Metal-Chalcogenide Nanocomposites: Fundamentals, Properties and Industrial Applications 9780443188091

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Metal-Chalcogenide Nanocomposites

Woodhead Publishing Series in Composites Science and Engineering

Metal-Chalcogenide Nanocomposites Fundamentals, Properties and Industrial Applications

Edited by

Mohammad Ehtisham Khan Jeenat Aslam Series Editors

Suresh G. Advani Leif Asp Yuris A. Dzenis Ing. Habil. Bodo Fiedler Adrian Mouritz Chun H. Wang Editor-in-Chief

Costas Soutis

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2024 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-443-18809-1 (print) ISBN: 978-0-443-18808-4 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Rafael Guilherme Trombaco Production Project Manager: Kamesh R. Cover Designer: Vicky Pearson Esser Typeset by MPS Limited, Chennai, India

Contents

List of contributors About the editors Preface 1

2

3

4

Chalcogenides and their nanocomposites: fundamental, properties and applications Ho Soonmin, Pronoy Nandi, Immanuel Paulraj, Dilawar Ali and Rakesh K. Sonker 1.1 Introduction 1.2 Metal chalcogenide thin film-based solar cells 1.3 Antibacterial applications of thin films 1.4 Future perspective/outlook 1.5 Conclusion Acknowledgment References Chalcogenides and their nanocomposites in environmental remediation Chilukoti Srilakshmi 2.1 Introduction 2.2 Experimental 2.3 Results and discussion 2.4 Conclusion and future perspectives References Chalcogenides and their nanocomposites in photocatalytic reactions Nagaraju Kerru and Suresh Maddila 3.1 Introduction 3.2 Chalcogenides for the photocatalytic hydrogen evolution 3.3 Conclusions and perspectives References Metal chalcogenides and their nanocomposites in water purification systems Mahmoud H. Abu Elella, Safaa S. Hassan, Heba M. Abdallah, Mervat S. Mostafa and Nedal Y. Abu-Thabit 4.1 Introduction

ix xiii xv

1

1 12 15 18 19 19 19

29 29 30 32 40 40 45 45 46 51 52

59

59

vi

Contents

4.2

Removal of synthetic dyes using metal chalcogenide nanocomposites 4.3 Removal of toxic heavy metal ions using metal chalcogenides nanocomposites 4.4 Removal of residual antibiotics using metal chalcogenides nanocomposites 4.5 Future perspective/outlook 4.6 Conclusion References 5

6

7

8

Metal chalcogenides and their nanocomposites in industrial effluents treatments R. Elancheran, V.L. Chandraboss, B. Karthikeyan and S. Kabilan 5.1 Introduction 5.2 Role of metal chalcogenides 5.3 Conclusion References Heterostructured transition metal chalcogenides photocatalysts for organic contaminants degradation Aarti Sharma, Gagandeep Kaur, Madhvi Garg and Dhiraj Sud 6.1 Introduction 6.2 Methods for wastewater treatment 6.3 Transition metal chalcogenides 6.4 Synthesis methodologies 6.5 Characterizations 6.6 TMCs as heterogeneous photocatalysts 6.7 Application for photocatalytic degradation of organic pollutants 6.8 Conclusion References Chalcogenides and their nanocomposites in heavy metal decontamination Tshimangadzo S. Munonde, Shirley Kholofelo Selahle and Philiswa Nosizo Nomngongo 7.1 Introduction 7.2 Traditional heavy metal treatment 7.3 Photocatalytic heavy metal treatment 7.4 Conclusion and future perspectives Acknowledgments Declaration of competing interest References Chalcogenides and their nanocomposites in oxygen reduction Theivasanthi Thirugnanasambandan 8.1 Introduction

62 65 70 71 76 76

83 83 83 91 92

95 95 95 98 99 100 102 102 111 112

117

117 118 122 127 127 127 128 135 135

Contents

9

10

11

vii

8.2 Molybdenum based electrocatalysts 8.3 Ruthenium based electrocatalysts 8.4 Cobalt based chalcogenides 8.5 Rhenium based electrocatalysts 8.6 Iridium based electrocatalysts 8.7 Other electrocatalysts 8.8 Conclusion References

137 138 139 144 145 146 148 148

Nanocomposites of chalcogenides as super capacitive materials Muhammad Sajjad and Zhiyu Mao 9.1 Introduction 9.2 Chalcogenides as promising electrodes for SCs 9.3 Conclusion References

153

Metal-chalcogenides nanocomposites as counter electrodes for quantum dots sensitized solar cells Xie Zou, Zhe Sun and Zhonglin Du 10.1 Introduction 10.2 QD sensitizers 10.3 Counter electrodes 10.4 Interface modification layer 10.5 Conclusion Acknowledgments Author Contributions Notes References II
VI semiconductor metal chalcogenide nanomaterials and polymer composites: fundamentals, properties, and applications Vikas Lahariya, Pratima Parashar Pandey and Meera Ramrakhiani 11.1 Introduction 11.2 Structure and chemical properties of II
VI chalcogenide nanomaterials 11.3 Different properties of II
VI chalcogenide nanomaterials 11.4 Chemical Synthesis of II
VI chalcogenide nanomaterials and polymer composites 11.5 Applications of chalcogenides nanomaterials 11.6 Summary and future scope References

153 154 161 161

167 167 170 174 177 180 181 181 181 181

187 187 189 192 197 202 212 213

viii

12

Contents

Challenges and opportunities of chalcogenides and their nanocomposites Aleem Ansari, Rashmi A. Badhe and Shivram S. Garje 12.1 Introduction 12.2 Synthesis of metal chalcogenide and their nanocomposites 12.3 Preparation of chalcogenide nanocomposites 12.4 Application of chalcogenide and their nanocomposites 12.5 Future prospects of chalcogenides 12.6 Conclusion References

Index

221 221 227 233 237 247 249 250 261

List of contributors

Heba M. Abdallah Polymers and Pigments Department, Chemical Industries Research Institute, National Research Center, Giza, Egypt Mahmoud H. Abu Elella Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Nedal Y. Abu-Thabit Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City, Saudi Arabia Dilawar Ali Department of Physics, Government College University Lahore, Lahore, Pakistan Aleem Ansari Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz (East), Mumbai, Maharashtra, India Rashmi A. Badhe Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz (East), Mumbai, Maharashtra, India V.L. Chandraboss Department of Chemistry, Bharath Institute of Higher Education and Research, Bharath University, Chennai, Tamil Nadu, India Zhonglin Du Institute of Hybrid Materials, National Center of International Joint Research for Hybrid Materials Technology, National Base of International Science & Technology Cooperation on Hybrid Materials, College of Materials Science and Engineering, Qingdao University, Qingdao, P.R. China R. Elancheran Department of Chemistry, Annamalai University, Annamalai Nagar, Tamil Nadu, India Madhvi Garg Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, Deemed University, Sangrur, Punjab, India Shivram S. Garje Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz (East), Mumbai, Maharashtra, India Safaa S. Hassan Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt

x

List of contributors

S. Kabilan Department of Chemistry, Annamalai University, Annamalai Nagar, Tamil Nadu, India B. Karthikeyan Department of Chemistry, Annamalai University, Annamalai Nagar, Tamil Nadu, India Gagandeep Kaur Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, Deemed University, Sangrur, Punjab, India Nagaraju Kerru Department of Chemistry, GITAM School of Science, GITAM University, Bengaluru Campus, Bengaluru, Karnataka, India Vikas Lahariya Department of Physics, Amity School of Applied Sciences, Amity University Haryana, Gurugram, Haryana, India Suresh Maddila Department of Chemistry, GITAM School of Science, GITAM University, Visakhapatnam, Andhra Pradesh, India Zhiyu Mao College of Chemistry, and Life Sciences, Zhejiang Normal University, Jinhua, P.R. China Mervat S. Mostafa Science and Technology Center of Excellence (STCE), Ministry of Military Production, Cairo, Egypt Tshimangadzo S. Munonde Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa; Department of Science and Innovation-National Research Foundation South African Research Chair Initiative (DSI-NRF SARChI) in Nanotechnology for Water, University of Johannesburg, Doornfontein, South Africa Pronoy Nandi Department of Energy Science (DOES) (N-Center), Sungkyunkwan University (SKKU), Suwon, Korea Philiswa Nosizo Nomngongo Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa; Department of Science and Innovation-National Research Foundation South African Research Chair Initiative (DSI-NRF SARChI) in Nanotechnology for Water, University of Johannesburg, Doornfontein, South Africa Pratima Parashar Pandey School of Science & Technology, IILM University, Greater Noida, Uttar Pradesh, India Immanuel Paulraj Department of Physics, National Changhua University of Education, Changhua, Taiwan

List of contributors

xi

Meera Ramrakhiani Department of Physics and Electronics, Rani Durgawati University, Jabalpur, Madhya Pradesh, India Muhammad Sajjad College of Chemistry, and Life Sciences, Zhejiang Normal University, Jinhua, P.R. China Shirley Kholofelo Selahle Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa; Department of Science and Innovation-National Research Foundation South African Research Chair Initiative (DSI-NRF SARChI) in Nanotechnology for Water, University of Johannesburg, Doornfontein, South Africa Aarti Sharma Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, Deemed University, Sangrur, Punjab, India Rakesh K. Sonker Department of Physics, Acharya Narendra Dev College, University of Delhi, Delhi, India Ho Soonmin Centre for Green Chemistry and Applied Chemistry, INTI International University, Putra Nilai, Negeri Sembilan, Malaysia Chilukoti Srilakshmi Department of Chemistry, GITAM School of Science, GITAM (Deemed to be University), Bengaluru, Karnataka, India Dhiraj Sud Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, Deemed University, Sangrur, Punjab, India Zhe Sun Institute of Hybrid Materials, National Center of International Joint Research for Hybrid Materials Technology, National Base of International Science & Technology Cooperation on Hybrid Materials, College of Materials Science and Engineering, Qingdao University, Qingdao, P.R. China Theivasanthi Thirugnanasambandan International Research Centre, Kalasalingam Academy of Research and Education (Deemed University), Krishnankoil, Tamil Nadu, India Xie Zou Institute of Hybrid Materials, National Center of International Joint Research for Hybrid Materials Technology, National Base of International Science & Technology Cooperation on Hybrid Materials, College of Materials Science and Engineering, Qingdao University, Qingdao, P.R. China

About the editors

Dr. Mohammad Ehtisham Khan is working as an assistant professor and the head of the Department of Chemical Engineering Technology at Jazan University, Kingdom of Saudi Arabia. Dr. Khan completed his PhD in the Department of Chemical Engineering at Yeungnam University and later worked as a postdoctoral research associate in the Department of Chemical Engineering at Yeungnam University, Republic of Korea. At present, Dr. Khan is teaching several courses in the Chemical Engineering Department and handles various final-year graduation projects. Dr. Khan is working in the cuttingedge area of nanosciences and nanotechnology, especially in the field of inorganic materials such as biogenic synthesis of metal
metal oxide nanoparticles and carbon materials based on nanostructures/nanocomposites and their applications in photocatalysis, photoelectrodes, photocurrent, sensing, and antimicrobial performances. Dr. Khan is the author of several good-quality research and review articles published in peer-reviewed international journals of Elsevier, Nature, RSC, ACS, Springer, Bentham Science, and so on. He received several national and international awards for his research and academic achievements.

Dr. Jeenat Aslam is currently working as an associate professor in the Department of Chemistry at College of Science, Taibah University, Yanbu, Al-Madina, Saudi Arabia. She earned her PhD degree in surface science/ chemistry from the Aligarh Muslim University, Aligarh, India. Materials and corrosion, nanotechnology, and surface chemistry are the primary areas of her research. Dr. Jeenat has published a number of research and review articles in peer-reviewed international journals of ACS, Wiley, Elsevier, Springer, Taylor & Francis, Bentham Science, and others. She has authored more than 30 book chapters and edited more than 20 books for the American Chemical Society, Elsevier, Springer, Wiley, De-Gruyter, and Taylor & Francis.

Preface

More dependable and efficient products and devices for energy and environmental monitoring and purification applications are being made with metal and metal chalcogenide nanocomposites. This book describes the development of metal chalcogenides, new fabrication techniques, and environmentally responsible strategies for utilizing a variety of metal chalcogenide
based nanocomposites in creative ways. It serves as an essential information source for materials scientists and engineers who are developing the newest products and technology for use in the energy and environmental fields. Metal chalcogenides have outstanding qualities, such as the ability to effectively use their narrowing band gaps for the photocatalytic degradation of environmental pollutants, particularly in the field of wastewater treatment, electronic devices, environmental sensing devices, energy storage, electrode materials, fuel cell, membranes, and environmental pollutants in general. The current industrial-scale applications of metal chalcogenides are outlined in the book. There are 12 chapters in the book, which cover fundamentals, properties, and applications of chalcogenides and their nanocomposites in various areas, such as environmental remediation, photocatalytic reactions, water purification systems, industrial effluents treatments, heavy metal decontamination, oxygen reduction, heterostructure transition metal chalcogenide photocatalysts for organic contaminant degradation, supper capacitive materials, counter electrodes in solar cells, II
VI semiconductor metal chalcogenide nanomaterials, and polymer composites, as well as challenges and opportunities. The goal of this book is to provide the most recent developments in metal chalcogenides for large-scale industrial applications. The book is intended for academic and professional researchers who work in the domains of chemistry, energy, environmental science, materials science and engineering, and nanotechnology. Libraries in academic and professional settings, organizations, autonomous research institutions, government units, and scientists will all find this book to be an invaluable source of reference information. Overall, this book is intended to be a helpful resource for advanced undergraduate and graduate students, researchers, and scientists looking for industrial-scale applications for metal chalcogenides for modern research demands. All chapters in the book were edited by eminent academic and professional researchers, scientists, and subject matter specialists.

xvi

Preface

We extend our sincere gratitude to all contributors on behalf of Elsevier for their superb work. Sincere appreciation is extended to Dr. Gwen Jones (Senior Acquisitions Editor), Mr. Rafael G. Trombaco (Editorial Project Manager), and the whole Elsevier editorial staff for their unwavering assistance and support during this project. Last but not least, thanks to Elsevier for releasing the book. Mohammad Ehtisham Khan Jeenat Aslam

Chalcogenides and their nanocomposites: fundamental, properties and applications

1

Ho Soonmin1, Pronoy Nandi 2, Immanuel Paulraj3, Dilawar Ali 4 and Rakesh K. Sonker 5 1 Centre for Green Chemistry and Applied Chemistry, INTI International University, Putra Nilai, Negeri Sembilan, Malaysia, 2Department of Energy Science (DOES) (N-Center), Sungkyunkwan University (SKKU), Suwon, Korea, 3Department of Physics, National Changhua University of Education, Changhua, Taiwan, 4Department of Physics, Government College University Lahore, Lahore, Pakistan, 5Department of Physics, Acharya Narendra Dev College, University of Delhi, Delhi, India

1.1

Introduction

Generally, metal chalcogenide is defined as a chemical compound consisting of chalcogen (selenium, sulfur, oxygen, and tellurium) anion and a more electro-positive element. The term chalcogenide could be referred to oxide, sulfide, telluride, and selenide. Metal chalcogenide has gained huge attention because of its potential significance in industrial and technological applications in the past few decades. Thin filmbased nanostructure and nanocomposite materials have been recently used in a variety of sustainable applications [1,2]. Their distinct structural properties enable researchers to investigate functionalities in a variety of directions, such as thin film-based solar cells [3,4], light-emitting devices [5,6], transistors [7], sensors [8,9], supercapacitors (SCs), perovskite solar cell, and antibacterial applications. The world relies on nonrenewable energy sources such as fossil fuels for most of its energy requirements, which are fast depleting. Due to the ever-increasing demand for energy for growth, development, and sustainability, we are heading toward a global energy crisis. Search for alternative and renewable energy sources such as wind, rain, tides, waves, geothermal heat, converting biomass to energy, and solar energy has gained momentum [10]. Renewable energy such as solar energy could be used to reduce the carbon footprint. The advantages of solar energy include pollution-free technology, lower prices of installation costs, and cheaper prices of materials [11]. In today’s world energy scenario, solar energy has become more popular in commercial, residential, and utility applications. Energy storage devices and their growth have attracted the scientific community’s and society’s attention, due to the increased demand for portable devices and hybrid automobiles. As a result, the development of novel high-performance energy Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00001-8 © 2024 Elsevier Ltd. All rights reserved.

2

Metal-Chalcogenide Nanocomposites

storage materials and methods is critical. Recently, researchers mainly focusing to develop high-performance energy storage devices, for example, capacitors, SCs, and batteries. Nevertheless, the facts of fossil fuel shortage, human health risks, and environmental and ecosystem damage must all be considered in this effort [12,13]. It is commonly known that batteries suffer from a number of flaws, including slow charge and discharge, storage reactions, low power, and low rate capability, all of which limit their applicability in the long run. However, the well-known ecofriendly energy storage devices of the SC device with remarkable attributes such as longer life cycles, quick charging, safe operation, and high power density, all of which are achieved by fine-tuning the surface characteristics of the electrodes. Rapid growth in microelectronics and advancements in technology made machines more autonomous and intelligent day by day. It aroused a demand for artificial sensing organs that perform independently. Thus, people developed devices according to their requirements and now sensors are ubiquitous in our daily lifestyle [14]. Adhesion of microbial infection diseases on the surface of the material poses a serious threat to human health, which leads to social and economic impacts. In hospitals and intensive care units, life-threatening microbial infections are referred to as healthcare-associated infections or nosocomial infections [15,16]. More than 90% of nosocomial infections are caused by bacteria followed by viruses and fungi. Healthcare centers need to be diligently sterilized to remove these nosocomial infections [17]. Traditionally antimicrobial chemical agents were used to disinfect these areas, but these chemicals are extremely toxic and irritant to humans, moreover, they were applied to surfaces every week, which increases the cost [18]. To address these health-related issues, nanotechnology-based treatment has been developed to inhibit the growth and reproduction of harmful bacteria. In this chapter, the properties of the various types of thin films have been studied by using different common tools. Many functional nanomaterial-based devices with exceptional sensing demonstration to several chemical and biological species in both solid circumstances and solution phases have been demonstrated. Nanostructure materials in particular are accomplished by sensing a slight gas molecule such as CO, H2S, CO2, LPG, SO2, NH3, NO2, O2, and others [19
25]. This chapter expands on the sensing application of nanostructure material with various cations, dopants, and composites [26]. Furthermore, the fundamental mechanisms and electron transport properties that are basic and aimed at understanding sensing phenomenon will be talked about. To promote such functional materials-based molecular designs, they give manufactured strategies, structural data, alterations, and continuous detecting applications. Finally, we summarize our findings and suggestion attainable procedures for the forthcoming development of new nanostructure-based chemo and biosensors with continuous demonstration. Rapid growth in microelectronics and advancements in technology made machines more autonomous and intelligent day by day. It aroused a demand for artificial sensing organs that perform independently. Thus, people developed devices according to their requirements and now sensors are ubiquitous in our daily lifestyle. The 3S factor plays a significant character in remarking a sensor as a good sensor and these three factors are sensitivity, selectivity, and stability. Sensors can be classified into three

Chalcogenides and their nanocomposites: fundamental, properties and applications

3

different types such as electrochemical gas sensors, metal oxide gas sensors, and colorimetric gas sensors, but usually they were large. Around the 1980s OSHA worked on it to reduce the size of the electrochemical sensor. It is also recognized as an electrochemical concentration cell that is used to calculate the fractional pressure of each gaseous class. The physical appearance, geometry, and other various construction depend upon the intended use. Generally, people are miscommunicated about some electrochemical sensors that all are the same, but it is wrong. It may resemble physically but they vary depending upon the electrolyte and gel used. An electrochemical gas sensor consists of two major parts first is the electrolyte/gel used and another one is the anode and cathode terminal used. As we know that anode terminal is responsible for all the oxidation processes while the cathode is responsible for the reduction process. Due to this phenomenon, a current is created and positive and negative ions are developed. The positive ions flow toward the cathode rod, while the negative ions flow toward the anode rod. We get oxidizing gas at the anode and reducing gas at the cathode and the concentration is measured. The result is straightforwardly corresponding to the attention or fractional pressure of the gaseous species. Nowadays in the electrochemical gas sensor, the fluid electrolyte is swapped for a solid-state electrolyte but the whole working is the same. The acoustic gas sensor is based on sound efficiency. These types of sensors consist of a piezoelectric substrate containing interdigital electrodes. When RF voltage of a particular frequency is given then the mechanical waves are produced in the piezoelectric substrate. These rayleigh waves propagate, and the acoustic waves are produced. The mechanical energy is changed over into electrical RF voltage. Depending on this method, we can observe the properties and procedures for chemical species in the vapor stage, fluid phase, emptiness, or thin solid films. Acoustic gas sensor devices are already used in the transportable industry, phone industry, and sensor evolution. Acoustic wave devices are extremely sensitive to their surface perturbation. There are various kinds of acoustic wave gas sensors including surface acoustic wave, shear horizontal SAW, shear horizontal acoustic plate mode, flexural plate wave or Lamb wave mode, and thickness shears mode devices. The nanomaterial gas sensor is also called a chemo-resistor gas sensor. Semiconductors are found to be very sensitive to very low concentrations of gas. This type of sensor requires stain gauze, thick film, thin film, etc. In this sensor, the resistance of the film changes when the gas interacts with the thin film and it gets adsorbed on the surface of the material [27]. Then the resistance of the film changes depending on the type of material used and the type of gas used. Thin film interacts with the adsorbed oxygen molecule and the free electrons get attached to these molecules forming oxygen species, and when any gas interacts then there is a change in its parameters [28]. The solid and gas interaction changes the resistance of the material and their sensing properties are studied. Response to the gas can be increased by adding a dopant/additive or by increasing the temperature of the substrate through the heater [29]. Nowadays there are numerous categories of nanomaterial gas sensors available in the market for both reducing gas and oxidizing gas. Adsorption and chemisorption are the two phenomena which are responsible for all the mechanisms in nanomaterial gas sensors.

4

Metal-Chalcogenide Nanocomposites

The literature Table 1.1 reviews various nanostructure materials for various gases, and it is perceived that a great deal of work is finished in the field of gas sensors yet numerous irritating problems exist including higher response and selectivity, fast response and recovery speed, minimum ecological deficiency, lowtemperature operation, minimum power consumption, and low cost, etc. Therefore, there is an essential need for efficient thin film gas sensors that can be worked at a low temperature preferably at room temperature with enhanced sensitivity and improved stability so that any accidental explosion due to leakage could be avoided. For the further revolution of nanocomposite gas sensors, however, an essential examination survey is seeming to be necessary on significant characteristics of such as the gas-sensing principle and sensor strategy parameters, which have been to a great extent overlooked up to this point. Gas sensors play a pivotal character in domestic and industrial frontages and furthermore help to keep a cleaner climate by giving alerts of excessive emission of poisonous gases from time to time.

1.1.1 Thin film-based supercapacitor In the field of energy storage technologies, the flexible SC is a rising star due to its easy fabrication process, lightweight, compact design, and excellent power density (very rapid charging-discharging rate). The energy storage devices of the SCs can be divided into three parts [49
51] depending on their mechanisms, which is electric double layer capacitors (EDLC), pseudocapacitor, and hybrid SCs (HSCs) as indicated in Fig. 1.1. Generally, EDLC relies on the charges that are electrostatically adsorbed mostly on the electrode-electrolyte interface, the term pseudocapacitor relies on a device that stores energy through a redox process, and charge storage using a combination of the two methods is referred to as HSCs. The EDLC included carbon nanotubes (CNT), activated carbons (AC), and carbon aerogels such as carbon nanosheets (NSs), porous carbon nanowhiskers, porous CNT, and graphene; pseudocapacitor included the conducting polymers (CP) and metal oxides (MO) such as CoCl2 nanostructure, CuO nanowires (NWs), and NiO nano blocks, which have two types of reactions: (1) ion intercalation and (2) surface redox reactions. The combination of the HSCs includes composites hybrids, asymmetric hybrids, and battery-type hybrids, such as NiO/graphene, PANI/RGO, and Ni/graphene/ CNT. Modern research into flexible thin films for ethanol sensors, photocatalytic, thermoelectric (TE), and SC applications is particularly exciting [52
56]. Recently flexible SCs more attracted young researcher, which can have prepared many techniques such as physical and chemical methods. The classification of thin film deposition techniques is presented in Fig. 1.2. Li and coworkers [57] studied the AC nanotubes as a double-layer capacitors electrode material, which materials are prepared by the acid treatment. Activated CNT and CNT materials are used as an EDLC electrode. The acid-treated activated CNT achieved the maximum specific capacitance (56 F/g) than untreated CNTs and the acid-treated activated CNT were 2.5 times higher. CP and transition metal oxides SC electrode materials are commonly used for pseudocapacitors. Pang and coworkers [49] have prepared the electrodeposited manganese dioxide thin films,

Table 1.1 Brief literature review on various nanostructure materials for various gases. S. No.

Nanomaterial

Catalyst

Temp. ( C)

Gas Conc. (ppm)

Response

Response/recovery time

References

1. 2. 3. 4.

SnO2 thin film SnO2 thin film TiO2 thin film ZnO thin film

180 550 250 27

500 2000 1000 1000

56 17 37 0.58

161/142 s
/

/
3.7 /3.1 min

[29] [30] [31] [32]

5. 6. 7. 8. 9. 10. 11. 12.

ZnO thin film h-BN thin film PANI BaTiO3 thin film CuO thin film CdO thin film PEDOT thik film La12xSrxFeO3 thin film ZnO thin film ZnO thin film SnO2 thin film TiO2 thin film Fe2O3 thin film SnO2 thin film SrFe12O19 ZnO thin film

NiO CuO Ni PEDOT: PSS MWCNT

CuO/Ag CuxFe32xO4
BPEI


30 RT RT 250 250 250 RT 380

1500 3.0 vol.% 100 5000 3000 5000 1000 2000

61 6.17 12.10 0.28 0.50 0.01 0.03 0.25

5.8 /3 min 55 /40 s 11 /07 s 15 /10 min 9.5/
min 3.33/5 min
/60 min 11/15 min

[33] [34] [35] [36] [37] [38] [39] [40]


La PANI/Ag Zn PANI

Ag

200 400 30 RT RT RT RT RT

3000 5000 1000 1.5 vol.% 20 0.5 5 vol.% 5

0.03 0.65 67 2.92 229 0.11 602 142

8/40 s 90/38 s 1000/900 s 120/
s 2.35/3.8 min
/
20/40 s
/


[41] [42] [43] [44] [45] [46] [47] [48]

13. 14. 15. 16. 17. 18. 19. 20.

6

Metal-Chalcogenide Nanocomposites

Figure 1.1 Classification of supercapacitors.

indicating pseudocapacitive behavior and specific capacitance of about 698 F/g. Dubal and coworkers [58] have synthesized the Mn3O4 thin films using the SILAR method, which showed the super capacitance of 314 F/g (scan rate 5 5 mV/s). Liu and coworkers [59] have prepared flexible and HSCs using micromolding methods. The flexible HSCs (Ag/porous and Ag/NixFeYOZ@reduced graphene oxide electrodes) successfully achieved the highest areal capacitance of 282.1 μF/cm2 (when current density 5 3 μA/cm2). In this chapter, we discussed thin film based SC applications. Flexible thin filmbased SCs are attractive because of their considerable cost and lightweight, and they have shown promise and drawn a lot of interest. In this chapter, we were mainly focusing on the thin film preparation methods, fundamental mechanism, and electrochemical properties of the materials. We hope that this chapter on thin-film SCs will aid readers in greater understanding of how to choose materials systems and thin-film manufacturing methods.

Chalcogenides and their nanocomposites: fundamental, properties and applications

7

Figure 1.2 Thin film deposition techniques classification.

Fugare and coworkers [60] have prepared RuO2 thin films using the spray pyrolysis method for SCs application. To prepare the RuO2 thin films, stainless steel (SS, 1.5 5 cm2) and RuCl3.X H2O were used as a substrate and source material, respectively, during the experiment. The 0.005 M of the source material (RuCl3.X H2O) dissolved into the double distilled (DD) water and sprayed on the SS substrate with the spray rate, gas flow rate, and temperatures of 10 mL/min, 12 L/min, and at 673K, respectively. The spray pyrolysis nozzle and substrate distance were maintained at 22 cm, and direction movement of the x and y positions was kept at 40 and 70 cm, respectively. The prepared thin films were black gray and completely adhered to the SS substrates. Immanuel and coworkers [61] improved Mn3O4 thin films for SCs applications via Cr doping. The pristine and chromium-doped Mn3O4 films were prepared by SILAR method. To prepare the Mn3O4 thin films, the manganese (II) sulfate and sodium hydroxide were used as source materials while the stainless steel was used as a substrate. Fig. 1.3(ii) shows the thin films preparation method: for example, 0.1 M of MnSO4 dissolved into the 100 mL DD water, and the resulting solution was kept in the glass beaker (named a beaker A), DD water was kept in beaker B and D, sodium hydroxide dissolved into the100 mL DD water, and the resulting solution kept in the beaker C, respectively. Beakers A and D

8

Metal-Chalcogenide Nanocomposites

Figure 1.3 (i) Schematic diagram of RuO2 thin film preparation method using the spray pyrolysis methods [60]. (ii) Schematic diagram of Mn3O4 thin film preparation methods using the SILAR [61]. (iii) Fe2O3
rGO composite preparation method using (a) spray pyrolysis method and (b) photothermal processing methods are shown schematically [62]. (iv) Schematic diagram of MoS2-Cu3N composite thin films preparation method using the DC magnetron sputtering [63].

contain the cations and anionic solutions. There are two main steps are followed to prepare the Mn3O4 thin films. (I) The precleaned SS substrate was dipped into beaker A for 20s, which contain the cations and the films absorbed Mn21 ions, then the film was dipped into beaker B to remove the loosely bounded ions, (II) again the film was dipped into the beaker C for 20s, which contains the anions and the film reached with the OH2 ions, then the film was dipped into beaker D to remove the loosely bounded ions. This process is called a one SILAR cycle, the same procedures are followed to prepare the Cr-doped Mn3O4 films Gaire and coworkers [62] reported the Fe2O3 flexible thin films for SC applications through a photonic process. The Fe2O3 flexible thin films preparation method is shown in Fig. 1.3(iii-a). To prepare the Fe2O3 flexible thin films, 0.5 g of Fe(III)acetylacetonate was used as source material. Firstly, Fe (III)-acetylacetonate was dissolved in 20 mL of acetone and then the prepared solution was kept in the

Chalcogenides and their nanocomposites: fundamental, properties and applications

9

ultrasonication for 20 min. The resulting solution was sprayed on the ITO-coated PET substrate at room temperature. A unique photonic processing technique was used to process the as-sprayed film. The detailed photonic process is presented in Fig. 1.3(iii-b). Sharma and coworkers [63] prepared the MoS2-Cu3N composite thin films on the flexible SS substrate by using the DC magnetron sputtering for SC application. MoS2 and Cu are used as target materials, which were kept in the vacuum chapter with the pressure of 2.94 1026 Torr. To get the Cu3N thin films, the sputtering power (100 W) was used and the substrate temperature was maintained at 250 C. To prepare the MoS2 thin films on Cu3N, the sputtering power (45 W) was used. The sputtering mechanisms are shown in Fig. 1.3(iv). Cyclic voltammetry was used to examine the supercapacitive performance (RuO2 thin films) over a wide potential range [ 2 0.6 to 1.0 V (Ag/AgCl)]. The CVs of RuO2 electrodes at various scan rates, shown in Fig. 1.4(i-a), revealed the scan rate is exactly proportional to the voltammetric current. While elevating

Figure 1.4 Electrochemical performance: (i-a) Cyclic voltammetry of the RuO2 thin films at various scan rates, (i-b) GCD curve at various current densities, (i-c) Tafel plot of the prepared thin films, (i-d) stability of the films at 100 mV/S in 0.5 M H2SO4 [60]. (ii) chargedischarge curve of the Cr-Mn3O4 thin films at a current density of 1 Ag21 [61] (iii-a) chargedischarge curve at different current densities, (iii-b and c) areal and specific capacitance at various current density, (iii-d) specific capacitance (F/g), capacitance retention (%) versus number of cycles [62].

10

Metal-Chalcogenide Nanocomposites

the scan rate, the specific capacitance drops monotonically because of the presence of core active sites. At the scan rate of 2, 5, 10, 50, 100, and 200 mV/s, the specific capacitances were 2192, 1923, 1718, 1425, 1128, and 1050 F/g, respectively. In 0.5 M of sulfuric acid, a galvanostatic charge/discharge (GCD) investigation was performed on a RuO2 electrode at different current densities (4
14 mA/cm2) as shown in Fig. 1.4(1-b). The pseudocapacitance generated from the redox reaction may explain the nonlinear behavior of GCD curves, which differs from the conventional linear variation of potential with time. Fig. 1.4(i-c) shows the logarithmic scale of the films. Fig. 1.4(i-d) indicates the stability of the prepared films, which shows the specific capacitance versus cycle number up to 2000. The prepared RuO2 films are more stable material. The use of Cr as a dopant increased the electrochemical performance of SILAR-deposited Mn3O4 thin films as shown in Fig. 1.4(ii). Electrochemical investigations demonstrate that a 3-electrode study with 1 M Na2SO4 as the electrolyte and a 3% Cr-doped Mn3O4 electrode has an excellent specific capacitance (181 F/g). Fig. 1.4 shows the GCD curves of with and without Cr electrodes at constant current densities of 1 A/g. The charging time increases linearly in Region I. The discharge duration of the initial region may have some internal resistance (IR), which increases linearly as the current drop out. Region III depicts the charge separator at the electrode
electrolyte interface, which shows the drop depending on the time potential. Fig. 1.4(iii-a) shows the GCD studies were carried out at various current densities to further investigate the electrochemical characteristics. The pseudocapacitive charge storage mechanism is further confirmed by the nonlinear nature of the GCD curves and the distinctive voltage decreases at the onset of the discharge operations. The appearance of such voltage decreases due to the electrode’s internal IR. Fig. 1.4(iii-b) shows the areal capacitance versus current density. Increasing current density, the areal capacitance decreased, at 0.6 mA/cm2, the electrode indicated an initial areal capacitance of about 53 mF/cm2 and a gravimetric capacitance was 179 F/g (2 A/g constant current density). Fig. 1.4(iii-c) shows the current density versus specific capacitance, while elevating the current density from 2 to 10 A/g, the specific capacitance decreased. Decreasing the specific capacitance is due to the higher current densities. Fig. 1.4 (iii-d) shows the specific capacitance (F/g) and capacitance retention (%) versus number of cycles. The measured GCD for 5,000 cycles (when constant current density was 2 A/g), to see how long the Fe2O3
rGO could last. The electrode retains 70% of its initial capacitance after these many cycles, demonstrating its exceptional capacitance retention.

1.1.1.1 Thin film-based perovskite solar cells Solar energy is an example of renewable energy. To date, various types of solar cells have been developed and they can be categorized: First generation: single-crystalline Si (present efficiency B27.6%), GaAs (efficiency B28.8%), and their multijunctions (efficiency B46%); Second generation: copper indium gallium diselenide (CIGS) materials (efficiency about 22.6%), amorphous silicon (efficiency about 13%), and cadmium telluride (CdTe) films (efficiency about 22.1%); Third generation: thin-film solar

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cells, polymer cells (efficiency B10%), dye-sensitized cells (efficiency B11.9%); and Fourth generation: Organic-inorganic hybrid perovskite (OIHP) solar cells (efficiency B25.6%). Recently, OIHP photovoltaics received a swift surge since the first report in 2012 due to easy fabrication method (spin-coating), low processing/materials cost, and high power conversion efficiency [64,65]. The OIHPs are an exotic class of hybrid materials that combines the potentials of organic and inorganic semiconductors: long charge carrier diffusion length, large mobilities, high absorption coefficient across the solar spectrum, and tunable bandgap by changing the composition with the possibility of easy device fabrication [66]. Due to superior properties, such as flexibility, semitransparency, and vivid colors, OIHP became attractive for niche applications such as LEDs, photodetectors, phototransistors, portable lightweight chargers, Lasers, and x-ray detectors [67]. OIHP is a class of ABX3 perovskites. A represented monovalent cations such as methylammonium (MA1) and formamidinium(FA1), and B represented divalent metallic cations such as Pb21 and Sn21. X represented halide anions such as I2, Br2, Cl2, or mixtures of them [68]. Here, monovalent organic cation is confined in PbX6 octahedral, in which most of the semiconducting properties such as carrier generation and carrier transport arise from the inorganic BX3 component, whereas fundamental structure of perovskite and crystallographic phase transition is influenced by the orientation of organic cation [69]. Hybrid perovskite solar cell architecture is quite simple and has five components, namely transparent conductive oxide, electron transport layer (ETL), hole transport layer (HTL), perovskite absorber layer, and metal-based cathode. Thus a standard OIHP solar cell structure consisting of FTO glass/TCO/TiO2 (ETL)/mesoporous TiO2 (mp-TiO2)/perovskite (B500 nm)/HTL/metal showed a quite high PCE as high as 25% without any complicated processing steps [64,65]. The photoelectrons (holes) will be produced by light absorption, and transported toward the anode (cathode) and block holes (electrons) in the ETL (HTL). Thus, materials used in ETL showed higher LUMO and HOMO than that of OIHP, whereas HOMO of HTL should be higher than OIHP. PCE of OIHP-based solar cells has been significantly increased in 2009 (3.8%) to 25.5% (in 2020) for single-junction and 29.15% for the perovskite/silicon (Si) tandem. Initially, iodine-based liquid electrolyte in Dye-sensitized solar cells (DSSC) was replaced by OIHP, and 3.8% PCE was achieved by Miyasaka and coworkers [70]. Subsequently, Snaith et al. could increase the PCE to 9.7% by using spiroMeOTAD as a solid-state HTL [71]. By using a double layer of mesoporous TiO2, PCE reached 12.8% with reliable stability ( . 1000 h) in ambient conditions under sunlight [72]. In the above devices, OIHP was either spin-coated/deposited on hightemperature processed mesoporous TiO2 or Al2O3. In 2013, planar structure (solution-processed TiO2 as an ETL) was employed by Snaith et al., and 15.4% PCE was recorded [73]. Thereafter, Y. Yang at UCLA deposited OIHP on Yttriumdoped TiO2 (ETL), and a reverse scan PCE of 19.3% was claimed [74]. In November 2014, researchers from KRICT achieved a PCE of 20.1% by using an FAI solution during spin coating of MAPbI3 thin films [75]. In March 2017, researchers from KRICT and UNIST prepared a solid solution of MA12xFAxPb

12

Metal-Chalcogenide Nanocomposites

(I12yBry)3, which yielded a PCE [76] of 22.1% and the world record PCE is 25.2% with Jsc 5 25.1 mA/cm2, Voc 5 1.18 V, and fill factor of 84.8%. In parallel with the development of OIHP-based polycrystalline films, research work in OIHP-based nanocrystals (NCs) has been accelerated [77] due to their quantum-confinement effect since their first report in 2014. Depending on their different shapes, they can be categorized into NWs, NSs, quantum dots (QDs), nanorods, nanocubes, nanoplates (NPs), and nanoplatelets with size less than 100 nm in one or more dimensions. As reported by the researcher, the MAPbI3 was first used in DSSCs in 2009 and indicated a PCE [70] of about 4%. Swarnkar et al. first used CsPbI3 QDs (Eg 5 1.73 eV) as the photoactive material to fabricate photovoltaic cells and achieved a PCE over 10% with Jsc 5 13.4 mA/cm2 and Voc 5 1.23 V in ambient conditions [78]. Later, researcher has been actively involved in improving the PCEs of nanocrystalline perovskite-based solar cells using cation or anion exchange, posttreatment of QDs film, and component engineering [78
80]. CsPbI3-based QD photovoltaic cells showed power conversion efficacy [79] about 13.4% and higher Voc than its thin-film counterpart, which surpassed the PCE of PbS QDSCs. However, it is observed that surface defects (due to moisture and polar solvent) in both CsPbI3 and FAPbI3 QDs caused poor performance of solar cells and limited the carrier transport as well. Further, the dynamic disorder of FA1 in PbX6 octahedra resulted in longer carrier lifetimes and reduced nonradiative recombination. Research findings revealed that solar cells made from colloidal mixed cation Cs0.5FA0.5PbI3 QDs indicated PCE of about 16.6% (due to entropic stabilization), outperforming other QDSC technologies [80]. In order to reduce the defects and improve photo-generated carriers, surface ligand engineering and A-site cation alloying are some of the beneficial approaches [81]. Superior optoelectronic properties of OIHP-based NCs have motivated researchers to explore the synthetic approach through composition engineering, and device designing for better PV applications. Although NCs OIHP-based devices showed unprecedented progress over the past few years, it has several limitations too. Degradation of NCs OIHP-based PV cells under a range of environmental factors such as humidity, oxygen, illumination, and thermal stress is one of the significant drawbacks. We believe efforts directed toward overcoming these limitations and enhancing the stability of solar cells like A-site cation alloying, surface ligand engineering and the encapsulation of devices will improve device efficiency, lifetime, and stability of NCs OIHP-based PV devices.

1.2

Metal chalcogenide thin film-based solar cells

Solar energy is an example of renewable energy [82]. It employs the sun’s light to produce electrical energy [83,84]. Silicon-based materials known as first-generation solar cells [85] contributed to large shares of photovoltaic markets due to matured technologies. The advantages of this solar cell, including low maintenance costs

Chalcogenides and their nanocomposites: fundamental, properties and applications

13

[86], can reduce electricity bills and is known as renewable energy source if compared to fossil. However, some issues observed, such as higher production costs, could generate a lot of waste materials when the silicon was removed, and the performance could be degraded at high temperatures [87]. The second-generation solar cells consisted of CIGS and CdTe indicated low cost-effectiveness and toxicity problems. These absorber materials showed very suitable band gaps [88] and high absorption coefficient values, so they can adsorb the highest number of photons [89]. Generally, p-type and n-type materials could be produced via different deposition techniques. According to the world photovoltaic market, market shares of silicon-based solar cells dropped significantly (92%
73.3%), however thin film-based solar cells increased (7%
10.4%) from 2014 to 2020. Nanostructured CdTe films have been synthesized under different deposition techniques (Table 1.2). These films showed excellent absorption coefficient value and suitable band gap (1.5 eV) to absorb sunlight. Currently, there are several solar power plants that have been built by using CdTe films. The photovoltaic power stations in Topaz Solar Farm (California, United States), Desert Sunlight Solar Farm (California, United States), and Templin Power plant (Germany) have installed 9 million, 8.8 million, and 1.5 million CdTe film modules, respectively. The disadvantages of the CdTe thin films such as toxicity of cadmium, availability of the tellurium element (very rare in the earth), and lower PCE (9.59%
19%) if compared to silicon-based solar cells. Researchers have suggested some strategies to improve quality of films and PCE of solar cells. Selection of appropriate deposition technique and substrate greatly influences the properties of CdTe films. Larger grain sizes could be produced by using CSS method at high temperatures and the stability of films. Electrodeposition method could be used to produce CdTe films onto metallic foils and TCO substrate at low temperatures. The Voc could be improved by the better layer coverage and removal of pinholes in samples. Enhancement in PCE and current density could be achieved by controlling the pressure (in chamber). Quaternary compound such as CIGS thin films indicated p-type behavior [98], has a direct band gap (1.07
1.7 eV), and could be deposited onto flexible substrates [99]. Researchers observed that the band gap for CuInSe2 and CuGaSe2 was found to be 1 and 1.7 eV, respectively. The advantages of CIGS films include a higher performance ratio, better resistance to heat environments, cheaper cost, and lower energy recovery time if compared to silicon-based solar cells. Also, it is worth indicating that CIGS-based solar cells showed better stability than organic solar cells. Researchers also point out some limitations, such as complex structures, which caused higher production cost [100], and less PCE (21.08%
26.4% [98
101]) than silicon-based solar cells. The Cu2ZnSnS4 (CZTS) films have been used as absorber layer materials to replace rare elements (In and Ga). Several unique properties including direct band gap, excellent absorption coefficient, and p-type conductivity were reported. Hironori and coworkers proposed production of thin films via vapor phase sulfurization on the substrate (soda lime glass) [102]. The band gap values were 1.45 to 1.6 eV, and PCE of solar cells (Al/ZnO:Al/CdS/CZTS/Mo-SLG) reached 2.62%.

14

Metal-Chalcogenide Nanocomposites

Table 1.2 The properties of cadmium telluride thin films prepared under different deposition methods. References

Highlighted results

Razykov and coworkers [90]

Chemical molecular beam deposition technique was used to produce 4
5 μm thick films. The highest Jsc (19.7 mA/cm2) and efficiency values (8.5%) could be found when the composition of Cd/Te is about 0.7, because of an increase in the concentration of shallow acceptors. The use of copper in the back contact influences the performance of solar cells. Remarkably low Voc (733 mV), power conversion efficiency (10%), and fill factor (62.3%) were obtained in Cu-free MoO3/Te buffer materials. High power conversion efficiency (11.3%), fill factor (68.6%), and Voc (768 mV) could be found in the back contact layer containing Cu, Te, and MoO3. Ultrathin CdTe films have been synthesized via magnetron sputtering method. Thinner films (less than 1 μm) indicated good carrier collection, excellent light trapping, and higher power conversion efficiency (8%
11%). Photovoltaic behaviors could be improved in CdCl2 1 CdF2 treated CdTe films due to the unique properties of fluorine. The observed conversion efficacy successfully increased in CdCl2-treated films (1%
3%) and CdCl2 1 CdF2-treated samples (5%
7%). The dichlorofluoromethane (ozone-depleting agent) was used to improve the quality of back contact. However, it could be replaced by using a mixture of HCl, Ar, and CHF3. Annealed films provided a higher fill factor (0.63), Voc (800 mV), and power conversion efficiency (13.82%) if compared to as-deposited films. Influence of various types of antireflection layers on the performance of solar cells was studied. Aluminum trioxide served as a single antireflection coating layer, resulting in the best power conversion efficiency (17.8%) with Voc of 0.74 V and Jsc of 2.89 A if compared to tin oxide, magnesium oxide, and magnesium fluoride. CdCl2-treated CdTe films (annealed at a temperature of 400 C) indicated the highest refractive index, largest crystallite size, and excellent optical absorption coefficient. The photovoltaic parameters such as Jsc (13.6 mA/cm2), fill factor (53.9%), efficiency (4.9%), and Voc (668.4 mV) were highlighted. High-efficiency CdTe solar cells could be obtained via chlorine treatment under annealing process. Significantly, relative air humidity successfully reduces spatial uniformity by using CdCl2-treated solar cells than CHClF2-treated solar cells.

Gretener and coworkers [91]

Paudel and coworkers [92]

Echendu and coworkers [93]

Romeo and coworkers [94]

Devendra and coworkers [95]

Kumarasinghe and coworkers [96]

Lopez and coworlers [97]

Chalcogenides and their nanocomposites: fundamental, properties and applications

15

Spin coating was used to produce CZTS films as described by Eka and coworkers [103]. The formation of inhomogeneities and apparent secondary phases could affect deteriorating solar cell performance. Solar cells consisting of Mo/ Cu2ZnSnS4/CdS/ITO/Ag showed an efficiency of about 7.5% with a band gap of 1.43 eV. Thermal evaporation deposition technique has been used to produce CZTS films onto molybdenum-coated soda lime glass [104]. Solar cell was fabricated by using Mo/CZTS/CdS/ZnO, photovoltaic properties including FF (59.8%), open circuit voltage (541 mV), PCE (4.1%), and short circuit current density (13 mA/cm2) were reported. Tsukasa and coworkers claimed that the open atmosphere-type chemical vapor deposition technique was the best method to produce thin films [105]. The highest efficiency was achieved at 6.03% from the solar cell composed of Al/Al-doped ZnO/CdS/CZT(S,O)/Mo/soda lime glass. Jonathan and coworkers [106] demonstrated synthesis of thin films by using the electrodeposition method. Research findings showed more uniform morphology could be observed and the PCE reached 3.2%. Successive ionic layer adsorption and reaction technique has been used to prepare nanostructured films onto fluorine-doped tin oxide glass substrate [107]. Photovoltaic behaviors of the obtained films such as FF (0.62), efficiency (0.396%), open circuit voltage (390 mV), and short circuit current density (636.9 μA/cm2) were studied.

1.3

Antibacterial applications of thin films

At nanometer level, the dimensions are typically in the range from tens to a few hundred nanometers, the material structures exhibit considerably enhanced physiochemical properties, which significantly enhances their activity as compared to their bulk counterpart. The improvement in antibacterial activity is due to an increase in surface area of nanoparticles. Metals oxide nanoparticles retain good antimicrobial properties and have been used in surface coatings and release system [108
110]. Various metal chalcogenide colloidal NPs (ZnO, CdO, and CuO) showed significant antibacterial activity [111
118] against bacteria (gram-positive and gram-negative species). The bacterial toxicity mechanism due to most of the antibiotics is very well established, however, toxicity due to colloidal NPs is not completely identified. A general mechanism of NPs induced bacterial toxicity is the adhesion ability of NPs on the bacterial cell wall and disrupt the cell membrane. Another mechanism is the production of reactive oxygen species (ROS), which induces oxidative stress because of the formation of free radicals [111]. Cadmium oxide (CdO) cannot be used as an antibacterial agent due to its highlevel toxicity to human health. Many studies have been done to study the influence of inhalation of CdO on humans, animals, and plant health. All the investigations confirm that CdO is carcinogenic. It is also classified as a cancer-causing agent to human health by International Agency for Research on Cancer [119]. Copper and its oxide (CuO) hold good antibacterial properties. Since Cu has been widely used as doorknobs, bed handles, and parts of windows in hospitals, the surface become

16

Metal-Chalcogenide Nanocomposites

oxidized with the passage of time. According to Hassan and coworkers [120] the antibacterial activity of Cu2O films was high and can be comparable to that of the pure copper films. This investigation clearly suggests that oxide formation on copper objects would not significantly deteriorate the antibacterial activity. It has also been reported that bacteria causing nosocomial infections have been able to develop resistance to Cu and CuO in a similar way as they do to antibiotics. ZnO film has been reported as a promising and versatile material that could be employed in various applications. Some well-known applications in health and disease prevention include cosmetic, healing creams, ointments, sunscreen protector, and mouthwashes [111,112]. The US Food and Drug Administration (21CFR182.8991) demonstrates [121] that ZnO NPs are nontoxic and safe and promote their application in health care system and biomedical materials. However, according to European Union Regulation 10/2011, the ZnO NPs also have a negative effect on human health if utilize above critical concentration [122]. The situation becomes more critical when ZnO NPs overcome the barrier such as the skin, intestine, and lungs and are transported in mitochondria via cell membranes results in damaging the human cells and tissue. The ZnO NPs also have an adverse effect on the environment and ecosystem biosustainability. These NPs have a good chance to become deposited in aquatic ecosystems presenting an extensive threat to living organisms. The threat posed by ZnO NPs to human health and environment has been overcome by the development of antimicrobial coatings based on zinc oxide thin films [121,123]. The ZnO thin film may exhibit various microstructures such as morphology, surface roughness, porosity and crystallite/grain size, and different chemical compositions (stoichiometric ratios). The wide range of ZnO thin film properties can be achieved by modifying the above-mentioned parameters. All these parameters have a huge impact on the antibacterial activity. Such functional antibacterial coatings are helpful in lowering the microbial loads on the surface without any intervention from outside and hence will play a very important role in strengthening the disinfection regime of hospital environment [123
125]. So, by coating the surface with functional thin films of ZnO, it may be possible to break the nosocomial infection loop. The only problem that will be left is the transmission of bacteria from person to person, which can easily be fixed by using alcohol-based sanitizers and handwashing by healthcare workers. The functional antibacterial coatings should have good adhesion to the surface and be hard-wearing. There are a number of techniques for the deposition of ZnO thin films such as magnetron sputtering, chemical vapor deposition, atomic layer deposition, spray pyrolysis, and sol-gel methods [123,124]. Variety of surfaces can be coated with thin films of ZnO depending upon the type of application. For example, hospitals are a major source of most of the nosocomial infections, more than 80% of infections are caused by touching contaminated surfaces [123]. The most contaminated surfaces are doorknobs, bed handles, and parts of windows. They are generally made of copper (Cu) metal. The antibacterial properties of Cu are comparable with silver (Ag) and the advantages to use Cu are low cost and accessibility. It is reported that bacteria have been able to develop resistance to Cu and other metals in a similar way as they do for antibiotics [120]. So, the functional coatings of ZnO on those parts can enhance

Chalcogenides and their nanocomposites: fundamental, properties and applications

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the antibacterial activity without affecting the bulk properties (general mechanical behavior). The display glass panels can be easily contaminated with numerous kinds of bacteria and present a severe risk to patients, visitors, and healthcare workers. Since ZnO thin films are highly transparent in the visible region, so by coating ZnO one can easily improve the antibacterial properties of glass panels without compromising on the transparency [124]. For biomedical applications such as burn dressing, surgical masks, and wounds, there is a requirement for flexible substrates such as cotton fabrics with good surface antibacterial properties. These types of substrates are mostly unstable and can have harsh chemical environments, and it will be very difficult to coat them under different methods such as conventional chemical methods and physical coating techniques. The ZnO thin films can be grown on cotton fabrics using special techniques, these experiments were either carried out by solution methods or make use of external forces. Bioinspired method has also been used for the fabrication of functional coatings on cotton fabrics [126]. The antibacterial activity of ZnO thin films arises due to the electrostatic interaction of ZnO with bacteria cell wall, [127] release of Zn21 ions, and the formation of ROS. One of the possible antimicrobial mechanisms is the interaction of antimicrobial Zn12 ions with the media-containing microorganisms. It has been found that contribution of Zn12 ions in the antibacterial activity is minimum in dark conditions because the concentration of Zn species released from ZnO dissolution is very low. The insolubility of ZnO in the medium also restricts the distribution of Zn ions, which also affects the antibacterial activity. According to the investigation carried out by Pasquet and coworkers [128], the release of Zn21 ions from ZnO has been influenced by two most important parameters: (1) The chemistry of medium (UV illumination, exposure time, the pH, and presence of additional elements), (2) physiochemical properties (concentration, morphology, particle size and porosity). The production of ROS is also responsible for the enhancement in antibacterial activity by producing oxidative stress and directly destroying the unsaturated fatty acids of bacterial cell membranes [124,125]. The concentration of ROS increases rapidly on illuminating ZnO with photons of UV radiations. Since ZnO is a direct wide bandgap semiconductor with an optical bandgap of around 3.25 eV. On illuminating the surface of ZnO with photons having energy more the optical band gap results in the formation of electron-hole pair. These photogenerated electron-hole pairs will further react with H2O and O2 for the formation of ROS according to the reactions given below [129]: 1 ZnO 1 hv ! ZnO 1 e2 cb 1 hvb

(1.1)

d 1 h1 vb 1 H2 O ! OH 1 H

(1.2)

d2 e2 cb 1 O2 ! O2

(1.3)

1 d2 Od2 2 1 H ! HO2

(1.4)

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Metal-Chalcogenide Nanocomposites

d2 HOd2 2 1 HO2 ! H2 O2 1 O2

(1.5)

1 2 HOd2 2 1 H 1 ecb ! H2 O2

(1.6)

d 2 H2 O2 1 Od2 2 ! OH 1 OH 1 O2

(1.7)

d

OH 1 microbialcell ! Intermediates ! CO2 1 H2

(1.8)

d Superoxide (Od2 2 ) and hydroxyl radical ( OH) disintegrate microbial cells into CO2 and H2 O. However, H2 O2 could easily destroy the microbe by penetrating into the cells. The production of H2O2 primarily lies on ZnO’s surface, causing a large number of oxygen species on the surface that have higher antimicrobial activity [129].

