several conventional methods have previously been applied to remove Ni, but since, these methods have certain limitations. With high operating costs and low efficiency of removal, nanotechnology provided a more efficient and less expensive solution to their remediation. Thus, for Ni remediation, reactive nanomaterials, including nanoscale zeolites, metal oxide–based NPs, carbon nanotubes, and metal-based nanoparticles, are used, and the wide range of nanomaterials that have been researched and used to date for their Ni removal efficiency are shown in Table 7.5.

7.4 Futuristic Research The use of nanomaterials to remove heavy metals has excellent potential and numerous advantages over traditional technologies due to high performance, cost-effectiveness, etc. [181, 182]. However, after accumulating water resources, the bulk use, and the reuse of nanomaterials, the application of nanotubes can cause environmental hazards. Some studies have stated that higher concentrations of nanomaterials change the water quality [183–187]. Nanocomposites are considered, out of all nanomaterials, as an essential cum convenient strategy for boosting the regenerative and sorptive capabilities of nanomaterials, and the synergistic combination of nanocomposites with many supporting materials creates a new field of research in heavy metal remediation. Despite their outstanding results, the researchers are worried about most of the deficiencies with their economic synthesis, recyclability, and toxicity. Also, comprehensive research is desirable for producing environmentally sound nanoparticles for environmentally sustainable water treatment systems. Thus, the environmental and health-related concerns with the application of nanotechnology are currently of major concern, requiring the creation of better technologies [188, 189]. Therefore, the future research strategy should resolve the above concerns, which are still familiar with various nanotechnologies.

7.5 Conclusion Nanotechnology for water treatment is emerging as the new field with a lot of scope and technical requirements as various nanotechnologies were

210  Functionalized Nanomaterials for Catalytic Application investigated with heavy metal detection cum removal from polluted water of toxic metals. Indeed, recent studies of remediation of heavy metalbased nanoparticles revealed that the nanocomposite adsorbent-based membrane has intense potential in the remediation sector due to its better properties investigated at the in vitro stage. However, there is still the toxicological issue of applying nanotechnology to environmental health and human exposure, and such limitations hamper these innovations in reaching out to ordinary citizens. The study should therefore be excellent in order to reduce the above-mentioned shortcomings of this innovative technology so that the common citizens will stay protected from polluted drinking water consumption.

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224  Functionalized Nanomaterials for Catalytic Application 160. Hua, M., Jiang, Y., Wu, B., Pan, B., Zhao, X., Zhang, Q., Fabrication of a new hydrous Zr(IV) oxide-based nanocomposite for enhanced Pb(II) and Cd(II) removal from waters. ACS Appl. Mater. Interfaces., 5, 12135–12142, 2013. https://doi.org/10.1021/am404031q. 161. Anirudhan, T.S. and Shainy, F., Adsorption behaviour of 2-mercaptobenzamide modified itaconic acid-grafted-magnetite nanocellulose composite for cadmium(II) from aqueous solutions. J. Ind. Eng. Chem., 32, 157–166, 2015. https://doi.org/10.1016/j.jiec.2015.08.011. 162. Chen, L., Wu, P., Chen, M., Lai, X., Ahmed, Z., Zhu, N., Dang, Z., Bi, Y., Liu, T., Preparation and characterization of the eco-friendly chitosan/vermiculite biocomposite with excellent removal capacity for cadmium and lead. Appl. Clay Sci., 159, 74–82, 2018. https://doi.org/10.1016/j.clay.2017.12.050. 163. Salah, T.A., Mohammad, A.M., Hassan, M.A., El-Anadouli, B.E., Development of nano-hydroxyapatite/chitosan composite for cadmium ions removal in wastewater treatment. J. Taiwan Inst. Chem. Eng., 45, 1571–1577, 2014. https://doi.org/10.1016/j.jtice.2013.10.008. 164. Sharma, P. and Amiralian, N., Nanocellulose from Spinifex as an Effective Adsorbent to Remove Cadmium(II) from Water. ACS Publ., 6, 3279–3290, 2018. https://doi.org/10.1021/acssuschemeng.7b03473. 165. Hossein Beyki, M., Ghasemi, M.H., Jamali, A., Shemirani, F., A novel polylysine–resorcinol base γ-alumina nanotube hybrid material for effective adsorption/preconcentration of cadmium from various matrices. J. Ind. Eng. Chem., 46, 165–174, 2017. https://doi.org/10.1016/j.jiec.2016.10.027. 166. Awual, M.R., Khraisheh, M., Alharthi, N.H., Luqman, M., Islam, A., Rezaul Karim, M., Rahman, M.M., Khaleque, M.A., Efficient detection and adsorption of cadmium(II) ions using innovative nano-composite materials. Chem. Eng. J., 343, 118–127, 2018. https://doi.org/10.1016/j.cej.​2018.02.116. 167. Cao, C.Y., Qu, J., Wei, F., Liu, H., Song, W.G., Superb adsorption capacity and mechanism of flowerlike magnesium oxide nanostructures for lead and cadmium ions. ACS Appl. Mater. Interfaces., 4, 4283–4287, 2012. https://doi. org/10.1021/am300972z. 168. Tabesh, S., Davar, F., Loghman-Estarki, M.R., Preparation of γ-Al2O3 nanoparticles using modified sol-gel method and its use for the adsorption of lead and cadmium ions. J. Alloys Compd., 730, 441–449, 2018. https://doi. org/10.1016/j.jallcom.2017.09.246. 169. Zhang, Z., Li, M., Chen, W., Zhu, S., Liu, N., Zhu, L., Immobilization of lead and cadmium from aqueous solution and contaminated sediment using nano-hydroxyapatite. Environ. Pollut., 158, 514–519, 2010. https://doi. org/10.1016/j.envpol.2009.08.024. 170. World Health Organization, Trace elements in human nutrition and health. Geneva, 1996. https://doi.org/10.1002/food.19970410323. 171. Masindi, V. and Muedi, K.L., Environmental Contamination by Heavy Metals. Heavy Met., 115–132, 2018. https://doi.org/10.5772/intechopen.76082.

Nanotechnology for Heavy Metals  225 172. Wuana, R. A., and Okieimen F. E. Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation. ISRN Ecol., 2011, 1–20, 2011. https://doi.org/10.5402/2011/402647 173. Srivastava, V., Weng, C.H., Singh, V.K., Sharma, Y.C., Adsorption of nickel ions from aqueous solutions by nano alumina: Kinetic, mass transfer, and equilibrium studies. J. Chem. Eng. Data., 56, 1414–1422, 2011. https://doi. org/10.1021/je101152b. 174. Lee, L.Z., Zaini, M.A.A., Tang, S.H., Porous nanomaterials for heavy metal removal, in: Handb. Ecomater., Springer International Publishing, 1, 469– 494, 2019. https://doi.org/10.1007/978-3-319-68255-6_27. 175. Al-Saad, K.A., Amr, M.A., Hadi, D.T., Arar, R.S., Al-Sulaiti, M.M., Abdulmalik, T.A., Alsahamary, N.M., Kwak, J.C., Iron oxide nanoparticles: applicability for heavy metal removal from contaminated water. Arab  J. Nucl. Sci. Appl., 45, 335–346, 2012. https://www.researchgate.net/publica​ tion/273137805 (accessed September 1, 2020). 176. Rathor, G., Chopra, N., Adhikari, T., Remediation of nickel ion from soil and water using nano particles of Zero-Valent Iron (nZVI). Orient. J. Chem., 33, 1025–1029, 2017. https://doi.org/10.13005/ojc/330259. 177. Ulucan-Altuntas, K., Debik, E., Gungor, S., Nano zero-valent iron supported on activated carbon: Effect of ac/nzvi ratio on removal of nickel ion from water. Glob. Nest J., 20, 424–431, 2018. https://doi.org/10.30955/gnj.002718. 178. Lasheen, M.R., El-Sherif, I.Y., El-Wakeel, S.T., Sabry, D.Y., El-Shahat, M.F., Heavy metals removal from aqueous solution using magnetite Dowex 50WX4 resin nanocomposite. J. Mater. Environ. Sci., 8, 503–511, 2017. 179. Zhang, X. and Wang, X., Adsorption and desorption of Nickel(II) ions from aqueous solution by a lignocellulose/montmorillonite nanocomposite. PLoS One, 10, 1–21, 2015. https://doi.org/10.1371/journal.pone.0117077. 180. Mobasherpour, I., Salahi, E., Pazouki, M., Removal of nickel (II) from aqueous solutions by using nano-crystalline calcium hydroxyapatite. J. Saudi Chem. Soc., 15, 105–112, 2011. https://doi.org/10.1016/j.jscs.2010.06.003. 181. Yang, W., Kan, A.T., Chen, W., Tomson, M.B., PH-dependent effect of zinc on arsenic adsorption to magnetite nanoparticles. Water Res., 44, 5693–5701, 2010. https://doi.org/10.1016/j.watres.2010.06.023. 182. Prucek, R., Tuček, J.,Kolařík, J.,Filip, J.,Marušák, Z., Sharma, V.K., Zbořil, R., Ferrate(VI)-induced arsenite and arsenate removal by in situ structural incorporation into magnetic iron(III) oxide nanoparticles. Environ. Sci. Technol., 47, 3283–3292, 2013. https://doi.org/10.1021/es3042719. 183. Zheng, Y., Pretreatment of pharmaceutical wastewater by catalytic wet air oxidation(CWAO), ISWREP 2011 - Proc. 2011 Int. Symp. Water Resour. Environ. Prot., 2, 1316–1318, 2011. https://doi.org/10.1109/ISWREP.2011.5893261. 184. Shah, S.N.A., Shah, Z., Hussain, M., Khan, M., Hazardous Effects of Titanium Dioxide Nanoparticles in Ecosystem. Bioinorg. Chem. Appl., 2017, 2017. https://doi.org/10.1155/2017/4101735.

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8 Nanomaterials in Animal Health and Livestock Products Devi Gopinath*, Gauri Jairath and Gorakh Mal ICAR-Indian Veterinary Research Institute Regional Station, Palampur, Himachal Pradesh, India

Abstract

Nanotechnology is an emerging branch of science which offers great prospective in the animal healthcare especially in the drug delivery system and in development of functional animal products. A large variety of nanosystem can have wide application in animal health care, targeted delivery of bioactive compounds, functional meat products with improved bioavailability, antimicrobial effects, sensory acceptance, and improved quality assurance to the ultimate stakeholders, i.e., consumers. Animal health can be well assured with the help of nanomaterial-mediated drug delivery system, and food safety can be addressed through biosensors and rapid detection methods for contaminants. Moreover, animal health and safe food are also important in the current scenario of socio-­ economic conditions and increasing consumer awareness. Even the emerging issue of antibiotic resistance can be taken care off through nanotechnology. Though the regulatory bodies with respect to use of nanoparticles in animals are still debatable, the functionality in the field cannot be overlooked. Further, a holistic approach to study the overall effects of nanofoods or nanomedicines in animals, humans and environment is needed to be launched globally so as to utilize this emerging technology with minimal adverse effects. Keywords:  Functional nanomaterials, animal health, livestock produce, safety concerns

*Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (227–250) © 2021 Scrivener Publishing LLC

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8.1 Introduction Nanotechnology is a fast-emerging branch of science that gathered its importance due to great deal of applications in wide variety of fields. Their greatest applicability is attributed due to nanoscale which gave them the name functional nanomaterials. Various fields of its use include health sciences and food industry. Nanotechnology has potential applications right from the farm of the farmer to the plate of the consumer owing to different sized nanoparticles/nanomaterials (Figure 8.1). The nanomaterials are the main players in the field of nanotechnology which ranged from 1 to 100 nm in size, i.e., shorter than the wavelength of visible light and possess large surface-volume ratio that enhances their area of contact [1]. Their great penetrability is also helpful in achieving targeted drug delivery and advanced disease diagnosis. These nanomaterials are insoluble or bio-persistent, synthesized, and functionalized through various routes like non-covalent or covalent bonding to obtain complex hybrid systems [2] and can be utilized in different sectors including medicine, electronics, agriculture, and food industries [3]. They have already conquered the market in the form of cosmetic and skincare products [4]. While addressing their application in animal health, they have promising opportunities in disease diagnostics, therapeutics, advanced drug delivery systems, etc. Many studies are being conducted around the world exploring their multifunctionality and intricate applications at various levels. Studies are progressing in their application in wound healing, cancer therapy, and antibacterial and antifungal properties. Scientists are exploring the possibilities of revolutionizing therapeutics by their role in countering antimicrobial resistance (AMR). Day-by-day increase in their use makes their human contact inevitable, which can turn out challenging if we are not exploring their negative impact on living systems. So, an elaborate Nanomaterials

Nano drug delivery Nano therapeutics Nanoencapsulation Nano emulsion

Figure 8.1  Application fields of nanomaterials.

Nano packaging

Nanomaterials in Animal Health  229 and intensive research is essential to study their toxicities, accumulation in body systems, and excretions as well. Nanoparticles of silver, gold, zinc oxide, titanium dioxide, and carbon are the top most used among all nanomaterials due to their potential antimicrobial characteristics, being used in air-filters, food storage containers, deodorants, bandages, tooth-pastes, paints, and other home appliances [5]. Similarly, copper oxide nanoparticles are widely used for commercial production of nano-biocide products owing to their potent antibiotic activities [6]. In food sector, for the production and processing of functional, value added, and high quality safer foods, different sized nanoparticles are used in nanotechnologies of food science [7]. Nutrient delivery, food processing, food monitoring, and safety are the areas where nanotechnology can be utilized to a great extent. Some nano-sized particles are naturally found in foods, for instance, casein micelles in milk. However, some are intentionally added for the nutraceuticals delivery or to fulfill the purpose of intelligent and active packaging which may be termed as engineered nanoscale materials. Smart food packaging is gaining momentum because of their innovative concept of packaging material itself giving indication regarding food quality [8]. These engineered nanoscale materials are specifically and critically designed to accomplish one or the other specific targeted attribute. Their composition, size, shape, and interfacial properties vary according to their function. In food sector, nanomaterials may be used to amend the texture, appearance, or stability of foods [9]. Food packaging and food safety aspects of food industry represent the most potential fields where nanomaterials may play an imperative and important role and researchers are quite focused over the same. For instance, under, new NanoPackHorizon 2020 research program, the encapsulation of natural nanomaterials like halloysite nanotubes [10, 11] for improved food packaging has drawn the attention of EU funding. Improvement of mechanical properties of packaging materials through nanocomposites, improved barrier properties through nanocoatings, maintaining hygienic conditions through surface biocides, active and intelligent packaging, use of clay exfoliates to enhance biodegradability, use of nanoliposomes in the edible films, and construction of biosensors are the main applications of nanomaterials in the food packaging industry [12]. This book chapter will focus on the role of nanomaterials in animal health, i.e., role in disease diagnostics, drug-delivery systems, therapeutics, toxic effects and associated risk factors, and potential application of nanomaterials in livestock produce during their processing and packaging. In addition, associated safety concerns and regulatory frameworks to be required have been briefly addressed.

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8.2 Nanomaterials The novel functions associated with nanomaterials are contingent on the type of materials and their sizes [10] and produced through two broad approaches, i.e., top-down and bottom-up. The top-down approach involves conventional methods like milling, grinding, sieving, and chemical reactions, and inorganic materials are processed through this approach [13]. Homogenization process that utilizes pressure to reduce the size of materials such as fat globules is the simple example of top-down approach [14]. The bottom-up approach employs the basics of self-­organization where supra-molecular structures comprised of novel functionalities are formed through assembly of small molecules. Examples of bottom-up approach like solvent evaporation and layer-by-layer (lbl) deposition have important role in food applications using components such as phospholipids [13]. Nanomaterials can be manufactured into one-, two-, and three-­ dimensional structures and thin films, nanotubes, and nanoparticles are the respective examples of the same. Though, due to the complexity and diverse properties, these nanomaterials cannot be strictly classified, but general classification has been summarized in Table 8.1 [15].

8.3 Nanomaterials and Animal Health 8.3.1 Role in Disease Diagnostics Bioimaging techniques have an irreplaceable role in disease diagnosis especially tumor imaging. Early non-invasive molecular imaging for tumor detection is advantageous in controlling tumor growth and metastasis. Targeted delivery of nano-sized molecular probes can facilitate visualization of selected sites. Quercetin-Gold-Nano-Composite (Qu-GNC) is one such example where the anticancer property of quercetin is combined with bio-imaging property of Gold nanoparticle [16]. It facilitates visualization of fluorescent cancer cells by various imaging techniques. Harmless optical imaging using near-infrared fluorescent probes (NIRFs) is gaining attention because of high sensitivity and multicolor imaging [17]. A new branch of science has evolved “nanotheranostics” which combines diagnostics with nanotherapeutics. It helps in the delivery of drugs during diagnostic procedure itself avoiding multiple interventions. It has become popular because of its non-invasiveness and its ability to visualize metastasis and progress of cancer therapy. They can visualize remaining cancer tissue after performing surgery. It is a package which helps us to understand the outcome of treatment

Nanomaterials in Animal Health  231 Table 8.1  General classification of nanomaterials and their applications. Category

Nanomaterials

Applications

Inorganic nanoparticles

Iron

Improved bioavailability

Silver

Improved bioavailability and antimicrobial activity

Iridium

Improved bioavailability

Platinum Zinc Organic nanoparticles

Liposomes

Bioactive agent, nanoencapsulation, improved solubility and bioavailability, cell-specific targeting

Proteins

Nanoencapsulation of hydrophobic nutraceuticals, improved functionalities (gelation, heat stability)

Polymeric

Nanoencapsulation and improved functionalities (delivery, antimicrobial)

Nanofibers

Globular proteins

Improved functionalities (thermal stability, thickening agent, shelf life)

Nanoemulsions

Oil in water (o/w)

Nanoencapsulation and regulated release of bioactive agents and nutrients

Water in oil (w/o) Nanodispersions

Beta-Carotene

Improved solubility and addition levels

Nanoclays

Montmorillonite (mmt)

Improved properties in packaging (barrier, thermal, durability)

in real-time. In tumor imaging, greater permeability of tumor blood vessels causes passive accumulation of nanoparticles and their subsequent visualization. Nanoparticles can be combined with radioisotopes for using in radio nuclide-based imaging techniques. Applicability of nanoparticles can be enhanced by their combination with various bioactive molecules [18].

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8.3.2 Role in Drug Delivery Systems Nanoparticle-based drug delivery system has received greater attention because of their selective drug delivery and targeted activities. They act as vehicles for drugs as well as genes in modern medicine. Mostly, they are biodegradable and synthesized natural as well as synthetic. This property qualifies them as ideal vehicle of drugs. They help in the targeted drug delivery system because of their interaction with target cell-surface receptors. Their nanosize has eased their greater penetration, even they cross blood brain barrier. So, they can revolutionize brain cancer therapy by drug delivery to previously unreachable parts of brain. Their smaller size associated with larger surface area can reduce the drug dosage which in turn can minimize side effects also. Nanoparticles as vehicles can either integrate with drugs or can attach them with surface. Rather than simply acting as transporter of drug molecules they interfere with target cell interactions, drug uptake, and complex drug metabolism. Multiple drugs can be delivered efficiently using nanovehicles in an attempt to address the problem of drug resistance. Multiple drugs delivered together can reduce the dose of individual drugs [19] and associated toxicity. Sustained release of drugs is also possible with nanovehicles by integrating drugs in different layers. Drugs in outermost layers will be absorbed first followed by drugs in internal layers. This helps in predicting the time of drug release and we can arrange drugs on vehicles based on their required time of action. Multitargeted drug delivery is another new concept where active pharmaceutical principles having different targets are incorporated in nanovehicles. This is applied in cancer therapy where chemotherapeutic agents and siRNA are incorporated in biodegradable metal nanovehicles [20]. Even though enough studies have already been conducted to prove the efficacy of nanovehicles, their poor translation rate is a major problem. For that, more studies need to be conducted to find out the differences in their properties in physiological systems [21].

8.3.3 Role in Therapeutics Nanoparticles are having varied properties from their bulk counterparts because of their large surface area. Metal nanoparticles have received greater attention nowadays especially silver nanoparticles, because of their innate antimicrobial activity. Antibacterial property is attributed by their interaction with bacterial cell membranes and they tend to accumulate inside the cytoplasm. They further cause disruption of them by cytotoxicity. Metal nanoparticles interact with Gram-positive

Nanomaterials in Animal Health  233 and Gram-negative organisms differently because of the differences in membrane structures. So, because of complex membrane structure peptidoglycan, Gram-positive bacteria are reported to be resistant to toxic nanoparticles [22]. Recently, nanoparticles are reported to have anticancer property against HeLa cells [23]. Viricidal property of silver nanomaterials by interfering with their cell signaling pathways is a hopeful finding having potential future application in combating viral infections [24]. Silver nanoparticles are having antifungal properties also [25], a major challenge of chronically ill patients. Nanotherapy has been found useful against filariasis, a major concern of veterinary parasitology. Chitosan-based gold nanomaterials are reported to have antifilarial property [26]. In cancer therapy, mostly, nanocarriers are used for targeted activity. Nano-oncology found to reduce side effects caused by chemotherapy because they tend to accumulate in cancer tissue by inhibiting drug efflux mechanisms [27]. Dogs are very good models for oncology studies because of their greater incidence of cancer at old age. Two-dimensional graphenes are functional nanoparticles working as an interface between biological and electronic systems. The concept of biosensing could be better utilized in tumor diagnostics by means of tumor bio-marker sensing [28]. Graphenes have been identified as free from toxicities, having applicability as smart nanovehicles as well as bioimaging [29]. Biosynthesized magnesium oxide metal nanomaterial has shown in vitro anti-bacterial, anti-arthritic, and anticancer properties, which needs to be further examined for in vivo activity [30]. Zinc oxide nanoparticles have shown anti-gout property in mice gout models by their anti-inflammatory activity [31]. Biosynthesized nanomaterials are reported to have lesser toxicity than their laboratory synthesized counter parts. Various bacterial species can be exploited for this purpose. In immunotherapy nanomaterials having immunomodulatory potential are used other than their applicability as vaccine adjuvants. When gold nanoparticles are used as vaccine adjuvants, their surface functionalities can be further utilized to alter immune response [32]. Even in treating multidrug resistant E. coli, antibiotic coated metal nanoparticles are found effective.

8.3.4 Toxicity and Risks With their proven toxicity to microbes, there is greater possibility that the nanoparticles can be harmful to biological systems also. A new branch has evolved “nanotoxicology” to address the toxicities caused by nanomaterials when they interact with biological systems. Nanoparticles are

234  Functionalized Nanomaterials for Catalytic Application synthesized by three methods, namely, physical, chemical, and biological. Of these, biologically synthesized nanoparticles were proclaimed as less harmful while studies are conducted to identify their toxicity. In vitro cytotoxicity study and microscopic examination of fibroblast cells exposed to biologically synthesized gold nanoparticles proved that they caused varied degree of toxicity [33]. Owing to their smaller size and the greater degree of exposure, they can be potential risk for humans, animals, and even plants. Their abilities to cross all biological barriers can be a greater threat. So, people are eager to know about these functional particles having magical properties. Their production has increased manifold all over the world and their applicability is increasing in biology and engineering fields day by day. They can get in to the biological system by various means like inhalation, ingestion, skin exposure, etc. Either they get accumulated in respective systems or can cause adverse effects like cancer, DNA damage, cytotoxicity, etc. Their carcinogenic potential need to be explored further even though some evidences of gene mutations and tumorigenesis are already established in mice models. The degree of toxicity is dependent on their reactive surface area rather than their mass [34]. The mechanism of damage involves inflammatory changes and oxidative stress. The damage is usually subjective and has no scale. They are reported to cause other health implications like asthma, allergy, and even reach brain because of their ultra-penetration capacity through nasal passage and associated olfactory nerves. There are evidences of nephrotoxicity associated with gene expression changes in rats challenged with nanoscale copper. But, it was surprising that microscale copper did not cause any toxicity [35]. A greater understanding is necessary regarding their absorption, metabolism, and excretion from living systems also. Environmental impact and risks caused by nanoparticles are another field requiring attention. Their accumulation in physical environment can be another potential threat to all if knowledge about degradability is unknown. Toxicological studies should go hand in hand with toxicity studies of differently synthesized nanoparticles.

8.4 Nanomaterials and Livestock Produce 8.4.1 Nanomaterials and Product Processing Enhancing bioavailability and sensory attributes, modification of the particle size, its distribution, and charge or cluster formation are the potential benefits of incorporating nanomaterials in post-harvested processing technology

Nanomaterials in Animal Health  235 [36]. Further, nutrient smart delivery through nanoencapsulation, proteins’ bio-separation, and color or texture modification in food systems are some aspects where nanotechnology can show its emerging potential [9]. In this context, different nanodelivery vehicles have been developed such as associated colloids, lipid-based nanoencapsulators/nanocarriers, nanoemulsions, biopolymeric nanoparticles, nanolaminates, and nanofibers, which can improvise the bioavailability of desired bioactive compounds by different pathways [37]. Therefore, nanotechnology may be exploited in research and development of such functional or interactive foods which can effectively respond to body requirements and smartly release nutrients and various research units are also developing new on-demand foods, which will only get activated during a need of hour. Here, consumer can trigger the release of added nutritional elements (such as vitamins) encapsulated in nanomaterials whenever needed, other­wise would remain in dormant state [38]. In this regard, “Tip-Top” Up bread was developed by one bakery in Western Australia in which tuna fish oil (rich repository of ω-3 fatty acids) nanocapsules were incorporated to avoid unpleasant taste of fish oil as these nanocapsules are specially designed to release only when they reach the stomach [39]. In addition to nutrient delivery, enhancing the nutrient absorption of the existed food article with aid of the nanoparticles incorporation is also the area of development in nanotechnology. In this aspect, food and cosmetic companies are already working together to develop newer mechanisms to deliver vitamins directly to the skin. For instance, Unilever Ltd. is focused to develop low fat ice creams by working on emulsion particles. The idea is to use 90% of less emulsion with the hope to reduce fat percentage to nearly 1% from 16% [38]. Furthermore, removal of lactose from milk to replace it with other sugars or the elimination of microbes from milk without any thermal treatments are some applications of nanomaterials in dairy sector, where milk is filtered through nanosieves developed from nanomaterials [40].

8.4.1.1 Nanoencapsulation Nanoencapsulation in food processing is a process to develop smart interactive food where desired substance/nutrient is packed in miniature forms (nanocapsules) by using techniques like nanoemulsification or nanostructuration that provide functionality to the finished product. Functionality may be enhanced in terms of controlled/sustainable release of the nanoencapsulated bioactive substances and flavor enhancers, improved solubility of desired substance, bypassing the digestion at desired level, and by providing the environment protection [41].

236  Functionalized Nanomaterials for Catalytic Application Nutrient delivery and targeted delivery system in accordance to body needs are the novel contributions of nanotechnology. In this aspect, various bioactive/essential compounds, like vitamins, iron, calcium, and coenzyme Q10 (CoQ10), have already been widely tested [42]. A nutrition delivery system or nanocarrier that delivers nutrition to specific places and chitosan is the representative example of a nanocarrier [43]. Protection of flavor during processing or storage and release of flavor during oral digestion or enhancing the nutrient bioavailability can be achieved through nanoencapsulation technology. Nanosize helps to enhance the solubility of the substances that are difficult to dissolve due to exposure of large surface area. Further, if any desired active substance in the food to be protected from oral or stomach digestion or to be released during intestinal digestion, then pH oriented/nanoencapsulation is the best way to provide protective layers. It may also provide the environment protection by sustainable release of antioxidants, while keeping the concentration to the minimum possible effective level. Besides, nanocapsules can easily be incorporated in almost all foods, irrespective of color owing to their much smaller size than the wavelength of light [44]. Moreover, addition of nanoparticles to existing food can also enable increased absorption of these nutrients [45]. Thus, transportation of the bioactive compounds to the desired site, sustainable release of bioactive compounds in response to body requirements, and maintenance of taste and texture while processing of product and enhancing shelf life of finished product by providing protection against chemical or biological degradation are highlighted benefits of nanoencapsulations. Though, there are several delivery systems in field of nanotechnology, association colloids, nanoemulsions, and biopolymeric nanoparticles are mostly commonly used in food industry to encapsulate the bioactive ingredients to enhance their bioavailability.

8.4.1.1.1 Association Colloids

Those colloids which are made up of very small nano-scaled well-dispersed molecules are referred as association colloids. The molecules usually falls in the range of 5–100 nm and usually forms a transparent, much stable solution and may deliver any type of functional ingredients like polar, nonpolar, and amphiphilic with the objective of improving shelf life of the food [46]. However, they may affect/alter the flavor of the ingredients and spontaneously dissociate on dilution. Micelles and reverse micelles are good examples of this type of colloidal system. Yusop et al. [47] applied micelles to chicken breast fillets, where the use of nanoparticle paprika oleoresin as an ingredient seemed to enhance the effects of marination and the sensory qualities of the fillets.

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8.4.1.1.2 Nanoemulsions

Nanoemulsions are the emulsions that possess unique characteristics like smaller size, larger surface area, and low sensitivity to physical and chemical changes, making them ideal formulas in meat industry for improving digestibility, efficient encapsulation, increasing bioavailability, and targeted delivery [48]. A typical nanoemulsion is made up of three constituents, i.e., water phase, an oil phase, and a stabilizer [49]. Though, formation and stabilization of nanoemulsions depend upon the physico-chemical properties of these three constituents, but major role of emulsifier is to stabilize the nanoemulsion by reducing the interfacial tension between other two phases [45].

8.4.1.1.2.1 Common Food Grade Constituents of Nanoemuslions

Free fatty acids, monoacylglycerols, diacylglycerols, triacylglycerols (TAG), waxes, mineral oils, or various lipophilic nutraceuticals may be used as oil phase for production of food grade nanoemulsion. The economic, nutritional, and easy available source of oil phase is TAG oils from soybean, safflower, corn, flaxseed, sunflower, olive, algae, or fish. Whereas, polar molecules, carbohydrates, proteins, acids, minerals, or alcoholic cosolvents can be used as aqueous phase to prepare food-grade nanoemulsions. Stabilizers can be emulsifiers, ripening retarders, texture modifiers, and weighting agents. Sucrose monopalmitate, sorbitan monooleate, and phospholipids are the common examples of stabilizers [45].

8.4.1.1.2.2 Production of Nanoemulsions

High-energy and low-energy approaches are the two ways to fabricate nanoemulsions, where former approach employs mechanical devices like high-pressure valve homogenizers, microfluidizers, and sonication methods that produce disruptive forces to mix and disrupt oil and water phases leading to the formation of the tiny oil droplets [50, 51]. Here, droplets size depends upon homogenization intensity or duration, emulsifier concentration, and the viscosity ratio [51–53]. As the intensity or duration of energy increases, interfacial tension decreases, the emulsifier adsorption rate increases, the disperse-to-continuous phase viscosity ratio falls within a certain range (0.05 < ηD/ηC < 5), and the droplet size decreases [54, 55]. Further, the oil characteristics and type of emulsifier used also contribute toward the size of formed particle. This approach is most suitable for food industry as can be easily applied on various types of oil phase (triacylglycerol oils, flavor oils, and essentials oils as the oil

238  Functionalized Nanomaterials for Catalytic Application phase) and emulsifiers (proteins, polysaccharides, phospholipids, and surfactants) as well. Small droplets can easily be produced by using flavor oils, essential oils or alkanes as oil phase as they possess low viscosity and/or interfacial tension. However, the later approach (low-energy methods) operates on the basis of internal chemical energy of the system [54, 56]. Here, phase transition that occurs during emulsification process due to the change in the environmental conditions such as temperature or composition results in formation of nanoemulsion. Thus, size of emulsion depends upon two factors, i.e., composition of the emulsion such as surfactant-oil-water ratio, surfactant type, and ionic strength, and second environmental conditions like temperature, time history, and stirring speeds. Although, this approach is able to produce comparably smaller droplets than the high energy approach, it can only be applied to limited types of oils and emulsifiers which limit its use in food. For instance, proteins or polysaccharides cannot be used as emulsifiers in this approach [45].

8.4.1.1.2.3 Applications of Nanoemulsions

Key application of nanoemulsions in meat processing industries includes encapsulation and release of lipophilic components like vitamins, flavors, and nutraceuticals. Bioactive compounds like antioxidants, ω-3 fatty acids, can be well encapsulated in nanoemulsions. Besides this, nanoemulsions can be used to enhance the digestibility of the product. Ginger essential oil (6%) containing nanoemulsion-based edible sodium caseinate coating has been found to be better preservative for chicken breast fillets than those coated with emulsions in terms of color scores, i.e., appearance [57]. Nanoemulsions encapsulated various amounts of oregano (Origanum vulgare) have also been incorporated in chicken pâté previously inoculated with Escherichia coli and found that contamination on the last day of analysis was significantly lower than that with direct oregano essential oil treatments. Thus, it is helpful in enhancing shelf life of chicken [58]. Nanoemulsions have also been used in food packaging with objectives to control food pathogens and release flavor without affecting appearance [59].

8.4.1.1.3 Biopolymeric Nanoparticles

Biopolymeric nanoparticles may be employed to enhance the shelf life of foods by encapsulating the functional bioactive ingredients and polylactic acid (PLA); a food grade synthetic nanoparticle is the best representative of the same. However, polylactic-co-glycolic acid (PLGA) and polyethylene glycol also fit under the same roof. The antimicrobial activities of PLGA

Nanomaterials in Animal Health  239 incorporated phenolic compounds have been well documented in raw and cooked chicken. Further, such encapsulated phenolic compounds like benzoic and vanillic acids have reported to show inhibitory action against Salmonella typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes at very low concentrations [36]. Chitosan nanoparticles’ incorporation directly in fish fingers and indirectly through edible films coating have also shown shelf life extension of the product [60].

8.4.2 Nanomaterials and Sensory Attributes Color, flavor, and texture of a finished product are two most important sensory factors that attract consumers. Office of Cosmetics and Colors in the Center for Food Safety and Applied Nutrition and the USFDA has approved some nanomaterial products and their limits to be used as food color additives. For instance, TiO2 has been approved as a food color additive with permissible limit, i.e., not greater than 1% w/w if incorporated individually and not be more than 2% of the total if used with SiO2 and/ or Al2O3 as dispersing aids [61]. Similary, SiO2 nanomaterials act as carrier fragrances or flavors in food as well as nonfood products [62] and for this nanoencapsulation technique is commonly used for sustainable release and retention of flavor [63]. In addition, SiO2 is also used to maintain flow properties in powdered products as an anticaking agent along with as a carrier of fragrances or flavors in food and nonfood products [43].

8.4.3 Nanomaterials and Packaging Maintenance of bloom, reduction of weight loss, protection from bacterial contamination, and subsequently spoilage are the main objectives during packaging of the meat and meat products [64]. Food packaging materials must offer proper mechanical, thermal and optical properties, antimicrobial, and barrier functions so that food can be protected from external factors like gases, aroma, and contamination, and such properties can be improvised by employing nanomaterials [65]. Here, nanoparticles are used as carriers of bioactive compounds that may be antioxidants, antibrowning agents, flavors to extend the shelf life even after opening of package [66–68] and nanocomposite polymers are used to enhance thermal resistance, mechanical strength, barrier properties against O2, CO2, moisture, UV radiation, and volatiles to provide low weight [69, 70], making the packaging as active packaging. In addition, moreover, biodegradability of the packaging materials can also be improved. Further, nanosensors like devices can also be used in packaging materials to detect spoilage, toxins,

240  Functionalized Nanomaterials for Catalytic Application or pesticides [71] and to monitor food and surrounding environment and to track the history of time-temperature and expiration date, and such packaging is termed as intelligent/smart packaging [72].

8.4.3.1 Nanocomposite Polymer matrix reinforced with nanofillers to get nanocomposites of improved packaging properties [73] and fillers may be carbon nanoparticles, nanoclays, polymeric resins, nanoscale metals, or oxides which are incorporated at 2%–8% in polymer matrix to attain targeted results. Here, polymers may include polyamide, polystyrene, nylon, polyolefins, etc. [73]. Nanocomposites have great potential of improved packaging, e.g., elimination of CO2 or unpleasant flavors accumulation shall be avoided through incorporation of carbon nanotubes in polymer matrix. Gas barrier properties of bottles or packaging materials may also be enhanced through nonclay incorporation in the nanocomposites [74, 75], e.g., oxygen barrier was found to be improved when clay nanoparticles were incorporated into the ethylene–vinyl alcohol copolymer and PLA biopolymer. In addition, nanoclays incorporation into polyamide 6 have been found to have several beneficial effects when were used to vacuum package beef loins. Elevated O2 barrier properties, capability to block UV, and improved stiffness were the beneficial effects [76]. Similarly, gas and water vapor barrier properties are found in Durethan, a meat-packaging product of Bayer which is manufactured by reinforcing polyamide plastic film with clay nanoparticles [77]. Furthermore, bio-nanocomposites resulting from nanofillers incorporation in biodegradable PLA exhibit more rapid bio-degradation than its counterpart PLA without nanofillers [78]. Nanocomposites with metallic and metallic oxide nanoparticles make the packaging material an active one. For instance, antimicrobial activity can be enhanced by silver-based nanocomposite [79, 80] as active silver nanoparticles into the polymer matrices increase the performance of the food packaging material by the mechanism of cellular and structural damage of contaminants [81–83]. Moreover, silver nanoparticles are effective at very low concentrations, i.e., 2 to 4 μg/ml even against resistant one [84]. Titanium oxide (TiO2) nanoparticles are well-known UV protectors in addition to antimicrobials one [81]. Zinc oxide (ZnO) nanoparticles in low density polyethylene nanocomposites are also potent antimicrobials and reported to be efficient against Escherichia coli, Pseudomonas aeruginosa, and Listeria monocytogenes found in chicken breasts [85]. Antibacterial and antifungal properties of silver nanoparticles have also been found when incorporated in cellulose casings and collagen films [86].

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8.4.3.2 Nanosensors Nanosensors have a great potential in meat industry owing to rapid pathogen detection techniques as muscle foods are quite perishable, and despite of all good hygienic practices during slaughter, processing, and storage, they are quite prone to microbial contamination [87]. Package integrity and maintenance of cold chain are important factors in meat industry and sensor-based indicators had been used to address the same [64]. Nanosensors can either be used in packaging materials or directly in food package with many objectives like identification and quantification of pathogens, spoilage substances, and proteins that cause allergies [73] and to reduce the time of detection. The detection of food spoilage is based on detecting the chemicals released during spoilage, where nanosensors work either as “electronic tongue” or “noses” [88, 89] and pathogen detection involves the use of nano-based microfluid devices which work with high sensitivity and specificity and give real-time check on pathogen [90]. The quite popular devices based on nanoelectromechanical system (NEMS) used in analytical food industry can also be employed in food preservation and analysis. For instance, NEMS system technology–based digital transform spectrometer (DTS) is used to detect trans-fat content in foods [91]. NEMS also have potential application in quality control devices owing to advanced transducers for specific detection of chemical and biochemical signals. Moreover, micro- and nanotechnologies (MNTs) may be employed to design portable economic instruments with high specificity and sensitivity and quick response and have potential to detect and monitor any adulteration in packaging and storage conditions [92]. Furthermore, biosensors like nanocantilever devices which are capable to detect contaminant chemicals, toxins, and antibiotic residues are quite popular in food world [93]. As per literature, project named Biofinger funded by European Union has developed a nanocantilever device which runs on the principle of sensing of ligand-receptor interactions and is capable to diagnose cancer and detect pathogen in food and water [94].

8.4.4 Safety and Regulations Despite of all above mentioned applications of nanotechnology, interaction of nanoparticles with the food as well as with human system is the area that is still unexplored and unknown which raises the safety concerns. To employ nanotechnology in the food industry, following areas of concerns need to be addressed [95].

242  Functionalized Nanomaterials for Catalytic Application a. Where processed (natural) food nanostructures, which are either digested or solubilised in the gastrointestinal (GI) tract and are not biopersistent. This is the least area of concern. b. Where food products contain encapsulated food additives in nano-sized carriers which may not be biopersistent but may carry the encapsulated substances across the GI tract. This is the area of little concern. c. Where food products contain insoluble, indigestible, and potentially biopersistent nano-additives (e.g., metals or metal oxides), or functionalized nanomaterials. This is the major area of concern. d. Where ADME profile (adsorption, distribution, metabolism, and elimination) and toxicological properties of which are not fully known at present. Though the risk of nanoparticle migration from polymer matrix to food article is less, it cannot be overlooked and can be predicted by a model suggested by Simon. As per model, rate of migration of a system increases with decreasing polymer dynamic viscosity and nanoparticle size [96]. The impact of nanoparticles is not just related to humans or animals but also related to environment [97]. Thus, the disposal of nanoparticles is also an issue as nanoparticles like silver nanoparticles may accumulate in the environment leading to killing of microorganisms being antimicrobial in nature and may continue and would disturbed balance in natural microflora, especially in aquatic system [84]. Therefore, regulatory framework is prerequisite to the use nanoparticles in food industry. The nanofoods and nanopackaging in USA falls under USFDA regulations, however, in Australia, regulated by Food Standards Australia and New Zealand (FSANZ) [98, 99], and in European Union, Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) accesses the risks associated with nanotechnology and demands safety assessments of such foods prior to authorization of human use [100]. European Union Novel Foods Regulation (EC 258-97) covers nanofoods and food ingredients. Though, China and Japan are major producers of nanomaterials, do not have any nanotechnology-­specific regulations [101]. However, now, many countries are demanding a regulatory framework for use of nanomaterials in food industry, but information of exposure, availability, and toxicity to human is very less and a holistic approach is required for the same. Therefore, to fully utilize the benefits of nanotechnology, globally accepted body/organization need to be established along with complete guidelines and legislations [102, 103].

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8.5 Conclusion Thus, nanotechnology is a promising technology having vista of properties in various field of engineering, medical, veterinary, and livestock produce processing and packaging industries. Unique properties of nanoscale materials are attributed to their higher aspect ratio. Though, scientists in different fields are trying to explore their magical properties and the mechanisms underlying them, published numerous literatures, still the lacunae exists in their approval for use in public. The missing block in the jigsaw puzzle is their safety concepts, especially, in nanotherapy and food industry. So, the sooner the doubts regarding their metabolism in living systems, toxicological aspects, excretion and accumulation, adverse effects, mutagenicity, etc., are cleared, their lab to land transfer will happen, and it is going to be revolutionary.

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9 Restoring Quality and Sustainability Through Functionalized Nanocatalytic Processes Nitika Thakur1* and Bindu Mangla2 Department of Biotechnology, Shoolini University of Biotechnology and Management Sciences, Solan, India 2 J.C. Bose University of Science and Technology, YMCA, Faridabad, India 1

Abstract

A walk toward safety and security in terms of sustainability is being initiated to resolve the issues related to challenges of energy sources and environmental criteria. Keeping in view the increasing demand of a sustainable technology which can reduce the burden in terms of risks associated with environment and climate, nanotechnology is now emerging as a vital technology that provides ready solutions in terms of cost effectiveness, environmental acceptability, and specificity, respectively. The innovative aspect of nanotechnology has been emphasizing on cutting edge processes like nanofibers/membranes/channels/chambers for water filtration and distillation. Electrospun nanofilters and electrospinning strategies with upgraded surface modifications are emerging as the most promising nanocomponent offering a lightweight, high porosity, cost-effective, and lower energy consumption criteria for water treatment. The present chapter highlights the sustainable approach of nanotechnology through “Nanoremidiation”, energy conservation, and water purification processes with advanced approaches toward sustainability. Keywords:  Nanotechnology, functionalized nanomaterial, nanocatalysts, sustainability, water quality, energy conservation

*Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (251–260) © 2021 Scrivener Publishing LLC

251

252  Functionalized Nanomaterials for Catalytic Application

9.1 Introduction 9.1.1 Nanotechnology Toward Attaining Global Sustainability Nanotechnology is emerging as the most promising venture for addressing challenges related to sustainability at a global platform [1–3]. The functional materials thus make it an attractive policy to proceed forward sustainably and efficiently. Though there are many queries and challenges regarding nanotechnology in sustaining parameters related to energy, food, water, shelter, healthcare, and transport in reference to the societal benefits. The extension in deterioration and degradation of environment, energy sources, and water quality are some of the issues faced by the 21st century. This threat to sustainability issues can be dedicated to various factors like the increasing population, extensive climatic change, and fluctuating pattern of microclimatic situations, respectively. Many researches have been initiated in order to address these burning issues through new innovation and technology, but no technology could resolve these risks due to the constrains related to increasing costs However, nanotechnology provided a significant contribution in contributing toward both societal and industrial makeover [4]. Nanomaterials are generally providing benefits in targeting particular metabolic pathways of various viruses and bacteria and also providing a platform to develop various functional materials that may range from optics to magnetic and finally to catalytic components. These functional

Mesoporous Silica Nanomaterials (MSN’s) Magnetic Nanoparticles Layered Double Hydroxides Micro/Nanobubbles Semiconductor Quantum Dots Hybrid Nanomaterials Metal - Organic Frame works Catalytic Nanomotors

Figure 9.1  An insight through the advances in the catalytic field of nanotechnology: A triad for the 5-year development.

