Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science 9781774911679

Table of contents :
Cover
Half Title
Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science
Copyright
About the Book Series: Innovations in Agricultural and Biological Engineering
Other Books on Agricultural and Biological Engineering by Apple Academic Press, Inc.
About Senior-Editor-in-Chief
About the Editor
Contents
Contributors
Abbreviations
Preface
Part I: Green Nanomaterials in Energy Production and Storage
1. Innovative Graphene Thermoelectric Nanomaterials for Production and Conversion of Green Energy
2. Eco-Friendly Applications of Graphene Nanomaterials in Energy Storage Devices: A Review
3. Advances in High Volumetric Density Carbon Nanotubes for Electrochemical Energy Storage
4. Application of Nanomaterial-Modified Membrane Filters for Separation of Heavy Metal Pollutants
Part II: Scope of Green Nanomaterials in Biomedical Applications
5. Green Nanotechnology for Renovating Phytomedicines
6. Green Nanomaterials as a Boon to Biomedical Engineering
7. Trends in Green Synthesis of Carbon-Based Nanostructures
Absrtact
7.1 Introduction
7.2 Allotropes of Carbon
7.2.1 Fullerenes
7.2.2 Carbon Dot (C Dot)
7.2.3 Carbon Nanotubes (CNTs)
7.2.4 Graphene
7.3 Purpose of Green Carbob Nanostructures
7.4 Preparation of Carbon Nanostructures
7.4.1 Preparation of Fullerene
7.4.2 Preparation of Carbon Dots
7.4.3 Preparation of Carbon Nanotubes (CNTs)
7.4.4 Preparation of Graphene
7.5 Latest Developmets and Challenges in Green Carbon Nanostructures
7.6 Applications of Carbon Nanostructures
7.6.1 Applications of Fullerenes
7.6.2 Applications of Carbon Dots
7.6.3 Applications of CNTs
7.6.4 Applications of Graphene
7.7 Conclusion
Keywords
References
8. Applications of Biodegradable Polymeric Biomaterials in Biomedical Science
Abstract
8.1 Introduction
8.2 Biodegradable Polymer
8.2.1 Polymers of Natural Origin
8.2.1.1 Chitin/Chitosan
8.2.1.2. Alginic Acid/Alginate
8.2.1.3 Starch
8.2.1.4 Hyaluronic Acid
8.2.1.5 Fibrin
8.2.1.6 Collagen
8.2.1.7 Gelatin
8.2.1.8 Silk Fibers
8.2.1.9 Polyhydroxyalkanoates (PHA)
8.2.2 Polymers of Synthetic Origin
8.2.2.1 Saturated Aliohatic Ployesters
8.2.2.2 Polyanhydrides
8.2.2.3 Polyurethane (PUR)
8.2.2.4 Polyphosphazenes
8.3 Deriving Biomaterials from Biodegradable Polymers
8.3.1 Biomaterials from Natural BiodegradablE Polymers
8.3.1.1 Biopolymers as Smart Biomaterials
8.3.1.1.1 Biodegradable Thermo-Responsive Biopolymers
8.3.1.1.2 Biodegradable pH-Responsive Biopolymers
8.3.1.1.3 Biodegradable Physicochemical Responsive Biopolymers
8.3.2 Biomaterials from Synthetic Biodegradable Polymers
8.4 Role of Nanotechnology in Designing Biodegradable Polymeric Biomaterials
8.4.1 Micelle
8.4.2 Dendrimers
8.4.3 HydrogeL Nanoparticle Composites
8.4.4 Lipid Nanoparticles
8.4.5 2D Inorganic Quantum Dots (QDS)
8.5 Biomedical Applications of Biodegradable Polymeric Biomaterials
8.5.1 Drug Delivery
8.5.2 Tissue Engineering
8.5.2.1 Skin Tissue Engineering
8.5.2.2 Nerve Tissue Engineering
8.5.2.3 Bone Tissue Engineering
8.5.2.4 Wound Healing
8.5.2.5 Temporary Implants
8.6 Conclusion and Future Prospects
Keywords
References
Part III: Innovations in Green Nanotechnology for Agricultural and Environmental Sustainability
9. Role of Green Nanomaterials in Sustainable Agriculture
10. Green Agricultural Waste-Derived Silica Nanoparticles as Catalyst Support Materials
11. Green Synthesis of Nanomaterials for Photocatalytic Biodegradation of Pollutants in Wastewater
Index

Citation preview

ADVANCES IN GREEN AND

SUSTAINABLE NANOMATERIALS

Applications in Energy, Biomedicine,

Agriculture, and Environmental Science

Innovations in Agricultural and Biological Engineering

ADVANCES IN GREEN AND

SUSTAINABLE NANOMATERIALS

Applications in Energy, Biomedicine,

Agriculture, and Environmental Science

Edited by Megh R. Goyal, PhD

Shrikaant Kulkarni, PhD

First edition published 2024 Apple Academic Press Inc. 1265 Goldenrod Circle, NE, Palm Bay, FL 32905 USA 760 Laurentian Drive, Unit 19, Burlington, ON L7N 0A4, CANADA

CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 USA 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN UK

© 2024 by Apple Academic Press, Inc. Apple Academic Press exclusively co-publishes with CRC Press, an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the authors, editors, and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors, editors, and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library and Archives Canada Cataloguing in Publication Title: Advances in green and sustainable nanomaterials : applications in energy, biomedicine, agriculture, and environmental science / edited by Megh R. Goyal, PhD, Shrikaant Kulkarni, PhD. Names: Goyal, Megh R., editor. | Kulkarni, Shrikaant, editor. Series: Innovations in agricultural and biological engineering. Description: First edition. | Series statement: Innovations in agricultural and biological engineering | Includes bibliographical references and index. Identifiers: Canadiana (print) 20230133576 | Canadiana (ebook) 20230133606 | ISBN 9781774911662 (hardcover) | ISBN 9781774911679 (softcover) | ISBN 9781003328322 (ebook) Subjects: LCSH: Nanostructured materials—Environmental aspects. | LCSH: Nanostructured materials—Industrial applications. Classification: LCC TD196.N36 A38 2023 | DDC 620.1/15—dc23 Library of Congress Cataloging-in-Publication Data

CIP data on file with US Library of Congress

ISBN: 978-1-77491-166-2 (hbk) ISBN: 978-1-77491-167-9 (pbk) ISBN: 978-1-00332-832-2 (ebk)

ABOUT THE BOOK SERIES: INNOVATIONS IN AGRICULTURAL AND BIOLOGICAL ENGINEERING Under this book series, Apple Academic Press Inc. is publishing book volumes over a span of 8–10 years in the specialty areas defined by the American Society of Agricultural and Biological Engineers (). Apple Academic Press Inc. aims to be a principal source of books in agricultural and biological engineering. We welcome book proposals from readers in areas of their expertise. The mission of this series is to provide knowledge and techniques for agricultural and biological engineers (ABEs). The book series offers high-quality reference and academic content on agricultural and biological engineering (ABE) that is accessible to academicians, researchers, scientists, university faculty and university-level students, and professionals around the world. Agricultural and biological engineers ensure that the world has the necessities of life, including safe and plentiful food, clean air and water, renewable fuel and energy, safe working conditions, and a healthy environment by employing knowledge and expertise of the sciences, both pure and applied, and engineering principles. Biological engineering applies engineering practices to problems and opportunities presented by living things and the natural environment in agriculture. ABE embraces a variety of the following specialty areas (): aquaculture engineering, biological engineering, energy, farm machinery and power engineering, food, and process engineering, forest engineering, information, and electrical technologies, soil, and water conservation engineering, natural resources engineering, nursery, and greenhouse engineering, safety, and health, and structures and environment. For this book series, we welcome chapters on the following specialty areas (but not limited to): • • • •

Academia to industry to end-user loop in agricultural engineering; Agricultural mechanization; Aquaculture engineering; Biological engineering in agriculture;

vi

About the Book Series

• • • • • • • • • • • • • • • • • • • • • • • • • •

Biotechnology applications in agricultural engineering; Energy source engineering; Farm to fork technologies in agriculture; Food and bioprocess engineering; Forest engineering; GPS and remote sensing potential in agricultural engineering; Hill land agriculture; Human factors in engineering; Impact of global warming and climatic change on agriculture economy; Information and electrical technologies; Irrigation and drainage engineering; Nanotechnology applications in agricultural engineering; Natural resources engineering; Nursery and greenhouse engineering; Potential of phytochemicals from agricultural and wild plants for human health; Power systems and machinery design; Robot engineering and drones in agriculture; Rural electrification; Sanitary engineering; Simulation and computer modeling; Smart engineering applications in agriculture; Soil and water engineering; Micro-irrigation engineering; Structures and environment engineering; Waste management and recycling; Any other focus areas.

For more information on this series, readers may contact: Megh R. Goyal, PhD, PE Book Series Senior Editor-in-Chief: Innovations in Agricultural and Biological Engineering E-mail: [email protected]

OTHER BOOKS ON AGRICULTURAL AND BIOLOGICAL ENGINEERING BY APPLE ACADEMIC PRESS, INC. Management of Drip/Trickle or Micro Irrigation Megh R. Goyal, PhD, PE, Senior Editor-in-Chief Evapotranspiration: Principles and Applications for Water Management Megh R. Goyal, PhD, PE and Eric W. Harmsen, PhD Editors Book Series: Research Advances in Sustainable Micro Irrigation Senior Editor-in-Chief: Megh R. Goyal, PhD, PE • Volume 1: Sustainable Micro Irrigation: Principles and Practices • Volume 2: Sustainable Practices in Surface and Subsurface Micro Irrigation • Volume 3: Sustainable Micro Irrigation Management for Trees and Vines • Volume 4: Management, Performance, and Applications of Micro Irrigation Systems • Volume 5: Applications of Furrow and Micro Irrigation in Arid and Semi-Arid Regions • Volume 6: Best Management Practices for Drip Irrigated Crops • Volume 7: Closed Circuit Micro Irrigation Design: Theory and Applications • Volume 8: Wastewater Management for Irrigation: Principles and Practices • Volume 9: Water and Fertigation Management in Micro Irrigation • Volume 10: Innovation in Micro Irrigation Technology Book Series: Innovations and Challenges in Micro Irrigation Senior Editor-in-Chief: Megh R. Goyal, PhD, PE • Engineering Interventions in Sustainable Trickle Irrigation: Water Requirements, Uniformity, Fertigation, and Crop Performance • Management Strategies for Water Use Efficiency and Micro Irrigated Crops: Principles, Practices, and Performance

viii

Other Books on Agricultural and Biological Engineering

• Micro-Irrigation Engineering for Horticultural Crops: Policy

Options, Scheduling, and Design

• Micro-Irrigation Management: Technological Advances and

Their Applications

• Micro-Irrigation Scheduling and Practices • Performance Evaluation of Micro-Irrigation Management: Principles and Practices • Potential of Solar Energy and Emerging Technologies in Sustainable Micro-Irrigation • Principles and Management of Clogging in Micro-Irrigation • Sustainable Micro-Irrigation Design Systems for Agricultural Crops: Methods and Practices Book Series: Innovations in Agricultural and Biological Engineering Senior Editor-in-Chief: Megh R. Goyal, PhD, PE • Advanced Research Methods in Food Processing Technologies • Advances in Food Process Engineering: Novel Processing,

Preservation and Decontamination of Foods

• Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science • Advances in Sustainable Food Packaging Technology • Analytical Methods for Milk and Milk Products, 2-volume set: o Volume 1: Sampling Methods, Chemical and Compositional Analysis o Volume 2: Physicochemical Analysis of Concentrated, Coagulated and Fermented Products • Biological and Chemical Hazards in Food and Food Products:

Prevention, Practices, and Management

• Bioremediation and Phytoremediation Technologies in Sustainable Soil Management, 4-volume set: o Volume 1: Fundamental Aspects and Contaminated Sites o Volume 2: Microbial Approaches and Recent Trends o Volume 3: Inventive Techniques, Research Methods, and Case Studies o Volume 4: Degradation of Pesticides and Polychlorinated Biphenyls • Dairy Engineering: Advanced Technologies and Their Applications • Developing Technologies in Food Science: Status, Applications, and Challenges

Other Books on Agricultural and Biological Engineering

• • • • • • • • • • • • • • •

• • •

ix

Emerging Technologies in Agricultural Engineering Engineering Interventions in Agricultural Processing Engineering Interventions in Foods and Plants Engineering Practices for Agricultural Production and Water Conservation: An Interdisciplinary Approach Engineering Practices for Management of Soil Salinity: Agricultural, Physiological, and Adaptive Approaches Engineering Practices for Milk Products: Dairyceuticals, Novel Technologies, and Quality Enzyme Inactivation in Food Processing: Technologies, Materials, and Applications Field Practices for Wastewater Use in Agriculture: Future Trends and Use of Biological Systems Flood Assessment: Modeling and Parameterization Food Engineering: Emerging Issues, Modeling, and Applications Food Process Engineering: Emerging Trends in Research and Their Applications Food Processing and Preservation Technology: Advances, Methods, and Applications Food Technology: Applied Research and Production Techniques Functional Dairy Ingredients and Nutraceuticals: Physicochemical, Technological, and Therapeutic Aspects Handbook of Research on Food Processing and Preservation Technologies, 5-volume set: o Volume 1: Nonthermal and Innovative Food Processing Methods o Volume 2: Nonthermal Food Preservation and Novel Processing Strategies o Volume 3: Computer-Aided Food Processing and Quality Evaluation Techniques o Volume 4: Design and Development of Specific Foods,

Packaging Systems, and Food Safety

o Volume 5: Emerging Techniques for Food Processing, Quality, and Safety Assurance Modeling Methods and Practices in Soil and Water Engineering Nanotechnology and Nanomaterial Applications in Food, Health, and Biomedical Sciences Nanotechnology Applications in Agricultural and Bioprocess Engineering: Farm to Table

x

Other Books on Agricultural and Biological Engineering

• Nanotechnology Applications in Dairy Science: Packaging, Processing, and Preservation • Nanotechnology Horizons in Food Process Engineering, 3-volume set: o Volume 1: Food Preservation, Food Packaging and Sustainable Agriculture o Volume 2: Scope, Biomaterials, and Human Health o Volume 3: Trends, Nanomaterials, and Food Delivery • Novel and Alternative Methods in Food Processing: Biotechnological, Physicochemical, and Mathematical Approaches • Novel Dairy Processing Technologies: Techniques, Management, and Energy Conservation • Novel Processing Methods for Plant-Based Health Foods: Extraction, Encapsulation and Health Benefits of Bioactive Compounds • Novel Strategies to Improve Shelf-Life and Quality of Foods: Quality, Safety, and Health Aspects • Phytochemicals and Medicinal Plants in Food Design: Strategies and Technologies for Improved Healthcare • Processing of Fruits and Vegetables: From Farm to Fork • Processing Technologies for Milk and Milk Products: Methods, Applications, and Energy Usage • Quality Control in Fruit and Vegetable Processing: Methods and Strategies • Scientific and Technical Terms in Bioengineering and Biological Engineering • Soil and Water Engineering: Principles and Applications of Modeling • Soil Salinity Management in Agriculture: Technological Advances and Applications • State-of-the-Art Technologies in Food Science: Human Health, Emerging Issues and Specialty Topics • Sustainable and Functional Foods from Plants • Sustainable Biological Systems for Agriculture: Emerging Issues in Nanotechnology, Biofertilizers, Wastewater, and Farm Machines • Sustainable Nanomaterials for Biomedical Engineering: Impacts, Challenges, and Future Prospects • Sustainable Nanomaterials for Biosystems Engineering: Trends in Renewable Energy, Environment, and Agriculture • Technological Interventions in Dairy Science: Innovative Approaches in Processing, Preservation, and Analysis of Milk Products

Other Books on Agricultural and Biological Engineering

xi

• Technological Interventions in Management of Irrigated Agriculture • Technological Interventions in the Processing of Fruits and Vegetables • Technological Processes for Marine Foods, From Water to Fork: Bioactive Compounds, Industrial Applications, and Genomics • The Chemistry of Milk and Milk Products: Physicochemical Properties, Therapeutic Characteristics, and Processing Methods

ABOUT SENIOR-EDITOR-IN-CHIEF

Megh R. Goyal, PhD, PE Retired Professor in Agricultural and Biomedical Engineering, University of Puerto Rico, Mayaguez Campus; Senior Acquisitions Editor, Biomedical Engineering and Agricultural Science, Apple Academic Press, Inc. Megh R. Goyal, PhD, PE, is a Retired Professor in Agricultural and Biomedical Engineering from the General Engineering Department in the College of Engineering at the University of Puerto Rico–Mayaguez Campus. During his professional career, he has worked as a Soil Conservation Inspector; Research Assistant at Haryana Agricultural University and Ohio State University; Research Agricultural Engineer/Professor at the Department of Agricultural Engineering of UPRM; and Professor of Agricultural and Biomedical Engineering in the General Engineering Department of UPRM. He spent a one-year sabbatical leave in 2002–2003 at the Biomedical Engineering Department of Florida International University, Miami, USA. Dr. Goyal was the first agricultural engineer to receive the professional license in agricultural engineering from the College of Engineers and Surveyors of Puerto Rico. In 2005, he was proclaimed the “Father of Irrigation Engineering in Puerto Rico for the Twentieth Century” by the American Society of Agricultural and Biological Engineers, Puerto Rico Section, for his pioneering work on micro irrigation, evapotranspiration, agroclimatology, and soil and water engineering. During his professional career of 52 years, he has received many awards, including Scientist of the Year, Membership Grand Prize for the American Society of Agricultural Engineers Campaign, Felix Castro Rodriguez Academic Excellence Award, Man of Drip Irrigation by the Mayor of Municipalities of Mayaguez/Caguas/Ponce and Senate/Secretary of Agriculture of ELA, Puerto Rico, and many others. He has been recognized as one of the experts “who rendered meritorious service for the development of [the] irrigation sector in India” by the Water Technology Centre of Tamil Nadu Agricultural University in Coimbatore, India, and ASABE who bestowed on him the 2018 Netafim Microirrigation Award.

xiv

About Senior-Editor-in-Chief

Dr. Goyal has authored more than 200 journal articles and edited more than 100 books. AAP has published many of his books, including Management of Drip/Trickle or Micro Irrigation; Evapotranspiration: Principles and Applications for Water Management; ten-volume set on Research Advances in Sustainable Micro Irrigation. He has also authored the textbooks Elements of Agroclimatology (Spanish) by UNISARC, Colombia, and two Bibliographies on Drip Irrigation. Dr. Goyal has also developed several book series with AAP, including Innovations in Agricultural & Biological Engineering (with over 60 titles in the series to date), Innovations and Challenges in Micro Irrigation; and Innovations in Plant Science for Better Health: From Soil to Fork. Dr. Goyal received his BSc degree in Engineering from Punjab Agricultural University, Ludhiana, India, and his MSc and PhD degrees from the Ohio State University, Columbus, Ohio, USA. He also earned a Master of Divinity degree from the Puerto Rico Evangelical Seminary, Hato Rey, Puerto Rico, USA. Readers may contact him at [email protected].

ABOUT THE EDITOR

Shrikaant Kulkarni, PhD Adjunct Professor, Department of Science & Technology,

Vishwakarma University, Kondhwa Budruk,

Pune 411048 (M.S.), India

Shrikaant Kulkarni, PhD, has 37 years of teaching and research experience at both the undergraduate and postgraduate levels. He is currently Adjunct Professor in the Faculty of Science and Technology at Vishwakarma University, Pune, India. He has been teaching subjects such as engineering chemistry, green chemistry, nanotechnology, analytical chemistry, catalysis, chemical engineering materials, industrial organization, and management, to name a few during his long career. He has published over 100 research papers in national and international journals and conferences. He has authored 30 book chapters in CRC, Springer, and Elsevier books. He has edited four books on green engineering and renewable materials from Apple Academic Press/CRC Press. Dr. Kulkarni has coauthored four textbooks on chemistry as well. His areas of interests are analytical and green and sustainable chemistry. He is a reviewer and editorial board member of many journals in green and analytical chemistry and nanotechnology. He has been invited by UNESCO to give a talk on “Green Chemistry Education for Sustainable Development” at the IUPAC international conference on green chemistry held at Bangkok (Thailand), which was well received. He is an esteemed team member of the United Nations Conference on Sustainable Development (UNCSD) working for the attainment of sustainable development goals. He was appointed as an innovation summit judge in a Conrad challenge competition for teams from across the world, sponsored by NASA. He has been instrumental in formulating and coordinating RIO & COP programs dedicated to sustainable development at his institute by UNCSD and has worked as a resource person for various national and international events. Dr. Kulkarni possesses master’s degree in Chemistry, Business Management, Economics, Political Science and MPhil and PhD degrees in Chemistry. He also holds diplomas in human resources, industrial psychology, higher education, population education, etc.

CONTENTS

Contributors........................................................................................................... xix

Abbreviations ......................................................................................................... xxi

Preface ................................................................................................................ xxvii

PART I: Green Nanomaterials in Energy Production and Storage .......1

1.

Innovative Graphene Thermoelectric Nanomaterials for

Production and Conversion of Green Energy ..............................................3

Nisha Verma, Sonal, and Rohidas Bhoi

2.

Eco-Friendly Applications of Graphene Nanomaterials in

Energy Storage Devices: A Review..............................................................25

Shrut M. Desai and Shrikaant Kulkarni

3.

Advances in High Volumetric Density Carbon Nanotubes for

Electrochemical Energy Storage..................................................................89

Shrut M. Desai, Amol N. Joshi, and Shrikaant Kulkarni

4. Application of Nanomaterial-Modified Membrane Filters for Separation of Heavy Metal Pollutants.......................................................... 119

Pallavi Mahajan Tatpate, Supriya Dhume, Yogesh Chendake, and Sachin Chavan

PART II: Scope of Green Nanomaterials in

Biomedical Applications..................................................................137

5.

Green Nanotechnology for Renovating Phytomedicines.........................139

Garima Shandilya, Yogesh Chendake, and Sachin Chavan

6.

Green Nanomaterials as a Boon to Biomedical Engineering ..................161

Nidhi Jain

7.

Trends in Green Synthesis of Carbon-Based Nanostructures ................201

Divya Neravathu Gopi and N. K. Athira

8.

Applications of Biodegradable Polymeric Biomaterials in

Biomedical Science......................................................................................229

Saswati Mishra and Anindita Behera

xviii

Contents

PART III: Innovations in Green Nanotechnology for Agricultural

and Environmental Sustainability..................................................279

9.

Role of Green Nanomaterials in Sustainable Agriculture ......................281

Hemlata Karne and Shrikaant Kulkarni

10. Green Agricultural Waste-Derived Silica Nanoparticles as

Catalyst Support Materials........................................................................ 311

Sakshi Kabra Malpani and Deepti Goyal

11. Green Synthesis of Nanomaterials for Photocatalytic

Biodegradation of Pollutants in Wastewater ............................................339

Akash Balakrishnan and Mahendra Chinthala

Index .....................................................................................................................367

CONTRIBUTORS

N. K. Athira Vishnu Ayurveda College, Government Press P.O. Shoranur 679122, Kerala, India; E-mail: [email protected]

Akash Balakrishnan Process Intensification Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela 769008, Odisha, India; E-mail: [email protected]

Anindita Behera School of Pharmaceutical Sciences, Siksha O. Anusandhan Deemed to be University, Campus II, Kalinga Nagar, Bhubaneswar 751003, Odisha, India; E-mail: [email protected]

Rohidas Bhoi Department of Chemical Engineering, Malaviya National Institute of Technology,

F06 UG Lab Complex, JLN Marg, Jaipur 302017, Rajasthan, India;

E-mail: [email protected]; [email protected]

Sachin Chavan Department of Mechanical Engineering, Bharati Vidyapeeth (Deemed to be) University, College of Engineering, Pune 411043, Maharashtra, India; E-mail: [email protected]

Yogesh Chendake Department of Chemical Engineering, Bharati Vidyapeeth (Deemed to be) University,

College of Engineering, Pune 411043, Maharashtra, India; E-mail: [email protected]

Mahendra Chinthala Process Intensification Laboratory, Department of Chemical Engineering,

National Institute of Technology, Rourkela 769008, Odisha, India; E-mail: [email protected]

Shrut M. Desai Department of Chemical Engineering, Vishwakarma Institute of Technology, Bibvewadi, Pune 411037, Maharashtra, India; E-mail: [email protected]

Supriya Dhume Department of Chemical Engineering, Bharati Vidyapeeth (Deemed to be) University,

College of Engineering, Pune 411043, Maharashtra, India; E-mail: [email protected]

Divya Neravathu Gopi Department of Physics, Cochin University of Science and Technology, South Kalamassery 682022, Kerala, India; E-mail: [email protected]

Deepti Goyal Department of Applied Chemistry, School of Vocational Studies & Applied Sciences, Gautam Buddha University, Gautam Budh Nagar, Greater Noida 201312, UP, India; E-mail: [email protected]

Megh R. Goyal Agricultural and Biomedical Engineering from College of Engineering at University of Puerto Rico Mayaguez Campus; and Senior Technical Editor-in-Chief in Agricultural and Biomedical Engineering for Apple Academic Press Inc.; PO Box 86, Rincon, Puerto Rico, 00677-0086, USA; E-mail: [email protected]

xx

Contributors

Nidhi Jain Department of Engineering Science, Bharati Vidyapeeth College of Engineering, Lavale, Pune 411043, Maharashtra, India; E-mail: [email protected]

Amol N. Joshi Department of Chemical Engineering, Vishwakarma Institute of Technology (VIT), Bibvewadi, Pune 411037, Maharashtra, India; E-mail:[email protected]

Hemlata Karne Vishwakarma Institute of Technology, Bibwewadi, Pune 411037, Maharashtra, India; E-mail: [email protected]

Shrikaant Kulkarni Department of Science & Technology, Vishwakarma University, Kondhwa Budruk, Pune 411048, Maharashtra, India; E-mail: [email protected]

Sakshi Kabra Malpani Department of Chemistry, Jyoti Nivas College Autonomous, Kormangala, Hosur Road, Bengaluru 560095, Karnataka, India; Mobile; E-mail: [email protected]

Saswati Mishra Center of Biotechnology, Siksha O. Anusandhan Deemed to be University, Campus II, Kalinga Nagar, Bhubaneswar 751003, Odisha, India; E-mail: [email protected]

Garima Shandilya Department of Nanotechnology, Bharati Vidyapeeth (Deemed to be) University, College of Engineering, Pune 411043, Maharashtra, India; E-mail: [email protected]

Sonal Department of Chemical Engineering, Malaviya National Institute of Technology,

F06 UG Lab Complex, JLN Marg, Jaipur 302017, Rajasthan, India; E-mail: [email protected]

Pallavi Mahajan Tatpate School of Petroleum, Polymer and Chemical Engineering, Polymer Department,

MIT-World Peace University, Pune 411038, Maharashtra, India; E-mail: [email protected]

Nisha Verma Materials Research Center, Malaviya National Institute of Technology, F06 UG Lab Complex, JLN Marg, Jaipur 302017, Rajasthan, India; E-mail: [email protected]

ABBREVIATIONS

0D 1D 2D 3D AFM Ag AgNPs AGNR AgNW AOP’s ATRP Au BDC bFGF BTC BUP BUP HCl C Dot CD C-GP CMC CMC CMF CNM CNP CNT CNTC CNTs CQDs CRP CS CT CTAB CuNPs

zero-dimensional one-dimensional two-dimensional three-dimensional atomic force microscopy silver silver nanoparticles armchair graphene nanoribbons silver nanowire advanced oxidation processes atom transfer radical polymerization gold benzene-1,4-dicarboxylic acid basic fibroblast growth factor 1,3,5-benzenetricarboxylic acid bupivacaine bupivacaine hydrochloride carbon dot current density chitosan–β-glycerophosphate carboxymethyl cellulose critical micelle concentration carbon nanotube microscopic film carbon nanomaterial charge neutrality point carbon nanotubes carbon nanotube carpet carbon nanotubes carbon quantum dots controlled radical polymerization chitosan chitin cetyltrimethylammonium bromide copper nanoparticles

xxii

CuO2 CV CVD CVDs DAE DC DFT DMF DOX DSSC ECM ED ED EDLC EDX ELPs ESD FAO FESEM FTIR GCNT GelMA GINC GNW GO HA HCF HDPE hMSC HPCNTs HPEI HUMO ITO kDa KHz LA LCST LDPE LEDs LIBs

Abbreviations

copper oxide cyclic voltammetry chemical vapor deposition cardiovascular diseases diarylethene discharge capacity density functional theory N, N-dimethylformamide doxorubicin dye-sensitized solar cell extracellular matrix energy density electrodialysis electric double layer capacitors energy dispersive X-ray spectroscopy elastin-like polypeptides energy storage devices Food and Agriculture Organization field emission scanning electron microscopy Fourier transform infrared spectroscopy graphene-nanomesh carbon nanotube gelatin methacrylate graphene-iron oxide nano composite graphene nanowiggles graphene oxide hyaluronic acid hexacyano ferrate high-density polyethylene human mesenchymal stem cells hierarchically porous carbon nanotubes hyperbranched polyethyleneimine highest occupied molecular orbital Indium Tin Oxide kilo-Dalton kilo hertz longitudinal acoustic lower critical solution temperature low-density polyethylene light emitting diodes lithium-ion batteries

Abbreviations

LSH LUMO MB MC ME MFP MGNR MHz MNPs MO MOF mPEG-CL mPEG-LA MRI MSC MWCNTs NADP NASA NCM NDDSs NDs NF NMEG NPK NPs OLC PAA PAI PAMAM PAN PANI PC PCL PCM PD PDLA PDMS PDT PE PEDOT

xxiii

linseed hydrogel lowest unoccupied molecular orbital methylene blue methylcellulose microemulsion mean free path mixed graphene nanoribbons mega hertz metal nanoparticles methyl orange metal-organic framework poly(ethylene glycol-block-caprolactone) poly(ethylene glycol-block-lactide) magnetic resonance imaging micro-supercapacitor multi-walled carbon nanotubes nicotinamide adenine dinucleotide phosphate National Aeronautics and Space Administration, U.S.A. nickel, cobalt, and manganese oxides nano-drug delivery systems nanodiamonds nanofiltration N2-modified microwave exfoliated graphite oxide nitrogen, phosphorous, potassium nanoparticles onion-like carbon polyacrylic acid polyamide-imide poly-amidoamine polyacrylonitrile polyaniline phosphatidylcholine polycaprolactone phase change materials power density poly(d-lactic acid) polydimethylsiloxane photodynamic therapy phosphatidylethanolamine poly(3,4-ethylenedioxythiophene)

xxiv

PEG–PDGA PEG–PLGA PEI PEM PEN pero-HSC PET PF PPGA PGEC pH PPHA PL PLA PPLGA PLGA PLLA PMMA PNIPAAm PNVCL PPD PSF PSS PSSH PTT PUR PUUR PVA PVAc PVDF PVF PVP QDs RA RAFT RC rGO RhB RO RTG

Abbreviations

poly(ethylene glycol)-poly-d-lactic acid-co-glycolic acid poly(ethylene glycol)-poly-l-lactic acid-co-glycolic acid polyethylenimine polymer electrolyte membrane polyethylene naphthalate perovskite hybrid solar cell polyethylene terephthalate power factor poly(glycolic acid) phonon glass electron crystal potence of hydrogen poly(hydroxyalkanoates) photoluminescence poly(lactic acid) poly(lactic-co-glycolic acid) poly(lactic-co-glycolic acid) poly(l-lactic acid) poly(methyl methacrylate) poly(N-isopropylacrylamide) poly(N-vinylcaprolactam) poly-para-dioxanone polysulfone polystyrene sulfonate polystyrene sulphonic acid photothermal therapy polyurethanes polyurethane urea polyvinyl alcohol polyvinyl acetate polyvinylidene fluoride polyvinylidene fluoride polysulfone quantum dots rheumatoid arthritis reversible addition fragmentation chain transfer reversible capacity reduced graphene oxide Rhodamine B reverse osmosis radioisotope thermoelectric generators