1.4

Future perspective/outlook

From the above discussion, it is clear that the incorporation of semiconductor NCs with perovskite films showed potential for photovoltaic advancement. However, the fabrication and application of perovskite nanocrystalline-based solar cells are still in the early stage and need systematic investigations to understand the mechanism of optoelectronic properties, colloidal stability, and morphology of the NCs toward the PCE and stability of the fabricated solar cell devices. Further, systematic and comprehensive investigations from different perspectives such as surface chemistry, surface passivation, interfacial engineering, and device architecture are also essential in this promising but relatively new research field. On the other hand, thin filmbased solar cells will receive great attention in the future due to lower production costs and installation fees. However, researchers should work harder to improve the performance of solar cells. New metal chalcogenide materials should be produced by using various deposition methods. The photovoltaic properties of these films should be studied under various experimental conditions. The antibacterial activity induced by metal chalcogenide coatings/nanoparticles is not yet clear. Numerous investigations propose that ROS or oxidative stress is responsible for antibacterial activity while some other studies suggest that mechanism associated with antibacterial activity might not be related to bacterial metabolism. So, in future, it is worth investigating the antibacterial mechanism induced by metal chalcogenide. Moreover, very few studies have addressed the effect of metal chalcogenide on protein synthesis, gene expression, and bacterial cell metabolism. There are also very limited investigations related to intracellular inhibitory mechanisms. SCs have gained significant interest because of excellent power density and very fast charge-discharge rate. SCs are commonly utilized in portable/wearable electronics applications alongside batteries and other energy storage devices. Furthermore, flexible SCs have addressed the growing demands for flexible wearable electronics

Chalcogenides and their nanocomposites: fundamental, properties and applications

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systems in the energy storage industries owing to their special advantages including flexibility and lightness. In recent years, there has been significant progress in the development of sensors (micro and nanotechnologies). This new technology proposes high surface-tovolume ratio manufacturable frameworks with small dimensions, lower power utilization and high reliability. The progression of future tendencies resulted from the market-economical features, and very specific requirements of the board application.

1.5

Conclusion

Chalcogenide nanomaterials have received great attention because of some very unique properties. In this work, several applications of thin films were reported. Based on the experimental results, a thin film-based SC could be used to store the electrical energy in order to stabilize the power supply in all electronic devices. Perovskite solar cells and thin film-based solar cells have been studied. These films showed a suitable band gap, higher PCE, and large absorption co-efficiency value, which could be used to replace silicon-based solar cells. MO nanoparticles retain good antimicrobial properties and have been used in surface coatings and release system. Many functional nanomaterial-based devices with exceptional sensing demonstration to several chemical and biological species in both solid circumstances and solution phases have been demonstrated. Nanostructure materials in particular are accomplished by sensing a slight gas molecule such as CO, H2S, CO2, LPG, SO2, NH3, NO2, and O2.

Acknowledgment The author (HO SM) gratefully acknowledges the financial support provided by the INTI International University.

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2860. [121] P. Silva, A. Barbosa, D. Costa, M. Rodrigues, P. Carvalho, J. Borges, et al., Antifungal activity of ZnO thin films prepared by glancing angle deposition, Thin Solid Films (2019). Available from: https://doi.org/10.1016/j.tsf.2019.137461. [122] Commission Regulation (EU) No 10/2011 of 14 January 2011 on plastic materials and articles intended to come into contact with food. [123] D. Ali, M. Butt, I. Muneer, F. Bashir, M. Hanif, T. Khan, et al., Synthesis, characterization and antibacterial performance of transparent c-axis oriented Al doped ZnO thin films, Surf. Interfaces (2021). Available from: https://doi.org/10.1016/j.surfin.2021.101452. ´ . Ramı´rez, R. Rosales, R. Nava, C. Jota, New infrared-assisted method [124] B. Penguelly, A for sol-gel derived ZnO:Ag thin films: Structural and bacterial inhibition properties, Mater. Sci. Eng. C Mater. Biol. Appl. 78 (2017) 833
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[125] K. Park, G. Han, K. Neoh, T. Kim, J. Shim, Antibacterial activity of the thin ZnO film formed by atomic layer deposition under UV-A light, Chem. Eng. J. 328 (2017) 988
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Chilukoti Srilakshmi Department of Chemistry, GITAM School of Science, GITAM (Deemed to be University), Bengaluru, Karnataka, India

2.1

Introduction

Chalcogens (O, S, Se, and Te) have attracted considerable attention for materials applications in the last few decades. A binary compound of a chalcogen with a more electropositive element or radical is called chalcogenide. Metal chalcogenides exhibit narrower band gap values in contrast with most of the binary metal oxides, which makes them responsive to visible light absorption. Metal sulfides are the most common chalcogenide materials studied because of their narrow band gap values (1.3
2.40 eV) that are advantageous due to their capability for solar energy utilization, dye degradation, heavy metal removal, and CO2 reduction. Chalcogenides are classified according to the number of components in binary, ternary, and quaternary systems. Both binary and ternary chalcogenides have been more thoroughly studied than quaternary analogs despite their interesting properties [1]. These chalcogenides with nanocomposites are used for various applications such as photocatalytic degradation, heavy metal removal, CO2 reduction, and Electrocatalysis. In addition, nanodots (quantum dots)/nanocomposites of metal chalcogenides show stronger edge effects and the quantum confinement effect that makes it possible to utilize under environmental remediation. Photodegradation of aqueous organic pollutants is a very promising strategy to address environmental issues and energy problems. Among all reported photocatalysts, crystalline metal chalcogenides not only possess diverse architectures that can be enriched by integrating different metal ions and templates but also have narrower band gaps (visible light adsorption) and suitable band positions that can be tuned through composition regulation. Therefore, the application of crystalline metal chalcogenides as efficient photocatalysts has attracted much attention. However, the limited synthetic methods, low degradation efficiency, and poor chemical stability are major challenges to impede their practical application. In this chapter, the recent progress on employing transition metal chalcogenides (TMCs) as visible-lightdriven catalysts for the photodegradation of organic contaminants is summarized.

Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00002-X © 2024 Elsevier Ltd. All rights reserved.

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Besides that, some of the synthetic methods to prepare metal chalcogenides are also discussed [2
12]. Another major problem is that the environment has been deeply affected by many pollutants, of which toxic heavy metal ions such as Hg21, Pd21, and Cr61 have become a serious environmental issue to our society because of their low biodegradability and easy accumulation. The hexavalent Cr (VI) is viewed as a representative pollutant among these heavy metal ions, which typically exhibit high toxicity and solubility in water. These pollutants result from numerous industry areas including leather tanning, steel manufacturing, and electroplating [13
15]. To solve this contamination problem of heavy metal, different types of chalcogenides and nanocomposites such as MoSe2 materials are used [16]. The photoreduction of CO2 has also fascinated research interest due to the enhanced rate of CO2 emission, which results in global warming. To avoid such consequences, photocatalytic CO2 reduction is being researched by several authors. Various materials are used but the best results are found from the semiconductor chalcogenides and their nanocomposites, which are discussed in this chapter. Recent publications on the chalcogenides as electrode materials, environmental remediation of toxic pollutants, photocatalytic dye degradation, and electrocatalysis have been highlighted [17
19]

2.2

Experimental

Several methods are reported for the synthesis of transitional metal chalcogenides [17
28], including the hydrothermal method, ultrasonic chemical method, sonication method, simple microwave-assisted solvothermal process, reverse micelle process, and surface modification methods, respectively. Some of them are discussed here as follows.

2.2.1 Hydrothermal method Pourabbas et al. in 2008 reported the synthesis of chalcogenides and their nanocomposites by the modified hydrothermal method as follows [20]. An aqueous solution of ammonium heptamolybdate, ((NH4)6Mo7O24  4H2O, 1 g) and thiourea (H2NCSNH2, 0.9 g), was prepared by dissolving in 50 mL distilled water. 1Octanol (1.5 mL) and sodium lauryl sulfate (SDS, 0.7 g) were added into a 50 mL Teflon-lined autoclave reaction vessel. The autoclave was filled with the aqueous solution up to 75% of its total volume and sealed for annealing under continuous stirring for 5 h at 180 C. The autoclave was then gradually cooled down to room temperature; and the black precipitate was separated by filtration, washed with ethanol, and then washed with distilled water several times. Finally, it was dried for 6 h at 80 C under reduced pressure.

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2.2.2 Sonication method Sonication is the act of applying sound energy to agitate particles in a sample for various purposes. Nanosheets of WS2 and MoS2 synthesized by Mishra et al. in 2015 from bulk WS2 and MoS2 using the sonication method via a solid probe sonicator (Sonics Vibra Cell Sonicator VC 750
1/8v tapered micro tip, 750 W maximum power, and 20 kHz frequency) is given as below [26]: Bulk WS2 and MoS2 (both from Sigma Aldrich) were stirred separately in 50 mL distilled water mixed with 25 mg of household detergent (ultratide) for 20
30 min at 40 C
50 C. Thereafter the solutions were sonicated for 3 h by a solid probe sonicator. The resultant solutions contain TMD nanosheets, micronthick sheets, and residue bulk materials. The solutions were allowed to sediment for 3 h to remove the residue bulk and further centrifuged at 1000 rpm for 30 min to remove the thick sheets. Resultant solutions contained well-dispersed WS2 and MoS2 nanosheets. Powder samples were obtained by vacuum filtration and repeated washing, followed by drying in air.

2.2.3 Microemulsion method This emulsion contains three components oil, water, and surfactant, which form a thermodynamically stable, single-phase isotropic transparent solution. The reacting reagents are present in nano water droplets surrounded by surfactant molecules. These water droplets containing reagents coalesce rapidly allowing the mixing and precipitating of the nanoparticles. Monodispersed nanoparticles of different morphologies and sizes can be synthesized by this technique. The synthesis of Ag2Se by the microemulsion method by Buschmann et al. is discussed as follows [27]. The method involves mixing of microemulsion containing AgNO3 as precursor salt and a selenium salt solution, Aerosol OT (AOT or sodium bis(2-Ethylhexyl) sulfosuccinate), acting as a surfactant, and water. An aqueous solution of AgNO3 is added to the AOT solution in heptane. The microemulsion formed in this way contains the precursor salt in its water droplet. A solution of bis(trimethyl)selenium in heptane is added to the initial microemulsion and stirred until a stable state is reached. Different concentrations of AgNO3 (0.25,0.125 and 0.063 M) are used to influence the final size of the nanocrystals. The concentration of bis(trimethyl)selenium in heptane is required to adjust for a stoichiometric matching with respect to the AgNO3 concentration.

2.2.4 Solvothermal method Qian and coworkers [28
30] reported a novel solvothermal approach to prepare a series of metal chalcogenides by reactions between metal oxalates and chalcogens in organic solvents at temperatures ranging from 120 C to 180 C. Nanocrystalline

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Metal-Chalcogenide Nanocomposites

Ag2E [28], CdE [29], PbE, and Bi2E3 [30] (E 5 S, Se, Te) have been synthesized via this route. The reactions can be expressed as follows:

Both temperature and solvent have significant effects on the synthesis of the metal chalcogenides. By changing the preparation conditions, control over the size and morphology of the products could be realized.

2.2.5 Sol-gel method Stanic et al reported the sol-gel synthesis of ZnS as follows: In the method, zinc tert-butoxide in butanol and hydrogen sulfide gas were used as precursors, and toluene was used as a solvent. The zinc tert-butoxide and toluene were mixed under the dried nitrogen atmosphere of a glove box. The concentration of the solution was maintained 0.024 M. Before H2S gas was introduced into the solution, it was passed through a column filled with drierite. The gas was bubbled through the toluene/butoxide solution until complete gelation had occurred. The obtained gel enclosed in the reactor was aged for 24 h and then dried in a vacuum oven at room temperature [31].

2.3

Results and discussion

2.3.1 Sulfides (S) chalcogenides Pourabbas et al. in 2008 studied the photo-oxidation of phenol over MoS2/TiO2 hybrids using visible light, and the corresponding graphs are shown in Fig. 2.1. It was observed that neither Tiona nor P25 (types of TiO2) was active in removal of phenol under the applied conditions even after 240 min of reaction time because the band gap, 3.2 eV, of TiO2 falls into the ultraviolet (UV) region, 385 nm. Therefore, it is not expected for TiO2 to be an active photocatalyst in visible wavelengths. On the other hand, the same authors reported no photocatalytic activity in rutile form of TiO2. However, it was observed that hybrid catalysts P25/MM/2 and T/MM/2 were able to activate the photo-oxidation decomposition of phenol under the illumination of visible light as shown in Fig. 2.1. They explained this due to the synergic effect of MoS2 and TiO2 in the photo-activity of the hybrid catalysts. However, the phenol concentration declined from 20 ppm initial concentration to almost zero after 25 min by using P25 or P25/H/2 [20].

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Figure 2.1 Concentration-irradiation time diagrams for photo-oxidative removal of phenol under illumination of visible light (A) in the case of using P25 and P25/MM/2 hybrid catalyst and (B) in the case of using Tiona and Tiona/MM/2.

Next, Mishra et al. worked on sulfide chalcogenides and their nanocomposites. They demonstrated the visible light-responsive photocatalytic activity of WS2 and MoS2 nanosheets on the decomposition of brilliant green (BG) dye. BG aqueous solution was treated with nanosheets in the dark and under white light illumination without stirring or sonication. They observed that the physical adsorption mainly takes place within 1 h and reaches saturation in 24 h treatment in the dark, resulting in nearly 33% and 39% of BG reduction with WS2 and MoS2 nanosheets, respectively. At the same time, they worked on the visible light irradiation for 10 h and found 95% and 91% BG reduction with WS2 and MoS2 nanosheets, respectively (Fig. 2.2). It was concluded that active stirring/sonication of the solution under visible light irradiation may facilitate the contact between BG and nanosheets, further increasing the decomposition rates [26]. In another study, Zhu et al. synthesized graphene-like MoS2/Ag3VO4 composites in a simple and facile two-step method and studied the photocatalytic degradation of methylene blue (MB) and rhodamine B (RhB) degradation using visible light and compared the results with the pure Ag3VO4 [32]. Among all the samples, one with 7 wt.% graphene-like MoS2/Ag3VO4 showed excellent photocatalytic activity, and it was regarded as the optimal proportion. The better photocatalytic activity of MoS2/Ag3VO4 composites compared to Ag3VO4 alone is mainly due to synergistic effects and electron divert between graphene-like MoS2 and Ag3VO4. These chalcogenide nanocomposites are also explored as photocatalysts in sewage treatment. Pathania et al. in 2011 fabricated CdS nanoparticles by using bovine serum albumin as a stabilizing agent at 70 C to decompose MB dye, which showed a decomposition rate of up to 70% and 65% in two hours in the presence of visible sunlight and sodium lamp and found that the photocatalytic activity of CdS nanoparticles is more in presence of visible sunlight than sodium lamp irradiations [33]. Zhang et al. in 2011, developed size-controlled SnS2 nanoparticles and studied the photocatalytic degradation of methyl orange dye. And they reported approximately 100% MO degradation in 60 min under visible light (k . 420 nm) indicating that it has excellent photocatalytic degradation capability [34].

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Metal-Chalcogenide Nanocomposites

Figure 2.2 Photocatalytic effect of WS2 and MoS2 nanosheets. Absorbance spectra and concentration variation of BG solution treated with (A, C) WS2 and (B, D) MoS2 nanosheets. Inset images show the corresponding untreated and treated samples.

2.3.2 Selenide (Se) chalcogenides Muzakkar et al. in 2019 studied Chalcogenide material used for the RhB degradation. They have synthesized the selenium (Se) doped TiO2/Ti electrode as a novel chalcogenide material by anodizing and dip-coating methods. Se-TiO2/Ti electrode has honeycomb structure with the element composition of Ti (51.09%), O (36.91%), and Se (12.00%), respectively. Optical properties demonstrated that Se-TiO2/Ti electrode was active under visible light, meanwhile, the TiO2/Ti was active under UV light [35]. According to their studies, as shown in Fig. 2.3, photolytic degradation, that is without the use of a catalyst, in the presence of UV light showed degradation of 20.74%. Subsequently, the photocatalytic process using a catalyst showed 49.31% degradation under UV light. Photocatalytic degradation test was observed by applying an electrode to RhB in photoelectrocatalytic with multipulse amperometry technique, which resulted as 5 ppm RhB corresponded to the degradation of 73.27%.

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Figure 2.3 Comparison of percent degradation by photolytic, photocatalytic, and photoelectrocatalytic activity.

Further, Kaur et al. in 2019 studied SnSe-/SnO2-based nanocomposites synthesized by chemical coprecipitation method. These nanocomposites were vacuumannealed at 400 C to improve crystallinity. Photocatalytic activities were carried out using industrial waste dyes such as MB and RhB. The photodegradation of these dyes under exposure to sunlight is found to be . 90% in 90 min [36]. Comparable, S. Kumar et al. in 2021 studied chalcogen (Se21)-doped ZnO nanoparticles (SeZO-NPs) synthesized by using sol-gel precipitation method, and the photocatalytic degradation of the RhB was measured under the illumination of UV light. The double donor (Se) played an important role in the photodegradation of RhB by reducing the recombination of charge carriers. It showed the highest photocatalytic degradation of 98.23% and mineralization for the sample 5-SeZO (Se: 5 wt.%). They reported that the improved photocatalytic performance of 5-SeZO was attributed to the optimum Se dopant concentration for the production of more reactive oxygen species because of the effective separation of charge carriers in UV light [37,38].

2.3.3 Tellurides (Te) chalcogenides Even though Te as a chalcogenide has less research publication; recently, K. Mahendra et al. presented a successful synthesis of Si and Sn doped tellurium alloy (Te90Si5Sn5) in 2022 [39]. The Si
Sn doping in Te is confirmed from PXRD (Fig. 2.4) and EDAX spectra and the elemental mapping shows uniform distribution

36

Metal-Chalcogenide Nanocomposites

Figure 2.4 PXRD spectrum of the synthesized Te90Si5Sn5 alloy.

of the dopants. Characterization of the sample was done by BET for surface area calculated as 155.608 m2/g and TEM measurements observed spherical particles with a spacing of 0.54 nm. Photodegradation of MB and crystal violet dyes were 89% and 95% at time intervals of 100 and 40 min, respectively, indicating Te90Si5Sn5 alloy as an efficient visible-light photocatalyst. Apart from this, there is one more element that is compatible with all these three chalcogens (S, Se, and Te) known as lead (Pb). Qiao. L.N et al. in 2016 studied lead chalcogenides (PbX, X 5 Te, Se, S) synthesized via a simple hydrothermal method. As lead chalcogenides display efficient absorption in the UV-visible light range, their photocatalytic properties were evaluated for the photodegradation of Congo red. The photocatalytic experiments of these powder samples were performed with 0.1 g powder samples suspended in 100 mL aqueous Congo red (110 mg/L). When PbS powder was used as a catalyst, it was observed that 15% Congo red decomposition in 60 min under UV light irradiation, while around 90% degraded under visible light in 120 min (especially in the former 60 min). Likewise, the degradation of Congo red by PbTe particles under UV and visible light was about 20% and 70%, respectively. In contrast, PbSe exhibited very low activity and only 15% and 30% decomposition of Congo red observed under UV and visible light, respectively. In general, the photocatalytic activities of these samples decrease in the series PbS . PbTe . PbSe [40]. Correspondingly, the band gaps of lead chalcogenides change in the same way as shown in Fig. 2.5C and the photocatalytic capabilities of PbX (X 5 S, Te, Se) increase with increasing the band gap. Lead chalcogenides showed lower photocatalytic efficiencies under UV light because of the weak light intensity as they have used UV light, which comprises a single wavelength of 365 nm, resulting in significantly weaker light intensity. Comparatively, all of these powder samples showed effective photocatalytic degradation capability under visible light. They concluded that Congo red solution can be efficiently degraded under visible light in the presence of lead chalcogenides nanoparticles as the photocatalytic activities increase with increasing band gaps. Their result showed no appreciable

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Figure 2.5 Photocatalytic degradation of Congo red solution in the presence of powder samples under different irradiation (A) UV light, (B) visible light, and (C) the schematic diagram of the band structures of lead chalcogenides.

loss after repeated cycles, which were useful for developing new photocatalyst systems responsive to visible light among narrow band gap semiconductors.

2.3.4 CO2 reduction CO2 reduction using Chalcogenides and their nanocomposite for environmental remediation was experimented by Liu et al. in which Defect-free Pebax-MoS2 membranes were fabricated by the solution casting method, and the MoS2 loading was varied from 0 to 5.66 wt.%. CO2 permeability and CO2/N2 selectivity increased as MoS2 addition increased. As in this study, a Pebax membrane with 4.67 wt.% MoS2 nanosheet doping was found to have the highest CO2 permeability of 67.05 Barrer and CO2/N2 selectivity of 90.61. Using the metropolis Monte Carlo molecular simulations, it was shown that the solubility selectivity of CO2 and the CO2/N2 in the Pebax membranes were significantly improved after the incorporation of MoS2 nanosheets owing to the high affinity of CO2 to MoS2 nanosheets [41]. In another study, Shi. L et al. in 2020, reported Atomic Cobalt Species (ACS) anchored 2D tellurium nanosheet (Te NS), which can act as a highly active single-atom cocatalyst for boosting photocatalytic H2 production and CO2 reduction reactions under visible light irradiation. Te NS serves as the ideal support material to

38

Metal-Chalcogenide Nanocomposites

Figure 2.6 (A) CO and H2 evolution from photocatalytic CO2 reduction under different conditions (the reaction time is 2 h). (B) Cycling test of [Ru(bpy)3] Cl2/Te NS-ACS for photocatalytic CO2 reduction, and each cycle is conducted for 2 h.

bridge the light absorbers and ACS catalytic sites for efficient electron transfer. For promoting photocatalytic reactions, strong mutual interaction between the Te NS and ACS changes the electronic structure of Te NS by inducing the intermediate energy states, which act as trap sites to accommodate the photogenerated electrons [42]. CO and H2 evolution performances from photocatalytic CO2 reduction under various reaction conditions for 2 h are summarized in Fig. 2.6. The bare [Ru(bpy)3] Cl2 only exhibits the CO evolution amount of 4.5 μmol and H2 evolution amount of 6.7 μmol, and the bare Te NS is inactive in photocatalytic CO2 reduction reaction. When Te NS was added into the system, the evolution amount of H2 slightly increased while the generation of CO was suppressed. Their result indicates that the Te NS is not a promising cocatalyst for photocatalytic CO2 reduction because of its low CB position, which cannot trigger the CO2 to CO conversion reaction. It was reported that CO and H2 evolutions were observed, reaching 104.5 and 84.9 μmol within 2 h, respectively. The obtained CO evolution rate overtakes the catalysts under comparable conditions [39
41], whereas CO and H2 evolutions were completely terminated in the absence of [Ru(bpy)3]Cl2, indicating the catalytic effect of Te NS-ACS in the photocatalytic CO2 reduction reaction. Apparently, it is the ACS that acts as the active site for superior CO and H2 evolution. Hence, as shown in Fig. 2.6B, the recycled Te NS-ACS cocatalyst retains 93.3% of the original activity after four runs, indicating good stability for photocatalytic CO2 reduction (Shi et al., 2020).

2.3.5 Heavy metal removal Chalcogenides and their nanocomposites also showed an excellent result in removal of heavy metals [43,44]. Fang et al. in 2018 studied the photocatalytic activities of the ZnFe2O4/CdS nanorods composites by the photocatalytic reduction of Cr(VI) in an aqueous solution under visible light (λ . 420 nm) irradiation at room temperature as shown in Fig. 2.7. They obtained good results with pH 2 and observed

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Figure 2.7 Photocatalytic activities of 7%-ZnFe2O4/CdS composite for the Cr (VI) reduction in the aqueous solution with different pH values under simulated sunlight.

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Metal-Chalcogenide Nanocomposites

that under acidic conditions, the surface of the photocatalyst becomes highly protonated and more positive, which gives better access to accumulate HCrO42 ions. Whereas, under alkaline conditions, the photocatalyst surface becomes negative, which leads to the repulsion of Cr2O722 and thus reduces its photocatalytic activity. Furthermore, at higher pH, Cr (OH)3 deposited on the surface of the photocatalyst is another possible factor reducing photocatalytic activity [16]. In another work by Chu et al. in 2016, MoSe2 samples were successfully synthesized via a facile solvothermal method, and the photocatalytic activity in the reduction of Cr (VI) under UV, visible, and NIR light irradiation was studied. The results showed that MoSe2 exhibits excellent photo-absorption in the whole light region, which displays good photocatalytic activity with Cr(VI) reduction rates of 99%, 91%, and 98% at 180 min under UV, visible, and NIR light irradiation, respectively, and the enhanced photocatalytic activity is attributed to the comparatively higher light absorption, and efficient charge separation and transfer, as well as the relatively large number of surface active sites. Lastly, the photo-generated electrons play a crucial governing this photocatalytic process [45].

2.4

Conclusion and future perspectives

This chapter summarizes the progress of metal chalcogenides as visible-light-driven photocatalysts for various pollutants’ degradation. A brief discussion on the traditional synthetic methods is also included. Among all materials employed as photocatalysts for dye degradation, some of these tests quickly decolorized even if a small amount of metal chalcogenide photocatalyst was used for investigation, and some photocatalysts displayed highly-selective photocatalytic ability to one specific dye. Although reasonable research progress has been made on crystalline metal chalcogenides as photocatalysts for environmental remediation, there are still many issues to be addressed. For example, most photocatalysts need several-hour visible light illumination to complete degradation of pollutants, indicating their low photocatalytic efficiency. Hence, it is an urgent need to enhance the photodegradation efficiency of chalcogenides. There are two solutions: (1) the nanolization of known chalcogenide-based photocatalysts and (2) the design and preparation of novel metal chalcogenides composites with more suitable band gap and band position. In light of the promising properties of metal chalcogenides as photocatalysts as well as their recent achievements, there is plenty of room left to realize the photodegradation. Clearly, the intensive research on the degradation of pollutants over chalcogenide photocatalysts will be continued and their advances should bring more benefits to the environment and energy utilization.

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[37] S. Kumar, S.K. Sharma, R.D. Kaushik, L.P. Purohit, Chalcogen-doped zinc oxide nanoparticles for photocatalytic degradation of Rhodamine B under the irradiation of ultraviolet light, Mater. Today Chem. 20 (2021) 100464. [38] G.C. Hoover, D.S. Seferos, Photoactivity and optical applications of organic materials containing selenium and tellurium, Chem. Sci. 10 (40) (2019) 9182
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10.

Chalcogenides and their nanocomposites in photocatalytic reactions

3

Nagaraju Kerru1 and Suresh Maddila2 1 Department of Chemistry, GITAM School of Science, GITAM University, Bengaluru Campus, Bengaluru, Karnataka, India, 2Department of Chemistry, GITAM School of Science, GITAM University, Visakhapatnam, Andhra Pradesh, India

3.1

Introduction

The rapid development of material science in the scientific community is increasing broadly day by day, and exploring new methods [1], which are involved in mild operations for making nanosized composite materials with significant properties and well-behaved morphologies and dimensions, is a great challenge [2,3]. Over the years, chalcogenide and chalcogenide-based semiconductor materials have drawn substantial attention due to their thermal stability, low toxicity, narrow band gap, and tunable optical and surface properties [4,5]. These materials encompass one chalcogen ion (S22, Se22 and Te22) and one or more electropositive atoms, which are well-experienced for their narrow band gap energies [6
9]. The convention of chalcogenides composite materials in photocatalytic studies has been acquired broadly due to their narrow band gap that facilitates the competent mowing of visible light [10]. In recent years, hydrogen (H2) has been cutting edge as a highly efficient energy vector. It is generated by employing a diversity of technologies and feedstocks [11,12]. The damaging influence of standard fossil fuel work on environmental pollution, the rising demand for energy, and the production of hydrogen energy is a prospective tenacity to the sustainable environment with the existing energy systems [13,14]. Chalcogenides have drawn attention in photocatalyst-catalyzed hydrogen evolution reactions [15
17]. The production of H2 through photocatalysts is a highly viable approach for the water-splitting process and has been receiving growing attention in clean energy technologies [18]. Metal-based chalcogenides as efficient photoactive semiconductors are considered great candidates for developing appropriate photocatalytic hydrogen production systems [19
22]. Bahti et al., synthesized and characterized the aluminum-based mono-chalcogenides Al(Ga, In)STe and showed high photocatalytic activity [23]. Inta and coworkers have developed the interfacial structure of Ni0.85Se/MoSe2 photocatalyst by solvothermal method, which delivers a current density of 10 mA/cm2 at a cell potential of 1.7 V [24]. The development of 2D MoSe2 can significantly enhance the photocatalytic Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00003-1 © 2024 Elsevier Ltd. All rights reserved.

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Metal-Chalcogenide Nanocomposites

Figure 3.1 Last decade’s publication of data on chalcogenides and their nanocomposites. Metal chalcogenides in photocatalyst applications, Available online: https://www.scopus. com/sources.uri (accessed on 30 January 2023).

performance of hydrogen production with a current density of 10 mA/cm2 at 0.130 V [25]. The effect of NiSe on the MoSe2 nanohybrid is found to be an outstanding catalytic activity with a low onset potential of 2150 mV [26]. The challenge is the growth of narrow bandgap chalcogenide photocatalysts capable of driving water splitting under irradiation methods [27,28]. Substantial development has been achieved in metal chalcogenide composite materials (Fig. 3.1). This chapter aims to emphasize an overview of the metal chalcogenide nanocomposites and their photocatalytic studies.

3.2

Chalcogenides for the photocatalytic hydrogen evolution

Metal chalcogenide composites with chalcogen anion (S, Se, and Te) are an intriguing class of nanomaterials and are extensively used for the water-splitting photocatalytic process. Water splitting is involved the H2O particles being fragmented into hydrogen and oxygen, in which a photocatalytic activity of chalcogenides plays a vital role in the water splitting photocatalytic method. The following stages are occupied in this reaction; (1) the photocatalyst absorbs photons and creates photoexcited electron and hole pairs (e2/h1); (2) the individual and drift to the catalyst surface; and (3) photoproduced transporters are adsorbed on the catalyst surface and are oxidized to develop oxygen and reduced to develop hydrogen [29,30]

Chalcogenides and their nanocomposites in photocatalytic reactions

47

Figure 3.2 Steps occupied in the water splitting photocatalytic method.

(Fig. 3.2). Further, the cocatalyst doped on the shallow chalcogenide photocatalyst offers active sites and reduces the activation energy needed for H2/O2 development.

3.2.1 General synthesis approaches of chalcogenides Synthesizing chalcogenide materials with potent photo catalytically active is challenging, and various methods have been reported [31
38]. Table 3.1 emphasizes the distinct protocols of metal chalcogenide synthesis. Hydrothermal implies under an aqueous medium at higher pressure and temperature. For example, the

Table 3.1 Different preparation methods of chalcogenides. Precursors and media

Nanocomposite

Method (References)

Cu(NO3)2, Na2WO4, L-Cysteine Water. Cu(NO3)2, Cd(CH3COO)2Na2S2O3, (CH2OH)2. Cd(CH3COO)2, Sulfur powder, dodecylamine. Cd(CH3COO)2, Zn(CH3COO)2, Na2S, Water. Copper(II) acetylacetonate, Sn(OAc)4, Zinc acetate. Copper acetate, Thiourea, Water Ni(NO3)3, Thioacetamide, Water Zn(NO3)2, Na2S, Water

Cu2WS4 CuCdS2

Hydrothermal [32] Solvothermal [39]

CdS Cd0.5Zn0.5S Cu2ZnSnS4

Solvothermal [37] Hydrothermal [40] Hot injection [36]

CuS NiS2 ZnS

Zn(NO3)2, Cd(NO3)2, Thioacetamide, Water

Zn0.5Cd0.5S

Hydrothermal [34] Hydrothermal [38] Coprecipitation [31] Combustion [41]

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Metal-Chalcogenide Nanocomposites

Cd0.5Zn0.5S material was synthesized at various ranges of temperatures (120 C
200 C) from starting compounds of Cu(NO3)2, L-Cysteine, and Na2WO4 for 24 h by Kannan and coworkers. A low-temperature liquid technique is used to synthesize flower-like morphology Mn12 doped AgInS2 nanocomposite. The photocatalyst has proven remarkably with an increase in the H2 production rate. The chemical vapor deposition (CVD) method has been reported by Gopalakrishnan and coworkers, which was used for the synthesis of VS2 catalyst aligned with Si. Similarly, Wang et al. has been synthesized CdS photocatalyst by the solvothermal method.

3.2.2 Chalcogenides and their photocatalytic activities 3.2.2.1 Molybdenum-based chalcogenides A two-dimensional molybdenum disulfide (MoS2) chalcogenide material is trending to display more incredible photocatalytic characteristics than its corresponding individual entities [42,43]. Li et al. explored the photocatalytic study of MoS2 and Ag-modified MoS2 nanomaterials. As synthesized, Ag-modified MoS2 chalcogenide showed superior photocatalytic activity for the H2 production (2695 mol h/g), which compared to the MoS2 chalcogenide, which may be enhanced visible-light assimilation with improving Ag substance [44]. In another study, Tang and coworkers investigated the production of H2 under visible light irradiation by MoS2/ Zn0.5Cd0.5S/g-C3N4 as a photo corrosion-resistant composite. The maximum rate of H2 production is found to be 4917 mol h/g in the presence of a sacrificial solution, Na2S-Na2SO3. The two-dimensional photocatalytic MoS2-CuZnIn2S4 was developed using the solvothermal protocol by Yuan and coworkers [45], which is used to produce H2, and the evolution rate is obtained at 5463 mol h/g under visible radiation conditions. The superb execution is attained due to the excellent absorption of visible light and ample more active sites, applied for the H2 production reaction. An improved photocatalytic (MoS2/C) H2 development was accomplished by Yin et al., with a rate of evolution is 872.3 mol h/g by applying MoS2/ C chalcogenide [46]. The superior presentation was ascribed to the effective photogenerated charge carrying and disconnection among the MoS2 and Erythrosin-B. Hassan and coworkers have investigated the efficiency of another photocatalyst (MoS2/GaN2) under visible light conditions [47]. As-synthesized photocatalyst attained active light collecting by a current density is 5.2 mA/cm2, which is B2.6 cycles more excellent than the pure GaN. Additionally, MoS2/ GaN2 composite demonstrated increased conversion effectiveness of 0.91%, compared to the bare GaN (0.32%). The MoS2 catalyst has occupied considerable interest in photocatalytic studies because of efficient charge carriers and partition of the generation of the electron-hole pairs. Zhao and coworkers have reported a surface-modified photocatalyst (MoSe2) used for H2 production. Nickel-doped MoSe2 has shown efficient superb photocatalytic studies in both acidic and alkaline mixtures.

Chalcogenides and their nanocomposites in photocatalytic reactions

49

3.2.2.2 Zinc based chalcogenides Kurnia et al. explored the photocatalytic application of H2 production by using ZnS as a photocatalyst [48]. The photocatalyst ZnS exhibits superb activity for H2 production in visible radiation and the band gaps of B2.4 eV. Other heterostructure efficient photocatalysts, ZnO/ZnS and ZnS/ZnO/ZnS, have been developed, which are utilized for H2 production under visible radiation and have and significant band gap of B2.72 eV [49,50]. Comparison of ZnO coating with the ZnS photocatalyst exhibited improved water-splitting activity. Arai and team prepared Cu-loaded ZnS nano material photocatalysts under visible radiation for the decay of H2S to the production of H2. The Cu@ZnS material is highly efficient under irradiation visible light, and the evolution rate of H2 was improved at 2 wt.% Cu/ Zs photocatalyst.

3.2.2.3 Copper based chalcogenides Base and coworkers have developed a highly efficient photocatalyst, CuS/Au nanocomposite, which is proven for H2 production under visible light in medium acidic conditions [51]. As reported photocatalyst displayed a current density of 10 mA/ cm2 and was used for the degradation of organic pollutants. Chandra and coworkers say that metal oxide-supported CuS displayed improved photocatalytic applications [52]. As prepared heterostructure of CuS/TiO2 is used for the photocatalytic generation of H2 with a greater rate of visible light (1262 mol/hg), which is approximately 9.3 and 9.7 times faster in contrast to original TiO2 or CuS catalysts, the improved rate of H2 production is ascribed to enlarged light garnering and further effective charge division. At the same time, an ideal volume of CuS is placed upon the TiO2 and, therefore, boosts photocatalytic performance. In another efficient catalyst composite, CuS and Cu2O/CuO have developed by using an in situ growth technique by Dubale and coworkers [53]. As-synthesized chalcogenide is proven to be a potent and highly stable electrode for H2 production under visible radiation with 5.4 mA/ cm2 photocurrent density, which is B2.5 times higher than that attained by primary Cu2O/CuO material. Upon loading both CuS and Pt on Cu2O/CuO, a further upsurge of 5.7 mA/cm2 photocurrent density is due to the suppression of the charge carrier association. Ma et al. have described Au/CuSe/Pt photocatalyst by hydrothermal method. The synthesized catalysts displayed strong dual-plasmonic resonance and were active for photocatalytic H2 production in visible and near-IR radiation [54]. The photocatalyst Au/CuSe/Pt displayed remarkable production of H2, which is B7.8 (Au/CuSe) and 9.7 (Pt/CuSe) times more superb activity than their corresponding pure composites, respectively. Another efficient catalyst, CuS/ Ag2O/g-C3N4, was described by Mandari and coworkers using the hydrothermal method. The photocatalyst showed promising H2 generation activity with 1752 mol/ hg, which is significantly higher for individuals of the g-C3N4 and CuS catalysts. The improved production of H2 is attained may be attributed to the development of heterojunctions, which succeeded in the upsurge in visible radiation gathered and stifled the photogenerated recombination of charge carriers.

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Metal-Chalcogenide Nanocomposites

3.2.2.4 Vanadium based chalcogenides The ternary chalcogenides Cu3Nb12xVxS4 and Cu3Ta12xVxS4 have been described by Ikeda and coworkers by using a solid-state reaction [55]. As-prepared catalysts band gaps exhibited from a range of 1.6
1.7 eV and the Cu3M12xVxS4 composite band structure varies with their compositions, it was observed that the most significant alignments of the solid mixture and higher catalytic activity attained related to their desired composites Cu3Ms4 (M 5 V, Nb, and Ta). Li et al. studied the heterojunction of VS2@C3N4 catalyst by supramolecular self-assembly protocol [56] as obtained photocatalyst exhibited remarkable production of H2 with a greater rate (9628 mol/gh), which is B16.0 times superior to the C3N4 catalyst. Gopalakrishnan and coworkers described the core-shell heterostructures of VS2 dropped on silicon by using CVD. This is proven to produce H2 under solar irradiation [57]. The synthesized photocatalyst exhibited excellent H2 production with B23 mol/h, in another study developed by Shon and coworkers for the preparation of VS2 doped on Ni3S2 as a photocatalyst by using the hydrothermal method [58]. Without nitrogen doping of the composite catalyst, Ni3S2/VS2 displayed an enhanced oxygen evolution mechanism accomplished with a very small overvoltage of 227 mV due to significant active entities and outstanding edges. Additionally, nitrogen doping Ni3S2/VS2 displayed a powerful hydrogen evolution mechanism with a low overpotential of 151 mV owing to the occurrence of N2 doping conduction and rising the catalytic sites.

3.2.2.5 Cadmium based chalcogenides Zhou and coworkers have investigated the efficient photocatalyst, CdS/Cd, which is used for H2 production [59]. The photocatalyst displayed a more excellent photocatalytic production of H2 with a rate of 95.40 mol/h, which is B32.3 cycles more significant than the pure CdS catalyst. Another efficient photocatalyst, NiS/CdS, has been described by Li et al. [40]. Using a 2% NiS/CdS photocatalyst exhibited promising H2 production with an evolution rate of 18.9 mmol/gh. The amended activity ascribed to the existence of an interface among NiS and CdS, which proficiently encourages the charge separation, and the NiS composite assists with extremely active H2 production sites. Pan et al., have explored the H2 production rate (561.7 mol/h) by using CdS/CuS as a photocatalyst composite material [50]. The enhanced photocatalytic evolution rate of H2 production was achieved under visible-light irradiation. The pure CuS catalyst can progress the stability of CdS for H2 evolution in an aqueous medium. The increased stability of the CuS/CdS composite arises from the transfer of electron-hole pairs from CdS to CuS, which prevents the photo corrosion of CdS catalysts [60].

3.2.2.6 Tin based chalcogenides Lei and coworkers have developed the SnS/g-C3N4 nano photocatalyst by using ultrasound and microwave irradiation techniques [39]. The ideal catalyst demonstrated improved photocatalytic H2 generation from H2O. The use of 10% SnS/g-C3N4 displayed the excellent evolution rate of 818.93 mmol/hg, which is

Chalcogenides and their nanocomposites in photocatalytic reactions

51

B2.90 cycles superior to the pure g-C3N4, because of fitted energy band assembly among SnS and g-C3N4, which progresses the partition efficacy of charge carriers and delays the reamalgamation of electron-hole sets. Another photocatalyst (SnS2) has been investigated by Liu and coworkers [61]. The 5% of Cu/SnS2-x photocatalyst demonstrates excellent photocatalytic studies with an H2 production yielded 1.37 mmol/h, which are B6 cycles more significant than the pure SnS2 catalyst. The SnS2/ZnIn2S4 composite photocatalyst has been efficaciously fabricated by loading SnS2 with ZnIn2S4 by Geng and coworkers [62], which exhibited superior photocatalytic properties for H2 production in association with pure ZnIn2S4. The use of 2.5% of SnS2/ZnIn2S4 displayed superb photocatalytic H2 generation with a rate of 769 mol/gh, which is approximately 10.5 times higher activity of pure ZnIn2S4. The boosted photocatalytic activity may be due to the development of heterojunction between SnS2/ZnIn2S4, which allows vastly effective charge partition and moves on the interface within SnS2 and ZnIn2S4. A highly efficient photocatalyst, SnO2/SnS2, has been described by Li and his team for H2 production in visible radiation [63]. The photocatalyst showed a great H2 generation with a rate of 50 mol/h, which is B4.2 cycles more significant than the pure SnO2 catalyst.

3.2.2.7 Titanium-based chalcogenides The tri-chalcogenide photocatalyst (TiS3) is an active photoanode used in photoelectrochemical cells. The photocatalyst showed superb performance for the generation of H2, and the evolution rate was found to be 1.800.05 mol/min, and B7% of photoconversion was obtained [64]. Other efficient photocatalysts are Nb-rich (TixNb12xS3) and T-rich (NbxTi12xS3) as photoanodes [65]. Compared to Ti-rich, the polycrystalline structure of photocatalysts used for the generation of H2 showed superb activity than the Nb-rich and the evolution rate of 2.20.1 mol/min cm2.

3.3

Conclusions and perspectives

Chalcogenides and their composite materials have been broadly explored for watersplitting photocatalytic applications over the years. Due to the abundance, nontoxicity, cheaper starting substrates, more excellent thermal stability, and charge carriers to oxidize and reduce the H2O molecules, this work emphasizes the scope of photocatalytic applications of chalcogenides composite materials in water-splitting photocatalytic hydrogen evolution. Photocatalysis-engaging chalcogenide composites have received considerable attention over the years. However, some challenges desirables need to be addressed in endorsing and growing the photocatalysis field. Reports reveal that metal chalcogenide composites performed excellent excited photocatalytic studies under visible radiation. It is significant to encompass these catalysts by UV-light absorption and enhance a wide range of degradation applications. In addition, optimized and developed new surface modification and morphology methods on the composite catalysts for more outstanding performance and efficiency excite with a narrow band gap. Also, developing efficient semiconductor photocatalysts contributes to improved stability and reusability.

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Metal chalcogenides and their nanocomposites in water purification systems

4

Mahmoud H. Abu Elella1, Safaa S. Hassan1, Heba M. Abdallah2, Mervat S. Mostafa3 and Nedal Y. Abu-Thabit4 1 Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt, 2Polymers and Pigments Department, Chemical Industries Research Institute, National Research Center, Giza, Egypt, 3Science and Technology Center of Excellence (STCE), Ministry of Military Production, Cairo, Egypt, 4Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City, Saudi Arabia

4.1

Introduction

Environmental contamination is the most serious ecological concern because of the paucity of drinking water, water pollution has harmed both human health and the health of other living organisms. Water purification is considered a crucial topic in recent years. Rapid industrialization and significantly growing population resulted in the flowing of very dangerous industrial effluents and domestic sewage into water sources. They include many toxic materials that deteriorate and poison the water surfaces [1
3]. In this issue, there are different pollutants present in industrial effluents such as toxic heavy metals, synthetic dyes, and residual antibiotics, which are nonbiodegradable materials and do not degrade easily via conventional treatment techniques. As a result, they accumulate in living cells and cause fatal diseases. To solve this problem, advanced oxidation approaches have great interest to mineralize the pollutants and improve energy conversions [4
6]. In the past few decades, various material formulations are widely used to capture many contaminants from contaminated water including carbonaceous materials, zero-valent iron particles, nanocomposites (NCs), hydrogels, metallic oxides, quantum dots, noble metal nanoparticles, and so on. These materials are employed for several conventional techniques: membrane filtration, Fenton-like processes, electrocatalysis, batteries, photocatalysis, solar cells, etc. [3,7]. Out of the previously reported materials, the transition metal chalcogenides (TMC) have gained widespread attention worldwide because of their remarkable properties catalytic stability, easy availability, visible light activity, optical absorption, electron mobility, and modifiable elemental composition [8].

Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00004-3 © 2024 Elsevier Ltd. All rights reserved.

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4.1.1 Background of chalcogenides The chalcogenide word is a Greek word for “χαλκoς” expression, referring to copper, and “γενvω” expression, implying birth. Chalcogenide term was stated by Wilhelm Blitz and Werner Fische, who were immediately accepted amongst German scientific experts, and by Heinrich Remy who suggested its official use in 1938, while he was a working member of the International Union of Pure and Applied Chemistry. It was classified chalcogenide into various elements such as sulfur, selenium, tellurium, and oxygen [9,10]. The chemistry of both the chalcogen bond and metal
chalcogen bond was extensively studied for sulfur compounds during the time of early 1970 and later continued for tellurium and selenium as well. The history of sulfur-metal chemistry has been started in the 1960s, specifically for the catalytic property study [11]. Following theoretical studies, the past few years have reported the utilization of chalcogenide in various applications based on TMC, owing to its accelerated optical and electrical properties. A report studied by Cui and team based on electronic and optical properties of graphene-like gallium nitride doped with transition metal dichalcogenides heterostructures showcased accelerated results for water splitting [12]. The same optical spectra and electronic properties for electron-doping schemes were studied for transition metal dichalcogenides by Zhao and the team [13]. Hence, the footprint of chalcogenide makes it promising for wide functionalities. TMC are soft metals that have a great affinity to soft chalcogenides elements in group 16 in the periodic table such as sulfur and selenium. Most TMCs and their NCs have fabulous physicochemical properties such as outstanding affinities for many pollutants, magnetism, catalytic properties, high surface area, high optical properties, and being environmentally friendly. These remarkable advantages award great priority for TMC for the removal of several pollutants than other used materials [9,10]. Moreover, transition metal sulfide and transition metal selenide are vastly utilized for the most promising remediation strategy known as the adsorption strategy in few past years owing to their excellent adsorption characteristics, for example, cost-effectiveness, amenable properties, and surface modulational. Their characteristics are attributed to enhancing their remediation efficiency [11].

4.1.1.1 Methods of preparation There are several techniques that have been reported for fabricating TMC and their NCs to adjust their size diameter and thickness and surface area as well. Moreover, expanding the scope of the practical applicability of the TMCs can be achieved by modulating their properties through synthetic strategies. Fig. 4.1 includes different approaches, such as hydrothermal, solvothermal, electrospinning, and coprecipitation methods, which will be discussed in the sections below. The hydrothermal approach is one of the most common methods for the synthesis of TMC and their NCs. Therein the formation of nanoparticles carries out in a sealed Teflon autoclave under high pressure and elevated temperature, which calcinated later to get nanomaterials. The chemical reaction includes different parameters: pressure,

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61

Figure 4.1 Schematic diagram of the preparation methods of transition metal chalcogenides (TMC) and their nanocomposites (NCs).

pH, concentration, organic additives, reducing agents, time, and temperature [12]. The hydrothermal method is used to fabricate 3D TMC radially oriented nanospheres and corrugated nanoflowers. Finally, it is utilized to synthesize the transition metal of only group VI, because both group IV and V oxidize to their metal oxides during the preparation process [13
16].

Solvothermal method On the other hand, the Solvothermal approach happens at a high temperature above boiling point and 1 bar pressure. In this method, there are different solvents that can be used such as water and organic or inorganic solvents [17]. Recently, many reported studies have reported hydrothermal method-based TMC and their NCs synthesis higher than solvothermal technique [8]. For instance, Murugadoss et al. [18] reported the preparation of cobalt
nickel selenide/graphene NCs via the solvothermal method. The findings showed the distribution of graphene nanosheets onto nanohybrid to employ as a counter electrode for removing toxic dyes using solar cells. Moreover, Akyu¨z and his teamwork represented the preparation of TMC NCs as Cd(1
x
y)ZnxMoyS/reduced graphene oxide/Cd(1
x
y)ZnxNiyS and Cd(1
x
y)ZnxCuyS/ reduced graphene oxide NCs using the solvothermal approach for photoelectrochemical and photocatalytic processes [19]. In another study, Lui et al. [20] mentioned the preparation of four TMC NCs based on quaternary silver/sulfides Ba2Ag2XS4 (X 5 In, Sn, and Ge) by solvothermal technique for semiconducting characteristics.

Electrospinning method The electrospinning technique is considered a versatile and resourceful preparation approach for designing TMC and their NCs. In this method, intensive voltage/current is applied to the syringe tip, which is inserted with desired viscous precursors. For example, Wang and his coworkers reported the preparation of electrospun fibers-based cobalt (II) selenide nanocomposite in the presence of carbon nanofibers via the electrospinning technique for lithium-ion batteries [21].

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Metal-Chalcogenide Nanocomposites

Coprecipitation method The coprecipitation approach is a simple and low-cost technique for the preparation of TMC and their NCs. In this method, the metal plate is heated at a moderate temperature, which ensures the formation of metal oxides on the outer surface of the hot plate. The metal plate is applied as a source and as a substrate for TMC. Zhu and Sow [22] reported that the general mechanism for the formation of TMC using the hot-plate method depends on the growth temperature and vapor formation. Moreover, various controlled factors that are used for the preparation of TMC by hot-plate techniques including time, molar ratios, capping agents, and temperature, which are important parameters for preparing layered TMC nanostructures [23]. For example, Chen and his team reported the preparation of CdS-doped MoS2 by precipitation approach. Here, various precursors such as metal carboxylate, metal acetylacetone, and metal carbonate were used [24]. Therefore, this approach is the most different chemical preparation method because it can be used to prepare size- and shape-maintained nanostructures and single-crystalline nanostructures, such as TMCs [8].