Restoring Quality and Sustainability  253 nanocomponents/materials can be processed to various components like supramolecular hosts, fiber/membranes and channels in water sources. In addition, these processes/methodologies/devices/modeling tools can be effectively incorporated and utilized for generating the sustainable tools and products for the future (Figure 9.1) [5–7].

9.2 Nano Approach Toward Upgrading Strategies of Water Treatment and Purification Clean water is the need of the hour, so an important strategy should be employed for addressing issues related to water safety and quality [8]. Water safety and quality is an important criterion as it is being used for consumption, agricultural use, food industries, hydro-electric power generation, etc. However, risks related to water purity are being continuously increasing due to the burden imposed by the needs of growing population [9, 10]. These risks have been resolved by nanotechnology in terms of water treatment process, like desalinization and finally reuse strategy [11, 12]. The key components of nanomaterials include membranes incorporated or driven by pressure such as reverse osmotic membranes, ultra/microfiltration nanocomponents (polyvinylidene-coated fluoride nanofibrous channels with cross linked polyethyleneimine) exploited for advanced treatment of water sources, desalinization process, and finally recycling and resuse of treated water [11, 12]. The new designed membranes “biometric membranes” have been employed for desalination process of water sources. In addition to it, additional micro/nanofluidic channels having 3D configuration are being employed for sustainable chemical separation processes related to energy and other sensing conversions.

9.2.1 Nanoremediation Through Engineered Nanomaterials The innovative concept of “Nanoremidiation” with engineered nanomaterials (ENMs) specially designed for bioremediation purpose provides a new insight through an environmentally friendly, socially, and economically stable form of technology. However, the conventional technologies like treatment involving thermal processes, sparging, and bioremediation are known to be expensive, insufficient, time consuming, and laborious [13–16]. The nano-based decontaminating strategies thus offer a stable pathway and process which further reduces the addition of chemicals and proceeds effectively toward clean-up process. Indeed, nanotechnologies allow treatment of contaminated media in situ and minimize

254  Functionalized Nanomaterials for Catalytic Application the addition of further chemicals in the clean-up process [17–19]. The nanoengineered materials thus provides efficient reactive surface area, significantly reduction in time frame and costs (70%–80%) related to operations [20, 21].

9.2.2 Electrospun-Assisted Nanosporus Membrane Utilization The conventional methods of water treatment include obsolete methods for water purification process like coagulation, precipitation, and sedimentation, which lack the ability to remove the organic pollutant. Nanotechnology provides an innovative approach through electrospunassisted nanofiber membrane which stands as possible solution for delivery of portable water with minimum cost expenses. These fabricated electrospun-assisted nanosporus membranes have been developed through various polymer interactions like polyvinylchloride, polytetrafloroethylene, and polypropylene. They have been known to reduce problems like water turbidity (Figure 9.2). Although there are several processes related to fabricating of nanomembranes, but electrospinning is being preferred the most due to ease of production, cost effectiveness, and easy processing with minimum investment and input. The nanoelectrospun fibers still requires some lab tests for finally getting in to the stage of commercialization for large-scale production [22]. However, these nanomembranes do face challenges related to compatibility, inadequate infrastructure, associated environmental and human risk factors, and high-cost factors related to various operations. Furthermore, these issues can be resolved through an integrated approach of all the sectors (governmental agencies/institutions/industries and stakeholders), so that,

Plasma enhanced chemical vapor deposition

CNF’s

Electrospinning

Thermal Chemical vapor deposition

1. The substrate method 2. The spray method 3. The gas phase flow catalytic method

Figure 9.2  Different methods for wastewater treatment through nanotechnology-assisted innovations.

Restoring Quality and Sustainability  255 finally, nanotechnology can march forward for providing uninterrupted directions for water safety management [24, 25].

9.2.3 Surface Makeover Related to Electrospun Nanomaterials A surface modification of electrospun nanomaterials is needed to maintain water productivity by alleviating membrane fouling [24, 25]. Fouling is the process by which solutes can deposit on the pores of the membranes and can finally choke them [26]. The problem is being encountered in polymers with active surface which lacks mechanical and chemical stability. As a result, they get easily distorted and are unable to support the base membrane [25]. Therefore, a change or alteration is needed to provide suitable chemistry and mechanical transformation [26]. These alterations are provided through different methods of surface coating, grafting, and differential polymerization [27]. An approach of copolymerization includes “plasma-induced graft copolymerization” which is a superb and effective technique for synthesizing a layer of polymer on the hydrophobic membrane [28]. It basically results in ENM surface pore reduction without affecting its bulk porosity. ENMs in nanotechnology offer a cost-effective, lightweight, and energy-­ saving solution than the conventional technologies in terms of water treatment and management. They possess high porosity (70%–80%) and a large surface area–to-volume ratio. The essential requirements are the adequate pore size, fiber diameter, and removal and recovery parameters [29]. Wastewater is contaminated by a heavy microbial pool containing bacteria, algal blooms, viruses, etc. The nanoelectrospuns [31–34] can efficiently result in removing pathogens, excessive salt deposits, minerals, and various suspended materials, thus rendering pure and safe water for use [30]. Similarly, nanofiltration process can potentially eliminate bacterial flora/coliforms/viral flora effectively [35–37].

9.2.4 Restoring Energy Sources Through Nanoscience Nanotechnology provides opportunities for meeting the growing demands of energy. The utilization of solar photovoltaic cells is emerging as the most effective technology and an attractive source of clean technology, with minimal environmental impacts. The incorporation of fabricated photoanode TiO2 films are promising sources of nanoscience which provides higher porosity and less interfacial resistance between the electrolyte and anode respectively. Polymer electrolyte membrane fuel cell is the new innovation in the field of nanotechnology for green and clean generation

256  Functionalized Nanomaterials for Catalytic Application of energy which is also initiating multiscale modeling procedure using fuel cells [22–24].

9.3 Conclusion and Future Directions Nanotechnology can be a promising venture toward extracting benefits from appropriate strategies related to nanosolutions [38–41] for reducing the uncertainties related to sustainability. The application would hence reduce the uncertainties associated with environment and human health. Various upcoming tools like Nanoremidiation, electrospun nanofilters, and electrospinning provide easy solutions to the problems related to sustainability. Efforts should be made to focus on newer innovations related to greener, cleaner, and smart-nanosolutions for attaining goals of sustainability.

References 1. Diallo, M.S. and Brinker, J.C., Nanotechnology for sustainability: environment, water, food, minerals and climate, in: Nanotechnology research directions for societal needs in 2020: retrospective and outlook, M.C. Roco, M.C. Mirkin, M. Hersham, (Eds.), pp. 221–259, Science Policy Reports, Springer, Dordrecht, 2011. 2. Diallo, M.S. et al., Implications: convergence of knowledge and technology for a sustainable society, in: Convergence of knowledge, technology, and society: beyond conver-gence of nano-bio-info-cognitive technologies, M.C. Roco, W.S. Bainbridge, B. Tonn, G. Whitesides, (Eds.), pp. 311–356, Science policy reports. Springer, Dordrecht, 2013. 3. Brinker, J.C. and Ginger, D., Nanotechnology for sustainability: energy conversion, storage, and conservation, in: Nanotechnology research directions for societal needs in 2020: retrospective and outlook, M.C. Roco, M.C. Mirkin, M. Hersham, (Eds.), pp. 261–303, Science Policy Reports, Springer, Dordrecht, 2011. 4. Brundtland, H., Toward sustainable development. In our common future (chap 2). From A/42/427. Our commonfuture: report of the world commission on environment and development. Available online: http://www.un-​ documents.net/ocf-02.htm, 1987. 5. Buha, J., Fissan, H., Wang, J., Filtration behavior of silver nanoparticle agglomerates and effects of the agglomerate model in data analysis. J. Nanopart. Res., 15, 1709, 2013. 6. Gallard, H. and Majewski, P., Fabrication of amine functionalized magnetite nanoparticles for water treatment processes. J. Nanopart. Res., 14, 828, 2012.

Restoring Quality and Sustainability  257 7. Chen, Z.X., Cheng, Y., Chen, Z., Megharaj, M., Naidu, R., Kaolin-supported nanoscale zero-valent iron for removing cationic dye-crystal violet in aqueous solution. J. Nanopart., 14, 899, 2012. 8. Choi, J., Qu, Y., Hoffmann, M.R., SnO2, IrO2, Ta2O5, Bi2O3, and TiO2nanoparticle anodes: electrochemical oxidation coupled with the cathodic reduction of water to yield molecular H2. J. Nanopart. Res., 14, 983, 2012. 9. Chung, P.S., So, D.S., Biegler, L.T., Jhon, M.S., Nanotechnology convergence and modeling paradigm of sustainable energy system using polymer electrolyte membrane fuel cell as a benchmark example. J. Nanopart. Res., 14, 853, 2012. 10. Davis, S.C., Diegel, S.W., Boundy, R.G., Transportation energy data book, Ed 27, Oak Ridge National Laboratory (ORNL-6981, Oak Ridge, TN, USA, 2008. 11. Savage, N. and Diallo, M.S., Nanomaterials and water purification: opportunities and challenges. J. Nanopart. Res., 7, 331–342, 2005. 12. Shannon, M.A., Bohn, P.W., Elimelech, M., Georgiadis, J., Marinas, B.J., Mayes, A., Science and technology for water purification in the coming decades. Nature, 54, 301–310, 2008. 13. Vaseashta, A., Vaclavikova, M., Vaseashta, S., Gallios, G., Roy, P., Pummakarnchana, O., Nanostructures in environmental pollution detection, monitoring, and remediation. Sci. Technol. Adv. Mater., 8, 47–59, 2007. 14. Shevah, Y. and Waldman, M., In-situ and on-site treatment of groundwater (Technical Report). Pure Appl. Chem., 67, 1549–1561, 1995. 15. Otto, M., Floyd, M., Bajpai, S., Nanotechnology for site remediation. Remediat. J., 19, 99–108, 2008. 16. U.S. Environmental Protection Agency, Superfund: National Priorities List (NPL), Available online: https://www.epa.gov/superfund/superfund-nationalpriorities-list-npl, 2018. 17. Holland, K.S.A., Framework for Sustainable Remediation. Environ. Sci. Technol., 45, 7116–7117, 2011. 18. Karn, B., Kuiken, T., Otto, M., Nanotechnology and in Situ Remediation: A Review of the Benefits and Potential Risks. Environ. Health Perspec., 117, 1813–1831, 2009. 19. Grieger, K.D., Hjorth, R., Rice, J., Kumar, N., Bang, J., Nano-Remediation: Tiny Particles Cleaning up Big Environmental Problems, vol. 201, IUCN: Gland, Switzerland, 2015. 20. Sánchez, A., Recillas, S., Font, X., Casals, E., González, E., Puntes, V., Ecotoxicity of, and remediation with, engineered inorganic nanoparticles in the environment. TrAC Trends Anal. Chem., 30, 507–516, 2011. 21. Map of Contaminated Sites Utilizing Nano-Remediation (Oil, Manufactur­ ing, Military, Private Properties, Residences). Available online: http://www.​ nanotechproject.org/inventories/remediation_map, 2018.

258  Functionalized Nanomaterials for Catalytic Application 22. Chung, P.S., So, D.S., Biegler, L.T., Jhon, M.S., Nanotechnology convergence and modeling paradigm of sustainable energy system using polymer electrolyte membrane fuel cell as a benchmark example. J. Nanopart. Res., 14, 853, 2012. 23. Taylor, G., Electrically driven jets. Proc. R. Soc. London A313, 313, 453–475, 1969. 24. Zhao, Z., Zheng, J., Wang, M., Zhang, H., Han, C.C., High performance ultrafiltration membrane based on modified chitosan coating and electrospun nanofibrous PVDF scaffolds. J. Membr. Sci., 394–395, 209–217, 2012. 25. Yoon, K., Hsiao, B.S., Chu, B., A Formation of functional polyethersulfone electrospun membrane for water purification by mixed solvent and oxidation processes. Polymer, 50, 2893–2899, 2009. 26. Mei, Y., Yao, C., Fan, K., Li, X., Surface modification of polyacrylonitrile nanofibrous membranes with superior antibacterial and easy-cleaning properties through hydrophilic flexible spacers. J. Membr. Sci., 417–418, 20–27, 2012. 27. Wu, H., Tang, B., Wu, P., Preparation and characterization of anti-fouling b-cyclodextrin/polyester thin film nanofiltration composite membrane. J. Membr. Sci., 428, 301–30, 2013. 28. Tadanaga, K., Kitamuro, K., Matsuda, A., Minami, T., Formation of super hydrophobic alumina coating films with high transparency on polymer substrates by sol-gel method. J. Sol-Gel Sci. Technol., 26, 705–708, 2003. 29. Adamson, A.W. and Gast, A.P., Physical Chemistry of Surfaces, 6th edn, John Wiley and Sons, New York, USA, 1997. 30. Schiffman, J.D. and Schauer, C.L., Cross-linking chitosan nanofibers. Biomacromolecules, 8, 594–601, 2007. 31. Yoshikawa, M., Yoshioka, T., Fujime, J., Murakami, A., Pervaporation separation of MeOH/MTBE with hydrophilic polymer/agarose blended membranes. Membrane, 26, 259–264, 2007. 32. Thakur, N., Organic Farming, Food Quality, and Human Health: A Trisection of Sustainability and a Move from Pesticides to Eco-friendly Biofertilizers, in: Probiotics in Agroecosystem, V. Kumar, M. Kumar, S. Sharma, R. Prasad, (Eds.), Springer, Singapore, 2017. 33. Thakur, N., Heat stability and antioxidant potential of beta-carotene isolated from a fungal isolate. Bulg. J. Agric. Sci., 24, 5, 891–896, 2018. 34. Thakur, N., In Silico Modulation Techniques for Upgrading Sustainability and Competitiveness in Agri-food Sector, in: In Silico Approach for Sus­ tainable Agriculture, D. Choudhary, M. Kumar, R. Prasad, V. Kumar, (Eds.), Springer, Singapore, 2018. 35. Thakur, N., Integrated Approach for the Management of Differential Patterns of Diseases and Pest Incidence in Lychee, in: Lychee Disease Management, M. Kumar, V. Kumar, N. Bhalla-Sarin, A. Varma, (Eds.), Springer, Singapore, 2017.

Restoring Quality and Sustainability  259 36. Kashyap, A.S. and Thakur, N., Problems and Prospects of Lychee Cultivation in India, in: Lychee Disease Management, M. Kumar, V. Kumar, N. BhallaSarin, A. Varma, (Eds.), Springer, Singapore, 2017. 37. Thakur, N., Increased Soil-Microbial-Eco-Physiological Interactions and Microbial Food Safety in Tomato Under Organic Strategies, in: Probiotics and Plant Health, V. Kumar, M. Kumar, S. Sharma, R. Prasad, (Eds.), Springer, Singapore, 2017. 38. Choudhary, M., Shukla, S.K., Narang, J., Kumar, V., Govender, P.P., Niv, A., Hussain, C.M., Wang, R., Mangla, B., Babu, R.S., Switchable Graphene-Based Bioelectronics Interfaces. Chemosensors, 8, 2, 45, 2020. 39. Choudhary, M., Shukla, S.K., Kumar, V., Govender, P.P., Wang, R., Chaudhery, M.H., Mangla, B., 1D Nanomaterials and Their Opto-Electronic Applications, in: Nanomaterials for Optoelectronic Applications, CRC, Taylor and Francis, Florida, USA, 2020. 40. Hussain, C.M., Handbook of Functionalized Nanomaterials for Industrial Applications, Elsevier, UK, 2020. 41. Hussain, C.M., Handbook of Nanomaterials for Industrial Applications, Elsevier, UK, 2018.

10 Synthesis and Functionalization of Magnetic and Semiconducting Nanoparticles for Catalysis Dipti Rawat1†, Asha Kumari2† and Ragini Raj Singh1* Nanotechnology Laboratory, Department of Physics and Materials Science, Jaypee University of Information Technology, Waknaghat, Solan, India 2 Department of Chemistry, Career Point University, Hamirpur, India

1

Abstract

Catalysis using nanoparticles (NPs) is a swiftly mounting field for numerous applications in homo or hetero-catalysis. NPs of metals/oxides/semiconductors and other binary/ternary/multinary compounds are promising. In catalysis, high ­surface/volume ratio provides higher surface catalytic reactivity. NPs have large surface/volume ratio; therefore, even small quantity can provide high catalytic activity. Unfortunately, NPs have the tendency to agglomerate; hence, functionalization of NP’s surface is a must. These functionalities should not hinder/diminish the surface properties and hence the catalytic activity. Therefore, selection of the method of synthesis and functionalization is significant for magnetic and semiconducting NPs. Catalytic processes of magnetic NPs include photocatalytic degradation, oxidation, and alkylation. Moreover, functionalized NP catalysts can be effortlessly recovered from reaction matrix and are reusable without losing catalytic activity. The semiconducting NPs, on the light-driven processes, participate in photocatalytic redox reaction. Functionalization of these NPs is possible using stabilizers/cappents or passivation of particle surface with a shell. Synthesis of NPs must be environment friendly, faster, and economic to accommodate bulk preparation. As catalyst are being used to reduce the time of reaction process and to make the process environment friendly. This chapter covers the synthesis and functionalization of magnetic and semiconducting NPs for various catalytic applications. *Corresponding author: [email protected] † Joint first authors. Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (261–302) © 2021 Scrivener Publishing LLC

261

262  Functionalized Nanomaterials for Catalytic Application Keywords:  Catalysis, magnetic nanoparticles, semiconducting nanoparticles, synthesis, functionalization

10.1 Functionalized Nanomaterials in Catalysis 10.1.1 Magnetic Nanoparticles An active catalyst requires some parameters such as active surface sites which facilitate bond formation and bond breaking with other units, adsorption of the reactants, and desorption of the final products [1–5]. Moreover, an exceptional structural strength is principally crucial to permit the catalytic activity that must be competent for an extended duration. Therefore, metals and its oxides are the testified catalytic materials in varied industrial catalytic progressions. Yet, the metal and metal oxide–based catalysts every so often suffer through multiple demerits, including their low selectivity, high cost, and poor durability. Sometimes, catalyst residual and the bi-products obtained through the catalytic reaction also proved to have unfavorable environmental effects. Therefore, inexpensive and metal-free catalysts of high performance are needed to be developed [6]. Sometimes, the particle aggregation of magnetic nanoparticle (NP) reduces the catalytic activity by lowering the surface area of the NP. Therefore, to overcome this factor, functionalization of NPs with some polymer or oligomers is better option, which helps in protecting (passivating) the surface of NP and also prevent the particle agglomeration with one another. Functionalizing a NP, sometimes, increases the activity sites and also the stability of the NPs [7–10]. In this chapter, we are discussing magnetic NPs which can be covered with the layer of carbon, silica, quantum dots, etc., or else uncovered, for instance, those comprising only molecular stabilizers [11]. Uncoated magnetic NPs possibly simply synthesized and functionalized by tactics like alkoxyorganosilane and bi-functional ligands (containing phosphonate, carboxylate), etc. But, maximum among these can be easily oxidized in open atmosphere, which makes the NPs less stable in acid media, and alter the magnetic parameters of original NP. While coating the NPs externally is an approach to save the magnetic content when applied to particular applications. A defensive shell evades the straight contact of the core (magnetic) with supplementary materials linked to the shell’s material externally, thus shun undesirable interactions of the core and shell. Moreover, the magnetic core coated with other oxides allows the immobilization of catalysts via a covalent bond. Generally, coated magnetic nanomaterials are thermally stable and strong. In this chapter, we have summarized the

Magnetic and Semiconducting Nanoparticles  263 (a)

(b)

nitrates chlorides sulphates carbonates

organometallic compounds

precursors

drug peptide ligand antibody fluorophore RNA/DNA

functionalization

peptide nucleic acid antibody ligand fluorophore radioactive label

synthesis high-temperature synth. sol-gel reaction coprecipitation microemulsion sonolysis

modification of surface organic compounds surfactants polymers silica gold

drug polymer

Figure 10.1  Magnetic particles design workflow (a) possible modification and (b) functionalization of magnetic particles (with permission from [12]).

various methods for the synthesis of functionalized magnetic NPs for various catalytic applications through distinct “uncoated” and “coated” magnetic nanostructures [11]. Latest scientific approaches for the development of surface-modified or functionalized magnetic nanostructures along with the widespread synthesis approaches to affix catalytic (non-magnetic in nature) phases (active) on magnetic nanostructures. Figure 10.1 represents the design and synthesis of magnetic NPs to real-world application [12].

10.1.1.1 Heterogeneous and Homogeneous Catalysis Using Magnetic Nanoparticles The fixation of catalysts on the surface of support materials which integrate the prevalence of heterogeneous/homogeneous catalysis is the field of enduring research in current catalysis procedures and techniques. MNPs possesses high surface/volume ratios, negligible level of toxicity, higher thermal stability, higher activity, and the possibility of surface engineering or functionalization along with the trouble-free dispersal hence considered as a leading candidate in catalysis. Recoverable and recyclable magnetic nanocatalysts can be effortlessly taken apart from reaction media by applying an external magnetic field using external magnet, without filtration, centrifugation, or other tiresome procedures. Recently, immobilized gold nanocatalysts on MNPs unlock the age of gold catalysis. “The catalytic potential of these nanostructured systems have been evaluated in a many organic chemical reactions such as synthesis of propargyl amines, coupling reactions, and oxidation reactions [13]”. Figure 10.2 represents the recoverable and recyclable magnetic nanocatalysts or nanocatalyst-loaded magnetic NPs.

264  Functionalized Nanomaterials for Catalytic Application Substrate or reactants

Reaction in presence of catalyst (MNPs or catalyst loaded MNPs)

Product information and separation of magnetic nanoparticles

cy cla

bi

lit

y

External Magnet

Removal/collection of magnetic nanoparticles

Re

Substrate or reactants Magnetic Nanoparticles for catalysis Product

Figure 10.2  Recoverable and recyclable loaded and unloaded magnetic nanocatalysts.

10.1.1.2 Organic Synthesis by Magnetic Nanoparticles as Catalyst In chemistry, prospective use of magnetic NPs is as a catalyst or support for catalyst. A “catalyst support” substance is generally a solid material which has high surface/volume ratio on which a catalyst can be immobilized. The heterogeneous catalytic reactivity occurs at the surface atoms. Accordingly, there are many efforts are going on to increase the surface/volume ratio of a catalyst by allocating it on the support. However, the support is generally chemically inactive or it can take part in catalytic reactions. Affixing the catalytic materials on the surface of MNPs with a large surface/volume ratio can deal with this crisis. Figure 10.3 represents “catalytic activity of Fe3O4 NPs in homocoupling reaction of aryl25 boronic acids” [14, 15].

10.1.2 Semiconducting Nanoparticles Semiconductors (SCs) possess conductivity capability in between insulators and conductors. Valence band (VB) completely filled up with electrons, and the conduction band (CB) is vacant in case of SCs. The current is either transferred through the electron flow via faction of positivelycharged holes. In the last decade, nanomaterials in the range of 1–20 nm have gained a lot of attention of researchers because of their small size and wide application in fields of biomedical, industrial, and electronic

Magnetic and Semiconducting Nanoparticles  265 2 Cu2+ + β-CD + FeCl3 FeCl2 Cu2β-CD(OH)22– NH4OH

6 OH–

Cu2β-CD(OH)22– + 4H2O O

Fe3O4

O

Cu

O O

Cu

O O

Fe3O4-b-CD-Cu2 Cu2β-CD(OH)22–

B(OH)2

Fe3O4-Cu2-β-CD DMF, RT- 70ºC Yields: 31–90% 19 examples

Figure 10.3  Fe3O4-β-CD-Cu2-catalyzed homocoupling reaction of aryl25 boronic acids (with permission from [14]).

appliances. Two major important factors for nanomaterials are surface and interface whereas in case of bulk material a small portion of atoms are near to surface or interface. Small size of nanomaterials gives the advantage of having more than half of materials atoms near to its interface. Surface properties like electronic structure, energy levels and reactivity may vary from interior state which gives rise to different material properties. There has been a considerable rise in the synthesis, characterizations and applications of SC NPs due to their crucial role in recently developed technologies. Large surface/volume ratios of SC nanomaterials drastically change their physical and chemical properties. Optical properties and conductivity of SC material can be modified. SC materials are still getting developed as they are having encouraging applications in the fields of superabsorbents, parts of automobiles, components of armor, waveguide, biosensors, packaging films, laser technology, nanoscale light-emitting diodes, electronic devices, solar cells, and catalysis. SC development will achieve new milestones with the incorporation of nanomaterials research. SCs covers wide range of devices and appliances like the silicon controlled rectifier, numerous type of diodes like light-emitting diode, various types of transistors, solar cells, and digital and analog incorporated circuits. Few of the SC nanomaterials like AlInGaP, Si-Ge, Si, InP, AlGaAs, ZnS, InGaAs, CdSe, GaN, ZnSe, GaAs, AlGaN, CdS, and HgCdTe possess outstanding applications in laptops, computers, cell phones, pagers, palm pilots, CD players, TV remotes, traffic signals mobile terminals, fiber networks, satellite dishes,

266  Functionalized Nanomaterials for Catalytic Application car taillights, etc. Group II-VI or III-VI composite SCs illustrate quantum confinement. In the current book chapter, we are focusing on the synthesis routes and use of SC nanomaterials in catalysis [16]. In the recent past, efforts have been made to use metal nanomaterials as heterogeneous catalysts. Metal NPs have high catalytic potential and selectivity for certain reactions. Different types of reaction routes may be followed by nanocatalyst to catalyze a chemical reaction which includes coupling reaction and the photocatalytic reaction, oxidation reaction, and reduction reaction. Catalysis can be portrayed as the wonder where the pace of a procedure or response quickened because of the support of an additional material called impetus, typically present in little amounts [17]. Catalyst associates with the reactants in a few different ways which change from giving a surface to the response to happen. Catalyst additionally offers an effective route for the response with lower actuation vitality, in this way permitting response to happen at conditions which have been beforehand not great for the response. A catalyst does not have any impact on thermodynamics and harmony part of a response; it just influences the pace of response. The key property of a catalyst lies in the way that in spite of collaborating with the response the substance comes back to its unique state toward the finish of the response which permits it to catalyze the ensuing responses too. Figure 10.4 represents the main SC-based structures which have been used since last 5 years in heterogeneous/homogeneous catalysis, industrial/

2016

Ox id Se e/C m i

2019

2017 2018

Metal/O xid e Semic o n d uct or s

tructures nos Na lysis d a ri Cat yb in

2015

iem is y S alys t Ca

Bina ry/T cond uct erna ors r in

e nid ge rs o lc to ha duc n o c

H Nanosheets for Catalysis

Figure 10.4  Semiconductor-based structures used in catalysis since last 5 years.

Magnetic and Semiconducting Nanoparticles  267 engineering catalysis, multiphase/nanocatalysis, and supported/bio catalysis and supported by their computation studies.

10.1.2.1 Homogeneous Catalysis In homogeneous catalysis, as the name suggest, reactants and catalysts both are present in the same phase called homogeneous catalysis. Homogenous catalysts generally react with one or more reactants to form a reaction intermediate and then react to form the end product, in the process restoring the catalyst.

10.1.2.2 Heterogeneous Catalysis In heterogeneous catalysts, reactants and products are there in a different physical state. For example, the catalyst may be solid at the same time as the reactants and products are in liquid phase. In nanotechnology, a broad range of SC nanomaterials have been geared up which are utilized as photocatalysts and electro catalysts [18, 19]. The heterogeneous catalysis is greatly useful rather than the additional conventional processes, the reasons are as follows: 1. This photocatalysis process is “green and environmentally benign” since the degraded products are less toxic. 2. Low concentration is necessary for the process as the contaminants are sturdily adsorbed on the catalyst surface, even in parts per million concentrations. 3. The catalysts are cost-effective, stable, chemically and biologically inert, easily separable, and recyclable. Because of all these advantages of heterogeneous photocatalysis, this offers an economical and well-organized alternate to clean wastewater and environmental remediation.

10.1.2.3 Photocatalytic Reaction Mechanism Type of chemical reaction which occurs by the combined combat of light and the photocatalyst is referred as photocatalytic reaction. These reactions have numerous advantages, like complete deprivation of pollutants, environmental protection, and no consequential effluence. Figure 10.5 illustrates the general principle of a photocatalytic process [20]. In case of SC NPs photochemistry, photocatalytic activity is the light stimulated redox reaction. A forbidden band is present between a high

268  Functionalized Nanomaterials for Catalytic Application O2

ORGANIC POLLUTANT e–

hv Light

CB

e–

O2·–

·

TiO2 VB

h+

h+

· OH·

+ H2O

H+

H2O

+ CO2

Figure 10.5  Schematics showing principle of a photocatalytic reaction (with permission from [20]).

energy CB and a low energy VB in SC material. Electrons excitation from VB to CB takes place only when incident light has greater energy than energy bandgap of the SC. This process leads to production of holes in VB. On alienated by an electric field, these photogenerated electrons and holes travel to surface of SC particles. Electrons on CB budge to the surface of the catalyst and involve in a reduction reaction, while holes on the VB disseminate to the photocatalyst surface and take part in an oxidation reaction [21].

10.2 Types of Nanoparticles in Catalysis 10.2.1 Magnetic Nanoparticles Most of the nanomaterial catalysts are heterogeneous in nature which broken up into separate metal NPs, in turn, to increase the catalytic rate of the process. NPs usually have high surface/volume ratio, which can boost the catalytic activity. NP catalysts can be easily removable and recycled, therefore, characteristically used under gentle experimental conditions to avoid disintegration of the NPs. NPs of oxides, metals, SCs, and other binary, ternary or multicomponent compounds have been used in various fields and are discussed in this section.

10.2.1.1 Ferrites Ferrite NPs or iron oxide NPs are the most explored and the oldest magnetic NPs up to date [22]. They have ferrimagnetic properties and have iron oxide as their main contents. Ferrites can be classified depending upon

Magnetic and Semiconducting Nanoparticles  269 their crystal structure: spinel (MFe2O4), garnet (M3Fe5O12), and hexagonal (MFe12O19), where M is (Mn, Ni, Cu, Co, and Zn). Great advantage of ferrites for catalysis purpose is the easy recovery of NP after the completion of the reaction by using the external magnetic field [23, 24]. Ferrites are also separated into two groups as hard ferrites and the soft ferrites which are being discussed in detail in this section.

10.2.1.1.1 Hard Ferrites

Hard ferrites are permanent magnets having large value of coercivity and retentivity. These ferrites have high value of magnetic saturation. High value of magneto crystalline anisotropy energy is the most significant property of hard ferrite. Areas where the permanent magnetism is required opted for the hard ferrites as they cannot be easily magnetized or demagnetized. SrFe12O19 and BrFe12O19 are some examples of permanent magnet. These permanent magnets when functionalized with silica, polymers, quantum dots, etc., have find their usefulness in cell catalysis and other applications such as cell sorting and targeted drug delivery applications.

10.2.1.1.2 Soft Ferrites

Soft ferrites are also called temporary magnets which possess low coercivity and high resistivity. They can be easily magnetized and demagnetized with small application of field. The low energy loss of these materials makes it promising candidates for high-frequency applications. Most commonly synthesized soft ferrites are manganese-zinc ferrite, cobalt-zinc ferrite, nickel-zinc ferrite, etc.

10.2.1.2 Ferrites With Shell The surface of oxide ferrites is comparatively inactive and generally allows strong covalent bond formation with functionalization molecules, which can improve the NP surface by forming a shell or encapsulating the magnetic surface by the layer of gold, polymer, some fluorophores, etc. (Figures 10.6 and 10.7). One such example of ferrite with a shell is the modification of silica shell with various “surface functional” groups through covalent bonding between “silica shell” and “organosilane” molecules. In addition, this core-shell–type system is further processed by functionalizing the silica shell surface with some fluorescent dyes. Core-shell system of magnetic NPs with fine particle size distribution consisting of super-paramagnetic oxide NPs with silica shell yield quite a lot of recompenses over metallic NPs [15, 25] few of which are listed below:

270  Functionalized Nanomaterials for Catalytic Application

Direct Au Coating

Indirect Au Coating

MNPs

Glue material (G) G

MNPs

Au

Au G

MNPs

MNPs Au Coating

MNP-Au core@shell

Figure 10.6  Schematic of ferrite with the shell of Au nanoparticle. Carbonic acids Polymer

SiOx Au Nanoscale Fe3O4/γFe2O3

Size-dependent properties structure and surface-to-volume-ratio

Ni2+

Linker Ni2+

Protein

Au

Surface O hydroxide canted spins Fe

Fe3O4 + 2H+

γFe2O3 + Fe2+ + H2O

Migration Fe2+ Amino acid

Anti body

O

Phase transformation

Tag

OH

Diffusion O2

Enzyme

Catalyst

e–

hν O2·– H2O2 HO· O Fenton reaction HOO· 2

Figure 10.7  Schematics of diverse applications, surface interactions, and structural transformations of magnetic (iron oxide) nanoparticles (with permission from [25]).

• Narrow size distribution (which plays vibrant role in diverse field of biomedical applications) • Higher chemical stability with the formation of silica shell • Higher colloidal stability due to shell formation as they do not magnetically agglomerate

Magnetic and Semiconducting Nanoparticles  271 • Super-paramagnetic properties are reserved • Covalent functionalization is possible on silica surface because of suitability, which provides the stability to the core-shell system. • Ferrites comprising metal ions, in addition to iron, are extensively much more used in the nanocatalysis; metallic NP size can be varied to wide-ranging, from ultra-small particles size (5–10 nm) to 100 nm or more [26]. Due to high magnetic moment, metallic NPs benefit some technological applications as compared to oxides (maghemite, magnetite) which have many applications [27, 28].

10.2.1.3 Metallic Higher magnetic moment of metallic NPs allows them to possess same moment, for the smaller size as compared to their oxide counterparts. Conversely, metallic NPs suffer from the drawback of “pyrophoricity” and “reactivity” to oxidizing mediators to numerous grades. Therefore, this led to the tricky handling and the formation of unnecessary by products which results in less appropriateness of metallic NPs in sensitive applications. To overcome this difficulty, metallic NPs with core-shell structures are being formed and discussed.

10.2.1.4 Metallic Nanoparticles With a Shell The passivation of the metallic magnetic NPs by surfactants, creating mild oxidation, over coating with precious metals, and polymers is promising [29]. For example, “in an oxygen environment, Co NPs form an antiferromagnetic CoO layer on the surface of the Co NP. Recent work has explored the synthesis and exchange bias effect in these Co core CoO shell NPs with a gold outer shell [30]”. Magnetic core which is either simple iron or cobalt can be passivated with an unreactive shell made of grapheme, etc., which is a new trend. Formation of shell over the metallic core has numeral superiorities as compared to oxides or fundamental NPs which have higher magnetization of the core shell as compared to the bare metallic core and advanced constancy in basic and acidic solution as well as organic solvents.

10.2.2 Semiconducting Nanoparticles Photocatalysis is processes which convert plentifully existing photonic energy to valuable chemical energy. Through a brisk rise of flow photoreactors in

272  Functionalized Nanomaterials for Catalytic Application the last decade, the design and development of novel SC photocatalysts is occurring at a sweltering rate. At this time, various synthesis approaches are being utilized to develop modified/unmodified SC materials show evidence of superior performances in heterogeneous photocatalysis. SC nanomaterials play good role in photocatalysis because of high surface to volume ratio and better mono-dispersity in case of SC nanomaterials. For transfer of solar energy to chemical energy, SC nanomaterials with tunable absorption and elevated excitonic coefficients are being widely considered as photocatalysts [31]. The different classes of SC nanocrystal in catalysis beside their utilization in photochemical reactions are discussed below; these include binary SCs and ternary SCs, nanocomposite (NC) photocatalyst, produced by the conjunction of binary, ternary and other SC materials.

10.2.2.1 Binary Semiconducting Nanoparticles in Catalysis Semiconducting materials have a subclass which consists of two elements, for example, GaAs or InP. The binary SCs have a range of enviable properties that can be exploited in electronic sensors and circuits. Binary SCs further include nitrides, oxides, and chalcogenides.

10.2.2.2 Oxide-Based Semiconducting Nanoparticles, for Example, TiO2, ZrO2, and ZnO Teoh et al. stated synthesis of TiO2 NPs through Flame Spray Pyrolysis (FSP) technique and those NPs display better movement for sucrose photomineralization in comparison to commercially accessible photocatalysts. Accordingly, Teoh et al. published an exhaustive analysis of efforts to synthesize various binary nano-photocatalysts (ZnO, WO3, etc.), binary doped SCs, mixed complex metal oxides, NCs, and noble metal loaded structures [32]. This unusual ZrO2 nanoflower shows enhanced photocatalytic efficiency for dye degradation when compared to the higher surface area containing tetragonal ZrO2 NPs. ZnO is a material of oxide SC which can have strong ultraviolet absorption along with interesting piezoelectric properties due to quantum size effect. Studies on ZnO have found an assured relationship between its structure and photocatalytic properties. ZnO surface has oxygen vacancies which tend to absorb photogenerated electrons, which shows an excellent relation between oxygen vacancies, leads to oxygen absorption, and supports the oxidation reactions. WO3 is a photocatalytic-activated metal oxides, it displays other benefits also such as large “specific surface area”

Magnetic and Semiconducting Nanoparticles  273 and brilliant absorbing capability. Previous reports confirmed that WO3 can be used as both a primary catalyst and as a co-catalyst [33, 34].

10.2.2.3 Chalcogenide Semiconducting Nanoparticles for Catalysis Binary chalcogenides were measured on same intensity with binary oxide SCs. In the last four decades, nano-photocatalysts for ZnS and CdS have been tremendously researched for their effective application in reactions to environmental purification and also in the reduction of carbon dioxide (CO2), aldehydes, water splitting, and reduction of benzene derivatives [35]. ZnS Photocatalyst: ZnS is a material from II–VI group and is a large bandgap (3.6 and 3.8 eV) SC. ZnS possesses distinctive precious structures—zinc blende and wurtzite—in which zinc blende that is known as β-ZnS may present steadily at lower temperatures, while wurtzite or α-ZnS may present steadily at temperatures more than 1,000°C. Moreover, ZnS is difficult to oxidized and hydrolyzed. More notably, when the size of ZnS decreases to the nanometer scale, all these properties are there and improve. The ZnS nanomaterials exhibit great photocatalytic action in this way [36]. Moreover, ZnS is handily manufactured, non-harmful, and broadly utilized. Until now, many exploration bunches have effectively synthesized low-dimensional nanomaterials, for example, ZnS NPs [37– 39], nanowires [40], nanotubes [41], and nanosheets [42]. CdS Photocatalyst: CdS is an important, established, and studied SC material with a bandgap of ~2.42 eV and have a maximum 514-nm retention height. CdS can thus maintain visible or bright light at a frequency of 20 MPa), high temperature (>5,000 K), and high cooling

Magnetic and Semiconducting Nanoparticles  279 rate (1,010 Ks1), and the NPs are created by the coalesce of formed bubbles go along with by powerful shock waves and micro-jets at the speed 400 km/h [74]. These tendencies can put a stop to the “nuclear growth” and “secondary nucleation”. Sono-chemical technique is proved to be much operative, convenient, reasonable, and environmentally friendly as compared to other methods [75]. Therefore, the sono-chemical route could be a capable technique that offer the enhanced magnetic response of magnetic NPs and also facilitate the superior control over particle size distribution.

10.3.1.5 Sol-Gel Method The sol-gel method is an appropriate aqueous route for the synthesis of magnetic NP. The procedure is based on the “hydroxylation and condensation” of molecular precursors in solution, instigating a “sol” of nanometric units. Firstly, the nitrate or the chloride salts were dissolved in the aqueous solvent to form the solution. The “sol” formed is then dried or “gelled” by removing the solvent by chemical reaction to get viscous type gel. Properties of gel depend very much upon the structure created during the sol formation stage of the sol-gel process. Basic catalysis persuades the formation of a colloidal gel, while the acidic catalysis yields polymeric form of the gel. Complete sol-gel reactions processor take place at room temperature; to attain the final crystalline state heat treatments are desired. In the sol-gel process, ratio of salt used, pH of the solution, reaction temperature, etc., are some of the parameters to which the reaction kinetics, growth of the reaction, structure, and properties of the gel formed completely depend [76, 77]. Magnetic properties in the sol-gel system depend upon the nature of formed phases (such as hematite or magamite in oxide ferrite) and the particle volume fraction and are very delicate to the size dispersal and distribution of the magnetic NPs. Some of the merits of sol-gel method are as follows: (i) predetermined structures that are possible to obtain according to the opted experimental conditions; (ii) great possibility of obtaining pure amorphous or crystalline phases, monodispersity, and controlled particle size; and (iii) controlled consistency of the reaction products. Sol-gel synthesis is basically a low temperature synthesis process which is one of the key points and the advantage of the method. Because of the low synthesis temperature, the method is biocompatible. Sol-gel method is thus very useful and frequently used method for the preparation of NP which was further biologically processed. The only problem with the synthesis route is that it generates three-dimensional oxide networks, and hence, the method is

280  Functionalized Nanomaterials for Catalytic Application in adequate in its efficiency toward the formation of independent, disengaged nanosized units [78].

10.3.1.6 Biological Method Emergence of green chemistry has grasped a great interest from the researchers around the world in nanotechnology [79]. The main aim of biological approach is to either eliminate or to reduce the toxic materials by chemical manipulations [80]. Biological synthesis consists of formation of metallic nanomaterials via plants extract is presently under advancement and high priority and vibrant research areas under assessment. Plant-mediated synthesis involves the use of different components of plants including tissue, extracts, exudates, and other parts of the plants. Biological route to synthesize the magnetic NPs is the non-toxic and the safest method for the growth of eco-friendly and unfailing nanomaterials which have prodigious value in biological applications. The preparation of NPs includes fungi, enzymes, microbes, and the plant extracts and has been emerged as the ecological tool. It is reported in the literature that the plants and their parts demonstrate to be valuable over the other known biological procedures opted. For the green synthesis, plants are the resources comprehending reducing mediators, for example, ascorbic acid, flavones, citric acid, and crude enzymes like dehydrogenases, extracellular electron shuttles, and reductases, which play the significant part in the green synthesis of magnetic NPs.

10.3.2 Semiconducting Nanoparticles SCs such as germanium and silicon are from group IV; GaN, GaAs, GaP, InAs, and InP are form group III-V, while semi-conductors such as ZnS, ZnO, CdSe, CdS, and CdTe are from group II-VI [81]. NPs are known as 10–100 nm ranging partitions or solid particles [82]. Nanotechnology is presently an active field of scientific study due to the large diversity of prospective applications in the field of optoelectronics, in the biological world. Owing to strong optical and electronic properties there, the study of SC NPs over the past few years has attracted a lot of interest. NPs (PCs), such as CdSe, CdS, and PbS, are narrow bandgap SC quantum dots that have been used in recent years as photocatalysts. Silver sulfide (Ag2S) NPPs are useful as a photocatalyst, among others [83]. Nanosynthetic metallic particles can vary considerably in their chemical, physical, and biological properties owing to their volume-to-­ surface ratio. Therefore, these NPs have been used for a variety of purposes

Magnetic and Semiconducting Nanoparticles  281 Synthesized semiconducting nanoparticles used as catalyst

Synthesized semiconducting nanoparticles used as catalyst in hydrogenation of hydrogen peroxide

Tollen Method

Synthesized semiconducting nanoparticles used as photocatalyst

Methods for synthesis of Semiconducting Nanoparticles

Sol Gel Method

Laser Ablation Synthesized semiconducting nanoparticles used as photocatalyst

Microwave synthesis

Hydro Thermal Method

Synthesized semiconducting nanoparticles used as photocatalyst

Wet Chemical Gas Phase Method Method

Synthesized semiconducting Au loaded ZnO nanoparticles for catalytical degradation of Rhodamine

Synthesized semiconducting nanoparticles used as photocatalyst

Figure 10.10  Illustration representing methods of synthesis of semiconducting nanoparticles for catalysis.

including catalysis  [84]. Figure 10.10 represents the various synthesis methods of NPs for catalysis.

