Metal and Metal Oxides for Energy and Electronics [1st ed.] 9783030530648, 9783030530655

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Environmental Chemistry for a Sustainable World 55

Saravanan Rajendran Jiaqian Qin Francisco Gracia Eric Lichtfouse  Editors

Metal and Metal Oxides for Energy and Electronics

Environmental Chemistry for a Sustainable World Volume 55

Series Editors Eric Lichtfouse , Aix-Marseille University, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France Jan Schwarzbauer, RWTH Aachen University, Aachen, Germany Didier Robert, CNRS, European Laboratory for Catalysis and Surface Sciences, Saint-Avold, France

Other Publications by the Editors

Books Environmental Chemistry http://www.springer.com/978-3-540-22860-8 Organic Contaminants in Riverine and Groundwater Systems http://www.springer.com/978-3-540-31169-0 Sustainable Agriculture Volume 1: http://www.springer.com/978-90-481-2665-1 Volume 2: http://www.springer.com/978-94-007-0393-3 Book series Environmental Chemistry for a Sustainable World http://www.springer.com/series/11480 Sustainable Agriculture Reviews http://www.springer.com/series/8380 Journals Environmental Chemistry Letters http://www.springer.com/10311

More information about this series at http://www.springer.com/series/11480

Saravanan Rajendran • Jiaqian Qin Francisco Gracia • Eric Lichtfouse Editors

Metal and Metal Oxides for Energy and Electronics

Editors Saravanan Rajendran Department of Mechanical Engineering University of Tarapacá Arica, Chile Francisco Gracia Department of Chemical Engineering and Biotechnology and Materials University of Chile Santiago, Chile

Jiaqian Qin Metallurgy and Materials Science and Research Institute Chulalongkorn University Bangkok, Thailand Eric Lichtfouse CNRS, IRD, INRAE, Coll France, CEREGE Aix-Marseille University Aix-en-Provence, France

ISSN 2213-7114 ISSN 2213-7122 (electronic) Environmental Chemistry for a Sustainable World ISBN 978-3-030-53064-8 ISBN 978-3-030-53065-5 (eBook) https://doi.org/10.1007/978-3-030-53065-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

If you want to find the secrets of the universe, think in terms of energy, frequency and vibration. Nikola Tesla

Energy is a major challenge for humans in the context of climate change and increasing population. In particular, global warming is partly due to CO2 emissions from the excessive use of fossil fuels such as oil, coal and gas. Research has thus recently focused on sustainable biofuels and energy storage. In particular, nanomaterials and metal oxides are enhancing the efficiency of batteries, supercapacitors, fuel cells and electronics. For instance, optoelectronic devices consume much less power than classical devices. Materials for rechargeable batteries are presented in Chap. 1 by Balaji Sambandam et al., with emphasis on electrochemical properties of metal oxide-based electrode materials for energy storage. In Chap. 2, Akhila Das et al. review molybdenum disulphide supercapacitors and high performance electrodes. Chapter 3 by Ryan D. Corpuz et al. highlights the development of manganese oxide as cathode material in rechargeable zinc ion batteries (Figure). Shanmuga Sundar et al. review conductive oxides in the fabrication and application of flexible electronic devices in Chap. 4. Chapter 5 by Ramarajan et al. summarizes major findings on SnO2 doped with Sb and Sb-Ba, Nb and Ta using spray pyrolysis.

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Figure. MnO2-based zinc ion battery, from Chap. 3. A typical MnO2-zinc ion battery consists of a Zn anode and MnO2 cathode. A layered or tunnel structure MnO2 as cathode is used together with an aqueous electrolyte such as ZnSO4 solution

Advances in optical communication are discussed in Chap. 6 by Samuel Paul David et al., who review metal oxide semiconductors and optoelectronic applications such as light emitting diodes, solar cells, photodetectors, gas sensors and heat mirrors. Energy applications of zinc oxide in biomedicine, energy conversion and electrochemical sensing are presented by Barbosa et al. in Chap. 7. Chapter 8 by Dutta reviews metal oxides and sulfides-based gas sensors. In Chap. 9, Chintagunta et al. explain the role of metallic nanomaterials for algal biofuel production. Chapter 10 by Khan et al. discusses the fabrication of humidity sensors based on nanostructured Al2O3 using the thin-film sol-gel method. Muneeswaran et al. present in Chap. 11 the multiferroic properties of rare earth doped BiFeO3 and their spintronic applications. The main credit for this book goes to the contributing authors. We thank them very much for their high quality chapters. Arica, Chile Bangkok, Thailand Santiago, Chile Aix-en-Provence, France

Saravanan Rajendran Jiaqian Qin Francisco Gracia Eric Lichtfouse

Acknowledgements

We would like to first and foremost thank God to show blessings on us by involving in this book work with good health and coming out of a successful task Our deep sense of gratitude and heartfelt thanks goes to the series editors. We sincerely express gratitude to Springer for accepting this book as part of the series Environmental Chemistry for a Sustainable World. We also extended our thanks to contributing authors and reviewers for their valuable involvement throughout this book. We express our heartfelt gratitude to researchers and publisher for permitting us to use their figures and tables. Even though every effort has been made to obtain copyright permissions from respective owners to include citation for reproduced materials, we would like to offer our apologies to any copyright holder if unknowingly their right is being infringed. Saravanan Rajendran is thankful to the financial support from the SERC (CONI CYT/FONDAP/15110019), FONDECYT, Government of Chile (Project No.: 11170414), and Faculty of Engineering, Department of Mechanical Engineering, Universidad de Tarapacá, Arica, Chile. He also extends gratitude to Prof. Lorena Cornejo Ponce (Universidad de Tarapacá) and Prof. Rodrigo Palma (Director, SERC) for their constant support and encouragement which helped complete this book. Jiaqian Qin would like to acknowledge the support from Thailand Research Fund (RSA6080017), the Energy Conservation Promotion Fund and the Energy Policy and Planning Office, Ministry of Energy, Thailand. F. Gracia gratefully acknowledges the financial support from the ConicytFondecyt (Proj. 1171193) and MINECON-Chile through project Millennium Nucleus MULTIMAT – ICM/MINECON.

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Contents

1

Metal Oxides for Rechargeable Batteries Energy Applications . . . . Balaji Sambandam, Samuel Paul David, Tamilselvan Sakthivel, Anandhi Sivaramalingam, Ananthakumar Soosaimanickam, and Jaekook Kim

2

Molybdenum Disulfide (MoS2) and Its Nanocomposites as High-Performance Electrode Material for Supercapacitors . . . . . . . Akhila Das, Asha Paul, Nikhil Medhavi, Neethu T. M. Balakrishnan, M. A. Krishnan, Jou-Hyeon Ahn, Jabeen Fatima M. J., and Raghavan Prasanth

3

Manganese Dioxide (MnO2): A High-Performance Energy Material for Electrochemical Energy Storage Applications . . . . . . . Ryan D. Corpuz, Lyn Marie De Juan-Corpuz, and Soorathep Kheawhom

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4

Conductive Oxides Role in Flexible Electronic Device Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Shanmuga Sundar Dhanabalan, Arun Thirumurugan, Muniyandi Muneeswaran, Sitharthan R, Karthikeyan Madurakavi, Sivanantha Raja Avaninathan, and Marcos Flores Carrasco

5

Indium-Free Alternative Transparent Conducting Electrodes: An Overview and Recent Developments . . . . . . . . . . . . . . . . . . . . . 149 R. Ramarajan, D. Paul Joseph, K. Thangaraju, and M. Kovendhan

6

Thin Film Metal Oxides for Displays and Other Optoelectronic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Samuel Paul David, Ananthakumar Soosaimanickam, Tamilselvan Sakthivel, Balaji Sambandam, and Anandhi Sivaramalingam

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7

Zinc Oxide as a Multifunctional Material: From Biomedical Applications to Energy Conversion and Electrochemical Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Helliomar Pereira Barbosa, Diele Aparecida Gouveia Araújo, Lauro Antonio Pradela-Filho, Regina Massako Takeuchi, Renata Galvão de Lima, Jefferson Luis Ferrari, Márcio Sousa Góes, and André Luiz dos Santos

8

Metal Oxide- and Sulfide-Based Gas Sensors: Recent Trends and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Kingshuk Dutta

9

Contribution of Metallic Nanomaterials in Algal Biofuel Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Anjani Devi Chintagunta, Ashutosh Kumar, S. P. Jeevan Kumar, and Madan L. Verma

10

Fabrication of Nanostructured Metal Oxide Thin Film Capacitive Humidity Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 Anwar Ulla Khan, Lokesh Kumar, Tarikul Islam, and Mohammad Ehtisham Khan

11

Multiferroic Properties of Rare Earth-Doped BiFeO3 and Their Spintronic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Muniyandi Muneeswaran, Mayakrishnan Gopiraman, Shanmuga Sundar Dhanabalan, N. V. Giridharan, and Ali Akbari-Fakhrabadi

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

About the Editors

Saravanan Rajendran received his PhD in Physics – Material Science in 2013 from the Department of Nuclear Physics, University of Madras, and Chennai, India. He was awarded the University Research Fellowship (URF) during the years 2009–2011 by the University of Madras. After working as an Assistant Professor at Dhanalakshmi College of Engineering, Chennai, India, from 2013 to 2014, he was awarded SERC and CONICYT-FONDECYT postdoctoral fellowship by the University of Chile, Santiago, Chile, in 2014–2017. He has worked (2017–2018) in the research group of Professor John Irvine, School of Chemistry, University of St Andrews, UK, as a Postdoctoral Research Fellow within the framework of a EPSRC-Global Challenges Research Fund for the removal of Blue-Green Algae and their toxins. Currently, he is currently working as a Research Scientist in the Faculty of Engineering, Department of Mechanical Engineering, University of Tarapacá, Arica, Chile, and. also as Research Associate in SERC, University of Chile, Santiago, Chile. He is Associate Editor of International Journal of Environmental Science and Technology (Springer). His research interests focus in the area of nanostructured functional materials, photophysics, surface chemistry and nanocatalysts for renewable energy and waste water purification. He has published in 60+ (5600+ citation and h index 30) international peer-reviewed journals and 13+ book chapters and edited 8 books for renowned international publishers. Jiaqian Qin is presently working as a Researcher in the Metallurgy and Materials Science Research Institute, Chulalongkorn University, Thailand. He obtained his PhD Degree in Physics from Sichuan University, China, in 2010. After graduation, he was awarded JSPS Postdoctoral Fellowship at Ehime University, Japan, in the years 2010–2012. His current research interest focuses on the development of advanced nanostructured materials for energy storage and conversion applications like batteries, supercapacitors and nanocatalysts. He holds several Chinese patents, over 90 publications in the international journals of repute. He is the Editor/Editorial Member of several reputed journals like Scientific Report (Nature), Current Smart Materials and Journal of Metals, Materials and Minerals (JMMM). xi

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About the Editors

Francisco Gracia received a PhD in Chemical Engineering from the University of Notre Dame, USA. He is currently Associate Professor in the ChEBM Department, Universidad de Chile, and Deputy Director at the Multifunctional Materials Millennium Nucleus (MultiMat). His research interests are in the area of heterogeneous catalysis, nanostructured functional materials, gas–solid interphase reactivity with emphasis on CO2 activation and utilization, H2 generation, photocatalysis and energy-related applications. He is a co-author of 50+ research publications and a book chapter and edited a book on subjects related to nanotechnology, photocatalysis and environmental catalysis. Eric Lichtfouse (PhD), born in 1960, is an Environmental Chemist working at the University of Aix-Marseille, France. He has invented carbon-13 dating, a method allowing to measure the relative age and turnover of molecular organic compounds occurring in different temporal pools of any complex media. He teaches scientific writing and communication and has published the book Scientific Writing for Impact Factors, which includes a new tool – the Micro-Article – to identify the novelty of research results. He is founder and chief editor of scientific journals and series in environmental chemistry and agriculture. He was awarded the Analytical Chemistry Prize by the French Chemical Society, the Grand Prize of the Universities of Nancy and Metz, and a Journal Citation Award by the Essential Indicators.

Contributors

Jou-Hyeon Ahn Department of Chemical and Biological Engineering and Engineering Research Institute, Gyeongsang National University, Jinju, Republic of Korea Ali Akbari-Fakhrabadi Advanced Materials Laboratory, Department of Mechanical Engineering, University of Chile, Santiago, Chile Diele Aparecida Gouveia Araújo Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Sivanantha Raja Avaninathan Department of ECE, Alagappa Chettiar Government College of Engineering & Technology, Karaikudi, Tamilnadu, India Helliomar Pereira Barbosa Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Marcos Flores Carrasco Laboratorio de superficies y Nanomaterials, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Anjani Devi Chintagunta Vignan Foundation for Science, Technology and Research, Guntur, Andhra Pradesh, India Ryan D. Corpuz Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand Nanolabs LRC Co. Ltd., Quezon City, Philippines Akhila Das Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India Lyn Marie De Juan-Corpuz Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand Nanolabs LRC Co. Ltd., Quezon City, Philippines Department of Chemical Engineering, Faculty of Engineering, University of Santo Tomas, Manila, The Philippines xiii

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Contributors

Renata Galvão de Lima Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Instituto de Ciências Exatas e Naturais do Pontal, Universidade Federal de Uberlândia, Tupã, Ituiutaba, MG, Brazil Shanmuga Sundar Dhanabalan Laboratorio de superficies y Nanomaterials, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Márcio Sousa Góes Instituto Latino-Americano de Ciências da Vida e da Natureza, Universidade Federal da Integração Latino-Americana, Foz do Iguaçu, PR, Brazil André Luiz dos Santos Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Instituto de Ciências Exatas e Naturais do Pontal, Universidade Federal de Uberlândia, Tupã, Ituiutaba, MG, Brazil Kingshuk Dutta Advanced Polymer Design and Development Research Laboratory (APDDRL), School for Advanced Research in Polymers (SARP), Central Institute of Petrochemicals Engineering and Technology (CIPET), Bengaluru, India Jefferson Luis Ferrari Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil N. V. Giridharan Advanced Functional Materials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, India Mayakrishnan Gopiraman Department of Applied Bioscience, College of Life & Environment Science, Konkuk University, Seoul, South Korea Francisco Gracia Department of Chemical Engineering and Biotechnology and Materials, University of Chile, Santiago, Chile Tarikul Islam Department of Electrical Engineering, Jamia Millia Islamia, New Delhi, India Jabeen Fatima M. J. Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India S. P. Jeevan Kumar ICAR-Indian Institute of Seed Science, Mau, Uttar Pradesh, India Anwar Ulla Khan Department of Electrical Engineering Technology, College of Applied Industrial Technology (CAIT), Jazan University, Jazan, Kingdom of Saudi Arabia Mohammad Ehtisham Khan Department of Chemical Engineering Technology, College of Applied Industrial Technology (CAIT), Jazan University, Jazan, Kingdom of Saudi Arabia

Contributors

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Soorathep Kheawhom Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand Research Unit of Advanced Materials for Energy Storage, Chulalongkorn University, Bangkok, Thailand Jaekook Kim Department of Materials Science and Engineering, Chonnam National University, Gwangju, South Korea M. Kovendhan Department of Nuclear Physics, University of Madras, Chennai, Tamilnadu, India M. A. Krishnan Department of Mechanical Engineering, Amrita Vishwa Vidyapeetham, Kollam, Kerala, India Ashutosh Kumar ICAR-Indian Institute of Seed Science, Mau, Uttar Pradesh, India Lokesh Kumar Department of Physics and Astrophysics, University of Delhi, North Campus, New Delhi, India Eric Lichtfouse CNRS, IRD, INRAE, Coll France, CEREGE, Aix-Marseille University, Aix-en-Provence, France Karthikeyan Madurakavi School of Electronics Engineering, Vellore Institute of Technology, Vellore, Tamilnadu, India Nikhil Medhavi Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India Muniyandi Muneeswaran Advanced Materials Laboratory, Department of Mechanical Engineering, University of Chile, Santiago, Chile Neethu T. M. Balakrishnan Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India Asha Paul Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India Samuel Paul David HiLASE Centre, Institute of Physics of the Czech Academy of Sciences, Dolni Brezany, Czech Republic Department of Physics, Kalasalingam Academy of Research and Education, Krishnankoil, Tamil Nadu, India D. Paul Joseph Department of Physics, National Institute of Technology, Warangal, India Lauro Antonio Pradela-Filho Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil

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Contributors

Raghavan Prasanth Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India Department of Chemical and Biological Engineering and Engineering Research Institute, Gyeongsang National University, Jinju, Republic of Korea Jiaqian Qin Metallurgy and Materials Science and Research Institute, Chulalongkorn University, Bangkok, Thailand Saravanan Rajendran Department of Mechanical Engineering, University of Tarapacá, Arica, Chile R. Ramarajan Department of Physics, National Institute of Technology, Warangal, India Tamilselvan Sakthivel Department of Materials Science in Engineering, University of Central Florida, Orlando, FL, USA Balaji Sambandam Department of Materials Science and Engineering, Chonnam National University, Gwangju, South Korea Sitharthan R School of Electrical Engineering, Vellore Institute of Technology, Vellore, Tamilnadu, India Anandhi Sivaramalingam Department of Physics, School of Science and Humanities, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India Ananthakumar Soosaimanickam Institute of Materials (ICMUV), University of Valencia, Valencia, Spain Regina Massako Takeuchi Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Instituto de Ciências Exatas e Naturais do Pontal, Universidade Federal de Uberlândia, Tupã, Ituiutaba, MG, Brazil K. Thangaraju Department of Physics, National Institute of Technology, Warangal, India Arun Thirumurugan Advanced Materials Laboratory, Mechanical Engineering, University of Chile, Santiago, Chile Madan L. Verma Department of Biotechnology, School of Basic Sciences, Indian Institute of Information Technology Una, Una, Himachal Pradesh, India

Chapter 1

Metal Oxides for Rechargeable Batteries Energy Applications Balaji Sambandam, Samuel Paul David, Tamilselvan Sakthivel, Anandhi Sivaramalingam, Ananthakumar Soosaimanickam, and Jaekook Kim

Contents 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Rechargeable Batteries for Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Lithium Ion Batteries in Non-aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Principles of Lithium Ion Batteries: Pursuit for a Cathode . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Metal Oxides as Cathode Materials for Lithium Ion Batteries . . . . . . . . . . . . . . . . . . . . 1.3.3 Principles of Lithium Ion Batteries: Pursuit for an Anode . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Metal oxides as Anode Materials for Lithium Ion Batteries . . . . . . . . . . . . . . . . . . . . . . 1.4 Lithium Ion Batteries in Aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Metal Oxides for Aqueous Lithium Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Sodium Ion Batteries in Non-aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Principles of Sodium Ion Batteries: Pursuit for a Cathode . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Principles of Sodium Ion Batteries: Pursuit for an Anode . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Metal Oxides as Anode Materials for Sodium Ion Batteries . . . . . . . . . . . . . . . . . . . . . .

2 5 6 6 7 16 17 21 21 24 24 35 36

B. Sambandam (*) · J. Kim (*) Department of Materials Science and Engineering, Chonnam National University, Gwangju, South Korea e-mail: [email protected] S. Paul David HiLASE Centre, Institute of Physics of the Czech Academy of Sciences, Dolni Brezany, Czech Republic Department of Physics, Kalasalingam Academy of Research and Education, Krishnankoil, Tamilnadu, India T. Sakthivel Department of Materials Science in Engineering, University of Central Florida, Orlando, FL, USA A. Sivaramalingam Department of Physics, School of Science and Humanities, Sathyabama Institute of Science and Technology, Chennai, Tamilnadu, India A. Soosaimanickam Institute of Materials (ICMUV), University of Valencia, Valencia, Spain © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 S. Rajendran et al. (eds.), Metal and Metal Oxides for Energy and Electronics, Environmental Chemistry for a Sustainable World 55, https://doi.org/10.1007/978-3-030-53065-5_1

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1.6 Sodium Ion Batteries in Aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Metal Oxides for Aqueous Sodium Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Potassium Ion Batteries in Non-aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.1 Principles of Potassium Ion Batteries: Pursuit for a Cathode . . . . . . . . . . . . . . . . . . . . . 1.7.2 Metal Oxides as Cathode for Potassium Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Multivalent Ion Batteries for Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Nearly three decades of significant academic and commercialization progress, appreciations have to be credited for Li+ ion-based rechargeable secondary batteries, which conquered the entire world. The Li+ ion batteries dictate the consumer battery market and are considered crucial for the practical realization of plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and electric vehicles (EVs). Recently, post-lithium–ion batteries, particularly Na, K, Mg, and Zn, and Al–ion batteries have also been intensively explored for various energy storage tenders due to their natural abundance, low cost, and environmental safety of these materials. The utilization of metal oxides in battery application is tremendous and, an example, the first commercial lithium ion batteries by Sony Co. with LiCoO2 as a cathode. Recently, Ni-rich layered oxide-based lithium ion batteries are on an edge of commercialization. The focus on battery research had increased drastically from 2010, and still metal oxide-based cathodes/anodes are researched exclusively due to their significant physicochemical properties. This chapter emphasizes electrochemical properties of various metal oxide-based electrode materials for various secondary rechargeable energy storage applications including sodium ion batteries (SIBs), potassium ion batteries (PIBs), and zinc ion batteries (ZIBs). Keywords Metal oxides · Rechargeable batteries for energy storage · Lithium ion batteries · Post lithium ion batteries · Non aqueous and aqueous electrolytes

1.1

Introduction

Energy storage/conversion technologies have become a crucial research topic toward sustainable living in present-day society. The existing issues of oil price instability, fossil fuel resource depletion, geopolitical concerns, energy scarcity, and global warming indicate that renewable energy derived from geothermal, solar, hydro, and wind power will remain vital to mitigating climate change and replacing natural resources (Larcher and Tarascon 2015). The electrochemical energy production is an alternative serious consideration for energy/power source with respect to fossil sources, as long as this energy consumption is considered to be more sustainable and more environmentally friendly. Energy storage systems (ESSs) have

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become critically important for the effective utilization of the electricity generated from these renewable sources. Electrical energy storage systems (EESs) are broadly classified into electrochemical systems (batteries, fuel cells, and super capacitors), kinetic energy storage systems (flywheel systems), and potential energy storage systems (hydro/compressed air storage) (Gür 2018). Among the electrochemical renewable energy resources, battery is an electrochemical device where chemical energy is converted into electrical energy and vice versa. A simple battery consists of an anode, a cathode, and an electrolyte. The anode is a source of ions being reversible stripping/plating with electrolyte. The cathode is a sink for the ions reversible intercalate, and it can be chosen by optimizing a various parameters. The electrolyte offers for the separation of ionic/electronic transports, and it can be liquid either aqueous or organic-based (non-aqueous) liquid (Whittingham 2004). The cell potential can be derived by the difference between the chemical potential of the anode and cathode, ΔG ¼  nEF, where ΔG is the net useful energy available from a given reaction; n, E, and F represent number of electrons involved, Faraday constant, voltage of the cell with specific chemical reaction, respectively. Thus, batteries are closed systems, in which both anode and cathode being the charge transfer medium and main process of “redox reaction” occur on cathode. Batteries are categorized mainly into two different classes: (i) primary batteries are used for only one time, and these batteries are discarded after they discharged fully as they are assembled in charged state; (ii) secondary batteries, on the other hand, can be discharge/charged continuously for several times; thus the active material restores its original condition. This can be done by reversing the current flow through the cell after discharged. The best examples for primary batteries are the Daniell cell (zinc and copper electrodes), carbon–zinc batteries, and zinc–air batteries. On the other hand, lead–acid batteries, nickel–cadmium batteries, and lithium ion batteries are best examples for rechargeable secondary batteries. The history of batteries is important to understand how the timeline improves the technology in the electronic market. The first practical battery is the voltaic pile in 1800 introduced by Alessandro Volta, using stacked discs of Cu and Zn separated by cloth soaked in salty water, which produces 0.76 V per set of Cu/Zn in presence of an electrolyte. In 1866, Leclanché battery was developed by using Zn anode, and a MnO2 cathode wrapped in a porous material immersed in NH4Cl solution as an electrolyte. In 1899, Junger invented first Ni-Cd rechargeable battery containing nickel/cadmium electrodes soaked in an aqueous solution of KOH. This is the first battery with an alkaline electrolyte, which in turn performs better than the lead–acid battery. Later, a most important finding of lead–acid battery, a first practical rechargeable battery, was introduced in 1959, and this technology is still being utilized in internal combustion engine cars. Although the battery cost is low and high specific power of 180 W Kg-1 with nearly 60–90% efficiency, the major disadvantages are the loss of H2SO4 in the electrolyte and the formation of water at the end of discharge, low energy-to-weight ratio, freezing problem of electrolyte during winter, which make this technology not suitable for higher end applications. Later in the 1950s, alkaline electrolyte-based primary and secondary batteries dominated in the field of energy storage devices; the first modern primary alkaline battery was patented by the Canadian scientist Lew Urr in 1959. This battery

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technology was utilized by Ever Ready and Duracell between 1968 and 1970, and still they occupy the electronic market. An important breakthrough of rechargeable lithium ion battery technology in energy storage field was introduced by Whittingham (1974–1976) using TiS2 as cathode and Li metal as anode (Whittingham 1976) followed by Godshall (Godshall et al. 1980) and Goodenough (Mizushima et al. 1980) using LiCoO2 as cathode and graphite anode, as this technology mitigates relative disadvantages from the earlier invention of all types of batteries. Later, alkali ions (Na+, K+) and alkaline earth metal ions (Ca2+ and Mg2 + ) are introduced. In addition, other important rechargeable secondary batteries including Mg2+, Zn2+, and Al3+ ions batteries are also introduced in energy storage applications in later 2000. Supercapacitors, on the other hand, may or may not work through redox reaction (Faradaic process through oxidation/reduction reactions); however, by specific orientation of electrolyte ions at the electrolyte/electrode interface, electrical double layers (EDLs) are continuously formed and released, which produce electrons in the external circuit, as an example for non-Faradaic process (Winter and Brodd 2004). In specific, electrochemical supercapacitor is an electrochemical device capable of storing and delivering high power electricity quickly and for a long running time. This device attains greater energy densities while still maintaining the characteristic high power density of conventional capacitors. The taxonomy of supercapacitors, depending on the Faradaic processes, can be classified as electrical double-layer capacitor, pseudocapacitors, and hybrid capacitors. Each above said class is categorized by its mechanism, respectively, as non-Faradaic, Faradaic, and the combination of two. In an observation of two different separate charge layers on the surface of the electrodes, supercapacitors are often called as double-layer electrical capacitors or electrical double layers. The most common electrolytes for supercapacitors are aqueous electrolytes such as H2SO4 and KOH and organic electrolytes in organic medium including acetonitrile. In contrast to the closed systems of batteries and supercapacitors, fuel cells are open systems where the anode and cathode mimic as charge transfer media and the redox reaction delivered from the outside of the cell, for example, it takes oxygen from air. It infers that as long as there is a fuel and oxygen, the fuel cells continuously produce electricity. Thus a battery/supercapacitor can store energy, while a fuel cell produces energy by converting available fuel. To evaluate the practical suitability, energy density (amount of energy be stored in a given mass) and power density (amount of energy given off in a given mass) need to be addressed in the form of a plot, known as Ragone plot. Thus conventional capacitors/supercapacitors have relatively very high power densities as shown in Fig. 1.1 but relatively low energy densities compared to electrochemical devices of batteries and fuel cells. In specific, battery can store more energy than a capacitor/supercapacitor; however, it cannot deliver the energy quickly, revealing a low power density of the former. Therefore batteries are able to store more energy, but capacitors/supercapacitors can give off the stored energy more quickly. Transition metal-based oxides have played a vital role in energy storage technologies. All electrochemistry research breakthroughs are demonstrated through metal oxides due to ease of reducible/oxidizable ions within them. This chapter is focused

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on how transition metal oxides are involved in a modern secondary rechargeable ion battery for energy storage application including lithium ion batteries, sodium ion batteries, potassium ion batteries, and zinc ion batteries.

1.2

Rechargeable Batteries for Energy Applications

Though the primary batteries still occupy the market, they do not defiantly fulfil the current requirement to run electric vehicles. In specific, electrical energy storage is critical to support electronic, vehicular, and load-levelling applications, renewable solar, and wind power. Energy storage systems (ESSs) become crucially important for an effective utilization of the electricity generated from these renewable sources. Rechargeable batteries are most prominent for ESSs due to a large number of portable electronic devices depend on exploiting the chemical energy stored in them. Especially, lithium ion batteries are currently occupying the portable electronic market devices such as cellphones, laptops, and other digital tool recorders and are competitive power sources for vehicular transport, such as pure electric vehicles, plug-in hybrid electric vehicles, and hybrid electric vehicles. Whittingham, the pioneer of lithium ion batteries technology, showed layered crystal structure of TiS2 and Li metal, respectively, utilized as cathode and anode, for Li+ ions storage. He called this storage phenomenon as “the intercalation mechanisms.” Later, TiS2 cathode paired with Li or Li-Al alloy as anode forming Li/TiS2 system in a well known non-aqueous electrolyte medium, which was commercialized by Exxon in later 1970s with specific energy of 130 Wh Kg-1. Although it shows high intercalation capacity (240 mAh g-1), the voltage was

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relatively low (~2 V vs Li+/Li) and instability as it spontaneous releases H2S gas from TiS2 upon contact with moisture (Whittingham 2004). Thus safety and stability of Li metal-based anode were still in great concern. Later, in 1980, Godshall and Goodenough independently demonstrated a high voltage LiCoO2, in specific, of the latter utilized by Akira, who described Li metal-free anode such as graphite for first time (Mizushima et al. 1980). Thus, in 1991 Sony commercialized first high power demsity, small size rechargeable lithium ion batteries. This is a crucial period in the development of lithium ion batteries. Finally, these scientists, Whittingham, Goodenough and Akira were won the Nobel prize in chemistry in 2019 for this outstanding invention to make a renewable energy world. Alkali metal ions such as Li and Na with monovalent charge carriers are reversibly inserted/extracted during discharge/charge, respectively, with corresponding ions containing electrolytes and respective metal anode, to make electrochemical half-cells. These cells named as lithium ion batteries and sodium ion batteries, respectively, for lithium and sodium ions containing electrolytes. Similarly K (monovalent), Mg, Zn with divalent charge carriers, and Al with trivalent charge carriers, respectively, are named as potassium ion batteries, magnesium ion batteries, zinc ion batteries, and aluminum ion batteries. In comparison to fuel cells and supercapacitors, rechargeable batteries have found more commercial applications and established well in the electronic market.

1.3 1.3.1

Lithium Ion Batteries in Non-aqueous Electrolytes Principles of Lithium Ion Batteries: Pursuit for a Cathode

Lithium is the lightest alkali metal among the elements in the periodic table. Lithium-based lithium ion battery is a type of rechargeable secondary battery in which lithium ions move from the anode (negatively charged electrode) to the positive electrode (cathode) during discharge and reverse process taken when charging. The first commercial lithium ion batteries were developed by Sony Co., Japan, about 30 years ago, with LiCoO2 as the cathode and graphite as anode in a non-aqueous Li+ ion containing electrolyte. In specific, the half-cell reaction at cathode (LiCoO2 $ CoO2 + Li+ + e) and half-cell reaction at anode (C6 + Li+ + e $ LiC6) in presence of non-aqueous electrolyte containing Li+ ions. During this process, Li+ ion diffusion can be noted within the solid electrode, charge transfer occurred at the interface between the electrodes and electrolyte, the ions transport through the electrolyte, and this process is completely reversible when reversed. The electrolyte of lithium ion batteries, in general, may be in a form of crystalline solid, a liquid, a glass, or a polymer. Among these electrolytes, liquid or polymer electrolyte is preferred due to poor interface contact in solid electrolytes (Goodenough 2013). The liquid-based electrolytes can be classified as (i) non-aqueous and (ii) aqueous electrolytes. The most common non-aqueous

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electrolyte is lithium hexafluorophosphate (LiPF6) dissolved in organic carbonates; a mixture of ethylene carbonate with either dimethyl carbonate, propylene carbonate, diethyl carbonate, and/or ethyl methyl carbonate. This combined ratio can withstand high voltage, an added advantage of non-aqueous electrolytes in compared with aqueous electrolytes, which are made up of common Li-based salts such as LiSO4 in water. Since its introduction into the market, the lithium ion batteries technology in non-aqueous electrolytes opened up a new door for portable/mobile electronics applications owing to high energy density, less weight, long service life, and high efficiency (Goodenough and Kim 2010; Whittingham 2008). Since now, metal oxides alone are commercially utilized for mobile phone, and to the best knowledge of all electrochemical research, lithium ion batteries technology have occupied the entire communication system till to the date.

1.3.2

Metal Oxides as Cathode Materials for Lithium Ion Batteries

Cathode materials mimic as a heart of a human body; the major role is to select the cathode material and electrolyte for functioning a better battery life. Metal oxides used as cathode for lithium ion batteries are generally categorized by two types: (i) lithium containing metal oxides like LiCoO2, LiMn2O4, and LiNi1/3Co1/3Mn1/3O2 (NCM) and (ii) transition metal oxides such as V2O5. The lithium containing metal oxides are further classified based on their crystallographic structures, layered metal oxides LiMO2, and spinel oxides with general formula of LiM2O4.

LiMO2 (M = Co, Ni, Mn)-Layered Cathode LiMO2 (where M ¼ Co, Ni and Mn), in general, has space group R3m with lithium and metal ions located in octahedral 3a and 3b sites, respectively, separated by layers of cubic close-packed oxygen ions as shown in Fig. 1.2a The unit cell of the layered form consists of three slabs of edge-sharing CoO6 (if M ¼ Co) octahedra separated by interstitial layers of Li. LiCoO2 is the first successful cathode material for lithium ion batteries because of its high ionic and electronic conductivities, though the practical capacity is nearly half of the theoretical value of 274 mAh g-1. This is due to phase changes from hexagonal to monoclinic when nearly 50 % of the lithium has been extracted from the parent structure (Whittingham 2004) while charging. The layered structure obtained at high temperature displays better electrochemical performance than a cubic spinel structure, which is achieved at low temperature (Czyżyk et al. 1992). During charging, lithium ions extracted from the layered LiCoO2 crystal lattice, a nonstoichiometric Li1-xCoO2 compound is formed, and the oxidation state of Co3+ changes with respect to charge compensation in the potential window of 3.5–4 V vs Li+/Li in propylene carbonate non-aqueous electrolyte. However, in order to

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Fig. 1.2 (a) Crystal structure of LiCoO2. The structure is common with general formula of LiMO2, where M ¼ Co, Mn, and Ni (the figure was obtained from Daniel et al. 2014); (b) discharge–charge profiles of bare LCO (LiCoO2, top) and 2% LAF-LCO (Li-, Al-, and F-modified LiCoO2, bottom) electrodes at selected cycles within a voltage range of 3.0–4.6 V (vs Li+/Li) at current density of 27.4 mA g1 for lithium ion batteries; (c) corresponding cyclability comparison at a given applied current density of 27.4 mA g1 . (Reprinted with permission from Qian et al. 2018)

completely remove the Li+ ions during charging, the potential window should be widened up to high voltage of 5 V vs Li+/Li, which is quite difficult due to oxidation of organic electrolyte and instability the cathode. Ohzuku and Udea (1994) succeeded with nearly 85% lithium from LiCoO2, revealing a monoclinic phase transformation. Furthermore, Reimers and Dahn (1992) have mentioned that excessive amount of Co4+ (CoO2) on successful removal of lithium during charge process, which destroys the structure of the parent. A complete lithium extraction and nearly 95% (at 4.2 V) insertion have been witnessed, when cycled between 5.2 and 3.0 V vs Li+/Li in 1 M LiPF6 (with 1:1 ration ethylene carbonate and dimethyl carbonate) electrolyte (Amatucci et al. 1996), which reveals nearly 5% irreversible capacity loss for every cycle. Though the output remained half the theoretical capacity, the initial research on LiCoO2 opens a wide door for a free selection of anode materials due to its lithiated phase. Very recently, Jeong et al. (2019) demonstrated a high discharge capacity of 172.5 mAh g-1 at 14 mA g-1 (0.1C) current density in a potential domain of 3.5–4.4 V (vs Li+/Li) at 25 oC. Furthermore, in order to extend the upper potential, capacity, and structural stability, electrochemistry researchers often do surface coating with metallic

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compounds (metal oxides, phosphates, fluorides) or lithium compounds (Li2Co3, LiAlO2, LiNi0.5Mn1.5O4), doping with electrochemically inactive elements on cathode materials. These processes can directly hinder the contact between the cathode and electrolyte solution, thereby delaying a structural degradation. Qian et al. (2018) reported modified LiCoO2 with Li, Al, and F (denoted as LAF-LCO) as cathode and Li metal as an anode in 1 M LiPF6 in ethyl carbonate/diethylene carbonate (ethylene carbonate/diethyl carbonate, volume/volume ¼ 1:1, 45 μL) with enhanced performance at high cut-off voltage of 4.6 V. The electrochemical charge–discharge profile of LAF-LCO in Fig. 1.2b (top) shows uniform egress/ingress mechanism when compared with bare LCO (Fig. 1.2b (bottom)), revealing the role of surface modification, their corresponding cyclability patterns, in which a capacity of 81.8% after 200 cycles as shown in Fig. 1.2c was noted for surface modified LAF-LTO cathode. LiNiO2 is another important layered structure like LiCoO2, widely used for lithium ion batteries as cathode since from 1990s (Dahn et al. 1990; Rougier et al. 1996; Bianchini et al. 2018). It has own advantage due to high operating voltage of 4 V, high theoretical capacity of 274 mAh g-1 and energy density, and especially low production cost compared to LiCoO2. The major obstacle of LiNiO2 for commercialization as it requires extreme care for preparation, else it suffers from cation mixing since both Ni2+ and Li+ have almost similar ionic radii, which leads to a non-stoichiometric composition of Li1-xNi1+xO2 (0 < x < 1), exhibiting lower initial capacity and severe capacity loss upon cycling (Dahn et al. 1990). In other words, presence of stable lithium oxide and Ni2+ in LiNiO2, leading to a non-stoichiometry phase. In other words, occupation of Ni2+ ions in the lithium sites strongly hinders the Li+ diffusion; thereby the electrochemical performances decreased significantly. And it is almost a tough job to prepare stoichiometry LiNiO2 because of the ease of formation of cubic rock salt NiO domain. Additionally, formation of NiO2, an electrochemically inactive phase irreversibly formed when charging LiNiO2 above 4 V, which is further easily transformed to a more stable phase of NiO. This unwanted reaction phase, leading to high interfacial impedance thereby a poor electrochemical performance (Arai et al. 1995). Unlike LiCoO2, several reversible phase transitions in LiNiO2 during charge– discharge process can lead to severe capacity fading. Figure 1.3a explains the galvanostatic charge–discharge skeletons of three different voltage windows for LiNiO2 at a low current density of 18 mA g-1 (0.1 C) with corresponding longterm cyclability over 100 cycles at 0.5 C rate as represented Fig. 1.3b. There are four phase transitions including three individual transitions, namely, a rhombohedral phase (H1), a monoclinic phase (M), and a rhombohedral phase (H2); one two-phase coexistence region; and two rhombohedral phases (H2 + H3). Especially H3 phase formation is responsible for degradation due to large change in c axis parameter in H2 and H3 phases as H3 phase is solely brought by Ni4+ (Tabuchi et al. 2016; Ohzuku et al. 1993). This has been explained through the derivatives curve analysis (differential capacity vs voltage curves as shown in Fig. 1.3c–e. The electrochemical charge–discharge profiles and corresponding cyclability pattern at different potential windows clearly infer that the structural damage on LiNiO2 when extended to the upper cut-off potential above 4.1 V, although the registered

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Fig. 1.3 (a) Galvanostatic charge–discharge curves for LiNiO2 cathode at different upper cut-off voltages at 0.1 C (18 mA g–1) rate for lithium ion batteries (b) corresponding cycling performance at 0.5 C (90 mA g–1) for the above said three different voltage windows, differential capacity vs voltage curves with upper cut-off voltages of (c) 4.1, (d) 4.2, and (e) 4.3 V. H and M represent, respectively, for rhombohedral and monoclinic phases (Reprinted with permission from Yoon et al. 2017)

capacities are of quite high (4.2 and 4.3 V). Thus the registered initial discharge capacities were 179, 227, and 247 mAh g-1, respectively, at 4.1, 4.2, and 4.3 V in their specified cut-off voltages (Yoon et al. 2017). Interestingly, after 100 cycles, the retained discharge capacities of 95, 81, and 75%, respectively, for 4.1, 4.2, and 4.3 V were registered, revealing a structural stability when cut-off upper potential window was at 4.1 V. However, Li et al. (2018a) carefully analyzed the structure and electrochemical stability in 1 M LiPF6 (most common electrolyte for lithium ion batteries in ethylene carbonate: diethyl carbonate (1:2v/v) ratio) through operando XRD investigation. It was found that slow H1 single-phase and H1+M phase transition during first charge and the development of kinetic hindrance, most probably due to a slow H1 phase Li+ diffusion, in extended charge–discharge cycling was noted. Through this study, the phase transition (H1+M) was found to cause kinetic hindrance which is quite different from H3 phase transition s solely responsible for degradation mechanism as suggested by Yoon et al (2017). These different situations need to be studied in detail in order to eliminate the hindrance. Furthermore, to attain a maximum capacity and to shift the upper cut-off voltage above 4.3 V for gaining high energy/power densities, an excess of Li needs to be employed during preparation (Tabuchi et al.

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2016; Ohzuku et al. 1993). Both LiCoO2 and LiNiO2 tend to undergo phase transition from hexagonal to cubic phase, which is electrochemically inactive. Interestingly, this phase transition in LiCoO2 is almost reversible, whereas it is partially reversible and slow for the latter. LiMnO2, yet another candidate with theoretical capacity of 285 mAh g-1 based on a Mn3+/Mn4+ redox couple, registered highest among the layered structures. Depending on irreversibility and cation mixing, the registered capacity varies, and only 60–70% of its theoretical capacity has been achieved. Layered LiMnO2 has two crystal structures: a monoclinic phase which is less thermodynamic stable than an orthorhombic phase. In addition, the effect of (Jahn-Teller) J-T distortion of Mn3+ in LiMnO2 can strongly undergo disproportion reaction of 2Mn3+! Mn4+ + Mn2+, in which Mn2+ highly soluble in the electrolyte. A one-step flux method synthesis of LiMnO2, by Tong et al. (2018), exhibits a very high discharge capacity of ~191 mAh g-1 and reversible capacity of 162 mAh g-1 after 50 cycles at a rate of 0.1 C. In addition, the layered zigzag orthorhombic phase easily translates into a spine-like phase due to trivalent Mn migration during electrochemical charging, which creates an unfavorable environment (Sato et al. 2018). The two-voltage plateaus around 3 and 4 V during cycling defines a characteristic feature of spinel phase formation. Chemical substitution at the metal site, an alternate to the above said cathodes, has been intensively studied up to date and succeeded as well. In order to improve energy and power densities of these candidates (LiCoO2, LiNiO2, and LiMnO2), substitution approach in which electrochemically active metal ions are being partially replaced by either electrochemically active/inactive metal ions without loss of their (typical α-NaFeO2) layered frame, to stabilize the structure as well as gaining a maximum capacity. To achieve this, one must be aware of fundamental chemistry of oxidation states with ionic radius of the metal ions. Among these materials, LiNiO2 has played a crucial role because of the high potential of Ni3+/Ni4+ vs Li+/Li. Thus based on substation chemistry on LiNiO2, a new candidate, LiNi1/3Co1/3Mn1/3O2, coined as NCM111 with valences of the Ni2+, Co3+, and Mn4+, was first introduced as an alternate positive electrode material in 2001 by Tsutomu and Yoshinari (2001). In other words, a combination of LiNiO2, LiCoO2, and LiMnO2 forms a layered structured. Thus when cycled between 2.5 and 4.3 V vs Li+/Li, NCM333 registered a capacity of ~ 150 mAh g-1, while increasing cut-off voltage to 4.6 V, it utterly gives 200 mAh g-1 capacity; however, it costs the cyclability. In this material, only the divalent nickel and trivalent cobalt ions are electrochemically active, comprising Ni2+/Ni4+ and Co3+/Co4+ redox couples, whereas Mn4+ supports the layer structure without involving the electrochemical reaction. Interestingly, few of these candidates, depending on the Ni content, already employed in electric vehicles, especially LiNi0.5Co0.2Mn0.3O2 (NCM523). Interestingly, Kim et al. (2019) studied different cathodes of LiNi0.6Co0.2Mn0.2O2 (NCM622), LiNi0.8Co0.1Mn0.1O2 (NCM811), and LiNi0.9Co0.05Mn0.05O2 (0.9:0.5:0.5) at different upper cut-off potential of 4.3, 4.4, and 4.5 V. Typical charge–discharge curves for these cathodes at 0.1 C applied current density for different cut-off potentials are given in Fig. 1.4a, b. The cycling stability of these coined cells at three different potential windows of 2.7–4.3, 2.7–4.4, and 2.7–4.5 V run at 0.5 C is shown in Fig. 1.4c–e, revealing their stability during initial 100 cycles.

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Fig. 1.4 (a) Galvanostatic charge–discharge comparison of NCM-622 (4.5 V) and NCM-811 (4.3 V) at 0.1 C; (b) comparison of NCM-811 (4.5 V) and NCM-90 (4.3 V) at same current density of 0.1 C (18 mA g1), cycling performance of NCM cathodes tested vs Li+/Li cycled at lower fixed voltage of 2.7 V and upper varied voltages of (c) 4.3 V, (d) 4.4 V, and (e) 4.5 V at a current density of 0.5C (90 mA g1); (f) relationship between the capacity retention and discharge capacity of all NCM cathodes with different upper cut-off voltages of 4.3, 4.4, and 4.5 V (Reprinted with permission from Kim et al. 2019); (g) evolution of the layered cathode begins from LiCoO2 in the early 1980s to cationic substitution within the metal layers (left) with (i) partial replacement of Co with Ni and Mn (NMC phase) within the metal layer (purple) and (ii) more recently with Li (yellow) to form Li-rich NMC phases. NCM-622, LiNi0.6Co0.2Mn0.2O2; NCM-811, LiNi0.8Co0.1Mn0.1O2; NCM-90, LiNi0.9Co0.05Mn0.05O2 (Reprinted with permission from Rozier and Tarascon 2015)

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It is highlighted that LiNi0.6Co0.2Mn0.2O2 cycled at an upper cut-off voltage of 4.5 V and LiNi0.8Co0.1Mn0.1O2 at 4.3 V remarkably show a similar initial reversible capacity of ~ 200 mAh g-1 and capacity retention of 93% after 100 cycles. LiNi0.9Co0.05Mn0.05O2, on the other hand, registered highest initial capacities in all three potential windows, due to high Ni content with respect to other counterparts of NCM622 and NCM811, and deliberately displays a drastic capacity fading. The overall discharge capacity and capacity retention of all coined cells at three different working potential are related by a graph as shown in Fig. 1.4f. Eventually, LiNi0.8Co0.1Mn0.1O2 has displayed highest capacity retention in the wide potential window of 2.7–4.5 V, suppressing LiNi0.6Co0.2Mn0.2O2 and LiNi0.9Co0.05Mn0.05O2. Unfortunately, the implementation of high nickel containing NCM still needs to overcome a number of challenges for a practical application. Although NCM has their own demerits, this composite will certainly become a gifted material for electric vehicles applications. Figure 1.4g shows how this battery technology improved in the timeline of 1990–2014 (Rozier and Tarascon 2015). If excess of Li purposely added during preparation of NCM, use to call as Li-rich NCM, where some Li ions are residing in the transition metal layers containing Ni, Co, and Mn, in addition to the Li present in the van der Waals gap, exhibit very high capacity of more than 270 mAh g-1. This situation demonstrates a poor kinetic reaction and thereby causes a deprived cycling performance. In addition, O2 migration from the bulk to the surface, transition metal ions move from surface to bulk, and their over oxidation and reversible oxygen oxidation and transition are major controversial issues for their capacity fade (Boulineau et al. 2013). These unwanted reactions are partially addressed by surface coating with different materials such as ZrO2, carbon, TiO2, MgO, Al2O3, and Ni-Mn-based electrochemically inactive composite oxides which have been extensively studied to improve the performance which eventually retains the crystal structure during the course of state of charge (SoC) and depth of discharge (DoD) (Shi et al. 2013). In addition, AlF3, one of the most effective coating agent, stands against HF, which is formed frequently from LiPF6, a most common electrolyte used for battery testing (Yang et al. 2012). Thus Li-rich NCM of Li(Li0.17Ni0.29Mn0.58)O2 with AlF3 coating extensively delivered high capacity (Li et al. 2012a, b) than without Li-rich NCM, as shown in Fig. 1.5a. The corresponding cyclability for the initial 50 cycles at 0.2 C rate. This can be clearly seen in Fig. 1.5b. LiM2O4 (M = Co, Ni, Mn) Spinel Cathode A high energy density can be derived from either by high voltage and/or high capacity. Metal oxide-based spinel with general formula of LiM2O4, where M represents transition metals, for lithium ion batteries application, provides a high energy density as they operate at very high operating voltage of ~ 4.7 V vs Li+/Li. The interstitial space of the spinel network offers 3D channels for high-rate Li+ accommodation, thereby ease of Li+ insertion/de-insertion during electrochemical reaction.