4.2

Removal of synthetic dyes using metal chalcogenide nanocomposites

The dye-based industries (bleaching, textile, medicine, and paper industries) release about 10% of nonbiodegradable dye, which are harmful to the ecological system [25]. This impedes the penetration of required light for the photosynthesis process of aquatic plants. Additionally, a lot of organic dyes are carcinogenic, such as azo dyes, Congo red, and methylene blue (MB). Also, a list of toxic dyes degradation will be studied, for instance, MB, trichlorophenol (TCP), crystal violet (CV) dye, and congo red dye (CO). Moreover, organic pollutants (dyes) removal is categorized into three types, including physical technologies, Chemical processes, and Biological processes [25]. In this concern, photo-catalytic using nanocomposites has huge importance owing to their viability and inexpensive [25]. Likewise, photo-catalysts are functional in ecosystem treatment. Nevertheless, photocatalysts’ main limitation is their poor stability [26]. Saravanan et al. improve dye removal Photocatalytic technique by replacing UV radiation with visible light [27]. Thus, the improvement of active compositions in UV or/and visible light occupied a powerful concern owing to their technological potential environmentally. The core usage is the degradation of organic contamination in water [28]. Metal chalcogenides (especially sulfides and selenide) exhibit promising behavior for both electronic and optical claims [29]. Iron sulfide (FeS2), copper sulfide (CuS), Cadmium sulfide (CdS), and nickel sulfide (NiS2) are plentiful and nontoxic compositions, besides attributing to narrow band gaps (starts from 1.3 to 1.8 eV) and significant absorption coefficients, these characteristics boost photoactive applicability in visible light [30]. Metal sulfides are oxidized via photo-generated holes famed for photo-corrosion. Amongst them, several narrow band-gap semiconductors, CdS (B2.4) could absorb light

Metal chalcogenides and their nanocomposites in water purification systems

63

in the visible region (B550 nm), which promoted it to be used as a photo-catalyst. Pure CdS introduces a fast recombination rate that negatively influences the photo-catalytic process [25]. FeS2 displays the photo-catalytic degradation capability of numerous organic waste products, for instance, MB, and methyl orange (MO) [29]. The CuS composition is widely utilized in MB and congo red decomposition [29]. Whilst NiS proficiently removed MB, and CV [29]. On the other hand, Cadmium selenide (CdSe) is a type of Metal chalcogenide with a 1.75 eV band gap. As well, it attributed to a high charge recombination rate too [26]. Rajendran R et al., utilized PVA/Bi2WO6
CdS nano-composite for enhancing photo-catalytic potential, besides inhibition of photocorrosion. Pure CdS, and pure Bi2WO6 exhibit relatively low degradation potential (72% and 86%, respectively), as well Bi2WO6
CdS composite shows almost similar behavior. Although, Polyvinyl alcohol (PVA)/Bi2WO6
CdS nano-composite decomposition potential reaches 92% against MB dye after 100 min and showed good recyclability [25]. Additionally, the poly (vinyl alcohol) (PVA)-Bi2WO6
CdS nano-composite is prepared by hydrothermal technique [31]. The polymeric addition hydrophilic polymer-polyvinyl alcohol (PVA)-boosts photo-catalytic activity via retiring photo-generated charges recombination, also it is vital in a wide range of applications. Further, they demonstrated facile recycling afterward photo-catalytic degradation routine of the tested organic dyes [25]. El-Barbary et al. [26], utilized Ag nanoparticles/CdSe/graphene oxide @cellulose acetate (CA) nano-composite, the degradation of MB was reaching B90% upon exposure to visible light for 40 min. Aiming to avoid CdSe fast recombination, the produced photoelectrons must be eliminated to join in the loop current. The two-dimensional GO offers enlargement in the mean electronic free path. Additionally, it enhances electronic slip to dyes, thus obstacles recombination consequences. Further, CA hydrophilic property is promoted it for such applications. Further, Electro-spun CA filaments offer remarkable porosity (reaching 90%), This configuration introduces a proper structure for water treatment, as shown in (Fig. 4.2) [26]. Alenizia et al. [32] reported that nickel sulfide-reduced graphene oxidetitanium dioxide (NiS/RGO/TiO2) is capable to decompose TCP under solar light irradiation. The decomposition of TCP hits 95% after 6 h under sunlight exposure. The NiS/RGO composition displays relatively lower activity in visible light while adding TiO2 composition boosts the ternary nano-composite activity in visible light. Ashraf et al. reported NiS (nickel sulfide)
In2O3 (indium oxide)/GO (graphene oxide) nanocomposite for MB degradation under a UV lamp. At pH 9, it achieves 98.25% after 40 min. The presence of the NiS and GO causes narrowing in the band gap, which enhances photo-degradation capacities. The band gap of the NiS/In2O3/GO ranged from 2.30 to 3.00 eV. Additionally, the raising of NiS ratio enhances charge separation, generation of hydroxyl radicals, and photocatalytic behavior. Whilst, In2O3 offers a wide band gap (3.0 eV) that introduces significant optical and electrical characteristics and stability [33]. Ahmad et al. [34] offered a new catalyst of bismuth
iron selenide (Fe3Se4 and Bi2Se3)/chitosan microspheres (BISe-CM) for photocatalytic degradation of a CV dye. The degradation efficiency is executed under solar irradiation, using 0.2 g of BISe-CM, and the complete degradation of 30 ppm CV dye is done in 150 min at pH 8.0. The prepared nanocomposite could be recovered and reused. Chandra et al. showed

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Metal-Chalcogenide Nanocomposites

Figure 4.2 Photocatalytic degradation of MB using nanofibrous membranes of CA containing modified CdSe [31]. Source: Reproduced with permission from Ref. X. Lv, H. Cheng, S. Cao, Z.Y. Zhao, Y. Chen, W.F. Fu, Robustly photogenerating H2 in water using FeP/CdS catalyst under solar irradiation, Sci. Rep. 6 (2016) 19846. Copyrights 2021 Elsevier.

that nitrogenated graphene oxide (NrGO)/copper sulfide (CuxS) NCs (CuxSNrGO, where x 5 1 and 2) are incorporated in polysulfone (PSF). It exhibits an efficient CR dye removal of 93% owing to the electrostatic repulsion CuxSNrGO/PSF membrane (Fig. 4.3) [35]. Gadisa et al. [36] presented the preparation of the amorphous phase of iron sulfide (FeS2) nanowires for its excellent adsorption performance to organic dyes (CR and MB). The results showed that the adsorption efficiency reached up to around 99.0% with theoretical adsorption potential of 118.86 and 48.82 mg/g for CR and MB dyes, respectively. In this study, Nasseh et al. [37] have studied the ability of dye adsorbent nanocomposite FeNi3/SiO2/CuS for CR removal from contaminated water. The study illustrated various variables on the adsorption efficiency such as pH, time, adsorbent amount, initial CR concentration, and temperature. Regarding the conditions that achieved full CR adsorption include 200 min, an adsorbent concentration of 2 g/L, pH 5, and an initial CR concentration of 30 mg/L at 25 C. Iqbal et al. mentioned the preparation of CuSe/GO nanocomposite by hydrothermal route. The catalytic activity of CuSe/GO was monitored against methyl green (MG) dye. Additionally, the effect of irradiation time, catalyst amount, dye starting concentration, and pH were investigated (Fig. 4.4). CuSe/GO showed significant photocatalytic activity reaching up to 89% dye removal, while unmodified CuSe achieved approximately 81% at pH 5, 0.025 g CuSe amount, 30 mg/L dye starting concentration within 45 min of irradiation [38].

Metal chalcogenides and their nanocomposites in water purification systems

65

Figure 4.3 Displays a congo-red dye rejection onto nitrogenated graphene oxide/copper sulfide [35].

Figure 4.4 Photocatalytic mechanism of CuSe/GO NC [39]. Source: Reproduced with permission from Ref. D. Bejan, N.J. Bunce, Erratum to: Acid mine drainage: electrochemical approaches to prevention and remediation of acidity and toxic metals, J. Appl. Electrochem. 46 (2016) 423. Copyrights 2021 Elsevier.

4.3

Removal of toxic heavy metal ions using metal chalcogenides nanocomposites

The aqueous solutions of heavy metal ions in industrial effluents represent a considerable problem throughout the world because they adversely impact human health and the environmental system [11]. TMC and their NCs often have a preferable

66

Metal-Chalcogenide Nanocomposites

affinity for heavy metal ions. The nanoscale materials provide an efficient property relative to bulk materials. Therefore, the nanoscale metal sulfides can be used as potent materials for the environmental remediation processes. Regardless of the technique, the goal of nanoparticle synthesis generally focuses on minimizing and controlling particle size, maintaining a narrow size distribution, control of particle morphology, and controlling crystallinity. One of the nanoscale materials that showed removal with high efficacy for heavy metal ions is metal sulfides. The divalent metal ions can be removed through chemical precipitation, ion exchange, and complexation [40,41]. The precipitation of metal sulfides was considered an effective method for removing heavy metal ions due to the lower solubility of these compounds considered one of their main advantages. Some metal sulfides have solubility values in pure water [39,42
44]. Sulfide precipitates observed a high degree of metal removal from water in a shorter time over a wide pH range. Metal sulfides can be precipitated by using various precipitating agents such as ammonium sulfide, sodium sulfide, calcium sulfide, sodium hydrosulfide, calcium sulfide, barium sulfide, iron sulfide, and sodium thiosulfate [39,43,45,46]. The precipitation process is highly sensitive to the dosing of the precipitating agents but during the precipitation of metal sulfides, colloidal precipitates are formed, which cause problems in further separation processes. So, it forced us to search for new and effective other types of processes as adsorption processes of the metal sulfide compounds that show efficient removal of the heavy metals, especially in the nanoscale. The important properties of the nanoscale adsorbents are their large surface area to volume ratios, the absence of their internal diffusion resistance, and their enhanced structural properties with their high porosity [47
52]. So, it offers many advantages for heavy metal pollutants removal as the selectivity and the faster and more efficient adsorption processes of removal with a better capacity [50]. It may be due to the properties observed for the nanoscale adsorbents. These previous properties enable the nano adsorbents to be highly selective and have the capacity to attain efficient adsorption of many different types of water pollutants including heavy metals. There are many effective metal chalcogenides that succeeded as nano adsorbents for the removal of heavy metals, we will mention some of these compounds. One of the most investigated metal sulfides as adsorbents for the removal of heavy metals is iron sulfide (FeS) known as mackinawite. FeS removes many divalent metal ions such as Cd21, Ni21, Mg21, Ca21, and Mn21 ions [53]. FeS has a high capacity for nonredox active metals such as Cd(II), in which highly insoluble cadmium sulfides (CdS(s)) form by favorabe exchanging of Fe in FeS [54]. Daryoush Afzali et al. [55] reported the increased efficiency of the removal of Cd(II) using low-cost adsorbents such as river sand, granite particles, ceramic particles, and iron filings by the formation of NCs using Fe-S nanoparticles. The greatest adsorption capacity was observed by using the iron nanocomposite. It achieved cadmium removal above 98.5%. Arsenic (As) can be removed by conversion of As(V) to the more reduced forms of As(III) using FeS as an effective reductant and subsequently removing it by adsorption or formation of mixed-metal sulfide phases [54]. Also, the inorganic

Metal chalcogenides and their nanocomposites in water purification systems

67

(oxyanions) of Se(VI), Cr(VI), and As(VI) ions can be removed by the reduction properties of FeS [11]. Hg21 was scavenged using FeS to form β-HgS even with the concurrent of other different pollutants as the organic matter [41]. The modified FeS with carboxymethyl cellulose (CMC) provides a sorption capacity of 3449 mg/ g for Hg21 and this was attributed to the dual-mode adsorption mechanism where CMC-FeS adsorbs Hg21 through concurrent precipitation [41]. Liping Lou et al. [56] succeeded in the removal of Cu21 and Ni21 from water with a removal efficiency of Cu21 reached 98.3%
99.7% after 2 min, which is consistent with the research of Zhang et al. [57], but the removal efficiency of Ni21 reached 97.4% after 2-min reaction. He succeeded to remove Ni21 completely within 5 min. Liping Lou et al. used sulfide-modified nanoscale zerovalent iron (S-nZVI). The processes observed in the removal of Ni21 were adsorption and reduction, while Cu21 was predominantly by reduction. Zinc sulfide (ZnS) nanocrystals were another metal chalcogenide material that shows efficacy remediation capacity for several metal ions. It shows high selectivity toward Hg21 and Cu21 metal ions than Pb21 and Cd21 as investigated by Fang et al. [50]. Also, Xu et al. proved the high selectivity of ZnS nanocrystals as a novel sorbent for copper removal from wastewater. It succeeded to remove the copper with efficiency is over 99.0% in 1 min. The Cu21 removal efficiency is still over 99.0% in 5 min. In the presence of HgCl2, Pb(NO3)2 and CdSO4 were added into the CuCl2 solution although the presence of Hg21 decreased the Cu21 removal reaction rate. The suggested mechanism was cation exchange. It was found that the heat treatment improved its adsorption performance on copper by removing the organic ligand on sorbent and increasing its surface area [58]. A sorbent of ZnS nanocrystals showed extraordinary performance for the removal of Hg21, Cu21, Pb21, and Cd21 with removal efficiencies of 99.9%, 99.9%, 90.8%, and 66.3%, respectively. This can be interpreted due to the soft acid-soft base interaction theory. Pala and Brock [40] succeeded to remove Pb21 with a remediation capacity of 2950 mg/g by using ZnS gel with ion exchange and adsorption processes. Thokozani Xaba et al. [59] reported the preparation of ZnS through precipitation method in the presence of 1-methyl-2-pyrrolidone and thiourea according to Fig. 4.5A, which was subsequently modified by chitosan and PVA as a capping agent to prepare ZnS/chitosan nanocomposite. The prepared nanocomposite was employed for removing Cr(VI) ion from wastewater through the adsorption process. Fig. 4.5B showed that the obtained nanocomposite achieved superior adsorption above 95% better than chitosan alone, which adsorbs just 60% at pH 9. Kezheng Chen et al. [60] mixed ZnS with Fe3O4 to form the core-shell nanostructured material that revealed dual properties of the adsorption affinity of ZnS and the magnetic properties of the iron oxide Fe3O4 that increase the efficiency of the metal selectivity and separation. The adsorption capacity for removal of Hg21 from water reached 129.9 mg/g due to the magnetic nanoparticle’s behavior, which is better than the nonmagnetic nanoparticles. Several other modified ZnS nanoparticle adsorbents were used to remove the heavy metals as the wurtzite ZnS nanorods can catalyze the adsorption of various cations due to the negative polarization present. The ZnS nanorods were considered a withdrawer for the toxic Pb-ions. Every mole of ZnS is

68

Metal-Chalcogenide Nanocomposites

Reaction 1

(A) S

N

O N

H2N

CH3 1-methyl -2-pyrrolidone

Reflux for

C

NH2 Thiourea

N

2 hrs @70°C

C

S

NH2

CH3 (Z)-1-methyl -2-(pyrrolidin2-ylidene) thiourea

+ Reflux for 1 hrs @70°C ZnS

Zn (Acetate) Reaction 2

(B) 100 (i)

90 80

% Removal

70 (ii)

60 50 40 30 20 10

(i) ZnS-Chitosan (ii) Chitosan

0 3

4

5

6

8

7

9

10

11

pH

Figure 4.5 (A) Schematic preparation of ZnS in the presence of thiourea and zinc acetate via precipitation method and (B) the relation between the adsorption % of Cr(VI) and pH of solution for ZnS /chitosan nanocomposite and native chitosan.

capable of removing nearly 10 times this amount of Pb21 from water. Also, ZnS nanorods significantly facilitated Fe21 withdrawal from water. ZnS nanorods catalyze the process of removal of Fe3 1 ions with a withdrawal percentage of 65%
75% from water [61]. Nano-sized ZnS functionalized with dioxa-dithio ligands (S2O2
ZnS) was investigated for lead removal from an aqueous solution. Results indicated that S2O2
ZnS was proved as a good reusable adsorbent. The results achieved lead removal efficiency reached 99.6% after 120 min agitation time. Alamolhodaei et al. [62] mentioned that the optimum pH corresponding to the maximum adsorption of Pb(II) ions was 6. Also, the contact time necessary for maximum adsorption was found to be 120 min. S2O2
ZnS is a reusable adsorbent with 97% recovery. Some of the previous materials based on ZnS for lead removal efficiency were found to be 82%, 90.1%, and 41.68% for CdS/ZnS core-shell

Metal chalcogenides and their nanocomposites in water purification systems

69

nanoparticles, ZnS nanoparticles aerogels, and ZnS QDs impregnated chitosan films, respectively [63
65]. Magnetic biochar/ZnS composites successfully removed Pb (II) ions with a high maximum adsorption capacity of up to 367.65 mg/ g, which was 10 times higher than that of reported magnetic biochar. The superparamagnetic properties of the composite facilitate the separation of the lead-laden magnetic biochar/ZnS composites by using a magnet after adsorption. Therefore, the magnetic behavior was efficient behavior to increase the efficiency of separation as mentioned before in many composites [66]. Recently, transition metal dichalcogenide materials with widespread formula MX2, where M can denote molybdenum (Mo), and X can denote chalcogens, such as sulfur (S) for removal of Cr(VI)/Cr(III) from aqueous solutions. These (TMD) materials produce potent properties as it stabilizes the nanoparticles, ultimately inhibit their oxidation, and reveal a large surface area and high porosity of the synthesis product [67]. Polyaniline (PANI) was modified onto MoS2 to produce a new adsorbent PANI@MoS2 to remove Cr(VI) ions from aqueous solutions. The new adsorbent achieved a maximum removal capacity of Cr(VI) with values of 526.3 and 623.2 mg/g at pH 3.0 and 1.5, respectively. The interpretation of adsorption on PANI@MoS2 was due to the complexation between the amine and imine groups on the surface of PANI@MoS2 with Cr(VI). The results revealed that the new hybrid material PANI@MoS2 has a huge potential as an adsorbent for Cr(VI) removal from large volumes of aqueous solutions [68]. Kanatzidis et al. [69] intercalated the Mg/Al layered double hydroxide (Mg/AlLDH) with MoS422 (MoS4-LDH) to produce new material. It was found that the new MoS4-LDH material was highly selective for different metal ions especially Cr (VI) and As(III) in the presence of many competing nontoxic anions such as sulfate, nitrate, and chloride. The adsorptions of As(V) and Cr(VI) are exceptionally rapid, showing . 93% removals within 1 min and . 96% removal within 5 min. the highly polluted concentration of As(III) and Cr(VI) reached lower values than the permitted level for drinking water , 10ppb. The removal of chromate anion (Cr (VI)) is enabled by the redox reaction with MoS422 to form Cr(III) while the HAsO322 (As(III)) and HAsO422 (As(V)) are attributed to As 2 S interactions. Researchers mixed two of the known efficient materials MoS2 and magnetic nanoparticles (Fe3O4NPs) producing adsorption capacities for Cr(VI)/Cr(III) removal higher than the values related to Fe3O4NPs [67]. The superior magnetic property enhances the adsorption beside the numerous surface hydroxyl groups showing uniform size and shape and excellent water-dispersibility that improved the removal efficiency. Aregay et al. [70] demonstrated the various possible mechanisms for Cr(VI) removal using the prepared FeMgAl-MoS4 LDH where LDH means layered double hydroxide symbolized by H-3. The prepared adsorbent material showed high adsorption capacity and selectivity to Cr(VI) removal with fast kinetics. H-3 was removed more than 99.99% of Cr(VI) during 180 min with a removal capacity reached 135.59 mg/g. These results added the investigated and prepared adsorbent to the top of the materials list known for Cr(VI) removal. The (\S\)22 present in MoS422 participated in the reduction of Cr(VI) to Cr(III) and simultaneously was oxidized to sulfate ion. Moreover, it succeeded to achieve

70

Metal-Chalcogenide Nanocomposites

significant stability and excellent reusability due to the unique memory effect of layered double hydroxides. We can conclude depending on the obtained results that the sulfide embedded into hydrotalcite of Fe/Mg/Al hydroxides revealed hybrid adsorbents can be considered one of the preferred candidates for the removal of hexavalent chromium.

4.4

Removal of residual antibiotics using metal chalcogenides nanocomposites

Our environment has a variety of water contaminants from which residual pharmaceutical antibiotics [71,72]. Every year, a massive number of antibiotics are manufactured and consumed. There are two categories of antibiotics based on the presence of the β-lactam ring in their structure: β-lactam and non-β-lactam groups. Antibiotics that have not been utilized or digested are discharged into the environment, which introduces resistance into natural bacterial ecosystems [73
75], according to water analysis data, ciprofloxacin and tetracycline have been found at high concentrations in surface water, groundwater, and sediment [76,77]. Various chemical and physical methods have been used to eliminate organic compounds including adsorption, biodegradation, chemical oxidation, ion exchange, and membrane approaches [78
83]. Metal-organic framework and its derivatives [84,85], graphene oxide [86], graphene hydrogel [87], glass microspheres [88], and chitosan-based composite materials [89] are currently being studied as adsorbents for sorption antibiotics from the aqueous environment. The Fenton reaction is a cost-effective, simple, and environmentally sustainable wastewater treatment process because of its production of hydroxyl OHd radicals that are effective, highly oxidative, and nonselective for wastewater discharges. An effective Fenton-like catalyst (CNTs/FeS) based on carbon was produced from a preparative carbon nanotube (APCNT) via a simple solid technique using the iron nanoparticles in APCNTs without the need for additional purification. Then, in comparison to pyrite, the sulfur treatment was applied to broaden its optimum range of pH and equipped it with in-situ H2O2 production for the removal of ciprofloxacin pollutants [90]. An area of high concentration of ciprofloxacin antibiotic was formed on the surfaces of carbon nanotubes via adsorption, the antibiotic was then efficiently eliminated using a surface
modified improved oxidation catalyst. In wastewater treatment studies, carbon nanotubes created an area of high antibiotic concentration on their surface by adsorption. The antibiotic was then efficiently removed using an improved oxidation catalyst with a surface modification. At optimal conditions, the removal efficiency is 91.26%. Because of their thermal stability, electrical conductivity, and substantial redox performance, metal sulfides, particularly nickel-cobalt sulfide, are widely used in a variety of applications [91]. Furthermore, the larger band gap of nano-sized cobalt-nickel sulfide materials can be used as photocatalysts to degrade organic pollutants [92]. Wu et al. [93] prepared an innovative magnetic nickel cobalt sulfide/sodium dodecyl benzene sulfonate (FSNCS/SDBS) adsorbent via the hydrothermal method for removal of ciprofloxacin CIP using thioacetamide, ferroferric oxide, metal precursors, tetraethyl

Metal chalcogenides and their nanocomposites in water purification systems

71

orthosilicate, and SDBS. The produced FSNCS/SDBS materials exhibit high adsorption capability over a broad range of pH (6
11). The highest adsorption efficiency of CIP is defined to be 625 mg/g, outperforming a wide range of previously reported adsorbents in research. Furthermore, FT-IR, zeta potential, and XPS studies revealed that the adsorption processes were electrostatic interaction, π-π interaction, hydrogen bonding, and metal coordination. The regeneration efficiency of FSNCS/SDBS is greater than 80%, confirming its reusability. Penicillin G (PG) is a medication that dissolves in water, as well as its method of action is to damage the bacterial cell wall by blocking the synthesis of peptidoglycans [94]. It has the highest antibacterial activity of any natural antibiotic and is classified as a weak acid (pKa 5 2.75) [95]. The photocatalytic process transforms pollutants into nonharmful compounds such as CO2 and H2O. Furthermore, it is very efficient, has a moderate reaction mechanism, and has a short reaction time, it may also supplement and enhance current technologies [96]. Naghizadehc et al. [97] fabricated a magnetic CoFe2O4@CuS nanocomposite for removal of PG using photocatalytic degradation in aqueous solutions. The results showed that the PG maximal degradation by UV radiation was 70.7% under optimal conditions. Furthermore, the findings revealed that the catalyst efficiency was not considerably decreased after five consecutive runs. Also, VSM study demonstrated that the CoFe2O4@CuS magnetic nanocomposite retains magnetic characteristics (Ms 5 7.76 emu/g). Jia et al. [98] synthesized a new hierarchical heterostructure NiSe2/ MoS2 using a consecutive hydrothermal procedure and in-situ thermal injection technique for removal of tetracycline hydrochloride by photocatalytic degradation. Under light irradiation, the 5.4% NiSe2/MoS2 composite demonstrates significantly increased photocatalytic activity toward an H2 evolution rate of 2473.7 mol/h/g. Furthermore, after 120 min, the 5.4% NiSe2/MoS2 sample has a mineralization capacity and photodegradation efficiency of TCH over 58.1% (Table 4.1). .

4.5

Future perspective/outlook

The progress in the usage of TMC in wastewater treatment especially under advanced techniques such as adsorption, electrocatalysis, photocatalysis, and photoelectrocatalysis is growing intensively. The challenge is to apply TMC-based applications to real-life applications such as on a large scale with efficacy, stability, and cost-effectiveness. This challenge turns out to have a large-scale synthesis of material and further sustainable-based application. Such an accomplishment will help solve global energy and environmental problems toward the consciousness of a sustainable society. With a precise review of TMC, it can be stated that it is a “classic” material that stands for various applications in the field of environmental science. The future of water treatment remediation can be made using TMC-based composites by photocatalytic, electrocatalytic, or clubbed together, utilizing solar energy and power supply. This opens a chapter on the multifunctionality usage of TMC, which can be useful in resolving various environmental issues and getting a realization of a sustainable environment.

Table 4.1 Various metal chalcogenides nanocomposites for removal of dye, toxic heavy metal, and antibiotics. Metal chalcogenides nanocomposites

Preparation method

Pollutants

Method of removal

Main results

References

PVA
Bi2WO6
CdS nano-film

Hydrothermal

MB

Photocatalytic degradation

[25]

Ag NPs/CdSe/GO @ CA films

The hydrothermal method and then electrospinning technique to be merged into CA Hydrothermal method

MB

Photocatalytic degradation

After 100 min of photo-degradation PVA
Bi2WO6
CdS (92%) CdS (72%) Bi2WO6 (86%). MB degradation B90% Conditions (40 min /visible light irradiation).

TCP

Photocatalytic degradation

[32]

NiS
In2O3 /GO nano-composite

Hydrothermal method

MB

Photocatalytic degradation

Bismuth
iron selenide (Fe3Se4 and Bi2Se3) /chitosan microspheres (BISe-CM) CuxS-NrGO, where x 5 1 and 2 @ polysulfone (PSF) membranes

Solvothermal preocess

Crystal violet

Photocatalytic degradation

Disintegration of TCP hits 95% Conditions (after 6 h under sunlight exposure). MB degradation achieves 98.25% Conditions (after 40 min, at pH 9, and under UV irradiation). using 0.2 g of BISe-CM, the complete degradation of 30 ppm crystal violet dye is done in 150 min at pH 8.0.

Graphene oxide (Hummers’ Method) CuxSNrGO (Coprecipitation)

Congo red

Filtration

CR dye rejection of 93%.

[35]

NiS/RGO/TiO2

[26]

[33]

[34]

Congo red (anionic), and methylene blue (cationic) Congo Red (CR)

Adsorption

The adsorption capacity was 118.86 and 48.82 mg/g toCO and MB dyes, respectively.

[36]

Adsorption

[37]

Hydrothermal

Methyl green dye

Photocatalytic degradation (PC)




Hg21 ions

Adsorption

Coprecipitation

Cr(VI) ions

Adsorption

Polyaniline/MoS2

Hydrothermal

Cr(VI) ions

Adsorption

Magnetic biochar/ ZnS composites




Pb (II) ions

Adsorption

A maximum CR adsorption of 100% Conditions (after 200 /adsorbent concentration of 2 g/L/a pH of 5 /initial CR concentration 30 mg/L/a temperature 25 C). CuSe (81% dye removal) and CuSe/ GO (89%) Conditions (pH 5/ 0.025 g of catalyst 5 /30 mg/L dye concentration /45 min). The maximum sorption capacity was 3449 mg/g Nanocomposite achieved superior adsorption above 95% better than chitosan alone, which adsorbs just 60%. Nanocomposite achieved maximum removal capacity of Cr(VI) with values 526.3 and 623.2 mg/g at pH 3.0 and 1.5, respectively. Maximum adsorption capacity up to 367.65 mg/g, which was 10 times higher than that of reported magnetic biochar.

FeS2 nanowires @PVP

Solvothermal reaction

FeNi3/SiO2/CuS

Coprecipitation

CuSe/GO

FeS /carboxymethyl cellulose ZnS/chitosan

[38]

[41] [59]

[68]

[66]

(Continued)

Table 4.1 (Continued) Metal chalcogenides nanocomposites

Preparation method

Pollutants

Method of removal

Main results

References

CNTs/FeS Fentonlike catalyst

Solid method

Ciprofloxacin

Adsorption

[90]

FSNCS/SDBS

Hydrothermal method

Ciprofloxacin

Adsorption

CoFe2O4@CuS nanocomposite

Hydrothermal method

Penicillin G

photocatalytic degradation

NiSe2/MoShetero structure

Hydrothermal method

Tetracycline hydro chloride TCH

Photocatalytic degradation

The antibiotic was efficiently removed using a modified oxidation catalyst on its surface. The Removal rate at optimized conditions is 91.26%. The highest adsorption efficiency of CIP is defined to be 625 mg/g. The regeneration efficiency of FSNCS/ SDBS is greater than 80%. PG maximal degradation by UV radiation was 70.7% under optimal conditions. The catalyst efficiency was not considerably decreased after five consecutive runs. The 5.4% NiSe2/MoS2 composite demonstrates significantly increased photocatalytic activity toward an H2 evolution rate of 2473.7 mol/h/g. The 5.4% NiSe2/MoS2 sample has a mineralization capacity and photodegradation efficiency of TCH over 58.1%.

[93]

[97]

[98]

Ni-Co-S/activated carbon nanoparticle

Hydrothermal method

Ciprofloxacin CIP, Tetra cycline hydro chloride TC

Adsorption

MoS2QDs/BiOBr heterostructures

Hydrolysis& ultrasoundexfoliation method

Ciprofloxacin

Photocatalytic degradation

Nickel sulfide nanomaterial

Precipitation method

Ciprofloxacin

Adsorption

AC-NCS has a maximal sorption capacity (Qmax) of 962.21 and 744.70 mg/g for TC and CIP, respectively. The adsorption performance was determined to be higher than 82%, and the reusability of AC-NCS with TC was evaluated five times. Under visible light irradiation, the maximum photocatalytic degradation efficiency of MoS2QDs/BiOBr-2 for ciprofloxacin is about 2.3 times that of pure BiOBr. The heterojunction has good recyclability and high stability after 4 cycles. The adsorption studies of Nickel sulfide nanomaterial 1 toward CIP revealed a high adsorption capacity of about 971.83 mg/g. By using a mixture of 10% acetic acidethanol2, the adsorbent was efficiently recycled up to four times without considerable loss of efficiency.

[99]

[100]

[101]

76

4.6

Metal-Chalcogenide Nanocomposites

Conclusion

Metal chalcogenides and their NCs have gained high attention for removing various toxic pollutants from contaminated water surfaces. Fundamentally, they have been used as efficient materials thanks to their outstanding properties such as low toxicity, catalytic stability, abundance, excellent conductivity, low cost, high surface area, and optoelectrical properties. This chapter focuses on TMC and their NCs for removing toxic synthetic dyes, heavy metals, and residual antibiotics. Also, it showed different environmental remediation methods including adsorption, photocatalytic degradation, precipitation, and filtration methods.

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R. Elancheran1, V.L. Chandraboss2, B. Karthikeyan1 and S. Kabilan1 1 Department of Chemistry, Annamalai University, Annamalai Nagar, Tamil Nadu, India, 2 Department of Chemistry, Bharath Institute of Higher Education and Research, Bharath University, Chennai, Tamil Nadu, India

5.1

Introduction

Water is the most important substance for all life on earth and a precious resource for human survival and development. Providing clean safe water is a challenge and a needy thing in the current world [1]. Every day a large amount of unconsumed dye produced by textile and printing industries is discharged into the environment. The phenomenon of self-assembly nanomaterials is the spontaneous formation of a bigger functional unit by the system’s component parts as a result of direct particular contact, group effects, and environment [2]. Basic dyes have high intensity of color and are extremely toxic even in a very low concentration. Dye effluents are very difficult to degrade because of their stability to light. Increasing the availability of processes to control pollutants is a challenge. To treat the dye effluents discharged from textile industries, various chemical, physical, and biological methods have been employed (Fig. 5.1), but the search for a cost-effective approach still gets away from the scientific community. However, using microorganisms, microbial enzymes, or combining these with a physicochemical technique delivers a better outcome that is also more viable financially. Microbes can decolorize extremely complicated synthetic dyes, in addition to guaranteeing a nontoxic process [3]. Therefore, there is a need to develop a novel treatment that is more effective in eliminating dye effluents from industrial wastewater.

5.2

Role of metal chalcogenides

Metal chalcogenides are an inorganic chemical compound group consisting of at least one chalcogen anion and at least one more electropositive metal element. Transition metal chalcogenides have a wide range of compositions, diverse lattice structures, and distinctive electronic structures. The transition metal chalcogenides have promising applications in a variety of energy applications, including electrochemical catalysis, photocatalysis, metal-air batteries, and other energy conversion processes. This is due Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00005-5 © 2024 Elsevier Ltd. All rights reserved.

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TREATMENT METHODS FOR TEXTILE WASTEWATER

PHYSICO CHEMICAL METHODS FILTRATION

CHEMICAL METHODS ADVANCED OXIDATION PROCESSES (AOP)

FLOCCULATION

FENTON

ADSORPTION

PHOTOCATALYSIS

REVERSE OSMOSIS

OZONATION

BIOLOGICAL METHODS ENZYMES MICROBIOLOGICAL TREATMENT ACTIVATED SLUDGE CULTURE BACTERIAL CONSORTIUM

Figure 5.1 Treatment methods for dye removal from textile effluent [3]. Source: From R. Jamee, R. Siddique, Biodegradation of synthetic dyes of textile effluent by microorganisms: an environmentally and economically sustainable approach, Eur. J. Microbiol. Immunol. 9 (4) (2019)114
118. https://doi.org/10.1556/1886.2019.00018.

to their exceptional characteristics. The transition metal chalcogenides demonstrate enhancing performance for water splitting, particularly because of their numerous flaw sites, tunable electronic structure, and diverse shape. However, they still have significant drawbacks that prevent them from being widely used in industry, such as poor conductivity, activity, and stability in water splitting. It is still quite difficult to make active and stable transition metal chalcogenides, which is necessary for widespread application [4]. Modified metal sulfides are the most excellent choice for heterogeneous photocatalysis to remove various pollutants, dyes, and phenolic compounds. Hence, dye removal is of enormous value. Here, we have highlighted and evaluated recent progress in the development of the photocatalytic activity of metal chalcogenides nanocomposites. For example, photocatalysis has been effectively used to degrade a wide range of organic compounds, including dyes [5,6] and endocrine and mutagenic damaging chemicals [7] and for degradation of pollutants in petroleum refinery wastewater [8,9]. The enhanced photocatalytic activity of metal chalcogenides is caused by the reduction of the recombination of electron-hole pairs, a decrease in the band gap energy, an increase in surface area, an increase of dOH formation, improvement in the electron transfer, an increase in absorption of light intensity. Methylene blue (MB) is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. The chemical structure of this MB dye is shown in Fig. 5.2. At room temperature, it appears as a solid, odorless, dark green powder that yields a blue solution when dissolved in water. It has many medical, biological, and chemical applications. More recently, significant efforts have been made to improve advanced or modified metal oxides as a photocatalyst that are capable of using UV or visible light, including doped or supporting materials. Doping of metal ions, nonmetallic elements, transition metals, and rare earth elements has been primarily studied to enhance the

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N

CH3

H3C N

S+

N

C1CH3

CH3

Figure 5.2 Chemical structure of MB.

Figure 5.3 The degradation process of dye effluents.

photocatalytic activity under UV-light irradiation. Among these doping methods, doping with transition metals is one of the most effective methods. In recent years, extensive research works have focused on visible-light-induced photocatalysis by dope metal oxides with metal ions. The growing interest in metal chalcogenides is due to their significant dynamic strength and environmentally relevant applications as efficient photocatalysts. The organic substrates from dye effluents diluted in an aqueous medium are considered to be first adsorbed on the surface of the supporting material (usually activated charcoal), where they migrate to the semiconductor metal oxide with doped metal ion and are oxidized in the vicinity of the composite material by radical species such as hydroxyl radicals (dOH) and superoxide radical anions (O22), which are formed by the reaction with photogenerated holes (h1) and electrons (e2), respectively. Hence, a good understanding of the physicochemical properties of the support material is very important to achieve efficient photodegradation. The development using crystalline metal chalcogenides as UV-visible-lightdriven catalysts for the photodegradation of organic pollutants is a recent development for the breakdown of several color compounds (Fig. 5.3). Metal chalcogenides are excellent adsorbents of countless pollutants. It is very promising for two reasons: (1) metal chalcogenides can adsorb the dye molecules and then release them onto the surface of the catalysts and (2) the intermediates produced during degradation can be also adsorbed by metal chalcogenides and then further oxidized. Their industrial applications involve the adsorptive removal of odor, color, taste, and other undesirable organics from industrial wastewater.

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Photodegradation and decolorization of MB under solar light irradiation were studied to evaluate the photocatalytic performance of the synthesized metal chalcogenide nanocomposite material. Fig. 5.4 shows the time course of a decrease in the absorbance of MB under solar light irradiation. UV
vis spectra and the color of MB aqueous solution as a function of solar-light irradiation with different time intervals in the presence of materials (undoped metal oxide, nano metal doped metal oxides, and metal chalcogenides nanocomposites) are illustrated in Fig. 5.4A
C. It can be seen from UV
visible spectra changes, the adsorption peak of MB solution at 663 nm steadily decreased with increasing the light irradiation time, and the initial dark blue color of the solution gradually turned to light colored. Comparative studies of the photocatalytic activity of the materials for photodegradation as well as decolorization of MB were shown in Fig. 5.4D. Metal chalcogenides exhibited excellent photocatalytic activity when compared to undoped ones. Illustration of metal chalcogenides for photocatalytic MB is shown in Fig. 5.5.

Figure 5.4 Absorption spectral changes of MB solution (1 3 1024 M, 40 mL) under solar light ( . 400 nm) irradiation (A) undoped metal oxide, (B) nano metal doped metal oxides, (C) metal chalcogenides nanocomposites at 20 min interval, (D) comparative study of photocatalytic activity [absorbance Vs time (min)] of undoped metal oxide, metal ion doped metal oxide, and metal chalcogenides for photodegradation of MB (catalyst loading: 0.08 g and pH 7).

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Figure 5.5 Schematic illustration of metal chalcogenides for photocatalytic degradation of MB.

Upon irradiation of metal chalcogenides with solar light, electrons are excited from the valence band (vb) to the conduction band (cb) generating holes in the vb (h1vb) and the electrons in the cb (e2cb) according to Eq. (5.1). The chemisorbed H2O molecules interact with the h1vb forming dOH (Eq. 5.2). Furthermore, the cb electrons (e2cb) interact with dissolved O2, producing superoxide radical anion (O22) as shown in Eq. (5.3). On the other hand, HO2d could interact with H2O forming highly oxidizing dOH radicals (Eqs. 5.4 and 5.5), which degrade the dye molecule completely into simpler molecules (Eq. 5.6). Metal chalcogenides 1 hv s h1 vb 1 e2 cb 
1 d H2 O s H1 1 OH
1 h1 vb s H 1 OH

(5.1) (5.2)



2 1
s HOd2 1 2 OH O2 1 e2 cb s O2 1 H 1 OH

(5.3)

HOd2 1 H2 O s d OH 1 H2 O2 

(5.4)

H2 O2 s 2 d OH

(5.5)

d

OH 1 MB s Degradation products ðCO2 & H2 OÞ

(5.6)

The photogenerated electrons could react with the oxygen molecule adsorbed on the surface of metal chalcogenides to yield O2
. On the other hand, photogenerated holes react with H2O molecules to produce dOH, the photogenerated holes degrade

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Figure 5.6 Chemical structure of methyl violet and rhodamine B dyes.

the MB dye absorbed on the surface of metal chalcogenides. Thus they enhanced the photocatalytic activity. The photocatalytic dye degradation efficiency of GO, NiS, and GO/NiS nanocomposites was investigated using crystal violet (CV) dye. The GO/NiS nanocomposite exhibited good photocatalytic activity as compared to NiS, as well as GO. The optimum condition obtained for the effective photocatalytic degradation of CV is pH 5 8.0, CV 5 2.0 3 1025 M, and nanocomposite 5 0.30 g. The rate of degradation of CV with the composite was found to be 2.39 3 1024 s21 [10]. The synthesized rGO@NiO nanocomposites exhibited improved photocatalytic performance than the pure GO and NiO nanoparticles. The photocatalytic activity of prepared rGO@NiO nanocomposite was evaluated by using rhodamine B and methyl violet dyes. Significantly, by virtue of these characteristics, the rGO@NiO nanocomposite is a promising material for the development of high-performance supercapacitors and the remediation of polluted water [11]. The structures of methyl violet and rhodamine B dyes are shown in Fig. 5.6. There are several new developments for the metal chalcogenides. Excellent physical and chemical properties of two-dimensional (2-D) materials have drawn unheard-of interest to them, making them a hotbed of research in disciplines, including physics, chemistry, and materials [12]. The characteristics of 2-D materials can be extensively regulated and optimized by physically or covalently connecting organic molecules to 2-D materials. These materials can be used for industrial effluent treatment (Fig. 5.7). Due to their size-dependent photoemission properties and quantum confinement, metal chalcogenide quantum dots (QDs) have received a lot of attention. One of the first nanotechnology products to be launched for imaging cells and tracking macromolecules is QDs. To create various QDs, a variety of physical, chemical, and biological techniques have been devised. By adhering to green chemistry principles throughout the biological manufacture of QDs, harmful chemicals, high temperatures, high pressures, and the formation of byproducts are either limited or altogether avoided. Fig. 5.8 shows the future directions for improving biological metal chalcogenide synthesis [13].

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Exfoliation and then organic modification (E-M) (reported strategy) Organic functional group

Exfoliation Postmodification

Bulk materials

2D materials

Organically modified 2D materials with partially and disorderly arranged functional groups

Organic modification and then exfoliation (M-E) (new strategy)

Electron-donating O

OH F

SH

SH

O

SH

OH

SH

= -COOH, -F, -OCH3, -OH, -NH2 NH2

SH

_

= Cu , Ag+, Au+

Exfoliation

Coordination

=S

Electronegativity

Functional group presynthetically designed MOFs

Organic Metal Chalcogenides (OMCs) with periodically arranged functional groups

Figure 5.7 Schematic illustration of the 2-D metal chalcogenides [12]. Source: From Y. Li, X. Jiang, Z. Fu, Q. Huang, G.-E. Wang, W.-H. Deng, C. Wang, Z. Li, W. Yin, B. Chen, G. Xu, Coordination assembly of 2D ordered organic metal chalcogenides with widely tunable electronic band gaps, Nat. Commun. 11 (1) (2020). https://doi.org/ 10.1038/s41467-019-14136-8.

Figure 5.8 Diagram of metal chalcogenide quantum dots application [13]. Source: From J. Mal, Y. V. Nancharaiah, E. D. Van Hullebusch, P.N.L. Lens, Metal chalcogenide quantum dots: biotechnological synthesis and applications, RSC Adv. 6 (47) (2016) 41477
41495. https://doi.org/10.1039/c6ra08447h.

Table 5.1 Comparison of azo dye degradation by using various photocatalysts [15]. Catalyst

Dye

Concentration

Catalyst loading

Light source

Lamp

Time (min)

Degradation (%)

CdS

Reactive red 141 Reactive red 141 Reactive red 141 Congo red Congo red

10 mg/L

50 mg

Visible

13 W

240

95

30 mg/L




UV

90

90

10 mg/L

50 mg

UV

125 W

240

95

2.3 3 1025 M 50 mg/L

50 mg 50 mg

UV Visible

60 180

98 90

10 mg/L

50 mg

Visible

100 W 250 W Halogen lamp 15 W

240

100

10 mg/L

50 mg

Visible

15 W

80

96

TiO2 ZnO Pd/ZnO W/TiO2 CdS CdS

Reactive red 141 Congo red

Source: Data from T. Senasu, K. Hemavibool, S. Nanan, Hydrothermally grown CdS nanoparticles for photodegradation of anionic azo dyes under UV-visible light irradiation, RSC Adv. 8 (40) (2018) 22592
22605. https://doi.org/10.1039/c8ra02061b.

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Figure 5.9 Structures of RR141 and CR dyes.

According to magnetic studies, Fe and Mn doping transforms diamagnetic CuSe into paramagnetic material. The synthesized nanostructures’ ability to detect gases was evaluated using NO2 gas. The most sensitive nanodiscs were hexagonal, reaching 25% at an NO2 concentration of 40 ppm. Under UV
visible light exposure, MB dye was photodegraded using the nanostructures. The degradation rate increased from 59.4% to 64.5% as a result of the addition of the transition metal dopants, which improved the material’s photocatalytic activity [14]. The reactive red (RR141) and Congo red (CR) azo dyes were further degraded using the CdS semiconductor as an efficient photocatalyst. RR141 and CR dyes both degrade with a high photodegradation efficiency of 99.8% and 99.0% from the catalyst, respectively. According to the chemical kinetics analysis, both azo dyes are degraded by photocatalysis using first-order kinetics. The photodegradation of the azo dye was mostly caused by the photogenerated hole. After photodegradation, the CdS photocatalyst’s chemical composition remained constant. After the sixth cycle of reusing it, the CdS photocatalyst still operates at its initial efficiency. This demonstrates the benefits of durability and stability. For the purpose of protecting the environment, highly toxic and dangerous organic materials can be removed using CdS nanostructures [15]. The comparison of azo dye degradation by using various photocatalysts is shown in Table 5.1. The structures of RR141 and CR dyes are shown in Fig. 5.9.

5.3

Conclusion

Metal chalcogenides (metal sulfide, selenide, and telluride) have received attention due to their photodegrading features to treat wastewater. It retains its original efficiency after five or six cycles of reuse. This indicates the advantages of stability and reusability. Photocatalysis needs to be competitive with other advanced oxidation process technologies to advance water treatment systems. Before further modification to produce a high-activity photocatalyst suited for industrial applications, a main strategy in photocatalytic degradation study should be to compare photocatalysts systematically. The extent of treatment should take large pollutants into account.

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References [1] B. Karthikeyan, R. Elancheran, K. Sivasankari, Nanoscale self-assembly, Nanoscale Processing, Elsevier, India, 2020, pp. 237
245. Available from: https://www.elsevier. com/books/nanoscale-processing/thomas/978-0-12-820569-3, http://doi.org/10.1016/ B978-0-12-820569-3.00009-8. [2] B. Karthikeyan, R. Govindhan, M. Amutheesan, M. Gundhavi devi, R. Elancheran, Advanced nanostructured membranes, Nanomaterials for Air Remediation, Elsevier, India, 2020, pp. 295
308. Available from: https://www.elsevier.com/books/nanomaterials-for-air-remediation/amrane/978-0-12-818821-7, http://doi.org/10.1016/B978-0-12818821-7.00015-4. [3] R. Jamee, R. Siddique, Biodegradation of synthetic dyes of textile effluent by microorganisms: an environmentally and economically sustainable approach, Eur. J. Microbiol. Immunol. 9 (4) (2019) 114
118. Available from: https://doi.org/10.1556/ 1886.2019.00018. [4] J. Yin, J. Jin, H. Lin, Z. Yin, J. Li, M. Lu, et al., Optimized metal chalcogenides for boosting water splitting, Adv. Sci. 7 (10) (2020). Available from: https://doi.org/ 10.1002/(ISSN)2198-3844.10.1002/advs.201903070, http://onlinelibrary.wiley.com/ journal/. [5] A. Shoneye, J. Sen Chang, M.N. Chong, J. Tang, Recent progress in photocatalytic degradation of chlorinated phenols and reduction of heavy metal ions in water by TiO2-based catalysts, Int. Mater. Rev. 67 (1) (2022) 47
64. Available from: https://doi.org/10.1080/ 09506608.2021.1891368, http://www.tandfonline.com/loi/yimr20#.VwHbh01f1Qs. [6] S. Dong, J. Feng, M. Fan, Y. Pi, L. Hu, X. Han, et al., Recent developments in heterogeneous photocatalytic water treatment using visible light-responsive photocatalysts: a review, RSC Adv. 5 (19) (2015) 14610
14630. Available from: https://doi.org/ 10.1039/c4ra13734e, http://pubs.rsc.org/en/journals/journalissues. [7] M.A. Al-Ghouti, M.A. Al-Kaabi, M.Y. Ashfaq, D.A. Da’na, Produced water characteristics, treatment and reuse: a review, J. Water Process Eng. 28 (2019) 222
239. Available from: https://doi.org/10.1016/j.jwpe.2019.02.001, http://www.journals.elsevier.com/journal-of-water-process-engineering/. [8] A.G. Rinco´n, C. Pulgarin, Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: implications in solar water disinfection, Appl. Catal. B: Environ. 51 (4) (2004) 283
302. Available from: https://doi.org/ 10.1016/j.apcatb.2004.03.007. [9] I.J. Ani, U.G. Akpan, M.A. Olutoye, B.H. Hameed, Photocatalytic degradation of pollutants in petroleum refinery wastewater by TiO2- and ZnO-based photocatalysts: recent development, J. Clean. Prod. 205 (2018) 930
954. Available from: https://doi.org/10.1016/j.jclepro.2018.08.189, https://www.journals.elsevier.com/journal-of-cleaner-production. [10] V. Manikandan, R. Elancheran, P. Revathi, P. Suganya, K. Krishnasamy, Efficient photocatalytic degradation of crystal violet by using graphene oxide/nickel sulphide nanocomposites, Bull. Mater. Sci. 43 (1) (2020). Available from: https://doi.org/ 10.1007/s12034-020-02227-y, http://www.ias.ac.in/matersci/. [11] V. Manikandan, R. Elancheran, P. Revathi, U. Vanitha, P. Suganya, K. Krishnasamy, Synthesis, characterization, photocatalytic and electrochemical studies of reduced graphene oxide doped nickel oxide nanocomposites, Asian J. Chem. 33 (2) (2021) 411
422. Available from: https://doi.org/10.14233/ajchem.2021.22979, http://www.asianjournalofchemistry.co.in/User/SearchArticle.aspx?Volume 5 33&Issue 5 2&Article 5 &Criteria 5 .

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[12] Y. Li, X. Jiang, Z. Fu, Q. Huang, G.-E. Wang, W.-H. Deng, et al., Coordination assembly of 2D ordered organic metal chalcogenides with widely tunable electronic band gaps, Nat. Commun. 11 (1) (2020). Available from: https://doi.org/10.1038/s41467019-14136-8. [13] J. Mal, Y.V. Nancharaiah, E.D. Van Hullebusch, P.N.L. Lens, Metal chalcogenide quantum dots: biotechnological synthesis and applications, RSC Adv. 6 (47) (2016) 41477
41495. Available from: https://doi.org/10.1039/c6ra08447h, http://pubs.rsc.org/ en/journals/journalissues. [14] S. Masrat, R. Poolla, P. Dipak, M.B. Zaman, Rapid hydrothermal synthesis of highly crystalline transition metal (Mn & Fe) doped CuSe nanostructures: applications in wastewater treatment and room temperature gas sensing, Surf. Interfaces 23 (2021). Available from: https://doi.org/10.1016/j.surfin.2021.100973, http://www.journals.elsevier.com/surfaces-and-interfaces. [15] T. Senasu, K. Hemavibool, S. Nanan, Hydrothermally grown CdS nanoparticles for photodegradation of anionic azo dyes under UV-visible light irradiation, RSC Adv. 8 (40) (2018) 22592
22605. Available from: https://doi.org/10.1039/c8ra02061b, http:// pubs.rsc.org/en/journals/journal/ra.

Heterostructured transition metal chalcogenides photocatalysts for organic contaminants degradation

6

Aarti Sharma, Gagandeep Kaur, Madhvi Garg and Dhiraj Sud Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, Deemed University, Sangrur, Punjab, India

6.1

Introduction

The industrial and agricultural activities of the 20th century grew at a breakneck pace, resulting in major increases in water pollution. The run-offs from industries have many toxic and nondegradable contaminants that deteriorate the quality of drinking water. It includes synthetic dyes, pesticides, pharmaceuticals, plasticizers, heavy metals, cosmetics, detergents, etc., that are usually released into the aquatic system (Fig. 6.1). Though the agencies for environment safety have organized programs to aware people adopting greener eco-friendly strategies for production and reduce consumption of resources. However, the release of organic contaminants into the environment metrics has become one of the global critical issues that impact the survival of living beings [1].

6.2

Methods for wastewater treatment

Several treatment methods involving ion exchange, membrane separation, coagulation, flocculation, ultrasonic/bio-degradation, adsorption, and advanced oxidation processes (AOPs) [2,3] have been used for the exclusion of organic contaminants from wastewater (Fig. 6.2). Among them, AOPs are one the sustainable solutions for the degradation of organic contaminants including photocatalysis, ozonolysis, electrocatalysis, and Fenton reaction. Photocatalysis is a discipline of chemistry that studies chemical processes that occur when light and a photocatalyst are present. AOPs depend on the photo-assisted production of highly reactive free radicals such as hydroxyl (dOH), superoxide (dO22), and positively charged holes (h1) that support the complete degradation of organic pollutants [4]. The pictorial representation of the mechanism of photocatalytic degradation of contaminants is shown in Fig. 6.3. A large variety of photocatalysts are available for AOPs, which are further categorized as G

G

Homogeneous photocatalysis Heterogeneous photocatalysis

Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00006-7 © 2024 Elsevier Ltd. All rights reserved.

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Antiinflammatory

Soaps and shampoos

Antibiotics

UV Filters

Pharmaceuticals and Personal care products (PPCS)

Emerging contaminants

Organic compounds

Plasticizers

Artificial sweeteners

Chemical household products

Industrial chemicals Inorganic compounds

Trace metals Flame retardants

Insect repellents

Microplastics Particulates

Nanoplastics

Figure 6.1 An overview of emerging contaminants in wastewater. Source: Reproduced with permission from reference H.E. Dahlke, G.T. LaHue, M.R. Mautner, N.P. Murphy, N.K. Patterson, H. Waterhouse, et al. Advances in chemical pollution, Environ. Manag. Protect. (2018). Copyright American Chemical Society.

Figure 6.2 Common methods for wastewater treatment.

6.2.1 Homogeneous photocatalysis Homogeneous photocatalysis is comprised of a single type of soluble molecular catalysts, including a light energy capturing system commonly known as a photosensitizer, oxidation catalytic sites, and reduction catalytic sites (solution). Common homogeneous photocatalysts are transition-metal complexes due to their adequate stability and energy

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97

Figure 6.3 Mechanism of photocatalytic degradation.

band gap for homogeneous photocatalysis. In the chemical sector, which includes the production of polymers, medicines, and the treatment of wastewater, the contribution of homogeneous catalysts is approximately 20%. Ozone production and the photoFenton process (also known as the Fe1 and Fe1/H2O2 systems) are two of the most common reactions carried out by homogeneous photocatalysts. These reactions are like a chain reaction that follows the generation of OHd radicals that occurs during the ozone process [5]. Fe21 1 H2 O2 ! HOd 1 Fe31 1 OH2 Fe21 1 HOd ! Fe31 1 OH2 Fe31 1 H2 O2 ! Fe21 1 HOd2 1 H1 These processes are utilized to convert the solar energy that is collected into an easily available and environmentally friendly fuel in the form of molecular hydrogen [6]. In the field of photo-induced catalytic H2 synthesis using homogeneous molecular catalysts, significant progress has been made in recent years [7,8].

6.2.2 Heterogeneous photocatalysis In heterogeneous photocatalysis, the photocatalyst is often a semiconducting substance that is able to take in the photons that are entering the system. The photocatalytic processes, which include the redox reactions of donors and acceptors, can take place at the junction between the heterogeneous photocatalyst and the gaseous or liquid medium (usually aqueous), or in the proximity of the interface in the fluid [9]. In recent decades, heterogeneous photocatalysts [especially transition metal

98

Metal-Chalcogenide Nanocomposites

Figure 6.4 Differences between homogeneous and heterogeneous photocatalysts.

chalcogenides (TMCs)] such as TiO2, ZnO, Fe2O3, V2O5, CdO, and CdS [10] have been extensively studied for the removal of organic pollutants. In this chapter, the synthesis, characterization, and application of TMCs as photocatalysts have been discussed briefly (Fig. 6.4).

6.3

Transition metal chalcogenides

TMCs are a class of materials composed of transition metal and chalcogen atoms (O, S, Se, and Te), which are of considerable interest due to their unique physiochemical characteristics including low covalent behavior, poor electronegativity, and less thermal conductivity. They are being utilized in several fields such as hydrogen production, photocatalysis, sensor, and electrochemistry, etc., Moreover, metal chalcogenides are readily available in a variety of configurations. In 1970, extensive research efforts have been made to study the chemistry behind the chalcogen bonds, especially for sulfur-containing compounds, also known as metal
chalcogen bonds. It was further extended to tellurium and selenium as well. Each transition metal forms a covalent link with six atoms of chalcogen (chalcogen can be sulfur, selenium, or tellurium), and the different layers form bonds with one another by the interaction of van der Waals forces. Chalcogens possess empty d orbitals, which are associated with readily available energy. Therefore, chalcogen utilized their empty d-orbitals to take part in the creation of bonds with transition

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99

metals, which has the effect of reinforcing the link between metal and chalcogenide and favoring the trigonal prismatic coordination of the chalcogen [11].

6.4

Synthesis methodologies

The synthesis of TMCs and their nanocomposite has been reported to be accomplished through the use of a variety of distinct procedures (Fig. 6.5). These procedures include the hot-plate method, the one-pot heat-up method, a hydro/ solvothermal method, sonochemical electrospinning, and a few other random procedures (Fig. 6.3) [12].

6.4.1 Hot-Plate method This method used a hot metal plate having a specific temperature to carry out a reaction resulting in the growth of metal oxides on the hot surface of the metal plate. For the synthesis, the low-growth temperature points were employed [13]. The parameters such as reaction time, temperature, and precursor ratio must be optimized for getting nanostructured TMCs. Chen and his coworker synthesize the CdS@Mo using metal carbonyl as a precursor on the hot plate [14].

Figure 6.5 Pictorial representation of synthesis methods of TMCs.