10.3.2.1 Tollens Method This method is very effective in a one-step process for the synthesis of controlled size AgNPs of SCs. Tollens processes showed that the smallest particles were produced at the lowest concentration of ammonia. It is well known that Au, Ag, and Pt metal ion semiconducting material prepared by tollen method can catalyze the loss of hydrogen peroxide oxide [85].

10.3.2.2 Microwave Synthesis The interaction of dielectric substances and liquids or solids by means of microwaves is commonly known as dielectric heating. The electrical dipoles which are present in such materials respond to the applied electrical field. The use of MW-assisted synthesis opens the doors to new possibilities in the production of uniformly small size nanomaterials, which is not readily attainable by other synthesis techniques. In the electron exchange, reaction between hexacyanoferrate(III) and sodium borohydride the silver NPs was

282  Functionalized Nanomaterials for Catalytic Application used as a catalytic, resulting in the arrangement of hexacyanoferrate(II) ions and dihydrogen borate ions. Some authors stated that copper NPs were synthesized using microwave synthesis and that these semiconducting NPs were further used as catalysts [86].

10.3.2.3 Hydrothermal Synthesis Hydrothermal synthesis is normally achieved by reacting with an aqueous solution in an autoclave. Hydrothermal synthesis is commonly used to produce NPs of metal oxide, which can be simply attained by hydrothermal treatment of a metal precursor’s peptide residues in presence of water. The hydrothermal method is helpful in tailoring grain and particle size and crystalline phase, and can regulate surface chemistry, solution composition, temperature of reaction, heat, additives, aging, and solvent properties [87]. The hydrothermal approach can also achieve uniform NC doping and preparation by changing the product proportion or by adding a surfactant. The hydrothermal method has many advantages like little environmental pollution by catalytical reactions and low cost [88].

10.3.2.4 Gas Phase Method The gas phase techniques are suitable for thin film production. Synthesis is possible physically or chemically at the gas level. Chemical vapor deposition (CVD) is an industrialized technique that is commonly used and can cover vast areas in very short time. During the cycle the metal oxide is formed in a chemical reaction or gas phase from the decomposition of the precursor. Physical vapor deposition (PVD) is an added technique for thin-film deposition. The method is similar to CVD excluding that the precursors/raw materials, i.e., the substances to be deposited, arrive in solid form, while precursors are added in the gaseous state in the reaction chamber for CVD. NPs synthesized by gas phase reveal tremendous catalytic properties, perfect in electro-catalysis and gas-sensing [89].

10.3.2.5 Laser Ablation It is a simple and versatile way to manufacture NPs in an exact and selectable size. This method involves the removal of a material from a solid surface by Laser Beam irradiation. The material is heated when the laser energy is absorbed at a low laser flux; then, it finally evaporates or tolerates, while at a higher laser flux, the material is converted into plasma. This technique has been applied to manufacture various inert nanosilicate NPs

Magnetic and Semiconducting Nanoparticles  283 (silver, gold, silver, and gold). The key advantage of this approach is its high yield, fast processing time, and flexible output combined with the ability to monitor the scale, shape, and composition of NPs. As catalyst are used silver-based NPs synthesized by this method [90].

10.3.2.6 Wet-Chemical Approaches This approach includes synthesis of polar and nonpolar solvent nanomaterials. This is a kind of traditional form of precipitation, and it includes nucleation and growth steps. NPs of desired shape, composition, and size can be achieved by varying the parameter of synthesis, such as temperature and pH [91–93]. Because of the low cost and high-yield growth, wet-­chemical synthetic methods are desirable alternatives. Most specifically, they provide more possibilities for hetero-structural engineering. Photocatalytic degradation of rhodamine by using SC Au-ZnO hybrid prepared by wet chemical method was reported previously [94].

10.3.2.7 Sol-Gel Method The sol-gel process is a flexible soft chemical method commonly used in the manufacture of products from metal oxides, porcelain, and glass. To obtain the metal oxide nanopowder, the process engrosses converting a system from a liquid state to a solid “gel” level, and then, aeration of the gel persuaded by calcification at various temperatures. The form, composition, and textual properties of the final materials can be regulated with the sol-gel process. In previous reports, NP TiO2 was prepared with the pure anatase step in acid pH 3. The minimum particle size demonstrates the maximum reactivity for the photocatalytic nitric oxide reduction [95].

10.4 Functionalization of Nanoparticles for Application in Catalysis 10.4.1 Magnetic Nanoparticles Magnetic NPs, in general, and SPION (superparamagnetic iron oxide NPs), in specific, have to face ample number of challenges in in vivo processes such as lack of biodegradability, poor biocompatibility, and chemical volatility in physical environment. Coating the magnetic NP with some biocompatible material is an essential feature for attaining the efficacious drug nanovehicles in order to achieve colloidal stability of NP and for longer

284  Functionalized Nanomaterials for Catalytic Application blood circulation and to elude the toxicity of cell and to prevent oxidation. A number of methodologies are reported for the modification of magnetic NP surface out of which main methods that can be utilized for surface engineering or modification are ligand exchange and ligand addition [96]. Ligand addition involves the physical adsorption of polymers on the surface of the NPs either by hydrogen bonding or hydrophobic interaction or through electrostatic interactions [97]. In ligand exchange, original surface is reinstating with the carboxylic acids, thiols, diol, and amines functional groups [98]. Some peptides, enzymes, and oligonucleotides also played an important role for the specific cell or tissue attachment and targeting with the addition of the specific functional group on the outside layer [99]. Depending upon the end use of magnetic NP, particular surface functionalization techniques are opted (Figure 10.11) [100, 101]. Polymer-based functionalization of magnetic NPs is also a widely used technique for the surface passivation. Surface modification of SPIONs is classified into two main categories: polymer encapsulation and surface exchange [102]. In surface exchange technique, specific polymer substitutes the original magnetic NP similarly as in the case of ligand surface addition. While in the encapsulation method, original magnetic NP is encapsulated directly through the polymer by the interactions of polymer with magnetic NP. Coating via both the processes is completed by the mean of either natural or synthetic polymers. Broadly considered polymers for the surface coatings of SPION are polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), and poly (lactic-co-glycolic acid) (PLGA), while the natural polymers comprise of gelatine, chitosan, dextran, and starch [103]. Chitosan polymer in specific has put on the attention of researches due to its innate copiousness and the occurrence of

Nanoparticles Functionalization-Surface Coating Polymers

Organic Surfactants

Inorganic Compounds

Bioactive Molecules

Figure 10.11  Functionalization of magnetic nanoparticle by surface coating.

Magnetic and Semiconducting Nanoparticles  285 many functional groups in its backbone structure. Chitosan is a biocompatible, bio-degradable, hydrophilic, natural, and non-hazardous copolymer. Chitosan affixes easily and sturdily to the surface of SPION by simply forming the complexes with these specific groups. The positively charged amine groups intermingle with the negatively charged nucleic acid for gene delivery-based therapeutics using advanced techniques as MRI [104, 105]. Chitosan, however, lacks in solubility which make the polymer difficult choice for some of the biological applications. To alleviate this, glycol chitosan a derivative of chitosan has been used as an alternative. Glycol chitosan is completely soluble and self-assembled in acidic and neutral pH due to the occurrence of glycol detachments which helps in increasing the aqueous solubility and the steric stabilization.

10.4.2 Semiconducting Nanoparticles Different issues are confronted while utilizing SC NPs for catalysis. To upgrade the catalytic redox response of semiconducting NPs different strategies can be used. Decrease in the recombination of photogenerated electrons and gaps can develop the photocatalytic skills. Here, in this part, we will examine the distinctive functionalization ways to deal with improved catalytic exercises. These methodologies will accommodate to build explicit surface territory of NPs by changing the morphology. The raised explicit surface territory can intensify the dynamic site on the NP surface. These eventually increment the contact region among NP and the reactant, consequently, expanding the photocatalytic action. Catalytic action of SC NPs can likewise be improved by alteration of the band structure and hence the electronic properties SC by doping with diverse metals and nonmetallic components. Heterojunction made various SCs or a hybrid structure with worthy metals [106] to further boost the execution of SC NPs through photocatalysis. Moreover, there are other functionalization methods to improve catalytical application of NPs. These include the following.

10.4.2.1 Noble Valuable Metal Deposition The basic theory of deposition of the expensive metals on the SC surface considers that when the SC and the noble metal make contact with each other, in that case, the SC’s work function is lesser than the metal’s work function, such that the electron travels continuously from the SC to the metal till the energy levels are identical in both the metal and the SC. The SC surface and the metal sheet both acquire excessive positive and negative

286  Functionalized Nanomaterials for Catalytic Application charges; it includes the bending of bands at the boundary and hence forms a Schottky barrier. The Schottky barrier can trap photogenerated electrons on the surface of SC and prevent the electron hole pairs’ recombination [107], hence escalating the performance of photocatalyst.

10.4.2.2 Functionalization by Ion Doping: Metal or Non-Metal Performance of the SCs in terms of catalytic redox reaction can be improved by the introduction of impurity by ion doping into the prohibited band. It is among the most popular of all efficient methods to boost the efficiency of the SCs. Borgarello [108] mixed chromium particles (Cr5+) into TiO2 as early as 1982 to note visible light splitting water. At the one side, particle doping will create the grouping of SC bearers, in any case, then again, it is conceivable to generate particle entrap to capture electrons and that the recombination of electron gap sets. Particle doping facilitates the TiO2 photocatalyst to shift red, and the photocatalyst’s photographic reaction may be extended to the visible locale. Particulate doping may be classified as metal particle doping and non-metal particle doping, according to the types of doped particles. Metal Ion Doping: Widely used approach for photocatalyst’s manipulation is metal ion doping. The metal ion doping in the catalyst is accomplished by firing at high temperatures or auxiliary deposition. Metal ion doping influence on the photocatalyst is mainly due to suppression of recombination of photogenerated electrons and holes, and expansion of the photocatalyst’s range of response spectrum. For example, the excitation wavelength range of TiO2 extended to the visible region (about 600 nm away) on chromium and vanadium ions doping in TiO2. However, sometimes, metal ion inclusion turns out to be the recombination centre for electrons and holes which affects the photocatalytic effect. Non Metal Ion Doping: The doping of V3+, V4+, Mn3+, Cr3+, Co3+, Fe3+, 2+ Ni , Ga3+, Zn2+, Zr4+, Nb5+, etc., will extend TiO2’s spectral range en route for the visible region. Moreover, a few TiO2 doped with metal are not stable thermodynamically and create a natural raise in recombination centers, in that way, dropping the efficiency of light absorption for TiO2. Consequently, a number of researchers have preferred the doping of nonmetal atoms which form covalent bonds with titanium atoms in TiO2 system [109]. Kitano et al. [110] recorded the radio frequency (RF) magnetron sputtering (RFMS) deposition method for the preparation of N-TiO2 films, in which a N2/Ar mixture was used. It has been found that the N-TiO2 photocatalyst’s absorption band was shifted to the visible region and greatly improved the N-TiO2 photocatalyst’s catalytic efficiency under visible light.

Magnetic and Semiconducting Nanoparticles  287

e– e–

R O

e–

e–

CB

e–

CB

hν hν VB

VB h+ h+ Semiconductor B

h+ h+ h+ Semiconductor A

R O

Figure 10.12  Semiconductors heterojunction design for photocatalysis (with permission from [113]).

10.4.2.3 Semiconductor Composite or Coupling of Two Semiconductors Catalytic properties of SC nanomaterials are improved by combining two forms of band matched SCs to prepare a composite. The methods involved in recombining SCs contain simple arrangement such as doping, multilayer structure formation, and out of phase permutation [111, 112]. The quintessence of the blending of SCs is to perk up photocatalytic efficiency such that the photogenerated electrons or holes produced by a SC shift to separate photogenerated electrons and holes into the CB or another SC’s VB, effectively suppressing carrier recombination. Figure 10.12 shows SCs hetero-junction design for photocatalysis [113].

10.5 Application-Based Synthesis 10.5.1 Magnetic Nanoparticles 10.5.1.1 Silica-Coated Nanoparticles Magnetic NPs either coated or functionalized with silica have gained much attention during the last decades, especially for their extensive scope of applications in biomedicine field and catalysis. Easy surface functionalization and the virtuous stability of silica in aqueous media make the silica coating very advantageous. From the practical point of view, iron oxide NPs are needed to be covered with a consistent silica shell. This helps in providing the better stability to the NP and also prevents the magnetic NP

288  Functionalized Nanomaterials for Catalytic Application Magnetic Core made of Ni, Co, Mn, Fe their oxides and alloys (MNPs)

Dendrimeric Shell

MNP

2

MNP

NH2 NH

NH

Gold Shell

NH2

NH2

2

MNP

MNP

Polymeric Shell

NH

2

Silane Shell

2

NH2

NH

Figure 10.13  Magnetic nanoparticles with an assortment of envelops.

from getting oxidized. Core-shell nanostructure of magnetic core conjugated with silica shell has great application in magnetic separation. The formation of core-free or we can say bare silica NPs must be avoided, and because they do not contain any magnetism of their own, they cannot be separated by the externally applied magnetic field during the magnetic separation process. Moreover, disparity into the silica shell thickness and core number results in an irregular response to the externally applied magnetic field besides compromising on the competence of separation process. Thus, a magnetic core passivated with silica shell of proper thickness is required for the magnetic separation application. Magnetic NPs with mesoporous silica coating (Fe3O4@mSiO2) got noticed since they possess large surface area and magnetic separation capability in a single entity. The principal tactic engrosses the concurrent sol-gel polymerization of “n-octa decyl trimethoxy silane (C18TMS)” and “tetra ethoxy silane (TEOS)” to eradicate the organic groups of C18TMS and in order to generate the mesoporous silica shell. Figure 10.13 presents the possibility of magnetic NPs with various shells for several applications [114, 115].

10.5.1.2 Carbon-Coated Magnetic Nanoparticles Bare magnetic NP, sometimes, became very useless in many applications because of its low thermal and chemical stability, inertness or because of the segregation of toxic materials during the chemical reaction with the provided environment. Therefore, coating the magnetic NP with some

Magnetic and Semiconducting Nanoparticles  289 polymer, carbon-based material, SC shell, etc., are better alternates according to the need of application. Carbon materials have innovative thermal and chemical stability and are resistant to most chemicals, thus providing complete protection to the magnetic core. Well-built and developed graphitic carbon layers offer a valuable fence to stop oxidation and acid erosion. Myriad of iron-based and carbon-coated binary alloys (FeCo, FeCu, FeNi) were synthesized by controlled pyrolysis, sequential spraying, and chemical precipitation at higher temperature ranges. However, the synthesis of carbon-coated magnetite is further complicated due to the facile reduction of Fe3O4 to FeO and carbide under characteristic reaction settings. A significant quantity of carbon passivated ferromagnetic NPs using a thin graphene layer around 1 nm is possible [11]. Moreover, fundamentally cores of pyrophoric metal demonstrate exceptionally elevated thermal and chemical stability. The carbon layer formed on the magnetic core does not have any disadvantageous consequence on the magnetic properties.

10.5.1.3 Polymer-Coated Magnetic Nanoparticles Polymer-coated magnetic NPs have diverse field of applications. Organic polymer material coatings are being considered very useful for functionalization of magnetic NP’s surface and can be categorized into “synthetic and natural polymers”. Polymers such as poly(vinyl pyrrolidone) (PVP), poly(ethylene-co-vinyl acetate), poly(lactic-co-glycolic acid) (PLGA), poly(vinyl alcohol) (PVA), and poly(ethylene glycol) (PEG) are characteristic examples of synthetic polymer. However, innate polymers comprise dextran, gelatin, pullulan, starch, chitosan, etc. Therefore, in most cases, the coating methods used for magnetic NPs’s surface through surfactants/ polymers are different which are being used for inorganic oxides. The polymers or surfactants attached chemically or adsorbed physically onto the magnetic NP’s surface to construct a single layer or double layer. Therefore, this surface adsorption generates steric repulsive forces to stabilize and protect the magnetic NPs and to eliminate precipitation. Polymers those contain carboxylic acids, phosphates, sulphates, etc., as functional groups are found appropriate to passivate iron oxide magnetic nanomaterials. The polymers used often are poly (alkylcyanoacrylates), poly (pyrrole), poly(aniline), polyesters, poly (methylidene malonate), poly(lactic acid), poly (e-caprolactone), poly(glycolic acid), and their copolymers. The polymer-coated surfaces of the magnetic NPs present superior thermal, chemical stability, and recyclability. For instance, the poly (ionic liquid) coated magnetic NPs create a noble set of heterogeneous catalyst and are predominantly appropriate for organic synthesis in an eco-friendly approach. The catalyst have

290  Functionalized Nanomaterials for Catalytic Application been designed as the shield for aldehydes by converting into 1,1-diacetate in a solvent free ambient condition. Moreover, the catalyst confirms excellent activity for the deprotection reaction of acylals [11].

10.5.1.4 Semiconductor Shell Formation Over the Magnetic Nanoparticle A core-shell nanostructure where the magnetic core is functionalized with SC quantum dot is a much revolutionized field these days. These core-shell nanostructures having magnetic core and luminescent shell have many biological applications. Such as in catalysis, magnetic separation, targeted drug delivery, and in hyperthermia applications were the magnetic core is used as a drug delivery vehicle and the fluorescence of the quantum dot shell is used as the monitoring medium [78].

10.5.2 Semiconducting Nanoparticles The analysis of semiconducting NPs is an area with the most primitive beginning that has accomplished rich achievement. Nanomaterials and nanostructures play the most important sustaining role in catalytic activity applications such as photocatalysis, electro-catalysis, solar cells, fuel cells, and photovoltaic due to their high volume-to-volume ratio and optimized optical and conductivity properties. Various applications of nanomaterials in catalysis are discussed in this section and some are presented in Figure 10.14.

10.5.2.1 Semiconductor Nanomaterials in Solar Cell Sunlight-generated energy is the most perceptible “clean” and inexhaustible elective resource of energy (1,000 Wm−2 is the radiation power on a level surface, under ideal conditions). Photovoltaic cells logically transform photon energy to power by isolating the energy-intensive electron gap sets in photovoltaic materials. The energy-energized electrons and holes are used by photoelectrochemical cells to catalyze redox responses, where water or CO2 can be used to generate electricity. Apparently, photoelectrochemical and photovoltaic cells due to their poor change efficiencies have not rendered a firm contribution to the energy gracefully. Nanostructured photovoltaic materials can appreciably boost the effectiveness of gadgets based on sun energy. A few methodologies, including the installation of quantum dots [116] or quantum wells in nanostructures [117] and the use of multiple exciton generation saw in SC nanomaterials [118], were proposed to effectively

Magnetic and Semiconducting Nanoparticles  291

ENERGY SECTOR ENVIRONMENTAL APPLICATIONS

ELECTRO CATALYSIS

GLOBAL WARMING REDUCTION

APPLICATIONS OF SEMICONDUCTING NANOCATALYST

ENERGY EFFICIENCY

MINIMUM CHEMICAL WASTE

PHOTO CATALYSIS WATER PURIFICATION

Figure 10.14  Applications of semiconducting nanomaterials in catalysis.

maintain daylight across the entire range. When compared with their cell partners relying on silicon sunlight, sun-based cells reliant on QDs are more realistic. Approximately, 9 × 1022 J of power from the sun hits the earth surface every day and only part of that energy is consumed daily by mankind. Sun-oriented cell’s greatest thermodynamic efficacy is 31%. There is therefore a need for a sun-driven cell that shows higher productivity for improvement. Semiconducting quantum dot determines to give a crucial amplification in ability by using different sizes of quantum dots with the larger bandgaps on top of each other. Approaching photons are transmitted until they enter a layer with a littler bandgap than the energy of the photons. With enough layers, each photon will energize a bandgap electron close to its own energy and squander a small amount of energy along these lines.

10.5.2.2 Batteries and Fuel Cells Batteries and fuel cells and, for instance, polymer-electrolyte-membrane, solid-oxide fuel cells, and lithium batteries are electrical-chemical devices

292  Functionalized Nanomaterials for Catalytic Application intended for the transfer of energy from chemical energy and electricity [119]. Both are composed of an anode and a cathode, separated by an electrolyte. The efficiency of fuel cells and the energy storage capacity of batteries can be greatly impacted by nanomaterials applications. One big problem restricting the efficacy of electrochemical devices is electrolyte ion conductivity. The use of NCs or nanostructures can be useful in enhancing ionic transport [120]. On plummet, the thickness of the nanostructured composite in space charging layer increases the intensity of surface charging, and the tightly packed structure allows the nanoscale space charging regions aligned to offer ion transport alleyway. For instance, LiI is infiltrated into mesoporous Al2O3 in composites that display signs of conductivity 100-fold higher in comparison to innate LiI at room temperature [121]. Moreover, this improvement in ion conductivity has also been observed in a non-aqueous liquid electrolyte subsequent to the inclusion of SiO2 NPs. Nanomaterials affixed on electrodes of electrochemical cells augment the barrier load storage capability and reaction rates of redox reactions in fuel cells. Due to the nanoionic effect at interfaces, NCs may possibly have a very high charge storage capacity [122].

10.5.2.3 Semiconducting Nanomaterials for Environment Metal oxides NPs also take part in as a noteworthy candidate as catalyst in various oxidation-reactions. These NPs demonstrate excellent catalytic reactivity toward pollutants and alter certain polluting agents to suitable products for the environment [123]. There are some special properties to these nanomaterials such as nanoscale, high reactivity, and a greater region of the earth. TiO2 photocatalysis particularly plays a crucial role in eliminating a range of impurities from surface water.

10.5.2.4 Challenges for Water Treatment Using Nanomaterials The developing nanomaterials presently face several obstacles in the wastewater field therapy [124]. Nanomaterial offers many treatment options for wastewater and they include various types of materials that are different, depending on the morphology of the NPs. Expansion of industrial nanomaterial applications is too late, and nanomaterial development is growing globally. Nanomaterials are used to purify dirty water, using various methods such as photocatalysis, adsorption, and nanosorbents [125]. To function more efficiently, these approaches need some modifications.

Magnetic and Semiconducting Nanoparticles  293

10.6 Conclusion and Outlook A number of new and interesting progresses in catalytic systems immobilized on magnetic NPs surfaces have evolved. Silica-coating surface of magnetic core is the oldest and the most viable technique to decorate the surface of magnetic core. Sometimes, silica shell formation is followed by functionalization by means of suitable alkoxysilane derivatives. Moroever, some other functional polymeric groups are significantly developed in the previous year’s showing the great applications in number of fields. In addition to the formation of mesoporous silica shells, noteworthy advancement accomplished during the development of innovative mesoporous zeolite-like and metal-organic framework (MOF) shell on magnetic NP’s surface that allowed the amalgamation of the magnetic, porous, and the molecular catalytic arrangement in one single entity. This provides multi­ functional catalysts with high selectivity and reactivity. Thrive in of magnetically immobilized enzymatic catalysts is another leading feature despite having the number of challenges and hitches in the field, these magnetically immobilized enzymatic catalysts have great potential in biomedical field, principally for biopharmaceutical applications. Last of all, we think that hyperthermia proficiencies of magnetic substrates had better be explored in the upcoming time. The field is still not very much explored despite having the great potential prospects to associate catalysis with the selective and restricted heating of reagents on the substrate. The successful dispensation of the imprint should provide significant energy savings and cost, particularly for elevated temperature catalytic reactions. The role of the semiconducting NPs in catalysis has been discussed. Because of their small size and large application in various fields, SC NPs have gained much interest. Tiny size of nanomaterials offers the advantage of having atoms near its interface with more than half of the materials. In photocatalysis, the role of binary and ternary oxide-based, chalcogenide-based, and nitride-based SC NPs has been discussed in detail. In contrast to binary SCs, it was concluded that ternary oxide/chalcogenide SCs were extensively studied in the heterogeneous field of photocatalysis because of the stable nature and ability to support specific chemistries. Along with this various techniques of synthesis of SC, NPs for catalysis have been discussed in this book chapter. It has been found that we can synthesize NPs of the desired form, composition, and size by modifying the synthesis parameters, such as temperature and pH, while various problems are being solved when using SC NPs for catalysis. Most recent applications of these materials in switchable graphene-based bioelectronics interfaces [126], opto-electronic

294  Functionalized Nanomaterials for Catalytic Application applications [127], and functionalized nanomaterials for industrial application [128] are promising and open an avenue for future research and development. This book chapter addresses diverse methods such as noble metal deposition, metal and non-metal ion doping, etc., to facilitate the catalytic redox response of semiconducting NPs. Comprehensive overview of various applications of catalytic activity such as photocatalysis, electro-catalysis, solar panels, fuel cells, and photovoltaic has also been discussed.

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11 Green Pathways for Palladium Nanoparticle Synthesis: Application and Future Perspectives Arnab Ghosh1, Rajeev V. Hegde1, Sandeep Suryabhan Gholap2, Siddappa A. Patil1 and Ramesh B. Dateer1* Center for Nano and Material Sciences, Jain University, Bangalore, India KAUST Catalysis Centre, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia 1

2

Abstract

A simple green chemical method for the one step synthesis of palladium nanoparticles (PdNPs) and its application in organic synthesis has been described. Among the metallic NPs, Pd has a variety of applications in the field of homogeneous and heterogeneous catalysis, and therefore, greener pathways for its synthesis are focused. In particular, described herein is the recent development in synthesis of PdNPs through a greener setup in which phytochemicals present in plant extract will acts as a heterogeneous support and thus the need of external ligand is avoided. The detailed analysis of utilization of various plant extract for PdNPs synthesis and its characterization and application is specifically elaborated. The PdNPs have been synthesized from extracts of various portions of the plants in which presence of terpenes, citric acid, flavonoids, phenols, ascorbic acid, alkaloids, and reductase acts as potential reducing agents and stabilizers in nanoparticles synthesis. More importantly, higher surface area and optimum porosity are important factors in PdNPs to exhibit good catalytic activity. Notably, in a regular approach, reduction of Pd(II) to Pd(0) requires the expensive ligands, while in case of PdNPs’ catalyzed transformation, it is achieved by phytochemicals present in the plant extract. In the later stage of the chapter, utilization of synthesized PdNPs for various organic transformations (e.g., Suzuki, Kumada, Sonogashira Heck, SuzukiMiyaura, Negishi, Stille, or Kumada) is mainly described. Impertinently, the strategy of catalyst recyclability by various techniques is emphasized. *Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (303–328) © 2021 Scrivener Publishing LLC

303

304  Functionalized Nanomaterials for Catalytic Application Keywords:  Biogenic synthesis, palladium nanoparticles, green pathways, plant extract, phytochemicals, coupling reaction, recyclability

11.1 Introduction Green chemistry is the design, development, and implementation of chemical products and processes to reduce the generation of substances which are potentially hazardous to health and the environment [1]. The green chemistry methods have received tremendous interest among scientists, since the important aspect of green synthesis is its ease of preparation, cost effectiveness, potential for large-scale production, and environmentally friendliness. In this line, discovery of nanoparticles (NPs) has been introduced from the field of nanotechnology during the last two decades showcasing novel methods for synthesis of nanocatalysts and its subsequent application in diverse fields [2]. On the other hand, existing methods to synthesize metal NPs using different reducing agents, ligands are hazardous to nature and are not cost effective [3]. To defeat these challenges, synthesis of metal NPs using bio-inspired, eco-friendly, and greener methods is one of the most attractive aspects of current nanoscience and nanotechnology [4–6]. Nevertheless, 21st century has witnessed vast improvement in the biogenic synthesis of nanocatalysts, which has numerous advantages over the other methods (Figure 11.1). In connection with these aspects, this chapter emphasizes the scope of biogenic/greener pathways for the synthesis of transition metal (mainly Pd) NPs and further highlights its application in terms of synthesis perspectives.

Easy available

Clinically adaptable

Ecofriendly High stability

Green IMPORTANCE OF BIOGENIC SYNTHESIS

Biocompatible Rapid synthesis

Figure 11.1  Importance of biogenic synthesis.

One pot Simple Cost effective

Green Pathways for PdNP Synthesis  305

11.1.1 Methods for Metal Nanoparticle Synthesis The conventional methods for synthesizing metal NPs often involve the use of toxic chemicals and expensive technologies that possess increased environmental risks and render its clinical translation despite its fascinating properties [7]. Nevertheless, numerous metallic and bimetallic NPs, such as Au, Ag, Pd, Pt, Cu, Ag/Au, Au/Pd, and Cu/Au, have been successfully synthesized using a large variety of plant materials [8–11]. However, careful literature survey indicates that, among the metallic NPs, palladium NPs (PdNPs) have shown extensive application in heterogeneous and homogeneous catalysis due to their high surface to volume ratio [12–14]. Therefore, there is immense interest in the development of environmentally friendly and sustainable methods for the preparation of PdNPs [15]. It can be synthesized by physical, chemical, and biological approaches based on the availability and feasibility of protocols by adopting different techniques and methods [16] (Figure 11.2). Particularly, chemical reduction technologies have been well researched in the past decade, involving extreme operational conditions such as high temperature and pressure in which numerous toxic chemicals makes the process environmentally destructive. Therefore, for the sustainable development, the synthesis of PdNPs via eco-friendly, plant extract–based bio-reduction approach using non-toxic and biocompatible precursors is highly desired.

Synthesis of Pd Nanoparticles

Chemical

Physical

• Alcohol reduction • Reduction by chemical reductant • Microwave/UV Laser irradiation • Sonochemical methods • Polylol process • Electrochemical deposition

• Vapour Deposition • Sputtering • Laser ablation • Plasma Arc/ Vaporization

Figure 11.2  Various methods for synthesis of PdNPs.

Biological • Plant extract based

bioreduction

• Bacterial Synthesis • Fungal, algae

mediated process

• Biobased Chemicals,

Polymers/products as a reducing agents • Reduction by Marine products

306  Functionalized Nanomaterials for Catalytic Application

11.1.2 Biogenic Synthesis of PdNPs The careful literature survey designates that plant extract–mediated biological process is found to be a simple and versatile method for the synthesis of different types of metal NPs [17]. Extensive research efforts have been made in utilizing various biological systems such as bacteria, fungus, and plant extracts for the synthesis of metal NPs [15]. Among these methods, plant extracts have attracted significant attention due to easy sampling and cost effectiveness which can further facilitate the largescale biosynthesis of NPs [18]. Considering the fact that plant systems are reported as reliable green, eco-friendly approach for metal NPs synthesis, PdNPs has been synthesized from extracts of various portions of the plants such as stem, leaves, flower, seed, fruit, and pectin (Figure 11.3). There are numerous reports available for the biosynthesis of PdNPs that

S. M. Sarkar ChemistrySelect, 2016

Sk. Khadeer Pasha et al. Appl Nanosci 2015

ag. pal B B. Go et al. no Lett, a Int N 2017

H. M. R. t al. ui e Siddiq Trans, n Dalto 3, 9026 ,4 2014

g le pp rA Sta

i

el pe

j S. A. Patil et al. Catal Lett. 2018

ate

k

ra

a

ve

Cin n ca amo mp m ho um ra

po me gra n

e Alo

an be ya So leaf

Cinnamom zeylanicum

Pd(0) Nanoparticle

l Z. Ap Hab Ch pl O ibi e em rg t al . 2 ano . 01 m 8 eta l

Figure 11.3  Various biosynthesis routes of PdNPs.

l. et a 2019 ora a U. B Omeg ACS

Y.-S. Yun, et al. J. of Hazardou s Materials 2009

b

Lis J N han an Jia op e 20 art t al. 10 Re s

ion

On

Black pepper

Terminalia chebula

c N. Satyanarayana, et al. J. of Biomaterials and Nanobiotech, 2012

h

n

d

Pu glu licaria tin osa

lo me ter Wa rind

e

Corn Cob

f B. K Sp . Ma Pa ectro ndal r an t A: M chim et al. d Sp Bio ole ica A c e 20 ctro mole ular cta 13 c s 10 copy ular 21 28 , .

F. Wu et al. Int. J. Electrochem. Sci. 2017

Green Pathways for PdNP Synthesis  307 effectively utilizes Cinnamon zeylanicum [19], Cinnamomum camphora [20], Soybean Leaf [21], Terminalia chebula [22], Pulicaria glutinosa [23], Watermelon [24], Corn cob [25] Star apple [26], Onion peel [27], Black pepper [28], Pomegranate [29], and Aloe vera [30].

11.1.3 Phytochemicals: Constituent of Plant Extract Plants are known to be an important component of different ecosystems [31, 32]. In the plant extract, presence of common phytochemicals such as terpenes, citric acid, flavonoids, phenols, ascorbic acid, alkaloids, and reductase acts as potential reducing agents and stabilizers in NPs synthesis [33] (Figure 11.4). The presence of phytochemicals facilitate the reduction of Pd(II) to  Pd(0) leading to the formation of stabilized PdNPs (Scheme 11.1). These phytochemicals plays dual role acting not only as a reducing agent but also as a stabilizers, and thus, role of external ligand can be avoided [34, 35].

O HO

O HO

OH

OH

O Succinic acid

HO Caffeic acid

HO

p-Coumarnic acid

OH

O OH

OH O Kaempferol

O OH OH

OH O

6-hydroxyhexane-3-1

Syringic acid O

O

OH Gentisic acid

O

O

Salicylic acid

OH

OH OH Gallic acid

HO

OH HO

HO

O

OH

HO

O

OH HO

OH

O

OH

OH

O OH

Ascorbic acid

O

O

OH

O

O

HO

phthalic acid O

O

OH

HO

O Ferulic acid

Daidzein

OH OH

3-Alyl-6-methoxyphenol

OH

O

O O O

tridecan-1-ol

H

diisobutyl phthalate HO

HH beta-Sitosterol

Figure 11.4  Common phytochemicals present in plant extract.

308  Functionalized Nanomaterials for Catalytic Application

O OH HO O

PdII

Pd0 2e– + 2H+

Plant extract with phytochemicals

O O O O

O O

Pd0

O O

stabilized Pd NPs

stable solution of Pd NPs

Scheme 11.1  Plausible reduction mechanism of Pd(II) to Pd(0).

11.1.4 Techniques for Characterization of Metal NPs The characterization of nano catalysts in terms of size, shape, nature, etc., is an important aspect which determines its reactivity and behavior associated with its real-time application. There are various techniques available for characterization of PdNPs and few commonly used key techniques are summarized along with its properties (Table 11.1).

11.2 Biosynthesis of PdNPs and Its Applications The application of heterogeneous catalytic system for the synthesis of medicinally important and biologically active drug molecules is one of the hot topics in recent years [36, 37]. Considering the fact that palladium has a variety of applications in the field of both homogeneous and heterogeneous catalysis, in this context, various catalytic reactions have been explored using PdNPs including hydrogenations, oxidations, ­carbon-carbon coupling and electrochemical reactions [38–40] (Figure 11.5). Importantly, the catalytic activity changed marginally even after successive catalytic cycles without incorporating palladium metal impurity in the pharmaceutically significant organic products. Therefore, among various transition metal NPs, the PdNPs have been widely utilized by the scientific community because of its efficiency as a catalyst for C-C coupling reactions which is one of the most powerful transformations in organic chemistry [41–47]. In this line, few reports highlighting the synthesis of PdNPs and its applications are elaborated below in detail.

11.2.1 Synthesis of PdNPs Using Black Pepper Plant Extract In this work, Patil and co-workers [28] demonstrated the synthesis of PdNPs by using palladium acetate and aq. ethanolic extract of black

Green Pathways for PdNP Synthesis  309 Table 11.1  Common characterization techniques for PdNPs. S.N

Techniques

Information derived

1

ATR-IR (Attenuated Total ReflectanceInfrared Spectroscopy)

It gives the information regarding the surface composition, ligand binding and functional groups present in the material. In case of PdNPs, ATR-IR displays the aromatic and aliphatic stretching vibrations of phytochemicals absorbed on the surface of PdNPs. during the reduction process. Thus, clearly providing the information concerning interactions between phytochemicals and palladium.

2

UV–Vis (Ultra violet-Visible spectroscopy)

The optical properties of NPs are sensitive to size, shape, concentration, agglomeration state and refractive index near the surface. These properties make UV-Vis spectroscopy an important tool to identify, characterize and investigate these materials and evaluate the stability of NPs in colloidal solutions. In case of PdNPs, UV-Vis spectroscopy is used to monitor the reduction of Pd(II) to Pd(0). The characteristic peak observed for Pd(II) gets completely disappeared wherein the conversion of Pd(II) to Pd(0) is indicative of completion of reduction process.

3

FE-SEM (FieldEmission Scanning Electron Microscopy)

It is used to determine the surface morphology, dispersion of NPs in cells and other matrices/supports, precision in lateral dimensions of NPs and examination of elemental composition. It is a commonly used method for the high-resolution imaging of surfaces that can be employed to characterize nano scale materials. In case of PdNPs, FE-SEM is carried out to confirm its surface morphology. (Continued)

310  Functionalized Nanomaterials for Catalytic Application Table 11.1  Common characterization techniques for PdNPs. (Continued) S.N

Techniques

Information derived

4

TEM (Transmission Electron Microscopy)

TEM analysis shows the size and morphology of material and generally, gives clearer image than FE-SEM.

5

XRD (X-ray Diffraction)

XRD furnishes information related to the crystalline structure, nature of the phase, lattice parameters etc. The average particle size can be calculated using Debye Scherrer equation, which is further compared with TEM results.

6

XPS (X-ray photoelectron spectroscopy)

This technique works based on the principle of photoelectric effect and useful to elucidate electronic structure, elemental composition and oxidation states of elements in a material. It can also identify the bonding mode of ligand. In case of PdNPs, XPS technique determines the oxidation state of palladium present in the catalyst.

7

BET (Brunauer– Emmett–Teller surface area analysis)

BET analysis is carried out for the surface area analysis of synthesized material as well as to understand its porosity.

8

TGA (Thermogravimetric analysis)

Thermogravimetric TGA analysis is carried out from low temperature to very high temperature and determines the stability of the synthesized material. When PdNPs were subjected to TGA analysis, one can observe the weight loss at different temperature which can be attributed to removal of moisture and volatile organic moiety, thus confirms the presence of any phytochemicals absorbed on the surface of PdNPs. (Continued)

Green Pathways for PdNP Synthesis  311 Table 11.1  Common characterization techniques for PdNPs. (Continued) S.N

Techniques

Information derived

9

ICP-OES (Inductively Coupled PlasmaOptical Emission Spectroscopy)

This technique is used to determine the percentage content of a specific element in material. In case of PdNPs the palladium content is usually quantified by ICP-OES analysis.

10

EDS (Energy-Dispersive X-ray Spectroscopy)

It is used to identify the elemental composition of any material. It is a technique to analyze near surface element and estimate their proportion at different position and thus, giving the mapping of whole sample. In case of PdNPs it is used to determine the elements (e.g. O, N and C) present along with palladium.

11

AFM (Atomic force microscopy)

It is a powerful technique facilitates the imaging of diverse types of surface, including polymers, ceramics, composites, glass, biological samples and NPs.

C-C Coupling

C-Hete roato couplin m g Electrochem ical reaction Oth

n

icat

n

ion

io

nt

e og

ppl

er A

atio

id Ox

PdNPs

s

dr

Hy

Starting material

Figure 11.5  Schematic representation of PdNP applications.

312  Functionalized Nanomaterials for Catalytic Application pepper (Piper nigrum, Figure 11.6). The phytochemicals such as phenols, acids, pellitorine, ethyl piperonyl cyanoacetate, piperine, and N-isobutyltetradeca-2,4-dienamide present in the black pepper extract facilitate the reduction of Pd(II) to Pd(0), and it is confirmed by various characterization techniques. In particular, formation of agglomerated spherical morphology was confirmed using FE-SEM and TEM analysis. The synthesized catalyst showed excellent reactivity for cyanation reactions (Scheme 11.2, left) and Hiyama cross-coupling reaction (Scheme 11.2, right) with aryl halide and catalyst recyclability was studied. The various polyfunctional biaryls were synthesized in good to excellent

Black pepper plant

Black pepper seeds

Black pepper powder aq. EtOH

20 nm

75–80°C 1h

Pd(OAc)2

TEM image

85°C, 2 h palladium nanoparticles [Pd NPs]

Pepper extract

SEM image

Figure 11.6  Synthesis of PdNPs using black pepper plant extract and its characterization (Source: Reprinted with permission from Kandathil, V., Dateer, R.B., Sasidhar, B.S., Patil, S.A., Patil, S.A., Green synthesis of palladium nanoparticles: applications in aryl halide cyanation and hiyama crosscoupling reaction under ligand free conditions. Catal. Letters, 148, 1562, 2018).

Cyanation reaction

R

CN

R = H, 4-NO2, 4-CHO, 4-OCH3, 4-COCH3

O Si O O

K4FeCN6.3H2O Pd NPs (0.5 mol%) Na2CO3, DMF 120°C

X R [X = I, Br, Cl]

Hiyama cross-coupling

Pd NPs (0.2 mol%) NaOH, Ethylene glycol 100°C

R R = H, 4-CH3. 4-NO2, 4-CN, 4-COOH, 4-CHO, 2-COOH

Scheme 11.2  PdNPs’ catalyzed cyanation and Hiyama cross-coupling reaction.

Green Pathways for PdNP Synthesis  313 yields Interestingly, in a cyanation reaction, K4Fe(CN)6·3H2O was used as a cyanide source for performing cyanation of aryl halides, while Hiyama cross-coupling reaction was achieved without fluoride source, highlighting the importance of methodology. In continuation, Dateer et al. [48] recently reproduced the synthesis of PdNPs and developed denitrogenative coupling reaction by subjecting it in reaction with aryl halides and arylhydrazines (Scheme 11.3). The commercially available substrates were subjected for the synthesis of symmetrical and unsymmetrical biaryls in good to excellent yield under external ligandfree conditions. The plausible reaction mechanism was proposed based on series of control experiments and literature precedents [48]. Importantly, catalyst recyclability (>5 cycles), turnover number, and turnover frequency studies were successfully performed.

11.2.2 Synthesis of PdNPs Using Papaya Peel The worldwide consumption of Papaya (Carica papaya L, family: Caricaceae) generates almost 20%–25% of natural waste after processing of papaya peels and seeds. General reaction: R1

R2 Pd NPs (2 mol%) K2CO3, CH3CN, 50°C

NHNH2.HCl + X

(X = I, Br) R1 = R2 = H, 16 h, 82% 1 2 R = H, R = 4-OMe, 16 h, 65%

R2

R1

R1 = 4-F, R2 = 4-CN, 8 h, 71% R1 = 2,4-dimethyl, R2 = 4-OMe, 24 h, 56%

Reaction mechanism: na

Ar

no

reduction

Pd(0)

particles

I na

no

X

grinding C

oxidative addition

B

cultivation

X= Br, I

Pd(II)

Ar

extraction

D Pd(OAc)2

particles

na

no

Pd(II)

V PdNPs form Bio-reducing agent

N2

na

Ar

N

no

particles

transmetallation

Pd(II)

N

particles

IV

black pepper tree A Ar NHNH2

II

base.HX

Ar N NH + base III

atm O2 base

Scheme 11.3  PdNPs’ catalyzed denitrogenative coupling reaction.

314  Functionalized Nanomaterials for Catalytic Application Nevertheless, Dewan and co-workers [49] synthesized PdNPs by utilizing water extract of fresh papaya peel in absence of reducing agent at room temperature. The newly synthesized PdNPs were thoroughly characterized using various microscopic and spectroscopic techniques (Figure 11.7). The reduction of Pd(II) to Pd(0) was confirmed by UV-Vis analysis as change in color of solution from pale yellow to black was observed. Moreover, the absorption bands were observed in UV-vis graph with an overlapping of the peaks in the range at ~269 nm. Further, FE-SEM and TEM analysis showed the presence of highly crystalline spherically shaped PdNPs of size 1–5 nm, while BET analysis showed a specific surface area 18.2 m2/g with a pore diameter of 3.7 nm. The reactivity of PdNPs was tested by employing it for Suzuki-Miyaura reaction, wherein the reaction of aryl bromides and arylboronic acid led to the formation of new C-C coupling products under aqueous conditions at room temperature (Scheme 11.4). Nevertheless, Sonogashira cross-­ coupling reactions were developed with aryl halides and terminal alkynes

Pd(OAc)2 Papaya peel

Filtered Peel extract

Grinded peel

2 mL N2

Stirred 2 days

Centrifuged (d)

Pd Nano

Characterized by UV, SEM EDX, TEM, PXRD, BET

b

c

c 1

b

269

388

a 0 200

Pd NPs

SEM image of Pd NPs

a 2

Pa Ext. Pa ext. + Pd(OAC)2

Pd NPs

Absorbance (a.u)

Dried in vacuum

300

400

500

Wavelength(nm)

600

UV-vis absorption spectra

Figure 11.7  Synthesis of PdNPs using papaya peel and its characterization (Source: Reprinted with permission from Dewan, A., Sarmah, M., Thakur, A.J., Bharali, P., Bora, U., Greener biogenic approach for the synthesis of palladium nanoparticles using papaya peel: An eco-friendly catalyst for C-C coupling reaction. ACS Omega., 3, 5327, 2018).