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LiMn2O4, a most attractive spinel cathode for lithium ion batteries with a high working voltage of 4 V, was first proposed by Michael Thackeray in the early 1980s (Thackeray et al. 1983). Due to its low cost, environmental friendly, and safer alternative, LiMn2O4 spinel was studied well. A unique MnO2 framework in the spinel structure provides a 3D diffusion pathway for Li+ ions and forms a stable cubic [Mn2]O4 framework when charging LixMn2O4. However, it was very hard to extract all lithium from the framework practically. Compared to layered cathodes, manganese-based spinel oxides show severe capacity fading due to the following reasons: (i) by the disproportion reaction 2Mn3+! Mn4+ + Mn2+, in which Mn2+ highly soluble in the electrolyte, and (ii) by the factor of Jahn–Teller distortion of effect of Mn3+; an irreversible structural transformation from a spinel to a tetragonal structure is highly feasible (Gummow et al. 1994). Although LiMn2O4 cathode is suitable for both electric vehicles and plug-in hybrid electric vehicles, due to its low

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energy density and instability behaviour at high working temperatures, making this material not suitable for commercial applications. Various strategies were utilized to overcome the above issues including the Jahn–Teller distortion, and transition metalbased Ni doping is a good option. Thus LiNi0.5Mn1.5O4 with high operating voltage of ~ 4.7 V vs Li+/Li and high theoretical capacity of 148 mAh g-1 is interested by many researchers (Ma et al. 2016; Kim et al. 2014). By substituting transition metal ions, Chunsheng Wang and co-workers reported a high reversible 5.3 V battery with average discharge voltage of 4.8 V, using LiCoMnO4 cathode in 1 M LiPF6 in fluoroethylene carbonate/bis(2,2,2trifluoroethyl) carbonate/hydrofluoroether (FEC/FDEC/HFE, 2:6:2) with 0.02M LiDFOB (lithium difluoro(oxalate)borate) additive (Chen et al. 2019). Significantly, an energy density of 720 Wh Kg-1 over 1000 cycles, and as a full cell, LiCoMnO4| graphite, an energy density of 480 Wh kg1 for 100 cycles were noted. The recovered capacity of ~ 152 mAh g-1 was higher than the theoretical capacity of 148 mAh g-1 originated from LiMnO3 coating on the surface to avoid any dissolution. Figure 1.6a shows the galvanostatic charge–discharge profile for Mn3+-free LiCo3+Mn4+O4, in which Co3+/4+ is a major redox reaction. The corresponding cyclability patterns for Li metal (half-cell, Fig. 1.6b top) and pre-lithiated graphite as anodes (as full cell, Fig. 1.6b bottom), depict the stability of the electrode in a standard electrolyte with new additive. The structural investigation on LiNi0.5Mn1.5O4/Li half-cell during first charge by Lin et al (Lin et al. 2015), revealing a local atomic-level migration of transition metal ions into tetrahedral Li sites to form a Mn3O4 spinel. Interestingly, subsurface particles display migration of transition metal ions into empty octahedral sites to form a rocksalt-like structure. This migration leads to dissolution of Mn/Ni ions and building up high charge transfer resistance, which suggestively contributes for capacity degradation. This reveals unbalanced charge–discharge capacity of 147/136 mAh g-1 during first cycle with limited coulombic efficiency of 92% as shown in Fig. 1.6c; later the degradation is controlled and effectively shows a good cycling performance over 100 cycles (Fig. 1.6d). Very recently, Ariyoshi et al. (2019) developed a nickel substituted Li2Co2O4 (Li2Ni0.2Co1.8O4) spinel with a reversible capacity of 100 mAh g-1. This cathode exhibits a zero strain during electrochemical reaction and provides oxidation/reduction couple at 3.69–3.48 V vs Li+/Li with a plateau at 3.5 V, in which both Co and Ni ions are electrochemically active. Although spinel-based metal oxides are stabilized well for lithium ion batteries as cathode, the registered specific capacity is not up to the level of layered metal oxide-based cathodes. In specific, spinel cathodes provide higher average voltage than layered cathodes; indeed, the specific capacity of the latter competes over the former.

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Fig. 1.6 (a) Galvanostatic charge–discharge pattern for LiCoMnO4 at a current rate of 0.1 A g-1 cycled between 3 and 5.3 V vs Li+/Li with an inset of initial two cycle cyclic voltammetry curves at 0.3 mV s-1; (b) the cyclability performance of this cathode against Li anode (Li+/Li, half-cell) in an electrolyte of 1 M LiPF6 + 0.02 M LiDFOB in fluoroethylene carbonate/bis(2,2,2-trifluoroethyl) carbonate/hydrofluoroether at the current rate of 1 A g-1 over 1000 cycles (top) and performance against pre-lithiated graphite as full cell (bottom) at 1C (reprinted with permission from Chen et al. 2019); (c) initial charge–discharge profile of LiNi0.5Mn1.5O4/Li half-cell; (d) related cyclability domain for LiNi0.5Mn1.5O4/Li half-cell over 100 cycles (reprinted with permission from Lin et al. 2015)

1.3.3

Principles of Lithium Ion Batteries: Pursuit for an Anode

In a full cell diagram, the negative electrode called as “anode” being separated by a membrane which is soaked in a lithium containing electrolyte with the positive electrode called as “cathode.” During electrochemical charge reaction, at the anode side, lithium ions move out from the cathode and inserted on the anode when electrical current applied. On the other hand, during discharge, a reverse reaction is occurred in which those inserted lithium ions come out from anode and travel back to cathode through the pool of electrolyte containing lithium salt(s).

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Metal oxides as Anode Materials for Lithium Ion Batteries

In order to control the cost and a need to replace the Li metal as anode, researchers focused on suitable candidates after the introduction of lithiated graphite as anode in 1980 to replace Li metal by Mizushima, Goodenough, and co-workers (Mizushima et al. 1980). Notably, due to low lithiation potential of graphite, relatively low capacity of 372 mAh g-1 with intercalate only one lithium per six carbon as LiC6 and slow diffusion rate, intentionally researchers were fond of looking for alternative anode materials. Later in the 1990s to till date, a plenty of anode materials were subjected for electrochemical performance. Among them, metal oxides are continuously considered for lithium ion batteries application due to their variable oxidation states that can be reversibly reduced and oxidized by the following general equation. M x Oy þ 2yLiþ þ 2ye , xM þ yLi2 O Unlike cathode for lithium ion batteries, the electrochemical mechanism of these anodes can be considered into three different classes based on metal oxides crystallographic structure. (i) Conversion reaction: metal oxides such as Co3O4, MoO2, Fe2O3, and NiO with moderate theoretical capacities (ii) Intercalation reaction: Intercalated compounds such as Li3VO4, LiV3O8, Li4Ti5O12, and TiO2 with high theoretical capacities (iii) Alloy reaction: oxides such as SiO2, SnO2, and GeO2 and their mixed oxide derivatives with very high theoretical capacities Among the above said mechanisms, the conversion-based mechanism is well followed by most of the metal oxides as anode for lithium ion batteries application (Cao et al. 2017). However, major factors such as low electronic conductivity, voltage hysteresis, and volume expansion affected their performances. In specific, the low conductivity limits the electron transfers, voltage hysteresis hinders energy efficiency which is caused by voltage difference between discharge and charge depths, and volume expansion damages the crystal structure of the active materials. Rather, it is important to optimize the desired morphology which controls the volume expansion and to optimize the fabrication method of the electrode with carbon and binder sources to improve the structural stability during electrochemical reaction. A first conversion mechanism-based CoO anode for lithium ion batteries was introduced by Poizet, Tarscon, and co-workers in 2000 (Poizot et al. 2000). As an example, Liu et al. (2016) reported Fe3O4@carbon nanocubes prepared through an etching-in-a-box strategy with different time by HCl. These yolk–shell cubes exhibit a very high retained capacity of 470 mAh g-1 after prolonged 8000 cycles at 10 A g-1 as represented in Fig. 1.7a This is an example for improved cycling lifespan of the anode through tuned morphology, which suppresses the volume expansion during electrochemical reaction. A new synthesis route called MOF-C

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Fig. 1.7 (a) Prolonged cycling ability and the corresponding coulombic efficiency of Fe3O4/carbon at current density of 10 A g-1 (Reprinted with permission from Liu et al. 2016), (b) cyclic voltammetry of NiO anode with initial five cycles run in a voltage window of 0.02–3 V at a scan rate of 0.1 mV s-1, (c) corresponding galvanostatic discharge–charge depths at an applied current rate of 0.2 C (Reprinted with permission from Sun et al. 2013), (d) average potential/capacity comparison of metal oxides anode and other available non-oxides in the literature for lithium ion batteries application. (Reprinted with permission from Lu et al. 2018)

(metal organic framework combustion) is introduced to describe the electrochemical performance of mixed metal oxide of Co3V2O8 with microsphere morphology (Sambandam et al. 2016). From this one pot technique, the as-prepared Co3V2O8 anode delivers high specific discharge capacity of 940 mA h g-1 after 100 cycles at 1 A g-1 and a retained known capacity of 650 mA h g-1 at 5 A g-1 after 400 cycles. A

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detailed mechanism through the CV curves in Fig. 1.7b for NiO anode infers that a peak at 0.5 V in a cathodic scan of first cycle is attributed to reduction of NiO into Ni as well as the formation of solid electrolyte interphase (SEI), a layer grown up at the interface between electrode and the electrolyte due to electrolyte decomposition. In subsequent cathodic scans, this peak is shifted to 1.07 V. On the other hand, the first and subsequent anodic scans established by two broad peaks at 1.4 and 2.2 V could be ascribed to an oxidation of Ni to NiO and decomposition of solid electrolyte interface layer, respectively (Sun et al. 2013). This surface layer is common for all anode materials for both lithium and sodium ion batteries. The observation of this strong peak in CV corroborates well with a well-known plateau during first discharge, inferring a formation of stable layer of Li2O. The electrochemical discharge– charge curves for NiO nano-membrane in the initial five cycles at a current rate of 0.2 C within a voltage of 0.02–3.0 V was given in Fig. 1.7c. It displays a long plateau around 0.7 V during initial discharge, which was replaced by a sloped curve from 1.7 to 0.9 V in remaining cycles, well corroborating with cyclic voltammetry counterpart analysis. A huge loss of capacity in the first and remaining cycles due to formation of interfacial reaction between NiO electrode and the electrolyte, which is a most common phenomenon in anode materials. Similarly binary metal oxides with general formulas of AB2O4 (where A and B are transition metals), ABO4, ABO2, ABO3, and A2B3O8 (where A and B are suitable oxidation state of transition metals) are well documented for high capacity anodes for lithium ion batteries. For making full cell with suitable cathode, these anodes play a crucial role for their contribution. Thus among the anode materials used for lithium ion batteries, metal oxides infer high average voltages and relatively good capacity compared to phosphides, nitrides, alloys (Al, Zn, and Ag), all carbonaceous materials, graphene, and graphite. Figure 1.7d reveals a few exceptional cases such as of Si-, Ge-, Sn-, and Sb-based alloys deliberately providing very high specific capacities higher than metal oxides, but their average voltages are lower than the metal oxides, resulting a vital role of metal oxides in energy storage applications (Lu et al. 2018). In addition, Table 1.1 Table 1.1 Electrochemical properties of selected metal oxides as anode materials for lithium ion batteries Anode materials for lithium ion batteries Fe2O3-carbon Fe3O4-carbon CuO-carbon Co3O4-carbon Mn3O4-carbon

Electrochemical performance Retained capacity of 1027 mAh g1 after 50 cycles A reversible capacity 612 mAh g1 at 1 C with retained coulombic efficiency of 98% after 50 cycles Reversible capacity of 600 mAh g1 retains after 50 cycles at 100 mA g1 A reversible capacity of 910 mAh g1 after 100 cycles at 100 mA g1 High specific capacity of >900 mAh g1 at 40 mA g1 without capacity loss up to 50 cycles

Reference Zhu et al. (2011) Bhuvaneswari et al. (2014) Zhou et al. (2014) Lai et al. (2014) Li et al. (2012a, b) (continued)

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Table 1.1 (continued) Anode materials for lithium ion batteries ZnO-carbon MnO-carbon

NiO-carbon

MoO2

CoO

WO3 MoO3-rGO

V2O5 CoNiO2

MnMoO4 FeMnO3 NiFe2O4

MnCo2O4

Co3V2O8

Zn3V2O8

Electrochemical performance Reversible capacity of 460 mAh g1 retained after 50 cycles at 1 C 820 mAh g1 at a current density of 100 mA g1 after cycling for 1000 times at 1A g1, the specific capacity increases to 1625 mAh g1 High reversible specific capacity (1144 mAh g1) and excellent cyclability (nearly no capacity loss after 1000 cycles) Retaining 1051 mAh g1 over 100 cycles at a rate of 0.5 A g1 and 719 mAh g1 over another 100 cycles at a high rate of 5 A g1 At a rate of 1 C (716 mA g1), a reversible capacity as high as 1516.2 mA h g1 is obtained, and even when the current density is increased to 5 C, a capacity of 1330.5 mA h g1 could still be maintained Retained capacity of 803 mAh g1 at 180 mA g1 after 100 cycles After 100 cycles at a current density of 500 mA g1, it delivered 1115 mA h g1, with corresponding capacity retention of 92% 542 mAh g1 after 600 cycles at 200 mA g1 Storage capacity of 449.3 mA h g1 after 50 cycles with high coulombic efficiency at a current rate of 0.1 A g1, with good cycling stability and rate capability Reversible capacity of 1050 mAh g1 can be retained after 200 cycles at a current density of 100 mA g1 984 mA h g1 at 1.0 A g1 after 500 cycles Reversible capacity of 381.8 mA h g1 after 100 cycles at a constant current rate of 1.0 C, and when the current rate is increased to a high current rate of 5.0 C, a reversible capacity of 263.7 mA h g1 is retained Capacity as high as 610 mA h g1 even at a higher current density of 400 mA g1 with excellent electrochemical stability after 100 cycles High reversible capacity of 760 mA h g1 over 200 cycles at 200 mA g1, and 500 mA h g1 can remain after 500 cycles at 1000 mA g1 Specific capacity of 1128 mA h g1 after 200 cycles at 0.3 A g1. Retained capacity of 370 mAh g1 over 2000 cycles at 5 A g1

Reference Hsieh et al. (2013) Kang et al. (2016) Zou et al. (2016) Ni et al. (2015)

Cao et al. (2015)

Li et al. (2016) Park et al. (2015) Du et al. (2019) Liu et al. (2014) Guan et al. (2016) Cao et al. (2016) Preetham et al. (2016)

Li et al. (2013)

Wu et al. (2017) Sambandam et al. (2017)

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shows few selected metal oxides (simple and complex structures) as anode materials for lithium ion batteries application including their electrochemical performances.

1.4

Lithium Ion Batteries in Aqueous Electrolytes

Cost and safety are comparatively more important for large-scale stationary energy storage systems such as smart grids. The organic-based non-aqueous electrolytes are highly toxic, ease of combustion, and could therefore questionable about their safety while charge–discharge the batteries. Furthermore, lithium ion batteries are quite expensive and require anhydrous environment while assembling, making the system so complicated irrespective of their performances. To address these problems, aqueous rechargeable lithium ion batteries are widely researched nowadays. The first battery with the name of “Rechargeable lithium batteries” was introduced by Li, Dahn, and Wainwright in 1994 (Li et al. 1994). The idea is about to replace the costly Li sources of anode and electrolytes. Moreover, Li salts in water as solely provide a very high ionic conductivity are about 100 times higher than that of organic electrolytes, thereby ensuring a high rate capability/high specific power.

1.4.1

Metal Oxides for Aqueous Lithium Ion Batteries

Wainwright and co-workers (Li et al. 1994) have developed the first aqueous battery with LiMn2O4 cathode and VO2 (B) anode using a 5 M LiNO3 in water as the electrolyte, which provides a safe and cost-effective technology to compete with Ni-Cd and Pb-acid batteries on the basis of stored energy per unit of weight. It shows excellent reversibility in which Li+ was extracted from LiMn2O4, producing Li1+ xMn2O4; at the same time, Li was inserted into the anode, resulting in the formation of LixVO2(B), while charging. The reverse reaction was noted while discharging the cell, leading to an average voltage of ~ 1.5 V. In 2006, Wang et al. (2006) fabricated aqueous full cell with LiCoO2 cathode, LiV3O8 anode in a saturated electrolyte solution of LiNO3 with an output voltage of 1.05 V, a suitable system for safe electric vehicles. Figure 1.8a shows typical CV curves for LiCoO2 and LiV3O8 electrodes in LiNO3 electrolyte. A pair of redox peaks located at -0.19 (denoted as red. 1) and 0.098 V (ox. 1) with average redox potential of -0.046 V versus standard calomel electrode for LiV3O8 is still below the range of -1.0 V vs SCE (standard calomel electrode) for hydrogen evolution reaction, revealing a suitable stable anode. Similarly, LiCoO2 exhibits pair of Li+ intercalation/de-intercalation redox peaks at 0.8 (red. 2) and 1.35 V (ox. 2) with an average voltage of 1.075 V standard calomel electrode, standing in a comfortable position and much lower than the oxygen evolution peak at 1.8 V vs standard calomel electrode. Thus the average potential of the electrode (1.075 V  (0.046 V)) 1.121 V is much higher than 1.05 V derived from typical electrochemical charge–discharge profile, which required further investigation. Likewise, Tang et al. (2010a) projected nanosized LiCoO2 cathode for aqueous lithium ion batteries. This cathode delivers very high

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Fig. 1.8 (a) Cyclic voltammogram profile of LiV3O8 and LiCoO2 in a saturated aqueous LiNO3 electrolyte at a scan rate of 2 mV s1 and standard calomel electrode and nickel pellet were used as reference and counter electrodes, respectively (Reprinted with permission from Wang et al. 2006), (b) galvanostatic discharge–charge profiles of LiMn2O4 cathode at different applied current densities in 0.5 mol/l Li2SO4 aqueous electrolyte using Ni mesh and standard calomel electrode as a counter reference electrodes, respectively, (Reprinted with permission from Tang et al. 2011a), (c) voltage–capacity profile of LiMn2O4 versus Li metal at 0.3 C in water-in-salt aqueous battery. (Reprinted with permission from Yang et al. 2017)

capacity of 143 mAh g-1 at 1 A g-1 (7 C) and retained a stable capacity of 133 mAh g-1 at 10 A g-1 (70 C) in 0.5 M Li2SO4 solution with a three-electrode system, in which Ni grid and standard calomel electrode were used as counter and reference electrodes, respectively. Furthermore, the impact of crystalline morphology on the electrochemical performance is also noteworthy factor for aqueous batteries as the surface reaction is strongly dependent on the structural morphology. LiMn2O4 spinel has been researched intensively in both non-aqueous and aqueous electrolytes. By a facile route of preparation, LiMn2O4 nanorods based on working electrode yield a good capacity of 110 mAh g-1 at 4.5 C (0.5 A g-1) and retain 88% at a very high applied current rate of 90 C for aqueous rechargeable lithium ion batteries in 0.5 M Li2SO4 electrolyte with activated carbon as counter electrode by a two-electrode system (Tang et al. 2011a). Interestingly, after 1200 cycles at 4.5 C, this cathode displays a retained capacity of 103 mAh g-1 with ~ 94% capacity retention, demonstrating a superior cathode, which can compete over LiFePO4 cathode for non-aqueous lithium ion batteries. This system shows an initial discharge capacity of 108 mAh/g at a current density of 1A g-1 (9 C), 105 mAh g-1 at

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5000 mA g-1 (45 C) and 97 mAh g-1 even at 10000 mA g-1 (90 C) between 0 and 1.05 V (vs standard calomel electrode) as shown in Fig. 1.8b. Their different scan rates in CV profile maintain the redox peaks even at a high rate of 100 mV/s, indicating a stable electrochemical behavior when compared with its counterpart non-aqueous lithium ion batteries. Aforementioned electrochemical conditions except by replacing nanorods of LiMn2O4 with nanochains of LiMn2O4, Tang et al. (2011b) showed good reversible capacity of 110 mAh g-1 at 4.5 C and 95 mAh g-1 at 91 C. Interestingly, the same group prepared LiMn2O4 nanotube using carbon nanotube as a scarifying agent. As mentioned earlier nanotube morphology of the cathode plays a major role for the electrochemical performances, a more number of Li sites are exposed to the aqueous electrolyte due to preferred exposed (400) planes in LiMn2O4 nanotubes. Despite the presence of some impurity phases and most common Jahn–Teller distortion due to Mn3+, LiMn2O4 nanotube signposts a high charge capacity of 59.3 mAh g-1 with nearly 54% capacity retention at a high charge rate of 600 C (nearly 6 seconds). Yet another scarifying agent, polystyrene, is used to prepare porous LiMn2O4, which shows a prolonged cycling ability over 10000 cycles at 9 C (1 A g-1) rate with 93% retained capacity using a three-electrode system and consists of activated carbon electrode as counter electrode and saturated calomel electrode (standard calomel electrode, 0.242 V vs normal hydrogen electrode (NHE) as reference electrodes in 0.5 M Li2SO4 electrolyte (Qu et al. 2011). Recently, a high potential of 4.0 V aqueous lithium ion batteries was developed by Yang et al (2017) with a new class of aqueous electrolyte, termed as “water-insalt electrolyte” (WiSE) with very high salt concentration. This water-in-bisalt electrolyte has been prepared by dissolving 21 M LiTFSI (lithium bis (trifluoromethane)sulfonyl imide) and 7 M LiOTF (lithium trifluoromethane sulfolate) in water. This new electrolyte with LiMn2O4 cathode and a pre-coated Li anode or graphite with LiTFSI-HFE (highly fluorinated ether) anode to make an attractive high voltage aqueous lithium ion battery is known to be a ground breaking research work. This has been achieved by expanding the water stability window by in situ solid electrolyte interphase layer formation during the charge, offering a stable potential window > 3.0 V. This fluorinated-based solid electrolyte interphase layer protects and expands the window of more than 1.5 V against water oxidation– reduction stability of window of 1.23 V. Figure 1.8c shows a typical electrochemical charge–discharge profile at 1st and 50th cycles for LiMn2O4 cathode, pre-coated Li metal anode, and water-in-salt electrolyte. This “cathodic challenge” opens a wide door for achieving high energy/power operating at high potential regions with high safety environments.

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Sodium Ion Batteries in Non-aqueous Electrolytes

The concept of sodium ion batteries was introduced in the early 1980s, the same time period of lithium ion batteries. The principle and the electrochemical behaviors are discussed in detail in the following sections.

1.5.1

Principles of Sodium Ion Batteries: Pursuit for a Cathode

Incessant effort has been devoted for the development of high power/energy performance. Lithium ion batteries have become promising technology for being part of grid scale energy; however, lithium in the earth’s crust is unevenly distributed and their source on the earth becomes scarcity in near future. In contrast to lithium, the ore of sodium is unlimited in the earth’s crust and sea. Sodium is one among the most abundant element in the earth. Hence electrochemists focused on sodium ion batteries as an alternative to lithium ion batteries. The mechanism behind electrochemical reaction and battery components of sodium ion batteries is basically similar with lithium ion batteries except ion carriers. Sodium salt dissolved in organic electrolyte is used as an electrolyte. In addition, a larger ionic size of Na+ (1.02 Å) than Li+ (0.76 Å) affects the phase stability, higher standard electrode potential of -2.71 V vs standard calomel electrode as compared to -3.05 V vs standard calomel electrode for lithium. Also since the atomic weight of Na is heavier than Li (23 g mol-1 compared to 6.9 g mol-1), this makes this system always falls short in terms of energy density than lithium ion batteries (Kulbota and Komaba 2015). However, an unlimited ore in the earth’s crust and cheaper aluminum current collector for a negative electrode add advantages for sodium ion batteries over lithium ion batteries. The breakthrough era of 1980s for lithium ion batteries is also accounted for sodium ion batteries; the first reversible sodium insertion at room temperature was developed by Newman and Klemann (1980) using TiS2 as a cathode, followed by Braconnier et al. (1980) using NaxCoO2 cathode. The sodiated metal oxides are in general highly hygroscopic in nature with exposure to air; thereby more caution is required to avoid hydration; otherwise, an ease of formation of NaOH on the surface degrades electrode performances (Hwang et al. 2017). However, by partial substitution of suitable transition metals such as Cu2+, the stability in air is greatly improved (Zhang et al. 2019a). This will be discussed in the later part of this section. Layered cathode metal oxide structures based on their crystallographic framework are categorized as O3 and P2-type for sodium ion batteries. These two types of layered metal oxides were studied in detail in the last two decades.

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O3-Type Layered Oxides (NaxMO2, x = 1) Early investigation of 2D layered oxides was analyzed by Delmas and Hagenmuller in the early 1980s. Depending on sodium ions position between layers, the stacking sequence of the crystal structure is affected. Any sodiated transition metal oxide with general formula of Na1-xMO2 (M: transition metal) can be categorized into either of these two types of O3 and P2 stable crystal framework. Sodium ions occupy octahedral (O) and prismatic (P) sites with the difference in oxygen stacking sequence in the structure of ABCABC and ABBA, respectively (Kulbota and Komaba 2015). In addition to O3 and P2, few other sites including O2 and P3, where the numeric indicates the packing number of Na-ion in octahedral or prismatic layers within each unit cell. Furthermore, with prime symbol (’), O’3 and P’3 sites are signposted as monoclinic distortion in octahedral/prismatic structures. The O3 type is quite stable when the x value in Na1-xMO2 is close to 0, in which the average oxidation of M becomes +3. Thus O3-type layered compounds such as LiFeO2 and LiCrO2 are electrochemically inactive for lithium ion batteries, but they are quite active for sodium ion batteries. This is because de-intercalation does not occur in Li cells due to Fe ion distribution, creating a large irreversible capacity during charging of the layered α-LiFeO2. To avoid electrolyte decomposition at high voltage and reversibly available Na+ extraction in NaMO2, one must be aware of gaining maximum capacity and energy density. Most of the time, only 0.5 moles of Na+ extraction are from NaFeO2 from the available capacity, or else, on further Na+ extraction, these O3-type oxides show large irreversible capacity due to structure change from O3 to P3. Nearly 70% Na+ ions could be extracted when charging to 4.5 V, but no capacity was registered during discharge as trivalent Fe3+ ions migrate toward sodium vacancies sites, which was created while charging. This uncomfortable situation leads to an irreversible structural change in Na1-xFeO2 when x > 0.5, making the limited capacity of 80–100 mAh g-1 within the limited composition of x ¼ 0–0.45 (Yabuuchi et al. 2012a). Interestingly, by partial substitution of Fe sites with different transition metal ions as two components of NaFe0.5Co0.5O2 (Yoshida et al. 2013) and NaFe0.3Ni0.7O2 (Wang et al. 2014) and three components of NaFe1/ 2Co1/2O2 (Yoshida et al. 2013), NaNi1/2Fe1/3Co1/3O2 (Vassilaras et al. 2014), and NaNi1/3Fe1/3Mn1/3O2 (Yabuuchi et al. 2013), cathodes display not only large reversible capacity than NaFeO2 but also suppress the irreversible migration of trivalent Fe3+ ions. For example, a highly Fe-diluted NaFe0.3Ni0.7O2 shows increased capacity of 135 mAh g-1 with nearly 74% capacity retention after 30 cycles via Fe3+/4+ and Ni3+/4+ redox couples, which is certainly higher than a partially diluted (NaFe0.5Ni0.5O2) and without diluted mother phase (NaFeO2) as shown in Fig. 1.9a. On the other hand, NaNi1/2Fe1/3Mn1/3O2 based on three components can deliver a discharge capacity of 120 mAh g-1 within the voltage domain of 2–4 V vs Na+/Na half-cell, in which Mn4+ provides structural stability without involving in the electrochemical reaction (Kim et al. 2012a). Oh et al (2014a) provided a full cell battery consisting of Fe3O4 anode with three component cathodes, Na [Ni0.25Fe0.5Mn0.25]O2, delivering nearly 130 mAh g-1 with nearly 76% capacity

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Fig. 1.9 (a) Voltage–capacity profiles comparison for O3-NaFeO2, O3-NaFe0.5Ni0.5O2, O3-NaFe0.3Ni0.7O2 at a specific current of 30 mA g-1 for sodium ion batteries (Reprinted with permission from Wang et al. 2014), (b) cycle performance for the Fe3O4/Na[Ni0.25Fe0.5Mn0.25]O2 full cell at 0.5 C rate with coulombic efficiency curve cycled between 0.5 and 3.6 V (Reprinted with permission from Oh et al. 2014a), (c) sodium/NaNi0.5Mn0.5O2 cell charged to 3.8 or 4.5 V and then discharged to 2.2 V at a low rate of 1/50 C (4.8 mA g–1) to distinguish single and multi-phases reactions (Reprinted with permission from Komaba et al. 2012), (d) initial charge–discharge curve for radially assembled hierarchical columnar structure (RAHC) of {(Na[Ni0.75Co0.02Mn0.23]O2) -

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retention after 150th cycle (Fig. 1.9b). Quite interestingly, the same group had a different approach by diluting Fe concentration, thereby activating Mn3+/4+ upper redox reaction, as Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 cathode provides a large capacity of 180 mAh g-1 at 0.1 C rate in the voltage window of 1.7-4.4 V. The Li-O bond in the above cathode can stabilize the crystal structure (Oh et al. 2014b). NaNiO2 has two different polymorphs: a low temperature O3 layered with monoclinic distortion (metastable) and a high temperature rhombohedral phase. The Na-ions lie between the slabs of NiO2 and are isostructural with NaMnO2. At high temperature of around 450 K, the metastable phase turns to a stable rhombohedral phase; thus the phase is isostructural with LiNiO2 or NaFeO2. In general, NaNiO2 undergoes multistep oxidation/reduction during charge–discharge depths (Vassilaras et al. 2013). When cycled between 2.0 and 4.5 V vs Na+/Na, this cathode can be de-intercalated/intercalated with 0.85/0.62 Na+ with corresponding very high charge–discharge capacity of 199/147 mAh g-1. Whereas when it is cycled at cut-off potential of 1.25–3.75 V, nearly 0.63/0.52 Na+ de-intercalated/intercalated with capacity of 147/123 mAh g-1 and comparatively showing a stable cycle performance than that of cycled at 2.0–4.5 V window, without any significant structural changes. As expected, partial transition metal ions substitute into Ni site, an example of O3-Na[Ni0.5Mn0.5]O2, in which the presence of Mn4+ stabilizes Ni as 2+ oxidation state (Komaba et al. 2012), thus latter solely activated by the redox reaction of Ni2+/4+ with high discharge capacity of 185 mAh g-1, when cycled between 2.5 and 4.5 V vs Na+/Na as shown in Fig. 1.9c. In addition, the phase change sequence O3$O’3$P3$P’3 was reversible when cycled at 2.5–3.8 V with a reduced capacity of 100 mAh g-1. This sequence is greatly affected while further substituting with Fe, a gradual evolution of P3 to OP2 due to migration of Fe3+ ions toward transition metal layers or Na layers (Yuan et al. 2015). A well-known three components of NCM, an attractive Ni-rich-layered material currently commercialized for lithium ion batteries, is also subject for sodium ion batteries due to high capacity and high energy density. In 2015, Hwang et al (2015) developed a radially-aligned hierarchical columnar structure in which a core/shell network of {(Na[Ni0.75Co0.02Mn0.23]O2) -(Na[Ni0.58Co0.06Mn0.36]O2)} with electrochemically active Ni2+/3+/4+. Fig. 1.9d exhibits a first charge–discharge curves with reversible phase transition toward the monoclinic P3 phase, typically observed in all O3-type compounds, aiding a decent discharge capacity of 157 mAh g-1. As a full cell assembled with hard carbon as anode, nearly 125 mAh g-1 capacity and almost 80% capacity preserved after 300 cycles, added advantage for commercialization (Fig. 1.9e) within the potential range of 1.5-3.9 V. Furthermore, various three components system in which two components substitutions on Ni were reported with moderate reported capacity: NaNi0.5Sn0.5O2 (100 mAh g-1),  ⁄ Fig. 1.9 (continued) (Na[Ni0.58Co0.06Mn0.36]O2)} cathode with sodium metal anode, (e) as a full cell of RAHC with carbon cycled between 1.5 and 3.9 V at a current density of 75 mA g-1. For a comparison, performance for bulk Na[Ni0.60Co0.05Mn0.35]O2 is given (Reprinted with permission from Hwang et al. 2015 with modification)

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NaNi0.5Sn0.4Mn0.1O2 (110 mAh g-1) (Sathiya et al. 2018), NaNi0.5Mn0.3Ti0.2O2 (150 mAh g-1) (Wang et al. 2017a). Among the sodium based structures for sodium ion batteries, manganese containing O3-type compounds are quite important because of their costeffectiveness. To support further, an expected high theoretical capacity sort these materials for high energy/power density application. With a high sodium content, two types of phases occur: α-NaMnO2 is stable at low temperature and β-NaMnO2 is quite stable at high temperature. The average oxidation state of Mn is 3+, the JahnTeller distortion deprives the crystal structure, revealing a fast fading. Ceder and co-workers reported that O’3-NaMnO2, working in a voltage range of 2–3.8 V as shown in Fig. 1.10a, could deliver charge and discharge capacities as high as of 210 and 197 mAh g-1, respectively. However, due to a large number of charge– discharge plateaus with unverified transition from Na0.93MnO2 to orthorhombic or P2 Na0.7MnO2 at 2.63 V on SoC, and Mn dissolution with nearly 26% capacity loss in the initial 10 cycles, infer that they are not suitable for commercialization although it provided a high capacity (Ma et al. 2011). Similarly, the typical electrochemical O3- type NaCoO2 Na+ intercalation/de-intercalation was initially developed by Delmas et al (1981). The crystal structure underwent reversible phase transitions (O3$O’3$P’3) within the range of NaxCoO2 (x ¼ 0 - 0.2), nevertheless, the reported capacity was lower than the counterpart candidate of P2-type layered NaxCoO2, the detailed discussion can be found in next section. O3-type NaCrO2 showed moderate capacities by different groups although the theoretical capacity is as high as of 250 mAh g-1, in which Na0.85CrO2, Na0.4CrO2, and Na0.5CrO2 were the end products during de-sodiation (Broconnier et al. 1982; Komaba et al. 2010). Among them, the latter exhibits nearly 110 mAh g-1 capacity in the voltage range of 2–3.6 V vs Na+/Na due to large inter-slab distance provided by Na+ ions in the mother crystal structure. Recently, Liang et al. (2019) developed one-dimensional nanowires of NaCrO2, exhibiting a stable performance at different temperatures ranging from 55, 25, and -15  C with notable capacity of ~109, 87, and 60 mAh g-1 at 10 C rate, respectively, for sodium battery application. In specific, overlapped electrochemical charge–discharge patterns for initial three cycles in Fig. 1.10b reveal the charge plateaus located at 3.08 and 3.3 V along with discharging counterparts at 3.28 and 2.95 V which could be assigned oxidation/ reduction between Cr3+ and Cr4+ in the voltage range of 2–3.6 V. A moderate energy density of 161 Wh Kg-1 was obtained as a full cell with pre-sodiated hard carbon as an anode. Certainly, an important drawback is the toxicity of Cr element which makes it not suitable for large-scale production and commercialization. From the above studies, it is concluded that O3 NaMO2 materials follow a common feature of two phase transitions via (a) O3 NaMO2 ,P3 Na0.5MO2 and (b) P3 Na0.5MO2 ,O3 NayMO2 (y < 0.5 and varies with the nature of M) subjected to voltage window used for cycling (Mariyappan et al. 2018). The origin of phase transition is governed to minimize the O-O, Na-Na, and Na-M interlayer repulsions so as to retain the most thermodynamically stable intermediates, which were predicated by theoretical approach (Vinckevičiu̅tė et al. 2016). For example, higher energy is required to remove more sodium from P3 Na0.5MO2 as represented by

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Fig. 1.10 (a) Voltage–capacity profile of O3-NaMnO2 cycled between 2 and 3.8 V vs Na+/Na at a rate of C/30 for sodium ion batteries (Reprinted with permission from Ma et al. 2011), (b) initial three charge–discharge profiles of NaCrO2 nanowire cathode cycled between 2.0 and 3.6 V at 0.1 C rate (1 C ¼ 100 mA g–1) (Reprinted with permission from Liang et al. 2019), (c) two major structural evolutions (a) O3 NaMO2 , P3 Na0.5MO2 and (b) P3 Na0.5MO2 , O3 NayMO2 (where y < 0.5 and varies with the nature of M), widely observed in O3 NaMO2 materials during sodium de-insertion. The blue, red, yellow, and white circles denote the transition metal (TM), oxygen, sodium atoms, and vacancies, respectively. O3, O2, octahedral sites; P3, prismatic site. (Reprinted with permission from Mariyappan et al. 2018)

sudden potential jump in the cycling curve. A continuous decrease of negative charge on metal ions due to oxidation, leading to a gliding of the metal layers (MO2) in order to minimize the energy (Fig. 1.10c), resulting in ease of phase shift to another O3 structure. At the same time, close to the complete oxidation, the remaining Na+ ions do not screen the O-O repulsive interactions. Thus, for governing a better stability of the structure, the high-voltage phase transition of P3 to O3 should be avoided; this may reflect in their energy/power density premises.

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P2-Type Layered Oxides (Na1-xMO2, x < 1) In an off-stoichiometry of sodium and difference in oxygen stacking with arrangement notation of ABAB, P2-type layered oxides exhibit higher working voltage and hence higher energy density, when compared to O3-type layered oxides. Relatively high suppression of irreversible structural changes and possible to extract almost all sodium ions from the structure without loss of the crystal backbone, P2-type layered oxides are quite interesting. The crystal structure is quite stable if sodium content is in the range of x ¼ 0.3–0.7 in Na1-xMO2; hence the average oxidation state of transition metal ion is above +3.3. Interestingly, in the presence of vacancies, if any, the crystal structure suffers strong repulsion of oxygen in the sodium layers resulting in an expansion of the interlayer distance, leading to sodium ions occupying the prismatic sites. This leads to sodium ions occupying two different types of trigonal prismatic sites: Na1 contacts two MO6 octahedra units of the adjacent slabs along its face, whereas Na2 contacts with six MO6 octahedra along its edges, resulting in adjacent Na1 and Na2 sites too close to each other simultaneously (Hwang et al. 2017). Unlike O3-type layered oxides, P2-type oxides show a different phase transition mechanism in which transition occurs via P2 to O2 conversion during de-sodiation. Due to the instability of tetravalent Fe in P2-Na0.7FeO2 framework, a partial substitution with manganese (Nax[Fe0.5Mn0.5]O2) can deliver very high reversible capacity of 190 mAh g-1, in which Mn3+/4+ oxidation at 3.8 V without disturbing the P2 phase, and the phase transformation of P2 to OP4 (O2) at 4.2 V during de-sodiation, was noted (Yabuuchi et al. 2012b). As mentioned earlier that the corresponding O3 type certainly delivers less capacity than P2 type, Na [Fe0.5Mn0.5]O2 delivered less than 120 mAh g-1. Although P2-type layered oxide provides high capacity in the first cycle, capacity fading was inevitable in both cases. For further partial replacement of Fe by Co in the interlayers as Na0.7[(Fe0.5Mn0.5)1xCox]O2, the corresponding capacity increased to give a stable cycling performance; nevertheless, it underwent a phase transition to O2 when de-sodiated, which was transformed into P2 phase upon sodiation (Jung et al. 2015). The phase transition of P2 to O2 and reverse path has witnessed from operando XRD analysis in Fig. 1.11a, during de-sodiation and sodiation depths. This simple phase transition in Co-doped P2-type layered oxide could deliver better capacity retention in comparison with corresponding O3-layered materials, adding advantages to preserve the mother phase during cycling. In addition, expected high capacity can be achieved after first cycle due to sodium deficient in P2 structure, which is an intrinsic issue. The situation can be suppressed by addition of sodiumrich additive of NaN3 on the electrolyte, suggested by Singh et al. (2013). This additive certainly controls the irreversible capacity loss in the initial cycle. Like O3-NaCoO2, P2-type NaxCoO2 also exhibits similar electrochemical behavior in a voltage range of 2.7–3.5 V; in contrast, the phase transition appeared to be more complicated below 2.7 V in P2 type, in which considerable amount of Na+ is included at trigonal sites (Shacklette et al. 1988). Later, Berthelot et al. (2011)

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Fig. 1.11 (a) Operando XRD analysis for P2-Na0.7Fe0.4Mn0.4Co0.2O2 capillary electrode was charged (de-sodiation) at a current of 2 μA to 4.5 V and discharged (sodiation) at a current of 4 μA to 2.0 V (reprinted with permission from Jung et al. 2015); (b) cycling stability comparison between Na0.74CoO2 and Na5/8Ca1/24CoO2 cathodes, the applied current was frequently changed (Reprinted with permission from Matsui et al. 2015); (c) initial 50 cycles voltage–capacity curves for Na0.67Mg0.1Ni0.2Mn0.7O2 cathode cycled between 2 and 4.5 V vs Na+/Na (reprinted with permission from Singh et al. 2016); (d) prolonged cyclability of P2-Na0.67Co0.5Mn0.5O2 cathode over 2000 cycles at 30 C rate for sodium ion batteries (reprinted with permission from Zhu et al. 2016)

revisited the phase transition in P2-NaxCoO2; there are nine potential drops in the second discharge depth, revealing a complicated electrochemical regulation of Na+/ vacancies and two phase reactions addressed through operando analysis. This complex stepwise voltage plateau can be largely suppressed by introduction of Mn in the cobalt layers. This leads to disappearance of stepwise voltage profile in a range of 0.5 x  0.83 in Nax[Co2/3Mn2/3]O2. It was further found that Co3+/2+ and Mn4+/3 + reactions involved in the low voltage plateau, in addition with high oxidation/ reduction of Co4+/3+ at high-voltage region (Carlier et al. 2011). These reductions of Co3+/2+ and Mn4+/3+ were dominated in a voltage range of 1.5–2.1 V. Matsui et al. (2015) showcased P2-type calcium-doped sodium cobalt oxide for sodium ion batteries as cathode. Since the ionic radii of both calcium and sodium are almost same, the calcium ions occupy the sodium ions layer and thereby suppress the lattice mismatches of the two phases in Na2/3CaxCoO2. Although the substitution suppresses the specific capacity due to availability of sodium ions, the structural stability improved a lot, even at 5 mA current density; Na5/8Ca1/24CoO2 exhibits not only a

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stable cycling as show in Fig. 1.11b but also relatively high specific capacity than without calcium-doped P2-Na0.74CoO2 layered cathode. P2-Na0.7MnO2 was initially analyzed by Caballero et al. (2002) stating that a reversible capacity of 150 mAh g-1 obtained in a voltage range of 2–3.8 V; but due to Na+ intercalation in several steps, the progressive capacity fading was noted upon successive cycling tests. With the intention of structural stability as well as improved capacity, Ni substation was reported in 2001 by Lu and Dahn et al. P2-Na2/3[Ni2+1/ 4+ 3Mn 2/3]O2 was quite stable in air and showed an average voltage of 3.5 V with a 2+/4+ Ni redox reaction , delivering ~ 160 mAh g-1 in a voltage window of 2–4.5 V. However, controlling of the upper voltage limit to 4. 1 V to circumvent phase transition P2-O2 is responsible for increased volume change (Lu and Dahn 2001), resulting in better cycling stability. Interestingly, Li+ substitution in the layer, Na0.80[Li0.12Ni0.22Mn0.66]O2, strongly perturbed P2-O2 phase transition up to 4.4 V, revealing excellent capacity retention of ~ 110 mAh g-1 (91% over 50 cycles) without any voltage plateau between 4.1 and 4.4 V (Xu et al. 2014). Although the capacity is reduced by a substitution of divalent elements such as Zn2+ and Mg2+, an enhanced structural integrity is governed through this process. Yabuuchi et al (2014) showed P2-type Na2/3[Mg0.28Mn0.72]O2 cathode in a voltage range of 1.5–4.4 V for sodium ion batteries, delivering a high capacity more than 200 mAh g-1 by Mn3+/Mn4+ redox governed by Mg2+ in the layers. Nevertheless, a reversible charge plateau at 4.2 V could be originated from the contribution of oxide ions (potentially reversible solid-state reaction of oxide ions and/or partial loss of oxygen induced by Mg2+ ions, similar to Li[Li1/3Mn2/3]O2based electrode materials), responsible for plateau at 4.2 V and high registered capacity. Figure 1.11c signposts electrochemical charge–discharge curves for Na0.67[Ni0.2Mg0.1Mn0.7]O2, with a noted capacity of ~ 120 mAh g-1 after 50 cycles, and capacity drop was about only 6 mAh g-1 when compared with Na2/3[Ni1/3Mn2/3] O2. This substitution suppresses P2-O2 phase transition and Na+/vacancy ordering (Singh et al. 2016). Thus P2-O2 or OP4 phase transition is extremely reduced as Na+ ions are partially extracted from Na0.67[Ni0.2Mg0.1Mn0.7]O2 structure assisted by Mg2+ in the transition metal layers. Further replacement with trivalent 3D transition metal ions such as Co and Fe, the electrochemical performance was greatly enhanced. A P2-type Na0.7[Mn0.6Ni0.3Co0.1]O2 cathode reported by Yoshida et al. (2014) in a voltage window of 1.5– 4.3 V, in which the oxidation state of Mn is +3.4, exhibiting an improved capacity of ~185 mAh g-1 in a first cycle. Another P2-type cathode, P2-Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) prepared at three different temperatures of 800, 850, and 900  C through modified Pechini method by Kaliyappan et al. (2018), reveals a better reversible capacity of 148 mAh g-1 obtained from Na2/  3(Mn0.54Ni0.13Co0.13)O2 , prepared at 850 C, in the voltage range of 2–4.5 V. The observed plateaus around 2.3, 3.6, and 4.2 V are ascribed to the redox reactions, respectively, for Co3+/4+, Mn3+/4+, and Ni2+/4+, which were well matched with their corresponding cyclic voltammetry profiles. On the other hand, Talaie and co-workers reported (Talaie et al. 2017) that no high-voltage redox reaction is

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observed above 4 V. An oxide ion redox at high voltage may responsible for capacity contribution in Na0.67[Mn0.66Fe0.20Cu0.14]O2 , where Mn3+/4+ redox was noted below ~ 3.4 V. Thus, this cathode delivers a high capacity of ~ 176 mAh g-1 in a wide voltage window of 1.5–4.3 V, cycled at a rate of C/20 (13 mA g-1). Nevertheless, charging above 4.1 V results in a high-voltage polarization and capacity fading. The role of copper substation here was not only to achieve high capacity due to Cu2+/3+redox reaction but also provides structural stability during P2-layered oxide preparation and electrochemical reaction. Among the various candidates, a P2-Na0.67Co0.5Mn0.5O2-layered cathode (Zhu et al. 2016) cycled between 1.5 and 4.3 V exhibits excellent cycling stability of 2000 cycles with 75% capacity retention at 30 C (Fig. 1.11d). Water and CO2 molecules in air can react with NaxTMO2, forcing negative influence on its morphology and crystal structure and thereby affecting its overall electrochemical performance. These water molecules inserted in the Na layer result in the expansion of interlayers by Na+ and H+ exchange. This uncomfortable circumstance leads to (i) capacity decay and large polarization due to the interaction of water molecules with electrolytes, (ii) insulating NaOH and/or Na2CO3 formed on the electrode surface, and (iii) surface dissolution activated by the acid attack of proton, which is released by water molecules. In order to avoid these difficulties, sample storage, cell assembling, and electrochemical performance have been conducted under moisture-free environment (You et al. 2019). For example, Franger et al. (2000) analyzed hydration effect on α-Na0.7MnO2 through XRD measurement in which the material was soaked in water at different time. For a 60 min soaking time, the XRD phase of α-Na0.7MnO2 completely changed to Na0.45MnO20.6H2O, revealing insertion of water molecules. This is an important issue in sodium ion batteries for application and can be resolved by various techniques including surface coating with insulating materials and other suitable transition metal ions doping thus mainly include Cu2+. The protective surface coating may increase the stability without involving in the electrochemical reaction. You et al. (You et al. 2019) used a ZrO2 layer to protect the NaNi0.7Mn0.15Co0.15O2 surface. This layer maintains significant cycling stability at different current densities against moisture atmosphere, improving surface charge transfer kinetics by the presence of ZrO2 layer. Partial substitution of transition metal such as Cu2+ (ionic radius 0.73 Å with VI coordination number) could decrease the Na+ interlayer distance from 3.45 to 3.35 Å with Cu2+ doping, respectively, certainly preventing the insertion of water molecules. However, the presence of Cu2+ may increase the valence of Ni2+ in nickel-rich oxide and electrochemically active of Cu2 +/3+ redox also been observed in many P2-type layered oxides for sodium ion batteries (Yao et al. 2017). Overall, Table 1.2 describes the electrochemical properties of different metal oxide-based cathodes for sodium ion batteries application. It is found that both sodium salts of NaClO4 and NaPF6 are regularly used as an electrolyte.