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Metal-Chalcogenide Nanocomposites

6.4.2 One-pot heat-up method The one-pot-heat-up method is a process that goes in favor of controllable syntheses and is highly amenable for large-scale production. Chen and coworkers reported this method to have less sensitivity to detail mixing as compared to other methods such as hot-injection methods and, therefore, favorable for large-scale production for varied applications [15].

6.4.3 Hydro/solvothermal method Autoclave made of stainless steel with a Teflon coating for the purpose of solvothermal and hydrothermal synthesis. The metal intermediates and solvents are inserted here after being well mixed. Next, a minimum range of temperature and pressure is asserted on the steel container. After being heated and compressed, the solutions are filtered, cleaned, and dried to produce powdered composites. The dispersion of cobalt
nickel selenide over graphene nanosheets was done by Murugadoss and his team, which was further acquired as a counter-electrode for dye-sensitized solar cells [16]. Akyuz et al. solvothermally synthesized the reducedCd(12x2y)ZnxMoyS, reduced graphene oxide with Cd(12x2y)ZnxNiyS, and reduced graphene oxide-Cd(12xy)ZnxCuyS and employed for photocatalytic applications [17].

6.4.4 Electrospinning The manufacture of TMC or the heterostructure compounds based on them can be accomplished using the inventive and adaptable synthesis technique known as electrospinning. If the syringe is filled with the required viscous precursors, desired high-voltage current can be transmitted to the syringe tip during electrospinning up nanofibers. Wang et al. demonstrate the electro-spun preparation of cobalt (II) selenide@ carbon nanofibers, which was further utilized in lithium-ion batteries as an anodic material having a high reversible capacity of 1405 mAh/g and a current density of 200 mA/g [18].

6.4.5 Sonochemical Highly intense ultrasonic waves can be used to create novel materials such as TMCs while also providing a strange route to various materials without the need for high temperatures and pressures or long reaction times. Sonochemistry, and specifically the production or modification of nanomaterials during ultrasonic irradiation (Table 6.1).

6.5

Characterizations

TMCs were characterized using different physiochemical techniques to evaluate their physical, structural, and other chemical properties. The utilization of

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Table 6.1 Merits and demerits of various synthesis techniques. Synthesis techniques

Merits and demerits

Hydro/ solvothermal

Produce massive, high-quality crystals under conditions of high pressure and low temperature, but the expense of the necessary equipment is expensive. The process takes less time and moves quickly; size can be managed A quick, one-step procedure that produces uniform, highly pure crystals at the electrochemical system’s cathode for the coating process. Higher photon energy is needed; it is possible to create materials with many active areas and a big surface area. Used for large-scale synthesis that is simple and efficient

Microwaveassisted Electrodeposition method Photoreduction Refluxing method

Figure 6.6 Characterization techniques.

TMCs as photocatalysts require information regarding their electronic and photophysical response. The most frequently used analytical techniques are X-ray diffraction analysis (powder X-ray diffraction), morphology studies (scanning electron microscopy), Fourier Transform infrared ray, nuclear magnetic resonance, elemental analysis, surface area analyzer (Brunauer
Emmett
Teller), surface charge analysis (Zeta potential) and thermal analysis [19] (Fig. 6.6).

102

6.6

Metal-Chalcogenide Nanocomposites

TMCs as heterogeneous photocatalysts

For the production of energy and environmental sustainability, it is crucial to develop effective and economical photocatalysts. Due to their quantum confinement effect, optoelectronic activity, and stability, TMCs with nano-sized particles (1
100 nm) have been employed for a variety of applications, including photocatalysis, photovoltaic, and energy storage. TMCs and related heterostructures, in particular, show enormous potential as a new, affordable, and environmentally friendly hydrogen evolution catalyst [20]. Properties, such as low gaps and favorable band potentials, correspond with the visible light portion of the solar spectrum, and numerous TMCs (sulfides and selenides) are regarded as efficient heterogeneous photocatalysts for the degradation of resistant pollutants. CdS and ZnS, with respective band gaps of 2.4 and 3.6 eV, were initially the most-investigated materials for the photocatalytic degradation of harmful organic pollutants [21
23]. The band gap of some TMCs semiconductors is mentioned in Table 6.2.

6.7

Application for photocatalytic degradation of organic pollutants

Due to rapid industrialization, an aging population, and ongoing economic development, the hazards from various water contaminants with different concentrations to the environment and human health have increased. The population growth, rising demand for water supply, and declining water quality aggravated the need for decontamination of drinking water, which is the ultimate concern or motto. Organic chemicals, hazardous inorganic elements, and microorganisms are the three major categories of water contaminants. Amongst these, organic contaminants are briefly discussed here. The organic contaminants are further classified into pharmaceuticals, pesticides, herbicides, Table 6.2 Band gap of transition metal chalcogenides semiconductors. S.No.

Semiconductor

Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11

TiO2 (Anatase) TiO2 (Rutile) WO3 Fe2O3 ZnS CdS ZnSe CdSe ZnTe CdTe CdZnTe

3.2 3.0 2.8 2.3 3.6 2.4 2.8 1.7 2.2 1.6 1.6

Photocatalytic degradation of organic contaminants

103

fertilizers, phenols, detergents, surfactants, and dyes. Over than billion organic compounds, including those used in medications, personal care items, insecticides, and dyes have been discovered, manufactured, or generated. The existence of high concentrations of environmentally hazardous organic pollutants in water sources necessitates the design of sophisticated technology to cleanse the water. Thus, the need for ingenuity in water purification is critical. The commonly used methods for eliminating organic water contaminants are adsorption, AOPs, and photocatalytic degradation [24]. The adsorption technology is efficient at lower concentrations only and it becomes inefficient after blockage of all adsorption sites present on the adsorbent [25]. In AOPs, the major drawback is the potential health concern posed by improperly degraded byproducts [24]. Photocatalytic degradation or photocatalysis is a safe, efficient, and ecofriendly technique that entirely degrades harmful compounds or transforms them into quasistate. Mostly the metal chalcogenides-based photocatalysts (TiO2, ZnO, Fe2O3, ZnS, CdS, SnO2, ZrO2, and WO3) have been investigated for degrading organic pollutants so far. Herein, we discuss the photocatalytic degradation of organic water contaminants via TMC photocatalysts [26].

6.7.1 Dyes

Dyes

Dyes are organic aromatic compounds that have devastating effects on human health, mainly including mutagenic and cancer-causing effects. In the textiles, paper, and plastic sectors, dyes are the usually encountered water contaminants, which can be categorized into acidic or anionic and basic or cationic dyes [24] (Fig. 6.7). Azo dyes constitute the largest class of dyestuffs (60%
70%), therefore, are mainly focused on by various research groups [27]. Bansal et al. have utilized TiO2 as a photocatalyst for the degradation of azo dye viz. acid orange 7 under UV light Type

Example

Azo dyes

Methyl Orange (MO)

Xanthene dyes

Rhodamine B (RhB)

Anthraquinone dyes

Alizarin Red S (ARS)

Triphenylmethane dyes

Methyl Violet (MV)

Thiazine dyes

Methylene Blue (MB)

Nitro dyes

Martius yellow

Figure 6.7 Classification of dyes with their example.

104

Metal-Chalcogenide Nanocomposites

and they have determined the degradation products and reaction mechanism via various analytical techniques (liquid chromatography-Mass spectrometry and ion chromatography) [28]. Lim et al. have synthesized TMC cocatalyst (ZnIn2S4/MoS2, ZnIn2S4/CuS, and ZnIn2S4/Ag2S) using a photo-assisted deposition approach. They have proved that this technique is more beneficial because it does not impair the crystal structure of the host catalyst (ZnIn2S4) as well as exceptional methyl orange dye degradation efficiency under photo-irradiation [29]. Kansal et al. has utilized various TMCs (TiO2, ZnO, ZnS, CdS, and SnO2) as photocatalysts for the remediation of methyl orange and rhodamine 6G. The photocatalytic experiments were performed to optimize the reaction parameters (catalyst dosage, concentration, and pH of dye solution) under a UV/solar system and the maximum efficiency obtained was .90%. It was also investigated that the degradation rate was high in the case of solar light [30]. Reactive dyes constitute 12% of all synthetic dyes. Numerous efforts have been made for the photocatalysis of these reactive dyes. Bansal et al. have applied transition metal oxides (TiO2 and ZnO) for the photocatalysis of reactive dye (procion blue HERD) under UV irradiation. They have observed that ZnO and TiO2 decolorize 100% of the reactive dye at pH 7 with 1 g/L and pH 4 with 1.5 g/L photocatalyst dosage, respectively. They have also extended their studies to textile effluents for testing the feasibility of the approach on real wastewater [31]. The gas chromatography-mass spectrometry analytical technique has been applied to determine intermediates and to investigate which reaction pathway contains which reactions [32]. Chauhan et al. have synthesized the nanoparticles of ZnS and Cu-doped ZnS with various 3, 5, and 10 mol.% Cu doping concentrations via the chemical precipitation method. Their photocatalytic activity was investigated for decolorization of methylene blue and optimum results were obtained with 3 mol.% Cu doping under visible light in 180 min [33]. Atlaf et al. have developed transition metal (V, Nb, and Ta) selenides using solid-state hydrothermal technique and used them as photocatalysts for the remediation of methylene blue under the radiation of wavelength 400
700 nm [34]. Some other reports on the photocatalytic degradation of dyes via TMCs are compiled in Table 6.3.

6.7.2 Pesticides and endocrine disruptors In order to meet the rising global need for food, particularly, crops, fruits, and vegetables, pesticides are essential. However, the extensive use of pesticides in the agriculture sector is inextricably linked to the severe environmental crisis. Pesticides can be categorized as insecticides, herbicides, and fungicides. Further, the classification of pesticides is given in Fig. 6.8. Pesticides are increasingly causing water pollution, particularly in agricultural nations. They have a negative impact on human health after coming in contact with and eventually getting into the human food chain via its remnants in fruits and vegetables. Similarly, endocrine disruptors (ED) have also aroused issues to human health due to their partial remediation by wastewater treatment (WWT) plants [24]. Therefore, various review articles have emphasized photocatalytic degradation studies of diverse classes of pesticides [42]. The classification of pesticides is given in Fig. 6.8.

Table 6.3 Transition metal chalcogenides-based nanomaterials for the photocatalysis of dyes. Transition metal chalcogenides

Synthesis method

Targeted dye

Light irradiation

Photocatalytic efficiency

References

TiO2 ZnIn2S4/MoS2


Photo-assisted deposition technique


Acid orange 7 Methyl orange

UV Visible

92% 84%

[28] [29]

Methyl orange Rhodamine 6 G Procion blue HERD Methylene blue

UV UV UV

98.5% (pH 8) 97.9% (pH 10) 100% (pH 4) 99.78% (pH 7) 100%

[30]

90% 99.9% 98.5% 90.3% 92.4% 94.5% 93.3%

[35] [36] [37]

ZnO TiO2 ZnO Cu doped ZnS CdS TiO2 Immobilized TiO2



Chemical precipitation method Greener synthesis
Acid catalyzed sol-gel method

Fe3O4/V2O5 graphene oxide nanosheets

Chemical coprecipitation method

MoS2/C3N4 heterostructures

Hydrothermal method

FeS2 Fe2O3 MoS2/TiO2 WS2/TiO2

Hydrothermal method

Methylene blue Malachite green Methyl orange Methylene blue Indigo carmine Bismarck brown Acid orange 7 Rhodamine B Methyl orange Methylene blue

Hydrothermal method

Methylene blue

UV UV UV UV-Vis

sunlight

[31] [33]

[38]

Visible




[39]

Visible

[40]

Visible

75% 65%


[41]

106

Metal-Chalcogenide Nanocomposites

Organochlorines eg. DDT

Pesticides

Organophosphate eg. Malathion Organic Carbamates eg. Propoxur

Synthetic Inorganic Plant based eg. Pyrethrum

Pyrethroids eg. Deltamethrin

Natural Mineral oil

Figure 6.8 Classification of pesticides with their examples.

Many research reports have shown the synthesis of transition metal oxides and sulfides doped metal oxides, their composites with other metal oxides and their immobilized versions, and their utilization in photocatalysis of different frequently used pesticides and chlorophenols (ED) compiled in Table 6.4. Ayodhya et al. used the coprecipitation technique for the manufacture of zinc sulfide nanoparticles (ZnS NPs), with Schiff base as a capping agent (2-[(4-methoxy-phenylimino)-methyl]-4-nitrophenol) and the synthesized ZnS NPs have the potential to be both a sensitive probe sensor for heavy metal ion detection and even a preferential photocatalyst for the destruction of harmful contaminants. Sol-gel approach was used by Abdennouri et al. to create titanium dioxide and titanium dioxide photocatalysts that had been modified with tungsten. They discovered that tungsten-doped TiO2 significantly improved the photocatalytic activity for the photocatalysis of the two chosen organochlorine herbicides when compared to untreated TiO2. Alhaddad et al. used the Sol-gel method to create hexagonal CuS nanomaterials in a single step. Then, to create colloidal CuS@rGO heterogeneous catalysts, CuS nanostructures are aggregated onto varying numbers of graphene oxide nanosheets. The enhanced surface qualities of the produced CuS and the addition of rGO, which suppresses photoexcited charge recombination, are credited with the higher photocatalytic activity. Avasarala et al. construct Mg-doped TiO2 with various Mg concentrations by sol-gel process. The catalyst absorbs light in the visible area when doped with the Mg21 ion, according to UVvisible spectroscopic analysis. Degradation of the pesticide monocrotophos has been used to assess the photocatalytic performance of doped TiO2. Ayodhya et al. successfully made ZnxAg12xS ternary semiconductor composites with a quick and inexpensive hydrothermal technique without the need for any surfactants. The ternary composite nanomaterials had well-developed crystalline frameworks, wide surface areas of 15
70 m2/g, diameters of 10
30 nm, and exceptional UV light absorption properties, according to combined investigations employing XRD, N2 sorption, SEM, TEM, and UV-vis Drs. Bhoi et al. using CuS/BiFeO3 heterojunction materials, a long-lasting visible light-assisted photocatalytic pathway has been created for the degradation of the

Table 6.4 Transition metal chalcogenides-based nanomaterials for the photocatalysis of pesticides. Transition metal chalcogenides

Synthesis method

Targeted pesticides or endocrine disruptors

Light irradiation

Photocatalytic efficiency

References

Schiff base coordinated ZnS W/TiO2

Coprecipitation method

Chlorpyrifos

UV

85.29%

[43]

Sol-gel method

2,4-dichlorophenoxyacetic acid 2-(2,4-dichlorophenoxy) propanoic acid Atrazine

UV

. 90% . 80%

[44]

Visible

100%

[45]

Monocrotophos Malathion Monocrotophos Chlorpyrifos Alachor

Visible UV

[46] [47]

Visible


96.4% 92.51% 91.38% .95%

Chlorpyrifos Atrazine Atrazine

Solar Visible Visible


.99% 95.5%

[49] [50] [51]

Postsynthesis hydrolysis Ultrasonic-assisted impregnation method Photodeposition method
In situ growth method

Dimethoate Diazinon

Solar Visible

100% 95.07%

[52] [53]

Malathion Quinalphos Bisphenol A

UV UV Solar

98% 89% 88.12%

[54] [55] [56]

Ultrasound-assisted method


2-Chlorophenol 2,4,6-Trichlorophenol

Visible UV

79.8% 88%

[57] [58]

CuS assembled graphene oxide Mg-doped TiO2 ZnxAg1-xS

CuS/BiFeO3 The doped TiO2 CuFe2O4 In, S codoped TiO2 graphene oxide TiO2/SBA-15 TiO2/Fe2O3 nanopowder Zn doped TiO2 TiO2 ZnO/ TiO2 graphene oxide N doped TiO2 TiO2

Sol-gel method Sol-gel method Hydrothermal method

Two-step Hydrothermal method Sol-gel method Sol-gel combustion method

[48]

108

Metal-Chalcogenide Nanocomposites

pesticide alachlor. In two steps, the heterojunctions were created and characterized. Devi et al. used V51, Mo61 (ions of transition metal), and Th41 (ion of inner transition metal) and doped them into TiO2 to create photocatalysts. Chlorpyrifos (CP) was used as a target analyte in the investigation of these catalysts’ photocatalytic activity. The findings of the X-ray diffraction experiment exclusively revealed the anatase phase. When exposed to UV light, the photocatalytic efficiency of undoped TiO2 was more effective than that of doped catalysts. The Th41 (0.06%)-TiO2 showed the highest degree of activity for the degradation of CP when illuminated by solar light. Li et al. synthesized magnetic nanoparticles, CuFe2O4 (CFO), and applied a 3D electrochemical method for atrazine (ATZ) degradation as both a catalyst and particle electrode for persulfate (PS). Researchers investigated how operating factors, such as CFO dosage, PS concentration, starting pH, and current density, affected the degradation of ATZ. Under ideal circumstances, the CFO magnetic nanoparticle electrodes demonstrated satisfactory stability over the course of the five runs. Quenching tests and an electron paramagnetic resonance analysis showed that free radicals were responsible for the destruction of ATZ. Khavar et al. applied a novel ultrasound solvothermal technique to create the In,S-TiO2@rGO nanoparticle that is visible-light activated. When TiO2 was combined with rGO and codoped including In and S elements, their visible-light catalytic performance was greatly improved. The key mechanism that led to the destruction of ATZ in the established photocatalytic process was a reaction with hydroxy free radicals at the catalyst’s surface. The key mechanism that led to the degradation of ATZ in the established photocatalytic process was a reaction with radical dOH at the catalyst surface.

6.7.3 Pharmaceuticals

Pharmacological classification of drugs

Pharmaceuticals include medicines, prescription treatments, and veterinary medications, which are either natural or synthetic organic compounds. Pharmacologically drugs are classified as neurologically active, antihistamines, antacids, antifertility drugs, and antimicrobials (Fig. 6.9). Examples of some categories are antibiotics (chloramphenicol, sulfamethoxazole, tetracycline, amoxicillin, ofloxacin, enrofloxacin), analgesics

Neurologically active drugs

Tranquilizers Analgesics

Antihistamines Antacids Antifertility drugs Antimicrobials

Figure 6.9 Pharmacological classification of drugs.

Antibiotics Antiseptics and disinfectants

Photocatalytic degradation of organic contaminants

109

(acetaminophen and paracetamol) antihistamines (diphenhydramine and salbutamol), antiinflammatory (diclofenac). The origin of pharmaceutical contamination occurs from various elements such as inappropriate drug disposal, human defecation, clinical effluents, and drug industry waste. It has been found that several antibiotics and analgesics still retain 90% of their constituents, 30
40 years after their manufacturing. Consequently, there is a need for their elimination from aqueous streams because prolonged exposure to medications in water poses a risk to humans [25]. Numerous studies have been reported on the photocatalysis of pharmaceutical drugs using TMCs and their modified nanomaterials such as TiO2 (Degussa) for sulfamethoxazole [59], acetaminophen [60], diclofenac [61], salbutamol [62], and chloramphenicol [63] degradation. Various groups have checked the photocatalytic efficiency of TiO2 (Degussa) for the degradation of different pharmaceutical drugs shown here. Abellan et al. have used titania for the degradation of antibiotic and sulfamethoxazole and 82% degradation were observed with catalyst dosage 0.5 g/L. The LC-Ms analysis has been used to determine the mechanism of photocatalysis reactions and intermediates. The reduction in the aromatic content was also confirmed by SUVA parameter [59]. Radjenovic et al. have utilized P-25 TiO2 for the photocatalysis of an antiinflammatory drug, acetaminophen in a pilot plant under natural illumination. With the reaction conditions of photocatalysis 10 mg/L initial concentration of acetaminophen, 0.2 g/L catalyst dosage, and 5 mg/L of Fe dosage, 84% photocatalytic efficiency was obtained [60]. Calza et al. have applied an aqueous suspension of TiO2 Degussa for photocatalytic degradation of the antiinflammatory drug, diclofenac, under solar light. The RSM software was used to get the optimum conditions for the photo mineralization process. The LC-Ms analysis was applied for the elucidation of the mechanism and to identify the intermediates. The ecotoxicity of the diclofenac solution was determined by applying a microtox bioassay [61]. Then some researchers immobilized the TiO2 nanoparticles on support materials such as graphite or graphene oxide to avoid the filtration issue in the photocatalysis application of drugs. Vaiano et al. have synthesized TiO2-graphite nanocomposites (TiO2-G) using various amounts of graphite powder (G), and the resulting samples were assessed by UV-vis diffuse reflectance spectra, it was clear that TiO2-G nanocomposites exhibited absorption in the visible range, but the band-gap energy value was comparable to that of pure TiO2. The best results were obtained with 0.026% G loading in TiO2-G nanocomposite, that almost complete degradation of paracetamol occurred within 120 min of UV light illumination [64]. Pastrana Martinez et al. have prepared nanocomposites of TiO2 nanoparticles with graphene oxide using the liquid phase deposition method and then partial reduction of graphene oxide was done at different temperatures. The synthesized nanocomposites were utilized as photocatalysts for the degradation of a pharmaceutical drug, diphenhydramine, under UV-Vis light. The optimum nanocomposites consist of 3.3
4 wt.% graphene oxide loading and given thermal treatment at 200 C show complete degradation of diphenhydramine [65]. Several research groups have made the composites of TiO2 with other TMCs or metal chalcogenides such as WO3 [66,67], Bi2O3 [68], CdS [69], MoS2 [70], SnS2 [71], CuS [72], and Fe2O3 [73] for the photocatalysis of different pharmaceuticals (Table 6.5). Mugunthan et al. have synthesized TiO2-WO3 composites using

Table 6.5 Transition metal chalcogenides-based nanomaterials for the photocatalysis of pharmaceuticals. Transition metal chalcogenides

Synthesis method

Targeted pharmaceuticals

Light irradiation

Photocatalytic efficiency

References

TiO2 TiO2 TiO2 TiO2 TiO2 ZnO TiO2-graphite composite TiO2-reduced graphene oxide TiO2-WO3 TiO2-WO3 Bi2O3-TiO2 CdS-TiO2 MoS2-TiO2 TiO2/SnS2 films CuS-TiO2 nanobelts








Sulfamethoxazole Acetaminophen Diclofenac Salbutamol Chloramphenicol

UV UV Solar UV UV

[59] [60] [61] [62] [63]

Coprecipitation method

Paracetamol

UV

82% 84% .95% 93% 100% 90% 88%

Liquid phase deposition

Diphenhydramine

UV-Vis




[65]

Hydrothermal method Sol-gel method Hydrothermal method Hydrothermal method Wet chemical approach Solid-state ion exchange method Successive ionic layer adsorption and reaction (SILAR) method Ultrasound-assisted method

Diclofenac Amoxicillin Ofloxacin Ofloxacin Acetaminophen 17β-estradiol Enrofloxacin

Visible solar Solar Visible Solar Solar Solar

91% 64.4% 92% 86% 40% 51% 85.5%

[66] [67] [68] [69] [70] [71] [72]

Tetracycline

Visible

89.41%

[73]

One pot hydrothermal method

Tetracycline

Solar

76%

[74]

TiO2-Fe2O3 Carbon nanotubes MoS2/CuBi2O4

[64]

Photocatalytic degradation of organic contaminants

111

hydrothermal method and evaluated their photocatalytic performance for the diclofenac degradation under visible light. The results revealed that 91% photocatalytic degradation of diclofenac occurs in 4 h of illumination and intermediates were identified through LC-Ms analysis [66]. Sood et al. have synthesized heterostructures of Bi2O3TiO2 and analysis revealed that it constitutes anatase phase of TiO2 and monoclinic α-Bi2O3. They selected an antibiotic drug, ofloxacin as a model pollutant, and full degradation was observed within 2 h under solar light [68]. Kaur et al. have synthesized CdS/TiO2 nanocomposite via the hydrothermal method and confirmed the formation of nanoparticles using different spectroscopic analyses. The photocatalytic efficiency of nanoparticles was assessed for ofloxacin under visible light and 86% degradation of ofloxacin occurred within 180 min of illumination [69]. Kumar et al. have developed nanocomposite of MoS2-TiO2 using a wet chemical approach and utilized them as a photocatalyst for the removal of pharmaceutical pollutant, paracetamol, under visible light [70]. Kovacic et al. have synthesized TiO2 and SnS2 nanoparticles and immobilized them on iron-exchanged zeolites (FeZ). The results of XRD analysis confirmed the formation of the crystalline phase of TiO2 and SnS2 nanoparticles. The Dr’s spectra show the lowering of the band gap of nanoparticles on increasing loading % of SnS2 and the amount of FeZ in the nanocomposites. The nanocomposites show 78.1% degradation of an estrogenic hormone, 17β-estradiol, under solar light and it was compared with bare TiO2 (Degussa), TiO2/SnS2, and TiO2/SnS2/H2O2 [71]. Jiang et al. have synthesized nanobelts of CuS/TiO2 and immobilized them on titanium foil. The synthesized photocatalyst exhibited enhanced efficiency for the degradation of the antibiotic drug, enrofloxacin, under solar light due to improved light harvesting capability in the visible region [72]. Lu et al. have successfully developed nanocomposites of TiO2/Fe2O3 and carbon nanotubes and Guo et al. have synthesized CuBi2O4/MoS2 composites having distinctive heterostructure using the one-pot hydrothermal method for the photocatalysis of antibacterial drug, tetracycline under visible light [73].

6.8

Conclusion

The existence of high concentrations of environmentally hazardous organic pollutants in water sources necessitates the design of sophisticated technology to cleanse the water. Thus, the need for ingenuity in water purification is critical. Transition metal-based chalcogenides have a narrow-band gap and have been extensively explored as photocatalysts. These narrow-band gap semiconductors absorb 50% of solar light in the visible range. These materials generate e2/h1 pairs in the presence of visible light. Photo-generated e2/h1 pairs have been utilized to degrade synthetic dyes, pesticides, pharmaceuticals, plasticizers, heavy metals, cosmetics, detergents, etc., that are usually released into the aquatic system. Photocatalytic degradation is a safe, efficient, and eco-friendly technique that entirely degrades harmful compounds or transforms them into quasi-state in this perspective chapter, we also highlight the synthesis of TMCs accomplished through the use of a variety of distinct procedures along with their characterization techniques.

112

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94.

Chalcogenides and their nanocomposites in heavy metal decontamination

7

Tshimangadzo S. Munonde1,2, Shirley Kholofelo Selahle1,2 and Philiswa Nosizo Nomngongo1,2 1 Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa, 2Department of Science and Innovation-National Research Foundation South African Research Chair Initiative (DSI-NRF SARChI) in Nanotechnology for Water, University of Johannesburg, Doornfontein, South Africa

7.1

Introduction

The mitigation of heavy metals (HMs) contamination remains one of the challenges of the 21st century. As a result, the HMs accumulation within the environmental compartments has become a major threat to public and environmental health [1,2]. Owing to their persistence, nonbiodegradability, and environmental stability, HMs pose a serious threat to the water quality and the food chain [3,4]. To some extent, HMs accumulation within the environment results from natural activities such as bedrock weathering, volcanism, erosion, atmospheric precipitation, and bioturbation, amongst others [5]. However, anthropogenic activities, such as mining, rapid industrialization, urbanization, smelting, agricultural waste runoff, and transport, amongst others, are the major sources of HMs accumulation in the environment, which, in turn, impact water and food security [6,7]. This phenomenon yielded the development of many methods that aim to address the HMs contamination within the environment. Subsequently, various physical and chemical methods such as photocatalysis [8,9], adsorption [10,11], ion exchange [12,13], and coagulation-flocculation [14,15], amongst others, have been used for the removal of HMs in environmental water samples. Amongst these methods, adsorption and photocatalysis have emerged as cheaper and more effective methods than chemical or other physical methods due to their simplicity in design, cost-effectiveness, and energy efficiency [16]. However, adsorption often leads to secondary pollution due to the excessive waste products generated during the process [17], which has promoted the use of photocatalysis that often reduces toxic pollutants to their less toxic counterparts, with no secondary pollution left by the degraded substances [17
19]. Subsequently, the process of photocatalytic degradation primarily involves light-induced oxidation/reduction of compounds in the presence of a photocatalyst, which leads to the generation of photoexcited electron (e2) and hole (h1) pairs in the conduction band (CB) and valence band (VB), respectively [20]. Mechanistically, it can be observed that the photocatalytic reactions are based on the generation and recombination of reductive (e2)/oxidative (h1) process Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00007-9 © 2024 Elsevier Ltd. All rights reserved.

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across a photocatalyst (usually a semiconductor material), leading to a chain of redox reactions and finally the degraded products, as shown in Fig. 7.1. The photocatalyst, a semiconductor material used in photocatalytic reactions is a key component of the process, enabling visible light-driven reactions to take place, as shown in Fig. 7.1. Recently, photocatalytic degradation of HMs using various chalcogenide nanomaterials has emerged as a potential method for HMs decontamination. Interestingly, HMs have been reported to act as chalcophiles, as they display a strong specific affinity toward soft chalcogenides such as S, Se, and Te [22]. Thus, this strong affinity has resulted in the emergence of various chalcogenide nanomaterials and chalcogenide nanocomposites for the decontamination of HMs through the photocatalytic degradation process. This chapter, therefore, gives an overview of the application of various chalcogenide nanomaterials and chalcogenide nanocomposites in heavy metal decontamination from various environmental water matrices. The reported ion exchange, adsorption, and photocatalysis methods for the decontamination of HMs using chalcogenides and chalcogenide nanocomposites have been reviewed.

7.2

Traditional heavy metal treatment

7.2.1 Ion exchange methods In recent years, the ion exchange process has been increasingly used for the removal of HMs, as well as the recovery of precious metals [23,24]. Ion exchange is a versatile separation process with the potential for broad applications in wastewater treatment fields [25]. For instance, a novel ion-exchange oxy-sulfide material with a three-dimensional (3D)

Figure 7.1 Schematic representation of the oxidative and reductive photocatalytic reactions involved in heavy metal decontamination. Adopted from Gao et al. (2021), MDPI, open access. Steps involved in the semiconductor surface: (1) photon absorption; (2) generation of excited charge carriers; (3) charge separation and migration to the surface; charge recombination of excited charge carriers (4) in the bulk and (5) on the surface; and surface (6) reduction and (7) oxidation reactions [21]. Source: Adopted with permission from X. Li, J. Yu, C. Jiang, Principle and surface science of photocatalysis, in: Interface Science and Technology, vol. 31. Elsevier B.V., 2020, pp. 1
38. https://doi.org/10.1016/B978-0-08-102890-2.00001-4; X. Gao, X. Meng, Photocatalysis for heavy metal treatment: a review. processes 9 (2021) 1729.

Chalcogenides and their nanocomposites in heavy metal decontamination

119

open framework comprising the pseudo-T4 supertetrahedral [In4Sn16O10S34]12
cluster was reported elsewhere [26]. The material was reported to have large pores and it was a fast ion exchanger. Due to the selective bonding interactions of the soft Lewis basic (S22) sites to heavy metal ions, the material exhibited high selectivity in sequestering heavy metal ions from aqueous solutions including solutions containing heavy concentrations of sodium, calcium, ammonium, magnesium, zinc, carbonate, phosphate, and acetate ions. Furthermore, the ion-exchange efficiency in competitive ion-exchange experiments involving mixtures of metal ions was significantly higher than for solutions of single metal ions. The oxychalcogenide clusters were reported to influence the stability of the material and the authors concluded that the incorporation of more oxygen atoms in sulfide networks will create oxysulfide open-framework materials that can combine the semiconductor properties of chalcogenide materials and good stability of oxide materials. Layered nanostructured materials have also demonstrated interesting features when used in the treatment of HMs using the ion exchange process. To this end, Li et al. [27] demonstrated a flowerlike microspheres nanocomposite synthesized by a facile and scalable one-pot solvothermal method. The nanocomposite was composed of layered transition metal dichalcogenides, MX2, where M represents a transition metal (such as W, Mo, and Nb) and X is a chalcogen (S, Se, and Te). These components of nanocomposites have received much attention and research interest due to their unique electronic, mechanical, and chemical properties. The flowerlike WSe2 and WS2 microspheres demonstrated outstanding uptake capacities for Pb21 (288 and 386 mg/g, using WSe2 and WS2, respectively) and Hg21 (1512 and 1954 mg/g, using WSe2 and WS2, respectively) in aqueous samples, displaying great potential in heavy metal remediation. The results in Fig. 7.2 showed that both WSe2 and WS2 were very effective at removing Hg21 at various concentrations, whilst demonstrating moderate removal of Pb21.

7.2.2 Adsorption methods Adsorption is one of the widely used applications adapted to remove HMs from various matrices [28,29]. The metal chalcogenide materials are flexible and can be tuned and incorporated with other materials to form nanocomposites for the decontamination of HMs [30,31]. The adsorption efficiency of the nanocomposites showcases available sorption sites, which, in turn, increases the adsorption efficiency [32]. Various studies have been conducted for heavy metal decontamination using chalcogenide nanomaterials, with some studies demonstrating very high adsorption capacities, selectivity, ion exchange, and high efficiencies. For instance, Celik and colleagues [33] reported the design and synthesis of an economically viable layered double hydroxides—stannic sulfide and LDH
[Sn2S6] (Fig. 7.3) that exhibits a rapid, efficient, selective, and concurrent removal of Cu21, Ag1, Cd21, Pb21, and Hg21. Moreover, the LDH
[Sn2S6] shows exceptionally high removal efficiencies of the above metals in acidic, neutral, and basic conditions. The LDH
[Sn2S6] also demonstrates enormous sorption capacities of 378, 978, 332, 579, and 666 mg/g for Cu21, Ag1, Cd21, Pb21, and Hg21, respectively. Additionally, the LDH
[Sn2S6] shows pseudo-second-order sorption kinetics suggesting a chemisorption adsorption mechanism involving M 2 S bonding. Altogether, the regeneratable LDH
[Sn2S6]

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Figure 7.2 The sequestration results from 0.5 g/L of (A) Pb21 and (B) Hg21 using the flowerlike microspheres of WSe2 and WS2 in water samples [27]. Source: Reproduced with permission from the Royal Society of Chemistry (open access). From W. Li, D. Chen, F. Xia, J.Z.Y. Tan, J. Song, W.G. Song, R.A. Caruso, Flowerlike WSe2 and WS2 microspheres: one-pot synthesis, formation mechanism and application in heavy metal ion sequestration, Chem. Commun. 52 (24) (2016) 4481
4484. https://doi.org/ 10.1039/c6cc00577b

Figure 7.3 Schematic diagram showing the interlayer gallery of ion exchange of NO3
with the Sn2S64- anions for the synthesis of LDH
[Sn2S6] and the proposed Mn1 concentrations driven adsorption phenomena [33]. Source: Reproduced with permission from A. Celik, D.R. Baker, Z. Arslan, X. Zhu, A. Blanton, J. Nie, S. Yang, S. Ma, F.X. Han, S.M. Islam, Highly efficient, rapid, and concurrent removal of toxic heavy metals by the novel 2D hybrid LDH
[Sn2S6], Chem. Eng. J. 426 (2021). https://doi.org/10.1016/j.cej.2021.131696.

becomes an exceptional material that shows ultrahigh removal, unprecedented selectivity, rapid adsorption kinetics, wide pH stability, and a massive adsorption capacity. The integration of these features places LDH
[Sn2S6] at the top of all adsorbents known to date and thus could be used for wastewater purifications for HMs. In another study, the highly efficient and effective removal of mercury from water, using a nanocomposite that had a chalcogenidomelalate was reported [34].

Chalcogenides and their nanocomposites in heavy metal decontamination

121

They presented [TAEAH][TAEAH2]0.6Ga2.2Sn1.8S8  H2O (GaSnS-1; TAEA 5 Tris (2-aminoethyl)amine). GaSnS-1 features a three-dimensional (3D) zeolite-typed (RWY) framework structure of [Ga2.2Sn1.8S8]n20.2n
that was constructed by cornersharing of supertetrahedral [Ga2.2Sn1.8S10]6.2
T2 clusters. The equilibrium model study indicated that the maximum Hg21 saturation capacity of the nanocomposite was 213.9 mg/g and possessed extremely rapid adsorption kinetics following the pseudosecond-order model with a k2 of 5.65 3 102 g/mg/min. Particularly, GaSnS-1 exhibited excellent selectivity for Hg21 ions with a high distribution coefficient Kd value of 1.62 3 107 mL/g and high removal efficiency of close to 100%. The superior Hg21 ion adsorption performance was also impressive despite the presence of excessive competing cations and the acidic/basic conditions. The designed nanocomposite made it a promising candidate for the fast and efficient remediation of Hg21 contaminated water sources. Layered nanomaterials appear to be more promising for heavy metal decontamination than their bulky counterparts as they possess high surface areas and interesting coordination chemistry, as well as high specificity. To this end, Sarma et al. [35] reported on a rapid and efficient capacity of a layered metal sulfide material, K2xSn42xS82x (x 5 0.65
1, KTS-3), for heavy metal ion removal from water. The effect of concentration, pH, kinetics, and competitive ions such as Na1/Ca21 on the heavy metal ion removal capacity of KTS-3 was systematically investigated. The heavy metal removal by using KTS-3 follows the Langmuir
Freundlich model with high adsorption capacities of 205 mg/g for Cd21, 372 mg/g for Hg21, and 391 mg/g for Pb21. KTS-3 retains excellent heavy metal ion exchange capacity even in very high concentrations (1 M) of competing ions (Na1/Ca21) and also over a broad pH range (2
12). KTS-3 also exhibits a very good ion exchange capacity for precious Ag1 and toxic As31 ions. The kinetics of heavy metal ion adsorption by KTS-3 are rapid (absorbs all ions within a few minutes). These properties and the environmentally friendly character of KTS-3 make it a promising candidate for the sequestration of heavy metal ions from water. Magnetic chalcogenides have also been applied for heavy metal decontamination due to the easy separation of the adsorbent with the adsorbate solution before elution. For instance, Li and coworkers [36] demonstrated a facile strategy of designing a magnetic layered chalcogenide Fe3O4/KMS-1 (FK) composite adsorbents combining the advantages of rapid magnetic separation and satisfactory adsorption performance under vigorous stirring in ethanol solution at room temperature. The dispersion degree of Fe3O4 in ethanol was superior to that in water, which gave a better result with the preparation in ethanol. Characterization of the designed nanocomposite was carried out using SEM-EDS, X-ray powder diffraction (XRD), N2 physical adsorption, and TG-DTG. The results showed that the iron oxide microspheres could be well distributed over the surface of KMS-1 by electrostatic attraction in ethanol, and the obtained FK could be easily separated using a magnet from the water after adsorption. It was also observed that as the amount of the loaded iron oxide decreased, the adsorption capacity of the composite adsorbents increased. The FK nanocomposite was considered an excellent recoverable adsorbent in the removal of zinc in water samples. The above-reported methods of decontamination of HMs using chalcogenides containing nanocomposites are indicated to be effective and efficient for the adsorption of HMs.

122

Metal-Chalcogenide Nanocomposites

These methods were also reported to be environmentally friendly and demonstrated to have high removal efficiency. Celik and colleagues [33] reported a method that had high adsorption capacities of 666 mg/g of Hg21 and had the highest number of other HMs that were concurrently removed in water compared to the studies by Zhang et al. [34] and Sarma et al. [35]. Table 7.1 indicates some of the reported nanocomposites containing chalcogenides for the adsorption of HMs. Li et al. [37] reported a high adsorption capacity of 1053 mg/g for Pb21. Ma et al. [38] and Li et al. [39] demonstrated methods with the highest removal efficiency of 97.3%
99.7% and 100%, respectively. The challenge related to the disposal of ion exchange adsorbed HMs or secondary pollution has paved way for the photocatalytic degradation process that ensures the oxidation or reduction of HMs to their less or nontoxic states. This process has emerged as a potential solution to the drawbacks experienced when using the adsorption methods. Thus, the next section introduces various photocatalytic degradation studies that were done to degrade HMs into their less toxic states.

7.3

Photocatalytic heavy metal treatment

Photocatalysis usually employs a semiconducting photocatalyst, which can work efficiently under visible light, generating electron-hole pairs to attack the target pollutant [41]. In recent years, improved photocatalysts have emerged ascribing to their tunable bandgaps that are active in the visible region of the solar spectrum [42]. In addition, photocatalysis has been of great interest in water treatment processes because different classes of nanomaterials, carbon, polymers, and clays are compatible to constitute the photocatalyst [43]. Likewise, transition metal chalcogenides have shown excellent properties under visible light irradiation [44]. Owing to their structural dimension, chalcogenides possess exclusive features that can be used to design novel materials and nanocomposites for the decontamination of HMs. In one study, Madkour and colleagues [45] reported a military chalcogenide-based nanoheterostructures such as ZnS/SnIn4S8, and nanoheterostructures with different loading amounts were prepared. The prepared nanoheterostructures were utilized as photocatalysts for chromium (Cr(VI)) photoreduction to the less toxic Cr(III). The prepared nanoheterostructures were characterized by XRD, transmission electron microscopy, UV-Vis spectroscopy, dynamic light scattering, X-ray photoelectron spectroscopy, and Brunette Emmet Teller (BET) measurements. The absorption spectra of the prepared nanoheterostructures revealed that they are widely absorbed in the visible range with bandgap values of 2.4
3.5 eV. The photocatalytic activities of prepared nanoheterostructures were studied toward the photoreduction of heavy metal, chromium (Cr(VI)), under irradiation of natural solar light. The ZnS/SnIn4S8 (with a ZnS molar ratio of 20%) nanoheterostructures results showed high photocatalytic activity (92.3%) after 120 min, which could be attributed to its enhanced charge carrier separation with respect to the bare ZnS and SnIn4S8 nanoparticles. The recyclability tests revealed a beneficial role of the surface charge in the efficient regeneration of the photocatalysts for repeated use. In another study, Shahzad and coworkers [46] developed the unique CuCo2S4 modified Z-scheme MoSe2/BiVO4 hybrid composites prepared through the hydrothermal method for the efficient removal of HMs under visible light. The prepared samples showed remarkable

Table 7.1 Summary of reported chalcogenides nanocomposite for adsorption of heavy metals. Nanocomposites

Types of metals

Adsorption capacities (mg/g)

Removal efficiency

References

[CH3NH3]2xMnxSn32xS6  0.5H2O (x 5 0.5 2 1.1) (CMS) MgAl-MoS4.LDH LDH-[Sn2S6]

Cd21, Pb21

515, 1053

# 100

[37]

Cu21 Pb21 Ag1 Hg21 Cd21, Cu21 Ag1, Cd21, Pb21, Hg21 Cu21

450
500 378, 978, 332, 579, 666,321 152.7

97.3
99.7 96.4
99.6

[38] [33]

100

[39]

Cs1, Sr21

338.18, 220.12

89.1
99.8

[40]

Layered-chalcogenide, K2xMnxSn32xS6 (x 5 0.5
0.95) K2xMnxSn32xS6/r-GO

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Metal-Chalcogenide Nanocomposites

photocatalytic properties for the efficient removal of HMs. SEM, XRD, EDX, and BET were used to determine the flower and needle-like morphology (Fig. 7.4), structure and elemental composition, and surface area. Whereas UV
Vis absorption and PL emission spectroscopy were found to be useful to observe the optical properties of samples. The monoclinic phase was observed and the crystallite size of the prepared photocatalyst was calculated to be in the range of 15
25 nm. The band gap energy was found to be decreased due to the incorporation of composite materials. This decrease in band gap could facilitate to absorption of more energy in the visible range. The photocatalytic activity of the method was found at 94.30%
98.99%. This outstanding photocatalytic performance of the composite sample could be attributed to unique morphology, enriched absorption of visible part, large surface area, and inhibited recombination of chargecarriers to a synergistic effect of CuCo2S4 and MoSe2/BiVO4. Exfoliated chalcogenide-based materials often demonstrate higher photocatalytic activities than their unexfoliated counterparts, however, the challenge of restacking the exfoliated layers reduces the photocatalytic activity when the materials are

Figure 7.4 SEM micrographs of (A) 0.5% CuCo2S4
MoSe2/BiVO4, (B) 1.0% CuCo2S4
MoSe2/BiVO4, (C) 1.5% CuCo2S4
MoSe2/BiVO4, (D) MoSe2/BiVO4 nanocomposite [46]. Source: Reproduced with permission, Copyright 2019, Elsevier. From K. Shahzad, M.B. Tahir, M. Sagir, M.R. Kabli, Role of CuCo2S4 in Z-scheme MoSe2/BiVO4 composite for efficient photocatalytic reduction of heavy metals, Ceram. Int. 45 (17) (2019) 23225
23232. https://doi.org/10.1016/j.ceramint.2019.08.018.

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125

applied [47
50]. However, Ref. [51] reported a composite heterostructure with line-to-face contact made of few-layer metal chalcogenides (MoSe2) in situ tightly grown on TiO2 nanoribbon (TiO2@MoSe2) that was synthesized via a facile hydrothermal route. In contrast to the bare MoSe2, the growing MoSe2 array avoided the aggregation and face-to-face self-restacking phenomenon, making it a reality for the transmission of photo-generated carriers mainly in the basal plane, rather than between the interlaminations. The line-to-face heterointerface could significantly facilitate the photo-generated electron-hole pair’s fast separation and transport, which is verified by the analysis of photoelectrochemical performance tests. The Cr (VI) photoreduction experiments under visible light further demonstrated the photocatalytic capability of TiO2@MoSe2, which was superior to both MoSe2 and TiO2. The photocatalytic process was carried out for 100 mins with outstanding photocatalytic performance. The combination of adsorption and photocatalysis has also been reported, taking advantage of photodegrading the adsorbed substances to enable high efficiencies with minimal secondary pollution emergence [52,53]. Recently, Saiz et al. [54] reported on the dual photocatalytic and adsorptive functionality of archetypal Zr and Ti-based metal-organic frameworks (UiO-66-NH2 and MIL-125 MOFS) over the simultaneous Cr(VI) to Cr(III) photoreduction and Cr(III) immobilization. The authors reported that the study opens new avenues for other HMs such as As(III) to As(V) arsenic photooxidation or emerging pollutant photodegradation coupled with the adsorption of the photogenerated intermediate products. It is highly evident that Cr(VI) is the most studied heavy metal for its degradation using chalcogenide-based materials. Thus, the photoreduction mechanism has been deduced from some of these studies as shown in Eqs. (7.1)
(7.4) [21]. CrðVIÞ 1 e2 ! CrðVÞ

(7.1)

CrðVÞ 1 e2 ! CrðIVÞ

(7.2)

CrðIVÞ 1 e2 ! CrðIIIÞ

(7.3)

Cr2 O7 22 1 6e2 1 14H1 ! 2Cr31 1 7H2 OðEo 51 1:33VÞ

(7.4)

The above-reported photocatalysis methods using various chalcogenide nanocomposites for the decontamination of HMs were demonstrated to be effective and rapid. Various lights for the degradation of HMs were employed. Wang et al. [55] reported the degradation of Cr(VI) using solar light and photocatalyst shows a 99.8% reduction efficiency for Cr(VI) within 120 mins whilst still maintaining a 98.1% removal efficiency after 5 cycles at the same condition. However, Madkour et al. [45] reported a longer period of degradation 120 mins using multitenancy chalcogenide-based nanoheterostructures such as ZnS/SnIn4S8 nanoheterostructure with different loading as the nanocomposite and natural solar light. A lower photocatalytic activity of 92.30% was also obtained. Furthermore, Zhang et al. [56] demonstrated degradation of Cr(VI) within 100 mins and this time was less than the time reported by Wang et al. [55] (Table 7.2).

Table 7.2 Summary of reported chalcogenides nanocomposites for the photocatalytic degradation of heavy metals. Material

Pollutant

The band gap (eV)

Removal efficiency

References

BC/MoS2 hybrid aerogel ZnO/Se nanocomposite ZnS/Zn2Ti3O8/ZnO ternary photocatalyst CoS2/g-C3N4 heterostructure Ternary Cu2WS4

Cr(VI) (pH 5 5.3)

1.97
2.17

Displayed 88% removal in 120 mins

[57]

Cu, Pb, Ni, Zn, Cd and Cr, Ni and Zn (pH 4,7 and 10) Cr(VI) ions

2.5

Cu, Pb, Ni, Zn, Cd, and Cr were reduced, but Ni and Zn were completely removed (100%) Cr(VI) solution reduces from 41% to 29% after four cycles in 150 mins

[58]

Cr(VI)

2.15

[55]

Cr(VI)

2.15

99.8% reduction efficiency for Cr(VI) within 120 min at pH 5 2 Exhibited high removal of Cr(VI) up to 99% after 150 mins Exhibited the highest photocatalytic activity of nearly 99% after 40 mins Demonstrated a removal efficiency of 92.77% Indicated the highest removal of Cr(VI) up to 86.90% after 50 mins 88.52% removal efficiency within 90 mins




SnS2/SnO2 heterostructure Cu3.21Bi4.79S9/g-C3N4 Bi2S3

Cr(VI)

2.27

Cr(VI) Cr(VI)


1.11

CuS/MoS2

Cr(VI)

1.48

[59]

[60] [56] [61] [62] [18]

Chalcogenides and their nanocomposites in heavy metal decontamination

7.4

127

Conclusion and future perspectives

In this chapter, we have explored the current state of chalcogenide-based materials research for HMs decontamination. The traditional ion exchange and adsorption methods frequently used for the decontamination of HMs were explored, with some key studies in both methods deliberated. Although both these methods are important, they still face challenges for widespread application as they mainly cause secondary pollution and incur high costs, thus the emerging photocatalytic decontamination of HMs using chalcogenide nanocomposites has recently attracted extensive research interest. Chalcogenides nanocomposites demonstrate outstanding photocatalytic capabilities but still require a long time to degrade the HMs from a high to a less toxic state. This is due to their shortcomings related to visible light harvesting, with materials showing a high or a lower recombination rate than the feasible recombination rate. Thus, research on the band gap optimizations for chalcogenide nanomaterials to enable a more effective photocatalytic degradation process is still required. The combination of photocatalysis with adsorption can also be considered to enable the elimination of matrices and preconcentration of the heavy metal analyte of interest before degradation. Furthermore, most of the research that has been done on heavy metal decontamination involved the photoreduction of Cr(V) to Cr(III) or Cr metal, however, more research still needs to be done on other toxic HMs such As(III), Pb(II), Sb (III), Cd(II), Cu(II), and Ni(II) amongst others. The green synthesis of the chalcogenide and chalcogenide nanocomposites that can be used as visible light photocatalysts can also be explored to enable the safe practice of the photocatalysis process, especially from the environmental and energy points of view. Given the discussed factors, key factors such as low efficiency, difficult separation, and difficult catalyst regeneration still hinder the photocatalytic degradation of HMs, and the technology still requires some improvements.

Acknowledgments The authors wish to acknowledge the Department of Science and Innovation /National Research Foundation South African Research Chairs Initiative (DSI/NRF SARChI) (grant no. 91230) for funding and the University of Johannesburg, Chemical Sciences department, for the provision of research facilities.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter.

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41643. Available from: https://doi.org/ 10.1021/acsami.0c14137, http://pubs.acs.org/journal/aamick. [58] L.S. Shyni, K. Jagadish, S. Srikantaswamy, M.R. Abhilash, Photocatalytic degradation and removal of heavy metals in pharmaceutical waste by selenium doped ZnO nano composite semiconductor, J. Res. 2 (2016). [59] F. Chen, C. Yu, L. Wei, Q. Fan, F. Ma, J. Zeng, et al., Fabrication and characterization of ZnTiO3/Zn2Ti3O8/ZnO ternary photocatalyst for synergetic removal of aqueous organic pollutants and Cr(VI) ions, Sci. Total Environ. 706 (2020) 136026. Available from: https://doi.org/10.1016/j.scitotenv.2019.136026. [60] Q. Jia, Y.C. Zhang, J. Li, Y. Chen, B. Xu, Hydrothermal synthesis of Cu2WS4 as a visible-light-activated photocatalyst in the reduction of aqueous Cr(VI), Mater. Lett. 117 (2014) 24
27. Available from: https://doi.org/10.1016/j.matlet.2013.11.110.

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[61] T.O. Ajiboye, O.A. Oyewo, R. Marzouki, D.C. Onwudiwe, Photocatalytic reduction of hexavalent chromium using Cu3.21Bi4.79S9/g-C3N4 nanocomposite, Catalysts 12 (10) (2022) 1075. Available from: https://doi.org/10.3390/catal12101075. [62] Y. Liu, H. Wan, M. Islam, K.M. Faridul Hasan, S. Cao, Z. Wang, et al., Liquid exfoliated Bi2S3 nanosheets as photocatalysts for degradation of azo dyes and detoxification of hexavalent chromium, Mater. Sci. Eng. B 285 (2022) 115898. Available from: https://doi.org/10.1016/j.mseb.2022.115898.

Chalcogenides and their nanocomposites in oxygen reduction

8

Theivasanthi Thirugnanasambandan International Research Centre, Kalasalingam Academy of Research and Education (Deemed University), Krishnankoil, Tamil Nadu, India

8.1

Introduction

Electrocatalysts find applications in energy devices. When these energy devices are used on a large scale, the cost and efficiency of the electrocatalysts are to be considered. Electrocatalysts such as platinum, ruthenium oxide, and iridium oxide are well known for hydrogen evolution and oxygen evolution reactions (OERs) as they possess low overpotential. At the same time, their cost and durability are not suitable for use as commercial electrocatalysts. Metal chalcogenides are efficient electrocatalysts since they have more active sites and high electrical conductivity. Their performance can be enhanced in many ways. The number of edge active sites can be increased and the materials can be doped with various heterogeneous atoms. The substrates can be modified with graphene, carbon cloth, and nickel foam. A large surface area of the materials helps with the quick transmission of electrons in electrochemical reactions [11]. Transition metal chalcogenides include sulfides, selenides, and tellurides with the metals may be Co, Mo, W, Ni, Mn, Cu, and Zn. When they are used as electrode materials for energy storage devices such as batteries, supercapacitors, and fuel cells, they offer more stability and a high- power density. This is because of their crystalline structure (layer and spinel), morphology, and composition [10]. Chalcogenides can replace platinum catalysts for oxygen reduction reactions (ORRs) since they are more economical, have high catalytic activity, have more methanol tolerance, and can be synthesized easily [2]. Chalcogenides are now emerging as suitable materials for ORR activity. This is because of their selection and tolerance to small organics present in the electrolyte. They find applications in low-power systems. Chalcogenides possess a chevre phase with the structure T6X8 where T is a transition metal such as Mo, Re, Ru, Ir, and Co and X is a chalcogen such as S, Se, or Te. The ORR catalytic activity of chalcogenide materials can be improved by decorating them with other functional nanoparticles. These strategies will help to develop nanoparticles of required structures, composition, and shape. This will make the chalcogenides materials suitable for commercial fuel cells. There exists a Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00008-0 © 2024 Elsevier Ltd. All rights reserved.