Green Pathways for PdNP Synthesis  315 Pd NPs, K2CO3 H2O, RT Method A X + (HO)2B

R1

R2

Method B

R2

R1

Pd NPs, K2CO3 EtOH:H2O (1:1), RT X

R1

R2

Br

H

Br

Method A

Method B

time (h)

yield (%)

time (min) yield (%)

H

1

98

15

98

4-OCH3

4-Cl

1

90

30

95

Br

4-CH3

4-OCH3

2

95

20

98

Cl

4-CH3

H

12

40

12

50

Scheme 11.4  PdNPs’ catalyzed Suzuki-Miyaura coupling reaction.

X R1

+

R2

PdNPs (0.01 mmol) EtOH, 60°C

R1

R2

R1 = H, R2 = Ph, Yield = 98% (X = I, 5h) and 90% (X = Br, 8h) R1 = 4-NO2, R2 = Ph, Yield = 98% (X = I, 4h) R1 = 4-CH3, R2 = cyclohexyl, Yield = 85% (X= I, 8h)

Scheme 11.5  PdNPs’ catalyzed Sonogashira cross-coupling reaction.

in presence of ligand, copper and amine-free system (Scheme 11.5). The efficient synthesis of biaryl and acetylenyl derivatives under ligand-free conditions and aerobic atmosphere makes this process more viable and green. Overall, natural wastes such as papaya peel were successfully implemented for the synthesis of PdNPs following a greener protocol without adjusting pH and temperature. This approach opens a door as a potential step-up approach for industrial applications.

11.2.3 Synthesis of PdNPs Using Watermelon Rind In continuation of similar approach for utilizing natural and agricultural waste for developing nanocatalysts for Suzuki coupling reaction, Pasha and co-workers [24] synthesized PdNPs by using aqueous extract of watermelon rind in absence of external reducing agent at room temperature (Figure 11.8). The watermelon rind played a dual role acting not only as a capping agent but also as a reducing agent and biogenic synthesis of PdNPs

316  Functionalized Nanomaterials for Catalytic Application

a) Washed with water b) Cut into small pieces c) Dried at 85 °C for 48 h & grind it

Watermelon rinds (WR)

a) Stirred for 24 h at 30 °C

PdCl2 + WR extract

a) Added 50 mL distilled water in 1 g of sieve WR b) Boiled in water bath for 30 mints

PdCl2 (20 mL 1mM) 10 mL WR aq. extract

WR aq. extract

556.65

1631.88 1396.33

b) Centrifused & Dried at 85 °C

1049.42

3446.23

2924.76 2487.67

%Transmittance

Pd NPs

Oven dried WR powder

Wavenumbers (cm–1)

FTIR spectra

AFM images

TEM images

Figure 11.8  Synthesis of PdNPs using watermelon rind and its characterization (Source: Reprinted with permission from Lakshmipathy, R., Reddy, B.P., Sarada, N.C., Chidambaram, K., Pasha, S.K., Watermelon rind-mediated green synthesis of noble palladium nanoparticles: catalytic application. Appl. Nanosci., 5, 223, 2015).

was achieved. The visual observation of change in color from pale yellow to dark brown indicates the formation of PdNPs which were further confirmed by UV-Vis spectroscopy. Further, the synthesized PdNPs were characterized by XRD, FT-IR, AFM, and TEM techniques. The FT-IR peaks of PdNPs revealed that the presence of polyhydroxyl groups in aqueous solution of watermelon rind helped in successful fabrication. The formation of shape-controlled spherical NPs with an average particle size of 96 nm was confirmed using TEM analysis. Furthermore, AFM studies confirmed that PdNPs formed strong aggregates in water. After the successful synthesis and characterization of watermelon rind derived PdNPs, it was further tested for Suzuki coupling reaction using aryl halides and boronic acids at room temperature under aqueous condition (Scheme 11.6). The new C-C bond formation occurred and variety of polyfunctional biaryls was synthesized in good to excellent yields. This study revealed that polyhydroxyl groups in watermelon rind were capable of fabricating PdNPs for potential catalytic applications in organic transformations.

11.2.4 Synthesis of Cellulose-Supported PdNs@PA In another report, Sarkar and co-workers [25] synthesized waste corn cob cellulose supported poly(amidoxime) palladium nanoparticles (PdNs@PA)

Green Pathways for PdNP Synthesis  317 2 mol% PdNPs

X + (HO)2B

R1

K2CO3, RT, H2O

R2

R2

R1

Selected examples

H 3C O2N

H3COC

2 h, 98% [X= I] 3 h, 96% [X= Br]

2 h, 95% [X= Br]

3 h, 83% [X= Br]

Scheme 11.6  PdNPs’ catalyzed Suzuki coupling reaction.

by the surface modification of waste corn cob cellulose. The two step synthesis of PdNs@PA was achieved through graft co-polymerization followed by subsequent amidoximation (Figure 11.9). Initially, the waste corn cob cellulose supported poly(amidoxime) chelating ligand was prepared and subsequently mixed with aq. (NH4)2PdCl4 solution at room temperature to give light brown colored poly(amidoxime) Pd-complex. ICP-AES analysis confirmed anchoring of 0.45 mmol/g of Pd onto the poly(amidoxime) ligand. Lastly, the cellulose supported palladium complex was treated with hydrazine hydrate giving rise to a dark black colored PdNs@PA catalyst.

(a)

NH2

poly(amidoxime) ligand

HO O

OH O OH

Cellulose

waste corn-cob

H2C

O n

H C

C NOH CH 2 CHCHCH 2

n C NOH C NOH

n

NH2

NH2

H C CH2

OH

HO O

C NOH

O

O OH

NH2

n

(NH4)2PdCl4

(c)

PdNs@PA

hydrazine

HO O

OH O OH

(b)

H2C

O n

CH

n N

H 2N O O Pd NH 2 N C CH2 n H

HO O

OH O OH

O n

TEM image

Figure 11.9  Synthesis of PdNs@PA using waste corn cob and its characterization (Source: Reprinted with permission from Sultana, T., Mandal, B.H., Rahman, M.L., Sarkar, S.M., Bio-Waste Corn–cob cellulose supported poly (amidoxime) palladium nanoparticles for Suzuki-Miyaura cross-coupling reactions. Chemistry Select, 1, 4108, 2016).

318  Functionalized Nanomaterials for Catalytic Application

R1

X + (HO)2B

PdNs@PA (300 mol ppm) R2

K2CO3 (2 mol equiv) EtOH:H2O (1:1) 80°C

R2

R1

Selected examples Me

O2N 2.5 h, 92% [X= Br] 5 h, 90% [X= Cl]

MeO 2.5 h, 92% [X= Br] 6 h, 96% [X= Cl] MeOC

Me

OMe 2.5 h, 97% [X= Br] 6 h, 90% [X= Cl]

3 h, 90% [X= Br] NC

OMe 3 h, 87% [X= Br]

2 h, 97% [X= Br] 7 h, 87% [X= Cl]

2 h, 97% [X= Br] 7 h, 80% [X= Cl]

Scheme 11.7  Suzuki coupling reaction catalyzed by cellulose-supported PdNs@PA.

TEM analysis displayed the successful dispersion of the PdNPs on the surface of cellulose matrix with an average size of 2–3 nm. After confirming the formation of PdNs@PA, it was subjected for the Suzuki-Miyaura cross-coupling of aryl bromides/chlorides with organoboronic acids (Scheme 11.7). The reactions were performed in aqueous ethanol and the corresponding biaryl products were obtained in good to excellent yield with high turnover number (TON) of 19,777 and turnover frequency (TOF) of 4,944 h−1. Interestingly, the PdNs@PA catalyst could be easily recovered from the reaction mixture and showed recyclability for six recycles with a marginal loss in catalytic performance.

11.2.5 PdNPs Synthesis by Pulicaria glutinosa Extract Herein, Siddiqui and co-workers [23] reported an eco-friendly synthesis method for the synthesis of PdNPs, utilizing an aqueous solution of abundantly found Saudi Arabian plant named as Pulicaria glutinosa, which acted as an efficient bioreductant (Figure 11.10). The color change from pale yellow to dark brown followed by subsequent UV-vis analysis confirmed the reduction of palladium Pd(II) to Pd(0). Further, FT-IR spectroscopy confirmed the phytochemicals present in the aqueous solution of Pulicaria glutinosa plant acted both as a natural reducing agent as well as a capping agent on the surface of PdNPs. XRD analysis revels the presence of highly crystalline face centered cubic (fcc) structured lattice with distinctive peaks at 40.02° (111), 46.49° (200), 68.05° (220), 81.74° (311), and 86.24° (222). The morphological characteristics of the synthesized PdNPs were studied using HR-TEM analysis which displayed the spherically shaped PdNPs with an average diameter of ~20–25 nm.

Green Pathways for PdNP Synthesis  319

Diluted Soln of PE

Plant extract

Pulicaria glutinosa Plant

Bioreductant & Capping agent

8000

Aqueous Soln of PdCl2

90 °C, 2 h

Plant Extract Residue

(111) 7000

b Intensity

6000 (200)

5000

(220)

(311)

4000 (222)

3000

10 nm

2000

HRTEM image

30

40

50

60

2θ[°]

70

80

90

100

XRD graph

Figure 11.10  Synthesis of PdNPs using Pulicaria glutinosa extract and its characterization (Source: Reprinted with permission from Khan, M., Khan, M., Kuniyil, M., Adil, S.F., Al-Warthan, A., Alkhathlan, H.Z., Tremel, W., Tahir, M.N., Siddiqui, M.R.H., Biogenic synthesis of palladium nanoparticles using Pulicaria glutinosa extract and their catalytic activity toward the Suzuki coupling reaction. Dalton Trans., 43, 9026, 2014).

Br

+

OH

Pd Nps (5 mol%)

OH

H2O, SDC, K3PO4 100°C, 4 min

B

100%

Scheme 11.8  PdNPs’ catalyzed Suzuki coupling reaction.

Further, PdNPs were tested as potential nanocatalysts for Suzuki coupling reaction using bromobenzene and phenylboronic acid in presence of water (Scheme 11.8). Gas chromatography analysis was performed to monitor the reaction, wherein it was found that 100% conversion was obtained in a short reaction time (5 cycles) were studied by simple filtration and recycled without any significant loss of efficiency.

11.2.8 PdNPs Synthesis Using Gum Olibanum Extract Herein, Aruna and co-workers [51] developed a synthesis of PdNPs by employing renewable and non-toxic glucuronoarabinogalactan polymer; gum olibanum (Boswellia serrata) (Figure 11.13). The polymer played dual role of acting not only as a reducing as well as a stabilizing agent. The synthesized PdNPs were further characterized, in which the TEM analysis displayed spherical and polydispersed morphology with average particle

Green Pathways for PdNP Synthesis  323 Powdered in a Prestige Deluxe–Vs high speed mechanical blender

Stirring overnight at r.t. in ultrapure water Centrifuged (5500 × g, 10 min)

(a)

(b)

Gum olibanum tears

(c)

38 µm sized gum powder

0.5% (w/v) gum solution Autoclaving at 121 °C & 103 kPa for 30 min.

PdCl2

Characterization

Pd Nps

20 nm TEM image

5 1/nm

SAED pattern

Figure 11.13  Synthesis of PdNPs using gum olibanum and its characterization (Source: Reprinted with permission from Kora, A.J., Rastogi, L., Catalytic degradation of anthropogenic dye pollutants using palladium nanoparticles synthesized by gum olibanum, a glucuronoarabinogalactan biopolymer. Ind. Crop Prod., 81, 1, 2016).

sizes in the range of 6.6 ± 1.5 nm (Figure 11.13). Further, XRD analysis confirmed the presence of face centred cubic structure. The presence of functional groups such as hydroxyl, carboxylate, proteins present in the polymer gum facilitated the reduction of Pd(II) to Pd(0) and confirmed using IR analysis. The synthesized catalyst was tested for potential antioxidant activity as well as degradation of anthropogenic dye pollutants. The catalyst was nontoxic and shows excellent efficiency toward both Gram-positive and Gramnegative bacteria at higher loadings. Further, dye degradation studies were performed to investigate the catalytic activity of PdNPs. Synthetic dyes such as coomassie brilliant blue G-250 (CBB), rhodamine B (RB), methylene blue (MB), and 4-nitrophenol (4-NP) with sodium borohydride were reduced in presence of PdNPs. Finally, it can be concluded that the dye degradation capability of PdNPs can be applied for various applications for environmental sustainability and wastewater management by removal of harmful and toxic components.

11.3 Conclusion and Future Perspectives In conclusion, the greener protocols for the synthesis of PdNPs are described in which environmentally friendly and non-toxic materials were used. A great number of bio-reduction processes have been explored for the

324  Functionalized Nanomaterials for Catalytic Application synthesis of metal NPs including palladium as the active metal participant in most of the cases. Among various processes, the plant-mediated bioreduction process has become more popular due to its operational simplicity, abundance of plant resources, and possibility of making different types of NPs. In greener pathways, plant extract contains a phytochemicals which acts as reducing gent as well as stabilizing agents and thereby reduces an overall cost of catalyst synthesis. Therefore, the biogenic synthesis is devoted as gateway and offers powerful alternative tool for NPs synthesis in large scale. Nevertheless, the catalyst recyclability is possible up to several cycles and large-scale production of the catalyst can be achieved. More importantly, due to high surface area and porosity, NPs exhibit exciting activity toward various organic applications like Suzuki-Miyaura, Hiyama cross-coupling, hydro-dechlorenation, denitrogenative coupling, and dye degradation. In future, morphologically controlled PdNPs synthesis using the phytochemicals still remains a big challenge. Therefore, despite of recent reports [52–59], more studies are required to demonstrate the exact mechanistic aspects for bio-reduction and stabilization of the NPs by phytochemicals. Nevertheless, mechanistic investigation pertaining to the involvement of PdNPs in a synthetically important organic transformation such as C-H activation, cyclization reactions, and cascade reaction under mild reaction conditions to construct the new heterocyclic molecules is still demanding. In fact, considering an availability of computational techniques, area of green synthesis could be used in the field of medicine such as targeted drug delivery, cancer treatment, gene therapy, DNA analysis, antibacterial agents, separation science, and magnetic resonance imaging (MRI). Further, NPs can also be used in the field of energy and environmental concerns such as bio-hydrogen production, carbon monoxide bioconversion, pre-water treatments to detect the toxic ions found in water. Moreover, due to the exclusive chemical and physical properties NPs, they play diverse roles in different sensing systems for constructing electrochemical sensors and biosensors [60, 61].

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12 Metal-Based Nanomaterials: A New Arena for Catalysis Monika Vats1*, Gaurav Sharma1, Varun Sharma2, Varun Rawat1, Kamalakanta Behera1 and Arvind Chhabra3† 1

Department of Chemistry, Biochemistry & forensic Sciences, Amity School of Applied Sciences, Amity University Haryana, Gurugram, India 2 Vorick Analytical Research Pvt. Ltd., Delhi 3 Stem Cell Institute, Amity University Haryana, Gurugram, India

Abstract

Nanomaterials have large surface area to volume ratio as compared to their macromolecular counterparts, which imparts unique properties to them. The size, structure, selectivity, activity, productivity, and electronic changes of metal-based nanomaterials distinguish them from the bulk materials and make them useful for various applications. Recently, nanocatalysis has emerged as dynamic and valuable field for metal-based nanomaterials. This chapter focuses on fundamental properties, methods for fabrication of metal-based nanocatalysis, types of nanocatalysis, role and types of metal-based nanocatalyst, catalysis-structure correlation, and future aspects. Keywords:  Catalysis, nanomaterials, nanocatalyst, metal-based nanocatalysts

12.1 Introduction Catalytic reactions are vital components of our daily life. The area extends from enzymatic reactions occurring in a living system to the catalytic transformation of a research laboratory or chemical industry [1]. In 1998, Anastaset et al. suggested a set of 12 basic principles to lessen or eradicate *Corresponding author: [email protected] † Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (329–354) © 2021 Scrivener Publishing LLC

329

330  Functionalized Nanomaterials for Catalytic Application the use of chemicals and chemical processes that may lead to adverse environmental impacts [2]. Among them, catalytic reagents were preferred for use over others. The catalyst has advantages such as small amounts of catalyst can carry out a single reaction many times, reduce the temperature of a transformation, reagent-based waste, reaction activation energy, undesirable side reactions, and enhance the selectivity of the reaction. The fabrication of several useful products like automobile fuel, lubricants, refrigerants, polymers, drugs, which are needed for ease living would not have been possible without the catalysts [3]. Catalysts also been used in environmental remediation, and air and water pollution reduction [4]. The catalysis also provides development of inexpensive, less toxic, and sustainable commercial products [5]. Among three categories catalysis, namely, homogeneous catalysis, heterogeneous catalysis, and enzymatic catalysis, the natural enzymatic catalysis is considered as efficient and green [2]. Nature clearly provided hints to us to carry out environmentally benign reactions by using micro-organisms and/or enzymes [6]. However, there always been a continuous need to develop new catalysts as both homogeneous and heterogeneous catalytic system have merits and demerits [7]. In this direction new types of catalysts have emerged, known as nanocatalysts which have high-surface-area and very small size. They are typically composed of metal nanoparticles having dimensions in nanometers and exhibit high activity, selectivity, and stability [8]. Size reduction leads to increase in active catalytic sites, increment in surface to volume ratio which results in imparting unique properties to the nanocatalysts such as improved selectivity, activity, productivity, and electronic changes in comparison to the bulk and make their use suitable for catalytic applications [9]. The literature suggests that nanocatalysis can potentially provide efficient, selective, stable, cost-effective, less toxic, and environmentally sustainable solutions for agricultural and industrial applications. There are various benefits of nanocatalyst such as they are less toxic, selective, eco-friendly, energy efficient, and economical and they help in reducing global warming and wastes. Some of the benefits of nanocatalysts are shown in Figure 12.1. The nanocatalysis is also known as semi-heterogeneous catalysis as they have advantages of homogeneous as well as of heterogeneous catalytic systems. The nanocatalytic system when coupled with catalyst separation and recovery processes, delivers excellent yield in faster and selective mode [10]. There are many types of nanocatalysts known that can be divided into three categories as shown in Figure 12.2. The different catalytic materials have their own merits and demerits which are further responsible for their application in the field of catalysis. The enzyme-mimicking artificial single chain catalysts [11], conducting

Metal-Based Nanomaterials As Catalysts  331 N A N O C A T A L Y S I S

Energy Efficiency Reduced Global Warming Optimum Feedstock Utilization Low Toxicity Reduced Wastes Selective Economic

Figure 12.1  Benefits of nanocatalysts.

TYPES OF NANOCATALYST

Carbon based (e.g. Carbon nanotubes) Metal based (e.g. Nano-Ni and Nano-ZnO) Others (e.g. Nano-clay)

Figure 12.2  The three categories of nanocatalyst.

polymers [12], polymer/inorganic hybrid nanomaterials [13], graphenebased bioelectronics materials [14], and one dimensional nano-materials [15] have unique properties which can be utilized in the field of catalysis. The comparison between the polymeric, carbon-based, and metal-based nanomaterials has been discussed in Table 12.1. Nanocatalysts resembles in behavior to homogeneous catalyst as they have high surface area which increases their number of active sites as well as contact between reactants and the catalyst. The nanocatalysts can be easily separated and recovered from the catalytic reaction as they are in different phase and insoluble in the reaction media, this phenomenon resembles the behavior of a heterogeneous catalytic reaction [10, 16]. The nanomaterials exhibit different

332  Functionalized Nanomaterials for Catalytic Application Table 12.1  The comparison between the different types of nanomaterials used in catalysis. Metal nanomaterial

Polymer nanomaterial

Carbon based nanomaterial

High temperature strength

Moderate

Very Poor

Good

Ease of fabrication

Good

Very good

Difficult

Conduction

Good

Poor

Good

Resistance to chemical attack

Poor

Inert

Inert

Dimensional stability

High

Poor

High

Density

Very high

Very Low

Moderate-high

Lustre

Excellent

Poor

Poor

Elastic modules

High

Low

High

Melting point

Moderate

Low

High

Heat capacity

High

Low

Moderate-high

Hardness

Moderate

Low

`High

Toughness

High

Low

-

Coefficient of thermal expansion

Moderate

High

Positive

Compressive strength

Moderate

Low

Moderate

Tensile strength

High

Low

High

Dielectric constant

Infinity

Very low

-

Magnetism

High

Very low

Very low

Band gap

Very low

High

Low

Wear rate

Moderate

High

-

Coefficient of friction

High

Very low

Low

Properties

Metal-Based Nanomaterials As Catalysts  333 size, structure, and remarkable properties. The unique characteristics of nanomaterials lead to wide range of applications together with catalysis [16, 17]. Recently, nanocatalysis has emerged as dynamic and valuable field for metal-based nanomaterials. The metal-based nanoparticles offer several advantages such as facile synthesis, selective reaction, porous structures, physical stability, and ease of separation [13]. Extensive research has been done on metal-based nanocatalysts. The particle shapes, size, structures composition, and other physio-chemical properties of metal-based nanoparticles can be controlled through designing and optimizing synthesis conditions and experiment [9, 18–20]. The metal-based nanoparticles as supported or non-supported metal catalyst, metal oxide, etc. display wide range of catalytic applications [20, 21]. The ease of fabrication is also one of the advantages of metal-based nanomaterials for widespread use in catalysis. There are various methods to synthesize the metal based catalytic materials like co-precipitation, chemical vapor deposition, green synthesis, laser ablation, etc. Among the different synthetic methods, green synthesis is environmentally friendly approach which utilizes various plants, animals, microorganisms and other biomolecules for the synthesis of less metal nanoparticles [22]. The importance and wide range of applications of nanoparticles in the field of catalysis made it indispensable for us to compile and discuss about these remarkable materials in detail. This chapter focuses on fundamental properties and methods for fabrication of nanocatalysis, types of metal-based nanocatalysis, role of metals-based nanocatalyst, catalysisstructure correlation, and future prospects.

12.2 Fabrication Methods of Nanocatalysts There are various fabrication methods of nanocatalysts described in literature. The most popular are ligand displacement, chemical reduction, electro-chemical reduction, condensation-vapor, and nanofabrication. Designing and developing ideal catalysts is one of the very important aspects which involves two major approaches (Figure 12.3) [23, 24]: (i) Top-down technologies (ii) Bottom-up technologies On one hand, we have the top-down methods, where nanomaterials are obtained from a bulk by removing material, until the desired nanomaterial

334  Functionalized Nanomaterials for Catalytic Application Synthetic Approaches

Top-down Mechanical Grinding Thermal degradation Sputtering

Bottom-up Chemical Vapor deposition Sol-Gel Coprecipitation

Figure 12.3  Two major approaches used for fabrication of nanoparticles.

is obtained. On contrary, bottom-up methods are just the reverse; the nanomaterial is derived by gradually assembling atomic or molecular level, until the desired material is obtained [25]. The main application of top-down methods is in semiconductor industry and for development of computer chips. These methods, called lithography, removes layers of material, from a precursor material, selectively using a light or electron beam, and thanks to the advances of the lithography fabrication methods, has been possible to reduce the size of electronic devices. Bottom-up methods has been divided into two groups; first group consists of gas-phase methods while the second group consists of liquid phase methods. In bottom-up approach a controlled fabrication route is used for synthesis of nanomaterial from single atom or molecule. Additionally, sintering and surface-­stabilizing are also been used in synthesis process [26]. The size, separation, and solubility of nanoparticles sometimes become a problem in their application as catalyst. Another problem is aggregation of nanoparticles and thus it becomes difficult to recover them from reaction mixture. To solve these problems, measure like anchoring of nanoparticles to a solid inert or active support surface like silica, zeolite, alumina, metal oxides, activated charcoals, magnetic materials, etc., can be taken [24]. Electrostatic and steric methods can also be used for stabilization via support-free nanocatalysts [27]. It has been reported that stability and dispersibility of metal nanocatalysts depends on the chosen method and chemicals and on oxidation-reduction and calcination treatments. There are three fundamental phases in fabrication of supported catalyst: (a) primary solid preparation: precipitation, co-precipitation, deposition, gel formation, and selective removal can be used along with the control on reaction parameters like support, carrier, concentrations, temperature, and stirring time to prepare the intermediate solid; (b) intermediate solid treatment: the processes like air drying, spray drying, calcination, sintering, thermal decomposition, etc., are useful in the thin phase along with exposure time and quantity of gas/liquid in the reactor. Slow process rate is generally preferred for better results and catalyst consistency; and

Metal-Based Nanomaterials As Catalysts  335 (c)  precursor activation to flourish active catalyst: hydrodesulfurization, deammoniation, acidic zeolites, and hydrogenation are used for activation of precursor to obtain active catalyst [28–30]. There are many methods available in literature to prepare supported metal nanoparticles; (i) old plasma method: it is a fast, very simple, and environmentally friendly fabrication method for heterogeneous nanocatalysts. The supported metal nanocatalysts are synthesized by controlling condition like temperature (low gas and high electron temperature) and combining with methods like deposition and co-precipitation impregnation method. The metal precursors are subjected to calcination and reduction process [31]. (ii) Laser electrodispersion method: it is a favorable fabrication for preparing stable supported metal nanocatalysts. The laser torch is used for irradiation of metal a then the metal that results in droplets splashing out from the metal surface. The splashed metal droplets are then uniformly deposited on the external surface of a support with the help of cascade fission [32]. (iii) Immobilization: the methods used for fabrication of ultrafine metal nanoparticles with high-surface-area [33]. Different other nanofabrication techniques have also been reported for preparation of stable metal nanoparticles such as (a) using reducing agent, (b) creating nanogaps or (c) thermal dewetting, (d) hydrothermal method, (e) formation of nanocomposite, (f) encapsulation (g), and multi-metallic approach. The instrumentation techniques like ultra-violet and visible spectroscopy, infra-red spectroscopy, nuclear magnetic resonance spectroscopy, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, extended X-ray adsorption fine structure spectroscopy photoluminescence spectroscopy, atomic force microscopy, etc., have been used for characterization of the nanoparticles.

12.3 Application of Metal-Based Nanocatalysts The metal-based nanocatalysts show wide applications such as water purification, fuel cell, energy storage, rocket propellants, organic synthesis, bio diesel production, medicine, agriculture, etc. The metal-based nanocatalysts are currently being used in fields (Figure 12.4) such as agriculture, cancer therapy, water treatment, bio-oil generation, photocatalysis, fuel cell, environmental remediation, and energy generation. Wastes from different industries and agriculture consist of solid wastes, atmospheric pollutants, and huge amount of contaminated water. Agricultural waste products lead to problems such as unpleasant smell, degradation, and change in soil pH [34]. These wastes can be minimized

336  Functionalized Nanomaterials for Catalytic Application

Environmental remediation (a) Bio-fuel generation

(a) Energy generation Metal based nanocatalyst

(a) Disease diagnosis

Agriculture solution (a) Water treatment

Figure 12.4  Some applications of metal-based nanocatalyst in various fields.

by using different nano metal-based catalysts and converting waste to better useful products via heating, gasification, trans-esterification, hydrogenation, etc. [35]. Agri-biomass residues, byproducts, and energy crops have been used for production of biofuel [36]. The nano metal oxides of alkali earth and transition have emerged as heterogeneous catalysts which enhance the trans-esterification process of oils. The ZnO nanostructures have been found to be capable of improving the catalytic activity for biofuel synthesis [37]. The KF/CaO nanocatalyst has been used to produce biodiesel from Chinese tallow seed oil. The nanocrystalline MgO and NiO have been reported as potential catalyst for bio-fuel production [38, 39]. The metal-based nanocatalysts have also been used for environmental remediation like degradation of pollutants. For the degradation of organic pollutants and wastewater treatment, sulfate radical-based advanced oxidation technology is emerging rapidly. The sulfate salts used in the technology are activated with the help of transition metal oxidesbased catalysts. It has been observed that introduction of nano-scaled metal oxides rather than use of bulk materials for activation of sulfate salts enhanced the catalytic activities. This can also help in easy recovery and removal of catalyst from the reaction. Nanocatalyst of mixed oxide (Fe-Co) has been reported for activation of peroxymono-sulfate used for degradation of 2,4-dichlorophenol [40]. N-doped TiO2 nanoparticles have also been reported to have high catalytic activity for wastewater treatment as well as disinfection [41].

Metal-Based Nanomaterials As Catalysts  337 Fuel cells (particularly hydrogen fuel cells) have potential of electricity generation and considered as renewable fuel. The metal-based nanocatalysts have found to be better candidates for fuel cells. Palladium pellets are the nanocatalysts used in fixed bed reactors for hydrogenation [42]. For ex situ groundwater treatment nano-sized magnetite found to be highly active catalysts [43]. The gold-palladium core-shell nano-structures catalyzed hydrogen generation in small fuel cells from formic acid have also been reported [44]. Nanocatalyst governed glucose powered fuel cells have also been reported by Wang et al. [45]. Metal-based nanocatalysts have also affected the field of biosensing. They have been reported to have diagnostic or therapeutic applications [46]. The bionic approach is considered for designing nano catalysts-based biosensors which mimic enzymes. Metal nanoparticle based chemodynamic therapy in combination with other therapies have been used for cancer treatment [47]. The use of magnetic nanorobots as mobile nanocatalysts to activate prodrugs (chemotherapeutic) has been reported for biorthogonal targeted treatment of cancer [48]. Metal nanoparticles have large surface area, high bio recognition, high degree of reaction catalysis, immobilization ability, biocompatibility, unique structure, and shape, which make them perfect for their utilization in biosensing [49]. The metal oxide-based nanomaterials can be used for both target recognition as well as for signalling. Their ease toward modification influences and enhances their catalysis, optical, and adsorption activities, thus making them promising candidates for biosensors [50]. The different applications of nano-metal catalysts have been compiled in Table 12.2.

12.4 Types of Nanocatalysis Nanocatalysis is a rapidly developing field involving either one or both homogeneous and heterogeneous catalysis. The conventional catalysis (homogenous and heterogenous catalysis) has advantages as well as disadvantages [7]. Thus, there is an immediate need of new advanced catalysis technique which allows easy recovery of catalyst like heterogenous and is easily miscible like homogeneous catalyst. The nano size and high surface to volume ratio increase the contact between reactants and catalyst just like homogeneous catalysis. The insolubility of nanocatalyst and their easy separation form reaction mixture is just like heterogeneous. Nanocatalysis provides a better solution with advantages of both homogeneous as well as heterogeneous catalysis [61]. Nano metal-based catalysis is also known as green catalysis, as they are energy efficient and results in reduction in waste

338  Functionalized Nanomaterials for Catalytic Application Table 12.2  The different application nano-metal catalysts. S. no.

Nanocatalyst

Application

Ref.

1

Li-CaO

Bio-diesel production from the Oils

[51]

2

CuO/ZnO/Al2O3

Production of hydrogen via methanol

[52]

3

Au-MoS2

Au-MoS2 nano catalyst excellently destroy excellently reduce 99.99% E. coli within 15 minutes

[53]

4

Pd/hydroxyapatite/Fe3O4

Pd/hydroxyapatite/Fe3O4 effectively Degradation of Azo dye and can be recovered after use

[54]

5

Pt3Co

Pt3Coused in low cost dye sensitized solar cells.

[55]

6

Ag-ZnO/CNT

Wastewater purification from acid orange

[56]

7

Cu7S4-Au@S-MoS2

Detection of mercury(II)

[57]

8

Mn3O4-MnCo2O4-CNT

Used in electrochemical sensors for the determination of thioridazine

[58]

9

Fe-Ni hydroxide

Works as promising catalyst in oxygen evolution reaction

[59]

10

Nano γ- Al2O3

Adsorptive desulfurization and bio-desulfurization of fossils oils

[60]

production [62]. The unique properties of nano metal-based catalysis allow their usage in multiphase/multicomponent catalysis [63]. Different types of nano metal catalysis are shown in Figure 12.5.

12.4.1 Green Nanocatalysis The nanocatalysis is considered as green because their use in synthetic process involves low energy consumption and the catalysts can be

Metal-Based Nanomaterials As Catalysts  339

Heterogenous Catalysis

Green Catalysis

Nano Metal Catalysis

Multiphase Catalysis

Homogenous Catalysis

Figure 12.5  Different types of nano-metal catalysis.

easily recovered and reused. A catalytic system containing magnetic nano-­ structures in ionic liquids has been considered as green [64]. Ionic liquid or biphasic fluorous media along with nanocatalyst system can be used even at low vapor pressure. The system is also useful the systems where organic solvents cannot be applied or isolation of highly volatile components, also propose the system as promising technique for green catalysis [65]. The concept of green chemistry is adoption of any step or process which leads to (a) reduction and elimination of hazardous and toxic substances, (b)  sustainability, (c) decreases energy consumption, (d) lower downs waste production and (e) exerts minimum environmental effect [66]. Moreover, greener processes should also be economic especially for industrial applications. The use of nanocatalysts helps in achieving the principles of green chemistry [67, 68]. The principle of surface science, heterogeneous catalysis, homogeneous catalysis, and materials science governs the nanocatalysis. These “green” nanocatalysts use abundant and inexpensive metals rather than rare and expensive metals [69].

12.4.2 Heterogeneous Nanocatalysis Heterogeneous metal nano-metal catalysts are prepared by adsorption of metal nanomaterial on a functionalized support. The nano metal based heterogeneous catalysts are thermally stable, reusable, sustainable, and selective [70]. The metal nanocatalysts are can be recovered from reaction mixture and reused which makes them highly useful in heterogeneous catalysis.

340  Functionalized Nanomaterials for Catalytic Application Transition metal (Pt, Rh, Au, Ag, Ni, and Cu) nanocatalysts are known for their application chemical transformations [71]. The nanoparticles have started replacing the conventional catalysts, availability of more surface area for reaction. Metal nanoparticles are available in various size, shape, and composition, which make them highly selective and reactive [72].

12.4.3 Homogeneous Nanocatalysis The colloidal solutions of metal nanoparticles are used in homogeneous catalysis or quasi homogenous catalysis [73]. In homogeneous catalysis, the colloidal metal nanoparticles (generally nano forms transition metals) are finely dispersed in an organic or aqueous solvent mixture. It is important to stabilize colloidal of nanoparticles firstly to prevent aggregation of the nanoparticles and secondly to make them recyclable. Metal colloids of transition metal nanoparticles are considered very efficient catalysts [74].

12.4.4 Multiphase Nanocatalysis The metallic oxide and bi-metallic nano-structures in ionic liquids solvent system has recently been studied for their application for multiphase catalysis [74]. The high polarity of ionic liquids solvent system is perfect for multiphase or multicomponent catalysis. In the system, the catalyst can easily recovered and reloaded, hence making the system sustainable and reusable. The multiphasic catalytic system containing transition metal nanoparticles, bimetallic alloys, or metal oxides have been reported for efficiently catalyzing transformation reactions, hydrogenation reactions, condensation reactions, etc. [75]. Though the research is in initial stage, it possesses high potential for future catalysis especially for multiphase systems. The synergic progression in between the phases can be explained on the basis of three proposed models for metal-based multiphasic catalysis: (a) support effect (core and shell configuration), (b) remote control mechanism (two metal oxides co-existence, one as donor and other as acceptor), and (c) interfacial effect (described by epitaxy and have coherent interfaces) [63].

12.5 Different Types of Metal-Based Nanoparticles/ Crystals Used in Catalysis The catalysts have main aim to accelerate the rate of a reaction by lowering the activation energy. It should also have high activity, selectivity, stability,

Metal-Based Nanomaterials As Catalysts  341 Late transition metals

Noble metals

IB metals Metal-based nanocatalysts

Multimetallic

Perovskitetype metal oxide

Figure 12.6  The major types of metal nanoparticles used for catalysis.

and durability. The nanomaterials have great potential as new generation catalysts with optimized properties. The nanoparticles of metal, their compounds or their composites are extensively studied for catalytic activities. The metal-based nanomaterials include nanostructures of late transition metal, IB metals, noble metals, Perovskite-type metal oxide, multimetallic, alloys, doped metal nanoparticles, etc. The major types of metal nanoparticles used for catalysis are represented in Figure 12.6.

12.5.1 Transition Metal Nanoparticles The nanoparticles of transition metal are well known for catalytic activities. The late transition metals such as iron, cobalt, and nickel have been used for large scale industrial catalysis like methane transformation to syngas and syngas to hydrocarbons. Fe3O4, Fe, and Co nanoparticles alone or dispersed on Al2O3 and/or SiO2 have been reported for industrial catalysis used for converting syngas into hydrocarbons [76]. The iron nanoparticles catalyzed synthesis of carbon nanomaterials via cracking of hydrocarbons has also been acknowledged [77, 78]. Co3O4 supported on Al2O3 has been reported to get reduced to CoO and CoRu or Ru nanoparticles at 400°C, which are already known for their catalytic activities [79]. Ni nanoparticles have been known to catalyze number of industrial reactions like methane reformation into syngas, cracking of methane to carbon nanomaterials. Ni nanoparticles

342  Functionalized Nanomaterials for Catalytic Application supported by SiO2 or Al2O3 have also been reported for catalyzing cracking, where hydrocarbons get converted to carbon nanomaterials [80]. IB metals such as copper, silver, and gold have excellent potential as heterogeneous catalysts. They are industrially very important. Copper based catalysts are industrially important IB metal catalyst. They have been used for water gas shift reaction and methanol synthesis at low temperature. Ag-Cu catalysts have been reported for their application in ethene epoxidation [81]. Cu/ZnO/Al2O3 catalysts have also exhibited their application as industrial catalyst for synthesis of methanol from syngas [82–84]. The size dependent Au nanocatalysts have been found to be useful for low temperatures water gas shift and CO oxidation [85–88]. The Cu/CeO2 catalysts have been found effective for CO/CO2 hydrogenation of methanol and watergas shift and CO oxidation [89]. The Ag nanoparticles have been used for ethylene epoxidation, selectivity in chemical industry [90]. Ag/ Al2O3 nanoparticle/clusters found to be active for coupling reaction of primary alcohol or secondary alcohol and amine [91]. The noble metals like Rhodium, Palladium, and Platinum are important and expensive industrial catalysts. Thus, they need to be utilized to their maximum to reduce the cost of the process. This can be done by maximizing the surface area as well lowering the particle size. Rh and Pd metal catalysts are commonly used in the chemical industry for oxidation and hydrogenation [92]. Pt nanoparticles have explored extensively for heterogeneous catalysis. The mobility of Pt nanoparticles is responsible for their high catalytic activity [93]. The Pt nanoparticlesbovine-bone powder nanocomposite displayed catalytic activity with 83% selectivity and 100% conversion toward 2-butene-1,4-diol [94]. Pt nanocatalysts with γ-Al2O3 support found to be active for the oxidation of 2-propanol [95].

12.5.2 Perovskite-Type Oxides Metal Nanoparticles The perovskite oxides have ABO3, where A is an alkaline-earth or rareearth metal (coordination no: 12), and B is a transition-metal (e.g., Mn, Co, Fe, Ni, Cu, Ti) with coordination number 6 [96]. Additionally, perovskite oxides have flexible redox active sites and tunable physical and chemical properties, oxygen vacancies and structures. Perovskite oxides are thermally stable, and exhibit high oxygen mobility, high redox potential, and conductivity. The morphology also affects the physical and chemical properties of perovskite oxides [96–98]. Nano-porous perovskite oxides display catalytic activity for the reduction of exhaust gases [98–101], catalytic combustion CO, toluene

Metal-Based Nanomaterials As Catalysts  343 [102,  103], and soot oxidation [104, 105]. Nano-porous La0.6Ca0.4CoO3 has been reported to display high bi-functional electrocatalysis in alkaline medium and photocatalytic activity [106].

12.5.3 Multi-Metallic/Nano-Alloys/Doped Metal Nanoparticles These new era nanocatalyst can also be bi-metallic, multi-metallic, alloy, doped metal, mixed metal composites, etc. The core/shell Pd/FePt nanoparticles have been known for oxygen reduction reactions [107]. The Au/Pt-bimetallic nano electrocatalyst have also been known for catalyzing oxygen reduction reaction in fuel cells [108]. Controlled catalytic activity has been shown by Pt-Au mixed nanoparticles [109]. The carbon-­ supported platinum-based alloy nano particles can exhibit oxygen reduction reaction at low temperature [110]. The AgM type nano-alloys (where M is metal like Pd, Pt, and Au) are known for peroxidase like catalytic activity [111]. The Au doped Ni-based catalysts have been reported for their catalytic hydrogenation [112]. The TiO2-supported, Au-Pd, and Ru-Pd nano-alloys have been reported for their potential as heterogeneous catalysis in synthesis of imine N-benzylideneaniline and amine N-benzylaniline [113].

12.6 Structure and Catalytic Properties Relationship It has been observed that catalytic activity, stability, and selectivity of the metal nanocatalysts are related to the size and morphology of nanoparticles. The metal support has also been found to be responsible for the catalytic activity of the metal nanoparticles. The structure-reactivity relationship has provided the fundamental understanding for surface and catalysis science [114]. Though, it should be noted that metal-based catalysis is dynamic in nature and the catalysts may undergo structural as well as chemical changes during a catalytic reaction [115–117]. The dynamic nature depends on reaction conditions like temperature, pressure, etc. Thus, in situ assessments and characterizations of metal catalysts during reaction may directly evaluate the active sites and reaction mechanism [4, 118–121]. This helps in better understanding of catalytic process and structure-activity of nanocatalysts at the atomic and molecular level [122– 124]. High surface area leads to increased active catalytic sites and thus enhanced activity. It is reported in literature that by changing the material density and available active sites may results in enhanced activity of a

344  Functionalized Nanomaterials for Catalytic Application

Structure Size

Composition

Activity of Catalyst

Figure 12.7  Pictorial representation of size, structure, and composition dependence of catalytic potential of a catalyst.

catalyst [125]. The modification in size, structure, and composition may affect electronic properties and number of available active sites for catalysis (presented in Figure 12.7). The functionalization and immobilization may lead to increased stability, adsorption potential, and electronic properties. Anisotropy in shape like surface density, edges, and corners of nanoparticles affects the catalytic properties of nanocatalysts. The coordination number is known for affecting catalytic properties of nanomaterials via substrate-catalyst interactions. Nanocatalyst such as alloy, bimetallic, or multi-metallic compositions results in inexpensive industrial oriented nanomaterials with synergic catalytic potential [125]. Different studies found a correlation between the structure and electronic state with the catalytic activity of nanocatalyst suggesting a direct link between the two [9, 125–127] and supports the confinement effect in nanocatalysis [127].

12.7 Conclusion and Future Prospects Nanomaterials have remarkable properties such as small size, unique structure, selectivity, activity, and productivity. Further, electronic changes of metal-based nanomaterials distinguish them from the bulk materials and make them useful for various applications. The metal-based nanocatalysts are currently being used in fields such as agriculture, cancer therapy, water treatment, bio-oil generation, photocatalysis, fuel cell, environmental remediation, and energy generation. Nanocatalysis has emerged as

Metal-Based Nanomaterials As Catalysts  345 dynamic and valuable field for metal-based nanomaterials and is considered as green catalysis which involves any one of the nano-homogeneous, heterogeneous, and multiphase catalysis. There are different types of metalbased nanomaterials which are used in the field of catalysis and include nanostructures of transition metal, noble metals, Perovskite-type metal oxide, multi-metallic, alloys, and doped metal nanoparticles. The ­structure-reactivity relationship has provided the fundamental understanding for surface and catalysis science. It has been observed that catalytic activity, stability, and selectivity of the metal nanocatalysts are related to the size and morphology of nanoparticles. Metal-based nanomaterial offers numerous prospects as impetuses to satisfy future needs in the catalytic process. The high mobility and better selectivity of metal-based nanomaterial over traditional catalysts are credited to their huge surface region. The higher level of surface molecules and extraordinary crystal structure of metal or their derivatives makes them suitable for catalytic applications. The development of novel metal-based nanomaterial as future catalysts is progressively upheld by advances in preparation, characterization, and testing of catalysts.