Abbreviations

SAED SC SEI SLG SLN SPIONs SSG STSG SWCNTs SWNHs TA TCP TE TeCNT TEM TENG TEP TES TFC TFN THF TLR-2 TMS UCST UF UHMWPE UV VEGF VP VPGVG WHO XRD YSC ZA ZGNR ZnO ZT

xxv

selected area electron diffraction specific capacitance solid electrolyte interface single-layer graphene solid-lipid nanoparticles super paramagnetic iron oxide nanoparticles self-stacked solvated graphene split-thickness skin graft single-walled carbon nanotubes single-walled carbon nanohorns transverse acoustic tricalcium phosphate thermoelectric tellurium carbon nanotube transmission electron microscopy triboelectric nanogenerator thermoelectric power thermal energy storage thin film composite thin film nanocomposite tetrahydrofuran toll-like receptor 2 transition metal sulfides upper critical solution temperature ultrafiltration ultrahigh molecular weight polyethylene ultraviolet vascular endothelial growth factor volumetric performance val–pro–gly–val–gly World Health Organization X-ray diffraction yarn supercapacitors in-and-out of plane acoustic zigzag graphene nanoribbons zinc oxide thermoelectric figure of merit

PREFACE

Sustainable development is gaining momentum in the modern world. It has been a driving force in bringing about the development and growth of generations to come by striking a balance between human beings and the ecosystems by utilizing the resources for sustenance. There has been substantial degradation of ecology due to overexploitation of resources as well as use of chemical-based materials on a mass scale. This piquant situation demands remedial measures like dematerialization and material substitution. Therefore, the need of the hour is to open new vistas in the advancement of novel materials not only in tune with the requirements but also for green, sustainable, or eco-benign ways to preserve the sanctity of the ecology and sustenance of the planet Earth to make it livable for the forthcoming generations. In fact, attainment of sustainable development goals has become a globally adopted movement that includes design and development of synthesis protocols for green and eco-friendly materials and practices. These initiatives demonstrate capabilities of the natural resources bestowed upon us by the nature in conservation and preservation of resources for the better cause of generations of humanity. Development of green and sustainable nanomaterials cover synthesis, process technologies, fabrication, and optimization of processes, as well as testing, their functional behavior and stability, reproducibility, durability, etc. Moreover, future prospects of such materials will heavily rely upon how far we are committed to the sustenance in research and development. This volume provides an overview of synthesis, characterization, and applications in fields like agriculture, environmental sustainability, biomedicine, energy harvesting, and storage of some representative nanomaterials that are developed by employing green and sustainable methods of synthesis. It also covers advances in green and sustainable nanomaterials. This book also sets a tone for future directions in greening and sustaining nanoscience. Sustenance in innovative nanomaterials is achieved through innovative synthesis protocols or by modifying incumbent materials possessing distinct and unique property profiles. Such materials meet growing demands for the making of equipment, instruments, and gadgets with better attributes for a broad spectrum of applications in areas that include biomedicine, textiles, construction, the paper industry, etc. Many of the latest nanomaterials known

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Preface

for their novelty have been developed over the years in the form of bionanomaterials, ceramics, polymeric materials, etc. The genesis of some of these lie in nature, namely, plants, animals, ores, etc. or derived products from plants and found in various geometrical shapes that are suitable to develop composites with innumerable materials that are application specific. While some of them are produced synthetically with requisite geometrical shapes and sizes in congruence with the requirements. The outstanding properties qualities of new nanomaterials cater to the needs of innumerable applications in place of numerous configurations. The major goal of this volume is to take a stock of sophistication in the development of a spectrum of recent nanomaterials that are eco-benign and sustainable. Keeping in perspective the meaningful knowledge base of newer materials obtained by translating conceptual understanding into reality, and to bring about massive production of nanomaterials, the book consists of two parts with each part comprising many chapters therein. Chapter 1 provides an overview of using graphene as a thermoelectric nanomaterial for the production and conversion of green energy. Chapter 2 deals with eco-benign applications particularly in devices used for energy harvesting and storage applications. Chapter 3 provides an overview of advances in high volumetric carbon nanotubes in storing electrochemical energy. Chapter 4 encompasses details about nanomaterial-embedded membranes as filtering media to get rid of heavy metal pollutants. Chapter 5 refers to the use of green nanotechnology for reinventing the phytomedicine concept for using it to advantage. Chapter 6 is an extensive overview of how green nanomaterials are looked upon as boon in the frontier area of biomedical engineering. Chapter 7 is all about trends that are developed in green synthesis of carbon-based nanostructures. Chapter 8 extensively discusses applications of biodegradable polymeric materials in the field of biomedical science and technology. Chapter 9 gives an account of the role of green nanomaterials for bringing sustainability to agriculture at large. Chapter 10 presents the results obtained in the form of silica nanoparticles derived from agricultural waste as a renewable source to be used as a novel catalyst support material. Chapter 11 discusses futuristic applications of green and sustainable nanomaterials in the remediation of contaminants in the environment and thereby cleaning the environment. Overall, the book can be considered and used as a reference book for academicians, research scholars, and the scientific community who are endeavoring to develop novel, advanced, and sustainable nanomaterials. The authors of this book are renowned academicians, green material scientists, and engineers. We express our sincere gratitude to all the chapter

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contributors for their encouraging and energetic support in taking this book project to reality. Finally, we thank the editorial staff at Apple Academic Press, Inc. for their valuable assistance, great support, and guiding in the right direction all the way through this project. We also thank our friends and families for their unlimited support, encouragement, love, and affections during the editing of this volume. Finally, and most importantly we would like to commend our spouses and families for their understanding and patience throughout this project. We appeal to the reader to suggest your feedback that may benefit to improve the subsequent edition of this book. —Editors

PART I

GREEN NANOMATERIALS IN ENERGY

PRODUCTION AND STORAGE

CHAPTER 1

INNOVATIVE GRAPHENE THERMOELECTRIC NANOMATERIALS FOR PRODUCTION AND CONVERSION OF GREEN ENERGY NISHA VERMA, SONAL, and ROHIDAS BHOI

ABSTRACT Energy crisis of the world is daunting and solution is the sustainable inexhaustible energy alternatives. Practically, most of the power generation/ conversion technology offers efficiency far below the theoretical limit set by Carnot efficiency, leading to generation of waste heat. Harvesting this waste heat and converting into useful electrical energy can not only be a means to boost power generation efficiencies, but it is potentially the most viable solution for increasing demand for energy. Thermoelectric material can directly convert heat flux into electricity through the Seebeck effect, thus providing a green technology for power production and conversion. Whether thermoelectric material can find a place in real application or not depends upon its conversion efficiency. A highly efficient thermoelectric material should display high electrical conductivity and Seebeck coefficient. At the same time to maintain thermal gradient across its ends, material should demonstrate low thermal conductivity. A vast amount of research on thermoelectric materials in recent times is focused on either discovering new high efficiency materials or improving the efficiency of incumbent materials by means of microstructure engineering. Recently, carbon-based 2D material (such as graphene) has shown promising outcomes. It has gained its merit over other conventional thermoelectric materials, mainly owing to its high Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science. Megh R. Goyal and Shrikaant Kulkarni (Eds.) © 2023 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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strength, flexibility, and fracture toughness, which are highly beneficial in preventing thermo-mechanical failure of components due to thermal shock during the service. Despite all the mechanical benefits, graphene being gapless semiconductor exhibits high thermal conductivity. 1.1 INTRODUCTION Sustainable energy sources mostly cultivate towards world energy immunity and cut down the utilization of fossil fuel resources, thus providing a carbonfree environment. Many renewable sources have been exploited for harvesting energy, that is, solar, wind, and biomass. However, along with exploiting above-mentioned natural resources, another approach explored is to make our existing systems more efficient for energy production, management, and conservation. Thermoelectric (TE) material can play a key role in this scenario, where the material can directly convert heat into electricity or extract heat by supply of electricity.4,53 Even though Seebeck effect (electric voltage generation by temperature gradient across a material) and Peltier effect (appearance of temperature gradient by application of voltage) existed since 1821 and 1834, respectively; their credibility was realized by thermoelectric devices, whereby applying the Seebeck effect they can function as generators (convert heat to electricity). Same material when configured to operate in reverse, manifesting Peltier effect, can function as a cooler, with the application of a voltage drop across the device to generate temperature gradient. Cooling effects are highly beneficial for thermal management of electronic devices, where overheating of small components can lead to their failure during service. Thermoelectric technology has various advantages, such as: (1) being in solid state with no movable parts, making thermoelectric devices noisefree and highly reliable; (2) no greenhouse gases emission making it a green energy resource; and (3) it is easily scalable, making it compatible with multiple applications and finally, it is highly flexible. Thermoelectric utilization in dual application is presented below, where a thermoelectric module is seen to be composed of n-type semiconductor connected with p-type semiconductor electrically in series through electrical contact pads for power generation. When current is passed through the device, heat can be pumped from one end and rejected at other end to obtain cooling effects.15 •

N-type semiconductor

Innovative Graphene Thermoelectric Nanomaterials

• • • • •

5

P-type semiconductor Metal connector Electric insulator, (+) and (−) Load (cold end) Heat sink (hot end)

Despite being so promising, till date the thermoelectric technology could make place in few niche applications, that is, luxury car seats (climate control), radioisotope thermoelectric generators (RTG) used to power NASA spacecrafts, temperature control of microelectronic devices, where economic constraints are unimportant.4 A wider spectrum of applications can be unlocked for thermoelectric technology only by designing thermoelectric devices out of high efficiency thermoelectric materials, which is currently limited to around ~10% (calculated using eq 1.1 with ZT value of unity). Conversion efficiency (η) for a thermoelectric device operating between TH (temperature of heat source) and TC (temperature of heat sink) is defined by the formula given in eq 1.1. It can be noticed that it is a function of temperature gradient and a material property defined as figure of merit ZT, which is scaled with Carnot efficiency. ZT = [(S2σTavg) /к]

(1.1)

where Tavg is the average temperature of thermoelectric device that is defined as Tavg = (TH + TC)/2; S is Seebeck coefficient in microvolts per Kelvin (μVK−1), σ is electrical conductivity in siemens per cm (S cm−1), and к is thermal conductivity in Watts per meter-Kelvin (W m−1 K−1). High efficiency can be achieved by a high value of ZT, implying high Seebeck coefficient (S) and electrical conductivity (σ) along with low thermal conductivity (к). To summarize, the best thermoelectric material should be “phonon glass electron crystal” (PGEC), well stated by Slack.41 In other words, large temperature gradient should be maintained without any heat flux flow from one end to other, demanding low thermal conductivity. High currents and high voltage drop should be created by small temperature differences, implying high power factor (PF) that is defined by S2σ in the same material. Numerically, if we can get thermoelectric materials with ZT = 4, the conversion efficiency could reach up to 30%, creating more striking prospects in real-world applications. However, improving conversion efficiencies is quite challenging as ZT engross material properties which are interrelated.47 For example, increasing charge carrier’s density would inevitably increase the thermal conductivity as heat and charge; both are conducted by same carrier.

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Similarly, Seebeck coefficient (S) varies with mobile charge carriers (n) and effective mass m*, such as S ~ m*/n2/3. This relationship implies that increasing both σ and S, simultaneously, is challenging. The ZT is dependent on Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (к).47 There is a dependence on electrical conductivity (σ) and thermal conductivity (к), which affects the peak value of ZT. For widely used metals and semiconductors, this peak occurs between 1019 to 1021 carriers per cm−3. Despite all these challenges, two main approaches have been adopted for improving the ZT. One methodology is to increase the power factor and another one targets for reducing the thermal conductivity. Generally, power factor or numerator is maximized by microstructure and band structure engineering,34,50 whereas thermal conductivity is reduced by heavy metal doping and hierarchical microstructure.6 One common approach that sounds promising for optimization of both parameters is low dimensional materials. More focus was built on nanostructured low dimensional 1D and 2D materials after pioneered research articles published by Hicks and Dresselhaus in 1993,21 where theoretically as well as experimentally some nanostructured materials are shown to achieve significant improvement of ZT for quantum dots, nanowires, and layered structures.47 The higher ZT was mostly credited to two factors: (1) quantum confinement that enhances the electronic degrees of freedom near fermi level, improving power factor and (2) enhanced phonon scattering due to increased interfaces and surfaces. The state-of-the-art bulk thermoelectric materials that have been developed and gained researchers’ attention are skutterdites (MX3; M = Co, Rh, Ir; X = P, As, Sb),37,46 Oxides (NaxCoO2, ZnO, and RuddlesdenPopper homologous series),27,48,60 Bi2Te3/Sb2Te3, SnSe, SiGe, half Heusler alloys,7,22,51,52,69 lead/copper chalcogenides (PbX/Cu2-xX; X = S, Se, Te),11 clathrates.3 All these thermoelectric materials are of interest due to their low thermal conductivity, but most of them lack good thermo-mechanical properties. During service, thermoelectric devices will be subjected to thermo-elastic stress, thermal shock, and mechanical vibrations, where most of these mentioned thermoelectric materials will crack or fracture during cyclic thermal loading, owing to their inherently brittle nature. Carbon is the 15th most abundant element on earth and exhibits many allotropes due to its ability to hybridization into sp1, sp2, and sp3 bonds, which permit it to form multidimensional structures ranging from 0D fullerenes, 1D carbon nanotubes, 2D graphene, and 3D diamond. Additionally, carbonbased materials exhibit high flexibility, fracture toughness, high mechanical strength, and high temperature stability, in comparison to conventional thermoelectric materials. Diamond is ruled out for being considered a

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thermoelectric material due to its high thermal conductivity. The thermal conductivity of carbon-based materials depends on their purity, quality, density, grain size, layer size, and edge scattering. The thermal conductivity of selected carbon-based materials is as follows2: Type

Thermal conductivity

Low

1 W mK−1

High

2300 W mK−1

Amorphous carbon

2000 W mK−1

Polycrystalline diamond: sp3

1000 W mK−1

Single crystal diamond: sp3

100 W mK−1

Extrinsic graphene: sp

10 W mK−1

Carbon nanotubes: sp2

0.1 W mK−1

Intrinsic graphene: sp2

0.01 W mK−1

Diamond like carbon

3000 W mK−1

2

Similarly, graphene in its pristine state cannot be used for thermoelectric applications due to its high thermal conductivity,2 however many approaches have been proposed and experimentally applied to enhance the phonon scattering. This chapter focuses on: (1) “2D allotrope of carbon graphene,” with its physical properties (such as electrical, optical, thermal, and thermoelectric properties); (2) its advantages and disadvantages; (3) its thermo-mechanical robustness; (3) comparative assessment of graphene with other state-of-theart thermoelectric materials; (4) most recent approaches to improve its ZT by nano-structuring, alloying, nanocomposites designs; (5) introduction of various defects; and (6) future prospects of graphene. 1.2 PROPERTIES OF 2D ALLOTROPE OF CARBON GRAPHENE Graphene is a one-atom thick layer of carbon atoms bonded through sp2 hybridization, arranged in a hexagonal lattice structure. The name is derived from graphite and prefix-ene, indicating the fact that the basic building units for graphite are graphene layers stacked together.49 Since its discovery in 2004, graphene has been extensively researched due to its remarkable physical properties, making it a promising material for electronic applications. The most striking properties of graphene which have attracted much

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attention of researchers since its discovery are high electrical conductivity, high electrical mobility, high thermal conductivity, optical transparency, high mechanical strength, and flexibility.71 Additionally, one active field for graphene which is currently being explored is to its excellent electrical conductivity and large Seebeck coefficient for thermoelectric device fabrication. Recently, progress in advanced synthesis methods of nanomaterials has permitted the realization of low dimensional materials, that is, graphene to design thermoelectric devices.26 1.2.1 ELECTRICAL PROPERTIES OF CARBON GRAPHENE The particular band structure is responsible for the electrical property of the graphene. Each lattice of graphene is made of three σ bonds, and the p-orbital of carbon atom makes a delocalized π bond.16 The presence of the π bond makes graphene a good conductor at room temperature as the electrons associated with the π bond are free to move. Graphene exhibits excellent electric properties at room temperature and shows the Quantum Hall effect with superconductivity and high carrier rate. The transfer of an electron from the valence band to the conduction band creates a positively charged hole. This electron-hole pair is free to move into free space and its unidirectional motion under the influence of the external electric field creates macroscopic current. The Quantum Hall effect is a classical phenomenon that occurs when a conductor placed in the magnetic field experiences the flow of electric current.57 The direction of the electrical current is perpendicular to the direction of the magnetic field. The lattice structure of graphene is such that it supports the phenomena. Each lattice unit of graphene consists of two carbon atoms. The atom makes a tapered intersection point in the Brillouin zone at which the energy is zero. Therefore, the bandgap of graphene is zero. The electrons in the graphene experience the Kelvin tunneling effect. This effect describes the capacity of the electron to pass the energy barrier. In the graphene structure, the electrons are tunneled in such a way that it possesses a 100% probability to cross the energy barrier. At the time of the crossing barrier, the electron exhibits a massless property that refrains it from any deviation or reflection due to the presence of the barrier. In most of the lattice structure, there may be some scattering effect due to the lattice defect or presence of other electrons. As the electron in the graphene can move like a massless particle, it does not encounter

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any scattering effect during its transport.17 The velocity of the electron is faster at room temperature compared to any other conductor. Due to the negligible scattering effect on the movement of an electron, the free path of the electron is around 1 mm, whereas in other materials, it remains in nanometers. These unique properties of graphene result in the Quantum Hall effect in the material.40,42 Due to its excellent electrical properties, graphene has applications in various fields of electronic devices. It has been used frequently in making lithium-ion batteries, supercapacitors, and transportation devices. 1.2.2 OPTICAL PROPERTIES The optical property of graphene originates from its peculiar band structure and massless nature of charged carriers. It is often evaluated experimentally using spectroscopic ellipsometry from ultraviolet to infrared spectral region. Among the σ and π bonds, π-bond is responsible for optical and electrical properties of graphene. Graphene has a simple band structure with zero bandgap, but its optical response is nontrivial. The electrical and optical properties of graphene depend on the method of synthesis, such as mechanical exfoliation or chemical vapor deposition (CVD). The pristine graphene has high degree of perfection in the atomic lattice, whereas other forms of graphene generate defects during its growth and processing steps. The defects in graphene may exhibit minimal change in electrical properties, but the optical properties can be significantly different from its pristine form. Graphene exhibits a particularly unique but simple optical absorption spectrum. Its absorbance is independent of frequency of electromagnetic radiation in infrared to visible spectral range. Photoluminescence (PL) is not possible in case of graphene due to lack of an electronic bandgap.24 The optical properties of the graphene can be controlled by electrical and magnetic fields, layer-layer interactions, and nanostructuring. The lightmatter interactions in graphene can be controlled by interaction between graphene layers, electrical gating and manipulating its structure. This controllability proves to be an efficient tool for understanding the physics of graphene and its technological application in photonics and optoelectronics. The important tunable optical properties of graphene are: interband and intraband transitions in electrically gate graphene, Landau level transitions under a magnetic field, plasmon excitation, and transition in bilayer and multilayer graphene.55

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1.2.3 THERMAL PROPERTIES Thermal conductivity of a material refers to its ability to conduct heat. By definition, heat transfer rate per unit area (called heat flux (q) = Wm−2) divided by the temperature gradient is called thermal conductivity (k). The negative sign will indicate that the direction of heat flow is opposite to the temperature gradient (heat flow from hot end to cold end). Heat in most of the solids is conducted by vibrating ion cores attached to each lattice site (also called acoustic phonons), and electrons. The overall thermal conductivity can be expressed as k = ke + kp, where ke is electronic contribution and kp is phonon contribution. Copper is the best thermal conductor with k ~ 400 Wm−1 K−1, where ke >> kp, thus implying thermal conductivity is dominated by electrons. Graphene, on a relative scale, shows comparatively high thermal conductivity ranging from 2000 to 6000 Wm−1K−1, where kp >> ke. This high range of k values is experimentally determined for suspended single layer and indicative of in-plane thermal conductivity at room temperature.2,49 The conductivity is increased with increasing length due to excitation of acoustic phonons of low frequency. The conductivity on substrate is lower ~370–600 Wm−1 K−1 (due to discharge of phonons at the interface), but is still higher than the best known conducting metals.44 Unlike typical metals, where most of the heat is conducted by electrons, measurement on graphene indicates dominance of phonons for thermal conduction. Using Wiedemann-Franz (K/σ = [π2/3] × [kB/e2]T) law, it could be shown that it is only 1% electronic contribution to thermal conductivity. As compared to room temperature values, the k reduces to 700–1500 Wm−1K−1 at 500 K, mostly due to increased phonon scattering in addition to Umklapp scattering.19 The exceptionally high thermal conductivity of graphene is attributed to quantum effects, such as: weak localization due to quantum interference and size effect that affects the various scattering mechanisms.35 Temperature dependence of thermal conductance varies as T1.5 (originates from out of plane acoustic ZA branch) as compared to electronic conduction which increases with temperature as T2.43 The phonon frequency of vibration (ω) for graphene is quite high ωmax ~ 1600 cm−1 as compared to other materials (ωmax ~ 500 cm−1 for Si).62 This unusual high value is credited to strong sp2 bond and graphene’s small atomic mass. In a unit cell of graphene, carbon atoms of a single layer are paired, forming total six phonon bands out of which three are optical and three are acoustic, which constitutes in-plane, in-andout of plane, and transverse modes each with acoustic and optical branch.

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Graphene shows linear dispersion close to center of Brillouin zone (small wave vector) for both transverse acoustic (TA) and longitudinal acoustic (LA) modes, that is, ωLA ~ νLA(q) and ωTA ~ νTA (q). But, the in-and-out of plane acoustic (ZA) mode is the one that mainly gives rise to the unusual thermal conductivity of graphene, which is shown to exhibit quadratic dispersion, ωZA ~ q2.19 Here, the q is wavevector. The main reason for high thermal conductivity of graphene is its large thermal conductance per unit area (Kball/A, where Kball is ballistic thermal conductance) in ballistic range [L (Transport length) >> λ  (Phonon mean free path)] and long mean free path (MFP). Density functional theory (DFT) could predict the temperature dependence of Kball/A, which is ~0 Wm−1k−1 at 0 K, and goes to 4.2 × 109 Wm−1 k−1 at 300 K, which is significantly high.63 On the other hand, going to small dimensions, as the boundary effects and wavelength are strongly dependent on A, therefore, Kball/A is greatly influenced by the material dimensions. It is shown that Kball/A for graphene nanoribbons decreases with increasing W (widths of nanoribbons). Kball/A for zigzag graphene nanoribbons is 4.2 × 109 Wm−1K−1 at room temperature, which scales well with graphene and carbon nanotubes (CNTs). On the other hand, armchair graphene nanoribbons (AGNRs) have low value of thermal conductance due to anisotropy in ballistic thermal conductance. Due to strong dependence of mean free path of phonon on sample dimensions, thermal conductivity of graphene increases with sample length.28 1.2.4 THERMOELECTRIC PROPERTIES Seebeck coefficient (S), which is also defined as thermoelectric power (TEP), is represented as ΔV/ΔT (the thermoelectric voltage (ΔV) generated per unit of thermal gradient of ΔT). Thermoelectric power is mainly contributed by diffusion TEP (Sd) and the phonon drag TEP (Sg). During diffusion TEP, Sd, an electric potential, is generated through thermal gradient, as charge carrier diffuses from hot-end to cold-end. An additional potential (Sg) can be generated by dragging electrons via momentum transfer through phonons, which comes about due to strong coupling between electrons and phonons, defined as phonon drag TEP. The key to the high TEP value is asymmetric energy distribution of electrons near fermi energy. Thermoelectric power is greatly influenced by the quality of the graphene and its environment, as that dictates the amount of scattering charge carriers will experience. Quality of sample significantly impacts the scattering, either

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intrinsically through phonons or extrinsically through point defects (charged or neutral), surfaces and edges, structural disorders, and charged impurities (substrates). Phonon scattering is governed by electronic transition created by the acoustic/optical phonons. Diffusion TEP strongly depends on the scattering process details and type, whereas phonon drag TEP is influenced only by electron phonon (acoustic) coupling. Experimentally and theoretically many attempts have been made to evaluate the thermoelectric power of graphene. Zuev et al.70 indicated that thermoelectric power shows a strong dependence on gate voltage within the range of 10 K to 300 K. Across charge neutrality point (CNP), the thermoelectric power changes sign indicating switching of majority charge carriers. A linear temperature (T) dependence for gate voltage of −3 V and −5 V also indicates that thermoelectric power has only diffusive components. Nonexistence of drag component is indicative of weak electron phonon interaction. Single-layer graphene exhibits linear dispersion, as a result of that S ~ 1/nc1/2 in case of carrier density is high. Wang et al.54 examined the single-layer graphene (SLG) close to charge neutrality point (CNP), mainly focusing on the charge impurities effect on S. Mott’s relation (which was validated for low carrier mobility at high temperatures, where experimental value of TEP matches well with calculated value using Mott’s relation), where a slight deviation occurred at high temperature. Experimentally, the correlation between thermoelectric transport with band structure has been established.43,61 In the presence of magnetic field (B = 8T), the Sxy reaches 50 μV/K and Sxx reaches 39 μV/K at dirac point (Vg = VD = 10 V). With the change in the sign of thermoelectric voltage Vth, it could be interpreted that charge carrier switches to electrons from holes at dirac point. In graphene, the dirac particles exhibit linear dispersion, concluded from the variation of Vth/Sxx with ns−1/2. Sxx is also shown to show linear dependence on T (for holes) and flat for electronic parts. The unique thermoelectric transport for graphene is correlated with non-linearity of T at low temperatures. Nissimogoudar et al.36 had formulated the dependence of Sd (diffusion contribution to thermopower) on sub-band structure and ribbon width for arm chair nanoribbons. With the width of 5 nm, diffusive thermopower showed dependence on temperature till 75 K, noticeable contribution coming from layer impurities. This relationship was derived considering that the active scatterers were layer impurities, impurities over volume, optical phonons, acoustic phonons, and edge roughness. Acoustic phonons contribution was seen at low temperatures, whereas at high temperatures, optical phonons dominate, indicated by change in sign of Sd, achieves a value of 42 μV/K (for

Innovative Graphene Thermoelectric Nanomaterials

13

T = 300 K). With the width of 3 nm, Sd reaches higher values, accompanied by the changes in scattering mechanisms contributions, where edge roughness scattering becomes significant at lower ribbon widths. Also, presence of singularity in density of state due to the presence of second sub band for ni = 3 × 108 m−1 (Fermi energy, Ef = 278 meV) was reported. On the other hand, Sg (phonon drag contribution to thermopower) can be calculated using effect of force. The overall contribution of phonon drag effect to total thermoelectric power at temperatures below 10 K is mainly due to phonon-phonon process; whereas at higher temperatures it is mainly diffusive, validating the contribution of phonon drag effect at very low temperatures. 1.3

SCOPE FOR IMPROVEMENT TO ATTAIN HIGH OVERALL ZT

To attain high overall ZT, a material should exhibit high electrical conductance (σ), high Seebeck coefficient (S), and low thermal conductivity (K).47 This ideal combination is rare in naturally occurring material and optimization either is not trivial due to interdependence of these properties. The relationship between Seebeck coefficient and electrical conductivity depends on kβ = Boltzmann constant, m* = effective mass, h = planck’s constant, e = electronic charge, n = the carrier concentration, and μ = carrier mobility. Seebeck coefficient is inversely related to carrier concentration (n), and electrical conductivity is directly related to carrier concentration. Hence, simultaneous increase of S and σ by modulating carrier concentration is unrealizable. Also, high electrical conductivity is accompanied by increase in thermal conductivity, undesirable for improving thermoelectric performance. Generally, semiconductors/insulators show high S, metals show high σ, still the optimum combination is achieved only in heavily doped semiconductors with carrier concentration of 1019 to 1021 atoms.cm−3 and narrow band gap of ~1 eV. Fortunately, thermal conduction is contributed by phonons and electrons, and alteration of phonons alone can bring in great boost in ZT values. The lattice thermal conductivity (phonon contribution) is a function of Cv = heat capacity, vs = sound velocity, and λph = phonon mean free path. Many methodologies have been employed for improving conversion efficiency, and one way is by suppressing thermal conductivity due to phonon; and another one is band structure engineering. In the next section, we discuss the strategies to simultaneously improve kl and Seebeck coefficient (S).

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1.3.1 FUNDAMENTALS AND METHODOLOGY FOR IMPROVING ZT 1.3.1.1 NANOSTRUCTURING While restoring the high mobility of graphene, nanostructuring can be beneficial in improving Seebeck coefficient from the size quantization effect and reduced thermal conductivity by enhanced phonon scattering due to interface effects. Many nanostructure variants of graphene have been investigated, experimentally and theoretically, that is, nanoribbons, single molecule and quantum dot nanojunctions, and nanomeshes (anti-dot lattices). Graphene nanoribbons (GNRs) is 1D material derived from graphene by truncation into finite sheets with smooth edges.23 It exists in two variants, namely due to its edge geometry, zigzag graphene nanoribbons (ZGNRs), and armchair graphene nanoribbons (AGNRs). First unsuccessful attempt to explore the unique properties of graphene nanoribbons was made by Ouyang et al.,38 as it could not show that graphene nanoribbons can improve the value of ZT, but these acted as a trigger for other investigators to explore it further. It was shown that the benefit of improved thermopower due to edge effects and lattice vacancies was counter compensated by reduced electrical conduction. Contrary to this, later, other researchers showed promising results. Moreover, zigzag graphene nanoribbons (a gapless variant of graphene) showed insignificant ZT. But, the armchair graphene nanoribbons with finite band gap showed 0.1 value of ZT at room temperature.32 Another approach to enhance ZT was to apply strain along the main axis of graphene nanoribbons, which increased the band gap by reducing the counter contribution of electrons and holes toward S,64 due to enhanced transmission of electrons near band edges. Other reports are indicative of improvement in ZT by designing complex graphene nanoribbons structures, that is, arrangement into multi-junction graphene nanoribbons with alternate sections of different chirality and width, different chirality, or different width.9 Also, mixed graphene nanoribbons (MGNR), consisting of alternate sections of armchair graphene nanoribbons and zigzag graphene nanoribbons with varying widths, was investigated for thermoelectric performance. Phonon thermal conductance of mixed graphene nanoribbons in comparison to armchair graphene nanoribbons showed a drastic reduction along with improvement in thermopower.32 Reduced thermal conductivity results due to mismatch of phonon modes for different sections, whereas the high electron conduction and thermopower are due to resonant tunneling of electrons across these mixed sections.