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great challenge in developing an ORR catalyst to use at the cathode of PEMFCs, which can be the best alternative to platinum catalysts. The chalcogenide materials not only possess an effective ORR activity but also have a high methanol tolerance in fuel cells. A more efficient catalytic activity of Co-S and Co-NiS in an acid medium is attributed to the minimal energy difference between the oxygen 2p orbital and the highest occupied d orbital of sulfides. Among the ferromagnetic materials such as iron, nickel, and cobalt, the performance of cobalt is found to be greater. In the chalcogens, the performance of Sulfur is superior to selenides, tellurides, and even oxides. As a result, cobalt sulfide is considered the best ORR catalyst in transition metal chalcogenides both in acidic and basic solutions. A four-electron ORR activity is exhibited more effectively by Co3S4 and Co9S8. Similarly, the thin films of cobalt selenide, iron sulfide, and cobalt iron sulfide show extraordinary oxygen reduction and high stability. The heating of the material to high temperatures during synthesis also decides the ORR performance of the material. For example, cobalt selenide, when heated to 800 C under an ammonia atmosphere, shows a reversible potential of 0.5 V whereas the nonheated material is not showing any activity toward ORR s. Further, transition metal chalcogenides show a high methanol tolerance and are used as effective cathode materials for direct-methanol fuel cells. Particularly, Fe and Co follow a four e2 pathway for oxygen reduction to water as their redox potentials are the same as for this reaction [2]. Transition metal chalcogenides form a cluster-like crystal structure in which a metal ion surrounded by many nonmetal ions is present. In the compound M6X8, the metal cluster is formed by six metal ions with eight chalcogen ions. Similar to this structure, some ternary and pseudo-binary compounds also exhibit some ORR activity. Among the mixed transition metal cluster compounds, pseudo binaries containing Ruthenium and Molybdenum are found to be effective electrocatalysts with a high ORR activity. The metal cluster structure supports the efficient transfer of electrons from the electrode to coordinated oxygen on the clusters. But ruthenium cannot be made feasible because of its high cost and its low abundance. So, it cannot replace platinum as an electrocatalyst. The ORR activity is also affected by methanol crossover that is, methanol permeates between the two electrodes through the membrane. It can react with oxygen at the cathode resulting in a mixed potential. Thus, the performance of the cathode is greatly lowered. Therefore, it is needed to have a cathode material that shows inertness toward methanol. Ruthenium chalcogenides possess a high methanol tolerance and hence a high ORR activity in acid electrolytes. The main drawback of platinum catalysts is that they show a very poor methanol tolerance. Chalcogenides such as cobalt selenides CoSe2 are more selective in oxygen reduction with large methanol tolerance. So, these materials find applications as cathode materials in direct methanol fuel cells [14]. Commercialization of fuel cells becomes possible when transition metal chalcogenides can replace platinum as ORR catalysts. Transition metal chalcogenides are becoming prominent candidates for ORR activity because of their constituent materials and their electronic structure. For example, dichalcogenides of metals such as niobium, tantalum, and molybdenum have been explored for their ORR activity. Their over-potentials seem to be very low when compared to other materials.

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Table 8.1 Overpotentials for various materials in ORR. Material

Over-potential V

NbS2 TaTe2 NbTe2 NbSe2 TaS2 TaSe2

0.54 0.54 0.55 0.70 0.73 0.82

NbS2 and TaTe2 show an over-potential of 0.54 V with a volcano-type plot on the adsorption energies of  OH. An over-potential of 0.43 V is obtained during the activity of strain engineering. This is attributed to the shift of the p orbital center of the chalcogens when subjected to strain. The ORR activity is very low at the cathode of proton exchange membrane (PEM) fuel cells, which decides the performance of the fuel cells. The ORR performance of the chalcogenide materials can be improved by various doping methods. For example, P- and O-doping are able to increase the ORR performance of molybdenum sulfide [15]. A high ORR activity can be obtained with a very low over-potential. The increase in overpotentials for various materials is tabulated below in Table 8.1.

8.2

Molybdenum based electrocatalysts

Three-dimensional nickel molybdenum oxy selenide nanoparticles are synthesized with more oxygen vacancies and so more diffusion networks for oxygen species are present. This material is found to be multifunctional in terms of electrocatalytic activities such as ORR, OER, and hydrogen evolution reaction (HER). It also provides a high-power density of 166.7 mW/cm2 with durability for 300 h for Zn-air batteries. The material possesses large electroactive sites with quick charge-transfer ability and thus excels in electrochemical performance than platinum and IrO2 catalysts [8]. Molybdenum is doped with nickel selenide to improve the electrocatalytic activity. Improved performance is achieved for the composition of Ni0.5Mo0.5Se. The catalyst with more active sites possesses a high areal capacitance of 1.201 mF/cm2 [9]. The ORR kinetics is improved by the modification of the cluster structure of Mo by selenium. Mo6Se8 exhibit the ORR overpotential to be 0.9 V in the acid medium. The selectivity of the metal center in chalcogenides offers a high tolerance for them toward methanol, ethanol, formic acid, and carbon monoxide [1]. When the electrodes such as EPPG, GC, and SPEs are modified with molybdenum di sulfide, the onset potential is decreased to 0.10 V approaches the performance of the platinum electrodes. The reaction follows a four-electron pathway in the acid electrolyte [31]. Nitrogen-doped MoS2/carbon provides more active sites responsible for ORRs. The defects present in the material offer high oxygen permeability and ORR

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performance, which is needed for microbial fuel cells. Nitrogen doping is performed by doping molybdenum di sulfide with melamine. The electrocatalyst is obtained using ammonium molybdate, thiourea, and Pluronic F127 by evaporation-induced self-assembly and carbonization at high temperatures. The microbial fuel cells show a high-power density of 0.815 W/m2, which is very much greater than with platinum catalyst. The same condition exists even after 1800 h of operation of fuel cells. The electrocatalyst follows a four-electron pathway for oxygen reduction, which results in the effect of oxygen on the active sites. A remarkable ORR activity is sponsored by the presence of more defects, nitrogen doping, and high electrical conductivity [32]. Hybrid catalysts are developed using MoS2 and iron phthalocyanine using pyridyl as a linker to use as an electrocatalyst for ORRs. The obtained electrocatalyst possesses more Fe-N4 sites and the intrinsic activity of iron phthalocyanine is maintained. High durability is provided by the bonding of iron and nitrogen. The advantage of using linker hybridization is that the electrocatalyst exhibits a half-wave potential of 0.88 V versus RHE and a Tafel slope of 26 mV/dec. The performance is retained even after 50,000 CV cycles that show the durability of the material [33].

8.3

Ruthenium based electrocatalysts

Ruthenium is identified as an effective material for ORR with all three chalcogens. Ruthenium-based chalcogenides are reported as effective electrocatalysts. RuSe and RuS are reported not only more efficient for oxygen reduction but also exhibit a high inertness to methanol in fuel cells. The reason is that the metallic active centers in these materials are activated with the support of chalcogens. These transition metal chalcogenides possess a high inertness to methanol because a high energy barrier is provided for the dehydrogenation of methanol that is not available in pure transition metals [19]. Ru85Se15/C catalyst shows excellent electrocatalytic performance with a high methanol tolerance. A membrane electrode assembly with Ru85Se15/C as the cathode is constructed catalyst for a PEM fuel cell. A maximum power density of 400 mW cm2 at the current density of 1300 mA cm2 is achieved in the fuel cell [28]. The ORR activity is demonstrated for chalcogen-modified ruthenium materials such as Ru/C, Se/Ru/C, S/Ru/C, Se/Ru Mo/C, S/Ru, and Mo/C. The surface oxidation of ruthenium is prevented by the presence of chalcogen and so oxygen can adsorb easily on its surface. In the absence of chalcogen, ruthenium follows the two-electron pathway and reduces the oxygen that leads to the formation of peroxides. The chalcogen reduces HO2̅
to OH
and the overpotential is also decreased. The increase in ORR activity is more prominent in acid electrolytes rather than alkaline electrolytes. The stability of the chalcogen-modified ruthenium is greatly improved by the addition of ternary additives such as Mo [29] Fig. 8.1. Transition-metal chalcogenides possess more attractive physical properties that make them useful in functional devices. Many molybdenum and tungsten-based chalcogenides are prepared by molten-salt-assisted chemical vapor deposition [36].

Chalcogenides and their nanocomposites in oxygen reduction

(A)

139

950

(B) Intensity (a.u)

900

850

800 360

3.0µm

380

400

420

440

Raman shift (cm-1)

10

Bi2Te3

Intensity (a.u.)

WSe2

8

z [A]

Figure 8.1 AFM image and Raman spectrum of MoS2.

6 4 2

x [nm] 0 220 240 260 280 300 -1

Raman shi (cm )

(A)

320

50µm

0

(B)

50

100

150

100 µm

Figure 8.2 Optical microscopy images of WSe2 and Bi2 Te2 with the insets showing the Raman spectrum and AFM line cut.

Transition-metal chalcogenides are mostly two-dimensional materials and are separated as single-layer thin sheets by mechanical method. Since this method offers a low yield, this technique with chemically enhanced adhesion is reported [37] Fig. 8.2.

8.4

Cobalt based chalcogenides

The orthorhombic structure of cobalt selenide CoSe2 is transformed into monoclinic-phase Co3Se4 by adding Cu (II) ions during synthesis and so the material becomes more electro-catalytically active. This is achieved by a hydrothermal process followed by thermal annealing at 300 C. Graphene and carbon nanotubes

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are used as substrates to support the catalyst. The oxygen reduction occurs with a half-wave potential of 10.782 V in the KOH medium [6]. Cobalt selenide is found to possess more electrochemical activity than cobalt oxides. When compared with the platinum catalyst, the onset potential of cobalt selenide/carbon nanofiber catalyst is low. At the same time, the catalyst has a current density of 5 mA/cm2, which is closer to the one for platinum catalyst. The halfwave potential of the cobalt selenide/carbon nanofiber catalyst is 0.82 V, which confirms that this novel catalyst can replace platinum for ORR. The mechanism of the four-electron pathway is more important for ORR activity. The number of electrons for cobalt selenide/carbon nanofiber catalyst is calculated as 3.84 from the slope of Koutechy-Levich (K-L) plots. A faster ORR is confirmed by a very low Tafel slope of 69 mV/dec for cobalt selenide/carbon nanofiber catalyst [7]. Cobalt sulfides are identified as the best ORR catalyst with more electrocatalytic activity than other chalcogenides in acid electrolytes. Theoretical predictions confirm the electrocatalytic activity of Co9S8 as comparable to that of platinum through four e2 pathways for the reduction of oxygen. The ORR performance of the Co1xS/ graphene hybrid is demonstrated both in oxygen versus argon saturated electrolyte. The cyclic voltammetry curves exhibit an onset potential of 0.8 V and a current density of 4.2 mA/cm2 at 0.18 V versus the reversible hydrogen electrode (RHE). The current density is close to that needed for four e2 transfers in 0.5 M sulfuric acid electrolyte. Koutecky
Levich plots give the value of the electron-transfer number as 3, which confirms that both two- and four-electron reductions occur in this hybrid material. The ORR catalytic activity of the Co1xS/graphene hybrid is also tested in 0.1 M KOH electrolyte. The rotating disk electrode (RDE) measurements on the hybrid catalyst at 1600 rpm exhibit an onset potential of 0.87 V and a current density of 3.8 mA cm2 versus RHE with an electron-transfer number of 4 [13]. Even though cobalt sulfide actively participates in ORRs, it possesses some disadvantages such as less electrical conductivity, poor cyclability, capacity fading, and structural modifications. To overcome these difficulties, graphene is added with Co3S4, which can render effective electrochemical coupling, more electrical conductivity, high mobility of ions and electrons, and a stable structure due to its elasticity. Thus, the activity of Co3S4 becomes greater with the onset potential close to platinum with more stability and a high methanol tolerance. Fuel cells are used to power electric vehicles and electronic devices. Their advantages include high energy conversion, less operating temperatures, zero carbon emission, and high energy and power density. The cost of the fuel cells will become more economical if the platinum catalyst is replaced with non-noble metals. Cobalt sulfides in various forms such as Co1xS, Co S, CoS2, Co9S8, and Co3S4 show effective ORR performance with high thermal stability and better electrical conductivity. In addition, the pulverization of electrodes results in a decrease in specific capacity, as well as the formation of polysulfide anions which dissolve in the electrolyte, resulting in a low conductivity. Then poor cyclability occurs due to the movement of the polysulfide anions through the separator to the negative electrode. The formation of a cobalt sulfide/graphene hybrid can give a solution to these problems. Graphene not only participates to improve conductivity but also acts as a

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141

buffering substrate. Since graphene has more surface area, it can interact with the electrolyte more effectively. Graphene prevents the dissolution of the polysulfide anions in the electrolyte. Co3S4 is found to be multifunctional due to its tubular morphology. A strong electrochemical coupling between cobalt sulfide and graphene results in improved electrical conductivity. The ORR performance of the Co3S4/G hybrid is checked by electrochemical measurements both in acid and alkaline medium. The cyclic voltammetry is performed in oxygen- and nitrogen-saturated environments. The electrocatalytic activity of the hybrid material is confirmed by a low current density in nitrogen. From the polarization curves of the hybrid and platinum electrode, the onset potential of Co3S4/G is found to be 0.05 V at 1600 rpm, which is close to the onset potential of platinum at 0.02 V at 1600 rpm. The ORR performance of Co3S4/G is enhanced by leaching the materials in the acid electrolyte at 60 C for 8 h. This is because of the removal of ORR inactive material from the electrode by the acid medium. A current density of 4.5 mA/cm2 is obtained for 1.1 V versus Ag/AgCl in alkaline electrolyte thus confirming the four e2 pathways for oxygen reduction. The current density at the electrode can be increased with more diffusion of oxygen at a high rotation speed. Kentucky
Levich plots are used to determine the number of electrons thereby confirming the reduction of oxygen to the water, which is more vital in fuel cells. If the potential is more negative, the electron transfer number will be equal to four. The following table gives the values of the electron transfer number for various potentials versus Ag/AgCl Table 8.2. The composite is more durable than platinum even in H2SO4 for more than 20 000 s. Such high durability is offered by graphene in the hybrid composite. So, the constant current galvanostatic discharge shows the relative current of the composites even after 20 000 s as 60%, which is only 10% for the platinum. The commercial platinum catalyst is not having any methanol tolerance. If 2% (w/ w) of methanol is added, the hybrid composite behaves inert and its performance is not at all affected by methanol [16]. Platinum has been the conventional standard ORR catalyst. The ORR is a great challenge for the commercialization of fuel cells. The mechanism of this process deals with multielectron and the steps include several elementary, intermediate, and sluggish kinetics. Transition metal chalcogenides are not only superior to metal oxides but also competitive with noble metals such as platinum because of their high electronic conductivity. Particularly, pentlandites also possess chemical stability such as corrosion resistance. The traditional catalysts such as platinum and iridium oxide are active either in OER or ORR. Some chalcogenide materials are bifunctional. But Table 8.2 Electron transfer number for various potentials. Potential versus Ag/AgCl

Electron transfer number

0.5 V in 0.1 M KOH 1.1 V in 0.1 M KOH 0.20 V in 0.5 M H2SO4 0.10 V in 0.5 M H2SO4

3.2 3.9 2.0 3.2

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Table 8.3 Materials with lower overpotential are preferred to have better electrocatalytic activity. Material

Overpotential V

Pt/C RuO2 IrO2 N-Co9S8/G Co9S8/NSGg-C3N4 Fe5Ni4S8

0.94 1.10 1.32 0.78 0.76 0.72

at the same time, these materials possess high overpotential and the stability is low. The performance of the catalyst can be improved through three methods viz nitrogen doping of the material, etching of the electrode, and making a hybrid of the active material with graphene. These three strategies make the halfwave potential close to that of the platinum catalyst. The production of hydrogen peroxide is also very less thus confirming the four-electron pathway. The catalyst Fe5Ni4S8/carbon mesh is bifunctional and exhibits both OER and ORR activity. The following table gives the values of overpotentials for various materials as given in many research articles Table 8.3. Making a hybrid composite of transition metal chalcogenides with graphene has become a trend to harvest the electrocatalytic performance of chalcogenides. The high electrical conductivity and the large surface area of graphene support this enhanced activity. The catalytic activity of chalcogenides is increased by doping the materials with other heteroatoms. Ni3S2/11.8% Fe gives 253 mV to attain 100 mA/cm2 with a Tafel slope of 66 mV/dec and can provide 1000 mA/cm2 at an overpotential of 269 mV in 30 wt.% KOH. The properties of the electrocatalyst can also be tuned during the synthesis process. For example, the overpotential of Ni3Se2 is reduced by annealing the material at 300 C for 5 min [17]. In fuel cells, methanol is transmitted between the electrodes across the separating membrane. At the cathode, it reacts with the oxygen, which results in a drop in potential. The performance of ORRs is reduced because of methanol even in noble metal catalysts such as platinum. Ruthenium-based chalcogenides show more ORR activity both in acid and alkaline electrolytes. The polarization curves for IrxS12x/C confirm its electrocatalytic activities as measured from RDE tests at 5 mV/s and 2000 rpm in 0.5 M H2SO4. Transition metal selenides such as nickel selenide and cobalt selenide are coated on glassy carbon electrodes by electrodeposition to check their ORR performance. Their ORR activity and methanol tolerance are clearly demonstrated with onset potentials of 0.85 and 0.9 V (vs RHE) in an alkaline medium [21]. Co9S8/nitrogen-doped hollow carbon sphere has been demonstrated as an effective electrocatalyst for ORRs. ORR performance of the catalyst is studied using RDE measurements in the electrolytes such as 0.1 M KOH and 0.1 M HClO4. The catalyst exhibits a high onset potential and a high half-wave potential that are close to that of a platinum catalyst. It also possesses an ORR current density of 5.8 mA/cm2 at

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0.75 V at 1600 rpm. Thus, from all these measurements, it is confirmed that the Co9S8/nitrogen-doped hollow carbon sphere is superior to all the transition metal sulfide catalysts. The K-L plots of Co9S8/nitrogen-doped hollow carbon sphere in ORR are linear thus obeying first-order reaction kinetics corresponding to the dissolved oxygen concentration with a broader potential window. The slope of the curve gives the electron transfer number as 3.95 for the KOH electrolyte and 3.65 for the H2SO4 electrolyte, thereby following a four-electron pathway for an alkaline medium. Water is the main product when Co9S8/nitrogen-doped hollow carbon sphere is used as an electrocatalyst for ORR reactions. This is because the disk current is larger than the ring current at 1600 rpm. The yield of hydrogen peroxide is less than 2% for the potential window from 0.9 to 0.2 V in KOH and 24.4%
16.0% for the acid electrolyte. Co9S8/nitrogen-doped hollow carbon sphere is analyzed at 0.8 V for 10000 s in 0.1 M KOH and 0.1 M HClO4 that are saturated with O2 at a rotation rate of 1600 rpm. The current density of up to 87.1% and 60.9% is retained, which is higher than platinum. Thus, the Co9S8/nitrogen-doped hollow carbon sphere is more durable than the noble metal catalysts. Since the material also possesses a high methanol tolerance, it is suitable for commercial applications. When methanol is added, a slight decrease in the current is observed for this catalyst whereas for platinum the current decreases more abruptly [23]. Iron-cobalt sulfide is formed by the adsorption of metal ions on the sulfur source ammonium persulfate (NH4)2S2O8. The electrocatalyst iron-cobalt sulfides/N, S-doped mesoporous carbon exhibits a four-electron pathway for ORRs, which is comparable to the commercial catalysts. It is also highly selective, durable, and has a high methanol tolerance. This outstanding ORR performance is attributed to the fact that the sample with an optimal iron/cobalt molar ratio (Fe 0.5Co 0.5S) is pyrolyzed at 1000 C. The sample shows a high positive onset potential, half-wave potential, and a large limiting current density. It also follows a four-electron pathway for oxygen reduction as evidenced by the RDE method. The bimetallic nature creates more active centers by suitably placing Fe atoms on Co9S8/CoS. The material obtained is highly porous and it offers very good ORR activity and high electrochemical stability. Cobalt sulfide is superior among the other transition metal sulfides. This is due to the small bandgap between the orbit (2p) of oxygen and orbit (d) of Co9S8 [24]. Cobalt nickel sulfides with spinel structure exhibit a high electrocatalytic activity. Their interelement synergistic coupling makes them excel over monometallic materials in oxygen evolution and reduction reactions. They also possess a multivalent oxidation state and high electronic conductivity with exposed octahedral active sites [25]. A catalyst for ORR is developed with an electron-transfer number of 3.994 using selenocyanate/cobalt/ nitrogen-doped carbon. The material is more durable even after 30,000 cycles, which is demonstrated in a fuel cell with a Fumapem FAA-3 membrane. The selenocyanate is well bonded with the active sites of cobalt selenide, which results in quick electron transfer. The bonding between cobalt and nitrogen provides more active sites for the ORRs and increases the rate of reaction [26] Fig. 8.3. The XRD and SEM images of CoFeS2@CoS2 nanocubes/carbon nanotubes that are demonstrated to be effective ORR catalysts are shown in the following images. The structural and morphological analyses are shown in the figure. A high onset

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Metal-Chalcogenide Nanocomposites

Figure 8.3 XRD and SEM analyses of CoS2/CNTs and CoFeS2@CoS2/CNTs.

potential of 0.976 V, half-wave potential of 0.871 V, and a low Tafel slope of 24.43 mV/dec for ORR are attributed to the core-shell structure of the electrocatalyst. The average number of the transferred electrons per oxygen molecule as given from Koutecky´
Levich (K-L) plots is found to be 4.3. Thus, complete oxygen reduction is guaranteed by this four-electron pathway. The polarization curve of CoFeS2@CoS2/CNTs confirms the existence of current even after 2000 cycles at a scan rate of 10 mV/s is shown in the figure. The materials such as CoFeS2@CoS2 are transformed into CoO and CoFe2O4 after the ORR testing. In general, the sulfides are converted into oxides during ORRs [34] Fig. 8.4.

8.5

Rhenium based electrocatalysts

Rhenium disulfide nanosheets are developed as an efficient ORR catalyst. The material possesses high current density and stability similar to the carbon-supported platinum catalyst. It follows a four-electron pathway to reduce oxygen into water and shows a high methanol tolerance. The ORR is more significant for all energy conversion devices, particularly fuel cells. Transition metal dichalcogenides exhibit high electrocatalytic activity in the acid medium rather than metal oxides. Some of the recently explored catalysts are Ru, Co, Ni, Cu, and Mo-based nanosheets. Molybdenum disulfide performs well as an ORR catalyst mostly in KOH electrolytes. When compared to molybdenum disulfide, rhenium disulfide exhibits layer-independent electrocatalytic properties. Rhenium disulfide is bifunctional with its ability as a catalyst for HERs. This material shows a high over potential but at the same time, it is able to attain a high current density and stability than platinum. The electron transfer number of ReS2 is 3.9, which is just 3.3 for MoS2 and 3.4 for platinum. The production of hydrogen peroxide is also less than 10% for ReS2 and for MoS2 82% of the hydrogen peroxide is produced. Chronoamperometry provides the decay in current density as a function of time. The durability and the methanol tolerance of the

Chalcogenides and their nanocomposites in oxygen reduction

145

Figure 8.4 Polarization curve of CoFeS2@CoS2/CNTs.

material can be assessed using chronoamperometry. The analysis is performed by purging oxygen to 0.1 M H2SO4 after 2000 seconds. A reduction peak appeared for both ReS2 and Pt/C exists up to 4000 s. If methanol is introduced into the electrolyte after 4000 s, the oxidation peak appears only for platinum and not for ReS2. Methanol tolerance of ReS2 can also be demonstrated in the same way using linear sweep voltammetry. A fast decay in the current is observed up to 1400 s and the current is stabilized at 0.08 mA/cm2 for ReS2. These results confirm the durability of the catalyst ReS2 [22]. The ORR performance is tested for rhenium, ruthenium, and iridium with the chalcogens in an electrolyte containing 0.1 M lithium bis(trifluoromethanesulfonyl) imide salt in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid and dimethyl sulfoxide. Among all the catalysts, rhenium sulfide is found to have a high current density for ORRs [30].

8.6

Iridium based electrocatalysts

The electrocatalysts Ir/C and IrxS12x/C exhibit limiting currents below 0.2 V thus confirming an effective ORR performance. The IrxS12x/C catalysts are superior in their ORR performance than Ir/C. Among all IrxS12x/C catalysts, the Ir0.7S0.3/C catalyst shows the highest ORR performance. The kinetic current density of Ir0.7S0.3/C is very high. The chalcogen S modifies the Ir active sites of the catalyst and its loading decides the level of ORR performance of the catalyst. For an average loading of sulfur, the catalyst exhibits a high ORR performance. Here, the Ir surfaces modified by sulfur atoms act as the active centers for ORR activity. When the loading of sulfur increases above a certain level, Ir atoms become inactive thus reducing

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the ORR catalytic activity. The average Tafel slope of Ir/C and IrxS12x/C is found to be 130 mV/dec with an n value of 0.5. The methanol tolerance of Ir/C, Ir0.7S0.3/C, and Pt/C are analyzed using polarization curves in 0.5 M H2SO4. Ir0.7S0.3/C has a diffusion limiting region below 0.4 V versus RHE without methanol, which is not shown for pure Ir/C catalysts. The ORR performance is greatly reduced for both platinum and iridium if methanol is present. Ir0.7S0.3/C shows a high methanol tolerance and so the onset potential for platinum is 0.85 V versus RHE, which is reduced to 0.5 V if methanol is present. This reduction is due to a mixed potential caused by methanol oxidation and oxygen reduction. Ir0.7S0.3/C shows a high methanol tolerance and so the ORR polarization curves remain the same for the catalyst with and without methanol [20]. Metal chalcogenides are available in many forms, and after the synthesis of the material, the form of the material can be found using techniques such as Auger electron spectroscopy. As an example, the chemical formula of iridium selenide is found to be Ir4Se as evidenced by Auger electron spectroscopy. Ir4Se exhibits a strong electrocatalytic activity in ORRs with a high methanol tolerance. The material also follows a strong four-electron pathway to reduce oxygen to water with a very low-level production of hydrogen peroxide. Thus, the catalyst exhibits a superior ORR performance than pure Ir. The enhanced electrocatalytic activity is attributed to the interaction between the two metals [18].

8.7

Other electrocatalysts

Chalcogenide nanocomposites are prepared by adding them with carbon nanomaterials. The charge transfer and electronic conductivity of the MnSe @ MWCNT are increased by carbon nanotubes. This composite also shows a very low overpotential of 290 mV at a current density of 10 mA/cm2 and a high onset potential of 0.94 V. This electrocatalyst is even superior to platinum as an ORR catalyst because of its methanol tolerance [3]. The catalytic activity in ORR is determined by the interaction between the surface of the catalyst and oxygen molecules. The incorporation of selenium atoms with rhodium results in rhodium selenide Rh3Se4, which is a very good candidate for the ORR. In this reaction, the interaction between rhodium and oxygen is reduced by the addition of selenium atoms. Since the material is having a high oxophilicity, it is able to break the  OOH molecule into  O species. This electrocatalyst is not degraded even after 5000 cycles in a zinc-air battery and is superior to the conventional platinum catalyst [4]. The exfoliated β-Ag2Se reduces oxygen more effectively by showing an onset potential of 0.88 V/RHE in an alkaline medium. A limiting current of 3 mA/cm2 and a Tafel slope of 68.5 mV/dec makes this two-dimensional material suitable for fuel cells and metal-air batteries [5]. NiSSe nanoporous composites are developed with an overpotential of 342 mV at 100 mA/cm2. This composite is superior in performance when compared to its counterparts such as NiS and NiSe. The Tafel slope of 175.7 mV/dec is obtained for OER thus allowing more mass and electron transport. This is due to the large

Chalcogenides and their nanocomposites in oxygen reduction

147

surface area and high electronic conductivity of the composite. The ORR to water is preferred in fuel cells because the current involved in this reaction is higher [12]. Tungsten disulfide on carbon nanotubes exhibits a high electrocatalytic activity for oxygen electrocatalysis because of more active sites provided by tungsten disulfide and the conducting channels offered by carbon nanotubes. Tungsten disulfide and carbon nanotubes are connected with the help of tungsten carbide bonding. This high electrocatalytic performance is attributed to the transfer of electrons by the spin coupling, which can provide solutions for stability-related problems. Bifunctional performance that is, for both oxygen evolution and oxygen reduction exhibited by four to five layers of tungsten disulfide with carbon nanotubes. This is comparable to the performance of noble metal electrocatalysts. In addition to the electrocatalytic activity, chalcogenide materials are durable and more economical [27]. Microwave-synthesized FeSe2/C exhibits a superior ORR performance with a potential of 0.814 V. This becomes possible because the ratio of Se/Fe is from 2 to 4 and the iron and selenium atoms are placed on the surface of the catalyst. Se:Fe tends to affect the degree of graphitization in carbon support, which results in more active sites for ORRs. The cyclic voltammograms for FeSe2/C analyzed in HClO4 are shown in the figure. The oxygen-saturated CV curve shows two reduction peaks. The CV curves and RDE polarization curves for FeSe2/C in the alkaline medium are shown in the figure. The cyclic voltammetry curves exhibit two reduction peaks that confirm a high ORR performance of the catalyst Fig. 8.5.

(A)

(B)

0

0.0 -2.5

-1

Se/Fe=2.0

0

Se/Fe=2.0

100 200 400 600 900 1200 1600 2000

-2.5

-1

j (mA cm-2)

-5.0 0.0

100 200 400 600 900 1200 1600 2000

Se/Fe=2.5

0

-5.0 0.0

Se/Fe=2.5

100 200 400 600 900 1200 1600 2000

-2.5

-1

Se/Fe=3.0

-5.0

Se/Fe=3.0

0.0 -2.5

0 -1

Se/Fe=3.5

-5.0

100 200 400 600 900 1200 1600 2000

Se/Fe=3.5

0.0 -2.5

0 -1

Se/Fe=4.0

0.4

0.6

0.8 1.0 1.2 E (V VS. RHE)

1.4

-5.0

100 200 400 600 900 1200 1600 2000

Se/Fe=4.0

0.2

0.4 0.6 0.8 E (V VS. RHE)

1.0

Figure 8.5 CV curves at 50 mV/s and RDE polarization curves at 5 mV/s for FeSe2/C.

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Metal-Chalcogenide Nanocomposites

1.0

0.0 0 (B) -0.5

0.6 0.4

Se/Fe=2.0 Se/Fe=2.5 Se/Fe=3.0 Se/Fe=3.5 Se/Fe=4.0

2e

0.2 4e

0.0 0.00

0.02

0.04

0.06

ω -1/2(rpm-1/2)

0.08

0.10

j (mA.cm-2)

j-1(cm2 mA-1)

(A) 0.8

-1 -2 -3 -4 -5 -6

-1.0 -1.5 -2.0 -2.5 0.7

EORR 0.8

0.9

1.0

Se/Fe=2.0 Se/Fe=2.5 Se/Fe=3.0 Se/Fe=3.5 Se/Fe=4.0 20% Pt/C

0.2

0.8 0.4 0.6 E (V vs. RHE)

1.0

1.2

Figure 8.6 Kentucky-Levich plots at 0.3 V and RDE polarization curves at 1600 rpm for FeSe2/C.

The electron transfer number ranges from 2 to 4 as shown in the KouteckyLevich plots. In the figure, RDE polarization curves for FeSe2/C and platinum at 1600 rpm are given. The inset of the figure shows the values of ORR potential for the catalyst [35] Fig. 8.6.

8.8

Conclusion

ORRs are more important in fuel cells and batteries. The high electrocatalytic performance of chalcogenides and their nanocomposites makes them a suitable candidate to replace high-cost platinum catalysts. In addition, these materials possess a high methanol tolerance and follow a four-electron pathway for oxygen reduction. The structure of chalcogenides consists of more active sites that are essential for enhanced electrocatalytic activity in alkaline and acid media. The ORR activities of various chalcogenides are discussed in this chapter using cyclic voltammograms, RDE measurements, and Koutecky´
Levich plots. Some of the chalcogenide materials are identified as efficient ORR catalysts by analyzing various literature. They include cobalt sulfide, ruthenium-based catalysts, molybdenum sulfide, and iridium-based chalcogenides. Their performance can be improved by doping with other materials and also by using substrates made of carbon nanomaterials, which are having high electrical conductivity. Thus, transition metal chalcogenides render opportunities for the commercialization of fuel cells and other electrochemical devices.

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5.

Nanocomposites of chalcogenides as super capacitive materials

9

Muhammad Sajjad and Zhiyu Mao College of Chemistry, and Life Sciences, Zhejiang Normal University, Jinhua, P.R. China

9.1

Introduction

The upsurge of the global economy has resulted in a significant increase in the depletion of fossil fuels, causing two major concerns: the first is the expenditure of existing fossil fuel reserves, and the second is linked with an increase in the emissions of greenhouse gas, in particular, and environmental pollution. As a result, it is critical to urgently develop and market sustainable, environmentally friendly energy sources and related technologies [1
6]. It is worth noting that present energy storage systems might use some improvement in terms of efficiency and capacity. From this perspective, supercapacitors (SCs) are the best option, as they overtake power delivery and cycling stability more than batteries and solar cells but limited energy storage ability. More specifically, high safety, wide operating temperature range, high efficiency, and environmental friendliness make an appealing candidate for the next-generation modern electronic market [5,7,8]. Electrode materials, nanostructured design, and an appropriate electrolyte are essential to attain an enhanced performance for SCs. Metal oxides [9
11], carbon materials [12,13], polymers [14,15], metal sulfides [16], nitrides [17,18], oxides [19], and selenides [1,6,20,21] are useful electrodes to achieve maximum performance. Meanwhile, each electrode material has its merits and demits depending on its class selection. Chalcogenides-based functional nanomaterials turn the widespread attention of material scientists and engineers due to their fascinating advantages of anisotropic properties and rich redox chemistry. Transition elements from groups IV to VII B, such as S, Se, and Te, mix with VI A group elements to generate binary stable layered crystalline formations [22]. Chalcogenides-based nanomaterials boost electrochemical performance, largely originating from their excellent electrical conductivity, electrical conductivity, and thermal and mechanical stability. On the contrary, the performance of functional electrode materials largely depends on their structural, morphological features, crystalline size, phase, and composition characteristics [23]. Relative to sulfides, chalcogenides are not reported in the existing literature. This needs in-depth theoretical, experimental, and computational investigations to understand their structural and morphological depending feature better and more elaborately. Chalcogenides are used in many diverse applications, such as solar cells, batteries, LEDs, sensors, catalysis, and energy storage applications, owing to their (1) Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00009-2 © 2024 Elsevier Ltd. All rights reserved.

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longer life cycle, (2) more flexibility, (3) more reactive sites and catalytic activity, (4) increased conductivity and ohmic loss, (5) shorter electron transport channel lengths, and (6) quantum-sized effects make these materials attractive for industrial-grade applications in the future. However, a more in-depth theoretical and computational study should be carried out to correlate with their morphological-dependent application in a specific direction. This chapter discussed the appealing future area, specifically for SCs applications, in detail with recent achievements, challenges, and possible solutions. Generally, there are various strategies to boost the performance of SCs, as thoroughly presented in Fig. 9.1.

9.2

Chalcogenides as promising electrodes for SCs

Chalcogenide exhibits chemical characteristics similar to sulfide and oxides and has more excellent metallic qualities since selenium, sulfur, and oxygen are members of the same main group element. Chalcogenides, such as oxides, are gaining popularity because of their high theoretical specific capacity, low cost, high crust content, and ease of manufacture, as shown in Fig. 9.2. Chalcogenides, novel batterylike electrode materials, are gaining traction as viable supercapacitor electrode materials, with rich redox chemistry, improved electronic conductivity, and mechanical and thermal durability. However, there are some problems in exploring chalcogenides as electrode materials for SCs, such as the complex synthesis steps. After the continual exploration of the researchers, a pronounced impact and progress have been made in the investigation of chalcogenides-based active electrodes for the utilization and diverse applications for SCs. There are fewer reports on chalcogenides in the existing literature survey than on metal sulfides. This section focuses solely on selenium-based metal chalcogenides for SC applications. Selenium has the same valence electrons and oxidation number as sulfur in the VI A group [24]. As a result, metal selenide’s chemical and electrochemical activities

Figure 9.1 Strategies to boost the performance of SCs.

Nanocomposites of chalcogenides as super capacitive materials

155

Figure 9.2 Illustration of the flow chart of this chapter.

are almost identical to those of a metal sulfide, implying that metal selenides might be helpful in SCs [25]. This chapter focuses on a comprehensive, thorough, and chalcogenides discussion.

9.2.1 Nickel-based chalcogenides and their composites for SCs Nickel is particularly interesting among the transition metal chalcogenides (TMCs) researched because of their flexible electronic configuration, high conductivity, theoretical capacitance, low cost, and oxidation states. They also have a resistance of fewer than 103 Ohm/cm due to their paramagnetic nature, making them good candidates for energy storage devices, particularly SCs [6]. Due to their relatively complex synthetic pathways, few reports have been on NiSe2-based SCs so far. Multiple processes are required to synthesize NiSe2, resulting in higher capital costs in large-scale preparations. In recent years, COMSATs University reported [26] a facile cost-effective hydrothermal-assisted synthesis of a NiSe2-based rGO composite with different ratios for the first time. It explored its morphological-dependent electrochemical investigation for SCs. Among the other rGO contents, we found that a lower rGO content showed an admirable capacitive performance with high capacitance and decent stability coupled with excellent rate performance. An asymmetric SC was assembled, which achieved good energy and power densities of 31.25 Wh/kg and 6779.2 W/kg after adding an optimum voltage of 1.8 V in an aqueous electrolyte solution. The excellent energy storage and conversion can be attributed to its multiple oxidation states and high conductivity, which shorten the diffusion pathways and accelerate rapid electrolyte transport channels for redox reactions. A honeycomb-like NiSe nanostructure has been prepared and directly grown on Ni foam via a one-step hydrothermal method and studied for SCs [27]. By developing a binary metal-organic framework, researchers created a nickel-cobalt (CU)-selenium-carbon layered nanostructured material with high pseudocapacitance and used it in a

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lithium-ion battery. It holds 2061 mAh/g capacity after 30 cycles [28]. Goli Nagaraju and coworkers [29] reported the successful synthesis of cauliflower-like Ni3Se2 nanostructures on conductive fabrics as an electrode delivers a capacity of as high as 119.6 mAh/g in potassium hydroxide electrolyte at a current rate of 2 A/ g. Additionally, remarkable power was given at a high energy density with incredible cycling stability and lightened several LEDs, showing promising potential for real-life applications. More strikingly, Lu’s group [30] reported the synthesis of pure NiS2 and NiSe2 with rGO and studied the impact of rGO on their capacitive energy storage performance. When compared to pure NiS2 and NiSe2, NiS2/GO and NiSe2/rGO had significantly higher specific capacitance (1020 F/g and 722 F/g, respectively, at a minor discharge current rate in a three-electrode mode) and rate capability (specific capacitance remained at 569 and 302 F/g, respectively, after a fivefold increase in the current discharge rates). Other nickel-based various distinct morphologies are briefly outlined in Table 9.1.

9.2.1.1 Copper-based selenides and their composites for SCs Recent research on Cu-based materials are widely assessed for SCs due to their unique properties and rich structural chemistry. Cu possessed a relatively high conductivity and a theoretical capacity of 300
800 mAh/g, and accelerated electron transmission during the charge-transport process during the electrochemical process. However, due to the shuttle effect, pulverization caused due to significant volume changes during the insertion/desertion process leads to fast capacity decay during long-term cycling. A CuSe nanosheet was grown on an Au substrate with open channels in PVA-LiCl gel electrolyte for SC applications. The free-standing CuSe film electrode yields a high capacitance of 209 F/g, further utilized to construct flexible all-solid-state SCs. The brilliant energy storage performance of the CuSe electrode can be attributed to the well-oriented skeleton, which expands the surface area for redox reactions, resulting in increased diffusion of electrolyte ions during the extraction/insertion process [41]. The following report by Li and coworkers proposes a hybridization strategy as a means of addressing the demerits of a material effectively [42]. CuSe-FeOOH and CuSe-MnOOH for hybrid SCs via electrodeposition strategy. The CuSe-FeOOH displayed relatively higher capacitance than CuSe-MnOOH with remarkable stability. Also, CuSe@Ni(OH)2 nanosheets were developed via templet free process to boost the capacitive performance of pristine CuSe electrode in a gel electrolyte. A remarkable performance (38.9 F/cm3), with high volumetric energy and power densities, was well achieved due to the high conductivity offered by the Ni (OH)2 sheets, and the integration of CuSe prevents the restacking of Ni (OH)2, which further speedup the electron transmission pathways during redox reactions [43]. These results demonstrated that Cu-based nanostructures could be effectively utilized as appealing and robust electrode materials for high-energy SCs.

9.2.1.1.1 Manganese-based chalcogenides and their composites for SCs Manganese (Mn), a well-known pseudocapacitive material, has been widely employed as an electrode material for advanced SCs for decades. During electrochemical activities, the multiple oxidation states (Mn21, 31, 41) offer more

Nanocomposites of chalcogenides as super capacitive materials

157

Table 9.1 NiSe-based nanostructures for SCs application. Materials

Morphology

Capacity/ capacitance

Current density

References

NiSe2 NiSe2 NiSe NiSe2 NiSe NiSe@Ni3Se2 NiTe/NiSe NiSe/ZnSe NiSe-Se NiSe-CoSe

Nanosheets Hexapod-like Microsphere Truncated cube Nanoflowers Nanostructure Nanocomposite Nanostructures Nanotubes Nanoparticles

466 F/g 75 F/g 492 F/g 1044 F/g 1244 F/g 1580 F/g 1868 F/g 651.5 mAh/g 2447.46 F/g 368 C/g

3 A/g 1 mA/cm2 0.5 A/g 3 A/g 2 A/g 2 A/g 1 A/g 1 A/g 1 A/g 1 A/g

[31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

electrolyte ion diffusion to participate in faradaic redox reactions, leading to excellent charge storage properties. From the literature, the best performance of Mnbased materials is in aqueous electrolytes. Therefore, extensive progress has been made to construct high-energy and capacity SCs. In practice, the poor electrical conductivity and low surface area are the two significant factors restricting the high capacitance of Mn-based SCs. To solve this problem, Sahoo et al. [44] reported the successful synthesis of the -MnSe electrode via a low-cost and straightforward hydrothermal route. A 96.76 F/g capacitance was achieved at a low discharge current rate in a three-electrode mode in an aqueous electrolyte. A reasonable nanostructure and rational design architecture are the two pillars of enhancing the poor electrochemical properties of Mn-based selenide electrodes. In this quest, NiSe2/ MnSe nanostructures were developed with different morphologies with rGO-coated Ni foam as a battery-grade material. Among the other electrode materials, the composite prepared at 140 C, denoted as (NiSe2/MnSe-140), achieved a high capacity of 1450.7 C/g at a lower discharge rate of 1 A/g with a durable cycling lifespan. A two-electrode cell was assembled for the practical aspect, which manifests a high power delivery with an excellent energy density, showing promising future perspectives [45]. A -MnSe with a single-phase was recently synthesized by solvothermal route as an SC electrode, which attained remarkable capacitive behavior with 56% contribution and 44% from surface redox reactions with good electrochemical behavior in a three-electrode mode in an aqueous electrolyte. When tested as an electrode for asymmetric SCs, the -MnSe//AC exhibits a decent capacitance of 200 F/g at a high energy density of 55.4 Wh/kg at 1 A/g with excellent longer cycling stability. The outstanding capacitive performance can be attributed to its single-phase structure, mesoporous morphology with high surface area facilitating electrolyte ion diffusion for fast faradaic redox reactions, and the high capacitive feature makes it a promising alternative material for robust SCs [46]. Molybdenum (Mo)-based selenides and their composites for SCs: Due to their electrical conductivity, optical characteristics, and electrochemical properties,

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two-dimensional TMCs have attracted increased interest in energy harvesting and storage industries [18,47]. The various Molybdenum-based selenides displayed high redox surface properties, high electronic conductivity, and diffusivity arising due to Se22 compared to their oxide analog [48]. MoSe2 is notable for its sheet-like structures, with Se-Mo-Se individual nanosheets separated through van der Waals contact similar to MoS2, allowing enough room for ion intercalation and deintercalation processes [49]. Based on the charge storage mechanism, MoSe2 can store charge in two ways as follows; that is, nonfaradaic manner, the charge can be stored between the electrode and electrolyte interface owing to the following reaction: ðMoSe2 Þsurface 1K 1 1e2 ðMoSe2 2KÞsurface On the other hand, the faradaic process charges can be stored via redox reactions due to intercalation/deintercalation as the following: ðMoSe2 Þsurface 1K 1 1e2 K-MoSe2 A MoSe2 was successfully prepared, exhibiting a capacitance of 198 F/g at a high mass loading of 4 mg/cm2 with asymmetric SC, revealing a capacitance of 49 F/g in 0.5 M H2SO4 electrolyte with great cycling life after long-term cycling durability. The enhanced energy storage performance can be concluded by providing a large surface area and optimal circumstances for H1 ion transport in quick reversible redox processes [50]. The impacts of graphene content on the structure of MoSe2-graphene composites are studied. The findings show that an appropriate mix of MoSe2 and graphene (7:1) is more helpful to charge transport and ion transfer due to the production of a unique porous layered structure. Graphene nanosheets cover the surface and interspace of MoSe2 bars uniformly in this composite, resulting in a large specific surface area and many macropores. The composites have a high specific capacitance of 1422 F/g and keep a specific capacitance of 100.7% after 1500 cycles as the electrode material for SCs. After charging, the composites can light a tiny bulb for more than 70 s [51]. Tin-based selenides and their composites for SCs: Tin selenide (SnSe), a layered metal chalcogenide family member, has been thoroughly investigated as one of the essential binary IV
VI compounds for various energy storage applications. Without any binder, two-dimensional hexagonal tin selenide nanosheets were produced using a one-pot colloidal process and evaluated as active electrodes for SCs. Due to excellent ion transport between electrolyte and active electrode and charge transport between the electrode and current collector, the as-prepared SnSe electrode displays a capacitance of 617.9 F/g at a scan rate of 2 mV/s with high electrochemical stability [52]. SnSe nanorods were made using a one-step solvothermal process, and their modification by reduced graphene oxide (rGO) was investigated. SnSe/rGO composites with various rGO ratios were made, and electrochemical studies revealed that SnSe/7rGO had the most excellent electrochemical

Nanocomposites of chalcogenides as super capacitive materials

159

performance, with a capacitance of 568 F/g at a nominal discharge current of 1 A/g and excellent electrochemical efficiency. To further study the electrochemical efficiency of this material, a hybrid supercapacitor device was made with SnSe/7rGO as a positive electrode and activated carbon as a negative electrode. At an extraordinary power of 1.007 kW/kg, the device produced 30.5 Wh/kg with outstanding power density [53]. Other selenides and their composites for SCs: Several other chalcogenides are reported to be favorable electrode materials for SCs compared to their sulfide counterparts. Their enhanced performance can be effectively outlined in the following paragraph as follows. Recently, an N-doped rGO decorated NiSe2 nanoparticles (NrGO/NiSe2) were synthesized using a straightforward two-step procedure that included hydrothermal production of Ni(OH)2 precursor and subsequently solvothermal synthesis of N-rGO/NiSe2 composites with variable N-rGO content [30]. The findings reveal that N-rGO supports NiSe2 nanoparticles, preventing them from aggregating and increasing the material’s specific surface area and electrical conductivity. At a smaller discharge current of 1 A/g, the optimized N-rGO/NiSe2 composite may produce a high capacitance of 2451.4 F/g. In thin-film form, CdSe@PbS core-shell surface architecture for electrochemical supercapacitor application. Onedimensional (1D) Cd(OH)2 nanowires (NWs) were grown using horizontal chemical bath deposition (H-CBD), followed by ion exchange to produce 1D CdSe NWs. PbS nanoparticles were also encapsulated on the surface of CdSe NWs utilizing the SILAR process (successive ionic layer adsorption and reaction). Surface architecture has been confirmed and investigated using structural, compositional, and morphological research. The design CdSe@PbS core-shell surface architecture provides maximal active sites and a smooth and quick electron transport channel, resulting in a substantial electrochemical performance with a specific capacity of 82 mAh/g and increased long-term stability [54]. We were able to show that employing WSe2 as a binder in supercapacitor electrodes is feasible. We were also able to use WSe2 to replace conductive additives and polymer binders. Overall performance was improved: capacitance was increased by 35%, charge transfer resistance was lowered by 73%, and self-discharge potential was increased by 9% [55]. MoSSe/rGO has an excellent cyclic stability characteristic of up to 1000 cycles. MoSSe/rGO composite has a specific capacitance of 373 F/g in supercapacitor experiments. The composite material is as strong due to the synergistic impact between MoSSe and rGO nanosheets via electrostatic contact [56]. As a result, tremendous advances were made to improve the electrochemical performance of the electrode materials. ZnSe, MoSe2, and the composite of the mixed creation of ZnSe@MoSe2 have been examined, and the electrochemical performances for supercapacitor applications have been researched. In the charge/discharge profile, the binary composite of the ZnSe@MoSe2 electrode produced a high specific capacitance of 450 F/g and extended cycle life of 99.6% after reaching up to 2000 cycles. Furthermore, the presence of flower-like ZnSe@MoSe2 composites improves electrochemical performance by providing extra electrolyte ions and electron mobility. Similarly, a 1.5 V complete cell design with ZnSe@MoSe2/AC was built, yielding 43 Wh/kg with high power delivery [57]. However, other chalcogenide electrodes reported so far are summarized in Table 9.2.

Table 9.2 Other chalcogenides for SCs application. Method

Electrode materials

Capacitance (F/g)/ capacity

Current density (A/g)

Rate performance (%)

References

Hydrothermal Hydrothermal Chemical reaction Hydrothermal Not reported SILAR Chemical bath deposition Not reported

Zn-Ni-Se@NiCo2S4 NiSe2/CoSe2 MXene/WSe2 WSe2/Se FeSe@C FeS La2Se3 NiCo2Se4

1417.3 C/g 171.5 mAh/g 840 1650 611 281 311 258.1 mAh/g

1 1 2 6 1 0.8 mA/cm2 5 mV/s 1

[58] [59] [60] [61] [62] [63] [64] [65]

Potentiostatic deposition method Hydrothermal

Co2NiSe4

602 C/g

1

NiCo2Se4/CNT

1394

0.5

1T-WSe2/graphene

1735

1

93% after 10K 92.2% after 5K Not reported Not reported 92% after 10K 90% after 10K 81% after 1000K 92.2% after 14000K 98.30% after 5000K 81.095% after 10,000K Not reported

1T-VSe2 Zn
Mn
Ni
S@NiSe

4.167 mF/cm2 358 mAh/g

1 mA/cm2 2

78% after 2000K 86.6% after 10,000K

[69] [70]

Ammonia assisted synthesis Chemical vapor deposition Hydrothermal

[66] [67] [68]

Nanocomposites of chalcogenides as super capacitive materials

9.3

161

Conclusion

The energy issue is one of the issues that the national society places a high priority on. The rising usage of nonrenewable energy has shifted the focus of attention to developing innovative and efficient energy storage technologies. Electrochemical energy storage systems have focused on the conflict between economic progress and environmental issues getting more significant. Scientists have focused their research on the SC’s benefits, such as high specific capacity, high power density, quick charging speed, strong ultra-low temperature properties, extended cycle life, and environmental friendliness. Many researchers have worked on nickel selenide nanoparticles as electrodes for the SC and have achieved significant progress in recent years. In the future, energy storage will pay more attention to the SC. More scientific understanding and efforts are required to investigate and enhance its electrode materials’ performance and practical application. According to the author, further research into nickel-based selenide as an SC cathode material has a promising future in the following directions: 1. Nanostructured chalcogenides have received much attention because of their exceptional chemical stability, electrical characteristics, and unique structure. Transition metal sulfides have been shown to outperform their bulk counterparts in electrochemical performance because of their distinctive features related to reduced size, unusual shape, and faulty nature. Researchers should combine double-layer capacitor materials with carbon-based nickel-based selenide electrode materials based on their study (graphene, carbon nanotubes, etc.). By exploiting the good cycle performance of the electric double-layer capacitor material and the synergy between the electric double-layer capacitor material and the chalcogenides, the chalcogenide rate and cycle stability performance may be further enhanced. 2. Wearable and foldable electronic devices have drawn much interest in recent years. As a result, chalcogenides nanoarrays constructed on flexible substrates (such as carbon cloth) will have a bright future. Furthermore, because the electrode material is flexible, the flexible substrate can improve its electrical characteristics. 3. Nanoscale structures can significantly enhance electrochemical reaction efficiency and active material use with higher energy and power densities. Extraordinary inquiry is necessary to create fresh electrode materials for SCs and new concepts and design tactics in this sector. When designing and building electrode materials, researchers should consider that they should be abundant, inexpensive, and environmentally favorable for clean technology and possibly valuable for many applications.

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Metal-chalcogenides nanocomposites as counter electrodes for quantum dots sensitized solar cells

10

Xie Zou, Zhe Sun and Zhonglin Du Institute of Hybrid Materials, National Center of International Joint Research for Hybrid Materials Technology, National Base of International Science & Technology Cooperation on Hybrid Materials, College of Materials Science and Engineering, Qingdao University, Qingdao, P.R. China

10.1

Introduction

Seeking renewable and clean energy to replace fossil fuels is the first choice for sustainable social development. Solar energy is considered one of the most environmentally friendly and low-carbon resources, with the great advantages of sustainable development, less environmental pollution, and no geographical restrictions [1]. Nowadays, as the most direct solar power generation technology, solar photovoltaics (PVs) are widely used in basic research and industrialization [2]. In 1954, the first solar cell was successfully assembled in Bell Laboratory, and since then various types of solar cells have been invented [3]. At present, solar PVs can be divided into three categories: silicon-based solar cells (first generation), thin-film solar cells (second generation), and emerging thin-film PV technologies (third generation) [4]. The first and second-generation solar cells dominate the PV market with more than 95% of the total market share, with both types of photoelectric conversion efficiency (PCE) being more than 20%. The third generation PV technology has emerged, followed by huge research and development efforts in material discovery and equipment engineering. Depending on the materials for harvesting sunlight, the third-generation emerging PVs mainly include dye-sensitized solar cells, organic solar cells, perovskite solar cells, and Quantum dot-sensitized solar cells (QDSCs) [5]. Currently, emerging PV has potential properties that pave the way for applications such as visual transparency, high power-to-weight ratio, and portability. In the long run, if the high PCE and long-term stability of the devices are achieved, the emerging solar PV will overcome some of the commercial technology limitations in a low-cost and easy-to-manufacture manner. As a result, much research and development have been invested to develop the emerging solar PV in the past few years. Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00010-9 © 2024 Elsevier Ltd. All rights reserved.