Acknowledgment The authors would like to acknowledge the support provided under the DST-FIST Grant No.SR/FST/PS-I/2019/68 of Government of India.

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13 Functionalized Nanomaterials for Catalytic Application: Trends and Developments Meena Kumari1*, Badri Parshad2, Jaibir Singh Yadav3 and Suresh Kumar4† Department of Chemistry, Govt. College for Women, Badhra, Charkhi Dadri, Haryana, India 2 Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, United Kingdom 3 Department of Chemistry, AIJHM PG College Rohtak, Haryana, India 4 Department of Chemistry, Kurukshetra University, Kurukshetra, Haryana, India 1

Abstract

Catalysis by functionalized nanomaterials is the contemporary discipline of nanoscience which is expanding exceptionally to meet the upcoming global demands of mankind. Nanocatalysts, being lying at the frontier of homogeneous and heterogeneous catalysts, offer multiple benefits of atom economy, remarkable stability, enhanced activity, better selectivity, recoverability, reusability, and energy efficiency, thereby allowing optimum feedstock utilization and minimal chemical waste. However, with time, it was diagnosed that some of these very active nanocatalysts suffer with the limitation of stability causing them to agglomerate during catalysis, which was later resolved to a great extent by modifying their surface composition via functionalization. The functionalization of these nanocatalysts with various biocompatible and active species serving as weak ligands not only enhances their stability and selectivity but also facilitates their easy separation along with preventing their undue coagulation during catalysis. Besides these, the functionalization of nanomaterials also has considerable effect reflected in their structure, morphology, optical, electrical, magnetic, and other properties owing to the novel theory of quantum effects, enabling a control of their catalytic activity. This Chapter will cover nanocatalysis, factors affecting catalytic performance, different functionalization strategies and application of these functionalized nanocatalysts in various fields. *Corresponding author: [email protected] † Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (355–416) © 2021 Scrivener Publishing LLC

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356  Functionalized Nanomaterials for Catalytic Application Keywords:  Nanocatalyst, magnetic, nanostructured, carbon dioxide (CO2), carbon nanotube (CNT), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), Faradaic efficiency

13.1 Introduction Nanocatalysis is the process of evolving the environment friendly and cost effective chemical conversions via lower energy paths and higher atom utilization efficiency with the help of nanocatalysts. The field of nanocatalysis has enabled researchers to increase the efficiency of various conversions using nanoengineered catalysts via molecular level understanding of catalytic mechanisms. Nanocatalysts have played a prominent role in making various catalytic conversions cleaner and greener by relaxing the requirements for various transformations which otherwise require harsh conditions with traditional catalysts [1]. The increasing number of research publications (Figure 13.1) and patents related to catalysis by nanomaterials depict its exponential growth for developing nanocatalysts with improved activity, selectivity, and recoverability toward various chemical processes and making them greener [2]. Though the field of nanocatalysis involves extensive research on several parameters, yet development of efficient catalysts with high catalytic activity, selectivity and stability as well as the advancement in characterization techniques needs 4000

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Figure 13.1  Publications 2011–2020 containing “nanocatalyst” as topic using SciFinder® (till September 2020).

Functionalized Nanomaterials  357 to be upgraded regularly. The great influence of size, shape, morphology, and support on the catalytic efficiency and stability of the nanocatalysts necessitated the development of advanced methods for synthesis, characterization, and postsynthetic modifications. Though, significant progress has been made in the preparation of nanocatalysts yet hydrothermal method, sol gel technique, wet chemical method, microwave irradiation, thermal decomposition, and sonochemical method are most commonly used for synthesizing nanomaterials possessing catalytic activity [3]. Further, the characterization techniques not only help in determining the various parameters like shape, size, morphology, and dominating facets of the nanocatalyst but also help in determining the mechanism of the catalysis. Although the catalytic role of transition metal-based nanomaterials was recognized in the 19th century, the field of nanocatalysis gained momentum after the discovery of electron microscopy and various other characterization techniques.

13.1.1 Nanocatalysis Although research work pointing the role of substances as supporters and accelerators of some chemical transformations started in the late 18th century, the process was interpreted and the term “catalysis” was coined by Jöns Jacob Berzelius in 1835. Catalysis by functionalized nanomaterials is the contemporary discipline of nanoscience which is expanding exceptionally to meet the upcoming global demands of mankind. Nanomaterials are the substances having at least one of its dimensions or the pore size in between 1 and 100 nm; however, most of the nanomaterials utilized as catalysts were in its smallest size domain with very few exceptions. Though the production of chemicals and materials of immense utility, with the help of homogeneous and heterogeneous catalysis by various species, is well established, still cutting down these catalytic species in the nano range have led to a distinctive and substantial field of “Nanocatalysis”. The nanometer scale dimensions of nanocatalysts not only facilitate the enhanced diffusion rates but also fasten the electron transfer kinetics, thereby revolutionizing the field of catalysis. Nanocatalysts being lying at the frontier of homogeneous and heterogeneous catalysts possess multiple benefits of fast and selective chemical processes with magnificent product yield integrated with the ease of catalyst separation and recovery. The large surface area of these nanocatalysts allows better exposure of catalytic centers ultimately leading to increased activity. The usage of various external and internal triggers leading to insolubility in the reaction solvent facilitates the separation and recovery

358  Functionalized Nanomaterials for Catalytic Application of these nanocatalysts. Recovery from the reaction media and repeated usage over multiple cycles without significant loss in catalytic activity, are among the most acceptable and qualifying criteria for usage in green industrial applications. However, with time, it was diagnosed that some of these very active nanocatalysts suffer with the limitation of stability causing them to agglomerate during catalysis which was later resolved to a great extent by modifying their surface composition via functionalization. The functionalization of these nanocatalysts with various biocompatible and active species serving as weak ligands not only enhances their stability and selectivity but also facilitates their easy separation along with preventing their undue coagulation during catalysis. Besides these, the functionalization of nanomaterials also have considerable effect reflected in their structural, morphological, optical, electrical, magnetic, and other properties owing to the novel theory of quantum effects, enabling a control of their catalytic activity. To conclude, these nanocatalysts being lying at the frontier of homogeneous and heterogeneous catalysts offer multiple benefits of atom economy, remarkable stability, enhanced activity, better selectivity, recoverability, reusability, and energy efficiency, thereby allowing optimum feedstock utilization and minimal chemical waste. Here, in this chapter, in the first section, we will discuss nanocatalysis, the factors affecting nanocatalysis, characterization techniques, principles of green chemistry involved in nanocatalysis and role of functionalization and support materials in improving the catalytic activity. The second section will summarize some of the frequently explored classes of nanomaterials possessing catalytic activity such as metal nanoparticles, alloys and intermetallic compounds, single atom catalysts (SACs), magnetically separable nanocatalysts, metal organic framework (MOF)–based catalysts, and carbocatalysts. In the third section, applications of functionalized nanomaterials will be discussed in the area of organic transformations, electrocatalysis, photocatalysis, and conversion of biomass into fuels among various others. The fourth section will conclude the above discussion with the future outlook.

13.1.2 Factors Affecting Nanocatalysis The proper engineering of the various parameters of the catalyst like size, shape, and morphology plays a determining role in achieving their better efficiency (activity and selectivity) and higher stability which further lead to designing a greener, cleaner, and economical process. The size and shape

Functionalized Nanomaterials  359 of the nanocatalyst is also known to be affected by the method of preparation [4, 5]. Besides these, the electronic structure and stability of the catalyst can also be altered by the nature of the support and functionalizing ligands [6].

13.1.2.1 Size Size of the nanocatalyst plays a significant role in determining its catalytic activity and selectivity. The size of the catalyst not only changes the surface area but also the electronic structure by changing the relative ratio and position of surface atom types [7]. The influence of size of nanoparticles on their catalytic properties was realized in 1989 after the findings of Haruta and coworkers, depicting that cutting down the size of gold nanoparticles to 5 nm or less and supporting them on Fe2O3 significantly enhanced the oxidation potential of CO [8]. The increased catalytic activity with decreasing size is perfectly illustrated by gold nanoparticles. The activity and selectivity of these nanoparticles was found to decrease significantly when the diameter of these gold nanoparticles increases beyond 10 nm [9]. However, some nanocatalysts require an optimum size for maximum activity and the catalytic activity decreases with both increase and decrease in their size as revealed by Pt nanocatalysts for generation of photochemical hydrogen which display optimum activity for 3-nm catalytic diameter [10]. While partially hydrogenating cyclopentadiene using Pt nanoparticles, the maximum selectivity was observed for less than 2-nm catalytic size which decreases for increasing size which might be due to steric effects [11].

13.1.2.2 Shape and Morphology Shape and morphology combinedly determine the surface structure of the nanocatalyst which is one of the most important parameters affecting the catalytic efficiency. The surface structure of the nanocatalyst not only changes the electronic structure but also the number and positioning of active sites, thereby altering their catalytic activity. Further, the morphology of the nanocatalyst also plays a determining role in enhancing the selectivity toward a particular product by controlling the energy and formation of various reaction intermediates [12]. The selectivity of the catalyst for different reactions also alters with the change in compositions of different facets along with structure as Pt(111) facets are three to seven times more efficient for aromatization reactions as compared to Pt(100) facets [13]. The impact of varying shapes of transition metal nanoparticles

360  Functionalized Nanomaterials for Catalytic Application (MNPs) on their catalytic properties and recycling potential was highlighted by the associated changes in the composition of surface atoms at corners and edges as well as by varying the kind of crystallographic facets by Narayanan and coworkers [14, 15].

13.1.2.3 Catalytic Stability The catalytic stability is another significant parameter determining the catalytic activity of nanocatalysts. The role of stability further increases with the decreasing size of the catalytic species as it is associated with the increase in surface free energy. This increased free energy makes the nanocatalytic species more susceptible toward aggregation and agglomeration. The stability of these nanocatalytic species might be increased for achieving the high catalytic efficiency of the catalysts either by proper functionalization or by the use of suitable stabilizers in the colloidal dispersions [16], or by the use of suitable supports in case of heterogeneous catalysts. The supramolecular assembly formed by self-organization of the polymers/dendrimers is also exploited for increasing the stability of nanocatalytic species. In one such example, Klingelhöfer et al. have stabilized the colloidal dispersions of palladium-based nanocatalysts in micelles, formed by a block copolymer (polystyrene-b-poly-4-vinylpyridine) [17].

13.1.2.4 Surface Modification The surface of the nanocatalyst can be modified either by functionalizing these with various capping agents or by supporting them on different supports. The surface modification of the nanocatalyst is further known to alter its catalytic efficiency. Therefore, with the proper selection of capping agents and nature of support, the significant increase in catalytic efficiency can be achieved. The increase in catalytic efficiency might be explained either through the generation of low coordination sites which might facilitate the binding of the reactant as well as intermediate reactive species to their active sites or by the enhanced stability. Nature of the support and its composition significantly affect the catalytic activity in case of heterogeneous catalysts. The catalytic potential of supported MNPs depends on various factors which include size, composition and morphology of MNPs, composition of the support [18], interactions of MNPs with the support material, orientation of catalytically active facets at the surface of supported MNPs [19]. Jackson and coworkers have studied the impact of support by studying the interaction of nickel

Functionalized Nanomaterials  361 catalysts possessing metal in +2 oxidation state and octahedral coordination on alumina, silica, and molybdenum oxide supports. With the help of various characterization techniques, it was suggested that the metal ions interact differently with different supports as depicted by varying ligand spheres around metal ions. The different behavior of Ni/ MoO3 catalyst toward hydrogenation of buta-1,3-diene further revealed its different electronic structure as compared Ni/SiO2 and Ni/Al2O3 catalytic systems. Gould and coworkers have shown that the thermal stability of silver nanoparticles increases significantly by embedding these in amorphous silicon making them robust catalysts [20]. All the abovementioned factors exert their effect via changes in the electronic structure of the catalytic system. The impact of these parameters on the catalytic potential of supported MNPs is so profound that even small changes can highly alter the catalytic efficiency. The impact of support material on the catalytic potential of dispersed MNPs was elegantly demonstrated by Matsumura and co-workers by dispersing NiNPs on various supports and evaluating their influence on steam reforming of methane [21]. Also, Haruta and coworkers developed supported MNPbased catalysts by supporting Au Nanoparticles on alumina and carbon surfaces and observed that complete conversion for oxidizing ethylene glycol was found at lower than 4-nm size for AuNPs/alumina and at size between 7 and 8 nm for AuNPs/carbon [8]. The authors also reported that the catalytic activity of the prepared AuNPs/alumina reduces to 40% with an increase in AuNPs size beyond 5  nm. Besides these, the doping of the surface with suitable chemical species also increases the catalytic potential of the designed catalyst [22].

13.1.3 Characterization Techniques The recent advances in characterization techniques for nanoscopic catalytic systems have enabled researchers to easily tune their activity and selectivity by determining even the small changes in composition, size, and morphology of the catalytic species. The small-angle X-ray scattering (SAXS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), liquid transmission electron microscopy, scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and thermogravimetric analysis (TGA) are some of the most commonly used characterization techniques.

362  Functionalized Nanomaterials for Catalytic Application

13.1.4 Principles of Green Chemistry Although catalysis and green chemistry are closely associated since they allow us to perform various reactions at mild conditions of temperature and pressure via lowering of activation energy, thereby making the conversion much efficient and economical. However, the growth of material science and nanotechnology has also revolutionized the field of catalysis by the emergence of nanoscale catalytic species as well as advanced functionalization strategies. The increased surface area of nanocatalysts with proper functionalization has not only enabled the efficient interactions between active sites of the catalyst and reactant leading to improved activity but also increased their stability and selectivity. Further, the development of nanocatalytic species which are responsive to magnetic fields have greatly enhanced their recovery from the reaction mixture and allowed their reuse over multiple cycles making the process further greener and economical. Nanocatalysts not only improve the atom economy but are also being employed for generating renewable sources of energy by utilizing the various byproducts ultimately playing a significant role in environmental remediation. Thus, the field of nanocatalysis has allowed the sustainable development of highly efficient catalytic species which will enable the optimum utilization of energy resources with minimal waste and easier recycling contributing toward a safer, cleaner, and greener environment for mankind. In a recent report, Li et al. have

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Figure 13.2  Smart paper transformation approach for catalytic separation and reusability (with copyright permissions from ref 23).

Functionalized Nanomaterials  363 developed a smart paper transformer approach involving phase conversion among paper and pulp for facilitating the separation and reusability of the catalyst (Figure 13.2) which ultimately increases the efficiency of catalytic transformation [23].

13.1.5 Role of Functionalization The functionalization of nanocatalysts involves surface modification using various biocompatible species like polymers, oligomers, dendrimers, and various other ligands [24]. While functionalizing, it should be taken due care that ligands are decorated in such a way that they should not have any hindrance to active sites which might lower its catalytic activity. Nanocatalysts are functionalized for enhancing their stability in the solutions by minimizing their aggregation from interaction between naked surfaces. Functionalization not only increases the surface area but also changes the electronic structure and morphology of the nanocatalysts, resulting in increased catalytic activity [25]. The functionalization of nanocatalysts also affects the shape and morphology; hence regulates the exposure of different crystallographic facets, edges, and corners which further determine the catalytic activity and selectivity [26]. The formation of enantioselective products can also be enhanced by using chiral capping agents. In addition to functionalization, the nanocatalysts are also being embedded in porous skeletons formed by zeolite/MOFs as well as anchored over various solid supports [27]. The assemblies formed by nanostructured 2D materials like graphene, MXene, and transition metal dichalcogenides (TMDs) have also been utilized for various catalytic applications [28].

13.1.6 Frequently Used Support Materials Carbon nanotubes (CNTs) and graphene/graphene oxide are the carbon materials which are being used tremendously for functionalizing various catalytic species for enhancing their stability and activity. Though, the CNTs are recognized as efficient support material due to availability of higher surface area, good conductivity, and excellent chemical stability, yet it is difficult to achieve high dispersion of catalytic species on pristine CNTs due to very low availability of binding sites [29]. The inefficiently anchored catalytic species will not be stable and tend to undergo aggregation as a result of the self-graphitization tendency of pristine CNTs [30]. The number of binding sites on the surface of CNTs are increased either

364  Functionalized Nanomaterials for Catalytic Application by functionalizing these with suitable polymeric skeletons [31] /silica [32] /other compounds having pendant functional groups [33] or by oxidizing chemically at the site of defects at the surface of CNTs [34]. Ma et al. synthesized the size-controllable nanoparticles of SmCo5 by chemical method and further demonstrated that coating of these magnetic nanoparticles with a 10-nm-thick N-doped graphitic carbon increases their stability toward air oxidation. The functionalized nanomaterials retained their magnetic properties and were found to be stable over a broad temperature range (up to 100°C) [35].

13.2 Different Types of Nanocatalysts 13.2.1 Metal Nanoparticles MNPs are the most widely studied class of nanomaterials explored for their catalytic potential [36]. The research on MNPs started with the synthesis of finely divided colloidal gold particles (ruby fluid) by Michael Faraday in 1857. However, it was only revealed by Turkevich and coworkers that the ruby colored colloidal solution prepared by following the procedure reported by Michael Faraday contained gold particles with size in the range of 6 ± 2 nm, by determining the size measurements with the help of electron microscopy [37]. The MNPs possessing catalytic applications in various fields are often found with the size between 1 and 30 nm which is comparatively larger than a single metal atom and much smaller than bulk metal species. The MNPs are very efficient catalysts owing to their unique electronic structure and increased surface exposure of catalytically active sites, yet for most of the commercial applications, it is preferred to have supported MNPs to facilitate their easy recovery after completion of the reaction. Besides easy recovery, the dispersing of MNPs on various supports further increases their stability toward self-aggregation and also helps in modulating their activity and selectivity for particular catalytic processes. The use of supported metal-based catalysts started more than 100 years ago with the research work by Sabatier and Senderens for the application of supported nickel catalysts for hydrogenation of CO to methane [38]. The catalytic conversion was then applied in various industrial processes and Paul Sabatier shared the Noble Prize in Chemistry in 1912 with Victor Grignard for the use of finely disintegrated metals for hydrogenating various organic compounds [39]. These finely disintegrated metals mentioned in the

Functionalized Nanomaterials  365 research work by P. Sabatier are well understood these days as supported MNPs. Ni, Au, Ag, Ru, Rh, Pd, and Pt are among the most widely used metals in designing the supported MNPs for catalyzing various industrial processes which include hydrogenation, dehydrogenation, oxidation, C-C coupling, petroleum refining, and exhaust cleaning from automobiles [40].

13.2.2 Alloys and Intermetallic Compounds The mixing of two or more kinds of metals yields either alloys or the intermetallic compounds depending upon the nature of interactions present among them. The presence of different types of metals not only changes the electronic structure but also the surface parameters like shape, size, and morphology, which ultimately determine the catalytic properties. The nano-sized bimetallic catalysts are among the most extensively explored nanomaterials for catalyzing various conversions owing to their controlled synthesis and easy tuning of desired parameters. Bimetallic catalysts are the class of heterogeneous catalysts possessing unique structure and electronic properties. The detailed study of the impact of valence electronic configuration of metal species and their surface parameters on their catalytic activity also helped in exploring the potential of bimetallic catalysts for various applications [41]. Based on the composition of the metals involved, their arrangement with relative bond strengths and surface energies, the bimetallic catalysts can have various nanoengineered structures which might include alloy or intermetallic compound or core-shell structures among various others [42]. The effect of their unique structures is further known to be translated into their unique optical, electronic, and catalytic properties in addition to the properties of the participating metals [43]. The homogeneous distribution of Pt-based intermetallic nanoparticles on various conducting support materials further enhances their catalytic potential. Yoo et al. designed intermetallic alloy nanoparticles, MPt where, M can be either Fe, Co, or Ni and dispersed these homogeneously on reduced graphene oxide (rGO) resulting in highly effective catalytic systems for oxygen reduction reaction (ORR) [44]. The homogeneous dispersion of nanoparticles was achieved through the decomposition of [M(bpy)3] [PtCl6], a bimetallic complex on the surface of support material. The L10-FePt catalyst having 37 wt% dispersion of FePt on the reduced graphene oxide support was found to possess 18–19 times higher catalytic activity for ORR for even up to 20,000 cycles as compared to commercial catalyst containing Pt/C.

366  Functionalized Nanomaterials for Catalytic Application

13.2.3 Single Atom Catalysts

Surface Free Energy

Although a lot of progress had been made in developing the highly dispersed metal-nanostructure-based catalytic systems supported over diverse supports, these nanostructures were found to have different sizes and morphologies (nanoparticles/nanoclusters) on detailed analysis by modern characterization techniques. The presence of these catalytically active nanomaterials in varying sizes not only lowers the selectivity but also reduces the efficiency of the catalyst. In the last few years, various research articles came across reporting the increased catalytic performance either by decreasing the particle size of the catalyst or by ordered arrangement of catalytically active sites with suitable support. The reduction of size of catalytic species generates coordinatively unsaturated states leading to the increase in surface free energy. Although, this increased surface energy is known to enhance their self-aggregation, thereby lowering their stability and activity. However, with proper supports and functionalization strategies, this surface free energy might be utilized in increasing their catalytic potential. In continuing efforts for increasing the optimum atom utilization efficiency with decreasing size for supported metal-based catalytic systems, an extreme state of the dispersion of isolated metal atoms on the support, known as single-atom catalysts (SACs), is achieved (Figure 13.3). These SACs, with well-defined single active catalytic centers, not only maximize their efficiency by increasing the catalytic activity and selectivity but also help in studying the reaction mechanisms. The unique catalytic efficiency of SACs might be accounted for, by their increased surface

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Figure 13.3  Journey from bulk materials to single atom catalyst (with copyright and permissions from ref 45).

Functionalized Nanomaterials  367 energy, highly active valence electrons, and sparse quantum levels with quantum confinement of the electrons owing to their extremely small size [45].

13.2.4 Magnetically Separable Nanocatalysts The efficient recovery of the catalyst is one of foremost requirements for commercialization so that the process could be made environmentally benign and cost-effective. Earlier, the colloidal metallic nanoparticles were considered with the advantageous edge of being separable by the process of precipitation, centrifugation, and filtration; however, these separation techniques, besides adding extra cost to the process, also make it tedious. The separation of colloidal metallic nanoparticles with size dimension lower than 50 nm becomes more entangled as a result of increasing stability of colloidal solution with smaller dispersed phase [46]. To make the catalytic process more economical, the incorporation of magnetism by using magnetic supports for anchoring the catalytic species resolves the issue of separation to some extent [47]. The anchoring of homogeneous catalysts on magnetic nanoparticles not only increases their stability and repeated usage over multiple cycles but also retains the high catalytic activity of homogeneous species [48]. The resulting magnetic nanocatalyst also increases the efficiency of catalysis by improved dispersity of these in a wide range of solvents. Magnetic nanomaterials possess unique magnetic behavior utilizing the quantum size effect due to nano-size which further enables the tuning of magnetism which thereafter determines the properties of nanomaterials. The coating of magnetic nanoparticles with suitable surfaces (carbon/ silica/polymer) not only increases the stability of the resulting catalyst toward diverse reaction conditions by protecting the magnetic core but also facilitates the anchoring of catalytic species. Gao and coworkers developed another approach by functionalizing the Pd-based homogeneous catalyst with N-heterocyclic carbene (organic ligand) and then immobilizing these on gamma-Fe2O3 nanoparticles for analyzing their catalytic potential for Suzuki, Heck, and Sonogashira cross-coupling reactions [49]. The optimum conversion for different cross coupling reactions was observed at 50°C and Na2CO3 as base in addition to the designed catalyst (Pd-NHC/ gamma-Fe2O3). The small size of the catalyst and its capping with organic ligands helped in achieving the reasonably good solubility in various organic solvents which further enabled them to exhibit higher catalytic activity. The observed higher catalytic activity might be explained by the presence of suitable linkers (N-heterocyclic carbene) which facilitate the interaction between active sites of the catalyst and reactants/intermediates

368  Functionalized Nanomaterials for Catalytic Application by allowing their easy access to the active sites. Encouraged by the above results, the same research group also studied the effect of change in the support functionalization over the activity and selectivity of the catalyst. The immobilization of Pd-NHC catalyst on core-shell magnetic nanoparticles consisting of Fe2O3 core and 2-nm-thick polymer shells increased its selectivity toward Suzuki cross-coupling with catalytic conversion reaching more than 80% [50]. Tsang and coworkers reported the deposition of 5 wt% Pd on Fe-Ni magnetic nanoparticles coated with graphitic C surface for hydrogenation of nitrobenzene with catalytic activity twice to that of the commercial catalyst [51]. Jin and coworkers also reported the Suzuki, Sonogashira, and Stille coupling reactions of unreactive aryl chlorides in water at 50–60°C by using a magnetic core-shell catalyst formed by (b-oxoiminato)(phosphanyl) Pd catalyst supported over Fe3O4-SiO2 nanoparticles [52]. The size of nanostructures formed on the surface of functionalized magnetic nanoparticles is also affected by the mediating ligands/linkers/shells which further alter the catalytic activity. Yi and coworkers dispersed Pd on the core-shell architecture formed by amine- and thiol-functionalized Fe2O3/SiO2 nanoparticles resulting in Pd/H2N- SiO2/Fe2O3 and Pd/HS-SiO2/Fe2O3, which exhibited enhanced catalytic activity for hydrogenation of nitrobenzene as compared to Pd/C [53]. The functionalization of Fe2O3/SiO2 nanoparticles with amine and thiol groups was achieved in toluene by refluxing with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) and (3-mercaptopropyl) trimethoxysilane (MPS), respectively. TEM studies showed the formation of smaller size Pd nanoclusters with narrow size distribution on the amine functionalized Fe2O3/SiO2 nanoparticles via AAPS ligand as compared to thiol functionalized magnetic nanoparticles via MPS ligand depicting stronger interactions leading to larger stabilization via AAPS.

13.2.5 Metal Organic Frameworks MOFs are the crystalline materials possessing porous skeletons which are formed by interactions between multidentate organic structures with inorganic species. The suitable nano-sized pores of the MOFs allow them to embed MNPs/nanoclusters/SACs possessing catalytic activity, thereby increasing their catalytic performance via shape selectivity and higher stability. The catalytic activity and selectivity of the resulting composite catalysts may also be enhanced synergistically depending upon the composition of the MOFs and their interactions with embedded nanocatalysts [54]. Also, the MOF-derived carbon materials have been reported as excellent nanocatalysts toward electrocatalytic reduction of oxygen and evolution

Functionalized Nanomaterials  369 of hydrogen owing to their better electron conductivity and high porosity [55]. The MOFs with embedded nanocatalysts have also been utilized for generating hydrogen efficiently from liquid-phase chemical hydrogen storage materials like formic acid, sodium borohydride, ammonia borane, and various others [56].

13.2.6 Carbocatalysts Carbocatalysts comprise a novel field of catalysis: carbocatalysis which involves exploring the catalytic applications of various carbon-based advanced materials. The nanostructured carbon materials like partially oxidized CNTs and graphene oxide itself play catalytic role for various processes owing to their unique skeleton and properties. Graphene oxide is one among the functionalized graphene derivatives which is found to possess good catalytic properties particularly toward oxidation reactions owing to its unique structural, electronic, and mechanical properties [57]. Besides being inexpensive, the GO also offers the advantage of easy recovery via filtration on completion of the reaction. In the last few years, the extensive growth in methods of preparation like micromechanical exfoliation, vapor deposition, epitaxial growth and advanced oxidation/ reduction processes among various others have also facilitated the preparation of graphene/functionalized graphene for carbocatalysis as well as support materials [58–60]. Dreyer et al. have studied the oxidation of benzyl alcohol in presence of GO under ambient conditions yielding >90% conversion to benzaldehyde with minimal amount (7%) of benzoic acid (Scheme 13.1) [61]. The GO also catalyzes the oxidation of cis-stilbene to benzil [62] and alkynes to ketones depicting its broad catalytic potential. GO

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Scheme 13.1  GO catalyzes the oxidation of alcohols and alkynes, as well as the hydration of alkynes. The reactions shown were generally performed at 100°C under neat conditions. R, R’ = aryl, alkyl, H [59].

370  Functionalized Nanomaterials for Catalytic Application The doping of graphene sheets with transition metal clusters and heteroatoms like nitrogen, and boron facilitate the ORR by lowering the activation energy as depicted by Tafel slope [63]. The ORR activity of these doped graphene sheets was found to be comparable to Pt based catalysts. Also, Zhang et al. demonstrated the dehydrogenation of n-butane to 1-butene with modest conversion yield of 93% Faradaic efficiency at very low over potential for electrocatalytic reduction of CO2 to formate [99]. The high electrocatalytic activity was explained on account of comparative lower interactions between CO•2− and nanostructured tin, subsequently leading to kinetic activation of the intermediate toward protonation which drives the reaction path toward formate.

13.3.2.1.2 Copper-Based Electrocatalysts

Copper metal lies in separate group owing to its unique ability to generate C2-C3 hydrocarbons [100], including alcohols, aldehydes, and various hydrocarbons in addition to C1 products like CO and formate. However, the selectivity of the copper-based electrocatalysts is very low due to its moderate binding affinity for CO and optimum binding of hydrogen, leading to a mixture of products. The selectivity of the copper-based nanocatalyst toward any particular product could be enhanced by controlling the binding of reactants/intermediates to the active center which is further determined

Functionalized Nanomaterials  377 by the surface electronic structure and morphology of the catalyst [101]. The electronic composition of surface atoms also affects nature of products which is further determined by various parameters including shape, size, chemical state, morphology of the electrocatalyst, composition of the electrocatalyst, pre-functionalization of the electrode as well as pH and composition of the electrolyte [102–104]. Ager and coauthors reported optimized study for generating C-C coupled products using oxide- derived copper catalysts from electrocatalytic reduction of CO2 and found significant role of surface area, pH and nature of the electrolyte [105]. The designed electrocatalysts showed increased selectivity toward C2+ products as compared to C1 owing to the moderately high (not too high) pH near catalyst surface at overpotential of −1 V. The extremely high pH near the catalyst surface will reduce the catalytic activity due to decrease in the local concentration of CO2. The selectivity of the reported catalysts toward C2+ products also changed with the change in nature of electrolyte being ~70% and ~56% with CsHCO3 and KHCO3, respectively. Merino-Garcia et al. reported a suitable example of varying the selectivity in response to the changing sizes of copper nanoparticles anchored over electrodes for electrocatalytic reduction of CO2 toward different products. The results revealed that with the decreased particle sizes, the selectivity of the nanocatalyst increased for CO production but decreased for hydrocarbons (CH4 and C2H4) [106]. Choi and coworkers studied the impact of morphology on the electrocatalytic reduction of CO2 by synthesizing unique star decahedron Cu nanoparticle-based electrocatalysts with twin defects and surface stacking faults [107]. The designed nanostructured catalyst not only demonstrated highly enhanced potential for ethylene formation with a Faradaic efficiency of 52.43 ± 2.72% but also comparatively lower onset potential for methane production than commercial Cu nanoparticle-based electrocatalysts by a value of 0.149 V. Li and coworkers have evaluated the electrocatalytic activity and selectivity of ultrathin 5-fold twinned copper nanowires with graphene oxide for CO2 reduction. The designed catalytic system was found to be highly selective for methane with Faradaic efficiency of 55% at −1.25V, thereby highlighting the role of morphology [108]. Further, Li et al. highlighted the importance of preparation method depicting that the electrochemically reduced Cu2Oderived Cu-based nanocatalyst produced multi carbon oxygenated liquid fuels (ethanol, n-propanol and acetate) with Faradaic efficiency of 57% as compared to Cu nanoparticles prepared by traditional vapor condensation yielding hydrogen with Faradaic efficiency of 96% from electroreduction of CO [109]. Cuenya and coworkers have shown that the prism shaped Cu nanocatalysts produced four times higher current density for ethylene production from electrocatalytic reduction of CO2 as compared to planar

378  Functionalized Nanomaterials for Catalytic Application Cu nanocatalysts which was attributed to increased defect sites at the prism shapes Cu nanocatalyst [110]. Gao et al. have very efficiently shown that atomically thin layers of co-existing cobalt and cobalt oxides possess higher catalytic activity and selectivity for electroreduction of CO2 to formate production as compared to cobalt atoms in bulk materials depicting the role of correct morphology and oxidation state in determining the electrocatalytic activity [111]. In current years, the generation of nanostructured activated electrocatalysts is also achieved using plasma treatments which involve making changes on the surface of preformed catalysts by creating defects, embedding atoms, removing capping ligands at room temperature [112]. Mistry et al. reported that the nanostructured Cu(I) catalysts synthesized by the reduction of copper oxides using plasma treatment exhibited improved CO2 reduction efficiency at lower overpotentials and increased selectivity toward ethylene production at higher overpotentials [113]. The selectivity of developed electrocatalyst toward ethylene might be explained by the changes achieved during nanocatalyst generation by oxide reduction which include the generation of low coordination active sites [114], as well as the roughened surface leading to high current density and increase in local pH [115]. Gao et al. have reported that Cu nanocube-based catalytic systems with tunable catalyst shape and ion stability, determined by Cu (100) facet were synthesized through plasma activation as revealed by scanning electron microscopy and combining the energy dispersive X-ray spectroscopy with quasi–in situ X-ray photoelectron spectroscopy techniques [116]. The authors demonstrated that these nanocube morphology designed catalysts exhibit higher selectivity for C2-C3 products (ethylene, ethanol, and n-propanol) reaching a maximum Faradaic efficiency (FE) of 73% along with lower overpotentials, highlighting the presence of oxygen in surface and sub-surface regions for increased activity and selectivity of the catalysts. In another report, Scholten et al. emphasized the role of overall catalyst morphology compared to copper oxide species in the low-pressure oxygen plasma treated dendritic Cu catalyst for selectivity toward ethylene and ethanol, with Faradaic efficiency of 45% [117]. Yang and coworkers depicted that the confinement of the intermediate species formed from CO2 electroreduction in the nano-sized cavities of copper oxide–based functionalized catalyst also protects copper in +1 oxidation state which is further translated into increased selectivity and stability of the catalyst toward the formation of C2+ products with a Faradaic efficiency of approx 75.2%. The presence of Cu+ species during the process of CO2 electrocatalytic reduction was confirmed by X-ray absorption spectroscopy and operando Raman spectroscopy [118]. Lange and coworkers have reported the RuNi-based bimetallic catalysts obtained by partially replacing

Functionalized Nanomaterials  379 the ruthenium metal with nickel and supporting these over zirconia to achieve complete electrocatalytic conversion of CO2 into synthetic natural gas with selectivity nearly equal to 100% [119]. In an another report, Zhao and coauthors designed Cu-SA/NPC catalysts by anchoring the single atom copper on N-doped porous carbon yielding acetone as a major product with a Faradaic efficiency of 36.7%, with minor amounts of acetic acid and ethanol [120]. Density functional theory (DFT) revealed that the coordination of Cu with four pyrrole-N atoms of N-doped carbon assisted in the CO2 activation and C-C coupling, thereby producing acetone.

13.3.2.1.3 Ni/Other Metal–Based Single Atom Catalysts

In the last few years, SACs of transition metals have emerged as effective alternatives to enhance the selectivity of various catalytic systems toward electrocatalytic CO2 reduction due to its maximum atom utilization efficiency, unique electronic properties, and unsaturated coordination environment developed through atomically dispersed metals on suitable supports [121]. In case of coordinatively functionalized SACs on various supports, the support architectures also play a very significant role in designing effective catalytic systems. Graphene, being a single atomic layer of carbon, fulfills the criteria of suitable support to SAC’s as it is highly stable, provides large surface area, forms strong chemical bonds to guest atoms, and possesses high conductivity [122]. Although various literature reports reveal that Ni-SACs supported on nitrogen doped graphene have been utilized for electrochemical reduction of CO2 [123], yet their varying performance dependence on the carbon or nitrogen bonds to Ni is studied kinetically by Luo and coworkers [124]. The authors studied the kinetics of nitrogenated graphene-supported Ni-SACs to determine the number of carbon or nitrogen bonds to Ni as active centers via utilizing the grand canonical potential kinetics (GCP-K) formulation of quantum mechanics. These varying numbers of carbon or nitrogen bonds to Ni were found to further affect the Faradaic efficiency, applied potential, etc., ultimately determining the potential for electrochemical reduction of CO2 to CO. The GCP-K formulation of quantum mechanics enables studying the continuous changes in geometry of the transition states and the charge transfer from electrode to adsorbed species with the changing potential. From the above study, the onset potentials for Ni-N2C2, Ni-N3C1, and Ni-N4 sites determined experimentally were found to be in agreement with the theoretically predicted values and the highest current (40 mA cm−2) was found for the Ni-N4 for CO2 reduction leading to its Faradaic efficiency of nearly 100% at −1.05 V potential. The electrocatalytic reduction of CO2 is found

380  Functionalized Nanomaterials for Catalytic Application to be dependent on both the diffusion and adsorption of CO2 molecules as well as desorption of reduced product from the catalyst surface, making it a three-phase interface [125]. This kind of three-phase interface is also easily generated by using porous catalytic systems. Yeng and coworkers have developed a NiSA/PCFM catalyst by supporting the Ni single atoms on the porous carbon-based nanofibrous membranes via electrospinning method, establishing an extremely stable three-phase interface for electrocatalytic reduction of CO2 [126]. The designed porous architecture with well distributed catalytic sites generated an integrated membrane with substantial channels allowing CO2 diffusion and electron transport yielding the Faradaic efficiency of 88% and partial current density of 308.4 mA cm−2 for up to 120 h for electrocatalytic reduction of CO2 to CO. Pan et al. developed an atomically dispersed CO-N5 site anchored via N-coordination strategy on the polymer derived hollow N-doped porous carbon spheres which exhibited nearly 100% selectivity for electrochemical reduction of CO2 to CO at −0.73 and −0.79 V [127]. Pan et al. have developed atomically dispersed iron-nitrogen sites by oxidation-­induced partial unzipping of CNTs, thereby generating GNR nanolayers which remain attached to the remaining fibrous CNT frameworks resulting in a hierarchical mesoporous network possessing high electrochemical active surface area. The anchoring of the Fe-N4 sites on CNT and graphene nanoribbon (GNR) yields a Fe-N/CNT@GNR catalytic system for highly selective electrocatalytic reduction of CO2 to CO with Faradaic efficiency of 96% at a partial current density of 22.6 mA cm−2 and low overpotential of 650 mV [128]. Tang and coworkers studied the mechanism of transition metal-based single-atom catalysts anchored on nitrogen-doped porous carbon, M–N–C (M = Ni, Fe, Co, and Cu) for electrocatalytic reduction of CO2 [129]. The study depicted that the Ni-based electrocatalyst exhibited the highest Faradaic efficiency of up to > 97% for CO formation followed by iron-based nanocatalyst with Faradaic efficiency of 86.8%. The observed Faradaic efficiencies of Ni- and Fe-based catalytic systems might be accounted for the moderate and strong binding of *CO, with the respective metal-based catalytic sites. The strong binding of *CO by Fe-based catalytic system enabled the further reduction of adsorbed CO to more reduced products. The electrocatalysts based on iron metal have been shown to possess selectivity toward acetic acid. As, R. Arrigo and coworkers studied the electrocatalytic reduction of carbon dioxide by anchoring the nanostructured iron (III) oxyhydroxide-based catalyst on nitrogen-doped carbon support with the help of DFT simulations, electron microscopy and operando X-ray spectroscopic techniques [130]. The fabricated catalytic system yielded acetic acid at very low potential with

Functionalized Nanomaterials  381 Faradaic efficiency and selectivity of 97.4% and 61%, respectively, which was explained by the formation of nitrogen-coordinated iron (II) sites as single/polyatomic catalytic centers.

13.3.2.1.4 Macrocyclic Transition Metal Complexes

Since the process of electrocatalytic reduction resembles natural photosynthesis, it was thought to mimic the chlorophyll (a natural catalyst for converting CO2 in presence of sunlight) kind of architecture for designing the catalyst for the electroreduction of CO2. The presence of porphyrins in chlorophyll pigment aroused the curiosity of scientists for the macrocyclic ligands possessing a highly conjugated surface. The phthalocyanines and porphyrins are among the macrocyclic ligands explored initially for studying their electrocatalytic potential by coordinating with transition metals having variable oxidation states. Although, the first report for studying the electrocatalytic potential of cobalt phthalocyanines for electroreduction of CO2 appeared around 1974 when Meshitsuka and coworkers adsorbed these on graphite electrodes in aqueous electrolytes yet they failed to identify the product [131]. Lieber et al. continued the above approach by adsorbing cobalt phthalocyanine on carbon cloth and got success in electrocatalytic reduction of CO2 to CO with a Faradaic efficiency of 50% in 1984 [132]. The incorporation of cobalt porphyrins into the porous covalent organic frameworks changed their electronic structure, thereby increasing both the catalytic efficiency and stability of the catalyst for the electroreduction of CO2 to CO in water [133]. In further attempts, the effect of nanostructured changes in the catalytic species was exploited by anchoring the functionalized electrocatalyst on the conducting surface of nanoscopic materials like CNTs and graphene by utilizing the strong π-π interactions between the functionalized electrocatalyst and nanoscopic surface [134]. These conducting surfaces not only facilitate the electron transfer process but also provide increased surface area and high mechanical strength. The grafting of metal coordinated macrocycles on the surface of conducting CNTs was achieved both covalently as well as by utilizing the non-covalent interactions [135]. The molecular phthalocyanine grafted electrocatalytic species were also reported to undergo aggregation due to their lower stability and durability however attempts were also made to solve this problem by using the polymerized phthalocyanine instead of molecular one [136]. In one such attempt, Han et al. developed electrocatalytic species by sheathing CNTs with cobalt polyphthalocyanine (CoPPc) for electrocatalytic reduction of CO2 to Co with 90% Faradaic efficiency and much enhanced stability [137].

382  Functionalized Nanomaterials for Catalytic Application The authors utilized the template-directed polymerization for preparing cobalt polyphthalocyanine and achieving its thin and uniform polymer coating around CNTs. Beside these, in a recent report, Zhang et al. have developed highly dispersed nickel phthalocyanines on CNTs by molecular engineering which yielded >99.5% selectivity for electrocatalytic reduction of CO2 to CO along with excellent stability [138].

13.3.2.2 Hydrogen Evolution Reaction Hydrogen is one of the most important chemical fuels which can be utilized directly or can be converted into electrical energy via fuel cells besides various other applications. The production of hydrogen from electrolysis of water in presence of nanocatalysts has received significant attention during the last few decades. Jang et al. have recently reported the highly efficient 2D electrocatalytic interface for achieving the high utilization efficiency of platinum toward HER [139]. The 2D electrocatalytic interface was generated by the lateral growth of (2D)-Pt with nanometer thin and nanoporous structure on 2D nanosheets of NiFe-hydroxide laminates. The imposed lamellar nanoplatform in Pt/M(OH)x-based electrocatalyst not only exhibited enhanced HER potential but also with increased stability. Zhang and coauthors studied the effects of functionalizing the Pd-based nanocatalysts on the emerging two-dimensional high-performance electrode material, MXene (transition metal carbides/nitrides/carbonitrides), Nb2C-Tx in this case, thereby resulting in Pd/Nb2C-Tx [140]. The developed catalytic systems were then explored for studying the HER potential and compared with the same catalytic species without MXene functionalization. The electronic distribution of Pd-based nanocatalysts was modulated by tuning the surface functional groups and it was revealed that higher functional group density in Pd/Nb2C-Tx lowers the charge-transfer resistance, thereby weakening the hydrogen adsorption which ultimately leads to enhance HER potential. Song and coworkers developed highly efficient electrocatalysts by doping single-atom Ru into the metal vacancies of nickel hydroxides followed by phosphorization resulting in Ni5P4-Ru toward alkaline water electrolysis for enhanced hydrogen production via localized structure polarization [141].