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Both effects combined boost the figure of merit (ZT) to values higher than 1, which is a significant improvement. Further improvements were noticed for kinked graphene nanoribbons or graphene nanowiggles (GNWs), with a value of ZT ~ 0.6 at room temperature, and a significant reduction in thermal conductivity was noticed.9 Theoretically, Liang et al. demonstrated that thermal conductance in isolation can be degraded without affecting the electronic conduction.29 Similar to other cases, the electrical conductivity was maintained by the resonant tunneling effect between the oblique and parallel sectors, attaining ZT = 0.79 at room temperature, which could be further improved (ZT = 1) by introducing structural dislocation.30 Sevincli et al. have proposed geometrical structuring and isotope cluster engineering with 12C and 14C as a possible route for increasing ZT.45 It was concluded: (1) for the length ranging from 100 nm to 1 μm, the ZT ~ 2 was achievable; (2) ZT ≥ 2 was made for length ~75 nm. Maximum ZT was achievable, when Anderson type disorder is introduced in the electronic Hamiltonian. When the variation of onsite energies is set equal to the temperature, σ = kBT: the maximum ZT values were realized inside the band gap, i.e, |μ| 1 at T = 300 K. 1.3.1.2 DOPING Pristine single-layer graphene is a semimetal with zero bandgap, where the valence band and conduction band meet at Dirac points. For undoped graphene, the fermi level lies at the point of singularity in the density of states, where valence band is completely filled and conduction band is completely empty.13 Graphene displays linear energy dispersion, giving rise to massless chiral particles called Dirac Fermions. The energy band engineering and fermi level can be tuned by doping, thereby increasing the S.33 The most common ways of doping are classified based on their tuning capabilities, that is, heteroatom doping and chemical modification methods are used to tune fermi level and energy band, whereas electrostatic field tunes the fermi level alone. Modulation doping is a promising way to achieve high carrier mobility and enhance S. These benefits stem from the fact that through modulation doping charge carriers can be spatially separated from the parent impurity atoms, which reduces the scattering through impurity and hence gives an enhanced ZT.66

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Most suitable dopants for graphene are Boron (B) and Nitrogen (N), not only due to perfect size match; but also they accept holes and donate electron, respectively.56 It is easy to incorporate B and N into graphene lattice as binding energy of N-N and B-B bonds is smaller than the C-N/C-B bonds.57 Reports on chemical vapor deposition (CVD) grown doped graphene showed doping with N bond with pyrrolic and pyridinic nitrogen, while for doping of B demonstrate graphitic boron and boron silane configuration of bonding.59 Wei et al.61 used NH3 and CH4 for N and C sources, respectively, and for CVD grown doped graphene, the resulting structure mimicked the honeycomb graphitic structure. The carrier mobility of 500 cm2V−1s−1 was achieved, which no way is comparable to the conductivity achieved in suspended (200,000 cm2V−1s−1) or exfoliated graphene (5000 cm2V−1s−1). Zang et al. also showed that N-doped graphene with n-type characteristics can be obtained with a bandgap of 0.16 eV in the CVD technique.67 Graphene nanoribbons have also been successfully doped with N by the electrothermal reaction using NH3.58 Another method that has shown successful results is N+ ion irradiation followed by annealing.20 The carrier mobility is shown to be strongly dependent on the method used for doping, becuase CVD-doped graphene showed lower mobility compared to the ion irradiation method. Wang et al. used the CVD method to dope boron (B) in monolayer of graphene using phenylboronic acid as a precursor.56 Graphene B-doping was successfully achieved, which displayed p-type characteristics and mobility of 800 cm2V−1s−1 was recorded. Another method used for B-doping is Wurtz-type reduction coupling reaction.31 Conjugated B-N-doping has also been done in a CVD method, where copper foil acted as a catalyst and nitrogen gas, methane, and boric acid powder as precursors.5 Small domains of B-N-C were seen scattering in graphene metric. Theoretically, it has been shown that band gap opens due to C and N doping, which manifests a linear dispersion relation similar to that of pristine graphene. Fermi level was shifted toward valance band for B doping and toward conduction band for N doping, where the shift was related to a dopant concetration ~n1/2.39 Bandgap opening was noticed in both the cases. But, the introduction of B-N domains into graphene displays wider bandgap as dispersion relation is affected at a wider energy region due to dopant concentration.18 Yang et al. examined B-N doped graphene theoretically, where ZT was greatly improved due to incorporation of h-B-N in graphene nonoribbons periodically.23,39 Other variants, i.e, armchair-edged B-N-doped graphene showed ZT enhancement up to an order of 10–20 times, while ZT was increased

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by a factor of 2–3 for Zigzag-edged graphene. The noticeable improvement in ZT was attributed to enhancement of S and reduction in kl. The h-B-N domains act as discontinuity in the transport channels, reducing thermal and electrical conductivity, but these also create small gaps in the transmission spectrum giving rise to high thermoelectric power. Theoretically, it is shown that the ZT value of 1.6 can be achieved for zigzag graphene nanoribbons with boron or nitrogen doping, which is a noticeable improvement compared to best known thermoelectric, Bismuth telluride with ZT ~ 1.18 Most of these results are related to the reduction in thermal conductivity due to extinction of phonons with different frequencies. There are other elements, which have been explored for doping of graphene. Si and Ge addition is shown to change the semimetal character to metallic and also show spin polarization at the fermi level.1 Molecular dynamic simulations have shown that thermal conductivity can be greatly reduced by introducing Si impurity into single-layer graphene, almost 30% reduction was noticed for 0.63% substitution.12 The reduction in thermal conductivity is collectively due to increased scattering of both acoustic phonons and optical phonons, which comes across due to mass fluctuation, as a heavy element Si replaces light element C. Additionally, replacement of C-C sp2 bonds by Si-C sp3 bonds reduces in-plane kl. 1.3.1.3 NANOCOMPOSITES In this section, polymer-based composites are presented. Polymers are eligible candidates for thermoelectric application due to their low thermal conductivity, which is desired for high ZT.12 Contrarily, the electrical conductivity of polymers is not comparable to the other state-of-the-art thermoelectric materials, which is attempted to improve by doping, which inevitably compromises the thermopower. Recently, much interest has been gained by conjugated polymer composites, where decoupling of interdependent factors affecting the ZT can be realized. Two main factors contribute toward high ZT:68 (1) enhanced phonon scattering; and (2) energy filtering. Graphene makes the best choice for filler material for polymer, owing to its exceptional electrical conductivity. These nanocomposites show improved thermoelectric properties in comparison to the pristine polymer. The main reason for improved efficiency due to the presence of graphene as discontinuity in the thermal conductivity path and as prevailed continuity in the electrical path. Typical methods used for graphene incorporation into polymer matrix are melt mixing, solution mixing, and in-situ polymerization.

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Zhang et al.68 synthesized polymer-based nanocomposite using fullerenedecorated reduced graphene oxide (rGO) as filler for poly(styrene sulfonate) (PEDOT: PSS). The electrical conductivity showed a strong dependence on rGO-fullerene ratio; with optimized ratio, 7-fold increase in electrical conductivity was achieved along with 4-fold increase in S, accompanied by increase in thermal conductivity from 0.2 to 2, which accounted for ZT of 0.067 with 30% wt filler and 7:3 ratio of rGO to fullerene. Again, the improvement was accomplished by phonon scattering and energy filtering. Kim et al.25 prepared PEDOT: PSS - rGO composite films using the solution spin coating method. Overall ZT of 0.021 was achieved due to an increase in S (2.03 to 11.09 μVK−1) and reduction in thermal conductivity (0.24 to 0.14 Wm−1K−1). Porous structure of composite resulted in reducing thermal conductivity due to increased phonon scattering, whereas strong interaction between polymer and rGO facilitates charge transfer, enhancing the thermopower. Yoo et al. used in situ polymerization to synthesize PEDOT: PSS-rGO, which was highly conductive.65 Structural modification, where rGO sheets organize themselves in an ordered manner to facilitate the charge transfer, favors high electrical conductivity. Dey et al.14 used ultra-sonication followed by hot-press for compaction to synthesize polyvinyl acetate (PVAc)/ grapheneiron oxide nanocomposite (GINC). Use of graphene-iron oxide increased the power factor by 27-folds in comparison to graphene alone, and overall ZT reached was 0.0031, and most of this effect resulted from the discontinuity in thermal conduction path due to the presence of oxide of iron on graphene. To summarize, most of the enhancement in thermoelectric conversion efficiency is related to the nature of graphene, its dispersion, size, aspect ratio, orientation, and functionalization. Mostly, the π-π stacking that links the polymer matrix with graphene filler acts as the channel for charge transfer giving rise to high electrical conductivity. Functionalization for graphene works for dual purpose, one it helps in uniform dispersion of graphene into matrix and second is its rough interface enhances phonon scattering to reduce thermal conductivity. In fact, synthesis by in situ polymerization turns out to be a better method for attaining high power factor. 1.4 SUMMARY AND FUTURE PROSPECTS This chapter presents a comprehensive overview of experimental and theoretical reports of electrical, optical, thermal, and thermoelectric properties of graphene. Exceptional properties for graphene are high electrical conductivity

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and high optical absorption and transparency. All the properties of graphene are strongly dependent on the quality of the sample, which also depends on the synthesis method used. Thermoelectric properties of graphene show promises, due to exceptional electrical mobility and tunable band-gap structure. High thermal conductivity is the only limitation for realizing graphene into thermoelectric applications. Many methods have been implemented to reduce the thermal conductivity and improve thermoelectric power, such as doping, nanostructuring, and nanocomposites. Most of these methods can increase the phonon scattering and create band gap, reducing thermal conductivity and increasing thermoelectric power, simultaneously. Doping alone can serve three purposes, such as: it increases carrier concentration along with band gap opening; it increases phonon scattering; and confining dimension through nanostructuring also enhances phonon scattering and hence improves ZT. Another effective way of improving thermoelectric efficiency is by dispersing graphene into polymer. Merger of low thermal conductivity of polymer with high electrical conductivity of graphene displays energy filtering and increases phonon scattering, resulting in improved ZT. Overall, graphene makes a promising candidate for thermoelectric applications. However, the ZT value can still be improved with efforts in developing novel synthesis methods and tailoring the microstructure. Theoretical prediction will be needed to guide the experimental efforts. KEYWORDS • • • • • •

green technology microstructure engineering sustainable graphene thermoelectric figure of merit (ZT) thermoelectric materials

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CHAPTER 2

ECO-FRIENDLY APPLICATIONS OF GRAPHENE NANOMATERIALS IN ENERGY STORAGE DEVICES: A REVIEW SHRUT M. DESAI and SHRIKAANT KULKARNI

ABSTRACT The increasing energy demand requires high-performance energy storage devices (ESD), which depend on their power density (PD), energy density (ED), operating temperature ranges, and life-cycle. PD, ED, and lifetime of supercapacitors rarely fulfill needs. The high density of ESD can be attained by using nanomaterials. Morphological advantages of graphene can be utilized to improve the device performance. This chapter discusses about the modifications that can be brought about in the currently used batteries which paves a way towards implementation of next-generation sustainable storage devices. It gives an in-depth review of the use of graphene-based nanomaterials in lithium-ion batteries and thermal-driven supercapacitors. Graphene modified by transition metals/metal oxides and sulfides/heteroatoms/polymers can greatly enhance the device performance. Analysis of how morphology, structure, porosity, defects, functionalization, and synergy influence the performance is discussed. The low-temperature failure of supercapacitors, solar energy intermittency, and conversion efficiency issues can be over-turned by employing thermal-driven supercapacitors. This inspires to new-concept devices and to a sustainable future. This chapter gives an in-depth review of graphene-based nano-material utilization for high-performance ESDs.

Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science. Megh R. Goyal and Shrikaant Kulkarni (Eds.) © 2023 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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Various forms of energy (such as light radiation, chemical energy, thermal energy) can be captured and stored in the form of a sensible heat, latent heat, electrical potential, gravitational potential, and kinetic energy in the energy storage devices (ESDs) which can help to convert the energy forms that are more difficult to store, to the forms that can be conveniently stored. Most used energy storage devices are the rechargeable batteries that store energy converted from chemical energy to electrical energy. Earliest research by Harris demonstrated the use of lithium for electrodepositing and this gave origin to the Li-ion batteries.60 Li-ion batteries have been extensively researched and commercialized as an efficient energy source for compact and lightweight electronics, electric vehicles, and sensors.141,214,248 The battery performance depends upon characteristics like energy and PD, specific capacitance, cycle stability, and cycle life. In all types of energy-storage devices, the materials and the technology play a crucial role. The secondary rechargeable batteries rely upon intercalation mechanism, where ions through the porous channels intercalate a crystal lattice back and forth.137 These batteries exhibit excellent performances giving high capacitance, ED, and cycle stability. Li-ion batteries (LIB) have played an intrinsic role in powering the modernday society and are being used in almost all aspects of life. However, these have drawbacks over their low PD. Various materials are being researched as electrode materials for these secondary batteries. Many such secondary batteries have come up, such as metal-air, sodium-ion, potassium-ion, nickel-zinc batteries. However, these do not perform adequately to supplant the Li-ion (Lithium-ion) batteries. Hence, various electrode additives are being researched to improve its performances. Graphene with its novel properties is widely being researched as an electrode additive that can enhance the battery performance. Graphene can be used both as an anode and a cathode material. Graphene additives have shown promising results with potential to improve the performance of the conventional batteries and can greatly contribute toward the performance of the energy grids. However, the environmental impact of the LIB is questionable. As these batteries are not recyclable, the lithium residues of the battery pose a huge environmental threat. Hence, alternative power sources that can perform equivalent/better than the LIB and suffice the energy demands while being environmentally benign are under examination. The ever-increasing energy demands cannot be fulfilled by the traditionally used techniques. Hence, the utilization of renewable inexhaustible power

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sources is of prime interest. Huge attempts have been made in reducing the carbon footprints and the environmental impact. To address the everincreasing energy demands, an efficient device should deliver power whenever it is needed. The most reliable, abundant, and ubiquitous form is the solar energy. Various energy harvesters like solar cells, thermal-driven devices have come up, but we cannot solely depend upon these harvesters due to their inherently low efficacies. Supercapacitors are the third-generation evolutions of the conventional capacitors. These can store electrical energy up to the 106 Farads. These deliver higher power and ED than the batteries and capacitors. They have a longer cycle life than any other storage device. These supercapacitors can be used as complementary to batteries, or they even have the potential to completely replace them. The supercapacitors are reported to have the most optimized performance and they bridge the gap between batteries and capacitors. However, supercapacitors fail to operate at extremely low temperatures and hence a way to address these issues needs to be identified. Thermal-driven supercapacitors are most viable and sustainable alternative. These hybrid supercapacitors harvest the solar energy and have performance equivalent to that of supercapacitors. These storage devices can even solve the intermittency issues related to solar energy. Nanotechnology is a pivotal tool that highly amplifies the device performance by exploiting material properties at the nanoscale. This chapter presents an in-depth review of advancements in the currently used LIB and the next-generation hybrid thermal-driven supercapacitors. It first provides till-date research on the use of graphene and graphene derivatives in LIB. It also gives an overview of thermal-driven devices and supercapacitors and then dives deep into the applications of thermal-driven supercapacitors, and how graphene-based nanomaterials can be efficiently utilized for powering these devices. Further, it also gives an analysis of the literature presented, summary on the effects of temperature on supercapacitors and provides prospects that can aid in further research. 2.2 PROPERTIES AND SYNTHESIS OF GRAPHENE Since the discovery of graphene in 2004 by Novoselov and Geim, by micromechanical exfoliation of bulk graphite, it has seized a lot of attention by researchers.136 Graphene can be beneficially utilized as an alternative material for the electrode in energy storage applications. It can be suitably used

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for electrochemical functionality due to its reported novel properties, such as excellent electro-catalytic activity, high surface area, electric, ionic, and thermal conductivities, high current density (CD), chemical inertness14,25,54 Graphene is at the core of advanced material research due to its unprecedented physical attributes, which bestow astonishing performances. Utilization of graphene in electronics can highly aid technological advancements. With the implementation of graphene, pioneering research in the field of electronics can be achieved while it is both energy efficient and environmentally benign. 2.2.1 PROPERTIES OF GRAPHENE An atomic layer thick, two-dimensional planar graphene sheets contain sp2 bonded atoms in a hexagonally structured carbon lattice. It has in-plane sigma bonds and pi-orbitals that lie perpendicular to the plane. Graphene can be considered a building block of all carbon forms as it could be either encased into 0D Buckyballs or tumbled up into 1D Carbon Nanotubes (CNT). Graphene and derived materials possess unique properties. Most peculiar property is its highly accessible surface area, which consequently results in excellent performances. Graphene is reported to have a theoretical surface area of 2630 m2/g. This is greater than CNTs and graphite that possess a surface area of 1315 m2/g and 10 m2/g, respectively.148 Attributed to its highly conjugated sp2 lattice, it possesses an electrical conductivity of 64 mS/cm for wide temperature ranges.54,109 This value is 60 times greater than the value of conductivity of single-walled CNTs.208 It also possesses excellent thermal conductivity of 3000 W/mK, a Fermi velocity (vf) of 106 m/s and exhibits a quantum hall effect. These properties of graphene are even evident at room temperature.62,162,174 Its tunable bandgap distinguishes it from its bulk counterparts. This renders the graphene quasiparticles into massless Dirac Fermions.15 Their charge density can be tuned by the gate electrode.62 These superior electronic properties of graphene can be ascribed to the ballistic movement of free electrons.14 Even in the presence of impurities, this unwavering movement has been observed. It was reported that ultra-high mobility of 200,000 cm2/Vs at 2 × 1011 cm−2 electron density was obtained when graphene was suspended on Si/SiO2 gate electrode. This is reported to be 200 times than that of silicon.10 Graphene is speculated to carry a super-current and exhibits an ambipolar electric field effect.14 Intrinsic graphene is characterized as zero bandgap semiconductor or as a semi-metal.81

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Graphene sheet has a resistivity of 106 Ω-cm, which is less than that of silver. This resistivity of graphene sheets is the lowest at room temperature among all the material resistivity known.54 The C–C bonds in graphene are small and strong and hence any external fluctuations/noises do not destabilize it. Graphene also possesses high mechanical stability. The properties of graphene can be easily tuned by changing its morphology or functionalizing it with other molecules. 2.2.2 SYNTHESIS OF GRAPHENE Graphene was widely believed to be an unstable structure and that if it would be scaled down to nm it would scroll and buckle.158,168 The ability to isolate free-standing graphene has created the current research boom. The stability of 2D crystals in the nanometer (nm) regime can be attributed to the intrinsic microscopic roughening.46 Graphene can be produced using the top-down and the bottom-up approaches. To separate the individual sheets of graphite, the Van der Waals weak interlayer force needs to be overcome.33 The top-down approach involves breaking graphite into atomic layers, while the carbon molecules are dealt as building blocks in bottom-up approach. While top-down approaches are tedious and offer low yields, bottom-up approaches are not viable for large-scale productions.33,46 However, bottom-up approaches can be used to synthesize graphene nanoribbons/ nanoflakes in large quantities. After the scotch tape/peel-off method was initially discovered, vapor deposition, exfoliation by chemical/mechanical, or thermal treatments have come up to synthesize pure, defect-free graphene. Exfoliation/cleavage method to synthesize graphene utilizes chemical/ mechanical/thermal energy to break the interlayer bonds. Novoselov and Geim first discovered graphene via micromechanical exfoliation.136 Jayasena et al. reported another diamond wedge aided by the ultrasonic waves method to scrape off graphene from the source.130 This method is identical to the method, where graphene can be obtained by sonication in an aqueous medium using surfactants. Zhang et al. thermally treated graphene oxide (GO) under high pressures to improve the quality of graphene obtained from GO.48 Via this method, highly crystalline sheets that were of reduced graphene oxide (rGO) were obtained. Stankovich et al. attempted reduction of graphite oxide and exfoliated GO sheets by ultrasonication in an aqueous suspension.175 This method utilized the hydrophobic nature of graphene to form a suspension. This method resulted in partial rGO sheets. Further, Nandez et al. came up with

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the use of N-methyl pyrrolidone media for exfoliation of graphite, which resulted in dispersed few-layered graphene.63 The energy needed to rupture the Van der Waals forces was countered by graphene-solvent interaction, where the solvent has a surface energy that is like the surface energy of graphene. Parvez et al. presented a method to produce graphene via electrochemical exfoliation of graphite, where graphite would act as an anode and would get exfoliated. Various electrolytes for electrochemical exfoliation have been tried and tested, such as various mineral acids, inorganic and organic salts.9,93,119,133,143,190 The exfoliation technique is easy and can be implemented for large-scale production. Chemical vapor deposition (CVD) technique for the production of graphene is being widely researched. It yields comparatively high-quality graphene and has potential for large-scale production. Although it requires sophisticated equipment, yet the process is quite straightforward and yields high-quality graphene. CVD is simply deposition of gaseous reactants on a substrate. Using the carrier gases in a reaction chamber, gas molecules combine and get deposited on the substrate. The chamber is heated, and a reaction takes place that creates a thin film of the required material on the substrate, that is, the solid compound first vaporizes and then condenses on the substrate via chemical reaction.69 Depending on the operating pressure inside the chamber, CVD processes are categorized into two types: low-pressure CVD and ultrahigh vacuum CVD. Low pressures are usually under sub-atmospheric pressure, while ultrahigh vacuum conditions are under extremely low pressure in the order 10-6 Pa. These low-pressure conditions help to prevent impurities from entering and yield high-quality pristine graphene. However, the carrier gases required for the process need to be highly volatile in nature and these form toxic by-products. Thus, the residue gases from the chamber need to be properly disposed off. Much research is ongoing to make this process efficient with the use of benign precursors.66,73,85,86,118,179 Apart from these techniques, milling, sonication, and radiation-based techniques are being extensively researched. Some bottom-up methods like the dry ice method and proliferation on substrates like metal-carbon melts and silicon carbide are also being used.33,165 Carbon is the most copious element in our ecosystem. Among the advanced functional materials, carbon is pivotal in the development of highperformance and sustainable materials. The use of green synthetic methods to produce environmentally benign graphene-based materials is an emerging area in the field of nanotechnology. This route offers environmental as

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well as economic benefits and is an alternative to the conventional routes of synthesis. Kartick et al. used Cocos nucifera L. as an eco-friendly and nontoxic reducing agent in green synthesis of graphene.74 Use of nontoxic reducing agents like bovine serum albumin, vitamin C, and reducing sugar for the conversion of graphite oxide (GO) to reduced graphene oxide (rGO) has been delineated in the literature.13 All the above reducing agents successfully eliminated the toxic effects, but these produced highly agglomerated rGO. To eliminate the agglomeration effect, Vusa et al. made the use of carotenoids that are available in vegetable extracts.192 They demonstrated a green, facile, and cost-effective method for direct reduction of graphite oxide by a vegetable extract. Every method holds its own advantages and disadvantages. During the synthesis of biocompatible graphene, the commonly addressed issue is the preparation of its stable suspension. Further, research needs to be done to find a method that forms stable suspensions and is feasible for mass manufacturing. The graphene properties can be tuned depending upon the functionality required. And accordingly, a method of synthesis needs to be chosen,because the synthesis method heavily influences the end-product of graphene and its performance. Graphene has excellent potential to be applied as an active material in field-effect transistors, sensors, energy harvesting, and storage devices. Future technologies can highly depend upon graphene for higher speeds, better stability, and functionality while still being eco-friendly. The upcoming sections in this chapter and Chapter 3 will emphasize upon how graphene can be utilized in batteries and supercapacitors as a material for the electrode. 2.3 GRAPHENE-BASED NANOMATERIALS FOR LITHIUM-ION BATTERIES (LIB) The development of the new-generation secondary batteries started in the 1970s.145 Lithium is a light weighted material with a low resistance and a high reduction potential. This makes it attractive to be utilized as an electrode. The LIB have many advantages over the conventional batteries. The major advantages of Li-ion batteries are high volumetric and gravimetric energy densities, high coulombic and energy efficiencies, high open-circuit voltage, long cycle-life, and design flexibility. It has promising applicability and is among the commercially highly used batteries. As a result, great focus is being put to enhance the performance of these batteries in a cost-effective way.

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The Li-ions move from the negative to the positive electrode through the charging-discharging process and hence they are also referred to as rocking chair batteries.21 Both the electrodes of the Li-ion batteries are intercalation compounds, where Li+ ion is inserted into the crystal lattice in a topotactic manner.161 The electrolyte window is the difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HUMO). The electrochemical potentials of the anode should be above the LUMO and that of the cathode should be below the HUMO.57 In the electrochemical cell, the anodes and cathodes must be selected so that they lie between the electrolyte’s electrochemical potential window. The development of a passive solid-electrolyte interface (SEI) takes place either on the cathode or on the anode. SEI functions as a barricade for the reaction between the electrode and the electrolyte. This SEI layer induces kinetic steadiness in the electrochemical cell. The anodic reduction and the cathodic oxidation reactions take place as follows: At cathode: LiCoO2 —> Li1–xCoO2 + xLi+ + xe– At anode: xLi+ + xe– + C6 —> LiC6

(2.1)

The commonly used cathodic materials are metal oxides (such as lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxides). Although tin-based cathode materials have a good cycle performance, but its first cycle shows a highly irRC loss.260 Various spinel metal oxide electrodes (such as lithium iron phosphate and lithium manganese oxides) are also under research and show promising results. Transition metal oxides, lithium titanate oxide, tin-based oxides, and graphite are typically used negative electrodes in commercial batteries.21 There are various advantages and limits for the use of these materials. The carbonaceous materials show a good cycle life and stability but low preliminary charge-discharge efficacies. The limitations of these materials are poor capacity retention upon the runs, limited Li storage capabilities, high reversible capacity (RC) loss, and low charge-discharge rate capacities.76 Thus to meet the augmenting power demands, there is a requirement for new electrode materials to address these issues. Graphene with its novel properties shows promising results. The rich porousness of graphene provides plenty of active sites for Li adsorption, which enhances the power and energy densities. Throughout the chargingdischarging in the LIB, Li+ ions are interpolated and detached in the intercalation material and the material swells and shrinks in the process. This leads to a quicker breakdown of the material. Addition of the graphene to the electrodes

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prevents this from happening. The graphene basal plane provides Li-ion diffusivity as high as 10−7 to 10−6 cm2/s.21 As the addition of graphene would enhance the electrical conductivity, its resistive heating within the electrode would decrease and hence the battery could also be operated at lower temperatures to improve the battery safety.237 Batteries with graphite as an anodic material have a theoretical specific capacity (SC) of 372 mAhg-1.197 Graphite stores one Li+ ion for six carbon atoms, forming an intercalation compound LiC6. This limited storage of ions within the sp2 hexagonal carbon structure shows low Li storage capacity. This capacity increases to 500–1100 mAhg−1 using graphene. The Li-ions on the graphene films are stored both on the edge and on the surface of sheets. During the intercalation, 2s electrons are transferred to the carbon host. These increased capacities can be ascribed to: the ion insertion within the cavities, ion adsorption on both the sides of the sheet, ion binding on the covalent sites, and hydrogen-terminated edges.37,116,161,182 It is hypothesized that Li-ions can be stocked on both the sides of the graphene sheet; this results in two layers of lithium on a single layer of graphene. By forming Li2C6 intercalation compounds, the theoretical SC was found to be 744 mAhg−1.107 Also, a highly reversible storage capacity of 540 mAhg−1 has been reported via graphene nanosheets.129 The capability of graphene when doped with Li-ions for storing hydrogen becomes essential.198 The adsorption energy of graphene is cluster size-dependent. For enhanced battery performance, a small cluster size between two larger clusters has been proposed.195 Introduction of C60 and CNT in the graphene lattice enhances its capacity, due to electron affinities of CNT and C60, which expands the space between the graphene.37 The crumpling and uneven morphology of graphene sheets also leads to the formation of nano-cavities, increasing the number of active sites for Li adsorption.99,197. However, the use of pure graphene as an anode is reported to be difficult for practical purposes. The restacking of graphene layers into a turbo-stratic structure degrades the battery performance by reducing the PD and SC.123 Small Li clusters may form on the surface of graphene that may lead to nucleation and eventual formation of Li dendrites.200 The capacity of lithium absorption is largely influenced by interactive forces between lithium and graphene. The absorption capacity is reduced due to the repulsive forces between lithium and graphene, as it becomes difficult for the formation of LiC6 intercalation compound.108 Principle computations show that Li clustering and phase segregation can limit the Li absorption capacity of pure graphene. This capacity is even lower than Li intercalation

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via graphite.67 Thus to enhance the characteristics of the batteries, graphene can be functionalized using electron-withdrawing groups, p-type, and n-type dopants. 2.3.1 GRAPHENE-BASED ANODIC MATERIALS Depending on the reaction mechanism at the anode, the anode material can be classified into three types: conversion, insertion, and alloying. Most used are the insertion type carbonaceous materials, while transition metal oxides undergo conversion reactions. Metal oxides, metal sulfides, and heteroatoms have much higher storage capacities than carbon materials. But these materials suffer from low electrical and ionic conductivities and low cycle stability. Hence, a composite of these dopants with graphene and its derivatives are being heavily researched. Integration of graphene with these dopants creates uneven morphology and defects on the surface of graphene, which greatly contributes to enhance its ion storage capabilities. Also, the performance of a material is highly dependent on the synthesis method as documented in the literature. This section of chapter includes discussion on: (1) research studies on various transition metal oxides and sulfides, singleatom and dual-atom dopants, and silicon doping; and (2) how its synthesis technique, structure, morphology, and size affect the performance. 2.3.1.1 TRANSITION METAL OXIDE-BASED GRAPHENE MATERIALS Commonly used transition metal oxides as secondary battery electrodes are: TiO2, MnO2, NiO, Fe3O4, Fe2O3, Co3O4, etc. These transition metal oxides have shown high-energy densities because of their high theoretical SC. The Li-ions in these transition metal oxide materials are stored via the following conversion reaction: Li + MaOb < – > aM + bLixC

(2.2)

where M is the transition metal, O is the oxygen atom, x is the valence number, and a, b are the stoichiometric coefficients. However, these metal oxide electrodes are subjected to pulverization and electrical disconnection, due to substantial volumetric changes through the conversion reaction. Therefore, these transition metal oxide anodes suffer from limited cycle life.203 Due to the formation of SEI, ion diffusion becomes a kinetically limited process. Hence, nanosizing of the material becomes

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necessary.108 Along with large-volume changes, low electrical conductivity and kinetic limitations of materials present an obstacle toward their applications in lithium-ion batteries. It is well established that graphene can function as a barrier to the volume expansions and contractions through the conversion reactions, enhancing the conductivity and improving the cycling stability of these materials.67,259 Also, transition metal oxides inhibit restacking of graphene materials, which increases its stability and cyclability. Zhang et al. demonstrated that α-Fe2O3 + rGO mixture synthesized through microwave autoclave can deliver a SC up to 300 mAhg−1 for 140 runs.251 Modafferi et al. compared the specific capacities of bare hematite and Fe2O3-rGO nanocomposite. In agreement with the above theories, the SC demonstrated by bare hematite is of 129.2 mAhg−1 while that of Fe2O3-rGO nanocomposite is 331.4 mAhg−1. However, this initial SC of Fe2O3-rGO nanocomposite is not fully reversible. Post 10 runs, with a discharge rate of 20C, the capacity drops down to 181.4 mAhg−1. However, at the same rate for the last 10 runs, capacity retention was observed. A steady SC of 166 mAhg−1 was recovered, which was still 40% greater than that of bare hematite. This shows the importance of the linkage between the graphene support and the oxygen anchored on it. In the initial runs, the capacities of both α-Fe2O3 + rGO mixture and Fe2O3-rGO nanocomposite are comparable. However at higher current rates, nanocomposite shows a better stability.132 Using electrostatic co-assembly, graphene-encapsulated Co3O4 (rGOCo3O4) after 130 runs gave a SC of 1000 mAhg1 as demonstrated by Yang et al.231 Su et al. prepared carbon-coated rGO-metal oxide nanosheets (G-MOC), using hydrolysis, polymerization, and carbonization techniques. After 100 runs at the CD of 200 mAhg−1, G-Fe3O4-C electrodes yield a capacity of 920 mAhg−1 while the G-SnO2-C yields a capacity of 800 mAhg−1.178 Jiang et al. presented that via ball milling, rGO-NCM nanocomposites (Nickel, Cobalt, and Manganese oxides) relinquished a capacity of 153 mAhg1 while only NCM particles yield a capacity of 138 mAhg−1.61 The same nanocomposites synthesized by spray drying and heat treatment furnished a composite capacity of 88.5 mAhg−1 and bare NCM furnished a capacity of 6 mAhg−1.154 Mai et al. demonstrated that after 50 runs CuO-graphene nanocomposite synthesized by hydrothermal techniques rendered a capacity of 583.5 mAhg−1 while it possessed a high retention percentage of 75.5%.126 TiO2-graphene composites with a rate of 100 mAg−1 have a Li-extraction potential of 499 mAhg−1. This capacity drops to a value of 150 when its CD goes up to 3000.19 Li et al. produced an anisotropic anode, with TiO2 nanosheets parallel to that of graphene. This composite showed a 112 mAhg−1 capacity at an ultrahigh rate of 100C.95 A MXene-derived TiO2/