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Table 1.2. Electrochemical properties of selected cathodes for sodium ion batteries

Cathode materials for sodium ion batteries O3-NaCoO2

P2-Na0.7CoO2

O3-NaMnO2

P2-Na0.6MnO2

O3-NaNiO2

O3-NaNi0.5Mn0.5O2

O3-NaFeO2

O3-NaFe1/2Mn1/2O2

P2-Na0.78Ni0.23Mn0.69O2 P2-Na0.67Mn1-xMgxO2

O3-NaNi 1/3Mn1/3Co1/3O2

Al2O3-coated P2-Na2/ 3(Mn0.54 Ni0.13Co0.13)O2

P2-Na0.66Ni0.17Co0.17Ti0.66O2

Electrolyte 1M NaClO4 in PC 1M NaClO4 in EC:DEC (1:1, v/v) 1 M NaPF6 in EC: DMC (1:1, v/v) 1M NaClO4 in PC 1 M NaPF6 in EC: DMC (1:1, v/v) 1M NaClO4 in EC:PC 1M NaClO4 in PC + 2 vol % FEC 1M NaClO4 in PC 1 M NaPF6 in PC 1M NaClO4 in EC:PC (1:1, w/w) 1M NaClO4 in EC:DMC (1:1, v/v) 1M NaClO4 in EC:DEC (1:1, v/v) 1M NaClO4 in

Potential window (V) 2.0–3.5

Discharge capacity (mAh g1) @applied current density (mA g1) -

2.0–3.8

125 @ 5 (0.04 C)

Fang et al. (2017)

2.0–3.8

197 @ 24 (0.1 C)

Ma et al. (2016)

2.0–3.8

140/0.21 mA cm2

Caballero et al. (2002)

1.25–3.75

123 @ 23.5 (0.1 C)

Vassilaras et al. (2013)

2.0–4.0

141 @ 12 (0.05 C)

Wang et al. (2016)

2.5–3.4

80 @ 12

Yabuuchi et al. (2012a)

1.5–4.3

~110 @ 12

2.0–4.5

138 @ 12 (0.1 C)

1.5–4.0

175 @ 12

Yabuuchi et al. (2012b) Ma et al. (2017) Billaud et al. (2014)

2.0–3.75

120 @ 0.24 mA cm2 (0.1 C)

Sathiya et al. (2012)

2.0–4.5

123 @ 160 (0.1 C)

Kaliyappan et al. (2015)

2.0–4.0

55 @ 0.2 C

Guo et al. (2015)

Reference Delmas et al. (1981)

(continued)

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Table 1.2. (continued)

Cathode materials for sodium ion batteries

O3NaNi0.45Cu0.05Mn0.4Ti0.1O2

O3NaMn0.25Fe0.25Co0.25Ni0.25O2 P2-Na0.6Li0.2Ni0.2Mn0.6O2

P2-Na2/3Ni1/3Mn5/9Al1/9O2

P3/P2/O3Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2

Electrolyte PC + 2 vol % FEC 1M NaClO4 in PC + 5% FEC 1 M NaPF6 in EC:DEC (1:1, v/v) 1 M NaPF6 in EC:DEC (1:2) 1M NaClO4 in EC:PC (1:1, v/v) 1 M NaPF6 in PC

Potential window (V)

Discharge capacity (mAh g1) @applied current density (mA g1)

2.0–4.0

124 @ 0.1 C

Yao et al. (2017)

1.9–4.3

180 @ 0.1 C

Li et al. (2014)

2.0–4.4

115 @11.8 (0.5 C)

Xu et al. (2014)

1.6–4.0

118 @ 0.1 C

Zhang et al. (2016)

2.0–4.3

155 @ 18

Keller et al. (2017)

Reference

DEC diethyl carbonate, DMC dimethyl carbonate, EC ethylene carbonate, PC propylene carbonate, FEC fluoroethylene carbonate.

1.5.2

Principles of Sodium Ion Batteries: Pursuit for an Anode

Similar with lithium ion battery system, there are three important mechanisms involved in sodium storage for anode materials: intercalation/de-intercalation reaction, conversion reaction, and alloying reaction. Thus, graphitic carbon, a most used anode for lithium ion batteries, may not work for sodium ion batteries due to large ionic radius of Na+ with 1.02 Å. Later, hard carbon plays a crucial role as anode for sodium ion batteries due to its larger layer space of 0.352 nm, which is benefited for Na+ storage. Nevertheless, there are some disadvantages such as large irreversible capacity and poor capacity retention which were noted. Interestingly, metal oxides are well capable for sodium intercalation/de-intercalation such as Na2Ti3O7 and Li4Ti5O12 which were well studied and show low storage capacity of ~ 300 mAh g-1, much lower than hard carbon, which may not be suitable for high energy sodium ion batteries. However, unlike intercalation-type anode materials, some metal oxides can store Na+ through the alloying and conversion mechanism, with high theoretical capacity almost 2 to 3 times higher than intercalation based metal oxides, making them suitable for high energy materials for sodium ion batteries (Zhang et al. 2015).

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1.5.3

B. Sambandam et al.

Metal Oxides as Anode Materials for Sodium Ion Batteries

The transition metal oxides with significant lithium storing properties are also subjected for sodium ion batteries as anode materials for conversion-based mechanism. These metal oxides produce higher specific capacities than intercalation-type metal oxides and satisfactory cycling than metal alloy compounds used for sodium ion batteries application. A spinel-type NiCo2O4 was reported in 2002 with high initial discharge capacity of 618 mAh g-1 (Alcántara et al. 2002). Fe3O4 was investigated by Balaya and his team, delivering a discharge capacity of 643 mAh g-1, but it shows poor capacity retention (Hariharan et al. 2013). Efforts have been given for an improvement of their cyclability by many research groups. 3D porous γ-Fe2O3 embedded in porous carbon matrix delivered a high discharge capacity of 740 mAh g-1 at 200 mA g-1 after 200 cycles. Jiang et al. (2014) fabricated half-cell for sodium ion batteries with different metal oxides such as Fe2O3, NiO, Mn3O4, and Co3O4. Among them, a high capacity of 386 mAh g1 at 100 mA g1 could be sustained by Fe2O3 after 200 cycles. In a typical galvanostatic charge discharge curve, a low voltage hysteresis of Fe2O3 was found near 0.75–1.0 V, revealing a fast sodium reaction kinetics characterized redox plateau as shown in Fig. 1.12a. A small variation for its counterpart CV profile as shown in Fig. 1.12b confirms that both analyses cannot be operated under similar condition. There might probably be a small variation between charge–discharge plateau and CV peak position. The strong plateau around 0.54 V while discharging could be attributed to the reduction of Fe (III) to Fe(0). A shift of this plateau to 0.65 V in a subsequent second discharge with an intensity drop is due to decomposition of liquid electrolyte and formation solid electrolyte interface layer, likely observed in initial cycle in anode materials for lithium ion batteries. Very recently, NiCo2O4 yolk–shell spinel structure was used as anode for sodium ion batteries. The spinel is served as a working electrode, while sodium foil is used as counter and reference electrode. A 1M NaClO4 in a mixture of ethylene carbonate/dimethyl carbonate at a ratio of 1:1 was utilized as an electrolyte. Both cyclic voltammetry and charge–discharge profiles of first cycle are completely different from subsequent cycles, accompanied by the solid electrolyte interface generation of amorphous Na2O on the first cycle. During discharge, both Co3+ and Ni2+ are reduced to form Co(0) and Ni(0) at the end of discharge, while reverse mechanism registered as Ni(0) to Ni2+ and Co(0) to Co2+ and Co2+ to Co3+ during charge depths. Yet another spinel of FeV2O4 (FVO) as an anode for sodium ion batteries is proposed by Maggay and co-workers (Maggay et al. 2018) in 1M NaClO4 with 1:1 volume ratio of ethylene carbonate/dimethyl carbonate electrolyte

 Fig. 1.12 (continued) batteries application. The notation of C, N-C, CC, CNFs, CNTs, G, NG, GO, and rGO is denoted as amorphous carbon, N-doped carbon, carbon cloth, carbon nanofibers, carbon nanotubes, graphene, N-doped graphene, graphene oxides, and reduced graphene oxides, respectively. (Reprinted with permission from Wang et al. 2019)

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Fig. 1.12 (a) Charge–discharge curves for Fe2O3 anode with initial three cycles measured in the voltage range of .005 to 3 V; (b) corresponding cyclic voltammetry profile at a scan rate of 0.1 mVs-1 (reprinted with permission from Jiang et al. 2014); (c) cycling life of FeV2O4 anode electrode prepared through two different binders of polyvinylidene fluoride (PVdf) and CMC/SBR (carboxymethyl cellulose/styrene-butadiene rubber); (d) corresponding rate performance at different applied current densities for FeV2O4/Pvdf and FeV2O4/CMC-SBR (reprinted with permission

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within the potential range of 0.01 to 3.0 V against sodium metal as a counter/ reference electrode. Two different methods were used for making electrode, namely, FVO-PVdF (polyvinylidene fluoride) and FVO-CMC/SBR (carboxymethyl cellulose/styrene–butadiene rubber), for sodium ion batteries, and their cycling performance/rate capability was monitored in a given voltage window at a fixed current density. Figure 1.12c shows cycling test for those electrodes prepared in a different method, in which initial three cycles run at low current density of 100 mA g-1 and rest of the cycles at 200 mA g-1. It was found that FVO-PVdF electrode exhibits rapid capacity fading, whereas FVO-CMC/SBR eventually delivered a more stable capacity even after 200 cycles. Apart from this cycling test, FVO-PVdF electrode has given an average discharge capacities of 85, 66, 52, 36, and 22 mAh g1 at current densities of 200, 400, 800, 1600, and 3200 mA g1, respectively, through a rate test as shown in Fig. 1.12d. After reducing the applied current density to 200 mA∙g1, it retains an average capacity of 63 mAh∙g1, which is nearly 25% less than the initial discharge cycle. However, FVO-CMC/SBR electrode gives better rate performance as it delivered average discharge capacities of 86, 82, 77, 65, and 46 at 200, 400, 800, 1600, and 3200 mA g1, respectively. When revert to 200 mA g1, it retained an average of 90 mAh g1 which is literally higher than the initial discharge. Almost all metal oxides including NiO, Nb2O5, CuO, SnO2, CaV4O9, ZnV2O4, and SbOx have been extensively investigated for sodium ion batteries. Morphology, crystalline nature, and particle size have played a crucial role for their performance. Further tailoring particle size below 10 nm (quantum dots) or embedded in a two-dimensional/three-dimensional matrix may enhance the capacity and cycle lifespan (Li et al. 2018a, b). A recent review by Wang et al. (2019) clearly emphasizes different metal oxides used as anode materials for sodium ion batteries with their merits and demerits. Figure 1.12e displays various metal oxides along with suitable carbon sources for sodium ion batteries application. It was found that metal oxides/graphene hybrid anodes with suitable electrolyte will govern this application for large energy storage application.

1.6

Sodium Ion Batteries in Aqueous Electrolytes

For a safe, green through low-cost power sources, especially for grid scale applications, aqueous batteries are great optional. An alternative option is the use of organic electrolyte-based lithium/sodium ion batteries, which use highly toxic/flammable solvent. Aqueous sodium ion batteries give multiple advantages including low cost and wide abundance in the earth’s crust. In addition, strong alternative to nonaqueous-based sodium ion batteries due to non-flammability, fast ion transportation, and lower manufacturing cost, thus aqueous sodium ion battery is seemingly most suitable for large-scale energy storage system applications (Kim et al. 2012b). Commonly, unlike in organic-based non-aqueous electrolytes, the electrochemical regulation in aqueous electrolyte is quite complex; thereby restriction in selection of

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suitable electrode material and other notable factors is as follows: (i) the redox potentials of electrodes should lie within or near the electrolysis potentials of water, beyond which the electrolysis of water, leading to H2 or O2 evolution; (ii) proton co-insertion into the host electrodes and electrode materials dissolution; and (iii) unavoidable side reactions by the consequence of electrode materials in contact with water molecules. There are plenty of materials that have been intensively studied for aqueous sodium ion batteries including metal oxides and polyanionic compounds. The following section is devoted for metal oxides for aqueous sodium ion batteries applications.

1.6.1

Metal Oxides for Aqueous Sodium Ion Batteries

Simple metal oxides including V2O5 and MnO2 were screened for aqueous sodium ion batteries as cathode materials in the initial period of late 1985s. Regrettably, these metal oxide electrodes typically have capacitance behavior, where the surfacecontrolled adsorption/desorption occurred instead of Na-ion insertion/extraction process. Among the electrochemically active elements, manganese-based oxides are thoroughly studied because of its abundance and cost. Recently, Shan et al. (2016) show a high-voltage aqueous Na-ion full cell system using surface hydroxylated Mn5O8 pseudocapacitor electrode, which shows a wide potential window between -1.7 (0.64 V overpotential for hydrogen evolution reaction) and 0.8 V vs Hg/HgSO4 (0.64 V overpotential for oxygen evolution reaction) in 0.1 M Na2SO4 electrolyte. This system exhibits a stable 2.5 V potential window in which it offers two-electron charge transfer via Mn2+/4+ redox couple with 85% retained capacity (61 mAh g1) over 25,000 cycles. A registered capacity of ~116 mAh g1 was obtained at 5 A g1 and retains 20 mAh g1 and was maintained at 50 A g1. Sodium metal oxides are promising cathode materials for aqueous Na+ energy storage because of theoretical capacities and undergo redox reactions at anodic potentials. A tunnel-structured Na0.44MnO2 (Na4Mn9O18) cathode shows three redox potentials through a cyclic voltammetry study as shown in Fig. 1.13a, in the voltage regime of -0.3 to 0.3 V vs HgSO4 in 0.1 M Na2SO4 electrolyte, indicating a multiphase Na-ion extraction/insertion mechanism. It supplies low specific capacity of ~ 35 mAh g1 at C/5 rate and still maintains capacity of 20 mAh g1 at 18 C rate (Whitacre et al. 2010). As a full cell, the working electrode is combined with activated carbon as a counter electrode, providing a good potential window of 0.4–1.8 V vs standard Hg/HgSO4 as a reference electrode, over 1000 charge– discharge cycles (Fig. 1.13b). Because of low received capacity, researcher tuned the structure by partial substitution with other transition metals. This improves the redox potential, rising the discharge potential of the oxide above 2.7 V vs Na+/Na. As a result, tunnel-type Na0.44Mn0.44Ti0.56O2 cathode exhibited a capacity of 45 mAh g1 at 2 C in a Na2SO4 aqueous electrolyte, which is almost similar with the absence of substitution. However, by increasing Na content, Na0.66Mn0.66Ti0.34O2 cathode delivered 76 mAh g1 at 2 C (Wang et al. 2015).

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Fig. 1.13 (a) Cyclic voltammetry profile analyzed through three electrode cells with Na0.44MnO2, Hg/HgSO4, and activated carbon used as working, reference, and counter electrodes, (b) corresponding cycle lifespan at 4 C. inset: charge–discharge curve at 500th test cycle (Reprinted with permission from Whitacre et al. 2010)

Two important factors were involved for the structural stabilization during cycling: (i) the Ti substitution suppressed many phase transitions of Na0.44MnO2, and (ii) the higher content of Na (0.66 Na+) in above said cathode partially nullifies the solubility issues of Mn2+; thereby it retains 89% of capacity after 300 cycles at 2 C. Layered O3- and P2-type structures are intensively studied for non-aqueous electrolyte in the past few decades. Attention is recently given for these layered structures for aqueous sodium ion batteries; however, most of the P2 and O3 layered structures are highly air sensitive, which strongly affects the electrochemical reaction in a non-aqueous electrolyte (Boyd and Augustyn 2018). This further worsens in the case of aqueous electrolyte. Layered P2-NaxMnO2 and NaxMn0.66 Ni0.22Co0.11 coined as NCM-621 rapidly form hydrated phase upon exposure to air, leading to a de-intercalation of Na content (reduced Na content). The water molecules sit in the Na layers, thereby creating electrostatic repulsion between MO2 layers, experiencing a layer glide upon cycling. Nevertheless, by engineering with transition metal substitutions, air- and water-stable-layered cathodes are shown significant performances for aqueous battery. Yu et al. (2017) reported that P2 Na0.67Ni0.25Mn0.75O2-layered cathode delivered a capacity of 62 mAh g-1 at 1 C in a mixed electrolyte containing Na2SO4 and Li2SO4. Although the capacity is improved by the addition of Li2SO4, the oxide shows a poor cycling performance. It can be further concluded that due to the reactivity with moisture/air, most of the P2-layered oxides are not promising for aqueous Na+ energy storage. O3-layered oxides including NaMnO2 and NaNi0.33Mn0.33Co0.33O2 are air sensitive; they experience spontaneous oxidation, interlayer hydration, and Na2CO3 coating on surface and phase transition from O3 to P3 phase. A mixed phase of NaMnO2 has recently been reported. In a full cell with activated carbon as an anode and 0.5 M Na2SO4 as an electrolyte, the mixed phase material delivered a capacity of 43 mAh g-1 with a mere 3% capacity loss over 10,000 cycles at 10 C (Qu et al. 2009).

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Recently, a new concept of “water-in-salt” (WiS) with very high concentration of Na-ion salts in minimum amount of water can successfully expand the voltage window up to 2.5 V (Suo et al. 2017). This leads to a suppression of hydrogen evolution on anode with a creation of Na+-conducting solid electrolyte interface layer and thereby reduce the electrochemical action of water on cathode.

1.7

Potassium Ion Batteries in Non-aqueous Electrolytes

Due to the abundance in the earth’s crust, the monovalent potassium ion batteries are interested recently. The heavy atomic mass and a relatively higher ionic radius of K+ than that of its counter-partners of lithium and sodium ions, the technology was not developed by researchers in the 1980s. Although the theoretical capacity of cathodes used for potassium ion batteries relatively again smaller than lithium/sodium ion batteries, the compensation can be achieved by increased energy densities as potassium ion batteries produce higher voltages than their counter-partners. For example, the theoretical capacities for Li3V2(PO4)3, Na3V2(PO4)3, and K3V2(PO4)3, respectively, of 132, 118, and 106 mAh g-1, in which the calculated capacity of potassium ion batteries is 19.7% less than that of lithium ion battery cathode. However, with respect to V3+/4+ redox couple, the reaction occurred around 4 V for lithium ion batteries, less than 3.5 V for sodium ion batteries and greater than 3.5 V for potassium ion batteries, respectively, inferring a high energy out from potassium ion batteries than the sodium ion batteryes.

1.7.1

Principles of Potassium Ion Batteries: Pursuit for a Cathode

Mechanism of potassium ion batteries operates through a similar rocking chair principle similar to that of lithium ion batteries, in which K+ ions are shuttled between the cathode and anode using an intercalation/de-intercalation through an electrolyte, which contains K+ ions (Zhang et al. 2019b). The working principle is similar to lithium ion batteries; during charging, the cathode, for example, KxMnO2, experiences an oxidation reaction through extraction of K+ ions from the lattice (de-intercalation) and electron loss to becomes Kx-1MnO2 or MnO2. Simultaneously, the anode undergoes a reduction reaction (either metallic K or graphite) with K+ ions insertion (intercalation) and electron acquisition. The charge carriers of K+ ions and electrons are moving to the anode side through the internal electrolyte and external conduction circuit, respectively. In addition, unlike lithium ion batteries, whose Li foils are commercially available (Xu et al. 2019), both sodium and potassium ion batteries did not utilized their corresponding metal as readymade or commercially available foils because of more reactivity with moisture in the order of K > Na > Li.

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Metal Oxides as Cathode for Potassium Ion Batteries

Manganese-based oxides are extensively studied for potassium ion batteries as of lithium and sodium ion batteries due to low costs, high stability, and theoretical capacities. The first report by Vaalma et al. (2016) reported that a P2-type layered birnessite manganese oxide, K0.3MnO2, delivers a high discharge capacity 136 mAh g-1 for potassium ion batteries within the voltage range of 1.5–4.0 V; nevertheless, the capacity retention was poor as it holds only 58% after 50 cycles. Two oxidation plateaus in this window at 3.7 and 3.9 V reveal a two-phase reaction. In contrast, the same cathode registers 100 mAh g-1 in a voltage region of 2–4.0 V with retained capacity of 73% after 50 cycles. The authors here pointed out some serious of issues, however, no supporting proof, for capacity fading due to (1) a coulombic repulsion in the adjacent oxygen layers while extracting K+ in a high-voltage region and (2) partial irreversibility due to volume change during electrochemical reaction. By increasing potassium content, a different crystal structure of P3-layered K0.5MnO2 was obtained by Kim et al. (2017a), in which well redox pairs retain in the voltage boundary of 1.5–3.9 V against 1.5–4.2 V. An initial charge–discharge capacity of 53/106 mAh g-1 was obtained with notable phase transition of P3 ! twophase reaction ! O3 ! two-phase reaction ! X through in situ X-ray diffraction analysis (O3, stacking in the order of ABCABC; X, a new phase at a high state of charge near 3.8–3.9 V), which was completely reversible upon K+ intercalation during discharge process. Despite the high-voltage advantages, their capacity and cyclability of potassium ion batteries are not yet to the mark of counterpart of lithium and sodium ion batteries. Hence more attention is required in near future. Cobalt-based P2-type K0.6CoO2, with an average voltage of ~2.7 V, was reported by Kim, Ceder, and co-workers (Kim et al. 2017b) and shows discharge capacity of 80 mAh g-1 at 2 mA g-1 current density in a voltage space of 1.7–4.0 V, which was lesser than corresponding manganese based counter-partner. Figure 1.14a displays the galvanostatic charge–discharge curves at different current densities of 2, 10, 70, 100, 120, and 150 mA g-1, in which a large number of plateaus make this cathode not suitable for commercialization. The corresponding cyclability pattern in Fig. 1.14b reveals structural instability over 120 cycles at 100 mA g-1, implying severe side reactions of the electrolyte with the cathode; however, after refilling with fresh electrolyte, it recovers initial capacity ~ 60 mAh g-1. Vanadium-based K0.5V2O5 reported by Deng et al. (2018) for PIBs exhibits a reversible capacity of 90 mAh g-1at 10 mA g-1 in a voltage range of 1.5–3.8 V (vs K+/K) due to V4+/5+ redox reaction. This cathode shows several pseudo-voltage plateaus (Fig. 1.14c) centered at 2.95 and 3.25 V right after first charge as only 32% K+ ions are extracted with very less charge capacity. The corresponding cyclability profile in Fig. 1.14d, at 20 mA g-1 current density over 80 cycles, inferring structural stability. Multimetal-layered oxides, on the other hand, delivered very high capacity as like lithium/sodium ion batteries. Wang, Mai, and co-workers (Wang et al. 2017b) studied a Fe/Mn-based layered K0.7Mn0.5Fe0.5O2 nanowires cathode deployed in a voltage range of 1.5–4.0 V vs K+/K for potassium ion batteries. This cathode depicts

Fig. 1.14 (a) Charge–discharge curves for P2-type K0.6CoO2 at different applied current densities; (b) cyclic ability of P2-type K0.6CoO2 at a low current rate of 100 mA g1 performed in the voltage range of 4.0–1.7 V versus K/K+ with refreshed electrolyte (reprinted with permission from Kim et al. 2017b); (c) selected

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 ⁄

Fig. 1.14 (continued) galvanostatic charge–discharge curves for K0.5V2O5 cathode cycled between 1.5 and 3.8 V (vs K+/K) at 20 mA g-1; (d) corresponding cycling curve over 80 cycles (reprinted with permission from Deng et al. 2018); (e) initial 70 cycles charge–discharge profile of K2Ni2TeO6 prototype K half-cell cathode in an ionic liquid (0.5 M KTFSI (potassium trifluoromethanesulfonimide) in Pyr13TFSI) electrolyte at a rate of C/20. The gray line indicates calculated voltages (reprinted with permission from Masese et al. 2018), (f) initial three cycles voltage–capacity profile K2NiCoTeO6 in K half-cells using ionic liquid (0.5 M KTFSI in Pyr13TFSI) measured at C/20. Inset: scanning electron microscopy image of pristine sample (reprinted with permission from Masese et al. 2019)

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two discharge voltages at 2.20 and 1.88 V and exhibit high first charge–discharge capacity of 214/178 mAh g-1, the cycling performance, interestingly, stabilized over 200 and 450 cycles, respectively, for 20 and 100 mA g-1, current densities. As a full cell with soft carbon, the nanowire cathode supplied a good reversible capacity of 82 mAh g-1 at 40 mA g-1 (based on the mass of a cathode) and retained nearly 90% capacity over 50 cycles run between 0.5 and 3.5 V. In 2017, Liu et al. (2017) testified a layered K0.67Ni0.17Co0.17Mn0.66O2 for potassium ion batteries, delivering a reversible capacity of 76.5 mAh g-1 with an average voltage of 3.1 V and retained nearly 87% after 100 cycles at 20 mA g-1 current density when the cell was operated between 2 and 4.2 V in 0.8 M KPF6 in ethylene carbonate/diethylene carbonate (1:1 by volume) electrolyte. Recently, Masese et al. (2018) introduced a new class of cathode, K2Ni2TeO6 (or K2/3Ni2/3Te1/3O2), which interestingly gains a reversible capacity of 65 mAh g-1, demonstrating the ability to ingress/egress of K+ ions from/ to the framework with an average voltage of 3.6 V within a voltage domain of 1.3–4.7 V vs K+/K in an ionic liquid (0.5 M KTFSI (potassium trifluoromethanesulfonimide) in Pyr13TFSI) electrolyte. The presence of more electronegative moiety of [TeO6]6- (Te6+) which is higher than O22- and an electrochemical active nickel (Ni2+/4+) redox couple are solely responsible for high-voltage premise. Figure 1.14e represents a typical galvanostatic charge–discharge profile for this cathode for initial 70 cycles. Interestingly, a partial substitution of Co or Mg (1/4 th of Ni content) on the above said cathode delivered a low and high capacity with respect to K2Ni2TeO6, respectively, of ~ 50 and 70 mAh g-1, for K2Ni1.5Co0.5TeO6 and K2Ni1.5Mg0.5TeO6 with little altered average voltages. The same group, later in 2019, modified the above mother cathode by replacing 50% of Ni with Co. Masese et al. 2019, demonstrated a high-voltage honeycomblayered P2-type K2/3Ni2/3Co1/3Te1/3O2 (or K2NiCoTeO6) cathode for PIBs, in which a main redox peak appears at around 4.3 V, suggesting a high-voltage material reported so far. On the other hand, the registered capacity is not to the level of mark as it delivers 30 mAh g-1 at a current density of C/20 in a voltage scan of 1.3–4.7 V (vs K+/K) as shown in Fig. 1.14f.

1.8

Multivalent Ion Batteries for Energy Storage

Apart from monovalent ion batteries such as lithium, sodium, and potassium ion batteries, multivalent ion-based rechargeable batteries with abundant elements (Mg2+, Ca2+, Zn2+ and Al3+) exhibit significant attractions. Due to multi-ions involved in the electrochemical reaction, this strategy provides a combined individual merit of multiple ions as primary ions in rechargeable batteries (Liu et al. 2019). Among the multi-ion rechargeable batteries, zinc ion batteries (ZIBs) are focused briefly here, due to almost similar ionic radius of lithium. For a safety and economic concern, aqueous rechargeable zinc ion batteries (ARZIBs) functioning in mild acidic medium with a reversible Zn2+ intercalation cathode are promising. Comparatively, Zn has a high redox potential (0.76 V against a standard hydrogen electrode) and a

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high kinetic overpotential for hydrogen evolution, thus rendering them stable in water than those of other multivalent batteries of Mg2+, Ca2+, or Al3+ (Manalastas Jr et al. 2019). Furthermore, the currently known electrolytes for Mg2+/Ca2+/Al3+ batteries corrode the metal anode and/or the current collector and reduce electrochemical reactivity due to the formation of unreactive layers (Canepa et al. 2017). As an anode, Zn metal displays high volumetric and gravimetric capacities (5851 mAh mL1 and 820 mAh g-1) and efficient reversible metal plating/stripping, demonstrating that a smaller and lighter battery system could be realized. The ionic radius, as mentioned early, is 74 pm (Zn2+) comparable to those of 72 pm (Mg2+), 76 pm (Li+), and lesser than 102 pm of Na+, thus levitating the scope for exploration for advancement in this battery. Both manganese- and vanadium oxide-based cathodes are intensively researched for zinc ion battery applications. Among the manganese-based oxides subjected for battery application, MnO2, a well-known candidate since from the birth of alkaline battery in early 1950s, is still being utilized for producing high energy and power densities in aqueous electrolyte. Feiyu Kang and co-workers (Xu et al. 2012) introduced a new energy storage concept, coined as “zinc ion batteries” when MnO2 reversibly intercalate/de-intercalate Zn2+ ions during discharge–charge processes, with Zn metal as anode in Zn(NO3)2 or ZnSO4 aqueous electrolyte as shown in Fig. 1.15a and typical electrochemical charge–discharge curve (Alfaruqi et al. 2015) within the voltage range of 1–1.8 V vs Zn2+/Zn for α-MnO2/ZnSO4/Zn battery is given in Fig. 1.15b. The electrochemistry regulation is still an open discussion in which different mechanisms were proposed in the past couple of years. The proposed mechanism includes complete conversion reaction and intercalation reaction with/without proton co-intercalation, which again is a controversy argument. This issue needs to be addressed initially. Informatively, due to better stability and high capacity among the electrolytes used for aqueous rechargeable zinc ion batteries, sulfate counter-ion-based ZnSO4 gained special attention compared with other electrolytes. The other electrolytes such as Zn(NO3)2, Zn(CH3COO)2, ZnCl2, ZnF2, and bulky Zn(CF3SO3)2 (Zn(tfl)2) and Zn(C2F6O4N)2 (Zn(tfsi)2) were utilized for energy storage application. Among these, Zn(tfl)2 electrolyte received special attention. Vanadate-based cathodes, on the other hand, have better cycling stability than manganese oxide-based cathodes, and nevertheless, the voltage range is lesser than its counter-partner, manganese oxide. Sambandam et al. (2018) reported α-Zn2V2O7 cathode for zinc ion batteries in 1M ZnSO4 electrolyte in the voltage window from 0.4 to 1. 4 V, which retains similar charge–discharge skeleton even after 1000 cycles at 4 A g-1 current density, revealing a good reversibility of Zn2+ ingress/egress properties as shown in Fig. 1.15c. The corresponding cyclability pattern at 4 A g-1 over 1000 cycles initiated with low current density of 100 mA g-1 for three cycles in Fig. 1.15d demonstrates the structural stability with retained capacity of 138 mAh g1 after 1,000 cycles. The electrochemical regulation mostly follows intercalation/de-intercalation of Zn2+ ions in vanadium oxide-based cathodes, in which Zn metal is utilized as an anode in a Zn2+ ions containing electrolyte. Since it is in an aqueous medium, the water-based side reaction is highly feasible as a major drawback in this technology.

Fig. 1.15 (a) Schematic representation of zinc ion battery, in which Zn2+ ions migrate between a cathode (MnO2 here) and Zn anode. The inset shows a basic unit of MnO6 octahedron of MnO2 (reprinted with permission from Xu et al. 2012), (b) discharge–charge voltage profiles of α-MnO2 for initial two cycles at a current density of 83 mA g-1 for zinc ion batteries (reprinted with permission from Alfaruqi et al. 2015); (c) selected discharge–charge depths for α-Zn2V2O7 cathode for aqueous zinc ion batteries at 4A g-1 current density; (d) corresponding cycle life at same current density (reprinted with permission from Sambandam et al. 2018); (e) charge–discharge profile of Na2V6O163H2O cathode for aqueous zinc ion batteries with selected points for operando analysis; (f) corresponding operando X-ray diffraction analysis of selected 2θ regions with selected points from (e) (reprinted with permission from Soundharrajan et al. 2018)

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In a mid-acidic medium of ZnSO4 electrolyte, a new phase of zinc basic sulfate (Zn4(OH)6 SO4. nH2O) reversibly formed during discharge and is dissolved when charged. This is due to cathode metal ion dissolution, leading to pH variation in the electrolyte buffers H+ intercalation. This basic sulfate is thus formed from available Zn2+, H2O, SO42- from the electrolyte. This parasitic reaction may alter the reaction kinetics a major setback for electrochemical diffusivity of Zn2+ ions in the layered cathodes. Irrespective of metal oxides used, this parasitic reaction is a most common phenomenon as clearly evident from operando X-ray diffraction analysis which is clearly projected in Fig. 1.15e during electrochemical reaction. Although a layered cathode Na2V6O16. 3H2O exhibits (de) intercalation of Zn2+ ion, resulting from a shift of XRD peaks (see Fig. 1.15f (ii-iv)), a well-defined unshifted 2θ peak around 8.1 (Fig. 1.15f(i)), confirming the new phase, as denoted by parasitic reaction, which is reversibly formed/dissoluted during cycling (Soundharrajan et al. 2018). The electrochemical mechanism is still an open debate, irrespective of metal oxides to be used. Since this technology is in the beginning level of research, more in-depth works on electrochemical regulations are to be expected in near feature.

1.9

Conclusion

The Tesla S Model (2017) electric car has a large (100 kWh) battery weighing 625 kg (~ 1,400 lbs) which generates energy at a density of 160 Wh kg1 at a consumption rate of 210 Wh km1. This equals to 330 Wh mi1 and can be driven for 539 km (335 mi), a longest distance achieved through electric vehicles on a single charge. This can be attained through metal oxide-based electrodes in lithium ion batteries, revealing the importance of this technology and the role of metal oxides. Nevertheless, there are still some bottlenecks include large irreversible capacity, poor cycle life, etc., to be addressed for achieving high energy/power density. Thus lithium ion battery technology can attain practical energy density around 120–250 Wh Kg1 with respect to the theoretical density of 400–600 Wh Kg1. The deployment of high conductivity electrolytes, surface coating with passive layer, and nanostructured cathode/anode materials are suggested to be key factors for effectively solving above issues, resulting to achieve high desired voltage with high energy density with long cycle life. Alternatively, metal oxide-based sodium ion batteries can be documented as a primary contender for an alternate to lithium ion batteries in terms of cost saving with minimum sacrifice of electrochemical performance and scarcity of lithium sources. Nonetheless, the energy/power density output is still not to the level of lithium ion batteries. Metal oxide-based aqueous rechargeable batteries, on the other hand, afford clean, environmentally safe and cost-effectiveness, and hence novel rechargeable batteries including Zn2+- and Al3+-based multi-ion batteries have become a hot topic in the last one decade. Especially, though aqueous Zn battery provides high power density, due to several challenges still remain including sluggish kinetics, insufficient energy density and poor reversibility. Importantly, focus is required to widen the operating voltage

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window of aqueous rechargeable batteries to above 1.23 V without interference of water oxidation/reduction. Through electrolyte architecture, one can not only unravel the aforementioned issues but also extend the window to high voltage. In this circumstance, these multi-ion-based aqueous ion batteries are to be commercialized for better economy and clean energy for next generation. Further, all solidstate batteries in which solid electrolyte enhances energy/power density compared with liquid-based electrolyte are currently pursued in the last few years. Nevertheless, the performance of solid-state electrolyte-based lithium ion batteries has not yet attained the level of liquid electrolyte counterpart. Hence research could be focused in this arena, especially to improve the ionic conductivity of solid electrolyte to provide better/safe energy storage applications. The success or failure of battery performance revealed especially by cathode materials, and the crucial role of metal oxides as cathode is quintessential. Acknowledgment The authors dedicated the work to the deceased eminent Professor Dr. P. T. Manoharan, INSA Senior Scientist, Department of Chemistry, Indian Institute of TechnologyMadras, for his valuable scientific research contributions. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2018R1A5A1025224). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government MSIT (NRF-2020R1A2C3012415).

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Billaud J, Singh G, Armstrong AR, Gonzalo E, Roddatis V, Armand M, Rojo T, Bruce PG (2014) Na0.67Mn1xMgxO2 (0  x  0.2): a high capacity cathode for sodium-ion batteries. Energy Environ Sci 7:1387–1391. https://doi.org/10.1039/C4EE00465E Boulineau A, Simonin L, Colin JF, Bourbon C, Patoux S (2013) First evidence of manganese– nickel segregation and densification upon cycling in li-rich layered oxides for lithium batteries. Nano Lett 13:3857–3863. https://doi.org/10.1021/nl4019275 Boyd S, Augustyn V (2018) Transition metal oxides for aqueous sodium-ion electrochemical energy storage. Inorg Chem Front 5:999–1015. https://doi.org/10.1039/C8QI00148K Braconnier JJ, Delmas C, Fouassier C, Hagenmuller P (1980) Comportement electrochimique des phases NaxCoO2. Mater Res Bull 15:1797–1804. https://doi.org/10.1016/0025-5408(80)901993 Broconnier JJ, Delmas C, Hagenmuller P (1982) Etude par desintercalation electrochimique des systemes NaxCrO2 et NaxNiO2. Mater Res Bull 17:993–1000. https://doi.org/10.1016/00255408(82)90124-6 Caballero A, Hernan L, Morales J, Sanchez L, Santos Pena J, Aranda MAG (2002) Synthesis and characterization of high-temperature hexagonal P2-Na0.6MnO2 and its electrochemical behaviour as cathode in sodium cells. J Mater Chem 12:1142–1147. https://doi.org/10.1039/ B108830K Canepa P, Sai Gautam G, Hannah DC, Malik R, Liu M, Gallagher KG, Persson KA, Ceder G (2017) Odyssey of multivalent cathode materials: open questions and future challenges. Chem Rev 117:4287–4341. https://doi.org/10.1021/acs.chemrev.6b00614 Cao K, Jiao L, Liu Y, Liu H, Wang Y, Yuan H (2015) Ultra-high capacity lithium-ion batteries with hierarchical CoO nanowire clusters as binder free electrodes. Adv Funct Mater 25:1082–1089. https://doi.org/10.1002/adfm.201403111 Cao K, Liu H, Xu X, Wang Y, Jiao L (2016) FeMnO3: a high-performance Li-ion battery anode material. Chem Commun 52:11414–11417. https://doi.org/10.1039/C6CC04891A Cao K, Jin T, Yang L, Jiao L (2017) Recent progress in conversion reaction metal oxide anodes for Li-ion batteries. Mater Chem Front 1:2213–2242. https://doi.org/10.1039/C7QM00175D Carlier D, Cheng JH, Berthelot R, Guignard M, Yoncheva M, Stoyanova R, Hwang BJ, Delmas C (2011) The P2-Na2/3Co2/3Mn1/3O2 phase: structure, physical properties and electrochemical behavior as positive electrode in sodium battery. Dalton Trans 40:9306–9312. https://doi.org/ 10.1039/C1DT10798D Chen L, Fan X, Hu E, Ji X, Chen J, Hou S, Deng T, Li J, Su D, Yang X, Wang C (2019) Achieving high-energy density through increasing the output voltage: a highly reversible 5.3 V battery. Chem 5:896–912. https://doi.org/10.1016/j.chempr.2019.02.003 Czyżyk MT, Potze R, Sawatzky GA (1992) Band-theory description of high-energy spectroscopy and the electronic structure of LiCoO 2. Phys Rev B 46:3729. https://doi.org/10.1103/ PhysRevB.46.3729 Dahn JR, Sacken UV, Michal CA (1990) Structure and electrochemistry of Li1yNiO2 and a new Li2NiO2 phase with the Ni (OH)2 structure. Solid State Ionics 44:87–97. https://doi.org/10. 1016/0167-2738(90)90049-W Daniel C, Mohanty D, Li J, Wood DL (2014) Cathode materials review. AIP Conference Proceedings 1597:26–43. https://doi.org/10.1063/1.4878478 Delmas C, Braconnier JJ, Fouassier C, Hagenmuller P (1981) Electrochemical intercalation of sodium in NaxCoO2 bronzes. Solid State Ionics 3–4:165–169. https://doi.org/10.1016/01672738(81)90076-X Deng L, Niu X, Ma G, Yang Z, Zeng L, Zhu Y, Guo L (2018) Layered potassium vanadate K0.5V2O5 as a cathode material for nonaqueous potassium ion batteries. Adv Fun Mater 28:1800670. https://doi.org/10.1002/adfm.201800670 Du L, Lin H, Ma Z, Wang Q, Li D, Shen Y, Zhang W, Rui K, Zhu J, Huang W (2019) Using and recycling V2O5 as high performance anode materials for sustainable lithium ion battery. J Power Sources 424:158–164. https://doi.org/10.1016/j.jpowsour.2019.03.103

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Li H, Zhang N, Li J, Dahn JR (2018a) Updating the structure and electrochemistry of LixNiO2 for 0  x  1. J Electrochem Soc 165:A2985–A2993. https://doi.org/10.1149/2.0381813jes Li L, Zheng Y, Zhang S, Yang J, Shao Z, Guo Z (2018b) Recent progress on sodium ion batteries: potential high-performance anodes. Energy Environ Sci 11:2310–2340. https://doi.org/10.1039/ C8EE01023D Liang L, Sun X, Denis DK, Zhang J, Hou L, Liu Y, Yuan C (2019) Ultralong layered NaCrO2 nanowires: a competitive wide-temperature-operating cathode for extraordinary high-rate sodium-ion batteries. ACS Appl Mater Interfaces 11:4037–4046. https://doi.org/10.1021/ acsami.8b20149 Lin M, Ben L, Sun Y, Wang H, Yang Z, Gu L, Yu X, Yang X, Zhao H, Yu R, Armand M, Huang X (2015) Insight into the atomic structure of high-voltage spinel LiNi0.5Mn1.5O4 cathode material in the first cycle. Chem Mater 27:292–303. https://doi.org/10.1021/cm503972a Liu Y, Zhao Y, Yu Y, Li J, Ahmad M, Sun H (2014) Hierarchical CoNiO2 structures assembled from mesoporous nanosheets with tunable porosity and their application as lithium-ion battery electrodes. New J Chem 38:3084–3091. https://doi.org/10.1039/C4NJ00258J Liu Z, Yu XY, Paik U (2016) Etching-in-a-box: a novel strategy to synthesize unique yolk-shelled Fe3O4@carbon with an ultralong cycling life for lithium storage. Adv Energy Mater 6:1502318. https://doi.org/10.1002/aenm.201502318 Liu C, Luo S, Huang H, Wang Z, Hao A, Zhai Y, Wang Z (2017) K0.67Ni0.17Co0.17Mn0.66O2: a cathode material for potassium-ion battery. Electrochem Commun 82:150–154. https://doi.org/ 10.1016/j.elecom.2017.08.008 Liu Q, Wang H, Jiang C, Tang Y (2019) Multi-ion strategies towards emerging rechargeable batteries with high performance. Energy Storage Mater. https://doi.org/10.1016/j.ensm.2019. 03.028 Lu Z, Dahn JR (2001) In situ X-ray diffraction study of P2 Na2/3[Ni1/3Mn2/3]O2. J Electrochem Soc 148:A1225–A1229. https://doi.org/10.1149/1.1407247 Lu J, Chen Z, Pan F, Cui Y, Amine K (2018) High-performance anode materials for rechargeable lithium-ion batteries. Electrochem Energy Rev 1:35–53. https://doi.org/10.1007/s41918-0180001-4 Ma X, Chen H, Ceder G (2011) Electrochemical properties of monoclinic NaMnO2. J Electrochem Soc 158:A1307–A1312. https://doi.org/10.1149/2.035112jes Ma J, Hu P, Cui G, Chen L (2016) Surface and interface issues in spinel LiNi0.5Mn1.5O4: insights into a potential cathode material for high energy density lithium ion batteries. Chem Mater 28:3578–3606. https://doi.org/10.1021/acs.chemmater.6b00948 Ma C, Alvarado J, Xu J, Clément RJ, Kodur M, Tong W, Grey CP, Meng YS (2017) Exploring oxygen activity in the high energy P2-type Na0.78Ni0.23Mn0.69O2 cathode material for Na-ion batteries. J Am Chem Soc 139:4835–4845. https://doi.org/10.1021/jacs.7b00164 Maggay IVB, Marie Z, De Juan L, Lu JS, Nguyen MT, Yonezawa T, Chan TS, Liu WR (2018) Electrochemical properties of novel FeV2O4 as an anode for Na-ion batteries. Sci Rep 8:8839. https://doi.org/10.1038/s41598-018-27083-z Manalastas W Jr, Kumar S, Verma V, Zhang L, Yuan D, Srinivasan M (2019) Water in rechargeable multivalent-ion batteries: an electrochemical pandora’s box. Chem Sus Chem 12:379–396. https://doi.org/10.1002/cssc.201801523 Mariyappan S, Wang Q, Tarascon JM (2018) Will sodium layered oxides ever be competitive for sodium ion battery applications? J. Electrochem Soc 165:A3714–A3722. https://doi.org/10. 1149/2.0201816jes Masese T, Yoshii K, Yamaguchi Y, Okumura T, Huang ZD, Kato M, Kubota K, Furutani J, Orikasa Y, Senoh H, Sakaebe H, Shikano M (2018) Rechargeable potassium-ion batteries with honeycomb-layered tellurates as high voltage cathodes and fast potassium-ion conductors. Nat Commun 9:3823. https://doi.org/10.1038/s41467-018-06343-6 Masese T, Yoshii K, Kato M, Kubota K, Huang ZD, Senoh H, Shikano M (2019) A high voltage honeycomb layered cathode framework for rechargeable potassium-ion battery: P2-type K2/ 3Ni1/3Co1/3Te1/3O2. Chem Commun 55:985–988. https://doi.org/10.1039/C8CC07239F

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Matsui M, Mizukoshi F, Imanishi N (2015) Improved cycling performance of P2-type layered sodium cobalt oxide by calcium substitution. J Power Sources 280:205–209. https://doi.org/10. 1016/j.jpowsour.2015.01.044 Mizushima K, Jones PC, Wiseman PJ, Goodenough JB (1980) LixCoO2 (085 150 Vg ‐VTH ð6:2Þ 2L

where W is the channel width, μ is the channel mobility, Cox is the capacitance per unit area of gate dielectric layer (Cox ¼ C/A ¼ kε0/d where k is the dielectric constant of the gate dielectric material, ε0 is electric permittivity of vacuum, and d is the layer thickness), and L is the channel length. Figure 6.4 shows the accumulation of charge carriers in n-type TFT in an enhancement mode when biased in saturation regime, i.e., for VDS > > (Vg - VTH). By grounding the source electrode and applying a positive gate voltage (Vg), charge carriers accumulate along the interface of semiconductor and dielectric. The performance of a TFT is evaluated on some several metrics such as mobility, current on/off ratio (Ion/Ioff), threshold voltage or turn-on voltage (VTH), and sub-threshold swing (SS) which can be extracted from the above curves. It is necessary to define these characteristics along with the current-voltage curves to help us understand the characteristics of TFT devices.

Carrier Mobility in Thin Film Transistors The ability of charge carriers such as electrons or holes to move through the crystal is termed as carrier mobility. This is an important parameter that defines the electrical properties of a device. In a solid material, the electrons experience two kinds of velocity: thermal (u) and drift velocity (vd). With no external field, the drift velocity

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of an electron is zero due to the random motion of the electrons in all directions. However, drift velocity increases linearly with the external electric field (E). vd ¼ ‐μe E

ð6:3Þ

where μe is the drift mobility. In a bulk semiconductor, the mobility of the carriers is reduced due to several scattering mechanisms. For instance, in an electronic device such as metal-oxide semiconductor field effect transistor (MOSFET), charge carriers are scattered along the semiconductor-oxide interface. Therefore the mobility can be mentioned as an effective carrier mobility (μeff) which can be defined by the scattering time (τ) and effective mass (meff) of an in-plane electron (Ye et al. 2017). μeff ¼

eτ meff

ð6:4Þ

As can be seen, carrier mobility can be increased by smaller effective mass of carriers. Effective mass is determined by the curvature of the energy band diagram. Electron’s effective mass is determined from the curvature of the conduction band minimum (CBM) whereas the hole effective mass is obtained by the curvature of valence band maximum (VBM). In metal oxide semiconductors, CBM is formed by unoccupied ns states of metal cations, whereas VBM is formed by occupied oxygen 2p anti-bonding bands. Most commonly, the value of such intrinsic or Hall mobility is determined from Hall effect measurement. In case of TFTs, channel mobility defines the semiconductor performance and therefore is an important electrical parameter to understand its potential for maximum switching frequency and current drive capability. However, an additional scattering source also needs to be considered in these devices. Narrow region along the dielectric-semiconductor interface limits the flow of charge carriers and causes scattering effect. Such narrow charge confinement leads to coulomb scattering from dielectric charges and surface roughness. Several kinds of TFT mobilities such as field effect (μFE), effective (μeff), and saturation mobility (μsat) can be determined using the two characteristic curves mentioned above using the following formulas: L dID WCox VDS dVg

ð6:5Þ

g L
m  WCox Vg  VTH

ð6:6Þ

μFE ¼ μeff ¼ and

6 Thin Film Metal Oxides for Displays and Other Optoelectronic Applications

μsat ¼

 p ffiffiffiffiffi2 2L d 2 ID WCox dVg

ð6:7Þ 

where gm is the transistor trans-conductance given by

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dID dVg

 .