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QDSCs began development in the 1980s when colloidal semiconductor microcrystals or QDs were first proposed as the materials for collecting sunlight into energy [6]. In 1998, the InP-sensitized TiO2 optical anode was first assembled by Nozik et al., contributing to the development of QDSCs devices [7]. However, until 2006, the reported PCE for QDSCs was below 1%. Nozik then went on to delve into InAs-based QDSC devices, achieving the PCE of 1.7% [8]. Later, Toyoda and colleagues fabricated a CdSe-sensitized TiO2 photoanode that achieved a highest PCE of 2.7% [9]. In 2008, Peng et al. Depositing CdS QDs into TiO2 nanotube arrays increases the PCE to 4.15% [10]. They further studied based on improving the deposition of QDs on the surface of the mesoporous film, which increased the PCEs to 4.92% [11,12]. However, the final PCE is never more than 5%, so the effect is far behind DSC before 2012 [13]. Until 2012, Kamat et al. used CdS/CdSe QDs as sensitizers, doped with Mn21, and obtained PCEs and QDSCs with a content of more than 5%. The Mn21-doped QDs possess the ability to promote charge separation and transfer. Furthermore, Klimov et al. used quaternary CuInSe12xSx QDs as sensitizers coated with a CdS passivation layer to achieve a PCE of 5.5% in QDSCs [14]. The most exciting thing is the grasping performance optimization strategy proposed by Zhong et al., which has witnessed in the past few years that the PCE of QDSC grows rapidly from 5% to nearly 14%, making it comparable to other emerging solar PV [15
18]. Despite the record certification, PCE is still theoretically 44% behind the highest thermodynamic conversion efficiency. After longterm efforts, QDSCs are expected to have higher PCE soon under various improvement schemes. The principle and structure of QDSCs are very similar to those of dye-sensitized solar cells, except that the dye molecules are replaced by QD materials to capture sunlight. As shown in Fig. 10.1, QDSCs are called “sandwich” structures, which mainly include four basic elements, namely, QD sensitizer, semiconductor metal oxide, electrolyte, and opposite electrodes (CEs) [19]. Under the illumination of sunlight, the QD sensitizer absorbs sunlight and forms electron-hole pairs, which are

Figure 10.1 Scheme of the structure and operation principle of a QDSC device (left) and classification of MCs nanocomposites (right).

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169

excited from the valence band (VB) to the conduction band (CB) of the QDs. After excitation, electrons are rapidly injected from the QDs into the CB of the TiO2 mesoporous film. The photocarriers accumulated in the photoanode are transferred from the external circuit to the CEs. Under the catalysis of CEs, the redox pairs in the electrolyte finally initiate electron transfer, which regenerates the oxidized QDs to the original ground state. As mentioned earlier, a large part of the PCE of QDSCs depends on the adaptability of the components and the close coordination between them. Among them, the ideal QD sensitizer should satisfy several requirements, such as owning a broad light absorption region, containing an energy gap that can be properly matched with the metal oxide, being firmly anchored to the metal oxide surface, and possessing excellent properties in the electrolyte. QD sensitizers are widely used in QDSCs, which can be divided into heavy metal-based sensitizers containing Cd and Pb, eco-friendly metal-based sensitizers containing Cu and Ag, and other metal ions-based sensitizers. The semiconducting metal oxide not only enables the loading of QDs to capture sunlight but also transfers photoinduced electrons to conductive substrates [20]. Metal chalcogenides (MCs) and their nanocomposites are widely regarded as excellent absorption materials in PV cells due to their large absorption coefficient, wide sunlight wavelength region, and high sunlight radiation stability. Almost all species of MCs such as binary, ternary, and quaternary materials have been used as QDSCs materials for collecting sunlight. The PCE of the resulting devices is mainly improved from the synthesis and operation of chalcogenides, the deposition process, and the surface design of doping and coating [21
23]. Initially, MCs were selected as sensitizers as CdS and CdSe QDs. Then, such as alloying CdSexTe12x QDs and core/shell CdSe/CdS [24,25]. In addition, binary QDs with narrow band gaps (PbS, PbSe, etc.) are also used as sensitizers in QDSCs [26
29]. Due to the high toxicity of cadmium and lead-heavy metal ion-based QDs, the environmental protection properties of copper and silver make them popular in “green” manufacturing such as CuInS2, CuInSe2, and AgInS2 QDSCs. As the QD sensitizer for capturing sunlight, MCs are usually synthesized in organic solvents by heating and thermal injection to ensure high crystallinity, tunable size, and phase composition. In addition, aqueous phase synthesis, ion exchange, successive ionic layer adsorption and reaction (SILAR), electrodeposition, and microwave heating were used to synthesize MCs as QD sensitizers in QDSCs [30
32]. At present, according to the elemental composition and configuration, MCs as sensitizers include binary compounds, ternary compounds, and quaternary compounds, which are composed of core/shell structures, alloy structures, and doping structures [22]. Meanwhile, the use of MCs in QDSCs is not only as sensitizers but nanocrystal-based MCs are also widely used as CEs in QDSCs. Due to the promising electrocatalytic activity, low cost, and excellent chemical stability to polysulfide electrolytes, CEs materials of MCs generally have a larger surface area and more catalytically active sites. All types of nanostructured MCs (such as compositional and morphological building blocks and composites with other catalytic materials) are aimed at finding QDSCs materials for highly electrocatalytic CEs [33
36].

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Furthermore, MCs are often used as interfacial modification layers, consisting of an interfacial passivation layer and a hole transport layer. One reason is that to passivate the high density of trap states on the surface of QDs, MCs (e.g., ZnS, CdS, or ZnSe) are often used as shell overgrowth and further induce possible charge recombination [37
40]. Another reason is that researchers often use some metal sulfide as a buffer layer to help transfer holes from the photoanode to the polysulfide electrolyte [41
43]. Through this thin interfacial modification layer, metal sulfide groups are fabricated using simple ion exchange and SILAR methods.

10.2

QD sensitizers

Size-dependent MCs are a very promising class of solar light-harvesting materials based on their large absorption coefficients (104
105 cm21), broad absorption thresholds, possessing continuous spectral bands, and easily size controllable effects. Due to the stable photochemistry ability, QDs have been widely used in PV technologies. In addition, it can be said that the vast majority of QD sensitizers are derived from MCs in QDSCs [44,45]. QD sensitizers are immobilized on the surface of a semiconducting metal oxide (usually TiO2 or ZnO). The excellent QD sensitizer should have a bandgap that matches the semiconductor oxide substrate to ensure efficient electron injection. Commonly used MCs as QD sensitizers mainly fall into the following three types: (1) Cd- and Pb-based heavy MCs, such as CdX, PbX (X 5 S, Se, and Te), and other mixed structures (alloys) or core-shell [46
48], (2) Cu- and Ag-based environmentally friendly MCs, such as Cu-In-X2, Ag-In-X2 (X 5 S, Se) [18,37], (3) other metal-based chalcogenides. Currently, Cd-based sulfide (e.g., CdS, CdSe) has become the first choice for QDSCs because of its simple synthesis method, easy surface modification process, and excellent stability [49]. Improving the efficiency of sunlight collection and electron injection simultaneously is contradictory for single-component binary sulfide. Overloaded reduction of the band gap to ensure that sunlight collection may cause the loss of VOC, which can decrease the performance. For example, the absorption wavelength range of CdSe can extend beyond 700 nm, but the electron injection efficiency of CdSe-based QDSCs is lower than that of CdS-based QDs. Therefore, how to design a rational QDs system with a wide absorption range and high electron injection efficiency is a key point for improving the performance of QDSCs. Lee et al. systematically investigated the effect of the deposition sequence of CdS/CdSe on the properties of TiO2. The PCEs of the cosensitized QDSCs were as high as 4.22% compared to the single CdS-sensitized devices. Furthermore, to obtain fast charge separation efficiency and transport channels, the researchers used a series of core/shell QDs as sensitizers in QDSCs for reducing the charge recombination rate and improving PCEs. For instance, Zhong et al. designed and experimented with the type-II core/shell CdTe/CdSe QDs and obtained a record-breaking 6.76% PCE [24]. The higher PCE is mainly due to the exciting complex formed between the VB of CdTe and the CB of CdSe. Later, Zhong et al. went on to synthesize ZnTe/CdSe core-shell QDs and exhibited better 7.17% PCE than CdTe/

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CdSe-based QDSCs. This is due to the larger CB shift in CdTe/CdSe (1.22 vs 0.27 eV) [16]. Although the obtained PCE has exceeded 7%, further improvement in QDSCs is still limited. This is because Cd-based core/shell sensitizers only have a rather narrow light absorption range. The band gaps of alloy-structured QDs can be easily tuned by simply changing their composition. The tuned alloy-structured QDs will obtain a wide solar collection range and bandgap arrangement favorable for efficient charge injection, thus better replacing the core-shell structure [50]. Zhong et al. first selected CdSexTe12x as a sensitizer with NIR absorption to fabricate efficient QDSCs. CdSexTe12x QDs have broader exciton absorption onsets above 800 nm than similarly sized CdSe and CdTe QDs. Oil-dispersed QDs were exchanged into a water-soluble phase and bound to TiO2 films using the bifunctional linking molecule mercaptopropionic acid (MPA). Finally, a record-breaking 6.36% QDs based on CdSexTe12x were achieved under one sunlight due to their inherently promising optoelectronic properties [25]. Fig. 10.2.

Figure 10.2 (A) Energy level of two different core/shell types of QD sensitizers (ZnTe@CdSe, CdTe@CdSe) and their J-V curves, respectively. (B) The energy level of the alloyed CdSeTe, CdSe, and CdTe QDs. The J-V curves of their three different types of QDs.

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Benefit from the narrow band gap (1.41 eV), relatively large absorption coefficient (up to 105 cm21), and large Bohr exciton radius (18 nm), Pb-based QDs are considered to be excellent sunlight collectors for full-spectrum absorption, especially in the near-infrared region [27
29]. For instance, Lee et al. fabricated PbS QDSCs and achieved high photocurrent density (Jsc) values by continuous ion-layer absorption and SILAR reaction methods [51]. Furthermore, by introducing Hg21 into the lattice gap of PbS QDs, Park et al. fabricated the QDSCs and achieved the highest Jsc of 30 mA/cm2. Although PbS and PbSe can achieve quite high Jsc values, the PCEs of the resulting QDSCs are much lower than that of other types of PCEs. Pb-based QDs with severe surface defects can act as photocarrier trapping states, thereby degrading the performance of heterojunction PbS QDSCs. Later, Mora-Sero et al. suppressed electronic recombination by coating a layer of CdS on the surface of PbS QDs, resulting in 4.2% PCE with higher Jsc and better stability [52]. In addition, they synthesized PbS/CdS core-shell structured QDs, which were then loaded, and finally, these PbS/CdS sensitized QDSCs achieved an optimal PCE of 7.19% under AM 1.5G sunlight illumination. This is attributed to the reduction in charge by the composite at the TiO2/QDs/electrolyte interface [26]. As an eco-friendly alternative to Cd and Pb-based QDs, Cu-based sulfides are considered to be the most promising sunlight harvesting materials in QDSCs. Especially CuInS2 and CuInSe2 [53
55]. Initially, simple binary Cu22xS (x 5 1,0.03) was grown on the surface of TiO2 photoanodic thin films by the SILAR method and assembled into QDSCs devices [56]. Although the obtained PCE is low, it can be effectively used as a sensitizer by modifying the Cu22xSbased QDs. Recently, a series of modified Cu22xS sensitizers such as alloys, coreshell, and doped structures were developed and used as sensitizers in QDSCs. Among them, CuInX2 (X 5 S, Se) has attracted extensive attention in QDSCs for the following reasons. It contains a high absorption coefficient (B105 cm21) and energy distribution close to the optimal bulk bandgap (1.5 eV). Several effective strategies have been introduced to improve the PCEs of such environmentally friendly QDSCs, for example, synthesis and deposition methods, size and composition tuning, the introduction of interfacial buffer layers, and cosensitization with other QDs. [57
60]. It is known that the size and structure of QDs can have a significant impact on their optical and electronic properties, thus determining the performance of QDSCs. Smaller-scale QDs have higher CB and stronger electron injection efficiency than large-scale QDs, for example, Kamat et al. nm CuInS2 QDs of different sizes. As the size of QDs increases, the PCEs of the QDSCs device increase before decreasing [61]. Meanwhile, the researchers synthesized a series of CuInS2 QDs of different sizes. The QDSCs device fabricated by Chen et al., through the J-V curve, saw that the QDs with CuInS2 size of 3.5 nm had higher PCEs than other QDs [62]. Therefore, the key to making efficient QDSCs is the selection of intermediate-sized QDs. On the other hand, due to the high surface atomic ratio and low synthesis temperature, there are many trap state defects inside and on the surface of CuInX2 QDs, which easily cause charge recombination, resulting in relatively poor performance of QDSCs. Therefore, it offers an opportunity to further improve PCE by maximizing the suppression of trap state

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defects in CuInX2 QDs. Currently, several effective methods have been developed to control charge recombination, thereby improving the PCE of CuInX2-based QDSCs [63]. Zhong et al. used a high-temperature cation exchange method to synthesize in the infrared region, and then covered it with a thin layer of ZnS to form I-type CuInS2/ZnS core-shell QDs. Benefiting from the wide absorption range and type I core/shell structure of CuInS2 QDs, the highest certified PCE of 6.66% was obtained under standard conditions. However, alloy-structured QDs may be superior to type I core-shell structures as sensitizers because the alloying process can create a fairly uniform electronic structure around the QDs surface [64]. Zhong et al. continued to incorporate Ga metal ions into CuInSe2 QDs, achieving a PCE of 11.49%. The incorporation of Ga ions can effectively increase the band gap of CuInSe2 QDs and further improve the JSC and VOC values in QDSCs devices [65]. Meng et al. achieved 8.54% PCE based on Cu/In nonstoichiometric CuInS2 QDs. The experimental results show that with the increase of the content of In, the surface defects of CuInS2 QDs are significantly reduced [66]. CuInSe2 QDs are considered more promising sensitization alternatives in QDSCs. This is due to the larger exciton Bohr radius and narrower band gap, extending the optical absorption range into the near-infrared region [39]. Then Zhong et al. explored the aborption range of Zn-Cu-In-Se QDs extending to 1000 nm, which achieved the high PCE of 11.61% [18]. The outstanding PV properties benefit from extensive light trapping, low charge recombination rates, and the ability to rapidly extract electrons from special alloy structures. Klimov et al. prepared CuInSe12xSx QDs, and 5.5% PCE was achieved by adjusting the amount of selenium to tune the band gap and perform surface cation exchange [67,68]. In addition to the CuInX2-based QD sensitizers, the abundant multicomponent alloys Cu2ZnSnS4 and Cu2ZnSnSe4 QDs are expected to be promising alternatives because of its proper band gap, high absorption coefficient, and excellent optical stability [69,70]. Wu et al. were the first to report defect-free quaternary Cu2ZnSnS4 QDs as sensitizers for assembled QDSCs devices, achieving a PCE of 3.29% [71]. Next, they set out to fabricate type II Cu2ZnSnS4/CdSe core/shell QDs into QDSCs with a higher PCE of 4.70% [72]. At the same time, Angaiah et al. prepared Cu2ZnSnSe4 QDs with an average size of about 5 nm and deposited them on the surface of porous TiO2 nanofibers, resulting in 3.61% PCE and significantly enhanced JSC [73]. Compared with the Cu-based QDs in question, Ag-based eco-friendly QDs were also used as sensitizers for QDSCs. Due to its optimal band gap of about 1.1 eV, binary Ag2X (X 5 S, Se) was previously used as a harvesting material for sunlight for PV devices [74,75]. Lee et al. were the first to fabricate Ag2S-based QDSCs and exhibit efficient sunlight responses over a broad spectral range [74]. The results are not satisfactory due to the narrow band gap, hot carrier cooling, and radiative recombination of Ag-based environmentally friendly QDs. Therefore, the development of ternary and quaternary silver-based sulfides with suitable metal ions is an effective method to improve PCEs in QDSCs. Prepared (AgInS2)x(ZnS)12x (x 5 0.67) QDs and used them as sensitizers in solar cells. The broad emission and strong interaction with TiO2 suggest that electron injection can be effectively

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promoted and a higher PCE can be obtained [76]. Initially, Zn-Ag-In-S QDs were used as sensitizers to immobilize them on ZnO nanorod substrates. Compared with Ag-In-S2 QDs, Ag-In-Se2-based QDs have higher sunlight absorption intensity and a wider sunlight range [77]. Bhattacharyya et al. synthesized Zn-Ag-In-Se QDs by varying the Zn/(Ag1, In1) ratio and finally achieved 3.57% PCE. This is because the Zn21 diffusion can effectively suppress the charge recombination at the TiO2/ QDs/electrolyte interface [78]. Despite our above efforts, Ag- and sulfide-based PCEs are still not very ideal. Limited by the internal atomic vacancies and lattice positions of Ag and other ions. These two factors will induce the existence of defect states, thereby reducing the crystallinity, the electrical conductivity of the electric ternary QDs, and the carrier mobility. At the same time, these high-density trap states can act as recombination centers, thereby reducing Voc and PCE. At present, the research on such eco-friendly QDs is still immature and needs to be further improved.

10.3

Counter electrodes

As high-energy components of QDSCs devices, CEs have a great influence on determining PV performance. CEs mainly collect electrons from the external circuit and then reduce the oxidizing species in the electrolyte. Therefore, suitable materials for CEs should have high electrical conductivity. This is to enable lower series resistance, excellent catalytic activity for the reduction of Sn22, and to ensure proper corrosion resistance in polysulfide electrolytes. Over the past few years, efforts have been devoted to presenting various candidates as CEs. MCs have become one of the most common and suitable CEs materials in QDSCs. MCs have attracted much attention due to their large specific surface area and high electrocatalytic activity toward polysulfide electrolytes [34
36]. In this section, the CEs and charge transfer resistance of all types of metal PV devices (Cu2S, CuS, CuSe, PbS, CoS, NiS, FeSx, and their complexes) will be introduced and discussed. Based on the excellent catalytic ability to reduce Sn22 to S22, nanostructured copper-based sulfides are the most popular and efficient CEs materials in QDSCs. In addition, the copper-based sulfide group meets the advantages of large surface area, low cost, and easy fabrication that people seek for CEs. At present, according to previous explorations, copper-based sulfides used as CEs in QDSCs mainly include CuxS, CuxSe, CuInS2, Cu2SnS3, or CuZnSnS, and their complexes with other catalytic materials [20,33,36]. Especially CuxS-type materials are the most widely used CEs on QDSCs due to their surprisingly high catalytic activity and good stability. In addition, the researchers experimented with different forms of copper-based sulfides (including nanowires, nanoplates, nanoflowers, and nanocages) and deposited them on conductive substrates. The widely used Cu2S CEs were obtained by immersing a brass foil in a polysulfide electrolyte for several minutes. However, these CEs are chemically unstable, which will further destroy the device. To solve this problem, the most-commonly fabrication method is to deposit

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these sulfides on the surface of conductive glass. For example, Zhong et al. developed flower-like Cu2S/FTO CEs by electrodeposition and sulfurization. The nanoflower-like Cu2S structure on FTO glass not only has a large surface area and high catalytic activity but also exhibits excellent electrolyte stability [79]. Hu et al. first designed hierarchical ITO nanowire arrays by CVD followed by surface overgrowth of Cu2S nanocrystals. Finally, the assembled QDSCs achieved a high PCE of 6.12% and a VOC of 0.688 V. This is because the small charge transfer resistance and high catalytic activity are provided by ITO@Cu2S CEs [80]. Fig. 10.3 According to previous reports, the Cu2S phase is unstable in stoichiometric compounds. Since it can form Cu vacancies, this will be easily degraded to Cu-deficient phases such as CuS, Cu1.8S, and Cu1.75S [81
84]. Logically, an in-depth analysis of the entire CuxS material is necessary. The purpose is to find special CuxS CEs with excellent catalytic activity and low charging resistance. Besides Cu2S, CuS is also used as CE material in QDSCs due to its good electrical conductivity and good electrical conductivity, stability. Several novel CuS structures, such as nanosheets, and nanorods, were prepared and introduced to further improve PCE. For example, Wang et al. fabricated layered CuS/FTO CE by an electrochemical deposition process [85]. Compared with Cu2S/brass and Pt CE, the CuS CE-based QDSCs showed higher PCEs and stability. For example, Prabakar et al. fabricated structured CuS

Figure 10.3 (A) Scheme of the preparation of quasi-2D porous Co,N 2 C and the corresponding SEM images of Co,N-C nanosheets, and the J-V curves of the different types of CEs. (B) Schematic image of the Co, N doped MOF derivatives based CEs and their resultant J-V curves.

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films for CE in QDSCs [86]. The researchers also explored CuxS materials with different nonstoichiometric structures and used them as CEs in QDSCs [87]. For example, Chen et al. designed novel Cu1.75S nanocages and used them as CE materials in QDSCs [88]. Electrochemical characterizations show that the 3D hollow structure of Cu1.75S nanocages has better catalytic activity and stability than Cu2S/ brass PCE. Currently, CuxS is considered the preferred CEs material in QDSCs, but other Cu group sulfides, such as CuxSe and CuZnSnS, have also been explored as CEs [89]. For example, Choi et al. investigated eight different types of MCs and fabricated the corresponding devices in PV cells and QDSCs [90]. The maximum 5.01% PCE indicates that the Cu1.8Se CEs have better catalytic activity than other metal selenide functional CEs such as PbSe and Ag2Se. Meanwhile, transition metal sulfide inks of Zhu et al., such as FeSe2, Cu1.8S, and CuSe, were deposited on conductive substrates by first dripping and subsequent heat treatment [91]. In particular, the prepared CuSe films exhibited better catalytic activity and achieved a higher PCE of 4.94%. In addition, Chen et al. layered Cu22xSe nanotubes and developed and used them as efficient CEs for QDSCs [92]. With the nanotube-based CEs, the PCE was relatively high at 6.25%, benefiting from the good catalytic activity and electrical conductivity. In addition, Bai et al. focus on the size and composition of Cu22xSe CE to optimize QDSCs performance. The nonstoichiometric Cu22xSe CEs showed better conductivity and yielded up to 7.11% of the PCEs, as attributed to the partial oxidation of the stoichiometric Cu2Se CEs. In addition to the abovementioned binary copper-based sulfides, the excellent catalytic activity and strong chemical stability of polysulfide electrolytes motivate researchers to continue to introduce ternary and quaternary sulfides into QDSCs as CEs materials. Zhang et al. also prepared copper-deficient Cu2ZnSnS4 NPs by thermal infusion and then assembled CdSeTe-sensitized QDSCs. The final result is an ultra-high PCE of 7.64% [93]. Earth-abundant MCs (usually CoS, NiSx, FeSx, and their mixed structures) have been exploited as polysulfide replacements [94]. For example, Chang fabricated low-cost CoS CEs and CdS/CdSe QDSCs via the CBD method [95]. According to the obtained results, CoS-based CEs exhibit better stability and higher reflectivity than CuS CEs. Anticatalytic CoS thin films deposited on ITO/PEN flexible substrates by Yuan et al. [96]. The obtained CoS/ITO or CoS/FTO CE showed remarkable electrocatalytic activity compared with the CBD method. Recently, a series of CoS-based flexible CEs, such as CoS nanorod arrays/graphite paper, CoS nanowires/Au networks, and CoS nanotube arrays/carbon fibers, are ready to design fully flexible QDSCs devices. These CoS-based CEs exhibit excellent catalytic activity and strong mechanical bending stability. Like CuxS, NiSx is a simple binary compound with a variety of stable crystal structures and stoichiometry, as well as abundant phase diagrams. Currently, many NiSx structures of different stoichiometry, such as NiS, Ni3S2, and NiS2, have also been widely applied in sensitized solar cells. This is mainly because of their excellent chemical activity, low cost, considerable electrical conductivity, easy fabrication, and the existence of various valence states. Focusing on improving the catalytic activity of polysulfide electrolytes

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[97
99], FeSx has also attracted much attention due to its special advantages including suitable band gap, earth abundance, high absorption coefficient, and high carrier mobility, making it a strong candidate for light harvesting in solar cells [100
102]. Currently, besides MCs, many excellent materials including metals, carbon derivatives, and polymers have also been developed and utilized as CEs in QDSCs. Often, different CEs cannot meet unlimited requirements and perform optimally. Therefore, the researchers considered integrating the composite material with other materials into integrated CEs. Different combinations of metal sulfide/metal, metal sulfide/carbon, metal sulfide/carbon derivatives, and metal sulfide/polymer were generated in the effective combination of two different types of CEs materials. For CEs integration with composite materials such as CuS/Pt, Cu2S/reduced graphene oxide (RGO), CuS/CoS, PbS/carbon black, CuS/metal-organic framework (MOF), CuS/graphene, and CuInS2/carbon composites [103
105], For example, Kamat et al. reported Cu2S/RGO as CEs and obtained a decent 4.4% PCE based on CdS/ CdSe QDSCs. This takes advantage of the material’s high surface area and ability to accelerate electron shuttles [103]. Zhang et al. trying to use CuS NPs being incorporated into graphene hydrogels (GHs) and fabricated into CuS/GHs CEs. The assembled device based on this novel composite CE yielded a PCE of 10.71%, which is much higher than the catalytic activity of pristine GHs/Ti mesh CE [104]. Meanwhile, Li et al. prepared high-efficiency QDSCs using bimetallic (Zn and Co) zeolite MOFs as CEs and obtained 9.12% PCEs. MOFs can uniformly disperse Co and N atom active dopants and possess large hydrophilic surface area and good electrical conductivity [106].

10.4

Interface modification layer

As a key component of QDSCs, QD sensitizers can absorb solar photons to generate electrons and inject photogenerated electrons into semiconducting metal oxides. These functions directly determine the PV performance. According to the device structure and working principle of QDSCs, QDs generate electron-hole pairs under the excitation of sunlight. The electrons are transferred to the titanium dioxide surface and the holes are reduced by the electrolyte. However, the surface separation and interfacial transport of QDs have serious recombination problems, which have become the bottleneck of current work development [23]. The charge recombination occurring in the device mainly includes four types: QDs interior and surface, QDs/TiO2 interface, QDs/electrolyte interface, and TiO2/electrolyte interface. Among them, internal charge recombination is the most serious [107,108]. Recombination occurs mainly due to a situation where there are severe defect states inside and on the surface of QDs, and photogenerated electrons are easily captured by the defect states and return through the defect states. On the other hand, holes migrate relatively slowly on the surface of QDs, orders of magnitude slower than electrons. The relatively slow transport rate leads to the accumulation of holes, and

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too many holes tend to trap electrons, thereby affecting the normal operation of the entire PV device. Until now, covering wide-bandgap MCs (often ZnS, ZnSe, and CdS) was an effective and essential surface passivation procedure [38]. The existence of the protective shell not only passivates the surface trap states but also hinders the recombination of charge carriers at the interface [26,39]. In addition, the outer shell prevents the polysulfide electrolyte from corroding the internal QDs. Bandgap ZnS shell overgrowth on the photoanode surface is considered the most widely used surface passivation process [109]. Since then, the ZnX (X 5 S, Se) processing strategy has been widely adopted, and the performance improvement mechanism has been intensively studied. According to their results, ZnS treatment can not only passivate the surface states of the QDs but also suppress the electron-charge recombination during surface trapping [110]. The photogenerated electrons are smoothly transferred to TiO2, which improves the final PCEs. Construction of ZnS or ZnSe surface passivation layers in QDSCs is to suppress surface recombination and improve interfacial charge transfer, including (1) formation of ZnX passivation layers on the surface; (2) design of type II core/shell QD structures; and (3) ZnS QD layers are grown directly on the QDs surface by partial cation exchange. Zhong designed a type I core/shell structure, using ZnS or ZnSe as the shell material. [15,39,64]. The enhanced PCE benefits from efficient passivation of surface defects and accelerated charge transfer from QDs to TiO2 films. In addition, Hyeon et al. improved the PCE of CuInSe2 QDSCs to 8.1% by controlling the thickness of the ZnS passivation layer, especially the JSC value was as high as 26.93 mA/cm2 [111]. The organic passivation layer also synergized with ZnS on the QDs-sensitized photoanode Suppress charging resistance and promote PCEs. Mora-Sero et al. explored surface modification methods for CdSe-sensitized photoanodes. By ZnS coating and molecular dipole treatment (DT). The final results show that the devices obtained based on the surface treatment sequence (DT1 ZnS) achieve the highest PCEs compared to the reference electrode. The combined DT/ZnS combined surface treatment can increase the charge injected from the QDs into the TiO2 matrix and reduce the recombination of the QDs-sensitized photoanode [112]. In2S3 is a typical n-type semiconductor with a band gap of 2.0
2.3 eV and is often used as a buffer layer coated on the surface of QDs. According to related studies, the In2S3 buffer layer significantly improves the level of PCE QDs and TiO2 [112
115]. By tuning the matching between the energies. For example, Chang et al. fabricated an In2S3/CIS/CdSe/ZnSe multilayer structure and achieved a PCE as high as 4.55%. The presence of an In2Se3 hole transport layer between TiO2 and CuInS2 significantly improves VOC and JSC [116]. For instance, Chang et al. synthesized drained Mn: In2S3/CuInS2 QDs in one pot with a PCE as high as 8.0%, which was attributed to the synergistic effect of In2S3 and Mn21. The combination of In2S3 and Mn21 accelerates the electron injection rate and prolongs the electron lifetime [58]. Today, CdS is not only used as shell coating QDs to passivate surface defects but also used as a cosensitizer to extend the sunlight absorption range. For example, a thin CdS layer was coated on CuInS2 QDs-sensitized TiO2 nanotubes by the

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SILAR method. The final PCE is 7.3%, which is much higher than pristine CdS/ TiO2 (3.3%), thanks to the energy barrier of the CdS shell [117]. Meng et al. prepared CuInS2 QDs, which were then coated with transition metal Mn21 ion-doped CdS layers, resulting in a PCE as high as 5.38% [63]. The Mn21 ion-doped QDs show longer carrier lifetimes than pristine DQs. Generally, to improve PCE, the passivation treatment of ZnS is carried out after depositing CdS on the surface of the photoanode by the SILAR method. According to the working principle of QDSCs, photoexcited electrons are injected from QDs into TiO2. Instead, the photogenerated holes are transported to the redox materials in the polysulfide electrolyte. Electron injection usually occurs on the picosecond scale, while hole transfer usually occurs on the nanosecond scale. The hole transport rate is almost two or three orders of magnitude slower than the electron injection rate. Moreover, an excessively thick passivation layer will further reduce the separation-transport rate of electron-hole pairs. Excessive accumulation of holes not only corrodes the anode of the QDs themselves but also causes more severe charge recombination, which negatively affects the device’s performance. Therefore, it is necessary to overcome the problem of slow hole transport, improve the stability of the QDs themselves and improve the PCEs of QDSCs [118]. This paper continues the discussion of methods and mechanisms for accelerating hole transport in QDSCs. As mentioned earlier, copper-based sulfides are considered to be the most promising candidates for CE of QDSCs. In addition, copper-based sulfides, usually CuxS, have also been used as hole-transporting materials for QDSCs due to their inherent p-type conductivity. Kamat et al. deposited a CuxS layer on the CdS/CdSe surface by the SILAR method, achieving a PCE of 6.6% [42]. The results suggest that the CuxS thin layer acts to facilitate the transfer of interfacial holes from the QDs to the polysulfide electrolyte and reduce charge recombination. Ghosh et al. prepared a CdSe-sensitized photoanode by introducing a p-type CuS hole-transporting layer by a partial cation exchange method. The resulting QDSCs with a CuS hole transport layer and a PCE of 4.03%, more importantly, increase VOC and FF [43]. The formation of the CuS layer can transfer the redox material into the smooth cavities in the polysulfide electrolyte, thereby reducing interfacial recombination. Therefore, distributing the hole transport layer with MCs is an effective strategy to accelerate the hole transport and further improve the final performance of QDSCs. As the most promising third-generation PV technology, QD-based solar cells have been systematically studied and made great progress over the past few decades. Extensive efforts have been made to design and synthesize QD absorbers, find existing efficient catalysts, and construct viable interface modification layers to enhance charge separation. Specifically, MCs and their nanocomposites play an important role in acquiring sunlight, catalyzing electrolyte reduction, and planning for modifying interfaces in the development of high-performance QDSCs. We introduce metal sulfide from three key modules: sensitizer, CEs material, and interface modification layer. Almost all QD-sensitizers were selected from MCs. According to the basic metal ions, MCs as sensitizers are divided into three types: Cd-and Pbheavy MCs, Cu-, Ag- eco-friendly MCs, and other MCs. Each type of MC has its

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advantages. QDs based on Cd- and Pb- heavy MCs are widely used as sensitizers for their unique optical properties, easily adjustable photoluminescence, strong sunlight absorption capacity, and excellent chemical stability. However, the high toxicity of Cd- and Pb- elements seriously restrict their practical application and market expansion. To overcome this problem, QDs based on Cu-, Ag- eco-friendly metal ions were developed and a higher PCE was achieved in QDSCs. However, some problems, such as a large number of internal and surface trap defects, slow electron injection rate, poor chemical stability, and so on, have hindered their further development. Second, MCs have been widely used as CEs materials in QDSCs and significantly demonstrate their electrocatalytic properties. The complexity of metal sulfide in composition, morphology, crystalline phase, and defects makes it difficult to deeply investigate the effects and mechanism of polysulfide electrolytes. Therefore, further improving QDSCs performance from the points of MCs remains a great challenge. Progress still poses great challenges in improving the PCEs in QDSCs. Based on the recent advances in QDSCs, it is urgently necessary to further improve the PCEs and stability of QDSCs, to develop efficient surface chemical reconstruction strategies to effectively resolve the contradiction between surface passivation and electronhole pair transport. Future research will draw more attention to the following aspects. Firstly, focus on the internal and surface defect-state passivation strategies of QDs. Secondly, research on electron-hole separation and transport law. Although the surface passivation technique of QDs can effectively inhibit the charge recombination, excessive passivation processes can easily reduce the electron-hole separation transport rate. Third, exploration of the combination of metal sulfur sulfide and perovskite QDs. Combining conventional QDs with perovskite QDs seems to be an excellent strategy to obtain high absorption capacity, inhibit composites and improve device stability. Moreover, research to improve device stability is crucial for the commercialization of QDSCs. It can be expected that the chapter provides effective theoretical and technical guidance for the final industrial applications of QDSCs, and provides a reference for other types of solar cells in the field.

10.5

Conclusion

A new generation of PVs has attracted increasing attention as a result of its easy fabrication, low cost, potential high efficiency, and other attributes. In order to maximize their PCE and device stability, materials and device architecture have been precisely optimized. Many components have been designed using MCs and their nanocomposites in QDSCs, including sensitizers, counter electrodes, and interface charge transport materials. Our purpose in this chapter was to review the recent progress of MCs from these three different points in QDSCs. Meanwhile, QDSCs were formally introduced as well as their fundamental structure and principles of operation. In conclusion, MCs for QDSCs were discussed along with their challenges and prospects.

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Acknowledgments Z. Du is grateful to the financial support from the National Natural Science Foundation of China (Grant 51802169), Natural Science Foundation of Shandong Province (Grant ZR2018BEM007), China Postdoctoral Science Foundation Funded Project (Grant 2018M632614), Qingdao Innovation Program Applied Basic Research Fund (18-2-2-8-jch), and Applied Research Project for Postdoctoral Researchers in Qingdao.

Author Contributions Z. Du conceived and supervised the project. X. Zou and Z. Sun wrote the draft, and Z. Du revised the manuscript. All authors discussed and commented on the manuscript.

Notes The authors declare no competing financial interest.

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VI semiconductor metal chalcogenide nanomaterials and polymer composites: fundamentals, properties, and applications

11

Vikas Lahariya1, Pratima Parashar Pandey 2 and Meera Ramrakhiani 3 1 Department of Physics, Amity School of Applied Sciences, Amity University Haryana, Gurugram, Haryana, India, 2School of Science & Technology, IILM University, Greater Noida, Uttar Pradesh, India, 3Department of Physics and Electronics, Rani Durgawati University, Jabalpur, Madhya Pradesh, India

11.1

Introduction

Nanostructure science and technology is a broad and interdisciplinary area of research and development activity that has been growing explosively worldwide in the past few years. This has tremendously contributed to almost all areas of science, that is, physics, material science, chemistry, biology, computer science, and engineering. The most promising results in recent years are in technology development, environmental remediation, and human health such as cancer treatment and hazardous metal detection. In the present time, nanomaterials have found an important role in technological advancements such as melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption, and scattering because their tunable physicochemical properties resulted in enhanced performance in comparison to their bulk counterparts. Thus, nanoscience breakthroughs in almost every field of science and nanotechnologies make life easier in this era. This is an expanding research area involving structures, devices, and systems with novel properties and functions. Nanomaterials are comprised of molecules and atoms, which can be used to make stable building blocks by combining them into more complex and larger useful materials. Nanomaterials are most interesting due to their unique optical, magnetic, electrical, and other properties. Nano-structured materials are having extremely small sizes with at least one dimension of 100 nm or less. These can be found in nano size in one, two, or three dimensions, for example, surface films, strands or fibers, and particles, respectively. Nanostructures can exist in single, fused, and aggregated or agglomerated forms with various sizes such as spherical, tubular, or irregular shapes. The general types of nanomaterials are nanotubes, dendrimers, quantum dots (QDs), and fullerenes. QDs are zero-dimensional Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00011-0 © 2024 Elsevier Ltd. All rights reserved.

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nanostructured semiconductors. Nanostructures of II
VI semiconductors have also been recognized for their unique properties and versatile applications. These are called chalcogenide semiconductors. The most widely used and popular class of chalcogenides are ZnS, CdS, ZnSe, and CdSe. They are efficiently used for optoelectronics applications. In the past few years, the chalcogenide nanomaterials and their heterostructures have been found to gain much attention as catalysis, sensors, and environmental remediation. The materials that contain one or more chalcogen elements, especially sulfides, selenides, and tellurides (S, Se, and Te), are called chalcogenides. These are the binary compounds that have at least one chalcogen anion with an additional electropositive element. The dichalcogenides contain two different chalcogens. The chalcogen is a chemical element in column VI of the periodic table, which is also known as the oxygen family. The term chalcogenide is used in reference to sulfides, selenides, and tellurides. The electropositive elements are arsenic and germanium, phosphor, antimony, bismuth, silicon, tin, lead, aluminum, gallium, silver, and lanthanum. The chalcogen elements and these electropositive elements are alloyed to form chalcogenide glasses (CG) that exhibit fascinating properties. The glasses made of silicates and quartz transmit radiation in the visible range of the electromagnetic spectrum. Several applications in optics, photonics, and optoelectronics domain require transmission of radiation in the infrared range of the wavelength of approximately 2 μm. The three types of special glasses are found to fulfill these requirements. These are fluoride glasses such as zirconium fluoride (ZrF4) and hafnium fluoride (HfF) and CG such As-S, As-Se, As-Se-Te, Ge-Se-Te, and Ge-As-Se. Heavy metal oxide glasses also fall in the same category such as GeO2-Pbo, and TeO2-Pbo. These glasses transmit both in the middle IR and far IR regions. The energies of phonon have lower values with higher values of refractive indices in comparison to SiO2 [1]. Mostly metal chalcogenides are semiconductor and crystalline in nature. However, in the case of nano regime below to exciton Bohr radius, these metal chalcogenides and semiconductor experience strong quantum confinement effects thus called QDs. All the elements of II
VI are semiconductors that exhibit unique properties useful for unique applications. In the view of a device application, the properties of the different phases needed to be compared. The property of one phase can be more suitable than another for several applications. Metal chalcogenide nanomaterials have been applied in different forms such as binary, ternary, and hybrid structures with organic/inorganic compounds, polymer composite, and doped and codoped with alkali metals or transition metals for various applications (as shown in Fig. 11.1). These are versatile materials in nano size regime. The popular QDs are cadmium selenide (CdSe), CdS, ZnS, and ZnSe. The induced higher energy of these materials shows a shift in the electronic band structure [2,3]. Recently, most of the research is carried out on QDs for biological imaging [4,5] and computing applications [6]. These QDs emit light due to the phenomenon of luminescence or fluorescence and hence detect and trace the biological targets inside the body. [4,7,8]. The application of QDs has an advantage over the organic dyes, which is currently used because QDs are brighter and more resistant to photobleaching [9]. Zinc sulfide also plays a key role in quantum dot-based

II
VI semiconductor metal chalcogenide nanomaterials and polymer composites

189

Figure 11.1 Representation of various types of metal chalcogenides.

nanosensors. They exhibit unusual fluorescence, tunable emission, size-dependent optical and electronic properties, and high photostability [10]. Because of their tunable band gap and size-selective emission in UV-visible radiation, they are potentially useful for photovoltaic and display applications. Recently, many researchers have realized improved efficiency and quantum yield with the combination of ZnS/ CdSe/polymer hybrid structure for display applications [11]. Similarly, the recent research work on the hybridization of CdS/CdSe/ZnS and its binary or ternary compounds with other transition metals explored its utilization for the degradation of hazardous pollutants from water and the environment. Hence the progress of the work and recent research on these chalcogenide nanomaterials are technologically important. This chapter discusses the properties, progress, and development of ZnS, ZnSe, CdSe, and CdS chalcogenide nanomaterials in powder, thin films, and heterostructure for various optoelectronics and photocatalysis applications. It is followed by the literature review and recent progress. Further, the chapter explored the possibilities and future challenges of chalcogenides nanomaterial (ZnS, ZnSe, CdSe, and CdS) for various optoelectronics and photocatalysis applications.

11.2

Structure and chemical properties of II
VI chalcogenide nanomaterials

Structural analysis is essential to understand the atomic arrangements and the properties of any material. The Greek word “chalcos” meaning ore and “gen” meaning formation are the origin of the word chalcogenide, which means ore forming [12].

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Metal-Chalcogenide Nanocomposites

A crystalline solid-state has long-range order of atomic distances. Chalcogenides have short range order and partially intermediate range order similar to oxide glasses, as both oxygen and chalcogen belong to group VI in the periodic table. Chalcogenides can be considered as “soft semiconductors” because their atomic structure is flexible, viscous and possesses band gap energy (B2 eV) comparable to the band gap of semiconductor materials (1
3 eV). Therefore, chalcogenide is material in between an oxide glass composed of 3-dimensional networks and an organic polymer possessing a one-dimensional chain structure [13]. The preparation of chalcogenide materials in amorphous form has profound effects on the band structure of these materials introducing several types of disorders in the material. The topological disorder occurs with variation in the bond length and angles, lack of long-range order in the material, a structural disorder due to bond breaking, and the presence of undercoordinated atoms at the end of chains (dangling bonds) creating large density of structural defects in the material. The most widely studied chalcogenides are zinc (ZnS, ZnSe, and ZnTe), cadmium (CdS, CdSe, and CdTe), and PbS, PbSe appears with various stoichiometries and different structures. Mainly they occur with stoichiometries, such as 1:1 and 1:2. They exhibit strong covalent, nonionic bonds and are polarizable. They represent zinc-blende or wurtzite structures. Zinc sulfide exists in two forms: a low-temperature cubic (sphalerite or zincblende) structure and a high-temperature hexagonal (wurtzite) structure. The sphalerite structure can be derived from a cubic close packing of ions, while the wurtzite structure is derived from a hexagonal close packing scheme. Fig. 11.2 shows each crystal structure [14]. In case of ZnS zinc blend, S atoms pack in cubic symmetry and the Zn21 ions occupy half of the tetrahedral holes resulting in FCC framework. In wurtzite ZnS, the Zn21 and S22 are connected in tetrahedral form in

Figure 11.2 Schematic structural representation of II
VI semiconductor chalcogenides.

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hexagonal crystal symmetry. The Zinc blend Zn (S and Se) exsist with cubic symmetry of space group, F-43 m. However, their wurtzite structure with hexagonal symmetry has been found with P63mc space symmetry. In the cubic structures, Zn is at the 4a position of (000), and S or Se is at the 4c position of (1/4, 1/4). The lat˚, tice parameters of ZnS and ZnSe in cubic symmetry are 5.4093 and 5.6676 A respectively. For the hexagonal phase, Zn, S, or Se are at the 2b positions of (1/3, 2/3, z) and (2/3, 1/3, 1/2þz). The lattice constants of ZnS and ZnSe in hexagonal ˚ , c 5 6.234 A ˚ and a 5 3.98 A ˚ , c 5 6.53 A ˚ , respectively symmetry are a 5 3.811 A [13]. CdSe is a solid, binary chalcogenides semiconductor. CdSe in its wurtzite crystal structure is an important II
VI semiconductor. In cadmium telluride, facecentered cubic CdTe has a group symmetry of F-43 m with the lattice constant a 5 ˚ . Bulk Pb (S, Se) exists in cubic (rock salt) crystal structures with a narrow 6.4827 A direct band gap of 0.28
0.41 eV at room temperature. These materials have fourfold degenerated at the L-point of the Brillouin zone. XRD is the best method for characterizing homogeneous and inhomogeneous strains. [15,16]. Homogeneous strain shifts the diffraction peak positions. From the shift in peak positions, one can calculate the change in “d” spacing, which is the result of the change of lattice constants under a strain. Inhomogeneous strains vary from crystallite to crystallite or within a single crystallite and this causes a broadening of diffraction peaks that increase with sin θ. Peak broadening is also caused by the finite size of crystallites, but the broadening is independent of sin θ. When both crystallite size and inhomogeneous strain contribute to the peak width, these can be separately determined by careful analysis of peak shapes. The atomic force microscope (AFM) is another versatile tool for determining the surface morphology. The AFM can also provide insight into the binding properties, and determine the specific interaction between two kinds of molecules. In AFM, the adhesion force between the tip and samples is a measure of the binding strength. Electron microscopy such as TEM and SEM are the most common microscopy to analyze the surface structure, topography, size, and size distribution of given samples. The resolution of these microscopy is in order of a few nm providing good images and reliable information about their different structural and morphological aspects. HRTEM is likely to be very powerful for revealing the atom distributions on chalcogenide nanocrystal surfaces even when they are passivated with polymers or other inorganic materials. To date, HRTEM is a versatile tool that provides not only atomic-resolution lattice images but also chemical information at a spatial resolution of 1 nm or better, allowing direct identification of the chemistry of a single nanocrystal as well [17,18]. As mentioned above, all these methods are used for investigating the structure and morphology of various semiconductor chalcogenide nanomaterials in binary, ternary, or heterostructure forms. Numerous data and studies are available for studying the different structural forms and various shapes of chalcogenide nanomaterials (nanoparticles, nanorods, thin films, star-shaped, diamond-shaped, nanoflakes, nanoribbons, etc.). Moreover, their shape and structure have influenced their optical and thermal stability and electrical properties and determine their applications in various optoelectronics, biological, and sensor applications. For example, CdS and ZnS are more stable in face-centered cubic structure form as compared to others hence they are the

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most suitable chalcogenide nanomaterials for photoconductivity, photovoltaic, and photocatalysis applications. Although wurtzite type structure predominates if the ionic bonding has primarily existed. While the more covalent systems favor the sphalerite form. Thus, the cubic phase of ZnS is not grown as easily as the hexagonal phase, thus making the hexagonal phase more appealing for photonic applications [3]. CdS is n-type semiconductor due to its electron affinity [19]. Besides, selenides of Zn, Cd, and Pb are P-type semiconductors.

11.3

Different properties of II
VI chalcogenide nanomaterials

As the chalcogenide anions are polarizable, most of the metal-chalcogen bonds are covalent in nature except for some alkali metals. The covalency exists because of a strong mixing of the valence 3s and 3p orbitals of the chalcogen with the outer 4s and 4p orbitals of the electropositive metal (Zn, Cd, Mn, W, etc.) elements. Such overlapping of molecular orbitals results in the formation of a wide valence and conduction band. The bonding and antibonding between the molecular orbitals create the band gap and determine their optical and electrical nature. For instance, copper monosulfides (CuS) and disulfides (Cu2S) behave either as semiconductors or as metallic conductors depending on their composition [20]. In case of metallic conductors, they also display surface plasmon resonance in the near-infrared (NIR) region. Most of the other metal chalcogenides can be regarded as semiconductors, where the energy gap is generally higher in sulfides than selenides and tellurides because sulfur is much more electronegative than selenium and tellurium [13]. Typically, for any metal chalcogenide, the gap in sulfides is in the range of 1
3 eV, whereas in selenides and particularly in tellurides, it tends to be much smaller. Also, this difference in band gap is mainly associated with the changes in the valence band, whereas the conduction band related to metal remains much less affected. The wide band gap binary metal chalcogenide semiconductor nanomaterials are recognized by MX (MX; M 5 Zn, Cd; X 5 S, Se, Te). The electronic structure of any material is necessary for the proper interpretation of experimental data on its electrical transport properties. The diagram of density of states (DOS) is used to predict and explain the properties of a material in the band theory. The Cohen-Fritzsche-Ovishnsky model was proposed for the multicomponent chalcogenides exhibiting switching properties [21]. In addition, these chalcogenide semiconductor nanoparticles in size smaller than exciton Bohr radius exhibit superior optical and electronic properties due to modified band structure formation [22]. In this size regime, they are called QDs most demanding nanomaterials because of their versatile application and superior response [22]. There are different types of nanoparticles such as CdS, ZnS, and CdTe, which are synthesized with various techniques for wide applications. Various chalcogenide binary compounds, their structure, energy band gap, and exciton Bohr radius are presented in Table 11.1. Among group II
VI semiconductors, CdS has a direct bandgap of 2.42 eV[19]. CdSe nanoparticles have an absorption edge of around 712 nm with a

Table 11.1 Various binary chalcogenides semiconductor are their properties. Binary chalcogenides compound

Lattice structure

Lattice ˚) parameters (A

Energy band gap (eV) at 300 K

Exciton Bohr radius (nm)

Type semiconductor

ZnS ZnS

Cubic, FCC Hexagonal, Wurtzite Cubic, FCC Hexagonal, Wurtzite Hexagonal, Wurtzite Cubic, FCC Cubic, FCC Cubic, FCC Cubic, FCC Cubic FCC

a 5 5.41 a 5 3.8, c 5 6.2

3.64 3.70

5 5

N type N type

a 5 4.20 a 5 4.1, c 5 6.7

2.58 2.42

3.6 3.6

N type N type

a 5 4.3, c 5 7.0

1.74

6.1

N type

a 5 5.66 a- 5 6.48 a 5 60.10

2.60 1.45 2.26 0.41 0.28

4.5 7.3 6.7 18

P type P type P type P type P type

CdS CdS CdSe ZnSe CdTe ZnTe PbS PbSe

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band gap of 1.74 eV [23]. The CdSe nanoparticles exhibit quantum confinement effect and size-dependent fluorescence; therefore they have been used in optical applications such as laser diodes, solar cells, and multicolor fluorescent markers [23]. It has a large exciton Bohr radius of 6.1 nm [23,24]. The II
VI semiconductors such as CdS and CdSe, as well as some of the III
V semiconductors such as GaAs, which have relatively large values of Bohr diameter, are found to be suitable systems for strong quantum confinement effect [19,23,25]. There are several theoretical efforts to account for the increase in the energy gap of a semiconductor as a function of particle size [26
28]. Profound changes observed in the optical properties of nanocrystal QDs, especially in direct gap II
VI semiconductors, have been the subject of intensive study due to possible photonic applications [23]. ZnSe is a binary solid material with an intrinsic direct band gap of 2.58 eV (480 nm) at 25 C. Wide band gap chalcogenides are ideal materials for studies of discrete states in the gap. ZnS (3.6 eV) is a direct wide band gap chalcogenide semiconductor that has been used for photonics due to its high transmittance in the visible range and high index of refraction (about 2.2). Due to the high surface area and quantum confinement effect in small nano-size regimes, these are utilized in many energy harvesting and catalysis fields. semiconductors Pb (S, Se, and Te) are narrow band gap binary chalcogenides, that provide unique properties for exploring the consequences of strong confinement on electrons and phonons. PbS, PbSe, and PbTe are zero-dimensional nanomaterials that show strong quantum confinement effects due to large Bohr radius ( PbS—18 nm and PbSe —46 nm), short stokes shift, and bright luminescence[29]. Crystalline Se displays a photovoltaic effect by conversion of optical energy into electrical through photoconductivity. Thus, it permits the application of Se for versatile photocells and photovoltaic cells [30,31]. Tellurium (Te) also represents photoconductivity and photovoltaic to a lower level. Both Se and Te are p-type semiconductors and are hence used in electronics and solid-state applications [29].

11.3.1 Electrical and optical properties The band gap of chalcogenides is equivalent to semiconductors, hence these behave as semiconductors [32]. Energy band gap is relatively smaller for Te-based compound semiconductor because it has good metallic nature. Whereas sulfur-based compound semiconductor exhibits the largest band gap and poorest conductivity. Thus, the band structure formulation of chalcogenide compound semiconductors is important to understand optical and electrical properties for optical switching and sensor applications [33,34]. In addition, its dielectric nature indicates a conduction mechanism with temperature. All these properties can be modified by structural defects, interstitial states, disorders, and impurities in chalcogenides because it creates localized centers into the forbidden band gap. The transition of electrons from these localized centers affects the optical and electrical properties of chalcogenide semiconductors due to low carrier mobility. All chalcogen atom has six valence electrons in the s2p4 configuration. The two half-filled p shells contribute to making covalent bonds, therefore chalcogen atoms are having twofold coordination, and a fully filled p shell is recognized as a lone pair (LP) orbital. The LP electrons

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possess higher energy and form the top of the valence band. The LP band lies between the σ and σ bands and the conduction band is formed by the σ band [35]. The metallic conductivity of chalcogenides ranges from 1022 to 10
16 Ω21/ cm [35]. The electrical mechanism can be understood by the Davis
Mott model and the small-polaron model [36]. As per Davis
Mott model, conduction can occur by one of the three possible paths depending on the temperature. At low temperatures, conduction occurs by electron hopping between mid-gap localized states. At higher temperatures, conduction is carried out by electrons excited into localized states at the band edges. At sufficiently higher temperatures, electrons can be directly excited to extended states. In the small-polaron model, the charge carriers are small-polarons and the conduction occurs by thermally activated hopping. In conduction behavior, chalcogenides exhibit smaller conductivities than the corresponding crystals because the electronic mobility in the band tail and localized states decreases due to the disorder present in the structures of these materials. The optical properties of the materials reveal their energy band structure, impurity levels, localized and exciton states, optical transitions, and defects. The absorption coefficient and density of charge carrier states decide the optical properties of amorphous solids. By knowing the absorption coefficient, the optical band gap, which is the difference in energy between the highest maximum of the valance band and the lowest minimum of the conduction band, can be easily calculated. The optical transition depends on the excitation of the lone-pair electrons into the conduction band. The nature and environment of the lone-pair electrons determined the optical behavior of the chalcogenides. The absorption, transmission, and reflection of the light depend critically on the DOS and the Fermi energy of the material. The gap in the DOS around the Fermi energy in transparent materials is such that there are no states available into which an electron absorbing a photon could move. The optical constants, that is, changes in refractive index and optical absorption coefficient, are responsible for many photo-sensitive processes observed in the chalcogenides. These lightinduced changes are favored in chalcogenides due to their structural flexibility. The photo-induced phenomena and optical nonlinearity are the main optical effects.

11.3.2 Thermal properties The various thermal properties like glass transition, crystallization and melting temperatures, thermal conductivity, and thermal diffusivity are associated with chalcogenides. A differential scanning calorimeter (DSC) thermograph is used to study the thermal properties of these materials. The two important phenomena that appear in the process are as follows: 1. Heating leads to the appearance of a new transition called the glass transition, where the material softens on heating at a specific temperature and the enthalpy of the material experiences changes. It can be observed in the DSC thermograph by changes in specific heat and viscosity [37]. This specific temperature is known as the glass transition temperature (Tg). 2. When the glass is maintained at a temperature lower than Tg, it will undergo structural relaxation due to a decrease in enthalpy.