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rGO composite was synthesized using the hydrothermal process. At a high rate of 1680 Ag−1, a capacity of 152 mAhg−1 with a retention capacity of 82% was obtained. At a CD of 1 Ag−1, 104 mAhg−1 capacity at 85% retention rate was reported after 1000 runs.50 Xianbo et al. decorated graphene sheets with hollow transition metal oxide nanoparticles,218 based on the Kirkendall effect. This technique enables us to control the interior voids and the shell thickness of the material, leading to smaller particle size and lattice uniformity. The initial cycle capacity of graphene-hollow iron oxide nanocomposite was 2247 mAhg−1 and had a Coulomb efficiency of 71.1%. With a Coulomb efficiency of 98% after 50 runs, the reversible SC of the anode was 1272 mAhg−1. This demonstrates fine cycle stability even at low current rates. Various initial discharge capacities at 200, 500, 1000, 2000 mAg−1 were studied. All these capacities were significantly greater than that of bare hematite. Graphene-hollow Co3O4 and NiO nanocomposites were also studied. Both these composites show structural feature like that of graphene-hollow iron oxide nanocomposite. Both the composites exhibited excellent reversible capacities with good cycle stability and rate capabilities even at higher current rates. Graphene-hollow Co3O4 and Graphene-hollow NiO nanocomposites demonstrated a high discharge capacity (DC) of 1346 mAhg−1 and 776.8 mAhg−1, respectively at high rate of 1000 mAg−1. These capacities are much higher than their counterpart metal oxide anodes, which displayed a capacity of 409.5 mAhg1 after 48 runs for pure Co3O4 and of 409 mAhg−1 after 88 runs for NiO, respectively. Zhou et al. demonstrated that an orderly stacked, self-assembled, packed structure has efficient ion-transport with no resistance conducting lattice.255 Superior power delivery and high capacity were observed using MnCO3-graphene hybrid, with the above-mentioned structure. Lavoie et al. synthesized and compared the performances of Mn3O4-graphene platelets and Mn3O4-reduced graphene oxide nanocomposites. Both displayed high gravimetric capacities and a high cycle stability of greater than 100 runs. The capacities for Mn3O4/graphene-platelets composite and Mn3O4/ rGO composite were 720 and 675 mAhg−1, respectively, representing that thicker graphene planes as in platelets can efficiently espouse with the metal oxides.84 Liu et al. demonstrated the use of graphene-modified nanostructured vanadium pentoxide (V2O5) hybrids with peculiar electrochemical rendition for LIB. The intra-particle conduction and the ion-diffusion of the V2O5 xerogel was significantly improved by the insertion of graphene nanosheets

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between the xerogel nanoribbons.114 The electrochemical performances are also affected by the metal oxide to rGO mass ratio.115 Improved performances of the batteries can be achieved by optimization of this ratio. Further investigation on this aspect needs to be done, to utilize a material to its maximum potential. 2.3.1.2 TRANSITION METAL SULFIDE-BASED GRAPHENE MATERIALS The transition metal sulfides attributed to their eminent specific capacities and excellent rate capabilities have dragged a lot of attention. These capacities are even greater than their oxide counterparts.26,167 Although transition metal oxides have shown high cycle stabilities and good reversible faradic reactions, yet the replacement of oxygen atom with a low electronegativity atom sulfur may improve the performance as compared to metal oxides.200 Like the volume changes occurring in metal oxides due to the conversion reaction, hybridization with different materials is required for metal sulfides as well.113,200 Thus, to enhance electrochemical performances, graphene/ graphene-derivatives are added to metal sulfides to form composites. It has also been demonstrated that to prepare TMS-graphene composites, two-dimensional graphene is one of the epitomes for support frameworks.65 The following reaction mechanism is like those of metal oxides: Li + MaSb < – > aM + bLixS

(2.3)

where M is the transition metal, S is the sulfur atom, x is the valence number, and a, b are the stoichiometric coefficients. As the performance of TMS-graphene nanocomposites is highly dependent on the structure, morphology, and synthesis techniques, therefore all recent literature was reviewed for all of above-mentioned parameters. Cobalt (Co) is a common transition metal that has various sulfide forms (CoS, CoS2, Co3S4, Co9S8), few of these forms show promising applications as LIB anodes. Xiao et al. fabricated the CoSx composite with different compositions of its various sulfides.221 It constituted of 48.1% of Co9S8, 39.7% of CoS, 12.2% of Co3S4, and 1.0% of graphene. This composite demonstrated a capacity of 1012 mAhg−1 after 100 runs, at a Coulomb efficiency of 500 mAg−1. Xie et al. using hydrothermal methods synthesized CoS2-graphene nanocomposite. The morphology of the graphene sheet was observed to be uniformly linked by flower-like

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CoS2 nanoparticles on both the sides. After 100 runs at 100 mAg−1, the capacity was reached to 900 mAhg−1.222 Different synthesis techniques were examined by Wang et al. for Co9S8rGO nanocomposites.202 By the introduction of ball-milling in the synthesis process, small-sized cubic-phase of Co9S8 nanoparticles was reported. High Li-extraction potential of 1415 mAhg−1 at high CD of 545 mAg-1 was seen. Even after 500 runs, the capacity was retained at 573 mAhg−1. CoS-graphene anode also rendered a capacity of 939 mAhg−1 after 100 runs at 100 mAg−1. This CoS-graphene was obtained by the hydrothermal process followed by sonication.183 Further, Guo et al. fabricated graphene nanosheets mono-dispersed CoS2 nanocages in a layer-by-layer assembly. These mono-dispersed nanocages helped tolerate volume changes, shortened Li diffusion path, and prevented aggregation that occurs during the cycling process. High potential of 800 mAhg−1 at 100 mAg−1 after 150 runs and 697 mAhg−1 at 500 mAg−1 after 300 runs was reported.58 With good retention and rate capacity, this material showed promising results. Lu et al. demonstrated that by the virtue of the in-situ formation of GO layers, Co1-xS-rGO composites exhibited a Li-extraction capacity of 969.8 mAhg−1 after 90 runs with a Coulomb efficiency of 96.5%. A high reversible capacity (RC) of 527.2 mAhg−1 at 2.5 Ag−1 after 107 runs was achieved. This potential is much higher than that observed in ex-situ preparation of Co1-xS-rGO composites.120 Lai et al. fabricated Co3O4 nanoparticles on N2 modified microwave exfoliated graphite oxide (Co3O4-NMEG). They investigated the effect of Co3O4 weight ratio on Co3O4-NMEG composite for RC, high CD, first cycle efficiency and cycle stability. The 70% Co3O4-NMEG rendered a reversible SC of 750 mAhg−1 and a first cycle irRC loss of 700 mAhg−1.82 The N modification of exfoliated graphite oxide greatly contributes to Co3O4 dispersion, first cycle efficiency and helps in the reduction of oxygen content in graphene. Much of the research in TMS is based on Manganese sulfide (MnS), which is a magnetic semiconductor with a 3.7 eV bandgap and shows promising potential as a material in anode in LIB. MnS has a high SC of 616 mAhg−1.135 MnS has three different polymorph structures, namely, α-MnS, β-MnS, and γ-MnS.149,194 Among these, α-MnS and γ-MnS show excellent properties and are promising candidates for Li-ion battery anodes.184,253 Chen et al. synthesized hollow α-MnS-rGO nanocomposite using a one-pot templatefree solvothermal method, which gave a stable SC of 430 mAhg−1 at 0.2 Ag−1 after 20 runs. This capacity was higher than that obtained by using pure MnS or graphene.25

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On the other hand, Xu et al. synthesized α-MnS hollow microsphere-rGO using the hydrothermal method. To increase the number of pores and to reduce GO, urea was added in the solution. After 400 runs, the capacity was retained to 640 mAhg−1.228 This comparison shows the difference in capacity attained by different synthesis methods. As compared to the expensive solvothermal method, the facile hydrothermal method proves to be efficient. A high rate of performance was demonstrated by γ-MnS-rGO nanocomposite, with a RC of 320 mAhg1 even at high CD of 1000 mAg−1. Also, stable SC of 600 mAhg−1 was observed after 100 runs at 200 mAg−1.155 Li et al. fabricated mesoporous manganese sulfide nanoparticles supported on nitrogen and sulfur co-doped graphene by the solvothermal method, which displayed a SC of 1004 mAhg−1 after 100 runs at 200 mAg−1.89 At a CD of 0.2 Ag−1, a RC of 1231 mAhg−1 was reported by Chen et al. using mesoporous MnS-graphene composite.38 This mesoporous microstructure helps to ameliorate the mechanical strain of the electrode exposed to charging/discharging processes and helps to improve the contact between the anode and the electrolyte. Hierarchical structure of Vanadium sulfide is identical to graphene and d-electrons; and this allows efficient intercalation and diffusion. Mikhaleva et al. fabricated freestanding two-layer heterostructure of VS2 and graphene sheets.131 They analyzed both the H and T configurations of VS2 monolayer. The SC of this composite was reported to be close to the theoretically expected SC of 569 mAhg−1. Further, Li et al., via the one-pot hydrothermal process, synthesized the VS4-rGO composite.96 It gave a SC of 898.8 mAhg−1 after 80 runs at a retention of 87%. This is significantly higher than bare VS2, which give a SC of only 88.3 mAhg−1 after 80 runs. Following a twostep process of the hydrothermal method followed by heat treatment, Yu et al. synthesized a Ni3S2-C/rGO composite.242 Various CDs of 500, 300, 200 mAg−1 were examined and corresponding capacities of 464.5, 490.7, and 520.2 mAhg−1 were reported. Further, Chen et al. synthesized NiS2-graphene using the same hydrothermal technique and demonstrated a high RC of 813 mAhg−1 even after 1000 runs.29 The NiS2-graphene composite along with high SC showed excellent cycling stability. A direct fabrication of Ni2S4 nanoprisms via the one-pot technique was demonstrated by Abdelhamid et al.1 The composite showed the best results after 50 runs of 980 mAhg−1. Molybdenum sulfide (MoS2) is extensively being researched for electronic applications due to its 2D-layered structure like that of graphene. It provides large storage capabilities and shortens the diffusion paths of ion

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and molecules. Although it is a single-layered, yet the electrical conductivity between the adjacent S-Mo-S layers is low and this constrains its direct applicability as an anode material. You et al. fabricated free-standing MoS2/N-doped graphene nanosheets, in a highly compact film-foam-film hierarchical structure.239 At 0.1 mAg−1, N-doped graphene/MoS2 gave a high coulombic SC of 1875 mAhg−1 with a Coulomb efficiency of 63% during the first cycle. Further, on the surface of graphene, 1T-MoS2 nanosheets were vertically synthesized via the solvothermal method.220 Xiang et al. observed that the composite performance was quite higher than that of pure MoS2. Wang et al. fabricated MoS2 nanoflakelets–graphene composite using ethanol, which facilitated the formation of nanoflakelets.204 Various weight ratios of graphene to flakelets were examined (7.5, 15, 30, and 60 wt%). Among the examined ratios, MoS2 nanoflakelets-graphene composite of 60 wt% graphene gave a high RC of 1309 mAhg−1 at 100 mAg−1 after 50 runs, while outstanding rate performance and cycle stability were found with 30 wt% graphene composites. Another test using a solution-based synthesis method assisted by cetyltrimethylammonium bromide (CTAB) followed by annealing was carried out.212 Wang et al. fabricated single-layer MoS2-graphene nanosheets. The graphene sheets and CTAB prevent MoS2 from restacking. While singlelayered MoS2 delivers a SC of 1091 mAhg−1 and a RC of 825 mAhg−1 after the first cycle, the single-layer MoS2-graphene nanocomposite delivered the capacities of 1367 mAhg−1 and 912 mAhg−1, respectively. This irreversibility was observed after every first cycle; this could be ascribed to the SEI layer formed, reduction of oxygen-containing groups, and decomposition of electrolyte.40 Utilizing the one-pot template-free solution technique, Fend et al. fabricated CuS nanowire-rGO nanocomposite, which gave a good RC.51 Then Li et al. synthesized the CuS nanospheres–graphene nanocomposite that exhibited outstanding cycling performance.90 At 2 Ag−1 even after 1000 runs, the SC was retained to 348 mAhg−1 and a Coulomb efficiency of 100% was observed. Li et al. created neoteric N-doped graphene-coated WS2 nanosheets. This innovative structure was based on graphene hollow spheres to provide a two-sided shielding framework for Li-ion deposit.97 This composite showed a high SC of 1309.4 mAhg−1 at 100 mAg−1. At a high CD of 1000 mAg−1, it gives a SC of 300 mAhg−1 after 320 runs. Youn et al. demonstrated a simple synthesis method of nanocrystalline SnS2-rGO composite, which shows a good cycling stability and retained a SC of 562 mAhg−1 after 200 runs at 0.2 Ag−1.240

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In quest of advanced materials for high cycle stabilities, high retention rates, and high reversible capacities, various multiple-metal graphene mixtures have been synthesized and tested for their performances. Zhang et al. fabricated a hierarchical rGO matrix.252 NiCo2S4 hollow nanoprisms were interconnected with reduced graphene oxide lattice by them. This interconnection of nanoparticles provides a higher density of pores and spaces, providing more surface area for reaction and intercalation. This feature makes it resilient to volume changes that occur during the reaction. This rGO-NiCo2S4 nanocomposite had an RC of 903 mAhg−1 at 0.5 Ag−1 after 80 runs. It further gave a splendid SC of 489.3 mAhg−1 even after 1600 runs. Wang et al. synthesized an in-situ N/S doped graphene-CuCo2S4 nanocomposite.207 This composite material by these authors had a capacitance equivalent to that of a pseudo-capacitor. After 500 runs, it had ultrahigh rate SC of 328 mAhg−1 at a CD of 20 Ag−1. This work further leads to more novel research on possessing pseudocapacitor storage capabilities in secondary batteries. Using the facile hydrothermal method, high-performance composites can be obtained according to Bai et al. using NiCo2S4/ Ni0.96S-graphene aerogel.4 In a water bath-hydrothermal-sulfurization-annealing technique, Yu et al. fabricated unique zinc-cobalt bimetallic sulfide-rGO (Zn0.76Co0.24S-rGO) nanomaterial.241 The Zn0.76Co0.24S-rGO nanocomposite exhibited a high RC of 989 mAhg−1 at 100 mAg−1 after 100 runs. The strong synergy between zinc-cobalt sulfide and rGO contributes toward significant enhancement in electrochemical performance. Authors of this chapter conclude from the above-mentioned review that several sulfide composites of varying properties have been synthesized and examined. Among all metal sulfides, layered Molybdenum and Vanadium are most suitable as a material for cathode in the LIB, due to large space for ion transportation, short diffusion paths, and a greater number of active sites. By controlling the grain size and morphology of the particle, better performance can be obtained. Thus, the synthesis method employed for the composite becomes very significant. 2.3.1.3 GRAPHENE DOPED WITH HETEROATOMS The performance of graphene, in terms of its physical and chemical properties, can be significantly improved by adding heteroatoms as dopants. The doping sites on graphene sheets can be at the edge, pores, vacancies, and strained regions.209 Doping of heteroatoms like N/S/B/F/combination on

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graphene causes electronic and structural disorders to result in changes in electronic state, Fermi energy level, bandgap, and thermal stability.196 These properties play an instrumental role toward the modified conductivity of the material. Improved properties can be achieved by varying the type and content of these dopants. Xing et al. demonstrated that N-doped graphene gives elevated SC, cycle performance, and superior rate capabilities.224 As the diffusion paths are shortened, these N-doped graphene sheets synthesized via one-pot hydrothermal method display a SC of 650 mAhg−1 at 100 mAg−1. The first cycle Coulomb efficiency is 67%, which reaches to near 98% in the following runs. On the other hand, Wu et al. followed thermal exfoliation of graphite to form graphene and then heat treatment to dope N and B atoms.217 At CD of 50 mAg−1, N-doped graphene displayed a first cycle SC of 1043 mAhg−1 and B-doped graphene of 1549 mAhg−1. After 30 runs, they demonstrated a steady RC of 872 and 1227 mAhg−1, respectively. The SC retention was increased from 66.8% of pure graphene to 83.6% and 79.2% of N-doped and B-doped, respectively. Using first principle of density functional theory (DFT) computations, Hardikar et al. studied the effects of B-doping on graphene.9 They confirmed that although N-doped defective graphene showed enhanced performance, yet its high absorption energy and barrier energy leave it ineffective to be used as a dopant. B-doping has a low energy barrier and has higher Li-storage SC making it a potential anode material. Yun et al. observed that the reversible capacities of S-doped graphene nanosheets were twice and thrice than the SC of pure graphene at 372 mAg−1 and 11,160 mAg−1.244 With good cycle stability, the SC was retained even after 500 runs at 1488 mAg−1. The enhanced rate capability is due to the increased conductivity of graphene nanosheets from 32 S/m for pure graphene to 1743 S/m of S-doped nanosheets. Solvo-thermally derived S-doped graphene by Quan et al. demonstrated a RC of 380 mAhg−1 at 100 mAg−1 after 300 runs.150 It showed excellent retention capacity at 2 Ag−1 even after 1000 runs. Ju et al. fabricated F-doped graphene foam via the direct solid-state method, and this composite exhibited 800 mAhg−1 at 100 mAg−1 after 50 runs; and 555 mAhg−1 at 500 mAg−1 after 200 runs.72 Shan et al. showed prominent improvement in capacity and its retention with a unique porous network of Sulfur-Nitrogen dual doped graphene aerogels.166 With the optimization of mass ratio of the composite to 1.5 wt% Nitrogen and 15.11 wt% Sulfur, excellent cycle life and rate retention were recorded. At a CD of 800 mAg−1 after 400 runs, 1109.8 mAhg−1

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was recorded. Luan et al. fabricated Nitrogen-Phosphorous dual doped multilayered graphene via the one-step arc-discharge process.122 This NPG (Nitrogen-Phosphorous Graphene) composite exhibited high commencing SC of 1383 mAhg−1 and a RC of 859 mAhg−1 at 100 mAg−1. At higher CD of 2000 mAg−1 a SC of 758 mAhg−1 was obtained. NPG showed outstanding rate performance along with good cycle stability. After 1000 runs, the SC is retained to 787 mAhg−1, which is 98.6% retention, at a fade of 0.0014% per cycle. Further, Nitrogen-Fluorine (NF) co-doped graphene (G) was prepared via hydrothermal reactions.68 The best performance was seen via NFG composite synthesized at 150ºC. A RC of 1075 mAhg−1 at 100 mAg−1, 305 mAhg−1 at 5 A/g, and SC retention of 95% at 5 A/g after 2000 runs were observed. These high-rate capabilities, long cycle stability, and superior DC were reported by Huang et al. Further, Ai et al. synthesized Nitrogen and Sulfur co-doped graphene.2 It displayed a high excellent RC of 1090 mAhg−1 even after 500 runs, which is greater than twice that of reduced graphene oxide, which displayed a SC of 420 mAhg−1. Varying the CDs to 400, 800, 2000, 5000 mAg−1, high capacities of 896, 882, 844, and 297 mAhg−1 after 700, 900, 1300, 1500 runs with a Coulomb efficiency of 99% were reported. Doping of graphene using heteroatoms has shown an exceptional increase in their performing capabilities in contrast to pristine graphene. Attributed to the introduction of these dopants (N, S, B, F, P), the bandgap opens and causes defects on the surface. This endows graphene-enhanced conductivity and increased number of active sites. Thus, these dopants present a pivotal strategy in improving the graphene effectuation. The performances of these dopants depend on highly synthesis method, dopant type, and content. Among all single dopants studied, N-dopant has demonstrated super-high capacity and rate capabilities. These affirmations were examined and demonstrated by Cia et al..20 They reported excellent performance from N-doping that was obtained when 7.04 at% of nitrogen was embedded in graphene via thermal annealing. Experiments show that both N and B dopants show high electrochemical performance even at low current densities and they also suppress the decomposition of the electrode. The N creates an electron-rich lattice, while B creates an electron deficit structure of graphene. Although N shows excellent capacities among all the dopants due to the high difference in electronegativity with carbon, yet N requires a high adsorption and barrier energy to be surpassed. On the other hand, B overcoming this drawback shows similar performances along with good structural integrity, maintaining the hexagonal lattice of graphene.

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When in a ratio of 1:3 with the carbon atom, it does not crystallize and shows good results even at high working temperatures.234 However, these single dopants inevitably face some hold-ups, which result in reduction in performance.125 To take out the best performances from these dopants, dual doping has been extensively worked out and has also exhibited performances that have promising potentials for practical applications. The strong synergy between these dopants when in a composite along with the increased active sites due to the creation of defects and pores has come forth as the dominant reasons behind these extraordinary performances. 2.3.1.4 SILICON-GRAPHENE NANOMATERIAL-BASED ANODES Silicon has seized significant attention due to its highest gravimetric capacity for Li-ion as compared to any other dopant. The challenges faced with Li-Si system are yet the volumetric changes in the system through the chargingdischarging process. It has been reported that Li15Si4 gives a theoretical SC of 3579 mAhg−1 but the system faces 300% volumetric changes.12 These changes decrease the cycle stability and lead to the weakening of mechanical properties of the electrode. It has also been reported that conventional batteries cannot resist a volumetric change of greater than 6-8%.49 Silicon combined with graphene has reduced these volumetric changes but at the expense of the volumetric capacity of the conventional materials used. Several experiments were done using solution-based or physical mixing methods to synthesize high-capacity silicon-graphene nanocomposites. However, these high capacities resulting from highly exposed surface area of the material had its consequences, such as irRC losses, low Coulomb efficiency, and low anodic stability.88,198 Evanoff et al. proposed an alternative method to synthesize Si-graphene composites.49 Via vapor deposition, they fabricated nanoporous silicon-coated graphene nanocomposite. This reduced the surface area exposed by 100 times but increased Coulomb efficiency up to 99%. This composite gave a SC of 2000 mAhg−1 and stable performances after long runs. As the exposed area was reduced, it resulted in decreased volumetric changes occurring, which helps SEI get stably formed. This method also enables tuning of Si layer thickness, which permits optimization of the composite to obtain higher performances. Wang et al. via the in-situ electroless deposition technique using Ni as a catalyst synthesized nano-silicon-coated 3D multilayer graphene sheets.206 This composite displayed good cycle stability and rate capability, with a

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RC of 1909 mAhg−1 at 0.2 Ag−1 after 100 runs. At 5 A/g, it had a discharge SC of 975 mAhg−1. The 3D multilayer structure provided copious diffusion pathway and protected the Si atoms from being in direct contact with the electrolyte, which helps in maintaining the stability of the SEI layer. Palumbo et al. demonstrated high SC of 2300 mAhg−1 at 350 mAg−1 with an efficiency of 99%.140 At 3.5 A/g, it displayed 1000 mAhg−1. This was attained using polycrystalline silicon nanoparticles anchored on few-layered graphene flakes. This method is cost-efficient and scalable ensuring practical applications. Wasalathilake et al. fabricated Si nanoparticles anchored on macroporous 3D rGO bubble film network using simple self-assembly approach.213 The bubble films help in making the material resilient for volume changes and introduce large void spaces in the structure. This Si-rGO composite exhibited exceptional rate capabilities and cycle stability. After 200 runs at 500 mAg−1 the SC was 1346 mAhg−1. After 1000 runs at 2.5 Ag−1, it maintained 996 mAhg−1 with an efficiency of 97%. By wet-jet milling, Malik et al. synthesized the layered-Si graphene heterostructure.127 It gave a RC value exceeding 1763 mAhg−1 after 450 runs and a SC retention of 98% at 358 mAg−1 and the SC fades at 0.005% per cycle after 450 runs. Physiochemical evaluation was examined in the performance for the microstructural dynamics. Various nanostructured silicon-graphene nanocomposites exhibited superior DC and cycle life compared to the conventional graphite anodes. Many research studies reviewed in this section have led a way to achieve high performances and carved a path to realize the potentials of Si-graphene nanostructured material to be used as an anode for LIB. These burgeoning efforts are taken toward developing scalable and practical electrodes for next-generation high-density secondary batteries. 2.3.2

GRAPHENE AND COMPOSITES AS A CATHODE MATERIAL

Lithium metal composites are used for cathodes in the modern batteries, as lithium delivers a quite high theoretical SC of 3860 mAhg−1.121 Among all the parameters that contributed toward the battery performance, ionic and electrical conductivity are of paramount importance. Also, cathode materials are one of the heaviest and most expensive parts of a battery. Lithium metal itself could be used as a cathode, but during the intercalations, there is possible nucleation taking place at multiple sites on the plane, which guides to the subsequent development of dendrites. Through the charging-discharging

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process, because of the formation of dendrites, there is a chance that these may pierce the diaphragm and the cathode comes in contact to the anode, leading to a short circuit. This dendrite formation can also lead to heating of electrode or even combustion of electrode causing safety issues or dendrites may fall off the cathode causing loss of active surface area, hence reducing battery life. LiCoO2 is conventionally used as a cathode material. But due to its cost, toxicity, and safety issues much work is being done on replacing cobalt with other metals, such as Mn, Ni, or Fe. However, these materials have low conductivities as compared to LiCoO2. Hence, carbonaceous materials can be thought of as an additive to improve its conductivity. Among conventional carbon additives, crystalline carbon additives are rendered ineffective due to their low melting points while carbon black or a carbon coating possesses a low electrical conductivity as its single layer would provide lower active sites.177 Hence, addition of graphene can be beneficial in improving its performance as its hexagonal 2D structure ensures high electrical conductivity and has a high melting point. Many researchers have also reported that the 3D nanostructured lattice of graphene or the addition of other dopants as conductive substrates to graphene will hinder the nucleation of lithium and prevent the formation of dendrites. 2.3.2.1 GRAPHENE–LiMPO4 COMPOSITES (M = Fe, Co, Mn, V) Substitute metals to cobalt like manganese, vanadium, and iron are being extensively studied due to their good conductivity, low cost, and low toxicity. The polyanion compound, LiMPO4, possesses a strong covalent bond of PO43−, which contributes to a stable lattice even at higher oxygen charged state and hence is safer.139 The performances of materials depend highly on their synthesized route, and this can be very well observed through research results. Many routes have been chosen to synthesize LiFePO4-graphene composites. Synthesis via sol-gel method, mechanical mixing, spray-drying, and co-precipitation method gave a range of capacity values.36 For over 100 runs, the composite exhibited SC of 60, 109, 130, 160 mAhg−1 at a discharge rate of 10C. Via solid-state route and with LiFePO4 anchored scattered graphene sheets, the composite showed SC of 161 mAhg−1 at discharge rates of 0.1C and 70 mAhg−1 at 50C.210 Via solvothermal route to synthesize LiFePO4-graphene, followed by annealing of LiFePO4-graphene and citric acid, a SC of 110 mAhg−1 at 5C discharge rate was reported.176

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Zhang et al. demonstrated similar capabilities as above, but LiFePO4graphene and sucrose were annealed instead of citric acid.249 Shi at al. synthesized the LiFePO4-C-graphene composite via the microwave-assisted hydrothermal method, which had a low-cost and high efficiency.169 Zhou et al. demonstrated a more effective method to synthesize LiFePO4-graphene nanocomposite, where the LiFePO4 nanocomposites were loosely and homogeneously anchored on 3D graphene structure with a microsized spherical secondary structure.257 With a high discharge rate of 60C, the composite demonstrated a SC of 70 mAhg−1. This high SC was due to this spherical structure that assisted Li-ion diffusion. Amorphous FePO4 has been reported to show excellent reversible characteristics.77 As compared to LiFePO4, FePO4 has some advantages like low-temperature synthesis, simple fabrication method, and low fabrication costs. The FePO4-graphene composite can deliver up to 120 mAhg−1 at 1250 mAg−1 and at 2500 mAg−1 a SC of >100 mAhg−1 was obtained. Li3V2(PO4)3 and LiMnPO4 compound grabs more attention than LiFePO4 as it requires intercalation voltage of only about 4 V and 4.1 V, respectively. Li3V2(PO4)3 has a theoretical SC of 197 mAh/g, while LiMnPO4 has low conductance, almost 5 times lower than LiFePO4.206 Cui et al. synthesized graphene and carbon nanotube modified Li3V2(PO4)3-C composite via the hydrothermal-assisted sol-gel technique.36 They observed a SC of 147 mAhg−1 after 200 runs at a rate of 20C. This 3D cross-linked network demonstrated a great cycle stability and high SC with a retention efficiency of 82.7%. Zhang et al. via the spray drying process synthesized Li3V2(PO4)3 3D graphene nanocomposite.246 A SC value of 131 mAhg−1 at a discharge rate of 10C after 100 runs was recorded. Fe-doped LiMnPO4 is substantiated to be a successful technique to enhance its performance.53 Zhou et al. fabricated LiMn0.75Fe0.25PO4-graphene nanocomposite with LiMn0.75Fe0.25PO4 nanorods anchored on graphene.256 These LiMn0.75Fe0.25PO4 nanorods interact with the graphene surface and are confirmed to modify its surface chemistry, hence giving ultra-high performance. Kim et al. prepared by one-step salt-assisted spray-drying followed by heat treatment a 3D LiMn0.75Fe0.25PO4/rGO composite.78 It displayed gravimetric and volumetric capacities of 161 mAhg−1 and 281 mAhcm−1, at 0.5C. At a high discharge rate of 60C, the SC value was close to 90 mAhg−1. This can be attributed to the 3D framework and the porosity of the material. A carbon double-coated LiMn0.9Mg0.1PO4-rGO composite also displayed excellent rate capacities as reported by Wi et al.215 Thus, these improved properties of phosphate-graphene composites can be ascribed to the addition of graphene as it enhanced the conductivity, cycle stability and provided good mechanical stability.