13.3.2.3 Fuel Cells Fuel cells are the electrochemical cells converting chemical energy of hydrogen/hydrogen-based fuels in presence of oxygen into electrical energy generating only water as a byproduct. Since the invention in

Functionalized Nanomaterials  383 1839, the generation of electrical energy via hydrogen fuel cells have not only revolutionized the transport sector but also contributed toward the greener and cleaner environment by cutting down the harmful emissions (CO2, CO, and unburnt carbon particles). The advancements in the fuel cell technology in the last few years have increased the potential of fuel cells for storing the excess wind and solar energy into electrical energy [142]. The diagrammatic illustration of the polymer electrolyte membrane fuel cell (PEMFC) is shown in Figure 13.4. In the PEMFCs, the oxidation of the fuel takes place at anode releasing protons and electrons which are then transported to cathode via polymer electrolyte membrane and external circuit, respectively. At the cathode, the reached electrons reduce the oxygen via ORR. The reactions at their respective electrodes generate a potential difference among them which is further facilitated by the catalytic role played by coating of nanocatalytic species over carbon electrodes. Platinum metal is one of the most commonly used metals for designing catalytic species, used in the petrochemical industry and various industrial processes since the 19th century yet nanostructured platinum-based catalysts have revolutionized the field of energy transformation via fuel cells [143]. The major limitations in

POLYMER ELECTROLYTE MEMBRANE (PEM) FUEL CELL

- HYDROGEN H2 - PROTONS H+ - OXYGEN O2

FLOW OF CURRENT

- ½ OXYGEN O2

GAS DIFFUSION LAYER

GAS DIFFUSION LAYER

FUEL CELL ANODE (–)

CATHODE (+)

POLYMER ELECTROLYTE MEMBRANE (PEM)

Figure 13.4  Illustration of modern polymer electrolyte membrane fuel cell (copyright and permission from ref 142).

384  Functionalized Nanomaterials for Catalytic Application designing the platinum-based catalytic systems are the high cost of metal as well as changes in the catalytic properties during fuel cell functioning due to dissolution, aggregation, and Ostwald ripening [144]. However, the recently developed core-shell structured catalytic systems with cores of less expensive metals and shells of platinum not only lower down the cost of the catalyst but also improve its catalytic potential. In addition to above, this limitation is also overcome to a certain extent by supporting the Pt-based nanocatalysts on efficient supports which either retain or increase their catalytic activity. CNTs (both single and multiwalled) possessing higher surface area, good conductivity, and excellent chemical and electrochemical stability are most commonly used support materials for nanoparticles of Pt as well as other noble metals. Su and coworkers have shown that dispersion of Pt nanoparticles on the surface of multiwalled CNTs (MWCNTs), functionalized with thiolated polyaniline enhanced the electrocatalytic oxidation of methanol under acidic conditions as compared to same catalytic species dispersed on the surface of unmodified/differently modified MWCNTs [145]. The functionalization of MWCNTs surface with thiolated polyaniline was achieved by coupling of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) with oxidized polyaniline (PANI) utilizing the microwave-assisted thiol-ene reaction resulting in thiolated polyaniline modified MWCNTs (TPANI-MWCNTs) as a nanocomposite material. The dispersion of Pt nanoparticles on the TPANIMWCNTs was also achieved under microwave conditions resulting in nanohybrids of PtNPs/TPANI-MWCNT. The modification of glassy carbon electrode (GCE) with PtNPs/TPANI-MWCNTs provided PtNPs/ PANI-MWCNTs/GCE with larger surface area. The higher electrocatalytic activity and stability of PtNPs/TPANI-MWCNTs/GCE toward oxidation of methanol as compared to PtNPs/PANI-MWCNTs/GCE and PtNPs/ MWCNTs/GCE was revealed by cyclic voltammetry and thermo gravimetric analysis, respectively. The higher stability of PtNPs/TPANI-MWCNTs/ GCE was also depicted by the 86% retention in its electrocatalytic activity even after going through 1,000-cycle CV under acidic conditions. He et al. have shown that the synergistic effects of porous carbon support, homogeneous distribution of gold (trace amount) in Au-PtFe/porous carbon catalyst, with a highly ordered intermetallic structure exhibited enhanced activity for electrocatalytic reduction of oxygen [146]. The nine times higher electrocatalytic potential and higher durability even in acidic conditions for ORR as compared to the commercial catalyst (Pt/C) not only improved the process conversion but could also help in lowering its overall cost. Li et al. reported the synthesis of highly efficient electrocatalysts based on iron-platinum (FePt) alloy for ORR and HER under acidic

Co-existing cobalt and cobalt oxides

8.

(Continued)

Gao et al. [111]

Cu nanocatalysts

7.

Reduction of CO2 to formate

Cu2O-derived Cu-based nanocrystals

6.

Cu nanowires with graphene oxide

Jeon et al. [110]

Li et al. [108]

CO2 reduction to hydrocarbons

Cu nanowires

5.

Cu nanoparticles anchored over electrodes

Reduction of CO2 to ethylene

MerinoGarcia et al. [106]

Reduction of CO2 toward different products

Cu nanoparticles

4.

Supported on carbon black

Li et al. [109]

Zhang et al. [99]

Reduction of CO2 to formate

SnO2 nanocrystals

3.

Reduction of CO to carbon oxygenated liquid fuels (ethanol, n-propanol and acetate)

Zhang et al. [97]

NPs are uniformly deposited on carbon support Conversion of CO2 to C2+ species

Ref.

Sn-based catalysts

2.

Applications Gao et al. [96]

Pd nanoparticles

1.

Functionalization Reduction of CO2

Nanocatalyst

S. No.

Table 13.2  Electro catalysis.

Functionalized Nanomaterials  385

Size-dependent activity

Plasma-activated Cu catalyst

Cu nanoparticles

Cu nanocube

Dendritic Cu catalyst

10.

11.

12.

13.

CO2 reduction

Atomically dispersed nickel on nitrogenated graphene

Pore size of 100–200 nm

Single atom Ni(I) catalyst

Nanoporous Cu

Ni single-atom

Single-Atom Co–N5

Atomically Dispersed Iron–Nitrogen

14.

15.

16.

17.

Pan et al. [127] Reduction of CO2 to CO Reduction of CO2 to CO

Anchored on hollow N-doped porous carbon spheres

Fe-N4 sites supported on hierarchical carbon nanotube and graphene nanoribbon networks

(Continued)

Pan et al. [128]

Yang et al. [126] Reduction of CO2 to CO

decorated on porous carbon membrane

Lv et al. [125]

Reduction of CO2 to CO

Yang et al. [121]

Scholten et al. [117]

CO2 reduction to hydrocarbons and alcohols

Dendritic Cu catalysts supported on Ag and Pt

Reske et al. [114]

Reduction of CO2 to hydrocarbons

Gao et al. [116]

Mistry et al. [113]

Ref.

CO2 reduction to ethylene

Applications

CO2 reduction to hydrocarbons and alcohols

Plasma-activated Cu catalyst

Nanostructured Cu(I) catalysts

9.

Functionalization

Nanocatalyst

S. No.

Table 13.2  Electro catalysis. (Continued)

386  Functionalized Nanomaterials for Catalytic Application

Covalent organic frameworks

CNTs with cobalt polyphthalocyanine

Co porphyrin

Co polyphtha­ locyanine on CNTs

Ni phthalocyanines on CNTs

(2D)-Pt on NiFe-LDH nanosheets

Pd nanocatalysts

Single-atom Ru

Pt nanoparticles

19.

20.

21.

22.

23.

24.

25.

Oxidation of methanol

Dispersion of Pt nanoparticles thiolated polyaniline modified MWCNTs

(Continued)

Su et al. [145]

He et al. [141]

Hydrogen evolution

Jang et al. [139]

Hydrogen evolution

Doping single-atom Ru into the metal vacancies of nickel hydroxides followed by phosphorization (Ni5P4-Ru)

Zhang et al. [138]

Reduction of CO2 to CO

Zhang et al. [140]

Han et al. [137]

Lin et al. [133]

Reduction of CO2 to CO Reduction of CO2 to CO

Menisa et al. [129]

Ref.

Reduction of CO2 to CO

Applications

Hydrogen evolution

Pd/Nb2C be modified by different surface functional groups

CNTs with nickel phthalocyanines

M-N4 structure supported on carbon

M-N-C electrocatalysts

18.

Functionalization

Nanocatalyst

S. No.

Table 13.2  Electro catalysis. (Continued)

Functionalized Nanomaterials  387

Electrodeposition of Pd nanoparticles on SWNTs

Mo-Bi bimetallic chalcogenide

PtPb/Pt core/shell nanoplate catalysts

Pd nanoparticles

Nanosheets

Iridium Oxide Nanoparticles

Pt-loaded TiC film

28.

29.

30.

31.

32.

33.

34.

Oxygen reduction reaction

Oxidized multiwalled CNTs

Reduced graphene oxide

Cobalt oxide/carbon nanotube

Cu2O/RGO composite

Oxygen reduction reaction

Support Pt catalyst for methanol electrooxidation

Highly active for water oxidation

Efficient production of methanol by electrochemical reduction of CO2.

Hydrazine oxidation

Oxygen reduction reaction

Pt nanoparticles

Deposited on glassy carbon electrodes

Exhibit large biaxial strains

Oxygen reduction reaction and hydrogen evolution reaction

Coated with MgO

Dumbbell-like fccFePt-Fe3O4 NPs

27.

Oxygen reduction reaction

Applications

Entrapped in a porous carbon

Au-PtFe particles

26.

Functionalization

Nanocatalyst

S. No.

Table 13.2  Electro catalysis. (Continued)

Yan et al. [153]

Liang et al. [152]

Ou et al. [151]

Zhao et al. [150]

Sun et al. [149]

Guo et al. [148]

Bu et al. [143]

Li et al. [147]

He et al. [146]

Ref.

388  Functionalized Nanomaterials for Catalytic Application

Functionalized Nanomaterials  389 conditions by removing the MgO from the dumbbell-like MgO coated FePt-Fe3O4 nanoparticles by thermal annealing and further washings [147]. The synthesized catalytic species were also found to have strong ferromagnetism as a result of highly ordered face-centered tetragonal (fct) arrangement. Bu et al. produced core/shell nanoplate catalysts (PtPb/Pt), comprising PtPb-based central intermetallic core surrounded uniformly by four layers of Pt as a shell for catalyzing the ORR. The developed coreshell architecture-based catalytic nanoplates were found to have stable ORR specific Pt (110) facets at the surface along with large biaxial tensile strains which assisted in strengthening of Pt-O bond. The high stability of the designed catalytic system was reflected in retaining its catalytic activity and structure even after 50,000 voltage cycles. Guo et al. have depicted that the electrodeposition of Pd nanoparticles on SWNTs not only increased their catalytic activity toward hydrazine oxidation but also cut down the loading of precious metal catalysts, thereby minimizing the process cost [148]. The electrocatalytic applications of some nanocatalysts with the functionalization are summarized in Table 13.2.

13.3.3 Photocatalysis Nano-photocatalysis involves the use of nanocatalysts in presence of light for catalyzing various chemical transformations. The nanostructured engineering allows the tuning of different types of activities like chemical, electrical, magnetical, and optical by changing various parameters is further enhanced owing to magnifying effects of quantum size [154]. The nano photocatalysts can increase the rate of advanced oxidation processes due to their increased surface area and unique electronic structure for photocatalysis [155]. MNPs, nano-sized bimetallic compounds, and various nanostructured semiconductors are some of the most extensively used nanomaterials for removing water pollution [156]. Although a large number of nanostructured metal oxides such as ZnO, TiO2, Al2O3, and SiO2 are used as photocatalysts, yet TiO2 is most extensively explored owing to its low cost, stability, and availability [157]. The photocatalyst chosen for photocatalytic conversions must possess some necessary requirements. The foremost requirement for a photocatalyst is the adequate energy gap between valence band and conduction band so that source of radiation can efficiently transfer electrons from valence band to the conduction band. As a result of this, a hole is generated in the valence band and a negative charge in the conduction band. These holes and electrons further initiate redox reactions, generating other reactive species (free radicals), thus catalyzing the photocatalytic conversions. The nanostructured titanium dioxide (TiO2) in the size domain

390  Functionalized Nanomaterials for Catalytic Application 10–20 nm is recognized as an excellent photocatalyst [158]. Among various forms of TiO2, its anastase form is most efficient [159]. TiO2 possesses good photocatalytic potential owing to its resistance toward photo-corrosion and various chemicals, low cost, insolubility in water, good quantum yields, etc [160]. In spite of these advantageous properties, pure TiO2 can’t be activated in visible light and require UV light at λ ≤ 380 nm due to its band gap of >3.0 eV [161]. The incorporation/doping of other metal ions/metal oxides/ carbon allotropes into the nanostructured TiO2 increases its photocatalytic efficiency via reduction of band gap and slowing down of recombination rates of e−-h+ pairs [162]. The doping also generates energy states in between the band gap, thereby enabling its activation and subsequent photocatalytic conversion in visible light region [163]. Further, the immobilization of the photocatalysts in polymeric membranes generates the photocatalytic-membrane reactors which enable recovery of the photocatalyst post catalytic conversion, thereby increasing its efficiency [164]. The polymeric membranes selected for designing these photocatalytic membrane reactors should be stable enough to avoid ultraviolet and oxidative degradation. Among the various studied polymeric membrane architectures, the polytetrafluoroethylene, and polyvinylidene fluoride are found to be the most stable. The major disadvantage of these photocatalytic membrane reactors is their lower stability leading to the degradation of its skeleton caused by close and direct contact between photocatalysts and polymeric membranes during the photocatalytic process. Besides the composition of the polymeric membrane, the selection of suitable pH values and formation of a protection layer in between membrane and photocatalyst during photocatalytic conversion avoids close interactions also play a determining role in reducing the deteriorating effects of polymeric membrane [165]. Attenuated total reflection-Fourier transform infrared (ATRFTIR) spectroscopy helps in studying the chemical changes to the skeleton of the polymeric membrane. The stability of the porous polymeric membranes toward mechanical degradation and fouling resistance could also be increased by the use of a composite layer which is generally formed by dispersing carbon-based material (graphene oxide/CNT) into the porous matrix [166]. The photocatalytic applications of nanostructured photocatalysts for degradation of wastewater, reduction of CO2 into useful chemicals and hydrogen evolution from water have received significant attention in the last few decades and some of the progress toward these conversions is discussed here. These photocatalytic applications involve the most advanced oxidation processes in presence of a suitable photocatalyst and light source and their efficiency increases especially when it is mediated via solar light.

Functionalized Nanomaterials  391

13.3.3.1 Photocatalytic Treatment of Wastewater The increasing water pollution as a result of inadequate treatment and improper disposal of industrial, agricultural, sewage and radioactive wastes, etc., is a critical issue of high concern causing adverse impact on environment and human health. The search for the development of highly efficient catalytic systems for resolving the problem of water pollution has gained immense attention and researchers are continuously making efforts. The use of nanostructured photocatalysts including nanostructured catalytic membranes and nanosorbents for purifying polluted/wastewater holds a great promise owing to their eco-friendly nature [167]. A photocatalyst on irradiation with suitable energy light in presence of water generates radicals of hydroxyl and radical anions of superoxide. These free-radicals then further oxidize various organic pollutants in water as these are very powerful oxidizing agents. The nanostructured TiO2 is commonly used for photodegradation of a wide series of compounds which include—halogenated hydrocarbons, aromatics, surfactants, herbicides, toxic metal ions, and various others [168]. Kerkez et al. depicted that doping of Cu2+ in TiO2 nanorod films increases their photocatalytic efficiency by 40% toward methylene blue degradation efficiency under visible light irradiation [169]. The doping of the nanostructured TiO2 with CdS yielded composite materials (CdS/TiO2) which revealed high photocatalytic activity in the visible region toward water purification by Chen and coworkers [170]. Although the use of polymeric nanocomposite membranes in designing highly efficient photocatalysts have also contributed notable improvements in the water purification process [171], yet the search of polymeric membrane architectures with better stability and integrity under photocatalytic conditions is still going on.

13.3.3.2 Photocatalytic Conversion of CO2 Into Fuels With the increasing CO2 emissions and depleting conventional fossil fuels, the conversion of CO2 into fuels and other value added compounds hold multiple benefits. Among various methods of CO2 reduction to fuels/useful chemicals, the photocatalytic conversion holds its foremost place due to its strong resemblance to photosynthesis. The photocatalytic reduction of CO2 into various fuels requires solar light, CO2, H2O, and a photocatalyst. Since the remarkable research work by Honda and coauthors in 1979 for CO2 conversion to formic acid and methanol via photocatalytic pathway, this field keeps on growing significantly in search of developing more efficient photocatalysts [172]. The major hurdle in CO2 reduction is its high

392  Functionalized Nanomaterials for Catalytic Application stability and multiple reduction pathways. In the last two decades, various semiconductors, transition metal chalcogenides [173], MXenes (Ti3C2) [174], MOFs [175], inorganic perovskite halides [176], metal free catalysts (graphene/graphene oxide/g-C3N4/others) [177], and nanostructured composite materials have been extensively explored for studying their photocatalytic potential for reducing CO2. The activity of the photocatalysts is also known to be enhanced by generating surface defects leading to increased interactions. These increased interactions further allow better adsorption and activation of CO2, thereby facilitating its reduction [178]. Also, the doping with other species, supporting on various conducting surfaces and usage of porous membrane [179] reactors [180], are other frequently used techniques for increasing the photocatalytic efficiency for CO2 reduction. Hsu and coauthors reported the photocatalytic reduction of CO2 to methanol in presence of graphene oxide at the rate of 0.172 micromolgcat−1 h−1 which is almost six times higher than a commercial photocatalyst containing TiO2 dispersions (commercial P25, Degussa) [181].

13.3.3.3 Photocatalytic Hydrogen Evolution From Water The requirement for hydrogen is continuously increasing as renewable and green fuel with the depleting fossil fuel resources. The hydrogen can be directly used or via fuel cells. The advantage of this reaction lies in utilization of excess renewable energy sources (solar and wind energy) and generation of hydrogen which can be stored and used in fulfilling the future energy requirements. Platinum is the most commonly used metal in designing various photocatalysts and its structural nanoengineering further allows further improvements in its catalytic potential. The functionalization of various photocatalysts with suitable capping agents and its influence on photocatalytic potential for hydrogen evolution has been reported by various research groups. Kiwi and coworkers studied the effect of colloidal dispersions of platinum as reduction catalysts on the visible light induced hydrogen evolution in presence of Ru( bpy )32+ as photosensitizer and EDTA as an electron donor and methyl viologen as electron mediator [182]. Hirakawa and coworkers applied various polymer-­protected Au, Pt, Pd, Rh, and Ru monometallic, and Au/Pt, Au/Pd, Au/Rh, and Pt/Ru bimetallic nanocluster catalysts and studied the rates of electron transfer kinetics for visible light induced hydrogen evolution under similar conditions [183]. The authors found higher hydrogen generation potential for the dispersions of bimetallic nanoclusters as compared to monometallic species which could be attributed to the higher electron transfer rate from methyl

Photodegradation of methyl orange under visible light irradiation Degradation of reactive brilliant red X-3B in an aqueous solution under solar irradiation

Conjugated with flavin

Prepared by liquid catalytic phase transformation and the sol-gel method

N-doped titania-coated γ-Fe2O3 magnetic activated carbon

Iron oxide nanoparticles

Polydopamine nanoparticles

TiO2/ZnFe2O4 powder

N-doped titania-coated γ-Fe2O3 magnetic activated carbon

4.

5.

6.

7.

Conjugated with flavin

Conjugated with flavin

Photooxidation and photoreduction

Azo dye degradation

Monooxygenase activity

Polydopamine nanoparticles

3.

(Continued)

Ao et al. [189]

Xu et al. [188]

Crocker et al. [187]

Nehme et al. [186]

Crocker et al. [185]

Hsu et al. [181]

Reduction of CO2 to methanol

Graphene oxide

2.

Kerkez et al. [169]

Methylene blue degradation

Doping of Cu in TiO2 nanorod

TiO2 nanorod

1.

Ref.

Applications

2+

Functionalization

Nanocatalyst

S. No.

Table 13.3  Photo catalysis.

Functionalized Nanomaterials  393

Nanowires exhibit strong UV emissions Degradation of amaranth and Brilliant Blue FCF dyes Degradation of amaranth Reduction of nitroarenes

Doped with tungsten

Immobilized on inert support

Supported on ZrO2

Carbon-doped titanium dioxide

Strontium-doped zinc oxide nanoparticles

ZnO nanowires

TiO2 nanoparticles

TiO2 nanoparticles

Au nanoparticles

10.

11.

12.

13.

14.

Hexagonal wurtzite structure

Supported onto filter paper

Photocatalyst to remove methylene blue

Photocatalyzes the gas-phase degradation of the atmospheric pollutants

Recyclable photocatalyst for wastewater treatment

9.

Fe3O4/SiO2/TiO2 core-shell photocatalysts

Core-shell structured

Applications

8.

Functionalization

Nanocatalyst

S. No.

Table 13.3  Photo catalysis. (Continued)

(Continued)

Zhu et al. [196]

Kumar et al. [195]

Shahmoradi et al. [194]

Wang et al. [193]

Yousefi et al. [192]

Sakthivel et al. [191]

Ye et al. [190]

Ref.

394  Functionalized Nanomaterials for Catalytic Application

Nanocatalyst

Ag nanoparticles

Au nanoparticles

Au nanoparticles

Pt, Pd and Au catalysts

CNT/TiO2 nanocomposites

CNT/TiO2 nanocomposites

S. No.

15.

16.

17.

18.

19.

20.

Degradation of methylene blue

Removal of various organic pollutants

TiO2 incorporated with MWCNT

CNT/TiO2 nanocomposites

Cao et al. [202]

Wongaree et al. [201]

Yang et al. [200]

Zhang et al. [199]

Oxidation of aromatic alcohols Hydrochlorination of olefin

Zhu et al. [198]

Christopher et al. [197]

Ref.

Oxidation of synthetic dye, phenol degradation and selective oxidation of benzyl alcohol

Catalytic oxidations reactions such as ethylene epoxidation, CO oxidation and NH3 oxidation

Applications

Supported on zirconia

Supported on zeolite support

Supported on ZrO2, SiO2, and zeolite Y

Functionalization

Table 13.3  Photo catalysis. (Continued)

Functionalized Nanomaterials  395

396  Functionalized Nanomaterials for Catalytic Application viologen cation radical to the bimetallic nanocluster. Oshima and coworkers demonstrated 420-nm visible-light driven splitting of water using Pt-intercalated HCa2Nb3O10 nanosheets, modified with amorphous Al2O3 clusters, which were photo-sensitized by Ru(II) tris-diimine dye along with a triiodide/iodide redox couple and WO3-based photocatalyst. This solar driven and dye sensitized water splitting resulted in a quantum yield of 2.4% depicting the huge potential of hybrid photocatalyst for H2 production [184]. The photocatalytic applications of some nanocatalysts are discussed in Table 13.3.

13.3.4 Conversion of Biomass Into Fuels The biomass from wood, crop residues, organic wastes, etc., need to be processed before using it as fuel energy due to the presence of low hydrogen carbon ratio and high moisture content. The conversion of biomass into fuel could be achieved either by thermochemical or biochemical processes. The thermochemical processing of biomass into fuels is generally performed in the presence of various catalytic species; however, use of nanocatalysts in these conversions not only increased the efficiency of the process but also enhanced the quality of the fuels. Gasification and pyrolysis of biomasses require nanocatalysts for efficient conversion rates [203]. Gasification of biomass involves preparation of syngas or producer gas along with formation of char and tar as by-products. Tar being a highly condensed mixture of aromatics and hydrocarbons with high boiling point is known to hinder the gasification process by blocking filters, pipes as well as poisoning of the catalyst [204]. The use of nanocatalysts for treating effluent from gasification via hot gases treatment reduces the tar formation [205]. Arandiyan and coworkers have also depicted that nanocatalysts reduce the fraction of CO in the syngas, thereby increasing its quality [206]. Also, nanocatalysts have been extensively used for the production of diesel from biomass. The biodiesel can either be produced via trans-esterification process to yield fatty acid methyl esters or via Fischer-Tropsch process. The use of nanocatalysts in trans-esterification esterification reaction allows proper mixing of reactants due to accelerated mass transfer as well as the facilitated separation [207]. The metal oxide-based nanocatalysts, nanohydrotalcites, nanozeolites, and magnetic composite nanomaterials are some of the most promising catalytic species for diesel production. Some of the methods for the production of diesel via trans-esterification are discussed here. Prado and coauthors designed nano-sized SiO2/ZrO2 nanocatalyst depicting high catalytic activity for biodiesel production with a yield of about 96.2 ± 1.4% from soybean oil and reusability up to six times [208].

Functionalized Nanomaterials  397 In  another report, Piraman and coworkers have also depicted that TiO2ZnO mixed oxide nanocatalyst provides biodiesel production from palm oil with a yield of 92% [209]. The nanohydrotalcite (nanostructured anionic clays of double metal hydroxides) represent another class of nanocatalysts explored for production of biodiesel. Deng et al. reported production of biodiesel with a yield of 95.2% using hydrotalcite-derived nanoparticles with Mg/Al oxides from transesterification of Jatropha oil [210]. Nanozeolites are the hydrophobic support materials with better dispersibility which are used for immobilizing catalytic enzymes. The resulting nanozeolite-enzyme complex offers advantages of increased stability and reusability over multiple cycles leading to increased catalytic efficiency. Nery and coworkers studied the immobilization of lipase enzyme to the glutaraldehyde crosslinked nanozeolites functionalized with (3-aminopropyl)trimethoxysilane for biodiesel production from microalgal oil [211]. The results revealed the yield of more than 93% for the transesterification which is higher than non-immobilized enzymes, and allowing reusability up to five cycles without any significant loss in catalytic activity. Zillillah and coauthors reported preparation of biodiesel using Phosphotungstic acid-functionalized magnetic nanoparticles, HPW-PGMA-MNPs providing biodiesel yield of 98% and reusability up to 10 cycles [212]. The HPW-PGMA-MNPs consisted of a magnetic core formed by iron oxide (MNPs) surrounded by poly(glycidyl methacrylate) (PGMA) shell with a surface coating of phosphotungstic acid, HPW. Diesel can also be prepared from biomass via Fischer-Tropsch process which is one of the most important processes for converting biomass and coal-based petroleum products into a mixture of linear and branched hydrocarbons by utilizing various catalytic processes. The major advantage of Fischer-Tropsch process lies in its potential to generate high quality fuel, free from sulphur and with a very low aromatic content, even economically [213]. Among the various catalytic species utilized for synthesis of diesel via Fischer-Tropsch process, the nanocatalysts of iron and Co in between 10 and 15 nm have highly improved the catalytic yield [214]. The selectivity of the Fischer-Tropsch process toward C10-C20 hydrocarbons has been achieved via anchoring of Co-based nanocatalysts over porous silica support [215]. The ruthenium-based catalysts supported over CNTs have been reported to provide enhanced selectivity even in presence of little moisture for the diesel hydrocarbons [216].

13.3.5 Other Applications Besides these applications, nanocatalysts are also used for miscellaneous applications in various fields [217]. Nanocatalysts are used in oil refineries

398  Functionalized Nanomaterials for Catalytic Application for catalyzing naphtha reforming, paraffin hydrogenation and hydrodesulfurization, etc. Nanocatalysts are also used for increasing the performance of various fuels by lowering down their ignition temperature [218]. Also the nanoengineered catalysts are also used in photoelectrochemical conversions for CO2 reduction as well as in fuel cells.

13.4 Conclusions The applications of the nanoengineered catalytic species have highly revolutionized the energy and pharmaceutical sector among others. The recent developments in various characterization techniques have achieved tremendous growth which enabled researchers to prepare highly efficient nanocatalysts with well-controlled shape, size, morphology, and electronic structure with improved activity and selectivity. Here in, we have highlighted the catalytic applications of nanomaterials for electrocatalytic, photocatalysis, organic transformations, and for conversion of biomass into renewable fuels. The roles of functionalized nanocatalysts in improving various conversions are discussed for the most frequently used classes of nanocatalysts like MNPs, alloy and intermetallic compound–based nanocatalysts, magnetic nanocatalysts, single atom nanocatalysts, MOF-based nanocatalysts, and carbocatalysts.

13.4.1 Future Outlook Though a significant progress is achieved for exploring the catalytic applications of nanocatalytic materials, designing of the nanocatalyst with increased stability and reusability are some of the factors requiring further improvement. The understanding of the mechanical aspects at the molecular level of the catalytic conversion will further assist in suitable designing of the nanocatalyst.

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14 Carbon Dots: Emerging Green Nanoprobes and Their Diverse Applications Shweta Agarwal* and Sonika Bhatia Department of Chemistry, Isabella Thoburn College, Lucknow, India

Abstract

Carbon dots (CDs) are excellent fluorescent nanoprobes exhibiting tunable physical and chemical properties. These properties can be modified by surface functionalization. CDs are made of carbon, a building block of life, and abundantly available on earth. Their ease of synthesis, water solubility, non-toxicity, and low cost have given them an edge over fluorescent semiconductor nanoparticles. The present chapter gives an overview of environmentally sustainable preparations of CDs from biomass and biowastes, their distinct physicochemical properties, and diverse analytical and biological applications of CDs. Keywords:  Bioimaging, biosensing, CO2 reduction, hydrogen production, photocatalysis, photoinduced electron transfer, photoluminescence, wastewater treatment

14.1 Introduction Fluorescent carbon dots (CDs) belong to zero-dimensional carbon nano­ particles (CNPs) family and are graced with excellent water solubility, tunable excitation, emission properties, high biocompatibility, and low toxicity [1–3]. Additional advantages of CDs are simple synthesis, easy functionalization and modification, chemical stability, inert nature, and resistance to photobleaching and photoblinking. Owing to these unique and exceptional properties, CDs have attracted much attention world over and have found applications in areas of photocatalysis, bioimaging, biosensing, drug *Corresponding author: [email protected] Chaudhery Mustansar Hussain, Sudheesh K. Shukla and Bindu Mangla (eds.) Functionalized Nanomaterials for Catalytic Application, (417–492) © 2021 Scrivener Publishing LLC

417

418  Functionalized Nanomaterials for Catalytic Application delivery, energy production, and environmental safety [4–10]. In comparison to inorganic semiconductor quantum dots (SQDs), CDs exhibit enhanced biocompatibility, water solubility, negligible cytotoxicity, inexpensive synthesis from readily available materials, and resistance to photobleaching and thus have a potential to replace the SQDs [11, 12]. Fluorescent CDs were discovered accidentally in 2004 by Xu et al. in the course of purification of single-wall carbon nanotubes (SWCNTs) prepared from arc discharge method [13]. However, CDs started garnering interest of researchers in 2006, after Sun et al. reported first synthesis of stable and photoluminescent CNPs which he named carbon quantum dots (CQDs) [14]. Later, he reported the synthesis of water soluble CDs passivated with poly-propionylethylenimine-co-ethylenimine [15]. These CDs were used for detection of human breast cancer MCF-7 cells. CDs are nanoparticles of size less than 10 nm. They are composed of a carbon core and a shell of functional groups and/or aggregates of polymers. The term “CDs” has been comprehensively used for graphene quantum dots (GQDs), CQDs, carbon nanodots (CNDs), and carbon polymer dots (CPDs). The classification of CDs into different classes is based on the nature of carbon core (crystalline or amorphous), quantum confinement effect (QCE), and surface structure [16, 17]. Core of GQDs is composed of only sp2-conjugated carbon framework, whereas the core of other types of CDs comprises of varying proportions of sp2 and sp3 hybridized carbon atoms. The surface of CDs have oxygen containing hydroxyl, carbonyl, and carboxylic functional groups which account for their hydrophilicity, easy surface modification, and binding with inorganic and organic moieties. Numerous carbonaceous materials and various synthetic methods have been used for the synthesis of CDs. Synthetic approaches for CDs are broadly divided into two categories: top-down and bottom-up. Top-down approach involves pruning of large graphitic carbonaceous materials, whereas bottomup entails formation of nanosized materials from non-graphitic small organic molecules and polymers [18–20]. Conventionally, CDs were synthesized from expensive and non-­renewable carbon resources under harsh reaction conditions, which were neither environmentally benign nor sustainable. With increasing demand of CDs for a wide array of applications, inexpensive, abundant, and renewable precursors are required which produce minimum waste and require eco-friendly reaction conditions. Use of biomass including every plant part, animal products, and biowastes offers inexpensive and environmental sustainable synthesis of CDs based on the principles of green synthesis wherein zero-value materials are converted into functional materials [21–25]. Additionally, biomass rich in various heteroatoms such as N and S is ready raw material for the

Carbon Dots: Emerging Green Nanoprobes  419 synthesis of surface functionalized and heteroatom-doped CDs in one step without the need of post synthesis modifications [26, 27]. The size, structure, and properties of CDs can be controlled by reaction time, temperature, methodology adopted for synthesis, and the precursor used. Most of the applications of CDs are based on their tunable optical properties. Among various optical properties, photoluminescence (PL) is most fascinating [28]. Several mechanisms have been proposed to explain the origin of PL in CDs but the exact mechanism is still unclear. Similar to traditional QDs, the PL in GQD is observed due to the QCE which is a signature characteristic of QDs. CQD exhibits PL due to the QCE and surface states. On the other hand, size quantization is absent in CNDs and CPDs and light emission is from the fluorescent molecules attached on the surface rather than the entire dot [29, 30]. This chapter focuses on clear classification of CDs, environmentally sustainable synthetic routes for production of CDs from renewable and abundant carbon resources, microscopic and spectroscopic characterization techniques, and important optical properties of CDs. An overview of the varied applications of CDs in wastewater treatment, solar light–driven hydrogen production from water, carbon dioxide sequestration, and emerging biomedical applications are covered. The chapter concludes with a discussion on ethical, legal and sociological implications of CDs and the way forward.

14.2 Classification of Carbon Dots The basic term “nanodot” refers to nanometer-scale-sized particles with unique optical and electrical properties. The term “quantum dots” (QDs) was first proposed in 1988 by Mark Reed [1] for semiconductor nanoparticles with dimensions less than the exciton Bohr’s radius, which result in the observed QCE. At this stage, it is important to understand the term QCE. In Bohr’s atomic model, the principal quantum number is associated with the energy levels of electrons. These energy levels are discrete and have fixed energy. In solids, many atoms come in close proximity and their inner electrons remain localized. However, the electrons of valence shell overlap with neighboring atoms and form energy bands (valence band and conduction band) which are separated by band gaps. These band gaps resemble energy level gaps of various orbitals of an isolated atom. On absorption of appropriate amount of energy, the electron jumps from the valence band to the conduction band and leaves a vacancy called “hole”. This “electron-hole” pair together is called exciton and the distance between them is called Bohr’s exciton radius (nm). In the bulk material,

420  Functionalized Nanomaterials for Catalytic Application the energy levels are continuous and the electrons move randomly in all directions. However, if the size of the bulk material is reduced to the size of Bohr’s exciton radius (i.e., nanometer range), the energy levels are not continuous, and motion of electron is confined to energy levels. The band gap increases as the size of particle decreases. This is known as QCE. Due to QCE, the electron energy levels are quantized as in the isolated atom. Thus, by adjusting the size and shape of QDs, these energy levels and therefore the band gap can be tailored without any modification in the underlying material or synthesis process. Any zero-dimensional nanodot with quantum confinements may be referred as QD. Now, the term QD has been used for “semiconductor quantum dots” as well as for carbogenic nanodots with quantum-confined regimens called “carbon quantum dots”. In 2016, Cayuela et al. [16] classified CDs in three categories: GQDs, CQDs, and CNDs (also named carbon nanoclusters, polymer dots, or CDs). His classification was based on the precursors used for the synthesis, intrinsic structure, shape, amorphous or crystalline nature, and emission properties. Later, in 2019, Chunlei et al. [17] have proposed another class of CDs called “Carbonized Polymer Dots” (CPDs) which have a polymeric/ carbon hybrid structure, instead of a carbon core. Thus, the umbrella term CD now includes: GQDs, CQDs, CNDs, and CPDs. This classification is based on size, shape, intrinsic structure, and surface groups (Figure 14.1). The GQDs consist of single or multiple sheets (200°C temperature is usually required. Importantly, biomaterials and biowastes are rich in N, S, and P; therefore, heteroatom-doped CDs with improved optical properties and quantum yields (QYs) can be obtained in a single step without the need of further doping, passivation, and functionalization steps. Use of biomass and biowastes as precursors is associated with waste minimization and waste management, thus reducing the load of environmental pollution. Following modified bottom-up approaches have been used for green synthesis of CDs from biomaterials and biowastes.

Method

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Precursor

Orange juice

Sugarcane juice

Coffee ground

Sweet pepper

Pomelo peel

Bamboo leaves

Milk

Onion waste

Garlic

200°C, 3 h, centrifugation, filtration, dialysis

Water extract, ethylene diamine, 180°C, 15 lbs pressure, 2 h, centrifugation, filtration, dialysis

180°C, 2 h, filtration

200°C, 6 h, filtration

200°C, 3 h, centrifugation

180°C, 5 h, centrifugation, dialysis

90°C, vigorous stirring, centrifugation, filtration

120°C, 3 h, extraction with dichloromethane, centrifugation

120°C, 2.5 h, extraction with dichloromethane, centrifugation

Reaction conditions

11

9

3

2–6

2–4

2–7

4.4

3

1.5–4.5

Size (nm)

17.5

28

12.0

7.1

6.9

19.3

5.5

5.7

26.0

QY (%)

Bioimaging, free radical scavenging

Bioimaging

Bioimaging

Detection of Cu2+

Detection of Hg2+

Bioimaging

Bioimaging

Bioimaging

Bioimaging

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes.

(Continued)

[61]

[87]

[60]

[86]

[85]

[84]

[83]

[82]

[81]

Ref.

426  Functionalized Nanomaterials for Catalytic Application

Method

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Hydrothermal

Precursor

Garlic

Apple juice

Oatmeal

Aloe

Coriander leaves

Paper waste

Ginger

Sugarcane molasses

Bagasse waste

180°C, 12 h, centrifugation, dialysis, vacuum-dried

250°C, 12 h, filtration, centrifugation, lyophilization

300°C, 2 h, centrifugation, dialysis

Sonication 30 min, 180°C, 10 h, centrifugation, dialysis

240°C, 4 h, filtration

180°C, 11 h, filtration

200°C, 3 h, centrifugation, filtration, dialysis

150°C, 12 h, filtration, washed with dichloromethane, centrifugation, dialysis

200°C, 6 h, centrifugation, dialysis, passivation with ethylene diamine

Reaction conditions

5.0

1.9

4.3 ± 0.8

3-7

1.5–2.98

5

20–40

4.5

1–3

Size (nm)

11.8

5.8

13.4

11.7

6.5

10.37

37.4

4.3

20.5

QY (%)

Trace Hg2+ detection

Sensing of Fe3+

Bioimaging

Bioimaging

Sensing, and boimaging

[95]

[94]

[93]

[92]

[62]

[91]

[90]

[89]

[88]

Ref.

(Continued)

Detection of tartrazine

Bioimaging

Bioimaging

Selective detection of Fe3+

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes. (Continued)

Carbon Dots: Emerging Green Nanoprobes  427

Method

Solvothermal

Solvothermal

Microwave-assisted synthesis

Microwave-assisted synthesis

Microwave-assisted synthesis

Microwave-assisted synthesis

Microwave-assisted synthesis

Precursor

Scindapsus

Rice husk

Eggshell membrane

Flour

Rose flowers

Sesame seeds

Yogurt

800 W, 200°C, 30 min, filtration

800 W, 10−15 min, ground, centrifugation

Dried, microwave, centrifugation, dialysis

Microwave 180°C, 20 min, centrifugation, vacuum-dried

400°C, 2 h, ash added to 1 M NaOH, microwave 5 min, diluted, centrifugation, dialysis

HNO3, 200°C, 6 h, neutralized, filtration, centrifugation, dialysis

HNO3, sonication, 180°C, 24 h, dialysis

Reaction conditions

2

5

4–6

1–4

~5

4–5

2–6

Size (nm)

1.5

8.02

13.45

5.4

14

9.3

-

QY (%)

Medical applications

Selective sensing of Fe3+

[100]

[69]

[99]

[71]

[98]

[97]

[96]

Ref.

(Continued)

Detection of tartrazine

Detection of Hg2+

Detection of Cu2+

Detection of volatile organic compounds

-

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes. (Continued)

428  Functionalized Nanomaterials for Catalytic Application

Method

Pyrolysis

Pyrolysis

Pyrolysis

Pyrolysis

Pyrolysis

Pyrolysis

Pyrolysis

Precursor

Sago waste

Watermelon peel

Konjac flour

Waste frying oil

Black soya beans

Citrus limetta

Pigeon feathers, egg white, egg yolk, and manure

300°C, 3 h, centrifugation, dialysis

190°C, 20 min

200°C, 4 h, centrifugation, dialysis, freeze-dried

100°C in 1 ml of H2SO4, 5 min, pH adjusted by NaOH, dialysis

470°C, 1.5 h, extraction

220°C, 2 h, centrifugation, dialysis

400°C, 1 h, homogenize, centrifugation

Reaction conditions

3.8 ± 0.5, 3.3 ± 0.5, 3.2 ± 0.5 and 4.2 ± 0.5 nm respectively

–4–7

5.16 ± 0.30

1–4

3.37

2.0

6–17

Size (nm)

24.87, 17.48, 16.34, and 33.5, respectively

63.3

38.7 ± 0.64

23.2

22.0

7.1

-

QY (%)

[105]

[104]

[103]

[25]

[102]

[101]

[77]

Ref.

(Continued)

Detection of Hg2+ and Fe3+

Photoelectro-catalytic, sensing and bioimaging

Free radical scavenging, Fe3+ sensing and Bioimaging

Cell imaging

Bioimaging, detection of Fe3+

Bioimaging

Detection of various metal ions in aqueous solution

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes. (Continued)

Carbon Dots: Emerging Green Nanoprobes  429

Method

Pyrolysis

Oxidation

Oxidation

Oxidation

Precursor

Water chestnut

Eucalyptus kraft wood pulp

Coconut activated carbon

Cow manure

300°C, 3 h, refluxing with HNO3, 72 h, centrifuge, filtration, functionalization with ethylenediamine

HNO3, amine passivation

Carbonaceous material obtained by H2SO4 treatment, HNO3 (2 M), refluxing, 12 h, neutralization, dialysis, functionalization with PEG1500N

PTFE reactor, 180°C, 3 h, filtration

Reaction conditions

4

4.2

1–3 (Unmodified); 1–5 (modified)

2–7

Size (nm)

0.65

12.2

1.2 (Unmodified); 3.2 (modified)

12.6

QY (%)

Bioimaging

Bioimaging

Bioimaging, photocatalysis

[108]

[46]

[107]

[106]

Ref.

(Continued)

Multicolor imaging in living cells

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes. (Continued)

430  Functionalized Nanomaterials for Catalytic Application

Method

Mixed acid oxidation

Oxidation

Oxidation

Precursor

Tobacco leaves

Manilkara zapota fruits

Hydrochar (waste from hydrothermal treatment of biomass)

Dried hydrochar, NaOH/H2O2, 8 h, room temperature, neutralization, dialysis

Freeze-dried, mixed with H2SO4, sonication, 100°C, 1 h, dialysis

Dried leaves, conc HNO3, H3PO4, 140°C, 12 h, neutralization, filtration and

Reaction conditions

~2.4

1.9 ± 0.3, 2.9 ± 0.7 and 4.5 ± 1.25 nm

2.34

Size (nm)

22.67

5.7, 7.9, and 5.2

-

QY (%)

Detection of Pb2+

Bioimaging

CQD-doped WO2.72 for bioimaging and cancer therapy

Applications

Table 14.2  Methods of preparation, properties, and applications of carbon dots obtained from biomass and biowastes. (Continued)

[22]

[110]

[109]

Ref.