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The physical quantities such as melting temperature, mean atomic volume, and width of the band tails in chalcogenide are also dependent on Tg. The exothermic crystallization peak demonstrates the production of excess free-volume, and the endothermic melting valley reflects the amount of energy that liberates due to the breaking of all types of existing bonds in solid alloy, that is, completely destroying the solid phase structure. Hence, the material’s crystallization temperature depends on the compositions of alloys. The transport of energy by the electrons or lattice vibrations is responsible for the thermal conductivity of the material. Thermal conductivity is related to phonon mean free path that is shorter for amorphous materials than crystals. Therefore, the thermal conductivity of Chalcogenides is low. The thermal conductivity of materials decreases slowly with decreasing temperature [37]. Thermal diffusivity affects the switching mechanism exhibited by CG. It decides the rate of dissipation of heat from the conducting channel. It has been reported that there is a strong correlation between thermal diffusivity and the switching behavior of CG. Chalcogenides with low values of thermal diffusivity are likely to exhibit memory behavior and those with high values of thermal diffusivity show threshold-type switching. Therefore, thermal diffusivity in switching glasses is important for identifying suitable materials for phase change memory applications. This is unique for each material like the optical absorption coefficient and is often ideal over conductivity measurements due to its insensitivity to radiate heat losses as the conductivity involves heat fluxes that are difficult to control.

11.3.3 Physical properties 11.3.3.1 Refractive index and dispersion The refractive indices of chalcogenides are relatively large between 2 to greater than 3. The increase in refractive index is achieved by replacing sulfur with the more polarizable selenium and tellurium. The high refractive index is advantageous for strong optical field confinement, which allows small waveguide bend radii and enhanced optical intensities. The additional advantage of a large index can potentially provide a complete bandgap for photonic crystals. The mid-IR region observed the zero dispersion wavelengths for chalcogenides. At communication wavelengths around 1.55 μm, these materials exhibit strong normal dispersion. A typical device length is short and hence this is not necessarily detrimental to performance as the order of centimeters and the dispersion sign ensures the possibility of using waveguide dispersion to engineer the chromatic dispersion, similar to what has been achieved in silicon [38].

11.3.3.2 Linear loss mechanisms Chalcogenides have low phonon energies due to large atomic masses and relatively weak bond strengths and hence the long wavelength cut-off lies in the mid-infrared. The transparency edge is 12 μm for sulfide-based glasses, 15 μm for selenide glasses, and 20 μm for telluride glasses. The extrinsic attenuation mechanisms are

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not noticeable in these materials, but Rayleigh scattering defines the minimum attenuation of the glass within the electronic and multi-photon absorption. Most of the chalcogenides have small band gaps, and they also exhibit low carrier mobility. In the case of low mobility, the free carrier in solids is proportional to the carrier mobility. Free carrier absorption is not important in chalcogenides. However, chemical impurities, particularly oxygen, can result in a drastic reduction in infrared transmission. Therefore, ultra-pure chemicals must be purified, by hydrogen distillation, before being melted either under a vacuum or within a nitrogen environment to make bulk glasses [38].

11.3.3.3 Photo-induced phenomena Photocrystallization, photopolymerization, photodecomposition, photocontraction, photovaporization, photodissolution of metals, and light-induced changes in local atomic configuration are the various types of photo-induced phenomena exhibited by chalcogenides. These changes are accompanied by the changes in the optical band gap, and optical constants are found in all these phenomena. These strong photo-induced properties are their inherent structural flexibility. The usually double covalent bonded chalcogen atom possesses a LP of nonbinding electrons that under illumination can alter the bond number. The application of photo-induced effects on the formation of various components, including waveguides and surface gratings [38].

11.4

Chemical Synthesis of II
VI chalcogenide nanomaterials and polymer composites

Synthesis of II
VI chalcogenides in powder as well as in thin film forms depends on their demand for various applications. Numerous top-down and bottom-up approaches have been developed for the synthesis of monodisperse chalcogenide compound semiconductors in nanocrystalline and 2D structures. Different chemical routes such as organometallic precursor, chemical bath deposition (CBD), chemical coprecipitations, and hydrothermal/solvothermal combustions have been widely used for creating high-quality nanoparticles and nanocomposites with tunable properties. This section will more focus on some most widely and effectively used chemical synthesis and their limitations for II
VI chalcogenide compound semiconductor nanomaterials.

11.4.1 Chemical route for preparation of II
VI chalcogenide nanocrystals in powder form Synthesizing these materials in the form of particles of different size and shapes provide an opportunity for exploring their properties beyond their bulk characteristics. The chemical route is used for the low-cost preparation of samples. It is most

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essential to restrict the size distribution in nanocrystalline samples. To reduce the agglomeration of the nanocrystallites, the crystallites are generally capped with organic/polymers [39
41] such as thiophenol, mercaptoethanol (ME), ethylene glycol, and mercapto-acetic acid. In this method, chalcogenide semiconductor nanoparticles can be synthesized by mixing metal salts solutions (aqueous or solvent) with chalcogenide (S, Se, and Te) compound salt, whereby each can carry out the chemical synthesis of binary semiconductors having one component. One main problem for the synthesis of sulfur and selenium chalcogen compound is stability in air and water environment. Sulfides and selenides are more likely to oxidize and decompose under ambient conditions than others, due to low-energy reaction paths with H2O to form H2S or H2Se. Hence during the synthesis, it requires specific gas environment and standard conditions. Although their metal compounds or complexes are more stable and involved in reactions with high energy, one of the effective syntheses is based on the high-temperature decomposition of organometallic precursors. Murray et al. prepared Cd-based chalcogenide compound semiconductors (CdS, CdSe, and CdTe) by hot injection organometallic precursor method with coordinating solvent (TOP). The TOPSe and TOPTe were used for selenium and tellurium sources, respectively. In this work, Me2Cd was chosen as the Cd source and (TMS)2S or TOPO (tri-n-octyl phosphine oxide) were used as chalcogen sources. The TOP/TOPO solvent coordinates the surface of the crystallites and allows uniform slow growth of particles at high temperatures above 280 C [42]. In chemical production, multiple processing parameters like reaction time, reaction temperature, the concentration of the capping agent, the concentration of precursor, molar ratio, pH value, and sonication time, control synthesis and affect the size, structure, and morphology of the chalcogenide nanomaterials. To obtain the desired material, the organic capping agent is to be added to the salts before mixing the two solutions. This organic capping agent controls the particle size. The particle size also depends on the concentration of salt solutions. The amine-thiol solvent system is the most sought because it is capable of dissolving various metal precursors such as metal oxides, halides, chalcogenides, and even pure metals for the synthesis of metal chalcogenide nanoparticles [43]. In most of the Cd-based chalcogenides, mercaptopropionic acid (MPA) and ME are widely used as capping agents. These thiol group capping agents strongly attached to the Cd cations on the surface and the carboxyl group on the other end fully charged at high pH improve the colloidal stability of the QDs and reduce the particle size [44
46]. Monodispersed CdSe nanoparticles were prepared by the chemical method described by Zhang et al. [47]. In our previous work, CdSe nanocrystalline powder of various particle sizes has been prepared by chemical route in the N2 atmosphere. The MPA works as a capping agent. Cadmium acetate dihydrate [Cd (CH3COO)2U2H2O] and sodium selenosulfite (Na2SeSO3) were chosen as metal ions and Se ion sources, respectively. Particle size was controlled by capping agent concentration [23]. An in-depth understanding of the chemical method was developed for the synthesis of a variety of nanoparticles, as well as controlled microassemblies, for applications in thermoelectric (lead chalcogenides) and photovoltaic (CuInxGa12xSySe22y) devices [48].

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Synthesis of lead (Pb) chalcogenide nanoparticles was realized by dissolution and room-temperature reaction between lead halide solution and chalcogen solution in amine-thiol solvent system [49]. The size and shape of the resulting nanoparticles and their self-assemblies were controlled with the appropriate selection of aminethiol pair and their relative ratios, without introducing any additional ligand or temperature variation. The volatile amine-thiol solvents are used in these reactions and the organic residue on the particles was also eliminated for making it attractive for use in electronic applications. This facile route demonstrated quantum confinements in nanoparticles as well as assembly structures in the preparation of PbS material. The solution-phase cation and anion exchanges in the PbTe system at room temperature resulted in controlled composition variation within the micro-structures. The PbSe nanoparticles demonstrated the thermoelectric performance, and the presence of a hollow core provides a possible route for a reduction in thermal conductivity and improvement in thermoelectric performance of the material. The other binary, ternary, and quaternary metal chalcogenides such as Cu2S, (InxGa12x)2S3, CuIn(S,Se)2, CuInxGa12xS2, and Cu2ZnSnS4 have been synthesized at higher temperatures (150 C
300 C) using a similar solvent system [50]. The dissolution of pure metals like Cu, In, Sn, Zn, Ga, and Se in amine-dithiol solution was used as precursor solutions for these reactions. This approach completely avoids anionic impurities due to the use of elemental metals. The metal chloride, iodide, acetate, nitrate, and acetylacetonate salts in this reaction make it a true impurity-free route for nanoparticle synthesis. These syntheses of nanoparticles were also carried out in setups like heat up, hot injection, and microwave-assisted solvothermal conditions, giving versatility of experimental settings for better control over particle properties like size, shape, and phase. ZnS nanoparticles can be prepared using a wet chemical route [51]. The powder of ZnS nanoparticles was prepared by using a chemical coprecipitation method with a dilute solution (0.01 M concentration) of zinc chloride (ZnCl2) and sodium sulfide (Na2S) mixed in the presence of various concentrations of capping agent ME (C2H5OSH) [52,53]. The particle size was controlled with the amount of ME in the resultant solution. Highly monodispersed, uniform spherical ZnS nanoparticles were found in the range of 2
4 nm [14]. Our previous reported work presented well-suited chemical method for the preparation of metal chalcogenide nanocrystal polymer composite (PVA/PVK) films in an aqueous medium as, well as dimethyl formaldehyde as a solvent with spherical symmetry in the range of 2
20 nm [14,23,24]. In the preparation of CdSe/PVA nanocomposite, highly viscous polyvinyl alcohol and its metal ion interaction by complex ion pair formation serve as surface modifiers [23]. Our recent study described the preparation of CdS nanoparticles using PVP and SHMP as surface stabilizing agents [54]. The agglomeration can be restricted by stabilizer because of the steric hindrance effect of the polymer. It provides high surface stability and alters the structure and morphology of the materials [54
56]. Previously, several researchers have used varrious polymers (PVA, PVP, SHMP, PEG, and PMMA) for preventing the agglomeration and synthesis of CdS, ZnS, CdSe and ZnSe nanoparticles in powder and thin films forms [54
59].

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11.4.2 Synthesis of thin films embedded in polymers Synthesis of chalcogenide thin films of desired structural, optical, surface morphology, and electrical properties is very important, especially when different layers have to be integrated. For desired properties of chalcogenide thin films, knowledge of various deposition techniques and synthesis parameters is required. These are further classified as vacuum-based methods and solution-based methods. All the techniques have the edge over the others [60]. The methods of thin film deposition are classified into two, physical and chemical, depositions. The various types of physical/chemical deposition methods are as follows: 1. Evaporation techniques a. Vacuum thermal evaporation b. Electron beam evaporation c. Laser beam evaporation d. Arc evaporation e. Molecular beam epitaxy f. Ion plating evaporation g. Sputtering 2. Chemical vapor deposition a. Low pressure (LPCVD) b. Plasma-enhanced (PECVD) c. Atomic layer deposition (ALD) 3. Solution-based film deposition a. Spray pyrolysis b. Electro deposition c. Chemical bath deposition (CBD) d. Spin coating e. Sol-gel method

The other techniques of film deposition are the sol-gel technique, CBD, spray pyrolysis technique, electroplating technique, and electrolysis technique. Among them, some need high vacuum with temperature. On the other hand, some methods do not require a vacuum, however, specific environment and temperature are desired depending on the metal salts and type of chalcogenide materials. For example, thin film preparation of PbSe, CdSe, ZnSe, and ZnTe has been done in specific gas environment with a high temperature. Although the purity, homogeneity, morphology, and crystallization are varied by other parameters such as rate of flow, deposition time, stoichiometry, temperature rate, and vacuum pressure [23,61
63]. Among these techniques, CBD is the oldest method to deposit the films on a substrate and is also known as the solution growth technique or controlled precipitations. This method is simple, low cost, and versatile for deposition over a large area and hence does not require sophisticated instruments like vacuum system or other expensive equipment. The required chemicals are readily available, work at lower temperatures, and are capable of yielding good-quality thin films. Here, the precursor solution of metal ions is a complex solution obtained with ammonia solution, citric acid, etc. Further, the addition of anions comes from the source of sulfur anions to deposit the chalcogenides. Aqueous or nonaqueous baths

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VI semiconductor metal chalcogenide nanomaterials and polymer composites

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Thermometer

pH Meter Substrate Aqueous/ Nonaqueous bath Composion reacon both

Srrer

Magnec srrer with hot plate

Figure 11.3 Experimental setup of chemical bath deposition [64]. From S. Kumar, K.P. Tiwary, Nano Trends J. Nanotechnol. Appl. (1) (2020) 19
27. https:// doi.org/10.37591/nanotrends. v22i1.829

with constant stirring are used to heat the chemical bath to the desired temperature under continuous stirring. Substrates are inserted in a vertical position inside the solution so that the desired film thickness is obtained. Depending upon deposition conditions such as temperature, stirring rate, and solution concentration, the film growth can take place by ion-by-ion condensation of materials on the substrate. The chalcogenide ZnCdS thin films deposited by CBD method are discussed in detail in Fig. 11.3 [64]. The preparation of thin films of CdS nanoparticles is reported to be carried out by the CBD method. The fibril-like nanowire structure can be obtained by chemical etching in the second step. The process was carried out by washing the substrates with double distilled water and etching was done with dilute 0.1 M hydrochloric acid for a few seconds. For the preparation of CdS nanocrystallites, 1 M of CdSO4 and 1.9 MNH4OH were taken as initial precursors [37]. In addition, CdS/PVA, CdS/PVK, and CdSe/PVA nanocomposite films were prepared by solution growth method where PVA is used as a polymer matrix with aqueous solubility and high viscosity [19,23]. In all these methods, cadmium chloride was used as Cd21 and H2S gaseous form was used for S22 ions [19], and for Se sodium selenosulfite solution and nitrogen gas were used [12]. The molar concentration of the samples 1:1,2:1, 3:1, and 4:1 influenced the morphology, crystal structure, and band gap of chalcogenides nanocomposites films, respectively [23]. Similarly, CdSe/PVK (Polyvinyl carbazole) nanocomposite thin films were prepared with selenium powder and CdCl2 and PVK solution in N2 atmosphere [65].

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For Zn metal, the source ZnCl2 was used and for Cd, CdCl2 was used as a metal ion source. The choice of polymer host matrix is governed by the fact that some polymeric compounds serve as capping agents and surface modifiers. In which the coordination between the surface functionalized group and metal ions restricts the surface area by steric hindrance effect or Columbia repulsion force between them. In addition, some polymeric surfactants cum capping agents including PVA and PMMA, offer good optical, electrical properties, and thermal stability in 2D nanostructure form. Also these properties are thickness-dependent [66,67]. Nanocrystalpolymer samples of ZnS have also been prepared by the solution casting method [52]. Other studies have been reported to synthesize ZnS/PVK (Poly N-vinyl carbazole) nanocomposite [68]. Pulse electro-deposition technique was used to prepare CdSe nanocrystal films on a titanium substrate. For pulsed voltage, an ON-OFF IC555 in a stable mode was used [69].

11.5

Applications of chalcogenides nanomaterials

In modern times, the chalcogenides semiconductors and their heterostructures are technologically important for optoelectronics applications. In these applications, the semiconducting properties of metal chalcogenides are enormously important. Charge transport ability, high optical conductivity, electron affinity, and high oscillator strength facilitate the design of specific metal chalcogenides for the extensive use of different optoelectronic applications like photovoltaic, phosphor, photocatalytic, photodetectors, and sensors. Especially, thin film metal chalcogenide semiconductor materials have received great attention due to their superior electronic properties and promising application as solar cells and photodetectors. In this section, we will briefly discuss some most demanding applications of II
VI metal chalcogenide semiconductor QDs, core/shell nanoparticles, and heterostructures.

11.5.1 Applications of chalcogenide nanomaterials and heterostructures for quantum dot LEDs Light emitting diode (LED) has emerged as the most efficient energy and environment saver technology. It is worked on P-N junction diode principle, in which optical energy is illuminated by irradiation by electrical energy. It is based on the electroluminescence mechanism. Color purity, high quantum yield, and stability are prerequisites for the use of all light-emitting applications. LED-based solid-state lighting has been promised to offer high-quality and energy-efficient light sources [70]. With continuous improvement in efficiency and cost, LED lighting is on track to be the dominant form of lighting in our daily life. In 1996, Yang et al. [71] showed that various II
VI metal chalcogenide semiconductor nanocrystals can be used as light-emitting material in electroluminescence layered structures. Colvin et al. [5] have prepared highly monodisperse CdSe nanocrystals deposited onto

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hole-conductive polymer films, which produce visible light with spectra corresponding to the excitonic photoluminescence (PL). Artemyev et al. [72] observed electroluminescence from a thin film structure containing close-packed CdS particles size of 1
2 nm with Schottlkey configuration, ITO/CdS/Ag. At low forwarded bias, electroluminescence was obtained in the red region, but as the forward bias is increased from 20 to 36.5 V, the electroluminescence is blue-shifted. At a voltage above 36.5 V, the electroluminescence spectrum similar to the PL was obtained, which suggests similar localized states participating in the luminescence process. The emission of light by a phosphor material resulting from the recombination of electric field-injected, opposite-sign charge carriers will be called “recombination electroluminescence.” Electrons and holes are capable of direct recombination by the mechanism of photon emission. This process can compete successfully with the nonradiative mechanism in materials of sufficiently high purity and high carrier concentration. Electroluminescence of QDs is of great importance because it may give an insight into quantum confinement and surface effects in nanocrystals and may be used for various light-emitting device applications. II
VI metal chalcogenides are the most commonly used nanomaterials in the QDs -based LEDs. In quantum, dotbased LED or heterostructure, various II
VI metal chalcogenides (CdS, CdSe, ZnS, ZnSe, ZnTe, and CdTe,) are commonly used as an emitting layer. Among these, the metal chalcogenide QDs based on cadmium are the most common ones. It offers better charge carrier recombination. Also, low scattering losses, tunable energy band gap, and absorption in UV-visible region are the some other characterstics. Emission tunability and high quantum yield make it suitable for distinct color emission and white light generation. Hence, they are appropriate nanomaterials for LEDs (QD-LEDs) for next-generation devices. In the past decades, numerous efforts have been made to enhance the efficiency of emissions using QDs and their heterostructures [73
76]. The heterostructure consists of two or more organic/ inorganic materials. In QDLED, two or more metal chalcogenide semiconductor nanoparticles have been used as emissive core and outer shell layers. These core/ shell nanoparticles support the carrier charge injection process and radiative electronic transitions. Passivation of the emissive core with a thick inorganic shell layer provides high PL QY and photostability [77]. CdSe-based QDs have QYs of 70%
80% [78,79] and the QYs of CdTe-based QDs are .90% [80]. Colloidal cadmium sulfide QDs in combination with CdSe/ZnS/ZnO/ZnSe heterostructure and coreshell structure have been widely used in LED applications [78
83]. Studies on the cadmium-based (CdS, CdSe, CdTe) core-shell nanostructured with silica shells have been also used for emitting layers. For the CdSe QDs, a number of interesting optical properties such as band edge luminescence and excitonic radiative decay have been related to the surface state [83]. Thus, from the literature review, it is found that the purpose of enhancing the external quantum efficiency (EQE) with optimum value of CIE coordinates in warm light emission has been achieved by using II
VI metal chalcogenides semiconductor QD heterostructures or core/shell nanostructure with multishell as emitting layer [84,85]. Recent reports on CdSe nanocrystals dispersed in a polymer matrix showed enhanced and efficient

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electroluminescence and PL for white light emission [86,87]. In which the binary semiconductor chalcogenides QDs dispersed in a polymer matrix such as polydimethylsiloxane (PDMS) or PMMA, or any other conducting polymer for surface functionalization and charge transfer layer [86,87]. Moreover, it improves the dispersion of particles and increases the photostability and thermal stability of hybrid QD structures for QDLEDs. In multilayer QD-based LED devices, polymers such as PVK, PMMA, TPD, and PEDOTS:PSS play key roles, they work as charge carrier injection layers to improve mobility of electrons or holes in junction. Moreover, by encapsulation of II
VI semiconductor chalcogenide QDs in polymer for fabrication of electroluminescence device structure, it subsequently down the turn-on voltage and increases the stability of the light illumination [14,19,23,85]. These layers can be deposited over the ITO-coated glass plates layer by layer via different solution-processed methods or thin film preparation methods. Fig. 11.4 shows a schematic representation of a multilayered EL device structure with polymer hybrid structures. recent report claimed the hybridization with polymeric structure siginificantly improve the life span and quantum efficiencey of the emitted light. Thus, it explores the possibility of multicolor displays in compact sets or large-area displays for commercial purposes. In CdSe-based LED, emission is occurred due to exciton recombination and charge carrier recombination via surface states, due to a smaller energy band gap and large Bohr exciton radius. Hence, cadmium and zinc-based chalcogenides have presented considerable achievements in terms of high color purity, color rendering index, and PL quantum efficiency (more than 70%) in the visible range with warm light emission. In the recent past, researchers have developed the LED using Mn-doped ZnS [88], ZnTe/ZnS, or ZnSe/ZnS core-shell nanostructures for white light generation [89]. In such a way, band position plays an important role in the transportation of charges inside the emitting layer and their recombination. The overlapping of the energy states and radiative electronic transition in type II heterostructures avoid the lattice mismatch because the shell has a relatively larger band gap. Thus, high emission with quantum efficiency was observed in the case of Type II heterojunction core/shell nanomaterials. Hahm et al. presented a design and principle of InP/ZnSexS12x heterostructure QDs for bright emission. By band gap engineering of the emissive core and thick outer shell layer, electrons can be confined in the emissive layer while the shell passivates the core and support the charge accumulation in the core [90]. This heterostructure QDs consist of InP emissive core, ZnSexS12x inner shell, and ZnS outer shell system. A comprehensive spectroscopic analysis and study on the geometry and composition expressed photophysical properties and high photochemical stability of QDs [90]. The large band gap ZnS (3.6 eV) and thicker inner ZnSexS12x shell effectively confine charge carriers within the emissive core. In addition, metal chalcogenides’ outer shell serves as the protective layer, improving the lattice strain originating from lattice mismatch at the interface between InP core and the outermost shell [87
91]. Recently, Omata et al. reported broad emission via defect states in ZnTe/ZnS core/shell nanostructures. They prepared ZnTe12xSx alloy system and found broad emission intensity to the exciton recombination emission was determined by the composition of the core/shell nanostructure [91].

_

PVK Poly-TPD

+

PEDOT:PSS ITO

Figure 11.4 EL device structure of multilayered CdS/CdSe QD LED [22].

-5 -6 -7 -8 -9

Ag ZnO

QD

-4

PVK CdS CdSe CdS PMMA

ZnO PMMA

ITO

Energy (eV)

-3

Poly-TPD

-2 Ag

PEDOT:PSS

-1

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The most exciting property of semiconducting metal chalcogenide colloidal QDs is PL. The narrow emission width and high oscillator strength offer good optical stability and have shown their potential for various lighting purposes. In the case of PL, the quality of metal chalcogenide QDs is decided by the factor called PL quantum yield (PLQY), which is measured on a scale from 0 to 1.0 (100%, often represented as a percentage). PLQY is a measure of the efficiency of photon emission as defined by the ratio of the number of photons emitted to the number of photons absorbed. The high radiative recombination rate improves the PLQY. Although poor PLQY derives from the trapping of photo-excited electrons from the surface states. Hence the passivation of surface states is most important in the case of semiconductor QDs since the surface is highly sensitive. While white-light emission for cadmium Se-based chalcogenides QDs is because of the integration of the exciton recombination and the electron-hole pair recombination via surface states [92
94].

11.5.2 Applications of chalcogenide nanomaterials and their heterostructures for photocatalysis Photocatalysis is a chain reaction to convert optical energy to chemical energy. Semiconductor chalcogenide QDs have also been explored to improve existing or create new photocatalysts. Such photocatalysts have been applied for the degradation of pollutants from surroundings in water and the environment. The excitation of atoms in their radiating materials leads to radicals that alter the environment [95]. The reduction and oxidation phases (Fig. 11.5) of the photocatalysis process may be classified into two categories. When a material is bombarded with light with an energy equal to or greater than its bandgap, electrons in the conduction band jump to the valence band across the bandgap, leaving positive holes, which is known as reduction. The produced electrons and holes result in the creation of reactive oxygen species (ROS) such as O2 and OH as a result of oxidation as shown in Fig. 11.5. The type of ROS produced is determined by the substance and the

Figure 11.5 Schematic illustration for photocatalytic process.

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amount of energy irradiated. To address harmful environmental impacts, photocatalysis is one of the highly demanding investigation areas. It is used for the remediation of the ecosystem by destroying toxic micro-organisms, degradation of dyes, and contaminants in the air, respectively [96
98]. Chalcogenides have triggered great interest in photo-catalysis [99
101]. Moreover, in comparison to TiO2 and CuO, the metal chalcogenides have small optical bandgaps, hence they can effectively apply in near UV
visible portion of the electromagnetic spectrum [102,103]. Crystalline metal chalcogenides possess exceptional characteristics of charge carriers, conductivity, antioxidation, and long-time stability in air and water medium [103
105]. The slower decay rate of organic dyes and nonreusability under UV visible-light illumination are the main drawbacks of various metal chalcogenides photocatalysts [29,98,106,107]. Moreover, higher radiative recombination rate and photo corrosion are also needed to overcome II
VI semiconductor chalcogenides as photocatalysts [98]. Thus, it is important to develop the degradation efficiency and capabilities of chalcogenides for efficient photocatalysis. To date, extensive efforts have been made for band gap engineering to modify the structural and electronic properties of chalcogenides to enhance their photocatalytic activity [105]. Cadmium-based (S, Se, Te) chalcogenide materials possess inadequate photoactivity and stability for H evolution reaction, a cathodic reduction of water. This can be assigned to the fast recombination of charge carriers and mismatched valence and conduction band potentials [108]. Besides, the higher specific surface area of cadmium-based nanocrystal chalcogenide semiconductors delivers more value for photocatalytic reactions. Although sulfides suffer from fast photo corrosion by oxidizing holes and chemical decomposition of the semiconductor into sulfur and metal ions [109]. Zhou et al. fabricated carbon and nitrogen codoped CdS for impeding the photo corrosion and improving photostability [110]. In addition, doping and codoping create the trap states and are helpful for band gap engineering to enhance charge carrier separation efficiently. This problem can be overcome by encapsulating sulfide into the oxide’s nanoparticle shell. On the contrary, CdS has superior photocatalytic activity and water detoxification to decompose water pollutants and to photoreductively degrade the halogenated benzene derivatives and hazardous metal ions under visible light irradiation [111]. Encapsulating the CdS photocatalysts into oxide shells substantially improves their thermal stability and efficiency. Gupta et al. fabricated CdS/SiO2 photocatalyst in which the outer silica shell provides better photostability and photochemical activity against the industrial dye methyl orange (MO) under visible light. An efficiency of 91% for the photodegradation of MO was found [112]. In such a core-shell nanostructure system, the high surface area, smaller core particle size, and selection of outer shell improve the charge separation and redox potential activity. Researchers have applied CdS-SiO2 or CdS-TiO2 core-shell structure with polymer composite for hydrogen production and photocatalytic activity [113,114]. In which the dispersion of nanoparticles in the contaminated source can be improved by considering the polymer layer, and it also works as a surface coordinating agent to release metal ions. Liu et al. studied bilayer CdS/SiO2 hybrid systems for solar photocatalyst and solar thermal storage. In which, the paraffin core is encapsulated by CdS/SiO2 shell. The n-eicosan SiO2/CdS bilayer heterostructure was used

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for the photodegradation of methyl blue (MB) dye under sunlight illumination. It was observed that the n-eicosan SiO2/CdS microcapsules had made about 90% of MB photo decompose after sunlight illuminated for 240 min. Hence, degradation rate influenced by illuminating time. Moreover, photocatalytic efficacy was considerably dependent on the number of CdS in the shell. Consequently, the higher amount of CdS generates more superoxide and hydroxyl radicals and then leads to a higher degradation rate [115]. Zinc and cadmium-based (S, Se) chalcogenides are advantageous for water splitting. Because the holes in the VB have a lower chemical potential than EO2/H2O and undergo an oxygen evolution reaction (OER), the photoinduced electrons in the conduction band contribute to hydrogen evolution reaction (HER). In the family of chalcogenides photocatalysts, 2D planar chalcogenides with one or a few layers are emerging materials because they have a number of advantageous properties, including an increased specific surface area for species absorption, more exposed surfaceactive sites for catalytic redox reactions, proper band edges alignment, and improved charge separation efficiency. [116,117]. Furthermore, the bandgap values of these compounds vary between 1.6 and 2.4 eV, which is excellent for visiblelight-driven photocatalysis. In the realm of electrocatalysis for the OER, HER, and water splitting, zinc and cadmium-based metal chalcogenides hetrostructure system have proven the several advantages [118]. This type of material is thought to be a promising photo as well as electrocatalytic material with the potential to replace noble metals because of its strong catalytic activity. Numerous catalytic strategies, including increasing the specific surface area to promote the number of active sites exposed, altering the electronic structure of active sites, raising the intrinsic activity of active sites, enhancing conductivity, and optimizing oxygen-containing intermediate absorption and desorption energies, have been designed and used to further promote its electrocatalytic performance [118]. As a result, several metal chalcogenide compounds are gaining popularity because they function as efficient photocatalytic and more electrocatalytically than the standard noble metals.

11.5.3 Applications of chalcogenides nanomaterials heterostructures for solar cell Solar energy is one of the best renewable energy sources. This is the precious gift of nature to humankind as the whole world is going through energy crisis. There is no harmful waste, and no fumes are emitted by solar energy as this provides clean, low-cost electricity in abundance. The second generation of solar cells has used thin films that are based on amorphous-Si, cadmium telluride (CdTe), copper indium gallium and selenium (CIGS), and copper zinc tin sulfide (CZTS). Thus, these photovoltaic devices added a new dimension as a relatively sustainable energy source. The large production of these cells makes them eco-friendly and costeffective. II
VI semiconductor chalcogenide materials are chosen as good absorbers. They display remarkable absorption and coefficient and appropriate optical band gap for absorbing the maximum amount of solar radiation. The direct

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conversion of solar radiation into electrical energy is done in the photovoltaic cell for developing voltage. In this regard, semiconductor chalcogenide materials exhibit many advantages like lower transmission loss, flexibility, affordable, and light weight. Such chalcogenides are either n-type or P-type. By the combination of semiconductor chalcogenides, optical to electrical energy conversion is happened.

11.5.3.1 Thin film photovoltaic cell Semiconductor devices are playing an important role in the generation of power as photovoltaics. The manufacturing technology of these semiconductor-based photovoltaic cells is very efficient and cost-effective. The large-scale production of photovoltaic cells is very cost-effective with thin films. These solar cells are prepared by depositing a number of layers of photovoltaic materials on a substrate made of metal, plastic, or glass. The lightweight and flexibility of these thin film-based photovoltaic solar cells can be applied in various technologies. The use of the material in the manufacturing of devices in film technology is very less and hence is cheaper compared to other technologies [119]. Light absorbance properties of semiconductor materials play a pivotal role in thin film photovoltaic technologies. Metal chalcogenide materials have been found to be very good absorber materials for photovoltaic cell applications [120]. In comparison to crystalline materials, the absorption coefficient of chalcogenides with suitable band gap so as to absorb all the possible number of photons from electromagnetic radiation [121,122]. For effective absorption of sunlight, a very thin layer of the material is enough. In the past, thin film technology was totally based on amorphous silicon. Earlier, amorphous silicon, cadmium telluride, and copper indium gallium selenide were the three common thin film materials. In the present stage, chalcogenides nanomaterials such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) are used even commercially, in thin film devices because of their high efficiency and good performance. The characteristics such as flexibility, lightweight, less drag, and a very thin layer (from nanometers to micrometers) of materials make them advantageous. Although the power conversion efficiency of these cells is 18% only. Commercially, this is even less, 14%
16% [123]. The suitable band gap of the CdTe films with a high absorption coefficient in the visible light region makes them good solar absorbers [124] Further, these can be deposited on the glass substrate. The thin films of quaternary materials such as copper indium gallium selenide can be deposited by the coevaporation method. There is great demand globally for CdTe and CIGS films since 2004 onwards [124]. For CdS/CdTe structures, a substrate configuration produced an efficiency of 13.6%, whereas a superstrate structure was claimed to have a 21.5% [125]. Due to the exposed CdTe surface for contact, a superstrate arrangement is preferred for CdTe devices. The lattice mismatch between CdTe and CdS is lessened by this characteristic of CdS diffusion during manufacture. In which, n-type CdS has a large band gap and P-type CdTe works as a good absorber for solar radiation. A typical solar cell configuration is fabricated by assembling the transparent conducting layer as the front electrode with P-N junction diodes and the active electrode as the back contact.

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11.5.3.2 Polymer/quantum dot hybrid organic
inorganic solar cell When semiconductor chalcogenide QDs are applied in photovoltaic devices, these need to be incorporated with the charge transport layer to accumulate charges in the active layer. Some conjugated polymers are used as charge transport layer along with active layer and metal electrodes in hybrid solar cells. The polymer and (S-, Se-, and Te-based) binary metal QD mixtures are proved to be the best organic/inorganic hybrid material where inorganic has the high electronic transport properties and ability of home polymers processed in solution. These have been found to be advantageous over traditional polymer/fullerene systems [126]. The advantages are as follows: (1) The inorganic acceptor has double advantages, one as a generator of photoinduced charge carriers, and the other as a light absorber better than fullerene derivatives. (2) Modification in size and shape of nanoparticles offer a broad spectral range of the absorption of QDs. (3) Pathways, which simultaneously can be achieved by tailoring the physical dimensions of some of the inorganic/organic mixtures, can be done in a manner to achieve efficient excitonic dissociation and electron transporting path. Further, these provide ultrafast photo-induced charge carrier transfer to host semiconductors to carry efficient charge transfer between organic and inorganic materials. (4) Electron mobility is higher. (5) Photo- and chemical stability are good. From the year 2002, chalcogenide nanoparticle-based heterojunction polymer/QD mixtures became popular for application in solar cells. The maximum EQE has been achieved by CdSe spherical nanoparticles (7 nm diameter) or CdSe nanorods (7
60 nm dimensions) with poly(3-hexylthiophene) (P3HT) from 20% to 55% [127]. A homogeneously distributed polymer film has been found better with nanorods than spherical nanoparticles [128]. Further, the tetrapods and highly branched CdSe, CdTe, and CdSexTe12x, nanocrystals combined with P3HT or poly [2-methoxy-5(3,7- dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) could not yield many efficiencies only around 1%
2%. More efficient devices can be achieved by branched nanoparticles due to more electric pathways provided by them [129]. Therefore, branched, complex polymer/chalcogenide-based devices are successful in the present time. The hybrid solar cells are also formed by combining other metal chalcogenides. The optical properties can be varied by changing the size of the metal QD from 6 to 2 nm yielding the energy gap from 2.6 to 3.1 eV in CdS. In PbS, the band gap can be varied from 0.7 to 1.3 eV by changing the size from 8 to 4 nm [126]. Even PbS and PbSe have low bandgap and absorption properties near the IR region, and makes it possible to use in QD based solar cells. Hence, Pb-based chalcogenides are candidates for the formation of hybrid devices. Heavy metals (Cd and Pb) have been found to be environmentally unsafe. Therefore, efforts are focused on developing technologies based on environmentally friendly QDs. Hence, efforts are on to prepare other hybrid polymer/QD devices. In the recent past, devices based on the combination of SnS [130], FeS2 [131
133], and

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Sb2S3 [134,135] nanoparticles with MEH-PPV, MDMO-PPV, or P3HT were prepared, the efficiencies have been less in comparison to Cd- and Pb-based devices. The cost and availability of pyrite (FeS2) are more attractive than other materials [136]. The indirect band gap of 0.95 eV of this material gives it a large optical absorption coefficient. This is good for photovoltaic applications due to increased light absorption in the INR region. In spite of having suitable optical properties and being more environmentally friendly, the electrical properties of these QDs might be difficult to control properly due to the formation of surface defects arising from Sulfur vacancies.

11.5.3.3 Chalcogenides nanostructures for hybrid photovoltaic cell The chalcogenides have been found with a key role in the PV field as photovoltaic absorbers. The main chalcogenides used in hybrid cells are Cd (CdS, CdSe, CdTe) and Pb (PbS, PbSe) along with other compounds, such as SnS, FeS2, CuInS2, and CuInSe2 [137]. The hybrid layers for PV applications can be developed by mixing different organic semiconductors with colloidal semiconductor nanocrystal QDs [137,138]. The unique size of these colloidal semiconductor nanomaterials controls their optoelectronic properties (based on the quantum confinement effects). The synthesis of these simple solution-based materials is very easy with great potential, hence these are low-cost and low-energy consumption. To have an accurate control on size, shape, crystal structure, and size distribution, the experimental parameters such as precursor type, legend type, reactants concentration, temperature, and time should be controlled [139]. Therefore, these chalcogenide nanocrystals are promising substitutes over decades-old fullerene acceptors. The chalcogenides are at an advantage due to their features such as tuning of the band gap, strong and broad absorption at energies higher than the band edge, and high dielectric constant Also, these exhibit usefulness by overcoming the strong exciton binding energy of the conjugated polymers and electron mobility higher compared to the organic materials. The efforts are to obtain a continuous percolation network between the colloidal chalcogenide’s nanocrystals and the organic compound for the fabrication of highly efficient PV devices. A strong impact on charge generation and transportation can be achieved by the presence of surface traps, which further influenced the contact area of polymer nanostructures [137,140]. The adequate band gap of B2.42 eV and high absorption of Cd-chalcogenides made them promise photovoltaic materials [141]. These colloidal nanostructures are suitable for various fields like solar cells, LEDs, or photodetectors because the energy gap of these materials falls within visible light of the solar energy spectrum. The colloidal nanostructures of CdSe with thiophene-based polymers (e.g., P3HT) can be used because of its band gap energy, B1.74 eV [138] in the photovoltaic cells. The band gap of CdTe is B1.54 eV, which has an absorption in the visible range of light [142]. Its preparation is aqueous solution-based, and hence its approach is environmentally friendly. Pb-chalcogenide plays a significant role in the PV field as its band gap can be tuned from 0.3 eV to .1.5 eV during the synthesis in the NIR region [143]. This way, efficiencies of colloidal nanostructures

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with active layers of embedded PbS or PbSe are improved. The ternary chalcogenides such as CuInS2 or CuInSe2 have been found less-toxic materials with adequate optical and electrical properties [144]. The PbI2 and FeS2 can also be introduced in the PV cells for improving the absorption properties of the active layer [145]. A large surface area and a direct path for charge transport can be provided by embedding silicon nanostructures (nanowires or nanoparticles) in the active organic material [146]. All these efforts enhanced the performance of the PV devices.

11.6

Summary and future scope

Chalcogenides are amorphous compound semiconductors. These are important materials for photonic and optoelectronic devices because they exhibit fascinating photo-induce properties and a wide visible spectrum [1]. The chapter reviews the various properties, synthetic methods, and theoretical and experimental results with possible applications of II
VI metal chalcogenides and polymer composite hybrid structures. Different approaches and strategies help to understand their behavior, mechanism, and potential application with limitations. The Cd and Zn-based chalcogenide nanomaterials and core/shell heterostructure is a developing field for phosphor, photocatalytic, and photovoltaic devices. CdSe and CdS QD-based heterostructures with polymer are most widely used due to their ease of fabrication and characterization. CdS as a buffer layer has been applied for optoelectronic devices. It increases efficiency by lower reflection losses. Although scattering losses, reabsorption, and carrier recombination are the major issues to be addressed. Although Se and Te-based chalcogenide solar cells have exhibited improved efficiency, their toxic nature confines their mass production. Thus, research needs to be focused on green photovoltaic technology with excessive efforts to address all kinds of important issues and pave a way toward the ultimate successful realization of low-cost and high-efficiency solar cells. In this way, chalcogenide perovskites based on S and Se can be realized a major revolution in photovoltaic materials. The sulfide, selenide, and telluride-based chalcogenides are available in abundance. Their binary, ternary, and quaternary compounds find applications in many areas such as sensors, electroluminescent devices, photovoltaic, batteries, fuel cells, and photocatalysts. Such inherent properties are further enhanced in lowerdimensional structures. There are numerous chalcogenides nanomaterials and their hybrid structures with polymers that have been studied extensively and progressively to be used for commercialization. In recent decades, photocatalysis employing chalcogenide materials has made considerable strides. However, some realistic obstacles such as larger scattering losses, reabsorption, relatively larger band gap, excitation limitations, and poor thermal diffusivity are needed to be addressed in promoting and expanding its utilization in industries and commercialization. Hence, special attention is to be paid to developing new methods of surface sensitization or hybrid structures with suitable band gap materials to improve its utilization over a wider regime of the electromagnetic spectrum [2]. One of the biggest challenges to

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improving the performance of photocatalysts is to maximize the absorption of the excitation energy and mitigate the recombination of photo-induced electron-hole pairs. In this direction, several approaches have been implemented to enhance the photocatalytic activity of chalcogenides. Moreover, theoretical calculations and mathematical modeling are beneficial for deep understanding and regulating the activity of chalcogenides with higher photocatalytic output. Cost-effective and environment-bengin synthesis routes to manufacture high-quality, large-scale, lowcost chalcogenides materials are the future challenges. Development in twodimensional chalcogens will accelerate the photocatalysis field, to reach the commercial level early.

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IV semiconductors for optoelectronics: CdS, CdSe, CdTe, in: S. Kasap, P. Capper (Eds.), Handbook of Electronic and Photonic Materials, Springer, Cham, Switzerland, 2017. [143] A. Shrestha, M. Batmunkh, A. Tricoli, S. Dai, S.Z. Qiao Angew, Chem. Int 58 (16) (2018) 5202
5224. Available from: https://doi.org/10.1002/anie.201804053. Ed. [144] J. Kolny-Olesiak, in: A. Tiwari, R. Boukherroub, M. Sharon (Eds.), Solar Cell Nanotechnology, John Wiley & Sons, Inc, Hoboken, NJ, 2013, pp. 97
115. [145] P. Yu, S. Qu, C. Jia, K. Liu, F. Tan, Mater. Lett. 157 (2015) 235
238. Available from: https://doi.org/10.1016/j.matlet.2015.05.033. [146] S.B. Dkhil, R. Ebdelli, W. Dachraoui, H. Faltakh, R. Bourguiga, J. Davenas, Synth. Met. 192 (2014) 74
81. Available from: https://doi.org/10.1016/j.synthmet.2014.03.016.

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Aleem Ansari, Rashmi A. Badhe and Shivram S. Garje Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz (East), Mumbai, Maharashtra, India

12.1

Introduction

Metal chalcogenide (MC) nanocomposites are an important class of nanomaterials and they have a special place in the development of nanotechnology, due to their diverse applications in various fields such as photocatalysis, electrochemical sensors, supercapacitors, and batteries and in environmental remediation, etc. This chapter describes the present status of these materials with respect to two aspects, that is, their synthetic methods and applications. An attempt has been made to identify challenges in the synthesis of these materials as well as factors that affect their properties, which makes them useful for various applications. Further, this chapter will highlight opportunities in the field of MCs, their nanocomposites, and their applications. Nanomaterials are made up of multiple atoms or molecules of varying sizes and morphologies. Theoretically, nanomaterials are materials having at least one dimension less than 100 nm. The production of chalcogenides at the nanoscale has received a lot of attention in recent years. The formation of nanoparticles has been studied for a wide range of materials, and chalcogenides are no exception. Man-made nanomaterials are created specifically to take benefit of unique properties due to the nanoscale of the materials.

12.1.1 Introduction to chalcogens Group 16 or VI(A) of the modern periodic table includes oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po), which are known as chalcogens (“ore builders”) [1,2]. The ability to form new bonds and new compounds is the most valuable and fundamental characteristic of these elements [3
5]. In this aspect, the variation of the properties in this group is not random but rather depends on the structure of the element and changes in a systematic way with the atomic number [1,2]. Chalcogens are soft elements in geochemistry and so are also known as chalcophiles [6,7]. Chalcophiles are the soft elements in terms of geochemistry (based on hard/soft acids and bases terminology) [1,8]. The term “chalcogen” comes from two Greek words, chalcos or khalkos, which imply “ore formers” [7]. Metal-Chalcogenide Nanocomposites. DOI: https://doi.org/10.1016/B978-0-443-18809-1.00012-2 © 2024 Elsevier Ltd. All rights reserved.

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These two terms gave rise to the term “khalkos-genes,” or chalcogens as we know them today [9,10]. Chalcophiles were previously assumed to have an abundance of electrons [1]. They are known as the low-valent elements of Group 16 (O, S, Se, Te, and Po) or primarily electronegative elements because they frequently attach to the functional groups of organic molecules and/or serve as electron donors for transition metals [11,12]. This feature has resulted in a new field of research known as electrophilic and polycationic chalcogen chemistry [13]. The general electronic configuration of chalcogens is ns2np4, with two electrons short of a filled valence shell [14,15]. Table 12.1 shows the characteristics and properties of O, S, Se, Te, and Po. This chapter focuses on chalcogenides and chalcogenide-based nanomaterials, this includes materials where oxygen, sulfur, selenium, and tellurium combine with metals to form binary, ternary, and quaternary composite materials. The synthetic methods and modifications in them to give desired structure, morphology, and chemical composition have been discussed. Further main emphasis has been given to MC nanocomposites as they show enhanced properties, thereby increasing their use in a large number of applications.

12.1.2 Introduction to chalcogenides Chalcogenides are a broad group of compounds with diverse physical and chemical properties. Chalcogenides have lower band gap energies, making them ideal for visible-light harvesting and related applications. Due to their low band gap energy, efficient light harvesting, and many uses, metal sulfides are the most studied chalcogenides [16,17]. Due to their anisotropic properties, MCs (S, Se, and Te) are industrially important and scientifically significant materials [18]. This chapter describes general features, various synthetic methods, and applications of chalcogenides, such as metal sulfides, selenides, and tellurides [17].

12.1.3 Classification of chalcogenides based on the number of elements Based on the number of elements present, chalcogenides can be classified into various types such as binary, ternary, and quaternary systems. In comparison to quaternary chalcogenides, both binary and ternary chalcogenide molecules have been thoroughly studied. Except for noble gases, all of the main group elements form chalcogen derivatives [19].

12.1.3.1 Binary chalcogenides Binary chalcogenides are made up of two ions: a metal ion and a chalcogen anion. Cadmium sulfide (CdS), for example, is one of the most investigated chalcogenides. It has a band gap of 2.4 eV. Due to its numerous uses in laser light-emitting diodes, solar cells, and optoelectronic devices based on nonlinear characteristics, significant efforts have recently been taken to prepare and study the optical properties of CdS nanoparticles and quantum dots [20]. Palve et al. [21] have used cadmium(II)

Table 12.1 Selected properties of the Group 16 elements: oxygen, sulfur, selenium, tellurium and polonium [14]. Property

Oxygen

Sulfur

Selenium

Tellurium

Polonium

Atomic symbol Atomic number Atomic mass (amu) Atomic radius (pm) Density (g/cm3) at 25 C Electron affinity (kJ/mol) Electronegativity First ionization energy (kJ/mol) Melting point/boiling point ( C) Ionic radius (pm)

O 8 16.00 48 1.31 2141 3.4 1314 2219/ 2 183 140 (22)

Normal oxidation state(s) Standard reduction potential (E , V)(E ⇢ H2 E in acidic solution) Type of oxide Valence electron configurationa

22 1 1.23

S 16 32.07 88 2.07 2200 2.6 1000 115/445 184 (22), 29 (16) 16, 14, 22 1 0.14

Se 34 78.96 103 4.81 2195 2.6 941 221/685 198 (22), 42 (16) 16, 14, 22 2 0.40

Te 52 127.60 123 6.24 2190 2.1 869 450/988 221 (22), 56 (16) 16, 14, 22 2 0.79

Po 84 209 135 9.20 2180 2.0 812 254/962 230 (22), 97 (14) 12(14) 2 1.00


2s22p4

Acidic 3s23p4

Acidic 4s24p4

Amphoteric 5s25p4

Basic 6s26p4

a

Configuration shown in this table does not include filled d- and f-subshells.

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acetophenone thiosemicarbazone complex as a single source molecular precursor for the preparation of CdS nanoparticles by the solvothermal route.

12.1.3.2 Ternary chalcogenides Three different ions, including a chalcogen anion, constitute ternary chalcogenides. The property of these materials can be tuned by controlling the stoichiometric ratio between two metal ions. Disale et al. [22] have reported the hexagonal phase of the ternary chalcogenides M12xFexS (M 5 Cd and Zn) by changing the preparation method, that is, pyrolysis and solvothermal. Similarly, Ansari et al. [23] have reported ternary chalcogenide (CuFeS2) material using a single source precursor. Further, this chalcopyrite CuFeS2 material has been used as an electrode material for supercapacitor applications.

12.1.3.3 Quaternary chalcogenides Four distinctions, including a chalcogen anion, constitute quaternary chalcogenides. Quaternary chalcogenides are a broad family of materials made up of earthabundant and harmless elements that may be used for a variety of purposes, including solar cell absorbers, photocatalysts for water splitting, and so on. Quaternary materials, unlike binary and ternary chalcogenides, may contain a high number of different types of elements, resulting in relatively complicated electrical and structural properties. Quaternary CuInGaSe2, whose band gap can be adjusted by the amount of indium incorporation, has been reported by Devany et al. [24]. In order to boost the volatile organic compounds and reduce the infrared absorption losses in ZnO, the quaternary CuInGaSe2 layer with a larger bandgap was used. This led to a 12.5% efficiency. Similarly, Gabor et al. [25] have reported the preparation of quaternary CuInxGa(12x)Se2 solar cells using (Inx, Ga1x)2Se3 precursor films, which gave an efficiency of 15.9%. Thus, there are opportunities to prepare binary, ternary, and quaternary nanocomposites having desired elements in appropriate stoichiometry to obtain novel properties for specific applications.

12.1.4 Classification of chalcogenides based on the number of chalcogen ions MCs can also be classified based on the number of chalcogen ions present in them.

12.1.4.1 Mono-chalcogenides Monochalcogenides can be represented as MX, where M is a metal and X is a chalcogenide anion of elements such as S, Se, or Te with 1:1 metal to chalcogen stoichiometry [26]. Safari et al. [27] have reported the structural, electronic, and optical properties of mono-chalcogenides ZnX and CdX (X 5 S, Se, and Te) monolayer. Similarly, Ferahtia et al. [28] have reported the structural, electronic, and optical properties of ZnS and ZnSe mono-chalcogenides. Mono chalcogenide compounds are widely used in optical waveguides [29], photovoltaic devices [30], blue

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light-emitting diodes [31], photodetectors [32], solar cells [33], and blue-lasing materials [29].

12.1.4.2 Dichalcogenides These are represented as MX2, where M is a metal and X is a chalcogen. Some important dichalcogenides are TiO2 [34], MoS2 [35,36], WS2 [37,38], TaS2 [39], MoSe2 [40], etc. The stoichiometry is exactly 1:2 and the crystals are formed in two-dimensional (2D) structures. Recent studies have demonstrated the use of 2D transition metal dichalcogenides (2D TMDs) as affordable catalysts for the hydrogen evolution reaction (HER) in water electrolysis [41].

12.1.4.3 Trichalcogenides Trichalcogenides are formed by several early metals, including Ti, V, Cr, and Mn. Recently, the ternary chromium-based tritellurides CrSiTe3 and CrSnTe3 [42
44] and the ternary chalcogenides MnPX3 [45] have been studied by several research groups. Wang et al. [46] have demonstrated the photocatalytic hydrogen evolving activity of ultrathin NiPS3 nanosheet, revealing the potential characteristic of this family toward water splitting.

12.1.5 Chalcogenide nanomaterials At nanodimensional level, MCs acquire interesting properties that make them useful for a large number of applications, such as nanocatalysis, environmental remediation, and energy storage devices. For example, CdS has emerged as the most promising material for photocatalytic activity, particularly in photocatalytic H2 generation. However, due to the rapid recombination of photogenerated charges, CdS has poor photocatalytic activity [19]. For convenience, MCs have been divided into three types.

12.1.5.1 Metal-based chalcogenides This includes MCs in which a non-noble metal is incorporated. Kumar et al. have prepared CdS nanoparticles codoped with Cu and Ni using a wet chemical synthesis route [47]. The effective wastewater treatment procedure is possible by doping TiO2 with various metals, including Ce, Co, Cu, Fe, Mn, Mo, Ni, Y, V, Cr, and Zr, which have higher degradation efficiency under visible light [48].

12.1.5.2 Noble metal-based chalcogenides Noble metals such as Pt are known to work as an effective cocatalysts to improve the photocatalytic activity of chalcogenides. Choi et al. have described the preparation of Au and Ag-loaded ZnS [49]. Yao et al. have reported synthesis of an efficient Pt/CdS photocatalyst by photo technique approach. It is found that the

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photocatalytic activity of this material improves effectively for solar hydrogen production [50].

12.1.5.3 Chalcogenide composites In general, it has been observed that the properties of MC nanocomposites are much better than the bare MCs, due to the synergistic effect in the former. There are various ways of synthesizing such nanocomposites, for example, Pawar et al. and Badhe et al. have reported the preparation of CdS@MWCNTs and CdS@TiO2 nanocomposites via solvothermal route, respectively [20,51]. These materials show enhanced catalytic activity over bare MCs. He et al. used a simple solvothermal approach to make MoS2/CdS nanocomposite. The photocatalytic activity of the synthesized MoS2/CdS is reported to be five times greater than that of bare CdS [52]. Khan et al., have prepared CdS-graphene by employing a simple and one-step precipitation method. CdS-graphene nanocomposites demonstrated improved photocatalytic activity when subjected to visible light [53]. Chalcogenide and chalcogenidebased nanomaterials are well known for their important applications (Fig. 12.1). Because of the unusual physical and chemical phenomena revealed in these nanochalcogenides, they continue to stimulate the interest of researchers and engineers as a diverse group of interesting solids, paving the way for new inventions in science and technology. Novel applications of MC-based materials, such as photocatalysts for degradation of organic pollutants, electrode materials, water splitting, nanocatalysts, sensors, and CO2 activation for environmental remediation, are described below in more detail.

Figure 12.1 Schematic representation of metal chalcogenides and their potential applications.