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2.3.2.2 GRAPHENE–LiMOX COMPOSITES (M = Mn, Co, Ni, Fe) LiMn2O4 can be used as an alternative to LiCoO2, but due to Mn dissolution into the electrolyte, it suffers from high-capacity losses and Jahn-Teller distortion, leading to conductivity losses. Thus, the addition of 2D graphene was tested. Via two different methods of preparing LiMn2O4-graphene composite, both the composites should result in good increase in SC, but unexpectedly these composites suffered from poor controllability and repeatability.70,106 But, 3D graphene-LiMn2O4 nanocomposite displayed outstanding rate capacities and cycle stabilities.80 Additionally, ZnO and Y2O3 were introduced in the composite to suppress the dissolution of Mn3+. As LiMn2O4 has a spinel structure, lithium holds the tetrahedral sites and manganese holds the octahedral sites. The intercalation of lithium ions takes place on the 3D network rather than planes. Hence, this enhanced performance can be seen while using 3D graphene. Li2FeSiO4 shows a high theoretical SC 332 mAhg−1 and as both Si and Fe are found abundantly on earth’s crust, the material cost is low. The 2D Li2FeSiO4 nanorods-rGO composite synthesized by Yang et al. exhibited a SC of 300 mAhg−1 at 0.1C and of 134 mAhg−1 at 12C.229 It shows a retention efficiency of 95% after 200 runs. Yang et al. prepared a 3D hierarchical flower-like Li2FeSiO4 with secondary nanopetals that was graphene-coated. This composite demonstrated a SC of 327.2 mAhg−1 at 0.1C and 100.5 mAhg−1 at a discharge rate of 20C discharge rate.230 Zhu et al. compared various types of graphene-LiMn2O4 composites at the same discharge rate.258 3D graphene-Li2FeSiO4/C, 2D graphene-Li2FeSiO4/C, and Li2FeSiO4/C exhibited specific capacities of 150, 135, and 80 mAhg−1, respectively, at a discharge rate of 10C. Gong et al. synthesized Li2MnSiO4 nanospheres anchored on the 3D nest-like carbon network.56 The Li2MnSiO4-graphene-C composite had a high SC of 215.3 mAhg−1 at 0.05C discharge rate. With enhanced safety, low cost, high ED, and good cycle stability, LiNi1/3Mn1/3Co1/3O2-graphene composite was prepared by Jiang et al. using mechanical ball milling.154 A RC of 150 mAhg−1 at 5C was demonstrated. Replacing the metal constituents, a high SC of 180 mAhg−1 was exhibited by LiNi0.8Co0.15Al0.05O2-graphene composite synthesized by mechanical mixing.238 Further, Song et al. synthesized Li (Li0.2Mn0.54Ni0.13Co0.13) O2-reduced graphene oxide composite via the sol-gel technique accompanied by the thermal annealing.173 This composite exhibited an elevated SC of 313 mAhg−1 at 12.5 mAg−1 and at a CD of 2500 mAg−1 a SC of 201 mAhg−1. Such high

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performances can be ascribed to the coexistence of rGO and spinel-like structure with recrystallized particles on primary particles. 2.3.2.3 TRANSITION METAL OXIDE–GRAPHENE COMPOSITE Transition metal oxides have a high theoretical SC ranging from 600 mAhg−1 to 1100 mAhg−1. These metal oxides have low voltage and low costs as they are abundantly available in the earth’s crust and are environmentally benign. Transition metal oxide-graphene composite resolves the issues of material damage due to volumetric changes and low conductivities. Wang et al. synthesized Co3O4 nanoparticles dispersed upon graphene sheets.193 This Co3O4-graphene composite demonstrated a RC of 722 mAhg−1, after 50 runs and the SC was retained to 631 mAhg−1. A sandwich structure of GO-Co3O4-GO on mesoporous silicon wafer (template) was prepared by Yang et al.232, and this structure significantly improved the electrode stability, which is represented by excellent rate capabilities. The first cycle SC was 915 mAhg−1, with 85% SC retention after 30 runs. A metastable monoclinic polymorph of VO2 via a hydrothermal method was fabricated by Nethravathi et al., which demonstrated a SC of 450 mAhg−1.134 V2O5 due to its low price, intercalating structure, and high ED has great potential for cathode materials. But due to its torpid electron kinetics and ion diffusion, it is limited from attaining high capacities and good cycle stabilities. Hence, V2O5-graphene composites are probed for their performances. Wang et al. via thermal decomposition by using (NH4)2V2O6 and graphene synthesized V2O5-graphene composites. After 50 runs at 300 mAg−1, the SC of 178 mAhg−1 was reported.211 Further, V2O5. nH2O nanoribbons were anchored on graphene nanosheets via a simplistic hydrothermal technique.44 Du et al. using this compound reported a RC of 190 mAhg−1. Liu et al. using a facile green approach synthesized ultralong single crystalline V2O5 nanowire/graphene composite.112 This composite showed excellent cycle stability with a SC of 100 mAhg−1 after 100,000 runs at 10 Ag−1. By immersing graphene nanosheets into KMnO4 solution, Li et al. prepared MnO2-graphene composite utilizing sodium alginate as a binder.92 After 150 runs, the composite still maintained a SC value of 230 mAhg−1. When polyvinylidene fluoride was used as a binder, a poor performance was reported. Different polymorphs α-, β-, and γ-MnO2 were fabricated by Ozcan et al. by microwave hydrothermal synthesis.138 Via the vacuum filtration process,

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freestanding graphene-MnO2 composite was synthesized. At 0.1 mA/cm2 current, DC of 321, 198, and 251 mAhg−1 were recorded for α-, β-, and γ-MnO2, respectively. Among the MnO2 polymorphs, α-MnO2 exhibited good capacities and cycle stabilities ascribed to its structural properties that offer low resistance to diffusion. 2.3.2.4 SULFUR–GRAPHENE AND METAL SULFIDE–GRAPHENE COMPOSITES As compared to the anodes, all the materials studied as a potential material for cathode have quite low capacities, which limit a batteries ED. Sulfur demonstrates an excellent theoretical capacity of 1675 mAhg1. The dissolution of polysulfides in the electrolyte decreases the active material and hence the capacity. Therefore to amend over the low capacity and low conductivity, graphene has been used as an additive. Wang et al. were the first to prepare a mixture of S-graphene205 by heat treatment of the mixture. The mixture exhibited a SC of 600 mAhg−1 at a CD of 50 mAg−1 after 40 runs. To improve the capacity, Cao et al. improved the method and mixed graphene with a solution of sulfur.22 The sandwich structure of the composite could deliver a SC of 750 mAhg−1 at 168 mAg−1 after 50 runs. Wang et al. fabricated a wrapped structure of S-graphene composite and the capacity was further increased.202 This again demonstrates that the method of synthesis is of paramount importance. To amend the graphene-Sulfur interface, Sulfur was embedded in graphene by using Na2Sx as a Sulfur source. On a similar basis, Na2S2O3 was also used as a source to prepare S-graphene mixture.94 Graphene was treated with HF that would eliminate the impurities and would create defects allowing a greater number of active sites on graphene.142 This composite exhibited a high 830 mAhg−1 at 168 mAg−1 after 50 runs. The same activation method was also done using KOH in a thermal process.39 Chang et al. fabricated MoS2-graphene composites and tested them for CDs ranging from 0.1 Ag−1 to 1 Ag−1, the corresponding specific capacities varied between 1290 and 1100 mAhg−1.219 Via the one-step hydrothermal method, Zhang et al. fabricated hexagonal SnS2 nanorods-rGO nanosheets composite.247 The discharge SC was 1005 mAhg−1 at 100 mAg−1 after 200 runs and with a reversible potential of 612 mAhg−1 at 2000 mAg−1. When SnS2 nanorods were loaded on graphene sheets the SC was 500 mAhg−1 at 0.5C after 200 runs, with a RC of 168 mAhg1.71 Large discharge rates and high capabilities were demonstrated by Lyu et al. by using CoS2-graphene nanocomposite.124

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The capacity and stability of cathode materials were significantly improved when Sulfur-graphene or metal sulfide-graphene composites are used. This can be ascribed to the presence of Sulfur that provides high capacity and graphene that allows high conductivity and diffusion rates. Graphene also prevents the dissolution of Sulfur into the electrolyte, making the battery life better. This proves that the synergy between materials highly influences its properties and hence its performance. 2.3.3 ADVANTAGES OF GRAPHENE AND ITS COMPOSITES IN PLACE OF SYNOPSIS Different composites of Graphene and its derivatives can be used both as an anode and a cathode material. Conventional materials possess high theoretical specific capacities for lithium storage, but their inherent kinetics and structure lowers its conductivity and material strength, consequently reducing the battery performance, stability, and safety. On addition of graphene, it improves the electric and ionic conductivity of the material. Graphene acts as a buffer layer to the volumetric changes, hence improving its mechanical strength. It also provides a larger number of active sites (bare or via doping), which improves the capacities. Overall, the composite’s performance, namely, rate capabilities, cycle stability, battery life, and range of operating conditions have remarkably improved. These improved performances are ascribed not only to the properties of individual materials but also to the synergetic effects of these materials when blended into a composite. To obtain the best synergy their morphology, grain size, mixing patterns, and concentration need to be controlled. Thus, the synergy between materials would highly depend upon the composite fabrication methods. Different fabrication methods yield different performances. While some yield higher capacities, the others yield high cycle stabilities. Thus, optimizing the porosity of the composite, mass ratios of the components, type of dopants, and the degree of doping becomes essential. Depending on the mechanism involved, some composites would best perform when they possess a 3D structure while some with a layered 2D sheet. Hence, according to the performance required, the synthesis method should be selected, and the process parameters should be optimized. Figure 2.1 shows graphenebased composites as an anode and cathode material and this chart covers all

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the composites covered in the literature and illustrates the associated main characteristics.

FIGURE 2.1 Graphene-based composites as an anode and cathode material. “” Indicates the range of capacitance exhibited; “” the main features exhibited; and “•” displays the commonly used synthesis method.

2.4 NEXT-GENERATION SUSTAINABLE ENERGY STORAGE DEVICES 2.4.1 SUPERCAPACITORS Supercapacitors (also known as ultracapacitors) have shown potential to complement or even replace the conventional batteries. In 1957 after the first patent on supercapacitors by General Electric Corp., which reported fast power delivery and long stable cycle life, extensive work has been conducted to achieve commercial applicability of these devices.6 Supercapacitors have the potential to overpass the differences between capacitors and batteries based on the Ragone plot=225, and these provide higher power density (PD) and high energy density (ED) compared to conventional batteries and capacitors.

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Depending on the mechanism involved, supercapacitors are of two kinds: Electric Double Layer Capacitors (EDLC) and Pseudocapacitors. The former depends upon the ion adsorption (accumulation of charges occurs at the electrolyte and electrode interface) and the latter is based on fast redox faradic reactions (faradic charge transfer between electrode–electrolyte). Supercapacitors should exhibit optimistic performances, such as high PD while maintaining high ED, low self-discharge rates, high cycle rates, long life, and stability. Pseudocapacitors can exhibit capacities 10–100 times higher than EDLC. Supercapacitors can also be categorized into two kinds depending on the type of electrodes: symmetric and asymmetric supercapacitors. When both the electrodes are of same material, it is a symmetric type and when two different materials are used for two electrodes it is asymmetric type. An evident advantage of using asymmetric type is that both EDLC and faradic transfer can be made to occur simultaneously, thus giving better outputs than symmetric supercapacitors.75 Electrolytes play a pivotal role in the capacitor performances. An electrolyte with broad voltage window (between HUMO and LOMO), high electrochemical stability, high ionic radius and low volatility, viscosity, toxicity, and cost would be an epitome of kind of properties that an electrolyte should possess.199 The two types of electrolytes are the liquid and the solid-state electrolyte. Solid-state electrolytes offer an edge over conventionally used electrolytes. Solid-state electrolytes offer super-high ionic conductivities, low leakage currents, low self-discharge rates, lightweighted, and flexible. With high available surface area (~1000 m2/g), low cost, easy processing, inter- and intra-particle conductivity, and highly accessible intra-pore regions, graphene has shown applicability as a supercapacitor material.35 Graphene with its tunable properties via different structures has shown promising results. Various microstructures of graphene-like 0D free-standing graphene, graphene dots, 1D fiber-type graphene, 2D graphene nanosheets and thin films, 3D graphene composites, aerogels, and foams have been extensively studied and reported.75 2.4.2 THERMAL ENERGY STORAGE DEVICES Among the renewable sources like hydro energy, wind energy, biogas, and solar radiations, solar energy has led researchers to develop techniques that would store this energy. However, the challenge in using solar energy as a

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source is in its intermittent nature. The drawback is its nonavailability during the night-time. Thus, the ways in which a device can supersede this problem are being scrutinized. Thermal Energy Storage (TES) works on a mechanics that captures the thermal energy and makes it available for later times. By using these TES devices, the intermittency issues of solar radiations are overcome.159 The advantages of these devices include better efficiency and reliability, reduction in running costs, and lesser environmental impact due to fewer CO2 emissions. TES provide better efficiencies than photovoltaics, but its backup during minimum or no radiations needs to be improved. New materials are being analyzed and composites are being fabricated to enhance thermo-physical properties, so that these devices become capable of serving for 24 hours. TES can store energy in the form of sensible heat, latent heat, thermochemical reactions, or in any combination. Sensible heating is the most facile method of storing energy, wherein heat is stored via heating/cooling of the storage media. The system utilizes the changes in temperature and heat capacity of the media during its charging– discharging runs. Water tank and packed bed storage are commonly used for this purpose. Latent heat is stored in the form of Phase Change Materials (PCM). By realizing/absorbing energy, these materials undergo a change in their physical state. As this phase change process occurs at a constant temperature, it is directly dependent on the latent heat of the material. There are advantages of using PCMs over sensible heat storage, which involve higher capacity over the same temperature range, high ED, isothermal process, and better efficiency. Thermochemical devices depend on bonding strength between atoms/ molecules, energy is stored and liberated as completely reversible chemical reactions. Although thermochemical storages provide higher capacities and efficiencies as compared to latent or sensible heat devices, yet the cost involved in using thermochemical storages is much higher. Thus, based on the kind of the process and the energy requirements, the storage medium should be selected accordingly. Phase Change Materials (PCMs) greatly contribute toward bridging the gap between demand and supply, supplanting the issues of intermittency of solar energy and meeting to the energy requirements at all durations. These PCMs are commonly available and possess good thermal, physical, and chemical properties. These are classified into three categories: Inorganic, Organic, and Eutectic materials. Organics consists of paraffinic and nonparaffinic compounds: salt hydrates and metallics under inorganic and

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organic–organic, inorganic–inorganic, and organic–inorganic mixtures under eutectic. All these types have their pros and cons. Hence, depending on the application required, the PCM material should be selected. An important criterion for the selection of material is its suitability in each temperature range. Based on the differences in melting points, they can also be classified as low-temperature PCMs (ice and water gels), moderate temperature PCMs (organics, polymers, and hydrated salts), and high-temperature PCMs (molten salts, metal alloys, and paraffins).160 The PCM materials should possess good thermo-physical, chemical, and kinetic properties and should also have an economic advantage. The characteristics like high thermal conductivity, high transitional latent heat, high-density variation, and low volume variation during phase change, good chemical stability and compatibility, no environmental hazards, low cost, and scalability are ideal for a PCM. To prevent the composition changes when in contact with the surrounding, these materials are contained. Bulk storage, macro-encapsulation, and micro-encapsulation are types of containments. Major disadvantage of using the conventional PCMs is its low thermal conductivity. It limits the rate of energy released/absorbed, and consequently reduces the thermal response of the material.181 Hence, several additives like metals, metal oxides, expandable graphite, carbon nanotube, and graphene derivatives are examined for their thermo-physical properties. 2.4.3 THERMAL PROPERTIES OF GRAPHENE Thermal properties of materials can be largely categorized as: thermal conductivity and thermal rectification characteristics. The strong in-plane bonding, highly anisotropic nature, and low mass give graphene its peculiar thermal properties. Graphene and its composites should have features that can help achieve ballistic scattering-free thermal flow. Ballistic flows can be achieved when the material dimensions (length) are equivalent to or greater than the photon’s mean free path. As the thermal properties are dependent upon the lattice vibrations, lattice structure and defects highly affect the materials thermal properties. Thus, investigations of vibrational modes (phonons) of the material are crucial. It is also reported that disorders, defects, or any other residue significantly reduce the thermal conductivity due to the phonon scattering.146 Using Raman tests, Balandin et al. reported a thermal conductivity of 5000 W/mK of suspended graphene.5 This conductivity performance is higher than many other known materials like diamond, carbon nanotubes, reduced graphene oxide, copper, silicon carbide, silicon, gold, or even silver, which exhibit conductivities of about 2300, 3500, 1390, 400, 490, 150, 318,

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and 420 W/mK, respectively.98 When graphene’s thermal conductivity on silicon dioxide was studied, the thermal conductivity was found to be drastically reduced to 600 W/mK.164 This was the effect of the phonon leak. This discharge through the substrate results in a strong interfacial scattering. Yet this value is higher than that obtained by the conventional IC copper material (400 W/mK). It is also observed that the substrate type affects the thermal properties. Unexpectedly, with an increase in interactions in the substrategraphene, the conductivity was more efficient.98 Specific heat determines the amount of thermal energy stored and the rate of heat transfer. Specific heat is stored in the form of phonons (vibrations) and free electrons (conduction electrons). At all temperatures, phonons dominate the specific heat of graphene. As temperature increases, specific heat increases, and at very high temperatures it becomes almost constant.7 At room temperature, the specific heat of graphite is 30% higher than that of diamond.186 This is due to the high state densities at lower phonon frequencies because of weak coupling between the graphite layers. Similar trend is expected for single layers of graphene. Highly anisotropic 2D nature of graphene gives excellent in-plane conductivity of greater than 1000 W/mK, but its conductivity is limited because of the out-of-plane weak Van der Waals coupling leading to thermal dissipation. On the other hand, 3D structures with their interconnections and networks between graphene sheets yield greater thermal conductivities. This enables tuning of thermo-mechanical properties along with enhanced thermal properties. The interlayer distances and inter-junction distances also play a pivotal part in deciding thermal properties of graphene composites.191 2.4.4 THERMALLY ENABLED SUPERCAPACITORS Predictions say the energy consumption by 2040 will hike up by 48%.191 With the increasing energy demands and decline in the availability of fossil fuels accompanied by the increasing environmental threat, there have been vast research studies on new sustainable energy storage techniques. To reform this unbalanced scenario of demand and supply, while also probing for ways in which these methods can be sustainable and efficient, various means of storing energy from renewable energies are being investigated. Supercapacitors demonstrate an efficient alternative to the conventional batteries with their high performances. Akin to all the other devices, at low temperatures the performances of these supercapacitors degrade or may even fail under extremely cold conditions.236 As discussed earlier, TES devices, being a

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sustainable alternative, can be employed as complementary devices to the main energy supply grids. But these devices, by harvesting energy from heat mediums with temperatures below 300–400ºC, render into low-grade heat storages. Due to these low-temperature heat radiations, lower thermal energy is provided. Furthermore, there are other losses and consequently the thermal to electrical efficiency highly drops down. Hence, only a small proportion of heat from the source can be utilized and converted, which becomes economically infeasible. Thus, this calls for new mechanisms, methods, and concepts that can be as competent as the conventional devices, while being sustainable and economical. Like fabricating material composites by combining two materials where advantages of both materials in synergy can be obtained, the combination of technologies can render a device that could exhibit unprecedented performances. Fabricating photothermally self-powered devices with high capabilities is a futuristic technology. Thermal-driven supercapacitor is a new-concept technology, which combines TES and supercapacitor methodology. Temperature change effects on supercapacitors are usually detrimental, causing changes in electrode potential and output voltage. But in thermaldriven supercapacitors, the inverse process is employed. Here, two half supercapacitors are at dissimilar temperatures. These are connected by means of a salt bridge, with electrodes immersed in the electrolyte. Electrode potentials are different at different temperatures. With an external resistance, two electrodes complete the circuit and current is generated giving a high output voltage and efficient electrical conductivity. The device performance mainly depends on the surface charge and electrode’s thermal sensitivity.103 It has also been reported that smaller cation size, large anion size along with high dielectric constant of solvent helps to improve the device sensitivity.101,102,105 Electrode materials also play a pivotal role. When the material’s work function is low, the variance of electrode potential is haphazard, even over large temperature span. While when material’s function is high, electrode potentials rise swiftly with temperature. This results in large output voltage for even tiny deviations in temperature.104 2.4.5

GRAPHENE-BASED MATERIALS

The operating temperature range of a supercapacitor depends upon the type of electrolyte used. As next-generation devices need to be light-weighted, flexible, and easy to handle, solid-state electrolytes commensurate the

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requirements. Among all the well-exploited materials, nanostructured carbon materials have shown excellent performances. Due to excellent conductivity, light absorption properties, flexibility, surface area, chemical stability, low molar specific heat, high Debye temperature,104 graphene has great potential as a photothermal material. Much of the potentials of these thermal-driven supercapacitors are yet to be explored. All the research till now has been reviewed and presented, and the analysis would give the crux of the matter and would aid further research. 2.4.5.1 SELF-CHARGING TYPE SUPERCAPACITOR Owing to the photothermal effect under solar radiations the capacitance, power, and ED are remarkably amplified. Yi et al. devised a pseudosupercapacitor using two parallel PEDOT:PSS coated 3D-hierarchical graphene electrodes, with a PVA/H3PO4 solid-state electrolyte.236 3D-hierarchical graphene was fabricated using plasma-enhanced CVD, with a nickel foam substrate. PEDOT:PSS was anchored on three-dimensional graphene using a dip-and-dry technique. The supercapacitor exhibited a low transmittance ( Pb > Hg; because of an increase in ionic radius, removal efficiency decreases. Thus, the minimum ionic radius was observed for Zn.31 The chromium rejection was studied by Mahajan et al.18,19 using polysulfone/ZnO nanoparticle membrane. It was observed that the Cr rejection efficiency was increased from 66 to 80% when ZnO nanoparticle was modified by HCl. This implies that the ionization of Zn in Cl enhances the charge on the membrane and hence according to the Donnan exclusion principle, it improves the rejection. But when ZnO was modified with HNO3, Cr rejection was reduced from 66 to 38%, which implies the membrane was masked with O3 and results in a reduction in the interaction behavior and hence rejection. ZnO nanoparticles were completely dispersed in polyvinylidene fluoride (PVDF) (14 wt %) and polyvinylpyrrolidone (PVP) (2 wt %) in 100 ml of N,N-dimethylformamide (DMF). Using dip coater, the film is casted on the glass plate. Rejection of copper on pristine PVDF and ZnO nanoparticle-embedded PVDF/PVP membrane was noted. It was observed that the nanoparticle-embedded membrane has an adsorption capacity of 87.4 μg/cm2 which is almost 9 times that of a pure PVDF membrane (9.83 μg/cm2).33 The 5 g of carbon nanomaterial (CNM) was added to 10 ml deionized water in which a 10-ml solution of hyperbranched polyethyleneimine (HPEI) of concentration 1 g/L was added and sonicated for 30 min. This solution was then vacuum filtered through the cellulose acetate support membrane and the formed membrane was dried at 50°C for 2 hours. These membranes have shown excellent rejection up to 95.65, 94.10, 92.56, 92.34, 90.51% for Zn, Cd, Cu, Ni, and Pb, respectively.28 Apart from this data metal ion, removal efficiency of more nanomaterial-embedded membranes is shown in Table 4.1.

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TABLE 4.1

Heavy Metal Removal Efficiency of Nanomaterial-Embedded Membrane.

Heavy Membrane metal material

Nanomaterial

% removal Reference efficiency

As(III) PSF (20–100%) MWCNT/Amide treated (0–1 % CNT) 10.9–79.4 PSF

Oxidized MWCNT (0–1%)

83.6

17

PSF

SWCNT

87.6

5

GO (0–2%)

25.87–83.65 24

As(V) PSF

Cd(II) PSF (20–100%) MWCNT/Amide treated (0–1 % CNT) 9.9–78.2 Oxidized MWCNT

71.6

17

PEI

Carboxylated GO

92.4

17

PSF

Oxidized MWCNT

86.2

PSF

SWCNT

96.8

Cu(II) PSF (20–100%) MWCNT/Amide treated (0–1 % CNT) 10.1–93.1

Pb(II)

Zn(II)

1

PSF

Cr(VI) PSF (20–100%) MWCNT/Amide treated (0–1 % CNT) 10.2–94.2

Ni(II)

1

1 17 5 1

PSF

Oxidized MWCNT

79.3

17

PAI

GO framework

99.7

17

PEI

Carboxylated GO

95.5

17

PAA/PVDF

Oxidized MWCNT

53.1

17

PSF (20–100%) MWCNT/Amide treated (0–1 % CNT) 10.5– 90.1

1

PSF

Oxidized MWCNT

41.3

17

PAI

GO framework

95.8

17

PSF

SWCNT

94.2

5

PAI

GO framework

98.1

17

Polysulfone (PSF), Polyamide-imide (PAI), Polyethylenimine (PEI), Polyacrylicacid (PAA), Polyvinylidenefluoride (PVDF), Graphene Oxide (GO), and Multi/Single-walled carbon nanotubes (MWCNT/SWCNT)

4.11

FUTURE PROSPECTS

Nanomaterials used for heavy metal separation is the promising approach in the separation area. But its environmental impact needs to be considered for a sustainable approach toward industrial implementation. The use of nanocomposite for separation is the hopeful development to resolve the issue related to separation, recovery, and reuse of nanomaterials in conventional processes.

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The incorporation of an extremely small amount of nanomaterials in the membrane matrix plays an extraordinary role in enhancing heavy metal separation efficiency. The modification of nanomaterial with the functional group and their interaction is essential to be carefully studied or else instead of positive impact on separation it will result in a reduction in separation. The selection of the correct separation method for the recovery of pollutant (heavy metal) in its nascent form is highly desirable for its further use in various applications. The design and synthesis of the membrane by the use of modifying agent (nanomaterial) by maintaining selectivity and transport properties is crucial. The use of port-forming agents as nano-additives for forming the membranes need to be used. The post-modification using grafting, coating method should be avoided to overcome the major challenge of life span and fouling phenomena of the membrane. 4.12

CONCLUSION

Air, water, and food are the basic needs of human beings. Day by day its quality is depleting due to rapid growth in population, industrialization, and human activities. This growth, development, and progress are the major and main causes of the natural calamities-earth quake, tsunami, hurricanes, global warming, etc. Currently, the World’s biggest challenge: Corona is one among these. It is predicted that the next world crisis will be due to a shortage of fresh potable water. Human/industrial activities and natural calamities are polluting the existing freshwater resource. The effluent from industry is discharged into rivers, lakes without maintaining the defined standard of WHO. According to the survey, only 10% of wastewater is treated to recycle and reuse. This challenge is the motivation for the researcher to work in this area to compensate for the shortage of water. The wastewater is contaminated with various pollutantsviruses, bacteria, organics, heavy metal ions, etc. In India, water bodies of Kabini river-Karnataka, Kolleru lake-Andhra Pradesh, etc. are found to be contaminated with different metal ion pollutants Cr, Mn, Fe, Cu, Cd, Ni, Zn, etc. These heavy metal ions are nonbiodegradable and persistent in nature. It can cause an adverse impact on the environment and human beings. It can irregulate the normal body functions, damage the central nervous system, cardiovascular function, digestive system, skeletal growth, fertility, etc. Also, it can alter the photosynthesis process, enzyme activities, germination, etc. However, these metal ions are also required for the normal growth of human beings. It is also used as raw material for various industrial processes

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such as leather tanning, cement industry, die, pigment, metal processing industry. Hence, it necessitates not only to remove but also to recover these metal ions from wastewater. Various conventional methods, namely, chemical precipitation, coagulation, ion exchange, adsorption and membrane-based methods, namely, reverse osmosis, nanofiltration, ultrafiltration are used for the separation of these metal ions from wastewater. They have their own benefits and limitations. Surface-modified membrane-based methods are reported to be sustainable with the techno-economical application. It was observed that the use of nanomaterial for heavy metal separation using conventional and membrane-based methods has shown improvement in its performance. Various nanoparticles like silver, gold, zinc oxide, titanium dioxide, salicylate-alumoxane, carbon nanomaterials are reported for this objective. Nanomaterial-modified membranes have shown promising separation efficiency with excellent thermal, chemical, mechanical, etc. stability of the membrane. Additionally, it improves transport rate, selectivity, fouling resistance properties of the membrane. The current chapter emphasizes details about the use and impact of nanomaterials in the separation processes of heavy metal ions. ACKNOWLEDGMENT The authors thank for the financial support from DST-Nano-Mission (Sanction No. SR/MN/NT/-1029/2015). They also thank the “Bharati Vidyapeeth (Deemed to be) University, College of Engineering, Pune, Maharashtra, India,” and “MIT-World Peace University, Pune, Maharashtra, India” for the valuable resources to accomplish this work. KEYWORDS • • • • •

heavy metals membranes nanomaterials separation efficiency wastewater

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PART II

SCOPE OF GREEN NANOMATERIALS IN

BIOMEDICAL APPLICATIONS

CHAPTER 5

GREEN NANOTECHNOLOGY FOR

RENOVATING PHYTOMEDICINES

GARIMA SHANDILYA, YOGESH CHENDAKE, and SACHIN CHAVAN

ABSTRACT Bioinspired advanced nanomaterials are high potential materials and emerging research field for designing unprecedented solutions for human disease management. In the design of such biomedicines, nanotechnologies hold huge potential, because human biological systems are derived from self-assembled nano units to perform the life processes. The nanomaterials are characterized shape, size, chemistry, and surface charge properties of cellular units and hence their use in biomedicine can alter the fate of current medical treatments. This chapter reviews the nature-nanotechnology amalgamation for innovation in nanomaterial synthesis and their applications in medicine. It presents development of plant-derived nanomaterials as the theragnostic solutions to transform biomedicine. Advantages of utilizing plants as source for nanomaterial synthesis, their pharmaceutical benefits, and effectiveness in treating human diseases are also highlighted. Chapter is aimed at considering plants as renewable, sustainable, and biocompatible resources for the production of nanomaterials with potential applications in the field of biomedicine. 5.1 INTRODUCTION The term plant-medicine refers to use of plants in herbology as tools for medicinal treatment or primary health care. It could also be termed as traditional medicine, ancient medicine, or herbal medicine. Applications of Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science. Megh R. Goyal and Shrikaant Kulkarni (Eds.) © 2023 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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plants for medicinal purpose are well-known methods since prehistoric times. Ancient Ayurveda, Chinese medicine, Unani hakim manuscripts, Egyptian papyrus, and Mediterranean cultures have systematically documented the use of herbs and plants for healing rituals and medicine for ailment. Traditionally parts of medicinal plant (such as fruits, stem, seeds, bark, leaves, flowers, root, stigma) or nonwoody section; or their extracts with or without secondary treatments have been used for different purposes, such as, food, flavonoid, perfume, medicines, and healing agents.6 These medicinal compounds are harmless with no or minimal aftereffects. They can synchronize perfectly with nature and can provide best remedy to use for any age group and gender. This makes the plants living factory for therapeutic chemicals and healing agents that are rich resources for drug development, which could be pharmacopeial, nonpharmacopeial, or a synthetic portion. Transformation of these plant-based components to medicinal composition and maintaining their composition with purity is one of the major challenges in biomedicine.28 Among different formulations used in biomedicine, the nanoparticle forms have proved to possess certain benefits above other forms. Nanobiomedicine or nanotechnology in biomedicine is a term used to describe the area of science that combines nanotechnology with the diagnostic or therapeutic aspects of medicine.70 Green nanotechnology uses methods and processes inspired by nature for synthesis and separation of chemicals with due consideration toward sustainability of the environment.86 It is aimed to encourage the use of green sources and minimize environmental pollution, human health risks caused by chemical methods. Green nanotechnology is the development of clean technologies to retrench contamination of environment, risks to human health in response to the manufacturing, and employing nanotechnology products along with vitalization of new eco-friendly products throughout their lifetime. It uses postulates of green chemistry and green engineering for production of nanomaterials without use of toxic ingredients. The synthesis is carried out at low temperature. Thus, these possess benefits of low energy requirements and renewable inputs considering whole lifetime of product from designing, engineering, and usage to disposal. All these three components of plant origin, nanobiomedicine, and green nanotechnology can be combined during synthesis and formulations of advanced drugs for human and animal applications with larger benefits. Imparting benefits of nature along modern science for treating and curing diseases has coined a new science arena called green medicine97 or plant-nanotechnology.32 Green medicine opens the extremities of physical medicine by building bridge between conventional medicine and complementary medicine. It

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aims to benefit human beings by encouraging plants and nanotechnology amalgamation. Plant-nanotechnology is a passionate approach of utilizing knowledge, experiences, and information toward natural health for all and greener environment for the planet Earth. This chapter provides an overview of paradigm shift from chemically synthesized drugs to awareness, evolution, and use of parts of plant as pharmaceutical resources with amalgamation of nanotechnology as potential tool in biomedicine. 5.2 APPLICATIONS OF GREEN NANOTECHNOLOGY IN PHYTOMEDICINE Nanotechnology is more of a tool than a discipline. Nanometric sizes aid in interaction of nanostructures with cells or tissues, thus enhancing their applicability and selective effectiveness in biomedicine. Many research studies have shown that nanostructures are treated as microorganisms by body’s immune system. Hence, they can be easily absorbed by cells and accessible through blood-brain barrier.95 These nanomaterials could integrate well in biomedicine also because most biological systems are nanometric. Though use of nanomaterials in medicine has its own advantages, yet its production and chemical fabrication schemes are expensive, in addition to harsh effects on environment during synthesis. Thus, to prevent complications of health risks to humans as well as ecosystem, and taking advantage of nanotechnology’s benefits for medicine, scientists are incorporating green methods and processes to design safer materials with increased performance along with reducing environmental impact and production costs. These engineered green nanomaterials can result in improved drug loading, targeted delivery, detecting malady, therapeutic competency, diagnostic potential, and multifunctionality to be a pharmacy tool. Therefore, in designing greener combinations of nanotechnology and biomaterials, main focus is laid on following four aspects7: • • • •

Use of safe solvents, reagents, and source materials. Reducing waste production by improving atom alignment efficiency. Eliminating requirement for purification of end products. Neutralizing health risks by use of biomaterials.

Green strategies aid in engineering bio-multifunctional nanomaterials for various applications in science, technology, and engineering. Green nanoparticles could be delivered safely to specific position orchestrating multifunctionally in comparison to synthetic drug elements.

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5.3 GREEN NANOMATERIALS FOR MULTIFUNCTIONAL PHYTOMEDICINE APPLICATIONS Green routes are new perspective for the fabrication of novel biomedicine instruments derived from plants. Green nanomaterials are based on the theories of green chemistry and engineering tools. These nanobiomaterials are new, eco-friendly, and safer resolutions for the diagnosis, imaging, and treatment of major health issues. It would follow the modification of available components to improve their applicability, performance, and targeting, in addition to reduced production costs, improved biocompatibility, enhanced biological activities, and obtaining desired results. Greener technologies for engineering nanopatterned structures consist of two methods: • •

Biofunctionalization of nanostructures using plants. Enfolding plant extracts using nanostructures.