High-definition flat panel displays (FPDs) are made of a large number of pixels, each with a layer of thin film transistors (TFTs) to control the brightness of the pixel color. Increasing the full high definition of such a display requires TFTs with very high electron mobility. Conventional a-Si has a lower electron mobility of 3 V respectively. Inset shows EL spectra for several currents. (Reprinted with permission from Ohta and Hosono 2004)

suitable to reduce the localization of the upper edge of VB which is critical to achieve p-type behavior. With the introduction of such p-type materials, Ohta et al. demonstrated the first-ever metal oxide-based UV-LEDs using p-SrCuO2 (SCO) and n-ZnO heterojunction deposited by pulsed laser deposition process (Ohta et al. 2000a, b). Figure 6.13 shows the schematic device structure of the first UV LED made of SrCuO2 and ZnO heterojunction. Using YSZ single crystalline substrate and ITO epitaxial layer as the transparent bottom contact, the pn junction of SCO and ZnO is successively deposited in the same PLD chamber. A thin Ni layer on top of the deposited films acts as the top contact. Using potassium (K+) ion as the dopant, the carrier concentration is tuned to be the same value for both the layers. With a turn-on voltage of 3 V, the device emitted at 382 nm which corresponds to electroluminescence due to electron-holeplasma recombination in ZnO. Apart from copper based p-type oxides, efforts were made to achieve p-type ZnO layer also. Presence of vacancies and interstitials of both oxygen and zinc ions make ZnO semiconductor, a natural electron donor (n-type). n-ZnO is mostly used to fabricate LEDs forming heterostructures with n-GaN or p-SrCuO2 layers as mentioned above forming a heterostructure. Having a pn junction made of only ZnO can simplify device fabrication and reduces contamination and heat dissipation. Considering its importance, several groups managed to achieve p-type ZnO widely by adding dopants such as N, P, As, or Sb. Such homojunction LEDs based on ZnO were fabricated using PLD, MOCVD, and MBE techniques. The emission wavelength of such devices was mostly centered at around 380 nm. LEDs have also been fabricated using different nanostructures of ZnO such as nanowires, nanorods, nanoplates, and quantum dots (Ji et al. 2017). These devices were found to generate

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luminescence in various wavelength regions based on the structure and bias voltages. In addition, such nanostructures of metal oxide heterojunctions have also been used to fabricate flexible LEDs which are useful for flexible electronics as bendable displays or even in clothing (Zhang and Rogers 2019). Biswas et al. fabricated fully inorganic flexible LEDs using p-CuO/n-ZnO heterojunctions with nanostructures (Biswas et al. 2016). Using flexible polyimide substrates, Al bottom contact layer and Ga-doped ZnO (GZO) seed layer, ZnO nanorods were grown on the seed layer by conventional low-temperature solution method. By immersing GZO-coated substrates in an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine (HMT), 500 nm long ZnO nanorods were grown at 90  C for 1 h. Similarly CuO nanorods were also synthesized and then deposited on ZnO layer by polydimethylsiloxane-transfer-and-rubbing technique. Au layer was then deposited to act as Ohmic contact. Figure 6.14a shows the schematic illustration of the fabrication process of p-CuO/n-ZnO flexible LEDs. Figure 6.14b shows the electroluminescence spectra for various bending radius of curvature and for various bending cycles with 5 mm radius of curvature. The red EL emission centered at 710 nm was attributed to the transition of native point defects of ZnO, i.e., transition between deep energy levels related to oxygen vacancies in this case. Following, Kim et al. reported emission at 610 and 400 nm wavelengths using flexible heterojunction LEDs based on p-type cobalt oxide (CO3O4) nanoplates and n-type ZnO nanorods (Kim et al. 2017). The low temperature aqueous solution process for the fabrication of these device layers is promising for flexible optoelectronics in industrial scale. Nanoplates of cobalt oxide were synthesized using cobalt (II) chloride hexahydrate and hexamethylenetetramine (HMTA), whereas nanorods of ZnO were synthesized using zinc nitrate hexahydrate and HMTA. Figure 6.15a shows the schematic of overall fabrication process of n-ZnO nanorods and p-Co3O4 nanoplates junction LED. Figure 6.15b shows the bright EL emission at 610 nm and a weak EL emission at around 400 nm from this heterojunction LED. The observed emission peaks can generally be explained based on band-to-band transition or transition between various point defects (Zni, Oi, VZn, VO) in ZnO. For instance, the peak at 400 nm was attributed to transition between conduction band minimum (CBM) to valence band maximum (VBM) or transition between Zni to VZn. The peak at 610 nm was related to the transition between CBM and VO or Zni to Oi. However, commercial LEDs based on ZnO remain a challenge because of defect level emission. A high-quality p-ZnO may help to realize the goal of commercial all-ZnO LEDs in the near future (Rahman 2019).

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Fig. 6.14 (a) Multi-step fabrication of flexible inorganic LEDs – A schematic representation. A 100 nm thick Al layer is deposited on 20 μm thick polyimide (PI) substrate. On the Al/PI structure, 100 nm thick 5 wt.% Ga-doped ZnO (GZO) layer is deposited followed by 1.5 μm thick patterned photoresist (PR) using optical lithography. ZnO nanorods were then grown on the patterned GZO seed layer. Using polydimethylsiloxane (PDMS)-transfer-and-rubbing technique, CuO nanorods were then deposited. A layer of Au acts as an Ohmic contact with CuO (b) Electroluminescence (EL) spectra for various bending radii of curvature and for various bending cycles at 5 mm radius of curvature. No significant change in EL intensity confirms the robust nature of the device. (Reprinted with permission from Biswas et al. 2016)

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Fig. 6.15 (a) Schematic representation of overall LED fabrication process based on heteroepitaxial junction of n-type zinc oxide (ZnO) nanorods and two dimensional p-type cobalt oxide (Co3O4) nanoplates. It shows the shapes of nanomaterials, a two-dimensional array of nanoparticles and nanoparticles size change during thermal decomposition. A free-standing heterojunction between vertically grown n-ZnO nanorods/p-Co3O4 nanoplates is also shown (b) Electroluminescence (EL) spectra of the above mentioned heterojunction LED under different forward bias voltages. The spectra show intense peak at 610 nm (reddish-orange light) and a relatively weak peak around 400 nm (violet light). Inset shows the magnified EL spectra around 400 nm. (Reprinted with permission from Kim et al. 2017)

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6.4 6.4.1

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Thin Film Metal Oxides for Other Optoelectronic Applications Transparent Conducting Oxides

For devices such as displays, organic light-emitting diodes (OLEDs) and photovoltaic cell modules, passive devices such as electrochromic windows and electrodes need to be transparent or semitransparent to allow the light to pass through. For device applications, such electrodes should have a transparency of over 80% in the visible region (400 nm < λ > 700 nm), but at the same time maintaining a low electrical resistivity value of less than 10‐3 Ω. cm. Transparent conducting oxides (TCOs) of semiconductors with a bandgap of more than 3.1 eV have been major choices for such electrodes due to their high transparency in the visible region along with high conductivity. In general, one or two layers of TCOs are coated onto glass by vacuum deposition technique. Both p-type and n-type semiconducting metal oxides that are transparent have been used as TCOs. The figure of merit (ΦTC) of TCO is used to evaluate the device performance and is given by Haacke (Haacke 1977). ΦTC ¼

T 10 RS

ð6:9Þ

where RS is the sheet resistance and T10 is ratio of optical transmission which is given by: T ¼ eαt

ð6:10Þ

where α is the absorption coefficient and t is the film thickness. Sheet resistance for a square substrate is calculated by the following equation: RS ¼

π R þ RVertical  Horizontal f ln 2 2

ð6:11Þ

where RHorizontal and RVertical are the average resistance values measured along the horizontal and vertical direction of the substrate and f is the correction factor. Figure of merit can also be defined in terms of material properties as follows: ΦTC ¼

πc 2 2 2 nv μ m e

ð6:12Þ

where n is the carrier concentration, ν is the frequency of incident light, μ is the mobility of the carrier, and m* is the effective mass. It can be seen that figure of merit can be increased by increasing carrier concentration and mobility. However, this parameter can only give a rough estimation for the choice of TCO. Depending on the applications, other factors such as mechanical,

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chemical, and thermal stability also play a major role. For applications such as OLEDs, work function of TCO is another factor that influences the performance of OLED. Work function is the minimum energy required to remove the electron from the conduction band. Whether TCO acts as anode or cathode determines the value for work function: high work function (> 5 eV) for anode TCO and low work function (< 4 eV) for anode TCO. One can alter the work function even by cleaning process thereby making them as anode and cathode electrodes. For instance, cleaning ITO by ultraviolet ozone treatment increases the work function of ITO to 5 eV thereby making them an anode for OLEDs. The most commonly investigated metal oxides for TCO applications are cadmium oxide (CdO), indium tin oxide (ITO), zinc oxide (ZnO), indium oxide (In2O3), and tin oxide (SnO2) because of their required characteristics. These materials can also be fabricated in large dimensions which add to their choices. Other multicomponent oxides such as cadmium tin oxide (CdSnO4), gallium indium oxide (GaInO3), yttrium-doped cadmium antimonates (Y:CdSb2O6), and so on also exhibit required properties for TCOs. Because of its low resistivity (~2  10‐4 Ω. cm) and optical transparency (~90%), indium tin oxide (ITO)-based TCOs have been dominant for optoelectronic devices such LCDs and thin film solar cell applications. ITO is typically a solid solution of 90 wt.% In2O3 and 10 wt.% SnO2. Thin films of pure and doped ITO have been deposited by several techniques such as vacuum evaporation, sputtering (DC and RF), spray pyrolysis, PLD, and sol-gel techniques. Almost two decades ago, Ohta et al. reported highly electrically conductive ITO thin films deposited on yttria-stabilized zirconia substrate by PLD technique using a KrF excimer laser beam emitting at 248 nm with a pulse duration of 20 ns at a repetition rate of 10 Hz. Using an ITO ceramic target with various SnO2 concentration, these epitaxial ITO films were fabricated in which the carrier concentration increases with an increase in SnO2 concentration and then saturates. A resistivity value of 7.7  10‐5 Ω. cm, carrier mobility of 42 cm2/Vs, and a carrier density of 1.9  1021cm‐3 were obtained for ITO films with 5.7 wt.% SnO2 (Ohta et al. 2000b). Over conventional fabrication techniques such as evaporation and sputtering, wet chemical methods such as sol-gel have become attractive over the last few years to fabricate ITO thin films in large volumes because of its simplicity and cheaper price. Sunde et al. reported fabrication of ITO thin films by spin coating of an aqueous precursor solution containing indium nitrate hydrate and tin acetate as the precursors (Sunde et al. 2012). The precursor solution is then mixed constantly with acetic acid and glycol which act as the complexing and esterification agents, respectively. The final solution was then spin coated on a spinning glass substrate (3000 rpm) for 45 s to form a single ITO layer of 17 nm thickness. The deposition was repeated to fabricate films with multiple layers as the electrical conductivity increases with number of depositions. The coated film was then further calcined for 1 h at 530  C under vacuum. The deposited films showed a transmittance of over 80% and a sheet resistance of 4.59  10‐3 Ω. cm. Figure 6.16a shows the TEM image of ITO film with 10 deposited layers. More than 80% transmittance for films with different layer numbers is demonstrated as shown in Fig. 6.16b.

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Fig. 6.16 (a) Cross-sectional transmission electron microscopic (TEM) image of indium tin oxide (ITO) thin film prepared with 10 deposited layers. Each deposited layer is around 17 nm thick. (b) Transmittance of ITO films prepared with various number (3, 5, 7, and 10) of deposited layers. Thicker films show maxima and minima due to the interference between the deposited layers. (Reprinted with permission from Sunde et al. 2012)

Nevertheless, indium metal has become scarce and expensive that pushes researchers to identify for other oxide materials either with less indium or without indium. Thin films based on zinc oxide became more popular because they are cheaper, highly durable, and less toxic. When doped with metal ions such as Ga and Al, these thin films have been found to exhibit similar conductivity and transparency as that of ITO. They are widely used as transparent cathode electrodes for transparent OLEDs, crystal displays, and solar cell windows (Wang et al. 2013). Liu et al. studied the effect of various doping and processing conditions on the performance of ZnO based TCOs (Liu et al. 2013). Similarly Sb- or F-doped tin oxide (SnO2) is

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another common TCO material for thin film photovoltaic solar cells. By adding fluorine ion with SnO2 increases the carrier concentration and mobility by a factor of 2, thereby reducing the resistivity by a factor of 4. Two phase binary systems such as ZnO-In2O3, ZnO-SnO2, and CdO-SnO2 have also been developed but with poor performance. One can also achieve p-type TCOs by adding dopant ions to conventional metal oxide materials. By adding Mg or N or In in ZnO, one can get p-type TCOs. Similarly, NiO, lithium-doped NiO, Cu2SrO2, CuGaO2, and CuAlO2 have also been fabricated with p-type semiconducting characteristics for TCO applications (Afre et al. 2018).

6.4.2

Solar Cells

A solar cell is a device that employs photovoltaic effect to convert the energy of sunlight into electricity. Silicon was used for building the first photovoltaic module and dominates the solar cell devices for several decades. Second generation photovoltaic cells were developed using thin films of direct bandgap semiconductor materials such as cadmium sulfide, cadmium telluride, copper indium gallium selenide (CIGS), gallium arsenide, copper zinc tin sulfide, and so on. This was followed by third generation of photovoltaic cells based on nanocrystals, polymers, dye sensitizers, and so on. Dye sensitized solar cells use dye molecules to generate electron/hole pairs under the illumination of light. Metal oxide-based films are utilized to collect the generated electrons. Metal oxides have gained attention for solar cells because of their relatively cheaper material cost, economical fabrication, low toxicity, and high chemical stability with considerable high conversion efficiency. As mentioned in the previous section, transparent conducting oxide (TCO) layers using such metal oxides act as front electrodes or as electron transport layers. On the other hand, metal oxides such as vanadium pentoxide (V2O5), tungsten trioxide (WO3), nickel oxide (NiO), and molybdenum trioxide (MoO3) act as hole transport materials. Solar cells based on metal oxides are built with a general structure of n-transparent layer (TCO) and metal oxide p-absorber layer forming a pn-junction. Figure 6.17 shows a typical representation of a thin film solar cell structure. The absorber layer should be a direct bandgap semiconductor having a strong absorption for sunlight even with a material of small thickness. One of the most widely studied p-absorber material is cuprous oxide (Cu2O), a direct bandgap semiconductor with a bandgap of about 2.1 eV. Heterojunction solar cells of Cu2O with n-type semiconductor layer such as ZnO with a TCO electrode have been built with different combinations to improve the efficiency (Dumitru et al. 2018). Efficiency was further improved by Minami et al. using heterojunction of p-type Cu2O layer with multi-component oxide n-type semiconductors such as ZnGa2O4, MgIn2O4, Zn2SnO4, Zn2GeO4, CuGaO2, and so on (Minami et al. 2016). A 200 nm thick aluminum-doped ZnO (AZO) thin film was used as the transparent conducting electrode. All these oxide layers were deposited by pulsed

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Fig. 6.17 Schematic of a typical thin film solar cell structure. The device is formed on a substrate such as glass or polyimide (PI) foils followed by a metallic back contact such as Ti, W, and so on. A p-type absorber layer is a direct bandgap material such as cuprous oxide (Cu2O). A pn-junction is formed between semiconductor materials. An n-type semiconductor layer acts as the window followed by an antireflection layer. Transparent conducting oxide (TCO) layer is then finally deposited to allow maximums sun light to pass through. (Modified after Mathur et al. 2014)

laser deposition technique using a 193 nm KrF excimer laser with a pulse width of 20 ns at a repetition rate of 20 Hz. Cuprous oxide sheets were formed by oxidizing highly pure copper sheets by heat treating the metal sheet at 1015  C. The solar cell then forms a structure AZO/n-oxide semiconductor/p-Cu2O. By illuminating the sunlight through AZO, the performance of the solar cells was evaluated. A highest efficiency of 5.36% was achieved using ZnGa2O4 as the active layer in AZO/ZnGa2O4/p-Cu2O heterojunction solar cell. The resistivity of Cu2O can be further lowered to 10‐1 Ω. cm by doping sodium metal ion in post-annealing treatment under Ar atmosphere. One can also increase the solar cell efficiency by adding an additional antireflecting layer such as magnesium fluoride (MgF2) to reduce the reflection of incident light. Such changes improve the efficiency to 6.25% with an open circuit voltage (Voc) of 0.84 V using a heterojunction layers of MgF2/AZO/n(Ga0.975Al0.025)2O3/p-Cu2O:Na. Another way to improve the efficiency of such heterojunction solar cells is to add a buffer layer between Cu2O and n-semiconductor. The buffer layer helps to improve the charge carrier transport properties by reducing the band misalignment. An addition of buffer layer between the pn-junction helps to reduce the band gap energy offset. As an example, a heterojunction of Al-doped ZnO (AZO) and Cu2O causes a misalignment between conduction and valence band because of their difference in electron affinity and bandgap values. By adding a buffer layers such as Ga2O3, TiO2, ZnS, Ta2O5, GaN, and ZnO alloys, one can minimize this band misalignment and improve the power conversion efficiency and Voc significantly. In the case of AZO/Cu2O heterojunction solar cells, addition of a buffer layer with lower electron affinity can increase the solar cell performance as shown in Fig. 6.18a. Figure 6.18b shows the simulated I-V characteristics of AZO/Cu2O heterojunction solar cells as a function of the buffer

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Fig. 6.18 (a) Schematic of aluminum-doped zinc oxide (AZO)/cuprous oxide (Cu2O) heterojunction solar cell structure. (b) I-V characteristics for AZO/Cu2O solar cell with various buffer layer electron affinity values – simulated values. Highest slope efficiency of 12.5% was observed for barrier with an electron affinity of 3.7 eV. (c) Energy band diagram of AZO/Cu2O heterojunction with buffer layers of two different electron affinity values (3.7 and 4.4 eV). (Reprinted with permission from Nordseth et al. 2018)

layer electron affinity values of the buffer layer. An efficiency of 7.2% was found to increase up to 12.5% just by changing the buffer layer of 4.4 eV electron affinity into a buffer layer having an electron affinity of 3.7 eV. Figure 6.18c shows the energy band diagram with two different buffer layers (Nordseth et al. 2018). Spinel oxides with a general formula AB2O4 is another popular material for metal oxide-based heterojunction solar cells. Kupfer et al. fabricated heterojunction solar cells using thin films of p-type Co3O4 spinel as the absorber layer on TiO2 window layer (Kupfer et al. 2015). With two optical bandgaps of 1.5 eV and 2.2 eV related to direct energy transitions, Co3O4 has been studied for solar cells because of their abundance and low toxicity. Thin films of titanium diode (TiO2) were first deposited on fluorine doped tin oxide (F-doped SnO2) TCO coated glass substrate using spray pyrolysis process. Pulsed laser deposition technique was then followed to deposit Co3O4 thin films on top of TiO2 using KrF excimer laser and Co3O4 target at different deposition temperatures. Au back contacts were made by sputtering, whereas Sn/Pb common front contacts were made by ultrasonic soldering. This spinel based solar cell showed an open circuit voltage of 430 mV for the films deposited at 600  C. Yan et al. obtained an improved Voc of 534 mV using Co-Fe-O composite spinel oxides through structural inversion (Yan et al. 2016). By varying the composition of Fe to Co, the optical bandgap and electrical conductivity changes along with the crystallinity. Compared to Co, addition of Fe was found to improve the power conversion efficiency and Voc. Metal oxide thin films are also used as transparent conducting oxide (TCO) layers for organic photovoltaic cells which is made of organic semiconductors as the active layer. These organic cells are built with heterojunction of organic semiconductors sandwiched between top metal electrode and a bottom TCO. Transparency and conductivity of the TCO layer are the deciding factors of power conversion efficiency of these photovoltaic cells. Indium tin oxide, the most common TCO electrode for solar cells has been used widely for these devices. However, these

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Fig. 6.19 (a) Sheet resistance as a function of bending radius of amorphous zinc-indium-tin-oxide (a-ZITO)/Arylite electrodes. Measurement was done after fully relaxed. The film does not show significant change in conductivity with bending radius (b) polycrystalline-ITO/arylite and a-ZITO/ arylite optical and scanning electron microscopic images after bent at a radius of 5 and 1.5 mm respectively (c) and (d) power conversion efficiency and current density of two flexible polymer solar cells (PSCs) (P3HT:PC61 BM and PTB7PC71 BM PSC) at various bending radii values. The device performances degraded only a very little for bending radii from 101 to 5 mm. (Reprinted with permission from Zhou et al. 2014)

metal oxide thin films suffer from degradation and poor mechanical stability such as cracking along the grain boundaries thereby reducing the electrical performance of these organic-based devices. Mixed metal oxides with ITO help to solve these problems with much better mechanical stability. Zhou et al. reported ultra-flexible organic solar cells using 250 nm thick thin films of amorphous zinc indium tin oxide (a-ZITO) as the TCO electrode (Zhou et al. 2014). Deposited by PLD technique using ZITO targets and a 248 nm KrF excimer laser, these a-ZITO electrodes showed an excellent transparency of more than 80% and a sheet resistance of ~20 Ω/sq. Polymer solar cells based on spin coated P3HT: PC61BM and PTB7:PC71BM active layers using these a-ZITO TCO electrodes performed with a power conversion efficiency of 3.63 and 6.42%, respectively. Under bending, these PCE values varied only very slightly for various bending radii ranging from 100 to 5 mm confirming the stability of electrical performance of these TCO layers. Figure 6.19a and b show the sheet resistance as a function of

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bending radius of a-ZITO/Arylite electrodes and the microscopic images of polycrystalline-ITO/arylite and a-ZITO/arylite substrate after bending, respectively. Figure 6.19c and d show the performances polymer solar cell (PSC) based on two different structures for various bending radii. Thin films of metal oxides have also been used as interfacial layers in organic solar cells for collecting charge carriers, both holes and electrons. In general, organic solar cell employs a configuration with absorbing/charge separating bulkheterojunction layers sandwiched between two metal electrodes. Another interfacial layer mostly based on transition metal oxides is added to the electrodes to enhance the carrier collection efficiency. Metal oxides such as NiO, RuO2, Ag2O, CuO, Fe3O4, and so on with higher work function are used as a buffer layer to inject holes for anodes, whereas the metal oxides such as TiO2, ZnO, and ZrO2 with lower work function are used as electron-injection layer for cathodes (Greiner and Lu 2013). Acidic PEDOT:PSS buffer layer placed between the active layer and ITO has been widely used for hole collection. Thin films of p-NiO have been used as the hole extracting interfacial layer for P3HT:PCBM based solar cell with a power conversion efficiency of 5.16%. In this heterojunction solar cell, P3HT acts as the electron donor, whereas PCBM acts as the acceptor. NiO thin films are commonly deposited by pulsed laser deposition even though low temperature solution-based process has also been attempted. Zilberberg et al. fabricated sol-gel processed vanadium pentoxide (V2O5) thin films as a hole extraction interfacial layer in polymer/fullerene solar cells (Zilberberg et al. 2011). Using isopropanol solution of vanadium oxitriisopropoxide, V2O5 layers are spin coated with different thickness from 10 to 45 nm. A maximum power conversion efficiency of 3% was achieved using 10 nm thick V2O5 layer which is found to be better than that of conventional PEDOT:PSS extraction layer. Compared to PEDOT:PSS, V2O5 based solar cells are also found to perform without degradation even several hundreds of hours. Other transition metal-based metal oxides with larger work function such as molybdenum (III) oxide (MoO3) and tungsten (III) oxide (WO3) with larger work function up to 6.9 eV have also been studied as hole extraction layers. Similar to hole extraction layers, n-type semiconducting metal oxides such as TiO2, PbO, and ZnO have been extensively used as electron extraction layers. Yusoff et al. achieved a PCE of 11.83% using a lithiumdoped ZnO as the electron extraction layer in triple junction polymer solar cell containing PSEHTT:IC60BA and PTB7:PC71BM as the absorbing layers along with TCO layers and electrodes (Yusoff et al. 2015). A 40 nm thick lithium-doped zinc oxide (LZO) layer was spin coated onto ITO substrate which was cleaned and treated with UV-ozone before the coating. Titanium dioxide is introduced as an electron collecting material by Kim et al. for polymer solar cell of P3HT:PCBM active layer (Kim et al. 2006). A solution of titanium isopropoxide precursor with 2-methoxyethanol and ethanolamine mixture was prepared in isopropyl alcohol. The precursor solution is then spin coated onto the photo active polymer layer. The incorporation of TiO2 layer significantly improved the cell performance compared to the cell without TiO2 layer. Lead monoxide (PbO) was used as the electron extraction layer by depositing a PbO

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layer onto ITO substrate. An addition of PbO has been found to decrease the work function of ITO from 4.7 eV to 3.8 eV. PbO thin films with thicknesses of 0.5–2.5 nm were deposited onto ITO by thermal evaporation technique. Solar cell made of 230 nm thick P3HT:PCBM active layer was found to perform with a PCE of 4% and Voc of 0.59 V. Several other metal oxide layers are being explored to improve the performance of solar cells with a better PCE and stability over long usage.

6.4.3

Ultraviolet Photodetectors

For daily life, industries and in laboratories, photodetection of ultraviolet (UV) light ranging from 10 nm to 400 nm has gained extensive attention for solar UV monitoring, space exploration, gas sensing, and so on. Different materials are used to detect different regions of UV which can be divided into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). These detectors are based on semiconductor in which the absorption of incoming photons generates electron-hole pair. The generated carriers, i.e., both electrons and holes are then collected by electrodes before their recombination. An efficient photodetector requires an active layer with a significant absorption to the incoming photons and an electrode to collect the generated pairs (Hoon et al. 2013). The semiconductors are placed between the two metal electrodes. Silicon with a bandgap of 1.1 eV was initially used but suffered from degradation by high energy photons. Devices using Si also require long-pass optical filters to filter out the low energy photons. Nitride-based wide bandgap semiconductors such as GaN, AlGaN, and InGaN showed better thermal stability for UV light. Materials such as silicon carbide (SiC) and diamond have also been explored. Wide bandgap semiconductors based on thin film metal oxides such as ZnO, In2O3, SnO2, CuO, Co3O4, TiO2, Ga2O3, and so on have attracted attention because of their high efficiency and sensitivity in UV photodetection. Figure 6.20 shows the simple UV-detector structure using metal oxide semiconductors. In addition, nanostructured metal oxides improve the overall performance in terms of sensitivity. As an example, indium oxide (In2O3) with a bandgap of ~3.6 eV is an efficient material for UV photodetector whose sensitivity is significantly enhanced by one-dimensional nanostructures. In2O3 nanowires deposited by laser ablation technique showed significant increase in conductance when the device was illuminated by UV light (Zhang et al. 2003). Zinc oxide with an excellent transmission in UV wavelengths, a direct wide bandgap of ~3.37 eV at room temperature, large exciton binding energy of 60 meV, low cost, and a strong response to UV makes it a potential candidate for UV photodetector. High thermal, mechanical, and chemical stability make ZnO a major choice for harsh environments. Thin films of ZnO for these detectors have been deposited by several thin film processing techniques. Sputtering technique offers advantages of depositing of ZnO onto large area substrates with uniformity and at moderate deposition temperature. Hoon et al. reported the deposition of a

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Fig. 6.20 Schematic structure of self-powered ultraviolet (UV) photodetector based on photovoltaic effect which transforms incident UV radiation into electrical energy. The device in general is a homojunction of p and n-type semiconductor such as zinc oxide (ZnO). (Modified after Chen et al. 2015)

350 nm thick ZnO films on glass substrates using highly pure ZnO ceramic target in Ar atmosphere (Hoon et al. 2013). The structural properties of deposited films can be varied by varying the substrate temperature between 50 and 450  C. Aluminum electrodes were then deposited on ZnO active layer to fabricate the detector in a metal-semiconductor-metal (MSM) structure. The ZnO detectors function as per the principle of oxygen molecule adsorption under atmospheric condition on the ZnO surface, thereby increasing the thin film resistance. Under the illumination of UV light with an energy greater than the energy gap of ZnO thin film, the electrons are excited to the conduction band generating electron-hole pair resulting in an increase in conductivity. When a bias voltage is applied along the metal electrodes, the generated carriers are collected by the electrodes before their recombination. The measurements of photocurrent or I-V characteristics under UV light and white light are used to evaluate the device. The sensitivity of the detectors are increased by forming nanostructures such as nanowires, nanorods, nanobelts, or nanotetrapods of ZnO on the surface. With continuous research on this area, efficient and compact devices have been developed with fast response time and high gain based on the advantages of nanostructures. The detector responsivity can be defined as (Zeng et al. 2016): R¼

Ip Popt A

ð6:13Þ

where Ip is the photocurrent, Popt is the incident optical power, and A represents the active area. The ability of the detector to detect weak optical signals can be defined as detectivity given by:

6 Thin Film Metal Oxides for Displays and Other Optoelectronic Applications

D¼

R ð2qJ Þ1=2

229

ð6:14Þ

where J is the background current density. A low background current increases the detecting ability of the detector. Park et al. demonstrated ultrafast response of photodetector based on ZnO nanostructures as the active layer sandwiched between Ti/Au electrodes (Park et al. 2015). ZnO nanoline pattern was deposited by ultraviolet nanoimprint lithography (UV-NIL) using a ZnO precursor resin. The resin was prepared by dissolving zinc acetate dehydrate, monoethanolamine, and 2-nitrobenzaldehyde in 2-methoxyethanol. The precursor was coated onto SiO2/Si substrate by spin coating process. Nanoline-patterned polyurethane acrylate mold is then attached to the substrate followed by UV light illumination for resin curing. By position-controlled patterning method, ZnO was produced in a grating structure with a period 1 μm. Using UV-NIL technique, the photodetector was fabricated in metal-semiconductormetal (MSM) architecture with a responsivity of 22.1 A/W. The device performed with an impressive rising and falling times of 43 and 56 μs respectively, which are much better than other ZnO-based photodetectors. Flexible photodetectors based on vertically aligned ZnO nanorods on ultrathin nylon substrate have been fabricated by Mohammad et al. at a very low cost (Mohammad et al. 2015). Zeng et al. demonstrated good performing flexible photodetector using ZnO nanowires deposited onto polyethylene terephthalate (PET) substrates coated with indium tin oxide layer (Zeng et al. 2016). ZnO seed layer was first deposited on the ITO layer by pulsed layer deposition at room temperature. This was followed by the immersion into a solution of zinc acetate and hexamethylenetetramine solution kept in a stainless steel autoclave with a Teflon liner. The container was sealed and maintained at 82  C for 5–6 h to grow ZnO nanowires by hydrothermal process. Once the growth is finished, the substrate was cleaned with ethanol and deionized water and then dried. To enhance the light concentration in the active layer, silver nanoparticles are deposited on ZnO nanowires by photoreduction method. Platinum film deposited by e-beam evaporation acts as the counter metal electrode for the device. Figure 6.21 shows the schematic representation of UV-photodetector structure made of Ag nanoparticle-modified ZnO nanowires. Figure 6.21b and c show the I-V characteristics of UV-photodetector working in the flat and bending status in darkness and in illumination. Figure 6.21d shows the variation of photo current density when the device is flat, flexed, and then reverted. Figure 6.21e shows the photocurrent response for different Ag concentration (in μM). Deposition of Ag nanoparticles having a size of ~30 nm, smaller than that of ZnO nanowire diameter has been found to significantly increase the absorption of the light, thereby increasing the sensitivity and efficiency of the detectors. Heterojunctions of two different metal oxide thin films such as ZnO/Cu2O and NiO/ZnO have been investigated to cover different regions of UV and with high response time. For instance, heterojunction of n-ZnO and p-NiO has been used to develop highly sensitive UV detectors for long wavelength UV light ranging from

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Fig. 6.21 (a) Schematic of UV-photodetector structure based on ZnO nanowires (NWs) modified with Ag nanoparticles that act as an active electrode. A 20 nm thick platinum film acts as the counter electrode. Both the electrodes are bonded together with sealant based on polyethylene terephthalate (PET). (b) and (c) I-V characteristics of ZnO nanowires detector in flat and bending statuses respectively when the device is in darkness and under illumination. The results confirm the device normal performance even under bent or flat (d) Photocurrent response of the device when the device is flat, bent, and then reverted back. (e) Photocurrent density for different Ag concentration in ZnO NWs modified with Ag nanoparticles. The device performance is enhanced by modification with Ag nano particles. (Reprinted with permission from Zeng et al. 2016)

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380 to 400 nm. Patel et al. fabricated transparent UV photodetectors using NiO/ZnO heterojunctions fabricated on fluorine-doped tin oxide (FTO) coated glass substrate (Patel et al. 2015). NiO/ZnO structure was deposited onto the substrate by a sputtering process. ZnO thin film was first deposited using ZnO target and then Ni metallic film was deposited on ZnO by DC sputtering method. By flowing oxygen, NiO layer is formed by rapid thermal process. Aluminum films deposited by DC sputtering method on NiO film acts the electrodes. The detector was found to show high sensitivity and repeatability for long operation time with a fast rising time of 24.2 ms. A highest responsivity and detectivity of 3.85 A/W and 9.6  1013 Jones have been determined for these devices, respectively. Other metal oxides that are common for UV photodetectors are Ga2O3, SnO2, In2O3, and multi-component oxides such as In2Ge2O7, ZnGeO, and so on. Gallium oxide semiconductor with a bandgap of ~4.9 eV is preferred for deep UV light at 254 nm wavelength. Ga2O3 nanowires show good response and high conductivity for UV light.

6.4.4

Gas Sensors

Detection and quantification of toxic, flammable, and contaminant gases is essential in industries and for environmental protection. Compact gas sensors with high sensitivity have been attractive for such applications. Several metal oxide materials are used as gas sensors to detect gases such as carbon monoxide (CO), nitrous oxide (NO), oxygen (O2), and so on. The basic principle of gas sensor is based on the change in electrical properties such as conductivity of the sensor material during the interaction between metal oxide and gas species. When surface of metal oxide is exposed to gas molecule, the gas molecules are adsorbed on the surface either by chemical bonds or just physical attachment. During this adsorption, electrons are either drawn to the surface or pushed inside metal oxide depending on adsorbed gas molecules. This results in the change of charge carrier density on the surface which lead to band bending. A change in conductivity of the sensor material is therefore observed which can be detected by the change in current flow through the sensor. For instance, when a metal oxide layer is in contact with O2 gas, the electrons are extracted from the conduction band and become oxygen anions. The presence of these oxygen anions on the sensor surface result in band bending with an electron depleted space charge layer as shown in Fig. 6.22. When the target gas molecules come in contact with this surface, the oxygen anions react with the target gas which then changes the charge carrier density of sensor material (metal oxide). A change in conductivity is then detected as the sensor signal. Based on the target gas, the electron concentration increases or decreases. For oxidizing gases such as nitrogen dioxide (NO2) and carbon dioxide (CO2), the electrons are trapped on the surface which decreases the electron concentration in the metal oxide. If the sensor material is an n-type semiconductor whose majority carriers are electrons, this process increases the resistance. However if the sensor is made of p-type semiconductor having holes as the majority carriers, interaction with

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eVsurface EC

–

e

e–

e–

e–

O2, gas

e–

EF

– Osurface

Λair

EV bulk

surface

gas

Fig. 6.22 Band bending due to chemisorption of O- ions. Adsorption of O2 molecules on the surface of metal oxides extracts electrons from conduction band and trap the electrons in the form of ions at the surface. This results in band bending with an electron depletion layer. Reacting gases in contact with such oxygen species can reverse the band bending and increase the conductivity (Ec Conduction band, Ev Valence band, EF Fermi level, ^air thickness of space-charge layer, e conducting electrons, + donor sites). (Reprinted with permission from Wang et al. 2010)

oxidizing gas decreases the resistance thereby increasing the conductivity. The process is opposite for reactive gases such as carbon monoxide (CO) and nitrous oxide (NO) in which the electrons are injected into the metal oxide during the reaction with the pre-adsorbed oxygen anion. In such cases, carrier concentration of n-type semiconductor material increases and hence the conductivity of the sensor. The magnitude of the signal depends on concentration of the target gas. In general the conductance of the sensor is linearly proportional to the logarithm of gas concentration. Sensitivity (S) also depends on the temperature, test gas concentration and the nature of gas also. For reducing gas, S can be defined as Ra/Rg, whereas for oxidizing gas S is Rg/Ra. The sensor sensitivity is widely defined as follows (Lee et al. 2001): Sð % Þ ¼

Ra  Rg  100 Ra

ð6:15Þ

where Ra and Rg are the sensor resistances in clean air and in test gas environment. Even though several materials including conducting polymers are explored for this application, metal oxides are more attractive because of their simple device structure with excellent physical and chemical stability which is essential for harsh gaseous environment and temperature. Metal oxides such as Cu2O, CeO2, In2O3, TiO2, ZnO, SnO2, and so are being used for different gas species including volatile organic compounds. Gas sensors based on SnO2 have been widely investigated for several decades followed by ZnO and V2O5 materials. Compared to sensors based on bulk materials, thin films or thick films offer low power consumption,

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compactness, cheaper price, and stability. Most of the metal oxide thin films for gas sensors are based on n-type semiconductors. Nevertheless cupric oxide (CuO) which is used to sense H2, O2, ethanol, and CO is a p-type semiconducting material. By mixing with other oxides, the sensitivity of CuO can be extended to other gases also. Wollenstein et al. reported single chip thin film SnO2 sensor array using four sensing elements to detect CO, O3, and NO2 gases (Wollenstein et al. 2000). High-dense SnO2 thin films were prepared by sputtering followed by sintering at 700  C. To study the effect of a catalyst, a layer of platinum layer was deposited on SnO2 both using evaporation and sputtering technique. Sensors with the Pt layer showed higher sensitivity at low operating temperatures below 325  C, whereas pure SnO2 layers showed higher sensitivity to CO gas with the maximum sensitivity at 340  C. Suchea et al. used transparent ZnO thin films to detect O3 gas (Suchea et al. 2006). Using high pure metallic Zn and ZnO ceramic targets, ZnO films were deposited by DC magnetron sputtering method under O2/Ar gas atmosphere. An exposure to ozone gas results in change in ZnO conductivity value. It was found that the films prepared from Zn metallic target showed higher sensitivity toward ozone and also a significant change in conductivity almost four order of magnitude better than that of the films prepared from ZnO ceramic target. For high operating temperatures, thin films based on metal oxides such as gallium oxide (Ga2O3), titanium dioxide (TiO2, strontium titanate (SrTiO3) and ferric oxide (Fe2O3) are used. Thin films of Ga2O3, an n-type semiconductor with a melting point of 1900  C is used for O2 detection even at high temperatures of above 900  C due to their high melting point (Fleischer and Meixner 1992). These sensors are prepared by depositing polycrystalline Ga2O3 thin films on a ceramic substrate by sputtering technique using a Ga2O3 ceramic target. The thin films are annealed to increase the crystallinity. By using platinum electrodes for electrical contacts, these sensors are used for oxygen detection at various temperatures and pressures. For detecting ammonia, sensors have been made using TiO2 thin films deposited on a Si substrate by DC magnetron sputtering (Karunagaran et al. 2007). Highly pure Ti was used as the sputtering target, whereas oxygen was used as the reactive gas. Ar gas was used as sputtering gas to maintain the sputtering pressure. The deposited films are subsequently annealed at 600  C which helps to significantly increase the sensitivity of the sensor. By injecting ammonia gas inside an airtight chamber containing a heater and the sensor, the electrical resistance of the sensor is measured for various heater temperatures up to 500  C. The device was found to perform with high sensitivity at a higher concentration of ammonia and at an operating temperature of 250  C. Similarly, α ‐ Fe2O3 thin films deposited by plasma enhanced CVD technique was used for isobutane (i-C4H10) detection. Gas sensors with different nanostructures also play a major role in sensing performance. To detect NO2 gas, sensors based on ZnO thin films with nanorod structures. The sensor contains 100 nm thick platinum electrodes deposited on the front side of Al2O3 substrate by DC sputtering method. The back side of the sensor contains screen-printed resistive heater as shown in Fig. 6.23a. On this sensor substrate, 40 nm thick thin film of Zn was deposited using rf sputtering technique at an rf power of 100 W for 2 min (Fig. 6.23b). The deposited substrate was then

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Fig. 6.23 (a) Picture of sensor substrate with platinum (Pt) electrodes and a resistive heater. Pt electrodes are comb shaped. (b) Thin film of Zn sputtered on sensor substrate. (c) Schematic diagram of ZnO nanorod arrays with vertical alignment on a sensor substrate grown by sonochemical process. (d) Isothermal response curve of ZnO gas sensor under 10 ppb NO2 gas measured at 250  C. A maximum response of 37%, response time of 4.5 min, and a recovery time of 4 min were found during the performance. (Reprinted with permission from Oh et al. 2009)

subsequently immersed in an aqueous solution of 0.01 M of zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and 0.01 M of hexamethylenetetramine ((CH2)6N4). After ultrasonification of the solution for 1 h at an ultrasonic frequency of 20 kHz and an intensity of 39.5 W/cm2, ZnO nanorod arrays were formed on the substrate (Oh et al. 2009). Figure 6.23c shows the schematic illustration of vertically aligned ZnO nanorods synthesis by sonochemical process, grown in sensor substrate. Electrical characteristics of ZnO changes under the exposure of NO2 gas due to the change in concentration of charge carriers (electrons) on ZnO nanorod arrays. The reaction that involves in this process can be defined as follows: NO2 ðgasÞ þ e‐ ! NO2‐ ðadsÞ ‐

‐

ð6:16Þ

NO ðadsÞ þ O ðadsÞ þ 2e ! NOðgasÞ þ 2 O ðadsÞ 2‐

2‐

ð6:17Þ

It is well known that oxygen molecules are adsorbed onto ZnO surface to form O2, O and O2 ions depending on the temperature. Below 100  C, O2 ions are stable, whereas O ions are stable between 100 and 300  C. Above 300  C, O2 ions are stable. When NO2 gas comes in contact with adsorbed oxygen ions (O) or with ZnO nanorods, NO2 ions are adsorbed and collects electrons from ZnO nanorod arrays. This results in decrease in electron concentration on ZnO nanorod surface and thereby decreases the conductance measured as the sensor signal. These sensors were found to detect up to 10 ppb with an excellent response time in few tens of seconds as shown in Fig. 6.23d. Similarly sensors based on tungsten oxide (WO3) thin films with nanostructures such as nanorods and nanowires have been developed for the detection of NO2 gas (Qin et al. 2010). These spin coated thin films showed a detection response of less than 20 s to detect 5 ppm of NO2 gas. Because of the low processing temperature of these materials, thin films with nanostructures are gaining

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popularity for gas sensors. Apart from pure metal oxides, doped materials, mixed metal oxides, and hybrids of metal oxides with other semiconductors are also used for sensing applications.

6.4.5

Heat Mirrors

Heat mirrors consist of dielectric or metallic multi-layers of thin films that transmit visible part of the electromagnetic spectrum but reflect the infrared (IR) region of the spectrum. Because of its high reflection at the thermal IR radiation, windows with such heat mirror coatings act as thermal insulator reducing the cooling requirement significantly in buildings during warmer days. With proper design, similar mirrors can also reflect back thermal radiation back inside the building during the winter while allowing the light and near IR radiation. Another most common use of such mirrors is to spectrally select wavelengths for solar cells. By separating thermal radiation from the solar radiation, one can enhance the overall efficiency of solar cells. By placing a heat mirror on to the backside of the solar panel cover plate, solar radiation passed to the solar absorber, whereas the emitted IR radiation by the absorber are reflected back by heat mirrors thereby increasing the efficiency by the reducing the radiation losses (Fan and Bachner 1976). These mirrors are generally made with two types of materials: multilayers of metal/dielectric films and doped semiconductor films. In the former case, a semitransparent metallic thin film is incorporated between the dielectric films that can transmit the visible light with a high reflectance for IR from the metallic films. Metal films such as Au, Cu, Ag, Al, Ni, and Ti with a thickness of less than 100 Å are transparent to the visible region and highly reflecting in the IR. The film architecture can generally be X/M and X/M/ X where X can be a dielectric, semiconductor, or polymer and M is the metal. In the latter case, heat mirror materials based on semiconducting thin films have high reflectance for the IR radiation. Earlier materials are doped semiconductors such as F:SnO2, Sb:SnO2, Sn:In2O3, and CdSnO4. Using such materials, heat mirrors are developed as MOS/M/MOS where MOS is metal oxide semiconductor and M is the metal. Several other metal oxide materials are used for thermal heat mirror. Al-Kuhaili et al. fabricated NiO/Ag heat mirrors based on 30 nm thick nickel oxide thin films deposited by thermal evaporation technique (Al-Kuhaili et al. 2015). These mirrors had an IR reflectance of 77% and a visible to solar transmittance ratio of 1.52. Ramzan et al. fabricated heat mirrors as HfO2/Ag/HfO2 structure with a reflection of more than 80% at near infrared (NIR) region (Ramzan et al. 2015). Compared to a single HfO2 thin film, multilayer thin films along with an additional metal layer such as Ag increase the reflectance >80% which is suitable for heat mirrors. Recently Wiatrowski et al. reported high-quality HfO2 thin films deposited by mediumfrequency magnetron sputtering (Wiatrowski et al. 2019). Using Hf target, HfO2 films were sputtered in a pure oxygen for various magnetron power (200, 400, and 600 W) with a working pressure of 2  10‐2 mbar. Based on the magnetron power,

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the coated films showed a transmission between 80 and 90% in the visible region. The hardness of HfO2 thin films increased with increasing sputtering power with a maximum hardness of 12 GPa. Overall, such coatings are suitable for heat mirrors and other optoelectronic applications.

6.5

Thin Film Fabrication Techniques

Metal oxide thin films have been deposited by several techniques based both on chemical and physical routes. Chemical routes such as sol-gel, spray pyrolysis, and various chemical vapor deposition methods have gained interest for large-scale production. Similarly physical vapor deposition methods based on evaporation and sputtering are also widely employed. Deposition methods are chosen based on the semiconductor and substrate materials. Here we discuss briefly different thin film deposition techniques commonly employed for the fabrication of metal oxide thin films.

6.5.1

Physical Vapor Deposition Techniques

Physical vapor deposition (PVD) method enables to achieve high pure thin films of several inorganic materials with improved stability and with an extremely low interface roughness. In this technique, thin films are deposited onto the substrate by direct condensation of the source material. The source material is first vaporized either by evaporation or sublimation from its initial solid form and then followed by vapor transport onto the substrate under vacuum. Unlike chemical vapor deposition that involves chemical reaction of gaseous precursors to form stable compound, PVD is purely a physical process that involves solid starting material without any chemical reaction involved between the reactants. However, in some cases, reactions occur between source material with reactive gases (nitrogen, oxygen, and methane) during the transport stage. The vaporized precursor material is then deposited onto the substrate under vacuum. Thin film deposition by PVD is achieved by several physical processes activated either at high or low temperatures. In vacuum evaporation, one of the PVD processes deposition occurs at higher temperature, whereas the condensation of atomized matter occurs on the substrates maintained at relatively lower temperatures. PVD techniques are widely classified into evaporation and sputtering. The evaporation techniques are further classified into thermal and electron beam evaporation based on the source of evaporation. Similarly sputtering technique is further divided into DC, DC magnetron, and rf sputtering based on the source of sputtering device. Other common PVD techniques such as cathode arc deposition and pulsed laser deposition are also being used to fabricate metal oxide thin films.

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Thermal Evaporation Evaporative deposition is the process of vaporizing the solid source material using a heat source and then allowing the vapor to condense onto the substrate to form a solid thin film. Based on different heat sources, evaporation method can be further classified. Thermal evaporation is the simplest form of deposition and has been in practice for more than a century. In this process, resistive heating is widely used to evaporate the source placed in the crucible, commonly made of refractory metals such as tantalum, molybdenum or tungsten. To avoid contamination of the crucible or to evaporate the refractory metals itself, electron gun heating is used in which the electron beam sources are generated either by field cathodes or by thermionic emission. The deposition chamber is maintained under high vacuum to avoid oxidation of sensitive materials.