Carbon Dots: Emerging Green Nanoprobes  431

432  Functionalized Nanomaterials for Catalytic Application

14.3.1 Hydrothermal Treatment This is an economical, simple, and scalable method and allows the carriage of heteroatoms from precursors to product. Owing to its low cost, versatility, eco-friendly, and non-toxic nature, hydrothermal treatment is the most explored method for the synthesis of CDs. As water is used as solvent, the method is said to be hydrothermal process. This process usually avoids multistep passivation, use of expensive reagents, and sophisticated instruments. In a typical hydrothermal process, the precursor in water is placed in a teflon-coated autoclave and heated in an oven or furnace at 100°C–250°C temperature and high pressure for a required period of time. Formation of CDs is indicated by a color change of solution to yellow or brown. After hydrothermal treatment, the CQDs are separated, purified, and concentrated by centrifugation, filtration, and dialysis respectively to obtain pure CDs. A schematic illustration for the formation of carbons dots by hydrothermal treatment is shown in Figure 14.3. Hydrothermal treatment includes four steps: dehydration, polymerization, carbonization, and passivation. At high temperature, small organic molecules dehydrate to give cross-linked polymers and form carbogenic cores and finally grow to form CDs of different sizes. As mentioned earlier, the size and, therefore, the PL can be modulated by varying the precursor, reaction temperature, and time. Wang et al. prepared N-doped, monodispersed, fluorescent CDs of 2- to 4-nm size from milk by hydrothermal treatment at 180°C for 2 h followed by filtration [60]. These water soluble CDs exhibited excitation dependent PL with 12% QY. Due to their non-toxicity and excellent photostability, these CDs were used as optical cell-imaging probes. Zhao et al. synthesized CDs through hydrothermal treatment of garlic at 200°C for 3 h. Subsequent purification involves centrifugation, filtration, and dialysis [61]. Hydrothermal treatment of coriander leaves at 240°C for 4 h yield CDs of size ranging from 1.5 to 2.98 nm with 6.48% QY [62].

BIOMASS SEPARATION

BIO WASTE

PURIFICATION DISPERSION

H 2O HEAT Solution

Carbon Dots

Figure 14.3  Schematic illustration of hydrothermal treatment for preparation of carbons dots.

Carbon Dots: Emerging Green Nanoprobes  433 Even though hydrothermal synthesis of CDs is eco-friendly, inexpensive, non-toxic, and easy to use, it has disadvantages like poor size control and long reaction time.

14.3.2 Solvothermal Treatment This method resembles the hydrothermal method except that the organic solvents are used instead of water for the formation of CDs. In this method, a precursor and a suitable organic solvent are sealed in a teflon-lined stainless steel autoclave and heated at high temperature and high pressure in an oven or furnace followed by isolation and purification of CDs. Despite being inexpensive, environmental friendly, and non-toxic, the main drawback of solvothermal technique is formation of high quantity of carbonaceous aggregate during the carbonization, poor control on particle size, and reduced solubility of CDs. Shaikh et al. [63] have reported synthesis of amorphous water soluble C-dots, from the fruit juice of Citrus limetta by solvothermal method. A mixture of fruit juice and ethanol in a teflon-lined stainless steel autoclave was heated at 120°C for 12 h to give a yellow solution. The fluorescent CDs of 2- to 4-nm size were collected from the solution by filtration, extraction, and dialysis. Solvothermal process for preparation of CDs from juice of Citrus limetta is shown schematically in Figure 14.4. Detailed analysis of CDs revealed the presence of conjugated C=C bonds, -COOH, -OH and -NH groups. These CDs displayed excitation dependent fluorescence spectra and posses antibiofilm activity against the infection caused by biofilm forming organisms. Long et al. [64] synthesized dual-emission CDs from white pepper through one pot solvothermal method. Typically, white pepper was dried at 60°C and grounded into powder and dispersed in anhydrous ethanol in a round-bottom flask followed by refluxing in a water bath for 24 h in an

Filtration EtOH 120°C 12 h

Extraction, Dialysis Solution

Carbon Dots

Figure 14.4  Schematic representation of solvothermal treatment for preparation of carbon dots from Citrus limetta.

434  Functionalized Nanomaterials for Catalytic Application inert atmosphere. After filtration, H2N-PEG-NH2/anhydrous ethanol solution was mixed into the filtrate and refluxed at 75°C for 4 h. The resulting passivated CDs were further filtered and freeze-dried to obtain powder CDs.

14.3.3 Microwave-Assisted Method Microwave-assisted technique is the second most utilized method for the synthesis of CDs after the hydrothermal treatment due to ease of synthesis, cost-effectiveness, and significant reduction of reaction time by increasing the reaction rate. Use of microwave has many advantages over the conventional heating techniques which include both convection and conduction modes. Microwave irradiations heat the substance uniformly, thus eliminating the effect of temperature gradients and produce CDs of uniform and small size in high yield, with high purity and in very short reaction time. However, microwave-assisted synthesis of CDs can be used only for small scale reactions and the solvents of low boiling point cannot be used [65]. In microwave-assisted synthesis of CDs either water is used as solvent (microwave-assisted hydrothermal synthesis) [26, 66, 67] or can be carried out in absence of solvent (microwave-assisted pyrolysis) [21, 23, 68–70] or simply microwave-assisted method [71–74]. However, the microwaveassisted hydrothermal technique is less commonly used as compared to hydrothermal treatment. Using tender coconut water, Purbia et al. [66] demonstrated a 1-min synthesis of CDs via single-step microwave-assisted hydrothermal method without the addition of any other chemical. In a typical experiment, a sealed glass vessel containing 10 ml of coconut water and ethanol (1:1 v/v) was heated in a microwave for 1 min at three temperatures (140°C, 160°C, and 180°C), cooled to room temperature, and centrifuged at 10,000 rpm (rotation per minute) for 10 min to yield fluorescent CDs in the size range from 2 to 4 nm. It was observed that the size and crystalline nature of CDs increases with increase of reaction temperature or time. Monte-Filho et al. [75] synthesized CDs from lemon and onion juices using a domestic microwave (Figure 14.5). In this work, onion was used as S-dopant, while NH4OH was used as a N-dopant. Lemon juice (20 ml) and onion juice (20 ml) were dissolved in 8.0-ml deionized (DI) water and 10 ml of 25% NH4OH solution was added. The mixture was then irradiated in a domestic microwave oven at 1450 W followed by separation and purification of CDs. Wang and coworkers [23] have used wool as a precursor to prepare CDs of average size (2.8 nm) by using microwave-assisted pyrolysis. The wool

Carbon Dots: Emerging Green Nanoprobes  435 Lemon

Onion

Microwave

Filtration Centrifugation

Dialysis

Ammonia 1450 W, 6 min.

6000 rpm

24 h

400 500

Figure 14.5  Microwave-assisted synthesis of carbon dots from lemon and onion juices [75].

(0.3 g) pieces of 3-5 mm length were mixed with 40 ml water and heated at 200°C for 60 min by microwave irradiation. The solution was cooled to room temperature and centrifuged at 10,000 rpm for 15 min to remove the large particles. Subsequently, the suspension was filtered through a 0.22mm microporous membrane to collect the CDs.

14.3.4 Pyrolysis Treatment The process of thermal decomposition of a substance at elevated temperature in an inert atmosphere is termed as pyrolysis. Although preparation of biomass derived CDs by pyrolysis is one of the most implemented methods and provides a rapid, simple, and eco-friendly route, it is difficult to scale up and produce CDs of broad size distribution. Reaction conditions of pyrolysis including type of reactor, temperature, pressure, catalysts, and reaction time have a profound effect on the composition and yield of CDs [76]. In this technique, a carbon source is heated at a very high temperature and under controlled pressure to form a solid black carbon residue which on separation and purification yields CDs. Tan et al. [77] have synthesized CDs from sago waste and studied the effect of carbonization temperature on the fluorescent property and yield. They placed pre-dried and ground sago waste in a crucible and heated it in a furnace (temperature range of 250°C–450°C) for 1 h at the set temperature followed by cooling to room temperature, homogenization in sonicator, and centrifugation. The optimum carbonization temperature was found to be 400°C at which the prepared CDs exhibited strongest fluorescence emission. Chatzimitakos et al. [78] reported synthesis of highly fluorescent CDs from Citrus sinensis and Citrus limon peel. The fresh peels were heated separately in porcelain crucibles at 180°C for 2 h. The homogeneous black residue was crushed into fine powder, mixed in double distilled water, and sonicated for 10 min. The dispersion was separated and purified followed by freeze-drying.

436  Functionalized Nanomaterials for Catalytic Application Moreover, Deka et al. [79] have used fine powder of rinsed and sundried water hyacinth leaves impregnated with 50% phosphoric acid as a natural source for synthesis of CDs by pyrolysis. Powder was heated in a furnace from room temperature to 160°C at the rate of 10°C/min to give a sticky solution which on subsequent purification yielded CDs. Later, Prathumsuwan et al. [80] have also utilized a fine powder of rinsed and sun-dried water hyacinth leaves for synthesis of CDs by acid assisted pyrolysis. The powder was dispersed in 1.5 M nitric acid and DI water, followed by refluxing at 110°C under stirring at 900 rpm for 24 h. The brown solution was heated at 250°C for 6 h. The resulting black solid was dispersed in DI water and underwent separation and purification to obtain the CDs. Despite the same source, the CDs obtained under different reaction conditions exhibited different properties. For instance, the size of CDs obtained at higher temperature was below 10 nm and contained −COOH and −OH groups on the surface with crystalline core. The highest emission intensity of 370 nm was found to be at an excitation wavelength of 300 nm with 17.02% QY. These CDs elicited emission enhancement in presence of herbicide pretilachor [79]. On the other hand, the CDs prepared at low temperature were of small size (3.7 nm), exhibited high QY (27%), and contained −OH, −NH, and >C=O functionalities at the surface with crystalline core. The CDs thus prepared have been explored for sensing of borax [80]. Disposal of waste frying oil is an important waste management concern. Hu et al. [25] developed a one-step method to obtain sulfur-doped CDs from waste frying oil. In a simple procedure, to 5 ml of waste frying oil in a flask, 1 ml of conc H2SO4 was added and the mixture was heated at 100°C for 5 min with vigorous stirring. Subsequent dilution, neutralization, and filtration gave a dark yellowish-brown solution of S-CDs which was further purified by dialysis for 4 days.

14.3.5 Chemical Oxidation Chemical oxidation is defined as gain of oxygen or loss of electrons by a species while an oxidizing agent accepts electrons and gets reduced. This process of synthesis of CDs involves carbonization of precursor, treatment with oxidizing agent with subsequent isolation, and purification of CDs. The CDs thus prepared are decorated with hydrophilic groups such as −OH and −COOH groups on the surface. These groups increase the water dispersibility and PL properties [22]. Most common oxidants used are HNO3, H3PO4, H2SO4, and NaOH/H2O2. Gunjal et al. [111] synthesized CDs by the oxidation of mahogany fruit shell powder using H2SO4 and HNO3. The activated carbon was produced

Carbon Dots: Emerging Green Nanoprobes  437 from mahogany fruit shell by the reaction of dried shell powder and conc H2SO4, followed by neutralization. The activated carbon was dispersed in 1.0 M HNO3 solution and sonicated for 10 min. The mixture was refluxed for 12 h, showing a color change to brownish-yellow, confirming the formation of CDs. Sodium carbonate (Na2CO3) was employed for the neutralization process. The fluorescence of these CDs could be tuned by surface modification with amines. Later, N-CDs derived from waste tea residue were reported by the same research group [112]. Souza and coworkers [107] have prepared CDs of 1-3 nm diameter from carbonaceous material obtained from eucalyptus kraft wood pulp by nitric acid oxidation. Surface of “so-prepared” CDs is surrounded by a large number of −COOH groups and displayed very low (1.2%) QY. The size and QY of particles were increased to 1–5 nm and 3.2%, respectively, on reaction with PEG1500N. Although, production of CDs from biomass and waste materials via chemical oxidation method renders easy, sustainable, large-scale, and inexpensive synthesis as no expensive reagents and experimental setups are required. The major drawback of the present method includes use of environmental damaging strong oxidizing agents which violates the rules of green synthesis. In addition to this, use of strong oxidants generate a Table 14.3  Summary of merits and demerits of different methods used for biomass and biowastes derived carbon dots. Method

Merits

Demerits

Hydrothermal/ solvothermal treatment

Inexpensive, versatile, simple, eco-friendly, non-toxic, large-scale synthesis

Long reaction time, poor control of particle size

Microwave-assisted synthesis

Very fast, eco-friendly, easy and inexpensive

Difficult to scale up, poor control of particle size

Pyrolysis

Fast, simple, eco-friendly

High temperature, poor control of particle size, require chemical modification, and passivation

Chemical oxidation

Simple, reproducible, inexpensive, large-scale synthesis

Use of strong oxidants, tedious isolation

438  Functionalized Nanomaterials for Catalytic Application large no of functional residues on the surface which can be further modified by means of chemical reactions but sometimes the residue may stay in the CDs, which can impart biological toxicity. So far, chemical oxidation has been scarcely used for the preparation of CDs from biomass. From the above discussion, it is clear that the various modified bottomup approaches under milder reaction conditions have been adopted to produce the CDs from the renewable natural carbonaceous materials and waste materials. However, every method has its own advantages and disadvantages. A detailed summary of merits and demerits of different modified bottom-up approaches used for the synthesis of CDs from biomass and biowastes is presented in Table 14.3.

14.4 Characterization of Carbon Dots Detailed characterization of CDs is crucial for desired applications and fabrication of devices. CDs consist of a core and surface decorated with various functionalities. Therefore, structure elucidation of CDs involves characterization of the core as well as the surface functional groups [37, 113]. A brief description of broad range characterization of CDs by numerous microscopic and spectroscopic techniques is discussed in this section. High-resolution transmission electron microscopy (HRTEM) is exploited to determine the periodicity of graphitic core associated with crystalline nature of CDs. The crystalline nature of CDs is revealed by the presence of two types of lattice fringes namely: interlayer spacing at about 0.34 nm and in-layer spacing centered at 0.24 nm in HRTEM image [114]. X-ray diffraction (XRD) is an important technique to obtain the characteristic features of crystalline CDs including state of crystallinity, size of unit cell, and interlayer spacing. Purity of CDs sample is often assessed from the XRD pattern [115–118]. In addition to evidence of crystalline nature obtained from HRTEM, appearance of a sharp peak at around 2θ = 26° in XRD indicates a highly crystalline structure, whereas a broad peak suggests the amorphous nature of CDs. The XRD pattern of N- and S-doped CDs obtained from one step hydrothermal treatment of garlic displayed two broad peaks at 25° and 44° corresponding to (002) and (001) diffraction patterns of low-graphitic carbon structure, respectively [119]. Raman spectroscopy is a rapid, non-destructive technique to evaluate the state of carbon including the crystalline or amorphous nature, crystallite size, edge-type, and extent of defects in CDs [120–124]. Raman spectrum of CDs is characterized by the presence of two bands centered around 1,360 and 1,580 cm−1 called D and G bands, respectively. The D band (D for defects

Carbon Dots: Emerging Green Nanoprobes  439 or disorder) arises due to defects in the graphitic lattice (i.e., sp3 C), whereas the G-band (G for graphitic) is attributed to vibrations of sp2 hybridized carbon atoms of graphitic network. The intensity of D band also reflects the defects on surface or heteroatoms on the surface of CDs. The ratio of intensities of these two bands (i.e., ID/IG) is an important criterion to decide the purity (or degree of disorder or graphitization) of CDs. A higher value of ID/IG is indicative of amorphous sample; however, a lower value of ID/IG is suggestive of high degree of graphitization. Tunistra et al. suggested that ID/ IG ratio can be used to ascertain the crystallite size [125]. Primary morphological and microstructure information of CDs such as particle size, shape, and dispersion can be revealed by surface electron microscopy (SEM) and transmission electron microscopy (TEM) [25, 77, 120, 126]. SEM provides three-dimensional topographical image with low resolution of CDs by using electrons reflected from the surface of the sample, whereas TEM with comparatively higher resolution power provides two-dimensional image by the electrons transmitted through the sample. Atomic force microscopy (AFM) is used to determine the size of CDs and surface morphologies to the resolution of a fraction of nanometers. X-ray photoelectron spectroscopy (XPS) is based on irradiation of a sample by X-rays with simultaneous measurement of the number of ejected electrons and kinetic energy. X-rays spectra can be used for the assessment of carbon bonding configuration and elemental composition on the surface of CDs. Doping of CDs with nonmetals as well as with metals can also be evaluated by XPS [25]. Infrared (IR) and NMR spectroscopies can be used as complementary techniques for the qualitative elucidation of functional groups. Apart from the evaluation of commonly present functional groups such as −OH, >C=O, −COOH, −C-O-C- groups on CD’s surface, IR spectroscopy is quite helpful for determination of doping with nonmetals like N, S, P, B, and Si but it fails to detect the metal atoms. Evidence for doping with nonmetal atoms in CDs framework is achieved by the presence of peaks corresponding to amine (−NH2), amide (−CONH2), thioalkyl (C-S), phosphate (P=O and P-OR), organosiloxane (Si-O-C), and boronic acid (B-O, B-N) functionalities in IR spectrum [25, 127–131]. IR spectroscopy is helpful to detect surface passivation treatments and the interaction of metal oxides with CDs in metal oxide-conjugated CDs. Characterization of CDs by 1H NMR is less common as they are deficit in protons. However, 13C NMR can be employed to identify the differently hybridized carbons and to confirm different functional groups present on the surface. In 13C NMR, aliphatic carbons (sp3) resonate in the range of 8–80 ppm, whereas aromatic and carbonyl carbons (sp2) resonate in the range of 90–180 ppm. CDs composed

440  Functionalized Nanomaterials for Catalytic Application of aromatic carbon core (sp2 carbon) show absence of peaks below 120 ppm in 13C NMR [37, 127, 132]. CDs exhibit characteristic UV-visible absorption spectrum which is often used to measure the band gap required for their semiconducting property and to know the excitation wavelength for fluorescence emission spectrum [133, 134]. Generally, four types of transitions are observed in UV-visible spectrum and the order of energy requirement for these transitions is σ-σ*>n-σ*>π-π*>n-π*. CDs exhibit a broad and strong absorption in the UV region (230–320 nm) with a weak tail extending in the visible region. The band in UV region is attributed to π-π* transitions of the aromatic C=C bond along with a shoulder peak ascribed to n-π* transitions from functional groups with non-bonding electrons. The intensity of shoulder peak increases on increasing the functional groups on the surface. The absorption pattern is affected by surface passivation, functional groups on the surface, hybridization, and size of CDs [133–136].

14.5 Optical and Photocatalytic Properties of Carbon Dots The optical properties of CDs can be evaluated from the combined study of their chemical and electronic structures. Important optical properties of CDs include absorption, PL, QY, upconversion PL (UCPL) (or anti-Stokes effect), and photoinduced electron transfer (PET). The optical properties of CDs depend on their size and surface states. Due to small size, the phenomenon of QCE is exhibited by a certain category of CDs, this is very crucial in understanding the effect of size of CDs on the optical properties. Investigation of electronic structure of CDs revealed that with the increase in size of CDs with extended π-conjugation, the energy of highest occupied molecular orbital (HOMO) increases, whereas energy of lowest unoccupied molecular orbital (LUMO) decreases, thus HOMO-LUMO band gap decreases and absorption band appeared at higher wavelength. Similarly, the functional groups also affect the band gap, however, the effect is not remarkable. Electron withdrawing groups narrow the HOMO-LUMO gap, while the electron donating groups widen the energy band gap and the optical properties of CDs vary accordingly [37]. The inherent optical properties of CDs, especially the PL and QY can be remarkably tuned by the heteroatom doping. Doping is done with metals (Zn, Cu, and Gd are most commonly used), non-metals (N, S, P, and B are commonly used), and a combination of both. Heteroatoms either provide

Carbon Dots: Emerging Green Nanoprobes  441 the extra electrons (n-type dopants) or extra holes (p-type dopants). Therefore, doping associated tuning of optical properties is attributed to modification of surface active sites and thus alters the electronic environment of CD. However, heteroatom doping reduces the crystallinity of CDs and the presence of metal atoms imparts toxicity to CDs [30, 137, 138].

14.5.1 Absorbance In UV-visible absorption spectroscopy, the electrons in a molecule are excited from lower to higher energy level by the absorption of light in UV-visible range. Amount of light absorbed by the sample at a certain wavelength can be calculated and used further to investigate electronic structure of the molecule and concentration of the sample. Absorption wavelength can also be used to assess the band gap. UV-visible absorption characteristics of CDs have already been discussed in the last section. The CDs prepared from different methods exhibit strong absorption band in the UV region, but the position and intensity of absorption bands are entirely different for CDs obtained from different techniques. The absorption position and intensity can be further significantly improved post synthesis surface modification and heteroatom doping.

14.5.2 Photoluminescence PL is defined as the emission of photons of longer wavelength upon absorption of photons in visible region, whereas fluorescence refers to emission of photons in visible to near IR region upon absorption of radiations of UV range. The fluorescence phenomenon can be well understood from Jablonski energy diagram (Figure 14.6). When a molecule with conjugated system absorbs electromagnetic radiation, the π electrons jump from ground state (S0) to first excited state (S1). These electrons come back from higher vibrational energy level to lower vibrational energy level (called vibrational relaxation) and dissipate some energy in the form of non-radiative transitions and intersystem crossing and finally return to ground state accompanied by emission of radiation. While coming from higher vibrational energy level to lowest vibrational energy level, a part of energy is lost which accounts for low energy of emission compared to that of absorption energy. From the viewpoint of applications of CDs, PL/fluorescence is one the most fascinating optical properties. PL in CDs results from the excitation of valence electrons of definite energy leaving behind holes and the emission of photons of lower energy as the excited electrons return to ground

442  Functionalized Nanomaterials for Catalytic Application Vibrational relaxation S1 Intersystem crossing T1

Absorption

Fluorescence Phosphorescence

S0

Figure 14.6  Jablonski energy diagram.

state and recombination of electron-hole pairs takes place. CDs exhibit excitation wavelength dependent PL in UV to near IR region wherein the emission peak position and intensity changes with the excitation wavelength [14, 30, 37, 44]. CDs are also known to exhibit electroluminescence and chemiluminescence. Electroluminescence is emission of light in response to a strong electric field, whereas chemiluminescence is emission of light as a result of chemical reaction. CDs exhibit PL emission in blue to green region of the visible spectrum which can be shifted to red and near IR region by modifications in CDs. Owing to the large size distribution, most PL spectra are symmetrical and broad. Moreover, diverse electron transitions are responsible for large Stokes shift [37]. Stokes shift is the difference in the wavelengths (band maxima) of absorption and emission peaks. The excitation wavelength dependent emission behavior is responsible for the appearance of multiple PL spectra of a single CD. This unique property of CDs is exploited in multicolor imaging applications. The PL and therefore color of CDs is determined by the band gap which can be tuned by modifying the size and surface states of CDs. CDs of different sizes produce different emission colors. The smaller the size of CDs, higher the band gap energy and therefore excitation and emission wavelength belong to lower wavelength (toward blue region) and vice versa, e.g., CQDs of 1.2 nm, 1.5–3.0nm, and 3.8 nm display emissions at 300 nm (UV region), 400–700 nm (visible region), and 800 nm (near IR region),

Carbon Dots: Emerging Green Nanoprobes  443 respectively. Upon excitation of a sample of CDs of varying sizes, the emission spectrum shifts with the excitation wavelength [4]. The PL of CDs can be further improved by surface defects, surface groups, surface passivation, and heteroatom doping. The oxygen atoms and other functional groups create defects in CDs and shift the emission toward longer wavelength [29]. Red shift in emission is observed with increasing number of oxygenated groups including hydroxyl groups, epoxy groups, carboxylic groups, etc. Electron donating groups increase the band gap and the emission wavelength is shifted toward blue region. In contrast, electron withdrawing groups bring about red shift in emission. Surface passivation with polymers led to an increase in the emission of CDs. Passivated CDs also possess excellent photostability and do not photobleach after longtime continuous exposure to light, i.e., their PL emission intensity does not change on prolonged excitation. The photostability of CDs is usually due to the large π-conjugated structure achieved by surface passivation. Doping of CDs with elements of higher electronegativity (e.g., N), led to blue shift of emission band. On the other hand, elements of electronegativity values lower than the C atom (e.g., S) led to a red shift of emission. In addition, PL of CDs is significantly affected by external circumstances such as solvent, temperature, and pH of the medium [137].

14.5.3 Quantum Yield Evaluation of QY of CDs is important for application of CDs for bioimaging, sensing, and photocatalysis. QY is defined as ratio of photons absorbed to photons emitted and can be evaluated using quinine sulfate as standard according to Eq. (14.1)



ϕ=

no. of photons emitted ϕ = ϕ r × (II /I r ) × ( Ar /A) × (n 2 /nr2 ) no. of photons absorbed (14.1)

where φ is the QY, I is the integrated PL emission intensity, A is absorbance, n is the refractive index, and subscript “r” refers to standard. In order to minimize the reabsorption effects, absorbance value of the individual solution should be below 0.10 at the excitation wavelength. Similar to other optical properties, QY of CDs varies with precursor, preparation method, size, and surface structures. QYs of CDs obtained from top down methods are usually low as compared to those obtained from bottom up approaches. The QY of CDs of 4-nm size, prepared from

444  Functionalized Nanomaterials for Catalytic Application electrochemical oxidation of graphite electrodes using ethanol/NaOH/ H2O as electrolyte by applying a potential of 5 V for 3 h was found to be 11.2% [44]. Sun et al. [14] synthesized CDs from refluxing of CNPs (prepared from laser ablation) with aqueous solution of nitric acid for 12 h followed by passivation with PEG. The QYs of passivated CDs of ~5 nm size at 400 nm excitation wavelength were about 4%–10%. Whereas, ZNOdoped CDs and ZnS-doped CDs obtained from laser ablation of CNPs and nitric acid treatment followed by doping, showed 45% and 50% QYs, respectively [139]. As heteroatom doping modifies the band gap and electronic structure of CDs, therefore QYs and chemical reactivity of doped CDs increased considerably. The QYs of unpassivated CDs prepared from solution chemistry and microwave-assisted acidic oxidation treatment lie in the range between 2% and 30%. These unpassivated CDs have electron withdrawing carboxylic and epoxide groups, which reduce the π-electron density and act as centers of non-radiative electron-hole recombination. Passivation of CDs by conversion of carboxylic group into amide and amine groups, and epoxide into amine group increases QY. Increase of QY on passivation is due to reduction in non-radiative electron-hole recombination. Electron donating groups also improves the QY of CDs. The electron donating groups increase π-electron density and facilitate radiative electron-hole recombination and thus increase the QY [140]. To the best of our knowledge, the highest QY is 94% reported for the CDs prepared from folic acid by hydrothermal treatment [141].

14.5.4 Up-Conversion Photoluminescence (Anti-Stokes Emission) In contrast to traditional PL (which involves emission of photons of longer wavelength than the excitation wavelength), UCPL involves emission of photons of shorter wavelength than the excitation wavelength. It is also known as anti-Stokes emission. In UCPL, the electron is excited by simultaneous absorption of multiple photons of near IR (NIR) or longer wavelength (500–1,000 nm) and returned to ground state with the emission of photons of lower wavelength in UV, visible or near IR region. Thus, the up-conversion luminescent materials can convert low energy photons to high energy photons. Besides down-converted PL, the phenomenon of UCPL is also exhibited by the CDs [142]. It is believed that UCPL is attributed to simultaneous absorption of two or more photons. Other theories have also been put forward to explain the UCPL in CDs which include anti-Stokes PL (ASPL)

Carbon Dots: Emerging Green Nanoprobes  445 and auger process or thermal effect. In all of these proposed mechanisms, an intermediate is essential for up-conversion of carriers [143–145]. UCPL of CDs is associated with other optical properties such as high fluorescence efficiencies, resistance to photobleaching and photoblinking. As CDs are non-toxic and biocompatible, therefore CDs can be photoexcited by deep penetrating longer wavelengths and can be used in deep tissue imaging without damaging the tissues under the visible light [15, 142, 146]. CDs showing UCPL generally show emission at fixed wavelength. The appearance of fixed emission on varying the excitation wavelength suggested that the emission occurs from the lowest single state irrespective of the mode of excitation (difference with PL) [147]. Cao et al. in 2007 reported that water soluble surface passivated CDs of quantum size displayed luminescence in the visible region on excitation in the NIR region. They claimed that the two-photon absorption crosssections of the CDs are comparable to those of SQDs. Moreover, these CQDs were used for in vitro imaging of human cancer cells [15]. The CQDs synthesized by heating aqueous solution of ascorbic acid at 90°C also displayed upconversion fluorescence. The “so prepared” CDs exhibited nearly constant emission at 540 nm when excited in the NIR region from 805 to 1,035 nm [148]. Some studies revealed that the observed up-converted emission in CDs is an artificial technical error. Appearance of UCPL is attributed to excitation by a second-order diffraction of light. The apparent UCPL disappeared upon adding a filter to prevent the long wavelength excitation [145, 149]. Therefore, it is important to be careful while interpreting UCPL.

14.5.5 Photoinduced Electron Transfer Upon absorption of photons by a molecule, an electron is excited from a lower energy ground state to higher energy excited state. The excited electron can come back to ground state with the emission of energy in the UV-visible to NIR region and the process is called fluorescence or phosphorescence. However, when an electron acceptor or donor is present in the vicinity of a photoexcited molecule, the process of electron transfer can take place. This phenomenon is called PET which is observed with concomitant quenching of fluorescence. Such photoexcited molecules can act as oxidizing and reducing agents. Electrons and holes are generated within the CDs upon irradiation with light, and the photogenerated electrons can be transferred to a species, thus CDs possess outstanding ability to accept or donate electrons leading to quenching of fluorescence of CDs [12, 150, 151]. Wang et al.

446  Functionalized Nanomaterials for Catalytic Application [150] prepared PEG1500N surface passivated CDs of 4.2 nm size. The PL of these surface passivated CDs was quenched in presence of well-known electron acceptors 2,4-dinitrotouene and 4-nitrotouene with appreciably high Stern-Volmer and bimolecular quenching constants, thus emphasizing the high efficiency of underlying electron transfer and presence of static quenching contribution. These CDs are also efficient electron acceptors and quench the luminescence of N, N-diethylaniline. Owing to PET features, CDs offer the opportunity of their potential use in light energy conversion and as photocatalysts for degradation of organic pollutants, evolution of hydrogen, and reduction of CO2 [9, 10, 152]. Moreover, such types of CDs have also been extensively utilized for sensing, bioimaging, and related applications [9, 151].

14.5.6 Photocatalytic Property Photocatalysis is defined as the change in rate of a photochemical reaction in the presence of a catalyst. The catalyst absorbs light and is involved in the photoreaction thus aptly referred as photocatalyst. The process of photocatalysis involves four important steps: (1) generation of electron-hole pairs (charge carriers) by absorption of light energy equivalent to band gap; (2) separation of charge carriers by transfer of electron to conduction band (higher energy state) leaving behind the hole; (3) transfer of charge carriers to the surface of photocatalysts; and (4) utilization of charge carriers for subsequent redox reaction [153]. The efficiency of a photocatalyst primarily depends upon the band gap, separation of charge pairs, and the slow recombination of charge pairs in presence of electron and/or hole acceptors. The energy of band gap should be equal to or lower than the energy of photon required for excitation of electron. An ideal photocatalyst should be inexpensive, eco-friendly, and able to operate in visible and/or near UV light. The energies of photons corresponding to visible region are in range of 2.43–3.2 eV [154]. Therefore, for a photocatalyst to be energy efficient and to harness the full solar spectrum, the band gap energy of photocatalysts should be less than aforementioned photon energies. The modified CDs with tunable optical properties and their tendency to combine with other materials make them promising candidates to be used at photocatalysts. The photocatalytic property of CDs has been utilized for energy production, removal of greenhouse gas via carbon sequestration, organic transformations, degradation of water pollutants and wastewater treatment. Photocatalytic performance of CDs can be improved by size-control, surface modification, doping, and composite formation. Surface modification

Carbon Dots: Emerging Green Nanoprobes  447 of CDs assists in separation of electron-hole pairs, inhibits electron-hole pair recombination and promotes migration of charge carriers by trapping the photogenerated electrons and thus improving their photocatalytic performance. Surface modification can be achieved by covalent bond formation or non-covalent interactions. Oxygenated functional groups on the surface of CDs cause recombination of charge carriers. On the other hand, amino groups on the surface are related to high QY. Hu et al. [5] investigated the effect of surface groups on photocatalytic activity of CDs and revealed that the coexistence of O and Cl results in high photocatalytic activity of Cl-CQDs and overall photocatalytic performance of Cl-CQDs is better than that of N-CQDs and O-CQDs. This is attributed to different directions and degrees of energy band bending induced by O-, Cl-, and N-containing groups. Doping of CDs with heteroatoms will lower the work function by adjusting the band gaps, rendering easy electron transfer for chemical reactions. Local electronic structure and charge carrier transfer property of CDs varies with doping elements. Doping with electron rich dopants such as N, P, and O and (n-type) can inject electrons into CDs and accelerates the electron transfer while electron deficient dopants such as B (p-type) slows down the electron transfer and creates a hole transfer process. N-doped CDs are most commonly studied and can be synthesized easily via onepot synthesis. As the size of nitrogen atom is comparable to carbon atom, therefore it binds effectively in the carbon framework. Due to their lower work function, N-CDs exhibited improved photocatalytic activity. N-CDs with oxygenated functional groups on the surface were reported as excellent photocatalysts for splitting of water due to the formation of p-n junctions by N-doped cores (as the n-domain) and oxidative edges (as the p-type domain). Similarly, S,N-CDs displayed broad absorption in the visible region due to π- π* and n- π* transitions of C=S and S=O groups [155–157]. Despite broad absorption range, efficient electron transfer in pure CDs is not observed. To enhance the electron transfer, transition metals/CDs nanocomposites were fabricated. Presence of transition metals in these nanocomposites affects the ability to donate or accept the electron. Wu et al. [158] revealed that the photocatalytic efficiency of Cu-N-doped CDs (Cu-CDs) to oxidize 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate was 3.5 times higher than pristine CDs. The redox property of CDs was also confirmed from the reduction of metal ions from their aqueous solution in the presence of CDs. Sun et al. [159] prepared gold-coated PEGfunctionalized CDs (Au-CDs) by photoirradiation of an aqueous solution of the CDs solution in the presence of gold salt HAuCl4. The metal ions get

448  Functionalized Nanomaterials for Catalytic Application reduced by photoexcited electrons of CDs and deposited on the surface of CDs. The doped metal atoms in CDs function as co-catalyst and further enhance the photocatalytic action of CDs by trapping the charge carriers and preventing charge recombination. The Au-CDs were successfully used as an efficient photocatalyst for reduction of CO2 into acetic acid which require many more electrons than that for formic acid. Photocatalytic performance of CDs can be further improved many fold by combining it with other semiconductors to form composites. Semiconductors are known to absorb in the UV region and CDs exhibit properties of UCPL, PET, and electron reservoirs, therefore coupling of CDs with semiconductors not only extends the photoabsorption range of composite but also promotes charge separation. In composites, CDs can be employed in two ways for photochemical reactions [160]. CDs can act as photosensitizers and transfer the “up-converted” radiations to the coupled semiconductor, resulting in increased light absorption and efficiency of the composite system. CDs can also act as electric sinks for semiconductors. The CDs facilitate transfer of electrons from the surface of semiconductors to the substrate and thus suppress the recombination of charge-pairs and increase the overall catalytic efficiency of the system. Based on these distinctive characteristics, various composites such as CDs/TiO2, CDs/SiO2, CDs/Fe2O3, and CDs/Cu2O have been synthesized and used as efficient photocatalysts for various types of reactions induced by visible light [4, 161–163]. Li et al. [4] have synthesized TiO2/CQDs and SiO2/CQDs. SEM, TEM, and HRTEM analysis of composites confirmed that CDs are attached on the surface of semiconductor nanoparticles. CDs absorb radiations from visible-NIR region and convert them into UV light due to UCPL which, in turn, is absorbed by TiO2 nanoparticles and generates electron-hole pairs (Figure 14.7). Moreover, the CDs permit electron transfer from

TiO2 TiO2

CQDs

λ >500nm

λ 0.5CQDs/UBW > 3CQDs/UBW > 5CQDs/UBW [208]. Very recently for the first time a covalent organic framework (COF) composed of nickel porphyrin-based CDs has been reported for the photoreduction of CO2 under visible light. The catalytic system efficiently converted CO2 into CO, with a small amount of H2. The selectivity of CO production over H2 generation was found to be 98%. The promising product selectivity highlighted the importance of COFs as platforms to accommodate the CDs for photoreduction of CO2 [209]. The conversion of CO2 by photocatalysis has not been investigated on a large scale. Thus, more and more attention should be given to this field to solve the greenhouse effect and it is of great potential to produce new energy resources. The photoreduction of CO2 may lead to formation of a mixture of products. Separation of target product from the mixture is quite challenging.

Carbon Dots: Emerging Green Nanoprobes  459 Furthermore, for reduction of CO2, protons are supplied from water; therefore, HER is a competitive reaction which consumes a significant portion of energy required for reduction of CO2. The production of H2 further reduces the yield of the target product. The inhibition of HER is highly desirable to increase the yield of the target product. Therefore, more research is needed to develop an understanding of the mechanism of catalytic system (in terms of mobility of charge-carriers) used for photoreduction of CO2 to improve the product selectivity and yield of the target product.

14.7.3 Photocatalytic Synthetic Organic Transformations In general, UV and/or visible light of short wavelength are reported to bring about photochemical organic transformations, while NIR and IR region of solar spectrum, which constitutes the major portion, remained unexplored for carrying out the organic transformations. The unique optical properties make CDs promising candidates to be used as photocatalysts for photochemical reactions under green reaction conditions. Further, due to the broad absorption range from UV to NIR region, CDs can also harness the NIR light to enhance their photocatalytic performance. Despite the applications of CDs in various fields, reports on CD-based photocatalysts carrying out the organic transformations are few and far between. Li et al. [210] achieved the NIR light driven photooxidation of benzyl alcohol into benzaldehyde in 92% yield with 100% selectivity in presence of simple CDs. Due to strong UCPL and PET property, CDs decompose H2O2 into HO. (hydroxy radical) which subsequently converts benzyl alcohol into benzaldehyde. Incorporation of metal nanoparticles into CDs improves the catalytic performance of CDs. This is attributed to synergistic effect of components which includes enhanced light absorption and PET. Yang et al. [211] reported that CDs in combination with silver nanoparticles catalyze the selective oxidation of cyclohexane into cyclohexanone in presence of H2O2 upon irradiation in visible light under solvent free conditions. Conversion of cyclohexane into cyclohexanone is achieved in 58% yield with 84 % selectivity which was about five times higher than that reported for silver nanoparticles alone. The surface plasmon resonance of Ag nanoparticles enhanced the light absorption of Ag/CDs nanocomposites. In presence of light, H2O2 is decomposed into hydroxyl radicals, which oxidizes the cyclohexane into cyclohexanone. Similarly, the Au/CDs composite system oxidizes cyclohexane into cyclohexanone using H2O2 with 63.8 % conversion efficiency and 99.9% selectivity under visible light at room temperature [212].

460  Functionalized Nanomaterials for Catalytic Application Electron transfer ability and therefore catalytic property of CDs can be improved by doping. Wu et al. [158] synthesized Cu (II)-CDs from Na2[Cu(EDTA)] by pyrolysis and investigated the photocatalytic efficiency of doped CDs for aerobic oxidation of 1,4-dihydro-2,6-­dimethylpyridine3,5-dicarboxylate (1,4-DHP) under UV light. The formation of product in 68% yield, which is 3.5 times higher than that observed with undoped CDs, confirmed the increased electron transfer and electric conductivity of doped-CDs. Recently, Cu (I)-CDs have been synthesized from the thermolysis of Na2[Cu(EDTA)] and ascorbic acid, where ascorbic acid is used as a reducing agent to convert Cu (II) to Cu (I) [213]. Liu et al. demonstrated Cu(I)-CDs catalyzed, Cu(I)-mediated Husigen 1,3-dipolar cycloaddition reaction between terminal azides and alkynes upon irradiation at 365 nm. It was proposed that the Cu(I)-CDs on excitation at 365 nm generate excitons. The electrons escape from CDs leaving holes. The holes thus produced, release the Cu(I) from doped CDs and the free Cu(I) further reacts with the reactants to give the product. CD-mediated photocatalytic reduction has also been carried out. Reduction of nitrobenzene derivatives in aqueous solution under visible light irradiation in presence of CDs/CdS multilayered films has been reported with conversion rate as high as 99% [214]. Efficient reduction of 2-tert-butyl-1,4-benzoquinone (TBBQ) into tert-butylhydroquinone (TBHQ) was reported in presence of Cu1.8S-passivated CDs [215]. Formation of product was monitored by recording the UV spectrum of reaction mixture. Formation of product in 90% and 55% in presence Cu1.8S-passivated CDs and CDs, respectively, indicating the enhancement of catalytic performance by introducing Cu1.8S. Besides oxidation and reduction reactions, CDs have also been explored for photoinduced typical acid-mediated organic transformations such as esterification, Beckmann rearrangement, and aldol condensation [216].

14.8 Biomedical Applications of Carbon Dots Semiconducting QDs (II-VI QD) have been used as probes for bioimaging, biosensing, and drug delivery [217–219]. However, various studies have reported cytotoxicity on extended usage due to heavy metal migration from the QDs as a result of photolysis and oxidation [220, 221]. Scientists have been on a look out for benign photoluminescent materials. In 2006, Sun et al. [14] for the first time reported PL in PEG surface functionalized CDs providing the much needed breakthrough. Photoluminescent CDs are biocompatible, nontoxic, water soluble, and stable at body pH, possess

Carbon Dots: Emerging Green Nanoprobes  461 broad excitation spectra, tunable emission spectra, and exceptional photostability, and thus have great potential to serve as green probes for biomedical applications. Lately, innumerable studies have been carried out wherein green CDs have been synthesized from biological sources and used for bioimaging, biosensing, and carriers for site-specific drug delivery. For CDs to function as good and versatile bioimaging probes, both the excitation and emission lights of CDs should be able to penetrate the tissues. The NIR light possesses higher penetration power than visible light. Thus, methods for synthesis of CDs which exhibit emission in the NIR range are being developed. To achieve red and NIR emissive CDs, like semiconductors QDs, the CDs can also be doped with heteroatoms (N, O, P, S, Se). This brings about changes in the valence band and conduction band positions, yielding a narrow band gap [222] and thus the emission shifts toward red light.

14.8.1 Bioimaging According to WHO figures, each year, 8.8 million people die from cancer. Many cancer cases are diagnosed at an advanced stage leading to unnecessary suffering and early death. CDs have shown potential for early diagnosis of cancer cells. Various cells carry different biomarkers on their membrane. Thus, tailor made surface fictionalized CDs have been synthesized as the recognition mechanism depends on the interaction between the biomarkers on the cancer cells and the functional groups on the CDs. In 2016, Bhunia et al. [6] obtained fluorescent CDs from folic acid in a single step. It was observed that these dots carried folic acid on the surface thus were specifically taken up by HeLa and SKOV3 cells which overexpress folate receptors on their surface. While cell lines like HepG2, MCF-7, A549, and CHO exhibited low fluorescence indicating presence of fewer surface folate receptors. Ruan et al. [223] synthesized CDs using glucose and glutamic acid as the carbonaceous source. The surface of the CDs was further functionalized with PEG and Angiopep-2 ligand. These An-PEGCDs could specifically accumulate on the glioma cells due to overexpression of low density lipoprotein receptor-related protein-1 (LRP1) receptors on the surface of glioma cells. Alam et al. [224] developed a green synthesis for fluorescent CDs derived from cabbage using water as solvent. The in vitro bioimaging results displayed immense potential for biomedical applications. Li et al. [225] synthesized CDs which were effectively picked up by breast cancer cells (MIDA-MB-23 and MIDA-MB-68) via endocytosis into the cytoplasm. Eutrophic algal [226] blooms were utilized as a carbon source for synthesis of CDs which exhibited excellent luminescence and

462  Functionalized Nanomaterials for Catalytic Application cell permeability. They were easily uptaken by MCF-7 cells showing excellent potential as biomarkers. Agricultural straw burning commonly practiced by farmers to clear their fields of excess crop residues has a huge impact on urban air quality [227]. Yuan et al. [228] were able to solve two problems in one go by effectively utilizing wheat straw to synthesize luminescent CDs and utilizing them for biolabeling, imaging, and sensing, while Wang et al. utilized rice husk to synthesize CDs and grafted them on silica nanoparticles [229]. These silica-CDs with inherent advantages of both silica and CDs showed potential for various biochemical applications. Fengyi Du and co-workers [230] derived CDs from bagasse waste and evaluated them successfully for biolabeling and bioimaging of human lung adenocarcinoma of A549 cells. One of the most promising approaches in medicine today is stem cell therapy. Stem cells are either directly introduced into the target organ or are given intravenously. These cells then interact with a tissue microenvironment (niche), differentiate into different cell types, and facilitate tissue formation leading to healing. For understanding the role of stem cells in regenerative medicine, their in vivo visualization is necessary. Shao et al. [231] developed CDs from citric acid and utilized them for labeling and tracking bone marrow mesenchymal stem cells and their osteogenic differentiation. Malina et al. [232] effectively utilized quaternized CQDs for in vivo bioimaging of transplanted human mesenchymal stromal cells. The positively charged CQDs were taken up by the negatively charged cell membrane. Upto a concentration of 100 mg/ml, the CDs did not show any negative effect on the function and behavior of the cells, both in vivo and in vitro environment. To gain in-depth understanding into the functioning of the nervous system and visualizing the neuron activity, CDs can play a vital role. Zhou et al. [233] developed a CD-cholera toxin B conjugate (CTB-CDs) fluorescent tracer and utilized it effectively for retrograde tracing in the peripheral nervous system of rats. These conjugates exhibited better optical properties and negligible toxicity when compared to the traditional retrograde tracers (fluorescent dyes and fluorescent labeled compounds). In 2015, Zheng and co-workers [234] developed CDs using D-glucose and L-aspartic acid. The fluorescent dots were able to target C6 Glioma cells. Much stronger fluorescence was detected from glioma site as compared to normal brain, indicating that the CDs had successfully crossed the blood brain barrier. Xue and co-workers developed a photodynamic therapy involving a NIR fluorescent imaging nanoprobe for cancer cells, utilizing lychee exocarp for CDs and loading transferrin and a photosensitizer chlorin e6 on the surface. Due to presence of transferrin on the surface, the nanoprobes

Carbon Dots: Emerging Green Nanoprobes  463 gained access to the cancer cells easily and the surface photosensitizer chlorin e6 produced singlet oxygen through a photodynamic reaction, when irradiated with 650 nm thereby killing the cancer cells in mice [235]. A novel bioimaging technique known as photoacoustic imaging involves conversion of light waves into acoustic waves due to absorption of electromagnetic waves and localized thermal excitation. This technique can be effectively used to monitor in vivo tumor angiogenesis, mapping blood oxygen, measuring methemoglobin, and detecting skin melanoma [236]. In the past few years CDs have been utilized for photoacoustic imaging. Wu et al. [237] for the first time utilized CDs for photoacoustic imaging of sentinel lymph nodes (SLN). This can be used for monitoring progression of cancer. The CDs were synthesized from honey and surface passivated with organic macromolecules. Owing to their strong optical absorbance in NIR region and small size, they underwent rapid lymphatic transport and exhibited an exceptional signal enhancement (~2 min) of the SLN.