This chapter encourages multidisciplinary research on converting natural materials into nanomaterials with potential applications in biomedicine by laying emphasis on above-mentioned issues. 5.3.1 BIOFUNCTIONALIZATION OF NANOSTRUCTURES Conjugation of nanotechnology with biological entities involves application of living systems from prokaryotic and eukaryotic origins, such as, fungi, viruses, yeast, algae, cyanobacteria, and plants. It promotes use of multicell and single-cell biological entities, such as: bacteria,38,51,68,78 actinomycetes,4,5,41,80 fungi,3,14,65,66 plants,23,48,75 viruses,,42,50 and yeasts.20,30,31,46 These different biological entities have different metabolic activities and enzymatic processes, which result in bio-reduction of salts along with stabilization of synthesized nanoparticles. They can be adopted for synthesis and modification of bionanomaterials for medicinal application. Green nanostructures synthesis technology using plants is cost-effective, biologically safe, and eco-friendly. Biosynthesized nanoparticles have immeasurable biomedical applications, such as, ailment diagnosis, targeted drug delivery, tissue engineering, and therapeutics. All these synthesis methods are supported by nanotechnology, which combines all disciplines of life sciences in addition to physics, chemistry, biology, engineering, and material science for the formulation of nanostructures.84 Formed nanostructures can be tuned with the size, shape, chemistry, atomic distribution, morphology,

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and texture.58,92,93 Such tuning of nanomaterials during synthesis and extraction can be utilized in medicine as nanoscale drug carrier, specific targeting, and passive delivery of genes/proteins/drugs, nanometric diagnostic tools, and therapeutic elements. Biological synthesis acts as a reliable, eco-friendly, and clean technique with broad spectrum for shape, size, composition, assembly, and physiological properties for the organization of atoms to form nanostructures. A nanoscale drug functions as one assembly with respect to its fabrication, characteristics, and transfer. Plants being nonpathogenic and rich in phytochemicals contribute to a broad spectrum of synthesized nanoparticles with unique electrical, physical, optical, magnetic, thermal, and chemical properties.21,39,61,69 These properties are highly beneficial with numerous applications of bionanomaterials in phytomedicine. 5.3.1.1 PHYTOFABRICATION OF NANOPARTICLES Greener methods of synthesis have proven to be better and effective due to their slow kinetics and good control over crystal growth reduction, manipulation, and stabilization.2 Plants are abundantly available natural resources, and are biocompatible, nontoxic, eco-friendly, and cost-effective. They serve as better platform for synthesizing nanoparticles due to possession of variety of secondary metabolites, nontoxic reducing agents, natural capping agents, and availability of stabilizing agents. Furthermore, plant extracts do not require expensive and extensive processing, and the cost of culture media and isolation of microorganisms is also eliminated. Nanoparticles phytofabrication can be obtained through two different methods: • •

Nanofabrication using extracts of leaves, fruits, stem, and roots. Growing nanoparticles in plants.

5.3.1.1.1 Nanofabrication Using Plant Extracts Naturally occurring reagents like vitamins, secondary metabolites, leaves, seeds and stem extracts, sugars, polysaccharides, phenolic groups, terpenoids, and alkaloids in plants act as reducing agents and capping molecules for green synthesis of nanoparticles. Plant leaf extract plays the role of solution to bioreducing the ions into nanoparticles. Plant extract synthesis is a single-step, rapid, and cost-effective approach with capability of large-scale

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production62 of nanoparticles (Figure 5.1). Bio-fabrication of numerous nanoparticles (such as gold, silver, copper, iron oxide, titanium oxide, zinc oxide, platinum, etc.)23 is possible through plant systems.

FIGURE 5.1

Synthesis of nanoparticles using plant extracts.

5.3.1.1.2 Growing Nanoparticles in Plants Some research studies have documented the growing of nanoparticles in plants. Many plants could actively uptake and bioreduce metal ions from soils and solutions during detoxification, thus forming insoluble complexes with metal ions in the form of nanoparticles. This is based on the concept of exposing plants to aqueous solution of metal salts; and due to photosynthesis, it is possible to sequestrate metal ions from solution and deposit them as intracellular nanoparticles in plants.27 A report on synthesizing nanoparticles in live plants was demonstrated on gold nanoparticles with approximate size of 2–20 nm inside alfalfa seeds.26 Also, alfalfa was able to form silver nanoparticles, when it was immersed in silver-rich medium.45 Dead biomass could also be good synthesizing agents for nanoparticles. Camphor tree, when exposed to silver or gold precursors at ambient conditions, was able to fabricate triangular silver or spherical gold nanoparticles with size between 55 and 80 nm.13 This method for synthesizing nanoparticles in plants is still unexplored and need further research to create some breakthroughs in synthesis of green nanoparticles.

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5.3.1.2 GREEN METHODS FOR SYNTHESIS OF NANOPARTICLES Green synthesis conditions demonstrate a research expecting to focus on sustainable techniques for material production and responding to the chemical synthesis methods by implying rules for safety, guarding health and risks of human beings and environment.36 5.3.1.2.1 Mechanochemical Methods It is a process, where high-energy grinders perform abrasive crushing of metal ions along with plant extracts and other materials to yield nanoparticles as final product. Mechanochemistry11,12,36 of metal ions provides following benefits: • Shorter reaction time for completing the reaction. • Reactions are performed at room temperature without use of solvents thus reducing cost of production. • Metal salts are the primary sources for initiating the process to produce water or acetic acid as by-product, which could easily be evaporated through heating. • Cost-effective and eco-friendly. 5.3.1.2.2 Sonochemistry (Ultrasound-based Methods) Ultrasound radiations are one of the effective green methods for organic synthesis.35,46 Ultrasound has potential to enhance rate of reaction and maintaining execution of reaction. The science behind initiating chemical reaction depends on ultrasound irradiation (20 KHz–10 MHz). The process of cavitation occurs due to growth, generation, and implosion of bubbles in liquid.40 It would establish hot spots with high-pressure conditions to reduce metal salt and to aid in the formation of the nanoparticles.57,84,89 Chemical and physical changes occur due to prevailing harsh surroundings helping the metal ions to devise nano-size structures through crystallization of nuclei. Research studies indicate efficiency of ultrasounds in developing nanoregions as well as nanostructures or greener nanotechnologies.

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5.3.1.2.3 Microwave-Assisted Synthesis Microwaves are being used for fabricating nanostructured materials. Interaction of the electromagnetic radiations with dipole movement of polar solvents leads to activation of solid material as the final product. The solvent molecules collide with each other through irradiation by microwaves, thus increasing temperature of reagents. It would allow better control and designing of nanomaterials through nucleation separation and nanomaterial growth phase. Microwave-assisted synthesis is a thermal treatment procedure leading to high rate of volumetric heating of reaction matter.22,87 Gradual decrease is observed, when the reagents approach to final nanostructure formation due to less absorption of microwaves by reagents.72 5.3.2 POTENTIAL OF PLANT-BASED PHYTOCHEMICALS USING ENCAPSULATING NANOSTRUCTURES Phytomedicine is concerned with the role of isolation, characterization, and discovery of phytochemicals from plants to serve as therapeutic drugs to cope up with diseases. The active components from plants (such as, secondary metabolites, terpenoids, alkaloids, steroids, flavonoids and nonflavonoids, and phenolic groups) have robust therapeutic value but with some cons of poor stability, bioavailability, solubility, and performance.99 Nano-plant medicine is the savior to aid plant medicine in reaching their full potential as pharmaceutical tools. Engineered nanostructures (such as micelles, liposomes, solid lipid nanoparticles, organic–inorganic nanoparticles, microemulsions, dendrimers, and polymer nanoparticles) could help in the formulation, encapsulation, and passive release of the active ingredients of phytochemicals derived from natural resources for targeted drug delivery. 5.3.2.1 ROLE OF PLANT-BASED PHYTOCHEMICALS Plants possess immense capability of curing diseases.76 The plants possess certain biochemicals, which are in response to their environmental stress management but not crucial for plant’s routine function. The identification of these derived chemical molecules is of medicinal value to cope with many ailments. During any stress situations, plants protect themselves by initiating some hormonal imbalances and producing few agents, such as, antioxidants,

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terpenes, polyphenolics, alkaloids, terpenoids, flavonoids, and nonflavonoids.54 In return, chemical reactions in plant—cells manifest biologicalstructural activities as well as dose-effect correlation with persons.82 It is also considered that plants bioactive compounds prove to be good reducing as well as stabilizing agents. 5.3.2.2 PHARMACEUTICAL FUNCTIONS OF PHYTOCHEMICALS The bioactivities of plants are natural sources of molecules that can be employed as therapy agents for numerous health problems. Innate mixture of chemicals has biological activities from plants leaves, stems, fruits, roots in form of essential oils, powder, tea, or salves. Plants show properties for antifungal, antioxidants, antibiotic, antiparasitic, anticancer, hypoglycemic, antidepressants, and antihypertensive.16,18,19,29,59,85,88 With advance scientific technologies, innovation is made by engineering phytoplasma therapies based on supplemented pharmaceutical products in flexible and well-developed dosage forms. Plant extracts have their bioactivities related to the compounds, such as, fibers, vitamins, minerals, phytosterols, carotenoids, sulfur-containing complexes, terpenoids, phenolic groups, and other organic acids like anions.73 Plant medicine dosages have poor stability and bioavailability along with bitter taste. They need to be properly addressed along with formulations to maintain their effectiveness for longer period before applications. Encapsulation strategy could help to design plant- based medicines to cope up with such issues. 5.3.2.3 PLANT-DERIVED NANOMATERIALS FOR MEDICINAL PURPOSE Nanotechnology-based colloidal carriers can form a coating surrounding the natural drugs, active components, and other proteins or peptides to be used in biomedicine with enhanced performance, bioavailability, controlled release of drugs, and targeted delivery. This novel drug delivery system of utilizing nanostructures to improve the performance of traditional drugs and therapies has brought new way of engineering pharmaceuticals. It has been studied that phytochemicals can be easily decomposed by acidic nature of the stomach and enzymatic action of liver fluids. Due to this

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degradation, optimum number of extracts may not get ply into bloodstream. It would reduce the potency of drug to fully elucidate the therapeutic effect. Welding nanosystems for the delivery of herbal extracts or plant essences increase the optimum amount of drug reaching effective site of action. It would be tuned to dodge all the blockades of pH of stomach, liver enzymes, systematic drug distribution, and circulation through bloodstream.74,98 5.3.2.4 COMMON ENCAPSULATION PROCESSES The encapsulation will protect the active substance in the core by a superficial layer or coating with an intention for target delivery.101 When considering plant medicine, the plant extracts can be encapsulated within the coating of biodegradable particles tuned with nanoproperties. Natural sources (such as chitosan, gelatin, albumin) and biopolymers are major elements used to design outer covering of the core element with engineered property of surface activation.10 Such encapsulation of the drug or active therapy enhances mechanism and effectiveness of drug by providing additional stability, bioavailability, and proper circulation in the bloodstream. Physiochemical attributes (such as zeta potential, hydrophobicity, drug release in form of triggered, delayed, prolonged, biological behavior of uptake mechanism, bioadhesion, absorption, or adsorption) and all modifications of a novel drug system could be engineered by using polymeric materials for encapsulations.24,64,71 Poly(lactic acid), poly(glycolic acid), and poly(lacticco-glycolic acid (PLGA)) are some of the main biodegradable encapsulation elements. These polymers play effective role in drug release kinetics, which is based on the mechanics of degradation of polymer. Encapsulation of active ingredients could be done in two ways: • •

Chemistry-related process by using polymerization of monomers. Physiochemical-based process using dispersion of covering polymers.

These systems are based upon active interactions with physicochemical sorption of molecules aided by evaporation of solvent, nanoprecipitation, diffusion and dialysis of solvent, radical polymerization, micro and miniemulsion, interfacial polymerization. These techniques are used to engineer microparticles, nanoparticles, liposomes, and niosomes for numerous applications.49 This drug encapsulation needs careful control on composition along with drug loading. Such drug encapsulation depends on the affinity nature of drug hydrophobic or hydrophilic, hydrogen bonding, ionic interaction, and other secondary bondings.77

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Polymeric encapsulation biomaterials with different physiochemical characteristics are employed to impart nontoxic, biodegradable, noninflammatory, biocompatibility of the drug delivery, and stability. The encapsulation process for plant extracts is shown in Figure 5.2. Encapsulation techniques are divided into nanoprecipitation and emulsification. Advantages of encapsulation of drugs are • Formulations with different characteristics could be enclosed together. • Protection of active drug elements from outer environment and biological enzymes in digestive tract. • Control over drug release. • Specific targeting. • Elimination of organic solvents.

FIGURE 5.2

Nanoencapsulation of plant extracts using nanostructures.

5.3.2.4.1 Nanoprecipitation Nanoprecipitation is also known as solvent displacement, interfacial deposition, solvent injection, spontaneous emulsification, solvent diffusion, and mixing-induced levels. Nanoprecipitation is considered by dissolving the active ingredients into film-forming solvents to form a coat on active drug component. Nanoprecipitation happens through three steps when two miscible solvents9 begin to nucleate, grow and aggregate.75 In this technique, active molecules are incorporated into colloidal drug delivery systems to help developing pharmaceutical products.25,52,60

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5.3.2.4.2 Emulsification • Solvent removal induced encapsulation technique: In this, encapsulation occurs following elimination of organic solvents. Conditioned on method of solvent removal, it has two different techniques:23 (1) Emulsion-solvent evaporation where emulsification is followed by particle hardening due to evaporation of solvent and precipitation of polymer; and (2) Emulsion diffusion where emulsification is backed by dilution and then deposition of polymer surrounding droplets of active ingredients of phytochemicals. • Layer-by-layer encapsulation: Water dispersed oil microdroplets can coat the oppositely charged polymers45,81,94,96 to create smart and multifunctional micro and nanocapsules.56,79 • Emulsion polymerization: The monomer or polymer of nonaqueous medium is attracted by the ions or radicles of aqueous medium, where polymer acts as the template for forming functional shell of nanoparticles.7 • Emulgels: Emulgels also called emulsion hydrogels are specially designed hydrogels for specific shape, geometry, size, and hardness of hydrogel component.34 These constitute polymer assemblies of microdroplets of dispersed oil within the water-abundant hydrogel. 5.3.2.5 NANOSTRUCTURES TO ENTRAP PLANT EXTRACTS (Figure 5.2) 5.3.2.5.1 Micelles, Liposomes, and Phytosomes Micelles are amphiphiles aggregates of spherical structures of nanoscale dimension with core as hydrophobic for polar media, and hydrophilic for nonpolar media.33 By accompanying natural active molecules inside the core, micelles are dynamic carriers to increase shelf-life and stability of the drug. Liposomes are spherical vesicles made up by disturbing the lipid films in aqueous medium. They are bilayer lipid membrane micelles90 with ability to encapsulate polar and nonpolar drugs, to enhance stability and shelf-life of active molecule, size, charge, and other functional properties along with biodegradability and compatibility. Phytosomes are mainly liposomes that are covalently attached to phytochemicals obtained from plants. These drug delivery agents are mostly used for enhanced delivery of active phytoagents to their targeted location for curing the disease.37

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5.3.2.5.2 Solid Lipid Nanoparticles Solid lipid nanoparticles are spherical structures composed of lipid core that surround surfactants to create a nanocarrier for hydrophilic drugs and multifunctional processes. These lipophilic structures could easily encapsulate insoluble drugs within its polymeric shell-coat protecting the drug from its outer environment. These nanostructures are not only biocompatible, reduce toxicity to body but also increase targeting with help of functional markers along with ability to provide slow and steady release of drug profiles. 5.3.2.5.3 Polymer Nanoparticles Polymer nanoparticles are aggregated natural and biodegradable protein structures to increase the stability and circulation within blood. Poly lactidec-glycolic acid and poly lactic acid85 are common forms of polymeric nanoparticles with other candidates, such as, sugar,44 proteins,101 albumin,1 gelatin nanoparticles;43 and other naturally occurring macromolecules, such as, agar-agar, chitosan. Dendrimers are branched molecules that repeat themselves in tree-like manner in symmetry around its core characterized by their structure, water solubility, nondispersity. Dendrimers are excellent encapsulating candidates by hiding hydrophobic drugs into their nonpolar core. 5.3.2.5.4 Microemulsions (ME) The combination of oil water and cosurfactant spontaneously forms the microemulsion. These drug carriers are transparent, thermally stable, and with size of 1–100 nanometer can improve penetration capacity up to cellular levels. These could be optimized suiting the drug. Presence of nonpolar and polar components adds good solubility to ME in their applications.55 5.3.2.5.5 Inorganic Nanoparticles Inorganic nanoparticles are mainly categorized into metallic nanoparticles, ceramic nanoparticles, and carbon nanoparticles. Metals, ceramic, and carbon at nanoscale impart unique property to them, making them powerful catalysts triggering desired reactions and specific targeting, imaging as well as diagnosis.15,47,67,102

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5.4 APPLICATIONS OF PHYTONANOPARTICLES AND ENCAPSULATED PLANT EXTRACTS IN BIOMEDICINE Owing to higher bioactivity of phytonanoparticles in comparison to physiochemical nanoparticles, they are suggested for several biomedical applications. Multimodal biomedical applications of phytonanoparticles and encapsulated plant extracts are summarized in Figure 5.3.

Therapeutic

• Anticancer • Antimicrobial • Neurogenerative • Anticoagulation • Antiinflamatory • Antiangiogenic

• Targeting • Passive release • Enhanced shelf life

Drug delivery

Diagnostic

FIGURE 5.3

• Bioimaging • Biosensing

Applications of phytonanoparticles in biomedicine.

Photosynthesized nanoparticles (such as, silver, gold, copper, zinc, and many more) are mostly used for their antimicrobial, antifungal and antibacterial activities.53 These phytonanoparticles are also used as pharmaceutical and healing agents in wound healing, wound therapy as well as tissue engineering. Zinc, titanium-based nanoparticles are used as UV-blocking candidates. Moreover, phytonanoparticles are used in development of sensors and functionalized targeting molecules for specific deliveries. Different metal and metal-oxides are also being explored as the gene delivery, cell-delivery, imaging, sensing, photoimaging, photothermal therapies, and magnetically induced drug delivery agents.17,93 Enfolding nanostructures are also helpful in increasing the shelf-life, bioavailability,

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controlled release, performance, and targeting of plant extracts to desired position for extraordinary results. 5.5

CONCLUSION AND FUTURE PROSPECTS

Enrichment of science and technology has improved sophistication of modern healthcare but plant medicine has its benefits. Plant medicine has been playing role of vital drugs due to their efficiency, safe consumption, environment-friendly, and diversity in their applications. Numerous resources of information, knowledge, cultivation, production, methods, and quality control techniques of plant medicine must be incorporated into advanced analytical processes of mainstream healthcare. This amalgamation would bring out extraordinary medicinal tools for transforming biomedicine. Besides benefiting humans, it would also contribute to the safety and good health of our planet. The nanoparticle synthesis by green process using biological systems has applications in imaging, detecting, and treating disease. This chapter encourages future research in green routes as well as use of medicinal plants in biomedicine. KEYWORDS • • • • • •

bioinspired nanomaterials biomedicine green nanotechnology green synthesis nanostructures plant extracts

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CHAPTER 6

GREEN NANOMATERIALS AS A BOON TO BIOMEDICAL ENGINEERING NIDHI JAIN

ABSTRACT To protect our precious life in the ocean of diseases, new materials and applications are utmost important. In this chapter, various green synthesis methods of nanomaterials have been discussed. The nanomaterials have been synthesized taking the advantages of green Chemistry principles, and limitations of nanomaterials are reviewed with profound intensity and depth. The foremost significance of the green synthesis is nonexistence of harmful and deadly toxic chemicals during the synthesis, which helps in protecting human body and at-large environment. Various applications in the health and biomedical sciences have been discussed in this chapter. The chapter discusses about the toxicity generated using nanomaterials and highlights greener pathways as the potential solution. It also covers the applications of biosynthesized nanomaterials in drug delivery, cancer treatment, and other biomedical frontiers. 6.1 INTRODUCTION The demand for the newer biomaterials in the medical sector is increasing due to onset of new diseases in the recent years. Nanomaterials have attracted huge interest in various multidisciplinary fields because of their unique properties. It is aiding in creating the hygienic, safe, and eco-friendly world. The green nano-synthesis helps in developing and reducing toxic chemicals in the environment. It helps in promoting less use of venomous chemicals Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science. Megh R. Goyal and Shrikaant Kulkarni (Eds.) © 2023 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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and energy during the synthesis process. This chapter mainly spotlights the large-scale application of nanomaterials, which are employed to fabricate greener and safe environment. The opening out of eco-friendly technologies in material amalgamation has boosted biomedical application. These eco-friendly technologies are more accepted by the human body because of the less side effects. Green nanoparticles (NPs) have been synthesized, and nowadays are available in various chemical compositions, structure, and sizes. These green nanomaterials have vivid benefits of green synthesis along with some limitations.33,53,66 Nowadays, especially NPs (e.g., silver (Ag) and gold (Au), etc.) are being used in various fields of science from development of material science to biotechnology.34,131 The use of toxic chemicals in the production process of nanomaterials restricts their usage in the clinical field. To increase the application of the nanomaterials, scientists are looking for the synthesis of nontoxic, biodegradable, and eco-friendly NPs, which could be used in the extracellular and intracellular activities. Still there are major gaps in the technology transfer of nanomaterials from the laboratory scale to the industrial scale. The voids of the technology transfer can be fulfilled by developing better understanding of the hazards produced, right from the synthesis to the application of nanotechnology. The thirst for the novel materials has been developed and generated based on the green nanoscience concept. Green nanoscience is in the purview of the philosophy of Green Chemistry. These voids have been supported by the nanotech society, which tries to use three 3R (reduce, recycle, reuse) concept effectively.10,31,101,103,128 This chapter focuses on green nanomaterials, preparation, advantages, and limitations of available methods. It also provides a comprehensive overview of present status and has also suggested future guidelines for using green nanomaterials in biomedical, electronics, catalysis, sensing and storage devices, environmental, etc. 6.2

MULTIFUNCTIONAL NPs

The multifunctional NPs can be classified as Liposomes, Super-paramagnetic NPs, Fullerenes, Carbon nanotubes (CNTs), Dendrimers, Quantum dots, etc. Table 6.1 shows list of NPs with their composition and applications. • Liposomes are spherical-shaped vesicles having phospholipid bilayers. Liposomes have a wide range of applications in pharmaceutical, cosmetics, agriculture sectors, etc. Liposome is useful in the

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assets of cell membrane and its related functions. It is used in pharmaceutical industries for drug delivery, cancer treatment, antifungal, and vaccines. Liposomes are very suitable for biomedical research as these are biocompatible and biodegradable. They are mostly used as vehicles in the diagnostics. The only issue with this is the physical stability of the liposome.4,52,134 • Super-paramagnetic iron oxide nanoparticles (SPIONs) are small synthetic inorganic-based NPs to encompass an iron oxide in the core. They are covered with either inorganic or organic materials, such as, silica, gold, polysaccharides, peptides, surfactants, and polymers, respectively. They are used for the determination of tumor through magnetic resonance imaging and they are various applications in cardiovascular diseases, etc. SPION is very useful tool for magnetic separation techniques and opens new paths for discriminating treatment of local tissues, which are helpful in avoiding side effects.6,18,28,35,41 • Fullerenes is one of the allotropes of carbon, and the molecule is connected to single and double bonds to form a unique cage structure made up of carbon. They are also present in vacant spheres, ellipsoids, or various shapes of tubes. Sphere-shaped ball-like structure of fullerenes is called as buckyballs. Fullerene is soluble in organic solvents. They have a wide application in the medical, electronics, energy and storage, environment, and water treatment fields. The unique structure of the fullerenes is responsible for their extraordinary chemical properties. In fullerenes, each carbon atom is linked by three carbon atoms on the apex of a polyhedron, with presence of two single bonds and one double bond. The distinctive physical and chemical properties of fullerenes have numerous applications in areas, such as, antioxidants/biopharmaceuticals, antibacterial/antimicrobial/ antiviral activities, diagnostics, drug delivery photovoltaics, water purification/environment, hydrogen storage, gas adsorption/storage, supercapacitors, etc.91,92,115,150 • CNTs are made up Graphene sheets in rolled manner. These are present in the form of singly rolled, single-walled carbon nanotubes, mutlirolled, and multiwalled carbon nanotubes (MWCNTs). MWCNTs are made-up of two or more concentric graphene sheets arranged around a center hollow area with gaps between the layers. Whereas singlewalled carbon nanotubes are composed of a single cylinder graphite sheet joined together by Vander-Waal forces. These have a huge application in biomedicine science, such as, in drug delivery, cancer

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treatment, tissue-engineered scaffolds, biosensors; and in waste water treatment, water purification, air pollutant removal, and electronic devices. • Dendrimers: The size, shape, and length of the branches, etc. of the dendrites are controlled in the synthesis phase. They are generally circular, highly branched structures. They are in the form of layers and all the layers are covalently bonded. The density of the dendrimers increases from the core to the periphery. The synthesis of the dendrites was first done in 1980 and but was finally developed in 2000’s. These are currently used in (1) the drug delivery system (DDS) because of their high stability with biomolecules and receptors; and (2) nanomedicine, fluorescent dyes, enzymes cell-identification tags, trapping, or encapsulating drug molecules.158 • Quantum Dots (QDs) are one-dimensional semiconductor NPs. Based on size, these emit lights. When the size of the quantum dots decreases, these show blue-end of the spectrum; and when it increases, then QDs show red end. Quantum dots range in size from 2 to 10 nm, and 10 to 50 atoms. They have wide applications in Optical Storage, LEDs, organic dyes, quantum computing, and solar power. These are important tools for the cellular imaging and labeling.104,142,143 • Metal NPs have important applications due to their huge surface area to volume ratio. Some of the most widely used metal NPs are nanogold, nanosilver, and titanium dioxide. These NPs are derived from metals and are responsive to surface modification to accomplish target functionality. The outcome of the metal NPs could be developed in the form of rod, dots, or cubes. They have mammoth applications in the biochemistry field because of optical properties like fluorescence; size and magnetic properties act as upright drug delivery agents. At the low concentration, these are easily traceable. These serve as good catalysts and promising biological and chemical sensors, storage devices, etc.12,112,113 6.3 SYNTHESIS OF NPs There are two basic advances for the formulation of NPs: first one is top-down approach and another one is bottom-up approach. The top-down approaches involve the construction of nanoscale substance via bulk counterparts that break down the bulk materials systematically bit by bit into fine NPs. The techniques involve leaching out process with externally controlled tools. The

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Name of Nanoparticles Composition Applications Reference 150 Graphene sheets rolled in both the forms Biomedical purpose such as drug delivery, cancer treatCarbon nanotube of Singly rolled, Single-walled carbon ment, tissue-engineered scaffolds, biosensors, wastewater

treatment, water purification, air pollutant removal, and

nanotubes (SWCNTs), mutli rolled

multiwalled carbon nanotube (MWCNTs) electronic devices.

They are used prevalently used for the drug delivery system 158 They are generally circular, highly

Dendrimer as they are highly stable with biomolecules and the receptors.

branched structure

These are also used in nanomedicine, fluorescent dyes,

enzymes cell-identification tags, trapping or encapsulating

drug molecules, etc.

91,92,115, Fullerenes have numerous medical application such as

Fullerene is one of the allotropes of

Fullerenes carbon. Spherical ball-like structure of antioxidant/ biopharmaceutical, antibacterial/ antimicrobial/ 150 antiviral activities, diagnostics, drug delivery, photovoltaic,

fullerenes is referred to as buckyballs.

water purification/ environment, hydrogen storage, gas

adsorption/ storage, supercapacitors, etc.

4,52,134 Liposomes are spherical shaped vesicles It is used in pharmaceutical industries for drug delivery,

Liposomes cancer treatment, antifungal, and vaccines.

having phospholipid bilayers.

They are derived from metals.

They have mammoth applications in the biochemistry field 12,112,113 Metal Nanoparticles because of optical properties like fluorescence, size; and

magnetic properties act as upright drug delivery agents.

Quantum dots are 1-D semiconductor

They have wide applications in the fields of optical storage, 104,142, Quantum Dots nanoparticles

LEDs applications, various types of organic dyes, quantum 143 computing and solar power. They are important tools for the

cellular imaging and labeling.

Super-paramagnetic iron oxide nanopar- They are used in various applications in cardiovascular

6,18,28, Super-paramagnetic 35, 41. nanoparticles ticles called as SPIONs. They are small disease.

synthetic inorganic-based nanoparticles

encompass an iron oxide in the core.

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TABLE 6.1

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laser ablation methods are electron beam lithography, mechanically milling procedure, anodization, ion, and plasma etching. The bottom-up approaches involve the merger or accumulating of atoms and molecules to create wide range of NPs. Some of the common methods for bottom-up approaches are sol–gel processing, laser pyrolysis, chemical vapor, etc. Generally, synthesis of NPs can be done by three methods, such as (1) physical methods; (2) chemical methods; and (3) green synthesis or bioinspired methods. This chapter is focused only on green synthesis or bioinspired method.132 Figure 6.1 shows two approaches of production of NPs, i.e., top-down approach and bottom-up approach. It also represents physical, chemical, and biological methods of formulation of NPs. Top down approach Bulk

PHYSICAL

METHOD

• • • • •

Mechanical milling Laser ablation Electron beam Evaporation Sputtering electro spraying

CHEMICAL

METHOD

• • • • •

Sol Gel synthesis Hydrothermal synthesis Co procipitation Micro emulsion Chemical Vapour deposition

Fine particles METHODS OF SYNTHESIS OF NANOPARTICLES

Atoms/ molecules Nanoparticles

BIOLOGICAL

METHOD

Nuclei

• • • • • • •

Synthesis by Bacteria Synthesis by yeast Synthesis by Fungi Synthesis by Virsus Synthesis by Algae Synthesis by Biomolecules

bottom-up approach FIGURE 6.1

Diverse methods of formulation of metallic nanoparticles.

6.3.1 GREEN OR BIOINSPIRED SYNTHESIS Green chemistry is related to sustainable development that covers all aspects of present and future generations in a balancing manner. It is concerned with the pollution control, excess use of natural resources, and its implications on

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the individual needs. Green chemistry acts as a powerful tool to minimize excess use of toxic chemicals thus providing an alternative eco-friendly tool to protect the environment and ecosystems. Therefore, designing inoffensive supportable chemicals and procedures are need of the hour to reduce danger of accidents and damages.131 Chemical process of production of NPs produces toxic chemicals, which are harmful to humans. The bioassisted method or bioinspired method or green synthesis method provides less toxic, eco-friendly, easily available raw materials, and cost-effective alternative to produce the NPs. The Green synthesis or bioinspired method is animal-based methods of synthesis, plantbased method of synthesis, biomolecules, etc. Schematic illustration of green formulation of metallic NPs is given in Figure 6.2.

FIGURE 6.2 Green formulation of metallic NPs. The image represents the plant parts from which phytochemicals are extracted that are further combined with metal salts to produce NPs. It also represents the overview of microorganism used from getting enzymes, which can be further used for production of NPs.