Sputtering Sputtering is a process of using energetic ions as the energy sources to eject atoms by bombarding a solid target. These high energy ions are generated by a directed ion source or accelerated from a plasma source. The bombarding ions physically penetrate into the surface of the target and create a cascade collision on the surface atoms. With the momentum transfer from these ions to the target, the atoms are the ejected from the target and then deposited as a thin film onto the substrate (Depla et al. 2010). Compared to evaporation technique, substrate is kept relatively closer to the target source material during sputtering process. By maintaining a low pressure of less than 5 mTorr, collisions between gases or other ions can be avoided in the space between the target and substrate inside the chamber. Based on the energy sources required to create plasma and processes involved, sputtering operations are divided into several varieties such as direct current (DC), radio frequency (RF), ion-beam, cathodic, diode, and reactive sputtering. In cathodic DC sputtering, an electrical discharge of a gas generates plasma with positive ions which are then used to bombard the target (cathode) to generate sputtered atoms. DC sputtering is used to deposit electrically conductive materials like metals. Metals as well as insulating materials such as dielectrics can be deposited using alternating current (ac) or rf sputtering. In such cases, the applied potential on the target is periodically reversed at a very high frequency. When rf power is applied and capacitively coupled to the target, an alternating potential develops on the surface. At high rf frequency of around 14 MHz, the ions are accelerated to the target during each half-cycle and create sputtered atoms. The efficiency of dc and ac sputtering processes can be further increased by using a magnetron. By applying a magnetic field around the target/cathode surface, the electrons are locally trapped around the cathode surface. This increases the ionization efficiency and decreases plasma impedance. This results in increase of current density at the target and sputtered atoms. Magnetron-assisted sputtering therefore

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Fig. 6.24 Continuous DC and pulse-DC magnetron sputtering – Process diagrams. DC magnetron sputtering continuously produces new ions from the target, whereas pulse-DC magnetron sputtering produces ions in pulses resulting in slower sputtering speed. This helps pulse-DC magnetron sputtering process for efficient bonding of metal ions (In, Ga, Zn) with oxygen ions for every pulsing period. A reduction in dangling bonds and defect density enhances the electrical performance of the devices prepared by pulse-DC magnetron sputtering process. (Reprinted with permission from Zheng et al. 2017)

increases the deposition rate and efficiency. Using RF magnetron sputtering technique, multi-component oxide films such as zinc tin oxide (ZTO) have been deposited for thin film transistor applications (Chiang et al. 2005). ZnO:SnO2 (1:1 M ratio) target is prepared by sintering at 1100  C, and then thin films are deposited on Si substrate maintained at 175  C in Ar/O2 (90/10%) atmosphere. The popular IGZO thin films have been fabricated both by DC and rf sputtering techniques using In2Ga2ZnO7 as the target (Deng et al. 2012). Figure 6.24 shows the schematic representation of the room temperature fabrication of IGZO layer using DC and pulse-DC magnetron sputtering for flexible TFT devices. Compared to DC magnetron sputtering, in which the ions are continuously produced from the target, pulseDC magnetron sputtering process produces ions in every pulsing period. With such a slower process, metal ions such as In, Ga, and Zn can be efficiently bonded with oxygen ions. Such bonding reduces dangling bonds and defect density. This in turn increases the electrical performance of the devices prepared by pulse-DC magnetic sputtering. There is another special case of sputtering process called reactive sputtering in which a reactive gas is used along with a carrier gas. Using this technique, dielectric thin films are deposited using oxygen as reactive gas. By controlling the reactive gas flow-rate, one can control the composition of the desired compound which is formed both at the magnetron surface and at the substrate. As an example, ZnO thin films

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have been deposited using RF magnetron sputtering using Zn metal as the target in an O2/Ar mixed gas atmosphere (Ondo-Ndong et al. 2003).

Pulsed Laser Deposition Pulsed laser deposition (PLD) is another popular PVD technique which utilizes high power laser pulses to deposit high-quality metal oxide thin films. This technique has been attempted immediately after the invention of first commercial ruby laser. Today, PLD is one of the most promising and easy methods to fabricate thin films of metal oxides semiconductors and complex oxide heterostructures and nanostructures. In PLD process, a beam of laser pulses with a nano/femto second pulse duration is focused onto the target kept in vacuum. Under such a powerful laser irradiation in a limited target volume, the target material is vaporized forming a plume, i.e., a plasma cloud of ions, molecules, and small particles. This process is also termed as ablation because of the dramatic explosion of the material under laser irradiation. The generated plasma is then condensed onto the substrate forming a layer of target material. By repeated laser irradiation and vapor plumes, thin films are formed onto the substrate by subsequent layer deposition. The substrate is maintained at a high temperature at a distance of 3–5 cm away from the target. Since laser operates outside the deposition chamber, high quality and purity can be maintained in this process compared to sputtering or evaporation. The processing parameters such as laser wavelength, pulse width, laser energy, pulse repetition rate, background gas, substrate temperature, and type depend on the material to be deposited. Laser wavelengths range from 10.6 μm from a CO2, 1.064 μm from Nd:YAG, second harmonic wavelength of 532 nm, or ultraviolet. More common PLD systems also work on excimer lasers emitting pulses at UV wavelengths below 300 nm, e.g., 248 nm from KrF laser. Figure 6.25 shows a schematic of pulsed laser deposition system using a KrF excimer laser. As shown, Shin et al. used KrF excimer laser to deposit ZnO thin films on various substrates for TFT application (Shin et al. 2008). By varying the oxygen pressure during the deposition, properties of the films were varied to achieve the best TFT performance. PLD is widely used for fabricating doped thin films of zinc oxide. Taabouche et al. reported the fabrication of aluminum-doped ZnO thin films onto a glass substrate using a KrF excimer laser (Taabouche et al. 2016). The target was prepared by sintering a mixture of ZnO and Al2O3 at 500  C for 3 h. A beam of laser pulses with a wavelength of 248 nm, pulse duration of 25 ns, energy fluence of 2 J/cm2, and a repetition rate of 5 Hz was irradiated on the rotating target kept at an angle of 45 and at a distance of 4 cm from the substrate. The substrate was maintained at 450  C and the chamber was filled with O2 gas at 1 Pa pressure. PLD technique can deposit thin films without any change in stoichiometry of the target material. In addition, different materials also can be deposited in sequence in a single vacuum chamber just by changing the targets within the chamber (carousel system).

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Fig. 6.25 Schematic of KrF excimer pulsed laser deposition system. Substrates are ultrasonically cleaned using ethanol and acetone and are positioned few cm away from the target inside an evacuated chamber. The target is then ablated using a KrF excimer pulsed laser. The properties of the deposited films can be controlled by varying the ambient oxygen pressure and substrate temperature. (Reprinted with permission from Shin et al. 2008)

6.5.2

Thin Film Deposition by Chemical Processes

Chemical Vapor Deposition Chemical vapor deposition (CVD) technology is used to prepare high pure nanopowders and thin films from the gaseous phase reactants. A CVD system consists of a precursor supply system, a reactor, and an exhaust system. The precursors are introduced into the reaction chamber in gaseous or vapor phase, commonly diluted with an inert gas. The reaction chamber is generally activated by temperature, light, or plasma. The gaseous reactants undergo reaction in reactor chamber and then deposited onto a heated substrate kept inside the chamber which can be maintained either at low pressure or at atmospheric pressure. The precursors are initially taken in liquid form which are then heated to a medium temperature of less than 200  C and then mixed with other precursors to form chemical mixtures. There are two types of reactions possible in CVD: homogenous and heterogeneous reactions. In the former case, reactions occur only in the gas phase, whereas in the latter case, reactions can occur between the gaseous chemicals and solid substrate. Once the thin film deposition of the compound is finished, the by-products are removed from the chamber using the exhaust system. This technique is widely employed for deposition of metal oxide thin films because of its ability to achieve

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materials having complex stoichiometries with a good reproducibility without compromising the homogeneity. There are several variants of CVD techniques based on precursors such as metal organic CVD (MOCVD) and based on reaction activating energy sources such as plasma-enhanced CVD (PECVD), laser induced CVD, photo-assisted CVD, and so on. Several metal oxide thin films for gas sensing applications have been fabricated using CVD technique (Vallejos et al. 2016). ZnO thin films have been deposited by CVD process using metallic zinc or zinc nitrate as the precursors and oxygen as the reactive carrier gas. Organometallic complex such as zinc ketoiminate has also been used as a precursor. Deposition temperatures of the reactor vary between 350 to 450  C. Similarly for tin oxide thin films, metallic tin, or chloride salts of tin are widely used as precursors with oxygen as the carrier gas. Atomic layer deposition (ALD) is a special case of CVD which allows controlling thickness of the film in an atomic level. In ALD, precursors react individually with a surface either spontaneously or in sequence. By repeating this reaction of precursors, thin films are eventually deposited onto the growth surface. Ultrathin ZnO thin films have been deposited using ALD process onto a glass substrate maintained at 200  C (Iqbal et al. 2016). Diethyl zinc was used as the precursor, whereas water was used as the oxidant. Thin films were deposited by repeated layer by layer growth as per the following reaction. Diethyl zinc þ water ! Zinc oxide þ ethane:

ð6:18Þ

Sol-Gel Sol-gel process has attracted significant attention among the material scientists to develop thin films, nanoparticles, and ceramics because of its low-processing temperature and reproducibility. Metal oxide thin films can be deposited at relatively lower temperature compared to other deposition methods. In this process, colloidal solution (sol) of metal oxide materials are deposited on the substrate by spray coating or dip coating techniques. The sol is formed either by hydrolysis or polymerization of homogeneously mixed precursor solution along with some reagents. Subsequent condensation process by adding polymers converts sol into gel which can then be deposited as thin films using coating techniques such as spin coating or dip coating. The properties of the material can be controlled by various processing parameters such as pH of the solution, concentration of the precursor, aging of the solution, nature of solvent and polymer, thermal treatment of gel, and other additives added during the process. Figure 6.26 shows a typical sol gel spin coating process used to coat metal oxide thin films. Using sol-gel process, ZnO thin films have been widely prepared for photonic applications. Znaidi et al. used zinc acetate dehydrate as the precursor material which was dissolved in a mixture of ethanol and monoethanolamine which acts as the complexing agent (Znaidi et al. 2012). Using spin coating technique, precursor

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Fig. 6.26 Typical sol-gel spin coating process – A schematic representation. Precursor solution is first prepared by dissolving precursor chemicals in solvent along with stabilizer. After proper mixing and aging, the precursor solution is then deposited on substrate such as glass, rotated in a spin coater. The films are then dried and the process is repeated several times to achieve the desired thickness. Eventually, crystalline thin films are obtained by further annealing at higher temperature. (Modified after Yahia et al. 2018)

solution was deposited as thin films on glass substrates for 30 s at a rotation rate of 3000 rpm. As-synthesized films were preheated for 10 min and the final deposited films were thermally treated at 550  C for 2 h yielding crystallized ZnO thin films. Thickness of the films can be increased by repeating coating of the films after every preheating step as can be seen in Fig. 6.26. This technique is attractive to fabricate large area thin films at low temperature. Schematic diagram of this deposition process is given in Fig. 6.26. Similarly, dip coating process has been widely employed to coat thin films of materials prepared by sol-gel process.

Spray Pyrolysis Another common way to deposit thin films using wet chemical route is spray pyrolysis. This technique offers advantages for large area deposition, especially for transparent conducting oxide layers for photovoltaic applications. In this technique, a precursor solution of chemicals and appropriate solvent is sprayed onto a heated substrate maintained at a temperature required for precursor decomposition. An atomizer or nebulizer is used to atomize the solution into aerosol droplets by flowing high pressure air or oxygen through the liquid chemical solution. On reaching the hot substrate or even sometimes before arriving at the substrate, the solvent in the droplet evaporates quickly leaving the solid/solute that undergoes condensation, drying, and then thermal decomposition. Figure 6.27 shows a schematic of typical spray pyrolysis technique to grow thin films of metal oxides. In this process, substrate temperature, the choice of solvent, and the concentration of precursor in the droplet are critical parameters to control during the first stage. The deposited precursor undergoes chemical reaction on the hot substrate forming thin films. The crystallinity and nanostructure of the deposited films can be easily controlled by varying the spray conditions. Another advantage of spray pyrolysis technique is that the stoichiometry is maintained even at the droplet scale and is useful for deposited mixed metal oxide thin films. Spray coating also produces thin films with high uniformity

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Fig. 6.27 Schematic representation of a spray pyrolysis system. The precursor solution containing metal oxides undergoes pyrolytic decomposition and deposits as crystalline thin films on the substrate which is maintained at a higher temperature. Glass, ceramics, and metals are widely used as substrates. Compressed air acts as a carrier gas to spray the precursor solution on the hot substrate. (Modified after Park et al. 2016)

and quality compared to that of spin coated films. Also, spray coating does not require intermediate processing such as multi-layer coating in the case of spin coating method. Highly crystalline nickel oxide thin films have been prepared by spraying an aqueous solution of nickel chloride by a nebulizer based on hydraulic pressure onto a glass substrate maintained at 400  C (Gowthami et al. 2014). Similarly, mixed metal oxide thin films such as Cd:ZnO thin films have been prepared using a mixture of zinc acetate and cadmium acetate dissolved in a mixture of ethanol and deionized water.

6.6

Conclusions

Metal oxides based semiconductors with binary, ternary, or quaternary systems have been instrumental in building several electronic, optical, and optoelectronic devices. Most of these materials exhibit n-type semiconducting properties while p-type materials also being explored with relatively poorer performance. Thin films of such metal oxides have contributed significantly to manufacture optoelectronic devices in large volumes at a cheaper price with more stability. This chapter discussed in detail the role of metal oxides and their properties required to develop efficient optoelectronic devices. Main focus was given to the development of large area displays using thin film transistors based on metal oxide thin films. Lightemitting diodes used in displays are also discussed with various metal oxides being used in their development. Other optoelectronic applications of metal oxide films for devices such as transparent conducting oxides, photovoltaic cells, photodetectors,

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gas sensors, and heat mirrors are discussed as well. With a brief discussion on operating principle of each device, the use of both binary and multi-component metal oxides for such devices has been discussed. A brief overview of several fabrication techniques employed to growth thin films of metal oxides has also been given with an example each. Acknowledgment The author (S. P. D) acknowledges the support of Antonio Lucianetti and Tomas Mocek from HiLASE research center, Czech Republic, during this work.

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Chapter 7

Zinc Oxide as a Multifunctional Material: From Biomedical Applications to Energy Conversion and Electrochemical Sensing Helliomar Pereira Barbosa, Diele Aparecida Gouveia Araújo, Lauro Antonio Pradela-Filho, Regina Massako Takeuchi, Renata Galvão de Lima, Jefferson Luis Ferrari, Márcio Sousa Góes, and André Luiz dos Santos

Contents 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Improvement of ZnO as Photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Biomedical Application of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Photodynamic Therapy: Antitumoral Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Antimicrobial Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Energy Conversion and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Solar Energy Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Electrochemical Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Conclusion and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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H. P. Barbosa · D. A. G. Araújo · L. A. Pradela-Filho · J. L. Ferrari Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil R. M. Takeuchi · R. G. de Lima · A. L. dos Santos (*) Instituto de Química, Universidade Federal de Uberlândia, Santa Mônica/Uberlândia, MG, Brazil Instituto de Ciências Exatas e Naturais do Pontal, Universidade Federal de Uberlândia, Tupã, Ituiutaba, MG, Brazil e-mail: [email protected] M. Sousa Góes Instituto Latino-Americano de Ciências da Vida e da Natureza, Universidade Federal da Integração Latino-Americana, Foz do Iguaçu, PR, Brazil © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 S. Rajendran et al. (eds.), Metal and Metal Oxides for Energy and Electronics, Environmental Chemistry for a Sustainable World 55, https://doi.org/10.1007/978-3-030-53065-5_7

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Abstract Zinc oxide (ZnO) is a very attractive material which has received growing attention in the academic and technological areas. This metal oxide shows several advantageous properties such as facile and inexpensive synthesis, low toxicity, high surface area, and rich surface chemistry. However, the most impressive property of ZnO is the possibility of obtaining ZnO nanoparticles with different morphologies and crystal size by merely changing the synthetic parameters, such as temperature, pH, or the solvent. Thus, the combination of the attractive chemical, optical, and electrical properties of ZnO to the possibility of easily producing ZnO nanoparticles with different sizes and morphologies makes this metal oxide an extremely versatile material. Because of this versatility, ZnO has found several applications, including the development of electronic and optoelectronic devices, energy conversion in solar cells and supercapacitors, sensing and electrochemical sensing, besides several biomedical applications in photodynamic therapy, disease diagnoses, and microbial killing. Therefore, the objective of this chapter is to highlight the main approaches used to achieve the efficient application of ZnO in biomedical, energy conversion, and electrochemical sensing fields. Keywords Nanomaterials · Photodynamic therapy · Supercapacitors · Solar cells · Electrochemical sensing

7.1

Introduction

Zinc oxide (ZnO) nanoparticles have demonstrated great potential in a wide range of multimodal applications like gas sensors (Cheng et al. 2004), biosensors (Wang 2004), solar cells (Hames et al. 2010), superconductors (Sharma et al. 2003a), varistors (Jun et al. 2002), photodetectors (Sharma et al. 2003b), photocatalyst (Sahdan et al. 2009), optoelectronic devices (Shao et al. 2013), and cosmetics (Venkataprasad-Bhat et al. 2011). ZnO is distinctive electronic and photonic wurtzite n-type semiconductor with a wide direct bandgap of 3.437 eV at 2 K and a high exciton binding energy (60 meV) and deep violet/borderline ultraviolet (UV) absorption of the solar spectrum when compared to TiO2 (Look et al. 1999; Qiu et al. 2008; Madhusudhana et al. 2011; Zhou et al. 2011). A variety of techniques such as sputtering (Gabás et al. 2011), sol-gel (De La Olvera et al. 2002; Alias et al. 2010; Ba-Abbad et al. 2013), vapor–liquid–solid growth (Huang 2001), physical vapor deposition (Kato et al. 2002), zinc–air (Zn–air) system (Yap et al. 2009), co-precipitation (Chen et al. 2013c), micro-emulsion (Kumar and Rani 2013), thermal evaporation (Utlu 2019), microwave-assisted hydrothermal synthesis (Hasanpoor et al. 2015), metal-organic chemical vapor deposition (Park et al. 2005), molecular beam epitaxy (Tien et al. 2007), solvothermal (Šarić et al. 2015), sonochemical (Ghosh et al. 2014), wet chemical (Zhou et al. 2008; Samanta and Mishra 2013), and electrochemical deposition (Yang

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et al. 2007) have been developed to prepare ZnO. Among these methods, the wet chemical has a promising potential for device applications because of its very simple ambient conditions, low temperature, no catalyst, low cost, and high yield (Boukos et al. 2012). An overall view of the main properties and applications of ZnO nanoparticles prepared by different methods is shown in Table 7.1. ZnO exhibits a tetrahedral geometry and large ionicity at the borderline between that of covalent and ionic semiconductors (Ong et al. 2018). A ZnO crystal can exhibit three different forms: hexagonal wurtzite, cubic zincblende, and rock salt. ZnO hexagonal wurtzite is the thermodynamically most stable structure. Cubic zincblende, however, can be stabilized by growing ZnO on cubic substrates. ZnO will exist in the rock salt structure only at higher pressures (Özgür et al. 2013). ZnO is generally an n-type semiconductor with the presence of intrinsic or extrinsic defects such as oxygen vacancies (VO), zinc interstitials (Zni), and zinc vacancies (VZn), which will affect its optical properties and electrical behavior (Boukos et al. 2012). The intrinsic point defects of ZnO (i.e. zinc vacancy, zinc interstitial, oxygen vacancy and oxygen interstitial) drawing a fundamental role in the electrical behavior of this material since this type of defect is more promising to decrease the ZnO bandgap. The n-type semiconductor character of ZnO arises from its non-estimation interstitial oxygen and zinc vacancies. ZnO has a dense anionpacking hexagonal structure, in which half of the tetrahedral sites are occupied by zinc ions. All octahedral interstitials are empty, and therefore ZnO can even include intrinsic defects and extrinsic dopants (Fig. 7.1), which affords different electronic levels, providing different forms structure for application in different systems (Schmidt-Mende and MacManus-Driscoll 2007). When ZnO is photo-induced by solar radiation with photonic energy (hv) equal to or higher than the excitation energy (Eg), electrons from the filled valence band (VB) are promoted to an empty conduction band (CB) (Ong et al. 2018). ZnO has been shown to exhibit higher absorption efficiency across a larger fraction of the solar spectrum compared to TiO2. The photoactivity of a catalyst is governed by its ability to create photogenerated electron/hole pairs. The major constraint of ZnO as a photocatalyst, however, is the rapid recombination rate of the photogenerated electron/hole pairs, which disturbs the photodegradation reaction. Additionally, it has also been noted that the solar energy conversion performance of ZnO is affected by its optical absorption ability, which has been associated with its large bandgap energy. Therefore, researches have been devoted to improving the optical properties of ZnO in order to minimize bandgap energy and inhibit the recombination of photogenerated electron/hole pairs.

7.1.1

Improvement of ZnO as Photocatalyst

ZnO is usually an n-type semiconductor mainly due to the oxygen vacancies (VO), which can provide more electron charge carriers. The drawback fabrication of ZnO

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Table 7.1 Properties and applications of ZnO nanoparticles obtained from different synthetic methods Method of preparation Solid-state reaction Hydrothermal

Shape and dimensions Nanorods 20 nm Nanorods 20 nm

Application PDT/drug release PDT/drug release

Hydrothermal

Variable

Antibacterial

Hydrothermal Hydrothermal

Spherical 20 nm Nanoflowers 21–43 nm Spherical 6.0 nm

Antibacterial Gas sensor

Solution phase technique Aqueous solution precipitation Solvothermal processing Hydrolysis in polyol medium Hydrothermal Microwave-assisted Chemical decomposition of ZnHCF Hydrothermal Co-precipitation Chemical batch deposition Ultrasound-assisted Hydrothermal Aqueous chemical growth Chemical deposition Hydrothermal Hydrothermal

Spherical 24 nm

Photocatalysis and anticancer therapy Antimicrobial and environmental sanitation Dye-sensitized solar cells

Spherical 100 nm

Dye-sensitized solar cells

Nanorods 90 nm

Photocatalysis/phenol degradation Photocatalysis/malachite green degradation Supercapacitor

Pillars 1–10 μm

Spherical 15–25 nm Nanoflowers 1–2 μm Nanorods 80–150 nm Spherical 25 nm Hexagonal 2.2 μm Spherical 100 nm Nanowires 100–150 nm Nanorods 50 nm Film 90–120 μm roughness Sheets 100 nm

Optically sensitive supercapacitor Supercapacitor Flexible supercapacitor Supercapacitor

References Zhang et al. (2011) Hariharan et al. (2013) Stanković et al. (2013) Akbar et al. (2019) Agarwal et al. (2019) Mitra et al. (2018) Yi et al. (2019) Chou et al. (2007) Zhang et al. (2008a) Hezam et al. (2018) Rabieh et al. (2014) Subramani and Sathish (2019) Boruah and Misra (2019) Yadav et al. (2018) Liu et al. (2018)

ES of DNA hybridization

Jayachandiran et al. (2018) Zhang et al. (2019)

ES of estradiol

Singh et al. (2019)

ES of catechol

Maikap et al. (2019) Saritha et al. (2019) Paik et al. (2018)

ES of quercetin

Solvothermal

Microspheres 1–3 μm Tetrapod

Glucose biosensor ES of Helicobacter pylori

Electrodeposition

Rods 90 μm

ES of methyl parathion

Electrodeposition

Bamboo-like 5 nm

Photoelectrochemical detection of H2O2

Chauhan et al. (2018) Thota and Ganesh (2016) Li et al. (2016)

PDT photodynamic therapy, ZnHCF zinc hexacyanoferrate, ES electrochemical sensing

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Fig. 7.1 Energy levels of native defects in ZnO. The donor defects are Zni•• , Zni• , Znxi, V o•• , V o• , and the acceptor defects are Zn00i , Zn0i . (Modified after Schmidt-Mende and MacManus-Driscoll 2007)

semiconductor is the difficulties in obtaining a stable and reproducible p-type ZnO. The high purity of p-type ZnO is optimal for various applications due to its high radiative stability. Doping has been a strategy adopted to improve the physical and chemical properties of ZnO, incorporating impurities such as metals or nonmetals, to shift energy from the ZnO valence band upward and decrease the bandgap energy to the ultraviolet-visible region. Metal doping of ZnO can improve the photoactivity of catalysts by increasing the trapping site of the photo-induced charge carriers and thus decrease the recombination rate of photo-induced electron/hole pairs (Rezaei and Habibi-Yangjeh 2013). This phenomenon can occur without causing any significant lattice distortion. Besides, energy bandgap decreases, and the ZnO-doped material could be applied in dye degradation and solar cells. However, highly active photocatalysts can be obtained by coupling two semiconductors having different bandgaps. Higher efficient charge separation may be achieved due to photo-induced electrons that are transferred away from the photocatalyst. Therefore, the heterostructure of nanomaterials acts as an attractive alternative for enhancing the photoactivity of photocatalysts. For ZnO, coupled with other semiconductors, TiO2/ZnO, SnO2/ZnO, SnO2/ZnO/TiO2, and Co3O4/ZnO are the most investigated materials for photocatalytic processes (Ong et al. 2018). In the

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ZnO/TiO2-xNy array, the ZnO electrons are transferred from the conduction band to the TiO2-xNy conduction band, while conversely the photogenerated holes are transferred from the valence band of TiO2-xNy to the valence band of ZnO (Huang et al. 2012). The occurrence of such phenomena suppresses the recombination of the electron/hole (e/h+) pairs, increasing in this way the charge carriers. In this system, ZnO could increase the concentration of free electrons in the CB of TiO2; this implies that charge recombination is drastically reduced in the electron transport process. An improvement in the redox processes is expected in this combination of matrices since the separation of charges increases the lifetime of the charge carriers.

7.1.2

Solar Cells

Reducing the use of fossil fuels and their consequent production of gaseous pollutants has been the main target for the search for clean and environmentally friendly sources of energy. Thus, ways to convert sunlight into electrical energy have become the focus of the global energy field. Zinc oxide has recently been explored as an alternative material in photoelectrochemical cells such as dye-sensitized solar cells with great potential. Dye-sensitized solar cells were introduced by O’Regan and Grätzel (O’Regan and Grätzel 1991) in 1990; the photoconversion efficiency of these devices reached 11–15% (Shi et al. 2010). Unlike conventional solar cells, such as silicon-based solar cell, photoelectrochemical cells have as their primary function a kinetic competition between the interfaces that make up the device. For example, the process in dye-sensitized solar cells is based on the principle that light absorption and charge separation processes occur differently. Light absorption is done by a monolayer of a chemically adsorbed dye on the surface of the semiconductor material. The general kinetic mechanism of dye-sensitized solar cells is shown in Fig. 7.2. All the processes of charge transport – photogeneration, separation, and recombination – occur primarily or exclusively at the dye-sensitized solar cell interfaces during the creation of an electron/hole pair. Thus, unlike conventional n-p cells, interface properties are of prime importance, i.e., the bulk property of the semiconductor is less critical (Gregg 2003). According to Retamal et al., ZnO can be manufactured in various morphologies with a high surface area. Morphology is also related to electron transport in ZnO (Retamal et al. 2012). The main reasons for this increase in research surrounding ZnO material include (1) a bandgap similar to that for TiO2 at 3.2 eV and (2) a much higher electronic mobility 115–155 cm2 V1 s1 than that for anatase titania (TiO2), which is reported to be 105 cm2 V1 s1 (Zhang et al. 2008a). On the other hand, ZnO may be easily nanoarchitectured in a wide range of shapes that should be able to improve light harvesting and electron transport properties leading to better overall efficiencies. Table 7.2 shows the different ZnO nanostructures used in dye-sensitized solar cells. Comparing the nanostructures, one

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Fig. 7.2 Dye-sensitized solar cells scheme of operation. D D+ and D * are, respectively, the ground-state oxidized and excited dye. ECB and EVB are the semiconductor conduction and valence band energies, respectively. EFn is the Fermi level of the semiconductor conduction band. Eredox is the redox energy of the electrolyte. eoV is the maximum potential

can notice a relationship between the photoelectrode morphology and the conversion efficiency of dye-sensitized solar cells. For example, the highest current efficiencies and densities were obtained in nanoparticles, followed by hierarchical structures and nanoparticle aggregates, reaching efficiencies of up to 6.58%. There are reports in the literature describing the higher state of nanocrystallite aggregation induces a more effective photon capture in the visible region, as well as a strong dispersive effect of light. This phenomenon optimizes the absorption of the incident light, weakening the transmittance of thin films, which may present substantial interference in the solar conversion process (Zhang et al. 2008a). ZnO nanocrystallites have been demonstrated as a practical approach to generate light scattering within the photoelectrode film of DSCs while retaining the desired specific surface area for dye molecule adsorption. The maximum energy conversion efficiency of 5.4% was achieved on photoelectrode films that consisted of polydisperse ZnO aggregates of nanocrystallites. This efficiency is more than 100% increased over 2.4% achieved for films only including nanosized crystallites. The

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Table 7.2 Dye-sensitized solar cells performance using different ZnO nanostructures

Branched structure nanowires

η (%) 0.40 2.80 1.32 4.70 1.50 0.84 0.40

Voc (mV) 550 550 570 710 710 500 620

Pillar structure nanowires

0.34

Nanospheres

Nanostructure Nanocrystalline film Nanosticks Nanowires

Nanotetrapods

Nanoflowers Nanoflowers with Au particles Nanoparticles

Nanosheet Thin film mesoporous air gel Nanoparticle aggregate Board aggregate Network structure Hierarchical structure Nanospheres/film composite Nanowire/nanoparticle composite Nanowire ZnO/TiO2 (core/ shell)

jsc (mA. cm2) 1.22 9.10 7.00 10.70 5.85 3.40 1.84

FF 0.66 0.57 0.33 0.62 0.38 0.49 0.40

690

1.26

0.39

1.59 2.60 1.02

668 557 580

5.43 12.30 3.76

0.44 0.48 0.47

3.27

614

9.71

0.55

1.60 0.30 2.50

580 535 500

8.75 1.10 15.00

0.32 0.54 0.33

Synthesis method Sol-gel Hydrothermal Precipitation Hydrothermal Wet chemical Wet chemical Chemical vapor deposition Chemical vapor deposition Wet chemical Hydrothermal Physical vapor deposition Chemical vapor transport Hydrothermal Hydrothermal Hydrothermal

0.87 6.58 0.75 1.55 2.40 5.40 1.90 1.34 6.51 2.25 2.20

670 621 573 593 600 595 554 600 670 718 610

2.25 18.11 1.20 2.06 8.32 9.70 8.40 3.58 10.90 6.13 6.30

0.58 0.59 0.51 0.55 0.48 0.45 0.41 0.62 0.48 0.51 0.58

Precipitation Sol-gel Hydrothermal Hydrothermal Sol-gel Polyol Hydrothermal Electrospinning Solvothermal Wet chemical Wet chemical

2.27 2.00 2.00

800 705 704

4.78 5.30 5.30

0.60 0.54 0.53

Wet chemical Hydrothermal Hydrothermal

Source Data: (Retamal et al. 2012) η conversion efficiency, Voc open-circuit potential, jsc short-current density, FF fill factor

polydispersity in the size distribution of ZnO aggregates was indicated to be positive in causing light scattering in a broad wavelength region and, therefore, in enhancing the light harvesting capability of the photoelectrode film. The presence of nanometric material promotes enhanced light scattering for increased photon absorption.

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In general, the short circuit current density is proportional to the amount of dye absorbed. However, one of the factors that are the cause of the low efficiency of dye-sensitized solar cells applied ZnO is the excessive dye aggregation on the ZnO surface, which causes its degradation and then its slow dye electron injection to the ZnO (Boschloo et al. 2006). The slow injection of the dye (kinetic process) into the ZnO photoelectrode determines the charge transfer process and the ability of these materials as active electrodes in DSCs. Pauporté stated that the low efficiency of most ZnO dye-sensitized solar cells prepared using N719 dye (adsorbed acid dye) is probably caused by the dissolution of ZnO into Zn2+, followed by the formation of a Zn2+ insulating layer and ruthenium dye molecule. This causes the injected electrons from the dye molecules to the semiconductor to be blocked by the insulating layer (Pauporté 2018). Nevertheless, ZnO nanotube type as photoanodes may have an effective diffusion coefficient with three orders of magnitude higher than TiO2-based photoanodes, which may enable better electrical charge collection (Martinson et al. 2009). The coating methods or other photosensitizing types can also be used to reduce the recombination interface and improve the photon absorption range, respectively.

7.2

Biomedical Application of ZnO

Nanotechnology applied to the field of biology allows the development of new materials in the nanosize range for applications and improvements in medical diagnosis and therapies (Zak et al. 2011; Cao et al. 2018). Zinc oxide, ZnO, is an important material for the construction of new nanomaterial because of its vast area of applications and flexibility of preparation in different morphologies with different properties (Jiang et al. 2018). ZnO nanoparticles exhibit low cost, facile synthesis, and low toxicity; this material has attracted tremendous interest in various biomedical fields, including anticancer (Sivakumar et al. 2018), antibacterial (Kadiyala et al. 2018; Soren et al. 2018), antioxidant (Soren et al. 2018), antidiabetic (El-Gharbawy et al. 2016), and anti-inflammatory activities (Nagajyothi et al. 2015), as well as for drug delivery and bioimaging applications (Zhang and Xiong 2015; MartínezCarmona et al. 2018) (Fig. 7.3). Several products have ZnO in their formulation due to its properties like deodorant and antibacterial action, and this metal oxide is also used in cotton fabric, rubber, food packaging, and UV solar blockers (Smijs and Pavel 2011; Lu et al. 2015; Jiang et al. 2018). Furthermore, properties like nanosize provide to ZnO a large surface area enabling it to interact with biomolecules on the surface and inside to cells due to their small size (about 100–10,000 times smaller than human cells) (Ashraf et al. 2018). The durability, selectivity, and heat resistance of ZnO are higher than other organic and inorganic materials because ZnO shows weak covalent character and a very strong ionic bonding at the Zn–O bond (Padmavathy and Vijayaraghavan 2008). In a biological environment, the ZnO nanostructured materials are generally

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Fig. 7.3 Applications of ZnO nanoparticles in different fields of medicine and biology. ROS reactive oxygen species

insoluble but can dissolve at the stomach pH (Mitra et al. 2018). ZnO particle size (nanoparticles or microparticles) and human serum albumin are factors that influence the Zn2+ ion bioavailability (Yang and Xie 2006). The zinc is an indispensable trace element for humans and plays a crucial role in regulating cellular metabolism and homeostasis (Sivakumar et al. 2018). However, the excess of zinc may cause cell death due to apoptosis or necrosis (Rana et al. 2016). Many aspects such as size, shape, source of Zn2+ ions, and reactive oxygen species (ROs) production were investigated to improve the knowledge of the behavior of ZnO in the cell culture media (Meißner et al. 2014). However, in general, the ZnO is classified as a “GRAS” safe substance by the Food and Drug Administration (FDA) (Cancer &Therapy). ZnO nanoparticles can be synthesized by several methods (Fig. 7.4) under controlling parameters such as the solvent type, precursors, pH, and temperature (Sirelkhatim et al. 2015).

7.2.1

Photodynamic Therapy: Antitumoral Effect

Photodynamic therapy is a non-invasive therapy, differently from the radiotherapy and surgery, and is based on photosensitizing agents which induce oxidative stress by reactive oxygen species generation when activated by visible light. Tetrapyrrole class is photosensitizing molecules like porphyrins, chlorins, bacteriochlorin, and phthalocyanines that have already been approved by authorities such as the US FDA and are currently used with success in cancer therapy (Weidinger and Kozlov 2015).

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Fig. 7.4 Relevant morphologies and techniques of obtaining ZnO nanoparticles applied in the biological area. Reprinted with permission of [Powder Technol., Ren Y, Yang L, Wang L, et al., Copyright 2015, Elsevier] from (Ren et al. 2015); [CrystEngComm, Hezam A, Chandrashekar BN, Cheng C, Sadasivuni KK, Copyright 2017, RSC] from [(Hezam et al. 2017)]; [Sensors Actuators B Chem, Agarwal S, Rai P, Gatell EN, et al., Copyright 2019, Elsevier] from (Agarwal et al. 2019)

The nanotechnology expansion has been stimulated the photodynamic therapy impact in other areas of biomedicine. In addition to molecular photosensitizers, nanostructured materials can be used as the photosensitizers absorbing light and producing reactive oxygen species, as in the case of quantum dots and metal– semiconductor nanocomposite based in titanium dioxide and zinc oxide (Abrahamse and Hamblin 2016). The aqueous suspensions of ZnO nanoparticles under light irradiation can produce reactive oxygen species such as hydroxyl radicals (●OH) and hydrogen peroxide (H2O2) (Fig. 7.5), which promote the efficient decomposition of the organic compounds. The ZnO photophysical mechanism involving the photoexcitation of semiconductor and initially the electrons (e) are excited from the valence band (VB) to the conduction band (CB), generating positive holes (h+). When the photophysical reaction occurs in an aqueous environment, the photogenerated e can reduce oxygen molecules, forming superoxide radical anion (●O2). On the other hand, the h+ can oxidize water molecules and hydroxide ions, generating hydroxyl radicals and hydrogen peroxide (H2O2) molecules. Furthermore, the recombination of the electron/hole pair can produce the emission of a photon (radiative recombination), which can excite ground-state oxygen, generating singlet oxygen (1O2). The reactive oxygen species, when produced in the cellular medium, can exert highly cytotoxic effects, which are very useful for killing tumor cells (Martínez-Carmona et al. 2018).

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Fig. 7.5 Generation of reactive oxygen species (ROs) from ZnO surface. VB valence band, CB conduction band, h+ positively charged holes, e– electrons

An overproduction of intracellular reactive oxygen species promotes oxidative stress (Ren et al. 2015), altering the cell cycle and promoting cell death through apoptosis or autophagy. Moreover, reactive oxygen species can promote lipid peroxidation, associated with impairment of cell membrane structure, protein denaturation, and different types of DNA damage (Ghosh et al. 2018). The size, shape, morphology, and orientation are characteristics that affect the optical and piezoelectrical properties of ZnO and, therefore, influence its performance as a photosensitizer in photodynamic therapy and drug delivery system (Tripathy et al. 2015; Martínez-Carmona et al. 2018). When the ZnO nanoparticles act as a carrier for the delivery of photosensitizers, three types of radiation can be used to promote excitation: (a) UV-A to excite ZnO nanoparticles, (b) UV/visible to excite photosensitizers, and (c) X-ray for fluorescence resonance energy transfer (FRET) between ZnO nanoparticles and photosensitizers (Youssef et al. 2017). ZnO nanoparticles, such as nanoflowers and nanorods (NRs), are used not only as a potential photosensitizer in photodynamic therapy but also as an efficient light extractor in photonic devices (Özgür et al. 2013; Firdous 2018b). Among the forms for ZnO nanoparticles, the nanorods are a good candidate as photosensitizer because of their high area/volume ratio and biocompatibility. Characteristics, as well as light absorption, scattering, and fluorescence, provide a large number of applications in medical science (Atif et al. 2011). However, there are still some drawbacks for ZnO nanostructures in clinical use, such as the need for high concentrations, poor water stability, and facile agglomeration (Chen et al. 2017). Hence, it is necessary to modify ZnO nanostructures with other molecules leading to combined cancer therapy modalities. This approach can overcome the low efficiency of single modality therapy and improve the outcome of anti-tumor (Zhang et al. 2011; Hariharan et al. 2013). Zhang et al. studied a new strategy of the combined application of ZnO nanorods (Fig. 7.6) and the anticancer drug daunorubicin in photodynamic therapy (Zhang et al. 2011). The results demonstrated that the encapsulation and loading efficiency of daunorubicin into the ZnO nanorods are dependent on the pH. At pH 7.4, the drug release was slow and sustained, with releasing ratio of about 19% within 36 h. The results showed that daunorubicin-loaded ZnO nanorods might efficiently act as the

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Fig. 7.6 In vitro daunorubicin (DNR) release behaviors at pH 7.4, 6, and 5.0 and cytotoxic effect of daunorubicin or daunorubicin-ZnO nanocomposites in the absence or presence of ultraviolet (UV) irradiation against SMCC-7721 cells. Inset: the inhibitory concentration (IC50) of daunorubicin, daunorubicin-ZnO nanocomposites in the absence or presence of UV irradiation for SMCC7721 cells (B). Reprinted with permission of [Biomaterials, Zhang H, Chen B, Jiang H, et al., Copyright 2011, Elsevier] from (Zhang et al. 2011)

Fig. 7.7 Scanning electron microscopy images of ZnO nanoporous (a) and percentage viability of control and treated A-549 cells in the presence of 100 μg/ml Photofrin and exposed to 640 nm laser light (b). Reprinted with permission of [Med Sci, Fakhar-E-Alam M, Ali SMU, Ibupoto ZH, et al., Copyright 2012, Springer Nature] from (Fakhar-E-Alam et al. 2012)

anticancer drug delivery carrier. ZnO nanorods can carry more daunorubicin molecules for internalization and increase the accumulation of daunorubicin in SMMC7721 cells. Additionally, UV irradiation could further enhance the growth inhibition of cancer cells by photocatalysis of ZnO nanorods. The synergistic effect could induce a remarkable improvement in anti-tumor activity. This same group coordinated by Zhang in 2010 has explored the effect of UV irradiation combined with different sized ZnO nanoparticles (ZP5, ZP6, and ZP7 with the average diameters of about 20, 60, and 100 nm) conjugated to daunorubicin. This system was used to inhibit cancer SMMC-7721 cell proliferation under UV irradiation compared to that without UV irradiation (Fig. 7.7). The effect of the

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different sized ZnO nanoparticles in the absence/presence of UV irradiation on SMMC 7721 cell lines was evaluated. The results have shown that all the studied ZnO nanoparticles had a similar inhibition capacity to target cancer cells (Li et al. 2010). Nanosized ZnO particles can promote a decrease in the cell viability after UV irradiation because they efficiently induce the formation of reactive oxygen species on the cell membrane. This can lead to the membrane destruction resulting in changes in the permeability on the target cell membrane, which causes the efflux of cytoplasm and the apoptosis or death of the target cancer cells (Li et al. 2010). Using the same concept of synergy between ZnO nanoparticles and photosensitizing molecules, Fakhar-E-Alam studied the synergic effect between ZnO nanorods and 5-ALA (5-aminolevulinic) as photosensitizers (Fakhar-e-Alam et al. 2011). The fluorescence spectra for ZnO nanorods exhibited four peaks, at 501, 517, 560, and 575 nm (Fakhar-e-Alam et al. 2011). After 5-ALA conjugation, three peaks were observed at 320 nm (UV region), 400, and 470 nm. The work suggested that white light emission from ZnO nanorods was transferred by absorption to the 5-ALA, which in turn results in their excitation leading to the production of reactive oxygen species resulting in HeLa cell necrosis. The same group in 2012 developed a ZnO nanoporous material with a high surface/volume ratio due to its porosity for Photofrin drug delivery system for human lung cancer cells (A-549) (Fig. 7.7) (Fakhar-E-Alam et al. 2012). The reduction of 92–95% in the viability of the A-549 cells was founded using laser light (630 nm) dose 60 J/cm2 applied along with 100 μg/ml of Photofrin. In general, the reported studies involving ZnO nanoparticles in photodynamic therapy use the UV/visible light irradiation. However, the UV light irradiation exhibits limitations that include (i) a limited treatment depth due to shallow penetration in tissue, (ii) nonspecific tissue damage by natural light exposure to a photosensitizer, and (iii) photo-damage of healthy tissues by harmful UV light. The use of the up-conversion nanoparticle system has shown promising applications over the traditional photodynamic therapy (Wang et al. 2013a). The up-conversion nanoparticle therapy, the photon up-conversion, is known to be a stepwise process that converts NIR light to visible or ultraviolet (UV) emission. The NIR radiation penetrates significantly deeper than visible light in the tissue since most of the body molecules strongly absorb the visible light, which can also be scattered at a higher level by these molecules. Zhang and co-works developed an up-conversion ZnO nanoparticles core/shell composite (Fig. 7.8), based in NaYF4:Yb,Tm core/shell (Dou et al. 2015). The breast cancer cells (MDA-MB-231 and 4 T1) exposed to up-conversion ZnO core/ shell nanoparticles showed a significant release of reactive oxygen species as measured by aminophenyl fluorescein and ARE-FLuc luciferase assays and 45% cancer cell death as measured by MTT assay when illuminated with 980 nm infrared light (Dou et al. 2015). Furthermore, the water has noticeable absorption at and beyond 980 nm, resulting in tissue heating during photodynamic therapy treatment, which is undesirable. The development of up-conversion nanoparticles that can be effectively

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Fig. 7.8 Hydrophilic up-conversion ZnO core/shell nanoparticles and their therapeutic route in producing the reactive oxygen species upon the excitation at 980 nm infrared light

excited by other NIR wavelengths (e.g., 915 nm, as shown by Zhang and Xiong (2015)) may help to solve this problem (Qiu et al. 2018). Hexagonal phase NaYF4:Yb3+/Er3+ or NaYF4:Yb3+/Tm3+ up-conversion nanoparticles are commonly used as NIR-to-visible nanotransducers, with the crystalline matrix providing the highest photon up-conversion efficiency (Qiu et al. 2018). The Er3+-doped up-conversion nanoparticles offer an up-conversion emission peak at ~540 and ~ 660 nm, enabling the activation of organic photosensitizers such as merocyanine 540 (MC 540), zinc phthalocyanine, tetraphenylporphyrin, pheophorbide a, and Rose Bengal. Another application of ZnO nanoparticles besides the light therapeutic is like markers in disease diagnosis and the theranostic nanoparticle application. Theranostic is a new therapeutic technique with fewer limitations than the conventional therapy nanotechnology and has been emerged as an attractive tool for developing multifunctional and targeting strategies, leading simultaneously toward the detection, diagnosis, and treatment of cancer (Figueiredo et al. 2018). Apart from this typical UV range excitonic emission, the photoluminescence spectrum of ZnO nanocrystals also displays a broad visible emission, more suitable for biological imaging. This extended emission has been ascribed to point defects such as O and Zn vacancies or interstitials and related to surface oxygen-containing moieties, such as OH groups (Nagajyothi et al. 2015). Firdous reported the synthesis of ZnO nanorods (ZnO NRs) using a hydrothermal method, which produced a highly crystalline structure with particle diameter in the range of 80–120 nm. The fluorescence spectroscopy of ZnO nanorods (Fig. 7.9) allowed the excitation at 488 and 514 nm wavelength of these nanoparticles at the cancer cells and determination through laser scanning confocal microscopy. The surface profile at a fluorescence of 500–635 nm and 635–750 nm, a differential contrast image, and combined laser scanning confocal microscopy image indicate fluorescence conformation, surface morphology, and a diameter of 100–150 nm of cancer cells (Firdous 2018a). Among the main semiconductors applied in theragnostic applications for diagnosis and therapy are quantum dots. A very attractive property of the ZnO quantum dot (ZnO quantum dots) is its ability to exhibit the photoluminescence in the range

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Fig. 7.9 Scanning electron microscopy image indicating the diameters of the ZnO nanorods (left) and laser scanning confocal microscopy fluorescence image of ZnO nanorods: (a) fluorescence from 635 to 750 nm, (b) fluorescence from 500 to 635 nm, (c) differential interference contrast image, and (d) combined image of images (a)–(c). Reprinted with permission of [Laser Phys Lett, Firdous S, Copyright 2018, IOP PUBLISHING LIMITED] from (Firdous 2018a)

from blue to yellow (depending on particle diameter), under ultraviolet (UV) excitation. The photoluminescence of the ZnO quantum dots, at room temperature, basically consists of two competitive emissions: one in the UV region and the other in the visible region, in the green to the yellow range (Sandri et al. 2017). ZnO quantum dots slowly dissolve at physiological pH ¼ 7.4 (Martínez-Carmona et al. 2018), producing small changes in the extracellular zinc concentrations that cause minimal cytotoxicity. However, nanoparticles are preferentially internalized in the tumor cells because of the enhanced permeability and retention effect. Once inside and because of electrostatic interactions, ZnO quantum dots present certain cytotoxicity due to a higher intracellular release of dissolved zinc ions due to the acidification of the media, followed by increased reactive oxygen species induction. This situation results in the loss of protein activity balance mediated by zinc as well as in an oxidative stress environment that finally produce cell death. Chen and co-works developed ZnO quantum dots conjugated to gold nanoparticles loaded with the anticancer drug camptothecin (Chen et al. 2013d). As gold nanoparticles can destroy tumor cells via photo-thermal therapy by converting the absorbed light energy into localized heat, both blank and drugloaded nanocarriers exhibited potent cytotoxicity toward HeLa cells. On the other hand, dual-modal imaging modality could be achieved by hybridization between paramagnetic Gd ion and CuInS/ZnS quantum dots to enable dual fluorescence/ magnetic resonance-mediated diagnosis. Yang and co-works prepared Cd-free Gd-doped ZnS quantum dots in lipid vesicles with optimized fluorescence and excellent colloidal stability properties (Yang et al. 2015). Similarly, super-

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paramagnetic Fe3O4were separated from the fluorescent graphene–CdTe quantum dots by SiO2 shell to avoid fluorescence quenching of quantum dots (Ou et al. 2014). Quantum dots have several advantages in bioimaging over conventional dyes which enable quantum dots to attract tremendous interest in various biological fields. However, there are many challenges to the biological applications of quantum dots, such as cellular toxicity due to reactive oxygen species generation and small changes in extracellular ions concentrations. However, it has been reported different techniques used to alleviate quantum dots toxicity via hybridization with different biocompatible polymers, proteins, lipids, or inorganic nanoparticles (Zayed et al. 2019).