14.8.2 Carbon Dots as Biosensors, pH Sensors, and Temperature Sensors CDs exhibit PL in the NIR on being excited with NIR light. This property makes CDs ideal candidates for in vivo biosensing due to transparency of body tissue to light in the NIR range. Also these photoluminescent CDs are good electron donors and acceptors, so their fluorescence can be easily quenched by both electron donor and acceptor molecules. In order to bind with biomolecules of interest, the CDs surfaces need to be functionalized with targeting molecules that can latch on to the biomolecules or metal ions of interest. CDs have extensively been utilized as biosensors for biomolecules like proteins, carbohydrates, vitamins, DNA, metal ions, and also at biological pH. The overall mechanism of fluorescence quenching with all metal ions is similar and involves an interaction between the functional groups on the surface of CDs and the metal ions. Electrons are transferred from the CDs to the metal ions, and this results in quenching of fluorescence. CDs have been utilized to detect a variety of metal ions like Cu+2, Fe+3, Hg+2, Ag+, Cr+3, and H+. Detection of Fe3+ is important because of the important role of Fe3+ in binding to various regulatory proteins and cellular metabolism. Deficiency or excess of Iron in the body can lead to diseases like anemia, Alzheimer’s, and Parkinson’s disease. Zhu et al. [7] synthesized CDs using citric acid and ethylene diamine. The group observed different fluorescent intensities on varying the ratio of the two precursors used. They further observed that

464  Functionalized Nanomaterials for Catalytic Application the fluorescence of CDs was quenched in presence of Fe+3 and the detection limit for Fe+3 being 1 ppm. A hydrothermal synthesis of CDs from coffee beans was developed by Zhang and co-workers [238]. These dots showed the presence of nitrogen and oxygen functional groups on the surface and were used for intracellular sensing and imaging of Fe3+ ions. In the presence of Fe3+ ions, quenching of CDs is noticed, this probably happens due to chelation of Fe3+ ions with carbonyl and hydroxyl groups present on the surface of CDs. Thus, CDs can be developed as probes for diagnosis of Fe3+ related disorders. Sun et al. [239] synthesized GQDs and treated them with ammonia to yield amino functionalized CDs with high QY. The presence of N and O on the surface led to high selectivity for Cu2+ ions as Cu2+ has higher chelating power to N and O than other transition metals. Thus, these CDs can be used to sense Cu2+in living cells. Yang and co-workers [240] synthesized oxidized CDs from Chinese ink and heavily doped them with heteroatoms (N, S, or Se). The doped CDs exhibited higher QY, longer life time, and good photostability. The N- and S-doped CDs were very sensitive for detection of Cu2+ and Hg2+, respectively. Highly fluorescent boron-doped CDs were prepared from ascorbic acid and boric acid by Wang et al. [241] Their fluorescence was quenched by Cu2+ and Pb2+ ions due to formation of metal complexes between the chelating oxygen atoms on the surface of the dots and the metal ions. The fluorescence of the dots was reverted by addition of pyrophosphate, which preferentially formed chelates with the metal ions, thus facilitating metal ion removal from the CD surface and subsequent restoration of fluorescence. Zan and coworkers [242] recently developed an on-off-on fluorescence assay utilizing silicon-doped CDs (Si-CDs) for detection of Cu2+ ions and L-cysteine in the body. Due to presence of abundant amino groups on the surface of these Si-CDs, they could bind to copper ions forming cupric amine complexes and the fluorescence of Si-CDs was quenched due to electron transfer. On the addition of L-cysteine, the Si-CDs regained their fluorescence as the L-cystine itself complexes with Cu2+, removing it from the surface of Si-CDs. These CDs can thus be used for tracing Cu2+ and L-cystine in living systems. Zhang et al. [243] developed bacteria derived fluorescent CDs which were used for detection of p-nitrophenol which is a pollutant widely dispersed in soil and water. The fluorescence is quenched with a detection limit of 0.11 µM for p-nitrophenol. CDs have also been used to detect the presence of bacteria and thus prevent diseases. Zhong and co-workers [244] in 2015 for the first time utilized vancomycin-conjugated CDs for detection of gram positive Staphylococcus

Carbon Dots: Emerging Green Nanoprobes  465 aureus. The vancomycin was able to bind on the bacteria surface which resulted in quenching of the fluorescence of CDs. Ahmadian et al. [245] synthesized green CDs from lemon, grapefruit, and turmeric extract and combined them with Fe3O4 NPs so that the CDs can be manipulated by applying a magnetic field. Fluorescence quenching was observed in presence of E. coli. bacteria. Choi et al. [246] utilized Förster resonance energy transfer (FRET) to detect bacteria. They synthesized phenyl boronic acid functionalized fluorescent CDs and attached them to a diol modified probe molecule. This led to quenching of fluorescence of the CDs. Boronic acid is known to chemoselectively bind reversibly to diols on the surface of bacteria. In the presence of bacteria, the CDs preferentially got attached to the bacterial surface, dislocating the diol probe. The fluorescence of the CDs was regained. Nickel ferrite–CD nanocomposites were used to detect Pseudomonas aeruginosa. With increasing concentration of the bacteria, the fluorescence of the CDs decreased. Antibacterial properties of these dots were also investigated in this study [247]. pH sensing is a very important property of CDs and can be utilized very effectively for detection of pH changes associated with pathogens and physiological changes occurring during prognosis of disease like apoptosis, inflammation, tumor growth, and muscle contraction. Thus, this can facilitate early detection of fatal diseases like cancer and save lives. Jin and co-workers [248] developed pH sensitive CDs from threonine. The fluorescence of these dots decreased with increasing pH. Li et al. [249] have also synthesized pH sensitive CDs from ethanol and sodium hydroxide. The pH sensitivity in both the above methods was due to fluctuation of a single fluorescence emission which are influenced by factors like stability of light source and photo bleaching. These shortcomings were overcome by developing ratiometric fluorescence probes wherein the ratio between the intensities of the two fluorescence maxima is measured rather than the intensity of a single fluorescence emission [8]. Zhou et al. [250] developed pH stimulated fluorescent GQDS as imaging probes with fluorescence transition. The dots appeared blue at normal blood pH of 7.4 and became green at 6.8 pH which is the pH of acidic extracellular microenvironment in a solid tumor. Uthaman et al. developed pH responsive CDs for in vivo tumor diagnosis and treatment. The CDs were surface functionalized with Zwitterion molecules which were positively charged in acidic pH and became negatively charged in basic pH (6.5–6.9). This led to a change in conformation of the CDs and consequently the release of therapeutic agent was triggered. Though CDs can be used to detect DNA but they are not effective at lower concentrations [251].

466  Functionalized Nanomaterials for Catalytic Application Temperature variation has an important role to play in biological processes. Thus, much research is being done to develop non-invasive temperature monitoring probes. CDs have been used for sensing in vivo temperatures. Macairan et al. [252] developed fluorescence-based ratio metric temperature probe utilizing CDs possessing dual fluorescence in the blue to red region of spectra. In the temperature range 5°C–60°C, a linear response was observed with thermal resolution of 0.048 K−1 and thermal sensitivity of 1.97%. With increase in temperature the fluorescence intensity of blue component decreases and red component increases. Liu et al. [253] synthesized CDs with dots in zeolites strategy. The PL intensity of the zeolite-CDs decreased near linearly across the temperature range of 223°K–333°K. This study has demonstrated that CDs in Zeolite strategy can be utilized to synthesize photoluminescent materials with widespread applications. Zhang et al. [254] have recently developed blue luminescent CDs with 56% QY. The CDs were doped with epoxy resin and exhibited exceptional temperature response probably due to high dielectric constant of the resin. A temperature dependent fluorescence was observed from the CD-epoxy composite decreasing linearly from 25°C–95°C.

14.8.3 Carbon Dots for Drug Delivery Most anticancer drugs are associated with many side effects as they cause extensive damage to both the cancer cells as well as the healthy somatic cells. Continuous efforts are being made to improve the target efficiency of the anticancer agents. Nanocarriers conjugated with targeting ligands have been used for selective delivery to cancer cells. Thus, receptor targeted nanocarrier delivery has shown improved therapeutic response with fewer side effects [255]. Another edge that conjugated nanocarriers have over traditional therapeutic agents is that they usually gain entry into the cells via receptor-mediated endocytosis, thus are able to accumulate in the cell without being recognized by P-glycoproteins. It is well known that P-glycoproteins are the main cause of development of drug resistance [256]. In the last few years, CD-ligand conjugates have been effectively used for treatment of various cancer types. They have been conjugated with ligands like transferrin, folate, hyaluronan, and peptides and have been effectively used for drug delivery. Table 14.4 summaries various CD-ligand conjugates, drugs carried, target cells, and characterization techniques utilized. With long-term usage of a drug, there is a likelihood of drug resistance therefore to improve therapeutic efficacy Hettiarachchi et al. [257] developed a triple conjugate of CDs with transferrin and two anticancer

Ligand attached

Tranferrin

Hyaluronan

Nuclear localization signal peptide

hyaluron

Transferrin

Folic Acid

Dots

CDs

CDs

CDs

Mesoporous silica nanoparticle coated with GQDs

CDs

cGQDs Mitoxantrone

Epirubicin Temozolomide

DOX

DOX

Gene

DOX

Drug loaded

Table 14.4  Carbon dot-ligand conjugates used for drug delivery.

HeLa

SJGBM2, CHLA266, CHLA200, U87

HeLa

A549

HeLa

CHLA-266, SJGBM2,

Cell targeted

UV-Visible, FTIR, TEM, fluorescence

UV-Visible fluorescence, FTIR, ATR, TEM

UV-Visible, fluorescence, TEM, IR

FTIR, TEM, UV-visible, XPS, NMR, fluorescence

FT-IR, NMR, UV, TEM, XRD

UV-Visible, FTIR, XPS, TEM

Characterization

[262]

[257]

[261]

[260]

[259]

[258]

Ref

Carbon Dots: Emerging Green Nanoprobes  467

468  Functionalized Nanomaterials for Catalytic Application drugs epirubicin and temozolomide with an average size of just 1.5–1.7nm and tested it against various glioblastoma brain tumor cells lines. The triple conjugate was much more effective than the double conjugates of both the corresponding drugs.

14.8.4 Carbon Dots as Carriers for Neurotherapeutic Agents The biggest challenge for development of neurotherapeutic agents is achieving sufficient penetration of the blood brain barrier which is composed of endothelial cells, closely interconnected by tight intercellular junctions. These tight junctions possess minute nanosize gaps of the order of 4–6 nm; thus, in the last few years, nanoparticle carriers have been used as drug delivery systems for crossing the blood brain barrier [263]. These carriers are usually conjugated to surface ligands which bind to their specific receptors located on the endothelial cells. Transferrin-conjugated CDs have been earlier used for drug delivery to cancer HeLa cells and are known to suppress insulin fibrillation [264] and amyloid beta peptides [263]. As transferrin receptors are found on the endothelial cells, Li et al. [265] developed human transferrin-conjugated CDs as nanocarriers. The dots were injected in the vasculature of zebrafish and they were found to reach the CNS after crossing the blood brain barrier. The group further utilized transferrin-conjugated dots for doxorubicin delivery to pediatric brain tumors. The in vitro tests showed increased uptake of drug as compared to when doxorubicin was used alone though the conjugate exhibited increased cytotoxicity [258]. In 2016, Lu et al. [266] synthesized N-CDs hydrothermally, utilizing citric acid in the presence of polyethylenimine. These dots with negligible cytotoxicity exhibited strong blue luminescence under UV light with a high QY of 51% and were able to cross the blood brain barrier, thus exhibiting potential to be used as traceable drug delivery carriers to the brain. In the last couple of years, CDs have been extensively utilized for neurological imaging, drug delivery in brain, treatment of tumors, and various neurological disorders such as Parkinson’s, Alzheimer’s, prion disease, and amyotrophic lateral sclerosis. Mintz et al. [267] synthesized N-CDs utilizing ethylenediamine as nitrogen source and tryptophan as a carbonaceous precursor for CDs. Since tryptophan is naturally required by the brain, it has specialized transporter LAT1 which facilitates tryptophan to cross the barrier. The CDs produced were found to be surface functionalized with tryptophan and were easily able to migrate in CNS of zebrafish without the need to conjugate with receptors. Cyclodextrins as carriers for antibiotics alginate beads coated with CDs were synthesized and used as drug delivery carriers for tetracycline.

Carbon Dots: Emerging Green Nanoprobes  469 β-Cyclodextrin was used to facilitate drug attachment by inclusion complex formation with tetracycline. The nanocarriers were able to release 70% of tetracycline after 96 h at pH 1 showing its potential use as sustained release treatment for gastrointestinal infections [268]. With so many advantages like easy synthesis, tunable phosphorescence, biocompatibility, water solubility, and exceptional photo stability, CDs have great potential as bioimaging probes and theranostic agents. In the past decade, much work has been done on surface functionalization and doping techniques to lower the band gap and synthesize NIR emissive CDs for utilization in biomedical applications. Various types of biomass wastes have been utilized to synthesize CDs. These methods now need to move on from pilot scale to large-scale industrial production. Further work also needs to be done to improve the fluorescence QY for obtaining a better contrast image in bioimaging with CDs.

14.9 Ethical, Legal, and Sociological Implications of Carbon Dots In the past decade, mammoth number of papers has been published on CDs focusing on their synthesis from every possible carbonaceous raw materials and their wide array of applications ranging from being used as a photocatalyst for energy production to energy storage, biosensors, bioimaging, and tools for drug delivery to wastewater treatment. This has consequently led to a spike in the release of CDs into the environment around us. The ELS debate about CDs like any other nanoparticle circles around three major issues: (1) Public hype and interest in these comparatively environmentally benign CDs: In the recent years, there has been a huge interest of researchers in the synthesis of CDs from biowaste materials. It has been touted as a revolutionary idea of killing two birds with a single arrow. This being an environmentally benign methodology of managing biowaste and converting it into wonder CDs. But, a major hurdle in the process has been non standardized techniques which are difficult to replicate on a large scale. Thus, commercialization of technologies from research papers to industry has by far been very few. (2) Unclear definition, classification and mechanisms: Though a class of CDs is referred as QDs but their structure, synthetic chemistry, and optical properties are very different from the traditional SQDs. This has led to confusion in understanding about the structure and properties of CDs.

470  Functionalized Nanomaterials for Catalytic Application Unlike traditional SQDs, CDs possess excellent water solubility, negligible toxicity and good conductivity. Another aspect is that the word CD is used for a diverse group of carbonaceous nanodots ranging from amorphous to crystalline nature of the core. Across literature varied terms for fluorescent CDs such as CNDs, CQDs, polymeric dots and GQDs have been used adding to confusion ambiguity. Despite the huge number of publications describing the optical properties of CDs, the exact mechanism of PL is still debatable. Moreover, often C dots do not show a size dependent QCE and most of the time the observed colors are related to surface states and molecular states. Thus, there is a need for an in-depth mechanistic study of the factors associated leading to aviation in optical properties. (3) Nascent stage of technology: The standardized procedures for preparation and characterization of CDs are still in the developmental stage. Very few toxicity studies of CDs have been reported in animals, plants, and aquatic environment. In 2013, Gao et al. injected CDs in rats through the tail vein. At high doses, oxidative damage and proliferation in lymphocytes was observed. The proliferative capacity was found to be directly dependent on CD dosage [269]. An in vivo cytotoxicity study for citric acid derived CDs on mice reported negligible toxicity on low concentrations of CDs [270]. The LD50 of CDs calculated by linear interpolation was found to be above 350 mg/ kg. Male mice were found to be more sensitive to higher concentration of CDs as compared to female mice. A few in vitro cytotoxicity studies of CDs on human bronchial epithelial cells (16HBE), human lung carcinoma cells (H1299 andA549), and kidney epithelial cells of African green monkey have reported toxicity at high CD concentrations [271]. Toxic effects of CDs were studied by Xio et al. [272] on rare minnow, a fish in China with a short life cycle. The group reported that the rare minnow embryos exhibited increase in heart rate, decrease in size and movement, edema, and morphological malformations. Biochemical analysis confirmed that exposure of CDs inhibited the activity of Na+/K+, ATPase, Ca2+, ATP, and increased malondialdehyde-a marker of oxidative stress. The decrease in activity of first line defense antioxidants (superoxide dismutase, catalase, glutathione peroxidase) was also reported. This led to embryonic cellular damage. A comparative study of CDs toxicity on various aquatic species including zebrafish, zooplanktons, and phytoplanktons showed that zooplanktons were more sensitive to CDs and exhibited oxidative stress, nutrient insufficiency, water acidification, and inhibited photosynthesis [273].

Carbon Dots: Emerging Green Nanoprobes  471 Recently, CDs have been used in agriculture for improving the growth of plants and yield of food crops. But, some studies have reported that in higher concentrations of the order of 1,000–2,000 mg/L inhibition of growth in maize plants was observed [274]. CDs at such high concentrations resulted in oxidative stress. Wang et al. have demonstrated that CDs over concentrations of 125 mg/L reduced root elongation in Arabidopsis thaliana and activity of glutathione reductase was significantly increased [275]. Thus, it may be concluded that despite exceptional suitability of CDs for plant growth, water remediation, bioimaging, biosensing, and other biomedical applications, their biosafety is still unclear and long-term toxicological and pharmacokinetic investigations involving metabolism, excretion, and persistence of CDs in body and their effects on immunological response needs to be extensively researched. Accordingly, regulatory frameworks, for CDs permissible limit in air, water, and soil, need to be formalized at both the national and international level and protocols for disposal of CDs need to be established.

14.10 Conclusion and Future Outlook In the present chapter, we have tried to summarize environmental sustainable synthesis of CDs, characterization, optical properties, and their diverse applications. Since their discovery in 2004, the field of CDs is evolving rapidly in terms of development of synthetic protocols, understanding of properties and their applications in the area of energy production, eradication of pollutants, water purification, photocatalysis, biomedical applications, fabrication of solar cells, and energy storage devices. Synthesis of CDs from any carbonaceous material present around us including kitchen waste such used frying oil and tea leaves or fruit and vegetable peels, agricultural wastes, as well as animal by-products is a promising concept of “waste to wealth”. Various cost-effective synthetic techniques are available which utilize these precursors and produce fluorescent CDs under environmental benign reaction conditions. However, the synthetic protocols developed, so far, lack consistency in terms of control on the structural and physicochemical properties of CDs as well as the reproducibility of the procedures are not reliable. Therefore, high yielding, cost-effective, and eco-friendly synthetic techniques are required which are reproducible and can produce stable CDs, with precise control on their structural and physicochemical properties. Efforts are also needed to develop synthetic protocols employable at the commercial level.

472  Functionalized Nanomaterials for Catalytic Application The unique structure of CDs provides immense opportunities for chemical and structural modifications in the core and as well as on the surface. The surface structure and therefore optical properties of CDs can be modified by chemical treatment involving surface doping, passivation, and/or surface functionalization which not only enhance the PL but also can increase their QYs and stability. As properties of CDs are controlled by their structure, a clear “structure-property correlation” needs to be established. CDs exhibit unique PL which not only depends on the size, composition, and surface structure but is also dependent on the excitation wavelength. The exact mechanism for the origin of PL in different classes of CDs is unclear and controversial. The unique optical properties of CDs coupled with their non-toxic, water soluble, and biocompatible nature make them promising luminescent probes to replace organic dyes and inorganic SQDs. Therefore, it is extremely important to address the issues raised here to tap the full potential of CDs in various fields of its applications.

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Index 2,4-diclorophenol, 454 4-nitrophenol (4-NP), 323 Abamectin, 94 Ablation, 170 Accumulation in body systems, 229 Acid orange 7 dye, 454 Active and intelligent packaging, 229 ADME profile (adsorption, distribution, metabolism, and elimination), 241 Advanced disease diagnosis, 228 Advanced drug delivery systems, 228 AFM (atomic force microscopy), 311t, 316 Ag nanoparticles, 92, 99 Agglomeration, 174 Agriculture, 228 Aloe vera, 97, 307 Amorphous, 171–172 Animal healthcare, 227, 229 Anodic, 7 Antibacterial, 228 Antibiotic activities, 229 Antibiotic resistance, 227 Antifungal, 228 Antimicrobial, 30, 229 Antimicrobial effects, 227 Antimicrobial resistance, 228 Anti-stokes, 440, 444 Application of metal-based nanocatalysts, 335 Applications of carbon dots,

bioimaging, 426–431, 443, 446, 460–463, 469 biolabeling, 462 biosensing, 460, 461, 463, 471 drug delivery, 460, 466, 467 hydrogen evolution reaction (HER), 455 optical switch, 450 oxygen evolution reaction (OER), 455 pH sensing, 465 photoacoustic imaging, 463 photocatalysis, 443, 446, 449, 458 photoreduction of CO2, 458, 459 removal of dyes, 452 temperature sensing, 463 Applications of nanoemulsions, antioxidants, 238 Escherichia coli, 238 flavors, 238 food packaging, 238 food pathogens, 238 ginger essential oil, 238 meat processing industries, 238 nutraceuticals, 238 oregano, 238 shelf life, 238 sodium caseinate, 238 vitamins, 238 ω-3 fatty acids, 238 Arsenic, 181 Arsenic, removal by magnetic nanoparticles, 74–75

493

494  Index Ascorbic acid, 58 Association colloids, micelles and reverse micelles, 236 paprika oleoresin, 236 ATR-IR (attenuated total reflectanceinfrared spectroscopy), 309t Bandages, 229 BET (Brunauer–Emmett–Teller surface area analysis), 310 Bioactive compounds, 227 Bioactive molecules, 231 Bioavailability, 227 Biodegradability, 229 Biogenic synthesis, 304, 306–307, 319f, 324 Biointensive integrated pest, 91 Biological-based pest management, 96 Biomaterial-based FNMs, 154–156 chitosan/cellulose-based FNMs as heterogeneous catalyst, 155–156 Bionanotechnology, 90, 91 Biopersistent, 228 Biopesticides, 91, 92, 93, 96 Biopolymeric nanoparticles, antimicrobial, 238 benzoic and vanillic acids, 239 chitosan nanoparticles, 239 edible films, 239 Escherichia coli O157:H7, 239 fish fingers, 239 Listeria monocyto­genes, 239 polyethylene glycol, 238 polylactic acid, 238 polylactic-co-glycolic acid, 238 Salmonella typhimurium, 239 Biosensors, 31, 227, 229 Black pepper plant extract, 307, 308, 312–313 Bottom-up, 170 Bug’s vermin, 98 Built nanoparticles, 95

Cadmium, removal by magnetic nanoparticles, 75 Camellia sinesis, 179 Cancer therapy, 228 Carbon, 229 Carbon dioxide (CO2), 371, 374–383, 385–393, 398, 400, 404–412 Carbon nanodots (CND), 418–423, 470 Carbon nanoparticles, 417 Carbon nanotube (CNT), 56, 65, 67, 363, 364, 369, 370, 380–382, 384, 387–388, 395, 397, 399, 401, 403, 409–412, 414 Carbon polymer dots (CPD), 418–423 Carbon quantum dots (CQD), 418–422, 432, 442 Carbon sequestration, 446, 457 Carbonaceous, 5, 9, 32 Carbon-based FNMs as heterogeneous catalyst, 145–147 Carbon-based nanomaterials (CNMs), functionalization of, 65–67 synthesis of functionalized carbon nanotubes and graphene, 67 Casein micelles, 229 Cathodic, 7–9, 13 Ceramic foam, 15 Cetyl trimethyl ammonium bromide, 58 Chalcogenides, 272–274 Characterization of carbon dots, atomic force microscopy, 439 high-resolution transmission electron microscopy (HRTEM), 438, 448 infrared spectroscopy, 439 NMR spectroscopy, 439 raman spectroscopy, 438 surface electron microscopy (SEM), 439, 448 transmission electron microscopy (TEM), 439, 448, 467

Index  495 UV-visible spectroscopy, 440, 441 x-ray diffraction (XRD), 438, 467 x-ray photoelectron spectroscopy (XPS), 439, 467 Chemical pesticides, 93 Chitosan, 181 Chromium, removal by magnetic nanoparticles, 75–76 Chrysophyllum cainito, 307, 319–321 Cinnamomum camphora, 307 Cinnamon zeylanicum, 307 Classification of carbon dots, 419 Classification of nanomaterials, beta-carotene, 231 globular proteins, 231 inorganic nanoparticles, 231 iridium, 231 iron, 231 liposomes, 231 nanodispersions, 231 nanoemulsions, 231 nanofibrers, 231 organic nanoparticles, 231 platinum, 231 polymeric, 231 proteins, 231 silver, 231 zinc, 231 Clay, 181 Coleoptera, 99, 101 Comparison table, 157–158 Complex hybrid systems, 228 Condensation, 172 Contaminants, 227 Coomassie brilliant blue G-250 (CBB), 323 Copper oxide, 229 Coprecipitation, 170, 275–276 Core-shell, 271, 288, 290 Corn cob, 307 Cosmetic, 228 Co-surfactant, 172 Cyanation reactions, 312

Cyfluthrin, 94 Cytotoxic activities, 100 Deactivation, 175 Degradation, 179, 181 Deodorants, 229 Deposition, 170 Diamond NP, 15 Different types of metal-based nanoparticles/crystals used in catalysis, 340 multi-metallic/nano-alloys/doped metal nanoparticles, 343 Perovskite-type oxides metal nanoparticles, 342 transition metal nanoparticles, 341 Disease diagnostics, 228, 229 DNA-tagged CdS, 100 DNA-tagged nanogold, 100 Doping, 420, 439–441, 443, 447, 460 Drug delivery, 227, 229 Drug delivery systems, active pharmaceutical principles, 232 biodegradable, 232 biodegradable metal nanovehicles, 232 blood brain barrier, 232 brain cancer therapy, 232 cancer therapy, 232 chemotherapeutic agents, 232 complex drug metabolism, 232 drug resistance, 232 drug uptake, 232 genes, 232 multiple drugs, 232 multitargeted drug delivery, 232 nanosize, 232 nanovehicles, 232 selective drug delivery, 232 sustained release of drugs, 232 target cell interactions, 232 target cell-surface receptors, 232 targeted activities, 232

496  Index transporter, 232 vehicles, 232 Drugs, 2, 31, 32 Dyes, 179 ECMR, 2, 7 Eco-accommodating, 95 Eco-friendly chemical, 93 Edible films, 229 EDS (energy-dispersive x-ray spectroscopy), 311t Electrocatalyst, 2, 6, 7 Electrochemical, 170, 178 Electronics, 228 Electrospun-assisted nanosporus membrane, 254, 255 Engineered nanomaterials, 253, 256 Engineered nanoscale materials, 229 Environment, 241 EPA, 2, 5 Epoxidation, 176–177 Ethylene glycols, 100 Eutrophic, 181 Excretions, 229 Extract, 179–181 Faradaic efficiency, 376, 381 Fenton, 180 Ferrites, 268–269, 271 FE-SEM (field-emission scanning electron microscopy), 309 Ficus benghalensis, 97 Ficus religiosa, 97 Flocculation, 175 Fluorescence/phosphorescence, 440, 441, 445 FNMs for various other applications, 156–157 Food industries, 228 Food industry, 228 Food packaging industry, 229 Food processing, 229 Food safety, 227 Food science, 229

Food storage containers, 229 FT-IR, 99 Fullerene, 25, 32 Functional, 229 Functional animal products, 227 Functionalization, 169–170, 174 Functionalized, 228 Functionalized nanomaterials (FNMs), application of, 67, 69–70, 74 removal of arsenic by magnetic nanoparticles, 74–75 removal of cadmium by magnetic nanoparticles, 75 removal of chromium by magnetic nanoparticles, 75–76 removal of mercury by magnetic nanoparticles, 76 Functionalized nanomaterials (FNMs), synthesis of, chemical methods, 57–58 grafting of synthetic polymers, 58 ligand exchange process, 58 miscellaneous methods, 58–59 Functionalized nanomaterials in environmental applications, alumina, 121–124 cellulose, 117–121 chitosan, 114–117 mixed composites, 124–126 other nanocomposites for environment, 126–130 Functionalized nanomaterials, 143–145, 228 carbon-based functionalization, 143–145 covalent functionalization, 145 non-covalent functionalization, 145 Gas chromatography analysis, 319 Gaseous/air pollutants, 54 Geraniol, 176–177 Gold, 229 Graphene, 65, 67

Index  497 Graphene quantum dots (GQD), 418, 420–422, 464, 467, 470 Gum olibanum (Boswellia serrata), 322–323 Halloysite nanotubes, 229 Health sciences, 228 Heavy metal detective nanotechnology, 187 Heavy metals, health problems due to, 71t–72t removal of (see functionalized nanomaterials (FNMs), application of) Heck coupling reaction, 320 Helicoverpa armigera, 99 Hematite, 177–178 Herceptin, 65 Heterogeneous, 2, 14, 169–171, 174, 176 Heterogeneous catalysis, 267 History of nanotechnology, 186–187 Hiyama cross-coupling reaction, 312–313 Home appliances, 229 Homogeneous, 170–171, 174, 176 Homogeneous catalysis, 263, 266–267 HR-TEM analysis, 320 Hydroelectric power, 253, 256 Hydrogen evolution reaction (HER), 374, 375, 382, 384, 389, 409 Hydrolysis, 172–174 Hydrothermal, 179, 276, 282 Hyperthermia, 178 ICP-OES (inductively coupled plasma-optical emission spectroscopy), 311t, 317 Impregnation, 170 In situ, 253 Insectpest, 91, 92, 97 Interfacial resistance, 255 Jablonski energy diagram, 441, 442

Keggin-type, 176 Leaf senescence accelerators, 93 Lepidoptera, 102 Light-weight stealers, 93 Livestock produce, 229 Maghemite, 59, 76, 177 Magnetic, 355, 358, 362, 364, 367, 368, 371, 373, 374, 393, 396–398, 401, 402, 404, 410, 414 Magnetic nanoparticles, characterization of, 60 functionalization of, 63–65 magnetite, 59, 76 mercury, removal by magnetic nanoparticles, 76 synthesis of, 59–60 Magnetite, 15, 177, 179 Medicine, 228 Mesoporous, 177 Meso-prous, 8, 9 Metabolites, 178 Metal and metal oxide-based FNMs, 147–154 functionalization techniques of metal oxides, 147–148 palladium-based FNMs as heterogeneous catalyst, 153 platinum-based FNMs as heterogeneous catalyst, 150–153 silver-based FNMs as heterogeneous catalyst, 148–150 zirconia-based FNMs as heterogeneous catalyst, 153–154 Methods followed for fabrication of nanomaterials, 137–142 chemical vapour deposition, 141 co-precipitation, 138 hydrothermal, 142 immobilization/encapsulation, 140–141 impregnation, 139 ion-exchange, 139–140

498  Index microemulsion, 141–142 thermal decomposition, 142 Methods for the functionalization of nanomaterials, 110–112 Methyl orange, 452 Methylene blue (MB), 323, 452 Micelle, 172, 174 Microemulsion, 172, 174–175, 178 Microemulsion method, 277–278 Microporous, 9 Mineralization, 181 Monodisperse, 172 Multifunctionality, 228 Multiscale modeling, 256 Nano-additives, 241 Nano-based pest management, 96 Nano-biocide, 229 Nanobiopesticide, 89, 90, 92, 96, 100, 101 Nanobiosensors, 90 Nanocatalyst, 355–360, 362–364, 367–371, 377, 378, 382, 384–385, 387, 389, 396, 397, 398, 400, 402, 403, 407, 409, 414 Nanocoatings, 229 Nanocomposite, 447 antibacterial, 240 antifungal, 240 bionanocomposites, 240 carbon nanoparticles, 240 carbon nanotubes, 240 cellulose casings, 240 collagen films, 240 Durethan, 240 Escherichia coli, 240 ethylene–vinyl alcohol copolymer, 240 gas barrier, 240 Listeria monocytogenes, 240 nanoclays, 240 nanofillers, 240 nanoscale metals, 240 nylon, 240 PLA biopolymer, 240

polyamide, 240 polymer matrix, 240 polymeric resins, 240 polyolefins, 240 polystyrene, 240 Pseudomonas aeruginosa, 240 silver-based nanocomposite, 240 titanium oxide, 240 zinc oxide, 240 Nanocomposites, 229 Nanoemulsions, diacylglycerols, 237 emulsifiers, 237 free fatty acids, 237 interfacial tension, 237 lipophilic nutraceuticals, 237 mineral oils, 237 monoacylglycerols, 237 oil phase, 237 phospholipids, 237 physico-chemical properties, 237 ripening retarders, 237 sorbitan monooleate, 237 stabilizers, 237 sucrose monopalmitate, 237 texture modifiers, 237 triacylglycerols (TAG), 237 water phase, 237 waxes, 237 weighting agents, 237 Nanoencapsulation, calcium, 236 chitosan, 236 coenzyme Q10, 236 controlled/sustainable release, 235 flavor enhancers, 235 iron, 236 nanoemulsification, 235 nanostructuration, 235 nutrient delivery, 236 pH oriented/nanoencapsulation, 236 shelf life, 236 sustainable release of antioxidants, 236

Index  499 targeted delivery system, 236 vitamins, 236 Nanofiber membrane, 254 Nanofiltration, 254, 255 Nanofoods, 227 Nanoliposomes, 229 Nanomaterial-functional group bonding type, 112–114 Nanomaterials, 228, 229 bottom-up, 230 grinding, 230 homogenization, 230 milling, 230 nanoparticles, 230 nanotubes, 230 self-organization, 230 sieving, 230 supra-molecular structures, 230 thin films, 230 top down, 230 Nanomaterials and animal health, anticancer, 230 bioimaging, 230 disease diagnostics, 230 fluorescent cancer cells, 230 nanotheranostics, 230 near-infrared fluorescent probes, 230 non-invasive, 230 quercetin-gold-nano-composite, 230 targeted delivery, 230 tumor, 230 visualization, 230 Nanomaterials and livestock produce, bioavailability, 234 biopolymeric nanoparticles, 235 bioseparation, 235 colloids, 235 color, 235 dairy sector, 235 lipid-based nanoencapsulators/ nanocarriers, 235 milk, 235 nanoemulsions, 235

nanoencapsulation, 235 nanofiber, 235 nanolaminates, 235 nanosieves, 235 nutrient smart delivery, 235 post-harvested processing technology, 234 sensory attributes, 234 texture, 235 tip-top up bread, 235 Nanomaterials and packaging, active packaging, 240 anti-browning agents, 240 antimicrobial, 240 antioxidants, 240 barrier functions, 240 flavors, 240 intelligent/smart packaging, 240 meat and meat products, 240 mechanical, 240 nanocomposite polymers, 240 nanosensors, 240 optical properties, 240 pesticides, 240 thermal, 240 Nanomaterials and sensory attributes, Al2O3, 239 anticaking agent, 239 center for food safety and applied nutrition, 239 color, 239 flavor, 239 office of cosmetics and colors, 239 SiO2, 239 texture, 239 TiO2, 239 USFDA, 239 Nanomaterials, different types of, one-dimensional (1D) nanostructures, 56 three-dimensional (3D) nanostructures, 56 two-dimensional (2D) nanostructures, 56

500  Index zero-dimensional (0D) nanostructures, 55–56 Nanomedicines, 227 Nanoparticles, 227–229, 261, 263–264, 268, 275–279, 283, 287–289 Nanoremediation, 253 Nanoscale, 228 Nanoscience, 255 Nanosensors, antibiotic residues, 241 biofinger, 241 digital transform spectrometer, 241 electronic tongue or noses, 241 micro- and nanotechnologies (MNTs), 241 microbial contamination, 241 nanocantilever, 241 nanoelectromechanical system, 241 rapid pathogen detection, 241 real-time check, 241 Nanostructured, 363, 369–371, 375–378, 380, 381, 383, 388–392, 397, 405, 406, 411 Nanostructured aluminium oxide, 92, 97, 99, 100 Nanosystem, 227 Nanotechnology, 169, 227–229, 252–256 Nanotechnology for arsenic (As) removal, 187–197 Nanotechnology for cadmium (Cd) removal from water, 200 Nanotechnology for lead removal from water, 197–200 Nanotechnology for nickel (Ni) removal, 200–209 Nano-TiO2, 99, 100 Natural nanomaterials, 229 Non-leaching, 175 Nutraceuticals, 229 Nutrient delivery, 229 Ocimun sanctum (tulsi), 321–322 Oleic acid, 58

Oleylamine, 58 Onion peel, 307 Oolong, 180 Oxygen reduction reaction (ORR), 365, 370, 374, 383–384, 388, 389, 403, 410 Paints, 229 Palladium nanoparticles (PdNPs) synthesis, applications, 308–323 biogenic synthesis of PdNPs, 306–307 common characterization techniques for, 309t–311t methods for, 305 phytochemicals: constituent of plant extract, 307 techniques for characterization of metal NPs, 308 Papaya peel, 313–315 PDMS, 454 Pest management, 90, 92, 94, 96, 102 Pesticide, 4, 8, 31, 32 Photoblinking, 445 Photocatalyst, 267–268, 272–274, 280, 286 Photocatalytic, 267–268, 273–274, 278, 283, 285–287 Photogeneration, 19 Photoinduced electron transfer (PET), 445, 446 Photoluminescence (PL), 419–421, 441, 442, 463, 472 Photosynthetic rate reducers, 93 Photothermal mediators, 450 Photovoltaic cells, 255 Phytochemical, 94, 96, 100, 307 Phytopathogens management, 102 Pollutant, 181 Polyacrylamide, 58 Polycarboxylic acids, 59 Polycondensation, 173 Polydisperse, 181

Index  501 Polyoxometalated, 176 Polystyrene, 58 Polytetrafloroethylene, 254 Polyvinyl alcohol, 58 Polyvinyl pyrrolidone, 58 Pomegranate, 307 Precipitation, 170, 175, 178 Processing and packaging, 229 Production of nanoemulsions, high-energy and low-energy approaches, 237 high-pressure valve homogenizers, 237 ionic strength, 238 microfluidizers, 237 phospholipids, 238 polysaccharides, 238 proteins, 238 sonication, 237 surfactant type, 238 surfactant-oil-water ratio, 238 surfactants, 238 Pulicaria glutinosa extract, 307, 318–319 Quality assurance, 227 Quantum confinement effect (QCE), 418–420, 440, 470 Quantum yield (QY), 426, 427–432, 436, 440, 443, 444, 447 Radio nuclide-based imaging techniques, 231 Recyclability, 171, 312, 313, 318, 322, 324 Regulatory bodies, 227 Regulatory frameworks, 229 Rhodamine B (RB), 323 Risk factors, 229 Role in therapeutics, antiarthritic, 233 anticancer, 233 anticancer property, 233 anti­filarial, 233 antifungal, 233

antigout, 233 antiinflammatory, 233 bio­sensing, 233 cell signaling pathways, 233 chitosan-based gold nanomaterials, 233 cytotoxicity, 232 dogs, 233 drug efflux, 233 filariasis, 233 gold nanoparticles, 233 gram negative, 233 gram positive, 233 immunomodulatory potential, 233 immunotherapy, 233 in vitro anti-bacterial, 233 innate antimicrobial activity, 232 large surface area, 232 magnesium oxide metal nanomaterial, 233 multidrug resistant E. coli, 233 nanocarriers, 233 nano-oncology, 233 old age, 233 peptidoglycan, 233 silver nanoparticles, 232, 233 targeted activity, 233 tumor biomarker sensing, 233 two-dimensional graphenes, 233 vaccine adjuvants, 233 viricidal, 233 zinc oxide nanoparticles, 233 Safety concerns, 229, 241 SEM, 97 Semiconductor(s), 13, 19, 27, 29 Sensory acceptance, 227 Silanol, 174, 176 Siloxane, 174 Silver, 229 Silver salt, 98 Skincare products, 228 Smart food packaging, 229 Sodium borohydride, 323

502  Index Sol-gel, 170, 172–173, 178 Sol-gel method, 279, 283 Sonochemical, 170, 178 Soybean leaf, 307 Stand reducers, 93 Star apple, 307, 319–321 Stokes shift, 442 Structure and catalytic properties relationship, 343 Styrene, 177–178 Surface biocides, 229 Surface functionalziation, 424, 425, 469 Surface passivation, 422, 424, 439, 440, 443 Surface-active, 176 Surfactant, 172, 174, 179 Sustainability, 253 Suzuki coupling reaction, 315–316, 319 Suzuki-Miyaura reaction, 314 Synthesis of carbon dots, bottom-up, 418, 421, 422, 424, 425, 443 chemical oxidation, 430, 431, 436–438 hydrothermal treatment, 426, 427, 432, 433, 437, 444, 451 microwave assisted method, 428, 434, 435, 437, 444, 451, 452 pyrolysis, 429, 435, 436, 456, 460 solvothermal treatment, 424, 428, 433, 437 top-down, 418, 421, 422, 424, 443 Synthetic pesticides, 97

Therapeutics, 228, 229 Tissue customers, 93 Titanium dioxide, 229 Tooth-pastes, 229 Top-down, 170 Toxic effects, 229 Toxicities, 229 Toxicity and risks, allergy, 234 asthma, 234 biological barriers, 234 cancer, 234 cytotoxicity, 234 DNA damage, 234 environmental impact, 234 gene mutations, 234 in vitro cytotoxicity study, 234 nanotoxicology, 233 nephro­toxicity, 234 oxidative stress, 234 tumorigenesis, 234 Trioctylphosphine, 58 Types of nanocatalysis, 337 green nanocatalysis, 338 heterogeneous green nanocatalysis, 339 homogeneous green nanocatalysis, 340 multiphase nanocatalysis, 340

Targeted drug delivery, 228 TEM (transmission electron microscopy), 97, 310t, 316, 317, 322–323 Terminalia chebula, 307 TGA (thermogravimetric analysis), 310t

Van der Waals, 24, 31

UCPL, 440, 444, 445, 448, 459 Ultrafilteration, 253 Unhydrolyzed, 174 UV–Vis (ultra violet-visible spectroscopy), 309

Water purification, 253 Water quality, 253 Watermelon rind, 307, 315–316 Wavelength, 228 WHO, 2, 5, 31

Index  503 Wound healing, 228 XPS (x-ray photoelectron spectroscopy), 310, 321–322

XRD (x-ray diffraction), 310, 316, 318, 320 Zinc oxide, 229