Numerous studies have reported the fabrication of NPs using plants and microorganisms. The brief methodology of green synthesis involves the use of microorganisms or plant extracts merely reacted with a metallic salt to produce metal NPs. Such NPs are easily available for use after appropriate characterization. Microorganism fabrication of NPs exploits bacteria, fungi, viruses, and yeast and actinomycetes for the creation of NPs. These microorganisms sometime play the role of templates for getting distinct, structured of NPs. Plant-based synthesis of NPs is simpler and more convenient for production of NPs. Phytochemicals for the synthesis of NPs can be effortlessly scaled up. Numerous phytochemicals are present in plants, which can act as

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reducing as well as stabilizing agents. Some of the known phytochemicals are flavonoids, alkaloids, polyphenols, and organic alcoholic substances. Various organic compounds present in plants are also used for NP synthesis, such as, polysaccharides, proteins, amino acids, etc. 6.3.2 MICROORGANISMS-ENABLED SYNTHESIS OF NPs The synthesis of NPs from microorganism can be both extracellular and intracellular. The various types of NPs, such as, gold, copper, nickel, and silver could be synthesized using microorganisms. During extracellular synthesis, the metallic salt is mixed in culture filtrate of microorganism to get desired NPs, whereas in intracellular synthesis, microorganisms are incubated in metal ion solution under optimum growth conditions. Completion of NPs synthesis can be identified by color change. Finally, NPs could be obtained by ultrasonication, centrifugation, and washing.3,48,63,73,148 6.3.3 BACTERIA-ENABLED SYNTHESIS OF NPs Bacteria can survive under adverse conditions, such as, highly saline water in deep-sea or in hot and icy freezing temperatures, heavily contaminated water, etc. The examples of bacteria, which can survive under adverse conditions are Pseudomonas stutzeri survive in extreme concentration of metal ions; Sulfolobus acidocaldarius and Thiobacillus thiooxidans can live on elemental sulfur to convert ferric to the ferrous ion. Bacteria are entitled as biofactories for production of metallic NPs.56 Scientists are able to synthesize gold and silver NPs from microorganisms (Table 6.2), which have way of growth, survival, reproduction, and multiplication. They are generating energy for themselves by reducing the metallic substances. Silver NPs can be successfully synthesized inside or outside the cells, which are effective against pathological organisms. Some of the commonly used bacteria are138 as follows: • • • • • •

Achromobacter sp., Aeromonas hydrophila, Bacillus licheniformis, Bacillus siamensis, Bacillus thuringiensis, Pseudomonas poae,

• • • • • •

Pseudomonas rhodesia, Pseudomonas sp., Sargassum wightii, Streptomyces capillispiralis, Streptomyces griseus, Streptomyces spp.

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The most extensively putative process for the fabrication of silver NPs from P. stutzeri AG259 is the formation of nitrate reductase enzyme, which converts nitrate into nitrite by providing electrons to oxidize NADH to NAD+. The electron contribution is the major path for biological reduction of Ag ions to AgNPs.80 Generalized schematic diagram of green production of silver nanoparticles (AgNPs) from plant sources is given in Figure 6.3.

FIGURE 6.3 Generalized diagram of green fabrication of silver nanoparticles (AgNPs) from plant sources.

6.3.4 ACTINOMYCETES Filamentous bacteria group is known as Actinomycetes. They are the bacteria, which are physiognomies of fungi and prokaryotes. Actinomycetes have gathered attention because of the ability to produce antibiotics. These bacteria can survive under adverse conditions by exploiting bioactive potential. These are known to survive at thrilling habitats and produce enzyme. These bacteria work both outside and inside the cells to synthesize NPs but extracellular synthesis of NPs is more attainable method for commercial gain. A total of 70% are from of actinomycetes out of 22,000 discovered microbial species. The examples of synthesis of gold NPs by Actinomycetes are kalothermophilic Actinomycete, Thermomonospora sp. They can successfully fabricate gold NPs outside the cells using gold salts under alkaline conditions. Similarly, Streptomyces zaomyceticus Oc-5

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TABLE 6.2

Microorganisms used for the Synthesis of Metal Nanoparticles.

Microorganisms

Product Size (nm)

Shape

Applications

Reference

Aeromonas hydrophila

ZnO

57–72

Crystalline

—

82

Bacillus licheniformis

Ag

Bacillus siamensis

Ag

77–92

Polymorphic

Antiviral agent

155

25–50

Spherical

Antifungal agent

123

Bacillus thuringiensis

Bacteria

10–20

Polymorphic

Antiviral agent

77

20–50

Spherical

Antifungal agent

58

Pseudo monaspoae

Ag

20–45

Spherical

Antifungal agent

79

Pseudo monasrho Desiae

Ag

20–50

Spherical

Antifungal agent

145

Sargassum wightii

Au

8–12

Planar

Antibacterial agent

71

Streptomyces capillispiralis

Cu

4–59

Spherical

Antifungal agent

89

Streptomyces spp.

CuO2

78–80

Spherical

Antifungal agent

46

Streptomyces griseus

Cu

5–50

Spherical

Antifungal agent

152

Fungus Alternata

Ag

20–60

Spherical

Antifungal agent

162

Aspergillus favus

Ag

1–8

Spherical

Isotropic

60

5–25

Spherical

Used in electrical batteries for coating; for 42 solar energy absorption

Aspergillus fumigates Aspergillus niger

Ag

20

Spherical

Antibacterial agent

121

Aspergillus terreus

ZnO

8

Spherical

Used as catalysis, biosensors, drug delivery, molecular diagnostic.

78,141

Cariolus versicolor

Ag

25–75

Spherical

as metallic catalysts, and also in labeling of living cells and tissues

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Ag

Pseudomonas sp., and Achromobacter sp. Ag

(Continued)

Microorganisms

Product Size (nm)

Shape

Applications

Reference

Cladosporium cladosporioides

Ag

10–100

Spherical

Fusarium oxysporum

Au, Ag

8–14

Spherical

Biomedical

72

Fusarium semitectum

Ag

10–60

Spherical

Biolabeling

96

Fusarium solani

Ag

5–35

Spherical

Used in biolabeling, and sensors and also 129 in drug delivery

Penicillium brevicompactum

Ag

23–105

Crystalline spherical

Antimicrobial agent

108

Penicillium fellutanum

Ag

5–25

spherical

as thin film and as surface coating

21

Phanerochaete chrysosporium

Ag

50–200

Pyramidal

for medical textiles, and antimicrobial activity

36

97

Phoma glomerata

Ag

60–80nm

Spherical

Antimicrobial agent

161

Rhizopus nigricans Silver

Ag

35–40

Round

Used as Bactericidal, catalytic

165

Rhizopus stolonifer

Au,Ag

25–30, 1–5 Spherical

Trichoderma viride

Ag

5–40

Spherical

Trichothecium sp.

Au

10–25

Spherical, rod-like and triangular

Verticillium

Ag

21–25

Spherical

Catalysis

81

Verticillium luteoalbum

Au

25 million people by novel SARS-CoV-2 and around approximately one million deaths have been caused around the world. Modern vaccines developed by several nations merely act as stopgap measures to halt advancement of this virus.146 Nanotechnology can overcome the contemporary challenges. For example, direct approach can be applied to utilize NPs to deliver antiviral drugs against SARS-CoV-2. Many drugs are known for noble treatment of coronaviruses (such as remdesivir, dexamethasone, lopinavir-ritonavir, and EIDD-2801, among many others) can be combined with NPs to improve their pharmacokinetic profiles to increase circulation time, increase specific targeting at tissue sites, and increase bioavailability. Nano-delivery vehicles have virus-neutralizing properties. All parts of the viruses are effectively used in nanotechnology. The viruses are made of outer capsid with genome materials inside. The outer capsid proteins act as a container for the drug delivery and act as highly reactive surface for synthesis of NPs. NPs also help in bio-imaging when incorporated with virus in-cap side. Therapeutics use of the drug with nontoxic viruses can lengthen therapeutic window of the drug in all aspects.7,50,64,86,94,102,114,116,153,159,164 6.3.8 PHYTOCHEMICALS-ENABLED SYNTHESIS OF NPs (TABLE 6.2) Out of all green synthesis methods by other sources such as algae, fungi, viruses, and other microorganisms, plants are safest and widely accepted processes. The synthesis of NPs by microorganism needs high maintenance in conservation and preserving the microbial cultures. No side effects of hazardous and toxic chemicals are there during synthesis of NPs through plants. It is also eco-friendly, cost-effective, and readily available. The usage of plant materials for NPs synthesis requires less energy, easy to make, and huge implications. It is more favorable than microbial or chemical methods as there are no consequences of microbes or harmful chemical contamination.1

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Whereas in animal-generated viruses, there is always threat of deadly viruses. Plants are also capable of producing plants viruses, which can be helpful in producing multipurpose NPs. In recent years, many in vitro studies have documented production of NPs using plants extracts. Plant extracts are being used as a reducing as well capping agents during synthesis of NPs. Different types of NPs (such as platinum, gold, silver, copper, and iron) have been successfully synthesized in vitro by using plant extracts. All fractions of the plants (such as roots, shoots, leaves, flowers, fruits, and seeds) could be used to produce NPs. These parts of the plants can produce certain types of phytochemicals like terpenoids, polysaccharides, vitamins, amino acids, proteins, saponin, aldehydes, ketones, acids, phenols, enzymes, amides, and flavonoids.62 The contemporary in vitro approaches have been developed to use plant extracts as reducing agents for production of NPs. Green synthesis of NPs such as Palladium, copper, gold, silver, platinum, iron, etc. could easily use extracts of different plant parts along with a variety of acids and metal salts (Figure 6.4). The green synthesis of NPs mediated by plant extracts can be accomplished because of the presence of organic biomolecules, such as, phenols, terpenoids, ketones, carboxylic acids, aldehydes, enzymes, amides, and flavonoids. The plant extracts act as reducing and stabilizing agents in the whole process of green synthesis of NPs.62 6.3.8.1

Cu NPs

Copper NPs (Figure 6.4) can be easily obtained by the reduction of copper ions by using plant parts, such as, Aloe vera flower, C. reflexa leaf extracts. The initial characterization can be done by ~579-nm peak with the help of UV–Visible spectrometer. The average size of Cu NPs is 40 nm. C. reflexa leaf extract contains numerous phytochemicals, which are responsible for formation of NPs; and these are98:

Green Nanomaterials as a Boon to Biomedical Engineering

• • • • • • • • • •

Amarbelin, Azaleatin, Bergenine, Beta-sitosterol, Coumarin. Cuscutalin, Cuscutin, Dulcitol, Isorhamnetol, Kaempferol-3-O-galactoside,

• • • • • • • • • •

177

Kaempferol, Linoleic acid, Linolenic acid, Luteolin, Maragenin, Myricetin glucoside, Myricetin, Oleic acid, Palmitic acid, and Stearic acid.

FIGURE 6.4 Green synthesis of copper nanoparticles (CuNPs) from plant sources. Copper ions are reduced in the presence of Cuscuta relfexa, which acts as capping and stabilizing agent, which converts Copper-II into Copper-I and provides electrons to oxidize. The electron donation is the major path for bioreduction of copper ions to copper nanoparticles.

6.3.8.2 Pd AND Pt NPs Palladium and platinum metals are silver and white in color. These metal NPs are biosynthesized by using plant extracts (Figure 6.5). The examples of the plants for synthesis of palladium and platinum are Cinnamomum camphora, Pinusresinosa, Pulicaria glutinosa, Doipyros kaki, Musa paradisiaca, etc. We use daily tea leaves (Camellia sinensis), which are nontoxic and renewable in nature. These leaves act as reducing and

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stabilizing agents to produce NPs that act as catalysts in the Suzuki coupling reaction and are effective in reduction of 4-nitrophenol. The tulsi leaf broth (Ocimum sanctum: sweet basil) at 100 °C is used to obtain platinum NPs. The tulsi leaf has ample amount of phytochemicals, such as, terpenoids, saponins, ascorbic acid, plant proteins, gallic acid, and amino acids, which help in the synthesis of NPs. Other phyto sources for producing platinum NPs are Anogeissus latifolia, Chlorella vulgaris, Azadirachta indica (neem), Diopyros kaki, Prunus×yedoensis tree gum extract, and Terminalia.9

FIGURE 6.5 Green synthesis method of Pd nanoparticles by using leaf extract of Camellia sinensis (black tea); it indicates the Suzuki coupling reaction, and reduction of 4-nitrophenol. Source: Reprinted with permission from Ref. [88]. © 2017 Elsevier.

6.3.8.3 SYNTHESIS OF SILVER NPs Synthesis of Silver Ag NPs by chemical and physical methods is costly (Figure 6.6). Plentiful chemicals and energies are consumed during the synthesis process, which leads to side lethal effects on administration. Hence, an alternate method is required to surpass these poisonous effects. Biosynthesis not only provides the safety and effectiveness of the NPs being produced but it also harnesses the availability of cheaper, nontoxic NPs. Production of silver NPs using plants is eco-friendly and inexpensive method. Silver NPs can be synthesized by using ~10.5 g leaves of Nelumbo nucifera

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boiled in 110 ml of distilled water. The 12 ml of this solution after filtration and approximately 1.5 mM aqueous solution of AgNO3 (88 ml) are kept in a closed shadowy room at normal temperature. The formation of brownlike yellow color solution represents the formation of silver nanoparticles (AgNPs). Hibiscus rosa sinensis leaf extract and solution of AgNO3 (25 ml) are heated at 300 K temperature; and after reduction within ~35 min, there appear light-brown or yellow silver NPs. Silver NPs can also be fabricated by Jatro phacurcas seed extract and aqueous solution of AgNO3 (20 ml) at 80 C for 15 min. Completion and formation of reddish color indicate the synthesis of silver NPs. Various numerous other examples are given in the literature.17,120,137 Figure 6.6 gives overview of green synthesis of Ag NPs using various parts of a plant source.

FIGURE 6.6

The green synthesis method of Ag nanoparticles by various plant sources.

Green synthesis of metal NPs has several advantages and disadvantages over the conventional methods (Figure 6.7). The advantages of the fabrication of metal NPs by green process are enormous, easy process, cost-effective,

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and eco-friendly. The synthesis could be possible with available bioresources that are widely present in nature, such as, plants, micro-organisms, bacteria, and fungi. These bioresources can decrease the dependence on harmful chemicals. The functionalization of the green nanomaterials by biomolecules makes them biocompatible with different therapeutic and diagnostic agents. Nanobio conjugates act as theragnostic agents on the surface of NPs. Disadvantages of green route synthesis involve overutilization of bioresources, only some of nanomaterials like gold and silver are promptly used. It is not easy to segregate the active biomolecules existing in the bioresources. Sometimes overutilization of bioresources also cause ecological imbalance. It also causes limitation of bioresources because of the seasonal availability.

FIGURE 6.7

Pros and cons for green synthesis of metal nanoparticles.

6.4 APPLICATIONS OF GREEN NPs IN BIOMEDICAL SCIENCES Biomedical applications of NPs are versatile. The exceptional properties of green NPs (such as enormously small size and elevated surface-to-volume

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ratio) make them strong candidates for biomedical applications. Metal nanomaterials (1–100 nm in size) are used in medicine. Size, shape, crystal morphology, and structure are responsible for catalytic, electronic, electrical, and optical properties.53,68 In this chapter, various applications of green NPs in medicine (Figure 6.8) are presented, such as • • • • • •

Biosensors,131 Cancer therapy and diagnosis,66 Catalysts,34 Drug delivery,33 Labeling,128 Wound healing,68 etc.

FIGURE 6.8

Various biomedical applications of metal nanoparticles via greener route.

Advancement of DDSs using nanomaterials becomes a novel tool for the several highly sensitive diseases, such as, cancer and cardiovascular diseases, etc. Nanodrug delivery systems (NDDSs) are being developed to provide water soluble, stable, increased life cycle drug, which reduces the risk through reducing enzyme deprivation, safety, and efficacy of the drug. These NDDSs drugs could be administered in the body by inhalation or by oral administration or by injection.55 Cancer diagnosis and treatment is an emerging field. Sensitive and correct diagnosis of the same is required to decrease the mortality rate. Conventional method of the treatment and diagnosis is lengthy and time-consuming process. The use of the nanomaterials opens new avenues in the detection

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methods, such as, use of biomarker. The applications of the targeted drug delivery, molecular changes detection with small number of cells, and multidrug delivery are possible by multifunctional NPs. Catalysis application, biosensor technology, labeling, and wound healing are some of the other major highlights of this section. The specific nature of NPs enables them to easily combine with biomolecules of the body thus increasing biocompatibility. Current challenge is to identify the nanotoxicological effects of its applications. The results of the nano-toxicological research could be incorporated with the new nanomaterials to engineer the materials in a better way. Green nanobiotechnology has initiated the sustainable and innocuous nanotechnology. Green nanotechnology provides novel routes for biological toxicology, biocompatibility expansion. Green efforts can address the potential negative impacts of nanomaterials and develop a novel technology, providing significant benefits, and reduces costs to human health and we find paradigm shift in this technology. • Drug delivery: Role of nanomedicine brings a major advancement in the DDS. Wide range of DDSs based on NP formulations have been developed and are presently playing an imperative role in the treatment, diagnosis, and mapping of the disease. NPs are being combined with the therapeutic agents to provide desirable results. An improved understanding of the biological processes and modifications, advancement, and refinement of nanotechnology will positively help to develop more harmonious nanomedicine for the treatment of various diseases in future. • Green nanotechnology: It helps in the DDS by increasing the solubility and biocompatibility of the drug. The treatment of cancer can be effective by having the drug with good bioavailability, specificity, biocompatibility, etc. Compared to conventional methods, nanotechnology delivers multiple drugs directly at times to the tumor cells. Nanotechnology helps in nucleic acid delivery and gene therapy. Nanomaterials can enable the nondrug therapies, such as, photothermal and photodynamics, etc.27 • Targeted DDS: It is of two types, passive targeting and active targeting. The aim of both types of the targeting is to release the drug at the desired location. The passive targeting involves the release of the drug through EPR (enhanced permeability retention) in which leaky vasculature of tumor allows uptake of the NPs into the tumor tissue. Once the drug enters the tumor cells, it kills the cancer cells. Whereas in the active targeting, the NPs are conjugated with different ligands

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(such as, peptides, proteins, antibodies, etc.) and which ultimately bind to specific cell-surface receptor and deliver the drug.13,32 • Reproducible release of drugs: NPs of 200 nm size are unable to escape from the liver and spleen, which can release the drug slowly in the body as well as remain for the longer period. The DDS is developed effectively by the help of NPs ranging from 70 to 200 nm.30 Several scientists worked on green fabrication of biocompatible gold nanoparticles (AuNPs) by using polysaccharide gum karaya. The synthesized biocompatible gold nanoparticles (AuNPs) are conjugated with the anticancer drug (which is approved by FDA) gemcitabine to form DDS. The application of DDS prepared by AuNPs and anticancer drug gemcitabine is very effective in lung cancer casing cells in humans. A549 leads reserve of cell-spread in a dose-reliant manner by enhancing the fabrication of reactive oxygen species (ROS) inside the cells. Another example of green fabrication of AuNPs is by using pectin polysaccharide encapsulation of anticancer drug doxorubicin (DOX) with the targeting ligand folic acid (FA) effectively in HT-29 colon cancer cells. In several other illustrations of the development of green-fabrication of AuNPs, sulfated polysaccharide fucoidan acts as reducing as well as capping agent conjugated with DOX to form DOX-Fu AuNPs.38,95,124 6.4.1 SELECTIVITY AND SPECIFICITY OF GREEN NANOMATERIALS IN DRUG DELIVERY Eco-friendly nature of the cell and development of nanoscale drug delivery causes modicums side effects on the healthy cell. Nanoscale drugs act precisely on the cancerous tissue.65,67 Due to their biological nature, nanomaterials can easily cross cell barriers.29 Drug delivery benefits of nanotechnology are given in Figure 6.9. 6.4.1.1 CANCER DIAGNOSIS AND THERAPY Cancer is not only painful but an emotionally draining disease. Cancer is a disease caused by uncontrolled growth of the cells; it starts from one part of the body and spreads to the entire body. Cancer cells replicate and do not convert into specialized cells. They are malignant in nature, which grow indefinitely, spread, and kill the healthy host cells.

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NPs are successfully able to diagnose and treat the tumor cells because of the unique properties of the NPs. For example, NPs are quantum dots, superparamagnetic iron, etc. These NPs with optical, physical, and magnetic properties are extremely helpful in the detection and treatment of tumor cells.

FIGURE 6.9

Drug delivery benefits of nanotechnology.

Diverse antitumor drugs, antibodies, and biomolecules including peptides incorporated with NPs to label highly specific tumors, which are not only helpful in early detection but also helpful in screening of cancer cells. Earlystage detection of the tumor tissue can be possible by NPs. Recent studies on SPIONs have reported that it can be used for treatment of lung cancer cells in human by using magnetic resonance imaging imaging. Tomographic imaging technology developed with the help of magnetic powder imaging illustrates a high resolution to cancer tissues. It helps in prior recognition and handling of cancer to reduce disease spread and deaths.149,163 Biomedical applications of green-synthesized NPs for cancer treatment and diagnosis are given in Figure 6.10. Cancer biomarkers play noteworthy role in the early detection of the cancer thus decreasing the mortality caused by the cancer. Cancer biomarkers are biochemical molecules, such as, nucleic acids, proteins, sugars, small metabolites, tumor cells whose expression specifies the presence or state of

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a tumor. Cancer biomarkers help in diagnosis, prognosis, treatment, toxicity, and recurrence. Nanotechnology and nanoscience provide a high sensitivity and selectivity in early detection and treatment of the cancer. NPs are very operational in cancer biomarkers. Proteins, protein fragments or DNA can be used as biomarkers. There should be high sensitivity (>75%) and specificity (99.6%). Blood, urine, or saliva samples are used for the biomarkers to monitor individuals for cancer risk.26,90

FIGURE 6.10

6.4.2

Applications of green-synthesized NPs for cancer treatment and diagnosis.

CATALYSIS

NPs are being used as catalysts in numerous fields. The surface of the NPs acts ideally in comparison with bulk materials. NPs have elevated surface area to volume ratio to provide media for attachment of the reactant and the

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product. Decomposition of H2O2 to oxygen by Au, Ag, and Pt is well known. The formations of C2H2 to ethylene oxide and CH3OH to HCHO by use of Silver NPs are known for their controlled catalytic activity, which is not found in bulk materials.53 Pt nanocrystals are of different shapes, such as, cubes, tetrahedral, and spheres. It shows different shape-dependent effect. Gold NPs also exhibit catalytic properties and more suitable for use because of low-cost and high stability. Gold nanoparticles (AuNPs) with dimensions of 5 nm on metal oxide or carbon have established catalytic properties. AuNP-based gas sensors are efficiently used for perceiving presence of gases, such as, CO and NOx. AuNPs show shape-dependent effect as they can show catalytic activity in Au nanocage.70 The fabrication of green NPs has been broadly eased because of easy availability of the raw materials with high yields for the synthesis of NPs. Moreover, these NPs as catalysts are nontoxic in nature as compared to synthesized by the chemical and physical methods. Bioinspired hybrid materials are also being developed using metal NPs, such as, Au, Cu, Ag, etc. and diverse substrates like CNTs, proteins, etc. These metal NP-based catalysts can also be encapsulated with biopolymers, such as, chitosan, cellulose, etc. These green catalysts are used for removal of chlorine in the water, hydrogen sorption, environmental remediation, and fuel cell usage.85,156,157 6.4.3 BIOSENSORS Several NPs act as excellent biosensors because of high sensitivity and lower detection limits at atomic or molecular level. NPs, such as, gold NPs, CNTs, nanodiamonds, and graphene are widely used. The advantage of nanomaterials over bulk materials is their high specific surface, which helps in immobilization of an improved amount of bioreceptor units. Enzymes play major role in conjugation of bio-specific entity onto nanomaterials. The gold NPs are extensively utilized due to less modification, electronic properties, optical properties, and biocompatibility nature. The gold NPs could be turned, tuned, and adjusted according to desired application. Their shapes and sizes can be modified by means of suitable synthesis technique. These diverse structural arrangements can modify behavior of these gold NPs in the optical, catalytic, and electronic fields. AuNP changes its absorption spectra when number of particles come adjacent to each other. To visualize the biological processes, metal particles are used for labeling of bioassays and the staining of biological tissues. The change in the spectral shifts by

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modification of structural composition, shape and size can open new avenues for optical biosensors with biomolecule–NP hybrid systems.11 Quantum Dots (QDs), luminescent semiconducting nanocrystals, are another nanomaterials, which can be used as biosensors. Prominently used are colloidal QDs with cadmium chalcogenides (S, Se, Te), which provide huge absorption spectrum and narrow emission spectrum.23,44,130 6.4.4 LABELING Metal NPs show optical properties with the help of absorption coefficient, index of refraction, which is useful in photothermal or photoacoustic imaging. Metal NPs have electron absorbing properties due to its size, shape, specific surface area, aspect ratio, and functionalizing with antibodies suitable for biotagging or labeling. These properties are widely used in biological research for sensitive detection in cancer diagnosis and treatment. AuNPs provide tremendously high spatial resolution, which is helpful in labeling applications.133 6.4.5 WOUND HEALING When wound is open, its skin is cut or broken; and in close wound blunt force shock causes a bruise. The healing of such wound is a biological process and involves wide range of cells like fibroblast, epithelial, and endothelial cells. In wound healing, the tissues need ample care to recuperate themselves. Different cytokines play significant role in the wound healing (Figure 6.11). Cytokines are sometimes having high cost and not highly effective. To heal the wound more effectively, nanomaterials are highly used. Scientists are working on various types of the wound healing materials, which can be combined with nanomaterials to form more effective wound healing materials. Fabricated through green route polysaccharide-based metal NPs are one of such examples. Gold NPs can be synthesized by several green route methods, such as, linseed hydrogel (LSH); and the NPs (~10–35 nm) formed on the LSH were stable for 6 months. The synthesis of AuNPs glucuronoxylan (GX) is isolated from Mimosa pudica seeds, etc.40,57,107 The developed AuNPs—LSH showed abundant wound healing ability in terms of collagen deposition on the wound and tensile strength of the LSH. Apart from the NPs from the plants, abundant examples can be found in the

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literature to prepare NPs by fungi and microorganisms that are endophytic fungi (e.g., Penicillium spinulosum OC-11). The NPs were tested on the rat model and good results were observed. Thus, we can conclude that the NPs synthesized by nontoxic, eco-friendly methods can be used in biomedical field without side-effects.74–76,107 Biomedical applications of green-synthesized NPs for wound healing are given in Figure 6.11.

FIGURE 6.11

Applications of green-synthesized nanoparticles for wound healing.

6.5 CONCLUSION Several scientists and researchers are constantly trying to increase the production of nanomaterials from small-scale laboratory to large-scale industrial production, but the process has been quite slow and challenging. Out of various encounters that have slackened the development of the nanomaterials is deprived knowledge of the novel danger caused due to nanomaterials & dearth of right guiding principle to cope up with new-fangled

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risks. Many of the field experts took it as a challenge and move forward to over these obstacles by managing and producing to the larger sector. Green nanoscience or green nanotechnology has drawn a considerable attention due to its benefits that significantly overshadow the side effects/toxic effects of the nanotechnology. This new emerging green nanotechnology or science avoid following the footprints of the conventional technologies that have been in use from the past several years. The conventional technologies had wide problems and the problem does not unimagined costs to human race and at large surroundings. The expansion of feasible green nanotechnologies is griming task and require sensible efforts. By knowing the benefits of the green nanomaterials and technologies, scientist and researcher sooner and later mold their research and invention in green direction. In the initial part of this chapter, exceptional focused on the various types of NPs and diverse technique for the synthesis of the nanomaterials, pro and cons of the nanomaterials, in the second part biomedical applications of metal NPs have been discussed, with particular emphasis on drug delivery, cancer therapy and diagnosis, catalysis, sensor, labeling, and wound healing. KEYWORDS • • • • • • • •

biological engineering biomedical applications green synthesis gold nanoparticles magnetic nanoparticles nanoparticles nanotubes quantum dots

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137. Santhoshkumar, T.; Rahuman, A. A.; Rajakumar, G. Synthesis of Silver Nanoparticles using Nelumbo Nucifera Leaf Extract and its Larvicidal Activity against Malaria and Flariasis Vectors. Parasitol. Res. 2011, 108, 693–702. 138. Saratale, R. G.; Karuppusamy, I.; Saratale, G. D.; Pugazhendhi, A.; Kumar, G.; Park, Y.; Ghodake, G. S.; Bharagava, R. N.; Banu, J. R.; Shin, H. S. Comprehensive Review on Green Nanomaterials using Biological Systems: Recent Perception and their Future Applications. Colloids Surf. B Biointerfaces 2018, 170, 20–35. 139. Sastry, M.; Ahmad, A.; Khan, I. M.; Kumar, R. Biosynthesis of Metal Nanoparticles using Fungi and Actinomycete, Curr. Sci. 2003, 85 (2), 162–170. 140. Senapati, S.; Ahmad, A.; Khan, M. I.; Sastry, M.; Kumar, R. E B B Au--Ag A N. Small J. 2005, 1, 517–520. 141. Shaligram, N. S.; Bule, M.; Bhambure, R.; Singhal, S. R. Biosynthesis of Silver Nanoparticles using Aqueous Extract from the Fungal Strain. Process Biochem. 2009, 44, 939–943. 142. Sharma, V. K.; Yngard, R. A.; Lin, Y. Silver Nanoparticles: Green Synthesis and their Antimicrobial Activities. Adv. Colloid Int. Sci. 2009, 145, 83–96. 143. Sharma, V. K. Aggregation and Toxicity of Titanium Dioxide Nanoparticles in Aquatic Environment: A Review. J. Environ. Sci. Health A 2009, 44 (14), 1485–1495. 144. Sharmila, G.; Thirumarimurugan, M.; Sivakumar, V. M. Optical, Catalytic and Antibacterial Properties of Phytofabricated Cu Nanoparticles using Tecomacastanifolia Leaf Extract. Optik 2016, 127 (19), 7822–7828. 145. Shivaji, S.; Madhu, S.; Singh, S. Extracellular Synthesis of Antibacterial Silver Nanoparticles using Psychrophilic Bacteria. Process Biochem. 2011, 46, 1800–1807. 146. Siddique, S.; Rovina, K.; Al-Azad, S.; Naher, L.; Suryani, S.; Chaikaew, P. Heavy Metal Contaminants Removal from Wastewater using the Potential Filamentous Fungi Biomass: A Review. J. Microb. Biochem. Technol. 2015, 7, 384–393. 147. Sindhura, K. S.; Prasad, T. N. V. K. V.; Selvam, P. P.; Hussain, O. M. Synthesis, Characterization and Evaluation of Effect of Phytogenic Zinc Nanoparticles on Soil Exo-Enzymes. Appl. Nanosci. 2014, 4 (7), 819–827. 148. Singh, P.; Kim, Y. J.; Zhang, D.; Yang, D. C. Biological Synthesis of Nanoparticles from Plants and Microorganisms. Trends Biotechnol. 2016, 34, 588–599. 149. Singh, R. Nanotechnology Based Therapeutic Application in Cancer Diagnosis and Therapy. 3 Biotech. 2019, 9, 415–420. 150. Snow, S. D.; Park, K.; Kim, J. H. Photochemical and Photophysical Properties of Sequentially Functionalized Fullerenes in Aqueous Phase. Environ. Sci. Techno. Lett. 2014, 1, 290–294. 151. Song, J. Y.; Jang, H. K.; Kim, B. S. Biological Synthesis of Gold Nanoparticles using Magnolia Kobus and Diopyros Kaki Leaf Extracts. Process Biochem. 2009, 44 (10), 1133–1138. 152. Southam, G.; Beveridge, T. J. The i n-Vitro Formation of Placer Gold by Bacteria. Geochimcosmochim Acta 1994, 58, 4527–4530. 153. Sportelli, M. C.; Izzi, M.; Kukushkina, E. A.; Hossain, S. I.; Picca, R. A.; Ditaranto, N.; Cioffi, N. Can Nanotechnology and Materials Science Help the Fight Against Sars-Cov-2? Nanomaterials, 2020, 10, 802–810. 154. Stan, M.; Popa, A.; Toloman, D.; Dehelean, A.; Lung, I.; Katona, G. Enhanced Photocatalytic Degradation Properties of Zinc Oxide Nanoparticles Synthesized by using Plant Extracts. Mater. Sci. Semicond. Process 2015, 39, 23–29.