7.2.2

Antimicrobial Effect

ZnO nanoparticles can adsorb on the bacteria surface due to electrostatic forces, or it can be internalized by bacterial cells. In both cases, the antibacterial activity will be provided by the reactive oxygen species generated under light irradiation, which can be formed at the surface or in the interior of the bacteria, in this case induced by Zn2+ ions spontaneously formed. It is also possible that the antibacterial effects are a result of the combination of the processes mentioned above (Song et al. 2010; Sivakumar et al. 2018). Another possible mechanism of the antibacterial activity of ZnO particles is the generation of hydrogen peroxide (H2O2) by UV and visible light stimulation (Xie et al. 2011). The negatively charged hydroxyl radicals and superoxides cannot penetrate the cell membrane and probably remain on the cell surface, whereas H2O2 penetrates bacterial cells (Sirelkhatim et al. 2015) (Fig. 7.10). ZnO nanoparticles have a wide range of antibacterial activities against both Gram-positive and Gram-negative bacteria, including primary foodborne pathogens like Escherichia coli (Şahin et al. 2017), Listeria monocytogenes (Kim and Song

Fig. 7.10 Antibacterial activity of ZnO nanoparticles

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2018), Salmonella typhimurium, and Staphylococcus aureus (Akbar et al. 2019). ZnO nanoparticles have shown antimicrobial activity on skin-specific bacteria (Aditya et al. 2018), Streptococcus mutans, Streptococcus pyogenes, Vibrio cholerae, and Shigella flexneri. It has also shown antimicrobial activity against methicillin-resistant Staphylococcus aureus (Kadiyala et al. 2018). Generally, the antibacterial tests are performed in aqueous media or cell culture media. ZnO is almost insoluble in water; it agglomerates immediately with water during synthesis due to the high polarity of water leading to deposition. In order to avoid this effect, some alternatives are studied (Stanković et al. 2013; Sirelkhatim et al. 2015), such as the addition of poly-vinyl alcohol, polyvinylpyrrolidone, and poly(α, γ, L-glutamic acid) as stabilizers which have affected the ZnO morphology and size enhancing its antibacterial activity. Zhang and co-works have addressed the problem by adding dispersants polyethylene glycol and polyvinylpyrrolidone (10% of the amount of ZnO nanoparticles) which improved the stability of ZnO and resulted in ZnO nanofluids, well suited for the antibacterial tests (Zhang et al. 2007). Other capping agents (Padmavathy and Vijayaraghavan 2008) or deflocculants (sodium silicate Na2SiO3 or sodium carbonate Na2CO3) have also been used. The ZnO nanoparticle morphology and size are essential characteristics of nanoparticle in the function of the optoelectronic results and consequence antimicrobial effect (Shoeb et al. 2013). Therefore, developing a shape-controlled ZnO nanoparticles synthesis method is crucial for exploring the potential of ZnO nanoparticles as functional materials. Some procedures using distinct molecules such as tri-n-octyl phosphine oxide, sodium dodecyl sulfate, polyoxyethylene stearyl ether, bovine serum albumin, and citric acid have been used to control the size and shape of ZnO nanoparticles during synthesis (Moazzen et al. 2012). Green synthesis strategies of ZnO nanoparticle obtention have been used, allowing to avoid toxic chemicals reagents and producing safe routes (Lee and Kim 2019). Ali et al. synthesized small size ZnO nanoparticles using Aloe vera extract and showed their antibacterial activity cells before and after cell damage of Escherichia coli and MRSA cells before and after ZnO nanoparticle treatment (Ali et al. 2016). Santoshkumar et al. synthesized ZnO nanoparticles using Passiflora caerulea extract against urinary tract infection pathogens. The ZnO nanomaterials showed a reasonable zone of inhibition of various pathogens (Santhoshkumar et al. 2017). In addition to the bactericidal activity, ZnO nanoparticles have presented antifungal results (Yi et al. 2019). ZnO spherical nanoparticles were synthesized by an eco-friendly green combustion method using citrate containing Artocarpus gomezianus fruit extract. The zone of the inhibition method showed that the spherical ZnO nanoparticles also exhibit significant antibacterial activity against Staphylococcus aureus and antifungal activity against Aspergillus niger (Anitha et al. 2018).

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Energy Conversion and Storage

The evolution of human civilization has drastically increased the global demand for energy. Not only the absolute but also the per capita energy demand has been increased since currently, there is a larger population using a higher number of portable electronic devices such as smartphones, tablets, and personal computers. According to the International Energy Agency report (IEA: Directorate of Global Energy Economics 2014), the global energy consumption in 2015 was 108% higher than 2010, which is an impressive increase in only 5 years. The continuous expansion of agriculture, industrial, and economic activities combined with the fast technological development is expected to more than double the global energy demand by 2050 (Lewis 2005). Fossil fuels are the primary energy source and supplied more than 80% of the global demand for energy in 2015 (Nguyen and Montemor 2019). However, the non-renewable nature of fossil fuels means that this energy source could be scarce in the future. Also, fossil fuels are not environmentally friendly, since their use has substantially increased the greenhouse gas emissions and the level of atmospheric pollution in urban and/or industrialized areas. In this scenario, it is evident that the development of sustainable and renewable energy sources will be a crucial issue for human society in the next decades. We are already experiencing remarkable improvements in renewable energy technologies which have brought substantial advances in hydropower energy, wind electricity, and photovoltaic energy conversion (Nguyen and Montemor 2019). Electricity is currently one of the most used forms of energy, and the development of technologies to store electricity is so important as conversion technologies since most of the renewable energy sources as wind and solar are intermittent. The most known electrical energy storage devices are batteries, dielectric capacitors, and supercapacitors, and the performance of these devices has continuously increased as a result of intensive researches involving different areas, such as physics, chemistry, materials, and engineering (Liu and Liu 2019). Therefore, coupling sustainable renewable energy sources with efficient storage devices seems to be the most reasonable way to deal with the future global demand for energy. Solar energy and supercapacitors are, respectively, promising conversion and storage energy technologies which are supposed to play a central role in energy production in the future (Lewis 2005; Salinas-Torres et al. 2019). Solar energy is abundant, clean, and fully renewable, while supercapacitors are reliable, efficient, and durable devices. ZnO is a very attractive material which has been successfully used in both the conversion of solar energy and the preparation of electrodes for supercapacitors. This oxide can be inexpensively and safely synthesized according to several procedures leading to a great variety of particle size and morphology, making ZnO an extremely versatile material. Moreover, its low toxicity makes ZnO an environmentally friendly material which is very important regarding its potential use to the high-scale production of supercapacitors and solar cells. Therefore, these desirable features of ZnO have been allowed its extensive use in solar energy conversion and supercapacitor preparation.

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Solar Energy Conversion

Solar energy is one way to humanity solve the problems of its energy demands in order to contribute to technological development sustainably. In this sense, studies about this goal have significantly increased. Searching in the Web of Science database using the keywords: “Solar Energy conversion” between the years of 2010 and 2019, there are 5215 records, being articles, review, editorial material, letter, proceedings paper, meeting abstract, book chapter, correction, note, and early access. Figure 7.11 shows these results. There is a considerable increase of works published on these aims, and the number of studies about many kinds of materials for this type of application also follows the same behavior. Worldwide various Research Groups have been contributed with research within this segment. Numerous works can be found in the literature dealing with many types of materials in the most varied ways possible for use in solar energy conversion systems. However, in this case, ZnO is highlighted, mainly due to its physical and chemical properties in which have been previously presented. Recently, Rajan and Cindrella (2019) reported a comparison of the use of TiO2 and ZnO as nanorods as photoanode. In this work, conversion efficiency of 0.027% was observed for the system containing ZnO. According to the authors, the formation of Zn2+ dye species can contribute to a decrease in the performance of the parameters, thus requiring optimization for this field of application. Parallel the ZnO in nanowires shape have been reported as an essential point for energy conversion systems (Law et al. 2005). The particle shapes contribute with the increase in surface area and consequently improve the charge effect on the dyes, improving the electron transport and the quantum efficiency of the whole system.

Fig. 7.11 Results obtained through a search in the Web of Science database between 2010 and 2019, using the following information in the site: keywords – “Solar Energy conversion” between the year 2010 and 2019 accessed August 26, 2019, at 10:48 a.m. Adapted from Web of Science

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Specifically, particles in the form of nanowires have new properties related to the shape of sunlight absorption in the orthogonal direction as well as the light scattering effect and increase of the photon carrier effect (Bierman and Jin 2009) in contrast to the materials based on the film deposited onto a planar surface. The scattering of light from the surface of nanowire materials can be absorbed by the other close surface present in the path of light. In this sense, the conversion of energy in devices made by nanowires materials is adequate due to the absorption process be improved due to the architecture of matted formed by the wires. In this sense, the system with these shapes can contribute to better efficiency in the conversion of solar energy to electrical energy. Other points of view also have been reported and need to be considered. Ueno et al. (2011) have reported the obtaining of ZnO particles with macroporous prepared through the spray method with ZnO dispersed in water containing spheres with around 200 nm of diameter. The size of the particle contributed to the light scattering, enhancing the harvesting efficiency improving in 4.8% of the energy conversion efficiency. Dye-sensitized solar cells based on ZnO nanoforest materials as photoanode obtained by hydrothermal growth with high density have been reported. Higher harvesting and dye loading with significant contribution to avoiding the charge recombination in crystalline ZnO nanotree are reported by Zhang et al. (2015b). The efficiency values from 0.45 to 2.63% were reported as a function of the configuration of nanotree that was lengthwise growth and branched growth. In the same way, the presence of defects on the surface of the ZnO particles can contribute or promote the effect of photon-generated electron/hole pair in which compromise the quality of the solar cells devices. To avoid this problem, Rahman et al. (2016) prepared the ZnO doped with 1, 2, 5, and 7.5% wt of Ti4+ and observed superior of harvesting of light and efficiency in conversion of the energy of 5.6%. The authors reported that the improvements observed in the system are associated with the Fermi level and the large surface area, contributing to the enhancement of dye loading. In this sense, it is necessary to take into account that the many ways of solar energy harvesting are one of the most crucial points for energy conversion (Han et al. 2012). The particle of ZnO tetrapod also has been reported in the literature as another way to improve the efficiency in solar cells. In accordance to Yan et al. (2015), this kind of architecture improves the quality of the connections among the particles improving formation of porous and consequently increasing the superficial area. However, the control of tetrapod size still is an important point to be improved. Hybrid systems to be used in photovoltaic cells can also be the basis of nanoparticles of ZnO with poly(3-hexylthiophene) on the surface (Beek et al. 2006). The poly(3-hexylthiophene) is widely used as carrier holes, thus improving the efficiency of cells (Huynh 2002; Huynh et al. 2003). The presence of ZnO together poly(3-hexylthiophene) has fundamental importance to enlarge the absorption region, due to the absorption band positioned around 350 nm attributed to the ZnO. The enlargement of the absorption band can contribute to the improvement of

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efficiency in energy conversion, and the absence of this oxide decreases the absorption of photons mainly in the region below 400 nm (Beek et al. 2006).

7.3.2

Supercapacitors

Supercapacitors or electrochemical capacitors are energy storage devices able to provide ultrahigh power density, typically on the order of 100 kW kg1, which is 100–1000 times higher than batteries (Bockris and Reddy 2000). However, contrasting with batteries which store charge by faradaic reactions taking place in the bulk of the electrodes, supercapacitor energy storage is based on interfacial capacitive or pseudocapacitive processes, leading to very low energy densities (Xiong et al. 2019), usually lower than 10 Wh kg1 which is 3–30 times lower than batteries (Nguyen and Montemor 2019). This feature makes supercapacitors adequate for applications in which high power density and fast charging are more important than energy density (Bockris and Reddy 2000; Liu and Liu 2019). Despite the limited energy density, supercapacitors show several attractive characteristics such as fast charge–discharge cycle, high durability, simplicity, and reliability, besides their very high power density (Dubey and Guruviah 2019; Nan et al. 2019). These characteristics suggest that supercapacitors will have a crucial role in energy storage in the future. Contrasting with conventional capacitors, in which the charge storage is based on electrical charge separation on two metal sheets separated by a dielectric (Bockris and Reddy 2000), supercapacitors store charge at the interfaces of two electrodes (one positive and another negative) and the electrolyte solution (Dubey and Guruviah 2019) as depicted in Fig. 7.12. The most straightforward equation for the capacitance (C) of a conventional parallel plate capacitor is presented in Eq. 7.1 (Bockris and Reddy 2000).

Fig. 7.12 Charge storage at a conventional capacitor and supercapacitor

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Fig. 7.13 (a) General representation of a supercapacitor assembly. (b) Charge storage on an electric double-layer capacitor and pseudocapacitor (c). Ox oxidized form of a redox couple. H-Red reduced form of a redox couple, e electron

C¼

εA 4πδ

ð7:1Þ

where ε is the dielectric constant, A is the area of the plates, and δ is the distance between the plates. As can be seen in Eq. 7.1, the capacitance, and therefore the energy storage capacity, can be increased by either increasing the area of the electrodes and/or decreasing the distance between the charged plates. This equation defines the overall characteristics to design high-performance capacitors, which must use electrodes with the highest electrochemical active area and keep the charged plates as close as possible each other. Thus, the electrodes used in supercapacitors are based on nanostructured porous materials, and as shown in Fig. 7.12, the distance between the charged plates is on the order of molecular diameters, typically 5 Å (Bockris and Reddy 2000). These features are responsible for the ultrahigh power density presented by supercapacitors. Usually, supercapacitors are assembled in sealed packages as schematically shown in Fig. 7.13a, in which the positive and negative electrodes are immersed in the same electrolyte solution, but a separator is used to prevent short circuits (Liu and Liu 2019). Charge storage at supercapacitors is achieved by one of the two mechanisms: (a) the development of electric double layers at electrode/electrolyte interface and (b) the occurrence of pseudocapacitive processes (Dubey and Guruviah 2019; Zhao et al. 2019; Liu and Liu 2019). The capacitance on electric double-layer capacitors (Fig. 7.13b) comes from adsorption, desorption, or ion exchange of cations and anions at the electrode/electrolyte interface. Therefore, charge storage at electric double-layer capacitors occurs only by physical non-electrochemical processes (purely capacitive phenomena). On the other hand, as shown in Fig. 7.13c, pseudocapacitors store charge through reversible faradaic processes occurring at the electrode surfaces changing the oxidation states of the pseudocapacitive materials (Conway and Pell 2003; Brousse et al. 2017). The electrochemical performance of pseudocapacitors is often superior to electric double-layer capacitors (Wang et al. 2013b, 2014), and it is strongly dependent on

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the electrode materials (Zhao et al. 2019). Carbon-based electrodes are the most used in electric double-layer capacitors, while transition metal oxides and conducting polymers are the most used materials for pseudocapacitor electrodes (Zhao et al. 2019). Among pseudocapacitive transition metal compounds used as electrodes for supercapacitors, RuO2, V2O5, and MnO2, besides redox-active transition compounds, such as oxides and hydroxides of Ni, Co, Ti, Fe, and Mo, are the most common (Nguyen and Montemor 2017, 2019; Zhao et al. 2019; Nan et al. 2019). ZnO is regarded as a promising material to be used to prepare electrodes for supercapacitors, mainly due to its good electrochemical activity, environmental friendliness, high abundance, and the possibility to be obtained with different morphologies and particles size (Majeed et al. 2016; Wang et al. 2018). It was demonstrated that flower-like mesoporous ZnO could be successfully synthesized by a hydrothermal method at 180  C for 12 h in the presence of CTAB, producing a material with a grain size of 17 nm (Saranya and Selladurai 2018). The as-obtained material was used as electrodes for supercapacitors and provided a specific capacitance of 322 F g1, determined by cyclic voltammetry in 3 mol L1 KOH at 5 mV s1. ZnO tetrapod with “arms” about 170 nm and shorter than 4000 nm was synthesized by an oxidative metal vapor transport method and produced a specific capacitance of 160 F g1 at 1 A g1 in 1.0 mol L1 Na2SO4 (Luo et al. 2017). In another study, ZnO nanorods produced by a chemical route with NH3 as complexing agent provided a maximum area capacitance of 29 mF cm2 at 5 mV s1 (Deshmukh et al. 2017). The performance of ZnO-based electrodes is strongly dependent on the morphology and the synthetic method. Lee et al. evaluated the effect of the morphology of ZnO nanostructures on their specific capacitance and observed that flower-like ZnO provided the highest specific capacitance (2.75 F g1) compared with rod-like ZnO (Lee et al. 2017). Flower and rod-like ZnO nanostructures were prepared by a precipitation route, and the morphology was controlled by changing the precursor, reaction time, and the pH of the reaction mixture. The effect of the precursor in the hydrothermal synthesis of ZnO was evaluated by Alver et al., who studied zinc acetate, chloride, and nitrate as precursors (Alver et al. 2016). These authors found out that zinc nitrate and acetate had produced a hexagonal wurtzite-like structure, while the simonkolleite structure was produced when zinc chloride was used as the precursor. The highest specific capacitance (5.87 F g1) was obtained with zinc nitrate as the precursor. Despite the high potentiality of ZnO to be used in supercapacitors, this material has some drawbacks such as low specific capacitance and its short life cycle caused by dendrite formation with sequential cycling (Wang et al. 2018). Hybrid materials, such as ZnO/ZnS (Zhang et al. 2015b), ZnO/polypyrrole (Sidhu and Rastogi 2014) and Ni-doped ZnO (Reddy et al. 2018), have been used to overcome the limitations of ZnO-based electrodes. All these hybrid materials brought improvements in the specific capacitance and life cycle compared with ZnO alone. Figure 7.14 shows the values of the specific capacitance obtained with some ZnO and ZnO-hybrid electrodes, which provide superior performance.

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Fig. 7.14 Specific capacitance for ZnO electrodes (cyan bars) and ZnO-hybrid electrodes (magenta bars). (a) RZ rod-like ZnO. FZ flower-like ZnO. The numbers 1 and 3 are the time of synthesis in hours. Data from (Lee et al. 2017). (b) Ac acetate. Data from (Alver et al. 2016). (c) PPy-ONT – polypyrrole open nanotubes. PPy-NT – polypyrrole nanotubes. ZnO/PPy – hybrid ZnO/PPy. Data from (Sidhu and Rastogi 2014). (d) The numbers correspond to the Ni percentage on the hybrid ZnO/Zn. Data from (Reddy et al. 2018)

Another approach to improve the performance of ZnO-based supercapacitors is to use nanocomposite materials containing ZnO. This approach leads to high ZnO dispersion and stabilization, producing higher electrochemical active areas, life cycling, and specific capacitance. Several ZnO-based nanocomposites have been studied, and the application of these materials as electrodes for supercapacitors was recently revised (Wang et al. 2018). Figure 7.15 summarizes the main ZnO nanocomposites used to develop electrodes for supercapacitors (Wang et al. 2018). ZnO-activated carbon composites have received growing attention as electrodes for supercapacitors since activated carbon is an abundant and low-cost material which has high electrical conductivity, specific area, and chemical stability (Wang et al. 2018). Therefore, the combination of these advantageous properties with the attractive features of ZnO has a great potential to produce high-performance and cost-effective supercapacitors. The main approaches adopted to prepare the ZnO-activated carbon composites include the co-precipitation method (Kalpana et al. 2006; Yadav et al. 2018), electrospinning using zinc acetate and polyacrylonitrile (Kim and Kim 2015), in situ precipitation (Lee et al. 2018a), and microwave-assisted precipitation (Selvakumar et al. 2010). Despite the great variety of morphologies obtained for these composites, the reported average particle size

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Fig. 7.16 Evolution of the specific capacitance for supercapacitors based on ZnO-activated carbon composites. The specific capacitances correspond to the maximum reported value obtained by cyclic voltammetry. Values extracted from (Kalpana et al. 2006; Selvakumar et al. 2010; Kim and Kim 2015; Yadav et al. 2018)

Specific capacitance / F g–1

Fig. 7.15 ZnO-based composites used for the fabrication of electrodes for supercapacitors

350 300 250 200 150 100 50 0 2006

2015

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Year was always close to 100 nm with grain size lower than 30 nm. ZnO-activated carbon composites have brought impressive advances for supercapacitors, allowing an increase for more than ten times in the specific capacitance in just over 10 years, as shown in Fig. 7.16. As can be observed in Fig. 7.16, ZnO-activated carbon composite electrodes provided higher specific capacitance than ZnO-hybrid materials (Fig. 7.14), which can be ascribed to synergistic effects introduced by the interactions between ZnO and the activated carbon. Besides activated carbon, other carbon-based materials such as graphene, reduced graphene oxide, and carbon nanotubes have also been applied to prepare electrodes for supercapacitors. Usually, these advanced carbon materials have even better performance than the composite ZnO/activated carbon. Graphene is a material with amazing electrical and mechanical properties which are ascribed to the 2D nature of this material since the thickness of graphene is in the

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atomic scale. This bidimensional array provides strong electron delocalization and, therefore, extremely high electric conductivity. Moreover, the strong covalent bonds in the aromatic rings lead to a high mechanic strength and chemical stability. Also, graphene shows a very high surface area. Therefore, graphene combines the most desirable properties to be applied in supercapacitors (Li et al. 2015; Sahu et al. 2015; Wang et al. 2018). Despite that, there are strong, attractive forces between the graphene sheets due to the van der Waals interactions which favor agglomeration, decreasing the surface area and the capacitance and compromising the performance of this material for applications in supercapacitors (Li et al. 2013). The introduction of metal oxide nanoparticles to the interlayer of graphene sheets is an efficient strategy to avoid/minimize graphene agglomeration (Li et al. 2013). ZnO/graphene composites have been widely used to develop high-performance supercapacitors. Besides the minimization of the graphene agglomeration, ZnO effectively contributes to increasing the overall performance of the supercapacitor (Kumar et al. 2015a). The reduction of graphene oxide is the most successful approach to prepare graphene-based materials (Dubey and Guruviah 2019). This method is inexpensive and has high production capacity since graphene oxide is obtained from the chemical oxidation of graphite, which is an inexpensive and highly available material (Lee et al. 2018b; Wang et al. 2018). Graphene oxide has a layered structure containing oxygen functional groups (epoxide, carboxyl, and hydroxyl), which increase the hydrophilicity of this material compared with graphene (Chaudhary et al. 2017). Therefore, graphene oxide can be easily dispersed in water and other polar solvents, facilitating the processing of this material. One the other hand, these hydrophilic functional groups sharply decrease the electric conductivity, so graphene oxide is an electric insulator (Lee et al. 2018b). The primary strategy to restore the electrical conductivity of this material is to perform a reduction step in which the oxygenated groups are partially removed, forming the called reduced graphene oxide (Dubey and Guruviah 2019). Reduced graphene oxide holds almost the same attractive properties of graphene, and it has also been widely used for supercapacitors design, usually combined with conducting polymers or metal oxides, such as RuO2, MnO2, V2O5, and ZnO (Jayachandiran et al. 2018). Table 7.3 summarizes some studies in which ZnO/graphene-based composites were used as electrodes in supercapacitors. It can be observed that the highest specific capacitance values obtained with ZnO/graphene composites are much superior to the obtained with ZnO or ZnO-hybrid materials. The maximum specific capacitance reported was 875 F g1 (Chaudhary et al. 2017), which is more than two times higher than the reported for ZnO/activated carbon. This result demonstrates the high potentialities of ZnO/graphene composites to develop high-performance supercapacitors. Several synthetic procedures have been used to produce the ZnO/graphene composites. In a general way, it can be observed that the highest specific capacitance values are obtained when the synthesis is assisted by microwave irradiation. Besides providing high electrochemical performance, microwave irradiation significantly decreases the time of reaction and produces highly crystalline and pure materials (Kumar et al. 2015a; Guo et al. 2016; Sreejesh et al. 2017).

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Table 7.3 Synthetic and electrochemical features of different supercapacitors prepared with ZnO/graphene nanocomposites

Material Gr/ZnO-NC ZnO/rGO

Synthesis Hydrothermal 160  C, 12 h scCO2-assisted

v or j 5 mV s1 5 mV s1

ZnO/rGO/ ZnO-sandwich ZnO/rGO

Solid-state reaction 1200  C, 2 h Solvothermal 45 or 145  C

5 mV s1

ZnOL@MpEG

5 A g1

ZnO/GNS

Microwaveassisted CVD/ hydrothermal Co-precipitation method Electrodeposition

Gr/ZnO-NC

Solvothermal

5 mV s1

Gr/ZnO-NC

Microwaveassisted method Hydrothermal 180  C, 12 h Microwaveassisted hydrothermal Hydrothermal 150  C, 12 h Microwaveassisted precipitation Ultrasonicassisted Precipitation method Chemical decomposition of ZnHCF over rGO

1 A g1

3DG/ZnO ZnO/GNR

ZnO/rGO-NC Au/ZnO/rGO

Gr/ZnO-NC ZnO/rGO-NC

ZnO/rGO-NC ZnO/rGO ZnO-NFs/rGO

50 mV s1

5 mV s1 5 mV s1 3 A g1

– 1 A g1 5 mV s1 5 mV s1 5 mV s1 0.5 A g1 1 A g1

Electrolyte 0.5 mol L1 Na2SO4 2.0 mol L1 KOH 1.0 mol L1 Na2SO4 1.0 mol L1 KCl 1.0 mol L1 Na2SO4 1.0 mol L1 KOH 0.5 mol L1 Na2SO4 1.0 mol L1 KOH 6.0 mol L1 KOH 1.0 mol L1 Na2SO4 1.0 mol L1 KOH 2.0 mol L1 KOH

Specific capacitance/ F g1 156 314 275 108

347 554 450 291 122 201 313 875

3.0 mol L1 KOH 1.0 mol L1 KOH

719

1.0 mol L1 Na2SO4 1.0 mol L1 Na2SO4 3.5 mol L1 KOH

312

635

97 203

References Li et al. (2013) Haldorai et al. (2014) Li et al. (2014b) Prakash and Bahadur (2014) Kumar et (al. 2015a) Li et al. (2015) Sahu et al. (2015) Zhang et al. (2015c) Saranya et al. (2016) Guo et al. (2016) Majeed et al. (2016) Chaudhary et al. (2017) Rajeswari et al. (2017) Sreejesh et al. (2017) Jayachandiran et al. (2018) Lee et al. (2018b) Subramani and Sathish (2019)

v scan rate, j current density, Gr graphene, NC nanocomposite, rGO reduced graphene oxide, scCO2 supercritical CO2, ZnOL@MpEG ZnO layer on microwave exfoliated graphene, 3DG 3D graphene nanorods, CVD chemical vapor deposition, GNR graphene nanoribbon, GNS graphene nanosheets, NFs nanoflowers, ZnHCF zinc hexacyanoferrate

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ZnO/carbon nanotube composites have also been used to develop supercapacitors. Functionalized multiwalled carbon nanotubes are preferably used for these applications, mainly due to the lower cost and higher surface area of multiwalled carbon nanotubes. Usually, the functionalization is achieved by refluxing the multiwalled carbon nanotubes in concentrated HNO3 and/or H2SO4 (Aravinda et al. 2013; Suroshe and Garje 2015). ZnO/carbon nanotube composites have been produced by dip coat-based method (Sankapal et al. 2016), solvothermal decomposition of precursors (Suroshe and Garje 2015), magnetron sputtering (Aravinda et al. 2013), chemical batch deposition (Al-Asadi et al. 2017), precipitation (Lee et al. 2018c), and hydrothermal method (Ramli et al. 2017). The average particle size of ZnO found in these composites was lower than 30 nm (Suroshe and Garje 2015; Sankapal et al. 2016) showing that nanotubes are efficient to avoid the growth of ZnO nanoparticles. ZnO/carbon nanotube composites showed attractive electrochemical performances, with specific capacitance ranging from 48 (Aravinda et al. 2013) to 306 F g1 (Ramli et al. 2017). Other carbon-based materials used to produce ZnO composites for applications in supercapacitors are carbon nanofibers (Shi et al. 2013; Kim and Kim 2014; Pant et al. 2018), organic non-conducting (Tripathi and Kumar 2018) and conducting polymers (Dubey and Guruviah 2019), porous carbon (Madhu et al. 2016; Yu et al. 2016), and chitosan (Pandiselvi and Thambidurai 2014). A particularly exciting application of carbon-based ZnO composites is the production of flexible supercapacitors. These devices are highly desirable from a technological viewpoint since they can be constructed in a miniaturized and portable design and compatible with wearable devices, which will undoubtedly play an essential role in communication, environmental monitoring, clinical diagnosis, and entertainment. Flexible supercapacitors have been produced by using ZnO composites with polydimethylsiloxane (Cauda et al. 2015), polypyrrole/graphene (Chee et al. 2015a, b), polyvinylidene fluoride– tetrafluoroethylene (Sami et al. 2017), and graphene polyaniline (Liu et al. 2018). All these composites showed satisfactory performance regarding specific capacitance, life cycling, and flexibility. The application of composites between ZnO and other metal oxides to supercapacitors has also been reported. MnO2 is often studied due to its high theoretical capacitance, low cost, and low toxicity. However, MnO2 has low electric conductivity, and its low surface area leads to specific capacitance much lower than the theoretical capacitance (Raj et al. 2017; Wang et al. 2018). Therefore, the production of ZnO/MnO2 composites intends to overcome these limitations by synergistic interactions between both metal oxides and/or by increasing the MnO2 dispersion, leading to a higher surface area. A three-dimensional nanostructured ZnO/MnO2 composite was used as electrode material in a planar supercapacitor, which is schematically represented in Fig. 7.17A (Raj et al. 2017). These authors obtained well-aligned hexagonal ZnO nanorods with approximately 200 nm diameter (Fig. 7.17B-a, b) which were grown onto fluorine-doped tin oxide by a hydrothermal procedure. MnO2 was deposited onto ZnO originating almost spherical particles (~50 nm diameter) uniformly distributed along to the ZnO nanorods

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Fig. 7.17 (A) ZnO/MnO2 planar supercapacitor. (B) Low and high magnification scanning electron microscopy images of (a) and (b) vertically aligned bare ZnO nanorods (c) and (d) MnO2 decorated ZnO nanorods. FTO fluorine-doped tin oxide. Reprinted with permission of [Electrochim. Acta, Raj CJ, Rajesh M Manikandan R, et al., Copyright 2017, Elsevier] from (Raj et al. 2017)

(Fig. 7.17B, c-d). This composite showed a promising performance providing an area capacitance of 14 mF cm2 at 0.1 mA cm2 in 1.0 mol L1 Na2SO4. Huang et al. produced a hierarchical ZnO@MnO2 core/shell composite by a hydrothermal method, and this material was used as an electrode for supercapacitors. The as-prepared composite showed a promising electrochemical performance with a specific capacitance of 423.5 F g1 at 0.5 A g1 in 1.0 mol L1 Na2SO4. Moreover, this composite has presented very high cycling stability, retaining 92% of its capacitance after 3000 cycles (Huang et al. 2015). ZnO/MnO2 composites have also been used to construct flexible supercapacitors (Li et al. 2014a). These authors produced three-dimensional MnO2 nanowire/ZnO nanorod composites onto a carbon cloth substrate. ZnO nanorods were obtained by a seeded low-temperature solution method followed by MnO2 nanowire deposition, as schematized in Fig. 7.18a. This composite has shown excellent performance with a specific capacitance of 746.7 F g1 (at 2 mV s1 in 1.0 mol L1 Na2SO4) and retention of 93.5% of its capacitance after 1000 cycles (Fig. 7.18b). Moreover, it was observed that the voltammetric profile of these electrodes was independent on the bending angle (Fig. 7.18c). Therefore, this composite efficiently combined high specific capacitance with high cycling stability and flexibility. The sol-gel method has also been used to prepare ZnO/MnO2 composites. Gao et al. produced a porous microstructure of zinc–manganese oxide (ZnMn2O4) by calcination at 450  C of a precursor obtained by a sol-gel reaction (Gao et al. 2015). The as-obtained oxide was used as an electrode in a supercapacitor and provided specific capacitance values as high as 1093 F g1 at 1 A g1 in 1.0 mol L1 Na2SO4. Chen et al., also using the sol-gel method, prepared the spinel-like compound ZnxMn3  xO4 (Chen et al. 2013b). The highest specific capacitance (301 F g1, at 100 mV s1 in 1.0 mol L1 Na2SO4) was obtained for the material calcinated at 300  C and prepared with the addition of 10% (m:m) of ZnO. The method of spray and pyrolysis was also used to prepare ZnO/MnO2 composites (Chen et al. 2013a).

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Fig. 7.18 (a) Synthesis of the MnO2 nanowire/ZnO nanorod composite. (b) Capacitance retention vs cycle number with some representative cyclic voltammograms in the inset, v ¼ 100 mV s1. (c) Cyclic voltammograms recorded under different bending angles. Reprinted with permission of [J. Power Sources, Li S, Wen J, Mo X, et al., Copyright 2014, Elsevier] from (Li et al. 2014a)

These composites were prepared by spray pyrolysis at 500  C from their acetate salts. The highest specific capacitance obtained with these materials was 230 F g1 at 25 mV s1 in 1.0 mol L1 Na2SO4 for the composite containing 5% (m:m) of ZnO. Besides manganese, other metals have been used together with ZnO to develop supercapacitors such as cobalt (Karthikeyan et al. 2009; Deka-Boruah et al. 2017; Ng et al. 2017; Boruah and Misra 2019), cobalt–nickel (Shakir et al. 2014b; Boruah and Misra 2017), nickel (Shakir et al. 2014a), cerium (He et al. 2016), titanium (Ray et al. 2015), and aluminum (Iglesias et al. 2015). Usually, these metals are used in the form of oxides, mixed oxides, or hydroxides, and they bring improvements in the specific capacitance and life cycling compared with ZnO alone. Figure 7.19 shows the specific capacitance values reported for some ZnO/metals composites. Figure 7.19 shows the prevalence of ZnO/Mn-based composites to develop supercapacitors. On average, the highest capacitance values are obtained in more recent studies. A remarkably high specific capacitance was reported in 2014 for a ZnO/Ni(OH)2 composite (Shakir et al. 2014a). These authors found a specific capacitance of 3150 F g1 at 5 mV s1 in 1.0 mol L1 LiOH for Ni(OH)2-coated ZnO nanowires obtained by a solution method. Besides the excellent specific capacitance, the proposed capacitor was designed as a highly flexible capacitor. These same authors, also in 2014, achieved a specific capacitance of 1927 F g1 at 2 A g1 in 6.0 mol L1 KOH for a flexible supercapacitor prepared with Ni-Co double-layered hydroxides/ZnO nanowires (Shakir et al. 2014b). Therefore, the use of ZnO/metal composites as electrodes is a promising approach to develop highperformance supercapacitors, and researches aiming to develop new composites are highly desirable and necessary to ensure more efficient energy storage in the future.

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ZnO/Ni(OH)2 - 2014 ZnO/Ni-Co - 2014

3000

ZnO/TiO2 - 2015

ZnO/CeO2 - 2016

ZnO/Co2O4 - 2017

ZnO/Co3O4 - 2017

500

ZnO/MnO2 - 2015

1000

ZnO/MnO2 - 2014

1500

ZnxMn3-xO4 - 2013

2000

ZnO/Mn2O4 - 2015

2500

ZnO/Mn3O4 - 2013

Specific capacitance / F g–1

3500

0

Electrode Material Fig. 7.19 Specific capacitance values reported for some ZnO/metal compounds composites

7.4

Electrochemical Sensing

The attractive mechanical, optical, electrical, and electrochemical properties of metal oxide semiconductors allied to their high abundance, low toxicity and low cost make them key materials for the development of several kinds of sensors. The attractive physicochemical properties of metal oxides are usually enhanced when they are used as nanosized materials due to the increased surface/volume ratio provided by nanomaterials (Nunes et al. 2019). Gas sensors and humidity sensors are the most common sensing devices based on metal oxides. Due to the simple chemisorption operating mechanism of these devices, metal oxide semiconductor-based gas and humidity sensors show high sensitivity, durability, and reliability which is combined with high mechanical resistance and low cost (Witkowski 2018; Nunes et al. 2019). Metal oxide nanoparticles have also been used in a large variety of more sophisticated sensors including UV/visible photodetectors, flame and smoke detectors, temperature sensors, and several kinds of biosensors (Witkowski 2018; Nunes et al. 2019). Electrochemical sensors are advantageous analytical devices since they provide high sensitivity and a fast response coupled with relatively inexpensive and straightforward instrumentation. Moreover, electrochemical sensors are usually prepared by inexpensive and straightforward procedures, and they can be efficiently designed as portable devices. Additional attractive features of the electrochemical sensors include the possibility of simultaneous, online, and in situ determinations (Lim and Gao 2015). This set of advantageous characteristics enables the application of the electrochemical sensors to various fields of interest, including clinical, environmental, pharmaceutical, and industrial. Metal oxide nanoparticles have brought impressive advances in the development of high-performance electrochemical

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sensors since these materials are cost-effective and able to remarkably improve the sensitivity and selectivity (George et al. 2018). Metal oxide nanoparticles show significantly different physicochemical properties compared to their corresponding bulk material. These differences have been attributed to the high surface/volume ratio of nanoparticles, which enhances the chemical reactivity compared to bulk materials. Moreover, the electronic properties of metal oxide nanoparticles are also different from their bulk form due to the quantum confinement effect (Lim and Gao 2015; George et al. 2018). Thus, the introduction of metal oxide nanoparticles to prepare electrochemical sensors can provide several benefits such as the improvement of the electron transfer kinetics and the increase of the active electrochemical area; in both cases, an improvement in the sensitivity is obtained. Moreover, the enhanced surface reactivity of the metal oxide nanoparticles can be exploited to improve the selectivity via immobilization of enzymes or other recognition molecules (Lim and Gao 2015; George et al. 2018). ZnO nanoparticles deserve a special place in the development of sensors and electrochemical sensors due to its low cost, high availability, the possibility of obtaining different morphologies, and low toxicity compared to other metal oxides (Asif et al. 2015; Hahm 2016; Chaudhary et al. 2018). The versatility of the morphology of ZnO nanoparticles is achieved by varying the synthetic procedure and/or the operational parameters of the synthesis such as the pH, the concentration of zinc precursor, the temperature, and even the solvent. Thus, both the size and morphology of ZnO nanoparticles can be extensively variated by conveniently changing these parameters (Nunes et al. 2019). ZnO nanoparticles have been successfully used to design potentiometric sensors (Willander et al. 2014; IsrarQadir et al. 2017) and several kinds of electrochemical biosensors (Shi et al. 2014; Izyumskaya et al. 2017), which have found exciting applications such as healthcare (Kumar et al. 2015b) and intracellular analyses (Asif et al. 2015). Therefore, the contributions of ZnO to the electroanalysis field are significant and numerous, and they seem to continue growing, regarding the impressive advances obtained in both the synthesis of ZnO nanoparticles and in the immobilization of these nanoparticles onto the electrode surface. The main properties of ZnO exploited to develop electrochemical sensors are its high surface area and rich surface chemistry instead of its electrochemical properties. While the high surface area is desirable to provide high sensitivity, the enhanced surface chemistry is essential to provide the efficient immobilization of electroactive or recognition groups, improving the selectivity (Izyumskaya et al. 2017). Thus, the rich surface chemistry of ZnO enables the immobilization of a great variety of chemical modifiers by exploring different types of interactions such as van der Waals, electrostatic interactions, and covalent bonding (Izyumskaya et al. 2017). Another fascinating application of ZnO nanoparticles is to use them as a template to produce nanostructured chemical modifiers, giving rise to chemically modified electrodes with very high analytical performance (Cheng et al. 2014; Lin 2015). Glassy carbon is the most common substrate used to prepare ZnO-based electrochemical sensors, and the drop-casting method is the main procedure to immobilize ZnO nanostructures onto the glassy carbon surface. The advantages of this

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immobilization method are its simplicity, low cost, and prompt electrode preparation. On the other hand, the possibility of losing the ZnO-modifier layer by leaching and the lack of reproductivity are the main disadvantages of drop-casting. Several ZnO-casting suspensions have been proposed in the literature. Wahab et al. coated a paste prepared with 30% of butyl carbitol and 70% of ZnO nanorods onto the glassy carbon surface to achieve ZnO immobilization. This electrochemical sensor was applied to determine hydrazine, and it was very stable, indicating that the immobilization method was efficient (Wahab et al. 2019). Carbon nanotubes/ dimethylformamide (Fang et al. 2009; Wang et al. 2010a), carbon nanotubes/ chitosan composites (Zhang et al. 2008b), and Nafion®/isopropyl alcohol (Ahmad et al. 2010) are additional examples of nanostructured ZnO casting suspensions. Besides drop-casting, the electrodeposition from Zn2+ aqueous solutions is also often adopted to immobilize ZnO nanostructures onto the glassy carbon surface (Hallaj et al. 2009; Wang and Zheng 2010). Alternative electrode substrates used to prepare ZnO-based electrochemical sensors include Pt (Devi et al. 2011; Narang and Pundir 2011; Paik et al. 2018), Au (Nesakumar et al. 2014), indium tin oxide (Saha et al. 2009; Wang et al. 2010b), fluorine-doped tin oxide (Lin et al. 2010), and screen-printed electrodes (Balamurugan et al. 2015). Carbon paste electrodes chemically modified with ZnO nanostructures have also been used (Afsharmanesh et al. 2013; Tashkhourian et al. 2014; Afkhami et al. 2015; Shafiee and Shabani-Nooshabadi 2018; Saritha et al. 2019). These electrodes are prepared from a mixture of graphite powder and a waterimmiscible binder agent, and they have several advantageous features such as low cost, simplicity, easiness of preparation, modification, and miniaturization (Švancara et al. 2009; Svancara et al. 2012). Several ZnO-based electrochemical sensors are prepared with ZnO composites aiming to obtain synergistic effects able to improve both the sensitivity and selectivity. Composites between ZnO and graphene-based materials have been successfully used for the determination of acetaminophen and phenacetin (Jiang et al. 2014), ethyl acetate (Ameen et al. 2014), hydrogen peroxide (Xie et al. 2013), and the estrogen receptor raloxifene (Shafiee and Shabani-Nooshabadi 2018), besides heavy metal ions (Yukird et al. 2018). Other examples of ZnO composites widely used to prepare electrochemical sensors are ZnO/carbon nanotubes (Jain and Dhanjai 2013), ZnO/chitosan (Narang and Pundir 2011; Wang et al. 2015), ZnO/Fe3O4 (Mikani et al. 2019), and ZnO with conducting polymers such as polyaniline (Jain et al. 2014) and polypyrrole (Chawla and Pundir 2012; Lin 2015). These composites have shown a very high analytical performance for an infinity of analytes. The extensive range of ZnO-based composites is a demonstration of the versatility of this metal oxide, which has been successfully used for the electrochemical sensing of analytes of great importance in several fields such as clinical, environmental, and pharmaceutical. Regarding clinical applications of ZnO-based electrochemical sensors, they have been widely applied for the determination of physiologically essential compounds such as glucose (Ahmad et al. 2017a), cholesterol (Wu et al. 2016), uric acid (Ghanbari and Hajian 2017), and lactate ion (Lei et al. 2012), besides metallic ions

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Fig. 7.20 Preparation of the ZnO/graphite microfiber DNA sensors (top). Scanning electron microscopy images of unmodified graphite microfibers (a and b) and ZnO nanowires (NWs)/ graphite microfibers (c to d). High resolution SEM image of ZnO nanowires (e). Transmission electron microscopy images of the ZnO nanowires/graphite microfibers (f). Nyquist plots recorded in the presence of the electrochemical probe [Fe(CN)6]3/4- at different concentrations of the complementary DNA target (g). Calibration plot (h). Reprinted with permission of [Bioelectrochemistry, Zhang J, Han D, Yang R, et al., Copyright 2019, Elsevier] from (Zhang et al. 2019)

such as Ca2+ (Ahmad et al. 2018), Mg2+, and K+ (Israr-Qadir et al. 2017). Thus, ZnO chemically modified electrodes have a high potential to detect abnormal levels of these critical biological species in human blood, which is necessary for the diagnosis of a large variety of diseases. Most of the ZnO-based electrochemical sensors used to detect these biologically relevant species are enzymatic biosensors, and ZnO has demonstrated to provide a completely biocompatible microenvironment in which several enzymes can be successfully immobilized without loss of their biological activity (Zhao et al. 2010). The development of reliable DNA and RNA sensors is extremely desirable regarding their broad range of important applications such as detection of genetic disorders and diseases and medical bioengineering, besides several applications in forensic sciences (Low et al. 2017; Mohammed et al. 2017). The use of ZnO-based electrochemical sensors for DNA and RNA detection is a relatively new application of ZnO, and literature brings only a few examples of these devices (Low et al. 2017; Mohammed et al. 2017; Zhang et al. 2019). Zhang et al. (2019) have used ZnO nanowires/graphite microfiber electrodes to detect DNA hybridization. The ZnO nanowires were synthesized onto graphite microfibers by a hydrothermal method, and the resulting composite was immobilized onto a fluorine-doped tin oxide electrode. The schematic representation of the sensor preparation is presented at the top of the Fig. 7.20. Scanning electron microscopy and transmission electron microscopy images of the proposed sensor showed that the

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smooth nature of the graphite microfiber is remarkably changed when the ZnO nanowire is immobilized, demonstrating the efficiency of the synthetic procedure (Fig. 7.20a–f). DNA hybridization was detected by using electrochemical impedance spectroscopy and the electrochemical sensor linearly responded in the range of 1  1014 to 1.0  107 mol L1 for determination of the target DNA fragment (Fig. 7.20g–h). Therefore, ZnO has a great potential to contribute to the DNA detection field, and this application must be intensified in the very near feature regarding the fantastic performance of the ZnO-based electrochemical sensor proposed by Zhang et al. (2019). ZnO-based electrochemical sensors have also been widely used for environmental analyses, and several organic and inorganic pollutants have been determined by electrochemical methods using working electrodes chemically modified with ZnO nanostructures. Hydrazine (N2H4) is an organic pollutant which has several applications in different industrial activities and, therefore, combines the two key factors to become a severe environmental problem, i.e., high toxicity and intense utilization. Several studies have proposed new ZnO-based electrodes for hydrazine quantification, and different approaches to electrode preparation have been adopted, including the use of ZnO nanorods (Wahab et al. 2019), polythiophene/ZnO nanocomposites (Faisal et al. 2018), Sn/ZnO nanoparticles (Rahman et al. 2016), vertically aligned ZnO nanorods (Ahmad et al. 2017b), Au/ZnO nanocomposites (Ismail et al. 2016), and ZnO/graphene nanocomposites (Ding et al. 2015). All these approaches were described as efficient and provided fast, reliable, sensitive, and selective quantification of hydrazine in environmental samples. Other organic pollutants determined with ZnO-based electrodes are catechol (Maikap et al. 2016, 2019), hydroquinone (Balram et al. 2018), bisphenol A (Akilarasan et al. 2018), nitrophenol (Singh et al. 2015; Alam et al. 2017; Thirumalraj et al. 2017), polychlorobiphenyls (Rather et al. 2014), and different pesticides, such as acephate (Pabbi and Mittal 2017), methyl parathion (Thota and Ganesh 2016), and paraoxon (Sinha et al. 2010). Thus, ZnO-based chemically electrodes have significantly contributed to the field of environmental monitoring regarding the detection of organic pollutants. Figure 7.21 shows schematic representations of electrode preparation and detection of some selected organic pollutants. Regarding inorganic pollutants, toxic transition metal ions have received growing attention due to their high toxicity and non-biodegradable nature. Thus, once toxic metal ions are introduced into the environment, they are bioaccumulated in the sediments, soils, and biosphere. Anodic stripping voltammetry is a well-established electroanalytical technique for transition metal determination, and some authors have demonstrate the suitability of ZnO-based electrodes as working electrodes for the anodic stripping quantification of some toxic transition metal ions. Yukird et al. used ZnO nanorods obtained from a thermal decomposition method to chemically modify a carbon screen-printed electrode using a drop-casting procedure. The authors applied these electrodes for the simultaneous determination of Pb2+ and Cd2+, which are very toxic metal ions widely used in the electronics and battery industry. The proposed electrode showed a satisfactory performance with limits of detection lower than 1 μg L1 for both metal ions (Yukird et al. 2018). The literature

Fig. 7.21 Prepare of ZnO-based electrodes used for the determination of different organic pollutants. F-127 – Pluronic® F-127. PTh/ZnO – polythiophene/ZnO nanocomposite. GCE glassy carbon electrode. MWCNT – multiwalled carbon nanotubes. f-MWCNT – functionalized multiwalled carbon nanotubes. r-GO – reduced graphene oxide. SPCE screen-printed carbon electrode. CHT chitosan. OHP overhead projector polyester sheets. MPTMS – (3-mercaptopropyl)trimethoxysilane. RE reference electrode. WE working electrode. CE counter electrode. Reprinted with permission of [Mater Chem Phys, Faisal M, Harraz FA, Al-Salami AE, et al., Copyright 2018, Elsevier] from (Faisal et al. 2018); [Inorg. Chem Front, Balram D, Lian KY, Sebastian N, Copyright 2018, RSC] from (Balram et al. 2018); [Ecotoxicol Environ Saf, Akilarasan M, Kogularasu S, Chen SM, et al., Copyright 2018, Elsevier] from (Akilarasan et al. 2018), [J Colloid Interface Sci, Thirumalraj B, Rajkumar C Chen S-M, Lin K-Y, Copyright 2017, Elsevier] from (Thirumalraj et al. 2017), [Sensors Actuators B Chem, Thota R, Ganesh V, Copyright 2016, Elsevier] from (Thota and Ganesh 2016)

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has also demonstrated that ZnO quantum dot-based electrodes are useful for the voltammetric quantification of mercury ions, providing limits of detection as low as 5 ppb (Bhanjana et al. 2015). Inorganic toxic anions have also been determined in environmental and food samples with ZnO-based electrochemical sensors. Nitrite, whose concentration in the environment is continuously growing due to its utilization as a corrosion inhibitor and food additive (Pradela-Filho et al. 2015) was quantified with a ZnO/Pt nanocomposite electrodeposited on a glassy carbon electrode (Zhang et al. 2015a). These authors obtained a limit of detection of 82 nmol L1 and applied the proposed electrode for the determination of nitrite in food samples such as pickled vegetable, beef, and chicken with peppers with good recovery percentage (from 96.8% to 103.8%). Cheng et al. have used ZnO micro-flowers as a template to obtain a film of poly(3,4-ethylenedioxythiophene) hollow micro-flowers, which was prepared by electrodeposition on a fluorine-doped tin oxide electrode (Cheng et al. 2014). The as-prepared electrochemical sensor was successfully used as an amperometric sensor for nitrite, and it was applied to quantify this anion in natural and tap water samples. Additional examples of toxic anions determined in environmental and food samples with ZnO-based electrochemical sensors are nitrate (Gumpu et al. 2017) and bromate (Vilian et al. 2016). ZnO-based electrochemical sensors also hold a special position in the determination of pharmaceutical compounds. Drug analysis is crucial, and it is required in several stages in the pharmaceutical industry such as stability evaluation of raw materials, formulation studies, and in the quality control of the final product (Santos et al. 2009). Several pharmaceutical compounds have been determined with ZnO-based electrochemical sensors such as the hormone estradiol (Singh et al. 2019), paracetamol (Kumar et al. 2019), dopamine (Kumar et al. 2019), folic acid (Kumar et al. 2019), morphine (Afsharmanesh et al. 2013), ascorbic acid (Ghanbari and Bonyadi 2018), carbamazepine (Dhanalakshmi et al. 2018), atorvastatin (Bukkitgar et al. 2018), captopril (Karimi-Maleh et al. 2016), pyrazinamide (Kalambate et al. 2016), and nilutamide (Temerk et al. 2015). These ZnO-based electrochemical sensors have allowed not only the drug determination in dosage forms, which is quite relevant for the quality control of pharmaceutical formulations, but also in some biological fluids which are important for pharmacodynamics and pharmacokinetics studies. Contrasting with the diversified procedures to prepare ZnO-based electrochemical sensors and the vast range of analytes determined with these sensors, the detection mode is restricted to a few electroanalytical techniques. Pulsed voltammetry, mainly differential pulse, and square wave voltammetry are often coupled with non-enzymatic electrochemical sensors for determination of drugs, organic and inorganic pollutants. On the other hand, amperometric detection is the preferred technique when electrochemical biosensors are used. This can be justified by the quite attractive features of amperometric detection, i.e., instrumental simplicity, rapid response, and high sensitivity. However, the lack of selectivity is the main drawback of amperometry. Thus, when highly selective biosensors are used, it is possible to explore all the advantageous characteristics of the amperometric

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detection without losing selectivity. For DNA and RNA detection, the impedimetric detection is widely used since hybridization process usually leads to an extraordinary increase in the electrochemical impedance and in the charge transfer resistance which can be conveniently used as a remarkably sensitive analytical signal. Therefore, ZnO-based electrochemical sensors have immensely contributed to the electroanalysis field, and this contribution is expected to grow more and more, regarding the impressive recent advances introduced in both ZnO nanoparticles production and immobilization on the electrode surface. The improved surface chemistry of ZnO has enabled the immobilization of several chemical modifiers, which have improved both the sensitivity and selectivity. Thus, ZnO is an extremally versatile material that efficiently combines superior analytical performance with low cost and low toxicity which is highly desirable for analytical/electroanalytical purposes.