Challenges and opportunities of chalcogenides and their nanocomposites

12.2

227

Synthesis of metal chalcogenide and their nanocomposites

Various applications of MC nanocomposites depend on their properties that, in turn, rely on size and shape of these materials. Synthetic methods play an important role in controlling size and shape of the nanomaterials. Therefore, it is necessary to adopt appropriate methods. If required newer methods should be developed or the existing methods should be modified in order to overcome the challenges. There are a variety of methods available for the preparation of chalcogenide and chalcogenide-based nanomaterials. An appropriate method can be used with the objective to prepare a material with a specific application. The progress in the synthesis of new nanostructured materials of various sizes and morphologies has improved the wide range of applications of MCs and their nanocomposites. Due to the well-known quantum size effect, when MCs are reduced to the nanoscale, new physical and chemical properties emerge [54
61]. Apart from nanoscale synthesis of MCs, the host material can also be modified to achieve new, optimized, and enhanced properties (e.g., with noble metals, metal oxides, carbon materials, and so on). Bare nanomaterials may not be able to match the stringent requirements of future applications in various fields, and their inherent physical properties, such as conductivity, mechanical and thermal stability, and recyclability. The rational introduction of one or more distinct materials in contact with single-phased nanomaterials holds great promise for addressing the existing challenges [62
64]. The resulting modified MCs have been demonstrated to have synergetic abilities, allowing them to reach their maximum performance potential [62,65]. It is urgently required to further encourage MC and their nanocomposite-related research and developmental activities to address the actual chalcogenide challenges. Novel MC nanomaterials with defined shapes, sizes, compositions, and structures manufactured on a massive scale are the core of their practical applications. In this context, a few synthetic methods are described below.

12.2.1 Hot-injection method This method has been extensively used for the preparation of MC nanomaterials and their nanocomposites. The hot-injection approach is particularly effective for producing high-quality nanocrystals with high crystallinity and narrow particle size distributions. Monodisperse nanocrystals of semiconductors and metal oxides/sulfides can be produced via thermal decomposition of organometallic compounds in hot coordinating solvents [54,59,60,66]. Bawendi et al. were the first to use the hotinjection approach to make monodispersed CdSe nanocrystals [67]. Since then, this approach has been used to successfully produce a variety of high-quality semiconductor nanocrystals, including II
VI, III
V, and IV
VI semiconductor nanocrystals [59,68]. This approach has been used to successfully synthesize a variety of MC nanocrystals, such as GeTe [69,70], ZnS [71], Bi2S3 [72], Bi2Te3 [73], Ag2E (E 5 S, Se, Te) [74,75], β-FeSex [76], FeS2 [77], In2S3 [78,79], SnSe [80], Cu2S

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Metal-Chalcogenide Nanocomposites

Figure 12.2 Temporal shape evolution of rice-shaped CdSe nanocrystals. Reprinted from Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 3343
3353 with permission from American Chemical Society.

[81], GaSe [82], Sb2S3 [83], and so on. Liu et al. [84] have reported the synthesis of CuInS2 nanocrystals with tunable structure and band gap using hot-injection method by altering the dose of oleylamine. Through a multiple-injection reaction, the morphology of rice-shaped CdSe nanocrystals evolved after a certain amount of time (Fig. 12.2) [85]. The solvents are usually organic compounds in hot-injection synthesis, while the capping agents are frequently chain organic compounds containing phosphine moieties such as trioctylphosphine and tributylphosphine. These organic chemicals are frequently poisonous, unstable at high temperatures, and costly. As a result, less hazardous, more stable at high temperatures, and comparatively less expensive solvents are needed. The main disadvantage of this method is that it necessitates fast injection and, as a result, rapid cooling, thereby limiting industrial production of the materials. Therefore, large-scale manufacturing of the materials using hot-injection synthesis is not feasible. Furthermore, it has a low percentage of synthetic repeatability [86].

12.2.2 Hydrothermal method The hydrothermal approach has been widely employed for several decades for the synthesis of a wide range of functional nanomaterials with particular sizes and shapes. Water is used as the reaction medium in sealed steel pressure containers with Teflon liners, which are subsequently heated to a predetermined temperature

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to accelerate the chemical reaction [59,68,87
90]. The large-scale synthesis of various functional nanomaterials using this method under appropriate reaction parameters, which is difficult to achieve using the previously discussed hot-injection method, is possible. This is essential for the practical applications of chalcogenide materials. Along with water other tiny coordinating molecules such as ethylenediamine, hydrazine hydrate, ethylenediamine tetraacetic acid, and polyvinylpyrrolidone can be used to control the ultimate nanocrystal development [54]. The hydrothermal approach has been quite successful in the synthesis of MCs such as ZnSe hollow microspheres [91], MoS2 hollow cubic cages [92], β-In2S3 nanoflowers [93], CuS microtubules [94], SnS2 nanocrystals [95], NiSe nanowires [96], CoTe and NiTe nanowires [97], Sb2S3, Sb2Se3 and Sb2Te3 nanobelts [98
100], Ag2Te nanotubes [101], Ag2Se nanoparticles [102], and so on. Gau et al. have used a simple hydrothermal method to prepare specific octahedral and cubic NiS2 nanocrystals with orientation along 111 and 100 facets (Fig. 12.3) [103]. Despite the fact that the method has several advantages, the particle size distribution, purity of nanophase, and morphology variation still remain a challenge to overcome.

12.2.3 Solvothermal method The solvothermal method is similar to the hydrothermal process, except that the solvent is nonaqueous. Different organic solvents can often be used as reaction media, which control the shapes, sizes, and phases of the final materials. The size and shape distributions, as well as the crystallinity of nanoparticles, are usually better controlled by the solvothermal approach than by the hydrothermal method [54]. The solvothermal technique has been used to synthesize the wire-like Fe1xS(en)0.5 [104], FeS2 [105], Cu2Te [106], Ag2Te [107], and Bi2S3 [108]; rod-like α-MnSe [109] and MnS [110]; belt-like ZnSe [111]; flower-like FeSe2 [112], γ-In2Se3 [113] and CoS1.097 [114]; dendrite-like Cu2xSe [115] and Cu2S [116]; plate-like Sb2Te3

Figure 12.3 SEM, TEM, and SAED on NiS2 octahedrons (A, B, and C) and cubes (D, E, and F). SAED patterns (C and F) have been obtained from the corresponding particles with the electron beam along the [111] and [001] directions, respectively. Reprinted from J. Zheng, W. Zhou, Y. Ma, W. Cao, C. Wang, L. Guo, Chem. Commun. 51 (2015) 12863
12866 with permission from Royal Chemical Society.

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Metal-Chalcogenide Nanocomposites

[117] and CuS [118]; and sphere-like In2S3 [119], FeS2 nanowebs [120], and CoTe nanotubes [121]. The selection of an adequate solvent is important as it often functions as a “shape controller,” which is especially important for MC nanocrystals. Pawar et al. have reported that the morphology of CdS depends on the reaction temperature (Fig. 12.4) [122]. Qian et al. [123] and Cho et al. [124] have synthesized ultrathin β-In2S3 nanobelts and graphene-like MoS2 nanoplates using pyridine and xylene as solvents, respectively. There are tremendous opportunities to prepare desired MC nanocomposite in the required morphological form by selecting an appropriate solvent and other synthetic parameters in this method.

12.2.4 Microwave method Microwave synthesis has been widely used for the liquid phase synthesis of diverse inorganic nanomaterials due to its unique advantages such as rapid reaction rate, inexpensive processing costs, high yields, and side reaction inhibition [54,125
133]. Dı´az-Cruz et al. [134] have reported the microwave synthesis of Bi2S3 with various morphologies, such as nanofibers (Fig. 12.5A), nanorods (Fig. 12.5B), and nanohedgehogs (Fig. 12.5C). The type of solvents used, the pH of the solution, the reaction temperature, and the concentration of the solution are all significant parameters for tuning the morphologies of Bi2S3 nanostructures. Panda et al. [128] have reported the synthesis of an organically passivated uniform, ultra-thin, crystalline, and highly aligned rods and wires of ZnS, ZnSe, CdS, and CdSe, etc. using a microwave-assisted approach. Ge et al. [135] have reported ternary MC (CoIn2S4, CuInS2, AgInS2, and CuFeS2) nanocrystalline materials using nonaqueous (mixed solvents), that is, amine (octadecylamine or benzylamine) and benzyl alcohol by microwave-assisted route. This method can be extended for the preparation of a variety of MC nanocomposites.

Figure 12.4 TEM images of CdS nanoparticles synthesized from Bis(cinnamaldehyde thiosemicarbazone)cadmium(II) Chloride (I) and Bis(4-fluoroacetophenone Thiosemicarbazone)cadmium(II) Iodide (II) at (A) 190 C (B) 230 C and (C) 270 C. (D) HRTEM image of nanoparticles prepared at 270 C (inset: FFT of CdS nanoparticles (I)). Reprinted from A.S. Pawar, S.C. Masikane, S. Mlowe, S.S. Garje, N. Revaprasadu, Eur. J. Inorg. Chem. (2016) 366
372 with permission from Wiley.

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Figure 12.5 SEM micrographs of (A) Bi2S3 nanofibers, (B) Bi2S3 nanorods and (C) Bi2S3 nano-hedgehogs. Reproduced from E.B. Dı´az-Cruz, O.A. Castelo-Gonza´lez, C. Martı´nez-Alonso, Z. MontielGonza´lez, M.C. Arenas-Arrocena, H. Hu, Mater. Sci. Semicond. Process. 75 (2018) 311
318 with permission from Elsevier.

12.2.5 Sonochemical method In the chemical synthesis of diverse crystalline nanostructures with narrow size distribution and relatively large areas, ultrasound method has been widely employed. Ultrasound accelerates a chemical reaction by acoustic cavitation rather than direct interaction with precursors. When a liquid is subjected to intense ultrasound stimulation, the bubbles undergo the formation, growth, and implosive collapse processes [54,136
138]. The ability to manufacture extremely pure nanocrystals with uniform shapes and narrow size variations is one of the most significant benefits of the sonochemical approach. The radical species formed from water molecules by the absorption of ultrasonic radiation is thought to be involved in the formation of α-HgS nanoparticles [139]. The following is a summary of the possible reaction mechanism for the sonochemical production of α-HgS nanoparticles in aqueous solution: H2 O☽☽☽☽☽☽☽☽☽☽ H d 1 OH d 22 d 22 2H 1 S2 O3 ! S 1 2H 1 1 SO3 22 Hg21 1 S22 ! HgS nHgS ! ðHgSÞn The sonochemical synthesis is used to prepare various MC NPs, such as ZnSe [140], CuSe [141], PbSe [142], NiS [143], HgSe [144], Cu3Se2 [141], Bi2S3 [137], PbS [145], CdS [146] and Sb2S3 [147]. Wang et al. have also reported the selective production of α-HgS and β-HgS nanoparticles in an aqueous solution by a sonochemical approach [139]. Sakthivel et al. [148] have reported the irregular nanorodlike structure for bare Bi2S3 and rod-like structures for Ni-doped Bi2S3 as shown in Fig. 12.6. Similarly, Gupta et al. have prepared irregular shape ZnO NPs [149]. The main advantages of sonochemical technique are its low cost, control over the reaction conditions, high reaction rate, small particle size distribution, low reaction temperature, high purity, scalability, and environmentally friendly nature [148,149].

Figure 12.6 The FESEM images of Bi2S3 (A), lower and higher magnification view of Ni-Bi2S3 (B and C). EDX spectrum of the Ni-Bi2S3 (D) and inset shows the pie chart. Reprinted from R. Sakthivel, S. Kubendhiran, S.-M. Chen, Ultrason. Sonochem. 54 (2019) 68
78 with permission from Elsevier.

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As a result, when compared to other traditional chemical synthesis methods, the sonochemical methodology is a more feasible alternative [150,151]. This method can be used further for the preparation of binary, ternary, and quaternary MC nanocomposites.

12.2.6 Growth of metal chalcogenide nanostructure arrays on substrates The importance of nanostructured MC arrays on the substrate in understanding quantum size effects and specific applications has received much interest. Many well-aligned MC nanostructures have now been successfully produced on a variety of substrates using various techniques. SnS nanowires on tin metal foils [54,152]; CuS nanoneedles on carbon nanotubes (CNTs) [153]; ZnS nanobelts on a zinc substrate [154]; Bi2S3 nanowires on W foils [155]; and Cu2xSe nanosheets [156], nanoribbons, and heterostructures on Cu substrates are only a few examples. Xiong et al. [157] have described the synthesis of BiSI nanorod arrays on tungsten (W) substrates using as-prepared Bi2S3 precursor on tungsten foil, which is immersed in the autoclave and heated at 180 C for 5 h (Fig. 12.7). Hu et al. have demonstrated deposition of MoS2 nanofilms on Mo foils [158].

12.3

Preparation of chalcogenide nanocomposites

12.3.1 Preparation of metal chalcogenide nanocomposites with carbon materials The use of carbon materials, particularly activated carbon, CNTs, and graphene, has been explored extensively as substrates for different active nanomaterials for various applications, due to their high conductivities and large surface areas. It has been demonstrated that MC nanoparticles can be loaded onto carbon substrates

Figure 12.7 (A
C) SEM image of the Bi2S3 precursor grown on the W substrate at different magnification. Reprinted from J. Xiong, Z. You, S. Lei, K. Zhao, Q. Bian, Y. Xiao, et al., Chem. Eng. 8(35) (2020) 13488
13496 with permission from American Chemical Society.

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[54,159
164]. Gedanken et al. have shown that 1D carbon-coated WS2 (WS2/C) composite may be synthesized by thermolysis of W(CO)6 with elemental S at 750 C in an inert environment in a closed Swagelok reactor [162]. Park et al. have reported that by reacting Cu(acac)2 with elemental S at 190 C, β-Cu2S nanocrystals can be produced in situ on CNTs in an oleylamine solution (Fig. 12.8) [165]. Chi et al. have reported a low-temperature hydrothermal method to prepare Co3O4/ graphene nanocomposite using GO nanosheets and cobalt nitrate as precursors (Fig. 12.9A
C). These Co3O4 particles range in size from 100 to 250 nm and have a consistent polyhedral shape (Fig. 12.9) [166]. Prabhu et al. have also used a solvothermal process to develop TiO2/rGO nanocomposites and have demonstrated that rGO improves the photovoltaic performance of TiO2 nanostructures [167]. Thus, the future scope lies in the preparation of carbon-based MC nanocomposites with improved properties for various applications. Further, MC nanocomposites with phosphorous, nitrogen-doped carbon nanospheres, carbon dots, carbon onions, etc. can be prepared and studied for their various applications.

Figure 12.8 TEM and HRTEM images of the (A) Cu2S-DWCNT and (B) Cu2S-MWCNT. Reprinted from Y. Myung, D.M. Jang, Y.J. Cho, H.S. Kim, J. Park, J.U. Kim, et al., J. Phys. Chem. C. 113 (2009) 1251
1259 with permission from American Chemical Society.

Figure 12.9 (A
C) SEM images of the Co3O4/graphene composite at different magnification. Reprinted from X. Chi, L. Chang, D. Xie, J. Zhang, G. Du, Mater. Lett. 106 (2013) 178
181 with permission from Elsevier.

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12.3.2 Preparation of chalcogenide nanocomposites with noble metals The MC nanocomposites with doped noble metals leading to modified structures can be divided into two types, that is, heterostructures and core-shell structures. There are many nanocomposite systems wherein the combination of noble metals such as Au, Ag, Pt, and Pd and metal oxides such as TiO2, CeO2, and ZrO2 have been reported in literature [54,168
177]. This has become one of the prominent research areas because the nanocomposites having a combination of noble metal and MCs have enhanced catalytic activities, due to synergistic effects [178]. Xiong et al. [179] have reported one-dimensional (1D) Ag@ZnO core-shell hetero-nanowires synthesized by a simple solution process (Fig. 12.10).

12.3.3 Preparation of chalcogenide nanocomposites with metal oxides In general, metal oxides are important active components due to their mixed electronic and ionic conductivity. Many research groups have used metal oxides with other MCs to expand and deepen the applications of MC nanostructures

Figure 12.10 (A and B) SEM images of the as-prepared Ag@ZnO core-shell heteronanowires. (C) TEM image and (D) HRTEM image of an individual Ag@ZnO core-shell hetero-nanowire. Reprinted from J. Xiong, Q. Sun, J. Chen, Z. Li, S. Dou, CrystEngComm. 18 (2016) 1713
1722 with permission from Royal Chemical Society.

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[54,180
187]. Several metal oxide-MC nanocomposites, such as TiO2/WS2 nanotubes [183], ZnO/WS2 nanotubes (Fig. 12.11) [184], and many others, have been studied. Badhe et al. have reported the preparation of CdS@TiO2 nanocomposites by a two-step low-temperature solvothermal decomposition technique (Fig. 12.12) [51]. Majhi et al. have used hydrothermal method for the synthesis of transition MC nanocomposites, that is, CoTe2@CuTe, FeTe2@CuTe, and CoSe2@CuSe2. Among these sphere-like CoTe2@CuTe nanocomposites have excellent electrocatalytic activity for HER at all pH levels [188]. The above discussion proves that there is tremendous scope for synthesizing a variety of MCs and their nanocomposites using an appropriate method of synthesis. Further, synthetic methods can be modified to obtain materials in desired size and shape. Future scope lies in the optimization of synthetic parameters to get MC nanocomposites for specific applications. Moreover, suitable methodologies can be adopted to prepare binary, ternary, and quaternary nanocomposites with enhanced properties.

Figure 12.11 ZnO colloids immobilized on WS2 nanotubes: (A) overview TEM, (B) HRTEM, and (C) overview SEM. Reprinted from M.N. Tahir, A. Yella, H.A. Therese, E. Mugnaioli, M. Pantho¨fer, H.U. Khan, et al., Chem. Mater. 21 (2009) 5382
5387 with permission from American Chemical Society.

Figure 12.12 TEM images of as-synthesized (A) CdS@TiO2 (CT1, TiO2 5 50 mg) and (B) CdS@TiO2 (CT2, TiO2 5 100 mg) nanocomposites. Reprinted from R.A. Badhe, A. Ansari, S.S. Garje, Bull. Mat. Sci. 44 (2021) 11 with permission from Springer Nature.

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12.4

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Application of chalcogenide and their nanocomposites

The following section describes major applications of MCs, their nanocomposites, and future opportunities.

12.4.1 Photocatalysts The application of semiconductor materials as photocatalysts in the degradation of dyes is presently a keenly researched area, due to its environmental and economic advantages over other wastewater treatment processes. The photocatalysis process involves the use of UV or visible light [54,189
192]. Many MC semiconductors such as Cu2S [193], SnS [194], WS2 [195], and MoS2 [196] have been explored. Xu et al. [193] have reported the synthesis of a highly efficient CuS photocatalyst by the dealloying method. Evaluation of the photocatalytic activity against methyl orange, methylene blue, and rhodamine B dyes shows that the materials have good photocatalytic activity under visible light irradiation (Fig. 12.13A). The narrow bandgap of these materials makes them suitable for absorption of visible light of solar radiation, making the process much more cost-effective and environmentally benign. However, the high rate of recombination of charge carriers generated by photo irradiation seriously limits the application of MC semiconductors [197]. One way to overcome this is to use MC nanocomposites wherein the electron-hole recombination rate is reduced. GO/MoS2 nanocomposite synthesized via a hydrothermal method shows 99% degradation of methylene blue dye after 60 min exposure to sunlight. This is due to improved charge transfer as confirmed by electron (A)

(B) 1.0

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Figure 12.13 (A) At/A0 curves for photocatalytic degradation of dyes in the absence of CuS NPs under solar irradiation. (B) Degradation efficiency of MB dye catalyzed by GH and MGH composite. (A) Reprinted from W. Xu, S. Zhu, Y. Liang, Z. Li, Z. Cui, X. Yang, et al., Sci. Rep. 5 (2015) 18125 with permission from Springer Nature. (B) Reprinted from Y. Ding, Y. Zhou, W. Nie, P. Chen, Appl. Surf. Sci. 357 (2015) 1606
1612 with permission from Elsevier.

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impedance spectroscopy analysis (Fig. 12.13B [198]). There are many reports available wherein the coupling of TiO2 with suitable semiconductors such as ZnO, WO3, and MoO3 has been reported [199
201]. Moreover, an electrical field-assistance concept in p-n heterojunction diode photocatalyst composites such as Cu2O/TiO2, BiOI/g-C3N4, Ag2O/Bi2O2CO3, BiVO4/Bi2O3, and Ag2O/TiO2 for the efficient separation of electron and hole pairs has been studied to enhance the photocatalytic activity [202
206]. Zhou et al. and Wang et al. have reported Ag2O/TiO2 nanobelts and Ag2O/TiO2/V2O5 heterostructured materials, respectively, for photodegradation of dyes [207,208]. There are great opportunities to prepare MC nanocomposites with a variety of other materials to give efficient photocatalysts.

12.4.2 Environmental remediation The MCs have been used to convert hazardous chemicals into less hazardous or nonhazardous substances. Hexavalent chromium Cr(VI) is the most hazardous pollutant found in soil, wastewater, and groundwater. High toxicity of Cr(VI) at extremely low concentrations can cause risk to humans and another living being. For example, it can raise the risk of DNA mutation and lung cancer when inhaled over an extended period of time (Fig. 12.14) [209
212]. Several methods have been used to treat Cr(VI), including ion exchange, bacterial degradation, chemical reduction, and adsorption [213
215]. However, usage of these conventional methods is limited since they are expensive and produce secondary waste as they involve use of large amounts of reducing agents, such as ferrous sulfate, sodium hydrogen sulfite, sodium pyrosulfite, hydrazine hydrate, or sulfur dioxide [216
218]. Due to its ease of use and effectiveness, formic acid (HCOOH) has recently been reported as a reducing agent for reducing Cr(VI) to Cr(III) [212,219
222]. Various MCs and their nanocomposites, such as TiO2 [223], NiO@TiO2 [224], ZnO@TiO2 [225], ZnO [226], Fe3O4@graphene [227], Pd@SiO2-NH2 (Fig. 12.15A) [228], ZnOTiO2-OCNT [229], CdS/RGO [230], and Pd@CdS@TiO2 [231] have been reported for the reduction of Cr(VI) to Cr(III). Wu et al. [232] have reported the synthesis of Pt NCs/g-C3N4 by a one-step hydrothermal process and used as a catalyst for the reduction of Cr(VI) to Cr(III) (Fig. 12.15B). Liu et al. have reported an enhanced photocatalytic reduction of Cr(VI) by ZnO-TiO2-CNTs [229] and by ZnO-reduced graphene oxide composites [233]. The rate of Cr(VI) reduction increased with ZnO content in the ZnO/TiO2 composite, and the sample with 2.0 mol.% ZnO showed the greatest photocatalytic reduction of Cr(VI) in aqueous solution because the efficient transfer of electrons from ZnO to TiO2 prevented charge carrier recombination [234]. Our group has reported conversion of Cr(VI) to Cr(III) at room temperature using a simple reducing agent without any external source of light [231].

12.4.3 Reduction of nitroaromatic compounds Nitrobenzene derivatives (NB) are extremely harmful to humans and the environment, while their reduced products, aminobenzene derivatives, exhibit low toxicity and better biodegradability. Moreover, aniline compounds are usually used as important

Challenges and opportunities of chalcogenides and their nanocomposites

m

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Figure 12.14 Schematic diagram of toxicity and mutagenicity of Cr61. Reprinted from K.H. Cheung, J. Gu, Int. Biodetor. Biodeg. 59 (2007) 8
15 with permission from Elsevier.

reaction intermediates in the manufacturing of many organic compounds and in the production of dyes, pharmaceuticals, and agricultural chemicals [235
242]. Noble metal nanoparticles are used as catalysts, although this increases the cost of the material and makes large-scale industrial uses difficult due to the aggregation of the catalysts. The selection of appropriate components is essential to such a transformation because it affects the conversion reaction conditions, efficiency, and selectivity [236,243
245]. This issue can be solved by using MCs based on noble metals, which are proven to be extremely selective for converting nitro compounds into their equivalent amino derivatives. Many composites such as Pd/TiO2 [246], Au/TiO2 [247], Ag2O/TiO2 [248], Cu2O/Au [205], Fe3O4@Cu [249], Au/Fe3O4 [250] and polymer stabilized metal nanoparticles have been used for this purpose. Mandlimath et al. [251] have reported that the reduction of 4-nitrophenol (4-NP) is accelerated by CuO, Co3O4, Fe2O3, and NiO, while TiO2, V2O5, Cr2O3, MnO2, and ZnO are inactive toward reduction of 4-NP. Lee et al. and Huang et al. have also used a cuprous oxidebased catalyst for the reduction of 4-NP [252,253]. Jiang et al. [242] have demonstrated the preparation of p-n heterojunction, that is, SiO2/Ag2O@TiO2 by sol-gel method. It is investigated as a catalyst for the conversion of 4-NP into 4-aminophenol

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Figure 12.15 (A) UV-vis spectra for the reduction of Cr(VI) to Cr(III) by using Pd@SiO2-NH2. (B) UV-vis spectra of Cr(VI) recorded in the presence of formic acid at room temperature catalyzed by Pt NCs/g-C3N4. (A) Reprinted from M. Celebi, M. Yurderi, A. Bulut, M. Kaya, M. Zahmakiran, Appl. Catal. B: Environ. 180 (2016) 53
64 with permission from Elsevier. (B) Reprinted from J.-H. Wu, F.-Q. Shao, S.-Y. Han, S. Bai, J.-J. Feng, Z. Li, et al., J. Colloid Interface Sci. 535 (2019) 41
49 with permission from Elsevier.

(4-AP) at room temperature. Similarly, Ansari et al. [254] have reported the use of CdS-TiO2/Pd nanocomposite for the reduction of various nitroaromatic compounds in a very short span of time at room temperature (Fig. 12.16). Naeem et al. [255] have proposed the mechanism for the reduction of 2-nitroaniline via the Langmuir Hinshelwood process, which occurs on the surface of gold nanoparticles embedded in graphene oxide sheets (Fig. 12.17). In conclusion, the use of MC nanocomposites has several advantages over MCs such as reduced band gap, slow charge transfer carrier recombination, and enhanced catalytic activity. There is the possibility of doping bare MCs with various other constituents such as carbon-based materials, noble metals to give enhanced properties.

12.4.4 Supercapacitors Electrochemical capacitors, also known as supercapacitors, have attracted researchers’ interest in recent years, owing to their high power density, quick charging and discharging, extended lifespans, wide thermal operating range, and high safety [54,256]. A material should possess a high specific area and show good electrochemical activity in order to be a good electrode material for supercapacitors. Therefore, MCs are subjected to a variety of modifications in order to increase their electrochemical activity and specific surface area (SSA) for use in highperformance supercapacitors [54]. Tang et al. used a solvothermal method to make a CoS2/rGO nanocomposite, which showed a higher specific capacitance (331 F/g) and cyclic stability (97% for 2000 cycles) compared to pristine CoS2 (197 F/g, 90%) [257]. Azad et al. have developed NiS2-coated MWCNTs and investigated the influence of MWCNT concentration on their electrochemical behavior. NiS2/MWCNT,

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241

Figure 12.16 UV-Vis spectra of catalytic reduction of nitroaromatic derivatives such as 2-NP (A), 4-NP (B), 2-NA (C), 4-NA (D), and ONPDA (E), using ternary CdS 2 TiO2/Pd heterogeneous nanocatalyst. Reprinted from A. Ansari, R.A. Badhe, S.S. Garje, ACS Omega 4 (12) (2019) 14937
14946 with permission from American Chemical Society.

which contains 20% MWCNT, has a strong capacitive response, with a specific capacitance of 2054.28 F/g at 2 A/g [258]. NiCo2S4 anchored carbon fibers have been reported by Ma et al. with a superior specific capacitance of 1169 F/g at 1 A/g (Fig. 12.18) [259]. Ansari et al. have constructed MoS2/g-C3N4 heterostructures with a

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Figure 12.17 Langmuir Hinshelwood mechanism for reduction of 2-nitroaniline. Reprinted from H. Naeem, M. Ajmal, S. Muntha, J. Ambreen, M. Siddiq, RSC Adv. 8 (2018) 3599
3610 with permission from Royal Chemical Society.

specific capacitance of 240.85 F/g [260]. Li et al. have prepared MoS2/rGO@PANI heterostructures and got the ultrahigh specific capacitance of 1224 F/g at 1 A/g [261]. Ansari et al. [262] have synthesized NiFeS2, which has an excellent specific capacitance of 1051 F/g at 1 A/g. Recent advancement in the energy storage sectors is the use of transition metal carbides, nitrides, and phosphides owing to their intriguing properties such as high electronic conductivity, SSA, and chemical stability. Ni2P/ NiS2 hollow sphere composite has been fabricated by Gou et al. [263]. Zhao et al. have synthesized a 2D-Ni3S2/d-Ti3C2 heterostructure in which the superior specific capacitance of 2204 F/g at 1 A/g has been achieved [264].

12.4.5 Lithium-ion batteries Lithium-ion batteries (LIBs) are currently the most widely used devices to power portable electronics and electric vehicles because of their high energy densities and long life spans [265,266]. LiCoO2 (LCO) introduced by Goodenough [267] is the first and the most commercially successful form of layered transition metal oxide cathodes. LCO cathodes are expensive because of the high cost of Co [268,269]. Continuous research efforts on developing cathode material less expensive than LCO resulted in the formulation of the Li(Ni0.5Mn0.5)O2 (NMO) cathode. NMO could be an attractive material because it can maintain a similar energy density as that of LCO while reducing cost by using lower-cost transition metals [54,270]. MCs have been considered promising electrode materials for LIBs because of their high lithium storage capacity and long life cycle [271,272]. For example,

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243

Figure 12.18 (A) Galvanostatic charge-discharge curves of NiCo2S4/CF electrode at different current densities. (B) Nyquist plots. (C) Specific capacity of NiCo2S4/CF, NiCo2S4 and NiCo2O4/CF at various current densities. (D) The cycling performance of NiCo2S4/CF. Reprinted from Z. Ma, Z. Sun, H. Jiang, F. Li, Q. Wang, F. Qu, Appl. Surf. Sci. 533 (2020) 147521 with permission from Elsevier.

Cu2S [273], Na/Cu2S [274], NiS2 [275], Ni3S2 [276], TiS2 [277], VS2 [278] and others have all been investigated as cathode materials. Combining MCs with other popular LIB materials might result in more successful electrode materials for LIBs, as proven by several typical examples, including In2S3/graphene [279], FeS/C [280], MoS2/graphene [281], MoS2/CNTs [282], MoS2/amorphous carbon [283], SnS2/graphene [284], SnS2/SiO2 [285], CoS2/C [286] and so on. Recently, 2D titanium carbide (Ti3C2), termed as MXene (Mn11XnTx) due to its graphene-like morphology, discovered in 2011 from ceramic phase MAX (Mn11AXn), is being explored. It has a high electrical conductivity that enhances rate performance and abundant surface areas that facilitates ions diffusion for redox reaction [287]. The common challenges for MCs include severe volume expansion and insufficient charge mobility, leading to inferior rate capability and cycling span [288]. Researchers have reported composites containing MC and MXene to obtain enhanced properties due to synergistic effects. For example, Li3VO4/Ti3C2Tx having a fast charging feature at a high rate has been prepared using sol-gel method by Huang and coworkers [289]. Wu et al. [290] have reported a hierarchical

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MoS2/Ti3C2-MXene@C hybrids with ultra-high specific capacity of 580 mAh/g at 20 A/g and 95% cycling retention after 3000 cycles. Tang et al. [291] have reported the Ti3C2@C@SnS electrode, which shows remarkably high reversible capacity of 1473 and 640 mAh/g at 0.1 and 5.0 A/g, respectively. It also has excellent cycling performance with 1050 mAh/g at 1 A/g over 350 cycles (Fig. 12.19).

Figure 12.19 (A) CV curves of Ti3C2 MXene@C@SnS electrode at scan rate of 0.1 mV/s. (B) The first three galvanostatic charge and discharge curves of Ti3C2 MXene@C@SnS electrode at a current of 0.1 A/g. (C) Rate performance of Ti3C2 MXene@C@SnS and C@SnS electrodes at different current densities. (D) Charge and discharge curves of Ti3C2 MXene@C@SnS electrodes at different current densities. (E) Long-term cycling stability of Ti3C2 MXene@C@SnS electrode at 1 A/g. (F) Mechanism diagram of electrolyte transfer during charging and discharging of Ti3C2 MXene@C@SnS electrode. Reprinted from H. Tang, R. Guo, M. Jiang, Y. Zhang, X. Lai, C. Cui, et al., J. Power Sources 462 (2020) 228152 with permission from Elsevier.

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The aforementioned analysis indicates that transition metal (Fe, Co, Mo) chalcogenides coupled with MXene shows superior electrode properties. The MXene sheets can ultimately reduce the negative effects such as large volume expansion, poor reaction kinetic, and agglomeration of MCs. Thus, these kinds of MC composites possess high potential as electrode materials in LIBs [292].

12.4.6 Water splitting MC-based electrocatalysts for electrochemical energy conversion have attracted significant attention during the last several years owing to their high activity, low cost, and facile tunability. Among these, electrocatalytic water splitting, which includes hydrogen evolution and oxygen evolution reactions (HER and OER, respectively), has been the focus of many research groups [54,293,294]. The challenge for practical implementation of water splitting technology, however, depends on identification of suitable and stable catalysts, which can lower reaction energy barrier and increase Faraday efficiency for both reactions. Traditionally, Pt metal is most active as HER catalyst, whereas oxides of iridium (Ir) and ruthenium (Ru) exhibit higher OER performance. However, high price and low stability of these precious metal-based catalysts make commercialization difficult [54]. Umanga et al. [295] have compared the electrochemical activity of Ni3Te2 with Ni3E2 (E 5 S, Se, Te). Further, NiOx.Ni3Te2 composite showed an overpotential of 180 and 212 mV at 10 mA/cm2 for OER and HER, respectively, exhibiting one of the lowest overpotentials for OER. This study further confirmed that a decrease in electronegativity of the chalcogenide anions [Te (2.1) vs Se (2.55), O (3.44)] lead to increased catalytic efficiency (Fig. 12.20) [295]. Fig. 12.20A
G represents the effect of change in chalcogenide anions on OER and HER activity for Ni-based material, which

Figure 12.20 OER and HER polarization curves of (A, D) Nickel sulfide, (B, E) Nickel selenide. (C) Comparison of OER polarization curves for NiOX, Ni3S2, Ni3Se2, and Ni3Te2 in 1.0 M KOH depicting effect of anion electronegativity on OER activity. Reprinted from U. De Silva, J. Masud, N. Zhang, Y. Hong, W.P.R. Liyanage, M. Asle Zaeem, et al., J. Mater. Chem. A 6 (2018) 7608
7622 with permission from Royal Chemical Society.

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shows an order of Ni3Te2 . Ni3Se2 . Ni3S2 toward the OER and NiSe . Ni3S2 . Ni3Te2 toward the HER. Cao et al. [294] have synthesized a bimetallic spinel structure CuCo2Se4 electrocatalyst that requires a low overpotential of 320 mV to obtain a current density of 50 mA/cm2 for OER and 125 mV to achieve a current density of 10 mA/cm2 for HER. Wan et al. have reported that the p-CoSe2/CC can serve as an efficient bifunctional electrocatalyst for both OER and HER in alkaline electrolyte, with a current density of 10 mA/cm2 at an overpotential of 243 mV for OER and 138 mV for HER, and Tafel slope of 82 and 83 mV/dec for OER and HER, respectively [296]. Although MXene materials are proven to be promising materials for some important applications, they have low electrocatalytic performances owing to their compositions and surface terminations [297
299]. Therefore, a new area has emerged wherein a composition of MXene with MCs is being explored to enhance their electrocatalytic properties. Tie et al. [300] have prepared Ti3C2Tx/Ni3S2/NF via one-pot hydrothermal method. This self-supported architecture possesses an impressive overpotential (72 mV) at 10 mA/cm2 and a Tafel slope of 45 mV/dec. MoS2 crystallites could be formed in situ on the surface of MXene roll. Thus, a novel hybrid material MoS2/Ti3C2-Tx with 0.152 V overpotential at 10 mA/cm2 and 70 mV/dec for Tafel slope [301] has been reported. The use of MXenes coupled with MCs can promote reaction kinetic in HER and OER, due to high conductive surface areas in the former. This research area is still in primitive stage and such kinds of MC nanocomposites have a profound future in energy storage and conversion applications [292].

12.4.7 CO2 activation The consumption of fossil fuels and rapid industrialization have resulted in emission of large amount of carbon dioxide (CO2) and air pollution [302
306]. Carbon dioxide is a major greenhouse gas and it has the most significant impact on the environment [307
309]. Converting CO2 into useful chemicals and fuels (e.g., carbon monoxide, methane, formic acid, formaldehyde, and methanol) using the appropriate method to minimize the harmful effects on environmental needs to be given top priority along with a reduction in CO2 emission [310
312]. However, CO2 has a dissociation energy of 750 kJ/mol and it is a thermodynamically very stable molecule. However, it is possible to convert CO2 to other products using a variety of methods, including direct or catalyzed chemical conversion, thermochemical conversion, electrochemical conversion, and photochemical conversion [313
315]. A photocatalyst-based method is the simplest and most widely used for the reduction of CO2 to fuels as the reaction conditions are quite mild (low temperature and atmospheric pressure) [315]. Park et al. have reported mesopores and macropores Ga2O3 for the conversion of CO2 to CH4 [316]. Compared to the standard β-Ga2O3 nanoparticles, highly porous Ga2O3 produces more CH4 due to its high surface area. Tsuneoka et al. [317] have proposed a Langmuir-Hinshelwood-type mechanism for the photocatalytic reduction of CO2 over Ga2O3 in the presence of H2 (Fig. 12.21). The further increase in CO2 adsorption and enhanced photocatalytic conversion of MC nanocomposites doped with noble metals, carbon-based

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247

Figure 12.21 Schematic mechanism of photocatalytic conversion of CO2 over Ga2O3 in the presence of H2. Reprinted from H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. C. 114 (19) (2010) 8892
8898 with permission from American Chemical Society.

materials, etc. can be used [318]. Wang et al. [319] have reported CO2 reduction reaction with water using CeO2-TiO2 wherein conversion efficiency of 46.6 μmol/g for CH4 and 30.21 μmol/g for CO is observed. Li et al. [320] have reported hydrothermal synthesis of Bi2S3, CdS, and Bi2S3/CdS using their corresponding salts and thiourea. Bi2S3/CdS shows excellent photo absorption of CO2 and produces higher methanol yield over Bi2S3 than CdS, due to large surface area and high pore diameter (Fig. 12.22). Zhang et al. [321] have reported the synthesis of zinc oxide/ reduced graphene oxide nanocomposites (ZnO-rGO), which has good activity in photoreduction of CO2 into CH3OH. It has five times higher activity as compared to pure ZnO because the rGO has a greater surface area providing more active sites to facilitate CO2 adsorption and reduction. Tan et al. [322] have reported the use of rGO-TiO2 as a catalyst wherein they could obtain 0.135 μmol/g-cat/h CH4 by CO2 reduction.

12.5

Future prospects of chalcogenides

Materials based on chalcogenides such as sulfides, selenides, and tellurides are abundant in nature. Their derivatives such as binary, ternary, and quaternary chalcogenide materials are useful in many areas such as photovoltaics, photocatalysts, fuel

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Figure 12.22 The yields of CH3OH in the photocatalytic reduction of CO2 with H2O over various photocatalysts under visible light irradiation. Reprinted from X. Li, J. Chen, H. Li, J. Li, Y. Xu, Y. Liu, et al., J. Nat. Gas. Chem. 20 (2011) 413
417 with permission from Elsevier.

cells, supercapacitors, and batteries. Chalcogenides can be prepared by many methods such as hydrothermal, solvothermal, microwave-assisted, sonochemical, electrospinning, and photochemical methods. The morphology of chalcogenide materials is also affected by the composition of raw materials, methods of synthesis, and treatment. Sulfide, selenide, and telluride-based chalcogenides have their own unique characteristics, structure, and physical as well as chemical properties making them one of the most studied nanostructures. One of today’s most scary issues, water contamination is mostly the outcome of fast industrialization, economic expansion, and even population growth. As a result, the role of chalcogenides in wastewater treatment is very crucial to improve water quality. One of the finest techniques for degradation of toxic organic contaminants from wastewater is photocatalysis using MCs. Further, MCs are useful to convert highly toxic metal ions in the water into low toxic metal ions. The advantages of photocatalytic technology include high efficiency, cheap cost, avoiding secondary pollutants, and the direct utilization of solar energy. Future development of MC photocatalysts lies in tuning their band gap for efficient absorption of visible light and hence the increased catalytic activity. Enhancing charge carrier transfer and preventing recombination of electron-hole pairs by preparing suitable MC for the increased photocatalytic performance of MC nanomaterials for their use in hydrogen generation by water splitting under visible light are very important aspects, and they will play a key role in the production of clean energy in future.

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The global photovoltaic industry is now expanding quickly, setting a record in 2019 with 176 GW according to the International Renewable Energy Agency. The majority of photovoltaic thin films are prepared using copper indium gallium sulfoselenide and cadmium telluride, and they account for less than 5% of manufacturing. However, nearly 95% of all solar cells are produced using silicon wafers. Although thin-film technologies show potential, their standing is gradually declining because of the cheap cost production of competing technologies. Due to the narrow band gap and high absorption coefficient values, binary, ternary, and quaternary MC semiconductor materials are receiving increasing attention. For example, thin films of CuInSe2 [323], CuInGaSe2 [324] CdZnS [325], CdS/CdTe [326], and CdTe [327] have been utilized for this purpose. Due to Faradic charge transfer process, MC nanocomposites are finding increasing applications in energy storage devices such as supercapacitors as well as batteries and fuel cells. Because of high thermal stability, strong optical absorption, electron conductivity, and low recombination rate, MC nanocomposites have become viable materials for the photocatalytic reduction of CO2. Thus, a systematic and strategic approach for development of MC nanocomposites with tuned properties has tremendous scope for various applications in the future.

12.6

Conclusion

The chalcogens can readily combine with various metals resulting in binary, ternary, and quaternary MCs. MCs have diverse structures and properties making them useful for a large number of applications. The nanodimensional MCs have applications in photovoltaics, sensors, fuel cells, water splitting, photocatalysis, nanocatalysis, and as battery and electrode materials in energy storage devices. Further, nanocomposites of these materials wherein bare MCs doped with various dopants enhance their properties due to synergistic effect. The SSA, size, and phase purity of the MC and its nanocomposites are important considerations. More active sites may be available in higher specific areas. Porous MCs, MC thin films, single-layer or multilayer MCs, and amorphous MCs have drawn a lot of attention. The composition of raw materials, synthesis processes, and processing methods all have an impact on the morphology of chalcogenides and their nanocomposites. It is crucial to control growth of MC nanocrystals by choosing the right crystal size. Smaller sizes frequently suffer from faster crystalline structure collapse and reduced stability, whereas larger crystal sizes will result in smaller SSAs and hence decreased activity. The MC nanocomposites with band gap engineering and high surface area are crucial for future applications. Therefore, in the future, apart from conventional synthetic methods such as hot-injection, hydrothermal, solvothermal, microwave, electrospinning, sonochemical, and photochemical methods, modified synthetic methods need to be developed. A large variety of novel MC nanocomposites that not only incorporate the characteristics of the individual MC but also add new collective and synchronic functionalities might be accessed with the use of modified approaches.

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Metal-Chalcogenide Nanocomposites

Though, recent development has demonstrated successful use of MC nanocomposites containing carbon-based materials, noble metals, non-noble metals, and metal oxides, a wide variety of alternative MC-based nanocomposites needs to be developed for novel applications. In view of this, the MC nanocomposites containing rare earth metals, metal-organic frameworks, nitrogen and phosphorous codoped nanospheres, carbon dots, and onions, surface-modified nanocomposites, and layered MCs need to be explored further.

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Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Acoustic gas sensor, 3 Activated carbons (AC), 4 Adsorption methods, 119
122 Advanced oxidation processes (AOPs), 95 Amorphous silicon, 10
11, 208
209 Aqueous organic pollutants, photodegradation of, 29
30 Arsenic (As), 66
67 Atomic Cobalt Species (ACS), 37
38 Atomic force microscope (AFM), 191
192 Atrazine (ATZ), 106
108 Azo dyes, 103
104 B Beta-lactam antibiotics, 66
67 Binary chalcogenides, 222
224 C Cadmium-based chalcogenides, 50 Cadmium oxide (CdO), 15
16 Cadmium telluride (CdTe), 10
11, 13, 14t, 208
209 Carbon aerogels, 4 Carbon nanotubes (CNTs), 4
6, 233 Chalcogenide(s) cobalt based, 139
144 CO2 reduction using, 37
38 in diverse applications, 153
154 future perspectives, 40 heavy metal removals, 38
40 issues of, 30 nanocomposites, 29 overview of, 29 photocatalytic activities cadmium, 50 copper, 49 molybdenum disulfide, 48

tin, 50
51 titanium, 51 vanadium, 50 zinc, 49 pollutants, 30 preparation methods of, 47t selenide, 34
35 sulfides, 32
33 synthesis approaches of, 47
48 tellurides, 35
37 transition metal hydrothermal method, 30 microemulsion method, 31 sol-gel method, 32 solvothermal method, 31
32 sonication method, 31 Chalcogen ions, 222
224 Chemical vapor deposition (CVD) method, 47
48 Chemo-resistor gas sensor.Nanomaterial gas sensor Chlorpyrifos (CP), 106
108 Cobalt selenides, 139
144 Conducting polymers (CP), 4 Conduction band (CB), 117
118 Congo red (CR) azo dyes, 91, 91f Copper oxide (CuO) nanoparticles, 15
16 Copper-based chalcogenides, 49 Copper-based selenides, 156
160 Copper indium gallium and selenium (CIGS), 10
13, 208
209 Copper monosulfides (CuS), 192 Copper zinc tin sulfide (CZTS), 208
209 Coprecipitation approach, 62 D Disulfides (Cu2S), 192 Dye-sensitized solar cells (DSSC), 10
12

262

E Electric double layer capacitors (EDLC), 4
6 Electrocatalysts, 95, 135 chalcogenide nanocomposites, 146 iridium based, 145
146 molybdenum based, 137
138 NiSSe nanoporous composites, 146
147 rhenium based, 144
145 ruthenium based, 138
139 tungsten disulfide, 146
147 Electrochemical capacitors, 240
242 Electrochemical energy storage, 153 Electroluminescence, 203
204 Electron microscopy, 191
192 Electron transport layer (ETL), 11 Electrospinning method, 61, 100 Endocrine disruptors, 104
108 External quantum efficiency (EQE), 203
204 F Fenton reaction, 95 G Galvanostatic charge/discharge (GCD), 9
10 H Heavy metals (HMs) decontamination mitigation of, 117
118 photocatalytic heavy metal treatment, 122
126 traditional heavy metal treatment adsorption methods, 119
122 ion exchange methods, 118
119, 127 Heterogeneous photocatalysis, 97
98 Hole transport layer (HTL), 11 Homogeneous photocatalysis, 96
97 Hot-plate method, 99 Hybrid catalysts, 138 Hybrid organic/inorganic solar cells, 210
211 Hybrid photovoltaic cell, 211
212 Hybrid supercapacitors (HSCs), 4 Hydrogen evolution reaction (HER), 137 Hydro/solvothermal method, 100 Hydrothermal approach, 60
61, 228
229 Hydrothermal method, 30

Index

I II
VI metal chalcogenides, 187
188 applications of photocatalysis, 206
208 quantum dot LEDs, 202
206 solar cells, 208
212 chemical synthesis of, 197 nanocrystals in powder form, 197
199 thin films embedded in polymers, 200
202 future goals, 212
213 nanomaterials, properties of, 192
194 electrical and optical, 194
195 physical, 196
197 thermal, 195
196 structure and chemical properties of, 189
192 types of, 189f zinc and cadmium-based, 208 Industrial effluent treatment, metal chalcogenides dye removal, treatment methods for, 84f role of, 83
91 Ion exchange methods, 118
119, 127 Iridium, 145
146 Iron-cobalt sulfide, 143 K Kentucky
Levich plots, 141 L Light emitting diode (LED), 13, 202
206 Linear loss mechanisms, 196
197 Lithium-ion batteries, 242
245 M Manganese (Mn), 156
157 antibacterial activity, 18 definition of, 1 thin film-based solar cells, 12
15 Metal chalcogenides (MCs) application of carbon dioxide activation, 246
247 environmental remediation, 238 lithium-ion batteries, 242
245 nitrobenzene derivatives, 238
240 photocatalysts, 237
238 supercapacitors, 240
242 water splitting, 245
246

Index

classification of chalcogen ions, 222
224 elements, 222
224 composites with chalcogen anion, 46
47 earth-abundant, 176
177 future goals, 247
249 irradiation of, 86
87 in industrial effluent treatment, 83
91 and nanocomposites, 169 nanocomposites, preparation of, 233
236 overview of, 222 in photocatalyst applications, 46f quantum dots application, diagram of, 89f synthesis of, 227
233 water splitting method, 45
47 Metal oxides (MO), 4, 235
236 Methylene blue (MB), 83
84 chemical structure of, 85f solar light irradiation, 86 Microemulsion method, 31 Microwave method, 230 Molybdenum (Mo), 48, 136
137, 157
158 N Nanocomposites (NCs), 47t, 118
119 heavy metals decontamination, 118
119 preparation methods, 60
62, 233
236 residual antibiotics removal using, 70
71, 72t synthesis of, 227
233 synthetic dyes removal, 62
64, 72t toxic heavy metal ions removal, 65
70, 72t types of, 187
188, 192
194 Nanomaterial gas sensor, 3 Nanostructure materials, for various gases, 4, 5t Nickel-based chalcogenides, 155
160 NiO/RGO nanocomposite, 88 NiSSe nanoporous composites, 146
147 Nitrobenzene derivatives (NB), 238
240 Nitrogen-doped MoS2/carbon, 137
138 Noble metal-based chalcogenides, 225
226, 235 O One-pot heat-up method, 100 Organic capping agent, 198 Organic-inorganic hybrid perovskite (OIHP), 10
12

263

Organic pollutants, photocatalytic degradation application, 102
103 dyes, 103
104 pesticides and endocrine disruptors, 104
108 pharmaceuticals, 108
111 Oxidation catalytic sites, 96
97 Oxychalcogenide clusters, 118
119 Oxygen reduction reactions (ORRs), 135 chalcogenide materials, catalytic activity of, 135
136 chalcogenides, cobalt based, 139
144 electrocatalysts, 135 chalcogenide nanocomposites, 146 iridium based, 145
146 molybdenum based, 137
138 NiSSe nanoporous composites, 146
147 rhenium based, 144
145 ruthenium based, 138
139 tungsten disulfide, 146
147 overpotentials for various materials in, 137t transition metal chalcogenides, oxygen evolution reactions, 135 Ozonolysis, 95 P Perovskite solar cells, 10
12 Pesticides, 104
108 Pharmaceuticals, 108
111 Photocatalysis, 29
30, 38
40, 118, 206
208, 237
238 cadmium, 50 copper, 49 of dyes, 105t heavy metal treatments, 122
124 hydrogen production, 45
46 molybdenum disulfide, 48 of pesticides, 107t of pharmaceuticals, 110t tin, 50
51 titanium, 51 vanadium, 50 water-splitting process, 45
47 zinc, 49 Photodegradation, 1, 86
88 Photoelectric conversion efficiency (PCE), 167
171

264

Photoluminescence (PL), 202
203 Photosensitizer, 96
97 Photovoltaics (PVs), 167 Pollutants, 30 Polymer cells, 10
11 Proton exchange membrane (PEM) fuel cells, 136
137 Pseudocapacitive materials, 156
157 Q Quantum dots (QDs), 88, 89f Quantum dot-sensitized solar cells (QDSCs), 167
168 counter electrodes, 174
177 description of, 170
174 interface modification layer, 177
180 principle and structure of, 168
169, 168f Quaternary chalcogenides, 224 R Reactive red (RR141) azo dyes, 91, 91f Reduction catalytic sites, 96
97 Refractive indices, 196 Residual antibiotics removal process, 70
71, 72t Rhenium, 136, 138
139 Ru85Se15/C catalyst, 138 Ruthenium, 138
139 S Sandwich structures, 168
169 Selenide (Se), 34
35 Selenium, 154
155 Sensor(s), 2
3 acoustic gas, 3 nanocomposite gas, 4 nanomaterial gas, 3 SILAR method, 7
8, 178
179 Silicon-based materials, 12
13 Solar cell(s), 208
212 organic-inorganic hybrid perovskite, 10
11 thin film-based, 10
12 metal chalcogenide, 12
15 Solar-light irradiation, 86 Sol-gel method, 32 Solvothermal method, 31
32, 45
46, 61, 229
230 Sonication method, 31

Index

Sonochemical electrospinning, 100 Sonochemical method, 231
233 Sonochemistry, 100 Sulfides, 32
33 Supercapacitors (SCs), 153, 240
242 chalcogenides, in applications, 153
154, 160t classification of, 6f electrode materials for, 154
155 copper-based selenides, 156
160 molybdenum-based selenides, 156
160 nickel-based chalcogenides, 155
160 other selenides and composites, 159 tin-based selenides, 158
159 thin film-based, 4
12 perovskite solar cells, 10
12 SUVA parameter, 109 Synthetic dyes removal process, 62
64, 72t T Tellurides, 35
37 Tellurium, 192
194 Ternary chalcogenides, 224 Thin film(s) antibacterial applications of, 15
18 based solar cells, metal chalcogenide, 12
15 based supercapacitor, 4
12 perovskite solar cells, 10
12 deposition techniques classification, 7f photovoltaic cell, 209 Tin-based chalcogenides, 50
51 Tin-based selenides, 158
159 Titanium-based chalcogenides, 51 Toxic heavy metal ions removal process, 65
70, 72t Transition metal chalcogenides (TMCs), 29
30, 59, 98
99. See also Chalcogenide(s) characterizations of, 100
101 future perspectives, 71
75 heterogeneous photocatalysts, 102 methods of hydrothermal, 30 microemulsion, 31 sol-gel, 32 solvothermal, 31
32 sonication, 31 nanocomposites

Index

residual antibiotics removal using, 70
71, 72t synthetic dyes removal using, 62
64, 72t toxic heavy metal ions removal using, 65
70, 72t oxygen evolution reactions, 135 preparation methods, 60
62 supercapacitors, 155 synthesis methodologies, 99 electrospinning, 100 hot-plate method, 99 hydro/solvothermal method, 100 one-pot heat-up method, 100 sonochemical, 100 Tungsten disulfide, 146
147

265

V Valence band (VB), 117
118 Vanadium-based chalcogenides, 50 W Wastewater treatment (WWT) methods, 95 heterogeneous photocatalysis, 97
98 homogeneous photocatalysis, 96
97 Water purification systems, 59, 83 Water splitting, 45
47, 245
246 Z Zinc sulfide (ZnS) nanocrystals, 67 nanoparticles, 106
108 ZnO nanoparticles, 15
18