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155. Sunkar, S.; Nachiyar, C. Biogenesis of Antibacterial Silver Nanoparticles using the Endophytic Bacterium Bacillus Cereus Isolated from Garcinia xanthochymus. Asian Pac. J. Trop. Biomed. 2012, 2, 953–959. 156. Thema, F. T.; Manikandan, E.; Dhlamini, M. S.; Maaza, M. Green Synthesis of ZnO nanoparticles via Agathosmabetulina Natural Extract. Mater. Lett. 2015, 161, 124–127. 157. Thompson, T. D. Using Gold Nanoparticles for Catalysis. Nano Today 2007, 2, 40–43. 158. Tomalia, D. A. Birth of a New Macromolecular Architecture: Dendrimers as Quantized Building Blocks for Nanoscale Synthetic Polymer Chemistry. Prog. Polymer Sci. 2005, 30 (3–4), 294–324. 159. Uskokov, I. Why have Nanotechnologies been Underutilized in the Global uprising Against the Coronavirus Pandemic. Nanomedicine (London) 2020, 15, 1719–1734. 160. Vanathi, P.; Rajiv, P.; Narendhran, S.; Rajeshwari, S.; Rahman, P. K.; Venckatesh, R. Biosynthesis and Characterization of Phytomediated Zinc Oxide Nanoparticles: A Green Chemistry Approach. Mater. Lett. 2014, 134, 13–15. 161. Vigneshwaran, N.; Ashtaputre, N. M.; Varadarajan, P. V.; Nachane, R. P.; Paralikar, K. M.; Balasubramanya, R. H. Biological Synthesis of Silver Nanoparticles using Fungus Aspergillus Favus. Mater Lett. 2007, 61, 1413–1418. 162. Vigneshwaran, N.; Kathe, A.; Varadarajan, P.; Nachane, R. P.; Balasubramanya, R. H. Biomimetics of Silver Nanoparticles by White Rot Fungus, Phaenerochaete chrysosporium. Colloids Surf. B Biointerfaces 2006, 53, 55–59. 163. Wan, X.; Song, Y.; Song, N.; Li, J.; Yang, L.; Li, Y. The Preliminary Study of Immune Superparamagnetic Iron Oxide Nanoparticles for the Detection of Lung Cancer by Magnetic Resonance Imaging. Carbohydrate Res. 2016, 419, 33–40. 164. Wang, M.; Cao, R.; Zhang, L.; Yang, X.; Liu, J.; Xu, M.; Shi, Z.; Hu, Z.; Zhong, W.; Xiao, G. Remdesivir and Chloroquine may Inhibit the Recently Emerged Novel Coronavirus (2019-Ncov) In Vitro. Cell Res. 2020, 30, 269–271. 165. Wen, L.; Lin, Z.; Gu, P. Extracellular Biosynthesis of Monodispersed Gold Nanoparticles by a Sam Capping Route. J. Nanoparticle Res. 2009, 11, 279–288. 166. Yurkov, A. M.; Kemler, M.; Begerow, D. Accumulation Curves and Incidence—Based Species Richness Estimators to Appraise the Diversity of Cultivable Yeasts from Beech Forest Soils. PLoS One, 2011, 1, 1–9.

CHAPTER 7

TRENDS IN GREEN SYNTHESIS OF CARBON-BASED NANOSTRUCTURES DIVYA NERAVATHU GOPI and N. K. ATHIRA

ABSTRACT The combination of nanotechnology with green chemistry has become a major factor to determine the future of nanotechnology. The use of natural products for the synthesis of nanomaterials, especially the green growth of carbon-based nanomaterials has extensively been explored and many of these products are now finding their way from the laboratory to various fields of engineering and technology. Nanostructures of carbon include tubes, spherical, or single-layered with fundamentally different properties. These carbon nanostructures have numerous technological applications in microelectronics and nanoelectronics, displays, durable batteries, biosensors, etc. However, the applications of these carbon-based nanomaterials still face significant challenges. This chapter presents the development of green carbon-based nanostructures, recent advances, and the challenges and applications of these nanomaterials. 7.1 INTRODUCTION Nanotechnology is one of the most significant scientific breakthroughs of the 21st century, which involves the manipulation of materials at atomic, molecular, and macromolecular levels, where the properties of nanomaterials widely differ from those at a larger scale. The main features of nanomaterials arise from their size-dependent functional properties due to the exceptional surface activities, and electrical, magnetic, and optical properties, as well as Advances in Green and Sustainable Nanomaterials: Applications in Energy, Biomedicine, Agriculture, and Environmental Science. Megh R. Goyal and Shrikaant Kulkarni (Eds.) © 2023 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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morphologies at nanosize. Nanomaterials entail great potential in various fields mainly because of a high ratio of surface atoms that can modify the physicochemical properties which increases their chemical reactivity. Nanomaterials have reached wide uses in numerous scientific and technical fields, such as catalysis, electrocatalysis, water treatment, energy applications, supercapacitors, and biomedical applications from cancer treatment to regenerative medicine. The recent advancements in nanotechnology have the potential to dramatically change lifestyles, jobs, and whole economies. However, some of the currently used materials are dependent on nonrenewable resources and produce hazardous wastes. These crises have led to the development of green nanotechnology to turn the resource constraints and the environmental issues into an economic opportunity that produces a double dividend of greater growth with lower environmental impact by improving the potential efficiency of resource usage. So, the combination of green chemistry techniques with nanotechnology applications has thus become a key component of the future of nanotechnology. Green nanotechnology is a new effort to employ the ability of nature to remove or decrease the environmental and human health risks caused by the persistent use of nanomaterials, and also to encourage the replacement of existing products with new nanoproducts that are more eco-friendly throughout their lifespans. Thus, the use of natural ingredients to synthesize nanomaterials and design environmentally benign synthetic processes has been extensively explored. The green nanostructures developed by combining nanotechnology with the principles and practices of green chemistry are the key to build an environmentally sustainable society in the 21st century. The present green approach toward nanotechnology practices involves the efficient use of natural resources, nonhazardous solvents, and energy-efficient synthesis processes by eliminating the use and generation of hazardous substances.7 The principal aim of employing green resources to produce nanomaterials is to minimize pollutions and thereby address the environmental issues, which should be conducted in such a way that encompasses the principles of green science by producing nanomaterials and products without harming human health, and the environment by using less energy and producing the least pollution. Green nanotechnology can help to solve environmental issues associated with the use of nanomaterials such as the use of fewer nonrenewable resources, reducing the emission of greenhouse gases, minimizing fossil fuel usage, and developing lightweight vehicles and equipment, flexible electronics, etc.

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Green nanoscience has had a significant effect on our world that helps substantially to reduce the release of hazardous waste into the environment. In this direction, green nanoscience incorporates various research fields, such as the study of green solvents, alternative energy science, nextgeneration catalyst design, molecular self-assembly, and molecular designs with minimum environmental hazards. Applications using the nanomaterials derived from this new science have led to a huge decrease in the release of toxic waste materials, such as hydrochloric acid, trichloroethylene, and methyl isobutyl ketone. Generally, all environmentally friendly inventions involving renewable resources, energy efficiency, recycling, safety and health concerns, etc., come under the umbrella of green nanotechnology. It is always essential to reduce or avoid the production of environmentally hazardous materials. Thus, this field involves both the production and consumption of materials by altering human activities in such a way that minimizes the damage to the environment. This branch of nanotechnology helps to control hazardous substances without entering the environment and thus improves the ecosystem. For energy saving and generation, solar panels and thermal discs are good examples of technologies that make the use of sustainable heat from the sun to produce electrical energy in a safe and environment-friendly manner. Sustainable and green chemical products such as cleaning agents, detergents, and insecticides can be made using green reagents, such as glycerin, orange, coconut, and peppermint oil, by avoiding the use of toxic or hazardous materials. Nowadays, the frequent use of many materials that are not eco-friendly such as plastics can also cause serious damage to the earth because of the large time span of these materials for biodegradation. Green technology makes use of only sustainable and recyclable materials and also solar-powered charging devices and thus the development of green science and technology provides more convenience and offers a safe and healthy environment for life on the earth. Among the different classes of nanomaterials such as carbon-based, inorganic-based, organic-based, and composite-based materials, carbon-based nanomaterials have been emerged as a more powerful tool due to the wide applications of these materials in industries. The different nanoallotropes of carbon include graphene, carbon nanotubes, fullerene, carbon dot (CD), etc. Though some exemptions exist, all the members of this carbon family generally consist of covalently bonded sp2 carbon atoms.1,9 Almost two centuries ago, the presence of carbon was identified in biomolecules and in natural carbon materials, such as diamond, graphite, and

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various amorphous carbon sources. The properties of diamond and graphite, such as hardness, conductivity, transparency, etc., are widely different from each other, though they consist only of carbon atoms. Graphite is a black, soft, and opaque material that is also well known for its good electrical conductivity, whereas diamond is the hardest material and transparent electrical insulator. These differences in their properties arise due to the differences in the way by which carbon atoms are interconnected with each other in these materials. Diamond is made up of tetrahedrally coordinated sp3-hybridized carbon atoms, whereas graphite is made up of stacked sp2-hybridized twodimensional single-layer graphene that is held together one above the other by a weak van der Waals force.4 Thus, the properties and applications of these diverse forms of carbon differ widely from each other. Different nanoallotropes of carbon have recently gained tremendous research interest due to their unique properties arises from size, morphology, and reactivity. The graphene sheet with 2-dimensionally (2D) arranged carbon atoms can be considered the universal building block of various carbon nanostructures. Depending on how the graphene layers are folded, it can yield different carbon nanostructures such as zero-dimensional (0D) fullerenes, one-dimensional (1D) single-wall or multiwall carbon nanotubes (CNTs), or a 2D graphene-based nanomaterials.1 The different nanoallotropes of carbon possess mechanical strength, good conductivity, and the capability to undergo chemical functionalization under different experimental conditions to tune their properties to meet the required practical applications. They even find applications in biomedicine and oncology such as innovative components in cancer therapy and cancer theranostics, biosensing, etc. Thus, these allotropic forms of carbon are now finding their way from the laboratory to commercial applications in diverse fields, though they still face some significant challenges. Despite decades of research efforts, there exist many concerns about the toxicity of carbon nanostructure and immunogenicity which offers unexploited opportunities and unresolved challenges, especially in nanomedicine. However, the development of green chemical routes for the sustainable production of carbon nanostructures is more advisable for the preservation of environment.1 This chapter focuses on (1) introduction to different allotropic forms of carbon nanomaterials, such as fullerene, CDs, graphene, etc.; (2) preparation methods of each of these allotropic forms of carbon with special attention to green routes, recent advances, and challenges in the synthesis of green carbon-based nanomaterials; and the potential applications of various carbon-based nanostructures in diverse fields of technology.

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7.2 ALLOTROPES OF CARBON The unique physicochemical properties of allotropic forms of materials arise from the variations in the structural geometry of constituent atoms and the nature of chemical bonds within the molecules. Carbon is one of the most attractive elements in that aspect, with the capability of forming different structures in different allotropic forms, from zero-dimension to threedimension, with unique and diverse physicochemical properties. The distinct ability of carbon atoms to form sp, sp2, or sp3 hybridized, covalent bonds with the neighboring carbon atoms or even with nonmetallic elements, and form unique structures, extending from small molecules to long chains.4 The carbon family comprises of several members including fullerenes, CDs, carbon nanotubes, and graphene-based materials (Fig. 7.1). Since the discovery of fullerenes (1985), carbon nanotubes (1991), and graphene (2004), the unique properties of carbon-based nanostructures have gained tremendous research interest. Carbon nanostructures with spherical morphology belong to the group of fullerenes or CDs (0D nanomaterials) and the tubular morphologies are termed as carbon nanotubes (1D nanomaterials). The single or multilayered sheets of carbon atoms belong to the group of graphene or graphite, another well-known allotropic form of carbon. 7.2.1 FULLERENES Fullerenes consist of sp2 hybridized carbon atoms arranged into 12 pentagons and a calculable number of hexagons that are arranged into closed hollow cages of carbon atoms. The internal voids of fullerene can accommodate guest molecules such as atoms or other nanostructures that can even tailor unique nanoenvironment to facilitate different chemical reactions or phenomena, thus making them more suitable for the tiniest known stable carbon nanostructures applications.4,11 Fullerenes are the tiniest, stable nanostructure of carbon that marks the boundary between nanomaterials and molecules.4 They belong to the group of zero-dimensional (0D) carbon nanostructures. The first carbon nanostructure discovered was the C60 molecule composed of 60 carbon atoms which belong to the fullerene family that was initially reported in the year 1985. The C60 is a spherical molecule with a diameter of around 0.7 nm. Each C60 molecule consists of 60 carbon atoms (sp2 hybridized) arranged in hexagons and pentagons to form a closed spherical structure (truncated

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icosahedral).4,6,11 This was followed by the discovery of other fullerenes such as C20, C70, and several other larger species. Among these fullerenes, C60 is the most widely studied to date.4 Their nanosize and novel electronic properties made them the focus of intense research for potential uses in various fields, such as nanomedicine, including drug delivery, antioxidant, antiviral therapy, etc.1

FIGURE 7.1 Schematic representations of: (a) diamond, (b) amorphous carbon, (c) carbon nanotubes, (d) graphene, (e) fullerene, (f) CDs. Sources: Reprinted with permission from a–d) Ref. [18]. © 2017 Elsevier; e & f) Ref. [14]. © 2021 Elsevier.

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CARBON DOT (C DOT)

Carbon dots or C-dots are quasispherical nanoparticles of carbon with 2–10 nm of diameter and lattice spacing of 0.34 and 0.21 nm attributed to (002) and (100) lattice planes of graphite, which consist of combinations of graphitic and turbo-stratic carbons in different volumetric ratios. They contain mixtures of sp3 and sp2 hybridized carbon atoms in different ratios.4 They can be classified into two types: one with excitation-dependent luminescence due to surface defects and the other is carbon quantum dots (CQDs) with obvious crystal lattices, which exhibit quantum confinement effect and intrinsic luminescence properties.19 The C-dots are usually amorphous. The unique property of C-dots is their strong photoluminescence, which generally depends on several parameters such as size, surface functionalization, and excitation wavelength.4 The C dots with particle sizes less than 10nm have a wide range of applications in various fields of bioimaging, biosensing, diagnosis, drug delivery, synthetic chemistry, and materials science.19 These photochemically and physicochemically stable carbon nanomaterials are usually water-soluble with comparatively low toxicity and production costs, which can offer tunable fluorescence emission and excitation with good biocompatibility. Thus, the synthesis, properties, and applications of C dots have drawn tremendous attention from the research community. 7.2.3 CARBON NANOTUBES (CNTs) Among the different carbon-based nanomaterials, carbon nanotubes (CNTs), found by Ijima in the year 1991, constitute a new allotropic form of carbon that derived special interest from the application point of view. In CNTs, each carbon atom is bonded by sp2 hybridized neighboring carbon atoms, much stronger than the sp3hybridized bonds in a diamond (Figure 7.2). This provides them an exceptional mechanical, optical, thermal, and electrical properties.9 The CNTs are layers of graphene sheets with honeycomb lattices of carbon atoms rolled up into hollow, cylindrical nanostructures.9 At the edges, the graphene sheets are fused to form cylindrical tubes with closed edges. According to the number of sheets in their structure, CNTs can be classified as single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs). The SWCNTs be made up of a single layer of graphene sheet with capped edges at both ends that are having a diameter that usually varies in the range of 0.4–40 nm. However, the MWCNTs consist of multiple layers of graphene

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sheets forming concentric cylinders with an interlayer distance of 0.35 nm with capped edges at both ends.9

FIGURE 7.2 Schematic representation of (a) single-walled (SWCNTs) and multiwalled (MWCNTs), (b) crystallographic configurations of armchair, zig-zag and chiral SWCNTs. Source: Reprinted with permission from Ref. [8]. © 2021 Elsevier.

The diameters of CNTs can be as low as 0.7 nm for SWNTs and 100 nm for MWNTs and lengths can be in the range of a few micrometers to several millimeters. The aspect of CNTs usually surpasses 10,000 and consequently, they are regarded as the most anisotropic materials ever produced. Apart from this, the chirality (angle between hexagons and nanotube axis) is another important parameter of CNTs, depending on which, the carbon atoms around the tube circumference can be arranged in different patterns like an armchair, zigzag, and chiral patterns (Figure 7.2).4 7.2.4 GRAPHENE Graphene is a recently isolated carbon structure, which can be widely interpreted as the building block of graphite. Its existence had been anticipated decades ago, however, identified experimentally in the year 1962 (Boehm et al.), and isolated and characterized in 2004 (Andre Geim and Konstantin Novoselov).4,10 It is an abundant material since it is the building block of natural graphite. It is currently the strongest known material.4 Graphene is one of the most important allotropic forms of carbon which consists of a single layer of carbon atoms arranged in a two-dimensional (2D) honeycomb lattice. Each of the constituent carbon atoms of graphene is sp2 hybridized, having three bonds with adjacent carbon atoms in the same plane.15 From a theoretical point of view, graphene can be viewed as

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the “building block” of graphitic/graphenic nanoallotropes. As an example, a graphene sheet could be wrapped up to form a 0D fullerene, rolled up in the form of 1D CNTs, or several sheets of graphene stacked up to form multilayered 2D graphite.4 It is the thinnest known material with zero bandgaps and is incredibly strong, almost 200 times stronger than steel. Moreover, graphene is a good conductor of heat and electricity with very interesting light absorption properties. It is a promising material that could change the world, with unlimited potential for wide industrial applications in various fields. All of the above-discussed, carbon nanoallotropes such as fullerene, C-dots, CNTs, graphene, etc., can be viewed as members of the same carbon family because these nanostructures are molded from carbon atoms arranged in a hexagonal honeycomb crystal lattice. Thus, these nanostructures have some common properties such as comparable electrical conductivity, mechanical strength, chemical reactivity, and optical properties. They also have significant differences due to different shapes and sizes. The biggest differences among them arise from their dispersibility in organic solvents. As an example, C60 is the single readily soluble nanostructure, whereas graphene is not soluble in water but dispersible in certain organic solvents. Many of the other carbon nanostructures are only slightly dispersible even in organic solvents, and the lack of dispersibility results in the formation of unstable suspensions.4 Apart from these well-known nanostructures of carbon, there exist other carbon structures also with unique shapes such as onion-like carbon spheres (Figure 7.3), single-walled carbon nanohorns, bamboo-like nanotubes, carbon nanofibers, graphene quantum dots, nanodiamonds (NDs), etc.4 Moreover, the different carbon nanostructures can be accumulated in 3D materials and superstructures.1 7.3

PURPOSE OF GREEN CARBON NANOSTRUCTURES

Humans have widely been adopted a synthetic approach for the development of nanomaterials without properly considering the ancillary consequences. Though synthetic nanomaterials have several benefits and applications, their synthesis and applications are expensive and in most situations, they are accompanied by hazardous by-products. There are several concerns about the effect of nanomaterials on human beings since most of the nanoparticles can easily get absorbed by living systems. Thus, the application of these nanomaterials is limited in several fields, especially in medicines.

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Onion-like carbon nanoparticles with (a) polyhedral and (b) spherical structures.

Source: Reprinted with permission from Ref. [17]. © 1995 Elsevier.

The potential impact of nanomaterials especially the use of engineered nanomaterials on the earth and its organisms lead to the development of another field of research known as eco-nanotoxicology.5 So far, there is no direct evidence for in-situ environmental harm by engineered nanomaterials, the uncertainty still persists around their fates and effects on the environment. However, this new field of research can assess and predict the effects of nanomaterials on ecology for a range of exposure scenarios. Considering all the harmful effects of nanomaterials especially the carbon-based nanomaterials on the environment and organisms, the use of green routes for their synthesis is more encouraged nowadays that can address several major environmental challenges, which may also help to reduce the harmful effects of these nanomaterials on human beings. In fact, the green approaches for the synthesis of carbon-based nanomaterials imply the use of nanotechnology to improve the environmental sustainability of carbon nanostructures by minimizing the cost of synthesis and potential environmental risks of these materials. Actually, plants are the main source of carbon for preparing carbon nanostructures. Plants draw CO2 from the air and utilize it for the formation of biomass via carbon respiration and a process of carbon fixation. Thus, the plants, fruits, and their extracts can be effectively used as the major sources of carbon for the synthesis of various carbon nanostructures. As an example,

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a simple oxidative treatment of carbon nanostructures derived from foodstuffs (bread), makes them water-soluble and fluorescent. This biomaterial has demonstrated good fluorescence over a wide range of excitation wavelengths which can be used for fluorescence imaging of human erythrocytes (red blood cells) under 488 and 561 nm bandpass filters, demonstrating around 6% hemolysis in 5 h on direct interaction with human erythrocytes.13 These fluorescent carbon nanostructures are novel types of biomaterials with promising biomedical applications.13 7.4

PREPARATION OF CARBON NANOSTRUCTURES

The preparation, properties, and applications of carbon nanoallotropes such as fullerenes, C-dots, CNTs, and graphene have been studied extensively. Graphene and multilayer carbon nanosheets can be isolated from the naturally available sources of carbon such as graphite. The graphitic carbon nanostructures can be prepared from various sources of carbon such as graphite, volatile organic compounds, or organic gases using instrumental procedures such as chemical vapor deposition (CVD), laser ablation, arc discharge, etc., to reorganize the carbon atoms in a specific pattern.4 However, researchers have recently been given considerable attention to adopt green routes for the development of these nanostructures due to their promising applications in biomedical fields and to minimize environmental hazards. 7.4.1 PREPARATION OF FULLERENE Fullerenes are usually produced from natural graphite by vaporization using hydrocarbon combustion, laser irradiation, arc and plasma discharges, pyrolysis of naphthalene, etc. Among the different methods, hydrocarbon combustion is the most commonly adopted method for the large-scale synthesis of fullerenes for commercial production. The products obtained using all of these methods usually contain a small proportion of fullerenes, with C60 being the most abundant. The fullerene mixture is then extracted from the products by an extraction procedure using some organic solvents, which is followed by column or liquid chromatography techniques to isolate the desired products.4,11 Since fullerene purification requires a large quantity of organic solvents such as benzene or toluene, due to their low solubility a complete green route for the preparation of fullerene is not yet trivial. So, several kinds of researches are still going on in this direction to develop more

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apt green procedures to minimize the environmental impacts and toxicity of the fullerene nanomaterials.1 Besides natural graphite, other precursors have also been successfully used to produce fullerenes by adopting a bottom-up approach. The use of hydrocarbons, such as corannulene, decacyclene, tribenzodecacyclene, and trinaphtodecacyclene, is regarded as the most suitable C60 precursor. However, the synthesis of fullerene by using such precursors is a complex process that involves several steps such as the preparation of precursors themselves and, the successive treatment to the product using flash vacuum pyrolysis to stimulate the ring-closure process in fullerenes. Thus, it is always preferable to use simple precursors.4,11 Plastic waste can also provide a source of carbon for carbon-based products such as hydrocarbons, activated carbon, fullerenes, carbon nanotubes, and graphene. The catalytic thermal decomposition of polyethylene terephthalate bottles (plastic bottles) is also exploited as a source of carbon to develop an economical route for the synthesis of functionalized magnetic fullerene nanocomposites.3 However, a complete environmental route for the synthesis of fullerene is not yet identified. Moreover, the main disadvantage associated with many of these green routes is the relatively low yield of fullerene. Another difficulty is associated with the purification and isolation of the desired products which most probably increases the cost and environmental impacts even with the use of cheap green or naturally abundant raw materials such as graphite.4 The formation of fullerenes has been explained with several models. The icospiral particle nucleation mechanism assumes a corannulene-like C20 molecule as the starting structure with a pentagon encircled by five hexagons. Due to its high reactivity, small carbon fragments accumulate to form nautilus-like open spiral shells. This will be followed by an edge bypass, and the final closure occurs on a statistical basis when a proper number of pentagons are combined to this structure.4 The formation of fullerene can also be explained by another mechanism involving four steps. The first step involves the vaporization of carbon atoms from graphite that form almost 10 carbon atoms long carbon chains. These chains grow into monocyclic rings which describes the second step. The third step involves the subsequent growth of this into three-dimensional carbon networks resembling the features of curved shells. Finally, due to the shell-closing mechanism, they grow into small fullerene cages. The formation of C60 and C70 can be alternatively explained by another mechanism called the ring-stacking model. This model assumes the C10 ring

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as the initial structure, and the deformation of which form a molecule with two hexagons and eight dangling bonds. This molecule is stacked with C18 such that the initially formed dangling bonds are eliminated. But simultaneously, new dangling bonds also arise, the number of which decreases upon stacking with C18, C12, and C2 carbon molecules, which results in the formation of the C60 cage. For the formation of molecule C70, C10 is stacked together with C18, C20, C16, and 3C2 carbon molecules. An annealing of carbon clusters was also proposed as another model to understand the formation of fullerene. The annealing of carbon atoms is usually accompanied by the emission of atoms or small carbon clusters. The fullerene structures are subsequently formed either by a sequential isomeric transformation of these clusters or crystallization of liquid-like clusters.4 The theoretical aspects of quantum chemical molecular dynamics predict the formation of fullerenes by the so-called shrinking hot giant road. This involves two steps: a size-up process when giant fullerenes form from the hot vapor of carbon which is followed by a size-down process where giant fullerenes shrink to C60 and/or C70 by eradication of C2 molecules owing to the vibrationally excited, defective state of cages of fullerenes.4 Though fullerene is a highly useful carbon allotrope, the high production costs of fullerene have prevented its widespread applications.11 There are numerous fullerene preparation methods introduced in the literature, each of which has its own merits, demerits, production costs, and several other parameters which should be improved. 7.4.2 PREPARATION OF CARBON DOTS C-dots were identified fortuitously by Xu et al. in the course of the purification of SWCNTs by gel electrophoresis purification process. Recently, a great variety of synthesis processes is widely adopted for the preparation of C-dots. They are prepared by using top-down approaches such as laser ablation of suitably treated graphite powder and cement mixtures, followed by treatment with an oxidative acid to enrich the surfaces with reactive oxygen. This is tailed by a surface-passivation process by which the organic molecules and oligomers are stuck on the dots. Other methods for the preparation of C-dots include decomposition or carbonization of organic compounds, ultrasonic carbonization, or solvothermal, hydrothermal, or microwave-assisted treatment of carbohydrates, organic compounds, polymers, organosilanes, or natural products.4

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Normally, the products of synthetic procedures contain unreacted organic masses of precursor materials and intermediates and carbon nanoparticles in wide size ranges which lead to a wide range of fluorescence quantum yields. By employing an appropriate purification process, the organic mass is removed which may cause fluorescence quenching of C-dots. More efficient isolation with a high quantum yield is also possible by adopting procedures such as size exclusion chromatography which separates molecules according to the difference in size as they pass through a resin packed in a column.4 A plethora of natural resources has been recently identified for the green synthesis of C-dots. The pyrolysis of citric acid and penicillamine produced nitrogen and sulfur codoped C-dots for the detection of mercury ions in living cells by fluorescence quenching process. The various plants and plant derived-materials, such as peel, flesh, seed extract, juice, and fruit waste, can act as efficient sources of carbon. The cellulose combined with caffeic acid also acts as an efficient green reducing agent. Some plant parts, turmeric leaves, palm fruit powder, red cabbage, soybean residues, sunflower seeds, mint, roasted chickpeas, green tea, wheat straw, and sandalwood powder. Some of these sources have been used with the view of recycling household kitchen waste. Moreover, the sugarcane bagasse pulp can be converted into antibacterial C-dots. In most cases, carbon quantum dots (CQDs) are used in solution, since they enter the cells by endocytosis and could be tailored to target the preferred subcellular organelles. But, more research is still going on to investigate their use in composites to reinforce thin films for tissue engineering applications, to yield luminescent hydrogels, or UV-responsive smart nanomaterials, and antibacterial composites. 7.4.3

PREPARATION OF CARBON NANOTUBES (CNTs)

Since the discovery of CNTs in 1990 by Iijima, the CNTs have been attracted immense research interest from the scientific community as well as industries due to their remarkable chemical, physical, and mechanical properties. However, many of these properties are predicted for ideal CNTs that are unlike the synthetic CNTs prepared and their properties are still incapable to meet various commercial requirements. The lack of knowledge of the CNT growth mechanism is the root cause of this problem.4 There are several well-established methods for the production of CNTs. The simplest among them is an arc discharge process that occurs between

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two carbon electrodes placed in an inert chamber in which the purity and quality of CNTs prepared hinge on the inert gas used in the chamber. In addition to inert gases, such as He, CH4, and H2, some volatile organic molecules can also be introduced into the chamber which influences the properties of the resultant CNTs.4 The MWNTs can be prepared by the arc discharges process in the absence of a catalyst. The same procedure produces SWNTs if the electrodes are made up of graphite encompassing catalytic metallic nanoparticles, such as Ni, Fe, Co, Pt, and Rh, or their alloys. The CNTs can also be prepared by pulsed-laser ablation (using either Nd: YAG or CO2 lasers) of natural graphite in the presence of catalytic particles of Co, Ni, or a mixture of these two in a chamber under a controlled atmosphere. However, the most powerful method for preparing CNTs is carbon vapor deposition using methane, ethane, acetylene, ethylene, an H2/CH4 mixture, or ethanol as the source of carbon over a metal catalyst, such as Fe, Co, or Ni, or their alloys. The properties, quality, purity, and yield of CNTs developed by the CVD method depend on the composition and morphology of the catalytic nanoparticles used, nature of substrate, decomposition temperature, and source of carbon. Some of the major disadvantages associated with CNT preparation are the probable presence of metallic impurities arising from the catalyst, amorphous carbon impurities, and lack of chirality control. The chirality is related to the structural characteristic of CNTs that determines the diameter and wrapping angle of CNTs, on which the electrical properties of the SWCNTs vary. A small difference in chirality indices of SWNTs can change their character from metallic to semiconducting nature, which reveals the significance of chirality indices of CNTs.4 The inorganic catalyst especially metal catalysts used for the growth of some CNTs’ harbor serious threat to living organisms since they are highly harmful, carcinogenic, and toxic because these catalyst particles usually remain in low concentration on the grown CNTs, and thus can be inserted in the human body while using them for medical applications and in the textile industry. However, the benefits of using CNTs in modern technologies were no longer questioned widely under the assumption that they are not as dangerous as graphite. Moreover, the devices made up of CNTs grown by using metal catalysts may produce serious impediments in their commercialization, as numerous reports flag the prominent dangers of CNTs during manufacturing, disposal, and commercialization. Thus, there is an immediate need to attend to its cones until it becomes too late.16 In agriculture,

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the nano-sized metal catalyst particles on the CNTs may enter plants and can cause misalignment of chromosomes at the metaphase plate which may result in cell division inhibition.16 The presence of a metal catalyst is also not adaptable with most semiconductor-based devices as it may reduce the quantum efficiency of photodetectors and can reduce the lifetime of devices. At high temperatures, the metal catalyst particles present on the CNTs melt or vaporize and get attached to the CNTs, thus occupy more surface area and block the lattice vibrations of CNTs which interfere with the lattice wave propagations, a key factor in a temperature sensor.16 Another important issue is the production cost of CNTs due to the requirements of expensive equipment and high growth temperature usually in the range of 700–1200°C depending on the choice of the metal catalyst. Under these circumstances, the development of nonmetallic and cheap catalysts is essential for the commercialization of CNTs in various fields, such as the agricultural, medical, textile, and electronics fields.16 Many reports are published on the growth of CNTs using nonmetal catalysts with several merits and demerits. In such a scenario, the green catalyst-assisted CNTs’ growth may overcome several threats as discussed so far. The major merits of green catalyst for the growth of CNTs include16: • Nontoxicity of green catalyst. • Green catalysts are organic compounds that help to grow CNTs free from metal impurities. • Sources of metal catalysts are limited while the green catalysts are abundant in nature. • Metal catalysts are costly while green catalysts can be available at a very low cost. • The use of green catalyst eliminates the requirements of expensive growth equipment, such as sputtering unit, dip coating, etc. • Green catalyst reduces growth temperatures and thus reduces power consumption. • Ultra-care is not required for CNTs’ growth as the green catalyst is nontoxic in nature. • Green catalyst reduces the complexity in the growth process since the green catalyst is easy to prepare. The potential use of green catalysts derived from plants, an abundant and cheap natural source, for the growth of CNTs is reported. The green sources for the growth of CNTs include garden grass (Cynodon dactylon), rose (Rosa), neem (Azadirachta indica), wall-nut (Juglans regia) plant extracts,

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etc., which are reported.16 This concept is unconventional but practically realized into existence by adopting several growth mechanisms, such as CVD, by keeping away the hazards caused by the metal catalyst on living organisms and the environment. The notable points to be mentioned of such green growth mechanisms are: the grown CNTs are free from the toxic metal catalyst, low growth temperature required (