7.5

Conclusion and Perspectives

ZnO has been extensively used in the academic area, and it has introduced remarkable advances in the fields of photodynamic therapy, energy conversion and storage, and electrochemical sensing. The main driven force behind this vast application is the facility in which ZnO nanoparticles are synthesized and the incomparable variety of morphologies and crystal size in which these nanoparticles are obtained. ZnO nanoparticles have significantly improved the efficiency of energy storage in supercapacitors and energy conversion in solar cells. Moreover, ZnO nanoparticles highly improve the potentialities of ZnO as an antimicrobial agent and active material in photodynamic therapy. Another fantastic property of ZnO nanoparticles is their ability to form nanocomposites with an uncountable number of materials, which increases, even more, the versatility of this metal oxide. Usually, the ZnO-based nanocomposites show synergistic effects which lead to highperformance devices. Despite the impressive developments introduced in the academic field by ZnO, practical or industrial applications of this material have not been observed yet. As pointed out by Witkowski (Witkowski 2018), a probable explanation for that could be the difficulties to scaling up the synthetic procedures used in the scientific laboratories. Therefore, new low-cost ZnO synthetic methods which could be easily implemented by the industries with low requirements for expensive instrumentation or high-purity chemicals are highly needed. The intensification of the studies concerning the biocompatibility of ZnO nanoparticles is also necessary. Although bulk ZnO is recognized as a safe material, there are doubts about the environmental impacts of the large-scale usage of ZnO nanoparticles. These are probably the most crucial challenges to be faced by the scientific community. Once they are overcome, ZnO will certainly skip from the “university walls” to the “real world,” and the society will be extremely benefited from the amazing properties and advances introduced by ZnO.

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Acknowledgments In this challenging time for the Brazilian science, the authors are deeply grateful to the scientific foundations FAPEMIG, CAPES, Araucaria Foundation, and CNPq not only for all the financial support to Brazilian’s researches but also for their commitment with the science and Brazil’s future.

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Chapter 8

Metal Oxide- and Sulfide-Based Gas Sensors: Recent Trends and Development Kingshuk Dutta

Contents 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Metal Oxide-Based Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Tungsten Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Tin Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4 Nickel Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5 Iron Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.6 Indium Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.7 Copper Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.8 Dual Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.9 Other Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Metal Sulfide-Based Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Zinc Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Tungsten Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Tin Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.4 Molybdenum Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.5 Cadmium Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.6 Other Metal Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Chemical substances in gaseous state are often extremely hazardous, especially when present in concentrations above the prescribed safe limit. Gaseous chemicals reach the open environment under various circumstances, including gas leakages, uncontrolled emission of gaseous effluents from industries, transportation vehicle exhausts, burning and combustion, etc. The situation gets even more serious in indoor environments and densely populated areas. Therefore, proper control and K. Dutta (*) Advanced Polymer Design and Development Research Laboratory (APDDRL), School for Advanced Research in Polymers (SARP), Central Institute of Petrochemicals Engineering and Technology (CIPET), Bengaluru, India © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 S. Rajendran et al. (eds.), Metal and Metal Oxides for Energy and Electronics, Environmental Chemistry for a Sustainable World 55, https://doi.org/10.1007/978-3-030-53065-5_8

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monitoring is essential to prevent gas exhaust and detect the trace of exhausted gases at the earliest possibility, in order to bring in immediate remediation and ensure safety of the environment and our health. In this scenario, gas sensors are playing a vital role. There are different categories of gas sensors, among which those based on metal oxides and sulfides have received very high attention owing to their highperformance level, cost-effectiveness, and wide applicability. This chapter will present different types of metal oxide- and sulfide-based gas sensors, and the interesting results obtained from their operations, that have been developed in the last 5 years (i.e., 2014–2019). This 5-year period has been chosen in order to present to the readers the very recent trends and developmental aspects in this particular area of research. Overall, this research area is very well-studied, as evidenced from the huge number of reports that get published every year for a period of over three decades. Therefore, this chapter does not aim to present exhaustive literature review. The main focus will be to dish out to the readers selective interesting research and developmental reports that have been published in the field of metal oxide- and sulfide-based gas sensors in the last 5-year period. Keywords Metal oxides · Metal sulfides · Gas sensing · Chemical sensors · Environmental protection

8.1

Introduction

We are all aware of the basic fact that in the gaseous state the molecules of a substance spread the fastest, owing to the negligible intermolecular attraction that exists between the molecules in the gaseous state. Now, two scenarios can arise out of this: (a) in a closed environment (e.g., a closed room) this spread is limited by the fixed volume of the room, which results in higher concentrations of the gas within a given volume; and (b) in an open environment this spread is not limited and can reach longer distances depending upon the source concentration of the gas, wind, etc.; however, owing to larger spread, the concentration gets reduced. We also know that any chemical substance is harmful above the prescribed safe concentration limit; and this limit is different for different materials. As a result, depending upon the nature of the substance, the environment (open or closed), the source concentration of the gas, etc., a gaseous substance can pose potential threat to both the environment and our health. This situation gets even more serious in cases of poisonous, flammable and/or explosive gases and in places with high population density. Therefore, the first step to prevent any alarming situation is to ensure that we do not exhaust any gases into our environment above the safety limit. For this purpose, we must know the main sources of gaseous contamination of the environment. These majorly include gas leakages from storage tanks, gas exhausts from industries and transportation vehicles, burning and combustion, etc. In cases where the first step of

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prevention is not possible or followed, we have to adopt the second step of early detection and remedy. Many gases do not have characteristic color or smell for easy detection. Even if they have, these are not easily detected at the concentration levels of their presence around safety limits. As a result, by the time we realize their presence at increased concentration, it gets too late. Therefore, for effective remediation, early detection is extremely important, and this can be ensured by the use of a chemical gas sensor. Owing to the wide variety of available gaseous substances that generally concerns us, and their varying nature (both chemical and physical), evolution of gas sensors has also been varied that ranges from gas-specific determination capability to nearversatile gas sensors (Mizsei 2016; Yang et al. 2017; Baharuddin et al. 2019; Lee 2017). Among the different types of gas sensors, those based on metal oxides and sulfides have attracted much attention owing to their high-performance level in terms of sensitivity, accuracy, selectivity, response, efficiency-to-cost-ratio, precision, etc. (Nazemi et al. 2019; Hung et al. 2017; Zhang and Gao 2019; Guidi et al. 2015). The metal oxides and sulfides used for the fabrication of these sensors are preferably nanomaterials, having very high available surface areas for interaction-based detection of gas molecules. In addition, the semiconducting nature of these materials helps in conducting electrical signal that gets generated upon interaction with the gas molecules. In this chapter, we will provide detailed information on the various metal oxides and sulfides, either in their native forms or with modifications, and the important results noted from their operations within the span of the last 5 yearperiod, i.e., 2014–2019. However, this chapter does not aim to provide an exhaustive literature review on this topic; therefore, readers interested in detailed literature review may refer to (Dey 2018; Gaiardo et al. 2016).

8.2

Metal Oxide-Based Gas Sensors

Metal oxides have emerged as a very promising and common gas sensors at present times owing to a large amount of focused research activities towards their development (Zhang et al. 2017a; Hoa et al. 2015; Mirzaei et al. 2016; Li et al. 2015; Righettoni et al. 2015). Metal oxide semiconductors sense the presence of gaseous molecules via change of its electrical conductivity/resistivity upon physical or chemical reaction with the molecules (Miller et al. 2014; Kim and Lee 2014). In essence, these molecules either behave as an acceptor or a donor of electron or holes from or to the metal oxide semiconductors, resulting in alteration of the latter’s conductivity/resistivity toward electrons or holes (Shankar and Rayappan 2015). Several transition and post-transition metal oxides, owing to their semiconducting nature, have been extensively used for the fabrication of sensors. Doping of these oxides are often performed in order to enhance the response sensitivity and signal (Luo et al. 2017; Korotcenkov and Cho 2017; Joshi et al. 2018; Chatterjee et al. 2015). In this section, we will present the developments made and efficiencies of

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some of the important transition and post-transition metal oxides, either in their native form or doped, in this very important area of application.

8.2.1

Zinc Oxide

Among the transition and post-transition metal oxides, zinc oxide (ZnO) has received the most attention from the researchers in recent years – as evident from the higher number of studies performed using this particular oxide. This high attention is owing to the higher exciton energy, larger bandgap, and higher stability (chemical and thermal) of ZnO compared to others (Kumar et al. 2015; Senthil and Anandhan 2014). In a typical example, Galstyan et al. (2015) fabricated polycrystalline nanoparticles of n-type semiconducting ZnO, having an average diameter of 25 nm, and successfully sensed three gases, namely, methane, hydrogen, and nitrogen dioxide. A schematic and a digital image of the sensing device have been presented in Fig. 8.1a. The sensing capability was found to be high as well as reversible and mostly depended upon the operating temperature (Fig. 8.1b). In another example, Han et al. (2016) demonstrated high n-butanol gas-sensing ability

Fig. 8.1 (a) A schematic (a) and a digital image (b) of the ZnO sensing device and (b) Response toward methane, hydrogen, and nitrogen dioxide at 50 ppm, 1000 ppm and 1 ppm, respectively, and working temperatures of 300
C, 400
C and 500
C with relative humidity of 40% at 20
C. (Reproduced from Galstyan et al. 2015, with permission from Elsevier)

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of high specific surface area hollow spheres of ZnO at 385
C. Hosseini et al. (2015a) was successful in fabricating a highly selective sensor for hydrogen sulfide gas that was operational at room temperature, using flower-like nanorods of ZnO. With this innovative sensing material, the responses toward hydrogen sulfide gas were determined to be 581 and 296 at concentrations of 5 ppm and 1 ppm, respectively. The authors could further modify the response using Au-sensitization of the ZnO nanorods, achieving values of 1270 and 475 at concentrations of 6 ppm and 3 ppm, respectively (Hosseini et al. 2015b). This enhancement was attributed to the formation of Schottky barriers induced by Au nanoparticle, as well as introduction of surface roughness-induced increased surface area. ZnO has been further utilized by Usha et al. (2015) for detecting hydrogen sulfide gas, via ZnO nanoparticles and thin films-coated fiber optic sensors, and has found inspiring results at room temperature analyses. Patil et al. (2017) showed that thin films of ZnO nanoparticles exhibit sensitivity to as low as 100 ppb concentration of nitrogen dioxide, with responses of ~5% for 100 ppb and ~ 13% for 200 ppb concentrations. Again, transparent and flexible films of ZnO have been successfully used for fabrication of light-controlling wearable electronic devices for detection of ethanol gas under light illumination and at room temperature (Zheng et al. 2015). In order to amplify the room temperature detection ability of ZnO and to further enhance upon its sensitivity and response time, doping with suitable materials or other modifications/functionalizations has been carried out frequently (Zhu and Zeng 2017). For example, Drobek et al. (2016) performed encapsulation of nanowires of ZnO within membranes based on metal organic framework, which performed as a sensor for hydrogen gas. The fabrication steps of nanowires and membrane-encapsulated nanowires have been schematically presented in Fig. 8.2. The response obtained with this modified sensor was found to be better than that of native nanowires of ZnO. Reduced graphene oxide (rGO), by virtue of its semiconducting nature, has also been used for modification of ZnO in order to fabricate efficient sensors for detecting nitrogen dioxide, hydrogen, and acetylene gases (Liu et al. 2014a; Abideen et al. 2015; Uddin et al. 2015); while graphene oxide (GO) has found use as a highly selective hydrogen sensor (Anand et al. 2014). A schematic of the sensing mechanism of rGO-modified nanograins of ZnO in hydrogen gas has been presented in Fig. 8.3. In another study, a combination of nanofibers of conducting polymer polyaniline and ZnO has been employed as a potential and selective sensor of ammonia gas, demonstrating a response of ~2.5 at 100 ppm gas concentration (Talwar et al. 2014). In other interesting works, metal-doped ZnO has been fabricated, including (a) Au-doped nanowires of ZnO with a response of ~34, a response time of 3 s and a recovery time of 1 s toward ethanol gas of 100 ppm concentration at 380
C (Guo et al. 2014); and (b) Al-doped ZnO nanoparticles with a response of 80%, a sensitivity of 1.6% per ppm, a response time of 7 s towards carbon monoxide gas of 50 ppm concentration at 300
C (Hjiri et al. 2014).

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Fig. 8.2 Schematic of the synthesis steps yielding the ZIF-membrane encapsulated ZnO nanowires (ZnO@ZIF-8 NWs). [IDE Interdigitated electrode; ZIF Zeolitic Imidazolate Frameworks]. (Reprinted with permission from Drobek et al. 2016. © 2016 American Chemical Society)

8.2.2

Tungsten Oxide

Tungsten oxide (WO3) has also found some important use as gas sensors. For example, two-dimensional nanoplates of WO3 have shown a highly selective sensing ability toward nitrogen dioxide gas, with a response of ~10 at a gas concentration of 5 ppm at 100
C (Shendage et al. 2017). Su and Peng (2014) have reported the use of a combination of WO3 and conducting polymer polypyrrole as a sensor for hydrogen sulfide gas operating at room temperature, demonstrating a response of 81%, a response time of 6 min and a recovery time of 210 min when employed at a gas concentration of 1 ppm. This sensor also demonstrated a performance stability of 54 days. Yang et al. (2015) fabricated nanofibers of WO3 functionalized with Au by electrospinning and showed that this composite can act as a highly effective sensor for volatile organic compounds (VOCs). The modification with Au resulted in a decrease in operating temperature from 300
C to 250
C, enhancement of the response by ~60 times along with marked reduction of the response and recovery times. On the other hand, Esfandiar et al. (2014) adopted a unique approach of using

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Fig. 8.3 Sensing mechanisms in which the surface of the ZnO nanograin is metallized. [rGO/RGO: Reduced graphene oxide]. (Reproduced from Abideen et al. 2015, with permission from the Royal Society of Chemistry)

partially reduced GO and GO incorporated with Pd-doped WO3, via a controlled hydrothermal method, for sensing hydrogen gas. The partial rGO was found to demonstrate the best performance in terms of response and recovery times (less than 60 s) at a temperature range of normal room temperature to 100
C at a gas concentration range of 20 ppm to 10,000 ppm. Schematic depictions of the synthesis of the sensing material and the sensing mechanism have been illustrated in Fig. 8.4.

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Fig. 8.4 Schematic depictions of (a) the hydrothermal synthesis of nanostructures of Pd-WO3 on GO and partial rGO, and the proposed mechanism for hydrogen sensing (in circle); and (b) the changes in the energy band diagram of the Pd/WO3 on a graphene sheet because of dissociation of hydrogen (before and after exposure to hydrogen gas). [rGO/RGO: Reduced graphene oxide]. (Reproduced from Esfandiar et al. 2014, with permission from the Hydrogen Energy Publications and Elsevier)

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315

Tin Oxide

n-type semiconductor tin oxide (SnO2), having properties of transparency and wide bandgap, has been extensively used for fabrication of gas sensors (Das and Jayaraman 2014). For instance, SnO2 nanoparticles with worm-like mesoporous structures have been used by Wang et al. (2014) to detect n-butanol with high selectivity within the concentration range of 5 ppm to 400 ppm (showing maximum responses of 435 and 3389, respectively) at a temperature of 150
C. rGO-modified SnO2 sensors have been used to detect acetone, humidity, and nitrogen dioxide. rGO modification has been invariably proved to be advantageous over that of neat SnO2. Xiao et al. (2016) could detect satisfactorily as low as 1 ppm concentration of nitrogen dioxide at 75
C, with a response of 696. The detection limit of this sensor was found to be 50 ppb. A schematic depiction of the sensing mechanism and electronic conduction at the grain and heterojunction boundaries have been presented in Fig. 8.5. Zhang et al. (2016) was able to successfully employ rGO-modified SnO2 sensor for detecting room temperature relative humidity within the broad range of 11% to 97%, demonstrating rapid response and recovery times and ultrahigh sensitivity. The same group further demonstrated the efficacy of rGO/SnO2 sensor towards detecting acetone gas at room temperature within the concentration range of 10 ppm to 2000 ppm with corresponding response of 2.19% to 9.72% (Zhang et al. 2015a). The synthesis and adsorption mechanism of this sensor has been illustrated in Fig. 8.6. Other important works in this category include nanoparticles of SnO2-modified graphene transistor for sensing hydrogen gas (Zhang et al. 2015b), films of carbon nanotube-modified SnO2 to detect hydrogen sulfide, methanol, and ethanol at room temperature (Mendoza et al. 2014) and nanoparticles of noble metal-doped SnO2 for sensing VOCs (namely, tetrahydrofuran, methanol, acetone, and ethanol (Liu et al. 2015a).

8.2.4

Nickel Oxide

p-type semiconductor nickel oxide (NiO) has been used as sensor after doping with metals, such as tungsten, aluminum, etc. Tungsten-doped hierarchical NiO hollow spheric flower-like nanostructures-based sensor has been fabricated by Wang et al. (2015a) for ultrahigh sensing of acetone with low limit of detection (in ppb). This sensor exhibited a response of ~198 at a gas concentration of 100 ppm and a temperature of 250
C. Upon doping with 4 at% of tungsten, concentration of hole carriers, specific surface area, and size of the crystallites got altered, resulting in improvement in the sensing properties of the doped NiO over that of the neat NiO by ~139 times. The same group further showed that flower-like nanorods of NiO doped with Al(III) (2.15 at%) hold promising potential to be utilized as a sensor for ethanol, operating at 200
C with a gas response of 12 for 100 ppm ethanol concentration (Wang et al. 2016).

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Fig. 8.5 (a) Sensing mechanism of the NO2 sensor based on SnO2/rGO and (b) model of a potential barrier to electronic conduction at heterojunction and grain boundaries. [rGO: Reduced graphene oxide]. (Reproduced from Xiao et al. 2016, with permission from Elsevier)

8.2.5

Iron Oxide

Among the iron oxides, α-Fe2O3 is an n-type semiconducting material, having a favorable room temperature bandgap of 2.1 eV. Huang et al. (2015) utilized this material for selectively sensing hydrogen sulfide gas and observed a sensitivity of 38.4 at room temperature at a gas concentration of 100 ppm as well as a low detection limit of 50 ppb. The proposed sensing mechanism has been presented schematically in Fig. 8.7. Spinel ferrite oxides have been extensively used for gas sensing. However, we will not cover this aspect in this chapter. The readers may

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Fig. 8.6 (a) Preparation of the SnO2-rGO nanocomposite and (b) adsorption of acetone molecules on the SnO2-rGO film. [rGO/RGO: Reduced graphene oxide]. (Reproduced from Zhang et al. 2015a, with permission from the Royal Society of Chemistry)

Fig. 8.7 A schematic representation of the proposed sensing reaction mechanism between hydrogen sulfide gas molecule and α-Fe2O3 film. (Reproduced from Huang et al. 2015, with permission from Elsevier)

consult Šutka and Gross (2016) for relevant information on this interesting class of semiconductor sensors for gases.

8.2.6

Indium Oxide

Indium oxide (In2O3) is characterized by a room temperature band gap of 3.6 eV. Gu et al. (2015) utilized an rGO-modified In2O3 nanocomposite as an n-type sensor for nitrogen dioxide. They observed that at 30 ppm gas concentration, the fabricated sensor produced a response of 8.25 at room temperature with a response time of 4 min and a recovery time of 24 min. On the other hand, Zhao et al. (2014) employed Fe-doped In2O3 for highly selective detection of nitrogen dioxide with a response of >70 at a gas concentration of 1 ppm and a temperature of 150
C.

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Copper Oxide

Oxides of copper (Cu2O and CuO) are p-type semiconducting materials with narrow bandgaps of ~2.1 eV and ~ 1.2 eV, respectively. These two oxides have started to find some use in sensing of gases. For example, Part et al. (2014) used nanocubes of CuO to detect formaldehyde gas in closed/indoor environment, with a detection limit of 6 ppb at 250
C. Hsu et al. (2016) demonstrated the applicability of composite nanowires of CuO and Cu2O in sensing of humidity and ethanol. The corresponding sensing mechanisms and associated band diagrams have been presented schematically in Fig. 8.8. Similarly, Wang et al. (2015b) used high-surface-area cages (~150 m2g1) of CuO/Cu2O, synthesized using MOF templates based on Cu, in order to sense ethanol gas at an operating temperature of 150
C. Adopting a separate

Fig. 8.8 Space charge, electric field, and band diagram of the adsorption of (a) O species, water and (b) ethanol molecules onto the surface of CuO/Cu2O composite nanowires. (Reproduced from Hsu et al. 2016, with permission from Elsevier)

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approach, Zhang et al. (2017b) fabricated composite sensor arrays based on graphene and nanoflowers of CuO for sensing of ammonia gas with high sensitivity and selectivity at room temperature.

8.2.8

Dual Metal Oxides

In order to improve the sensing and selectivity, researchers have also tested dual metal oxide sensors. Individually, ZnO and SnO2 are the two most used metal oxide for sensor application. Based on this fact, and to utilize the positive attributes of these two semiconducting n-type metal oxides, Mondal et al. (2014) fabricated a composite sensor of SnO2 and ZnO to detect hydrogen gas with high selectivity at a temperature of 150
C. This hybrid sensor also exhibited high response as well as reproducibility, with a maximum response value of 90% at a gas concentration of 10,000 ppm. Adopting a similar approach, Fu et al. (2016) devised a heteronanostructured ZnO/SnO2 sensor for hydrogen sulfide gas. This net-like two-dimensional structured sensor was found to be able to successfully detect hydrogen sulfide gas concentrations as low as 10 ppb at a temperature of 100
C. On the other hand, for the purpose of sensing hydrogen gas, Liu et al. (2014b) employed thin films of ZnO/WO3 hybrid. This Schottky diode showed a response time of 105 s and a recovery time of 25 s at an operating temperature of 50
C, compared to the corresponding values of 125 s and 35 s obtained for pristine WO3based sensor diode.

8.2.9

Other Metal Oxides

Among the other transition metal oxides used for gas sensing applications, the two most important are molybdenum trioxide (MoO3) and cobalt tetraoxide (Co3O4). MalekAlaie et al. (2015) used a composite MoO3/rGO sensor for selective sensing of hydrogen sulfide gas at 160
C at a gas concentration of 50 ppm and a response value of 4120. Overall, this sensor could efficiently perform between a temperature range of 70
C and 350
C and a gas concentration range of 50 ppm and 500 ppm. On the other hand, Feng et al. (2016) showed that composite nanofibers of Co3O4 encapsulated in rGO can behave as a promising p-type sensor for ammonia gas at room temperature, with high selectivity, at a gas concentration range of 5 ppm to 100 ppm. At a gas concentration of 50 ppm, this composite sensor demonstrated a sensitivity of ~54%, a response time of 4 s and a recovery time of 5 min at room temperature.

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Metal Sulfide-Based Gas Sensors

The next class of gas sensors under the scope of this chapter is based on transition metal and post-transition metal sulfides. However, this class of sensor materials has been comparatively new and less studied than the metal oxides. Nevertheless, metal sulfides do possess many suitable attributes required in a gas sensor (Gaiardo et al. 2016; Guidi et al. 2015); especially they are intrinsically higher electrically conductive and chemically more stable compared to the metal oxides. In this section, we will present the recent developments that have taken place in this particular area of gas sensor research.

8.3.1

Zinc Sulfide

Zinc sulfide (ZnS) possesses a wide bandgap, high thermal stability, and good electron mobility, which are suitable for application as sensors. Utilizing these properties, Hussain et al. (2017) developed a zero-dimensional nano ZnS-based formaldehyde gas sensor having high sensitivity and selectivity at room temperature. Additionally, the sensor also exhibited high performance level within a temperature range of 100
C to 400
C. In another study, Zhang et al. (2017c) fabricated a versatile sensor based on hollow spheres of ZnS decorated with gold nanoparticles. This sensor was shown to detect a number of VOCs, including butanol, ethanol, npropanol, benzene, toluene, formaldehyde, and acetone with response values of ~90%, ~98%, ~90%, ~38%, ~47%, ~76%, and ~ 84%, respectively, at 260
C operating temperature. A schematic depiction of the proposed sensing mechanism has been presented in Fig. 8.9.

Fig. 8.9 A schematic depiction of the proposed sensing mechanism of VOCs (or VOPs) by the Au/ZnS sensor. [VOC: Volatile organic carbon; VOP: Volatile organic pollutant; HS: Hollow sphere]. (Reproduced from Zhang et al. 2017c, with permission from Elsevier)

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321

Tungsten Sulfide

p-type tungsten sulfide (WS2) sensor has been shown to demonstrate ultrasensitive sensitivity towards hydrogen sulfide gas (Asres et al. 2018). This hybrid nanowirenanoflake sensor material showed a sensitivity of 0.043 per ppm. Gu et al. (2018) fabricated chemiresistive sensors based on microflakes of WS2 in order to selectively sense ammonia under light illuminated conditions and at a low temperature of 40
C. The light-enhanced mechanism of sensing has been illustrated schematically in Fig. 8.10. In another interesting work, nanoflakes of WS2 arranged in multilayers was used for the fabrication of n-type field effect transistors that functioned as a photoresponsive gas sensor for oxygen, ammonia, and ethanol under room temperature (Huo et al. 2014).

8.3.3

Tin Sulfide

Tin (IV) sulfide or tin disulfide (SnS2) has often been found to be more stable compared to SnO2 and, therefore, has been used regularly to fabricate gas sensors. In addition, this semiconductor has a bandgap in the mid-range of 2.35 eV. Apart from SnS2, tin sulfide (SnS) has also been tested as a potential gas sensor material. Giberti et al. (2016) demonstrated the use of SnS2 as a sensor for ketones and aldehydes, via selection and sensing of the carbonyl groups present in these molecules. At an operating temperature of 300
C, the nanorod-structured SnS2 was able to selectively sense acetaldehyde and acetone separately with concentration between 1 ppm and 10 ppm. However, under simultaneous presence of both the gases, the sensor showed selective sensing of acetone without interference from acetaldehyde. Afsar et al. (2017) noticed the sensing affinity of nanoflakes of SnS toward acetone and butanol. The maximum efficiency and stability were observed for acetone, with a demonstrated response of ~1000% at an operating temperature of 100
C. Again, Li et al. (2016) presented the efficacy of SnS2, having two-dimensional flake-based layered structure, towards sensing of oxygen gas at 150
C under dark conditions. It was further observed that the performance of sensor improved under reduced operating temperature when exposed to UV light. In order to incorporate the positive attributes of tin oxide, researchers have fabricated composites of SnO2 and SnS2. As an example, Gu et al. (2017) designed a nanocomposite of SnO2 and nanosheets of SnS2 for selective sensing of nitrogen dioxide at a temperature of 80
C and within a gas concentration range of 1 ppm and 8 ppm. This hybrid sensor demonstrated a response of 5.3 (when tested at a gas concentration of 8 ppm), with a response time of 159 s and a recovery time of 297 s. The sensing mechanism has been schematically presented in Fig. 8.11. Xu et al. (2015) further used this SnO2/SnS2 hybrid combination to sense ammonia gas under room temperature conditions. The strong sensing efficiency observed was attributed to the number of heterogeneous interfacial bonds formed between the two tin compounds, which resulted in lowering of the interfacial state density.

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Fig. 8.10 A possible light-enhanced mechanism of ammonia gas sensing by the WS2-based sensor under 940 nm light and at 40
C. (Reproduced from Gu et al. 2018, with permission from Elsevier)

8.3.4

Molybdenum Sulfide

Molybdenum disulfide (MoS2) has been used either in conjunction with a dopant or in the form of a hybrid for application as a sensor material. For example, nickeldoped MoS2 has been found to show potential efficiency as a sensor for sulfur

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Fig. 8.11 (a) The proposed gas sensing mechanism of SnS2- and SnO2/SnS2-based sensors, and (b) electron transfer at the SnO2/SnS2 interface. (Reproduced from Gu et al. 2017, with permission from Elsevier)

dioxide gas at room temperature. Doping of MoS2 with metallic nickel produced better results compared to the cobalt-doped, iron-doped, and undoped versions. Zhao et al. (2016) performed calculated incorporation of copper within MoS2 monolayers in order to enhance its sensing efficiencies towards ammonia, nitrogen dioxide, nitric oxide, and oxygen. Layer-by-layer fabricated films of ZnO-modified MoS2 nanocomposite has been tested as a potential sensor material for ammonia gas under room temperature conditions. This sensor demonstrated a response of 46.2% at a gas concentration of 50 ppm, with high stability, repeatability, selectivity, sensitivity, response, and recovery. Zhang et al. (2017f) designed a ternary hybrid sensing material composed of SnO2, MoS2, and Pd for sensing of hydrogen gas at room temperature. The hydrothermally synthesized sensing material showed a response of 18% at a gas concentration of 5000 ppm. A schematic of the proposed sensing mechanism and the associated changes in the band gap have been presented in Fig. 8.12.

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Fig. 8.12 (a) Hydrogen-sensing mechanism and (b) modulation of potential barrier of the Pd-SnO2/MoS2 film. (Reproduced from Zhang et al. 2017f, with permission from Elsevier)

8.3.5

Cadmium Sulfide

Known otherwise as an excellent light-detecting material, cadmium sulfide (CdS) has also found some use as a gas sensing material. As an example, in a work by Liu et al. (2017), it was realized that ultrathin nanoflakes of CdS, with high surface area, could effectively sense VOCs (like ethanol and isopropanol). Similarly, Zhu et al. (2014) reported on the gas sensing behavior of nanowires of CdS. This single-

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crystalline sensing material demonstrated high gas sensing capability toward ethanol, with response and recovery times of 0.4 s and 0.2 s, respectively, when subjected to 100 ppm of the gas. The response was found to be 14.9 at a temperature of 206
C.

8.3.6

Other Metal Sulfides

Among other important studies based on metal sulfide gas sensors is a hydrogen sulfide gas sensor material, composed of colloidal quantum dots of lead sulfide, that exhibited a response of 4218 when exposed to a gas concentration of 50 ppm and a temperature of 135
C (Liu et al. 2015b). In another study, Zhu et al. (2015) showed that nanowires of Zn1-xCdxS performed as an excellent gas sensing material for detecting ethanol, with a sensor response value of 12.8 (when x ¼ 0.4) at a temperature of 206
C and a gas concentration of 20 ppm. This unique sensor material further demonstrated a response time of 2 s and a recovery time of 1 s, with good sensing selectivity.

8.4

Conclusion

In summary, this chapter has presented the very recent developments in the field of metal oxides and sulfides-based gas sensors. The recent trends in the choice of materials for rapid and often selective sensing of a wide variety of toxic, explosive, flammable, and/or carcinogenic has been presented with examples and obtained results. It has been realized that zinc- and tin-based oxides and sulfides have been the most preferred choice of materials owing to their high sensor response values at low concentrations of gases. Also, materials developed for practical room temperature and ppb-level sensing have been specially mentioned. Other factors, like recovery time and reusability, have been particularly stressed, keeping in mind the commercial aspects of the developed materials. Future research shall concentrate more on development of dual and multi-component hybrid oxide-oxide, sulfidesulfide and oxide-sulfide combination materials for realizing the synergistic impact of the individual components. Also, increased use of high performing semiconducting materials like carbon nanomaterials, graphene, and conducting polymers will be achieved for obtaining increased level of sensor performance.

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Chapter 9

Contribution of Metallic Nanomaterials in Algal Biofuel Production Anjani Devi Chintagunta, Ashutosh Kumar, S. P. Jeevan Kumar, and Madan L. Verma

Contents 9.1 9.2 9.3 9.4 9.5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle Application in Lipid Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle Application in Microalgae Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle Application in Microalgae Lipid Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle-Immobilized Lipase for Biodiesel Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.1 Nanoparticles in Enzyme Immobilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.2 Lipase Nanoparticles in Biodiesel Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Massive utilization of the fossil fuels has caused the unwanted release of many harmful pollutants to the environment. The consequent increment of the environmental pollutions has raised many environmental issues to the escalating populations. The impressing issue can be minimized by adopting alternative renewable biofuel resources. However, the bioprocessing of biofuels production such as biodiesel, bioethanol, biobutanol, biohydrogen, and biogas needs to be efficient and improved. Nanotechnology-based metallic nanomaterials are providing tangible advancement to the process of the algal biofuel production. Metallic nanoparticles have been incorporated in various crucial steps of algal biofuel production, namely, algal harvesting, algal lipid biomass enrichment, algal cell separation, and biocatalyst A. D. Chintagunta Vignan Foundation for Science, Technology and Research, Guntur, Andhra Pradesh, India A. Kumar · S. P. Jeevan Kumar ICAR-Indian Institute of Seed Science, Mau, Uttar Pradesh, India M. L. Verma (*) Department of Biotechnology, School of Basic Sciences, Indian Institute of Information Technology Una, Una, Himachal Pradesh, India e-mail: [email protected] © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 S. Rajendran et al. (eds.), Metal and Metal Oxides for Energy and Electronics, Environmental Chemistry for a Sustainable World 55, https://doi.org/10.1007/978-3-030-53065-5_9

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carrier for performing the transesterification process. Considerable enhancement of photosynthesis, biomass, and lipid productivity in the algal is established with the incorporation of nanoparticles such as silver, magnesium, and titanium oxide. Hundred percent recovery of algal cell is achieved using magnetic metallic nanoparticle. Hundred percent of transesterification for biodiesel production is obtained using metallic nanoparticles. Thus, it may conclude with discussion that metallic nanoparticle application for biofuel production will feasible at the commercial scale. Keywords Nanoparticle · Algae · Harvesting · Lipids · Lipase · Biofuel · Biodiesel

9.1

Introduction

Nanotechnology is most rapid growing field. Its applications have been increasing over the year throughout the world in the different sectors ranging from medicine to biotechnology fields (Dhanya et al. 2020; Arora et al. 2020; Verma et al. 2017, 2019a, b, 2020; Verma 2018). Nanotechnology application is expanding tremendously in all the sectors ranging from the food to agriculture. Thus, nanotechnology market is expanding at the fast pace and expected to exceed US$125 billion of the world nanotechnology market by 2024 (Research and Market 2018). Nanomaterials are the tiny materials having one of the dimensions in the range of 1–100 nanometer (nm). Nanomaterial types and synthesis routes have been improved through the advancement in the nanobiotechnological methodology (Chamundeeswari et al. 2019; Verma 2017a, b, c). Biological and chemistry-based routes are being used to synthesize the desired size and shape of the nanomaterial. Different forms of nanomaterials such as nanoparticle, nanofiber, nanotube, and nanocomposite are being used in the biotechnology domain. Nanomaterial can be composed of organic, inorganic, or in combination of inorganic–organic hybrid material (Abraham et al. 2014; Verma 2017a, b, c, Verma et al. 2012, 2008a, b). Recently metallic-based nanomaterials are getting more attention in bioenergy production (Abraham et al. 2014). The improved algal biofuel production is primarily dependent on the incorporation of the metallic nanomaterials due to the unique properties of the nanoparticles. Different forms of the nanomaterials have contributed the specific function on the bioprocessing of biofuel (Verma 2018; Puri et al. 2013). For example, different forms of metallic nanoparticles have been employed for facilitate the harvesting, lipid biomass enrichment, algal cell separation, and biocatalyst carrier for performing the transesterification process, couple of valuable steps involved in algal biofuel production (Fig. 9.1). The area of biofuel research is one of the topmost agenda in most of the countries to mitigate the environmental issues and bioenergy needs (Verma et al. 2013a, b, c, d). Recently biofuel production such as biodiesel, bioethanol, biobutanol, biohydrogen, biogas, and bioelectricity is driven with the intervention of nanobiotechnological routes due to ease of improved

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Fig. 9.1 Metallic nanoparticle use in algal biofuel production. Metallic nanoparticle enhances the growth of algae, helps in algae separation from the media, and enhances the lipid extraction from the algae, used as nanocarrier for microalgal bioactive and enzyme to perform the transesterification process of algal biodiesel production

bioprocess (Banerjee et al. 2019; Althuri et al. 2017; Chintagunta et al. 2016, 2017; Jacob et al. 2016; Verma and Barrow 2015). Keeping the pressing need of biofuel production, the present chapter is focused on the role of different forms of metallic nanoparticles in algal biofuel production. All the processes of algal biofuel production facilitated by a variety of metallic nanomaterials are critically discussed in the present chapter.

9.2

Nanoparticle Application in Lipid Production

Biodiesel from microalgae consists of several steps such as cultivation, dewatering and harvesting of biomass, drying of biomass, lipid extraction, transesterification, and purification. Among oleaginous microbes, cultivation of microalgae is highly advantageous owing to tremendous potential to sequester the atmospheric carbon dioxide into higher lipid content, which concomitantly utilized for biodiesel synthesis (Kumar et al. 2020). Since microalgae is autotrophic in nature, it can be cultivated either in raceway ponds utilizing atmospheric carbon dioxide as carbon source or through heterotrophic/mixotrophic modes. Although microalgae cultivation is quite beneficial due to shorter incubation time, potential to alleviate food vs fuel dilemma and greenhouse gas (CO2) mitigation, meager lipid content is one of the setbacks for development of commercially viable technologies. To increase the lipid content in microalgae, several approaches have been deployed like inducing stress conditions (salt, nutrient, pH), co-cultivation of antimicrobial agents, and optimization of physical parameters (Shaheena et al. 2019; Raju et al. 2019; Kumar and Banerjee 2018; Farooq et al. 2013; Kim et al. 2013; Takagi et al. 2006; Fig. 9.2). However, altering one-variable-at-a-time has failed to optimize the process parameters, which has necessitated to look for alternative techniques (Kumar and Banerjee 2013). Recent studies illustrate that the nanoparticles have been used for cultivation purpose rather than harvesting and

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Fig. 9.2 Enhanced lipid production of algal biomass. Different forms of nanoparticles improve the algal lipid content. Higher lipid content is responsible for higher yield of biofuel production

Table 9.1 Metallic nanoparticles effect on algal. Enhanced photosynthesis, biomass, and lipid productivity in the algal are documented with the incorporation of nanoparticles such as silver, magnesium, and titanium oxide Mode Micronutrient supplement Inducer of lipid production Micronutrient supplement Oxidative stress Micronutrient supplement

Nanoparticles MgSO4

Microalgae C. vulgaris

Mgaminoclay nanoparticles Silica nanoparticles Titanium di oxide Nanoscale zero-valent iron

C. vulgaris

Usage in photo bioreactor Usage in photo bioreactor

Silver nanoparticles Silver nanoparticles

Inferences Enhanced photosynthesis with glycerol consumption Increase in intracellular lipid content

Reference Sarma et al. (2014) Lee et al. (2015b)

C. vulgaris

Increase of cell growth

C. vulgaris

Increased lipid productivity

Isochrysis galbana, Tetraselmis suecica, and Pavlova lutheri Chlamydomonas reinhardtii

Although growth rates are similar, lipid productivity was observed in P. lutheri and T. suecica

San et al. (2014) Kang et al. (2014) Kadar et al. (2012)

C. vulgaris

Significant increase of chlorophyll and carotenoid pigments

Enhanced the cell growth >30%

Torkamani et al. (2010) Eroglu et al. (2013)

lipid extraction (Banerjee et al. 2018, 2019). Nanoparticles have been employed either in direct or indirect mode that stimulate the photosynthetic cell growth and intracellular lipid accumulation. Besides, it has supplemented as micronutrient that showed enhanced biomass and lipid productivity. Studies pertinent to increase of photosynthesis and biomass content have been illustrated in Table 9.1.

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9.3

335

Nanoparticle Application in Microalgae Harvesting

In the past few decades, there has been an increased interest in microalgae investigation for the purpose of health (nutrition) and biofuel, and application of microalgae is not restricted to medicinal and cosmetic development and pharma industries, but its uses have been extended to more refined products manufacturing like pigments, omega-3 fatty acids, recombinant proteins, antioxidants, or vitamins (Kargozar and Mozafari 2018; Kollati et al. 2017; Gao and Yan 2016; Hussein 2015; Kumar and Nazeer 2012). Working with microalgae, there two prime benefits as microalgae do not require any complex medium for its cultivation and growth like other microbes (Kumar et al. 2016) and hardly have any competitors in food crop production (Singh et al. 2019; Kumar et al. 2019; Wang et al. 2016a, b). However, there is lack of efficient process for dewatering of microalgae at large scale (industrial), concentrating and separating (Kumar et al. 2017a, b). Problem of dewatering of microalgae from growth media is mainly encountered due to small size of the microalgae, their small concentration of biomass availability after cultivation, and the presence of high charge density in their cell walls (Chokshi et al. 2016) provides stability to suspensions. Hence, down steaming process of compounds extraction from microalgae in industrial reaches to a critical level and enhances the manufacturing cost of product up to 20–60% of total production costs (Rodríguez-Couto 2019; Rizwan et al. 2018; Fig. 9.3). In the present scenario, the conventional methods for harvesting/extraction of valuable products from microalgal biomass are sedimentation, flotation, and flocculation for the down steaming process, while centrifugation and filtration are two main processes for water expulsion (Amaro et al. 2012). Enormous research being conducted for microalgae production and significant results have been gained in enhancing the production potential with various approaches (Kumar et al. 2017a, b; Menetrez 2012; Mercer and Armenta 2011). Recently, material scientists have emphasized their studies on the ease and different production methodologies of valuable and important compounds derived from microalgae through uses of nanomaterials made of metals oxides or alloy. Metal and metal oxide of nanorange have more benefits potentially to improve the biological activities of different organisms (de Morais et al. 2018; Hariskos and Posten 2014). Some nanoparticles

Fig. 9.3 Role of nanoparticle in algal biofuels. Nanoparticle helps in microalgae cultivation along with higher lipid production. Nanoparticle facilitates the microalgae harvesting. Nanoparticle acts as nanocarrier for biofuel production

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can also potentially toxic to microalgae as nanoparticles cause the elevation in reactive oxygen species (ROS) generation which induces oxidative stress (Kumar et al. 2015, 2017a, b; Gujjalla et al. 2017; Manier et al. 2013), shading effect (Pagliolico et al. 2017; Schwab et al. 2011), and agglomeration (Röhder et al. 2014). However, nanoparticles which generate oxidative stress could be an option to enhance algal cultivation and secondary metabolite deposition (Agarwal et al. 2019).The importance of magnetic nanoparticles in lipid harvesting and its interaction with different biomolecules like amino acid has been revealed in several studies (Schwaminger et al. 2015; Walkey and Chan 2012; Wang et al. 2009b; Kane and Stroock 2007). Uses of high gradient magnetic separation for downstream purposes of microalgae have been documented in Table 9.2. Magnetic nanoparticle-based microalgae harvesting has economic benefits along with lowering in operating cost, coupled with ease and fast processing. In the last two decades, magnetic nanoparticle-based microalgae harvesting has gained high popularity. The principle of magnetic nanoparticles harvesting lies over the add on positively charged magnetic nanoparticles which interact with negatively charged microalgae cells and later separation of agglomerated portion from the culturing medium through an external magnetic field architected magnetic nanoflocculant (Lee et al. 2015a). Wang et al. (2016a, b) have reported the magnetic iron oxide nanoparticles harvesting efficiency with 95% at the rate of 0.6 g per gram of Chlorella cell. Another study conducted by Seo et al. (2014) has used 3-aminopropyl triethoxysilane grafted BaFe12O19 magnetic particles to harvest Chlorella sp. and found 99% efficient in 2–3 min under the influence of magnetic field. Similarly, the magnetic flocculant prepared with iron oxide coupled with 0.1 mg/mL cationic polyacrylamide consequenced in harvesting efficiency of around 95% within 10 min for using a dosage of 25 mg/l for Botryococcus braunii (25 mg/l) and Chlorella ellipsoidea (120 mg/l) (Wang et al. 2014). Another study carried out by Zhu (2019) has used flocculants made up of iron oxide (Fe3O4) and yttrium iron oxide (Y3Fe5O12) nanoparticles for the harvesting of Chlorella vulgaris biomass and found that Y3Fe5O12 nanoparticles were more efficient in microalgal biomass harvesting than Fe3O4 nanoparticles with 90% of harvesting efficiency at concentration of 10 and 2.5 g/L Fe3O4 and Y3Fe5O12, 6.2 and 7.3 pH values, respectively. Hu et al. (2014) used Fe3O4 nanoparticles for the efficient harvesting of microalgae, and nanoparticles were attached with polyethylenimine having high amount of amine groups. The functional magnetic nanocomposites were in diameter of 12 nm and 69.77 emu/g of saturation magnetization. Fe3O4 nanoparticles (20 mg/ L) were used for harvesting Chlorella ellipsoidea cells, and the adopted nanoparticle gives the harvesting efficiency of 97% within 2 min. The elevated temperature improved the harvesting efficiency of Fe3O4 nanoparticles. Finally, the nanocomposites hold an efficient material for microalgae harvesting with direct benefits as rapid execution process, minimized the energy consumption and water sustainability in the algal harvesting process. There are several magnetic nanomaterials (MxO3, where M ¼ Cu, Co, Fe, Mn, Ni, Zn, and metal ferrites like CoFe2O4, CuFe2O4, and ZnFe2O4) which can consider for

12,100 5 5 40 1200

0.20 107 cell/ml 107 cells/mL 107 cells/mL 0.1

Silica-coated magnetic particles

Carbon nanotubes α-Fe2O3 nanoparticles MgO nanoparticles Fe2O3 nanoparticles

20–30

120

200

0.2

300–1400

1–1.5

Silica-coated magnetic particles

28–98

Not defined

0.2–0.9

300–1300

0.2–1.3

Fe3O4 nanoparticles,

20–300

0.7–0.9

1.0

25–75

0.8–1.8

Poly(diallyldimethylammonium chloride)– Fe3O4, chitosan–Fe3O4, aminoclay–zero-valent Fe (ZVFe) Fe3O4–chitosan

Nanoparticles Fe3O4 nanoparticles, positive charged polyacrylamide–Fe3O4 Fe3O4 nanoparticles, positive charged polyacrylamide–Fe3O4 Positive charged polyacrylamide–Fe3O4, polyaziridine–Fe3O4 Fe3O4 particles, polyacrylamide magnetic beads, diethylaminoethyl magnetic beads Fe3O4SiO2 nanoparticles

Hundred percent recovery of algal cell is achieved using magnetic metallic nanoparticle

Scenedesmus dimorphus

Chlorella sp. KR-1 Nemania maritima Nannochloropsis salina Chlamydomonas reinhardtii Scenedesmus obliquus

Microalgae Botryococcus braunii Botryococcus braunii Corymbia ellipsoidea Chlorella vulgaris Chlorella pyrenoidosa Chlorella sp.

Magnetic particle (mg/L) 25–75

Microalgae biomass (g/L or cell/ml) 0.8–1.8

Table 9.2 Effect of various magnetic nanoparticle on microalgae harvesting efficiency

73 84 80 80

90–95

40–60

60–90

98–99

96–99

9–83

83–99

96–98

95–98

Recovery efficiency (%) 95–98

Ge et al. (2015)

He et al. (2017)

Cerff et al. (2012)

Cerff et al. (2012)

Hu et al. (2013)

Regazzoni et al. (1981), Eboibi et al. (2014), Lee et al. (2014b) and Hu et al. (2014) Lee et al. (2013c)

Amaro et al. (2011), Prochazkova et al. (2013) and Lee et al. (2014a) Vashist et al. (2019)

Reference Yadidia et al. (1977) and Hu et al. (2013) Yadidia et al. (1977) and Hu et al. (2013) Toh et al. (2014)

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microalgae harvesting and have the properties like biocompatibility, chemical stability, and potential to lower the production cost (Solano et al. 2012; Ning et al. 2011). Metal ferrites showed higher magnetization than iron oxide, but metal ferrites are easily accessible to oxidation and highly bioincompatible, and for all these reasons, iron oxide is widely used in biological applications (Borlido et al. 2013). The alloy magnetic particles composed of iron–platinum, iron–lead, and iron–cobalt are considered to have higher magnetization and chemical high stability (Hao et al. 2010). Conversely, high manufacturing cost of alloy-type magnetic particles restricts its application primarily in biomedical sector only. Various aminoclays coupled with cationic metals like Al3+, Ca2+, Mg2+, and organic materials as3-aminopropyltriethoxysilane and 3-[2-(2-amino-ethylamino) ethylamino] propyltrimethoxysilane have been examined for harvesting of oleaginous Chlorella sp. (Lee et al. 2014b). Al-APTES and Mg-APTES aminoclays exhibited 97% harvesting potential within 30 min near neutral pH in culture medium. The high efficiency is contributed due to presence of high amine groups protonated by delamination of aminoclay in cultivation medium over extended range of pH (2–10). However, lower harvesting efficiencies for Ca-3aminopropyltriethoxysilane and Mg-3-[2-(2-amino-ethylamino) ethylamino] propyltrimethoxysilane aminoclays were noticed mainly due to large particle size distribution. Besides, aminoclay quick microalgae harvesting is not popular due its high manufacturing cost (Lee et al. 2015b).

9.4

Nanoparticle Application in Microalgae Lipid Extraction

Cell wall of algae exhibits high degree of diversity related to changing in molecular components, linkages of molecular compounds within and outside of the cell wall, and finally the overall morphology of microalgae cell structure (Work et al. 2013). The most common features pertaining to the presence of biomolecules in the microalgae cell are lipid, cellulose, protein, glycoprotein, and polysaccharide, whereas the cell wall component is basically consisting of microfibrillar network of protein matrix that looks like gel (Yap et al. 2014). However, majority of microalgae cell wall are protected by silica frustules or calcium carbonate (Bolton et al. 2016). Praveenkumar et al. (2015) have reported that the chemical composition and degree of thickness of microalgal cell wall vary significantly in response to the growth conditions and medium composition. Generally, the lipid extraction step covers cell wall disruption mainly through physicochemical and biological methods followed by the lipid recovery by different solvent systems. Some microalgal species have rigid, multilayered cell walls which inhibit efficient lipid recovery by conventional organic solvents (such as chloroform, methanol, and hexane); organic solvent entry in the cell was inhibited (Praveenkumar et al. 2015). Consequently, high energy input required for the microalgae cell disruption and cost-effective

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downstream processes needed for lipid extraction can be integrated by nanotechnology coupled with convention method. Development in the area of nanotechnology gives reliability, safety, and cost-effectiveness. Contrary to lower lipid extraction through conventional solvents systems, nanomaterials offer eco-friendly approaches of lipid extraction. Further, the conventional lipid extraction method is tedious and adds additional cost to the downstream process, which can be excluded with the use of nanomaterials (Zhang et al. 2013). Recently, metallic nanoparticles have received attention as agent of microalgal cell wall disrupting. Mechanism behind the application of metal for cell disruption that it causes the elevation in the generation of reactive oxygen species (ROS) and hence physical damage takes place in the microalgae cell (Huang and Kim 2013; Castro-Bugallo et al. 2014). Nanoparticle of nickel oxide causes high level of toxicity to cells, causing to plasmolysis, breakage in cytomembrane, and thylakoid distraction by attachment of nanoparticle of nickel oxide aggregates to the algal cell wall (Gong et al. 2011). Simultaneously, Huang and Kim (2013) have demonstrated that the harvesting and disruption of Chlorella vulgaris cells using nickel oxide of dimension 300 135–155

RFE Relaxor ferroelectric, FE ferroelectric, AFE antiferroelectric, AFM antiferromagnetic, FM ferromagnetic, and Ferri ferrimagnetic (Banerjee and Tyagi 2012)

finite polarization (ferroelectrics) and/or a finite magnetization (ferro- and ferrimagnets). With respect to the definition, multiferroics correspond to the intersection of the ellipses or the circles. The smallest circle represents the systems exhibiting magnetoelectric coupling (Bea et al. 2008). The well-studied multiferroic materials are listed in Table 11.1.

Types of Multiferroic Materials With respect to the mechanism and its nature (phases and effect), multiferroic materials are classified under various subgroups such as type I and type II. The important details of both multiferroic materials are given in Table 11.2. Heterostructures and composites that show multiferroic behavior have two different origins, even though they exhibit coupled effects so that they can be treated as a separate class. In type I the different active “subsystems” of a material lead to multiferroics ferroelectricity (FE) and magnetism at various origins. In such type I multiferroics, the ferroelectric order parameter, breaking spatial inversion symmetry, and the magnetic order parameter, breaking time reversal symmetry, coexist and have a certain coupling between them.

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Table 11.2 General features of type I and type II multiferroic materials (Banerjee and Tyagi 2012) Type Type I

Type II

Mechanism Magnetism and FE exist independently

Magnetism causes ferroelectricity and vice versa

Nature Weak coupling

Strong coupling but net ferroelectric polarization is often very small

Origin Lone pair

Example BiFeO3, BiMnO3, PbVO3

Charge ordered

RFe2O4, RFeMO4 RGaCu2O4, (R ¼ Yb), Lu (M ¼ Co, Cu)

Geometric fustration

Hexagonal YMnO3, BaMF4 (M ¼ 3d transition metal ions)

Spiral or collinear magnetic ordering

Ortho-RMnO3 (R ¼ Tb, Tm, Yb, etc.), hex-RMnO3 (Y, Er, Tm, etc.) RMn2O5 (R ¼ Yb, Er, Tm, Y, Tb, Dy, Bi, etc.)

Remarks The lone pairs of Bi or Pb bonds cause dangling bonds, which cause high polarizability Coexistence of crystallographic different sites with different charges of transition metal ions causes non-equivalent bonds which results in ferroelectricity The geometrical effect of increasing the packing density can also result in permanent dipoles and ferroelectricity (in YMnO3, the off-centering displacement of Mn ions also has a role in ferroelectricity) Ferroelectricity is due to the magnetic ordering of the material. In general, the ferroelectricity is due to the spiral or collinear magnetic structures

In type II the magnetically ordered state leads to multiferroics ferroelectricity, and ferroelectricity stabilizes in the same temperature similar to certain type of magnetic ordering and is proceeded by it, for example, spiral magnetic ordering multiferroicity (Katsura et al. 2005; Mostovoy 2006).

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Properties of Multiferroic Materials

The basic study on multiferroic properties of a material is by independent polarization (P–E) and magnetic hysteresis (M–H) loop measurements. Such studies represent the existence of the above properties, but have no information on their coupled nature. However, the analyses of polarization in availability of magnetic fields provide huge information on multiferroic properties. In addition, dielectric properties are also studied as an indirect characteristic of multiferroic properties. The appearance of anomalies by changing the temperature depends on magnetic (dielectric) susceptibility at the ferroelectric (magnetic) transition temperature represents the coupling behavior. Also, by studying dielectric properties in the absence or presence of a magnetic field, the change represents the coupled magnetoelectric properties of multiferroic materials. Beyond that, the direct structural studies equipping resonant X-ray scattering or neutron diffraction provide most important information on multiferroic properties. Theoretical crossing of the density of state of the materials is also an important characterization method for multiferroic materials.

11.2.1 Ferroelectric Hysteresis Property In ferroelectric materials, a group of same directional electric dipoles form a region called as ferroelectric domain. There would be multiple domains in a material separated by interfaces called domain walls. A single domain can be derived by domain wall motion created by the application of an appropriate electric field. A very strong field could lead to the reversal of the polarization in the domain, known as domain switching (Matthias and Hippel 1948; Hippel 1950). This behavior can be explained by the P-E hysteresis loop as shown in Fig. 11.2. As the electric field strength increases in the positive direction, the domain starts to align in the positive direction giving rise to a rapid increase in the polarization (OA). The polarization reaches a saturation value (Psat) at very high field levels. When the external field is removed, the polarization does not fall to zero. At zero external fields, some of the domains remain aligned in the positive direction, and it is called as remnant polarization (Pr). Further increase of the electric field strength in negative direction (OC) brings the domains to its initial position. This field strength is called as the coercive field (Ec). If the negative field strength is maximum in the negative direction, then the direction of the polarization changes leading to a hysteresis loop. The value of the spontaneous polarization Ps (OG) is obtained by extrapolating the curve onto the polarization axis (AG).

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Fig. 11.2 Schematic diagram of P-E hysteresis loop

Fig. 11.3 Schematic representation of a magnetization hysteresis loop showing the saturation magnetization, Ms; coercive field, Hc; and remnant magnetization, Mr (Kittel 1996)

11.2.2 Ferromagnetic Hysteresis Property Ferromagnetism is a very strong magnetic response compared to diamagnetic and paramagnetic behavior. It is featured by a transition temperature (Curie temperature, Tc). When the temperature is above and below, the material is paramagnetic and ferromagnetic, respectively. It is characterized by hysteresis response in the external magnetic field, as shown in Fig. 11.3. The remnant magnetization (Mr), coercive field (Hc), and saturation magnetization (Ms) are all shown in the figure. With respect to the value of coercive field, the magnetic materials are differentiated into hard and soft magnets. To demonstrate the spontaneous alignment of the spins and the hysteresis loop developed in the ferromagnetic materials, Weiss proposed that (1) there is a strong internal magnetic field which aligns the dipoles even without an external field and

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(2) macroscopically domains are magnetized spontaneously. The magnetic moment of the entire specimen equals the vector sum of the magnetic moment of each domain. Since the direction of each domain may not be parallel, certain domain configurations lead to zero net moment. The change in the domain arrangement is obtained by the application of a relatively small field and hence an appreciable difference in net magnetization. Hysteresis loop is produced on switching the domains under external field.

11.2.3 Piezoelectric Property The piezoelectricity was discovered in 1880 by Jacques and Pierre Curie. It has been defined as the capacity to generate an electric potential by applied mechanical stress (direct piezoelectric effect). The piezoelectric effect is reversible, that is, a material exhibiting the direct piezoelectric effect also exhibits the converse piezoelectric effect, the production of stress when electric field is applied. The schematic representation of piezoelectric effect is shown in Fig. 11.4. In general, piezoelectricity can be demonstrated by the following equation:
fSg ¼ sE fTg þ ðdt Þ fEg
fDg ¼ ðdÞ fTg þ εT fEg

ð11:1Þ ð11:2Þ

where D is electric displacement, ε permittivity, E electric field strength, S strain, s compliance, T stress, and d piezoelectric constant; the superscript E indicates a zero

Fig. 11.4 Schematic representation of piezoelectric effect

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or a constant electric field; the superscript T represents a zero, or constant, stress field; and subscript t indicates a transposition of a matrix. Generally, D and E are vectors, T and S are rank-2 tensors, and piezoelectric coefficient is a rank-4 tensor (Izyumskaya et al. 2007).

11.2.4 Ferroelastic Property Ferroelasticity is a phenomenon in which a material exhibits a hysteresis in the stress–strain behavior that is characterized by coercive stress and remnant strain, while the “paraelastic” crystals exhibit a linear stress–strain behavior with no hysteresis loop. There are two ingredients that make a crystal ferroelastic: 1. A phase transition between the ferroelastic low temperature low symmetry phase and the paraelastic high temperature high symmetry phase. This transition in ferroelastic phase creates a lattice distortion. 2. An external mechanical stress can reorient the lattice distortion. This type of phase transition, which includes a change in the point-group symmetry, is called a ferroic phase transition. Ferromagnetic and ferroelectric materials are examples of ferroelastic and ferroic materials which are simply the mechanical analogues of ferroelectrics and ferromagnetics (Orlovskaya et al. 2003).

11.2.5 Dielectric Property A dielectric is an insulating material in which all the electrons are tightly bound to the nucleus of the atom. The electrons are not free to move under the influence of an external field. In principle, all dielectrics are electrical insulators. But all electrical insulators need not be dielectric. The difference between an insulator and a dielectric material depends on the application to which one is employed. The flow of current through insulating materials and it is resisted, when a potential difference is applied across its ends. On the other hand, the electrical energy is stored by dielectric materials. An applied electric field can polarize dielectric materials. On placing a dielectric in an electric field, electric charges do not flow through the material as they do in a conductor, but only a slight shift from their equilibrium position causing dielectric polarization. Dielectric materials can be solids, liquids, or gases. Dielectric materials are used in many applications, from simple electrical insulators to sensors and circuit components.

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Dielectric Constant The dielectric constant (εr) of a material is defined as the ratio of the permittivity of the medium (ε) to the permittivity of the free space (ε0): εr ¼

ε ε0

ð11:3Þ

where εr is the dielectric constant and is a dimensionless quantity. The measure of dielectric constant or relative permittivity gives the properties of a dielectric material.

Dielectric Loss On applying an AC field to a dielectric material, some quantity of electrical energy is absorbed and is dissipated as heat. This is known as dielectric loss. Dielectric loss is especially high around the relaxation or resonance frequencies of the polarization mechanisms as the polarization lags behind the applied field, causing an interaction between the field and the dielectric’s polarization that results in heating. In an ideal dielectric, the voltage is led by the current at an angle of 90 as shown in Fig. 11.5 (a, b, and c). But in a commercial dielectric, the current does not lead exactly to the voltage at 90 . It leads by an angle less than 90 . The angle φ ¼ 90 – θ is called as the dielectric loss angle. For a dielectric, having voltage V and capacitance C applied to it at a frequency (Hz), the dielectric power loss is: P ¼ VIcosθ

ð11:4Þ

since I ¼ XVc where Xc is the capacitive reactance and is equal to 1/jωC.

a

b

I

I

c

ωε0εr’E J

φ

θ

90° V

ωε0εr’’E0 V

Fig. 11.5 Relationship between current and voltage in dielectrics

E0

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Multiferroic Properties of Rare Earth-Doped BiFeO3 and Their Spintronic. . .

P¼

V2 cos ð90  φÞ Xc

P ¼ V 2 jωC sin φ

385

ð11:5Þ ð11:6Þ

Since θ is very small, sinφ ¼ tan φ: P ¼ jV 2 ωC tan φ

ð11:7Þ

where tanφ is the power factor of the dielectric. The power loss depends only on the power factor of the dielectric as long as the applied capacitance, frequency, and voltage are kept constant.

11.2.6 Magnetoelectric Coupling The definition of the magnetoelectric effect has demonstrated the coupling between the ferroelectric and magnetic ordering. Schmid (1994) and Rivera (1994) explained Landau theory by the free energy F of the system in terms of an applied magnetic field H whose ith component is denoted Hi, and applied electric field E whose ith component is denoted Ei within a material encodes the resultant field that a test particle would experience in single-crystal materials. It is represented as an infinite, homogeneous, and stress-free medium by writing F under the Einstein summation convention in SI units as: 1 1 F ðE, HÞ ¼  F 0 þ ε0 εij E i E j þ μ0 μij H i H j þ αij E i H j 2 2 1 1 þ βijk E i H j H k þ γ ijk H i E j H k 2 2

ð11:8Þ

which can be explained by the first term on the RHS that describes the contribution resulting from the electrical response to an electric field, where ε0 denotes the permittivity of free space and the relative permittivity εij is a second-rank tensor that is typically independent of Ei in non-ferroic materials. The second term is the magnetic equivalent of the first term, where μij is the relative permeability and μo is the permeability of free space. The last term describes linear magnetoelectric coupling via αij; the third-rank tensors βijk and γ ijk represent higher-order (quadratic) ME coefficients. All ME coefficients include the field independent material response functions εij and μij. The ME effects can be recognized in the form of Pi(Hj) or Mi(Ej). The former is obtained by differentiating F with respect to Ei and then setting Ei ¼ 0. A complementary operation involving Hi establishes the latter. One obtains:

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1 Pi ¼ αij H j þ βijk H j H k þ . . . 2

ð11:9Þ

1 μ0 M i ¼ αij E j þ γ ijk E j E k þ . . . 2

ð1:10Þ

and

In ferroic materials, the above analysis is less rigorous because εij and μij display field hysteresis. Moreover, ferroics are better parameterized in terms of resultant rather than applied fields (Lines and Glass 1977). In practice, resultant electric and magnetic fields may sometimes be approximated (Lottermoser et al. 2004) by the polarization and magnetization, respectively. Here, αij is magnetoelectric effect corresponding to the induction of polarization by applied magnetic field or electric field and vice versa. However, several materials exhibited the magnetoelectric effect like BiMnO3, Cr2O3, YMnO3, TbMnO3, Cr2BeO4, Sr(Fe2/3W1/3)O3, CdCr2S4, and BaNiF4. Multiferroic materials usually show that the values of magnetoelectric effect are very small to be practically applicable as term αij is limited by the relation: α2 ij  εii μjj :

11.3

ð11:11Þ

Perovskite Structure

Perovskite oxides, which have a chemical formula ABO3, are materials that have been extensively researched and investigated due to their wide-ranging structures and physical properties as a function of composition and synthesis parameters. Electrical conductivity of the perovskite oxides, for instance, ranges from insulator, semiconductor, conductor, to even superconductor depending on their chemical composition and temperature. Perovskite oxides are of great interest in many applications due to their multifunctional (semiconducting, electrochromic, magnetoresistive, dielectric, multiferroic, etc.) properties (Lantto et al. 2004). Until recently, perovskite oxides in electronic devices were mostly used in non-volatile ferroelectric random access memories (NVFRAM), tunable microwave devices, and sample stages in various characterization instruments that require precise and delicate movement (Haertling 1999). A simultaneous combination of ferromagnetism and ferroelectricity, the so called multiferroism, is provided by perovskite oxides. Multiferroics, one of the most recently found classes of materials, exhibit unique properties, mechanisms, and atomic scale interactions, some of which have not been fully established (Ramesh and Spaldin 2007). In principle, the coexistence of both ferroelectricity and magnetism in a single phase could be achieved through an alternative mechanism for

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magnetism or an alternative mechanism for ferroelectricity. Overall, the exact mechanism is yet to be established. Consequently, the interest in identifying the mechanism on how multiferroics work in comparison with the observed phenomena in either ferroelectrics or ferromagnets alone has motivated a lot of research in this field. With regard to applications in nanotechnology, integration of perovskite oxides is expected to provide remarkable impacts in scaling down the sizes of electronic devices, thus enhancing capability and performance. The scaling down of perovskite oxides, however, has demonstrated numerous challenges, due to the presence of strain, defects, and compatibility issues with adjacent layers of different materials, particularly at interface regions.

11.3.1 An Overview of Bismuth Ferrite (BiFeO3) Recently, the multiferroic bismuth ferrite (BFO) has been considered as a significant material into developing the new type of multifunctional applications; it has exhibited the ferroelectric, piezoelectric, magnetism, and optical properties. The multiferroic ordering temperature of the BFO, high Currie temperature of ferroelectricity (Tc ~ 825  C), (Venevtsev et al. 1960), high Néel temperature of antiferromagnetism (TN ~ 360–400  C), (Kiselev et al. 1963). It is applicable in the field of lead-free piezoelectricity, and large flexibility in the wavelength of visible light region, large spontaneous polarization, magnetic properties and magnetoelectric effect. In addition, the cross correlation of these properties can be expected above room temperature (RT) is as shown in Fig. 11.6. The ferroelectric performance of BFO is comparable to that of well-known ferroelectric material such as PbZrTiO3 (PZT) because BFO exhibits excellent spontaneous polarization at RT. Theoretically, spontaneous polarization corresponds to crystal symmetry, wherein the rhombohedral and tetragonal BFO structures are expected to show spontaneous polarizations. The large spontaneous polarization values has been observed by theoretically in BFO and at same time, the experimentally predicted the spontaneous polarization values also almost consist with theoretically. These results are most favorable for developing new ferroelectric devices like ferroelectric Fig. 11.6 The manifestation of multiferroic nature by BiFeO3 (BFO)

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random-access memory (FeRAM) and voltage-controlled random-access memory (V-RAM). However, it has some drawbacks for practical application of BFO by high coercive field (Quan et al. 2008) and large leakage current density. When a large leakage current is passed through, the BFO easily undergo electrical breakdown which is done before the change in polarization. Therefore, the researchers are still finding the proper solution for BFO thin films to avoid the electrical breakdown for using practical applications, and also the leakage current and coercive field or both of these films must be reduced. The same situation exists for bulk BFO also. In terms of magnetic properties, BFO is antiferromagnetic with a G-type spin configuration (Tang et al. 2010); that is, nearest neighbor Fe moments are aligned nonparallel to each other, and a sixfold degeneracy, resulting in an efficient “easy magnetization plane” for the direction of the magnetic moments. In mechanism, the electric field-controlled magnetization gives useful information of the study about ferroelectric domain switching and ferroelectric domain wall motions in the direction of applied electric filed. It has applications related to V-MRAM. In further, V-MRAM can strongly reduce electrical consumption in comparing to spin-MRAM, which is operated by spin-polarized current.

11.3.2 Structure of BFO Bismuth ferrite (BFO) compound belongs to ME materials that exhibit a coexistence of mutually coupled magnetic and ferroelectric ordering. Researchers can focus toward ME materials due to their prospective applications in future electronic, multi-state memory devices and sensors. BFO has a perovskite-type crystal structure that is crystallized in the space group R3c and rhombohedral distorted in the [111] direction. This is equivalent to the hexagonal setting often used by crystallographers, which has hexagonal unit cell with lattice constants of ahex ¼ 5.579 Å and chex ¼ 13.869 Å as shown in Fig. 11.7 (Kubel and Schmid 1990). It is exhibited excellent ferroelectric spontaneous polarization materials, when comparable with Pr(Zr,Ti)O3 (PZT) at room temperature. Fig. 11.7 Schematic drawing of the crystal structure of perovskite BiFeO3 (space group: R3c). Two crystals along [111] direction are shown in the figure

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However, it has been reported that the tetragonal and rhombohedral BFO structures are likely to enhance spontaneous polarization values of ~100 μC/cm2 in the [111] direction and ~ 150 μC/cm2 in the [001] direction, respectively. Multiferroic materials have coupling between magnetic and ferroelectric ordering parameters and are single-phase materials, and they are useful for electric field-controlled magnetism. Electric fields can be generated such as local fields, voltages, and disparate magnetic fields, which are nonlocal and can adversely influence neighboring bits (Hill 2000; Rondinelli et al. 2009; Smolenskii and Chupis 1982; Curie 1894; Kimura et al. 2003). The creation of ferroelectricity due to empty‘d’ shells and magnetism generated from partially filled ‘d’ shells, which is main rule in mautiferrouic oxides. However, the ordering temperature has been found to be Néel temperature around TN  625–643 K and ferroelectric ordering temperature to be Curie temperature around T C  1083–1103 K for bulk BFO (Muneeswaran et al. 2014; Wang et al. 2003), and also BFO exhibits a magnetoelectric coupling at room temperature. Seidel et al. (2009) reported and explained the electronic conductivity of ferroelectric domain in BFO thin films at room temperature. These results are potential to perform a developing new multifunctional applications in future.

11.3.3 Applications of Spintronics Bismuth Ferrite (BFO) for Spintronic Devices

Table 11.3 Applications of rare earth doped bismuth ferrite (BFO) for spintronic devices S. no. 1

Applications Electric field control of magnetic anisotropy

2

Photovoltaic device

3

Optoelectronics

4

Non-volatile ferroelectric random-access memory ( NVFRAM)

References Kleemann (2007), Mishra et al. (2008), Suresh and Srinath (2014), Sati et al. (2014), Jeon et al. (2011), Lebeugle et al. (2009) Ji et al. (2010), Biswas et al. (2019), Peng et al. (2017), Arti et al. (2019), Biswas et al. (2019), Kan et al. (2011)), Lotey and Verma (2013), Muneeswaran and Giridharan (2014), Muneeswaran et al. (2014, 2015a, b, 2018) Katherine Hayles (Intellect Books 2004), Yuan et al. (2017), Ghosh et al. (2019), Zhu (2005) Simoe et al. (2009), Scott (2000), Setter et al. (2006), Lee and Wu (2007), Catalan and Scott (2009) FRAM Guide Book (2005), Takashima et al. (2011), Fujitsu (2005), Vendik (2009), Maruyama et al. (2007)

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Electric Field Control of Magnetization In multiferroic materials have exhibited coupling between magnetic and ferroelectric order parameters. Recently electric fields have been demonstrated to direct magnetic properties and in controlling magnetic spin with electric fields. The exchange coupling between transition metal ferromagnets to the BFO and single-domain in bulk and multi-domain in thin films of the BFO materials is not observed any differences in the magnetic state of the canted moment. The velocity of magnetic domain wall motion is given to various rare earth-doped BFO materials (magnetic anisotropy, saturation magnetization, damping constant, etc.), which gives rise to developing new electronic device applications (Kleemann 2007; Mishra et al. 2008, Suresh and Srinath 2014; Sati et al. 2014; Jeon et al. 2011). It has various demonstrations of electric field control over the trapping strength and the changes in the magnetic anisotropy by voltage induced in the magnetic domain wall motion. Lebeugle et al. (2009) described the exchange coupling of transition metal ferromagnets with BFO crystals, as well as the electric field control of these exchangecoupled spin. However, the use of ferroelectric thin films and their applications such as dielectric gates to manipulate the magnetic anisotropy, band shifting or charge modulation, Utilize strain coupling to electric field-induced ionic diffusion, piezoelectricity. Voltage-controlled domain wall traps and gates based on these concepts provide new prospects for memory technologies and magnetic logic.

Photovoltaic Devices Last decades, the ferroelectric PV effect has been studied in the few system, and also explained the theoretical and experimental point of view both bulk and thin film materials. However, the net polarization and depolarization field plays a vital role, for the ferroelectric PV effect. Despite such an anomalous PV effect, the ineffective performance of ferroelectric PV in device applications is related with the large bandgap. Recently, the new PV effect developed from the BFO-based multiferroic oxide materials and to tuning of the bandgap energy by doping rare earth elements substitution into Bi site. PV effect is dependent on the polarization of the BFO, and it has been reported in epitaxial BFO thin films but with very low VOC (0.3 V) (Ji et al. 2010). Recently Biswas et al. (2019) reported PV response in the rare earth-doped BFO samples of open-circuit voltage of 1.30 V for the x ¼ 0.25. For example, the morphotropic phase boundary composition can be gained in Bi1-xRExFeO3 through A-site substitution of trivalent rare earth metals (RE ¼ Dy3+, La3+, Gd3+ and Nd3+) and improved electromechanical properties. Works are done to engineer the bandgap of known ferroelectric oxides with the substitution of chemical (Peng et al. 2017; Biswas et al. 2019; Kan et al. 2011; Lotey and Verma 2013; Muneeswaran et al. 2014, 2015a, b, 2018).

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Optoelectronic Devices Spintronic applications can be divided into two parts: (i) semiconductor spintronics and (ii) metal spintronics. Rather than the rapid advances in metal-based spintronic devices, researchers focus on finding some new ways to utilize and generate spin-polarized currents in semiconductors (Katherine Hayles, Intellect Books 2004). Today, the semiconductor spintronics is the most interesting application due to the bandgap energy in the materials. Semiconductor spintronics is frequently concerned with materials that become magnetic when doped with magnetic 3D transition metal elements (Zhu 2005). Hence, they are called as dilute magnetic semiconductors. In particular, BFO has antiferromagnetic behavior and also weak magnetism, so researchers are focused to the enhancement of magnetism by doped different concentration ratio of rare earth elements in Bi site (Yuan et al. 2017; Ghosh et al. 2019). This is important as semiconductor-based spintronic devices can be easily integrated with traditional semiconductor technology; they can also serve as multifunctional devices such as spin-FETs (field effect transistors), spin-LEDs (light-emitting diodes), spin-RTDs (resonant tunneling devices), optical switches operating at terahertz frequencies, modulators, quantum computation, etc.

Non-volatile Ferroelectric Random-Access Memory (NVFRAM) BFO is a ferroelectric compound with its lead-free composition, and having uniquely high temperatures of magnetic and electric ordering makes it a very prospective material for the applications in dynamic random-access memory, sensors, microactuators (Simoe et al. 2009), portable electric devices, and smart cards, utilizing its low electric consumption and non-volatility (Lee and Wu 2007). Because of its magnetoelectric coupling, data can be read magnetically and written electrically. Magneto electric coupling exploits the best ways of developing new type of ferroelectric random-access memory (Catalan and Scott 2009). In a ferroelectric random-access memory (FeRAM) cell, it has reduced the heat losses sharply (Scott 2000; Setter et al. 2006). In particular, whereas the current needed for semiconductor dynamic random-access memory (DRAM) on silicon reaches 1000 μA and should be maintained constantly, that for FeRAM is just 5 μA (FRAM Guide Book 2005; Takashima et al. 2011; Fujitsu 2005), with non-volatile memory. Ferroelectric reader technologies applications are phased arrays and microwave high-electron mobility transistors (HEMTs) (Vendik 2009; Maruyama et al. 2007).

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Conclusion and Perspectives

In recent years, BFO finds applications as photocatalytic material due to its small bandgap. This small bandgap also allows carrier excitation in BFO with commercially available femtosecond laser pulses, hence enabling us to develop ferroelectric ultrafast optoelectronic devices as widely demonstrated in semiconductor (Gao et al. 2007). Solar energy being the most promising alternative resource, attention has been turned toward dye-sensitized solar cells (DSSCs) in both industrial and academic platforms as an alternative sustainable energy resource because of their comparatively high energy conversion efficiency, cost efficiency, ease of fabrication and flexibility, and low manufacturing toxicity (Hagfeldt et al. 2010; Hardin et al. 2012; Nielsen et al. 2010). Recently, there has been a growing research interest in multiferroic materials due to their vast usage in a variety of multifunctional devices (Wang et al. 2003). There are a number of materials that possess multiple ferroic properties, but the reports of coupling between these properties are significantly more limited. The potential dearth of interesting multiferroic structures that may be grown under equilibrium conditions underlines the significance of a scientific search for a prototype multiferroic material. Bismuth ferrite (BFO) is identified as one of the rare compounds that show the multiferroic behavior even at room temperature, thus attracting extensive attention. BFO prepared several synthesis methods, and it has advantage of low temperature processing, and the co-precipitation method was extensively adopted to prepare ferroelectric or magnetic materials (Muneeswaran et al. 2014). The task becomes intriguing and hard due to the narrow temperature range that makes BFO stable, and there are many other phases of Fe and Bi, which appear in accurate temperature. At the time of synthesis, the kinetics of formation always proceeds to a mixture of BFO as an important phase combined with impurity phases (Munoz et al. 1985; Teague et al. 1970). It therefore becomes a challenge to prepare phase to make a systematic effort on A-site (Bi3+) substitution of the BFO by a series of lanthanide ions in preferred concentrations whose ionic radii are smaller than that of the Bi3+ ion such as dysprosium (Dy) with ionic radii of 0.912 Å, praseodymium (Pr) with ionic radii 0.99 Å, and terbium (Tb) with ionic radii 0.923 Å and analyze the effect of its modification to yield desirable physical properties and functions. Acknowledgments The authors acknowledge FONDECYT Postdoctoral Research Project No. 3180055, Government of Chile, for the financial support.

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Index

A Afsar, M.F., 321 Ahn, J.-H., 62–84 Ajayan, P.M., 74 Akbari-Fakhrabadi, A., 376 Algae, 333, 338–340 Ali, K., 268 Al-Kuhaili, M.F., 235 Aluminum oxide (Al2O3), 13, 96, 134, 203, 204, 210, 212, 213, 233, 239, 357–359, 363, 370 Alver, U., 274 Alzoubi, K., 139 Amaro, H.M., 337 Aqueous electrolytes, 4, 6, 7, 21–23, 38–40, 46, 72, 104, 105, 107, 108 Araújo, D.A.G., 252–289 Ariyoshi, K., 15 Arti, S.K., 389

B Babar, A.R., 171 Badeker, K., 152 Bandgaps, 67, 69, 152, 157, 159–163, 170, 174, 176, 187, 189, 200, 206, 214, 219, 222–224, 227, 231, 253, 255, 256, 390 Barbosa, H.P., 252–289 Behtash, M., 170 Berthelot, R., 30 Bevacqua, S.F., 188

Binary metal oxides, 19, 177, 204 Biodiesel, 332, 333, 339–344 Biofuels, 332–335, 341, 343 Bismuth ferrite (BiFeO3), 387–389, 392 Bisset, M.A., 72 Biswas, P., 216 Biswas, P.P., 389, 390 Braconnier, J.J., 24

C Caballero, A., 32 Capacitive sensors, 370 Carcia, P.F., 202 Catalan, G., 389 Ceder, G., 28, 42 Cerff, M., 337 Chen, C.Y., 280 Chen, J.H., 65 Chen, T., 266 Chen, Y.Z., 342 Cheng, Y., 288 Chiang, H.Q., 205 Chintagunta, A.D., 332–343 Chronopoulou, L., 342 Chung, W., 131 Cindrella, L., 270 Clerici, F., 72 Colegrove, J., 154 Cook, H.B., 153 Corpuz, R.D., 92–113

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 S. Rajendran et al. (eds.), Metal and Metal Oxides for Energy and Electronics, Environmental Chemistry for a Sustainable World 55, https://doi.org/10.1007/978-3-030-53065-5

397

398 D Dahn, J.R., 8, 21, 32 Dang, H.P., 171 Das, A., 62–84 Das, S., 167 David, S.P., 2–49, 186–244 De Juan-Corpuz, L.M., 92–113 de Lima, R.G., 252–289 Delmas, C., 25, 28 Deng, L., 42 de Sousa Góes, M., 252–289 Ding, X., 131 dos Santos, A.L., 252–289 Drobek, M., 311 Dumri, K., 342 Dutta, K., 308–325

E Eboibi, B.E., 337 Elangovan, E., 171 El-Batal, A.L., 342, 344 Electrochemical energy storage (EES), 93–95, 100, 112 Electrochemical sensing, 284, 289 Energy storage, 2–5, 19, 21, 38–40, 46, 49, 62, 65, 66, 68, 69, 71, 73, 74, 81, 82, 84, 92–113, 269, 272, 273, 281, 289 Environmental protection, 231 Eroglu, E., 334 Esfandiar, A., 312 Esro, M., 171

F Fakhar-E-Alam, M., 264 Fan, L.Q., 71 Feng, Q., 77, 319 Ferrari, J.L., 252–289 Feynman, R., 100 Flexible displays, 122, 125, 129, 159, 188, 192, 199, 209–214 Franger, S., 33 Fraser, D.B., 153 Fu, D., 319 Fujitsu, D., 389

G Galstyan, V., 310 Gandhi, T.I., 171 Gao, Y., 280 Gas sensing, 227, 241, 316, 319, 322–325

Index Ge, J., 340 Ge, S., 337 Ghosh, A., 389 Giberti, A., 321 Giridharan, N.V., 376 Godshall, N.A., 4, 6 Goodenough, J.B., 4, 6, 17 Gopiraman, M., 376 Gordon, R.G., 153 Grätzel, M., 256 Gross, K.A., 317 Gu, D., 321 Gu, F., 317 Gupta, M.N., 340 Gürakar, S., 171

H Haacke, G., 219 Hagenmuller, P., 25 Han, B., 310 Harvesting, 151, 155, 256, 258, 271, 332, 333, 335–339, 343 He, M., 337 Heiras-Trevizo, A., 171 Holland, L., 153 Holonyak, N., 188 Hoon, J.W., 227 Hosono, H., 206 Hosseini, Z.S., 311 Hsu, C.-L., 318 Hsu, C.M., 208 Hsu, H.H., 207 Hu, Y.R., 336, 337 Huang, H., 70 Huang, M., 280 Huang, S.H., 342 Huang, W.C., 339 Huang, Y., 316 Hussain, S., 320 Hwang, J.Y., 27

I Islam, T., 356–372

J Jabeen Fatima, M.J., 62–84 Jayaraman, V., 167 Jeon, N., 389 Jeong, S., 8 Ji, W., 389

Index Jiang, Y., 36 Joseph, D.P., 151–177

K Kadar, E., 334 Kaempgen, M., 65 Kalantari, M., 343, 344 Kaliyappan, K., 32 Kan, D.C.-J., 389 Kaneko, H., 171 Kang, F., 46 Kang, N.K., 334 Kang, Y., 209 Kasar, R.R., 171 Katherine Hayles, N., 389 Kawazoe, H., 214 Khan, A.U., 356–372 Khan, M.E., 356–372 Kheawhom, S., 92–113 Kikuchi, N., 171 Kim, D., 42 Kim, H., 42 Kim, J., 2–49 Kim, J.D., 339 Kim, J.H., 11 Kim, J.W., 216, 226 Kim, M.G., 199 Kim, M.I., 342 Klemann, L.P., 24 Koo, B.R., 171 Korvin, J., 356 Kovendhan, M., 151–177 Krishnamoorthy, K., 79 Krishnan, M.A., 62–84 Kumar, A., 332–343 Kumar, L., 356–372 Kumar, S.P.J., 332–343 Kuo, Y., 344 Kupfer, B., 224

L Lamberti, A., 82 Lebeugle, D., 389 Lebeugle, D.A., 390 Lee, B., 337 Lee, C.C., 389 Lee, K., 334, 339 Lee, K.S., 274 Lee, S.J., 137 Li, H., 10 Li, K., 344

399 Li, W., 21 Li, Y., 321 Liang, L., 28 Lim, H., 131 Lin, M., 15 Lin, X., 169 Lipases, 340–344 Lipids, 262, 266, 267, 332–334, 336, 338–341, 343 Lithium ion batteries (LIB), 3, 5–10, 13–25, 27, 35, 36, 41, 48, 49 Liu, C., 45 Liu, G.J., 70 Liu, X.-H., 324 Liu, Y., 319 Liu, Y.L., 221 Liu, Z., 17 Lokanc, M., 154 Lu, H., 74 Lu, Z., 32

M Ma, N., 344 Maggay, I.V.B., 36 Mai, L., 42 MalekAlaie, M., 319 Manganese oxide (MnO²), 42, 46, 280 Maruyama, K., 389 Masese, T., 45 Matsui, M., 31 Matuoog, N., 342 McMaster, H., 152 Medhavi, N., 62–84 Mehrasbi, M.R., 344 Meng, X., 342 Metal oxides, 2–49, 63, 65, 77, 153, 157, 159–161, 165, 166, 169, 174, 177, 187, 188, 190–192, 196, 197, 199, 200, 204–206, 209–211, 213, 214, 216, 219, 220, 222, 224–227, 229, 231–233, 235, 236, 239–244, 259, 274, 277, 279, 282–284, 289, 309, 310, 319, 320, 325, 335, 356–372 Metal sulfides, 67, 320, 325 Miletia, N., 342 Mishra, R., 389 Mizushima, K., 17 Mohammad, S.M., 229 Mondal, B., 319 Multiferroics, 375–392 Muneeswaran, M., 376, 389

400 N Nanomaterials, 218, 255, 259, 268, 282, 309, 325, 332, 333, 335, 336, 339, 340, 343 Nanoparticles, 78, 218, 229, 230, 252–254, 257–268, 271, 277, 279, 282, 283, 286, 289, 310, 311, 315, 320, 332–344 Neethu, T.M., 62–84 Newman, G.H., 24 Nomura, K., 206 Non aqueous and aqueous electrolytes, 22 Non-aqueous electrolyte, 6, 40, 113

O Ogo, Y., 199 Oh, S.M., 25 Ohta, H., 215, 220 Ohzuku, T., 8 Omura, K., 171 O’Regan, B., 256

P Pandey, R., 171 Park, H.J., 318 Park, J.S., 211, 229 Patel, M., 231 Patil, K., 72 Patil, V.L., 311 Paul, A., 62–84 Peng, Y.T., 389 Peng, Y.-T., 312 Peng-Fei, L., 161 Petti, L., 214 Photodynamic therapy, 260–262, 264, 289 Poizot, P., 17 Pradela-Filho, L.A., 252–289 Prasanth, R., 62–84 Praveenkumar, P., 338 Prins, M.W.J., 198 Prochazkova, G., 337

Q Qian, J., 9

R Rahman, M.M., 271 Raita, M., 344 Rajan, A.K., 270 Ramachandran, R., 65 Ramarajan, R., 151–177

Index Ramzan, M., 235 Rare earth elements, 391 Ravichandran, K., 171 Ray, S.C., 171 Razack, S.K., 339 Rechargeable zinc-ion battery, v, 45, 46 Regazzoni, A.E., 337 Reimers, J.N., 8 Remashan, K., 202, 203 Retamal, J.R.D., 256 Ritchie, H., 94 Rivera, J.P., 385 Roser, M., 94

S Sakai, S., 340 Sakthivel, T., 2–49, 186–244 Sambandam, B., 2–49, 186–244 San, N.O., 334 Sarma, S.J., 334 Schmid, H., 385 Schmitt, C., 65 Scott, F., 389 Scott, J.C., 137 Scott, J.F., 389 Seidel, J., 389 Seo, J.Y., 336 Setter, N., 389 Shah, S., 340 Shan, X., 39 Shanthi, E., 169 Shanthi, S., 171 Sheet resistance, 133, 138, 152, 153, 155, 164, 171, 174, 175, 219, 220, 225 Shin, D.H., 100 Shin, P.K., 239 Sibin, K.P., 132 Simoe, A.Z., 389 Singh, G., 30 Sivaramalingam, A., 2–49, 186–244 Sol gel, 202, 220, 226, 236, 241, 242, 252, 258, 280, 357, 363, 366, 368, 370–372 Solar cells, 122, 154, 159, 187–190, 202, 220–227, 235, 252, 254–259, 269, 271, 289, 392 Soosaimanickam, A., 2–49, 186–244 Srinath, S., 389 Stjerna, B., 171 Su, P.-G., 312 Suchea, M., 233 Sun, W., 107 Sun, X., 79, 81

Index Sundar, D.S., 376 Sunde, T.O.L., 220 Supercapacitors, 4, 6, 62–72, 74, 75, 77, 78, 81–84, 94, 113, 254, 269, 272–281, 289 Suresh, P., 389 Surface work function, 165, 172, 176

T Taabouche, A., 239 Takashima, D., 389 Takeuchi, R.M., 252–289 Talaie, E., 32 Tang, W., 21, 23 Tarscon, J.M., 17 Thackeray, M.M., 14 Thangaraju, K., 151–177 Thin films, 70, 99, 122, 124, 130, 132, 137–139, 152, 153, 160, 170–176, 187–193, 198–203, 205–209, 211, 212, 214, 215, 220–229, 231–244, 257, 311, 319, 356–372, 377, 388–390 Thin film transistors (TFTs), 123–125, 190–192, 194, 196–200, 202–214, 243 Tian, Y., 71 Tin oxide (SnO2), 69, 122, 131–139, 152–155, 166, 198, 205, 215, 220, 221, 224, 225, 229, 231, 238, 241, 279, 280, 284, 285, 288, 315, 321 Toh, P.Y., 337 Tong, W., 11 Torkamani, S., 334 Tran, D.T., 344 Transmittance, 69, 128, 132, 133, 137, 140, 152, 153, 155, 161, 162, 164, 171, 173, 174, 220, 221, 235, 257 Transparent conducting oxides (TCO), 122, 123, 131–133, 153, 155, 157–166, 169–171, 176, 177, 190, 219, 220, 222–226, 243 Tsutomu, O., 11 Turgut, G., 170, 171

U Udea, A., 8 Ueno, S., 271 Urr, L., 3 Usha, S.P., 311

401 V Vaalma, C., 42 Vashist, V., 337 Veluchamy, P., 171 Vendik, O.G., 389 Verma, M.L., 332–343 Volta, A., 3

W Wahab, R., 284 Wainwright, D.S., 21 Wang, C., 15, 315 Wang, F., 336 Wang, G., 21 Wang, H., 315 Wang, J., 340 Wang, L., 38, 171 Wang, X., 42 Wang, Y., 318 Weimer, P.K., 192, 198 Whittingham, M.S., 4, 5 Wiatrowski, A., 235 Witkowski, B.S., 289 Wollenstein, J., 233 Wu, J.M., 389 Wu, Y., 342

X Xiao, Y., 315 Xie, W., 344 Xiong, H.M., 265 Xu, K., 321

Y Yabuuchi, N., 32 Yan, L., 271 Yan, Z., 224 Yang, C., 23 Yang, L., 73 Yang, M.H., 83 Yang, W., 171, 266 Yang, X., 312 Yoldas, B.E., 363, 372 Yoon, C.S., 10 Yoshida, J., 32 Yoshinari, M., 11

402 You, Y., 33 Yu, C.Y., 344 Yu, F., 40 Yuan, X., 389 Yukird, J., 286 Yusoff, A.B., 226

Z Zeng, Y.Y., 229 Zhang, C., 77 Zhang, D., 315, 318, 323 Zhang, H., 262, 263 Zhang, J., 285

Index Zhang, L., 268, 320 Zhang, S., 271 Zhang, W., 344 Zhang, Z.Y., 264, 265 Zhao, B., 323 Zhao, J., 317 Zheng, Z.K., 213 Zhou, N.J., 225 Zhu, L., 324, 325 Zhu, L.D., 336 Zhu, Y., 389 Zilberberg, K., 226 Znaidi, L., 241